1. Bart Johnson

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    1. Mentioned In 18 Articles

    2. Temperature stabilized phase reference for MEMS based swept sources

      Temperature stabilized phase reference for MEMS based swept sources
      MEMS tunable lasers are not inherently phase stable because Brownian motion and drive electronics noise make the starting wavelength of the sweep unstable with respect to the electrical sweep trigger. A typical solution to the problem is to use a fiber Bragg reflector wavelength trigger. That is a sub-optimal solution since environmental changes can move both the Bragg peak and the k-clock phase. We have packaged temperature controlled trigger and ...
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    3. Tunable 1060nm VCSEL co-packaged with pump and SOA for OCT and LiDAR

      Tunable 1060nm VCSEL co-packaged with pump and SOA for OCT and LiDAR
      A 1060 nm optically pumped tunable VCSEL was formed from an InGaAs/AlGaAs/GaAs half-VCSEL bonded to a MEMS movable mirror on a silicon substrate. The VCSEL was co-packaged in a 14-pin butterfly module with an 825 nm pump laser and a 1060 nm semiconductor optical amplifier. The co-packaged device exhibited shot-noise-limited sensitivity with up to 50 mW output power and 75 nm tunability. Ophthalmic OCT, especially whole-eye imaging and ...
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    4. OCT swept laser with cavity length compensation

      OCT swept laser with cavity length compensation
      An optical coherence tomography system utilizes an optical swept laser that has cavity length compensator that changes an optical length of the laser cavity for different optical frequencies to increase the length of the laser cavity for lower optical frequencies. Specifically, a spectral separation between longitudinal cavity modes of the laser cavity is shortened or alternatively lengthened as a passband of a cavity tuning element sweeps through a scanband of ...
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    5. Long-to-short wavelength swept source

      Long-to-short wavelength swept source
      Most swept external cavity diode lasers tune in the short-to-long wavelength direction (red tuning). Lower relative intensity noise (RIN) and higher output power are typically possible in this direction. We show here that long-to-short tuning (blue tuning) is possible for a short, linear cavity laser that has both low noise and high power. This mode of operation is made possible by nonlinear frequency broadening in the semiconductor optical amplifier (SOA ...
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    6. Integrated optical coherence analysis system

      Integrated optical coherence analysis system
      Optical coherence tomography (OCT) probe and system designs are disclosed that minimize the effects of mechanical movement and strain to the probe to the OCT analysis. It also concerns optical designs that are robust against noise from the OCT laser source. Also integrated OCT system-probes are included that yield compact and robust electro-opto-mechanical systems along with polarization sensitive OCT systems.
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    7. Integrated dual swept source alternating same scan band for OCT medical imaging

      Integrated dual swept source alternating same scan band for OCT medical imaging
      An optical coherence analysis system comprising: a first swept source that generates a first optical signal that is tuned over a first spectral scan band, a second swept source that generates a second optical signal that is tuned over a second spectral scan band, a combiner for combining the first optical signal and the second optical signal to form a combined optical signal, an interferometer for dividing the combined optical ...
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    8. Coherence properties of short cavity swept lasers

      Coherence properties of short cavity swept lasers
      It has been shown theoretically and experimentally that short cavity swept lasers are passively mode locked. We develop a mathematical model of these lasers and the light field solutions are used to predict the coherence length and coherence revival behavior. The calculations compare favorably with data from a 9901100 nm laser swept at 100 kHz suitable for optical coherence tomography applications.
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    9. Regenerative mode locked laser swept source for OCT medical imaging

      Regenerative mode locked laser swept source for OCT medical imaging
      An optical coherence analysis system uses a laser swept source that is constrained to operate in a mode locked condition using regenerative mode-locking. This is accomplished by synchronously changing the laser cavity's net gain and/or phase based on time varying intensity of the swept optical signal generated by the laser. This produces a stable pulsation behavior, which is associated with smooth tuning (low optical frequency reference clock jitter ...
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    10. Optical teardown of a Kindle Paperwhite display by OCT.

      Optical teardown of a Kindle Paperwhite display by OCT.
      An optical teardown, or reverse engineering, of an Amazon Kindle Paperwhite electrophoretic display was performed by Optical Coherence Tomography at 1060 nm. The display incorporates an optical diffuser, lightguide and scattering layers for white light illumination, capacitive touch sensing, and an electrophoretic display. All these layers can be imaged by OCT as well as the thin film transistor array on the back side for driving the pixels. Phase sensitive OCT ...
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    11. SNR of swept SLEDs and swept lasers for OCT

      SNR of swept SLEDs and swept lasers for OCT
      A back-to-back comparison of a tunable narrow-band-filtered SLED (TSLED) and a swept laser are made for OCT applications. The two sources are similar in terms of sweep speed, tuning range and coherence length. A fundamental issue with a TSLED is that the RIN is proportional to 1/linewidth, meaning that the longer the coherence length, the higher the RIN and clock jitter. We show that the TSLED has an SNR ...
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    12. IEEE Boston Photonics Society Talk on Swept Source Lasers by Bart Johnson

      IEEE Boston Photonics Society Talk on Swept Source Lasers by Bart Johnson
      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 ...
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    13. Spectral filtering of k-clock signal in OCT system and method

      Spectral filtering of k-clock signal in OCT system and method
      A system and method for spectral filtering of a k-clock signal in a swept-source Optical Coherence Tomography ("OCT") system to remove artifacts in the k-clock signal. The system synchronizes sampling of the k-clock and interference signals generated from scanning a sample. Using a filtered k-clock signal, the system resamples an interference dataset of the interference signals. The system then performs Fourier transform based processing upon the resampled interference dataset to ...
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    14. Multi-speed OCT swept source with optimized k-clock

      Multi-speed OCT swept source with optimized k-clock
      An optical coherence tomography system utilizes an optical swept source that frequency scans at least two different sweep rates. In this way, the system can perform large depth scans of the sample and then the same system can perform shorter depth high precision scans, in one specific example. In order to optimally use the analog to digital converter that samples the interference signal, the system further samples the interference signals ...
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    15. System Method For Resampling Optical Coherence Tomography Signals In Segments

      System Method For Resampling Optical Coherence Tomography Signals In Segments
      A system and method for resampling interference datasets of samples in segments, in a swept- source based Optical Coherence Tomography (OCT) system. The resampling is preferably performed within a Field Programmable Gate Array (FPGA) of the OCT system, the FPGA preferably having Fourier-transform based signal processing capabilities such as Fast Fourier Transform (FFT) cores. Resampling the interference datasets in segments provides a flexible approach to resampling that makes efficient use ...
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    16. 1-15 of 18 1 2 »
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  2. About Bart Johnson

    Bart Johnson

    Bart Johnson has been employed, since 2000, 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 Director of Electro-Optics Systems Engineering at Axsun involved in laser-based NIR spectroscopy and Optical Coherence Tomography.  He is actively working in the areas 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.