Femtosecond laser inscription of optical waveguide-based devices on lithium niobate

Abstract

This work explored the use of high-repetition-rate femtosecond laser pulses for the direct writing of optical waveguide-based devices in a sample of z-cut lithium niobate crystal. The effects of inscribing parameters including pulse energy, writing speed, writing direction and focus depth on the optical and physical properties of a laser-induced straight structure were revealed, and systematically investigated for the optimal regime of low-loss waveguide fabrication. Also, the impacts of optical aberrations due to refractive index mismatch and birefringent astigmatism on the laser focus were numerically studied and discussed with the results obtained from the experiments. To test the thermal-dependent characteristics of an inscribed sample, a series of heat treatments in a temperature range between 250 ºC and 950 ºC was applied. It was found that the heats up to the temperature of 500 ºC enhanced the overall refractive index contrast of a Type-II laser modification, and also reduced the residual stress. For the temperatures greater than 500 ºC, the annealing resulted in the deterioration of an inscribed structure. In the part of straight waveguide fabrication, an optical-lattice-like geometry which consisted of multiple damaged tracks arranged in a multi-layer hexagonal packing was implemented for writing the depressed-cladding waveguide with various sets of inscribing parameters. To address the laser-focusing issue stemmed from the spherical aberration effects, the pulse-energy variation schemes were applied – resulting in more circular and symmetric optical mode-fields. The lowest propagation losses of (0.4 ± 0.1) dB/cm and (3.5 ± 0.2) dB/cm for TE and TM polarised light, respectively, at 1550 nm were achieved after thermally annealed at 350 ºC for 3 hours. In addition, the waveguides were found to be thermally stable and showed the low-loss guiding up to the temperature of 700 ºC. The fundamental guiding mode was observed over a wide range of spectral from 500 nm to 1550 nm. Our laser inscription technique was also applied to fabricate the s-bend structures and power splitters which were based on the multi-mode interference. The computer simulation in COMSOL was used to optimize the interference pattern inside the waveguides such that the high intensity transmission could be obtained. The lowest insertion losses in TE mode of (4.23 ± 0.14) dB and (4.31 ± 0.20) dB for the two-output and three-output splitters, respectively, at 1550 nm were measured after the sample was annealed at 250 ºC for 3 hours. Besides, the integration of s-bends and the splitter to allow the wider separation (293.7 μm) between two splitter’s output branches was demonstrated. The insertion loss of this structure was found to be (4.96 ± 0.17) dB, and the splitting ratio of 0.48:0.52 was achieved for the TE propagation mode.