Large-modulation-depth dynamic saturable absorber mirror and preparation and working methods thereof

Abstract

The invention provides a large-modulation-depth dynamic saturable absorber mirror and a preparation and working method thereof, the large-modulation-depth dynamic saturable absorber mirror comprises a vanadium dioxide nano film and a gold film silicon substrate high-reflection mirror, and the film is deposited on the upper layer of a gold film of the gold film high-reflection mirror through a direct current magnetron sputtering technology to form a dynamic saturable absorber mirror device; the core mechanism is that when the dynamic saturable absorption mirror is driven by enhanced light intensity in a cavity, phase transformation and optical property dynamic transformation occur, starting from low reflectivity and high loss of a vanadium dioxide insulating phase, the state is converted into a vanadium dioxide mixed phase saturable absorption state through phase transformation equivalent nonlinear absorption, and then the state is converted into a metallic state saturable absorption state; and finally, the metal-state saturable absorption is completely saturated, so that the large-modulation-depth dynamic saturable absorption mirror is realized. According to the invention, the modulation depth of the saturable absorber mirror can be effectively improved and flexibly adjusted; and the basic temperature of the device can be regulated and controlled in a passive / active manner, so that the universality and adaptability of system application are enhanced.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of laser, in particular to a large modulation depth dynamic saturable absorber mirror and its preparation and working method. BACKGROUND

[0002] Short pulse fiber lasers have the characteristics of high peak power and high pulse energy, and can rapidly emit strong laser pulses in a very short time scale (ns-fs). Portable integrated mid-infrared pulse fiber lasers with high peak power and high pulse energy are widely used in many fields such as medical treatment, industry and national defense applications. Passive Q-switched pulse modulation technology using saturable absorber is widely used in high pulse energy mid-infrared pulse fiber lasers due to its characteristics of independence from external driving signal, high stability, easy integration, and pulse characteristics of high pulse energy and ns-μs pulse width.

[0003] The absorption coefficient of saturable absorber material decreases with the increase of incident light intensity, and the amplitude of its absorption change is called modulation depth, which is a key parameter for evaluating the pulse modulation ability of saturable absorber. For reflective saturable absorber, its saturable absorption phenomenon is that the reflectivity of the device increases with the increase of incident light intensity. Passive Q-switching realizes pulse modulation by periodically modulating the total loss in the resonant cavity through the saturable absorber. In a fixed resonant cavity, a saturable absorber with large modulation depth can quickly saturate at the pulse front, causing the cavity loss to decrease rapidly and the energy to be released quickly, thereby forming a pulse output with shorter pulse width and higher peak energy. In actual engineering and scientific research applications, saturable absorber materials are usually integrated on optical lenses to make saturable absorber mirror devices. When the light intensity increases, for transmissive saturable absorber mirror devices, the loss reduction is reflected in the increase of transmittance, and for reflective saturable absorber mirror devices, the loss reduction is reflected in the increase of reflectivity.

[0004] At present, most of the basic researches on saturable absorber use materials with fixed single energy band structure, and the modulation depth is mostly less than 40%. The passive Q-switched fiber laser pulse width that can be achieved is mostly in the order of microseconds to hundreds of nanoseconds, and it is difficult to realize passive Q-switched fiber laser narrow pulse width pulse modulation with large modulation depth. Moreover, for the demand of different saturable absorption performance parameters in applications, it is often necessary to replace saturable absorber mirror devices with different materials or different structure parameters to solve the problem, and it is difficult to realize flexible adjustment of saturable absorption performance parameters of a single saturable absorber mirror device.

[0005] The existing saturable absorber has low modulation depth, so that high peak power, high pulse energy and narrow pulse width of mid-infrared fiber laser passive Q-switched pulse output cannot be realized. The saturable absorber-based fiber laser passive Q-switched pulse modulation technology can output high pulse energy of muJ level, but it is difficult to realize narrow pulse width of tens of nanoseconds and passive Q-switched pulse output of peak power of more than one hundred watts due to the low modulation depth of the saturable absorber and the long resonant cavity length of the fiber. The saturable absorption performance parameters of the single-parameter fixed saturable absorption mirror are fixed, and it is difficult to control. The saturable absorption performance of the traditional single fixed physical structure and physical property material saturable absorber is usually controlled by optimizing the size, composition and other specifications of the saturable absorption mirror material, so different saturable absorption mirrors with different specifications are required for different saturable absorption performance. SUMMARY

[0006] In view of the problems in the prior art, the present application provides a large modulation depth dynamic saturable absorption mirror and its preparation and working method, which is a large modulation depth and nonlinear absorption performance parameter adjustable vanadium dioxide dynamic saturable absorption mirror, applied to pulse modulation of high peak power, high pulse energy and narrow pulse width of mid-infrared fiber laser. The present application relates to the design principle, preparation method and working principle of vanadium dioxide nanometer film dynamic saturable absorption mirror, and a new type of vanadium dioxide dynamic saturable absorption mirror design is proposed, which can greatly improve the modulation depth of mid-infrared saturable absorber. The vanadium dioxide dynamic saturable absorption mirror of the present application can realize the control of saturable absorption performance parameters such as modulation depth and saturation light intensity by simply controlling the temperature.

[0007] The present application provides a design and preparation method of a large modulation depth and nonlinear absorption performance parameter adjustable vanadium dioxide dynamic saturable absorption mirror. The vanadium dioxide dynamic saturable absorption mirror utilizes the dynamic continuous transformation between the reflectivity rise caused by the photothermal effect driven phase change and the saturable absorption effect driven by strong light to realize the purpose of greatly increasing the modulation depth, and the nonlinear absorption performance parameter control is realized by controlling the initial phase change state of the vanadium dioxide dynamic saturable absorption mirror at low light intensity through temperature. The vanadium dioxide nanometer film dynamic saturable absorption mirror is prepared by depositing vanadium dioxide nanometer film on a gold mirror. The present application is applied to pulse modulation of high peak power, high pulse energy and narrow pulse width of mid-infrared fiber laser, and aims to solve the inherent problem of low modulation depth of traditional saturable absorber devices.

