The generation of a variable diameter annular flat-top laser beam in the far field based on an adaptive weight FFT-based iterative algorithm (AWFFT-IA) and a phase only liquid crystal spatial light modulators (poLCSLM) is demonstrated. The iterative algorithm is used to design the needed phase distribution written on the poLCSLM for the target diameter annular flat-top laser beam shaping. The experimental results show that the method proposed can concentrates about 71% of the incident laser energy into the desired region and the root mean square error (RMSE) of the tailored flat-top intensity profile is more or less 12%.
Multi-beam technology is one of the key technologies in optical phased array systems for multi-object treatment and
multi-task operation. A multi-beam forming and steering method was proposed. This method uses isosceles triangle
multilevel phase grating (ITMPG) to form multiple beams simultaneously. Phase profile of the grating is a quantized
isosceles triangle with stairs. By changing the phase difference corresponding to the triangle height, multiple beams can
be steered symmetrically. It took 34 ms to calculate a set of parameters for one ITMPG, namely one steering. A liquid
crystal spatial light modulator was used for the experiment, which formed 6 gratings. The distortion of which had been
compensated with the accuracy of 0.0408 λ. Each grating included 16 phase elements with the same period. Steering
angle corresponded to the triangle height, which is the phase difference. Relative diffraction efficiency for multiple
beams was greater than 81%, intensity nonuniformity was less than 0.134, and the deflection resolution was 2.263 mrad.
Experimental results demonstrate that the proposed method can be used to form and steer symmetrical multiple beams
simultaneously with the same intensity and high diffraction efficiency in the far field, the deflection resolution is related
to the reciprocal of grating period.
The discreteness of driving electrodes in liquid crystal optical phased array (LCOPA) device causes phase valley in the
region between two adjacent electrodes. When a one-dimensional transmission-type LCOPA device was used, there was
a pair of diffraction sidelobes with considerable intensity on either side of the deflected beam, which decreased the
diffraction efficiency of deflected beam greatly. An analytical numerical model of phase valley was proposed for the
quantitative analytic analysis on diffraction efficiency. Results showed that phase valley is the cause for diffraction
sidelobes. The diffraction efficiency of deflected beam decreases steeply as the phase valley depth increases. When
valley depth decreases close to zero, the main factor having affect on diffraction efficiency turns to be flyback region
size. The influence of electrode space on diffraction efficiency was quantitatively analyzed as well. An effective way to
reduce or even eliminate the phase valley is to reduce the space between electrodes.
The structure of a storage capacitor in nematic liquid crystal optical phased array causes the data refresh rate to be higher
than the liquid crystal switching frequency, which demands a high data bandwidth. A novel driving method was
therefore proposed to reduce the data refresh rate. Without using a storage capacitor, the proposed method uses digital
scanning instead of conventional analog scanning. Furthermore, digital scanning is able to drive all pixels in parallel and
transmit driver data only once for one frame. 1-D nematic liquid crystal optical phased array was used to test the novel
driving method. During the experiment two frames of driver data were alternately transmitted at a refresh rate of 10 Hz,
and diffraction patterns were acquired. Experimental results demonstrate that the proposed method can be used to keep
the data refresh rate lower than the liquid crystal switching frequency.
In order to realize laser beam steering, a driving and control system based on Voice Coil Actuator (VCA) is designed for
linear motion of micro-lens array. According to the feedback displacement signals acquired from displacement sensor,
controller of the system sends control signals to Purse Width Modulator (PWM) driver which is used to power the VCA.
VCA provides driving force for the support structure of the system which connects the micro-lens array, and then the
linear motion of micro-lens array is achieved with the movement of support structure. The effectiveness of the system
was verified through experiment. Experiments results show that the displacement resolution of the linear driving and
control system is 10&mgr;m , which satisfies the requirement for laser beam steering.
Diffractive beam direction needs to be controlled agilely and precisely in beam steering and some diffraction
applications. Far field fringe generation, wave front distortion correction by employing programmable diffractive element
is a new interesting research area. A phase-only liquid crystal spatial light modulator is used to produce arbitrary
For the practical application, the main technology we tried includes: 1) the fast recovering speed from arbitrary fringe
to the phase distribution of liquid crystal spatial light modulator, 2) the expected fringe light intensity information is
delivered by phase information. The previous G-S phase recovering algorithm demands more iterative calculations,
which is not suitable for real time purpose. According to the statistics of light field distribution of expected fringes, a
pseudorandom phase encoding method has been used in this work. The expected fringe light intensity information is used
to limit the initial value for phase recovering calculation. By this way, the iterative time has been reduced largely, and
the phase recovering is optimized as well. The expected diffraction fringe can be obtained through 3-5 iterative
The experiment results show that the technology is satisfactory for fast applications.
