Optical signal processing device
The optical signal processing device with a tapered waveguide structure addresses manufacturing errors in waveguide-type polarization control elements, enhancing tolerance and facilitating miniaturization of optical transceivers.
Patent Information
- Authority / Receiving Office
- WO · WO
- Patent Type
- Applications
- Current Assignee / Owner
- NT T INC
- Filing Date
- 2024-11-12
- Publication Date
- 2026-05-21
AI Technical Summary
Waveguide-type polarization control elements in optical transceivers are susceptible to manufacturing errors, leading to degraded characteristics and hindering miniaturization efforts due to their small size.
An optical signal processing device with an optical converging unit and a tapered waveguide optical mode conversion unit, featuring at least two tapered sections with different rates of change in waveguide width along the light propagation direction, to enhance tolerance to manufacturing errors and reduce device size.
The device provides improved tolerance to manufacturing errors while maintaining effective polarization and mode state conversion, enabling miniaturization of optical transceivers.
Smart Images
Figure JP2024040230_21052026_PF_FP_ABST
Abstract
Description
Optical signal processing device
[0001] The present invention relates to an optical signal processing device used in optical communication, and particularly to an integrated polarization control element.
[0002] With the expansion of data communication demand such as the Internet, higher capacity is required for optical communication networks. The transmission capacity of optical communication has been increased by multiplexing technologies such as wavelength division multiplexing (WDM), and further development of multiplexing technologies is required in the future.
[0003] Among multiplexing technologies, attention has been focused on polarization multiplexing, which focuses on the polarization state of light and can double the transmission capacity by simultaneously transmitting and receiving two orthogonal polarizations. Polarization multiplexing has already been put into practical use in coherent optical communication used for medium- and long-distance communication, and its introduction is also being studied in IMDD optical communication used for short-distance communication.
[0004] FIG. 1 shows an optical front end 100 of a general transceiver in coherent optical communication used for medium- and long-distance communication. The optical front end 100 of a coherent transceiver consists of a local light source 101, an optical transmission unit 102, and an optical reception unit 103.
[0005] The optical front end 100 of a coherent transceiver operates as follows. First, on the transmission side, continuous light (CW: Continuous Wave) of a desired wavelength output from the local light source 101 is branched by an optical splitter 110, and a part of it is input to the optical transmission unit 102. The input light is branched into two and input to two orthogonal (IQ: In-Phase / Quadrature-Phase) modulators 111 shown by shading, and is modulated into QPSK (Quadrature Phase Shift Keying), QAM (Qudrature Amplitude Modulation), etc. by the applied electrical signal. By a polarization beam combiner rotator (PBCR: Polarization Beam Conbiner Rotator) 112, one of the modulation signals output from the two IQ modulators 111 is rotated by 90° in polarization and polarization combined with the other and output as a modulation signal from output 120.
[0006] On the receiving side, the other CW light branched at the optical splitter 110 is input to the optical receiver 103. Simultaneously, the signal to be received is input from input 121 and input to the receiver 103. The received signal is separated into orthogonal polarizations by the polarization beam splitter (PBS) 113, and one of these is rotated by the polarization rotator (PR) 114 so that it is polarized by 90°. At this time, the polarization of the polarized received signal is rotated so that it is polarized in the same direction as the CW light. These signals are optically demodulated by the 90° optical hybrid mixer 115, and the IN-Phase component and Quadrature-Phase component are extracted for each orthogonal polarization component, and the information is recovered as a signal by being received by the photodiode (PD) 116.
[0007] The PBCR 112, a component of the optical transmitter 102, and the PBS 113 and PR 114, components of the optical receiver 103, are elements for realizing polarization multiplexing and are collectively called polarization control elements. The form of polarization control elements can be broadly classified into two types. One is a spatial optical system using microoptics, which has low loss and little crosstalk between polarizations, but has the disadvantage of being large in size. The other is a waveguide type that can be integrated into an optical chip, and although its characteristics are inferior to those of a spatial optical system, it can be integrated into an optical chip and enables miniaturization of the entire optical front end.
