Precision Temperature Control for the Laser Gyro Inertial Navigation System in Long-Endurance Marine Navigation.

In the Ring Laser Gyro Inertial Navigation System (RLG INS), the temperature characteristics of the accelerometer can directly influence the measurement results. In order to improve navigation accuracy in long-endurance marine navigation, the operating temperature of the accelerometer should be precisely controlled. Based on thermal studies on the accelerometer, temperature control precision should be better than 0.01 °C to achieve 1 × 10-5 m/s2 output accuracy of the accelerometer. However, this conclusion is obtained by approximate calculations and cannot be directly applied to different inertial navigation systems. In order to verify this thermal conclusion and broaden its application, the Back Propagation Neural Network (BP-NN) algorithm is adopted to validate the feasibility of temperature control in this paper. In addition, a multi-level temperature control system is also set up and carefully designed to support the validation and experiments under different conditions. Test results of the temperature control system prove that operating temperature variation can be reduced to 0.01 °C. Meanwhile, the standard deviation per hundred seconds of the accelerometer outputs, after temperature control, reaches 1 × 10-5 m/s2. Static altitude and navigation results were improved by 41.97% and 62.91%, respectively, with the precision temperature control system, which meets the long-endurance marine navigation requirements.

Phase-generated-carrier-modulated laser interferometer for measurement of ground vibration on an absolute gravimeter.

This paper presents an innovative interferometer based on the phase-generated-carrier method to meet the requirement of micro-vibration detection on an absolute gravimeter. Our results, including those of both simulation and experiments, verified the feasibility of the interferometer. Based on this interferometer, continuous micro-vibrations are obtained and synchronously compared with the results detected by a 991B seismometer. The interferometer's accuracy reaches 10-8m, and the detected vibration frequency is less than 100 Hz. These findings can help broaden the application of interferometers and provide new guidelines for vibration measurement.

Application of Near-Infrared Optical Feedback Cavity Enhanced Absorption Spectroscopy (OF-CEAS) to the Detection of Ammonia in Exhaled Human Breath.

The qualitative and quantitative analysis to trace gas in exhaled human breath has become a promising technique in biomedical applications such as disease diagnosis and health status monitoring. This paper describes an application of a high spectral resolution optical feedback cavity enhanced absorption spectroscopy (OF-CEAS) for ammonia detection in exhaled human breath, and the main interference of gases such as CO2 and H2O are approximately eliminated at the same time. With appropriate optical feedback, a fibered distributed feedback (DFB) diode laser emitting at 1531.6 nm is locked to the resonance of a V-shaped cavity with a free spectral range (FSR) of 300 MHz and a finesse of 14,610. A minimum detectable absorption coefficient of αmin = 2.3 × 10-9 cm-1 is achieved in a single scan within 5 s, yielding a detection limit of 17 ppb for NH3 in breath gas at low pressure, and this stable system allows the detection limit down to 4.5 ppb when the spectra to be averaged over 16 laser scans. Different from typical CEAS with a static cavity, which is limited by the FSR in frequency space, the attainable spectral resolution of our experimental setup can be up to 0.002 cm-1 owing to the simultaneous laser frequency tuning and cavity dither. Hence, the absorption line profile is more accurate, which is most suitable for low-pressure trace gas detection. This work has great potential for accurate selectivity and high sensitivity applications in human breath analysis and atmosphere sciences.

Two-dimensional laser Doppler velocimeter and its integrated navigation with a strapdown inertial navigation system.

In the field of land navigation, a laser Doppler velocimeter (LDV) can be used to provide the velocity of a vehicle for an integrated navigation system with a strapdown inertial navigation system. In order to suppress the influence of vehicle jolts on a one-dimensional (1D) LDV, this paper designs a split-reuse two-dimensional (2D) LDV. The velocimeter is made up of two 1D velocimeter probes that are mirror-mounted. By the different effects of the vertical vibration on the two probes, the velocimeter can calculate the forward velocity and the vertical velocity of a vehicle. The results of the vehicle-integrated navigation experiments show that the 2D LDV not only can actually suppress the influence of vehicle jolts and greatly improve the navigation positioning accuracy, but also can give high-precision altitude information. The maximum horizontal position errors of the two experiments are 2.6 m and 3.2 m in 1.9 h, and the maximum altitude errors are 0.24 m and 0.22 m, respectively.

