Polarization recording

With normal holographic techniques, the amplitude and phase of the object wavefront are recorded accurately, but information on its state of polarization is lost. The polarization of the reconstructed wave is determined by the polarization of the light used to illuminate the hologram.

Orthogonally polarized reference beams

Two basic methods for recording the state of polarization of the object wave have been described. The first method was proposed by Lohmann [1965a] and subsequently demonstrated by Bryngdahl [1967]. The experimental arrangement for this method, which is shown in fig. 11.1, uses two orthogonally polarized reference waves which interfere with the corresponding polarized component of the light from the object, so that two holograms are recorded on the same plate. After processing, when the plate is illuminated once again with the same reference beams, it yields two superimposed images that reproduce the polarization of the object wave. Care must be taken to adjust the angles of incidence of the beams so that the cross-talk images formed by diffraction of each of the reference beams at the hologram formed with the orthogonally polarized component do not overlap the desired image.

Coded reference beams

The other method, which is due to Kurtz [1969], uses a single reference beam in which, as shown in fig. 11.2, an opal glass diffuser is inserted. The light transmitted by this diffuser is depolarized, and the complex amplitudes of the two orthogonally polarized components of the reference beam at any point exhibit little or no correlation.

Introduction

The use of electromagnetic wave theory and geometrical optics to describe propagation of light is satisfactory when there is no exchange of energy between radiation and matter. All observable characteristics – direction of propagation, polarization, diffraction, interference, the energy of radiation, and so forth – are accurately described by one of these so-called classical theories. However, with the exception of absorption by lossy media (eqn. 4.36) or the effects of dispersion (eqn. 4.22), classical theories fail to give reasons for many phenomena that have practical consequences in modern technology and that involve interaction between radiation and matter. Even the partially successful descriptions of dispersion or loss cannot explain the existence of the multitude of spectrally distinct absorption lines; cannot predict the wavelengths for absorption or the wavelengths for anomalous dispersion; cannot account for all the absorbed energy; and, most importantly, cannot predict or describe the effect of optical gain, which is closely related to absorption. Thus, the principles of operation of many important devices of modern optics such as lasers, photodiodes, electronic cameras, and television screens cannot be explained by classical theories. Even the emission by the sun or an incandescent lamp are beyond the scope of these theories. It is evident therefore that a new, modern theory is necessary – one that is either more complete than, or simply complements, the classical theories.

Introduction

In the introduction we saw that explaining the concept of light may require more than just one theory. However, before we begin our journey through the disciplines of optics, we must identify the physical parameters needed to quantify the phenomena that are associated with light. But even before that, we should recognize that the phenomenon we call light is only a part of the broader phenomenon of radiation. If we consider radiation to be

then light may be defined (American National Standard 1986) as

(This definition is sometimes extended to include ultraviolet and infrared radiation.) Note that, by the present definition, radiation that is spectrally detectable to the eye will be called light even if it is too faint to be seen. Although these definitions include terms (such as electromagnetic waves and spectra) that are yet to be explained, it is evident that light represents a subcategory of radiation. Thus, once the physical quantities that specify radiation are defined they may also be used for the quantification of light. Furthermore, the measurement of radiation – or radiometry – does not depend on how we define what is detectable by the eye. Therefore, radiometry is more objective than photometry, which is the process of measuring light.

An obvious application of holography is in displays. With the availability of suitable photographic emulsions coated on glass plates in sizes up to 1.5 m × 1.0 m, as well as film in rolls up to 1 m wide, several striking displays have been made using holograms. Some of the problems involved and techniques used to solve them have been described by Fournier, Tribillon and Vienot [1977] as well as by Bjelkhagen [1977a]. Even larger displays are possible with projection techniques using a reflecting lenticular screen (see, for example, Okoshi [1977]).

However, conventional holograms have several drawbacks [Benton, 1975], such as the limited angle over which the image can be viewed, low image luminance, and the need to use a monochromatic source to illuminate the hologram, as well as the necessity to illuminate the subject with laser light when recording the hologram. This chapter discusses some of the techniques developed for holographic displays that have overcome these limitations (see also Benton [1980]).

