A method of inducing supercontinuum local dispersion wave generation in a waveguide

Abstract

The application discloses a method for inducing supercontinuum local dispersion wave generation in a waveguide. The method is embedding a grating structure with a limited length in an optical waveguide structure. The grating structure realizes periodic modulation of the effective refractive index of the waveguide, so as to induce supercontinuum local dispersion wave generation. The wavelength and intensity of the dispersion wave can be controlled by changing the period length, modulation depth, period number and other parameters of the grating structure. The method for inducing supercontinuum local dispersion wave generation in a waveguide can accurately control and realize supercontinuum spectrum power promotion at any specified wavelength, so as to provide a wider space for the practical application of the supercontinuum light source.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of optoelectronic technology, in particular to a method for inducing supercontinuum local dispersion wave generation in a waveguide. BACKGROUND

[0002] Supercontinuum generation (SCG) is a process of converting laser into broadband spectrum. On-chip waveguide SCG can excite wider spectrum at low power, with the advantages of high efficiency, high coherence, high dispersion, high nonlinearity, high integration, etc., and is widely used in spectral measurement, optical coherence imaging, optical communication, molecular spectroscopy, medical and biochemical sensing, etc.

[0003] Supercontinuum spectrum generation in a waveguide usually uses a nonlinear optical waveguide with uniform or slowly varying structure. Due to the limitation of waveguide dispersion and material dispersion, the supercontinuum spectrum usually has significant power fluctuation, and it is difficult to achieve power balance in the whole spectrum. Changing the waveguide structure can only adjust the overall spectrum envelope, and cannot realize local spectrum regulation. Therefore, there is an urgent need for a technical means to locally regulate the supercontinuum spectrum, which can increase the power of the low-power band in the supercontinuum spectrum without affecting other bands. The present application provides a method for inducing supercontinuum local dispersion wave generation in a waveguide, which embeds a finite length grating structure in the optical waveguide structure to realize periodic modulation of the effective refractive index of the waveguide, and can produce tunable local power enhancement effect, thereby precisely regulating the local area of the supercontinuum spectrum. SUMMARY

[0004] The technical problem to be solved by the present application is that the existing on-chip waveguide supercontinuum spectrum usually has significant power fluctuation. The present application provides a method for inducing supercontinuum local dispersion wave generation in a waveguide, which aims to precisely regulate the local power enhancement effect of the supercontinuum spectrum.

[0005] Technical scheme of the present application:

[0006] A method for inducing supercontinuum local dispersion wave generation in a waveguide, which embeds a finite length grating structure in the optical waveguide structure, the grating structure makes the effective refractive index of the waveguide periodically change, and in the process of supercontinuum spectrum generation, the grating structure causes the formation of local dispersion wave, thereby increasing the intensity of the local spectrum.

[0007] Further, the effective refractive index of the grating periodically changes, which is realized by changing the structure of the waveguide or by changing the material of the waveguide.

[0008] Further, the core layer of the optical waveguide structure and the grating structure is made of high refractive index material, and the optical waveguide structure and the grating structure are encapsulated in a low refractive index material cladding layer.

[0009] Further, the structure of the waveguide is changed periodically, i.e. the cross-sectional area of the waveguide core layer is changed periodically, including the change of the width or height of the waveguide core layer, so as to realize the periodic change of the effective refractive index of the waveguide; the length of the change period of the effective refractive index is denoted as ; the modulation depth of the effective refractive index is denoted as , i.e. the change amount of the width or height of the waveguide core layer.

[0010] Further, the waveguide with the grating structure is used for supercontinuum generation and realization of dispersion wave generation of specific wavelengths, and the position of the grating should be set at the first compression point of the optical pulse on the waveguide and the best spreading position of the supercontinuum spectrum, without covering the entire waveguide, so as to optimize the intensity of the dispersion wave generation of specific wavelengths; the distance between the starting position of the grating and the input end of the waveguide is denoted as .

