Gain dynamics in quantum
experimental comparison
well lasers and optical amplifiers:
An
N. Tessler, J. Mark,a) G. Eisenstein, J. Mdrk,a) U. Koren,b) and C. A. Burrusb)
Electrical Engineering Department Technion, Haifa 32000, Israel
(Received 6 December 1993; accepted for publication 7 February 1994)
We describe an experimental comparison of gain dynamics in quantum well lasers and optical
amplifiers. We demonstrate an approximately 30% increase in the time constant describing the gain
recovery on the -1 ps time scale in a laser above threshold. The increase is due to the high rate of
stimulated emission which modifies the relative significance of the various mechanisms contributing
to the gain recovery. We suggest an explanation based on the coupling of two processes: Carrier
capture and carrier cooling. We conclude that laser gain dynamics contain details that cannot be
revealed in experiments on optical amplifiers due to the vast differences in operating conditions
between a laser and an optical amplifier.
Carrier and gain dynamics are fundamental properties
that govern operation of diode lasers. In addition to the obvious interest in the nature of these basic properties, they are
important due to their relationship to the laser gain
nonlinearity.‘*’ It is well known that the gain nonlinearity
dictates3 limitations to the modulation capability as well as
.~
many static properties.
Gain dynamics are usually studied experimentally in the
small signal regime. Namely, the dynamics are measured in
experiments which for a particular operating condition, introduce a small gain perturbation to a static operating point.
Common experiments include modulation response measurements (by electrical4 or optical5 gain modulation), short
pulse pump-probe,6V7and nondegenerate wave mixing.8 Such
experiments are interpreted using a variety of models56,7710
that are based on the assumption of a linear behavior. These
experiments4-7 can be divided into two groups. Modulation
response measurements are performed on lasers and their
aim is to extract parameters of the gain nonlinearity.’ Pumpprobe and wave mixing experiments, on the other hand,
serve for the identification and characterization of physical
mechanisms that contribute to the gain nonlinearity.27’*0 Reported pump-probe and wave mixing experiments have used
optical amplifiers’+8 and not lasers. The fact that the two
groups of experiments have been performed under fundamentally different operating conditions (stemming from, the
differences between lasers and optical amplifiers) causes difficulties in quantifying the implications to gain nonlinearity
of the details of the relevant physical processes.
The differences between optical amplifiers and lasers operating above threshold result primarily from stimulated
emission. Stimulated emission causes: (a) Shortening of the
carrier lifetime, (b) a deviation from equilibrium at the lattice
temperature of the carrier density distribution, and (c) near
pinning of the carrier density and Fermi level. In structures
based on a quantum well (QW) gain region, the differences
are significantly sharpened since the shortening of the carrier
lifetime (in the QWj causes an accumulation of carriers in
the barrier and confinement regions.‘r’*’ This effect, known
as the carrier injection bottleneck effect, has recently been
demonstrated under static injection conditions.r3 This letter
reports measurements comparing gain dynamics under different operating conditions, namely: (a) a QW laser above
threshold, (b) a QW laser below threshold, and (c) a QW
optical amplifier. In order for the comparison to be reliable,
all measurements were performed using a single, specially
designed, device.14 This ensures that the observed differences are exclusively due to the different operating conditions.
The specially designed laser diode is depicted schematically in Fig. 1. It is a two-section distributed Bragg reflector
(DBR) laser width, a QW gain region, and an antireflection
(AR) coating on the back of the Bragg mirror. The laser has
a short gain section and a narrow-band Bragg mirror. The
length of the gain and Bragg sections as well as the Bragg
coupling coefficient are such that the cavity mode spacing is
larger than the Bragg mirror bandwidth. Current injection
‘)Tele Danmark Research, DK-2970 Horsholm, Denmark.
“AT&T Bell Laboratories, Holmdel, New Jersey 07733.
FIG. 1. Schematic description of the special DBR laser, the cavity modes,
and the Bragg filter reflection function.
2050
Appl. Phys. Lett. 64 (16), 18 April 1994
1
--q,y,,,/j
I,
I
~~~__I__..._
Bragg
Reflection
Cavity
/
i,=5mA
0003-6951/94/64(16)/2050/3/$6.00
\
I,=0
Mode
h
Q 1994 American Institute of Physics
Downloaded 27 Jan 2004 to 132.68.1.29. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jsp
1
1,=4mA
(b)
I,=4mA
t
t -77-77
-2
3
b
3
2
Laser
0
0
IO.
