Semiconductor Lasers

Semiconductor Lasers:

Semiconductor Lasers was discovered in 1962 that a gallium arsenide diode, such as the one shown in Figure 12-41, is capable of producing laser action. This occurs when the diode is forward-biased, so that effective dc pumping is needed (a very convenient state, of affairs). Depending on its precise chemical composition, the GaAs laser is capable of producing an output within the range of 0.75 to 0.9 μm, i.e., in the near infrared region (light occupies the 0.39 to 0.77 μm range).

Semiconductor Lasers

Briefly, the device is an injection laser, in which electrons and holes originat­ing in the GaAIAs layers cross the heterojunctions (between dissimilar semiconductor materials, GaAlAs and GaAs in this case) and give off their excess recombination energy in the form of light. The heterojunctions are opaque, and the active region is constrained by them to the p-layer of GaAs, which is a few micrometers thick, as shown.

The two ends of the slice are very highly polished, so that reinforcing reflection takes place between them as in other lasers, and a continuous beam is emitted in the direction shown. The laser is capable of powers in excess of 1 W, which is far higher than the 1 mW, or so, necessary to send along optic fibers.

The indium gallium arsenide phosphide laser, also illustrated in Figure 12-41, is a much more recent development than the GaAs device, having been evolved during the late 1970s.

The motive force was a desire to produce laser outputs at wavelengths longer than those which the GaAs laser is capable of producing, to take advantage of “windows” in the transmission spectrum of optic fibers. Consequently, the InGaAsP lasers are less well developed at the time of writing, and so many of the world’s optic fiber communications systems still operate at wavelengths of about 0.85 μm, whereas, transmissions at wavelengths of 1.3 or 1.55 μm incur significantly less attenuation than at 0.85 μm in optic fibers.

By the early to mid-1980s, the teething problems with the new laser materials were being solved, and all new light-wave systems were being designed for wavelengths of 1.3 μm or greater.

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