Optical device based on double-resonance PPLN

Abstract

The invention belongs to the field of integrated optics, optical communication and nonlinear optics, and particularly discloses an optical device based on a double-resonance PPLN. Through the two cascaded FP cavities, the resonant FP inner cavity only resonates frequency doubling light to improve the difference frequency efficiency, and the resonant FP outer cavity only resonates pump light to improve the frequency doubling efficiency; in the frequency doubling process, the pump light and the frequency doubling light are respectively enhanced by resonance due to the outer cavity and the inner cavity; in the difference frequency process, the frequency doubling light is enhanced by resonance, so that the nonlinear frequency mixing efficiency can be greatly enhanced through triple resonance enhancement in the optical parametric amplification process. Signal light in the structure does not resonate in the inner cavity and the outer cavity, so that wavelength transparent operation can be realized in a large range. The optical parametric amplification or wavelength conversion function is realized based on the frequency doubling and difference frequency principle, the signal light is not limited by the FSR of the resonant cavity, broadband operation can be realized, and the realization of high-speed all-optical signal processing is expected to be promoted.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of integrated optics, optical communication and nonlinear optics, and more particularly, relates to an optical device based on double-resonant PPLN. BACKGROUND

[0002] All-optical signal processing refers to processing signals directly in the optical domain, which has a much higher processing bandwidth than electronic devices, and also has the advantages of format transparency and rate transparency. Optical parametric amplification refers to using second-order or third-order nonlinear optical effects to transfer the energy of a strong pump light to a weak signal light through the interaction of the pump light and the signal light, thereby realizing the direct amplification of the signal light, and in addition, an idler light is generated. All-optical wavelength conversion refers to a technology of directly loading information carried on one frequency to another frequency in the optical domain.

[0003] Among many schemes, if a common straight waveguide structure is used to realize the above functions, although the conversion bandwidth and wavelength transparent operation can be realized, in order to ensure the conversion efficiency, the system is difficult to integrate and has high power consumption; if a resonant cavity scheme is used, although the experimental power consumption can be reduced, the system is easy to integrate, but the signal operation bandwidth is affected by the resonant wavelength, which leads to the inability to realize large conversion bandwidth and wavelength transparent operation. The following are two examples of wavelength conversion experiments realized by PPLN (Periodically Poled Lithium Niobate) straight waveguide. Scheme 1. A ridge waveguide with a waveguide length of 5 mm is made on a thin film lithium niobate platform on an insulator, and through second-harmonic generation and difference frequency generation, all-optical wavelength conversion of a 92 Gb / s 16QAM signal with a conversion bandwidth of 151 nm and a modulation format is realized. Compared with the traditional PPLN straight waveguide, the thin film lithium niobate straight waveguide has the advantages of high nonlinear efficiency and small size, but the structure needs high power consumption. Experiments show that when the maximum conversion efficiency is -10 dB, about 24 dBm of on-chip pump light power is needed. Scheme 2. A ridge waveguide with a waveguide length of 12.3 mm is made on a thin film lithium niobate platform on an insulator, and by using ion beam modification, the thickness variation of the wafer is suppressed to ensure uniform phase matching on the long waveguide. Through second-harmonic generation and difference frequency generation, a signal light on-chip gain of 13.9 dB and wavelength conversion function are realized at a pump light power of 33.6 dBm, and the bandwidth is about 110 nm. This scheme needs additional process steps, which increases the complexity of device manufacturing, and the required pump light power consumption is high.

[0004] At present, the schemes of optical parametric amplification and all-optical wavelength conversion have the problems of large device structure, complex system, difficult to tune, and the conversion efficiency and conversion bandwidth are difficult to be compatible. SUMMARY

[0005] In view of the defects of the prior art, the purpose of the present application is to provide a kind of optical device based on double resonance PPLN, to solve the problems of large equipment structure, complex system, difficult to tune, conversion efficiency and conversion bandwidth are difficult to be compatible.

