In advanced sensing photonics it is of great importance to explore the detection limits of extremely weak optical signals and imaging using state-of-the art technology. In this paper we describe recent progress in photonic sensing technology achieved in practice in the standard quantum limit of optical detection imposed by the signal-limited shot noise, which can be realized by both the optical heterodyne detection and photon counting techniques. Then, with particular attention on imaging of ultraweak photonic signals by these techniques, their applications in developing new frontiers in the field of biophotonics such as laser computed tomography, and the imaging and characterization of ultraweak biophoton emission phenomena are described and discussed as one of the typical examples of future trends in this field.

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This paper describes photonic sensing methods which are applied to a fiber maintenance system. They are designed for fault sensing in fiber lines, water sensing in cable joints, and future technologies.

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Photonic sensing technologies are an integral part of Remote Fiber Test Systems used for maintaining optical fiber networks. Optical components used for this purpose such as branching components, filters, switches,water sensors and fiber identifiers are overviewed.

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This paper reviews the basics of the interferometric fiber-optic gyroscope: fundamental principle based on Sagnac effect, importance of reciprocity and single-mode propagation, analysis of coherence and polarization problems, signal processing techniques. It also describes the technological progresses of guided-wave components (single-mode optical fiber, semi-conductor diode source, integrated optics, in-line fiber components). Finally, recent trends like multi-axis configurations and rare-earth doped fiber source are presented.

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Recent research on future evolution of fiber optic gyros is discussed. Research factors to improve the functions of interferometer fiber optic gyros in regard to light-source characteristics, temperature change induced drift, and cost reduction are reviewed, and resonator fiber optic gyros and Brillouin fiber optic gyros are evaluated as next generation configurations.

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Bulk-optic current sensors have been developed for electric power systems. This current sensor has a glass ring type Faraday cell, where a light beam circulates around a current-carrying-conductor to precisely measure the current. Fundamental performances and practical applications are described.

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This paper describes development of the optical current transducers using flint glass fiber as the Faraday effect sensing element. Excellent polarization properties of the fiber with low birefringence are described, and the design and test data of a current transducer using the fiber manufactured for use in electric power facilities are reviewed. Experimental results on flexible characteristics of a scheme with round trip light transmission in the fiber is also reported.

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We introduce applied examples (already put to practical use) of power distribution fields of optical fiber current sensors using iron garnet crystal that have high sensitivity, high linearity and stable temperature characteristics. We also describe two types of new optical fiber current sensors developed recently aimed at cost reduction and high performance.

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Two types of optical current transducers (OCTs) have a bulk Faraday sensor inserted into the gap of an iron core and a porcelain insulator with optical fiber. The sensor consists of Bi₁₂SiO₂₀ (BSO) single crystal, a polarizer, and an analyzer. The OCTs satisfied the target performance requirement for fault location and metering and demonstrated maintained performance at some power utilities in Japan and the US. We have developed a fault location system that immediately detects the fault current with the OCTs, there by locating the fault section. The OCT can easily replace the existing support insulators for the disconnecting switch without any modifications to structure height or bus-bar. For metering requiring 0.3% class accuracy, use of a BSO with right optical rotatory power combined with BSO with left optical rotatory power results in a Faraday sensor with improved temperature characteristics. The OCT demonstrated 0.3% class accuracy for metering described in the current transformer Specifications of IEEE C57-13, 1993.

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This paper presents a new method for writing and erasing a birefringence in optical fibers by polarized ultra violet side-exposure. This experiment gives new insight into the origin of the induced birefringence and some applications are outlined.

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A novel reciprocal Brillouin fiber optic current sensor has been developed and tested. The sensor is based on two counter-propagating Brillouin lasers circulating in a fiber ring cavity. It shows a sensitivity of 126 Hz/A that is very close to the maximum sensitivity obtained by using fibers without linear birefringence. The limitations due to the Kerr effect and optical feedbacks in the system are also investigated.

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An alternative and simple fiber-optic backreflectance sensor method for accurate measurement of either effective focal lengths of optical elements and short distances using a single-mode optical fiber and cw laser is presented. The method is based on the intensity sensing of focused backreflectance laser emission and the spatial location of three specific points: the focal point of the focusing optical element and two object points. The single-mode fiber is a key optical element and serves simultaneously as a point laser source for testing, an object for projecting and a highly sensitive point receiver of the focused backreflectance emission. The experimental and analytical results demonstrate the potential of the method for locating the spatial points and determining the effective focal length with accuracies exceeding 1 μm and 0.5%, respectively, as well as for short distance measurement with submicron accuracy.

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We propose a simple, highly sensitive fiber-optic autocollimation method for refractive-index dispersion measurement of solid-state and liquid bulk optical materials using a double-pass fiber Raman laser with Littrow-prism-tuned emission. The optical fiber is a key element of the scheme and serves simultaneously as a point laser source for the test, as a highly sensitive point receiver (or spatial filter) of the autocollimation backreflectance signal and as a medium for nonlinear frequency conversion and generation of a broadband continuum spectrum. When the Raman medium is a graded-index multimode fiber with powerful pumping (over 100 kW) using the second harmonic of a Q-switched Nd:YAG laser (λ_p=532 nm), we obtain widely tunable (0.54-1.01 μm) generation in both the visible and near-IR ranges. The results obtained in the refractive-index dispersion measurements are fitted to the Sellmeier dispersion equation and the standard deviation of the experimental data from the analytical curve does not exceed 50^-5.

