Interference-based multilayer film saturable absorption mirror and optimization method

Abstract

The invention provides a multilayer film saturable absorption mirror based on interference and an optimization method. The multilayer film saturable absorption mirror comprises a saturable absorption material film, a middle interference layer, a gold film and a silicon substrate, the thin film is a flat thin film with a saturable absorption characteristic; the middle interference layer is a ZnS thin film layer deposited on the gold reflector substrate; and the gold film and the silicon substrate jointly form a gold reflector substrate. The thin film interference cancellation condition is achieved by adjusting the thicknesses of the thin film and the middle interference layer, so that the linear reflectivity is extremely low when the light intensity is low; as the light intensity is enhanced, the refractive index of the material is changed and the absorption coefficient is reduced until the material is bleached by strong light, the optical lengths of the thin film and the middle interference layer do not meet destructive interference any more, so that the upper limit of the modulation depth cannot be greatly reduced due to the introduction of the middle interference layer. According to the invention, the linear reflectivity or transmittance of the thin film type saturable absorption mirror at low light intensity can be effectively reduced to enlarge the modulation range, the effect of improving the modulation depth is achieved, and the method is simple to operate, high in flexibility and universal.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of laser, in particular to a multi-layer thin film saturable absorber based on interference and an optimization method. BACKGROUND

[0002] The development of sensing detection, photoelectric countermeasure, minimally invasive medical treatment, high-precision industrial processing and other technologies puts forward higher requirements on the performance parameters of pulsed fiber lasers such as working wavelength, pulse width, peak power and pulse energy, especially for passive Q-switched pulsed fiber lasers with high pulse energy. Saturable absorber is a key pulse modulation device of passive Q-switched pulsed fiber lasers. In the passive Q-switched pulse modulation technology of lasers, the saturable absorber uses its saturable absorption characteristics of high loss state under low light intensity and low loss state under high light intensity to periodically modulate the loss in the resonant cavity, thereby realizing passive Q-switched pulse laser output. The change amount of the loss of the saturable absorber is called modulation depth, which is represented as the change amount of transmittance or reflectance in the device. For passive Q-switched pulsed fiber lasers, when the resonant cavity structure and length are fixed, the pulse width of the output pulse is inversely proportional to the modulation depth of the saturable absorber, and a saturable absorber with large modulation depth is more likely to realize narrow pulse width pulse output. However, limited by the energy band structure of the material, the modulation depth of the saturable absorber is mostly less than 50%, which further makes the pulse width of the passive Q-switched pulsed fiber laser only reach the order of hundreds of nanoseconds to tens of microseconds, which seriously limits the peak power level of the passive Q-switched pulsed fiber laser. And limited by the narrow working bandwidth of the saturable absorber material, different saturable absorber materials with superior performance need to be found for pulse modulation of broadband wavelength lasers.

[0003] The saturable absorption property of a material saturable absorber originates from the photo-generated carrier transition between the optical energy bands of the material. Light with energy higher than the band gap energy can excite carriers from the valence band to the conduction band. With the increase of light intensity, the valence band carriers are depleted, and the conduction band is gradually filled due to the Pauli effect, so that the absorption reaches saturation, the material is bleached, and the low-loss state of high transmittance to light is presented. Therefore, the saturable absorption performance such as the modulation depth of the saturable absorber is determined by the physical properties of the material band structure, carrier motion, etc., and can be controlled by methods such as impurity doping, defect introduction, heterojunction structure design, band gap regulation, and layered structure design. However, such atomic-scale and microscale control methods for the microstructure of the material lattice structure, composition, band structure, etc. have difficulties such as high difficulty in material synthesis, complex preparation process, inability to accurately control, difficulty in characterization, and susceptibility to multi-scale physical processes, making it difficult to design saturable absorbers with large modulation depth for different wavebands and directions. Therefore, there is an urgent need for a macroscopic and saturable absorber performance control method with strong operability, so that the saturable absorber has high modulation depth performance for wideband wavelength laser to meet the needs of various applications for different wavebands and pulse characteristics. The existing saturable absorber material has the technical problems of low modulation depth, difficult performance control method implementation, and limited response bandwidth. SUMMARY

[0004] In view of the above problems in the prior art, the present application provides an interference-based multilayer film saturable absorption mirror and an optimization method. The present application provides an optical optimization design method for improving the modulation depth of a thin film saturable absorption mirror based on the principle of thin film interference, which is applied to the field of laser pulse modulation.

