We report the observation of room-temperature optical gain at 1.3 μm in electrically driven dilute nitride vertical cavity semiconductor optical amplifiers. The gain is calculated with respect to injected power for samples with and without a confinement aperture. At lower injected powers, a gain of almost 10 dB is observed in both samples. At injection powers over 5 nW, the gain is observed to decrease. For nearly all investigated power levels, the sample with confinement aperture gives slightly higher gain

A Aho

A Erol

A Forchel

A Mircea

AH Clark

B Bonnefont

B Royall

D Alexandropoulos

E Luna

ES Björlin

FAI Chaqmaqchee

Gabor Mezosi

J Hashimoto

JM Hopkins

K Schires

M Guina

M Kondow

Marc Sorel

Michael John Adams

Mircea Guina

N Balkan

N Bouché

N Laurand

N Suzuki

Naci Balkan

Nadir Ali Khan

S Calvez

Sefer Bora Lisesivdin

Simone Mazzucato

T Jouhti

Ville-Markus Korpijärvi

Nanoscale Research Letters

PubMed

Lisesivdin, Sefer Bora

Khan, Nadir Ali

Mazzucato, Simone

Balkan, Naci

Adams, Michael John

Korpijärvi, Ville-Markus

Guina, Mircea

Mezosi, Gabor

Sorel, Marc

Enlighten: Publications

  
 
 
Lisesivdin, S. B., Khan, N. A., Mazzucato, S., Balkan, N., Adams, M. J., 
Korpijärvi, V.-M., Guina, M., Mezosi, G., and Sorel, M. (2014)Optical gain 
in 1.3-μm electrically driven dilute nitride VCSOAs. Nanoscale Research 
Letters, 9 (1). p. 22. ISSN 1556-276X 
 
Copyright © 2014 The Authors 
 
 
 
http://eprints.gla.ac.uk/96239/ 
 
 
 
 
Deposited on: 20 August 2014 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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Lisesivdin et al. Nanoscale Research Letters 2014, 9:22
http://www.nanoscalereslett.com/content/9/1/22NANO EXPRESS Open AccessOptical gain in 1.3-μm electrically driven dilute
nitride VCSOAs
Sefer Bora Lisesivdin1,2*, Nadir Ali Khan1,5, Simone Mazzucato1, Naci Balkan1, Michael John Adams1,
Ville-Markus Korpijärvi3, Mircea Guina3, Gabor Mezosi4 and Marc Sorel4Abstract
We report the observation of room-temperature optical gain at 1.3 μm in electrically driven dilute nitride vertical
cavity semiconductor optical amplifiers. The gain is calculated with respect to injected power for samples with and
without a confinement aperture. At lower injected powers, a gain of almost 10 dB is observed in both samples. At
injection powers over 5 nW, the gain is observed to decrease. For nearly all investigated power levels, the sample
with confinement aperture gives slightly higher gain.
Keywords: Dilute nitride; Optical injection; Gain; Vertical cavity semiconductor optical amplifiersBackground
InxGa1−xAs1−yNy semiconductor alloy was first proposed
by Kondow et al. in 1996 [1], and considerable research at-
tention has been devoted to this alloy system due to its
possible optoelectronic applications at an operating wave-
length of 1.3 μm. With the addition of small amounts of
nitrogen into the (In)GaAs lattice, a strong electron con-
finement and bandgap reduction are obtained. Further-
more, addition of N allows band engineering, allowing the
device operating wavelength range to extend up to 1.6 μm
[2]. An extensive set of different devices based on this
alloy has been fabricated and demonstrated [3]. Examples
of these devices are vertical cavity surface-emitting lasers
(VCSELs) [4-6], vertical external cavity surface-emitting
lasers [7,8], solar cells [8,9], edge-emitting lasers [10], pho-
todetectors [11], semiconductor optical amplifiers (SOAs)
[12], and vertical cavity semiconductor optical amplifiers
(VCSOAs) [13,14].
VCSOAs can be seen as the natural evolution of SOAs,
which, owing to their fast response, reduced size, and low-
threshold nonlinear behavior, are popular in applications
such as optical routing, signal regeneration, and wave-
length shifting. Within these fields, VCSOAs have been* Correspondence: sblisesivdin@gmail.com
1School of Computer Science and Electronic Engineering, University of Essex,
Wivenhoe Park CO4 3SQ, UK
2Department of Physics, Faculty of Science and Arts, Gazi University,
Teknikokullar, Ankara 06500, Turkey
Full list of author information is available at the end of the article
© 2014 Lisesivdin et al.; licensee Springer. This
Attribution License (http://creativecommons.or
in any medium, provided the original work is pused as optical preamplifiers, switches, and interconnects
[15-17]. Their geometry provides numerous advantages
over the edge-emitting counterpart SOAs, including
low noise figure, circular emission, polarization insensi-
tivity, possibility to build high-density two-dimensional
arrays of devices that are easy to test on wafer, and low-
power consumption that is instrumental for high-
density photonic integrated circuits. Generally speaking,
a VCSOA is a modified version of a VCSEL that is driven
below lasing threshold. The first experimental study of an
InxGa1−xAs1−yNy/GaAs-based VCSOA was reported in
2002 [18], with a theoretical analysis published in 2004
[19]. Several studies on optically pumped InxGa1−xAs1−yNy
VCSOAs have been published [14,20-23], while electrically
driven VCSOAs have been demonstrated only in ‘Hellish’
configuration [24]. The present contribution builds on
these technological developments to focus on an electric-
ally driven multifunction standard VCSOA device operat-
ing in the 1.3-μm wavelength window.Methods
The amplification properties of InxGa1−xAs1−yNy VCSOAs
were studied using a 1,265- to 1,345-nm tunable laser (TL;
TLM-8700-H-O, Newport Corporation, Irvine, CA, USA),
whose output was sent to the sample using the setup
shown in Figure 1a. The TL signal was split via a 10/90is an Open Access article distributed under the terms of the Creative Commons
g/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction
roperly cited.
Figure 1 Experimental setup (a) and the layer structure of the investigated samples (b).
Lisesivdin et al. Nanoscale Research Letters 2014, 9:22 Page 2 of 5
http://www.nanoscalereslett.com/content/9/1/22coupler to a power meter and to the sample, respectively.
Back reflections were avoided using an optical isolator
while the TL power was changed from 0 to 7 mW using
an optical attenuator. A lens-ended fiber (SMF-28 fiber,
conical lens with cone angle of 80° to 90° and radius of
6.0 ±1.0 μm) was used to focus the TL light to the sample
surface as well as to collect its reflected/emitted/amplified
light, which was then directed to an optical spectrum
analyzer (OSA). The VCSOA was electrically DC biased
up to 10 mA and stabilized in temperature at 20°C via a
Peltier cooler.
The investigated VCSOA structure with a 3.5λ cavity
is shown in Figure 1b. The structures were grown by a
solid source molecular beam epitaxy reactor with a1100 1200 130
Waveleng
R
efl
ec
tiv
ity
0
0.2
0.4
0.6
0.8
1
1
2
Figure 2 Room temperature photoluminescence (red) and reflectance
reflectivity spectra of the studied VCSOA structure are shown in black andradio frequency plasma source for incorporating nitro-
gen. The growth was carried on an n-type GaAs(100)
substrate, and the bottom and top distributed Bragg
reflectors (DBRs) were doped with silicon (n-type)
and beryllium (p-type), respectively. The two DBRs
comprised 21 and 24 pairs of AlxGa1−xAs/GaAs layers
for the top and bottom DBR, respectively. The Al con-
centrations were x = 0.8 and 0.98 in the top and bottom
DBRs, respectively. The confinement aperture, which is
required for better carrier and light confinement, was
defined in the uppermost layer of the bottom DBR. The
active region contains three stacks of three 7-nm-thick
