Octave-crossing on-chip super-continuum spectrum generation method of near-zero dispersion engineering

Abstract

The invention provides a cross-octave on-chip super-continuum spectrum generation method for near-zero dispersion engineering, and the method employs the excellent third-order nonlinear characteristic of a Z-cut film lithium niobate material doped with 5% MgO, achieves the integrated dispersion characteristic of near-zero distribution through the optimization design of the geometric dimension of an optical waveguide. Secondly, by means of a low energy barrier between the pump wave and the dispersion wave, cross-octave super-continuum spectrum broadening and flat broadband short-wave band dispersion wave can be achieved no matter whether the pump wavelength is in a normal group velocity dispersion condition or an abnormal group velocity dispersion condition; finally, the experimental test result further verifies the effectiveness of the designed waveguide structure, and an on-chip super-continuum spectrum of which the 30dB spectral range covers one octave and flat short-wave band dispersion waves of which the 3dB spectral bandwidth reaches 70.1 THz and 77.3 THz are respectively obtained.

Description

Technical Field

[0001] This invention belongs to the field of integrated photonics and nonlinear optics, and particularly relates to a method for generating on-chip supercontinuum across octave bands using near-zero dispersion engineering. Background Technology

[0002] Supercontinuum generation (SCG) is a nonlinear optical process in which ultrashort light pulses on the femtosecond / picosecond scale propagate through a nonlinear medium, achieving significant spectral broadening through a third-order nonlinear optical process, ultimately forming an ultrawideband coherent light source. This process involves the synergistic effects of multiple nonlinear and dispersion mechanisms, including self-phase modulation, four-wave mixing, stimulated Raman scattering, self-steepening, and higher-order dispersion. Benefiting from its wide spectral width and high coherence, SCG demonstrates significant application value in trace gas detection, optical frequency combs, and biomedical imaging.

[0003] Traditional SCG schemes are primarily based on photonic crystal fibers or highly nonlinear fibers. However, due to the relatively large effective mode area of ​​the fiber (typically >10 μm²) and the limited third-order nonlinear coefficient (γ ~ 10 W), - ¹km - ¹), this approach typically requires pump energy on the order of nanojoules and an operating length of tens of centimeters, resulting in high system power consumption, increased cost, and difficulty in maintenance. With the rapid development of integrated photonics technology, on-chip micro / nano waveguide (SCG) solutions based on integrated photonic platforms such as silicon, silicon nitride, aluminum nitride, and thin-film lithium niobate have emerged. These micro / nano waveguides possess submicron-level cross-sectional areas, compact optical mode field confinement, and extremely high third-order nonlinear coefficients (γ>1000 W). - ¹km - ¹), high-efficiency, wide-spectrum SCG can be achieved with pump pulse energy in the order of hundreds of picojoules and waveguide length in the order of millimeters.

[0004] Currently, there are two main typical implementation schemes for on-chip SCGs. Scheme one involves pumping in the anomalous dispersion region, where the fundamental soliton undergoes soliton fission under the combined effect of higher-order dispersion. When the phase-matching condition is met, the soliton energy is transferred to short-wavelength and long-wavelength bands through dispersive wave radiation, thus achieving an ultra-wideband SCG covering the visible to mid-infrared region. However, this process involves complex four-wave mixing and cross-phase modulation, introducing significant noise and phase perturbations, enhancing modulation instability, and reducing the temporal coherence of the output spectrum. Scheme two involves pumping in the normal dispersion region, relying on self-phase modulation to achieve spectral broadening. Since normal dispersion can suppress modulation instability and avoid soliton dynamics, this scheme can better maintain the temporal waveform and phase coherence of the pump pulse, achieving high coherence and low-noise spectral output, making it particularly suitable for coherent light sources and precision measurement applications. However, under normal dispersion conditions, the spectral broadening bandwidth of the SCG is limited, and waveguide lengths of tens of centimeters are typically required to achieve a spectral range spanning multiple octaves.

[0005] One existing method for achieving flat supercontinuum output pumped by a high-repetition-rate femtosecond laser involves automatically obtaining the geometry and pumping conditions of a photonic crystal fiber through target spectrum setting and parameter optimization. In the experiment, a Yb: fiber laser with a 1 GHz repetition rate, a center wavelength of 1064 nm, a pulse width of approximately 150 fs, and an energy of 450 pJ was used as the pump, resulting in a flat and broadened supercontinuum output in the visible band. The results show that the output covers 510 nm to 850 nm (bandwidth of approximately 340 nm) within a 3 dB intensity fluctuation range. Figure 1 As shown, a smoother spectral output with a fluctuation of 1.72 dB in the 530 nm to 790 nm range can be further obtained through polarization adjustment. This technique requires no additional spectral shaping and can directly obtain broadband, flat, and highly coherent supercontinuum spectra under low-energy pump conditions, making it particularly suitable for applications such as high-precision calibration of astronomical spectrometers, multispectral imaging, and optical coherence tomography.

