Coherent FMCW Reflectometry Using a Piezoelectrically Tuned Nd:YAG Ring Laser

Abstract

Coherent optical implementations of the FMCW technique [1] depend on the interferometric mixing of an optical signal probing the network under test with a reference signal obtained from the same source, whose carrier frequency is linearly, and phase-continuously, chirped [2]. Limitations on measurement range may be reduced by the choice of a laser source of long coherence length, as demonstrated in Ref. 3, but the temperature tuning employed in that particular case is relatively slow and inconvenient. A more serious problem is spatial resolution degradation due to residual nonlinearities in the frequency chirp, but this may be significantly reduced by incorporating a reference interferometer in the measurement system to track the nonlinearity and compensate for it [4]. This paper describes a coherent optical FMCW system that uses piezoelectric rather than thermal tuning of a source of long coherence length, in a fiberoptic network which includes a reference interferometer, triggering data capture by a system with a Fast Fourier Transform capability.