[0008] The application relates to a design method, structure and preparation method of a vanadium dioxide dynamic saturable absorption mirror based on a vanadium dioxide nanofilm, and a working principle, and solves the problems of low modulation depth, difficulty in flexible control of performance parameters, limited response bandwidth and the like of an existing middle infrared saturable absorption mirror. The core principle of the vanadium dioxide dynamic saturable absorption mirror is as follows: equivalent nonlinear absorption caused by light-induced phase transition is used to cause the reflectivity of the vanadium dioxide nanofilm to rise, the vanadium dioxide nanofilm is dynamically switched from phase transition equivalent nonlinear absorption to mixed phase and metal state intrinsic saturable absorption through light intensity control, the modulation depth of the saturable absorption mirror device is improved, and therefore the vanadium dioxide dynamic saturable absorption mirror device with total equivalent saturable absorption and large modulation depth is realized. Through control of the initial phase transition state of the vanadium dioxide nanofilm under low light intensity of middle infrared laser, flexible adjustment of performance parameters such as modulation depth can be realized.

[0009] The application discloses a dynamic saturable absorption mirror with large modulation depth, which comprises a vanadium dioxide nanofilm and a gold film silicon substrate high reflector mirror in an embodiment.

[0010] In an embodiment, when strong light acts on the monoclinic phase vanadium dioxide dynamic saturable absorption mirror, the vanadium dioxide phase transition is driven through the photo-thermal effect, the reflectivity of the vanadium dioxide dynamic saturable absorption mirror is caused to rise, the loss is caused to decrease, and the equivalent nonlinear absorption effect with light intensity is presented. When the vanadium dioxide nanofilm is phase transitioned to a mixed phase in which two phases coexist, the band gap is gradually closed to the band gap width of the middle infrared waveband laser response, nonlinear optical absorption response is generated, and the saturable absorption property is presented; when the vanadium dioxide nanofilm of the saturable absorption mirror is completely phase transitioned and is in the metal state at high temperature (>68 DEG C), the nonlinear absorption effect of saturable absorption on the middle infrared waveband laser is presented.

[0011] In one embodiment, when gradually increasing light intensity acts on the dynamic saturable absorption mirror, a dynamic transition occurs, starting from a low reflectivity and high loss state of the vanadium dioxide insulating phase, through the phase transition equivalent nonlinear absorption, to the saturable absorption state of the vanadium dioxide mixed phase, and further to the saturable absorption state of the metal state, until the final saturation of the vanadium dioxide metal state saturable absorption; the dynamic transition has an overall total equivalent saturable absorption effect with a large modulation depth, which includes the phase transition equivalent nonlinear absorption, the saturable absorption of the vanadium dioxide mixed phase, and the saturable absorption of the vanadium dioxide metal phase; the maximum modulation depth of the total equivalent saturable absorption is 1 minus the linear reflectivity of the vanadium dioxide insulating state under low and medium infrared laser light intensity minus the non-saturated loss of the vanadium dioxide metal state of the dynamic saturable absorption mirror.

[0012] In one embodiment, the in-cavity focused laser light intensity is controlled by adjusting the pump light power of the laser, thereby controlling the temperature difference caused by the photothermal effect; and a basic temperature is provided for the vanadium dioxide dynamic saturable absorption mirror to make up for the insufficient temperature difference caused by the photothermal effect.

[0013] In one embodiment, when the modulation depth of the vanadium dioxide saturable absorption mirror needs to be maximum, the basic temperature is set below the phase transition temperature, at which time the vanadium dioxide nanofilm is in an insulating state, and the phase transition factor is the light intensity focused on the surface of the vanadium dioxide nanofilm; when the temperature rises to the initial phase transition state of the vanadium dioxide dynamic saturable absorption mirror under low and medium infrared laser light intensity to the metal state, the total equivalent saturable absorption is the saturable absorption of the metal state of vanadium dioxide, at which time the modulation depth reaches a minimum value.

[0014] Importantly, the "dynamic" of the vanadium dioxide dynamic saturable absorption mirror includes three aspects: first, in the total equivalent saturable absorption, the physical properties of the material such as crystal lattice structure and energy band distribution are not fixed but dynamically change with the increase of light intensity; second, in the total equivalent saturable absorption, the optical effects that trigger the change of optical properties are not a single fixed optical effect but a dynamic transition of multiple optical effects; third, the total equivalent saturable absorption performance of the dynamic saturable absorption mirror is not fixed, and the control of the initial phase transition state of the vanadium dioxide dynamic saturable absorption mirror under low and medium infrared laser light intensity through temperature can realize the regulation of the total equivalent saturable absorption performance, which is specifically manifested as the regulation of the modulation depth, the saturation light intensity and the non-saturated loss of the total equivalent saturable absorption.

[0015] In one embodiment, the dynamic saturable absorption mirror uses a material with intrinsic saturable absorption characteristics and reversible equivalent nonlinear absorption effects affected by light intensity instead of vanadium dioxide nanofilms, including but not limited to various reversible phase change materials with different induction mechanisms, thermochromic liquid crystal materials, metal organic framework materials, perovskite materials and ferroelectric materials. The dynamic saturable absorption mirror uses a transmissive saturable absorption mirror structure, fiber end face coating, and fiber adapter docking structure instead of a reflective saturable absorption mirror structure.

