In our SS-OCT system, the free spectral range (FSR) of the MZI is set at 30 GHz, corresponding to an imaging depth of 2.5 mm (free space).
shows the MZI interference signal during forward wavelength scanning with a 40 kHz sinusoidal driving waveform applied to the FFP-TF. The total number of fringe cycles of the forward scan signal is 409 and the corresponding number of the sampled data points per A-scan is therefore 818. It is noted that the frequency of the MZI signal varies a lot, e.g., the frequency of the slow fringe region (~25 MHz) with a zoomed-in version shown in the upper inset of is less than half of the fast fringe region (~55 MHz) with a zoomed-in version shown in the lower inset. The external clock signal is shown in which was to trigger the high-speed digitizer in a point by point fashion. The trigger frequency in the middle portion of the clock signal is about 110 MHz, while the clock frequency slows down to about 50 MHz at the edge of each A-scan. The frequency of the “dummy” clock should be high enough to fill in enough clock cycles in order to keep the digitizer working continuously, and in our case it was chosen to be 28 MHz or higher. It should be noted that the duty ratio of the external clock over the entire A-scan is close to 50% as shown in the insets in , which is required for maintaining a good FSDR for a high-speed digitizer as mentioned previously.
Fig. 3 (A) MZI interference signal. The upper and lower insets show the MZI signal at the beginning (or end) and the center of an A-scan, respectively, indicating that the MZI signal frequency varies during wavelength scanning. (B) External clock signal during (more ...)
The measured axial resolution is ~9.3 µm in air as shown in
for a FDML laser of a 130-nm FWHM at a center wavelength 1310 nm (measured by an optical spectrum analyzer), and the axial resolution remains almost the same throughout the entire imaging depth of 2.5 mm (). The measured detection sensitivity was greater than 120dB at the imaging depth of 1.0 mm.
(A) Point-spread function of an FDML-based SS-OCT system equipped with the real-time linear K-space sampling method. (B) Point-spread function versus imaging depth revealing no axial resolution degradation throughout the imaging depth of 2.5 mm.
One attractive advantage of the real-time linear K-space sampling method is the reduced requirement for the speed of data digitization, processing and storage by at least a factor of 2.5. Only 2 data points need to be digitized per MZI fringe cycle with the new method while at least 5 data points per cycle need to digitized when using the conventional numerical calibration method. With our current hardware implementation, only about 800 points per A-scan need to be digitized as opposed to 2048 points per A-scan required by the conventional numerical calibration algorithm. Consequently, the new calibration method enables a digitizer of a given speed to handle a higher A-scan rate. Or equivalently, the new method can also handle a broader wavelength sweeping range and thus a light source of a broader spectrum bandwidth can be used in an SS-OCT system to achieve a better axial resolution. Furthermore, a higher A-scan rate can be supported with data acquisition, transfer, processing, display and storage performed in real time. Our current hardware can support an A-scan rate up to ~100,000 A-scans/sec with real-time linear K-space sampling without any interruption of data acquisition, transfer, processing or storage. In comparison, the conventional numerical calibration algorithm could only support up to ~10,000 A-scan/sec with spline interpolation or up to ~50,000 A-scans/sec with nearest neighbor interpolation. More importantly, our hardware-based real-time linear K-space sampling method provides a universal solution for an SS-OCT system with any A-scan rates provided that the clock signal is within the working frequency range of the clock-generation circuit, e.g. our prototype can handle an A-scan rate from 20 to 100 kHz without any adjustment. It is also noted that, unlike the numerical calibration algorithm which normally obtains the MZI calibration signal only once at the beginning (or the end) of OCT imaging and is thus sensitive to any instability or drifting of the laser spectrum and the FFP-TF scanning, the new calibration algorithm permits OCT interferometric fringes to be digitized against the corresponding real-time MZI calibration signal for all A-scans at all time.
summarizes the major improvements of the linear K-space sampling method over numerical calibration.
Comparison between real-time linear K-space sampling and numerical calibration