[0006] Another existing method for achieving coherent supercontinuum output in fully normal dispersive (ANDi) silicon nitride waveguides is as follows: Figure 2 As shown, this scheme designs a silicon nitride waveguide with a cross-sectional area of ​​2700nm × 690nm and a length of approximately 20cm, employing a helical geometry to achieve low loss and flat group velocity dispersion characteristics. Experiments show that, when pumped by high-energy pulses with a pulse width of 50fs and a repetition frequency of 250MHz, SCG broadening can be achieved through a self-phase modulation mechanism, obtaining a cross-octave range covering 1010nm-2020nm, while maintaining good coherence throughout the entire bandwidth.

[0007] The existing third method for generating mid-infrared broadband SCG is as follows: Figure 3 As shown in (a), this scheme employs a segmented optical waveguide structure, including a 1.5 mm long normal dispersion waveguide (4.0 µm wide), a 1.2 mm long tapered waveguide, and a 1.5 mm long anomalous dispersion waveguide (2.0 µm wide, wavelength range 1500-2600 nm). First, the pump light is broadened in the normal dispersion waveguide, and then compressed into an ultrashort time pulse (<10 fs) in the anomalous dispersion section to obtain a broad spectral output. The experiment used a femtosecond pulse pump with a center wavelength of 1560 nm, a pulse width of 55 fs, and an on-chip energy of 527 pJ. This structure, with a length of 5 mm, can achieve a cross-octave spectrum covering a range of 0.87–3.9 µm, and exhibits a flat and coherent spectral output in the mid-infrared band, such as... Figure 3 As shown in (b) above, this flat mid-infrared spectrum exhibits good coherence and uniform energy distribution within the 3 to 4 µm molecular fingerprint region, making it particularly suitable for gas absorption detection and molecular spectral analysis, and providing an effective approach for the engineering application of on-chip mid-infrared optical combs.

[0008] It can be seen that on-chip supercontinuum generation, as an important research direction in nonlinear optics, has been verified in many integrated photonic platforms. However, there are still some shortcomings in the existing technology: (1) Poor spectral flatness: Although the supercontinuum in anomalous dispersive waveguides can be broadened to span octaves, the spectral energy distribution is uneven and there are obvious fluctuations. In addition, the spectral flatness of the dispersive wave is insufficient, which limits its application in high-precision spectral measurement; (2) Large barrier to dispersive wave generation: When the waveguide dispersion design is not ideal, the energy transfer from solitons to dispersive waves is hindered, resulting in weak dispersive wave energy and difficulty in obtaining octave spectral broadening; (3) Limited pump power: Although the supercontinuum in normal dispersive waveguides has good flatness, high-energy femtosecond laser pumping is usually required to achieve octave spectral broadening, so there is a problem of high-power pumping requirement. Summary of the Invention

[0009] To address the aforementioned problems, this invention provides a method for generating octave band on-chip supercontinuum using near-zero dispersion engineering. Based on near-zero dispersion modulation, it achieves octave band spectra and flat, broadband short-wavelength dispersive waves through a simpler straight waveguide structure.

[0010] A method for generating octave band on-chip supercontinuum in near-zero dispersion engineering is proposed, which couples a femtosecond laser into a thin-film lithium niobate waveguide chip and generates octave band supercontinuum and flat broadband short-band dispersive waves through third-order nonlinear interactions. The thin-film lithium niobate waveguide chip consists of, from bottom to top, a silicon substrate, a thermally oxidized silicon dioxide layer, and a Z-cut thin-film lithium niobate doped with 5% mol MgO.

[0011] Furthermore, the Z-cut thin-film lithium niobate waveguide has a thickness of 800 nm. The Z-cut thin-film lithium niobate waveguide is divided into a waveguide substrate layer and a waveguide channel layer with a width of 590 nm and a length the same as the waveguide substrate layer. The waveguide channel layer is obtained by directly etching the surface of the waveguide substrate layer to a depth of 425 nm. The angle between the sidewall of the waveguide channel layer and the surface of the waveguide substrate layer is 68°.

[0012] Furthermore, the Z-cut thin-film lithium niobate waveguide has a thickness of 600 nm. The Z-cut thin-film lithium niobate waveguide is divided into a waveguide substrate layer and a waveguide channel layer with a width of 2030 nm and a length the same as the waveguide substrate layer. The waveguide channel layer is obtained by directly etching the surface of the waveguide substrate layer to a depth of 450 nm. The angle between the sidewall of the waveguide channel layer and the surface of the waveguide substrate layer is 75°.

