Page 108 - Optical Communications Essentials

Light Sources and Transmitters

98 Chapter Six

6.3.3. Tunable lasers
In many networks there is a need to tune the wavelength of a laser transmitter.
This is especially important in a multiwavelength network that uses many laser
transmitters operating on a grid of closely spaced wavelengths on the same
fiber. Since each laser needs to emit at a precise wavelength (within a fraction
of a nanometer), the ability to tune the laser precisely is essential. A number of
different technologies have been considered for making tunable lasers, each
having certain advantages and limitations with respect to tuning range, tuning
speed, power output, and control complexity. Table 6.3 lists some example tech-
nologies and typical performance parameter values. Tuning is achieved by
changing the temperature of the device (since the wavelength changes by
approximately 0.1nm/°C), by altering the injection current into the gain region
(yielding a wavelength change of 0.01 to 0.04nm/mA), or by using a voltage
change to vary the orientation of a MEMS (microelectromechanical system)
mirror to change the length of a lasing cavity.
The DFB laser has been around for a long time and is very reliable, but its
tuning time is slow. The distributed Bragg reflector (DBR) laser has a simple
structure, a high output power, and a modest tuning range. The GCSR (grating-
assisted codirectional coupler with sampled reflector) laser and the SG DBR
(sampled grating DBR) laser have fairly complex structures, but both have a
large tuning range of 40nm. The VMPS (VCSEL MEMS pump laser solid-
state optical amplifier) offers simple tuning over a large range, but the hybrid
packaging structure is complex. For further details on these lasers and other
structures, see the referenced article by R.-C. Yu.

6.3.4. Vertical cavity surface emitting laser
A vertical cavity surface emitting laser (VCSEL) consists of stacks of up to 30
thin mirroring layers placed on both sides of a semiconductor wafer to form a

TABLE 6.3. Tunable Laser Technologies and Typical Parameters (T Temperature;
C Current; V Voltage)
Tuning Tuning Output
Technology Tuning range, nm time, ms power, mW Advantages Limits
DFB T 3–9 10,000 2 Proven Slow;
reliability small range
DBR C 8–10 10 5 Simple device; Intermediate
high power out tuning range
GCSR C 40 10 2 Wide tuning Complex
range control
SG DBR C 40 10 2 Wide tuning Complex
range control
VMPS V 32 10 10–20 Wide tuning; Complex hybrid
high output packaging

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