Efficient Raman Conversion Research Articles

Multiple visible wavelength switchable cascaded self-Raman laser based on selective wave-mixing mechanism

A diode-pumped solid-state laser with watts-level five switchable wavelengths spanning green to red is first experimentally demonstrated. The selective wave-mixing mechanism was introduced for multiple visible wavelength switchable output. The five visible wavelengths are generated by wave-mixing of the fundamental wave, the first-Stokes wave, and the second-Stokes wave in a Nd:YVO4 self-Raman laser. A β-barium borate crystal with a critical phase-matching angle cut is used for the wavelength conversion, and the angle-tuning method was used to achieve fast wavelength switching. A diffusion bonded YVO4/Nd:YVO4/YVO4 crystal was used to reduce the thermal lens effect and increase the effective length of the Raman gain medium for high output power and efficient Raman conversion. Under a pump power of 19.5 W, five visible laser output powers achieved for the green at 532 nm, lime at 559 nm, yellow at 588 nm, orange-red at 620 nm, and deep red at 657 nm are 4.01, 1.76, 2.54, 1.41, and 2.82 W, respectively. This laser system provides a convenient way for visible wavelength-switchable coherent light generation and may find potential applications where multiple visible wavelengths are required.

Highly efficient, high-energy, picosecond, synchronously pumped Raman laser at 1171 and 1217 nm based on PbMoO4 crystals with single and combined Raman shifts.

For the first time to our knowledge, the operation of a synchronously pumped ultrafast Raman laser that uses a PbMoO4 crystal as the active medium has been demonstrated. We achieved efficient Raman conversion in PbMoO4 from pumping 1063 nm into 1171 and 1217 nm, respectively, at single and combined frequency shifts on stretching and bending Raman modes. The output pulse energy (up to 160 nJ) and peak power (up to 11 kW) of the output picosecond radiation is the highest among all-solid-state synchronously pumped Raman lasers published to date. The strongest pulse shortening at 1217 nm down to 1.4 ps was obtained that is close to the bending mode dephasing time.

Mid-infrared 10-µJ-level sub-picosecond pulse generation via stimulated Raman scattering in a gas-filled revolver fiber

A sub-picosecond mid-infrared laser based on hollow-core silica fiber is demonstrated for the first time. By using deuterium-filled revolver fiber as an active medium, we realized efficient two-cascade Raman conversion 1.03 → 1.49 → 2.68 µm pumped by chirped pulses of a femtosecond ytterbium laser. The gas fiber Raman laser generates ∼920 fs pulses at 2.68 µm with output pulse energy as high as 10 µJ. It is shown that SRS can dominate other nonlinearities even in highly transient regime implemented in the mid-IR. The approach used may be applied to develop mid-IR laser sources of various types, such as frequency combs, supercontinuum and few-cycle pulse sources.

Two-wavelength Tm:YLF/KGW external-cavity Raman laser at 2197 nm and 2263 nm.

This paper presents a KGW Raman laser with an external-cavity configuration at the 2 µm region. The Raman laser is pumped by an actively Q-switched Tm:YLF laser, especially designed for this purpose emitting at 1880 nm. Due to the KGW bi-axial properties, the Raman laser is able to lase separately at two different output lines, 2197 nm and 2263 nm. The output energies and pulse durations that were achieved for these two lines are 0.15 mJ/pulse at 21 ns and 0.4 mJ/pulse at 5.4 ns, respectively. To the best of our knowledge, this is the first time that the KGW crystal, which is well known for its wide use in shorter wavelengths, is demonstrated in a Raman laser in the 2 µm region. According to the achieved results and due to the KGW properties, it appears to be a suitable crystal for energy scaling and efficient Raman conversion in this spectral range. An estimation of the Raman gain coefficient for this wavelength is provided as well.

Off-Peak Dual-Wavelength Multimode Diode-Laser- Pumped Fiber Raman Laser

We demonstrate a fiber Raman laser directly pumped by two spectrally combined multimode diode lasers at ~950 and ~976 nm. The emission wavelength becomes 1020 nm, which corresponds to the first Stokes of 976 nm. The pump separation of 270 cm−1 is both much smaller than the Raman peak shift of 440 cm−1 and much larger than the Raman linewidth of around 100 cm−1, which is unfavorable for efficient Raman conversion. Nevertheless, the addition of the pump at 950 nm triples the output power compared to pumping only at 976 nm. We reach 23 W of output power with slope efficiency of 51% and beam quality M2 of 5.2 in a germanosilicate graded-index fiber.

