Pulse trains with a 42GHz repetition rate were generated by monolithic InGaAs/AlGaAs double quantum well lasers at a wavelength of 985 [angstroms]. The cavity was electrically divided into three regions, one providing gain and the other two providing saturable absorption. The optical modulation has a depth greater than 98% and full-width at half-maximum under 6ps, and bias conditions for sustained mode-locking are determined

Eng, L.

Sanders, S.

Yariv, A.

English

Caltech Authors - Main

support of our work. They are also grateful to C. Brehm from 
Centre de Recherches de la CGE (Marcoussis) and M. 
Monerie from Laboratoires CNET (Lannion) for providing 
the optical fibres. This work has been supported by CNET 
under contract 88 6B 101. 
L. CHUSSEAU 4th June 1990 
I. M. XIE 
L. DUVILLARET 
J. M. LOURTIOZ 
Institut d’Electronique Fondamentale, U R A  22 du CNRS 
Uniuersite Paris-Sud, Bdtiment 220 
91405 Orsay Cedex, France 
A. ACCARD 
J-P. HEBERT 
Centre de Recherches de ia CGE 
Route de Noray 
91460 Marcoussis, France 
References 
1 HAWKINS, R. T. : ‘Generation of < 3 ps optical pulses by fibre com- 
pression of gain-switched InCaAsP DFB laser diode pulses’, Elec- 
tron. Lett., 1990,26, pp. 292-294 
2 TSKADA, ., IWATSUKI, ., and SARUWATARI, M.: ‘Picosecond laser 
diode pulse amplification up to 12 W by laser diode pumped 
erbium-doped fiber’, IEEE Photonics Technol. Lett., 1990, 2, pp. 
122-124 
3 CHUSSEAU, L., BOUCHON, D., O U V I L L ~ T ,  L., and LOURTIOZ, 1.-M.: 
‘Autocorrelation at 1.3-1-5pm using POM crystal’, Electron. Lett., 
1990,26, pp. 589-590 
LAU, K. Y.: ‘Short-pulse and high-frequency signal generation in 
semiconductor lasers’, J .  Lightwave Technol., 1989, LT-1, pp. 
W 1 9  
LIU, H.-F. et al .:  ‘Gain-switched picosecond pulse (<lops) gener- 
ation from 1.3pm InGaAsP laser diodes’, IEEE J. Quantum Elec- 
tron., 1989, QE25, pp. 1417-1425 
KUROBORI, I., CHO, v., and MATSUO, Y.: ‘An intensity/phase autocor- 
relator for the use of ultrashort optical measurements’, Opt. 
Commun., 1981,40, pp. 15&160 
4 
5 
6 
PASSIVE MODE-LOCKING OF MONOLITHIC 
InGaAs/AIGaAs DOUBLE QUANTUM WELL 
LASERS AT 42GHz REPETITION RATE 
Indexing terms: Lasers and laser applications, Semiconductor 
lasers 
Pulse trains with a 42GHz repetition rate were generated by 
monolithic InGaAs/AIGaAs double quantum well lasers at a 
wavelength of 9850A. The cavity was electrically divided into 
three regions, one providing gain and the other two provid- 
ing saturable absorption. The optical modulation has a depth 
greater than 98% and full-width at half-maximum under 6ps, 
and bias conditions for sustained mode-locking are deter- 
mined. 
Monolithic GaAs/AIGaAs semiconductor lasers have been 
passively mode-locked at repetition rates around lOOGHz by 
dividing the cavity into regions of gain and saturable absorp- 
tion, and pumping the lasers with low-frequency electrical 
 source^.'^^ Such devices may eventually find applications in 
communications, synchronisation of optoelectronic systems or 
as sources for spectroscopy and sampling systems. Great sta- 
bility in the output will be required and devices with various 
lasing wavelengths will be necessary. Lasers made from 
strained InGaAs/AlGaAs double quantum wells can be passi- 
vely mode-locked to produce pulses at 9850A. When the repe- 
tition rate is reduced to 42GHz, by cleaving longer devices, 
the observed pulse trains are substantially more stable than 
those from a I08GHz GaAs/AlGaAs triple quantum well 
laser.’ 
The laser, illustrated in Fig. 1, is grown by molecular beam 
epitaxy on an n-type GaAs substrate and consists of a 1.5pm 
ELECTRONICS LETTERS 5rh Julv 1990 Vol. 26 No. 14 
n-type A1,,Ga0.,As cladding layer, the graded index and 
double quantum well region, a 1.5pm p-type AI,.,Ga,.,As 
absorber 
RIN and DQW region 
distance from cladding. I\ 
5A~OL1000 2000 3000 7 
:m”)  
In ,Go , . ,As  
