Mode switch

Abstract

To solve the problem that some input mode is not exchanged with the other input mode in a mode exchanger on PLC substrate and thereby facilitate reducing a mode dependent loss (MDL), as well as eliminate the need for introducing a large number of mode exchangers.SOLUTION: The present disclosure is a mode exchanger M2 on a PLC substrate that is inserted into a multi-mode optical fiber in which an optical signal having a plurality of modes propagates, for performing an exchange between the plurality of modes. The mode exchanger M2 includes a side tapered waveguide 21 and a side grated waveguide 22, whose thickness (width) in a direction parallel to the PLC substrate changes in the direction of optical signal propagation, and a top grated waveguide 23 and a top tapered waveguide 24, whose thickness (height) in a direction perpendicular to the PLC substrate changes in the direction of optical signal propagation, in the order stated and in the direction of optical signal propagation.SELECTED DRAWING: Figure 4

Description

Technical Field

[0001] The present disclosure relates to a mode converter on a PLC substrate that is inserted into a multimode optical fiber through which an optical signal having a plurality of modes propagates and performs an exchange between the plurality of modes.

Background Art

[0002] A mode converter on a PLC substrate that is inserted into a multimode optical fiber through which an optical signal having a plurality of modes propagates and performs an exchange between the plurality of modes is disclosed in Patent Document 1 and Non-Patent Document 1. In order to increase the capacity of optical fiber communication, a multimode optical fiber is used, and in order to equalize the mode characteristics, a mode converter on a PLC substrate is used. Here, the mode converter on a PLC substrate is a waveguide element on a PLC substrate in which a SiO film is formed by a flame deposition method and processed by etching, and further SiO is deposited thereon and a SiO waveguide is fabricated. 2 film is formed by a flame deposition method and processed by etching, and further SiO 2 is deposited and SiO 2 waveguide is fabricated.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Non-Patent Documents

[0004]

Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0005] The configuration of the prior art mode converter M1 is shown in FIG. 1. The processing of the prior art mode converter M1 is shown in FIG. 2. The mode converter M1 includes a tapered waveguide 11, a grating waveguide 12, and a tapered waveguide 13 in this order in the optical signal propagation direction.

[0006] The thickness (width) of the tapered waveguide 11 in the direction parallel to the PLC substrate varies in the optical signal propagation direction. And it exchanges the optical fiber mode to the rectangular waveguide mode. Specifically, the LP01, LP11a, LP11b, and LP21a modes are respectively transmitted to the LP01, LP11a, LP11b, and LP21a modes. And the LP21b and LP02 modes are mixed (the reason will be described later with reference to FIG. 3.) and exchanged to the E13 and E31 modes.

[0007] The grating-shaped waveguide 12 has a thickness (width) in the direction parallel to the PLC substrate that varies in the optical signal propagation direction. Then, it performs an exchange between desired rectangular waveguide modes. Specifically, the LP01, LP11a, LP11b, LP21a, and E31 modes are exchanged to the E31, LP01, LP21a, LP11b, and LP11a modes, respectively. However, the E13 mode cannot be exchanged to other modes and is transmitted in the E13 mode.

[0008] The tapered waveguide 13 has a thickness (width) in the direction parallel to the PLC substrate that varies in the optical signal propagation direction. Then, it exchanges the rectangular waveguide mode to the optical fiber mode. Specifically, the LP01, LP21a, LP11b, and LP11a modes are transmitted to the LP01, LP21a, LP11b, and LP11a modes, respectively. And the E31 and E13 modes are exchanged to the LP02 and LP21b modes, respectively (there is no mode mixing).

[0009] The efficiency of the prior art mode converter M1 is shown in FIG. 3. The LP01, LP11a, LP11b, and LP21a input modes are exchanged to the LP02, LP01, LP21a, and LP11b output modes, respectively, with an efficiency of almost 100%. The LP21b input mode is exchanged to the LP11a and LP21b output modes with an efficiency of approximately half. The LP02 input mode is exchanged to the LP11a and LP21b output modes with an efficiency of approximately half.

[0010] In this way, the LP21b input mode is exchanged to other output modes with an efficiency of approximately half, but it is not exchanged to other output modes with an efficiency of approximately half. Therefore, it is difficult to reduce the MDL (Mode Dependent Loss). And in order to reduce the MDL, it is necessary to introduce a large number of mode converters.

[0011] Therefore, in order to solve the above problems, the present disclosure aims to facilitate the reduction of MDL (Mode Dependent Loss) by solving the problem that a certain input mode is not exchanged for another output mode in a mode converter on a PLC substrate, and to eliminate the need to introduce a large number of mode converters.

Means for Solving the Problems

[0012] In order to solve the above problems, a grating waveguide in which the thickness (width) in the parallel direction of the PLC substrate varies in the optical signal propagation direction and a grating waveguide in which the thickness (height) in the perpendicular direction of the PLC substrate varies in the optical signal propagation direction are combined.

