Micro-spectrometry using tapered air-core waveguides
Integrated devices for spectral discrimination and measurement are widely sought for applications in sensing, metrology, and communications. A wide variety of chip-scale devices - including those based on diffraction gratings, Fabry-Perot etalons, photonic crystals, dispersive plasmonic structures, and standing-wave interferometry - have been proposed in the literature. Typically, these devices suffer from on or more drawbacks, such as low resolving power, low light throughput, or low operational bandwidth.
Our research employs tapered air-core waveguides, clad by omnidirectional-dielectric-reflector mirrors, as a unique and powerful dispersive element. These waveguides can be viewed as side-coupled Fabry-Perot interferometers - light launched into the wide end of the tapered waveguide is adiabatically transformed into a vertical-cavity resonance as it approaches the narrow end of the waveguide. At a specific and wavelength-dependent cutoff point, light is radiated in an out-of-plane direction and can be captured by an image sensor. The spectrum of the input light can thus be extracted from the spatial pattern of light collected by the image sensor.
This approach combines the advantages of a Fabry-Perot etalon (high resolving power without the need for large size) with the advantages of a diffraction grating (spectral-to-spatial mapping without the need for moving parts). It is particularly well-suited to applications that involve small emitters (e.g. lab-on-a-chip, cytometry) or which are waveguide-based (e.g. fiber sensors). We have employed these devices both for reconstructing broadband spectra and for interrogating pm-scale wavelength shifts in optical sensing.
References
- C. Potts, T. W. Allen, A. Azar. A. Melnyk, C. R. Dennison, and R.G. DeCorby, "Wavelength interrogation of fiber Bragg grating sensors using tapered hollow Bragg waveguides," Opt. Lett. 39, 5941-5944 (2014).
https://doi.org/10.1364/OL.39.005941 - B. Drobot, A. Melnyk, M. Zhang, T.W. Allen, R.G. DeCorby, "Visible-band dispersion by a tapered air-core Bragg waveguide," Opt. Express 20, 23906-23911 (2012).
https://doi.org/10.1364/OE.20.023906 - R.G. DeCorby, N. Ponnampalam, E. Epp, T. Allen, J.N. McMullin, "Chip-scale spectrometry based on tapered hollow Bragg waveguides," Opt. Express 17, 16632-16645 (2009).
https://doi.org/10.1364/OE.17.016632