Effects of Sinusoidal Phase Modulation on the Signal-to-Noise Ratio in a Digital Holography System

Digital holography (DH) has been demonstrated to be an effective wave-front sensor in low signal-to-noise ratio (SNR) environments due to the use of a strong reference. However, since DH relies on the interference of a signal with the mutually coherent reference, the coherence properties of the master-oscillator (MO) laser can degrade system SNR at long ranges. In this paper, a DH system in the off-axis image plane recording geometry was assembled and used to measure the effects of the MO coherence properties on the SNR. The coherence properties of the MO laser were degraded using sinusoidal phase modulation, which imparted maximum phase shifts of 0.38π, 0.55π, and 0.73π, at modulation frequencies of 20MHz to 100MHz. The measured coherence efficiency losses were closely predicted by the square of the fringe visibility, and deviated from the theoretical predictions by root-mean-squared errors of 0.0397, 0.0373, and 0.1007 for the three depths of modulation, respectively. The empirical data and models presented in this work may be used to assess efficiency losses in a DH system due to coherence effects.