[0013] Specifically, the present disclosure is a mode converter on a PLC substrate that is inserted into a multimode optical fiber through which an optical signal having a plurality of modes propagates and performs exchange between the plurality of modes, wherein the thickness in one of the parallel direction and the perpendicular direction of the PLC substrate varies in the optical signal propagation direction, and the first tapered waveguide and the first grating waveguide, and the thickness in the other of the parallel direction and the perpendicular direction of the PLC substrate varies in the optical signal propagation direction, and the second grating waveguide and the second tapered waveguide are provided in this order in the optical signal propagation direction.

[0014] Further, in the present disclosure, the first tapered waveguide exchanges an optical fiber mode for a rectangular waveguide mode, the first grating waveguide exchanges between desired rectangular waveguide modes, the second grating waveguide exchanges between a rectangular waveguide mode that is not exchanged by the first grating waveguide and other rectangular waveguide modes, and the second tapered waveguide exchanges a rectangular waveguide mode for an optical fiber mode.

[0015] According to these configurations, in the mode converter on the PLC board, by solving the problem that a certain input mode is not exchanged for another output mode, it becomes easier to reduce the MDL and it is possible to eliminate the need to introduce a large number of mode converters.

[0016] Further, the present disclosure includes a third tapered waveguide inserted between the first tapered waveguide and the first grating-shaped waveguide, wherein the thickness in one direction varies in the propagation direction of the optical signal, and a fourth tapered waveguide inserted between the second grating-shaped waveguide and the second tapered waveguide, wherein the thickness in the other direction varies in the propagation direction of the optical signal, and is characterized in that it is a mode converter.

[0017] Further, the present disclosure is characterized in that the third tapered waveguide and the fourth tapered waveguide each exchange a rectangular waveguide mode having three or more peaks in one direction or the other direction for a rectangular waveguide mode having three or more peaks in the other direction or the one direction, respectively, and is a mode converter.

[0018] According to these configurations, in the mode converter on the PLC board, by preventing a certain input mode from being mixed with another input mode, it becomes even easier to reduce the MDL and it is possible to eliminate the need to introduce a large number of mode converters.

[0019] Further, the present disclosure is characterized in that the first grating-shaped waveguide and the second grating-shaped waveguide are each designed such that the wavefront of the forward propagation field with respect to the input field from each input end and the wavefront of the backward propagation field with respect to the output field from each output end coincide, and the thicknesses in the one direction and the other direction vary in the propagation direction of the optical signal, and is a mode converter.

[0020] According to this configuration, in the first grating-shaped waveguide and the second grating-shaped waveguide, it is possible to design so as to perform an exchange between desired rectangular waveguide modes.

[0021] In addition, in the present disclosure, the length of the first grating-shaped waveguide in the propagation direction of the optical signal is 10,000 μm or more and 20,000 μm or less, and the length of the second grating-shaped waveguide in the propagation direction of the optical signal is 14,000 μm or more and 20,000 μm or less, and it is a mode converter characterized by this.

[0022] According to this configuration, even with only one mode converter, it is possible to easily reduce the MDL, and thus it is not necessary to introduce a large number of mode converters.

Effects of the Invention

[0023] Thus, the present disclosure facilitates reducing the MDL (Mode Dependent Loss) by solving the problem that a certain input mode is not exchanged for another output mode in the mode converter on the PLC board, and it is possible not to introduce a large number of mode converters.

Brief Description of the Drawings

[0024]

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Embodiments for Carrying Out the Invention

[0025] Embodiments of the present disclosure will be described with reference to the accompanying drawings. The embodiments described below are examples of the implementation of the present disclosure, and the present disclosure is not limited to the following embodiments.

[0026] (Configuration of the Mode Converter of the First Embodiment) The configuration of the mode converter M2 of the first embodiment is shown in FIG. 4. The processing of the mode converter M2 of the first embodiment is shown in FIG. 5. The mode converter M2 includes a side-tapered waveguide 21, a side-grating waveguide 22, a top-grating waveguide 23 (newly added), and a top-tapered waveguide 24 (newly added) in this order in the optical signal propagation direction. Here, the mode converter M2 of the first embodiment is manufactured as follows. First, the side-tapered waveguide 21, the side-grating waveguide 22, the top-grating waveguide 23, and the top-tapered waveguide 24 are fabricated by the waveguide element manufacturing method on a normal PLC substrate, that is, all are separately fabricated as side-tapered waveguides and side-grating waveguides. Next, the top-grating waveguide 23 and the top-tapered waveguide 24 are rotated 90° with the optical signal propagation direction as the rotation axis and butt-jointed and coupled with the side-tapered waveguide 21 and the side-grating waveguide 22.

[0027] The side-tapered waveguide 21 has a thickness (width) in the direction parallel to the PLC substrate that varies in the optical signal propagation direction. And it exchanges the optical fiber mode to the rectangular waveguide mode. Specifically, the LP01, LP11a, LP11b, and LP21a modes are respectively transmitted as the LP01, LP11a, LP11b, and LP21a modes. And the LP21b and LP02 modes are mixed (similar to FIG. 2) and exchanged to the E13 and E31 modes.

