Nonlinear frequency multiplier and frequency multiplication method

Abstract

The non-linear frequency multiplier comprises a waveguide layer, a coupling hole is formed in the top of the waveguide layer, the cross section of the coupling hole is square, and the area, except for the coupling hole, of the outer surface of the waveguide layer is totally coated with a thick metal layer; a light emitting hole is formed in one end part of the waveguide layer; the surfaces of the coupling hole and the light emitting hole are covered with thin metal layers; the center position of the coupling hole and the center position of the light outlet hole are both located on a plane formed by the incident light and the reflected light. Meanwhile, the invention discloses a frequency multiplication method of the nonlinear frequency multiplier. According to the invention, interaction between light and substances in a nonlinear medium can be enhanced, a direction-independent automatic phase matching process is realized, high-efficiency frequency conversion within a wide pumping wavelength range covering a visible light area to an infrared area is realized, and the light field transmission efficiency and the nonlinear frequency doubling efficiency are effectively improved.

Description

Technical Field

[0001] The invention belongs to the technical field of nonlinear optics and photonics, and in particular relates to a nonlinear frequency multiplier and a frequency multiplication method. Background Art

[0002] Nonlinear optics is a branch of modern optics that studies the nonlinear phenomena and their applications produced by media under the action of strong coherent light. The study of nonlinear optics is of great significance to the development of laser technology, spectroscopy, and material structure analysis. In 1961, Peter Franken and others at the University of Michigan observed the generation of frequency doubling for the first time, and this achievement opened a new era of nonlinear optics research. In the following years, scientists have successively discovered a series of nonlinear optical phenomena such as sum frequency, difference frequency, optical parametric amplification, multiphoton absorption, self-focusing, stimulated Raman scattering and Brillouin scattering. These studies have gradually expanded the application range of nonlinear optics, from solids to gases, liquids, atomic vapors, liquid crystals and other media, and introduced higher-order nonlinear effects, extending from second-order effects to third-order, fifth-order and even higher-order responses.

[0003] With the progress of materials science and photonic technology, many new nonlinear optical materials have been developed and put into use, such as barium borate (BBO), potassium titanyl phosphate (KTP), lithium triborate (LBO), lithium niobate (LN) and potassium dihydrogen phosphate (KDP). These materials have become the core materials of nonlinear optical devices such as frequency doubling generators due to their excellent nonlinear properties. In laser technology, nonlinear optical frequency conversion can extend the wavelength of laser light sources to a wider range, which is suitable for laser display, precision measurement, biomedical imaging and other fields. In cutting-edge fields such as optical communication, quantum communication and quantum computing, nonlinear optics can not only amplify and modulate optical signals to improve system performance, but also can be used to generate entangled photon pairs to meet technical requirements such as quantum key distribution.

[0004] Frequency doubling technology is to increase the frequency of light waves to twice its original frequency, thereby achieving nonlinear optical effects. This technology is widely used in photonic devices and related equipment. Traditional frequency doubling methods have great limitations in structural design and material selection, and it is difficult to achieve efficient frequency conversion on a large scale. Materials such as lithium niobate (LN) have become ideal choices for frequency doubling applications due to their excellent electro-optical and nonlinear properties, but they have high requirements for fine processing and stability of the structure. At the same time, metal cladding structures can effectively enhance light field confinement and improve the transmission efficiency of light fields in waveguides, but they place higher requirements on overall design and material selection.

[0005] In optical frequency conversion, achieving phase matching is crucial to improving conversion efficiency and quality. The basic principle of phase matching technology is to ensure that the fundamental frequency light and the doubled frequency light have consistent phase velocities in the medium to satisfy the conservation of momentum and energy. Traditional phase matching technologies include birefringence phase matching (BPM) and quasi-phase matching (QPM). The birefringence phase matching relies on the natural birefringence characteristics of nonlinear crystals and adjusts the direction of light waves and the orientation of the medium to achieve phase matching effects. It is mainly used for frequency conversion of crystals such as BBO, KTP, and KDP. The quasi-phase matching is achieved by constructing a periodic structure (such as a periodically polarized crystal) in a nonlinear medium, which expands the applicable wavelength range and frequency conversion efficiency of nonlinear optical materials.

