Thin film lithium niobate based optical frequency comb generation apparatus and method

Abstract

The application provides a thin-film lithium niobate-based optical frequency comb generation device and method, and belongs to the technical field of optical frequency combs, and comprises a pumping unit for providing single / multi-frequency laser; a microwave modulation unit for performing electro-optic modulation on a normal dispersion double-micro-ring resonant cavity unit to generate modulation sidebands; and the normal dispersion double-micro-ring resonant cavity unit generates an optical frequency comb through normal dispersion, electro-optic effect and third-order nonlinear effect. The single / multi-frequency laser is coupled to the normal dispersion double-micro-ring resonant cavity unit based on thin-film lithium niobate, and the normal dispersion, electro-optic effect and third-order nonlinear effect are combined to realize optical frequency comb spectrum widening and spectrum envelope flattening.

Description

Technical Field

[0001] This invention relates to the field of optical frequency comb technology, and specifically to an optical frequency comb generation device and method based on thin-film lithium niobate. Background Technology

[0002] Optical frequency combs have important applications in wavelength division multiplexing passive optical networks, ultra-high capacity high-speed coherent optical communication, optical signal processing, lidar, and microwave photonics. Currently, there are several methods for generating optical frequency combs, which can be broadly categorized as: generation using mode-locked lasers, generation using electro-optic modulation, generation using pulse-pumped cavity-less optical waveguides based on supercontinuum broadening, and generation using Kerr microring resonators.

[0003] Mian Zhang proposed generating an optical frequency comb using the electro-optic effect in a lithium niobate microring resonator (Mian Zhang, et al. Broadband electro-optic frequency comb generation in a lithium niobate microring resonator[J]. Nature, 2019). The spectral span of the electro-optic modulated optical frequency comb exceeded the entire L-band. However, in this single-cavity structure, most of the light is transmitted through a common straight waveguide and is not coupled into the resonator. The energy conversion efficiency of the optical frequency comb is limited to 0.3%, and the spectral envelope of the optical frequency comb is triangular. Yaowen Hu proposed using two coupled microring resonators to improve the pump conversion efficiency of the integrated electro-optic modulated frequency comb by 100 times (Yaowen Hu, et al. High-efficiency and broadband electro-optic frequency combs enabled by coupled micro-resonators[J]. Nature Photonics, 2022). However, because it operates at near-zero dispersion, the spectral envelope of the generated optical frequency comb is still triangular, and the flatness of the envelope is still not ideal. Many practical applications require a certain flatness of the optical frequency comb's spectral envelope, so improving the flatness of the optical frequency comb envelope is an urgent problem to be solved.

[0004] When generating electro-optic modulated optical frequency combs based on thin-film lithium niobate microring resonators using the electro-optic effect, electro-optic phase modulators are often implemented on straight waveguides in racetrack-shaped microcavities. However, thin-film lithium niobate optical waveguides are multimode optical waveguides, and ordinary circular bending can lead to complex inter-mode cross-coupling and higher-order mode excitation problems in multimode microcavities, resulting in increased fundamental mode loss and reduced Q value of the microcavity.

[0005] In both normal-dispersion and anomalous-dispersion microring resonators, the third-order Kerr nonlinear effect can be used to generate optical frequency combs. Frequency comb generation in the normal-dispersion region has attracted much attention due to its high pump-conversion efficiency. However, the normal-dispersion region lacks modulation instability effects similar to those in the anomalous-dispersion region, requiring mode cross-coupling to provide local anomalous dispersion to generate four-wave mixing sidebands or multi-frequency sidebands for pump-assisted startup. In the normal dispersion region, there are several implementation schemes for generating optical frequency combs using single-frequency laser pumping. Xiaoxiao Xue et al. used a single-frequency laser to pump a single microcavity, introducing anomalous dispersion locally based on the cross-coupling of the fundamental mode and higher-order modes to form a four-wave mixing sideband, thus generating an optical frequency comb (Xiaoxiao Xue, et al. Mode-locked dark pulse Kerr combs in normal-dispersion microresonators[J]. Nature Photonics, 2015); Elham Nazemosadat et al. used the cross-coupling of the fundamental mode and higher-order modes to generate mode splitting and form sidebands, thus generating an optical frequency comb (Elham Nazemosadat, et al. Switching dynamics of dark-pulse Kerr frequency comb states in optical microresonators[J]. Phys. Rev. A, 2021); Warren Jin et al. used the forward and reverse propagation of light in a microring to generate mode splitting and form sidebands, thus generating an optical frequency comb (Warren Jin, et al. Hertzlinewidth semiconductor lasers using CMOS-ready ultra high Q Microresonators [J]. Nature Photonics, 2021). Local anomalous dispersion is generated by the cross-coupling of the fundamental and higher-order modes to form sidebands, thus enabling a single-frequency laser to generate an optical frequency comb in the normal dispersion region. However, a serious drawback is that the location of mode splitting or cross-coupling is difficult to control, requiring complex modulation. Alternatively, dual-frequency lasers or high-repetition-rate pulsed lasers can be used for pumping, but high-repetition-rate pulsed sources often require cascaded electro-optic modulators and pulse compression units; these pulse sources are bulky and complex to control. Summary of the Invention

[0006] The purpose of this invention is to provide an optical frequency comb generation device and method based on thin-film lithium niobate to solve at least one of the technical problems existing in the background art.

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

[0008] On one hand, the present invention provides an optical frequency comb generation device based on thin-film lithium niobate, comprising a pump unit, a microwave modulation unit, and a normal dispersion dual microring resonator unit, wherein:

[0009] The pumping unit is used to provide laser pumping;

[0010] The microwave modulation unit is a coplanar waveguide traveling-wave electrode electro-optic modulator, consisting of electrodes, a drive signal, and a load resistor. The signal output port of the drive signal is connected to the outer electrode of the racetrack-shaped lithium niobate microcavity, and the common ground port of the drive signal is connected to the inner electrode of the racetrack-shaped lithium niobate microcavity. The traveling-wave electrode structure allows light waves and microwaves to propagate in the same direction along the coplanar electrodes, and the signal is applied to the thin-film lithium niobate crystal in the form of a traveling wave, so that the high-frequency electric field interacts fully with the light wave in the form of a traveling wave, thereby realizing the electro-optic modulation of the racetrack-shaped lithium niobate microcavity unit.