[0028] The side-grating waveguide 22 has a thickness (width) in the direction parallel to the PLC substrate that varies in the optical signal propagation direction. And it performs the exchange between desired rectangular waveguide modes. Specifically, the LP01, LP11a, LP11b, LP21a, and E31 modes are respectively exchanged to the E31, LP01, LP21a, LP11b, and LP11a modes. However, the E13 mode cannot be exchanged to other modes and is transmitted as the E13 mode.

[0029] The top grating-shaped waveguide 23 has a thickness (height) in the direction perpendicular to the PLC substrate that varies in the propagation direction of the optical signal. Then, it exchanges between the rectangular waveguide mode that does not exchange and other rectangular waveguide modes in the side grating-shaped waveguide 22. Specifically, it exchanges the LP01 and E13 modes to the E13 and LP01 modes, respectively. Here, the E13 mode is the above-mentioned non-exchanging rectangular waveguide mode, and the LP01 mode is the above-mentioned other rectangular waveguide mode. And it transmits the E31, LP21a, LP11b, and LP11a modes through respectively in the E31, LP21a, LP11b, and LP11a modes.

[0030] The top tapered waveguide 24 has a thickness (height) in the direction perpendicular to the PLC substrate that varies in the propagation direction of the optical signal. Then, it exchanges the rectangular waveguide mode to the optical fiber mode. Specifically, it transmits the LP21a, LP11b, LP01, and LP11a modes through respectively in the LP21a, LP11b, LP01, and LP11a modes. And it mixes the E31 and E13 modes (the reverse process of waveguide 21.) and exchanges them to the LP21b and LP02 modes.

[0031] The design of the side grating-shaped waveguide 22 of the first embodiment is shown in FIG. 6. The side grating-shaped waveguide 22 has a thickness (width) in the direction parallel to the PLC substrate that varies in the propagation direction of the optical signal so that the wavefront of the forward propagation field of the input field from the input end and the wavefront of the backward propagation field of the output field from the output end match.

[0032] That is, when the input modes are the LP01, LP11a, LP11b, LP21a, E13, and E31 modes, respectively, the ideal output modes are the E31, LP01, LP21a, LP11b, E13, and LP11a modes, and the side grating-shaped waveguide 22 is designed so that the wavefront of the forward propagation field and the wavefront of the backward propagation field match at all positions in the propagation direction of the optical signal (refer to the wavefront matching methods of Patent Document 2 and Non-Patent Document 2).

[0033] In FIG. 6, as the structural parameters of the side grating-shaped waveguide 22, the thickness (width) in the direction parallel to the PLC substrate is 10 μm ± variation width, the thickness (height) in the direction perpendicular to the PLC substrate is 10 μm, the length in the propagation direction of the optical signal is 10000 μm, and the relative refractive index difference Δ is 1.1%.

[0034] The design of the top grating-shaped waveguide 23 of the first embodiment is shown in FIG. 7. In the top grating-shaped waveguide 23, the thickness (height) in the direction perpendicular to the PLC substrate varies in the propagation direction of the optical signal so that the wavefront of the forward propagation field for the input field from the input end and the wavefront of the backward propagation field for the output field from the output end coincide.

[0035] That is, when the input modes are E31, LP01, LP21a, LP11b, E13, and LP11a modes, the top grating-shaped waveguide 23 is designed so that the wavefronts of the forward propagation field and the backward propagation field coincide at all positions in the propagation direction of the optical signal in a state where the ideal output modes are E31, E13, LP21a, LP11b, LP01, and LP11a modes, respectively (see the wavefront matching methods of Patent Document 2 and Non-Patent Document 2).

[0036] In FIG. 7, as the structural parameters of the top grating-shaped waveguide 23, the thickness (height) in the direction perpendicular to the PLC substrate is 10 μm ± variation width, the thickness (width) in the direction parallel to the PLC substrate is 10 μm, the length in the propagation direction of the optical signal is 10000 μm, and the relative refractive index difference Δ is 1.1%.

[0037] Note that, as the structural parameters of the side tapered waveguide 21, the thickness (width) in the direction parallel to the PLC substrate is tapered from 10 μm to 11 μm, the length in the propagation direction of the optical signal is 1000 μm, and the relative refractive index difference Δ is 1.1%. Also, as the structural parameters of the top tapered waveguide 24, the thickness (height) in the direction perpendicular to the PLC substrate is tapered from 11 μm to 10 μm, the length in the propagation direction of the optical signal is 50 μm, and the relative refractive index difference Δ is 1.1%.

[0038] The efficiency of the mode converter M2 of the first embodiment is shown in FIG. 8. The LP01 input mode is exchanged with the LP02 and LP21b output modes with a somewhat biased efficiency. The LP11a input mode is exchanged with the LP02 and LP21b output modes with a somewhat biased efficiency. The LP11b and LP21a input modes are each exchanged with the LP21a and LP11b output modes with an efficiency of approximately 100%. The LP02 input mode is exchanged with the LP01 and LP11a output modes with a somewhat biased efficiency. The LP21b input mode is exchanged with the LP01 and LP11a output modes with a somewhat biased efficiency.

