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RSS2008 Vol. 32, No. 1
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2008, 32(1): 1-3,7.
[Abstract]
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Abstract:
Double-clad scheme has provided an effective solution to take use of high power semiconductor lasers(or laser arrays) as the pump source.However,with the increase of the fiber laser power,stimulated Raman scattering may reach its threshold and finally limit the further increase of the fiber laser powers.After adopting justified approximation,it was analytically solved.The rate equation of the double-clad fiber lasers without the spontaneous emission terms,and expressions for the fiber laser power and pump power were derived when the Stokes radiation inside fiber reached its threshold.Based on these expressions,dependence of the threshold on the fiber laser parameters were studied.
Double-clad scheme has provided an effective solution to take use of high power semiconductor lasers(or laser arrays) as the pump source.However,with the increase of the fiber laser power,stimulated Raman scattering may reach its threshold and finally limit the further increase of the fiber laser powers.After adopting justified approximation,it was analytically solved.The rate equation of the double-clad fiber lasers without the spontaneous emission terms,and expressions for the fiber laser power and pump power were derived when the Stokes radiation inside fiber reached its threshold.Based on these expressions,dependence of the threshold on the fiber laser parameters were studied.
2008, 32(1): 4-7.
Abstract:
A new-style lidar,synthetic aperture lidar was introduced.The principle and progress of synthetic aperture lidar were described in detail and the signal processing methods were introduced.The key techniques and the difference at home and abroad in developing synthetic aperture lidar system were briefly introduced.Finally,the developmental perspective in military,scientific research and industrial production was put forward.
A new-style lidar,synthetic aperture lidar was introduced.The principle and progress of synthetic aperture lidar were described in detail and the signal processing methods were introduced.The key techniques and the difference at home and abroad in developing synthetic aperture lidar system were briefly introduced.Finally,the developmental perspective in military,scientific research and industrial production was put forward.
2008, 32(1): 8-10,22.
Abstract:
In order to overcome the decline of side-coupler under high-power pumping conditions,in which the index-matching materials cannot endure high-power light and will decompose,a novel method using CO2 laser to make fused side-coupler was introduced.The experimental equipments and making processes for such fused side-coupler was included.This kind of fused side-coupler can endure high-power light without index-matching materials.It acquired coupling power 7.23W and coupling efficiency better than 70.5%,when the fused side-coupler was pumped by a high-power laser diode arrays in experiments.These results showed that this kind of fused side-coupler was able to used in high-power side-pumped double-clad fiber lasers practically.
In order to overcome the decline of side-coupler under high-power pumping conditions,in which the index-matching materials cannot endure high-power light and will decompose,a novel method using CO2 laser to make fused side-coupler was introduced.The experimental equipments and making processes for such fused side-coupler was included.This kind of fused side-coupler can endure high-power light without index-matching materials.It acquired coupling power 7.23W and coupling efficiency better than 70.5%,when the fused side-coupler was pumped by a high-power laser diode arrays in experiments.These results showed that this kind of fused side-coupler was able to used in high-power side-pumped double-clad fiber lasers practically.
2008, 32(1): 11-14.
Abstract:
To present preference to design of pointing,acquisition and tracking link in space optical communication,time-averaging effect of the atmospheric scintillation in the strong turbulent regime was studied experimentally.The mathematical model for the fade probability in space optical communications was provided in which effect of the pointing jitter and atmospheric scintillation on intensity fluctuations was taken into account.Based on the above,the improvement of links' fade fluctuation by time-averaging effects was analyzed.Experimental results show that the probability density function of irradiance in the strong turbulent regime is governed by the lognormal distribution,when exposure time reaches several decades of milliseconds;the required fade budget of the link at a lost-tracking probability of 10-3 is improved by the least of 15dB~20dB,when the method of time-averaging effects of the atmospheric scintillation is used in space optical communications.
To present preference to design of pointing,acquisition and tracking link in space optical communication,time-averaging effect of the atmospheric scintillation in the strong turbulent regime was studied experimentally.The mathematical model for the fade probability in space optical communications was provided in which effect of the pointing jitter and atmospheric scintillation on intensity fluctuations was taken into account.Based on the above,the improvement of links' fade fluctuation by time-averaging effects was analyzed.Experimental results show that the probability density function of irradiance in the strong turbulent regime is governed by the lognormal distribution,when exposure time reaches several decades of milliseconds;the required fade budget of the link at a lost-tracking probability of 10-3 is improved by the least of 15dB~20dB,when the method of time-averaging effects of the atmospheric scintillation is used in space optical communications.
