11.1.1 âLASERâ versus âLOSERâ
A photon, unlike an electron, can stimulate an excited atom to emit a second photon into the same electromagnetic mode. This stimulated emission process is the foundation for light amplification and oscillation (i.e., self-generation). Initially, the term LASER referred to Light Amplification by Stimulated Emission of Radiation. Nowadays laser often means Light Oscillation by Stimulated Emission, which should literally be called âLOSERâ instead of âLASER.â To distinguish the above two devices, the former is called a laser amplifier, the latter a laser oscillator (Siegman 1986).
In a laser amplifier, input light is amplified when the net gain coefficient geff = ga â Î±r > 0, where ga and Î±r represent gain and absorption coefficients, respectively. In the absence of input light, photons spontaneously emitted by excited atoms are multiplied, giving amplified spontaneous emission (ASE).
Laser oscillation occurs when the photon generation rate exceeds the photon loss rate in a system. If gain saturation were absent, the photon number in a laser oscillator would diverge in time. In other words, the rate equation for the photon number would acquire an unstable solution above the oscillation threshold. In reality, gain saturation reduces the photon generation rate to the photon loss rate so that the number of photons in the oscillator remains at a finite value.
11.1.2 Random lasers
For a long time, optical scattering was considered to be detrimental to lasing because such scattering removes photons from the lasing modes of a conventional laser cavity. However, in a disordered medium with gain, light scattering plays a positive role in both laser amplification and laser oscillation. Multiple scattering increases the path length or dwell time of light in an active medium, thereby enhancing laser amplification. In addition, strong scattering increases the chance of light (of wavelength Î») returning to a coherence volume (~ Î»3) it has visited previously, providing feedback for laser oscillation.
Since the pioneering work of Letokhov and coworkers (Ambartsumyan et al. 1966), lasing in disordered media has been a subject of intense theoretical and experimental studies. It represents the process of light amplification by stimulated emission with feedback mediated by random spatial fluctuations of the dielectric constant. There are two kinds of feedback: one is intensity or energy feedback, the other is field or amplitude feedback (Cao 2003). Field feedback is phase sensitive (i.e., coherent) and therefore frequency dependent (i.e., resonant). It requires light scattering to be elastic and the spatial distribution of the dielectric constant to be time-invariant. The intensity feedback is phase insensitive (i.e., incoherent) and frequency independent (i.e., nonresonant). This can occur, e.g., in the presence of inelastic scattering, mobile scatterers, dephasing, and nonlinearity.