Heinz K. Philipp

Costas Loop Experiments for a 10.6 µm Communications Receiver

We have transferred the principle of the Costas-type nonlinear phase-locked loop to the \lambda = 10.6 \mu m wavelength of CO 2 lasers. The ability of the optical Costas loop to regenerate the carrier of a binary phase-shift-keyed input signal and to perform coherent demodulation was demonstrated at a data rate of 20 Mbits/s.

Frequency synchronization and phase locking of CO2 lasers

Employing the principle of a phase‐locked loop (PLL) we have synchronized the frequency and phase of two CO2 lasers. The laser acting as voltage controlled oscillator is tuned both by electro‐optic and piezoelectric means. A cooled photodiode serves as the phase detector. The main loop parameters, natural frequency ωn and damping factor ζ, have been measured. The small phase jitter in the error signal obtained will allow application of such an optical PLL in homodyne receivers at 10 μm.

Realization of a 10-µm homodyne receiver

We have realized a homodyne receiver for phase-modulated laser radiation at \lambda = 10.6 \mu m, based on a dc-coupled optical phase-locked loop (PLL). A control loop maintains the output power of the CO 2 laser local oscillator at 0.7 mW. Fine frequency tuning of this laser is achieved by an acoustooptic frequency shifter whose inherent time delay has been taken into account in the design of both the PLL and the power control loop. The PLL rms phase error is some 3° for a received optical power of 4 nW. The HgCdTe photodiode serving as phase detector and as demodulator, as well as the preamplifier are operated at 80 K by means of a Stirling cooler. Clear domination of shot noise is achieved for the data rate of 140 Mbit/s. A microcomputer performs dc-offset compensation and frequency acquisition. The system is part of a breadboard of an intersatellite data link.

Mechanically Cooled Receiver Front-End For High Data Rate CO 2 Laser Communication

A cooler/detector/preamplifier unit to be used as the front end in coherent optical receivers for X = 10pm was designed, fabricated, and tested. Both the HgCdTe-detector and the preamplifier were cooled to 80 K with the aid of a Stirling cooler. The three-stage high-impedance preamplifier realized employs field effect transistors and can hence be operated at low temperatures, too. Receiver tests were performed at a data rate of 140 Mbit/s using a quasi-homodyne receiver setup, with the bit-error-rate as the criterion of performance. Receiver operation very close to the quantum limit was demonstrated.