Coherent detector for the non-separability measurement of vectorial structured light

In free-space optical communication scenarios, atmospheric turbulence distorts the complex wavefront of signal-carrying light beams, leading to bit errors and even communication interruptions. Note that the spatial-polarization non-separability of vectorial structured light remains invariant in unitary and one-sided channels, and atmospheric turbulence is a typical kind of unitary and one-sided channels. This provides a convincing and new solution for turbulence-resilient communications based on structured light—packing information into the non-separability of vectorial structured light. To establish such an efficient free-space optical communication link based on non-separability coding method, precise and fast detection of the non-separability value of the received vectorial structured light is urgent required.

 

In a new paper published in Light: Science & Applications, a team of scientists, co-led by Professor Xiaopeng Shao from Xidian University, and Professor Jian Wang from Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, and co-workers have proposed and demonstrated a coherent detector to efficiently detect the non-separability of vectorial structured light, which addresses the mentioned requirement. The coherent detector splits two circularly polarized components of vectorial structured light and reconstructs their complex phasefronts based on off-axis digital holography technique. Then they digitally calculate the overlapping degree (inner product) of between measured complex wavefronts and ideal basic modes, which allows efficient non-separability characterization through digital processes. The coherent detector enables single-shot detecting the non-separability of vectorial structured light with low spatial complexity.

 

They show that their coherent detector can be utilized in two non-separability measurement scenarios, i.e., 1) detecting the non-separability values of different vectorial structured light beams with the same mode index of 2; 2) characterizing the non-separability contributions of different modal indices for vectorial structured light superposition states. For each scenario, they accomplish 50 times of independent measurements to evaluate the accuracy and robustness of this coherent detector. Experimental results indicate that such a coherent detector is capable of detecting the non-separability values of vectorial structured light beams with high efficiency, low complexity and enough stability.

 

Different from traditional non-separability detection methods that directly measure the light intensity, the coherent detector proposed in this work detects the phasefronts of light beams to characterize the non-separability. Moreover, this non-separability coherent detector forgoes demodulating basic spatial states using bulky spatial light modulators (SLMs) or digital micromirror devices (DMDs) yet instead sorting them in digital processing. Due to the completely different principle, the proposed coherent detector features lots of advantages compared to traditional direct detection methods, including single-shot detection, low spatial complexity, and does not require bulky and high-cost SLM/DMD.

 

These scientists summarize the future visions of their technique:

“We believe our findings may pave the way for turbulence-resilient optical communications based on vectorial structured light beams. For example, the coherent detector can be applied to establish a free-space optical communication system that loads signals onto the non-separability of vectorial structured light beams. In this case, our coherent detector is placed at the receiver to achieve efficient detection of the non-separability values of received beams, thus demodulates the transmitted signal.”

 

“Our work brings new insights into the characterization of vectorial structured light beams. It suggests that the modal tomography process, which is required in detecting the non-separability of vectorial structured light, can be accomplished through a digital way. Thus, many traditional modal tomography processes, which widely exist in structured light detection scenarios, can be simplified to digital calculations.”

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