Mach-Zehnder interferometer cascade comb filter and wavelength division multiplexer / demultiplexer

Abstract

The present invention relates to a Mach-Zehnder interferometer cascade comb filter and a wavelength division multiplexer / demultiplexer to solve the technical problems of wavelength sensitivity introduced by the wavelength division multiplexer of the existing cascade Mach-Zehnder interferometer and its unsuitability for transverse magnetic waves. The filter comprises n-stage comb filter units connected in series, and the m-th stage comb filter unit comprises 2 m A comb filter is provided, each comb filter includes a Mach-Zehnder interferometer, and the Mach-Zehnder interferometer includes a first optical coupling component, a transmission waveguide, and a second optical coupling component arranged in sequence along the transmission direction of the light wave; the first transmission waveguide in the transmission waveguide is provided with a phase delay waveguide, and after TE polarized light and TM polarized light pass through the phase delay waveguide, the phase difference between the TE polarization and the TM polarization is 2πA, where A is a positive integer, and the group refractive index difference between the TE polarization and the TM polarization is within 15%.

Description

Technical Field

[0001] The present invention relates to a wavelength division multiplexer / demultiplexer, in particular to a Mach-Zehnder interferometer cascade comb filter and a wavelength division multiplexer / demultiplexer. Background Art

[0002] With the popularization of network terminal equipment, people generate a huge amount of information in their daily lives. With the development of more network applications, people's demand for information will continue to surge. Wavelength division multiplexing technology is one of the important technical means to expand the communication capacity of optical networks.

[0003] Compared with wavelength division multiplexers / demultiplexers based on arrayed waveguide gratings and waveguide Bragg gratings, wavelength division multiplexers / demultiplexers based on cascaded Mach-Zehnder interferometers have low crosstalk, low insertion loss and high integration. However, traditional wavelength division multiplexers based on cascaded Mach-Zehnder interferometers use directional couplers with straight waveguide coupling, such as Figure 1 Figure 01 shows a curved directional coupler. This type of coupler introduces wavelength sensitivity, hindering wide-bandwidth wavelength division multiplexing. Furthermore, conventional schemes only multiplex transverse electric waves (TE waves) and are not applicable to transverse magnetic waves (TM waves), significantly limiting their application scenarios. Therefore, developing a wavelength- and polarization-insensitive wavelength division multiplexer based on cascaded Mach-Zehnder interferometers is crucial for achieving large-scale applications. Summary of the Invention

[0004] The present invention aims to solve the technical problems of wavelength sensitivity introduced by the wavelength division multiplexer of the existing cascaded Mach-Zehnder interferometer and its unsuitability for transverse magnetic waves, and proposes a Mach-Zehnder interferometer cascaded comb filter and a wavelength division multiplexer / demultiplexer.

[0005] The technical solution provided by the present invention is:

[0006] A Mach-Zehnder interferometer cascade comb filter, which has the following special features:

[0007] It includes n-stage comb filter units connected in series, and the m-th stage comb filter unit includes 2 m-1 comb filters, each comb filter in the m-1th stage comb filter unit has two output terminals connected to the input terminals of two corresponding comb filters in the mth stage comb filter unit, n ≥ 1 and is an integer, m = 2, ..., n;

[0008] The comb filter includes a Mach-Zehnder interferometers, where a≥1 and is an integer;

[0009] The Mach-Zehnder interferometer includes a first optical coupling component, a transmission waveguide, and a second optical coupling component arranged in sequence along the transmission direction of the light wave; the transmission waveguide includes a first transmission waveguide and a second transmission waveguide, the first transmission waveguide is provided with a phase delay waveguide, and after the TE polarized light and the TM polarized light pass through the phase delay waveguide, the phase difference between the TE polarization and the TM polarization is 2πA, where A is a positive integer, and the group refractive index difference between the TE polarization and the TM polarization is within 15%;

[0010] The first optical coupling assembly includes a first coupler and two first coupler input waveguides and two first coupler output waveguides provided at both ends of the first coupler. The second optical coupling assembly includes a second coupler and two second coupler input waveguides and two second coupler output waveguides provided at both ends of the second coupler. The two first coupler output waveguides are connected to the input ends of the first transmission waveguide and the second transmission waveguide, respectively. The output ends of the first transmission waveguide and the second transmission waveguide are connected to the two second coupler input waveguides, respectively.

