High performance swept sources for optical coherence tomography

Abstract

Optical coherence tomography (OCT) proposed by D. Huang in 1991 finds applications in various scenes and ignites the passion of numerous researchers to work on OCT. Compared with time domain OCT (TD-OCT) which is hampered by the slow mechanical movement of the reference mirror and the spectral domain OCT (SD-OCT) which is limited by resolution and the low acquisition rate of the spectrometer, swept source OCT (SS-OCT) could provide much higher axial scan (A-scan) rate and is the most promising OCT technique in development to realize the volumetric imaging at video rate. The core technology of SS-OCT system is the swept laser source, sweep rate, sweep range and instantaneous linewidth of which directly determine the SS-OCT imaging performance. In this thesis, I focus on two high performance swept sources, which are discrete Fourier domain mode locked (FDML) laser and swept laser based on time stretching of the pulses from a mode locked fiber laser in dispersive modules, and their applications in SS-OCT imaging systems. In conventional FDML laser cavities, the fiber dispersion and nonlinearity of the semiconductor optical amplifier (SOA) jointly induce high frequency fluctuations and limit the coherence length to a few millimeters only. The discretization of the swept signal by an intracavity Fabry-Pérot (F-P) comb filter is proven as an effective method to improve the coherence length. However, it is challenging to reduce the linewidth of bulky fused silica F-P comb filter to sub-GHz level for further improvement of the performance of such frequency comb swept laser. Compared with F-P comb filter, whispering gallery mode (WGM) based high quality (Q) microring filters could provide much higher finesse and narrower linewidth. In this thesis, we demonstrate a highly coherent discrete FDML laser by adopting a microring comb filter with a Q factor of >106, which greatly narrows the instantaneous linewidth of the swept signal and extends the imaging range of SS-OCT to 53 mm and 100 mm with 6 dB and 10 dB sensitivity roll off, respectively. However, using comb filters to realize discrete FDML lasers lacks the flexibility to vary the free spectral range (FSR) and linewidth. To improve the flexibility of signal manipulation in discrete FDML lasers, we propose and demonstrate a novel discrete FDML laser by time domain modulation using an optical modulator, which can be easily reconfigured with variable driving pulse trains. Discrete swept signals with pulses uniformly distributed in the time domain and the frequency domain are respectively demonstrated with different parameters and features when they are applied in the SS-OCT system. In the study of discrete FDML laser with time domain modulation, we find that the flexibility is not limited to varying the swept signal in a single sweep. We propose and demonstrate another configuration of FDML laser with grouped mode hopping modulation to discretize the swept signals in three adjacent sweep periods by different modulations to effectively increase the density of the spectral comb lines. The demonstration shows that one can either use the swept signals with a high sweep rate or combine the adjacent three sweeps to obtain a spectrally dense swept signal. Time stretching of broadband ultrashort pulses generated by mode locked lasers could generate swept signals with tens of megahertz sweep rate because of the inertia free configuration. In a time stretching setup, a highly dispersive element is adopted to sequentially distribute the different spectral components of a broadband ultrashort pulse to different temporal positions to obtain a swept signal. The performance of such time stretched swept signals depends dominantly on the generation of the ultrashort pulse train with a high repetition rate and a highly coherent broadband spectrum. In this thesis, we demonstrate an ultrahigh speed time stretched swept source with a sweep range of more than 100 nm and a 100 MHz sweep rate. To the best of our knowledge, this is the first demonstration of a swept laser source for OCT with simultaneously a 100 MHz sweep rate and a >100 nm sweep range. By using the SS-OCT based on the 100 MHz swept source, we imaged an encoding disk and a hard disk rotating at ~17,000 rpm. Because of the ultralong coherence length of the swept signal, the imaging range of the SS-OCT depends only on the total dispersion used in the time stretching and the bandwidth of the detection subsystem. By incorporating an optical modulator and utilizing appropriate dispersive modules, a reconfigurable swept source is implemented. We demonstrate three different repetition rates of 100 MHz, 25 MHz and 2.5 MHz. Chirped fiber Bragg grating (CFBG) and dispersion compensation fiber (DCF) are respectively used as the dispersion elements in experiments. The sensitivity roll off length of the SS-OCT with a 2.5 MHz sweep rate is extended to 111 mm by using a DCF with a total nominal dispersion of ‒3,306 ps/nm. This is also the first time that the imaging range of an MHz SS-OCT is extended to >100 mm. We then demonstrate a 400 MHz swept source by using the buffering technique and then applied in OCT imaging. The ultrahigh speed OCT will enable the application of 3-D imaging of ultrafast motion or reactions with ultrashort temporal duration. The enhancement in the performance of SS-OCT will extend the use of SS-OCT into industrial applications besides the applications such as endoscopic OCT imaging of large scale lumen.