Intra-coupler beam splitting device

Abstract

The application relates to the technical field of semiconductor optoelectronic device integration, and provides a multi-mode interference coupler internal beam splitting device, a core layer high-refractive index waveguide is embedded in a low-refractive index cladding layer, and the core layer comprises an input straight waveguide, a tapered input waveguide, a multi-mode interference region, an air slit, a tapered output waveguide and an output straight waveguide which are sequentially connected. The tapered input waveguide is connected with one end of the multi-mode interference region, and the tapered output waveguide can be connected with multiple positions of the multi-mode interference region according to use requirements. The application adds a 45-degree air slit at an MM I image point, realizes light emission in horizontal and vertical directions, and different slit widths have different beam splitting ratios. The beam splitting device provided by the application avoids adding other devices for changing the light path direction and the beam splitting ratio outside the MM I, reduces the loss of the whole integrated device, makes the whole device more compact, and improves the overall performance and stability.

Description

Technical Field

[0001] This invention belongs to the field of semiconductor optoelectronic device integration technology, and relates to optical waveguide beam splitting devices, especially to devices that can realize beam splitting within a multimode interference (MMI) coupler, i.e., beam splitting devices within a multimode interference coupler. Background Technology

[0002] With the rapid development of modern communication technology, traditional microelectronic devices can no longer meet people's requirements for communication performance. Optoelectronic integrated devices have attracted widespread attention, and optical waveguide beam splitters are important integrated devices, which places increasingly higher demands on optical waveguide devices. A good optical waveguide device can improve the performance of the entire optoelectronic integrated device, thereby improving the stability and integration of the entire system. Therefore, simple design, low loss, low cost, and diverse functions have become the goals pursued by researchers for optical waveguides.

[0003] Multimode interferometers (MMIs) are important passive optical waveguide devices. A typical MMI consists of three parts: an input waveguide, a multimode interference section, and an output waveguide. Based on the self-imaging principle, for any input light field, one or more images of the input light field will periodically appear in the multimode waveguide along the direction of light field propagation. Therefore, MMIs can be used as multiplexed or multiplexed beams. Multiple input and output waveguides can be designed according to the actual application. The width and length of the multimode interference region, as well as the positions of the input and output waveguides, can be calculated based on the number of waveguides and their materials, thus achieving N×N MMI port output. Compared to common Y-branch couplers and directional couplers (DC), MMIs offer advantages such as low insertion loss, insensitivity to polarization, high operability, compact structure, relatively simple fabrication process, and good fabrication tolerance. MMIs are widely used in various optoelectronic systems such as Mach-Zehnder interferometers, optical switches, ring oscillators, and wavelength division multiplexers. Therefore, MMIs have attracted much attention due to their many advantages, and have successfully replaced Y-branch couplers and directional couplers in many devices.

[0004] However, conventional multimode interference devices (MMIs) can only achieve a fixed beam splitting ratio and can only split the beam horizontally, which is a significant disadvantage for highly integrated devices. Initially, beam splitting ratio control was achieved by changing the position of the input waveguide, but this only allows for a few fixed ratios. Later, methods were developed to arbitrarily change the beam splitting ratio by altering the geometry of the MMI's multimode interference waveguide (e.g., butterfly, angular, introducing holographic patterns), cascading MMIs of different widths, adding waveguide gratings to the multimode waveguide, and through electrical tuning. However, these methods result in complex designs, large space requirements, and limited large-scale applications. To achieve a vertical beam output direction, most methods involve externally attaching a curved waveguide or a mirror to the MMI, leading to significant light loss. Therefore, designing an MMI with high compactness, low loss, simple structure, capable of achieving both horizontal and vertical beam splitting within the MMI and obtaining arbitrary beam splitting ratios is of great practical significance. Summary of the Invention

[0005] To overcome the shortcomings of the prior art, the present invention provides an internal beam splitting device for a multimode interference coupler, which splits light into horizontal and vertical directions at the internal image point position and can obtain output light with any beam splitting ratio.

