Introduction

In the previous two chapters we have discussed the basic principle behind acoustooptic interaction. In this chapter we will discuss some important acoustooptic devices. We have seen in the previous two chapters that the intensity of the diffracted light depends on the acoustic power. Thus by changing the acoustic power one can correspondingly modulate the intensity of the diffracted light beam. This is the basic principle behind an acoustooptic modulator. We have also seen that the angle of diffraction depends on the acoustic frequency. Thus if we have an acoustic beam whose frequency is changed, the corresponding diffracted light beam will appear along different directions. This is the basic principle behind the acoustooptic deflector. If the acoustic transducer is given an input signal whose frequency increases with time, then the corresponding diffracted light beam will scan along different directions leading to an acoustooptic scanner. If on the other hand the acoustic transducer is fed simultaneously with a signal containing different frequencies then corresponding to each frequency, the diffracted light appears along different directions and this principle is used in the acoustooptic spectrum analyser. In this chapter we shall discuss the operation of a Raman–Nath modulator, a Bragg modulator, a deflector and a spectrum analyser.

Raman–Nath acoustooptic modulator

Fig. 19.1 shows an acoustooptic modulator based on Raman–Nath diffraction.

Introduction

In this chapter we shall discuss some specilc laser systems and their important operating characteristics. The systems that we shall consider are some of the more important lasers that are in widespread use today for different applications. The lasers considered are:

1. (a) solid state lasers: ruby, Nd:YAG, Nd: glass;

2. (b) gas lasers: He–Ne, argon ion and CO2;

3. (c) liquid lasers: dyes;

4. (d) excimer lasers;

5. (e) semiconductor lasers.

Ruby lasers

The irst laser to be operated successfully was the ruby laser which was fabricated by Maiman in 1960, Ruby, which is the lasing medium, consists of a matrix of aluminium oxide in which some of the aluminium, ions are replaced by chromium ions. It is the energy levels of the chromium ions which take part in the lasing action. Typical concentrations of chromium ions are ∼0,05% by weight. The energy level diagram of the chromium ion is shown in Fig. 10.1. As is evident from the figure this is a three level laser. The pumping of the chromium ions is performed with the help of flash lamp (e.g., a xenon or krypton flashlamp) and the chromium ions in the ground state absorb radiation around wavelengths of 5500 Å and 4000 Å and are excited to the levels marked E1 and E2. The chromium ions excited to these levels relax rapidly through a nonradiative transition (in a time ∼ 10-8–10-9s) to the level marked M which is the upper laser level.

Introduction

In Chapter 3 we studied light propagation through anisotropic media aod found that, in general, the state of polarization of the light beam may change as it propagates through the medium. In the present chapter we shall discuss light propagation through crystals in the presence of an externally applied electric field. This field can in general, alter the refractive indices of the crystal and thus could induce birefringence in otherwise isotropic crystals, or could alter the birefringence property of the crystal. This effect is known as the electrooptic effect. If the changes in the refractive indices are proportional to the applied electric field, such an effect is known as the Pockels effect or the linear electrooptic effect. If the changes in indices are proportional to the square of the applied electric field, the effect is referred to as the quadratic electrooptic effect or the Kerr effect.

In this chapter we shall study in detail the Pockels effect and obtain expressions for the phase shift suffered by a beam propagating through a crystal which is being acted upon by an external electric field. We will show that under certain geometrical configurations, the applied electric field acts differently on two linearly polarized light waves passing through the crystal and thus one can introduce an electric field dependent retardation between the two polarizations. We shall show in Sections 15.2 and 15.3 that such an effect can be used to control the amplitude of the light beam in accordance with the applied field.

Introduction

In the last chapter it has been shown how photographic processing may be used to form speckle pattern correlation fringes (Section 3.3). The resolution of the recording medium used for this technique need be only relatively low compared with that required for holography since it is only necessary that the speckle pattern be resolved, and not the very fine fringes formed by the interference of object and holographic reference beams. The minimum speckle size is typically in the range 5 to 100 μm (Section 3.1) so that a standard television camera may be used to record the pattern. Thus video processing may be used to generate correlation fringes equivalent to those obtained photographically. This method is known as Electronic Speckle Pattern Interferometry or ESPI and was first demonstrated by Butters and Leendertz (1). Similar work has since been described by Biedermann et al. (2) and Løkberg et al. (3,4). The major feature of ESPI is that it enables real-time correlation fringes to be displayed directly upon a television monitor without recourse to any form of photographic processing, plate relocation etc. This comparative ease of operation allows the technique or speckle pattern correlation interferometry to be extended to considerably more complex problems of shape measurement (Chapter 5) and deformation analysis (Chapter 7).

Intensity correlation in ESPI is observed by a process of video signal subtraction or addition. In the subtraction process, the television camera video signal corresponding to the interferometer image-plane speckle pattern of the undisplaced object is stored electronically.

In preparing this second edition the authors have taken the opportunity to modify Chapter 4 and Chapter 7. The purpose of this work has been to present the theory for the optimization of ESPI in a more general and exact form (Chapter 4) and to extend considerably the range of applications discussed (Chapter 7). The latter is representative of the wide range of problems to which holographic and speckle techniques are now applied. Chapter 7 also includes a brief review of techniques for automatic fringe interpretation.

Introduction: basic techniques and the general problem

It is the opinion of many scientists involved in the complex subject of surface deformation analysis that one of the most important techniques to emerge from the principle of holography (Section 1.7) is that of holographic interferometry. This enables the static and dynamic displacements of an optically rough surface to be measured interferometrically. First reports of the method appeared during the mid 1960s (for example, references 1–5) and were soon followed by numerous papers describing new general theories and applications (Section 2.9). One of the main reasons for such interest is that holographic interferometry clearly removes the most stringent limitation of classical interferometry (Section 1.5.5) i.e. that the object under investigation be optically smooth. Thus the advantages of interferometric measurement, for example, high sensitivity and non-contacting field view, can be extended to the investigation of numerous materials, components and systems previously outside the scope of optical study.

Let us first consider qualitatively how the holographic recording of a scattering surface can be used to detect the displacement of that surface. It has been shown in Section 1.7.3 that the developed hologram reconstructs a virtual image of the original object. If one views the precise superposition of the light from the reconstructed image and the real object through the hologram, then the interference of the two identical wavefronts results in a uniform field of view.

Introduction

Conventional shape measuring instruments use mechanical probes and give either point-by-point or line-scan information about shape (1). An optical method of measuring shape has the advantage of being non contacting and can also give a field view of the surface under investigation. Thus, there has been considerable effort directed towards the development of optical shape-measurement techniques.

Holographic methods of measuring surface shape are based on a two-wavelength technique first reported by Hildebrand and Haines (2). The two wavelengths can be produced by using two laser lines of different frequencies, or alternatively by altering the refractive index of the medium surrounding the object. The fringes represent the intersection of the object surface with a set of surfaces which in general are hyperboloids, but may be a set of equispaced planes in which case the fringes represent true depth contours. A new hologram must be made each time a new component is inspected.

ESPI can be used to compare the shape of test components with a master wavefront. The fringes obtained represent the difference in depth along the viewing direction between the master wavefront and the test component. The master wavefront may be produced by conventional optical components (i.e. flat, spherical or cylindrical) or may be generated holographically using a master component. The system enables components to be inspected in rapid succession.

When two beams of light which interfere to form a fringe pattern are projected onto the surface of an object, the form of fringes observed on the object surface depends on the shape of the surfaces (3).