Raman Conversion Research Articles

Multiple Competition Processes Between Stimulated Brillouin and Raman Scattering in a Sulfate Aqueous Solution

Stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) processes from the aqueous solution of MgSO4 have been observed. The experimental results show that there are three different processes in aqueous solution of sulfate: SBS, SRS attributed to the OH bond stretching vibration of water molecules, and SRS attributed to the stretching vibration of sulfate ion (SO42–). The strong and weak sulfate ions determine which one is dominant among the three competition processes. In saturated aqueous solution, it is shown that the SRS process of sulfate ion clearly dominates over the other two stimulated scattering processes. Meanwhile, we have also observed the thermal defocusing phenomenon in saturated aqueous solution at higher pump energy. The results can aid in enhancing the Raman conversion efficiency in liquid scattering media by improving the experimental process.

Diamond‐based concept for combining beams at very high average powers

AbstractEver since the laser's invention, there has been great interest in increasing beam output power without detriment to its coherence. Despite great advances having been obtained through the use of a diverse range of approaches, steady‐state beam powers above ten kilowatts remain a significant challenge for solid‐state lasers due to the heightened impact of detrimental nonlinear effects such as thermal lensing. Multiplexing several lasers using beam combination represents a method for surpassing the power barriers of single lasers. Here we propose and demonstrate a novel approach to beam combination and power scaling based on Raman conversion in diamond. Power from multiple non‐collinear pump beams is efficiently transferred onto a single Stokes beam in a single‐pass amplifier. Using three mutually‐independent nanosecond pulsed beams from a free‐running‐linewidth 1064 nm laser, 69% of the total peak pump power of 6.7 kW was transferred onto a TEM00 Stokes seed pulse at 1240 nm in a 9.5 mm long diamond crystal. Compared to other beam combination techniques, diamond beam combination has advantages of relaxed constraints on pump beam mutual coherence, while enabling narrowband output. Thermal considerations for extending from low duty‐cycle to continuous wave operation and higher power levels are discussed. image

Eye-Safe KGd(WO4)2:Nd Laser: Nano- and Subnanosecond Pulse Generation in Self-Frequency Raman Conversion Mode with Active Q-Switching

The shape of the multimode Stokes pulse generated by an eye-safe KGd(WO4)2:Nd laser with self-frequency Raman conversion and active Q-switching was shown to depend on the inhomogeneity of the active-medium pump. The laser generated a short and undistorted Stokes pulse of length ~2.5 ns that increased with increasing laser cavity length for a moderately inhomogeneous pump characterized by a higher population inversion in the center of the active element. The energy of the Stokes pulse (~11.5 mJ) varied little as the output-mirror reflectivity varied in the range 5–45%. The Raman pulse became distorted if the inhomogeneity of the pump was increased considerably. The degree of pump inhomogeneity was negligible with fundamental TEM00 mode selection. The laser generated subnanosecond Stokes pulses with peak power in the MW range.

Low-threshold 1.9 µm Raman generation in microstructured hydrogen-filled hollow-core revolver fibre with nested capillaries

Efficient Raman laser based on the revolver hollow-core fibre (HCF) with a cladding formed by one layer of double nested capillaries was realized for the first time. Using hydrogen gas as a nonlinear medium we demonstrated single-pass single-stage 1064 nm–1907 nm Raman conversion with quantum efficiency as high as 40%. Improved mode confinement in the hollow core and low optical losses of the fibre enabled us to achieve Raman threshold as low as 270 W of peak power, which is 4–7 times improvement compared to previously reported results on 1.9 µm Raman generation in other types of HCFs. Further progress in revolver fibre technology could be a promising way to develop low-threshold, efficient, compact gas fibre lasers for various applications in the mid-infrared spectral range.

