JP2022078865A - Mode converter and its manufacturing method - Google Patents

Abstract

To provide an optical device for achieving broad-band mode conversion with a simple configuration.SOLUTION: A mode converter comprises: an input waveguide 1 that can propagate a mode; an output waveguide 2 that can propagate a mode; and a third waveguide 3 that is connected between the input waveguide 1 and the output waveguide 2. A waveguide width W3 of at least part of the third waveguide 3 is different from a waveguide width W1 of a connection portion of the input waveguide 1 and a waveguide width W2 of a connection portion of the output waveguide 2. In at least part of the third waveguide 3, a wave front in a normal propagation direction of an input field being overlap of one or more specific modes input from the input waveguide 1 matches a wave front in a reverse propagation direction of an output field being overlap of modes different from the specific modes and output from the output waveguide 2.SELECTED DRAWING: Figure 1

Description

There is an application for exception of loss of novelty

The present invention relates to a technique for converting a plurality of propagating modes in a mode multiplex transmission system using a multimode optical fiber.

In the optical fiber communication system, the non-linear effect generated in the optical fiber and the fiber fuse become problems, and the increase in the transmission capacity is limited. In order to alleviate these restrictions, spatial multiplexing techniques using multi-core and multi-mode fibers are being studied.

In the mode multiplex transmission technique, in order to combine and demultiplex a plurality of modes, it is necessary to convert the basic mode used in the conventional optical communication system to a higher-order mode and combine the waves. Further, in the transmission line, the loss differs depending on the propagating mode, and the loss difference between modes caused by the loss reduces the transmission capacity of the mode multiplex transmission line.

G. Labroil et al, "Efficient and mode Selective spital mode multiplexer based on multi-plane light combination," Opt. Express, vol. 22, p. 15599 (2014) T. Fujisawa et al. , "One chip, PLC three-mode exchanger based on symmetry and symmetry digital directional coupler with integrated mode rotor," OFC2017 paper. 2 (2017) Y. Sakamaki et al. , “New optical waveguide design based on wavefront matching method,” IEEE JLT, vol. 25 pp. 3511-3518 (2007). T. Fujisawa et al. , "Wide-bandwidth, low-waveguide-width-sensitivity InP-based multimode interference coupler designed, by wavefront matchingE, EL." I. 8, pp. 2100-2105 (2011).

A wide variety of mode demultiplexers and mode converters (for example, Non-Patent Documents 1 and 2) have been proposed for mode demultiplexing and mode loss difference compensation, but any method has been proposed. It has a complex optical system or wavelength dependence, which increases insertion loss or limits the operating band.

The present disclosure has been made in view of the above problems, and provides an optical device for realizing wideband mode conversion with a simple structure.

The mode converter of the present disclosure is
An input waveguide capable of propagating n (n is an integer of 1 or more) or more modes,
An output waveguide capable of propagating a mode of m (m is an integer of 2 or more) or more,
A th-order that can propagate a mode of l (l is an integer larger than m), has a larger waveguide width than the input waveguide and the output waveguide, and is connected between the input waveguide and the output waveguide. 3 waveguides and
Equipped with
The width of at least a part of the third waveguide is different from the width of the connecting portion of the input waveguide and the width of the waveguide of the connecting portion of the output waveguide.
In at least a portion of the third waveguide, a wavefront in the forward propagation direction of the input field, which is a superposition of one or more specific modes input from the input waveguide, and the output to the output waveguide. The wavefront in the backpropagation direction of the output field, which is a superposition of modes different from the specific mode, matches.

The method for manufacturing the mode converter of the present disclosure is as follows.
An input waveguide capable of propagating n (n is an integer of 1 or more) or more modes,
An output waveguide capable of propagating a mode of m (m is an integer of 2 or more) or more,
A th-order that can propagate a mode of l (l is an integer larger than m), has a larger waveguide width than the input waveguide and the output waveguide, and is connected between the input waveguide and the output waveguide. 3 waveguides and
It is a manufacturing method of a mode converter provided with
It is a superposition of a wavefront in the forward propagation direction of an input field, which is a superposition of one or more specific modes input from the input waveguide, and a mode different from the specific mode output to the output waveguide. The width of at least a part of the third waveguide is set so that the wavefront in the back propagation direction of the output field matches.

According to the present disclosure, it is possible to provide an optical device for realizing wideband mode conversion with a simple structure.

The reference structure of the mode converter of this disclosure is shown. The input / output characteristics of the mode in the reference structure which does not modulate the waveguide width of the waveguide 3 in the propagation direction are shown. The result of modulation of the waveguide width in this disclosure is shown. An example of modulation of the waveguide width in the present disclosure is shown. A specific process flowchart is shown. The number of wavefront matching iterations and the input / output transmittance (loss) are shown. The calculation result of the wavelength-dependent tax of transmittance is shown.

Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. The present disclosure is not limited to the embodiments shown below. Examples of these implementations are merely examples, and the present disclosure can be implemented in various modified and improved forms based on the knowledge of those skilled in the art. In the present specification and the drawings, the components having the same reference numerals indicate the same components.

The first embodiment of the present disclosure will be described. FIG. 1 shows a reference structure of the mode converter of the present disclosure. It is composed of a waveguide 1 on the input side, a waveguide 2 on the output side, and a reference structure of a waveguide 3 arranged between them. The waveguide 1 can propagate a mode of n (n is an integer of 1 or more) or more, the waveguide 2 can propagate a mode of m (m is an integer of 2 or more) or more, and the waveguide 3 can propagate l (n). l is an integer larger than m) or more modes can be propagated. In this embodiment, n = m = 2, and the waveguide 1 and the waveguide 2 show an example of a two-mode waveguide that guides up to the LP11 mode. The reference structure of the waveguide 3 has a waveguide width W ₃ larger than the waveguide widths W ₁ and W ₂ of the waveguides 1 and 2, and the waveguide height H is the same.

The function realized in the present disclosure is that when a specific mode is input from the waveguide 1, it is converted into a specific different mode and output from the waveguide 2. The specific mode is at least a part of the n or more modes that can be propagated in the waveguide 1. The specific different modes are at least some of the multimodes of the waveguide 2 over m or more propagable. Hereinafter, the propagation direction of light from the waveguide 1 to the waveguide 2 will be described as being the z direction.

FIG. 2A shows the input / output characteristics of the mode in the reference structure in which the waveguide width of the waveguide 3 is not modulated in the propagation direction. FIG. 2B shows the input / output characteristics of the mode in the reference structure in which the waveguide width of the waveguide 3 is modulated in the propagation direction. In FIGS. 2A and 2B, the white line shows the shape of the waveguide 3. In the example of the present disclosure shown in FIG. 2B, the connection positions of the waveguides 1 and 2 to the waveguide 3 are designed so that the same mode can be obtained from the waveguide 2 when the basic mode is input from the waveguide 1. .. Further, in this example, the connection positions of the waveguides 1 and 2 to the waveguide 3 differ on the z-axis in the propagation direction, and the waveguide is symmetric from the center position of the waveguide 3 in the waveguide width direction (x direction). 1 and 2 are connected.

FIG. 3 shows an example of modulation of the waveguide width in the present disclosure. The particular mode input from the waveguide 1 is input to the waveguide 3 to form an input field. Since the specific mode has n or more modes, the input field is a superposition of n or more modes. On the other hand, the specific different modes output from the waveguide 3 to the waveguide 2 have m or more modes, and the output field is a superposition of m or more modes. In the present disclosure, the waveguide for each minute section Δz at the position z of the waveguide 3 is such that the wave surface of the cross-sectional direction field in the forward propagation direction of the input field and the wave surface of the cross-sectional direction field in the reverse propagation direction of the output field match. The width W ₃ is changed.

FIG. 4 shows a specific process flow chart.
First, a structure having a constant waveguide width W ₃ is defined as a reference structure (S11).
To determine the waveguide width W ₃ at any position z in the light propagation direction
Set the optimization position z (S12),
The field when the input field propagates forward and reaches the position z and the field when the output field propagates backward and reaches the position z are calculated (S13).
The waveguide width W3 is changed so that these fields match ₍ S14).
These steps S12 to S14 are repeated by sequentially scanning the position z from the connection end with the waveguide 1 in the waveguide 3 to the output end connected to the waveguide 2.

As a result, the structure of the waveguide 3 is determined so that the forward propagation field from the input waveguide 1 and the back propagation field from the output waveguide 2 match at an arbitrary position z of the waveguide 3. By manufacturing a planar light wave circuit having this structure, it is possible to realize an optical device that obtains a desired output field from a desired input field. Here, the method for manufacturing the planar waveguide is arbitrary, but for example, a pattern mask of the designed waveguide structure can be manufactured, and the waveguide can be formed by patterning using the pattern mask. It should be noted that such field matching may be at least a part of the waveguide 3.

As the above-mentioned waveguide width optimization method, the same method as the design of the single-mode device described in Non-Patent Documents 3 and 4 can be used. For example, wavefront matching so that the wavefront of the forward-propagated field of the input field from the input end of the input waveguide 1 matches the wavefront of the back-propagated field of the output field from the output end of the output waveguide 2. The _wavefront width W3 is varied along the light propagation direction using the method to improve the characteristics. However, its application is limited to the case where the input / output waveguide is a single-mode waveguide, and the improvement of the characteristics when the input / output is a pair of multi-mode waveguides as in the present disclosure has not been considered at all. Moreover, no studies have been made on a mode converter that produces mode conversion by utilizing multimode interference in the waveguide 3, as in the present disclosure.

