**5. References**


**6** 

*Russia* 

**Multi-Channel Adaptive Interferometers** 

Roman Romashko and Yuri Kulchin

*Institute of Automation and Control Processes FEB RAS* 

**Based on Dynamic Hologram Multiplexing** 

Optical interferometry was always considered as one of the most flexible and sensitive techniques for measuring mechanical vibrations (Hariharan, 1990; Osterberg, 1932). The number of applications of this method tremendously increased after the discovery of lasers (in 1960) and the development of low-loss optical fibres (in 1966). A laser as a source of coherent radiation assures high sensitivity of an interferometer while an optical fibre guarantees compactness, light weight, immunity to electromagnetic influence, and capability of the measuring system to operate in hazardous conditions (high temperature, radiation, etc.). As known, the phase of a light field cannot be directly detected. It is an interferometer, which provides the detection of the phase difference by combining the signal wave (which bears information about the measurand) with the coherent reference wave and measuring changes of the resultant light flux by a photodetector. The sensitivity of an interferometer to phase difference is limited by shot noise of the photoelectrons and can be extremely high: the theoretical minimum detectable displacement (corresponding to the phase difference) is 1.110-16 m·Hz-1/2 for 10 mW of detected laser power at the wavelength of 500 nm (Forward, 1978; Wagner & Spicer, 1987). However, two main problems should be overcome to achieve such high sensitivity in non-laboratory conditions. The first is the necessity of precise adjustment of the object and the reference wavefronts interfering at the photodetector. This requirement makes difficult, in particular, the use of multimode optical fibres in interferometric sensors. The second is the need to keep constant the average phase shift between the interfering wavefronts. This is more fundamental than the first one because the induced phase shift (which is assumed to be proportional to influence of the measurand on the optical paths) is nonlinearly transferred into the light-flux change at the

A simple and elegant solution of both these problems was achieved when a conventional beam-splitter, which serves for combining the reference and the object waves, was replaced by a dynamic hologram continuously recorded in a photorefractive crystal (PRC). Since the configuration in this case involves only two waves that interfere inside the crystal, it was called two-wave mixing, TWM (Huignard & Marrakchi, 1981). Considering that the photorefractive dynamic hologram adapts not only the signal wavefront to the reference one but it is also self-adapted to slow temporal variations of the phase difference, this type of optical system is referred to as adaptive interferometry (Stepanov, 1991). The idea of using

**1. Introduction** 

detector via the familiar cosine interference function.