[0005] The present application provides an optical optimization design method for improving the performance of a thin film saturable absorption mirror based on the principle of thin film interference, which aims to improve the modulation depth performance of a thin film saturable absorption device to obtain high-energy, high-peak-power, and narrow-pulse-width pulse laser modulation. The thin film saturable absorption mirror includes a saturable absorption material film, an intermediate interference layer, a gold film, and a silicon substrate. By introducing a material with weak absorption to the modulation laser wavelength as an intermediate interference layer between the saturable absorption mirror substrate lens and the saturable absorption material film, the thickness of the saturable absorption material film and the intermediate interference layer is adjusted to meet the condition of thin film interference cancellation, thereby reducing the linear reflectivity of the thin film saturable absorption mirror at low light intensity, increasing the range of optical property changes of the thin film saturable absorption mirror, and improving the modulation depth of the saturable absorption mirror device.

[0006] An interference-based multilayer film saturable absorption mirror, in one embodiment, the interference-based thin film saturable absorption mirror includes a saturable absorption material film, an intermediate interference layer, a gold film, and a silicon substrate.

[0007] Wherein, the saturable absorption material film is a flat film made of material with saturable absorption characteristics, with refractive index n1, n1>1, and thickness h1; the saturable absorption material film and air form an interface a, and the refractive index of air is n0=1;

[0008] The intermediate interference layer 2 is a ZnS film layer deposited on the gold mirror substrate, with refractive index n2 and thickness h2, and the extinction coefficient is less than 0.1 in the 0.35-28 μm wave band; the saturable absorption material film and the intermediate interference layer form an interface b at the junction;

[0009] The silicon substrate is a silicon wafer with a thickness of 3 mm and a diameter of 25.4 mm, used as a gold film deposition substrate; the gold film is deposited on the surface of the silicon substrate, providing a high reflectivity >95% for broadband laser; the gold film and the silicon substrate together form the gold mirror substrate of the saturable absorption mirror; the intermediate interference layer and the gold film form an interface c at the junction.

[0010] In one embodiment, the intermediate interference layer of the thin film saturable absorption mirror uses a material with no absorption loss and no effect on the reflectivity and transmissivity of the substrate to replace ZnS, which includes but is not limited to silicon dioxide.

[0011] An optimization method for any of the above saturable absorption mirrors, in one embodiment, the incident light I0, when the saturable absorption mirror is in a low light intensity condition, its linear reflectivity R lin The reflected light I 反 The reflected light I1 reflected by the thin film interface a, the reflected light I2 reflected by the interface b after twice absorption by the thin film, and the reflected light I3 reflected by the interface c after twice absorption by the thin film;

[0012] The optical path difference includes twice the optical length of the saturable absorption material film and the intermediate interference layer, and the half-wave loss x times due to the refractive index difference in the transmission process; the optical path difference OPD of the reflected light I1 and I3 with central wavelength λ is calculated as follows:

[0013]

[0014] When the optical path difference satisfies m is the interference order, m=0, 1, 2,...; the interference cancellation condition is reached, and the linear reflectivity R lin reaches the minimum value;

[0015] When strong light bleaches the saturable absorption material film, due to the decrease of absorption loss of the saturable absorption material film, the reflected light I3 is enhanced, so that there is a significant light intensity difference between the reflected light I1 and I3, the interference contrast is reduced, and the interference effect is extremely weak in strong light, and the thin film saturable absorption mirror maintains the original high reflectivity Rmax .

[0016] In one embodiment, a reflective thin film type saturable absorber mirror is provided, and a material with weak extinction coefficient <0.1 for the modulation laser wavelength is introduced as an intermediate interference layer between the saturable absorber mirror substrate and the thin film of saturable absorber material, and the thickness of the thin film of saturable absorber material and the intermediate interference layer is adjusted to satisfy the condition of thin film interference cancellation, so that the linear reflectivity R lin of the saturable absorber material thin film reaches the minimum value at low light intensity; with the increase of light intensity, the refractive index of the material changes and the absorption coefficient decreases until the strong light bleaching, and the optical length of the thin film of saturable absorber material and the intermediate interference layer no longer satisfies the interference cancellation condition, so that the upper limit of the modulation depth does not decrease significantly due to the introduction of the intermediate interference layer.

