Design method of polarization-insensitive wavelength division multiplexing demultiplexer

Abstract

The invention discloses a design method of a polarization-insensitive wavelength division multiplexing demultiplexer, which comprises the following steps of: selecting the thickness of a core layer of the wavelength division multiplexing demultiplexer to obtain a change rule of an effective refractive index along with a waveguide width in a polarization mode TE and a polarization mode TM; the diffraction series of the waveguide width and the polarization mode TE and the diffraction series of the polarization mode TM are solved through the grating equation, so that the polarization mode TE and the polarization mode TM meet the grating equation under the same central wavelength; and if the ratio of the wavelength interval in the polarization mode TE to the wavelength interval in the polarization mode TM is detected to be 0.98-1, the waveguide width, the diffraction series of the polarization mode TE and the diffraction series of the polarization mode TM are used as design parameters of the polarization-insensitive wavelength division multiplexing demultiplexer, and the design is completed. The wavelength division multiplexer can work under TE polarization and TM polarization at the same time, and the polarization sensitivity problem of the wavelength division multiplexer can be solved.

Description

Technical Field

[0001] The invention relates to the technical field of multiplexing, and in particular to a design method for a polarization-insensitive wavelength division multiplexing demultiplexer. Background Art

[0002] Wavelength division multiplexing (WDM) technology greatly improves the transmission capacity of optical fiber by transmitting multiple independent data streams in the same optical fiber, meeting the growing communication needs. The advancement of WDM technology not only provides a solid communication foundation for fields such as high-speed Internet, video transmission and telemedicine, but also promotes the rapid development of the information society and lays the foundation for the future of optical communication technology.

[0003] In the application of WDM system, single-mode fiber is the first choice for long-distance communication. However, the polarization state of light is unstable, which may cause polarization-dependent loss and wavelength error, thus affecting signal transmission. Therefore, WDM devices must work properly in any polarization state to avoid energy loss and signal loss.

[0004] Although the planar light waveguide (PLC) platform based on silica can realize polarization-independent WDM devices, its device size is large and it is difficult to achieve monolithic integration with detectors and modulators. In contrast, although the InP platform can achieve monolithic integration and is polarization-insensitive, it still faces size and cost limitations.

[0005] Silicon-based photonic integration is considered to be an ideal platform for realizing wavelength division multiplexing systems due to its advantages such as low cost, high density, monolithic integration and compatibility with CMOS processes. However, for silicon-based optical integrated devices, the high refractive index difference between the silicon layer and the cladding will lead to a significant birefringence effect, which will cause serious polarization-related problems. This makes typical planar integrated wavelength division multiplexing devices (such as etched diffraction gratings, arrayed waveguide gratings, Mach-Zehnder cascade structures, microring resonant filters, etc.) only able to operate in a single polarization state. Therefore, building a polarization-compatible receiving-end demultiplexing architecture has become a difficult problem to be solved in realizing monolithic integrated silicon-based wavelength division multiplexing systems.

[0006] In silicon-based platforms, due to the large refractive index difference between TE (Transverse Electric Mode) and TM (Transverse Magnetic Mode) modes, the demultiplexing devices are usually polarization-dependent.

[0007] To address this problem, planar integrated demultiplexing devices usually have the following solutions:

[0008] The first is the polarization-independent solution. Increasing the thickness of the waveguide core layer will reduce the birefringence effect of the waveguide, making the effective refractive index difference under the two polarizations smaller. By using thick waveguide core layers, square waveguides and other methods, the effective refractive indexes under the two polarizations are made as close as possible, and polarization-insensitive demultiplexing devices are constructed to achieve wavelength demultiplexing of TE and TM polarizations. However, this method often requires a thicker waveguide section, which is inconsistent with the thickness size of general conventional waveguides, has low adaptability and poor integration.

[0009] The second is the polarization diversity solution: one is to separate the TE mode and TM mode of the input signal through a polarization separation rotator (PSR), and rotate the TM0 mode to the TE0 mode, and divide the two polarizations into two paths of light with the same polarization and separate them through a demultiplexer designed for the TE mode. The PSR device in this solution often requires secondary etching, which increases the manufacturing steps and has a high process complexity.