[0011] The second optical coupling component of the b-1th Mach-Zehnder interferometer also serves as the first optical coupling component of the bth Mach-Zehnder interferometer, b=2, ..., a;

[0012] The two second coupler output waveguides of the ath Mach-Zehnder interferometer are connected to the first optical coupling component (11) of the first Mach-Zehnder interferometer of the two corresponding comb filters in the next stage comb filter unit.

[0013] Furthermore, the first coupler and the second coupler are segmented curved directional couplers or multimode interference couplers;

[0014] The segmented bent directional coupler or the multimode interference coupler has the same splitting ratio in the TE polarization mode and the TM polarization mode.

[0015] Furthermore, the segmented curved directional coupler includes a first coupling region and a second coupling region, wherein both TE polarized light and TM polarized light are coupled in the first coupling region, and TM polarized light is coupled in the second coupling region; the first coupling region comprises two co-centric first coupling region curved waveguides, and the radii of the two first coupling region curved waveguides are unequal; the second coupling region comprises two co-centric second coupling region curved waveguides, and the radii of the two second coupling region curved waveguides are unequal; the spacing between the two second coupling region curved waveguides is greater than the spacing between the two first coupling region curved waveguides, and the first coupling region and the second coupling region are connected by a straight waveguide or an S-shaped waveguide;

[0016] The bending directions of adjacent segmented curved directional couplers are opposite to each other, so as to offset the phase difference between the first transmission waveguide and the second transmission waveguide caused by the segmented curved directional couplers.

[0017] Furthermore, the multimode interference coupler comprises one or two input sections, a multimode interference region and two output sections connected in sequence;

[0018] The input section is connected to the first coupler input waveguide or the second coupler input waveguide, the input section is a first strip waveguide, and the width of the first strip waveguide is the same as the width of the first coupler input waveguide or the second coupler input waveguide; the output section is connected to the first coupler output waveguide or the second coupler output waveguide, the output section is a second strip waveguide, and the width of the second strip waveguide is the same as the width of the first coupler output waveguide or the second coupler output waveguide; wherein the cross-sections of the first strip waveguide, the first coupler input waveguide, the second coupler input waveguide, the second strip waveguide, the first coupler output waveguide, and the second coupler output waveguide perpendicular to the direction of light wave transmission are rectangular, and the width of the rectangle is the width of the corresponding waveguide;

[0019] The multimode interference region is reversely calculated using a neural network algorithm to obtain structural parameters of the multimode interference region.

[0020] Furthermore, the comb filter includes one Mach-Zehnder interferometer, two Mach-Zehnder interferometers, or three Mach-Zehnder interferometers.

[0021] Furthermore, the comb filter includes a Mach-Zehnder interferometer, and the coupling coefficients of the two optical couplers of the Mach-Zehnder interferometer are both between 0.45 and 0.55;

[0022] Alternatively, the comb filter comprises two Mach-Zehnder interferometers, and the coupling coefficients of the couplers arranged sequentially along the light wave transmission direction are in the ranges of 0.45-0.55, 0.12-0.18, and 0.12-0.18, respectively;

[0023] Alternatively, the comb filter includes three Mach-Zehnder interferometers, and the coupling coefficients of the couplers arranged sequentially along the light wave transmission direction are in the ranges of 0.45-0.55, 0.17-0.23, 0.17-0.23, and 0.03-0.07, respectively.

[0024] Furthermore, the 2 in the n-th stage comb filter unit n-1 Comb filters form 2 n-1 Wavelength division multiplexing channels, and the spacing between adjacent wavelength division multiplexing channels is equal;

[0025] The free spectrum range of the comb filter in the m-th stage comb filter unit is 2 times the wavelength division multiplexing channel spacing. mtimes, the center wavelength of each comb filter is set to the middle wavelength of the corresponding comb filter wavelength division multiplexing channel to improve the extinction ratio of the final filter response. Generally, the position of the center wavelength can be adjusted by the length of the delay waveguide;

[0026] The phase delay waveguide is used to meet the requirements of polarization insensitivity and the free spectrum range of the Mach-Zehnder interferometer by lengthening the waveguide structure or setting a heater or performing ion implantation.

[0027] Furthermore, each waveguide section of the Mach-Zehnder interferometer includes a core layer and a cladding layer, the core layer is silicon, silicon nitride, or lithium niobate, and the cladding layer is silicon dioxide, air, or SU-8 photoresist.