Current
20
(mA)
-
I,=0
-
II
1545
1555
Wavelength
1565
(nm)
FIG. 2. Measured L-Z curves (a) and emission spectra (b) for the laser and
the amplifier. The emission spectra were measured for a bias level of 20 mA.
into the Bragg section causes the reflection to tune away
from the cavity mode (without reaching the next mode).
Consequently, the two conditions needed for oscillations
(proper round-trip phase accumulation and gain loss balance)
are not satisfied at the same wavelength and the laser operates as an optical amplifier.14 The AR coating on the Bragg
side prevents multiple reflections at wavelengths outside the
Bragg mirror bandwidth. Measured L-1 curves and spectra
under the two different operating conditions are shown in
Fig. 2. The spectra are shown near 1550 nm where we note
that the power contained in the laser oscillating line is reduced by four orders of magnitude when the laser is turned
into an optical amplifier. The spectra near 1490 nm are not
shown but they contain no ripples since the output reflectivity is close to zero outside the Bragg reflection region.
Gain dynamics in the special device were measured using a cross polarized pump-probe technique. Pump-probe
measurements in a laser require special care in the choice of
pulse energy which must be kept extremely low in order for
the device to remain in the linear regime. The reason for the
strong limitation on pulse energy lies in the inherent nonlinearity of the laser oscillation which takes place simultaneously with the pulse propagation in the cavity. The pump
and probe energies in the present experiment were 40 and 4
fJ, respectively. The pulse wavelength is chosen to be 1490
and 60 nm shorter than the DBR laser wavelength. This ensures that in the laser case, the dynamics are examined sufficiently far from the oscillating line so as to not be affected
by the spectral hole caused by the DBR laser (since spectral
hole burning has a limited spectral extent2). It also ensures
that the output (Bragg section) reflectivity is approximately
zero.
The experimental setup contains a standard cross polarized pump-probe apparatus. The pulses used were generated
by an additive pulse mode-locked (APM) color center lasersV7
and are of 120 fs duration. In order to reduce the background
light generated by the DBR laser, we used, in addition to the
usual polarizer, a 20-nm-wide optical filter centered around
the probe pulses, at the laser output (before the detection
Appl. Phys. Lett., Vol. 64, No. 16, 18 April 1994
Amplifier
-
I,=5
i
0
2
Pump
4
6
Probe
8
0
2
Delay
4
6
8
ips)
FIG. 3. Measured gain evolution for the laser (a) and the optical amplifier
(b).
system). The output probe signal was measured using a standard lock-in technique.
?tyo sets of pump-probe measurements are described in
Fig. 3. In the first, described in Fig. 3(a), the Bragg contact
was grounded so the device operating condition was that of a
laser. In the second, described in Fig. 3(b), a 5 mA bias was
injected into the Bragg section changing to an optical amplifier operating condition. For each case, the gain dynamics
were measured for several drive currents to the gain section
ranging from 4 to 25 mA. The laser threshold current was 8
mA. Although the APM laser was tuned 60 nm shorter than
the laser oscillating wavelength, all but the curve for the 4
mA case, exhibit a decrease in the long lived gain, typical of
operation in the gain regime.6T7The time constant describing
the gain recovery during the first few ps was extracted for
each of the measured traces, some of which are shown in
Fig. 3. In order to obtain accurate time constants, it was
necessary to extend the measurements to a time delay of at
least 30 ps. The time constant was extracted iu the following
manner: First the final value was subtracted. Next, the subtracted curves were displayed on a logarithmic scale where
an exponential recovery appears as a straight line whose
slope represents the recovery time constant. The data were
sufficiently clear to identify a single time constant in the 1 ps
time scale.
The dependence of this time constant on operating condition is obtained from its bias dependence in both the optical amplifier and laser modes of operation. The results are
depicted in Fig. 4. The laser L-f curve is superimposed on
Fig. 4 for reference. At low currents, below threshold, the
laser and amplitude exhibit essentially the same time constant -0.7 ps, consistent with Refs. 1, 6, and 7. The fact that
the time constants are similar assures that there are no artifacts produced by the current injection into the Bragg section. At higher currents, the time constant of the amplifier
remains essentially unchanged while in the laser case, it increases sharply above threshold to -0.9 ps. This result
shows that the high rate of stimulated emission in the laser
above threshold slows down the gain recovery during the
first one picosecond following the gain perturbation. This is
Tessler et a/.
2051
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Laser
i
I
I
\
/’ Amplifier
I’
I,=5mA
/!
0
10
Current
20
(mA)
FIG. 4. Gain recovery time constant for a laser and an amplifier. The laser
L-I curve is superimposed on the plot for reference.
most important since it suggests that the nonlinear gain coefficient is power dependent. The limited accuracy of the
measurement (seen by the displayed error bars) prevented us
from observing this bias dependence above threshold. Only
the large differences between below and above threshold
conditions are clearly observable as seen in Fig. 4.
