A third harmonic generation system and method

Abstract

The embodiment of the present invention discloses a third harmonic generation system and method. It includes a wavelength tunable light source, a polarization controller, an optical fiber and an optical microcavity; the wavelength tunable light source provides pump light, and the pump light is coupled into the optical fiber; the optical fiber extends from the output end of the polarization controller to the optical microcavity, and the optical fiber extending to the optical microcavity is coupled with the optical microcavity through a tapered structure; the optical microcavity includes a substrate, a support column and a microdisk cavity; the pump light is coupled into the optical microcavity through a tapered structure; the polarization controller adjusts the polarization state of the pump light in the optical fiber; the wavelength, power, polarization state of the pump light and the distance between the tapered structure and the optical microcavity are adjusted so that the pump light satisfies the phase matching condition of the third harmonic when transmitted in the optical microcavity, thereby generating the third harmonic. The technical solution of the embodiment of the present invention, after obtaining a high-quality factor optical microdisk by dry etching, uses continuous light to directly pump the optical microdisk to generate the third harmonic, thereby simplifying the complex process of generating the third harmonic.

Description

Technical Field

[0001] The embodiments of the present invention relate to laser technology, and more particularly to a third harmonic generation system and method. Background Art

[0002] Whispering gallery mode optical microcavity has an ultra-high quality factor and a small volume mode, which can greatly enhance the interaction between light and matter and is an ideal platform for studying nonlinear frequency conversion. Third harmonic generation is a typical third-order nonlinear effect, and the frequency of the generated light is three times the frequency of the incident pump light. The third harmonic can directly establish a connection between the near-infrared communication band and the visible light band, so it is widely used to expand the emission wavelength of laser light sources.

[0003] Silicon oxide is considered a good carrier for third harmonic generation due to its durability and low optical loss in a wide range of visible spectrum in the communication band. However, in the existing on-chip integrated silicon oxide microcavities, there are problems of low third harmonic conversion efficiency and high threshold. Among them, the types of silicon oxide microcavities can be divided into micro-ring core cavities, micro-sphere cavities and micro-disk cavities. The preparation of micro-ring core cavities and micro-sphere cavities requires the introduction of laser reflux, and their size cannot be controlled, which is not conducive to dispersion control and cannot achieve accurate phase matching, resulting in low conversion efficiency. The current quality factor of the micro-disk cavity is not high and the threshold is high. It is difficult to accurately control the size of the micro-disk cavity using a wet etching process. Summary of the invention

[0004] The embodiments of the present invention provide a third harmonic generation system and method, so as to generate the third harmonic without adding additional modulation devices, thereby simplifying the process flow of generating the third harmonic.

[0005] In a first aspect, an embodiment of the present invention provides a third harmonic generation system, including a wavelength tunable light source, a polarization controller, an optical fiber, and an optical microcavity;

[0006] The wavelength tunable light source is used to provide pump light, and the pump light is coupled into the optical fiber;

[0007] The optical fiber is connected to the input end of the polarization controller;

[0008] The optical fiber extends from the output end of the polarization controller to the optical microcavity, the optical fiber extending to the optical microcavity comprises a tapered structure, and the optical fiber is coupled to the optical microcavity through the tapered structure;

[0009] Wherein, the optical microcavity comprises a substrate and a support column and a microdisk cavity located on one side of the substrate;

[0010] The pump light is coupled into the optical microcavity through the tapered structure;

[0011] The polarization controller is used to adjust the polarization state of the pump light in the optical fiber;

[0012] The wavelength, power, polarization state of the pump light and the distance between the tapered structure and the optical microcavity are adjusted so that the pump light satisfies the phase matching condition of the third harmonic when transmitting in the optical microcavity, thereby generating the third harmonic.

[0013] Optionally, the micro-disk cavity is in the shape of a truncated cone;

[0014] The included angle between the generatrix of the truncated cone and the bottom surface of the truncated cone is greater than or equal to 30° and less than or equal to 60°.

[0015] Optionally, the third harmonic generation system further includes a first coupler, a photodetector, an oscilloscope, a first spectrometer and a second spectrometer;

[0016] The optical fiber extending from the optical microcavity is connected to the input end of the first coupler, the first output end of the first coupler is connected to the photodetector, the photodetector is connected to the oscilloscope, the second output end of the first coupler is connected to the first spectrometer, and the third output end of the first coupler is connected to the second spectrometer;

[0017] The oscilloscope is used to output the time domain waveform detected by the photodetector, the first spectrometer is used to measure the spectrum of the third harmonic, and the second spectrometer is used to measure the spectrum of the pump light.

