We present a new displacement probe for a gantry-type profilometer to measure the ground surface form. The system is based on heterodyne interferometry with an acousto-optic modulator (AOM) and I/Q demodulation scheme. When the light from the single-frequency laser diode passes through the AOM following the Bragg condition angle, it is divided into the non-diffracted zero-order and a diffracted first-order beam with a frequency shift as the amount of the AOM driving frequency. One beam is used for the test and the other for the reference beam. Orthogonally reflected beams pass through the AOM one more time, and these beams make a heterodyne signal with a double modulation frequency. In this case, the only backscattered beam from the ground surface satisfies the Bragg condition, producing the modulated interference. Therefore, the AOM tends to reduce the stray light noises from the other incidence angles, acting as a transmission filter for backscattered light. The interference light arrives at the high-speed photodiode and it is then demodulated using the I/Q demodulation to extract the phase value. Although the backscattered ray from the rough glass surface has a very low intensity, the I/Q interferometer can detect the signal because the phase can be acquired regardless of the intensity change. Preliminary experiments confirmed that the system can measure from backscattered light of ground glass surface form.
Free space optical wireless communication is an attractive way of connecting vast numbers of urban area customers to the fiber optic communication network. We have designed and tested a prototype 2 km long 1.2 Gb/s optical wireless link operating at 1550 nm. An EDFA amplified signal from a standard fiber optic transmitter unit was sent via a small telescope to a 5 inch corner cube mounted on the roof of a building located over 1 km from the transmitter. An estimated 10 mWatt incident on the corner cube was reflected back to the transmitter/receiver unit, where the signal was successfully recovered. Using this test range we have tested the two-fold time-delayed diversity scheme. Diversity delays of 5 ms, and 10 ms show significant reductions in the probability of a joint fade at a particular level. Delays beyond about 10 ms do not significantly improve link performance. The system we have developed allows straightforward DWDM and polarization diversity extensions. Design issues for such optical wireless systems are discussed. We believe that such optical wireless transmitter/receiver units, which operate as an extension of the fiber network, offer a reliable and inexpensive solution for the 'last mile' problem in optical communications.
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