CN108037311B - High-precision seawater flow velocity measurement method based on acousto-optic effect - Google Patents
High-precision seawater flow velocity measurement method based on acousto-optic effect Download PDFInfo
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Abstract
The invention discloses a high-precision seawater flow velocity measuring method based on an acousto-optic effect. Combining the first-order diffracted light of the measuring light with the acousto-optic effect with a reference light path to obtain the change of the light frequency; when the emitted ultrasound generates acousto-optic effect with the measuring light again because the suspended particles in the seawater scatter, the change of the light frequency can be obtained through the same method, and then the Doppler frequency shift of the ultrasound generated because of the moving particles is obtained, so that the size and the direction of the flow velocity of the seawater are obtained. The invention detects the flow velocity profile of the seawater by taking the infrared laser acousto-optic effect as the basic principle, and aims to realize high-precision and high-stability flow velocity measurement.
Description
Technical Field
The invention relates to the field of seawater flow velocity measurement, in particular to a high-precision seawater flow velocity measurement method based on an acousto-optic effect.
Background
At present, the measurement of the flow rate of seawater mainly comprises: mechanical current meter, electromagnetic current meter, acoustics doppler current meter and ship-borne acoustics doppler current profiler, above-mentioned sea water current velocity of flow measuring instrument all has comparatively extensive application according to the needs of difference, but the measurement accuracy and the scope of above-mentioned instrument can not satisfy the needs of ocean scientific development completely under many circumstances.
Disclosure of Invention
The invention provides a high-precision seawater flow velocity measuring method based on acousto-optic effect, which detects a seawater flow velocity profile by taking infrared laser acousto-optic effect as a basic principle and aims to realize high-precision and high-stability flow velocity measurement, and the method is described in detail as follows:
a high-precision seawater flow velocity measurement method based on an acousto-optic effect comprises the following steps:
after being modulated by an electro-optical modulator, an infrared light source sequentially passes through a erbium-doped fiber optical power amplifier, a frequency doubling crystal, a first 1/2 wave plate and a first polarization beam splitting cube, and is reflected by a first reflector, two beams of light are ensured to enter a water tank in parallel at the same height, a pulse generator is started, and pulse ultrasound is excited;
when the measuring light path and the reference light path are adjusted, a reflector capable of adjusting pitching deflection is placed at a position as far as possible from the front ends of the two beams of light, the reflector is adjusted to enable the reference light path to be perpendicular to the reflector, namely, a pinhole diaphragm is placed at the front end of the reference light path, and light reflected by the reflector is ensured to pass through the same diaphragm;
adjusting the position of the ultrasonic probe to enable the ultrasonic to vertically fly through a reference light path, calculating the sound pressure of the ultrasonic at the reference light path according to the ratio of the obtained diffraction light intensity to the original substrate light intensity, calculating the sound pressure at the measuring light path according to the attenuation coefficient of the ultrasonic in water, calculating the light intensity ratio at the measuring light path reversely, and adjusting the position of the ultrasonic probe;
combining two beams of parallel light which is emitted from a water tank and generates an acousto-optic effect, receiving optical signals by a first photoelectric detector and a second photoelectric detector at two sides of a second polarization beam splitting cube respectively, converting the optical signals into electric signals, and acquiring flight distance information and time information by a frequency counter and an oscilloscope so as to obtain the speed of sound of seawater;
combining first-order diffracted light and reference light into a beam to perform beat frequency after an acousto-optic effect is generated between ultrasonic without Doppler frequency shift and the measured light, converting the beam into an electric signal to detect after the beam is received by a photoelectric detector, and acquiring frequency shift of the light frequency caused by the acousto-optic effect;
then when the emitted ultrasound touches suspended particles in the seawater, the ultrasound with Doppler frequency shift is generated, and then the frequency shift of the optical frequency is also obtained after the acousto-optic effect is generated between the emitted ultrasound and the measuring light, so that the acoustic Doppler frequency shift is obtained through two times of optical frequency shift, and the flow velocity of the seawater is obtained.
