Optical waveguide device and optical communication system including the same

Abstract

The present invention relates to an optical waveguide device capable of performing single-mode connection between cores to be connected by controlling the intensity variation of an optical signal caused by MPI. The optical waveguide device has a first device end face, a second device end face, a waveguide, and a cladding. The waveguide has a first waveguide end face and a second waveguide end face, and guides light of multiple modes with different orders. In addition, the waveguide has one or more bent portions. The cladding has a refractive index lower than that of the waveguide. The waveguide has a waveguide length L of 5×10<supgt;6< / supgt> [nm] or more and 100×10<supgt;6< / supgt> [nm] or less, and has a structure in which the inter-mode group delay time difference Δβ1 satisfies the condition given by |Δβ1|≤1 / 2×10<supgt;-12< / supgt> [s] / L.

Description

Technical Field

[0001] The present invention relates to an optical waveguide device and an optical communication system including the same.

[0002] This application claims priority based on Japanese Patent Application No. 2020-038906 filed on March 6, 2020, and the content thereof is incorporated herein by reference in its entirety. Background Art

[0003] As one of the optical waveguide devices, for example, as disclosed in Patent Document 1 and Patent Document 2, there is known a FIFO (Fan-in / Fan-out) device that changes the pitch of a plurality of arranged waveguides (cores) in the light propagation direction. By using the above-mentioned FIFO

[0004] device, it is possible to connect cores with different core pitches, for example, each core of a plurality of single-core optical fibers (hereinafter referred to as "SCF") arranged side by side on the same plane and the corresponding cores of a multi-core optical fiber (hereinafter referred to as "MCF") with low loss.

[0005] In addition, Patent Document 1 discloses a FIFO device having a three-dimensional waveguide structure and a mode field diameter (hereinafter referred to as "MFD") conversion function. Further, Patent Document 2 discloses a structure that suppresses connection loss between cores to be connected by a lens.

[0006] Patent Document 1: International Publication WO2018-135411

[0007] Patent Document 2: Japanese Unexamined Patent Application Publication No. 2014-178628 Summary of the Invention

[0008] The optical waveguide device of the present invention has a first device end face, a second device end face opposite to the first device end face, a waveguide, and a cladding. The waveguide has a first waveguide end face that coincides with the first device end face and a second waveguide end face that coincides with the second device end face, and guides light of a plurality of modes with different orders. In addition, the waveguide is provided with one or more bending portions on the optical path from the first waveguide end face to the second waveguide end face. The cladding has the waveguide provided inside or on the surface. The cladding has a refractive index lower than that of the waveguide. In particular, the waveguide has a waveguide length L of 5

[0009] ×10 6 [nm] or more and 100×10 6 [nm] or less, and the group delay time difference (DMD: Differential Mode Delay, hereinafter referred to as "inter-mode group delay time difference") Δβ1 between a plurality of modes satisfies the following formula:

[0010] |Δβ1| ≤ 1 / 2×10 －12 [s] / L

[0011] The structure of the given conditions. Description of the Drawings

[0012] Figure 1 It is a diagram for explaining a structural example (system structure) of the optical communication system of the present invention.

[0013] Figure 2 It is a diagram for explaining a structural example (device structure) of the optical waveguide device of the present invention.

[0014] Figure 3 It is a diagram for explaining the control operation of the optical signal intensity variation caused by MPI.

[0015] Figure 4 It is a curve showing the relationship between the intermodal group delay time difference Δβ1 (absolute value) for various modulation speeds and the number of divisions N. Detailed Description of the Invention

[0016] [Problems to be Solved by the Present Invention]

[0017] An object of the present invention is to provide a solution to the problems that are inevitable in the structure of existing FIFO devices, namely, interference between low-order modes and high-order modes within the frequency band required for signal modulation in optical signal transmission (Multipass Interference, hereinafter referred to as "MPI")

[0018] By controlling the optical signal intensity variation caused by MPI, an optical waveguide device capable of performing single-mode connection between the cores of a pair of optical fibers to be connected and an optical communication system including the optical waveguide device can be provided.

