GB/T 42559-2023Acoustics - Measurement of phase-shifted sensitivity of interferometric fiber-optic hydrophones (English PDF)

1 Scope

The document describes the measurement principle, the measuring set-up, the measurement conditions, the measurement procedure and the measurement uncertainty for measuring the sound pressure phase-shifted sensitivity of interferometric fiber-optic hydrophones by the interferometric fringe counting method, the Bessel function ratio method, the phase generated carrier demodulation method, the differential delay heterodyne demodulation method and the phase demodulation method using a 3 x 3 coupler.

The document applies to the measurement of the phase-shifted sensitivity of interferometric fiber-optic hydrophones in the frequency range 10 Hz to 20 kHz.

Note: where the phase-shifted sensitivity levels given by the above methods differ by more than 3 dB, the result obtained by the interferometric fringe counting method is taken as correct.

2 Normative references

The content of the documents listed forms indispensable provisions of this document through normative reference in the text. For dated references only the edition corresponding to that date applies; for undated references the latest edition, including all amendments, applies.

GB/T 3947-1996 Acoustical terminology; GB/T 4130-2017 Acoustics - Low frequency calibration methods for hydrophones; GB/T 7965-2002 Acoustics - Measurement of underwater acoustic transducers; GB/T 14733.12-2008 Terminology for telecommunications - Optical fibre communication; JJG 449-2014 Octave and fractional octave filters.

3 Terms and definitions

The terms and definitions given in GB/T 3947-1996, GB/T 4130-2017, GB/T 7965-2002 and GB/T 14733.12-2008 apply, together with those below.

3.1 interferometric fiber-optic hydrophone: hydrophone made with optical fibre as the sensing element and working on the principle of optical interference. Note: its characteristic is that the phase shift of the interfering light bears a fixed proportional relation to the change of sound pressure in the sound field.

3.2 phase-shifted value of optical interference: phase difference between the interfering light signals of the sensing arm and the reference arm of the interferometric fiber-optic hydrophone caused by the action of an external signal.

3.3 interferometric fringe counting method: method of obtaining the phase-shifted value of optical interference of an interferometric fiber-optic hydrophone caused by the action of an external signal by acquiring the number of interference fringes output by the hydrophone.

3.4 Bessel function ratio method: method in which the interference signal output by the interferometric fiber-optic hydrophone is expanded in a Bessel series and the ratio of the third harmonic to the fundamental of the signal, or of the fourth harmonic to the second harmonic, is used. Note: the phase-shifted value of optical interference measured by the Bessel function ratio method is also called the Bessel function argument.

3.5 phase generated carrier demodulation method: method of obtaining the phase-shifted value of optical interference of an interferometric fiber-optic hydrophone by demodulating the interference light signal output by the hydrophone after phase carrier modulation.

3.6 differential delay heterodyne demodulation method: method of obtaining the phase-shifted value of optical interference of an interferometric fiber-optic hydrophone by generating a differential delay heterodyne optical pulse pair which, after passing through the hydrophone, forms a heterodyne interference light signal that is then demodulated.

3.7 phase demodulation method using 3 x 3 coupler: method of obtaining the phase-shifted value of optical interference of an interferometric fiber-optic hydrophone by using the property that the interference light signals output at the three ends of a 3 x 3 coupler differ in phase by 2 pi/3, and demodulating the phase of the light signal output by the hydrophone.

3.8 standard hydrophone: transducer used for underwater acoustic measurement, stable in performance and absolutely calibrated; commonly used as the standard for establishing the underwater sound pressure reference and for transferring the sound pressure value. The definition is taken from GB/T 3947-1996, 7.82.

3.9 sound pressure phase-shifted sensitivity: ratio of the phase-shifted value of optical interference of the interferometric fiber-optic hydrophone caused by the action of a sound pressure signal to the free field sound pressure that existed, before the hydrophone was introduced into the sound field, at the position of the reference centre of the hydrophone. Note: the unit of the sound pressure phase-shifted sensitivity is the radian per pascal.

