JP2023021851A - Optical unitary converter and optical transfer system - Google Patents

Abstract

To reduce the element length of an optical unitary converter which converts an arbitrary unitary matrix.SOLUTION: The present disclosure relates to an optical unitary converter having N(N-1)/2 power distributors and N(N+1)/2 phase shifts for N input waveguides. The power distributors have two Y-branch waveguides with 2×1 ports in which two parallel waveguides become closer to each other at one end. The two Y-branch waveguides for one port and for one port are connected to each other by a 2-mode waveguide.SELECTED DRAWING: Figure 9

Description

The present disclosure relates to an optical unitary converter that optically unitarily converts a plurality of input optical signals.

N(N−1)/2 power dividers whose optical distribution ratio is electrically controllable from 1:0 to 0:1, and N(N) whose phase shift amount is electrically controllable from 0 to 2π By combining N+1)/2 phase shifters, for the complex amplitude sequence {a}=(a1, a2, . , a complex amplitude sequence {b} multiplied by an arbitrary N-row N-column unitary matrix U can be output to N output waveguides. In the present disclosure, this is referred to as an optical unitary converter [see, eg, Non-Patent Documents 1-3. ].

In Non-Patent Document 1, a case of adopting a Mach-Zehnder interferometer as a power divider is taken up. A unitary transformation using

In order to change the phase by π with an infrared wavelength phase shifter of a silica-based optical waveguide, a Cr heater with a length of 2 mm or longer and an optical waveguide are required [see, for example, Non-Patent Documents 4-6. ]. Also, two 2×2 optical equal splitters included in one Mach-Zehnder interferometer each require an optical waveguide with a length of about 1 mm [see, for example, Non-Patent Documents 4-6. ].

As reported in Non-Patent Document 7, the device length required to configure the unitary converter must be at least N times the length L, which is the sum of the element lengths of the Mach-Zehnder interferometer and the phase shifter. In a conventional element using a silica-based optical waveguide for infrared wavelengths, L=8 mm at the minimum.

As described above, by connecting Mach-Zehnder interferometers and phase shifters in multiple stages, it is possible to configure an optical unitary converter that performs an arbitrary N-row N-column unitary matrix U. However, in order to arbitrarily change the transformation matrix U by electrical control, the element lengths of the Mach-Zehnder interferometer and the phase shifter become long, and there is a problem that it is difficult to increase the order of the optical unitary transformation.

M. Reck, A. Zeilinger, H.; J. Bernstein, andP. Bertani, "Experimental realization of any discrete unitary operator," Phys. Rev. Lett. , vol. 73, no. 1, pp. 58-61, Jul. 1994. J. Carolan, C.E. Harrold, C.I. Sparrow, E. Martin-Lopez, N.G. J. Russell, J.; W. Silverstone, P.S. J. Shadbolt, N.; Matsuda, M.; Oguma, M.; Itoh, G. D. Marshall, M.; G. Thompson, J.; C. F. Matthews, T.; Hashimoto, J.; L. O'Brien, and A. Laing, "Universal linear optics," Science. , vol. 349, no. 6249, pp. 711-716, Aug. 2015. A. Annoni, E. Guglielmi, M.; Carminati, G.; Ferrari, M. Sampietro, D.; A. Miller, A. Melloni, andF. Morichetti, "Unscrambling light-automatically undoing strong mixing between modes," Light Sci. Appl. , vol. 6, no. 12, pp. e17110-1-e17110-10, 2017. Q. Lai, W. Hunziker, and H. Melchior, "Low-power compact 2×2 thermooptic silica-on-silicon waveguide switch with fast response," IEEE Photonics Technol. Lett. , vol. 10, no. 3, pp. 681-683, 1998. J. -K. Hong and S. -S. Lee, “Silica-Based MMI-MZI Thermo-Optic Switch with Large Tolerance and Low PDL,” Journal of the Optical Society of Korea, vol. 9, no. 3. pp. 119-122, 2005. P. Samadi, I. A. Kostko, A.; Jain, L.; R. Chen, P. Dumais, C.; L. Callender, S.; Jacob, andB. Xia, "Tunable 6-stage lattice-form Mach-Zehnder interferometer for arbitrary binary code generation at 40 GHz," Conf. Opt. Fiber Commun. Tech. Dig. Ser. , vol. 28, no. 21, pp. 3070-3078, 2009. W. R. Clements, P.S. C. Humphreys, B.; J. Metcalf, W.; S. Kolthammer, and I.M. A. Walsmley, "Optimal design for universal multiport interferometers," Optica, vol. 3, no. 12, pp. 1460-1465, Dec. 2016.