[0039] The MDL of the mode converter M2 of the first embodiment is shown in FIG. 9. A multimode optical fiber with a total length of 150 km is divided into multimode optical fibers F-0, F-1, ···, F-N of equal length, and the mode converters M2-1, M2-2, ···, M2-N shown in FIG. 4 are inserted. When no mode converter was used, an MDL of about 12 dB was measured. In the prior art, when one mode converter M1 was used, an MDL of about 8 dB was measured. On the other hand, in the first embodiment, when one mode converter M2 was used, an MDL of about 5 dB was measured, and a reduction in MDL of about 3 dB was achieved (see the MDL calculation method in Non-Patent Document 3).

[0040] In this way, by solving the problem that a certain input mode is not exchanged for another output mode in the mode converter M2 on the PLC substrate, it becomes easy to reduce the MDL and it is possible to eliminate the need to introduce a large number of mode converters M2.

[0041] And in the side grating waveguide 22 and the top grating waveguide 23, it is possible to design so as to perform the exchange between desired rectangular waveguide modes. Furthermore, even with only one mode converter M2, since it becomes easy to reduce the MDL, it is possible to eliminate the need to introduce a large number of mode converters M2.

[0042] In the first embodiment, on the input side of the mode converter M2, the PLC substrate is provided with a waveguide whose thickness (width) in the parallel direction varies in the optical signal propagation direction, and on the output side of the mode converter M2, the PLC substrate is provided with a waveguide whose thickness (height) in the vertical direction varies in the optical signal propagation direction. In a modified example, on the input side of the mode converter M2, the PLC substrate may be provided with a waveguide whose thickness (height) in the vertical direction varies in the optical signal propagation direction, and on the output side of the mode converter M2, the PLC substrate may be provided with a waveguide whose thickness (width) in the parallel direction varies in the optical signal propagation direction.

[0043] In the first embodiment, a glass-based material is applied as the waveguide of the mode converter M2, and a communication wavelength band (about 1.3 μm to 1.7 μm, particularly 1.55 μm) is applied as the propagation wavelength of the multimode optical fiber. In a modified example, a semiconductor such as Si or InGaAsP or an organic substance such as a polymer may be applied as the waveguide of the mode converter M2, and a mid-infrared region (2 μm or more) or a visible light band or the like may be applied as the propagation wavelength of the multimode optical fiber.

[0044] (Configuration of the mode converter of the second embodiment) The configuration of the mode converter M3 of the second embodiment is shown in FIG. 10. The processing of the mode converter M3 of the second embodiment is shown in FIG. 11. The mode converter M3 includes a tapered waveguide 31 (newly added), a tapered waveguide 32 (newly added), a grating waveguide 33, a grating waveguide 34 (newly added), a tapered waveguide 35 (newly added), and a tapered waveguide 36 (newly added) in this order in the optical signal propagation direction. Here, the mode converter M3 of the second embodiment is manufactured as follows. First, the tapered waveguide 31, the tapered waveguide 32, the grating waveguide 33, the grating waveguide 34, the tapered waveguide 35, and the tapered waveguide 36 are fabricated by the waveguide element manufacturing method on a normal PLC substrate, that is, all are separately fabricated as side-tapered waveguides and side-grating waveguides. Next, the grating waveguide 34, the tapered waveguide 35, and the tapered waveguide 36 are rotated 90° with the optical signal propagation direction as the rotation axis and butt-jointed and coupled with the tapered waveguide 31, the tapered waveguide 32, and the grating waveguide 33.

[0045] The thickness (width) of the tapered waveguide 31 in the direction parallel to the PLC substrate varies in the optical signal propagation direction. And it exchanges the optical fiber mode to the rectangular waveguide mode. Specifically, the LP01, LP11a, LP11b, and LP21a modes are respectively transmitted as the LP01, LP11a, LP11b, and LP21a modes. And the LP21b and LP02 modes are respectively exchanged to the E13 and E31 modes (without mode mixing).

[0046] The tapered waveguide 32 has a thickness (width) in a direction parallel to the PLC substrate that varies in the propagation direction of the optical signal. Then, a rectangular waveguide mode having three or more peaks in a direction parallel or perpendicular to the PLC substrate is exchanged for a rectangular waveguide mode having three or more peaks in a direction perpendicular or parallel to the PLC substrate, respectively. Specifically, the E13 and E31 modes are exchanged for the E31 and E13 modes, respectively. Here, the E13 mode is the mode having three peaks in the above-mentioned vertical direction, and the E31 mode is the mode having three peaks in the above-mentioned parallel direction. Then, the LP01, LP11a, LP11b, and LP21a modes are transmitted through the LP01, LP11a, LP11b, and LP21a modes, respectively.

