Zhen Qu

500 Gb/s free-space optical transmission over strong atmospheric turbulence channels

We experimentally demonstrate a high-spectral-efficiency, large-capacity, featured free-space-optical (FSO) transmission system by using low-density, parity-check (LDPC) coded quadrature phase shift keying (QPSK) combined with orbital angular momentum (OAM) multiplexing. The strong atmospheric turbulence channel is emulated by two spatial light modulators on which four randomly generated azimuthal phase patterns yielding the Andrews spectrum are recorded. The validity of such an approach is verified by reproducing the intensity distribution and irradiance correlation function (ICF) from the full-scale simulator. Excellent agreement of experimental, numerical, and analytical results is found. To reduce the phase distortion induced by the turbulence emulator, the inexpensive wavefront sensorless adaptive optics (AO) is used. To deal with remaining channel impairments, a large-girth LDPC code is used. To further improve the aggregate data rate, the OAM multiplexing is combined with WDM, and 500 Gb/s optical transmission over the strong atmospheric turbulence channels is demonstrated.

RF-subcarrier-assisted four-state continuous-variable QKD based on coherent detection

We theoretically investigate and experimentally demonstrate a RF-assisted four-state continuous-variable quantum key distribution (CV-QKD) system. Classical coherent detection is implemented with a simple digital phase noise cancelation scheme. In the proposed system, there is no need for frequency and phase locking between the quantum signals and the local oscillator laser. Moreover, in principle, there is no residual phase noise, and a mean excess noise of 0.0115 (in shot-noise units) can be acquired experimentally. In addition, the minimum transmittance of 0.45 is reached experimentally for secure transmission with commercial photodetectors, and the maximum secret key rate (SKR) of >12  Mbit/s can be obtained. The proposed RF-assisted CV-QKD system opens the door of incorporating microwave photonics into a CV-QKD system and improving the SKR significantly.

Coded Orbital Angular Momentum Based Free-space Optical Transmission In the Presence of Atmospheric Turbulence

An LDPC-coded OAM-based FSO transmission system is experimentally studied in the presence of emulator-induced-scintillation. Coding gains >6.8dB are obtained at BER=10-4 for single OAM mode and dramatic improvement is found in case of OAM multiplexing.

Coded Orbital Angular Momentum Modulation and Multiplexing Enabling Ultra-High-Speed Free-Space Optical Transmission

To simultaneously achieve high throughput and low energy consumption, in this chapter we advocate the use of energy-efficient N-dimensional coded orbital angular momentum (OAM)-based modulation and multiplexing for ultra-high-speed optical transmission over free-space optical (FSO) links. OAM is associated with the azimuthal phase dependence of the complex electric field. Because its eigenvectors are orthogonal, they can be used as basis functions for multidimensional signaling. Because the information capacity is linear in the number of dimensions, we can dramatically improve the overall optical channel capacity through multidimensional signal constellations. On the other hand, energy efficiency can be achieved by properly designing the N-dimensional signal constellation. To deal with time-varying FSO channel conditions, OAM modulation can be combined with rateless coding. Atmospheric turbulence effects can be handled, in addition to FSO-MIMO, through the use of the azimuthal phase correction method, similar to the Gerchberg-Saxton (GS) phase retrieval algorithm. Moreover, distortions introduced by turbulence will be compensated for via adaptive optics approaches. Additionally, LDPC-coded OAM-based free-space optical FSO transmission system is experimentally studied in both the absence and presence of emulator-induced atmospheric turbulence.

High-speed continuous-variable quantum key distribution over atmospheric turbulent channels

We experimentally demonstrate a RF-assisted four-state continuous-variable quantum key distribution (CV-QKD) system in the presence of turbulence. The atmospheric turbulence channel is emulated by two spatial light modulators (SLMs) on which two randomly generated azimuthal phase patterns are recorded yielding Andrews’ azimuthal phase spectrum. Frequency and phase locking are not required in our system thanks to the proposed digital phase noise cancellation (PNC) stage. Besides, the transmittance fluctuation can be monitored accurately by the DC level in this PNC stage, which is free of post-processing noise. The mean excess noise is measured to be 0.014, and the maximum secret key rate of >20Mbit/s can be obtained with the transmittance of 0.85, while employing the commercial PIN photodetectors.

Non-uniform signaling based LDPC coded modulation for high-speed optical transport networks

Different non-uniform (probability shaping) signaling schemes are introduced in this invited paper. By transmitting symbols with different probabilities, energy efficiency (shaping gain) of traditional schemes can be improved. With constellation points selected according to Maxwell-Boltzmann distribution, ultimate shaping gain of 1.53dB can be achieved.

Experimental evaluation of LDPC-coded OAM based FSO communication in the presence of atmospheric turbulence

A low-density parity check (LDPC)-coded orbital angular momentum (OAM)-based free-space optical (FSO) transmission system is experimentally studied in both the absence and presence of emulator-induced atmospheric turbulence. In the presence of turbulence, the coding gains larger than 6.8 dB are obtained at BER of 10-4 for single OAM mode and dramatic improvement is found in case of LDPC-coded OAM multiplexing of states ±2, ±5 as uncoded case enters into an early error floor phenomenon.

High-speed Orbital Angular Momentum Multiplexed Free-space Optical communication systems

We review recent progress in high-speed orbital angular momentum (OAM) multiplexed free-space optical communication systems. The outdoor atmospheric turbulence is emulated by an indoor turbulence emulator, which is based on split-step beam propagation method. Adaptive optics, channel coding, Huffman coding combined with LDPC coding, and spatial offset are used for turbulence mitigation; while OAM multiplexing and wavelength-division multiplexing (WDM) are applied to boost aggregate capacity.

Beyond 1 Tb/s free-space optical transmission in the presence of atmospheric turbulence

In this invited paper, we review the approach to achieve beyond 1 Tb/s free-space optical (FSO) transmission in the presence of atmospheric turbulence, which is enabled by wavelength division multiplexing, orbital angular momentum multiplexing, and polarization division multiplexing; in combination with higher order modulation formats.

Experimental study of nonlinearity tolerant modulation formats based on LDPC coded non-uniform signaling

Nonlinearity tolerant 5-QAM and 9-QAM are experimentally studied for long-haul WDM transmission. Compared to QPSK and 8-QAM, the transmission reach is extended for 12% and 222% by using LDPC-coded non-uniform 5-QAM and 9-QAM, respectively.