Simultaneous frequency and spatial tracking using a Fabry-Perot interferometer for free-space laser communications

Jackie S. C. Fung

doi:10.1117/12.184664

16 August 1994 Simultaneous frequency and spatial tracking using a Fabry-Perot interferometer for free-space laser communications

Jackie S. C. Fung

Author Affiliations +

Proceedings Volume 2123, Free-Space Laser Communication Technologies VI; (1994) https://doi.org/10.1117/12.184664
Event: OE/LASE '94, 1994, Los Angeles, CA, United States

Abstract

Before a communication link can be established between free-space coherent optical communication terminals, the frequency of the optical signal and spatial direction of the transmitter or receiver must be acquired. During the communication period, the operations of frequency tracking and spatial tracking must be maintained. The frequency of the optical signal will deviate from its nominal value due to Doppler shift and laser frequency instability. The frequency tracking system must be able to track out the frequency variation to maintain the detected signal within the passband of the communication receiver. A coherent detection spatial tracking system based on a Fabry-Perot Interferometer (FPI) had been designed. Electro-optic material was used for the cavity of the FPI. In this paper, a scheme is presented to use the same electro-optic FPI (EO-FPI) to track the frequency of the optical signal. The method is to apply a small sinusoidal modulation voltage to the electro-optical cavity of the FPI. In doing so, the coherently detected signal has the amplitude and phase modulated at the same frequency. The amplitude of the coherently detected signal is used to generate the frequency error signal to minimize the frequency difference between the signal and local oscillator (LO) optical field. While the phase of the coherently detected signal is low- passed filtered to generate the angular error signal. The angular error signal is proportional to the angular difference between the signal and LO beams. The length of the cavity and the angular bias of the EO-FPI is chosen to optimize the frequency and spatial tracking performance of the system.

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Jackie S. C. Fung "Simultaneous frequency and spatial tracking using a Fabry-Perot interferometer for free-space laser communications", Proc. SPIE 2123, Free-Space Laser Communication Technologies VI, (16 August 1994); https://doi.org/10.1117/12.184664

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KEYWORDS

Signal detection

Receivers

Electro optics

Free space optical communications

Free space optics

Modulation

Optical tracking

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