[0018] Optionally, the third harmonic generation system further includes an optical amplifier disposed on the optical path between the wavelength tunable light source and the polarization controller, for amplifying the pump light.

[0019] Optionally, the optical amplifier is a semiconductor optical amplifier;

[0020] The third harmonic generation system also includes a first collimator, an optical isolator and a second collimator;

[0021] The first collimator, the semiconductor optical amplifier, the optical isolator and the second collimator are arranged in sequence along the optical path between the wavelength tunable light source and the polarization controller;

[0022] The input end of the first collimator is coupled to the output end of the wavelength tunable light source, and is used to collimate the pump light and then input it into the semiconductor optical amplifier;

[0023] The semiconductor optical amplifier is used to amplify the pump light;

[0024] The optical isolator is used to enable the amplified pump light to be transmitted unidirectionally;

[0025] The output end of the second collimator is connected to the optical fiber connected to the input end of the polarization controller, and the second collimator is used to couple the amplified pump light into the optical fiber.

[0026] Optionally, the optical amplifier is a fiber amplifier;

[0027] The wavelength tunable light source is connected to the optical fiber amplifier via the optical fiber;

[0028] The optical fiber amplifier is connected to the polarization controller via the optical fiber.

[0029] Optionally, the optical system further comprises a second coupler and a power meter;

[0030] The input end of the second coupler is connected to the output end of the polarization controller through the optical fiber, the first output end of the second coupler is connected to the power meter, and the optical fiber extends to the optical microcavity through the second output end of the second coupler.

[0031] Optionally, the optical system further comprises an attenuator, the input end of the attenuator is connected to the output end of the polarization controller through the optical fiber, and the output end of the attenuator extends to the optical microcavity through the optical fiber.

[0032] Optionally, the substrate material of the optical microcavity includes silicon, and the material of the microdisk cavity includes silicon dioxide;

[0033] The optical microcavity is prepared by using a dry etching process.

[0034] In a second aspect, an embodiment of the present invention further provides a third harmonic generation method, which uses the third harmonic generation system to output a third harmonic, comprising:

[0035] The wavelength-tunable light source outputs pump light, and the pump light is coupled into the optical microcavity through the optical fiber;

[0036] The wavelength, power, polarization state of the pump light and the distance between the tapered structure and the optical microcavity are adjusted so that the pump light satisfies the phase matching condition of the third harmonic when transmitting in the optical microcavity to generate the third harmonic.

[0037] The third harmonic generation system provided by the embodiment of the present invention includes a wavelength tunable light source, a polarization controller, an optical fiber and an optical microcavity; the wavelength tunable light source can provide pump light, and the pump light is coupled into the optical fiber; the optical fiber passes through the polarization controller and then passes through the optical microcavity, wherein the optical microcavity includes a substrate, a support column and a microdisk cavity, the optical fiber includes a tapered structure, the pump light and the tapered structure are coupled into the optical microcavity, and the polarization controller can adjust the polarization state of the pump light in the optical fiber. By applying a 2Vpp, 10Hz electrical signal to the laser, a continuous wavelength scanning signal of the wavelength tunable light source in a small range near the central wavelength is obtained, thereby realizing scanning from short wave to long wave, changing the central wavelength in the wavelength tunable light source, and further obtaining a high quality factor mode; then adjusting the wavelength, power, polarization state of the pump light and the distance between the tapered structure and the optical microcavity, so that the pump light meets the conditions of high quality factor and strong matching degree of the pump mode and the third harmonic mode during the transmission of the optical microcavity, thereby generating the third harmonic in the third harmonic system. The third harmonic generation system provided in this embodiment obtains a high-quality factor optical micro-disk by dry etching, and then uses continuous light in the infrared band to directly pump the optical micro-disk to generate the third harmonic. The third harmonic can be generated without the need for additional modulation devices, which simplifies the complex process of generating the third harmonic and reduces the theoretical threshold of the third harmonic and the cost of generating the third harmonic. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 A schematic diagram of the structure of a third harmonic generation system provided by an embodiment of the present invention;

[0039] Figure 2 A schematic diagram of the structure of an optical microcavity provided by an embodiment of the present invention;

[0040] Figure 3 A schematic diagram of the structure of another third harmonic generation system provided by an embodiment of the present invention;

[0041] Figure 4 A schematic diagram of the structure of another third harmonic generation system provided by an embodiment of the present invention;

