Athermal operation of silicon waveguides for the TM and TE mode is achieved using the bridged subwavelength grating (BSWG) waveguide geometry. For the TM mode the experimental results show that the temperature-induced wavelength shift (dλ/dT) is an order of magnitude smaller for the BSWG waveguides with grating duty cycle, waveguide and bridge widths of 42%, 490 nm and 220 nm, respectively, as compared to standard photonics wires (PW). For the TE mode similar results are achieved by using the bridge width of 200 nm and similar duty cycle and waveguide width. A temperature-induced shift of only -2.5 pm/°C is reported for the TM polarized light. Propagation losses of BSWG waveguides for both polarizations were measured to be about 8 dB/cm, comparable to that of PWs.
In this paper, athermal subwavelength grating (SWG) waveguides are investigated. Both numerical simulations and
experimental results show that a temperature independent behaviour can be achieved by combining two materials with
opposite thermo-optic coefficients within the waveguide. SU-8 polymer with a negative thermo-optic coefficient (dn/dT
= -1.1x10-4 K-1) is used in our silicon SWG waveguides to compensate for silicon's positive thermo-optic coefficient of
1.9x10-4 K-1. The grating duty ratio required to achieve an athermal behavior is reported to vary as a function of the
operating wavelength and the waveguide dimensions. For example, for athermal waveguides of 260 nm in height, duty
ratios of 61.3% and 83.3% were calculated for TE and TM polarized light respectively for a 450 nm wide waveguide,
compared to ratios of 79% and 90% for a 350 nm wide waveguide. It is also reported that with increasing width, and
increasing height, a smaller grating duty ratio is necessary to achieve an athermal behaviour. A smaller fraction of silicon
would hence be needed to compensate for the polymer's negative thermo-optic effect in the waveguide core.
Subwavelength sidewall grating (SWSG) waveguides are also proposed here as alternatives to high duty ratio SWG
waveguides that are required for guiding TM polarized light. Assuming a duty ratio of 50%, the width of the narrow
segments for temperature-independent behavior is found by numerical simulations to be 125 nm and 143 nm for TE and
TM polarized light, respectively.
Multiple Aperture Transform Chip Heterodyne (MATCH) spectrometers have been developed for targeted remote
sensing applications in harsh environments. These waveguide-based Fourier Transform Spectrometers (FTS) offer
significant improvements in resource efficiency over monolithic glass implementations, but are relatively limited in
terms of input coupling efficiency and fill factor of the input facet. Integrated optics spectrometers have significant
resource advantages for space applications. Monolithic Spatial Heterodyne Spectrometers are insensitive to
vibration and do not require frequent calibration. In addition, Fourier Transform Spectrometers are known to
provide significant performance advantages for emission spectroscopy. Ongoing work will improve the MATCH
spectrometer input coupling efficiency from free space. This paper discusses the signal to noise improvements
expected by incorporation of surface gratings, or back-thinning and stacking of slabs. We show that the use of
surface gratings can increase the throughput over coupling to bare waveguides alone (in a single polarization), and
provide close to 100% fill factor, albeit with limited field. Étendue improvements associated with stacked slabs are
limited only by the sensing area available, but the fill factor of the input facet is limited to ~10%. The impact of
these improvements is assessed in the context of two space-based applications: 1) Atmospheric remote sensing in
the context of Spatial Heterodyne Observations of Water (solar occultation absorption spectroscopy) near 1.3 μm
and 2) Point emission spectroscopy (LIBS/Raman/fluorescence) for mineral identification on a planetary rover.
We review recent advances in subwavelength and diffractive structures in planar waveguides. First, we present a new
type of microphotonic waveguide, exploiting the subwavelength grating (SWG) effect. We demonstrate several
examples of subwavelength grating waveguides and components made of silicon, operating at telecom wavelengths. The
SWG technique allows for engineering of the refractive index of a waveguide core over a range as broad as 1.5-3.5
simply by lithographic patterning using only two materials, for example Si and SiO2. This circumvents an important
limitation in integrated optics, which is the fixed value of the refractive indices of the constituent materials in the absence
of an active tuning mechanism. A subwavelength grating fibre-chip microphotonic coupler is presented with a loss as
low as 0.9 dB and with minimal wavelength dependence over a broad wavelength range exceeding 200 nm. It is shown
that the SWG waveguides can be used to make efficient waveguide crossings with minimal loss and negligible crosstalk.
We also present a diffractive surface grating coupler with subwavelength nanostructure, that has been implemented in a
Si-wire evanescent field biological sensor. Furthermore, we discuss a new type of planar waveguide multiplexer with a
SWG engineered nanostructure, yielding an operation bandwidth exceeding 170 nm for a device size of only 160 μm ×
100 μm.
