Mitigation for turbulence effects in a 40-Gbit/s orbital-angular-momentum-multiplexed free-space optical link between a ground station and a retro-reflecting UAV using MIMO equalization.

We experimentally demonstrate turbulence effect mitigation in a 100-m round-trip orbital-angular-momentum (OAM)-multiplexed free-space optical communication link between a ground transmitter and a ground receiver via a retro-reflecting hovering unmanned aerial vehicle (UAV) using multiple-input-multiple-output (MIMO) equalization. In our demonstration, two OAM beams at 1550 nm are transmitted to the UAV through emulated atmospheric turbulence, each carrying a 20-Gbit/s signal. 2×2 MIMO equalization is used to mitigate turbulence-induced crosstalk between the two OAM channels. Bit error rates below the 7% overhead forward error correction limit of 3.8×10-3 are achieved for both channels.

Performance enhancement of an optical high-order QAM channel by adding correlated data to robust neighboring BPSK or QPSK channels.

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.

Raman-assisted phase sensitive amplifier using a fiber Bragg grating-based tunable phase shifter.

A low-loss Raman-assisted phase sensitive amplifier (PSA) with a ∼20 dB signal net gain is experimentally demonstrated. The amplitude and phase adjustment for PSA are achieved by using non-uniform Raman gain and a tunable fiber Bragg grating (FBG), respectively. The total component loss of the system is measured to be ∼8 dB. By tuning the FBG central wavelength, (1) an up to 5.6 dB signal gain improvement is obtained; and (2) a ∼4 dB receiver sensitivity enhancement is observed for 20 and 25 Gbaud quadrature phase shift keying signals and a 10 Gbaud 16-quadrature amplitude modulation signal.

High-Capacity Free-Space Optical Communications Between a Ground Transmitter and a Ground Receiver via a UAV Using Multiplexing of Multiple Orbital-Angular-Momentum Beams.

We explore the use of orbital-angular-momentum (OAM)-multiplexing to increase the capacity of free-space data transmission to moving platforms, with an added potential benefit of decreasing the probability of data intercept. Specifically, we experimentally demonstrate and characterize the performance of an OAM-multiplexed, free-space optical (FSO) communications link between a ground transmitter and a ground receiver via a moving unmanned-aerial-vehicle (UAV). We achieve a total capacity of 80 Gbit/s up to 100-m-roundtrip link by multiplexing 2 OAM beams, each carrying a 40-Gbit/s quadrature-phase-shift-keying (QPSK) signal. Moreover, we investigate for static, hovering, and moving conditions the effects of channel impairments, including: misalignments, propeller-induced airflows, power loss, intermodal crosstalk, and system bit error rate (BER). We find the following: (a) when the UAV hovers in the air, the power on the desired mode fluctuates by 2.1 dB, while the crosstalk to the other mode is -19 dB below the power on the desired mode; and (b) when the UAV moves in the air, the power fluctuation on the desired mode increases to 4.3 dB and the crosstalk to the other mode increases to -10 dB. Furthermore, the channel crosstalk decreases with an increase in OAM mode spacing.

Using a complex optical orbital-angular-momentum spectrum to measure object parameters.

Light beams can be characterized by their complex spatial profiles in both intensity and phase. Analogous to time signals, which can be decomposed into multiple orthogonal frequency functions, a light beam can also be decomposed into a set of spatial modes that are taken from an orthogonal basis. Such decomposition can potentially provide a tool for spatial spectrum analysis, which may enable stable, accurate, and robust extraction of physical object information that may not be readily achievable using traditional approaches. As a proof-of-concept example, we measure an object's opening angle using orbital-angular-momentum (OAM) -based complex spectrum, achieving a >15 dB signal-to-noise ratio. Moreover, the dip (i.e., notch) positions of the OAM intensity spectrum are dependent on an object's opening angle but independent of the opening's angular orientation, whereas the slope of the OAM phase spectrum is dependent on the opening's orientation but independent of the opening angle.