2007 Vol. 28, No. 4
A new laser transverse mode demonstration system based on the loss adjustment of a cat-eye reflector is presented in this paper. A cat-eye corner reflector made up of a convex lens and a concave mirror adjustable in five degrees of freedom was used to form a lasing chamber capable of adjusting the loss. The change of the state of the laser transverse mode was realized by changing the loss factor of the lasing chamber through adjusting the cat-eye corner reflector. When one or some position parameters of the convex lens or the concave mirror were changed, the power loss and the symmetry of the laser resonator produced the relative changes of diffraction losses. Over twenty clear highorder transverse mode images were collected by CCD and stored in computer.
To overcome the limitation of the internal memory space and calculation complexity for the asymptotic image transmission algorithms, an improved algorithm, the static image coding method, is proposed. The algorithm carries out the high-frequency section subdivision of the wavelet coefficients after the wavelet transform, sets the thresholds for the image blocks of different frequency respectively, adopts a new discriminating criterion, and improves the framework of the algorithm. This concept is adopted in JPEG2000. The improved coding method is simulated with MATLAB 6.5. The result shows that the occupation of the internal memory space is reduced and the computational complexity is reduced, and the compression effect meets the expectation.
In order to achieve the velocity measurement for initiating explosive device, a new double-slit light screen was designed to realize low speed measurement for the part in initiating explosive device. The light screens designed are different from the traditional single-slit light screens. Two transmitters and receivers were installed into a common mechanical frame so that two screens were formed, which detected the moving objects going through their sensing area. Two identical devices designed are fixed on one mechanical frame with sliding rail. The distance between two devices can be easily adjusted by operator. The device can be used to measure the momentum of initiating explosive device. Compared to the traditional method with two pieces of copper as the sensor, this method improves the precision of measurement. The experiment with gas gun and real product proves that the double-slit light screen device is reliable and the result of measurement data is accurate.
Based on the theory of blackbody radiation, the influencing factors in detecting spatial target with a hyperspectrometer were analyzed. The radiation from the sun to the surface of an object was calculated. The sky diffusion radiation from visible to near infrared was analyzed with MODTRAN software. The calculation method for the target and background radiant energy received by the imaging spectrometer is given. The general expression of SNR was deduced. The feasible means to get the high SNR is put forward by analyzing the effect of all the factors on SNR.
The planar-array spatial light modulator and multi-pixel parallel scanning mode were put forward to meet the requirements of the mechanical rotating mirror scanning and the line-field synchronization of the beam and horizontal signal. The planar-array spatial light modulator was adopted for the laser beam gating and light-intensity modulation, such that the light intensity of video images was modulated when the laser beam covering multi pixels scanned the planar-array spatial light modulator in high speed. Therefore, the laser video images were formed. Since the multi-pixel parallel scanning, a novel 2-D scanning rotating mirror and an integrator beam path were used, the scanning speed was reduced from 30000rpm to 3000rpm. This technology solved the difficulties in scanning laser display and eliminated the laser interference fringes.
A novel method based on photorefractive (PR) two-wave mixing to detect air turbulence is presented. The air turbulence is modulated on the input pump and signal beams of a PR crystal, thus creating a phase fluctuation for the input beams. Such a phase fluctuation results in an intensity fluctuation in the output beams and the intensity fluctuation can be detected by measuring the differential signals between the two output beams. Our experimental demonstration shows that air turbulence can be detected effectively based on the process of PR two-wave mixing and an electrical differential detection. This method may be widely used in future detecting applications.
A multi-band optical system was designed for the push-broom space camera covering visible，medium infrared and far infrared wavebands. The system has large aperture and low F number due to the requirement of large FOV. With the reference to various scanning and push-broom cameras at home and abroad, an optimized compact optical system was achieved by an effective beam-splitting approach, in which the aberrations of three channels were corrected respectively according to the aberration theory and system requirement. The field-of-view of the optical system is 2.93°×0.3°, and F number for the visible light is 3.8 while F number for both of medium infrared and far infrared is 1.9. The design result indicates that the optical modulation transfer functions of the three bands within the cut-off frequency are all close to the diffraction limitation.
