Types of Optical Fibers
A fiber optic sensor is a sensor that uses optical fiber either as the sensing element ("intrinsic sensors"), or as a means of relaying signals from a remote sensor to the electronics that process the signals ("extrinsic sensors"). Fibers have many uses in remote sensing. Depending on the application, fiber may be used because of its small size, or the fact that no electrical power is needed at the remote location, or because many sensors can be multiplexed along the length of a fiber by using different wavelengths of light for each sensor, or by sensing the time delay as light passes along the fiber through each sensor. Time delay can be determined using a device such as an optical time-domain reflectometer.
Intrinsic sensors
Optical fibers can be used as sensors to measure strain, temperature, pressure and other quantities by modifying a fiber so that the quantity to be measured modulates the intensity, phase, polarization, wavelength or transit time of light in the fiber. Sensors that vary the intensity of light are the simplest, since only a simple source and detector are required. A particularly useful feature of intrinsic fiber optic sensors is that they can, if required, provide distributed sensing over very large distances.
Temperature can be measured by using a fiber that has evanescent loss that varies with temperature, or by analyzing the Raman scattering of the optical fiber. Electrical voltage can be sensed by nonlinear optical effects in specially-doped fiber, which alter the polarization of light as a function of voltage or electric field. Angle measurement sensors can be based on the Sagnac effect.
Optical fibers are used as hydrophones for seismic and sonar applications. Hydrophone systems with more than one hundred sensors per fiber cable have been developed. Hydrophone sensor systems are used by the oil industry as well as a few countries' navies. Both bottom-mounted hydrophone arrays and towed streamer systems are in use. The German company Sennheiser developed a laser microphone for use with optical fibers.
A fiber optic microphone and fiber-optic based headphone are useful in areas with strong electrical or magnetic fields, such as communication amongst the team of people working on a patient inside a magnetic resonance imaging (MRI) machine during MRI-guided surgery.
Optical fiber sensors for temperature and pressure have been developed for downhole measurement in oil wells. The fiber optic sensor is well suited for this environment as it functions at temperatures too high for semiconductor sensors (distributed temperature sensing).
Optical fibers can be made into interferometric sensors such as fiber optic gyroscopes, which are used in the Boeing 767 and in some car models (for navigation purposes). They are also used to make hydrogen sensors.
Fiber-optic sensors have been developed to measure co-located temperature and strain simultaneously with very high accuracy using fiber Bragg gratings. This is particularly useful when acquiring information from small complex structures. Brillouin scattering effects can be used to detect strain and temperature over larger distances (20–30 kilometers).
Other examples
A fiber-optic AC/DC voltage sensor in the middle and high voltage range (100–2000 V) can be created by inducing measurable amounts of Kerr nonlinearity in single mode optical fiber by exposing a calculated length of fiber to the external electric field. The measurement technique is based on polarimetric detection and high accuracy is achieved in a hostile industrial environment.
High frequency (5 MHz–1 GHz) electromagnetic fields can be detected by induced nonlinear effects in fiber with a suitable structure. The fiber used is designed such that the Faraday and Kerr effects cause considerable phase change in the presence of the external field. With appropriate sensor design, this type of fiber can be used to measure different electrical and magnetic quantities and different internal parameters of fiber material.
Electrical power can be measured in a fiber by using a structured bulk fiber ampere sensor coupled with proper signal processing in a polarimetric detection scheme. Experiments have been carried out in support of the technique.
Fiber-optic sensors are used in electrical switchgear to transmit light from an electrical arc flash to a digital protective relay to enable fast tripping of a breaker to reduce the energy in the arc blast.
Extrinsic sensors
Extrinsic fiber optic sensors use an optical fiber cable, normally a multimode one, to transmit modulated light from either a non-fiber optical sensor, or an electronic sensor connected to an optical transmitter. A major benefit of extrinsic sensors is their ability to reach places which are otherwise inaccessible. An example is the measurement of temperature inside aircraft jet engines by using a fiber to transmit radiation into a radiation pyrometer located outside the engine. Extrinsic sensors can also be used in the same way to measure the internal temperature of electrical transformers, where the extreme electromagnetic fields present make other measurement techniques impossible.
Extrinsic fiber optic sensors provide excellent protection of measurement signals against noise corruption. Unfortunately, many conventional sensors produce electrical output which must be converted into an optical signal for use with fiber. For example, in the case of a platinum resistance thermometer, the temperature changes are translated into resistance changes. The PRT must therefore have an electrical power supply. The modulated voltage level at the output of the PRT can then be injected into the optical fiber via the usual type of transmitter. This complicates the measurement process and means that low-voltage power cables must be routed to the transducer.
Extrinsic sensors are used to measure vibration, rotation, displacement, velocity, acceleration, torque, and twisting.
Intrinsic (all-fiber) fiber optic sensors
Intrinsic fiber optic sensors rely on the light beam propagating through the optical fiber being modulated by the environmental effect either directly or through environmentally induced optical path length changes in the fiber itself.
In this case an optical fiber leads up to a ‘‘black box’’ that impresses information onto the light beam in response to an environmental effect. The information could be impressed in terms of intensity, phase, frequency, polarization, spectral content, or other methods. An optical fiber then carries the light with the environmentally impressed information back to an optical and=or electronic processor.
In some cases the input optical fiber also acts as the output fiber. The intrinsic or all-fiber sensor shown in image above uses an optical fiber to carry the light beam, and the environmental effect impresses information onto the light beam while it is in the fiber. Each of these classes of fibers in turn has many subclasses with, in some cases, sub-subclasses that consist of large numbers of fiber sensors.
Extrinsic (hybrid) fiber optic sensors
Fiber optic sensors are often loosely grouped into two basic classes referred to as extrinsic, or hybrid, fiber optic sensors and intrinsic, or all-fiber, sensors. The below image illustrates the case of an extrinsic, or hybrid, fiber optic sensor.
Extrinsic fiber optic sensors consist of optical fibers that lead up to and out of a ‘‘black box’’ that modulates the light beam passing through it in response to an environmental effect.
Extrinsic Fabry Perot Interferometer (EFPI)
EFPI sensors generally work as follows: Two fibers are inserted into a hollow core material. One of the fibers is connected to an instrumentation system. Light from the instrumentation system is sent down the length of the fiber, and some of this light reflects off of the end of the fiber (R1). The rest of the light continues through the gap, and some of the that light reflects off of the front face of the second fiber (R2). R1 and R2 travel back down the length of the fiber to the instrumentation system, forming an interference pattern. As the gap between the two fibers change, the interference pattern changes, as well. If the hollow core material expands or contracts, the gap size will change, and thus can be measured by the instrumentation system. Thus anything that can change the length of the hollow core material can be measured.
Interference:
– The diagrams below illustrate the effect of light interference. When R1 and R2 (see previous slide) are in phase, the two signals will constructively interfere, producing a signal with a larger amplitude, as shown in Figure 1a. When R1 and R2 are out of phase, they will destructively interfere, making the combined signal have a smaller amplitude. This is shown in Figure 1b.
– Principles of interference are classically studied with an interferometer, such as the Michelson Interferometer of Figure 2.
