A deep ultraviolet optical frequency conversion element and converter capable of realizing adiabatic mode conversion

Abstract

The application discloses a kind of deep ultraviolet optical frequency conversion elements and converters that can realize adiabatic mode conversion, belong to the technical field of optoelectronic devices, wherein, deep ultraviolet optical frequency conversion element includes quartz crystal, quartz crystal is internally provided with the periodic phase grating structure along the light direction arrangement, the surface of quartz crystal is provided with the ridge type optical waveguide structure along the y direction of quartz crystal, the light direction of the ridge type optical waveguide structure is along the y direction of crystal, the ridge type optical waveguide structure is divided into two portions of front and back, the front half of the ridge type optical waveguide structure is consistent and penetrates the periodic phase grating structure;The back half of the ridge type optical waveguide structure is located outside the periodic phase grating structure and gradually narrows in width size.The application combines surface ridge waveguide structure and periodic phase grating structure, and can realize the function integration of nonlinear frequency conversion and adiabatic mode conversion under deep ultraviolet band.

Description

Technical Field

[0001] This invention relates to the field of optoelectronic device technology, and more specifically to a deep ultraviolet optical frequency conversion element and converter capable of adiabatic mode switching. Background Technology

[0002] Researchers have demonstrated that by precisely controlling the processing parameters of femtosecond lasers, periodic phase grating structures conforming to phase-matching theory can be induced within nonlinear optically transparent materials. These artificially designed periodic structures can provide the necessary additional phase compensation, enabling successful optical frequency conversion at specific wavelengths even in materials where phase matching cannot be achieved using conventional methods. Furthermore, adiabatic mode conversion of the optical field on-chip can be achieved using ridge waveguides.

[0003] Chinese patent document CN119087725A discloses a method for fabricating an infrared optical frequency conversion element based on nonlinear optical polycrystalline transparent ceramic and its application. This method addresses the lack of birefringence in nonlinear optical polycrystalline ceramics by utilizing APP phase-matching technology. Through laser processing and ion etching, periodic refractive index-modulated phase gratings are fabricated within the ceramic to compensate for phase mismatch, thereby achieving difference frequency output in the infrared band. This method offers flexible fabrication, is applicable to various infrared nonlinear ceramic materials, and the fabricated infrared optical frequency conversion element has a simple overall structure, exhibiting good process adaptability and potential for large-scale fabrication.

[0004] However, this method, based on nonlinear optical polycrystalline ceramics, is only applicable to frequency conversion in the infrared band and cannot achieve frequency conversion in the deep ultraviolet band. Moreover, due to the presence of internal grain boundaries and micro-defects, this material significantly increases the optical damage threshold and the non-uniformity of thermal stability in its microscopic distribution, severely restricting the reliability of the device at high power. In addition, the bulk discrete device fabricated by this method is difficult to be compatible with modern integrated photonics, making it impossible to achieve on-chip high-integration frequency conversion photonic device fabrication, and also difficult to obtain the high power density and nonlinear enhancement effects brought by waveguide structures. Finally, the device design focuses on achieving phase matching to complete frequency conversion, and its structure does not have the ability to perform mode conversion of the optical field. This functional singularity and large size of the structure make it difficult for this device to meet the urgent requirements of advanced photonic integrated systems for multifunctional and highly integrated optoelectronic devices.

[0005] Therefore, how to provide a deep ultraviolet band optical frequency conversion device that has more uniform resistance to optical damage and thermal stability, is compatible with modern integrated photonics, and can realize on-chip optical field adiabatic mode conversion is a problem that those skilled in the art still need to explore and solve. Summary of the Invention

[0006] In view of the above problems, the present invention is proposed to provide a deep ultraviolet optical frequency conversion element and converter that can realize adiabatic mode switching, which overcomes or at least partially solves the above problems.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] In a first aspect, embodiments of the present invention provide a deep ultraviolet optical frequency conversion element capable of adiabatic mode switching, comprising: a quartz crystal, The quartz crystal has a periodic phase grating structure arranged along the light transmission direction, consisting of several modified regions and several unmodified regions. The surface of the quartz crystal has a ridge-shaped waveguide structure arranged along the y-direction of the quartz crystal. The light transmission direction of the ridge-shaped waveguide structure is along the y-direction of the crystal. The ridge-shaped waveguide structure is divided into front and rear parts. The front half of the ridge-shaped waveguide structure has a uniform width and penetrates the periodic phase grating structure. The rear half of the ridge-shaped waveguide structure is located outside the periodic phase grating structure and its width gradually narrows.