[0006] This invention is achieved through the following technical solution:

[0007] The beam splitting device inside the multimode interference coupler of the present invention has a high-refractive-index waveguide in the core layer embedded in a low-refractive-index cladding. The core layer includes an input straight waveguide, a tapered graded-ratio input waveguide, a multimode interference region, an air slit, a tapered graded-ratio output waveguide, and an output straight waveguide connected in sequence. The tapered graded-ratio input waveguide is connected to one end of the multimode interference region. The tapered graded-ratio output waveguide can be connected to multiple locations in the multimode interference region as needed. The air slit is placed at different image points of the MMI according to the length of the MMI and actual needs. It can be one air slit or multiple air slits can be placed at multiple image points.

[0008] The input straight waveguide is a single-mode waveguide, used to input light into the beam splitter in single-mode form.

[0009] The output straight waveguide is a single-mode waveguide, used to transmit light in single-mode form.

[0010] The cross-sectional width of the tapered input waveguide increases linearly in the direction of light propagation.

[0011] The cross-sectional width of the tapered output waveguide narrows linearly in the direction of light propagation.

[0012] The length of the multimode interference region is:

[0013] ;

[0014] Where L π Let β0 and β1 be the beat lengths of the two lowest-order modes, N be the number of output waveguides, β0 and β1 be the propagation constants of the fundamental and first-order modes in the multimode interference region, and n be the number of output waveguides. r W is the effective refractive index of the core layer. e W represents the effective width of the multimode interference region, λ0 represents the input light wavelength, and W represents the wavelength of the input light. m n is the actual width of the multimode interference region. c denoted as the refractive index of the waveguide cladding material.

[0015] Air slits are placed at image points in the multimode interference region, allowing for the placement of air slits at multiple image points.

[0016] The tapered, gradually tapered output waveguide for horizontal light output is placed at the corresponding image point on the right side of the multimode interference region.

[0017] A tapered, gradually increasing output waveguide for vertical light output is placed on the upper or lower side of the multimode interference region at the corresponding image point.

[0018] The waveguide core is composed of strips of Si embedded in the cladding SiO2.

[0019] The beneficial effects of this invention are as follows: This MMI beam splitter is a novel MMI that can achieve 90° beam splitting. Compared with traditional MMIs that can only emit light horizontally, it achieves light emission in both horizontal and vertical directions without changing the original size of the MMI. The vertical direction is at any image point in the multimode interference region, and it can also obtain any beam splitting ratio. This avoids the need to add other devices to the outside of the MMI to change the optical path direction and beam splitting ratio, reduces the loss of the entire integrated device, makes the entire device more compact, and thus improves the overall performance and stability.

[0020] By adding a 45° air slit at the image point in the MMI, light can be emitted in both horizontal and vertical directions. Different slit widths have different beam splitting ratios. The structure is simple and compact, and its application in optical networks will greatly reduce the complexity of the system. Attached Figure Description

[0021] The attached figure is a schematic diagram of the structure of the present invention.

[0022] Figure 1 , 1×1 internal bundled MMI structure diagram.

[0023] Figure 2 , Light intensity diagram of the cross section at point A-A' of the 1×1 input straight waveguide.

[0024] Figure 3 , 1×1 internal beam MMI intensity distribution diagram.

[0025] Figure 4(a) shows the relationship between different slit widths of the 1×1 internal beam split MMI and the transmittance and reflectance of each port. Figure 4(b) shows the relationship between the transmittance and reflectance of each port when the air slit is semi-transparent and semi-reflective.

[0026] Figure 5 The relationship between the maximum width of a 1×2 tapered waveguide and its port transmittance and reflectance.

[0027] Figure 6(a) shows the beam splitting structure of a 1×2 MMI at three image points.

[0028] Figure 6(b) shows the intensity distribution of the beam split at the three-image point of the 1×2 MMI.

