Compact, Low Threshold Nd^3＋：YVO4 Self-Raman Laser at 1178 nm

A compact low-threshold Raman laser at 1178 nm is experimentally realized by using a diode-end-pumped actively Q-switched Nd^3＋ ：YVO4 self-Raman laser. The threshold is 370mW at a pulse repetition frequency of S kHz. The maximum Raman laser output is 182 m W with the pulse duration smaller than 20 ns at a pulse repetition frequency of 30kHz with 1.8 W incident power. The optical efficiency from the incident power to the Raman laser is 10% and the slope efficiency is 13.5%.     

High Power Photonic Crystal Fibre Raman Laser

A cw Raman laser based on a 100-m photonic crystal fibre is demonstrated with up to 3.8 W output power at the incident pump power of 12 W, corresponding to an optical-to-optical efficiency of about 31.6%.. The second order Stokes light, which is firstly reported in a cw photonic crystal fibre Raman laser, is obtained at 1183nm with an output power of 1.6 W and a slope efficiency of about 45.7%.     

Polarization-resolved analysis of the characteristics of a Raman laser made with a polarization maintaining fiber

We derive a simple model describing steady characteristics of Raman fiber lasers made with polarization maintaining fibers. We show both theoretically and experimentally that this kind of laser simply consists of two independent Raman lasers linearly polarized along the fiber birefringence axes. The output power characteristics of the laser are shown not to be influenced by optical Kerr effect. Finally, we use our model to propose answers to questions recently raised about efficiency of Raman lasers made with polarization maintaining fibers.     

Ring-down spectroscopy for studying properties of CW Raman lasers

We describe theoretically a simple technique for analyzing the conversion efficiency and the oscillation threshold of all-resonant intracavity Raman lasers. The method is based on a dependence of the ring-down time of the pump cavity mode on the energy accumulated in the cavity.     

Low threshold, actively Q-switched Nd:YVO4 self-Raman laser and frequency doubled 588 nm yellow laser

We reported an actively Q-switched, intracavity Nd³⁺:YVO₄ self-Raman laser at 1176 nm with low threshold and high efficiency. From the extracavity frequency doubling by use of LBO nonlinear crystal, over 3.5 mW, 588 nm yellow laser is achieved. The maximum Raman laser output at is 182 mW with 1.8 W incident pump power. The threshold is only 370 mW at a pulse repetition frequency of 5 kHz. The optical conversion efficiency from incident to the Raman laser is 10%, and 1.9% from Raman laser to the yellow.     

Sub-nanosecond microchip laser with intracavity Raman conversion

Efficient sub-nanosecond pulse operation of microchip lasers with intracavity Raman conversion and pulse compression is presented for the first time. The microchip lasers were composed of Nd:LSB or Nd:YAG laser crystals, Cr⁴⁺:YAG saturable absorber, and Ba(NO₃)₂ Raman medium. The pulse duration obtained at the Stokes wavelength (1196 nm) was as short as 118 ps. Optical conversion efficiency of laser-diode pump power to the Stokes power of 8% was reached. Pulse energy and peak power of Stokes emission were 1.2 μJ and 5.4 kW, correspondingly. Numerical calculations are in good agreement with obtained experimental results. Received: 20 December 2002 / Revised version: 6 March 2003 / Published online: 5 May 2003 RID="*" ID="*"Corresponding author. Fax: +1-716/645-6945, E-mail: ankuzmin@acsu.buffalo.edu RID="**" ID="**"Present address: University at Buffalo, SUNY, The Institute for Lasers, Photonics, and Biophotonics, 458 NSC, Buffalo, NY 14 260-3000, USA     

Efficient cw sodium dimer Raman laser operation in a high-temperature sapphire cell

cw Raman lasing of Na₂ molecules generated in a heated, sealed-off, all-sapphire cell is demonstrated. Being not damaged by highly corrosive alkaline vapours, this type of cell enables operation without buffer gas in contrast to the normal heatpipe operation of these lasers. This allows us to study Raman lasers in alkaline vapours in new regimes and under ideal conditions. With an argon ion pump laser at 488 nm, Raman laser operation at 525 nm with more than 10% efficiency and thresholds below 0.2 mW for a cell without buffer gas (length 9 cm) have been obtained so far. The low thresholds, being a factor of 10 less than for comparable heatpipe operation, gives us the chance to use low-power diode lasers as pump sources and to realize compact reliable Raman laser systems. Received: 17 May 1999 / Published online: 25 August 1999     

