Introduction
Within the Circles of Light project, the coupling between silicon photonic structures fabricated on Silicon-on-Insulator (SOI) material and the outside world, mostly in form of fibre-to-chip coupling, has been a key issue. Due to the high refractive index contrast between silicon and silica the waveguide size on SOI can be scaled down to several hundreds of nm to support a single guiding mode at 1550nm. However, the mode size of optical fibres is typically several orders of magnitude larger, also the effective index difference between silica fibre and silicon waveguide is quite large. This mode and index mismatch causes high coupling losses. Different techniques are proposed to overcome this issue, ranging from simple adiabatic tapers to sophisticated taper structures [2-5] and complex grating coupler designs [6]. Prior to the start of the project the partners had expertise in the use of adiabatic taper structures. As this type of coupling structures only allows coupling efficiencies below 10% per coupler and is limited with respect to the reproducibility due to the intricate facet preparation process the consortium has agreed to investigate two different coupling techniques which promise high efficiency coupling and high reproducibility as well as a cost-effective fabrication:
1. a basic but none the less effective grating coupler approach
2. a coupling structure which consists of a inversely tapered silicon waveguide end cladded with a fibre-scale polymeric waveguide
Grating Coupler
The grating coupler approach allows polarization-dependent out-of-plane coupling with coupling efficiencies up to 70%. For the Circles of Light project focus was set to a cost-effective and CMOS-compatible fabrication process, limiting the achievable coupling efficiencies to about 50% per coupler.
Based on simulations done by partner IBM very basic coupler structures, only consisting of simple gratings with high duty cycles which are fully etched into the top silicon layer of the SOI substrates (tsi=340nm, of tox=2µm) have been chosen as promising candidates for high-efficiency, cost-effective and CMOS-compatible couplers. Due to the high duty cycles needed the trenches between the gratings have to be very narrow. Therefore, partner AMO used its well-established electron beam lithography for the definition of the gratings. For passive photonic components like resonator structures no further lithography is needed, as these devices can be defined in the same lithographic step as the couplers. The whole device, including the couplers, has then been transferred into the substrate by means of reactive ion etching. In Fig. 1 an exemplary coupling structure is presented which has been fabricated as described above.
- Fig. 1: SEM micrograph of a grating coupler for TE polarization
To evaluate the potential of this coupling structures some test chips consisting of couplers with different design parameters (grating pitch, duty cycle) have been fabricated by AMO and characterized by IBM. Fig. 2 shows a plot of the coupling efficiency against the wavelength for two different coupling structures suitable for TE and TM polarization, respectively. For both TE and TM polarization the maximum coupling efficiency measured for those devices was only slightly slightly (<0.5dB for TE, <1dB for TM) below the theoretical limit of this type of structures, given by the index contrast. The 3dB bandwidth of those couplers is about 50nm, which is sufficient for a wide variety of applications.
- Fig. 2: Coupling efficiency vs. wavelength for both TE and TM grating couplers
Inversely tapered silicon waveguide end cladded with a fibre-scale polymeric waveguide
This type of coupling structure, suitable for TE and TM polarizations and with almost no bandwidth limitation, is schematically depicted in Fig 3.The light is first coupled from a micrometer-scale fibre core into a waveguide of approximately the same size and refractive index. A nano-scale tapered silicon strip waveguide is embedded into this larger cladding waveguide. The light mode is continuously transferred from the cladding waveguide to the silicon strip waveguide with the refractive index gradually changing from that of silica to that of silicon.
A detailed theoretical investigation of both the fibre-to-cladding waveguide coupling and the mode transfer from the cladding waveguide into the silicon waveguide has been carried out by partner RWTH. The results of those simulations lead to an optimized choice of both cladding material (SU8) and design parameters (taper tip width Wtip, taper length Ltaper, SU8 waveguide height HSU8). As already described above, the devices are fabricated using SOI material with a top silicon thickness of tsi=340nm. Again, partner AMO has been responsible for the fabrication of the devices. Photonic structures and at the same time markers used to align further lithographic layers are defined by electron beam lithography. An ICP-RIE process is used to transfer the structures into the substrate, resulting in silicon photonic structures and positive markers.
- Fig.: Schematic of the fabricated coupler structure
Finally, the cladding waveguide layer is defined by a second electron beam lithography step. It is worth to note that the alignment between the silicon waveguide and the SU8 cladding waveguide and the lithographic definition of smooth SU8 waveguides with low losses in a 3µm thick polymer layer by means of electron beam lithography is a demanding task. With the marker trapped between the isolating silica layer and the non-conducting SU8 polymer a precise pre-alignment of the samples and a fine-tuning of the marker search algorithm of the electron beam writer are needed for a reliable high-precision alignment with an overlay accuracy of about 50nm. Fig. 4 shows exemplary a silicon waveguide embedded into the SU8 waveguide.
For a taper tip width of 60nm and a taper length of 200µm, efficiencies for the modal transformation from the SU8 cladding to the silicon waveguide of above 85% per coupler for both TE and TM polarization over a 3-dB bandwidth of more than 100nm have been experimentally determined. This transformation efficiency corresponds to a total coupling efficiency from the fibre to the waveguide of up to 60% per coupler, as additional losses during coupling from the fibre to the SU8 waveguide have to be taken into account, mainly caused by a not optimized facet preparation.
- Fig.: SEM micrograph of a silicon waveguide embedded into 3 µm thick SU8 waveguide
