Showing papers on "Fiber optic sensor published in 1982"

Optical fiber sensor technology

Abstract: The current state of the art of optical fiber sensors is reviewed. The principles of operation are detailed and the various types of fiber sensors are outlined. Achievable performance and limitations are discussed and a description of technology used to fabricate the sensor is presented. The characteristics of acoustic, magnetic, gyro, laser diode, and other sensors are described. Trends in the development of this sensor technology and expected application areas are briefly outlined.

Homodyne demodulation scheme for fiber optic sensors using phase generated carrier

Abstract: A method of homodyne demodulation using a phase generated carrier is described and experimentally demonstrated. The method has a large dynamic range, good linearity, and is capable of detecting phase shifts in the microradian range. The detection scheme obviates the phase tracker resetting problem encountered in active homodyne detection schemes. Two methods of producing the carrier are presented, one employing a piezoelectric stretcher, the other using current induced frequency modulation of the diode laser source. These two methods are compared. The origins of the noise limiting the system are briefly discussed.

Homodyne Demodulation Scheme for Fiber Optic Sensors Using Phase Generated Carrier

Abstract: A method of homodyne demodulation using a phase generated carrier is described and experimentally demonstrated. The method has a large dynamic range, good linearity, and is capable of detecting phase shifts in the microradian range. The detection scheme obviates the phase tracker resetting problem encountered in active homodyne detection schemes. Two methods of producing the carrier are presented, one employing a piezoelectric stretcher, the other using current induced frequency modulation of the diode laser source. These two methods are compared. The origins of the noise limiting the system are briefly discussed.

Compression of femtosecond optical pulses

Abstract: We describe the generation and measurement of optical pulses as short as 30 fs. The pulses are produced using self‐phase modulation in a short 15‐cm optical fiber followed by a grating compressor.

Passive stabilization scheme for fiber interferometers using (3×3) fiber directional couplers

Abstract: A passive stabilization scheme using a (3×3) fiber directional coupler in an all fiber Mach–Zehnder interferometer and suitable signal processing has been successfully demonstrated. A stable output with large signal dynamic range and a minimum detectable phase shift in the microradian range has been achieved.

Optical Fiber Sensor Technology

Abstract: The current state of the art of optical fiber sensors is reviewed. The principles of operation are detailed and the various types of fiber sensors are outlined. Achievable performance and limitations are discussed and a description of technology used to fabricate the sensor is presented. The characteristics of acoustic, magnetic, gyro, laser diode, and other sensors are described. Trends in the development of this sensor technology and expected application areas are briefly outlined.

Fiber-Optic Fabry-Perot Interferometer and its Sensor Applications

Abstract: Fiber-optic Fabry-Perot interferometers using monomode fibers are fabricated and their basic properties of finesse, polarization, and thermal response are studied. Fiber-optic Fabry-Perot interferometers are applied to the sensors of temperature, mechanical vibration, acoustic wave including human voice, ac voltage, and ac and dc magnetic fields. It has been demonstrated that a fiber-optic Fabry-Perot interferometer can simplify the interferometric fiber sensor sys-tem and that high measurement sensitivity can be obtained by using a high-finesse and/or long-distance fiber Fabry-Perot interferometer.

Development of low- and high-birefringence optical fibers

Abstract: The polarization properties of single-mode optical fibers are easily modified by environmental factors. While this can be exploited in a number of fiber sensor devices, it can be troublesome in applications where a stable output polarization-state is required. Fibers with both low and high birefringence have been developed to enhance or diminish their environmental sensitivity, and recent progress in each area is reviewed. Low-birefringence fibers are described which are made by spinning the preform during the draw. In addition, developments in high-birefringence fibers which maintain a polarization state over long lengths are summarized. The effect on performance of external factors such as bends, transverse pressure, and twists is analyzed. Consideration is also given to polarization mode-dispersion as a potential limiting factor in ultrahigh bandwidth systems.