[0008] Optical transceivers, which handle communication inside and outside data centers, face the challenge of miniaturization to improve space utilization efficiency, and miniaturization of the optical front end is essential to achieving miniaturization of optical transceivers. Waveguide-type polarization control elements are only a few millimeters in size overall and can be integrated into optical chips, so they can be implemented in a smaller space compared to polarization control elements using spatial optics, making them suitable for miniaturizing optical transceivers. However, waveguide-type elements integrated into optical chips have the problem that, due to their small size, they are highly susceptible to manufacturing errors during processing, and their characteristics are easily degraded by manufacturing errors.
[0009] Figure 2 is a schematic diagram illustrating the overall optical waveguide structure of a conventional optical signal processing device that constitutes a polarization control element.
[0010] In Figure 2, the optical signal processing device consists of an optical confluence unit 201 made of a lithium niobate waveguide and an optical mode conversion unit 202. Note that in Figure 2, only the shape and arrangement of the optical waveguide core are shown, and the insulating substrate on which the optical waveguide is formed is omitted.
[0011] The optical confluence section 201 has input waveguides 211a and 211b, and an optical confluencer 212 formed in the portion where input waveguides 211a and 211b are located adjacent to each other.
[0012] Light with polarization and mode state SOP1 incident from input waveguide 211a is input to optical mode conversion unit 202 while retaining polarization and mode state SOP1. Light with polarization and mode state SOP1 incident from input waveguide 211b is converted from polarization and mode state SOP1 to polarization and mode state SOP3 in optical confluencer 212, merged in input waveguide 211a, and then input to optical mode conversion unit 202.
[0013] The light with polarization and mode state SOP3 input to the optical mode conversion unit 202 gradually changes polarization and mode state as it propagates through the optical mode conversion unit 202, eventually becoming polarization and mode state SOP2, which is then input to the output waveguide 214 and output.
[0014] The light with polarization and mode state SOP1 input to the optical mode conversion unit 202 is input to the output waveguide 214 without changing the polarization and mode state, and is output simultaneously with the light with polarization and mode state SOP2.
[0015] Note that polarization and mode states SOP1 and SOP3 have the same polarization and their modes are orthogonal to each other. Also, mode state SOP2 is a different mode state from SOP1 and SOP3.
[0016] Overall, the light with polarization and mode state SOP1 input to input waveguide 211a and input waveguide 211b is simultaneously output to the output waveguide as light with polarization and mode state SOP1 and light with polarization and mode state SOP2. While this explanation uses the case where the polarization and mode state SOP1 input to input waveguide 211b is converted to mode state SOP3, the system may also be configured so that the polarization and mode state SOP1 input to input waveguide 211a is converted to mode state SOP3.
[0017] To describe the optical mode conversion unit 202, for the polarization and mode state SOP3 to be converted to polarization and mode state SOP2, the effective refractive indices of the polarization and mode state SOP3 and polarization and mode state SOP2 of the waveguide constituting the optical mode conversion unit 202 must be sufficiently close. Such waveguide cross-sectional structures are limited, and the conversion of polarization and mode states can only be achieved when the height and width of the waveguide are within a range of specific dimensions.
[0018] On the other hand, when performing a desired polarization and mode state conversion, it is necessary to precisely set the length of the waveguide cross-sectional structure in which the polarization and mode state conversion occurs. This is because, in a waveguide with a waveguide cross-sectional structure within a specific dimensional range, if the effective refractive indices of polarization and mode state SOP3 and polarization and mode state SOP2 are sufficiently close, mutual conversion between the two polarizations and mode states occurs as light propagates through the waveguide.
[0019] For example, consider the case where light with polarization and mode state SOP3 is input, and light with polarization and mode state SOP2 is output. When light with polarization and mode state SOP3 is input into a waveguide with a waveguide cross-sectional structure of a specific size that allows sufficient polarization and mode state conversion to occur, the polarization and mode state are gradually converted from SOP3 to SOP2. After the polarization and mode state SOP3 has been converted to SOP2, SOP2 will then be converted back to SOP3, and in order to obtain the output of SOP2, it is necessary to take the output before SOP2 is converted back to SOP3.
