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We utilize aperture diversity combined with multiple-mode receivers and multiple-input-multiple-output (MIMO) digital signal processing (DSP) to demonstrate enhanced tolerance to atmospheric turbulence and spatial misalignment in a 10 Gbit/s quadrature-phase-shift-keyed (QPSK) free-space optical (FSO) link. Turbulence and misalignment could cause power coupling from the fundamental Gaussian mode into higher-order modes. Therefore, we detect power from multiple modes and use MIMO DSP to enhance the recovery of the original data. In our approach, (a) each of multiple transmitter apertures transmits a single fundamental Gaussian beam carrying the same data stream, (b) each of multiple receiver apertures detects the signals that are coupled from the fundamental Gaussian beams to multiple orbital angular momentum (OAM) modes, and (c) MIMO DSP is used to recover the data over multiple modes and receivers. Our simulation shows that the outage probability could be reduced from > 0.1 to < 0.01 . Moreover, we experimentally demonstrate the scheme by transmitting two fundamental Gaussian beams carrying the same data stream and recovering the signals on OAM modes 0 and + 1 at each receiver aperture. We measure an up to ∼ 10 d B power-penalty reduction for a bit error rate (BER) at the 7% forward error correction limit for a 10 Gbit/s QPSK signal.

Sinc-shaped temporal pulse trains have a spectrally efficient, rectangular Nyquist spectrum. We demonstrate the simultaneous and reconfigurable optical generation of multiple Nyquist-shaped wavelength-division-multiplexed (WDM) channels having temporal sinc-shaped pulse trains as data carriers. The channels are generated through the insertion of coherent lines using cascaded continuous-wave amplitude modulation around the spectral lines of a microresonator-based Kerr optical frequency comb. For each of nine Kerr frequency comb lines, we insert sub-groups of uniform and coherent lines to generate nine WDM channels. The deviations from ideal Nyquist pulses for the nine channels at repetition rates of 6 and 2 GHz are between 4.2%-6.1% and 2%-4.5%, respectively. Each WDM channel is modulated with on-off keying (OOK) at 6 Gbit/s. In addition, we show the reconfigurability of this method by varying the number of WDM channels, the generated sinc-shaped pulse train repetition rates, the duration, and the number of zero-crossings. (C) 2019 Optical Society of America

We experimentally investigate the scattering effect on an 80 Gbit/s orbital angular momentum (OAM) multiplexed optical wireless communication link. The power loss, mode purity, cross talk, and bit error rate performance are measured and analyzed for different OAM modes under scattering levels from ballistic to diffusive regions. Results show that (i) power loss is the main impairment in the ballistic scattering, while the mode purities of different OAM modes are not significantly affected; (ii) in the diffusive scattering, however, the performance of an OAM-multiplexed link further suffers from the increased cross talk between the different OAM modes.

We experimentally demonstrate a scalable and reconfigurable optical tapped-delay-line (TDL) for multichannel equalization and correlation of 20-Gbaud quadrature-phase-shift-keyed (QPSK) signals using nonlinear wave mixing and a microresonator Kerr frequency comb. The optical TDL mainly consists of two stages: one being a multi-casting of the original signals in a periodically poled lithium niobate (PPLN) waveguide with Kerr comb lines functioning as mutually coherent pumps, while the other is a coherent multiplexing of the delayed and weighted signal replicas in a second PPLN. A two- or three-tap optical TDL is demonstrated to simultaneously equalize a distorted QPSK data signal, reducing the error vector magnitude (EVM) from 22.5% to either 19.9% or 18.2%, and search two- or three-symbol patterns on another QPSK signal. (C) 2018 Optical Society of America

We experimentally demonstrate simultaneous turbulence mitigation and channel demultiplexing in a 200 Gbit/s orbital-angular-momentum (OAM) multiplexed link by adaptive wavefront shaping and diffusing (WSD) the light beams. Different realizations of two emulated turbulence strengths (the Fried parameter r 0 = 0.4 , 1.0 m m ) are mitigated. The experimental results show the following. (1) Crosstalk between OAM l = + 1 and l = − 1 modes can be reduced by > 10.0 and > 5.8 d B , respectively, under the weaker turbulence ( r 0 = 1.0 m m ); crosstalk is further improved by > 17.7 and > 19.4 d B , respectively, under most realizations in the stronger turbulence ( r 0 = 0.4 m m ). (2) The optical signal-to-noise ratio penalties for the bit error rate performance are measured to be ∼ 0.7 and ∼ 1.6 d B under weaker turbulence, while measured to be ∼ 3.2 and ∼ 1.8 d B under stronger turbulence for OAM l = + 1 and l = − 1 mode, respectively.

We numerically simulate and experimentally demonstrate an approach to potentially enhance the performance of a high-order quadrature amplitude modulation (QAM) channel by adding correlated data to other robust binary-phase-shift-keyed (BPSK) or quadrature-phase-shift-keyed (QPSK) channels. The correlated data are introduced by optically multiplying the BPSK or QPSK channels, already modulated with their own data, by the target high-order QAM data of the same baud rate. After joint detection and signal processing, a ∼3 dB optical signal-to-noise (OSNR) improvement is observed in simulations by comparing the performance of the target QAM channel (from 4QAM to 256QAM) with and without the use of channel correlation. We also experimentally demonstrate the scheme for a QPSK or a 16QAM target channel using BPSK correlated channels. A ≥2 dB OSNR improvement is observed.

We experimentally demonstrate a 10 Mbit/s free-space quantum communication link using data encoding on orthogonal Laguerre–Gaussian (LG) modes with the same azimuthal index but different radial indices. Data encoding on two LGlp modes (i.e., for l=0, we encode [“0”, “1”] as [p=0, p=1], and for l=1, we encode [“0”, “1”] as [p=0, p=1]) is demonstrated by employing directly modulated laser diodes and helical phase holograms. The quantum symbol error rate (QSER) of