Providing primary standard calibrations beyond 20 MHz

Abstract

The number of applications of medical ultrasound utilising frequencies in excessof 20 MHz has shown a consistent increase over recent years. Coupled with the commercialavailability of wide-bandwidth hydrophones whose response extends beyond 40 MHz, this hasdriven a growing need to develop hydrophone calibration techniques at elevated frequencies.The current National Physical Laboratory primary standard method of calibratinghydrophones is based on an optical interferometer. This has been in operation for around 20years and provides traceability over the frequency range of 0.3 to 20 MHz. More recently,calibrations carried out using the interferometer have been extended to 60 MHz, although theuncertainties associated with these calibrations are poor, being in excess of ±20% at highfrequencies. Major contributions to the degraded calibration uncertainties arise from poorsignal-to-noise at higher frequencies, the frequency response of the photodiodes used and thenoise floor of the instrument.To improve the uncertainty of hydrophone calibrations above 20 MHz, it has beennecessary to build and commission a new interferometer. Important features of the newprimary standard are its use of a higher power laser to improve the signal-to-noise ratio, alongwith photodiodes whose greater bandwidth to improve the overall frequency response.This paper describes the design of key aspects of the new interferometer. It also presentssome initial results of the performance assessment, including a detailed comparison ofcalibrations of NPL reference membrane hydrophones, undertaken using old and newinterferometers for calibration up to 40 MHz.