Calibration of a three-dimensional laser Doppler velocimeter in a land integrated navigation system.

In the land navigation field, a laser Doppler velocimeter (LDV), which is able to measure the speed of a carrier, can be combined with a strapdown inertial navigation system (SINS) to form an integrated navigation system. To realize the integrated navigation positioning of a free motion carrier on the ground accurately, this paper introduces a split-reuse three-dimensional (3D) LDV. For the error parameters during application, a Kalman filtering calibration method with the assistance of a differential global positioning system (DGPS) is put forward in this paper. Two groups of integrated navigation experiments are designed to validate the effectiveness of this method and the universality of the obtained parameters. The experimental results show that the calibration method proposed in this paper is effective and the 3D LDV after compensation can greatly improve the positioning accuracy of the integrated navigation. The maximum horizontal position errors of the two experiments calculated by the dead reckoning of the 3D LDV and the gyroscopes are 4.2 m and 2.9 m, and the maximum altitude errors are 0.8 m and 0.9 m, respectively.

Response of a Bell⁻Bloom Magnetometer to a Magnetic Field of Arbitrary Direction.

The Bell⁻Bloom magnetometer in response to a magnetic field of arbitrary direction is observed theoretically and experimentally. A theoretical model is built from a macroscopic view to simulate the magnetometer frequency response to an external magnetic field of arbitrary direction. Based on the simulation results, the magnetometer characteristics, including the signal phase and amplitude at resonance, the linewidth, and the magnetometer sensitivity, are analyzed, and the dependencies of these characteristics on the external magnetic field direction are obtained, which are verified by the experiment.

Temperature compensation method using readout signals of ring laser gyroscope.

Traditional compensation methods using temperature-related parameters have little effect when the ring laser gyroscope (RLG) bias changes rapidly. To solve this problem, a novel RLG bias temperature compensation method using readout signals is proposed in this paper. Combined with the least squares support vector machine (LS-SVM) algorithm, the novel method can improve the precision of the RLG bias. Experiments show that by utilizing the readout signals in the LS-SVM model, the RLG bias stability can be significantly raised compared to the original data. The novel method proposed in this paper is shown to be feasible, even when the RLG bias changes rapidly.

Multiple-Point Temperature Gradient Algorithm for Ring Laser Gyroscope Bias Compensation.

To further improve ring laser gyroscope (RLG) bias stability, a multiple-point temperature gradient algorithm is proposed for RLG bias compensation in this paper. Based on the multiple-point temperature measurement system, a complete thermo-image of the RLG block is developed. Combined with the multiple-point temperature gradients between different points of the RLG block, the particle swarm optimization algorithm is used to tune the support vector machine (SVM) parameters, and an optimized design for selecting the thermometer locations is also discussed. The experimental results validate the superiority of the introduced method and enhance the precision and generalizability in the RLG bias compensation model.

Spatial coating inhomogeneity of highly reflective mirrors determined by cavity ringdown measurements.

The inhomogeneity of high-reflectivity mirror coatings is a potential error source in the application of the cavity ringdown technique. Here, the ringdown times for different transverse modes were recorded. Together with the observed spatial distribution of these modes the ringdown times can be used to approximately locate the position of coating defects. A simple model based on a weighted sum of Hermite-Gaussian mode functions is used to explain the experimental results.

Angle measurement with laser feedback instrument.