[0006] To achieve the above object, the present application provides a kind of optical device based on double resonance PPLN, comprising: waveguide core layer, waveguide substrate layer and substrate layer;The waveguide core layer includes first Bragg grating to fourth Bragg grating and first straight waveguide to fifth straight waveguide, wherein, third straight waveguide constitutes resonant FP inner cavity, third Bragg grating and fourth Bragg grating are located at the two ends of resonant FP inner cavity, as the mirror of resonant FP inner cavity, resonant FP inner cavity is used to generate idle frequency light based on difference frequency nonlinear effect, frequency doubling light and signal light;Second straight waveguide to fourth straight waveguide and third Bragg grating and fourth Bragg grating constitute resonant FP outer cavity, first Bragg grating and second Bragg grating are located at the two ends of resonant FP outer cavity, as the mirror of resonant FP outer cavity, resonant FP outer cavity is used to generate frequency doubling light based on frequency doubling nonlinear effect, pump light;Second straight waveguide between first Bragg grating and third Bragg grating, fourth straight waveguide between second Bragg grating and fourth Bragg grating jointly constitute tunable phase shift region, for changing the effective refractive index of phase shift region waveguide, thereby adjusting the resonant wavelength of resonant FP outer cavity;First straight waveguide and fifth straight waveguide as the two ends of waveguide core layer, respectively for guiding pump light and signal light, guiding signal light and idle frequency light.

[0007] Preferably, the third straight waveguide is periodically polarized under the thin film lithium niobate to meet the quasi-phase matching condition.

[0008] Preferably, the center wavelength of resonant FP outer cavity and resonant FP inner cavity is consistent with the fundamental frequency and frequency doubling light of PPLN.

[0009] Preferably, the reflectivity R2 of the second Bragg grating is 0.99-0.999, and the reflectivity R1 of the first Bragg grating is 0.999-0.9999. Wherein, a is the loop loss, the reflectivity R3 of the third Bragg grating is greater than 0.999, and the reflectivity R4 of the fourth Bragg grating is greater than 0.999.

[0010] Preferably, the middle part of all Bragg gratings is uniformly distributed, and the part connected with straight waveguide is transitioned by using chirped grating.

[0011] Preferably, the band gap width of resonant FP outer cavity is 6 nm-8 nm, and the band gap width of resonant FP inner cavity is 2 nm.

[0012] Preferably, the waveguide core layer is made based on the thin film lithium niobate platform without etching.

[0013] Preferably, the waveguide core layer is obtained by exposing the device pattern on a photoresist with a refractive index between 1.444 and 2.21.

[0014] Preferably, the optical device further includes a filter for filtering the output of the fifth straight waveguide as needed, retaining either the signal light or the idler light.

[0015] Preferably, if the filter retains signal light, the optical device is an optical parametric amplifier; if the filter retains idler light, the optical device is an all-optical wavelength converter.

[0016] Overall, the technical solutions conceived in this application have the following beneficial effects compared with the prior art: This application proposes an optical device based on a dual-resonant PPLN, employing two cascaded FP cavities. The inner FP cavity resonates only with the frequency-doubled light to improve difference frequency efficiency, while the outer FP cavity resonates only with the pump light to improve frequency doubling efficiency. During frequency doubling, the pump light and the frequency-doubled light are amplified by resonance in the outer and inner cavities, respectively. During difference frequency doubling, the frequency-doubled light is amplified by resonance. Therefore, this optical parametric amplification process undergoes triple resonance enhancement, significantly improving nonlinear mixing efficiency. In this structure, the signal light does not resonate in either the inner or outer cavities, thus enabling wavelength-transparent operation over a wide range. This application achieves optical parametric amplification or wavelength conversion based on the principles of second harmonic and difference frequency, and the signal light is not limited by the FSR of the resonant cavity, enabling broadband operation and potentially advancing the realization of high-speed all-optical signal processing. Attached Figure Description

[0017] Figure 1 This is a cross-sectional schematic diagram of an optical device based on a dual-resonance PPLN provided in an embodiment of this application.

[0018] Figure 2 This is a schematic diagram of the waveguide core structure provided in the embodiments of this application.

[0019] Figure 3 This is a schematic diagram of the transmission spectrum of the dual resonator device provided in the embodiments of this application. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0021] First, the technical terms involved in the embodiments of this application will be introduced.

[0022] The difference-frequency nonlinear effect refers to the phenomenon where two light waves of different frequencies (frequency 1, frequency 2, frequency 3, frequency 4, frequency 5, frequency 6, frequency 7, frequency 8, frequency 9, frequency 10, frequency 11, frequency 12, frequency 13, frequency 14, frequency 15, frequency 16, frequency 17, frequency 18, frequency 19 ... and When incident on a medium with nonlinear optical properties, a new light wave is generated through nonlinear interaction, the frequency of which is equal to the difference between the frequencies of the two incident light waves. .