[0009] Preferably, the quartz crystal is a Z-cut quartz crystal.

[0010] Preferably, in the periodic phase grating structure, the width ratio of the modified region to the unmodified region within a single period is 1:1.

[0011] Secondly, embodiments of the present invention provide a quartz crystal ultraviolet band frequency converter capable of adiabatic mode switching, comprising an ultraviolet pulsed laser, an optical fiber coupled input end, a quartz crystal surface ridge waveguide structure element, an optical fiber coupled receiving end, and a filter arranged sequentially along the beam transmission direction, wherein the quartz crystal surface ridge waveguide structure element is any one of the deep ultraviolet optical frequency conversion elements capable of adiabatic mode switching described in the first aspect.

[0012] Thirdly, embodiments of the present invention provide a method for fabricating a deep ultraviolet optical frequency conversion element capable of adiabatic mode switching, comprising the following steps: S1. A femtosecond laser Bessel beam is translated along the x-axis of the quartz crystal to scan the quartz crystal and obtain a femtosecond laser irradiation area with modified refractive index. S2. The quartz crystal is translated several times along the y-direction by a fixed distance, and step S1 is repeated after each translation to obtain a periodic phase grating structure arranged along the y-direction of the quartz crystal. S3. Based on femtosecond laser scanning, the near-surface of the periodic phase grating structure region of the quartz crystal is scanned and ablated along the y-direction of the quartz crystal to prepare two parallel air slots that penetrate the periodic phase grating structure, thus obtaining the first half of a set of undetermined optical waveguides of the ridge-type optical waveguide structure; scanning is continued at the end of the air slots to prepare two air slots with gradually decreasing spacing, thus obtaining the second half of the undetermined optical waveguides; S4. Translate the quartz crystal along the x-direction of the quartz crystal, and repeat step S3 after each translation to obtain a ridge-type optical waveguide structure composed of multiple sets of undetermined optical waveguides; S5. In each group of undetermined optical waveguide regions, several refractive index-reducing modified grooves parallel to the corresponding air grooves are prepared based on femtosecond laser scanning to obtain the prepared optical waveguide regions.

[0013] Furthermore, the above method also includes: S6. Perform optical polishing on the two end faces perpendicular to the y-direction of the quartz crystal.

[0014] Furthermore, in step S1, the femtosecond laser Bessel beam travels along the quartz crystal during the scanning process. z The light is incident perpendicularly onto the quartz crystal.

[0015] Further, in step S1, the femtosecond laser Bessel beam is obtained through the following steps: A femtosecond laser Gaussian beam is shaped into a femtosecond laser Bessel beam using a femtosecond laser shaping system based on a conical lens, and the femtosecond laser Bessel beam is then focused through a microscope objective to obtain the final femtosecond laser Bessel beam.

[0016] As can be seen from the above technical solution, compared with the prior art, the present invention discloses a deep ultraviolet optical frequency conversion element and converter that can realize adiabatic mode switching, which has the following beneficial effects: The deep ultraviolet optical frequency conversion element disclosed in this invention integrates deep ultraviolet frequency conversion and adiabatic mode conversion. Its core application value lies in integrating on-chip generation of high-performance deep ultraviolet light source with high-efficiency, low-loss on-chip optical field modulation. On the one hand, deep ultraviolet light can be used for on-chip sensing, quantum light source pumping, or high-precision processing; on the other hand, through adiabatic mode conversion, the light transmission path and mode distribution can be flexibly and stably controlled to adapt to different back-end functional units (such as detectors, modulators, or second-stage nonlinear crystals), ultimately realizing the miniaturization, integration, and high performance of deep ultraviolet photonic devices.

