We will review the development in the last decade of discrete beam combiners (DBC), phase sensors based on the propagation of light in photonic lattices. The latest results on the development of DBC for astronomical applications will be presented, along with a new application for the complete tomography of modes at the tip of a multi-mode fiber. The possible use of the DBC in monitoring and controlling modal instabilities in high power lasers will be discussed.
We analyze the use of bursts of ultra-short pulses in order to improve drilling efficiency and quality. Silicon is used as a non-transparent model material, in which the behavior of laser percussion drilling with 1030 nm bursts consisting of 200 fs pulses separated by a time delay between 1 ps and 4 ns was investigated. The deep drilling process is directly imaged perpendicular to the drilling direction using a CCD camera and an illumination beam at 1064 nm, where the silicon sample is transparent. The results are compared to drilling without bursts for different pulse energies. The efficiency of the drilling process, hole quality, as well as reproducibility of the hole shape are analyzed.
Pulse separation times within the burst from 1 ps to 8 ps result in deeper holes with a larger silhouette area, however equal or reduced hole quality and reproducibility compared to drilling with individual pulses. In contrast with pulse separation times from 510 ps to 4 ns a quality and reproducibility improvement is visible. For these delay times the achieved depth was equal or higher compared to micromachining without bursts.
We report on an experimental investigation of ultrafast laser ablation of silicon with bursts of pulses. The pristine 1030nm-wavelength 200-fs pulses were split into bursts of up to 16 sub-pulses with time separation ranging from 0.5ps to 4080ps. The total ablation threshold fluence was measured depending on the burst features, finding that it strongly increases with the number of sub-pulses for longer sub-pulse delays, while a slowly increasing trend is observed for shorter separation time. The ablation depth per burst follows two different trends according to the time separation between the sub-pulses, as well as the total threshold fluence. For delays shorter than 4ps it decreases with the number of pulses, while for time separations longer than 510ps, deeper craters were achieved by increasing the number of subpulses in the burst, probably due to a change of the effective penetration depth.
Metal mirrors are an attractive solution for scan mirrors working with ultra-short pulse lasers. Small mechanical inertia and a small mirror mass are required. Therefore, the mirrors have to be very stiff and a high quality optical surface has to be provided. This can be achieved with lightweight AlSi based mirrors with diamond-turned NiP polishable plating.
Different coating options were evaluated in order to provide the necessary high reflectivity and a satisfactory laser damage threshold for ultrashort laser pulses in the few ps to fs regime at λ = 1030 nm. High-reflective metal layers enhanced by dielectric HfO2/SiO2 stacks were found to be the most advantageous coating option due to their comparatively small thickness and measured damage thresholds above 1 J/cm2@8ps.
Nulling interferometry has been identified as a competitive technique for the detection of extrasolar planets. The technique consists in combining out-of-phase pairs of telescopes to null effectively the light of a bright star an reveal the dim glow of the companion. We have manufactured and tested with monochromatic light an integrated optics component which combines a linear array of 4 telescopes in the nulling mode envisaged by Angel&Wolf.1 Our testbench simulates the motion of a star in the sky. The tests have demonstrated a nulling scaling as the fourth power of the baseline delay.
We present a compact setup based on a three-dimensional integrated optical component, allowing the measurement of spectrally resolved complex-visibilities for three channels of polychromatic light. We have tested a prototype of the component in R band and showed that accurate complex visibilities could be retrieved over a bandwidth of 50 nm centered at 650 nm (resolution: R=130). Closure phase stability in the order of λ/60 was achieved implying that the device could be used for spectro-interferometry imaging.
Topological insulators are a new phase of matter, with the striking property that conduction of electrons occurs
only on the surface. In two dimensions, surface electrons in topological insulators do not scatter despite defects
and disorder, providing robustness akin to superconductors. Topological insulators are predicted to have wideranging
applications in fault-tolerant quantum computing and spintronics. Recently, large theoretical efforts were
directed towards achieving topological insulation for electromagnetic waves. One-dimensional systems with
topological edge states have been demonstrated, but these states are zero-dimensional, and therefore exhibit no
transport properties. Topological protection of microwaves has been observed using a mechanism similar to
the quantum Hall effect, by placing a gyromagnetic photonic crystal in an external magnetic field. However,
since magnetic effects are very weak at optical frequencies, realizing photonic topological insulators with scatterfree
edge states requires a fundamentally different mechanism - one that is free of magnetic fields. Recently, a
number of proposals for photonic topological transport have been put forward. Specifically, one suggested
temporally modulating a photonic crystal, thus breaking time-reversal symmetry and inducing one-way edge
states. This is in the spirit of the proposed Floquet topological insulators, where temporal variations in solidstate
systems induce topological edge states. Here, we propose and experimentally demonstrate the first external
field-free photonic topological insulator with scatter-free edge transport: a photonic lattice exhibiting topologically
protected transport of visible light on the lattice edges. Our system is composed of an array of evanescently coupled
helical waveguides arranged in a graphene-like honeycomb lattice. Paraxial diffraction of light is described by
a Schrödinger equation where the propagation coordinate acts as ‘time’. Thus the waveguides' helicity breaks zreversal
symmetry in the sense akin to Floquet Topological Insulators. This structure results in scatter-free, oneway
edge states that are topologically protected from scattering.
