Interaction with External Fields
193
It is the properties of narrow frequency range, coherence, directionality
and high intensity which make the laser beam extremely useful. Some of the
uses and applications of lasers are discussed below.
1. Tunable dye lasers allow the excitation and analysis of atomic and
molecular energy levels with a high accuracy. Indeed, they have
revolutionized the field of optical spectroscopy. In particular, Lamb shift
has been observed optically, two-photon transitions have been observed
in atomic and molecular systems, Rydberg stateshave been analysed,
and significant tests of unified theories of electromagnetic and weak
interactions have been made.
2. The coherence and the high intensity of laser beams enable us to measure
small changes in the frequencies of radiation resulting from Raman
scattering by measuring beats produced by the interference between the
scattered and the original beams.
3. The narrow frequency width and coherence of lasers makes them very
useful in precision measurements. Interferometers with laser beams allow
measurement of distances to a very high accuracy, as also surface
variations, refractive index, etc. For measuring the velocity of fluids, a
laser beam is scattered by the fluid. This Doppler-shifted beam then
interferes with the original beam producing beats. The beat frequency
enables us to measure the velocity of the moving medium.
4. The well-defined directionality of a laser beam makes it valuable in
communications, surveying and tracking systems.
5. Some lasers are capable of producing narrow beams of extremely high
intensity. Such beams find use in precision cutting and boring, soldering
and welding. They are used in tumor destruction and in eye surgery for
‘welding’ detached retina. There is also the possibility that they can induce
controlled thermonuclear fusion.
6. Lasers have important applications in nonlinear optics and in holography.
These are discussed in some detail.
Nonlinear Optics
For ordinary light sources, the electric field is so small that the induced
polarization P is approximately proportional to the electric field E, and the
various properties of the medium such as polarizability a, refractive index, etc.
are independent of the field intensity (here, for simplicity the vector nature of
P and E is neglected). Thus, we have what is known as linear optics for which
the superposition principle holds i.e., P 1 = α E 1 , P 2 = α E 2 implies P 1 + P 2 = α
(E 1 + E 2 ). However, with laser fields of high intensity, there is no longer a linear
relation between P and E, and the description is in terms of nonlinear optics.
193
It is the properties of narrow frequency range, coherence, directionality
and high intensity which make the laser beam extremely useful. Some of the
uses and applications of lasers are discussed below.
1. Tunable dye lasers allow the excitation and analysis of atomic and
molecular energy levels with a high accuracy. Indeed, they have
revolutionized the field of optical spectroscopy. In particular, Lamb shift
has been observed optically, two-photon transitions have been observed
in atomic and molecular systems, Rydberg stateshave been analysed,
and significant tests of unified theories of electromagnetic and weak
interactions have been made.
2. The coherence and the high intensity of laser beams enable us to measure
small changes in the frequencies of radiation resulting from Raman
scattering by measuring beats produced by the interference between the
scattered and the original beams.
3. The narrow frequency width and coherence of lasers makes them very
useful in precision measurements. Interferometers with laser beams allow
measurement of distances to a very high accuracy, as also surface
variations, refractive index, etc. For measuring the velocity of fluids, a
laser beam is scattered by the fluid. This Doppler-shifted beam then
interferes with the original beam producing beats. The beat frequency
enables us to measure the velocity of the moving medium.
4. The well-defined directionality of a laser beam makes it valuable in
communications, surveying and tracking systems.
5. Some lasers are capable of producing narrow beams of extremely high
intensity. Such beams find use in precision cutting and boring, soldering
and welding. They are used in tumor destruction and in eye surgery for
‘welding’ detached retina. There is also the possibility that they can induce
controlled thermonuclear fusion.
6. Lasers have important applications in nonlinear optics and in holography.
These are discussed in some detail.
Nonlinear Optics
For ordinary light sources, the electric field is so small that the induced
polarization P is approximately proportional to the electric field E, and the
various properties of the medium such as polarizability a, refractive index, etc.
are independent of the field intensity (here, for simplicity the vector nature of
P and E is neglected). Thus, we have what is known as linear optics for which
the superposition principle holds i.e., P 1 = α E 1 , P 2 = α E 2 implies P 1 + P 2 = α
(E 1 + E 2 ). However, with laser fields of high intensity, there is no longer a linear
relation between P and E, and the description is in terms of nonlinear optics.