In0.35Ga0.65As0.975 N0.025 quantum wells separated by
20-nm thick GaAs spacers. A set of several VCSOA0 1400 1500
th (nm)
PL Intensity (a.u.)
spectra of the studied structure. Experimental and simulated
blue lines, respectively.
1100 1150 1200 1250 1300 1350
90
85
80
75
70
Wavelength (nm)
Po
w
er
 (d
Bm
)
0mA
1mA
2mA
4mA 
6mA 
8mA 
10mA
Figure 3 Power spectra of VCSOA without confinement
aperture obtained for different bias currents.
Lisesivdin et al. Nanoscale Research Letters 2014, 9:22 Page 3 of 5
http://www.nanoscalereslett.com/content/9/1/22samples was fabricated, having different dimensions of
the top DBR mirror radius (R1), confinement aperture
radius (R2), and bottom DBR radius (R3) for cases with
and without the confinement aperture. In this paper, we
compare the results obtained for two samples with and
without confinement aperture, with R1 = 5 μm, R2 =
25 μm, and R3 = 50 μm.
Results and discussion
Room-temperature reflectivity and photoluminescence
(PL) measurements were performed on the as-grown
sample, and the results are shown in Figure 2. Simulated1240 1260 1280 1300 1320 134
1.0 10 12
0.0 100
1.0 10 12
2.0 10 12
3.0 10 12
4.0 10 12
5.0 10 12 ASE-ASE0
1240 1260 1280 1300 1320 134
1.0 10 12
0.0 100
1.0 10 12
2.0 10 12
3.0 10 12
4.0 10 12
Wavele
Po
w
er
 (W
)
AOS + ASE - ASE0
)a(
)c(
Figure 4 Results of various power spectra for λinj = 1,279 nm. (a) ASE −
power spectra.reflection is also shown in the figure. Two resonances
λR1 and λR2 are observed within the DBR stop band as a
result of the relatively long cavity length [25]. The
principle resonance, which is designed for 1.3-μm oper-
ation, is observed at λR1 = 1,282 nm, while the other un-
wanted resonance at lower wavelength is observed at
λR2 = 1,235 nm. Figure 3 shows the VCSOA amplified
spontaneous emission (ASE) spectra obtained with no
optical injection at different applied bias currents of 0 to
10 mA for the sample without confinement aperture.
The highest ASE power peak appears at 1,288 nm and is
blue-shifted with respect to that of the lasing cavity
mode wavelength [26,27]. The other modes are also con-
sistent with the PL spectra. Figure 3 shows that with in-
creasing the bias current, the amplitude of each mode
increases and also slightly shifts towards higher wave-
lengths. This shift is associated with local temperature
increase in the device. A similar result was observed in
the VCSOA with the confinement aperture.
Since no significant change in the spectrum amplitude
above 7 mA was observed, we investigated the devices
up to this current value. ASE power spectra taken at 7
and 0 mA (ASE0) are used to extract the spectrum
ASE −ASE0, which is the spectrum where the background
noise is minimized. To find the amplified optical signal
(AOS), we injected light sweeping the TL wavelength
(λinj) from 1,266 to 1,310 nm with a 7-mA current bias.
Figure 4 shows results for injection at λinj =1,279 nm
only. We could not investigate the second resonance
peak λR2 because of the wavelength limit of the TL. In0 1240 1260 1280 1300 1320 1340
1.0 10 12
2.0 10 12
3.0 10 12
4.0 10 12
5.0 10 12
6.0 10 12
AOS + ASE
0
ngth (nm)
1240 1260 1280 1300 1320 1340
0.0 100
1.0 10 12
2.0 10 12
3.0 10 12
4.0 10 12
AOS - ASE0
)b(
)d(
ASE0level, (b) AOS + ASE, (c) AOS + ASE − ASE0, and (d) AOS − ASE0
018642
0
5
10
15
Power (nW)
G
ai
n 
(dB
)
Without confinement aperture
With confinement aperture
Figure 6 Power-dependent gain for the samples with and
without confinement aperture.
Lisesivdin et al. Nanoscale Research Letters 2014, 9:22 Page 4 of 5
http://www.nanoscalereslett.com/content/9/1/22Figure 4a,b,c,d, the results for ASE −ASE0, AOS + ASE,
AOS + ASE −ASE0, and finally AOS −ASE0spectra are
shown, respectively.
As the gain is small, the amplified signal cannot be
easily discerned in Figure 4d. Hence, the gain was calcu-
lated using the simple relation
Gain ¼ AOSþ ASE‐ASE0
AOS‐ASE0
ð1Þ
for each wavelength after obtaining AOS and ASE data.
Results are shown as a function of the injected wavelength
in Figure 5 for a specific laser power (Pinj) of 2.25 nW. A
maximum gain of 3 dB with a very broad peak is observed
at the maximum ASE wavelength of 1,288.5 nm. In the
study, measured signal levels are very near to limits of the
OSA; therefore, larger bandwidth wavelength values are
used, which can be the reason of the broadness of the gain
peak.
Having verified that the gain peak corresponds to the
ASE peak wavelength, we investigated the Pinj depend-
ence by varying it from 1.5 nW to a few milliwatts for
the single wavelength of 1,288.5 nm. Results are pre-
sented for both samples with and without confinement
aperture in Figure 6 for power values below 10 nW.
For injected laser powers over 5 nW, the gain falls rap-
idly. At the lowest injected power, the sample with
confinement aperture exhibits 10 dB of gain, which is
observed near the maximum ASE wavelength. For the
investigated injected power range, the sample with the
confinement aperture showed a higher gain because of
the better carrier and light confinement in the VCSOA.
Conclusions
In this paper, we report the observation of gain in an
electrically driven dilute nitride VCSOA device operated1260 1270 1280 1290 1300 1310
0
0.5
1
1.5
2
2.5
3
3.5
Wavelength (nm)
G
ai
n 
(dB
)
Figure 5 Gain versus injected laser wavelength with Pinj = 2.25 nW.at 1.3-μm in reflection mode. Two different types of sam-
ples with and without confinement aperture are investi-
gated. The ASE power peak is found to be at 1,288.5 nm
with additional modes, which are caused by the length of
the cavity. Optical gain is found to occur at low optical in-
jection values. Above 5 nW of optical injection, the gain is
found to fall rapidly. The maximum observed optical gain
is observed at 1,288.5 nm at room temperature. The max-
imum observed optical gain at 7-mA current at room
temperature is around 10 and 6 dB for samples with and
without confinement aperture, respectively. It is important
to mention that despite the small gain, the device is very
promising because it requires very small currents com-
pared with in-plane SOAs.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
SBL, NAK, and SM carried out the measurements and data analysis. VMK and
MG performed the growth of structures. GM and MS carried the fabrication
of devices. MJA performed the theoretical studies and analysis. NB is the
project leader. SBL and NB wrote the paper. All authors read, corrected, and
approved the final manuscript.
Acknowledgements
Support from EPSRC under grant EP/G023972/1 is gratefully acknowledged.
Sefer Bora Lisesivdin also acknowledges partial support from the Turkish
Scientific and Technological Research Council (TUBITAK) 2219 coded
scholarship. COST Action MP0805 is also gratefully acknowledged.
Author details
1School of Computer Science and Electronic Engineering, University of Essex,
Wivenhoe Park CO4 3SQ, UK. 2Department of Physics, Faculty of Science and
Arts, Gazi University, Teknikokullar, Ankara 06500, Turkey. 3Optoelectronics
Research Centre (ORC), Tampere University of Technology, P.O. Box 692,
Tampere 33101, Finland. 4School of Engineering, University of Glasgow,
Glasgow GG12 8QQ, UK. 5Kohat University of Science and Technology
(KUST), Kohat, Khyber Pakhtoonkhwa, Pakistan.
Received: 17 October 2013 Accepted: 17 December 2013
Published: 13 January 2014
Lisesivdin et al. Nanoscale Research Letters 2014, 9:22 Page 5 of 5
http://www.nanoscalereslett.com/content/9/1/22References
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English