[0042] Figure 5 A schematic diagram of the structure of another third harmonic generation system provided by an embodiment of the present invention;

[0043] Figure 6 A schematic diagram of the structure of another third harmonic generation system provided by an embodiment of the present invention;

[0044] Figure 7 A schematic diagram of the structure of another third harmonic generation system provided by an embodiment of the present invention;

[0045] Figure 8A schematic diagram of the structure of another third harmonic generation system provided by an embodiment of the present invention;

[0046] Fig. 9 A schematic diagram of the quality factor of an optical microcavity provided by an embodiment of the present invention;

[0047] Fig.10 A structural schematic diagram of another third harmonic generation system is provided for an embodiment of the present invention;

[0048] Fig.11 A schematic diagram of a flow chart of a third harmonic generation method provided by an embodiment of the present invention;

[0049] Fig.12 Schematic diagram of the spectrum of pump light and third harmonic in an embodiment of the present invention;

[0050] Fig.13 An optical micrograph of a microdisk cavity when generating third harmonic provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0051] The present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. It is to be understood that the specific embodiments described herein are only used to explain the present invention, rather than to limit the present invention. It should also be noted that, for ease of description, only parts related to the present invention, rather than all structures, are shown in the accompanying drawings.

[0052] The terms used in the embodiments of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention. It should be noted that the directional words such as "upper", "lower", "left", "right" and the like described in the embodiments of the present invention are described at the angles shown in the accompanying drawings and should not be understood as limitations on the embodiments of the present invention. In addition, in the context, it is also necessary to understand that when it is mentioned that an element is formed "on" or "under" another element, it can not only be directly formed "on" or "under" another element, but also indirectly formed "on" or "under" another element through an intermediate element. The terms "first", "second" and the like are only used for descriptive purposes and do not indicate any order, quantity or importance, but are only used to distinguish different components. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to the specific circumstances.

[0053] Efficient generation of third harmonics in an optical microcavity requires phase matching conditions, which can be further expressed as momentum conservation and energy conservation. The condition for momentum conservation is to find a mode with a wave number k TH is the pump mode wave number k p One third of k TH =3k p, but in the case of momentum conservation, because there is usually material dispersion in the optical microcavity, if only the fundamental mode is considered, the energy conservation condition is difficult to meet, that is, ω TH =3ω p , where ω TH and ω p Represent the angular frequencies of the third harmonic and pump light, respectively. In order to meet the energy conservation condition, the material dispersion needs to be compensated by the cavity mode dispersion. Therefore, the higher-order modes of the third harmonic are usually used to achieve phase matching with the pump light mode. Seeking optical microcavities with high quality factors and small volume modes can effectively reduce the threshold of third harmonic generation and achieve on-chip integration.

[0054] In the existing technology, the end of the semi-tapered optical fiber is melted by arc discharge to produce a silica microsphere cavity with a diameter of 57±1μm and a quality factor of about 107. Three different wavelength third harmonic signals are obtained in different microspheres by changing the pump wavelength. However, it is difficult to accurately control the dispersion of the microsphere cavity, and the fiber-based manufacturing process cannot achieve on-chip integration.

[0055] In another prior art, a micro-ring core cavity is made by laser reflow method, so that the 1553.9nm pump light generates a 517.4nm third harmonic signal in the micro-ring core cavity. By simultaneously inputting the 1553nm and 1674nm pump lights into the micro-ring core cavity, a 542nm third-order sum frequency signal is generated. However, the micro-ring core cavity also has the problem of being unable to accurately control dispersion, and requires an additional laser reflow process, which increases the complexity of the process.

[0056] In another prior art, a micro-disk cavity (quality factor of about 10) is prepared by a wet etching process. 6 ), the continuous light output by the infrared tunable laser is modulated into a pulse light with a low duty cycle and a pulse width of 20ns, and then the micro-disk cavity is pumped after amplification. Different third harmonic signals with wavelengths ranging from 512nm to 520nm are obtained (the interval between different signals is about 2.5nm). However, the quality factor of the micro-disk cavity using wet etching is low and it is difficult to accurately control the sample size. The introduction of additional modulation devices also greatly increases the complexity of the system, which is not conducive to on-chip integration and increases costs.

[0057] In order to solve the shortcomings of the third harmonic generation method based on silicon oxide microcavity, an embodiment of the present invention provides a third harmonic generation system. The silicon oxide micro-disk cavity prepared by dry etching process overcomes the above problems. First, on-chip integration is achieved without the need for laser reflow and other steps to change the size of the microcavity. Compared with the wet etching process, the quality factor is improved on the basis of small size, and the theoretical threshold of the third harmonic is reduced.