Optical Fiber Sensor for Measuring Refractive Index

Abstract: An optical fiber sensor for measuring a liquid refractive index is proposed. When an optical fiber is bent and part of its cladding is stripped off, the light energy (E) emerging from the fiber depends on the refractive index of the surrounding medium (nm). The change in nm can be found from E. The light output energy and the measuring sensitivity are calculated numerically as a function of nm for several values of the bending radius R. The fundamental characteristics of the sensor, made of a plastic fiber, are investigated using lamp oil and light oil as specimens, and a measurement accuracy of nm to the third decimal place is easily obtained. As applications of this sensor, measurement of liquid density and detection of oil are described.

Optical techniques to solve the signal fading problem in fiber interferometers

Abstract: In single-mode optical fiber interferometer sensors environmental effects such as ambient temperature fluctuations and static pressure changes result in signal fading. Three different optical techniques which eliminate the signal fading problem are presented and experimentally verified. Comparison of the three techniques in terms of their technical merits is presented.

Source statistics and the Kerr effect in fiber-optic gyroscopes

Abstract: We show theoretically that polarized thermal light as generated by a superluminescent diode has the properties required to compensate for Kerr-effect drift in fiber gyroscopes. Light from some multimode lasers has related properties that can substantially reduce Kerr-effect errors. An experimental gyroscope using a multimode laser diode demonstrates no Kerr-effect error and provides significantly improved long-term stability.

Fiber-optic instrument for temperature measurement

Abstract: A practical fiber-optic measurement instrument for temperature was constructed consisting of a small sensor responding to optical absorption change in a semiconductor, and a unique signal processing system with two different-wavelength light emitting diodes (LED's). The fiber-optic sensor with a semiconductor chip is quite small, very sensitive, highly reliable, and easy to manufacture at low cost. The most outstanding feature of this system is that it is free from optical-stray-loss. The accuracy of about \pm1\deg and the response time of about 2 s were obtained in the temperature range from -10°C to 300°C.

Phase compensation in interferometric fiber-optic sensors

Abstract: A new method of keeping interferometric fiber-optic sensor systems in quadrature by tuning the emission frequency of the laser-diode source is described. The compensation scheme has a better than 10−5-rad sensitivity and a usable tracking range.

Development of Low- and High-Birefringence Optical Fibers

Abstract: The polarization properties of single-mode optical fibers are easily modified by environmental factors. While this can be exploited in a number of fiber sensor devices, it can be troublesome in applications where a stable output polarization-state is required. Fibers with both low and high birefringence have been developed to enhance or diminish their environmental sensitivity, and recent progress in each area is reviewed. Low-birefringence fibers are described which are made by spinning the preform during the draw. In addition, developments in high-birefringence fibers which maintain a polarization state over long lengths are summarized. The effect on performance of external factors such as bends, transverse pressure, and twists is analyzed. Consideration is also given to polarization mode-dispersion as a potential limiting factor in ultrahigh bandwidth systems.

Birefringence and polarization effects in fiber gyroscopes

Abstract: A formalism is presented for treating birefringence and polarization in fiber optic sensors. This formalism is applied to study theoretical characteristics of fiber gyroscopes which are operated with various states of polarization of the input light including nonpolarized and partially polarized light, which have potentially useful characteristics. Measurements supporting the theoretical predictions are described.

Passive fiber optic multiplexer

Abstract: A passive, frequency selective, fiber optic multiplexer, comprises a directional coupler in which a pair of single mode optical fibers are accurately positioned to provide evanescent field coupling, typically by polishing a portion of the cladding from each of said fibers to place the respective cores of said fibers within the evanescent field of light in the other fiber. The coupling efficiency of a coupler constructed in this manner is wavelength dependent, and provides over-coupling, that is, the capability of transferring light, virtually entirely, back and forth between the fibers within the coupler. The wavelength dependent nature of the evanescent field coupling permits multiplexing, specifically between a pair of wavelengths, one of which is coupled in its entirety from a first fiber to a second fiber, and the other of which is essentially uncoupled. By increasing the number of total transfers of the light signals between the pair of fibers, the frequency resolution of the multiplexer may be optimized for light signals of virtually any frequency separation.

Video signal transmission system for endoscope using solid state image sensor

Abstract: A video signal transmission system for an endoscope converts an electrical video signal produced by a solid state image sensor into an optical signal and transmits the optical signal to a control section through a fiber optic cable or like light carrying medium. Because the fiber optic cable used for the present invention transmits light indicative of a time serial signal from the solid state imaging sensor, it will be clear that the cable may be constituted by a single length of optical fiber and, in this respect, substantially differs from usual light carrying fiber bundles which are installed in general endoscopes as image guides for transmitting light images simply as light.