[0011] The normal dispersive dual microring resonator unit is a thin-film lithium niobate microcavity, which consists of a single-chip integrated input-output coupling waveguide and two ring racetrack-shaped microcavity waveguides. The input-output coupling waveguide and the two ring racetrack-shaped microcavity waveguides adopt a pulley coupling structure to achieve effective excitation of the fundamental mode.

[0012] Optionally, the input coupling waveguide is used to input the pump laser, and the output coupling waveguide is used to output the generated optical frequency comb; it includes a first annular microcavity and a second annular microcavity based on Euler bending, wherein the perimeter of the first annular microcavity is smaller than that of the second annular microcavity, and the first annular microcavity is used to improve the coupling efficiency of the pump laser.

[0013] Optionally, the second annular microcavity is a racetrack-shaped microcavity based on Euler bending, used to smoothly transition the fundamental mode from the straight waveguide to the curved waveguide, suppress the excitation of higher-order modes, thereby suppressing mode cross-coupling between the fundamental mode and higher-order modes and improving the Q value of the microcavity; the straight waveguide part of the racetrack-shaped microcavity performs electro-optic modulation on the fundamental mode transmitted in the waveguide. First, the electro-optic modulation generates sidebands. As the fundamental mode optical field is transmitted around the microcavity multiple times, the optical frequency comb spectrum broadening and spectrum envelope flattening are achieved through the combined action of normal dispersion, electro-optic effect and third-order nonlinear effect.

[0014] Optionally, the pumping unit is used to provide single-frequency / multi-frequency laser pumping into the waveguide, with a pump power range of 1mW to 10W.

[0015] Optionally, the microwave modulation unit is a coplanar waveguide traveling wave electrode electro-optic modulator, and its electrode material is gold.

[0016] Optionally, the normal dispersion dual-microring resonant cavity unit is a single-chip integrated normal dispersion thin-film lithium niobate microcavity optical waveguide, or an optical waveguide formed of materials with electro-optic effects and third-order nonlinear effects; the normal dispersion dual-microring resonant cavity unit includes two parts: an input-output coupling waveguide and a dual-microring resonant cavity waveguide, with a nonlinear coefficient of approximately 0.3 W. -1 m -1 Up to 500W -1 m -1 .

[0017] Optionally, the normal dispersive thin-film lithium niobate microcavity is implemented using a microring resonator based on Euler bending / or similar Euler bending; a pulley coupling structure is used between the dual microring resonator and the input / output coupling waveguide; a metal heating electrode is located above the cladding of the microring resonator for tuning the resonant wavelength of the microcavity.

[0018] Optionally, the normal dispersive thin-film lithium niobate microcavity waveguide is a ridge-structured multimode thin-film lithium niobate waveguide.

[0019] Optionally, the optical frequency comb generating device is used to generate a tunable flat coherent optical frequency comb in the 1550nm band.

[0020] Secondly, the present invention provides a method for generating an optical frequency comb using the device described above, comprising the following steps:

[0021] The pump unit pumps single-frequency / multi-frequency lasers into the input-coupled thin-film lithium niobate waveguide;

[0022] Single-frequency / multi-frequency lasers are coupled into a small-perimeter ring microcavity based on Euler bending by an input coupling waveguide, and then coupled into a long-perimeter racetrack-shaped microcavity based on Euler bending, thereby improving the optical frequency comb pump power conversion efficiency.

[0023] In a racetrack-shaped microcavity based on Euler bending, an electro-optic modulation unit with coplanar waveguide traveling-wave electrodes is used to achieve spectral broadening and spectral envelope flattening of the optical frequency comb through the combined effects of normal dispersion, electro-optic effect and third-order nonlinear effect.

[0024] The beneficial effects of this invention are as follows: By pumping single-frequency / multi-frequency lasers into an input-coupled thin-film lithium niobate waveguide, and then coupling it to a normal-dispersion dual-micro-ring resonator unit, the fundamental mode is effectively excited based on Euler bending, while higher-order modes are suppressed. Sidebands are generated by electro-optic modulation, thus producing multi-frequency lasers. The combined effect of normal dispersion, electro-optic effect, and third-order nonlinear effect achieves spectral broadening and spectral envelope flattening of the optical frequency comb. This avoids complex mode cross-coupling control and realizes the generation of a high-efficiency optical frequency comb with a relatively flat normal dispersion region.

[0025] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and will become apparent from the description or may be learned by practice of the invention. Attached Figure Description

[0026] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0027] Figure 1 This is a schematic diagram of the structure of an optical frequency comb device based on the electro-optic effect and third-order nonlinear effect of thin-film lithium niobate as described in an embodiment of the present invention.

[0028] Figure 2 This is a schematic diagram illustrating the generation of an optical frequency comb in the normal dispersion region in the 1550nm band based on the electro-optic effect and third-order nonlinear effect of thin-film lithium niobate, as described in an embodiment of the present invention.

[0029] Figure 3 This is a schematic diagram showing the time-domain and frequency-domain correspondence of an optical frequency comb with an optical pulse as the Fourier transform limit according to an embodiment of the present invention.

[0030] Figure 4 This is a schematic diagram of a normal dispersion flat thin-film lithium niobate ridge structure optical waveguide according to an embodiment of the present invention.

[0031] Figure 5 This is a schematic cross-sectional view of the electro-optic modulator according to an embodiment of the present invention.

[0032] Figure 6 This is a schematic diagram of the 180° Euler bending structure in the racetrack-shaped microcavity according to an embodiment of the present invention.

