Inter-mode loss difference compensation device and design method for inter-mode loss difference compensation device

Abstract

To provide an inter-mode loss difference compensation device capable of suppressing wavelength dependency of an inter-mode loss compensation quantity for mode multiplex transmission, and a design method for the inter-mode loss difference compensation device.SOLUTION: An inter-mode loss difference compensation device has a first waveguide and a second waveguide, and a coupling part which has a parallel waveguide structure where the first waveguide and second waveguide are closed to each other, and in which a LP01 mode of propagation in the first waveguide and a LP01 mode of propagation in the second waveguide are coupled together. The first waveguide and second waveguide have a same structure at the coupling part, and the coupling part has a parallel waveguide length for preventing a LP11a mode and a LP11b mode of propagation in the first waveguide shifting to the second waveguide. The first waveguide and second waveguide vary in a cross-sectional area along a propagation direction of a light signal so that a wave front of an electromagnetic field distribution of light obtained by propagating incident light from one end of the first waveguide forward meets a wave front of an electromagnetic field distribution of light obtained by propagating emission light from the other end of the second waveguide backward.SELECTED DRAWING: Figure 1A

Description

[Technical field]

[0001] The present disclosure relates to a modal loss difference compensation device and a method for designing a modal loss difference compensation device. [Background technology]

[0002] In mode-division multiplexing transmission, which increases communication capacity by utilizing multiple modes propagating through optical fiber, a mode-loss difference compensation device that improves transmission quality by suppressing the loss difference between modes that occurs in the transmission fiber or in a mode multiplexer / demultiplexer is disclosed in Non-Patent Documents 1 and 2 and Patent Document 1.

[0003] Non-Patent Document 1 discloses a modal loss difference compensation device using a spatial optical system that sets the amount of modal loss compensation by an interpolating spatial filter.

[0004] In Non-Patent Document 2, a PLC (Planar Lightwave Circuit) technology is used to realize an LP using a parallel waveguide with two coupling parts. 11 While suppressing the loss to the fundamental mode, LP 01 A modal loss differential compensation device is disclosed that provides loss to the modes.

[0005] In Patent Document 1, the LP is achieved by changing the waveguide length or the refractive index between two waveguides. 11 Almost no loss to the mode, 01 Mode and LP 11 A modal loss difference compensation device is disclosed that provides a modal loss difference between modes. [Prior art documents] [Non-patent literature]

[0006] [Non-Patent Document 1] T. Mizuno et al., "Mode dependent loss equalizer and impact of MDL on PDM-16QAM few-mode fiber transmission", ECOC2015, P5.9 (2015) [Non-Patent Document 2] T. Fujisawa et al., "Silica-PLC based mode-dependent-loss equalizer for two LP mode transmission", OFC2022, paper M4J.4 (2022) [Patent documents]

[0007] [Patent Document 1] JP 2020-148968 A Summary of the Invention [Problem to be solved by the invention]

[0008] However, when mode multiplexing transmission and wavelength multiplexing transmission are combined, if the modal loss difference compensation devices disclosed in Non-Patent Documents 1 and 2 and Patent Document 1 are used, there is a problem that the bandwidth for compensating characteristics is limited.

[0009] The present disclosure has been made in view of the above-mentioned problems, and has an object to provide a modal loss difference compensation device and a method for designing a modal loss difference compensation device that can suppress the wavelength dependency of the modal loss compensation amount for mode multiplexing transmission. [Means for solving the problem]

[0010] In order to solve the above-mentioned problems, a mode loss difference compensation device according to one embodiment of the present disclosure has an LP 01 Mode, LP 11a Mode and LP 11bThe LP has a first waveguide and a second waveguide capable of propagating a mode, and a parallel waveguide structure in which the first waveguide and the second waveguide are close to each other, and the LP propagates through the first waveguide. 01 Mode and LP propagating in the second waveguide 01 The first and second waveguides in the coupling section have the same structure, and the coupling section couples the LP mode propagating through the first waveguide. 11a Mode and LP 11b The first waveguide has a parallel waveguide length that prevents the mode from shifting to the second waveguide. The cross-sectional areas of the first waveguide and the second waveguide vary along the propagation direction of the optical signal so that the wavefront of the electromagnetic field distribution of light obtained when the light incident from one end of the first waveguide is propagated in the forward direction coincides with the wavefront of the electromagnetic field distribution of light obtained when the light output from the other end of the second waveguide is propagated in a reverse direction opposite to the forward direction, assuming that the direction in which the optical signal input to one end of the first waveguide shifts to the second waveguide and exits from the other end of the second waveguide is the forward direction.

