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.
We demonstrate low-loss photonic wire waveguides, in both the straight and bent waveguide configurations, fabricated
by the LOCal Oxidation of Silicon (LOCOS) process, using the standard optical lithography. The oxidation in the
LOCOS process produces waveguides in submicron dimensions with ultra-smooth sidewalls. The Full-Width Half-
Maximum (FWHM) of the fabricated LOCOS wire waveguide is approximately 650 nm and the height is 280 nm. We
used the cut-back method to measure the propagation loss of the TE (x-polarized) mode. The average propagation loss
measured by the cut-back method was 8.78 dB/cm, while the minimum measured propagation loss achieved was 7.18
dB/cm for simple straight waveguides. The propagation loss is expected to be lower, as we include the scattering loss in
the measurements. The measured bending loss of the LOCOS wire waveguide with a bending radius of 5 um is as low as
0.0089 dB/90° bend for the TE mode. To the best of knowledge, this is the first direct measurement in propagation loss
and bending loss for LOCOS wire waveguides.
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.