Optical mixer based on multimode interference coupler

Abstract

The invention is suitable for the technical field of optical communication, and provides an optical mixer based on a multimode interference coupler. The 2 * 2MMI is adopted, the phase modulator is introduced in the transmission process to provide the 90-degree absolute phase difference, the mode that the phase difference is obtained by increasing the optical path in the past is thoroughly changed, loss is reduced, and the application range of the wavelength is enlarged. In the design, the optical signal and the intrinsic signal are respectively input, so that the maximum power distribution of the optical signal and the intrinsic signal can be realized in the frequency mixing process, and the overall efficiency is improved. In addition, the optical mixer is compact in design, integration on a chip can be easily achieved, and powerful support is provided for various mixing application scenes. The characteristics of low loss and high broadband of the optical mixer enable optical signals to maintain high signal quality and stability in the transmission process, meet the requirements of a modern optical communication system for high-speed and large-capacity transmission, and show huge use potential in the field of optical communication.

Description

Technical Field

[0001] The invention belongs to the technical field of optical communication, and in particular relates to an optical mixer based on a multi-mode interference coupler. Background Art

[0002] Compared with traditional optical communication technology, coherent optical communication technology has the advantages of higher receiving sensitivity, larger communication capacity and more modulation modes. It mainly includes two parts: coherent modulation and heterodyne detection. At the transmitting end, the optical signal is used to modulate the frequency, phase, amplitude and other information of the carrier signal, and then at the receiving end, the local oscillator signal is mixed with the received signal in the optical mixer to obtain the frequency, phase, amplitude and other information of the input optical signal.

[0003] Optical multi-level modulation formats for digital coherent detection are considered to be promising candidates for building next-generation high-capacity transmission systems. In their receivers, 90-degree optical mixers are absolutely necessary to retrieve the in-phase and quadrature (I and Q) components from optical signals. The basic principle of a 90-degree optical mixer is that the intrinsic light and the signal light are injected from two channel entrances as linearly polarized light, and the intrinsic light is converted from linearly polarized light to circularly polarized light after passing through a quarter wave plate (due to the fast and slow axes, the O light and the E light have a phase difference of π / 2, and the light intensity is the same), and then it is split by a polarization beam splitter (PBS), and the P light and the S light are separated and enter two optical paths respectively. At the same time, the signal light is also split by the PBS, and the P light and the S light are separated from each other, and are mixed with the S light and the P light of the intrinsic light to generate new light beams. At this time, the two optical paths have a phase difference of π / 2. After passing through a half wave plate, a π phase difference is generated in the mixed optical path, and then separated by the PBS, and finally two beams with a phase difference of π are generated.

[0004] An optical mixer is an optical device that can mix multiple optical signals together and output a composite signal. In optical communications, it is mainly used to realize modulation, frequency conversion, wavelength conversion and time domain conversion of optical signals to meet the needs of different communication systems for optical signals. Coherent reception uses the mixing of local oscillator signals and received signals, which can effectively increase the power of the transmitted signal and provide a higher OSNR than the incoherent system, which is very advantageous for improving the transmission distance. However, the optical mixer in the prior art has the following disadvantages: the traditional 90-degree optical mixer relies on the precise positioning of optical components, such as polarization beam splitters (PBS) and wave plates, and the positions of these components need to be very precise to maintain the required phase difference. Any slight offset will affect the accuracy of the phase difference, thereby reducing the system performance; the traditional design contains multiple optical components, each of which will introduce additional losses, reducing the overall system efficiency; the method based on physical delay lines can usually only work in a narrow wavelength range, which limits its application in multi-wavelength systems. For this reason, an optical mixer based on a multimode interference coupler (MMI) is proposed. Summary of the invention

[0005] The object of the present invention is to provide an optical mixer based on a multimode interference coupler, aiming to solve the problems raised in the above background technology.

[0006] To achieve the above object, the present invention provides the following technical solutions:

[0007] Optical mixer based on multimode interference coupler, including:

[0008] A substrate, wherein the substrate is a silicon layer;

[0009] A substrate, which is a silicon dioxide layer, located on the base;

[0010] A silicon waveguide layer, wherein the silicon waveguide layer comprises four 2×2 MMIs, all of which are located on a substrate; the four MMIs are respectively MMI No. 1, MMI No. 2, MMI No. 3 and MMI No. 4, and transmission waveguides are connected between MMI No. 1 and MMI No. 3, between MMI No. 1 and MMI No. 4, between MMI No. 2 and MMI No. 3, and between MMI No. 2 and MMI No. 4; an input waveguide No. 1 is arranged above MMI No. 1, and signal light is injected from the input waveguide No. 1; an input waveguide No. 2 is arranged below MMI No. 2, and intrinsic light is injected from the input waveguide No. 2; two output waveguides are arranged on MMI No. 3, namely output waveguide No. 3 and output waveguide No. 4; two output waveguides are arranged on MMI No. 4, namely output waveguide No. 1 and output waveguide No. 2;

[0011] The electrode layer includes a phase modulator, which is located on the transmission waveguide between the second MMI and the third MMI and is used to provide an absolute phase shift of 90°.