[0020] The length required for polarization and mode state conversion varies depending on the dimensions of the waveguide cross-sectional structure. Therefore, if the waveguide cross-sectional structure changes due to manufacturing errors, the characteristics will deteriorate. In order to eliminate this deterioration of characteristics due to manufacturing errors, the optical waveguide forming the optical mode conversion section is a tapered waveguide 213 in which the waveguide width changes along the direction of light propagation.
[0021] If the optical mode conversion unit 202 is a tapered waveguide 213 whose waveguide width changes along the direction of light propagation, as shown in Figure 3, in accordance with the dimensional change of the optical waveguide cross-sectional structure along the direction of light propagation, the effective refractive indices of the polarization and mode state SOP2 and polarization and mode state SOP3 of the optical waveguide gradually approach each other, become close enough for polarization and mode state conversion to occur, then gradually move apart, and finally the effective refractions move apart to the point where polarization and mode state conversion no longer occurs.
[0022] Thus, by controlling the effective refractive index of each polarization and mode state by setting the shape of the waveguide forming the optical mode conversion unit 202 to a tapered shape in which the waveguide width gradually changes along the direction of light propagation, it is possible to prevent the polarization and mode state from being converted again once they have been converted, thereby improving tolerance to manufacturing errors (Non-Patent Literature 1).
[0023] However, when the optical mode conversion unit 202 is formed using a tapered waveguide 212, regions are required before and after the region where the effective refractive indices of each mode are close enough for polarization and mode state conversion to occur, and before and after the region where the effective refractions are far enough apart that polarization and mode state conversion no longer occurs. In order to include these regions before and after, the length of the waveguide forming the optical mode conversion unit becomes large. Thus, the conventional method still has problems from the standpoint of miniaturization.
[0024] Z. Chen, J. Yang, W. Wong, E. Pun, and C. Wang, "Broadband adiabatic polarization rotator-splitter based on a lithium niobate on insulator platform," Photon. Res. 9, 2319-2324 (2021).
[0025] This disclosure has been made in view of the above issues, and its purpose is to provide an optical signal processing device that constitutes a polarization control element suitable for miniaturizing optical transceivers, in a form with excellent tolerance to manufacturing errors.
[0026] One embodiment for achieving this objective is an optical signal processing device including an optical converging unit and an optical mode conversion unit, wherein the optical converging unit has an optical converger that converts the optical mode of one of two input signals to a mode orthogonal to the optical mode of the other input signal and converges it with the other light, and the optical mode conversion unit consists of a tapered waveguide whose waveguide width changes along the direction of light propagation, which converts only one of two mutually orthogonal modes of light input from the optical converging unit to a mode different from the optical mode of the other input signal and the mode orthogonal to it, and the tapered waveguide includes at least two tapered sections with different rates of change in waveguide width along the direction of light propagation.
[0027] According to this embodiment, an optical signal processing device constituting a polarization control element suitable for miniaturizing optical transceivers can be provided in a form with excellent tolerance to manufacturing errors.
[0028] Figure 1 is a schematic diagram of the optical front end of a typical transceiver used in coherent optical communication for medium- and long-distance communication. Figure 2 is a schematic diagram illustrating the overall optical waveguide structure of an optical signal processing device that constitutes a conventional polarization control element. Figure 3 is a diagram showing the relationship between the waveguide width of a tapered waveguide and the effective refractive index of each mode. Figure 4 is a schematic diagram of the optical waveguide structure of an optical signal processing device according to Embodiment 1 of this disclosure. Figure 5 is a schematic diagram of the optical waveguide structure of an optical signal processing device according to Embodiment 2 of this disclosure.
