A frequency modulated semiconductor laser and an interferometer are used as a source of very high frequency amplitude modulation to measure the response of optical detectors. This new technique does not require a laser with a flat, or even known, frequency response, and measures the detector response at frequencies well above the modulation frequency applied to the laser. The response of several InGaAs p-i-n detectors has been measured to 22 GHz using 1.3-and 1.55-pm semiconductor lasers modulated at only 500 MHz. These measurements were not limited by the measurement method, which may be capable of measuring bandwidths substantially in excess of 20 GHz.
The optical path difference (OPD) in a classical heterodyne interferometer is derived from the phase of a sinusoidally oscillating signal[l]. Thus surfaces with discontinuities > A cannot be measured due to ambiguities of 2% in phase. Similarly, target distances > A cannot be measured except by counting fringes under translation from a known reference point. If the optical beam is interrupted the reference point can be lost, making heterodyne interferometers difficult to use for robotic applications.
The fabrication of long wavelength opto-electronic devices on GaAs substrates is an attractive method for the monolithic integration of optical and electronic devices on a single chip for applications in telecommunications. InGaAs strained layer quantum wells provide one way of engineering the bandgap needed for 1.3 µm devices at room temperature. We have grown and characterized quantum well structures with bandgap of .96 eV using a novel technique, called spatial strain separation, to avoid the stringent limitation of critical thickness which limits the achievable bandgap in this material system. Moreover we have studied the stability of such structures with respect to thermal processing, including oven and rapid thermal annealing. We have observed no degradation of the quantum wells after thermal annealing as judged from the PL spectra shown here.
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