On-chip f-2f self-referencing method for a periodic-cone thin-film lithium niobate optical waveguide

Abstract

The application provides a kind of on-chip f-2f self-referencing method of period taper type thin film lithium niobate optical waveguide, period taper waveguide structure is introduced to widen the phase matching bandwidth of second harmonic, make the broadband dispersion wave of supercontinuum spectrum generated by second harmonic spectrum fully cover, so as to reduce the demand of f-2f self-referencing waveguide device to pump pulse energy. ceo Beat signal; that is, the application can effectively widen the phase matching bandwidth of second harmonic, so that the second harmonic generated and the dispersion wave of supercontinuum process have wideband spectrum overlap, and on-chip f-2f self-referencing can be realized under lower pump energy, which can reduce the energy consumption by nearly 40% compared with traditional straight waveguide device.

Description

Technical Field

[0001] This invention belongs to the fields of integrated photonics and nonlinear optics, and particularly relates to an on-chip f-2f self-reference method for a periodic conical thin-film lithium niobate optical waveguide. Background Technology

[0002] Optical frequency combs (hereinafter referred to as "optical combs") are indispensable tools in the fields of precision metrology and timekeeping. In these applications, the traceability of absolute frequency is crucial. Therefore, it is necessary to extract the carrier-envelope offset frequency (f0) of the optical comb. ceo This process involves completely locking the octave band optical comb. This is achieved using f-2f self-reference technology, which involves frequency doubling the long-wavelength portion of the octave band optical comb and heterodyne-beating its corresponding short-wavelength portion. Traditional methods for realizing octave band optical combs rely on supercontinuum generation (SCG) in bulk nonlinear crystals or highly nonlinear fibers. However, this method has inherent drawbacks such as high energy consumption, difficulty in on-chip integration, and large size, making it difficult to meet the miniaturization and integration requirements of modern precision instruments. In recent years, micro / nano waveguides, with their subwavelength structural advantages, have exhibited extremely strong optical field confinement capabilities and flexibly tunable dispersion characteristics. Pumped by pulsed energies of several hundred pJ and below, optical waveguides can realize ultra-wideband SCGs in millimeter-scale chips, providing an integrated and energy-efficient solution for the generation of octave band optical combs.

[0003] Furthermore, while simultaneously possessing second-order (χ) (2) ) and third order (χ (3) In micro / nano optical waveguides with nonlinear effects, femtosecond pulse pumping can simultaneously achieve SCG and second-harmonic generation (SHG), thus enabling on-chip f-2f self-reference without the need for additional frequency doubling devices. This approach has been validated in various photonic integration platforms, including aluminum nitride (AlN), gallium nitride (GaN), silicon nitride (SiN), and thin-film lithium niobate (TFLN). Among these materials, TFLN exhibits strong second-order nonlinearity (d... 33 =19.5pm / V, d 31 =3.2pm / V @1.3 μm) and ferroelectric effect, which can be utilized by cascaded χ (2) The interaction between them was achieved at femtosecond pulse pump energies ranging from a few pJ to sub-hundred pJ. ceo Detection. However, the fabrication process of high-quality periodically polarized lithium niobate (PPLN) waveguides is complex, requiring precise control of waveguide geometry, film thickness, and polarization period, resulting in high process costs, low yield, and hindering large-scale applications.

[0004] In contrast, mode-phase-matched (MPM)-based SHGs eliminate the need for a polarization process, significantly simplifying device fabrication. This method optimizes the waveguide cross-sectional dimensions to achieve the desired χ² value.(3) Dispersive waves (DW) and χ generated during the dominant SCG process (2) The dominant SHG spectra overlap in the frequency domain, enabling direct on-chip f-2f self-reference. However, in uniform straight waveguides, SHGs implemented using MPM are typically limited by narrow phase-matching bandwidths, making it difficult to fully cover the broadband DW generated during SCG. This reduces the spectral overlap between SHG and DW, thereby weakening the f-2f self-reference. ceo The signal-to-noise ratio of the signal. Meanwhile, the phase-matching wavelength of the SHG is highly sensitive to fabrication deviations in waveguide parameters (such as waveguide width, film thickness, etching depth, and sidewall tilt). This makes it difficult to guarantee consistency between theoretical design and fabrication, significantly reducing the robustness and practical potential of on-chip f-2f self-referenced waveguide devices.

