Fiber-Optic Sensors - Applied Optics Fundamentals and Device Applications - page 186

noise, and resonance within the laser structure itself [46]. Some control of
excess phase noise is possible through implementation of a feedback control
system in the laser drive. Tight control of the laser drive current is necessary
because the laser resonance is a function of the drive current. Typically, a
change in drive current of 1 mA will result in a several gigahertz change
in lasing frequency. Other enhancements such as modification of the laser
emission linewidth through the use of an external cavity and the use of ther-
moelectric cooling of the laser body provide only minimal improvements.
Laser source stability improvements are necessary for some of the emerging
applications in fiber sensing. Further developments are also necessary before
laser manufacturers offer the advanced laser structures in the wide range of
needed operating frequencies; therefore, external stabilization are employed
as an interim solution. One such approach to achieving stable operation of
the laser source involves the use of an external electronic feedback circuit
and the fiber ring resonator.

4.17.9 Achieving Laser Stability through External Control

The external cavity structure was developed for reduced linewidth and
improved stability, but these structures are still susceptible to instability
due to thermal and mechanical influences on the optical feedback and laser
operation [47]. These temperature stability and mechanical effects are a direct
result of the external structures being spread over a wider surface area, which
is more vulnerable to external influence. The system shown in Figure 4.22 was

Diff erential
amp (1)

Directi onal
coupler

Peak
hold

Integrator

Detector
(1)

PZT plate
Mirror
No filter
Beam
splitter
Lens

Forr (1)

Current
supply

LD

Inversion
amp

V ref

B uffer

Dir ectional
co upler

Monitor

Phase
modulator
Oscillator

~

Optic al
fiber

Inversion
amp

Direction al
coupler

Grin

Fa raday
isolator

Monitor Buffer

Temperat ure
controller

Forr (2)

Detector
(2)

V ref

Filter
differential
amp (2)

FIGURE 4.22
Schematic configuration of experimental arrangement for the stabilization of spectral line-
width and oscillation frequency of the external-cavity laser diode.