Crystalline Raman Laser Research Articles

High-power and narrow-linewidth nanosecond pulsed intracavity crystalline Raman laser operating at 1.7 µm

We report on a high-power and narrow-linewidth nanosecond pulsed intracavity crystalline Raman laser at 1.7 µm. Driven by an acousto-optically Q-switched 1314 nm two-crystal Nd:YLF laser, the highly efficient cascaded YVO4 Raman laser at 1715nm was obtained within the well-designed L-shaped resonator. Thanks to the absence of spatial hole burning in the stimulated Raman scattering process, significant spectral purification of second-Stokes radiation was observed by incorporating a fused silica etalon in the high-Q fundamental cavity. Under the repetition rate of 4 kHz, the highest average output power for single longitudinal mode operation was up to 2.2 W with the aid of precision vibration isolation and precision temperature controlling, corresponding to the pulse duration of ∼2.8 ns and the spectral linewidth of ∼330 MHz. Further increasing the launched pump power, the second-Stokes laser tended toward be always multimode, and the maximum average output power amounted to 4.8 W with the peak power of ∼0.8 MW and the spectral linewidth of ∼0.08 nm. The second-Stokes emission was near diffraction limited with M2 < 1.4 across the whole pump power range.

Modelling of synchronously pumped crystalline Raman laser with a gain reduction

In this paper, a model of synchronously pumped picosecond Raman laser based on a solid state Raman material is presented. The commonly used set of equations describing Raman interaction in transient regime has been enhanced by a gain reduction according to the simplified theory of spectral effects in stimulated Raman scattering. This approach fundamentally modifies previously published numerical results. In addition, the effect of group delay dispersion is commented.

High beam quality yellow laser at 588 nm by an intracavity frequency-doubled composite Nd:YVO4 Raman laser.

High beam quality 588 nm radiation was realized based on a frequency-doubled crystalline Raman laser. The bonding crystal of YVO4/Nd:YVO4/YVO4 was used as the laser gain medium, which can accelerate the thermal diffusion. The intracavity Raman conversion and the second harmonic generation were realized by a YVO4 crystal and an LBO crystal, respectively. Under an incident pump power of 49.2 W and a pulse repetition frequency of 50 kHz, the 588 nm power of 2.85 W was obtained with a pulse duration of 3 ns, corresponding to a diode-to-yellow laser conversion efficiency of 5.75% and a slope efficiency of 7.6%. Meanwhile, a single pulse's pulse energy and peak power were 57 µJ and 19 kW, respectively. The severe thermal effects of the self-Raman structure were overcome in the V-shaped cavity, which has excellent mode matching, and combined with the self-cleaning effect of `Raman scattering, the beam quality factor M2 was effectively improved, which was measured optimally to be Mx 2 = 1.207, and My 2 = 1.200, with the incident pump power being 49.2 W.

Power Scaling of CW Crystalline OPOs and Raman Lasers

Optical parametric oscillators (OPOs) and Raman lasers are two nonlinear-based laser technologies that extend the spectral range of conventional inversion lasers. Power and brightness scaling of lasers are significant for many applications in industry, medicine, and defense. Considerable advances have been made to enhance the power and brightness of inversion lasers. However, research around the power scaling of nonlinear lasers is lacking. This paper reviews the development and progress of output power of continuous-wave (CW) crystalline OPOs and Raman lasers. We further evaluate the power scalability of these two laser technologies by analyzing the cavity architectures and gain materials used in these lasers. This paper also discusses why diamond Raman lasers (DRLs) show tremendous potential as a single laser source for generating exceedingly high output powers and high brightness.

Nanosecond pulsed deep-red laser source by intracavity frequency-doubled crystalline Raman laser.

We demonstrated a deep-red laser source by intracavity frequency-doubled crystalline Raman laser for the first time, to the best of our knowledge. The actively Q-switched 1314 nm Nd:LiYF4 laser was first converted to the eye-safe Raman laser using a KGd(WO4)2 (KGW) crystal, which was subsequently frequency-doubled in a bismuth borate crystal. Benefiting from the KGW bi-axial properties, the deep-red laser source was able to lase separately at two different spectral lines at 730 and 745 nm. Under an optimal repetition rate of 4 kHz, the maximum average powers of 1.7 and 2.0 W were attained with good beam quality of M2≈1.7. The corresponding pulse durations were determined to be 3.0 and 2.8 ns with the peak powers up to approximately 140 and 180 kW, respectively.

