210 OPTICS LETTERS / Vol. 20, No. 2 / January 15, 1995

Heterodyne nondegenerate pump–probe measurementtechnique for guided-wave devices

C.-K. Sun

Division of Applied Sciences, Harvard University, and Research Laboratory of Electronics,Massachusetts Institute of Technology, Cambridge, Massachusetts 02139

B. Golubovic and J. G. Fujimoto

Department of Electrical Engineering and Computer Science and Research Laboratory of Electronics,Massachusetts Institute of Technology, Cambridge, Massachusetts 02139

H. K. Choi and C. A. Wang

Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, Massachusetts 02173-9108

Received July 15, 1994

We describe a new heterodyne nondegenerate pump–probe waveguide measurement technique that permitsindependent control of wavelengths, pulse widths, and polarizations of the pump and probe pulses in acollinear geometry. This technique provides both time- and spectral-domain information and can be appliedto transmission and index measurements alike. We demonstrate this technique for the measurement of gaindynamics in a strained-layer single-quantum-well diode laser.

Nonlinear transient gain and index dynamics inwaveguide devices influence laser linewidth and mod-ulation bandwidth, signal processing, amplification,and short-pulse generation in diodes.1 – 4 Opticalpump–probe measurements provide a direct wayto investigate ultrafast transient behavior in thetime domain. During the past few years, severalsingle-wavelength pump–probe techniques such asbias lead monitoring,5 time-domain interferometry,6,7

and heterodyne detection8 have been developed.Multiple-wavelength pump–probe techniques9 – 11

were also demonstrated by several different groups,permitting measurements with different pump andprobe frequencies. However, to permit the probe tobe distinguished from the pump in a collinear wave-guide geometry, the pump and probe must be eitherorthogonally polarized or spectrally separated.

In this Letter we describe a new heterodyne non-degenerate pump–probe technique that permits mea-surements to be performed with arbitrary pump andprobe wavelengths in arbitrary polarization states.This technique is suitable for performing gain/loss aswell as index measurements at various transition en-ergies. Moreover, the independent pump and probewavelength control allows for the recovery of bothtime-domain and spectral-domain information.

Figure 1 shows a schematic of the experimen-tal arrangement for heterodyne nondegeneratepump–probe measurements. A Kerr-lens mode-locked Ti:Al2O3 laser (Coherent Mira-900) generated120-fs pulses at a 76-MHz repetition rate with a10-nm bandwidth, tunable from 880 to 1068 nm,with an output power of 0.3 W (1068 nm) to 1.8 W(900 nm). The laser output was coupled into a7-cm-long, 4-mm core-diameter, non-polarization-preserving single-mode optical fiber. With 500 mWof average power in the fiber, self-phase modulation

0146-9592/95/020210-03$6.00/0

broadened the spectral bandwidth to 70 nm FWHM.After the fiber, the beam was split into a pump anda probe and directed into separate spectral window-ing assemblies.12 The spectral windowing assemblyconsisted of a pair of lenses arranged as a telescope.The distance between the second lens and gratingwas adjusted to compensate or precompensate forpositive dispersion in the fiber, the lenses, and theacousto-optic modulators (AOM’s) used later. Thedifferent pulse spectral components were spatiallyseparated at the midplane between the lenses, andarbitrary bandwidth and wavelength were selectedwith a slit of adjustable width and position. The slitswere moved away from the midplane (focal plane)to defocus the image and yield a smoother-shapedspectrum. With a 50-nm input bandwidth, the pulsewidth was tunable from 30 fs up to several picosec-onds, and the wavelength separation between thepump and the probe could be up to 70 nm.

To distinguish the probe from the pump pulses inthis collinear geometry, we used a heterodyne detec-tion technique.8 The probe and reference pulses

Fig. 1. Heterodyne nondegenerate pump–probe setup.

1995 Optical Society of America

January 15, 1995 / Vol. 20, No. 2 / OPTICS LETTERS 211

were frequency shifted by means of fused-silicaAOM’s with 50- and 51-MHz modulation frequen-cies, respectively. The pump was directed through astandard optical delay line, then combined with theprobe and reference collinearly and coupled into thewaveguide. The reference pulse was 1 ns ahead ofthe pump and the probe so that it would experiencethe same conditions. Polarization control elementscan be inserted into the paths of the pump and theprobe to control the polarization independently.

The end facet of the waveguide was imaged, and aniris was used to select the light from the guiding re-gion only. The beam was then recollimated and sentto an imbalenced Michelson interferometer to delaythe reference and interfere it with the probe at thedetector. For the nonlinear transmission measure-ments, an AM radio receiver was used to select theprobe signal by monitoring the 1-MHz beat frequencybetween the probe and reference pulses. By chop-ping the pump at 400 to 1000 Hz and using lock-in de-tection, we achieved a background-free measurement.For nonlinear index measurements, the only changesin the setup are the use of an FM instead of an AMreceiver and the addition of a piezoelectric transducerto one of the interferometer mirrors to calibrate theabsolute phase change.13 Because the pump and theprobe pulses are distinguished by heterodyne detec-tion, the pump and the probe can be of any arbitrarypolarization or wavelength, thus permitting time-resolved measurement of different tensor elements ofthe nonlinear response at various frequencies.

This technique was demonstrated on measure-ments of the femtosecond nonlinear gain dynamicsin laser diodes. The devices used were InGaAsyAlGaAs strained-layer single-quantum-well diodelasers.14 The single quantum well had 10-nm-thickIn0.13Ga0.87As active layers and 2.5-nm-thick GaAsbounding layers. The tested device was 300 mmlong, with uncoated facets, and had a thresholdcurrent of ,20 mA. The heavy-hole band gapwas ,960 nm. All measurements were performedwithin the perturbative limit, so that differentialtransmission changes were directly proportional tochanges in the gain spectrum. The pump and probepulses were chosen to be 200 fs in duration with an8.2-nm bandwidth.

