Miniature laser and optical imaging system

Abstract

The invention provides an optical imaging system. The optical imaging system comprises a micro laser, a filter, a second focus lens, a third focus lens, a reflector, an objective lens, an electric displacement platform, an acrylic box, an ultrasonic transducer, an amplification filter, an oscilloscope and a computer which are arranged in sequence, the miniature laser comprises a programmable pulse power supply, a laser diode, a collimating mirror, a first focusing mirror and a laser resonant cavity which are arranged in sequence, wherein the laser resonant cavity is composed of a laser crystal, a saturable absorber, a nonlinear crystal and an output coupling mirror; the programmable pulse power supply is connected with the laser diode, pulse pump light generated by the laser diode passes through the collimating focus lens and then enters the laser resonant cavity, and stable pulse laser output is achieved by controlling the repetition frequency, the pulse width and the current of the programmable pulse power supply. The output multi-wavelength nanosecond pulse laser is used as an excitation light source of the photoacoustic imaging system, and a modulation signal of the programmable pulse power supply is directly used as a trigger source of the photoacoustic imaging system.

Description

Technical Field

[0001] The invention relates to the technical field of solid lasers and photoacoustic imaging, and in particular to a micro laser and an optical imaging system. Background Art

[0002] Photoacoustic imaging is a new imaging technology that combines the advantages of high resolution of optical imaging and high penetration depth of ultrasonic imaging, and has broad application prospects in the biomedical field. Different biological tissues have high optical absorption coefficients for light of specific wavelengths. The optical nonlinear effect can expand the spectrum of the output laser, which helps to improve the photoacoustic imaging effect.

[0003] Traditional photoacoustic imaging systems often use actively Q-switched solid-state lasers as excitation light sources, such as acousto-optic Q-switching and electro-optic Q-switching. These active Q-switching methods are large in size and high in cost; electro-optic Q-switching requires a high driving voltage, which is difficult to practicalize and miniaturize.

[0004] However, existing passively Q-switched solid-state lasers still face some challenges in their application in photoacoustic imaging systems due to their large output pulse jitter amplitude and relatively low output stability. Therefore, it is of great significance to develop pulse-stable, portable, and low-cost passively Q-switched microchip lasers. Summary of the invention

[0005] The purpose of the present invention is to provide a micro laser and an optical imaging system to address the drawback of existing passive Q-switched solid-state lasers due to large output pulse jitter amplitude. A programmable pulse power supply is used to adjust the energy, repetition rate and pulse width of pulse pump light to achieve stable pulse laser output, and the output laser repetition rate is consistent with the repetition rate of the pump light modulation signal. The modulation signal output by the programmable pulse power supply can be directly used as a trigger for a photoacoustic imaging system, thereby realizing a low-cost portable photoacoustic imaging system.

[0006] In order to solve the above technical problems, the present invention provides a micro laser, comprising a programmable pulse power supply, a laser diode, a collimator, a first focusing mirror, a laser crystal, a saturable absorber, a nonlinear crystal and an output coupling mirror arranged in sequence; the laser crystal, the saturable absorber, the nonlinear crystal and the output coupling mirror are clamped and fixed together to form a laser resonant cavity, and the laser crystal is arranged toward the first focusing mirror;

[0007] The programmable pulse power supply is connected to the laser diode, and the output end face of the laser diode, the collimating mirror, the first focusing mirror, the laser crystal, the saturable absorber, the nonlinear crystal and the output coupling mirror are arranged on the same horizontal line and are all placed vertically;

[0008] The programmable pulse power supply is used as a trigger source of the photoacoustic imaging system; the programmable pulse power supply provides a pulse current, and the laser diode generates a pulse pump light synchronized with the pulse current;

[0009] The collimator and the first focusing mirror are used to collimate and focus the pulsed pump light respectively, and the focal spot size of the pulsed pump light after focusing is set at the order of tens to hundreds of microns; the pulsed pump light is incident on the laser resonant cavity after passing through the collimator and the first focusing mirror;

[0010] The side of the laser crystal away from the saturable absorber is coated with a first anti-reflection film and a first high-reflection film, and the side close to the saturable absorber is coated with a second anti-reflection film; both side surfaces of the nonlinear crystal are coated with a third anti-reflection film; a partial reflection film is coated between the output coupling mirror and the nonlinear crystal, and the partial reflection film serves as an output mirror of the laser resonant cavity.