[0017] In one embodiment, for the reflective saturable absorber mirror, the modulation depth of the saturable absorber can be represented as

[0018] △R=R max -R min

[0019] wherein the upper limit of the modulation depth is R max =1-R ns , R ns is the non-saturated loss, which is the inherent loss when the material is saturated, and is caused by material defect state absorption and surface scattering effect; the lower limit of the modulation depth is R min =R lin , i.e. the linear reflectivity R lin of the saturable absorber under low light intensity; the interference cancellation makes the linear reflectivity R lin reach the minimum value at low light intensity, and the reflectivity of the thin film saturable absorber mirror maintains the original high R max at strong light, so as to realize the improvement of the modulation depth.

[0020] In one embodiment, the thin film saturable absorber mirror adopts a transmissive structure design instead of a reflective structure design.

[0021] The above technical features can be combined in various suitable ways or replaced by equivalent technical features, as long as the purpose of the application can be achieved.

[0022] The present application provides an interference-based multilayer thin film saturable absorber mirror and an optimization method, which has at least the following advantages compared with the prior art:

[0023] Compared with the performance of the existing saturable absorber, the design method provided by the application can effectively reduce the linear reflectivity or transmittance of the thin film type saturable absorption mirror at low light intensity by adjusting the thickness of the saturable absorption material film layer and the intermediate interference layer, thereby increasing the optical loss modulation range of the thin film type saturable absorption mirror, and achieving the effect of improving the modulation depth of the thin film type saturable absorption mirror.

[0024] The design method provided by the application is suitable for different wavelengths of laser: for a saturable absorption mirror using a saturable absorption film material with a wideband response, based on the design method of the application, high modulation depth can be achieved for pulse modulation of different center wavelengths, which can greatly improve the pulse width and peak power performance of the wideband passive Q-switched pulse laser.

[0025] The method of the application is simple to operate and has high flexibility: based on the thin film interference principle, the optical macroscopic regulation of the thin film saturable absorption mirror can be realized by only changing the thickness of the thin film and the intermediate interference layer, without the need for microscopic regulation of the saturable absorption material.

[0026] The saturable absorption material film layer and the intermediate interference layer used in the application can be applied to different applications and use different materials, which are universal. BRIEF DESCRIPTION OF DRAWINGS

[0027] The application will be described in more detail below based on the embodiments and with reference to the accompanying drawings. Among them:

[0028] Figure 1 The schematic diagram of the thin film saturable absorption mirror of the application is shown;

[0029] Figure 2 The interference schematic diagram of the thin film saturable absorption mirror of the application is shown;

[0030] Figure 3 The structure schematic diagram of the vanadium dioxide nanofilm saturable absorption mirror of the application is shown;

[0031] Figure 4 The relationship between the optical path difference and the thickness of the vanadium dioxide nanofilm of the application is shown. DETAILED DESCRIPTION

[0032] The application will be further described below with reference to the accompanying drawings.

[0033] The application provides a multilayer thin film saturable absorption mirror based on interference and an optimization method.

[0034] In one embodiment, the optical property of the saturable absorber mirror changes with the light intensity, for the reflective saturable absorber mirror, the optical property is reflected in the increase of reflectivity with the increase of light intensity, for the transmissive saturable absorber mirror, the optical property is reflected in the increase of transmissivity with the increase of light intensity, the maximum amplitude of the change of reflectivity or transmissivity is defined as the modulation depth. Taking the reflectivity as an example, the modulation depth of the saturable absorber can be expressed as

[0035] △R=R max -R min

[0036] Wherein, the upper limit of the modulation depth is R max =1-R ns , R ns is the non-saturated loss, which is the inherent loss of the material when saturated, resulting from the material defect state absorption and surface scattering effect; the lower limit of the modulation depth is R min =R lin , that is, the linear reflectivity R lin of the saturable absorber under low light intensity.

[0037] The modulation depth of the traditional saturable absorber except SESAM is derived from the change of the material absorption coefficient, and for the reflective thin film type saturable absorber mirror, the linear reflectivity under low light intensity is also affected by the thin film interference effect. A material with extremely weak absorption to the modulation laser wavelength is introduced as an intermediate interference layer between the base mirror of the saturable absorber mirror and the thin film of the saturable absorber material, and by adjusting the thickness of the thin film of the saturable absorber material and the intermediate interference layer to meet the condition of thin film interference cancellation, the linear reflectivity under low light intensity is extremely low; with the increase of light intensity, the refractive index of the material changes and the absorption coefficient decreases until the material is bleached by strong light, and the optical length of the thin film of the saturable absorber material and the intermediate interference layer no longer meets the interference cancellation condition, so the upper limit of the modulation depth will not decrease significantly due to the introduction of the intermediate interference layer. By adjusting the thickness of the thin film and the thickness of the intermediate interference layer, the linear reflectivity of the reflective thin film type saturable absorber mirror under low light intensity is extremely low, and the range of the change of the optical property of the thin film saturable absorber mirror is increased, thereby improving the modulation depth of the device.