2008, 32(1): 15-17.
Abstract:
In order to analyze the factors affecting the contrast-ratio of a electro-optic switch with double crystals,the index-ellipsoid method is adopted.The experiments show that there are close dependence of the difference of light direction of double crystals and the polarization direction of the polarizers on contrast-ratio.A good Q-swtiching effect could be obtained if the above two factors are more precisely adjusted.
In order to analyze the factors affecting the contrast-ratio of a electro-optic switch with double crystals,the index-ellipsoid method is adopted.The experiments show that there are close dependence of the difference of light direction of double crystals and the polarization direction of the polarizers on contrast-ratio.A good Q-swtiching effect could be obtained if the above two factors are more precisely adjusted.
2008, 32(1): 18-22.
Abstract:
In order to improve the anti-interference ability of spatial optic communication,a novel optic differential modulation device based on two-wave mixing was adopted.Experimental results proved that the common-mode noise caused by cloud cluster,fog,etc,could be eliminated effectively with the help of differential mode spatial optic communication.The quality of spatial optic communication can be markedly improved by taking differential method.
In order to improve the anti-interference ability of spatial optic communication,a novel optic differential modulation device based on two-wave mixing was adopted.Experimental results proved that the common-mode noise caused by cloud cluster,fog,etc,could be eliminated effectively with the help of differential mode spatial optic communication.The quality of spatial optic communication can be markedly improved by taking differential method.
2008, 32(1): 27-29,56.
Abstract:
In order to apply semiconductor laser to measure cavity loss with continuous-wave cavity ring-down technology,the laser diode's driving circuit was designed.The circuit was composed of different modules,such as modulation,driver and temperature controller.Based on theory of passive cavity's multi-beam interference,modules' parameter demand was suggested after analysis and numerical simulation of ring-down signal.The measurement system of cavity loss was built with the circuit.Under the guarantee of the semiconductor laser trouble-free,tests indicated that the shutoff time of laser diode was about 60ns and the precision of temperature controller was better than 0.01℃.The measuring accuracy was better than 1% for a cavity whose single pass loss is about 50×10-6.Results indicate that the circuit meet the theory design demands and can be applied in high-accuracy measurement of cavity loss.
In order to apply semiconductor laser to measure cavity loss with continuous-wave cavity ring-down technology,the laser diode's driving circuit was designed.The circuit was composed of different modules,such as modulation,driver and temperature controller.Based on theory of passive cavity's multi-beam interference,modules' parameter demand was suggested after analysis and numerical simulation of ring-down signal.The measurement system of cavity loss was built with the circuit.Under the guarantee of the semiconductor laser trouble-free,tests indicated that the shutoff time of laser diode was about 60ns and the precision of temperature controller was better than 0.01℃.The measuring accuracy was better than 1% for a cavity whose single pass loss is about 50×10-6.Results indicate that the circuit meet the theory design demands and can be applied in high-accuracy measurement of cavity loss.
2008, 32(1): 30-32,36.
Abstract:
In order to study the measure error of detecting Brillouin scattering based on edge detecting technology,the measure error formula of detecting the Brillouin scattering signal was educed after analyzing the underwater channels.At the same time,the Brillouin scattering signal was estimated based on the formula.The frequency drift of Brillouin scattering was simulated under the condition of different depth and the different water quality parameters.Under good quality water,the measure error of the Brillouin scattering frequency drift was less than 5MHz at 120m underwater.Based on the above results,a final conclusion is concluded that higher signal-to-noise ratio and smaller measure error could be acquired based on the edge detecting technology.
In order to study the measure error of detecting Brillouin scattering based on edge detecting technology,the measure error formula of detecting the Brillouin scattering signal was educed after analyzing the underwater channels.At the same time,the Brillouin scattering signal was estimated based on the formula.The frequency drift of Brillouin scattering was simulated under the condition of different depth and the different water quality parameters.Under good quality water,the measure error of the Brillouin scattering frequency drift was less than 5MHz at 120m underwater.Based on the above results,a final conclusion is concluded that higher signal-to-noise ratio and smaller measure error could be acquired based on the edge detecting technology.
2008, 32(1): 33-36.