[0019] [Effects of the Present Invention]

[0020] According to the optical waveguide device of the present invention, it is possible to control the optical signal intensity variation caused by MPI within the required frequency band during signal modulation in optical signal transmission. As a result, it is possible to perform single-mode connection between the cores of a pair of optical fibers to be connected.

[0021] First, the structural characteristics required for optical waveguide devices such as FIFO devices will be described.

[0022] In a FIFO device, the pitch at which a plurality of cores (waveguides) are arranged is changed in the light propagation direction, so each core usually has a bent structure. Therefore, bending loss occurs during the propagation of light in the core within this FIFO device. Thus, in order to connect between a pair of optical fibers (substantially between cores corresponding one-to-one) with low loss, it is necessary to suppress the bending loss. In addition, within the FIFO device, the minimum pitch between adjacent cores is 50 μm or less, and the increase in crosstalk between adjacent cores becomes a noise factor for optical signal transmission. Therefore, it is also necessary to suppress crosstalk.

[0023] If the suppression of the above-mentioned bending loss and crosstalk is studied, it is effective to increase the relative refractive index difference between the core and the cladding within this FIFO device (for example, at a wavelength of 589 [nm], the relative refractive index difference of the core based on the refractive index of the cladding) and strengthen the confinement of light in the core. However, an increase in the relative refractive index difference Δ of the core causes a decrease in the MFD (mode field diameter) of this FIFO device and an increase in the coupling loss with a single-mode optical fiber (SMF). Therefore, in this FIFO device, along with an increase in the relative refractive index difference Δ of the core, it is necessary to increase the core width (or core diameter).

[0024] However, when increasing the relative refractive index difference Δ of the core and expanding the core width, not only the fundamental mode that is desired to guide light can guide light, but also the high-order modes that are not desired to guide light can guide light. In optical signal transmission, the light transmission of this high-order mode usually becomes a problem. That is, in an optical waveguide device such as a FIFO device for single-mode transmission, when the structure allows the high-order mode to guide light, the occurrence of interference (MPI) between the fundamental mode and the high-order mode becomes a problem.

[0025] Therefore, the waveguide device of the present invention does not actively eliminate the above-mentioned MPI, but controls it in such a way as to reduce the light signal intensity variation caused by MPI within the frequency band required for signal modulation, thereby ensuring single-mode connection between a pair of optical fibers to be connected.

[0026] [Description of Embodiments of the Present Invention]

[0027] First, the content of the embodiments of the present invention will be separately listed and described.

[0028] (1) The optical waveguide device of the present invention is for a frequency of 25×10 9 [Hz] or more and 1000×10 9An optical device for single-mode transmission of an optical signal of wavelength λ0, modulated at a modulation speed defined by an arbitrary frequency below [Hz]. Specifically, the optical waveguide device, as one mode thereof, has a first device end face, a second device end face opposite to the first device end face, a waveguide, and a cladding. The waveguide has a first waveguide end face consistent with the first device end face and a second waveguide end face consistent with the second device end face, and guides light of multiple modes of different orders. In addition, the waveguide is provided with one or more bends on the optical path from the first waveguide end face to the second waveguide end face. The cladding is provided with a waveguide inside or on the surface.

[0029] The cladding has a refractive index lower than that of the waveguide. In particular, the waveguide has a refractive index of 5×10 6 [nm]

[0030] Above and 100×10 6 The waveguide length L is less than [nm], and has a waveguide length L that satisfies the following equation:

[0031] |Δβ1|≤1 / (N·2Δf·L)

[0032] In addition, the waveguide length L is defined by the optical path length from the first waveguide end face to the second waveguide end face. In addition, in the above formula, the parameter

[0033] "Δβ1" is the group delay time difference between modes. The parameter "N" is an integer value for dividing the vibration period Tf of the optical frequency corresponding to the optical signal intensity variation caused by MPI between multiple modes, and the number of divisions is defined by an arbitrary integer greater than 10 and less than 100. Parameter

[0034] "Δf" is calculated by 25×10 9 [Hz] or more and 1000×10 9 The modulation speed is defined at an arbitrary frequency below [Hz] (for example, Δf=25 [GHz] is equivalent to 25 [GBaud]).

