Polarization Enables High-Speed Spectroscopy Measurements

Researchers at Tampere University and their collaborators have shown how the speed of spectroscopic measurements can be made much faster. By correlating polarization to the color of a pulsed laser, the researchers tracked changes in the spectrum of the light by simple and extremely fast polarization measurements. The method opens new possibilities to measure spectral changes on a nanosecond timescale over the entire color spectrum.

Spectroscopic measurements usually rely on separating the different color components to different positions, where the spectrum can then be read out by a detector array. While this approach enables a direct inspection of the spectrum, it is rather slow due to the limited speed of the large read-out array.

The new method the researchers implemented circumvents this limitation by generating a more complex state of laser light.

“Our work shows a simple way to have different polarizations for all color components of the laser. By using this light as a probe, we can simply measure the polarization to gain information about changes in the color spectrum,” said Lea Kopf, lead author of the paper and a doctoral researcher at Tampere University.

Kopf and her team performed a modulation into the temporal domain by coherently splitting a femtosecond laser pulse into two parts — each having a different polarization slightly delayed in time with respect to each other.

“Such a modulation can easily be done using a birefringence crystal, where differently polarized light travels at different speeds. This leads to the spectrally changing polarization required for our method,” said Robert Fickler, associate professor and leader of the Experimental Quantum Optics group at Tampere University.

Conceptual of the method of using spectrally varying polarization states for high-speed spectroscopic measurements. Courtesy of Frederic Bouchard/National Research Council of Canada.

The researchers not only demonstrated how such complex states of light can be generated in the lab; they also tested their application in reconstructing spectral changes using only polarization analysis. As the latter only requires up to four simultaneous intensity measurements, a few very fast photodiodes can be used.

Using this approach, the researchers reported they can determine the effect of narrowband modulations of the spectrum at a precision that is comparable to standard spectrometers, but at high speed. Still, Kopf said, they could not push their measurement scheme to its limits in terms of possible read-out states. The researchers remain limited by the speed of their modulation scheme to a few million samples per second.

Future tasks that will build on the initial result will apply the idea to more broadband light, such as supercontinuum light sources, and to apply the scheme in spectroscopic measurements of naturally fast, varying samples to use its full potential.

“We are happy that our fundamental interest in structuring light in different ways has now found a new direction which seems to be helpful for spectroscopy tasks which are usually not our focus,” Fickler said. “As a quantum optics group, we have already started discussing how to apply and benefit from these ideas in our quantum photonics experiments.”

The research was published in Optica (www.doi.org/10.1364/OPTICA.424960).