Uniform Frequency Sample Clocking for SS-OCT Phase Sensitivity
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Solution Overview
Problem
High-speed Swept-Source Optical Coherence Tomography (SS-OCT) systems face challenges due to nonlinear wavenumber vs. time characteristics of tunable laser sources, requiring software remapping that introduces phase sensitivity errors and computational inefficiencies, and necessitate accurate recalibration for reliable imaging, which is complicated by stringent requirements on digitizer clocking and potential disruptions during laser sweeps.
Innovation Solution
The implementation of a Uniform Frequency Sample Clocking method that provides external clocking for swept laser sources, using auxiliary wavemeters and analog or digital processing to generate a uniform-frequency sample clock signal, allowing direct Fourier transformation of OCT data in the wavenumber domain, thereby bypassing the need for time-intensive remapping and improving image quality and display rates.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If software remapping is used to correct nonlinear wavenumber vs. time characteristics, then uniform sampling in wavenumber domain is achieved, but phase sensitivity errors and computational inefficiency increase
Solution Approach 1:
The patent applies preliminary action by pre-calculating and storing the nonlinear wavenumber vs. time calibration data in a lookup table during system initialization. This allows the system to retrieve pre-computed correction factors during operation, eliminating the need for real-time software remapping and its associated phase sensitivity errors and computational delays.
2Productivity
If high-speed A/D converter cards are used for fast frequency sweep speeds, then imaging speed increases, but timing jitter of the clocking signal degrades image quality
Solution Approach 1:
The patent introduces an intermediary clock conditioning circuit between the A/D converter card and the laser sweep trigger. This circuit includes buffer amplifiers, voltage regulators, and timing synchronization elements that condition the clock signal to meet the stringent requirements of high-speed digitizers, reducing timing jitter while maintaining fast imaging speeds.
3Measurement precision
If external clocking is implemented for swept laser sources, then uniform sampling in wavenumber domain is achieved, but stringent requirements on clock continuity and amplitude make implementation difficult
Solution Approach 1:
The system implements self-service by using the laser's own sweep trigger signal as the basis for generating the external clock. The clock conditioning circuit automatically adjusts the trigger signal's amplitude, duty cycle, and timing characteristics to meet digitizer requirements, eliminating the need for manual calibration and making the system robust against laser sweep variations.
4Measurement precision
If calibration is performed between laser sweep and digitizer sampling frequency, then accurate wavenumber sampling is achieved, but disruptions in wavemeter signal during laser idle periods disable the digitizer
Solution Approach 1:
The patent applies beforehand cushioning by implementing a hold circuit that captures and maintains the last valid clock signal parameters during laser idle periods. This ensures continuous clocking to the digitizer even when the wavemeter signal is disrupted, preventing digitizer disablement while maintaining calibration accuracy during active sweeps.
Data Source
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Figure 4A~4C
AI summary
A method and a system for Uniform Frequency Sample Clocking to directly sample the OCT signal with a temporally-non-linear sampling clock derived from a k-space wavemeter on the external sample clock input port of a digitizer.