Handbook for Sound Engineers

Transmission Techniques: Fiber Optics 467

wavelength of 820 nm and a spectral width of 30 nm, its

output ranges from 805 nm to 835 nm from the spectral

width curve specs. The spectral width of a laser diode is

about 0.5 nm to 6 nm; the spectral width of LEDs is

much wider—around 20 nm to 60 nm.

15.5.5.4 Speed

A source must turn on and off fast enough to meet the

bandwidth requirements of the system. The source

speed is specified by rise and fall times. Lasers have rise

times of less than 1 ns, whereas LEDs have slower rise

times of about 5 ns. A rough approximation of band-

width for a given rise time is

(15-18)

where,

BW is the bandwidth in hertz,

tr is the rise time in seconds.

15.5.5.5 Lifetime

The expected operating lifetime of a source runs into
the millions of hours. Over time, however, the output
power decreases due to increasing defects in the
device’s crystal-line structure. The lifetime of the
source is normally considered the time where the peak
output power is reduced 50% or 3 dB. In general LEDs
have a longer lifetime than laser diodes. As an example,
an LED emitting a peak power of 1 mW is considered at
the end of its lifetime when its peak power becomes
500 μW or 0.5 mW.

15.5.5.6 Safety

There are a few main precautions to take in the field of
fiber optics. Most important is to never look directly
into an LED or laser diode! Generally, the light emitted
by LEDs is not intense enough to cause eye damage,
however, it is best to avoid looking at all collimated
beams emitted from LEDs or lasers. Be familiar with
the sources used. For more safety information, you can
contact the Laser Society of America or OSHA.

15.6 Detectors

The detector performs the opposite function from the

source: it converts optical energy to electrical energy.

The detector can be called an optoelectronic transducer.

The most common detectors in fiber optics are PIN pho-

todiodes, avalanche photodiodes (APD), and integrated

detectors-preamplifiers (IDP).

The PIN photodiode is the simplest type of detector,

useful for most applications. It is a three-layer semicon-

ductor device having a layer of undoped (or intrinsic)

material sandwiched between a layer of positively

doped material and negatively doped material. The

acronym PIN comes from this ordering: positive,

intrinsic, negative. Light falling on the intrinsic layer

causes electron-hole pairs to flow as current. In a

perfect photodiode, each photon will set an elec-

tron-hole pair flowing. In real PIN photodiodes, the

conversion from light to electric current is not perfect;

only 60% (or less) of the photons reaching the diode

causes current flow.

This ratio is the detector’s responsitivity. A photo-

diode has a responsitivity of about 0.6 A/W; in practical

terms, an electrical current of 60μA results for every

100 μW of optical energy striking the diode. Respon-

sivity (R) is the ratio of the diode’s output current to

input optical power and is given in amperes/watt (A/W).

The responsivity also depends on the wavelength of

light. Being the simplest device, the PIN photodiode

offers no amplification of the signal. Even so, it has

several virtues: it is inexpensive, easy to use, and has a

fast response time.

The avalanche photodiode (APD) provides some

gain and is more sensitive to low-power signals than the

PIN photodiode. A photon striking the APD will set a

number of electron-hole pairs in motion, which in turn

sets other pairs in motion, a phenomenon known as the

avalanche effect. A photon initiates an avalanche of

current. A typical APD has a responsivity of 15μA/μW.

An additional advantage of the APD is that it is very

fast, turning on and off much faster than a photodiode.

The drawback to the APD is its complexity and

expense. It requires high voltages for operation and is

sensitive to variations in temperature. Like the laser as a

source, the APD is only used where speeds and distance

require it.

The integrated detector-preamplifier (IDP) is a

photodetector and transimpedance amplifier in the same

integrated circuit. The advantage is that the signal can

be amplified or strengthened immediately, before it

meets the noise associated with the load resistor. This is

important since any following amplifier stages will

boost not only the signal but the noise as well. The IDP

amplifies the light induced current and provides a

usable voltage output. The responsivity of an IDP is in

volts/watt (V/W). The responsivity of a typical IDP is

about 15 mV/μW. Again, the device has provided gain

to overcome noise and provide a suitable SNR.

BW 0.35

tr

= ----------