Fig. 1 Device structure 
! 
a Three-section 42GHz InGaAsIAIGaAs laser 
b Enlargement of graded-index and double quantum well region 
layer, and a 2000A heavily p-doped cap layer. The double 
quantum well structure consists of two 50A InGaAs layers 
spaced by a lOOA GaAs barrier, between two 40A GaAs 
layers. Each graded index region is 2000A thick. 
The laser mesa is formed by etching 3pm wide stripes to a 
depth of 1.2pm. The top metallisation is defined by liftoff, 
leaving 20pm gaps between the sections, and electrical iso- 
lation of 1 kR is effected by etching 7pm gaps through the cap 
layer. The gain region is 595pm long, and the front and rear 
absorbers are 140 and 180pm long, respectively. The gain 
region is driven by 50ps electrical pulses, while the front 
absorber is left open-circuited and the rear absorber DC 
biased through a lOOR series resistor, across which the 
current is measured by sample-and-hold amplifiers while the 
gain section current is on. Measurements are typically per- 
formed 25ps after the start of the gain section pulse. 
The average output power as a function of current is shown 
in Fig. 2 for two bias conditions. In Fig. 2 4  all three sections 
of the laser are driven in parallel, and the threshold current is 
28mA. There is a linear dependence of output power on 
current above threshold and a differential external quantum 
efficiency of 60%. When the rear absorber is reverse biased, a 
light jump ocurs at threshold. At higher gain currents, where 
the absorber is more saturated, there is another large differen- 
os&- ~ 
0 2 5  50 75 100 125 150 
current, m A  
Fig. 2 Light output against currentfor three-section laser 
a All three sections connected in parallel 
b Centre section pumped, rear absorber with - 2.2 V bias through 
IOOR series resistance, front absorber open-circuited 
1087 
tial increase in light output, after which conditions where 
passive mode-locking can be observed are reached. At gain 
section currents above 145mA, the power drops, and streak 
camera measurements show self-pulsations. 
The output from the front facet is measured using a Hama- 
matsu C1537 streak camera, with an S-1 photocathode, in its 
single-shot mode. Fig. 3 shows typical pulse trains near 
optimum bias conditions on the two fastest streak scales. The 
0 ISI
time, p s  
a 
1204 I 
0 100 200 300 LOO 
1507/3) time, ps 
b 
Fig. 3 Outputfromfront facet 
a Time resolution better than 2 ps 
b 450ps streak 
pulsewidths average 5 . 9 ~ ~  and have a 42GHz repetition rate. 
The average peak power is estimated to be 80mW, and the 
modulation depth exceeds 98%. Integrating under the peaks 
in Figs. 3a and 36 gives standard deviations in the pulse ener- 
gies of 9% and 14%, respectively. Photocathode shot noise 
can account for about a 3% deviation. Other sources of the 
variations include lower frequency intensity relaxation oscil- 
lations, spontaneous emission noise and inhomogeneities in 
the streak camera system. Only a small number of pulses are 
compared, and more accurate noise characterisation will 
require measurements over longer time periods, possibly using 
a very fast photodetector and spectrum analyser. The optical 
spectra were typically very broad, with full-widths at half- 
maximum of 400GHz or higher, and centred near 9850A.’ 
The ranges of both gain and absorber current where mode- 
locking is consistently observed are shown in Fig. 4. To  
perform these measurements, the gain section current is fixed, 
while the absorber bias is swept in the negative direction. The 
output is unmodulated or shows some unstable structure at 
901 
-30 -2s -20 -1 s -10 
rear absorber current, m A  
Fig. 4 Absorber currents for sustained mode-locking ai fixed gain 
section currents 
frequencies near the cavity resonance. At stronger bias, the 
current flowing out of the absorber continues to increase and 