In this invited paper we focus on the discussion of two recent unique applications of the Finite-Difference Time-Domain
(FDTD) simulation method to the design and modeling of advanced nano- and bio-photonic problems. We will first
discuss the application of a traditional formulation of the FDTD approach to the modeling of sub-wavelength photonics
structures. Next, a modified total/scattered field FDTD approach will be applied to the modeling of biophotonics
applications including Optical Phase Contrast Microscope (OPCM) imaging of cells containing gold nanoparticles (NPs)
as well as its potential application as a modality for in vivo flow cytometry configurations. The discussion of the results
shows that the specifics of optical wave phenomena at the nano-scale opens the opportunity for the FDTD approach to
address new application areas with a significant research potential.
As a result of the evolution semiconductor fabrication tools and methods over several decades, it now possible to
routinely design and make optical devices with features comparable to or smaller than the wavelength of the light that
propagates through these structures. This paper will review some silicon optical structures with critical features at these
extremely short length scales. For example it becomes possible to create segmented waveguide structures with optical
properties that can be tuned continuously between those of the cladding and waveguide core, using lithographic
patterning rather than varying etch depth. Using thin high index contrast waveguides and the correct polarization, the
optical electric field profiles can be shaped to maximize the coupling to molecular monolayers or cladding layers with
specific functionality. Examples are given from our recent work on optical biosensors chips which employ grating
couplers made by sub-wavelength digital patterning, and use waveguides optimized for coupling to molecular
monolayers.
This paper introduces a compact 90º optical hybrid, built on small size SOI waveguide technology .This optical hybrid
is a critical component of a potentially low-cost coherent optical receiver design developed within the frame of our
Optical Coherent Transmission for Access Network Extensions (OCTANE) project. In previous recent work, 90º
optical hybrids were realized in SOI rib waveguide technology with 4 μm top silicon and a rib height of approximately
2 μm. In this paper, we introduce a compact 90º optical hybrid, built on small size SOI waveguide technology (1.5 μm
SOI -based rib waveguide, with 0.8μm rib height). The proposed device consists of multimode interferometers (MMIs)
connected in such a way that four different vector additions of a reference signal (local oscillator) and the signal to be
detected are obtained. At the outputs, the hybrid provides four linear combination of the signal with the reference
which differs by a relative phase shift of the reference of 90º. The four output signals are detected by a pair of
balanced receivers to provide in-phase and quadrature (I&Q) channels. The phase differences arise naturally from the
self imaging property of a MMI.
The key elements of the 90º optical hybrid, including a 2×2 MMI, a 4×4 MMI, and polarization diversity
configuration have been designed and simulated, using the numerical mode solving tool FIMMPROB. The 2×2 and
4×4 MMI had overall lengths of 701μm and 3712.5μm lengths respectively. Tapers are used to couple adiabatically
single mode waveguides to the entrance and exit ports of the MMI to assure correct operation by avoiding coupling to
the higher order transverse modes allowed at the entrance and exit ports of the MMI. The simulation results at 1550nm
show polarization independence and phase errors between the ports of less than 0.03 degrees. Currently the design is
in fabrication at the Canadian Photonics Fabrication Center with the support of CMC Microsystems and experimental
results will be subject to a further report.
We discuss several applications of both resonant and non-resonant subwavelength gratings (SWGs) for silicon
photonics. We present results of evanescent field molecular sensing using the transverse magnetic mode of a 0.22 μm
thick silicon slab waveguide with a resonant SWG, which couples a free space laser beam to the silicon waveguide
mode. The optical readout of this configuration is almost identical to the established surface plasmon resonance sensing
technology. Using calibrated sucrose solutions, we demonstrate a bulk refractive index sensitivity of 111 nm/RIU in
good agreement with rigorous coupled wave analysis calculations. The binding of a monolayer of streptavidin protein on
the waveguide surface is monitored in real time with a signal-to-noise ratio of ~500. In another application, non-resonant
SWGs are used to create effective dielectric media with a refractive index that can be tuned between the values of silicon
(3.48) and SU-8 polymer used for the cladding (1.58). For example, we present SWG waveguides with an effective core
index of approximately 2.65, which exhibit lower propagation loss than photonic wire waveguides of similar
dimensions. We use these SWG waveguides to demonstrate highly efficient fiber-chip couplers.
