Quantum-limited performance is achievable.

A method of coherent detection of highrate pulse-position modulation (PPM) on a received laser beam has been conceived as a means of reducing the deleterious effects of noise and atmospheric turbulence in free space optical communication using focal plane detector array technologies. In comparison with a receiver based on direct detection of the intensity modulation of a PPM signal, a receiver based on the present method of coherent detection performs well at much higher background levels.

Figure 1. A Coherent Optical Receiver as it is set up for experiments at NASA’s Jet Propulsion Laboratory.

Figure 2. Results of the Study show (a) convergence of the LMS algorithm in the presence of simulated atmospheric turbulence (combined output with a step-size of µ=22), and (b) phases of the combining weights for µ=22.

As a result of the inherent squaring operation of each photodetector, the output current includes an IF component that contains the signal modulation. The amplitude of the IF component is proportional to the product of the local-oscillator signal amplitude and the PPM signal amplitude. Hence, by using a sufficiently strong local oscillator signal, one can make the PM modulated IF signal strong enough to over-come thermal noise in the receiver circuits: this is what makes it possible to achieve near quantum limited detection in the presence of strong background.

Following quantum limited coherent detection, the outputs of the individual photodetectors are automatically aligned in phase by use of one or more adaptive array compensation algorithms [e.g., the least mean square (LMS) algorithm]. Then the outputs are combined and the resulting signal is processed to extract the high rate information, as though the PPM signal were received by a single photodetector.

In a continuing series of experiments to test this method (see Fig. 1), the local oscillator has a wavelength of 1,064 nm, and another laser is used as a signal transmitter at a slightly different wavelength to establish an IF of about 6 MHz. There are 16 photodetectors in a 4×4 focal-plane array; the detector outputs are digitized at a sampling rate of 25 MHz, and the signals in digital form are combined by use of the LMS algorithm. Convergence of the adaptive combining algorithm in the presence of simulated atmospheric turbulence for optical PPM signals has already been demonstrated in the laboratory; the combined output is shown in Fig. 2(a), and Fig. 2(b) shows the behavior of the phase of the combining weights as a function of time (or samples). We observe that the phase of the weights has a sawtooth shape due to the continuously changing phase in the down-converted output, which is not exactly at zero frequency.

Detailed performance analysis of this coherent free space optical communication system in the presence of simulated atmospheric turbulence is currently under way.

This work was done by Victor Vilnrotter and Michela Muñoz Fernández of Caltech for NASA’s Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Electronics/Computers category. NPO-40974

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