[0033] Figure 7 The image shows the second-order dispersion curve of the fundamental mode of the quasi-transverse electric (TE) bend waveguide (bending radius 212.31 μm) with a normal dispersion flat thin film lithium niobate ridge structure as described in the embodiment of the present invention.

[0034] Figure 8 The figures show the D2 and D3 curves of the fundamental mode microring dispersion of the quasi-transverse electric (TE) fundamental mode of the curved waveguide with a ridged structure of lithium niobate ridge-shaped thin film with normal dispersion, as described in the embodiments of the present invention; wherein, Figure 8 (a) is the D2 curve of the fundamental mode microring dispersion. Figure 8 (b) is the D3 curve of the fundamental mode microring dispersion.

[0035] Figure 9This is a time-domain and spectral evolution diagram of the optical frequency comb generated in the micro-ring resonator according to an embodiment of the present invention under different pump detuning. Figure 9 (a) shows the intracavity spectral evolution; Figure 9 (b) represents the intracavitary temporal evolution.

[0036] Figure 10 The following diagrams illustrate the typical time-domain and spectral envelope of a pulse within a microring cavity, corresponding to the optical frequency comb generated after multiple cyclic propagations of the laser in the microring resonant cavity as described in this embodiment of the invention, as well as the evolution of the typical intracavity average power with detuning; wherein, Figure 10 (a) is the pulse time-domain plot. Figure 10 (b) is the spectral envelope diagram. Figure 10 (c) shows the evolution of the typical intracavity average power with detuning.

[0037] Figure 11 This invention relates to the pulse time-domain and spectral envelope diagrams of the microring cavity pulses generated after multiple cyclic propagations of laser light in a microring cavity under different detuning conditions, as described in this embodiment of the invention. Figure 11 (a) is the pulse time-domain plot. Figure 11 (b) is the spectral envelope diagram.

[0038] Figure 12 As described in this embodiment of the invention, by changing the multiple relationship between the modulation frequency Ω and the FSR, the laser generates a pulse time-domain envelope map of the micro-ring cavity corresponding to the optical frequency comb after multiple cyclic transmissions in the micro-ring resonant cavity.

[0039] Figure 13 As described in this embodiment of the invention, by changing the multiple relationship between the modulation frequency Ω and the FSR, the laser generates a pulse spectrum diagram in the micro-ring cavity corresponding to the optical frequency comb after multiple cyclic transmissions in the micro-ring resonant cavity.

[0040] Figure 14 This invention presents the pulse time-domain and spectral envelope diagrams of the microring cavity pulses generated after multiple cyclic propagations of the laser in the microring cavity under different pump input powers, as well as the evolution of the average cavity power with detuning. Figure 14 (a) is the pulse time-domain plot. Figure 14 (b) is the spectral envelope diagram. Figure 14 (c) shows the evolution of the average power in the cavity as detuning occurs.

[0041] Figure 15The diagram shows the evolution of the average pulse power in the microring resonator with detuning under different normal dispersion values ​​as described in the embodiments of the present invention, as well as the time-domain and spectral envelope diagrams of the pulses in the microring resonator corresponding to the optical frequency comb generated after multiple cyclic transmissions of the laser in the microring resonator. Detailed Implementation

[0042] Embodiments of the present invention are described in detail below, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0043] It will be understood by those skilled in the art that, unless otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0044] It should also be understood that terms such as those defined in general dictionaries should be understood to have meanings consistent with their meanings in the context of the prior art, and should not be interpreted in an idealized or overly formal sense unless defined as here.

[0045] Those skilled in the art will understand that, unless specifically stated otherwise, the singular forms “a,” “an,” “the,” and “the” used herein may also include the plural forms. It should be further understood that the term “comprising” as used in this specification means the presence of the stated features, integers, steps, operations, elements, and / or components, but does not exclude the presence or addition of one or more other features, integers, steps, operations, elements, and / or groups thereof.

[0046] In the description of this specification, references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the present invention. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of those different embodiments or examples.

[0047] In the description of this specification, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this invention, "a plurality of" means two or more, unless otherwise explicitly specified.

[0048] In the description of this specification, the terms “center,” “upper,” “lower,” “front,” “rear,” “left,” “right,” “vertical,” “horizontal,” “top,” “bottom,” “inner,” and “outer,” etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing this technology and simplifying the description, and do not 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 this technology.

[0049] Unless otherwise expressly specified and limited, the terms "installation," "connection," "linking," and "setting" should be interpreted broadly. For example, they can refer to a fixed connection or setting, a detachable connection or setting, or an integral connection or setting. Those skilled in the art can understand the specific meaning of these terms in this art according to the specific circumstances.

[0050] To facilitate understanding of the present invention, the present invention will be further explained and described below with reference to the accompanying drawings and specific embodiments, and the specific embodiments do not constitute a limitation on the embodiments of the present invention.

[0051] Those skilled in the art should understand that the accompanying drawings are merely schematic diagrams of embodiments, and the components in the drawings are not necessarily essential for implementing the present invention.

[0052] Example 1

[0053] In this embodiment 1, an optical frequency comb generation device and method based on thin-film lithium niobate is first provided. Single-frequency laser / multi-frequency laser is coupled to a normal dispersion thin-film lithium niobate dual micro-ring resonator. Through the combined effect of normal dispersion, electro-optic effect and third-order nonlinear effect, the optical frequency comb spectrum is broadened and the spectrum envelope is flattened, with good flatness.

