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Waveguide grating mirror in a fully suspended 10 meter Fabry-Perot cavity

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Abstract

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 ≈ 20 nm thin etch stop layer made of Al2O3 that allowed us to define the grating depth accurately and preserve the waveguide thickness 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.

©2011 Optical Society of America

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Figures (6)

Fig. 1
Fig. 1 (a) Principle of a waveguide grating mirror under normal incidence in a ray picture. The incoupled first order diffracted rays are guided via total internal reflection at the substrate with index of refraction n L < n H. The outcoupling at the grating structure is a constructive interference if all grating parameter are designed properly, hence, providing 100% reflectivity. (b) Equivalent waveguide grating architecture realized in this work, including an Al2O3 layer as etch stop in order to define the grating depth and waveguide layer thickness in the fabrication process.
Fig. 2
Fig. 2 (a) First order diffraction efficiency from a tantala grating (n H = 2.186, g = 390 nm, f = b/d = 0.38, TE-polarization) into a material with varying index of refraction n L, illustrating the range of the grating period that allows for resonant excitation as predicted by Eq. (1). (b) If an etch stop layer with index of refraction n M = 1.66 is implemented, the first diffraction orders can propagate through the etch stop layer for dλ 0/n M independent of its thickness s.
Fig. 3
Fig. 3 (a) SEM image of a fabricated waveguide grating structure. Except of the fill factor all parameters are the same for the sample investigated here. (b) Calculated reflectivity (RCWA) of a waveguide grating under normal incidence for the parameters marked in the SEM image and a fill factor of f = 0.38. The white lines mark an area of reflectivity ≥ 99%.
Fig. 4
Fig. 4 (a) Schematic of the prototype facility including the laser bench, vacuum system and waveguide cavity having a length of ≈ 10m. The cavity was stabilized using the Pound-Drever-Hall scheme also depicted at the laser bench. (b) Intermediate and lower stage of the triple suspension system used for the cavity mirrors. Behind the main test mass a second triple suspension carries the so-called reaction mass, which is used to act on the main test mass. (c) Test mass with the waveguide grating mirror (area of 10 × 15mm) attached.
Fig. 5
Fig. 5 Cavity scan via tuning of the laser frequency. Reflected power (red trace) and Pound-Drever-Hall error-signal (green trace) detected with the photodiodes PDR and PDE, respectively. The reflected signal for a stabilized cavity (blue trace) indicates a visibility of ≥ 0.6.
Fig. 6
Fig. 6 Typical measurements of the transmitted light of a swept cavity resonance for different mirror velocities v (red trace). Theoretical results for the transmitted power |a T|2 are based on Eq. (3) with r 1 r 2 = 0.996 (black line), which corresponds to a cavity finesse of 790. Hence, a lower boundary for the waveguide grating power reflectivity is |r 1|2 ≥ 0.992.

Equations (4)

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487 nm = λ 0 / n H d λ 0 / n L = 734 nm ,
v = 2 Ω FSR λ 2 1 Δ τ ,
a T ( τ ) = a 0 m = 1 t 1 t 2 ( r 1 r 2 ) m 1 exp ( i ϕ m ( τ ) ) ,
ϕ m ( τ ) ( 2 m 1 ) 2 π L ( τ ) λ m ( m 1 ) 2 L 0 c v 2 π λ ,
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