Jan 1, 1992 - Picosecond and sub-picosecond pulse generation in .... have the evident advantage of being compatible with integrated electronic circuits.
Picosecond and sub-picosecond pulse generation in semiconductor lasers J.-M. Lourtioz, L. Chusseau, N. Stelmakh
To cite this version: J.-M. Lourtioz, L. Chusseau, N. Stelmakh. Picosecond and sub-picosecond pulse generation in semiconductor lasers. Journal de Physique III, EDP Sciences, 1992, 2 (9), pp.1673-1690. .
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Phys.
J.
III
France
(1992)
2
1673-1690
SEPTEMBER
1992,
PAGE
1673
Classification
Physics
Abstracts
42.60F
42.55P
42.80W
Picosecond sub-picosecond and semiconductor lasers Lourtioz,
J.-M.
Chusseau
L.
d'Electronique Orsay Cedex,
Institut 91405
(Received
5
and
N.
Fondamentale,
generation
pulse
in
Stelmakh
URA
22
CNRS,
du
Universitd
Paris
XI,
220,
Bit.
France
I99J,
December
accepted
20
February
1992)
g6n6ration
d'impulsions dans les semiconducteurs d'int£rEt lasers h courtes est d'applications. aussi intdrdt EIIe pr6sente fondamental de pour un un relation 6troite dtudes les de la dynamique milieu semiconducteur. du Apr6s une par sa avec introductive, les performances des techniques de commutation de gain, de revue nous comparons commutation de penes de cavitd et de blocage de modes. illustr6e La comparaison est aussi bien r6sultats exp£rimentaux que par des r6sultats de moddlisation. Certains rdsultats de ces par des Un particulier est mis sur [es aspects de modulation de phase dans Ie milieu sont accent nouveaux. semiconducteur. Nous discutons finalement quelques perspectives de d6veIoppements nouveaux. R4s~1m4.
La
pratique
nombre
croissant
short optical pulses in semiconductor lasers is of practical interest for applications. It is also of fundamental interest because of its tight relation with the field of semiconductor laser dynamics. After introductory review, we the an compare performances of gain-switching, Q-switching and mode-locking techniques. The comparison is illustrated by experimental results as well as by laser modelling, of which A special some are new. emphasis is given on the aspects of phase modulation in the Finally, semiconductor laser medium. discuss perspectives for further developments. we some
Generation
Abstract.
an
1.
increasing
Introductory
High-speed picosecond demand exists
for a
need
review.
light
semiconductor time
scales
picosecond of
extemally systems [4, 5].
rate
of
of
number
sources
modulated
are
laser for
sources
essential diodes soliton
transmission
for
a
generating short pulses in the picosecond large variety of applications. To date, the
remains transmission
systems
in
field
the
in and
optical for
of
optical
fibers
time-division
communications
[1-3]
as
well
multiplexed
as
for
sub-
and
strongest where
high
bit
transmission
(Sub)-picosecond laser diodes are also expected to serve as clock generators in other hand, semiconductor all-optical parallel On the high-power pulsed lasers processors. sizes YAG, interesting altemative conventional of larger (e.g. laser to systems are an more dye and time-resolved solid-state laser lasers) for electro-optic sampling [6, 7], spectroscopy, time-domain. addition their semiconductor ranging and metrology in the In to compactness, electronic circuits. lasers have the evident advantage of being compatible with integrated
JOURNAL
1674
PHYSIQUE
DE
III
N°
9
pulse characteristics desired according to the application which is envisaged. are soliton long-distance transmission in fibers require Fourier transform-limited pulses of adjustable length, while studies of nonlinear phenomena in various materials essentially need high peaks powers. For each application, different techniques and optical schemes be explored. As for other lasers, there basically three techniques for must are operating semiconductor lasers in pulsed regime : I) gain-switching (I.e. pulsed pumping), it) Q-switching, iii) mode-locking. These techniques were investigated early on in the three development of semiconductor lasers. Moreover, laser the first emission from an homojunction diode achieved in 1962 with the injection of pulses [8]. Two years structure current was Lasher later, proposed an inhomogeneous pumping of the active medium realize the to conditions for regularly pulsed emission [9]. Q-switching was effectively demonstrated in 1966 by Kumosov al. with a two-section laser [10]. Mode synchronisation in an et geometry injection semiconductor GaAs laser with an extemal cavity was claimed by Morozov et al. in 1968 [1il. All these first experimental investigations performed with homojunction were below operated low 77 The shortest K. pulses were already found temperature structures at a 100 ps, less than with pulse repetition of gigahertz [12]. several to be rates Different
instance,
For
Progress accomplished in the of semiconductor developments 1970,
measurements.
In
semiconductor
laser
Q-switched
of
for
search P.T.
lasing materials reported in laser operated
heterostructure
mode-locking
with
a
Better
same
pulses by
laser
saturable
well
as
1978
al.
et
decade
materials. Harris
[13]. In the
GaAs
new
Ho
E.P.
following
at
clearly year, Glodge
followed
accuracy demonstrated
also
was
active
technological in pulse mode-locking in a the
achieved
reported accurate measurement autocorrelation [14]. After a long period devoted to the improvement of CW laser spectral properties, as to the the
absorber
and
Lee
mode-locking
first
[15]
temperature
room
obviously
:
demonstrated
was
20 ps three
experiments
with
pulses
obtained.
were
after
years
double-
a
by Van
Passive
der
Ziel
et
by proton implantation through the diode facets. For the : was first time, subpicosecond pulses (0.65 ps) were measured in semiconductor lasers. This was performances were achieved (0.58 ps) followed by the work of Yokoyama et al., where similar GaAs with aged laser [17]. Gain-switching experiments in double lasers heterostructure an which experimental carried out in parallel, output pulses of 15 ps [18]. All these were gave improvements in terms of short pulse duration. represented decisive results [16]
al.