Abstract:
The optical band-pass filter based on cascaded Mach-Zehnder interferometers with chirped path differences was designed.The filter had wide free spectral range(FSR) and narrow 3dB bandwidth.With theoretical model established by transfer matrix method,its band-pass filtering characteristics(i.e.,3dB bandwidth and FSR) were investigated under different structure parameters(i.e.,number of stages and diffraction order).Using optimized chirp coefficient,the FSR was expanded and the 3dB bandwidth was almost fixed.Meanwhile the side-mode-suppuration-ratio(SMSR) during whole FSR was excellent.For a design example,the 3dB bandwidth was specified less than 4.5nm and the FSR was specified more than 100nm(the corresponding SMSR was required to be more than 35dB).The derived structure parameters of filter were as follows:number of stages was 16;the diffraction order was 45;and the optimal chirp coefficient was 5(the corresponding SMSR was 65.90dB).
The optical band-pass filter based on cascaded Mach-Zehnder interferometers with chirped path differences was designed.The filter had wide free spectral range(FSR) and narrow 3dB bandwidth.With theoretical model established by transfer matrix method,its band-pass filtering characteristics(i.e.,3dB bandwidth and FSR) were investigated under different structure parameters(i.e.,number of stages and diffraction order).Using optimized chirp coefficient,the FSR was expanded and the 3dB bandwidth was almost fixed.Meanwhile the side-mode-suppuration-ratio(SMSR) during whole FSR was excellent.For a design example,the 3dB bandwidth was specified less than 4.5nm and the FSR was specified more than 100nm(the corresponding SMSR was required to be more than 35dB).The derived structure parameters of filter were as follows:number of stages was 16;the diffraction order was 45;and the optimal chirp coefficient was 5(the corresponding SMSR was 65.90dB).
2008, 32(1): 37-39,43.
Abstract:
In order to study the distribution of residual stress and distortion,a FEM model of marking by means of laser shock waves was established based on ANSYS/LS-DYNA.By loading the shock waves induced by laser,a numerical simulation of marking was carried out.The simulated results showed that the mesh of marking area acted by laser shock waves formed a crater with dimension similar to the diameter of the load,and its residual stress was compressive stress.As the amount of the deformation increased gradually,the residual compressive stress in the center of marking area reached the maximum;the residual compressive stress along the thickness of the material declined as the thickness increased,and the loading effect was not obvious in the depth of 1mm~1.4mm.The results are applicable in theoretical analysis and experimental research of residual stress field of 3-D nondestructive marking based on laser shock wave.
In order to study the distribution of residual stress and distortion,a FEM model of marking by means of laser shock waves was established based on ANSYS/LS-DYNA.By loading the shock waves induced by laser,a numerical simulation of marking was carried out.The simulated results showed that the mesh of marking area acted by laser shock waves formed a crater with dimension similar to the diameter of the load,and its residual stress was compressive stress.As the amount of the deformation increased gradually,the residual compressive stress in the center of marking area reached the maximum;the residual compressive stress along the thickness of the material declined as the thickness increased,and the loading effect was not obvious in the depth of 1mm~1.4mm.The results are applicable in theoretical analysis and experimental research of residual stress field of 3-D nondestructive marking based on laser shock wave.
2008, 32(1): 44-46,49.
Abstract:
For obtaining narrow pulse wide and high power fiber laser,a series of experiments were carried about stimulated Brillouin scattering pulse-pumped Q-switched fiber laser.The lineal cavity,including Bragg fiber grating,Yb3+-doped double-clad fiber(Yb3+ -DCF) and SM fiber,were designed.Yb3+-DCF and pump module were connected by taper fiber,achieving all-fiber laser.Nonlinearity of the fiber-stimulated Brillouin scattering Q-switched fiber laser through pulse-pump,400ns pulse wide 2.53W average power,the repetition rate of 15kHz were achieved.The result shows that through pulse-pump,stimulated Brillouin scattering can reduce pulse's line width and realize high power output.
For obtaining narrow pulse wide and high power fiber laser,a series of experiments were carried about stimulated Brillouin scattering pulse-pumped Q-switched fiber laser.The lineal cavity,including Bragg fiber grating,Yb3+-doped double-clad fiber(Yb3+ -DCF) and SM fiber,were designed.Yb3+-DCF and pump module were connected by taper fiber,achieving all-fiber laser.Nonlinearity of the fiber-stimulated Brillouin scattering Q-switched fiber laser through pulse-pump,400ns pulse wide 2.53W average power,the repetition rate of 15kHz were achieved.The result shows that through pulse-pump,stimulated Brillouin scattering can reduce pulse's line width and realize high power output.