[0005] The following are some solutions that are closest to this invention:

[0006] Option 1: Implement on-chip f-2f self-reference based on x-cut TFLN straight waveguide. Figure 1 (a) illustrates a traditional approach: implementing SCG using micro / nano waveguides, implementing SHG using frequency-doubling crystals, and completing f-2f self-reference using free-space delay optical paths. Due to the need for additional frequency-doubling devices and free-space delay structures, the system is quite complex overall. Figure 1 (b) Demonstrates an on-chip integration solution: employing a solution that simultaneously possesses χ (2) and χ (3) An x-cut TFLN waveguide was constructed, simultaneously implementing SCG and SHG within the same device, thus achieving on-chip f-2f self-reference. Under the condition of an on-chip pump pulse energy of 107 pJ, SHG and SCG were simultaneously obtained in this TFLN waveguide. The DW phase matching condition changes accordingly with variations in the incident pump wavelength. Figure 2 As shown, the DW component exhibits a blue shift towards the SHG component as the pump wavelength increases. At a pump wavelength of 1530 nm, due to the high intensity of the DW and SHG components and good spectral overlap, a signal-to-noise ratio of 39 dB was achieved. ceo Signal. This scheme verifies the feasibility of implementing on-chip f-2f self-reference based on TFLN optical waveguide.

[0007] Option 2: Use an x-cut TFLN waveguide for on-chip f-2f self-reference to achieve f ceo Signal detection and locking. Figure 3 (a) is a schematic diagram of the principle of this scheme, in which the output light of the TFLN waveguide chip is directly detected by a detector to obtain f. ceo Signal. Figure 3 (b) is a photograph of the experimental test. The TFLN optical waveguide is in the middle and the silicon-based photodetector is on the left. Figure 3(c) is a scanning electron microscope image of the waveguide, showing a waveguide width of 1300 nm, a thickness of 600 nm, and an etching depth of 400 nm. Under pump wavelength of 1555 nm and optical pulse energy of 140 pJ, the waveguide output spectrum is as follows: Figure 3 As shown in (d). In the wavelength range of 600–900 nm, the spectra of the DW excited by the SCG process and the SHG process have good overlap, which is beneficial for achieving f-2f self-reference. ceo Detection. Based on this waveguide, researchers achieved f ceo The signal is frequency locked, achieving a signal-to-noise ratio as high as 56 dB. This research provides core optical device support for realizing miniaturized, fully locked fiber frequency combs.

[0008] Although the on-chip F-2F self-reference already possesses X-ray... (2) and χ (3) This can be achieved in nonlinear micro / nano optical waveguides, but existing schemes generally use uniform straight waveguide structures, which have the following three limitations: 1) The phase-matching bandwidth of the second harmonic is relatively narrow, making it difficult to fully cover the broadband DW spectrum excited by the SCG. This results in a limited spectral overlap between the two, thereby reducing the available optical power of the DW participating in the on-chip f-2f self-reference process. This limitation makes the scheme unsuitable for applications with low pulse pump energy, restricting its widespread application in low-power optical frequency comb systems.

[0009] 2) The phase-matching wavelength of SHG in a uniform straight waveguide is highly sensitive to waveguide parameter processing errors (such as thin film thickness, etching depth, sidewall tilt angle and waveguide width), making it difficult to guarantee the consistency between theoretical design and fabrication process, thereby weakening the robustness and engineering potential of on-chip f-2f self-referenced waveguide devices.

[0010] 3) The damage threshold of pure TFLN waveguides used in existing studies is too low. They are prone to photo-induced damage and failure under high-energy femtosecond laser pumping, which limits the long-term operational stability of the devices. Summary of the Invention

[0011] To address the aforementioned issues, this invention provides an on-chip f-2f self-reference method for a periodic conical thin-film lithium niobate optical waveguide. By utilizing the second- and third-order nonlinear effects of magnesium oxide-doped thin-film lithium niobate, broadband second harmonics and octave band supercontinuum are generated, thereby achieving low-power, highly robust on-chip f-2f self-reference.

[0012] An on-chip f-2f self-reference method for a periodic tapered thin-film lithium niobate optical waveguide is disclosed. This method optimizes the cross-sectional geometry of the tapered lithium niobate waveguide to achieve broadband second harmonic and dispersive wave generation at a selected pump wavelength. The optimized cross-sectional geometry of the tapered lithium niobate waveguide is then width-tuned, and a periodic tapered waveguide structure is fabricated. The periodic tapered waveguide structure is composed of multiple cascaded tapered lithium niobate waveguides, each with a wide end and a narrow end. When adjacent tapered lithium niobate waveguides are cascaded, the narrow ends are connected to the narrow ends, and the wide ends are connected to the wide ends. In the experiment, a rotating fiber collimator was used to adjust the polarization state of the pump light emitted from the femtosecond laser to the horizontal direction. The pump light was then coupled to a periodic tapered waveguide structure. The tapered lithium niobate waveguide, combined with second- and third-order nonlinearity, achieved second-harmonic generation and supercontinuum generation. The optical signal output from the periodic tapered waveguide was converted into an electrical signal by a photodetector. A spectrum analyzer was used to extract the carrier-envelope offset frequency f from the electrical signal. ceo This enables on-chip F-2F self-reference.

[0013] Furthermore, any conical lithium niobate waveguide consists of, from top to bottom, 5 mol% MgO-doped lithium niobate, silicon dioxide, and a silicon substrate.

