Tunable picosecond optical parametric amplifier pumped by 1 ps pulses at 1†µm for coherent anti-Stokes Raman scattering.

We report an optical parametric amplifier (OPA), providing a maximum pulse energy of ∼200 µJ at 700-950 nm and a pulse duration of ∼1 ps. The OPA is driven by a ∼1 ps pulse with ∼2.5 mJ energy at 1 kHz, provided by a commercial thin-disk based laser. Using the output pulse of the OPA as pump, the thin-disk laser pulses at 1030 nm as Stokes, and the second harmonic (515 nm) as probe, we investigate the coherent anti-Stokes Raman scattering (CARS) of N2 and CO2 at various temperatures.

Lasing of N2+ induced by filamentation in air as a probe for femtosecond coherent anti-Stokes Raman scattering.

We investigated ultrashort pulse filamentation and lasing action of N2+ for pump-probe experiments in gases. Using femtosecond coherent anti-Stokes Raman scattering, the white-light supercontinuum generated in the filament was used to excite ro-vibrational Raman transitions in air, CO2 and CH4. We show that the lasing pulse acts as a probe for the excited levels by detecting the corresponding anti-Stokes Raman spectroscopy signals. This feature may be applied to remote sensing applications, as the temporal and spatial alignment of the probe beam and the filament is intrinsically provided.

CARS-imaging guidance for fs-laser ablation precision surgery.

Due to ageing populations the number of tumors is increasing worldwide. Successful surgical treatment requires complete resection of tumors to reduce recurrence rates. To reach this goal, novel methods combining in vivo tumor and tumor margin detection with low invasive precision surgical tools are needed. Coherent anti-Stokes Raman scattering (CARS) imaging is a highly promising optical tool for visualizing tumors based on characteristic changes in tissue morphology and molecular composition, while fs-laser ablation is to date the most precise surgical tool established in ophthalmology. In this contribution, CARS imaging has been combined with fs-laser ablation as a new approach for image-guided precision surgery for the first time. CARS guided fs-ablation has been applied to ablate brain, liver, skin, muscular and vascular tissues with µm-precision using sub-100 fs pulses of µJ level. We demonstrate superior imaging performance and contrast as well as detection of tissue margins by coherent Raman microscopy in comparison to laser reflectance imaging. The combination of CARS-image-guided tissue ablation is a promising tool for minimally invasive surgeries particularly in the vicinity of functional structures in the future.

Ultrabroadband two-beam coherent anti-Stokes Raman scattering and spontaneous Raman spectroscopy of organic fluids: A comparative study.

Spontaneous Raman spectroscopy is a well-established diagnostic tool, allowing for the identification of all Raman active species with a single measurement. Yet, it may suffer from low-signal intensity and fluorescent background. In contrast, coherent anti-Stokes Raman scattering (CARS) offers laser-like signals, but the traditional approach lacks the multiplex capability of spontaneous Raman spectroscopy. We present an ultrabroadband CARS setup which aims at exciting the full spectrum (300-3700 cm-1) of biological molecules. A dual-output optical parametric amplifier provides a ~7 fs pump/Stokes and a ~700 fs probe pulse. CARS spectra of DMSO, ethanol, and methanol show great agreement with spontaneous Raman spectroscopy and superiority in fluorescent environments. The spectral resolution proves sufficient to differentiate between the complex spectra of L-proline and hydroxyproline. Moreover, decay constants in the sub picosecond range are determined for individual Raman transitions, providing an additional approach for sample characterization.

An in vitro study on focusing fs-laser pulses into ocular media for ophthalmic surgery.

BACKGROUND AND OBJECTIVE: To investigate femtosecond (fs)-laser patterns within ocular media for ophthalmic surgery. METHODS: Vitreous and crystalline lens tissue from porcine eyes were treated with 2-dimensional fs-laser patterns and inspected under the optical microscope. Time resolved pump-probe experiments were conducted on vitreous tissue and gelatin, which should act as a model for crystalline lens tissue. RESULTS: Within crystalline lens tissue, pulse overlap leads to the formation of large bubbles, which is caused by subsequent energy input from the surrounding plasma channel. This effect can be used for bubble size control. Vitreous tissue behaves similar to water under fs-laser treatment, but it still allows fs-laser cutting. CONCLUSION: Bubble size control by laser bursts may reduce optical side-effects of fs-laser treatment. Furthermore, fs-laser treatment could be used for vitreoretinal applications.

Analysis of optical side-effects of fs-laser therapy in human presbyopic lens simulated with modified contact lenses.

BACKGROUND: In a single-blinded study, optical side-effects of a potential femtosecond (fs)-laser therapy in presbyopic human lenses were tested. Simulation of this therapy was carried out by applying fs-laser patterns into standard contact lenses (CL). METHODS: In the first part of the study, the influence of the numerical aperture on optical side-effects was investigated by comparing a typical fs-LASIK configuration to a fs-presbyopia treatment (n = 11). The second part focused on a possible improvement of visual performance by comparing a regular grid pattern to a randomly chosen spacing of the laser spots (n = 16). Visual acuity was measured with ETDRS charts, contrast sensitivity with F.A.C.T. charts and mesopic vision with Mesotest II. Forward scattered light was measured with the C-Quant (both instruments: Oculus Optikgeräte GmbH, Germany). A questionnaire detected subjective quality of vision. Differences between laser-treated and untreated CL and among the modifications were analyzed. RESULTS: The laser-treated and standard CL indicated no significant difference in visual acuity, contrast sensitivity and mesopic vision without glare. While wearing modified lenses with a regular grid, quality of vision decreased significantly by means of mesopic vision with glare and subjective straylight. These modifications also caused an impairment of subjective quality of vision. In contrast, there was no significant difference between the random pattern and standard CL. CONCLUSION: The increase of optical side-effects was reproducibly dependent on the geometry of the laser-structure. A randomized grid induced the least limitation. The study results are useful for planning possible laser-patterns in fs-laser therapy of the presbyopic lens.

