Effect of an increasing number of input signals on the dynamic range of a coherent optical spectrum analyzer with a liquid-crystal spatial modulator at the input

Subject of study . The dynamic range of a signal-processing coherent optical spectrum analyzer, employing a liquid-crystal spatial light modulator at the input, is investigated. Aim of study . The aim of the study is to theoretically assess the level of false signals at the output of a spectrum analyzer with an increasing number of input harmonics. In addition, the possibility of reducing the level of these false signals using automatic gain control is examined. Furthermore, the calculated ratios are experimentally verified, and the dynamic range of the spectrum analyzer with a liquid-crystal spatial light modulator at the input is evaluated. Method . To theoretically analyze the level of false signals in the output of the optical spectrum analyzer, the nonlinear transmission characteristic of the liquid-crystal input device is represented as a power-series expansion in the vicinity of the operating point, expressed in terms of the amplitude of the input optical control signal. The influence of the second-, third-, and higher-order nonlinear expansion terms on the dynamic range of the spectrum analyzer is evaluated for different numbers of input signals, both with and without an automatic gain-control system. Experimental investigations are performed using an operating prototype of the spectrum analyzer. Main results . When the number of input harmonics exceeds three—the number typically used in the analysis of devices with nonlinear characteristics—the dynamic range may decrease due to the increased level of spurious signals. To address this issue, the implementation of an automatic gain-control system that limits the maximum level of these signals is proposed. For a small number of input signals, using automatic gain control is impractical, and the dynamic-range values for one, two, and three signals are related by simple ratios. Practical significance . The proposed approach and the corresponding results can be used to analyze the dynamic ranges of spectrum analyzers, other types of optical signal-processing devices, and holographic devices employing liquid-crystal spatial light modulators.