The method comprises the following steps:
adjusting the first 1/2 wave plate, the second 1/2 wave plate and the third 1/2 wave plate to enable the reference light path and the measurement light path to be combined through the second polarization beam splitting cube;
starting the first signal generator to output continuous signals, namely the second ultrasonic probe sends out continuous ultrasonic waves; adjusting a fourth reflector to enable the first-order diffracted light and the reference light to be combined at a third polarization beam splitting cube, and measuring the change of the frequency of the diffracted light due to the acousto-optic effect through beat frequency;
changing the output of a first pulse generator into a pulse signal with fixed frequency, setting the pulse time interval to be larger than the time required by an echo, and detecting suspended particles moving in water, namely the change of optical frequency caused by the acousto-optic effect of ultrasonic wave generating Doppler frequency shift and measuring light; finally obtaining the flow velocity of the seawater.
The technical scheme provided by the invention has the beneficial effects that:
1. the method can solve the contact measurement problem of the mechanical current meter and avoid the influence on the flow of the seawater; the problem that the mechanical current meter rotor cannot be started when the flow rate is low and has the lowest starting speed is solved; in addition, the method is based on the acousto-optic effect, has very high sensitivity, and can effectively avoid the problems of large inertia of the propeller and lower measurement sensitivity;
2. the method can solve the problems that the sensor structure of the electromagnetic current meter influences the measured current field, the interference of the sea water magnetic field and the change of the sea water conductivity can also cause the zero drift of the voltage measured by the electromagnetic current meter, and the current measuring precision is not high, and simultaneously, the method can effectively solve the problem that the electromagnetic current meter is not suitable for the flow velocity measurement in the large turbulence intensity occasions such as near-bottom shallow water environment, near-shore sediment transportation and the like;
3. the method can be used for replacing the probe of the ADCP sold on the market at present;
4. the method can play an important direct or indirect role in hydrogeology, anti-submergence, cable laying and geological investigation, mining, geophysical exploration, water microstructure analysis, hydrowater channel measurement, marine investigation and national defense application, and provides high-precision data guarantee for operation tasks in related fields.
Drawings
FIG. 1 is a flow chart of a high-precision seawater flow velocity measurement method based on acousto-optic effect;
fig. 2 is an optical path diagram of high-precision seawater flow velocity measurement based on acousto-optic effect.
In the drawings, the components represented by the respective reference numerals are listed below:
1: an infrared light source; 2: an electro-optic modulator;
3: an erbium-doped fiber optical power amplifier;
9: a first ultrasonic probe; 10: a water tank;
11: a carrying groove; 12: a first signal generator;
13: a second ultrasonic probe; 14: a first sound-transmitting panel;
15: a second sound-transmitting panel; 16: a second reflector;
17: a second polarizing beam splitting cube; 18: measuring a light path;
19: a reference optical path; 20: a first photodetector;
21: a frequency counter; 22: a second signal generator;
23: a second photodetector; 24: an oscilloscope;
25: a first aperture diaphragm; 26: a second aperture diaphragm;
27: a third reflector; 28: a second 1/2 wave plate;
29: a third 1/2 wave plate; 30: a fourth mirror;
31: a third polarization beam splitting cube; 32: first order diffracted light;
33: a power amplifier;
the first 1/2 wave plate 5, the second 1/2 wave plate 28 and the third 1/2 wave plate 29 are all the same in type and are Thorlabs, WPH 10E-780;
the models of the first sound transmission plate 10 and the second sound transmission plate 11 are the same;
the first signal generator 12 and the second signal generator 22 are of the same type, and are both Rigol, DG 4162;
the first polarization beam splitting cube 6, the second polarization beam splitting cube 17 and the third polarization beam splitting cube 31 are all of the same type, and are Thorlabs and PBS 252;
the first reflector 7, the second reflector 16 and the fourth reflector 30 are all the same in type and are Thorlabs, PF 10-03-P01;
the models of the first small hole observation billows 25 and the second small hole diaphragm 26 are the same, and are Thorlabs, SM1D 12;
the model of the infrared light source 1 is NP Photonics,1560 nm; the erbium-doped fiber optical power amplifier 3 is of type YM, GA 81;
the first ultrasonic probe 9 and the second ultrasonic probe 13 are of the type Olympus, V3337; the first photodetector 14 is of the type Thorlabs, FPD 310-FV; the second photodetector 16 is of the type Thorlabs, APD 430A/M; the frequency counter 15 is of the type Keysight, 53230A;
the oscilloscope 17 is model Tektronix, MDO 3104; the pulse generator 18 is of the type Olympus, 5073 PR.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, embodiments of the present invention are described in further detail below.
