Reversal of crystallization in cryoprotected samples by laser editing

Advances in cryobiology techniques commonly target either the cooling or the warming cycle, while little thought has been given to ≪repair≫ protocols applicable during cold storage. In particular, crystallization is the dominant threat to cryopreserved samples but proceeds from small nuclei that are innocuous if further growth is forestalled. To this end, we propose a laser editing technique that locally heats individual crystals above their melting point by a focused nanosecond pulse, followed by amorphization during rapid resolidification. As a reference, we first apply the approach to ice crystals in cryoprotected solution and use Raman confocal mapping to study the deactivation of crystalline order. Then, we examine dimethyl sulfoxide trihydrate crystals that can germinate at low temperatures in maximally freeze concentrated regions, as commonly produced by equilibrium cooling protocols. We show how to uniquely identify this phase from Raman spectra and evidence retarded growth of laser-edited crystals during warming.

Numerical investigation of the energy limit in a picosecond fiber optic parametric oscillator

Numerical simulation of a fiber optic parametric oscillator to produce picosecond narrowband pulses for coherent anti-Stokes Raman spectroscopy has been performed by an open source Python-based library using an extremely wide range of parameters, such as the pump pulse duration, parametric frequency shift, spectral bandwidth of the pump, and the parametric pulses. It required an extremely large calculation window, both in time and spectral domains. We managed to speed up the simulation 50 times using a graphic processor unit that allowed us to define the areas of stability for different lengths of standard passive (5-100 m) and photonic crystal (23-100 cm) fibers used in the external linear oscillator cavity. It was shown that highly chirped dissipative solitons at a wavelength about 800 nm can be generated with energy up to 55 nJ, which is limited by the pump depletion and self-phase modulation effects.

Frequency doubling of multimode diode-pumped GRIN-fiber Raman lasers

Frequency doubling of multimode diode-pumped GRIN-fiber Raman laser with improved beam quality (M²=1.9-2.6 depending on configuration) in a simple single-pass scheme with 5-mm PPLN crystal is studied. After scheme optimization and elimination of back reflection and crystal heating effects, an efficient conversion into blue spectral range with output power of about 0.4 W@488 nm and 0.64 W@477 nm has been demonstrated.

2nd-order random lasing in a multimode diode-pumped graded-index fiber

Raman lasing in a graded-index fiber (GIF) attracts now great deal of attention due to the opportunity to convert high-power multimode laser diode radiation into the Stokes wave with beam quality improvement based on the Raman clean-up effect. Here we report on the cascaded Raman generation of the 2nd Stokes order in the 1.1-km long GIF with 100-μm core directly pumped by 915-nm diodes. In the studied all-fiber scheme, the 1st Stokes order is generated at 950-954 nm in a linear cavity formed at GIF ends by two fiber Bragg gratings (FBGs) securing beam quality improvement from M2 ≈ 30 to M2 ≈ 2.3 due to special transverse structure of FBGs. The 2nd Stokes wave is generated either in linear (two FBGs) or half-open (one FBG) cavity with random distributed feedback via Rayleigh backscattering. Their comparison shows that the random lasing provides better beam quality and higher slope efficiency. Nearly diffraction limited beam (M2 ≈ 1.6) with power up to 27 W at maximum gain (996 nm), and 17 W at the detuned wavelength of 978 nm has been obtained, thus demonstrating that the 2nd-order random lasing in diode-pumped GIF with FBGs provides high-efficiency high-quality beam generation in a broad wavelength range within the Raman gain spectral profile.

Generation of Raman dissipative solitons near 1.3 microns in a phosphosilicate-fiber cavity

An external-cavity generation of powerful ultrashort pulses in an all-fiber scheme by using a new type of phosphosilicate polarization maintaining fiber is investigated. The phosphorus-related Stokes shifted Raman pulse near 1.3 microns is observed. Optimization of Stokes output spectrum depending on pump pulse duration (chirp), energy and output coupling ratio of the cavity is performed. As result, the output energy of highly-chirped pulses compressible to 570 fs reaches 1.6 nJ.