Theoretical analysis of 2D acceleration laser sensor and several design parameters

Having adopted several sets of new technology and techniques, we have developed a new type of laser angle sensor that can be used to measure two-dimensional (2D) angle of moving target. On this basis, a new design for 2D acceleration laser sensor has been advanced. The optics-type sensor can be used to measure 2D acceleration of motion object, and it has high sensitivity, high precision and longer measuring range. Under these design parameters that are given in this paper, the measuring range will achieve 177 g, while its sensitivity is 13.5 s². The experimental results show that the measuring error is not greater than 0.1 g. In this paper, several design parameters and measuring results for 2D acceleration laser sensor are given.     

Collimating of diverging laser diode beam using graded-index optical fiber

This paper presents a technique for collimating a diverging beam from a laser diode using graded-index optical fiber. The optical set-up is relatively simple and easy to operate. The results show that laser beam of large divergent angle in the range of 10–35° can be reduced to a fully collimated beam with a divergent angle of less than 0.05°. The theory of the method as well as some experimental results is presented.     

Performance of an optical stand-off control system for laser materials processing

This paper presents an experimental investigation into a high-bandwidth optical range sensor for laser materials processing stand-off control. Radiation from a low-power laser beam is focused onto a workpiece surface and light reflected from the surface is collected through a main lens and directed into an imaging lens which focuses the signal to two positions after being split by a beam splitter. The irradiances of the two beams are detected by photodiodes placed behind pinhole apertures positioned fore and aft of the two focal positions. A differential amplifier is used to generate an output signal that determines the magnitude and direction of any workpiece displacement. The system facilitates a measuring range of ±6 mm. A set of of experiments are performed and results are analysed for different setup configurations. The approximate range of instrument linearity is ±1 mm for the 75-mm focal length main less and ±2 mm for the 120-mm lens; in this linear range the optimal accuracy resolution is 1 μm. The system's effectiveness is controlling the stand-off distance of a laser cutting machine, and hence cut quality, is assessed.     

Folded resonator-dual polarization competition laser displacement sensor

The existing so-called “dual polarization competition laser displacement sensor” applies a cat’s eye resonator. The resonator could be disarranged when the cat’s eye reflector (CER) swings slightly with the rod of the slide rail in measuring the procedure and droops due to its deadweight in long-time operation. This influences the stability of the sensor significantly. In this paper, a novel cat’s eye folded resonator was built, based on which our system named “folded resonator-dual polarization competition laser displacement sensor” was set up. It has a high resolution of λ/16 (39.6 nm) and large measurement range of up to 17 mm. The misalignment sensitivities of traditional cat’s eye resonator and the folded one are both analyzed; and comprehensive experiments are carried out, through which a conclusion can be drawn out that the maximal adjusting angle and the maximal cross displacement of the CER in folded resonator are both about 5 times of those in cat’s eye cavity. Furthermore, the measurement results from this new sensor prove its high stability.     

A novel fiber Bragg grating high-temperature sensor

A novel fiber Bragg grating (FBG) sensor for the measurement of high temperature is proposed and experimentally demonstrated. The interrogation system of the sensor system is simple, low cost but effective. The sensor head is comprised of one FBG and two metal rods. The lengths of the rods are different from each other. The coefficients of thermal expansion of the rods are also different from each other. The FBG will be strained by the sensor head when the temperature to be measured changes. The temperature is measured basis of the wavelength shifts of the FBG induced by strain. A dynamic range of 0–800 °C and a resolution of 1 °C have been obtained by the sensor system. The experiment results agree with theoretical analyses.     

All-fibre optical system for exciting and detecting the vibration of silicon microresonator sensors

An all-fibre optical system for optical interrogation and detection of the vibrations of a silicon microresonator is reported. Metal-coated silicon microresonators are excited by intensity modulated laser light delivered through an optical fibre, while the vibration of the resonators is detected by an optical fibre interferometer. Measurements have shown that an average optical power of 10 μW is sufficient to maintain the flexural vibration of the resonator. When the resonator is used as a pressure sensor, its resonant frequency changes from 62 kHz to 130 kHz as the pressure varies from -0°6 bar to 1 bar (gauge). A silicon resonator with 700 nm aluminium coating functions as a temperature sensor, showing a frequency shift from 262 kHz to 251 kHz when the temperature changes from 25 °C to 80 °C.     