In the embodiment of the invention, the ultrasonic wave is pulse ultrasonic and has fixed frequency, so that the ultrasonic grating cannot exist all the time, namely diffraction does not occur all the time, and the zero-order light intensity is obviously weakened compared with incident light intensity when diffraction occurs, so that when the ultrasonic wave flies through a light beam region, a combined light intensity signal detected on an oscilloscope is obviously sunken, and the time difference between the two sunken parts is the time required by the ultrasonic wave to fly through two beams of light.
Although the two parallel beams carry the same phase information when being transmitted, the phase information carried by the two beams also has great difference when being detected finally due to different optical paths, and the distance information can be obtained by reading the difference between the phases.
Regarding echo measurement, the acousto-optic effect is that when ultrasonic waves pass through a medium, local compression and elongation of the medium are caused to generate elastic strain, the elastic strain periodically changes along with time and space, so that the medium has a phenomenon of density and density alternation, and the refractive index periodically changes at the same time, like a phase grating, which is called an ultrasonic grating.
When light passes through the medium which is disturbed by ultrasonic waves, namely the ultrasonic grating, diffraction phenomenon occurs, and since sound moves in water, which is equivalent to the moving grating, when light passes through the moving grating, the frequency of each level of diffracted light except zero-level diffracted light changes. Because the change of diffraction light frequency caused by acousto-optic effect generated by ultrasonic without Doppler frequency shift and ultrasonic with Doppler frequency shift twice and measuring light is required to be detected so as to obtain the Doppler frequency shift generated by the ultrasonic hitting moving particles suspended in water, according to the difference of the measured seawater depths, the embodiment of the invention needs to select the pulse ultrasonic with adjustable frequency, thereby ensuring that the pulse interval time is longer than the ultrasonic echo time, and avoiding interference and influencing the final measuring result.
The embodiment of the invention uses the acousto-optic effect as a basic measurement principle, and when the ultrasonic wave flies over the laser, the frequency of the diffracted light is changed due to the acousto-optic effect. Combining the first-order diffracted light of the measuring light with the acousto-optic effect with a reference light path to obtain the change of the light frequency; when the emitted ultrasound generates acousto-optic effect with the measuring light again because the suspended particles in the seawater scatter, the change of the light frequency can be obtained through the same method, and then the Doppler frequency shift of the ultrasound generated because of the moving particles is obtained, so that the size and the direction of the flow velocity of the seawater are obtained.
Example 1
The embodiment of the invention introduces a new method for detecting a seawater flow velocity profile by using good properties of sound and light, high speed and high resolution and taking a sound and light effect as a basic principle, aiming at realizing high-precision, high-stability and quick seawater flow velocity profile measurement, and referring to fig. 1 and fig. 2, the method comprises the following steps:
101: the infrared light source 1 is subjected to intensity modulation by the electro-optical modulator EOM2, and light is emitted by the infrared light source 1 and then is directly connected into the electro-optical modulator EOM 2;
the infrared light source 1 is modulated mainly to avoid the distance ambiguity problem in the phase method distance measurement process, that is, if the modulation signal has only one fixed frequency when measuring the phase, no matter which phase difference measurement method is adopted, only the phase difference less than 2 pi can be measured, and the whole period number contained in the actual phase difference can not be obtained.
In order to solve the problem of distance ambiguity, a simple method is to make the wavelength of the modulation signal larger than the distance to be measured, and obtain the wavelength of the modulation signal meeting the measurement requirement after modulating the infrared light source 1.
102: after the infrared light source 1 is modulated, as shown in fig. 2, the infrared light source is sequentially passed through an erbium-doped fiber optical power amplifier EDFA3, a frequency doubling crystal 4, a first 1/2 wave plate 5, a first polarization beam splitting cube 6 and a first reflector 7, two beams of light are ensured to enter a water tank 10 in parallel at the same height, a first pulse generator 8 is started, and pulse ultrasound is excited;
through multiple experiments, water has a strong absorption effect on 1560nm light emitted by the infrared light source 1, so that the frequency doubling crystal 4 can play a role only by carrying out frequency doubling treatment on the light, which is required by frequency doubling, with high power, and therefore modulated light needs to pass through the erbium-doped fiber optical power amplifier EDFA3 and the frequency doubling crystal 4 in sequence.