[0023] Frequency doubling nonlinearity refers to the phenomenon where a beam of monochromatic light (with a frequency of 1000 Hz) exhibits frequency doubling nonlinearity. When incident on a medium with nonlinear optical properties, a new light wave is generated through nonlinear interactions, with a frequency twice that of the incident light. This phenomenon is also known as second harmonic generation.

[0024] The embodiments of this application are described below with reference to the accompanying drawings.

[0025] like Figure 1 As shown, this application provides an optical device based on a dual-resonance PPLN, which includes, from top to bottom, a waveguide core layer, a waveguide substrate layer, and a substrate layer.

[0026] like Figure 2 As shown, the waveguide core layer includes a first to a fourth Bragg grating and a first to a fifth straight waveguide. The third straight waveguide forms the inner cavity of a resonant FP (Fabry-Perot cavity). The third and fourth Bragg gratings are located at both ends of the resonant FP inner cavity, serving as mirrors. The resonant FP inner cavity is used to generate idler light from the frequency-doubled light and the signal light based on the difference-frequency nonlinear effect. The second to fourth straight waveguides, along with the third and fourth Bragg gratings, form the outer cavity of the resonant FP. The first and second Bragg gratings... Located at both ends of the resonant FP external cavity, serving as mirrors of the resonant FP external cavity, the resonant FP external cavity is used to generate frequency-doubled light from pump light based on the frequency doubling nonlinear effect; the second straight waveguide between the first and third Bragg gratings and the fourth straight waveguide between the second and fourth Bragg gratings together constitute a tunable phase shift region, which is used to change the effective refractive index of the phase shift region waveguide, thereby controlling the resonant light wavelength of the resonant FP external cavity; the first and fifth straight waveguides serve as the two ends of the waveguide core layer, respectively used to introduce pump light and signal light, and to export signal light and idler light.

[0027] The first and second Bragg gratings reflect the pump light, while the third and fourth Bragg gratings reflect the frequency-doubled light.

[0028] Preferably, the thin-film lithium niobate below the third straight waveguide is periodically polarized to satisfy the quasi-phase matching condition.

[0029] Preferably, the center wavelengths of the resonant FP outer cavity and the resonant FP inner cavity are consistent with the fundamental frequency and harmonic frequency of the PPLN.

[0030] Preferably, the reflectivity R2 of the second Bragg grating is between 0.99 and 0.999, and the reflectivity of the first Bragg grating is... Where a is the cyclic loss, the reflectivity of the third Bragg grating R3>0.999, and the reflectivity of the fourth Bragg grating R4>0.999.

[0031] It should be noted that by specially designing the reflectivity of the first and second Bragg gratings, the pump light is critically coupled in the external cavity, thereby improving the conversion efficiency. Furthermore, by specially designing the reflectivity of the third and fourth Bragg gratings, the frequency-doubled light is confined to resonance within the internal cavity, thus enhancing the conversion efficiency. The reflectivity can be controlled by changing the length of the Bragg gratings.

[0032] Preferably, all Bragg gratings are uniformly distributed in the middle, and the parts connected to the straight waveguide are transitioned using chirped gratings to ensure that no additional loss is introduced into the cavity surface.

[0033] Preferably, the bandgap width of the outer cavity of the resonant FP is 6 nm to 8 nm, and the bandgap width of the inner cavity of the resonant FP is 2 nm, so that the signal light can achieve broadband transparent processing.

[0034] Preferably, the waveguide core layer is fabricated based on an etching-free thin-film lithium niobate platform.

[0035] It should be noted that etching-free fabrication can reduce device transmission losses to achieve high Q characteristics, thereby enhancing nonlinear processes and reducing experimental power consumption. Thin-film lithium niobate possesses excellent characteristics such as ultra-high second-order nonlinear coefficient, flexible quasi-phase matching technology, wide-spectral transparency window, and high damage threshold, which can be used to improve nonlinear efficiency. Furthermore, it can compensate for the difficulty in achieving phase matching due to the etching-free structure. As an alternative, normal etching or fabrication on a lithium tantalate platform can also be used.

[0036] Preferably, the waveguide core layer is obtained by exposing the device pattern on a photoresist with a refractive index between 1.444 and 2.21 to achieve the function of confining the light field.

[0037] Preferably, the waveguide substrate is a thin-film lithium niobate, and the substrate layer is silicon dioxide, silicon, or lithium niobate.

[0038] Preferably, the optical device further includes a filter for filtering the output of the fifth straight waveguide as needed, retaining either the signal light or the idler light.

