JP2023032196A - Optical mode converter and optical mode multiplexer/demultiplexer - Google Patents

Abstract

To provide an optical mode converter using a tapered waveguide, which enables easy and efficient design of a taper structure.SOLUTION: Provided herein is an optical mode converter with a waveguide having a tapered shape width-wise. Defining a Z-axis as a tapering direction, a rate of change dw/dz of the waveguide width at an anti-crossing coordinate z0 corresponds to a maximum value -amin (amin≠0), and the rate of change of the waveguide width becomes minimum near the coordinate z0.SELECTED DRAWING: Figure 2A

Description

The present disclosure relates to optical mode converters in multimode transmission.

Space-division multiplexing technology is being actively studied to expand optical communication capacity. Among them, technology using orthogonal modes that exist independently within one optical waveguide (core) is called multimode transmission technology. Various methods have been investigated to excite and multiplex different modes within a single core. An optical mode converter is used (see, for example, Non-Patent Documents 1 and 2).

For example, in silica-based planar lightwave circuits, the cross section of the core is generally rectangular. Conversion between degenerate modes can be performed.

The principle of optical mode conversion can be broadly divided into two types. One is the adiabatic type, in which conversion to a different mode is suppressed by setting a sufficiently gently tapered structure, and the other is the adiabatic type, in which degeneracy is resolved. It is an interference type that equally excites two modes that form a pair to cause them to interfere appropriately and excites the mode before the degeneracy is resolved. (For example, see Non-Patent Documents 3 to 7.)

In the adiabatic type, if the taper length is short, conversion to a different mode occurs, so a sufficiently long taper length is required, and there is a trade-off between conversion efficiency and taper length. The interference type, in principle, does not require a tapered structure, but it excites two non-degenerate modes evenly with low loss, and sets the taper length according to the degree of interference between degenerate modes. Precise waveguide parameter setting is required. It also tends to be more wavelength dependent than the adiabatic tapered type.

On the other hand, a curved taper structure using an optimization algorithm, which cannot be classified as either adiabatic or interference, has been reported, but implementation of the algorithm requires complicated variable settings.

K. Saitoh et al. , "PLC-based mode multi/demultiplexers for mode division multiplexing," Opt. Fiber Technol. , vol. 35, pp. 80-92, 2017. D. Dai et al. , "Mode conversion in tapered submicron silicon ridge optical waveguides," Opt. Express, vol. 20, no. 12, pp. 13425-13439, 2012. M. Galarza et al. , "Compact and highly-efficient polarization independent vertical resonant couplers for active-passive monolithic integration," Opt. Express, vol. 16, no. 12, pp. 8350-8358, Jun. 2008. M. Shirata et al. , "Design of small mode-dependent-loss scrambling-type mode (de) multiplexer based on PLC," Opt. Express, vol. 28, no. 7, pp. 9653-9665, Mar. 2020. K. Saitoh et al. , "PLC-based LP_11 mode rotator for mode-division multiplexing transmission," Opt. Express, vol. 22, no. 16, pp. 19117-19130, Jul. 2014. Y. Yamashita et al. , "PLC-based LP11 mode rotor with curved trench structure developed from wavefront matching method," IEEE Photonics Technol. Lett. , vol. 29, no. 13, pp. 1063-1066, Jul. 2017. Z. Chen et al. , "Ultra-compact broadband in-line mode converter based on a width-modulated silicon wave guide," IEEE Photonics J. Am. , vol. 13, no. 2, p. 2700107, 2021.

An object of the present disclosure is to enable simple and efficient design of a tapered structure in an optical mode converter using a tapered waveguide.

In the present invention, attention is focused on the structural change rate of the tapered structure, and an arbitrary function (for example, polynomial function) that minimizes the structural change rate in a state where degeneracy is resolved is set. As a result, the present disclosure can obtain only a tapered structure with a smooth structural change rate with a small number of parameters, and can design the tapered structure simply and efficiently.

Specifically, the optical mode converter according to the present disclosure includes:
comprising a waveguide having a tapered waveguide width,
When the direction in which the tapered shape is tapered is taken as the z-axis, the waveguide width change rate dw/dz at the anti-intersecting coordinate z ₀ has a maximum value −a _min (a _min ≠0), and the coordinate It is characterized by the minimum rate of change of the waveguide near _z0 .

