On-chip supercontinuum light source based on cascaded polarized ferroelectric material and design method thereof

Abstract

The application discloses an on-chip supercontinuum light source based on a cascade polarized ferroelectric material and a design method thereof. The application comprises: a pump light source for providing pump light; the pump light is incident to a supercontinuum generation waveguide; the supercontinuum generation waveguide satisfies that group velocity dispersion is close to 0 at the wavelength of the pump light, group velocity mismatch of the pump light wavelength and the frequency-doubled light wavelength is close to 0, and the polarization period of the supercontinuum generation waveguide satisfies phase matching of frequency doubling. The polarization period of the supercontinuum expansion waveguide varies along the propagation direction, and the supercontinuum expansion waveguide outputs supercontinuum. A tapered transition waveguide is used to connect the supercontinuum generation waveguide and the supercontinuum expansion waveguide and eliminate the mode mismatch between the supercontinuum generation waveguide and the supercontinuum expansion waveguide. The system of the application has the advantages of low cost, high integration and wide spectral coverage, and is suitable for fields such as environmental monitoring, medical diagnosis and spectral analysis.

Description

Technical Field

[0001] This invention relates to a supercontinuum light source in the field of integrated photonics and nonlinear optics, specifically to an on-chip supercontinuum light source based on cascaded polarized ferroelectric materials and its design method. Background Technology

[0002] Supercontinuum, through the propagation of ultrashort pulses in nonlinear media that induce dramatic spectral broadening, is a core technology for obtaining broadband light sources. Among these, mid-infrared supercontinuum has attracted significant attention due to its coverage of the molecular fingerprint region, making it suitable for applications such as environmental gas detection, imaging, and spectral sensing. In particular, integrated and miniaturized supercontinuum light sources hold great promise for applications in wearable medical detection devices, deep space exploration, and laboratory research. Traditional methods for generating supercontinuum typically employ bulk crystals or optical fibers. However, these methods often result in bulky and difficult-to-integrate supercontinuum light sources.

[0003] With the development of science and technology, integrated nanophotonic waveguides have become important materials for supercontinuum generation. Integrated nanophotonic waveguides possess advantages such as high integration density, easily tunable dispersion, strong mode field confinement, easily tunable nonlinear effects, and low processing cost, making them a very promising supercontinuum generation scheme. Replacing the previously bulky and cumbersome crystal or fiber supercontinuum generation devices with a chip occupying only a square millimeter has significant commercial and research value.

[0004] Existing integrated nanophotonic waveguides (such as silicon-based waveguides and silicon nitride waveguides) mainly rely on third-order nonlinear effects. However, its bandwidth is limited by the waveguide dispersion characteristics, and the bandwidth generated by the supercontinuum is relatively limited.

[0005] In 2011, Bart Kuyken et al. achieved supercontinuum generation from 1535 nm to 2525 nm in a 2 cm long silicon waveguide using a femtosecond pulse with a peak power of only 12.7 W. In 2014, Leo et al. first achieved SCG from 1.1 to 1.7 μm using a standard silicon-based insulator (SOI) waveguide combined with 150 fs pulse pumping, but its bandwidth was limited by the anomalous dispersion properties of silicon, making it difficult to extend to longer or shorter wavelengths. Especially for generating mid-infrared supercontinuum, the effectiveness of nanophotonic waveguide schemes is limited.

[0006] Singh et al. (2015) achieved a 2–6 μm supercontinuum on a silicon-sapphire platform using 3.7 μm pumping, but this required a complex suspension structure and an expensive mid-infrared femtosecond source.

[0007] Grassani et al. (2019) achieved 1.3–4 μm supercontinuum generation in silicon nitride waveguides via 2 μm pumping, but its bandwidth remains insufficient for many applications, including multi-gas sensing. Existing lithium niobate waveguide supercontinuum systems (Yu et al., 2019), although utilizing… Nonlinearity achieves 2 octave bands, but the band range is not large enough, especially in the mid-infrared.

[0008] In 2019, Davide Grassani et al. designed and fabricated a mid-infrared supercontinuum light source based on silicon nitride waveguides. Combined with a non-integrated commercial gas absorption cell and a Fourier spectrometer, they achieved the detection of absorption spectral lines of C2H2 gas from 2800 nm to 3300 nm.

[0009] Although the above-mentioned research has achieved on-chip integration of light sources, it generally suffers from problems such as narrow spectral range and low output power, and still needs to rely on non-integrated commercial spectrometers, resulting in a large overall system size.

