2012 Vol. 33, No. 3
Due to the lower detection and recognition probability of camouflage target by current opto-electrical imaging systems, it is important to develop a new technology to detect and recognize the camouflage target. This paper introduces an infrared polarization imaging system, describes the principle, constitute and characteristics of the system. The waveband range of the developed infrared polarization imaging system is 3 m ~5 m/8 m ~12 m, the degree of linear polarization is 95%, and the extinction ratio is above 100∶1. The results of analysis data and polarization fusion image are given. These results indicate that the infrared polarization imaging system can effectively realize the detection and recognition of camouflage target. The results can also be extended into the detection and reorganization of artificial false objects and aerial stealth targets.
he target location accuracy evaluation index of electro-optical reconnaissance system was investigated. Based on the research of root-mean-square error (RMSE), average error, and confidence probability, the accuracy evaluation methods of location, attitude, direction, distance were discussed and the optimal selection of various equipments was given, which would standardize the design and acceptance of the electro-optical reconnaissance system.
The characteristics, contents and methods of the design of optoelectronic products were discussed by using systematic engineering method, and the optimization design analytic decision based on the value criterion was described, as well as the method and process for achieving the full digitalized simulation platform of optoelectronic product by utilizing the virtual product development (VPD) technique in the design phase of the optoelectronic system development. An optoelectronic tracking and measurement product was developed based on the systematic engineering method in combination with the VPD technology .By using models and simulation technique the designer obtained the 3-D visual interactive integrated development environment for the concept formation ,design, manufacture and realization of the product. Combining VPD with the practical experience and according to the digital main model, the virtual prototype was designed and the simulation tests such as identification of dynamic model were carried out. The forecast design was achieved substituted for the experience design to finish the whole process for virtually-manufactured driver designing.
As a medium of spatial azimuthal information, modulated polarized light can be used to measure the azimuthal information. The technique can be widely applied in military, aerospace and biomedicine areas. The principle of measuring azimuth with magneto-optic modulation polarized light was expatiated, the misalignment azimuth measurement error from Bessel functions expansion was analyzed, and the formulae of calculating azimuth under different Bessel functions expansions were deduced in detail. The simulation results showed that with the increase of the items of Bessel functions expansions, the azimuth measurement error became smaller; the measurement error values of the frequency doubling signal and the tripling one were same, but the sign was different; after the items including fourfold frequency signal, the error kept unchanged basically. So when the modulated polarized light was used to measure the azimuth, the items of Bessel functions expansions subter-fourfold frequency signal were better.
Angle measurement accuracy plays an important role in photoelectric theodolite positioning and tracking. In order to further improve the accuracy of photoelectric theodiolite, the angle measurement errors were analyzed. Based on the overall design of photoelectric theodolite, the error sources of angle measurement accuracy which effect data acquisition, transformation and synthesis were found, and a detailed analysis of the size and properties was made. This paper first found out the main error sources by analyzing the working principle and structure of photoelectric theodolite. Then, it analyzed and calculated the rack system error, angle measurement system error, electrical system error, off target error, atmospheric refraction correction error, and distributed the error for each system. Finally, this paper calculated the root mean square of photoelectric theodolite-s angle measurement accuracy. The results show that the careful design, machining, testing and correcting could reduce and even ignore errors, but the infl ence on the angle measurement accuracy of the rack system error, angle measurement system error and off target error is still significant; it is possible to develop the photoelectric theodolite accuracy of spatial point less than 2 under the current level of technology.
Telescope is important to measure atmosphere optical parameters by tracking star or beaconing to measure data in real time. Based on the moving characteristics of beaconing at different speeds, design of a continuous zoom measurement system for atmospheric coherence length was required. The designed focal length of the system was 3 m~6 m, the wavelength was 486 m~656 m, the charge coupled device (CCD) size was 11mm and the entrance pupil diameter was 300mm. By using optical design software, a catadioptric zoom telescope system was designed which consisted of a Cassegrain telescope and three-component mechanically compensated zoom system. The optical zoom system has high quality, simple structure, low cost and smooth cam curve, and all the pressure rise angles are no more than 45. It can output real-time zoom data of focus length and is suitable for measuring the atmosphere coherent length real-timely on the occasion of high and low speeds.