A MONTE CARLO STUDY OF FLUX RATIOS OF RAMAN SCATTERED O vi FEATURES AT 6825 AND 7082 Å IN SYMBIOTIC STARS

ABSTRACT Symbiotic stars are regarded as wide binary systems consisting of a hot white dwarf and a mass losing giant. They exhibit unique spectral features at 6825 and 7082 Å, which are formed via Raman scattering of O vi λλ 1032 and 1038 with atomic hydrogen. We adopt a Monte Carlo technique to generate the same number of O vi λ1032 and λ1038 line photons and compute the flux ratio of these Raman scattered O vi features formed in neutral regions with a simple geometric shape as a function of H i column density N H i . In cylindrical and spherical neutral regions with the O vi source embedded inside, the flux ratio shows an overall decrease from 3 to 1 as N H i increases in the range . In cases of slab geometry and other geometries with the O vi source outside the H i region, Rayleigh escape operates to lower the flux ratio considerably. For moderate values of the flux ratio behaves in a complicated way to exhibit a broad bump with a peak value of 3.5 in the case of a sphere geometry. We find that the ratio of Raman conversion efficiencies of O vi λλ 1032, 1038 ranges from 0.8 to 3.5. Our high resolution spectra of “D” type HM Sge and “S” type AG Dra obtained with the Canada–France–Hawaii Telescope show that the flux ratio of AG Dra is significantly smaller than that of HM Sge, implying that “S” type symbiotics are characterized by higher N H i than “D” type symbiotics.

Polarization-Resolved Raman Study of Bulk-like and Davydov-Induced Vibrational Modes of Exfoliated Black Phosphorus.

Owing to its crystallographic structure, black phosphorus is one of the few 2D materials expressing strongly anisotropic optical, transport, and mechanical properties. We report on the anisotropy of electron-phonon interactions through a polarization-resolved Raman study of the four vibrational modes of atomically thin black phosphorus (2D phosphane): the three bulk-like modes Ag1, B2g, and Ag2 and the Davydov-induced mode labeled Ag(B2u). The complex Raman tensor elements reveal that the relative variation in permittivity of all Ag modes is irrespective of the atomic motion involved lowest along the zigzag direction, the basal anisotropy of these variations is most pronounced for Ag2 and Ag(B2u), and interlayer interactions in multilayer samples lead to reduced anisotropy. The bulk-forbidden Ag(B2u) mode appears for n ≥ 2 and quickly subsides in thicker layers. It is assigned to a Davydov-induced IR to Raman conversion of the bulk IR mode B2u and exhibits characteristics similar to Ag2. Although this mode is expected to be weak, an electronic resonance significantly enhances its Raman efficiency such that it becomes a dominant mode in the spectrum of bilayer 2D phosphane.

Achieving a 1.5 μm fiber gas Raman laser source with about 400 kW of peak power and a 6.3 GHz linewidth.

We have demonstrated for the first time, to the best of our knowledge, a novel and effective method to produce a 1.5 μm fiber source by means of Raman wavelength conversion in a gas-filled hollow core fiber. An ethane-filled, anti-resonance, hollow core fiber is pumped with a high peak power pulsed 1064 nm laser, generating a 1552.7 nm Stokes wave by pure vibrational stimulated Raman scattering of ethane. A maximum peak power of about 400 kW is achieved with a 6 m fiber length at 2 bars of pressure. The maximum Raman conversion efficiency is about 38%, and the corresponding laser slope efficiency is about 61.5%. The linewidth of the Stokes wave is 6.3 GHz. If a tunable pump laser is used, this kind of fiber source can easily achieve a broad tuning range near 1.5 μm.

Parametric second Stokes Raman laser output pulse shortening to 300 ps due to depletion of pumping of intracavity Raman conversion

A new effect of the pulse shortening of the parametrically generated radiation down to hundreds of picosecond via depletion of pumping of intracavity Raman conversion in the miniature passively Q-switched Nd:\(\hbox {SrMoO}_{4}\) parametric self-Raman laser with the increasing energy of the shortened pulse under pulsed pumping by a high-power laser diode bar is demonstrated. The theoretical estimation of the depletion stage duration of the convertible fundamental laser radiation via intracavity Raman conversion is in agreement with the experimentally demonstrated duration of the parametrically generated pulse. Using the mathematical modeling of the pulse shortening quality and quantity deterioration is disclosed, and the solution ways are found by the optimization of the laser parameters.