[0006] With the development of nanophotonics and metamaterial technology, a variety of new phase matching methods have emerged. Through mode coupling and local field enhancement in nanostructures, as well as the design of metamaterial structures, light waves achieve more ideal phase matching between different frequencies. The above innovative designs significantly improve the efficiency of nonlinear frequency conversion, providing more possibilities for the efficiency and applicability of optoelectronic systems. Summary of the invention

[0007] In order to solve the problems of low efficiency and poor structural stability existing in the existing frequency doubling technology, the purpose of the present invention is to provide a nonlinear frequency doubler and a frequency doubling method, which have a high quality factor and an effective refractive index close to zero, which helps to enhance the interaction between light and matter in nonlinear media; can form ultra-high-order guided modes, with the number of guided modes reaching thousands, so that in the process of nonlinear wavelength conversion, traditional phase matching is not required, and a direction-independent automatic phase matching process can be achieved; at the same time, the frequency doubling method of the above-mentioned nonlinear frequency doubler is disclosed, which realizes high-efficiency frequency conversion in a wide pump wavelength range covering from visible light to infrared regions, effectively improving the light field transmission efficiency and nonlinear frequency doubling efficiency.

[0008] In order to achieve the above-mentioned invention object, the present invention adopts the following technical scheme:

[0009] A nonlinear frequency multiplier comprises a waveguide layer, a coupling hole is provided on the top of the waveguide layer, the cross section of the coupling hole is square, and the outer surface of the waveguide layer except the coupling hole is completely covered with a thick metal layer; a light exit hole is provided at one end of the waveguide layer; the surfaces of the coupling hole and the light exit hole are both covered with a thin metal layer; the center position of the coupling hole and the center position of the light exit hole are both located in the plane formed by the incident light and the reflected light.

[0010] Furthermore, the thickness of the waveguide layer is 0.8-10 mm.

[0011] Furthermore, the thickness of the thick metal layer is 300-500 nanometers.

[0012] Furthermore, the thickness of the thin metal layer is 30-80 nanometers.

[0013] Furthermore, the depths of the light exit hole and the coupling hole are 220-280 nanometers.

[0014] Furthermore, the coupling hole is a square hole, and the ratio of the side length to the length of the waveguide layer is 0.5-1:10-30.

[0015] Furthermore, the light output hole is a rectangular hole, the ratio of the long side to the wide side is 0.5-6:0.5-1, and the ratio of the long side to the width of the waveguide layer is 0.5-6:10-15.

[0016] Furthermore, the ratio of the horizontal distance between the center of the coupling hole and the end face where the light exit hole is located to the length of the waveguide layer is 6-20:10-30.

[0017] Furthermore, the above-mentioned waveguide layer materials include but are not limited to lithium niobate, potassium niobate, sodium niobate, barium bismuthate, barium metaborate, and aluminum gallium arsenide.

[0018] Furthermore, the above-mentioned thick metal layer materials include but are not limited to gold, silver, aluminum, nickel, and chromium; the thin metal layer materials include but are not limited to gold, silver, aluminum, nickel, and chromium.

[0019] Furthermore, the thick metal layer is deposited on the outer surface of the waveguide layer by using a magnetron sputtering method.

[0020] Furthermore, the thin metal layer is formed by evaporation coating on the surface of the coupling hole of the waveguide layer.

[0021] A frequency doubling method for a nonlinear frequency multiplier comprises the following steps: a light beam emitted by a stable light source is shaped and spatially filtered by a pinhole and a polarizing plate, and then incident on a coupling hole of the nonlinear frequency multiplier; when the incident light beam meets the coupling condition, it enters a waveguide layer through a thin metal layer and excites the intrinsic mode of the waveguide.

[0022] Furthermore, the incident angle of the incident light beam is precisely controlled by a rotator to ensure that light beams at different angles can enter the nonlinear frequency multiplier and meet the coupling condition.