3.10 sound pressure phase-shifted sensitivity level: twenty times the logarithm to base ten of the ratio of the phase-shifted sensitivity to the reference phase-shifted sensitivity, the reference phase-shifted sensitivity being 1 rad/µPa. Note: the unit of the sound pressure phase-shifted sensitivity level is the decibel.

4.1 Interferometric fringe counting method - measurement principle

The intensity of the interference light signal output by an interferometric fiber-optic hydrophone under the action of an underwater sound wave is expressed by formula (3), whose symbols are: the interference light intensity voltage signal output by the hydrophone, in volts; the direct current component of the interference light signal, in volts; the alternating current component of the interference light signal, in volts; the phase-shifted value of optical interference output by the hydrophone, in radians; the angular frequency of the sound signal, equal to two pi times the frequency of the sound signal, that frequency being in hertz; the phase difference produced by external environmental factors, in radians; and time, in seconds. The equation itself is not reproduced here.

Figure 1 gives the time domain waveform of a typical interference signal of an interferometric fiber-optic hydrophone; the signal is clearly periodic.

Whenever the phase-shifted value of optical interference is a whole number n of pi radians, a crossing straight line exists along the time axis between the crests and the troughs of the interference signal fringes, as the broken line in Figure 1 shows; the number of fringes within half a period of the interference signal then corresponds to the whole number n. Formula (4) gives the phase-shifted value of optical interference output by the hydrophone under the action of the sound field as n times pi, with n not less than 1 and n a whole number.

In the frequency range 10 Hz to 1 kHz the measurement is made by the method recommended in GB/T 4130-2017. As Figure 2 a) shows, the interferometric fiber-optic hydrophone and the standard hydrophone are placed at the same depth in a standing wave tube, the output voltage of the standard hydrophone and the interference phase shift of the fiber-optic hydrophone are measured, and the phase-shifted sensitivity level of the fiber-optic hydrophone is obtained from formula (5), in which the amplitude of the output voltage of the standard hydrophone is in volts and the sound pressure sensitivity level of the standard hydrophone is in decibels with a reference value of 1 V/µPa.

In the frequency range 1 kHz to 20 kHz the measurement is made by the method recommended in GB/T 7965-2002. As Figure 2 b) shows, the interferometric fiber-optic hydrophone and the standard hydrophone are placed at the same depth under water, the distance between the standard hydrophone and the transmitting transducer and the distance between the fiber-optic hydrophone and the transmitting transducer being noted; the output voltage of the standard hydrophone and the interference phase shift of the fiber-optic hydrophone are measured, and the phase-shifted sensitivity level is obtained from formula (6), in which the two distances, from the reference centre of the transmitting transducer to the reference centre of the fiber-optic hydrophone and to the reference centre of the standard hydrophone, are in metres.

Note to Figure 2: the other methods recommended in GB/T 4130-2017 and GB/T 7965-2002 also apply to the measurement of underwater sound pressure where they meet the measurement requirements; when the phase-shifted sensitivity of an interferometric fiber-optic hydrophone is measured, the measurement uncertainty is evaluated according to the method used.

4.2 Interferometric fringe counting method - measuring set-up

Figure 3 gives the block diagram of the measuring set-up of the interferometric fringe counting method for an interferometric fiber-optic hydrophone. The set-up consists mainly of a standard hydrophone, a laser, a photodetector, a digital oscilloscope, a filter, a signal source and a power amplifier. During the measurement the transmitting transducer, driven by the signal source and the power amplifier, produces in the water the sound signal needed for the measurement. The laser produces the light signal that is injected into the fiber-optic hydrophone; under the action of the sound wave in the water interference fringes of the kind shown in Figure 1 are produced, which the photodetector converts into the corresponding electrical signal, displayed and counted by the oscilloscope, so that the phase-shifted value of optical interference is obtained. At the same time the set-up is controlled by a computer to acquire and process the output voltage signal of the standard hydrophone, whose output voltage amplitude is measured with the digital oscilloscope. Normally the measurement frequency range in the standing wave tube is not less than 100 Hz to 1 kHz and the measurement frequency range in the free field is not less than 1 kHz to 20 kHz.