Optical gain in 1.3-μm electrically driven dilute nitride VCSOAs

Sefer Lisesivdin

Nadir Khan

Michael Adams

Springer - Publisher Connector

Lisesivdin et al. Nanoscale Research Letters 2014, 9:22
http://www.nanoscalereslett.com/content/9/1/22NANO EXPRESS Open AccessOptical gain in 1.3-μm electrically driven dilute
nitride VCSOAs
Sefer Bora Lisesivdin1,2*, Nadir Ali Khan1,5, Simone Mazzucato1, Naci Balkan1, Michael John Adams1,
Ville-Markus Korpijärvi3, Mircea Guina3, Gabor Mezosi4 and Marc Sorel4Abstract
We report the observation of room-temperature optical gain at 1.3 μm in electrically driven dilute nitride vertical
cavity semiconductor optical amplifiers. The gain is calculated with respect to injected power for samples with and
without a confinement aperture. At lower injected powers, a gain of almost 10 dB is observed in both samples. At
injection powers over 5 nW, the gain is observed to decrease. For nearly all investigated power levels, the sample
with confinement aperture gives slightly higher gain.
Keywords: Dilute nitride; Optical injection; Gain; Vertical cavity semiconductor optical amplifiersBackground
InxGa1−xAs1−yNy semiconductor alloy was first proposed
by Kondow et al. in 1996 [1], and considerable research at-
tention has been devoted to this alloy system due to its
possible optoelectronic applications at an operating wave-
length of 1.3 μm. With the addition of small amounts of
nitrogen into the (In)GaAs lattice, a strong electron con-
finement and bandgap reduction are obtained. Further-
more, addition of N allows band engineering, allowing the
device operating wavelength range to extend up to 1.6 μm
[2]. An extensive set of different devices based on this
alloy has been fabricated and demonstrated [3]. Examples
of these devices are vertical cavity surface-emitting lasers
(VCSELs) [4-6], vertical external cavity surface-emitting
lasers [7,8], solar cells [8,9], edge-emitting lasers [10], pho-
todetectors [11], semiconductor optical amplifiers (SOAs)
[12], and vertical cavity semiconductor optical amplifiers
(VCSOAs) [13,14].
VCSOAs can be seen as the natural evolution of SOAs,
which, owing to their fast response, reduced size, and low-
threshold nonlinear behavior, are popular in applications
such as optical routing, signal regeneration, and wave-
length shifting. Within these fields, VCSOAs have been* Correspondence: sblisesivdin@gmail.com
1School of Computer Science and Electronic Engineering, University of Essex,
Wivenhoe Park CO4 3SQ, UK
2Department of Physics, Faculty of Science and Arts, Gazi University,
Teknikokullar, Ankara 06500, Turkey
Full list of author information is available at the end of the article
© 2014 Lisesivdin et al.; licensee Springer. This
Attribution License (http://creativecommons.or
in any medium, provided the original work is pused as optical preamplifiers, switches, and interconnects
[15-17]. Their geometry provides numerous advantages
over the edge-emitting counterpart SOAs, including
low noise figure, circular emission, polarization insensi-
tivity, possibility to build high-density two-dimensional
arrays of devices that are easy to test on wafer, and low-
power consumption that is instrumental for high-
density photonic integrated circuits. Generally speaking,
a VCSOA is a modified version of a VCSEL that is driven
below lasing threshold. The first experimental study of an
InxGa1−xAs1−yNy/GaAs-based VCSOA was reported in
2002 [18], with a theoretical analysis published in 2004
[19]. Several studies on optically pumped InxGa1−xAs1−yNy
VCSOAs have been published [14,20-23], while electrically
driven VCSOAs have been demonstrated only in ‘Hellish’
configuration [24]. The present contribution builds on
these technological developments to focus on an electric-
ally driven multifunction standard VCSOA device operat-
ing in the 1.3-μm wavelength window.Methods
The amplification properties of InxGa1−xAs1−yNy VCSOAs
were studied using a 1,265- to 1,345-nm tunable laser (TL;
TLM-8700-H-O, Newport Corporation, Irvine, CA, USA),
whose output was sent to the sample using the setup
shown in Figure 1a. The TL signal was split via a 10/90is an Open Access article distributed under the terms of the Creative Commons
g/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction
roperly cited.
Figure 1 Experimental setup (a) and the layer structure of the investigated samples (b).
Lisesivdin et al. Nanoscale Research Letters 2014, 9:22 Page 2 of 5
http://www.nanoscalereslett.com/content/9/1/22coupler to a power meter and to the sample, respectively.
Back reflections were avoided using an optical isolator
while the TL power was changed from 0 to 7 mW using
an optical attenuator. A lens-ended fiber (SMF-28 fiber,
conical lens with cone angle of 80° to 90° and radius of
6.0 ±1.0 μm) was used to focus the TL light to the sample
surface as well as to collect its reflected/emitted/amplified
light, which was then directed to an optical spectrum
analyzer (OSA). The VCSOA was electrically DC biased
up to 10 mA and stabilized in temperature at 20°C via a
Peltier cooler.
The investigated VCSOA structure with a 3.5λ cavity
is shown in Figure 1b. The structures were grown by a
solid source molecular beam epitaxy reactor with a1100 1200 130
Waveleng
R
efl
ec
tiv
ity
0
0.2
0.4
0.6
0.8
1
1
2
Figure 2 Room temperature photoluminescence (red) and reflectance
reflectivity spectra of the studied VCSOA structure are shown in black andradio frequency plasma source for incorporating nitro-
gen. The growth was carried on an n-type GaAs(100)
substrate, and the bottom and top distributed Bragg
reflectors (DBRs) were doped with silicon (n-type)
and beryllium (p-type), respectively. The two DBRs
comprised 21 and 24 pairs of AlxGa1−xAs/GaAs layers
for the top and bottom DBR, respectively. The Al con-
centrations were x = 0.8 and 0.98 in the top and bottom
DBRs, respectively. The confinement aperture, which is
required for better carrier and light confinement, was
defined in the uppermost layer of the bottom DBR. The
active region contains three stacks of three 7-nm-thick
In0.35Ga0.65As0.975 N0.025 quantum wells separated by
20-nm thick GaAs spacers. A set of several VCSOA0 1400 1500
th (nm)
PL
 Intensity (a.u.)
spectra of the studied structure. Experimental and simulated
blue lines, respectively.
1100 1150 1200 1250 1300 1350
90
85
80
75
70
Wavelength (nm)
Po
w
er
 (
dB
m
)
0mA
1mA
2mA
4mA 
6mA 
8mA 
10mA
Figure 3 Power spectra of VCSOA without confinement
aperture obtained for different bias currents.
Lisesivdin et al. Nanoscale Research Letters 2014, 9:22 Page 3 of 5
http://www.nanoscalereslett.com/content/9/1/22samples was fabricated, having different dimensions of
the top DBR mirror radius (R1), confinement aperture
radius (R2), and bottom DBR radius (R3) for cases with
and without the confinement aperture. In this paper, we
compare the results obtained for two samples with and
without confinement aperture, with R1 = 5 μm, R2 =
25 μm, and R3 = 50 μm.
Results and discussion
Room-temperature reflectivity and photoluminescence
(PL) measurements were performed on the as-grown
sample, and the results are shown in Figure 2. Simulated1240 1260 1280 1300 1320 134
1.0 10 12
0.0 100
1.0 10 12
2.0 10 12
3.0 10 12
4.0 10 12
5.0 10 12
ASE-ASE
0
1240 1260 1280 1300 1320 134
1.0 10 12
0.0 100
1.0 10 12
2.0 10 12
3.0 10 12
4.0 10 12
Wavele
Po
w
er
 (
W
)
AOS + ASE - ASE
0
)a(
)c(
Figure 4 Results of various power spectra for λinj = 1,279 nm. (a) ASE −
power spectra.reflection is also shown in the figure. Two resonances
λR1 and λR2 are observed within the DBR stop band as a
result of the relatively long cavity length [25]. The
principle resonance, which is designed for 1.3-μm oper-
ation, is observed at λR1 = 1,282 nm, while the other un-
wanted resonance at lower wavelength is observed at
λR2 = 1,235 nm. Figure 3 shows the VCSOA amplified
spontaneous emission (ASE) spectra obtained with no
optical injection at different applied bias currents of 0 to
10 mA for the sample without confinement aperture.
The highest ASE power peak appears at 1,288 nm and is
blue-shifted with respect to that of the lasing cavity
mode wavelength [26,27]. The other modes are also con-
sistent with the PL spectra. Figure 3 shows that with in-
creasing the bias current, the amplitude of each mode
increases and also slightly shifts towards higher wave-
lengths. This shift is associated with local temperature
increase in the device. A similar result was observed in
the VCSOA with the confinement aperture.
Since no significant change in the spectrum amplitude
above 7 mA was observed, we investigated the devices
up to this current value. ASE power spectra taken at 7
and 0 mA (ASE0) are used to extract the spectrum
ASE −ASE0, which is the spectrum where the background
noise is minimized. To find the amplified optical signal
(AOS), we injected light sweeping the TL wavelength
(λinj) from 1,266 to 1,310 nm with a 7-mA current bias.
Figure 4 shows results for injection at λinj =1,279 nm
only. We could not investigate the second resonance
peak λR2 because of the wavelength limit of the TL. In0 1240 1260 1280 1300 1320 1340
1.0 10 12
2.0 10 12
3.0 10 12
4.0 10 12
5.0 10 12
6.0 10 12
AOS + ASE
0
ngth (nm)
1240 1260 1280 1300 1320 1340
0.0 100
1.0 10 12
2.0 10 12
3.0 10 12
4.0 10 12
AOS - ASE
0
)b(
)d(
ASE0level, (b) AOS + ASE, (c) AOS + ASE − ASE0, and (d) AOS − ASE0
018642
0
5
10
15
Power (nW)
G
ai
n 
(d
B
)
Without confinement aperture
With confinement aperture
Figure 6 Power-dependent gain for the samples with and
without confinement aperture.
Lisesivdin et al. Nanoscale Research Letters 2014, 9:22 Page 4 of 5
http://www.nanoscalereslett.com/content/9/1/22Figure 4a,b,c,d, the results for ASE −ASE0, AOS + ASE,
AOS + ASE −ASE0, and finally AOS −ASE0spectra are
shown, respectively.
As the gain is small, the amplified signal cannot be
easily discerned in Figure 4d. Hence, the gain was calcu-
lated using the simple relation
Gain ¼ AOSþ ASE‐ASE0
AOS‐ASE0
ð1Þ
for each wavelength after obtaining AOS and ASE data.
Results are shown as a function of the injected wavelength
in Figure 5 for a specific laser power (Pinj) of 2.25 nW. A
maximum gain of 3 dB with a very broad peak is observed
at the maximum ASE wavelength of 1,288.5 nm. In the
study, measured signal levels are very near to limits of the
OSA; therefore, larger bandwidth wavelength values are
used, which can be the reason of the broadness of the gain
peak.
Having verified that the gain peak corresponds to the
ASE peak wavelength, we investigated the Pinj depend-
ence by varying it from 1.5 nW to a few milliwatts for
the single wavelength of 1,288.5 nm. Results are pre-
sented for both samples with and without confinement
aperture in Figure 6 for power values below 10 nW.
