Polarization electro-optic control is realized by femtosecond laser writing and liquid crystal modulation

Researchers in Germany have developed a novel method of optical signal control by combining femtosecond laser writing and liquid crystal electro-optic modulation. By embedding liquid crystal layer into the waveguide, the electro-optical control of the beam polarization state is realized. The technology opens up entirely new possibilities for chip-based devices and complex photonic circuits made using femtosecond laser writing technology. The research team detailed how they made tunable wave plates in fused silicon waveguides. When a voltage is applied to the liquid crystal, the liquid crystal molecules rotate, which changes the polarization state of the light transmitted in the waveguide. In the experiments conducted, the researchers successfully completely modulated the polarization of light at two different visible wavelengths (Figure 1).

Combining two key technologies to achieve innovative progress in 3D photonic integrated devices
The ability of femtosecond lasers to precisely write waveguides deep inside the material, rather than just on the surface, makes them a promising technology to maximize the number of waveguides on a single chip. The technology works by focusing a high-intensity laser beam inside a transparent material. When the light intensity reaches a certain level, the beam changes the properties of the material at its point of application, just like a pen with micron accuracy.
The research team combined two basic photon techniques to embed a layer of liquid crystals in the waveguide. As the beam travels through the waveguide and through the liquid crystal, the phase and polarization of the beam change once an electric field is applied. Subsequently, the modulated beam will continue to propagate through the second part of the waveguide, thus achieving the transmission of the optical signal with modulation characteristics. This hybrid technology combining the two technologies enables the advantages of both in the same device: on the one hand, the high density of light concentration brought about by the waveguide effect, and on the other hand, the high adjustability of the liquid crystal. This research opens up new ways to use the properties of liquid crystals to embed waveguides in the overall volume of devices as modulators for photonic devices.

Application and advantages of liquid crystal in femtosecond laser waveguide modulation

Although optical modulation in femtosecond laser writing waveguides was previously achieved primarily by applying local heating to the waveguides, in this study, polarization was directly controlled by using liquid crystals. “Our approach has several potential advantages: lower power consumption, the ability to process individual waveguides independently, and reduced interference between adjacent waveguides,” the researchers note. To test the device’s effectiveness, the team injected a laser into the waveguide and modulated the light by varying the voltage applied to the liquid crystal layer. The polarization changes observed at the output are consistent with theoretical expectations. The researchers also found that after the liquid crystal was integrated with the waveguide, the modulation characteristics of the liquid crystal remained unchanged. The researchers stress that the study is merely a proof of concept, so there is still a lot of work to be done before the technology can be used in practice. For example, current devices modulate all waveguides in the same way, so the team is working to achieve independent control of each individual waveguide.