Optronic (Sweden)

A microwave photonic phase detector is proposed to synchronize a microwave to a mode-locked laser. The phase noise of an 8.058-GHz signal is improved by more than 40 dB at 1-Hz offset frequency.

Results obtained in a study of spectrometric characteristics of arrays of four detectors based on 4H-SiC ion-implantation-doped p +-n junctions in the temperature range 25–140 °C are reported for the first time. The junctions were fabricated by ion implantation of aluminum into epitaxial 4H-SiC layers of thickness ≤45 μm, grown by chemical vapor deposition with uncompensated donor concentration N d − N a = (4–6) × 1014 cm−3. The structural features of the ion-implantation-doped p +-layers were studied by secondary-ion mass spectrometry, transmission electron microscopy, and Rutherford backscattering spectroscopy in the channeling mode. Parameters of the diode arrays were determined by testing in air with natural-decay alpha particles with an energy of 3.76 MeV. The previously obtained data for similar single detectors were experimentally confirmed: the basic characteristics of the detector arrays, the charge collection efficiency and energy resolution, are improved as the working temperature increases.

The spectral characteristics of a series of different VCSELs emitting at 850 nm and 762 nm have been examined, to assess the potential of this new kind of semiconductor laser for use in laser spectroscopy and fundamental metrology, particularly for environmental and energy monitoring of combustion processes. Adequate polarization control, which is necessary for laser spectral purity, was provided through manufacture of a dumbbell or 'figure-of-eight' shaped active region of the lasers at 850 nm. Spectral lineshape and linearity of the frequency tuning have been examined with a variety of methods such as optical interferometry and heterodyning. Optical heterodyning was used to measure a minimum linewidth of 90 MHz FWHM for a 762 nm VCSEL during single mode operation. Deviations from linearity of up to some GHz of frequency tuning with current were observed with an original application of a Twyman-Green interferometer, with basic current tuning rates of 58 GHz/mA and 130 GHz/mA for an 850 nm and a 762 nm VCSEL, respectively. The frequency stability for a 762 nm device was measured to be approximately plus or minus 40 MHz. A possible problem for the use of VCSELs in wavelength modulation spectroscopy (WMS) is that due to their extremely high tuning rate with current: ultra-low noise current drivers must be used in order to achieve high sensitivity detection. The WMS absorption spectrum with VCSEL of a number of lines of the oxygen molecule at 760 nm have been registered and studied.

There is a trend today towards a reduction in target signatures, the signatures becoming increasingly adapted to the background in which the targets operate. In addition, new types of countermeasures are making the task for optical seekers increasingly difficult. One way to increase the capability of detecting low-signature targets in a countermeasure environment is to utilize not only the magnitude of the signature but also its distribution over the spectrum. For collection of information regarding the spectral signatures of targets, countermeasures and backgrounds, a multispectral imaging MWIR sensor has been developed by us. This device utilizes the high frame rate made possible by modern FPA arrays. Such an array has been combined with a rapidly rotating filter wheel, thereby producing images of 128 by 128 pixels in six wavelength bands in the 2 - 5 micrometer region at a frame rate exceeding 30 Hz in each band. The sensor has a field-of-view of 3.7 degrees and a pixel resolution of 0.5 mrad. The sensor has the capability to perform two point correction in real time, thereby compensating for the different dynamic ranges in each spectral band. An extensive measurement program is in progress for gathering data for targets, countermeasures and backgrounds. Selected results from this program are presented.