We report on a channel drop filter with a mode gap in the propagating mode of a photonic crystal slab that was
fabricated on silicon on an insulator wafer. The results, simulated with the 3-dimensional finite-difference time-domain
and plane-wave methods, demonstrated that an index-guiding mode for the line defect waveguide of a photonic crystal
slab has a band gap at wave vector k = 0.5 for a mainly TM-like light-wave. The mode gap works as a distributed Bragg
grating reflector that propagates the light-wave through the line defect waveguide, and can be used as an optical filter.
The filter bandwidth was varied from 1-8 nm with an r/a (r: hole radius, a: lattice constant) variation around the
wavelength range of 1550-1600 nm. We fabricated a Bragg reflector with a photonic crystal line-defect waveguide and
Si-channel waveguides and by measuring the transmittance spectrum found that the Bragg reflector caused abrupt dips
in transmittance. These experimental results are consistent with the results of the theoretical analysis described above.
Utilizing the Bragg reflector, we fabricated channel dropping filters with photonic crystal slabs connected between
channel waveguides and demonstrated their transmittance characteristics. They were highly drop efficient, with a flattop
drop-out spectrum at a wavelength of 1.56 μm and a drop bandwidth of 5.8 nm. Results showed that an optical adddrop
multiplexer with a 2-D photonic crystal will be available for application in WDM devices for photonic networks
and for LSIs in the near future.
Reconfigurable optical add/drop multiplexers (R-OADMs) are indispensable devices in wavelength division
multiplexing (WDM) network systems, since they can be used for dynamically wavelength routing and for replacing any
failed OADM unit. Here, we propose a photonic integrated R-OADM device based on silicon photonic crystal (PhC)
slab waveguides, which is controlled through thermo-optic effect. The R-OADM device was composed of a tunable
wavelength multiplexer/demultiplexer and a 2×2 optical switch, which were both formed with Mach-Zehnder
interferometers (MZIs). The device was compact with a net footprint of 500 μm × 140 μm, excluding its electrode pads.
The dropping central wavelength of the R-OADM can be tuned through thermo-optic effect, and the output port of the
drop signal can be selected between the THROUGH and DROP ports with the 2x2 optical switch. A maximum 10.8 nm
dropping wavelength tuning was obtained with a heating power of 0.9 W. The 3-dB channel-dropping bandwidth was 5
nm and the extinction ratio at the dropping wavelength for the port THROUGH was as high as 40 dB. The tuning
response speed was about 100 μsec.
We fabricated various microscopic optical devices using photonic crystal slab and Si-wire waveguides and demonstrated their fundamental characteristics. We demonstrated a channel-dropping filter with a photonic crystal slab point-defect optical cavity. Wavelength resolution of less than 1.5 nm and signal dropping efficiency of more than 90 % were obtained for a 20-μm-square device. We also demonstrated an optical add/drop multiplexer with Bragg grating reflectors made from Si-wire waveguides. Its dropping wavelength bandwidth was less than 2 nm, and the center wavelength of the dropped optical signal could be tuned by thermo-optic control using a microheater formed on the Bragg reflector. Using Si-wire waveguide, we also demonstrated thermo-optic switches by forming a micro heater on a branch of a Mach-Zehnder interferometer constructed from the waveguides. In this switching operation, we observed an extinction ratio of more than 30 dB, switching power of less than 100 mW, and switching response speed of less than 100 μs using a 1 × 2 optical switch with an 85 × 30 μm2 footprint. Using the 1 × 2 optical switch elements, we also fabricated a compact 1 × 4 optical switch and demonstrated its fundamental operation.
We report on a channel-drop filter (CDF) with a mode gap of propagating mode for a photonic crystal slab that was fabricated on silicon on an insulator wafer. The results simulated with 3-dimensional finite-difference time-domain and plane-wave methods demonstrated that an index-guiding mode for a line defect waveguide of a photonic crystal slab has a band gap at wave vector k = 0.5 for a mainly TM-like light-wave. The mode gap works as a distributed Bragg grating reflector for the propagating light-wave through the line defect waveguide, and it can be used as an optical filter. The filter bandwidth was varied from 1 - 8 nm with an r/a (r: hole radius, a: lattice constant) variation around the wavelength range of 1550 - 1600 nm. We fabricated a Bragg reflector with a photonic crystal line-defect waveguide and with Si-channel waveguides and obtained results of transmittance abrupt dips that come from the Bragg reflector that were measured in the transmittance spectrum. The experimental results are consistent with our theoretical analysis.
The tuning of the light propagation and localization properties in photonic crystal (PhC) slabs by using microactuators was demonstrated numerically and experimentally. A micromechanical actuator controls the position of the exterior structural element, which is located close to the PhC slab, and modulates the PhC properties through the change of the evanescent interaction of light confined in the PhC slab with the exterior element. When the exterior structural element approaches to a line-defect PhC waveguide, intensity and phase modulations occur. In the preliminary experiment using a line-defect PhC waveguide, we demonstrated the optical switching operation with an extinction ratio of ~10 dB at a wavelength of 1.55 μm. The localized state of light in a point-defect cavity can also be controlled. The tuning of the resonant wavelength over the spectral range of ~60nm at around the wavelength of 1.55 μm was numerically demonstrated by combining two PhC slabs. The approach discussed here can be widely employed for realizing functional and tunable PhC slab devices.
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