Optical system component of Fourier telescope field imaging experiment system was proposed, optical alignment and test progress were introduced particularly, and imaging experiments for simulation target were carried out. The simulation target was irradiated by laser beams with 3 different frequencies and interference fringes were formed on it. Then the scattering echo energy was received by the segmented primary mirror, and collected by the secondary mirror again, finally come into the compress lens group and was detected by photomultiplier. The experiment result indicates that the stability of support truss excels 0-075 mrad and the angle adjustment accuracy excels 0-05 mrad. The co-focus experiment for the assembled 61 segmented mirrors indicates that the superposition accuracy of the spots- centers of mass is less than 20 mm. The results of field imaging experiment for 3mm simulation target show that the field imaging effect is well consistent with the laboratory and PC emulations. The results provide references for the subsequent experiments of reflected simulation target.
A novel high-accuracy star simulator with simple structure and the function of optical feedback was designed to satisfy the practical requirements of highaccuracy test and calibration of star sensor, which was a device used for celestial navigation. LED was selected as the light source. The parallel output beam was formed by means of focusing first and then collimating. The optical feedback system of the star simulator automatically adjusted the luminous intensity of its light source to keep the output intensity stable within a quite long time when the output intensity was changing.The parallelism of output beam of the star simulator reached 8, the uniformity of its output beam achieved 80% and the stability of output intensity of the star simulator reached 89% under the continuous working condition of at least 8 hours. Because the stable output intensity reduces the influence of magnitude change of the star simulator, it is very useful for improving the accuracy of test and calibration of star sensor.
According to the alignment demand of the large aperture aspheric mirror(hereinafter to be referred as the main mirror), the common detection methods for the large aperture aspheric surface were comparatively analyzed, and the best method for this sort of surface shape detection was put forward. In the alignment procedure of the main mirror, the two key links of the main mirror's fixation and deformation compensation were expounded, the difficulties of the fixation and the reasons for the deformation were summarized, and then a fire-new set of project was established by using measurement method with circumrotation counteracting gravity, real-time center position controling with particular tools, assistant material jointing method for the main mirror fixation and finally adjusting fixation with assistant upholders. The result showed that the method obtained good experimental effect, and the surface shape error of the main mirror was less than or equal to 0.03（＝632.8nm）.
According to the development of missile technology, a scheme of IR/laser dual-mode common aperture optical system which could obtain the dual-mode information of objects was proposed. The basic parameters of the optical system were analyzed, the design method of the dual-mode common aperture system was researched, and the optical system was designed. The system achieved common-aperture imaging at long wave infrared and laser wavebands. The relative aperture of the IR subsystem is 1.26, and the modulation transfer function approaches the diffraction limit. The relative aperture of the laser subsystem is 0.63, which approaches the theoretical value. The energy distribution of the imaging spot is uniform, and the spot size hardly changes in the range of linearity.
A design of low-light-level(LLL) night vision objective with long focal length and high relative aperture is introduced. The design parameters are as follows: the effective focal length is 100mm, the F-number is 1.4 and the field of view is 10.To improve the visual range, reduce the size and weight of the objective and get achromatic result in wide spectral range, the all-spherical catadioptric structure is selected. Catadioptric system is considerably shorter and lighter than the refractive lens with the same focal length and F-number. The Mangin mirror is introduced into the design, which gives the optical system extra degrees of freedom without adding another element. By optimizing the aberrations, good image quality of the objective is obtained. At the spatial frequency of 50lp/mm, the on-axis modulation transfer function (MTF) of the objective is not less than 0.4 and the off-axis MTF is not less than 0.2, which match with the limiting resolution of image intensifier. The distortion of objective is less than 2% in full field of view, and the total length of objective is 68mm.