[0014] Furthermore, the total thickness of the 5 mol% MgO-doped lithium niobate is 600 nm, and a 450 nm etch is used to form a ridge waveguide structure with a sidewall tilt angle of 75°.

[0015] Furthermore, based on the phase matching condition between the dispersive wave and the broadband second harmonic, the width of the ridge waveguide tip is optimized and determined to gradually increase from 1330 nm at the narrow end to 1440 nm at the wide end.

[0016] Furthermore, the length of a single-segment tapered lithium niobate waveguide is 1 mm, and the total length of the waveguide after 6-segment periodic modulation is 6 mm. The thickness of the 5 mol% MgO-doped lithium niobate is 600 nm, the etching depth is 450 nm, the thickness of the silicon dioxide is 2 μm, and the thickness of the silicon substrate is 525 μm.

[0017] Furthermore, the input and output ends of the periodic tapered waveguide are respectively provided with an input coupling waveguide and an output coupling waveguide, and the width of any coupling waveguide is 3μm to reduce coupling loss; wherein, the input coupling waveguide is coupled with free space light to receive pump light, and the output coupling waveguide is coupled with lens fiber to output optical signal to photodetector.

[0018] Furthermore, a rotating fiber collimator is first used to adjust the polarization state of the pump light emitted from the femtosecond laser to the horizontal direction. Secondly, to avoid the influence of the tuning femtosecond laser current on the femtosecond pulse width, a variable optical attenuator is used to adjust the pulse power of the pump light emitted from the femtosecond laser, thereby obtaining the carrier-envelope offset frequency f corresponding to pump light with different pulse powers. ceo Beat frequency signal.

[0019] Furthermore, a polarization-maintaining lens fiber is used to collect the output optical signal of the periodic tapered waveguide. Then, a 780nm single-mode fiber is used to suppress the spectrum above 1000nm in the output optical signal. The output optical signal collected by the 780nm single-mode fiber is coupled into a photodetector, and finally characterized by a spectrum analyzer. ceo Beat frequency signal.

[0020] Furthermore, a metallized polarization-maintaining lens fiber with a working wavelength of 1550nm and a spot diameter of 2.5μm is fixed to the input and output ends of the periodic tapered waveguide by laser welding, and then the periodic tapered waveguide is encapsulated inside a sealed tube.

[0021] Beneficial effects: 1. This invention provides an on-chip f-2f self-reference method for a periodic tapered thin-film lithium niobate optical waveguide. In terms of device structure, a periodic tapered waveguide structure is introduced to effectively broaden the phase-matching bandwidth of the SHG (Shrink-Shrink-Growth Gaussian) waveguide, allowing the SHG spectrum to fully cover the broadband DW (Difference-Wide Wavelength) generated by the SCG (Shrink-Shrink-Growth Gaussian) waveguide, thereby reducing the pump pulse energy requirement of the f-2f self-reference waveguide device. Experimental results show that this device requires only 100 pJ of on-chip pump pulse energy to achieve an f-2f signal-to-noise ratio as high as 34 dB. ceo Beat frequency signal. In contrast, a uniform straight waveguide fabricated using the same process requires pulse energy of over 160 pJ to achieve a comparable f-frequency signal. ceo Signal-to-noise ratio. In other words, the periodic conical waveguide structure proposed in this invention can effectively broaden the phase-matching bandwidth of the second harmonic, thereby enabling the generated second harmonic to have broadband spectral overlap with the dispersive wave of the supercontinuum process. On-chip f-2f self-reference can be achieved at lower pump energy, reducing energy consumption by nearly 40% compared to traditional straight waveguide devices. Simultaneously, this structure has a large fabrication tolerance and is suitable for other integrated photonic platforms with second- and third-order nonlinearities.

[0022] 2. This invention provides an on-chip f-2f self-reference method for a periodic tapered thin-film lithium niobate optical waveguide. In terms of material selection, 5 mol.% magnesium oxide (MgO)-doped z-cut TFLN material (hereinafter referred to as MgO:LN) is chosen. Its optical damage threshold is approximately 130 times higher than that of undoped pure TFLN, significantly improving the waveguide's damage threshold and ensuring long-term stable operation of the device under high-energy pulse pumping. Simultaneously, this invention optimizes the structural parameters of the MgO:LN uniform straight waveguide, enabling the second harmonic and dispersive waves to simultaneously satisfy the phase-matching condition and achieving spectral overlap between the dispersive wave and the second harmonic in the output spectrum, thereby realizing the on-chip f-2f self-reference process.