For injected laser powers over 5 nW, the gain falls rap-
idly. At the lowest injected power, the sample with
confinement aperture exhibits 10 dB of gain, which is
observed near the maximum ASE wavelength. For the
investigated injected power range, the sample with the
confinement aperture showed a higher gain because of
the better carrier and light confinement in the VCSOA.
Conclusions
In this paper, we report the observation of gain in an
electrically driven dilute nitride VCSOA device operated1260 1270 1280 1290 1300 1310
0
0.5
1
1.5
2
2.5
3
3.5
Wavelength (nm)
G
ai
n 
(d
B
)
Figure 5 Gain versus injected laser wavelength with Pinj = 2.25 nW.at 1.3-μm in reflection mode. Two different types of sam-
ples with and without confinement aperture are investi-
gated. The ASE power peak is found to be at 1,288.5 nm
with additional modes, which are caused by the length of
the cavity. Optical gain is found to occur at low optical in-
jection values. Above 5 nW of optical injection, the gain is
found to fall rapidly. The maximum observed optical gain
is observed at 1,288.5 nm at room temperature. The max-
imum observed optical gain at 7-mA current at room
temperature is around 10 and 6 dB for samples with and
without confinement aperture, respectively. It is important
to mention that despite the small gain, the device is very
promising because it requires very small currents com-
pared with in-plane SOAs.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
SBL, NAK, and SM carried out the measurements and data analysis. VMK and
MG performed the growth of structures. GM and MS carried the fabrication
of devices. MJA performed the theoretical studies and analysis. NB is the
project leader. SBL and NB wrote the paper. All authors read, corrected, and
approved the final manuscript.
Acknowledgements
Support from EPSRC under grant EP/G023972/1 is gratefully acknowledged.
Sefer Bora Lisesivdin also acknowledges partial support from the Turkish
Scientific and Technological Research Council (TUBITAK) 2219 coded
scholarship. COST Action MP0805 is also gratefully acknowledged.
Author details
1School of Computer Science and Electronic Engineering, University of Essex,
Wivenhoe Park CO4 3SQ, UK. 2Department of Physics, Faculty of Science and
Arts, Gazi University, Teknikokullar, Ankara 06500, Turkey. 3Optoelectronics
Research Centre (ORC), Tampere University of Technology, P.O. Box 692,
Tampere 33101, Finland. 4School of Engineering, University of Glasgow,
Glasgow GG12 8QQ, UK. 5Kohat University of Science and Technology
(KUST), Kohat, Khyber Pakhtoonkhwa, Pakistan.
Received: 17 October 2013 Accepted: 17 December 2013
Published: 13 January 2014
Lisesivdin et al. Nanoscale Research Letters 2014, 9:22 Page 5 of 5
http://www.nanoscalereslett.com/content/9/1/22References
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Lisesivdin et al. Nanoscale Research Letters 2014, 9:22http://www.nanoscalereslett.com/content/9/1/22NANO EXPRESS Open AccessOptical gain in 1.3-μm electrically driven dilutenitride VCSOAsSefer Bora Lisesivdin1,2*, Nadir Ali Khan1,5, Simone Mazzucato1, Naci Balkan1, Michael John Adams1,Ville-Markus Korpijärvi3, Mircea Guina3, Gabor Mezosi4 and Marc Sorel4AbstractWe report the observation of room-temperature optical gain at 1.3 μm in electrically driven dilute nitride verticalcavity semiconductor optical amplifiers. The gain is calculated with respect to injected power for samples with andwithout a confinement aperture. At lower injected powers, a gain of almost 10 dB is observed in both samples. Atinjection powers over 5 nW, the gain is observed to decrease. For nearly all investigated power levels, the samplewith confinement aperture gives slightly higher gain.Keywords: Dilute nitride; Optical injection; Gain; Vertical cavity semiconductor optical amplifiersBackgroundInxGa1−xAs1−yNy semiconductor alloy was first proposedby Kondow et al. in 1996 [1], and considerable research at-tention has been devoted to this alloy system due to itspossible optoelectronic applications at an operating wave-length of 1.3 μm. With the addition of small amounts ofnitrogen into the (In)GaAs lattice, a strong electron con-finement and bandgap reduction are obtained. Further-more, addition of N allows band engineering, allowing thedevice operating wavelength range to extend up to 1.6 μm[2]. An extensive set of different devices based on thisalloy has been fabricated and demonstrated [3]. Examplesof these devices are vertical cavity surface-emitting lasers(VCSELs) [4-6], vertical external cavity surface-emittinglasers [7,8], solar cells [8,9], edge-emitting lasers [10], pho-todetectors [11], semiconductor optical amplifiers (SOAs)[12], and vertical cavity semiconductor optical amplifiers(VCSOAs) [13,14].VCSOAs can be seen as the natural evolution of SOAs,which, owing to their fast response, reduced size, and low-threshold nonlinear behavior, are popular in applicationssuch as optical routing, signal regeneration, and wave-length shifting. Within these fields, VCSOAs have been* Correspondence: sblisesivdin@gmail.com1School of Computer Science and Electronic Engineering, University of Essex,Wivenhoe Park CO4 3SQ, UK2Department of Physics, Faculty of Science and Arts, Gazi University,Teknikokullar, Ankara 06500, TurkeyFull list of author information is available at the end of the article© 2014 Lisesivdin et al.; licensee Springer. ThisAttribution License (http://creativecommons.orin any medium, provided the original work is pused as optical preamplifiers, switches, and interconnects[15-17]. Their geometry provides numerous advantagesover the edge-emitting counterpart SOAs, includinglow noise figure, circular emission, polarization insensi-tivity, possibility to build high-density two-dimensionalarrays of devices that are easy to test on wafer, and low-power consumption that is instrumental for high-density photonic integrated circuits. Generally speaking,a VCSOA is a modified version of a VCSEL that is drivenbelow lasing threshold. The first experimental study of anInxGa1−xAs1−yNy/GaAs-based VCSOA was reported in2002 [18], with a theoretical analysis published in 2004[19]. Several studies on optically pumped InxGa1−xAs1−yNyVCSOAs have been published [14,20-23], while electricallydriven VCSOAs have been demonstrated only in ‘Hellish’configuration [24]. The present contribution builds onthese technological developments to focus on an electric-ally driven multifunction standard VCSOA device operat-ing in the 1.3-μm wavelength window.MethodsThe amplification properties of InxGa1−xAs1−yNy VCSOAswere studied using a 1,265- to 1,345-nm tunable laser (TL;TLM-8700-H-O, Newport Corporation, Irvine, CA, USA),whose output was sent to the sample using the setupshown in Figure 1a. The TL signal was split via a 10/90is an Open Access article distributed under the terms of the Creative Commonsg/licenses/by/2.0), which permits unrestricted use, distribution, and reproductionroperly cited.Figure 1 Experimental setup (a) and the layer structure of the investigated samples (b).Lisesivdin et al. Nanoscale Research Letters 2014, 9:22 Page 2 of 5http://www.nanoscalereslett.com/content/9/1/22coupler to a power meter and to the sample, respectively.Back reflections were avoided using an optical isolatorwhile the TL power was changed from 0 to 7 mW usingan optical attenuator. A lens-ended fiber (SMF-28 fiber,conical lens with cone angle of 80° to 90° and radius of6.0 ±1.0 μm) was used to focus the TL light to the samplesurface as well as to collect its reflected/emitted/amplifiedlight, which was then directed to an optical spectrumanalyzer (OSA). The VCSOA was electrically DC biasedup to 10 mA and stabilized in temperature at 20°C via aPeltier cooler.The investigated VCSOA structure with a 3.5λ cavityis shown in Figure 1b. The structures were grown by asolid source molecular beam epitaxy reactor with a1100 1200 130WavelengReflectivity00.20.40.60.8112Figure 2 Room temperature photoluminescence (red) and reflectancereflectivity spectra of the studied VCSOA structure are shown in black andradio frequency plasma source for incorporating nitro-gen. The growth was carried on an n-type GaAs(100)substrate, and the bottom and top distributed Braggreflectors (DBRs) were doped with silicon (n-type)and beryllium (p-type), respectively. The two DBRscomprised 21 and 24 pairs of AlxGa1−xAs/GaAs layersfor the top and bottom DBR, respectively. The Al con-centrations were x = 0.8 and 0.98 in the top and bottomDBRs, respectively. The confinement aperture, which isrequired for better carrier and light confinement, wasdefined in the uppermost layer of the bottom DBR. Theactive region contains three stacks of three 7-nm-thickIn0.35Ga0.65As0.975 N0.025 quantum wells separated by20-nm thick GaAs spacers. A set of several VCSOA0 1400 1500th (nm)PL Intensity (a.u.)spectra of the studied structure. Experimental and simulatedblue lines, respectively.1100 1150 1200 1250 1300 13509085807570Wavelength (nm)Power (dBm)0mA1mA2mA4mA 6mA 8mA 10mAFigure 3 Power spectra of VCSOA without confinementaperture obtained for different bias currents.Lisesivdin et al. Nanoscale Research Letters 2014, 9:22 Page 3 of 5http://www.nanoscalereslett.com/content/9/1/22samples was fabricated, having different dimensions ofthe top DBR mirror radius (R1), confinement apertureradius (R2), and bottom DBR radius (R3) for cases withand without the confinement aperture. In this paper, wecompare the results obtained for two samples with andwithout confinement aperture, with R1 = 5 μm, R2 =25 μm, and R3 = 50 μm.Results and discussionRoom-temperature reflectivity and photoluminescence(PL) measurements were performed on the as-grownsample, and the results are shown in Figure 2. Simulated1240 1260 1280 1300 1320 1341.0 10 120.0 1001.0 10 122.0 10 123.0 10 124.0 10 125.0 10 12 ASE-ASE01240 1260 1280 1300 1320 1341.0 10 120.0 1001.0 10 122.0 10 123.0 10 124.0 10 12WavelePower (W)AOS + ASE - ASE0)a()c(Figure 4 Results of various power spectra for λinj = 1,279 nm. (a) ASE −power spectra.reflection is also shown in the figure. Two resonancesλR1 and λR2 are observed within the DBR stop band as aresult of the relatively long cavity length [25]. Theprinciple resonance, which is designed for 1.3-μm oper-ation, is observed at λR1 = 1,282 nm, while the other un-wanted resonance at lower wavelength is observed atλR2 = 1,235 nm. Figure 3 shows the VCSOA amplifiedspontaneous emission (ASE) spectra obtained with nooptical injection at different applied bias currents of 0 to10 mA for the sample without confinement aperture.The highest ASE power peak appears at 1,288 nm and isblue-shifted with respect to that of the lasing cavitymode wavelength [26,27]. The other modes are also con-sistent with the PL spectra. Figure 