JP2018063333A - Mode multiplexing/demultiplexing optical circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a mode multiplexing/demultiplexing optical circuit with which it is possible to have the mode field shapes of light matched at the connection end face of planar lightwave circuits.SOLUTION: A mode multiplexing/demultiplexing optical circuit 1 includes, on each of two planar lightwave circuits 10, 20, a directional coupler comprising two waveguides 11, 12 (21, 22) mutually differing in waveguide width. The two waveguides of each of the planar lightwave circuits have their waveguide widths set so that the effective refractive indices are equal, and the two planar lightwave circuits are connected so that the substrate directions intersect at an angle of 90 degrees, the wide waveguides out of the two waveguides of the respective planar lightwave circuits are optically joined together at the connection end face of the planar lightwave circuits, and the wide waveguides at the connection end face assume the form of a square cross section that is equal in width and height.SELECTED DRAWING: Figure 6

Description

  The present invention relates to a mode multiplexing / demultiplexing optical circuit using a planar lightwave circuit (PLC).

  In recent years, many optical communication systems utilizing the degree of freedom of modes have been proposed. For example, with the aim of increasing the transmission capacity per fiber, a technique for placing a signal for each mode used in spatial multiplexing transmission, or a higher-order mode test light is incident on the measurement target fiber, and the intensity of the backscattered light A method for monitoring the state of the laid fiber is proposed (Patent Document 1).

  When monitoring the state of the laid fiber, it is necessary to multiplex / demultiplex all the modes generated in the fiber, so a mode coupler using a planar lightwave circuit is used.

  FIG. 1 shows an asymmetric directional coupler 100 having waveguides 101 and 102 having different waveguide widths using a quartz-based planar lightwave circuit (PLC) as such a mode coupler.

  The PLC is a circuit that can be connected to an optical fiber on a flat substrate. This is because an optical waveguide including a core and a clad having a refractive index smaller than that of the core is manufactured on a flat substrate by patterning and etching by photolithography or the like, and a plurality of basic optical circuits (for example, Various functions are realized by combining a directional coupler and a Mach-Zehnder interferometer.

In FIG. 1, the mode coupler {β (λ, W _m , Δ) relating to 0th-order mode (also referred to as LP01 mode in the following description) light that is a fundamental mode propagating through the narrower waveguide 102. _{m = 0} } and a higher-order mode (also referred to as LP11 mode in the following description) propagating through the wider waveguide 101 {β (λ, W _m , Δ) _{m = 1} } relating to light And the length of the directional coupler 100 is appropriately set. Thus, the LP01 mode and LP11 mode light can be converted through the directional coupler 100.

For the above-mentioned values, β: propagation constant, λ: wavelength, W _m : waveguide width, Δ: refractive index difference between core and clad, m: mode order.

  The above-described propagation constant β is expressed as β = n · k (n: effective refractive index, k: 2π / λ), so that the effective refractive index n of each mode is matched with the propagation constant. Matching β is synonymous. Since the effective refractive index n is a function determined by “λ”, “Wm”, and “Δ”, the propagation constant β is referred to as “β (λ, Wm, Δ)”. The LP01 mode light propagating through the waveguide 101 passes through as it is because the propagation constant does not match that of the LP01 mode light propagating through the waveguide 102 (FIG. 1). As a result, mode multiplexing / demultiplexing using a mode coupler is possible.

  Although one type of LP11 mode is shown in FIG. 1, for example, in the case of 1 μm band light, there are two types of LP11 modes.

  FIG. 2 shows two types of LP11 modes propagating through the laying fiber 130: (a) an LP11a mode in which there are two peaks in the x direction of the electric field component d; and (b) an LP11a mode in which the electric field component d is in the y direction. LP11b mode in which there are two peaks is shown. Accordingly, at this time, there are three modes of light propagating through the laying fiber 130 in addition to the LP01 mode shown in FIG. 1, the LP11a mode in FIG. 2A, and the LP11b mode in FIG. 2B. There are modes.

  In general, in a PLC, since only a mode parallel to the PLC substrate surface can be converted, only one of the above-described LP11a mode or LP11b mode can be converted. Therefore, the conventional PLC converts both of the two types of LP11a mode and LP11b mode by changing the orientation of the two PLC substrates by 90 degrees and optically connecting the waveguides of the respective PLCs. (Patent Document 2).

