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A laser interferometer has been designed and constructed for the calibration of hydrophones at the National Physical Laboratory to an accuracy of 1% in the frequency range 0.5 to 15 MHz. Factors influencing the optical and electronic design and the choice of detectors are discussed. Compensation for low frequency background vibration is made by a Pockels cell phase shifting unit controlled by feedback from the interferometer signal. The behaviour of this unit is discussed. The coherence properties of the lasers used and their intermode beat signals are investigated. For measurements of the accuracy required, the linearity and frequency response of the detectors and amplifiers are very important. Avalanche photodiodes have a frequency response up to 100 MHz but some diodes used were found to have an anomalous response at low frequencies. This could affect the calibration technique which depends on a flat response down to low frequencies. A simple experiment was performed to check the correction for the refractive index variation of the water due to the passage of ultrasonic waves. The effective refractive index found agreed with the calculated value to within the experimental error of 3%.

Additional information

Authors: DRAIN L E, AERE Harwell, Didcot (GB);MOSS B C, AERE Harwell, Didcot (GB)
Bibliographic Reference: EUR 12944 EN (1990) 60 pp., MF, ECU 4
Availability: (2)
Record Number: 199011085 / Last updated on: 1994-12-01
Original language: en
Available languages: en
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