Acousto-optic mode locker capable of improving working frequency and diffraction efficiency

Abstract

The utility model relates to an acousto-optic mode locker capable of improving working frequency and diffraction efficiency, and belongs to the field of photoelectrons. The acousto-optic mode locker comprises an acousto-optic medium which adopts a quartz crystal, the X axis of the quartz crystal serves as an ultrasonic transmission direction, the Y axis serves as a light transmission direction, and the Z axis serves as an optical axis and is perpendicular to the light transmission direction; two parallel polished surfaces are arranged in the X-axis direction of the quartz crystal, one polished surface serves as a sound transmission surface, a welding layer is arranged on the sound transmission surface, a transducer is mounted on the welding layer, and a surface electrode is arranged on the transducer; and the other polished surface is used as an ultrasonic reflecting surface. Fused quartz is replaced by the high-purity quartz crystal, and the tangential direction of the quartz crystal is optimized, so that the influence of the optical rotation of the quartz crystal on the mode-locked laser is reduced to the minimum. The sound wave attenuation coefficient of the quartz crystal is lower, and the acousto-optic merit figure is higher, so that the working frequency and diffraction efficiency of the acousto-optic mode locker can be greatly improved, and the development requirement of an ultra-short pulse laser towards a smaller size is met.

Description

Technical Field

[0001] The present invention relates to an acousto-optic mode locker for improving the working frequency and diffraction efficiency, belonging to the field of optoelectronics. Background Art

[0002] Acousto-optic mode lockers have been widely used in ultrashort pulse (femtosecond) lasers and are key devices for making the longitudinal mode intervals of lasers equal and in phase. The working principle of an acousto-optic mode locker is to fabricate a transducer on the acoustic wave passing surface of an acousto-optic medium. The transducer converts the absorbed radio frequency signal (with a frequency of f) into ultrasonic waves. The ultrasonic waves are transmitted to the ultrasonic wave reflecting surface of the acousto-optic medium and are reflected back to the acoustic wave passing surface by the ultrasonic wave reflecting surface (parallel to the acoustic wave passing surface). When the conditions are appropriate, the reflected ultrasonic waves form a standing wave.

[0003] When the acousto-optic mode locker is placed in a laser cavity, the standing wave in the acousto-optic medium will have an acousto-optic interaction with the laser, causing periodic loss to the laser, making the longitudinal mode intervals of the laser equal and in phase, and outputting an ultrashort pulse laser with a frequency of 2f. The length of the laser cavity (optical path length) is set as L, and the relationship between the laser cavity length L and the radio frequency signal frequency f is:

[0004] f = C / (4L) (1)

[0005] In formula (1), C is the speed of light. It can be seen from formula (1) that the working frequency f of the acousto-optic mode locker is inversely proportional to the laser cavity length L. To reduce the laser cavity length L, the working frequency f must be increased. Obviously, the acoustic wave attenuation coefficient is proportional to the square of the working frequency. As the working frequency increases, the acoustic wave attenuation coefficient will increase sharply, thereby reducing the intensity of the reflected acoustic waves and reducing the diffraction efficiency, which limits the working frequency of the acousto-optic mode locker. Fused silica has excellent optical properties and no optical activity and is a commonly used acousto-optic mode locker material. The acoustic wave attenuation coefficient of fused silica is 12 dB / cm·GHz 2 . Taking the products of Brimrose Corporation of America as an example, they use fused silica as the acousto-optic medium material to make acousto-optic mode lockers. Under the same conditions, the diffraction efficiency of an acousto-optic mode locker with a working frequency of 80 MHz (model FSML-80-20-BR1064) is 60%, while the diffraction efficiency of an acousto-optic mode locker with a working frequency of 125 MHz (model FSML-125-20-BR1064) is only 30%. To meet the requirements of the development of ultrashort pulse lasers towards higher frequencies, it is necessary to seek and develop an acousto-optic mode locker that can improve the working frequency and diffraction efficiency. Summary of the Invention

[0006] In view of the above technical problems, the present invention provides an acousto-optic mode locker for improving the working frequency and diffraction efficiency.

