CN110243763B - Non-contact photoacoustic imaging device and method - Google Patents

Non-contact photoacoustic imaging device and method Download PDF

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CN110243763B
CN110243763B CN201910587193.8A CN201910587193A CN110243763B CN 110243763 B CN110243763 B CN 110243763B CN 201910587193 A CN201910587193 A CN 201910587193A CN 110243763 B CN110243763 B CN 110243763B
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王毅
胡瀛心
彭斌扬
周红仙
马振鹤
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Northeastern University China
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Abstract

The invention provides a non-contact photoacoustic imaging device and method, and belongs to the technical field of photoacoustic imaging. The photoacoustic imaging device comprises a detection light source, a fiber isolator, a2 x 2 fiber coupler, a fiber circulator, a3 x 3 fiber coupler, a collimator, a lens, a reflector, a dichroic mirror, a laser, a photoelectric detector, a high-pass filter, a data acquisition card and a computer. The invention detects the in-situ sound pressure, and utilizes the high-pass filter and the 3 multiplied by 3 optical fiber coupler to demodulate the photoacoustic signal, thereby improving the sensitivity and stability of the system; the invention solves the limitation of using water layer to practical application, and realizes complete non-contact photoacoustic imaging.

Description

Non-contact photoacoustic imaging device and method
Technical Field
The invention belongs to the technical field of photoacoustic imaging, and particularly relates to a non-contact photoacoustic imaging device and method.
Background
Photoacoustic imaging (PAI) is a non-invasive imaging modality that can be used for structural, functional imaging of biological tissues. PAI, combined with optically high contrast and acoustically high resolution, has recently become a major research area for biomedical imaging. PAI is based on the photoacoustic effect, and uses pulsed laser to irradiate biological tissue, which absorbs light energy and generates thermoelastic expansion, thereby generating ultrasonic waves, and the ultrasonic waves are detected to obtain an absorption distribution image of the tissue.
Current photoacoustic detection techniques can be divided into contact photoacoustic imaging and non-contact photoacoustic imaging. The contact type photoacoustic imaging uses the piezoelectric transducer to detect ultrasonic sound pressure, when a probe is in contact with a sample, an air layer exists between the probe and the sample, ultrasonic waves have different acoustic impedances in different media, and strong reflection can be generated at an interface of the two media with the different acoustic impedances, so that the fact that an acoustic coupling medium needs to be used between the transducer and the sample is determined in principle, loss is reduced, and sensitivity is improved. However, the practical application of photoacoustic imaging is greatly limited by the use of coupling agents, many medical tests need to be performed without physical contact, such as burn diagnosis, brain surgery tests, etc., and non-contact detection is needed to reduce infection.
As an alternative to piezoelectric transducers, various non-contact detection methods based on optical interference have been proposed in succession, which methods detect ultrasound-induced displacements and vibrations of the sample surface non-contact instead of using an ultrasonic transducer to detect the ultrasonic sound pressure. Compared with the detection method based on the ultrasonic transducer, the optical interference method has the following advantages: non-contact, device miniaturization, optical transparency, large bandwidth and high sensitivity. However, the existing photoacoustic imaging method based on optical interference has the defects that: firstly, the surface of a tissue sample is rough, so that the intensity of reflected detection light is weak and the phase changes randomly, so that the sensitivity of the system is reduced, in order to solve the problem, a water layer is usually coated on the surface of the sample and generates a uniform reflecting surface, but the method needs to add the water layer on the surface of the sample, so that the method is not completely non-contact and is very inconvenient to apply; secondly, the photoacoustic signal is waited to be transmitted from the photoacoustic excitation origin to the surface of the sample for detection, and the loss of the photoacoustic signal is large in the process. The same problem exists in the patent "non-contact photoacoustic detection method and apparatus based on optical interference method" (patent No. 201510881786.7).
Disclosure of Invention
In order to solve the defects of the prior art, the invention provides a non-contact photoacoustic imaging system and a non-contact photoacoustic imaging method based on 3 x 3 optical fiber coupler demodulation, the invention uses an optical interference method to directly detect the reflected light intensity change of a photoacoustic signal excitation point in a sample, the focus of the excitation light and the sample light is superposed (positioned in the sample), an absorber at the focus absorbs laser energy to generate sound pressure change, the optical refractive index at the position is changed, the reflection coefficient is further increased, and the light intensity of the backscattered sample light is increased; the light intensity change is measured by an interference method based on 3 multiplied by 3 optical fiber coupler demodulation, three paths of interference signals output by the 3 multiplied by 3 optical fiber coupler are detected at the same time, the reflected light intensity change of the photoacoustic signal excitation point is obtained through demodulation, and random interference of the outside is eliminated by combining a high-pass filter.