Fiber-optic systems for temperature and vibration measurements in industrial applications

Fiber-optic systems for temperature and vibration measurements have been developed. It is shown that using the principle of wavelength conversion in the sensor, measurement systems, in which only one fiber is used for the connection of the sensor to the instrument, can be realized. The measuring accuracy is practically independent of the properties of the fiber-optic system such as fiber length, fiber diameter and type of connectors used. These properties are very important, reducing installation problems and costs. The adoption of semiconductor technology allows batch fabrication of sensor elements. For the prototype temperature instrument an accuracy of 0·5°C was obtained in the temperature range 0 to +200°C. Fiber-optic accelerometers for vibration measurements with a resolution of 0·05 g and a dynamic range of 70 dB have been realized. The systems have been tested in different industrial environments and have properties that make them suitable for many industrial applications. Advantages of fiber-optic systems are their noise immunity, galvanic isolation and inherent intrinsic safety.     

Fiber grating sensors for high-temperature measurement

Two fiber grating sensors for high-temperature measurements are proposed and experimentally demonstrated. The interrogation technologies of the sensor systems are all simple, low cost but effective. In the first sensor system, the sensor head is comprised of one fiber Bragg grating (FBG) and two metal rods. The lengths of the rods are different from each other. The coefficients of thermal expansion of the rods are also different from each other. The FBG will be strained by the sensor head when the temperature to be measured changes. The temperature is measured based on the wavelength-shifts of the FBG induced by the strain. In the second sensor system, a long-period fiber grating (LPG) is used as the high-temperature sensor head. The LPG is very-high-temperature stable CO₂-laser-induced grating and has a linear function of wavelength–temperature in the range of 0–800 °C. A dynamic range of 0–800 °C and a resolution of 1 °C have been obtained by either the first or the second sensor system. The experimental results agree with theoretical analyses.     

激光位移传感器的光学系统设计

针对目前国内自主研制的激光位移传感器精度低,测量范围小等问题,提出了一种采用光学设计软件预先仿真整个激光位移传感器光学系统的方法。在分析系统各部分的光学特性的基础上,结合具体要求设计了一个激光位移传感器的光学系统,其工作范围为（50±10）mm。采用系统分割的方法,将整个光学系统分为两部分进行设计,第一部分是激光束的整形透镜,要求在有效的工作范围内得到小而均匀的出射光斑,设计结果表明,在测量范围内,光斑大小能够控制在10-1mm量级;另一部分是被测面散射光接收的成像物镜,该系统的特点是物面和像面相对于光轴都有一定的角度,实验结果表明其成像满足Scbeimpflug条件。     

Intrinsic integrated optical temperature sensor based on waveguide bend loss

Curvature of a multimode integrated optical waveguide reduces the numerical aperture and induces radiation losses. In this paper, we study this phenomenon and we present a geometrical approach to calculate the local numerical aperture and the intensity attenuation. We exploit the bending effect on the local numerical aperture to make a new intrinsic temperature sensor. The simulation results are validated for the silica/silicone integrated optical case. The principal performances of the silica/silicone temperature sensor are the extended temperatures range (−50 to 200 °C) with an excellent linearity response (1%) between 20 and 200 °C.     

A multi-parameter optical fiber sensor with interrogation and discrimination capabilities

A multi-parameter and multi-function, but low-cost, optical fiber grating sensor with self-interrogation and self-discrimination capabilities is presented theoretically and experimentally. The sensor bases on three fiber Bragg gratings (FBG) and one fiber long period grating (LPG). Strain, vibration, pressure, ordinary temperature (−10 to 100 °C) and high temperature (100–800 °C) can be measured by the sensor. When high temperature (100–800 °C) is measured, the LPG is used as a high temperture sensor head and FBG₁ is used as an interrogation element. Alternatively, when one of the other four measurands is measured, FBG₁ (or FBG₂) is used as a sensor head and LPG is used as an interrogation element. When two of the other four measurands are measured simultaneously, FBG₁ and FBG₂ are used as sensor heads and LPG is used as a shared interrogation element. FBG₃ is used as a reference element to eliminate the errors resulted from light source fluctuation and the cross-sensitivity between measurand and environmental temperature. The measurands can be interrogated according to the signals of the photodiodes (PDs), which are related to the relative wavelength shift of the LPG and the FBGs. Experimental results agree well with theoretical analyses. The interrogation scheme is immune to light source fluctuation and the cross-sensitivity between measurands and enviromental temperature, and also the dynamic range is large.     