[0007] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0008] An acousto-optic mode locker for improving the working frequency and diffraction efficiency, comprising an acousto-optic medium; the acousto-optic medium is a quartz crystal, the X-axis of the quartz crystal is used as the ultrasonic transmission direction, the Y-axis of the quartz crystal is used as the light transmission direction, and the Z-axis of the quartz crystal is used as the optical axis perpendicular to the light transmission direction; two mutually parallel polished surfaces are arranged in the X-axis direction of the quartz crystal, one polished surface is used as the sound transmission surface, a welding layer is arranged on the sound transmission surface, a transducer is installed on the welding layer, and surface electrodes are arranged on the transducer; the other polished surface is used as the ultrasonic reflection surface.

[0009] Further, when the length between the sound transmission surface and the reflection surface is an integer multiple of the ultrasonic wavelength, standing waves are formed in the quartz crystal.

[0010] Further, a Brewster angle is arranged on the light transmission surface corresponding to the light transmission direction.

[0011] Further, an antireflection film is plated on the light transmission surface corresponding to the light transmission direction.

[0012] Further, the antireflection film material is hafnium oxide and aluminum oxide.

[0013] Further, the quartz crystal is a high-purity material, and the content of metal element impurities is less than 1 ppm. Further, the ultrasonic reflection surface is in a suspended state.

[0014] The positive effects of the present invention are:

[0015] The present invention uses a high-purity quartz crystal to replace fused quartz, and preferentially cuts the quartz crystal to minimize the optical rotation and maximize the acousto-optic figure of merit, minimizing the influence of the optical rotation of the quartz crystal on the mode-locked laser, thereby improving the light output efficiency of the acousto-optic mode locker. Since the quartz crystal has a lower acoustic wave attenuation coefficient and a higher acousto-optic figure of merit, the working frequency and diffraction efficiency of the acousto-optic mode locker can be greatly improved, meeting the development requirements of ultra-short pulse lasers towards smaller volumes. The present invention makes the ultrasonic reflection surface suspended, which can avoid the attachment reducing the ability of the ultrasonic reflection surface to reflect sound waves, thereby effectively improving the standing wave effect and further improving the diffraction efficiency. Description of the Drawings

[0016] Figure 1 is the front view of the present invention;

[0017] Figure 2 is the top view of the present invention in the working mode with a Brewster angle;

[0018] Figure 3 is the top view of the present invention in the working mode without a Brewster angle;

[0019] In the figure: 1. Acousto-optic medium; 2. Sound-transmitting surface; 3. Transducer; 4. Surface electrode; 5. Welding layer; 6. Ultrasonic wave; 7. Ultrasonic wave reflecting surface; 8. First light-transmitting surface; 9. Second light-transmitting surface; 10. Third light-transmitting surface; 11. Fourth light-transmitting surface. Detailed implementation mode

[0020] The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0021] An Acousto-Optic Mode Locker (AOML) uses the acousto-optic effect to achieve mode locking of laser pulses. In an acousto-optic mode locker, the selection of the acousto-optic material is crucial for its performance. Fused quartz is a common acousto-optic medium and is widely used due to its good acousto-optic performance, high mechanical strength, and thermal stability. However, as the operating frequency increases, the acoustic wave attenuation coefficient will increase sharply, thereby reducing the intensity of the reflected acoustic wave, which limits the operating frequency and diffraction efficiency of the acousto-optic mode locker.

[0022] The inventor found through retrieval that quartz crystal is also a commonly used acousto-optic medium material. Generally, an acousto-optic modulator or an acousto-optic Q-switch is made with the optical axis (Z-axis) as the light-transmitting direction. The advantage of this manufacturing method is that when light propagates along the Z-axis direction, there is no birefringence phenomenon, and both the extraordinary ray (e-ray) and the ordinary ray (O-ray) are transmitted along the same optical path, which is convenient for optical path design. This tangential (with the optical axis (Z-axis) as the light-transmitting direction) quartz crystal has optical activity, but some optical path systems can tolerate this property, so it can be used to make an acousto-optic modulator or an acousto-optic Q-switch; however, in an acousto-optic mode locker, this design will increase the loss of the laser cavity and reduce the light output efficiency. For the above reasons, the prior art avoids using quartz crystal to make an acousto-optic mode locker and instead uses fused quartz without optical activity to make an acousto-optic mode locker; therefore, how to improve the operating efficiency, diffraction efficiency, and light output efficiency of the acousto-optic mode locker is the problem to be solved by the present invention.

