Principles and design of a planar waveguide Fourier transform spectrometer for remote-sensing applications
Bibliographic record
Abstract
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.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.001 | 0.001 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.001 |
| Scholarly communication | 0.001 | 0.001 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.001 | 0.001 |
| Insufficient payload (model declined to judge) | 0.001 | 0.001 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".