Demonstration of a cavity coupler based on a resonant waveguide grating

Abstract

Abstract:Thermal noise in multilayer optical coatings may not only limit the sensitivity of future gravitational wave detectors in their most sensitive frequency band but is also a major impediment for experiments that aim to reach the standard quantum limit or to cool mechanical systems to their quantum ground state. Here, we present the experimental realization and characterization of a cavity coupler, which is based on a surface relief guided-mode resonant grating. Since the required thickness of the dielectric coating is dramatically decreased compared to conventional mirrors, it is expected to provide low mechanical loss and, thus, low thermal noise. The cavity coupler was incorporated into a Fabry-Perot resonator together with a conventional high quality mirror. The finesse of this cavity was measured to be F = 657, which corresponds to a coupler reflectivity of R = 99.08 %.

Lasers and optics: looking towards third generation gravitational wave detectors

“…The exact optical properties of such a device can be found with rigorous coupled wave analysis (RCWA) [86]. We experimentally realized grating waveguide structures and demonstrated a reflectivity >99% in a cavity setup [87], which was in good agreement with RCWA calculations (see Fig. 3).…”

“…The incoupled ±1st transmitted orders are totaly reflected at the substrate or thin ridges, respectively, and coupled out at the grating structure. For proper chosen dimensions of the nanostructured surface and waveguide one can achieve constructive interference and thus 100% reflectivity Right spectral transmittance under normal incidence of a fabricated waveguide mirror that had reflectivity of >99% at a wavelength of 1,064 nm, [87] a waveguide layer and partly coupled out. The waveguide mirror dimensions can be adjusted to achieve constructive interference of the outcoupled and specular reflected light and thus 100% reflectivity.…”

Toward a third generation of gravitational wave observatories

“…A promising R&D activity [54] is devoted to produce high reflectivity mirrors with just one dissipative layer of dielectric coating material on the substrate or even without any additional layer, realizing the so-called resonant waveguide grating [55] by nano-structuring the surface of the Silicon substrate.…”

Challenges in thermal noise for 3rd generation of gravitational wave detectors

“…Another recent proposal is to use resonant waveguide mirrors. Here, reflection is produced by coupling the light to a waveguide by means of a grating structure [132]. If designed properly high reflectivity can be achieved using only one dielectric layer with a thickness of about 400 nm.…”