Fiber-based arrayed waveguide grating for spectral sensing

Microwave photonics can be used to conduct instantaneous spectral imaging of incoming microwave signals. Here, we demonstrate the use of a fiber-based arrayed waveguide grating (FAWG) to detect the microwave spectrum over a 25 GHz bandwidth with sub-2-GHz resolution. The received microwave signal, amplified by the device radio-frequency (RF) front end, is upconverted to the optical domain using an electro-optic modulator and sent through an array of optical fibers of different lengths. Like an arrayed waveguide grating (AWG), the different fiber lengths produce observable frequency dispersion. Fine frequency resolution is enabled by large path length differences among the optical fibers. Since fibers are susceptible to environmental fluctuations such as mechanical vibrations or temperature changes, active phase control is utilized to compensate for phase variations in real time. After optical filtering, the spectrum is captured by a linear IR camera that is placed at the Fourier-transform plane of the free-space optical-detection system. The FAWG has possible applications for microwave sensing as well as optical communications and can be adapted for different frequency ranges and resolutions. Here, it is discussed in the context of atmospheric sounding to measure remotely the temperature and water-vapor content.