[0017] This invention utilizes a shaped femtosecond laser Bessel beam to prepare a periodic phase grating structure on the near-surface of a quartz crystal, enabling faster and more efficient preparation of the periodic phase grating structure and achieving nonlinear frequency conversion at a specific wavelength.

[0018] This invention integrates a ridge waveguide structure into a femtosecond laser-induced periodic phase grating structure, significantly improving nonlinear frequency conversion efficiency while also enabling on-chip adiabatic mode conversion of the optical field. The invention employs APP phase matching technology, which uses methods such as femtosecond laser direct writing to fabricate a periodically distributed refractive index phase grating structure within a nonlinear optical material, thereby providing the additional phase required to compensate for phase mismatch. This contrasts with traditional birefringence phase matching techniques (which strictly rely on crystal anisotropy) and quasi-phase matching techniques (which rely on precise domain inversion to achieve periodic sign flipping of nonlinear coefficients). In contrast, this technology does not require the material itself to possess birefringence or ferroelectricity, and does not require complex polarization processes. Phase matching can be achieved simply by forming a periodic phase grating in any nonlinear material through methods such as laser processing or ion etching. This allows for greater flexibility in material selection and process implementation. Furthermore, this invention uses a quartz crystal material platform, which not only has excellent optical uniformity, strong resistance to light damage, and excellent thermal and chemical stability, but also exhibits good light transmittance in the ultraviolet band. This provides an ideal carrier for fabricating high-stability and highly integrated ultraviolet nonlinear frequency conversion devices. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0020] Figure 1 A top view of a frequency conversion element with a ridge-type optical waveguide structure on the surface of a quartz crystal, provided by the present invention; Figure 2 A front view of a frequency conversion element based on a ridge-type optical waveguide structure on the surface of a quartz crystal, provided by the present invention; Figure 3 A left view of a frequency conversion element based on a ribbed optical waveguide structure on the surface of a quartz crystal, provided by the present invention; Figure 4 A schematic diagram of the quartz crystal periodic phase grating structure provided by the present invention; Figure 5 A schematic diagram illustrating a method for fabricating a frequency conversion element based on a ridge-type optical waveguide structure on the surface of a quartz crystal, provided by this invention; Figure 6 A schematic diagram of a frequency converter based on a ridge-type optical waveguide structure on the surface of a quartz crystal is provided by the present invention. Among them, 1-phase grating structure, 2-air groove near the crystal surface etched by femtosecond laser, 3-quartz crystal, 4-ridge waveguide structure for direct writing by femtosecond laser to realize the adiabatic mode conversion region, 5-ridge waveguide structure for direct writing by femtosecond laser to realize the frequency conversion region, 6-refractive index reduction modification region of quartz crystal induced by femtosecond laser, 7-ultraviolet pulsed laser, 8-fiber coupled input end, 9-fiber coupled receiver end, 10-filter. Detailed Implementation

[0021] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0022] Terminology Explanation: Frequency doubling: A nonlinear frequency conversion process that increases the frequency of fundamental light by two times.

[0023] Mode control: Manipulating the spatial distribution of light within an optical waveguide by controlling its structural parameters.

[0024] Adiabatic mode switching: By designing a slowly changing optical waveguide structure, the optical field mode evolves slowly and continuously during transmission, thereby achieving near-lossless energy conversion between different modes.

[0025] Example 1 Embodiment 1 of this invention discloses a deep ultraviolet optical frequency conversion element capable of adiabatic mode switching, used for nonlinear optical frequency conversion capable of adiabatic mode switching, such as... Figures 1-4 ,include: z A platform for cutting quartz crystal materials, the light transmission direction of which is along the crystal. y Towards; z Cutting the interior of a quartz crystal along y The direction is divided into two parts, the front part is equipped with a... y The periodic phase grating structure induced by femtosecond lasers is arranged in a direction, and the latter half is left unprocessed; Along the quartz crystal y The structure is a ridge waveguide with direct writing via femtosecond laser. The ridge waveguide is also divided into two parts, the front half of which has a waveguide width of ( x The direction remains unchanged, and the width of the waveguide in the second half remains unchanged. x The width of the waveguide gradually decreases, and the two waveguide sections are connected end-to-end. The width of the latter waveguide is equal to the width of the former waveguide, and the light transmission direction is along the quartz crystal.y Towards.