Efficient Raman frequency conversion by coherent feedback at low light intensity

We experimentally demonstrate efficient Raman conversion to respective Stokes and anti-Stokes fields in both pulsed and continuous modes with a Rb-87 atomic vapor cell. The conversion efficiency is about 40-50% for the Stokes field and 20-30% for the anti-Stokes field, respectively. This efficient conversion process is realized with coherent feedback of both the Raman pump and the frequency-converted fields (Stokes or anti-Stokes). The experimental setup is simple and can be applied easily to produce light sources with larger frequency shifts using other Raman media with long coherence time. They may have potential applications in nonlinear optics, Raman spectroscopy and precision measurement.

A high power diode-side-pumped Nd:YAG/BaWO4 Raman laser at 1103 nm

Pulsed operation at 1103 nm of a diode-side-pumped Nd:YAG laser with intracavity Raman shifting in BaWO4 is reported. The first Stokes wavelength at 1103 nm was generated by a Raman shift of 332 cm−1 from the fundamental wave (1064 nm). A maximum power at 1103 nm of 9.4 W was obtained for a diode pump power of 115 W at a pulse repetition rate of 15 kHz. The pump-to-Stokes conversion efficiency was up to 8.2%. When the output power at 1103 nm was over 7 W, a second Stokes line at 1145 nm was also observed in the experiment. Our research indicates that efficient Raman conversion can be realized by a Raman frequency shift at 332 cm−1 in BaWO4 Raman lasers.

Optical frequency comb generation in gas-filled hollow core photonic crystal fibres

The efficiency of gas-based nonlinear processes is often limited by the diffraction of the pump laser as it propagates through the nonlinear medium. As a consequence, phenomena with strong nonlinear response requirements, such as high harmonic generation or Raman sideband generation, lack the required laser–matter interaction to fulfil their potential. Indeed, the conversion efficiency of these techniques is usually low and the experimental set-up cumbersome. The advent of hollow core photonic crystal fibre technology drafts new territories for nonlinear optics, and in particular offers new alternatives for sub-femtosecond pulse generation. The air-guiding fibre combines unprecedented laser confinement over long interaction lengths and, when filled with an adequate nonlinear gas, offers improved conversion efficiency and up to a million-fold reduction of the pump power threshold. This paper presents a review of the types of hollow core PCF available for nonlinear applications and the results obtained for efficient Raman conversion in H2-filled hollow core PCF that led to the observation of a multi-octave frequency comb spanning from ~325 to ~2300 nm using a single pump laser with relatively low power. The generated ultra-broad spectrum creates a simple route towards a compact source of attosecond pulses.

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.

Compact efficient self-frequency Raman conversion in diode-pumped passively Q-switched Nd:GdVO4 laser

I report the first demonstration of the generation of efficient sub-nanosecond self-stimulated Raman pulses by a diode-pumped passively Q-switched Nd:GdVO4/Cr4+:YAG laser. The conversion efficiency for the average power is 7% from pump diode input to self-Raman output and the slope efficiency is up to 14%. At an incident pump power of 2.0 W, the pulse duration, pulse energy, and peak power for the Stokes wavelength of 1175.6 nm were found to be 750 ps, 6.3 μJ, and 8.4 kW, respectively, with a pulse-repetition rate of 22 kHz.

Efficient rotational Raman conversion of XeCl laser radiation in H 2

is investigated with a XeCl laser. It is shown that energy conversion to rotational Stokes radiation can be efficiently obtained by properly choosing the focusing geometry of pump radiation and the pressure of the Raman medium even at moderate pulse energies. Energy conversion to the first rotational Stokes at 313.8 nm with efficiencies as high as 38% is obtained with a circularly polarised XeCl pump beam of 10-mJ energy focused in 30 bar of H2. The spectral and optical characteristics of the pump and the rotational Stokes radiation are analysed.

Superluminal-source-induced emission

Raman emission by an axicon-generated superluminal source is demonstrated. The anti-Stokes radiation is emitted at an angle defined by a Cerenkov-type condition. This technique of superluminal-source generation has advantages in both low pump self-phase modulation and compensation of temporal walk-off caused by group-velocity dispersion, rendering the scheme useful for efficient Raman conversion, pump-probe measurements, and harmonic and optical parametric generation with ultrashort intense pulses.

Efficient Raman conversion through backward stimulated Brillouin scattering

We report a new scheme for efficient Raman conversion in high-pressure CH(4) gas. Through the use of backward stimulated Brillouin scattering as a resonator mirror for the pump wave at a wavelength of 1.06 µm, Raman laser generation at the eye-safe wavelength of 1.54 µm has been obtained from a passively Q-switched Nd:YAG laser. At a pressure of 600 psi, we obtained Raman conversion efficiencies of up to 48%.