[0054] The optical frequency comb generation device in this embodiment includes a pump unit, a modulation unit, and a dual-micro-ring resonant cavity unit. The pump unit provides single-frequency / multi-frequency laser pumping. The modulation unit is a coplanar waveguide traveling-wave electrode electro-optic modulator, consisting of electrodes, a driving signal, and a load resistor. The signal output port of the driving signal is connected to the outer electrode of the racetrack-shaped lithium niobate microcavity, and the common ground port of the driving signal is connected to the inner electrode of the racetrack-shaped lithium niobate microcavity. The traveling-wave electrode structure allows light waves and microwaves to propagate in the same direction along the coplanar electrodes, and the signal is applied to the thin-film lithium niobate crystal in the form of a traveling wave, enabling the high-frequency electric field to fully interact with the light wave in the form of a traveling wave, thereby achieving electro-optic modulation of the racetrack-shaped lithium niobate microcavity unit.

[0055] The normal dispersive dual-microring resonator unit is a thin-film lithium niobate microring resonator, consisting of a single-chip integrated input / output coupling waveguide and two ring-shaped racetrack-type microcavity waveguides. The input / output coupling waveguide and the two ring-shaped racetrack-type microcavity waveguides are pulsed together to achieve fundamental mode excitation. The input coupling waveguide is used to input the pump laser, and the output coupling waveguide is used to output the generated optical frequency comb. A smaller-perimeter ring-shaped Euler microcavity is used to improve the pump laser coupling efficiency; the other, longer-perimeter ring-shaped racetrack microcavity is a racetrack microcavity based on Euler bending / or similar Euler bending. Euler bending or similar Euler bending, compared to conventional... The circular bend has a gradually changing bending radius, with a larger bending radius at the connection with the straight waveguide. This allows the fundamental mode to smoothly transition from the straight waveguide to the curved waveguide, suppressing the excitation of higher-order modes. This suppresses mode cross-coupling between the fundamental mode and higher-order modes, thereby improving the Q value of the microcavity. The straight waveguide part of the racetrack-shaped microcavity, combined with the above-mentioned modulation unit, performs electro-optic modulation on the fundamental mode optical field transmitted in the waveguide. First, the electro-optic modulation generates sidebands, producing multi-frequency lasers. As the fundamental mode optical field propagates multiple times within the microcavity, the combined effects of normal dispersion, electro-optic effect, and third-order nonlinear effect achieve spectral broadening of the optical frequency comb and flattening of the spectral envelope.

[0056] The modulation unit is a coplanar waveguide traveling wave electrode electro-optic modulator, and its electrode material is gold.

[0057] The normal dispersion dual-microring resonant cavity unit is a single-chip integrated normal dispersion thin-film lithium niobate microcavity optical waveguide (optical waveguides formed of materials with electro-optic modulation effect and third-order nonlinear effect are all acceptable, such as lithium tantalate LiTaO3, silicon carbide SiC, aluminum nitride AlN, gallium nitride GaN, gallium arsenide GaAs, gallium phosphide GaP, indium phosphide InP, etc.), including input and output coupling waveguides and dual-microring resonant cavity waveguides, with a nonlinear coefficient range of 0.3W. -1 m -1 Up to 500W -1 m -1 (Achieve a suitable normal dispersion value based on the width and height of the waveguide).

[0058] The normally dispersive thin-film lithium niobate microcavity optical waveguide is implemented using a microring resonator based on Euler bending / or similar Euler bending. A pulley coupling structure is used between the dual microring resonators and the input / output coupling waveguides to ensure effective excitation of the fundamental mode and suppress higher-order modes. A heating electrode is located in a portion of the area above the cladding of the dual microring resonators for tuning the microcavity resonant wavelength. The dual microring resonators have different perimeters, ranging from tens of micrometers to several centimeters.

[0059] The normal dispersive thin-film lithium niobate microcavity waveguide is a ridge-structured multimode thin-film lithium niobate waveguide. The width of the ridge top of the ridge-structured multimode thin-film lithium niobate waveguide is on the order of several micrometers, the thickness of the ridge and the thickness of the film below the ridge are on the order of several hundred nanometers, and the tilt angle is between 50° and 80°.

[0060] The optical frequency comb generating device is used to generate a tunable flat coherent optical frequency comb in the 1550nm band.

[0061] In this embodiment 1, the optical frequency comb generation method is implemented using the above-described device, including the following steps:

[0062] Single-frequency / multi-frequency lasers are emitted from the pump unit and incident on the input-coupled thin-film lithium niobate optical waveguide;

[0063] To improve the power conversion efficiency of the optical frequency comb, a single-frequency laser is coupled to a small-perimeter annular Euler-bent microcavity waveguide via an input coupling thin-film lithium niobate optical waveguide, and then coupled to a longer-perimeter annular runway microcavity waveguide based on Euler bending / or similar Euler bending.

[0064] In a long-circumference annular racetrack-shaped microcavity waveguide based on Euler bending, an electro-optic modulation unit with coplanar waveguide traveling-wave electrodes is used to achieve optical frequency comb spectral broadening and spectral envelope flattening through the combined effects of normal dispersion, electro-optic effect and third-order nonlinear effect.

[0065] In summary, this embodiment uses a dual-ring microcavity structure to improve the pump conversion efficiency of the optical frequency comb by incidenting a qualified single-frequency / multi-frequency laser into an input-coupled thin-film lithium niobate waveguide. The Euler bending / or similar structure within the racetrack-shaped microcavity effectively suppresses mode cross-coupling, allowing the fundamental mode to smoothly transition from the straight waveguide to the curved waveguide. The straight waveguide portion, combined with the aforementioned modulation unit, electro-optically modulates the fundamental mode light field propagating in the waveguide. First, electro-optic modulation generates sidebands, producing multi-frequency lasers. As the fundamental mode light field propagates multiple times within the microcavity, normal dispersion, electro-optic effects, and third-order nonlinear effects achieve spectral broadening and spectral envelope flattening of the optical frequency comb. Normal dispersion maintains the coherence of the optical frequency comb, resulting in an optical frequency comb with tunable frequency spacing, good coherence, and a flat spectral envelope.

[0066] By changing the relationship between the modulation frequency Ω of the modulation signal and the multiple of the microcavity FSR, it can be seen that changing the modulation frequency to a multiple of the FSR results in multiple pulses appearing in one time domain cycle of the microcavity, that is, multiple pulses appearing within one revolution of the microcavity.