Height gauge based on dual polarization competition laser

Based on dual polarization competition laser, we develop a new height gauge. By inserting a quartz crystal plate into the laser cavity, one laser beam splits into two orthogonally linear polarized laser beams, which appear one after another with the change in cavity length. After detecting their intensities, we obtain two power-turning curves. The longitudinal mode spacing is divided into 4 equal zones and each one has different intensity phenomena, which provides a new method of height measurement. According to experiments, the direct measuring range of this new method is 12 mm with resolution of 79.1 nm, and the indirect measuring range can be increased using first grade gauge blocks and a liftable platform. Compared to other height gauges such as vernier gauge, inductance sensor and gauge block interferometer, this new instrument has the advantage of self-calibration and simple structure without frequency stabilization system.     

Design and operation of a fiber optic sensor for liquid level detection

Design and construction of an optical fiber sensor for liquid level detection are reported. This sensor operates based on light intensity modulation, and such modulation results from alteration of total internal reflection into partial reflection at the interface. The modulated intensity has been measured by using a pair of fibers, one transmitting source light, another acting as receiving fiber, and a glass prism providing the total and partial reflections. During the level measurements, when a liquid in a vessel touches the 45° faces of the 45-90-45° prism, the total internal reflection is disturbed, and the reflected light is modulated. The performance of this sensor is tested with different source lights including a light emitting diode (LED), a diode laser, and a He–Ne laser. Extinction ratio has been measured for different liquids, and compared. This ratio for water using LED source is about 0.03, for diode laser is 0.006 and for He–Ne laser is 0.003. Although this device was tested as a liquid level sensor, but the distinct results obtained for samples with different index of refractions demonstrate that the reported sensor can also be used as a liquid refractometer.     

Superhydrophobic surfaces fabricated by microstructuring of stainless steel using a femtosecond laser

Fabrication of superhydrophobic surfaces induced by femtosecond laser is a research hotspot of superhydrophobic surface studies nowadays. We present a simple and easily-controlled method for fabricating stainless steel-based superhydrophobic surfaces. The method consists of microstructuring stainless steel surfaces by irradiating samples with femtosecond laser pulses and silanizing the surfaces. By low laser fluence, we fabricated typical laser-induced periodic surface structures (LIPSS) on the submicron level. The apparent contact angle (CA) on the surface is 150.3°. With laser fluence increasing, we fabricated periodic ripples and periodic cone-shaped spikes on the micron scale, both covered with LIPSS. The stainless steel-based surfaces with micro- and submicron double-scale structure have higher apparent CAs. On the surface of double-scale structure, the maximal apparent CA is 166.3° and at the same time, the sliding angle (SA) is 4.2°.     

Crystallographic contribution to the formation of the columnar grain structure in cobalt films deposited at oblique incidence

The geometric and crystallographic structures for cobalt films deposited at 75° and 45° by sputtering were investigated on the basis of optical and magnetic measurements and X-ray analysis. The substrate temperature was 332 K and the film thickness ranged from 20 to 900 nm. The alignments of columnar grains below and above 50 nm are perpendicular and parallel to the incidence plane, respectively, and the packing density of columnar grains constituting the parallel alignment decreases with increasing thickness at both incidence angles. At the high incidence angle of 75° the parallel alignment is more well-defined and its packing density is lower. The conclusions from these results are as follows. (1) Above 50 nm the crystal habit induces the two-degree orientation of the HCP phase through the geometric selection. Higher incidence angle enhances the geometric selection. (2) The crystal size necessary to exhibit the crystal habit is independent of the incidence angle.     

