Broadband nonlinear frequency converter and preparation method

Abstract

The invention discloses a broadband nonlinear frequency converter which comprises a silicon dioxide coating layer, an x-cut lithium niobate film layer, a silicon dioxide buffer layer and a silicon substrate layer from top to bottom, the x-cut lithium niobate film layer is provided with a weak bending ridge waveguide, and the bending angle of the weak bending ridge waveguide is not larger than 5 degrees. According to the invention, birefringence phase matching is realized on the lithium niobate film in a weak bending waveguide form, so that broadband second harmonic generation, birefringence phase matching wavelength and matching angle correlation are realized, and an included angle between a wave vector of transmitted fundamental frequency light and a crystal optical axis is continuously and slightly changed through the weak bending film, so that the broadband second harmonic generation is realized. Therefore, birefringence phase matching of different wavelengths can be realized at different matching angles, and the purpose of broadband matching is achieved.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of nonlinear optics and optical communications, and in particular to a broadband nonlinear frequency converter and a preparation method. BACKGROUND

[0002] Due to the working wavelength band and bandwidth limitations of the gain medium, the laser can directly output laser wavelengths are very limited. Nonlinear optical frequency conversion is one of the important means to realize special laser wavelengths, and thus has very important scientific significance and application value. At present, as an important physical mechanism for realizing wavelength conversion, the second-order nonlinear optical process (such as second-harmonic generation, SHG) has been widely used in optical communication, infrared photon generation, laser radar, spectral detection, and entangled photon pair source. In high-end photonics applications such as ultrafast pulsed laser, supercontinuum generation, and optical frequency comb, broadband and efficient frequency conversion technology is irreplaceable for realizing all-optical signal processing, spectral coverage expansion, and precise measurement.

[0003] Lithium niobate thin film, or Lithium Niobate on Insulator (LNOI), has a large second-order nonlinear coefficient, a wide optical transparent window, excellent electro-optic effect, and a high refractive index contrast. It can realize strong light field confinement in sub-micron scale waveguides, thereby significantly improving the efficiency of nonlinear interaction. With these advantages, lithium niobate thin film has become one of the ideal material platforms for realizing on-chip integrated optical frequency conversion in recent years.

[0004] In order to realize efficient second-order nonlinear process, in addition to energy conservation, a good phase matching condition (i.e. momentum conservation) must be met. At present, the common phase matching methods in lithium niobate thin film waveguides include birefringent phase matching (BPM), mode phase matching (MPM), and quasi-phase matching (QPM). Among them, the quasi-phase matching technology can achieve phase compensation at a specific wavelength by introducing a periodic polarization structure in the lithium niobate thin film waveguide, and use the maximum nonlinear coefficient of lithium niobate material to obtain the highest normalized second-harmonic conversion efficiency. In order to realize broadband quasi-phase matching, a chirped polarization structure is generally used, that is, a gradually changing polarization period is used to achieve quasi-phase matching at different wavelengths. However, quasi-phase matching has very high requirements for polarization period, duty cycle and domain inversion quality. The preparation process requires high-voltage pulse polarization and precise lithography and etching steps, which is complex and has limited yield, and significantly increases the cost and process difficulty in waveguide manufacturing, and the corresponding device cost is also high.

[0005] In contrast, birefringent phase matching and mode phase matching utilize the dispersion effect between materials or modes, respectively, by making the effective refractive indices of the fundamental light and the frequency-doubled light equal, but due to the inability to utilize the maximum nonlinear coefficient of lithium niobate material, the theoretical peak conversion efficiency is lower than quasi-phase matching. The advantage is that the structure design is relatively simple, without the need for periodic polarization treatment, the manufacturing process is mature and highly repeatable, the corresponding device cost is low, and it is easy to achieve phase matching through temperature tuning or waveguide geometry adjustment, so it is more practical and scalable in some broadband frequency conversion application scenarios.

[0006] However, under the existing technical conditions, there is a clear "bandwidth-efficiency" constraint relationship between the waveguide length and the dispersion characteristics. For lithium niobate waveguides with a length of centimeters, due to the combined limitations of material dispersion and waveguide dispersion, the common phase matching bandwidth in the optical communication band is usually about 1 nm, which is difficult to meet the application requirements of broadband frequency conversion. In order to solve this problem, researchers have proposed a variety of bandwidth expansion schemes, such as introducing a temperature gradient in the waveguide, optimizing the waveguide cross-section through dispersion engineering, using a chirped poling structure, using multi-mode mixing or coupled waveguides to achieve phase compensation, etc. These schemes have expanded the bandwidth to some extent, but often require higher process precision, more complex device structure design, or additional external control means, inevitably increasing the manufacturing cost, reducing the processing tolerance, and posing challenges to mass production.