From was
of
the
the
certainly realizing
carried
InGaASP
year the
1982,
lasers
research
the
investigation
coherent
in
out
created
absorber
with
emitting
in
was
pursued
effects
optical development spectral regions near
emission
parallel
of
work
chirping
for
the
in
in
new
modulated
directions. and
[19-21]. single-mode
communication of 1.3
~m
and
1.5
pulsed These
The lasers
main with
direction the
investigations
distributed-feedback ~cm.
In
goal
1986,
Takada
were
(DFB) et
al.
picosecond pulse generation at 1.3 ~m by fiber compression of further pulses [22]. The efficiency of this novel technique was for the spectral region 1.5 [23, 24]. Fiber compression confirmed by several authors near ~m Q-switched laser where strong chirping effects also successfully applied to a single-mode was and powerful (3W) pulses [25]. Other generation of very short (~lps) led the to for and interferometers used compression schemes based gratings compensate to were on obtained by Silberberg Transform-limited chirping effects in mode-locked lasers. pulses were active mode-locking, respectively. Kuhl et al. [27] using passive and and et al. [26] transform-limited reported gain-switched DFB laser
A
second
concemed
locking, repetition repetition
important
improvement of (sub)-picsecond laser diodes during the last decade pulse repetition rate. For gain-switching as well as for active moderesulted the of modulation bandwidths. A from lasers with large progress use of 12 GHz 1986 [22]. achieved with gain-switched DFB laser in Pulses rate was a scaling from 16 GHz to 40 GHz were reported in different active works rates on the
increase
in
(SUB)-PICOSECOND
9
N°
mode-locking monolithic
[28-31], in
demonstrated
350
to
up
InGaASP
without
lasers
SEMICONDUCTOR
IN
highest performance being
the
cavity
extended
PULSES
[31].
laser
extended
cavities
1675
LASERS
in 1989 by Tucker et al. with a recently, passive mode-locking was repetition rates [32, 34], leading to ultimate achieved
More
[34].
GHz
of studies directed peak-power optimization. In a few at was exceeding I W were directly obtained from solitary diodes [25, 35attained by complementing the 37]. However, in most high-power performances cases, were laser diode with several amplifier sections and nonlinear compression stages. A peak power of mode-locked elaborate 70 W reported by Delfyett et al. for an system comprised of a was AlGaAs laser diode with external cavity, grating and travelling-wave two compressor an a amplifiers [38]. Peak powers scaling from 100 W to I kW were achieved semiconductor in two experiments by using high-order soliton effects in fiber compression stages : the laser recent gain-switched DFB laser in one case [39] and a synchronously pumped surfacewas source a emitting InGaAs [40]. Extremely short pulse durations of 0.46ps, laser in the other case in references [38, [40] respectively, the 0.2ps and 21fs reported 39] and but were
Finally, an increasing experiments, peak powers
number
characteristics performances. laser for a little in these were Following this introductory review, the rest of the paper is organized in four sections. The techniques for short pulse generation, I-e-, gain-switching, Q-switching and modethree 4, locking, presented in sections 2, 3 respectively. Experimental results and and are calculations from Special is laser modelling used illustrate these techniques. emphasis to are (chirping) which are of prime importance in given on the aspects of phase modulation semiconductor finally discuss lasers. We perspectives of further developments in some semiconductor
5.
section
Gain-switching.
2.
Gain-switching is population
strong turn,
as
with and
lasing takes level rapidly
fast
a
excitation
high gain
thus
places,
as
soon
laser
upper
achieved inversion
decrease
below
is
threshold.
medium.
laser be
can
light
emitted
the
of the
level
reached intense
very The laser
Under before
and
the
emission
the
this laser
condition,
then
ceases
In
starts.
populations
of the and
an
optical pulse is obtained. Semiconductor lasers characterized by high gain values and short are cavity roundtrip times (w10ps). These features give the possibility of short pulse two generation at high repetition rates by gain-switching. The simplest mathematical description of the gain-switching process is provided by the two familiar equations for the carrier density N and the photon density S : rate ~ ~~
=
~
~
~~
~n
" ~~ The the
three
terms
on
Those
on
the
numerical case
step before
of
solutions simulations
side
are
side
rate
the
in
table
a
(2)
shown
figures
I
steady-state
value
respectively
total
the rate.
injection rate, respectively. cavity-loss rate, the Parameters entering
carrier
rate,
I.
and
in
are
the
recombination
stimulated-emission
(Fig. la) corresponds to an ideal step amplitude exceeds threshold current,
reaching
(2j
s.
represent
stimulated
the
equations (I) are
Noj
es j (N
equation (I)
of
and
equation (2)
of
listed
(1)
S
~n
and
rate
(i
rA
+
~p
right-hand
the
right-hand
spontaneous-emission equations (I) and (2) Exact
+
No)
ES) (N
(I
~~~
~ =
recombination
spontaneous
A
different
only
can
and 2 for excitation the
from
laser zero.
be found numerically. of Results particular cases of excitation. The first with an infinitely risetime. short If the
output
exhibits
Actually,
the
relaxation laser
response
oscillations is that
of
1676
JOURNAL
Table
I.
of parameters entering into figures1, 2 and 3 (right).