The technical scheme of the invention is as follows:
a non-contact photoacoustic imaging device comprises a detection light source 1, a fiber isolator 2, a2 x 2 fiber coupler 3, a fiber circulator A4, a fiber circulator B8, A3 x 3 fiber coupler 12, a collimator A13, a lens A14, a reflector A15, a collimator B16, a dichroic mirror 17, a reflector B18, a reflector C19, a lens B20, a laser 22, a reflector D23, a photodetector A24, a photodetector B25, a photodetector C26, a high-pass filter A27, a high-pass filter B28, a high-pass filter C29, a data acquisition card 30 and a computer 31.
The detection light source 1, the optical fiber isolator 2 and the 2 x 2 optical fiber coupler 3 are sequentially connected through optical fibers; the output end of the 2 × 2 optical fiber coupler 3 is respectively connected with an optical fiber circulator A4 through an optical fiber circulator A1 port 5 and connected with an optical fiber circulator B8 through an optical fiber circulator B1 port 9;
the optical fiber circulator A4 is connected with a collimator A13 through a port 6 of an optical fiber circulator A2 and is connected with the input end of A3X 3 optical fiber coupler 12 through a port 7 of an optical fiber circulator A3; the collimator A13, the lens A14 and the reflector A15 are coaxially arranged in sequence;
the optical fiber circulator B8 is connected with a collimator B16 through a port 10 of an optical fiber circulator B2 and is connected with the input end of a 3X 3 optical fiber coupler 12 through a port 11 of an optical fiber circulator B3;
the laser emitted by the laser 22 sequentially passes through the reflector D23 and the dichroic mirror 17, is converged with the probe light passing through the collimator B16, and then sequentially passes through the reflector B18, the reflector C19 and the lens B20 to be focused inside the sample 32 arranged on the objective table 21; the trigger output end of the laser 22 is electrically connected with the trigger input end of the data acquisition card 30;
the output end of the 3 × 3 optical fiber coupler 12 is respectively connected with the photodetector a24, the photodetector B25 and the photodetector C26 through optical fibers; the photoelectric detector A24, the photoelectric detector B25 and the photoelectric detector C26 are respectively connected with a high-pass filter A27, a high-pass filter B28 and a high-pass filter C29; the three high pass filters are electrically connected to the input of a data acquisition card 30 in a computer 31.
Further, 3 × 3 division of the fiber coupler 12Light ratio of K1:K2:K3。
Further, the position where the mirror C19 is disposed is replaced with a two-dimensional galvanometer to realize two-dimensional imaging.
A non-contact photoacoustic imaging method comprising the steps of:
step 1 photoacoustic excitation Process
The laser 22 emits laser light, the exciting light is focused inside the sample 32, the sample absorbs energy to generate ultrasonic waves, the ultrasonic waves cause the optical refractive index change of the exciting point, the optical refractive index change causes the optical reflectivity of the point to be larger, and the reflected light intensity of the detection light is increased.
Step 2 photoacoustic detection Process
The detection light emitted by the detection light source 1 enters the optical fiber isolator 2 through the optical fiber and then enters the 2 x 2 optical fiber coupler 3 to be divided into the sample light and the reference light. The sample light enters the optical fiber circulator B8 through the port 9 of the optical fiber circulator B1, is output from the port 10 of the optical fiber circulator B2, is collimated into parallel light through the collimator B16, is combined with the laser into a beam of light through the dichroic mirror 17, and is focused inside the sample 32 on the objective table 21 through the reflector B18, the reflector C19 and the lens B20 in sequence; the backscattered light returns back and enters the 3 x 3 fiber coupler 12 through port 11 of fiber circulator B3. The reference light enters the optical fiber circulator A4 through a port 5 of the optical fiber circulator A1, is output from a port 6 of the optical fiber circulator A2, sequentially passes through a collimator A13, a lens A14 and a reflector A15, returns in the original path, and enters the 3X 3 optical fiber coupler 12 through a port 7 of the optical fiber circulator A3.
The two paths of light respectively enter the 3 × 3 optical fiber coupler 12 and then output three paths of signals, the three paths of signals respectively enter the photoelectric detector A24, the photoelectric detector B25 and the photoelectric detector C26 to generate interference and convert the interference into electric signals, and the electric signals are filtered by the high-pass filter A27, the high-pass filter B28 and the high-pass filter C29 to enter the computer 31 and then are collected by the data collection card 30.
