Shalini Venkatesh

Phase noise considerations in coherent optical FMCW reflectometry

An analysis of coherent optical FMCW is presented. It shows the limitations imposed on measurements of discrete reflections and of Rayleigh backscattering due to phase noise originating in the finite coherence length of the optical source. The dependence of the range for accurate measurements on particular parameters of the optical system is discussed in the general case and for three specific laser sources. >

Incoherent frequency modulated cw optical reflectometry with centimeter resolution

An incoherent, frequency modulated continuous wave optical reflectomer is described which utilizes a broadband LiNbO(3) electrooptic modulator and slow photodiode for optical downconversion and detection, and a waveform recorder for digital spectrum analysis of the incoming signal. A two-point resolution of 6.8 cm in fiber is achieved. The dynamic range, defined as the location of the system noise floor relative to the maximum producible signal, is 50-dB optical.

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

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.