Ultra-Wideband Photonic Spectrometer for PBL Sensing

Within the Planetary Boundary Layer (PBL) most of the Earth's energy exchange occurs, yet remote sensing observations of the PBL are severely lacking. PBL is notoriously difficult to measure via traditional measurement techniques. The key to PBL characterization is maximizing spectral information to enable accurate atmospheric retrievals near the PBL. For a space-based microwave radiometry application, much narrower channel widths over much wider overall bandwidths are desired in order to measure the upwelling microwave radiance of the sky over varying PBL depths and shapes. Channel widths of 1 GHz or less are desirable over 40-50 GHz bandwidth in order to increase altitude resolution of microwave sounders. To improve our understanding of PBL phenomenon, we will develop an advanced arrayed waveguide grating (AWG) device based on photonic integrated circuit (PIC) technology, which we'll use to construct an RF photonic spectrometer for a millimeter wave radiometer back-end. The high spectral resolving power of the AWG will enable unprecedented sampling and wideband operation for improved retrieval accuracy across the temperature and humidity sounding bands, allowing never achieved before ability to spectrally resolve the shape and magnitude of the sounding channel lines. The AWG is a dispersive integrated photonic device commonly employed to spectrally separate optical signals in telecommunications applications. Commercial devices are available with typical channel widths in the range of 50 to 200 GHz. In partnership with Sandia National Lab and JPL we propose a silicon based photonic AWG that allows channelization of a 40 GHz RF bandwidth into less than 1 GHz channels in a compact footprint. Sandia National Labs has recently demonstrated a silicon photonic arrayed waveguide grating that allowed channelization of an 11 GHz RF bandwidth into 1 GHz channels in a compact (1.1 cm2) footprint. The high index contrast of silicon waveguides allows for tight waveguide bends which enables this compact footprint. To achieve high cross-talk suppression, we will combine this technology with active tuning of the optical phase of each waveguide, using integrated thermal phase shifters. The resultant technology will provide capabilities to perform microwave spectrometry over unprecedented bandwidths in a small form-factor and extremely low power. PSI has developed high speed, electro-optic phase modulators as a core product, with extremely wide bandwidth operation from DC to 500 GHz, limited only by test capability. By using these modulators to "optically upconvert" millimeter wave signals, we have demonstrated passive, video-rate W-band imaging receivers using optical back-end processing for real-time image reconstruction. Our modulator provides the bridge between the RF front-end and the spectrometer back-end, converting RF signals in the range of 118 - 183 GHz to optical signals for processing and detection. The upconversion is efficient and low-noise, and allows powerful coherent processing to be performed with optical lenses and filters. We possess considerable experience implementing phase locking techniques for such coherent optical systems, which we'll leverage to provide phase locking and phase tuning of the arrayed waveguide grating and thus generate the desired filter functions for the spectrometer. JPL through its Micro Devices Laboratory will lend its expertise in testing the Photonic chip. JPL will also perform system integration and testing with a front-end low noise 183 GHz radiometer HAMSR as well as phase modulator. Testing will also involve upward looking atmospheric sounding using the wideband photonic spectrometer. JPL has a long and successful heritage in millimeter wave radiometry, and has several low noise millimeter wave radiometers in lab such as HAMSR, GeoSTAR, and MASC.

Accelerate mission development and reduce risk by developing critical components and subsystems for advanced instruments and observing systems

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