2008, 32(1): 47-49.
Abstract:
In order to find the achromatic scope of three-element compound λ/4 wave-plate,mathematical analysis was performanced and experiments at the center wavelength of 600nm was executed.In the experiment the spectrum range was between 540nm~680nm when the deviation is less than 2%.It is concluded that the achromatic scope is only near the center wavelength,which is very important for practiceal prism selection.
In order to find the achromatic scope of three-element compound λ/4 wave-plate,mathematical analysis was performanced and experiments at the center wavelength of 600nm was executed.In the experiment the spectrum range was between 540nm~680nm when the deviation is less than 2%.It is concluded that the achromatic scope is only near the center wavelength,which is very important for practiceal prism selection.
2008, 32(1): 50-52,112.
Abstract:
In order to study the supercontinuum generation in a photonic crystal fiber pumped at the anomalous dispersion region,a theoretical investigation with the method of split-step Fourier is presented on the nonlinear propagation and supercontinuum generation of a femtosecond laser pulse in a photonic crystal fiber.Finally,a conclusion can be drawn that the initial pulse power and initial pulse chirp have the impact on the spectral shape and band width of the supercontinuum generation at the anomalous dispersion region in a photonic crystal fiber,which provides significant reference for supercontinuum generation in photonic crystal fibers.
In order to study the supercontinuum generation in a photonic crystal fiber pumped at the anomalous dispersion region,a theoretical investigation with the method of split-step Fourier is presented on the nonlinear propagation and supercontinuum generation of a femtosecond laser pulse in a photonic crystal fiber.Finally,a conclusion can be drawn that the initial pulse power and initial pulse chirp have the impact on the spectral shape and band width of the supercontinuum generation at the anomalous dispersion region in a photonic crystal fiber,which provides significant reference for supercontinuum generation in photonic crystal fibers.
2008, 32(1): 53-56.
Abstract:
In order to further study characteristics of cosh-Gaussian beam which passing through an astigmatic lens,starting from the generalized Huygens-Fresnel diffraction integral,the relative errors of the beam parameters,such as the beam width and far-field divergence angle of cosh-Gaussian beams after passing through an astigmatic lens are studied.It is shown that the relative errors of the beam width and far-field divergence angle introduced by the astigmatism mainly depend on the astigmatic coefficient C2,decentered parameter α,Rayleigh range z0 and focal lengths f of the lens.The relative errors of the beam widths in the x and y directions at the geometrical focal plane are equal,while at the waist plane in the image space they are different,and dependent on the astigmatic coefficient C2 and decentered parameter α.In addition,the relative errors of the beam width are dependent on the propagation distance z.The far-field divergence angles in the x and y directions are not equal due to the astigmatism and dependant on the decentered parameter α.
In order to further study characteristics of cosh-Gaussian beam which passing through an astigmatic lens,starting from the generalized Huygens-Fresnel diffraction integral,the relative errors of the beam parameters,such as the beam width and far-field divergence angle of cosh-Gaussian beams after passing through an astigmatic lens are studied.It is shown that the relative errors of the beam width and far-field divergence angle introduced by the astigmatism mainly depend on the astigmatic coefficient C2,decentered parameter α,Rayleigh range z0 and focal lengths f of the lens.The relative errors of the beam widths in the x and y directions at the geometrical focal plane are equal,while at the waist plane in the image space they are different,and dependent on the astigmatic coefficient C2 and decentered parameter α.In addition,the relative errors of the beam width are dependent on the propagation distance z.The far-field divergence angles in the x and y directions are not equal due to the astigmatism and dependant on the decentered parameter α.
2008, 32(1): 57-60.
Abstract:
The progress in the application of ion beam assisted deposition on the preparation of high laser films was reviewed.Some key problems of this project were analyzed such as too high packing density,new defects introduced by ions,increased compressive stress etc.,and some ways of this project for the future were presented.
The progress in the application of ion beam assisted deposition on the preparation of high laser films was reviewed.Some key problems of this project were analyzed such as too high packing density,new defects introduced by ions,increased compressive stress etc.,and some ways of this project for the future were presented.
2008, 32(1): 61-63.