[0010] The other method is to separate the TE mode and TM mode of the input signal through a polarization separation device (PSR) and input them into WDM devices operating in the TE mode and TM mode respectively.

[0011] This solution has obvious disadvantages. First, it requires additional integrated polarization separation devices. Second, this solution requires the design of WDM devices for TE and TM modes respectively. The design is complex and requires two WDM devices, resulting in a large layout area, low efficiency and low integration.

[0012] In addition, the use of polarization beam splitters can also separate the TE0 mode and the TM0 mode in real lines, input TE and TM signals from different input angles, and output them in a wavelength division multiplexer, so as to realize EDG, AWG and other devices that can work in TE and TM modes at the same time. However, this design still requires a front-end cascade polarization beam splitter, and it is difficult to realize polarization-independent wavelength division multiplexing with only a single device.

[0013] The above solutions often require additional components, have complex processes, and are not very universal. Summary of the invention

[0014] The purpose of the present invention is to provide a design method for a polarization-insensitive wavelength division multiplexing demultiplexer. By utilizing the different effective refractive indices of TE mode and TM mode waveguides, the polarization sensitivity problem of wavelength division multiplexing devices can be solved by selecting appropriate TE mode diffraction orders, TM mode diffraction orders and adjusting the optical path difference.

[0015] A polarization-insensitive wavelength division multiplexing demultiplexer design method is disclosed. The polarization-insensitive demultiplexer is designed to adjust the effective refractive index of each polarization and the diffraction order of different polarizations to construct the optical path difference according to the different effective refractive indexes under different polarization states and wavelengths, so that the two polarizations satisfy the grating equation under different diffraction orders and the same central wavelength, and achieve consistent wavelength separation effect under different polarizations.

[0016] The design is universal and applicable to arrayed waveguide gratings, microring resonators, Mach-Zehnder interferometers, etc. The optical path difference of arrayed waveguide gratings, microring resonators, Mach-Zehnder interferometers, etc. mainly comes from straight waveguides or curved waveguides, and the optical path difference is constructed by adjusting the thickness, width and length difference of the waveguide.

[0017] A design method for a polarization-insensitive wavelength division multiplexing demultiplexer comprises the following steps:

[0018] S1. Select the core thickness of the wavelength division multiplexing demultiplexer to obtain the variation law of the effective refractive index with the waveguide width in polarization mode TE and polarization mode TM;

[0019] S2, according to the variation law of the effective refractive index with the waveguide width in the polarization mode TE and the polarization mode TM obtained in step S1, the waveguide width and the diffraction order of the polarization mode TE and the diffraction order of the polarization mode TM are obtained by solving the grating equation, so that the polarization mode TE and the polarization mode TM both satisfy the grating equation at the same central wavelength;

[0020] S3. The wavelength interval under polarization mode TE and polarization mode TM is tested through the waveguide width obtained in step S2 and the diffraction order of polarization mode TE and the diffraction order of polarization mode TM. If the ratio of the wavelength interval under polarization mode TE and polarization mode TM is 0.98-1, the waveguide width and the diffraction order of polarization mode TE and the diffraction order of polarization mode TM obtained in step S2 are used as design parameters of the polarization-insensitive wavelength division multiplexing demultiplexer to complete the design of the polarization-insensitive wavelength division multiplexing demultiplexer.

[0021] The present invention utilizes the different effective refractive indices of TE mode and TM mode waveguides. By selecting appropriate TE mode diffraction orders, TM mode diffraction orders and adjusting the optical path difference, TE polarization and TM polarization satisfy the grating equation at the same wavelength, thereby enabling a wavelength division multiplexer to work simultaneously under TE polarization and TM polarization, and solving the polarization sensitivity problem of wavelength division multiplexing devices.

[0022] In step S1, the wavelength division multiplexing demultiplexer is an arrayed waveguide grating, a micro-ring resonant cavity or a Mach-Zehnder interferometer.