Step 3 Signal acquisition Process
In step 1, while the laser 22 emits laser light, the laser 22 emits a trigger signal, and the data acquisition card 30 performs synchronous acquisition of photoacoustic signals.
Further, in step 2, the process of performing three-phase demodulation by the 3 × 3 fiber coupler 12 is as follows:
the interference caused by backscattered light at different depths of the sample is expressed as:
Figure BDA0002114850800000051
wherein, IRRepresenting the light intensity from the reference arm; i iss,iIs the light intensity from the ith depth of the sample; delta Is(t) is the detected light intensity variation generated at the photoacoustic excitation location;
Figure BDA0002114850800000052
is at IRAnd Δ Is(t) a time-varying phase difference therebetween;
Figure BDA0002114850800000053
is IRAnd Is,iTime varying phase difference therebetween;
Figure BDA0002114850800000054
is Δ Is(t) and Is,iTime varying phase difference therebetween;
Figure BDA0002114850800000055
and
Figure BDA0002114850800000056
representing random environmental interference. And Δ Is(t) the other terms in equation (1) are all slowly varying compared to the pulse variation, filtered out by a high pass filter, and Is,iMuch less than Δ Is(t) thus, with Is,iWith the correlation term ignored, the measured signal is approximated as:
Figure BDA0002114850800000057
the formula (2) shows that the measurement signal is composed of
Figure BDA0002114850800000058
Modulation, demodulation of the change in reflection intensity Δ I by means of the 3 × 3 fiber coupler 12s(t) of (d). The stable photoacoustic signal can be demodulated by using the three-phase demodulation method, the photoacoustic signal is not interfered by the outside, and the stability of the system is improved. The three interference signals collected by the data collection card 30 are represented as:
Figure BDA0002114850800000059
wherein,
Figure BDA00021148508000000510
is shown in equation (1)
Figure BDA00021148508000000511
Summing;
Figure BDA00021148508000000512
and
Figure BDA00021148508000000513
representing the phase difference between the three output signals; the splitting ratio of the optical fiber coupler is K1:K2:K3；
The three-way interference signal of the formula (3) is obtained:
Figure BDA00021148508000000514
wherein G ═ K2/K1；H＝K3/K1；
Figure BDA00021148508000000515
Figure BDA00021148508000000516
Further, in step 2, two-dimensional imaging is realized by continuously moving the object stage 21 horizontally; or mirror C19 may be replaced with a two-dimensional galvanometer to achieve two-dimensional imaging.
The invention has the beneficial effects that: firstly, the invention does not need to use a water layer, solves the limitation of the water layer on practical application and realizes complete non-contact photoacoustic imaging; secondly, the invention detects the in-situ sound pressure, demodulates the photoacoustic signal by utilizing the high-pass filter and the 3 multiplied by 3 optical fiber coupler, and improves the sensitivity and the stability of the system.
Drawings
FIG. 1 is a schematic view of the apparatus of the present invention.
In the figure: 1, detecting a light source; 2 an optical fiber isolator; a 32 × 2 fiber coupler; 4, a fiber optic circulator A; 5 fiber optic circulator a1 port; 6 fiber optic circulator a2 port; 7 fiber circulator a3 port; 8, a fiber circulator B; 9 fiber optic circulator B1 port; 10 fiber optic circulator B2 port; 11 fiber optic circulator B3 port; 123 × 3 fiber optic couplers; 13 a collimator A; 14, a lens A; 15 mirror A; 16 a collimator B; a 17 dichroic mirror; 18 mirror B; 19 mirror C; 20 lenses B; 21 an object stage; 22 a laser; 23 mirror D; 24 photo detector A; 25 photodetectors B; 26 a photodetector C; 27 a high-pass filter A; 28 a high-pass filter B; 29 a high-pass filter C; 30, a data acquisition card; 31 a computer; 32 samples.
Detailed Description
The following detailed description of the embodiments of the invention refers to the accompanying drawings.
