Changjing Bao

Optical communications using orbital angular momentum beams

Orbital angular momentum (OAM), which describes the “phase twist” (helical phase pattern) of light beams, has recently gained interest due to its potential applications in many diverse areas. Particularly promising is the use of OAM for optical communications since: (i) coaxially propagating OAM beams with different azimuthal OAM states are mutually orthogonal, (ii) inter-beam crosstalk can be minimized, and (iii) the beams can be efficiently multiplexed and demultiplexed. As a result, multiple OAM states could be used as different carriers for multiplexing and transmitting multiple data streams, thereby potentially increasing the system capacity. In this paper, we review recent progress in OAM beam generation/detection, multiplexing/demultiplexing, and its potential applications in different scenarios including free-space optical communications, fiber-optic communications, and RF communications. Technical challenges and perspectives of OAM beams are also discussed.

Nonlinear conversion efficiency in Kerr frequency comb generation

We analytically and numerically investigate the nonlinear conversion efficiency in ring microresonator-based mode-locked frequency combs under different dispersion conditions. Efficiency is defined as the ratio of the average round trip energy values for the generated pulse(s) to the input pump light. We find that the efficiency degrades with growth of the comb spectral width and is inversely proportional to the number of comb lines. It depends on the cold-cavity properties of a microresonator only and can be improved by increasing the coupling coefficient. Also, it can be increased in the multi-soliton state.

Crosstalk mitigation in a free-space orbital angular momentum multiplexed communication link using 4×4 MIMO equalization.

We demonstrate crosstalk mitigation using 4×4 multiple-input-multiple-output (MIMO) equalization on an orbital angular momentum (OAM) multiplexed free-space data link with heterodyne detection. Four multiplexed OAM beams, each carrying a 20  Gbit/s quadrature phase-shift keying signal, propagate through weak turbulence. The turbulence induces inter-channel crosstalk among each beam and degrades the signal performance. Experimental results demonstrate that with the assistance of MIMO processing, the signal quality and the bit-error-rate (BER) performance can be improved. The power penalty can be reduced by >4  dB at a BER of 3.8×10-3.

Phase correction for a distorted orbital angular momentum beam using a Zernike polynomials-based stochastic-parallel-gradient-descent algorithm.

A stochastic-parallel-gradient-descent algorithm (SPGD) based on Zernike polynomials is proposed to generate the phase correction pattern for a distorted orbital angular momentum (OAM) beam. The Zernike-polynomial coefficients for the correction pattern are obtained by monitoring the intensity profile of the distorted OAM beam through an iteration-based feedback loop. We implement this scheme and experimentally show that the proposed approach improves the quality of the turbulence-distorted OAM beam. Moreover, we apply phase correction patterns derived from a probe OAM beam through emulated turbulence to correct other OAM beams transmitted through the same turbulence. Our experimental results show that the patterns derived this way simultaneously correct multiple OAM beams propagating through the same turbulence, and the crosstalk among these modes is reduced by more than 5 dB.

Experimental demonstration of 16-Gbit/s millimeter-wave communications link using thin metamaterial plates to generate data-carrying orbital-angular-momentum beams

We present the design and performance characterization of a thin metamaterial plate for generation of orbital angular momentum (OAM) modes of a millimeter-wave beam, which can carry independent data streams over the same physical medium. The plate has a thickness of 1.56 mm, and consists of 3.06 × 0.68 mm rectangular apertures with spatial variant orientations. It generates OAM beams l = +1 and l = +3 with mode purity larger than 77.5% over a bandwidth of 6 GHz (25-31 GHz). We then use these streams to experimentally demonstrate a 16-Gbit/s millimeter-wave wireless communications link using two multiplexed OAM modes, each carrying a 2-Gbaud 16-QAM signal. A channel crosstalk less than -20 dB over a bandwidth of 4 GHz (26-30 GHz) and biterror-rates (BER) less than 3.8 × 10-3 are achieved.

Experimental measurements of multipath-induced intra- and inter-channel crosstalk effects in a millimeter-wave communications link using orbital-angular-momentum multiplexing

This paper reports on an experimental measurement and analysis of multipath-induced intra- and interchannel crosstalk effects in a mm-wave communications link using orbital angular momentum multiplexing at 28 GHz. The reflection is from an ideal reflector parallel to the propagation path. The intra-channel crosstalk effect is measured when a single OAM beam is transmitted, and inter-channel crosstalk effect is measured when 2 multiplexed OAM beams are transmitted. Both simulation and experimental results show that OAM channels with larger OAM number ℓ tend to have stronger intra-channel crosstalk because less power is received from the direct path and more power is received from the reflected path. This effect is caused by OAM beam divergence, as OAM beams with larger ℓ spread into a larger beam size and have less power in the beam center. For the same reason, OAM beams of larger ℓ lead to stronger inter-channel crosstalk with the other OAM channels.

Experimental demonstration of a 200-Gbit/s free-space optical link by multiplexing Laguerre-Gaussian beams with different radial indices.

We demonstrate a 200-Gbit/s space-division multiplexing system using two Laguerre-Gaussian (LG) beams with different radial indices (LG_ℓ=0,p=0 and LG_ℓ=0,p=1). With a proper design of the radial change of the demultiplexing pattern, the channel crosstalk could be minimized and both channels could achieve a bit error rate of 3.8×10^-3. Moreover, the multiplexing of four LG beams with different azimuthal indices and different radial indices (e.g., LG_ℓ=0,p=0, LG_ℓ=0,p=1, LG_ℓ=1,p=0, and LG_ℓ=1,p=1 beams) is also demonstrated with a <-12  dB channel crosstalk, potentially enabling a 400-Gbit/s data transmission.

Orbital-angular-momentum-multiplexed free-space optical communication link using transmitter lenses

In this paper, we explore the potential benefits and limitations of using transmitter lenses in an orbital-angular-momentum (OAM)-multiplexed free-space optical (FSO) communication link. Both simulation and experimental results indicate that within certain transmission distances, using lenses at the transmitter to focus OAM beams could reduce power loss in OAM-based FSO links and that this improvement might be more significant for higher-order OAM beams. Moreover, the use of transmitter lenses could enhance system tolerance to angular error between transmitter and receiver, but they might degrade tolerance to lateral displacement.

Analysis of aperture size for partially receiving and de-multiplexing 100-Gbit/s optical orbital angular momentum channels over free-space link

In this paper, we present a free-space link utilizing orbital angular momentum (OAM) multiplexing, where only a proportion of the beam is received. Partially receiving an OAM mode could lead to crosstalk between neighboring modes. Here, we analyzed the effect of the aperture size on the system crosstalk both numerically and experimentally. We also demonstrated an optical data link with a total capacity of 200 Gbit/s using receiver with partial aperture. Bit error rate (BER) performance of the partial receiver is measured for different receiver aperture sizes.

Tunable generation and angular steering of a millimeter-wave orbital-angular-momentum beam using differential time delays in a circular antenna array

In this paper, the generation and steering of beams carrying orbital angular momentum utilizing a custom-designed circular antenna array has been demonstrated at 28 GHz. A steering angle as large as 30 degrees for an orbital angular momentum (OAM) beam has been achieved. The effect of number of antennas and the distance from antennas to the array center to the quality of beam generation and beam steering is investigated through both experiments and simulations. Our results indicate that: (1) As the steering angle increases, the mode purity of the generated OAM beams decreases; (2) Increasing the number of antennas improves the OAM mode purity; (3) For a fixed number of antennas, high mode purity is observed for lower order OAM modes; (4) Placing the antennas farther away from the array center allows for reduced divergence of the generate OAM beams.