  FIG. 3A shows the conventional PLC described above, in which the directions of the substrates of the two PLCs 200 and 300 are changed by 90 degrees, and the linear waveguides 210 and 310 provided in the PLCs 200 and 300 are optically end faces. A planar lightwave circuit 1000 is shown which is connected at. In FIG. 3A, the waveguides 220 and 320 include a bent portion.

  FIG. 3B shows that the higher-order mode LP11b is converted to the 0th-order mode LP01 in the directional coupling section of the PLC 300, and the higher-order mode LP11a is converted to the 0th-order mode LP01 in the directional coupling section of the PLC 200. It is.

JP2015-152399A JP 2013-152272 A

  In general, in the PLC, the waveguide width of the input / output unit is designed so as to minimize the connection loss with the fiber. However, when connecting the PLC and the fiber, the conductor formed in the PLC is designed. Since it is difficult to increase the thickness (height direction) of the core of the waveguide, the PLCs may be connected only in the direction of the waveguide width rather than the height direction. For this reason, the waveguide cores formed in the PLCs may be displaced in the height direction, resulting in connection loss.

  FIG. 4 shows a connection state between PLCs when such a shift occurs. In this case, the connection between the waveguides 210 and 310 of the PLC does not match the mode field shape of the light, and connection loss may occur.

  An object of the present invention is to provide a mode multiplexing / demultiplexing optical circuit capable of matching the mode field shape of light at the connection end faces of PLCs.

  To achieve the above object, the present invention provides a first planar lightwave circuit, a second planar lightwave circuit, and each of the first planar lightwave circuit and the second planar lightwave circuit. A first directional coupler composed of two waveguides having different waveguide widths, and a second directional coupler, wherein the first planar lightwave circuit and the second planar lightwave circuit Each of the two waveguides has a waveguide width set so that the effective refractive index is equal, and the first planar lightwave circuit and the second planar lightwave circuit are orthogonal to each other by 90 degrees. The wide waveguides of the two waveguides of the respective planar lightwave circuits are optically coupled to each other at the connection end surfaces of the first planar lightwave circuit and the second planar lightwave circuit. And the wide waveguide at the connection end face It is formed so as to be equal square cross-sectional width and height.

  The wide waveguide may be changed in steps from the directional coupler to the connection end face with a narrow width.

  The first directional coupler and the second directional coupler may be configured to perform a plurality of mode conversions.

  The width and height of the wide waveguide may be 5 μm or more and 7 μm or less on the connection end face.

  According to the present invention, it is possible to match the mode field shape of light at the connection end faces between PLCs.

It is a figure which shows the structure of the conventional mode coupler. It is a figure for demonstrating two general high-order modes. It is a figure for demonstrating the conventional PLC which connected different PLCs. It is a figure for demonstrating the connection part of different PLC in the conventional PLC. It is a figure for demonstrating the waveguide width | variety which can propagate high order mode, and the waveguide width which cannot propagate high order mode. It is a figure which shows the structural example of the mode multiplexing / demultiplexing optical circuit of one Embodiment.

  Hereinafter, the mode multiplexing / demultiplexing optical circuit in the present embodiment will be described.

  First, the waveguide provided in this mode multiplexing / demultiplexing optical circuit will be described with reference to FIGS.

  FIG. 5 is a diagram for explaining the width of a waveguide in which higher-order mode propagation is possible and the width of a waveguide in which higher-order mode propagation is impossible.

  In FIG. 5, the horizontal axis represents the waveguide width through which light having a wavelength of 1030 nm propagates, and the vertical axis represents the effective refractive index of the waveguide.

In the example of FIG. 5, the refractive index difference Δ between the core and the clad in the waveguide is 0.45%, and the core height is 7 μm. At this time, in order to convert between the zero-order mode and the first-order mode, the relational expression {β (λ, W _m , Δ) _{m = 0} −β (λ, W _m , Δ) _{m = 1} = 0} is obtained. It is necessary to satisfy.

In the above relational expression, β: propagation constant, λ: wavelength, W _m : waveguide width, Δ: refractive index difference between core and clad, m: mode order.

