Electronic demultiplexing for dual-polarization (DP) intensity-modulated direct detection (IMDD) optical communication systems

1 . A dual-polarization (DP) intensity-modulated direct detection (IMDD) receiver comprising:

an input port configured to receive input light;

an optical polarization splitter configured to split the input light into a pair of optical transmission paths;

a pair of photodetectors comprising (i) a first photodetector configured to detect light at a first carrier frequency and output an electrical signal into a first electrical transmission path, and (ii) a second photodetector configured to detect light at a second carrier frequency and output an electrical signal into a second electrical transmission path, wherein the first carrier frequency and the second carrier frequency are separated by a frequency difference, wherein the frequency difference is configured to cause the light detected at the first photodetector and the light detected at the second photodetector to be heterodyne-multiplexed; and

an electrical 2×2 multi-input-multi-output (MIMO) polarization demultiplexer configured to receive signals from the first electrical transmission path and the second electrical transmission path and, based on the light detected at the first photodetector and the light detected at the second photodetector being heterodyne-multiplexed, perform electrical 2×2 MIMO polarization demultiplexing to output a first demultiplexed signal and a second demultiplexed signal,

wherein the DP-IMDD receiver is configured to output the first demultiplexed signal and the second demultiplexed signal using intensity-modulated direct detection.

2 . The DP-IMDD receiver of claim 1 , wherein the electrical 2×2 MIMO polarization demultiplexer comprises four adjustable gain elements interconnected in a butterfly configuration.

3 . The DP-IMDD receiver of claim 2 , wherein the electrical 2×2 MIMO polarization demultiplexer comprises:

a first splitter configured to split the first electrical transmission path into a third electrical transmission path and a fourth electrical transmission path;

a second splitter configured to split the second electrical transmission path into a fifth electrical transmission path and a sixth electrical transmission path;

a first gain element configured to apply a controllable gain on the third electrical transmission path;

a second gain element configured to apply a controllable gain on the fourth electrical transmission path;

a third gain element configured to apply a controllable gain on the fifth electrical transmission path;

a fourth gain element configured to apply a controllable gain on the sixth electrical transmission path;

a first summing element configured to sum an output of the first gain element and an output of the third gain element; and

a second summing element configured to sum an output of the second gain element and an output of the fourth gain element.

4 . The DP-IMDD receiver of claim 3 , further comprising at least one additional electrical transmission path configured with time delays and gains.

5 . The DP-IMDD receiver of claim 1 , wherein the electrical 2×2 MIMO polarization demultiplexer is configured to perform the electrical 2×2 MIMO polarization demultiplexing in an analog electronic portion of the DP-IMDD receiver.

6 . The DP-IMDD receiver of claim 1 , wherein the electrical 2×2 MIMO polarization demultiplexer is configured to perform the electrical 2×2 MIMO polarization demultiplexing in a digital electronic portion of the DP-IMDD receiver.

7 . The DP-IMDD receiver of claim 6 , further comprising:

a first analog-to-digital converter (ADC) configured in the first electrical transmission path between the first photodetector and the electrical 2×2 MIMO polarization demultiplexer; and

a second ADC configured in the second electrical transmission path between the second photodetector and the electrical 2×2 MIMO polarization demultiplexer.

8 . The DP-IMDD receiver of claim 1 , further comprising:

a first transimpedance amplifier (TIA) configured in the first electrical transmission path between the first photodetector and the electrical 2×2 MIMO polarization demultiplexer; and

a second TIA configured in the second electrical transmission path between the second photodetector and the electrical 2×2 MIMO polarization demultiplexer.

9 . The DP-IMDD receiver of claim 1 , wherein the frequency difference between the first carrier frequency that the first photodetector is configured to detect and the second carrier frequency that the second photodetector is configured to detect is greater than a signal symbol rate.

10 . The DP-IMDD receiver of claim 1 , further comprising an optical 2×2 MIMO demultiplexer configured to receive light in the pair of optical transmission paths from the optical polarization splitter, and perform optical 2×2 MIMO polarization demultiplexing to output a pair of optical signals.