[0029] Embodiments of this disclosure will be described in detail below with reference to the drawings. Note that the following description is illustrative, and embodiments with some configurations modified are possible without departing from the gist of this disclosure. Identical or similar reference numerals indicate identical or similar elements, and repeated descriptions may be omitted. Numerical values in the following description are illustrative, and other numerical values may be used in carrying out this disclosure without departing from the gist of this disclosure.
[0030] Furthermore, due to the reciprocity of light, the input and output are commutative, and it is clear that a configuration with the input and output reversed is also acceptable.
[0031] (Embodiment 1) The optical mode conversion unit of the conventional optical signal processing device described in Figure 2 controls the effective refractive index of each polarization and mode state by using a tapered waveguide in which the waveguide width changes along the direction of light propagation, thereby improving tolerance to manufacturing errors.
[0032] In this tapered waveguide, the actual polarization and mode state conversion occurs in the central region where the effective refractive indices are close enough to cause the conversion. Before and after this central region, there needs to be a range of regions where the effective refractive indices are close to a range where the effective refractions are far enough apart that the polarization and mode state conversion no longer occurs. In these regions where the effective refractive indices gradually approach and gradually diverge, the taper gradient (the inclination of the straight line of the tapered waveguide's outer shape relative to the direction of light propagation) should be such that adiabatic optical coupling occurs.
[0033] In other words, the taper shape of the region where the effective refractive index is close enough for polarization and mode state conversion to occur, and the regions before and after that region, do not need to have the same gradient. The gradient of the outer shape of the taper of the waveguide constituting the optical mode conversion section will not degrade performance even if the region where polarization and mode state conversion does not occur is made steeper.
[0034] Therefore, by increasing the rate of change of the waveguide width along the direction of light propagation in at least one of the regions where polarization and mode state conversion does not occur, it becomes possible to compress the length of the tapered waveguide that forms the optical mode conversion section.
[0035] Based on this understanding, the optical signal processing device of this embodiment 1 includes at least two tapered sections in the tapered waveguide forming the optical mode conversion section, each with a different rate of change in the waveguide width along the direction of light propagation.
[0036] Hereinafter, this embodiment will be described with reference to Figure 4. Figure 4 is a schematic diagram illustrating the overall optical waveguide structure of the optical signal processing device 400 according to this embodiment. In Figure 4, only the shape and arrangement of the optical waveguide core are shown, and the insulating substrate on which the optical waveguide is formed is omitted.
[0037] The optical signal processing device 400 of this embodiment consists of an optical confluence unit 201 made of a lithium niobate waveguide, an optical mode conversion unit 401, and an output waveguide 214. In the following description of this embodiment 1, an example using a lithium niobate waveguide will be given, but it is clear that any of the following materials suitable for realizing the optical confluence unit 201 and the optical mode conversion unit 401 may be used, such as a quartz waveguide, an InP waveguide, a lithium niobate waveguide, a PLZT waveguide, or a polymer waveguide.
[0038] The optical converging unit 201 has the same structure as the optical converging unit 201 of the optical signal processing device 200 shown in Figure 2, and its operation is also the same.
[0039] The optical mode conversion unit 401 is composed of a tapered waveguide 402 consisting of a first tapered section 411, a second tapered section 412, and a third tapered section 413. Note that the tapered waveguide shape shown by the dotted line in Figure 4 is shown for reference to make the difference from the conventional tapered waveguide shown in Figure 2 easier to understand, and does not represent the waveguide actually provided by the optical signal processing device 400.
[0040] In the second tapered section 412 of the optical mode conversion unit 401, the input light with polarization and mode state SOP3 gradually changes polarization and mode state as it propagates through the optical mode conversion unit 401, eventually becoming polarization and mode state SOP2. The light with polarization and mode state SOP1 input to the optical mode conversion unit 401 is input to the output waveguide 214 without changing polarization and mode state, and is output simultaneously with the light with polarization and mode state SOP2.
[0041] Note that the polarization and mode states SOP1 and SOP3 have mutually orthogonal modes. Also, mode state SOP2 is a different mode state from SOP1 and SOP3.