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A pressure-sensitive device using an optical fiber coupler is proposed, and is integrated into an array system to measure sound field distributions. The optical coupling between two optical fibers is modulated by the applied sound field with the use of a membrane mechanically connected to the coupler. In the array system, a large number of the optical coupler sensors are connected two-dimensionally, and sound pressure at each sensor can be read out by switching the input light sources in turn. A prototype of the basic array system based on this principle was made, and its performance was demonstrated. The signal discrimination between each element was confirmed in a 2 array; then, using a four-element linear array, we demonstrated the steering of beam based on the phased array method.

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Low coherence multiple-quantum well edge-emitting light-emitting diodes were obtained using selective-area metalorganic vapor-phase epitaxial growth, which utilized growth rate enhancement on an open stripe region between mask stripes. An optical absorption region, which was controlled by selective-area growth, was introduced to suppress optical feedback. At a driving current of 100 mA and an ambient temperature of 25°C, a power of 55 μW was coupled into a single-mode fiber, and a broad spectrum without spectral ripple was observed. Low coherence characteristics and very small temperature dependence were obtained in the temperature range from -40°C to 85°C. The modulation bandwidth was 210 MHz at a bias current of 100 mA.

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An optical output power exceeding 210 mW has been achieved using 1.625-μm strained multiple quantum well lasers at a forward current of 800 mA under pulsed operation. We introduced tensile-strained barrier layers to increase internal quantum efficiency. High quantum efficiency is attributed to improved of hole injection efficiency and suppressed electron overflow from wells. The 1.625-μm high-power lasers are expected to be applied to optical time-domain reflectometers, which enable regular communication light to be used.

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An analytical model for the pressure and strain sensitivity of highly-birefringent fibers is proposed which is in excellent agreement with numerical calculations. Results of polarimetry and coherence-multiplexing experiments are presented and a novel method for accurately splicing this kind of fibers is demonstrated.

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An optical fiber multi-function device consisting of a single gradient-index-rod lens and a multi-facet blazed reflection grating is proposed to simultaneously realize functions of wavelength demultiplexing and optical signal distribution in a multimode optical fiber transmission system. We analyzed the demultiplexing characteristics and the tolerance of optical components using the ray trace method. This device can realize not only low loss optical signal distribution but also offers improved demultiplexing characteristics in comparison with the previously proposed demultiplexer-multiposition switch. The following characteristics are expected from the design using commercially available optical components: a working band of 0.64-0.88 μm, channel separation of 34-36 nm, 3 dB bandwidth of 27-28 nm, channel cross-talk of less than -40 dB and minimum excess insertion loss of 0.9-2.1 dB.

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Glass capillaries with a chemically sensitive coating on the inner surface are used as optical sensors for medical diagnostics. The capillary simultaneously serves as a sample compartment, a sensor element, and an inhomogeneous optical waveguide. Different optical setups have been investigated and compared regarding its waveguiding properties.

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A fluorescence intensity ratio technique has been applied to Pr³⁺: ZBLAN glass, realising a point temperature sensor. We present data for a blue light emitting diode-pumped prototype which provides accurate and self- referenced measurements.

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Novel birefringent single-mode fibers are fabricated with a vacant hole on either side of the central elliptical core, referred to as side-hole fiber, and their intrinsic and tension-induced modal birefringences are measured to know their characteristic properties for the purpose of making a strain sensor. Five sampled side-hole fibers are prepared for measurements. The sensitivity of the strain sensor is primarily determined by two parameters, tension-induced and intrinsic modal birefringences. Tension-induced modal birefringence is controllable by optimal arrangement of the size and location of the side-holes in the cross-section of the fiber. A sensitivity diagram for designating and fabricating side-hole fibers is presented.

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A double fiber-optic Mach-Zehnder interferometer for sensing magnetic fields is devised using two polarization maintaining single-mode fibers. This sensor achieves not only highly sensitive performance but also excellent immunity from temperature disturbance. The sensitivity attains 243 deg/mT (mT=10^-3N/A) for the sensing part of the 5 cm long fiber.

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A fiber-optic transceiver which is particularly suitable for long range optical time-domain reflectometers is proposed. An active fiber ring employed in the proposed configuration has both transmitter and receiver functions. The active fiber ring acts as a Q-switched pulse laser source in the transmitter whereas it acts as an optical preamplifier in the receiver. Experimental results on the reflection detection of several lengths of optical fibers have confirmed the operation principle of the proposed scheme.