[0012] Furthermore, the MMI has a width of 6 μm, a length of 50 μm, a silicon-based refractive index of 3.95, a silicon dioxide refractive index of 1.45, and an effective refractive index of 2.45.

[0013] Furthermore, the input waveguide and the output waveguide of the MMI are both tapered in design, with a length of 10 μm, a maximum width of 1 μm, and a minimum width of 0.2 μm.

[0014] Furthermore, the MMI is a multi-mode interference coupler of the limited interference type.

[0015] Furthermore, the intersection of the four MMIs is a cross-coupled waveguide.

[0016] Furthermore, the transmission waveguide has a width of 0.5 μm and a height of 0.22 μm.

[0017] Furthermore, the optimal applicable wavelength bands of the signal light and the intrinsic light are both 1500-1600nm.

[0018] Further, the output waveguide No. 1, output waveguide No. 2, output waveguide No. 3 and output waveguide No. 4 generate relative phase differences of 180°, 0°, 270° and 90° respectively.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] The present invention adopts 2×2MMI and introduces a phase modulator to provide an absolute phase difference of 90° during the transmission process, which completely changes the previous method of relying on increasing the optical path to obtain the phase difference, not only reducing the loss, but also increasing the applicable range of the wavelength. In the design, the optical signal and the intrinsic signal are input separately, ensuring that the two can achieve maximum power distribution during the mixing process, thereby improving the overall efficiency. In addition, the optical mixer of the present invention is compact in design and can be easily integrated on the chip, providing strong support for a variety of mixing application scenarios. The low-loss and high-bandwidth characteristics of the optical mixer enable the optical signal to maintain high signal quality and stability during the transmission process, meeting the needs of modern optical communication systems for high-speed and large-capacity transmission, and showing great potential for use in the field of optical communications. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a schematic diagram of the connection relationship structure of four MMIs in the present invention.

[0022] Figure 2 The structural diagram of MMI in the present invention is

[0023] Figure 3 This is the output effect diagram of simulation in commercial software.

[0024] Figure 4 This is a simulation diagram of the output port phase difference of MMI in commercial software.

[0025] Figure 5 This is a diagram of the overall simulation in link simulation.

[0026] Figure 6 This is the signal output diagram of output waveguide No. 3 and output waveguide No. 4.

[0027] In the figure: MMI No. 1 1, MMI No. 2 2, MMI No. 3 3, MMI No. 4 4, phase modulator 5, input waveguide No. 1 6, input waveguide No. 2 7, transmission waveguide 8, output waveguide No. 1 9, output waveguide No. 2 10, output waveguide No. 3 11, output waveguide No. 4 12. DETAILED DESCRIPTION

[0028] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0029] The specific implementation of the present invention is described in detail below in conjunction with specific embodiments.

[0030] like Figure 1 and Figure 2 As shown, an optical mixer based on a multimode interference coupler provided by an embodiment of the present invention includes:

[0031] A substrate, wherein the substrate is a silicon layer;

[0032] A substrate, which is a silicon dioxide layer, located on the base;

[0033] A silicon waveguide layer, wherein the silicon waveguide layer comprises four 2×2 MMIs, all of which are located on a substrate; the four MMIs are MMI No. 1 1, MMI No. 2 2, MMI No. 3 3 and MMI No. 4 4, and transmission waveguides 8 are connected between MMI No. 1 1 and MMI No. 3 3, between MMI No. 1 1 and MMI No. 4 4, between MMI No. 2 2 and MMI No. 3 3, and between MMI No. 2 2 and MMI No. 4 4; an input waveguide No. 1 6 is arranged above MMI No. 1 1, and signal light is injected from the input waveguide No. 1 6; an input waveguide No. 2 7 is arranged below MMI No. 2 2, and intrinsic light is injected from the input waveguide No. 2 7; two output waveguides are arranged on MMI No. 3 3, namely output waveguide No. 3 11 and output waveguide No. 4 12; two output waveguides are arranged on MMI No. 4 4, namely output waveguide No. 1 9 and output waveguide No. 2 10;

