FIBEROPTIC SENSOR TECHNOLOGY HANDBOOK

the aingle-sideband phaae noiae intercepts the 120 dB
level at about 1 kHz. The acoustooptic modulator drive
requirement is approximately +33 dBm. Asauming an output
of +10 dBm from the oscillator the amplifier shown
must provide 23 dBm of gain.
Fig. 4.21
cy noise. Phase trackers and the associated circuits
are not required. An interferometer employing heterodyne
detection is ahown in Fig. 4.21. The aystem difr
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A interferornetric fiberoptic senaor employing
heterodyne detection.
fers from the usual heterodyne syatem in that uae is
made of two Bragg cells. The firat cell also serves
as a 3-dB coupler. By placing a star coupler (or plaited
3-dB fiber coupler) at points A and B it should be
possible for a single diode laser and one pair of Bragg
cells to serve as the optical source for several dozen
aenaors. For the Bragg frequencies indicated in Fig.
4.21, bulk Bragg cells are required. In thia caae GRIN
rods must be used to focus the light in and out of the
fiber. If surface acoustic wave (SAW) Bragg cella are
employed, they must be operated at approximately 600
klllz (with a difference of 100 kHz). If the lowest signal
frequency of interest is fs, the oscillator must
exhibit phase noise less than -120 dB/Hz at an offset
of fs from the carrier frequency. The plot of single
sideband phase noise versus displacement from the carrier
frequency for two oscillators is shown in Fig.
4.22. The curve that corresponds to 600 MHz shows that
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z -110
n
z -120
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flj -130
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m -140
y -150
60-100 MHz
a ~ -160
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101 102
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Fig. 4.22 Typical single-sideband phaae noise measured
relative to the carrier in a 1 Hz bandwidth
in a fiberoptic interferornetric
heterodyne sensor using fixed-frequency
crystal osallators as shown in Fig. 4.12.
4-11
The maximum optical power that a channel waveguide
will handle is about 120 uW. One pair of Bragg
modulators configured as shown in Fig 4.21 could supply
several sensors. However, with only 120 pW into the
first Bragg cell it would not be possible to provide
sufficient optical intensity to the detectors to insure
quantum-limited operation for more than four sensors.
Thua, for the operation of several dozen senaors from a
aingle laser and one pair of bulk Bragg cells, heterodyne
detection must be uaed.
The difference frequency constitutes a heterodyne
frequency of 100 kHz for the heterodyne detection
configuration shown in Fig. 4.21. This permits the use
of low-frequency, low-noise electronic circuits, such
as low-noise amplifiers and FM discriminators. Since
heterodyne detection is relatively insensitive to optical
intensity fluctuation and low frequency noise,
phase tracker circuits are not required. As in the
case of homodyne detection, there may be a need for
polarization-preserving fiber in order to prevent signal
loss. Also, Bragg cells that are highly atable relative
to each other, are required. Finally, in heterodyne
detection, aa in homodyne detection, optical feedback
into the diode laser greatly increases the laaer
noise. Thus it is necessary to employ the aame precautions
as were indicated above for homodyne detection.
A number of other detection schemes have been
suggested and are currently being considered. These
include simple homodyning employing a 3 x 3 coupler in
place of the input coupler (see Ref. 5 in Subsection
4.2.8). It can be ahown that this resulta in an operating
condition very close to quadrature. Synthetic
heterodyne operation is another. A high frequency dither
is employed on the phase stretcher. The output of
such an interferometer can be shown to exhibit the characteristics
of a heterodyne system.
In view of the significant effort being applied
to the detection problem further improvements can
be expected; however, such effort la also an indication
that the optimal detection acheme haa not been achieved.
Numerous trade-offs are required. Theae include frequency
range, sensitivity, dynamic range, cost, and complexity
of the detection circuitry.
4.2.8 References
1.
2.
3.
4.
5.
A. Dandridge, A. Tveten, R. Miles, D. Jackson, and
T. Giallorenzi, “Single-Mode Diode Laser Phaae
Noise”, Appl. Phya. Lett. — 38, 77 (1981).
A. Dandridge and A. Tveten, “Noise Reduction in
Fiber-Optic Interferometer Systems”, Appl. Opt.
20-, 2337 (1981).
R. Miles, A. Dandridge, A. Tveten, H. Taylor, and
T. Giallorenzi, “Feedback-Induced Line Broadening
in Cw Channel-Substrate Planar Laser Diodes”, Appl.
Physica. Lett. — 37, 990 (1980).
K. Fritsch and G. Adamovsky, “Simple Circuit for
Feedback Stabilization of a Singlemode Optical
Fiber Interferometer”, Rev. Sci. Instrum. ~, 996
(1981).
S. Sheem, “Fiber-Optic Gyroscope with [3x3] Directional
Coupler”, Appl. Phys. Lett. 37, 869 (1980). —