JP2025041213A - Modal loss difference compensation device and method for designing the modal loss difference compensation device - Google Patents

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

This disclosure relates to a modal loss difference compensation device and a method for designing a modal loss difference compensation device.

In mode-multiplexed 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.

Non-Patent Document 1 discloses a modal loss difference compensation device that uses a spatial optical system to set the amount of modal loss compensation using an interpolating spatial filter.

Non-Patent Document 2 discloses a modal loss difference compensation device that uses PLC (Planar lightwave circuit) technology to suppress loss to the _LP11 mode by a parallel waveguide having two coupling parts while imparting loss to the fundamental mode, _LP01 mode.

Patent Document 1 discloses a modal loss difference compensation device that imparts a modal loss difference between the LP _{01 mode} and the LP ₁₁ mode while imparting almost no loss to the LP 11 mode by changing the waveguide length or the refractive index between two waveguides.

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

JP 2020-148968 A

However, when combining mode multiplexing transmission and wavelength multiplexing transmission, the use of the mode loss difference compensation devices disclosed in Non-Patent Documents 1 and 2 and Patent Document 1 poses the problem that the bandwidth for compensating characteristics is limited.

The present disclosure has been made in consideration of the above problems. Its purpose is to provide a mode loss difference compensation device and a method for designing a mode loss difference compensation device that can suppress the wavelength dependency of the amount of mode loss compensation for mode multiplexing transmission.

In order to solve the above-mentioned problems, a modal loss difference compensation device according to one aspect of the present disclosure includes a first waveguide and a second waveguide capable of propagating an LP ₀₁ mode, an LP _11a mode, and an LP _11b mode, and a coupling portion having a parallel waveguide structure in which the first waveguide and the second waveguide are close to each other and in which the LP ₀₁ mode propagating in the first waveguide and the LP ₀₁ mode propagating in the second waveguide are coupled. The first waveguide and the second waveguide in the coupling portion have the same structure, and the coupling portion has a parallel waveguide length that prevents the LP _11a mode and the LP _11b mode propagating in the first waveguide from transitioning to the second waveguide. The cross-sectional area of the first waveguide and the cross-sectional area of the second waveguide vary along the propagation direction of the optical signal so that a wavefront of the electromagnetic field distribution of light obtained when light incident from one end of the first waveguide is propagated in the forward direction coincides with a wavefront of the electromagnetic field distribution of light obtained when light output from the other end of the second waveguide is propagated in a reverse direction opposite to the forward direction.

A method for designing a modal loss difference compensation device according to an aspect of the present disclosure includes a first waveguide and a second waveguide capable of propagating an LP ₀₁ mode, an LP _11a mode, and an LP _11b mode, and a coupling portion having a parallel waveguide structure in which the first waveguide and the second waveguide are close to each other and in which the LP ₀₁ mode propagating through the first waveguide and the LP ₀₁ mode propagating through the second waveguide are coupled. The structure of the first waveguide and the structure of the second waveguide in the coupling portion are set to be the same, and the coupling portion is provided with a parallel waveguide length that prevents the LP _11a mode and the LP _11b mode propagating through the first waveguide from migrating to the second waveguide. The cross-sectional area of the first waveguide and the cross-sectional area of the second waveguide are varied along the propagation direction of the optical signal so that a 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 a 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.

The present disclosure provides a mode loss difference compensation device and a method for designing a mode loss difference compensation device that can suppress the wavelength dependency of the amount of mode loss compensation for mode multiplexing transmission.

FIG. 1A is a schematic diagram (reference structure) of a modal loss difference compensation device employing a single coupler structure. FIG. 1B is a schematic diagram of a modal loss difference compensation device (after application of the wavefront matching method). FIG. 1C is an enlarged view of a main portion of FIG. 1B. FIG. 2 is a diagram showing an example of a transmission spectrum of a modal loss difference compensation device having a reference structure. 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. FIG. 4 is a diagram showing the relationship between the length of the parallel waveguide and the performance index. FIG. 5 is a schematic diagram (reference structure) of a mode loss difference compensation device employing a Mach-Zehnder structure. FIG. 6 is a diagram showing the relationship between wavelength and mode dependent loss.