[0026] In this invention, a surface ridge waveguide structure can be fabricated using femtosecond laser etching and femtosecond laser direct writing, thereby enabling adiabatic mode switching by designing the structural parameters of the ridge waveguide.

[0027] In this embodiment, the width ratio of the laser-processed area to the unprocessed area of ​​the periodic phase grating structure unit is close to 1:1. The closer this ratio is to 1:1, the higher the conversion efficiency.

[0028] In this invention, the periodic phase grating structure is obtained through the following steps: using a periodic phase matching technique, a phase grating with a periodic refractive index distribution is fabricated inside a quartz crystal using a femtosecond laser. In the periodic phase grating structure, the periodic phase compensation provided by the fabricated area effectively blocks energy "backflow" and achieves efficient optical frequency conversion in quartz crystals where phase matching is inherently difficult to achieve.

[0029] At room temperature, the period in the APP phase-matching structure during nonlinear frequency conversion is:

[0030] Where Λ is the quasi-phase matching period size, λ 2ω It is the wavelength of the emitted frequency-doubled light. n ω and n 2ω These are the refractive index values ​​of the material under fundamental and second-harmonic light conditions, respectively; the dispersion equation is used to calculate the refractive index of a quartz crystal at a certain incident fundamental light wavelength λ. z The refractive index in the direction is as follows:

[0031] In practical applications, in order to ensure the uniformity and stability of the prepared periodic phase structure, the period is usually set to an even multiple of the ideal period, that is, the widths of the laser-modified region and the unmodified region are odd multiples of the widths of the corresponding regions in the ideal period.

[0032] To further implement the above technical solution, the front half of the ridge-shaped optical waveguide... x The dimensions of the two parallel air slots arranged in the direction are 4 μm. x ) 12 μm ( z The lateral spacing between the two parallel air slots (along the crystal) x The distance between the two air slots is 12 μm. The ridge waveguide region 5 is located between two parallel air slots. Five 2 μm diameter ( ) strips are set in the bottom region between the two air slots. x ) 4 μm (z Femtosecond laser-induced refractive index reduction indentation is used to generate a ridge-shaped optical waveguide.

[0033] To further implement the above technical solution, the initial lateral spacing of the two air slots in the rear half of the ridge waveguide is 12 μm, and the end spacing is 4 μm. The ridge waveguide region 4 is located between the two air slots with gradually decreasing spacing. Five 2 μm ( x ) 4 μm ( z Femtosecond laser-induced refractive index reduction indentation is used to generate a ridge-shaped optical waveguide.

[0034] To further implement the above technical solution, a femtosecond laser-induced periodic phase grating structure is constructed along the quartz crystal. z The length of the direction is 30 μm.

[0035] To further implement the above technical solution and improve the frequency conversion effect, along y The number of periodic phase grating structures processed by femtosecond lasers is ≥300.

[0036] To further implement the above technical solution, the six sides of the quartz crystal are optically polished, and a femtosecond laser is used along the quartz crystal... z An incident induced phase grating structure is used, where the incident fundamental frequency light and the outgoing frequency-doubled laser light travel along the quartz crystal. y Transmission.

[0037] The deep ultraviolet optical frequency conversion element disclosed in this invention has the characteristics of adjustable mode, high output power, high conversion efficiency, good stability and high integration. It can realize nonlinear frequency conversion in the ultraviolet band and will have wide applications in the fields of laser technology, integrated optics and nonlinear optics.

