Steady-state Stimulated Raman Scattering Research Articles

Efficient picosecond traveling-wave Raman conversion in a SrWO4 crystal pumped by multi-Watt MOPA lasers at 1064 nm

Raman conversion with a 50-mm-long SrWO4 crystal in a single-pass, traveling-wave setup has been investigated in both purely steady-state and transient stimulated Raman scattering (SRS) regimes. For steady-state SRS experiment, we employed as a pump source a Q-Switched master oscillator power amplifier (MOPA) laser system at 1064 nm, delivering 325 μJ, 550-ps-long pulses with diffraction limited beam quality and high spectral purity. At 2-kHz repetition rate, we obtained up to 90 μJ pulse energy and 250 ps pulse duration at 1180 nm, with a conversion slope efficiency close to quantum limit. To approach the transient SRS regime, we pumped the same crystal with 16-ps-long pulses from a hybrid MOPA laser system based on a mode-locked Yb-fiber oscillator followed by a diode-pumped bulk Nd:YVO4 power amplifier. At the maximum incident pump average power of 3.75 W, we obtained 1.4 W at the first Stokes Raman-shifted wavelength of 1180 nm (37 % optical-to-optical conversion efficiency), with 15 ps pulse duration and 70 % conversion slope efficiency.

Steady-state Raman gain in visible and near-infrared waveband of SrWO4 and BaWO4 crystals

The steady-state stimulated Raman scattering (SRS) gain with different excitation wavelengths ranging from 400 to 1100 nm of tungstate crystals, SrWO4 and BaWO4, is systematically researched. As excitation frequency is close to electronic transition frequency, molecular polarizability is not a constant, which has to be taken into account in our work. The experiment and theory agree well with each other and show that SRS gain is not only proportional to Stokes light frequency, but is also inversely proportional to biquadratic excitation frequency.

SRS in di‐glycine nitrate (NH2CH2COOH)2 · HNO3 crystals: Observation of high‐gain many‐phonon nonlinear interactions

AbstractEfficient high‐gain many‐phonon steady‐state stimulated Raman scattering (SRS) was excited in the novel χ(3)‐active organic crystal of di‐glycine nitrate (DGN), (NH2CH2COOH)2 · HNO3, at room temperature under picosecond laser pumping in the visible and near‐IR regions. Besides high‐order Stokes and anti‐Stokes (comb) generation in this centrosymmetric crystal, several cascaded and cross‐cascaded χ(3) ↔ χ(3) nonlinear photon–phonon interactions were observed. The recorded Raman‐induced lasing wavelengths were identified and attributed to the SRS‐promoting optical vibration modes ωSRS1 ≈ 1058 cm−1, ωSRS2 ≈ 2985 cm−1, ωSRS3 ≈ 895 cm−1, and ωSRS4 ≈ 3000 cm−1. The measured large Raman frequency shifts (≈3000 cm−1) and estimated moderately high steady‐state Raman gain coefficients ($g_{{\rm ssR}}^{{\rm St}_{1} } $ ≥ 6.5 cm · GW−1) make this DGN crystal attractive for a number of new applications in modern laser physics and nonlinear optics.

Non-centrosymmetric Y(HCOO)3 � 2 H2O crystal. A new inorganic material for Raman lasers with large frequency shift of three promoting vibration modes of its [O-CH-O]- formate anions: effective high-order Stokes and anti-Stokes generation and cascaded self-frequency [?(3)(SRS) ??(2)(SHG, SFM)] and [?(2)(SHG, SFM) ??(3)(SRS)] conversions

Efficient multi-phonon steady-state stimulated Raman scattering (SRS) was excited in the novel χ(3)-active crystal of orthorhombic yttrium formate dihydrate, Y(HCOO)3 · 2 H2O, at room temperature under picosecond laser pumping in the visible and near-IR regions. Besides high-order Stokes and anti-Stokes generation in this non-centrosymmetric crystal, several cascaded second (χ(2))- and third (χ(3))-order nonlinear interactions were observed under 1 μm pumping. Among them are self-frequency χ(2) χ(3) conversion effects by simultaneous second harmonic generation (SHG) and SRS, as well as χ(3) χ(2) lasing action by successive SRS + SHG and SRS + SFM (sum-frequency mixing). All the recorded Raman-induced lasing wavelengths were identified and attributed to the SRS-promoting optical vibration modes wSRS1 ≈ 1395 cm–1, wSRS2 ≈ 1377 cm–1 and wSRS3 ≈ 2895 cm–1 of the [O–HC–O]– ionic groups of the crystal. The measured large Raman frequency shifts and estimated moderately high steady-state Raman gain coefficient for the first Stokes generation (gssR ≥ 7.5 cm GW–1) in the visible of the first SRS-promoting mode, as well as the fact that it is easy to grow large crystals of Y(HCOO)3 · 2 H2O make this material attractive for a number of new applications in modern quantum electronics and nonlinear optics. Y(HCOO)3 · 2 H2O offers the largest Raman frequency shifts among all known SRS-active inorganic crystals. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Picosecond stimulated Raman scattering in crystals

