High-performance integrated photonic spectrometers based on arrayed waveguide gratings in silica

Compact yet highly functional optical components are desired in modern astronomical instruments targeted at low system cost and reduced maintenance complexity. Integrated photonic spectrometers based on planar lightwave circuits are attractive as the planar miniature device can provide high spectral resolution but also great robustness and flexibility in the design of spectrograph systems. Arrayed waveguide gratings (AWGs) have the potential to be adapted and optimized to function as compact spectrometers in astronomical spectrographs. In this work high-resolution AWGs based on low-loss silica waveguides have been designed, fabricated and characterized. The measured spectral resolution exceeds 104 with Δλ = 150 pm at 1548 nm. The insertion loss (including two times fiber-chip coupling) is merely 2.07 dB, amounting to a peak throughput of 62%. Adiabatic fiber taper is developed to bring down the mode field diameter of a standard single mode fiber to match the mode size of the designed waveguide, resulting in almost lossless coupling from the fiber to the waveguide. The free-spectrum range is 48 nm and the side-band suppression is 22 dB. The AWG is also polarization-insensitive. Rotating the linearly polarized input light by 180° results in a slight shift of the central wavelength ~ 30 pm. The excellent overall performance makes this AWG an ideal candidate as the key building block for the development of an integrated astronomical spectrograph module.