**8. References**


The central wavelength of the filter based on a photorefractive grating can be additionally tuned electrically by application of external electric field to the photorefractive crystal. Figure 27 demonstrates such electrical tuning of the filter. This is an example of fast tuning via the electro-optically induced variations in the average refractive index. One can see that the application of voltages ranging from −614 to +655 V cm−1 provides tuning in the 0.55 nm range. The speed of tuning reached 2.2 nm *μ*s−1 and was limited only by the available

In this Chapter we considered different approaches to development of multi-channel adaptive measurement systems based on multiplexing of dynamic holograms in a photorefractive crystal. We show that such systems can provide (i) high sensitivity to detection of ultra-small physical quantities (close to the classical homodyne detection limit), (ii) cross-talk free performance, and (iii) adaptive properties which cancel uncontrollable environmental influence on the system. For development of such systems different approaches to dynamic holograms multiplexing, including spatial, angular, spectral and their combinations, can be effectively used. Moreover, dynamic gratings recorded in a photorefractive crystal can also be used for development of such elements of multi-channel systems as spectral tunable filters which can provide effective demultiplexing of signals

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**7** 

Kaige Wang

*China* 

**Incoherent Holographic Interferometry** 

The original meaning of coherence was attributed to the ability of light to exhibit interference phenomena. There are two types of coherence: temporal coherence and spatial coherence. Temporal coherence measures the ability of two relatively delayed beams to form interference fringe. Interference in a Michelson interferometer refers to temporal coherence. However, spatial coherence reflects the ability of a beam to interfere with a spatially shifted (but not delayed) version across the beam. Young's double-slit experiment

Since the first experimental realization of Young's double-slit interference, it has been known that the observation of interference-diffraction pattern of an object requires a spatially coherent source. The waves emitted from positions outside the coherent area possess independent irregular phases which may degrade the interference pattern. In the early days when coherent sources were unavailable, interference experiments were carried out with an extended thermal source restricted by a pinhole aperture, which can improve the spatial coherence. Holography is one of the most important applications of spatial interference. In the first holography experiment, Dennis Gabor stated that (Gabor, 1972) ''The best compromise between coherence and intensity was offered by the high-pressure mercury lamp, which had a coherence length of only 0.1 millimeter, ... But in order to achieve spatial coherence, we had to illuminate, with one mercury line, a pinhole 3 microns in diameter. '' The pinhole eventually reduced the power of the source and thus impeded the potential application of optical interferometric techniques such as holography. This barrier was overcome with the invention of the laser, whose intense and coherent beam was

Coherent sources are obtainable within a certain range of optical frequencies. However, various holographic techniques have been developed using incoherent sources such as Xrays, electrons, and radiation. To improve the coherence one has to pay the cost of decreasing the intensity of the source, as already pointed out in relation to Gabor's experiment. A challenging question would be whether coherence is absolutely necessary in holography, or can we bypass the coherence requirement? As a matter of fact, in the early days of holography, a technique using incoherent illumination was first proposed by Mertz and Young (Mertz & Young, 1963), and then extended by Lohmann (Lohmann, 1965), Stroke and Restrick (Stroke & Restrick, 1965), and Cochran (Cochran, 1966). The strategy is based on the fact that each point of a spatially incoherent object produces, through interference of

**1. Introduction** 

is an example which concerns spatial coherence.

ideal for performing interference.

*Department of Physics, Applied Optics Beijing Area Major* 

 *Laboratory, Beijing Normal University, Beijing* 