[0034] like Figure 2 As shown, the width W of MMI MMI 6μm, (length) L MMIIt is 50μm, the silicon-based refractive index is 3.95, the refractive index of silicon dioxide is 1.45, and the effective refractive index is 2.45. On this basis, this design can achieve the maximum output power. The input waveguide and output waveguide of the MMI are both tapered to ensure the lowest loss. The length is 10μm. To ensure that it can be connected to the waveguide, the widest part is 1μm and the narrowest part is 0.2μm. The transmission waveguide 8 has a width of 0.5μm and a height of 0.22μm to ensure that the transmission efficiency is optimal. The cross position is a cross-coupled waveguide. Since there is no optical path difference between each other, interference coupling does not occur. Its loss is mainly the waveguide transmission loss and MMI interference coupling loss during the transmission process. The optimal applicable band of signal light is 1500-1600nm. The available band of intrinsic light is consistent with that of signal light, and it can be input at the same frequency as the signal light.

[0035] The electrode layer comprises a phase modulator 5 , which is located on a transmission waveguide 8 between the second MMI 2 and the third MMI 3 and is used to provide an absolute phase shift of 90°.

[0036] In the embodiment of the present invention, this design can prevent the slight offset of the lens position. At the same time, different from the 4×4 MMI design, the 4×4 MMI design is more complicated than the 2×2, and the loss of light waves therein is greater than that of 2×2. In the transmission process, the phase modulator 5 is added to provide a 90° absolute phase difference, and the previous method of increasing the optical path to provide the absolute phase difference is abandoned, which not only reduces the loss but also increases the applicable range of wavelengths.

[0037] The 2×2 MMI and phase modulator 5 are arranged and combined to play the role of a quarter wave plate, a half wave plate and a PBS. The first MMI and the phase modulator 5 actually play the role of a quarter wave plate and a PBS, the second MMI 2 and the third MMI 3 play the role of a half wave plate and the front and rear PBS, and the fourth MMI 4 plays the role of a quarter wave plate and a PBS. The phase modulator 5 generates an absolute phase difference of π / 2, the second MMI 2 and the third MMI 3, the first MMI 1 and the fourth MMI 4 generate a π phase difference, and finally generate relative phase differences of 0°, 90°, 180°, and 270°.

[0038] As a preferred embodiment of the present invention, the signal light is injected from the input waveguide above the first MMI, the splitting ratio is 50:50, and the relative phase difference of the output port is π / 2; the intrinsic light is injected from the input waveguide below the second MMI 2, the splitting ratio is 50:50, and the relative phase difference of the output port is π / 2; after the signal light is divided into two beams, they are respectively directed to the third MMI 3 and the fourth MMI 4, and their phases are slightly different during the transmission process. Multimode interference coupling occurs in the MMI, and the equal division effect is achieved through the interference coupling in the middle of the multimode, and the π / 2 phase difference of the output port is achieved. After the intrinsic light is divided into two beams, they are respectively directed to the third MMI 3 and the fourth MMI 4. The optical path leading to the third MMI 3 will pass through the 90-degree absolute phase modulator 5, and a phase difference of 90° will be generated with the other path. The optical path leading to the fourth MMI 4 is not processed in any way, and finally four signals with relative phase differences of 0°, 90°, 180°, and 270° are generated. The details are as follows

[0039] The input signal light is expressed as The input eigenlight is expressed as Where A1 is the signal light intensity, A2 is the intrinsic light intensity, j is a complex number, w1 is the signal light frequency, w2 is the intrinsic light frequency, t is the time, is the initial phase of the signal light, is the initial phase of the intrinsic light; the four output signals are E0, E 90 、E 180 、E 270 , according to the transfer function, the signal value of each channel is:

[0040]

[0041] According to the output signal value, it can be known that the relative phase differences generated by the output waveguide No. 1 9, the output waveguide No. 2 10, the output waveguide No. 3 11 and the output waveguide No. 4 12 are 180°, 0°, 270° and 90° respectively; The phase error will be generated during the modulation process of phase modulator 5. Therefore, the subsequent signal output is connected to a balanced detector for filtering to remove noise interference.

[0042] In the embodiment of the present invention, the following additional equipment is required when conducting the experiment:

[0043] Semiconductor lasers are used to generate continuous wave signals. The wavelength range of the laser should be between 1500-1600nm to ensure that the wavelength can be less lost during data transmission.

[0044] The polarization controller is used to control the polarization of the signal light and the intrinsic light to ensure that the signal light and the intrinsic light are in a linear polarization state.

[0045] The constant voltage controller is used to control the phase modulator 5 in the mixer to ensure that its absolute phase shift is 90 degrees.