[0013] Furthermore, the thickness of the silicon substrate is 525 μm, and the thickness of the thermally oxidized silicon dioxide layer is 2 μm.

[0014] Furthermore, the method for generating transoctave supercontinuum and flat broadband short-wavelength dispersive waves by femtosecond laser pulses in a thin-film lithium niobate waveguide chip through third-order nonlinear interactions is as follows: In the initial stage of supercontinuum generation, a femtosecond laser pulse with a center wavelength of 1560 nm forms a normal dispersion broadened spectrum in the first normal dispersion region of the Z-cut thin-film lithium niobate waveguide through self-phase modulation, so that the femtosecond laser pulse gradually broadens in both frequency and time dimensions, forming a supercontinuum. As the leading edge of the femtosecond laser pulse gradually approaches the zero-dispersion point of the Z-cut thin-film lithium niobate waveguide, some of its energy leaks into the anomalous dispersion region of the Z-cut thin-film lithium niobate waveguide, thereby compensating for the nonlinear phase accumulated in the previous normal dispersion and forming an optical soliton in the anomalous dispersion region. As the propagation distance of femtosecond laser pulses in Z-cut thin-film lithium niobate waveguides increases, solitons in the anomalous dispersion region interact with the supercontinuum of the first normal dispersion region through cross-phase modulation effects, causing the solitons to move away from the pump direction in the spectrum. Ultimately, in the second normal dispersion region, a short-wavelength dispersive wave with high spectral power density and spectral flatness is formed, thereby expanding the broadening range of the supercontinuum.

[0015] Furthermore, the method for generating transoctave supercontinuum and flat broadband short-wavelength dispersive waves by femtosecond laser pulses in a thin-film lithium niobate waveguide chip through third-order nonlinear interactions is as follows: A femtosecond laser pulse with a wavelength of 1560 nm first forms a higher-order soliton through a self-phase modulation process during propagation; As the propagation distance of the femtosecond laser pulse in the Z-cut thin-film lithium niobate waveguide increases, the higher-order solitons undergo soliton splitting in the Z-cut thin-film lithium niobate waveguide, decomposing into multiple lower-order solitons. During propagation, each lower-order soliton is affected by both higher-order dispersion and nonlinearity, radiating energy into a specific wavelength region, exciting phase-matched short-wavelength and long-wavelength dispersive waves located at 1080nm and 3075nm, respectively, thereby achieving supercontinuum broadening across octave bands.

[0016] Furthermore, the light source for pumping the thin-film lithium niobate waveguide chip is an erbium-doped fiber femtosecond laser, which outputs femtosecond pulses with a center wavelength of 1560nm, a pulse width of approximately 80fs, and a repetition frequency of 100MHz; a tunable optical attenuator is used to adjust the power of the incident femtosecond laser; and the power-adjusted femtosecond laser is coupled into the thin-film lithium niobate waveguide chip through a collimating lens and a focusing lens. The broadband supercontinuum generated by the third-order nonlinear interaction of femtosecond laser pulses in the thin-film lithium niobate waveguide chip is collected by a lens fiber and sent to a near-infrared spectrometer and a mid-infrared spectrometer for real-time spectral detection and analysis.

[0017] Furthermore, femtosecond lasers in thin-film lithium niobate waveguide chips must meet the following phase-matching conditions:

[0018] in, Integrated dispersion for Z-cut thin-film lithium niobate waveguides The propagation constant of the Z-cut thin-film lithium niobate waveguide is... The propagation constant at the center angular frequency of the femtosecond laser pulse. ω is the angular frequency of the light wave. The center angular frequency of the femtosecond laser pulse. The group velocity of the Z-cut thin-film lithium niobate waveguide is given.

[0019] Beneficial effects: 1. This invention provides a method for generating a supercontinuum across octave bands in near-zero dispersion engineering. Utilizing the excellent third-order nonlinear characteristics of a 5% MgO-doped Z-cut lithium niobate thin film, near-zero dispersion characteristics are achieved through optimized design of the optical waveguide geometry. Secondly, by leveraging the low energy barrier between the pump wave and the dispersive wave, supercontinuum broadening across octave bands and flat, broadband short-wavelength dispersive waves can be achieved regardless of whether the pump wavelength is under normal or anomalous group velocity dispersion conditions. Finally, experimental results further verify the effectiveness of the waveguide structure designed in this invention, yielding a supercontinuum spanning one octave band in the 30 dB spectral range, and flat short-wavelength dispersive waves with 3 dB spectral bandwidths of 70.1 THz and 77.3 THz.