Next, an embodiment of the present disclosure will be described in detail with reference to the drawings. In the description, the same parts will be given the same reference numerals and duplicate explanations will be omitted.

[1. Structure of a mode loss difference compensation device using a single coupler structure]
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 portion CP.

Figure 1B is a schematic diagram of a mode loss difference compensation device (after application of the wavefront matching method). Figure 1C is an enlarged view of a main portion of Figure 1B. Figures 1B and 1C are diagrams in which the wavefront matching method is applied to the mode multiplexer/demultiplexer of Figure 1A, and the width of the waveguide is varied. In Figures 1B and 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 Figure 1A, in which no variation is added to the waveguide width by the wavefront matching method, is called the "reference structure."

The first waveguide WG1 and the second waveguide WG2 are configured to be capable of propagating the _LP01 mode, the _LP11a mode, and the _LP11b mode. Hereinafter, the _LP11a mode and the _LP11b mode are collectively referred to as the _LP11 mode.

The first waveguide WG1 has a first port P1 and a third port P3, with light entering through the first port P1 and exiting through the third port P3. The second waveguide WG2 has a second port P2 and a fourth port P4, with light entering through the second port P2 and exiting 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 path (not shown). The second port P2 and the fourth port P4 may be terminated to prevent light reflection.

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. At the coupling portion CP, the LP ₀₁ mode propagating through the first waveguide WG1 and the LP ₀₁ mode propagating through the second waveguide WG2 are coupled. Also, the LP ₁₁ mode propagating through the first waveguide WG1 and the LP ₁₁ mode propagating through the second waveguide WG2 are coupled. The distance between the first waveguide WG1 and the second waveguide WG2 at the coupling portion CP is shown as a parallel waveguide distance g. Note that, at the coupling portion CP, the structure of the first waveguide WG1 and the structure of the second waveguide WG2 are the same. The width of the first waveguide WG1 and the width of the second waveguide WG2 are shown as a waveguide width w.

The coupling part CP has a parallel waveguide length _Lc that prevents the _LP11a mode and the _LP11b mode propagating through the first waveguide WG1 from transferring to the second waveguide WG2. In the coupling part CP, the _LP01 mode is branched at an arbitrary ratio, and the parallel waveguide length _Lc is adjusted so that the _LP11a mode and the _LP11b mode do not transfer optical power from the waveguide through which they have been guided to the other waveguide.

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

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

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 _Ss relating to the bending of the second waveguide WG2 was set to 20 μm. The parallel waveguide length _Lc was adjusted as described above.

The branching ratio between the first waveguide WG1 and the second waveguide WG2 may be adjusted by parameter design.

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

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 exits 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 exiting 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 examples given here.

Then, the cross-sectional area of the first waveguide WG1 and the cross-sectional area of the second waveguide WG2 are changed so that the forward wavefront and the reverse wavefront coincide, and the structure of the modal loss difference compensation device 1 is optimized. 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.

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.

In this embodiment, the structure of the modal loss difference compensation device 1 is optimized for various parallel waveguide lengths _Lc using a wavefront matching method.

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 prohibits abrupt modulation of the waveguide width, and smooths the wavelength dependency of the loss difference compensation by the mode loss difference compensation device 1. As a result, it is possible to provide a mode 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.

[2. Transmission spectrum in modal 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.

FIG. 2 shows the result of calculating the transmission spectra of a modal loss difference compensation device having a reference structure (a modal loss difference compensation device without optimization using a wavefront matching method) when light in LP ₀₁ mode and light in LP ₁₁ mode are incident on the first port P1 and output from the third port P3.

FIG. 3 shows the result of calculating the transmission spectrum when light of LP ₀₁ mode and light of LP ₁₁ mode incident on the first port P1 are output from the third port P3 in the modal loss difference compensation device 1 obtained by applying the wavefront matching method.