In order to realize multi-target track initiation of infrared fisheye warning system, the technology of multi-target track initiation was studied for infrared fisheye warning system. The target moving characteristics on the image surface were analyzed, based on which an algorithm of multi-target track initiation proper to the infrared fisheye warning system was proposed. Using 3/4 logic of the logic method, the maximum velocity constraint method was firstly used to remove the uncorrelated measuring points in each sampling period so that the calculation amount of the Hough transformation could be reduced. The Hough transform method was used to generate initial target tracks, and the uniform velocity criterion was used to select the right tracks. The algorithm was verified with simulated targets. It is shown that, compared with the standard Hough transform and the logic method, the track initiation performance is obviously improved. For 450 clutters, the false alarm probability of track initiation is only 14%; the track initiation consuming time for continuous points is 3 frames while that for the non-continuous points is 4 frames. Further more this algorithm has less running time under the dense clutter environment.
Lateral shearing interferometry is an important optical measurement technology which obtains the wave-front phase distribution directly. Due to the ability of measuring surface shape of large deviation without high precision reference plane, it has been used in various fields;however, the selection of shearing displacement depends on work experience. In order to provide an explicit determination method, a shearing displacement determination method based on Zernike polynomial fitting is proposed, and the relation between shearing displacement and measurement error is given by computer simulation and experiment.
According to the redundant and complementary information among polarization characteristic images, a second polarization fusion method was proposed based on gradient characteristic and support value transformation. Firstly, the images of linear degree of infrared polarization and polarization angle were fused using gradient characteristic fusion. Then the support value transform was used to fuse the images of primary polarization fusion and total intensity second，and a fusion image with enhanced edge ,clear contour and high contrast was obtained. Experiment results show that compared with the polarization image, polarization linear image and total intensity image, the fused image increases the local standard deviation by 23.02%, 176.9% and 148.2% respectively, increases the contrast between the target and the background by 67.84%, 196.5%, 49.39% respectively and the average gradient by 46.09%, 164.1%, 214.5% respectively .The validity of the algorithm proposed is proved.
A design of video tracking and processing system based on ADSP-BF561 was proposed. This system took full advantage of the excellent performance of dual-core processing and the profile interfaces of encoding and decoding. The paper used a correlation tracking algorithm based on correlation coefficient. And many measures were put forward to improve the speed of the algorithm, in order to satisfy the requirement of object real-time tracking. The experiment result based on the target video in phase alternating line (PAL) shows that the method can achieve object tracking in a complicated background in real time.
Based on the ant colony optimization (ACO), a new algorithm for digital image correlation method is proposed. The procedure simulates the way that the real ants find the shortest path from their nest to food source and back. Modifications are made to the ACO algorithm in order to reduce the number of iterations and improve the quality of solutions. The accuracy and efficiency of the new method are demonstrated via application to computer-simulated images and comparison with widely used Newton-Raphson method. The feasibility and validity of the algorithm are also verified through experimentally obtained speckle images. The experimental results demonstrate the performance of our new algorithm.
Based on single scattering theory and diffusing wave spectroscopy theory, low-coher-ence dynamic light scattering was used to investigate the path-length resolved light field intensity spectra of backscattering lights from different radius particles of colloidal suspensions. The linewidths of the measured light field intensity spectra were compared with the predict results calculated from the corresponding theories. The results show that in short path-length region, the measured linewidths well agree with that of the single scattering after considering the influences of wall-drag effect close to the interface. For different radius particles, the region with the path length about 5 times of the mean-free path length can be regarded as a single scattering region, while the region with the propagated path length larger than 225 m can be regarded as a diffusing region. It is demonstrated that the low-coherence dynamic light scattering can realize resolved path-length light field intensity spectra measurement from single scattering to diffusive scattering regions for dense suspensions.
