Semiconductor laser |
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Semiconductor
laser is a laser in which semiconductor serves as photon
source.
Semiconductors (typically direct band-gap semiconductors)
can be used as small, highly efficient photon sources. Such
semiconductor photon sources find application in displays,
optical storage, communications, printing, surveying, and
as small efficient optical pumps for other optical processes.
Semiconductor photon sources come in two major categories
-- laser diodes and light-emitting diodes. Although both
photon sources are fundamentally the same (in other words,
a failed laser diode is an LED); application-based manufacturing
differences mean that laser diode and LED products are constructed
in fundamentally different ways -- and so possess fundamentally
different optical properties.
Light propagates in a laser diode somewhat differently
than in an LED. In an LED, the emitted photons can generally
be modeled as independent entities propagating through the
structure in a ray-like fashion.
In a laser diode, the photons must be modeled as collective
entities traveling in a confined fashion down a waveguide.
This gives a dramatically different character to the issue
of modeling light output from a laser diode.
Semiconductor photon sources can be "pumped"
by a variety of techniques. These include pumping with another
optical source (photopumping), pumping with an electron
beam, or pumping via a pn-junction. However, the most common
technique is via a pn-junction.
Semiconductor
lasers are the most basic of the existing laser types.
In their simplest form they consist of a small rectangular
slab of semiconductor material with two cleaved facets
to act as mirrors. The other facets are destroyed in some way (etched, ground, sawn, ion implanted...) in order
to avoid spurious laser modes. Additional information about lasers
Semiconductor lasers are quite different from
conventional lasers. In particular:
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The
gain of the laser material is very high and is generated
by a population inversion between the conduction and
valence bands of the semiconductors. In some sense,
a semiconductor laser is a two-state laser
system.
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Since
the electromagnetic mode is on the order of the size
of the laser device, then the transverse mode of the
semiconductor laser is quite different from
that of a conventional laser. In particular, the beam
is not Gaussian, the beam profile tends to be elliptical,
and the beam divergence tend to be large.
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The
gain spectrum is quite large (many THz or hundreds of
angstroms).
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The
short cavity (several hundred microns) means that the
longitudinal mode spacing is much larger than that of
a conventional gas or solid state laser (on the order
of GHz or angstroms).
Defenition
of semiconductor laser
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