Abstract
The design, modeling, micro-fabrication, and characterization of an ultra-broadband Ge-on-Si waveguide polarization rotator are presented. The polarization rotator is based on the mode evolution approach where adiabatic symmetric and anti-symmetric tapers are utilized to convert from the fundamental transverse magnetic to electric mode. The device is shown to be extremely fabrication tolerant and simple to fabricate. The fabricated devices demonstrate a polarization extinction ratio of ≥15 dB over a 2 μm bandwidth (9–11 μm wavelength) with an average insertion loss of <1 dB, which is an order of magnitude improvement compared to previously demonstrated devices. This device will provide polarization flexibility when integrating quantum cascade lasers on-chip for mid-infrared waveguide molecular spectroscopy.
Highlights
The mid-infrared (MIR) spectral region is of significant interest for several applications such as security,1 healthcare,2 drug identification,3 and environmental monitoring.4,5 This is due to how chemical compounds can be identified through their unique vibrational modes, which absorb in the molecular “fingerprint” region (6.7–20 μm wavelength).6 An integrated waveguide molecular spectrometer would significantly reduce the size and cost compared to commercially available spectrometers.7 A waveguide spectrometer requires the integration of a source and detector along with the passive waveguides for sensing
A Ge-on-Si rib waveguide polarization rotator that operates between 9 μm and 11 μm wavelengths with an average insertion loss
The polarization rotator is based on the mode evolution approach in order to convert the fundamental supported transverse magnetic (TM) mode to transverse electric (TE) mode
Summary
The mid-infrared (MIR) spectral region is of significant interest for several applications such as security, healthcare, drug identification, and environmental monitoring. This is due to how chemical compounds can be identified through their unique vibrational modes, which absorb in the molecular “fingerprint” region (6.7–20 μm wavelength). An integrated waveguide molecular spectrometer would significantly reduce the size and cost compared to commercially available spectrometers. A waveguide spectrometer requires the integration of a source and detector along with the passive waveguides for sensing. The mid-infrared (MIR) spectral region is of significant interest for several applications such as security, healthcare, drug identification, and environmental monitoring.4,5 This is due to how chemical compounds can be identified through their unique vibrational modes, which absorb in the molecular “fingerprint” region (6.7–20 μm wavelength).. By integrating several QCLs on-chip with tunable waveguide resonators, this would enable scanning across many molecular absorption lines with high power and would compensate for a non-cryogenically cooled detector (see Fig. 1). This would provide an alternative architecture compared to realizing an integrated Fourier-transform spectrometer.. The largest operational bandwidths demonstrated have been ≤200 nm with an IL ≤1 dB and a PER ≥10 dB. There has been limited investigation into migrating a waveguide polarization rotator to the MIR, where the majority of the work has been predominately simulation and where there have been two recent experimental demonstrations below 6.15 μm wavelength. Here, we demonstrate the design, modeling, microfabrication, and characterization of a Ge-on-Si waveguide polarization rotator operating between 9 μm and 11 μm wavelengths with an average IL
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