The TV system of opto-electric tracking and measuring device, affected by natural conditions, often cannot capture focused images. It is prone to extremely low contrast image. This paper presents a gray-scale transform method with the difference of neighbor pixel to determine the segment points. First, by calculating the difference between the gray scale pixel 55 neighborhoods, ND is taken as the sum of the minimum 12 values. Then, on the basis of a large number of experiments, the appropriate threshold T is determined, with which the ND is compared to determine the edge pixel in the image range. Finally, according to the scope, the segment points of segmentation gray-scale transform formula are determined. The results show that after processed by this algorithm, the contrast is 18.79 times of the original image which is 1.19%, 6.97 times of the histogram equalization and 41.41 times of the histogram specification. The contrast has reached 22.36%, and the image histogram is also highlighted by the single peak to a more balanced state. The algorithm has obtained good data and visual effects, and met the requirements of TV system, which has been used in optical tracking device. The algorithm has good practical value.
In order to solve the technical problem of the incorrect signal of optical screen due to disturbance of mosquitoes, impact wave or noise from photoelectric device, a digital signal processing algorithm was put forward. The algorithm used the data acquisition (DAQ) instrument to collect signals of the projectile passing through the screen, and then used 3 criterion to remove singular points. The triggering moment of the half peak could be selected correctively by means of the 4-power Gaussian curve fitting for the sampling data to achieve high-precision measurement with the lower sampling rate after the time frame of the projectile signal was determined by threshold comparison. The testing results of the computer simulation and the ball firing show that the algorithm is correct with high efficiency. The testing result of the ball firing of the XGK-2002 optical screen shows that the algorithm is eligible to measure the moment when various projectiles crossing through the screen and improves the application field and stability of the light screen target, the measurement relative error of the algorithm is less than 0.1％. The algorithm is also applies for the extraction and calculation of dynamic characteristics of this kind of signals such as sky screen target.
Regarding to the mistake that the focal plane array optoelectronic devices such as CCD were often placed on the exit port plane of integrating sphere source to perform calibration, the output irradiance uniformity of integrating sphere source was analyzed, and the basis on which the integrating sphere source was used rationally to calibrate the optoelectronic devices was obtained. Two theoretical output irradiance uniformity models based on numerical analysis method and Monte Carlo method were developed respectively. The models considered two fundamental situations of integrating sphere sources, namely ideal Lambertian source and non-ideal Lambertian source. The distribution regularity of irradiance uniformity was generalized by contrast of the theoretical data obtained by the models and the measured data obtained by two different actual integrating sphere sources. The results show that when the diameter of optoelectronic device is less than the half of the sphere exit port diameter and the ratio of the distance between device and exit port to the exit port diameter is 3~5, a 99% better output irradiance uniformity can be obtained.
We presented a technique for frequency calibration of terahertz time-domain spectrometers (THz-TDS) using absorption lines of carbon monoxide (CO). By comparing the peak positions of absorption lines between measured results and standard data, the linearity error caused by the optical delay line was modified so as to achieve the frequency calibration. After modifying, the mean error of the peak positions measured by our THz-TDS was 3.1 GHz. The results show that CO is suitable for frequency calibration of THz-TDS.
Aiming at the measurement of kinetic energy for flying fragments, a measuring method of kinetic energy for the flying fragment using single screen was presented. Two screens were used to measure the speed of the flying fragment in the method, the volume information was got by mean of the output waveform of the fragment passing through the screen, further the quality and the kinetic energy of the fragment were obtained. This paper analyzed a theoretical model of the relationship between the output waveform of the screen and the volume of the fragment. Three typical rotational symmetry objects simulations were used to verify the theoretical model, and a calibrating method was studied. The testing result of the ball firing proved that the proposed method could measure the kinetic energy of the rotational symmetry projectile or fragment, the measurement precision satisfied the requirement of the range test.