Limitation of CO<inline-formula><sup><roman>2</roman></sup></inline-formula> laser-induced gas breakdown

To obtain economical extraction of ²³⁵U in the molecular laser isotope separation (MLIS) process, 16-μm laser beams must be generated in a parahydrogen Raman cell with high repetition rates and sufficient intensity. Because the intensities of the 16-μm laser beams are dependent on the intensity of the incoming pump laser beams, the intensity of the CO₂ lasers must be kept as high as possible. The maximum intensity has, however, been found to be controlled by the onset of gas breakdown in the Raman cell at too low a level for efficient Raman conversion. Through tests, the origin of gas breakdown in a 2-kHz-repetition-rate Raman cell is identified as particle contamination. The effect of the degree of contamination is determined and compared with experimental results. Conditions are set and modifications implemented on the Raman cell to ensure efficient Raman conversion.

Efficient Raman conversion of high repetition rate, 193 nm picosecond laser pulses

Efficient generation of Stokes lines has been obtained in H2, CH4, N2, and O2 by 10 ps, 300 Hz, 193 nm laser pulses. A 60% energy conversion efficiency was recorded for the first Stokes of H2 and 40% in CH4. For the pressure range 1–30 atm the conversion efficiency did not change with pump pulse repetition rate up to 300 Hz. The higher order Stokes observed were generated mainly by means of four wave mixing.

Efficient vibrational Raman conversion in O_2 and N_2 cells by use of superfluorescence seeding

We report first-Stokes vibrational conversion efficiency of 21% and 35%, respectively, in high-pressure O2- and N2-stimulated Raman cells. Broadband superfluorescence is employed to seed these Raman cells, significantly increasing the conversion efficiencies with no measured effect on the Raman spectrum. The addition of helium buffer gas reduces competition from stimulated Brillouin scattering and improves the pulse-to-pulse stability and spatial mode quality by increasing the thermal conductivity. Further improvement of the spatial mode quality is achieved by use of gentle heating on the bottom of the cell to induce convection.

Efficient anti-Stokes Raman conversion by four-wave mixing in gases

We describe a means for efficient, energy-scalable generation of first-order anti-Stokes light by four-wave mixing, using collimated pump and first-order Stokes seed beams. We performed a one-dimensional, plane-wave analysis of the process, assuming steady-state conditions and monochromatic beams. A prescription for designing the optimum anti-Stokes converter with this technique is derived from this analysis, and we show that energy conversion efficiencies from pump light to anti-Stokes light of ~30% might be possible. We also report an unoptimized, proof-of-concept experiment that produced 40 mJ of anti-Stokes-shifted light at 432 nm from 1.2 J of 527-nm pump light in a 60-cm-long hydrogen cell.

Efficient anti-Stokes Raman conversion in collimated beams

We have demonstrated 10% energy-conversion efficiency of 0.5-nsec FWHM laser pulses at 248 to 225 nm by an anti-Stokes process in hydrogen, using collimated (unfocused) laser beams. To accomplish this, we simultaneously seeded the Raman cell with a Stokes pulse that was 2–6% as intense as the pump at the phase-matching angle with respect to the pump beam axis. Under these conditions, a fully transient plane-wave calculation suggests that 44% energy-conversion efficiency is possible. Phase-front imperfections on the pump and Stokes beams are thought to limit the observed conversion efficiency. We present a simple model describing the effect of phase distortion on anti-Stokes production that agrees with our observations. Experimentally observed dynamic effects are in good agreement with theoretical predictions.

Higher-stokes order Raman conversion to the near infrared: High efficiency and brightness via a capillary waveguide amplifier

Efficient Raman conversion in H 2 gas from 0.53 μm to third Stokes (S 3 at 1.58 μm has been demonstrated using an oscillator/ amplifier configuration. Seed radiation from the oscillator controls the divergence of the Stokes beams and a capillary amplifier configuration enhances the conversion. An overall 9% photon conversion efficiency to S 3 is achieved with the S 3 beam divergence twice the diffraction limit. Photon conversion efficiency within the amplifier is ≈ 20%.

High efficiency and high brightness Raman conversion of dye laser radiation

A technique for efficient Raman conversion of an excimer pumped dye laser is described, based on the use of a Raman oscillator/amplifier combination. A photon conversion efficiency of 90% to first Stokes is demonstrated, with diffraction-limited beam quality.