Radiation-balanced lasers

Bowman, S.R., Jenking, N.W., O Connor, S.R., Feldman, B.J., 2002. Sensitivity and stability of a radiation-balanced laser system. Quantum Electron. IEEE J. 38 (10), 1339-1348. 

It is perhaps worth while to point out that most of the attenuation of infrared radiation in the atmosphere is due to water-vapour absorption bands, the other major contributions coming from carbon dioxide and ozone (Hackforth, i960). The existence of wavelength windows of low absorption is of prime importance in the development of laser communication systems, while the presence of strongly absorbing bands is a major factor in determining the radiation balance of the earth s atmosphere. 

In a celebrated paper, Einstein (1917) analyzed the nature of atomic transitions in a radiation field and pointed out that, in order to satisfy the conditions of thermal equilibrium, one has to have not only a spontaneous transition probability per unit time A2i from an excited state 2 to a lower state 1 and an absorption probability BUJV from 1 to 2 , but also a stimulated emission probability B2iJv from state 2 to 1 . The latter can be more usefully thought of as negative absorption, which becomes dominant in masers and lasers.1 Relations between the coefficients are found by considering detailed balancing in thermal equilibrium... 

A force that is as large as the gravitational force can be used to suspend a particle against gravity, provided that it can be controlled and directed upward to balance gravity. One such force is the radiation pressure force or radiometric force. Ashkin and Dziedzic (1977), whose work is discussed in the next section, were the first to use the radiation pressure to levitate a microsphere stably. It was demonstrated by Allen et ai (1991) that the radiometric force can be measured with the electrodynamic balance, and they used the technique to determine the absolute intensity of the laser beam illuminating a suspended particle. This was accomplished in the apparatus displayed in Fig. 13. The laser illuminated the microparticle from below, and... 

As we saw in Section 5.5, the rate of absorption between two molecular energy levels E] and E2 is exactly equal to the rate of stimulated emission, for a given density of resonant photons, with co = E2 — E )/h. Whether there will be net absorption or emission depends on the relative populations, N and N2, of the two levels. At thermal equilibrium, when the populations follow a Boltzmann distribution, with the lower level E more populated than the upper level E2, a net absorption of radiation of frequency w can occur. If the two populations are equal, there will be neither net absortion or emission since the rates of upward and downward transitions will exactly balance. Only if we can somehow contrive to achieve a population inversion, with N2> N, can we achieve net emission, which amounts to amplification of the radiation (the A in LASER). Thus, to construct a laser, the first requirement is to produce a population inversion. Laser action can then be triggered by a few molecules undergoing spontaneous emission. 

To achieve a sustained oscillation in a laser, amplification in the gain medium must at least balance out with the optical loss during each round-trip of the cavity. Therefore, when the pump rate increases beyond a threshold value, an intense coherent laser beam is generated whose power rises linearly with the excess pump rate. At low pumping rates, the excitations in the gain medium are radiated in all directions as spontaneous emission. 

Another technique for measuring velocity is the use of the Doppler effect - both of ultrasonic and electromagnetic radiation. In these applications there is a balance between obtaining a reflection from the particles and penetration into the particle stream. Due to the short wavelengths associated with Laser Doppler techniques the velocity measurement is localised however the penetration, in all but low-density applications, is limited. 

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