SWEPT-WAVELENGTH LASERS: Akinetic all-semiconductor swept lasers boost OCT image quality

Optical coherence tomography (OCT) image quality continues to improve thanks to advances in detection systems and improved light sources such as wavelength-tunable vertical-cavity surface-emitting lasers (VCSELs). But a different light source that eliminates the mechanical tuning of VCSEL and external-cavity lasers is rapidly gaining momentum, as it could offer superior OCT image quality and potentially lower-cost manufacture: akinetic all-semiconductor swept laser sources from Insight (Lafayette, CO).¹

Nearly all swept-wavelength light sources use mechanical means to move optical filter elements or microelectromechanical systems (MEMS) mirrors to tune the output wavelength; unfortunately, these mechanical elements can exhibit hysteresis, cause cavity instabilities, or introduce extraneous reflections that produce noise and limit OCT imaging quality in swept-source OCT (SS-OCT) systems. Akinetic means “absence of movement”; an all-semiconductor akinetic laser is electrically tuned—the only moving parts are photons and electrons.

In OCT, a long coherence length facilitates increased working distances and allows for volumetric imaging of larger tissue features and greater distance variation between probe-light output and the tissue (see figure). Because the Vernier-tuned distributed Bragg reflector (VT-DBR) design of Insight’s akinetic laser (with a 2-mm-long semiconductor laser cavity) produces a very narrowband output and its small size reduces cavity length variations, the cavity has high finesse and minimized time-based shifting of the center wavelength, enabling a long coherence length on the order of 220 mm even at sweep repetition rates of hundreds of kilohertz.

In some OCT systems, multiple reflections can contaminate images. To reduce unwanted signals in OCT images, Insight is implementing “user-adjusted coherence length.”

For a 1310 nm akinetic laser sweeping over a 100 nm range, sweep rate can be programmed from 4000 up to 400,000 sweeps per second; akinetic tuning technology has even been tested up to sweep rates of one million sweeps per second to reduce blur from movement and increase measurement resolution. In addition, the akinetic laser automatically linearizes the sweep over optical frequency, regardless of sweep rate or tuning range.

Axial point-spread function (PSF) quantifies the linearity of a laser’s sweep and is an important contributor to image quality possible in an SS-OCT system. The PSF is defined as the power spectrum of the interferogram produced by the OCT interferometer with a single reference reflector. A PSF with low side lobes and correspondingly low background noise creates an image with excellent contrast and detail. Depending on the interferometer configuration, typical PSF values for the akinetic laser are 55–70 dB to the noise floor for the background signal and 45–60 dB for the side lobes, enabling sensitivity values of approximately 95 dB.

The mode-hop-free 0.5 pm wavelength repeatability of the akinetic laser contrasts sharply with typical values of hundreds of picometers for MEMS swept lasers and swept VCSELs, facilitating shot-noise-limited, phase-sensitive OCT without an external reference and related data processing. And the side-mode-suppression ratio of the akinetic laser is roughly 45 dB compared to approximately 30 dB for mechanically tuned lasers.

Finally, because the akinetic laser is all semiconductor and benefits from volume production, akinetic lasers should cost just $2,000–3,000 as the semiconductor price drops over the next several years. The only drawbacks to the technology are that wavelength ranges are presently limited to standard, popular semiconductor regions like 1310 nm and 1550 nm (though 1060 nm lasers are in development for ophthalmic OCT), the upper limit of sweep rate is roughly one million sweeps per second, and coherence length limit is around 2 m.

“Better images are a direct result of a more linear, lower noise, and faster swept laser without moving parts,” says Michael Minneman, CEO at Insight. Professor Wolfgang Drexler from the Medical University of Vienna adds, “Swept-source technology from Insight has the potential to revolutionize OCT in the future, especially regarding ease of use, flexibility, and unique OCT performance.”

REFERENCE
1. M. Bonesi et al., “Akinetic swept-source with centimeters coherence length for high speed OCT,” European Conferences on Biomedical Optics 2013, Hot Topics Postdeadline Session, Munich, Germany (May 14, 2013).