[0035] (2) As one embodiment of the present invention, the curved portion preferably has a curvature radius r of 40 mm or less. The optical waveguide device can be applied to a FIFO (Fan-In / Fan-Out) device, and can perform optical connection between optical fibers with different core pitches. However, the waveguide length L is preferably 5×10 6 [nm] or more so that the bending loss does not become too large (the curvature radius r does not become too small). On the other hand, in order to ensure the ease of handling of the device, the waveguide length L is preferably 100×10 6 [nm] or less.

[0036] (3) As one aspect of the optical communication system of the present invention, it has at least a pair of optical fibers and an optical waveguide device (the optical waveguide device of the present invention) having the above-described structure provided between the pair of optical fibers. By applying the optical waveguide device having the above-described structure, the pair of optical fibers can perform single-mode connection between the cores.

[0037] (4) As one aspect of the present invention, it is preferable that in this optical communication system, each of the pair of optical fibers arranged with the optical waveguide device therebetween includes a single-core optical fiber (hereinafter denoted as "SCF") or a multi-core optical fiber (hereinafter denoted as "MCF"). In particular, when one of the pair of optical fibers includes an MCF, efficient use of the wiring space of a base station or the like can be achieved.

[0038] As described above, each of the aspects listed in the column of [Description of the Embodiment of the Present Invention] can be applied to each of the remaining aspects, or can be applied to all combinations of these remaining aspects.

[0039] [Detailed Content of the Embodiment of the Present Invention]

[0040] Hereinafter, with reference to the drawings, the specific structures of the optical waveguide device and the optical communication system according to the embodiment of the present invention will be described in detail. In addition, the present invention is not limited to these examples, but is shown by the claims, and is intended to include all changes within the meaning and scope equivalent to the claims. In addition, in the description of the drawings, the same reference numerals are given to the same elements and repeated description is omitted.

[0041] Figure 1 is a diagram for explaining a structural example (system structure) of the optical communication system of the present invention. In Figure 1 the upper part, a general structural example of the optical communication system of the present invention is shown. In Figure 1 the middle part, a structural example of the fusion splice extension type optical waveguide device 50A that can be applied as the FIFO device of this optical communication system is shown. In addition, in Figure 1 the lower part, a structural example of the PLC (Planar Lightwave Circuit) type optical waveguide device 50B that can be applied as the FIFO device of this optical communication system is shown. In addition, the FIFO device of the optical communication system of the present invention may also be an optical waveguide device in which an optical circuit is three-dimensionally arranged.

[0042] This optical communication system includes: a plurality of optical transmitters (hereinafter denoted as "TX") 10; a plurality of optical receivers (hereinafter denoted as "RX") 20; an MCF (multi-core optical fiber) 40; a pair of FIFOs

[0043] Device 50 (the FIFO device located between multiple TXs 10 and MCF 40 is denoted as the "input-side FIFO device", and the FIFO device located between MCF 40 and multiple RXs 20 is denoted as the "output-side FIFO device"); and multiple SCFs (single-core fibers) 30 respectively arranged between multiple TXs 10 and the input-side FIFO device and between the output-side FIFO device and multiple RXs 20. The TX10 modulates optical signals at wavelengths such as 1310 [nm] and 1550 [nm] at any modulation speed Δf above 25×10 9 [Hz] and below 1000×10 9 [Hz] (for example, 25×10

[0044] (such as 25×10 9 [Hz], 50×10 9 [Hz], 100×10 9 [Hz], 200×

[0045] 10 9 [Hz], 500×10 9 [Hz], 1000×10 9 [Hz], etc.).