Wave front precision distortion correction and transformation using a Liquid Crystal Spatial Light Modulator (LC
SLM) is discussed. After passing through non perfect elements, a coherent beam becomes a distortional beam. The
distortion is expressed in either optical paths difference or phase difference way. The difference is measured
quantitatively. SLM modulates the phase of the beam in a pixel resolution. The beam is recovered to a plane wave
front again after the correction. The accuracy of the modulation is 0.0165λ.
A practical numerical model for liquid crystal cell is set up based on the geometry structure of liquid crystal optical phased arrays (LCOPA). Model parameters include width and space of electrodes, thickness of liquid crystal layer, alignment layers, electrodes and glass substrates, pre-tilted angles, dielectric constants, elastic constants, the refractive indexes and so on. Especially, the thickness of alignment layer is first considered to the best of our knowledge. According to electrostatic field theory and Frank-Oseen elastic continuum theory, two dimension (2D) electric potential distribution and 2D director distribution are calculated by means of the finite difference method on non-uniform grids. And the phase delay of LCOPA is derived from crystal optics. The influence of cell sizes on phase delay distribution is analyzed. The evaluation function of fringing field effect is provided. And the methods to decrease fringing field effect between electrodes are also discussed.
In order to obtain the phase modulation characteristics of liquid crystal spatial light modulator (LC SLM) a polarization interference method is introduced to measure the phase shift. In this method the polarized components of the light reflected from the LC SLM will interfere with each other after crossing two polarizers. The phase shift can be determined by analyzing the interference intensity. Since the two interference components of the light travel through the same path, the measurement setup is insensitive to vibrations and air currents, and then the accuracy of the measured data can be improved. The experiment results show that the phase modulations corresponding to different pixels of the LC SLM are relatively uniform across the aperture. The phase shifts will change linearly over 0 to 2π with gray levels when the custom look-up table (LUT) is used.
Along with the rapid development of wearable computer and multi-medium technology, many units need to be mounted on the helmet such as micro display, camera, voice input and output components, even some devices for safety and security purpose. If these units and components are controlled by the wearable computer directly, it would make the interface between helmet and wearable computer complicated. The better way is to add a controller to the helmet, then the wearable computer only need to interface with the controller. The helmet controller controls all of the functional components of helmet. Of cause, it should be noticed that the dimensions of the components must be small since the volume of helmet for the controller is very limited. The core of the helmet controller we designed is composed of a digital signal processor (DSP) and field programmable logical array (FPGA). The DSP carries out the function of encoding, decoding, compression, encryption, synthesis, and filtering of image and voice signals. FPGA drives and controls a micro display, controls the functional components, as well. All of these reduced the amount of elements, enlarged the integration level, which realized the helmet controller microminiaturization.
This paper focuses mainly on DMT transmission technology applying to downhole image transmission to extend bandwidth, also considers the details about using digital signal processor (DSP) to compress, process and reconfigure the image taken from infrared CCD camera and to process signals from multi-parameter sensors. The system is composed of multi-parameter sensors, infrared camera, TMS320c6x DSP for image compression and reconfiguration, ADSL modem chipset, ground computer, etc., which extends the bandwidth, decreases signal attenuation and abates the noise. Therefore system implements real time downhole imaging, multi-parameter monitoring and inspection by two-wire transmission over 7000 feet no repeaters.
In this paper, the principle of our depth-image based surface measurement sensor using the single-stripe pattern is introduced. The mathematical model of depth measurement based on structured-light method is given. According to the analysis of the depth measurement error of our sensor, the design rules of our sensor is put forward so as to achieve the aim of enhancing the surface measurement precision of our sensor. The designed example of our sensor is given.
At first, this paper introduces the principle of our section outline sensor using the single-stripe pattern. At second, the measurement error of our sensor is analyzed: the error factors causing the measurement error of our sensor are found out; the mathematical expressions of the measurements are deduced. At third, the measures to reduce the measurement error and the design rules of our sensor are given. At last, a design example is given.
