Optical Networks - _3_5 Transmitters_39

The upper mirror is curved to prevent beam walk off

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The upper mirror is curved to prevent beam walk-off in the cavity, leading to better stability of the lasing mode. To conduct the heat away from the bottom mirror, a hole is etched in the InP substrate. The design uses a 980 nm pump laser to pump the VCSEL cavity. Any pump wavelength lower than the desired lasing wavelength can be used to excite the semiconductor electrons to the conduction band. For example, the 980 nm semicon- ductor pumps used to pump erbium-doped fiber amplifiers can be used here as well. By designing the pump spot size to match the size of the fundamental lasing mode,
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3.5 Transmitters 187 the laser can be made single mode while suppressing the higher-order Fabry-Perot cavity modes. Using gain to perform this function is better than trying to design the cavity to provide higher loss at the higher-order modes. The high gain also allows the output coupling reflectivity to be reduced, while still maintaining sufficient inversion inside the cavity to prevent excessive recombination. The laser described in [Vak99] was able to put out about 0 dBm of power in continuous-wave (CW) mode over a tuning range of 50 nm. Two- and Three-Section DBR Lasers We saw earlier that we can change the refractive index of a semiconductor laser by injecting current into it. This can result in an overall tuning range of about 10 nm. The DFB laser shown in Figure 3.44 can be tuned by varying the forward-bias current, which changes the refractive index, which in turn changes the effective pitch of the grating inside the laser cavity. However, changing the forward-bias current also changes the output power of the device, making this technique unsuitable for use in a DFB laser. A conventional DBR laser also has a single gain region, which is controlled by injecting a forward-bias current I g , as shown in Figure 3.44(b). Varying this current only changes the output power and does not affect the wavelength. This structure can be modified by adding another electrode to inject a separate current I b into the Bragg region that is decoupled from the gain region, as shown in Figure 3.52(a). This allows the wavelength to be controlled independently of the output power. As in a conventional DBR laser, the laser has multiple closely spaced cavity modes corresponding to the cavity length, of which the one that lases corresponds to the wavelength peak of the Bragg grating. As the wavelength peak of the grating is varied by varying I b , the laser hops from one cavity mode to another. This effect is shown in Figure 3.52(a). As the current I b is varied, the Bragg wavelength changes. At the same time, there is also a small change in the cavity mode spacing due to the change in refractive index in the grating portion of the overall cavity. The two changes do not track each other, however. As a result, as I b is varied and the Bragg wavelength changes, the laser wavelength changes, with the laser remaining on the same cavity mode for some time. As the current is varied further, the laser hops to the next cavity mode. By careful control over the cavity length, we can make the wavelength spacing
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