[0011] In a preferred embodiment, the programmable pulse power supply is a constant current source, and the laser diode is a fiber-coupled laser diode;

[0012] The maximum output current of the programmable pulse power supply is 15A, the repetition frequency is adjustable from 0 to 20kHz, the minimum pulse width is 20μs, and it can output an internal modulation signal;

[0013] The central wavelength of the laser diode output light beam is 808 nm; the core diameter of the coupling optical fiber is 200 μm, and the numerical aperture is 0.22.

[0014] In a preferred embodiment, the focal lengths of the collimating lens and the first focusing lens are both 11 mm.

[0015] In a preferred embodiment, the reflectivity of the first anti-reflection film, the second anti-reflection film and the third anti-reflection film is greater than 99%; the reflectivity of the first high-reflection film is less than 1%; and the reflectivity of the partial reflection film is 50%-98%.

[0016] In a preferred embodiment, the laser crystal is a laser crystal doped with rare earth ions; the laser crystal is bonded with a saturable absorber to form a composite crystal.

[0017] In a preferred embodiment, the laser crystal is one of a neodymium-doped yttrium aluminum garnet crystal (Nd:YAG), a neodymium-doped yttrium vanadate crystal (Nd:YVO4) and a ytterbium-doped yttrium aluminum garnet crystal (Yb:YAG).

[0018] In a preferred embodiment, the saturable absorber is made of chromium-doped yttrium aluminum garnet crystal (Cr 4+ :YAG) or two-dimensional nanomaterial saturable absorber or semiconductor saturable absorber (SESAM).

[0019] In a preferred embodiment, the nonlinear crystal is one of yttrium vanadate crystal (YVO4), diamond crystal, barium nitrate crystal (BN), potassium gadolinium tungstate crystal (KGW), potassium titanyl phosphate crystal (KTP), β-barium borate crystal (BBO) and lithium triborate crystal (LBO).

[0020] In a preferred embodiment, the substrate of the output coupling mirror is made of fused quartz or sapphire.

[0021] The present invention also provides an optical imaging system, comprising the micro laser, wherein a filter, a second focusing mirror, a third focusing mirror, a reflector, an objective lens, an electric displacement platform, an acrylic box, an ultrasonic transducer, an amplifying filter, an oscilloscope, and a computer are sequentially arranged on the rear side of the micro laser; the filter is a long-pass filter;

[0022] The second focusing mirror and the third focusing mirror constitute a beam expansion system, which is used to enlarge the focal spot size after the pulse pump light is focused;

[0023] The electric displacement platform has XY axis movement; the bottom of the acrylic box is sealed by a cover glass, a sample is placed on the cover glass, and the sample is immersed in water;

[0024] The focusing mode of the ultrasonic transducer is point distance focus.

[0025] Compared with the prior art, the technical solution of the present invention has the following beneficial effects:

[0026] 1. The present invention provides a triggerable, handheld, multi-wavelength, low-cost micro-laser device suitable for photoacoustic imaging. Stable pulse laser output is achieved by controlling the repetition frequency, pulse width and current size of a programmable pulse power supply. The output multi-wavelength nanosecond pulse laser is used as an excitation light source of the photoacoustic imaging system, and the modulation signal of the programmable pulse power supply is directly used as a trigger source of the photoacoustic imaging system.

[0027] 2. The laser of the present invention generates nanosecond pulse laser through passive Q-switching technology, which is suitable as an excitation light source for a photoacoustic imaging system; at the same time, the laser has a simple structure and low cost, which is conducive to the miniaturization and integration of the photoacoustic imaging system.

[0028] 3. The present invention adopts pulse pumping technology, and the pulse laser sequence generated is stable and synchronized with the modulation signal of the pump light. The modulation signal output by the programmable pulse power supply can be directly used as the trigger of the photoacoustic imaging system, and there is no need to use a photodetector to detect the output laser.

[0029] 4. The laser of the present invention combined with nonlinear crystal can realize multi-wavelength laser, which is beneficial for functional photoacoustic imaging. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 It is a structural schematic diagram of an optical imaging system in a preferred embodiment of the present invention;

[0031] Figure 2 A schematic diagram of a modulation signal of a pump in a preferred embodiment of the present invention;

[0032] Figure 3 A pump pulse sequence diagram obtained by a laser diode in a preferred embodiment of the present invention;

[0033] Figure 4 This is a diagram of an output laser pulse sequence corresponding to pulsed pump light in a preferred embodiment of the present invention;