[0047] The grating waveguide 33 has a thickness (width) in a direction parallel to the PLC substrate that varies in the propagation direction of the optical signal. Then, an exchange between desired rectangular waveguide modes is performed. Specifically, the LP01, LP11a, LP11b, LP21a, and E31 modes are exchanged for the E31, LP01, LP21a, LP11b, and LP11a modes, respectively. However, the E13 mode cannot be exchanged for other modes and is transmitted through the E13 mode.

[0048] The grating waveguide 34 has a thickness (height) in a direction perpendicular to the PLC substrate that varies in the propagation direction of the optical signal. Then, an exchange is performed between the rectangular waveguide mode that is not exchanged in the grating waveguide 33 and other rectangular waveguide modes. Specifically, the LP01 and E13 modes are exchanged for the E13 and LP01 modes, respectively. Here, the E13 mode is the above-mentioned non-exchanged rectangular waveguide mode, and the LP01 mode is the above-mentioned other rectangular waveguide mode. Then, the E31, LP21a, LP11b, and LP11a modes are transmitted through the E31, LP21a, LP11b, and LP11a modes, respectively.

[0049] The tapered waveguide 35 has a thickness (height) perpendicular to the PLC substrate that varies in the propagation direction of the optical signal. Then, a rectangular waveguide mode having three or more peaks in the direction parallel or perpendicular to the PLC substrate is exchanged for a rectangular waveguide mode having three or more peaks in the direction perpendicular or parallel to the PLC substrate, respectively. Specifically, the E31 and E13 modes are exchanged for the E13 and E31 modes, respectively. Here, the E31 mode is the mode having three peaks in the above-mentioned parallel direction, and the E13 mode is the mode having three peaks in the above-mentioned perpendicular direction. Then, the LP21a, LP11b, LP11a, and LP01 modes are each transmitted as the LP21a, LP11b, LP11a, and LP01 modes.

[0050] The tapered waveguide 36 has a thickness (height) perpendicular to the PLC substrate that varies in the propagation direction of the optical signal. Then, the rectangular waveguide mode is exchanged for the optical fiber mode. Specifically, the LP21a, LP11b, LP11a, and LP01 modes are each transmitted as the LP21a, LP11b, LP11a, and LP01 modes. Then, the E13 and E31 modes are exchanged for the LP02 and LP21b modes, respectively (without mode mixing).

[0051] The design of the grating-shaped waveguide 33 of the second embodiment is shown in FIGS. 12 and 13. The grating-shaped waveguide 33 has a thickness (width) in the direction parallel to the PLC substrate that varies in the propagation direction of the optical signal so that the wavefront of the forward propagation field for the input field from the input end and the wavefront of the backward propagation field for the output field from the output end coincide.

[0052] That is, when the input modes are the LP01, LP11a, LP11b, LP21a, E31, and E13 modes, the grating-shaped waveguide 33 is designed so that the wavefronts of the forward propagation field and the backward propagation field coincide at all positions in the propagation direction of the optical signal in a state where the ideal output modes are the E31, LP01, LP21a, LP11b, LP11a, and E13 modes, respectively (refer to the wavefront matching methods of Patent Document 2 and Non-Patent Document 2).

[0053] In Fig. 12, as the structural parameters of the grating waveguide 33, the thickness (width) in the direction parallel to the PLC substrate is 10 μm ± variation width, the thickness (height) in the direction perpendicular to the PLC substrate is 10 μm, the length in the optical signal propagation direction is 16000 μm, and the relative refractive index difference Δ is 1.1%. The length L in the optical signal propagation direction grt1 is set to 16000 μm because, as shown in Fig. 13, the transmission efficiency from the E13 mode to the E13 mode is high (the loss is small).

[0054] The design of the grating waveguide 34 of the second embodiment is shown in Figs. 14 and 15. The thickness (height) in the direction perpendicular to the PLC substrate of the grating waveguide 34 varies in the optical signal propagation direction so that the wavefront of the forward propagation field for the input field from the input end and the wavefront of the backward propagation field for the output field from the output end coincide.

[0055] That is, when the input modes are the E31, LP01, LP21a, LP11b, LP11a, and E13 modes, the grating waveguide 34 is designed so that the wavefronts of the forward propagation field and the backward propagation field coincide at all positions in the optical signal propagation direction in a state where the ideal output modes are the E31, E13, LP21a, LP11b, LP11a, and LP01 modes, respectively (refer to the wavefront matching methods of Patent Document 2 and Non-Patent Document 2).

[0056] In Fig. 14, as the structural parameters of the grating waveguide 34, the thickness (height) in the direction perpendicular to the PLC substrate is 10 μm ± variation width, the thickness (width) in the direction parallel to the PLC substrate is 10 μm, the length in the optical signal propagation direction is 16000 μm, and the relative refractive index difference Δ is 1.1%. The length L in the optical signal propagation direction grt2 is set to 16000 μm because, as shown in Fig. 15, the exchange efficiency from the E13 / E11 mode to the E11 / E13 mode is high.

[0057] The designs of the tapered waveguides 31 and 36 of the second embodiment are shown in FIGS. 16 and 17. The tapered waveguide 36 is obtained by rotating the tapered waveguide 31 by 90° with the propagation direction of the optical signal as the rotation axis. In FIGS. 16 and 17, the tapered waveguide 31 will be described.