[0020] 2. This invention provides a method for generating on-chip supercontinuum across octave bands in near-zero dispersion engineering. It utilizes 5% MgO-doped Z-cut thin-film lithium niobate material, which effectively improves the damage threshold of the waveguide and suppresses the photorefractive effect, enabling the waveguide to remain stable and repetitive under high-power pumping conditions. This effectively avoids the problem of easy damage to traditional undoped thin-film lithium niobate waveguides under strong laser conditions, and can be extended to more practical application scenarios.

[0021] 3. This invention provides a method for generating on-chip supercontinuum across octave bands in near-zero dispersion engineering. By precisely designing the cross-sectional dimensions of the thin-film lithium niobate waveguide, a controllable group velocity dispersion curve distribution is achieved, resulting in near-zero integrated dispersion characteristics. This significantly reduces the potential barrier between solitons and dispersive waves, thereby improving the dispersive wave generation efficiency and obtaining broadband flat spectrum output. Attached Figure Description

[0022] Figure 1 The supercontinuum output of a photonic crystal fiber is shown in the prior art, where the purple and orange curves represent the results of dispersion optimization and non-dispersion optimization, respectively. Figure 2 This is a schematic diagram illustrating the coherent supercontinuum output achieved in a fully normal dispersive (ANDi) silicon nitride waveguide using existing technology 2; wherein, Figure 2 In the figure, (a) and (b) are silicon nitride spiral waveguides and their group velocity dispersion curves; (c) and (d) are SCG spectra of different input pulse energies in simulation and experiment. Figure 3 This is a schematic diagram of the existing third technique for generating mid-infrared broadband SCG based on a gallium nitride optical waveguide on a sapphire substrate; wherein, Figure 3 (a) shows the group velocity dispersion curves for gallium nitride waveguides with widths of 4µm (violet) and 2µm (red). Figure 3 (b) in the figure represents the cross-octave SCG spectrum and the normalized absorption spectra of different gases output from the segmented gallium nitride optical waveguide. Figure 4 This is a schematic diagram of the thin-film lithium niobate optical waveguide structure used in this invention; Figure 5 The first waveguide structure parameters designed and optimized in this invention and their corresponding TE00 mode field distribution diagram are shown. Figure 6 The second waveguide structure parameters designed and optimized in this invention and their corresponding TE00 mode field distribution diagram are shown. Figure 7 This is the variation of the second-order group velocity dispersion curve with wavelength calculated in this invention; Figure 8 The integrated dispersion curve calculated for this invention; Figure 9 The time-domain output pulses (top) of waveguide structure 1 at a propagation length of 4.3 mm (left) and waveguide structure 2 at a propagation length of 7 mm (right) provided for this invention, and the pulse evolution process (bottom) with waveguide length. Figure 10 A schematic diagram of the experimental setup for characterizing the optical waveguide performance provided by this invention; Figure 11 The broadened spectral output of the waveguide structure 1 provided by this invention under different on-chip pump pulse energies; Figure 12 The broadened spectral output of the waveguide structure 2 provided by this invention is obtained under different on-chip pump pulse energies. Detailed Implementation

[0023] To enable those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings.

[0024] This invention provides a method for generating a transoctave supercontinuum on-chip with near-zero dispersion engineering. A femtosecond laser is coupled into a thin-film lithium niobate waveguide chip, generating a transoctave supercontinuum and a flat, broadband short-wavelength dispersive wave through third-order nonlinear interactions; wherein, as... Figure 4 As shown, the thin-film lithium niobate waveguide chip consists of a silicon substrate, a thermally oxidized silicon dioxide layer, and a Z-cut thin-film lithium niobate doped with 5% mol MgO, from bottom to top.

[0025] The following simulation optimization of the dispersion characteristic curve of the thin-film lithium niobate optical waveguide demonstrates that the present invention can achieve dispersion control and realize the generation of flat, broadband short-wavelength dispersive waves and transoctave supercontinuum.

