
Semiconductor diode lasers
Last updated: September 12, 2009.
Lasers are the stuff of science fiction: big, heavy boxes that make blazing blasts
of light.
If you've ever seen an ordinary laser in a laboratory, you'll know
it's quite a hefty beast: typically about as long as your forearm,
fairly heavy, and capable of producing a very intense beam of light.
But if lasers are that big, how come we can use them in small things like
portable CD players and handheld barcode scanners?
The answer is that we don't! These things use a very different kind of laser that's
about the same size as (and works in a similar way to) an ordinary LED
(light-emitting diode). Known as
semiconductor lasers (also called diode lasers or injection lasers),
they were developed in the early 1960s and,
largely because they're so compact and inexpensive, are now the most
widespread lasers in the world. Let's take a closer look!
Photo: Laser beams bending (refracting) through a crystal.
Photo by Warren Gretz courtesy of US Department of Energy/National Renewable Energy Laboratory (DOE/NREL).
What is a semiconductor laser?
Chances are you've used a semiconductor laser in the last few days without even knowing
it. If you've watched a DVD you've "looked through" one; if
you've been into a grocery store and had a barcoded product swiped through the
checkout you've bought with one; if you've made a long-distance telephone
call by fiber-optic cable you've "talked through" one; and if
you've printed something with a laser printer your printout has passed very near one.
Semiconductor lasers make powerful, precise beams of light (like
ordinary lasers), but they're about the same size as simple LEDs—the
little colored lamps you see on electronic instrument panels.
Photo: The smaller circle on the bottom left of this photo is a semiconductor laser diode in a CD player. The larger, blue-tinted circle on the top right is a lens that reads the reflected light bouncing down off the CD. Never attempt to look at the laser light in a CD player: you can easily blind yourself.
If you've read our article on diodes, you'll already have an idea how LEDs work.
Essentially, an LED is a semiconductor sandwich with the "bread"
made from slices of two different kinds of treated silicon known as
p-type (rich in "holes" or, in other words, slightly lacking electrons, the tiny negatively charged particles inside atoms) and n-type
(with slightly too many electrons). Put the two slices together and you make what's
called a p-n junction diode that has all kinds of
interesting properties.
In an ordinary diode, the p-n junction works like a turnstile that allows
electric current to flow in only one direction (known as
forward-biased operation). As electrons flow across this
barrier, they combine with holes on the other side and give out
energy in the form of phonons (sound vibrations) that
disappear into the silicon crystal. In an LED, much the same process
takes place but the energy is given out not as phonons but as
photons—packets of visible light.
How the laser effect works
In a laser diode, we take things a stage further to make the emerging light more pure
and powerful. Instead of using silicon as the
semiconductor, we use a different material, notably an alloy of
aluminum and gallium arsenide (indium gallium arsenide phosphide is
another popular choice). Electrons are injected into the diode, they
combine with holes, and some of their excess energy is converted into
photons, which interact with more incoming electrons, helping to
produce more photons—and so on in a kind of self-perpetuating
process called resonance. This repeated conversion of incoming
electrons into outgoing photons is analogous to the process of
stimulated emission that occurs in a conventional, gas-based
laser.
Artwork: The basic setup of a laser diode. Laser light is produced when electrons and photons interact in a p-n junction arranged in a similar way to a conventional junction diode or LED. One end of the diode is polished so the laser light can emerge from it. The other ends are left roughened to help confine the light.
In a conventional laser, a concentrated light beam is produced by "pumping" the
light emitted from atoms repeatedly between two mirrors. In a laser
diode, an equivalent process happens when the photons bounce back and
forth in the microscopic junction (roughly one micrometer wide)
between the slices of p-type and n-type semiconductor, which is technically known
as a Fabry-Perot resonant cavity (a kind of interferometer). The amplified laser light eventually emerges from the polished end of the gap in a beam parallel to the junction. From there, it goes on to read music from your CD, scan the price on your cornflakes, print out your college dissertation, or do a thousand other useful things!