[0067] Example 2

[0068] This embodiment 2 provides an optical frequency comb generating device, aiming to obtain a high-efficiency broadband coherent optical frequency comb with tunable frequency spacing. Combined with... Figure 1 and Figure 2 The optical frequency comb generating device includes a pump unit, a normal dispersion dual microring resonator unit, and its modulation unit connected in sequence, wherein:

[0069] The pump unit is used to provide single-frequency / multi-frequency pump laser; the modulation unit is a coplanar waveguide traveling-wave electrode electro-optic modulator, consisting of electrodes, a drive signal, and a load resistor. The signal output port of the drive signal is connected to the outer electrode of the lithium niobate microcavity, and the common ground port of the drive signal is connected to the inner electrode of the lithium niobate microcavity. The traveling-wave electrode structure allows light waves and microwaves to propagate in the same direction along the coplanar electrodes, and the signal is applied to the crystal in the form of a traveling wave, so that the high-frequency electric field interacts fully with the light wave in the form of a traveling wave, thereby realizing the electro-optic modulation of the microcavity unit. The normal dispersive dual-microring resonator unit is a thin-film lithium niobate microcavity, consisting of a single-chip integrated input / output coupling waveguide and two ring-shaped racetrack microcavity waveguides. The input / output coupling waveguide and the two ring-shaped racetrack microcavity waveguides employ a pulley coupling structure to achieve effective excitation of the fundamental mode. The input coupling waveguide is used to input the pump laser, and the output coupling waveguide is used to output the generated optical frequency comb. A small-perimeter ring microcavity based on Euler bending is used to improve the pump laser coupling efficiency; the other long-perimeter ring-shaped racetrack microcavity is based on Euler bending / or similar Euler bending. The racetrack-shaped microcavity, with its Euler bending compared to ordinary circular bending, allows the fundamental mode to smoothly transition from a straight waveguide to a bent waveguide, suppressing the excitation of higher-order modes. This suppresses mode cross-coupling between the fundamental and higher-order modes, improving the Q value of the microcavity. The straight waveguide portion of the racetrack-shaped microcavity, combined with the aforementioned modulation unit, performs electro-optic modulation on the fundamental mode optical field transmitted in the waveguide. First, sidebands are generated by electro-optic modulation, producing multi-frequency lasers. As the fundamental mode propagates multiple times within the microcavity, the optical frequency comb achieves spectral broadening and spectral envelope flattening through normal dispersion, electro-optic effects, and third-order nonlinear effects.

[0070] In this embodiment, a novel architecture and method are adopted to couple single-frequency / multi-frequency lasers to a dual-micro-ring resonant cavity unit. Through the combined effects of normal dispersion, electro-optic effect, and third-order nonlinear effect, the optical frequency comb spectrum is broadened and the spectrum envelope is flattened.

[0071] This embodiment 2 also provides a method for generating an optical frequency comb, including the following steps: a single-frequency pump laser emitted from a pump unit is incident into an input-coupled thin-film lithium niobate optical waveguide; in order to improve the pump power conversion efficiency of the optical frequency comb, the single-frequency laser is coupled to a smaller-perimeter annular Euler-bent microcavity waveguide through the input-coupled thin-film lithium niobate optical waveguide, and then coupled to a longer-perimeter annular racetrack-shaped microcavity waveguide based on Euler bending / or similar Euler bending; in the longer-perimeter annular racetrack-shaped microcavity waveguide, a coplanar waveguide traveling-wave electrode electro-optic modulation unit is used to achieve time-domain broadening and spectral envelope flattening of the shaped pulse through the combined action of normal dispersion, electro-optic effect and third-order nonlinear effect.

[0072] The pump unit inputs a single-frequency laser with a peak power of 0.1W to 1W. Alternatively, it can input multiple lasers with significantly different frequencies (more than 100nm apart), each with the same parameters as described above. This generates multiple optical frequency combs in the frequency domain, each centered on the input laser frequency.

[0073] The modulation unit is a coplanar waveguide traveling-wave electrode electro-optic modulator. In this traveling-wave electrode electro-optic modulator, a single-frequency laser is transmitted in a thin-film lithium niobate waveguide, and a microwave modulation signal is transmitted in the electrode. The two transmit in the same direction at the same speed, completing their interaction (electro-optic modulation). However, as the transmission length increases, the light wave and microwave will drift away to a certain extent (velocity mismatch). Therefore, the coplanar waveguide traveling-wave electrode structure, in which the electrode width, electrode thickness, electrode spacing, and upper and lower cladding thickness have a significant impact on the characteristic parameters of the electro-optic modulator, is used. Preferably, in this embodiment, the electrode material is gold, the electrode width is 10 μm, the electrode thickness is 0.9 μm, and the electrode spacing is 7 μm, in order to achieve an electro-optic modulator that meets the requirements.

[0074] The normal dispersion dual-microring resonant cavity unit is a thin-film lithium niobate microcavity, which consists of a single-chip integrated input-output coupling waveguide and two annular racetrack microcavity waveguides. The width of the ridge top of the thin-film lithium niobate optical waveguide is on the order of several micrometers, the ridge thickness and the thickness of the film under the ridge are on the order of hundreds of nanometers, and the tilt angle is between 50° and 80°. In this embodiment, preferably, the ridge top width of the ridge-type multimode thin-film lithium niobate optical waveguide is 3000 nm, the ridge thickness is 450 nm, the thickness of the film under the ridge is 200 nm, and the tilt angle is 80°. The optical frequency comb spectrum broadening and spectrum envelope flattening are achieved through normal dispersion, electro-optic effect and third-order nonlinear effect.

