Anti-crosstalk multimode interference coupler for MZI (Mach Zehnder Interference) structure and application method

Abstract

The invention discloses an anti-crosstalk multimode interference coupler for an MZI (Mach Zehnder Interference) structure and an application method. The coupler specifically comprises a single-mode input waveguide, a multimode interference area waveguide, two single-mode output waveguides, two stray light output waveguides, two stray light interference area waveguides and two metal absorption areas, the metal absorption region, the stray light output waveguide, the multimode interference region waveguide and the single-mode output waveguide are connected in sequence, and the coupler is symmetric about the single-mode input waveguide.

Description

Technical Field

[0001] The present invention relates to the technical field of integrated optical devices, and particularly relates to an anti-crosstalk multimode interference coupler for an MZI structure and an application method thereof. Background Art

[0002] In recent years, thin-film lithium niobate (abbreviated as TFLN) has become a popular research direction. Due to a series of advantages such as high integration, low cost, and low power consumption, it has developed rapidly and is expected to replace traditional LN materials in multiple application fields in the future. Traditional fiber optic gyroscopes achieve mode polarization and filtering through bulk lithium niobate. However, due to the relatively large device structure size, operations such as beam splitting cannot be implemented on-chip. For TFLN, due to its higher integration, various functions can be implemented on-chip. The higher integration and the thin-film device structure design also bring new problems. For example, stray light leaked in the waveguide will interfere with other structures on the chip, resulting in a reduction in the on-chip polarization effect. This leakage situation is not serious in waveguide transmission; but in the MMI region, due to mode mismatch caused by the change in waveguide width, and problems such as all the coherent cancellation light in the reverse transmission being converted into stray light, it is necessary to optimize the structure of the MMI in application scenarios with high requirements for polarization degree. Summary of the Invention

[0003] In view of the above problems, the present invention provides an anti-crosstalk multimode interference coupler for an MZI structure. This design provides feasibility for reducing the crosstalk introduced by the multimode interference coupler of the MZI structure on TFLN, and lays a foundation for the large-scale integration development of lithium niobate on insulator (abbreviated as LNOI). The coupler includes: a base layer, a single-mode input waveguide, a multimode interference region waveguide, two single-mode output waveguides, two stray light output waveguides, two stray light interference region waveguides, and two metal absorption regions;

[0004] The metal absorption region, the stray light output waveguide, the multimode interference region waveguide, and the single-mode output waveguide are connected in sequence;

[0005] The single-mode input waveguide, the multimode interference region waveguide, the single-mode output waveguide, the stray light output waveguide, the stray light interference region waveguide, and the metal absorption region are all arranged on the base layer;

[0006] The coupler is a coupler that is axisymmetric with respect to the single-mode input waveguide.

[0007] Optionally, the input end of the stray light output waveguide is an S-shaped bent waveguide.

[0008] Optionally, the S-shaped bent waveguide includes a straight waveguide section and a bent waveguide section.

[0009] Optionally, the length of the straight waveguide is 5 - 50 μm, and the length of the bent waveguide is 15 - 20 μm.

[0010] Optionally, the substrate layer from bottom to top is successively: silicon substrate layer, buried oxide layer, lithium niobate slab layer, lithium niobate waveguide layer, metal absorption layer, and dielectric film material.

[0011] Optionally, the metal material of the metal absorption region includes gold, titanium, chromium, aluminum, molybdenum, and tungsten.

[0012] Optionally, the dielectric film material includes air cladding, silicon dioxide, aluminum oxide, silicon oxynitride, SU8 photoresist, and SOG spin - on glass.

[0013] Optionally, the waveguide output end of the stray light interference region outputs a spiky waveguide arrangement with a length of 2 - 10 μm.

[0014] The present invention also discloses an application method of a crosstalk - proof multimode interference coupler for an MZI structure, and the method includes:

[0015] Based on the waveguide parameters of the multimode interference region, according to the guided - mode transmission analysis method, calculate the transverse field distribution at the end of the waveguide in the multimode interference region;

[0016] Based on the transverse field distribution result at the end of the waveguide in the multimode interference region, obtain the light with coherent cancellation. The light with coherent cancellation is transmitted through the two stray light interference region waveguides to the stray light output waveguide, and a metal absorption region is covered at the spikes at the end of the stray light output waveguide to absorb the stray light, thereby completing the stray light absorption method of the crosstalk - proof multimode interference coupler.

[0017] Optionally, the content of calculating the transverse field distribution at the end of the waveguide in the multimode interference region includes:

[0018]

[0019] where ψ v (x) is the optical field distribution of the v - th mode, C v is the field excitation coefficient, λ0 is the wavelength in vacuum, n c is the refractive index of the waveguide, W is the width of the waveguide in the multimode interference region, L is the length of the waveguide in the multimode interference region, m is the number of modes of the waveguide in the multimode interference region, v is the v - th order mode, j is the imaginary unit, and L π is the beat length between the 0 - th order mode and the 1 - st order mode.