[0046] Among the multiple SCFs 30, the SCFs 30 respectively corresponding to multiple TXs 10 are arranged between multiple TXs 10 and the input-side FIFO device, and the SCFs 30 respectively corresponding to multiple RXs 20 are arranged between multiple RXs 20 and the output-side FIFO device. Optical signals are respectively output from multiple TXs 10, and multiple RXs 20 respectively receive the optical signals from multiple TXs 10. Therefore, the multiple SCFs 30 act as waveguides for respectively transmitting the optical signals from multiple TXs 10 to the input-side FIFO

[0047] devices, or act as waveguides for respectively transmitting the optical signals from the output-side FIFO device to multiple RXs 20. In addition, each of the multiple SCFs 30 has a single core extending along the central axis and a cladding surrounding the single core.

[0048] The MCF 40 arranged between a pair of FIFO devices 50 (input-side FIFO device and output-side FIFO device)

[0049] has multiple cores extending along the central axis and a common cladding surrounding each of the multiple cores. In addition, each of the pair of FIFO devices 50 is for making the cores of multiple SCFs

[0050] 30 respectively perform single-mode connection with the corresponding cores among the multiple cores of MCF 40

[0051] An optical device that transforms the waveguide pitch (core pitch) along the optical transmission direction.

[0052] In a pair of FIFO devices 50, for example, it can be applied Figure 1 The optical waveguide device

[0053] 50A shown in the middle. The optical waveguide device 50A has the following structure, that is, it is fused in a state where the front end portions of a plurality of SCFs 30 each having a core 31 and a cladding 32 are bundled, and

[0054] extends in such a way that the arrangement (pitch) of the cores at the end face 300 of the fused front end portion is consistent with the arrangement (pitch) of the cores of the MCF 40. The MCF 40 also has a plurality of cores 41 and a common cladding 42. At the end face of the optical waveguide device 50A (substantially the end face 300 of the SCF 30), each core 41 of the MCF 40 and each core 31 of the plurality of SCFs 30 are optically connected (for example, fusion-connected). Among the plurality of SCFs 30 constituting the optical waveguide device 50A, the core 31 of the SCF 30 located at the periphery at the cross-section of the fusion portion forms a bent portion with a smaller radius of curvature r than the bent portion provided in the core of the SCF 30 located at the center at the cross-section of the fusion portion. In this optical waveguide device 50A, each core 31 is integrally surrounded by the cladding 32 of the SCF 30. In addition, in the case of this optical waveguide device 50A, the position where the pitch between adjacent cores 31 among the fusion portions of the SCF 30 starts to change corresponds to the first device end face, and the position of the end face 300 of the SCF 30 corresponds to the second device end face. The waveguide length L of this optical waveguide device 50A is preferably 10 to 100 mm in order to suppress excessive loss caused by a sharp change in the core diameter of the extended portion. The upper limit of the waveguide length can be 80×10 6

[0055] [nm], 60×10 6 [nm], 40×10 6 [nm] or 20×10 6 [nm].

[0056] In addition, Figure 1 The PLC-type optical waveguide device 50B shown in the lower part of

[0057] can also be applied to a pair of FIFO devices 50. The optical waveguide device 50B has: a first device end face that fixes the end face 300 of each SCF 30 each having a core 31 and a cladding 32; and

[0058] The second device end face fixes the end face 400 of the MCF 40. Between the first device end face and the second device end face, device inner cores (waveguides within the optical waveguide device 50B) for single-mode connecting the cores 31 of the respective SCFs 30 to the corresponding cores 41 of the MCF 40 are formed near the surface or inside the cladding. The cross-sectional shape of the device inner core can be any of circular, semi-circular, and rectangular. In particular, in the case where the device inner core is a rectangular waveguide having a rectangular cross-section, as an example, after forming a rectangular waveguide on the base layer of the cladding, an upper layer of the cladding is provided so as to cover the rectangular waveguide. The waveguide length L of the optical waveguide device 50B is preferably 5 to 20 mm from the viewpoint of ease of processing.

[0059] Figure 2 FIG. is a diagram for specifically describing an example of a PLC-type optical waveguide device 50B as a structural example (device structure) of the optical waveguide device of the present invention.