We report on new types of sub-wavelength grating gradient index structures for efficient mode coupling in high index
contrast slab waveguides. Using sub-wavelength gratings (SWGs), an adiabatic transition can be formed at the interface
between slab waveguides of different core thicknesses. The SWG transition region minimizes both mode mismatch loss
and coupling to higher order modes. By creating the adiabatic gradient effective index region in the direction of
propagation, we demonstrate that vertical mode size transformation is readily achieved in structures that can be
fabricated using a single etch step. Using 3D finite-difference time-domain simulations we study the loss, polarization
dependence and the higher order mode excitation for two types (triangular and triangular-transverse) of SWG transition
regions between silicon-on-insulator slab waveguides of different core thicknesses. We also demonstrate two solutions to
mitigate polarization dependent loss, namely using a partial transverse SWG and a SWG in the region between the
triangular teeth. Our mode transformer designs are optimized for applications in polarization compensator in echelle
grating and arrayed waveguide grating (AWG) multiplexers and a gradient index interface between the ridge waveguide
and the slab combiner region in a curved waveguide grating demultiplexer.
We report on our recent progress in polarization control and polarization compensator designs in SOI-based planar
reflective gratings for a range of silicon core thicknesses of 0.1 μm to 10 μm. The dispersion property of the silicon
slab, without a compensator region, was found to limit the applicability of SOI gratings for achieving the polarizationinsensitive
performance of wavelength division multiplexing systems based on planar gratings. We have found that in
coarse wavelength division multiplexing systems, the birefringence of the uncompensated slab becomes impractical at
core thicknesses below 1.7 μm. Our findings clearly show that shallow etched polarization compensators can effectively
eliminate polarization dependence only in thick-core gratings and only in applications requiring free spectral ranges
(FSRs) of no more than 80 nm. In silicon cores with thicknesses of less than 1.0 μm, the significantly different value of
linear dispersion strength for the two polarization states make traditional compensator designs ineffective, since only the
central wavelength can be fully compensated. We used our findings to construct a procedure for building compensators
with a flat polarization response over wide FSRs (>80 nm). The results of our study were applied to the design of a
polarization compensator in an 18-channel multiplexer for use in coarse wavelength division multiplexing. Our
simulation results show that a careful selection of the silicon core thicknesses in the slab and compensator regions is
essential for achieving low-cross talk and low insertion loss devices. The application of thin core planar silicon gratings
to building silicon interconnects is discussed.
We propose an original diffraction grating demultiplexer (spectrometer) device with a very small footprint, designed for
the silicon-on-insulator waveguide platform. The wavelength dispersive properties are provided by a second-order diffraction grating lithographically defined and etched in the sidewall of a curved Si waveguide. The grating is blazed to maximize the -1st order diffraction efficiency. The diffracted light is coupled into the silicon slab waveguide via an impedance matching sub-wavelength grating (SWG) graded-index (GRIN) anti-reflective interface. The waveguide is curved in order to focus the light onto the Rowland circle, where different wavelengths are intercepted by different output waveguides. The phase errors were substantially reduced using an apodized design with a chirped grating pitch, which assures a constant effective index along the grating length. The simulated crosstalk is -30 dB. The device has 15 channels with a spacing of 25 nm, thus a broadband operational bandwidth of 375 nm. Its performance approaches the diffraction limit. The device layout size is 90 μm × 50 μm, which is the smallest footprint yet reported for a mux/dmux device of a similar performance.
We propose a new waveguide resonator device with a mirror cavity and a multimode interference (MMI) coupler. We present simulation results for the silicon wire MMI coupler with suppressed reflections and its use as a coupling element in the resonator cavity, built on the silicon-on-insulator waveguide platform. Tapering structures used in the reflection suppression were optimized, and the wavelength dependency of a conventional MMI was compared to that of the MMI with reflection suppression. Two-mirror MMI coupled resonator was studied using finite difference time domain simulation by both pulse and continuous wave excitation. The optimal resonator has a very small footprint of 3 μm × 29.7 μm, with a quality factor of 774.
By using two orthogonally-polarized pump beams, an ultrabroad tunable wavelength converter is demonstrated with uniform efficiency and equalized signal-to-noise ratio (SNR) through four-wave mixing (FWM) in an 1500-nm semiconductor optical amplifier (SOA). This device allows the conversion of the input data signal to lower or higher frequencies with nearly-constant conversion efficiency and SNR over a 10.66 THz tuning range. This result is a significant improvement of both the conversion efficiency and the SNR as compared with the conventional FWM-based wavelength converters. We have also investigated the effect of parameters of both input power and wavelength of pump P2 on conversion efficiency and SNR of the wavelength-converted signals.
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