Ensuring the parallel height of the two beams is an important premise for realizing high-precision measurement of the sound velocity profile of the seawater. After the optical path difference between the two optical paths is obtained according to the phase difference between the measuring optical path 18 and the reference optical path 19, only if the two beams are ensured to be parallel and equal in height, and half of the optical path difference between the two beams is the flying distance when the ultrasonic wave vertically flies through the two beams.
When the measurement optical path 18 and the reference optical path 19 are adjusted, a reflector 27 capable of adjusting pitch and yaw is placed at a position where the front ends of the two beams of light are far away as possible, the reflector 27 is adjusted to enable the reference optical path 19 to be perpendicular to the reflector 27, namely, a first pinhole diaphragm 25 is placed at the front end of the reference optical path 19, and the light reflected by the reflector 27 is ensured to pass through the first diaphragm 25; the first mirror 7 is adjusted such that the measuring beam path 18 is perpendicular to the mirror 27, i.e. it is ensured that the light reflected back via the mirror 27 passes the second diaphragm 26.
103: adjusting the position of the first ultrasonic probe 9 to ensure that the ultrasonic wave vertically flies through two parallel light beams;
the distance between the two beams can be ensured to be the ultrasonic flying distance only when the ultrasonic vertically flies through the two parallel beams, and the sea water sound velocity value can be accurately obtained only by obtaining the flying time of the corresponding distance through the acousto-optic effect.
The ultrasonic wave is adjusted to vertically fly through the reference light path 19, the sound pressure of the ultrasonic wave at the position of the reference light path 20 is calculated according to a correlation formula according to the ratio of the obtained diffraction light intensity to the original substrate light intensity, then the sound pressure at the position of the measuring light path 18 is calculated according to the attenuation coefficient of the ultrasonic wave in water, then the light intensity ratio at the position of the measuring light path 18 is inversely calculated by the correlation formula, and the position of the ultrasonic probe 9 is adjusted.
104: two beams of parallel light which are emitted from the water tank 9 and generate acousto-optic effect are combined, as shown in fig. 2, a first photoelectric detector 20 and a second photoelectric detector 23 are respectively used for receiving optical signals at two sides of a second polarization beam splitting cube 17 and are converted into electric signals, and then a frequency counter 21 and an oscilloscope 24 are used for acquiring flight distance information and time information, so that a sound velocity value is acquired;
105: combining first-order diffracted light and reference light into a beam to perform beat frequency after an acousto-optic effect is generated between ultrasonic without Doppler frequency shift and the measured light, converting the beam into an electric signal to detect after the beam is received by a photoelectric detector, and acquiring frequency shift of the light frequency caused by the acousto-optic effect;
106: when the emitted ultrasound touches suspended particles in seawater, the ultrasound with Doppler frequency shift is generated, and then the frequency shift of optical frequency is also obtained after the ultrasound and measuring light generate acousto-optic effect, so that acoustic Doppler frequency shift is obtained through two times of optical frequency shift, and the flow velocity of the seawater is obtained.
In summary, the embodiment of the invention detects the flow velocity profile of the seawater by using the acousto-optic effect as the basic principle, and the adopted flow velocity measuring method can finish high-precision, high-stability and rapid flow velocity measurement.
Example 2
The specific implementation method for measuring the flow velocity profile of seawater provided by the embodiment of the invention is described in detail below with reference to fig. 1 and 2.
