Optical Networks - _3_5 Transmitters_39

Several of the filters discussed in section 33 can be

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within the gain bandwidth, and we will have a single-mode laser. Several of the filters discussed in Section 3.3 can be used as wavelength-selective mirrors in external cavity lasers. We have already seen the use of the diffraction grating (Section 3.3.1) and Fabry-Perot filter (Section 3.3.5) in external cavity lasers. These laser structures are used today primarily in optical test instruments and are not amenable to low-cost volume production as SLM light sources for transmission systems. One version of the external cavity laser, though, appears to be particularly promising for this purpose. This device uses a fiber Bragg grating in front of a conventional FP laser with its front facet AR coated. This device then acts as an SLM DBR laser. It can be fabricated at relatively low cost compared to DFB lasers and is inherently more temperature stable in wavelength due to the low temperature- coefficient of the fiber grating. One disadvantage of external cavity lasers is that they cannot be modulated directly at high speeds. This is related to the fact that the cavity length is large. Vertical Cavity Surface-Emitting Lasers In this section, we will study another class of lasers that achieve single-longitudinal mode operation in a slightly different manner. As we saw in Figure 3.43, the frequency
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3.5 Transmitters 179 Figure 3.47 The structure of a VCSEL. spacing between the modes of an MLM laser is c/ 2 nl , where l is the length of the cavity and n is its refractive index. If we were to make the length of the cavity sufficiently small, the mode spacing increases such that only one longitudinal mode occurs within the gain bandwidth of the laser. It turns out that making a very thin active layer is much easier if the active layer is deposited on a semiconductor substrate, as illustrated in Figure 3.47. This leads to a vertical cavity with the mirrors being formed on the top and bottom surfaces of the semiconductor wafer. The laser output is also taken from one of these (usually top) surfaces. For these reasons, such lasers are called vertical cavity surface-emitting lasers (VCSELs). The other lasers that we have been discussing hitherto can thus be referred to as edge-emitting lasers. Since the gain region has a very short length, very high mirror reflectivities are required in order for laser oscillation to occur. Such high mirror reflectivities are difficult to obtain with metallic surfaces. A stack of alternating low- and high-index dielectrics serves as a highly reflective, though wavelength-selective, mirror. The reflectivity of such a mirror is discussed in Problem 3.13. Such dielectric mirrors can be deposited at the time of fabrication of the laser. One problem with VCSELs is the large ohmic resistance encountered by the injected current. This leads to considerable heating of the device and the need for efficient thermal cooling. Many of the dielectric materials used to make the mirrors have low thermal conductivity. So the use of such dielectric mirrors makes room temperature operation of VCSELs difficult to achieve since the heat generated by the device cannot be dissipated easily. For this reason, for several years after they
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