Specifically, the optical mode multiplexer/demultiplexer according to the present disclosure is
an optical mode converter according to the present disclosure;
2 ^N -mode scrambler;
Prepare.

According to the present disclosure, it is possible to easily and efficiently design a tapered structure in an optical mode converter using a tapered waveguide. Furthermore, according to the present disclosure, since the degree of design freedom is higher than that of tapered shapes such as arcs, it is possible to efficiently design the tapered structure of the optical mode converter, reducing the taper length and broadening the optical mode conversion. operation becomes possible.

An example of an adiabatic optical mode converter is shown. An example of an interferometric optical mode converter is shown. An example of an anti-intersection is shown. Correspondence between non-degenerate modes and degenerate optical modes at anti-crossing points is shown. 1 illustrates an example of an optical mode converter according to the present disclosure; 4 is an example of structural parameters of an optical mode converter according to the present disclosure; It is an example of a waveguide shape considering the change rate of the waveguide width, (a) shows the waveguide width w, and (b) shows the change rate w' of the waveguide width. A comparative example of taper length is shown. 1 shows an example of a waveguide shape of an optical mode converter according to the present disclosure; 1 shows an example of a waveguide shape of an optical mode converter according to the present disclosure; 1 shows an example of a waveguide shape of an optical mode converter according to the present disclosure; 1 shows an example of a waveguide shape of an optical mode converter according to the present disclosure; A comparison of the taper length and transmittance of the existing structure and the proposed structure is shown. An example of structural parameters is shown. It is an example of the n dependence of the transmittance of the proposed waveguide, (a) shows the case of a _min =0, and (b) shows the case of a _min =1/15000. It is an example of a _min dependence of an optical mode converter, (a) shows n=3 and (b) shows n=10. An example of wavelength dependence of an optical mode converter is shown. An example of a _min dependence of conversion efficiency and bandwidth at 1550 nm is shown. An example of a _min dependence of conversion efficiency and bandwidth at 1550 nm is shown. 1 shows an example of an optical mode multiplexer comprising an optical mode converter of the present disclosure; A configuration example of a 4-mode scrambler is shown. 2 shows a pickup diagram of 6 input ports; FIG. 4 shows a schematic configuration of a 4x4 unitary converter and a 4-mode scrambler; A specific configuration example of a 4x4 unitary converter and a 4-mode scrambler is shown.

Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. Note that the present disclosure is not limited to the embodiments shown below. These implementation examples are merely illustrative, and the present disclosure can be implemented in various modified and improved forms based on the knowledge of those skilled in the art. In addition, in this specification and the drawings, constituent elements having the same reference numerals are the same as each other.

(Embodiment example 1)
1A and 1B show a conventionally proposed optical mode converter. There are two main types: the adiabatic type and the interference type. In the adiabatic type, as shown in FIG. 1A, the mode is converted along the dispersion curve by a sufficiently moderate waveguide change (hereinafter sometimes referred to as a straight taper or straight taper structure). In the adiabatic type, a three-step taper structure has also been proposed in which the taper length in the adiabatic type is shortened while maintaining the principle of adiabatic insulation by forming a steep taper up to the vicinity of the anti-intersection (see, for example, Non-Patent Document 4). ). The interference type, as shown in FIG. 1B, has a structure that, in principle, does not require a tapered structure. The interferometric type excites two modes at opposite crossing points, and the modes are converted by interference action.

An example of an anti-intersection is shown in FIG. 1C. The anti-crossing point is the point where the modes are coupled due to the tilt of the optical axis. The anti-crossing points shown in FIG. 1C can convert the E31 and E13 modes as shown in FIG. 1D into LP02-like and LP21-like modes.

FIG. 2A shows the proposed optical mode converter. The optical mode converter of the present disclosure is characterized by having an intermediate structure between the adiabatic type and the interference type. A smooth waveguide shape can be easily obtained by setting the change rate of the waveguide width to a smooth function such as a polynomial function. Also, depending on how the function is given, it is possible to control the difference in rate of change between the vicinity of the center and the edge of the tapered structure. Depending on the setting of the difference in the rate of change, the adiabatic type or the interference type can be used. The optical mode converter of the present disclosure has a waveguide width change rate that is steep at both ends of the tapered structure and gradual near the center of the tapered structure. In particular, in a structure close to the interference type, by not setting the rate of change near the center of the tapered structure to 0, it is possible to obtain characteristics that are shorter than those of the adiabatic type and excellent in wavelength dependence.

