Influence of compressed gas circulation on SRS conversion of ultrashort laser pulses

Comparative studies on SRS conversion of 1 kHz ultrashort laser pulses (0.9–1.2 ps) at 401 and 802 nm in circulated and non-circulated compressed methane and hydrogen were carried out in SRS generation (SRSG) and stimulated Raman amplification (SRA) modes. It was shown that circulation of the gases weakened substantially the thermal lens, which facilitated reaching higher efficiencies of radiation frequency conversion and improved the quality of the obtained beams. This effect was especially pronounced in SRA mode in methane. In this case, the spectrum of the amplified beam practically coincided with that of the seed (beam to be amplified) and the quality of the first Stokes beam was close to the diffraction limit.     

Thermal aberrations and high power frequency conversion in a barium nitrate Raman laser

Raman frequency conversion of high average power pulsed Nd:YAG laser radiation into the near IR spectral region in a barium nitrate Raman laser was studied with the emphasis on thermal effects inside the Raman-active medium. The probe-beam technique together with numerical reconstruction, done by integrating the transient heat conduction and paraxial wave equations, revealed dynamics of the induced distortions featuring high-order optical aberrations. By utilizing the Zernike expansion of the reconstructed phase profile and implementing a special focusing geometry of the pump beam, partial compensation of the distortions was realized in a stable configuration of the Raman cavity. Generation of the first-, second-, and third-order Stokes radiation with output power of 17, 9.5, and 5.5 W corresponding to a quantum conversion efficiency of 32, 21, and 13% is reported.     

Self-mode locking at multiple Stokes generation in the Raman laser

A self-mode-locking at multiple Stokes generation in Raman lasers is reported. Trains of pulses with durations of down to 50 ps each have been obtained. The reasons for the self-mode-locking have been investigated numerically. Way to the further decreasing of pulse durations was formulated.     

Stationary generation of diode-pumped self-Raman Nd:YVO<Subscript>4</Subscript>/YVO<Subscript>4</Subscript> composite crystal laser

Output power dependences of composite Nd³⁺:YVO₄ Raman laser stationary generation on the longitudinal diode pump power are measured at different transmissions of the output mirror at the Stokes radiation frequency. The deviation of the measured dependences from linear is explained by the influence of thermal effects on both the overlap of the beams and diffraction losses. A method to estimate the laser and Stokes losses in the cavity and the parameters characterizing the overlap of the laser radiation with the pump and Stokes beams is proposed. A Stokes-component of power 2.1 W is obtained and corresponds to 12% diode-to-Stokes efficiency.     

Dynamics of generation of a pulsed Nd<Superscript>3+</Superscript>:KGd(WO)<Subscript>2</Subscript>/V<Superscript>3+</Superscript>:YAG laser emitting in the self-frequency Raman conversion mode at λ = 1.538 μm

It has been shown that the generation of the 1st Stokes component (λ = 1.538 μm) in an Nd:KGW laser with a passive V:YAG Q Q -switch is multimodal and its dynamics have a complex spatio-temporal character. The SRS-generation features the impact excitation manifested as the formation of a high-intensity peak at the beginning of the pulse, the peak position relative to the subsequent part of the pulse depending on the radius of curvature of the end cavity mirror. The SRS-conversion of the fundamental laser radiation (λ = 1.351 μm) starts in the central region of the Nd:KGW-element and then spreads towards its boundaries. The total integral SRS-pulse of duration 15–25 ns represents an envelope of shorter (1–2 ns) time-shifted pulses generated by separate local areas of the active medium cross-section. The multimode character of generation results in gradual damage to the V:YAG Q Q -switch at the attained SRS-radiation energy of 8–14 mJ.     

Multimode pumped continuous-wave solid-state Raman laser

We demonstrate the continuous-wave operation of a solid-state Raman laser containing a barium nitrate crystal as the Raman medium. The Raman laser, which has a singly resonant cavity, is pumped by multimode radiation. The Raman oscillation threshold is reached at approximately 2 W of pump power. As much as 500 kW/cm2 of Stokes power density at 60-kW/cm2 pump power density is obtained in the cavity.     

Observation of Raman conversion for 70-fs pulses in KGd(WO4)2 crystal in the regime of impulsive stimulated Raman scattering

Raman conversion of femtosecond pulses in a solid-state medium is reported. Stimulated Raman scattering (SRS) was observed in an impulsive SRS regime when the pump pulse's duration was comparable with the period of Raman-active vibration. The generation of super broadband radiation, which is a distinguishing feature of impulsive SRS, is shown.     

Stimulated Raman scattering in light and heavy water under picosecond laser excitation

The spectra of stimulated Raman scattering in ordinary and heavy water under excitation by second harmonic of a picosecond Nd:YAG laser were compared. It was shown that when stimulated Raman scattering is excited in water in cavities of a photonic crystal (synthetic opal matrix constructed of silica nanoglobules) or a photonic glass in the form of close-packed monodimensional millimeter-size amorphous-quartz balls, the efficiency of stimulated Raman scattering increases significantly compared to a uniform liquid medium.     

A method for reducing the stimulated Raman scattering threshold in liquids embedded into photonic crystals

We study stimulated Raman scattering (SRS) in liquids (water and ethanol) embedded into photonic crystals (artificial opals) under excitation with 60 ps laser pulses at 532 nm. We observe a substantial decrease of the SRS threshold when the focused laser beam excites the crystals near their surface. The spatial distance between the beam center and the surface of the crystals is close to their nanoscale structure. The decrease in the SRS threshold is due to a substantial increase in the photonic-state density near the edges of the photonic crystal stop zones and the so-called Mie resonances in active media near the nanosized globules. This leads to a considerable increase in the local-electric-field intensity, causing the SRS threshold reduction. Such a method of reducing the SRS threshold opens up a way of creating new efficient laser sources based on photonic crystals and their use for different applications in nanotechnologies.     

Stimulated Raman conversion of radiation from a YAG:Nd laser with wavelengths of 1.319, 1.338, and 1.357 µm in a barium nitrate crystal

We have studied stimulated Raman (SRS) conversion to the first Stokes component of multimode (M₂ = 8) radiation from a YAG:Nd laser with lasing wavelengths of 1.319 μm, 1.338 μm, and 1.357 μm in a barium nitrate crystal. We have obtained pulses of converted radiation with energies up to 120 mJ. We have achieved conversion efficiencies greater than 40% for each of the three Raman laser wavelengths of 1.530 μm, 1.556 μm, and 1.582 μm with divergence of the beams of converted radiation close to the diffraction-limited value (M₂ < 1.5). __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 3, pp. 330–334, May–June, 2006.