Light emitters are a key element in any fiber
optic system. This component converts the electrical signal into a
corresponding light signal that can be injected into the fiber. The light
emitter is an important element because it is often the most costly
element in the system, and its characteristics often strongly influence
the final performance limits of a given link.
|Figure 1 - Laser Diodes Convert
an Electrical Signal to Light |
Diodes are complex semiconductors
that convert an electrical current into light. The conversion process is
fairly efficient in that it generates little heat compared to incandescent
lights. Five inherent properties make lasers attractive for use in fiber
1. They are small.
2. They possess high radiance
(i.e., They emit lots of light in a small area).
3. The emitting area
is small, comparable to the dimensions of optical fibers.
4. They have
a very long life, offering high reliability.
5. They can be modulated
(turned off and on) at high speeds.
Table 1 offers a
quick comparison of some of the characteristics for lasers and LEDs.
These characteristics are discussed in greater detail throughout this
article and in the article on light-emitting
1 - Comparison of LEDs and Lasers|
||Linearly proportional to drive current
||Proportional to current above the threshold|
||Drive Current: 50 to 100 mA Peak
||Threshold Current: 5 to 40 mA|
||0.66 to 1.65 µm
||0.78 to 1.65 µm|
||Wider (40-190 nm FWHM)
||Narrower (0.00001 nm to 10 nm FWHM)|
|Ease of Use
Laser diodes are typically constructed of GaAlAs
(gallium aluminum arsenide) for short-wavelength devices. Long-wavelength
devices generally incorporate InGaAsP
(indium gallium arsenide phosphide).
|Laser Diode Performance Characteristics|
Several key characteristics lasers determine
their usefulness in a given application. These are:
Wavelength: This is the wavelength
at which the source emits the most power. It should be matched to the
wavelengths that are transmitted with the least attenuation
through optical fiber. The most common peak wavelengths are 1310, 1550,
and 1625 nm.
Width: Ideally, all the light emitted from a laser would be at the
peak wavelength, but in practice the light is emitted in a range of
wavelengths centered at the peak wavelength. This range is called the
spectral width of the source.
Emission Pattern: The pattern
of emitted light affects the amount of light that can be coupled into the
optical fiber. The size of the emitting region should be similar to the
diameter of the fiber core.
2 illustrates the emission pattern of a
Power: The best results are usually achieved by
coupling as much of a source's power into the fiber as possible. The key
requirement is that the output power of the source be strong enough to
provide sufficient power to the detector
at the receiving end, considering fiber attenuation, coupling
losses and other system constraints. In general, lasers are more
powerful than LEDs.
source should turn on and off fast enough to meet the bandwidth
limits of the system. The speed is given according to a source's rise
time, the time required to go from 10% to 90% of peak power. Lasers
have faster rise and fall times than LEDs.
Figure 2 - Laser Emission Pattern
Linearity is another important characteristic
to light sources for some applications. Linearity
represents the degree to which the optical output is directly proportional
to the electrical current input. Most light sources give little or no
attention to linearity, making them usable only for digital
applications require close attention to linearity. Nonlinearity
in lasers causes harmonic distortion in the analog signal that is
transmitted over an analog fiber optic link.
Lasers are temperature sensitive; the lasing
threshold will change with the temperature. Figure
3 shows the typical behavior of a laser diode. As operating
temperature changes, several effects can occur. First, the threshold
current changes. The threshold current is always lower at lower
temperatures and vice versa. The second change that can be important is
efficiency. The slope efficiency is the number of milliwatts or
microwatts of light output per milliampere of increased drive current
above threshold. Most lasers show a drop in slope efficiency as
temperature increases. Thus, lasers require a method of stabilizing the
threshold to achieve maximum performance. Often, a photodiode
is used to monitor the light output on the rear facet of the laser. The
current from the photodiode changes with variations in light output and
provides feedback to adjust the laser drive current.
Figure 3 - Temperature Effects on Laser Optical
Figure 4 - Emitters
shows the behavior of an LED, and Figure 4b
shows the behavior of a laser diode. The plots show the relative amount of
light output versus electrical drive current. The LED outputs light that
is approximately linear with the drive current. Nearly all LED's exhibit a
"droop" in the curve as shown in Figure 4b.
This nonlinearity in the LED limits its usefulness in analog applications.
The droop can be caused by a number of factors in the LED semiconductor
physics but is often largely due to self-heating of the LED chip. All
LED's drop in efficiency as their operating temperature increases. Thus,
as the LED is driven to higher currents, the LED chip gets hotter causing
a drop in conversion efficiency and the droop apparent in Figure 4a.
LED's are typically operated at currents to about 100 mA peak. Only
specialized devices operate at higher current levels.
