JP7510137B2 - Mode converter and method for manufacturing the same - Google Patents

Description

Applicable to Article 30, Paragraph 2 of the Patent Act. Presented at Microoptics Conference 2019 on November 19, 2019 under the title "A Broadband PLC-type Mode Converter Designed by Wavefront Matching Method."

The present invention relates to a technology for converting multiple propagating modes in a mode multiplexing transmission system that uses a multimode optical fiber.

In optical fiber communication systems, nonlinear effects and fiber fuses that occur in optical fibers are problems that limit the increase in transmission capacity. To alleviate these limitations, spatial multiplexing technology using multi-core or multi-mode fibers is being considered.

In mode multiplexing transmission technology, in order to multiplex and split multiple modes, it is necessary to convert the fundamental mode used in conventional optical communication systems into a higher-order mode and multiplex it. In addition, in the transmission line, the loss differs for each propagating mode, and the resulting difference in loss between modes reduces the transmission capacity of the mode multiplexing transmission line.

G. Labroille et al., "Efficient and mode selective spatial mode multiplexer based on multi-plane light conversion," Opt. Express, vol. 22, p. 15599 (2014) T. Fujisawa et al. , "One chip, PLC three-mode exchanger based on symmetric and asymmetric directional coupler with integrated mode rotor," OFC2017 paper, W1B. 2 (2017) Y. Sakamaki et al. , "New optical waveform 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 matching method," IEICE ELEX, vol. 8, pp. 2100-2105 (2011).

A wide variety of mode multiplexers/demultiplexers and mode converters (e.g., Non-Patent Documents 1 and 2) have been proposed to multiplex/demultiplex modes and compensate for mode loss differences, but all of these methods have complex optical systems or wavelength dependence, which increases insertion loss or limits the operating band.

This disclosure was made in consideration of the above problems, and provides an optical device with a simple structure that achieves broadband mode conversion.

The mode converter of the present disclosure includes:
an input waveguide capable of propagating n or more modes (n being an integer of 1 or more);
an output waveguide capable of propagating m or more modes (m being an integer of 2 or more);
a third waveguide capable of propagating l or more modes (l being an integer greater than m), having a waveguide width greater than the input waveguide and the output waveguide, and connected between the input waveguide and the output waveguide;
Equipped with
a waveguide width of at least a part of the third waveguide is different from a waveguide width of a connection portion of the input waveguide and a waveguide width of a connection portion of the output waveguide;
In at least a portion of the third waveguide, 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, coincides with a wavefront in the reverse propagation direction of an output field, which is a superposition of modes different from the specific modes output to the output waveguide.

The method for manufacturing a mode converter according to the present disclosure includes the steps of:
an input waveguide capable of propagating n or more modes (n being an integer of 1 or more);
an output waveguide capable of propagating m or more modes (m being an integer of 2 or more);
a third waveguide capable of propagating l or more modes (l being an integer greater than m), having a waveguide width greater than the input waveguide and the output waveguide, and connected between the input waveguide and the output waveguide;
A method for manufacturing a mode converter comprising:
The waveguide width of at least a portion of the third waveguide is set so that 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, coincides with a wavefront in the reverse propagation direction of an output field, which is a superposition of modes different from the specific modes output to the output waveguide.

This disclosure provides an optical device with a simple structure that can achieve broadband mode conversion.

1 shows a reference structure for a mode converter of the present disclosure. 1 shows input/output characteristics of a mode in a reference structure in which the waveguide width of the waveguide 3 is not modulated in the propagation direction. 1 shows the results of modulating the waveguide width according to the present disclosure. 1 illustrates an example of waveguide width modulation in accordance with the present disclosure. A specific process flow chart is shown below. The number of wavefront matching iterations and the input and output transmittance (loss) are shown. The calculation results of the wavelength dependence of transmittance are shown.

The following describes in detail the embodiments of the present disclosure 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 forms with various modifications and improvements based on the knowledge of those skilled in the art. Note that components with the same reference numerals in this specification and drawings are considered to be identical to each other.

A first embodiment of the present disclosure will be described. FIG. 1 shows a reference structure of the mode converter of the present disclosure. The reference structure is composed of an input side waveguide 1, an output side waveguide 2, and a waveguide 3 disposed therebetween. The waveguide 1 can propagate modes of n (n is an integer of 1 or more), the waveguide 2 can propagate modes of m (m is an integer of 2 or more), and the waveguide 3 can propagate modes of l (l is an integer larger than m). In this embodiment, n=m=2, and the waveguide 1 and the waveguide 2 are 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 _W3 larger than the waveguide widths _W1 and W2 of the waveguides 1 and ₂ , and the waveguide height H is the same.