[0023] Based on the above problems, the present invention proposes an acousto-optic mode locker that improves the working frequency and diffraction efficiency. It uses a quartz crystal instead of fused silica as the acousto-optic medium, thereby increasing the working frequency and diffraction efficiency of the acousto-optic mode locker. Due to its crystal structure, the quartz crystal has a higher acousto-optic figure of merit and a smaller acoustic wave attenuation coefficient. These characteristics may give the quartz crystal an advantage in the propagation of high-frequency acoustic waves. In addition, the improvement of the diffraction efficiency does not solely depend on the material itself. In practical applications, factors such as optical path design and the interaction between acoustic waves and light waves also need to be considered. Therefore, after changing the material of the acousto-optic medium from fused silica to quartz crystal, the present invention also makes corresponding adjustments and optimizations in aspects such as optical path design and acoustic wave loading method to reduce the optical activity of the quartz crystal and reduce the loss of the laser cavity of the acousto-optic mode locker, thereby ensuring the maximization of the working frequency, diffraction efficiency, and light output efficiency.

[0024] In an embodiment of the present invention, the present invention uses a high-purity quartz crystal with a relatively high acousto-optic figure of merit and a relatively small acoustic wave attenuation coefficient to replace fused silica as the acousto-optic medium, improving the working frequency and diffraction efficiency of the acousto-optic mode locker. The quartz crystal is an anisotropic crystal material. When in use, the X-axis of the quartz crystal is used as the ultrasonic wave transmission direction, the Y-axis of the quartz crystal is used as the light passing direction, and the Z-axis (optical axis) of the quartz crystal is perpendicular to the light passing direction. The purpose of such a design is to minimize the optical activity of the quartz crystal and eliminate the influence of the optical activity of the quartz crystal on the mode-locked laser as much as possible.

[0025] Two mutually parallel polished surfaces are designed in the X-axis direction of the quartz crystal. One polished surface serves as the acoustic wave passing surface, on which a welding layer is provided. A transducer is installed on the welding layer, and surface electrodes are provided on the transducer; the other polished surface serves as the acoustic wave reflection surface. The transducer converts the absorbed radio frequency signal into ultrasonic waves and transmits them into the quartz crystal, and then they are reflected back into the quartz crystal by the reflection surface. When the length between the acoustic wave passing surface and the reflection surface is an integer multiple of the ultrasonic wave wavelength, standing waves will be formed in the quartz crystal.

[0026] The acoustic wave attenuation coefficient of the high-purity quartz crystal is 4 dB / cm·GHz 2 , which is only one-third of the acoustic wave attenuation coefficient of fused silica, creating conditions for increasing the working frequency of the acousto-optic mode locker.

[0027] The acousto-optic figure of merit M of the quartz crystal 2 is 3.5×10 -15 S 3 / kg, while the acousto-optic figure of merit M of fused silica 2 is 1.51×10 - 15 S 3 / kg. According to the diffraction efficiency formula of the acousto-optic device:

[0028]

[0029] In formula (2), η is the diffraction efficiency, λ is the optical wavelength, P is the driving power, k is the electromechanical coupling coefficient of the transducer, L is the acousto-optic interaction length, and H is the optical aperture. According to formula (2), under the same conditions, due to the acousto-optic figure of merit M 2 being higher, the diffraction efficiency of the acousto-optic mode locker made of quartz crystal is nearly twice that of fused silica.

[0030] There are two working modes for making an acousto-optic mode locker with quartz crystal: One is the working mode with Brewster angle. The characteristic of this design is that the light-passing surface is designed with a Brewster angle. The vertically polarized light (o-light) incident along the Brewster angle can be transmitted without loss, and the transmittance is theoretically 100%. Therefore, the advantage of this device is that the light-passing surface does not need to be coated with an antireflection film, and it can optimize the polarization state of the laser to ensure that the mode-locked output laser is linearly polarized light. The disadvantage is that the light is transmitted along the Brewster angle and not in a straight line, making it less convenient to use; The other is the non-Brewster angle working mode, which requires an antireflection film to be coated on the light-passing surface. Its advantage is that the laser passing through the acousto-optic mode locker is transmitted in a straight line, making it convenient to use. The disadvantage is that it cannot optimize the polarization characteristics of the mode-locked laser. Both of these working modes are designed for o-light (vertically polarized light, with the polarization direction perpendicular to the plane formed by the acoustic wave direction and the light propagation direction). Mode-locked lasers are all in the linearly polarized light working mode. Therefore, when using an acousto-optic mode locker made of quartz crystal, the polarization state of the light of the mode-locked laser must be o-light (vertically polarized light, with the polarization direction perpendicular to the plane formed by the acoustic wave direction and the light propagation direction).