[0026] Step S1: The transoctave supercontinuum scheme implemented in this invention is as follows: Figure 4 As shown, the main approach utilizes the third-order nonlinear effect in thin-film lithium niobate optical waveguides to achieve near-zero flat dispersion and supercontinuum output across octave bands through optimized dispersion design. Figure 5 and Figure 6 To optimize the parameters of the two optical waveguide structures, Figure 5 In structure 1, the thickness of the Z-cut thin-film lithium niobate waveguide is 800nm. The Z-cut thin-film lithium niobate waveguide is divided into a waveguide substrate layer and a waveguide channel layer with a width of 590nm and a length the same as the waveguide substrate layer. The waveguide channel layer is obtained by directly etching the surface of the waveguide substrate layer to a depth of 425nm. The angle between the sidewall of the waveguide channel layer and the surface of the waveguide substrate layer is 68°. Figure 6In structure 2, the Z-cut thin-film lithium niobate waveguide has a thickness of 600 nm. It consists of a waveguide substrate layer and a waveguide channel layer with a width of 2030 nm and a length equal to the substrate layer. The channel layer is directly etched onto the surface of the substrate layer to a depth of 450 nm, with a 75° angle between the sidewalls of the channel layer and the surface of the substrate layer. In all these structures, the invention employs an air cladding, a core layer of 5% mol MgO-doped Z-cut thin-film lithium niobate, a 2 μm thick thermally oxidized silicon dioxide layer as the lower cladding, and a 525 μm thick silicon substrate. Compared to pure lithium niobate, MgO doping increases the damage resistance threshold by 130 times, thus ensuring the stability of the optical waveguide under high-power femtosecond pulse pumping.

[0027] Step S2: Using the finite element analysis method in the commercial software COMSOL Multiphysics, simulate the mode field distribution in the thin-film lithium niobate optical waveguide to obtain the transverse electric fundamental mode (TE00) distribution of structures 1 and 2, such as... Figure 5 and Figure 6 As shown.

[0028] Step S3: Analyze the dispersion characteristics of the optical waveguide, including group velocity dispersion. It is an important parameter representing the propagation constant. For frequency The result obtained by taking the second derivative is often used to describe the dispersive properties of materials and is an important factor affecting pulse broadening. It is usually expressed as: (1) in, Represents the corresponding wavelength The effective mode refractive index. >0 indicates the normal dispersion region, while <0 indicates the anomalous dispersion region. For example... Figure 7 As shown, at a pump wavelength of 1560 nm, structure 1 exhibits normal dispersion, while structure 2 exhibits anomalous dispersion.

[0029] Step S4: When the pump is a femtosecond laser pulse, the soliton dynamics perturbation caused by higher-order dispersion can further broaden the spectrum by generating a dispersive wave. This process requires the following phase-matching condition to be met: (2) in, Integrated dispersion for Z-cut thin-film lithium niobate waveguides The propagation constant of the Z-cut thin-film lithium niobate waveguide is... The propagation constant at the center angular frequency of the femtosecond laser pulse. ω is the angular frequency of the light wave. The center angular frequency of the femtosecond laser pulse. The group velocity of the Z-cut thin-film lithium niobate waveguide is given.

[0030] The wavelength that satisfies the above phase-matching condition is called the zero-dispersion wavelength. For example... Figure 8 As shown, both Structure 1 and Structure 2 exhibit near-zero integrated dispersion characteristics, with their short-wavelength dispersion waves located at 1000 nm and 1080 nm, respectively. Due to the design... The trend of change is gradual, and the dispersion barrier between the zero-dispersion wavelength and the pump wavelength is small, which enables efficient dispersive wave radiation when the pump pulse propagates in the waveguide. Specifically, near-zero dispersion wavelength... This allows for a better phase matching condition between the nonlinearity and the dispersive term, thus enabling the efficient generation of dispersive waves over a wider wavelength range. Simultaneously, this characteristic helps avoid significant spectral asymmetry during spectral broadening, thereby facilitating the formation of flat, broadband dispersive wave outputs over a wide wavelength range.

[0031] Step S5: In structure 1, this invention achieves directional supercontinuum broadening for the first time in a thin-film lithium niobate optical waveguide. The curve is characterized by an anomalous dispersion region (ADR) surrounded by two normal dispersion regions (NDR). When the pump wavelength is in the normal dispersion region ( This method can fully utilize dispersive wave radiation to achieve spectral broadening. The specific mechanism is as follows: In the initial stage of supercontinuum generation, the pulse is mainly subjected to self-phase modulation, forming a typical normal dispersion broadened spectrum, with the pulse gradually broadening in both frequency and time dimensions. As the pulse leading edge gradually approaches the zero dispersion point, some of its energy leaks into the anomalous dispersion region, thereby compensating for the nonlinear phase accumulated in the previous normal dispersion and forming a soliton in the anomalous dispersion region. As the transmission distance further increases, the soliton interacts with the supercontinuum in the normal dispersion region through cross-phase modulation, causing the soliton to move away from the pump direction in the spectrum. Finally, in the second normal dispersion region, around 1000 nm, a short-wavelength dispersive wave with high spectral power density and spectral flatness is formed, thus expanding the broadening range of the supercontinuum.