[0060] Figure 2 The PLC-type optical waveguide device 50B shown in the upper part of FIG. has: a first device end face 50a that fixes the end face 300 (refer to Figure 1 the lower part of FIG.) of the SCF 30 each having a core 31 and a cladding 32; and a second device end face 50b that fixes the end face 400 (refer to Figure 1 the lower part of FIG.) of the MCF 40. Between the first device end face 50a and the second device end face 50b, cores (waveguides) 51 for single-mode connecting the cores 31 of the SCF 30 to the corresponding cores 41 of the MCF 40 are formed near the surface or inside the cladding 52. The first end face (first waveguide end face) 51a of the core 51 coincides with the first device end face 50a

[0061] and the second end face (second waveguide end face) 51b of the core 51 coincides with the second device end face 50b

[0062] . The core 51 has at least one bent portion 55 provided between the first end face 51a and the second end face 51b, and the bent portion 55 has a curvature radius r of 40 mm or less. The relative refractive index difference of the core 51 with respect to the cladding 52 and the width of the core 51 are appropriately determined corresponding to the SCF 30 and the MCF 40. The lower limit of the relative refractive index difference of the core 51 with respect to the cladding 52 is, for example, 0.25%, 0.30%, or 0.35%. The upper limit is, for example, 0.6%, 0.7%, or 0.8%. In addition, the lower limit of the width of the core 51 is, for example, 6.5 μm, 7.0 μm, or 7.5 μm. The upper limit is, for example, 8.0 μm, 8.5 μm, or 9.0 μm. According to this structure, light of multiple modes with different orders is guided in the core 51. Preferably, the waveguide length L of the core 51 is 5×10 6 [nm]

[0063] The above is to ensure that the bending loss does not become excessive (the curvature radius r does not become too small). On the other hand, in order to ensure the ease of device processing, the waveguide length L is preferably 20×10 6

[0064] [nm] or less.

[0065] In addition, when the core 51 is formed on the surface of the cladding 52, the core 51 has a semicircular or rectangular cross-section. When the core 51 is formed within the cladding 52, the core 51 has a circular or rectangular cross-section. In any case, the width of the core 51 is given by the maximum width in the plane figure obtained by projecting the core 51 onto the surface of the cladding 52 along the direction orthogonal to the length direction of the core 51. In addition, when the core 51 is disposed within the cladding 52, the core 51 and the over cladding can be sequentially stacked on the under cladding, or the cladding 52 can be irradiated with a laser to draw in the cladding 52.

[0066] As Figure 2 shown in the lower part, generally, if light of a specified wavelength is input from one end face 510a of the waveguide 510 provided with the bending portion 550 having the curvature radius r, low-order mode light and high-order mode light are emitted during the transmission of the input light toward the other end face 510b with the waveguide length L. At this time, MPI occurs between the low-order mode and the high-order mode transmitted within the core 51. The occurrence of this MPI becomes a problem during signal modulation of the optical signal transmission (optical signal intensity fluctuations caused by MPI occur within the frequency band required for signal modulation). Therefore, the optical waveguide device of the present invention (equivalent to a pair of FIFO devices 50) is designed to enable single-mode connection between a pair of optical fibers that can optically connect the cores to each other.

[0067] Figure 3 is a diagram for explaining the control action for optical signal intensity fluctuations caused by MPI. In Figure 3 the upper part, the frequency dependence of the optical signal intensity caused by MPI is shown, and this frequency characteristic is characterized by the vibration period Tf with respect to the optical frequency of the optical signal intensity caused by MPI.

[0068] In the present invention, the influence of the interference mode (MPI) between the low-order mode and the high-order mode on the optical communication (modulation) system is quantified as the Figure 3 "variation rate of optical signal intensity fluctuations" shown in the middle and lower parts, and the condition that the "variation rate of optical signal intensity fluctuations" caused by this MPI does not become a problem within the required frequency band during signal modulation is defined as follows.