An instrument for angle measurement based on laser feedback has been designed. The measurement technique is based on the principle that when a wave plate placed into a feedback cavity rotates, its phase retardation varies. Phase retardation is a function of the rotating angle of the wave plate. Hence, the angle can be converted to phase retardation. The phase retardation is measured at certain characteristic points identified in the laser outputting curve that are then modulated by laser feedback. The angle of a rotating object can be measured if it is connected to the wave plate. The main advantages of this instrument are: high resolution, compact, flexible, low cost, effective power, and fast response.

Polarization flipping and hysteresis phenomenon in laser with optical feedback.

We realize polarization control in He-Ne laser by optical feedback. Polarization flipping occurs when the length of the anisotropy feedback cavity is modulated. The relationship between polarization flipping and phase retardation of birefringence component placed in feedback cavity is built. The hysteresis effect of polarization flipping is observed. We build a system to measure the size of hysteresis loop based on Laser Feedback Interferometer (LFI). The variation of hysteresis loop with phase retardation is measured. The width of hysteresis loop decreases when phase retardation increases.

Thickness and refractive-index measurement of birefringent material by laser feedback technique.

We introduce a technique for simultaneous measurement of thickness and refractive index of birefringent materials. The principle is based on the laser feedback effect that laser polarization states flip between two orthogonal directions when a birefringent material is placed into the external cavity. The position of polarization flipping is determined by the phase-retardation magnitude of the birefringent material. Some feature points in the laser intensity curve can be used to calculate phase retardation. We derive an expression for phase retardation and rotation angle of a birefringent material to calculate thickness and refractive index. This technique is noncontact and compatible with in situ thickness and refractive-index measurement. The measurement precision of thickness is 59 nm and of refractive index is 0.0006.

Optic axis determination based on polarization flipping effect induced by optical feedback.

For many materials, particularly biological tissues, optic axis orientation directly correlates with the materials performance, such as refractive index. In this Letter, a system measuring the optic axis azimuth was built using the laser feedback method that the polarization state of laser output is linearly polarized when optic axis azimuth is consistent with the initial direction of the laser polarization, otherwise elliptical polarization will be observed. The polarization state of the laser output is highly sensitive to the relative position of the optic axis and the initial direction of the laser polarization. This may be used to determine the optic axis azimuth of a material with a high precision.

Beam position controlling method for 3D optical system and its application in non-planar ring resonators.

A novel theoretical beam position controlling method for 3D optical system has been proposed in this paper. Non-planar ring resonator, which is a typical 3D optical system, has been chosen as an example to show its application. To the best of our knowledge, the generalized ray matrices, augmented 5 × 5 ray matrices for paraxial dielectric interface transmission and paraxial optical-wedge transmission, and their detailed deducing process have been proposed in this paper for the first time. By utilizing the novel coordinate system for Gaussian beam reflection and the generalized ray matrix of paraxial optical-wedge transmission, the rules and some novel results of the optical-axis perturbations of non-planar ring resonators have been obtained. Wedge angle-induced mismatching errors of non-planar ring resonators have been found out and two experimental beam position controlling methods to effectively eliminate the wedge angle-induced mismatching errors have been proposed. All those results have been confirmed by related alignment experiments and the experimental results have been described with diagrammatic representation. These findings are important to the beam control, cavity design, and cavity alignment of high precision non-planar ring laser gyroscopes. Those generalized ray matrices and their deducing methods are valuable for ray analysis of various kinds of paraxial optical-elements and resonators. This novel theoretical beam position controlling method for 3D optical system is valuable for the controlling of various kinds of 3D optical systems.

Novel coordinate system for Gaussian beam reflection.

A novel coordinate system for Gaussian beam reflection has been proposed in this Letter. Reflection from a spherical mirror is used to describe the novel coordinate system. One single segment of a general resonator is chosen to describe coordinate rotation in detail. Nonplanar ring resonators are chosen to show the application of the novel coordinate system. This novel coordinate system has been proved by two simple experiments and the problem existing in using the traditional coordinate system has been pointed out. This novel coordinate system is valuable for not only the designing of laser resonators but also Gaussian beam propagation analysis.

Internal stress measurement by laser feedback method.