  As described above, the propagation constant β is expressed as β = n · k (n: effective refractive index, k: 2π / λ), and the effective refractive index n of each mode is matched with the propagation constant β. Therefore, the propagation constant β is also referred to as “β (λ, Wm, Δ)” in this embodiment.

  According to the example shown in FIG. 5, if the waveguide width that propagates the LP01 mode is set to “4.0 μm”, and the waveguide width that propagates the LP11a mode and the LP11b mode is set to “10.6 μm”, Since the effective refractive indexes n of the modes are the same, the above-described relational expression is satisfied.

  In this case, the length of the directional coupler having the different waveguide widths described above may be optimized and set in advance.

  Next, the configuration of the mode multiplexing / demultiplexing optical circuit 1 that satisfies the above relational expression will be described with reference to FIG.

  FIG. 6A is a diagram illustrating a configuration example of the mode multiplexing / demultiplexing optical circuit 1 according to the present embodiment. FIG. 6B is an enlarged view of a connection portion of the PLCs 10 and 20 shown in FIG.

  As shown in FIG. 6A, the mode multiplexing / demultiplexing optical circuit 1 includes two PLCs 10 and 20. The PLCs 10 and 20 are connected orthogonally to each other by changing the direction of the substrate by 90 degrees.

  The PLC 10 includes two waveguides 11 and 12. The waveguide 11 is formed in a straight line and is wider than the waveguide 12. The waveguide 12 includes a bent portion. These waveguides 11 and 12 function as directional couplers and realize optical coupling with different modes.

  The PLC 20 includes two waveguides 21 and 22. The waveguide 21 is formed in a straight line and is wider than the waveguide 22. The waveguide 22 includes a bent portion. These waveguides 21 and 22 function as directional couplers and realize optical coupling with different modes.

  In FIG. 6B, the waveguide 12 has a width and a height that allow mode coupling between the LP01 mode and the LP11a mode and allow the LP11a mode and the LP11b mode to propagate.

  In FIG. 6B, on the connection end faces of the PLCs 10 and 20, the waveguides 11 and 21 of the PLCs have a width and a height that allow the LP11a mode and the LP11b mode to propagate. That is, the widths of the waveguides 11 and 21 are changed so as to be narrowed in steps, so that the shapes of the connection portions of the waveguides 11 and 21 are the same. In the example of FIG. 6B, the waveguides 11 and 21 are formed to have a square cross section having the same width and height. Thereby, the shift of the core portion at the connection end surfaces of the waveguides 11 and 21 is eliminated, and the mode field shape of the light can be matched at the connection end surfaces of the PLCs 10 and 20.

  For example, in the example of FIG. 6B, the height of the waveguide 11 is always “h”. On the other hand, the width of the waveguide 11 is changed from the directional coupling portion to the connection end face so as to gradually decrease in the order of “Wa” and “h” (h <Wa) (in FIG. 6B). (See Waveguide portions 11a-11c). As will be described later, the value of “h” is preferably 5 μm or more and 7 μm or less on the connection end face.

  The direction of the waveguide 21 is also different from that of the waveguide 11, but gradually decreases in the order of “Wa” and “h” (h <Wa) from the corresponding directional coupling portion to the connection end surface. It is changed (see the waveguide portions 21a to 21c in FIG. 6B).

[Connection cross section between waveguides]
Hereinafter, the values of “Wa” and “h” described above will be described again with reference to FIG. 5 in connection with the connection cross-sectional shape of the waveguides 11 and 21.

  In FIG. 5, the effective refractive index in the case of the LP11 mode (hereinafter including LP11a and LP11b) is shown after the waveguide width starts from about 5 μm. Is less than 5 μm or less, it means that the LP11 mode cannot be propagated in the waveguide, resulting in propagation loss.

  On the other hand, the thickness of the waveguide core is limited to about 7 μm even if it is thick. For this reason, in the waveguide, both modes of LP11 (LP11a, LP11b) can propagate, and in order to make the cross-sectional shape of the waveguide square, so as to eliminate the shift of the waveguide at the connection end surfaces of the PLCs 10 and 20, the core The film thickness is preferably 5 μm or more and 7 μm or less. Actually, the LP11 mode cannot be stably propagated in the waveguide when the core film thickness is just before the cutoff of about 5 μm, so a design with a margin of about 1 to 2 μm is required.