[0023] 3. This invention provides an on-chip f-2f self-reference method for a periodic tapered thin-film lithium niobate optical waveguide. In terms of practicality and mass production, the periodic tapered waveguide is insensitive to processing deviations in key parameters such as etching depth and waveguide width, which is beneficial for the engineering and mass production of f-2f self-reference waveguide devices. This invention further develops an fiber-coupled packaged f-2f self-reference waveguide module, making it portable and plug-and-play. This f-2f self-reference waveguide module can maintain stable operation under long operating times and temperature fluctuations, and supports f-2f self-reference lasers with high repetition rates. ceo Detection and locking, in which the module achieved a signal-to-noise ratio of 48 dB under fiber femtosecond laser pumping at a repetition rate of 500 MHz. ceo The beat frequency signal is used, and phase locking is achieved through servo feedback to meet the needs of precision metrology and other fields for stable control of optical combs. Attached Figure Description

[0024] Figure 1 (a) in the figure represents a traditional f-2f self-reference system: f is achieved through a micro / nano optical waveguide (SCG), an external frequency-doubling crystal (SHG), and free-space delay. ceo (a) To detect; (b) To realize SHG and SCG simultaneously based on x-cut TFLN waveguide, thereby directly completing f-2f self-reference on chip.

[0025] Figure 2 (a) shows the output spectra of the x-cut TFLN waveguide at different pump wavelengths (1470, 1490 nm, 1510 nm, 1530 nm); (b) shows the f-2f f obtained from the on-chip reference. ceo Signal.

[0026] Figure 3(a) is a schematic diagram of an optical frequency comb locking scheme based on f-2f self-reference on a TFLN waveguide sheet; (b) is an experimental photograph of the TFLN waveguide output directly coupled to a silicon-based photodetector; (c) is a scanning electron microscope image of the surface and end face of the TFLN waveguide; and (d) is the spectrum of the TFLN waveguide output under femtosecond laser pumping.

[0027] Figure 4 The diagrams show the principles of on-chip f-2f self-reference, comparing the traditional uniform straight waveguide in (a) with the periodic tapered waveguide proposed in this invention in (b).

[0028] Figure 5 This is a schematic diagram of the cross-sectional parameters of the z-cut MgO:LN waveguide used in this invention.

[0029] Figure 6 This is the phase matching result of the second harmonic mode calculated in Embodiment 1 of the present invention.

[0030] Figure 7 This refers to the integrated dispersion curve calculated in Embodiment 1 of the present invention and the result of simulating the corresponding supercontinuum.

[0031] Figure 8 This is the result of calculating the phase matching wavelength of the dispersive wave and the second harmonic as a function of the waveguide width in Embodiment 1 of the present invention, wherein the waveguide thickness is 600 nm, the etching depth is 450 nm, and the sidewall tilt angle is 75°. .

[0032] Figure 9 This invention enables the generation of supercontinuum in MgO:LN waveguides and f ceo Schematic diagram of the experimental setup for signal detection.

[0033] Figure 10 In the figure, (a) is the output spectrum of the test periodic tapered waveguide under different pump energies in Embodiment 1 of the present invention; (b) is the corresponding f ceo Beat frequency signal test results.

[0034] Figure 11 In the figure, (a) is the output spectrum of the uniform straight waveguide tested in Example 1 of the present invention under different pump energies; (b) is the corresponding f ceo Beat frequency signal test results.

[0035] Figure 12 (a) shows the fiber-coupled packaged waveguide module developed in Embodiment 2 of the present invention; (b) to (d) show the performance test results under 100MHz laser pumping; where (b) is the measured f of the module. ceo(c) shows the change in visible light power output by the module under 100MHz laser pumping as a function of time; (d) shows the f signal obtained by the module in Embodiment 2 of this invention. ceo Test results of temperature stability of the test module.

[0036] Figure 13 These are the test results of the waveguide module of Embodiment 2 of the present invention under pumping by a laser with a repetition frequency of 500MHz, where (a) is the module output spectrum at a pump power of 180mW; and (b) is the corresponding f... ceo Signal test results; (c) is f ceo Spectrum test results after signal locking. Detailed Implementation

[0037] 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.

[0038] Example 1 In this embodiment, steps S1-S8 are executed to obtain a uniform waveguide structure with consistent phase matching conditions between dispersive waves and second harmonics by optimizing the structural parameters of the z-cut MgO:LN waveguide. Based on this, a periodic tapered waveguide is proposed and designed to achieve low-power on-chip f-2f self-reference.

[0039] Step S1: Based on the z-cut MgO:LN photonic platform, the following were designed and fabricated: Figure 4 The diagram shows a uniform straight waveguide and a periodic tapered waveguide to achieve on-chip f-2f self-reference. Its working principle is as follows: femtosecond pulsed laser light is injected into both types of waveguides, thereby simultaneously exciting X-rays within the waveguides. (2) Nonlinear-dominated SHG, and by χ (3) Nonlinearly dominant SCG. The heterodyne beat frequency f can be obtained by using the SHG spectrum and the DW component generated by the SCG. ceo The signal enables on-chip f-2f self-reference.