[0010] In summary, existing supercontinuum generation chips have insufficient bandwidth and require high-quality pump sources. Many applications require systems with greater bandwidth and lower light source costs. Summary of the Invention

[0011] To address the problems and needs existing in the background technology, this invention provides an on-chip supercontinuum light source based on cascaded polarized ferroelectric materials and its design method. This invention employs cascaded supercontinuum generation waveguides and supercontinuum extension waveguides for supercontinuum generation and broadening, achieving ultra-wideband supercontinuum generation from visible light to mid-infrared through second-order nonlinear effects. Compared to previous supercontinuum generation systems, the system proposed in this invention has a simpler structure, which is more conducive to product miniaturization, weight reduction, and cost reduction.

[0012] The technical solution of the present invention is as follows:

[0013] I. An on-chip supercontinuum light source based on cascaded polarized ferroelectric materials

[0014] Pump light source, used to provide pump light;

[0015] A supercontinuum generation waveguide is formed, and pump light is incident on the supercontinuum generation waveguide. The supercontinuum generation waveguide satisfies the following conditions at the pump light wavelength: the group velocity dispersion is close to 0, the group velocity mismatch between the pump light wavelength and the frequency-doubled light wavelength is close to 0, and the polarization period of the supercontinuum generation waveguide satisfies the phase matching of the frequency-doubled light.

[0016] A supercontinuum extended waveguide, whose polarization period varies along the propagation direction, outputs a supercontinuum;

[0017] A tapered transition waveguide is used to connect the supercontinuum generation waveguide and the supercontinuum extension waveguide and to eliminate mode mismatch between the two.

[0018] The waveguide width of the supercontinuum extended waveguide is greater than the waveguide width of the supercontinuum generated waveguide.

[0019] The supercontinuum generating waveguide, the supercontinuum extended waveguide, and the tapered transition waveguide are all waveguides made of polarized ferroelectric materials.

[0020] The polarized ferroelectric material includes lithium niobate or lithium tantalate.

[0021] The supercontinuum generating waveguide, the supercontinuum extending waveguide, and the tapered transition waveguide are all substrate-hollowed-out waveguides.

[0022] The supercontinuum generation waveguide, the supercontinuum extension waveguide, and the tapered transition waveguide constitute a supercontinuum chip, and the supercontinuum chip is vacuum-sealed.

[0023] II. A Design Method for On-Chip Supercontinuum Light Sources Based on Cascaded Polarized Ferroelectric Materials

[0024] The waveguide width and etching depth of the supercontinuum generation waveguide are designed based on the pump light wavelength generated by the pump source. This ensures that the supercontinuum generation waveguide satisfies near-zero group velocity dispersion, near-zero group velocity mismatch between the pump light wavelength and the frequency-doubled light wavelength, and phase matching of the polarization period of the supercontinuum generation waveguide at the frequency-doubled wavelength. The pump light generated by the pump source is incident into the supercontinuum generation waveguide, generating a supercontinuum spectrum with wavelengths concentrated near the pump wavelength. The supercontinuum generation waveguide is connected to the supercontinuum extension waveguide via a tapered transition waveguide. The tapered transition waveguide is used to eliminate mode mismatch between the supercontinuum generation waveguide and the supercontinuum extension waveguide. The polarization period of the supercontinuum extension waveguide varies along the propagation direction, generating a supercontinuum spectrum with wavelengths far from the pump wavelength in the supercontinuum extension waveguide.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] This invention generates near-infrared supercontinuum spectra using a second-order nonlinear method and extends the spectrum to the mid-infrared using difference frequency generation. Compared to conventional mid-infrared supercontinuum systems, this invention's system is integrated on-chip, resulting in a smaller system size and simpler structure. Compared to traditional mid-infrared supercontinuum generation methods, this invention's system does not require expensive and bulky mid-infrared femtosecond light sources, offering a cost advantage. Furthermore, ferroelectric materials have relatively large second-order nonlinear coefficients, meaning the pump power required for supercontinuum generation in this invention is less than that required for supercontinuum spectra based on third-order nonlinearity.

[0027] Compared to spatial light systems, the system of this invention is smaller, lower in cost, and simpler in structure. This invention demonstrates innovation and feasibility in using ferroelectric material chips to improve the efficiency and performance of supercontinuum generation.

[0028] This invention can use lithium tantalate waveguides, which have the advantage of low cost. Attached Figure Description

[0029] Figure 1 This is an overall schematic diagram of the supercontinuum generation system of the present invention.

