High Precision Hybrid Pulse and Phase-Shift Laser Ranging System

With the rapid development of military, aerospace, and precision manufacturing technology, a multitude of situations need to carry out a large-range and high-precision distance measurement. The growth of measurement applications has led to a higher requirement for the laser ranging technology which can be accomplished by using different patterns. At present, the pulse laser ranging method is widely used for medium-range and long-range measurement because of the fast measurement speed and considerable measurement range. However, the ranging precision is low. The short-distance measurement mostly adopts the phase-shift laser ranging method which has high ranging accuracy but limited measurement range. Therefore, the research on lifting the accuracy of pulse laser ranging method and extending the measurement range of the phase-shift laser ranging method will be carried out. In this thesis, combining the existing pulse laser ranging system and phase-shift laser ranging system, dual-frequency and single-frequency hybrid pulse and phase-shift laser ranging systems are proposed. The basis for solving the current problems of poor measurement precision in pulse laser ranging method and short measurement distance in phase-shift laser ranging method are provided. Also, the designed structures have a broad application prospect in the fields of industrial production, military, and aviation. At the beginning of the thesis, the principle and characteristics of the current typical laser ranging methods are introduced and analyzed. According to the Fourier Series theory, the spectrum analysis of the pulse signal and the relationship between the pulse signal and the same-frequency sinusoidal signal, the idea of phase-shift laser ranging based on pulse modulation signal is generated. Instead of a continuous sinusoidal signal, the laser is modulated with a periodic pulse signal. Distance measurement by calculating the phase difference on the sinusoidal signal extracted from the pulse signal with the same frequency at the receiving end. Based on the principle of conventional dual-frequency phase-shift laser ranging method, a dual-frequency pulse laser ranging method is proposed. The distance to be measured is obtained by transmitting two periodic pulse signals with different frequency and then combining the implementation of rough and accurate measurement outcomes. Afterward, a single-frequency pulse laser ranging method is introduced. After receiving the pulse signal, the direct counter method is used to realize rough measurement and phase-shift of the co-frequency sinusoidal signal is utilized to improve the ranging accuracy. This proposed model has the advantages of high ranging precision and long-distance measurement without any other auxiliary frequency. The accuracy of the phase difference calculation is the most critical element in both the dual-frequency and single-frequency laser ranging systems. Currently, the commonly used phase difference calculation methods operated in phase-shift laser ranging system are digital synchronous detection, fast Fourier transform method, and all phase fast Fourier transform method. Published works have discussed the performance of frequency estimation and initial phase calculation using these approaches. In this thesis, the precision of phase difference measurement based on these methods above is compared. The effects of normalized frequency deviation, white Gaussian noise, harmonics are simulated in MATLAB. Simulation results show that all phase fast Fourier transform method has a superior anti-noise ability so that exceptional accuracy of phase difference measurement can be achieved. Furthermore, as the number of sampling points increases, all phase fast Fourier transform method will obtain a more accurate calculation consequence. Finally, this thesis carries on the co-simulation test of the designed dual-frequency and single-frequency hybrid pulse and phase-shift laser ranging systems in Optisystem and MATLAB. The transmitting frequencies of pulse signals operated in the dual-frequency method are 15 MHz and 150 KHz. The pulse used in the single-frequency method is set to 15 MHz. In the simulation, the performance of proposed methods is tested by setting various measuring distance. When the number of sampling points is 1024, the standard deviation and ranging error of the dual-frequency method are 3.72 cm and 13.6 cm within 963.15 meters. For the single-frequency method, the results show a 3.78 cm standard deviation and 14.6 cm ranging error. Simulation results illustrate that the proposed ranging methods have lower ranging error compared with recently published works. It means that the combination of the pulse method and the phase-shift method can achieve high-accuracy and long-range measurement

On the use of power reflection ratio and phase change to determine the geometry of a blockage in a pipe

Blockages may be detected in pipes by sending acoustic signals down the pipe and measuring the echo from the blockage. This presents a fast and efficient way of determining the presence of a blockage and this method is now being used, for example, to probe the integrity of sewer systems. In this article a method is presented for obtaining both the length and the equivalent cross-sectional area of a blockage using only a single microphone to capture the incident and reflected pulse. The method presented uses the change in phase between the incident and reflected acoustic signals caused by a blockage, as well as the difference in the amplitude of each pulse, to generate two independent equations from which the area ratio and the length of the blockage may be recovered. This requires measurements to be carried out in the plane wave region of the pipe, however it is shown that through appropriate processing of each signal in the frequency domain the area ratio and length of a relatively large number of blockages can be successfully recovered.UK Engineering and Physical Sciences Research Council

Characterization of chromatic dispersion in single mode fibre

In this dissertation, an investigation of chromatic dispersion is presented. The Pulse delay and phase Shift chromatic dispersion characterization techniques were used for conducting the measurements. The experiments were performed in a modern optical fibre research laboratory and chromatic dispersion measurements were carried out on several lengths of G.652 and G.655 single mode fibres. The pulse delay characterization technique measures the time of flight between two modulated optical signals whilst propagating along the fibre under test. During phase shift experiments, the group delay is obtained by measuring the relative phase difference as a function of wavelength, between adjacent sinusoidal light signals. The pulse delay and phase shift characterization techniques illustrated excellent agreement in the measured! chromatic Dispersion coefficients along the G.652 standard single mode! fibre as well as the G.655 positive and negative non-zero dispersion shifted fibre. It was found that the measurement accuracy improved as the fibre length increased. A periodic shift between the modulated optical signals, propagating along the fibre was experimentally observed. It is to be remarked that the longer wavelength signals propagated faster along the G.655 positive non-zero dispersion shifted fibre in comparison to its transmission within the G.655 negative non-zero dispersion shifted fibre. Furthermore, it was found that the sinusoidal signal shifted towards the left along the G.655 negative NZDSF fibre whilst the shift occurred towards the right along the G.655 positive NZDSF fibre. Generally, the shift arising along the G.655 fibres was found to be smaller than the shift seen throughout the G.652 fibres. Towards the end of this study, a chromatic dispersion compensation system was designed and tested. Once characterization of the compensation link was completed, it was experimentally illustrated that the chromatic dispersion across the system was successfully reduced. Finally, a sum of squares of error statistical test showed that the phase shift technique is more accurate in comparison to the pulse delay method. This result was found to be in good agreement with published work found in literature

