Introduction Learning Objectives On completion of this class you will be able to 1. Define fiber sensor 2. List the different types fiber sensors 3. Mech-Zender Fiber optic interferometer Fiber optic sensor 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 Material prepared by: < Physics faculty > Topic No: < 06 > Page 1 of 9
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 distances of up to one meter. Temperature can be measured by using a fiber that has evanescent loss that varies with temperature. 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. 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). Material prepared by: < Physics faculty > Topic No: < 06 > Page 2 of 9
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 microsensors. Fiber-optic sensors have been developed to measure co-located temperature and strain simultaneously with very high accuracy. This is particularly useful when acquiring information from small complex structures. Apply pressure Source Detector Transmitted intensity changes with pressure Microbend sensor 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. [3] 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. [4] With appropriate sensor design, Material prepared by: < Physics faculty > Topic No: < 06 > Page 3 of 9
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. 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 lowvoltage power cables must be routed to the transducer. Material prepared by: < Physics faculty > Topic No: < 06 > Page 4 of 9
Source Detector Extrinsic sensors are used to measure vibration, rotation, displacement, velocity, acceleration, torque, and twisting Mach-Zchnder Interferrometer The Mach-Zehnder interferometer (named after physicists Ludwig Mach (son of Ernst Mach) and Ludwig Zehnder) is a device used to determine the phase shift caused by a small sample which is placed in the path of one of two collimated beams (thus having plane wavefronts) from a coherent light source. Sourc A C Reference E G Detector B D Measuring F H Detector Test Eg.: Optical-Fiber Infrasound Sensors Optical-fiber infrasound sensors (OFISs) are being developed for detecting acoustic pressures in the frequency range from a few millihertz to a few hertz. As explained below, these sensors were conceived to overcome some of the limitations of prior infrasound sensors based on pipe filters connected to microbarographs. Material prepared by: < Physics faculty > Topic No: < 06 > Page 5 of 9
Figure 1. An OFIS includes a fiber-optic Mach-Zehnder interferometer that measures acoustic strain in a sealed hose. The length and diameter of the hose are not critical and can be chosen according to considerations of sensitivity, directionality, and suppression of noise. In one prototype OFIS, the hose is 2.5 cm in diameter and 89 m long An OFIS includes a sealed hose and a fiber-optic Mach-Zehnder interferometer that is sensitive to acoustically induced fluctuations in strain in the hose. The OFIS (see Figure 1) includes two optical fibers wrapped around and along the hose in slight tension in two spiral patterns having equal pitch. Both fibers are doubled back on themselves, but with different spacings, so that the two fibers undergo different amounts of strain when the hose expands or contracts in response to changing air pressure. Light from a laser is coupled into both fibers via a fiber-optic splitter and a piezoelectric modulator, which is excited at a suitable frequency. After traveling along the optical fibers, the laser beams are coupled out via another fiber-optic splitter and fed to a photodetector, wherein the beams interfere. The output of the photodetector is demodulated by a lock-in amplifier synchronized with the modulator excitation. The output of the lock-in amplifier consists of two interference signals in quadrature. These signals are sampled at a suitable rate (e.g., 200 Hz) and the samples are processed to obtain the strain-fluctuation signal and, hence, the desired acoustic-pressure signal. The principal source of acoustic noise in the infrasound frequency range is wind turbulence. In a typical prior infrasound sensor based on a pipe filter Material prepared by: < Physics faculty > Topic No: < 06 > Page 6 of 9
connected to a microbarograph, the sound is summed acoustically from multiple locations connected to the microbarograph via the pipe filter in order to obtain an averaging or smoothing effect that suppresses the relative contribution of noise. Unfortunately, the acoustic sum is not a true average and is affected by the frequency response of the pipe filter, which response is not flat across the entire infrasound frequency band and can be determined only with extreme difficulty. In contrast, the response of an OFIS depends on the optical response of the fiber but is substantially independent of the acoustic frequency. Temperature sensor Bimetal coated on the tip of fiber. As temperature increases the bimetal bends due to difference in thermal expansion of the 2 metals. Phase and hence intensity of reflection changes Major Fiber Sensors Point sensors: Bragg gratings, long period gratings, Fabry-Perot interferometers Maximum temperature can be as high as 1600C. Multiplexed sensors: wavelength and time division multiplexing for as many as 1000 sensor elements along a single fiber cable Distributed sensors: Raman and Brilluion nonlinear scattering systems for temperature and strain measurements with a span of 50km and a special resolution of 15cm Chemical, medical and biological sensors: Often based on the combined use of fiber optics, MEMS and nanotechnology Material prepared by: < Physics faculty > Topic No: < 06 > Page 7 of 9
Benefits of fiber-optic sensors for harsh environments Small, lightweight, contact measurements No electromagnetic interference No spark hazard Negligible thermal apparent strain Will survive up to 3000 F Check Your Understanding 1. Skin is a passive sensor a. True b. False 2. Optical fibers can be used as sensors to measure,, and other quantities by modifying a fiber Check the correct answers on page 9. Summary On completion of this class you have learned that: Definition of fiber sensor Lists of varios different types fiber sensors Mech-Zender Fiber optic interferometer Activity 1. list different fiber optical sensors and compare it with other sensors Material prepared by: < Physics faculty > Topic No: < 06 > Page 8 of 9
Suggested Reading 1. Engineering Physics, by P.K.Palaniswamy 2. Optical Fiber Communications Ghatak and Thyagarajan Answers to CYU. 1. a 2. strain, temperature, pressure Material prepared by: < Physics faculty > Topic No: < 06 > Page 9 of 9