1. Introduction

In order to determine amplitude and phase in a two-arm interferometer, a phase modulator is often required to clearly discriminate phase changes from amplitude changes. Such operation is straightforward when the phase modulation is a linear function of time. In this case, a standard Lock-in Amplifier (LIA) gives the required information with an optimal signal to noise ratio (SNR). Unfortunately, a number of phase modulators interesting in term

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© The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

distribution, and eproduction in any medium, provided the original work is properly cited.

of cost, achromaticity or integration offers non-linear responses, that are even sometimes coupled with unwanted amplitude modulation. A critical question that arises is "How can we extract phase and amplitude information in an optimal way when non-linear phase modulation is used?"

To solve this issue while keeping the benefits of high SNR, approaches have been proposed based on multiple lock-in detection at selected signal harmonics. These approaches were mainly employed in the case where the phase modulation is a sine function [1–6]. Such phase modulation is, for example, achieved using piezo-actuator, fiber stretchers, and other phase modulators where a sine excitation typically offers the best response. The multiple lock-in approach works fine but it is less direct and does not necessarily provide an optimal SNR or a straightforward implementation. Especially, if an amplitude modulation is present at the same frequency as that of the phase modulation. Alternately, the Generalized Lock-in Amplifier (G-LIA) technique was recently introduced [7] to solve this issue with a procedure similar to a single LIA operation. In this chapter, we first detail the principle of this method when operated in the simplest case where no amplitude modulation is present. Application is provided notably in the context of digital holography. Then we consider the case where there is an additional amplitude modulation in the signal field. The first case which is discussed is related to unbalanced interferometry where the phase modulation is achieved via a power modulation of the laser source. Finally, we also discuss the case of phasesensitive near-field imaging.