[0007] Researchers in the field have been committed to developing broadband, high-efficiency, and low-cost broadband nonlinear frequency conversion schemes. SUMMARY

[0008] In view of the above-mentioned defects of the prior art, the technical problem to be solved by the present application is that the existing second harmonic (or frequency-doubled) process in the waveguide on the lithium niobate thin film requires a wideband phase matching method that is difficult to implement and costly, and generally requires chirped poling.

[0009] To achieve the above-mentioned purpose, the present application provides a broadband nonlinear frequency converter, which comprises, from top to bottom, a silicon dioxide coating layer, an x-cut lithium niobate thin film layer, a silicon dioxide buffer layer, and a silicon substrate layer. The x-cut lithium niobate thin film layer has a weak curved ridge waveguide. The curved angle of the weak curved ridge waveguide is not greater than 5 degrees.

[0010] Further, the shape of the weak curved ridge waveguide is arc-shaped.

[0011] Further, the shape of the weak curved ridge waveguide is a circular arc.

[0012] Furthermore, the angle between the transmission direction of the weakly bent waveguide and the optical axis of the lithium niobate crystal gradually changes from 54° to 56.5°.

[0013] Furthermore, the length of the weak-bend waveguide is no more than 2 cm, and the curvature radius of each position of the weak-bend waveguide is no less than 45 cm.

[0014] Furthermore, the thickness of the weakly bent ridge waveguide is 3 μm, the etching depth is 2 μm, and the etching angle is 60°.

[0015] Furthermore, the thickness of the silicon dioxide coating layer is 0.3 μm.

[0016] Furthermore, the thickness of the silicon dioxide buffer layer is 2-5 μm.

[0017] Furthermore, the upper width of the weakly curved ridge waveguide is 3-6 μm.

[0018] The present invention also provides a preparation method for preparing the broadband nonlinear frequency converter, comprising the following steps: Step 1: A layer of photoresist suitable for UV exposure is evenly spin-coated on the surface of a 5μm thick x-cut lithium niobate film. The x-cut lithium niobate film coated with photoresist is placed in a UV exposure system and exposed with high precision according to the pre-designed waveguide structure layout, thereby defining the mask pattern required for etching in the photoresist layer. Step 2: Place the exposed x-cut lithium niobate film in a developer for development treatment to dissolve and remove the photoresist in the exposed area, thereby forming a design on the photoresist surface. Figure One After developing the mask pattern structure, the x-cut lithium niobate film is rinsed with deionized water and blown dry; Step 3: Place the x-cut lithium niobate film with the mask pattern structure into an inductively coupled plasma reactive ion etching (ICP-RIE) system. Selecting appropriate process parameters such as gas ratio, RF power, and chamber pressure, the pattern on the photoresist is precisely transferred to the x-cut lithium niobate film layer in a highly anisotropic manner, thereby achieving the etching process of the waveguide structure. Step 4: After etching, the x-cut lithium niobate film is ultrasonically cleaned with an acetone solution to completely remove the residual photoresist layer on the surface. The film is then rinsed with isopropyl alcohol and deionized water in sequence and dried under a nitrogen atmosphere to obtain an x-cut lithium niobate film layer with a ridge waveguide. Step 5: Place the x-cut lithium niobate thin film layer with a ridge waveguide in a plasma-enhanced chemical vapor deposition (PECVD) device, using silicon dioxide as the deposition material to uniformly deposit a silicon dioxide coating layer with a thickness of 0.3 μm on the surface of the waveguide.

[0019] The preparation of the silicon dioxide buffer layer and the silicon substrate layer can be completed by using the existing conventional manufacturing process.

[0020] Technical effects

[0021] The present application realizes birefringent phase matching through the form of weakly curved waveguide on the lithium niobate thin film, thereby realizing broadband second harmonic generation. The birefringent phase matching wavelength and the matching angle are related. The present application realizes continuous and weak change of the included angle between the wave vector of the transmission fundamental light and the optical axis of the crystal through the weakly curved thin film, thereby realizing birefringent phase matching of different wavelengths at different matching angles and achieving the purpose of broadband matching. The present application realizes broadband refractive index phase matching in the weakly curved ridge waveguide on the micron-level lithium niobate thin film platform through simple and ingenious improvement. Since the waveguide is slightly curved, high-order mode excitation will not be caused.