Efficient continuous-wave eye-safe Nd:YVO4 self-Raman laser at 1.5 µm.

Due to a high Raman threshold and serious thermal effect, a challenge is to achieve efficient continuous-wave (CW) operation of a crystalline Raman laser at 1.5 µm. Based on effective thermal management and the self-Raman effect, we demonstrate, to our knowledge, the first efficient CW operation of a Nd:YVO4 Raman laser at 1.5 µm. We achieve 685 mW of CW eye-safe emission at 1524.5 nm by the use of a 20-mm-long composite Nd:YVO4 and 300-µm pump beam radius, with a diode-to-Stokes conversion efficiency of 4.8%. Lasers operating at ∼1.5µm have found many important applications in various areas such as optical communication, laser radar, laser ranging, remote sensing, and spectral research.

Single-longitudinal-mode cascaded crystalline Raman laser at 1.7 µm.

A single-longitudinal-mode crystalline Raman laser in the 1.7 µm wave band was reported for the first time, to the best of our knowledge. The YVO4 Raman laser, which was intracavity-pumped by an actively Q-switched 1314 nm Nd:YLF laser, demonstrated the cascaded Stokes oscillation at 1715 nm. By inserting an etalon in the fundamental resonator, linewidth narrowing and power scaling of the second-Stokes laser were realized based on the spatial-hole-burning-free Raman gain. With an optimal pulse repetition frequency of 4 kHz, the maximum single-longitudinal-mode average output power of 1.8 W was acquired with the spectrum linewidth of ∼340MHz. Further increasing the incident pump power, the second-Stokes laser transitioned to multimode regime, and the maximum average output power reached 2.7 W with the peak power as high as ∼380kW.

50-µJ level, 20-picosecond, narrowband difference-frequency generation at 4.6, 5.4, 7.5, 9.2, and 10.8 µm in LiGaS2 and LiGaSe2 at Nd:YAG laser pumping and various crystalline Raman laser seedings

A comparative study of picosecond mid-IR difference frequency generation in a wide wavelength range of 4.6-10.8 µm in a simple down-converter based on the 8-mm long, high-damage-threshold crystals of LiGaS2 or LiGaSe2 under the 5-mJ, 20-ps, 1.064-µm Nd:YAG laser pumping and the single-pass crystalline (CaCO3, BaWO4, CVD-diamond) Raman laser seeding was presented. 10-µJ-level, narrowband (<2 cm−1) generation at discrete wavelengths of 4.6, 5.4, 7.5, and 9.2 µm with the optical frequencies equal to the vibrational Raman frequency or its second harmonic of various crystalline Raman seeders was demonstrated. Optimization of the pump spot and mode-matching between the pump and signal spots allowed to increase the output pulse energy up to 50 µJ in the case of LiGaSe2.

860 fs GdVO4 Raman laser at 1228 nm pumped by 36 ps, 1063 nm laser

The first femtosecond crystalline Raman laser pumped by a picosecond laser is demonstrated. The method consisted of a synchronously pumped cascade SRS process. This process is started with extracavity Raman conversion from pump radiation into the long-shifted Stokes component and continues with secondary intracavity Raman conversion from the long-shifted Stokes component into the Stokes component with combined long and short Raman shift. This secondary conversion shows strong pulse shortening down to the inverse width of the widest short-shift Raman line. We report characteristics of the all-solid-state extracavity Raman laser based on the 16 mm long a-cut GdVO4 crystal under synchronous pumping by the 1063 nm 36 picosecond 150 MHz Nd:GdVO4 laser. Cascade two-wavelength Raman generation in the external high-Q ring cavity has been obtained not only at the usual wavelength of 1174 nm corresponding to the long-shifted first Stokes component, but also at the unusual wavelength of 1228 nm having the combined long and short Raman shift. The individual pulse energy of the 1228 nm radiation was up to 10 nJ and slope efficiency of 5%. Significant shortening of the 36 ps pump pulse down to 860 fs at 1228 nm has been demonstrated without using any compressor.