the output appears similar to that in Fig. 3. The sustained 
pulse train remains until the absorber bias is increased to 
where the laser self-pulsates, at which point the absorber 
current drops by several milliAmperes. A second laser on the 
same cleaved bar had almost the same locking ranges. The 
locking ranges are qualitatively consistent with those predict- 
ed from a three mode coupling theory.4 
Self-sustained pulsation and large relaxation oscillations 
greatly limit the range of stable passive mode-locking in this 
and othr devices. A shorter, two-section laser, whose output is 
shown in Fig. 5, displays a 4GHz, 80% modulation in the 
envelope of the mode-locked pulse train, indicating that the 
modes remain coupled while their amplitudes oscillate. The 
300 
5 250 
+ 
> ; 200 
+ 
f 150 
6 100 
B so 
I 
200 400 600 800 1000 
time, p s  
a 
1001  I h  I 
b 
Fig. 5 Strong 4 G H z  modulation ufmode-locked pulse amplitudes 
L,,, = 545pm; L,,, = 135pm; I,,. = 84ma; I,,, = - 13.9mA 
a 12ps resolution, 1 nanosecond streak 
b 2 ps resolution, l a p s  streak 
conditions for self-sustained pulsation and passive mode- 
locking of inhomogeneously pumped lasers are similar, since 
lower saturation intensity of the absorption than the gain and 
short carrier lifetimes in the absorber facilitate both pro- 
 cesse^.^.^ 
InGaAs/AlGaAs double quantum well lasers have been 
passively mode-locked at 42 GHz. The output wavelength is 
centered around 9850 A, and large current ranges exist where 
sustained mode-locking is observed. Instabilities in the form of 
large modulation at a few gigaHertz severely limit the condi- 
tions where stable passive mode-locking can be achieved. 
This work was supported by the Air Force Offce of Scienti- 
fic Research, the Offce of Naval Research, and the National 
Science Foundation. One author (L. E.) acknowledges the 
support of an IBM graduate fellowship. 
S. SANDERS 
L. ENG 
A. YARIV 
Department of Applied Physics, 128-95 
California Institute of Technology 
Pasadena, California 91125, U S A  
References 
5th May 1990 
1 YASIL’EV, P. P., and SERGEEV, A. B.: ‘Generation of bandwidth- 
limited 2ps pulses with IOOGHz repetition rate from multi- 
segmented injection laser’, Electron. Lett., 1989,25, p. 1049 
E L E C T R O N I C S  LETTERS 5th July 1990 Vol .  26 N o .  14 1088 
2 SANDERS, s., ENC. L., PASLASKI, J., and YARIV, A.: ‘108GHz passive 
mode locking of a multiple quantum well semiconductor laser 
with an intracavity absorber’, Appl.  Phys. Lett., 1990,56, p. 310 
LAU, K. Y.: ‘Narrow-band modulation of semiconductor lasers at 
millimeter wave frequencies (> 100GHz) by mode locking’, IEEE 
J .  Quantum Electron., 1990, QE-26, p. 250 
4 LAU, K. v.: ‘Efficient narrow-band direct modulation of semicon- 
ductor injection lasrs at millimeter wave frequencles of l00GHz 
and beyond, Appl. Phys. Lett., 1988,52, p. 2214 
6 UENO, M., and LANG, R.: ‘Conditions for self-sustained pulsation 
and bistability in semiconductor lasers’, J. Appl .  Phys., 1985, 58, p. 
1689 
3 
INFLUENCE OF LEAKAGE CURRENTS 
BETWEEN ELECTRODES I N  TUNABLE 
D B R - LAS E RS 
Indexing term: Semiconductor lasers 
Wavelength tunable multi-section DBR lasers have been fab- 
ricated on a p-doped InP substrate by LPE. Maximum 
output power is 10.5mW and largest step-wise tuning range 
is 5.1 nm for a single-sided DBR laser. A double-sided laser 
has a continuous tuning range of 1.4nm. It  is shown that the 
low electrical isolation between the top contacts causes lower 
output power and a significantly larger laser linewidth. 
Wavelength tunable distributed Bragg reflector lasers are 
attractive as transmitters in optical fibre communication 
systems. They are also suitable as tunable local oscillators for 
channel selection in coherent frequency multiplexed systems 
or as tunable wavelength filters’ in direct detection systems. A 
wide tuning range together with a narrow spectral linewidth 
