Sebastian Kroesen

Electro–optical tunable waveguide embedded multiscan Bragg gratings in lithium niobate by direct femtosecond laser writing

optical tunable Bragg gratings in lithium niobate fabricated by direct femtosecond laser writing. The hybrid design that consists of a circular type-II waveguide and a multiscan type-I Bragg grating exhibits low loss ordinary and extraordinary polarized guiding as well as narrowband reflections in the c-band of optical communications. High bandwidth tunability of more than a peak width and nearly preserved electro-optic coefficients of r(13) = 7.59 pm V(-1) and r(33) = 23.21 pm V(-1) are demonstrated.

Electro-optical tunable waveguide Bragg gratings in lithium niobate induced by femtosecond laser writing

We report the fabrication of femtosecond laser-induced, first-order waveguide Bragg gratings in lithium niobate in the low repetition rate regime. Type-II waveguides are written into an x-cut lithium niobate wafer and structured periodically to achieve narrowband reflections at wavelengths around 1550 nm. Additionally, electrodes are employed to allow for electro-optic tuning of the spectral response. We demonstrate wavelength control of the central reflection peak by applying a static external electric field. A maximum shift of the reflection peak of Δλ = 625 pm is observed.

Monolithic fabrication of quasi phase-matched waveguides by femtosecond laser structuring the χ(2) nonlinearity

We demonstrate second harmonic generation in quasi phase-matched waveguide structures fabricated by direct laser writing. Circular waveguides are inscribed in z-cut lithium niobate that provide well confined guiding of the fundamental and second harmonic wave. In contrast to classic schemes that employ periodically poled crystals, quasi phase-matching is realized by a laser-induced modulation of the nonlinearity inside the waveguide core. The proposed design allows monolithic integration of buried frequency conversion devices with tailored nonlinear response and excellent compatibility to on-chip optical elements. Second harmonic generation of 1064 nm radiation is demonstrated for different grating periods and associated matching temperatures. A maximum conversion efficiency of 5.72% is obtained for a 6 mm long, laser-induced quasi phase-matching grating.

T-junction droplet generator realised in lithium niobate crystals by laser ablation

Abstract A femtosecond laser at 800 nm was used to create micro-fluidic circuits on lithium niobate (LiNbO3) substrates by means of laser ablation, using different scanning velocities (100-500 μm/s) and laser pulse energies (1-20 μJ). The T-junction geometry was exploited to create on y-cut LiNbO3 crystals a droplet generator, whose microfluidic performance was characterized in a wide range of droplet generation frequencies, from few Hz to about 1 kHz.

Optical group-velocity control in a phase-shifted narrowband filter

We demonstrate group-velocity control with an optically reconfigurable narrow-band filter at 1550 nm based on a volume-holographic Bragg grating. The filter is dynamically addressed to obtain different tunable magnitude and phase function by modification of the length, coupling strength, apodization, and phase discontinuities. We characterize the switching behavior, group-delay, electro-optical tuning, and Gaussian pulse propagation.

Lithium Niobate Micromachining for the Fabrication of Microfluidic Droplet Generators

In this paper, we present the first microfluidic junctions for droplet generation directly engraved on lithium niobate crystals by micromachining techniques, preparatory to a fully integrated opto-microfluidics lab-on-chip system. In particular, laser ablation technique and the mechanical micromachining technique are exploited to realise microfluidic channels in T- and cross junction configurations. The quality of both lateral and bottom surfaces of the channels are therefore compared together with a detailed study of their roughness measured by means of atomic force microscopy in order to evaluate the final performance achievable in an optofluidic device. Finally, the microfluidics performances of these water-in-oil droplets generators are investigated depending on these micromachining techniques, with particular focus on a wide range of droplet generation rates.

Fabrication of chirped and multi-period waveguide embedded Bragg gratings in lithium niobate

We demonstrate monolithic fabrication of tunable, waveguide embedded Bragg gratings (WBG) in lithium niobate by direct femtosecond laser writing. Complex refractive index modulation profiles such as chirped and multi periodic gratings are inscribed into the core volume of a circular, two-dimensional waveguide structure. The hybrid type-I/II design that consists of a type-II waveguide and a type-I multiscan grating exhibits low loss symmetric guiding of ordinary and extraordinary polarized modes and narrowband reflections in the c-band of optical communications. High bandwidth spectral tunability of more than a peak half width and nearly preserved electro{optic coefficients of r13 = 7:67pmV-1 and r33 = 24:7pmV-1 are realized.

Integrated optics on Lithium Niobate for sensing applications

In micro-analytical chemistry and biology applications, optofluidic technology holds great promise for creating efficient lab-on-chip systems where higher levels of integration of different stages on the same platform is constantly addressed. Therefore, in this work the possibility of integrating opto-microfluidic functionalities in lithium niobate (LiNbO3) crystals is presented. In particular, a T-junction droplet generator is directly engraved in a LiNbO3 substrate by means of laser ablation process and optical waveguides are realized in the same material by exploiting the Titanium in-diffusion approach. The coupling of these two stages as well as the realization of holographic gratings in the same substrate will allow creating new compact optical sensor prototypes, where the optical properties of the droplets constituents can be monitored.

Direct Inscription of Quasi Phase-Matching Waveguide Structures in Lithium Niobate

We report monolithic fabrication of quasi phase-matching waveguides in lithium niobate by direct femtosecond laser writing. The circular type-II waveguide with an embedded multiscan grating enables efficient second harmonic generation of 1064 nm radiation.

Polarization Independent, Tunable Waveguide Bragg Gratings in Lithium Niobate by Femtosecond Laser Micromachining

We report integration of two-dimensional waveguide Bragg gratings that support low loss, polarization independent guiding and tunable reflection spectra around λ=1.55 μm. Effective nonlinear coefficients of r13=7.73 pm/V and r33=25.62 pm/V are shown.