[0028] Furthermore, the first optical coupling component and the phase delay waveguide, as well as the phase delay waveguide and the second optical coupling component are connected by a tapered waveguide and a curved waveguide.

[0029] The present invention also provides a wavelength division multiplexer / demultiplexer, which is special in that it includes the above-mentioned Mach-Zehnder interferometer cascade comb filter.

[0030] Beneficial effects of the present invention:

[0031] 1. The present invention provides a Mach-Zehnder interferometer cascade comb filter. By matching the TE polarization mode and the TM polarization mode, the phase delay waveguide in the Mach-Zehnder interferometer introduces an integer multiple phase difference, and at the same time adopts a polarization-insensitive optical coupler, so that the cascade comb filter can realize wavelength division multiplexing of the same channel under different polarization conditions, and has the advantage of polarization insensitivity.

[0032] 2. Compared with the traditional solution of rotating TM polarization to TE polarization, the Mach-Zehnder interferometer cascade comb filter of the present invention does not require a polarization rotator, so it has lower requirements on the manufacturing process and lower device insertion loss.

[0033] 3. The wavelength division multiplexer / demultiplexer provided by the present invention provides a Mach-Zehnder interferometer cascade comb filter suitable for any waveguide processing technology without adding more functional devices. By designing the waveguide of the Mach-Zehnder interferometer, an optical coupler structure and a phase delay waveguide structure are formed, thereby achieving wavelength insensitivity and polarization insensitivity. This method has a wide applicability to processing technology.

[0034] 4. The segmented curved directional coupler or multimode interference coupler provided by the present invention has the advantage of being insensitive to wavelength. Its coupling coefficient varies less within the spectral range of wavelength division multiplexing than that of traditional optical couplers, making it easier to reduce crosstalk between different channels. Therefore, the wavelength division multiplexer / demultiplexer provided by the present invention has the advantage of low crosstalk. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 Schematic diagram of an existing bent directional coupler;

[0036] Figure 2 Schematic diagram of an embodiment of a Mach-Zehnder interferometer cascade comb filter of the present invention (wherein the first coupler and the second coupler are both segmented curved directional couplers);

[0037] Figure 3 Schematic diagram of the structure of a Mach-Zehnder interferometer in an embodiment of the present invention (wherein the first coupler and the second coupler are both multimode interference couplers);

[0038] Figure 4 Schematic diagram of the structure of a segmented curved directional coupler in an embodiment of the present invention;

[0039] Figure 5 Schematic diagram of the structure of a multimode interference coupler according to an embodiment of the present invention;

[0040] Figure 6 Spectral simulation diagram of the wavelength division multiplexer / demultiplexer of the present invention in TE polarization mode and TM polarization mode.

[0041] The reference numerals are as follows:

[0042] 01-Bent directional coupler;

[0043] 1-Mach-Zehnder interferometer, 11-first optical coupling component, 12-second optical coupling component, 13-segmented curved directional coupler, 131-first coupling region, 132-second coupling region, 14-phase delay waveguide, 15-multimode interference coupler, 151-input section, 152-multimode interference region, 153-output section, 16-transmission waveguide, 17-first coupler input waveguide, 18-second coupler output waveguide. DETAILED DESCRIPTION

[0044] Example 1

[0045] See also Figure 2 This embodiment provides a Mach-Zehnder interferometer cascade comb filter, which includes n-stage comb filter units connected together, and 2 of the n-stage comb filter units n-1 Comb filters form 2 n-1 There are wavelength division multiplexing channels, and the distances between adjacent wavelength division multiplexing channels are equal.

[0046] The m-th stage comb filter unit includes 2 m-1 A comb filter with a free spectral range of 2 times the wavelength division multiplexing channel spacing. mtimes, and the center wavelengths of the comb filters are different, and the center wavelengths of the comb filters are set to the middle wavelength of the wavelength division multiplexing channel of the corresponding comb filter; in this embodiment, two stages of comb filters are included, then the free spectral range of the first-stage comb filter is twice the wavelength division multiplexing channel interval, and the free spectral range of the second-stage comb filter is four times the wavelength division multiplexing channel interval; each comb filter in the m-1th stage comb filtering unit has two output ends, which are respectively connected to the input ends of the corresponding two comb filters in the mth stage comb filtering unit, n ≥ 1 and is an integer, m = 2, ..., n.