An object of the present disclosure is to shorten the element length of an optical unitary converter that converts an arbitrary unitary matrix.

In order to solve the above problems, the present disclosure employs an optical splitter composed of a Y-branch waveguide and a two-mode waveguide instead of using a Mach-Zehnder interferometer as a power splitter. Thus, the present disclosure provides a shorter element length optical unitary converter.

Specifically, the optical unitary converter of the present disclosure includes:
N (N-1)/2 power dividers and N (N+1)/2 phase shifters for N input waveguides;
The power divider comprises two 2×1 port Y branch waveguides in which two waveguides arranged in parallel are adjacent at one end,
1 ports of the two Y-branch waveguides are connected by a 2-mode waveguide.

Specifically, the optical transmission system of the present disclosure includes:
N optical transmitters (N is an integer equal to or greater than 2) that transmit signal light;
M (M is an integer equal to or greater than N) optical receivers that receive signal light from the optical transmitter;
an optical fiber connecting between the optical transmitter and the optical receiver;
An optical transmission system comprising
a transmitting-side optical unitary converter according to the present disclosure, which converts N signal lights transmitted from each optical transmitter;
a reception-side optical unitary converter according to the present disclosure, which converts the N signal lights propagated through the optical fiber with an inverse matrix of the transmission-side optical unitary converter;
Equipped with

The present disclosure can shorten the element length of an optical unitary converter that converts an arbitrary unitary matrix. This allows the present disclosure to enable order expansion of optical unitary transformations.

1 is a schematic diagram of an optical unitary converter; FIG. It is an example of a conventional optical unitary converter. It is an example of any unitary transformable configuration. 1 is an example of a conventional 4×4 optical unitary conversion configuration; It is an example of the proposed power divider. It is an example of structural parameters of a Y-branch waveguide. It is an example of power distribution ratio control by 2-mode waveguide length adjustment. It is an example of transmittance and insertion loss by Lc adjustment. This is an example of miniaturization of the phase shifter by the width modulation waveguide. It is an example of structural parameters of an optical unitary converter. It is an example of the amount of phase change due to the width modulated waveguide _wt . It is an example of a 2x2 optical unitary converter that realizes the proposed arbitrary unitary conversion. It is an example of a 4x4 optical unitary converter that implements the proposed arbitrary unitary conversion. It is an example of adjusting the phase change amount of the phase shifter by the Cr heater. It is a configuration example of a 4x4 optical unitary converter. It is a configuration example of a 4x4 optical unitary converter. It is an example of a multimode optical transmission system using an optical unitary converter. It is a configuration example of an optical transmission section using a non-selective mode demultiplexer. It is a configuration example of an optical receiver using a non-selective mode demultiplexer.

Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. Note that the present disclosure is not limited to the embodiments shown below. These implementation examples are merely illustrative, and the present disclosure can be implemented in various modified and improved forms based on the knowledge of those skilled in the art. In addition, in this specification and the drawings, constituent elements having the same reference numerals are the same as each other.

