Introduction

The acoustooptic effect is the change in the refractive index of a medium caused by the mechanical strain produced by an acoustic wave. Since the strain varies periodically in the acoustic wave, the refractive index of the medium also varies periodically leading to a refractive index grating. When a light beam is incident on such a refractive index grating, diffraction takes place and this produces either multiple order diffraction or only single order diffraction. The former is referred to as Raman–Nath diffraction and is usually observed at low acoustic frequencies. The latter is analogous to Bragg diffraction of X-rays in crystals and is referred to here also as Bragg diffraction; this is usually observed at high acoustic frequencies.

The interaction between acoustic waves and light waves is used in a number of applications such as in acoustooptic modulators, deflectors, frequency shifters for heterodyning, spectrum analysers, Q-switching and mode locking in lasers. In this chapter we will discuss the basic principle of Raman–Nath diffraction and in the next chapter we will discuss Bragg diffraction.

Raman–Nath and Bragg regimes of diffraction

As discussed in the previous section when an acoustic wave propagates in a medium, the periodic strain associated with the acoustic wave generates a periodic refractive index variation in the medium. This periodic refractive index grating has the same period as the acoustic wave and is also propagating at the same velocity as the acoustic wave.

Ever since the invention of the laser in 1960, there has been a renaissance in the field of optics and the field of optical electronics encompassing generation, modulation, transmission etc. of optical radiation has gained tremendous importance. With optics and optical electronics now finding applications in almost all branches of science and engineering, study of these subjects is becoming extremely important. The present book intended for senior undergraduate and first year graduate students is an attempt at a coherent presentation of the basic physical principles involved in the understanding of some of the important optoelectronic effects and devices.

The book starts with the basic formulation of the study of propagation of electromagnetic waves, reflection and refraction and propagation through anisotropic media. This is followed by diffraction and its application in the study of spatial frequency filtering and holography. Basic physics behind laser operation is treated next with a brief discussion on different laser types. The next four chapters deal with the subject of optical waveguides including fibre and integrated optics which are already revolutionizing the field of information transmission. The next five chapters deal with three very important effects which are used in many opto-electronic devices namely the electrooptic, acoustooptic and nonlinear optical effects.

The various concepts in the book have been derived from first principles and hence it can also be used for self study. A large number of solved and unsolved problems have been scattered throughout the book.

Introduction

In Chapter 1 we discussed wave propagation in isotropic media in, which the velocity of propagation of an electromagnetic wave is independent of the direction of propagation. In this chapter we will discuss wave propagation in anisotropic media in which the velocity of propagation, in general, depends on the propagation direction and also on the state of polarization and one observes the phenomenon of double refraction. Anisotropic media form the basis of a large number of polarization devices such as quarter wave and half wave plates, the Soleil–Babinet compensator, the Wollaston prism, etc. Their study is also very important for understanding various light modulators based on the electrooptic effect (see Chapter 15).

In Sec, 3.2 we will discuss the phenomenon of double refraction and in Sec. 3.3 we will discuss some important polarization devices based on anisotropic media. Sees. 3,4–3.7 will discuss the electromagnetics of anisotropic media. In Sec. 3.8 we will introduce the index ellipsoid and show how from the index ellipsoid one can obtain the velocities of propagation and the polarizations of the two waves which can propagate along any given direction.

Double refraction

If we place a crystal of calcite or quartz on a point marked on a piece of paper, we will, in general, observe two images of the point. This phenomenon is referred to as double refraction or birefringence. This happens because when a ray enters the crystal it splits up, in general, into two rays which propagate along different directions and which are orthogonally polarized.

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.