Based on the analysis for propagation characteristics of the light traveling in water, several essential factors in underwater photography, such as energy attenuation, field loss and water proof, are discussed. The regularity of image quality degradation due to defocus was analyzed when ordinary cameras installed with water proof windows were used under water. To achieve high quality images in deep water exploration, the design concept of the underwater zoom lens is proposed based on the focal power division formula, and an example is given for further explanation of the design consideration. The relative aperture of the zoom lens is F/1.6 in this design. The image quality of the zoom lens is improved, and the maximum FOV vignette is avoided.
Using pure ZnO and TiO2 grains, ZnO/TiO2thin films were fabricated on Si substrates by electron beam evaporation method in O2 environment. The structure and photoluminescence of ZnO/TiO2 thin films grown under different temperatures were investigated. The structural properties were investigated by X-ray diffraction and Raman spectra. The data indicate that the ZnO film deposited on TiO2 buffer layer are amorphous. Their optical properties were investigated by photoluminescence spectra. The photoluminescence measurements show that the amorphous ZnO/TiO2 thin film prepared at 250℃ emits strong UV at 389nm, strong violet light at 431nm and strong greenyellow light at 519nm.
The components of the space noise in an infrared image with the sky background were discussed. The reasons for forming all types of noises were analysed. The noise types of the IR image were classified and their effects on the image were investigated. The analysis indicates that the space nosie in the IR image mainly represents the Gauss noise and spicedsalt noise. An improved median filtering based on a simple threshold is introduced by considering the difference in the noise disposal of the averaging method and the median filtering. For the different thresholds, the operators are different too. The weight selection of every operator in the M×N area depends on the gray median in the area.The more the gray value is close to the median, the more the weight of operator is. The background of the image becomes smoother after the preprocessing.The SNR and improved PSNR increase by 1.2dB. It indicates that the improved median filtering works better than the traditional ones.
The total precipitable water in atmosphere is one of the most important parameters in astronomical observation and meteorology. A new instrument for measuring the total precipitable water in the atmosphere is introduced. The near infrared spectra of sunlight or moonlight passing through atmosphere were measured by a CCD spectrometer in the instrument, then the central residual intensity of the water vapor absorption at 935nm waveband was calculated and the total precipitable water in the atmosphere was derived by R=0.59W model. The calibration method, performance, advantages and disadvantages of the water vapor measurement instrument were discussed and measurement results were given. The measurement results indicate that this method is straightforward and its performance is stable.
At present, the acceleration sensor widely used in inertial navigation system of aviation industry can′t offer effective immunity towards the electro magnetic interference (EMI) and electro magnetic pulse (EMP), so a novel optical acceleration sensor based on a Fresnel diffractive microlens is proposed, which can solve the problems mentioned above effectively. The sensing principle of the sensor is that a reflecting membrane is put behind the microlens in parallel, and the acceleration is calculated by measuring the variation of the light intensity at the converging point in front of the microlens. The optical principle of the sensor is discussed in details by using Fresnel-Kirchhoff diffractive formula. The effect of the optical fiber deviation on the light intensity is analyzed with the computer simulation. The results prove that the light intensity received by the optical fiber has visible changes even for the nanometer level changes of the position of the reflecting membrane and is highly sensitive to the deviation of optical fiber in the direction of the focal plane. The light intensity will drop off to less than 50% of ideal light intensity when the deviation exceeds 2 microns. Moreover, the principle of this sensor was proved to be correct by a demonstration experiment.
