Broadband
femtosecond
pump-
robe setup operating
at 1300 and 1550 nm
J. Mark, N. Tessler,a) G. Eisenstein,a) and J. M&k
Tele Danmark Research, Lyngd Alli 2, DK-2970 Hdrsholm, Denmark
(Received 8 November 1993; accepted for publication 8 February 1994)
We describe a broadband, tunable femtosecond pump-probe measurement setup operating in the
1300 and 1550 nm wavelength range. We demonstrate measurements of carrier dynamics in the
barrier states of a multiple quantum well optical amplifier. The amplifier is excited (pumped) in the
gain region near 1510 nm and probed, with femtosecond time resolution, in the barrier region, at
1300 nm, yielding dynamical details which cannot be observed with conventional, singlewavelength pump-probe techniques.
Ultrafast dynamics in diode lasers and optical amplifiers
are governed by complicated gain nonlinearities and carrier
injection processes. The most common method to experimentally study these processes is the single-wavelength
pump-probe technique in which subpicosecond pulses are
used.rm3 Frequency domain measurements, based on four
wave mixing in the nonlinear gain medium have also been
used.4 However, some details of the nonlinearities cannot be
observed using those relatively simple methods. Rather,
broadband, wavelength tunable techniques in which the excitation and probing are done at different energies are required.
An important example, and one on which this letter concentrates, is the study of carrier injection and gain nonlinearities in a quantum well (QW) optical amplifier. It is well
known that the nonlinearities in a QW amplifier result from
coupling of structural effects (such as carrier transport and
carrier capture)“5’6 with material nonlinearities (spectral hole
burning and carrier heating).7 A large fraction of the injected
carriers in a QW optical amplifier occupy the barrier states
and do not contribute to gain.3’8*9 The relaxation of these
barrier state carriers into the QWs has a major influence on
the gain nonlinearities. Some features of the relaxation process have been studied in single-wavelength pump-probe3
and broadband staticlo experiments.
Broadband tunable pump-probe measurements with subpicosecond time resolution can be performed in several
ways. Nonlinear continuum generation,‘13’2 where short
pulses at different wavelengths are filtered out simultaneously and used for broadband pump-probe analysis is a
very convenient vehicle. Other common techniques use a
single short pulse which serves as the excitation (pump)
while its replica serves as the gate (usually by some nonlinear technique) for a broadband signal. That technique has
been applied to time resolved luminescence’3’14 as well as to
cw probe signals.r5*16
This letter describes a new experimental procedure enabling pump-probe measurements with 120 fs resolution in
the 1300-1550 nm wavelength range. The setup, which resembles the ones described in Refs. 15 and 16, is needed for
detailed studies of carrier dynamics in InGaAs/InGaAsP QW
lasers and optical amplifiers as well as for experimental dis-
tinction among the various gain nonlinearities in those devices and their spectral extent.7
The experimental setup described in Fig. 1 uses an additive pulse mode-locked (APM) color center laser to which
a wavelength tunable, mode-locked diode laser is synchronized. Detailed operation of the setup is as follows. The 120
fs pulses from the APM laser are divided into three. One part
serves as the reference, the second which travels through a
variable delay line acts as the pump, and a third is detected
and amplified forming a drive signal to the mode-locked external cavity diode laser, which serves as the probe source.
The length of the external cavity is adjusted to match the
repetion rate of the APM laser (76 MHz). The probe pulse
width is 22 ps FWHM and the average power (from the
single-mode fiber pigtail) is typically 100 pW, corresponding
to a peak power level of 60 mW. The probe beam is chopped,
combined with the pump, and injected into the amplifier under test. For convenience and in order to eliminate coherence
artifacts we use orthogonal pump (TM) and probe (TE) polarization. The probe signal at the amplifier output is gated
by the 120 fs, -1500 nm reference pulse in a background
free upconversion scheme with a POM nonlinear crystal, the
output of which is filtered and measured with a photomultiplier and lock-in technique. The insert in Fig. 1 shows the
relative timing of the pulses. The complete usable wave-
‘JAlso at Department of Electrical Engineering, Technion, Haifa 32000,
Israel.
FIG. 1. Schematic diagram of the tunable pump-probe setup. The insert
illustrates the relative timing of the pump, probe, and reference pulses.
Appl. Phys. Lett. 64 (15), 11 April 1994
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0 1994 American Institute of Physics
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FIG. 2. Pump-probe responses for a four-well Q W optical amplifier biased
at 40 mA. The pump wavelength is 1.510 nm and the probe wavelength is
1510 and 1310 nm. The probe response has been normalized to its value
before pump injection.
FIG. 3. Measured broadband pump-probe responses for a four-well Q W
optical amplifier by 1510 nm pump and 1310 mn probe at various bias
levels.
length range is 1490-1530 run for the pump and, depending
on the laser diode, 1280-1580 MI for the probe signal.