[0047] The comb filter includes a Mach-Zehnder interferometers 1, where a≥1 and is an integer; Figure 2 As shown, this embodiment performs wavelength division multiplexing on four channels of 1271 nm-1331 nm. In the figure, λ1, λ2, λ3, and λ4 correspond to central wavelengths of 1271 nm, 1291 nm, 1311 nm, and 1331 nm, respectively. The Mach-Zehnder interferometer 1 cascade comb filter includes two levels of comb filters, the first-level comb filter includes one comb filter, and the comb filter includes two Mach-Zehnder interferometers 1 connected end to end, with a total of three optical couplers. In the case of wavelength division multiplexing, after the light input of four channels with central wavelengths of 1271nm, 1291nm, 1311nm, and 1331nm passes through the first-level comb filter, the light with central wavelengths of 1271nm and 1311nm enters the upper second-level comb filter and is output to its two output ends respectively, and the light with central wavelengths of 1291nm and 1331nm enters the lower second-level comb filter and is output to its two output ends respectively.

[0048] See also Figure 3The Mach-Zehnder interferometer 1 includes a first optical coupling component 11, a transmission waveguide 16, and a second optical coupling component 12 arranged in sequence along the transmission direction of the light wave; the transmission waveguide 16 includes a first transmission waveguide and a second transmission waveguide, the first transmission waveguide is provided with a phase delay waveguide 14, and the phase delay waveguide 14 adopts a standard 220nm SOI (Silicon-On-Insulator) with a 2μm thick silicon dioxide cladding. After TE polarized light and TM polarized light pass through the phase delay waveguide 14, the phase difference between the TE polarization and the TM polarization is 2πA, where A is a positive integer, which is used to ensure that the filtering response of TE polarization and TM polarization is consistent, and the group refractive index difference between TE polarization and TM polarization is within 15%, so as to ensure that the free spectral range of the filter in the TE mode and TM mode is as equal as possible. It can be understood that as the phase difference between TE polarization and TM polarization increases, the group refractive index of the two polarizations also increases, resulting in an increase in the deviation of the free spectral range of the filter in the two polarization modes. Therefore, while satisfying the condition of an integer multiple phase difference, the group refractive index difference needs to be minimized. The phase delay waveguide 14 can be designed through waveguide structure, a heater, or ion implantation to meet polarization insensitivity and the free spectral range of the Mach-Zehnder interferometer 1.

[0049] In this embodiment, by changing the length, width, and curvature of the phase delay waveguide 14, the TE polarization and TM polarization effective refractive index n of the phase delay waveguide 14 are adjusted. TE 、n TM Satisfies the following formula:

[0050]

[0051] where n g is the mean of the group refractive index of TE and TM polarizations, FSR is the free spectral range, λ is the central wavelength, N is a positive integer, and n g The free spectral range (FSR) of the Mach-Zehnder interferometer 1 satisfies the following formula:

[0052]

[0053] Wherein, ΔL is the length of the phase delay waveguide 14. When designing the phase delay waveguide 14, it should be noted that the group refractive index of TE and TM polarizations should be as equal as possible under the condition that the phase difference between TE and TM polarizations is an integer multiple. Figure 2As shown, in this embodiment, the first comb filter has an FSR of 40 nm and a λ of 1301 nm, and includes two Mach-Zehnder interferometers 1 connected end to end, with two phase delay waveguides 14. The accumulated phase of the first phase delay waveguide is determined by the FSR and λ of the first comb filter, and the accumulated phase of the second phase delay waveguide is approximately twice that of the first phase delay waveguide. The length, width, and curvature of the two phase delay waveguides are then finely adjusted to ensure that the TE and TM phase difference period multiple A is 1. That is, after TE polarized light passes through the phase delay waveguide 14, the accumulated phase difference compared to the TM polarization is exactly 1 period. The second comb filter has an FSR of 80 nm and λ of 1291 nm and 1311 nm, respectively. The design steps of the phase delay waveguide 14 are the same as those of the first comb filter. It can be understood that the phase delay waveguide 14 needs to offset the phase difference between the two arms caused by the optical coupler.

[0054] Each waveguide segment of the Mach-Zehnder interferometer 1 includes a core layer and a cladding layer. The core layer is made of silicon, silicon nitride, or lithium niobate, and the cladding is made of silicon dioxide, air, or SU-8 photoresist. Each waveguide segment of the Mach-Zehnder interferometer 1 can be a strip waveguide. The cross-section of the strip waveguide can be square or rectangular, or a combination of a strip waveguide and a ridge waveguide. However, each waveguide segment cannot be a ridge waveguide.