Figure 2 shows the TE-mode gain spectra of thisdiode at bias currents of 2.5, 3.3, and 4.2 mA. Thespectra were measured with 200-fs optical pulsestuned from 900 nm (1.38 eV) to 970 nm (1.28 eV).Because of carrier temperature effects, the short-pulse gain spectrum will vary slightly from the cwgain spectrum. For 3.3 mA, the measurement showsa transparency point of ,935 nm and a gain peak of,945 nm.

Figure 3 shows the TE-mode transient at the gainpeak produced by pump pulses at different wave-lengths in the gain spectrum. We performed experi-ments by biasing the diode at 3.0 mA and placing theprobe at the 945 nm. The pump pulse was tuned inincrements of 5 nm, from 885 nm (,110 meV abovethe heavy-hole band gap in the loss region), throughthe transparency point at 935 nm, and up to the bandgap at 965 nm.

The sharp drop in transmission near zero time de-lay was generated by coherent two-photon absorp-tion effects. Spectral hole burning also was presentfor pump wavelengths close to the probe in the gainregion. For the pump in the gain region and in-termediate time delays, gain depletion is producedby carrier heating and a decrease in carrier popu-lation. Carrier heating effects relax on the ,0.8-pstime scale as the carrier temperature equilibratesto the lattice. After 1 ps, the gain depletion is pro-duced by the carrier population decrease, which re-covers on a much longer time scale through diffusionand carrier injection. When the pump is in the lossregion, absorption saturation is observed because of

Fig. 2. Experimentally measured short-pulse (200-fs)gain profiles for wavelengths ranging from 900 nm(1.38 eV) to 970 nm (1.28 eV) at 4.2, 3.3, and 2.5 mA.Fitted curves are also shown.

Fig. 3. Gain peak changes induced by different pumpwavelengths. Data were collected for I ­ 3.0 mA at afixed probe wavelength of 945 nm. Different pumps hadnormalized input powers with wavelengths tuned from885 to 965 nm. The vertical scale is inverted s2DTyT dfor better illustration.

212 OPTICS LETTERS / Vol. 20, No. 2 / January 15, 1995

Fig. 4. Transient gain dynamics with the pump in thegain region. The pump wavelength was 935 nm in thegain region, as indicated by an arrow. The probe wastuned from 900 to 960 nm with I ­ 4.2 mA. The verticalscale is inverted (2DTyT ) for better illustration.

carrier cooling15 and an increase in carrier popula-tion. After 1 ps, the cool carriers equilibrate backto lattice temperature, and the absorption saturationis produced by the carrier population increase, whichwill recover through carrier recombination on a muchlonger time scale.

The maximum gain depletion in the gain regionwas at ,945 nm and corresponded to the peak wave-length of the gain. With higher gain, stimulationemission is more intense and produces strongercarrier heating and carrier depletion effects. Con-versely, the maximum absorption saturation in theloss region in our study range was observed at,905 nm. This may be because light-hole bandtransitions become allowed for pump pulses of thiswavelength and contribute to the observed signal.

This experimental technique also permits transientspectral measurements by tuning the probe frequencythroughout the desired wavelength region. Figure 4displays the transient changes in the TE gain spec-trum (which are proportional to the differential trans-mission changes) induced by a pump pulse in thegain region. The diode was biased at 4.2 mA withthe transparency point at ,925 nm. The pump waschosen to be at 935 nm with TE polarization. Theprobe pulses were also TE polarized and tuned from900 nm in the loss region to 960 nm at the bottom ofthe gain region.

The transient measurement show a pump-inducedtransmission decrease in both the gain and lossregions. Near zero time-delay, a sharp transmis-sion decrease is observed with a spectral peak nearthe pump wavelength and with a time-resolution-limited recovery. This transient may be attributedto a combination of two-photon absorption, spectralhole burning, and coherent artifacts. Shortly afterthe pump pulse, a thermalized carrier distributionwith a higher temperature and a lower concentra-tion is established, and gain depletion (or increasein loss) throughout the investigated spectral regionis observed. For time delays longer than 1 ps, the

gain partially recovers, as the carrier temperaturereaches equilibrium with the lattice. The residualgain changes are produced solely by the decrease inthe carrier population and recover on a much longertime scale.

In summary, we have developed a new hetero-dyne nondegenerate pump–probe measurement tech-nique that permits independent tuning of the wave-length, pulse width, and polarization of the pump andprobe pulses by mean of an unamplified, cw, mode-locked Ti:Al2O3 laser. This technique can measureboth real and imaginary components of the nonlin-ear tensor for various wavelength and polarizationcombinations in a collinear waveguide measurementgeometry. This greatly enhances the versatility ofpump–probe measurements and can be applied toa wide range of studies of different materials anddevices. In this study we performed femtosecondtransmission measurements on an InGaAsyAlGaAsstrained-layer diode laser to obtain what is to ourknowledge the first direct femtosecond measurementof the wavelength dependence of the ultrafast gaindynamics in a diode laser.

We acknowledge scientific collaborations with E. P.Ippen, G. D. Sanders, and C. J. Stanton and discus-sions with D. Dougherty, G. Lenz, C. T. Hultgren,and K. L. Hall. This research is supported in part byU.S. Office of Naval Research FEL program N00014-91-J-1956, Joint Services Electronics Program con-tract DAAL03-92-C-0001, U.S. Air Force Office ofScientific Research contract F49620-91-C-0091, andthe U.S. Department of the Air Force.

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