The function realized by this disclosure is to convert a specific mode input from waveguide 1 into a specific different mode and output it from waveguide 2. The specific mode is at least a part of the n or more modes that can propagate through waveguide 1. The specific different mode is at least a part of the m or more multimodes that can propagate through waveguide 2. In the following description, the propagation direction of light from waveguide 1 to waveguide 2 is assumed to be the z direction.

Figure 2A shows the input/output characteristics of the mode in a reference structure in which the waveguide width of the waveguide 3 is not modulated in the propagation direction. Figure 2B shows the input/output characteristics of the mode in a reference structure in which the waveguide width of the waveguide 3 is modulated in the propagation direction. In Figures 2A and 2B, the white lines indicate the shape of the waveguide 3. In the example of the present disclosure shown in Figure 2B, the connection positions of the waveguides 1 and 2 to the waveguide 3 are designed so that when the fundamental mode is input from the waveguide 1, the same mode is obtained from the waveguide 2. In this example, the connection positions of the waveguides 1 and 2 to the waveguide 3 are different on the z-axis in the propagation direction, and the waveguides 1 and 2 are connected to symmetrical positions from the center position of the waveguide 3 in the waveguide width direction (x-direction).

FIG. 3 shows an example of the modulation of the waveguide width in the present disclosure. The specific 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 mode output from the waveguide 3 to the waveguide 2 has m or more modes, and the output field is a superposition of m or more modes. In the present disclosure, the waveguide width W3 is changed for each minute section Δz at the position z of the waveguide ₃ so that the wavefront of the cross-sectional field in the forward propagation direction of the input field and the wavefront of the cross-sectional field in the reverse propagation direction of the output field coincide with each other.

A specific process flow chart is shown in FIG.
First, a structure with a constant waveguide width _W3 is defined as a reference structure (S11).
To determine the waveguide width _W3 at any position z in the light propagation direction,
Set the optimization position z (S12);
Calculate the field when the input field propagates forward and reaches position z, and the field when the output field propagates backward and reaches position z (S13).
The waveguide width _W3 is changed so that these fields coincide (S14).
These steps S12 to S14 are repeated by sequentially scanning the position z from the connection end of the waveguide 3 with the waveguide 1 to the output end connected with the waveguide 2.

As a result, a structure of the waveguide 3 is determined in which the forward propagation field from the input waveguide 1 and the reverse propagation field from the output waveguide 2 coincide at any position z of the waveguide 3. By manufacturing a planar lightwave circuit having this structure, an optical device that obtains a desired output field from a desired input field can be realized. Here, the method of manufacturing the planar waveguide is arbitrary, but for example, a pattern mask of the designed waveguide structure can be produced and the waveguide can be formed by patterning using the pattern mask. Note that such coincidence of fields may be at least a part of the waveguide 3.

The above-mentioned waveguide width optimization method can use a method similar to that used for designing single-mode devices described in Non-Patent Documents 3 and 4. For example, the waveguide width W3 is varied along the light propagation direction using a wavefront matching method so that the wavefront of the forward-propagated field of the input field from the input end of the input waveguide ₁ coincides with the wavefront of the backward-propagated field of the output field from the output end of the output waveguide 2, thereby improving the characteristics. However, the application of this method is limited to the case where the input and output waveguides are single-mode waveguides, and there is no consideration at all of improving the characteristics in the case where the input and output are a pair of multimode waveguides, as in the present disclosure, and there is no consideration at all of a mode converter that generates mode conversion by utilizing multimode interference in the waveguide 3, as in the present disclosure.

2B shows the waveguide width shape as a result of optimization on the premise that the fundamental mode is output from waveguide 1 and the LP11 mode is output from waveguide 2. Here, the relative refractive index difference Δ of waveguides 1 to 3 is 1.0%, the width _W1 of waveguide 1 and the width _W2 of waveguide 2 are 11.0 μm, the waveguide height H is 10.0 μm, the waveguide width _W3 of waveguide 3 is 30.0 μm, and the lengths of the respective waveguides are _L1 = _L2 = 100.0 μm and _L3 = 4000.0 μm.

Figure 5 shows the number of wavefront matching iterations and the input/output transmittance (loss). It can be seen that by repeating the calculations of S12 to S14 approximately five times, a low loss of -2 dB or more was obtained over a wide wavelength band. This is because the wavefront matching method was applied to a structure in which the input field and output field are the same in the reference structure.

