Femtosecond direct laser writing has recently shown great potential for the fabrication of complex integrated devices in the cladding of optical fibers. Such devices have the advantage of requiring no bulk optical components and no breaks in the fiber path, thus reducing the need for complicated alignment, eliminating contamination, and increasing stability. This technology has already found applications using combinations of Bragg gratings, interferometers, and couplers for the fabrication of optical filters, sensors, and power monitors. The femtosecond laser writing method produces a local modification of refractive index through non-linear absorption of the ultrafast laser pulses inside the dielectric material of both the core and cladding of the fiber. However, fiber geometries that incorporate air or hollow structures, such as photonic crystal fibers (PCFs), still present a challenge since the index modification regions created by the writing process cannot be generated in the hollow regions of the fiber. In this work, the femtosecond laser method is used together with a pre-modification method that consists of partially collapsing the hollow holes using an electrical arc discharge. The partial collapse of the photonic band gap structure provides a path for femtosecond laser written waveguides to couple light from the core to the edge of the fiber for in-line power monitoring. This novel approach is expected to have applications in other specialty fibers such as suspended core fibers and can open the way for the integration of complex devices and facilitate miniaturization of optical circuits to take advantage of the particular characteristics of the PCFs.
Submicron surface-relief gratings were fabricated in ultrathin dielectric films by F2-laser ablation. Projection mask imaging by a Schwarzschild objective applying nanosecond duration pulses from a high-resolution 157-nm optical processing system generated 780-nm-period gratings in various thin oxide layers. The grating modulation depths were controlled within tens of nanometers by applying suitable energy densities and number of pulses. Thus, high-resolution laser ablation proves to be a promising alternative approach to well-known lithographic methods for the fabrication of submicron-period gratings in thin films.
Such gratings are the most critical component of grating waveguide structures (GWS) that comprise of a substrate, a thin waveguide, and a grating layer in a planar multilayer structure. Interference effects in a GWS will provide high reflection efficiency under resonance conditions for an ideal grating with no absorption losses. The resonance spectral responses of the F2-laser ablated gratings have been investigated using an ultrashort-pulse titanium-sapphire laser. Their potential for optical applications will be shown and discussed. GWS are attractive for optical switches or modulators, narrow-band spectral filters, high reflectivity mirrors, bio-sensor chips and many other applications.
The 157nm F2-laser drives strong and precisely controllable interactions with fused silica, the most widely used material for bulk optics, optical fibers, and planar optical circuits. Precise excisions of 10 to 40 nm depth are available that meet the requirements for generating efficient visible and ultraviolet diffractive optical elements (DOE). F2-laser radiation was applied in combination with beam homogenization optics and high-precision computer controlled motion stages to shape 16-level DOE devices on bulk glasses and optical fiber facets. A 128×128 pixel DOE was fabricated and characterized. Each level had distinguishable spacing of ~140 nm and surface roughness of ~38 nm. The far-field pattern when illuminated with a HeNe laser agreed well with the simulation results by an Iterative Fourier Transform Algorithm (ITFA). Improvements to increase the 1st order diffraction efficiency of 22% are offered.
F2-laser ablation at 157 nm was used for generating sub-micron surface relief structures on fused silica to define binary diffractive phase elements (DPE). A pattern array of 128 x 128 pixels was excised using the F2 laser in combination with a high resolution processing system comprising of CaF2 beam-homogenization optics and a high-resolution Schwarzschild reflective objective. A square projection mask provided precise excisions in less than 10 x 10 μm2 spots, having sub-μm depths that were controlled by the laser fluence and the number of laser pulses to provide for the required phase delay between ablated and non-ablated pixels. Thus a diffractive phase element (DPE) optimized for first order in the UV spectral range was made. A four-level DPE design computed by the Iterative Fourier Transform Algorithm (IFTA) will be described for generating an arbitrary irradiation pattern without the point symmetry of a two level design.