Hugh Podmore

Demonstration of a compressive-sensing Fourier-transform on-chip spectrometer

We demonstrate compressive-sensing (CS) spectroscopy in a planar-waveguide Fourier-transform spectrometer (FTS) device. The spectrometer is implemented as an array of Mach-Zehnder interferometers (MZIs) integrated on a photonic chip. The signal from a set of MZIs is composed of an undersampled discrete Fourier interferogram, which we invert using l1-norm minimization to retrieve a sparse input spectrum. To implement this technique, we use a subwavelength-engineered spatial heterodyne FTS on a chip composed of 32 independent MZIs. We demonstrate the retrieval of three sparse input signals by collecting data from restricted sets (8 and 14) of MZIs and applying common CS reconstruction techniques to this data. We show that this retrieval maintains the full resolution and bandwidth of the original device, despite a sampling factor as low as one-fourth of a conventional (non-compressive) design.

A compressive-sensing Fourier-transform on-chip Raman spectrometer

We demonstrate a novel compressive sensing Fourier-transform spectrometer (FTS) for snapshot Raman spectroscopy in a compact format. The on-chip FTS consists of a set of planar-waveguide Mach-Zehnder interferometers (MZIs) arrayed on a photonic chip, effecting a discrete Fourier-transform of the input spectrum. Incoherence between the sampling domain (time), and the spectral domain (frequency) permits compressive sensing retrieval using undersampled interferograms for sparse spectra such as Raman emission. In our fabricated device we retain our chosen bandwidth and resolution while reducing the number of MZIs, e.g. the size of the interferogram, to 1/4th critical sampling. This architecture simultaneously reduces chip footprint and concentrates the interferogram in fewer pixels to improve the signal to noise ratio. Our device collects interferogram samples simultaneously, therefore a time-gated detector may be used to separate Raman peaks from sample fluorescence. A challenge for FTS waveguide spectrometers is to achieve multi-aperture high throughput broadband coupling to a large number of single-mode waveguides. A multi-aperture design allows one to increase the bandwidth and spectral resolution without sacrificing optical throughput. In this device, multi-aperture coupling is achieved using an array of microlenses bonded to the surface of the chip, and aligned with a grid of vertically illuminated waveguide apertures. The microlens array accepts a collimated beam with near 100% fill-factor, and the resulting spherical wavefronts are coupled into the single-mode waveguides using 45& mirrors etched into the waveguide layer via focused ion-beam (FIB). The interferogram from the waveguide outputs is imaged using a CCD, and inverted via l1-norm minimization to correctly retrieve a sparse input spectrum.

Spectral retrieval techniques for high-resolution Fourier-transform micro-spectrometers

Spatial heterodyne Fourier transform (SHFT) spectroscopy is based on simultaneous interferometric measurements implementing linearly increasing optical path differences, hence circumventing the need for mechanical components of traditional Fourier transform spectroscopy schemes. By taking advantage of the high mode confinement of the Siliconon-Insulator (SOI). platform, great interferometric lengths can be implemented in a reduced footprint, hence increasing the resolution of the device. However, as resolution increases, spectrometers become progressively more sensitive to environmental conditions, and new spectral retrieval techniques are required. In this work, we present several software techniques that enhance the operation of high-resolution SHFT micro-spectrometers. Firstly, we present two techniques for mitigating and correcting the effects of temperature drifts, based on a temperature-sensitive calibration and phase errors correction. Both techniques are demonstrated experimentally on a 32 Mach-Zehnder interferometers array fabricated in a Silicon-on-insulator chip with microphotonic spirals of linearly increasing length up to 3.779 cm. This configuration provides a resolution of 17 pm in a compact device footprint of 12 mm2. Secondly, we propose the application of compressive-sensing (CS) techniques to SHFT micro-spectrometers. By assuming spectrum sparsity, an undersampled discrete Fourier interferogram is inverted using l1-norm minimization to retrieve the input spectrum. We demonstrate this principle on a subwavelength-engineered SHFT with 32 MZIs and a 50 pm resolution. Correct retrieval of three sparse input signals was experimentally demonstrated using data from 14 or fewer MZIs and applying common CS reconstruction techniques to this data.

Recent advances in on-chip Fourier transform spectrometers

We report our advances in development of on-chip Fourier-transform spectrometers. Specifically, we present applications of subwavelength engineered waveguides in a new type of a compressive-sensing Fourier-transform spatial heterodyne spectrometer chip. The spectrometer is implemented as an array of Mach-Zehnder interferometers (MZIs) integrated on a photonic chip, in a spatial heterodyne configuration [1]. The signal from a set of MZIs comprises an undersampled discrete Fourier interferogram, which is inverted using L 1 -norm minimization technique to retrieve a sparse input spectrum. To implement this technique we use a subwavelength-engineered Fourier-transform spectrometer on a chip comprising 32 independent MZIs [2]. We successfully demonstrate the retrieval of sparse input signals by collecting data from restricted sets (8 and 14) of MZIs and applying common compressive-sensing reconstruction techniques to this data. We show that this retrieval maintains the full resolution and bandwidth of the original device despite a sampling factor as low as 1/4th of a conventional (non-compressive) design [3].