11 . The DP-IMDD receiver of claim 10 , wherein the pair of photodetectors are configured to receive the pair of optical signals that are output from the optical 2×2 MIMO polarization demultiplexer, and

wherein the pair of photodetectors are configured to provide outputs to the first electrical transmission path and the second electrical transmission path which are connected to the electrical 2×2 MIMO demultiplexer.

12 . The DP-IMDD receiver of claim 11 , wherein the optical 2×2 MIMO demultiplexer provides only partial demultiplexing for one or both of the pair of optical signals that are output from the optical 2×2 MIMO demultiplexer.

13 . The DP-IMDD receiver of claim 10 , wherein the optical 2×2 MIMO demultiplexer comprises:

a first optical phase shifter configured to receive light from the pair of optical transmission paths from the optical polarization splitter and apply a first relative phase shift between the pair of optical transmission paths;

a first 2×2 optical coupler configured to combine light from the pair of optical transmission paths and output light to a second pair of optical transmission paths;

a second optical phase shifter configured to apply a second relative phase shift between the second pair of optical transmission paths;

a second 2×2 optical coupler configured to combine light from the second pair of optical transmission paths and output light to a third pair of optical transmission paths.

14 . The DP-IMDD receiver of claim 13 , wherein the optical 2×2 MIMO demultiplexer further comprises:

a third optical phase shifter configured to apply a third relative phase shift between the third pair of optical transmission paths;

a third 2×2 optical coupler configured to combine light from the third pair of optical transmission paths and output light to a fourth pair of optical transmission paths; and

a pair of MIMO outputs configured to output the pair of optical signals from the fourth pair of optical transmission paths.

15 . The DP-IMDD receiver of claim 14 , wherein the third optical phase shifter is configured to operate between 0 and +nπ, based on the value of the first relative phase shift being c−π/2, and is configured to operate between −nπ and 0, based on the value of the first relative phase shift being c+π/2, where n is an integer.

16 . The DP-IMDD receiver of claim 13 , wherein a value of the first relative phase shift applied by the first optical phase shifter is binary among c+π/2 and c−π/2, where c is a real number.

17 . The DP-IMDD receiver of claim 13 , wherein the second optical phase shifter is configured to apply a value of the second relative phase shift within a finite range that includes −nπ and +nπ, where n is an integer.

18 . The DP-IMDD receiver of claim 1 , wherein the DP-IMDD receiver is configured to output the first demultiplexed signal and the second demultiplexed signal without any optical MIMO demultiplexing.

19 . A method of performing dual-polarization (DP) intensity-modulated direct detection (IMDD) reception, comprising:

receiving input light through an input port;

splitting, through an optical polarization splitter, the input light into a pair of optical transmission paths; and

performing intensity-modulated direct-detection to output a first demultiplexed signal and a second demultiplexed signal, wherein performing the intensity-modulated direct-detection comprises:

(i) detecting light at a first carrier frequency with a first photodetector and outputting an electrical signal into a first electrical transmission path, and (ii) detecting light at a second carrier frequency with a second photodetector and outputting an electrical signal into a second electrical transmission path, wherein the first carrier frequency and the second carrier frequency are separated by a frequency difference, wherein the frequency difference is configured to cause the light detected with the first photodetector and the light detected with the second photodetector to be heterodyne-multiplexed, and

based on the light detected with the first photodetector and the light detected with the second photodetector being heterodyne-multiplexed, performing electrical 2×2 multi-input-multi-output (MIMO) polarization demultiplexing based on the first electrical transmission path and the second electrical transmission path, to output the first demultiplexed signal and the second demultiplexed signal.

20 . The method of claim 19 , wherein the frequency difference between the first carrier frequency detected by the first photodetector and the second carrier frequency detected by the second photodetector is greater than a signal symbol rate.

21 . The method of claim 19 , comprising:

performing optical 2×2 MIMO demultiplexing by receiving light in the pair of optical transmission paths from the optical polarization splitter, and performing optical 2×2 MIMO polarization demultiplexing to output a pair of optical signals.