Te core of the fibre is only about
8–10 microns. Tis is the standard single-mode fibre used for long-range communication like telephony and cable TV. Te optical system also includes
a transmitter that converts an electric signal into light and sends it down the fibre. Te transmitter is a laser diode designed to emit light pulses at wavelengths around 1,550 nanometres (nm). Te attenuation of light in an optical fibre is mainly caused
Figure 4.35: The optical fibre is composed of an inner core, a cladding and an outer buffer.
by scattering resulting from minor imperfections in the glass. Tis attenuation is strongly dependent on the wavelength of the light. By choosing wavelengths corresponding to low attenuation (1,550 nm), it is possible to transmit optical signals over tens of kilometres with little loss. Typical losses for single- mode fibres are of the order of 0.2 decibels (dB) per kilometre. (dB is measured as 10 times the logarithm of the output power divided by the input power.)
4.4.3 Fibre Optic Hydrophones and Geophones
Te sensors used with fibre optics are hydrophones and three-component accelerometers. A fibre optic hydrophone is composed of a cylinder of a plastic-like material, where the fibre is coiled around the cylinder. When a pressure wave is passing the cylinder, the length of the coil will be slightly changed, and this displacement can be measured with high resolution as a shift in reflection time or phase delay between two identical Bragg gratings situated on opposite sides of the hydrophone. Te wavelength of the incident light must be equal to the Bragg wavelength. In this way the hydrophone converts pressure
The Bragg Grating
A fibre Bragg grating (FBG) acts as a light reflector with maximum reflection at a very specific wavelength, while other wavelengths are transmitted. Te Bragg grating is ‘written’ into a short segment (about 1 cm) of the fibre by using an intensive ultraviolet laser that alters the refractive index of the fibre core. Te core is photosensitive with exposure to UV light, and its change in refractive index is a function of the intensity and duration of the exposure. Te grating will typically have a sinusoidal refractive
index variation. Te reflected wavelength λ, called the Bragg wavelength, is defined by the relationship λ=2nΛ, where n=(n3
+n2 )/2 is the average refractive index of the
grating in the fibre core and Λ is the grating period. Using an average refractive index of 1.55 and grating period of 500 nm, the grating reflects at λ=1,550 nm. Te reflection strength Rλ (peak reflectivity) is determined by the grating length L=NΛ, where N is the number of periodic variations, and the grating strength n3
-n2 . 175 Te refractive index is a measure for how much the speed
of light is reduced inside the fibre relative to a vacuum. For example, the glass core has a refractive index of 1.5, which means that in the core, light travels at 1/1.5 = 0.67 times the speed of light in a vacuum. Tis is fast – 200,000,000 metres per second. Note: Te refractive index of water is 1.33, corresponding
to a velocity of 225,000,000 m/s. If particles in water are accelerated to a velocity above this they will radiate, an effect called Cerenkov radiation. Cerenkov received the Nobel prize in Physics in 1958 for describing this intensive ‘blue glow’ observed in nuclear reactors. Particles travelling faster than the speed in vacuum are referred to as tachyons – hypothetical particles introduced by Sommerfeld.
Figure 4.37: Sketch of Bragg grated fibre. 0.8 0.6 OH absorption 0.4 Rayleigh scattering 0.2 IR absorption 1100 1200 1300 1400 1500 Wavelength (nm) Figure 4.36: The attenuation in the fibre is a minimum of about 1,550 nm.
waves into a change in the fibre length. Te change in the length is proportional to the amplitude of the seismic wave. Te fibre optic accelerometer similarly modulates the fibre
length as the sensor is exposed to acceleration. To achieve a directional measurement, a special design utilising two half-spheres at each side of a rod ensures that the coil length is changed only for a directional signal. If the seismic wave hits the optic accelerometer perpendicular to the rod, the coil length will not change. When the wave hits along the rod, the coil length will change, and a non-zero seismic signal is measured (Figure 4.37). An advantage of fibres is that a number of sensors can
be multiplexed on one fibre. In the Optowave system this is implemented through a combination of time division multiplexing and wavelength division multiplexing. All sensors within one 4C seismic station are multiplexed using time
1600 1700
Lasse Amundsen Loss (dB/km)
Lasse Amundsen and Martin Landrø
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