FIBEROPTIC SENSOR TECHNOLOGY HANDBOOK

cal leads in the vicinity of the vapor. The chopper
is operated at a frequency at which the acoustic aenaitivity
of the absorportion cell is high.
The light from the excitation laser excites
a characteristic absorption line in the molecular spectrum
of the vapor being detected. When the excited
molecules return to equilibrium the temperature of the
vapor Is increased resulting in a preaaure increase.
By chopping (interrupting) the light at a given rate
(frequency), the resultant fluctuation in preasure Is
detected as aound of the same frequency. Even more
desirable 1S to make use of a variable frequency laaer
aa the excitation source. The absorption coefficients
vs. frequency of ambient atmosphere and of methane
whose concentration 1S five times ambient are shown in
Figs. 5.17 and 5.18.
measure strains, electric fields, temperature, acceleration,
and rate of rotation. They differ from the
acoustic and magnetic senaors discussed above that rely
on specialized jacketa and utilize a Mach-Zehnder interferometer.
The electric current and trace vapor sensors
are secondary devices relying on the measurement
of the magnetic field or the temperature in the former
case and the aound associated with the absorption of
light in the latter case.
5.1.7 References
1.
2.
3.
D. Jackson, A. Dandridge, and S. Sheem, Opt. Lett.
~, 139 (1980).
B. Budiansky, D. Drucker, G. Rino, and J. Rice,
APP1. Opt. ~, 4085 (1979).
G. Hocker, Opt. Soc. Am. ~, 320 (1979).
lo”~
WAVELENGTH
Fig. 5.17 The absorption coefficient of ambient atmosphere
aa a function of wavelength.
Provide by D. Leslie, U.S. Naval Reaearch Laboratory.
4.
5.
6.
7.
8.
9.
10.
11.
N. Lagakos and J. Bucaro, Appl. Opt. — 20, 2716
(1981).
N. Lagakos, T. Hickman, J. Cole and J. Bucaro,
Opt. Lett. ~, 443 (1981).
N. Lagakos, private communication.
A. Yariv and H. Winsor, Opt. Lett. ~, 87 (1980).
J. Jarzynski, J. Cole, J. Bucaro and C. Davia,
Appl. Opt. ~, 3746 (1980).
J. Cole, N. Lagakos, J. Jarzynski, and J. Bucaro,
Opt. Lett. ~, 216 (1981).
K. Koo and G. Sigel, Technical Digest, Optical Fiber
Communication Meeting, Phoenix, Arizona (1982)
p. 72.
A. Dandridge, A. Tveten, and T. Giallorenzi, Electron.
Lett. ~, 523 (1981).
ABSORBERS
TYPE (TORR)
H20 14.260
C02 0,251
03 2.3x lC”5
~’”i
N20 2.1 X1 O-4
co 5.7 x10-5 I
CH4 12’1 0-31 1II I
159627
02
1~ II
WAVELENGTH
22.9°C
JJ
760 TORR
Fig. 5.18 The absorption coefficient of methane gas
as a function of wavelength for a concentration
of 7.6 x 10-3 torr (5 x ambient).
Provide by D. Leslie, U.S. Naval Research Laboratory.
12.
5.2
D. Lealie, G. Trusty, A. Dandridge and T. Giallorenzi,
Electron. Lett. ~, 581 (1981).
5.2.1 General
INTENSITY MODULATED FIBEROPTIC SENSORS
The second type of fiberoptic sensor to be
diacusaed is referred to as an intensity-modulated aensor.
Ita basic configuration is sketched in block diagram
form in Fig. 5-19. The output lightwave from an
OPTICAL
SOURCE
+ l~”T
SENSOR
OPTICAL
DETECTOR
.
‘OUT
5.1.6 Summary
The acoustic and magnetic field sensors described
above represent primary measurement devices.
Other primary measurement devices include sensors to
TIME
TIME
TIME
Fig. 5.19 A generalized intensity-type fiberoptic
senaing system.
5-7