[0069] In an optical waveguide device having a waveguide length L [mm], the group delay time difference between the low-order mode and the high-order mode, that is, the inter-mode group delay time difference Δβ1 [s / nm] and the vibration period Tf [Hz] of the optical frequency with respect to the frequency dependence of the optical signal intensity variation caused by MPI ( Figure 3 in the upper part), the following first relational expression holds:

[0070] Tf [Hz] = 1 / (|Δβ1|·L)

[0071] Here, the group delay time β1 of each mode is given by the following formula (1):

[0072]

Formula 1

[0073]

[0074] λ0: signal wavelength

[0075] c: speed of light

[0076] . The group delay time given by the above formula (1) is the first-order differential value of the transmission constant β of the center frequency ω0 and is the reciprocal of the group velocity Vg.

[0077] In order to prevent the optical signal intensity variation caused by this MPI from becoming noise during signal modulation, the vibration period Tf [Hz] of the optical frequency with respect to MPI needs to be sufficiently larger than the frequency band 2Δf required for signal modulation (Δf is the rate of the modulation speed and is 25 [GHz] or more). Specifically, as Figure 3 shown in the middle part of, it is assumed that modulation is performed within the frequency range obtained by dividing the vibration period Tf [Hz] by an integer N (the frequency band 2Δf required for modulation is set to be within Tf / N). By increasing the number N of divisions as Figure 3 shown in the lower part of, the variation rate of the optical signal intensity variation caused by MPI can be arbitrarily suppressed. In addition, in Figure 3 the middle part of, the vertical axis shows the normalized intensity with the variation width of the optical signal intensity set to 1 (the maximum intensity is set to 0.5 and the minimum intensity is set to -0.5). That is, the modulation speed Δf

[0078] is set to satisfy the following second relational expression with respect to the vibration period Tf of the optical frequency and its number N of divisions:

[0079] 2×Δf [Hz] ≤ Tf [Hz] / N

[0080] . This means that at (f 0 centered at the frequency f 0 -Δf) [Hz] or higher and (f 0 +

[0081] The range below Δf)[Hz] defines the influence on the optical signal intensity variation caused by MPI. In other words, it means taking the optical signal intensity variation with the maximum variation width A max as the variation width A min of the optical signal intensity for processing.

[0082] For example, as shown in the lower part of Figure 3 , when the number of divisions N is 10, the maximum variation rate of the optical signal intensity variation caused by MPI is 31%. When the number of divisions N is 20, the maximum variation rate is 16%. When the number of divisions N is 50, the maximum variation rate is 6%. When the number of divisions N is 100, the maximum variation rate of the optical signal intensity variation caused by MPI is 3%.

[0083] And by substituting the above first relational expression into the above second relational expression, the following formula is obtained:

[0084] |Δβ1| ≤ 1 / (N·2Δf·L)

[0085] . In addition, "L" is the waveguide length, "Δβ1" is the inter-modal group delay time difference, "N"

[0086] is an integer value obtained by dividing the vibration period Tf with respect to the optical frequency, and is the number of divisions defined by any integer from 10 or more to 100 or less. "Δf" is a modulation speed defined by any frequency from 25×10 9

[0087] [Hz] or more to 1000×10 9 [Hz] or less (for example, Δf = 25 [GHz]

[0088] corresponds to 25 [GBaud]). In an optical waveguide device satisfying |Δβ1| ≤ 2×10

[0089] －12 [s] / L, for a signal with a modulation speed of Δf = 25×10 9 [Hz], the number of divisions can be set to 10 or more, and the intensity variation of the modulation signal can be suppressed to 31% or less with respect to the maximum optical signal intensity variation caused by MPI.

[0090] Figure 4 is a graph showing the relationship between the inter-modal group delay time difference Δβ1

[0091] (absolute value) and the number of divisions N for the above various modulation speeds. The prepared optical waveguide device has an SI (Step Index) type refractive index distribution that allows higher-order modes as a waveguide, and has a FIFO device with a core having a rectangular cross-section. In Figure 4The upper part shows the relationship between Δβ1 (absolute value) and the number of divisions N of an optical waveguide device having a waveguide length L of 10 mm. Figure 4 The lower part shows the relationship between Δβ1 (absolute value) and the number of divisions N of an optical waveguide device having a waveguide length L of 40 mm.