[0011] In addition, a method for designing a modal loss difference compensation device according to an embodiment of the present disclosure includes: 01 Mode, LP 11a Mode and LP 11b The LP has a first waveguide and a second waveguide capable of propagating a mode, and a parallel waveguide structure in which the first waveguide and the second waveguide are close to each other, and the LP propagates through the first waveguide. 01 Mode and LP propagating in the second waveguide 01 The present invention relates to a method for designing a modal loss difference compensation device having a coupling section where a LP propagating through the first waveguide is coupled to a LP propagating through the first waveguide. 11a Mode and LP 11bA parallel waveguide length is provided to prevent the mode from migrating to the second waveguide. The cross-sectional areas of the first waveguide and the second waveguide are varied along the propagation direction of the optical signal so that the wavefront of the electromagnetic field distribution of the light obtained when the light incident from one end of the first waveguide is propagated in the forward direction coincides with the wavefront of the electromagnetic field distribution of the light obtained when the light output from the other end of the second waveguide is propagated in a reverse direction opposite to the forward direction, where the direction in which the optical signal incident on one end of the first waveguide migrating to the second waveguide and output from the other end of the second waveguide is defined as the forward direction. Effect of the Invention

[0012] According to the present disclosure, it is possible to provide a modal loss difference compensation device capable of suppressing the wavelength dependency of the modal loss compensation amount for mode multiplexing transmission, and a method for designing the modal loss difference compensation device. [Brief description of the drawings]

[0013] [Figure 1A] FIG. 1A is a schematic diagram (reference structure) of a modal loss difference compensation device employing a single coupler structure. [Figure 1B] FIG. 1B is a schematic diagram of a modal loss difference compensation device (after application of the wavefront matching method). [Figure 1C] FIG. 1C is an enlarged view of a main portion of FIG. 1B. [Diagram 2] FIG. 2 is a diagram showing an example of a transmission spectrum in a modal loss difference compensation device of a reference structure. [Diagram 3] FIG. 3 is a diagram showing an example of a transmission spectrum in a modal loss difference compensation device obtained by applying the wavefront matching method. [Figure 4] FIG. 4 is a diagram showing the relationship between the length of the parallel waveguide and the performance index. [Diagram 5] FIG. 5 is a schematic diagram (reference structure) of a mode loss difference compensation device employing a Mach-Zehnder structure. [Figure 6] FIG. 6 is a diagram showing the relationship between wavelength and mode dependent loss. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] Next, an embodiment of the present disclosure will be described in detail with reference to the drawings. In the description, the same components are designated by the same reference numerals and duplicated description will be omitted.

[0015] [1. Structure of the mode loss difference compensation device using a single coupler structure] Fig. 1A is a schematic diagram (reference structure) of a modal loss difference compensation device employing a single coupler structure. The modal loss difference compensation device 1 shown in Fig. 1A employs a single coupler structure and includes a first waveguide WG1, a second waveguide WG2, and a coupling part CP.

[0016] FIG. 1B is a schematic diagram of a mode loss difference compensation device (after application of the wavefront matching method). FIG. 1C is an enlarged view of the main part of FIG. 1B. FIG. 1B and FIG. 1C are diagrams in which the wavefront matching method is applied to the mode multiplexer / demultiplexer of FIG. 1A, and the width of the waveguide is varied. In FIG. 1B and FIG. 1C, the horizontal axis indicates the position coordinate (μm) in the z-axis direction, and the vertical axis indicates the position coordinate (μm) in the x-axis direction. The mode multiplexer / demultiplexer in FIG. 1A, in which no variation is added to the waveguide width by the wavefront matching method, is called the "reference structure."

[0017] The first waveguide WG1 and the second waveguide WG2 are LP 01 Mode, LP 11a Mode and LP 11b It is configured to be able to propagate the LP mode. 11a Mode and LP 11b Modes all together LP 11 is called a mode.

[0018] The first waveguide WG1 has a first port P1 and a third port P3, and light enters through the first port P1 and exits through the third port P3. The second waveguide WG2 has a second port P2 and a fourth port P4, and light enters through the second port P2 and exits through the fourth port P4. The first port P1, the second port P2, the third port P3, and the fourth port P4 are each connected to an optical transmission line (not shown). The second port P2 and the fourth port P4 may be terminated to prevent reflection of light.

