Optical frequency domain reflectometry based on real-time Fourier transformation

Area of Science:

• Photonics and Optical Engineering
• Metrology and Measurement Science
• Biomedical Imaging Technology

Background:

• Optical Frequency Domain Reflectometry (OFDR) is crucial for high-resolution depth profiling.
• Existing OFDR systems face limitations in acquisition speed, hindering real-time applications.
• Advanced signal processing and optical component integration are needed to overcome these limitations.

Purpose of the Study:

• To propose and demonstrate an ultrahigh-speed OFDR system utilizing optical frequency-to-time conversion.
• To develop a real-time Fourier transformation (OFDR-RTFT) approach for unprecedented axial line acquisition rates.
• To analyze system performance, including resolution, depth range, and sensitivity, and address phase distortion issues.

Main Methods:

• Implementation of optical frequency-to-time conversion via pulse time stretching using a linearly chirped fiber Bragg grating (LCFG).
• Development of an OFDR system based on real-time Fourier transformation (OFDR-RTFT).
• Mathematical analysis of OFDR-RTFT design equations and compensation of nonlinear phase variations using Hilbert transformation.

Main Results:

• Achieved unprecedented axial line acquisition rates up to the input pulse repetition rate (20 MHz).
• Demonstrated nearly transform-limited axial resolutions of approximately 92.8 µm over an 18 mm depth range.
• Obtained improved sensitivities up to -61 dB without balanced detection, using single-reflection depth profiles measured in ~50 ns time windows.

Conclusions:

• The LCFG-based OFDR-RTFT system offers ultrahigh acquisition speeds in the MHz range.
• The system shows potential for enhanced axial resolution, depth range, and sensitivity.
• This approach is highly attractive for optical coherence tomography (OCT) and other imaging applications requiring rapid, high-resolution depth profiling.