List
calculations
in
PHYSIQUE
DE
III
equations (left)
rate
1.5
photon
A F
carrier
o
p
spontaneous laser
filter
second-order second
pulse
case
emitted relaxation
is
first
ns
10-6 cm3/s
x
I
jransparency
at
x
I
x
10-~? cm3 ' 8 cm-3
10
0.5
emission
factor
2
10-5
x
factor
enhancement
4
efficiency
quantum
0.4
differential is govemed by two equations. The rectangular pulse excitation. single gain-switched A a of the pulse with nearly coincides the end of the current (Fig. 2) corresponds to a strong The last microwave case this situation being of practical interest for high-bit-rate current, time
whose
(Fig. lb)
2
coefficient factor
linewidth
a
a
coefficient
confinement
~m3
I
I ps
gain density
r
1~
lifetime
lifetime
nonlinear
N
for
laser
=
170 carrier
gain
linear
used
~m)
400
volume
spontaneous
r~
r~
DFB
~m
(L active
values
parameter
9
;
Parameters
V
N°
evolution
corresponds to the back edge
when
oscillation.
injection Figures 2a, 2b and 2c represent the simultaneous time-evolutions of and laser frequency, respectively. Note that figure 2a also represents output power of the density since the gain is assumed to linearly depend on time-evolution carrier the N in equations (I) and (2). Figure 2c will be discussed later. As the laser pulse is seen, emitted reached its maximum. This is the norrnal situation after the laser gain has where only pulse is emitted per period. The maximum-gain to threshold-gain ration is presently close one microwave 1.2. It rarely 1.5 under modulation [41]. exceeds to The laser frequency evolution reported in figure 2c merits further instantaneous comments. It is implicitely that the laser is single-mode. In the assumed semiconductor laser theory, the interrelated dimensionless gain and refractive index variations via parameter are a modulation
optical the gain,
of the
communication.
known
a,
linewidth
the
as
frequency
variations
(Av (t)
c
=
enhancement
(t)/n1),
An
Au
it)
also
are
the
laser
fi
Ag(t)
=
factor
directly frequency $ =
(rA
[19]
An
:
(t)
refractive
to
evolution
can
No)
by
the
variations
:
I/r~)
(3) ,
reference frequency is taken at the laser threshold. equation (3), a linear decrease of the gain is accompanied by a linear laser frequency (downchirp). This situation is illustrated in figures 2b and 2c. quasi-linear downchirp. from lasers exhibit Non-linearities semiconductor a beginning and at the end of the pulse. Consequently, pulse compression can passing the downchirped pulses through a linearly dispersive medium such as the
where
From
(Within I
Since
index
described
be
ES) (N
ii
(1/4 aT) Ag (t).
a
=
related
+
a~=
Figure
3
a
the
Gaussian
pulses,
the
maximum
compression
Pulses occur
be an
rate
of the emitted at
the
by optical fiber is given by realized
[42].
shows
gain-switched
assumption of
decrease
DFB
the
laser
results at
of
1.52
pulse ~m
and
compression a
I km
experiments
dispersion-shifted
recently fiber
with
performed with a dispersion a total
(SUB)-PICOSECOND
N° 9
PULSES
LASERS
SEMICONDUCTOR
IN
1677
400
~°°
E
'~
200
o 5
0.2
loo
0.35
0
~
(
~ _
~
§
0.25
o,1
(
0.7
Ins)
Time
j~
D-1
>
~
$
0
0
"
(psi
Time
O-OS
(
600
400
200
0
0 0
0.35
D-S
0.2
0.7
(ns)I
Time
i oo
2
I
Z
~
0.2Sfl
0.1
?
5 M
'
I
o
~
~
~
~~~~
I.
Fig.
2.
Laser
°.35
j
Fig.
Fig. pulse
o
600
~~o
0
Fig-
I.
under
responses
electrical
strong
single-step
a)
excitation.
0 7
Ins)
Time
2.
b)
excitation,
rectangular
excitation.
laser
Time-evolutions
under
amplitude
modulation
laser
modulation.
line
Calculations
(Tab. I) characteristics, the
parameters
current
frequency
resonance
Under
deduced
maximum). 120
GHz
pulse
tail
width
of
to
2a
represents
from
laser
spectra
from
to
be
over
the
I~=
those
under
a
threshold,
a
I.
and
1.5
~m
The
bias, =3GHz,
f
DEB
dc
comparison. The including the light-
presented
are
for
measurements
the
l~
for
table
l~=150mA
optical
of the
measurement
characteristics and
in
gain.
of
set
reported
80mA,
threshold
the
=
spread
are
frequency c)
emitted
are
equations (1-3)
rate
determined
are
b) and
parameters
small-signal modulation modulation ~f~ 2.I GHz, 120 GHz (Fig. 3a, bottom)
and
microwave
strong
measured
figure
in
power
Laser
frequency
modulation
and
dotted
ps/nm [24].