We present the experimental results of a 3D photonic component designed for the determination of coherence
properties of astronomical targets in optical interferometry. The component is based on the properties of two
dimensional arrays of evanescently coupled waveguides and has the potential of being scalable to arbitrary large
arrays of telescopes. Simulations and rst experimental results will be presented together with perspectives for
implementation of future instruments combining multiple telescopes.
We report on the fabrication of birefringent optical components based on so-called nanogratings. These selforganized
nanostructures with sub-wavelength periodicity are formed during femtosecond laser processing of
transparent materials, resulting in characteristic birefringent modifications. Nanogratings provide the means for
the direct inscription of customized birefringent elements with position-dependent retardation. We present our
investigations on the formation process of nanogratings in fused silica and the influence of fabrication parameters,
thereby identifying ways to systematically control the structural properties of the gratings. Consequently, we
were able to fabricate nanograting-based birefringent elements with specific retardations in bulk fused silica.
We report on the impact of topological defects on the formation of discrete spatial solitons in waveguide arrays.
The influence of defects, i.e. waveguides with detuned effective refractive index, is well understood within such
systems. They have been shown to support linear bound states and thus influence the formation of spatial
solitons in the surrounding sites. We show numerically and demonstrate experimentally how the presence of
topological defects caused by junctions within the otherwise periodical system similarly has a strong influence
on the surrounding sites.
Self-imaging in integrated optical devices is interesting for many applications including image transmission,
optical collimation and even reshaping of ultrashort laser pulses. However, in general this relies on boundary-free
light propagation, since interaction with boundaries results in a considerable distortion of the self-imaging
effect. This problem can be overcome in waveguide arrays by segmentation of particular lattice sites, yielding
phase shifts which result in image reconstruction in one- as well as two-dimensional configurations. Here, we
demonstrate the first experimental realization of this concept. For the fabrication of the segmented waveguide
arrays we used the femtosecond laser direct-writing technique. The total length of the arrays is 50mm with a
waveguide spacing of 16 μm and 20μm in the one- and two-dimensional case, respectively. The length of the
segmented area was 2.6mm, while the segmentation period was chosen to be 16 μm. This results in a complete
inversion of the global phase of the travelling field inside the array, so that the evolution dynamics are reversed
and the input field is imaged onto the sample output facet. Accordingly, segmented integrated optical devices
provide a new and attractive opportunity for image transmission in finite systems.
We report the realization of an evanescently coupled laser-written type II array in χ-cut Lithium niobate. Certain
processing parameters allow evanescent fields to extend beyond the regions of damage, while still increasing the
index sufficiently to guide light. An array consisting of eleven coupled waveguides was fabricated. Coupling
was evaluated by observing discrete diffraction patterns of single waveguide excitations at various array sites.
Homogeneous coupling was verified within the array, while the outermost guides are slightly detuned due to
being formed by just one damage structure.
For various applications it is interesting to directly visualize the propagation of light in waveguides. For this
purpose, we used special fused silica glasses with a high content of OH. This leads to the formation of color
centers when waveguides are written with fs laser pulses. When light is launched into the waveguides the color
centers are excited and the fluorescence can be directly observed. This is especially interesting in waveguide
arrays for the visualization of the evanescent coupling, since the discrete light evolution exhibits many features
which are in strong contrast to propagation in common isotropic media. As an example for the visualization
we will discuss here the possibility to excite a completely incoherent propagation within the waveguide array
although the sources are fully coherent. When multiple waveguides are excited, the light evolution in the array
can be described as a superposition of the single propagating amplitudes. The formula for the resulting intensity
contains an interference term. One can explicitly show that this interference term vanishes for certain excitation
patterns. When for instance two adjacent waveguides are excited the light propagates as there was no interference
term, which is equivalent to the simple sum of the two intensities of the single amplitudes. This suggests the term
"quasi-incoherent" for this new kind of propagation effect. In contrast a coherent superposition including the
interference term is obtained for an excitation of two waveguides when there is one waveguide located between
the two excited ones.
The evanescent coupling of femtosecond laser written waveguides with elliptical and circular shape is investigated
in detail. Elliptical waveguides are used to investigate directional tuning of the coupling properties in a square
array by tilting the elliptical waveguides. This allows to specifically pronounce diagonal coupling. In contrast,
directional insensitive coupling is demonstrated in a circular waveguide array based on circular waveguides.