Athermal planar-waveguide Fourier-transform spectrometer for methane detection

We demonstrate a passively thermally-stabilized planar waveguide Fourier-transform spectrometer for remote detection of atmospheric methane. The device is implemented as a spatial heterodyne spectrometer using an array of 100 Mach-Zehnder interferometers (MZIs) on an integrated photonic chip. The spectrometer is buffered against temperature fluctuations by using waveguides with a carefully engineered, athermal geometry. The achieved waveguide thermooptic optic coefficient is 3.5×10−6K−1. Effective entrance aperture is increased over dispersive element spectrometers, without sacrificing spectral resolution, by coupling light independently to each of the 100 MZIs. The output of each MZI is sampled in quadrature, to compensate for non-uniform illumination across the MZI input apertures. The spectrometer is validated using a methane reference cell in a benchtop setup: an interferogram is inverted via least-squares spectral analysis (LSSA) to retrieve multiple absorption lines at a spectral resolution of 50 pm over a 1 nm free spectral range (FSR) centered at λ0 = 1666.5 nm. The retrieved spectrum is compared against the Beer-Lambert absorption law and is found to provide a correct measurement of the volume mixing ratio (VMR) in the optical path.

Principles and design of a planar waveguide Fourier transform spectrometer for remote-sensing applications

This paper presents the design and operating principles of an advanced Fourier transform (FT) microspectrometer. The microspectrometer is a static Fourier transform instrument based on the principle of spatial heterodyne spectroscopy (SHS), affording high optical throughput (étendue) as compared with an arrayed waveguide (AWG) or planar waveguide echelle grating spectrometer. The instrument is realized as a densely-packed array of Mach-Zehnder interferometers (MZIs) with linearly increasing optical path delays (OPDs). Each MZI in the array constitutes a sampling point in the discrete Fourier-transform of the optical spectrum. The use of discrete MZIs in this device makes the selection of Fourier samples straightforward, and the throughput advantage permits the development of very high-resolution devices. Using this approach we have developed a 100-MZI FT chip with high resolution (0.05 nm) over a free spectral range (FSR) centered on the Q-branch absorption features of atmospheric methane (1667.75 nm-1665.25 nm). This FT chip is the central component of an integrated microspectrometer payload for measuring greenhouse gas emissions in the Canadian oil sands. The MZI array is realized in silicon nitride and has an overall footprint of 12mm × 22 mm with OPDs ranging from 0.32 mm to 32 mm. The two outputs of each MZI are re-balanced in multi-mode interference (MMI) devices but not combined, resulting in a system with 100 inputs and 200 outputs. Separating the outputs of the MZIs in this manner allows for normalization between MZI arms in order to correct for asymmetric loss in the MZI. High-resolution spectrometers of this type require highly unbalanced MZIs, which may be unstable with respect to temperature. Temperature induced variations may be corrected in postprocessing provided that the variations are made small, this can be accomplished either by active cooling via Peltier system, or passively by the use of athermal waveguides. We have designed athermal waveguides through the use of a cladding material with a negative thermo-optic coefficient (TOC). We balance the modal confinement between the core (positive TOC), lower cladding (positive TOC), and upper cladding (negative TOC) to lower the effective TOC of the device. By design of these athermal waveguides we eliminate the need for a precise active cooling system, reducing the mass, volume, and power requirements of the integrated payload.

Increasing solar cell power production on micro and nano satellites using sub-wavelength gratings

The application of 2-dimensional periodic arrays of sub-wavelength structures as anti-reflection surfaces at the interface between a vacuum and commercial solar cell coverglass is investigated. It is determined that compared to commercial anti-reflective coatings a single anti-reflective sub-wavelength grating will yield 3.7 to 7.1% total increase in power production during a 24 hour orbital period for a typical earth observing 3-U CubeSat. Sub-wavelength gratings are approximated as a many layered stack of thin films with refractive indices defined by their vertical position in the array and the grating fill factor at that position. The transmission of light between wavelengths of 350nm and 1800nm and incident angles 0-90 degrees is calculated by the transfer matrix method. The optimal periodicity, profile and height of the features for an antireflective sub-wavelength grating applied to a commercial triple junction solar cell in Air Mass 0 are explored by analysis of the transmission coefficient surfaces for various gratings, and two sub-wavelength grating surfaces are benchmarked against the performance of a MgF2 thin film anti reflective coating, as well as uncoated cover glass. The AM0 solar spectrum is propagated through the transmission coefficients of the SWG surface as well as a model for the external quantum efficiency and heat loss of commercially available Triangular Advanced Solar Cells. The amount of light reaching the cells is then applied to the photo response of each sub cell to determine the expected increase in solar cell power production at incident angles from 0-90 degrees. The incident angles for solar panels mounted on 3-U CubeSats in typical earth observation orbits are determined and used to calculate the expected increase in average power production over orbit which is found to be between 3.7 and 7.1% of the total.