[0058] Figure 1FIG. 1 is a schematic diagram of the structure of a third harmonic generation system provided by an embodiment of the present invention. Figure 1 The third harmonic system provided in this embodiment includes a wavelength tunable light source 10, a polarization controller 20, an optical fiber 30 and an optical microcavity 40; the wavelength tunable light source 10 is used to provide pump light, and the pump light is coupled into the optical fiber 30; the optical fiber 30 is connected to the input end of the polarization controller 20; the optical fiber 30 extends from the output end of the polarization controller 20 to the optical microcavity 40, and the optical fiber 30 extending to the optical microcavity 40 includes a tapered structure ( Figure 1 (not shown), the optical fiber 30 is coupled to the optical microcavity 40 through a tapered structure; wherein the optical microcavity 40 includes a substrate 41 and a support column 42 and a microdisk cavity 43 located on one side of the substrate; the pump light is coupled into the optical microcavity 40 through the tapered structure; the polarization controller 20 is used to adjust the polarization state of the pump light in the optical fiber 30; the wavelength, power, polarization state of the pump light and the distance between the tapered structure and the optical microcavity 40 are adjusted so that the pump light satisfies the phase matching condition of the third harmonic when transmitting in the optical microcavity 40, thereby generating the third harmonic.

[0059] Among them, the wavelength tunable light source 10 can output a continuously adjustable pump light within a preset wavelength range, such as a pump light in the 1550nm band. The wavelength tunable light source 10, the polarization controller 20 and the optical microcavity 40 can be connected by an optical fiber 30. The optical microcavity 40 is an on-chip integrated device, which can be integrated on a silicon wafer as a substrate. It can be understood that the light transmitted in the optical fiber 30 generates an evanescent field in the conical structure to achieve the coupling of the optical microcavity 40 and the optical fiber 30. The conical structure can be obtained by melting and tapering the optical fiber. By adjusting the state of the polarization controller 20, the coupling efficiency of the pump light and the optical microcavity 40 can be adjusted, wherein the polarization controller 20 can adopt a three-ring type or an embedded polarization controller, which is not limited in the embodiment of the present invention.

[0060] Optionally, the shape of the micro-disk cavity 43 is a truncated cone; the angle between the generatrix of the truncated cone and the bottom surface of the truncated cone is greater than or equal to 30° and less than or equal to 60°.

[0061] For example, Figure 2 FIG. 1 is a schematic diagram of the structure of an optical microcavity provided by an embodiment of the present invention. Figure 2 The optical microcavity 40 includes a substrate 41, a support column 42 and a microdisk cavity 43 located on one side of the substrate. In this embodiment, the microdisk cavity 43 is truncated cone-shaped, and the truncated cone is coupled with the optical fiber 30 through the conical structure of the optical fiber. The angle between the busbar of the truncated cone and the bottom surface of the truncated cone is greater than or equal to 30° and less than or equal to 60°.

[0062] Among them, by adjusting the wavelength, power, polarization state of the pump light and the distance between the cone structure and the optical microcavity, the pump light is transmitted in the optical microcavity to meet the phase matching condition of the third harmonic and generate the third harmonic. The wavelength tunable light source can be a laser. First, the pump light is output by the laser, and the pump light is set to be coupled into the optical microcavity at low power. By applying a 2Vpp, 10Hz electrical signal to the laser, the wavelength tunable light source is continuously scanned in a small range near the central wavelength; secondly, the central wavelength of the laser is scanned to obtain a mode with a high quality factor; thirdly, the polarization of the pump light and the coupling between the microcavity and the fiber cone are continuously adjusted until the mode is in a just coupled state; finally, the wavelength scanning of the pump light is turned off, and the laser wavelength is set at a blue detuning point far away from the pump mode. After increasing the power of the pump light, the wavelength of the laser is gradually increased until a stable third harmonic signal is generated.

[0063] Specifically, the high quality factor can be obtained by turning on the wavelength scanning of the laser, making the laser continuously and repeatedly scan in a small range near the central wavelength λ, maintaining the scanning frequency at 10Hz, and observing the transmission spectrum on the oscilloscope. Within the range of a rising edge, the laser can scan from short wave to long wave. When the wavelength and cavity mode match, the Lorentz transmission spectrum can be observed on the oscilloscope. Change the central wavelength to find a mode with a narrow half-width, so as to determine the mode with a high quality factor. Among them, the quality factor can be expressed by the Q value. The Q value can be expressed as the ratio of the resonant wavelength to the half-width at that location.