360° holograms

Holograms that give a 360° view of the object can be recorded using either four or more plates [Jeong, Rudolf & Luckett, 1966; Chau, 1970] or a cylinder of film to surround the object [Hioki & Suzuki, 1965; Jeong, 1967; Stirn, 1975; Upatnieks & Embach, 1980].

A very simple optical system for this purpose [Jeong, 1967] is shown in fig. 8.1. In this arrangement, the object is placed at the centre of a glass cylinder which has a strip of holographic film taped to its inner surface with the emulsion side facing inwards.

Introduction

Spectroscopic techniques are an excellent tool for thermodynamic measurements. Typically they do not require a direct contact with the tested medium and are therefore considered nonintrusive. This is a significant advantage over measurements that require physical probes, particularly when the environment at the point of measurement is hostile owing to such extreme conditions as high temperature, aggressive chemical reaction, or fast flows. In addition, many spectroscopic techniques can be adapted to image the two- or three-dimensional distribution of select properties. With the advent of electronic cameras, images can be digitized and stored almost instantaneously by computers. Exposure times of as short as 5 ns are possible with the use of CCD (charge-couple device) intensified cameras. Recording rates of up to 12 kHz can be achieved with image converter cameras, and storage of hundreds of images is possible on most desktop computers. These device characteristics enable such fast phenomena as explosions or detonation waves to be recorded electronically; depending on the selected spectroscopic technique, images of the distribution of thermodynamic parameters of such events can be obtained.

Spectroscopic techniques can be classified in several ways. A simplistic but useful classification includes only two groups: passive techniques, which depend on spontaneous emission by the tested object; and active techniques, which require an excitation of the tested object to induce detectable emission.

The ideal recording material for holography should have a spectral sensitivity well matched to available laser wavelengths, a linear transfer characteristic, high resolution, and low noise. In addition, it should either be indefinitely recyclable or relatively inexpensive.

While several materials have been studied [Smith, 1977; Hariharan, 1980b], none has been found so far that meets all these requirements. However, a few have significant advantages for particular applications. This chapter reviews the properties of some of these materials in the light of the general considerations discussed in Chapter 6 (see Table 7.1 for a summary of their principal characteristics).

Silver halide photographic emulsions

Silver halide photographic emulsions are widely used for holography because of their high sensitivity and their commercial availability. In addition, they can be dye sensitized so that their spectral sensitivity matches the most commonly used laser wavelengths.

An apparent drawback of photographic materials is that they need wet processing and drying; however, development is actually a process, with a gain of the order of 106, which amplifies the latent image formed during the exposure to yield high sensitivity as well as a stable hologram. Another advantage of the formation of a latent image is that the optical properties of the recording medium do not change during the exposure, unlike materials in which the image is formed in real time.

Over the last two decades, such new technologies as laser-aided material processing, optical communication, holography, and optical measurement techniques have advanced from laboratory curiosities to commercially available engineering tools. Faced with these developments, engineers must now apply in their practice many concepts of physics that in the past were considered outside the boundaries of classical engineering. Advanced levels of electromagnetic theory and of quantum mechanics are now required to answer fundamental questions about interference, diffraction, or polarization of light and their applications. For example: How does radiation interact with gases, liquids, and solids? How does one obtain optical gain? When should a laser be used, and when would an ordinary light source suffice? How can a laser beam be produced and focused? Answers to these questions are essential for selecting an optical technique for measuring the properties of gas, liquid, or solid phases, or when designing a laser system for material processing, surgery, communication, or entertainment. Although most engineering students attend two or three undergraduate courses in physics, they seldom acquire the proficiency required to fully understand the intricacies of modern optics or laser applications. Similarly, midlevel engineers who obtained their formal education before many new techniques were developed may find an increasing gap between the knowledge acquired in undergraduate physics classes and the requirements of professional practice.

