Fiber-Optic Sensors - Applied Optics Fundamentals and Device Applications - page 174

Splitter excess loss: −0.1 dB
Coupler-fiber connector loss (max): −1.0 dB
Fiber-combiner connector loss (max): −1.0 dB
Combiner excess loss (min): −0.1 dB
Combiner to detector: −1.5 dB
The total is 4.7 dB minus total fiber loss at 4-5 dB km −1 = 9.7 dB max, or
160 μW power at the detector, which even with the use of an Si p-n photodi-
ode, leaves a comfortable margin of power.
Several fibers are available commercially with 4-5/80 diameters, single-
mode operation at 850 nm, and optimum characteristics for fiber sensor
implementation. Although the various epoxy acrylates used as fiber jacket
coating are easily removed or chemically thinned accurately, a minimum
thickness can be specified which does not require removal for achieving the
desired mechanical coupling to transducer materials. Additionally, these
coatings provide minimal microbending losses and high survivability in
high-strain, high-environmental stress environments. The outstanding fea-
ture available in these fibers, however, is the polarization-preserving capa-
bility, such that the propagating light is maintained or “locked” into one
particular plan of polarization. Polarization preservation is extremely stable
under both radial and bending stresses and temperature variations, so that
superior fiber-sensing performance and sensitivity are afforded.
Diode laser problems that can result in a loss of sensor sensitivity include
amplitude noise, coherence length, phase noise due to small-frequency insta-
bilities, and optical feedback from the load. To avoid excess phase noise, path
lengths should be balanced to approximately 1 mm. By using amplitude
noise subtraction and ensuring optical feedback levels below 0.001%, 1 μrad
performance down to 10 Hz is achievable [1].
Several GaAlAs laser diodes are available that emit at approximately
830 nm a stable, fundamental transverse mode. These devices are charac-
terized by high-power outputs and by linear light output versus current
input. Recent advances in diode-to-fiber coupling permit ample power cou-
pling through a single-mode fiber pigtailed directly to the laser. Typical
threshold currents are 15-70 mA, with 1-5 mW/facet output at I th +25 mA,
maximum power of 10-15 mW per facet, and beam divergence half angles
of 10° × 30°. If the laser linewidth is very narrow and the frequency does
not change, the sensing and reference arms may differ in length by an
integer number of wavelengths without affecting the operation except
for aggravating the thermal drift problem. However, if the arms differ in
length and the light source frequency varies, the frequency shift will be
transformed into a differential phase variation and will contribute to the
noise from the detector. The phase variation Δϕ is Δ l /Δ f , where Δ l is the dif-
ference in the optical path length of the arms, and Δ f is the variation of the
light source frequency.