[0034] Figure 5 This is a pulse sequence diagram of a pulse power supply outputting laser at 20kHz in a preferred embodiment of the present invention;

[0035] Figure 6 This is the intensity distribution diagram of 1000 20kHz output laser pulses in the preferred embodiment of the present invention;

[0036] Figure 7 The distribution diagram of the spot and beam quality factor of the 20kHz output laser in the preferred embodiment of the present invention;

[0037] Figure 8 Schematic diagram of pulse width when the laser repetition rate is 20kHz in a preferred embodiment of the present invention;

[0038] Fig. 9 This is a schematic diagram of the power of the output laser with a repetition rate of 20kHz in a preferred embodiment of the present invention;

[0039] Fig.10 A spectrum diagram of the output laser obtained in a preferred embodiment of the present invention introduced into a photoacoustic imaging system;

[0040] Fig.11 A lateral resolution diagram of a photoacoustic imaging system in a preferred embodiment of the present invention;

[0041] Fig.12 This is an axial resolution diagram of the photoacoustic imaging system in a preferred embodiment of the present invention;

[0042] Fig.13 The imaging diagram of the resolution plate USAF 1951 by the photoacoustic imaging system in the preferred embodiment of the present invention under lasers with different repetition frequencies;

[0043] Fig.14 This is a spectrum diagram of the output laser in a preferred embodiment of the present invention;

[0044] Fig.151 is a photoacoustic image of a sample (beef tissue) obtained in a preferred embodiment of the present invention.

[0045] Explanation of the accompanying drawings: 1. Programmable pulse power supply; 2. Laser diode; 3. Collimator; 4. First focusing mirror; 5. Laser crystal; 6. Saturable absorber; 7. Nonlinear crystal; 8. Output coupling mirror; 9. Filter; 10. Second focusing mirror; 11. Third focusing mirror; 12. Reflector; 13. Objective lens; 14. Electric displacement platform; 15. Acrylic box; 16. Ultrasonic transducer; 17. Amplifying filter; 18. Oscilloscope; 19. Computer. DETAILED DESCRIPTION

[0046] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention; it is obvious that the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments, and all other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making creative work are within the scope of protection of the present invention.

[0047] In the description of the present invention, it should be noted that the terms "upper", "lower", "inner", "outer", "top / bottom" and the like indicate positions or positional relationships based on the positions or positional relationships shown in the drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific position, be constructed and operated in a specific position, and therefore cannot be understood as limiting the present invention. In addition, the terms "first" and "second" are used for descriptive purposes only and cannot be understood as indicating or implying relative importance.

[0048] In the description of the present invention, it should be noted that, unless otherwise clearly stipulated and limited, the terms "installed", "provided with", "mounted / connected", "connected", etc. should be understood in a broad sense. For example, "connection" can be a wall-mounted connection, a detachable connection, or an integral connection. It can be a mechanical connection or an electrical connection. It can be a direct connection or an indirect connection through an intermediate medium. It can be the internal connection of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0049] refer to Figure 1, this embodiment provides a micro laser, which is a passive Q-switched micro laser device based on pulse pumping. The micro laser comprises a programmable pulse power supply 1, a laser diode 2, a collimator 3, a first focusing mirror 4, a laser crystal 5, a saturable absorber 6, a nonlinear crystal 7 and an output coupling mirror 8 arranged in sequence; the laser crystal 5, the saturable absorber 6, the nonlinear crystal 7 and the output coupling mirror 8 are clamped and fixed together to form a laser resonant cavity, and the laser crystal 5 is arranged toward the first focusing mirror 4; the micro laser comprises a programmable pulse power supply 1, a laser diode 2, a collimator 3, a first focusing mirror 4, a laser crystal 5, a saturable absorber 6, a nonlinear crystal 7 and an output coupling mirror 8 arranged in sequence; the laser crystal 5, the saturable absorber 6, the nonlinear crystal 7 and the output coupling mirror 8 are clamped and fixed together by a copper clamp to form a laser resonant cavity, and the laser crystal 5 is arranged toward the first focusing mirror 4.

[0050] The programmable pulse power supply 1 has no clear parameter restrictions. The larger the output current and the adjustable repetition rate range, the better, and the narrower the pulse width, the better. The programmable pulse power supply 1 outputs a pulse modulation signal as a trigger source for the entire photoacoustic imaging system. The programmable pulse power supply 1 is a constant current source. The maximum output current of the programmable pulse power supply 1 is 15A, the repetition rate is adjustable from 0 to 20kHz, the minimum pulse width is 20μs, and it can output an internal modulation signal.