Raman Spectra of Crystalline Double Calcium Orthovanadates Ca10M(VO4)7 (M = Li, K, Na) and Their Interpretation Based on Deconvolution Into Voigt Profiles

Unpolarized spontaneous Raman spectra of crystalline double calcium orthovanadates Ca10M(VO4)7 (M = Li, K, Na) in the range 150–1600 cm–1 were measured. Two vibrational bands with full-width at half-maximum (FWHM) of 37–50 cm–1 were found in the regions 150–500 and 700–1000 cm–1. The band shapes were approximated well by deconvolution into Voigt profiles. The band at 700–1000 cm–1 was stronger and deconvoluted into eight Voigt profiles. The frequencies of two strong lines were ~848 and ~862 cm–1 for Ca10Li(VO4)7; ~850 and ~866 cm–1 for Ca10Na(VO4)7; and ~844 and ~866 cm–1 for Ca10K(VO4)7. The Lorentzian width parameters of these lines in the Voigt profiles were ~5 times greater than those of the Gaussian width parameters. The FWHM of the Voigt profiles were ~18–42 cm–1. The two strongest lines had widths of 21–25 cm–1. The vibrational band at 300–500 cm–1 was ~5–6 times weaker than that at 700–1000 cm–1 and was deconvoluted into four lines with widths of 25–40 cm–1. The large FWHM of the Raman lines indicated that the crystal structures were disordered. These crystals could be of interest for Raman conversion of pico- and femtosecond laser pulses because of the intense vibrations with large FWHM in the Raman spectra.

Tutorial on fiber-based sources for biophotonic applications.

Fiber-based lasers and master oscillator power fiber amplifier configurations are described. These allow spectral versatility coupled with pulse width and pulse repetition rate selection in compact and efficient packages. This is enhanced through the use of nonlinear optical conversion in fibers and fiber-coupled nonlinear crystals, which can be integrated to provide all-fiber pump sources for diverse application. The advantages and disadvantages of sources based upon supercontinuum generation, stimulated Raman conversion, four-wave mixing, parametric generation and difference frequency generation, allowing spectral coverage from the UV to the mid-infrared, are considered.

Ultrafast second-Stokes diamond Raman laser.

We report a synchronously-pumped femtosecond diamond Raman laser operating with a tunable second-Stokes output. Pumped using a mode-locked Ti:sapphire laser at 840-910 nm with a duration of 165 fs, the second-Stokes wavelength was tuneable from 1082 - 1200 nm with sub-picosecond duration. Our results demonstrate potential for cascaded Raman conversion to extend the wavelength coverage of standard laser sources to new regions.

Quasi-continuous-wave 589-nm radiation based on intracavity frequency-doubled Nd:GGG/BaWO4 Raman laser

Quasi-continuous-wave (QCW) 589-nm radiation was realized based on a frequency-doubled crystalline Raman laser. The fundamental wave with macro–micro-pulse trains was generated from an acousto-optically Q-switched QCW diode side-pumped Nd:GGG laser. Intracavity Raman conversion was accomplished by a BaWO4 crystal and the second harmonic generation was finished by a KTP crystal. Under a pumping power of 126.0W with a macro-pulse frequency of 300Hz and duration of 300µs, the maximum 589nm output power of 4.2W was obtained at a micro-pulse frequency of 33.3kHz. The micro-pulse width was 13.6ns.

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.

Intracavity CH 4 Raman laser using negative-branch unstable resonator

An intracavity Q-switched Nd:YAG/CH4 Raman laser is realized based on the configuration of a negative-branch confocal unstable resonator. A numerical model of the bare resonator was introduced to simulate the fundamental transverse mode and calculate the loss of the fundamental resonator. With different magnifications of the fundamental resonator, the first Stokes output energy was presented as a function of the discharge voltage. The influence of the Stokes resonator on Raman conversion was analyzed. With a fundamental resonator magnification of 1.1, a maximum output energy of 58 mJ was obtained, and the corresponding photon conversion efficiency was 48%.

Efficient diamond Raman laser generating 65 fs pulses.