The non-contact photoacoustic imaging device comprises a detection light source 1, a fiber isolator 2, a2 × 2 fiber coupler 3, a fiber circulator A4, a fiber circulator B8, A3 × 3 fiber coupler 12, a collimator A13, a lens A14, a reflector A15, a collimator B16, a dichroic mirror 17, a reflector B18, a reflector C19, a lens B20, a stage 21, a laser 22, a reflector D23, a photodetector A24, a photodetector B25, a photodetector C26, a high-pass filter A27, a high-pass filter B28, a high-pass filter C29, a data acquisition card 30 and a computer 31, wherein the detection light source is a light source, and the data acquisition card 30 is a light source; the 3 × 3 fiber coupler 12 in this embodiment has a splitting ratio of 1:1: 1.
The detection light source 1, the optical fiber isolator 2 and the 2 x 2 optical fiber coupler 3 are sequentially connected through optical fibers; the output end of the 2 × 2 optical fiber coupler 3 is connected with an optical fiber circulator A4 through a port 5 of an optical fiber circulator A1 and connected with an optical fiber circulator B8 through a port 9 of an optical fiber circulator B1.
The optical fiber circulator A4 is connected with a collimator A13 through a port 6 of an optical fiber circulator A2 and is connected with the input end of A3X 3 optical fiber coupler 12 through a port 7 of an optical fiber circulator A3; the collimator a13, the lens a14, and the mirror a15 are coaxially arranged in this order. The fiber circulator B8 is connected with the collimator B16 through a fiber circulator B2 port 10 and connected with the input end of the 3X 3 fiber coupler 12 through a fiber circulator B3 port 11.
Laser light emitted by the laser 22 is incident on the reflecting mirror D23 at an incident angle of 45 °, the dichroic mirror 17 is disposed in parallel at the same angle below the reflecting mirror D23, and the probe light passing through the collimator B16 and the laser light passing through the dichroic mirror 17 are combined into one beam; the reflecting mirror B18 and the dichroic mirror 17 are symmetrically arranged below the dichroic mirror 17 along the horizontal axis, the reflecting mirror C19 is arranged on one side of the reflecting mirror B18 in parallel at the same angle as the reflecting mirror B18, and light passing through the reflecting mirror C19 perpendicularly enters the lens B20 and is focused inside the sample 32 arranged on the objective table 21; the laser 22 is connected to a data acquisition card 30.
The output end of the 3 × 3 optical fiber coupler 12 is respectively connected with the photodetector a24, the photodetector B25 and the photodetector C26 through optical fibers; the photoelectric detector A24, the photoelectric detector B25 and the photoelectric detector C26 are respectively connected with a high-pass filter A27, a high-pass filter B28 and a high-pass filter C29; the three high pass filters are connected to a data acquisition card 30 in a computer 31.
The measuring process of the invention comprises the following steps:
step 1 photoacoustic excitation Process
The laser 22 emits laser (excitation light) to irradiate the sample, the laser is focused in the sample, the sample absorbs energy to generate ultrasonic waves, the ultrasonic waves cause the optical refractive index change of an excitation point, the optical refractive index change causes the optical reflectivity of the point to be increased, and the reflected light intensity of the detection light is increased.
Step 2 photoacoustic detection Process
The detection light emitted by the detection light source 1 enters the optical fiber isolator 2 through the optical fiber, and then enters the 2 x 2 optical fiber coupler 3 through the optical fiber to be divided into sample light and reference light. The sample light enters from a port 9 of the optical fiber circulator B1, is output from a port 10 of the optical fiber circulator B2, is collimated into parallel light by a collimator B16, is combined with the excitation light into a beam of light after passing through a dichroic mirror 17, and is focused inside the sample 32 on the objective table 21 after sequentially passing through a reflecting mirror B18, a reflecting mirror C19 and a lens B20; the backscattered light returns back and enters the 3 x 3 fiber coupler 12 through port 11 of fiber circulator B3. The reference light enters through a port 5 of the optical fiber circulator A1, is output from a port 6 of the optical fiber circulator A2, sequentially passes through a collimator A13, a lens A14 and a reflector A15, returns to the original path, and enters the 3 × 3 optical fiber coupler 12 through a port 7 of the optical fiber circulator A3.
The two paths of light respectively enter the 3 × 3 optical fiber coupler 12 and then output three paths of signals with phase difference of 120 degrees, the three paths of signals respectively enter the photoelectric detector A24, the photoelectric detector B25 and the photoelectric detector C26 to generate interference and convert the interference into electric signals, and the electric signals respectively enter the computer 31 to be collected by the data acquisition card 30 after the interference is filtered by the high-pass filter A27, the high-pass filter B28 and the high-pass filter 29.