[0055] When there are multiple Mach-Zehnder interferometers 1, the multiple Mach-Zehnder interferometers 1 are connected end to end. The more Mach-Zehnder interferometers 1 connected end to end, the larger the bandwidth of each channel, but the higher the insertion loss. Preferably, a is 1 or 2 or 3. Specifically, the comb filter includes one Mach-Zehnder interferometer 1, and the coupling coefficients of the two optical couplers of the Mach-Zehnder interferometer 1 are both between 0.45 and 0.55; or the comb filter includes two Mach-Zehnder interferometers 1, and the coupling coefficients of the couplers arranged in sequence along the light wave transmission direction are respectively in the ranges of 0.45-0.55, 0.12-0.18, and 0.12-0.18; in this embodiment, the splitting ratios of the three optical couplers along the light wave transmission direction are 0.5, 0.15, and 0.15, respectively. The comb filter comprises three Mach-Zehnder interferometers 1 , and along the light wave transmission direction, the splitting ratio ranges of the respective couplers arranged sequentially are 0.45-0.55, 0.17-0.23, 0.17-0.23, and 0.03-0.07 respectively.

[0056] See also Figure 1 , a commonly used curved directional coupler, has a coupling region consisting of two co-centered curved waveguides with different radii and central angles. The optical power in the two curved waveguides in a curved directional coupler can be calculated using the following formula:

[0057]

[0058]

[0059] Where P0 is the initial optical power, P1 is the optical power in the upper curved waveguide, and P2 is the optical power in the lower curved waveguide. is the maximum coupling coefficient, and K is defined by the following formula:

[0060]

[0061] The phase (K·L) of the curved directional coupler is π. It can be understood that under this condition, the variation of the splitting ratio is small, that is, the wavelength dependence is small. Due to the different bending radii, the effective refractive index n of the two curved waveguides is eff are also different, so the maximum coupling coefficient of the bent directional coupler is It will be reduced accordingly. By setting the coupling distance, bending radius and coupling length, a directional coupler with a specified splitting ratio can be obtained, and it has the advantage of being insensitive to wavelength. Since the bent directional coupler is a directional coupler, after the light passes through the coupling region, there is a 90-degree phase difference between the two arms, that is, a quarter of a cycle. At the same time, due to the asymmetric structure of the two arms of the bent directional coupler, the light will also accumulate additional phase difference after passing through the bent directional coupler.

[0062] In this embodiment, the first optical coupling component 11 includes two first coupler input waveguides 17, a segmented curved directional coupler 13, and two first coupler output waveguides connected in sequence. The second optical coupling component 12 includes two second coupler input waveguides, a segmented curved directional coupler 13, and two second coupler output waveguides 18. The segmented curved directional coupler 13 has the same splitting ratio in the TE polarization mode and the TM polarization mode. The two first coupler output waveguides are respectively connected to the input ends of the first transmission waveguide and the second transmission waveguide, while the output ends of the first transmission waveguide and the second transmission waveguide are respectively connected to the two second coupler input waveguides. Specifically, the first optical coupling component 11 and the phase delay waveguide 14, as well as the phase delay waveguide 14 and the second optical coupling component 12, are connected using tapered waveguides and curved waveguides to reduce mode adaptation loss across the waveguide widths. The second transmission waveguide includes tapered waveguides and curved waveguides with the same structure and number as the first transmission waveguide. The second optical coupling component 12 of the b-1th Mach-Zehnder interferometer 1 also serves as the first optical coupling component 11 of the bth Mach-Zehnder interferometer 1 , where b=2, . . . , a.