[0064]

[0065] The just-coupled state refers to a state in which the coupling position of the optical fiber and the micro-disk cavity and the polarization state of the pump light are adjusted so that the lowest point of the transmission peak is 0.

[0066] The technical solution of this embodiment is to provide pump light through a wavelength tunable light source. After the pump light is coupled into the optical microcavity through the tapered structure in the optical fiber, the polarization state of the pump light is adjusted by the polarization controller, so that the coupling efficiency between the pump light and the optical microcavity changes. By applying a 2Vpp, 10Hz electrical signal to the laser, a continuous wavelength scanning signal of the wavelength tunable light source in a small range near the central wavelength is obtained, thereby realizing scanning from short wave to long wave, changing the central wavelength in the wavelength tunable light source, and further obtaining a high quality factor mode; then adjust the wavelength, power, polarization state of the pump light and the distance between the tapered structure and the optical microcavity, so that the pump light meets the conditions of high quality factor and strong matching degree of the pump mode and the third harmonic mode during the transmission of the optical microcavity, thereby generating the third harmonic in the third harmonic system. The complex process of generating the third harmonic is simplified, and the theoretical threshold of the third harmonic and the cost of generating the third harmonic are reduced.

[0067] Based on the above technical solutions, Figure 3 FIG. 2 is a schematic diagram of the structure of another third harmonic generation system provided by an embodiment of the present invention. Figure 3 Optionally, the third harmonic generation system also includes a first coupler 50, a photodetector 60, an oscilloscope 70, a first spectrometer 80 and a second spectrometer 90; the optical fiber 30 extending from the optical microcavity 40 is connected to the input end of the first coupler 50, the first output end of the first coupler 50 is connected to the photodetector 60, the photodetector 60 is connected to the oscilloscope 70, the second output end of the first coupler 50 is connected to the first spectrometer 80, and the third output end of the first coupler 50 is connected to the second spectrometer 90; the oscilloscope 70 is used to output the time domain waveform detected by the photodetector 60, the first spectrometer 80 is used to measure the spectrum of the third harmonic, and the second spectrometer 90 is used to measure the spectrum of the pump light.

[0068] It is understandable that in order to verify whether the third harmonic system provided by the embodiment of the present invention generates the third harmonic, a test is required to determine whether the third harmonic is generated by observing the time domain waveform of the oscilloscope 70 and the spectrum measured by the first spectrometer 80, and to determine the signal of the pump light by the spectrum measured by the second spectrometer 90. Figure 3 The first coupler 50 can be a three-port optical fiber coupler, and the splitting ratios of the first output end, the second output end, and the third output end are not limited. Figure 4 FIG. 2 is a schematic diagram of the structure of another third harmonic generation system provided by an embodiment of the present invention. Figure 4 The first coupler 50 that can be replaced can be composed of two three-port sub-couplers, namely, a first sub-coupler 51 and a second sub-coupler 52. In the embodiment of the present invention, the first coupler 50 composed of the two methods has the same function in the specific implementation process.

[0069] Figure 5 FIG. 2 is a schematic diagram of the structure of another third harmonic generation system provided by an embodiment of the present invention. Figure 5 Optionally, the third harmonic system further includes an optical amplifier 11 disposed on the optical path between the wavelength tunable light source 10 and the polarization controller 20, for amplifying the pump light.

[0070] It is understandable that, in a specific implementation, the power of the pump light output by the wavelength tunable light source 10 may be relatively small and cannot reach the threshold power of the pump light that can excite the third harmonic. Therefore, an optical amplifier 11 may be provided in the optical path between the wavelength tunable light source 10 and the polarization controller 20 to amplify the power of the pump light to above the threshold power.

[0071] Figure 6FIG. 2 is a schematic diagram of the structure of another third harmonic generation system provided by an embodiment of the present invention. Figure 6 Optionally, the optical amplifier is a semiconductor optical amplifier; the third harmonic generation system also includes a first collimator 12, an optical isolator 13 and a second collimator 14; the first collimator 12, the semiconductor optical amplifier, the optical isolator 13 and the second collimator 14 are arranged in sequence along the optical path between the wavelength tunable light source 10 and the polarization controller 20; the input end of the first collimator 12 is coupled with the output end of the wavelength tunable light source 10, and is used to collimate the pump light and input it into the semiconductor optical amplifier; the semiconductor optical amplifier is used to amplify the pump light; the optical isolator 13 is used to make the amplified pump light unidirectionally transmitted; the output end of the second collimator 14 is connected to the optical fiber 30 connected to the input end of the polarization controller 20, and the second collimator 14 is used to couple the amplified pump light into the optical fiber 30.

