In the non-ablative regime, femtosecond laser pulse duration is known to affect the nature of the modification induced in the microstructure of fused silica. It has been demonstrated than below 200 fs, two different regimes are found, one at low energy, leading to bulk densification while the second one – for higher energy, leading to self-organized structure - nicknamed nanogratings - that induce a net and localized volume expansion of the material. The first regime is particularly interesting for waveguides fabrication, although, so far, the reported refractive index gain remains modest, typically within 10-3 relative net increase that limits the level of compactness for photonics circuits making use of it. Here, we investigate further how shorter pulses, i.e. in the sub-50 fs range, can increase the level of densification and in turn, the net refractive index gain, and possibly lead to an improve process for photonics device fabrication. First results show that indeed, higher level of densification can be obtained, level that we quantify, and that can be further correlated to a net increase of refractive index.
Simultaneous spatially and temporally focussing (SSTF) of ultrashort pulses allows for an unprecedented control of the intensity distribution of light. It has therefore a great potential for widespread applications ranging from nonlinear microscopy, ophthalmology to micro-machining. SSTF also allows to overcome many bottlenecks of ultrashort pulse micro-machining, especially non-linear effects like filamentation and self-focussing. Here, we describe and demonstrate in detail how SSTF offers an additional degree of freedom for shaping the focal volume. In order to obtain a SSTF beam, the output of an ultrafast laser is usually split by a grating into an array of copies of the original beam, which we refer to as beamlets. The ratio of the beamlet array width to the width of the invidual beamlet is the beam aspect ratio. The focal volume of the SSTF beam can now be tailored transversally by shaping the cross-section of the beamlets and axially by choosing the right beam aspect ratio. We will discuss the requirements of the setup for a successful implementation of this approach: Firstly, the group velocity dispersion and the third order dispersion have to be compensated in order to obtain a high axial confinement. Secondly, the beamlet size and their orientation should not vary too much spectrally. Thirdly, beamlet and SSTF focus should match. We will hence demonstrate how SSTF allows to inscribe tailored three-dimensional structures with fine control over their aspect ratio. We also show how the SSTF focus can be adapted for various glasses and crystals.
The Magdalena Ridge Observatory Interferometer is a 10-element 1.4 meter aperture optical and near-infrared
interferometer being built at 3,200 meters altitude on Magdalena Ridge, west of Socorro, NM. The
interferometer layout is an equilateral "Y" configuration to complement our key science mission, which is
centered around imaging faint and complex astrophysical targets. This paper serves as an overview and
update on the status of the observatory and our progress towards first light and first fringes in the next few
We report on the opto-mechanical design of the MROI fringe tracker. This instrument, currently under development in
collaboration with the University of Cambridge, will be a dedicated fringe tracking beam combiner and spectrographs. I
will utilize the "Y" geometry of the array to stabilize fringes on shorter "nearest neighbor" baselines, and thus allow for
increased integration times on the longer baselines and the buildup of signal to noise. The beam combiner has been
designed to accommodate light from a maximum of ten telescopes (three in each array arm, one at the "Y" vertex), but
can operate with fewer without having to change the overall layout. A single spectrograph will multiplex up to five
nearest neighbor combinations onto a single detector. Identical spectrographs are located at opposite sides of the
combiner outputs to simultaneously sample combination pairs that are Π radians out of phase with respect to one another.
We report on the design and performance of a 6-way multi-wavelength beam combining instrument for the MRO
Interferometer, allowing for fringe measurements at any of the J/H/K near-infrared bands at switchable spectral
resolution with high sensitivity. Three preliminary designs for the instrument are presented and compared. The
results of an ongoing evaluation performed on the performance, costs, and risks of each designs are analysed.
Signal-to-noise analyses confirm in particular the utility of one of the design at magnitudes as faint as K=13.
This report focuses on the design, application, and testing of custom beamsplitter and anti-reflection coatings for use in
the Magdalena Ridge Observatory Interferometer (MROI) beam combiners. The coatings were designed in collaboration
with Optical Surface Technologies, and the University of Cambridge. The fringe tracker and science combiners will
operate across the J, H, and K bands. The coatings were designed to achieve three optical characteristics critical to
optical interferometry: 1) minimized stress of the substrate (leading to induced wavefront errors), 2) high throughput,
and 3) high visibilities in broadband unpolarized light. The AR coating has mean reflection losses of less than 0.5%.
Beamsplitter coatings experienced visibility losses less than 1% due to group delay dispersion and s and p phase