20
of
The
gain a), output
of the
microwave
strong
respectively.
is
loo
0
0
various
at
200mA),
the
laser
=
the
pulsewidth
before
biases
laser
[43].
spectmm
is
compression
~
~18ps
from
autocorrelation
measurements
(FWHM,
full-width
half-
experiments. The calculated spectrum (Fig. 3a, top) is the low-frequency side, the latter being related to the wide and exhibits on a peak pulse (Fig. 3b) is asymmetrical and has a of the downchirped pulse. The calculated pulse after autocorrelation of the 18ps (FWHM). Figure 3c shows the trace Calculations
agree
with
1678
JOURNAL
DE
PHYSIQUE
al
9
N°
III
b) i
j ~
so
i
~~~~
0
E
~ =
fl
$
3
i.
~
~
So
0
0 loo
~
c) 3
]
0
200
loo
Frequency
100
200
(GHz)
300
(psi
Time
d) '00
2
~
p
E 200
~ c
f
$
~
y o
3
~
1
X ~ ~
~
0
50
100
200
(ps)
Time
Fig.
So
compression
T;me
300
(ps)
gain-switched DFB pulses at 1.5~m laser (experiments and reported in table modulation I. The dc bias, amplitude and are modulation 200 mA frequency are I~ 90 mA, and f 2. I GHz, I~ respectively. The fiber dispersion is -20ps/nm. a) laser spectrum calculations (bottom) and (top) b) measurements calculated pulse before compression ; c) autocorrelation of the compressed pulses trace measurecalculations (top) ; d) calculated pulse after compression. (bottom) and ments 3.
Fiber
calculations).
Laser
of
those
parameters
=
compression. curve).
Again,
The
trace
autocorrelation
=
=
experiments (bottom curve) are is 8ps (FWHM). The
width
reveals
curve
deviation
some
from
reproduced
well
existence
either
a
by
(top wings in the Gaussian hyperbolic secant or a compression give rise to small of
calculations
extended
in the pulse before shaped pulse. Actually, chirp non-linearities in the compressed pulse (Fig. 3d). Owing to the overall fit between side-lobes calculations and experiments, a pulsewidth of 5.5 ps can be estimated after compression, which leads to a timebandwidth product At. Au of ~0.6. The compression (~3) is close to the value rate for a (~ 4). The peak power at the fiber output (350 mW) is more estimated than one order of magnitude higher than the CW power delivered by the laser. Still shorter pulses scaling from 2 to 4ps have been obtained by applying the same technique to different [44]. Ultimate lasers performances of gain-switching can be evaluated from an analytical approach of rate equations. Ignoring emission and electrical spontaneous pumping in equations (I) and (2) during gain-switching, analytical expressions are found for the
[41, taken
total
45]. into
number The
of
pulsewidth
account,
photons,
emitted
it
is
is
then shown
S dt, deduced
that
for
and
the
from
these
very
high
peak photon density, S~~~, respectively
quantities.
If
gain
initial-inversion-ratios
non-linearities the
are
minimum
(SUB)-PICOSECOND
N° 9
pulsewidth
given by
is
PULSES
SEMICONDUCTOR
IN
1679
LASERS
:
f
~~~ r~ is
where
and
A
photon
the
entering
e,
semiconductor
into
laser
lifetime
f
and
f are a priori terminology, 4
~
~
P
~
e/Ar~.
=
material aT~ e/A
(4)
f-1
~
The
linear
parameters is
known
the
as
gain
nonlinear
and
while
r~ is K-factor
cavity
a
parameter.
f~
and
coefficients, Al
=
/
In
aT
the is
e
material [46]. achievable with a given laser easily understood. e/A is much larger than In most cases, pulsewidth is close to than I in Eq. (4)) so that the ultimate r~ (I,e., oscillations. related to the period of laser relaxation The e/A. This smallest lasers is 0.17 ns [47], which gives minimum date for long-wavelength to 4 ps. Still expected for short-wavelength lasers wich exhibit weaker F/A smaller values are gain-non-linearities. Complementing the gain-switching technique with fiber compression, sub-picosecond pulse generation thus appears to be feasible for semiconductor with lasers larger than 5. a different limit is predicted for lasers with very long cavities A since f becomes less than I in equation (4). The minimum identifies in pulsewidth with the photon lifetime, that case with since propagation effects be taken be neglected must cannot care r~. However, this result far in rate equation models. Indeed, the cavity roundtrip time, r~, appears to be the true so physical limit instead of r~. the
modulation
maximum
bandwidth
equation (4) f is much larger value is obviously K-value reported
limits
The
of
be
can
=
Q-switching.
3.
Q-switching
in
series
the
with
implantation
of
multi-section 5
semiconductor
gain ions
laser
is
lasers
medium
through geometries,
(Fig. 4). diode
the
where
by creating
achieved Two
main
facets some
[48-50].
of the
regions of
solutions
are
The
sections
second are
absorption
saturable
used.
first
The
one
unbiased
is or
one
is
realization
the
in the
of
[9,
reverse-biased
Ii-
saturable
Fig.
4.
the
absorber JOURNAL
Basic
DE
scheme
PHYSIQUE III
of
a
time,
relaxation -T
2,
N'9,
semiconductor the
active
absorber
gain
SEPTEMBER
laser
recovery 1992
with
a
time
and
saturable the
medium
absorber.
cavity
r~,
roundtrip
r~
and
r~
are
respectively
time. 59
JOURNAL
1680
mechanisms
The time.
latter
The
involved must
be
diode
in
always
PHYSIQUE
DE
laser
shorter
than
Q-switching the gain
pulsed emission. However, it may be shorter depending on the cavity length and the absorber mode-locking. The laser dynamics in the to switching. Let
focus
us
on
the
second
One
situation.