Note: the upper and lower frequency limits of the interferometric fringe counting method are usually affected by the conditions of the sound field and by the fiber-optic hydrophone itself. The measurement requires the sound pressure level in the sound field to be high enough for the modulation to reach more than pi radians. Moreover, for some interferometric fiber-optic hydrophones the interference fringe signal becomes distorted below 100 Hz and the waveform of the fringes is hard to count, so that the measurement cannot be carried out.

4.3 Interferometric fringe counting method - measurement conditions

4.3.1 Standing wave tube. The requirements are: a working frequency range not less than 10 Hz to 1 kHz; normally a vertical, thick walled, rigid cylindrical cavity opening upwards, the cavity being filled with the measuring medium, normally distilled water, the transmitting transducer being fitted at the bottom of the tube or the tube being driven by a vibration table; and a height of the liquid column inside the tube greater than the diameter of the liquid column and smaller than one quarter of the wavelength of sound in the liquid at the highest measuring frequency.

4.3.2 Free field. The requirements are: a working frequency range not less than 1 kHz to 20 kHz; a fully anechoic or semi-anechoic tank; and a measurement normally made with pulse signals, the dimensions of the tank, the pulse width and the measuring distance meeting the requirements of GB/T 7965-2002.

4.3.3 Electronic measuring instruments. The functional and performance requirements of the main instruments are as follows. Signal source: working frequency range not less than 10 Hz to 20 kHz, maximum output voltage not less than 10 V peak-to-peak, frequency indication error not greater than +/-0.5 %. Power amplifier: working frequency range not less than 10 Hz to 20 kHz, distortion factor not greater than 2 %, power not less than 100 W. Transmitting transducer: working frequency range not less than 10 Hz to 1 kHz for the standing wave tube and 1 kHz to 20 kHz for the free field, the sound pressure level at the position of the interferometric fiber-optic hydrophone and the standard hydrophone being not lower than 140 dB. Filter: filtering range not less than 10 Hz to 20 kHz, meeting the requirements of JJG 449-2014 for class 2 one-third octave filters. Digital oscilloscope: bandwidth not less than 500 MHz, number of channels not less than 2, signal amplitude measurement error not greater than +/-2 %. Laser: working wavelength range not greater than 1 200 nm to 1 600 nm, optical power not less than 10 mW, power stability better than 0.1 dB/h. Photodetector: working wavelength range not greater than 1 200 nm to 1 600 nm, bandwidth not less than 100 kHz. Standard hydrophone: working frequency range not less than 10 Hz to 20 kHz, difference between its sound pressure sensitivity level and that of the interferometric fiber-optic hydrophone not greater than 40 dB, measurement uncertainty not greater than 0.7 dB with a coverage factor of 2. Steel rule: length not less than 30 cm, maximum permissible error of the length measurement not greater than +/-1 mm. Tape measure: length not less than 3 m, maximum permissible error of the length measurement not greater than +/-1 mm.

Note to the standard hydrophone item: because the sensitivity of an interferometric fiber-optic hydrophone is relatively high, the standard hydrophone usually has its own preamplifier; if it has none, preamplification and impedance matching are usually needed when it is connected to the measuring set-up.

4.4.1.1 Preparation before measurement, for the standing wave tube comparison method: distilled water or another medium is poured slowly into the standing wave tube; the surfaces of the interferometric fiber-optic hydrophone and of the standard hydrophone are wiped with a non-corrosive detergent and the hydrophones are soaked in the water for at least 1 h; at the time of measurement the interferometric fiber-optic hydrophone and the standard hydrophone are fixed on the measuring frame so that they are at the same depth in the water. The remainder of this subclause continues beyond the pages available.