Optical MIMO processing
A technique is provided for an optical multiple input multiple output (MIMO) processing system. The optical (MIMO) processing system includes an interface to a multi-mode fiber at a first end of the optical processing system; M first single-mode waveguides coupled to the interface; wherein the first waveguides are arranged to carry M first optical signals, respectively; N second single-mode waveguides at a second end of the optical processing system, wherein the second waveguides are arranged to carry N second optical signals, respectively, wherein the N second optical signals are associated with N modes carried by the multi-mode fiber; and an optical processing core arranged to transform the first optical signals into the second optical signals such that a correlation between the second optical signals is decreased compared to a correlation between the first optical signals using a plurality of optical couplers for coupling each of the M first optical signals with one another, such that each of the N second optical signals is a linear combination of all of the M first optical signals.
1. An optical processing system, the optical processing system comprising:
an interface to a multi-mode fiber at a first end of the optical processing system;
M first single-mode waveguides, M>2, coupled to the interface; wherein the first waveguides are arranged to carry M first optical signals, respectively; each of the M first single-mode waveguides carrying its respective first optical signal in a basic mode;
N second single-mode waveguides, N>1, at a second end of the optical processing system, wherein the second waveguides are arranged to carry N second optical signals, respectively, and wherein each of the N second single-mode waveguides carries its respective second optical signal in a basic mode, and wherein the N second optical signals are associated with N modes carried by the multi-mode fiber, respectively; and
an optical processing core for transforming the first optical signals into the second optical signals such that a correlation between the second optical signals is decreased compared to a correlation between the first optical signals, wherein the optical processing core comprises two or more optical processing stages in sequence, each comprising one or more optical couplers arranged for processing one or more pairs of single-mode waveguides, respectively, thereby coupling some or all of the M first optical signals with one another, such that at least one of the N second optical signals is a linear combination of the some or all of the M first optical signals.
2. The optical processing system of claim 1 , wherein an optical processing stage of the two or more optical processing stages is traversed by a plurality of single-mode stage waveguides, and wherein the plurality of stage waveguides are coupled to at least three of the M first waveguides at a first end of the optical processing stage and to at least one of the N second waveguides at a second end of the optical processing stage, and wherein the one or more couplers are arranged to mix two input optical signals on the two of the plurality of stage waveguides at a first end of the coupler using a rotation value θ to provide two coupled optical signals on the two of the plurality of stage waveguides at a second end of the coupler.
3. The optical processing system of claim 2 , wherein if the two input optical signals and the two coupled optical signals are considered to form an x and y axis of a Cartesian coordinate system, the one or more optical couplers are arranged to perform a rotation of the two input optical signals by the rotation value θ to produce the two coupled optical signals.
4. The optical processing system of claim 2 , wherein the first optical processing stage comprises a phase shifter on one of the plurality of stage waveguides arranged to shift a phase of an optical signal on the one of the plurality of stage waveguides by a phase value cp.
5. The optical processing system of any of claim 4 , wherein the one or more phase values φ of the one or more phase shifters and/or the one or more rotation values θ of the one or more couplers, referred to as one or more parameters of the optical processing core, are adjustable.
6. The optical processing system of claim 5 , further comprising a control unit arranged to determine the one or more parameters of the optical processing core; wherein the one or more parameters are determined based on the N second optical signals.
7. The optical processing system of claim 6 , wherein
the N second optical signals are associated with training sequences transmitted via communication channels provided by the N modes of the multi-mode fiber; and
the control unit is arranged to determine the one or more parameters based on the training sequences and the N second optical signals.
8. The optical processing system of claim 1 , wherein
N>2;
the optical processing core comprises N−1 optical processing stages; and
each optical processing stage comprises M/2 or less couplers.
9. The optical processing system of claim 1 , wherein
M is greater or equal to N; and/or
N is greater than 3 or 4 or 50r 6 or 7 or 8 or 9 or 10 .
10. The optical processing system of claim 1 , wherein
the M first optical signals are associated with corresponding M samples of an optical field distribution at M different spatial positions on an endface of the multi-mode fiber, respectively; and
the N second optical signals represent the N modes carried by the multi-mode fiber.
11. The optical processing system of claim 10 , wherein under the condition that the M samples are associated with an optical field distribution of a single one of the N spatial modes carried by the multi-mode fiber, the optical processing core is arranged to
combine the M first optical signals to a single one of the N second optical signals, and/or
generate the M first optical signals from a single one of the N second optical signals.
12. The optical processing system of claim 11 , wherein, for each single one of the N spatial modes, the optical processing core is arranged to
combine the M first optical signals to a different single one of the N second optical signals; and/or
generate the M first optical signals from a different single one of the N second optical signals.
13. The optical processing system of claim 1 , wherein
M is pair; and
the optical processing system further comprises M/2 polarization beam splitters arranged to generate the M first optical signals from M/2 polarization multiplexed signals and/or vice versa.
14. The optical processing system of claim 1 , further comprising an optical receiver arranged to receive the N optical signals transmitted via N spatial modes of the multi-mode fiber, the N>1, the optical receiver comprising:
another optical processing system, arranged to process the M first optical signals to provide the N second optical signals, wherein the processing is performed subject to one or more parameters of one or more phase shifters and/or couplers comprised within the another optical processing system, and wherein the interface of the another optical processing system comprises a multi-mode fiber coupler arranged to extract the M first optical signals into the M single-mode waveguides from the multi-mode fiber, M≧N; and
N optical to electrical converters arranged to convert the N second optical signals into N corresponding received signals, wherein the one or more parameters are determined based on the N received signals.
15. The optical processing system of claim 1 , further comprising an optical spatial mode multiplexer/demultiplexer arranged to couple the N second optical signals from the N second single-mode waveguides to N spatial modes of the multi-mode fiber and/or vice versa, the N>1, the spatial mode multiplexer/demultiplexer comprising another optical processing system, arranged to generate the M first optical signals from the N second optical signals and/or vice versa, wherein the interface of the another optical processing system comprises a spatial sampler arranged to generate M samples of an optical field distribution at M different spatial positions on an endface of the multi-mode fiber from the M first optical signals carried within the M first single-mode waveguides, M>1 and/or vice versa.