There are two basic types of laser diode
structures: Fabry-Perot (FP) and distributed
feedback (DFB). Of the two types of lasers, Fabry-Perot lasers are the
most economical, but they are generally noisy, slower devices. DFB lasers
are quieter devices (e.g., high signal-to-noise),
have narrower spectral widths, and are usually faster
DFB lasers offer the
highest performance levels and also the highest cost of the two types.
They are nearly monochromatic (i.e. they emit a very pure single color of
light.) while FP lasers emit light at a number of discrete wavelengths.
DFB lasers tend to be used for the highest speed digital applications and
for most analog applications because of their faster speed, lower noise,
and superior linearity. Fabry-Perot lasers further break down into buried
hetero (BH) and multi-quantum
well (MQW) types. BH and related styles ruled for many years, but now
MQW types are becoming very widespread. MQW lasers offer significant
advantages over all former types of Fabry-Perot lasers. They offer lower
threshold current, higher slope efficiency, lower noise, better linearity,
and much greater stability over temperature. As a bonus, the performance
margins of MQW lasers are so great, laser manufacturers get better yields,
so laser cost is reduced. One disadvantage of MQW lasers is their tendency
to be more susceptible to backreflections.
See article "Laser
Backreflection - The Bane of Good Performance" for more
Figure 5 -
VCSELs are a new
laser structure that emits laser light vertically from its surface and has
vertical laser cavity. Figure
6 illustrates the structure of a VCSEL.
The VCSEL's principles of operation closely
resembles those of conventional edge-emitting semiconductor lasers. The
heart of the VCSEL is an electrically pumped gain region, also called the
active region, emits light. Layers of varying semiconductor materials
above and below the gain region create mirrors. Each mirror reflects a
narrow range of wavelengths back into the cavity causing light emission at
a single wavelength.
Figure 6 -
Basic VCSEL Structure
VCSELs are typically multi-quantum well (MQW)
devices with lasing occurring in layers only 20-30 atoms thick.
Bragg-reflectors with as many as 120 mirror layers form the laser
There are many
advantages to VCSELs. Their small size and high efficiency mirrors produce
a low threshold current, below 1 mA. The transfer function allows
stability over a wide temperature range, a feature that is unique to this
type of laser diode. These features make the VCSEL ideal for applications
that require an array of devices.
Actually, all lasers are susceptible to
backreflections. Backreflections disturb the standing-wave oscillation in
the laser cavity, and the net effect is an increase in the effective noise
floor of the laser. A strong backreflection can cause some lasers to
become wildly unstable and completely unusable in some applications. It
can also generate nonlinearities, called kinks, in the laser response.
Most analog applications and some digital ones cannot tolerate these
The importance of
controlling backreflection depends on the type of information being sent
and the particular laser. Some lasers are very susceptible to
backreflections due to the design of the laser chip itself. Most often the
determining factor is how tightly the fiber is coupled to the laser chip.
A low-power laser generally has weak coupling to the fiber. Perhaps only
5-10% of the laser power is coupled into the fiber. This means that only
5-10% of the backreflection would be coupled into the laser cavity, making
the laser relatively immune to backreflections. On the other hand, a
high-power laser may have 50-70% of the laser chip output coupled to the
fiber. This also means that 50-70% of the backreflection will be coupled
back into the laser cavity. This makes high-power lasers more susceptible
|Laser Driver Circuits
Analog Laser Drive
illustrates two common circuit configurations used to drive lasers for
analog applications. The simpler of the two, shown in figure 7a,
offers moderate linearity and good performance in frequencies up to 500
MHz. The analog signal path only involves C1, R1, Q1, R2, and D1, the
laser diode. Q1 acts as a transconductant stage in which voltage flows in
and current flows out. C1 passes only the AC portion of the analog input
signal. R1, usually only a few tens of Ohms, squelches any possible
oscillations in Q1. The AC portion of analog input voltage VIN
appears at the base of Q1 and also at the emitter of Q1. VIN,
the AC voltage at the emitter of Q1, imposes across R2 to create a
modulation current VIN/R2. U1 supplies DC current to the laser
through R3 and R1. U1 creates a servo loop that maintains a constant
photodiode current through the rear facet monitor PIN
7 - Analog Laser Drive Circuits
The circuit illustrated in Figure 7a
indirectly maintains constant laser optical output. The rear facet monitor
PIN diode receives light from one end of the laser chip while the other
end of the chip illuminates the optical fiber. While the light in the
fiber correlates to light in the monitor PIN diode, it never matches
exactly at all output and environmental conditions, an phenomenon called
shows a more advanced analog laser circuit, offering good to excellent
linearity at very high frequencies (GHz). The signal path of this circuit
only involves U2, Z1, C1, and the laser diode, D1. Amplifier U2 provides
input matching, gain and isolates the laser from outside conditions. The
block labeled Z1 can take on many functions. At a minimum, it interfaces
the output of the amplifier U2, usually 50 or 75Ohms, to the laser that
has an impedance ranging from 5 Ohms to 25 Ohms. As shown, sometimes the
laser package incorporates this impedance matching.