[0019] The coupling portion CP has a parallel waveguide structure in which the first waveguide WG1 and the second waveguide WG2 are close to each other. 01 Mode and LP propagating through the second waveguide WG2 01 In addition, the LP mode propagating through the first waveguide WG1 is coupled to the 11 Mode and LP propagating through the second waveguide WG2 11 The mode is coupled to the first waveguide WG1 and the second waveguide WG2 at the coupling portion CP. The distance between the first waveguide WG1 and the second waveguide WG2 at the coupling portion CP is shown as the parallel waveguide distance g. Note that the structure of the first waveguide WG1 and the structure of the second waveguide WG2 are identical at the coupling portion CP. The width of the first waveguide WG1 and the width of the second waveguide WG2 are shown as the waveguide width w.

[0020] The coupling point CP is a coupling point between the LP 11a Mode and LP 11b The parallel waveguide length L that prevents the mode from migrating to the second waveguide WG2 c At the joint CP, LP 01 The modes are split in any ratio, and the LP 11a Mode and LP 11b The mode is arranged so that there is no transfer of optical power from one waveguide to the other, and the parallel waveguide length L c shall be adjusted.

[0021] LP propagating through the optical transmission line 01 Mode and LP 11 The light in the LP mode is input from the first port P1 and is attenuated at the coupling point CP.01 Mode and LP 11 The light in the selected mode is again coupled to the optical transmission line and propagated.

[0022] The modal loss difference compensation device 1 may be installed together with a repeater or a receiver in an optical transmission line, or the modal loss difference compensation device 1 alone may be inserted into the optical transmission line as necessary. The modal loss difference compensation device 1 is fabricated using PLC (Planar Lightwave Circuit) technology, and is connected to an optical transmission line (optical fiber) by bonding optical fibers to both ends of the modal loss difference compensation device 1.

[0023] As parameters of the reference structure in this embodiment, the relative refractive index difference Δ between the first waveguide WG1 and the second waveguide WG2 was set to 0.55%. The waveguide height h (height in the y-axis direction in FIG. 1A) of the first waveguide WG1 and the second waveguide WG2 was set to 9.0 μm. The waveguide width w was set to 10.0 μm. The parallel waveguide spacing g was set to 3.0 μm. In addition, the length S related to the bending of the second waveguide WG2 was set to 10.0 μm. s The parallel waveguide length L c is adjusted as described above.

[0024] The branching ratio between the first waveguide WG1 and the second waveguide WG2 may be adjusted by designing parameters.

[0025] By applying a wavefront matching method to the structure of the modal loss difference compensation device 1 shown in FIG. 1A and varying the width of the waveguide as shown in FIG. 1B and FIG. 1C, broadband operation of the modal loss difference compensation device 1 is achieved.

[0026] First, the direction in which an optical signal incident on one end (first port P1) of the first waveguide WG1 moves to the second waveguide WG2 and is output from the other end (fourth port P4) of the second waveguide WG2 is defined as the forward direction. Then, the wavefront of the electromagnetic field distribution of light obtained when the light incident on the first port P1 is propagated in the forward direction by the beam propagation method (hereinafter, forward wavefront) is calculated. In addition, the wavefront of the electromagnetic field distribution of light obtained when the light output from the fourth port P4 is propagated in the reverse direction opposite to the forward direction by the beam propagation method (hereinafter, reverse wavefront) is calculated. The calculation of the wavefront of the electromagnetic field distribution may use a method other than the beam propagation method, and is not limited to the example given here.

[0027] Then, the cross-sectional areas of the first waveguide WG1 and the second waveguide WG2 are changed so that the forward wavefront and the reverse wavefront coincide with each other, thereby optimizing the structure of the modal loss difference compensation device 1. More specifically, the coupling coefficient between the first waveguide WG1 and the second waveguide WG2 at the coupling portion CP is calculated based on the forward wavefront and the reverse wavefront. Then, the process of calculating the coupling coefficient by changing the waveguide structures of the first waveguide WG1 and the second waveguide WG2 is repeated until the shape of the entire modal loss difference compensation device 1 converges to one shape.

