Optical Effects In Optical Sensing System

Chapter 4

OPTICAL EFFECTS IN OPTICAL FIBER SENSING SYSTEMS:

In an optical sensing system, the measurand can be any environmental parameters that may affect an optic signal. These parameters include temperature, strain, bending, and pressure. The optic signal guided by a fiber is either modulated by the measurands inside the fibre or outside the fibre at a sensing head through many kinds of optic effects. These optical effects include: optical Doppler effect, Sagnac effect, photoelectric effect, electro-optic effect, magneto-optical effect, and scattering including Raman scattering and Brillouin scattering. There are two different groups of scattering effects in the optical fiber, the linear scattering and the nonlinear scattering. The

In classical physics, the relationship between observed frequency f and emitted frequency f 0 is given by

f={(v+v r )/(v+v r)}*f_o

here v is the velocity of waves in the medium, vs is the velocity of the source relative to the medium, and v r is the velocity of the receiver relative to the medium. Because light is essentially an relativistic object and it has no medium, it is impossible to find absolute velocities as in the case of a material wave. Relativistic Relativity must be taken into account for the optical Doppler effect. Fortunately, it is even easier to find the relativistic Doppler effect than the classical one.

Assume the observer and the sources are moving away from each other with a relative velocity v. We consider the problem in the reference frame of the source. Suppose one wavefront arrives at the observer. The next wavefront is then at a distance λ / c f s from the observer (here λ is the wavelength, f s is the frequency of the wave the source emitted, and c is the speed of light). Because the wavefront moves to the observer with a velocity of c and the observe escapes with a velocity v, the next wavefront will meet the observer at the

Photoelectric effect takes place with photons with energies of about a few electronvolts. If the photon has sufficiently high energy, compton scattering (~keV) or pair production (~MeV) may take place.

The photons of a light beam have a characteristic energy determined by the frequency of the light. In the photoemission process, if an electron within some material absorbs the energy of one photon and thus has more energy than the work function (the electron binding energy) of the material, it is ejected. If the photon energy is too low, the electron is unable to escape the material. Increasing the intensity of the light beam increases the number of photons in the light beam, and thus increases the number of electrons emitted, but does not increase the energy that each electron possesses. Thus the energy of the emitted electrons does not depend on the intensity of the incoming light, but only on the energy of the individual photons. (This is true as long as the intensity is low enough for non-linear effects caused by multi-photon absorption to be