[0023] Due to the adoption of the above-mentioned technical solution, the present invention has the following advantages:

[0024] The nonlinear frequency doubler has a compact structure and high stability. The metal-fully-clad structural design improves the stability and durability of the waveguide structure and is suitable for long-term working environments. The coupling hole effectively improves the coupling efficiency of the incident light, with a coupling efficiency of 99%, significantly reducing light wave losses. By utilizing thousands of ultra-high-order guided modes and precisely controlling the waveguide geometry and material properties, automatic phase matching of fundamental frequency light and frequency doubled light is achieved without the need for external adjustments such as temperature or electric field in traditional methods, overcoming the reliance of existing phase matching technology on harsh external environmental conditions. The frequency doubler can achieve a frequency doubling efficiency of up to 2.6% under conditions of fundamental frequency light wavelengths of 1300 nanometers, 1064 nanometers and 860 nanometers. It can excite ultra-high-order standing wave fields at non-specific phase-matched incident angles, making it more widely applicable in high-efficiency frequency conversion applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is a main structural diagram of the nonlinear frequency multiplier of the present invention;

[0026] Figure 2 is a top view of the structure of the nonlinear frequency multiplier of the present invention;

[0027] Figure 3 is a side view structural diagram of the nonlinear frequency multiplier of the present invention;

[0028] Figure 4 1 is a schematic diagram of the coupling efficiency and quality factor variation of an embodiment of a nonlinear frequency multiplier of the present invention; wherein, Figure 4 a is the attenuated total reflection (ATR) coupling peak spectrum of the nonlinear frequency doubler; Figure 4 b is a schematic diagram of the change of the quality factor (Q value) of the nonlinear frequency multiplier;

[0029] Figure 5 It is an optical path diagram when the frequency doubling method of the nonlinear frequency multiplier of the present invention is implemented;

[0030] Figure 6 is a frequency doubled spectrum diagram generated when a nonlinear frequency doubler according to an embodiment of the present invention is implemented;

[0031] Figure 7 It is a schematic diagram of the change of the frequency-doubled signal of the nonlinear frequency multiplier of the present invention under different fundamental frequency optical powers and incident angles;

[0032] Figure 8 It is a schematic diagram of the double spectrum of the nonlinear frequency multiplier of the present invention at different fundamental frequency light wavelengths and the characteristic length change of different order guided modes; wherein, Figure 8 a is the frequency-doubled signal spectrum of different fundamental wavelengths; Figure 8 b is the characteristic length variation diagram of different order guided modes at different fundamental frequency wavelengths;

[0033] Fig. 9is a simplified structural diagram of an embodiment of a nonlinear frequency multiplier of the present invention;

[0034] Fig.10 is the theoretical ATR spectrum of the nonlinear frequency multiplier of the present invention;

[0035] Fig.11 It is a dispersion curve of TE / TM polarization and a characteristic length variation diagram of guided mode in the nonlinear frequency multiplier of the present invention;

[0036] In the figure: 1 (a, b) - thin metal layer; 2 - thick metal layer; 3 - waveguide layer; 4 - stable light source; 5 - small hole; 6 - polarizer; 7 - nonlinear frequency doubler; 8 - rotator. DETAILED DESCRIPTION

[0037] The technical solution of the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments.

[0038] like Figure 1 , 2 , 3, a nonlinear frequency doubler, which includes a waveguide layer 3, a coupling hole is provided on the top of the waveguide layer, the cross section of the coupling hole is square, and the area on the outer surface of the waveguide layer except the coupling hole is completely covered with a thick metal layer 2, the thick metal layer can effectively reflect the light field, enhance the constraint of the light field in the waveguide, and act as a protective layer to improve the stability of the waveguide. By controlling the thickness of the thick metal layer, the transmission characteristics of the light wave in the waveguide can be optimized; an output hole is provided at one end of the waveguide layer 3, the output hole is a frequency doubled light output port, which can ensure the stability of the incident light FMbeam and improve the extraction efficiency of the frequency doubled signal SH signal; the surfaces of the coupling hole and the output hole are respectively covered with thin metal layers 1a and thin metal layers 1b, and the thin metal layers are used to control the angle of the incident light and the coupling efficiency; the center position of the coupling hole and the center position of the output hole are both located in the plane formed by the incident light and the reflected light, so as to ensure the integrity of the transmission path and avoid beam deviation or mode mismatch.