Abstract:
In order to realize the miniaturization and integration of the optical correlation recognition system,an optical correlation recognition system was proposed with the help of computer generated hologram(CGH).Theory analysis and experimental results show that complex filter made of CGH instead of Fourier lens,integrates the second Fourier lens in the 4f system.So there is no need of Fourier lens behind the complex filter.The system has reduced the optics component,simplified optical section at the same time.It also has the character of shift invariability as 4f optical correlation recognition system and can be used as optical correlation recognition system.
In order to realize the miniaturization and integration of the optical correlation recognition system,an optical correlation recognition system was proposed with the help of computer generated hologram(CGH).Theory analysis and experimental results show that complex filter made of CGH instead of Fourier lens,integrates the second Fourier lens in the 4f system.So there is no need of Fourier lens behind the complex filter.The system has reduced the optics component,simplified optical section at the same time.It also has the character of shift invariability as 4f optical correlation recognition system and can be used as optical correlation recognition system.
2008, 32(1): 64-66,70.
Abstract:
In order to obtain the relationship between processing parameters and paint removing effects,a fast axis flow CO2 laser,whose wavelength is 10.6μm,was used to remove paint from aluminum plate surfaces.The relationships between laser power density,scanning velocity,scanning channel overlapping and cleaning effects were investigated.It is shown that there exist initial cleaning threshold,complete cleaning threshold and substrate damage threshold of laser power density for laser beam to remove the paint layer,higher cleaning efficiency and better cleaning effect can be achieved when laser power is higher,larger area paint can be completely removed when the overlap between scanning channels is larger than 40%.
In order to obtain the relationship between processing parameters and paint removing effects,a fast axis flow CO2 laser,whose wavelength is 10.6μm,was used to remove paint from aluminum plate surfaces.The relationships between laser power density,scanning velocity,scanning channel overlapping and cleaning effects were investigated.It is shown that there exist initial cleaning threshold,complete cleaning threshold and substrate damage threshold of laser power density for laser beam to remove the paint layer,higher cleaning efficiency and better cleaning effect can be achieved when laser power is higher,larger area paint can be completely removed when the overlap between scanning channels is larger than 40%.
2008, 32(1): 67-70.
Abstract:
To study theoretically the threshold characteristics of fibre Raman amplifier(FRA),at first,the course of FRA's threshold through the coupling equations of stimulated Raman scattering was presented and the numerical simulation results were given.Then,based on the study before,the theoretical expression of forward-pumped and backward-pumped FRA's threshold were obtained considering the practical factors and the effect of all parameters on the threshold was studied in detail under the condition of five typical kinds of fibres.The results are:threshold are different with different pumping ways and in the same pumping way,each parameter has different effects on threshold.Finally,conclusion has been got that the threshold of the amplifier can be reduced based on different effects of each parameter,which gives some advices for improving the pump efficiency and experiment work of the amplifier.
To study theoretically the threshold characteristics of fibre Raman amplifier(FRA),at first,the course of FRA's threshold through the coupling equations of stimulated Raman scattering was presented and the numerical simulation results were given.Then,based on the study before,the theoretical expression of forward-pumped and backward-pumped FRA's threshold were obtained considering the practical factors and the effect of all parameters on the threshold was studied in detail under the condition of five typical kinds of fibres.The results are:threshold are different with different pumping ways and in the same pumping way,each parameter has different effects on threshold.Finally,conclusion has been got that the threshold of the amplifier can be reduced based on different effects of each parameter,which gives some advices for improving the pump efficiency and experiment work of the amplifier.
2008, 32(1): 71-74.
Abstract:
In order to study the laser reflected character of "cat eye effect" of the target's optical system when the reflecting apparatus on the focal plane is focal shift,modeling and simulation for the "cat eye" aim were carried out by using the optical design and analysis software named ZEMAX.Adopting the above method,the reflecting characters were obtained including the energy transit ratio when the incident laser through the "cat eye" aim,the far field divergence,the far field facula and the energy distributing character of the back reflecting laser.Then the different effect from positive and negative focal shift on reflecting character was put forward.At last,the angle range of the incident laser beam was obtained which is necessary for the back reflecting beam of "cat eye effect" when the target optical system is focal shift.The results show that the size of focal shift has a decisive effect on the back reflecting light's characteristic,especially its far field light's intensity.These results provide reliable evidence for the analysis of detecting ability of the active laser detecting system.