III
depend
or
material.
The
with
difference
in
situation similar
are
gain-switching
relaxation
order
cavity
the
first
situation
second
time
than
absorber
the
on
relaxation
longer
9
N°
to
obtain
roundtrip may to
is the
a
those
priori of
a
time, lead
gain-
possibility
of
generating optical pulses with a dc laser bias. In this pure Q-switching case, the pulse periodicity is fixed by the recovery medium. difference time of the gain Another is related to than gain-switched pulses due to the pulse shape. Q-switched pulses exhibit shorter tails from the absorber end. However, the major interest of Q-switching at pulse stems recovery of very intense pulses under electrical excitation. saturation achievement pulse Absorber simultaneously effects combined with gain switching and the gain at maximum may be two are times higher than the saturated cavity losses. description of Q-switching is provided by rate mathematical As for gain-switching, a good equation models. Equations (I) and (2) of section 2 are accompanied by a third equation for population density N~. the absorber
N~
dN
N ~
~
~~
(5)
N~ S
A
=
~
~a
carrier right-hand side of this equation represent the spontaneous absorber respectively. N~ is the equilibrium absorption, absorber time and A~ is the relaxation population in the absence of light, r~ is the absorber stimulated absorption term must be included as a loss term in equation (2). cross-section. The reference [49]. been proposed in solutions of rate equations have Approximate The precise description of phase modulation effects in the Q-switching regime is somewhat difficult due to the of the saturable absorber. The simplest approach is to presence assume an value for the (Eq. (4) in Sect. 2). Under this assumption, the chirp a-parameter average amplitude during the pulse is proportional to the gain variations as well as to the electron density variations, a linear relation existing between g and N. Since the number of emitted photons is approximately equal to the number of lost electrons, the total chirp amplitude is expected to be proportional to pulse energy. time-resolved Figure 5 shows the results of spectroscopic experiments perforrned with Qof different switched AlGaAs lasers pulse energies [52]. The lasers are operated in singleradiation in the cavity [53]. The pulse monochromatic mode with the injection of a small CW ion-implantation conditions, I.e., by changing the spatial energy is varied by changing the Electrical pumping is achieved with 2 ns of the saturable absorber in the cavity. extension made with a streak pulses at a repetition rate of 40 MHz. Optical pulse measurements are increase with pulse energy. As seen, the chirp amplitude is found to regularly For the camera. highest energy (13 pJ), the chirp amplitude (15 A) is about four times the cavity mode The
two
terms
relaxation
spacing. Figure energy.
6
the
stimulated
results
shows
The
pulsewidth by streak
on
the
and
obtained
pulsewidth measured
measured
after
in
compression
fiber
by
autocorrelation
compression is
l.3
ps,
experiments 22ps
is
which
agrees
laser of highest compression. The previous measurements
with
the
before
with
semiconductor reported to date for a Q-switched laser with electrical pumping. Beyond the performance, notice the extremely high one may of the compression Such a result value (~17), the latter being directly related to a. rate which non-linearities with semiconductor laser models include gain and predict an agrees [54]. However, evolution forrn for high-power lasers of the I +P/P~ the a ao camera
[25].
This
result
=
is the
best
(SUB)-PICOSECOND
9
N°
Fig. with
a)
300keV
=
33
1400
HLP
implantation E
LASERS
1681
spectroscopic
Time-resolved ~m.
SEMICONDUCTOR
IN
images of single-mode gain-IQ-switched laser pulses at implantation b) HLP 1400 Hitachi E =1.2 pJ laser implantation -E=2.5 pJ c) HLP 1400 Hitachi laser with 11 MeV proton oxygen -E=5.8pJ; AlGaAs d) gain-guided with 20MeV implantation structure oxygen
5.
0.83
PULSES
Hitachi
without
laser
pJ.
0.8
d d
0.6
~'
0.4
2ps
I
38 ps 2
?
o 60
40
20
0
20
60
40
Delay, ps
Fig.
Second-harmonic
6.
AlGaAs
with
laser
compression. 24 ps/nm.
It A
20 becomes
factor
autocorrelation
MeV
of
2ps 0.65
oxygen after is
used
Q-switched
of
traces
implantation. The compression by a to
deduce
the
trace
laser
width
pulses
long pulsewidths
~200m
emitted
(FWHM) fiber in
the
by the
gain-guided
38 ps without extemal dispersion of with a total is
text.
contribution of the saturable absorber should be explicitly into taken for a account more complete analysis. The previous experimental illustrations clearly reveal the possibility of very large phasemodulation in single-mode Q-switched lasers. Due to high levels resulting in strong power laser non-linearities, the ultimate performances of Q-switching may be different from those previously established for gain-switched lasers. In principle, the ultimate limit of chirp
1682
JOURNAL
amplitude is given by the gain-bandwidth. be expected at 0.8 ~m for a pulse can compression. 240 fs pulses after
DE
PHYSIQUE
More
energy
N° 9
III
realistically, chirp amplitudes ~30pJ, which should of
of
about
40
correspond
h to
Mode-locking.
4.