The comparative values of the peak and integral cross sections of spontaneous Raman scattering and the optical dephasing time of molecular vibrations were determined for several oxide crystals by spontaneous Raman spectroscopy. The spectral, time, and energy parameters of stimulated Raman scattering (SRS) were measured for ten crystals using picosecond YLF: Nd laser pumping with a radiation wavelength of 1047 nm. An analysis of the experimental dependence of the threshold energy of pumping SRS on the integral and peak cross sections of spontaneous Raman scattering showed that the SRS gain increment explicitly depended on the integral cross section and was independent of the peak cross section of spontaneous Raman scattering as the ratio between the pumping pulse width (11 ps) and the time of optical dephasing of molecular vibrations changed from 0.42 to 9.3. The gain coefficients of steady-state stimulated Raman scattering under threshold stimulated Raman scattering conditions were determined for all the crystals studied on the basis of the measured threshold SRS pumping energies, the duration and width of the spectrum of pulses, the nonlinear interaction length, the intensity of pumping, and the theoretical dependences that relate the steady-state and transient SRS gain increments. The steady-state SRS gain coefficients obtained in this work fitted well a linear dependence on the peak cross sections of spontaneous Raman scattering, which substantiated the correctness of our analysis and measurements.

Stimulated Raman scattering in a spherical microparticle

An electrodynamic approach is developed to solve the problem of stimulated Raman scattering (SRS) in a transparent dielectric spherical particle. In this approach it is proposed that optical fields for time-dependent amplitudes of coupled waves at Stokes and pump frequencies in a spherical particle be represented as an expansion in terms of eigenfunctions of the stationary scattering problem in which the expansion coefficients determine the temporal behavior of the field and comply with inhomogeneous differential equations. Solutions of these equations for initial phase SRS and under steady-state conditions are analyzed. The SRS threshold is determined, and the threshold for steady-state SRS at a given intensity is found for when there is double resonance between the fields. It is shown that, to excite SRS, one should compensate for the loss of the Stokes wave that is due to absorption and emission through the particle surface. To provide steady-state SRS generation it is necessary to compensate additionally for the energy loss that is due to depletion of pump intensity.

Comparative spontaneous Raman spectroscopy of crystals for Raman lasers

A comparison of the spectroscopic parameters of Raman-active vibronic modes in various crystalline materials with a view to the use of these crystals for stimulated Raman scattering (SRS) is presented. It includes data on the Raman frequency shift, linewidth, integral, and peak Raman scattering cross sections. For steady-state SRS the highest Raman gain coefficient has been proved to be in barium nitrate and sodium nitrate crystals; for transient SRS it is expected to be in lithium niobate and tungstate crystals. Barium tungstate and strontium tungstate are proposed as new highly efficient Raman materials for both SRS cases.

Raman spectroscopy of crystals for stimulated Raman scattering

Raman frequency shift, line width, integral and peak Raman scattering cross sections were measured in various crystals using spontaneous Raman spectroscopy. The highest Raman gain coefficient in steady state Stimulated Raman Scattering (SRS) regime was proved to be in barium nitrate crystal; for transient SRS it is expected to be in lithium niobate and tungstate crystals. Barium molybdate crystal is proposed as a new highly efficient Raman material. OCIS: 300.6450; 290.5910; 190.2640

Statistics of stimulated stokes pulse energies in the steady-state regime

We calculate the probability distribution P( W) for the energy W generated by steady-state stimulated Raman scattering (SRS) during a time interval of duration T. For short intervals large energy flucuations are predicted, while for intervals longer than the coherence time the fluctuations disappear. The behavior of the probability distribution for interval durations comparable to the coherence time is governed by the shape of the SRS power spectrum, which is found to be lorentzian for small gains and gaussian for large gains. The results are also applicable to amplified spontaneous emission and the output of a cw laser with a large number of independent modes.

Stimulated Raman scattering: Unified treatment of spontaneous initiation and spatial propagation

A theory of stimulated Raman scattering (SRS) is presented which treats in a unified manner the buildup of SRS from spontaneous Raman scattering and the spatial propagation which leads to gain. Maxwell-Bloch equations describe the coupling between the Stokes field operator and the collective atomic operators, which are driven by a classical laser field under low-signal-gain conditions, where the atomic ground states and laser field remain undepleted. An analytic expression is derived for the forward-propagating Stokes field operator in a one-dimensional spatial approximation when the Fresnel number of the excited medium is unity. Steady-state and transient Stokes intensities are evaluated under low-gain (spontaneous) and high-gain (stimulated) conditions. The Stokes intensity is found to be exactly independent of a laser bandwidth arising solely from phase fluctuations. The power spectrum of steady-state SRS is evaluated, giving a gain-narrowed spectral profile when the laser is narrow band, and a spectrum identical to the laser spectrum when the laser is broad band. A strong analogy between SRS and two-level superfluorescence is found; in both cases the macroscopic, collective dipole moment is initiated by quantum fluctuations.