Optical coherence tomography (OCT) generates cross-sectional and three dimensional images of tissues in situ
with micron scale resolution by measuring time delay of backscattered or back reflected light [1
]. The recent development of Fourier domain OCT techniques with spectrometer based system (spectral/Fourier domain OCT, or SD/FD-OCT) or frequency swept laser sources based system (swept source OCT or SS-OCT) enables imaging speeds 10 to 100 times faster than standard time domain OCT techniques [2
]. Compared to SD/FD-OCT, SS-OCT can have improved overall sensitivity if operated in shot-noise-limit because there are no spectrometer losses and the photodetectors used are more sensitive than cameras [8
]. SS-OCT also enables operation at long wavelength ranges without the need for expensive InGaAs cameras. Improved system dynamic range can also be achieved in SS-OCT because it uses dual-balanced detection and higher bit depth data acquisition (DAQ) systems than cameras. In addition, swept source/Fourier domain detection can provide very large number of axial samples, as determined by the speed of DAQ system.
Typical swept lasers consist of a broadband gain medium with a tunable optical bandpass filter in the cavity. The tunable filter is swept so that the transmission frequency varies in time and sufficient time is needed to allow lasing in the transmission bandwidth to build up from spontaneous emission inside the cavity. This limits the maximum tuning rate of the laser and also results in lower power, broader instantaneous linewidth or shorter instantaneous coherence length. Fourier domain mode-locking (FDML) is a new mode of operation for frequency swept lasers that overcomes these problems and is especially promising for high speed OCT imaging [17
]. An FDML laser uses a cavity with a long fiber delay line and a fiber Fabry-Perot tunable filter (FFP-TF) whose sweep rate is synchronized with the round-trip time of light inside the cavity. The long fiber delay line stores the entire frequency sweep inside the laser and the different frequencies in the sweep return to the FFP-TF at the time when the filter is tuned to transmit them. The laser generates a sequence of optical frequency sweeps at the cavity repetition rate. However, the coherence length of standard FDML lasers is limited by the bandwidth of the FFP-TF and a 5dB–10dB drop in sensitivity over a 3mm depth range is typically observed [19
]. For OCT imaging applications, it is desirable to have uniform imaging sensitivity over a large depth range.
To improve the sensitivity roll off or ranging depth in OCT imaging systems, light sources with discrete frequency steps and narrow instantaneous bandwidth have been demonstrated. Amano et al.
demonstrated 1550nm superstructure-grating distributed Bragg reflector (SSG DBR) laser with the bandwidth of a few MHz for SS-OCT imaging [23
]; Bajrqaszewski et al.
used an external frequency comb filter with bandwidth of 1.62GHz to generate discrete frequency steps for SD-OCT imaging at 840nm [24
]; Jung et al.
also used an external fiber Sagnac comb filter to generate light with very narrow bandwidths [25
]. These methods showed that the ranging depth of the OCT system can be increased by reducing the instantaneous bandwidth of the light source, suggesting the use of frequency comb techniques can improve the sensitivity over a longer ranging depth.
At the same time, most Fourier domain OCT systems require re-sampling or recalibrating the OCT interference fringe signals in order to provide data evenly sampled in frequency domain prior to fast Fourier transformation (FFT). This recalibration process is computationally expensive and limits the real-time operation of OCT. For SS-OCT, Eigenwillig et al.
have successfully demonstrated an approach to linearize frequency sweeps in FDML lasers by incorporating the second and third harmonics of the drive waveform to FFP-TF [21
]. However, this approach requires that the FFP-TF have a high frequency response and is also sensitive to thermal drift in the FFP-TF.
In this manuscript, we demonstrate frequency comb (FC) lasers, a new type of swept laser incorporating a fixed narrowband frequency comb fiber Fabry-Perot (FFP-FC) filter inside the cavity of conventional swept lasers and FDML lasers. FC swept lasers generate a sweep of discrete steps in frequency, rather than a continuous sweep. The extremely narrow bandwidth of the frequency steps generated by FC lasers improves the sensitivity roll off in OCT compared to conventional swept source and FDML lasers, enabling imaging over a longer depth range. The comb frequencies are equally separated in k-space, providing a clock signal which can be used to trigger the OCT interference fringe acquisition. This self-clocking method outperforms standard frequency calibration methods using reference Mach-Zehnder interferometer signals. The general design principles of FC lasers will also be discussed, which will be important for the design of future high speed, short cavity FC lasers.