Characterization of birefringent Bragg gratings waveguides inscribed with the Femtoprint device

Abstract

Since their advent, femtosecond (fs) laser pulses have been effectively employed to create micro and nano structures within host materials, such as silica glass. Their application is of growing interest in the fields of photonics and optomechanics and numerous achievements have been obtained to date. The technology has matured to such a degree that automated production processes like the FemtoPrint machine are now commercially available. While dealing with such powerful tools, users need to optimize the energy per pulse and polarization state to create effective structures on purpose. Thereby, studying the effect of these parameters on the relative quality of the fs laser pulses-engineered structures is of prime importance and can bring assistance or even a useful methodology for the scientific community using this tool or an equivalent technique. In this study, our focus revolves around exploring the characteristics of optical waveguides and in-built Bragg gratings created with the Femtoprint process in a flat glass substrate to extract pertinent information regarding their physical and optical properties, such as dimensions, refractive index modulation and birefringence. To that aim, we rely on three advanced methodologies: Digital holographic Microscope (DHM) analysis, polarization-based spectral measurements and infrared camera imaging, respectively. This analysis reveals important findings about the actual implemented refractive index modulation. For the investigated pulse energy (130 nJ), repetition rate (1 MHz) and scanning speed of the fs laser pulses beam, we show that the refractive index modification in the waveguide determined by DHM analysis lies in the range of 10^-3. This value is the highest reported so far in waveguides at this relatively low energy and high repetition rate of the laser pulses. Besides, the Bragg grating inscribed in the waveguide shows a spectral separation between the Bragg modes (300 pm) corresponding to an effective birefringence of the waveguide of 1.36 10^-4. This value depends on the polarization of the writing beam with respect to the scanning direction.