[0040] Traditional solutions adopt Figure 4 The uniform straight waveguide shown in (a) has a narrow SHG phase-matching bandwidth, with only a portion of the SHG spectrum effectively overlapping with the broadband DW. This results in most of the DW power being located outside the overlap region and unusable, thus limiting the effective optical power participating in the f-2f self-reference process. Furthermore, the SHG phase-matching wavelength of this structure is easily affected by processing deviations in parameters such as film thickness, etching depth, and waveguide width, leading to reduced device yield and hindering large-scale mass production. To overcome these limitations, this invention proposes... Figure 4The periodic tapered waveguide structure shown in Figure (b) has its waveguide width periodically modulated around the optimal design value along the propagation direction. This structure enables broadband SHG phase matching, significantly increases the spectral overlap between the SHG and DW, and has higher process tolerance, thus providing a reliable way to achieve efficient and stable on-chip f-2f self-reference.

[0041] Step S2: Figure 5 This is a schematic diagram of the cross-section of the MgO:LN waveguide used in this invention, where the substrate is a 525 μm thick silicon (Si), the upper layer is a 2 μm thick silicon dioxide (SiO2) layer, and the upper layer is a 600 nm thick 5 mol% MgO-doped lithium niobate (MgO:LN). Compared with undoped pure lithium niobate, the optical damage threshold of MgO:LN is increased by approximately 130 times, thus enabling stable and long-term operation under high-energy optical pulse pumping. Specifically, the MgO:LN is etched for 450 nm to form a ridge waveguide structure with a sidewall tilt angle of 75°.

[0042] Step S3: Second-order tensor components d based on MgO:LN 31 Using the transverse electric fundamental mode TE 00 As a pumping mode, phase-matching design of the SHG mode is carried out in a homogeneous MgO:LN waveguide, such as... Figure 6 As shown. Given structural parameters of a waveguide width of 1330 nm, a film thickness of 600 nm, an etching depth of 450 nm, and a sidewall tilt angle of 75°, when the pump wavelength TE... 00 Model and corresponding SHG higher-order TM 20 Effective mode refractive index (n) eff When the phases are equal, the SHG mode phase-matching condition is satisfied. Due to the birefringence effect in MgO:LN materials, TM 20 Mode aliasing exists in the 700nm~800nm ​​wavelength range, resulting in its n eff The curves exhibit abrupt changes, intersecting with TE near 738nm and 787nm respectively. 00 Modulus n eff The curves intersect at two points. Therefore, there are two SHG mode phase-matched wavelengths (738 nm and 787 nm) in this waveguide structure.

[0043] Step S4: When the femtosecond optical pulse propagates in the straight waveguide, in addition to self-phase modulation, the soliton dynamics evolution induced by higher-order dispersion will induce DW generation, thereby further broadening the spectrum and ultimately achieving cross-octave SCG. The phase-matching condition for generating DW is determined by the integrated dispersion (β) of the waveguide. int )Decide:

[0044] in, β(ω) is the propagation constant of the waveguide, and ω0 is the center angular frequency. Let be the group velocity of the light pulse at ω0. When β int When = 0, the phase matching condition for the generation of dispersive waves is satisfied. Figure 7 The integrated dispersion curve and corresponding simulated SCG spectrum of a 1330 nm wide MgO:LN waveguide are presented. Calculation results show that the waveguide satisfies the DW phase-matching condition at 787 nm, and the simulated DW has a large spectral bandwidth, theoretically verifying the feasibility of realizing a cross-octave SCG.

[0045] Step S5: Simulate and analyze the effect of waveguide width on the phase matching conditions of DW and SHG. For example... Figure 8 As shown, under the conditions of a fixed waveguide thickness of 600 nm, etching depth of 450 nm, and sidewall tilt angle of 75°, when the waveguide width increases from 1300 nm to 1400 nm, the phase-matching wavelengths of both SH and DW shift to longer wavelengths, and the wavelength change rate of SH is significantly higher than that of DW. When the waveguide width is 1330 nm, the SH2 curve and the DW curve overlap at a wavelength of 787 nm, indicating that both simultaneously satisfy the phase-matching condition, and the corresponding spectral components will significantly overlap, facilitating on-chip f-2f self-reference. Furthermore, under strong pump pulse energy, the DW generated by the SCG process has a large spectral bandwidth (approximately 110 nm for a 20 dB bandwidth, corresponding to…). Figure 8 The shaded area further covers the phase-matching wavelength of the SH1 curve, thereby increasing the power of the optical signal participating in the f-2f self-reference. Based on the optimization results of the straight waveguide mode phase matching above, this example designs a periodic tapered waveguide structure: considering the waveguide width deviation caused by photoresist, the fabricated waveguide width varies linearly between 1330nm and 1440nm, and is repeated 6 times with a period of 1mm along the propagation direction. To improve the end-face coupling efficiency, both the uniform straight waveguide and the periodic tapered waveguide include a 3μm wide coupling waveguide at both the input and output ends, which is connected to the narrow middle waveguide through a linear tapered structure, resulting in a final total waveguide length of 7mm. Compared to a single linear tapered waveguide, this periodic width modulation design can effectively suppress the influence of factors such as MgO:LN film thickness inhomogeneity and waveguide etching depth deviation on device performance, thereby improving manufacturing tolerance.