[0020] Compared with the prior art, the beneficial effects of the present invention are:

[0021] 1. The present invention can realize the collection and conduction of the light with coherent cancellation passing through the multimode interference coupling region through an additional waveguide, with a compact structure and no additional loss introduced.

[0022] 2. The present invention absorbs stray light through a metal. The metal layer only has a high absorption of the output stray light and no absorption of the modes in the waveguide, reducing the interference of other on-chip devices to this structure.

[0023] 3. The structural design of the present invention has universality and can be used on various optical platforms with 1×2 multimode interference couplers, all having the effect of eliminating stray light in the waveguide.

[0024] 4. The process steps adopted have low cost and simple manufacturing processes. Brief Description of the Drawings

[0025] In order to more clearly illustrate the technical solutions of the present invention, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0026] Figure 1 It is a top view of an overall device of a MZI anti-crosstalk multimode interference coupler according to an embodiment of the present invention;

[0027] Figure 2 According to an embodiment of the present invention Figure 1 It is a schematic cross-sectional structure diagram at the dashed line in the embodiment;

[0028] Figure 3 It is a specific top view of the anti-crosstalk structure according to an embodiment of the present invention;

[0029] Figure 4 It is a schematic cross-sectional view of the stray light interference region according to an embodiment of the present invention;

[0030] Figure 5 It is a method step diagram of the application method of an overall device of a MZI anti-crosstalk multimode interference coupler according to an embodiment of the present invention;

[0031] Reference Numerals: 1 - single-mode input waveguide, 2 - multimode interference region waveguide, 3 - single-mode output waveguide, 4 - stray light output waveguide, 5 - stray light interference region waveguide, 6 - metal absorption region, 7 - S-shaped bent waveguide, 8 - cross-sectional view of stray light transmission waveguide, 9 - cross-sectional view of stray light scattering waveguide, 10 - cross-sectional view of metal absorption region, 10 - silicon substrate, 12 - buried oxide layer, 13 - lithium niobate flat layer, 14 - lithium niobate waveguide layer, 15 - metal absorption layer, 16 - dielectric film material. Detailed Embodiments

[0032] In order to make the above objects, features, and advantages of the present invention more obvious and understandable, the present invention will be further described in detail below with reference to the drawings and specific embodiments.

[0033] Embodiment 1

[0034] The present invention provides a crosstalk - resistant multimode interference coupler for an MZI structure, as Figure 1 , Figure 2 shown. The coupler includes:

[0035] a base layer, a single - mode input waveguide 1, a multimode interference region waveguide 2, two single - mode output waveguides 3, two stray - light output waveguides 4, two stray - light interference region waveguides 5, and two metal absorption regions 6;

[0036] The metal absorption region 6, the stray - light interference region waveguide 5, the stray - light output waveguide 4, the multimode interference region waveguide 2, and the single - mode output waveguide 3 are connected in sequence; the single - mode input waveguide 1, the multimode interference region waveguide 2, the single - mode output waveguide 3, the stray - light output waveguide 4, the stray - light interference region waveguide 5, and the metal absorption region 6 are uniformly arranged on the base layer; the coupler is a coupler symmetric about the single - mode input waveguide 3.

[0037] The input end of the stray - light output waveguide 4 is an S - shaped bent waveguide.

[0038] The S - shaped bent waveguide includes a straight waveguide section and a bent waveguide section.

[0039] The length of the straight waveguide section is 5 - 50 μm, and the length of the bent waveguide section is 15 - 20 μm.

[0040] As Figure 4 shown, the base layer from bottom to top is successively: a silicon substrate layer 11, a buried oxide layer 12, a lithium niobate flat layer 13, a lithium niobate waveguide layer 14, the metal absorption region 6, and a dielectric film material 15.

[0041] The metal material of the metal absorption region 6 includes gold, titanium, chromium, aluminum, molybdenum, and tungsten.

[0042] The dielectric film material 15 includes an air cladding, silicon dioxide, aluminum oxide, silicon oxynitride, SU8 photoresist, and SOG spin - on glass, and the dielectric film material is a dielectric film material with a refractive index less than 2.

[0043] The output end of the stray - light interference region waveguide 5 outputs a spike - shaped waveguide arrangement with a length of 2 - 10 μm.

[0044] The crosstalk - resistant multimode interference coupler for the MZI structure includes a silicon substrate layer 11, a buried oxide layer 12, a lithium niobate flat layer 13, a lithium niobate waveguide layer 14, the metal absorption region 6, and a dielectric film material 15 with a refractive index less than 2, which are stacked from bottom to top.