(Summary of this disclosure)
- The optical unitary converter of the present disclosure comprises N input/output waveguides, N(N-1)/2 power dividers, and N(N+1)/2 phase shifters. However, N is an integer of 2 or more.
• The input and output waveguides are single-mode optical waveguides.
- The power splitter is a 2x2 port optical splitter in which two Y-branch waveguides are connected by a 2-mode waveguide. A Y-branch waveguide is a 2×1-port waveguide in which two parallel-arranged waveguides are adjacent at one end. In the power divider, 1 ports of two Y-branch waveguides are connected by a 2-mode waveguide. Specifically, two input waveguides are connected to a 2-input 1-output Y-branch waveguide, the output end is connected to a 2-mode waveguide, and the output end is a 1-input 2-output Y-branch waveguide. is an optical waveguide element connected to the , and the output ends of which serve as two output waveguides.
The phase shifter may be a delay waveguide that lengthens the optical path length by meandering the optical waveguide, or a width modulation waveguide that lengthens the optical path length by widening the waveguide width.
- The above single-mode optical waveguide, Y-branch waveguide, two-mode waveguide, delay waveguide, and width modulation waveguide are conventionally used for general purposes.

FIG. 1 is a schematic diagram of an optical unitary converter that realizes conversion of incident light amplitude. As shown in FIG. 1, the device to which the present disclosure is applied has a complex amplitude sequence {a}=(a ₁ , a ₂ , _. ) has a function of outputting a complex amplitude sequence {b} obtained by multiplying ^T by an arbitrary N-row N-column unitary matrix U to N output waveguides. This function is the same as the conventional technology. (For example, see Non-Patent Documents 1 and 7.)

An optical unitary converter capable of arbitrary unitary conversion is constructed by regularly connecting N(N−1)/2 power dividers and arranging phase shifters in all the power dividers. .

FIG. 2 shows a configuration example of a conventional optical unitary converter when N=2. Power divider 100 uses a Mach-Zehnder interferometer. The power splitter 100 includes two optical equal splitters 110 and one phase shifter 120 . The optical splitter 110 is realized by a 2×2 MMI (Multi Mode Interference) or a directional coupler.

となる。 Light entering the unitary converter of FIG. 2 from the upper left waveguide passes through the power divider 100 after passing through the phase shifter 200A. Expressing this as a transfer matrix,

becomes.

Here, θ corresponds to the phase shift amount (0≦θ≦π) of phase shifter 120 and is a parameter for controlling the distribution ratio of power divider 100 . φ is a parameter for controlling the phase shift amount (0≦φ≦2π) of the phase shifter 200A arranged in the front stage of the Mach-Zehnder interferometer. When using a Mach-Zehnder interferometer as the power demultiplexer 100, the phase shifter 120 is placed in either the upper or lower or both waveguides. However, the 2×2 unitary converter shown in FIG. 2 cannot perform arbitrary unitary conversion.

As shown in FIG. 3, a 2×2 optical unitary converter capable of arbitrary unitary conversion has phase shifters in two input waveguides or two output waveguides in contrast to the 2×2 optical unitary converter. 200B and 200C are arranged.

FIG. 4 is a schematic diagram of a 4×4 optical unitary converter constructed using the 2×2 optical unitary converter and a phase shifter based on the method of Non-Patent Document 7. In FIG. In the case of a 4 × 4 complex matrix, the number of elements included in the matrix is 16, and since the complex number consists of a real part and an imaginary part, it can be said that there are 32 degrees of freedom, so the number of controllable parameters is 32 are required. Here, in order to realize the unitary transformation, by imposing the condition that the matrix is a unitary matrix, the number of elements excluding the diagonal term, that is, N(N−1 )/2 parameters for reproducing the real and imaginary parts (N(N−1)) and parameters for reproducing the phases of N diagonal elements (N) are required, A total of N(N−1)+N=N ² parameters are required. This parameter corresponds to the phase shifter 200 in the unitary converter. To realize 4×4 optical unitary conversion, 4×3=12 power dividers 100 and 4 ² =16 phase shifters 200 are required.

(Embodiment example 1)
FIG. 5 shows an example of a power divider of the present disclosure. The power divider of the present disclosure comprises 2×1 Y-branch waveguides 11 and 1×2 Y-branch waveguides 12 at the input and output of the two-mode waveguides, respectively. These Y branch waveguides 11 and 12 are connected at a coupling portion 13 . The coupling portion 13 is composed of a two-mode waveguide. In the present disclosure, any power distribution ratio is realized by controlling the length of the coupling portion 13 .

