CN110908215A - Optical waveguide dispersion regulation and control device for nonlinear process and design method thereof - Google Patents

Optical waveguide dispersion regulation and control device for nonlinear process and design method thereof Download PDF

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CN110908215A
CN110908215A CN201911184631.2A CN201911184631A CN110908215A CN 110908215 A CN110908215 A CN 110908215A CN 201911184631 A CN201911184631 A CN 201911184631A CN 110908215 A CN110908215 A CN 110908215A
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何赛灵
张健豪
刘杨
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Zhejiang University ZJU
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    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
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Abstract

The invention provides an optical waveguide dispersion regulation and control device for a nonlinear process and a design method thereof. By dividing the optical waveguide into several segments of waveguides with periodically changing widths, broadband phase matching under control of multiple degrees of freedom can be realized. Firstly, the relation between the wave vector quantity of light participating in the second-order nonlinear process and the waveguide width is obtained through numerical simulation, and the width and the length of each section of waveguide can be searched and optimized through an optimization algorithm according to overall planning. And calculating the phase mismatch amount brought by each section, and offsetting the phase differences introduced by each section of waveguide in an optimized nonlinear period to obtain a flat phase difference spectrum, so that the average wave vector difference of the nonlinear process in the concerned wavelength range is close to 0, and further the nonlinear phase matching with large bandwidth is realized. The invention can further improve the bandwidth of phase matching while ensuring the existence of the phase matching working point in the integrated optical waveguide.