[0032] Step S6: In structure 2, the present invention uses a pump wavelength located in the anomalous dispersion region ( The supercontinuum broadening mechanism of [structure 1] is described. Unlike structure 1, the pulse first undergoes self-phase modulation during propagation and rapidly forms higher-order solitons. As the transmission distance increases, the higher-order solitons undergo soliton fission in the waveguide, with the corresponding waveguide length denoted as [equation missing]. At this point, the pulse has been compressed to its limit, such as... Figure 9As shown, it then decomposes into multiple low-order solitons. During propagation, these solitons, influenced by both higher-order dispersion and nonlinearity, radiate energy into specific wavelength regions, exciting phase-matched short-wavelength and long-wavelength dispersive waves at 1080 nm and 3075 nm, respectively, thereby achieving supercontinuum broadening across octave bands.

[0033] Step S7: This invention further utilizes numerical simulation methods to theoretically analyze the nonlinear propagation dynamics of optical pulses in thin-film lithium niobate waveguides by solving the generalized nonlinear Schrödinger equation (GNLSE).

[0034] (3) Wherein, the electric field strength E(z,t) is expressed as a function of the propagation distance z and the delay time t. For propagation loss, for Dispersion coefficient of order, The model also incorporates Raman scattering and self-steepening effects to verify their influence on pulse propagation, using third-order nonlinear coefficients. To ensure computational accuracy, this invention employs a distributed Fourier algorithm to numerically integrate and solve the above equations, thereby obtaining the time-domain and frequency-domain results of the pulse at a specific propagation length.

[0035] From steps S1-S7, the two optical waveguide structures proposed in this invention, through reasonable design of their group velocity dispersion and integrated dispersion, can support cross-octave band and flat broadband dispersive wave output under both normal and anomalous dispersion pumping conditions, providing a reliable theoretical basis for subsequent experimental research. Based on the designed waveguide structures, this invention employs micro-nano fabrication techniques, including electron beam lithography, ion beam etching, wet cleaning, high-temperature thermal annealing, and end-face cleavage, to complete the fabrication of the device.

[0036] The following experimental study on nonlinear frequency conversion in a thin-film lithium niobate optical waveguide was conducted using a femtosecond laser as the pump source. Through testing and analysis of the output spectrum, it was shown that the waveguide of this invention can achieve the expected octave supercontinuum and flat width-dispersive wave under different pumping conditions, and the experimentally recorded spectral characteristics are basically consistent with the numerical simulation results.

[0037] Step 1: As Figure 10As shown, the light source is an erbium-doped fiber femtosecond laser, outputting femtosecond pulses with a center wavelength of 1560 nm, a pulse width of approximately 80 fs, and a repetition frequency of 100 MHz. A tunable optical attenuator is then used to control the incident light power. Subsequently, the light is efficiently coupled to a thin-film lithium niobate waveguide chip through a collimating lens and a focusing lens, with a double-ended coupling insertion loss of approximately 10 dB. The optical pulses generate a broadband supercontinuum spectrum through nonlinear interactions within the waveguide. The output is collected by a lensed fiber with a mode spot diameter of 2.5 μm and fed into a near-infrared spectrometer (350 nm–1750 nm) and a mid-infrared spectrometer (1000 nm–5600 nm) for real-time spectral detection and analysis.

[0038] Step 2: For structure 1, test the output spectrum under different input pulse energies. For example... Figure 11 As shown, when the input energy is low, the output spectrum broadens only slightly, mainly exhibiting a narrow band spectrum under self-phase modulation. With increasing input energy, the output spectrum gradually expands, and a significant dispersive wave component appears. When the on-chip pulse energy increases to 277 pJ, a flat and effectively broadened dispersive wave can be observed near a wavelength of 1000 nm, with a 3 dB spectral range covering 942 nm to 1208 nm, corresponding to a frequency bandwidth of 70.1 THz, while the 30 dB spectral range covers 890 nm to 1920 nm, approximately 1.1 octaves. The experimentally recorded spectral envelope shows good consistency with the numerical simulation results, verifying the rationality of the structural design.

[0039] Step 3: For structure 2, the output spectrum was tested under different on-chip optical pulse energies and compared with the simulation results. The results showed good consistency. Figure 12 As shown, when the on-chip pulse energy is gradually increased to 135 pJ, a spectrally flat dispersive wave is obtained through the interaction between the soliton and the dispersive wave, with a 3 dB bandwidth covering 998 nm to 1345 nm, corresponding to a frequency domain bandwidth of approximately 77.3 THz. Furthermore, the entire supercontinuum's 30 dB spectral range covers 952 nm to 2206 nm, approximately 1.2 octaves. Further increasing the on-chip pulse energy to 270 pJ results in further spectral broadening, but the flatness of the dispersive wave decreases. These results indicate that structure 2 (anomalous dispersion) can only achieve a spectrally flat dispersive wave at specific pump pulse energies.