The mathematic mode based on rate equations and power propagation equations of Tm3+-doped silica fiber lasers is established and calculated by Matlab software. Output mirror reflectivities of the laser power, optimization of the fiber length and dopant concentrations may significantly improve the slope efficiency of fiber lasers. The evolution of small signal gain coefficient without upconversion and with upconversion is showed for different pump powers. The distributions of the pump light and laser along the fiber and the ion concentrations at every energy level are analyzed. The relation between the small signal gain and incident pumping power, as well as the relation between the pumping light and laser powers with gain medium length are investigated at different dopant concentrations. The relation between output powers and different output coupling mirror reflectivities is studied at different pumping powers. And the further research is also carried out on the relation of slope efficiency and pumping threshold versus fiber lengths for different pump absorption coefficient. The results indicate that there are optimal fiber length and optimal coupling output transmissivity for the maximum of laser output power.
An optimization calculation of interleaver made of cascade Mach-Zehnder fiber interferometer with square wave is put forward. Based on transmit-matrix, the output spectrum from the cascade M-Z interferometer can be expressed as explicit function, the square wave output function of perfect interleaver can be expanded in a unique recursion way. Sometimes there is no accurate outcome when the terms of the two functions are compared, but approximate structureparameter of cascade Mach-Zehnder fiber interferometer can be obtained by optimizing calculation method with Matlab, which can make the output-spectrum an approximate square wave. The design example indicates that this method is convenient, flexible and effective.
Three techniques for measuring the pulse-to-pulse timing jitter of gainswitching semiconductor laser were experimentally compared. By experimental investigation, the advantages and disadvantages of these three measurement techniques were demonstrated and listed. It is found that the timing jitter measurement technique based on wide-band sampling oscilloscope is suitable for the measurement of the pulses with wide-band and big jitter, the resonance frequency spectrum analysis technique based on frequency spectrograph is applied to the optical pulse measurement of low repetition frequency, and the phase noise measurement technique based on the phase detector is applied to the measurement of high repetition frequency. It is pointed out that the phase noise measurement technique based on the phase detector has the advantages of high measurement accuracy, big dynamic measurement range and credible measurement result.
The influence of the one-stage single-lens optical system on the divergence angle of Gaussian laser beam is analyzed theoretically. The semiconductor laser in laser fuze was collimated with a plane-convex lens, and the result of the collimation met the requirement of the laser fuze. The divergence angle of light beam was measured with the method of CCD imaging and computed by second moment method. In the experiment, the horizontal divergence angle of 7.03°and the vertical divergence angle of 0.69° were obtained. Therefore, it is verified that the collimating system satisfies the specifications of θx＜10° and θy＜1°required by laser fuze.
The basic principle of the laser longitudinal mode splitting is introduced briefly, and the Nd∶YAG laser longitudinal mode splitting produced by placing one and two quarter-wave plates in the cavity of a Nd∶YAG laser is investigated experimentally. The results show that each longitudinal mode is split into two orthogonally and linearly polarized modes, and the splitting magnitude (i.e., frequency difference or wavelength difference) is just equal to one half of the laser longitudinal mode interval if the end surface of the quarter-wave plate is perpendicular to the laser beam when a quarter-wave plate is placed in the cavity of Nd∶YAG laser at 1064nm. The mode splitting phenomenon can also take place when the Nd∶YAG laser cavity contains two pieces of parallel placed quarter-wave plates along the direction normal to the laser beam, and its mode splitting magnitude is dependent on the angle between the fast (or slow) axes of both quarter-wave plates. The magnitude can be linearly tuned in one longitudinal mode interval when the angle is adjusted between 0 to 90 degrees. The results obtained experimentally agree with those of the theoretical analysis quite well.
Orbital angular momentum (OAM) of light beams attracted great attention recently due to its importance in theory and application. Most works on OAM of light beams were concenterated on coherent beams, and few efforts were made on partially (spatial) coherent beams (PCB). The OAM of PCB was deduced from their Wigner distribution function, and the OAM spectra of PCB were analysed. The Mercer expansion method was used to obtain the OAM spectra of PCB. For given PCBs, their OAM spectra were carried out. The dependence of OAM spectra on the partial coherence was also discussed. We found that the OAM of the PCBs takes the same form as that of coherent beams, the OAM spectrum of PCBs disperses more seriously with the worse coherence, and the OAM spectrum of PCBs with zero total OAM is more vulnerable to bad coherence than that of the PCBs with non-zero total OAM.