  In the following description, as an example, the film thickness of the core is described as 7 μm.

  In order to eliminate the connection loss in the connection portion between the PLCs 10 and 20, it is necessary to make the waveguide width and the waveguide height the same. For example, when the waveguide height is 7 μm, the minimum waveguide width that can be produced is about 4 μm. For this reason, even if the waveguide width is 7 μm, the cross-sectional shape of the waveguide can be made square.

  On the other hand, according to the example of FIG. 5, if the width and height of the waveguides 11 and 21 are 7 μm, if the mode coupler is designed with the waveguide width as it is, the narrower one (example of FIG. 6) Then, the waveguide width of the waveguides 12 and 22) is 4 μm or less. For this reason, a waveguide width that cannot be manufactured is required.

  In view of the above, in the mode multiplexing / demultiplexing optical circuit 1 of the present embodiment, when the waveguide width of the narrower one (the waveguides 12 and 22 in the example of FIG. 6) is 4 μm, the width of the mode conversion is possible. The waveguide width of the wider one (waveguides 11 and 21 in the example of FIG. 6) is 10.6 μm, and the waveguide width is adiabatically changed (FIG. 6B). Thereby, a manufacturable mode coupler is realized.

  As described above, according to the mode multiplexing / demultiplexing optical circuit 1 of the present embodiment, the directional coupler including the two waveguides having different waveguide widths on the planar lightwave circuits 10 and 20 respectively. including. Here, the waveguide widths of the two waveguides of each of the planar lightwave circuits 10 and 20 are set so that the effective refractive indexes are equal, and the planar lightwave circuits 10 and 20 are 90 degrees perpendicular to the substrate. So that they are connected. In the connection end faces of the planar lightwave circuits 10 and 20, the wide waveguides 11 and 21 of the two waveguides of the respective planar lightwave circuits 10 and 20 are connected so as to be optically coupled to each other. The wide waveguides 11 and 21 are formed to have a square cross section having the same width and height. Thereby, the shift of the core portion at the connection end faces of the waveguides 11 and 21 is eliminated, and the mode field shapes of light can be matched at the connection end faces of the PLCs 10 and 20.

  In addition, by making the connecting portion of the waveguides 11 and 21 have a square cross section, the LP11 mode has higher sensitivity than the LP01 mode, and as a result, the maximum sensitivity is obtained. Therefore, the use of the LP11 mode in the connection portion between the waveguides 11 and 21 results in highly sensitive light propagation.

1-mode multiplexing / demultiplexing optical circuit 10,20 Planar lightwave circuit (PLC)
11, 12, 21, 22 Waveguide

Claims (4)

A first planar lightwave circuit and a second planar lightwave circuit;
On each of the first planar lightwave circuit and the second planar lightwave circuit, a first directional coupler composed of two waveguides having different waveguide widths, and a second directional characteristic, respectively. Including a coupler and
Each of the two waveguides of the first planar lightwave circuit and the second planar lightwave circuit has a waveguide width set so that the effective refractive index is equal,
The first planar lightwave circuit and the second planar lightwave circuit are connected such that the orientation of the substrate is 90 degrees orthogonal,
The connection end faces of the first planar lightwave circuit and the second planar lightwave circuit are connected so that the wide waveguides of the two waveguides of the respective planar lightwave circuits are optically coupled to each other,
The mode multiplexing / demultiplexing optical circuit, wherein the wide waveguide on the connection end face is formed to have a square cross section having the same width and height.   2. The mode multiplexing / demultiplexing optical circuit according to claim 1, wherein the wide waveguide is changed in a stepwise manner from the directional coupler to the connection end surface in a stepwise manner.   The mode multiplexing / demultiplexing optical circuit according to claim 1, wherein the first directional coupler and the second directional coupler are configured to perform a plurality of mode conversions. .   4. The mode multiplexing / demultiplexing optical circuit according to claim 1, wherein a width and a height of the wide waveguide are 5 μm or more and 7 μm or less at the connection end face. 5.