[0092] Specifically, in the prepared FIFO devices, the relative refractive index difference Δ between the core and the cladding is 0.5% in Sample 1 (FIFO device as an embodiment of the present invention) and Comparative Example 1, and 0.7% in Sample 2 (FIFO device as an embodiment of the present invention) and Comparative Example 2. The core width is 8 μm in Sample 1 and Comparative Example 1, and 8.5 μm in Sample 2 and Comparative Example 2. In all the prepared FIFO devices, the minimum core pitch is 35 μm, and the radius of curvature r of the bent portion provided in the core is 20 mm. In addition, the waveguide length L of the core is 10 mm in Sample 1 and Sample 2 ( Figure 4 upper part), and 40 mm in Comparative Example 1 and Comparative Example 2 ( Figure 4 lower part). For this FIFO device, an optical signal with an input wavelength λ0 = 1310

[0093] [nm] is input.

[0094] Furthermore, in the FIFO device having the above structure, the theoretical bending loss is approximately 0 dB / mm in all the prepared FIFO devices. The measured crosstalk is -40 dB or less in Sample 1 and Comparative Example 1, and -50 dB or less in Sample 2 and Comparative Example 2. In addition, regarding the intermodal group delay time difference Δβ1, the group delay time β1 of the low-order mode (fundamental mode) is 4.904×10 －18 [s / nm] in Sample 1 and Comparative Example 1, and 4.914×10

[0095] －18 [s / nm] in Sample 2 and Comparative Example 2. The group delay time β1 of the highest-order mode guided in the waveguide is 4.905×10 －18 [s / nm] (first order) in Sample 1 and Comparative Example 1, and 4.919×10 －18 [s / nm] (second order) in Sample 2 and Comparative Example 2. At this time, the absolute value of the intermodal group delay time difference Δβ1 is 5.211×10 －22 [s / nm] (= |

[0096] (4.904 - 4.905)|×10 －18 ), and 4.717×10 －21 [s / nm] in Sample 2 and Comparative Example 2

[0097] (= |(4.914 - 4.919)|×10－18 )。

[0098] At Figure 4 the upper and lower parts of, curves G510A and G510B show the relationship between the number of divisions N and |Δβ1| when the modulation speed Δf = 25 [GHz], curves G520A and G520B show the relationship between the number of divisions N and |Δβ1| when the modulation speed Δf = 50 [GHz], curves G530A and G530B show the relationship between the number of divisions N and |Δβ1| when the modulation speed Δf = 100 [GHz], curves G540A and G540B show the relationship between the number of divisions N and |Δβ1| when the modulation speed Δf = 200 [GHz], curves G550A and G550B show the relationship between the number of divisions N and |Δβ1| when the modulation speed Δf = 500 [GHz], and curves G560A and G560B show the relationship between the number of divisions N and |Δβ1| when the modulation speed Δf = 1 [THz]

[0099] In addition, curve G410 is for the FIFO device with the structure of sample 1, showing the relationship between the number of divisions N and |Δβ1|

[0100] between them, and it can be confirmed that in all cases where the modulation speed Δf is from 25 [GHz] to 1 [THz], the formula: |Δβ1| ≤ 1 / (N·2Δf·L) is satisfied. Curve G420 is for the FIFO device with the structure of sample 2, showing the relationship between the number of divisions N and |Δβ1|, and it can be confirmed that in the case where the modulation speed Δf is from 25 [GHz] to 100

[0101] [GHz], the formula: |Δβ1| ≤ 1 / (N·2Δf·L) is satisfied. On the other hand, in the FIFO device (curve G430) with the structure of Comparative Example 1 where the waveguide length L exceeds 20 mm, the modulation speed Δf is limited to 200 [GHz], and in the FIFO device (curve G440) with the structure of Comparative Example 2, the modulation speed Δf is limited to 25 [GHz]

[0102] up to. In addition, the theoretical coupling loss between the connected optical fiber and this FIFO device is less than 0.05 [dB] for sample 1 and Comparative Example 1, and less than 0.08 [dB] for sample 2 and Comparative Example 2.

