Raman sensing and microscopy are among the most specific optical technologies to identify the chemical compounds of unknown samples, and to enable label-free biomedical imaging. Here we present a method for stimulated Raman scattering spectroscopy and imaging with a time-encoded (TICO) Raman concept. We use continuous wave, rapidly wavelength-swept probe lasers and combine them with a short-duty-cycle actively modulated pump laser. Hence, we achieve high stimulated Raman gain signal levels, while still benefitting from the narrow linewidth and low noise of continuous wave operation. Our all-fibre TICO-Raman setup uses a Fourier domain mode-locked laser source to achieve a unique combination of high speed, broad spectral coverage (750–3,150 cm−1) and high resolution (0.5 cm−1). The Raman information is directly encoded and acquired in time. We demonstrate quantitative chemical analysis of a solvent mixture and hyperspectral Raman microscopy with molecular contrast of plant cells.
We present a new 1060 nm Fourier domain mode locked laser (FDML laser) with a record 143 nm sweep bandwidth at 2∙ 417 kHz = 834 kHz and 120 nm at 1.67 MHz, respectively. We show that not only the bandwidth alone, but also the shape of the spectrum is critical for the resulting axial resolution, because of the specific wavelength-dependent absorption of the vitreous. The theoretical limit of our setup lies at 5.9 µm axial resolution. In vivo MHz-OCT imaging of human retina is performed and the image quality is compared to the previous results acquired with 70 nm sweep range, as well as to existing spectral domain OCT data with 2.1 µm axial resolution from literature. We identify benefits of the higher resolution, for example the improved visualization of small blood vessels in the retina besides several others.
Two-photon excitation fluorescence (TPEF) microscopy is a powerful technique for sensitive tissue imaging at depths of up to 1000 micrometers. However, due to the shallow penetration, for in vivo imaging of internal organs in patients beam delivery by an endoscope is crucial. Until today, this is hindered by linear and non-linear pulse broadening of the femtosecond pulses in the optical fibers of the endoscopes. Here we present an endoscope-ready, fiber-based TPEF microscope, using nanosecond pulses at low repetition rates instead of femtosecond pulses. These nanosecond pulses lack most of the problems connected with femtosecond pulses but are equally suited for TPEF imaging. We derive and demonstrate that at given cw-power the TPEF signal only depends on the duty cycle of the laser source. Due to the higher pulse energy at the same peak power we can also demonstrate single shot two-photon fluorescence lifetime measurements. Leproux, "Multicolor multiphoton microscopy based on a nanosecond supercontinuum laser source," J. Biophoton. In press (2016). 10. G. Donnert, C. Eggeling, and S. W. Hell, "Major signal increase in fluorescence microscopy through dark-state relaxation," Nat. Methods 4(1), 81-86 (2007). 11. M. Goeppert-Mayer, "Über Elementarakte mit zwei Quantensprüngen," Ann. Phys. 9(3), 273-294 (1931). ©2016 Optical Society of America #261064Received 11 12. S. Karpf, M. Eibl, W. Wieser, T. Klein, and R. Huber, "A Time-Encoded Technique for fibre-based hyperspectral broadband stimulated Raman microscopy," Nat. Commun. 6, 6784 (2015). 13. S. Tang, J. Liu, T. B. Krasieva, Z. Chen, and B. J. Tromberg, "Developing compact multiphoton systems using femtosecond fiber lasers," J. Biomed. Opt. 14, 030508 (2009). 14. F. Knorr, D. R. Yankelevich, J. Liu, S. Wachsmann-Hogiu, and L. Marcu, "Two-photon excited fluorescence lifetime measurements through a double-clad photonic crystal fiber for tissue micro-endoscopy," J. Biophotonics 5(1), 14-19 (2012). 15. K. Taira, T. Hashimoto, and H. Yokoyama, "Two-photon fluorescence imaging with a pulse source based on a 980-nm gain-switched laser diode," Opt. Express 15(5), 2454-2458 (2007). 16. R. H. Stolen and C. Lin, "Self-phase-modulation in silica optical fibers," Phys. Rev. A 17(4), 1448-1453 (1978). 17. E. P. Ippen and R. H. Stolen, "Stimulated Brillouin scattering in optical fibers," Appl. Phys. Lett. 21(11), 539-541 (1972 478-492 (2010). 21. N. S. Makarov, M. Drobizhev, and A. Rebane, "Two-photon absorption standards in the 550-1600 nm excitation wavelength range," Opt.
Surgical microscopes are vital tools for ophthalmic surgeons. The recent development of an integrated OCT system for the first time allows to look at tissue features below the surface. Hence, these systems can drastically improve the quality and reduce the risk of surgical interventions. However, current commercial OCT-enhanced ophthalmic surgical microscopes provide only one additional cross sectional view to the standard microscope image and feature a low update rate. To present volumetric data at a high update rate, much faster OCT systems than the ones applied in today’s surgical microscopes need to be developed. We demonstrate live volumetric retinal OCT imaging, which may provide a sufficiently large volume size (330x330x595 Voxel) and high update frequency (24.2 Hz) such that the surgeon may even purely rely on the OCT for certain surgical maneuvers. It represents a major technological step towards the possible application of OCT-only surgical microscopes in the future which would be much more compact thus enabling many additional minimal invasive applications. We show that multi-MHz A-scan rates are essential for such a device. Additionally, advanced phase-based OCT techniques require 3D OCT volumes to be detected with a stable optical phase. These techniques can provide additional functional information of the retina. Up to now, classical OCT was to slow for this, so our system can pave the way to holographic OCT with a traditional confocal flying spot approach. For the first time, we present point scanning volumetric OCT imaging of the posterior eye with up to 191.2 Hz volume rate. We show that this volume rate is high enough to enable a sufficiently stable optical phase to a level, where remaining phase errors can be corrected. Applying advanced post processing concepts for numerical refocusing or computational adaptive optics should be possible in future with such a system.
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