[0058] In FIG. 16, the tapered waveguide 31 includes the following elements in this order in the propagation direction of the optical signal: (1) a taper in which the thickness (width) in the direction parallel to the PLC substrate varies from W to W over the length L in the propagation direction of the optical signal 1 over a length L of 1 from W to tp1 W, and (2) a taper in which the thickness (width) in the direction parallel to the PLC substrate varies from W to W over the length L in the propagation direction of the optical signal. 2 over a length L of tp1 from W to tp2 W.

[0059] In FIG. 17, with W 1 = 10.0 μm, W tp1 = 10.2 μm, W tp2 = 11.0 μm, L 2 = 2000 μm, the thickness (height) H = 10.0 μm in the direction perpendicular to the PLC substrate, and the relative refractive index difference Δ = 1.1%, L 1 is explored in the range of 3000 μm to 16000 μm.

[0060] In FIG. 17, it is desirable that the tapered waveguide 31 exchanges the LP21b and LP02 modes into the E13 and E31 modes, respectively. Here, the conversion efficiency from the LP02 mode to the E31 mode increases in the range of L 1 = 3000 μm to 16000 μm, and the conversion efficiency from the LP02 mode to the E13 mode decreases in the range of L 1 = 3000 μm to 16000 μm. Therefore, L 1 is set to 5000 μm so as to minimize the MDL shown in FIGS. 20 and 21.

[0061] The designs of the tapered waveguides 32 and 35 of the second embodiment are shown in FIGS. 18 and 19. The tapered waveguide 35 is obtained by rotating the tapered waveguide 32 by 90° around the propagation direction of the optical signal as the rotation axis. In FIGS. 18 and 19, the tapered waveguide 32 will be described.

[0062] In FIG. 18, the tapered waveguide 32 includes the following elements in this order in the propagation direction of the optical signal: (1) a taper in which the thickness (height) in the direction perpendicular to the PLC substrate varies from W to W over the length L in the propagation direction of the optical signal 3 over a length L, (2) a taper in which the thickness (height) in the direction perpendicular to the PLC substrate varies from W to W over the length L in the propagation direction of the optical signal 2 over a length L, (3) a taper in which the thickness (height) in the direction perpendicular to the PLC substrate varies from W to W over the length L in the propagation direction of the optical signal tp3 over a length L, (4) a non-taper in which the thickness (height) in the direction perpendicular to the PLC substrate maintains W over the length L in the propagation direction of the optical signal, (5) a taper in which the thickness (height) in the direction perpendicular to the PLC substrate varies from W to W over the length L in the propagation direction of the optical signal. However, W in (1) and (5) has the same thickness (height). 4 over a length L, (3) a taper in which the thickness (height) in the direction perpendicular to the PLC substrate varies from W to W over the length L in the propagation direction of the optical signal tp3 over a length L, (4) a non-taper in which the thickness (height) in the direction perpendicular to the PLC substrate maintains W over the length L in the propagation direction of the optical signal, (5) a taper in which the thickness (height) in the direction perpendicular to the PLC substrate varies from W to W over the length L in the propagation direction of the optical signal. However, W in (1) and (5) has the same thickness (height). tp4 over a length L, (3) a taper in which the thickness (height) in the direction perpendicular to the PLC substrate varies from W to W over the length L in the propagation direction of the optical signal 5 over a length L, (4) a non-taper in which the thickness (height) in the direction perpendicular to the PLC substrate maintains W over the length L in the propagation direction of the optical signal, (5) a taper in which the thickness (height) in the direction perpendicular to the PLC substrate varies from W to W over the length L in the propagation direction of the optical signal. However, W in (1) and (5) has the same thickness (height). tp4 over a length L, (4) a non-taper in which the thickness (height) in the direction perpendicular to the PLC substrate maintains W over the length L in the propagation direction of the optical signal, (5) a taper in which the thickness (height) in the direction perpendicular to the PLC substrate varies from W to W over the length L in the propagation direction of the optical signal. However, W in (1) and (5) has the same thickness (height). tp5 over a length L, (4) a non-taper in which the thickness (height) in the direction perpendicular to the PLC substrate maintains W over the length L in the propagation direction of the optical signal, (5) a taper in which the thickness (height) in the direction perpendicular to the PLC substrate varies from W to W over the length L in the propagation direction of the optical signal. However, W in (1) and (5) has the same thickness (height). st over a length L, (4) a non-taper in which the thickness (height) in the direction perpendicular to the PLC substrate maintains W over the length L in the propagation direction of the optical signal, (5) a taper in which the thickness (height) in the direction perpendicular to the PLC substrate varies from W to W over the length L in the propagation direction of the optical signal. However, W in (1) and (5) has the same thickness (height). tp5 over a length L, (4) a non-taper in which the thickness (height) in the direction perpendicular to the PLC substrate maintains W over the length L in the propagation direction of the optical signal, (5) a taper in which the thickness (height) in the direction perpendicular to the PLC substrate varies from W to W over the length L in the propagation direction of the optical signal. However, W in (1) and (5) has the same thickness (height). tp3 over a length L, (4) a non-taper in which the thickness (height) in the direction perpendicular to the PLC substrate maintains W over the length L in the propagation direction of the optical signal, (5) a taper in which the thickness (height) in the direction perpendicular to the PLC substrate varies from W to W over the length L in the propagation direction of the optical signal. However, W in (1) and (5) has the same thickness (height). tp5 over a length L, (4) a non-taper in which the thickness (height) in the direction perpendicular to the PLC substrate maintains W over the length L in the propagation direction of the optical signal, (5) a taper in which the thickness (height) in the direction perpendicular to the PLC substrate varies from W to W over the length L in the propagation direction of the optical signal. However, W in (1) and (5) has the same thickness (height). 2 over a length L. However, W in (1) and (5) has the same thickness (height). 2 has the same thickness (height).