[0016] The preparation method of the large modulation depth dynamic saturable absorption mirror as described in any of the above, in one embodiment, the vanadium dioxide nanofilms are deposited on the gold film of the gold film high reflector by direct current magnetron sputtering technology to form a dynamic saturable absorption mirror device, specifically including using a V2O3 target with a diameter of 4 inches and a purity of 99.9%, depositing in an argon-oxygen mixed atmosphere with a volume ratio of 35:15; the total pressure is 7 mTorr, the deposition temperature is room temperature, and the sputtering power is 200 W, then the sample is subjected to one-step rapid thermal annealing treatment under the conditions of a vacuum degree of 5 Torr and a temperature of 400°C.

[0017] The working method of the large modulation depth dynamic saturable absorption mirror as described in any of the above, in one embodiment, when the dynamic saturable absorption mirror is placed at one end of a mid-infrared rare earth ion doped fluoride fiber laser resonator, in the process of forming a single Q-switched laser pulse, the initial state of the vanadium dioxide nanofilms in the dynamic saturable absorption mirror is an insulating state under a low mid-infrared laser intensity of less than 1 kW / cm 2 An insulating state under a low mid-infrared laser intensity of less than 1 kW / cm As the light intensity in the resonator gradually increases, the laser focused on the surface of the dynamic saturable absorption mirror causes the vanadium dioxide nanofilms to phase change to a metallic state, and the reflectivity of the dynamic saturable absorption mirror increases, and the resonator loss slowly decreases; As the light intensity in the resonator continues to increase, the intrinsic nonlinear saturable absorption of the vanadium dioxide nanofilms in the mixed phase and metallic state is excited, the reflectivity of the dynamic saturable absorption mirror continues to increase, and the resonator loss rapidly decreases; when the loss decreases to produce net gain in the resonator, the laser begins to produce a pulse corresponding to the rising edge of the pulse shape; When the light intensity in the resonator is high enough to fully saturate the saturable absorption of the metallic vanadium dioxide nanofilms, the vanadium dioxide nanofilms exhibit high transmittance, the reflectivity of the dynamic saturable absorption mirror reaches a maximum, the resonator loss reaches a minimum, and the pulse reaches a peak; After the pulse is released to the peak, the vanadium dioxide nanofilm gradually recovers to the insulating state, at the same time, the intrinsic saturable absorption of vanadium dioxide also recovers to the high loss state, the loss in the resonant cavity rises until it is completely recovered to the initial state; corresponding to the falling edge of the pulse shape and the pulse-off state after that; further, under the continuous pumping of the pump source, the inverted particle number, the photon number in the cavity, the gain and loss of the resonant cavity are rebuilt, the above-mentioned pulse period is repeated, and the passive Q-switched pulse sequence is output.

[0018] In one embodiment, the initial phase change state of the vanadium dioxide nanofilm is controlled by adjusting the base temperature; when the base temperature is controlled to make the vanadium dioxide nanofilm be in the insulating state, the reflectivity of the vanadium dioxide saturable absorption mirror under the low mid-infrared laser light intensity is the lowest, which is the linear reflectivity of the insulating vanadium dioxide saturable absorption mirror, and the modulation depth is the largest; When the base temperature is controlled to make the vanadium dioxide nanofilm be in the two-phase mixed phase in the phase change process, the reflectivity of the vanadium dioxide saturable absorption mirror under the low mid-infrared laser light intensity is increased, which is the linear reflectivity of the mixed-phase vanadium dioxide saturable absorption mirror, and the modulation depth is reduced; When the base temperature is controlled to make the vanadium dioxide nanofilm be in the metallic state, the reflectivity of the vanadium dioxide saturable absorption mirror under the low mid-infrared laser light intensity is the highest, which is the linear reflectivity of the metallic vanadium dioxide saturable absorption mirror, and the modulation depth is the smallest.

[0019] In one embodiment, the base temperature of the vanadium dioxide saturable absorption mirror can be controlled in an active way by a semiconductor heating sheet temperature control or in a passive way by the heat production of the absorption of the residual pump light of the laser by the vanadium dioxide nanofilm based on the photo-thermal effect.

[0020] 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.

[0021] The application provides a large-modulation-depth dynamic saturable absorption mirror and a preparation and working method thereof, and at least has the following beneficial effects compared with the prior art: Compared with the single fixed physical structure and physical property material saturable absorber, the vanadium dioxide dynamic saturable absorption mirror proposed in the application realizes the total equivalent saturable absorption with a large modulation depth by driving the phase change of the vanadium dioxide nanofilm through the light intensity, utilizing the equivalent nonlinear absorption in the phase change process and the dynamic change of the saturable absorption of multiple phase change states, which greatly expands the modulation depth of the saturable absorber. The proposed dynamic saturable absorption mirror has the characteristics that the crystal lattice structure, energy band distribution, linear optical property and nonlinear optical property of the saturable absorber material are dynamically changed, which is the key to realizing the large modulation depth and the adjustable modulation depth.

[0022] Compared to existing saturable absorbers with fixed physical structures and properties, this dynamic vanadium dioxide saturable absorber mirror allows for wide-range control over its saturable absorption performance, including modulation depth, saturation intensity, and unsaturation loss, by adjusting its base temperature and illumination intensity. This is achieved without requiring additional optimization of the mirror's dimensions or composition. Furthermore, the base temperature of the dynamic saturable absorber mirror can be controlled actively or passively, enhancing its versatility and adaptability in system applications. Attached Figure Description

[0023] The invention will now be described in more detail with reference to embodiments and the accompanying drawings. Figure 1 A schematic diagram of the physical mechanism of the vanadium dioxide dynamic saturable absorber mirror of the present invention is shown; Figure 2 A schematic diagram of the vanadium dioxide dynamic saturable absorber mirror structure of the present invention is shown; Figure 3 A schematic diagram of the active temperature control of the present invention is shown; Figure 4 The linear reflectivity of the vanadium dioxide dynamic saturable absorber mirror of the present invention for 2.8 μm laser light is shown as a function of temperature. Figure 5 The saturable absorption curve of the vanadium dioxide dynamic saturable absorber at 325 K is shown. Figure 6 The modulation depth of the present invention varies with temperature. Detailed Implementation

[0024] The invention will now be further described with reference to the accompanying drawings.

