Enabling High Speed Swept-Source OCT with Advanced Data Acquisition

Swept source OCT (SS-OCT) offers major technical advantages over spectral domain OCT (SD-OCT) because of its high speed swept laser source and single photodetector design, which deliver faster scan rates, deeper tissue penetration and far less sensitivity roll off with depth.
 
Its longer operating wavelength (~1050 nm) improves imaging through scattering tissues and enables superior visualization of the choroid and deeper ocular structures. SS-OCT also provides higher signal-to-noise performance and more stable phase information, making it especially powerful for high-density volumetric imaging and OCT angiography.
 
Historically, swept lasers were the most expensive part of SS-OCT systems. However, new semiconductor based tunable lasers (e.g., MEMS VCSELs) are being designed for low cost, high volume production, making SS-OCT instruments more competitive and affordable
 
SS-OCT Digitizer Requirements

Overall system performance is tightly constrained by the quality and capability of the digitizer. Developers are not simply looking for fast data acquisition; SS-OCT developers need digitizers that combine multi-GS/s sampling, low jitter, high analog bandwidth and high ENOB performance for the accurate capture of GHz interferometric fringes. They also require deterministic timing, real-time k space linearization and high throughput streaming to support real-time imaging pipelines. System performance is ultimately limited by the digitizer’s ability to acquire clean, precisely timed data at extremely high speeds.
 
Key developer requirements for SS-OCT digitizers typically include:

•  1 to 5 GSPS sampling rates to accurately capture rapidly swept interferometric signals. Imaging depth is proportional to digitizer sampling rate and laser coherence length. Increasing swept-source laser sweep rates, coherence length and k-clock frequencies are driving the need for higher speed digitizer technology.
•  Wide analog bandwidth (1 to 2 GHz) to support high-frequency fringe signals without distortion and k-clock frequencies in the range of 1 to 2 GHz.
• High dynamic range and low noise to preserve weak reflections from deeper tissue layers. 12-bit vertical resolution at up to 5 GSPS sampling rate.
• Real-time data handling, including peer-to-peer streaming and FPGA pre-processing, to avoid bottlenecks at high A-scan rates. Real-time streaming to CPU or GPU at gigabytes per second rates.
•  Low-latency processing, enabling immediate image reconstruction or feedback.

Teledyne’s ADQ3-series digitizers meet and exceed these requirements.
 
ADQ3 Digitizer Series for SS-OCT

Up to 5 GSPS capability ensures that even the fastest swept lasers can be sampled with high fidelity, capturing high-frequency fringe frequencies and preserving axial resolution. This high sampling rate, combined with excellent analog front-end performance up to 2.5 GHz, allows developers to fully exploit the bandwidth of modern SS-OCT light sources without compromise.
 
An essential part of the digitizer solution is dedicated application-specific firmware. FWOCT is firmware developed by Teledyne SP Devices to map the sampled k-clock to the OCT signal in SS-OCT imaging systems and perform all the other signal processing steps to produce OCT images. The k-clock and OCT signal are connected to a dual-channel digitizer. The k-clock signal is sampled by the digitizer and then processed to find the desired OCT sampling points. For the selected points, the corresponding value of the OCT signal input is estimated with high precision. This has many advantages compared to direct-clocking approaches – see Digitizer for swept-source OCT (SS-OCT) – Teledyne SP Devices for more details.

Integrated firmware moves key processing steps—such as resampling (k-linearization), digital filtering and potentially FFT preparation—onto the digitizer itself, and this has several advantages:

• Reduced host CPU/GPU load, enabling simpler system architecture.
•  Lower data transfer bandwidth requirements, since data can be pre-processed before streaming.
• Deterministic, real-time performance, essential for high speed imaging systems.
•  Improved phase stability due to tightly controlled, hardware-level processing.

FWOCT Highlights

• Flexible k-clock support (4 to 2000 MHz) for use with a wide range of lasers
• K-clock interpolation to support MZI-interferometers and low k-clock frequencies (4 to 2000 MHz k-clock supported)
• Programmable modes to select sampled k-clock points mapped to the OCT signal. For example, rising edge, rising and falling edge or interpolated configuration with multiple points per k-clock period
• High maximum OCT signal bandwidth (0 to 2000 MHz)
• Signal noise reduction by user-configurable FIR filters
• Timing adjustments between the k-clock and OCT signal paths
• FFT with flexible output formats – complex, squared magnitude, logarithmic.   
  •  FFT size scales with A-scan rate up to 32k bins
  • Lower latency than FFT implemented in GPU
• Background removal
• Dispersion compensation.

In contrast, many competing solutions rely heavily on software-based processing after acquisition, which introduces latency, increases system complexity and can limit achievable imaging speeds.
 
Taken together, the combination of up to 5 GSPS capable hardware and dedicated FWOCT firmware provides a tightly integrated acquisition and processing platform. For these reasons, the ADQ3-series stand out as a leading choice for SS-OCT developers seeking to push the limits of speed, sensitivity and real-time imaging capability.
 
ADQ32, ADQ33 and ADQ35 are available in either PCIe format or with a USB 3.2 interface (see ADQ3-USB – Teledyne SP Devices) while ADQ36 is available in PXIe.