[0039] Preferably, if the filter retains signal light, the optical device is an optical parametric amplifier; if the filter retains idler light, the optical device is an all-optical wavelength converter.

[0040] In this application, the optical parametric amplifier / wavelength converter is implemented in the following ways: (1) Based on the frequency doubling nonlinear effect, the pump light can generate frequency doubling light in the outer cavity; (2) Based on the difference frequency nonlinear effect, the frequency doubling light and the signal light generate idler light in the inner cavity. Due to the triple resonance enhancement, the signal light power is amplified; (3) Filtering is performed at the output end. If the signal light is retained, it is an optical parametric amplifier; if the idler light is retained, it is an all-optical wavelength converter.

[0041] The optical parametric amplifier amplifies the signal by combining frequency doubling and difference frequency with triple resonance. At the same time, this process generates idler light, which carries the same information as the signal light.

[0042] Example In this embodiment, the external cavity has a lattice constant of 443 nm, a duty cycle of 0.5, and a waveguide width of 2.0 μm / 1.2 μm, with a simulated band center wavelength of 1560 nm. The internal cavity has a lattice constant of 195 nm, a duty cycle of 0.5, and a waveguide width of 2.0 μm / 0.6 μm, with a simulated band center wavelength of 780 nm. With this design, the center wavelengths of the external and internal cavities are consistent with the fundamental and harmonic frequencies of the PPLN, achieving the highest conversion efficiency. A schematic diagram of the transmission spectrum of the dual resonant device is shown below. Figure 3 As shown, where, The frequency of the signal light, The frequency of the pump light, The frequency of idler light, The frequency of the frequency-doubled light.

[0043] Because the external cavity length is greater than the internal cavity length, the Free Spectral Range (FSR) is smaller, resulting in more resonant peaks within the bandgap. In the experiment, a tunable laser is first used to generate pump light, which is then injected into the resonant peak after passing through a polarizer and an EDFA (Erbium-doped fiber amplifier). The electrodes are adjusted to align this resonant peak with the resonant peak of the frequency-doubled light, simultaneously satisfying the double resonance condition and the quasi-phase matching condition, thus generating a highly efficient second harmonic. Next, a tunable laser is used to generate signal light (with continuous pump light injection). This signal light passes through a polarizer and an EDFA, and is then combined with the pump light by a 50:50 beam splitter and input onto the chip. Inside the device, the signal light and the frequency-doubled light generate a difference frequency, producing idler light. During this process, the frequency-doubled light is resonantly enhanced, resulting in significantly higher idler light power compared to idler light obtained based on a conventional straight waveguide. This structure, through triple resonance enhancement, greatly improves the conversion efficiency, enabling signal light enhancement and idler light generation for wavelength conversion.

[0044] According to photonic crystal theory, the center frequency of the energy band is inversely proportional to the lattice constant. When the energy band is in the communication band, the lattice constant is approximately 400 nm or higher. If the duty cycle is 0.5, the minimum feature size needs to be 200 nm. When the band reaches 780 nm, without considering dispersion, the lattice constant needs to be halved, and the minimum feature size needs to be 100 nm. This places high demands on the etching fabrication of lithium niobate platforms. On the one hand, there is a sidewall tilt angle during the etching of lithium niobate. On the other hand, multiple overlays can easily introduce additional losses and reduce device performance. Therefore, introducing an etching-free structure can reduce process requirements and realize devices with low loss and high Q characteristics.

[0045] In this application, the terms "first" and "second," etc., are used to distinguish different objects, not to describe a specific order of objects. For example, "first response message" and "second response message," etc., are used to distinguish different response messages, not to describe a specific order of response messages.

[0046] In the embodiments of this application, the terms "exemplary" or "for example" are used to indicate that something is an example, illustration, or description. Any embodiment or design that is described as "exemplary" or "for example" in the embodiments of this application should not be construed as being more preferred or advantageous than other embodiments or design. Specifically, the use of the terms "exemplary" or "for example" is intended to present the relevant concepts in a specific manner.

[0047] In the description of the embodiments of this application, unless otherwise stated, "multiple" means two or more, for example, multiple processing units means two or more processing units, multiple elements means two or more elements, etc.

[0048] It should be understood that expressions such as “comprising” and “may include” used in this application indicate the existence of the disclosed functions, operations, or constituent elements, and do not limit one or more additional functions, operations, and constituent elements. In this application, terms such as “comprising” and / or “having” are to be interpreted as indicating a particular characteristic, number, operation, constituent element, component, or combination thereof, but not to exclude the existence or possibility of adding one or more other characteristics, numbers, operations, constituent elements, components, or combinations thereof.

