We present ultra high speed optical coherence tomography (OCT) with multi-megahertz line rates and investigate the achievable image quality. The presented system is a swept source OCT setup using a Fourier domain mode locked (FDML) laser. Three different FDML-based swept laser sources with sweep rates of 1, 2.6 and 5.2MHz are compared. Imaging with 4 spots in parallel quadruples the effective speed, enabling depth scan rates as high as 20.8 million lines per second. Each setup provides at least 98dB sensitivity and approximately 10microm resolution in tissue. High quality 2D and 3D imaging of biological samples is demonstrated at full scan speed. A discussion about how to best specify OCT imaging speed is included. The connection between voxel rate, line rate, frame rate and hardware performance of the OCT setup such as sample rate, analog bandwidth, coherence length, acquisition dead-time and scanner duty cycle is provided. Finally, suitable averaging protocols to further increase image quality are discussed.

We describe a novel buffering technique for increasing the A-scan rate of swept source optical coherence tomography (SSOCT) systems employing low duty cycle swept source lasers. This technique differs from previously reported buffering techniques in that it employs a fast optical switch, capable of switching in 60 ns, instead of a fused fiber coupler at the end of the buffering stage, and is therefore appreciably more power efficient. The use of the switch also eliminates patient exposure to light that is not used for imaging that occurs at the end of the laser sweep, thereby increasing the system sensitivity. We also describe how careful management of polarization can remove undesirable artifacts due to polarization mode dispersion. In addition, we demonstrate how numerical compensation techniques can be used to modify the signal from a Mach-Zehnder interferometer (MZI) clock obtained from the original sweep to recalibrate the buffered sweep, thereby reducing the complexity of systems employing lasers with integrated MZI clocks. Combining these methods, we constructed an SSOCT system employing an Axsun technologies laser with a sweep rate of 100kHz and 6dB imaging range of 5.5mm. The sweep rate was doubled with sweep buffering to 200 kHz, and the imaging depth was extended to 9 mm using coherence revival. We demonstrated the feasibility of this system by acquiring images of the anterior segments and retinas of healthy human volunteers.

We report on the effect of chromatic dispersion on coherence length and noise of Fourier Domain Mode Locked (FDML) lasers. An FDML laser with a sweep range of 100 nm around 1550 nm has been investigated. Cavity configurations with and without dispersion compensation have been analyzed using different widths of the intra-cavity optical band-pass filter. The measurements are compared to non-FDML wavelength swept laser sources. Based on these observations, a simple model is developed providing a connection between timing, photon cavity lifetime and characteristic time constant of the filter. In an optimized configuration, an instantaneous laser linewidth of 20 pm is observed, corresponding to a 10x narrowing compared to the intra-cavity optical bandpass filter. A relative intensity noise of -133 dBc/Hz or 0.2% at 100 MHz detection bandwidth during sweep operation is observed. For optimum operation, the filter drive frequency has to be set within 2 ppm or 120 mHz at 51 kHz.

We report on a Fourier Domain Mode Locked (FDML) wavelength swept laser source with a highly linear time-frequency sweep characteristic and demonstrate OCT imaging without k-space resampling prior to Fourier transformation. A detailed theoretical framework is provided and different strategies how to determine the optimum drive waveform of the piezo-electrically actuated optical bandpass-filter in the FDML laser are discussed. An FDML laser with a relative optical frequency deviation ??nu/nu smaller than 8 x10(-5) over a 100 nm spectral bandwidth at 1300 nm is presented, enabling high resolution OCT over long ranging depths. Without numerical time-to-frequency resampling and without spectral apodization a sensitivity roll off of 4 dB over 2 mm, 12.5 dB over 4 mm and 26.5 dB over 1 cm at 3.5 mus sweep duration and 106.6 dB maximum sensitivity at 9.2 mW average power is achieved. The axial resolution in air degrades from 14 to 21 mum over 4 mm imaging depth. The compensation of unbalanced dispersion in the OCT sample arm by an adapted tuning characteristic of the source is demonstrated. Good stability of the system without feedback-control loops is observed over hours.

We present a 1300 nm Fourier domain mode locked (FDML) laser for optical coherence tomography (OCT) that combines both, a high 1.6 MHz wavelength sweep rate and an ultra-long instantaneous coherence length for rapid volumetric deep field imaging. By reducing the dispersion in the fiber delay line of the FDML laser, the instantaneous coherence length and hence the available imaging range is approximately quadrupled compared to previously published MHz-FDML setups, the imaging speed is increased by a factor of 16 compared to previous extended coherence length results. We present a detailed characterization of the FDML laser performance. We demonstrate for the first time MHz-OCT imaging of the anterior segment of the human eye. The OCT system provides enough imaging depth to cover the whole range from the top surface of the cornea down to the crystalline lens.