The embodiment of the invention is based on the acousto-optic effect, and the light path design is shown in figure 2. The measurement steps are as follows:
step 201: as shown in fig. 2, light emitted by an infrared light source 1 is directly connected to an electro-optical modulator 2 to obtain a modulated light source;
step 202: as shown in fig. 2, the light path is adjusted, so that the modulated light source sequentially passes through the erbium-doped fiber optical power amplifier EDFA3, the frequency doubling crystal 4, the first 1/2 wave plate 5 and the first polarization beam splitting cube 6, and is reflected by the first reflector 7 to be changed into two parallel light paths to be emitted into the water tank 10;
step 203: starting a first pulse generator 8 to excite pulse ultrasound, adjusting an ultrasonic probe 9 and ensuring that the ultrasound vertically flies through two parallel lights;
step 204: after a measurement light path 18 which is emitted from the water tank 10 and generates an acousto-optic effect is reflected by a second reflector 16, the measurement light path and a reference light path 19 which also generates the acousto-optic effect are combined at a second polarization beam splitting cube 17, as shown in fig. 2, light signals are received by a first photoelectric detector 20 and a second photoelectric detector 23 respectively at two sides of the second polarization beam splitting cube 17, converted into electric signals, and then received by a frequency counter 21 and an oscilloscope 24;
step 205: firstly, the first 1/2 wave plate 5 is adjusted to enable the light with the incident light in a specific polarization direction to be transmitted, then the second 1/2 wave plate 28 is adjusted to enable the light to be in the same state as the first 1/2 wave plate 5, and finally the third 1/2 wave plate 29 is adjusted to enable the polarization direction of the light passing through to be vertical to the polarization direction of the light transmitted by the first 1/2 wave plate 5 and the second 1/2 wave plate 28, so that only the reference light path 19 passes through, and the time reading t of the oscilloscope 24 is recorded1Simultaneously, the second signal generator 22 is started to make the output frequency of the second signal generator 22 the same as the modulation frequency of the infrared light source 1, and the phase difference phi between the second signal generator 22 and the reference light path 19 displayed by the frequency counter 21 is recorded1；
Step 206: firstly, the first 1/2 wave plate 5 is adjusted to enable the light with the incident light in a specific polarization direction to be transmitted, then the third 1/2 wave plate 29 is adjusted to enable the light to be in the same state as the first 1/2 wave plate 5, and finally the second 1/2 wave plate 28 is adjusted to enable the polarization direction of the light passing through to be vertical to the polarization direction of the light transmitted by the first 1/2 wave plate 5 and the second 1/2 wave plate 28, so that only the measuring light path 18 passes through, and the time reading t of the oscilloscope 24 is recorded2The phase difference phi between the second signal generator 22 and the measuring beam path 18, which is indicated by the frequency counter 212；
Step 207: let the modulation frequency be f1The wavelength of the modulated light source is λ ═ c/f1Finally, a sound speed measurement v1 is obtained as
Wherein S is
t is t2-t1。
Step 208: the first 1/2 wave plate 5, the second 1/2 wave plate 28 and the third 1/2 wave plate 29 are adjusted so that the reference optical path 19 and the measurement optical path 18 can both be combined by the second polarization beam splitting cube 17;
step 209: starting the first signal generator 12 to output a continuous signal, namely, the second ultrasonic probe sends out continuous ultrasonic waves; the fourth mirror 30 is adjusted to combine the first order diffracted light with the reference light at the third polarization beam splitting cube 31, and the change Δ f of the diffracted light frequency due to the acousto-optic effect is measured by beat frequency1；
Step 210: the output of the first pulse generator 12 is changed into a pulse signal with fixed frequency, the pulse time interval is set to be larger than the time required by an echo, and the change delta f of the optical frequency caused by the acousto-optic effect of detecting suspended particles moving in water, namely the ultrasonic wave generating Doppler frequency shift and the measuring light2；
Step 211: finally obtaining the seawater flow velocity as follows:
v2＝v1*(Δf2-Δf1)/Δf1
in conclusion, the embodiment of the invention detects the flow velocity profile of the seawater by taking the infrared laser acousto-optic effect as the basic principle, thereby realizing high-precision, high-stability and rapid flow velocity measurement; the method plays an active role in similar construction works such as an ocean standard field and the like, replaces the traditional seawater flow velocity measurement method with the advantage of direct on-site high-precision measurement, greatly improves the measurement precision, and contributes to the similar construction of the ocean standard field.
In the embodiment of the present invention, except for the specific description of the model of each device, the model of other devices is not limited, as long as the device can perform the above functions.
Those skilled in the art will appreciate that the drawings are only schematic illustrations of preferred embodiments, and the above-described embodiments of the present invention are merely provided for description and do not represent the merits of the embodiments.