[0049] Furthermore, in this application, the expression "and / or" includes any and all combinations of the associated listed words. For example, the expression "A and / or B" may include A, may include B, or may include both A and B.

[0050] In the description of the embodiments of this application, it should be noted that, unless otherwise explicitly specified and limited, the term "connection" should be interpreted broadly. For example, "connection" can be a detachable connection or a non-detachable connection; it can be a direct connection or an indirect connection through an intermediate medium. The directional terms mentioned in the embodiments of this application, such as "top," "bottom," "inner," "outer," "left," and "right," are only for reference to the directions in the accompanying drawings. Therefore, the directional terms used are for better and clearer explanation and understanding of the embodiments of this application, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the embodiments of this application.

[0051] Furthermore, the mathematical concepts mentioned in the embodiments of this application, such as symmetry, equality, parallelism, and perpendicularity, are limitations specific to the current technological level, rather than absolute and strict mathematical definitions. Slight deviations are permissible; approximations of symmetry, equality, parallelism, and perpendicularity are all acceptable. For example, "A and B are parallel" means that A and B are parallel or approximately parallel, and the angle between A and B can be between 0 and 10 degrees. "A and B are perpendicular" means that A and B are perpendicular or approximately perpendicular, and the angle between A and B can be between 80 and 100 degrees.

[0052] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. An optical device based on a dual-resonant PPLN, comprising: Waveguide core layer, substrate layer and substrate layer; characterized in that, The waveguide core layer includes a first to a fourth Bragg grating and a first to a fifth straight waveguide. The third straight waveguide forms the resonant FP inner cavity, and the third and fourth Bragg gratings are located at both ends of the resonant FP inner cavity, serving as mirrors. The resonant FP inner cavity is used to generate idler light from the frequency-doubled light and the signal light based on the difference-frequency nonlinear effect. The second to fourth straight waveguides, along with the third and fourth Bragg gratings, form the resonant FP outer cavity. The first and second Bragg gratings are located at the resonant FP... At both ends of the external cavity, serving as mirrors for the resonant FP external cavity, the pump light is used to generate frequency-doubled light based on the frequency-doubled nonlinear effect. The second straight waveguide between the first and third Bragg gratings and the fourth straight waveguide between the second and fourth Bragg gratings together constitute a tunable phase shift region, which is used to change the effective refractive index of the phase shift region waveguide, thereby controlling the resonant light wavelength of the resonant FP external cavity. The first and fifth straight waveguides serve as the two ends of the waveguide core layer, respectively used to introduce pump light and signal light, and to export signal light and idler light.

2. The optical device as described in claim 1, characterized in that, Periodic polarization of the thin-film lithium niobate beneath the third straight waveguide is performed to satisfy the quasi-phase matching condition.

3. The optical device as described in claim 2, characterized in that, The center wavelengths of the resonant FP outer cavity and the resonant FP inner cavity are consistent with the fundamental frequency and harmonic frequency of the PPLN.

4. The optical device as described in claim 1, characterized in that, The reflectivity R² of the second Bragg grating is between 0.99 and 0.999, while the reflectivity of the first Bragg grating is... Where a is the cyclic loss, the reflectivity of the third Bragg grating R3>0.999, and the reflectivity of the fourth Bragg grating R4>0.

999.

5. The optical device as described in claim 1, characterized in that, All Bragg gratings are uniformly distributed in the middle, and the parts connected to the straight waveguides are transitioned using chirped gratings.

6. The optical device as claimed in claim 1, characterized in that, The bandgap width of the external cavity of the resonant FP is 6 nm ~ 8 nm, and the bandgap width of the internal cavity of the resonant FP is 2 nm.

7. The optical device as claimed in claim 1, characterized in that, Waveguide cores are fabricated using an etching-free thin-film lithium niobate platform.

8. The optical device as claimed in claim 1, characterized in that, The waveguide core layer is obtained by exposing the device pattern on a photoresist with a refractive index between 1.444 and 2.

21.

9. The optical device according to any one of claims 1 to 8, characterized in that, The optical device also includes a filter, used to filter the output of the fifth straight waveguide as needed, retaining either signal light or idler light.

10. The optical device as claimed in claim 9, characterized in that, If the filter retains the signal light, the optical device is an optical parametric amplifier; if the filter retains the idler light, the optical device is an all-optical wavelength converter.

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