IEEE Photonics Society

Boston Photonics Society Chapter

Boston Chapter of the IEEE Photonics Society


April 7, 2016
6:30 PM

MIT Lincoln Laboratory Forbes Road

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Swept Source Lasers (Please Note: This meeting is on first Thursday of April) Slides

Dr. Bart Johnson, Axsun Technologies, Billerica, MA


Dr. Bart Johnson, Axsun Technologies, Billerica, MA

Abstract:  The development of swept source lasers has made high-speed optical coherence tomography (OCT) cost effective at 1060 and 1310 nm wavelengths.  Axsun Technologies manufactures this type of source and has made inroads into ophthalmology and cardiology applications.  The 1060 product, for example, tunes 990-1100 nm at a 100 kHz sweep rate.  This external cavity diode laser is tuned by a MEMS Fabry-Perot filter.  The act of rapidly sweeping the filter induces a mode-locking behavior.  This is because when the pulses deplete the gain medium they are red shifted due to the index rise in the semiconductor optical amplifier associated with the linewidth enhancement factor.  Instead of each wavelength building up anew from spontaneous emission, the pulses are nonlinearly red shifted to track the filter sweep.  A feedback mechanism built into the laser dynamics ensures that the nonlinear shift follows the MEMS filter sweep.  Axsun has created numerical models of this process and used it to design a new generation of low noise swept lasers.  The numerical model predicts mode locking and chaotic pulsation behaviors in laser designs intended for difference applications.  The agreement with experiment is striking.  The talk will focus on this laser physics to show how it makes these various OCT applications possible.


Biography:  Since 2000, Bart Johnson has been employed as an electro-optics engineer at Boston-area startups CoreTek (tunable VCSELs), Azna (chirp managed lasers for telecom) and Axsun Technologies.  His current position is Systems Engineering Fellow at Axsun involved in laser-based NIR spectroscopy and Optical Coherence Tomography.  He is actively working in the area of NIR spectroscopy, chemometrics and applications.  In OCT, he contributes to electronic systems design and testing as well as theoretical modeling of swept laser sources.  He holds a BEE from the University of Minnesota and a PhD from MIT in electrical engineering.


Location:  MIT Lincoln Laboratory Forbes Road