Picosecond-range single optical pulses with peak power in the range 10-100 W are fairly attractive for various practical applications. A laser diode structure has lately been suggested which produces powerful (~ 50 W) picosecond (~20 ps) optical pulses near the trailing edge of the current pulse by means of field-assisted gain control. Lasing onset is delayed in this diode by a few nanoseconds due to intendance-reduced pumping efficiency caused by the implementation of internal optical pumping. The ps operating mode is based on a compromise between the dynamics of carrier accumulation and of the transverse electric field, controlled by the efficiency of the internal optical pumping. The pumping efficiency is determined to a large extent by competition between stimulated and spontaneous radiative recombination at the source of optical pumping. An effect of the laser diode switching from the picosecond to the quasi-steady-state (ns) mode was observed when the length of the laser cavity was reduced from 400 μm to 200 μm. This phenomenon is studied and attributed to an increase in the fraction of spontaneous photons due to reduction in the density of the stimulated emission at the source of the optical pumping.
A measurement system for electro-optical characterization of photodetectors and photodetector arrays is presented. The system is intended for laboratory use where typically a few devices are characterized at a time. The instrument is designed for determining the photoresponse (dc and pulse) of a single photodetector or a photodetector array as a function of wavelength, position, and temperature. In case of photodetector arrays, the measurement of modulation transfer function (MTF) and fast determination of its photoresponse nonuniformity (PRNU) is also possible. The instrument setup and experimental results are presented.
The performance of a random target method for fast MTF measurement of a lens is evaluated. Although the method is well-known, its potential for fast lens testing has not been assessed in the open literature. To optimize speed, the simplest possible instrument setup with minimum amount of mechanical movements during measurement execution is used. The setup includes only a random target, lens under test and a CCD camera with focus adjustment. The target consists of a random black and white pattern of a flat spectrum. The MTF of the lens is acquired by imaging the random target on the CCD using the lens, and then analyzing the spatial frequency content of the image using an ordinary PC. It was found out that a suitable compromise between speed and precision is achieved using a matrix of 128*128 samples per measured field point. This provides better than 2% precision and a few second's total execution time per lens including best focus evaluation and the measurement of tangential and sagittal MTF curves of 5 field points. Using commercially available components, measurement frequencies up to 100 cycles/mm seem achievable using the simple instrument setup.
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