FIG. 2B shows an example of structural parameters. As shown in FIG. 2A, Δ is the refractive index difference of the waveguide, h is the height of the waveguide, w is the width of the waveguide, Δw is the amount of change in the width of the waveguide, and _LT is the taper length.

FIG. 3 shows the waveguide shape considering the change rate of the waveguide width.
The traveling direction is defined as the z-axis, and the width of the waveguide at the coordinate z is defined as w(z).
Let _w0 be the width of the anti-intersecting waveguide.
Let z ₁ (<z ₀ ) be the starting point coordinates of the tapered structure, and z ₂ (>z ₀ ) be the ending point coordinates.
At coordinate z ₀ , let the width of the anti-intersecting waveguide be w ₀ .
∴w( _z0 )= _w0
The waveguide width monotonously decreases in the traveling direction.
∴dw/dz<0

In this disclosure, we use these parameters to set w(z) such that the rate of change in waveguide width w′=dw/dz has a maximum a _min at coordinate z ₀ , so that the anti-crossing coordinates It is possible to set the width of the waveguide that is the loosest in the vicinity of _z0 . The present disclosure uses a high-order polynomial function as a function of waveguide width w(z) to achieve extremely high transmittance even with a short taper length compared to adiabatic optical mode converters.

FIG. 4 shows a comparison of the taper length when Δw is the same and a _min is the same at the opposite intersection. L _TP indicates a tapered structure of the present disclosure, L _TS indicates a straight tapered structure, and L _TT indicates a triple tapered structure. In the proposed structure, by increasing |w'| at a distance from the anti-intersection, the taper length _LTP becomes shorter than the conventional linear taper structure _LTS and the three-step taper structure _LTT .

ただし、ａ_ｍｉｎ＜Δｗ／Ｌ_Ｔである。 In this disclosure, the waveguide width function w(z) is represented by an n-th order polynomial function as follows.

However, a _min <Δw/ _LT .

Here, n is the degree of the polynomial and is a real number of 0 or more. α is a parameter corresponding to the change rate of the waveguide width expressed by the following equation, and is a real number of 0 or more.

5 to 8 show examples of proposed waveguide shapes (n=1, 2, 3, 10). 5 to 8, _z1 to _z2 are represented by z=0 to 5. FIG. 5(a) to 8(a) show the waveguide width w, and FIGS. 5(b) to 8(b) show the shape of the waveguide width. Order n, minimum change rate a _min of waveguide width, taper length L _T (=z ₂ −z ₁ ), waveguide width change amount Δw (=w(z ₁ )−w(z ₂ )) of tapered structure is determined, the change rate α of the waveguide width is automatically determined, and the tapered structure is uniquely determined. As n increases, the areas near both ends of the taper become steeper, and as a _min approaches 0, the area near the opposite intersection becomes linear.

FIG. 9A shows a comparison of taper length and transmittance between the existing structure and the proposed structure. Structural parameters are shown in FIG. 9B. The existing three-step taper structure has a taper length L _T =8.6 mm and a transmittance T=-0.085 dB. On the other hand, in the proposed structure, for example, when n=6, the transmittance T=-0.02 dB at the taper length L _T =4.5 mm. Therefore, in the present disclosure, when n=2 or more, it can be realized with about half the device length while improving the transmittance.

FIG. 10 shows the n dependence of the transmittance of the proposed waveguide. When a _min =0, as shown in FIG. 10(a), the larger n (the tapered end becomes steeper), the shorter the taper length becomes. descend. Therefore, it can be seen that when a _min =0, a taper length of 3.8 mm can be realized without lowering the transmittance when n=10.

On the other hand, when a _min =1/15000, as shown in FIG. 10(b), when n=3, for example, a flat region that was not seen when a _min =0 appeared, and when n=3 or more, a flat area appeared. It can be seen that there exists a structure with high efficiency. For example, when n=4, it can be seen that a taper length of 5 mm can be achieved.

From the above, it can be seen that the proposed optical mode converter can be realized with a device length about half that of the conventional optical mode conversion.