[0046] Step S6: Device Fabrication. First, an electron beam exposure system with an accelerating voltage of 100kV, along with positive ZEP520A photoresist, is used to expose and develop the waveguide pattern on a MgO:LN substrate. Then, oblique-incidence argon ion etching is used to transfer the photoresist pattern onto the MgO:LN layer. After etching, the waveguide chip is placed in a mixed solution of NH4OH: H2O2: H2O = 1: 1: 5 for wet processing to remove residual photoresist and etching byproducts. Finally, the waveguide chip is annealed at 500℃ for 3 hours to repair lattice damage introduced during processing and to obtain a flat waveguide end face through cleavage.

[0047] Step S7: Implement SCG and on-chip f-2f self-reference based on the optimized MgO:LN waveguide structure. Experimental test setup as follows: Figure 9 As shown, the pump source is an erbium-doped fiber femtosecond laser with a center wavelength of approximately 1560 nm, a repetition rate of 100 MHz, and an output pulse width of approximately 100 fs. During the test, the incident light pulse power was finely adjusted using a variable optical attenuator to avoid pulse width changes caused by directly altering the laser amplification current. Since the input end is a polarization-maintaining fiber, the pump light polarization state was adjusted to the horizontal direction by rotating the fiber collimator, thereby effectively exciting the TE waveguide within the waveguide. 00 The pump light undergoes nonlinear broadening in the MgO:LN waveguide to generate a supercontinuum and excite the SHG process. The output light is then collected by a lensed fiber and sent to an optical spectrum analyzer (OSA) for characterization. Finally, the optical signal is coupled through an optical fiber into a silicon-based amplified photodetector, and the f-value is extracted using an electronic spectrum analyzer (ESA). ceo Signal. Thanks to the high saturation optical power of the silicon-based amplified detector used in this embodiment, this embodiment does not require the use of traditional narrowband free-space optical filters. It can achieve f-2f self-reference by relying solely on 780nm single-mode fiber to filter out spectral components above 1000nm.

[0048] Step S8: Figure 10 and Figure 11 The output spectral evolution of periodic tapered waveguides and uniform straight waveguides at different on-chip pulse energies and the f-2f self-reference of the on-chip waveguides are shown respectively. ceo Beat frequency signal. The on-chip pulse energy is calculated using a free-space coupling insertion loss of 5 dB. For periodic tapered waveguides, no f was observed when the on-chip pump pulse energy was 60 pJ. ceoThe signal indicates that no DW was generated in the visible light band. At this time, the visible light output is mainly SHG, with a bandwidth exceeding 200nm, which is about an order of magnitude higher than that of a uniform straight waveguide under the same energy pump. By increasing the on-chip pulse energy to 100pJ, SHG and DW form broadband spectral overlap in the periodic tapered waveguide, enabling a signal-to-noise ratio of 34dB to be obtained at a lower pump energy. ceo Signal. In contrast, the f measured at the same pump energy for a uniform straight waveguide ceo With a signal-to-noise ratio of only 22dB, the pulse energy needs to be increased to 160pJ or higher to achieve a similar signal-to-noise ratio level. When the on-chip pulse energy is further increased to 125pJ and 160pJ, the f-value of the periodic tapered waveguide... ceo The signal-to-noise ratio (SNR) improvement is limited (maximum approximately 38 dB), indicating that the spectral overlap between the SHG and DW is nearing saturation at this point. Besides reducing the required pump pulse energy, the periodic tapered structure is relatively insensitive to changes in etching depth and film thickness. Based on this design method, this invention can repeatedly achieve SNRs exceeding 35 dB in periodic tapered waveguides fabricated in different batches. ceo The signal verifies that the method has good process reliability. It should be noted that the measured f in this embodiment... ceo The signal-to-noise ratio (SNR) is still limited by the noise performance of the femtosecond pump laser. With a lower-noise pump source, the SNR is expected to improve further.

[0049] Example 2 In this embodiment, steps S1 to S3 are performed to develop an fiber-coupled packaged f-2f self-reference waveguide module and to test and characterize its performance.

[0050] Step S1: Based on the prepared MgO:LN periodic tapered waveguide chip, the fiber-coupled packaged waveguide module developed in this invention is as follows: Figure 12 As shown in (a), this module has a compact structure with a package length of only 3.5 cm. It is used to excite the TE waveguide within the waveguide. 00 To ensure efficient fiber-waveguide coupling, this embodiment employs a polarization-maintaining lens fiber with a working wavelength of 1550 nm and a spot diameter of 2.5 μm for coupling and encapsulation with the waveguide. The lens fiber is equipped with a metal ferrule, and after precise coupling alignment, it is fixed to the metal substrate of the waveguide chip via laser welding, thus forming a component with good mechanical and vibration resistance. Test results show that the single-end insertion loss of the encapsulated waveguide module is approximately 4.9 dB, an increase of approximately 0.4 dB compared to the bare die test.