A new test for steep convex hyperboloidal secondary mirrors with large aperture and fast focus ratio is presented. It makes use of small correcting lens and an auxiliary ellipsoidal mirror which aperture is about 1.8 times as large as that of the convex mirror tested. This method is sutable for convex secondary asphere of very large aperture optical telescope. Taking TMT as an example whose surface is a convex shape with the diameter of 3.1m, fast focus ratio of F/1 and conic constant of -1.318 228, a null compensation system is successfully designed and optimized. The design results indicate that the surface quality of the steep convex hyperboloidal secondary mirror could be measured directly, in addition the aberration of the testing system is well corrected，PV is about /100 and the spot diameter is smaller than the diffraction limit. Analyses of the system-s wave-front error with respect to the change in the surface parameter of optical elements are described.
Noise always exists in photoelectric signal detected by laser Doppler velocimeter (LDV). A new method of signal processing was proposed in order to eliminate the noise interference and enhance the measurement accuracy of LDV. The least mean square(LMS) adaptive filter technology was applied in the signal detection system of LDV. By processing the Doppler signal, we greatly minished the interference of noise. The simulation and experiment results indicate that this technology effectively increases the signal to noise ratio(SNR) of Doppler signal, reduces the system-s demand for SNR of signal, and obviously improves the noise-proof ability, the sensibility,as well as the measurement accuracy of LDV system. This application creates conditions for the design of high accuracy LDV.
To obtain high-resolution iris images in the iris recognition system, a high-precision iris image acquisition camera lens is designed, whose focal length is 14.2 mm, distortion is less than 0.15%, image space numerical aperture (NA) is 0.26, image diameter is 5.35 mm, working distance is 130 mm and objectimage conjugate distance is 150 mm. This optical system can acquire high-resolution image with MTF 0.3 at 120 lp/mm over 0.7 field of view. The camera lens is composed of 4 spherical lenses, which is simple and suitable for industrial production. All the optical performances of productions are consistent with the predicted results analyzed by Zemax.
Aiming at the small-radius cylindrical lens proposed in a new infrared missile laser system, the traditional processing methods of cylindrical lens were analyzed comprehensively, the small-radius cylindrical lens was polished with selected processing techique and different polisher layers, and the results were compared.The process tests were carried out with several types of separator materials, seperators and different swing scopes.The minmun suface roughness of small-radius cylindrical lens achieved Ra 0.005 m,which was superior to the requirement of design drawing for Ra 0.01 m. Then the redundant half cylindrical lens was grinded off in order to meet the plane technical requirment.Finally all the technical specifications of small-raduis semicylindrical lens were satified, and the batch processing could be realized.
The multi-functional wireless optical mouse uses two image photography chips to realize cursor movement through the track ball rolling and the mouse base moving. First, we compared the percentages of luminous flux received by the two image photography chips in the original mouse optical path and the chip ADNS7530 which had smaller flux was remained only. Second, keeping the light path of track ball unchanged, we added a LED which had the same optical path as the track ball source in the mouse base optical path. After the desktop reflected light of mouse base was reflected through the track ball, prism and plane mirror respectively, we compared the flux percentages eventually reaching to the image photography chip. Finally, the design that the mouse base optical path was changed by the plane mirror was verified and the wireless air mouse using one image photography chip to operate both the track ball light path and the mouse base light path was achieved.
A novel optical element, combined triangular-section prism, for generating non-diffracting line-structured beam is proposed. This element is designed with gluing positive and negative isosceles triangular-section prisms. Its property is the same as the single positive isosceles triangular-section prism for beam transformation, and the equivalent bottom corner is decided by the bottom corner difference of positive and negative isosceles triangular-section prisms. Therefore, we can get smaller bottom corner by combining the positive and negative isosceles triangular-section prisms with comparatively large bottom corner for each triangular-section prism, and also get the non-diffracting line-structured beam with larger focus depth. The technical problem of the small bottom corner which is hard to machine is solved. The formation mechanism of the non-diffracting line-structured beam is analyzed by geometrical optics, and the relevant parameters are calculated. The intensity distribution is simulated by diffraction and interference theories. The results show that the nondiffracting line-structured beam is formed with large depth of focus at normal incidence of plane waves.