Mode-locking pulses from the gain- and well
known
as
sources.
Q-switching minimum
the
as
is laser
width
the
most
For
technique to produce extremely short optical mode-locking is effectively superior to concerning the amplitude and timing jitter [30, 55] as efficient
semiconductor
techniques of the
obtainable
lasers,
pulses [16, 17, 29, 38].
However,
it is
also
more
in large variety of effects observed mode-locked semiconductor lasers, still of them remain unexplained. some Basically, mode-locking is achieved by introducing a mechanism inside the laser cavity to the longitudinal interact with another thereby locking them in phase. One modes to cause one is to modulate the laser gain at a frequency close to multiple of the frequency means or spacing between the longitudinal modes. This technique is referred to as active mode-locking. However, for semiconductor laser devices, standard the cavity spacing is beyond mode 100 GHz and it is very difficult modulate the laser gain at these frequencies. modeActive to locking requires the extension of the laser cavity. The most solution consists of using current the diode facet being antireflection extemal reflector, inner (AR) coated (Fig. 7a). The an second solution is to realize special devices such as monolithic extended-cavity lasers where the gain region is coupled to a low-loss passive waveguide [31, 56]. In both the cases, minimum pulse repetition rate is fixed by the cavity mode spacing whereas the maximum repetition rate is limited by the laser bandwidth. modulation laser pulses is to use a The other of achieving mode-locked absorber in the saturable means laser cavity. This technique which is referred mode-locking is best described in to as passive the time domain. saturable As for Q-switching (see Sect. 3), the absorber sharpen to serves risetime while gain saturation effects combined with fast bring absorber up the pulse recovery about an abrupt termination difference with Q-switching lies in the of the pulse [57-59]. The supplementary conditions imposed to the absorber relaxation time, r~ and to the cavity roundtrip time, r~. The most favourable situation for passive mode-locking when : occurs Since r~ is of the order of I ns in time of the gain medium. recovery r~ r~ r~, r~ being the semiconductor lasers, the previous condition be only fulfilled if the cavity length exceeds a can few As for active mode-locking, is to use an centimeters. the experimental solution external cavity (Fig. 7b). On the hand, passive mode-locking other also when: may occur extemal cavity is needed in that case, but very fast absorbers no r~ m r~ r~. In principle, be used. Mode-locked pulses were recently reported in two experiments without using must extemal cavity [33, 34] region of saturable the absorption realized by ion an : was implantation in the first case [33] while a three-section laser with reverse-biased section one of in the second [34]. Interest such configurations from the achievement used stems was case of very high pulse repetition The major drawback is the level of emitted weak peak rates. fully pulses, the the gain medium between successive As cannot two recover power. inversion remains population always value. rate at a low mode-locking from the The complexity of the semiconductor lasers mostly results process in of extemal cavity. Even if residual reflectivity of the AR coating is the low, use an very say
complex.
Among
~
the
~
~
10-~,
laser
1980
in
presence
into
existence
the
locked
must
of
be
the
small
treated
as
laser a
[60, 61]. Figure 8 gives of the diode cavity, the
different
clusters,
each
cluster
diode
a
cavity
can
never
be
ignored.
In
fact,
the
mode-
analyzed by Haus schematic representation in frequency-domain. Due to the longitudinal modes of the extended cavity can be gathered being centered around of the diode cavity one resonances.
two-cavity
system.
This
situation
was
first
(SUB)-PICOSECOND
N° 9
PULSES
SEMICONDUCTOR
IN
LASERS
1683
a)
b)
absorber
Fig. 7. Basic schemes of a mode-locked locking, b) passive mode-locking.
semiconductor
h~~~~ """'j
with
laser
I
/
$~~)~
cavity. a)
active
mode-
~~~~
~
""""
-.,
_.-
extended
-.
' '
compound cavity gain
_/
cluster locked
of modes
J~l/~ext Fig.
Mode-locking of a nonideally antireflection laser coated diode in an extemal cavity. The the diode cavity is simulated by a periodic gain profile (compound gain) [60]. resonances time in the extemal cavity while r~ is the roundtrip time in the diode sub-cavity. r~~~ is the roundtrip Modes from neighboring clusters necessarily in phase with one another. not are 8.
influence
Using
of
simple picture,
different of a given easily that the modes cluster can neighboring clusters necessarily in phase with not are another. Mode-locking is then only partially realized and the emitted pulses are far to be one description proposed by Haus was based on a single master transforrn-limited. The theoretical longitudinal mode [60, 61]. The equation which gave the steady-state conditions for each diode cavity simulated using periodic profile for the laser gain (see by resonances were a Fig. 8) and the pulsewidth limits evaluated in this condition. were lock
this
together.