Laser Drive Circuits
Figure 8 illustrates
two common discrete component circuit configurations that function to
drive lasers for digital applications. However, a wide variety of highly
integrated ICs exist because of the high demand for digital laser drivers.
The discrete component circuit configurations illustrate the most commonly
used principles in commercially available laser driver ICs.
Figure 8 - Digital Laser Circuits
illustrates a simple circuit that is utilized at frequencies to several
hundred megahertz. "Digital data in" takes a relatively simple path. The
NAND gate, U2, buffers the signal and provides fast and consistent edges.
Potentiometer, R3, adjusts the amplitude of the laser's oncoming digital
signal, usually referred to as a modulation depth adjustment. Capacitor,
C2, block any DC component, allowing the AC component of the "digital data
in" to pass. Incidentally, nearly all digital laser drive circuits cannot
handle a DC component in the "digital data in" signal, meaning that the
"digital data in" signal must always have transitions present. Resistor,
R5, provides impedance matching into the laser, and feeds directly into
the cathode of the laser, D1. Inductor, L1, allows the AC component of the
"digital data in" signal to reach the laser, as well as a DC signal. The
rear facet monitor photodiode, D2, outputs a current proportional to the
laser output. The current out of D2 goes to a servo loop, ensuring that
the average optical output of D1 remains constant. U1 forms the heart of
the servo loop. Capacitor, C1, configures U1 as an integrator. The +input
of U1 remains at a positive voltage, VREF. The value of
VREF usually lies midway between ground and
Potentiometer, R4, adjusts the average optical
output power of the laser D1 by sinking a current out of the -input of U1.
This negative current causes the output of U1, referred to as
V2, to increase. As V2 increases, transistor Q1
turns on. This causes an increasing current to flow through both L1 and
D1. As the current through D1 increases, the average optical output of D1
increases, which causes the current from D2, the rear facet monitor
photodiode, to increase. This continues until the current out of D2
matches the current being sinked by potentiometer, R4. R4, usually
referred to as the "power adjust" in digital laser drive circuits, sets
the rear facet monitor photodiode current. The average optical output
power and the rear facet monitor photodiode current are nearly equal,
differing only by tracking error. Three components in the circuit, C2, L1,
and C1, function to limit the low-frequency, and thus limiting low data
rate operations. Normally, a digital laser driver circuit should handle
frequencies as low as 1/100th of the design data rate. Therefore, a laser
driver designed to handle a 622 Mb/s data rate must also handle
frequencies as low as 6.22 MHz.
The more complex
circuit shown in Figure 8b
allows very high, multi-gigabit speeds. With only the omission of L1, the
servo loop portion of the circuit matches the circuit in Figure 8a.
L1 is replaced in this circuit by Q4 a very fast , low capacitance
transistor. To not interfere with the modulation signal, Q4's collector
will appear as a current source. Potentiometer, R4, sets the rear facet
monitor photodiode current or average optical output power. The "digital
data in" signal first goes through the NAND gate, U2, as in the first
circuit. However, this circuit incorporates a NAND gate with the
differential outputs of U2 to drive a transistor-based differential
amplifier consisting of Q1 and Q2. Transistor Q3 forms a constant current
source. The potentiometer, R3, sets the current flowing in the collector
of Q3. The current flowing out of Q3 determines the amount of modulation
current that is switched to the laser in response to 1's and 0's. The
modulation current from the collector of Q3 oscillates between the +power
line (by Q1) and the laser, D1, (by Q2), as the outputs of U2 switch back
and forth. To avoid a circuit becoming slow, the digital laser circuit
must avoid saturation. Q1, Q2 and Q3 all operate in a linear mode in
circuit 8b allowing them to operate at very high speeds.
We have touched on the electrical and optical
characteristics of laser diodes. Other factors that are important are the
thermal and packaging characteristics. Laser diodes are available
pigtailed to fiber or mounted in active device mounts (ADMs). Lasers with
fiber pigtails require special handling precautions to prevent damage to
the fiber. See Handling
Fragile Optical Fibers and Fiber Pigtail Assemblies for more
Lasers are very sensitive to backreflection,
limiting their usefulness in the ADM
add/drop multiplexer configurations. Some recent lasers mounted in ADMs
incorporated a short length of single-mode
fiber that provides the interface to the fiber optic connector. This
technique not only enhances the launch stability, it also improves the