Measuring Polarization Simultaneously

This image shows the polarization of light emitting from each of 180 cores of an optical fiber. (Source: U Rochester / M. Alonso)

This image shows the polarization of light emitting from each of 180 cores of an optical fiber. (Source: U Rochester / M. Alonso)

Multi­core optical fibers — capable of trans­mitting hundreds of inde­pendent laser beams at the same time — will be more feasible thanks to researchers at the University of Rochester. They have found a simple, inex­pensive way to measure the polari­zation states of each of those beams simul­taneously. Doing so will make it easier to produce next-generation endo­scopes to diagnose cancer and high-power laser amplifiers for communi­cations.

The research by Miguel Alonso and Thomas Brown of the Institute of Optics and colla­borators at two universities in France is an example of more than 60 University of Rochester research projects that will be showcased at Frontiers in Optics (FiO) 2016. The 100th annual meeting of the Optical Society (OSA), which will bring together optical researchers, engineers, and industry leaders from across the United States and more than 40 countries, is being held in Rochester October 17 to 21 for the 25th time.

The annual meeting will be an oppor­tunity for University researchers from the Institute of Optics, the Laboratory for Laser Energetics, and from the Depart­ments of Physics, Biomedical Engi­neering, Electrical and Computer Engi­neering, Mechanical Engineering, Neuroscience, and Ophthal­mology to showcase their expertise in such areas as freeform optics, medical imaging, quantum optics, and terahertz imaging. “There’s a saying in a variety of different fields, but espe­cially in optics, that if you can’t measure it, you can’t make it,” says Brown. “Being able to do good measure­ments is always a pre­requisite to being able to do precise design and precise manu­facturing.”

Especially when trying to send beams of light through multiple optical fiber cores at once. The quality of what comes out the other end depends on being able to measure and control the polari­zation states — the orien­tation and ellip­tical properties of the “wiggle” in the electric field — in each of those beams. Otherwise, when the beams are combined at the end, “you’re going to get all kinds of random effects associated with the focal spot,” Brown says. For example, lens-less endo­scopes and high-power laser ampli­fiers using multi-core fibers require that the multiple beams have identical properties in order to generate highly intense, localized beams.

Alonso, Brown, and their colla­borators have come up with a relatively inexpensive device to capture the polari­zation states of laser beams as they are emitted from each of multiple cores. The device converts those polari­zation states into spatial shapes — or “polari­zation signatures” — that can be recorded on camera, then quanti­tatively analyzed. Called a stress engineered optical element (SEO), the device consists of a cylindrical glass window and an optical filter similar to those used in 3-D movies, placed between a fiber bundle and a camera. The researchers say it that can be inte­grated into any imaging system.

“To the best of our knowledge, this technique provides the first and fastest measurement of the polari­zation state of light traversing more than 100 inde­pendent fiber cores in a multicore fiber bundle,” Alonso says. “Meanwhile, this method allowed us to charac­terize how polari­zation is scrambled by the fibers, especially when the fibers are twisted or distorted. We found that the polari­zation state of light is surpri­singly robust to manipulation of the fibers.”

By measuring the polari­zation states of multiple beams at once, the device opens the door to creating a feedback loop to adjust polari­zation at the input, so that all the beams will be identical at output. “You could do real-time correc­tions,” Brown said. (Source: U Rochester)

Links: Frontiers in Optics (FiO) 2016, Rochester, New York, USA • Institute of Optics, University of Rochester, USA

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