Biomedical Engineering Reference
In-Depth Information
4.17 LaserDiodeFrequencyStabilityConsiderations
Experimental measurements of the output spectrum [18] produced by single-
mode semiconductor lasers resulted in observations of instabilities includ-
ing mode jumping, phase noise, mode partition noise, line broadening, and
frequency shifting. Unstable operation in fiber-optic interferometric sens-
ing systems has also been observed. Although stability problems exist for
certain applications, semiconductor lasers produce excellent performance
in communications and coherent transmission under carefully controlled
operating conditions.
In communications, the signal bandwidth can and usually does exceed the
laser linewidth [19]. For example, a modern communications system using a
directly modulated laser can support a signal bandwidth of 2-3 GHz. This
signal bandwidth is much wider than the laser linewidth (a laser linewidth of
10 kHz was reported through the use of a fiber-optic ring resonator). Because
of this wide bandwidth, some of the problems associated with frequency
instability are not critical to operation in noncoherent communications sys-
tems. In sensors and interferometers, however, the signal bandwidth may
be very small in comparison to the laser linewidth. For example, if the sen-
sor is designed to monitor ambient room temperature or perhaps the mag-
netic profile of a slow-moving object, the signal sensed may be in the near dc
range, while the linewidth of the laser used in the sensor itself may be in the
1 GHz range. If the laser linewidth suddenly broadens, or changes frequency
during operation, a resulting amplitude or phase fluctuation may be pro-
duced by the sensor, thus degrading performance due to the “random noise.”
The need for frequency stability and low noise in the light source for sensor
applications is extremely important and may even impose more stringent
requirements on laser operation than can be achieved with most devices.
Some laser structures, such as short cavity, external cavity, DFB, and the
rare-earth-doped heterostructure, offer various degrees of stability; but
in general, the ideal stabilized laser diode source is still in development.
Various stabilization techniques such as thermoelectric control of the laser
structure, feedback control of the laser injection current, the use of the exter-
nal fiber-optic ring resonator, etc., are an “after the fact” approach to solv-
ing the problem of laser instability, since these methods do not address the
source of the problem.
This section is concerned with frequency observations and stabilization
techniques applicable to single-mode lasers. Included in the discussions are
basic laser operation, the effects of high-frequency modulation and modula-
tion depth on laser operation and received noise, the effects of light reflec-
tions back into the laser cavity, stability requirements for an interferometer
applications, and laser and linewidth stabilization through the use of fiber-
optic resonators and the rare-earth-doped HRO laser.
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