[0028] As an example of the wavefront matching method, the “Solid pattern approach” disclosed in the document “Y. Sakamaki et al., “New optical waveguide design based on wavefront matching method”, IEEE JLT, vol. 25, pp. 3511-3518 (2007)” can be used.

[0029] In this embodiment, various parallel waveguide lengths L c For this, the structure of the modal loss difference compensation device 1 is optimized using a wavefront matching method.

[0030] The non-periodic variation of the waveguide width w of the first waveguide WG1 and the second waveguide WG2 is set to ±0.2 μm or less per 1 μm of length along the propagation direction of the optical signal (z-axis direction in FIG. 1A). This prevents abrupt modulation of the waveguide width, and smooths the wavelength dependency of the loss difference compensation by the modal loss difference compensation device 1. As a result, it is possible to provide a modal loss difference compensation device 1 with excellent reproducibility. The non-periodic variation of the waveguide width w is not limited to the examples given here, and may be steeper.

[0031] [2. Transmission spectrum of the mode loss compensation device] Fig. 2 is a diagram showing an example of a transmission spectrum in a modal loss difference compensation device of a reference structure, and Fig. 3 is a diagram showing an example of a transmission spectrum in a modal loss difference compensation device obtained by applying a wavefront matching method.

[0032] FIG. 2 shows the LP incident on the first port P1 in the reference structure modal loss difference compensation device (modal loss difference compensation device without optimization by the wavefront matching method). 01 Mode and LP 11 13 shows the results of calculating the transmission spectrum when the light of each mode is output to the third port P3.

[0033] FIG. 3 shows the LP incident on the first port P1 in the mode loss difference compensation device 1 obtained by applying the wavefront matching method. 01 Mode and LP 11 13 shows the results of calculating the transmission spectrum when the light of each mode is output to the third port P3.

[0034] 2 and 3, the wavelength 1530 nm at the lower end of the C-band (Conventional-band) in the optical communication wavelength band is shown as a straight line BC, and the wavelength 1625 nm at the upper end of the L-band (Long-wavelength-band) in the optical communication wavelength band is shown as a straight line BL.

[0035] In the example shown in Figure 2, the LP is 11a By optimizing the wavefront matching method, the LP is reduced across the entire C-band and L-band, as shown in Figure 3. 11a It can be seen that the modal loss approaches flatness, which confirms that the wavelength dependence of the modal loss compensation amount is suppressed by performing optimization using the wavefront matching method.

[0036] [3. Performance indices for modal loss compensation devices] 4 is a diagram showing the relationship between the length of the parallel waveguide and the performance index. The performance index F of the mode loss difference compensation device 1 can be defined by the following equation:

[0037]

number

[0038] Here, for light of wavelength λ, LP from one end (first port P1) of the first waveguide WG1 to the other end (third port P3) of the first waveguide WG1 is 01 Let the transmittance of the mode be T LP01 (λ), LP 11a Let the transmittance of the mode be T LP11a (λ), LP 11b Let the transmittance of the mode be T LP11b (λ). The performance index F is a function of a number of predetermined wavelengths λ k (where k = 1, 2, . . . , N, N is a natural number), it is calculated based on the transmittance of each mode.

[0039] The performance index F is 0 when it represents ideal characteristics. In other words, the smaller the performance index F is, the closer to 0 it is to the ideal characteristics of the mode loss difference compensation device 1. In the formula defining the performance index F, the value "0.5" appearing in the first term within the Σ symbol is a value set assuming a 3 dB coupler. A specified value may be set instead of the value "0.5".

[0040] According to FIG. 4, when the performance index F is the lowest (when the mode loss difference compensation device 1 has characteristics closest to the ideal), the parallel waveguide length L c In order to fabricate the inter-mode loss difference compensation device 1 with a small gain deviation in the band, the parallel waveguide length L c Within ±10% of the parallel waveguide length L c It may be possible for the above to be accommodated.

[0041] [4. Structure of the mode loss difference compensation device using Mach-Zehnder structure] Fig. 5 is a schematic diagram (reference structure) of a modal loss difference compensation device employing a Mach-Zehnder structure. The modal loss difference compensation device 1 shown in Fig. 5 employs a Mach-Zehnder structure, thereby realizing a higher mode extinction ratio, that is, the provision of modal loss difference compensation. The modal loss difference compensation device 1 includes a first waveguide WG1, a second waveguide WG2, and two coupling parts CP.