3 shows that with in-creasing the bias current, the amplitude of each modeincreases and also slightly shifts towards higher wave-lengths. This shift is associated with local temperatureincrease in the device. A similar result was observed inthe VCSOA with the confinement aperture.Since no significant change in the spectrum amplitudeabove 7 mA was observed, we investigated the devicesup to this current value. ASE power spectra taken at 7and 0 mA (ASE0) are used to extract the spectrumASE −ASE0, which is the spectrum where the backgroundnoise is minimized. To find the amplified optical signal(AOS), we injected light sweeping the TL wavelength(λinj) from 1,266 to 1,310 nm with a 7-mA current bias.Figure 4 shows results for injection at λinj =1,279 nmonly. We could not investigate the second resonancepeak λR2 because of the wavelength limit of the TL. In0 1240 1260 1280 1300 1320 13401.0 10 122.0 10 123.0 10 124.0 10 125.0 10 126.0 10 12AOS + ASE0ngth (nm)1240 1260 1280 1300 1320 13400.0 1001.0 10 122.0 10 123.0 10 124.0 10 12AOS - ASE0)b()d(ASE0level, (b) AOS + ASE, (c) AOS + ASE − ASE0, and (d) AOS − ASE0018642051015Power (nW)Gain (dB)Without confinement apertureWith confinement apertureFigure 6 Power-dependent gain for the samples with andwithout confinement aperture.Lisesivdin et al. Nanoscale Research Letters 2014, 9:22 Page 4 of 5http://www.nanoscalereslett.com/content/9/1/22Figure 4a,b,c,d, the results for ASE −ASE0, AOS + ASE,AOS + ASE −ASE0, and finally AOS −ASE0spectra areshown, respectively.As the gain is small, the amplified signal cannot beeasily discerned in Figure 4d. Hence, the gain was calcu-lated using the simple relationGain ¼ AOSþ ASE‐ASE0AOS‐ASE0ð1Þfor each wavelength after obtaining AOS and ASE data.Results are shown as a function of the injected wavelengthin Figure 5 for a specific laser power (Pinj) of 2.25 nW. Amaximum gain of 3 dB with a very broad peak is observedat the maximum ASE wavelength of 1,288.5 nm. In thestudy, measured signal levels are very near to limits of theOSA; therefore, larger bandwidth wavelength values areused, which can be the reason of the broadness of the gainpeak.Having verified that the gain peak corresponds to theASE peak wavelength, we investigated the Pinj depend-ence by varying it from 1.5 nW to a few milliwatts forthe single wavelength of 1,288.5 nm. Results are pre-sented for both samples with and without confinementaperture in Figure 6 for power values below 10 nW.For injected laser powers over 5 nW, the gain falls rap-idly. At the lowest injected power, the sample withconfinement aperture exhibits 10 dB of gain, which isobserved near the maximum ASE wavelength. For theinvestigated injected power range, the sample with theconfinement aperture showed a higher gain because ofthe better carrier and light confinement in the VCSOA.ConclusionsIn this paper, we report the observation of gain in anelectrically driven dilute nitride VCSOA device operated1260 1270 1280 1290 1300 131000.511.522.533.5Wavelength (nm)Gain (dB)Figure 5 Gain versus injected laser wavelength with Pinj = 2.25 nW.at 1.3-μm in reflection mode. Two different types of sam-ples with and without confinement aperture are investi-gated. The ASE power peak is found to be at 1,288.5 nmwith additional modes, which are caused by the length ofthe cavity. Optical gain is found to occur at low optical in-jection values. Above 5 nW of optical injection, the gain isfound to fall rapidly. The maximum observed optical gainis observed at 1,288.5 nm at room temperature. The max-imum observed optical gain at 7-mA current at roomtemperature is around 10 and 6 dB for samples with andwithout confinement aperture, respectively. It is importantto mention that despite the small gain, the device is verypromising because it requires very small currents com-pared with in-plane SOAs.Competing interestsThe authors declare that they have no competing interests.Authors’ contributionsSBL, NAK, and SM carried out the measurements and data analysis. VMK andMG performed the growth of structures. GM and MS carried the fabricationof devices. MJA performed the theoretical studies and analysis. NB is theproject leader. SBL and NB wrote the paper. All authors read, corrected, andapproved the final manuscript.AcknowledgementsSupport from EPSRC under grant EP/G023972/1 is gratefully acknowledged.Sefer Bora Lisesivdin also acknowledges partial support from the TurkishScientific and Technological Research Council (TUBITAK) 2219 codedscholarship. COST Action MP0805 is also gratefully acknowledged.Author details1School of Computer Science and Electronic Engineering, University of Essex,Wivenhoe Park CO4 3SQ, UK. 2Department of Physics, Faculty of Science andArts, Gazi University, Teknikokullar, Ankara 06500, Turkey. 3OptoelectronicsResearch Centre (ORC), Tampere University of Technology, P.O. Box 692,Tampere 33101, Finland. 4School of Engineering, University of Glasgow,Glasgow GG12 8QQ, UK. 5Kohat University of Science and Technology(KUST), Kohat, Khyber Pakhtoonkhwa, Pakistan.Received: 17 October 2013 Accepted: 17 December 2013Published: 13 January 2014Lisesivdin et al. Nanoscale Research Letters 2014, 9:22 Page 5 of 5http://www.nanoscalereslett.com/content/9/1/22References1. Kondow M, Uomi K, Niwa A, Kitatani T, Watahiki S, Yazawa Y: GaInNAs: anovel material for long-wavelength-range laser diodes with excellenthigh-temperature performance. Jpn J Appl Phys 1996, 35:1273–1275.2. Balkan N: The physics and technology of dilute nitrides. J Phys CondensMatter 2004. doi:10.1088/0953-8984/16/31/E01.3. Erol A: Dilute III-V Nitride Semiconductors and Material Systems: Physics andTechnology, (Springer Series in Materials Science 105). Heidelberg:Springer; 2008.4. Forchel A, Reinhardt M, Fischer M: A monolithic GaInAsN vertical-cavitysurface-emitting laser for the 1.3-μm regime. IEEE Photon Technol Lett2000, 12:1313–1315.5. Jouhti T, Okhotnikov O, Konttinen J, Gomes LA, Peng CS, Karirinne S,Pavelescu E-M, Pessa M: Dilute nitride vertical-cavity surface-emittinglasers. New J Phys 2003, 5:841–846.6. Schires K, Al Seyab R, Hurtado A, Korpijarvi V-M, Guina M, Henning ID, Adams MJ:Optically-pumped dilute nitride spin-VCSEL. Opt Exp 2012, 20:3550–3555.7. Hopkins JM, Smith SA, Jeon CW, Sun HD, Burns D, Calvez S, Dawson MD,Jouhti T, Pessa M: 0.6 W CW GaInNAs vertical external-cavity surfaceemitting laser operating at 1.32 μm. IET Electr. Lett 2004, 40:30–31.8. Guina M, Leinonen T, Härkönen A, Pessa M: High-power disk lasers basedon dilute nitride heterostructures. New J Phys 2009, 11:125019.9. Royall B, Balkan N: Dilute nitride n-i-p-i solar cells. Microelectron J 2009,40:396–398.10. Aho A, Tukiainen A, Polojärvi V, Salmi J, Guina M: High current generationin dilute nitride solar cells grown by molecular beam epitaxy. Proc. SPIE8620, Physics, Simulation, and Photonic Engineering of Photovoltaic Devices II,86201I 2013. doi:10.1117/12.2002972.11. Bonnefont B, Messant M, Boutillier O, Gauthier-Lafaye F, Lozes-Dupuy A,Sallet MV, Merghem K, Ferlazzo L, Harmand JC, Ramdane A, Provost JG,Dagens B, Landreau J, Le Gouezigou O, Marie X: Optimization andcharacterization of InGaAsN/GaAs quantum-well ridge laser diodesfor high frequency operation. Opt Quantum Electron 2006, 38:313–324.12. Luna E, Hopkinson M, Ulloa JM, Guzman A, Munoz E: Dilute nitride baseddouble-barrier quantum-well infrared photodetector operating in thenear infrared. Appl Phys Lett 2003, 83:3111–3113.13. Hashimoto J, Koyama K, Katsuyama T, Iguchi Y, Yamada Y, Takagishi S, Ito MM,Ishida A: 1.3 μm travelling-wave GaInNAs semiconductor optical amplifier.Jpn J Appl Phys 2004, 43:3419–3423.14. Alexandropoulos D, Adams MJ, Hatzopoulos Z, Syvridis D: Proposedscheme for polarization insensitive GaInNAs-based semiconductoroptical amplifiers. IEEE J Quantum Electron 2005, 41:817–822.15. Laurand N, Calvez S, Dawson MD, Bryce AC, Jouhti T, Kontinnen J, Pessa M:Performance comparison of GaInNAs vertical-cavity semiconductoroptical amplifiers. IEEE J Quantum Electron 2005, 41:642–649.16. Suzuki N, Ohashi M, Nakamura M: A proposed vertical-cavity opticalrepeater for optical inter-board connections. IEEE Photon Technol Lett1997, 9:1149–1151.17. Björlin ES, Geske J, Bowers JE: Optically pre-amplified receiver at 10Gb/susing a vertical-cavity SOA. Electron Lett 2001, 37:1474–1475.18. Bouché N, Corbett B, Kuszelewicz R, Ray R: Vertical-cavity amplifyingphotonic switch at 1.5 μm. IEEE Photon Technol Lett 1996, 8:1035–1037.19. Calvez S, Clark AH, Hopkins JM, Merlin P, Sun HD, Dawson MD, Jouhti T, Pessa M:Amplification and laser action in diode-pumped 1.3 μm GaInNAs vertical-cavity structures. In Proceedings of 2002 IEEE/Leos Annual Meeting Conference:10–14 Nov 2002. Glasgow; Piscataway: IEEE; 2002:165–166.20. Alexandropoulos D, Adams MJ: GaInNAs-based vertical cavitysemiconductor optical amplifiers. J Phys: Cond Matt 2004, 16:S3345–S3354.21. Calvez S, Clark AH, Hopkins J-M, Macaluso R, Merlin P, Sun HD, Dawson MD:1.3 μm GaInNAs optically-pumped vertical cavity semiconductor opticalamplifier. Electron Lett 2003, 39:100–102.22. Clark AH, Calvez S, Laurand N, Macaluso R, Sun HD, Dawson MD, Jouhti T,Kontinnen J, Pessa M: Long-wavelength monolithic GaInNAs vertical-cavity optical amplifiers. IEEE J Quantum Electron 2004, 40:878–883.23. Laurand N, Calvez S, Dawson MD, Kelly AE: Index and gain dynamics ofoptically pumped GaInNAs vertical-cavity semiconductor opticalamplifier. Appl Phys Lett 2005, 87:231115–231117.24. Laurand N, Calvez S, Dawson MD, Kelly AE: Slow-light in a vertical-cavitysemiconductor optical amplifier. Opt Express 2006, 14:6858–6863.25. Chaqmaqchee FAI, Balkan N: Gain studies of 1.3-μm dilute nitrideHELLISH-VCSOA for optical communications. Nanoscale Res Lett 2012,7:526–529.26. Calvez S, Hopkins J-M, Smith SA, Clark AH, Macaluso R, Sun HD, DawsonMD, Jouhti T, Pessa M, Gundogdu K, Hall KC, Boggess TF: GaInNAs/GaAsBragg-mirror-based structures for novel 1.3 μm device applications.J Cryst Growth 2004, 268:457–465.27. Mircea A, Caliman A, Iakovlev V, Mereuta A, Suruceanu G, Berseth C-A, Royo P,Syrbu A, Kapon E: Cavity mode—gain peak tradeoff for 1320-nm wafer-fused VCSELs with 3-mW single-mode emission power and 10-Gb/smodulation speed up to 70°C. IEEE Photonics Technol Lett 2007, 19:121–123.doi:10.1186/1556-276X-9-22Cite this article as: Lisesivdin et al.: Optical gain in 1.3-μm electricallydriven dilute nitride VCSOAs. Nanoscale Research Letters 2014 9:22.Submit your manuscript to a journal and beneﬁ t from:7 Convenient online submission7 Rigorous peer review7 Immediate publication on acceptance7 Open access: articles freely available online7 High visibility within the ﬁ eld7 Retaining the copyright to your article    Submit your next manuscript at 7 springeropen.com