Here, the 2-mode waveguide is an optical waveguide in which both the 0th-order mode (e.g., E11 mode) and the 1st-order mode (e.g., E21 mode) can propagate, and the 0th-order mode and the 1st-order mode are coupled. Instead, each mode propagates through the optical waveguide while undergoing a different phase change. Regarding the detailed operation of the 2×2 port power optical splitter, the light incident from one of the input waveguides is excited in the 0th-order mode and the 1st-order mode almost equally at the input part of the 2-mode waveguide. After each mode undergoes a different phase shift in the waveguide, it splits into two waveguides at the output of the two-mode waveguide with a power distribution ratio corresponding to the amount of phase shift.

By adopting this configuration, the present disclosure can realize a smaller element length than a conventional Mach-Zehnder interferometer using two equal splitters having two input/output waveguides. Δ is the refractive index difference between the Y branch waveguides 11 and 12, _w1 is the waveguide width of the input/output waveguides 111 and 112 constituting the Y branch waveguides 11 and 12, _w2 is the waveguide width of the coupling portion 13, h is the waveguide height, _Ls is the longitudinal length of the bent portions of the Y branch waveguides 11 and 12, _Lc is the longitudinal length of the 2-mode waveguide, and s is the Y branch waveguides 11 and 12. is the length in the width direction of the bent portion of the FIG. 6 shows an example of each structural parameter value.

FIG. 7 shows an example of the power distribution ratio when the waveguide length of the coupling section 13 is adjusted. When Lc is 51 μm, as shown in FIG. 7A, light input from port P1 is output to port P3. When Lc is 153 μm, light input from port P1 is output to both ports P3 and P4, as shown in FIG. 7(b). When Lc is 255 μm, light input from port P1 is output to port P4 as shown in FIG. 7(c). By changing the length of the coupling portion 13 in this way, it is possible to control the power distribution ratio of the power divider 10 . Structural parameters are as shown in FIG.

FIG. 8 shows the transmittance and insertion loss by adjusting Lc. Regarding the Y branch waveguides 11 and 12 shown in FIG. 5, when the E11 mode at a wavelength of 1550 nm is incident from the waveguide 111 of the upper left port P1, the emission to the upper right port P3 is Bar, and the emission to the lower right port P4 is Cross. The same applies to the emission (Bar) to the port P4 when entering from the port P2, and the emission (Cross) to the port P3 when entering from the port P2. By adjusting Lc, power splitting like a conventional Mach-Zehnder interferometer can be realized.

In this embodiment, the element length of the power distributor 10 is 2.3 mm or less. As described above, the optical unitary converter of this embodiment can realize a compact optical unitary converter in which the element length is reduced to half or less unlike the conventional one.

Note that the present disclosure may use the E12 mode instead of the 0th order mode (for example, the E11 mode). Also, the E22 mode may be used instead of the primary mode (for example, the E21 mode). In this way, the modes guided through the coupling section 13 are paired even functional modes such as the E(2p-1)q mode and the E(2p)q mode (where p and q are arbitrary natural numbers). A set of modes that can excite a normal mode and an odd function mode can be adopted.

(Embodiment example 2)
FIG. 9 shows a configuration example of the optical unitary converter of this embodiment. The optical unitary converter of this embodiment includes a phase shifter 20A before the input of the power divider 10. FIG. Phase shifter 20A comprises a width modulation waveguide.

The conventional phase shifter 200A shown in FIG. 2 requires a long straight waveguide of about 4 mm that can electrically control arbitrary phase shifts, assuming control by a heater. On the other hand, in the present embodiment, the length of the device is reduced by replacing the linear waveguide with the width-modulated waveguide. The figure shows an example of a width-modulated waveguide in which the waveguide width gradually widens toward the center of the phase shifter 20A. Instead of the width modulation waveguide, the phase shifter 20A may use a delay waveguide in which the waveguides 111 and 112 meander to lengthen the optical path length.

An example of each structural parameter of the optical unitary converter of this embodiment is shown in FIG. _Wt is the maximum waveguide width of the phase shifter 20, and _Lps is the length of the phase shifter 20 in the longitudinal direction.

