测量激光远场能量分布

Detector Array for Measuring Far-field Energy Density Distribution ofRepetitively Pulsed LaserYANG Pengling a,b, FENG Guobin a, WANG Qunshu a, WANG Jingjin b, CHENG Jianping ba Northwest Institute of Nuclear Technology, Xi’an, China, 710024b Department of Engineering Physics, Tsinghua University, Beijing, China,100084ABSTRACTA system based on detector array is developed to measure the far-field temporal and spatial distribution and absolute pulse energy density of the laser beam. In this experiment, the duration of the laser pulse is about 15ns, the repetition rate is 400Hz, and the diameter of the far-field beam is about 60cm. The detector array is composed of 112 Si-PIN photodiode detectors and arranged to be a disk with spatial sample rate of 0.4cm-1. Charge sensitive amplifiers and baseline restoration circuits are used to collect photocurrent of the detectors, and current-input AD converters with integrator front-end are used to digitalize the multi channel signals. The far-field laser beam profile is reconstructed with the spatial sample data using special arithmetic of spatial interpolation. The system is capable for absolutely measuring far-field energy density distribution of repetitively pulsed laser, with response wavelength between 400nm and 1100nm, minimal detectable pulse duration of about 10ns, and energy density of 0.1-100µJ/cm2.Keywords: Pulsed laser, Energy density distribution, Absolute pulse energy, Beam profile, Detector array, Charge sensitive amplifier, Analog-to-digital converter1. INTRODUCTIONBeam quality and intensity are critical characters of high power laser system. Accurately measuring the far-field laser irradiance distribution is an effective way to diagnose laser beam quality and study the atmospheric transmission effects for a high power laser system[1],[2]. Traditionally, witness plates and CCD cameras are used to fulfill this measurement. These methods can only give a relative intensity distribution. Thermopile detectors based on batteries of thermocouples are frequently employed for pulse energy determination, especially for high-energy lasers. However, such detectors are not accurate for low energy levels and have no spatial resolution. Thermopiles are also inadequate to perform direct energy determination at repetition rates exceeding few pulses per second. Pyroelectric ceramics are another important class of energy detector based on the temperature increment produced by the absorption of the laser radiation. Pyroelectric ceramics are more sensible than thermopile detectors. They can reliably determine pulse energies as low as few tens µJ without amplification electronics. In addition, the pulse repetition frequencies allowed by pyroelectric ceramics are also higher than those permitted by thermopiles. AtInternational Symposium on Photoelectronic Detection and Imaging 2007: Laser, Ultraviolet, and Terahertz Technology, edited by Liwei Zhou, Proc. of SPIE Vol. 6622, 66220T, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.790808Proc. of SPIE Vol. 6622 66220T-1present, ceramics for energy determination capable of working at repetition rates of up to 400 Hz are commercially available. However, common ceramics are accurate only at repetition rates below 20 Hz[3], and they haven’t good thermo-stability and uniformity. So they are insufficient for far-field laser energy measuring. Semiconductor Si-PIN photodiode detector with response spectrum from 400nm to 1100nm, having high response speed, acceptable uniformity and thermo-stability, are appropriate in high repetition pulsed laser energy density measuring. Moreover, with the size of about several millimeters, a Si-PIN detector cell is sufficient for arranging to be a compact detector array, which is capable to absolutely measure energy density distribution of a high repetition pulsed laser. In this paper, we developed a measurement system based on Si-PIN photodiode detector array for this purpose.2. PRINCIPLE OF THE MEASUREMENT SYSTEMIn detector array systems for laser beam profile measuring, hundreds of detectors are arranged in a plane to sample the laser beam profile with fixed interval. Thus the beam profile is converted to multi channel electric signals and digitalized. The beam profile is reconstructed with the sample data in the processing end. In the system, each detector cell has an amplifier and analog-to-digital(AD) converter. Particularly in a pulsed laser measurement system, output signals of the photodiode detectors have very high frequency, which has a high electromagnetic compatibility demand for amplifiers and speed demand for AD converters. Peak detect circuits are usually used to decrease the frequency of output signals [4]. The multi channel amplifiers, peak detect circuits, and converters make the system complicated and fallible, with low spatial resolution. In our system, charge sensitive amplifiers and current-input AD converters with integrator front-end are used to displace the traditional peak detect circuits and converters. A schematic block diagram of the system is shown in Fig.1.Ch112Fig.1 Schematic block diagram of the measurement systemProc. of SPIE Vol. 6622 66220T-2In all 112 Si-PIN detectors are used to sample the beam profile, and convert the pulsed laser energy density distribution to 112 channel of electronic signals. The photodiode detectors are arranged to be a disk with the spatial sample rate of 0.4cm-1. Fig.2 shows the schematic diagram of the detector array arrangement. The