Embedded Files
Joe Wiedemann (Class of '21) .
Scintillation of laser beams carrying orbital angular momentum propagating in a near-maritime environment
Joe Wiedemann, Charles Nelson, Svetlana Avramov-Zamurovic
Abstract
Laser beams carrying orbital angular momentum (OAM) were propagated in a near-maritime environment along an 890 meter link across the Severn River at the United States Naval Academy. The OAM beams were generated using green light ( λ = 532 nm) and etched spiral phase plates with different topological charges. The light intensity fluctuations of Gaussian and beams carrying OAM with topological charges of 1, 6, and 8 were measured and the scintillation indices of each beam were compared. Additionally, atmospheric turbulence was measured using a co-aligned scintillometer along the 890 meter propagation path. During testing, a wide range of atmospheric conditions occurred and the refractive index structure parameter measurements ranged from 5 × 10−14 down to 5 × 10−15 m −2∕3. Our measurements of transmitted voltage in the radial intensity across the maritime link, and calculated scintillation index, for the Gaussian beam and laser beams carrying OAM indicate a weak reduction in scintillation index for increasing topological charge. To reduce the measurement uncertainty and improve upon statistical significance of our findings, additional testing is needed.
Fig. 1. Experimental set up: (a) 890 m laser and scintillometer links at the US Naval Academy [21], (b) scintillometer transmitter (c) laser diode system set up, (d) scintillometer receiver (e) beam carrying OAM with topological charge 6 (540 m link) and (f) beam carrying OAM with topological charge 1 (540 m link).
Conclusion
We established an 890 m link across the Severn river for a Gaussian laser beam along with laser beams carrying OAM with 3 different topological charges 1, 6, and 8 in order to create a novel test to explore the behavior of these beams in a near maritime environment. As expected with testing in an open field environment, the experimentation condition variations were a significant challenge in obtaining data sets that were acceptable for the analysis. However, these variations also serve as a more comprehensive lens into the behavior of laser beams Fig. 6. 𝐶 2 𝑛 as a function of mean voltage measured using the photodetector. carrying OAM in a near maritime environment. The weak decrease in scintillation index for increasing topological charge still carries statistical uncertainty. Nonetheless, as our analysis has shown through comparison of the measured scintillation index and ratio of scintillation index to 𝐶 2 𝑛 , there is a noticeable possibility that the trend exists. Our outdoor experiment is an important bridge in the exploration of laser beams carrying OAM that warrants testing beyond laboratory experiments and simulations into further testing in real turbulence. Relevant literature has found that there is a reduction in SI in the vortex [23] of the laser beam carrying OAM for higher topological charges, and it is encouraging that our experimentation could suggest a similar trend for the radial beam intensity, which is actually used to transmit data. Future testing could consider the use of a beam splitter to simultaneously propagate a Gaussian beam with a beam carrying OAM in order to subject the two beams to nearly the same levels of optical turbulence, and ensuring capturing a uniform intensity at the detector point for various scenarios tested.
Page updated
Report abuse