This paper explains and demonstrates the unique properties of micron-size silicon-on-insulator (SOI) waveguides. It gives an overview of the silicon photonics research at VTT, as well as latest R&D highlights. The benefits of high mode confinement in rib and strip waveguides are described, reaching from low losses and small footprint to polarization independent operation and ultra-wide wavelength range from 1.2 to over 4 μm. Most of the results are from photonic integrated circuits (PICs) on 3 μm SOI, while a 25 Gbps link with a transceiver on 12 μm SOI is also reported. Wavelength multiplexing and filtering is demonstrated with some breakthrough performance in both echelle gratings and arrayed waveguide gratings. Lowest losses are below 1 dB and lowest cross-talk is below -35 dB. Progress towards monolithically integrated, broadband isolators is described, involving polarization splitters, reciprocal polarization rotators and nonreciprocal Faraday rotation in 3 μm SOI waveguide spirals. Quick update is presented about switches, modulators and Ge photodiodes up to 15 GHz bandwidth. Hybrid integration of lasers, modulators and photodiodes is also reported. The added value of trimmed SOI wafers and cavity-SOI wafers in Si photonics processing is addressed. Latest results also include up-reflecting mirrors with <0.5 dB loss, which support wafer-level testing and packaging.
We show theoretically and experimentally how a flat-top second-order response can be achieved with a self-coupled single add-drop ring resonator based on two couplers with different splitting ratios. The resulting device is a 1x1 filter, reflecting light back in the input waveguide at resonating wavelengths in the passbands, and transmitting light in the output waveguide at all other non-resonating wavelengths. Different implementations of the filter have been designed and fabricated on a micron-scale silicon photonics platform. They are based on compact Euler bends - either U-bends or Lbends - and Multi-Mode Interferometers as splitters for the ring resonators. Different finesse values have been achieved by using either 50:50 MMIs in conjunction with 85:15 MMIs or 85:15 MMIs in conjunction with 95:05 double MMIs. Unlike ordinary lowest order directional couplers, the MMIs couple most of the power in the cross-port which make them particularly suitable for the topology of the self-coupled ring, which would otherwise require a waveguide crossing. Experimental results are presented, showing good agreement with simulations. The proposed devices can find applications as wavelength-selective reflectors for relatively broad-band lasers or used as 2x2 add-drop filters when two exact replicas of the device are placed on the arms of a Mach-Zehnder interferometer.
Integrated circuits based on micron-scale silicon waveguides have the clear advantage of being tolerant to fabrication errors, thanks to the high mode confinement within the guiding core. Here we show how flat-top interleavers can be achieved on a micron-scale silicon photonics platform based on ring-loaded Mach-Zehnder Interferometers (MZIs), without the need for any thermal tuning. Robust designs are also guaranteed by resorting to Multi-Mode Interferometers (MMIs) as power splitters in both the MZIs and the ring resonators. A trade-off between in-band ripple and roll-off can be achieved by changing the ring splitting ratios. In particular rings with different finesse based on MMIs with 50:50, 72:28, and 85:15 splitting ratios have been designed, fabricated and successfully tested. In-band ripples as low as 0.2 dB and extinction ratios exceeding 15 dB have been measured from the fabricated samples. Repeatability of the performances from chip to chip and wafer to wafer is presented to show the tolerance of the devices to fabrication errors. Even though these particular devices have been designed for TE polarization only, polarization insensitive designs can be also achieved. All designs are based on strip waveguides and compact Euler-bends, leading to footprints in the order of 700x300 μm<sup>2</sup>, also thanks to an optimized configuration. They can find applications as interleavers as such or as stages in cascades of N interleavers to achieve flat-top 1x2<sup>N</sup> (de)multiplexers.
Integrated optical probes for detecting backscattered light in, e.g., Raman spectroscopy show desirable characteristics
compared to conventional optical fiber probes, although the latter ones may have better collection efficiency in many
cases. Major advantages of integrated probes include reduced size; reduced background noise due to scattering in the
probe because of reduced propagation length; potential for monolithic integration with filters and spectrometers; very
small collection volume, providing high spatial resolution; and polarization maintenance. We demonstrate that when
scattered light needs to be collected from a thin layer close to the probe surface, integrated probes can have better
collection efficiency than fiber probes do. We modeled a multimode integrated waveguide probe by adapting an
analytical model that had been developed for fiber probes. The model was extended in order to account for arbitrary
waveguide geometries and a low number of discrete waveguide modes compared to the quasi-continuum of modes in a
typical multimode fiber. Using this model we compared the collection efficiencies of integrated and fiber probes for a
thin scattering sample. We found that the integrated probe has a higher collection efficiency for scattering layer thickness
and probe-to-layer distance both smaller than ~100 μm.
Silicon oxynitride (SiON) is a highly attractive material for integrated optics, due to its excellent properties such as high
transparency, adjustable refractive index and good stability. In general, the growth of SiON layers by plasma enhanced
chemical vapor deposition (PECVD) is followed by a high temperature annealing step in order to remove hydrogen and
to achieve low propagation losses in the 1.5-μm wavelength window. The high annealing temperature (>1100°C)
required for sufficient hydrogen removal induces, however, side effects like significant inter-layer diffusion and micro-cracks
resulting in deterioration of the device performance.
In this paper compositional and optical properties of as-deposited and annealed boron (B) and phosphorous (P) doped
SiON layers were investigated. The doped layers have been fabricated by introducing PH<sub>3</sub> and B<sub>2</sub>H<sub>6</sub> gaseous precursors
into the PECVD process. Hydrogen contents of the samples have been studied by Fourier transform infrared (FTIR)
spectroscopy. Compared to undoped film, a 50% reduction of the hydrogen content was measured in as-deposited P-doped
SiON layers. Further reduction down to the FTIR detection limit was achieved upon annealing at temperatures as
low as 700°C.
Besides hydrogen reduction the reflow properties of B and P doped SiON are also highly relevant for the realization of
low-loss integrated optical circuits. Reactively ion etched channel waveguides have been reflown applying a temperature
of 900°C. Significant reduction of the sidewall roughness has been confirmed by scanning electron microscopy.