[0072] It can be understood that semiconductor optical amplifiers generally transmit light beams in free space, and the wavelength-tunable light source 10 can output pump light through the optical fiber 30. After passing through the first collimator 12, the transmission light in the optical fiber 30 is converted into parallel light in free space, and the pump light is gain amplified after the optical power is increased by the semiconductor optical amplifier. After passing through the optical isolator 13, the amplified pump light can only be transmitted in the forward direction to prevent the back-reflected light from damaging the semiconductor optical amplifier. After passing through the second collimator 14, the power-amplified free space parallel light is re-coupled into the optical fiber 30 for continued transmission.

[0073] In another embodiment, optionally, the optical amplifier 11 is a fiber amplifier; the wavelength tunable light source 10 is connected to the fiber amplifier via an optical fiber 30 ; and the fiber amplifier is connected to the polarization controller 20 via the optical fiber 30 .

[0074] It is understandable that the optical amplifier 11 may also be an optical fiber amplifier, and the optical path is only transmitted in the optical fiber 30, which reduces the coupling difficulty of the optical path. In other embodiments, other types of optical amplifiers may also be selected, and the embodiment of the present invention is not limited to this.

[0075] Figure 7 FIG. 2 is a schematic diagram of the structure of another third harmonic generation system provided by an embodiment of the present invention. Figure 7 Optionally, the third harmonic generation system also includes a second coupler 100 and a power meter 110; the input end of the second coupler 100 is connected to the output end of the polarization controller 20 through the optical fiber 30, the first output end of the second coupler 100 is connected to the power meter 110, and the optical fiber 30 extends to the optical microcavity 40 through the second output end of the second coupler 100.

[0076] It is understandable that the second coupler 100 is located after the polarization controller 20 and before the optical microcavity 40, and the output end of the second coupler can be connected to the power meter 110. The second coupler 100 can select different fiber couplers according to actual conditions, and the embodiment of the present invention does not limit this. The power meter 110 can detect the power of the pump light after passing through the second coupler 100.

[0077] Figure 8 FIG. 2 is a schematic diagram of the structure of another third harmonic generation system provided by an embodiment of the present invention. Figure 8 Optionally, the third harmonic generation system further includes an attenuator 120 , the input end of the attenuator 120 is connected to the output end of the polarization controller 20 through the optical fiber 30 , and the output end of the attenuator 120 extends to the optical microcavity 40 through the optical fiber.

[0078] It is understandable that the attenuator 120 can be used to adjust the output power of the pump light provided by the wavelength tunable light source 10 .

[0079] Optionally, the material of the substrate 41 of the optical microcavity 40 in the third harmonic generation system includes silicon, and the material of the microdisk cavity 43 includes silicon dioxide; the optical microcavity 40 is prepared by a dry etching process.

[0080] It is understandable that the specific low absorption energy consumption of silicon oxide material can obtain a better quality factor, reduce the threshold of third harmonic generation, and can be integrated on-chip, which is applied to the field of on-chip integrated silicon oxide microcavity. The microdisk cavity 43 is made of silicon dioxide material, which can be well matched with the frequency of the pump light provided by the wavelength tunable light source. The quality factor of the optical microcavity 40 prepared by the dry etching process can be improved by one order of magnitude, and the mode area is reduced to 2 / 3. Without considering the mode matching, the theoretical threshold of the third harmonic is reduced by more than three orders of magnitude; in addition, the dry etching process has strong repeatability, and the size of the optical microcavity 40 is controllable and consistent.

[0081] For example, Fig. 9 Schematic diagram of the quality factor of the optical microcavity provided by an embodiment of the present invention. Fig. 9 The silicon dioxide micro-disk cavity prepared by dry etching process has a thickness of 2μm and a diameter of 80μm. The angle between the truncated cone generatrix and the bottom surface of the truncated cone of the micro-disk cavity is 45°. The quality factor of the optical micro-cavity 40 (about 1.5×10 7 ), the black line is the curve obtained by Lorentz fitting.

