J. R. Karin

Short pulse generation using multisegment mode-locked semiconductor lasers

Mode-locked semiconductor lasers which incorporate multiple contacting segments are found to give improved performance over single-segment designs. The functions of gain, saturable absorption, gain modulation, repetition rate tuning, wavelength tuning, and electrical pulse generation can be integrated on a single semiconductor chip. The optimization of the performance of mode-locked lasers in terms of material parameters, waveguiding parameters, electrical parasitics, and segment length is discussed experimentally and theoretically. >

ULTRAFAST DYNAMICS IN FIELD-ENHANCED SATURABLE ABSORBERS

Absorption recovery dynamics of GaAs/AlGaAs field‐enhanced waveguide saturable absorbers are studied by pump‐probe differential transmission measurements. We compare the response of bulk and single quantum well absorbers at different reverse bias levels and pump powers, and find an ultrafast transient in the response, followed by a slower rise before the final recovery. The absorption fully recovers after a few picoseconds, which is an important result for mode‐locked lasers.

Picosecond dynamics of optical gain switching in vertical cavity emitting lasers

It is demonstrated that optically gain switched GaAs vertical lasers (VSELs) with short cavity lengths can generate pulses as short as 4 ps, with delays as short as 20 ps. A model of the large-signal response of VSELs to short optical input pulses is presented. The results demonstrate that the critical parameter of determining VSEL response time is the cavity length. A numerical solution to the rate equations is used to calculate the output pulsewidth and delay, given a specific cavity design and set of input conditions. Calculated results are compared to experimental data. Output pulse dependencies on input pulsewidth and power are examined for different cavity designs. The results of the calculations are used to generate design curves for future devices. >

Suppression of multiple pulse formation in external-cavity mode-locked semiconductor lasers using intrawaveguide saturable absorbers

Imperfect antireflection coatings in external-cavity mode-locked semiconductor lasers can cause multiple output pulse generation. The incorporation of an intrawaveguide saturable absorber segment into the laser suppresses this problem. Single pulse outputs of less than 2.8 ps and 0.7 pJ of energy are obtained using such devices with both quantum well and bulk active regions.<<ETX>>

Picosecond GaAs monolithic optoelectronic sampling circuit

An optoelectronic sampling circuit has been fabricated for direct measurement of picosecond optical waveforms. The monolithic device incorporates a GaAs Schottky photodetector and a high-speed sampling circuit gated by a nonlinear transmission line strobe pulse generator. Excited by a 850-nm mode-locked dye laser, a 5.6-ps FWHM impulse response is measured; the authors estimate a deconvolved impulse response of approximately 4.5 ps FWHM.<<ETX>>

High-frequency electrooptic Fabry-Perot modulators

Electrooptic modulators built from GaAs/Al/sub x/Ga/sub 1-x/As Fabry-Perot cavities operating up to 6.5 GHz are reported. The measured frequency response agrees well with the one predicted using an equivalent circuit model derived from high-speed electrical measurements. The parasitic capacitances have been reduced to approximately 30 fF by fabricating the devices on semi-insulating GaAs substrates and integrating them with on-wafer bound pads which have dimensions compatible with microwave coplanar probes.<<ETX>>

CARRIER HEATING DYNAMICS IN SEMICONDUCTOR WAVEGUIDE SATURABLE ABSORBERS

Pump‐probe measurements in GaAs/GaAlAs waveguide saturable absorbers reveal an ultrafast transient in the absorption recovery dynamics, and a second slower signal before the final recovery. We present a model of the differential transmission measurements which includes high field and space charge effects in addition to the spectral hole burning and carrier heating typically found in semiconductor amplifiers. The results of the model indicate that field‐induced carrier heating strongly influences the absorption dynamics in saturable absorbers used for mode‐locked diode lasers.