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Lisesivdin et al. Nanoscale Research Letters 2014, 9:22http://www.nanoscalereslett.com/content/9/1/22NANO EXPRESS Open AccessOptical gain in 1.3-μm electrically driven dilutenitride VCSOAsSefer Bora Lisesivdin1,2*, Nadir Ali Khan1,5, Simone Mazzucato1, Naci Balkan1, Michael John Adams1,Ville-Markus Korpijärvi3, Mircea Guina3, Gabor Mezosi4 and Marc Sorel4AbstractWe report the observation of room-temperature optical gain at 1.3 μm in electrically driven dilute nitride verticalcavity semiconductor optical amplifiers. The gain is calculated with respect to injected power for samples with andwithout a confinement aperture. At lower injected powers, a gain of almost 10 dB is observed in both samples. Atinjection powers over 5 nW, the gain is observed to decrease. For nearly all investigated power levels, the samplewith confinement aperture gives slightly higher gain.Keywords: Dilute nitride; Optical injection; Gain; Vertical cavity semiconductor optical amplifiersBackgroundInxGa1−xAs1−yNy semiconductor alloy was first proposedby Kondow et al. in 1996 [1], and considerable research at-tention has been devoted to this alloy system due to itspossible optoelectronic applications at an operating wave-length of 1.3 μm. With the addition of small amounts ofnitrogen into the (In)GaAs lattice, a strong electron con-finement and bandgap reduction are obtained. Further-more, addition of N allows band engineering, allowing thedevice operating wavelength range to extend up to 1.6 μm[2]. An extensive set of different devices based on thisalloy has been fabricated and demonstrated [3]. Examplesof these devices are vertical cavity surface-emitting lasers(VCSELs) [4-6], vertical external cavity surface-emittinglasers [7,8], solar cells [8,9], edge-emitting lasers [10], pho-todetectors [11], semiconductor optical amplifiers (SOAs)[12], and vertical cavity semiconductor optical amplifiers(VCSOAs) [13,14].VCSOAs can be seen as the natural evolution of SOAs,which, owing to their fast response, reduced size, and low-threshold nonlinear behavior, are popular in applicationssuch as optical routing, signal regeneration, and wave-length shifting. Within these fields, VCSOAs have been* Correspondence: sblisesivdin@gmail.com1School of Computer Science and Electronic Engineering, University of Essex,Wivenhoe Park CO4 3SQ, UK2Department of Physics, Faculty of Science and Arts, Gazi University,Teknikokullar, Ankara 06500, TurkeyFull list of author information is available at the end of the article© 2014 Lisesivdin et al.; licensee Springer. ThisAttribution License (http://creativecommons.orin any medium, provided the original work is pused as optical preamplifiers, switches, and interconnects[15-17]. Their geometry provides numerous advantagesover the edge-emitting counterpart SOAs, includinglow noise figure, circular emission, polarization insensi-tivity, possibility to build high-density two-dimensionalarrays of devices that are easy to test on wafer, and low-power consumption that is instrumental for high-density photonic integrated circuits. Generally speaking,a VCSOA is a modified version of a VCSEL that is drivenbelow lasing threshold. The first experimental study of anInxGa1−xAs1−yNy/GaAs-based VCSOA was reported in2002 [18], with a theoretical analysis published in 2004[19]. Several studies on optically pumped InxGa1−xAs1−yNyVCSOAs have been published [14,20-23], while electricallydriven VCSOAs have been demonstrated only in ‘Hellish’configuration [24]. The present contribution builds onthese technological developments to focus on an electric-ally driven multifunction standard VCSOA device operat-ing in the 1.3-μm wavelength window.MethodsThe amplification properties of InxGa1−xAs1−yNy VCSOAswere studied using a 1,265- to 1,345-nm tunable laser (TL;TLM-8700-H-O, Newport Corporation, Irvine, CA, USA),whose output was sent to the sample using the setupshown in Figure 1a. The TL signal was split via a 10/90is an Open Access article distributed under the terms of the Creative Commonsg/licenses/by/2.0), which permits unrestricted use, distribution, and reproductionroperly cited.Figure 1 Experimental setup (a) and the layer structure of the investigated samples (b).Lisesivdin et al. Nanoscale Research Letters 2014, 9:22 Page 2 of 5http://www.nanoscalereslett.com/content/9/1/22coupler to a power meter and to the sample, respectively.Back reflections were avoided using an optical isolatorwhile the TL power was changed from 0 to 7 mW usingan optical attenuator. A lens-ended fiber (SMF-28 fiber,conical lens with cone angle of 80° to 90° and radius of6.0 ±1.0 μm) was used to focus the TL light to the samplesurface as well as to collect its reflected/emitted/amplifiedlight, which was then directed to an optical spectrumanalyzer (OSA). The VCSOA was electrically DC biasedup to 10 mA and stabilized in temperature at 20°C via aPeltier cooler.The investigated VCSOA structure with a 3.5λ cavityis shown in Figure 1b. The structures were grown by asolid source molecular beam epitaxy reactor with a1100 1200 130WavelengReflectivity00.20.40.60.8112Figure 2 Room temperature photoluminescence (red) and reflectancereflectivity spectra of the studied VCSOA structure are shown in black andradio frequency plasma source for incorporating nitro-gen. The growth was carried on an n-type GaAs(100)substrate, and the bottom and top distributed Braggreflectors (DBRs) were doped with silicon (n-type)and beryllium (p-type), respectively. The two DBRscomprised 21 and 24 pairs of AlxGa1−xAs/GaAs layersfor the top and bottom DBR, respectively. The Al con-centrations were x = 0.8 and 0.98 in the top and bottomDBRs, respectively. The confinement aperture, which isrequired for better carrier and light confinement, wasdefined in the uppermost layer of the bottom DBR. Theactive region contains three stacks of three 7-nm-thickIn0.35Ga0.65As0.975 N0.025 quantum wells separated by20-nm thick GaAs spacers. A set of several VCSOA0 1400 1500th (nm)PL Intensity (a.u.)spectra of the studied structure. Experimental and simulatedblue lines, respectively.1100 1150 1200 1250 1300 13509085807570Wavelength (nm)Power (dBm)0mA1mA2mA4mA 6mA 8mA 10mAFigure 3 Power spectra of VCSOA without confinementaperture obtained for different bias currents.Lisesivdin et al. Nanoscale Research Letters 2014, 9:22 Page 3 of 5http://www.nanoscalereslett.com/content/9/1/22samples was fabricated, having different dimensions ofthe top DBR mirror radius (R1), confinement apertureradius (R2), and bottom DBR radius (R3) for cases withand without the confinement aperture. In this paper, wecompare the results obtained for two samples with andwithout confinement aperture, with R1 = 5 μm, R2 =25 μm, and R3 = 50 μm.Results and discussionRoom-temperature reflectivity and photoluminescence(PL) measurements were performed on the as-grownsample, and the results are shown in Figure 2. Simulated1240 1260 1280 1300 1320 1341.0 10 120.0 1001.0 10 122.0 10 123.0 10 124.0 10 125.0 10 12ASE-ASE01240 1260 1280 1300 1320 1341.0 10 120.0 1001.0 10 122.0 10 123.0 10 124.0 10 12WavelePower (W)AOS + ASE - ASE0)a()c(Figure 4 Results of various power spectra for λinj = 1,279 nm. (a) ASE −power spectra.reflection is also shown in the figure. Two resonancesλR1 and λR2 are observed within the DBR stop band as aresult of the relatively long cavity length [25]. Theprinciple resonance, which is designed for 1.3-μm oper-ation, is observed at λR1 = 1,282 nm, while the other un-wanted resonance at lower wavelength is observed atλR2 = 1,235 nm. Figure 3 shows the VCSOA amplifiedspontaneous emission (ASE) spectra obtained with nooptical injection at different applied bias currents of 0 to10 mA for the sample without confinement aperture.The highest ASE power peak appears at 1,288 nm and isblue-shifted with respect to that of the lasing cavitymode wavelength [26,27]. The other modes are also con-sistent with the PL spectra. Figure 3 shows that with in-creasing the bias current, the amplitude of each modeincreases and also slightly shifts towards higher wave-lengths. This shift is associated with local temperatureincrease in the device. A similar result was observed inthe VCSOA with the confinement aperture.Since no significant change in the spectrum amplitudeabove 7 mA was observed, we investigated the devicesup to this current value. ASE power spectra taken at 7and 0 mA (ASE0) are used to extract the spectrumASE −ASE0, which is the spectrum where the backgroundnoise is minimized. To find the amplified optical signal(AOS), we injected light sweeping the TL wavelength(λinj) from 1,266 to 1,310 nm with a 7-mA current bias.Figure 4 shows results for injection at λinj =1,279 nmonly. We could not investigate the second resonancepeak λR2 because of the wavelength limit of the TL. In0 1240 1260 1280 1300 1320 13401.0 10 122.0 10 123.0 10 124.0 10 125.0 10 126.0 10 12AOS + ASE0ngth (nm)1240 1260 1280 1300 1320 13400.0 1001.0 10 122.0 10 123.0 10 124.0 10 12AOS - ASE0)b()d(ASE0level, (b) AOS + ASE, (c) AOS + ASE − ASE0, and (d) AOS − ASE0018642051015Power (nW)Gain (dB)Without confinement apertureWith confinement apertureFigure 6 Power-dependent gain for the samples with andwithout confinement aperture.Lisesivdin et al. Nanoscale Research Letters 2014, 9:22 Page 4 of 5http://www.nanoscalereslett.com/content/9/1/22Figure 