Abstract We demonstrate that air-guiding photonic bandgap fibers filled with various gases, such as acetylene and methane provide optical wavelength references. The constructed devices are compact and cost-effective

H Ludvigsen

J C Petersen

J Tuominen

T Ritari

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Gas filled photonic bandgap fibers as wavelength references
J. Tuominen a,*, T. Ritari a, H. Ludvigsen a, J.C. Petersen b
a Fiber-Optics Group, Department of Electrical and Communications Engineering, Helsinki University of Technology,
P.O. Box 3500, FI-02015 TKK, Finland
b Danish Fundamental Metrology, Matematiktorvet Building 307, DK-2800 Kgs. Lyngby, Denmark
Received 30 March 2005; received in revised form 8 June 2005; accepted 8 June 2005
Abstract
We demonstrate that air-guiding photonic bandgap fibers filled with various gases, such as acetylene and methane
provide optical wavelength references. The constructed devices are compact and cost-effective.
 2005 Elsevier B.V. All rights reserved.
PACS: 33.20.Ea; 42.70.Qs; 42.81.Wg
1. Introduction
Molecular absorption lines are widely used as
absolute optical wavelength references for calibra-
tion of measurement equipment and frequency sta-
bilization of lasers [1,2]. Absolute wavelength
references have been proposed and realized based
on gases such as acetylene (C2H2), carbon monox-
ide (CO), hydrogen cyanide (HCN) and methane
(CH4) for the 1300 and 1550 nm regions [3–5].
The gas is typically enclosed in a 5–20 cm long
absorption cell. The employed gas pressure
depends on the gas species and the cell design.
Usually, moderately low pressure (50 mbar) is
desired to avoid the excess pressure broadening
and pressure shift of the absorption lines [6]. Thus
for weakly absorbing molecules, one might need to
increase the interaction length between light and
gas to obtain a sufficient signal level. The optical
path length can be increased by using a multi-pass
configuration but it will usually be limited to
around one meter when a portable wavelength cal-
ibration device is considered. Therefore, to assure
an adequate strength of the absorption lines, a
compromise between the pressure and the optical
path length is often needed.
The recently emerged air-guiding photonic
bandgap fibers (PBFs) provide a means to effec-
tively exploit the interaction of light and gas in a
fiber [7,8]. Such fibers do not guide light by total
0030-4018/$ - see front matter  2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.optcom.2005.06.021
* Corresponding author. Tel.: +358 9 4512338; fax: +358 9
4513128.
E-mail address: jesse.tuominen@tkk.fi (J. Tuominen).
Optics Communications 255 (2005) 272–277
www.elsevier.com/locate/optcom
internal reflection as in conventional optical fibers
but instead make use of the photonic bandgap
effect which confines certain light frequencies to
the hollow core. In these fibers more than 99% of
the light energy can be guided inside the empty
core and the nearest neighboring cladding holes
[9]. In addition, since PBFs have relatively low
bending and transmission loss, they can be coiled
on a small-sized spool to realize compact and
cost-effective artifacts with a very long optical path
length [10–12]. Filling the central voids of a PBF
with gas gives a possibility to obtain a large
mode-gas overlap and a long optical interaction
length at the same time while only a very small
quantity of gas is required to fill the core and clad-
ding voids. This allows for the detection of weak
absorption lines, which are more difficult to mea-
sure using conventional absorption cells due to
their limited optical path length.
In this paper, we demonstrate that air-guiding
PBFs filled with suitable gases can be used as
wavelength references in the 1550 and 1300 nm re-
gions. The fiber samples that are spliced at one end
to a single-mode fiber are filled with acetylene or
methane and, subsequently, sealed using index-
matching adhesive to realize wavelength refer-
ences. We make use of the concept to stabilize
the output frequency of a tunable laser to the cen-
ter frequency of a weak absorption line of acety-
lene by using lock-in technique. Moreover, we
