Low bend loss femtosecond laser written waveguides exploiting integrated microcrack

Abstract

We introduce the fabrication and use of microcracks embedded in glass as an optical element for manipulating light propagation, in particular for enhancing waveguide performance in silica integrated optics. By using a femtosecond laser to induce a strong asymmetric stress pattern in silica, uniform cracks with set dimensions can be created within the substrate and propagated along a fixed path. The smoothness of the resulting cleave interface and large index contrast can be exploited to enhance waveguide modal confinement. As a demonstration, we tackle the longstanding high bend-loss issue in femtosecond laser written silica waveguides by using this technique to cleave the outer edge of laser written waveguide bends, to suppress radiative bend loss. The microcrack cross section is estimated to be 15 μm in height and 30 nm in width, for the 10 $$\times$$ × 10 μm waveguides. At 1550 nm wavelength, losses down to 1 dB/cm at 10 mm bend radius were achieved, without introducing additional scattering. Both the cleave stress pattern and waveguide are fabricated with the same multiscan writing procedure, without requiring additional steps, and re-characterisation of the waveguides after 1 year confirm excellent long term performance stability.

Toward Higher Integration Density in Femtosecond-Laser-Written Programmable Photonic Circuits

“…Single-mode operation has been optimized for a wavelength of 940 nm, achieving propagation losses down to 0.14 dB/cm and negligible bending losses at curvature radii as low as R = 15 mm. This value shows an improvement of a factor of 2 or higher with respect to the one employed in previous laser-written UPPs [15,16], and it is in line with the state of the art for the FLM platform [25], where advanced techniques for the reduction in bending losses were introduced.…”

Laser-written Mach Zehnder interferometer in silica with surface-written sensing arms and sub-micron thick doped-polymer film for increased evanescent field interferometric methane sensing

“…The femtosecond laser writing system used to inscribe the waveguides uses 515 nm 200 fs pulses generated from the second harmonic of a 200 kHz 1030 nm Yb:KGW laser pump (PHAROS, Light Conversion Ltd.), see Lee et al 5 for further details. The beam is reflected by a dichroic mirror onto the objective lens (x25 0.4NA PLAN, Leitz Wetzlar) which focuses the light to the sample.…”

“…The bending radius can roughly be estimated as 3 cm which should be sufficient to suppress bending loss 5 and Fresnel reflection losses can be estimated as 0.36 dB. 10 Overall, it has been shown that waveguides written at or near the surface with estimated ∆n ≈ 3 × 10 −3 perform comparatively with bulk waveguides with α z > 0.6 dB cm −1 .…”

“…MZIs have also been demonstrated in coreless fiber using femtosecond laser inscription for refractive index sensing. 4 Writing in silica is attractive due to the considerable spatial overlap between the fundamental mode in SMF-28 and in previously demonstrated femtosecond laser inscribed waveguides, 5 which is attractive for integration.…”

Direct Writing of Channel Optical Waveguides in Er3+-Doped Aluminosilicate Glass by Low Repetition Rate Femtosecond Laser

“…The fact that waveguides can be fabricated in a large number of transparent dielectric materials, such as fused silica glass, phosphate glass, lithium niobate (LiNbO 3 ) crystal, and YAG crystal, − has been confirmed. Based on these advantages, various femtosecond laser inscribed photonic devices with passive and active functions, including waveguide coupler, waveguide laser, waveguide amplifier, etc., − have been successfully realized.…”