Si-PIN detectors have response spectrum from 400nm to 1100nm, response time of less than 1ns, and active area of 0.8mm2. The peak responsivity of the photodiode detectors is 0.45A/W at the wavelength of about 700nm, and the responsivity at 532nm is about 0.3A/W. In order to suppress interference of the sunlight, band-pass filters(BPF) are used in front of the Si-PIN detectors according to the measured laser wavelength. Charge sensitive amplifiers are used to amplify the pulsed signals of the Si-PIN detectors, and the output pulse duration of the amplifiers are also broadened from 15ns to about 100µs, thus frequency of the signals are reduced and speed demand of AD converters are decreased. Baseline restoration circuits are used to decrease the background noise of the amplifiers. To make the system more compact, 28 quad-input AD converters DDC114 are adopted to make the 112 channel of analog pulsed signals integrated and digitalized. The converters are controlled by a single chip microprocessor. The data are transmitted to the computer end using asynchronous RS-422 series interface and reserved at the same time by the data memory unit. The far-field laser beam is reconstructed in real time with the spatial sample data using special arithmetic of spatial interpolation. The system is calibrated using a standard pulsed laser, making the absolute energy density of the pulsed laser enable to be measured at the same time.Fig.2 Schematic diagram of the detectors arrangementThe simplified charge sensitive amplifier and baseline restoration circuit diagram is shown in Fig.3. The current signal I D of the detector is amplified by the charge sensitive amplifier A1. The amplifier A2 is used as a baseline restoration circuitto depress the DC voltage at the output of A1. The DC voltage of A1 could be produced by the dark current of the Si-PINdetector, or the leakage sunlight incident to the active area of the detector.Proc. of SPIE Vol. 6622 66220T-3Fig.3 Simplified charge sensitive amplifier and baseline restoration circuit diagramThe impulse response from the output current of Si-PIN detector I D to the output current of the charge sensitive amplifier I T is()F F 11-/-/F 2F F 11()=e -e -t R C t R C R R h t R C R C (1) And the current I T isT D ()=()*()I t h t I t (2)Adjust R F C F to be 100µs, which is much shorter than the integration time of AD converter, and set R 1C 1 to be 1µs, so that the rise edge of the current signal of the amplifier output is much quicker than the fall edge. Knowing that the pulse duration of the current input is about 15ns, which is several orders shorter than the characteristic time of the impulse response h (t ), we can consider the input signal as a Dirac delta function, and the amplitude of the impulse function can be described as+D D 0=()d Q I t t ∞∫(3) Here Q D represents the charge output of the Si-PIN detector, which is in proportion to the energy of the laser pulseincident to the active area of the detector.The output charge of the amplifier A 1 can be described as +T T 0=()d Q I t t ∞∫ (4)Here Q T represents the charge output of the charge sensitive amplifier A 1. The gain from Q D to Q T isT D F 2=/=/A Q Q R R (5)The output pulse duration of amplifier A 1 is broadened to R F C F , which is about 100µs. The output pulse duration is limited by integration time of the AD converter, and also limited by the output pulse duration and charge output of the Si-PIN detector. The amplifier has a wide bandwidth of 200MHz, enough for the 15ns pulse width of the input signal. The schematic shape of input signal and output signal of the charge sensitive amplifier are plotted in Fig.4.Proc. of SPIE Vol. 6622 66220T-4Fig.4 Schematic shape of input and output signal of the charge sensitive amplifierThe DDC114 converter is a 20-bit quad channel, current-input AD converter. It combines both current-to-voltage and AD conversion so that the photodiodes can be directly connected to its inputs and digitized. For each of the four inputs, the DDC114 provides a dual-switched integrator front-end. This allows for continuous current integration: while one integrator is being digitized by the onboard AD converter, the other is integrating the input current. The integration time of AD converter is set to 600µs, which is much shorter than the interval of pulsed signals (2.5ms), so there is at large one pulse coming in every two integration intervals of DDC114. For a system without a synchronization clock, there are two cases for the signals coming time. One is that when the signal lies in one integration clock, the energy of the laser pulse will be in proportion to the output of AD converter. The other is that when the signal lies on the boundary of the integration clock, the energy of the laser will be in proportion to the sum of the two adjacent integration periods output. The sketch is plotted in Fig.5.Fig.5 Scheduling sketch of pulse signals and AD converterAdd the output digital signals of the two adjacent integration clocks in the chip microprocessor program if both of them exceed the threshold of the amplifier noise output, then the output data of the single chip microprocessor represents the digitalized single pulse energy. The experiment result shows that the digital error of the system is less than 1%. The data are shown in Table.1.integrate transform transform tranform integrate integrateIntegrator B600µs 600µs600µs Integrator A R 1C 1＝1 µs R F C F ＝100µsQ T I D （t ） Q DI T （t ）Proc. of SPIE Vol. 6622 66220T-5Table.1 