[0029] Figure 7(a) shows the relationship between different slit widths of the 1×2 internal beam split MMI and the transmittance and reflectance of each port. Figure 7(b) shows the relationship between the transmittance and reflectance of each port when the air slit is semi-transparent and semi-reflective.

[0030] Figure 8 A beam splitting structure diagram with air slits added to the upper and lower image points of the trisection image point.

[0031] In the figure, 1 is a single-mode input straight waveguide, 2 is a tapered input waveguide, 3 is a multimode interference region, 4 is an air slit, 5 is a tapered output waveguide one, 6 is an output straight waveguide, 7 is a tapered output waveguide two, 8 is an output straight waveguide two, and 9 is a tapered output waveguide three. Detailed Implementation

[0032] The attached figures illustrate specific embodiments of the present invention.

[0033] The multimode interference coupler internal beam splitting device of the present invention has a high-refractive-index waveguide core embedded in a low-refractive-index cladding. The core includes an input straight waveguide 1, a tapered graded input waveguide 2, a multimode interference region 3, an air slit 4, tapered graded output waveguides 5 and 7, and an output straight waveguide 6 connected in sequence. The tapered graded input waveguide is connected to one end of the multimode interference region. The tapered graded output waveguide can be connected to multiple positions in the multimode interference region as needed. The air slits are placed at different image points of the MMI according to the length of the MMI and actual needs. One air slit can be placed, or multiple air slits can be placed at multiple image points.

[0034] The input straight waveguide is a single-mode waveguide, used to input light into the beam splitter in single-mode form.

[0035] The output straight waveguide is a single-mode waveguide, used to transmit light in single-mode form.

[0036] The cross-sectional width of the tapered input waveguide increases linearly in the direction of light propagation.

[0037] The cross-sectional width of the tapered output waveguide narrows linearly in the direction of light propagation.

[0038] The length of the multimode interference region is:

[0039] ;

[0040] Where L π Let β0 and β1 be the beat lengths of the two lowest-order modes, N be the number of output waveguides, β0 and β1 be the propagation constants of the fundamental and first-order modes in the multimode interference region, and n be the number of output waveguides. r W is the effective refractive index of the core layer. e W represents the effective width of the multimode interference region, λ0 represents the input light wavelength, and W represents the wavelength of the input light. m n is the actual width of the multimode interference region. c denoted as the refractive index of the waveguide cladding material.

[0041] Air slits are placed at image points in the multimode interference region, allowing for the placement of air slits at multiple image points.

[0042] The tapered, gradually tapered output waveguide for horizontal light output is placed at the corresponding image point on the right side of the multimode interference region.

[0043] A tapered, gradually increasing output waveguide for vertical light output is placed on the upper or lower side of the multimode interference region at the corresponding image point.

[0044] The waveguide core is composed of strips of Si embedded in the cladding SiO2.

[0045] The present invention discloses a beam splitting device within a multimode interference coupler. The MMI device includes an input waveguide region, a multimode interference region, an air slit, and an output waveguide region. The input waveguide region is connected to one end of the multimode interference region. The output waveguide region can be connected to multiple locations within the multimode interference region as needed. The air slit is placed at different image points of the MMI, depending on the length of the MMI and actual requirements; either one air slit can be placed at a time, or multiple air slits can be placed at multiple image points.

[0046] The input waveguide region inputs a single-mode optical field into the multimode interference region; the multimode interference region excites various modes from the input optical field, and under certain conditions, the modes constructively interfere with each other, resulting in one or more images of the input optical field appearing periodically along the direction of optical field propagation; the air slit at the image point of the multimode interference region splits the beam into horizontal and vertical directions, and the optical power of each output port can be controlled to achieve arbitrary beam splitting ratios; the output waveguide region transmits the beam from the multimode interference region and the beam split from the air slit.

