Page 165 - Photonics Essentials an introduction with experiments

Lasers

Lasers 159

achieved in a p-n junction. The other half of the requirement is to cre-
ate optical feedback. This is easy; it’s all done with mirrors.

7.5 Optical Feedback—Making a Laser
The simplest kind of optical resonator that you could think of is
formed by two parallel mirrors. In fact, the first lasers were made in
this way with metallized front-surface, flat mirrors. Improvements on
this simple beginning were to give the mirrors a concave surface so
that the light intensity would be focused to a maximum in the center
of the gain region. Replacing one of the metallized mirrors by a multi-
ple-coating interference reflector introduces improved wavelength se-
lectivity, so that only the wavelength range of interest is subject to
feedback. These features have all been applied to semiconductor
structures in order to make laser diodes. The majority of semiconduc-
tor lasers are fabricated using two plane, parallel mirrors formed by
cleaving the laser chip along parallel crystal planes. The mirror re-
flectivity is determined by the index difference between the semicon-
ductor material (n 3.4) and air (n = 1.0).
The good news about semiconductor lasers is the gain coefficient is
very large compared to that of gas lasers like He–Ne or solid state
laser materials like Nd–YAG. As a result, the mirrors at each end of
the cavity do not need to be as efficient as those required for other
kinds of laser materials. There are two big performance benefits: one
is that more power can be extracted from a semiconductor laser at
modest input power levels, and the other is that there is a much larg-
er tolerance in the design of the resonator needed to make a working
device. There is also a big space savings, too. This is why you can hold
a semiconductor laser in the palm of your hand, but you need a table
top to hold a gas laser or a YAG laser. These two features are impor-
tant reasons why semiconductor laser technology dominates the mar-
ket, a trend that is likely to accelerate.
The role of the resonator is easy to understand. A forward bias volt-
age applied to the diode creates excess concentrations of electron–hole
pairs. Electron–hole recombination generates photons that depart in
all directions by spontaneous emission. Some of these photons will
travel along the line that is perpendicular to the reflecting surface of
the two parallel mirrors. These photons will be reflected and will trav-
el back into the diode along the same path. Of course, there will be
some loss in this process. Some photons will be absorbed by impuri-
ties along the way. Others will be scattered out of alignment by de-
fects in the optical path. These events constitute the losses. Most im-
portant of all, some will traverse the mirror and be emitted into free
space. This “loss” constitutes the useful output of the device. At the
same time, the photons that traverse the gain region will stimulate

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