In order to study the laser reflected character of "cat eye effect" of the target's optical system when the reflecting apparatus on the focal plane is focal shift,modeling and simulation for the "cat eye" aim were carried out by using the optical design and analysis software named ZEMAX.Adopting the above method,the reflecting characters were obtained including the energy transit ratio when the incident laser through the "cat eye" aim,the far field divergence,the far field facula and the energy distributing character of the back reflecting laser.Then the different effect from positive and negative focal shift on reflecting character was put forward.At last,the angle range of the incident laser beam was obtained which is necessary for the back reflecting beam of "cat eye effect" when the target optical system is focal shift.The results show that the size of focal shift has a decisive effect on the back reflecting light's characteristic,especially its far field light's intensity.These results provide reliable evidence for the analysis of detecting ability of the active laser detecting system.
2008, 32(1): 75-76,79.
Abstract:
In order to see the transmission of electromagnetic wave in photonic crystals,the finite difference time domain method was used to calculate the band gap of two-dimension photonic crystals,and with the help of MATLAB,it could make the transmission of the electromagnetic wave in the vacuum and photonic crystals visual.At the meantime,the static map was shown.Numerical analysis showed that the wave in the forbidden band was controlled in photonic crystals,and its energy distributed in the media rod nearby,the localization of electromagnetic was observed.
In order to see the transmission of electromagnetic wave in photonic crystals,the finite difference time domain method was used to calculate the band gap of two-dimension photonic crystals,and with the help of MATLAB,it could make the transmission of the electromagnetic wave in the vacuum and photonic crystals visual.At the meantime,the static map was shown.Numerical analysis showed that the wave in the forbidden band was controlled in photonic crystals,and its energy distributed in the media rod nearby,the localization of electromagnetic was observed.
2008, 32(1): 77-79.
Abstract:
Modulation is a key technology of a fiber laser.Without polarization control,the laser is unable to be modulated.With strictly polarization control,a novel structure of stabile fiber laser was set up.The fiber laser was made of a twisted normal and Er-doped fiber and could be modulated well.The modulated laser was already used in an over 100km single mode fiber fiber transmission system.The result is accordance with that of the transmission system by using semiconductor laser.
Modulation is a key technology of a fiber laser.Without polarization control,the laser is unable to be modulated.With strictly polarization control,a novel structure of stabile fiber laser was set up.The fiber laser was made of a twisted normal and Er-doped fiber and could be modulated well.The modulated laser was already used in an over 100km single mode fiber fiber transmission system.The result is accordance with that of the transmission system by using semiconductor laser.
2008, 32(1): 80-83,87.
Abstract:
To study the influence caused by sampling,here a method,in which the comb function was introduced to sample the fringe patterns,was adopted.Combining with the knowledge of the convolution properties,the phase error caused by sampling in practical composite Fourier-transform propfilometry was discussed along two directions,one is along the phrase variation direction and the other is the vertical direction.The criterions for the selection of sampling frequency are derived respectively along both the directions,and the results indicate that the sampling frequency along the phrase variation direction must be at least four times as the fundamental frequency in order to avoid the spectrum overlapping,and along the orthogonal direction the sampling frequency must be at least three times as the larger frequency of the two carrier frequencies.The computer simulation and experiments verify the theories.
To study the influence caused by sampling,here a method,in which the comb function was introduced to sample the fringe patterns,was adopted.Combining with the knowledge of the convolution properties,the phase error caused by sampling in practical composite Fourier-transform propfilometry was discussed along two directions,one is along the phrase variation direction and the other is the vertical direction.The criterions for the selection of sampling frequency are derived respectively along both the directions,and the results indicate that the sampling frequency along the phrase variation direction must be at least four times as the fundamental frequency in order to avoid the spectrum overlapping,and along the orthogonal direction the sampling frequency must be at least three times as the larger frequency of the two carrier frequencies.The computer simulation and experiments verify the theories.
2008, 32(1): 88-91.
Abstract:
In order to study the feasibility of micro-electromechanical systems(MEMS) short processing technique with application of directly writing by means of femtosecond laser instead of traditional lithography,features of the material processing by means of femtosecond laser was analyzed and experiment was executed on 100 silicon wafers(SiN film thickness:350nm~500nm) by using a laser with 800nm wavelength and 50fs pulse width.Theoretic analysis and experimental result showed that femtosecond laser was superior for short process to nanosecond laser.MEMS shortened the processing flow and its cycle.Precise processing could be realized under precise control of laser pulse energy and workbench moving speed.
