Understanding Lasers

Chapter 9.6.3: External Cavity and Tunable Lasers

9.6.3 External Cavity and Tunable Lasers

Diode-laser emission also can be limited to a narrow linewidth by placing it in an external cavity with suitable tuning optics. For example, one facet may be coated to transmit nearly all light emerging from the active layer into external optics that select the wavelength, as shown in Figure 9-16. The figure shows a very short external cavity but, in practice, longer cavities are used to limit output to a narrow range of wavelengths.

The selected wavelength can be tuned by adjusting the optics; in Figure 9-16, the grating is turned to feed light at different wavelengths back into the active layer of the laser. Such tunability is also an attractive feature of external-cavity semiconductor lasers.

Other types of tunable diode lasers have also been developed, largely for fiber-optic systems for which it is desirable to be able to switch emission wavelengths. One example is the sampled grating, distributed Bragg reflector (SG-DBR) laser. Like the example in Figure 9-15B, it has gratings on both sides of the active region of

the laser, but in this case the spacings are different on the two sides. Slight adjustments of the two gratings tune them relative to each other, producing a disproportionately large change in the wavelength at which the laser oscillates.

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