J. H. Cole

Optical fiber sensor technology

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.

Improved Bend Loss Formula Verified for Optical Fiber by Simulation and Experiment

This paper presents an improved curvature loss formula for optical waveguides, which is shown to accurately predict the bend loss of both single-mode and multimode fibers. The formula expands upon a previous formula derived by Marcuse, greatly improving its accuracy for the case of multimode fiber. Also presented are the results of bent fiber simulations using the beam propagation method (BPM), and experimental measurements of bend loss. Agreement among simulation, formula and measurement support the validity of both theoretical methods. BPM simulations showed that the lowest order modes of the bent fiber were reduced to their linearly polarized constituents prior to the onset of significant bend loss. This implies that certain LP mode orientations should propagate with much lower loss than previously expected, and should impact the mode stripping ability of bent large mode area fibers, as employed in fiber lasers and amplifiers.

Optical Fiber Sensor Technology

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.

Frequency and temperature dependence of elastic moduli of polymers

The frequency and temperature dependence of the elastic moduli of a number of commercially available polymers has been studied in the temperature range of 0–35 °C and for frequencies 102–106 Hz. Away from transitions a significant new relationship has been obtained, i.e., the Young’s modulus of these polymers is proportional to log of frequency. Using this relationship, together with the low and high frequency data, transitions in some of the polymers were identified.

Single-mode fiber ultrasonic sensor

An acoustooptic ultrasonic sensor using a single-mode fiber is discussed. The sensor is based on acoustically induced modal birefringence which alters the polarization state of the optical beam.

Microbend fiber-optic sensor as extended hydrophone

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.

Optimizing fiber coatings for interferometric acoustic sensors

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.

Low noise remote optical fiber sound detector

An optical system for frequency-modulation heterodyne detection of an acoustic pressure wave signal. An optical beam is directed into a Bragg cell outside of the fluid medium in which acoustic signals are to be detected. The Bragg cell modulates the incident beam such that two beams of different frequency exit the cell. The two beams are directed into an input optical fiber and the resultant combined beam is transmitted over a desired distance to a fiber optic transducer disposed in the fluid medium. The transducer includes two coiled optical fibers, a reference fiber and a signal fiber, each of which has a different sensitivity to incident acoustic pressure wave signals. The transmitted beam is directed from the input optical fiber through a power divider which splits the beam into two equal parts, one part passing through the reference fiber, the other part passing through the signal fiber. A filter in the signal fiber transmits only a fraction of the light at one of the two frequencies. The two parts of the split beam exiting the coiled optical fibers are coupled into another optical fiber and transmitted to a photodetector from which the output signal is processed to indicate the detection of an acoustic pressure wave signal. In a modification of the system, different polarization states are imparted with a polarizer and a half-wave retardation plate to the two beams of different frequency produced by the Bragg cell. The power divider and filter are replaced by a polarization beam splitter and another half-wave plate.

Broad-band ultrasonic sensor based on induced optical phase shifts in single-mode fibers

A broad-band ultrasonic sensor based on induced optical phase shifts in single-mode fibers is demonstrated over a frequency regime of 0.5-50 MHz. In addition, a recently developed theory used to predict the magnitudes of acoustically induced strains in optical fibers is verified.

Optical-fiber sensors challenge the competition: Resistance to corrosion and immunity to interference head the list of benefits in detecting stimuli ranging from pressure to magnetism

Optical-fiber sensors are reviewed for the measurement of temperature, pressure, acceleration, flow of liquids and gases, and the level of a liquid in a container. Optical-fiber sensors are making their first commercial appearance in measuring instruments and controls where such attributes as sensitivity, resistance to hostile environments, and compactness are essential. Through alteration of light by external stimuli, these sensors can now detect virtually any stimulus the other sensors can-from pressure and magnetism to acidity and acceleration-and often more sensitively and accurately, and over a wider dynamic range. Optical-fiber sensors are more rugged and more resistant to corrosion than other sensors, immune to electromagnetic interference, and compatible with optical-fiber telemetry. Several types of fibre sensors that are in commercial production, being field-tested, and in advanced development are discussed.