In view of the convenience and effectiveness of a position sensitive detector (PSD) in sensing the local distortion of a light beam wavefront, a high-speed wavefront sensor based on PSD array is proposed, fabricated and characterized. As a prototype of this kind of wavefront sensor it consists of a matrix of 4×4 tetra-lateral PSDs, and each PSD unit measures 500 μm×500 μm with a 300 μm interval between them. Each unit has 4 electrodes, which can be used for photocurrent output. The local distortion of a light beam wavefront can be deduced from those 4 photocurrents. Besides 64 electrodes of 16 element units, the sensor also has another common electrode that can be used in applying a reverse bias. Primary tests of the device show that it has low dark current, high spectral sensitivity, very fast response speed, and quite small crosstalk between its neighbouring units. The dark current of an element unit is less than 1 nA at a reverse bias of 20 V. The peak spectral sensitivity of the sensor is over 0.6 mA / mW at 900 nm wavelength. Its response time is about 10 ns at 45 V reverse bias, and the crosstalk between its neighbouring units is as low as 1.6% at λ= 650 nm and U<sub>r</sub> = 5 V.
In a conventional, strip-type one-dimensional (1-D) position sensitive detector (PSD) the sensitive area and the position resistance area are combined together, the contact structure of the device is handicapped, so its accuracy and linearity is affected adversely. The sensitive area and the position resistance area in a new, comb-type 1-D PSD are separated to become the comb teeth and the comb ridge area respectively, moreover the position resistance area is made very narrow, thus the doping uniformity of that area is greatly improved. On the other hand, its position resistance can be largely increased owing to its narrower shape, if doping level is kept the same, so the restriction that is imposed on the contact structure previously can be lifted. Our measurements made on a strip-type 1-D PSD and a comb type 1-D PSD confirmed this difference, and showed that the accuracy and linearity of the comb type 1-D PSD has been increased markedly, especially the RMS nonlinearity of the comb type 1-D PSD is reduced to 0.090% from that of the strip-type 1-D PSD, 0.94%.
A new signal processing circuit for position sensitive detector (PSD) is proposed in this paper. Using this circuit, the photocurrents of PSD can be directly transformed into digital signals, and the signal-to-noise ratio can be greatly improved, but the measurement time is extended. The measurement results show that the agreement between theory and experiment is quite good.
ZnCdSe/ZnSe multiple quantum well (MQW) transmission and reflection photomodulators operating at room temperature were fabricated employing quantum-confined Stark effect on the exciton absorption. Samples were grown on p-type GaAs substrates by MBE with an i-Zn<SUB>0.87</SUB>Cd<SUB>0.13</SUB>Se/ZnSe MQW heterostructure sandwiched by a ZnSe p-n junction. The transmission modulator was constructed with a Zn<SUB>0.87</SUB>Cd<SUB>0.13</SUB>Se/ZnSe MQW glued onto a piece of ITO film-covered glass with silver paste and epoxy. To avoid absorption in GaAs substrates, a window with a diameter of about 2 mm was opened using a selective etch. For the reflective use an Al mirror was deposited on the glass back surface, the device then operates in reflection with the light to be modulated making a double pass through the active quantum well region, thereby increasing the modulation amplitude. Measurement results are given in this paper for transmission, reflection, differential transmission, differential absorption, and differential reflection as a function of the incident photon wavelength and the applied field.
Frequency doubling method has been applied to measure polar Kerr rotation and ellipticity. Magneto -optical (MO) constants of some GdCo and TbFeCo films were obtained. The optical performance of A1N TbFeCo / glass bilayer structure calculated with the measured MO constants agrees quite well with the experimental results.