The results shown in Fig. 4 can be explained by considering the local carrier capture phenomenon associated with
the gain recovery. There are two possible carrier capture
mechanisms; phonon mediated capturei and carrier-carrier
scattering mediated capture.‘5.16 In the case of phonon mediated capture, no extra heat is producedr’ and the capture
process simply serves as an additional channel for carrier
cooling. This increases the carrier density (and hence the
gain) at the probe wavelength and the measured time constant has some effective value that couples the two processes.
Since these two processes have very similar time constants,
they are inseparable in single wavelength pump-probe measurements. Above threshold, there is an increase of carriers
in the barrier and confinement (3D) states due to the injection bottleneck effect.‘“‘3 This increases the relative contribution of the capture process. If the gain recovery time that is
associated with capture is larger than the cooling time, the
increase in density of three-dimensional carriers results in an
2052
Appl. Phys. Lett., Vol. 64, No. 16, 18 April 1994
increase of the effective (measured) recovery time.
In the case of carrier-carrier scattering mediated capture,
the relevant time constant may be as short as a few tens offs
(on the scale of the spectral hole burning time constant). In
this case however, the capture process produces extra heat”
and slows down the cooling process.17 Since the carriercarrier scattering mediated capture time is density dependent,
the relative importance of the two capture mechanisms depends on the carrier density at threshold. The effective recovery time constant is expected to increase therefore for either
capture mechanism, as demonstrated in Fig. 4.
To conclude, we have shown that laser gain dynamics
are unique and contain details that cannot be revealed by
studying optical amplifiers. The fast gain recovery time (-1
ps) was found to increase by -30% above threshold. This
result is in good agreement with static gain spectrum measurements under similar conditions.13 It suggests that the
change in the time constant is indeed related to the carrier
capture phenomenon and the injection bottleneck associated
with it. However, further study is needed to clearly verify
and quantify the exact role of the carrier capture process.
‘M. P. Kcsler and E. P. Ippen, Appl. Phys. Len. 51, 1765 (1987).
‘M. Wileatzen, A. Uskov, J. M&k, H. Olesen, B. Tromborg, and A. P.
Jauho, IEEE Photon. Tech. Lett. 3, 606 (1991).
3 I. P. Kaminow and R. S. Tucker, Guided-Wave Optoelectronics, edited by
T. Tamir (Springer, Berlin, 1988). p. 211.
4J. E. Bowers and M. A. Pollack, Optical Fiber Telecommunications IZ?
edited by S. E. Miller and I. P. Kaminow (Academic, San Diego, 1988), p.
509.
5C. H. Lange and C. B. Su, Appl. Phys. Lett. 55, 1704 (1989).
“K. L. Hall, G. Lenz, E. P. Ippen, U. Karen, and G. Raybon, Appl. Phys.
Lett. 61, 2512 (1992).
7J. Mark and J. M&k, Appl. Phys. Lett. 61, 2281 (1992).
‘K. K&u&i, M. Kakui, C. E. Zah, and T. P. Lee, IEEE J. Quantum Elec-~
tron. 28, 151 (1992).
‘S. Murata, K. Kaniwae, J. Shimizu, M. Nido, A. Tomita, and A. Suzuki,
Electron. Lett. 28, 1456 (1992).
“B. N. Gomatam and A. P. DeFonzo, IEEE J. Quantum Electron. 26, 1689
(1990).
” N. Tessler and G. Eisenstein, IEEE J. Quantum Electron. 29, 1586 (1993).
‘aW. F. Shartin, J. Schlafer, W. Rideout, B. Elman, R. B. Lauer, J. Lacourse,
and F. D. Crawford, IEEE Photon. Tech. Lett. 3, 193 (1991).
13N. Tessler, R. Nagar, G. Eisenstein, S. Chandrasekhar, C. H. Joyner, A. G.
Dentai, U. Koren, and G. Raybon, Appl. Phys. Lett. 61, 2383 (1992).
14M Margalit, R. Nagar, N. Tessler, G. Eisenstein, and M. Grenstein, Opt.
Lit. 18, 610 (1993).
“P. W. M. Blom, J. E. M. Haverkort, P. J. Van Hall, and J. H. Walter, Appl.
Phys. Lett. 62, 1490 (1993).
16M. Svendsen, J. M&k, and H. Haug, Phys. Rev. B 49 (1994).
r7L. F. Lester and 8. K. Ridley, J. Appl. Phys. 72, 2579 (1992).
Tessler et a/.
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