In addition, in Figures 2 and 3, the wavelength of 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. Also, the wavelength of 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.

In the example shown in Fig. 2, the loss in the LP _11a mode becomes large at 1575 nm or more, but by performing optimization using the wavefront matching method, the loss in the LP _11a mode approaches flat over the entire range of the C-band and L-band, as shown in Fig. 3. In other words, it can be confirmed that the wavelength dependency of the inter-modal loss compensation amount is suppressed by performing optimization using the wavefront matching method.

[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 modal loss difference compensation device 1 can be defined by the following equation:

Here, for light of wavelength λ, the transmittance of the _LP01 mode from one end (first port P1) of the first waveguide WG1 to the other end (third port P3) of the first waveguide WG1 is T _LP01 (λ), the transmittance of the _LP11a mode is T _LP11a (λ), and the transmittance of the _LP11b mode is T _LP11b (λ). The performance index F is calculated based on the transmittance of each mode at a plurality of predetermined wavelengths λ _k (where k=1, 2, ..., N, N is a natural number).

The performance index F is 0 when it represents ideal characteristics. In other words, the smaller the performance index F is, the closer it is to 0, the closer the mode loss difference compensation device 1 is to ideal characteristics. In the formula defining the performance index F, the value "0.5" that appears 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".

4, it can be seen that the parallel waveguide length _Lc when the performance index F is the lowest (when the modal loss difference compensation device 1 has characteristics closest to ideal) is 13,520 μm. In order to fabricate the modal loss difference compensation device 1 with a small in-band gain deviation, the parallel waveguide length _Lc may be within a range of ±10% or less from the parallel waveguide length _Lc when the performance index F is the lowest, taking into account manufacturing errors.

[4. Structure of the mode loss difference compensation device using the 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 portions CP.

The modal loss difference compensation device 1 shown in FIG. 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.

The delay unit DL may include, for example, a refractive index control unit configured to change the refractive index of at least one of the first waveguide WG1 and the second waveguide WG2 and to vary the phase difference between the first waveguide WG1 and the second waveguide WG2 in the _LP01 mode within a range of 0 to less than π.

Various methods can be used to change the refractive index by the refractive index control section. For example, the refractive index control section may change the refractive index by applying heat to at least one of the first waveguide WG1 and the second waveguide WG2 to control the attenuation of the _LP01 mode.

Here, the phase difference between the waveguides, which corresponds to the delay amount of the second waveguide WG2 relative to the first waveguide WG1 caused by the delay unit DL, is defined as the phase difference Δθ. Here, in each coupling unit CP, the _LP01 mode is branched at a 1:1 ratio, and the parallel waveguide length _Lc of the coupling unit CP is adjusted so that the _LP11a mode and the _LP11b mode do not transfer optical power from the waveguide through which the wave has been guided to the other waveguide.

By applying the wavefront matching method to the modal loss difference compensation device 1 shown in Figure 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.

5, the length of the delay section DL is indicated as length _Lm , and the length of the bend of the second waveguide WG2 at the bent section connected to the delay section DL is indicated as length _Ld .

When the LP ₀₁ mode incident on the first port P1 of the first waveguide WG1 is not outputted from the third port P3 at all due to a change in the length _Lm , the phase difference between the waveguides of the LP ₀₁ mode in the delay unit DL is 0+(2π×j) (j is an integer). When all the incident power of the LP ₀₁ mode incident on the first port P1 of the first waveguide WG1 is outputted from the third port P3, the phase difference is π+(2π×k) (k is an integer). Therefore, in order to realize a transmission characteristic between the two cases, the phase difference in the delay unit may be set to 0+(2π×h) to π+(2π×h) (h is an integer).

Here, the relationship between the length _Lm and the phase difference Δθ can be obtained by "Δθ = Δβ · _Lm ", where "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".