Temperature Dependence of Raman Frequency Shift in SrWO4 Crystal Studied by Lattice Dynamical Calculations

The frequency shift of the Raman modes in strontium tungstate (SrWO4) was investigated in the temperature range from 15 to 295 K. The experimental temperature dependence of the shift was analyzed using both the lattice dynamical calculations and the lattice perturbative approach. We found that the quartic anharmonic term of the first-order perturbation and the cubic term of the second-order perturbation, as well as the thermal expansion, contribute to the temperature shift of the highest-frequency Ag(ν1) mode. The values of the temperature sensitivity of the frequency shift of the Raman modes at room temperature were measured, which is important for developing high-power crystalline Raman lasers and frequency shifters.

Cascaded crystalline Raman lasers for extended wavelength coverage: continuous-wave, third-Stokes operation

The development of high-power laser sources with narrow emission, tunable within the water transmission window around 1.7 μm, is of interest for applications as diverse as medical imaging and atmospheric sensing. Where suitable laser gain media are not available, operation in this spectral region is often achieved via nonlinear frequency conversion, and optical parametric oscillators (OPOs) are a common solution. A practical alternative to OPOs, to avoid birefringent- or quasi-phase-matching requirements, is the use of stimulated Raman scattering within a suitable material to convert a pump source to longer wavelengths via one or more Stokes shifts; however, as this is a χ(3) nonlinear process, such frequency conversion is usually the preserve of high-energy pulsed lasers. Semiconductor disk lasers (SDLs), on the other hand, have very high-finesse external resonators, suitable for efficient intracavity nonlinear conversion even in continuous-wave (CW) operation. Here we report, to the best of our knowledge, the first continuous-wave third-Stokes crystalline Raman laser and the longest emission wavelength from an SDL-pumped Raman laser, achieving high power, CW output, and broad wavelength tuning around 1.73 μm. The KGd(WO4)2 (KGW) Raman laser, which was intracavity-pumped by a 1.18 μm InGaAs-based SDL, demonstrated cascaded CW Stokes oscillation at 1.32 μm, 1.50 μm, and 1.73 μm with watt-level output achievable at each wavelength. The 1.73 μm Stokes emission was diffraction limited (M2<1.01) and narrow linewidth (<46 pm FWHM; measurement limited). By rotation of a birefringent filter placed within the fundamental resonator, we attained three tunable emission wavelength bands, one centred at each Stokes component, and achieved up to 65 nm tuning for the third-Stokes Raman laser from 1696 nm to 1761 nm. We have thus demonstrated a platform laser technology that takes well-developed InGaAs-based SDLs and provides spectral coverage and high performance in the near-infrared water transmission windows using commercially available components.

A pulsed single-frequency Nd:GGG/BaWO4 Raman laser

A single-frequency pulsed laser at 1178.3 nm was demonstrated in a crystalline Raman laser. A crystal combination of Nd:GGG and BaWO4 was selected to realize Raman conversion from a 1062.5 nm fundamental wave to a 1178.3 nm Stokes wave. An entangled cavity was specially designed to form an intracavity Raman configuration. Single-longitudinal-mode operation was realized by introducing two Fabry–Perot etalons into the Raman laser cavity. This laser operated at a pulse repetition rate of 50 Hz with 2 ms long envelopes containing micro pulses at a 30 kHz repetition rate. The highest output power was 41 mW with the micro pulse duration of 15 ns. The linewidth was measured to be less than 130 MHz.

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.

Characteristics of the temperature-tunable Nd:YAG/YVO_4 Raman laser

We demonstrate a tunable crystalline Raman laser by varying the temperature of Raman crystal. Nd:YAG and YVO(4) crystals were selected as the laser and Raman gain media, respectively. The center wavelength of this Nd:YAG/YVO(4) Raman laser was tuned over a 0.49 nm range from 1175.76 to 1175.27 nm when the temperature of the Raman crystal was adjusted from 5 °C to 150 °C. The characteristics of this Raman laser including tunability, output power, and beam quality factors (M(2)) dependent on temperature were also studied in this paper.