[0063] See also Figure 4The segmented curved directional coupler 13 includes a first coupling region 131 and a second coupling region 132. Both TE polarized light and TM polarized light are coupled in the first coupling region 131, and TM polarized light is coupled in the second coupling region 132. The first coupling region 131 is composed of two co-centered first coupling region curved waveguides, and the radii of the two first coupling region curved waveguides are unequal. The second coupling region 132 is composed of two co-centered second coupling region curved waveguides, and the radii of the two second coupling region curved waveguides are unequal. The spacing between the two second coupling region curved waveguides is greater than the spacing between the two first coupling region curved waveguides. The first coupling region 131 and the second coupling region 132 are connected by a straight waveguide or an S-shaped waveguide. After light containing TE and TM polarizations enters the segmented curved directional coupler 13, it first enters the TE coupling region, where both TE and TM polarizations are coupled. The coupling distance is then increased, and the light enters the TM coupling region. At this point, TE polarization exceeds the coupling distance and decouples, while TM polarization continues to couple. The design principle of each segmented coupling region is the same as that of the curved directional coupler 3. Since the segmented curved directional coupler 13 is a directional coupler, after light passes through the segmented curved directional coupler 13, there is a 90-degree phase difference, or one-quarter of a period, between the two arms. At the same time, due to the structural asymmetry of the two arms of the segmented curved directional coupler 13, light accumulates additional phase difference after passing through the segmented curved directional coupler 13.

[0064] If a comb filter includes an even number of segmented curved directional couplers 13, the bending directions of adjacent segmented curved directional couplers 13 are opposite, thereby canceling out the phase difference between the first and second transmission waveguides caused by the segmented curved directional couplers 13. If a comb filter includes an odd number of segmented curved directional couplers 13, the bending directions of adjacent segmented curved directional couplers 13 are opposite, but the phase delay waveguide 14 needs to be adjusted to cancel out the phase difference between the first and second transmission waveguides caused by the segmented curved directional couplers 13.

[0065] The output waveguides of the two second optical coupling components 12 of the ath Mach-Zehnder interferometer 1 are connected to the first optical coupling components 11 of the first Mach-Zehnder interferometer 1 of the two corresponding comb filters in the next stage comb filter unit; the first optical coupling component used in this embodiment is a segmented curved directional coupler 13, with Figure 2For example, the first-stage comb filtering unit includes a comb filter, which includes two Mach-Zehnder interferometers 1. The second-stage comb filtering unit includes a first comb filter and a second comb filter. Both comb filters include two Mach-Zehnder interferometers 1. The second optical coupling component 12 output waveguides of the second Mach-Zehnder interferometer 1 have two output waveguides. One output waveguide of the second optical coupling component 12 is connected to one of the first coupler input waveguides 17 of the first Mach-Zehnder interferometer 1 of the first comb filter in the second-stage comb filtering unit, transmits λ1 and λ3 through it, and after coupling through the segmented bent directional coupler 13, transmits the signal into the other waveguide; the other output waveguide of the second optical coupling component 12 is connected to one of the first coupler input waveguides 17 of the first Mach-Zehnder interferometer 1 of the second comb filter in the second-stage comb filtering unit, transmits λ2 and λ4 through it, and after coupling through the segmented bent directional coupler 13, transmits the signal into the other waveguide.

[0066] The Mach-Zehnder interferometer cascade comb filter provided in this embodiment can be used in a wavelength division multiplexer or a demultiplexer. When used in a demultiplexer, the first coupler input waveguide 17 of the first Mach-Zehnder interferometer 1 in the first stage comb filter unit receives a signal containing 2 n-1 The optical signal of the central wavelength is divided into two beams according to a specified ratio, and enters the first transmission waveguide and the second transmission waveguide respectively. The first transmission waveguide is provided with a phase delay waveguide 14 for generating a specified phase difference with the second transmission waveguide. The second optical coupling component 12 receives the two optical signals transmitted by the first transmission waveguide and the second transmission waveguide and combines them according to a preset ratio and generates interference, and then transmits them to the next stage comb filter unit until it contains 2 n-1 The optical signals with the same central wavelength enter 2 n-1 The wavelength division multiplexing channels are demultiplexed. When used in a multiplexer, the nth level comb filter will demultiplex 2 n-1 The optical signals corresponding to the three central wavelengths are transmitted in reverse order to achieve multiplexing.

[0067] See also Figure 6 The filtering response of this embodiment is simulated. The black curve is the TE polarization response curve, and the gray curve is the TM polarization response curve. The four wavelength division multiplexing / demultiplexing channels are represented by solid lines, dashed lines, dotted lines, and dot-dash lines respectively. The TE and TM polarizations do not completely overlap because the group refractive indices of the two polarizations of the phase delay waveguide 14 are not equal. It can be seen from the figure that the single-channel bandwidth of the filtering response of TE polarization and TM polarization in the embodiment of the present invention is greater than 8nm, the crosstalk at the center wavelength of each channel is less than -25dB, and the polarization-related loss is less than 1dB, which has the advantage of polarization insensitivity.