[0042] Light input from the optical confluence unit 201 is input to the first tapered section 411 of the tapered waveguide 402 of the optical mode conversion unit 401. The outer shape of the first tapered section 411 has a steep gradient such that adiabatic optical connection is maintained, and the waveguide width gradually narrows from the connection point with the optical confluence unit 201 along the direction of light propagation, until it reaches the second tapered section 412, which is the region where polarization and mode state conversion occurs. This first tapered section 411 is the region necessary to change the waveguide width from the width when the effective refractive indices of SOP1, SOP2, and SOP3 are far apart (the wider side of the tapered section in Figure 3) to the width on the side of the first tapered section 411 of the second tapered section 412 (when the effective refractive indices of SOP2 and SOP3 are close together).
[0043] The external shape of the second tapered section 412 has a gentler slope than the first tapered section, and gradually narrows along the direction of light propagation from the first tapered section 411 to the third tapered section 413, thus reducing the waveguide width. In the second tapered section 412, the effective refractive indices are close enough that polarization and mode state conversion occurs, so the light is converted to the desired polarization and mode state.
[0044] The third tapered portion 413 has a steep gradient such that adiabatic optical connection is maintained, similar to the first tapered portion 411, and gradually narrows along the light propagation direction from the second tapered portion 412 to reduce the waveguide width to the output waveguide 214 so that the effective refractive indices of the polarization and mode states are sufficiently separated. Since the first tapered portion 411 and the third tapered portion 413 can have a steep gradient to maintain adiabatic optical connection, the respective taper shapes can be set to have a larger rate of change of the waveguide width along the light propagation direction than the second tapered portion 412. This third tapered portion 413 is a region for changing the width from the narrower width of the second tapered portion 412 (the width at which the effective refractive indices of SOP2 and SOP3 are close) to the width of the output waveguide 214 (the width at which SOP3 cannot exist, the narrow side of the tapered portion in FIG. 3).
[0045] Thus, in this embodiment, the length in the light propagation direction can be compressed compared to the tapered waveguide forming the conventional optical mode conversion section. In the example shown in the figure, the gradients of the first tapered portion 411 and the third tapered portion 413 are set to be the same, but the respective gradients may be different.
[0046] Also, a tapered portion with a large rate of change of the waveguide width along the light propagation direction may be used as either one of the first tapered portion 411 and the third tapered portion 413 to make the length of the waveguide 402 in the light propagation direction of the optical mode converter 401 shorter than the conventional one.
[0047] When the optical mode conversion section is formed of a tapered waveguide, the waveguide width of the input section of the optical mode conversion section is determined by the waveguide width of the previous optical multiplexing section 403, and the waveguide width of the output section is determined by the waveguide width of the output waveguide 214.
[0048] Therefore, for example, as shown in FIG. 3, in the conventional tapered waveguide shape, the region where the effective refractive indices of the polarization and mode states are close enough for the conversion of the polarization and mode states to occur is near the center of the taper structure. Thus, compression is possible by changing the rate of change of the waveguide width along the light propagation direction in the region portions before and after that.
[0049] (Embodiment 2) As described above, optical mode conversion occurs in a region where the effective refractive indices of the polarization and mode states are very close together, and conversion to the desired polarization and mode state is possible when this region is of sufficient length. In the optical mode conversion section 402 of the optical signal processing apparatus of Embodiment 1 shown in Figure 4, the second tapered section 412 is the relevant section, and the conversion of polarization and mode state occurs almost entirely in the second tapered section 412.
[0050] Here, we consider the case where the dimensions of the waveguide cross-sectional structure of the second tapered portion 412 change due to manufacturing errors such as etching depth deviation and film thickness deviation.
[0051] If the dimensions of the waveguide cross-sectional structure of the second tapered section 412 change due to manufacturing errors, such as variations in etching depth and film thickness, the dispersion curve changes as a result of the change in cross-sectional structure. This causes the waveguide width in the region where the effective refractive index is very close to be different from the waveguide width in the region where the effective refractive index is very close to be when the cross-sectional cross-section is at the design dimensions.