[0040] From steps 1 to 3, this invention systematically experimentally verified the designed lithium niobate waveguide. Test results show that, under suitable pump power and pulse conditions, both designed waveguide structures can effectively achieve transoctave supercontinuum and flat dispersive waves, and the experimentally recorded spectral characteristics are basically consistent with the simulation results. These results demonstrate the feasibility and effectiveness of the waveguide structure and dispersive design, and are expected to provide support for its application in precision spectroscopy, biomedical imaging, and other fields.

[0041] In summary, the present invention has the following advantages compared with the prior art: 1. This invention utilizes 5% MgO-doped Z-cut thin-film lithium niobate material to effectively improve the damage threshold of the waveguide and suppress the photorefractive effect. This allows the waveguide to remain stable and repetitive under high-power pumping conditions, effectively avoiding the problem of easy damage to traditional undoped thin-film lithium niobate waveguides under strong laser conditions, and enabling its application in more practical scenarios. In other words, compared with photonic crystal fiber-based solutions, this invention employs a highly nonlinear micro / nano waveguide structure, avoiding the problems of high pump power and difficulty in dispersion control inherent in traditional fiber optic devices, greatly enhancing its practical application prospects.

[0042] 2. Compared with the fully normal dispersion silicon nitride waveguide scheme, the present invention is more flexible in terms of pumping conditions. It can achieve flat broadening under normal dispersion pumping and realize soliton fission and dispersive wave generation under anomalous dispersion pumping, thus broadening the spectral applicability range.

[0043] 3. This invention is the first to utilize directional supercontinuum in a thin-film lithium niobate optical waveguide to achieve octave-wide spectral broadening. By pumping in the normal dispersion region, a broadband and flat dispersive wave can be generated in a specific short-wavelength direction, with a 3dB bandwidth of 70.1 THz. Compared to gallium nitride waveguide schemes that use segmented waveguides to achieve flat mid-infrared spectrum output, this invention, based on a thin-film lithium niobate photonic platform, can achieve flat dispersive wave output in the short-wavelength band using only a simpler straight waveguide structure. Furthermore, this invention can fully utilize the second-order nonlinearity of lithium niobate material, potentially achieving the overlap of the second harmonic and dispersive wave in the output spectrum, thus enabling beat frequency analysis of the two components, suitable for on-chip f-2f self-reference technology.

[0044] 4. This invention systematically demonstrates, through a combination of theoretical simulation and experimental verification, that the designed waveguide structure can achieve high flatness, wideband flat dispersive waves and octave-band supercontinuum output under different dispersion conditions, providing a technical foundation for practical applications such as biomedical imaging, trace gas detection, and precision metrology. In other words, this invention combines theoretical feasibility with experimental verification results, ensuring the feasibility and scalability of the technical solution, and providing a new technical path for achieving near-zero dispersion octave-band spectra and flat broadband dispersive waves in thin-film lithium niobate optical waveguides.

[0045] The above description is merely a preferred embodiment of the present invention. The present invention is not limited to the examples themselves. Since it is impossible to exhaustively list all methods similar to the generation of on-chip octave band supercontinuum and flat broadband dispersive waves, all modifications to other forms are within the protection scope of the present invention.

Claims

1. A method for generating on-chip supercontinuum across octave bands using near-zero dispersion engineering, characterized in that, Femtosecond lasers are coupled into thin-film lithium niobate waveguide chips to generate transoctave supercontinuum and flat broadband short-band dispersive waves through third-order nonlinear interactions. The thin-film lithium niobate waveguide chip consists of, from bottom to top, a silicon substrate, a thermally oxidized silicon dioxide layer, and a Z-cut thin-film lithium niobate doped with 5% mol MgO.

2. The method for generating a transoctave on-chip supercontinuum in near-zero dispersion engineering as described in claim 1, characterized in that, The Z-cut thin-film lithium niobate waveguide has a thickness of 800 nm. It consists of a waveguide substrate layer and a waveguide channel layer with a width of 590 nm and a length the same as the waveguide substrate layer. The waveguide channel layer is obtained by directly etching the surface of the waveguide substrate layer to a depth of 425 nm. The angle between the sidewall of the waveguide channel layer and the surface of the waveguide substrate layer is 68°.