In
contrast,
one
imagines
modes
from
PHYSIQUE
DE
JOURNAL
1684
N° 9
III
mode-locking of semiconductor lasers in an extemal authors [30, 62-67]. Except for one case analyzed in time-domain by several directed active mode-locking and numerical simulations at were were equation models. For the long pulse regime (AT r~) (*), the basic rate identical feedback almost those given in section 2, except that a delayed to (**) in equation of form k. S (t included the photon density the term to account r~~~) was with observations, for the effect of the extemal cavity [64, 65]. In agreement experimental instabilities when the roundtrip time in the external cavity was laser where shown to occur for [64]. regime shorter period of the pulses For the short pulse than the current recurrence modified diode cavity (AT « r~), the rate equations include propagation effects in the to were conditions inner diode facet the external reflector together with the boundary both the and at [30, 66-68]. In gain non-linearities ignored for the sake of simplicity. The contrast, were
Following the cavity was further [63], analyses performed from equations were
of
existence
of
laser successive
work
non-ideal
a
Haus,
AR
coating
for
the
a
residual
facet
found
was
pulses, being the roundtrip [29, 30], pulse mode-locked
measurements
reflectivities
diode
inner
of
as
low
as
the time trains
to
in
in the
emission
separation
between
result
time
the
diode
consisting
of
cavity. In three pulses
10-~.
still models remain unadapted to predict long pulse regime to the short pulse regime, I-e- from partial modeExperimentally, this transition abruptly for a certain locking to full mode-locking. occurs tuning of the extemal cavity and/or for a careful adjustment of the feedback coupling [29]. active mode-locking. In view of results behaviour observed in passive and The is indifferently chirping effects play an important role in sections 2 and 3, it is clear that reported in models into semiconductor Improved then necessarily them lasers. take must account. experiments on Transverse effects importance as revealed by recent some may be also of Sapphire lasers [69, 70]. Experimental and theoretical mode-locked other systems such as Ti investigations carried this direction for better understanding of should be in out a dynamics. semiconductor laser results obtained mode-locked AlGaAs Figure 9 shows by the authors on a passively recent commercial The experimental scheme close that presented in figure 7b. A laser. is to HLP1400 Hitachi diode is used. The saturable absorber is by an lo MeV laser created implantation of oxygen ions through the rear facet of the diode. The other facet is used for coupling to the external cavity. The absence of AR coating in the present experiments places 30 fb. The cavity is 15 cm long and is the facet reflectivity to a standard value of extemal microscope objective for beam collimation and an end-mirror comprised of an AR coated
However,
the
in
transition
spite
at
regular given pulse train
instead
autocorrelation
with
calculated
of
~
pulse trains pulses of
agreement were
earlier
from
of
theoretical
these
fits,
laser
the
~
with
total
reflection.
Under
additionally
norrnal
conditions,
the
laser
diode
is
dc
biased.
A
modulation
synchronize the pulses on an extemal reference clock. discussion. autocorrelation reported in figure 9b well illustrate the previous It is The traces clear that the mode-locked laser output is composed of pulse trains, the pulse spacing being amplifier (~ 8 ps). For most of the experimental the roundtrip time in the semiconductor modulation ratio is less than 60 fb conditions, the between pulses is moderate : the contrast However, for certain tunings of the extemal (dotted curve). abrupt change is observed an increases 90fb Simultaneously, the cavity and the modulation ratio (solid curve). up to emitted towards shorter wavelengths and the spectral width is increased spectrum is shifted 45h (Fig. 9a). The minimum pulsewidth from 25 deduced in these conditions is to of modes corresponds to phase-locking of five clusters ~0.7ps, which approximately source
can
be
used
to
(~) The pulsewidth, AT, is larger than the roundtrip time in the diode (*~) z~~j is the roundtrip time in the external cavity while k determines
cavity,
r~.
the
of
rate
optical
feedback.
(SUB)-PICOSECOND
9
N°
PULSES
SEMICONDUCTOR
IN
1685
LASERS
io
8pS o-g
s
0.6
I
401
fi
6
~
4 0.4
4
0.2
2
0
0
835
837
839
Ml
Wavelength,
15
5
10
9.
with
a
Measured
15
different
(Al
long tunings of cm
A).
=15
a) and
autocorrelation
cavity. Dotted curve extemal cavity (see text).
extemal the
The
output
average
perforrnances
The
I W.
spectrum
solid
those
to
b) of
traces
and
is 4 mW,
power
comparable
are
5
10
15
ps
b)
a) Fig.
0
Delay,
nm
mode-locked AlGaAs passively b) respectively correspond to
a
laser
in
curve
thereby indicating peak-powers previously published by Van der
two
around Ziel
et
[16].
al.
Q-switching, the minimum pulsewidth by the inverse of the gain-bandwidth. pulses (~ 20 fs) since the gain-bandwidth typically for
As
by
obtainable
determined
mode-locking is, priori lead to a
should
This scales
from
20 to 60
rate-equation
nm
in
in
principle, femtosecond
semiconductor
placed the mode-locking [68]. Note that the finally authors case questioned existence of pure « active mode-locking ». A more direct estimation can be made from the spectral width of the laser emission. In pulsed regime, laser diodes emit can of reasonable amplitude (15 modes are obtained of in Fig. 9). In the up to 30 modes case AlGaAs lasers at 0.8 ~m, this approximately corresponds to a spectral width of 90 A and transform-limited pulses of 100 fs can be envisaged. lasers.
limit
5.
On
just
I ps about the
of
also
of
confirm
this
pulse
be
expected
pulses.
