We propose a large aperture static imaging spectrometer (LASIS) based on planar lightwave circuit (PLC) MZI array. The imaging spectrometer works in the push-broom mode with the spectrum performed by interferometry. While the satellite/aircraft is orbiting, the same source, seen from the satellite/aircraft, moves across the aperture and enters different MZIs, while adjacent sources enter adjacent MZIs at the same time. The on-chip spectrometer consists of 256 input mode converters, followed by 256 MZIs with linearly increasing optical path delays and a detector array. Multiple chips are stick together to form the 2D image surface and receive light from the imaging lens. Two MZI arrays are proposed, one works in wavelength ranging from 500nm to 900nm with SiON(refractive index 1.6) waveguides and another ranging from 1100nm to 1700nm with SOI platform. To meet the requirements of imaging spectrometer applications, we choose large cross-section ridge waveguide to achieve polarization insensitive, maintain single mode propagation in broad spectrum and increase production tolerance. The SiON on-chip spectrometer has a spectral resolution of 80cm-1 with a footprint of 17×15mm2 and the SOI based on-chip spectrometer has a resolution of 38cm-1 with a size of 22×19mm2. The spectral and space resolution of the imaging spectrometer can be further improved by simply adding more MZIs. The on-chip waveguide MZI array based Fourier transform imaging spectrometer can provide a highly compact solution for remote sensing on unmanned aerial vehicles or satellites with advantages of small size, light weight, no moving parts and large input aperture.
A novel method for designing a silica waveguide based visible etched diffraction grating (EDG) with uniform loss is proposed. The designed 1st-order EDG comprises 121 output waveguides with a 2.5 nm channel spacing at a wavelength range from 400 nm to 700 nm. Using the conventional flat-field design with two-stigmatic-points method, the simulated channel loss non-uniformity of a conventional EDG is 2.66 dB. By changing the central output waveguide position and rotating the angles of grating facets according to an appropriately designed distribution function, the loss non-uniformity is reduced to 1.36 dB and the highest loss of marginal channels is decreased from 2.69 dB to 2.13 dB simultaneously. With a total chip size of 30 mm×16 mm, this visible EDG is suitable for realization of spectrometer-on-chip. The proposed design method can achieve insertion loss uniformity in a wide wavelength range with no additional element or extra fabrication step.
We demonstrate an echelle diffraction grating (EDG) of 17 input waveguides and 33 output waveguides. For each input waveguide, only 17 of 33 output waveguides are used, receiving light ranging from 1520 nm to 1600 nm wavelength. The channel spacing of the EDG is 5 nm, with loss of -6dB and crosstalk of -17dB for center input waveguide and -15dB for edge input waveguides. Based on the 3 μm SOI platform the device is polarization insensitive. As a simple version of EDG spectrometer it is designed to be a part of the on-chip spectroscopic system of the push-broom scanning imaging spectrometer. The whole on-chip spectrometer consists of an optical on-off switch array, a multi-input EDG and detector array. With the help of on-off switch array the multiple input waveguides of the EDG spectrometer could work in a time division multiplexed fashion. Since the switch can scan very fast (less than 10 microseconds), the imaging spectrometer can be operated in push-broom mode. Due to the CMOS compatibility, the 17_channel EDG scales 2.5×3 mm2. The full version of EDG spectrometer is designed to have 129 input waveguides and 257 output waveguides (129 output channel for each input waveguide), working in wavelength ranging from 1250 nm to 1750 nm, and had similar blazed facet size with the 17_channel one, which means similar fabrication tolerance in grating facets. The waveguide EDG based imaging spectrometer can provide a low-cost solution for remote sensing on unmanned aerial vehicles, with advantages of small size, light weight, vibration-proof, and high scalability.
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