[0047] like Figure 1As shown, this invention relates to a controllable beam splitting ratio device within a multimode interference coupler (MMI) based on SOI (silicon-on-insulator) material. The MMI device includes an input waveguide region, a multimode interference region, an air slit, and an output waveguide region. To mitigate the adverse effects of the width mismatch between the input / output waveguide region and the multimode interference region, a tapered gradient waveguide structure is incorporated. The input waveguide region is connected to the left end of the multimode interference region, output waveguide one is connected to the right end of the multimode interference region, and output waveguide two is connected to the lower end of the multimode interference region. The air slit is placed at the image point of the MMI. The entire waveguide core is composed of strips of Si embedded in the cladding SiO2. When TE-polarized light is input, after passing through the tapered input waveguide and the multimode interference region, the light wave is split into horizontal and vertical beams at the image point by the air slit. The different slit widths result in different beam splitting ratios, which are then transmitted through their respective tapered output waveguides and connected to other subsequent devices. This achieves controllable beam splitting ratios for both horizontal and vertical light output within the MMI.

[0048] Figure 1 Since the narrow end of the downward tapered output waveguide 2 is already the width of the output straight waveguide, adding or not adding an output straight waveguide is optional.

[0049] Figure 8 In this method, multiple air slits are placed at multiple image points, for example, air slits are added between the two image points above and below the third image point.

[0050] according to Figure 3 As shown in Figure 6, after light enters the multimode interference region 3, it does not fly out in a straight line. Instead, the light will exhibit a self-image phenomenon after entering, and one or more image points will appear periodically in the waveguide.

[0051] The specific working principle of this invention is as follows:

[0052] The width of the air slit is on the nanometer scale. When it forms a 45° angle with the incident light, due to the evanescent field at the slit boundary, part of the incident light's energy continues to propagate horizontally through the evanescent field. The remaining portion of the light, because the refractive index of the waveguide core is much higher than that of air, easily undergoes total internal reflection at the interface between the air slit and the waveguide core, thus becoming reflected light in the vertical direction. By controlling the width of the air slit, different transmittance and reflectance can be achieved: a larger air slit results in lower transmittance and higher reflectance, leading to lower horizontal beam energy and higher vertical beam energy; conversely, a smaller air slit results in higher transmittance and lower reflectance, leading to higher horizontal beam energy and lower vertical beam energy.

[0053] When TE-polarized light with a wavelength of 1550nm is input, the input waveguide region inputs a single-mode optical field into the multimode interference region. The multimode interference region excites various modes from the input optical field. Under certain conditions, the modes constructively interfere with each other, and one or more images of the input optical field will periodically appear along the direction of optical field propagation. At the image point in the multimode interference region, the air slit splits the beam into horizontal and vertical directions within the MMI. By controlling the width of the air slit, different transmittance and reflectance of the beam at the image point can be achieved, thus realizing arbitrary beam splitting ratios in these two directions. Finally, the light in the horizontal and vertical directions is transmitted out after passing through the output waveguide region and connected to other subsequent devices. Thus, controllable beam splitting ratios for horizontal and vertical light output are achieved within the MMI.

[0054] To verify that the present invention can achieve the above-mentioned functions, Examples 1 and 2 are provided for verification and explanation.

[0055] Example 1

[0056] The main parameters used in Example 1 are: the entire waveguide layer thickness is 0.75 μm, n_ Si =3.476, n_ SiO2 =1.444; The multimode interference region is rectangular, with a width W_m = 1.6 μm and a length L_m = 5.75 μm; The air slit width is 112 nm, placed at the first single-image self-image point, at a 45° angle to the horizontal direction; The input / output straight waveguide is 0.5 μm wide and 2.5 μm long; The tapered input / output waveguide has a minimum width of 0.5 μm and a maximum width of 0.6 μm. (The rate of change of width is the maximum width minus the minimum width, then divided by the waveguide length). The absolute value of the width change rate of the tapered input waveguide and the tapered output waveguide 1 is 20 nm / um (based on a tapered waveguide length of 5 μm, (0.6-0.5) / 5=0.02 μm / um = 20 nm / um), and the absolute value of the width change rate of the tapered output waveguide 2 is 50 nm / um (based on a tapered waveguide length of 2 μm, (0.6-0.5) / 2=0.05 μm / um = 50 nm / um). The center of the port of the tapered input waveguide and the output waveguide 1 are located at the center of the left and right sides of the width of the multimode interference region, respectively, and the center of the tapered output waveguide 2 is on the same vertical line as the first single self-image point.