The advantage of using a mode-locked laser diode as a
probe source is twofold. First, careful selection of the external cavity laser diode ensures tuning over nearly any wavelength range of interest. Second, mode-locked operation reduces the average probe power loading. Saturation effects of
the sample under test are therefore lower, for a given upconversion efficiency, compared to that of a cw probe. From
measurements of the average probe power level before the
POM crystal, we have estimated the upconversion sensitivity
limit to be 7 FW of peak power at 1300 nm for 20 mW
average power in the 1500 nm reference beam. For an
equivalent 120-fs-wide probe pulse, this corresponds to a
pulse energy of 840X10-a’ J or 6 photons per pulse.
The setup was used to study the carrier dynamics in a
QW optical amplifier having four go-A-wide InGaAs wells
separated by 100 A InGaAsP (1300 nm band gap) barriers.
The amplifier had a total confinement region width of 300
nm and its length was 1 mm.17
For reference, we show in Fig. 2 the normalized pumpprobe response for the four-well QW amplifier, measured
with a conventional single frequency pump-probe setup with
the pump and probe wavelength at 1510 nm. The pump and
probe energies are 100 and 1 fJ, respectively. The amplifier
bias current is 40 mA and the observed response is typical
for an amplifier operating in the gain region.2
Near zero delay, the decrease and rapid recovery in
probe transmission is due to two effects. One is the twophoton absorption (simultaneous absorption of one probe
photon and one pump photon) and the other is spectral hole
burning. This rapid transient is followed by an increase in
transmission having a typical time constant of close to 0.8
ps. This recovery represents cooling of the carrier distributions which are first heated by free carrier absorption and
stimulated emission caused by the pump pulse. The carrier
distributions cool to the lattice temperature by emission of
LO phonons. The recovery also embodies the process known
as local carrier capture.3T5p14
In Fig. 2 we also show the transmission evolution probed
at 1310 nm (with a probe pulse energy of 20 fJ) for an
excitation (pump) at 1510 rmr (with a pump pulse energy of
600 W). The probe signal senses the high energy tail of the
distribution of the bound carriers as well as carriers from the
barrier states. We first note a reduction in transmission due to
TPA near zero delay followed by a rapid increase. The transmission decreases then with a time constant of about 0.8 ps.
This decrease is a measure of the evolution of high energy
carriers as they relax in energy due to cooling and carrier
capture into the wells. The two effects, carrier cooling and
carrier capture, are of a similar nature and are hard to distinguish in a pump-probe experiment. At a given drive current,
these two relaxation mechanisms may have the same or opposite signs, as described in Figs. 3 and 4 below. The evolution of these high energy carriers is complimentary to the
evolution observed when probing at 1510 nm, see Fig. 2.
The dependence of the 1310 nm probe response (obtained for 1510 nm pump) on amplifier bias level is depicted
in Fig. 3. For all currents, we tirst observe a reduced transmission near zero delay due to TPA. In addition, we observe
a reduced transmission at long delay times because the pump
signal, which is within the gain region of the amplifier, reduces the number of carriers by stimulated emission. This
pump-induced stimulated emission, as well as free carrier
absorption, gives rise to carrier heating effects under all bias
conditions. However, Fig. 3 shows that the probe transmission at 1310 nm can either increase (for bias currents larger
than 100 mA) or decrease (for bias currents below 100 mA).
The time constant associated with both the increase and the
decrease is approximately 1 ps.
The reason for the change of sign can be understood
with the aid of Fig. 4 where calculated equilibrium and
heated carrier distributions in the conduction band are shown
for two different carrier densities. The dotted line in Fig. 4
indicates the approximate energy probed by the 1.3 pm signal, when transitions from confined hh states to confined or
unconfined conduction band states are considered (see the
insert). We notice that the probed carrier density increases
1900
Appl. Phys. Lett., Vol. 64, No. 15, 11 April 1994
Mark et al.
Downloaded 27 Jan 2004 to 132.68.1.29. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jsp
E
3D 2D
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300K
I
4x10=
Density [m-3eV-I]
FIG. 4. Equilibrium (dashed lines) and heated (full lines) carrier distributions in the conduction band for different carrier densities in the QW, Now.
The insert (not drawn to scale) shows the possible optical transitions between confined hh states and confined or unconfined conduction band states
corresponding to the energy of the 1310 n m TE probe (dotted line).
for decreasing temperature for the high density case, whereas
the opposite holds true for the low density case. These trends
are in qualitative agreement with the measurements described in Fig. 3.
The various 1310 run probe responses, Fig. 3, describe
many more details which are not observable in single-
Appl. Phys. Lett., Vol. 64, No. 15, 11 April 1994
wavelength pump-probe measurements. These will be addressed in a future publication.
W e wish to thank C. P. Seltzer, BT Laboratories, for
supplying the laser diode and amplifier used in this experiment.
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