[0011] Further, the grating structure is used to cause the mutual coupling between the optical modes in the waveguide, so as to cause the optical phase fluctuation near the resonance wavelength of the grating, and further induce the generation of the dispersion wave; the resonance wavelength of the grating is proportional to the grating period length , and the wavelength of the dispersion wave can be controlled by changing the grating period length; the modulation depth of the effective refractive index and the number of the grating periods are used to control the mutual coupling efficiency between the optical modes in the waveguide, so as to realize the control of the intensity of the dispersion wave.

[0012] Further, the period length of the grating structure can be changed in a larger range compared with the wavelength, so as to selectively cause the coupling between different modes in the optical waveguide, including the forward mode, the backward mode and the spatial radiation mode; for the same design wavelength, there are more than at least one grating structure to realize the enhancement of the local dispersion wave.

[0013] Further, when the period length of the grating is greater than or much greater than the wavelength, the grating is a long-period grating, which mainly causes the coupling between the forward transmission optical modes in the waveguide, including the coupling between the fundamental mode and the high-order mode; when the period length of the grating is less than the wavelength, the grating is a Bragg grating, which mainly causes the coupling between the forward transmission optical mode and the backward transmission optical mode in the waveguide, including the coupling between the forward transmission fundamental mode in the waveguide and the backward transmission fundamental mode in the waveguide; when the actual waveguide structure of the grating is changed periodically, the spatial radiation will also be caused, i.e. the optical mode is coupled and guided to the outside of the waveguide.

[0014] Specifically comprising the following steps:

[0015] a) First set the wavelength of the supercontinuum local dispersion wave, that is, the resonance wavelength of the grating structure ;

[0016] b) Calculate the effective refractive index of the waveguide at the wavelength;

[0017] c) Obtain the period of the grating structure according to the dispersion wavelength and the effective refractive index of the waveguide ;

[0018] d) Obtain the transmission spectrum of the grating structure through mode calculation and analysis; according to the Kramers-Kronig relationship, the phase information of the grating is obtained from the transmission spectrum; adjust and optimize the grating parameters (A, N, T) , , ), optimize the transmission spectrum and phase of the grating; embed the transmission spectrum and phase of the grating into the optical waveguide to form the overall transmission and phase spectrum of the waveguide;

[0019] e) Numerical simulation calculation of supercontinuum spectrum, estimate the effect of supercontinuum spectrum generation and the local spectrum enhancement effect caused by the grating, and form a design scheme.

[0020] In step d: , and respectively represent the effective refractive index of the fundamental mode at wavelength and the effective refractive index of the high-order mode at wavelength ; after determining the value of T, adjust the values of A and N, and after adjusting the value of A or N each time, simulate and calculate the normalized transmission spectrum of the grating, and select the value when the phase change is maximum.

[0021] Compared with the prior art, the advantages of the present application are as follows:

[0022] 1. Compared with the traditional single dispersion control method, the embedding of the grating can provide precise control of local dispersion in different wavelength ranges, thereby realizing more precise supercontinuum spectrum design;

[0023] 2. This control method does not depend on extreme working conditions, so it can be applied to different waveguide materials and different working environments, and has wide applicability;

[0024] 3. The design and manufacturing process is compatible with existing micro-nano processing technology, which helps to realize the miniaturization and integration of the optical system, improve the stability and controllability of the system, and can be mass-produced and reduce costs. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1It is three-dimensional schematic view of silicon nitride waveguide containing grating structure provided by the embodiment of the present application. The reference signs are as follows: 1-silicon dioxide lower cladding layer, 2-silicon dioxide upper cladding layer, 3-silicon nitride grating structure, 4-silicon nitride uniform waveguide structure, 5-silicon nitride waveguide, and 6-silicon substrate.

[0026] Figure 2 It is a planar schematic view of the silicon nitride grating structure.

[0027] Figure 3 It is a normalized transmission spectrum diagram corresponding to different grating period numbers (N) in the embodiment of the present application.

[0028] Figure 4 It is a diagram of optical phase fluctuation near the grating resonance wavelength corresponding to different grating period numbers (N) in the embodiment of the present application.

[0029] Figure 5 It is a normalized transmission spectrum diagram corresponding to different grating modulation depths (A) in the embodiment of the present application.

