Coherent Separation Detection for Orbital Angular Momentum Multiplexing in Free-Space Optical Communications

Xiaoke Zhang,Yanliang He,Xinxing Zhou,Shuqing Chen,Ying Li,Yuanjiang Xiang,Yu Chen,Dianyuan Fan,Yao Cai,Lin Chen,Chenliang Su,Mingyang Su

doi:10.1109/jphot.2017.2694885

Xiaoke Zhang, Yanliang He + Show 10 more

Open Access

https://doi.org/10.1109/jphot.2017.2694885

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Abstract

Beams carrying orbital angular momentum (OAM) are very helpful in enhancing the information carrying capacity in free-space optical communications. However, atmospheric turbulence and energy attenuation will seriously affect communication quality and signal transmissions. Here, a novel coherent separation detection technology for OAM mode division multiplexing is proposed, and numerical simulation work is conducted. With the proposed structure, two light beams with different OAM states, each encoded with 16-quadrature-amplitude modulation orthogonal frequency-modulated (QAM-OFDM) signals, can be demultiplexed and allow two orders of magnitude bit error rate (BER) lower than direct separation detection. Moreover, we show the scalability of multiplexing four OAM beams, achieving a 20 m free-space transmission with BER below 3.8 × 10–3 for all channels at signal-noise ratio (SNR) 18 dB. Our results show that coherent separation detection has excellent antinoise performance and can effectively extend the communication distance. It also paves the way for entirely new coherent optical OAM communications.

Highlights

Angular momentum, which is one of the most fundamental physical quantities in both classical and quantum mechanics, comprises spin angular momentum (SAM) and orbital angular momentum (OAM) [1]–[4]
When the signal-noise ratio (SNR) is 18dB, the bit error rate (BER) performance of coherent separation is below 3.8 × 10−3 for all channels, while the SNR is 20 dB for traditional technologies when the BER is below 3.8 × 10−3 for all channels
As OAM was found to be a new multiplexing dimension, many studies had shown that high capacity communication can be achieved by employing OAM multiplexing, but the transmission distance is almost meter levels in these researches, and the BER is frustrated high because of the atmospheric turbulence and mode crosstalk [12]

Summary

Introduction

Angular momentum, which is one of the most fundamental physical quantities in both classical and quantum mechanics, comprises spin angular momentum (SAM) and orbital angular momentum (OAM) [1]–[4]. The intensity of a bright spot at the center of the beam that recovered by fork grating or spiral phase mask will gradually recede with increase in transmission distance and eventually vanish completely over the distance of 30 m. The phase of the recovered beam keep a Gaussian beam like without spiral phase at the different distance, which enable the coherent separation to detect signals. We can use the two technologies discussed above to keep a bright spot at the center, but the structure of improved spiral phase masks is only applicable for particular transmission distance of OAM beams, this limited the flexibility of OAM beam detection.

Atmospheric Turbulence for OAM Beam Distortion

Findings

Conclusion