FOCUS
MANUFACTURING with short pulse lasers
used for the fabrication of low-loss( down to 0.03 dB / cm in commercial borosilicate glass and optical fibers [ 3 ]) waveguide-based PICs mainly operating at telecom wavelength with various geometries such as splitters, Y-junctions, cascaded directional couplers for( de-) multiplexing, interferometers and complex arrangement of coupled waveguides. Near-surface waveguides can also be inscribed for evanescent sensing of the environment through absorption, dephasing and / or changes in modal coupling. As illustrated in Fig. 2( c-d), realization of dynamically controllable integrated interferometers embedding active thermal phase shifting now gives access to advanced reprogrammable light management both in the classical and quantum light regime. In the latter context, the great versatility of the direct-laser-written PICs allows for the realization of hybrid multi-material quantum photonic platforms where, for example, the nonlinear generation of entangled photon pairs occurs in a periodically-poled lithium niobate crystal while the advanced complex photonic circuitry takes place in laser-inscribed glass. Complex arrangement of waveguides with tailored couplings also provides a powerful photonic platform to mimic condensed matter configurations and has been pivotal in the topological photonics community. Moreover, advanced waveguide-based architectures are also expected to play a crucial role in astrophotonics, providing ultra-stable and miniaturized devices to be implemented in telescope flux-recombining interferometric systems( see e. g. [ 4 ] for a review on PICs applications).
Beyond optical waveguides fabrication, femtosecond laser inscription also allows for the realization of sub-wavelength periodic refractive index modulation for the inscription of Bragg gratings in waveguides( WBGs) [ 5 ], see Fig. 3a, or in the core
Figure 2. Waveguide-based photonic integrated circuits( PICs). a) Sketch of the writing setup for PICs fabrication. b) White light microscope image of a single waveguide( left) and the corresponding mode at 1.55 µ m. c) Sketch and actual picture of an integrated Mach-Zehnder interferometer( MZI). The dephasing is dynamically controlled with an electrode via thermo-optical effects. The normalized transmission of an MZI is plotted in d) as a function of the power dissipated by the electrode. The markers and solid line correspond to the data and fit respectively( Credits: A. Haykal, L. Bellando, I. Mouely Mouloungui, L. Raju Kalathil, L. Labonté, M. Bellec). of optical fibers( FBGs). The resulting high spectrally-selective reflectivity can be finely tuned by controlling the grating properties( apodization, linear period chirps or even adapted phase steps). WBGs and FBGs can then be used for filtering and high-precision remote sensing applications as well as for manufacturing optical resonators in passive and active material for bulk and mostly fiber lasers devices. Laser-written periodic media also give access to the management of group velocity and slow light topics with light velocities down to a few tens of km / s.
EXTENDING THE ENGINEER’ S TOOLBOX FOR 3D OPTICAL FUNCTIONS IN GLASS Direct-laser-writing in bulk glass is increasingly used to engineer 3D photonic microstructures that exhibit optical responses extending far beyond simple scalar refractive index modulation and conventional telecom-band applications.
Among the above-mentioned various optical contrasts offered by femtosecond laser structuring, tensorial optical responses( birefringence, fast-axis orientation, polarization-dependent scattering) are gaining strong interest. Such responses typically appear in silica when laser irradiation produces self-organized sub-wavelength structures( so called nanogratings) providing an effective uniaxial medium whose optical axis is linked to the writing laser polarization. In-volume retarders, polarization converters, and multiplexed high-dimensional optical data storage( 3D positions, retardance and axis orientation) have been realized, the latter now being close to industrialization [ 6 ]. More recently, ultralow-loss birefringent regime has been identified in silica, where birefringence arises from randomly distributed elongated nanopores rather than regular nanogratings [ 7 ]. This novel microstructure type is attractive for transmissive devices requiring strong polarization control with reduced scattering, including bulk geometric-phase elements, based on spatially patterned
40 www. photoniques. com I Photoniques 137