[0038] In the frequency conversion region of the deep ultraviolet optical frequency conversion element disclosed in this invention, the fundamental frequency light is efficiently converted into deep ultraviolet frequency-doubled light in a ridge waveguide of constant size. Immediately following this is a seamlessly connected adiabatic mode conversion region, which, through a gradient waveguide size, converts the generated deep ultraviolet light losslessly into other desired modes. This design not only allows the deep ultraviolet light to be guided onto its customized manipulation trajectory in real time before energy loss, but more importantly, the initial mode determined by the frequency conversion is precisely reshaped into the optimal mode distribution required for subsequent on-chip functions (such as second-order nonlinear processes, sensing, or quantum manipulation) through adiabatic conversion. Since the generation of deep ultraviolet light and mode manipulation are seamlessly integrated on the same waveguide chip and the adiabatic conversion process is theoretically lossless, the integrated architecture disclosed in this invention ensures that the mode purity and phase distribution of the output deep ultraviolet light field are maximized. This solves the problem of mode degradation caused by transmission loss and coupling mismatch in discrete systems, providing an ideal integrated optical field manipulation platform for achieving efficient on-chip cascaded nonlinear processes and precise quantum manipulation in the deep ultraviolet band.

[0039] Example 2 like Figure 5 As shown in Example 2, a method for fabricating a deep ultraviolet optical frequency conversion element capable of adiabatic mode switching is disclosed, comprising the following steps: Step 1. Using a femtosecond laser shaping system based on a conical lens, the femtosecond laser Gaussian beam is shaped into a femtosecond laser Bessel beam. After being focused by a microscope objective, the beam is then directed along the crystal. z A quartz crystal is incident perpendicularly to induce refractive index modification. A suitable grating structure unit is generated by adjusting the width of the modified region, and the result is obtained by repeated scanning along the [path / line]. y A periodic phase grating structure with directional distribution.

[0040] Step 2. Using a microscope objective, focus the femtosecond laser Gaussian beam onto the quartz crystal. Based on the processing parameters for ablation of the air grooves near the surface of the quartz crystal by the femtosecond laser, along the... y First, two parallel air slots are prepared through a periodic phase grating structure. Then, two air slots with gradually decreasing distances are prepared after the two parallel air slots, connecting to the two parallel air slots respectively. The overall air slot structure is symmetrical. Along the quartz crystal... y Based on the processing parameters of femtosecond laser-induced refractive index reduction modification inside quartz crystal, several parallel refractive index reduction modification grooves that precisely fill the gaps at the bottom of the air groove are scanned at the bottom of the air groove to form the waveguide region of the ridge-type optical waveguide.

[0041] Step 3. Apply the following to the two crystals of the quartz crystal. y The end face, i.e., the quartz crystal yOptical polishing is performed on the two perpendicular end faces to obtain a quartz crystal periodic phase grating structure element.

[0042] For a laser of a specific wavelength, this invention theoretically calculates the corresponding period width and adjusts the width ratio of the modified region to the unmodified region within the periodic structural unit to 1:1. By preparing a phase grating structure with hundreds or thousands of periods along the transmission direction of the "laser of a specific wavelength", efficient frequency conversion can be achieved.

[0043] In practical applications, the microscope objective has a magnification of 50 and a numerical aperture of 0.67.

[0044] To further implement the above technical solution, in step 1, the femtosecond laser pulse width is 400 fs, the wavelength is 1030 nm, the repetition frequency is 2.5 MHz, the single pulse energy is 1.92 μJ, and the femtosecond laser scanning speed is 10 mm / s. In step 2, the processing parameters for femtosecond laser ablation of the near-surface air grooves of the quartz crystal are: femtosecond laser pulse width of 400 fs, wavelength of 1030 nm, repetition rate of 25 kHz, single pulse energy of 8 μJ, and femtosecond laser scanning speed of 4 mm / s. The processing parameters for femtosecond laser-induced refractive index reduction modification inside the quartz crystal are: femtosecond laser pulse width of 400 fs, wavelength of 1030 nm, repetition rate of 25 kHz, single pulse energy of 0.5 μJ, and femtosecond laser scanning speed of 5 mm / s.

[0045] Example 3 like Figure 6 As shown, the present invention discloses a deep ultraviolet optical frequency converter that can realize adiabatic mode conversion. The converter is based on a surface ridge optical waveguide structure element of a quartz crystal as the core, and includes, in sequence along the beam transmission direction, an ultraviolet pulsed laser source, an optical fiber coupling input end, a quartz crystal surface ridge optical waveguide structure element, an optical fiber coupling receiver end, and a filter.