(Embodiment example 2)
FIG. 11 shows the a _min dependence of the proposed optical mode converter. In the case of n=3 shown in FIG. 11(a), as a _min increases from 0, the waveguide width variation near the taper center increases and the waveguide width at the taper end becomes moderate. The interference type gradually transitions to an intermediate conversion between the interference type and the adiabatic type. If a _min is too large, it corresponds to a state in which the taper length is insufficient in the adiabatic type, and the conversion efficiency decreases. Therefore, as an intermediate type between the interference type and the adiabatic type, a _min = around 1/15000 is desirable.

In the case of n=10 shown in FIG. 11(b), the behavior with respect to a _min is the same as that of n=3, and a _min of about 1/10000 is desirable as an intermediate type between the interference type and the adiabatic type.

From the above, it is desirable that a _min is 1/15000 to 1/10000 in order to reduce the loss and shorten the taper length of the proposed optical mode converter.

(Embodiment example 3)
FIG. 12 shows an example of the wavelength dependence of the proposed optical mode converter. In the figure, n = 1 indicates n = 1, a _min = 0, L _T = 8.75 mm, n = 3 indicates n = 3, a _min = 1/15000, L _T = 8.75 mm, n=6 indicates n=6, a _min =1/14000, L _T =8 mm, n=10 — 0 indicates n=10, a _min =0, L _T =3.8 mm, n=10 — 15000 indicates n= 10, a _min =1/15000, L _T =8 mm, n=10_10000 indicates n=10, a _min =1/10000, L _T =6.25 mm. A strong wavelength dependence is observed in the conventional three-step taper structure, and as shown in n=10_0, when a _min =0, even when n=10, wavelength dependence similar to that of the three-step taper structure is obtained. It is The three-step taper structure operates as an interference type because the taper length required for adiabatic operation is insufficient or mode coupling occurs at the steep taper portion. Therefore, in the case of a _min =0, even in a short region of L<10 mm, it is also an interference type, and it can be seen that even if the order n is changed, the wavelength dependence is not improved.

On the other hand, in the case of a _min =1/15000 to 1/10000, it can be seen that wideband operation can be achieved by selecting a combination of n and L _T with high conversion efficiency. It was difficult to improve the _band just by softening the waveguide width near the anti-crossing point, as in the conventionally reported three-step taper structure. The tapered shape obtained by the rate of change induces intermediate conversion between adiabatic and interference types, and wideband operation can be realized.

13 and 14 show a _min dependence of conversion efficiency and bandwidth at 1550 nm for n=6 and n=10, respectively. _LT is 8 mm. The bandwidth (maximum wavelength−minimum wavelength) calculated from the wavelength section where the change from the conversion efficiency at the center wavelength of 1550 nm is within ±0.1 dB is plotted. In the case of a _min =0 (1/a _min =∞), it operates as an interference type, and the order n and the taper length _LT at which high conversion efficiency is obtained at 1550 nm are limited. _T = 8.75 mm (x in the figure) and n = 10, L _T = 3.8 mm (+ in the figure) are plotted on the right end. Compared to a _min =0, the bandwidth is more than doubled while maintaining the conversion efficiency. When n=6, 1/a _min is 12000 to 20000 and the operating wavelength range is 100 nm or more. When n=10, 1/a _min is 12000 to 18000 and the operating wavelength range is 100 nm or more.

(Embodiment example 4)
FIG. 15A shows an optical mode multiplexer with the proposed optical mode converter. As shown in the figure, a 16×16 optical unitary converter 91 and a 4-mode scrambler 92 with a Y-branch waveguide are combined, and an E31/13 mode converter 93 (E31/13 Rotator) is used.

FIG. 15B shows a configuration example of a 4-mode scrambler. The 4-mode scrambler 92 can use a ^2N scrambler. Here, N is any integer greater than or equal to 0. As a characteristic of the Y-branch waveguides 31 to 33 of the 4-mode scrambler, only lateral peaks change at the coupling portion. When the E11 (LP01) mode is incident on the two input side of the Y branch, the E11 mode and the E21 (LP11a) mode are generated, and when the E21 mode is incident on the two input side of the Y branch, the E31 mode and the E41 mode are generated. Since the E13 mode has three vertical peaks, it cannot be excited unless the En3 mode having three vertical peaks is incident. Therefore, an optical mode converter that changes the number of vertical peaks in the mode is required. Here, the optical mode converter of the present disclosure is used as the E31/13 mode converter 93 shown in FIG. 15A.