[0075] In this embodiment, an optical frequency comb generating device can be implemented through the following process:

[0076] A single-frequency laser, exceeding the threshold conditions for electro-optic effects and third-order nonlinearity, is emitted from a pump unit and incident into an input-coupled thin-film lithium niobate optical waveguide. The laser is then coupled to a normal-dispersion dual-micro-ring resonator unit, where the two microcavities maintain a critical coupling state to improve pump conversion efficiency. The single-frequency laser generates modulation sidebands through electro-optic modulation in a racetrack-shaped microcavity based on Euler bending. Self-phase modulation in the normal-dispersion thin-film lithium niobate optical waveguide then broadens the laser's frequency domain. During transmission, third-order nonlinearity and normal dispersion interact simultaneously, further enhancing phase modulation in conjunction with electro-optic phase modulation. This results in optical frequency comb spectral broadening and spectral envelope flattening, with flatness improved to within 10 dB and pump conversion efficiency reaching a high level.

[0077] Specifically, the thin-film lithium niobate microcavity waveguide is a ridge-structure thin-film lithium niobate optical waveguide. The ridge-structure thin-film lithium niobate optical waveguide can achieve a suitable level of flat normal dispersion by adjusting the width and height of the optical waveguide.

[0078] After geometric optimization design, in this embodiment, the width of the top of the thin-film lithium niobate optical waveguide ridge is 3000nm, the ridge thickness is 450nm, the thickness of the thin film below the ridge is 200nm, and the tilt angle is 80°. It is a high power confinement factor optical waveguide. The lithium niobate optical waveguide is surrounded by silicon dioxide material. The refractive index equation of silicon dioxide material is shown in the following equation (1):

[0079]

[0080] The nonlinear refractive index coefficient of silicon dioxide at 1550 nm is calculated according to the following formula (2):

[0081] n² = 2.2 × 10 -20 m 2 / W (2)

[0082] The refractive index equation for lithium niobate is shown in equation (3) below:

[0083]

[0084] The nonlinear refractive index coefficient of thin-film lithium niobate material at 1550 nm is calculated according to the following formula (4):

[0085] n² = 1.8 × 10 -19 m 2 / W (4)

[0086] Simulation results show that the second-order dispersion β² of the quasi-TE fundamental mode at 1550 nm of a ridge-shaped thin-film lithium niobate waveguide with a bending radius of 212.31 μm is 226.6 ps. 2 / km, the nonlinear coefficient γ is approximately 0.540547W -1 m-1 Considering a waveguide loss of 3 dB / m for the quasi-TE fundamental mode, the second-order dispersion β2 curve of the quasi-TE fundamental mode of the ridge-structured thin-film lithium niobate optical waveguide is shown in the figure below. Figure 7 As shown in Figure D. k The values ​​of D2 / 2π and D3 / 2π at 1550 nm represent the k-th order dispersion parameters within the microring, respectively: -0.695319 MHz and 848.9 Hz. The fundamental mode microring dispersion curves for D2 and D3 are shown below. Figure 8 As shown.

[0087] In this embodiment, the ridge-shaped thin-film lithium niobate is actually a multimode optical waveguide. In practical applications, mode coupling in multimode optical waveguides often occurs during waveguide bending. This embodiment employs a 180-degree Euler bend structure in the racetrack-shaped microcavity, enabling the fundamental mode to smoothly transition from the straight waveguide to the bent waveguide, and effectively suppressing mode cross-coupling and higher-order mode excitation, ensuring that only the TE fundamental mode is effectively excited and propagated. The curvature of the Euler bend changes linearly with its length. In this example, the mathematical model for constructing an Euler bend with an angle θ = 180° is as follows, where the linear coefficient of the Euler bend is 'a', and we need to specify the bending radius R. min The corresponding Euler bending length s is given by s = 2R min θ represents, for a given Euler curve, This is certain. Every point on the curve can be characterized by a and length s, as shown in equation (5):

[0088]

[0089] Where L' = as, Formula (5) is the Fresnel integral. We use Python code to plot the Euler curve, specifically choosing θ = 180° and the diameter 2R of the Euler curve. min The length of the Euler bend is 60 μm, and the calculated length s of the Euler bend is 188.49 μm, as shown in the formula. Figure 6 As shown.

[0090] In this example, the free spectral range (FSR) of the microring resonator is 100 GHz, and the group refractive index n of the microring at a wavelength of 1550 nm is... g It is 1.93144. According to the formula... The calculated R0 of the racetrack-shaped microcavity is 246.89 μm, the length of the entire microcavity is L = 2πR0 = 1551.25 μm, and the length of the straight waveguide of the racetrack-shaped microcavity is (L-2s) / 2 = 587.135 μm.

[0091] A single-frequency laser is incident on an input-coupled thin-film lithium niobate optical waveguide, and then coupled to a first small-perimeter ring microcavity based on Euler bending, in order to improve the pump conversion efficiency. Under critical coupling conditions, it is then coupled to a longer-perimeter racetrack-shaped microcavity based on Euler bending with an electro-optic modulation unit. The modulation unit adopts a traveling-wave electrode structure, and the modulated microwave and the optical wave are parallel to each other and propagate in the same direction in the electrode and waveguide to achieve high-speed electro-optic modulation. In the early stage of transmission, electro-optic modulation generates modulation sidebands and generates multi-frequency lasers. Then, the equation of the electric field envelope in the resonant cavity under the combined action of electro-optic modulation, normal dispersion and third-order nonlinear effects is as follows (6):

[0092]

[0093] J s =Jcos(sD1t+θ) is the signal from the RF signal source of the modulation unit, J is called the modulation intensity, and s is the modulation frequency related parameter (s=1 means that the modulation frequency is the same as the microcavity FSR). If the modulation intensity J=0, then the equation is the ordinary LLE equation, which shows that the electro-optic effect can enhance the phase modulation in the resonant cavity and generate modulation sidebands (frequency components).

