Impacts of Laboratory Vibrations and Laser Flicker Noise on Digital Holography

Abstract

In this paper, we experimentally demonstrate the impacts of laboratory vibrations and laser flicker noise on digital holography (DH). Specifically, we measure both the vibration efficiency and the coherence efficiency of our DH system at various focal-plane array integration times and path-length differences between the signal and reference. These efficiencies, in practice, contribute to the overall mixing efficiency, which is a measure for how well the detected signal and reference interfere. The results show that when the integration time is ≤1 ms, the laboratory vibrations are negligible with a vibration efficiency of 100%; however, when the integration time equals 100 ms, the laboratory vibrations lead to a 94% vibration efficiency. In addition, the results show that the effective coherence length of the master-oscillator (MO) laser increases by 280% when the integration time decreases from 100 ms to 100 µs. To account for this outcome, we present a model of the coherence efficiency based on the frequency noise of the MO laser. The model fit to the DH data then shows that the frequency of the MO laser is flicker-noise dominated. As a result, decreasing the integration time improves the overall mixing efficiency because of highpass filtering in both the vibration efficiency and the coherence efficiency. Based on previous published efforts, these results have direct ties to the achievable signal-to-noise ratio of a DH system.

Spectral Broadening Effects on Pulsed-Source Digital Holography

“…Recording phasemodulated pulsed spectra is non-trivial and was not possible with the available equipment. Also, from previous experiments [14], [15], it was known that the fiber-amplifier stages used in this experiment significantly changed the spectral line shape from that of the MO laser. In practice, these stages introduced at least one additional longitudinal mode.…”

“…The coherence efficiency is a function of the effective complex degree of coherence,  eff , via the following relationship [14], [15]:…”

“…reference are derived from the same MO laser. Follow-on work then used the coherence efficiency to show that system performance degrades rapidly when the path-length difference between the scattered signal and strong reference exceeds the coherence length of the MO laser [14], [15]. Practically speaking, this outcome means that a CW configuration should not be used in such scenarios.…”

Digital-holography efficiency measurements using a heterodyne-pulsed configuration

“…Nevertheless, as the path-length differences between the signal and reference exceed the coherence length of the CW laser source, system performance degrades rapidly. 14,15 This outcome says that the effective ranges of DH systems in a CW configuration is limited by longitudinal coherence.…”

“…This understanding requires the quantification of individual sources of SNR loss, otherwise known as efficiencies. The efficiencies of DH systems using continuous-wave (CW) laser sources have been extensively studied, 13 – 17 and the success of these systems is largely due to the long coherence lengths of modern-day CW laser sources. Nevertheless, as the path-length differences between the signal and reference exceed the coherence length of the CW laser source, system performance degrades rapidly 14 , 15 .…”