Handbook for Sound Engineers

Transmission Techniques: Fiber Optics 465

1300 nm (second window) or around 1550 nm (third

window). The choice is dictated by:

1. Windows—i.e., loss minima, in optical fibers.


2. Availability of suitable detectors.


3. Cost.


4. Minimization of pulse spreading (dispersion) in a
   fiber.


5. Reliability.

Also the facility for wavelength-division multiplexing

(WDM) can also be a factor influencing choice.

15.5.2 Laser Diodes

Laser is an acronym for light amplification by the stim-

ulated emission of radiation. The main difference

between an LED and a laser is that the laser has an opti-

cal cavity required for lasing, see Fig. 15-22. This cav-

ity, called a Fabry-Perot cavity, is formed by cleaving

the opposite end of the chip to form highly parallel,

reflective mirrorlike finishes.

At low electrical drive currents, the laser acts like an

LED and emits light spontaneously. As the drive current

increases, a threshold level is reached, above which

lasing action begins. A laser diode relies on high current

density (many electrons in the small active area of the

chip) to provide lasing action. Some of the photons

emitted by the spontaneous action are trapped in the

Fabry-Perot cavity, reflecting back and forth from end

mirror to end mirror. These photons have an energy

level equal to the band gap of the laser material. If one

of these photons influences an excited electron, the

electron immediately recombines and gives off a

photon. Remember that the wavelength of a photon is a

measure of its energy. Since the energy of the stimulated

photon is equal to the original stimulating photon, its

wavelength is equal to that of the original stimulating

photon. The photon created is a clone of the first

photon. It has the same wavelength, phase, and direc-

tion of travel. In other words, the incident photon has

stimulated the emission of another photon. Amplifica-

tion has occurred, and emitted photons have stimulated

further emission.

The high drive current in the chip creates population

inversion. Population inversion is the state in which a

high percentage of the atoms move from the ground

state to the excited state so that a great number of free

electrons and holes exist in the active area around the

junction. When population inversion is present, a

photon is more likely to stimulate emission than be

absorbed. Only above the threshold current does popula-

tion inversion exist at a level sufficient to allow lasing.

Although some of the photons remain trapped in the

cavity, reflecting back and forth and stimulating further

emissions, others escape through the two cleaved end

faces in an intense beam of light. Since light is coupled

into the fiber only from the front face, the rear face is

often coated with a reflective material to reduce the

amount of light emitted. Light from the rear face can

also be used to monitor the output from the front face.

Such monitoring can be used to adjust the drive current

to maintain constant power level on the output.

Thus, the laser differs from an LED in that laser light

has the following attributes:

1. Nearly monochromatic: The light emitted has a
   narrow band of wavelengths. It is nearly mono-
   chromatic, that is, of a single wavelength. In
   contrast to the LED, laser light is not continuous
   across the band of its special width. Several distinct
   wavelengths are emitted on either side of the
   central wavelength.


2. Coherent: The light wavelengths are in phase,
   rising and falling through the sine-wave cycle at
   the same time.

Figure 15-21. The Burrus LED double heterostructure.

Figure 15-22. Semiconductor laser. Courtesy AMP Incorpo-
rated.

Light

Optical

fiber

Epoxy

resin

Various

GaAs

layers

Oxide

insulation

layer

Electrical contact

Etched

well

Heat sink

Electrical contact 50 Mmdia.

Oxide

Stripe Contact

Cleaved

End

Optical Cavity

Cleaved

Substrate End