[0046] In this embodiment, the working principle of a tunable quartz crystal frequency converter is as follows: an ultraviolet pulsed laser is used as a pump source to generate linearly polarized laser light. The laser light is then coupled into a ridge waveguide through an optical fiber coupling input end. Frequency doubling is achieved in the periodic phase grating structure distribution area, and adiabatic mode conversion is achieved in the ridge waveguide in the adiabatic mode conversion area. The optical fiber coupling receiver collects the pump light and signal light (such as frequency-doubled light) emitted from the ridge waveguide. Finally, the pump light is filtered by a filter to block the light and the signal light is transmitted, thus realizing the functions of adiabatic mode conversion and optical frequency conversion at a specific wavelength.

[0047] The advantages of this invention are as follows: First, it not only enables optical frequency conversion but also supports adiabatic mode conversion of the output frequency-doubled light while achieving frequency conversion, thus possessing multifunctionality. Second, by utilizing the excellent light transmittance of quartz crystal in the ultraviolet band, it can perform optical frequency conversion in the ultraviolet band to achieve deep ultraviolet laser output. In addition, by utilizing the ridge waveguide written directly by femtosecond laser, the device integration density can be improved while reducing the device's transmission loss. Finally, based on the strong resistance to optical damage and stable physicochemical properties of quartz crystal, it can achieve high-power, long-duration adiabatic mode conversion and nonlinear frequency conversion.

[0048] Example 4 Fabrication of a 320 nm tunable mode "frequency doubling" frequency converter, specifically: (1) Select z Quartz crystals are cut using a femtosecond laser shaping system based on a conical lens. The femtosecond Gaussian laser beam (pulse width 400 fs, wavelength 1030 nm, repetition rate 2.5 MHz, single pulse energy 1.92 μJ, femtosecond laser scanning speed 10 mm / s) is shaped into a femtosecond Bessel laser beam. This beam is then focused through a microscope objective (magnification 50, numerical aperture 0.67) and directed along the crystal. z A periodic phase grating structure is obtained by inducing refractive index modification by incident perpendicularly on a quartz crystal. The theoretically calculated phase-matching period corresponding to a wavelength of 320 nm is 0.46 μm. To ensure the uniformity of the periodic structure, the width of the refractive index-modified region is adjusted so that the width of both the modified and unmodified regions within a single period is three times the ideal calculated value, i.e., 1.38 μm. Repeated scanning forms a periodic phase grating structure, which can then be used to achieve the frequency doubling conversion process at 320 nm. (2) Experimental conditions for forming air grooves near the surface of a quartz crystal using femtosecond laser ablation (femtosecond laser pulse width 400 fs, wavelength 1030 nm, repetition rate 25 kHz, single pulse energy 8 μJ, femtosecond laser scanning speed 4 mm / s), first along the quartz crystal... y Two parallel air slots are fabricated to penetrate a periodic phase grating structure. Then, two more air slots with gradually decreasing distances are fabricated after the two parallel air slots, connecting to the two parallel air slots respectively. The overall air slot structure is symmetrical. Along the quartz crystal... y Based on the processing parameters of femtosecond laser-induced refractive index reduction modification inside quartz crystal, several parallel refractive index reduction modification grooves that precisely fill the gaps at the bottom of the air groove are scanned and prepared to form the waveguide region of a ridge-shaped optical waveguide. (3) Two quartz crystals yThe end faces are optically polished, removing approximately 0.5 mm from each face. The samples are then cleaned with a mixture of ethanol and acetone to obtain polished surfaces. (4) Integrate an ultraviolet pulsed laser (center wavelength 320 nm), an optical fiber coupling system, a quartz crystal surface ridge waveguide structure element and a filter together to achieve mode conversion and efficient “frequency doubling” conversion at a wavelength of 320 nm; the filter used blocks the 320 nm fundamental frequency light and transmits the 160 nm frequency doubling light after mode conversion.