In order to study the feasibility of micro-electromechanical systems(MEMS) short processing technique with application of directly writing by means of femtosecond laser instead of traditional lithography,features of the material processing by means of femtosecond laser was analyzed and experiment was executed on 100 silicon wafers(SiN film thickness:350nm~500nm) by using a laser with 800nm wavelength and 50fs pulse width.Theoretic analysis and experimental result showed that femtosecond laser was superior for short process to nanosecond laser.MEMS shortened the processing flow and its cycle.Precise processing could be realized under precise control of laser pulse energy and workbench moving speed.
2008, 32(1): 92-94.
Abstract:
To study the influence of the system optical parameters on the received signal,the profile of signal-to-range under several parameters were given after simulation.It turned out that the laser facula must be adjusted to be smaller than 25mm.Aiming at an object 1000m away,when the error of the angle between the transmitter and receiver axes was 2.2μrad,the emission angle was included in the received field of view of the telescope.
To study the influence of the system optical parameters on the received signal,the profile of signal-to-range under several parameters were given after simulation.It turned out that the laser facula must be adjusted to be smaller than 25mm.Aiming at an object 1000m away,when the error of the angle between the transmitter and receiver axes was 2.2μrad,the emission angle was included in the received field of view of the telescope.
2008, 32(1): 95-97.
Abstract:
In order to investigate the application of laser shock forming with half die in industry,experiments were executed with titanium alloy sheet.Inverse deformation of sample in its partial area was found in laser shock forming process,especially it was more obvious in the center of concave die.The inverse deformation heavily affected the shock forming quality because it made the sheet couldn't touch concave die closely.According to results of experiment and analyses,the sheet with great speed hits the concave die strongly when it touches concave die if laser energy is higher than the needs of deformation,so the sheet obtains an inverse velocity through hitting,and the inverse deformation take place if the inverse velocity beyond the speed threshold value of plastic deformation,which has great effect on the quality of shock forming.A good quality work piece was obtained after adjusting laser energy,which shows that inverse deformation can be avoided under lower laser energy.
In order to investigate the application of laser shock forming with half die in industry,experiments were executed with titanium alloy sheet.Inverse deformation of sample in its partial area was found in laser shock forming process,especially it was more obvious in the center of concave die.The inverse deformation heavily affected the shock forming quality because it made the sheet couldn't touch concave die closely.According to results of experiment and analyses,the sheet with great speed hits the concave die strongly when it touches concave die if laser energy is higher than the needs of deformation,so the sheet obtains an inverse velocity through hitting,and the inverse deformation take place if the inverse velocity beyond the speed threshold value of plastic deformation,which has great effect on the quality of shock forming.A good quality work piece was obtained after adjusting laser energy,which shows that inverse deformation can be avoided under lower laser energy.
2008, 32(1): 98-100,104.
Abstract:
Group velocity dispersion(GVD) and cubic nonlinearity effect can seriously affect the conversion efficiency of optical parametric amplification(OPA) pumped by ultra-short high intensity laser beams.Theoretical analysis and numerical calculation were used in order to increase the conversion efficiency of OPA in MgO:LiNbO3 crystal.Some numerical results with different pump intensity and GVD conditions were reported and the variation of pulse time shape and spectral were explained by cubic nonlinearity effect.It is found that pump pulse intensity is seriously decreased and pulse width is increased by GVD.Cubic nonlinearity can't be ignored when pump intensity is high,which reduces the conversion efficiency and makes saturated crystal length longer.Initial chirps of pump pulse are introduced to compensate GVD and increase the gain of parametric amplification.
Group velocity dispersion(GVD) and cubic nonlinearity effect can seriously affect the conversion efficiency of optical parametric amplification(OPA) pumped by ultra-short high intensity laser beams.Theoretical analysis and numerical calculation were used in order to increase the conversion efficiency of OPA in MgO:LiNbO3 crystal.Some numerical results with different pump intensity and GVD conditions were reported and the variation of pulse time shape and spectral were explained by cubic nonlinearity effect.It is found that pump pulse intensity is seriously decreased and pulse width is increased by GVD.Cubic nonlinearity can't be ignored when pump intensity is high,which reduces the conversion efficiency and makes saturated crystal length longer.Initial chirps of pump pulse are introduced to compensate GVD and increase the gain of parametric amplification.