show for the first time that lines of hot and mixed
isotope bands of acetylene can be utilized to accu-
rately reference wavelength-division multiplexing
(WDM) channels.
2. Fiber samples
Two different air-guiding PBFs (Crystal Fibre
A/S) were employed to realize absolute wavelength
references for the telecommunications bands at
1550 and 1310 nm. The wavelength ranges of the
photonic bandgap guidance in the fibers are
approximately 1250–1380 and 1510–1570 nm,
respectively. In both samples, the core was formed
by removing seven silica tubes from the center of
the preform. The characteristics of the two PBFs
labeled PBF1300 and PBF1550 are summarized
in Table 1 and the microscope image of the
PBF1550 is shown in Fig. 1. The samples are
spliced at one end to a standard single-mode fiber
terminated by a FC/PC fiber connector to allow
for easy light coupling and improved stability.
The splice loss was estimated to be 1 dB for
PBF1300 and 2 dB for PBF1550. The transmis-
sion losses for the fiber samples are less than 0.1
and 0.2 dB/m, respectively.
3. Molecular references
The absolute wavelength references commonly
used are based on the interaction of light and gas
in a low-pressure absorption cell. Each gas species
has a unique absorption spectrum consisting of
absorption bands, and the bands consisting of sev-
eral absorption lines. Many parameters need to be
considered when selecting the gas species to be
used for reference purposes: the location and
Table 1
Characteristics of the PBFs
PBF1300 PBF1550
Core size (lm) 11.6 8.5
Length (m) 10 5
Loss (dB/m) 0.1 0.2
Fig. 1. Microscope image of photonic bandgap fiber guiding
around 1550 nm.
J. Tuominen et al. / Optics Communications 255 (2005) 272–277 273
width of the bands, number of absorption lines,
strength of lines, pressure, absorption length,
availability, hazards, etc. The most utilized and
studied reference materials for the near infrared
1300 and 1550 nm bands are acetylene (12C2H2,
13C2H2) [12–15], carbon monoxide (
12C16O)
[5,16], carbon dioxide (12C16O2) [17], hydrogen
cyanide, (H13C14N) [16] and methane (CH4) [5].
The wavelength ranges of the absorption spectra
of these gases are presented in Fig. 2.
The WDM channels in fiber-optic communica-
tions systems are standardized by the International
Telecommunication Union (ITU-T) recommenda-
tion [19] to operate at frequencies ranging down
from 196 THz with a constant spacing of 100
GHz. The specified maximum wavelength drift
from the channel center is 20 GHz. Extensive re-
search has been carried out to find molecular
absorption lines that coincide with these channels
with an accuracy better than 2 GHz thus being
an order of magnitude better than the specified
maximum drift. Various acetylene lines and ITU-
T channels that nearly coincide are listed in Table
2. The four lines marked with an asterisk have not
been suggested, to our knowledge, to be used as
WDM channel references. All these lines are
located within 1 GHz from the nearest channel.
4. Realization of reference artifact
The experimental setup employed to realize a
compact wavelength reference is depicted in
Fig. 3. The open end of the PBF was butt-coupled
to a multi-mode fiber using a V-groove and was
subsequently placed inside a vacuum chamber.
The gap between the two fibers was around 100
lm. The fiber was then evacuated using a conven-
tional rotary pump for a time period of minimum
12 h before filling it with either 12C2H2 or
12CH4 to
the desired pressure. The evacuation times were
Fig. 2. Most common gas species for referencing the wave-
length range used for optical communication [5,16,18].
Table 2
Absorption lines of C2H2 coinciding with ITU-T WDM channels [14]
Line Molecule ITU channel Wavelength (nm) Frequency (THz) Difference to ITU channel (MHz)
P(5)a 12C13CH2 2 1531.1234 195.79901 1000
P(14) 12C2H2 5 1533.4613 195.50051 +500
P(13)a 12C13CH2 8 1535.8295 195.19905 950
P(18)a 12C13CH2 12 1538.9689 194.80086 +850
P(17)b 13C2H2 13 1539.7641 194.70025 +250
Pe(14)
a 12C2H2 13 1539.7623 194.70048 +500
P(13) 13C2H2 14 1540.5673 194.59874 1250
Note. The designation e is defined within [14].
a New candidates for WDM channel references.