[0023] In step S2, the grating equation is as follows:

[0024] m TM λ0=n TE (w)ΔL

[0025] m TM λ0=n TM (w)ΔL

[0026] Among them, m TE is the diffraction order of polarization mode TE, m TM is the diffraction order of polarization mode TM, λ0 is the central wavelength, w is the waveguide width, n TE is the effective refractive index of the waveguide in TE mode, n TM is the waveguide effective refractive index in TM mode, and ΔL is the waveguide length difference.

[0027] In step S3, the wavelength intervals under the polarization mode TE and the polarization mode TM are checked using the following formula:

[0028]

[0029] in, Refers to the wavelength interval ratio, Δλ TE With Δλ TM Respectively represent the wavelength interval output at the same output port in polarization mode TE and polarization mode TM, n sTE With n sTM is the effective refractive index of the slab waveguide in polarization mode TE and polarization mode TM, n gTE With n gTM is the group refractive index of the array waveguide in polarization mode TE and polarization mode TM.

[0030] Furthermore, the grating equation is

[0031] mλ0=n eff ΔL

[0032] Where m is the diffraction order of the wavelength division multiplexer, λ0 is the central wavelength, and n eff is the effective refractive index of the waveguide in TE mode, and ΔL is the waveguide length difference.

[0033] In an arrayed waveguide grating, the design steps are as follows:

[0034] 1) Select appropriate core layer thickness to obtain the variation of effective refractive index with waveguide width in TE and TM polarization modes;

[0035] 2) Calculate and select appropriate waveguide width and diffraction order of TE and TM polarization respectively by the following formula;

[0036] m TE λ0=n TE(w)ΔL

[0037] m TM λ0=n TM (w)ΔL

[0038] Among them, m TE is the diffraction order of polarization mode TE, m TM is the diffraction order of polarization mode TM, λ0 is the central wavelength, w is the waveguide width, n TE is the effective refractive index of the waveguide in TE mode, n TM is the waveguide effective refractive index in TM mode, and ΔL is the waveguide length difference.

[0039] 3) The diffraction order and waveguide width are selected through step 2), and the wavelength interval under TE polarization and TM polarization is tested. The two satisfy the following relationship:

[0040]

[0041] Among them, Δλ TE With Δλ TM Respectively represent the wavelength interval of TE polarization and TM polarization output at the same output port, n sTE With n sTM is the effective refractive index of the slab waveguide under TE polarization and TM polarization, n gTE With n gTM is the effective refractive index of the array waveguide under TE polarization and TM polarization.

[0042] The wavelength interval ratio under TE polarization and TM polarization was verified to be 0.98-1, and a polarization-insensitive device with good performance was obtained.

[0043] Compared with the prior art, the present invention has the following advantages:

[0044] The structure is simple in design and does not require any special design.

[0045] No need to cascade polarization separation devices, and the structure is simple.

[0046] It has wide applicability and is widely applicable to various wavelength devices without the need for redundant structural design. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 is the spectrum diagram of the output port;

[0048] Figure 2 The TE mode and TM mode in the straight waveguide vary with the effective refractive index and cladding thickness;

[0049] Figure 3 Schematic diagram of the structure of the polarization-insensitive AWG in the present invention;

[0050] Figure 4 The output of polarization-insensitive AWG in design case 1 under TE polarization and TM polarization;

[0051] Figure 5 The output of polarization insensitive AWG in TE polarization and TM polarization for design case 2; DETAILED DESCRIPTION

[0052] The implementation method, principle design and technical effects of the present invention will be further described in detail below in conjunction with the drawings and specific embodiments.

[0053] Among them, taking the polarization-insensitive arrayed waveguide grating in the silicon nitride (SiN) material platform as an example,

[0054] In principle, AWG consists of three main parts: input and output planar waveguides and array waveguides. The input and output planar waveguides follow the Rowland circle structure. The polarization problem mainly comes from the array waveguide part.

