Optical cavity losses

All the experimental techniques described here involve the determination of the delay time between initiation of the pumping pulse and laser-pulse onset, or the coincidence of two such delay times belonging to different transitions. An analytical model has been presented by Chester et al. 116> to describe the delay re between flashlamp initiation and the start of the laser signal in the flash-photolysis HF chemical laser. The model has been used to predict the functional dependence of re on pressure, flashlamp intensity, optical-cavity losses, and the absolute magnitude of rc. However, the possible extension of this work to a detailed vibrational energy-partitioning study has not been demonstrated so far. 

To achieve lasing, N must surpass the required threshold population difference (Mh) by having the gain due to stimulated emission exceed the optical cavity losses. 

The optical cavity losses can be evaluated by first noting that absorption of the lasing wavelength is negligible due to the large Stokes shift discussed earlier. Thus, only the mirror losses contribute. Assuming that both mirrors are identical (symmetric cavity), the distributed cavity loss coefficient (a) can be written as... 

Laser instabilities were experimentally investigated in many kinds of lasers (see an overview of early papers [14]), but the first experimental observation of the optical chaos was performed by Arecchi et al. [30] in 1982. They used a stabilized CO2 laser with modulated cavity loss = y(l + a cos fit) and by changing the frequency of modulation 2, they found a few period doubling oscillations of the output intensity, both numerically and experimentally. 

West and Berry (1037) have observed laser emissions due to the transition ( N(42n - Ar2 ) in the vacuum ultraviolet flash photolysis of C2N2, HCN, CICN, BrCN, and ICN. The CN(/42F1) radicals are produced within a low-loss optical cavity for effective laser action. 

The ringdown rate of an optical cavity depends on losses caused by transmission of the mirrors and scattering and/or absorbtion by species inside the cavity. 

We perform concrete calculations in the complex P-representation [Drummond 1980 McNeil 1983] in the frame of both probability distribution functions and stochastic equations for the complex c-number variables. We follow the standard procedures of quantum optics to eliminate the reservoir operators and to obtain a master equation for the density operator of the modes. The master equation is then transformed into a Fokker-Planck equation for the P-quasiprobability distribution function. In particular, for an ordinary NOPO and in the case of high cavity losses for the pump mode (73 7), if in the operational regime the pump depletion effects are involved, this approach yields... 

Two spectra are presented in Fig. 20 (a) was recorded with the optical cavity detuned (zero amplification) and (b) with a tuned cavity. Spectrum (b) shows that the laser operated at three wavelengths. Each of these wavelengths corresponds to a maximum in the gain curve as a function of wavelength. The dominant laser wavelength is at 6476 A. The number of laser wavelengths depends on the gain to loss ratio and would decrease with a smaller ratio. 

A simple all-fiber laser is shown schematically in Fig. 1. A pump laserprovides energy to the fiber amplifier. For the laser to exceed threshold and generate a coherent optical output, the amplifier must produce sufficient gain to overcome cavity losses, including the outpnt coupling loss. The absorbed pump power required to reach threshold is approximately... 

Such coherent nonlinear effects may also induce gain losses by the self-focusing effect, destroying the desired mode properties of the optical cavity. A power threshold has been estimated by Yariv (1967)... 

If the nonlinear crystal that is pumped by the incident wave Ep is placed inside a resonator, oscillation on the idler or signal frequencies can start when the gain exceeds the total losses. The optical cavity may be resonant for both the idler and signal waves (doubly-resonant oscillator) or for only one of the waves (singly-resonant oscillator) [5.292]. Often, the cavity is also resonant for the pump wave in order to increase 7p and thus the gain coefficient T. 

Mode-locking does not occur spontaneously in a simple laser cavity. Either it must be actively driven by a cavity element which introduces cavity losses with a period of exactly T/2 (i.e., one-half the optical round-trip time), or it must be... 

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