[0082] It should be noted that the above are only several illustrative embodiments of the present invention. When implementing the present invention, the combination of optical devices required can be selected according to actual needs to meet the actual application needs. Fig.10The present invention provides another structural diagram of a system for generating third harmonics. This embodiment is based on the above embodiment and provides a specific example. Fig.10 The third harmonic generation system includes a wavelength tunable light source 10, an amplifier 11, a polarization controller 20, an optical fiber 30, an optical microcavity 40, a first coupler 50, a photodetector 60, an oscilloscope 70, a first spectrometer 80, a second spectrometer 90, a second coupler 100, a power meter 110 and an attenuator 120. The wavelength tunable light source 10 is a 1550nm tunable external cavity diode laser (ECDL, New Focus TBL6328), the optical amplifier 11 is an erbium-doped fiber amplifier (EDFA), the pump light passes through the optical amplifier 11 to amplify the power of the pump light, passes through the polarization controller 20 (FPC), the attenuator 120 (VOA) and the optical microcavity 40, and the attenuator 120 divides the pump light into two paths through the second coupler 100, one path is connected to the power meter to collect the power of the pump light, and the other path enters the optical microcavity 40. The optical microcavity 40 is a silica microdisk cavity with a diameter of 80 μm, a thickness of 2 μm, and an inclination angle of about 45°. The pump light after passing through the optical microcavity 40 passes through the first coupler 50 and is output in three paths. Among them, the first path is detected by a photodetector 60 (PD), and the oscilloscope 70 (OSC) is used to monitor the power change of the transmitted pump laser. The second path collects the spectrum of the third harmonic through the first spectrometer 80, and the third path collects the pump light signal and the generated Raman laser signal through the second spectrometer 90.

[0083] Fig.11 FIG. 1 is a flow chart of a method for generating a third harmonic provided by an embodiment of the present invention. The third harmonic is generated by any third harmonic generating system provided by the above embodiments. Fig.11 , the third harmonic generation method comprises:

[0084] Step S110: The wavelength-tunable light source outputs pump light, and the pump light is coupled into the optical microcavity through the optical fiber.

[0085] Step S120, adjusting the wavelength, power, polarization state of the pump light and the distance between the tapered structure and the optical microcavity, so that the pump light satisfies the phase matching condition of the third harmonic when transmitting in the optical microcavity, thereby generating the third harmonic.

[0086] Exemplarily, in one embodiment of the present invention, first, a pump light is output by a laser, and the pump light is set to be coupled into an optical microcavity at low power, and an electrical signal of 2Vpp and 10Hz is applied to the laser so that the wavelength-tunable light source performs continuous wavelength scanning in a small range near the central wavelength; secondly, the central wavelength of the laser is scanned to obtain a mode with a high quality factor; thirdly, the polarization of the pump light and the coupling between the microcavity and the fiber taper are continuously adjusted until the mode is in a just coupled state; finally, the wavelength scanning of the pump light is turned off, the laser wavelength is set at a blue detuning position away from the pump mode, and the wavelength of the laser is gradually increased after increasing the power of the pump light until a stable third harmonic signal is generated. Fig.12 The figure shows the spectrum diagram of the pump light and the third harmonic in the embodiment of the present invention. At this time, the wavelength of the pump light is 1561.0 nm, and the wavelength of the third harmonic is 520.06 nm, which is only 0.05% different from the expected wavelength of the third harmonic (520.33 nm). Fig.13 An optical micrograph of a microdisk cavity when generating a third harmonic is provided for this embodiment. Part of the third harmonic can be observed in the figure.

[0087] Note that the above are only preferred embodiments of the present invention and the technical principles used. Those skilled in the art will understand that the present invention is not limited to the specific embodiments described herein, and that various obvious changes, readjustments and substitutions can be made by those skilled in the art without departing from the scope of protection of the present invention. Therefore, although the present invention has been described in more detail through the above embodiments, the present invention is not limited to the above embodiments, and may include more other equivalent embodiments without departing from the concept of the present invention, and the scope of the present invention is determined by the scope of the appended claims.