[0049] Fabrication of a 355 nm tunable mode "frequency doubling" frequency converter, specifically: (1) Select z To cut a quartz crystal, a commonly used femtosecond laser Gaussian beam (femtosecond laser pulse width of 400 fs, wavelength of 1030 nm, repetition rate of 2.5 MHz, single pulse energy of 1.92 μJ, and femtosecond laser scanning speed of 10 mm / s) is shaped into a femtosecond laser Bessel beam using a femtosecond laser shaping system based on a conical lens. This beam is then focused through a microscope objective (magnification of 50, numerical aperture of 0.67) and directed along the crystal. z The quartz crystal is incident perpendicularly, along the crystal z A periodic phase grating structure is obtained by inducing refractive index modification by incident perpendicularly on a quartz crystal. The theoretically calculated phase matching period corresponding to a wavelength of 355 nm is 0.7 μm. To ensure the uniformity of the periodic structure, the width of the refractive index modified region is adjusted so that the width of both the modified and unmodified regions within a single period is three times the ideal calculated value, i.e., 2.1 μm. Repeated scanning forms a periodic phase grating structure, which can be used to realize the frequency doubling conversion process at 355 nm. (2) Experimental conditions for forming air grooves near the surface of a quartz crystal using femtosecond laser ablation (femtosecond laser pulse width 400 fs, wavelength 1030 nm, repetition rate 25 kHz, single pulse energy 8 μJ, femtosecond laser scanning speed 4 mm / s), first along the quartz crystal... y Two parallel air slots are fabricated to penetrate a periodic phase grating structure. Then, two more air slots with gradually decreasing distances are fabricated after the two parallel air slots, connecting to the two parallel air slots respectively. The overall air slot structure is symmetrical. Along the quartz crystal... y Based on the processing parameters of femtosecond laser-induced refractive index reduction modification inside quartz crystal, several parallel refractive index reduction modification grooves that precisely fill the gaps at the bottom of the air groove are scanned and prepared to form the waveguide region of a ridge-shaped optical waveguide. (3) Two quartz crystals yThe end faces are optically polished, removing approximately 0.5 mm from each face. The samples are then cleaned with a mixture of ethanol and acetone to obtain polished surfaces. (4) Integrate an ultraviolet pulsed laser (center wavelength 355 nm), an optical fiber coupling system, a quartz crystal surface ridge waveguide structure element and a filter together to achieve mode conversion and efficient “frequency doubling” conversion at a wavelength of 355 nm; the filter used blocks the fundamental frequency light of 355 nm and transmits the frequency doubling light of 177.5 nm after mode conversion.

[0050] In this embodiment, a comparative experiment is conducted using three comparative assembly frequency converters and the quartz crystal ridge waveguide structure frequency converter prepared in Example 4: Comparative Example 2 differs from Example 4 in that a periodic phase grating structure is not fabricated, and a frequency converter is assembled in accordance with Example 4; Comparative Example 2 differs from Example 4 in that no waveguide is added, and a frequency converter is assembled in accordance with Example 4; Comparative Example 3 differs from Example 4 in that the fabricated air slots are two parallel lines, that is, the cross-sectional area of ​​the ridge waveguide does not change along the light transmission direction, and a frequency converter is assembled in accordance with Example 4.

[0051] In Comparative Example 1, the frequency doubling conversion efficiency is significantly reduced due to the lack of a periodic phase matching structure. In Comparative Example 2, the frequency doubling conversion efficiency and output power are significantly reduced due to the lack of a waveguide to confine the beam. In Comparative Example 3, the adiabatic mode conversion of the frequency-doubled light cannot be achieved because the cross-sectional area of ​​the ridge waveguide does not change along the light transmission direction.

[0052] Experiments show that this invention innovatively utilizes the shaped femtosecond laser Bessel beam in... z Periodic phase grating structures are fabricated on quartz crystals. Then, based on the femtosecond laser-induced refractive index reduction modification of the quartz crystal and the femtosecond laser ablation of the air grooves near the surface of the quartz crystal, ridge-type optical waveguide structures are fabricated along the direction of the periodic structure arrangement. This enables nonlinear optical frequency conversion and on-chip adiabatic mode conversion in the ultraviolet band, and can be used to fabricate multifunctional optical devices that support mode conversion and nonlinear frequency conversion.