FIG. 11 shows the amount of phase change due to the width modulated waveguide _wt . By adjusting _wt in the range of 5 to 13 μm, it is possible to obtain a phase change amount in the range of 0 to 2π.

FIG. 12 shows an example of a 2×2 optical unitary converter of the present disclosure. In the present disclosure, the 2×2 optical unitary converter capable of arbitrary unitary conversion shown in FIG. 3 is replaced with phase shifters 20A, 20B and 20C and a power divider 10 as shown in FIG.

FIG. 13 shows an example of a 4×4 optical unitary converter that implements any unitary conversion of this disclosure. In the present disclosure, an N×N optical unitary converter can consist of N(N−1)/2 2×2 power dividers 10 and N(N+1)/2 phase shifters 20 . When N=4, six power dividers 10 and ten phase shifters 20 can constitute an optical unitary converter.

In this embodiment, the length of the power divider 10 and the phase shifter 20 is 3.3 mm or less. As described above, the optical unitary converter of this embodiment can realize a compact optical unitary converter in which the element length is reduced to half or less unlike the conventional one.

(Embodiment example 3)
FIG. 14 shows a configuration example of the optical unitary converter of this embodiment. In this embodiment, the phase change amount in the phase shifter 20 can be finely adjusted by the heater 21 such as a Cr heater. The optical unitary converter of this embodiment has a mechanism for controlling the flow of an appropriate current to the heater 21 provided in each phase shifter 20 . For example, the optical unitary converter of the present embodiment includes a current control unit 23. The light intensity of each port provided in the optical unitary converter is read back by the current control unit 23, and the necessary current amount is determined by each ground 22. to the phase shifter 20. Thereby, the optical unitary converter of this embodiment can adjust the power of each port.

(Embodiment example 4)
15 and 16 show configuration examples of the optical unitary converter of this embodiment. The optical unitary converter of this embodiment realizes arbitrary N×N unitary conversion by arranging the phase shifters 20 and the power dividers 10 of FIG. 9 in multiple stages according to a certain rule. The arrangement rule of the elements is the same as the conventional one, and the arrangement position of the phase shifter 20 is arbitrary. (See Non-Patent Document 1)

In the case of the configuration of phase shifters 20-1 to 20-6 and power dividers 10-1 to 10-6 in FIG. 15, the element length of the optical unitary converter is
(Number 2)
( _LPS + _LPD )*N+ _LPS (2)
becomes.

In the case of the configuration of phase shifters 20-2 to 20-7 and power dividers 10-2 to 10-7 in FIG. 16, the element length of the optical unitary converter is
(Number 3)
(L _PS +L _PD )*(2*N−3)+L _PS (3)
becomes.

(Embodiment example 5)
FIG. 17 shows a configuration example of the optical transmission system of the present disclosure. The optical transmission system of this embodiment includes the optical unitary converter of the present disclosure in an optical transmitter and an optical receiver. Specifically, the optical transmission system of this embodiment includes N optical transmitters 91 (N is an integer equal to or greater than 2) in the optical transmission section, followed by an optical unitary converter 92 and a mode multiplexer 93. , a few-mode optical fiber 94, a mode demultiplexer 95, and an optical unitary converter 96. After that, the optical receiver 97 includes M coherent receivers 971 (M is an integer equal to or greater than N), a local oscillation light source 972, It is a multimode optical transmission system having a signal processor (DSP) 973 .

Each optical transmitter 91 transmits coherent modulated signal light.
The optical unitary converter 92 converts each coherent modulated signal light in the fundamental mode with a predetermined unitary matrix.
A mode multiplexer 93 converts each coherent modulated signal light into different modes and multiplexes them into a multimode optical fiber.
The few-mode optical fiber 94 is a multimode optical fiber having two or more propagation modes.
A mode demultiplexer 95 demultiplexes each mode and converts it into a fundamental mode.
The optical unitary converter 96 converts each coherent modulated signal light demultiplexed by the mode demultiplexer 95 . At this time, the optical unitary converter 96 uses the inverse matrix of the unitary matrix used in the optical unitary converter 92 .
The coherent receiver 971 functions as M optical receivers, and uses the local light from the local oscillation light source 972 to detect coherent modulated signal light in fundamental mode.
A signal processor (DSP) 973 receives the baseband signal transmitted from the coherent modulated signal light.
This system enables multimode optical transmission including phase information.