Hitherto, when the E12 (LP11b) mode was excited, the LP11a/b rotor was used, and in the subsequent Y branch, the E21 (LP11a) was newly excited. On the other hand, in the conventional 8-mode scrambler, the E31 mode is excited, and if the waveguide cross section is made square, it becomes the LP02 mode and the LP21a mode. Considering the rank of the transfer matrix, only one channel is divided into two modes, and it is necessary to add one more channel. Then, through the Y branch again, the E13 mode remains the E13 mode, and the E31 mode is excited from the E21 mode, thereby making it possible to independently excite two channels.

Normally, LP11b mode and LP21a mode come out, so E13 mode and E23 mode come out from E13 mode, so the preprocessing 16×16 optical unitary converter 91 suppresses unnecessary excitation. (For example, E13 is incident in phase).

FIG. 16A shows a pickup diagram of 6 input ports. The configuration of the unitary converter 91 is simplified and described. 16A, instead of the 16.times.16 optical unitary converter 91 of FIG. 15, a 2.times.2 unitary converter and a 4.times.4 unitary converter are combined to form a plurality of low-order (4.times.4, 2.times.2) ULOCs. A 4x4 unitary transform matrix is, for example, represented below.

In the figure, the unitary transformation matrix UA is applied to the unitary transformer indicated by (A), the unitary transformation matrix _UB is applied to the unitary transformer indicated by (B), and the unitary transformation matrix _UB is applied to the unitary transformer indicated by (C). Apply the unitary transformation matrix U _C to the unitary transformer. As described above, a 4LP mode multiplexer/demultiplexer can be realized.

FIG. 16B shows a schematic configuration of a 4×4 unitary converter and a 4-mode scrambler. The LP01 mode input to ports 1 to 4 is converted from port 5 to LP01, LP11a and LP11b, LP21 and output.

FIG. 16C shows a specific configuration example of a 4×4 unitary converter and a 4-mode scrambler. A 4×4 unitary converter and a 4-mode scrambler can be composed of a power divider 10 and a phase shifter 20, as shown in FIG. 16C(a).

Although the power divider 10 can adopt any configuration, for example, as shown in FIG. Two Y-branch waveguides of ports are provided, and one port of the two Y-branch waveguides is connected by a two-mode waveguide.

Although the phase shifter 20 can adopt any configuration, for example, as shown in FIG. 16C(c), it has a shape in which the waveguide width gradually increases toward the center of the phase shifter.

By providing these power distributor 10 and phase shifter 20, the present disclosure can shorten the element length of an optical unitary converter that converts an arbitrary unitary matrix.

(Effect of the present disclosure)
Only a tapered structure with a smooth structural change rate can be obtained with a small number of parameters, and the degree of design freedom is higher than that of tapered shapes such as circular arcs, making it possible to design an efficient tapered structure for an optical mode converter. , reducing the taper length and enabling broadband operation of optical mode conversion.

(Points of this disclosure)
In the present disclosure, by focusing on the structural change rate rather than the tapered shape of the optical mode converter itself, and by setting an arbitrary function that minimizes the structural change rate in a state where the degeneracy is resolved, a simple and efficient tapered structure can be created. enables design.

The present disclosure can be applied to the information and communications industry.

10: Power distributors 11, 12, 31, 32, 33: Y branch waveguides 111, 112: Waveguide 13: Coupling section 20: Phase shifter 91: 16×16 optical unitary converter 92: 4-mode scrambler 93: E31/13 mode converter

Claims (4)

comprising a waveguide having a tapered waveguide width,
When the direction in which the tapered shape is tapered is taken as the z-axis, the waveguide width change rate dw/dz at the anti-intersecting coordinate z ₀ has a maximum value −a _min (a _min ≠0), and the coordinate An optical mode converter characterized in that the rate of change of the waveguide is minimized near _z0 . The waveguide width w (z) at the coordinate z is represented by the following formula,
An optical mode converter according to claim 1.

The optical mode converter,
1/a _min =12000 to 18000,
3. An optical mode converter according to claim 2. an optical mode converter according to any one of claims 1 to 3;
2 ^N -mode scrambler;
An optical mode multiplexer/demultiplexer.

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