[0046] The balanced photodetector is used to detect the output light intensity in a balanced manner, remove the noise generated during the phase modulation process, ensure that the waveform is basically consistent with the data itself, and detect the final I and Q values.

[0047] The constant temperature control table is used to ensure that the temperature of the mixer is in a suitable range and that the phase modulator 5 can work at the set temperature to prevent errors in the experiment.

[0048] As a preferred embodiment of the present invention, MMI can obtain v guided modes according to the dispersion equation of the waveguide, and the effective width of each guided mode can be approximated by the effective width of the base film. When the input position of the input waveguide is offset by ±W relative to the center of the multimode interference region MMI / 6 [limited interference], the modes of order v = 2, 5, 8, ... are not excited, and the excited modes satisfy mod3[v(v+2)] = 0, so the imaging position will be reduced by 1 / 3, realizing a 2×2 MMI coupler. Therefore, the MMI used is a multimode interference coupler of the limited interference type, and the input waveguide and output waveguide positions are both within ±W MMI / 6, according to our design structure, it should be offset from the center by ±1μm.

[0049] Example 1: Simulation is performed in commercial software, and the output effect diagram is as follows Figure 3 shown. Figure 4 To simulate the output port phase difference of MMI in commercial software, the output port phase difference is -π / 2, indicating that the design meets the requirements and meets the conditions of use. Then the overall simulation is performed in the link simulation ( Figure 5 ) to verify the performance and mixing effect of the designed 90-degree mixer. Through simulation, the test shows that the 90-degree mixer can basically restore the input signal and reproduce the key points of the signal, which preliminarily meets the required requirements. Figure 6 The figure shows the signal output diagram of the third output waveguide 11 and the fourth output waveguide 12. It can be seen from the figure that except for the phase shift, the waveforms are basically the same.

[0050] The above are only preferred embodiments of the present invention. It should be pointed out that, for those skilled in the art, several modifications and improvements can be made without departing from the concept of the present invention. These should also be regarded as the protection scope of the present invention. These will not affect the effect of the implementation of the present invention and the practicality of the patent.

Claims

1. An optical mixer based on a multimode interference coupler, characterized in that: include: A substrate, wherein the substrate is a silicon layer; A substrate, which is a silicon dioxide layer, located on the base; A silicon waveguide layer, wherein the silicon waveguide layer comprises four 2×2 MMIs, all of which are located on a substrate; the four MMIs are respectively MMI No. 1, MMI No. 2, MMI No. 3 and MMI No. 4, and transmission waveguides are connected between MMI No. 1 and MMI No. 3, between MMI No. 1 and MMI No. 4, between MMI No. 2 and MMI No. 3, and between MMI No. 2 and MMI No. 4; an input waveguide No. 1 is arranged above MMI No. 1, and signal light is injected from the input waveguide No. 1; an input waveguide No. 2 is arranged below MMI No. 2, and intrinsic light is injected from the input waveguide No. 2; two output waveguides are arranged on MMI No. 3, namely output waveguide No. 3 and output waveguide No. 4; two output waveguides are arranged on MMI No. 4, namely output waveguide No. 1 and output waveguide No. 2; The electrode layer includes a phase modulator, which is located on the transmission waveguide between the second MMI and the third MMI and is used to provide an absolute phase shift of 90°.

2. The optical mixer based on the multimode interference coupler according to claim 1, characterized in that: The MMI has a width of 6 μm, a length of 50 μm, a silicon-based refractive index of 3.95, a silicon dioxide refractive index of 1.45, and an effective refractive index of 2.

45.

3. The optical mixer based on the multimode interference coupler according to claim 1, characterized in that: The input waveguide and the output waveguide of the MMI are both tapered in design, with a length of 10 μm, a maximum width of 1 μm, and a minimum width of 0.2 μm.

4. The optical mixer based on a multimode interference coupler according to claim 1, characterized in that: The MMI is a multi-mode interference coupler of the limited interference type.

5. The optical mixer based on the multimode interference coupler according to claim 1, characterized in that: The intersections of the four MMIs are cross-coupled waveguides.

6. The optical mixer based on a multimode interference coupler according to claim 1, characterized in that: The transmission waveguide has a width of 0.5 μm and a height of 0.22 μm.

7. The optical mixer based on a multimode interference coupler according to claim 1, characterized in that: The optimal applicable wavelengths of the signal light and the intrinsic light are both 1500-1600nm.

8. The optical mixer based on a multimode interference coupler according to claim 1, characterized in that: The output waveguide No. 1, output waveguide No. 2, output waveguide No. 3 and output waveguide No. 4 generate relative phase differences of 180°, 0°, 270° and 90° respectively.

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