Laser resonators, amplified spontaneous

There is a new class of laser system, frequently called mirrorless lasers, for which the gain is so high that they do not need a resonant cavity to emit bright and quasi-coherent beams. To achieve such performance, either of the following processes should be satisfied. First, there is a cooperative process, characterized by the emission of coherent radiation with peak intensity proportional to N, where N is the number of oscillating dipoles, and with the excited-state lifetime proportional to l/N. Second, there are collective processes, such as amplified spontaneous emission, where the spontaneous emission coming from a distribution of emitters is linearly amplified by the gain medium. 

An architecture very closely related to the cavity-dumped oscillator is the regenerative amplifier. The principle difference is that the laser oscillation in a regenerative amplifier does not build up from spontaneous emission, but is initiated by a signal injected into the resonator fi om the outside, as the Pockels cell is switched to transmission. This injected signal is then trapped in the cavity and amplified until it has reached maximum intensity, at which point it is ejected (dumped) from the cavity. 

The function of the optical resonator is the selective feedback of radiation emitted from the excited molecules of the active medium. Above a certain pump threshold this feedback converts the laser amplifier into a laser oscillator. When the resonator is able to store the EM energy of induced emission within a few resonator modes, the spectral energy density p(v) may become very large. This enhances the induced emission into these modes since, according to (2.22), the induced emission rate already exceeds the spontaneous rate for p(v) > hv. In Sect. 5.1.3 we shall see that this concentration of induced emission into a small number of modes can be achieved with open resonators, which act as spatially selective and frequency-selective optical filters. 

The total amplification /t//o has maxima for cp — lqn, which corresponds to the condition (5.53) for the eigenfrequencies of the resonator with the modification (5.57). For G(v) 1, the total amplification Ij/h becomes infinite for (j) = 2qn. This means that even an infinitesimally small input signal results in a finite output signal. Sueh an input is always provided, for instance, by the spontaneous emission of the excited atoms in the active medium. For G(v) = 1 the laser amplifier converts to a laser oscillator. This condition is equivalent to the threshold condition (5.7). Because of gain saturation (Sect. 5.3), the amplification remains finite and the total output power is determined by the pump power rather than by the gain. 

---