53 research outputs found
Cancellation of lateral displacement noise of 3-port gratings for coupling light to cavities
Reflection gratings enable light coupling to optical cavities without
transmission through substrates. Gratings that have three ports and are mounted
in second-order Littrow configuration even allow the coupling to high-finesse
cavities using low diffraction efficiencies. In contrast to conventional
transmissive cavity couplers, however, the phase of the diffracted light
depends on the lateral position of the grating, which introduces an additional
noise coupling. Here we experimentally demonstrate that this kind of noise
cancels out once both diffracted output ports of the grating are combined. We
achieve the same signal-to-shot-noise ratio as for a conventional coupler. From
this perspective, 3-port grating couplers in second-order Littrow configuration
remain a valuable approach to reducing optical absorption of cavity coupler
substrates in future gravitational wave detectors
Key signal contributions in photothermal deflection spectroscopy
We report on key signal contributions in photothermal deflection spectroscopy
(PDS) of semiconductors at photon energies below the bandgap energy and show
how to extract the actual absorption properties from the measurement data. To
this end, we establish a rigorous computation scheme for the deflection signal
including semi-analytic raytracing to analyze the underlying physical effects.
The computation takes into account linear and nonlinear absorption processes
affecting the refractive index and thus leading to a deflection of the probe
beam. We find that beside the linear mirage effect, nonlinear absorption
mechanisms make a substantial contribution to the signal for strongly focussed
pump beams and sample materials with high two-photon absorption coefficients.
For example, the measured quadratic absorption contribution exceeds 5% at a
pump beam intensity of about in Si and at
in GaAs. In addition, our method also
includes thermal expansion effects as well as spatial gradients of the
attenuation properties. We demonstrate that these effects result in an
additional deflection contribution which substantially depends on the distance
of the photodetector from the readout point. This distance dependent
contribution enhances the surface related PDS signal up to two orders of
magnitude and may be misinterpreted as surface absorption if not corrected in
the analysis of the measurement data. We verify these findings by PDS
measurements on crystalline silicon at a wavelength of 1550 nm and provide
guidelines how to extract the actual attenuation coefficient from the PDS
signal.Comment: 10 pages, 16 figures, submitted to Journal of Applied Physiv
Materials Pushing the Application Limits of Wire Grid Polarizers further into the Deep Ultraviolet Spectral Range
Wire grid polarizers (WGPs), periodic nano-optical meta-surfaces, are
convenient polarizing elements for many optical applications. However, they are
still inadequate in the deep ultraviolet spectral range. We show that to
achieve high performance ultraviolet WGPs a material with large absolute value
of the complex permittivity and extinction coefficient at the wavelength of
interest has to be utilized. This requirement is compared to refractive index
models considering intraband and interband absorption processes. We elucidate
why the extinction ratio of metallic WGPs intrinsically humble in the deep
ultraviolet, whereas wide bandgap semiconductors are superior material
candidates in this spectral range. To demonstrate this, we present the design,
fabrication and optical characterization of a titanium dioxide WGP. At a
wavelength of 193 nm an unprecedented extinction ratio of 384 and a
transmittance of 10 % is achieved.Comment: 21 pages, Advanced Optical Materials 201
Michelson interferometer with diffractively-coupled arm resonators in second-order Littrow configuration
Michelson-type laser-interferometric gravitational-wave (GW) observatories
employ very high light powers as well as transmissively- coupled Fabry-Perot
arm resonators in order to realize high measurement sensitivities. Due to the
absorption in the transmissive optics, high powers lead to thermal lensing and
hence to thermal distortions of the laser beam profile, which sets a limit on
the maximal light power employable in GW observatories. Here, we propose and
realize a Michelson-type laser interferometer with arm resonators whose
coupling components are all-reflective second-order Littrow gratings. In
principle such gratings allow high finesse values of the resonators but avoid
bulk transmission of the laser light and thus the corresponding thermal beam
distortion. The gratings used have three diffraction orders, which leads to the
creation of a second signal port. We theoretically analyze the signal response
of the proposed topology and show that it is equivalent to a conventional
Michelson-type interferometer. In our proof-of-principle experiment we
generated phase-modulation signals inside the arm resonators and detected them
simultaneously at the two signal ports. The sum signal was shown to be
equivalent to a single-output-port Michelson interferometer with
transmissively-coupled arm cavities, taking into account optical loss. The
proposed and demonstrated topology is a possible approach for future
all-reflective GW observatory designs
Waveguide grating mirror in a fully suspended 10 meter Fabry-Perot cavity
We report on the first demonstration of a fully suspended 10m Fabry-Perot
cavity incorporating a waveguide grating as the coupling mirror. The cavity was
kept on resonance by reading out the length fluctuations via the
Pound-Drever-Hall method and employing feedback to the laser frequency. From
the achieved finesse of 790 the grating reflectivity was determined to exceed
99.2% at the laser wavelength of 1064\,nm, which is in good agreement with
rigorous simulations. Our waveguide grating design was based on tantala and
fused silica and included a ~20nm thin etch stop layer made of Al2O3 that
allowed us to define the grating depth accurately during the fabrication
process. Demonstrating stable operation of a waveguide grating featuring high
reflectivity in a suspended low-noise cavity, our work paves the way for the
potential application of waveguide gratings as mirrors in high-precision
interferometry, for instance in future gravitational wave observatories
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