[0068] Example 2

[0069] In this embodiment, the first optical coupling assembly 11 includes two first coupler input waveguides 17, a multimode interference coupler 15, and two first coupler output waveguides connected in sequence. The second optical coupling assembly 12 includes two second coupler input waveguides, a segmented curved directional coupler 13, and two second coupler output waveguides 18. The multimode interference coupler 15 and the segmented curved directional coupler 13 have the same splitting ratio in the TE polarization mode and the TM polarization mode.

[0070] See also Figure 5 The multimode interference coupler 15 includes an input section 151, a multimode interference region 152 and two output sections 153 connected in sequence; the input section 151 is connected to the first coupler input waveguide 17 or the second coupler input waveguide, and the input section 151 is a first strip waveguide, and the width of the first strip waveguide is the same as the width of the first coupler input waveguide 17 or the second coupler input waveguide; the output section 153 is connected to the first coupler output waveguide or the second coupler output waveguide 18, and the output section 153 is a second strip waveguide, and the width of the second strip waveguide is the same as the width of the first coupler output waveguide or the second coupler output waveguide 18; wherein, the cross-sections of the first strip waveguide, the first coupler input waveguide 17, the second coupler input waveguide, the second strip waveguide, the first coupler output waveguide and the second coupler output waveguide 18 perpendicular to the direction of light wave transmission are rectangular, and the width of the rectangle is the width of the corresponding waveguide.

[0071] The multimode interference region 152 is surrounded by multiple rapidly changing curves, and rapid regulation of the effective refractive index of the waveguide is achieved by changing the cross-sectional structure. In this embodiment, the multimode interference coupler 15 is reverse-engineered based on a genetic neural network using edge polar coordinates as parameters, so that the multimode interference coupler 15 has the advantages of polarization insensitivity and wavelength insensitivity.

[0072] The other structures are the same as those in Example 1.

Claims

1. A Mach-Zehnder interferometer cascade comb filter, characterized in that: It includes n-stage comb filter units connected in series, and the m-th stage comb filter unit includes 2 m-1 comb filters, each comb filter in the m-1th stage comb filter unit has two output terminals connected to the input terminals of two corresponding comb filters in the mth stage comb filter unit, n ≥ 1 and is an integer, m = 2, ..., n; The comb filter includes a Mach-Zehnder interferometers (1), where a≥1 and is an integer; The Mach-Zehnder interferometer (1) comprises a first optical coupling component (11), a transmission waveguide (16), and a second optical coupling component (12) arranged in sequence along the transmission direction of the light wave; the transmission waveguide (16) comprises a first transmission waveguide and a second transmission waveguide, the first transmission waveguide is provided with a phase delay waveguide (14), after the TE polarized light and the TM polarized light pass through the phase delay waveguide (14), the phase difference between the TE polarization and the TM polarization is 2πA, A is a positive integer, and the group refractive index difference between the TE polarized light and the TM polarized light is within 15%; The first optical coupling component (11) includes a first coupler and two first coupler input waveguides (17) and two first coupler output waveguides arranged at both ends of the first coupler. The second optical coupling component (12) includes a second coupler and two second coupler input waveguides and two second coupler output waveguides (18) arranged at both ends of the second coupler. The two first coupler output waveguides are respectively connected to the input ends of the first transmission waveguide and the second transmission waveguide. The output ends of the first transmission waveguide and the second transmission waveguide are respectively connected to the two second coupler input waveguides. The second optical coupling component (12) of the b-1th Mach-Zehnder interferometer (1) also serves as the first optical coupling component (11) of the bth Mach-Zehnder interferometer (1), wherein b=2, ..., a; The two second coupler output waveguides (18) of the ath Mach-Zehnder interferometer (1) are connected to the first optical coupling components (11) of the first Mach-Zehnder interferometer (1) of the two corresponding comb filters in the next stage comb filter unit.

2. The Mach-Zehnder interferometer cascade comb filter according to claim 1, characterized in that: The first coupler and the second coupler are segmented curved directional couplers (13) or multimode interference couplers (15); The segmented curved directional coupler (13) or the multimode interference coupler (15) has the same light splitting ratio in the TE polarization mode and the TM polarization mode.