The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents, improvements and the like that fall within the spirit and principle of the present invention are intended to be included therein.
Claims (2)
1. A high-precision seawater flow velocity measurement method based on an acousto-optic effect is characterized by comprising the following steps:
after being modulated by an electro-optical modulator, an infrared light source sequentially passes through a erbium-doped fiber optical power amplifier, a frequency doubling crystal, a first 1/2 wave plate and a first polarization beam splitting cube, and is reflected by a first reflector, two beams of light are ensured to enter a water tank in parallel at the same height, a pulse generator is started, and pulse ultrasound is excited;
when the measuring light path and the reference light path are adjusted, a reflector capable of adjusting pitching deflection is placed at a position as far as possible from the front ends of the two beams of light, the reflector is adjusted to enable the reference light path to be perpendicular to the reflector, namely, a pinhole diaphragm is placed at the front end of the reference light path, and light reflected by the reflector is ensured to pass through the same diaphragm; adjusting the first reflector to enable the measuring light path to be vertical to the reflector, namely ensuring that the light reflected by the reflector passes through the second diaphragm;
adjusting the position of a first ultrasonic probe to ensure that the ultrasonic wave vertically flies through two parallel light beams, so that the distance between the two light beams is the ultrasonic flying distance, and acquiring the flying time of the corresponding distance by the acousto-optic effect to acquire the sound velocity value of the seawater; the sound pressure of the ultrasound at the reference light path is calculated according to the ratio of the obtained diffraction light intensity to the original substrate light intensity, the sound pressure at the measuring light path is calculated according to the attenuation coefficient of the ultrasound in water, the light intensity ratio at the measuring light path is calculated reversely, and the position of the ultrasonic probe is adjusted;
combining two beams of parallel light which is emitted from a water tank and generates an acousto-optic effect, receiving optical signals by a first photoelectric detector and a second photoelectric detector at two sides of a second polarization beam splitting cube respectively, converting the optical signals into electric signals, and acquiring flight distance information and time information by a frequency counter and an oscilloscope so as to obtain the speed of sound of seawater;
combining first-order diffracted light and reference light into a beam to perform beat frequency after an acousto-optic effect is generated between ultrasonic without Doppler frequency shift and the measured light, converting the beam into an electric signal to detect after the beam is received by a photoelectric detector, and acquiring frequency shift of the light frequency caused by the acousto-optic effect;
then, when the emitted ultrasound touches suspended particles in the seawater, the ultrasound with Doppler frequency shift and the measuring light generate acousto-optic effect, and then the frequency shift of the light frequency is obtained, so that the acoustic Doppler frequency shift is obtained through two optical frequency shifts, and the flow velocity of the seawater is obtained.
2. The method for measuring the seawater flow velocity with high precision based on the acousto-optic effect is characterized by comprising the following steps of:
adjusting the first 1/2 wave plate, the second 1/2 wave plate and the third 1/2 wave plate to enable the reference light path and the measurement light path to be combined through the second polarization beam splitting cube;
starting the first signal generator to output a continuous signal
The second ultrasonic probe sends out continuous ultrasonic waves; adjusting a fourth reflector to enable the first-order diffracted light and the reference light to be combined at a third polarization beam splitting cube, and measuring the change of the frequency of the diffracted light due to the acousto-optic effect through beat frequency;
changing the output of a first pulse generator into a pulse signal with fixed frequency, setting the pulse time interval to be larger than the time required by an echo, and detecting suspended particles moving in water, namely the change of optical frequency caused by the acousto-optic effect of ultrasonic wave generating Doppler frequency shift and measuring light; finally obtaining the flow velocity of the seawater.
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| CN111189528B (en) * | 2020-01-09 | 2022-04-08 | 天津大学 | High-precision underwater sound velocity measurement method based on femtosecond laser frequency comb |
| CN113433342B (en) * | 2021-08-26 | 2021-11-30 | 山东省科学院海洋仪器仪表研究所 | Ocean flow velocity detection system and detection method based on laser induced sound |
| CN114137251B (en) * | 2021-11-16 | 2024-01-16 | 安徽大学 | Integrated multipurpose laser Doppler flow velocity measuring instrument |
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