[0051] The laser diode 2 is a fiber-coupled laser diode 2; the input pulse current can realize pulse pump light output. The output power of the laser diode 2 is in the order of watts to tens of watts. The central wavelength of the output light beam of the laser diode 2 is 808nm; the core diameter of the coupled optical fiber is 200μm, and the numerical aperture is 0.22.

[0052] The programmable pulse power supply 1 is connected to the laser diode 2, and the programmable pulse power supply 1 provides a pulse current to power the laser diode 2, and the laser diode 2 generates a pulse pump light synchronized with the pulse current; the pulse pump light generated by the laser diode 2 is incident on the laser resonant cavity after passing through the collimating mirror 3 and the first focusing mirror 4, and the focal spot size of the pulse pump light after focusing is controlled to be in the order of tens to hundreds of microns.

[0053] The output end face of the laser diode 2, the collimator 3, the first focusing mirror 4, the laser crystal 5, the saturable absorber 6, the nonlinear crystal 7 and the output coupling mirror 8 are arranged on the same horizontal line and are all placed vertically; the collimator 3 and the first focusing mirror 4 are used to collimate and focus the pulse pump light respectively, and the focal spot size of the focused beam can be changed by changing the focal length and position of the collimator 3 and the first focusing mirror 4. In this embodiment, the focal lengths of the collimator 3 and the first focusing mirror 4 are both 11 mm.

[0054] In this embodiment, the side of the laser crystal 5 away from the saturable absorber 6 is coated with a first anti-reflection film and a first high-reflection film, and the side close to the saturable absorber 6 is coated with a second anti-reflection film; both side surfaces of the nonlinear crystal 7 are coated with a third anti-reflection film; a partial reflection film is coated between the output coupling mirror 8 and the nonlinear crystal 7, and the partial reflection film serves as an output mirror of the laser resonant cavity.

[0055] In actual experiments, the partial reflection film may be disposed on one end face of the output coupling mirror 8 close to the nonlinear crystal 7 or on the other end face of the nonlinear crystal 7 away from the saturable absorber 6 .

[0056] The first anti-reflection film and the first high-reflection film are arranged on the side close to the pump light, the pump wavelength corresponds to the first anti-reflection film, and the output laser wavelength corresponds to the first high-reflection film, the second anti-reflection film, the third anti-reflection film, and the partial reflection film. The reflectivity selected by various films needs to adapt to the parameters of other crystals in the resonant cavity, wherein the reflectivity of the first anti-reflection film, the second anti-reflection film, and the third anti-reflection film is greater than 99%; the reflectivity of the first high-reflection film is less than 1%; and the reflectivity of the partial reflection film is 50%-98%.

[0057] The laser crystal 5 is a rare earth ion doped laser crystal 5. The laser crystal 5 is a crystal including but not limited to neodymium-doped yttrium aluminum garnet crystal (Nd:YAG), neodymium-doped yttrium vanadate crystal (Nd:YVO4) and ytterbium-doped yttrium aluminum garnet crystal (Yb:YAG).

[0058] In this embodiment, the laser crystal 5 is selected from a neodymium-doped yttrium aluminum garnet crystal (Nd:YAG) with a doping concentration of 1 at.%, a thickness of 2.5 mm, and a diameter of 10 mm; the laser crystal 5 is coated with a first anti-reflection film with a wavelength of 808 nm and a first high-reflection film with a wavelength of 1064 nm.

[0059] The saturable absorber 6 is made of, but not limited to, chromium-doped yttrium aluminum garnet crystal (Cr 4+ :YAG) or two-dimensional nanomaterial saturable absorber 6 or semiconductor saturable absorber 6 (SESAM).

[0060] In this embodiment, the saturable absorber 6 is made of Cr-doped yttrium aluminum garnet crystal (Cr 4+ :YAG); in this embodiment, the laser crystal 5 and the saturable absorber 6 are bonded to form a composite crystal.

[0061] The nonlinear crystal 7 uses one of the crystals including but not limited to yttrium vanadate crystal (YVO4), diamond crystal, barium nitrate crystal (BN), potassium gadolinium tungstate crystal (KGW), potassium titanyl phosphate crystal (KTP), β-barium borate crystal (BBO) and lithium triborate crystal (LBO).

[0062] In this embodiment, the nonlinear crystal 7 is selected from yttrium vanadate crystal (YVO4) with a thickness of 2 mm and a diameter of 10 mm and a cut direction of a-cut; in this embodiment, the wavelength of the partial reflection film is 1064 nm and 1176 nm.