[0042] 5 includes a delay unit DL between one coupling unit CP and another coupling unit CP. The delay unit DL sets a waveguide length difference between the first waveguide WG1 and the second waveguide WG2.

[0043] The delay section DL may include, for example, a refractive index control section. The refractive index control section changes the refractive index of at least one of the first waveguide WG1 and the second waveguide WG2, thereby 01 The phase difference between the first waveguide WG1 and the second waveguide WG2 in the mode is variable within a range of 0 or more and less than π.

[0044] Various methods can be used to change the refractive index by the refractive index control section. For example, the refractive index control section applies heat to at least one of the first waveguide WG1 and the second waveguide WG2 to change the LP 01 The refractive index may be changed by controlling the attenuation of the mode.

[0045] Here, the phase difference between the waveguides, which corresponds to the delay amount of the second waveguide WG2 with respect to the first waveguide WG1 caused by the delay unit DL, is defined as the phase difference Δθ. 01 The mode is split 1:1, and LP 11a Mode and LP 11b The mode is connected to the parallel waveguide length L c is assumed to have been adjusted.

[0046] By applying a wavefront matching method to the modal loss difference compensation device 1 shown in FIG. 5, it is possible to realize a modal loss difference compensation device 1 that can handle a larger modal loss difference than a single coupler structure.

[0047] In FIG. 5, the length of the delay section DL is L m In addition, the length of the second waveguide WG2 at the bent portion connected to the delay portion DL is L d is shown as:

[0048] Length L m The change in LP 01 If no mode is emitted from the third port P3, the LP 01 In this case, the phase difference between the waveguides of the mode is 0+(2π×j) (j is an integer). 01 When all the input power of the mode is output from the third port P3, the phase difference is π+(2π×k) (k is an integer). Therefore, to realize a transmission characteristic between these two cases, the phase difference in the delay section should be 0+(2π×h) to π+(2π×h) (h is an integer).

[0049] Here, the length L m The relationship between the phase difference Δθ and the m"Incidentally, "Δβ" represents the propagation constant difference when the refractive index difference between the first waveguide WG1 and the second waveguide WG2 in the delay section DL becomes "Δn."

[0050] When the first waveguide WG1 and the second waveguide WG2 have the same structure, “Δn=0”, so the length L m On the other hand, when the structures of the first waveguide WG1 and the second waveguide WG2 are not the same, or when the refractive index variations are different between the first waveguide WG1 and the second waveguide WG2, the transmission characteristics do not change even if the length L m The change in the phase difference Δθ occurs.

[0051] Length L at the bend d Due to the presence of the bent portion, even if the first waveguide WG1 and the second waveguide WG2 have the same structure, a phase difference Δθ may occur in the delay portion DL. In addition, when the portion to which heat is applied is provided so as to include the bent portion, a phase difference occurs depending on the shape of the bent portion, so the bent portion may also be taken into consideration when designing.

[0052] The relative refractive index difference Δ between the first waveguide WG1 and the second waveguide WG2 was set to 0.55%. The waveguide height h (height in the y-axis direction in FIG. 5) of the first waveguide WG1 and the second waveguide WG2 was set to 9.0 μm. The waveguide width w was set to 10.0 μm. The parallel waveguide interval g was set to 3.0 μm.

[0053] Length L d The length S of the bent part was set to 4756 μm. d The length L of the delay section DL was set to 145 μm. m In addition, the length L of the second waveguide WG2 S and length S s The parallel waveguide length L c is adjusted as described above.

[0054] [5. Relationship between wavelength and mode dependent loss] FIG. 6 is a diagram showing the relationship between wavelength and mode dependent loss. In FIG. 6, the curve DFa is 01 Mode and LP 11a The curve DFb shows the difference between the modes. 01 Mode and LP 11b Indicates the mode difference.

[0055] Parallel waveguide length L c It was confirmed that the maximum mode dependent loss obtained by the mode loss difference compensation device 1 and the wavelength dependence of the mode dependent loss varied depending on the parallel waveguide length L c It was confirmed that the best characteristics were obtained for both the maximum mode dependent loss and the wavelength dependency of the mode dependent loss at around 13,520 μm. It was confirmed that a large mode dependent loss can be imparted to the mode loss difference compensation device 1 employing the Mach-Zehnder structure compared to the mode loss difference compensation device 1 employing the single coupler structure.