[0057] When TE-polarized light with a wavelength of 1550nm is input from the input waveguide, the cross-sectional optical field distribution of the light at the input straight waveguide A-A' is as follows: Figure 2 As shown, light propagates in single-mode within the input waveguide. The calculated light field distribution across the entire device is as follows. Figure 3As shown, light is split into horizontal and vertical beams at the air slit inside the MMI. By changing the width of the air slit from 10 nm to 200 nm, the beam splitting ratio of the output light in the horizontal and vertical directions is changed, as shown in Figure 4(a). It can be seen that as the width of the air slit increases, the transmittance of output port 1 decreases, that is, the output light power in the horizontal direction weakens; the transmittance of output port 2 increases, that is, the output light power in the vertical direction strengthens. Further investigation into the case of light being partially transparent and partially reflected in the air slit shows the relationship between the air slit width and the transmittance of each port, as shown in Figure 4(b). When the width of the air slit is 112 nm, the transmittance of output port 1 and output port 2 are equal, that is, the same power output light is achieved in the horizontal and vertical directions.

[0058] Substituting each parameter into the formula, the calculation is as follows:

[0059] Wm = 1.6 μm, λ0 = 1.55 μm, n r =3.3352, n c =1.444. W m λ0 is the actual width of the multimode interference region, and λn is the input light wavelength; r n is the effective refractive index of the core layer (the effective refractive index is calculated using model analysis in Comsol). c L is the refractive index of the waveguide cladding material. π Let β0 and β1 be the beat lengths of the two lowest-order modes, N be the number of output waveguides, β0 and β1 be the propagation constants of the fundamental and first-order modes in the multimode interference region, and W be the frequency of the output waveguides. e The effective width of the multimode interference region.

[0060] .

[0061] Example 2

[0062] The main parameters used in Example 2 are as follows: the entire waveguide layer is 0.3 μm thick, n_Si=3.476, n_SiO2=1.444; the multimode interference region is rectangular, with a width W_m=5 μm and a length L_m=25.3 μm; the air slit width is 127 nm, placed at the image point position under the first triple self-image, at a 45° angle to the horizontal direction; the input / output straight waveguide is 0.5 μm wide and 2.5 μm long; the center of the tapered input waveguide port is at the center of the width on the left side of the multimode interference region; the center distance between the tapered output waveguide one and the tapered output waveguide two (lateral waveguides) is 2.592 μm, located on both sides of the center of the width of the multimode interference region; the center of the output waveguide three (vertical waveguide) is on the same vertical line as the image point of the first triple self-image.

[0063] Example 2 is a 1×2 MMI internal beam splitter. To mitigate the adverse effects caused by the width mismatch between the input / output waveguide region and the multimode interference region, a tapered waveguide structure was added. The minimum width of the tapered waveguide is 0.5 μm. The relationship between the maximum width of the tapered waveguide and the port transmittance and reflectance was calculated, and the results are as follows. Figure 5 As shown, output port 1 and output port 2 have the same transmittance. It can be seen that the optimal width of the tapered waveguide is 0.85 μm, and this optimal width ensures... Figure 5 The minimum width at which the transmittance is maximum. (The width change rate is the maximum width minus the minimum width, then divided by the waveguide length). The absolute value of the width change rate for tapered input waveguide, tapered output waveguide one, and tapered output waveguide two is 70 nm / um (based on a tapered waveguide length of 5 μm, (0.85-0.5) / 5=0.07 μm / um = 70 nm / um), and the absolute value of the width change rate for tapered output waveguide three is 116.667 nm / um (based on a tapered waveguide three length of 3 μm, (0.85-0.5) / 3=0.116667 μm / um = 116.667 nm / um).