[0053] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on its differences from other embodiments. Similar or identical parts between embodiments can be referred to interchangeably. For the apparatus disclosed in the embodiments, since they correspond to the methods disclosed in the embodiments, the description is relatively simple; relevant parts can be referred to the method section.

[0054] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A deep ultraviolet optical frequency conversion element capable of adiabatic mode switching, characterized in that, include: Quartz crystal, The quartz crystal has a periodic phase grating structure arranged along the light transmission direction, consisting of several modified regions and several unmodified regions. The surface of the quartz crystal has a ridge-shaped waveguide structure arranged along the y-direction of the quartz crystal. The light transmission direction of the ridge-shaped waveguide structure is along the y-direction of the crystal. The ridge-shaped waveguide structure is divided into front and rear parts. The front half of the ridge-shaped waveguide structure has a uniform width and penetrates the periodic phase grating structure. The rear half of the ridge-shaped waveguide structure is located outside the periodic phase grating structure and its width gradually narrows.

2. The deep ultraviolet optical frequency conversion element capable of adiabatic mode switching according to claim 1, characterized in that, The quartz crystal is a Z-cut quartz crystal.

3. The deep ultraviolet optical frequency conversion element capable of adiabatic mode switching according to claim 1, characterized in that, In the periodic phase grating structure, the width ratio of the modified region to the unmodified region within a single period is 1:

1.

4. A quartz crystal ultraviolet band frequency converter capable of adiabatic mode switching, characterized in that, The device includes an ultraviolet pulsed laser, an optical fiber coupled input end, a quartz crystal surface ridge waveguide structure element, an optical fiber coupled receiver end, and a filter arranged sequentially along the beam transmission direction. The quartz crystal surface ridge waveguide structure element is a deep ultraviolet optical frequency conversion element capable of adiabatic mode conversion as described in any one of claims 1-3.

5. A method for fabricating a deep ultraviolet optical frequency conversion element capable of adiabatic mode switching, characterized in that, Includes the following steps: S1. A femtosecond laser Bessel beam is translated along the x-axis of the quartz crystal to scan the quartz crystal and obtain a femtosecond laser irradiation area with modified refractive index. S2. The quartz crystal is translated several times along the y-direction by a fixed distance, and step S1 is repeated after each translation to obtain a periodic phase grating structure arranged along the y-direction of the quartz crystal. S3. Based on femtosecond laser scanning, the near-surface of the periodic phase grating structure region of the quartz crystal is scanned and ablated along the y-direction of the quartz crystal to prepare two parallel air slots that penetrate the periodic phase grating structure, thus obtaining the first half of a set of undetermined optical waveguides of the ridge-type optical waveguide structure; scanning is continued at the end of the air slots to prepare two air slots with gradually decreasing spacing, thus obtaining the second half of the undetermined optical waveguides; S4. Translate the quartz crystal along the x-direction of the quartz crystal, and repeat step S3 after each translation to obtain a ridge-type optical waveguide structure composed of multiple sets of undetermined optical waveguides; S5. In each group of undetermined optical waveguide regions, several refractive index-reducing modified grooves parallel to the corresponding air grooves are prepared based on femtosecond laser scanning to obtain the prepared optical waveguide regions.

6. The method for fabricating a deep ultraviolet optical frequency conversion element capable of adiabatic mode switching according to claim 5, characterized in that, Also includes: S6. Perform optical polishing on the two end faces perpendicular to the y-direction of the quartz crystal.

7. The method for fabricating a deep ultraviolet optical frequency conversion element capable of adiabatic mode switching according to claim 5, characterized in that, In step S1, the femtosecond laser Bessel beam travels along the quartz crystal during the scanning process. z The light is incident perpendicularly onto the quartz crystal.

8. The method for fabricating a deep ultraviolet optical frequency conversion element capable of adiabatic mode switching according to claim 5, characterized in that, In step S1, the femtosecond laser Bessel beam is obtained through the following steps: A femtosecond laser Gaussian beam is shaped into a femtosecond laser Bessel beam using a femtosecond laser shaping system based on a conical lens, and the femtosecond laser Bessel beam is then focused through a microscope objective to obtain the final femtosecond laser Bessel beam.

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