Comb Light Source/Imaging spectrometer for advanced Spectral Domain Optical Coherence Tomography

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There is a growing interest in the development of functional imaging with Optical Coherence Tomography (OCT). Many of these approaches to functional imaging rely on the exquisite phase sensitivity of the OCT interferometer. These include well established techniques like Doppler OCT for measuring blood flow as well as emerging applications like OCT elastography for measuring mechanical properties, and Magnetomotive OCT and Photothermal OCT for molecular imaging. Our own interest lies in the area of vibrometry. Swept laser systems are advantageous because they enable the use of Mach-Zehnder type interferometers with balanced detectors. This provides for cancelation of common mode noise, the DC component, and autocorrelation artifacts. The most advanced commercially available swept lasers also have a very long coherence length (>1 m), hence signal roll-off as a function of depth is negligible. Taken together these qualities result in images that have fewer artifacts and increased signal-to-noise with concomitant high phase stability and vibrational sensitivity. Most commercially available swept lasers suffer from instabilities in the laser sweep such that every sweep needs to be calibrated in order to maintain high phase-stability. Ourselves and others have developed methods to accomplish this, but at the expense of substantial hardware complexity and sophisticated algorithms to ensure that wavenumber as a function of time, k(t), is known precisely for each sweep. In our experience, small changes in the system, environment, and the normal aging of the laser, forces frequent “tweaking” of the system to maintain the highest phase-stability. One company has developed a swept laser, where k(t) is linear and highly reproducible. We have shown that this source provides high phase- stability without the need of the complex hardware/software algorithms. The drawback to this laser system is its limited spectral bandwidth (typically 90-95 nm) and its limited availability. On the other hand, spectrometer based systems offer high-phase stability, but typically with relatively short coherence lengths (1-3 mm) and without the advantages of balanced detection. However, they can provide wide spectral bandwidth with concomitant higher resolution while maintaining high phase-stability. Here we propose to develop a novel comb light source spectrometer based system that has all of the advantages of a swept laser system, but with the inherent phase-stability of a spectrometer based system. Aim 1: Develop a comb laser source centered at ~1300 nm, with a bandwidth of 125-150 nm, and 1024 discrete lines over ~200 nm. The coherence length of each line will be at least 8 cm, providing a 4 cm 3 dB roll-off. Aim 2: Develop an imaging spectrometer with a magnification of 0.5 that disperses the light linearly (in k) over the 200 nm bandwidth using a custom compound prism, having a line rate of 147 kHz. Aim 3: Integrate the light source and spectrometer into a balanced spectral domain OCT system, validate performance, and develop FPGA code for pipelined real-time computation of the differential signal.