[0031] In the embodiment of the present invention, as Figure 1 shown, an acousto-optic mode locker for improving the working frequency and diffraction efficiency includes an acousto-optic medium 1. A welding layer 5, a transducer 3, and a surface electrode 4 are sequentially arranged on the acoustic wave passing surface 2 of the acousto-optic medium 1. The acousto-optic medium 1 is a quartz crystal. The quartz crystal has two mutually parallel polished surfaces. One of the polished surfaces serves as the acoustic wave passing surface 2. The normal of the acoustic wave passing surface 2 of the quartz crystal is the X-axis of the quartz crystal. The light passing direction of the acousto-optic medium 1 is the Y-axis of the quartz crystal. The direction perpendicular to the light passing direction of the acousto-optic medium 1 is the Z-axis of the quartz crystal; The other polished surface serves as the ultrasonic wave reflecting surface 7 of the quartz crystal.

[0032] In the embodiment of the present invention, it is necessary to preferentially cut the quartz crystal to minimize the optical activity and maximize the acousto-optic figure of merit; Common quartz crystal cutting methods include AT cut, BT cut, SC cut, GT cut, IT cut, and XY cut, etc. These cutting methods are cut according to different angles of the crystal principal axes (X, Y, Z axes), thereby endowing the crystal with different characteristics. In this embodiment, the X-cut is selected as the optimal choice because it hardly shows optical activity. This also makes the optical activity the weakest when the light propagation direction is perpendicular to the optical axis (Z-axis) of the quartz crystal.

[0033] In the embodiment of the present invention, the shape of the acousto-optic medium is a square column. When light propagates along the Y axis, two surfaces are formed when entering or leaving the quartz crystal, and these two surfaces are the light-transmitting surfaces.

[0034] In the embodiment of the present invention, Figure 2 is a top view of the working mode with Brewster angle, as Figure 2 shown. The first light-transmitting surface 8 is parallel to the second light-transmitting surface 9. The angle between the first light-transmitting surface 8 and the Y axis of the quartz crystal is the Brewster angle θ, and the Brewster angle θ is related to the refractive index of the crystal and the working wavelength. For a wavelength of 1.064 microns, the refractive index of the crystal is 1.53, and the Brewster angle θ is 56.86°. According to the Brewster angle law, the vertically polarized light incident at the Brewster angle can be transmitted without loss, and the theoretical transmittance is 100%.

[0035] In the embodiment of the present invention, Figure 3 is a top view of the working mode without Brewster angle, as Figure 3 shown. The third light-transmitting surface 10 is parallel to the fourth light-transmitting surface 11. An antireflection film needs to be deposited on the third light-transmitting surface 10 and the fourth light-transmitting surface 11 to increase the transmittance of the incident light.

[0036] In the embodiment of the present invention, the materials of the antireflection film are hafnium oxide and aluminum oxide. Among them, hafnium oxide has good chemical stability and is not easily affected by the external environment, which enables it to maintain stable optical properties for a long time. It can be used as a high refractive index layer in a multilayer antireflection film to reduce reflection in a specific wavelength range. Aluminum oxide has high durability and good optical properties and is commonly used in antireflection films that require high durability and is applicable to visible light and infrared light. When the hafnium oxide and the aluminum oxide are alternately arranged, reflection is reduced through the interference effect to achieve the best antireflection effect in a specific wavelength range. For example, a multilayer antireflection film can be constructed by depositing a thin layer of aluminum oxide first and then depositing a thick layer of hafnium oxide on it, and so on.

[0037] In some embodiments of the present invention, the present invention can also combine hafnium oxide with other materials. For example, a high refractive index hafnium oxide layer and a low refractive index silica layer can be alternately deposited to form a multilayer film structure to achieve effective antireflection in a specific wavelength range. Also for example, yttrium fluoride is deposited first and then hafnium oxide is deposited, effectively reducing the reflected light on the light-transmitting surface, thereby increasing the light transmission amount of the acousto-optic medium 1 and improving the light output efficiency of the acousto-optic mode locker.