[0056] [Effects of the embodiment] As described above in detail, the modal loss difference compensation device and the method for designing the modal loss difference compensation device according to the present embodiment are 01 Mode, LP 11a Mode and LP 11b The LP has a first waveguide and a second waveguide capable of propagating a mode, and a parallel waveguide structure in which the first waveguide and the second waveguide are close to each other, and the LP propagates through the first waveguide. 01 Mode and LP propagating in the second waveguide 01 and a coupling portion at which the mode is coupled to the coupling portion.

[0057] The structure of the first waveguide and the structure of the second waveguide at the coupling portion are the same, and the coupling portion is a LP propagating through the first waveguide. 11a Mode and LP 11bThe first waveguide has a parallel waveguide length that prevents the mode from shifting to the second waveguide. The cross-sectional areas of the first waveguide and the second waveguide vary along the propagation direction of the optical signal so that the wavefront of the electromagnetic field distribution of light obtained when the light incident from one end of the first waveguide is propagated in the forward direction coincides with the wavefront of the electromagnetic field distribution of light obtained when the light output from the other end of the second waveguide is propagated in a reverse direction opposite to the forward direction, assuming that the direction in which the optical signal input to one end of the first waveguide shifts to the second waveguide and exits from the other end of the second waveguide is the forward direction.

[0058] This makes it possible to suppress the wavelength dependency of the modal loss compensation amount for mode multiplexing transmission in the modal loss difference compensation device.

[0059] In the modal loss difference compensation device and the method for designing the modal loss difference compensation device according to the present embodiment, the parallel waveguide length is set to L c For light of wavelength λ, the LP from one end of the first waveguide to the other end of the first waveguide is 01 Let the transmittance of the mode be T LP01 (λ), LP 11a Let the transmittance of the mode be T LP11a (λ), LP 11b Let the transmittance of the mode be T LP11b (λ), and multiple predetermined wavelengths λ k (where k = 1, 2, . . . , N, N is a natural number), the performance index F is

number

[0060] This makes it possible to fabricate a modal loss difference compensation device 1 with small in-band gain deviation, taking into account manufacturing errors.

[0061] Furthermore, in the modal loss difference compensation device and the design method of the modal loss difference compensation device according to the present embodiment, the modal loss difference compensation device may include two coupling sections, and a delay section may be provided between one coupling section and the other coupling section, and the delay section may set a waveguide length difference between the first waveguide and the second waveguide. This makes it possible to realize a modal loss difference compensation device that can handle a larger modal loss difference than a single coupler structure. By employing a Mach-Zehnder structure, it is possible to realize a higher mode extinction ratio, that is, the provision of modal loss difference compensation.

[0062] In addition, in the modal loss difference compensation device and the design method of the modal loss difference compensation device according to the present embodiment, the delay section includes a refractive index control section, and the refractive index control section changes the refractive index of at least one of the first waveguide and the second waveguide, thereby 01 The phase difference between the first and second waveguides in the mode may be variable within a range of 0 to less than π. 01 The LP input to one end of the first waveguide is the same as the LP input to the other end, but the mode is not output from the other end at all. 01 Any transmission characteristic between these two can be achieved when all the power incident on the mode is output from the other end.

[0063] Furthermore, in the modal loss difference compensation device and the method for designing the modal loss difference compensation device according to the present embodiment, the refractive index control section applies heat to at least one of the first waveguide and the second waveguide to reduce the LP 01 The attenuation of the mode may be controlled by changing the refractive index of at least one of the first waveguide WG1 and the second waveguide WG2, thereby generating a phase difference between the first waveguide WG1 and the second waveguide WG2.

[0064] Furthermore, in the modal loss difference compensation device and the method for designing the modal loss difference compensation device according to the present embodiment, the variation in width of the first waveguide and the variation in width of the second waveguide may be ±0.2 μm or less per 1 μm length along the propagation direction of the optical signal. This prevents abrupt modulation of the waveguide width, and smooths the wavelength dependency of the loss difference compensation by the modal loss difference compensation device 1. As a result, it is possible to provide a modal loss difference compensation device 1 with excellent reproducibility.

[0065] Although the contents of the present disclosure have been described above along with the embodiments, the present disclosure is not limited to these descriptions, and various modifications and improvements are possible, which are obvious to those skilled in the art. The descriptions and drawings forming a part of this disclosure should not be understood as limiting the present disclosure. Various alternative embodiments, examples, and operating techniques will be apparent to those skilled in the art from this disclosure.