[0052] For example, in a ribbed lithium niobate waveguide, when the slab height is 300 nm and the rib height is 300 nm, the effective refractive index approaches very closely when the waveguide width is about 1600 nm, causing a transformation of polarization and mode state. However, when the slab height is 250 nm and the rib height is 350 nm, the effective refractive index approaches very closely when the waveguide width is about 1400 nm, causing a transformation of polarization and mode state.
[0053] In this case, if the dimensions of the second tapered section 412 are determined without considering manufacturing errors, the manufacturing errors in the cross-sectional structure may result in insufficient waveguide width, thickness, or taper length of the tapered section, making it impossible to obtain sufficient polarization and mode state conversion.
[0054] To avoid this, the dimensions of the second tapered section 412 can be set with sufficient margin. Specifically, by making the widest part of the waveguide width of the second tapered section 412 wider, the narrowest part narrower, and making the length of the taper longer than necessary and sufficient, it is possible to accommodate cases where the waveguide width shifts in the region where the effective refractive index is close, and further improve tolerance to manufacturing errors. However, the above method cannot avoid increasing the size of the second tapered section 412.
[0055] Therefore, in this embodiment, the outer shape of the second tapered portion 512 is made curved. Figure 5 is a schematic diagram illustrating the overall optical waveguide structure of the optical signal processing device 500 according to this embodiment. In Figure 5, only the shape and arrangement of the optical waveguide core are shown, and the insulating substrate on which the optical waveguide is formed is omitted.
[0056] The optical signal processing device 500 of this embodiment consists of an optical confluence unit 201 made of a lithium niobate waveguide, an optical mode conversion unit 501, and an output waveguide 214. In this second embodiment, an example using a lithium niobate waveguide will be described, but it is clear that any of the following materials may be used to realize the optical confluence unit 201 and the optical mode conversion unit 501, such as a quartz waveguide, an InP waveguide, a lithium niobate waveguide, a PLZT waveguide, or a polymer waveguide.
[0057] The optical converging unit 201 has the same structure as the optical converging unit 201 of the optical signal processing device 200 shown in Figure 2, and its operation is also the same.
[0058] The optical mode conversion unit 501 is formed by a tapered waveguide 502 consisting of a first tapered section 511, a second tapered section 512, and a third tapered section 513, the waveguide width of which changes along the direction of light propagation.
[0059] In this embodiment, the optical mode conversion unit 501 differs from Embodiment 1 in that the second tapered portion 512 is curved.
[0060] The length required when performing polarization and mode state conversion using a tapered waveguide is determined by how wide the region is relative to the waveguide width in which the effective refractive indices of the polarization and mode state SOP3 before conversion and the polarization and mode state SOP2 after conversion are close, and by the degree of mode overlap between the polarization and mode state SOP3 before conversion and the polarization and mode state SOP2 after conversion.
[0061] In the case of silicon waveguides, quartz waveguides, InP waveguides, lithium niobate waveguides, PLZT waveguides, and polymer waveguides, the length required for polarization and mode state conversion decreases when the waveguide width becomes narrow in the region where the effective refractive index approaches sufficiently for polarization and mode state conversion. Conversely, the length required for polarization and mode state conversion increases when the waveguide width becomes wide in the region where the effective refractive index approaches sufficiently for polarization and mode state conversion.
[0062] Therefore, in regions where the waveguide width narrows due to the proximity of effective refractive indices, the gradient of the taper, which is the rate of change of the waveguide width along the direction of light propagation, can be relatively steep. However, in regions where the waveguide width widens due to the proximity of effective refractive indices, the gradient of the taper, which is the rate of change of the waveguide width along the direction of light propagation, must be relatively gentle.
[0063] In the second tapered section 512 of this embodiment, the outer shape is a curved shape in which the slope of the curve is gentle in the thicker part and relatively steep in the thinner part. By adopting a waveguide with such an outer shape, the overall outer shape of the second tapered section 512 can be made to have a relatively long thicker section and a relatively short thinner section.
