Fourier domain mode-locked frequency-swept laser speeds OCT
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
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.
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