[0045] As Figure 3As shown, the input end of the stray light output waveguide 4 in the crosstalk - proof multimode interference coupler with MZI structure is a straight waveguide section, which is then connected to a bent waveguide, specifically an S - shaped bent waveguide. It is composed of an S - shaped bent waveguide with a gradually varying or non - varying cross - section width. The curve type of the arc includes sine curve, cubic curve, and Euler curve. The length of the straight waveguide is 5 - 50 μm.

[0046] There is no requirement for the shape of the metal absorption region 6 in the crosstalk - proof multimode interference coupler with MZI structure, and its width is greater than or equal to the width of the waveguide 5 in the stray light interference region.

[0047] The output end of the waveguide 5 in the stray light interference region of the crosstalk - proof multimode interference coupler with MZI structure is a spiky waveguide arrangement with a length of 2 - 10 μm.

[0048] Figure 3 It is a specific top - view of the crosstalk - proof structure. The light with coherent cancellation will be transmitted through the stray light transmission waveguides on both sides to the stray light scattering region. In the stray light scattering region, due to mode mismatch, the stray light reflected back to the waveguide can be reduced. And at the end of the stray light scattering region is a spiky structure, where the optical mode is more likely to scatter from the sharp waveguide, thereby further reducing the reflected power. A metal absorption region 6 is covered at the spiky output end to absorb the stray light. Thus, the stray light caused by multimode interference coupling will be absorbed without crosstalk.

[0049] The overall structure is compact. The stray light output waveguide is directly connected to the waveguide in the multimode interference region by a straight waveguide section. Through a straight waveguide, the stray light forms a mode for transmission in the waveguide. The length of the straight waveguide is 5 - 10 μm, the length of the bent waveguide is 15 - 20 μm. The length of the stray light scattering waveguide is 20 - 50 μm, and the thickness of the metal absorption layer 15 only needs to be greater than 500 nm to achieve complete absorption. The total length of the crosstalk - proof structure does not exceed 100 μm, which is relatively compact compared to thin - film lithium niobate devices and can be made even more compact on other optical platforms such as silicon platforms.

[0050] Embodiment 2

[0051] An application method for a crosstalk - proof multimode interference coupler for MZI structure, as Figure 5 shown, the method includes:

[0052] Based on the waveguide parameters in the multimode interference region, according to the guided - mode transmission analysis method, calculate the transverse field distribution at the end of the waveguide in the multimode interference region;

[0053] Based on the result of the transverse field distribution at the end of the waveguide in the multimode interference region, obtain the light with coherent cancellation. The light with coherent cancellation is transmitted through the two waveguides in the stray light interference region to the stray light output waveguide. A metal absorption region 6 is covered at the spiky end of the stray light output waveguide to absorb the stray light, completing the method for absorbing the stray light of the crosstalk - proof multimode interference coupler.

[0054] The width of the waveguide 2 in the multimode interference region of the 1×2 multimode interference coupler is relatively wide, which can support multiple high-order waveguide modes.

[0055] According to the guided-mode transmission analysis method, the transverse field distribution at the end of the multimode interference region waveguide with a length of L can be expressed as

[0056]

[0057] where, ψ v (x) is the optical field distribution of the v-th mode, C v is the field excitation coefficient, λ0 is the wavelength in vacuum, n c is the refractive index of the waveguide, W is the width of the multimode interference region waveguide. When L satisfies L = s(3L π / N), generally, N images of the input field are obtained at the end of the multimode interference region waveguide. Here, s and N are natural numbers and are relatively prime, m is the number of multimode interference region waveguide modes, v is the v-th order mode, j is the imaginary unit, L π is the beat length between the 0-th order mode and the 1-st order mode. The above formula can be simplified to the following mathematical expression:

[0058]

[0059] where C is the complex normalization coefficient, q is all natural numbers from 0 to N-1, x q and θ q are the relative imaging position and phase relationship of the q-th image.