[0030] Figure 6 It is a diagram of optical phase fluctuation near the grating resonance wavelength corresponding to different grating modulation depths (A) in the embodiment of the present application.

[0031] Figure 7 It is an overall phase matching diagram for regulating two different wavelengths provided by the embodiment of the present application.

[0032] Figure 8 It is a simulation comparison diagram of a long-period grating SCG spectrum corresponding to an overall phase matching diagram of a long-period grating and a straight waveguide without grating SCG spectrum provided by the embodiment of the present application.

[0033] Figure 9 It is a simulation comparison diagram of a Bragg grating SCG spectrum and a straight waveguide without grating SCG spectrum provided by the embodiment of the present application.

[0034] Figure 10 It is a method design flowchart for inducing supercontinuum local dispersion wave generation in a waveguide. DETAILED DESCRIPTION

[0035] The specific implementation mode of the present application will be described in detail below in combination with the drawings of the specification.

[0036] As shown in the drawings, Figure 10 the present application provides a method for inducing supercontinuum local dispersion wave generation in a waveguide, and the scheme flowchart comprises the following steps:

[0037] a) first set the wavelength of the supercontinuum local dispersion wave, i.e. the resonance wavelength of the grating structure ;

[0038] b) Effective refractive index of the waveguide at the wavelength ; By finite element method (FEM) simulation, the nonlinear parameters of the waveguide can be calculated, and the effective refractive index of different wavelengths of its optical modes (including the fundamental mode and high-order modes) can be further determined ;

[0039] c) According to the dispersion wavelength and the effective refractive index of the waveguide , the period length of the grating is obtained , that is, the modulation period of the effective refractive index, that is, the relationship between , and respectively represent the effective refractive index of the fundamental mode at wavelength and the effective refractive index of the high-order mode at wavelength ; The coupling of these two modes occurs at wavelength , which produces a dispersion wave enhancement local power;

[0040] d) Parameter optimization process: through mode calculation and analysis, the transmission spectrum of the grating structure is obtained, as shown in Figure 3 ; According to the Kramers-Kronig relation, the phase information of the grating is derived from the transmission spectrum, as shown in Figure 4 ; Adjust and optimize the grating parameters , , , optimize the transmission spectrum and phase of the grating, as shown in Figure 3 , 4 ; Embed the transmission spectrum and phase of the grating into the optical waveguide to form the overall transmission and phase spectrum of the waveguide;

[0041] e) Substitute the obtained phase value in d) into the nonlinear Schrödinger equation (NLSE) to perform numerical simulation calculation of supercontinuum spectrum, estimate the effect of supercontinuum spectrum generation, and the local spectrum enhancement effect caused by the grating, form a design scheme;

[0042] f) According to the design scheme, the optical waveguide chip is processed and manufactured, and the supercontinuum spectrum is tested experimentally to verify the local spectrum enhancement effect caused by the grating.

[0043] More specifically, the core layer of the optical waveguide and the grating structure is made of high refractive index silicon nitride (Si3N4) material, and the silicon nitride waveguide 5 and the grating structure are packaged in a low refractive index silicon dioxide (SiO2) material cladding layer. As Figure 2The diagram shows an on-chip silicon nitride waveguide 5 containing a grating structure. The silicon nitride waveguide 5 includes a section of silicon nitride grating structure 3, with the remaining portion being a uniform silicon nitride waveguide structure 4. A silicon dioxide underlayer 1 has a thickness of 4.0 mm. The silicon nitride waveguide 5, with a thickness of 0.8 mm, is placed on the silicon substrate 6. The width is 1.0. Up to 4.0 Within a variable range, it is placed on the lower silica cladding 1; the upper silica cladding 2 has a thickness of 6.6. Located on the silicon nitride waveguide 5 and together with the lower cladding layer, it covers the silicon nitride waveguide 5. More specifically, in this embodiment, the periodic structure of the grating is selected as a periodic square wave shape, such as... Figure 2 As shown. Among them The period length of the grating. The modulation depth of the grating. The number of periods in the grating. It is the total length of the grating structure, and the grating is positioned at the optimal broadening location of the supercontinuum. Nearby.