Laser operation and Raman self-frequency conversion in Yb:KYW microchip laser

We present a diode-pumped Yb:KYW microchip laser. The passive Q-switchedand CW regimes of operation for the Yb:KYW microchip laser have been investigated. An efficiency for CW operation of up to 10% with regard to incident pump has been obtained. Raman self-frequency conversion in the Q-switched regime has been observed. Received: 27 May 2002 / Revised version: 4 July 2002 / Published online: 15 November 2002 RID="*" ID="*"Corresponding author. Fax: +375-17/2840-879, E-mail: asg@dragon.bas-net.by     

Asymmetrical spectral broadening of SRS and its temperature dependence in a phenylethanol liquid-core wave-guide optical fiber

Spectral broadening of stimulated Raman scattering has been studied in a phenylethanol liquid-core wave-guide optical fiber by using a frequency-doubled Q-switched Nd: YAG laser. Four orders of Stokes lines were observed. All the Stokes lines were asymmetrically broadened favoring the lower frequencies. Both the red-shifted broadening and the blue-shifted broadening increased with the rise of the temperature of the Raman medium. The asymmetric broadening was discussed to be mainly due to the effect of the cascaded stimulated Rayleigh-wing scattering.     

Efficient generation of FIR radiation by optical pumping of D2 18O

In this paper we show that D₂ ¹⁸O vapour, optically pumped with a continuously tunable high pressure CO₂ laser, is an excellent source for far infrared radiation. Both high photon conversion coefficients and broad Raman gain regions were found for a large number of new laser transitions spread over the frequency range from 25 cm^–1 to 240 cm^–1. We demonstrate that these Raman gain regions can be used to generate far infrared laser pulses with high intensity and durations of about 100 ps.     

Stimulated Raman Scattering in a Weakly Polar Ⅲ-Ⅴ Semiconductor： Effect of dc Magnetic Field and Free Carrier Concentration

Using the hydrodynamic model of semiconductor plasmas, we perform an analytical investigation of stimulated Raman scattering （SITS） of an electromagnetic pump wave in a transversely magnetized weakly polar semicon- ductor arising from electron-density perturbations and molecular vibrations of the medium both produced at the longitudinal optical phonon frequency. Assuming that the origin of SItS lies in the third-order susceptibility of the medium, we investigate the growth rate of Stokes mode. The dependence of stimulated Raman gain on the external dc magnetic field strength and free carrier concentration is reported. The possibility of the occurrence of optical phase conjugation via SItS is also studied. The steady-state Raman gain is found to be greatly enhanced by the presence of the strong external dc magnetic field.     

Continuous-wave intracavity Raman laser at 1179.5 nm with SrWO<Subscript>4</Subscript> Raman crystal in diode-end-pumped Nd:YVO<Subscript>4</Subscript> laser

We report a continuous-wave intracavity Raman laser at 1179.5 nm with a SrWO₄ Raman crystal in a diode-end-pumped Nd:YVO₄ laser. The highest output power of 2.23 W is obtained at the laser diode power of 21.2 W corresponding to the slope efficiency of 17.3% and a diode-to-stokes optical conversion efficiency of 10.5%. The dependence of the Raman laser performance on the pump polarization is also studied. The measured Raman thresholds are about 9.3 and 8.3 W in the diode pump laser power for the a- and b-polarized configurations, respectively. The Raman gain coefficients of the c-cut SrWO₄ crystal for a- and b-polarized pumps are estimated to be about 4.9 and 4.7 cm/GW, respectively.     

Raman spectroscopy of SWNTs produced by a XeCl excimer laser ablation at high temperatures

Synthesis of single-wall carbon nanotubes (SWNTs) was carried out by an ablation method using a XeCl excimer laser. It was irradiated onto a graphite target containing Co and Ni at the temperatures of 1073, 1173, 1273, 1373, 1473, 1523 and 1623 K under the atmosphere (0.1 MPa) of Ar gas with the flow rate of 12 ml/min. The measurement by a scanning/transmission electron microscope and Raman spectroscopy found the formation of SWNTs with the diameter of about 1.3 nm and the length of about 2 μm in ablated carbonaceous soot. The ratio of peak intensity of 1590 cm⁻¹ (G band) to that of 1335 cm⁻¹ (D band) in the high frequency Raman spectra increased with increasing the ambient temperature. The radial breathing mode (RBM) in the low frequency Raman spectra shows that the mean diameter of SWNTs increased with increasing the ambient temperature.     

Passively Q-switched Nd:YAG pumped UV lasers at 280 and 374 nm

Pulsed UV lasers at the wavelengths of 374 and 280 nm are realized by cascaded second harmonic generation (SHG) and sum frequency generation (SFG) processes using a Nd:YAG laser at 1123 nm. The Nd:YAG laser is longitudinally pumped and passively Q-switched, and it has a high peak power of 3.2 kW. The UV peak powers at 280 and 374 nm are 100 and 310 W, with pulse lengths of 6 and 8 ns, respectively. Spectral broadening of 374 nm laser by stimulated Raman scattering is studied in single mode pure silica core UV fiber. Realizations of UV lasers enabling compact design at 280 and 374 nm wavelengths are demonstrated.     

