Equation 53.33, where n the refractive index of the ma- terial and L the length of the device. AA-242 (53.33) The number of the longitudinal modes which can be supported by the cavity is given by Equation 53.34. Mig = 2n 1/2 (53.34) For example at a = 0.850um for GaAs with n = 3.5 and device length, L, of say 200um the number of modes is 1647. However, only those modes for which the device has gain can oscillate and, in contrast to an LED spectrum, which is continuous the laser spec- trum is discrete. For the device used in the example, the mode Figure 53.12 The generation of the discrete laser spacing is a = 0.516nm. Depending on the design and driving spectrum: (a) device gain; (b) cavity modes; (c) laser conditions the number of laser modes range from one (single mode) spectrum to many (multimode). 'Many' usually implies 5 to 10 modes. 53/12 Fibre optic communications Distributed grating Active layer is continuous, Gaussian shaped and quite broad. Depending on the details of the structure it ranges from 25nm to 40nm for the 850nm window and from 50nm to 120nm for the 1.2um to 1.7um range. The spectral width quoted applies for the surface emitting (SE) structure. The spectral width of an edge emitting (EE) device is 0.6 that of a SE and that of a superluminence (SL) about 0.25 of the SE spectral width. The information bandwidth of LEDs is given by Equation 53.35, where the constant T is the total carrier lifetime. (53.35) Anti-reflection coated facets Figure 53.13 Schematic of DFB laser The modulation bandwidth, f, is given by Equation 53.36. (53.36) 53.2.3 Device characteristics Since the lifetime depends on the current density the bandwidth is The characteristics required to describe the operation of optical a function of the bias of the device and it drops as the power sources depend on the application. For fibre communication appli- increases. cations a minimum set of parameters which can describe the device There is are fair number of LED device but the typical parameters operation satisfactory is as follo