Precise information on dispersion of the nonlinear optical susceptibility of Raman active media is essential in order to get an insight into physics and chemistry of intra- and inter-molecular interactions. We propose and experimentally demonstrate a method that is capable of resolving both real and imaginary parts of third-order nonlinearity (χ(3)) in the vicinity of Raman resonances. Dispersion of χ(3) can be obtained from a medium probed within microscopic volumes with a spectral resolution of better than 0.1 cm-1 thus making our approach an essential tool in quantitative microscopic characterization of complex biological media. Time-domain CARS transients traced with femtosecond pulses within orders of magnitude in the signal decay can lead to resolution of fine spectral features in χ(3) dispersion that can not be reliably detected by frequency-domain Raman based spectroscopy/microscopy techniques, including coherent methods. We will present results of the method’s application in biological cells and tissue. Namely, we accessed a protein line at 1245 cm-1 in E-coli cell, major DNA and protein lines in red blood cells and triglyceride Raman active peaks in fat tissue.
Multiphoton microscopy is a powerful technique for high spatial resolution thick tissue imaging. In its simple version, it
uses a high repetition rate femtosecond oscillator laser source focussed and scanned across biological sample that contains fluorophores. However, not every biological structure is inherently fluorescent or can be stained without causing biochemical changes. To circumvent these limitations, other non-invasive nonlinear optical imaging approaches are currently being developed and investigated with regard to different applications. These techniques are: (1) second harmonic generation (SHG), (2) third harmonic generation (THG), and (3) coherent anti-Stokes Raman scattering
(CARS) microscopy. The main advantage of the above mentioned techniques is that they derive their imaging contrast
from optical nonlinearities that do not involve fluorescence process. As a particular application example we investigated
collagen arrays. We show that combining SHG-THG-CARS onto a single imaging platform provides complementary information about the sub-micron architecture of the tissue. SHG microscopy reveals the fibrillar architecture of collagen arrays and confirm a rather high degree of heterogeneity of χ(2) within the focal volume, THG highlights the boundaries between the collagen sheets, and CH2 spectroscopic contrast with CARS.
We report an application of the combined third order microscopy techniques to reveal structure and morphology of the
peripheral nerve in mice. The resonant Coherent Anti-Stokes Raman Scattering (CARS) and third harmonic generation
(THG) techniques have been applied to visualize structure of the myelinated peripheral axon. While CARS was quite
efficient in selective imaging of the cladding layer via characteristic Raman active vibrations of dense lipid structures
constituting the layers, the THG microscopy helped to clearly reveal the degree of optical and nonlinear optical
inhomogeneity of the axon core (that may have further important implications).
A new, synchronously pumped picosecond OPO for CARS microscopy is presented. It is based on non-critically
phasematched interaction in LBO pumped by a frequency-doubled modelocked Nd:Vanadat laser at 532 nm.
Within the parametric process a tuneable pair of two different wavelengths in the NIR range is generated (Signal <680
...990 nm, Idler 1150...>2450 nm). In this system they are extracted from the cavity at the same mirror and therefore
propagating collinear at the same beam path. Due to the mechanism of their generation there is no jitter between Signal
and Idler. Though the wavelengths are different the GVD is negligible for this picosecond pulse duration. As a result the
two pulse trains are spatially and temporally perfectly matched.
The pulses generated are close to transform limit with about 5-6 ps pulse duration, excellent beam quality (M2 < 1,1) and
high pointing stability. The output power for Signal and Idler is about 1 W each @ 4 W pump power. The tuning
mechanism is split into two parts - temperature tuning for rough variations and fast angular BRF tuning for the fine
adjustment of the output wavelength.
The perfect spatial and temporal overlap make the described OPO an ideal and nearly hands-free laser source for CARS
microscopy with a tuneable energy difference 1,400 ... >10,000 cm-1. The absolute wavelength range is resulting in high
penetration depth and low photo damage of the analyzed samples.
Finally some CARS-images are presented and the latest results and methods for further sensitivity enhancements are
shown.