[0063] The substrate of the output coupling mirror 8 includes but is not limited to high-quality fused quartz or sapphire, and a film system with a suitable reflectivity is plated on the surface of the output coupling mirror 8 .

[0064] The present embodiment also provides a low-cost portable optical imaging system, including the micro laser, and the imaging system is provided with a filter 9, a second focusing mirror 10, a third focusing mirror 11, a reflecting mirror 12, an objective lens 13, an electric displacement platform 14, an acrylic box 15, an ultrasonic transducer 16, an amplifying filter 17, an oscilloscope 18, and a computer 19 in sequence on the rear side of the micro laser.

[0065] The filter 9 selects an 808nm long-pass filter 9 to filter out the pump light; the focal lengths of the second focusing mirror 10 and the third focusing mirror 11 are 75mm and 125mm respectively, forming a beam expansion system to enlarge the spot size; the reflector 12 reflects the expanded light beam into the rear focal plane of the objective lens 13; the numerical aperture of the objective lens 13 is 0.1, and the focal length is 17.35mm; the electric displacement platform 14 can move in the xy axis, with a travel range of 50mm and a maximum speed of 5mm / s; the bottom of the acrylic box 15 is encapsulated by a cover glass, and a cover glass is placed on the cover glass. A sample (a beef sample) is placed and immersed in water; the bandwidth of the ultrasonic transducer 16 is 4.7 MHz, the focusing mode is point-distance focus, and the focal length is 25 mm; the amplification filter 17 can amplify the collected photoacoustic signal by 46 dB and perform 18 MHz low-frequency filtering; the oscilloscope 18 uses the modulation signal of the programmable pulse power supply 1 as a trigger source to collect the photoacoustic signal in real time, and transmits it to the computer 19 for processing; the computer 19 processes the collected photoacoustic signal in real time and reconstructs the image, and controls the electric displacement platform 14 to scan at the same time.

[0066] The energy, repetition rate and pulse width of the pulsed pump light are adjusted by the programmable pulse power supply 1 to achieve stable pulsed laser output, and the repetition rate of the output laser is consistent with the repetition rate of the pump light modulation signal. The modulation signal output by the programmable pulse power supply 1 can be directly used as the trigger of the photoacoustic imaging system. Figure 2 As shown, the pump pulse sequence obtained by laser diode 2 is as follows Figure 3As shown, the output laser pulse sequence of the corresponding pulse pump light is as follows Figure 4 As shown, the repetition rate of the output laser is adjustable.

[0067] The programmable pulse power supply has an adjustable repetition frequency of 0 to 20kHz. Figure 5 The pulse train of the 20kHz output laser is shown; Figure 6 The intensity distribution of 1000 20kHz output laser pulses is shown; Figure 7 The spot and beam quality factor of 20kHz output laser are shown; Figure 8 The laser output has a pulse width of 2.8ns when the repetition rate is 20kHz, which is suitable as a light source for photoacoustic imaging. Fig. 9 As shown in the figure, the power stability of the output laser with a repetition rate of 20kHz is less than 0.18% RMS, which is relatively stable. Fig.10 As shown, the output laser spectrum center is at 20kHz, and the signal-to-noise ratio is 41dB; the obtained output laser is introduced into the photoacoustic imaging system, Fig.11 and Fig.12 The lateral and axial resolutions of the photoacoustic imaging system were demonstrated to be 5.5m and 300m respectively; Fig.13 The imaging results of the resolution plate USAF 1951 under different repetition frequencies of the excitation light of the photoacoustic imaging system are shown, showing the stability of the photoacoustic imaging effect; Raman laser output can be obtained through the Raman frequency shift in the nonlinear effect, and the spectrum of the output laser is shown in Fig.14 As shown in the figure, the wavelength of the Raman light obtained is near the optical absorption peak of fat. Using it as the excitation light source of the photoacoustic imaging system can perform functional photoacoustic imaging. Fig.15 The photoacoustic image of beef tissue is obtained, wherein the white part is the fat in the beef tissue. Therefore, the present invention can be used to obtain a pulse-stable, triggerable, portable, multi-wavelength, nanosecond micro-laser suitable for photoacoustic imaging.

[0068] The above is only a preferred specific implementation of the present invention, but the design concept of the present invention is not limited to this. Any technician familiar with the technical field who uses this concept to make non-substantial changes to the present invention within the technical scope disclosed by the present invention shall be deemed to infringe the protection scope of the present invention.

