Efficient ultra-broadband low-resolution astrophotonic spectrographs

Area of Science:

• Photonics and Optical Engineering
• Astronomy and Astrophysics
• Materials Science

Background:

• Compact broadband spectrographs are essential for astronomical observations and sensing.
• Stringent requirements include high efficiency, broad operational bandwidth, and polarization insensitivity.

Purpose of the Study:

• To design, fabricate, and characterize broadband (1200-1650 nm) arrayed waveguide grating (AWG) spectrographs.
• To compare performance using two low-loss waveguide platforms: silicon nitride (Si3N4) and doped silicon dioxide (doped-SiO2).

Main Methods:

• Fabrication of AWG spectrographs on Si3N4 (rectangular waveguides) and doped-SiO2 (square waveguides).
• Experimental characterization of spectral performance, efficiency, and polarization dependence.
• Analysis of loss components, particularly fiber-to-chip coupling.

Main Results:

• Achieved resolving power (λ/Δλ) of ~200 and a free spectral range of 200-350 nm.
• Doped-SiO2 AWGs demonstrated peak efficiency of ~79% with negligible polarization-dependent shift.
• Si3N4 AWGs showed peak efficiency of ~50% (TE mode), with fiber-to-chip coupling as the primary loss factor.

Conclusions:

• Broadband AWGs on both platforms are suitable for compact astronomical spectrographs.
• These AWGs enable integration into multi-object spectrographs and astrophotonic devices.
• The doped-SiO2 platform shows superior efficiency and polarization performance for near-infrared applications.