[0051] Step S2: Perform performance testing on the packaged waveguide module. For example... Figure 12As shown in (b), the 100MHz fiber femtosecond laser is connected to the packaging module, and the output is connected to a silicon-based amplified photodetector for f-2f self-reference beat frequency measurement. The f-2f beat frequency is then measured using a spectrum analyzer. ceo The signal-to-noise ratio is 36dB. Figure 12 Image (c) shows the output power variation in the visible light band of the module during one hour of continuous pumping, verifying its high power stability. Furthermore, a temperature variation test was conducted by placing a thermoelectric cooler (TEC) at the bottom of the module, controlling the temperature to rise from 20°C to 35°C. Figure 12 As shown in (d), the insertion loss of the module changes by less than 0.3 dB during this process, and f ceo The signal-to-noise ratio remained consistently above 35 dB. This result demonstrates that the packaged module exhibits superior temperature robustness, meeting the requirements of complex environments such as those outside the laboratory.

[0052] Step S3: Apply the waveguide module to a fiber femtosecond laser with a repetition rate of 500MHz. ceo Detection and locking. For example... Figure 13 As shown in (a), when the laser output power is 180mW, the output spectrum obtained after connecting the waveguide module is basically consistent with the spectral characteristics of the aforementioned 100MHz repetition rate laser-pumped laser. The output spectrum was received by a silicon-based amplified photodetector, and a signal-to-noise ratio as high as 48dB was observed in the spectrum analyzer. ceo Signals, such as Figure 13 As shown in (b), this signal-to-noise ratio difference indicates that 100MHz and 500MHz pump lasers have different noise characteristics, thus directly affecting the generated f. ceo Signal quality. This embodiment further improves the detection of f. ceo Signal locking (frequency approximately 167MHz): The signal is extracted via a bandpass filter and amplified to 0 dBm before being input into a servo feedback system (D2-135) to achieve locking relative to a rubidium atom reference. During this process, the amplified f... ceo The signal is first divided by 8, then compared with a microwave source reference signal synchronized with a rubidium atomic clock. The resulting error signal is processed by the proportional-integral-derivative (PID) controller of the servo system and applied as a feedback signal to the laser drive current port, thereby completing the control of f. ceo Closed-loop locking. Figure 13 (c) shows the locked f. ceo Signal spectrum. The dips in the noise floor on both sides of the signal indicate that the noise within the lockout bandwidth has been effectively suppressed, while the servo bumps appearing on both sides of the dips reflect that the system's lockout bandwidth is approximately 180kHz.

[0053] Based on the above technical solution, compared with the existing on-chip f-2f self-reference scheme, the present invention has the following advantages: 1. In existing technologies, on-chip f-2f self-references utilize x-cut undoped TFLN optical waveguides. Regarding device design, no solution has yet been found that simultaneously optimizes the phase matching conditions of the second harmonic and dispersive waves to ensure both are satisfied at the same wavelength. Furthermore, undoped TFLNs have a relatively low damage threshold, which is detrimental to long-term device operation under high-energy pumping.

[0054] This invention uses 5 mol.% magnesium oxide-doped TFLN material, which has a damage threshold approximately 130 times higher than undoped TFLN, thus effectively improving the waveguide's damage threshold. Simultaneously, this invention optimizes the waveguide's structural parameters, enabling the second harmonic and dispersive waves to satisfy phase-matching conditions at the same wavelength, resulting in overlapping spectral peaks and thus improving the f-value. ceo Signal-to-noise ratio.

[0055] 2. Existing technologies typically employ uniform straight waveguides with both second- and third-order nonlinearity to achieve on-chip f-2f self-reference. However, the second harmonic process based on mode phase matching is usually limited by a narrow phase matching bandwidth, making it difficult to fully cover dispersive waves with broadband characteristics, thus limiting the effective optical signal participating in the f-2f self-reference process. Furthermore, the phase matching wavelength of the second harmonic in a uniform straight waveguide is highly sensitive to fabrication deviations, easily leading to low device yield, thereby limiting mass production.

[0056] The periodic tapered waveguide proposed in this invention can effectively broaden the phase-matching bandwidth of the second harmonic, realizing broadband second harmonic generation and its broadband spectral overlap with dispersive waves, thereby increasing the effective optical power participating in the f-2f self-reference process and reducing the required pump pulse energy. Simultaneously, the periodic modulation of the waveguide width can compensate for changes in phase-matching conditions caused by factors such as thin film thickness inhomogeneity and etching depth deviation, thereby improving the device's manufacturing tolerance. Furthermore, this invention develops a compact fiber-coupled f-2f self-reference waveguide module and verifies that the module has good output power stability and temperature stability. Based on this module, f-2f self-reference processes are further realized. ceo The detection and locking of signals are expected to provide key component support for the development of portable, miniaturized optical frequency comb systems.