2008, 32(1): 101-104.
Abstract:
Temperature field and thermal lens effect of laser diode end pumped composite Nd:YAG rod were investigated through numerical analysis methods.Numerical solutions of temperature field were obtained.The maximum temperature was 73.8℃ for composite laser crystal with an absorption coefficient of 3.5/cm,under pump power of 18W with 0.2mm beam radius.And also phase-retardation distribution ΔΦt(r)of the axial transmission light caused by radial temperature gradients was obtained.Comparing with conventional rod,the results show that composite rod leads a lower temperature rise in laser rod,as well as reduces distortion of pump end which brings on lens effect of laser crystal.Phase-retardation distribution ΔΦt(r) in composite rod is similar to that of conventional laser rod,but it doesn't accord with radius quadratic relationship,there is an aberration Δφ between the phase retardation of the thermal lens and referenced spherical lens.ΔΦt(r) corresponds with that of a spherical lens approximately at rp,and the equivalent foci fGRIN approaches to the same value of conventional rod,whereas Δφ increases quickly with the increase of laser beam radius at r>wp.
Temperature field and thermal lens effect of laser diode end pumped composite Nd:YAG rod were investigated through numerical analysis methods.Numerical solutions of temperature field were obtained.The maximum temperature was 73.8℃ for composite laser crystal with an absorption coefficient of 3.5/cm,under pump power of 18W with 0.2mm beam radius.And also phase-retardation distribution ΔΦt(r)of the axial transmission light caused by radial temperature gradients was obtained.Comparing with conventional rod,the results show that composite rod leads a lower temperature rise in laser rod,as well as reduces distortion of pump end which brings on lens effect of laser crystal.Phase-retardation distribution ΔΦt(r) in composite rod is similar to that of conventional laser rod,but it doesn't accord with radius quadratic relationship,there is an aberration Δφ between the phase retardation of the thermal lens and referenced spherical lens.ΔΦt(r) corresponds with that of a spherical lens approximately at rp,and the equivalent foci fGRIN approaches to the same value of conventional rod,whereas Δφ increases quickly with the increase of laser beam radius at r>wp.
2008, 32(1): 105-108.
Abstract:
The period of interference fringes is an important parameter in precision measurement.In order to detect the period of interference fringes simply and fast but not inaccurately,a simpler and practical digital image processing algorithm was adopted to detect the period of interference fringes.The image data processing software was designed on the MATLAB platform and the factors affecting the measurement accuracy of the period of fringes were analyzed.The demand of the fringes inclination was given.The algorithm was used in the detection of phase-plate,and the phase of the phase-plate was acquired.The precision of phase detection reached λ/20~λ/50 or π/10~π/25.This result is largely helpful to the thickness and refractive index detection of the phase object.
The period of interference fringes is an important parameter in precision measurement.In order to detect the period of interference fringes simply and fast but not inaccurately,a simpler and practical digital image processing algorithm was adopted to detect the period of interference fringes.The image data processing software was designed on the MATLAB platform and the factors affecting the measurement accuracy of the period of fringes were analyzed.The demand of the fringes inclination was given.The algorithm was used in the detection of phase-plate,and the phase of the phase-plate was acquired.The precision of phase detection reached λ/20~λ/50 or π/10~π/25.This result is largely helpful to the thickness and refractive index detection of the phase object.
2008, 32(1): 109-112.
Abstract:
To study the influence of hard-edge apertured and misaligned Lohmann's optical system on fractional Fourier transforms,the flat-topped multi-Gaussian beam model was adopted to simulate hard-edge apertures.The analytical expressions for transformation of plane wave in a hard-edge apertured and off-axis Lohmann's fractional Fourier transform optical system were derived.The numerical simulation for outputs when the system was apertured with aperture radius of 7.696cm and the lenses were off-axis was also given.It is shown that the two Lohmann's setups are not equivalent when they are apertured and misaligned.
To study the influence of hard-edge apertured and misaligned Lohmann's optical system on fractional Fourier transforms,the flat-topped multi-Gaussian beam model was adopted to simulate hard-edge apertures.The analytical expressions for transformation of plane wave in a hard-edge apertured and off-axis Lohmann's fractional Fourier transform optical system were derived.The numerical simulation for outputs when the system was apertured with aperture radius of 7.696cm and the lenses were off-axis was also given.It is shown that the two Lohmann's setups are not equivalent when they are apertured and misaligned.
