High-frequency calibration of hydrophones is becoming increasingly important, both for clinical and scientific applications of ultrasound, and user safety. At present, the calibrations available routinely to the user community extend to 60 MHz. However, hydrophones that can measure beyond this are available, and ultrasonic fields often contain energy at higher frequencies, e.g., generated through nonlinear propagation of high-amplitude ultrasound used for therapeutic applications, and the increasing use of higher frequencies in imaging. Therefore, there is a need for calibrations up to at least 100 MHz, to allow ultrasonic fields to be accurately characterized, and the risk of harmful bioeffects to be properly assessed. Currently, sets of focused piezoelectric transducers are used to meet the pressure amplitude and bandwidth requirements of Primary Standard calibration facilities. However, when the frequency is high enough such that the size of the ultrasound focus becomes less than the hydrophone element's diameter, the uncertainty due to spatial averaging becomes significant, and can be as high as 20% at 100 MHz. As an alternate to piezoelectric transducers, a laser-generated ultrasound calibration source was designed, fabricated, and characterized. The source consists of an optically absorbing carbon-polymer nanocomposite excited by a large-diameter 1064 nm laser pulse of 2.6 ns duration. Peak pressure amplitudes of several Mega-Pascal were readily achievable, and the signal contained measurable frequency components up to 100 MHz. The variation in the pressure amplitudes was less than 2% from its mean over a three-hour test period. The ultrasound beam was sufficiently broad that the uncertainties due to spatial averaging were negligible.

中文翻译：

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用于水听器校准的 100 MHz 带宽平面激光生成超声源

对于超声的临床和科学应用以及用户安全而言，水听器的高频校准变得越来越重要。目前，用户社区常规可用的校准扩展到 60 MHz。然而，可以测量超出此范围的水听器是可用的，并且超声场通常包含更高频率的能量，例如，通过用于治疗应用的高振幅超声的非线性传播以及在成像中越来越多地使用更高频率而产生。因此，需要至少高达 100 MHz 的校准，以便准确表征超声场，并正确评估有害生物效应的风险。现在，成套聚焦压电换能器用于满足初级标准校准设施的压力幅度和带宽要求。然而，当频率足够高以至于超声焦点的尺寸变得小于水听器元件的直径时，空间平均引起的不确定性变得显着，并且在 100 MHz 时可能高达 20%。作为压电换能器的替代方案，设计、制造和表征了激光产生的超声校准源。该光源由一种光吸收碳聚合物纳米复合材料组成，该复合材料由持续时间为 2.6 ns 的大直径 1064 nm 激光脉冲激发。几兆帕的峰值压力幅度很容易实现，并且信号包含高达 100 MHz 的可测量频率分量。在三小时的测试期间，压力幅度的变化与其平均值的差异不到 2%。超声波束足够宽，空间平均造成的不确定性可以忽略不计。