Internal stress in material detracts from its usefulness. In this Letter, a stress measurement instrument is reported. The instrument principle is based on a laser feedback effect where the polarization state of the laser with an anisotropic feedback cavity will flip between two orthogonal directions, while the feedback mirror is tuned by piezoelectric transducer sawtooth voltage. The position of polarization flipping in one period on curves reflects the birefringence or material internal stress of the feedback cavity. Hence, when a piece of internal stress material is placed in a feedback cavity, its internal stress can be measured by the polarization flipping position. The internal stress of the vacuum tube, Nd:YAG crystal, and GaN semiconductor are measured, which proved this instrument has very high precision.

Locking phenomenon of polarization flipping in He-Ne laser with a phase anisotropy feedback cavity.

In this article, the locking phenomenon of polarization flipping is reported. A model that integrates the model of the equivalent cavity of a Fabry-Perot interferometer and Lamb's semiclassical theory is built to explain the locking phenomenon. On the basis of analysis of the model, a method is proposed to release the locking of polarization flipping. After solving the problem of lock-in, the system in this paper can be used to measure small stress of the optical component and phase retardation of the birefringence component.

Generalized ray matrix for spherical mirror reflection and its application in square ring resonators and monolithic triaxial ring resonators.

To the best of our knowledge, the generalized ray matrix, an augmented 5×5 ray matrix for a spherical mirror reflection with all the possible perturbation sources including three kinds of displacements and its detailed deducing process have been proposed in this paper for the first time. Square ring resonators and monolithic triaxial ring resonators have been chosen as examples to show its application, and some novel results of the optical-axis perturbation have been obtained. A novel method to eliminate the diaphragm mismatching error and the gain capillary mismatching error in monolithic triaxial ring resonators more effectively has also been proposed. Both those results and method have been confirmed by related experiments and the experimental results have been described with diagrammatic representation. This generalized ray matrix is valuable for ray analysis of various kinds of resonators. These results are important for the cavity design, cavity improvement and alignment of high accuracy and super high accuracy ring laser gyroscopes.

Estimation of parameters of a laser Doppler velocimeter and their Cramer-Rao lower bounds.

Considering the influence of acceleration and the Gaussian envelope for a laser Doppler velocimeter (LDV), parameter estimation of a Doppler signal with a Gaussian envelope was investigated based on introducing acceleration. According to the theory of mathematics statistics, the Cramer-Rao lower bounds (CRLBs) of Doppler circular frequency and its first order rate were analyzed, formulas of CRLBs were given, and the power spectrum estimation with adjustment was discussed. The results of theory and the simulation show that the CRLBs are related to the data length, the signal-to-noise ratio (SNR), and the width of the Gaussian envelope, and they can be decreased by increasing the data length or improving the SNR; the larger the acceleration is and the narrower the Gaussian envelope is, the larger the CRLBs of Doppler circular frequency and its first order rate are; the gap between the variances of the measuring results and the CRLBs narrows when the SNR of the signal is improved, and is almost eliminated when the SNR is higher than 6 dB. It is concluded that the model presented is much more suitable for a LDV than that acquired by Rife and Boorstyn [IEEE Trans. Inform. Theory 20, 591 (1974)].

Precise wavelength calibration in continuous-wave cavity ringdown spectroscopy based on the HITRAN database.

We describe the wavelength calibration method of a narrowband laser diode in continuous-wave (CW) cavity ringdown spectroscopy (CRDS). The method uses known spectral lines as wavelength markers to calibrate and refine the wavelength-current relation of laser diodes, and their spectral positions are taken directly from the HITRAN 2004 database. We built a compact CW CRDS apparatus with a 1.517 microm (approximately 6594 cm-1) distributed feedback (DFB) laser diode as the light source and a 25 cm long glass ceramic as the cavity in which to demonstrate the method. A wavelength precision of approximately 0.8 x 10(-3) cm-1 was obtained by comparing the HITRAN 2004 database, which was approximately four times more precise than that of the conventional method.