[0094] Therefore, with the further increase in transmission distance, under normal dispersion conditions, the combined effect of electro-optic effect and third-order nonlinear effect causes the time-domain waveform to evolve into a rectangular shape, the spectrum to broaden, and the flatness to be further optimized, resulting in a broadband optical frequency comb with better flatness. The entire time-domain and frequency-domain evolution process of the optical frequency comb generated in the microcavity can be solved using equation (6).

[0095] Pump detuning is defined as the interval between the center frequency of the pump laser and the resonant frequency ω0 of the microring. Within a certain detuning range, a stable optical frequency comb can be generated. ξ0 = 2δω / κ is defined as pump detuning, where δω represents the difference between the pump center angular frequency and the microring resonant angular frequency, and κ represents the sum of internal and external losses. Due to the thermal effects and nonlinearity of the microring itself, when the pump frequency changes from high frequency to low frequency, the resonant frequency ω0 of the microring will drift, and the effective resonant frequency ω0 will decrease. eff The expression is

[0096]

[0097] Where α L Represents the coefficient of thermal expansion, α N The coefficient representing the relationship between the refractive index of the microring and the temperature difference, where n is the refractive index of the microring, and P... cav This represents the power within the microring, n2 represents the nonlinear refractive index coefficient, and A eff For the effective model area.

[0098] Typical time-domain and spectral envelope diagrams of the optical frequency comb within the micro-ring cavity are shown below. Figure 10 As shown. The parameters for generating the optical frequency comb were set as follows: racetrack-shaped microcavity FSR of 100 GHz, pump power of 100 mW, intracavity waveguide loss of 8 dB / m, microcavity dispersion D2 / 2π and D3 / 2π of -0.695319 MHz and 848.9 Hz, respectively; electro-optic modulator modulation intensity J = 6, s = 1; and the microring resonator and output coupling waveguide were critically coupled. Within a certain pump detuning range, a stable optical frequency comb could be generated. Figure 9 (b) illustrates the evolution of intracavity power with changing detuning. It can be seen that within a certain detuning range, the spectrum gradually broadens as detuning increases. Figure 9 a)) When detuning increases to a certain value, it exceeds the effective zero detuning region, and the power inside the cavity rapidly decreases to 0. The brighter the color in the figure, the higher the power. Figure 10 a) and b) show the typical time-domain and frequency-domain plots of the optical frequency comb generated at detuning=20, respectively. Figure 10 c) illustrates the evolution of intracavity power as detuning changes. Intracavity power refers to the average of the power at various locations within the cavity at a given detuning position. Figure 11 The diagram shows a comparison of the time domain and spectral envelope of the optical frequency comb in the micro-ring cavity with detuning of 17, 20, and 23. It can be seen that as the detuning increases, the corresponding spectrum becomes wider, while the pulse duration in the cavity becomes shorter.

[0099] By changing the relationship between the modulation frequency Ω and the FSR (in seconds), it can be seen that changing the FSR multiplier results in multiple pulses appearing within one time-domain period of the microcavity (corresponding to a larger spectral spacing in the optical frequency comb), such as... Figure 12 and Figure 13 As shown.

[0100] As the pump input power is changed, the evolution of the average power within the cavity with detuning is as follows: Figure 14 As shown in c), it can be seen that as the pump power increases from 0.03W to 0.16W, its detuning range continuously increases; Figure 14 a) Figure 14 b) are the time-domain and frequency-domain envelope diagrams of the optical frequency comb when the detuning is close to the effective zero-detuning region (with the same pulse time-domain duty cycle) under different pump powers. It can be concluded that when the pump power increases, the spectrum of the corresponding optical frequency comb becomes wider.

[0101] By altering the geometric dimensions of the thin-film lithium niobate optical waveguide, the magnitude of the normal dispersion β2 of the quasi-TE fundamental mode can be changed. This alteration of normal dispersion has a certain impact on the generation of the optical frequency comb, as shown in the following results. Figure 15 As shown. Figure 15 a) Shows the β2 dispersion curves corresponding to the width at the top of the ridge and the total thickness of the ridge as 3000nm×650nm, 2200nm×650nm, and 3000nm×600nm (of which the thickness of the film under the ridge is 200nm). Figure 15 (b) shows the evolution of the average power within the cavity as a function of detuning under different thin-film lithium niobate waveguide geometries. It can be seen that the β2 value is smaller at a width / height of 2200nm × 650nm, indicating the largest detuning range. The tuning ranges at width / height of 3000nm × 600nm and 3000nm × 650nm are essentially equivalent. Figure 15 c) and d) show that, under the same pulse time-domain duty cycle, the time-domain pulse power is basically the same when the width and height are 3000nm×650nm and 3000nm×600nm, while the time-domain pulse power is the highest when the width and height are 2200nm×650nm. At the same time, the closer the β2 dispersion value is to the near-zero dispersion region, the wider the spectral width.

[0102] In practical applications, the flatness of the optical frequency comb can be further improved by optimizing the microring resonator structure and loss, dispersion, or by re-pulse shaping.

[0103] Those skilled in the art should understand that Figure 1 The number of various elements shown for the sake of simplicity may be less than that of an actual optical frequency comb generating system, but such omissions are undoubtedly on the premise that they do not affect the clear and sufficient disclosure of the embodiments of the invention.

[0104] In summary, to address the issues of mode cross-coupling and higher-order mode excitation in multimode optical waveguides, this invention proposes using a 90° / 180° Euler bending structure in the microcavity to achieve bending. This allows the fundamental mode to smoothly transition from the straight waveguide to the bent waveguide, effectively suppressing mode cross-coupling and higher-order mode excitation, ensuring efficient excitation and propagation of only the quasi-TE fundamental mode, and improving the Q value of the microcavity. Regarding the need for multi-frequency sideband pumping to assist in generating the optical frequency comb in the normal dispersion region, this invention proposes using an electro-optic modulator to first form modulation sidebands, generating multi-frequency laser light, which is then propagated multiple times in a normal dispersion microcavity. Through the combined effects of normal dispersion, electro-optic effects, and third-order nonlinear effects, the optical frequency comb achieves spectral broadening and spectral envelope flattening.