[0063] In FIG. 19, with W = 11.0 μm, W = 10.4 μm, W = 9.8 μm, W = 9.0 μm, L = 100 μm, L = 500 μm, L = 300 μm, L = 300 μm, the horizontal thickness (width) H of the PLC substrate being 10.0 μm, and the relative refractive index difference Δ being 1.1%, L is explored in the range of 2000 μm to 17000 μm. 2 = 11.0 μm, W tp3 = 10.4 μm, W tp4 = 9.8 μm, W tp5 = 9.0 μm, L 3 = 100 μm, L 5 = 500 μm, L st = 300 μm, L tp3 = 300 μm, the horizontal thickness (width) H of the PLC substrate being 10.0 μm, and the relative refractive index difference Δ being 1.1%, L 4 is explored in the range of 2000 μm to 17000 μm.

[0064] In FIG. 19, it is desirable that the tapered waveguide 32 exchanges the E13 and E31 modes to the E31 and E13 modes, respectively. Here, the conversion efficiency from the E31 mode to the E13 mode becomes high at L 4 = 7000 μm to 13000 μm, and the transmission efficiency from the E31 mode to the E31 mode becomes low at L 4 = 7000 μm to 13000 μm. Therefore, in order to minimize the MDL shown in FIGS. 20 and 21, L 4 is set to 12000 μm.

[0065] The MDL of the mode converter M3 of the second embodiment is shown in FIGS. 20 and 21. In FIG. 20, in order to reduce the minimum MDL using only one mode converter M3, the length L grt1 in the propagation direction of the optical signal of the grating waveguide 33 is searched for from 6000 μm to 20000 μm, and the length L grt2 in the propagation direction of the optical signal of the grating waveguide 34 is searched for from 10000 μm to 20000 μm (see the MDL calculation method of Non-Patent Document 3).

[0066] In order to make the minimum MDL 3 dB or less using only one mode converter M3, it is desirable that the length L grt1 in the propagation direction of the optical signal of the grating waveguide 33 be 10000 μm or more and 20000 μm or less, and it is desirable that the length L grt2 in the propagation direction of the optical signal of the grating waveguide 34 be 14000 μm or more and 20000 μm or less.

[0067] In order to make the minimum MDL 2.5 dB or less using only one mode converter M3, it is desirable that the length L grt1 in the propagation direction of the optical signal of the grating waveguide 33 be 16000 μm or more and 20000 μm or less, and it is desirable that the length L grt2 in the propagation direction of the optical signal of the grating waveguide 34 be in the vicinity of 16000 μm.

[0068] In the upper part of Fig. 21, a multimode optical fiber with a total length of 150 km is divided into multimode optical fibers F-0, F-1, ···, F-N of equal length, and mode converters M3-1, M3-2, ···, M3-N shown in Fig. 10 are inserted (N may be 1).

[0069] In the middle part of Fig. 21, when no mode converter is used, a minimum MDL of about 12 dB was measured. In the prior art, when only one mode converter M1 was used, a minimum MDL of about 8 dB was measured. On the other hand, in the second embodiment, when only one mode converter M3 was used, a minimum MDL of 2.39 dB was measured, and a reduction of 1.60 dB in the system MDL was achieved (refer to the MDL calculation method in Non-Patent Document 3).

[0070] In the lower part of Fig. 21, when no mode converter is used, an average absolute loss of about -28.5 dB was measured. In the prior art, when two mode converters M1 were used, an average absolute loss of about -29.5 dB was measured. On the other hand, in the second embodiment, when only one mode converter M3 was used, an average absolute loss of about -29.5 dB was measured. In order to achieve a similar average absolute loss, only one mode converter M3 needs to be arranged.

[0071] In this way, by solving the problem that a certain input mode is not exchanged for another output mode in the mode converter M3 on the PLC board, it becomes easier to reduce the MDL and it is possible to eliminate the need to introduce a large number of mode converters M3. And by preventing a certain input mode from being mixed with another input mode in the mode converter M3 on the PLC board, it becomes even easier to reduce the MDL and it is possible to eliminate the need to introduce a large number of mode converters M3.

