Page 614 - Introduction to Information Optics

598 10. Sensing with Optics

this distributed fiber sensor is

L (10,24)
2ndf 2nd A'
where 6f and <5/l are the effective frequency resolution and wavelength
resolution, respectively.
For the semiconductor laser, the frequency tuning range can be 10 GHz
changing the driving current. Substituting this number, n = 1.5, and c =
8
3 x 10 m/s into Eq. (10.23), we obtain Ax = 1 cm. Similarly, if the effective
frequency resolution can be 1 kHz (this is possible with current diode laser
technology), then, based on Eq. (10.24), the sensing range L can be as long as
10 km.
In OFDR, coherent detection is needed. Since conventional single mode
fiber cannot hold the polarization state of the light propagating in the fiber,
polarization-insensitive detection is expected. Figure 10.19 shows polarization-
insensitive detection using the polarization-diversity receiver. Since the refer-
ence signal is equally divided between two detectors, squaring and summing
the two photocurrents produces a signal independent of the unknown polariz-
ation angle 9. If the returning test signal is elliptically polarized, the only
difference will be a phase shift in the fringe pattern on one of the detectors. This
phase shift is not important since the envelope detector eliminates any phase
information before the summing process. Therefore, the output signal is

Polareier

Frequency
Modulation

Output

Polarization Diversity Receiver
Fig. 10.19. An experimental setup for the implementation of a polarization-diversity receiver.