Figure 6 shows the calculation results for the wavelength dependence of transmittance. In designing the wavefront matching method, the wavelength range to be optimized is a parameter, and the results are shown for two assumed wavelength ranges of 1.5 to 1.6 and 1.3 to 1.7 μm. As can be seen from these results, widening the wavelength range increases the wavelength band accordingly, but a narrower wavelength band allows for locally superior loss characteristics to be obtained.

In this embodiment, the non-periodic modulation of the waveguide width _W3 has a maximum change width of ±0.2 μm in the direction perpendicular to the light propagation direction (x direction in FIG. 1) for a light propagation direction of 1 μm (z direction in FIG. 1). This is because prohibiting abrupt modulation of the optical waveguide width _W3 makes the wavelength dependency smooth, and an optical circuit with excellent reproducibility can be provided. However, the present disclosure is not limited to this example, and it is of course acceptable for the waveguide width _W3 to vary more abruptly.

In addition, in a structure in which the variation in the waveguide width _W3 along the propagation direction actually obtained by the wavefront matching method is smoothed by averaging the waveguide widths at 20 points before and after the propagation direction, equivalent characteristics can be obtained, and this can be a preferable structure in terms of fabrication.

In this specification, examples of planar lightwave circuits using glass-based materials are described, but the materials can of course be other materials. For example, similar effects to those of the examples described in this specification can be obtained with planar lightwave circuits using semiconductors such as Si or InGaAsP, or organic materials such as polymers.

In addition, the wavelength band used in the examples described in this specification is approximately 1.3 to 1.7 μm, but it can also be a longer wavelength band such as the mid-infrared region (2 μm or more) or the visible light region.

As for the waveguide structure, the examples described in this specification are directed to a rectangular embedded waveguide, but other waveguide structures, such as a ridge waveguide structure, may also be used.

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

1, 2, 3: Waveguide

Claims (5)

an input waveguide capable of propagating n or more modes (n being an integer of 1 or more);
an output waveguide capable of propagating m or more modes (m being an integer of 2 or more);
a third waveguide capable of propagating l or more modes (l being an integer greater than m), having a waveguide width greater than the input waveguide and the output waveguide, and connected between the input waveguide and the output waveguide;
Equipped with
the input waveguide and the output waveguide are disposed at positions symmetrically shifted from a center position in a waveguide width direction of the third waveguide,
a waveguide width of at least a part of the third waveguide is different from a waveguide width of a connection portion of the input waveguide and a waveguide width of a connection portion of the output waveguide;
a waveguide width is non-periodically modulated in a propagation direction so that, in at least a portion of the third waveguide, a wavefront in a forward propagation direction of an input field, which is a superposition of one or more specific modes input from the input waveguide, coincides with a wavefront in a reverse propagation direction of an output field, which is a superposition of modes different from the specific modes output to the output waveguide;
Mode converter. a waveguide width is set for each minute section Δz at a position z in the propagation direction so that, in at least a portion of the third waveguide, a wavefront in a forward propagation direction of an input field, which is a superposition of one or more specific modes input from the input waveguide, coincides with a wavefront in a reverse propagation direction of an output field, which is a superposition of modes different from the specific modes output to the output waveguide;
The mode converter of claim 1 . The specific mode is LP01 mode,
The mode different from the specific mode is an LP11 mode.
3. The mode converter according to claim 1 or 2 . an input waveguide capable of propagating n or more modes (n being an integer of 1 or more);
an output waveguide capable of propagating m or more modes (m being an integer of 2 or more);
a third waveguide capable of propagating l or more modes (l being an integer greater than m), having a waveguide width greater than the input waveguide and the output waveguide, and connected between the input waveguide and the output waveguide;
A method for manufacturing a mode converter comprising:
the input waveguide and the output waveguide are arranged to be shifted symmetrically from a center position in a waveguide width direction of the third waveguide,
a waveguide width of at least a part of the third waveguide is non-periodically modulated in a propagation direction so that a wavefront in a forward propagation direction of an input field, which is a superposition of one or more specific modes input from the input waveguide, coincides with a wavefront in a reverse propagation direction of an output field, which is a superposition of modes different from the specific modes output to the output waveguide.
A method for manufacturing a mode converter. a waveguide width of at least a part of the third waveguide is set for each minute section Δz at a position z in the propagation direction so that a wavefront in a forward propagation direction of an input field, which is a superposition of one or more specific modes input from the input waveguide, coincides with a wavefront in a reverse propagation direction of an output field, which is a superposition of modes different from the specific modes output to the output waveguide;
A method for manufacturing the mode converter according to claim 4 .

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