Active layer AIGaAs n-GaAs substrate Contact Figure 53.10 Structure of an edge emitting LED Optical sources 53/11 Active layer Mirror Mirror Contact +p InGaAsP P InP Active layer n Inp (InGaAsP) n Inp Substrate Contact Figure 53.11 Principles of optical feedback: (a) schematic of a laser; (b) structure of a long wavelength laser LEDs and lasers radiate. With LED the situation is simple. The In semiconductor lasers the mirrors are made by simply cleaving emitted radiation corresponds to the spontaneous emission. This can the device to the required length and leaving the facet un-coated. be quite broad 30um to 50um and it is centred at the For the laser The laser devices that depend for feedback on two mirrors on the the situation is very different. Being an oscillator there are two facets of the active layer is known as a Fabry Perot laser. This is the conditions for operation: internal gain to overcome losses in the simplest possible laser structure and naturally it has a number of resonator and positive feedback. disadvantages especially mode instability. To cure this problem advanced laser structures, such as distributed feedback lasers The gain, which corresponds to the range over which the material will amplify is characteristic of the material. The cavity however (DFB), are being introduced. The optical feedback for these devices imposes additional conditions through the feedback mechanism and is provided not by mirrors at the end of the cavity but by a grating selects the frequencies at which the device oscillates. Figure 53.12 built into the laser which provides continues feedback along the shows the basics of the operation. The device gain is g (2). The whole length of the device, as in Figure 53.13. The result is virtual frequencies at which the device can potentially oscillate are those single mode lasers because the other modes are 25-30dB below the fundamental. for which there is positive feedback. These wavelengths are called the longitudinal modes of the cavity. The spacing of the modes is given by 