[0039] The thickness of the waveguide layer is 0.8-10 mm; the depths of the light-emitting holes and the coupling holes are 220-280 nm; the ratio of the horizontal distance W between the center of the coupling hole and the end face where the light-emitting hole is located to the length of the waveguide layer is 6-20:10-30, so as to ensure that the positions of the two in the waveguide layer are accurately aligned; the coupling hole is a square hole, the ratio of the side length to the length of the waveguide layer is 0.5-1:10-30, the light-emitting hole is a rectangular hole, the ratio of the long side to the wide side is 0.5-6:0.5-1, and the ratio of the long side to the width of the waveguide layer is 0.5-6:10-15.

[0040] The thickness of the thick metal layer is 300-500 nanometers.

[0041] The thickness of the thin metal layer is 30-80 nanometers.

[0042] The waveguide layer materials mentioned above include but are not limited to lithium niobate (LN), potassium niobate (KTP), sodium niobate (KNbO3), barium bismuthate (BBO), barium metaborate (β-BBO), and aluminum gallium arsenide (AlGaAs).

[0043] The above-mentioned thick metal layer materials include but are not limited to gold, silver, aluminum, nickel, and chromium; the thin metal layer materials include but are not limited to gold, silver, aluminum, nickel, and chromium.

[0044] The thick metal layer is deposited on the outer surface of the waveguide layer by using a magnetron sputtering method.

[0045] The thin metal layer is formed by evaporation coating on the surface of the coupling hole of the waveguide layer.

[0046] The waveguide layer is precisely cut along the Z direction of its optical main axis to optimize the nonlinear frequency doubling response; the cross section of the waveguide layer is cut to ensure strict verticality to reduce the scattering of the emitted light; the surface of the waveguide layer is polished during the processing to reduce surface scattering and light loss; and the high parallelism control technology is used to ensure that the surfaces of the waveguide layer are parallel to the optical axis to further improve the optical performance and reduce beam deviation and power loss.

[0047] The following frequency multiplication test of the nonlinear frequency multiplier of the present invention is carried out by taking the waveguide layer made of lithium niobate (LN) and the thick metal layer and the thin metal layer made of silver as an example.

[0048] Use a single-frequency light source to test the frequency doubling effect, record the incident and output frequency doubling light power, and optimize the incident angle and intensity to achieve maximum frequency doubling efficiency.

[0049] Figure 4 a is the attenuated total reflection (ATR) coupling peak spectrum of the nonlinear frequency doubler, and the coupling efficiency is 99%. At a wavelength of 860 nanometers, the reflected light intensity of the nonlinear frequency doubler with a lithium niobate waveguide layer of 1 mm thickness is tested. The experimental results are shown in Figure 4 As shown in b, the quality factor (Q value) of the nonlinear frequency multiplier reaches 1.7×10 4 .

[0050] like Figure 5 As shown, the light beam emitted by the stable light source 4 is shaped and spatially filtered by the pinhole 6 and the polarizer 7, and then incident on the coupling hole of the nonlinear frequency doubler; when the incident light beam meets the coupling condition, it enters the waveguide layer through the thin metal layer and excites the intrinsic mode of the waveguide; in order to effectively excite the intrinsic mode, the incident angle is accurately controlled by the rotator 8 to ensure that light beams at different angles can enter the nonlinear frequency doubler, ensure the realization of the coupling condition, and thus excite the nonlinear effect of the waveguide layer.

[0051] Under coupling conditions, an incident light beam (fundamental frequency laser) with a wavelength of 1064 nanometers excites a guided mode in the waveguide layer and propagates along the waveguide with a propagation length of W millimeters. During the process, a frequency-doubled signal is generated; due to the limiting effect of the nonlinear frequency doubler on the light field, the generated frequency-doubled signal is coupled to the frequency-doubled eigenmode, achieving frequency-doubled mode enhancement under phase matching conditions, and output through the light-exit hole. Figure 6 The experimental setup and the generated 532 nm frequency-doubled optical signal are shown. Figure 6 In the experiment, FW laser is the laser, Spot is the pinhole, Attenuators is the attenuator, Polarizer is the polarizer, Chip is the waveguide, and spectrometer is the spectrometer. In the experiment, the rotator 8 is used to accurately adjust the incident angle of the light beam to ensure the realization of the coupling condition. By fine-tuning the rotator, the interaction between the fundamental frequency laser and the waveguide structure can be controlled, thereby optimizing the generation of the double frequency signal and the coupling efficiency.