Claims

1. A micro laser, characterized in that: It comprises a programmable pulse power supply, a laser diode, a collimator, a first focusing mirror, a laser crystal, a saturable absorber, a nonlinear crystal and an output coupling mirror which are arranged in sequence; the laser crystal, the saturable absorber, the nonlinear crystal and the output coupling mirror are clamped and fixed together to form a laser resonant cavity, and the laser crystal is arranged toward the first focusing mirror; The programmable pulse power supply is connected to the laser diode, and the output end face of the laser diode, the collimating mirror, the first focusing mirror, the laser crystal, the saturable absorber, the nonlinear crystal and the output coupling mirror are arranged on the same horizontal line and are all placed vertically; The programmable pulse power supply is used as a trigger source of the photoacoustic imaging system; the programmable pulse power supply provides a pulse current, and the laser diode generates a pulse pump light synchronized with the pulse current; The collimator and the first focusing mirror are used to collimate and focus the pulsed pump light respectively, and the focal spot size of the pulsed pump light after focusing is set at the order of tens to hundreds of microns; the pulsed pump light is incident on the laser resonant cavity after passing through the collimator and the first focusing mirror; The side of the laser crystal away from the saturable absorber is coated with a first anti-reflection film and a first high-reflection film, and the side close to the saturable absorber is coated with a second anti-reflection film; both side surfaces of the nonlinear crystal are coated with a third anti-reflection film; a partial reflection film is coated between the output coupling mirror and the nonlinear crystal, and the partial reflection film serves as an output mirror of the laser resonant cavity.

2. A micro laser according to claim 1, characterized in that: The programmable pulse power supply is a constant current source, and the laser diode is a fiber-coupled laser diode; The maximum output current of the programmable pulse power supply is 15A, the repetition frequency is adjustable from 0 to 20kHz, the minimum pulse width is 20μs, and it can output an internal modulation signal; The central wavelength of the laser diode output light beam is 808 nm; the core diameter of the coupling optical fiber is 200 μm, and the numerical aperture is 0.

22.

3. A micro laser according to claim 1, characterized in that: The focal lengths of the collimating mirror and the first focusing mirror are both 11 mm.

4. A micro laser according to claim 1, characterized in that: The reflectivity of the first anti-reflection film, the second anti-reflection film and the third anti-reflection film is greater than 99%; the reflectivity of the first high-reflection film is less than 1%; and the reflectivity of the partial reflection film is 50%-98%.

5. A micro laser according to claim 1, characterized in that: The laser crystal is a laser crystal doped with rare earth ions; the laser crystal is bonded with a saturable absorber to form a composite crystal.

6. A micro laser according to claim 5, characterized in that: The laser crystal is one of a neodymium-doped yttrium aluminum garnet crystal (Nd:YAG), a neodymium-doped yttrium vanadate crystal (Nd:YVO4) and a ytterbium-doped yttrium aluminum garnet crystal (Yb:YAG).

7. A micro laser according to claim 6, characterized in that: The saturable absorber is made of chromium-doped yttrium aluminum garnet crystal (Cr 4+ :YAG) or two-dimensional nanomaterial saturable absorber or semiconductor saturable absorber (SESAM).

8. The micro laser according to claim 1, characterized in that: The nonlinear crystal is one of yttrium vanadate crystal (YVO4), diamond crystal, barium nitrate crystal (BN), potassium gadolinium tungstate crystal (KGW), potassium titanyl phosphate crystal (KTP), β-barium borate crystal (BBO) and lithium triborate crystal (LBO).

9. The micro laser according to claim 1, characterized in that: The substrate of the output coupling mirror is made of fused quartz or sapphire.

10. An optical imaging system, characterized in that: The micro laser according to any one of claims 1 to 9 is provided with a filter, a second focusing mirror, a third focusing mirror, a reflector, an objective lens, an electric displacement platform, an acrylic box, an ultrasonic transducer, an amplifying filter, an oscilloscope, and a computer in sequence on the rear side of the micro laser; the filter is a long-pass filter; The second focusing mirror and the third focusing mirror constitute a beam expansion system, which is used to enlarge the focal spot size after the pulse pump light is focused; The electric displacement platform has XY axis movement; the bottom of the acrylic box is sealed by a cover glass, a sample is placed on the cover glass, and the sample is immersed in water; The focusing mode of the ultrasonic transducer is point distance focus.

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