[0022] In view of the low coupling efficiency in the thin film lithium niobate nanometer waveguide, the micron-level lithium niobate waveguide used by the present application solves the coupling problem between the optical fiber and the waveguide. In view of the broadband frequency doubling application requirement, the general commonly used method is to realize it through precise dispersion design or chiral polarization process, which greatly increases the preparation cost. The present application realizes it by using the weakly curved waveguide structure, without electrode polarization, and without changing the ultraviolet lithography and etching process, and the technical scheme has strong practicability. The target is realized at the minimum cost. The micron waveguide has better comprehensive performance than the nanometer waveguide. The present application has very direct application in pulsed laser, especially femtosecond pulse frequency conversion.

[0023] The concept, specific structure and technical effects of the present application will be further described below with reference to the accompanying drawings, so as to fully understand the purpose, features and effects of the present application. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 is a broadband nonlinear frequency converter structure diagram of the present application; Figure 2 is a phase matching angle change relation graph with wavelength under different waveguide wide conditions of the present application; Figure 3 is a theoretical result curve graph of normalized frequency doubling conversion efficiency under different wavelengths of the present application. DETAILED DESCRIPTION

[0025] The following reference description drawings introduce the plurality of preferred embodiments of the present application, so that the technical content thereof is more clear and convenient to understand. The present application can be embodied by many different forms of embodiments, and the protection scope of the present application is not limited to the embodiments mentioned in the text.

[0026] In the drawings, components of the same structure are denoted by the same reference numerals, and components similar in structure or function are denoted by similar reference numerals. The size and thickness of each component shown in the drawings are arbitrarily shown, and the size and thickness of each component are not limited in the present application. In order to make the drawing clearer, the thickness of some components is appropriately exaggerated in some places in the drawing.

[0027] As shown in Figure 1 The wideband nonlinear frequency converter based on the weakly curved waveguide of the x-cut lithium niobate thin film of the present application comprises a composite structure of an upper silicon dioxide coating layer, a middle lithium niobate crystal thin film waveguide layer, a middle lower silicon dioxide buffer layer, and a lower silicon substrate layer. The transmission direction of the ridge waveguide has a certain angle with the optical axis of the nonlinear crystal.

[0028] In the preferred embodiment of the present application, the upper width of the lithium niobate thin film ridge waveguide is 4.5 μm.

[0029] The present application also provides a preparation method of the wideband nonlinear frequency converter, which comprises the following steps: Step one: uniformly spin-coat a layer of positive photoresist suitable for ultraviolet exposure on the surface of a 3 μm thick lithium niobate thin film. Then, place the sample coated with the photoresist in an ultraviolet exposure system, and perform high-precision exposure according to the pre-designed waveguide structure layout, so as to define the mask pattern required for etching in the photoresist layer.

[0030] Step two: place the exposed sample in a developing solution for developing treatment, so that the photoresist in the exposed area is dissolved and removed, thereby forming a mask pattern structure on the surface of the photoresist corresponding to the design. After development, the sample is rinsed with deionized water and blown dry to avoid pattern deformation. Figure One

[0031] Step three: place the sample with the mask pattern into an inductively coupled plasma reactive ion etching (ICP-RIE) system, select appropriate gas ratio, radio frequency power, and cavity pressure and other process parameters, and accurately transfer the pattern on the photoresist to the lithium niobate thin film layer in a highly anisotropic manner, so as to realize etching processing of the waveguide structure.

[0032] Step four: after etching is completed, ultrasonic cleaning is performed on the sample with acetone solution to completely remove the residual photoresist layer on the surface, and then isopropanol and deionized water are used for rinsing in sequence, and the sample is blown dry in a nitrogen environment, thereby obtaining an etched waveguide sample with a clean surface and a complete structure.

[0033] Step five: place the cleaned sample in a plasma enhanced chemical vapor deposition (PECVD) device, and uniformly deposit a 0.3 μm thick silicon dioxide protective layer on the surface of the waveguide using silicon dioxide as the deposition material.

[0034] ​The preparation of the silicon dioxide buffer layer and the silicon substrate layer can be completed by using conventional manufacturing processes.

[0035] As shown in Figure 2 According to the calculation method of the frequency-doubling birefringence phase matching angle (wherein , , are the effective refractive indexes of the fundamental light and the frequency-doubled light in the waveguide, respectively), the mode effective refractive indexes under different wavelengths are calculated.