Description

Optical waveguide dispersion regulation and control device for nonlinear process and design method thereof
Technical Field
The invention relates to an optical waveguide dispersion regulation and control device for a nonlinear process and a design method thereof, belonging to the technical field of optical nonlinearity.
Background
Optical nonlinear processes are widely used for the generation of optical signals, by which the frequency of the laser can be greatly expanded at low cost, especially for the realization of long-span wavelength conversion. To realize the optical nonlinear process, the involved optical signals must satisfy two conditions of energy conservation and phase matching simultaneously. While both conditions are difficult to satisfy simultaneously since phase matching is limited by chromatic dispersion. One means to solve this problem is to use quasi-phase matching, which introduces wave vector perturbation by periodically polarizing the lithium niobate crystal material to compensate for the wave vector mismatch between lights participating in the nonlinear process, and this means is widely used in the second-order nonlinear process of lithium niobate crystal based on space optics, although this means can obtain higher nonlinear conversion efficiency, its perfect matching point is rather critical, and limited by the dispersion curve, its working bandwidth is very small, and can only realize the nonlinear conversion between specific wavelengths. The other solution is to use an advanced micro-nano processing technology to process the optical nonlinear material into a waveguide structure to introduce waveguide dispersion, and to realize a specific dispersion curve by accurately regulating and controlling the characteristics of the waveguide such as size and the like, so as to meet corresponding phase matching conditions. However, this method does not ensure the generation of a phase matching point, and even if it is generated, its tolerance to the operating wavelength is very low, and a small wavelength shift causes a large phase mismatch amount change, limiting the bandwidth in the non-linear process.
Disclosure of Invention
In order to overcome the defects of the prior art, the invention provides an optical waveguide dispersion regulation device for a nonlinear process and a design method thereof. By means of wave vector compensation and waveguide dispersion brought by periodically regulating and controlling waveguide width change, a dispersion curve can be regulated and controlled to ensure that phase matching is met to a certain extent in a larger wave band, and therefore the bandwidth of a nonlinear process is improved. Moreover, the method can be applied to an integrated optical platform, and polarization improvement of materials is not needed.
A design method of an optical waveguide dispersion regulation device comprises the following steps:
1) dividing each period of a waveguide with a period L into n sections of straight waveguides with uniform thickness, wherein n is a natural number greater than or equal to 1, and the length of the j-th section of straight waveguide is LjWidth of wjJ is a natural number with a value from 1 to n;
2) calculating the phase mismatch quantity of three light waves participating in the second-order nonlinear effect after transmitting a period in the waveguide;
3) the width, the length and the period length of each section of the n sections of waveguides are optimized through an optimization algorithm, so that the total phase mismatch of each wavelength combination in the waveband range needing to be optimized is lower than a set threshold value, and the needed optical waveguide dispersion regulation and control device can be obtained.
Step 2) the method for calculating the phase mismatch amount of three light waves participating in the second-order nonlinear effect after passing through one period comprises the following steps: firstly, the wave vectors of three light waves participating in the nonlinear process in the n-segment straight waveguides in a waveguide period are obtained, so that the accumulated phase difference of the three wavelengths after passing through one period is obtained
Figure BDA0002292106490000021
The average wavevector difference over the period of the set of wavelengths is thus
Figure BDA0002292106490000022
Generating wave vector first-order perturbation under consideration of periodic action
Figure BDA0002292106490000023
The final average wave vector difference for the set of wavelengths is then Δ kaverage＝Δk±kperturbationWhich is the amount of phase mismatch after one cycle.
Step 3) the calculation method for optimizing the width and length of each section of the n sections of waveguides and the period length by the optimization algorithm comprises the following steps: setting the three wavelengths participating in the second-order nonlinear process as lambda1,λ2,λ3Wherein λ is1Is a fixed value, specifies the band range to be optimized, in which m wavelength points are sampled, m being a natural number greater than 1, thereby obtaining m sets of wavelengths [ lambda ] participating in the nonlinear process1,λ2,i,λ3,i](ii) a Finding the proper period L, the width w of each straight waveguide section by an optimization algorithmjLength l of each straight waveguidejSo that the average wave vector difference delta k of each group of wavelengths is obtained within the specified wave band rangeaverage，i＝Δki±kperturbationThen the average value of the absolute values of the average wave vector differences after the first-order perturbation of the m groups of wavelengths is
Figure BDA0002292106490000024
Which is the total phase mismatch amount; optimizing to enable the total phase mismatch to be lower than a set threshold value, and obtaining the required dispersion regulation and control device.
The optimization algorithm is a genetic algorithm or a particle swarm algorithm.
The set threshold is one ten thousandth of the vacuum wave vector.
According to the design method, all the segmented waveguides are on the same axis, and the change of the waveguide structure is carried out along the light transmission direction.
A waveguide dispersion tuning device for nonlinear processes is obtained according to the design method.
The invention has the beneficial effects that: the optical waveguide dispersion regulation and control device designed by the invention can realize a second-order nonlinear effect with a certain bandwidth in a specified waveband, realize high-efficiency nonlinear optical wavelength conversion and is suitable for nonlinear application on a lithium niobate waveguide.
Drawings
Fig. 1 is a schematic structural diagram of an optical waveguide dispersion adjustment device according to the present invention;
fig. 2 is a schematic structural diagram of an embodiment of a waveguide dispersion adjustment device according to the present invention;
FIG. 3 is a schematic structural diagram of two periods of the structure of an embodiment of a waveguide dispersion modulating device according to the present invention;
fig. 4 is a graph showing a relationship between a wave vector difference and an optical wavelength in an embodiment of the waveguide dispersion adjusting device and a conventional lithium niobate straight waveguide according to the present invention;
