Page 632 - Introduction to Information Optics

616 10. Sensing with Optics

(a) Calculate the coefficient of finesse F.
(b) Draw the output light intensity as a function phase shift 0.
(c) Based on the curve arrived at in part (b), estimate the maximum
relative percentage change in output light intensity if the phase, 0,
shifts 0.1 radians.
10.7 Assume that the black-body radiation method is used to sense the
temperature of the sun's surface. If the maximum light intensity happens
at A = 550 nm, estimate the temperature of the sun's surface based on
black-body radiation.
10.8 Explain what is fluorescent light.
10.9 An optical time-domain reflectometer (OTDR) is used to detect the
fatigue location in optics networks. Assume that the refractive index of
the fiber is n = 1.5 and a reflection peak is detected at a time t — 10 jus
after launching the pulse. Where is the location of the fatigue relative to
the incident end of the fiber?
10.10 In Exercise (10.9), if the pulse width of OTDR is T - 5 ns, what is the
best spatial resolution of this OTDR?
10.11 Distributed temperature measurement is realized by using Raman scat-
tering. Assume that the pump wavelength is A = 514nm, the Stokes and
anti-Stokes wavelength shift in a particular location in terms of wave
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number is +400cm" , and the intensity ratio from the anti-Stokes line
to the Stokes line is 0.167. Calculate the temperature of that location.
10.12 A tunable diode laser is used as the light source of an optical frequency-
domain reflectometer. Assume that the total tuning range of the diode
laser is A/I = 1 nm, the frequency stability of the laser is <5v =
300kHz, and the refractive index of the fiber is n — 1.5. Estimate the
maximum sensing range and the spatial resolution of this distributed
fiber optic sensor.
10.13 A fiber Bragg grating is written by UV light. Assume that the maximum
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refractive index modulation of the grating is An = 3 x 10~ , the length
of the grating is L — 5 mm, the operating wavelength of the grating is
A — 1550nm, the effective refractive index of the fiber is n = 1.5, and
60% of light energy is inside the couple.
(a) What is the grating period?
(b) What is the maximum diffraction efficiency of this grating?
(c) What is the FWHM bandwidth of the grating?
10.14 A fiber Bragg grating is used to sense temperature change. Assume that
the wavelength shift induced by temperature change can be written as
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A A = / • a • AT; where a is the total thermocoefficient given by a = 10 ~ /°C
and the operating wavelength A = 1550nm. To achieve 0.1 °C tempera-
ture resolution, what is the required wavelength resolution of the wave-
length detector?