[0072] In the grating-shaped waveguides 33 and 34, it is possible to design them to perform the exchange between desired rectangular waveguide modes. Furthermore, even with only one mode converter M3, it is possible to eliminate the need to introduce a large number of mode converters M3 in order to facilitate the reduction of MDL.

[0073] In the second embodiment, on the input side of the mode converter M3, a waveguide is provided in which the thickness (width) in the direction parallel to the PLC substrate varies in the optical signal propagation direction, and on the output side of the mode converter M3, a waveguide is provided in which the thickness (height) in the direction perpendicular to the PLC substrate varies in the optical signal propagation direction. In a modified example, on the input side of the mode converter M3, a waveguide may be provided in which the thickness (height) in the direction perpendicular to the PLC substrate varies in the optical signal propagation direction, and on the output side of the mode converter M3, a waveguide may be provided in which the thickness (width) in the direction parallel to the PLC substrate varies in the optical signal propagation direction.

[0074] In the second embodiment, a glass-based material is applied as the waveguide of the mode converter M3, and a communication wavelength band (about 1.3 μm to 1.7 μm, particularly 1.55 μm) is applied as the propagation wavelength of the multimode optical fiber. In a modified example, a semiconductor such as Si or InGaAsP or an organic substance such as a polymer may be applied as the waveguide of the mode converter M3, and a mid-infrared region (2 μm or more) or a visible light band or the like may be applied as the propagation wavelength of the multimode optical fiber.

Industrial Applicability

[0075] The mode converter of the present disclosure facilitates the reduction of MDL by solving the problem that a certain input mode is not exchanged for another output mode when performing the exchange between a plurality of modes, and it is possible to eliminate the need to introduce a large number of mode converters.

Explanation of Reference Numerals

[0076] M1: Mode converter 11: Tapered waveguide 12: Grating-shaped waveguide 13: Tapered waveguide M2, M2-1, M2-2, M2-N: Mode converter F-0, F-1, F-N: Multimode optical fiber 21: Side-tapered waveguide 22: Side-grating waveguide 23: Top-grating waveguide 24: Top-tapered waveguide M3, M3-1, M3-2, M3-N: Mode converter F-0, F-1, F-N: Multimode optical fiber 31: Tapered waveguide 32: Tapered waveguide 33: Grating waveguide 34: Grating waveguide 35: Tapered waveguide 36: Tapered waveguide

Claims

1. A mode converter on a PLC substrate that is inserted into a multimode optical fiber through which an optical signal having a plurality of modes propagates and performs an exchange between the plurality of modes, a first tapered waveguide and a first grating-shaped waveguide in which the thickness in one of the parallel direction and the perpendicular direction of the PLC substrate varies in the propagation direction of the optical signal and the thickness in the other of the parallel direction and the perpendicular direction of the PLC substrate is constant; a second grating-shaped waveguide and a second tapered waveguide in which the thickness in the other direction varies in the propagation direction of the optical signal and the thickness in the one direction is constant; The mode converter is characterized by comprising these in this order in the propagation direction of the optical signal.

2. The first tapered waveguide exchanges an optical fiber mode into a rectangular waveguide mode, the first grating-shaped waveguide performs an exchange between desired rectangular waveguide modes, the second grating-shaped waveguide performs an exchange between a rectangular waveguide mode that is not exchanged by the first grating-shaped waveguide and other rectangular waveguide modes, and the second tapered waveguide exchanges the rectangular waveguide mode into an optical fiber mode. The mode converter according to claim 1, characterized by the above.

3. A third tapered waveguide that is inserted between the first tapered waveguide and the first grating-shaped waveguide and in which the thickness in the one direction varies in the propagation direction of the optical signal and the thickness in the other direction is constant; A fourth tapered waveguide that is inserted between the second grating-shaped waveguide and the second tapered waveguide and in which the thickness in the other direction varies in the propagation direction of the optical signal and the thickness in the one direction is constant; The mode converter according to claim 1 or 2, further characterized by comprising these.

4. The third tapered waveguide and the fourth tapered waveguide each exchange a rectangular waveguide mode having three or more peaks in the one direction or the other direction into a rectangular waveguide mode having three or more peaks in the other direction or the one direction. The mode converter according to claim 3, characterized by the above.

5. The thicknesses of the first grating-shaped waveguide and the second grating-shaped waveguide in the one direction and the other direction, respectively, vary in the propagation direction of the optical signal such that the wavefront of the forward-propagating field with respect to the input field from each input end and the wavefront of the backward-propagating field with respect to the output field from each output end coincide with each other. The mode converter according to any one of claims 1 to 4, characterized in that. **Claim 6** The length of the first grating-shaped waveguide in the propagation direction of the optical signal is 10,000 μm or more and 20,000 μm or less, and the length of the second grating-shaped waveguide in the propagation direction of the optical signal is 14,000 μm or more and 20,000 μm or less. The mode converter according to any one of claims 1 to 5, characterized in that.

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