The simplest type of diode laser resonator is the Fabry Perot cavity introduced in Chapter 3, a linear cavity with flat mirrors on the two ends. In a semiconductor laser, the linear cavity is the active stripe in the junction layer, and the flat mirrors are the facets on the edges of the chip. Typical diode laser resonators are only 300 to 500 micrometers long, but diode lasers have high gain, so milliwatt powers are easy and much higher powers are possible.

The cavity mirrors are formed by the boundary between the semiconductor crystal and the air. The refractive index of GaAs is 3.34 at 780 nm so according to Equation 5-4 roughly 30% of the light hitting the boundary is reflected back into the semiconductor, providing adequate feedback for laser oscillation.

A diode laser with such a simple cavity emits light from both facets. In practice, one facet is coated to reflect all or most of the incident light, and the beam emerges from the other facet. Light emerging from the rear facet can be monitored to control laser operation in devices such as fiber-optic transmitters.

Narrow-stripe lasers typically emit from an area less than a micrometer high and only a few micrometers wide, producing a single transverse mode, with a central intensity peak in the emerging beam. Wide-area lasers oscillate emit from a much wider stripe, up to about 100 μm, in multiple transverse modes. You will learn later in this chapter that these odd thin emitting areas strongly affect beam quality.

The resonators of Fabry Perot diode lasers are short compared to those of gas lasers, so cavity modes are spaced more widely in diode lasers-about 0.2 nm for an 800-nm GaAlAs laser, and 0.7 nm for a 1550-nm InGaAsP laser. That means that Fabry Perot lasers typically emit most of their light on one wavelength at any one time, but may have side bands. However, the gain curve of diode lasers is broad and the refractive index varies with temperature, which itself is a function of drive current. That means that a Fabry Perot laser might mode hop-shift to another wavelength-when its intensity is modulated, as shown in Figure 9-14. Mode hopping is undesirable because it can generate noise.

Fabry Perot lasers are widely used where low cost is important and precise wavelength is not critical, such as in CD players, laser pointers, and short-distance, low-speed communication links.