When the first waveguide WG1 and the second waveguide WG2 have the same structure, "Δn = 0", and therefore the transmission characteristics do not change even if the length _Lm is changed. On the other hand, when the first waveguide WG1 and the second waveguide WG2 have different structures or when the refractive index fluctuations differ between the first waveguide WG1 and the second waveguide WG2, a phase difference Δθ occurs due to the change in the length _Lm .

Due to the presence of the length _Ld in 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.

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 spacing g was set to 3.0 μm.

The length _Ld was set to 4756 μm. The length _Sd of the bent portion was set to 145 μm. The length _Lm of the delay portion DL was set to 4000 μm. In addition, the length _Ls and the length _Ss of the bent portion of the second waveguide WG2 were set to 500 μm and 20 μm, respectively. The parallel waveguide length _Lc was adjusted as described above.

5. Relationship between wavelength and mode dependent loss
6 is a diagram showing the relationship between wavelength and mode dependent loss, where curve DFa shows the difference between LP ₀₁ and LP _11a modes, and curve DFb shows the difference between LP ₀₁ and LP _11b modes.

It was confirmed that the maximum mode dependent loss and the wavelength dependency of the mode dependent loss obtained by the modal loss difference compensation device 1 vary depending on the parallel waveguide length _Lc . 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 when the parallel waveguide length _Lc was in the vicinity of 13,520 μm. It was confirmed that a large mode dependent loss could be imparted in the modal loss difference compensation device 1 employing the Mach-Zehnder structure, compared to the modal loss difference compensation device 1 employing the single coupler structure.

[Effects of the embodiment]
As described above in detail, the modal loss difference compensation device and the method of designing the modal loss difference compensation device according to the present embodiment relate to a modal loss difference compensation device including a first waveguide and a second waveguide capable of propagating the LP ₀₁ mode, the LP _11a mode, and the LP _11b mode, and a coupling portion having a parallel waveguide structure in which the first waveguide and the second waveguide are close to each other, and at which the LP ₀₁ mode propagating through the first waveguide and the LP ₀₁ mode propagating through the second waveguide are coupled.

The first and second waveguides in the coupling section have the same structure, and the coupling section has a parallel waveguide length that prevents the _LP11a mode and the _LP11b mode propagating through the first waveguide from migrating to the second waveguide. The cross-sectional areas of the first and second waveguides vary along the propagation direction of the optical signal such that, assuming that the direction in which an optical signal incident on one end of the first waveguide migrates to the second waveguide and is output from the other end of the second waveguide is defined as the forward direction, a wavefront of the electromagnetic field distribution of light obtained when the light incident on one end of the first waveguide is propagated in the forward direction coincides with a 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.

This makes it possible to suppress the wavelength dependency of the amount of inter-mode loss compensation for mode multiplexing transmission in the inter-mode loss difference compensation device.

によって定義した場合に、性能指標Ｆを最小化するＬ_ｃに対して、平行導波路長は、±１０％以下の範囲にあるものであってもよい。 Moreover, in the modal loss difference compensation device and the design method of the modal loss difference compensation device according to the present embodiment, the parallel waveguide length is _Lc , and for light of wavelength λ, the transmittance of the _LP01 mode from one end of the first waveguide to the other end of the first waveguide is _TLP01 (λ), the transmittance of the _LP11a mode is _TLP11a (λ), and the transmittance of the _LP11b mode is _TLP11b (λ), and for a plurality of predetermined wavelengths _λk (where k=1, 2, . . . , N, N is a natural number), the performance index F is expressed as

For L _c that minimizes the performance index F when defined by:

This makes it possible to produce a mode loss difference compensation device 1 with small gain deviation within the band, taking into account manufacturing errors.

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 have two coupling sections and a delay section 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 adopting a Mach-Zehnder structure, it is possible to realize a higher mode extinction ratio, that is, the provision of modal loss difference compensation.