[0036] As shown in Figure 3 The normalized frequency-doubling conversion efficiency under different wavelengths is simulated and calculated, and the effectiveness of the broadband birefringence phase matching mechanism realized by introducing the gradually-changing included angle design into the x-cut lithium niobate thin film waveguide is verified. In the vicinity of the 1550 nm central wavelength, the normalized frequency-doubling conversion efficiency is maintained at a high level, and the entire frequency conversion bandwidth exceeds 100 nm, which is significantly superior to the conventional straight waveguide structure. The results show that the proposed weakly-curved waveguide structure can realize high-bandwidth frequency conversion effect without the need for periodic polarization.

[0037] In the proposed scheme, the medium used is not limited to lithium niobate thin film, but can also be other nonlinear crystal thin films; the frequency conversion is not limited to the generation of second harmonic, but can also be other second-order nonlinear effects, such as sum frequency, difference frequency, optical parametric amplification, etc.; and the coupling mode is not limited to fiber end face coupling, but can also be spatial light coupling to the waveguide through an objective lens.

[0038] The above describes in detail the preferred embodiments of the present application. It should be understood that those skilled in the art can make many modifications and changes to the present application without creative labor based on the concept of the present application. Therefore, any technical solution obtained by logical analysis, reasoning or limited experiments based on the existing technology according to the concept of the present application shall be within the protection scope defined by the claims.

Claims

1. A broadband nonlinear frequency converter, characterized in that: From top to bottom, it includes a silicon dioxide coating layer, an x-cut lithium niobate film layer, a silicon dioxide buffer layer, and a silicon substrate layer. The x-cut lithium niobate film layer has a weakly curved ridge waveguide, and the bending angle of the weakly curved ridge waveguide is not greater than 5 degrees.

2. The broadband nonlinear frequency converter according to claim 1, wherein: The weakly bent ridge waveguide has an arc shape.

3. The broadband nonlinear frequency converter according to claim 2, wherein: The weakly curved ridge waveguide has an arc shape.

4. The broadband nonlinear frequency converter according to any one of claims 1 to 3, wherein: The angle between the transmission direction of the weak bending waveguide and the optical axis of the lithium niobate crystal gradually changes from 54° to 56.5°.

5. The broadband nonlinear frequency converter according to any one of claims 1 to 3, wherein: The length of the weak bending waveguide is no more than 2 cm, and the curvature radius of each position of the weak bending waveguide is no less than 45 cm.

6. The broadband nonlinear frequency converter according to claim 1, wherein: The thickness of the weakly bent ridge waveguide is 3 μm, the etching depth is 2 μm, and the etching angle is 60°.

7. The broadband nonlinear frequency converter according to claim 1, wherein: The thickness of the silicon dioxide coating layer is 0.3 μm.

8. The broadband nonlinear frequency converter according to claim 1, wherein: The thickness of the silicon dioxide buffer layer is 2-5 μm.

9. The broadband nonlinear frequency converter according to claim 1, wherein: The upper width of the weakly curved ridge waveguide is 3-6 μm.

10. A preparation method, characterized in that: The preparation method is used to prepare the broadband nonlinear frequency converter according to claim 1, comprising the following steps: Step 1: A layer of photoresist suitable for UV exposure is evenly spin-coated on the surface of a 5μm thick x-cut lithium niobate film. The x-cut lithium niobate film coated with photoresist is placed in a UV exposure system and exposed with high precision according to the pre-designed waveguide structure layout, thereby defining the mask pattern required for etching in the photoresist layer. Step 2: Placing the exposed x-cut lithium niobate film in a developer for development treatment to dissolve and remove the photoresist in the exposed area, thereby forming a mask pattern structure consistent with the design drawing on the photoresist surface. After development, the x-cut lithium niobate film is rinsed with deionized water and blown dry. Step 3: Placing the x-cut lithium niobate film with the mask pattern structure into an inductively coupled plasma reactive ion etching system, selecting appropriate process parameters such as gas ratio, RF power, and chamber pressure, and accurately transferring the pattern on the photoresist to the x-cut lithium niobate film layer in a highly anisotropic manner; Step 4: Ultrasonic cleaning of the x-cut lithium niobate film with an acetone solution to completely remove any residual photoresist on the surface. The film is then rinsed with isopropyl alcohol and deionized water, followed by drying under nitrogen to obtain an x-cut lithium niobate film layer with a ridge waveguide. Step 5: Place the x-cut lithium niobate thin film layer with a ridge waveguide in a plasma enhanced chemical vapor deposition device, use silicon dioxide as the deposition material, and evenly deposit a silicon dioxide coating layer with a thickness of 0.3 μm on the surface of the waveguide.

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