fig. 5 is a schematic structural diagram of a conventional lithium niobate straight waveguide.
Detailed Description
The present invention will be described in further detail with reference to the accompanying drawings and specific embodiments.
The optical waveguide dispersion regulation device designed by the invention is shown in fig. 1, and is a waveguide with periodically-changed width, each period comprises n sections of straight waveguides, extra wave vector disturbance is introduced through the period, and the phase matching bandwidth of the second-order nonlinear effect in a specified wave band is increased through the mutual accumulation relation between dispersion curves with different widths.
When in design, in the first step, each period of a waveguide with a period L is divided into n sections of straight waveguides with uniform thickness, wherein n is a natural number which is more than or equal to 1, and the length of the j-th section of straight waveguide is LjWidth of wjJ is a natural number with a value from 1 to n; secondly, calculating the phase mismatch quantity of three light waves participating in the second-order nonlinear effect after passing through the waveguide for one period; and thirdly, optimizing the width, the length and the period length of each section of the n sections of waveguides by an optimization algorithm, so that the total phase mismatch quantity of each group of wavelengths in the waveband range needing to be optimized is lower than a set threshold (such as one ten-thousandth of a vacuum wave vector), and the required dispersion regulation and control device can be obtained.
The second step of calculating the average wave vector difference of three light waves participating in the second-order nonlinear effect after passing through one period comprises the following steps: the three wavelengths participating in the second-order nonlinear process can be set to λ1,λ2,λ3By establishing a corresponding relation table k (w, λ) between the wave vector k and the waveguide width w and the optical wavelength λ, the j-th segment of the straight waveguide can be obtained that the light wave vectors corresponding to the three wavelengths in each group are respectively
Figure BDA0002292106490000034
The accumulated phase difference of each group of wavelengths in the section is
Figure BDA0002292106490000035
The accumulated phase difference after the set of wavelengths passes a period is
Figure BDA0002292106490000031
The average wave vector difference for this period of the set of wavelengths is then
Figure BDA0002292106490000032
Generating wave vector first-order perturbation under consideration of periodic action
Figure BDA0002292106490000033
The average wave vector difference for the set of wavelengths is finally Δ kaverage＝Δk±kperturbationThis is defined as the determined amount of phase mismatch.
The third step is to optimize the width and length of each segment of the n segments of waveguides and the calculation method of the period length by an optimization algorithm, and comprises the following steps: setting the three wavelengths participating in the second-order nonlinear process as lambda1,λ2,λ3Wherein λ is1Is a fixed value, specifies the band range λ to be optimized2∈[λ2,min,λ2,max]And λ3∈[λ3,min,λ3,max]Where m wavelength points are sampled, m being a natural number greater than 1, for any one lambda due to conservation of photon power by a non-linear process2,iOne and only one λ can be obtained3,iWherein i represents the ith wavelength, and is a natural number which is greater than or equal to 1 and less than or equal to m. Thereby obtaining m groups of wavelengths [ lambda ] participating in a nonlinear process in a specified waveband range1,λ2,i,λ3,i]. Finding out proper period L and width w of each straight waveguide by genetic algorithm or particle swarm algorithmjLength l of each straight waveguidejSo that the average wave vector difference delta k of all groups of wavelengths is obtained within the range of the specified wave bandaverage，i＝Δki±kperturbation. The average value of the absolute values of the average wave vector differences after the first-order perturbation of the m groups of wavelengths is
Figure BDA0002292106490000041
Which is the total phase mismatch amount; optimizing to make the total phase mismatch lower than the set threshold (one ten thousandth of the vacuum wave vector), and obtaining the required dispersion regulation device。
The above-mentioned segmented waveguides are on the same axis, and the change of the waveguide structure is also carried out along the light transmission direction.
Examples
Now take lithium niobate waveguide as an example. Lithium niobate is a crystal with very strong second-order nonlinear effect, and is widely used in various nonlinear light generation occasions. In this embodiment, the lithium niobate waveguide adopts x-cut y-pass, and the modes of light participating in the nonlinear process all adopt TE (transverse electromagnetic mode) fundamental modes and the refractive index of extraordinary light (e-light) in lithium niobate. The lithium niobate waveguide is implemented with silica as a cladding. Taking the difference frequency light generation as an example, the wavelength of the pump light is 1064nm, a set of structural parameters of the periodic waveguide structure needs to be obtained, so that when the signal light is within the wavelength range of 1.58um to 1.66um (the wavelength range of the corresponding idler light is about 2.953um to 3.258um), the average wave vector difference | Δ k-k after the first-order perturbationperturbation| is as close to 0 as possible.
The waveguide can be set to be divided into 5 sections, the height is set to be 1.6um, and the following group of solutions can be found through a genetic algorithm
First stage Second section Third stage Fourth stage Fifth stage
Width (um) 3.710 3.309 0.816 3.341 3.761
Length (um) 2.099 0.337 1.909 0.530 3.375
The corresponding period is 8.25um, and the width thereof changes periodically, which results in periodic variation of dispersion, so we call Periodic Dispersion Lithium Niobate (PDLN), fig. 2 and fig. 3 are schematic diagrams of a top view and a three-dimensional structure shown according to structural parameters in the table, wherein M +2 periods are included, and the position of each segment in the first period is indicated by letters i-v. The average wave vector difference delta k-k of the light after one period of the structure is obtained through calculationperturbationAnd the wavelength of light, as shown by the solid line in fig. 4. To compare the effects, a lithium niobate straight waveguide was used as a control. The straight waveguide can not generate wave vector disturbance kperturbationFor any size of lithium niobate straight waveguide, the phase matching (△ k is 0) in the specified waveband cannot be realized, so taking the lithium niobate straight waveguide with the height of 3um and the width of 5um as an example, the structural schematic diagram is shown in fig. 5, and the relationship between the average wave vector difference and the optical wavelength is calculated, which is shown by a dotted line in fig. 4.