Furthermore, in the modal loss difference compensation device and the design method for the modal loss difference compensation device according to the present embodiment, the delay section may include a refractive index control section, and the refractive index control section may change the refractive index of at least one of the first waveguide and the second waveguide to vary the phase difference between the first waveguide and the second waveguide in the LP ₀₁ mode in a range of 0 to less than π. This makes it possible to realize any transmission characteristic between a case in which no LP ₀₁ mode incident on one end of the first waveguide is output from the other end, and a case in which all incident power of the LP ₀₁ mode incident on one end of the first waveguide is output from the other end.

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 may apply heat to at least one of the first waveguide and the second waveguide to control the attenuation of the _LP01 mode. This makes it possible to change the refractive index of at least one of the first waveguide WG1 and the second waveguide WG2, and to generate a phase difference between the first waveguide WG1 and the second waveguide WG2.

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 variation in the width of the first waveguide and the variation in the width of the second waveguide may be ±0.2 μm or less per 1 μm of 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.

The contents of the present disclosure have been described above in accordance with the embodiments, but it will be apparent to those skilled in the art that the present disclosure is not limited to these descriptions, and that various modifications and improvements are possible. The descriptions and drawings that form 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.

Naturally, this disclosure includes various embodiments not described herein. Therefore, the technical scope of this disclosure is determined only by the invention-specific matters related to the scope of the claims that are appropriate from the above explanation.

1: Mode loss difference compensation device CP: Coupling section DL: Delay section WG1: First waveguide WG2: Second waveguide

Claims (7)

a first waveguide and a second waveguide capable of propagating an LP ₀₁ mode, an LP _11a mode, and an LP _11b mode;
a coupling portion having a parallel waveguide structure in which the first waveguide and the second waveguide are adjacent to each other, where an LP ₀₁ mode propagating through the first waveguide and an LP ₀₁ mode propagating through the second waveguide are 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 has a parallel waveguide length that prevents the LP _11a mode and the LP _11b mode propagating through the first waveguide 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 vary along a propagation direction of the optical signal so that a modal loss difference between the first waveguide and the second waveguide coincide with each other. The length of the parallel waveguide is _Lc ,
For light of wavelength λ, from one end of the first waveguide to the other end of the first waveguide,
The transmittance of the _LP01 mode is _TLP01 (λ),
The transmittance of the LP _11a mode is T _LP11a (λ),
The transmittance of the _LP11b mode is _TLP11b (λ),
For a plurality of predetermined wavelengths λ _k (where k=1, 2, . . . , N, N is a natural number), the performance index F is expressed as follows:

If defined by
2. The modal loss difference compensation device according to claim 1, wherein the parallel waveguide length is within a range of ±10% or less with respect to _Lc that minimizes the performance index F. The coupling portion includes two coupling portions,
A delay section is provided between the one connecting section and the other connecting section,
The modal loss compensation device according to claim 1 , wherein the delay section sets a waveguide length difference between the first waveguide and the second waveguide. The delay section includes a refractive index control section,
4. The modal loss difference compensation device according to claim _{3, wherein the refractive index control section changes a refractive index of at least one of the first waveguide and the second waveguide, and varies a phase difference between the first waveguide and the second waveguide in the LP 01} mode in a range of 0 or more and less than π. 5. The modal loss difference compensation device according to claim 4, wherein the refractive index control section applies heat to at least one of the first waveguide and the second waveguide to control an attenuation amount of the _LP01 mode. 6. The modal loss difference compensation device according to claim 1, wherein a variation in width of the first waveguide and a 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. a first waveguide and a second waveguide capable of propagating an LP ₀₁ mode, an LP _11a mode, and an LP _11b mode;
a coupling portion having a parallel waveguide structure in which the first waveguide and the second waveguide are adjacent to each other, where an LP ₀₁ mode propagating through the first waveguide and an LP ₀₁ mode propagating through the second waveguide are 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;
a parallel waveguide length is provided in the coupling portion to prevent the LP _11a mode and the LP _11b mode propagating through the first waveguide 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 are varied along a propagation direction of the optical signal so that the cross-sectional areas of the first waveguide and the second waveguide match.

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