[0038] In one embodiment, the present application reduces the linear reflectivity R lin of the saturable absorber mirror by thin film interference design to improve the modulation depth △R of the saturable absorber mirror.

[0039] The thin film saturable absorber mirror structure proposed by the present application is shown in Figure 1 , and the interference schematic diagram is shown in Figure 2 , the thin film saturable absorber mirror structure comprises: a thin film of saturable absorber material 1, an intermediate interference layer 2, a gold film 3, and a silicon substrate 4.

[0040] Wherein, the saturable absorption material film is a flat film made of material with saturable absorption characteristics, with refractive index n1 (n1>1) and thickness h1; the saturable absorption material film 1 and air form an interface a, and the refractive index of air is n0=1;

[0041] The intermediate interference layer 2 is a ZnS film layer deposited on the gold mirror substrate, which is an infrared commonly used material, with refractive index n2 and thickness h2, and the extinction coefficient is less than 0.1 in the 0.35-28 μm wave band, almost no absorption loss, no nonlinear optical response and no influence on the reflectivity of the gold mirror substrate (R>95%), the purpose is to increase the optical path difference of the reflected light of the substrate and the saturable absorption material film 1 interface under the premise of not introducing additional absorption loss, and to enhance the interference effect; the saturable absorption material film 1 and the intermediate interference layer 2 form an interface b;

[0042] The silicon substrate 4 is a silicon wafer with a thickness of 3 mm and a diameter of 25.4 mm, which is used as a gold film deposition substrate for easy lens clamping in actual use and does not participate in the optical process; the gold film 3 is deposited on the surface of the silicon substrate 4, which provides high reflectivity (>95%) for broadband laser; the gold film 3 and the silicon substrate 4 together form the gold mirror substrate of the saturable absorption mirror; the intermediate interference layer 2 and the gold film 3 form an interface c;

[0043] The incident light I0, when the saturable absorption mirror is in a low light intensity condition, its linear reflectivity R lin The reflected light Ireflected obtained by the film interface a reflected light I1, the film twice absorption reflected light I2 reflected by interface b, the film twice absorption reflected light I3 reflected by interface c;

[0044] When the saturable absorption mirror is placed in the resonant cavity, the laser is focused to be incident, because the beam radius in the fiber resonant cavity is very small, therefore, the laser irradiation of the saturable absorption mirror can be equivalent to the paraxial incidence, Figure 1 The schematic diagram is to facilitate the display, the incident, reflected and refracted light lines are drawn at a large angle with the normal line, and the actual is paraxial incidence and emission, which is calculated based on the paraxial condition.

[0045] Because the saturable absorption material film 1 is mostly high refractive index material, the light intensity of the light beam I2 reflected by the interface b is relatively weak, and the main interference effect is the reflected light I1 and I3, the optical path difference of which includes twice the optical length of the saturable absorption material film 1 and the intermediate interference layer 2, and the half-wave loss (x times) caused by the refractive index difference in the transmission process, the optical path difference (OPD) of the reflected light I1 and I3 with central wavelength λ is calculated by the following formula:

[0046]

[0047] When the optical path difference satisfies When m = 0, 1, 2,..., the interference cancellation condition is reached, and the linear reflectivity of the reflective thin film saturable absorber mirror is extremely low at low light intensity. When strong light bleaches the saturable absorber material film 1, the absorption loss of the saturable absorber material film 1 decreases, the reflected light I3 increases, and there is a significant difference in light intensity between the reflected light I1 and I3, the interference contrast decreases, and the interference effect is extremely weak at strong light, and the thin film saturable absorber mirror still maintains the original high R max By reducing the linear reflectivity at low light intensity through interference cancellation, the optical loss range of the thin film saturable absorber mirror can be effectively increased, and the purpose of enhancing the modulation depth of the saturable absorber mirror is achieved. For different wavelengths of laser, the thickness of the saturable absorber material film 1 and the intermediate interference layer 2 is adjusted to meet the interference cancellation condition.

