Use Cases

OCT based on a swept source laser

Challenge

Introduction with the Layperson's explanation:
Optical Coherence Tomography, or ‘OCT’, is a technique for obtaining sub-surface images of translucent or opaque materials at a resolution equivalent to a low-power microscope. It is effectively ‘optical ultrasound’, imaging reflections from within tissue to provide cross-sectional images.

The project:
MagentaSys, along with its partners Keysight and Exalos have built an OCT based on a swept source laser. This new technology enables to multiplies the scanning rate by ten compared with all the other previous OCT system. This is crucial for medical application such as Eye scanning.

Keysight has provided the acquisition board. An U5303A board with 2 acquisition channel having a sample rate at up to 1 GS/s.
Exalos is the provider of all the optical instrumentation like the swept source laser, the balanced-receiver and the sampled arm.

Major Issue:
Swept source lasers sampled with a common system are not giving a linear result in the frequency domain. This phenomena causes distortion to the processed FFT and avoid to see a proper final picture.
Another signal has to be provided to resample the OCT signal, in order to linéarise it and to improve the final result. The so-called k-clock reference is used for the data acquisition in two possible ways:

1. As an external clock for the ADC that captures the OCT signal at times of  equidistant frequency positions. 
2. Acquired in parallel to the OCT signal on a second ADC with both ADCs being clocked internally at a fixed and constant frequency.

The second approach is generally more robust in terms of data acquisition and is suitable to handle a greater variety of linear and nonlinear swept sources. But, it requires a fair amount of signal processing that can be accomplished in real time with a deterministic latency and reliable throughput using a DAQ card with an on-board FPGA.

Solution

The solution is described in depth on the documentation on the YellowSys website.

The resampling linéarisation process has been done by sampling in parallel the k-clock and OCT signals. It is then computed by performing an Hilbert Transform inside the FPGA. The U5303A board is very convenient for this kind of application as in owns indeed a powerful FPGA having a lot of processing units. 

The FPGA is also doing on the fly an FFT transform, and is performing a Cordic logarithm.

An API has been developed for a user to integrate easily this system, and an example GUI has been created to help the user to understand the different functionalities of it.

Products used

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