All-fiber single-mode fiber frequency-modulated continuous-wave Sagnac gyroscope

I systematically analyze the theory of optical frequency-modulated continuous-wave (FMCW) interference. There are three different versions of optical FMCW interference, discussed in detail: sawtooth-wave optical FMCW interference, triangular-wave optical FMCW interference, and sinusoidal-wave optical FMCW interference. The essential concepts and technical terms are clearly defined, the necessary simplifications are introduced according to the characteristics of optical waves, and the formulas used to calculate the signal intensities under two different situations (static and dynamic) are properly derived. Advantages and limitations of each version of optical FMCW interference are also discussed.

This paper introduces a coherence hypothesis, which is modified from the classical coherence theory and can explain the effect of the spectrum bandwidth of light source on the signal of optical frequency-modulated continuous-wave (FMCW) interference.

A singlemode fibre frequency-modulated continuous-wave Sagnac gyroscope is described. The rotation speed of the fibre coil can be determined simultaneously by detecting the phase shift of the beat signal produced by two counter-propagating laser beams in a singlemode fibre coil. A resolution of 0.02°/s with a 100 m fibre coil has been observed.

An all-birefringent-fiber frequency-modulated continuous-wave (FMCW) Sagnac gyroscope is described. The rotation velocity of the gyroscope is determined by measuring the phase shift of the beat signal produced by the two counter-propagating laser beams in the birefringent fiber coil. A resolution of 0.02 deg/sec with a 100-m fiber coil has been observed.

A multiband bandpass microwave photonic filter (MPF) whose passband number and position can be selected is theoretically analyzed and experimentally demonstrated. The proposed MPF is based on a wide-band optical source (WBOS) and a two-order high-birefringence fiber loop mirror (HB-FLM), which serves as a slicing filter. Two segments of high-birefringence fiber (HBF) with the lengths of 3 m and 6 m are used in the HB-FLM, and three typical spectral periods or their combinations can be independently achieved by simply adjusting the polarization controllers (PCs) in the HB-FLM. Subsequently, the light source is sliced with uniform or mixing wavelength spacing. A coil of single-mode fiber (SMF) is then used to act as a dispersive medium to introduce time delay between taps. Thus, a single or multiband bandpass response is obtained at the output of a high-speed photodetector (PD). In addition, the passband centered at dc frequency is removed due to the use of phase modulation. All of the radio frequency (RF) characteristics of the proposed MPF show good agreement with the theoretical prediction. It has the merits of good flexibility, high spectrum efficiency, and great potential of extension.

Polarization mode dispersion resulting from lateral stress by bending is measured as a function of bending curvature. The magnitude of the modal dispersion is found to be 286 psec/km for a bending radius of 2.8 mm. The experimental result is in good agreement with that predicted theoretically for bulk material.

To dynamically monitor the horizontal well, we studied the oil–water two-phase water holdup detection method based on transmission lines, and designed a micro-sensor and a sensor-array water holdup detection tool. We modeled the relationship of the dielectric constant of the transmission line filling medium and the amplitude and phase shift of the electromagnetic wave signal on the transmission line by using the time-domain analysis. We proposed a novel method to measure the water holdup of oil–water mixtures based on the phase shift of signals on the conical spiral transmission line. Furthermore, we simulated and optimized the structural parameters by software simulation, and developed a small conical spiral water holdup sensor suitable for arraying. The single sensor with detection circuits can achieve the full scale (water holdup from 0% to 100%) measurement with resolution better than 3%. On this basis, 12 sensors are used to develop a clock-like sensor-array water holdup detection tool, realizing the array detection of the distribution of the cross-section medium in horizontal wells.

A reflectometric singlemode elliptical-core fibre strain sensor is described. The sensor is based on optical frequency-modulated continuous-wave interference and can be used to measure remotely the strain variation of a distant large construction. The advantages of the sensor include high resolution, long dynamic range, long gauge length, and good stability.

