Page 168 - Photonics Essentials an introduction with experiments

Lasers

162 Photonic Devices

The mode spacing of the laser cavity is determined by the cavity
length. As the cavity length is reduced, the modes are spaced further
and further apart in frequency, and also in wavelength. The cavity
modes tell you which wavelengths (energies) will be reflected effi-
ciently in the cavity. The gain spectrum of the laser is independent
of the cavity modes. If you superpose the gain spectrum on the
spectrum of cavity modes, there should be a region of overlap.
This is diagrammed schematically in Fig. 7.10. These are the wave-
lengths at which amplification by stimulated emission will occur.
That is, laser action can occur at the wavelength of those cavity
modes that lie within the gain spectrum of the laser medium. Pho-
tons that have the wavelength of one of the cavity modes will be
reflected back into the cavity, provoking more emission at that wave-
length, creating more photons that will be reflected back into the
cavity, stimulating more emission, and so forth. Almost all the
photons in a cavity mode are the result of stimulated emission,
because the spontaneous emission occurs in all directions, but the
photons are stimulated only along the directions defined by the
cavity modes. Therefore, there is strong optical gain for the modes
of the resonant cavity, and very little gain for other directions or
wavelengths of light composing the spontaneous emission. Although
spontaneous emission diverts light from lasing modes, reducing
the laser efficiency, its presence is absolutely required to make the
laser work in the first place. The spontaneous emission “primes
the pump” in the beginning by filling all possible radiation modes
with photons. Gain and laser action then builds up out of the noise
in the much smaller number of modes that overlap in energy with
the gain spectrum and which are resonant modes of the reflecting
cavity.
In general, there are a number of modes that lie in the gain spec-
trum. The exact number can always be calculated if you know the
width of the gain spectrum, the cavity length, and the wavelength.
Because the gain of semiconductor materials is so large, lasing action
often occurs at several modes simultaneously. If you were trying to
design an audio oscillator, you would call this effect harmonic distor-
tion. In laser design, one often tries to design for single-mode oscilla-
tion also. It is easy to see that this could be achieved by making the
cavity much more wavelength selective, so that only one mode is pres-
ent in the gain spectrum.
At this point, we have assembled all the elements of a laser: gain,
population inversion, and a resonant cavity for feedback. It remains
only to determine the level of current injection into the diode that is
required in order to achieve laser action.

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