[0030] Figure 2 It is a graph showing the relationship between quasi-phase-matched wavelength and polarization period in the supercontinuum.

[0031] Figure 3 This is a schematic diagram of the near-infrared supercontinuum generated after the pump light provided by the C-band femtosecond seed source passes through the supercontinuum generation waveguide.

[0032] Figure 4 This is a schematic diagram of the mid-infrared supercontinuum generated after the pump light provided by the C-band femtosecond seed source passes through the supercontinuum extended waveguide.

[0033] Figure 5 This is a graph showing the relationship between the spectral lines of a supercontinuum light source and its propagation distance. Detailed Implementation

[0034] To make the structure, features, and advantages of the present invention clearer, the present invention will now be described in further detail with reference to the accompanying drawings, but this should not be construed as limiting the scope of protection of the present invention. It should also be noted that, for ease of description, only the parts relevant to the present invention are shown in the drawings, not the entire structure.

[0035] like Figure 1 As shown, the on-chip supercontinuum light source based on cascaded polarized ferroelectric materials proposed in this invention specifically includes:

[0036] Pump source 1 is a C-band femtosecond seed source, and its fiber end face is coupled to supercontinuum generation waveguide 2.

[0037] Supercontinuum generation waveguide 2 is constructed, and pump light is incident on it. The waveguide width and etching depth are specially designed for dispersion, ensuring that the group velocity dispersion of supercontinuum generation waveguide 2 is close to 0 at the pump light wavelength (specifically, less than 10). -11 The group velocity mismatch between the pump light wavelength and the frequency-doubled light wavelength is close to 0, specifically less than 200 fs. 2 / m; and the polarization period of the supercontinuum generation waveguide 2 satisfies the phase matching of frequency doubling, i.e., a uniform periodic structure. Under the combined effects of dispersion, third-order nonlinearity, and second-order nonlinearity of the supercontinuum generation waveguide 2, the pump light's spectrum broadens into a supercontinuum.

[0038] In one feasible implementation, an on-chip uniformly periodically polarized lithium niobate waveguide (PPLN) is used as the waveguide material for the supercontinuum generation waveguide 2. The pump wavelength for the C-band femtosecond seed source is selected as 1550 nm. Under these conditions, if the pump wavelength and the frequency-doubled wavelength satisfy quasi-phase matching, the waveguide polarization period should satisfy the following condition:

[0039]

[0040] Where Λ is the polarization period of the PPLN; β() is the propagation constant, which can be calculated and simulated using the finite element method; P s ω1 represents the peak power of the first emitted soliton; γ is the nonlinear coefficient; β1 is the first-order dispersion coefficient; and ω1 is the frequency of the pump light. In this example, the polarization period Λ of the PPLN is chosen to be 3.91 μm.

[0041] The supercontinuum extended waveguide 3 has a polarization period that varies along the propagation direction, meaning it must satisfy broadband difference-frequency phase matching to extend the frequency range of the spectrum. The supercontinuum extended waveguide 3 outputs a supercontinuum spectrum.

[0042] In one feasible implementation, the supercontinuum extension waveguide 3 extends the near-infrared supercontinuum generated by the uniformly periodically polarized lithium niobate waveguide to the mid-infrared. The waveguide width of the supercontinuum extension waveguide 3 is greater than the waveguide width of the supercontinuum generation waveguide 2. Optionally, the difference in waveguide width between the two is 3 μm.

[0043] The polarization period of the supercontinuum extended waveguide 3 varies along the propagation direction to extend the spectrum to the mid-infrared via difference frequency. To design the polarization period of the chirped lithium niobate, the relationship between the polarization period and the phase-matching wavelength needs to be established. For a quadratic nonlinear periodic polarization structure with a polarization period of Λ, the phase-matching condition for the difference frequency is:

[0044]

[0045] Power enhancement at the target wavelength can be achieved by changing the polarization period Λ. The propagation constant β(ω) of the supercontinuum extended waveguide 3 is calculated using finite element analysis to obtain the relationship between the phase-matching wavelength and the polarization period, as follows: Figure 2 As shown. Based on Figure 2 The calculation results show that selecting a polarization period of Λ = 3.6 μm to 4.7 μm can ensure that the phase matching wavelength covers 0.6–4.5 μm, which basically covers the transparent window range of lithium niobate material.