3. The Mach-Zehnder interferometer cascade comb filter according to claim 2, characterized in that: The segmented curved directional coupler (13) comprises a first coupling region (131) and a second coupling region (132); TE polarized light and TM polarized light are both coupled in the first coupling region (131), and TM polarized light is coupled in the second coupling region (132); the first coupling region (131) comprises two first coupling region curved waveguides with a common center, and the radii of the two first coupling region curved waveguides are unequal; the second coupling region (132) comprises two second coupling region curved waveguides with a common center, and the radii of the two second coupling region curved waveguides are unequal; the spacing between the two second coupling region curved waveguides is greater than the spacing between the two first coupling region curved waveguides, and the first coupling region (131) and the second coupling region (132) are connected by a straight waveguide or an S-shaped waveguide; The bending directions of adjacent segmented curved directional couplers (13) are opposite, and are used to offset the phase difference between the first transmission waveguide and the second transmission waveguide caused by the segmented curved directional couplers (13).

4. The Mach-Zehnder interferometer cascade comb filter according to claim 2, characterized in that: The multimode interference coupler (15) comprises one or two input sections (151), a multimode interference region (152) and two output sections (153) connected in sequence; The input section (151) is connected to the first coupler input waveguide (17) or the second coupler input waveguide, the input section (151) is a first strip waveguide, and the width of the first strip waveguide is the same as the width of the first coupler input waveguide (17) or the second coupler input waveguide; the output section (153) is connected to the first coupler output waveguide or the second coupler output waveguide (18), the output section (153) is a second strip waveguide, and the width of the second strip waveguide is the same as the width of the first coupler output waveguide or the second coupler output waveguide (18); wherein the cross-sections of the first strip waveguide, the first coupler input waveguide (17), the second coupler input waveguide, the second strip waveguide, the first coupler output waveguide and the second coupler output waveguide (18) perpendicular to the light wave transmission direction are rectangular, and the width of the rectangle is the width of the corresponding waveguide; The multimode interference region (152) is reversely calculated using a neural network algorithm to obtain structural parameters of the multimode interference region (152).

5. The Mach-Zehnder interferometer cascade comb filter according to any one of claims 1 to 4, characterized in that: The comb filter comprises one Mach-Zehnder interferometer (1) or two Mach-Zehnder interferometers (1) or three Mach-Zehnder interferometers (1).

6. The Mach-Zehnder interferometer cascade comb filter according to claim 5, characterized in that: The comb filter comprises a Mach-Zehnder interferometer (1), wherein the coupling coefficients of two optical couplers of the Mach-Zehnder interferometer (1) are both between 0.45 and 0.55; Alternatively, the comb filter comprises two Mach-Zehnder interferometers (1), and the coupling coefficients of the couplers arranged sequentially along the light wave transmission direction are in the range of 0.45-0.55, 0.12-0.18, and 0.12-0.18 respectively; Alternatively, the comb filter comprises three Mach-Zehnder interferometers (1), and the coupling coefficients of the couplers arranged sequentially along the light wave transmission direction are in the ranges of 0.45-0.55, 0.17-0.23, 0.17-0.23, and 0.03-0.07, respectively.

7. The Mach-Zehnder interferometer cascade comb filter according to claim 6, characterized in that: 2 in the nth stage comb filter unit n-1 Comb filters form 2 n-1 Wavelength division multiplexing channels, and the spacing between adjacent wavelength division multiplexing channels is equal; The free spectrum range of the comb filter in the m-th stage comb filter unit is 2 times the wavelength division multiplexing channel spacing. m times, the center wavelength of each comb filter is set to the middle wavelength of the wavelength division multiplexing channel of the corresponding comb filter; The phase delay waveguide (14) is used to meet the polarization insensitivity and the free spectrum range of the Mach-Zehnder interferometer (1) by changing the waveguide structure length or setting a heater or performing ion implantation.

8. The Mach-Zehnder interferometer cascade comb filter according to claim 7, characterized in that: Each section of the waveguide of the Mach-Zehnder interferometer (1) comprises a core layer and a cladding layer, wherein the core layer is silicon, silicon nitride or lithium niobate, and the cladding layer is silicon dioxide, air or SU-8 photoresist.

9. The Mach-Zehnder interferometer cascade comb filter according to claim 8, characterized in that: The first optical coupling component (11) and the phase delay waveguide (14), as well as the phase delay waveguide (14) and the second optical coupling component (12) are connected by tapered waveguides and curved waveguides.

10. A wavelength division multiplexer / demultiplexer, characterized in that: The invention comprises the Mach-Zehnder interferometer cascade comb filter as described in any one of claims 1-9.

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