[0044] More specifically, setting the dispersive wavelength The effective refractive index of the corresponding dispersive wavelengths of the fundamental mode and higher-order modes was obtained through simulation using COMSOL software, thus obtaining the period length of the grating. Set the modulation depth of the grating Furthermore, a grating with a period length greater than the wavelength is considered a long-period grating. Changing the number of periods in the grating... ,like Figure 3 As shown, number of cycles Number of cycles Number of cycles Simulations were performed to calculate the normalized transmission spectrum of the grating under three different numbers of periods. It is evident that the change in the number of grating periods affects the depth and aperture size of the transmission spectrum. The aperture size and depth of the normalized transmission spectrum directly affect the degree of mode coupling within the waveguide, i.e., the energy conversion efficiency, and the energy coupling between the two modes in the waveguide leads to a strong phase change. For example... Figure 4 The figure shows the phase change at the grating resonant wavelength for different numbers of periods. (The figure is incomplete and requires further context.) Below, the changes are most intense and related to Figure 3 The normalized transmission spectrum is corresponding.

[0045] More specifically, setting the dispersive wavelength To obtain the period length of the grating Set the number of periods for the grating. Furthermore, the period length is greater than the wavelength, making it a long-period grating. The refractive index modulation depth of the grating is changed. ,likeFigure 5 Modulation depth Modulation depth Modulation depth The normalized transmission spectrum of the grating is calculated by simulation under three different modulation depths. It can be seen that the change of modulation depth affects the depth of the transmission spectrum. The depth of the normalized transmission spectrum directly affects the degree of mode coupling in the waveguide, that is, the conversion efficiency of energy, and the energy coupling of the two modes in the waveguide brings strong phase change. As shown in Figure 6 The phase change at the resonance wavelength of the grating under different modulation depths. Under the modulation depth , the change is the strongest and Figure 5 The normalized transmission spectrum is corresponding. By analyzing it can be seen that under the given waveguide width (width is 1600 ), the number of grating periods and the modulation depth , the phase change can be maximized.

[0046] More specifically, the thickness of the silicon nitride waveguide is selected as 800 , and the width is 1600 . The dispersion wavelength (aluminum ion absorption line) is set, ( pulse frequency multiplication) to generate dispersion waves, and the spectral power is improved in this area. When , the parameters of the grating are , , ; when , the parameters of the grating are , , ; the grating position is set to be 3mm away from the input end of the silicon nitride waveguide (3mm ). The overall phase mismatch of the grating structure silicon nitride waveguide is obtained by simulation under the tuning of two different wavelengths (1068 , 775 ), as shown in the accompanying Figure 7 , the propagation constant is locally changed in the set wavelength range, so that the integral dispersion is equal to 0, and the phase matching is achieved, thereby generating a dispersion wave.

[0047] As shown in the accompanying Figure 8 , the SCG simulation results, compared with the silicon nitride waveguide without grating structure, it is found that the spectral power is improved at the set wavelength (1068 , 775 ).

[0048] More specifically, the dispersion wavelength To obtain the period length of the grating Set the number of periods for the grating. , Furthermore, the period length is less than that of a Bragg grating with a wavelength of [missing information]. (See attached image.) Figure 9 As shown in the SCG simulation results, compared with silicon nitride waveguides without grating structures, an increase in spectral power was achieved at the set wavelength (1068). Figure 8 The results of the long-period grating comparison show that both achieved an increase in spectral power at the set wavelength, but the improvement effect was different. For the same wavelength, there are two types of periodic structures that can achieve this.

[0049] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for inducing the generation of supercontinuous locally dispersive waves in a waveguide, characterized in that: A finite-length grating structure is embedded in an optical waveguide structure. This grating structure causes a periodic variation in the effective refractive index of the waveguide. During the generation of the supercontinuum, the grating structure induces the formation of local dispersive waves, thereby increasing the intensity of the local spectrum. The periodic variation in the effective refractive index of the grating is achieved by changing the waveguide structure or the waveguide material. The period length of the grating... That is, the modulation period of the effective refractive index; the modulation depth of the effective refractive index is denoted as... That is, the change in the width or height of the waveguide core layer; the number of grating periods is denoted as... ; The wavelength of the dispersive wave can be controlled by changing the grating period length. , and These are respectively represented as the fundamental mode at wavelength Effective refractive index and higher-order modes at wavelength Effective refractive index; The modulation depth of the effective refractive index and the number of grating periods It is used to control the mutual coupling efficiency between optical modes in the waveguide, thereby achieving the control of the intensity of dispersive waves.