This paper describes the principle of the spatial encoding- based rangefinder. Its parameters to be calibrate are found out. Mathematical expressions are deduced of its measurement errors caused by calibration errors. Analysis shows that it is difficult to calibrate the parameters separately, especially some parameters are unable to calibrate. So we present a new calibrating method based on table. In the normal operation of the rangefinder, it directly obtains the ranges the pixels of the image of the object from the table after giving its codes. Our calibrating method has the advantages of simple and high accuracy.
Space encoding is the effective method saving image- acquisition time in structure light 3D vision. In this paper, we propose calibration parameters, which do not include camera inner parameters, of the rangefinder using scanning-based spatial code pattern. We discuss the influence of calibration errors on rangefinder measuring. A model of the systematic calibration errors is developed based on the geometry of the rangefinder. According to given rangefinder accuracy, the limits to calibration errors are set from the model. An example is cited. Moreover, we discuss the influence of the shifting of the camera lens center and the polyhedral mirror reflecting point on the rangefinder precision.
In this paper, simultaneous frequency, power and beam direction stabilization of transverse Zeeman Laser is reported. The axial mode of He-Ne laser is collapsed into two polarized components perpendicular to each other in a transverse imagination field. By setting up a proper thermal field, taking a frequency difference, between the two composts as the stabilization parameter, the output angle floating is less than 2 X 10-4, the relative power stability of 0.089% and the qencystability of 4 X 10-10 is obtained.
In this paper, the principle of range-image based on image coding is introduced. According to the encoded images obtained by our laser scan-based image-encoder, the algorithms are presented to produce the threshold value, turn the encoded images into binary-state, calculate the scanning angle and find the range image. The corresponding utility programs are completed, and verified by our experiments.
Classical machine vision is difficult to be used as practical 3D vision because it loses depth when projecting 3D objects into 2D images and then has ambiguity when reconstructing 3D object. In contrast, range images, i.e. 3D images, keep the invariance properties of 3D objects, which makes the image processing tasks easier. This paper develops the prototype of spatial encoder using laser scanning method. Range images are obtained based on encoded images produced by our encoder.
The principle of structured light 3D vision sensing based on spatial encoding image is presented. The spatial encoded images is produced by the means of laser scanning and modulating according to time-sequences. Is realized its key technology, i.e. synchro control among the camera, laser diode modulation and scanning polyhedron. The spatial image encoder is developed.
This paper deals with a fast rangefinder based on laser scanning space-encoding method. In the first place the principle of image-encoding is introduced and the calculation formulas of measurement values in 3D rectangular coordinate system are given. In the second place the design rules are put forward at the angle of the measuring volume, shading light and received light power properties. At last the measurement errors caused by quantization errors, the resolutions and their errors are discussed through error analysis, and then the design rules to reduce effect of quantization errors are presented.
This paper deals with a laser scanning device used in a space-encoding rangefinder. At first the principle of space- encoding is introduced. And then the error expressions of the rangefinder are given on the basis of establishing its mathematical model. Furthermore are deduced the functional relations between the measurement errors and parameters of the laser scanning device as well as errors of the parameters. At last approaches to reduce the measurement errors caused by the laser scanning device are found out according to the above said error expressions and functional relations.
In 2D intensity image, the problems often cause wrong object recognition such as: los of depth information, shadows, overlapping objects. The main advantages of range image are the acquisition of depth information, which makes the separation easy, but it has blind areas; the boundary with special direction cannot be found. Using both intensity image and range image to do 3D object recognition can make full use of the advantages to avoid the shortcomings of each kind of image. Edge detection, grouping and linking, image segmentation, and outline determination are performed in both images. If the match between the two results is good, either one may be accepted, but if the match error is big, we need to confirm which one will be accepted or rejected. After examining each step, the criteria are used to form the outline of the object, then, the type, size, position, and orientation of each object can be calculated and determined.
This paper describes a laser scanning range finder that may produce a real time space encoding pattern. Laser diode is modulated by a programmable controller. So a space encoding sequential constructive laser beam is employed as an active illuminating source. Using less frames of space encoding patterns to produce a range image with high resolution is the main goal of this study. The calculation of 3D coordinates of each point on the measured object is explained briefly.
An instrument for fine-machined pipe (longer than 10 m) inspection and measurement is introduced in this paper. The non-contact interior wall roughness measurement system based on light reflecting and scattering theory has a wide range (Ra from 0.012 micrometer to 4 micrometer) and is suitable for different types of machined surface measurements. The interior diameter is measured by a pair of gratings which produce Moire fringes. The accuracy of which is better than 5 micrometer for a pipe diameter greater than 100 mm. A CCD video camera inspection system is also installed on the measuring head to check the interior wall quality.