[0025] This invention provides a large modulation depth dynamic saturable absorber mirror and its preparation and operation method.

[0026] In one embodiment, in a first aspect, the present invention provides a design method for a vanadium dioxide dynamically saturable absorber capable of achieving a large modulation depth and adjustable modulation depth, comprising: The vanadium dioxide nanofilm used exhibits thermally induced phase transition characteristics, with a phase transition temperature of approximately 68°C. At low temperatures, it exists as a monoclinic insulating state with a band gap of 0.68 eV, while at high temperatures, it exists as a tetragonal rutile metallic state with a zero band gap. When the vanadium dioxide nanofilm transitions from the insulating state to the metallic state, its refractive index and extinction coefficient in the mid-infrared band increase significantly. These changes reduce the transmittance of the vanadium dioxide nanofilm to mid-infrared laser light and significantly increase its reflectivity.

[0027] When strong light acts on a vanadium dioxide dynamic saturable absorber in a monoclinic phase, the vanadium dioxide nanofilm absorbs photons. Due to the photothermal effect, a large amount of heat is generated, driving a phase transition of vanadium dioxide. At the same time, the reflectivity increases, the loss of the saturable absorber decreases, and it exhibits an equivalent nonlinear absorption effect with light intensity.

[0028] When vanadium dioxide nanofilms undergo a phase transition to a mixed phase where two phases coexist, the band gap gradually closes to the band gap width of the mid-infrared laser response, thereby generating a nonlinear optical absorption response caused by electron transitions from the valence band to the conduction band, exhibiting saturable absorption properties. When the vanadium dioxide nanofilm of the saturable absorber undergoes a complete phase transition and is in a high-temperature metallic state, it exhibits a saturable nonlinear absorption effect for mid-infrared lasers due to surface plasmon resonance in the metallic state.

[0029] The equivalent nonlinear absorption effect during the phase transition mainly originates from the photothermal effect. When the saturable absorber has a certain base temperature (but not enough to trigger a phase transition), the light intensity required for its phase transition is lower than the saturation light intensity of the intrinsic nonlinear absorption of vanadium dioxide.

[0030] When gradually increasing light intensity is applied to a vanadium dioxide dynamic saturable absorber mirror, the following will occur: Figure 1 The dynamic transformation of the material structure and optical properties is shown: starting from the low reflectivity and high loss state of the vanadium dioxide insulating phase, it transforms into the saturable absorption state of the vanadium dioxide mixed phase through the equivalent nonlinear absorption of the phase transition, and then further transforms into the saturable absorption state of the metallic state, until the saturable absorption of the vanadium dioxide metallic state is completely saturated.

[0031] This dynamic transformation achieves an overall equivalent saturable absorption effect with a large modulation depth, which includes the equivalent nonlinear absorption of the phase transition, the saturable absorption of the vanadium dioxide mixed phase, and the saturable absorption of the vanadium dioxide metallic phase.

[0032] The maximum modulation depth of the total equivalent saturable absorption is (1 - linear reflectivity of the insulating state under low-to-mid-infrared laser intensity of the vanadium dioxide dynamic saturable absorber mirror - unsaturated loss of the metallic state of the vanadium dioxide dynamic saturable absorber mirror). By extending the loss variation of the intrinsic saturable absorption through the equivalent nonlinear absorption of the vanadium dioxide nanofilm phase transition, the reflectivity variation of the saturable absorber mirror is significantly improved, thereby significantly enhancing the modulation depth of the saturable absorber mirror.

[0033] Furthermore, the photothermal effect is caused by the laser focused on the surface of the vanadium dioxide saturable absorber mirror. The focused light intensity can be controlled by adjusting the pump light power of the laser, thereby controlling the temperature difference caused by the photothermal effect. Since the temperature difference caused by the photothermal effect is limited, it may not be sufficient to drive the vanadium dioxide nanofilm from room temperature to the phase transition temperature. Therefore, an appropriate base temperature is needed for the vanadium dioxide dynamic saturable absorber mirror as an auxiliary measure to compensate for the insufficient temperature difference caused by the photothermal effect. When the modulation depth of the vanadium dioxide saturable absorber mirror is maximized, this base temperature setting must be lower than the phase transition temperature. At this point, the vanadium dioxide nanofilm should be in an insulating state, ensuring that the main driving factor for the phase transition is the light intensity focused on the surface of the vanadium dioxide nanofilm within the cavity.

[0034] By controlling the temperature of the vanadium dioxide dynamic saturable absorber mirror, its initial phase transition state and corresponding optical properties can be determined, thereby adjusting the modulation depth of its total equivalent saturable absorption. When the temperature is increased to the point where the initial phase transition state of the vanadium dioxide dynamic saturable absorber mirror under low-to-mid-infrared laser light intensity is metallic, the total equivalent saturable absorption, i.e., the saturable absorption of metallic vanadium dioxide, reaches its minimum value.

