Gratings and couplers
Fabrication of grating couplers by direct laser ablation
Grating couplers are commonly used to couple free-space beams to thin-film waveguides on planar substrates. We use an F2-laser processing system working at a wavelength of 157 nm for grating fabrication. The extremely short wavelength allows precise structuring of transparent substrates like glass or fused silica that are usually required in life-science applications.
The laser ablation process allows a flexible choice of substrate materials, grating parameters and layout and can be used to tailor the grating couplers to specific applications. For example, the spectral acceptance can be matched to the spectral width of ultra short pulse lasers in applications requiring two-photon fluorescence excitation.
FEM Simulation of coupling efficiency
While theoretical considerations show that maximum coupling efficiencies in the regime of 80% are possible when using grating couplers, such strong coupling is usually not achieved in practical applications. In order to be able to predict the efficiency of grating couplers in specific applications, and to allow an optimization of geometry and grating parameters, the coupling process is analyzed by calculations using the finite element method (FEM).
For example, the optimum position and diameter of the coupled excitation beam depend on the modulation depth of the grating. For optimum efficiency, the beam size has to be larger in case of a shallow grating compared to a deeper grating. In addition, efficiency is usually larger when the excitation beam is incident on the grating coupler from the substrate side than when it is incident from the air side.
Direct fiber to waveguide coupling by an external grating
Most thin-film waveguide applications would greatly benefit from a simple and efficient method to couple light from an optical fiber to the planar waveguide. A new potential solution uses external gratings fabricated on the end face of a collimating micro lens or immediately on the exit face of an optical fiber to couple light from a single-mode fiber directly to planar or strip wave-guides. The gratings, which might be manufactured by direct laser processing, produce an effective index modulation on the surface of the waveguide and thus allow a part of the beam to be coupled to the waveguide. The external couplers can be repeatedly reused and eliminate the need for conventional internal grating couplers, which induce a major part of the production costs and impose certain restrictions on the waveguide devices. They are thus especially suited in conjunction with disposable waveguides in single-use biosensing applications.
The efficiency of the external couplers can be assessed by FEM simulation of the coupling process. In case of direct fiber to waveguide coupling, maximum efficiency is usually restricted to values below 10% due to the short coupling length comprising approximately 10 grating periods, only. On the other hand, this coupling scheme shows a huge angular resonance width and correspondingly a wide spectral acceptance. If a collimating lens with an external grating is employed, the coupling length is considerably increased. In this case, the FEM simulations reveal that the new technique can provide similar coupling efficiencies as common internal grating couplers. A deep grating and close proximity of external grating and waveguide surface are essential.
In an experimental realization, using a collimating GRIN lens equipped with a coupling grating of 500 nm period, which has been fabricated by direct laser ablation at 157 nm wavelength on the angle polished end face, a coupling efficiency of 13% could be achieved.
T. Fricke-Begemann, J. Ihlemann:
Direct light-coupling to thin-film waveguides using a grating-structured GRIN lens
Optics Express 18, 19860 (2010)
T. Fricke-Begemann, J. Ihlemann:
Coupling to planar and strip waveguides In: Planar Waveguides and other Confined Geometries, G. Marowsky, Ed. Springer Series in Optical Sciences 189 (2014)
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Laser-Laboratorium Göttingen e.V. (LLG)
Head of the Department
Dr. Peter Simon
"Short Pulses / Nanostructures"
Contact person for
Nano Structure Technology:
Dr. Jürgen Ihlemann