[0103] As described above, the optical waveguide device according to the present invention realizes low bending loss and low crosstalk, further realizes low coupling loss, and the inter-mode group delay time difference Δβ1 satisfies the formula:

[0104] |Δβ1| ≤ 1 / (N·2Δf·L). This indicates that the degradation of the optical signal caused by the interference of the low-order mode and the high-order mode (MPI) is not significant. Therefore, it is not a problem in the optical communication system of the present invention.

[0105] Description of reference numerals

[0106] 10…TX (optical transmitter), 20…RX (optical receiver), 30…SCF (single-core fiber), 31…core, 32…cladding, 300…end face, 40…MCF (multi-core fiber), 41…

[0107] core, 42…common cladding, 400…end face, 50…FIFO device, 50A, 50B…optical waveguide device (FIFO device), 50a…first device end face, 50b…second device end face, 51…

[0108] core (waveguide), 51a…first end face (first waveguide end face), 51b…second end face (second waveguide end face), 52…cladding, 55…bending section.

Claims

1. A Fan-in / Fan-out device that can connect each core of a plurality of single-core optical fibers arranged side by side on the same plane to the corresponding core of a multi-core optical fiber, comprising: A first device end face that is connected to the end faces of the plurality of single-core optical fibers; A second device end face that is connected to the end face of the multi-core optical fiber and is opposite to the first device end face; A waveguide having a first waveguide end face that coincides with the first device end face and a second waveguide end face that coincides with the second device end face, guiding light of multiple modes with different orders, and having one or more bending portions provided on the optical path from the first waveguide end face to the second waveguide end face; and A cladding provided with the waveguide inside or on the surface and having a refractive index lower than that of the waveguide, The waveguide has a waveguide length L defined by the optical path length from the first waveguide end face to the second waveguide end face, which is 5×10 6 [nm] or more and 100×10 6 [nm] or less, and The group delay time difference Δβ1 between the plurality of modes satisfies the condition given by |Δβ1| ≤ 2×10 －12 [s] / L.

2. A Fan-in / Fan-out device that can connect each core of a plurality of single-core optical fibers arranged side by side on the same plane to the corresponding core of a multi-core optical fiber, comprising: A first device end face that is connected to the end faces of the plurality of single-core optical fibers; A second device end face that is connected to the end face of the multi-core optical fiber and is opposite to the first device end face; A waveguide having a first waveguide end face that coincides with the first device end face and a second waveguide end face that coincides with the second device end face, guiding light of multiple modes with different orders, and having one or more bending portions provided on the optical path from the first waveguide end face to the second waveguide end face; and A cladding provided with the waveguide inside or on the surface and having a refractive index lower than that of the waveguide, The waveguide has a waveguide length L defined by the optical path length from the first waveguide end face to the second waveguide end face, which is 5×10 6 [nm] or more and 100×10 6 [nm] or less, and Having the following configuration, that is, for the number of divisions N and 25×10 9 [Hz] or more and 1000×10 9 [Hz] or less for any defined modulation speed Δf, the group delay time difference Δβ1 between the multiple modes satisfies the condition given by |Δβ1|≤1 / (N·2Δf·L), where the number of divisions N is an integer value obtained by dividing the vibration period Tf of the optical frequency with respect to the optical signal intensity variation caused by MPI between the multiple modes, and is defined as any integer value of 10 or more and 100 or less.

3. The Fan-in / Fan-out device according to claim 2, wherein, The waveguide length is 20×10 6 [nm] or less.

4. The Fan-in / Fan-out device according to any one of claims 1 to 3, wherein, The bending portion has a curvature radius r of 40 mm or less.

5. An optical communication system, comprising: At least a pair of optical fibers whose cores are connected by single-mode; and The Fan-in / Fan-out device according to any one of claims 1 to 4 arranged between the pair of optical fibers.

6. The optical communication system according to claim 5, wherein, Each of the pair of optical fibers includes a single-core optical fiber or a multi-core optical fiber.

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