b m1 þ v2 þ m14 þ m15 band.
Fig. 3. Measurement setup for realization of reference artifact.
Abbreviations: PBF – photonic bandgap fiber; MMF – multi-
mode fiber.
274 J. Tuominen et al. / Optics Communications 255 (2005) 272–277
investigated by first filling the fiber with acetylene
and subsequently evacuating it while measuring
the strength of the absorption line.
A special arrangement was used for sealing of
the gas-filled PBF. A thin tube for the application
of transparent adhesive and an UV guide were
built inside the vacuum chamber. After the fiber
had been filled with gas, a small amount of adhe-
sive was injected to the butt coupling. Subse-
quently, the chamber pressure was rapidly
elevated to around 20 mbar to prevent the adhe-
sive from boiling when exposed to UV-radiation.
The refractive index of the adhesive (Norland;
optical adhesive 61) was estimated to be 1.53 at
1.55 lm. The evaporation pressure at room tem-
perature was found to be 2 mbar before being
exposed to UV-radiation. We observed that the
insertion of the index-matching adhesive intro-
duces an additional transmission loss of 2 dB.
5. Measurement results
The absorption spectrum of the 5-m long
PBF1550 filled with C2H2 (1.1% natural abun-
dance of 13C) was measured using the experimental
setup described in Fig. 3. The fiber was filled to
various pressures to measure the different branches
of acetylene after it had been pumped empty for a
time period of minimum 12 h to effectively evacu-
ate the air holes inside the fiber. The standard
m1 + v3 P-branch was recorded for a pressure of 1
mbar by scanning the wavelength of the tunable
laser in wavelength steps of 1 pm. The measured
spectrum is presented in Fig. 4. Even at such a
low pressure, the absorption lines reached a maxi-
mum strength of 16 dB and lines up to P31 are
visible.
Encouraged by the high sensitivity of the gas-
filled PBFs, we investigate the applicability of
weak absorption lines as possible candidates for
wavelength references. Increasing the gas pressure
to 20 mbar allows study of the m1 þ v3 þ m14  m14
hot band, m1 þ v3 þ m15  m15 hot band, and the
m1 + v3
12C13CH2 mixed isotope bands in detail.
In this way, we were able to observe the four lines
P(5), P(13), P(18) and Pe(14) given in Table 2. A
part of the measured spectrum showing the three
latter lines is presented in Fig. 5. At this pressure,
the pressure shift and pressure broadening of the
lines are still small being 0.1 and 2 pm at maximum
[6] and thus allowing these lines to be used as
wavelength references [3] for WDM systems.
Subsequently, we applied the concept to stabilize
the output frequency of a commercially available
tunable external cavity laser (Nettest Tunics Plus)
by locking it to the center point of an absorption
line using a lock-in technique. For this purpose,
the lines marked in Fig. 5 were chosen. The center
Fig. 4. Absorption spectrum of the P-branch of 12C2H2
measured with a 5 m long PBF filled to 1 mbar with a
resolution of 1 pm.
Fig. 5. Absorption spectrum of acetylene in a 5 m long PBF
filled to 20 mbar showing the weak bands and the lines
coinciding with ITU-T WDM channels 8, 12 and 13. The
resolution used was 1 pm.
J. Tuominen et al. / Optics Communications 255 (2005) 272–277 275
point of the absorption line is preferred instead of
the slope as this makes the absorption line wave-
length values straightforward to compare with
values found in the literature [13,14]. The measure-
ment results are presented in Fig. 6. The laser fre-
quency stayed within 100 MHz for a time period
of 20 h with an integration time of 100 ms. The
performance is sufficient for future dense WDM
systems with channel spacing of only 25 GHz.
The values were measured with a wavemeter (Bur-
leigh WA-7600) specified to a resolution of 0.1 pm
and an accuracy of 0.3 pm.
It appears likely that WDM will be extended to
cover the entire useful transmission band of opti-
cal fiber. Therefore, to provide a wavelength refer-
ence for the 1300 nm region, we filled a 10-m long
piece of bandgap fiber guiding in this region with
12CH4. Methane has a congested spectrum of weak