[0038] In an embodiment of the present invention, the acousto-optic medium 1 is a high-purity quartz crystal, and the content of metal element impurities therein is less than 1 ppm. The metal element impurities here may include one or more of aluminum (Al), iron (Fe), potassium (K), sodium (Na), lithium (Li), titanium (Ti), magnesium (Mg), calcium (Ca), manganese (Mn), chromium (Cr), nickel (Ni), copper (Cu), boron (B), phosphorus (P), etc.

[0039] In an embodiment of the present invention, the transducer 3 is a lithium niobate material that excites longitudinal waves, and it has good frequency stability, which makes it very suitable for use in an acousto-optic mode locker that requires precise frequency control. In an embodiment of the present invention, the welding layer includes two electrode layers and a bonding layer in the middle, namely the upper electrode and the lower electrode. The bonding layer can be a thin film made of high-purity gold or high-purity silver material, which can improve the ability of the acousto-optic mode locker to withstand the driving electric power, and increasing the driving electric power can improve the diffraction efficiency of the acousto-optic mode locker.

[0040] In an embodiment of the present invention, the lower electrode can be formed of a conductive material such as molybdenum (Mo), ruthenium (Ru), tungsten (W), iridium (Ir), or platinum (Pt) or an alloy of molybdenum (Mo), ruthenium (Ru), tungsten (W), iridium (Ir), or platinum (Pt). The upper electrode is formed to at least partially cover the bonding layer, and similar to the lower electrode, the upper electrode can still be formed of a conductive material such as molybdenum (Mo), ruthenium (Ru), tungsten (W), iridium (Ir), or platinum (Pt) or an alloy of molybdenum (Mo), ruthenium (Ru), tungsten (W), iridium (Ir), or platinum (Pt).

[0041] During operation, an RF signal (RF) with a frequency of f is transmitted to the transducer 3 through the surface electrode 4. The transducer 3 absorbs the RF signal and converts it into ultrasonic vibration. The ultrasonic vibration is coupled into the acousto-optic medium 1 through the welding layer 2, and ultrasonic waves 6 are formed in the acousto-optic medium 1. When the ultrasonic waves 6 are transmitted to the ultrasonic reflecting surface 7, they are reflected back into the acousto-optic medium 1.

[0042] The distance between the sound-transmitting surface 2 and the ultrasonic reflecting surface 7 is set as M, and the ultrasonic wavelength is set as N. When M is an integer multiple of N, the ultrasonic waves 6 will form a standing wave (the frequency of the standing wave is 2f) in the acousto-optic medium 1. When the ultrasonic waves 6 are transmitted along the X-axis of the quartz crystal and the light is transmitted along the Y-axis of the quartz crystal, the acousto-optic figure of merit of the vertically polarized light ( Figure 1 the direction perpendicular to the paper surface in the -15 S 3 is relatively high, being 3.5×10 -15 S 3 / kg. Obviously, the acousto-optic figure of merit of quartz crystal is 2.3 times that of fused quartz. According to the acousto-optic interaction theory, the higher the acousto-optic figure of merit, the shorter the acousto-optic interaction length L required to achieve the same diffraction efficiency under the same conditions. When making an acousto-optic mode locker using quartz crystal in this way, the length W in the light passing direction can be made shorter. And the optical rotation is proportional to the length W of the optical transmission crystal. Reducing the length W can further reduce the optical rotation of the quartz crystal.

[0043] To improve the reflection efficiency of sound waves, the ultrasonic reflection surface usually needs to be specially treated, such as coating or using special coating materials. However, in the embodiment of the present invention, the ultrasonic reflection surface 7 opposite to the surface electrode 4 is in a suspended state without applying any object such as thermal conductive glue. The purpose is to avoid the attachment reducing the ability of the ultrasonic reflection surface to reflect sound waves, so as to effectively improve the standing wave effect and further improve the diffraction efficiency. The polished surface set in the embodiment of the present invention can significantly reduce the surface roughness, reduce the diffuse reflection, and improve the reflectivity of the surface.