4a,b,c,d, the results for ASE −ASE0, AOS + ASE,AOS + ASE −ASE0, and finally AOS −ASE0spectra areshown, respectively.As the gain is small, the amplified signal cannot beeasily discerned in Figure 4d. Hence, the gain was calcu-lated using the simple relationGain ¼ AOSþ ASE‐ASE0AOS‐ASE0ð1Þfor each wavelength after obtaining AOS and ASE data.Results are shown as a function of the injected wavelengthin Figure 5 for a specific laser power (Pinj) of 2.25 nW. Amaximum gain of 3 dB with a very broad peak is observedat the maximum ASE wavelength of 1,288.5 nm. In thestudy, measured signal levels are very near to limits of theOSA; therefore, larger bandwidth wavelength values areused, which can be the reason of the broadness of the gainpeak.Having verified that the gain peak corresponds to theASE peak wavelength, we investigated the Pinj depend-ence by varying it from 1.5 nW to a few milliwatts forthe single wavelength of 1,288.5 nm. Results are pre-sented for both samples with and without confinementaperture in Figure 6 for power values below 10 nW.For injected laser powers over 5 nW, the gain falls rap-idly. At the lowest injected power, the sample withconfinement aperture exhibits 10 dB of gain, which isobserved near the maximum ASE wavelength. For theinvestigated injected power range, the sample with theconfinement aperture showed a higher gain because ofthe better carrier and light confinement in the VCSOA.ConclusionsIn this paper, we report the observation of gain in anelectrically driven dilute nitride VCSOA device operated1260 1270 1280 1290 1300 131000.511.522.533.5Wavelength (nm)Gain (dB)Figure 5 Gain versus injected laser wavelength with Pinj = 2.25 nW.at 1.3-μm in reflection mode. Two different types of sam-ples with and without confinement aperture are investi-gated. The ASE power peak is found to be at 1,288.5 nmwith additional modes, which are caused by the length ofthe cavity. Optical gain is found to occur at low optical in-jection values. Above 5 nW of optical injection, the gain isfound to fall rapidly. The maximum observed optical gainis observed at 1,288.5 nm at room temperature. The max-imum observed optical gain at 7-mA current at roomtemperature is around 10 and 6 dB for samples with andwithout confinement aperture, respectively. It is importantto mention that despite the small gain, the device is verypromising because it requires very small currents com-pared with in-plane SOAs.Competing interestsThe authors declare that they have no competing interests.Authors’ contributionsSBL, NAK, and SM carried out the measurements and data analysis. VMK andMG performed the growth of structures. GM and MS carried the fabricationof devices. MJA performed the theoretical studies and analysis. NB is theproject leader. SBL and NB wrote the paper. All authors read, corrected, andapproved the final manuscript.AcknowledgementsSupport from EPSRC under grant EP/G023972/1 is gratefully acknowledged.Sefer Bora Lisesivdin also acknowledges partial support from the TurkishScientific and Technological Research Council (TUBITAK) 2219 codedscholarship. COST Action MP0805 is also gratefully acknowledged.Author details1School of Computer Science and Electronic Engineering, University of Essex,Wivenhoe Park CO4 3SQ, UK. 2Department of Physics, Faculty of Science andArts, Gazi University, Teknikokullar, Ankara 06500, Turkey. 3OptoelectronicsResearch Centre (ORC), Tampere University of Technology, P.O. Box 692,Tampere 33101, Finland. 4School of Engineering, University of Glasgow,Glasgow GG12 8QQ, UK. 5Kohat University of Science and Technology(KUST), Kohat, Khyber Pakhtoonkhwa, Pakistan.Received: 17 October 2013 Accepted: 17 December 2013Published: 13 January 2014Lisesivdin et al. Nanoscale Research Letters 2014, 9:22 Page 5 of 5http://www.nanoscalereslett.com/content/9/1/22References1. Kondow M, Uomi K, Niwa A, Kitatani T, Watahiki S, Yazawa Y: GaInNAs: anovel material for long-wavelength-range laser diodes with excellenthigh-temperature performance. Jpn J Appl Phys 1996, 35:1273–1275.2. Balkan N: The physics and technology of dilute nitrides. J Phys CondensMatter 2004. doi:10.1088/0953-8984/16/31/E01.3. Erol A: Dilute III-V Nitride Semiconductors and Material Systems: Physics andTechnology, (Springer Series in Materials Science 105). Heidelberg:Springer; 2008.4. Forchel A, Reinhardt M, Fischer M: A monolithic GaInAsN vertical-cavitysurface-emitting laser for the 1.3-μm regime. IEEE Photon Technol Lett2000, 12:1313–1315.5. Jouhti T, Okhotnikov O, Konttinen J, Gomes LA, Peng CS, Karirinne S,Pavelescu E-M, Pessa M: Dilute nitride vertical-cavity surface-emittinglasers. New J Phys 2003, 5:841–846.6. Schires K, Al Seyab R, Hurtado A, Korpijarvi V-M, Guina M, Henning ID, Adams MJ:Optically-pumped dilute nitride spin-VCSEL. Opt Exp 2012, 20:3550–3555.7. Hopkins JM, Smith SA, Jeon CW, Sun HD, Burns D, Calvez S, Dawson MD,Jouhti T, Pessa M: 0.6 W CW GaInNAs vertical external-cavity surfaceemitting laser operating at 1.32 μm. IET Electr. Lett 2004, 40:30–31.8. Guina M, Leinonen T, Härkönen A, Pessa M: High-power disk lasers basedon dilute nitride heterostructures. New J Phys 2009, 11:125019.9. Royall B, Balkan N: Dilute nitride n-i-p-i solar cells. Microelectron J 2009,40:396–398.10. Aho A, Tukiainen A, Polojärvi V, Salmi J, Guina M: High current generationin dilute nitride solar cells grown by molecular beam epitaxy. Proc. SPIE8620, Physics, Simulation, and Photonic Engineering of Photovoltaic Devices II,86201I 2013. doi:10.1117/12.2002972.11. Bonnefont B, Messant M, Boutillier O, Gauthier-Lafaye F, Lozes-Dupuy A,Sallet MV, Merghem K, Ferlazzo L, Harmand JC, Ramdane A, Provost JG,Dagens B, Landreau J, Le Gouezigou O, Marie X: Optimization andcharacterization of InGaAsN/GaAs quantum-well ridge laser diodesfor high frequency operation. Opt Quantum Electron 2006, 38:313–324.12. Luna E, Hopkinson M, Ulloa JM, Guzman A, Munoz E: Dilute nitride baseddouble-barrier quantum-well infrared photodetector operating in thenear infrared. Appl Phys Lett 2003, 83:3111–3113.13. Hashimoto J, Koyama K, Katsuyama T, Iguchi Y, Yamada Y, Takagishi S, Ito MM,Ishida A: 1.3 μm travelling-wave GaInNAs semiconductor optical amplifier.Jpn J Appl Phys 2004, 43:3419–3423.14. Alexandropoulos D, Adams MJ, Hatzopoulos Z, Syvridis D: Proposedscheme for polarization insensitive GaInNAs-based semiconductoroptical amplifiers. IEEE J Quantum Electron 2005, 41:817–822.15. Laurand N, Calvez S, Dawson MD, Bryce AC, Jouhti T, Kontinnen J, Pessa M:Performance comparison of GaInNAs vertical-cavity semiconductoroptical amplifiers. IEEE J Quantum Electron 2005, 41:642–649.16. Suzuki N, Ohashi M, Nakamura M: A proposed vertical-cavity opticalrepeater for optical inter-board connections. IEEE Photon Technol Lett1997, 9:1149–1151.17. Björlin ES, Geske J, Bowers JE: Optically pre-amplified receiver at 10Gb/susing a vertical-cavity SOA. Electron Lett 2001, 37:1474–1475.18. Bouché N, Corbett B, Kuszelewicz R, Ray R: Vertical-cavity amplifyingphotonic switch at 1.5 μm. IEEE Photon Technol Lett 1996, 8:1035–1037.19. Calvez S, Clark AH, Hopkins JM, Merlin P, Sun HD, Dawson MD, Jouhti T, Pessa M:Amplification and laser action in diode-pumped 1.3 μm GaInNAs vertical-cavity structures. In Proceedings of 2002 IEEE/Leos Annual Meeting Conference:10–14 Nov 2002. Glasgow; Piscataway: IEEE; 2002:165–166.20. Alexandropoulos D, Adams MJ: GaInNAs-based vertical cavitysemiconductor optical amplifiers. J Phys: Cond Matt 2004, 16:S3345–S3354.21. Calvez S, Clark AH, Hopkins J-M, Macaluso R, Merlin P, Sun HD, Dawson MD:1.3 μm GaInNAs optically-pumped vertical cavity semiconductor opticalamplifier. Electron Lett 2003, 39:100–102.22. Clark AH, Calvez S, Laurand N, Macaluso R, Sun HD, Dawson MD, Jouhti T,Kontinnen J, Pessa M: Long-wavelength monolithic GaInNAs vertical-cavity optical amplifiers. IEEE J Quantum Electron 2004, 40:878–883.23. Laurand N, Calvez S, Dawson MD, Kelly AE: Index and gain dynamics ofoptically pumped GaInNAs vertical-cavity semiconductor opticalamplifier. Appl Phys Lett 2005, 87:231115–231117.24. Laurand N, Calvez S, Dawson MD, Kelly AE: Slow-light in a vertical-cavitysemiconductor optical amplifier. Opt Express 2006, 14:6858–6863.25. Chaqmaqchee FAI, Balkan N: Gain studies of 1.3-μm dilute nitrideHELLISH-VCSOA for optical communications. Nanoscale Res Lett 2012,7:526–529.26. Calvez S, Hopkins J-M, Smith SA, Clark AH, Macaluso R, Sun HD, DawsonMD, Jouhti T, Pessa M, Gundogdu K, Hall KC, Boggess TF: GaInNAs/GaAsBragg-mirror-based structures for novel 1.3 μm device applications.J Cryst Growth 2004, 268:457–465.27. Mircea A, Caliman A, Iakovlev V, Mereuta A, Suruceanu G, Berseth C-A, Royo P,Syrbu A, Kapon E: Cavity mode—gain peak tradeoff for 1320-nm wafer-fused VCSELs with 3-mW single-mode emission power and 10-Gb/smodulation speed up to 70°C. IEEE Photonics Technol Lett 2007, 19:121–123.doi:10.1186/1556-276X-9-22Cite this article as: Lisesivdin et al.: Optical gain in 1.3-μm electricallydriven dilute nitride VCSOAs. Nanoscale Research Letters 2014 9:22.Submit your manuscript to a journal and beneﬁ t from:7 Convenient online submission7 Rigorous peer review7 Immediate publication on acceptance7 Open access: articles freely available online7 High visibility within the ﬁ eld7 Retaining the copyright to your article    Submit your next manuscript at 7 springeropen.com