FFT algorithm and Rung Kutta method were adopted to investigate the frequency doubling process of the ultrashort laser pulse in KDP crystal. The effects of group velocity mismatch and dispersion, third-order nonlinear phase modulation on pulse shape, spectrum distribution and conversion efficiency of second-harmonic pulse were discussed. The results indicate that the modulation effect of the third-order nonlinear phase may distort the pulse shape of the frequency doubling, broaden the second-harmonic spectrum obviously and reduce the conversion efficiency. The third-order nonlinear phase modulation becomes the dominant factor to influence the process of the frequency doubling when the fundamental intensity of incident luminous power is over 100GW/cm2.
The synthetic discriminant function (SDF) algorithm based on binary joint transform correlation is introduced. The iterative algorithm was further optimized with SDF obtained by the equally related peaks and the GramSchmidt expansion to acquire the SDF suitable for binary joint transform correlator. The synthetic discriminant filter made up of the algorithm has the advantages of high relevant peak value, sharp relevant peak, and can recognize the rotating images. The computer simulation shows that the signal-to-noise ratio, light efficiency, relevant peak sharpness and recognition ability of this correlator are increased, and the rotational invariant optical mode recognition with high recognizability is realized.
The development of microspherical cavity theory is introduced, and the microspherical cavity is analyzed with light quantum mechanics. The 1533nm fluorescence spectrum of the Er3+ doped phosphate glass microsphere (about 15μm in diameter) was measured under laser excitation of 488nm. The phenomenon of the morphology-dependent resonances of the fluorescence spectrum in the glass microsphere and its formation mechanism were observed and analyzed. The fluorescence spectra of the Er3+ doped phosphate glass microsphere were compared with those of corresponding bulk glass. The single-mode oscillation amplitude generated by the Er3+ doped phosphate glass microsphere was obtained in the experiment.
The bandgap structure of one-dimensional photonic crystal was calculated and derived with the plane wave method (PWM). The effects of the dielectric constants of high/low refractive films making up one-dimensional photonic crystal structure and filling ratio between thickness of high reflective film and crystal period on the bandwidth of forbidden band gap were analyzed. The function relation between bandwidth and dielectric constants or the function between bandwidth and filling ratio was obtained by least square curve and curved surface fitting to make sure the best value of bandwidth, and to design the periodic structure of one-dimensional photonic crystal. For a one-dimensional photonic crystal with dielectric constants of 13/1 and filling ratio of 0.16, its calculated bandwidth is 0.2564×2πc/Λ and central frequency is 0.3478×2πc/Λ. These results agree with experiment data.
By investigating the polarization states of object light and reference light in Fresnel hologram, It is found that the linearly polarized laser becomes “partially polarized light” after it is scattered and diffusely reflected by the objects with different material surfaces. Almost all the targets have obvious depolarization phenomenon, and the coherence degree of interference fringes formed by the linearly polarized reference light and “partially polarized object light”, which is actually recorded on the holographic plate, are lower. The great decrease of object-light polarization degree influences the contrast of interference fringes and the noise of hologram. An experiment scheme and testing beam path are presented in this paper. A 1/4 wave plate was inserted into the reference beam path to make full use of the luminous energy of nonpolarized object light to enhance the contrast of fringes, eliminate the polarization effect of object surfaces and improve the quality of holographic record.