Holographic diffraction gratings

Diffraction gratings formed by recording an interference pattern in a suitable light-sensitive medium (commonly called holographic diffraction gratings) have replaced conventional ruled gratings for many applications. While Burch and Palmer [1961] first showed that transmission gratings could be made by photographing interference fringes using silver halide emulsions, it was the use of photoresist layers coated on optically worked blanks which finally led to the production of spectrographic gratings of high quality [Rudolph & Schmahl, 1967; Labeyrie & Flamand, 1969]. After processing, the photoresist layer yields a relief image (see section 7.3) which can be coated with an evaporated metal layer and used as a reflection grating.

Holographic gratings have several advantages over ruled gratings. Besides being cheaper and simpler to produce, they are free from periodic and random errors and exhibit much less scattered light. In addition, it is possible to produce much larger gratings of finer pitch, as well as gratings on substrates of varying shapes, and gratings with curved grooves and varying pitch. This makes it possible to produce gratings with unique focusing properties and opens up the possibility of new designs of spectrometers [Namioka, Seya & Noda, 1976].

Against this, their main disadvantage is that the groove profile cannot be controlled as easily as in ruled gratings. While even a sinusoidal profile can give high diffraction efficiencies for small grating spacings (≈ λ) [Loewen, Maystre, McPhedran & Wilson, 1975], it is usually necessary to produce a triangular groove profile for maximum diffraction efficiency.

Photorefractive crystals

As described in section 7.7, photorefractive crystals can be used as recording materials for holography. In such crystals, exposure to light frees trapped electrons and induces a redistribution of charge within the crystal. The spatially varying electric field produced by this charge pattern modulates the refractive index of the crystal through the electro-optic effect, resulting in the formation of a volume phase hologram. If the incident light distribution changes, a new charge pattern and a new hologram are formed in a characteristic time τH. The dynamic and adaptive properties of holograms recorded in photorefractive crystals have opened up new and interesting possibilities in holographic interferometry.

Time-average interferometry of vibrating surfaces

Photorefractive crystals of the sillenite family (BGO, BSO, and BTO) have been used most commonly for holographic interferometry because of their relatively high sensitivity to light. One of the earliest applications of such photorefractive crystals to holographic interferometry made use of the ability of a BSO crystal to generate a phase-conjugate image in real time [Huignard, Herriau & Valentin, 1977]. A typical optical system used to study the vibration modes of a loudspeaker is shown in fig. 17.1.

In this case, time-average fringes are formed because the crystal integrates over the characteristic time τH required to build up the hologram, which is usually much longer than the period of the vibration. With such an arrangement, it is possible to observe the changes in real time, as the excitation frequency is changed.

Introduction

Optical frequencies are so high that a small relative bandwidth is a very large bandwidth in absolute terms. This fact is behind the success of optical short pulse generation. The first successful generation of short optical pulses via modelocking started in the 60s with Nd:glass. Figure 1.1 gives the history of the advances in short pulse generation. Steady progress in generating shorter and shorter pulses was achieved with dye lasers, principally because these were CW lasers that had much more predictable modelocking behavior than Q-switched lasers. After subpicosecond pulse generation was achieved with dye lasers, it was soon realized that the short pulses produced by the lasers were much shorter than the relaxation times of the dyes used for the gain and absorption media. It was recognized that the pulses were generated through combined action of the saturable absorber dye that, by saturating, opened the ‘shutter’ for the transmission of the pulse, and the gain, by saturating, closed the ‘shutter.’ This was an important discovery since there exist very few saturable absorbers with subpicosecond relaxation times appropriate for intracavity operation. The shortest pulses achieved were 27 fs in duration, and after spectral spreading in a fiber and subsequent compression 6 fs was achieved, still the record today.

In 1978 the first modelocking of semiconductor lasers was achieved. They are of prime importance in communications applications and are limited to about 100 fs pulse duration, a pulse width adequate for communications for many years to come.