[0064] When TE-polarized light with a wavelength of 1550nm is input through the input straight waveguide, it propagates in single-mode within the waveguide. The optical field distribution calculated for the entire device is shown in Figure 6. The light is split into horizontal and vertical beams at the air slit of the first triple image point of the MMI. By changing the width of the air slit from 10nm to 200nm, the beam splitting ratio of the output light in the horizontal and vertical directions is changed, as shown in Figure 7(a). It can be seen that as the width of the air slit increases, the transmittance of output port 2 decreases, while the transmittance of output port 3 increases. Further investigation into the case of half-transmission and half-reflection of light in the air slit reveals the relationship between the air slit width and the transmittance of each port, as shown in Figure 7(b). When the width of the air slit is 127nm, the transmittances of output port 2 and output port 3 are equal, thus achieving the half-transmission and half-reflection function of the air slit.

[0065] Substituting the parameters into the formula, the calculation is as follows:

[0066] Wm=5 μm, λ0=1.55 μm, n r =3.0427, n c =1.444. W m λ0 is the actual width of the multimode interference region, and λn is the input light wavelength; r n is the effective refractive index of the core layer. c L is the refractive index of the waveguide cladding material. π Let β0 and β1 be the beat lengths of the two lowest-order modes, N be the number of output waveguides, and β0 and β1 be the propagation constants of the fundamental and first-order modes in the multimode interference region, respectively. eThe effective width of the multimode interference region.

[0067] .

[0068] In summary, the beam splitting device inside the multimode interference coupler proposed in this invention achieves beam output in both horizontal and vertical directions by adding a 45° air slit at the image point in the MMI. Furthermore, different slit widths result in different beam splitting ratios. The structure is simple and compact, and its application in optical networks will greatly reduce the complexity of the system.

Claims

1. A multi-mode interference coupler in-bundle device, characterized by: The core layer high-refractive waveguide is embedded in a low-refractive cladding layer, and the core layer comprises an input straight waveguide, a tapered input waveguide, a multimode interference region, an air slit, a tapered output waveguide and an output straight waveguide connected in sequence; the tapered input waveguide is connected with one end of the multimode interference region, the tapered output waveguide can be connected with multiple positions of the multimode interference region according to the use requirement, the air slit is placed at different image points of the MMI according to the length of the MMI and the actual requirement, one air slit can be placed, or multiple air slits can be placed at multiple image points; By adding a 45° air slit at the image point in the MMI, horizontal and vertical light emission is realized, and different slit widths have different beam splitting ratios; The tapered output waveguide for horizontal light emission is placed at the corresponding image point on the right side of the multimode interference region; The tapered output waveguide for vertical light emission is placed at the corresponding image point on the upper side or lower side of the multimode interference region. The length of the multimode interference region is: ； where L π is the beat length of the two lowest order modes, N is the number of output waveguides, β0and β1are the propagation constants of the fundamental and first order modes of the multimode interference region, n r is the effective refractive index of the core, W e is the effective width of the multimode interference region, λ0is the input light wavelength, W m is the actual width of the multimode interference region; n c is the material refractive index of the waveguide cladding.

2. An intra-coupler beam splitting device according to claim 1, wherein, The input straight waveguide is a single-mode waveguide, which is used for inputting light into the beam splitter in a single-mode form.

3. An intra-coupler beam splitting device according to claim 1, wherein, The output straight waveguide is a single-mode waveguide, which is used for outputting light in a single-mode form.

4. A multi-mode interference coupler in-beam device according to claim 1, wherein, The cross-sectional width of the tapered input waveguide linearly widens in the light propagation direction.

5. A multi-mode interference coupler in-beam device according to claim 1, wherein, The cross-sectional width of the tapered output waveguide linearly narrows in the light propagation direction.

6. A multi-mode interference coupler in-beam device according to claim 1, wherein, The waveguide core layer is composed of a strip-shaped Si embedded in a cladding layer SiO2.

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