Optical Spectral Structure and Frequency Coherence

Abstract

To account for the various phenomena of light, two theories have been proposed: the corpuscular and the undulatory. The former assumes that the light is a stream of corpuscles, namely photons, discrete photons carrying packets of energy and momentum. The undulatory theory, on the other hand, requires that light consists of a series of wave trains (Michelson, 1927). Each wave train is followed by another which has a random change in phase (Mathieu, 1975). A single wave train is made up of monochromatic components, i.e. the wave train is polychromatic. Although wave trains are supposed not to be strictly monochromatic, experimental demonstration is extremely difficult and has not been reported to date (Diitchburn, 1963). In addition, many efforts have been made to measure the laser coherence length which is regarded as the length of the wave train. Among the methods for measuring the coherence length of lasers (Geng et al., 2005; Ryabukho et. al., 2005 ; Wheeler et al., 2003), Michelson interferometer-based method (Ryabukho et al., 2005) is the one widely used in the past. However, these methods suffer from mechanic vibration, thermal and acoustic fluctuations, and beam divergence, and errors in the observation of the spatial coherence are difficult to eliminate. For lightwave from a real laser source, the wave trains are neither identical nor of simple form (Born & Wolf, 1999). Unfortunately, a complete description of other properties, such as linewidth, intensity profile, and frequency spacings among wave trains, has not been explicitly given due to resolution limitations in both measuring techniques and instruments. Understanding the spectral structure of semiconductor laser is a fundamental issue. The spectral analysis, especially for the fine spectral structure, reveals the important properties of semiconductor laser, such as mode characteristics, atom emission behavior, highfrequency performances, and coherence features. Spectral lines of light are broadened by various processes. For semiconductor lasers, Henry’s model (Henry, 1982) can be used to explain linewidth broadening of the laser by two mechanisms: 1) the instantaneous phase change caused by the spontaneous emission results in the linewidth broadening of the light; 2) the instantaneous fluctuations of the field intensity through linewidth enhancement parameter ┙ results in a delayed phase change, which further broadens the linewidth. However, this model also can not describe the properties of wave trains mentioned above distinctly. Indeed, it implies that the spectral linewidth we observed, in fact, results from the