Pig lenses in a lens stretcher: implications for presbyopia treatment.

PURPOSE: To investigate porcine lenses in a lens stretcher with regard to presbyopia corrective procedures. METHODS: A lens stretching device was designed, which allows to simultaneously determine all relevant geometrical and optical parameters at each stretch position. The setup was used to compare the optical and geometrical lens properties of young slaughter pigs (n = 5) with older sows (n = 7). RESULTS: The change of optical power with stretching is about five times larger for young porcine lenses than for sows. For young pigs, the gradient index profile of the crystalline lens significantly contributes to the induced accommodation amplitude. CONCLUSIONS: We have shown that sow lenses are a suitable model for in vitro experiments on possible treatments for presbyopia. The rapid decrease in the induced accommodation amplitude with age may be explained by a reduced change of all geometrical lens parameters, which in turn leads to a smaller contribution of the gradient index profile to accommodation.

Femtosecond laser treatment of the crystalline lens: a 1-year study of possible cataractogenesis in minipigs.

BACKGROUND: To investigate the long-term stability and possible cataractogenesis after femtosecond laser treatment of the crystalline lens. METHODS: The crystalline lenses of ten Göttingen minipigs® underwent femtosecond laser treatment. During a subsequent 1-year follow-up, the pigs were monitored by means of slit-lamp examination of the anterior segment and Scheimpflug imaging of the lens. RESULTS: No laser-induced cataractogenesis was observed during the 1-year follow-up. The laser pattern showed a stable appearance under all examination devices. CONCLUSION: Femtosecond laser treatment seems to be no trigger for cataract formation. Moreover, the long-term stability of the laser pattern makes it suitable for applications such as presbyopia treatment.

Atomic-resolution mapping of transcription factor-DNA interactions by femtosecond laser crosslinking and mass spectrometry.

Transcription factors (TFs) regulate target genes by specific interactions with DNA sequences. Detecting and understanding these interactions at the molecular level is of fundamental importance in biological and clinical contexts. Crosslinking mass spectrometry is a powerful tool to assist the structure prediction of protein complexes but has been limited to the study of protein-protein and protein-RNA interactions. Here, we present a femtosecond laser-induced crosslinking mass spectrometry (fliX-MS) workflow, which allows the mapping of protein-DNA contacts at single nucleotide and up to single amino acid resolution. Applied to recombinant histone octamers, NF1, and TBP in complex with DNA, our method is highly specific for the mapping of DNA binding domains. Identified crosslinks are in close agreement with previous biochemical data on DNA binding and mostly fit known complex structures. Applying fliX-MS to cells identifies several bona fide crosslinks on DNA binding domains, paving the way for future large scale ex vivo experiments.

Electric events synchronized with laser filaments in thunderclouds.

We investigated the possibility to trigger real-scale lightning using ionized filaments generated by ultrashort laser pulses in the atmosphere. Under conditions of high electric field during two thunderstorms, we observed a statistically significant number of electric events synchronized with the laser pulses, at the location of the filaments. This observation suggests that corona discharges may have been triggered by filaments.

A feasibility study on femtosecond laser thrombolysis.

OBJECTIVE: In this feasibility study, we investigate possible femtosecond laser thrombolysis. BACKGROUND DATA: Because of low pulse energies, femtosecond laser surgery inherently minimizes side effects on the surrounding tissue. Moreover, current femtosecond laser sources as well as fiber technology allow consideration of catheter-based treatments. METHODS: Two femtosecond laser systems (λ=800 nm, λ=1030 nm) along with a three dimensional (3D) scanner system (NA ~0.1) were used in this study. In vitro experiments were performed on porcine thrombi and blood vessels. Ablation thresholds were determined in air, by determining the pulse energy at which single shot ablation was visible under the optical microscope. Ablation rates were determined in physiological saline. Additionally, ablation of thrombi and blood vessels was monitored by means of a fiber spectrometer. RESULTS: Depending upon the scan velocity, typical ablation rates for thrombi were ~0.04 mm(3)/sec. Ablation thresholds of thrombi and blood vessels differ by factors of 3 and 1.5 at laser wavelengths of 800 and 1030 nm, respectively. At a distance of 5 mm above the surface, second harmonic generation was observed in blood vessels, but not within thrombi. CONCLUSIONS: The results show that a typical thrombus volume can be destroyed within a reasonable time frame. Because of the higher threshold difference of thrombi and blood vessels, the use of a laser wavelength of 800 nm is preferable. Furthermore, the detection of the second harmonic could provide a feedback mechanism to protect the vascular wall from mechanical and laser damage.

Optical side-effects of fs-laser treatment in refractive surgery investigated by means of a model eye.

Optical side-effects of fs-laser treatment in refractive surgery are investigated by means of a model eye. We show that rainbow glare is the predominant perturbation, which can be avoided by randomly distributing laser spots within the lens. For corneal applications such as fs-LASIK, even a regular grid with spot-to-spot distances of ~3 µm is sufficient to minimize rainbow glare perception. Contrast sensitivity is affected, when the lens is treated with large 3D-patterns.