The three-phase demodulation process of the 3 × 3 fiber coupler is as follows:
the back-scattered light at different depths in the sample causes complex interference, expressed as:
Figure BDA0002114850800000081
wherein, IRRepresenting the light intensity from the reference arm; i iss,iIs the light intensity from the ith depth of the sample; delta Is(t) is the detected light intensity variation generated at the photoacoustic excitation location;
Figure BDA0002114850800000082
is at IRAnd Δ Is(t) a time-varying phase difference therebetween;
Figure BDA0002114850800000083
is IRAnd Is,iTime varying phase difference therebetween;
Figure BDA0002114850800000084
is Δ Is(t) and Is,iIn betweenA time-varying phase difference;
Figure BDA0002114850800000091
and
Figure BDA0002114850800000092
all represent random environmental interference. And Δ Is(t) the other terms in equation (1) are all slowly varying compared to the pulse variation, filtered out by a high pass filter, and Is,iMuch less than Δ Is(t) thus, with Is,iWith the correlation term ignored, the measured signal is approximated as:
Figure BDA0002114850800000093
equation (2) shows that the measurement signal consists of
Figure BDA0002114850800000094
Modulation, the invention uses a method based on a3 × 3 optical fiber coupler to demodulate the change of the reflection intensity Delta Is(t); the stable photoacoustic signal can be demodulated by using the three-phase demodulation method, the photoacoustic signal is not interfered by the outside, and the stability of the system is improved. The three interference signals collected by the data collection card 30 are represented as:
Figure BDA0002114850800000095
wherein,
Figure BDA0002114850800000096
is shown in equation (1)
Figure BDA0002114850800000097
Summing;
Figure BDA0002114850800000098
and
Figure BDA0002114850800000099
representing the phase difference between the three output signals, the splitting ratio for this embodimentA 1:1: 13 fiber optic coupler of 3 × 3,
Figure BDA00021148508000000910
and
Figure BDA00021148508000000911
120 deg. and 240 deg., respectively.
Obtaining from the three interference signals:
Figure BDA00021148508000000912
from equation (4), Δ I can be obtaineds(t)：
Figure BDA00021148508000000913
Step 3 Signal acquisition Process
In step 1, while the laser 22 emits laser light, the laser 22 emits a trigger signal, and the data acquisition card 30 performs synchronous acquisition of photoacoustic signals.
The detection method of this embodiment belongs to single-point detection, and when an image of one region is obtained, two-dimensional scanning is necessary. At the moment, the reflecting mirror C19 is replaced by a two-dimensional galvanometer, and the steps 1 to 3 are repeated to realize two-dimensional imaging; or by continuously moving the stage 21 horizontally in step 2 to achieve two-dimensional imaging.

Claims (1)

1. A non-contact photoacoustic imaging method is characterized in that the method is realized based on a non-contact photoacoustic imaging device, the device comprises a detection light source (1), a fiber isolator (2), a2 x 2 fiber coupler (3), a fiber circulator A (4), a fiber circulator B (8), a3 x 3 fiber coupler (12), a collimator A (13), a lens A (14), a reflector A (15), a collimator B (16), a dichroic mirror (17), a reflector B (18), a reflector C (19), a lens B (20), a laser (22), a reflector D (23), a photoelectric detector A (24), a photoelectric detector B (25), a photoelectric detector C (26), a high-pass filter A (27), a high-pass filter B (28), a high-pass filter C (29), a data acquisition card (30) and a computer (31);
the detection light source (1), the optical fiber isolator (2) and the 2 x 2 optical fiber coupler (3) are sequentially connected through optical fibers; the output end of the 2 x 2 optical fiber coupler (3) is connected with an optical fiber circulator A (4) through an optical fiber circulator A1 port (5) and is connected with an optical fiber circulator B (8) through an optical fiber circulator B1 port (9);
the optical fiber circulator A (4) is connected with the collimator A (13) through an optical fiber circulator A2 port (6) and is connected with the input end of the 3 x 3 optical fiber coupler (12) through an optical fiber circulator A3 port (7); the collimator A (13), the lens A (14) and the reflector A (15) are coaxially arranged in sequence;
the optical fiber circulator B (8) is connected with a collimator B (16) through a port (10) of an optical fiber circulator B2 and is connected with the input end of a3 x 3 optical fiber coupler (12) through a port (11) of an optical fiber circulator B3;
laser emitted by the laser (22) sequentially passes through a reflector D (23) and a dichroic mirror (17), is converged with detection light passing through a collimator B (16), and then sequentially passes through a reflector B (18), a reflector C (19) and a lens B (20) to be focused inside a sample (32) arranged on an objective table (21); the laser (22) is connected with the data acquisition card (30);