are important parameters. 
The BIG-DBRz laser is shown in Fig. 1. The fabrication 
includes four steps of liquid phase epitaxy. Reactive ion 
substrate 
Fig. 1 Double-sided BIG-DER laser 
etching and wet chemical etching are used to form the mesas, 
and the transverse current confinement is achieved using the 
FBH s t ru~ tu re .~  We have chosen to fabricate the lasers on a 
p-doped InP substrate because of better control of the pn- 
junction position. The electrical isolation between the top con- 
tacts is increased by etching l p m  deep and 15pm wide 
grooves in the top contact and cladding layers and implanting 
He’’ ions. We have chosen helium ions instead of protons 
because of their better thermal stability in the subsequent pro- 
cessing steps. Due to the limited resistivity achievable in the 
n-type cladding and waveguide materials: the resulting elec- 
trode separation resistance is only about 50Q in the present 
structure. Without the helium ion implantation the isolation is 
less than 20 Q. 
The threshold current of a three-electrode DBR laser 
having a 140pn long front grating repion, a 300pm long 
active region, and a 360pm long rear grating is measured to a 
low value of 19 mA. The light-current characteristic is shown 
in Fig. 2a, where the discontinuities in the curve are caused by 
mode-jumps. The low maximum output power of 0.9 mW can 
be explained by the high reflectivity (KL 1.8) of the output 
grating, and also by leakage currents from the active region to 
the passive grating regions which cause absorption losses.s 
ELECTRONICS LETTERS 5th Julv 1990 Vol. 26 No. 14 
A way to eliminate the leakage currents through the passive 
waveguides is to short-circuit the grating contacts to the 
50 100 
injection current, r n A  
a 
1533 1538 
wavelength, nm 
b 185112/ 
Fig. 2 Characteristics of three section DER laser 
a Ligh-current 
b Continuous tuning range 
common bottom electrode. The leakage currents from the 
active region will in this case increase since once the current 
has passed the electrode separation region, it is directed 
straight to the bottom contact and not through the pn- 
junction in the grating regions. This results in an increased 
threshold current; for this laser it increases from 19 to 46mA. 
In spite of this, the maximum output power increases to 
1.3mW because of the reduced number of free carriers in the 
passive waveguides and thereby the reduced waveguide 
absorption (Fig. 20). 
By increasing the resistance between the grating contacts 
and the bottom contact the leakage current is redirected to 
pass through the grating regions so that the refactive index in 
the passive waveguides is lowered due to the plasma effect. 
This results in a continuous tuning of 1.4nm towards shorter 
wavelengths when the total leakage current from the two 
grating electrodes is reduced from 47 to 6mA (Fig. 2b). At the 
same time the output power is reduced from 1.15 to 0.45mW 
due to the increased waveguide absorption. 
Two-electrode DBR lasers are obtained from the same 
wafer by cleaving the front grating region away. The best 
sample has a threshold current of 21mA and a maximum 
output power of 10.5mW, without any mode-jump. The 
quantum efficiency is 21% for the front facet. Continuous 
tuning is not possible with this configuration, having two sec- 
tions only. The output wavelength can be reduced in a step- 
wise manner by increasing the current through the grating 
1089 


Passive Mode-Locking of Monolithic InGaAs/AlGaAs Double Quantum Well Lasers at 42GHz Repetition Rate

Generation of bandwidthlimited 2ps pulses with IOOGHz repetition rate from multisegmented injection

http://authors.library.caltech.edu/485/1/SANel90.pdf

Passive Mode-Locking of Monolithic InGaAs/AlGaAs Double Quantum Well Lasers at 42GHz Repetition Rate

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