3. The method for generating a transoctave on-chip supercontinuum in near-zero dispersion engineering as described in claim 1, characterized in that, The Z-cut thin-film lithium niobate waveguide has a thickness of 600 nm. It consists of a waveguide substrate layer and a waveguide channel layer with a width of 2030 nm and a length the same as the waveguide substrate layer. The waveguide channel layer is obtained by directly etching the surface of the waveguide substrate layer to a depth of 450 nm. The angle between the sidewall of the waveguide channel layer and the surface of the waveguide substrate layer is 75°.

4. A method for generating a transoctave on-chip supercontinuum in near-zero dispersion engineering as described in any one of claims 1 to 3, characterized in that, The thickness of the silicon substrate is 525 μm, and the thickness of the thermally oxidized silicon dioxide layer is 2 μm.

5. The method for generating a transoctave on-chip supercontinuum in near-zero dispersion engineering as described in claim 2, characterized in that, The method for generating transoctave supercontinuum and flat broadband short-wavelength dispersive waves by femtosecond laser pulses in a thin-film lithium niobate waveguide chip through third-order nonlinear interactions is as follows: In the initial stage of supercontinuum generation, a femtosecond laser pulse with a center wavelength of 1560 nm forms a normal dispersion broadened spectrum in the first normal dispersion region of the Z-cut thin-film lithium niobate waveguide through self-phase modulation, so that the femtosecond laser pulse gradually broadens in both frequency and time dimensions, forming a supercontinuum. As the leading edge of the femtosecond laser pulse gradually approaches the zero-dispersion point of the Z-cut thin-film lithium niobate waveguide, some of its energy leaks into the anomalous dispersion region of the Z-cut thin-film lithium niobate waveguide, thereby compensating for the nonlinear phase accumulated in the previous normal dispersion and forming an optical soliton in the anomalous dispersion region. As the propagation distance of femtosecond laser pulses in Z-cut thin-film lithium niobate waveguides increases, solitons in the anomalous dispersion region interact with the supercontinuum of the first normal dispersion region through cross-phase modulation effects, causing the solitons to move away from the pump direction in the spectrum. Ultimately, in the second normal dispersion region, a short-wavelength dispersive wave with high spectral power density and spectral flatness is formed, thereby expanding the broadening range of the supercontinuum.

6. The method for generating a transoctave on-chip supercontinuum in near-zero dispersion engineering as described in claim 3, characterized in that, The method for generating transoctave supercontinuum and flat broadband short-wavelength dispersive waves by femtosecond laser pulses in a thin-film lithium niobate waveguide chip through third-order nonlinear interactions is as follows: A femtosecond laser pulse with a wavelength of 1560 nm first forms a higher-order soliton through a self-phase modulation process during propagation; As the propagation distance of the femtosecond laser pulse in the Z-cut thin-film lithium niobate waveguide increases, the higher-order solitons undergo soliton splitting in the Z-cut thin-film lithium niobate waveguide, decomposing into multiple lower-order solitons. During propagation, each lower-order soliton is affected by both higher-order dispersion and nonlinearity, radiating energy into a specific wavelength region, exciting phase-matched short-wavelength and long-wavelength dispersive waves located at 1080nm and 3075nm, respectively, thereby achieving supercontinuum broadening across octave bands.

7. The method for generating a transoctave on-chip supercontinuum in near-zero dispersion engineering as described in claim 1, characterized in that, The light source for pumping the thin-film lithium niobate waveguide chip is an erbium-doped fiber femtosecond laser, which outputs femtosecond pulses with a center wavelength of 1560nm, a pulse width of about 80fs, and a repetition frequency of 100MHz. A tunable optical attenuator is used to adjust the power of the incident femtosecond laser. The power-adjusted femtosecond laser is coupled into the thin-film lithium niobate waveguide chip through a collimating lens and a focusing lens. The broadband supercontinuum generated by the third-order nonlinear interaction of femtosecond laser pulses in the thin-film lithium niobate waveguide chip is collected by a lens fiber and sent to a near-infrared spectrometer and a mid-infrared spectrometer for real-time spectral detection and analysis.

8. The method for generating a transoctave on-chip supercontinuum in near-zero dispersion engineering as described in claim 1, characterized in that, Femtosecond lasers in thin-film lithium niobate waveguide chips must meet the following phase matching conditions: in, Integrated dispersion for Z-cut thin-film lithium niobate waveguides The propagation constant of the Z-cut thin-film lithium niobate waveguide is... The propagation constant at the center angular frequency of the femtosecond laser pulse. ω is the angular frequency of the light wave. The center angular frequency of the femtosecond laser pulse. The group velocity of the Z-cut thin-film lithium niobate waveguide is given.

Images