For example, FIG. 2B shows the waveguide width shape as a result of optimization on the premise that the basic mode is output from the waveguide 1 and the LP11 mode is output from the waveguide 2. Here, the specific refractive index difference Δ of the waveguides 1 to 3 is 1.0%, the width W 1 of the waveguide ₁ and the width W ₂ of the waveguide 2 are 11.0 μm, and the waveguide height H is 10.0 μm. The waveguide width W ₃ of the waveguide 3 is 30.0 μm, and each waveguide length is L ₁ = L ₂ = 100.0 μm and L ₃ = 4000.0 μm.

FIG. 5 shows the number of wavefront matching iterations and the input / output transmittance (loss). It can be seen that a low loss property of -2 dB or more is obtained in a wide wavelength band by repeating the calculation of S12 to S14 about 5 times. This is due to the application of the wavefront matching method to the structure in which the input field and the output field are the same in the reference structure.

FIG. 6 shows the calculation result of the wavelength-dependent tax on the transmittance. In the design of the wavefront matching method, the wavelength range to be optimized is a parameter, and the results when two types of wavelength ranges of 1.5 to 1.6 or 1.3 to 1.7 μm are assumed are also shown. .. As can be seen from this result, when the wavelength range is widened, the wavelength band increases by that amount, but it can be seen that the narrower the wavelength band, the better the loss characteristic can be obtained locally.

The aperiodic modulation of the waveguide width W3 in this embodiment is in the direction perpendicular to the light propagation direction ₍ x direction in FIG. 1) with respect to the light propagation direction (z direction in FIG. 1) of 1 μm. The maximum change width is ± 0.2 μm. This is because by prohibiting abrupt modulation of the optical waveguide width W3 _, the wavelength dependence becomes smooth and an optical circuit having excellent reproducibility can be provided. However, the present disclosure is not limited to this example _, and it is of course possible that there is a steeper variation in the waveguide width W3.

It should be noted that _, with respect to the fluctuation of the waveguide width W3 along the propagation direction actually obtained by the wavefront matching method, the same characteristics are obtained even in the structure smoothed by the value obtained by averaging the waveguide widths of 20 points before and after in the propagation direction. Can be obtained, and a structure preferable in terms of production can be obtained.

In this specification, an example relating to a planar light wave circuit using a glass-based material has been described, but the material may be other material as a matter of course. For example, even in a planar light wave circuit using a semiconductor such as Si or InGaAsP, or an organic substance such as a polymer, the same effect as that of the examples described in the present specification can be obtained.

Further, the wavelength band to be used is about 1.3 to 1.7 μm in the examples described in the present specification, but it may be a mid-infrared region (2 μm or more) having a longer wavelength or a visible light band. do not have.

As for the waveguide structure, in the embodiment described in the present specification, the one relating to the rectangular embedded waveguide is described, but another waveguide structure, for example, a ridge waveguide structure may be used.

This disclosure can be applied to the information and telecommunications industry.

1, 2, 3: Waveguide

Claims (4)

An input waveguide capable of propagating n (n is an integer of 1 or more) or more modes,
An output waveguide capable of propagating a mode of m (m is an integer of 2 or more) or more,
A th-order that can propagate a mode of l (l is an integer larger than m), has a larger waveguide width than the input waveguide and the output waveguide, and is connected between the input waveguide and the output waveguide. 3 waveguides and
Equipped with
The width of at least a part of the third waveguide is different from the width of the connecting portion of the input waveguide and the width of the waveguide of the connecting portion of the output waveguide.
In at least a portion of the third waveguide, a wavefront in the forward propagation direction of the input field, which is a superposition of one or more specific modes input from the input waveguide, and the output to the output waveguide. The wavefront in the backpropagation direction of the output field, which is a superposition of modes different from the specific mode, matches.
Mode converter. The input waveguide and the output waveguide are arranged symmetrically from the center position in the waveguide width direction.
The mode converter according to claim 1. The specific mode is the LP01 mode.
The mode different from the specific mode is the LP11 mode.
The mode converter according to claim 1 or 2. An input waveguide capable of propagating n (n is an integer of 1 or more) or more modes,
An output waveguide capable of propagating a mode of m (m is an integer of 2 or more) or more,
A th-order that can propagate a mode of l (l is an integer larger than m), has a larger waveguide width than the input waveguide and the output waveguide, and is connected between the input waveguide and the output waveguide. 3 waveguides and
It is a manufacturing method of a mode converter provided with
It is a superposition of a wavefront in the forward propagation direction of an input field, which is a superposition of one or more specific modes input from the input waveguide, and a mode different from the specific mode output to the output waveguide. The width of at least a part of the third waveguide is set so that the wavefront in the back propagation direction of the output field matches.
How to manufacture a mode converter.

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