Integrated Optical Transmitters And Receivers Using Multi-segment Laser Processes

Research on integrated optical devices has often focussed on complex structures and complicated processing techniques [1,2]. However, a variety of integmted optoelectrOnic devices can be fabricated fiom a simple semiconductor laser diode waveguide process. Several important functional components can be obtained by splitting the top contact in a laser process into several segments allowing for non-uniform pumping. When forward biased, the device has gain allowing far amplification, direct current modulation of the gain, and modulation of the index of refraction. When reverse biased, the segment functions as a p-i-n photodetector, saturable absorber, and electro-absorption modulator. With these simple functions, many useful integrated optoelectronic devices can be formed. Figures la and lb show example transmitter and receiver devices which have been fabricated with our process. Figure la shows a three-segment monolithic cavity mode-locked semiconductor laser 131 with a waveguide saturable absorber, gain modulation segment, and gain segment. Devices with repetition rates as low as 5.5 GHz (7mm device length) and as high as 80 GHz (0.5 mm device length) have been fabricated. Figure 2 shows an autocorrelation trace for a 1.55 pm wavelength monolithic cavity mode-locked laser operating at a repetition rate of 21 GHz. Device structures similar to that of Figure la have been used for a 3-section gain-switched laser in which two saturable absorbers near the laser facets are used to obtain shorter pulsewidths than are possible with single-section gain-switched lasers. The device of Figure la can also be used as a two-segment superluminescent LED. A short reverse-biased segment acts as an optical termination to absorb spontaneous emission in one direction with a power reflection coefficient below lo4. The reverse-biased optical termination segment also serves to monitor average power from the superluminescent LED. Figure lb shows a 3 segment pre-amplified photodetector in which one segment is used as an optical amplifier, the second segment is used as a switch to turn the signal to the detector on or off, and the third segment is the p-i-n photodetector. Figure 3 shows the impulse response of the waveguide photodetector with a mode-locked semiconductor laser as the pulse source. The full width at half of maximum is 33 ps limited by the capacitance of the photodetector. The switching segment can be tumed on and off in 200 ps using a step-recovery diode drive signal. The switching segment can also be used as a saturable absorber in pulse amplification applications. The entire spontaneous emission output from the amplifier does not reach the photodetector between optical pulses. Pumpprobe measurements of the saturable absorber section were made to measure the absorption recovery time constant. The saturable

Ultrafast dynamics in waveguide saturable absorbers

sorber can recover to the highly attenuating state in less than 10 ps after the passage of the optical pulse through the absorber denonsytting that the saturable absorber can be effective in reducing the spontaneous emission reaching the photodetector. An outline of a low capacitance multi-segment laser process is shown in Figure 4. A silicon nitride layer is deposited and patterned to outline the waveguide. The silicon nitride is used as an etch mask to define the waveguide for wet or dry chemical etching. Next, a polyimide layer is deposited and cured. In order to planarize the polyimide layer, a thick layer of photoresist is spun over the entire wafer. Both the polyimide and the photoresist layers are then etched in an oxygen plasma until the top of the m m slightly protrudes through the polyimide layer. The silicon nitride layer is then removed and the wafer is ready for metallization. The multi-segment p-metal is evaporated, lifted off and annealed. The metal contacts are used as a 3-way self-aligned mask to 1) etch the conmt layer between electrodes, 2) etch cleave marks for precise positioning of facet cleaves, and 3) remove the excess polyimide in order to promote good facet cleaving. Finally, the wafer is thinned, a backside metal is deposited and the devices are cleaved. [ 11 U. Koren, Indium Phosphide and Related Materials Conference, April, 1990. Paper TuC.1. [2] T.L. Koch, U. Koren, R.P. Gnall, F.S. Choa, F. Hernandez-Gil, C.A. B m , M.G. Young, M. Oron, and B.1 Miller, Indium Phosphide and Related Materials Conference, April, 1990. Paper TuC.2. [3] D. J. Derickson et al., 1992 Optical Fiber Conference, Paper ThB3. San Jose, Ca

High-speed III-V asymmetric Fabry-Perot modulators

Summary form only given. The results of a model of differential transmission measurements in GaAs-GaAlAs waveguide saturable absorbers indicate that high field effects, spectral hole burning and carrier heating are important mechanisms governing absorption dynamics.