Page 177 - Photonics Essentials an introduction with experiments

Lasers

Lasers 171

7.8 Summary
A laser is an amplifier with positive feedback. Amplification is gener-
ated by simulated emission of photons, and positive feedback is
achieved using mirrors. In a laser that is optically excited, the absorp-
tion rate of photons is equal to the emission rate. The emission rate is
composed of both spontaneous and stimulated emission. The absorp-
tion rate is due only to stimulated absorption. Optical gain will occur
if the stimulated emission rate exceeds the stimulated absorption
rate. Such a situation cannot occur in a two-level system because the
same two levels are responsible both for absorption and emission of
photons. In a three- or four-level system, the equilibrium between ab-
sorption and emission is maintained, but the absorption takes place
between one set of levels and the emission takes place between a dif-
ferent set.
Semiconductor lasers represent the largest class of lasers on the
market because of their low cost, small size, high efficiency and pow-
er, ease of use, and wide range of output wavelengths. A semiconduc-
tor laser is an example of a four-level system. Putting a forward cur-
rent through the diode causes recombination to occur, generating
photons. Some of these photons will be emitted into the resonant
modes of the cavity created by the mirrors. Only these photons will be
amplified by stimulated emission. As the current is increased, these
amplified modes will account for a greater percentage of the total re-
combination. Threshold is reached when the amplification per round
trip in the cavity exceeds the absorption and scattering losses for the
same round trip.

Bibliography
G. P. Agrawal and N. K. Dutta, Long Wavelength Semiconductor Lasers, Van
Nostrand Reinhold, New York, 1986.

Figure 7.12. This figure show a sequence of optical spectra taken at increasing levels of
2
current in a blue laser based on GaN. The threshold current density is 3 kA/cm , only
slightly larger than that measured in a typical GaAs-based laser at 920 nm. In the bot-
tom frame of the figure, the spontaneous emission is seen to be filtered by the many
longitudinal modes of the reflecting cavity. As the current is raised, the gain begins to
increase, and this results in a concentration of the emitted intensity in the modes
where the gain is largest, as well as a global increase of the light emitted. This process
continues until threshold is reached, as shown in the top frame. At threshold, all the
gain is concentrated in a few modes only. (Reproduced with permission from S. Naka-
mura, MRS Internet J. Nitride Semicond. Res. 4S1, G1.1, 1999.)

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