Stress induced birefringence tuning in femtosecond laser fabricated waveguides in fused silica

Abstract

Femtosecond laser exposure produces form and stress birefringence in glasses, mainly controlled by laser polarization and pulse energy, which leads to challenges in certain applications where polarization mode dispersion or birefringence splitting is critical for the desired responses from optical devices. In this paper, parallel laser modification tracks with different geometries were applied to preferentially stress the laser-written waveguides and explore the possibility of tuning the waveguide birefringence in devices fabricated in bulk fused silica glass. Polarization splitting in Bragg grating waveguides showed the laser modification tracks to controllably add or subtract stress to the pre-existing waveguide birefringence, demonstrating independence from the nanograting induced form birefringence and the contributions from material stress. Stressing bars are shown that offer tunable birefringence in the range from ~0 up to 4.35 × 10(-4), possibly enabling great flexibility in designing polarization dependent devices, as well as making polarization independent devices.

Compact Birefringent Waveplates Photo-Induced in Silica by Femtosecond Laser

“…By measuring the diffraction efficiency η = I1/I0 of the gratings (where I1 is the diffracted power in the first order and I0 the incident power of the He-Ne beam), we determined that 0.5 or 1 μm spacing allows suppression of this diffraction grating effect and ensures "homogenous" squares but leads also to higher retardance that is likely due to cumulative stress [23] arising from anisotropic shape of the confinement volume. Notice that recently, this stress-induced birefringence has been successfully engineering in femtosecond laser fabricated waveguides in fused silica [11]. In addition, the measurement of these thick Bragg holograms angular acceptances ΔθB (ΔθB ≈ 2Λ/teff) allow the determination of the effective thickness, teff, that is estimated to be around 30 μm for the conditions that achieve strong retardance (e.g., 250 nm @ λ = 546 nm) and thus a birefringence as high as B ~ (0.80 ± 0.05) × 10…”

“…Extremely large stresses and static pressures up to 10-20 GPa have recently been tentatively measured around these regions [10]; these are characteristic of extreme formation conditions that do not appear to have an analogous counterpart in nature and therefore open up a fascinating region of dynamic glass processing. Recently, Fernandes et al have used 3D waveguides to engineer the stress-induced birefringence in the nanogratings regime [11]. This offers tunable birefringence up to 4 × 10 −4 , possibly enabling great flexibility in designing polarization dependent devices, as well as making polarization independent devices.…”

Direct measurement of second-order coupling in a waveguide lattice

“…Therefore, it influences the propagation dynamics only slightly, which makes κ 2 hard to measure directly in light propagation experiments. In fact, the impact of κ 2 on the light propagation is indistinguishable from the one of the central site 2 being detuned with respect to the outer ones, a situation which can occur in laser-written waveguides due to stress fields 20,21 . For acquiring experimental access to the second-order coupling, which is not obstructed by nearest-neighbour coupling dynamics, we resort to stationary solutions of the coupled system.…”

Fabricating waveguide Bragg gratings (WBGs) in bulk materials using ultrashort laser pulses

“…WBG birefringence may arise from the choice of substrate [49], laser-induced stresses [62], asymmetric waveguide modes [23], or self-aligned nanogratings [63,64]. The ULI technique gives researchers access to pathways of birefringence manipulation, thus opening an avenue to creating 3D polarisation-dependent/independent optical circuits.…”

“…To preferentially stress and tune existing WBG birefringence, Fernandes et al fabricated parallel laser modification tracks, also known as stressors, around WBGs in fused silica [62] (Figure 13). Maximal change was obtained by placing the stressors as close to the WBG as possible without inducing coupling between the WBG and stressor tracks.…”