[0052] The frequency doubling efficiency of the nonlinear frequency doubler of the present invention was tested by using a pulsed laser (TE polarization) with a wavelength of 1064 nanometers, a pulse width of 10 nanoseconds, and an energy of 50 microjoules as the fundamental frequency light source, and observing the frequency doubling light signal generated by the waveguide, and the nonlinear conversion efficiency reached 2.6%. Figure 7 a shows the variation trend of the SHG intensity with the increase of incident pulse power, and the threshold power of the conversion process is about 1 microjoule; Figure 7 b shows the change of the SHG intensity at different incident angles, indicating that the SHG efficiency is affected by both the coupling rate and phase matching.

[0053] The frequency doubling effect of the nonlinear frequency multiplier of the present invention at different fundamental wavelengths was tested. The operation was as follows: three lasers with different wavelengths, 1300 nanometers, 1064 nanometers and 860 nanometers, were used as light sources. The pulse width of each laser was 10 nanoseconds, the energy was stabilized at 50 microjoules, and all were TE polarization. The frequency doubling spectrum obtained in the experiment was as follows: Figure 8 As shown in a; Figure 8 b shows the calculation results of the characteristic lengths of guided modes of different orders (L=2 / Δk). The data show that the fundamental waves of the above-mentioned wavelengths can achieve effective phase matching in the nonlinear frequency doubler, verifying that the nonlinear frequency doubler has good adaptability to lasers of different wavelengths and can effectively support wide-band frequency doubling.

[0054] like Fig. 9As shown, the simplified structure of the nonlinear frequency multiplier of the present invention consists of a coupling layer, a waveguide layer and a substrate layer, wherein the coupling layer is a thin metal layer and the substrate layer is a thick metal layer; by using free space coupling technology, the incident light beam is coupled into the waveguide layer after being irradiated by the coupling layer, thereby exciting a specific waveguide mode. The material layers of the nonlinear frequency doubler are assumed to be: air, thin Ag coupling layer, LN guided layer and thick Ag base layer from top to bottom, with corresponding dielectric constants of ε4, ε3, ε2 and ε1, respectively. The thicknesses of the coupling layer (thin Ag layer) and the waveguide layer (LiN guided layer) are s and h, respectively. It is assumed that the air layer and the thick Ag layer are infinitely extended, and the width of the LiN guided layer significantly exceeds its thickness, so that the electric field in the nonlinear frequency doubler is limited only in the z-axis direction and decays exponentially in the base layer (thick metal layer) and the coupling layer (thin Ag layer). At the same time, it is assumed that the geometric structure and refractive index of the nonlinear frequency doubler remain unchanged in the y-axis direction. Therefore, under the condition of TE (transverse electric) polarization, the electric field with a propagation constant β in the nonlinear frequency doubler is expressed as

[0055]

[0056] Among them, A1, A2, A3, A4 are the amplitudes of the electric field intensity function of each layer of light in the waveguide (along the negative direction of the z axis), B2, B3, B4 are the amplitudes of the electric field intensity function of each layer of light in the waveguide (along the positive direction of the z axis),

[0057]

[0058] p1, k2, p3, k4 are the coefficients of the solutions of the Helmholtz equation in each layer of the waveguide, β is the waveguide propagation constant, and k0 is the wave vector of light propagating in a vacuum.

[0059] Substitute E y and Under the continuous conditions at z = 0, z = s and z = -h, the reflection coefficient r is expressed as

[0060]

[0061] in,

[0062]

[0063] φ 21 ,φk 23 ,φp 43 is the phase shift of light when it is reflected at each interface.

[0064] Fig.10 The theoretical ATR coupling peak R of the nonlinear frequency multiplier of the present invention is shown as R = r·r* It can be observed that when the coupling conditions are met, the coupling peak appears periodically in a discrete form; the coupling efficiency is close to 100%, which is extremely important for improving the nonlinear conversion efficiency.

