Ultrafast laser machining of embedded glass stress/fatigue sensors based on whispering gallery mode resonators
Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
Final Report Abstract
In this project the fabrication of whispering gallery mode (WGM) waveguides by ultrafast laser writing in glass and their applicability as non-destructive stress-sensors were investigated. Such sensors consist of a ring-waveguide in which light is coupled from a straight waveguide and the out-put spectrum of the ring-resonator is measured. The measured modespacing (free spectral range) depends on the refractive index of the ring-waveguide. Therefore, a change in refractive index due to internal stresses can be monitored. The main challenge was the fabrication of single-mode waveguides with small enough propagation losses, especially bending losses, to allow for measuring the spectrum with a high signal-to-noise ratio. To increase the refractive index of waveguides, we investigated the influence of overwriting an existing waveguide. We found that by applying several overwriting tracks the mode field diameter (MFD) in both directions (along and orthogonal to the optical axis of the incident laser) can be influenced. While in the orthogonal direction only the total amount of deposited energy, described by the representative net fluence (RNF), is important for the change, along the laser the particular parameters play a role. We found that the achievable MFD depends on the writing speed with lower writing speeds yielding smaller diameters. As the rate of change in both directions is different, the ellipticity and therefore polarisation-sensitivity of the waveguides can be influenced. In addition, for round waveguides the coupling and bending losses will not depend on the particular 3D orientation of the WGM. For fabrication of arbitrarily orientated waveguides we also investigated a dynamic correction of the spherical aberration. Spherical aberrations depend on the depth of writing and will decrease the intensity in the focus spot and thus change the properties of waveguides. We used a liquid crystal on silicon spatial light modulator (SLM) for correcting the aberrations. As the refresh rate of our SLM was 30 Hz we were limited to apply a correction every 50 µm (already at slow writing speeds). We found that at the position, where the correction was changed, the waveguides were interrupted. This might be either due to a decreased diffraction efficiency during the change or due to a focus shift between two correction terms. Further investigation on the cause of these interruptions and possible ways for mitigation are needed the make 3D fabrication with a SLM viable. We investigated the coupling efficiency between two waveguides experimentally and with simulations for estimating the power that is transferred into the ring-resonator. The coupling per unit length is decreased for an increased refractive index, e.g. due to overwriting, which can be explained by a smaller mode field diameter and therefore smaller overlap of the evanescent field to the neighbouring waveguide. We investigated the coupling behaviour if the refractive index of only one waveguide is increased (e.g. of the ring to decrease bending losses) and found that this also leads to worse coupling per unit length. From these evaulations we estimate that the interaction length between the ring and straight part of the WGM has to be longer than 600 µm to achieve more than 20% coupling efficiency. Finally, we estimated the maximum change in refractive index in glass close to the point of failure to be ∆n = 10^-5 by simulations in ABACUS. From this and typically spectral resolutions of a detection system, we infer that the diameter of the ring-waveguide has to be on the order of a few hundred micrometre. Therefore, we conclude that the proposed stress-sensors will not work in a practical application due to tremendous propagation (bending) losses and small coupling efficiency.
Publications
- "Determination of laser beam focus position based on secondary speckles pattern analysis," Appl. Opt. 56, 7413-7418 (2017)
Ilya Alexeev, Ji Wu, Michael Karg, Zeev Zalevsky, and Michael Schmidt
(See online at https://doi.org/10.1364/AO.56.007413)