The mid-infrared bandpass filter is widely used as an observation channel in applications such as flight, meteorology and remote sensing. The peak transmission and full width at half maximum (FWHM) are key specifications of the bandpass filter, which depend on the film structure and its design.The midinfrared bandpass filter was successfully prepared using a new method with Ge and ZnS materials on Ge substrate. The selection of materials and the design principles of this method were introduced in detail, the film structure is also given. By using ZZSX-1100 device, this kind of filter was deposited with ion beam assisted process. The mea-surement results show that its peak transmission is more than 87% and the FWHM is 70 nm. The filter has good optical spectrum stability and adherence. The film structure is simple and easy to implement.
Infrared detectors have very important applications in space detection, target detection and tracking technology, the performances of which directly affect the accuracy and credibility of the acquisition of data. In order to improve the sensitivity of infrared signal analysis and reduce the detector noise level, using the electron beam and ion beam assisted deposition technology, the high-pass filter with 3 m～5 m high pass rate, 1.064 m～2.5 m band cutoff was prepared on the chemical vapor deposition (CVD) ZnS substrate. In order to effectively prevent the laser at 1.064 m band from injuring the detector and ensure the normal work of the detector, according to the laser output power, the highpass filter film series having 1%, 0.1%, 0.01% attenuation capability respectively for laser at 1.064 m wave band were prepared, which made the infrared detector more flexible and intelligent.
Three types of abnormal intensity noises of laser-diode(LD)-pumped Nd:YAG microchip laser were studied based on experimental data and theoretical analysis, including noises caused by the outside light feedback, the transmission of LD noise to solid laser and the mode coupling due to the deviation of end pumping. Corresponding methods were provided to eliminate these noises in order to suppress the intensity noise of the microchip Nd:YAG laser, improve the laser output characteristics and meet the demand of precision measurements.
In order to meet the demand of satellite laser altimeter (SLA) for high measurement accuracy, the design principles and requirements of receiving amplifier were analyzed. The parameters relationship and optimized parameters were given after some main performance parameters such as bandwidth, signalnoise ratio(SNR), detection sensibility and amplifier gain were analyzed and researched in terms of theory. The results show that SLA could achieve the measurement accuracy of false alarm rate of about 1% with the measuring range of 450 km~650km, pulse width of 8 ns~12 ns, working wavelength of 532 nm and 1064nm.
In order to investigate the effects of aperture averaging on the space laser communication system, the fading statistics of transmitted beam are deduced and evaluated, which include scintillation index, probability of fade, number of fade and mean fade time. The results show that the scintillation index, probability of fade, number of fade and mean fade time would be decreased significantly as the receiver aperture increases. The system performances can be improved, especially in the case of strong turbulence due to the small parameters. Based on the methods and the results, the effects caused by aperture averaging on the space laser communication systems can be predicted and estimated effectively.
Considering that the time delay signature has great threat to the security of chaotic communication, we proposed a novel method to directly identify the time delay signature of chaotic laser using a radio-frequency (RF) spectrum analyzer. Through recording and observing the RF power spectrum, it could be found that the power spectrum was actually modulated by the time delay signature; further performing an inverse Fourier transform to the RF power spectrum of chaotic laser, we could directly unveil the time delay signature. We experimentally studied two kinds of optical feedbacks: one was the single feedback cavity with a cavity length L=11-02 m and the other has two feedback cavities that the two cavity lengths were roughly equal to each other (L1=11-02 m and L2=11-25 m) or one was half of another (L1=11-02 m, L2=22-03 m). The results show that the time delay signatures can not be concealed when the two cavity lengths are roughly equal to each other; when one cavity is an integral multiple of another, the time delay signatures can still be extracted from the RF power spectrum of chaotic laser.
