基于声光调制的百MHz级光电探测器频响校准研究

It is the primary method to measure the sound pressure underwater with high precision based on the optical interferometry, which is crucial to ensure the accuracy of the diagnostic ultrasound equipments. The frequency distortion of the photodetector with the bandwidth about one hundred Mega Hertz in the optical interferometry is the main error to influence the measurement precision of the high frequency sound pressure, and the real difficulty of which is how to obtain the appropriate excitation signal to calibrate the frequency response and correct the frequency distortion induced error. The scheme of the excitation signal for the amplitude frequency response calibration of the mentioned photodetector is proposed based on the acoustic-optic modulation in this project, which aims to achieve the measurement uncertainty less than 3% (k=2) and provides the valid calibrated data to correct the frequency response of the photodetector. A novel scheme is proposed to calibrate the amplitude frequency response based on the acoustic-optic frequency modulation and the heterodyne interferometry. The parameter configuration and cascade of the acoustic-optic frequency modulator is investigated to cover the calibration frequency range and frequency resolution. In order to improve the calibration precision, it is necessary to investigate on the diffraction lights crosstalk in acoustic-optic effect induced measurement error and its elimination scheme. The calibration scheme for the phase frequency response is demonstrated based on the measurement of the pulse width in the time domain with single optical pulse or optical pulse train. And it is important to analyze the available repetition frequency of the optical pulse train restricted by the bandwidth of the photodetector to be calibrated. The modulated scheme to reduce the repetition frequency of the optical pulse train is also proposed based on the acoustic-optic intensity modulation. The investigation in the project is the promotion on the principle and the innovation on the technical method for the frequency response calibration of the photodetector.

激光干涉法是实现超声频段水声声压精确测量的主要技术手段，对于保障医疗超声设备的准确性至关重要。激光干涉仪中百MHz级光电探测器的频率失真是影响高频声压测量精度的主要误差源，构造性能稳定的激励信号实现探测器频响的高精度校准是频率失真误差修正的真正瓶颈。本项目将声光调制原理用于构造百MHz级光电探测器频响校准用激励信号，实现幅频响应校准结果的不确定度优于3% (k=2)，为探测器的频响修正提供有效数据。提出基于声光频率调制和外差干涉原理的幅频响应校准方法，研究声光移频器的参数配置和级联方法，以满足校准频率范围和频率分辨率，研究声光效应各衍射级次光场串扰的误差机理、误差模型和抑制技术，以提高校准精度；提出基于声光强度调制原理的光脉冲重复频率调制方法，解决百MHz级光电探测器对激励光脉冲重复频率的限制，基于光脉冲时域测量法实现相频响应的校准。本项目研究是对光电探测器频响校准原理的推广和技术方法的创新。

激光干涉法是高频、甚高频段水声声压复现和水听器原级校准的主要技术手段，已有研究表明激光零差干涉仪中光电探测器的非理想幅频响应是高频段的主要不确定度来源。项目的研究目标是基于声光调制原理实现百MHz 级光电探测器的频响校准，为激光干涉法水声声压复现中探测器频率失真的误差修正提供有效数据，主要研究内容及其取得的结果包括：. （1）基于外差干涉和声光频率调制提出了光电探测器的幅频响应校准方法，通过声光移频器的级联配置，解决了已有技术方案在MHz频段的校准频率分辨率低、校准频率范围有限且系统复杂的问题；为解决声光移频器在频率调谐过程中衍射光束传播方向的复位偏差引入的空间失配对校准装置测量重复性和测量效率的影响，借鉴凹面镜圆心入射的特性设计了反射式光路结构，所建立的幅频响应校准装置在500 kHz~150 MHz的频率分辨率优于500 kHz，校准结果的测量不确定度为3%（k=2）；. （2）建立了基于激光脉冲响应法的光电探测器相频响应校准装置，通过电光调制的方法将激光器的重复频率降低为1 Hz~500 kHz连续可调，解决了探测器响应电脉冲展宽导致的时域混叠；. （3）给出了光电探测器非理想频响对基于激光零差和外差干涉的水声声压复现影响的数值分析方法，以声质点振速解调结果的相对误差为评价指标，结合光电探测器的频响校准结果和不同应用需求对其符合性进行了评估。. 项目的研究成果可为百MHz级光电探测器的选型、非理想频响的不确定度分析及修正提供理论依据和技术参考，适用于光学法超水声声压基标准和其他光学测量系统。