[0057] Of course, the present invention may have other various embodiments. Without departing from the spirit and essence of the present invention, those skilled in the art can make various corresponding changes and modifications according to the present invention, but these corresponding changes and modifications should all fall within the protection scope of the appended claims.

Claims

1. An on-chip f-2f self-reference method for a periodic tapered thin-film lithium niobate optical waveguide, characterized in that, The cross-sectional geometry of a tapered lithium niobate waveguide is optimized to generate broadband second harmonics and dispersive waves at a selected pump wavelength. The optimized cross-sectional geometry of the tapered lithium niobate waveguide is then width-tuned, and a periodic tapered waveguide structure is fabricated. The periodic tapered waveguide structure is composed of multiple tapered lithium niobate waveguides cascaded together, and each tapered lithium niobate waveguide has a wide end and a narrow end. When adjacent tapered lithium niobate waveguides are cascaded, the narrow ends are connected to the narrow ends, and the wide ends are connected to the wide ends. In the experiment, a rotating fiber collimator was used to adjust the polarization state of the pump light emitted from the femtosecond laser to the horizontal direction. The pump light was then coupled to a periodic tapered waveguide structure. The tapered lithium niobate waveguide, combined with second- and third-order nonlinearity, achieved second-harmonic generation and supercontinuum generation. The optical signal output from the periodic tapered waveguide was converted into an electrical signal by a photodetector. A spectrum analyzer was used to extract the carrier-envelope offset frequency f from the electrical signal. ceo This enables on-chip F-2F self-reference.

2. The on-chip f-2f self-reference method for a periodic tapered thin-film lithium niobate optical waveguide as described in claim 1, characterized in that, Any conical lithium niobate waveguide consists of, from top to bottom, 5 mol% MgO-doped lithium niobate, silicon dioxide, and a silicon substrate.

3. The on-chip f-2f self-reference method for a periodic tapered thin-film lithium niobate optical waveguide as described in claim 2, characterized in that, The total thickness of the 5 mol% MgO-doped lithium niobate is 600 nm, and a 450 nm etch is used to form a ridge waveguide structure with a sidewall tilt angle of 75°.

4. The on-chip f-2f self-reference method for a periodic tapered thin-film lithium niobate optical waveguide as described in claim 3, characterized in that, Based on the phase matching condition between dispersive waves and broadband second harmonics, the width of the ridge waveguide tip is optimized and gradually increased from 1330 nm at the narrow end to 1440 nm at the wide end.

5. The on-chip f-2f self-reference method for a periodic tapered thin-film lithium niobate optical waveguide as described in claim 2, characterized in that, The length of a single-segment tapered lithium niobate waveguide is 1 mm. After 6 segments of periodic modulation, the total length of the waveguide is 6 mm. The thickness of the 5 mol% MgO-doped lithium niobate is 600 nm, the etching depth is 450 nm, the thickness of the silicon dioxide is 2 μm, and the thickness of the silicon substrate is 525 μm.

6. The on-chip f-2f self-reference method for a periodic tapered thin-film lithium niobate optical waveguide as described in claim 1, characterized in that, The periodic tapered waveguide has an input coupling waveguide and an output coupling waveguide at its input and output ends, respectively. The width of any coupling waveguide is 3μm to reduce coupling loss. The input coupling waveguide couples with free space light to receive pump light, and the output coupling waveguide couples with a lens fiber to output optical signal to a photodetector.

7. The on-chip f-2f self-reference method for a periodic tapered thin-film lithium niobate optical waveguide as described in claim 1, characterized in that, First, a rotating fiber collimator is used to adjust the polarization state of the pump light emitted from the femtosecond laser to the horizontal direction. Second, to avoid the influence of the tuning femtosecond laser current on the femtosecond pulse width, a variable optical attenuator is used to adjust the pulse power of the pump light emitted from the femtosecond laser. This allows the carrier-envelope offset frequency f corresponding to pump light with different pulse powers to be obtained. ceo Beat frequency signal.

8. The on-chip f-2f self-reference method for a periodic tapered thin-film lithium niobate optical waveguide as described in claim 1, characterized in that, The output optical signal of a periodic tapered waveguide is collected using a polarization-maintaining lens fiber. Then, the spectrum above 1000nm in the output optical signal is suppressed by a 780nm single-mode fiber. The output optical signal collected by the 780nm single-mode fiber is coupled into a photodetector, and finally characterized by a spectrum analyzer. ceo Beat frequency signal.

9. The on-chip f-2f self-reference method for a periodic tapered thin-film lithium niobate optical waveguide as described in claim 8, characterized in that, Metallized polarization-maintaining lens fiber with a working wavelength of 1550nm and a spot diameter of 2.5μm is fixed to the input and output ends of a periodic tapered waveguide by laser welding, and then the periodic tapered waveguide is encapsulated inside a sealed tube.

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