[0048] For a conventional thin film saturable absorber mirror, when the reflective saturable absorber mirror is in a low light intensity condition, its linear reflectivity R lin The reflected light obtained includes reflected light reflected by the film interface and reflected light absorbed by the substrate. Since the film thickness is thin, usually in the order of nanometers, the optical path difference between the two is small, which is not enough to produce obvious interference cancellation effect. The intermediate interference layer makes up for this problem and enhances the influence of interference, providing a more simple and flexible way to control the modulation depth of the thin film saturable absorber mirror.

[0049] Example 1: Preparation of a reflective vanadium dioxide nanometer film mirror device.

[0050] This embodiment provides a preparation of a reflective vanadium dioxide nanometer film mirror device based on vanadium dioxide nanometer film. Figure 3 This is a structural schematic diagram of the reflective vanadium dioxide nanometer film mirror device described in this embodiment, which includes a vanadium dioxide nanometer film 1, a ZnS interference layer 2, a gold film 3, and a silicon substrate 4.

[0051] The gold film is plated on the silicon substrate by a magnetron sputtering method, and the specific steps are as follows:

[0052] Pre-treatment of the substrate: the silicon substrate is sequentially placed in acetone, anhydrous ethanol and deionized water for ultrasonic cleaning, 10 minutes for each step, to remove organic contaminants and surface particle impurities. Then dry with high-purity nitrogen and place in an oven at 120°C for 30 minutes to ensure that the surface is free of residual moisture. Place the treated silicon substrate on the sample stage of the magnetron sputtering equipment, start the vacuum system, and evacuate the vacuum chamber to a base vacuum better than 5 x 10 -4Pa to exclude impurity gases in the air. High-purity argon gas (99.999%) is introduced into the vacuum chamber as a sputtering gas, and the working gas pressure is controlled in the range of 0.5-1.0 Pa to obtain a stable plasma. A high-purity gold target (purity ≥ 99.99%) is selected as the target material, and a direct current or radio frequency power source is turned on (depending on the equipment configuration) for 2-5 minutes of pre-sputtering to remove the oxidation layer and impurities on the surface of the gold target and stabilize the plasma state. The sample shielding plate is opened, and gold film deposition on the silicon substrate is started. The sputtering power (e.g., 50-100 W) and deposition time (e.g., 10-20 minutes) are controlled to obtain a gold thin film of the desired thickness (e.g., 10-50 nm). The substrate temperature can be controlled (e.g., room temperature or heated to 100-200°C) during deposition to improve the film density and adhesion. After deposition, the power and gas are turned off, and the sample is removed after the chamber naturally cools to room temperature.

[0053] The ZnS interference layer is deposited on the gold film by a thermal evaporation method, and the specific steps are as follows:

[0054] High-purity ZnS particles (purity ≥ 99.99%) are loaded into a tungsten boat or ceramic crucible and fixed at the evaporation source to ensure uniform material distribution and stability during evaporation. The vacuum system is started, and the evaporation chamber is pumped to a high vacuum state of better than 5×10 - 3 Pa, preferably better than 1×10 - 3 Pa to avoid interference from impurity gases during evaporation. The evaporation source temperature is slowly increased to heat the ZnS material to an evaporation temperature (typically about 900-1000°C), and the temperature rise rate is controlled to prevent spattering. After the evaporation reaches a stable state, the sample shield is opened, and deposition begins. The ZnS deposition thickness is monitored in real time by a quartz crystal monitor or other thickness control system to accurately control the desired interference layer thickness (e.g., 40-100 nm) and ensure that the optical performance meets the design requirements. After evaporation is complete, the power is turned off and the vacuum state is maintained until the sample naturally cools to room temperature. Then, the sample is removed after the gas is released.

[0055] The vanadium dioxide nanofilm is deposited on the ZnS interference layer, and the specific steps are as follows:

[0056] The VO2 thin film is deposited on the substrate by a direct current magnetron sputtering method, using a V2O3 target with a diameter of 4 inches and a purity of 99.9%, deposited in an argon / oxygen mixed gas atmosphere (volume ratio of 35:15) with a total pressure of 7 mTorr, a deposition temperature of room temperature, and a sputtering power of 200 W. Subsequently, the sample is subjected to one-step rapid thermal annealing at a vacuum of 5 Torr and a temperature of 400°C.