红外地球敏感器扫描镜摆角激光动态测试方法

为解决扫描镜摆角实时动态非接触测量问题,基于激光检测技术和CCD探测技术,提出一种红外地球敏感器扫描镜摆角激光动态测试方法,并研制了扫描镜摆角动态测试系统,其可实现扫描镜的摆动频率、零位角、幅值、峰峰值平均等参量的动静态激光非接触测量。介绍了系统的组成和总体结构,着重对扫描镜摆角动态测量理论和大视场、大相对孔径特殊线性扫描光学系统的设计方法进行了分析与探讨,通过建立系统的数学模型,解决了测量数据误差修正与图形处理问题。对测量系统的精度进行了验证,结果表明系统的摆角测量范围为0~±12°,分辨力为0.01°,动静态测量精度优于±0.04°。     

Computation of streamwise vorticity in a compressible flow of a winglet nozzle-based COIL device

Chemical oxygen iodine laser (COIL) is a high-power laser with potential applications in both military as well as in the industry. COIL is the only chemical laser based on electronic transition with a wavelength of 1.315 μm, which falls in the near-infrared (IR) range. Thus, COIL beam can also be transported via optical fibers for remote applications such as dismantling of nuclear reactors. The efficiency of a supersonic COIL is essentially a function of mixing specially in systems employing cross-stream injection of the secondary lasing (I₂) flow in supersonic regime into the primary pumping (O₂¹Δ_g) flow. Streamwise vorticity has been proven to be among the most effective manner of enhancing mixing and has been utilized in jet engines for thrust augmentation, noise reduction, supersonic combustion, etc. Therefore, a computational study of the generation of streamwise vorticity in the supersonic flow field of a COIL device employing a winglet nozzle with various delta wing angles of 5°, 10°, and 22.5° has been carried out. The study predicts a typical Mach number of approximately 1.75 for all the winglet geometries. The analysis also confirms that the winglet geometry doubles up both as a nozzle and as a vortex generator. The region of maximum turbulence and fully developed streamwise vortices is observed to occur close to the exit, at x/λ of 0.5, of the winglets making it the most suitable region for secondary flow injection for achieving efficient mixing. The predicted length scale of the scalloped mixer formed by the winglet nozzle is 4λ. Also, the winglet nozzle with 10° lobe angle is most suitable from the point of view of mixing developing cross-stream velocity of 120 m/s with acceptable pressure drop of 0.7 Torr. The winglet geometry with 5° lobe angle is associated with a low cross-stream velocity of 60 m/s, whereas the one with 22.5° lobe angle is associated with a large static and total pressure drop of 1.87 and 9.37 Torr, respectively, making both the geometries unsuitable for COIL systems. The experimental validation shows a close agreement with the computationally predicted values. The studies for the most suitable 10° lobe angle geometry show an observed Mach number of 1.72 with an improved mixing efficiency of 74% due to the occurrence of predicted streamwise vortices in the flow.     

Experimental determination of scattering matrices of dust particles at visible wavelengths: The IAA light scattering apparatus

We present a new apparatus for measuring the complete scattering matrix as a function of the scattering angle of dust irregular particles. The design is based on the well-known apparatus located in Amsterdam, The Netherlands. In this improved version we have extended the scattering angle ranging from 3° to 177°. Moreover, the measurements are performed with a tunable argon–krypton laser that emit at a wavelength (λ) of 483, 488, 520, 568, or 647 nm. The apparatus has been developed at the Instituto de Astrofísica de Andalucía (IAA), Granada, Spain. To measure the scattering matrix elements we use a number of different optical components such as polarizers, a quarter-wave plate, and an electro-optic modulator. These components are used to manipulate the polarization state of light. By using eight different combinations for the orientation angles of the optical components, all scattering matrix elements are obtained as functions of the scattering angle. The accuracy of the instrument is tested by comparing the measured scattering matrices of water droplets at 488, 520, and 647 nm with Lorenz–Mie calculations for a distribution of homogeneous water droplets.     

Sol–gel based anti-reflection coatings on wedged laser rods using a spin coater

Anti-reflection (AR) sol–gel coatings are deposited on wedge glass optics for high-power lasers using spin coating technique. Characterization of these coatings on BK-7 glass substrates is carried out in terms of thickness profile across the surface, thickness variation w.r.t. wedge angle, and its effect on AR coating reflectivity, at different wedge angles from 1° to 7°. Results of the study are used to deposit AR coatings on inclined end faces of Nd:phosphate glass laser rods.     

A 3 kW average power tunable TEA CO2 laser

A high average power line-tunable TEA CO₂ laser is described. Average output power up to 3 kW is achieved at repetition rate of 180 Hz. The maximum output power is almost equal among the four spectrum bands of CO₂ laser (the P and R branches of 00°1–10°0 and 00°1–02°0 transition bands). Several special technologies including rotating spark gap switching, PCB (printed circuit board) preionization and zeroth-order grating coupling are employed.