[0046] A tapered transition waveguide 4 is used to connect the supercontinuum generation waveguide 2 and the supercontinuum extension waveguide 3, and to eliminate mode mismatch between the supercontinuum generation waveguide 2 and the supercontinuum extension waveguide 3, as well as to reduce loss. Optionally, the waveguide length of the tapered transition waveguide 4 is 50 μm.

[0047] Supercontinuum extension waveguide 3 is a multimode wide waveguide. This widened waveguide can support longer wavelengths and increase mode overlap between the mid-infrared and pump wavelengths. The polarization period of the waveguide varies with position to satisfy phase matching conditions for different wavelengths. When the supercontinuum generated by supercontinuum generation waveguide 2 passes through this structure, the spectrum can be broadened to the mid-infrared due to the satisfaction of the difference frequency condition.

[0048] In one feasible implementation, the supercontinuum generating waveguide 2, the supercontinuum extending waveguide 3, and the tapered transition waveguide 4 are all thin-film lithium niobate waveguides.

[0049] In the on-chip supercontinuum light source, the supercontinuum generating waveguide 2 and the supercontinuum extending waveguide 3 have a thickness of 600 nm and are thin films.

[0050] This invention proposes a design method for an on-chip supercontinuum light source based on cascaded polarized ferroelectric materials. The design method includes the following steps:

[0051] The waveguide width and etching depth of the supercontinuum generation waveguide 2 are designed based on the pump light wavelength generated by the pump source, so that the supercontinuum generation waveguide 2 satisfies the following conditions at the pump light wavelength: group velocity dispersion is close to 0, group velocity mismatch between the pump light wavelength and the frequency-doubled light wavelength is close to 0, and the polarization period of the supercontinuum generation waveguide 2 satisfies the phase matching of the frequency-doubled light. The pump light generated by the pump source is incident into the supercontinuum generation waveguide 2, generating a supercontinuum spectrum with wavelengths concentrated near the pump wavelength. The supercontinuum generation waveguide 2 is connected to the supercontinuum extension waveguide 3 via a tapered transition waveguide 4, where the tapered transition waveguide 4 is used to eliminate the mode mismatch between the supercontinuum generation waveguide 2 and the supercontinuum extension waveguide 3. The polarization period of the supercontinuum extension waveguide 3 varies along the propagation direction, generating a supercontinuum spectrum with wavelengths far away from the pump wavelength in the supercontinuum extension waveguide 3. Finally, the output supercontinuum spectrum covers 0.6μm–4.5μm.

[0052] To further demonstrate the feasibility of this invention, a numerical simulation method is used to simulate the waveguide dispersion. This simulation employs a method involving second-order nonlinearity. and third-order nonlinear Modeling of ultrawideband nonlinear processes in lithium niobate waveguides using a single nonlinear envelope equation:

[0053]

[0054] Where E(z,t) is the electric field intensity; c is the speed of light; and n() is the effective refractive index; The total nonlinear polarization contains only non-negative frequency components, n0 is the effective refractive index of the pump light center wavelength, and β n Let ω1 be the nth order dispersion coefficient, α be the transmission loss, ω1 be the pump light frequency, and τ be the time in the moving coordinate system, satisfying τ = t - β1z, where t is the time in the fixed coordinate system and z is the propagation distance. Here, the equation is solved numerically using the split-step Fourier method, where the nonlinear step is solved in the time domain and the dispersion step is solved in the frequency domain.

[0055] Consider the nonlinear evolution of a pulse with a center wavelength of 1550 nm (peak power 6000 W, pulse width 50 fs, repetition frequency 100 MHz) in this example. After the pulse passes through the supercontinuum generation waveguide 2, the pulse undergoes a dramatic broadening in the frequency domain due to the combined effects of nonlinearity and dispersion. The final simulation yields the supercontinuum spectrum bandwidth of -30 dB (0.55–2.6 μm) after the pulse passes through the supercontinuum generation waveguide 2. The simulated relationship between wavelength and power after the pulse passes through the supercontinuum generation waveguide 2 is as follows: Figure 3 As shown in the figure. Analysis of the results shows that supercontinuum generation waveguide 2 can generate a supercontinuum for the pump pulse. Its energy is mainly concentrated near the pump wavelength, i.e., in the near-infrared region, part of the visible light region, and the mid-infrared region. After the supercontinuum enters the supercontinuum extension waveguide 3, the supercontinuum extends to the mid-infrared because supercontinuum extension waveguide 3 satisfies the self-difference frequency-quasi-phase matching condition in the mid-infrared band. The simulation results show the relationship between the wavelength and power of the pulse after passing through supercontinuum extension waveguide 3 as follows: Figure 4 As shown.