Faraday rotation measurement method and apparatus

A method and device for measuring Faraday rotation of a received RF signal is described. A simultaneous orthogonal polarization receiver compensates for a 3 db loss due to splitting of a received signal into left circular and right circular polarization channels. The compensation is achieved by RF and modulation arraying utilizing a specific receiver array which also detects and measures Faraday rotation in the presence or absence of spin stabilization effects on a linear polarization vector. Either up-link or down-link measurement of Faraday rotation is possible. Specifically, the Faraday measurement apparatus utilized in conjunction with the specific receiver array provides a means for comparing the phase of a reference signal in the receiver array to the phase of a tracking loop signal related to the incoming signal, and comparing the phase of the reference signal to the phase of the tracking signal shifted in phase by 90 degrees. The averaged and unaveraged signals, are compared, the phase changes between the two signals being related to Faraday rotation

Pulsed phase locked loop strain monitor

The RF output of a voltage controlled oscillator (VCO) is periodically gated to a transducer which produces acoustic waves in a bolt. The reflected acoustic waves are converted to electrical signals by the transducer and gated to a mixer which also receives the output from the VCO and produces an output which is filtered by a low pass filter. The output of filter is a dc signal proportional to the phase difference change from a fixed phase difference between the two input signals to the mixer. This dc signal is sampled at an instant and held by circuit in response to the "P" signal. The output of the circuit is integrated and then applied to the VCO to change the frequency of the VCO such that the phase difference between the two inputs to the mixer remains at the fixed phase difference. The frequency of the VCO is a measure of the change in strain of the bolt

Apparatus for using a time interval counter to measure frequency stability

An apparatus for measuring the relative stability of two signals is disclosed comprising a means for mixing the two signals down to a beat note sine wave and for producing a beat note square wave whose upcrossings are the same as the sine wave. A source of reference frequency is supplied to a clock divider and interval counter to synchronize them and to generate a picket fence for providing a time reference grid of period shorter than the beat period. An interval counter is employed to make a preliminary measurement between successive upcrossings of the beat note square wave for providing an approximate time interval therebetween as a reference. The beat note square wave and the picket fence are then provided to the interval counter to provide an output consisting of the time difference between the upcrossing of each beat note square wave cycle and the next picket fence pulse such that the counter is ready for each upcrossing and dead time is avoided. A computer containing an algorithm for calculating the exact times of the beat note upcrossings then computes the upcrossing times

Double reference pulsed phase locked loop

A double reference pulse phase locked loop is described which measures the phase shift between tone burst signals initially derived from the same periodic signal source (voltage controlled oscillator) and delayed by different amounts because of two different paths. A first path is from the transducer to the surface of a sample and back. A second path is from the transducer to the opposite surface and back. A first pulse phase locked loop including a phase detector and a phase shifter forces the tone burst signal delayed by the second path in phase quadrature with the periodic signal source. A second pulse phase locked loop including a second phase detector forces the tone burst signals delayed by the first path into phase quadrature with the phase shifted periodic signal source

Interferometric locating system

A system is described for determining the position of a vehicle or other target that emits radio waves and which is of the type that senses the difference in time of arrival at spaced ground stations of signals from the vehicle to locate the vehicle on a set of intersecting hyperbolas. A network of four ground stations detects the radio emissions from the vehicle and by means of cross correlation derives the relative signal delay at the ground stations from which the vehicle position is deduced. Because the signal detection is by cross correlation, no knowledge of the emission is needed, which makes even unintentional radio noise emissions usable as a locator beacon. By positioning one of the four ground stations at an elevation significantly above the plane of the other three stations, a three dimensional fix on the vehicle is possible

Laser induced surface acoustic wave combined with phase sensitive optical coherence tomography for superficial tissue characterization:a solution for practical application

Mechanical properties are important parameters that can be used to assess the physiologic conditions of biologic tissue. Measurements and mapping of tissue mechanical properties can aid in the diagnosis, characterisation and treatment of diseases. As a non-invasive, non-destructive and non-contact method, laser induced surface acoustic waves (SAWs) have potential to accurately characterise tissue elastic properties. However, challenge still exists when the laser is directly applied to the tissue because of potential heat generation due to laser energy deposition. This paper focuses on the thermal effect of the laser induced SAW on the tissue target and provides an alternate solution to facilitate its application in clinic environment. The solution proposed is to apply a thin agar membrane as surface shield to protect the tissue. Transient thermal analysis is developed and verified by experiments to study the effects of the high energy Nd:YAG laser pulse on the surface shield. The approach is then verified by measuring the mechanical property of skin in a Thiel mouse model. The results demonstrate a useful step toward the practical application of laser induced SAW method for measuring real elasticity of normal and diseased tissues in dermatology and other surface epithelia