Claims (7)

1. A design method of an optical waveguide dispersion regulation device for a nonlinear process is characterized by comprising the following steps:
1) dividing each period of a waveguide with a period L into n sections of straight waveguides with uniform thickness, wherein n is a natural number greater than or equal to 1, and the length of the j-th section of straight waveguide is LjWidth of wjJ is a natural number with a value from 1 to n;
2) calculating the phase mismatch quantity of three light waves participating in the second-order nonlinear effect after transmitting a period in the waveguide;
3) the width, the length and the period length of each section of the n sections of waveguides are optimized through an optimization algorithm, so that the total phase mismatch of each wavelength combination in the waveband range needing to be optimized is lower than a set threshold value, and the needed optical waveguide dispersion regulation and control device can be obtained.
2. The design method according to claim 1, wherein the calculation method of step 2) for calculating the amount of phase mismatch of three optical waves participating in the second-order nonlinear effect after passing through one period is: firstly, the wave vectors of three light waves participating in the nonlinear process in the n-segment straight waveguides in a waveguide period are obtained, so that the accumulated phase difference of the three wavelengths after passing through one period is obtained
Figure FDA0002292106480000011
The average wavevector difference over the period of the set of wavelengths is thus
Figure FDA0002292106480000012
Generating wave vector first-order perturbation under consideration of periodic action
Figure FDA0002292106480000013
The final average wave vector difference for the set of wavelengths is then Δ kaverage＝Δk±kperturbationWhich is the amount of phase mismatch after one cycle.
3. The method for designing a waveguide dispersion modulation device according to claim 1, wherein the calculation method for optimizing the width and length of each segment and the period length of n segments of waveguides in step 3) through an optimization algorithm comprises: setting the three wavelengths participating in the second-order nonlinear process as lambda1,λ2,λ3Wherein λ is1Is a fixed value, specifies the band range to be optimized, in which m wavelength points are sampled, m being a natural number greater than 1, thereby obtaining m sets of wavelengths [ lambda ] participating in the nonlinear process1,λ2,i,λe,i](ii) a Finding the proper period L, the width w of each straight waveguide section by an optimization algorithmjLength l of each straight waveguidejSo that the average wave vector difference delta k of each group of wavelengths is obtained within the specified wave band rangeaverage，i＝Δki±kperturbationThen the average value of the absolute values of the average wave vector differences after the first-order perturbation of the m groups of wavelengths is
Figure FDA0002292106480000014
Which is the total phase mismatch amount; optimizing to enable the total phase mismatch to be lower than a set threshold value, and obtaining the required dispersion regulation and control device.
4. The method of claim 1, wherein the optimization algorithm is a genetic algorithm or a particle swarm algorithm.
5. The method of claim 1, wherein the threshold is one ten thousandth of a vacuum wave vector.
6. The method of claim 1, wherein the segmented waveguides are on the same axis, and the change of the waveguide structure is performed along the optical transmission direction.
7. An optical waveguide dispersion tuning device for nonlinear processes obtained by the design method of claim 1.
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