[0057] Example 2: Interference optimization of vanadium dioxide nanofilm saturable absorber mirror for 2.78 μm wavelength laser

[0058] The vanadium dioxide nanometer film saturable absorber mirror described in Example 1 is used to pulse modulate a 2.78 μm wavelength laser. By adjusting the thickness of the vanadium dioxide nanometer film 1 and the ZnS interference layer 2, the linear reflectivity of the vanadium dioxide nanometer film saturable absorber mirror to the 2.78 μm wavelength laser is minimized.

[0059] The refractive index of the vanadium dioxide nanometer film 1 to the 2.78 μm wavelength laser is 3.1796, the refractive index of the ZnS interference layer 2 to the 2.78 μm wavelength laser is 2.267, the thickness of the ZnS interference layer 2 is 150 nm, and the gold film 3 provides total reflection. The theoretical calculation of the optical path difference between the reflected light from the vanadium dioxide nanometer film interface and the reflected light from the gold film total reflection to the air under the interference effect is shown in FIG. 1, and the relationship between the optical path difference and the thickness of the vanadium dioxide nanometer film is shown in FIG. 2. Figure 4 When the thickness of the vanadium dioxide nanometer film is about 110 nm, the interference cancellation condition is met, and the low light intensity linear reflectivity of the vanadium dioxide nanometer film saturable absorber mirror to the 2.78 μm wavelength laser should reach the minimum.

[0060] Based on the preparation method described in Example 1 above, vanadium dioxide nanometer film saturable absorber mirror devices with vanadium dioxide nanometer film thicknesses of 70 nm, 90 nm, 110 nm, 155 nm, and 190 nm and ZnS interference layer thicknesses of 150 nm are made. The linear reflectivity of the devices to the 2.78 μm wavelength laser at low light intensity is tested, which is 20.4%, 7.3%, 5.4%, 9.1%, and 24.1%, respectively. When the thickness of the vanadium dioxide nanometer film is 110 nm, the lowest linear reflectivity is reached. The modulation depth of the nonlinear absorption of the devices to the 2.78 μm wavelength laser is tested and calculated, which is 57.7%, 64.7%, 92.1%, 50.1%, and 18.4%, respectively. When the thickness of the vanadium dioxide nanometer film is 110 nm, the maximum modulation depth is reached.

[0061] In one embodiment, an optical optimization design method for improving the modulation depth of a thin film type saturable absorber based on the principle of thin film interference: an intermediate interference layer of a material with extremely weak absorption to the modulation laser wavelength is introduced between the saturable absorber mirror substrate and the saturable absorber material thin film. By adjusting the thickness of the saturable absorber material thin film and the intermediate interference layer to meet the thin film interference cancellation condition, the linear reflectivity of the thin film saturable absorber at low light intensity is reduced, the range of optical property change of the thin film saturable absorber is increased, and thus the device modulation depth is improved.

[0062] In one embodiment, a thin film saturable absorber mirror performance optimization design method, the intermediate interference layer of the thin film saturable absorber mirror provided by the application is not limited to ZnS material, and can be replaced by other materials such as silicon dioxide which has no absorption loss and does not affect the optical performance of the substrate.

[0063] The thin film saturable absorber mirror provided by the application is not limited to a reflective structure design, and can be replaced by a transmissive structure design, and correspondingly, the low light intensity transmittance effect is realized through the thickness design of the intermediate interference layer and the material thin film.

[0064] Although the application is described herein with reference to particular embodiments, it is to be understood that these embodiments are merely exemplary of the principles and applications of the present application. It is therefore to be understood that numerous modifications can be made to the illustrative embodiments and that other arrangements can be devised without departing from the spirit and scope of the present application as defined by the appended claims. It is to be understood that the different features that are described in connection with the separate embodiments can be combined with each other. It is to be understood that features described in connection with one embodiment can be combined with features described in connection with another embodiment. It is to be understood that features described in connection with one embodiment can be combined with features described in connection with another embodiment.