[0044] Embodiment

[0045] For a wavelength of 1.064 microns, the embodiment of the present invention made an acousto-optic mode locker with a Brewster angle working mode using quartz crystal. The working frequency of this acousto-optic mode locker is 215 MHz, and the diffraction efficiency reaches 30% at a driving power of 2W. The specific design parameters are: the Brewster angle θ is 56.86°, the length W in the light passing direction is 6 mm, the distance M between the sound passing surface 2 and the ultrasonic reflection surface 7 is 5 mm; a transducer 3 with a thickness of 16.5 microns is made on the sound passing surface 2, and then a surface electrode 4 is made on the transducer 3, and the length L of the surface electrode 4 is 5 mm.

[0046] According to the quartz crystal designed by the present invention, when the length W in the light passing direction is 6 mm, its optical rotation is very weak, and the influence on the transmittance is only 0.3%, which can be completely compensated by slightly increasing the pump power of the laser.

[0047] The above embodiments of the present invention are merely examples for illustrating the present invention, rather than limitations on the implementation manners of the present invention. For those of ordinary skill in the art, other different forms of changes and modifications can be made based on the above description. It is impossible to list all the implementation manners here. Any obvious changes or modifications derived from the technical solutions of the present invention still fall within the protection scope of the present invention.

[0048] In the description of the present invention, it should be understood that the orientation or positional relationship indicated by terms such as "coaxial", "bottom", "one end", "top", "middle", "the other end", "upper", "one side", "top", "inner", "outer", "front", "center", "both ends", etc. is based on the orientation or positional relationship shown in the drawings. It is 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 orientation, be constructed and operated in a specific orientation. Therefore, it should not be construed as a limitation to the present invention. In addition, the terms "first", "second", "third", "fourth" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. Thus, the features defined with "first", "second", "third", "fourth" may explicitly or implicitly include at least one of such features.

[0049] In the present invention, unless otherwise clearly defined and limited, terms such as "install", "set", "connect", "fix", "rotate", etc. shall be understood in a broad sense. For example, it may be a fixed connection, a detachable connection, or integrated; it may be a mechanical connection or an electrical connection; it may be directly connected or indirectly connected through an intermediate medium, and it may be the communication inside two elements or the interaction relationship between two elements. Unless otherwise clearly defined, for those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0050] Although the embodiments of the present invention have been shown and described, for those of ordinary skill in the art, it can be understood that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principle and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. An acousto-optic mode locker for improving operating frequency and diffraction efficiency, comprising an acousto-optic medium; characterized in that: The acousto-optic medium is a quartz crystal, the X-axis of the quartz crystal is used as the ultrasonic transmission direction, the Y-axis of the quartz crystal is used as the light transmission direction, and the Z-axis of the quartz crystal is used as the optical axis perpendicular to the light transmission direction; two parallel polished surfaces are arranged in the X-axis direction of the quartz crystal, one polished surface is used as the sound transmission surface, a welding layer is arranged on the sound transmission surface, a transducer is installed on the welding layer, and a surface electrode is arranged on the transducer; the other polished surface is used as an ultrasonic reflection surface.

2. The acousto-optic mode locker for improving operating frequency and diffraction efficiency according to claim 1, characterized in that: When the length between the sound-transmitting surface and the reflecting surface is an integer multiple of the ultrasonic wavelength, the ultrasonic wave forms a standing wave in the quartz crystal.

3. The acousto-optic mode locker for improving operating frequency and diffraction efficiency according to claim 1, characterized in that: A Brewster angle is arranged on the light-transmitting surface corresponding to the light-transmitting direction.

4. The acousto-optic mode locker for improving operating frequency and diffraction efficiency according to claim 1, characterized in that: The light-transmitting surface corresponding to the light-transmitting direction is coated with an anti-reflection film.

5. The acousto-optic mode locking device for improving operating frequency and diffraction efficiency according to claim 4, characterized in that: The anti-reflection film materials are hafnium oxide and aluminum oxide.

6. The acousto-optic mode locker for improving operating frequency and diffraction efficiency according to claim 5, characterized in that: The hafnium oxide and the aluminum oxide are arranged alternately.

7. The acousto-optic mode locker for improving operating frequency and diffraction efficiency according to claim 1, characterized in that: The quartz crystal is a high-purity material, and the metal element impurity content thereof is less than 1 ppm.

8. The acousto-optic mode locker for improving operating frequency and diffraction efficiency according to claim 1, characterized in that: The ultrasonic wave reflecting surface is in a suspended state.

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