[0064] As a result, the tapered waveguide forming the optical confluence section of this embodiment can have a shorter length in the optical propagation direction than the second tapered section 412 of Embodiment 1, which has a straight shape in which the outer shape of the taper has a constant rate of change in the waveguide width along the direction of optical propagation.
[0065] In this case, the slope of the curve, which is the outer shape of the taper, is such that the waveguide width needs to be gradually changed in order to maintain adiabatic optical connection. When viewed over the entire second tapered section 512, the slope of the outer shape changes so that it becomes gradually steeper from the wider part of the taper to the narrower part.
[0066] Specific examples of these external curves include quadratic and exponential curves. The choice between them depends on the length of the taper required for polarization and mode state transformation when the cross-sectional structure changes. When the difference between the length of the taper required for polarization and mode state transformation in a cross-sectional structure where the waveguide width narrows in the region where the effective refractive indices are very close and the length of the taper required for polarization and mode state transformation in a cross-sectional structure where the waveguide width widens in the region where the effective refractive indices are very close is large, it is preferable to use an exponential curve. When the difference is small, it is preferable to use a quadratic curve.
[0067] According to this disclosure, an optical signal processing device constituting a polarization control element suitable for miniaturizing optical transceivers can be provided in a form with excellent tolerance to manufacturing errors.
[0068] 100・・・・・・・・・・・・・・・・Optical front end 200, 400, 500・・・・・・・・Optical signal processing unit 201・・・・・・・・・・・・・・・・Optical confluence unit 202, 401, 501・・・・・・・・Optical mode conversion unit 211a, 211b・・・・・・・・・・Input waveguide 212・・・・・・・・・・・・・・・・Optical confluencer 213, 402, 502・・・・・・・・Tapered waveguide 214・・・・・・・・・・・・・・・・Output waveguide 411, 511・・・・・・・・・・・・First tapered section 412, 512・・・・・・・・・・・・Second tapered section 413, 513・・・・・・・・・・・・Third tapered section
Claims
1. An optical signal processing device comprising an optical converging unit and an optical mode conversion unit, wherein the optical converging unit includes an optical converger that converts the optical mode of one of two input signals to a mode orthogonal to the optical mode of the other input signal and converges it with the other light, and the optical mode conversion unit consists of a tapered waveguide whose waveguide width changes along the direction of light propagation, which converts only one of two mutually orthogonal modes of light input from the optical converging unit to a mode different from the optical mode of the other input signal and the orthogonal mode, and the tapered waveguide includes at least two tapered sections with different rates of change in waveguide width along the direction of light propagation.
2. The optical signal processing apparatus according to claim 1, wherein the tapered waveguide has a first tapered portion, a second tapered portion, and a third tapered portion along the direction of light propagation, and the rate of change of the waveguide width along the direction of light propagation of the first tapered portion and the third tapered portion is greater than the rate of change of the waveguide width along the direction of light propagation of the second tapered portion.
3. The optical signal processing apparatus according to claim 2, characterized in that the outer shapes of the first tapered portion, the second tapered portion, and the third tapered portion are straight lines.
4. The optical signal processing apparatus according to claim 2, characterized in that the outer shapes of the first tapered portion and the third tapered portion are straight lines, and the outer shape of the second tapered portion is curved.
5. The optical signal processing apparatus according to claim 4, characterized in that the slope of the curve of the outer shape of the second tapered portion changes so that it gradually becomes steeper from the thicker portion to the thinner portion.
6. The optical signal processing apparatus according to claim 5, characterized in that the curve of the outer shape of the second tapered portion is an exponential curve.
7. The optical signal processing apparatus according to claim 5, characterized in that the curve of the outer shape of the second tapered portion is a quadratic function curve.
8. The optical signal processing apparatus according to any one of claims 1 to 7, characterized in that the tapered waveguide of the optical mode conversion unit is one of a silicon waveguide, a quartz waveguide, an InP waveguide, a lithium niobate waveguide, a PLZT waveguide, or a polymer waveguide.