Claims

1. A third harmonic generation system, characterized in that: Including wavelength tunable light source, polarization controller, optical fiber and optical microcavity; The wavelength tunable light source is used to provide pump light, and the pump light is coupled into the optical fiber; The optical fiber is connected to the input end of the polarization controller; The optical fiber extends from the output end of the polarization controller to the optical microcavity, the optical fiber extending to the optical microcavity comprises a tapered structure, and the optical fiber is coupled to the optical microcavity through the tapered structure; Wherein, the optical microcavity comprises a substrate and a support column and a microdisk cavity located on one side of the substrate; The pump light is coupled into the optical microcavity through the tapered structure; The polarization controller is used to adjust the polarization state of the pump light in the optical fiber; Adjusting the wavelength, power, polarization state of the pump light and the distance between the tapered structure and the optical microcavity so that the pump light satisfies the phase matching condition of the third harmonic when transmitting in the optical microcavity to generate the third harmonic; The pump light is provided by the wavelength tunable light source. After the pump light is coupled into the optical microcavity through the tapered structure in the optical fiber, the polarization state of the pump light is adjusted by the polarization controller, so that the coupling efficiency between the pump light and the optical microcavity changes. By applying an electrical signal to the wavelength tunable light source, a continuous wavelength scanning signal of the wavelength tunable light source in a small range near the central wavelength is obtained, thereby realizing scanning from short wave to long wave, changing the central wavelength in the wavelength tunable light source, and further obtaining a high quality factor mode. Subsequently, the wavelength, power, polarization state of the pump light and the distance between the tapered structure and the optical microcavity are adjusted, so that during the transmission of the pump light in the optical microcavity, the pump light satisfies the conditions of high quality factor and strong matching degree of the pump mode and the third harmonic mode, thereby generating the third harmonic.

2. The third harmonic generation system according to claim 1, characterized in that: The shape of the micro-disk cavity is a frustum; The included angle between the generatrix of the truncated cone and the bottom surface of the truncated cone is greater than or equal to 30° and less than or equal to 60°.

3. The third harmonic generation system according to claim 1, characterized in that: Also includes a first coupler, a photodetector, an oscilloscope, a first spectrometer and a second spectrometer; The optical fiber extending from the optical microcavity is connected to the input end of the first coupler, the first output end of the first coupler is connected to the photodetector, the photodetector is connected to the oscilloscope, the second output end of the first coupler is connected to the first spectrometer, and the third output end of the first coupler is connected to the second spectrometer; The oscilloscope is used to output the time domain waveform detected by the photodetector, the first spectrometer is used to measure the spectrum of the third harmonic, and the second spectrometer is used to measure the spectrum of the pump light.

4. The third harmonic generation system according to claim 1, characterized in that: It also includes an optical amplifier arranged on the optical path between the wavelength tunable light source and the polarization controller, for amplifying the pump light.

5. The third harmonic generation system according to claim 4, characterized in that: The optical amplifier is a semiconductor optical amplifier; The third harmonic generation system also includes a first collimator, an optical isolator and a second collimator; The first collimator, the semiconductor optical amplifier, the optical isolator and the second collimator are arranged in sequence along the optical path between the wavelength tunable light source and the polarization controller; The input end of the first collimator is coupled to the output end of the wavelength tunable light source, and is used to collimate the pump light and then input it into the semiconductor optical amplifier; The semiconductor optical amplifier is used to amplify the pump light; The optical isolator is used to enable the amplified pump light to be transmitted unidirectionally; The output end of the second collimator is connected to the optical fiber connected to the input end of the polarization controller, and the second collimator is used to couple the amplified pump light into the optical fiber.

6. The third harmonic generation system according to claim 4, characterized in that: The optical amplifier is a fiber amplifier; The wavelength tunable light source is connected to the optical fiber amplifier via the optical fiber; The optical fiber amplifier is connected to the polarization controller via the optical fiber.

7. The third harmonic generation system according to claim 1, characterized in that: Also included is a second coupler and a power meter; The input end of the second coupler is connected to the output end of the polarization controller through the optical fiber, the first output end of the second coupler is connected to the power meter, and the optical fiber extends to the optical microcavity through the second output end of the second coupler.

8. The third harmonic generation system according to claim 1, characterized in that: It also includes an attenuator, the input end of the attenuator is connected to the output end of the polarization controller through the optical fiber, and the output end of the attenuator extends to the optical microcavity through the optical fiber.

9. The third harmonic generation system according to claim 1, characterized in that: The substrate material of the optical microcavity includes silicon, and the material of the microdisk cavity includes silicon dioxide; The optical microcavity is prepared by using a dry etching process.

10. A method for generating a third harmonic, characterized in that: Outputting a third harmonic using the third harmonic generation system according to any one of claims 1 to 9, comprising: The wavelength-tunable light source outputs pump light, and the pump light is coupled into the optical microcavity through the optical fiber; The wavelength, power, polarization state of the pump light and the distance between the tapered structure and the optical microcavity are adjusted so that the pump light satisfies the phase matching condition of the third harmonic when transmitting in the optical microcavity to generate the third harmonic.

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