4.4 Output Linearity of Light Sources
201
2ω 2 , etc.) are the most troublesome because they may fall within the bandwidth of
the channel. Of these, usually only the third-order terms are important, because the
amplitudes of higher-order terms tend to be significantly smaller. If the operating
frequency band is less than an octave, all other intermodulation products will fall
outside the passband and can be eliminated with appropriate filters in the receiver.
4.5 Summary
This chapter examines the basic operating characteristics of heterojunctionstructured light-emitting diodes (LEDs) and laser diodes. The first topic deals with the
basic structure of these sources, which is a sandwich type construction of different
semiconductor materials. These layers serve to confine the electrical and optical
carriers to yield optical sources with high outputs and high efficiencies. The principal materials of which these layers are composed include the ternary alloy GaAlAs
for operation in the 800-nm to 900-nm wavelength region and the quaternary alloy
InGaAsP for use between 1100 and 1700 nm.
An important source characteristic associated with the quantum efficiency of a
light source is the modulation capability and the response to transient current pulses.
By applying a small dc bias to the source, the time delay between the application of a
current pulse and the onset of optical power output can be reduced. This bias reduces
the parasitic diode space charge capacitance that could cause a delay of the carrier
injection into the active region. Such an injection delay otherwise could postpone
the start of the optical output.
When deciding whether to choose an LED or a laser diode source, a tradeoff
must be made between the advantages and limitations of each type of device. The
advantages that a laser diode has over an LED are as follows:
1. A faster response time, so that much higher modulation rates (higher data
transmission rates) are possible with a laser diode.
2. A narrower spectral width of the laser output, which implies less dispersioninduced pulse broadening during data transmission.
3. A much higher optical power level that can be coupled from a laser diode into a
fiber, thus allowing longer transmission distances.
Compared to LED sources some application issues of laser diodes are as follows:
1. The construction of laser diodes is more complex, mainly because of the requirement of current confinement in a small lasing cavity. This makes the laser diode
more expensive than an LED.
2. The optical output power level of a laser is strongly dependent on temperature.
If a laser diode is to be used over a wide external temperature range, then either
a cooling mechanism (such as a thermoelectric cooler) must be used to maintain
the laser at a constant temperature or a circuit that senses the lasing threshold can
be implemented to adjust the bias current with changes in temperature. Using a
thermoelectric cooler typically is the preferred temperature control method.
201
2ω 2 , etc.) are the most troublesome because they may fall within the bandwidth of
the channel. Of these, usually only the third-order terms are important, because the
amplitudes of higher-order terms tend to be significantly smaller. If the operating
frequency band is less than an octave, all other intermodulation products will fall
outside the passband and can be eliminated with appropriate filters in the receiver.
4.5 Summary
This chapter examines the basic operating characteristics of heterojunctionstructured light-emitting diodes (LEDs) and laser diodes. The first topic deals with the
basic structure of these sources, which is a sandwich type construction of different
semiconductor materials. These layers serve to confine the electrical and optical
carriers to yield optical sources with high outputs and high efficiencies. The principal materials of which these layers are composed include the ternary alloy GaAlAs
for operation in the 800-nm to 900-nm wavelength region and the quaternary alloy
InGaAsP for use between 1100 and 1700 nm.
An important source characteristic associated with the quantum efficiency of a
light source is the modulation capability and the response to transient current pulses.
By applying a small dc bias to the source, the time delay between the application of a
current pulse and the onset of optical power output can be reduced. This bias reduces
the parasitic diode space charge capacitance that could cause a delay of the carrier
injection into the active region. Such an injection delay otherwise could postpone
the start of the optical output.
When deciding whether to choose an LED or a laser diode source, a tradeoff
must be made between the advantages and limitations of each type of device. The
advantages that a laser diode has over an LED are as follows:
1. A faster response time, so that much higher modulation rates (higher data
transmission rates) are possible with a laser diode.
2. A narrower spectral width of the laser output, which implies less dispersioninduced pulse broadening during data transmission.
3. A much higher optical power level that can be coupled from a laser diode into a
fiber, thus allowing longer transmission distances.
Compared to LED sources some application issues of laser diodes are as follows:
1. The construction of laser diodes is more complex, mainly because of the requirement of current confinement in a small lasing cavity. This makes the laser diode
more expensive than an LED.
2. The optical output power level of a laser is strongly dependent on temperature.
If a laser diode is to be used over a wide external temperature range, then either
a cooling mechanism (such as a thermoelectric cooler) must be used to maintain
the laser at a constant temperature or a circuit that senses the lasing threshold can
be implemented to adjust the bias current with changes in temperature. Using a
thermoelectric cooler typically is the preferred temperature control method.