[0105] The optical frequency comb generating device of this invention mainly realizes a high-efficiency broadband coherent flat optical frequency comb with controllable frequency spacing in the 1550nm band, which is beneficial to ultra-high-speed coherent optical communication, microwave photonics and other fields.

[0106] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any person skilled in the art can make various changes and modifications without departing from the spirit and scope of the present invention. For example, other materials with electro-optic effects and high third-order nonlinearity, such as lithium tantalate (LiTaO3), silicon carbide (SiC), aluminum nitride (AlN), gallium nitride (GaN), gallium arsenide (GaAs), gallium phosphide (GaP), and indium phosphide (InP), can be selected as optical waveguides to operate in the normal dispersion region. Other wavelength bands, such as the visible light band or the 1300nm band, can also be selected for operation.

[0107] While the specific embodiments of the present invention have been described above in conjunction with the accompanying drawings, this is not intended to limit the scope of protection of the present invention. Those skilled in the art should understand that, based on the technical solutions disclosed in the present invention, various modifications or variations that can be made by those skilled in the art without creative effort should be included within the scope of protection of the present invention.

Claims

1. An optical frequency comb generation device based on thin-film lithium niobate, characterized in that, Includes a pump unit and a normal dispersive dual-microring resonator unit; wherein: The pumping unit is used to provide laser pumping; The normal dispersive dual-microring resonator unit is a thin-film lithium niobate microcavity, composed of a single-chip integrated input / output coupling waveguide and two ring microcavity waveguides. The input / output coupling waveguide and the two ring microcavity waveguides employ a pulley coupling structure to achieve effective excitation of the fundamental mode. The input coupling waveguide is used for inputting the pump laser, and the output coupling waveguide is used for outputting the generated optical frequency comb. The dual-microring resonator unit includes a first ring microcavity and a second ring microcavity based on Euler bending, wherein the perimeter of the first ring microcavity is smaller than the perimeter of the second ring microcavity. The first ring microcavity is used for... High pump laser coupling efficiency; the second annular microcavity is a racetrack-shaped microcavity based on Euler bending, used to smoothly transition the fundamental mode from the straight waveguide to the curved waveguide, suppress the excitation of higher-order modes, thereby suppressing mode cross-coupling between the fundamental mode and higher-order modes and improving the Q value of the microcavity; the straight waveguide part of the racetrack-shaped microcavity performs electro-optic modulation on the fundamental mode propagating in the waveguide. First, the electro-optic modulation generates sidebands. As the fundamental mode light field propagates around the microcavity multiple times, the optical frequency comb spectrum broadening and spectrum envelope flattening are achieved through the combined action of normal dispersion, electro-optic effect and third-order nonlinear effect. The normal dispersive dual-microring resonant cavity unit integrates a microwave modulation unit; the microwave modulation unit is a coplanar waveguide traveling-wave electrode electro-optic modulator, including electrodes and load resistors; the signal output port of the driving signal is connected to the outer electrode of the racetrack-shaped lithium niobate microcavity, and the common ground port of the driving signal is connected to the inner electrode of the racetrack-shaped lithium niobate microcavity; the traveling-wave electrode structure allows light waves and microwaves to propagate in the same direction along the coplanar electrodes, and the signal is applied to the thin-film lithium niobate crystal in the form of a traveling wave so that the high-frequency electric field interacts fully with the light wave in the form of a traveling wave, thereby realizing the electro-optic modulation of the racetrack-shaped lithium niobate microcavity unit; the traveling-wave electrode is set in the racetrack part, i.e., the straight waveguide part, of the second annular microcavity.

2. The optical frequency comb generation device based on thin-film lithium niobate according to claim 1, characterized in that, The pumping unit is used to provide single-frequency / multi-frequency laser pumping to the waveguide, with a pump power range of 1mW~10W.

3. The optical frequency comb generation device based on thin-film lithium niobate according to claim 1, characterized in that, The electrode material of the coplanar waveguide traveling wave electrode electro-optic modulator is gold.

4. The optical frequency comb generating device based on thin-film lithium niobate according to claim 1, characterized in that, The input / output coupling waveguide and the two annular microcavity waveguides are either normal dispersive thin-film lithium niobate microcavity waveguides or optical waveguides formed of materials with electro-optic effects and third-order nonlinear effects, with a nonlinearity coefficient of 0.3W. -1 m -1 Up to 500W -1 m -1 .

5. The optical frequency comb generating device based on thin-film lithium niobate according to claim 1, characterized in that, A metal heating electrode is located above the cladding of the microring resonant cavity to tune the resonant wavelength of the microcavity.

6. The optical frequency comb generating device based on thin-film lithium niobate according to claim 1, characterized in that, The normal dispersive thin-film lithium niobate microcavity waveguide is a ridge-shaped multimode thin-film lithium niobate waveguide.

7. The optical frequency comb generating apparatus based on thin-film lithium niobate according to any one of claims 1-6, characterized in that, The optical frequency comb generating device is used to generate a tunable flat coherent optical frequency comb in the 1550nm band.

8. A method for generating an optical frequency comb using the apparatus as described in any one of claims 1-7, characterized in that, Includes the following steps: The pump unit pumps single-frequency / multi-frequency lasers into the input-coupled thin-film lithium niobate waveguide; Single-frequency / multi-frequency lasers are coupled into a small-perimeter ring microcavity based on Euler bending by an input coupling waveguide, and then coupled into a long-perimeter racetrack-shaped microcavity based on Euler bending, thereby improving the optical frequency comb pump power conversion efficiency. In a racetrack-shaped microcavity based on Euler bending, an electro-optic modulation unit with coplanar waveguide traveling-wave electrodes is used to achieve spectral broadening and spectral envelope flattening of the optical frequency comb through the combined effects of normal dispersion, electro-optic effect and third-order nonlinear effect.

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