Limitations and noise in interferometric systems using frequency ramped single-mode diode lasers

Frequency ramped diode laser sensing and measurement systems suffer from a variety of limitations and noise sources. Nonlinearities in the frequency ramp produce unwanted sidebands in the frequency spectra of the system output and make accurate distance determination difficult in the frequency domain. Thermally induced drifts in the laser frequency prohibit long-term sensitive phase measurements even with a reference interferometer. It is shown that phase noise due to the fundamental linewidth of the diode laser and not bias current noise determines the noise floor of most FMCW systems in the regimes away from (1/
f
) noise. Time domain techniques suffer from low resolution because only a few data points can be taken during each frequency ramp and thus achieve poor averaging of the phase noise. The signal to noise ratio (SNR) of frequency ramped systems is shown to be lower (10-30 dB) than the theoretical prediction for an unmodulated heterodyne system, which was substantiated by showing that the minimum detectable phase is somewhat higher than that predicted by the idealized model.