the output end of the 3 x 3 optical fiber coupler (12) is respectively connected with a photoelectric detector A (24), a photoelectric detector B (25) and a photoelectric detector C (26) through optical fibers; the photoelectric detector A (24), the photoelectric detector B (25) and the photoelectric detector C (26) are respectively connected with a high-pass filter A (27), a high-pass filter B (28) and a high-pass filter C (29); the three high-pass filters are connected with a data acquisition card (30) in a computer (31);
the non-contact photoacoustic imaging method comprises the following steps:
step 1 photoacoustic excitation Process
The laser (22) emits laser, the laser is focused in the sample (32), the sample absorbs energy to generate ultrasonic waves, the ultrasonic waves cause the optical refractive index of an excitation point to change, further cause the optical reflectivity of the excitation point to increase, and cause the reflection intensity of the detection light to increase;
step 2 photoacoustic detection Process
The detection light emitted by the detection light source (1) is divided into reference light and sample light after passing through the optical fiber isolator (2) and the 2 multiplied by 2 optical fiber coupler (3) in sequence; the reference light enters the optical fiber circulator A (4) through an optical fiber circulator A1 port (5), is output from an optical fiber circulator A2 port (6), sequentially passes through a collimator A (13), a lens A (14) and a reflector A (15), returns to the original path, and enters the 3 x 3 optical fiber coupler (12) through an optical fiber circulator A3 port (7); the sample light enters the optical fiber circulator B (8) through a port (9) of the optical fiber circulator B1, is output from a port (10) of the optical fiber circulator B2, is collimated into parallel light by a collimator B (16), is combined with the laser into a beam of light after passing through a dichroic mirror (17), and is focused inside a sample (32) arranged on an object stage (21) through a reflector B (18), a reflector C (19) and a lens B (20) in sequence; the back scattered light returns to the original path and enters a3 multiplied by 3 optical fiber coupler (12) through a port (11) of an optical fiber circulator B3;
two paths of light enter the 3 x 3 optical fiber coupler (12) and then output three paths of signals, the three paths of signals respectively enter the photoelectric detector A (24), the photoelectric detector B (25) and the photoelectric detector C (26) to generate interference and convert the interference into electric signals, the electric signals respectively enter a computer (31) after being filtered by the high-pass filter A (27), the high-pass filter B (28) and the high-pass filter C (29), and a data acquisition card (30) is used for signal acquisition;
the three-phase demodulation process of the 3 x 3 optical fiber coupler (12) is as follows:
the interference caused by backscattered light at different depths of the sample is expressed as:
Figure FDA0002514655200000031
wherein, IRIs the light intensity from the reference arm; i iss，iIs the light intensity from the ith depth of the sample; delta Is(t) is the detected light intensity variation generated at the photoacoustic excitation location;
Figure FDA0002514655200000032
is at IRAnd Δ Is(t) a time-varying phase difference therebetween;
Figure FDA0002514655200000033
is IRAnd Is，iTime varying phase difference therebetween;
Figure FDA0002514655200000034
is Δ Is(t) and Is，iTime varying phase difference therebetween;
Figure FDA0002514655200000035
and
Figure FDA0002514655200000036
all represent random environmental interference; except for Delta IsThe other terms except (t) are filtered by a high-pass filter and are equal to Is，iNeglecting the correlation term of (c), the measured signal is:
Figure FDA0002514655200000037
demodulation of the change in reflection intensity Δ I by means of a3 × 3 fiber coupler (12)sAfter (t), the three interference signals collected by the data acquisition card (30) are expressed as:
Figure FDA0002514655200000038
wherein,
Figure FDA0002514655200000039
is represented by formula (1)
Figure FDA00025146552000000310
Summing;
Figure FDA00025146552000000311
and
Figure FDA00025146552000000312
representing the phase difference between the three output signals; the splitting ratio of the optical fiber coupler is K1∶K2∶K3；
Obtaining Delta I from the three interference signals of formula (3)s(t)：
Figure FDA00025146552000000313
Wherein G ═ K2/K1；H＝K3/K1；
Figure FDA00025146552000000314
Figure FDA00025146552000000315
Step 3 Signal acquisition Process
In the step 1, when the laser (22) emits laser, the laser (22) emits a trigger signal, and the data acquisition card (30) synchronously acquires photoacoustic signals.
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