Type II high-strength Bragg grating waveguides photowritten with ultrashort laser pulses

Manufacturing of Porous Glass by Femtosecond Laser Welding

Based on femtosecond laser glass welding, four different porous structures of welding spots were formed by the manufacturing processes of spatiotemporal beam shaping and alternating high repetition rate transformation. Compared with an ordinary Gaussian beam, the welding spot fabricated by the flattened Gaussian beam had smoother welding edges with little debris, and the bottom of the welding spot pore was flat. Instead of a fixed high repetition rate, periodically alternating high repetition rates were adopted, which induced multiple refractive indices in the welding spot pore. The welding spot pores manufactured by spatiotemporal beam shaping and alternating high repetition rate transformation have a special structure and excellent properties, which correspond to superior functions of porous glass.

Rapid and maskless nanopatterning of aluminosilicate glass surface via friction-induced selective etching in HF solution

A rapid and maskless nanofabrication method on glass was proposed, which consisted of scanning with a diamond tip and post-etching in HF solution. The masking effect on scan area is attributed to the formation of AlF₃during etching.

Laser-written photonic crystal optofluidics for electrochromatography and spectroscopy on a chip

Femtosecond laser processes were optimized for nonlinear interactions with various optical materials to develop a novel biophotonic lab-on-a-chip device that integrates laser-formed waveguides (WGs), microfluidic channels and photonic crystals (PCs). Such integration seeks the unique demonstration of dual PC functionalities: (1) efficient chromatographic separation and filtration of analytes through a porous PC embedded inside a microfluidic channel and (2) optofluidic spectroscopy through embedded WGs that probe PC stopband shifts as varying analyte concentrations flow and separate. The building blocks together with their integration were demonstrated, providing embedded porous PCs through which electrochromatography drove an accelerated mobile phase of analyte and an optical stopband was probed via integrated buried WGs. Together, these laboratory results underpin the promise of simultaneous chromatographic and spectroscopic capabilities in a single PC optofluidic device.

Femtosecond laser writing of optical edge filters in fused silica optical waveguides

The positional alignment of femtosecond laser written Bragg grating waveguides within standard and coreless optical fiber has been exploited to vary symmetry and open strong optical coupling to a high density of asymmetric cladding modes. This coupling was further intensified with tight focusing of the laser pulses through an oil-immersion lens to control mode size against an asymmetric refractive index profile. By extending this Bragg grating waveguide writing into bulk fused silica glass, strong coupling to a continuum of radiation-like modes facilitated a significant broadening to over hundreds of nanometers bandwidth that blended into the narrow Bragg resonance to form into a strongly isolating (43 dB) optical edge filter. This Bragg resonance defined exceptionally steep edge slopes of 136 dB/nm and 185 dB/nm for unpolarized and linearly polarized light, respectively, that were tunable through the 1450 nm to 1550 nm telecommunication band.

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

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.

Ultrafast laser inscription of Bragg-grating waveguides using the multiscan technique

We report the fabrication of high-strength (>30 dB) first order Bragg-grating waveguides in borosilicate glass substrates using ultrafast laser inscription. The cross section of each waveguide was controlled using the well known multiscan fabrication technique, where the desired waveguide cross section is constructed by scanning the sample through the laser focus multiple times. In order to fabricate high-strength gratings, it was therefore necessary to precisely control and spatially synchronize the refractive index modulations imprinted in the material by each scan. The Bragg-grating waveguides were inscribed using a femtosecond fiber laser that was externally modulated using an acousto-optic modulator. The required precision in the laser modulation was thus achieved by triggering the acousto-optic modulator using a position sensitive trigger signal supplied by the substrate translation stages themselves.

Ultrafast laser photoinscription of polarization sensitive devices in bulk silica glass

Ultrashort pulsed laser irradiation of bulk fused silica may result under specific energetic conditions in the self-organization of subwavelength material redistribution regions within the laser trace. The modulated structures have birefringent properties and show unusual anisotropic light scattering and reflection characteristics. We report here on the formation of waveguiding structures with remarkable polarization effects for infrared light. The photoinscription process using 800 nm femtosecond laser pulses is accompanied by third harmonic generation and polarization dependent anisotropic scattering of UV photons. The photowritten structures can be arranged in three-dimensional patterns generating complex propagation and polarization effects due to the anisotropic optical properties.

Three-dimensional optical sensing network written in fused silica glass with femtosecond laser

A single-step fast-writing method of burst ultrafast laser modification was applied to form a mesh network of multi-wavelength Bragg grating waveguides in bulk fused silica glass. Strain-optic and thermo-optic responses of the laser-written internal sensors are reported for the first time. A dual planar layout provided independent temperature- and strain-compensated characterization of temperature and strain distribution with coarse spatial resolution. The grating responses were thermally stable to 500 masculineC. To our best knowledge, the grating network represents the first demonstration of 3D distributed optical sensing network in a bulk transparent medium. Such 3D grating networks open new directions for strain and temperature sensing in optical circuits, optofluidic, MEMS or lab-on-a-chip microsystems, actuators, and windows and other large display or civil structures.

Transition from thermal diffusion to heat accumulation in high repetition rate femtosecond laser writing of buried optical waveguides

A variable (0.2 to 5 MHz) repetition rate femtosecond laser was applied to delineate the role of thermal diffusion and heat accumulation effects in forming low-loss optical waveguides in borosilicate glass across a broad range of laser exposure conditions. For the first time, a smooth transition from diffusion-only transport at 200 kHz repetition rate to strong heat accumulation effects at 0.5 to 2 MHz was observed and shown to drive significant variations in waveguide morphology, with rapidly increasing waveguide diameter that accurately followed a simple thermal diffusion model over all exposure variables tested. Amongst these strong thermal trends, a common exposure window of 200 mW average power and approximately 15-mm/s scan speed was discovered across the range of 200 kHz to 2 MHz repetition rates for minimizing insertion loss despite a 10-fold drop in laser pulse energy. Waveguide morphology and thermal modeling indicate that strong thermal diffusion effects at 200 kHz give way to a weak heat accumulation effect at approximately 1 microJ pulse energy for generating low loss waveguides, while stronger heat accumulation effects above 1-MHz repetition rate offered overall superior guiding. A comprehensive characterization of waveguide properties is presented for laser writing in the thermal diffusion and heat accumulation regimes. The waveguides are shown to be thermally stable up to 800 degrees C and can be written in a convenient 520 microm depth range with low spherical aberration.