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激光能量测试的原理主要包括两个部分:高速光电探测和激光参数测量。高速光电探测原理证明该激光能量测试方法的可行性,而脉宽测试原理主要保证该方法的真实性。
激光能量测试方法的整体流程如图1所示。
首先,观察接收到的光电压信号大小,如果光电压信号<1 mV,需要外接放大电路,再进行跨阻放大逆运算得到光电流信号。如果电压信号>1 mV,可直接经过跨阻放大电路的逆运算计算出光电流大小。
其次,光电流经过光电检测电路的逆运算得到光信号。因为射入光电探测器靶面的激光已经经过了光学衰减器的衰减,通常衰减精度是其衰减量的±0.1倍,所以经过光电探测器靶面探测到的激光能量还需乘以衰减倍数之后才等于激光发射能量。
最终,观察发射激光的参数与可调激光器设置参数是否保持一致确认该测试方法的真实有效性。激光能量测试的整体指标见表1。
Experimental parameters Main index requirements Voltage value displayed
by the oscilloscope> 1 mV One-stage voltage amplifier circuit Voltage gain more than 10 times Transimpedance amplifier circuit Gain 2 K, bandwidth 1 G High-speed photoelectric
detection circuitAPD conversion gain 105 V/W
PMT conversion gain 102-107 V/WAttenuator 1-99 dB Table 1. Overall index of laser energy test[10]
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光电探测技术是把不便量化分析的光信号转化为可在示波器中可见的电信号,然后根据后续信号处理电路所需的电压值进行放大处理。
文中设计的高速光电探测电路(见图2)由APD或PMT、跨阻放大器(TIA)、比较器、电流基准、输出驱动等模块构成[10]。
该电路的工作原理为:改变可调激光器的泵浦能量,当光脉冲信号照射到探测器光敏面上,实现光信号到电流信号的转换,电流信号输入TIA电路,实现电流信号到电压信号的转换,TIA输出的电压信号接入比较器,比较器充当模数转换器(ADC)的功能,所以当TIA输出达到比较器翻转阈值时,比较器就会翻转一次,从而表示光信号的探测结果[11]。
根据光电系统辐射源的发光强度、传输介质和目标的传输及调制损耗、接收光学系统接收孔径的限制及反射吸收等损失的影响,可以测试出入射到探测器光敏面上的实际辐射能量[12]。
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激光最主要的参数包括:脉宽、重复频率、泵浦功率。脉宽t是指激光时域脉冲上升时间tr和脉冲下降时间tf到它的50%的峰值功率点之间的时间间隔[13]。重复频率是指激光在某个时间周期内脉冲信号触发的次数,而泵浦功率主要表示脉冲信号所搭载的能量大小。
对于不同的脉冲激光的上升时间量级和时域量程,根据对测试结果的不同要求,采用的测试方法有两类[14]:(1)直接测试法,采用快速探测器,将光信号转换成电信号,通过存储示波器记录其波形;(2)采用相关函数将时间函数转换成空间函数。利用标准延迟器和光速c换算出其时域脉冲波形参数,并依据脉冲激光的时域波形测试获得脉冲宽度t等的时域参数[15]。
文中采用直接测试法,发射激光的同步信号如图3所示。
脉冲激光的参数量值的测试是否准确,将直接影响最终辐射模拟光源峰值功率的结果,是影响最终测试结果的一个重要因素,也是对测试结果进行不确定性评估的一个重要分量。
Testing method and experiment of large dynamic range energy of pulsed laser with wavelength of 1 064 nm and 532 nm
doi: 10.3788/IRLA20200417
- Received Date: 2021-04-08
- Rev Recd Date: 2021-06-04
- Publish Date: 2021-11-02
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Key words:
- pulsed laser /
- APD and PMT detector /
- energy test
Abstract: The range of the energy meter is limited in laser energy measurement, and it cannot take into account the measurement of energy with a large dynamic range. In the photoelectric measurement, a laser energy measurement method based on APD and PMT photodetectors was proposed. The measurement of laser energy in the 1 064 nm and 532 nm bands was compelted through analyzing the detection performance of the photodetector and its interaction with subsequent processing circuits by this method. The voltage signal changes were observed by changing the pulse width, repetition frequency, and pump power of the adjustable laser light source. The size of the photocurrent was tested according to the magnification of the transimpedance amplifier circuit. The relationship between photodetector sensitivity and laser wavelength was used to find the corresponding detector sensitivity, photoelectric conversion gain and laser attenuation multiple. The laser energy value was tested, and the pulse width, repetition frequency and pump power value of the emitted laser were compared to ensure the calculation the authenticity of the method. The experiment proves that the proposed method can complete the laser energy test with a large dynamic range nJ-mJ, and the test error is less than nJ compared with the energy meter test.