[0035] Importantly, the "dynamic" aspect of the vanadium dioxide dynamic saturable absorber mirror includes three aspects: First, in the total equivalent saturable absorption, the physical properties of the material, such as the crystal structure and band distribution, are not fixed but dynamically change with the light intensity as the light intensity increases; second, in the total equivalent saturable absorption, the optical effects that cause changes in optical properties with increasing light intensity are not single fixed optical effects but multiple adaptive dynamic transformations; third, the total equivalent saturable absorption performance of the dynamic saturable absorber mirror is not fixed. By controlling the initial phase transition state under low-to-mid-infrared laser light intensity through temperature, the total equivalent saturable absorption performance can be regulated, specifically manifested in the regulation of the modulation depth, saturated light intensity, and unsaturated loss of the total equivalent saturable absorption.

[0036] In one embodiment, in a second aspect, the structure and preparation method of the vanadium dioxide dynamic saturable absorber provided by the present invention include: The vanadium dioxide dynamic saturable absorber mirror structure proposed in this invention is as follows: Figure 2 As shown, it includes: vanadium dioxide nanofilm 1 and gold film silicon substrate high reflectivity mirror 2 (infrared band reflectivity is about 99%).

[0037] The vanadium dioxide nanofilm was deposited on the gold film of a gold high-reflectivity mirror using DC magnetron sputtering to form a dynamic saturable absorber mirror device. The specific steps included: depositing a 4-inch diameter, 99.9% pure V₂O₃ target in an argon / oxygen mixed atmosphere (volume ratio 35:15) at a total pressure of 7 mTorr, at room temperature, and with a sputtering power of 200 W. Subsequently, the sample underwent a rapid thermal annealing process at a vacuum of 5 Torr and a temperature of 400°C.

[0038] In use, the vanadium dioxide saturable absorber mirror is placed at the far end of the resonant cavity, and the laser in the cavity is focused onto the vanadium dioxide nanofilm of the vanadium dioxide saturable absorber mirror through a focusing lens. Its reflectivity changes with the intensity of the focused laser light due to the total equivalent saturable absorption.

[0039] In one embodiment, and in a third aspect, the working principle of the passive Q-switched pulse modulation of the vanadium dioxide dynamic saturable absorber provided by the present invention includes: When a vanadium dioxide dynamic saturable absorber mirror is placed at one end of the resonant cavity of a mid-infrared rare-earth ion-doped fluoride fiber laser, during the formation of a single Q-switched laser pulse, the initial state of the vanadium dioxide nanofilm in the vanadium dioxide dynamic saturable absorber mirror is an insulating state under low mid-infrared laser intensity, with the lowest reflectivity and the highest cavity loss. Further, as the light intensity within the resonant cavity gradually increases, the laser focused on the surface of the vanadium dioxide saturable absorber mirror induces a photothermal effect. The vanadium dioxide absorbs photons, generating heat, which raises the film temperature and triggers a phase transition of the vanadium dioxide nanofilm to a metallic state. Simultaneously, the reflectivity of the vanadium dioxide dynamic saturable absorber mirror increases, and the cavity loss slowly decreases. With continued increases in light intensity, the intrinsic nonlinear saturable absorption of the mixed phase and metallic state of the vanadium dioxide nanofilm is excited, further increasing the reflectivity of the vanadium dioxide dynamic saturable absorber mirror and rapidly reducing the cavity loss. When the loss decreases to the point where a net gain is generated within the resonant cavity, the laser begins to generate pulses, corresponding to the rising edge of the pulse shape. Furthermore, when the intracavity light intensity increases sufficiently to fully saturate the saturable absorption of the metallic vanadium dioxide nanofilm, due to the low unsaturated loss of the metallic vanadium dioxide nanofilm, the vanadium dioxide nanofilm exhibits high transmittance. Most of the incident light is reflected by the gold mirror, the reflectivity of the vanadium dioxide dynamic saturable absorber mirror reaches its maximum, the intracavity loss reaches its minimum, and the pulse reaches its peak. Further, when the pulse reaches its peak, the vanadium dioxide nanofilm exhibits high transmittance and extremely weak laser absorption. Moreover, the number of photons in the cavity is released with the pulse, without sufficient absorption to generate heat. Therefore, after the pulse reaches its peak, the vanadium dioxide nanofilm cannot maintain its metallic state and gradually recovers to an insulating state. Simultaneously, the intrinsic saturable absorption of vanadium dioxide also recovers to a high-loss state, and the intracavity loss increases until it fully recovers to the initial state. This process corresponds to the falling edge of the pulse shape and the subsequent pulse-off state. Further, under continuous pumping from the pump source, the particle number, intracavity photon number, intracavity gain, and loss are reversed and reconstructed, repeating the above pulse cycle to output a passively Q-switched pulse sequence.

[0040] Among them, when the vanadium dioxide dynamic saturable absorber is in the maximum modulation depth state, the pump power and the base temperature are a set of complementary control factors. When the base temperature is ensured to be within the temperature range that keeps the vanadium dioxide nanofilm stable in the insulating state, the higher the base temperature, the lower the pump power that maximizes the modulation depth of the vanadium dioxide saturable absorber.

[0041] By adjusting the base temperature to control the initial phase transition state of the vanadium dioxide nanofilm, the modulation depth of the vanadium dioxide saturable absorber mirror can be controlled: when the base temperature is controlled so that the vanadium dioxide nanofilm is in an insulating state, the reflectivity of the vanadium dioxide saturable absorber mirror under low to mid-infrared laser intensity is the lowest, which is the linear reflectivity of the insulating state vanadium dioxide saturable absorber mirror, and the modulation depth is the highest; when the base temperature is controlled so that the vanadium dioxide nanofilm is in a two-phase mixed phase during the phase transition process, the reflectivity of the vanadium dioxide saturable absorber mirror under low to mid-infrared laser intensity increases, which is the linear reflectivity of the mixed phase vanadium dioxide saturable absorber mirror, and the modulation depth decreases; when the base temperature is controlled so that the vanadium dioxide nanofilm is in a metallic state, the reflectivity of the vanadium dioxide saturable absorber mirror under low to mid-infrared laser intensity is the highest, which is the linear reflectivity of the metallic state vanadium dioxide saturable absorber mirror, and the modulation depth is the lowest.