[0066] Of course, the present disclosure includes various embodiments not described herein. Therefore, the technical scope of the present disclosure is defined only by the invention-specifying matters related to the scope of the claims appropriate from the above description. [Explanation of symbols]

[0067] 1. Mode loss difference compensation device CP joint DL Delay section WG1 1st waveguide WG2 2nd waveguide

Claims

1. LP ０１ Mode, LP １１ａ Mode and LP １１ｂ a first waveguide and a second waveguide capable of propagating a mode; The first waveguide and the second waveguide have a parallel waveguide structure in close proximity to each other, and an LP propagating through the first waveguide ０１ mode and the LP propagating in the second waveguide ０１ a coupling portion at which the mode is coupled; A mode loss difference compensation device comprising: a structure of the first waveguide and a structure of the second waveguide in the coupling portion are identical; The coupling portion is a LP that propagates through the first waveguide. １１ａ Mode and LP １１ｂ a parallel waveguide length that prevents the mode from transitioning to the second waveguide; a forward direction is a direction in which an optical signal input to one end of the first waveguide travels to the second waveguide and is output from the other end of the second waveguide, a wavefront of an electromagnetic field distribution of light obtained when light incident on one end of the first waveguide is propagated in the forward direction; and a wavefront of an electromagnetic field distribution of light obtained when the light output from the other end of the second waveguide is propagated in a reverse direction opposite to the forward direction; and a cross-sectional area of ​​the first waveguide and a cross-sectional area of ​​the second waveguide vary along a propagation direction of the optical signal so that A mode loss difference compensation device comprising:

2. The length of the parallel waveguide is L ｃ year, For light of wavelength λ, from one end of the first waveguide to the other end of the first waveguide, LP ０１ The transmittance of the mode is T ＬＰ０１ (λ), LP １１ａ The transmittance of the mode is T ＬＰ１１ａ (λ), LP １１ｂ The transmittance of the mode is T ＬＰ１１ｂ (λ), A plurality of predetermined wavelengths λ ｋ (where k = 1, 2, ..., N, N is a natural number), the performance index F is [0010] If defined by L that minimizes the performance index F ｃ The length of the parallel waveguide is within the range of ±10%.

2. The modal loss difference compensation device according to claim 1,

3. The coupling portion includes two coupling portions, A delay section is provided between the one connecting section and the other connecting section, The delay section sets a waveguide length difference between the first waveguide and the second waveguide.

2. The modal loss difference compensation device according to claim 1,

4. The delay section includes a refractive index control section, The refractive index control section changes the refractive index of at least one of the first waveguide and the second waveguide, ０１ The phase difference between the first waveguide and the second waveguide in the mode is variable within a range of 0 to less than π.

4. The modal loss difference compensation device according to claim 3,

5. The refractive index control section applies heat to at least one of the first waveguide and the second waveguide, ０１ Controlling the attenuation of a mode 5. The modal loss difference compensation device according to claim 4,

6. The variation in width of the first waveguide and the variation in width of the second waveguide are within ±0.2 μm per 1 μm of length along the propagation direction of the optical signal. The modal loss difference compensation device according to any one of claims 1 to 5,

7. LP ０１ Mode, LP １１ａ Mode and LP １１ｂ a first waveguide and a second waveguide capable of propagating a mode; The first waveguide and the second waveguide have a parallel waveguide structure in close proximity to each other, and an LP propagating through the first waveguide ０１ mode and the LP propagating in the second waveguide ０１ a coupling portion at which the mode is coupled; A method for designing a mode loss difference compensation device comprising: The structure of the first waveguide and the structure of the second waveguide in the coupling portion are set to be the same; The LP propagating through the first waveguide is １１ａ Mode and LP １１ｂ providing a parallel waveguide length that prevents the mode from transitioning to the second waveguide; a direction in which an optical signal input to one end of the first waveguide travels to the second waveguide and is output from the other end of the second waveguide is defined as a forward direction, a wavefront of an electromagnetic field distribution of light obtained when light incident on one end of the first waveguide is propagated in the forward direction; and a wavefront of an electromagnetic field distribution of light obtained when the light output from the other end of the second waveguide is propagated in a reverse direction opposite to the forward direction; and a cross-sectional area of ​​the first waveguide and a cross-sectional area of ​​the second waveguide are varied along a propagation direction of the optical signal so that A method for designing a mode loss difference compensation device comprising:

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