[0065] Considering the eigenmode propagating in the waveguide layer (A4 = 0), the dispersion equation is expressed as

[0066] exp(2i(κ2h-φ 21 -φ 23 ))-1

[0067] =[exp(2i(κ2h-φ 21 ))-exp(-2iφ 23 ]·exp(-2iφ 43 )exp(-2p3s)

[0068] For the eigenmode, we can obtain exp(-2p3s)<<1, so the first-order small quantity of the propagation constant of the eigenmode caused by radiation loss is expressed as

[0069]

[0070] in,

[0071]

[0072] Therefore, the propagation constant of the nonlinear frequency multiplier of the present invention can be obtained:

[0073] × L =β 0 +Δβ L

[0074] Among them, β 0 is the propagation constant of a symmetrical planar waveguide without traditional losses, and the intrinsic loss Im(β 0 )=αβ0' is obtained by perturbation theory

[0075]

[0076] Among them, ε 2i is the imaginary part of ε2, θ ATR is the incident angle of the eigenmode; therefore, the propagation constant β of the symmetric slab waveguide L Expressed as

[0077] β L =[Re(β 0 )+Re(Δβ L )]+[Im(β 0 )+Im(Δβ L )]

[0078] For incident light from air, the propagation constant β = k0n4sin(θ) is a real number, that is,

[0079] β=[Re(β 0 )+Re(Δβ L )]

[0080] At this time, the reflection coefficient r is re-expressed as

[0081]

[0082] in,

[0083] r 43 =exp(-2iφ 43 )

[0084] Substituting into exp(-2p3s)<<1, we get

[0085]

[0086] In Re(r 43 )=Re(r 43 -1 ) and Im(r 43 )=-Im(r 43 -1 ) under the condition that

[0087]

[0088] Then the reflectivity R can be obtained

[0089]

[0090] When the intrinsic loss Im(β) is equal to the radiation loss Im(Δβ), the coupling efficiency can be close to 100%.

[0091] In addition, the half-width of the coupling peak is derived from the following formula:

[0092]

[0093] The quality factor of the nonlinear frequency multiplier of the present invention, which can be calculated to have a waveguide layer with a thickness of 1 mm, is about 6.62×10 5 , the effective refractive index N of the higher-order guided modes eff Close to 0. In the nonlinear frequency doubler with a 1 mm thick waveguide layer, the N eff It is 0.0403, therefore, the slow wave effect can be achieved and the interaction between the fundamental frequency light and the nonlinear material can be greatly enhanced.

[0094] Regarding the frequency doubling phase matching in the nonlinear frequency multiplier of the present invention, frequency doubling generation requires that the propagation constants of the fundamental frequency light and the frequency doubling light satisfy the phase matching condition:

[0095] β SH =2β FW

[0096] For frequency doubling, the energy of the generated frequency doublet is given by:

[0097] P SH =γsinc 2 (ΔkL / 2)

[0098] Where γ is the characteristic coefficient; Δk = k SH -2k FW is the phase mismatch; L is the propagation length of the wave; when the phase mismatch Δk is close to zero, the double frequency signal increases rapidly.

[0099] In the nonlinear frequency doubler of the present invention, phase matching conditions can be achieved thanks to the large number of eigenmodes it possesses. Compared with the traditional all-dielectric optical waveguide, the nonlinear frequency doubler of the present invention has many outstanding optical properties, among which the most notable property is the existence of its ultra-high-order guided modes. In this submillimeter nonlinear frequency doubler, the number of eigenmodes can reach 10 3 Up to 10 4 When the coupling conditions are met, the incident fundamental frequency beam will be coupled to the guide layer, exciting a series of guided modes. During the propagation process, the nonlinear effect of the LN layer (lithium niobate layer) will produce a frequency-doubled signal. Due to the characteristics of the waveguide, the propagation constant is β SH (wavelength is λ SH ) in the guided mode, the phase mismatch Δk is equal to β SH -2β FW ; For the characteristic mode of order m, the simplified dispersion equation is

[0100]

