Nanophotonics Technology Center, Universidad Politecnica de Valencia, Camino ... Microsystem Technology Laboratory, School of Electrical Engineering, KTH ...
WP39 18:15 - 20:30
A high efficiency silicon nitride grating coupler L. Vivien, G. Maire, G. Sattler, D. Marris-Morini, E. Cassan, S. Laval
Institut d'Electronique Fondamentale, CNRS UMR 8622, Bat. 220, Universite Paris-sud XI, F-91405 ORSAY cedex - France
A. Kazmierczak, D. Giannone
Applied Photonics Departement, Multitel a.s.b.l., rue Pierre et Marie Curie, 2, B-7000 Mons, Belgium
B. Sanchez, A. Griol, D. Hill
Nanophotonics Technology Center, Universidad Politecnica de Valencia, Camino de Vera s/n, Valencia, Spain
K.B. Gylfason, H. Sohlstrom
Microsystem Technology Laboratory, School of Electrical Engineering, KTH - Royal Institute of Technology, Osquldas vag 10, SE-10044 Stockholm, Sweden
Abstract: The experimental demonstration of a high efficiency silicon nitride grating coupler is reported for wavelengths from X = 1.25 to 1.45 Mm for TE polarization. At the resonant angle, a coupling efficiency higher than 60% has been measured. Keywords: microphotonics, waveguide, silicon nitride, grating coupler
Silicon-based micro and nano-photonics have raised a growing interest over recent years for numerous applications. In particular high index contrast materials allow high electromagnetic field confinement leading to high integration density. Among such materials, both silicon nitride (Si3N4) for the core material and silicon oxide (SiO2) for the cladding material are good candidates for guided wave passive photonic devices. However, due to a high index contrast (An -0.5), light coupling into the micro structures with large tolerance alignment, is a real challenge. Compared to edge coupling, grating couplers are more alignment tolerant and can couple light anywhere on the chip, thus avoiding cleavage or polish. Here, experimental results on grating couplers in Si3N4 waveguides are presented. The gratings have been optimized for a wavelength of 1.3pm and characterized for a large wavelength range, 1.25 Mm to 1.5pm.
The main characteristics of grating couplers are the period, the etching depth and the duty ratio. Figure l(a) presents a schematic view of the cross-section of a fully etched grating coupler. To couple light using a grating coupler, the following phase matching condition has to be fulfilled:
kin' sin(6
+) p.
A
=,6', where k. = i
is the incident wave vector modulus, X the
2
wavelength, Or the input resonant angle, p the diffraction order (here p = + 1), A the grating period, ,S'= .2ne is the real part of the propagation constant, and ne. is the effective index of the guided mode. The coupling efficiency strongly depends on the thicknesses of the silicon nitride film (n=1.95), and of the top and bottom silicon oxide claddings (n=1 .45) thicknesses. The
1-4244-0935-71071$25.OO©C2007 IEEE
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84,=Lt-~ ~ .L|-!r.
principles of the simulation method and of the thickness optimization are described in references [1] and [2]. The Si3N4 film thickness is fixed at 300 nm. The grating coupler has been designed to couple light at 1.31pm for TE polarization. The grating period is then 1 Mm and a fully etched structure is also preferred for an accurate control of the etching depth and 50% duty ratio allows relaxing technology constraints (minimum size feature). Figure l(b) shows the coupling efficiency as a function of the bottom and top silicon silica cladding layer thicknesses for the considered grating coupler. Theoretically, a maximum coupling efficiency of about 60% is obtained for bottom and top cladding thicknesses of 3.26pm and 60 nm, respectively.
iiISiOr
Si
bS"te