absorption lines around this region [5]. For refer-
encing purposes, it is important to choose a single
line to obtain high accuracy. One particularly suit-
able single line belonging to the R-branch of the
m2 + 2m3 combination band near 1314 nm has been
recently measured using a heterodyne technique
[5]. Since methane is 100 times weaker absorbing
gas than acetylene, a longer fiber was chosen to in-
crease the strength of the absorption lines. With
conventional absorption cells, it is cumbersome
to achieve such a long path length. The spectrum
recorded for a pressure of 900 mbar is shown in
Fig. 7. The single line is marked in the figure by
an arrow.
Fig. 6. Frequency stability of laser locked to P(5) of 12C13CH2, P(18) of
12C13CH2 and Pe(14) of the m1 þ v3 þ m14  m14 transition of
12C2H2 lines of acetylene for 20 h.
Fig. 7. Absorption spectrum of CH4 at 900 mbar measured
with PBF1300 using a 5 pm resolution.
276 J. Tuominen et al. / Optics Communications 255 (2005) 272–277
6. Conclusion
We have demonstrated that PBFs filled with
acetylene or methane can provide optical wave-
length references for monitoring and calibrating
channels in WDM communication systems. With
the long natural interaction length of the fiber,
weak absorption lines can be used without the
need of long multi-pass absorption cells. By using
a lock-in technique, the output frequency of a tun-
able laser was successfully stabilized to three weak
absorption lines of acetylene, which coincide nicely
with WDM channels.
Acknowledgments
J.T., T.R. and H.L. are grateful to the Academy
of Finland. T.R. is also thankful to the Graduate
school of Modern Optics and Photonics for finan-
cial support.
References
[1] J.L. Hall, Laser Phys. 4 (1994) 306.
[2] A. Czajkowski, A.A. Madej, P. Dubé, Opt. Commun. 234
(2004) 259.
[3] W.C. Swann, S.L. Gilbert, J. Opt. Soc. Am. B 17 (2000)
1263.
[4] S.L. Gilbert, S.M. Etzel, W.C. Swann, in: Proceedings of
the Optical Fiber Communication Conference, paper
ThC1, Anaheim, USA, 2002.
[5] T. Dennis, E.A. Curtis, C.W. Oates, L. Hollberg, S.L.
Gilbert, J. Lightwave Technol. 20 (2002) 804.
[6] M. Kusaba, J. Henningsen, J. Mol. Spectrosc. 209 (2001)
216.
[7] R.F. Cregan, B.J. Mangan, J.C. Knight, T.A. Birks, P. St.
J. Russell, P.J. Roberts, D.C. Allan, Science 285 (1999)
1537.
[8] T. Ritari, J. Tuominen, J.C. Petersen, T. Sørensen, T.P.
Hansen, H. Simonsen, H. Ludvigsen, Opt. Express 12
(2004) 4080.
[9] G. Humbert, J.C. Knight, G. Bouwmans, P. St. J. Russell,
D.P. Williams, P.J. Roberts, B.J. Mangan, Opt. Express 12
(2004) 1477.
[10] T.P. Hansen, J. Broeng, C. Jakobsen, G. Vienne, H.R.
Simonsen, M.D. Nielsen, P.M.W. Skovgaard, J.R. Fol-
kenberg, A. Bjarklev, J. Lightwave Technol. 22 (2004) 11.
[11] P.J. Roberts, F. Couny, H. Sabert, B.J. Mangan, D.P.
Williams, L. Farr, M.W. Mason, A. Tomlinson, T.A.
Birks, J.C. Knight, P. St. J. Russell, Opt. Express 13 (2005)
236.
[12] T. Ritari, J. Tuominen, J.C. Petersen, T.P. Hansen, H.
Ludvigsen, in: Proceedings of the European Conference on
Optical Communication (ECOC 04), paper Mo3.3.2, 2004.
[13] K. Nakagawa, M. de Labachelerie, Y. Awaji, M. Kourogi,
J. Opt. Soc. Am. B 13 (1996) 2708.
[14] R.E. Hachtouki, J.V. Auwera, J. Mol. Spectrosc. 216
(2002) 355.
[15] J. Henningsen, J.C. Petersen, in: Proceedings of the 6th
Optical Fibre Measurement Conference (OFMC 01), 2001,
p. 183.
[16] S.L. Gilbert, T. Dennis, W.C. Swann, in: Proceedings of
the Workshop on Contemporary Photonic Technologies
(CPT 2001), Tokyo, Japan, 2001, p. 143.
[17] J. Henningsen, H. Simonsen, J. Mol. Spectrosc. 203 (2000)
16.
[18] J.C. Petersen, J. Henningsen, in: Proceedings of the
XXIVth URSI General Assembly, Maastricht, The Neth-
erlands, paper 1777, 2002.
[19] International Telecommunications Union (ITU), Recom-
mendation G.692 – Transmission media characteristics –
Characteristics of optical components and sub-systems,
1998.
J. Tuominen et al. / Optics Communications 255 (2005) 272–277 277


Gas filled photonic bandgap fibers as wavelength references

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Gas filled photonic bandgap fibers as wavelength references

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