[0055] Take the input position of light at the center of the Rowland circle as an example. Light enters the input slab waveguide area, disperses in the slab waveguide area, and couples into the array waveguide. Since the distance traveled by light in the first free transmission area is equal, when entering the array waveguide, the optical path difference of each light is 0; and the array waveguide is generally a group of waveguides with equal length difference and equal width, and light passing through these waveguides will produce equal intervals of phase difference. Light enters the output slab waveguide from the array waveguide port and diffracts. Light of different wavelengths converges on the Rowland circle. Since the optical path difference of different wavelengths when converging on the Rowland circle is different, light of different wavelengths converges at different positions of the Rowland circle.

[0056] If a beam of light is input from the center of the input slab waveguide, the output wavelength of an output port on both sides of the center satisfies the following equation:

[0057] mλ0=n eff ΔL+n s θ o d a

[0058] Where m is the diffraction order, n is eff is the effective refractive index of the array waveguide, n s is the effective refractive index of the slab waveguide, ΔL is the length difference of the array waveguide, θ o is the output angle of light, da is the spacing of the array waveguide. The grating equation satisfied by the light output from the center of the output slab waveguide is:

[0059] mλ0=n eff ΔL

[0060] It can be seen that due to the birefringence effect, the effective refractive index of different polarizations is different, and the optical path difference is different, resulting in different central wavelengths.

[0061] However, by using the polarization refractive index difference, we can select the appropriate TE mode diffraction order, TM mode diffraction order and appropriate waveguide width, so that the mi-th diffraction peak of the TM mode coincides with the m-th diffraction peak of the TE mode, thus achieving polarization insensitivity. The output spectrum diagram of a certain output port under polarization mode TE and polarization mode TM is shown in the figure below. Figure 1 As shown, the mth polarization mode TE TE The mth diffraction peak and the mth TM The diffraction peaks coincide with each other, which is a polarization-insensitive effect.

[0062] The variation of the effective refractive index of SiN straight waveguide with waveguide width in TE mode and TM mode was simulated.

[0063] like Figure 2 As shown, considering that the effective refractive index of the waveguide is a function of the waveguide width, for the central wavelength,

[0064] TE mode meets the following requirements:

[0065] m TE λ0=n TE (w)ΔL

[0066] TM mode meets the following requirements:

[0067] m TM λ0=n TM (w)ΔL

[0068] Among them, m TE is the diffraction order of the arrayed waveguide grating in TE mode, λ0 is the central wavelength, w is the waveguide width, n TE is the effective refractive index of the waveguide in polarization mode TE, n TM is the effective refractive index of the waveguide in polarization mode TM, and ΔL is the length difference between adjacent array waveguides.

[0069] Depend on Figure 2 It is found that the effective refractive index of the same waveguide in polarization mode TM is smaller than that in polarization mode TE. The waveguide width w and diffraction order m that satisfy the two equations are solved. TE With m TM , and then the difference in length of adjacent waveguides is obtained.

[0070] The AWG designed in this design has a saddle-shaped structure made of silicon nitride. The array waveguide part provides different optical path differences for adjacent optical paths. In this design, the optical path difference is provided by the straight waveguide part, which is a strip waveguide. After calculation, the central wavelength of the AWG designed in this design is selected as 1310nm. The diffraction order of case one under polarization mode TE is 54, the diffraction order under polarization mode TE is 51, and the width of the array waveguide is 2.022um. The calculated length difference of adjacent waveguides is 42.64um; the diffraction order of case two is 37, the diffraction order under polarization mode TE is 35, the width of the array waveguide is 1.6145um, and the calculated length difference of adjacent waveguides is 29.4221um; the obtained AWG structure diagram is as follows Figure 3 shown.

[0071] Since TE polarization and TM polarization share the same output port, the wavelength interval of each output channel under TE polarization is

[0072]

[0073] Lianlida

[0074]

[0075] In case one, When the wavelength interval of the TE mode is 3.2 nm, the wavelength interval of the TM mode is 3.172 nm, which meets the design requirements. With a limited number of channels, the error can be approximately ignored.