[0042] Furthermore, the base temperature of the vanadium dioxide saturable absorber mirror can be controlled by an active method of temperature regulation using a semiconductor heating element, or by a passive method of temperature rise through heat generation from the absorption of residual pump light by a vanadium dioxide nanofilm based on the photothermal effect.

[0043] In one embodiment, the fabrication of a reflective vanadium dioxide dynamic saturable absorber mirror device based on a vanadium dioxide nanofilm is described. This embodiment presents a structure and fabrication method for a vanadium dioxide dynamic saturable absorber mirror applied to a 2.8 μm passively Q-switched erbium-doped fiber laser. The vanadium dioxide dynamic saturable absorber mirror is fabricated using magnetron sputtering. The specific steps include: depositing a 135 nm thick vanadium dioxide nanofilm onto the gold film of a gold high-reflectivity mirror using DC magnetron sputtering to form a dynamic saturable absorber mirror device. A 4-inch diameter, 99.9% pure V₂O₃ target is used for deposition in an argon / oxygen mixture atmosphere with a 35:15 volume ratio, a total pressure of 7 mTorr, a deposition temperature of room temperature, and a sputtering power of 200 W. Subsequently, the sample undergoes a rapid thermal annealing treatment under a vacuum of 5 Torr and a temperature of 400°C.

[0044] In one embodiment, the saturable absorption performance parameters of the reflective vanadium dioxide dynamic saturable absorber mirror device are controlled by active temperature control.

[0045] Active temperature control device for reflective vanadium dioxide dynamic saturable absorber mirror devices, such as Figure 3As shown, the device includes: a vanadium dioxide nanofilm 1, a gold-coated silicon substrate high-reflectivity mirror 2, a semiconductor heating element 3, wires 4, and a DC regulated power supply 5. The semiconductor heating element 3 is used to control the temperature of the reflective vanadium dioxide dynamic saturable absorber mirror device composed of elements 1 and 2. The semiconductor heating element 3 is connected to the DC regulated power supply 5 via wires 4. The DC regulated power supply 5 applies a DC voltage to the semiconductor heating element 3, and the temperature of the semiconductor heating element increases with increasing voltage.

[0046] Figure 4 This describes the relationship between the linear reflectivity of a vanadium dioxide dynamic saturable absorber mirror (VAD) for a 2.8 μm laser and temperature. When the temperature increases from 300 K to 320 K, vanadium dioxide has not yet undergone a phase transition, and the linear reflectivity of the VAD for a 2.8 μm laser is approximately 8%. When the temperature increases to 320-330 K, the vanadium dioxide nanofilm is about to undergo a phase transition, and a slight increase in the absorption coefficient and refractive index causes the linear reflectivity of the VAD to decrease slightly to approximately 5%. As the temperature continues to rise, the vanadium dioxide nanofilm enters a rapid phase transition stage, and the linear reflectivity of the VAD increases significantly with increasing temperature. Until the temperature rises to approximately 352 K, the vanadium dioxide nanofilm completely transforms into a metallic phase, and the physical structure and optical properties gradually stabilize, with the linear reflectivity of the VAD VA dynamic saturable absorber mirror stabilizing at approximately 52%. Based on the design and working principle of the vanadium dioxide dynamic saturable absorber mirror, the temperature range corresponding to the lowest linear reflectivity of the vanadium dioxide dynamic saturable absorber mirror before phase transition is the optimal operating temperature range with the largest modulation depth. Figure 5 The curve is the saturable absorption curve in the 2.8 μm band when the vanadium dioxide dynamic saturable absorber is kept at 325 K. According to the theoretical fitting results, the maximum modulation depth of the vanadium dioxide dynamic saturable absorber with a film thickness of 135 nm is about 71.3%, and the corresponding unsaturated loss is about 23.7%.

[0047] According to passive Q-switching theory, the narrowest pulse width of a passively Q-switched pulse = 3.52 * resonant cavity round-trip time / modulation depth of the saturable absorber. Passive Q-switching pulse modulation of a 2.8 μm wavelength erbium-doped fluoride fiber laser was performed using a vanadium dioxide dynamic saturable absorber mirror. With the vanadium dioxide dynamic saturable absorber mirror maintained at 325 K, the resonant cavity round-trip time was approximately 8.13 ns. At a pump power of 0.999 W, the narrowest pulse width of the output 2.8 μm passively Q-switched laser was 43 ns, further confirming the large modulation depth of the vanadium dioxide dynamic saturable absorber mirror. As the temperature of the vanadium dioxide dynamic saturable absorber mirror increased, the pulse width of the output pulse gradually increased. Figure 6This paper describes the modulation depth of a vanadium dioxide dynamic saturable absorber (VAD) mirror in the 2.8 μm band as a function of temperature. The modulation depth was theoretically calculated by using the VAD mirror to passively Q-switched pulses in an erbium-doped fluoride fiber laser cavity under different operating temperatures. As the temperature gradually increases, the modulation depth of the VAD mirror decreases significantly. This is because the vanadium dioxide nanofilm undergoes a phase transition with increasing temperature, reducing the low-intensity linear reflectivity of the VAD mirror. Figure 4 As the temperature gradually increases, the modulation depth of the device gradually decreases. Furthermore, the increased carrier concentration at high temperatures prematurely occupies conduction band vacancies, leading to a decrease in photoinduced carrier concentration in the high-temperature metallic state under saturated absorption. This results in increased unsaturated loss and a lower modulation depth in the vanadium dioxide dynamic saturable absorber mirror at high temperatures.