Stable fiber-optic hydrophone

Abstract: An optical fiber hydrophone system in which a single optical fiber is used for all of the acoustical sensors in the system. A signal source and detector provides an optical signal in selected form, such as continuous or pulsed and detects and extracts an identifiable output signal. Each sensor is in the form of a sensing portion of the single optical fiber. Each sensing portion includes two optical reflectors separated one from another by a predetermined length of said optical fiber. Variations in acoustical pressure incident on the sensing portion causes a change in the predetermined length. This causes reflected portions of the optical signal to interfere with one another. Such interference is detectable for extraction of the identifiable output signal. In one form each sensing portion has two terminal branches of a mechanically deformable material, deformable in response to the fluctuations in acoustical pressure. Preferably, the optical fiber has two portions, a sensing portion thereof underwater and having a first optical cavity, and another portion thereof on board a vessel and having a second optical cavity, typically tunable with respect to the optical length thereof to maximize the interference in the detected optical signal.

An analysis of the performance of heterojunction phototransistors for fiber optic communications

Abstract: The theory of operation of the heterojunction phototransistor (HPT) is reviewed and the limitations on gain and speed-of-response are examined in the context of fiber optic systems requirements. The response of the base potential is shown to depend on the input optical power, and this dependence results in a power-dependent gain-bandwidth product, f T . Model calculations assuming optimized device structures suitable for multimode and single-mode systems operating in the 1.3-1.55-µm spectral region are used to demonstrate the consequences of this dependence. The results indicate that the HPT can have sufficient gain and speed-of-response for particular applications if a dc bias (optical or electrical) is used. The noise sources, S/N , and sensitivity of the optimized devices are discussed to clarify system applicability.

Analysis of fiber interferometer utilizing 3 × 3 fiber coupler

Abstract: An analysis of the properties of the 3 × 3 fiber coupler is made. The analysis reveals that four parameters are required to characterize the power transfer properties of this type of coupler. The performance of a fiber interferometer is analyzed in terms of these four parameters.

Characteristics of fiber-optic magnetic-field sensors employing metallic glasses

Abstract: A variety of extremely high-sensitivity, room-temperature magnetic-field sensors have been fabricated and tested using single-mode fibers in conjunction with highly magnetostrictive metallic glasses Minimum detectable fields of 5 x 10(-9) Oe/m fiber are reported The typical characteristics of the fiber magnetometers are summarized

Fiber optic rotation sensor.

Abstract: Rotation sensor having a loop of fiber optic material in which counter propagating waves are generated with a phase relationship corresponding to the rate at which the loop is rotated. All fiber optic components are employed in the system for directing the light to and from the loop and establishing, maintaining and controlling proper polarization of the light. In one particularly preferred embodiment, the loop and other components are formed on a single strand of fiber optic material which extends continuously through the system.

Integrated optical switch matrix for single-mode fiber networks

Abstract: Design analysis and experiments on optical directional coupler switch integration into an LiNbO 3 chip with arrayed fiber pigtails has been made at 1.3 μm wavelength. A limitation for high integration was discussed by taking into account radiation losses at connecting waveguides between switch elements and at the input/output curved waveguide, switching voltage, and crosstalk caused by applied electric field leakage. An optimum designed low-loss 4 × 4 switch matrix with arrayed fiber pigtails at 1.3 μm wavelength has been developed. Its insertion loss was measured to be as low as 6 dB.

High-power low-divergence superradiance diode

Abstract: Using a combination of antireflection coating and proton implantation techniques, GaAlAs double‐heterostructure lasers are fabricated to operate in superradiant mode. Light output, in excess of seven milliwatts, is obtained with spectral half‐power widths of 15–20 nm. This implies extremely short coherence lengths of less than 50 μm. Such a short coherence length light source should be very useful for many applications in fiber sensors and fiber optic communications. In particular, it has produced marked reduction (better than 15 dB) of coherent Rayleigh backscattering noise in a single‐mode fiber. Since the laser operates in superradiant mode, the output light is only partially polarized, another unique characteristic that is important for fiber gyroscopes. The typical light output angle of 30°×50° also provides easy coupling into fibers. Coupling efficiencies of 80% into multimode fiber and over 20% into single‐mode fiber have been achieved.