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An optical irradiation method for high-speed fiber coupler fabrications without contamination is proposed. In optical fiber taper fabrications we found an interesting effect that fiber elongation is self-arrested although laser irradiation is continued. This is a typical effect in the optical irradiation method, but is not seen in the flame method or the thermal heating method. The “self-controlled fiber taper shape effect” is theoretically explained by considering thermal energy flows, and optical fiber couplers are fabricated using this effect. From our experiments, we show that optical fiber coupler fabrications at high-speed without contamination are possible using the optical irradiation method.

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Theoretical analysis of measurement errors caused by the imperfection of the sensing optical loop in a glass block Faraday effect current sensor has been presented. The experimental results with the fabricated sensor which was designed to achieve a closed homogeneous and isotropic optical path show the importance of the perfectly isotropic and closed loop, in accordance with the theoretical consideration.

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This paper reports the development and characterisation of a multipoint quasi-distributed optical fiber sensor for pH measurement. The system is based on a 170 m length of 200 μm core diameter plastic cladding silica fiber where sections of cladding have been removed and replaced with dye immobilised sol-gel glass to form sensing points. Evanescent wave excitation of a dye, immobilised within 2 mm long sections of cladding, enables the pH value of any spillage material to be determined by optical time domain reflectometry along the length of the fiber. The results suggest a spatial resolution of better than 2.5 meters for this fiber system and indicate that this arrangement could form the basis of a practical sensor/actuator system for chemical spillage, provided that suitable dye/analyte combinations are available.

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A new distributed stress sensor, based on a birefringent fiber Sagnac ring configuration and frequency modulation continuous wave technology, has been investigated. The two forward-coupled mode beams in the Sagnac ring, induced by an applied stress at any given point along the fiber length, produce a beat frequency which can be analysed to determine, simultaneously, the magnitude and position of the stress. The system has a reasonable spatial resolution of 1 m in a dynamic sensing range of 50 m.

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Temperature sensing using 1.54 μm fluorescence at the transition between the energy levels ⁴I_15/2 (ground state) and ⁴I_13/2 generated in an erbium-doped fiber with 1.48 μm pumping is proposed. The fluorescence has a peculiar spectral profile that possesses two peaks around 1.530 μm and 1.552 μm wavelengths. The temperature-dependent fluorescence is investigated in the temperature range between -50°C and 90°C. The power ratio between the two peaks increases with an increase in temperature. The sensitivity of the ratio is 0.007/°C on average in the measured temperature range. The total fluorescence power and the absorption loss at λ=1480 nm in the fiber decrease as the temperature increases. Optical fiber temperature sensing immune from the fluctuation in pumping power can be performed using the peak power ratio and/or the absorption loss.

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An acoustic pulse propagating on a two-mode fiber can act as a beam splitter in a scanning interferometer. When this device is employed in white-light interferometry, the effects of distributed coupling and dispersive interferometer arms need to be considered. A theory suitable for treating acousto-optic interaction of partially coherent light in a moving interaction region was developed. It was found that differential optical dispersion should be negligible and the acoustic pulse length short. Also the coherence time should be short but long compared to the intermodal group delay difference over a pulse length. Experiments with long acoustic pulses were performed, and fairly good agreement with theory was obtained.

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The developed fiber-optic twin sensor allows in-vivo dose measurement in the tissue of the patie nt's body. It consists of two radiation-sensitive fibers. The radiation-induced attenuations of these fibers are used to determine the dose absorbed in tissue. The two signals have a linear dose response, but they depe nd differently on the energy of the ionizing radiation. Evaluation of the two sensor signals permits a nearly tissue-equivalent dose indication.

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A compact high-resolution optical heterodyne interferometer combining a two-frequency light module and a minute optical system is described. The light module, which generates two independent frequencies of light, is fabricated by proton exchange method on LiNbO₃ substrate. We report an experiment evaluating measurement accuracy using a micro-displacement measurement system which incorporates this interferometer. Results of the experiment with a standard thickness sample show high thermal stability with maximum measurement error of 1.8 nm at a temperature from 19°C to 33°C. The system was used to measure the hysteresis of a piezoelectric element for displacements of several nm, thereby making it possible to analyze the system quantitatively in practice.

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The signal to noise ratio (SNR) of a multiplexed fiber-optic sensor network can be improved by inserting fiber-optic amplifiers. Several configurations of amplifiers in the sensor network are analyzed. A single fiber-optic amplifier used as a preamplifier is a good solution with high SNR. However, by distributing amplifiers in the network even higher SNR can be obtained, and identical couplers can be used throughout the network.

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An accelerometer based on an in-fiber Bragg grating sensor has been fabricated and demonstrated. The grating sensor operates linearly up to accelerations of 170,000g_n. The design, testing and performance of the accelerometer are discussed.

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Tokimec has been developing high grade interferometric fiber-optic gyroscopes (I-FOGs) for useful applications. The I-FOG consists of a sensing coil module, a light emitting module and an electronics circuits module. We achieved the bias stability of 0.003 deg/h and the angular random walk of 0.001 deg/√h and the scale factor error of 10 ppm (1σ). Then the azimuth error of the strapdown gyrocompass system with the high grade I-FOGs was less than ±0.05 s λ (deg) under the environmental temperature condition 23±5(°C).

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