Liquid Crystals Control Polarization in Laser-Written Waveguides

Researchers in Germany have developed a way to control and manipulate optical signals by embedding a liquid crystal layer into waveguides created with direct laser writing. The work could lead to devices that enable electro-optical control of polarization. Such devices could open possibilities for chip-based devices and complex photonic circuits based on femtosecond-written waveguides.

According to Alessandro Alberucci, researcher from Friedrich Schiller University in Jena, the advance could be beneficial for other data-intensive applications in and beyond the data center. Alberucci added that the technology could also find application in the experimental realization of dense optical neural networks.

Liquid crystal-enabled control over polarization inside laser-written waveguides opens a path to possibilities for chip-based devices and complex photonic circuits based on femtosecond-written waveguides. Researchers used a femtosecond writing tool to embed a liquid crystal layer into a waveguide. Courtesy of Friedrich Schiller/University of Jena.

The researchers combined two fundamental photonic technologies by embedding a layer of liquid crystal inside a waveguide. When the beam propagating inside the waveguide entered the liquid crystal layer, it modified the light’s phase and polarization upon the application of an electric field. The modified beam then traveled through the second section of the waveguide, so that a beam with modulated properties was propagating. The fused silica waveguide contained a tunable waveplate. The researchers used the system to demonstrate full modulation of optical polarization at two visible wavelengths.

“Our work paves the way to integrating new types of optical functions into the whole volume of a single glass chip, enabling compact 3D photonic integrated devices that weren’t possible previously,” Alberucci said. “The unique 3D nature of femtosecond-written waveguides could be used to create new spatial light modulators where each pixel is separately addressed by one waveguide.”

Femtosecond lasers can be used to write waveguides deep within a material, as opposed to only on the surface like other methods, making it a promising approach to maximize the number of waveguides on a single chip. This approach involves focusing an intense laser beam inside a transparent material. When the optical intensity is high enough, the beam modifies the material under illumination, thus acting like a sort of pen with micrometer precision.

“The most important shortcoming of using femtosecond laser writing technology to create waveguides is the difficulty in modulating the optical signal in these waveguides,” Alberucci said. “Since a complete communication network needs devices capable of controlling the transmitted signal, our work explores new solutions to overcome this limitation.”

Researchers Alessandro Alberucci (left) and Kim Lammers injected laser light into waveguides and then varied the voltage applied to the liquid crystal layer to modulated the light. Courtesy of Friedrich Schiller/University of Jena.

Although optical modulation in femtosecond laser-written waveguides has previously been achieved by locally heating the waveguide, the use of liquid crystals, such as in the recent work, allows direct control of the polarization. Benefits of the approach, Alberucci said, include lower power consumption; the possibility to address single waveguides in the bulk independently; and decreased crosstalk between adjacent waveguides.

Further, although the use of liquid crystals for/as modulators is well established, the work helps to chart a course for the use of liquid crystal properties as a modulator in photonic devices that have waveguides embedded in their whole volume, Alberucci said.

As the research remains a proof of concept, more work will be needed before the technology is ready for practical applications, according to the researchers. For example, the current device modulates every waveguide in the same manner. So, the researchers are aiming to achieve independent control on each waveguide.

The research was published in Optical Materials Express (www.doi.org/10.1364/OME.507230).