[0048] In one embodiment, the present invention is a design method for a vanadium dioxide dynamic saturable absorber mirror based on vanadium dioxide nanofilm. By driving the phase transition of the vanadium dioxide nanofilm with light intensity, and utilizing the equivalent nonlinear absorption and dynamic transformation of saturable absorption of multiple phase transition states such as mixed phase and metallic state during the phase transition process, a total equivalent saturable absorption with a large modulation depth is achieved, which greatly expands the modulation depth of saturable absorber devices.

[0049] The saturable absorber operation method of the present invention enables wide-range control of the saturable absorption performance of a single saturable absorber by adjusting the base temperature and irradiation intensity of the vanadium dioxide dynamic saturable absorber. This control can be performed in an active or passive manner, enhancing the universality and adaptability of its system application.

[0050] The vanadium dioxide dynamic saturable absorber mirror design method of the present invention has three aspects in its "dynamic" feature: First, in the total equivalent saturable absorption, the physical properties of the material, such as the crystal structure and band distribution, are not fixed, but dynamically change with light intensity; second, in the total equivalent saturable absorption, the optical effects that cause changes in optical properties are not single fixed effects, but multiple optical effects that adaptively and dynamically transform; third, the total equivalent saturable absorption performance of the dynamic saturable absorber mirror is not fixed, and can be adjusted by controlling factors such as temperature. The first and second aspects are key to achieving a large modulation depth and adjustable saturable absorption performance.

[0051] In one embodiment, the materials used in the proposed dynamic saturable absorption mirror are not limited to vanadium dioxide nanofilms. The method can be extended to other materials that, in addition to intrinsic saturable absorption properties, also have reversible equivalent nonlinear absorption effects affected by light intensity. These include, but are not limited to, various reversible phase change materials with different induction mechanisms, thermochromic liquid crystal materials, metal-organic framework materials, perovskite materials, and ferroelectric materials. All of these materials can change their optical properties according to temperature or photothermal stimulation.

[0052] In one embodiment, the proposed structure of the dynamic saturable absorber is not limited to a reflective saturable absorber structure. For different materials with specific equivalent nonlinear absorption effects, appropriate structures are adopted, including but not limited to reflective saturable absorber structures, transmissive saturable absorber structures, fiber end-face coatings, and fiber optic adapter docking structures.

[0053] While the invention has been described herein with reference to specific embodiments, it should be understood that these embodiments are merely examples of the principles and applications of the invention. Therefore, it should be understood that many modifications can be made to the exemplary embodiments, and other arrangements can be designed without departing from the spirit and scope of the invention as defined by the appended claims. It should be understood that different dependent claims and features described herein can be combined in ways different from those described in the original claims. It is also understood that features described in conjunction with individual embodiments can be used in other described embodiments.

Claims

1. A large modulation depth dynamic saturable absorber mirror, characterized in that, The device includes a vanadium dioxide nanofilm and a gold-coated silicon substrate high-reflectivity mirror. The vanadium dioxide nanofilm is deposited on the gold film of the gold-coated high-reflectivity mirror by DC magnetron sputtering to form a dynamic saturable absorber mirror device. The film is a monoclinic insulating state with a band gap of 0.68 eV at low temperatures ≤25℃ and a tetragonal rutile metallic state with zero band gap at high temperatures >68℃.

2. The large modulation depth dynamic saturable absorber mirror according to claim 1, characterized in that, When strong light acts on a dynamic saturable absorber mirror of vanadium dioxide in the monoclinic phase, it drives the phase transition of vanadium dioxide and exhibits an equivalent nonlinear absorption effect with light intensity. When the vanadium dioxide nanofilm undergoes a phase transition to a mixed phase in which the two phases coexist, the band gap gradually closes to the band gap width of the mid-infrared laser response, generating a nonlinear optical absorption response and exhibiting saturable absorption properties. When the vanadium dioxide nanofilm of the saturable absorption mirror undergoes a complete phase transition and is in a high-temperature metallic state (>68℃), it exhibits a nonlinear absorption effect that can saturate absorb mid-infrared lasers.

3. The large modulation depth dynamic saturable absorber mirror according to claim 2, characterized in that, When the gradually increasing laser intensity acts on the dynamic saturable absorber mirror, a dynamic transition occurs. This dynamic transition begins in the low reflectivity and high loss state of the vanadium dioxide insulating phase, transforms into a saturable absorber state of the vanadium dioxide mixed phase through equivalent nonlinear absorption of the phase transition, and further transforms into a saturable absorber state of the metallic state, until the vanadium dioxide metallic state is completely saturated. The dynamic transition has a large modulation depth and an overall total equivalent saturable absorber effect. The total equivalent saturable absorber includes the equivalent nonlinear absorption of the phase transition, the saturable absorption of the vanadium dioxide mixed phase, and the saturable absorption of the vanadium dioxide metallic phase. The maximum modulation depth of the total equivalent saturable absorber is 1 minus the linear reflectivity of the insulating state under low-to-mid-infrared laser intensity of the vanadium dioxide dynamic saturable absorber mirror and then minus the unsaturated loss of the metallic state of the vanadium dioxide dynamic saturable absorber mirror.

4. The large modulation depth dynamic saturable absorber mirror according to claim 3, characterized in that, By adjusting the pump power of the laser, the intensity of the focused laser beam within the cavity is controlled, thereby controlling the temperature difference caused by the photothermal effect; this provides a base temperature for the vanadium dioxide dynamic saturable absorber mirror to compensate for the insufficient temperature difference caused by the photothermal effect.