CARS microscopy has emerged as a powerful tool in imaging of biological matter. In addition to a high-3D spatial resolution, the technique delivers an attractive set of properties such as chemical specificity, high sensitivity, and fast data acquisition rates thus making it very suitable for biomedical applications. However, these advantages come at a cost of complex tunable laser sources, beam guiding and delivery optics. In particular, two high intensity laser pulses, whose carrier frequencies ωp (pump) and ωs (Stokes) are separated by the corresponding Raman shift value, are required to interact with the imaged media. In this paper, we present experimental results corresponding to our first step towards an integrated CARS-microscope. We demonstrate optical fiber delivery of two color picosecond pulses before they interact in the focus of microscope objective in order to produce CARS image. Certain aspects related to the effect of the pump and Stokes pulse parameters on image quality (e.g. contrast, sensitivity) after the pulses' propagation in the fiber will be addressed. The incorporation of fiber delivery feature significantly improves the microscope performance and ease its operation. In addition, we are exploring certain approaches in further development of CARS-microscope as a biomedical tool towards fully functional endoscope for in vivo chemically sensitive imaging.
Omar Qasaimeh, Ferous Ganikhanov, E. Dean, Jong Yi, Lars Eng, R. Juluri, J. Stayt, S. Broutin, J. Johanson, Richard Bylsma, Leonard Ketelsen, John Johnson, David Ackerman, Mark Hybertsen, Scott Roycroft, G. Rao, T. Pinnington, Ciaran O'Cochlain
High power buried heterostructure 1.55micrometers tunable DBR lasers have been designed, fabricated and characterized. The laser consists of a gain section, a distributed Bragg reflector, a semiconductor optical amplifier and front photodetector for automated power control. The heterostructures were grown by MOCVD with the help of selective area growth techniques and dual waveguide heterostructure. Several advantages stem from this integration scheme which include simplicity of design and fabrication, increased reliability and low cost. The laser exhibits output power of 13dBm in the fiber and is tunable over 30(50GHz) ITU channels. The laser exhibits excellent performance and long-term control and reliability. The laser/transmitter also demonstrates significant increase of its functionality while its size remains small.
St.able pulses with a duration of 2ps derived from a special designed pulsed Nd:glass oscillator with negative feedback were amplified by the injection seeding of Q-swit.cned laser. Amplification fact.or of ~ 10• was achieved in this scheme. Chirped pulses amplification technique is used in further amplification in many staged laser system. In the works of recent. years to advance in the region of high power light. fields the Nd:glass lasers were suicessfully used and tfie highest J<OWer densities al. nearly 1020 W/cm level have been obt. ained1 • . Two problems would be solved for the beginning when such high PQwer picosecond laser system is creating.The first is the designing of J<icosecond oscillator with performed parameters. Compared witfi cw-raser system, pulsed mode-locked solia state laser offer the advantages of the increased out.put energy and short.er pulses. To overcome the shortcomings of a passive mode-locking concept the idea of feedback controllea generation was _proP.osed by many authors. N~at.ive feedback operation results in armost. 100% inode-Iocking and highly contrast. pulses at the out.put. due to controlled prolongation of the nonlinear stage of generation process. One of the main feature is that the pulse intensity in the cavity mar be maintained by negative feecffiack on the level when considerab e pulse shortening would occur. In this way, pulse duration which is close to the theoretical limit set. by the fluorescence linewidth of the laser material may be obtained. Generation of reproducible subpicosecond li~ht pulses in pulsed Nd: glass oscillator was perfectly realized in ' 4 • Second problem is concerned with the efficient preamplificat.ion of relatively weak pulse from the oscillator out.put.. One of the widely used method in preient is the ul t.rashort. pulses amplification in regenerative amplifier . However, one have to satisfy in this case to sufficiently severe reqqirement.s for the cavities length detuning Ct.o within 0.001- 0.01YJ bet.ween the oscillator and regenerative amplifier.
A device for duration measurements of the picosecond and subpicosecond optical pulses in single shot is described. The elaborated scheme allows us to measure autocorrelation trace in a dynamic range no less than 103. Operation is demonstrated by investigation of the temporal properties of the optical pulse on the output of the mode-locked Nd:glass laser.
Comparative analysis of main optical characteristics of KDP crystals grown by traditional technique at growth rates up to 1 mm/day and by rapid growth technique at growth rates ranging from 15 to 40 mm/day is carried out. The results on anomalous biaxiality absorption ard scattering of light effective nonlinear susceptibility and bulk laser damage threshold of crystals are given. It is shown that the increase of the crystal growth rate by more than one order does not deteriorate the crystal optical quality.
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