We report a synchronously-pumped femtosecond diamond Raman laser operating at 890 nm with a slope efficiency of 32%. Pumped using a mode-locked Ti:Sapphire laser at 796 nm with a pulse duration of 194 fs, the bandwidth of the Stokes output was broadened to enable subsequent pulse compression to 65 fs using a prism-pair. Modelling results provide an understanding of the physical mechanisms involved in the Raman conversion of femtosecond pulses, supporting an in-depth characterization of these ultrashort pulsed lasers.

A compact Raman converter for UV-VIS spectrometers.

A small form factor, easily constructed converter that adapts fiber coupled UV/VIS CCD detector-based spectrometers into a right angle scattering Raman spectrometer is described. Its design philosophy and design are discussed. An example measurement, the depolarization ratio of carbon tetrachloride, a classic Raman test compound, is presented. The unique instrument features a blue-violet (405 nm wavelength) diode laser that takes advantage of the inverse fourth power wavelength dependence of Raman scattering. The converter also features Glan-Thompson polarizing prisms that enable measurement of depolarization ratios. The spectrometer is also capable of measuring a standard Raman spectrum. A fiber optic link offers flexibility when adapting the converter to any spectrometer system that accepts a fiber optic input. The performance of the instrument is critically discussed in the context of an example measurement.

Eye-safe actively Q-switched diode-pumped lasers with intracavity Raman conversion in YVO4, KGd(WO4)2, PbWO4, and Ba(NO3)2 crystals

We have investigated the self-Raman and intracavity Raman frequency conversion of the end-diode-pumped acoustooptic Q-switched Nd:YVO4 laser radiation to eye-safe radiation. The 1st Stokes oscillation in YVO4 (Nd:YVO4), KGd(WO4)2, PbWO4, and Ba(NO3)2 crystals excited by the laser radiation at a wavelength of 1342nm permits obtaining oscillations at 1524, 1496 or 1527, 1527, and 1562nm wavelengths, respectively. We have obtained pulsed lasing with a repetition rate of 2–20kHz, an output energy of 37–95µJ, and a duration of 6–8ns. We propose a model of the actively Q-switched intracavity Raman laser taking into account the real switching off/on times of the actively Q-switch and the excited state absorption in the active medium. We used this model to simulate the dynamics of the self-Raman Nd:YVO4 laser. The proposed model gives results that agree fairly well with the experimental data.

Diode-pumped actively Q-switched Tm:YAP/BaWO(4) intracavity Raman laser.

We report an intracavity Raman laser based on BaWO(4) Raman conversion in a diode-pumped actively Q-switched Tm:YAP laser for the first time. With an incident diode power of 17.5 W and a pulse repetition rate of 1 kHz, the maximum average output powers of 880 mW and 306 mW for the fundamental laser at 1.94 μm and the first Stokes laser at 2.36 μm were obtained, respectively. The pulse width and pulse energy of the first Stokes laser were 14.1 ns and 0.31 mJ, respectively. The Raman gain coefficient of the BaWO(4) crystal was estimated to be 1.1 cm/GW at 1.94 μm.

5.2-W high-repetition-rate eye-safe laser at 1525 nm generated by Nd:YVO₄₋YVO₄stimulated Raman conversion.

We report herein an efficient eye-safe Raman laser, which is based upon Nd:YVO₄₋YVO₄ and in-band pumped by a wavelength-locked laser diode array at 878.6 nm. By virtue of mitigated thermal load and improved pump absorption, a maximum average output power of 5.2 W at 1525 nm is obtained under the incident pump power of 30.6 W with the pulse repetition frequency of 140 kHz, corresponding to an optical efficiency of 17.0%.

200 ps pulse generation at 1180 nm with a SrWO4 Raman crystal pumped by a sub-nanosecond MOPA laser system

We are reporting Raman conversion experiments with a SrWO4 crystal in a single-pass traveling-wave setup. As a pump source we employed a master oscillator power amplifier (MOPA) laser system at 1064 nm, delivering 200 µJ, 500 ps long pulses at a 9 kHz maximum repetition rate with limited diffraction beam quality. At 2 kHz we obtained up to 45 µJ pulse energy and 200 ps pulse duration at 1180 nm, with a conversion efficiency of 22%. Up to the maximum incident average pump power of 1.35 W no serious beam quality degradation was observed, and we measured a factor M2 ⩽ 1.5 for the Raman shifted beam.