Claims

1. A multilayer thin-film saturable absorber mirror based on interference, characterized in that, The interference-based thin-film saturable absorber mirror includes a saturable absorber material thin film, an intermediate interference layer, a gold film, and a silicon substrate. Among them, the saturable absorbent material film is a flat film made of a material with saturable absorbent properties, with a refractive index of n1, n1>1, and a thickness of h1; the saturable absorbent material film and the air form an interface a, and the refractive index of the air is n0=1; The intermediate interference layer 2 is a ZnS thin film layer deposited on the gold mirror substrate, with a refractive index of n2 and a thickness of h2. Its extinction coefficient is less than 0.1 in the 0.35-28μm wavelength range. The interface b is formed at the junction of the saturable absorber material film and the intermediate interference layer. The silicon substrate is a silicon wafer with a thickness of 3 mm and a diameter of 25.4 mm, which is used as the gold film deposition substrate. The gold film is deposited on the surface of the silicon substrate to provide high reflectivity (>95%) for broadband lasers. The gold film and the silicon substrate together constitute the gold reflector substrate of the saturable absorber mirror. The interface c is formed at the junction of the intermediate interference layer and the gold film.

2. The interference-based multilayer thin-film saturable absorber mirror according to claim 1, characterized in that, The intermediate interference layer of the thin-film saturable absorber mirror is replaced with a material that has no absorption loss and does not affect the reflectivity and transmittance of the substrate. The material that has no absorption loss and does not affect the reflectivity and transmittance of the substrate includes, but is not limited to, silicon dioxide.

3. An optimization method for a saturable absorber mirror as described in any one of claims 1-2, characterized in that, Incident light I0, when the saturable absorber is under low light intensity conditions, its linear reflectivity R lin The obtained reflected light I 反 Reflected light I1 is reflected from interface a of the thin film, reflected light I2 is reflected from interface b after secondary absorption by the thin film, and reflected light I3 is reflected from interface c after secondary absorption by the thin film. The optical path difference includes the optical lengths through the saturable absorber film and the intermediate interference layer twice, as well as the half-wave loss x times due to the refractive index difference during transmission. The optical path difference OPD of the reflected light I1 and I3 with center wavelength λ is calculated by the following formula: When the optical path difference satisfies m is the interference order. When m = 0, 1, 2, ..., the interference destructive condition is met, and the linear reflectivity of the reflective thin-film saturable absorber reaches its minimum value at low light intensity. When strong light bleachs the saturable absorber film, the absorption loss of the saturable absorber film decreases, and the reflected light I3 is enhanced. This results in a significant intensity difference between the reflected light I1 and I3, reducing the interference contrast. Consequently, the interference effect under strong light becomes negligible, and the thin-film saturable absorber mirror maintains its original high R-value. max .

4. A multilayer thin-film saturable absorber mirror based on interference, characterized in that, This includes reflective thin-film saturable absorber mirrors, in which a material with an extinction coefficient of <0.1 for modulated laser wavelength is introduced between the substrate lens of the saturable absorber mirror and the thin film of the saturable absorber material as an intermediate interference layer. By adjusting the thickness of the thin film of the saturable absorber material and the intermediate interference layer, the conditions for thin-film interference cancellation are achieved under low light intensity irradiation, so that the linear reflectivity reaches the minimum value under low light intensity. As light intensity increases, the refractive index of the material changes and the absorption coefficient decreases until it is bleached by strong light. The optical length of the saturable absorber material thin film and the intermediate interference layer no longer satisfies the interference destructive condition. Therefore, the upper limit of the modulation depth will not decrease significantly due to the introduction of the intermediate interference layer.

5. The interference-based multilayer thin-film saturable absorber mirror according to claim 4, characterized in that, For a reflective saturable absorber mirror, the reflectivity and modulation depth of the saturable absorber can be expressed as ΔR = R max -R min The upper limit of the modulation depth is R. max =1-R ns R ns This is unsaturated loss, an inherent loss when the material is saturated, originating from effects such as absorption by material defect states and surface scattering; the lower limit of the modulation depth is R. min =R lin That is, the linear reflectance R of a saturable absorber under low light intensity irradiation. lin By adjusting the thickness of the saturable absorber material thin film and the intermediate interference layer, the condition of destructive thin film interference can be achieved under low light intensity, thereby reducing the linear reflectivity R of the thin film saturable absorber mirror at low light intensity. lin This increases the range of optical properties variation of thin-film saturable absorber mirrors, thereby improving the modulation depth ΔR of the device.

6. The interference-based multilayer thin-film saturable absorber mirror according to any one of claims 1-2 and 4-5, characterized in that, Thin-film saturable absorber mirrors use a transmission-type structural design instead of a reflection-type structural design.

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