Structured light waves with spiral phase fronts include orbital angular momentum (OAM), which is linked to photon rotational motion. Light waves have recently been used with OAM by scientists, and these particular “helical” light beams have become highly essential in different modern technologies such as communication, imaging, and quantum information processing.
On-axis Kramers-Kronig interferometry retrieves the spectrum of orbital angular momentum in a single-shot. Image Credit: Z. Lin, J. Hu, et al., doi 10.1117/1.AP.5.3.036006
It is critical with these technologies to understand the precise structure of these unique light beams. This, however, has proven to be extremely difficult.
Using a camera, interferometry (superimposing a light field with a known reference field to extract information from the interference) can gather OAM spectrum information.
Since the camera only captures interference intensity, the measuring approach meets extra crosstalk known as “signal-signal beat interference” (SSBI), complicating the recovery process. It is like hearing multiple overlapping sounds that make distinguishing the original notes impossible.
Researchers from Sun Yat-sen University and École Polytechnique Fédérale de Lausanne (EPFL) employed a sophisticated mathematical tool called the Kramers-Kronig (KK) connection to aid in recognizing and resolving the problem in a recent breakthrough described in Advanced Photonics.
The method was used to decipher the complicated helical light pattern from the camera’s intensity-only information for single-shot recovery in simple on-axis interferometry. Also, the KK technique was utilised to examine diverse OAM fields, including Talbot self-imaged petals and fractional OAM modes, to investigate the duality between the time-frequency and azimuth-OAM domains.
The new measuring approach has the potential to significantly advance technology that relies on these unique light patterns.
The proposed method can also be generalized for OAM beams with complex radial structures, making it a powerful technique for real-time measurement of structured light fields, simply by a snapshot with a camera.
Jianqi Hu, Study Corresponding Author and Postdoctoral Scholar, Laboratoire Kastler Brossel
Compared to traditional on-axis interferometry, the KK approach revealed by the researchers not only speeds up the measurement but also makes it considerably more straightforward and less expensive.
Scientists now have a strong tool to explore the secrets of structured light waves with the help of this new approach. This invention can potentially transform different technologies, opening the door for new breakthroughs in the field of structured light in the near future.
Journal Reference:
Lin, Z., et al. (2023) Single-shot Kramers–Kronig complex orbital angular momentum spectrum retrieval. Advanced Photonics. doi:10.1117/1.AP.5.3.036006