Research
fundamental
interest
media.
already
300fs
below
of
from
models
active
developments. can
laser
calculations
recent
some
the
further
femtosecond
semiconductor
short
in
developments
towards is
hand,
other
below
Prospects
Future it
the
From
appear
[40].
assertion
generation,
but
to
in
several
in this for
a
discussions be
Mode-locking optimized AR
not
1, 2
sections Recent
remains
important direction is to go by applications, but of transient phenomena in and 3, it is clear that pulsewidths obtained by optical pumping efficient technique in terms of
One
only knowledge
is
better in
feasible.
directions.
field
results
the
most
motivated
(R~10~~)
coatings
are
needed
eliminate
to
switching in vertical gain or Q of VCSEL Interest (VCSEL) offers emitting lasers surface great promise [71]. (~ l ~m) leading in tum of the gain medium thickness from tile weak optical stems structures pulse-to-pulse timing jitters can be Sub-femtosecond short cavity lengths (~ 10 ~m). to very thickness. emission is localized in an active region of such a small expected when spontaneous parasitic cavity
effects
in
extended
Multiple-quantum-well
VCSEL
cavities.
On
structures
the
other
consisting
hand,
of
wells
of
different
widths
also
appear
JOURNAL
1686
be
to
interesting
an
solution
for
PHYSIQUE
DE
realization
the
of
N° 9
III
semiconductor
lasers
with
wide
gain-
bandwidth.
important
Another in
progress
require
field
the
Fourier
challenge
is
picosecond
of
optical
soliton
semiconductor
of
realization
the
lasers
Long-haul adjustable length at coherent
of
is
soliton
transmission.
pulses
transform-limited
surprising
most
evolution
of
1.55
dictated
by
transmission ~m.
semiconductor
in
Perhaps, laser
the
rapid fibers of the
one
pulses
with
repetition rates scaling from 2.5 to 10 Gbit/s. tens Actually, long pulses are needed to minimize pulse distorsions and dispersive radiation, wave regularly spaced along the optical link [72]. both resulting from the use of lumped amplifiers mode-locking in an extemal selective cavity One technique for realizing such pulses is to use [73], the pulsewidth being adjusted with spectral filtering in the cavity. However, solutions monolithic cavities preferable for optical communication applications. Investibased on are mode-locking gations are presently carried out in this direction. A first solution is to achieve monolithic cavities with integrated Bragg reflector [56]. A second solution is to in very long compression technique to the case of gain-switched pulses of long duration. extend the fiber Single-mode lasers with small phase-amplitude coupling factors be designed in order to must dispersive fiber needed for compression. Strained-layer multiplelimit the length of the quantum-well DFB lasers are good candidates [74]. picosecond pulses of high energy are desired Finally, semiconductor lasers able to deliver 200pJ pulses are typically required for of applications. For instance, for a large number circuits. electronic Up to now, the output energy of optical triggering of fast semiconductor rarely exceeds 30 pJ in the short-pulse regime and there is room for improvement. The lasers intensity which can propagate in the laser medium is determined by the two-photon maximum absorption threshold. A typical value is 109 W/cm2. For a laser stripe of standard transverse (~0.2 x 2.5 ~m~), the peak-power is then 5W dimensions maximum around and the emitted in a lo ps pulse is 50 pJ. Further maximum improvements require the use of energy with larger emitting surfaces. Several solutions lasers be considered broad stripe can : I) iii) lasers with stripe of large thickness (1-2 ~m), iv) vertical lasers, it) laser diode arrays, cavity surface emitting lasers (VCSEL). The two first solutions commercially available, are but mode control is difficult. Solution iii) presents the advantage of output transverse beams ellipticity. However, CW operation is excluded. with weak Solution iv) a priori transverse provides the largest emitting surfaces. A quick estimate indicates that peak powers up to 100 W should be obtained solutions I), it) or iii). Still one with order of magnitude could be gained with solution iv). durations
6.
lasers. some
several
Concluding
This
have
of
picoseconds
of
remarks.
reviewed the paper has The different methods of the been
and
results
also
presented
discussed.
current
used for
be
generate
illustration
However,
low-dimensionality could not of low-dimensionality lasers applications to short pulse performances are not related
understanding to
some
specifically
of short
pulse generation in semiconductor pulses have been analyzed in details, Possibilities of further improvements
short
being new. important aspects treated
in this
related context.
for
instance
to
lasers
of
Actually, the development necessarily motivated by
other developments is not recent generation high-speed modulation. and Moreover, the laser in a simple material instance, it is For to the parameters. manner established that well waveguide lasers do not exhibit better perforrnances than quantum now bulk lasers of the In acomplished in the growth of thin geometry. contrast, same progress semiconductor layers allows the realization of new geometries such as vertical cavity lasers which of great promise for high-power short-pulse generation. are as
(SUB)-PICOSECOND
N° 9
PULSES
1687
LASERS
SEMICONDUCTOR
IN
Acknowledgment. The
acknowledge
authors
valuable
the
and
assistance
expert
of
D.
in
Pascal
the
course
of
experiments.
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L.F.,
MOLLENAUER
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M.,
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Observation of 10 Gb/s optical soliton NISHI S., SARUWATORI M., over 17th Conference Optical gain-switched DFB-LD pulse European on a source, Communications ECOC'91 (Paris 1991) Post-deadline paper, pp. 88-91. time-division multiplication SARUWATARI M., 100 Gbit/s optical signal generation by TAKADA A., distributed feedback (DFB) laser modulated and compressed pulses from gain-switched of 1406-1408. diode, Electron Lett. 24 (1988) time-division multiplexing for very highTUCKER R. S., EISENSTEIN G., KOROTKY S. K., Optical CLEO 88 (Anaheim 1988), Conference Laser Electro-Optics bit-rate and systems, paper on IWATSUKI
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