[0060] x q =(2q - N)sW / N

[0061] θ q =(N - Q)sπ / N

[0062] For a 1×2 multimode interference coupler, that is, a multimode interference coupler with a single port on one side and a double port on the other side, the length of its multimode interference coupling region should be:

[0063]

[0064] At this time, two in-phase and equal-intensity images will appear at x = L, which are called double positive images or double negative images. Therefore, generally, the length of the multimode interference coupling waveguide can be obtained through the waveguide transmission equation. However, if two images with opposite phases are input from the double ports, they will interfere destructively and generate a large amount of stray light. At this time, the intensity of the overlapping light can be described by the following formula

[0065]

[0066] where r is the amplitude of the image output from the single port, r2 is the intensity. When the phase difference of the images input through the two ports is an odd multiple of π, destructive interference occurs, and the field strength r at the theoretical output end 2 is 0. However, according to the law of conservation of energy, the energy of destructive interference does not disappear out of thin air; in fact, these two beams of light form a standing wave at waveguide 2 in the multimode interference region. During the continuous energy input process, the energy of the standing wave does not continue to increase, but the energy radiates outward at both ends of the standing wave. This part forms a leakage mode in the waveguide, and this part of the light becomes stray light in application scenarios with high or low polarization requirements and crosstalk, and needs to be collected and absorbed. For example, in the on-chip Y-branch of an optical fiber gyroscope, due to the Sagnac effect, the two beams of coherent light input in the reverse direction will generate a phase difference, thus generating stray light. And the Y-branch is highly sensitive to stray light, so a special structure is needed to solve this problem.

[0067] The embodiments described above are only descriptions of the preferred embodiments of the present invention, and do not limit the scope of the present invention. Without departing from the design spirit of the present invention, various deformations and improvements made by those of ordinary skill in the art to the technical solutions of the present invention shall fall within the protection scope determined by the claims of the present invention.

Claims

1. A crosstalk - resistant multimode interference coupler for MZI structure, characterized in that, The coupler includes a base layer, a single-mode input waveguide, a multimode interference region waveguide, two single-mode output waveguides, two stray light output waveguides, two stray light interference region waveguides, and two metal absorption regions; The metal absorption region, the stray light output waveguide, the multimode interference region waveguide, and the single-mode output waveguide are connected in sequence; The single-mode input waveguide, the multimode interference region waveguide, the single-mode output waveguide, the stray light output waveguide, the stray light interference region waveguide, and the metal absorption region are all arranged on the base layer; The coupler is a coupler symmetric about the single-mode input waveguide.

2. The crosstalk - resistant multimode interference coupler for an MZI structure according to claim 1, wherein, The input end of the stray light output waveguide is an S-shaped bent waveguide.

3. The crosstalk - resistant multimode interference coupler for MZI structure according to claim 2, wherein, The S-shaped bent waveguide includes a straight waveguide section and a bent waveguide section.

4. The crosstalk - resistant multimode interference coupler for MZI structure according to claim 3, wherein, The length of the straight waveguide section is 5 - 50 μm, and the length of the bent waveguide section is 15 - 20 μm.

5. The crosstalk - resistant multimode interference coupler for MZI structure according to claim 1, characterized in that, The base layer includes, from bottom to top: a silicon substrate layer, a buried oxide layer, a lithium niobate slab layer, a lithium niobate waveguide layer, a metal absorption layer, and a dielectric film material.

6. The crosstalk-proof multimode interference coupler for MZI structure according to claim 5, characterized in that The metal materials of the metal absorption region include gold, titanium, chromium, aluminum, molybdenum, and tungsten.

7. The crosstalk - resistant multimode interference coupler for an MZI structure according to claim 6, wherein, The dielectric film material includes an air cladding, silicon dioxide, aluminum oxide, silicon oxynitride, SU8 photoresist, and SOG spin-on glass.

8. The crosstalk - resistant multimode interference coupler for MZI structure according to claim 7, wherein, The output end of the stray light interference region waveguide outputs a spiky waveguide arrangement with a length of 2 - 10 μm.

9. An application method of a crosstalk - resistant multimode interference coupler for an MZI structure, the application method using the crosstalk - resistant multimode interference coupler according to any one of claims 1 - 8, characterized in that, The method includes: Based on the parameters of the multimode interference region waveguide, calculate the transverse field distribution at the end of the multimode interference region waveguide according to the guided mode transmission analysis method; Based on the transverse field distribution result at the end of the multimode interference region waveguide, obtain the light with coherent cancellation. The light with coherent cancellation is transmitted through the two stray light interference region waveguides to the stray light output waveguide. A metal absorption region is covered at the spikes at the end of the stray light output waveguide to absorb the stray light, thereby completing the stray light absorption method of the anti-crosstalk multimode interference coupler.

10. The application method of the crosstalk - proof multimode interference coupler for MZI structure according to claim 9, characterized in that, The content of calculating the transverse field distribution at the end of the multimode interference region waveguide includes: where ψ v (x) is the optical field distribution of the v-th mode, C v is the field excitation coefficient, λ0 is the wavelength in vacuum, n c is the refractive index of the waveguide, W is the width of the waveguide in the multimode interference region, L is the length of the waveguide in the multimode interference region, m is the number of waveguide modes in the multimode interference region, v is the v-th order mode, j is the imaginary unit, and L is the beat length between the 0-th order mode and the 1-st order mode.

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