Annealing procedures that greatly reduce linear birefringence in single-mode fiber coils are described. These procedures have been successfully applied to coils ranging from 5 mm to 10 cm in diameter and up to 200 or more turns. They involve temperature cycles that last 3-4 days and reach maximum temperatures of about 850 degrees C. The residual birefringence and induced loss, are minimized by proper selection of fiber. The primary application of these coils is optical fiber current sensors, where they yield small sensors that are more stable than those achieved by other techniques. A current sensor with a temperature stability of +8.4*10/sup -5//K over the range from -75 to +145 degrees C has been demonstrated. This is approximately 20% greater than the temperature dependence of the Verdet constant. Packaging degrades the stability, but a packaged sensor coil with a temperature stability of about +1.6+10/sup -4//K over the range from -20 to +120 degrees C has also been demonstrated. >

A novel reciprocal-compensated structure is proposed and demonstrated for intensity-modulated intrinsic fiber-optic current sensors. Both the component losses and birefringence influences can be partly compensated using this configuration, and the sensor is sensitive only to nonreciprocal Faraday rotation. The dependence of the signal on circular birefringence drift can be reduced by a factor better than 10 for low birefringence fibers. The property of total reciprocity reduces the sensitivity of the current sensor to temperature changes and vibrations. The experimental results agree well with the theoretical predictions for the sensor, fabricated using a coil of low birefringence single-mode fiber.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Intermediate and Moderate grade Fiber Optic Gyroscope (FOG) have been developed for industrial applications such as Positioning system, Self-guided robots, and Bore-hole survey system. Those FOGs are based on the minimum reciprocal configuration with single mode fiber coil which reduces the production cost.

Dual-polarization interferometric fiber optic gyroscope (IFOG) is a novel scheme in which the polarization nonreciprocal (PN) phase error of the two orthogonal polarizations can be optically compensated. In this work, we investigate the effective of PN phase error compensation under varying temperature. It is proved that, the thermally induced strain deforms the fiber, and results in perturbations on the birefringence and polarization cross coupling which degrades the IFOG's stability. A wave propagation model and analytical expressions of PN phase error are derived by using coupled-wave equation and Jones matrix. We theoretically and experimentally verify that, although the single-mode (SM) and polarization-maintaining (PM) fiber coils behave different owing to their intrinsic properties of wave propagation, the thermal strain induced PN phase error can still be compensated under slow and adiabatic temperature variations. This could be a promising feature to overcome the temperature fragility of IFOG.

We investigated the nonreciprocity induced by an orthogonal magnetic field (OMF) in a single-mode fiber coil and expressions for the propagation constant of light waves in the bent fiber are deduced. Coils with different radiuses are put into depolarized interferometric fiber optic gyroscopes (D-IFOGs) for experimental verification. Theories, simulations and experimental results show that the OMF induced nonreciprocal phase error is closely related to the bending radius and the generated magnetic drift in D-IFOGs is linear to the amplitude of the OMF.

Remote platform-based laser positioning applications are severely affected by onboard vibration. To eliminate the effect, sensing and compensating the vibration by equal but opposite actuation is necessary. However, existing methods possess various limitations due to the technical challenges as follows: 1) the vibration has multiple time-variant dominant frequencies; 2) the bandwidth is broad; 3) there is a significant phase shift in signals caused by inherent delay; 4) the sensor (i.e., gyroscope) generates noise and integration drift disturbances. To overcome these challenges, we propose a novel sensing scheme in this paper, applying the Taylor series fore-casting and the Recursive Least Square (RLS)-based filter. Both techniques are designed to process signals with features listed in challenges 1) and 2). The Taylor series forecasting is a time series forecasting technique that eliminates the delay-induced phase shift with higher accuracy than existing methods. The RLS-based filter is an adaptive filter that applies a regression model to the input and achieves frequency domain separation with negligible phase shift. In this paper, both offline and online real-time (RT) experiments are conducted for validation, using the vibration signal sampled from a truck-based system. The proposed method shows higher accuracy than existing methods and achieves compensation rates over 70%.

Summary For purposes of shallow earth geophysical mapping, progressively higher frequencies are developed for helicopter electromagnetic (HEM) systems. However, concern has been expressed about the vulnerability of high frequency EM signals to the influence of the displacement current, especially the phase shift of the HEM signal due to the finite light speed that describes the propagation of the EM wave in the free space. In this paper, based on a full solution of EM field for a conductive, magnetically and dielectrically polarisable earth and an air of free-space dielectric permittivity, we investigate the influence of the displacement current and the finite speed of the light on HEM responses, including the amplitude change and the phase shift of HEM signal and the apparent resistivity. We find that the displacement current in the free air space has a small influence on the HEM signal, while substantial influences may occur when the earth is dielectrically polarisable. The finite speed of the EM propagation in the free apace does not result in significant phase change in HEM signal.