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    Lisesivdin, S. B., Khan, N. A., Mazzucato, S., Balkan, N., Adams, M. J., Korpijärvi, V.-M., Guina, M., Mezosi, G., and Sorel, M. (2014)Optical gain in 1.3-μm electrically driven dilute nitride VCSOAs. Nanoscale Research Letters, 9 (1). p. 22. ISSN 1556-276X  Copyright © 2014 The Authors    http://eprints.gla.ac.uk/96239/     Deposited on: 20 August 2014                         Enlighten – Research publications by members of the University of Glasgow http://eprints.gla.ac.uk Lisesivdin et al. Nanoscale Research Letters 2014, 9:22http://www.nanoscalereslett.com/content/9/1/22NANO EXPRESS Open AccessOptical gain in 1.3-μm electrically driven dilutenitride VCSOAsSefer Bora Lisesivdin1,2*, Nadir Ali Khan1,5, Simone Mazzucato1, Naci Balkan1, Michael John Adams1,Ville-Markus Korpijärvi3, Mircea Guina3, Gabor Mezosi4 and Marc Sorel4AbstractWe report the observation of room-temperature optical gain at 1.3 μm in electrically driven dilute nitride verticalcavity semiconductor optical amplifiers. The gain is calculated with respect to injected power for samples with andwithout a confinement aperture. At lower injected powers, a gain of almost 10 dB is observed in both samples. Atinjection powers over 5 nW, the gain is observed to decrease. For nearly all investigated power levels, the samplewith confinement aperture gives slightly higher gain.Keywords: Dilute nitride; Optical injection; Gain; Vertical cavity semiconductor optical amplifiersBackgroundInxGa1−xAs1−yNy semiconductor alloy was first proposedby Kondow et al. in 1996 [1], and considerable research at-tention has been devoted to this alloy system due to itspossible optoelectronic applications at an operating wave-length of 1.3 μm. With the addition of small amounts ofnitrogen into the (In)GaAs lattice, a strong electron con-finement and bandgap reduction are obtained. Further-more, addition of N allows band engineering, allowing thedevice operating wavelength range to extend up to 1.6 μm[2]. An extensive set of different devices based on thisalloy has been fabricated and demonstrated [3]. Examplesof these devices are vertical cavity surface-emitting lasers(VCSELs) [4-6], vertical external cavity surface-emittinglasers [7,8], solar cells [8,9], edge-emitting lasers [10], pho-todetectors [11], semiconductor optical amplifiers (SOAs)[12], and vertical cavity semiconductor optical amplifiers(VCSOAs) [13,14].VCSOAs can be seen as the natural evolution of SOAs,which, owing to their fast response, reduced size, and low-threshold nonlinear behavior, are popular in applicationssuch as optical routing, signal regeneration, and wave-length shifting. Within these fields, VCSOAs have been* Correspondence: sblisesivdin@gmail.com1School of Computer Science and Electronic Engineering, University of Essex,Wivenhoe Park CO4 3SQ, UK2Department of Physics, Faculty of Science and Arts, Gazi University,Teknikokullar, Ankara 06500, TurkeyFull list of author information is available at the end of the article© 2014 Lisesivdin et al.; licensee Springer. ThisAttribution License (http://creativecommons.orin any medium, provided the original work is pused as optical preamplifiers, switches, and interconnects[15-17]. Their geometry provides numerous advantagesover the edge-emitting counterpart SOAs, includinglow noise figure, circular emission, polarization insensi-tivity, possibility to build high-density two-dimensionalarrays of devices that are easy to test on wafer, and low-power consumption that is instrumental for high-density photonic integrated circuits. Generally speaking,a VCSOA is a modified version of a VCSEL that is drivenbelow lasing threshold. The first experimental study of anInxGa1−xAs1−yNy/GaAs-based VCSOA was reported in2002 [18], with a theoretical analysis published in 2004[19]. Several studies on optically pumped InxGa1−xAs1−yNyVCSOAs have been published [14,20-23], while electricallydriven VCSOAs have been demonstrated only in ‘Hellish’configuration [24]. The present contribution builds onthese technological developments to focus on an electric-ally driven multifunction standard VCSOA device operat-ing in the 1.3-μm wavelength window.MethodsThe amplification properties of InxGa1−xAs1−yNy VCSOAswere studied using a 1,265- to 1,345-nm tunable laser (TL;TLM-8700-H-O, Newport Corporation, Irvine, CA, USA),whose output was sent to the sample using the setupshown in Figure 1a. The TL signal was split via a 10/90is an Open Access article distributed under the terms of the Creative Commonsg/licenses/by/2.0), which permits unrestricted use, distribution, and reproductionroperly cited.Figure 1 Experimental setup (a) and the layer structure of the investigated samples (b).Lisesivdin et al. Nanoscale Research Letters 2014, 9:22 Page 2 of 5http://www.nanoscalereslett.com/content/9/1/22coupler to a power meter and to the sample, respectively.Back reflections were avoided using an optical isolatorwhile the TL power was changed from 0 to 7 mW usingan optical attenuator. A lens-ended fiber (SMF-28 fiber,conical lens with cone angle of 80° to 90° and radius of6.0 ±1.0 μm) was used to focus the TL light to the samplesurface as well as to collect its reflected/emitted/amplifiedlight, which was then directed to an optical spectrumanalyzer (OSA). The VCSOA was electrically DC biasedup to 10 mA and stabilized in temperature at 20°C via aPeltier cooler.The investigated VCSOA structure with a 3.5λ cavityis shown in Figure 1b. The structures were grown by asolid source molecular beam epitaxy reactor with a1100 1200 130WavelengReflectivity00.20.40.60.8112Figure 2 Room temperature photoluminescence (red) and reflectancereflectivity spectra of the studied VCSOA structure are shown in black andradio frequency plasma source for incorporating nitro-gen. The growth was carried on an n-type GaAs(100)substrate, and the bottom and top distributed Braggreflectors (DBRs) were doped with silicon (n-type)and beryllium (p-type), respectively. The two DBRscomprised 21 and 24 pairs of AlxGa1−xAs/GaAs layersfor the top and bottom DBR, respectively. The Al con-centrations were x = 0.8 and 0.98 in the top and bottomDBRs, respectively. The confinement aperture, which isrequired for better carrier and light confinement, wasdefined in the uppermost layer of the bottom DBR. Theactive region contains three stacks of three 7-nm-thickIn0.35Ga0.65As0.975 N0.025 quantum wells separated by20-nm thick GaAs spacers. A set of several VCSOA0 1400 1500th (nm)PL Intensity (a.u.)spectra of the studied structure. Experimental and simulatedblue lines, respectively.1100 1150 1200 1250 1300 13509085807570Wavelength (nm)Power (dBm)0mA1mA2mA4mA 6mA 8mA 10mAFigure 3 Power spectra of VCSOA without confinementaperture obtained for different bias currents.Lisesivdin et al. Nanoscale Research Letters 2014, 9:22 Page 3 of 5http://www.nanoscalereslett.com/content/9/1/22samples was fabricated, having different dimensions ofthe top DBR mirror radius (R1), confinement apertureradius (R2), and bottom DBR radius (R3) for cases withand without the confinement aperture. In this paper, wecompare the results obtained for two samples with andwithout confinement aperture, with R1 = 5 μm, R2 =25 μm, and R3 = 50 μm.Results and discussionRoom-temperature reflectivity and photoluminescence(PL) measurements were performed on the as-grownsample, and the results are shown in Figure 2. Simulated1240 1260 1280 1300 1320 1341.0 10 120.0 1001.0 10 122.0 10 123.0 10 124.0 10 125.0 10 12 ASE-ASE01240 1260 1280 1300 1320 1341.0 10 120.0 1001.0 10 122.0 10 123.0 10 124.0 10 12WavelePower (W)AOS + ASE - ASE0)a()c(Figure 4 Results of various power spectra for λinj = 1,279 nm. (a) ASE −power spectra.reflection is also shown in the figure. Two resonancesλR1 and λR2 are observed within the DBR stop band as aresult of the relatively long cavity length [25]. Theprinciple resonance, which is designed for 1.3-μm oper-ation, is observed at λR1 = 1,282 nm, while the other un-wanted resonance at lower wavelength is observed atλR2 = 1,235 nm. Figure 3 shows the VCSOA amplifiedspontaneous emission (ASE) spectra obtained with nooptical injection at different applied bias currents of 0 to10 mA for the sample without confinement aperture.The highest ASE power peak appears at 1,288 nm and isblue-shifted with respect to that of the lasing cavitymode wavelength [26,27]. The other modes are also con-sistent with the PL spectra. Figure 3 shows that with in-creasing the bias current, the amplitude of each modeincreases and also slightly shifts towards higher wave-lengths. This shift is associated with local temperatureincrease in the device. A similar result was observed inthe VCSOA with the confinement aperture.Since no significant change in the spectrum amplitudeabove 7 mA was observed, we investigated the devicesup to this current value. ASE power spectra taken at 7and 0 mA (ASE0) are used to extract the spectrumASE −ASE0, which is the spectrum where the backgroundnoise is minimized. To find the amplified optical signal(AOS), we injected light sweeping the TL wavelength(λinj) from 1,266 to 1,310 nm with a 7-mA current bias.Figure 4 shows results for injection at λinj =1,279 nmonly. We could not investigate the second resonancepeak λR2 because of the wavelength limit of the TL. In0 1240 1260 1280 1300 1320 13401.0 10 122.0 10 123.0 10 124.0 10 125.0 10 126.0 10 12AOS + ASE0ngth (nm)1240 1260 1280 1300 1320 13400.0 1001.0 10 122.0 10 123.0 10 124.0 10 12AOS - ASE0)b()d(ASE0level, (b) AOS + ASE, (c) AOS + ASE − ASE0, and (d) AOS − ASE0018642051015Power (nW)Gain (dB)Without confinement apertureWith confinement apertureFigure 6 Power-dependent gain for the samples with andwithout confinement aperture.Lisesivdin et al. Nanoscale Research Letters 2014, 9:22 Page 4 of 5http://www.nanoscalereslett.com/content/9/1/22Figure 4a,b,c,d, the results for ASE −ASE0, AOS + ASE,AOS + ASE −ASE0, and finally AOS −ASE0spectra areshown, respectively.As the gain is small, the amplified signal cannot beeasily discerned in Figure 4d. Hence, the gain was calcu-lated using the simple relationGain ¼ AOSþ ASE‐ASE0AOS‐ASE0ð1Þfor each wavelength after obtaining AOS and ASE data.Results are shown as a function of the injected wavelengthin Figure 5 for a specific laser power (Pinj) of 2.25 nW. Amaximum gain of 3 dB with a very broad peak is observedat the maximum ASE wavelength of 1,288.5 nm. In thestudy, measured signal levels are very near to limits of theOSA; therefore, larger bandwidth wavelength values areused, which can be the reason of the broadness of the gainpeak.Having verified that the gain peak corresponds to theASE peak wavelength, we investigated the Pinj depend-ence by varying it from 1.5 nW to a few milliwatts forthe single wavelength of 1,288.5 nm. Results are pre-sented for both samples with and without confinementaperture in Figure 6 for power values below 10 nW.For injected laser powers over 5 nW, the gain falls rap-idly. At the lowest injected power, the sample withconfinement aperture exhibits 10 dB of gain, which isobserved near the maximum ASE wavelength. For theinvestigated injected power range, the sample with theconfinement aperture showed a higher gain because ofthe better carrier and light confinement in the VCSOA.ConclusionsIn this paper, we report the observation of gain in anelectrically driven dilute nitride VCSOA device operated1260 1270 1280 1290 1300 131000.511.522.533.5Wavelength (nm)Gain (dB)Figure 5 Gain versus injected laser wavelength with Pinj = 2.25 nW.at 1.3-μm in reflection mode. Two different types of sam-ples with and without confinement aperture are investi-gated. The ASE power peak is found to be at 1,288.5 nmwith additional modes, which are caused by the length ofthe cavity. Optical gain is found to occur at low optical in-jection values. Above 5 nW of optical injection, the gain isfound to fall rapidly. The maximum observed optical gainis observed at 1,288.5 nm at room temperature. The max-imum observed optical gain at 7-mA current at roomtemperature is around 10 and 6 dB for samples with andwithout confinement aperture, respectively. It is importantto mention that despite the small gain, the device is verypromising because it requires very small currents com-pared with in-plane SOAs.Competing interestsThe authors declare that they have no competing interests.Authors’ contributionsSBL, NAK, and SM carried out the measurements and data analysis. VMK andMG performed the growth of structures. GM and MS carried the fabricationof devices. MJA performed the theoretical studies and analysis. NB is theproject leader. SBL and NB wrote the paper. All authors read, corrected, andapproved the final manuscript.AcknowledgementsSupport from EPSRC under grant EP/G023972/1 is gratefully acknowledged.Sefer Bora Lisesivdin also acknowledges partial support from the TurkishScientific and Technological Research Council (TUBITAK) 2219 codedscholarship. COST Action MP0805 is also gratefully acknowledged.Author details1School of Computer Science and Electronic Engineering, University of Essex,Wivenhoe Park CO4 3SQ, UK. 2Department of Physics, Faculty of Science andArts, Gazi University, Teknikokullar, Ankara 06500, Turkey. 3OptoelectronicsResearch Centre (ORC), Tampere University of Technology, P.O. Box 692,Tampere 33101, Finland. 4School of Engineering, University of Glasgow,Glasgow GG12 8QQ, UK. 5Kohat University of Science and Technology(KUST), Kohat, Khyber Pakhtoonkhwa, Pakistan.Received: 17 October 2013 Accepted: 17 December 2013Published: 13 January 2014Lisesivdin et al. 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Optical gain in 1.3-μm electrically driven dilute nitride VCSOAs

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