Читать книгу VCSEL Industry - Babu Dayal Padullaparthi - Страница 27

To achieve laser oscillation, a resonator that provides optical feedback to the gain medium is required. The laser resonator is formed by a pair of mirrors; a so‐called Fabry‐Pérot (FP) resonator is shown in Figure 1.8(a). In an edge‐emitting laser, the gain width is w, the cavity length is equal to L, and the mirror is usually made by simply cleaving the semiconductor crystal. In this case the refractive index of about 3.5 is higher than outside air, and the resonator edges look like open termination. (Here φ = 2 π . φ is defined in Eq. (1.1)).

In Table 1.1 we have touched on DFB and DBR structures for single‐mode operation of edge‐emitting lasers [20–23]. In both cases, we utilize a pair of Bragg mirrors having an electric field reflectivity expressed by that sandwich some space or active region. These wavelength‐selective cavities can provide single longitudinal‐mode operation. For using those lasers in optical pulse code modulation (PCM) for optical fiber communications, they should maintain single mode under high‐speed modulation (~100 Gb/s). Moreover, in the case of coherent digital communications, the laser should operate with narrow spectrum (~kHz). This kind of lasers is called a dynamic single‐mode laser [24].

In the case of VCSELs the mirrors are formed by semiconductor Bragg reflectors or dielectric mirrors, and therefore, we can design the resonator as open or short terminations. We can use its large free spectral range (FSR) for pure single longitudinal‐mode operation and wide‐range wavelength tuning. The details will be described in Chapters 2 and 8.