FIBEROPTIC SENSOR TECHNOLOGY HANDBOOK

(a)
&
A
FREE RUNNING
Av = 5MHZ
(c)
(b)
11
I
u
0.2 0 0.2 0.2 0 0.2
A
A
SATELLITE MODES
Av = 0.02GHz Av= .12GHz
0.04% 0.06%
FEEDBACK FEEDBACK
(
606
A
MULTIMODES
Av = 5GHZ
1.5%
FEEDBACK
Fig. 4.19 Influence of optical feedback on the modal
output of a Hitachi HLP 1400 diode laser.
Adapted from R. Miles et al., Appl. Phys. Lett. ~,
990 (1980).
Spectrum (d) shows the effect of 1.5% feedback.
Multimode laser operation results. In order to
eliminate this effect, the back reflections into the
laser must be maintained below approximately 0.06%.
This is achievable with care. To accomplish this, it
is necessary to assure that reflections from the end
of the fiber are avoided. This requires the use of
index-matching liquid between the laser and the fiber
and furthermore the end of the fiber must be cut at a
slight angle. All splices and couplers have to be
carefully fabricated in order to reduce insertion loss.
In the discussion of connections, it was shown that
fiber misalignment would lead to insertion loss. These
same misalignments will also lead to undesirable reflections.
4.2.6 Phase-Locked Loop Operation
The circuitry required to provide and insure
quadrature operation in the presence of low frequency
drift is shown schematically in Fig. 4.20 (see Ref. 4
Fig. 4.20
TWO STAGE INTEGRATOR,
UNBIASED
AMPLIFIER.
PHOTODIODES
CIRCUIT
RESET
KO
M
/)+“+ \
/ I
TO PZT PHASE SHIFTER
J L 1
-
w /
AMPLIFIER, HIGH PASS
/
..,FILTER. CUT OFFAT
KD
DIFFERENTIAL
LOW FREQUENCY
AMPLIFIER FOR
LIMIT OF SIGNAL RANGE
COMMON MODE
REJECTION
A phase-locked loop
Cuit.
t
BANDPASS
OUTPUT
homodyne detection cirin
Subsection 4.2.8). TWO photodiodea are shown on the
left. The photodiodes are operated in an unbiased condition
in order to eliminate dark current noise. Their
outputs are combined in a differential amplifier that
provides common-mode rejection as well as amplification.
This is followed by two stages of integration
that provide additional amplification. These two integrator-amplifiers
pass all signals from DC up to the
higheat frequency of interest. The output of the two
stage integrator-amplifier is applied to a phase shifter
located in the reference arm of. the interferometer.
The phase shifter consists of either a lead zirconatelead
titanate (PZT) cylinder around which the fiber in
the reference arm is rather tightly wound or a section
of polyvinylidene-floride (PVDF)-jacketed fiber. Both
PZT and PVDF are piezoelectric materials. The output
of the integrator-amplifier is just equal to the low
frequency noise and the signal of interest. The effect
then is to produce a phase ahift in the reference arm
equal to that in the sensing anm, causing the interferometer
to remain balanced, i.e., to phase-lock the system.
If the phase were exactly locked there would be
no output signal from the interferometer. However,
there must be an error signal at the photodetectors in
order to have a feedback signal. The amplification in
the feedback circuit thus increaaes the error signal
from the interferometer back up to the level of the
signal being detected. If the system is initially at
a bias (operating or quiescent) point away from quadrature
there is insuffient output from the interferometer
for the compensation circuit and the system till tend
to drift toward an increasing error signal and therefore
toward quadrature.
The signal out of the compensating circuit is
alao fed through a high-pass filter that has its low
frequency limit set at the lowest frequency of interest.
Therefore, the resulting output is a band of frequencies
corresponding to the frequency range of interest.
This constitutes the output of the interferometric sensor.
Operational amplifiers (OPAMPS) are used in
the feedback circuit and combined metal oxide semiconductor
(CMOS) components in the reset circuit. The
levels of voltage that can be applied by these circuits
to the phase shifter are the order of + 10 volts. On
the other hand, the range that the ph~se shifter can
accommodate ia hundreds or thousands of volts. Furthermore,
in many cases the amplitude of the phase drift
resulting from temperatuze or pressure changes is much
larger than the phase shift that would be generated by
applying ~ 10 volts to the phase shifter. Thus, it is
necesaary to keep track of how much voltage has been
applied to the phase shifter and if the limit of the
circuit begins to be reached it is necessary to rapidly
reset the circuits back to the initial condition from
which point it can start over. This is the purpose of
the reset circuit indicated in Fig. 4.20. The phaae
change associated with a large amplitude slow drift is
compensated by a number of saw toothed-like amall amplitude
phase changes. Care must be taken to minimize the
noise introduced during the reset process.
The upper frequency at which a measurement
can be made is limited by the resonant frequency in the
phase shifter itself. Phase-locked loop homodyne detection
is particularly useful for frequencies below
10 kHz. On the other hand, for casea where it is desired
to make measurements at higher frequencies, say
from a few kHz up to nearly a megahertz, heterodyne
detection may be desirable.
4.2.7 Heterodyne Detection
Heterodyne detection is relatively insensi -
tive to optical intensity fluctuations and low frequen-
4-10