Modal noise in an integrated photonic lantern fed diffraction-limited spectrograph.

Area of Science:

• Astrophysics
• Optical Engineering
• Photonics

Background:

• Developing streamlined astrophotonic instruments is crucial for efficient astronomical observations.
• Integrated photonic devices offer potential for compact and high-performance spectrographs.

Purpose of the Study:

• To demonstrate an all-photonic device for multimode to single mode conversion and spectral dispersion.
• To test the performance and identify potential issues of a monolithic photonic spectrograph on an 8-m class telescope.

Main Methods:

• The study utilized a monolithic photonic spectrograph combining an integrated photonic lantern and an arrayed waveguide grating.
• On-sky testing was performed on an 8-m class telescope.
• Extensive modeling of photonic components was conducted to analyze observed phenomena.

Main Results:

• An all-photonic device successfully achieved multimode to single mode conversion and spectral dispersion with efficient coupling.
• A previously unreported wavelength-dependent loss mechanism was identified as a source of noise.
• This loss was attributed to the mismatch between the feed fiber's near-field pattern and the photonic lantern's modal acceptance.

Conclusions:

• The discovered wavelength-dependent loss significantly complicated spectral extraction and calibration.
• Modeling confirmed the loss mechanism and its impact on spectral output.
• Direct injection into the integrated photonic lantern is proposed as a mitigation strategy for the identified noise.