SO FAR WE HAVE LEARNT,

Temporal coherence or coherence time of the source (c), which can be converted to coherence length via relation

The linewidth (Δv) and monochromaticity (spectral purity) (Δv/v)

The spatial coherence or lateral coherence width

wl

1

C

v

CL c

MICHELSON STELLAR INTERFEROMETER

Using the concept of spatial coherence, Michelson developed an ingenious method for determining the angular diameter of stars. The method is based on the result that for a distant circular source, the interference fringes will disappear if the distance between pinholes S1 and S2 (see Fig. 17.8) is given by

where θ is the angle subtended by the circular source as shown in Fig. 17.8.

For a star whose angular diameter is 10–7 rad, the distance ‘d’ for which the fringes will disappear is

where we have assumed λ 5000 Å. Obviously, for such a large value of d, the fringe width will become extremely small. Further, one has to use a big lens, which not only is difficult to make, but only a small portion of which will be used.

To overcome this difficulty, Michelson used two movable mirrors M1 and M2 as shown in Fig. 17.9, and thus he effectively got a large value of d. The apparatus is known as Michelson’s stellar interferometer. In a typical experiment the first disappearance occurred when the distance M1M2 was about 24 ft, which gave

for the angular diameter of the star. This star is known as Arctures. From the known distance of the star, one can estimate that the diameter of the star is about 27 times that of the Sun.

Note that a laser beam is spatially coherent across the entire beam. Thus, if a laser beam is allowed to fall directly on a double-slit arrangement (see Fig. 17.10), then as long as the beam falls on both the slits, a clear interference pattern is observed on the screen. This shows that the laser beam is spatially coherent across the entire wave front.

Figure 17.11 shows the interference pattern obtained by Nelson and Collins by placing a pair of slits of width 7.5 mm separated by a distance 54.1 mm on the end of the ruby rod in a ruby laser. The interference pattern agrees with the theoretical calculation to within 20%. To show that the spatial coherence is indeed due to laser action, they showed that below threshold (of the laser) no regular interference pattern was observed; only a uniform darkening of the photographic plate was obtained.

Since some of the energy is coupled back to the system, it is said to act as an oscillator. Indeed, in the early stages of the development of the laser, there was a move to change its name to LOSER which is an acronym for light oscillation by stimulated emission of radiation. Since it would have been difficult to obtain a research grant for LOSERs, it was decided to retain the name LASER.

LASER is an acronym for light amplification by stimulated emission of radiation.

Important Milestones

1917 The theory of stimulated emission was put forward by Albert Einstein.

1954 The phenomenon of stimulated emission was first used by Charles Townes in 1954 in the construction of a microwave amplifier device called the maser, which is an acronym for microwave amplification by stimulated emission of radiation. At about the same time, a similar device was also proposed by Prochorov and Basov in the U.S.S.R.

1958 The maser principle was later extended to the optical frequencies by Schawlow and Townes in 1958, which led to the realization of the device now known as the laser. Townes, Basov, and Prochorov were awarded the 1964 Nobel Prize in Physics for their “fundamental work in the field of Quantum Electronics, which has led to the construction of oscillators and amplifiers based on the laser-maser principle.”

1959 In a conference paper, Gordon Gould introduced the term LASER as an acronym for Light Amplification by Stimulated Emission of Radiation.

1960 The first successful operation of a laser device (λ 0.6943 mm) was demonstrated by Theodore Maiman in 1960 using a ruby crystal.

1961 Within a few months of the operation of the ruby laser, Ali Javan and his associates constructed the first gas laser, namely, the helium-neon laser

1961 The first fiber laser (barium crown glass doped with Nd3+ ions) was fabricated by Elias Snitzer.

1962 Semiconductor lasers (which are now extensively used in fiber-optic communication systems) were discovered by four independent groups.

1963 C. K. N. Patel discovered the CO2 laser (λ 10.6 mm).

1964 W. Bridges discovered the Ar-ion laser (λ 0.515 mm), and J. E. Geusic and coworkers discovered the Nd:YAG laser (λ 0.515 mm).

Since then, laser action has been obtained in a large variety of materials including liquids, ionized gases, dyes, and semiconductors.

The light emitted from a laser often possesses some very special characteristics. Some of these are

1. High power : Continuous wave lasers having power levels of ~ 105 W and pulsed lasers having a total energy of ~ 50,000 J can have applications in welding, cutting, laser fusion, etc.

2. Tight focusing : Because of highly directional properties of the laser beams, they can be focused to areas of approximately few micrometers squared—this leads to very high intensities and therefore leads to applications in surgery, material processing, compact discs, etc. Laser pulses having very small cross-sectional area (andhigh energy) can be guided through special fibers leading to very interesting nonlinear effects.

Levelling of ceramic tiles floor with a laser device

Laser sight used by the Defense Forces

3. Directionality : The divergence of the laser beam is usually limited by diffraction, and the actual divergence can be less than 10–5 rad; this leads to the application of the laser in surveying, remote sensing, etc.

Because of such unique properties of the laser beam, it finds important applications in many diverse areas, and indeed one can say that after the discovery of the laser, optics has become an extremely important field of study. For example, a 2 mW diffraction-limited laser beam incident on the eye can produce an intensity of about 106 W-m–2 at the retina—this would certainly damage the retina. Thus, whereas it is quite safe to look at a 500 W bulb, it is

The basic principle involved in the lasing action is the phenomenon of stimulated emission, which was predicted by Einstein in 1917.

4. Spectral purity : Laser beams can have an extremely small spectral width Δλ 10–6 Å. Because of high spectral purity, lasers find applications in holography, optical communications, spectroscopy, etc.

very dangerous to look directly into a 5 mW laser beam. Indeed, because a laser beam can be focused to very narrow areas, it has found applications in areas such as eye surgery and laser cutting.