Frequency-diverse structured light for optical gradient sensing using heterodyne interferometry

Area of Science:

• Photonics
• Optical Sensing
• Acoustic Metrology

Background:

• Current optical sensing methods often require reference arms or embedded elements.
• Detecting subtle refractive index changes in free space presents significant challenges.

Purpose of the Study:

• To develop a novel free-space optical sensing architecture for direct mapping of refractive index gradients.
• To establish a compact, reconfigurable platform for remote, non-contact acoustic sensing.

Main Methods:

• Utilized a frequency-diverse beamlet array for spatial sampling of the medium.
• Employed heterodyne modulation to map refractive index gradients into sidebands.
• Developed an analytic framework and experimentally validated the system using an eight-beamlet array at 532 nm.

Main Results:

• Achieved > 80 dB beat-to-sidelobe ratios and near-Pascal acoustic sensitivity.
• Demonstrated parallel sensing channels on a single detector without a reference arm.
• Established a general framework for non-contact sensing of refractive index gradients.

Conclusions:

• The developed architecture enables efficient, non-contact acoustic sensing with high sensitivity.
• The platform is reconfigurable and supports extension to higher acoustic bandwidths.
• Potential applications extend beyond acoustics to turbulence and thermal sensing.