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Fourier domain mode-locked frequency-swept laser speeds OCT

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Kevin Robinson

Optical coherence tomography (OCT) may one day be faster and more accurate, thanks to a new type of laser mode-locking developed by researchers at MIT in Cambridge, Mass. The technique, called Fourier domain mode-locking, yields a frequency-swept laser that can achieve a repetition rate of more than 200 kHz, a sweep range of 145 nm and an instantaneous linewidth of 0.07 nm.

OCT is commonly used to diagnose glaucoma and diabetic macular edema, and clinical studies are testing its use in cardiology and endoscopy. The new laser may help advance the technology in these areas, particularly because it allows quick collection of three-dimensional data rather than single cross-sectional images. It also could benefit metrology and other applications for which swept frequency generation is important.

Using Fourier domain mode-locking, researchers created a high-speed, frequency-swept laser, useful for optical coherence tomography (OCT) applications such as in vivo imaging of a human finger, as shown here.

“The Fourier domain mode-locking technology provides a unique combination of spectral purity, high sweep rate and wide sweep range,” explained Robert Huber of James G. Fujimoto’s group in MIT’s department of electrical engineering and computer science and Research Laboratory of Electronics.

To create the frequency sweep, the researchers placed a tunable filter in the laser cavity. Activated by fast electronics, the filter sweeps through the desired frequency band once per round trip (or some multiple thereof) of light in the cavity. Eventually, as the light makes multiple passes in the cavity, the frequency sweep becomes synchronized so that a particular wavelength returns to the filter at the same time that the filter will allow it to pass. The result is an output with a very narrow instantaneous linewidth and an extremely fast sweep.

Fourier domain mode-locking increases the speed of OCT, allowing three-dimensional images such as this reconstruction of an artery and acquiring 280 frames in 3.5 s. Courtesy of Lightlab Imaging Inc.

To create a laser that works well for OCT, Huber and his colleagues used a fiber ring laser configuration. They tested several combinations of fibers and components to build the laser sources, and also tested several fiber lengths. For the gain medium, they used a semiconductor optical amplifier from InPhenix, and they employed a Micron Optics Fabry-Perot filter for the tunable filter. The optical fiber was Corning SMF28e.

For OCT, Huber said, they typically need a sweep rate of between 10 and 400 kHz. The source could achieve a sweep rate of 290 kHz, which he said was a record imaging speed for OCT.

The researchers used the mode-locking method in an experimental setup. To demonstrate the speed of the method, they collected a 3-D data set for a 3.5 x 3.5 x 1-mm image of a human finger in less than a half second.

Huber said the group plans to improve the acquisition techniques and hardware to “provide a highly reliable, robust imaging module for future biomedical and clinical tests.

Optics Express, April 17, 2006, pp. 3225-3237.

Jun 2006
The science of measurement, particularly of lengths and angles.

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