Wavenumber and k-clock
The swept-source laser wavelength is incremented or decremented within each A-scan. For example, the wavelength may change from 1010 nm to 1110 nm in 4096 steps. The step size is non-linear and increase (or decrease) with wavelength. A specific wavelength is identified by its so-called wavenumber, denoted k, and many lasers offer an output signal called k-clock or k-trigger. This clock is often created using a Mach-Zehnder interferometer (MZI) and is utilized by the digitizer to acquire one sample for each wavelength. Due to its non-linear nature, the k-clock frequency varies during the A-scan and can for example span frequencies from 400 to 600 MHz.
Direct clocking versus k-clock remapping
Some SS-OCT systems utilize the k-clock as an external clock to the digitizer, however, this approach has many disadvantages:
- Analog-to-digital converters (ADCs) in high-performance digitizers require high-quality clock sources with very low jitter. The k-clock does not fulfill these requirements, and therefore ADC analog performance is degraded.
- The k-clock can exhibit noise or spikes and is sometimes even turned off completely during parts of the scan.
- The sample-and-hold circuit inside the ADCs is designed for using a clock with a constant duty cycle. Varying duty cycles may lead to bad ADC output samples or even occasional data loss.
- Direct clocking is only possible with older-generation ADCs that utilize parallel data interfaces such as Low Voltage Differential Signaling (LVDS). Such ADCs only support limited sampling rates and therefore limit achievable A-scan rates.