2. The method for inducing supercontinuous local dispersive waves in a waveguide according to claim 1, characterized in that: The core layers of both the optical waveguide structure and the grating structure are made of high-refractive-index materials, and the optical waveguide structure and the grating structure are encapsulated in a low-refractive-index material cladding.

3. The method for inducing supercontinuous local dispersive waves in a waveguide according to claim 1, characterized in that: The aforementioned change in waveguide structure involves periodically altering the cross-sectional area of ​​the waveguide core layer, including changing the width or height of the waveguide core layer, to achieve a periodic change in the effective refractive index of the waveguide.

4. The method for inducing supercontinuous local dispersive waves in a waveguide according to claim 1, characterized in that: The optical waveguide structure is used to generate supercontinuum and realize the generation of dispersive waves of a specific wavelength. The starting position of the grating structure is set at the first compression point of the optical pulse on the waveguide. The distance between the starting position of the grating structure and the input end of the waveguide is denoted as D.

5. The method for inducing supercontinuous local dispersive waves in a waveguide according to any one of claims 1-4, characterized in that: The aforementioned grating structure induces mutual coupling between optical modes in the waveguide, thereby causing optical phase fluctuations near the grating's resonant wavelength, and subsequently inducing the generation of dispersive waves; the resonant wavelength of the grating... With grating period length Proportional.

6. The method for inducing supercontinuous local dispersive waves in a waveguide according to claim 5, characterized in that: The period length of the grating structure varies over a wider range relative to the wavelength, thereby selectively inducing coupling between different modes within the optical waveguide, including forward mode, reverse mode, and spatial radiation mode; for the same design wavelength, at least one grating structure can achieve enhancement of local dispersive waves.

7. The method for inducing supercontinuous local dispersive waves in a waveguide according to claim 6, characterized in that: When the period length of the grating is greater than the wavelength, the grating is a long-period grating, causing coupling between forward propagation optical modes in the waveguide, including coupling between the fundamental mode and higher-order modes; when the period length of the grating is less than the wavelength, the grating is a Bragg grating, causing coupling between forward propagation optical modes and reverse propagation optical modes in the waveguide, including coupling between the forward propagation fundamental mode and the reverse propagation fundamental mode in the waveguide; when the actual waveguide structure of the grating changes periodically, it will cause spatial radiation, that is, the optical modes are coupled and guided to the outside of the waveguide.

8. The method for inducing supercontinuous local dispersive waves in a waveguide according to claim 5, characterized in that... Specifically, the following steps are included: a) First, define the wavelength of the supercontinuous locally dispersed wave. , that is, the resonant wavelength of the grating structure; b) Calculate the effective refractive index of the waveguide at the specified wavelength; c) Obtain the period length of the grating structure based on the dispersive wavelength and the effective refractive index of the waveguide. ; d) The transmission spectrum of the grating structure is obtained through mode calculation and analysis; the phase information of the grating is obtained from the transmission spectrum according to the Kramer-Krannicke relationship; the grating parameters T, A and N are adjusted to optimize the transmission spectrum and phase of the grating; the transmission spectrum and phase of the grating are embedded into the optical waveguide to form the overall transmittance and phase spectrum of the waveguide. e) Substitute the phase value obtained in d) into the nonlinear Schrödinger equation NLSE to perform numerical simulation calculations of the supercontinuum, estimate the effect of the supercontinuum and the local spectral enhancement effect caused by the grating, and form a design scheme.

9. The method for inducing supercontinuous local dispersive waves in a waveguide according to claim 8, characterized in that... In step d: after the T value is determined, the A and N values ​​are adjusted. After each adjustment of the A or N value, the normalized transmission spectrum of the grating is calculated by simulation, and the value at which the phase change is the largest is selected.

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