Fiber-Optic Sensors - Applied Optics Fundamentals and Device Applications - page 185

Line broadening of up to 40 GHz occurs when the laser is brought from
a condition of below threshold to threshold lasing operation. At one point
in the threshold, the gain is a minimum and the pulsations are strongly
damped, decaying to zero. The pulsations occur when the laser is operating
above threshold and the cavity exhibits gain [42]. Without external feedback,
as the carrier number increases, the gain increases. This increase in gain
results in an increase in light intensity, thereby increasing stimulated emis-
sion. The increase in stimulated emission in turn decreases the carrier num-
ber. The effect leads to a relaxation oscillation. The relaxation oscillation is
usually damped by other effects.
Instability arises because the laser seeks a steady-state operation that max-
imizes feedback. For example, the maximum feedback state occurs when
the stimulated emission is in phase with the reflected field. An increase in
the number of carriers occurs within the cavity, modulating the intensity
and the phase difference, and decreasing the intensity. Eventually the fluc-
tuations in carriers and phase become greater than the restoring forces and
instability occurs. In steady state with moderate feedback, the laser is phase
locked to the reflected field.
4.17.8 Laser Frequency Stability Considerations in Fiber-Optic Sensors
Fiber-optic sensing is a technology area in which applications continue to
emerge at a rapid pace. The sensing mechanisms in a fiber sensor can include
one or all of the following modulation techniques: intensity, phase, polariza-
tion, or frequency. The sensitivity of fiber sensors makes them attractive in
many applications where electronic sensors are inadequate. The interfero-
metric class of fiber sensors offers the highest sensitivity and dynamic range.
The parameters to be sensed cause a change in the phase of the interferom-
eter, eliminating the need for a separate modulator [43].
The major difficulty in implementing the interferometric fiber sensor
involves the requirement for a coherent and stable light source. Accuracy
of sensing is directly impacted by the laser source and any mode jumping,
phase noise, or mode partition noise that occurs. Laser structures with the
most stable operation possible are chosen for the fiber sensor system. For
applications in which size and power consumption are not limited, a gas
laser should be considered since the gain curve of the total linewidth is typi-
cally on the order of 1 GHz [44]. With the gas laser, it is relatively easy to
define the wavelength to parts in a million; in some cases, the HeNe laser
can be locked to the center-gain curve with the stability on the order of parts
in 100 million [45]. This parameter is very important because changes in the
operating frequency result in time delays due to fiber dispersion, even in the
low dispersion operating regions of the single-mode fiber.
Along with the effects of dispersion and frequency instability, the perfor-
mance of the sensing system can be affected by the presence of phase noise.
Phase noise occurs due to imperfections in the laser cavity, laser drive circuit