ABSTRACT
Optical frequency domain reflectometry (OFDR) is a powerful technique to investigate backscatter in waveguides. However, its use in Si photonics circuits has so far been limited to measuring the propagation loss and group index of a waveguide. We demonstrate that the transmittance (T) and reflectance (R) of elemental devices comprising a Si photonics circuit can be determined by OFDR. An analysis of Si wire waveguides with grating couplers (GCs) is described in detail. The wavelength dependence of T and R of the GCs were determined by using a backscatter model incorporating time-equivalent multiple-reflection paths and were well reproduced by a numerical simulation.
ABSTRACT
To achieve high spectral linearity, we developed a Fano-resonant graded-stub filter on the basis of a pillar-photonic-crystal (PhC) waveguide. In a numerical simulation, the availability of a linear region within a peak-to-bottom wavelength span was nearly doubled compared to that of a sinusoidal spectrum, which was experimentally demonstrated with a fabricated silicon-pillar PhC stub filter. The high linearity of this filter is suitable for optical modulators used in multilevel amplitude modulation.
ABSTRACT
We propose a double-stage guided-mode converter for pillar photonic-crystal (PhC) waveguide devices. The converter consists of a pillar-to-wire waveguide coupler and a transverse-magnetic-mode-selective spot-size converter. The former secures high-efficiency wide-band optical coupling of a pillar-PhC waveguide to a wire waveguide. The latter improves the coupling efficiency of the wire waveguide and an outside waveguide such as an optical fiber and also the signal-to-noise ratio of light guided in the pillar-PhC waveguide. The transmission band of a fabricated pillar-PhC waveguide having the converters on both ends was 88 nm in wavelength. The cutoff at the band edge was steep and deep with an extinction ration of 40 dB in a 4-nm wavelength range.
ABSTRACT
We present numerical demonstrations that anti-phase reflection coatings (APRCs) on the core layers of grating couplers (GCs) return anti-phase field into the core layers and cancel the downward scattering from the gratings by destructive interference to improve the upward directionality of the GCs while the output power per unit length is reduced. Investigating simplified models of GC, we reveal the effect of APRCs semi-analytically and quantitatively. The APRCs can be combined with other enhancement measures, like deep gratings and backside mirrors, to tailor an appropriate output power per unit length while achieving high upward directionalities cooperatively.
ABSTRACT
Highly integrated optical device technology based on square-lattice Si-pillar photonic-crystal-(PC) waveguides is described. The Si-pillar PC waveguides are now ready to use, since efficient optical coupling structures to Si-wire waveguides have been devised. Nanophotonic switches using the Si-pillar-PC waveguides were experimentally demonstrated. The nanophotonic switches make use of two of the features of Si-pillar photonic crystal waveguides. One is the property of slow-light and the other is the usability of zero-radius 90 degrees bends, both of which enable waveguide-based optical devices to be greatly miniaturized. Even apart from the cut-off wavelength, the group index of the pillar-PC waveguides was about 7.8, which was about twice that of a Si-wire waveguide for the entire C-band of telecommunications wavelengths. The 3-dB couplers we fabricated were only 3.2-microm long thanks to the 90 degrees sharp bends, and they operated throughout the entire C-band. Waveguide-cross operation was also demonstrated in the entire C-band. Asymmetric Mach-Zehnder interferometers (MZIs) were configured by using the 3-dB couplers in an area of 13.2 x 37.2 microm. An MZI with a Si-wire heater successfully operated with an extinction ratio of about 20 dB at a heating power of 17 mW. It is strongly suggested that Si-pillar PC photonic-waveguide technology should help us to achieve densely integrated optical-matrix switches demanded for future photonic-telecommunication systems.