[0056] Figure 5 This is a graph showing the relationship between the spectral power of a supercontinuum source and its propagation distance. This graph clearly illustrates the pulse dynamics of the pump source in this example. Figure 5 As shown, at z=0, the optical power is concentrated within a very small wavelength range around 1550 nm.

Claims

1. An on-chip supercontinuum light source based on cascaded polarized ferroelectric materials, characterized in that, include: Pump light source (1) is used to provide pump light; Supercontinuum generation waveguide (2), pump light is incident on supercontinuum generation waveguide (2); supercontinuum generation waveguide (2) satisfies group velocity dispersion close to 0 at the wavelength of pump light, group velocity mismatch between the wavelength of pump light and the wavelength of frequency doubling light close to 0, and the polarization period of supercontinuum generation waveguide (2) satisfies phase matching of frequency doubling. The supercontinuum extended waveguide (3) has a polarization period that varies along the propagation direction and outputs a supercontinuum. A tapered transition waveguide (4) is used to connect the supercontinuum generation waveguide (2) and the supercontinuum extension waveguide (3) and eliminate the mode mismatch between the supercontinuum generation waveguide (2) and the supercontinuum extension waveguide (3); The supercontinuum generation waveguide (2), the supercontinuum extension waveguide (3), and the tapered transition waveguide (4) are all waveguides made of polarized ferroelectric materials.

2. The on-chip supercontinuum light source based on cascaded polarized ferroelectric materials according to claim 1, characterized in that, The waveguide width of the supercontinuum extended waveguide (3) is greater than the waveguide width of the supercontinuum generating waveguide (2).

3. The on-chip supercontinuum light source based on cascaded polarized ferroelectric materials according to claim 1, characterized in that, The polarized ferroelectric material includes lithium niobate or lithium tantalate.

4. The on-chip supercontinuum light source based on cascaded polarized ferroelectric materials according to claim 1, characterized in that, The supercontinuum generation waveguide (2), the supercontinuum extension waveguide (3), and the tapered transition waveguide (4) are all substrate-hollowed waveguides.

5. The on-chip supercontinuum light source based on cascaded polarized ferroelectric materials according to claim 1, characterized in that, The supercontinuum generation waveguide (2), the supercontinuum extension waveguide (3), and the tapered transition waveguide (4) constitute a supercontinuum chip, and the supercontinuum chip is vacuum-sealed.

6. A design method for an on-chip supercontinuum light source based on cascaded polarized ferroelectric materials, characterized in that, The design method includes the following steps: The waveguide width and etching depth of the supercontinuum generation waveguide (2) are designed according to the pump light wavelength generated by the pump source, so that the supercontinuum generation waveguide (2) satisfies the following conditions at the pump light wavelength: the group velocity dispersion is close to 0, the group velocity mismatch between the pump light wavelength and the frequency doubling light wavelength is close to 0, and the polarization period of the supercontinuum generation waveguide (2) satisfies the phase matching of the frequency doubling. The pump light generated by the pump source is incident into the supercontinuum generation waveguide (2), and a supercontinuum with wavelengths concentrated near the pump wavelength is generated in the supercontinuum generation waveguide (2). The supercontinuum generation waveguide (2) is connected to the supercontinuum extension waveguide (3) via a tapered transition waveguide (4). The tapered transition waveguide (4) is used to eliminate the mode mismatch between the supercontinuum generation waveguide (2) and the supercontinuum extension waveguide (3). The polarization period of the supercontinuum extension waveguide (3) changes along the propagation direction, generating a supercontinuum spectrum with a wavelength far from the pump wavelength in the supercontinuum extension waveguide (3). The supercontinuum generation waveguide (2), the supercontinuum extension waveguide (3), and the tapered transition waveguide (4) are all waveguides made of polarized ferroelectric materials.

7. The method for generating an on-chip supercontinuum light source based on cascaded polarized ferroelectric materials according to claim 6, characterized in that, The waveguide width of the supercontinuum extended waveguide (3) is greater than the waveguide width of the supercontinuum generating waveguide (2).

8. The method for generating an on-chip supercontinuum light source based on cascaded polarized ferroelectric materials according to claim 6, characterized in that, The polarized ferroelectric material includes lithium niobate or lithium tantalate.

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