Continuous-wave Watt-level Nd:YLF/KGW Raman laser operating at near-IR, yellow and lime-green wavelengths

A Nd:YLF/KGW Raman laser has been investigated in this work. We have demonstrated CW output powers at six different wavelengths, 1147 nm (0.70 W), 1163 nm (0.95 W), 549 nm (0.65 W), 552 nm (1.90 W), 573 nm (0.60 W) and 581 nm (1.10 W), with higher peak powers achieved under quasi-CW operation. Raman conversion of the 1053 nm fundamental emission is reported for the first time, enabling two new wavelengths in crystalline Raman lasers, 549 nm and 552 nm. The weak thermal lensing associated with Nd:YLF has enabled to achieve good beam quality, M(2) ≤ 2.0, and stable operation in relatively long cavities.

Miniature wavelength-selectable Raman laser: new insights for optimizing performance

We report a miniature, wavelength-selectable crystalline Raman laser operating either in the yellow (588 nm) or lime (559 nm) selected simply by changing the temperature of an intracavity LBO crystal. Continuous-wave (CW) output powers are 320 mW and 660 mW respectively, corresponding to record diode-visible optical conversion efficiencies of 8.4% and 17% for such miniature devices. The complex laser behavior arising from interplay between nonlinear processes is studied experimentally and theoretically. We show that the interplay can lead to complete suppression of the first-Stokes field and that the phase matching conditions for maximum visible powers differ markedly for different length LBO crystals. By using threshold measurements, we calculate the round-trip resonator losses and show that crystal bulk losses dominate over other losses. As a consequence, Raman lasers utilizing shorter LBO crystals for intracavity frequency mixing can produce higher visible output power. These are new considerations for the optimum design of CW intracavity Raman lasers with visible output.

A continuous wave SrMoO_4 Raman laser

We demonstrate a cw, laser diode-pumped Nd:GdVO4/SrMoO4 crystalline Raman laser. First Stokes laser output at 1173.5 nm of 2.18 W was achieved with a diode-to-first Stokes efficiency of 8.7%. With intracavity frequency doubling in LiB3O5, 3.1 W of cw yellow emission at 586.8 nm was obtained with a 12.4% diode-to-yellow efficiency. The experimental results show that SrMoO4 is an excellent stimulated Raman scattering gain material for high-power cw near-IR Stokes and yellow lasers.

Side-pumped crystalline Raman laser

A crystalline Raman laser is pumped at 90° to the Raman laser axis by a single pass from a line-focused 532 nm pump laser of pulse duration 10 ns. The Raman laser threshold was 6.1 mJ, and at 12 mJ pump energy, a maximum output energy of 2.7 mJ was obtained with a slope efficiency of 46%. The threshold pump intensity is within a factor of 2 of the same device when end-pumped. The results highlight significant potential for coherent beam conversion and combination with enhanced degrees of flexibility and increased power.

Near-infrared and orange–red emission from a continuous-wave, second-Stokes self-Raman Nd:GdVO_4 laser

We report the first cw intracavity crystalline Raman laser operating at the second-Stokes wavelength. Approximately 950 mW emission is obtained at 1308 nm from a Nd:GdVO(4) self-Raman laser, with an overall (diode to 1308 nm) conversion efficiency of 6.8%. By intracavity sum-frequency mixing the first- and second-Stokes lines, 1027 mW emission at 620 nm is demonstrated with an overall (diode-to-visible) conversion efficiency of 4.9%.

Optical field dynamics in a wavelength-versatile, all-solid-state intracavity cascaded pulsed Raman laser

Monitoring the optical field energies in an intracavity cascaded crystalline pulsed Raman laser enables us to probe the dynamics and optimise the performance of the laser, which produces up to 2.1 W average output power at 4 wavelengths between 532 and 636 nm selectable by simple angle tuning.