Swept laser source non-linearity and k-clock instability compensation
Swept source OCT lasers are non-linear. This non-linearity must be compensated to improve both resolution and sensitivity.
Non-linearity correction can be done in two ways:
Direct ADC clocking: the ADC capturing the OCT interference is clocked directly with a clock signal that contains the information of the laser source non-linearity.
Digital Resampling: The ADC capturing the OCT interference is clocked at constant sampling period. The digitized OCT interference is digitally non-linearly resampled in a way to compensate the non-linearities computed on the K-clock and acquired with a second ADC channel. K-clock can be easily generated with a Mach-Zehnder interferometer (MZI) connected to the same laser source.
Current solution is based on the resampling approach. If it requires an additional channel, on the other hand, it provides more flexibility, and simpler implementation of the OCT system. For instance, it is not required to generate a “dummy” K-clock out of the source sweep. Besides, the digital resampling approach offers additional capabilities, such as:
-Compensation of the optical path difference between K-clock and OCT-signal
-Programmable maximum analysis depth.
U5303A High-Speed Data Acquisition Card for SS-OCT: Product features and specifications
Image clarity and high-resolution
Acquisition card signal integrity characteristics are critical to guarantee the best image quality. This unique solution offers:
- A best in class signal noise ratio for a 12 bits resolution digitizer (57-58 dB typical)
- Improved spurious free dynamic range (SFDR) thanks to x4 (SS2) or x2 (SS1) oversampling interpolation. For instance, with -SS2 option, artefacts are reduced by 14 dB due to interpolation.
- Better phase stability thanks to accurate synchronization for A-scan triggers
- SS1 option: ENOB = 9.3 (typical) at 100 MHz
- SS2 option: ENOB = 9.1 (typical) at 410 MHz
Flexible SS-OCT analysis depth
To allow deep analysis, the system needs a high sampling rate. The new SS2 option doubles the sampling rate from 500 MS/s to 1GS/s, allowing:
-either to double the maximum analysis depth with same A-scan rate.
-or to double the image speed (double A-scan rate), keeping the same analysis depth.
In addition, the maximum analysis depth is programmable by selecting the effective sampling rate of the ADC card.
Key advantages of this approach are:
– Flexibility from changing the A-scan rate.
– Independency from K-clock frequency
– Programmable through variable sampling rate (internal clock from 250 MHz up to 1 GHz).
Fast scan measurement and image restitution
3-D images are very long to acquire. The sustained scan rate is a crucial parameter to minimize unwanted samples movement in ophthalmic applications or to reduce the measurement time when the blood flow is stopped in cardio-vascular applications.
This solution enables up to 200 kHz A-scan rate (i.e. 200’000 A-scan per second), allowing fast acquisition of B-scans. The high scanning speed and fast image restitution are continuously guaranteed and not only for a limited period of time, thanks to:
– Continuous mode with simultaneous OCT signal acquisition and image data readout (see Signal acquisition sequence)
– Continuous acquisitions of B-scans without missing any trigger/data
– Real time OCT processing on FPGA
– U5303A sustained data throughput > 2 GB/s.
U5303A PCIe 12-bit High-Speed Digitizer
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