Intracavity Raman conversion for 16-μm Stokes pulse generation in para-hydrogen

16-μm Stokes pulses were directly generated for the first time to our knowledge by an intracavity configuration for the para-hydrogen Raman laser. We have analyzed Stokes field growth using a focused gain model and designed a pump/Stokes cavity to satisfy CO₂ pump power and pulse duration requirements for Raman oscillation. The CO₂ laser oscillation with circular polarization was realized by seeding externally circularly polarized CO₂ radiation. An output energy of 2.4 mJ was obtained with the output coupler of 0.5% transmittance, which indicated that 420 mJ of Stokes pulse energy was stored inside the cavity. This suggests that a much higher energy can be extracted by the optimization of cavity parameters. Received: 18 November 1998 / Published online: 26 May 1999     

Far-infrared laser-emission spectroscopy on ammonia isotopomers

In this paper we report the investigation of the isotopic ammonia gases¹⁴NH₃,¹⁴NH₂D,¹⁴NHD₂,¹⁵NH₃ and¹⁵ND₃ optically pumped by a continuously tunable 20 atm CO₂-laser. We found 267 far-infrared emission lines, produced by both inversion and Raman processes, in the frequency range from 15 cm^–1 to about 250 cm^–1. 200 of these were observed for the first time. We obtained far-infrared Raman tuning ranges exceeding 3 cm^–1, photon conversion coefficients up to 12% and maximum output energies of 0.9 mJ. Furthermore, the optimum gas pressure, the polarization and the transient behavior of optically pumped ammonia have been investigated.     

Nonlinear mirror based on raman interactions

We analyze a nonlinear mirror based on Raman interactions. The mirror is a combination of a Raman-active medium and a tetrachroic mirror. The reflections of the nonlinear mirror at two input frequencies differing by the Raman shift are calculated and its properties and possible applications are discussed.     

Highly efficient Raman conversion in O2 pumped by a seeded narrow band second-harmonic Nd:YAG laser

The first-Stokes conversion efficiency for a stimulated Raman scattering (SRS) is usually very low in gaseous oxygen media. In 3.0 Mpa O₂, a single longitudinal mode second harmonic Nd:YAG laser pump source gives a typical vibrational first-Stokes conversion efficiency of only 2.5%, In comparison, the accompanying stimulated Brillouin scattering (SBS) attains a reflectivity of 67%. However, by seeding an OPO beam into the Raman cavity, the first-Stokes photon conversion efficiency now attains a peak value of 54%, while the SBS reflectivity reduces to 5% in a 6.1 Mpa 41:59 O₂/ He mixture. This 54% efficiency was obtained for a seeder laser pulse-width less than one half that of pump laser (6.8 ns). A first-Stokes peak power conversion efficiency as high as 88% has been obtained when the pump and seeder pulse peaks coincide. So, we may expect a higher first-Stokes photon conversion efficiency if the seeder pulse-width can be made equal to or larger than that of the pump pulse. On the other hand, the beam quality of the first-Stokes in an O₂/ He mixture excels that of the pump laser for a seeder energy of 5 mJ and pump energy of 50 mJ. However, at pump energies higher than 105 mJ and a pump laser repetition rate of 10 Hz, the thermal defocusing effect worsens the first-Stokes beam quality. This thermal defocusing effect is a result of the Raman heat release and could be eliminated by fast circulating and cooling the Raman gas medium.     

High Efficient C6H12 Raman Laser Enhanced by DCM Fluorescence

We report the first-order Stokes output （wavelength of 627.6 nm） from C6H12 enhanced by DCM dye fluorescence with high energy conversion efficiency of 47.9%, quantum conversion efficiency of 56.5%. To our knowledge, it is the highest conversion efficiency of stimulated Raman scattering obtained from liquid Raman laser. A 532nm frequency doubled Nd：YAG laser with 8 Hz repetition rate is employed as the pump source, and the enhancement medium is DCM dye solution in ethanol. The conversion efficiencies at various pump energies and various pump repetition rates are measured and analysed. The enhancement mechanism of SRS together with its potential application is discussed.     

Threshold and conversion experiments on seeded stimulated Raman scattering

High energy-conversion efficiencies in Stimulated Raman Scattering (SRS) are demonstrated both in experiments and by simulations for pump powers below SRS threshold. The scattering is induced by a short seed pulse at the Stokes frequency, the pulse width of which is much shorter than the pump pulse width and which is comparable with the medium's dephasing time.