[0101] For TE polarization, η = 1; for TM polarization, η = ε2 / ε1; Fig.11 As shown in a, the dispersion curves of TE / TM polarization in the nonlinear frequency doubler are separated, which means that the doubled frequency signal can be coupled into the eigenmodes of TE and TM polarizations at the same time; Fig.11b shows the characteristic length of guided modes of different orders ΔL = 2 / Δk, where the maximum characteristic length can reach 18 cm, showing the potential for high conversion efficiency. For the fundamental frequency guided mode with a small characteristic length, as the propagation length increases, the intensity of the double frequency signal will oscillate like a sinc function, and the energy will be converted back to the fundamental frequency mode with different orders (including the mode with a large characteristic length), while the fundamental frequency mode with a large characteristic length will generate a double frequency signal. As a result, the energy will be reduced from the fundamental frequency mode with a short characteristic length and coupled to the fundamental frequency mode with a longer characteristic length, generating a double frequency signal during the propagation process.

[0102] The above description is only a preferred embodiment of the present invention, and is not intended to limit the present invention. Without departing from the spirit and scope of the present invention, all equivalent changes and modifications made within the scope of the patent application of the present invention shall fall within the scope of patent protection of the present invention.

Claims

1. A nonlinear frequency multiplier, characterized in that: It includes a waveguide layer, a coupling hole is opened on the top of the waveguide layer, the cross section of the coupling hole is square, and the outer surface of the waveguide layer except the coupling hole is completely covered with a thick metal layer; a light outlet hole is arranged at one end of the waveguide layer; the surfaces of the coupling hole and the light outlet hole are covered with a thin metal layer; the center position of the coupling hole and the center position of the light outlet hole are both located in the plane formed by the incident light and the reflected light.

2. The nonlinear frequency multiplier according to claim 1, characterized in that: Also includes any one or more of the following features: (1) The thickness of the waveguide layer is 0.8 to 10 mm; (2) The thickness of the thick metal layer is 300 to 500 nanometers; (3) The thickness of the thin metal layer is 30 to 80 nanometers; (4) The depth of the light output hole and the coupling hole is 220 to 280 nanometers.

3. The nonlinear frequency multiplier according to claim 1, characterized in that: Also includes any one or more of the following features: (1) The coupling hole is a square hole, and the ratio of the side length to the length of the waveguide layer is 0.5-1:10-30; (2) The light output hole is a rectangular hole, the ratio of the long side to the wide side is 0.5-6:0.5-1, and the ratio of the long side to the width of the waveguide layer is 0.5-6:10-15; (3) The ratio of the horizontal distance between the center of the coupling hole and the end face where the light output hole is located to the length of the waveguide layer is 6-20:10-30.

4. The nonlinear frequency multiplier according to claim 1, characterized in that: The waveguide layer material includes but is not limited to lithium niobate, potassium niobate, sodium niobate, barium bismuthate, barium metaborate, and aluminum gallium arsenide.

5. The nonlinear frequency multiplier according to claim 1, characterized in that: The thick metal layer material includes but is not limited to gold, silver, aluminum, nickel, and chromium; the thin metal layer material includes but is not limited to gold, silver, aluminum, nickel, and chromium.

6. The nonlinear frequency multiplier according to claim 1 or 5, characterized in that: The thick metal layer is deposited on the outer surface of the waveguide layer by using a magnetron sputtering method.

7. The nonlinear frequency multiplier according to claim 1 or 5, characterized in that: The thin metal layer is formed by evaporation coating on the surface of the coupling hole of the waveguide layer.

8. A frequency doubling method for a nonlinear frequency multiplier as claimed in any one of claims 1 to 7, characterized in that the steps are as follows: a light beam emitted by a stable light source is shaped and spatially filtered by a pinhole and a polarizer, and then incident on a coupling hole of the nonlinear frequency multiplier; when the incident light beam meets the coupling condition, it enters the waveguide layer through a thin metal layer and excites the intrinsic mode of the waveguide.

9. The frequency multiplication method of a nonlinear frequency multiplier according to claim 8, characterized in that: The incident angle of the incident light beam is precisely controlled by a rotator to ensure that light beams at different angles can enter the nonlinear frequency multiplier and meet the coupling conditions.

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