5. The large modulation depth dynamic saturable absorber mirror according to claim 4, characterized in that, When the modulation depth of the vanadium dioxide saturable absorber is required to be at its maximum, the base temperature is set below the phase transition temperature. At this time, the vanadium dioxide nanofilm is in an insulating state, ensuring that the driving factor for the phase transition is the laser intensity focused on the surface of the vanadium dioxide nanofilm within the cavity. When the temperature is increased to the point that the initial phase transition state under the low-to-mid-infrared laser intensity of the vanadium dioxide dynamic saturable absorber is in a metallic state, the total equivalent saturable absorption, i.e., the saturable absorption of metallic vanadium dioxide, reaches its minimum value.

6. The large modulation depth dynamic saturable absorber mirror according to any one of claims 1-5, characterized in that, The dynamic saturable absorption mirror uses a material with intrinsic saturable absorption characteristics and a reversible equivalent nonlinear absorption effect affected by light intensity to replace the vanadium dioxide nanofilm. The material with the reversible equivalent nonlinear absorption effect affected by light intensity includes, but is not limited to, various reversible phase change materials with different induction mechanisms, thermochromic liquid crystal materials, metal-organic framework materials, perovskite materials and ferroelectric materials. The dynamic saturable absorber can be replaced by a transmissive saturable absorber structure, fiber end face coating, and fiber adapter docking structure instead of a reflective saturable absorber structure.

7. A method for fabricating a large modulation depth dynamic saturable absorber mirror as described in any one of claims 1-5, characterized in that, The vanadium dioxide nanofilm is deposited on the gold film of a gold high-reflectivity mirror using DC magnetron sputtering technology to form a dynamic saturable absorber device. Specifically, it involves using a 4-inch diameter V2O3 target with a purity of 99.9% and deposition in an argon-oxygen mixed atmosphere with a volume ratio of 35:15; a total pressure of 7 mTorr; a deposition temperature of room temperature; and a sputtering power of 200 W. Subsequently, the sample undergoes a one-step rapid thermal annealing treatment under a vacuum of 5 Torr and a temperature of 400°C.

8. A method for operating a large modulation depth dynamic saturable absorber mirror as described in any one of claims 1-5, characterized in that, When the dynamic saturable absorber mirror is placed at one end of the resonant cavity of a mid-infrared rare-earth ion-doped fluoride fiber laser, the initial state of the vanadium dioxide nanofilm in the dynamic saturable absorber mirror during the formation of a single Q-switched laser pulse is kW / cm². 2 The insulating state under low to mid-infrared laser intensity below level 1 has the lowest reflectivity and the highest intracavity loss. As the light intensity inside the resonant cavity gradually increases, the laser focused on the surface of the dynamic saturable absorber mirror inside the resonant cavity induces the vanadium dioxide nanofilm to undergo a phase transition to the metallic state. At the same time, the reflectivity of the dynamic saturable absorber mirror increases, and the loss inside the resonant cavity slowly decreases. As the light intensity inside the resonant cavity continues to increase, the intrinsic nonlinear saturable absorption of the vanadium dioxide nanofilm mixed phase and metallic state is excited, and the reflectivity of the dynamic saturable absorption mirror continues to increase, and the loss inside the resonant cavity decreases rapidly. When the loss decreases to the point where a net gain is generated inside the resonant cavity, the laser begins to generate pulses, corresponding to the rising edge of the pulse shape. When the light intensity inside the cavity is increased to a level sufficient to fully saturate the saturable absorption of the metallic vanadium dioxide nanofilm, the vanadium dioxide nanofilm exhibits high transmittance, the reflectivity of the vanadium dioxide dynamic saturable absorption mirror reaches its maximum value, the loss inside the resonant cavity reaches its minimum, and the pulse reaches its peak value. After the pulse reaches its peak, the vanadium dioxide nanofilm gradually recovers to the insulating state. At the same time, the intrinsic saturable absorption of vanadium dioxide also recovers to the high-loss state, and the loss in the resonant cavity increases until it is completely restored to the initial state. This corresponds to the falling edge of the pulse shape and the subsequent pulse-off state. Furthermore, under continuous pumping from the pump source, the particle number, the number of photons in the cavity, the resonant cavity gain, and the loss are reconstructed, and the above pulse cycle is repeated to output a passively Q-switched pulse sequence.

9. The operating method of the large modulation depth dynamic saturable absorber mirror according to claim 8, characterized in that, The initial phase transition state of vanadium dioxide nanofilms is controlled by adjusting the base temperature. When the base temperature is controlled so that the vanadium dioxide nanofilm is in an insulating state, the reflectivity of the vanadium dioxide saturable absorber mirror under low to mid-infrared laser light intensity is the lowest, which is the linear reflectivity of the insulating vanadium dioxide saturable absorber mirror, and the modulation depth is the largest. When the base temperature is controlled to make the vanadium dioxide nanofilm in a two-phase mixed phase during the phase transition process, the reflectivity of the vanadium dioxide saturable absorber mirror under low-to-mid-infrared laser light intensity increases, becoming the linear reflectivity of the mixed-phase vanadium dioxide saturable absorber mirror, and the modulation depth decreases. When the base temperature is controlled to make the vanadium dioxide nanofilm in a metallic state, the reflectivity of the vanadium dioxide saturable absorber mirror under low to mid-infrared laser light intensity is the highest, which is the linear reflectivity of the metallic vanadium dioxide saturable absorber mirror, and the modulation depth is the smallest.

10. The operating method of the large modulation depth dynamic saturable absorber mirror according to claim 9, characterized in that, The base temperature of a vanadium dioxide saturable absorber mirror can be controlled actively by using a semiconductor heating element, or passively by using a vanadium dioxide nanofilm to absorb residual pump light and generate heat based on the photothermal effect.

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