[0076] In case 2, When the wavelength interval of the TE mode is 3.2 nm, the wavelength interval of the TM mode is 3.1718 nm, which meets the design requirements. With a limited number of channels, the error can be approximately ignored.

[0077] like Figure 4 The figure shows the simulated output of the AWG under TE polarization and TM polarization in Case 1. It can be seen that the output wavelengths of TE polarization and TM polarization are well matched, and the polarization-related wavelength offset of each channel is between 0.001nm and 0.08nm.

[0078] like Figure 5 The figure shows the simulated output of the AWG under TE polarization and TM polarization in Case 2. It can be seen that the output wavelengths of TE polarization and TM polarization are well matched, and the polarization-related wavelength offset of each channel is between 0.001nm and 0.08nm. At the same time, in order to meet the minimum polarization-related loss, the waveguide loss under different polarizations and the design of the wedge waveguide should be considered, so that light with different polarizations can have a small loss.

[0079] The simulation results are consistent with the above derivation.

[0080] At the same time, the waveguide structure used in the example is a strip waveguide. If a ridge waveguide structure is adopted, due to the asymmetry of the ridge waveguide in its vertical direction, at certain widths, the TM0 mode is easy to hybridize with the high-order TE mode, resulting in mode changes. In this case, the waveguide width should be avoided as much as possible within the hybridization range.

Claims

1. A design method for a polarization-insensitive wavelength division multiplexing demultiplexer, characterized in that: The following steps are involved: S1. Select the core thickness of the wavelength division multiplexing demultiplexer to obtain the variation law of the effective refractive index with the waveguide width in polarization mode TE and polarization mode TM; S2, according to the variation law of the effective refractive index with the waveguide width in the polarization mode TE and the polarization mode TM obtained in step S1, the waveguide width and the diffraction order of the polarization mode TE and the diffraction order of the polarization mode TM are obtained by solving the grating equation, so that the polarization mode TE and the polarization mode TM both satisfy the grating equation at the same central wavelength; S3. The wavelength intervals under polarization mode TE and polarization mode TM are tested by using the waveguide width obtained in step S2 and the diffraction order of polarization mode TE and the diffraction order of polarization mode TM. If the ratio of the wavelength intervals under polarization mode TE and polarization mode TM is between 0.98 and 1, the waveguide width and the diffraction order of polarization mode TE and the diffraction order of polarization mode TM obtained in step S2 are used as design parameters of the polarization-insensitive wavelength division multiplexing demultiplexer to complete the design of the polarization-insensitive wavelength division multiplexing demultiplexer.

2. The design method of a polarization-insensitive wavelength division multiplexing demultiplexer according to claim 1, characterized in that: In step S1, the wavelength division multiplexing demultiplexer is an arrayed waveguide grating, a micro-ring resonator or a Mach-Zehnder interferometer.

3. The design method of a polarization-insensitive wavelength division multiplexing demultiplexer according to claim 1, characterized in that: In step S2, the grating equation is as follows: m TF λ0=n TE (w)ΔL m TM λ0=n TM (w)ΔL Among them, m TE is the diffraction order of polarization mode TE, m TM is the diffraction order of polarization mode TM, λ0 is the central wavelength, w is the waveguide width, n TE is the effective refractive index of the waveguide in polarization mode TE, n TM is the effective refractive index of the waveguide in polarization mode TM, and ΔL is the waveguide length difference.

4. The design method of a polarization-insensitive wavelength division multiplexing demultiplexer according to claim 1, characterized in that: In step S3, the wavelength intervals under the polarization mode TE and the polarization mode TM are checked using the following formula: in, Refers to the wavelength interval ratio, Δλ TE With Δλ TM Respectively represent the wavelength interval output at the same output port in polarization mode TE and polarization mode TM, n aTE With n aTM is the effective refractive index of the slab waveguide in polarization mode TE and polarization mode TM, n gTE With n gTM is the group refractive index of the array waveguide in polarization mode TE and polarization mode TM.

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