A novel shaped annular beam heterodyne confocal measurement system (SABHCMS) is proposed. The shaped annular beam confocal measurement method for sharpening the main lobe of Airy spot is used to improve the lateral resolution of a laser probe measurement system (LPMS), and the heterodyne confocal measurement method is used to improve the axial resolution of LPMS, thereby the spatial resolution of LPMS is improved. Experimental comparison and analyses indicate that both lateral resolution and measurement range of SABHCMS are improved as the normalized radius of the annular beam inner aperture ε increases; the lateral resolution of SABHCMS is better than 0.2μm and the axial resolution of SABHCMS is better than 2nm when the wavelength (λ) of the incident beam is 632.8nm, the numerical aperture (NA) of the objective lens is 0.85 and the normalized inner radius (ε) of the annular beam is 0.5. It is therefore concluded that SABHCMS is a new approach to higher spatial resolution of LPMS and can be used for ultra-precise measurement of microstructural workpieces.
A laser feedback interferometric system for measuring both the displacement and absolute distance of non-cooperative objects is presented. The system is based on a laser-diode pumped Nd:YAG microchip laser，and it is comprised of a frequency-shifted laser feedback interferometer, a frequency-modulated microchip laser based on pumping modulation and a phase-sensitive detection device. The principle of frequency-shifted laser feedback interference is introduced. The principle and method of displacement and absolute distance measurements are elaborated. The measurement resolution was analyzed. The displacement and absolute distance of a blacken workpiece surface were detected experimentally in the laboratory. The results prove that the interferometric system has high sensitivity and relaxed requirement for the reflectivity of the UUT surface, and meets the need of both displacement and absolute distance measurements for uncoupled objects.
A method of multi-scale edge detection for colour images based on the wavelet was studied. By selecting perceptible YUV colour space, the tensor product of multi-resolution-analysis was substituted for the tensor product of the wavelet function to build up the two dimensional Mallat algorithm to implement the fast wavelet transform, according to wavelet analytical theory. A standard pattern was applied to the multi-scale wavelet decomposition and the edge-detection results were obtained in the case of each scale. The experiment result coincided with theory analysis in high multi-scale and low multi-scale detection.
In order to effectively evaluate the modulation transfer function of an objective or optical system, the pinhole image is scanned automatically by CCD, with consideration for the effects of the system and the environment on the pinhole image. The image processing for the system is performed by continuously collecting several pinhole images, averaging them and then subtracting their backgrounds from them. The proper integration area containing all target information is chosen, and the reliable MTF is achieved by deriving the onedimensional line spread function from the integration in the meridian and sagittal directions and then performing the discrete Fourier Transform with the processed data. The resulting data indicate that the MTFs of objectives under test can be measured quickly and accurately in this way.
As the sensitivity of laser energy meter is dependent on the environmental temperature, the sensitivity deviation could reach 16% in the temperature range of -50℃～70℃. In order to make accurate measurement under the different temperature conditions of open field, a novel laser energy meter for field application was developed. Base on the design of the current laser energy meter, laser materials with better transparence, thicker absorbing layer and thermal sensitivity surface unsusceptible to damage were chosen, the temperature sensor and the digital processing circuit in the energy meter were redesigned to meet the requirements of laser energy metrology in field.
Based on the analysis of LED luminous mechanism and packaging features, some of the optical parameters were selected for testing LED, such as luminous flux, luminance, luminous intensity, spatial light intensity distribution, relative spectrum power distribution and colorimetry. The features of these parameters were investigated. The test principles, the corresponding formulas and the measurement methods are proposed for these parameters. The luminous flux measurement system (LFMS), luminance luminescence intensity measurement system (LLIMS), divergence angle spatial light intensity distribution measurement system (DASLIDMS) and relative spectrum power distribution colorimetry measurement system (RSPDCMS) were designed based on LED luminous mechanism and packaging features. The practical application shows that these systems can meet the requirements of the engineering applications.
The content of the verification procedure for testers of optical film refractive index and thickness is described. The specifications of the instrument under test, the main verification parameters and the verification method are introduced. The procedure applies to the instruments measuring the optical film refractive index and thickness by means of spectrum ellipsometry method. It has a great potential in development, production and application of optical film.