Fiber Optic Tutorials
Coupled-Cavity Semiconductor Lasers
This is a continuation from the previous tutorial - tunable semiconductor lasers. Introduction Distributed-feedback mechanism can provide single-frequency semiconductor lasers with a high degree of side-mode suppression. The operating wavelength is relatively unaffected by external perturbations since it is determined by the spatial period of a permanently etched grating. Although wavelength stability is an attractive feature of distributed-feedback lasers, it is achieved at the expense of tunability. For some applications it is desirable to have a semiconductor laser whose wavelength can be tuned over a wide range, at least discretely. Coupled-cavity semiconductor lasers have the potential of offering mode...
Tunable Semiconductor Lasers
This is a continuation from the previous tutorial - DBR semiconductor lasers. Tunable semiconductor lasers are useful for many applications. Examples of their applications in fiber-optic communication systems are (i) wavelength-division multiplexed lightwave systems where optical signal at many distinct wavelengths are simultaneously modulated and transmitted through a fiber and (ii) coherent communication systems which require wavelength matching between the local oscillator and the transmitter laser. The simplest kind of tunable semiconductor lasers make use of an external cavity formed by placing a grating at some distance from a multimode laser. Such external-cavity semiconductor lasers can be tuned over...
DBR Lasers
This is a continuation from the previous tutorial - performance of DFB semiconductor lasers. The DBR laser provides an alternative scheme in which the frequency dependence of the distributed-feedback mechanism is utilized to select a single longitudinal mode of an FP cavity. In contrast to DFB lasers, the grating in a DBR laser is etched outside the active region [refer to Figure 7-1 in the DFB semiconductor lasers tutorial]. In effect, a DBR laser is an FP laser whose mirror reflectivity varies with wavelength; lasing occurs at the wavelength for which the reflectivity is maximum. In this tutorial we...
Performance of DFB Semiconductor Lasers
This is a continuation from the previous tutorial - DFB (distributed feedback) semiconductor lasers. In the previous tutorial, we briefly discussed the heterostructures used to make a DFB semiconductor laser. The purpose of this section is to describe their performance by considering the continuous wave (CW) and pulsed modes of operation. The features to be considered are the \(L-I\) curves and their temperature dependence, the longitudinal-mode spectra and the extent of side-mode suppression, and the modulation-response characteristics such as modulation bandwidth and frequency chirp. In the previous series tutorials we discussed these features for an FP semiconductor laser. We...
DFB (Distributed Feedback) Semiconductor Lasers
This is a continuation from the previous tutorial - effects of external optical feedback on semiconductor lasers. Introduction to distributed-feedback semiconductor lasers As we have seen in the previous tutorials a conventional semiconductor laser does not emit light in a single longitudinal mode. In general, the mode closest to the gain peak is most intense, and a few percent of the output power is carried by other longitudinal modes lying close to the gain peak. Furthermore, even when these side modes are reasonably suppressed under CW operation, their power content increases significantly when the laser is pulsed rapidly. Direct...