The experimental results of AD converterInput amplitude0.4V 0.3V 0.2V0.1V 11010 1010 0110 1000 0000 0001 0101 0110 0001 0010 1011 1111 21010 1010 0100 1000 0000 0010 0101 0110 00100010 1100 0000 31010 1010 0111 1000 0000 0100 0101 0110 0011 0010 1100 0001 41010 1010 0110 1000 0000 0110 0101 0110 0010 0010 1100 0001 51010 1010 0101 1000 0000 0001 0101 0110 0101 0010 1011 1111 61010 1010 0010 1000 0000 0011 0101 0110 0011 0010 1100 0000 71010 1010 0100 1000 0000 0110 0101 0110 0110 0010 1100 0000 Digitalized output data 81010 1010 0011 1000 0000 0011 0101 0110 0100 0010 1011 1110 average 2724.6 2051.3 1379.3 703.8Standard deviation0.138% 0.113% 0.09% 0.058% Nonlinear deviation±0.047% Each channel of the system is calibrated with a standard pulsed laser. The pulse energy of the standard laser is 2µJ, the wavelength is 532nm, the pulse duration is 10ns, and the repetition is 400Hz. The diameter of the laser is restricted by a tunable diaphragm. So the incident laser after the diaphragm can be completely collected by the active area of the detector. Adjust the energy incident to the active area of the photodiode detector with a series of attenuation filters and measure the output laser energy with a calibrated thermal-electric energy detector, and read the output data of the AD converter. The calibration curves of typical channels are shown in Fig.6. They are the calibration results of No.1, No.25, No.78, and No.107 channel. The x-axis is the incident energy of the pulsed laser, and the y-axis is the output unit of the AD converter. Fit results show that the nonlinearity of the energy density measuring is less than 0.1%. The response of each channel is uniformed by processing program in the computer end.A D o u t p u t (u n i t )Energy density(µJ/cm )A D o u t p u t (u n i t )Energy density(µJ/cm )Proc. of SPIE Vol. 6622 66220T-6A D o u t p u t (u n i t )Energy density(µJ/cm)A D o u t p u t (u n i t )Energy density(µJ/cm )Fig.6 Detector calibration curve of typical channelsThe system has a spatial resolution of 2.5cm. Cubic spline interpolation is used to reconstruct the whole laser beam profile with the spatial sample data. The refresh rate of the reconstructed image is 25Hz. It is limited by the transmission rate of asynchronous RS-422 series interface, and can be increased through displacing asynchronous series interface to synchronous series interface if needed. The beam quality and track of the beam center can also be calculated through the sample data. To improve the beam image quality, more sample photodiode detectors should be adopted to increase the spatial resolution. Ordinarily, the sample frequency should exceed two times of spatial frequency of the beam profile.3. APPLICATION OF THE SYSTEMWe measured the far-field beam profile and absolute energy density of the repetitively pulsed laser with the detector array system. In the experiment, the wavelength of the laser is 532nm, the repetition frequency is 400Hz, and the pulse duration is about 15ns. The measuring results are shown in Fig.7. We select six representative frames of the measured far-field laser beam profile. The time interval of the adjacent frame is 5 seconds. From the measured results, the energy density distribution evolvement of the far-field beam can be easily obtained. Using the system, other wavelength of 400nm to 1100nm, repetition frequency of 1Hz to 800Hz pulsed laser can also be measured.Proc. of SPIE Vol. 6622 66220T-7Fig.7 Representative frames of far-field repetitively pulsed laser beam profile4. CONCLUSIONIn this paper, we introduced a detector array system for measuring far-field energy density distribution of repetitively pulsed laser in real time. The measurement system has a dynamic amplitude range of above 103, a spatial resolution of 2.5cm, a refresh rate of 25Hz for the far-field laser beam image, along with a measurement uncertainty of less than 10% for energy density distribution. The system can absolutely measure far-field beam profile of repetitively pulsed laser, with response wavelength between 400nm and 1100nm, minimal detectable pulse width of about 10ns, repetition frequency of 1Hz to 800Hz, and energy density of 0.1-100µJ/cm2. The measuring repetition frequency can be broadened up to 1.25kHz through adjusting characters of AD converter. Near infrared pulsed laser with the wavelength up to 1700nm can also be measured with the system if displacing the Si-PIN detectors with InGaAs photodiode detectors. The spatial sample rate and the refresh rate of the frame should increase in the future according to the demand of laser measurement.5. ACKNOWLEGMENTSThe authors thank Mr. Wang Jianguo of the Risehood company for circuits design of high speed amplifier, Doctor Lin Yong for useful advice on the image processing software, Doctor Xue Tao of Tsinghua University for the single chip microprocessor and program supply, Wang Zhenbao, Shao Bibo and Yan Yan of the Northwest Institute of Nuclear Technology for their generous supports on system debugging and calibration experiments.REFERENCES1.Su Yi and Wan Min, High Energy Laser System, National Defense Technology Press, Beijing, 2004Proc. of SPIE Vol. 6622 66220T-82.Lv Baida, Propagation and Control of High-Power Lasers, National Defense Technology Press, Beijing, 19993.L. Baly Et al. Sensor for energy determination of nanosecond pulses of ultraviolet radiation, Sensors and ActuatorsA 102 (2003) 255–2604.M. W. Kruiskamp and D. M. W. 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