By using the optical unitary converter 92 in the optical transmitter or optical receiver 97, it is possible to equally assign modes to the channels, and to reduce the loss difference for each mode in the transmission path of the mode multiplexer 93 and the few-mode optical fiber 94. etc., can be compensated for.

FIG. 18 shows a configuration example of the optical unitary converter 92 and the mode multiplexer 93. As shown in FIG. The mode multiplexer 93 is a non-selective mode multiplexer using the Y branch waveguides 31 , 32 and 33 . By adjusting the waveguide widths of the Y-branch waveguides 31, 32, and 33, the waves can be combined while being converted into different modes. Signal lights input from the ports P1 to P4 of the optical unitary converter 92 are output from different waveguides. The signal lights output from each waveguide of the optical unitary converter 92 are converted into different modes and combined into a few-mode optical fiber 94 .

FIG. 19 shows a configuration example of the mode demultiplexer 95 and the optical unitary converter 96. As shown in FIG. The mode demultiplexer 95 is a non-selective mode demultiplexer using the Y branch waveguides 51 , 52 and 53 . By adjusting the waveguide widths of the Y-branch waveguides 51, 52, 53, it is possible to separate the waves for each mode and convert them into the fundamental mode. Each mode signal light input to the mode demultiplexer 95 is converted into a fundamental mode and input to different waveguides of the optical unitary converter 96 . The signal light input to each waveguide of the optical unitary converter 96 is output from different ports P1 to P4.

Although the mode multiplexer 93 and the mode demultiplexer 95 have mode conversion functions in this embodiment, the present disclosure is not limited to this. For example, optical unitary converter 92 may convert each optical transmitter 91 into a different mode, and optical unitary converter 96 may convert each mode into a fundamental mode.

(Effect of the present disclosure)
According to the present disclosure, the element length of the optical unitary converter can be shortened, and optical unitary conversion with a higher order of operation can be realized.

The present disclosure can be applied to the information communication industry.

10, 10-1 to 10-7: power dividers 11, 12, 31, 32, 33, 51, 52, 53: Y branch waveguides 111, 112: waveguide 13: coupling portions 20, 20A, 20B, 20C , 20-1 to 20-7: Phase shifter 21: Heater 22: Ground 23: Current controller 91: Optical transmitter 92: Optical unitary converter 93: Mode multiplexer 94: Several mode optical fiber 95: Mode demultiplexer Device 96: Optical unitary converter 97: Optical receiver 971: Coherent receiver 972: Local oscillation light source 973: Signal processor (DSP)

Claims (4)

N (N-1)/2 power dividers and N (N+1)/2 phase shifters for N input waveguides;
The power divider comprises two 2×1 port Y branch waveguides in which two waveguides arranged in parallel are adjacent at one end,
1 ports of the two Y-branch waveguides are connected to each other by a 2-mode waveguide,
Optical unitary converter. The phase shifter is characterized in that the waveguide width gradually expands toward the center of the phase shifter,
2. An optical unitary converter as claimed in claim 1. characterized by comprising a heater that changes the amount of phase change in the phase shifter,
3. An optical unitary converter according to claim 1 or 2. N optical transmitters (N is an integer equal to or greater than 2) that transmit signal light;
M (M is an integer equal to or greater than N) optical receivers that receive signal light from the optical transmitter;
an optical fiber connecting between the optical transmitter and the optical receiver;
An optical transmission system comprising
4. The transmission-side optical unitary converter according to any one of claims 1 to 3, which converts N signal lights transmitted from each optical transmitter;
4. The receiving side optical unitary converter according to any one of claims 1 to 3, which converts the N signal lights propagated through the optical fiber with an inverse matrix of the transmitting side optical unitary converter;
An optical transmission system comprising:

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