Optimization of femtosecond laser written waveguides in germanate glass for efficient lasing in near-IR

The development of low-loss optical waveguides holds paramount significance in the realm of monolithic photonic integrated circuits, enabling efficient light propagation and integration of various components on a single chip. Optical waveguides are integral optical sources for these photonic integrated circuits and are gaining importance in fields such as LiDAR, atmospheric monitoring, optical communication, etc. to count only a few. We present a novel approach to fabricate high-quality, low propagation loss waveguides in a lead-germanate glass (GPGN) possessing high linear and nonlinear refractive indices. This approach of waveguide fabrication is referred to as femtosecond laser direct writing (FLDW) technique. This technique enables short cavity (microscale level) laser operation in the near-IR spectral region. By employing different pulse energies and writing regimes (i.e. athermal (100 kHz), thermal (5 MHz), and intermediate (1 MHz)) a series of single line and double line waveguides (WGs) were inscribed in the sample to determine the optimal femtosecond laser (FSL) parameter set for inducing high quality 3D waveguides in GPGN glass. The high nonlinear refractive index (n2) of germanate glass produced strong self-focusing effects in the sample for all FSL repetition rates employed in the study. These self-focusing effects resulted in non-uniform guiding structures in the sample that added to the propagation losses of the WGs. The double-line waveguide fabricated with a 1 MHz pulse repetition frequency exhibited the lowest propagation loss of ~ 0.2 dB/cm at 1550 nm. To the best of our knowledge, this low-loss waveguide demonstrated the highest laser slope efficiency of 27%. These findings underscore the considerable potential of GPGN glass for efficient lasing applications within the near-IR spectral region.