Polarization fading in fiber interferometric sensors

Abstract: Mach-Zehnder interferometer sensors fabricated with conventional nonpolarization-preserving fibers are subject to polarization fading caused by temperature variations and minor positional changes in the sensor. For such sensors, we calculate the probability of a given decrease in sensitivity and in signal-to-noise ratio due to fading assuming the polarization of the light in the signal and reference legs is uncorrelated and drifts randomly. The resultant reduction of the signal-to-noise ratio may exceed 10 dB 10 percent of file time and exceed 20 dB more than 2 percent of the time.

Microbend fiber-optic sensor as extended hydrophone

Abstract: A novel microbend fiber-optic acoustic sensor has been studied, both analytically and experimentally. The sensor is simple mechanically, insensitive to acceleration, and achieves shape flexibility by utilizing fairly long fiber lengths for the sensing element. The acoustic sensitivity and minimum detectable pressure of the sensor were found to be significantly improved over previously reported microbend sensors. Further optimization of the sensor appears possible.

Fiber optic sensors for simultaneously detecting different parameters in a single sensing tip

Abstract: A fiber optic sensor for sensing pressure, temperature, voltage and other environmental parameters. A fiber is terminated in a series of semiconductor layers, each absorbing light as a function of the sensed parameters at a different wavelength. Light is transmitted down the fiber with plural frequencies and reflected by the termination to provide in the reflected light an attenuation representative of the parameters sensed. The reflected light intensity is detected and electronically processed to provide an indication of the magnitude of the parameters being sensed. With each frequency differently attenuated, the sensed parameters are separately detected. In one implementation two discrete frequencies or wavelengths of light are directed down the unitary fiber to its termination containing first and second semiconductor layers and reflective terminations that have an absorption characteristic which is a function of the sensed parameters such that the first frequency is partially absorbed in the first layer and reflected by its termination while the second frequency is transmitted substantially unaffected through to the second layer where it is partially absorbed and reflected. The two reflected wavelengths of radiation are retransmitted through the optical fiber and coupled into receivers which provide an output signal representing the reflected light magnitude. Parameter sensitive reflective layers may be substituted for the absorptive layers. The semiconductive layers may be employed in a detector configuration.

Multimode fiber-optic pressure sensor based on the photoelastic effect

Abstract: A multimode fiber-optic pressure sensor is described that is based on the photoelastic effect. The device was shown to be able to detect pressures as small as 95 Pa, to have a dynamic range of 86 dB, and to have hysteresis less than 0.01. Device sensitivity may easily be modified to coincide with required measurement sensitivity through careful choice of the active photoelastic material. The device is simple, rugged, electrically passive, and compatible with existing multimode technology.

Optimizing fiber coatings for interferometric acoustic sensors

Abstract: The pressure sensitivity of the phase of light propagating in an optical fiber is studied both analytically and experimentally The analysis, which takes into account the exact composition and geometry of multilayer fibers, is utilized to identify coating properties which optimize the fiber acoustic sensitivity In order to predict the fiber acoustic sensitivity, the elastic parameters of commonly used coating materials, thermoplastics, and UV curable elastomers have been studied in bulk samples as a function of frequency ( 10^{2}-10^{4} Hz) and temperature ( 0-35\deg C) The analytically predicted frequency dependence of the acoustic sensitivity is found to be in agreement with that obtained experimentally from fibers with coatings of various materials

Polarization Fading in Fiber Interferometric Sensors

Abstract: Mach-Zehnder interferometer sensors fabricated with conventional nonpolarization-preserving fibers are subject to polarization fading caused by temperature variations and minor positional changes in the sensor For such sensors, we calculate the probability of a given decrease in sensitivity and in signal-to-noise ratio due to fading assuming the polarization of the light in the signal and reference legs is uncorrelated and drifts randomly The resultant reduction of the signal-to-noise ratio may exceed 10 dB 10 percent of the time and exceed 20 dB more than 2 percent of the time