The statistical characteristic of the three important factors which are fading probability, mean fading number and mean fading time of satellite-to-ground optical communication link subject to weak atmospheric turbulence is researched, and the influence of zenith angle, light wavelength and altitude of receiver on the satellite-to-ground laser link is analyzed by numeric simulation. The result of research shows either decreasing the zenith angle or increasing the detection sensitivity of receiver can improve the fading characteristic of laser link; the 1.55 m laser is more suitable for satellite-to-ground optical communication than 0.85 m and 1.06 m; the receiver should be established in the region with altitude higher than 2 000 m.
The method for measuring the spectrum of ultrafast high energy pulsed laser in picosecond pulse width was presented with combination of photoelectrical spectrography and streak camera. The operating principle of streak camera and spectrophometric principle of plane diffraction grating were described respectively. Through thorough analysis, we pointed out that the spectral measurement of ultrafast high energy pulsed laser with wavelength in 300nm~1600nm and pulse width above 2 ps could be achieved by using our described facility. The streak image was obtained by utilizing ultrafast high energy pulsed laser in 1054nm wavelength, and the measured spectral curve was derived by processing the data of streak camera, then the actual spectral curve was also given by calibrating the related energy parameters, the method for measuring the spectrum of ultrafast high energy pulsed laser in picosecond pulse width was proved. The influence of coupling lens on measuring spectrum and grating fiber-s dispersion angle on streak image were analyzed finally, furthermore the role of spectrum for ultrafast high energy pulsed laser with picosecond pulse width was discussed. With the rapid development of the technology for manufacturing streak camera, the method will be used for measuring the spectral curve of laser with femtosecond pulse width.
We proposed a novel method for designing Raman fiber amplifier (RFA) which could achieve output signal gain spectra flattening by using pump lights with different wavelengths in the same fiber. We simplified the analytical solution of steady-state stimulated Raman scattering (SRS) equation by employing the linear model of leading and trailing edge of gain spectra, and obtained a constant output signal power at the output port finally. Simulation results demonstrate that the designed RFA has advantages of flattened gain spectra, high gain as well as simple configuration. It provides a new way for the designing of gain-flattened RFA.
Mode-field diameter (MFD) is a characteristic parameter of fundamental mode in fiber-optics waveguide. Power density in single-mode fiber is inversely proportional to the MFD. As the single-mode output power of fiber laser increasing, the power density in the fiber increases intensely. Much higher power density can cause optical damage and thermal damage of fiber waveguide. Based on the approximate mode-field distribution model, a character was found that the minimum MFDs in tapered fibers with different original fiber parameters corresponded to an invariable fiber normalized frequency. The relations between the core diameters and MFDs were simulated by the finite difference beam propagation method (FD-BPM) in different wavelengths and in different core numerical apertures (NA). The results demonstrated the supposition presented above and indicated that the minimum MFDs corresponded to the normalized frequency 1.8 nearly, without direct relations with the wavelength and fiber para-meters. The results provide theoretical reference for the quick determination of the point of maximum power density in fiber, especially in high power case, as well as add new content to the fiber optics theories.
The non-diffracting beam has smaller central spot than Gaussian beam and zero divergence angle, therefore it has significant prospect of application in freespace optical communication. A free-space optical communication experimental simulation system based on non-diffracting beam is introduced. The system is connected to the computer by serial port. A microprocessor is used for signal modulation and demodulation by adopting dual header-pulse interval modulation. An axicon is employed to generate non-diffraction beam. In the receiving terminal, a PIN photodiode is used to transfer the optical signal to electrical one. The electrical signal is amplified, shaped and demodulated. The data after demodulation are eventually transferred to the computer by serial port. The error rate is calculated by comparing the sending data and receiving data in the computer. The transmission performance of this system is also evaluated. The error rate is less than 610-5 within the maximum transmission distance when the bit rate is 9 600 bps.