Extracavity Raman Laser Research Articles

Investigation of high-energy extracavity Raman laser oscillator and single-pass Raman generator based on potassium gadolinium tungstate (KGW) crystal

Herein, a high-energy all-solid-state extracavity Raman oscillator and a high-energy single-pass Raman generator based on potassium gadolinium tungstate (KGW) crystal are theoretically and experimentally investigated. First, a high-energy 1064 nm nanosecond laser system with a maximum output energy of 7.03 J at a repetition rate of 1 Hz is built as the pumping source for stimulated Raman scattering (SRS). Then, the output spectra and energy of multiorder Stokes lasers from 1.16 to 1.49 μm are gradually detected as the 1064 nm pumping laser energy increases for both Raman laser structures. For the extracavity Raman oscillator, under a pumping energy of 1.8 J, three-order Stokes lasers with a maximum energy of 448 mJ are obtained when the electric field direction of the pumping laser is parallel to the Ng optical axis of the KGW crystal (E∥Ng). Moreover, to obtain a higher output of the Raman laser without damaging the cavity mirrors, a single-pass Raman generator is studied for comparison purposes. As the 1064 nm pumping laser energy of 2.8 J is injected, the measured maximum output energy of a two-order Stokes laser is ~ 676 mJ for the E∥Ng condition; this is the highest Stokes energy output of the nanosecond solid-state Raman lasers to the best of our knowledge.

Synchronously-pumped, all-solid-state, picosecond Raman laser at 1169 and 1222 nm on single and combined Raman modes in a Ca3(VO4)2 crystal with 30-times pulse shortening down to 1.2 ps

An all-solid-state extracavity synchronously pumped Ca3(VO4)2 Raman laser on broadened stretching (854 cm−1) and bending (354 cm−1) Raman modes with efficient stimulated Raman scattering (SRS) conversion at single frequency shift and the strongest 30-times pulse shortening down to 1.2 ps at combined frequency shift is demonstrated. The antireflection-coated, 78 mm long, a-cut Ca3(VO4)2 crystal was used as an active element under synchronous pumping by the 330 nJ, 1063 nm, 36 ps, 150 MHz mode-locked Nd:GdVO4 laser. SRS oscillation with slope efficiency of 34.2% and conversion of 18.5% at the wavelength of 1169 nm of the 854 cm−1-shifted first Stokes component with the pulse duration of 31 ps and output energy of up to 61 nJ in the external ring cavity with the output coupler reflectivity of 87% has been obtained. In the case of the second setup with high cavity Q-factor for the wavelength of 1169 nm, the additional cascade SRS component at 1222 nm with combined (854 cm−1 + 354 cm−1) frequency shift was observed. This component was generated with the slope efficiency of 8.1% and the output energy of 14.2 nJ with the output coupler reflectivity of 95%. Due to dynamics of SRS build-up in synchronously pumped extracavity Raman laser, the strongest pulse shortening at 1222 nm was achieved. The shortest pulse duration of 1.27 ps was measured at the cavity length detuning of +50 µm.

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.

Multi-wavelength picosecond BaWO4 Raman laser with long and short Raman shifts and 12-fold pulse shortening down to 3 ps at 1227 nm

In this paper, we demonstrate the generation of three (1179, 1227, and 1323 nm) Stokes components of stimulated Raman scattering with long (925 ) and short (332 ) Raman shifts in an all-solid-state, synchronously pumped, extra-cavity Raman laser based on a BaWO4 crystal excited by a quasi-continuous, 36 ps, diode side-pumped Nd:GdVO4 laser generating at the wavelength of 1063 nm. We achieved the strongest 12-fold pulse shortening down to 3 ps at the 925 + 332 shifted 1227 nm wavelength due to a shorter dephasing time (wider linewidth) of the short-shift 332 Raman line, resulting in a peak power of 2.5 kW.

Eye-Safe Extracavity Raman Laser: a Passive Way of Eliminating Optical Feedback with Double-Pass Pumping

A passive way of eliminating optical feedback between a pump laser and an extracavity two-pass pumped Raman laser in a compact arrangement was demonstrated. Feedback was suppressed by arranging the circuit elements in a configuration where the pump-laser 100% mirror and the Raman-laser output mirror formed an unstable cavity characterized by a substantially higher threshold for generating the fundamental radiation as compared with the stable pump-laser cavity. The method was demonstrated using an eye-safe KGd(WO4)2 Raman laser pumped by a pulsed KGd(WO4)2:Nd laser with the working transition $$ {}^{{}^4}F_{3/2}\to {}^4I_{13/2} $$ as an example. The Raman laser emitted pulses of energy ~11.5 mJ at an electrical pump energy of ~10 J, corresponding to a conversion efficiency of ~31%. The Stokes pulse width (~17 ns) was comparable to the pump pulse width (~20 ns) owing to the extracavity location of the Raman laser.

Extracavity pumped BaWO_4 anti-Stokes Raman laser

The characteristics of a barium tungstate (BaWO(4)) anti-Stokes Raman laser at 968 nm are studied theoretically and experimentally. The BaWO(4) Raman resonator is pumped by a Q-switched Nd:YAG laser at 1064 nm with its axis tilted from the pumping laser axis. The non-collinear phase matching for the generation of the first anti-Stokes wave in the same BaWO(4) crystal is achieved. The output energy, temporal and spectral informations are investigated. At a pumping laser energy of 128 mJ, the anti-Stokes laser energy obtained is 2.2 mJ. The second Stokes radiation at 1324 nm as well as the first and the third Stokes waves at 1180 nm and 1509 nm is also generated at the same time. The maximum total Stokes energy output is 42.5 mJ. In the theory, the anti-Stokes laser intensity expression as a function of the pumping and the first Stokes laser intensities for the extracavity anti-Stokes Raman laser is deduced. The properties of the anti-Stokes Raman laser are simulated theoretically by solving the rate equations of the extracavity Raman laser and using the derived expression. The theoretical results are in good agreement with the experimental results.

Theoretical models for the extracavity Raman laser with crystalline Raman medium

The rate equation model of an extracavity Raman laser is deduced for the first time to the best knowledge of the authors. The normalized radiation transfer equations of the extracavity Raman laser are given. The normalized radiation transfer equations and rate equations were solved numerically to find the optimum reflectivity of the output coupler to realize the maximum conversion efficiency at the first Stokes for both the single and double-pass pumping configurations. Also, the pulse duration of the first Stokes was investigated numerically. The rate equations are verified to be a good approximation to the radiation transfer equations for the regime of low Raman gain and long pump pulse duration. Furthermore, it is found that the optimum reflectivity of the first Stokes and the second Stokes threshold are influenced significantly by the minor feedback of the resonator mirror at the second Stokes.

Numerical optimization of the extracavity Raman laser with barium nitrate crystal

The radiation transfer equations of the extracavity Raman laser including up to the third Stokes beams and backward Raman scattering terms were deduced in detail from the wave equation and material equations of stimulated Raman scattering. The radiation transfer equations were solved numerically to optimize the performance of the extracavity Raman lasers with barium nitrate crystal as the nonlinear medium. The optimum reflectivity of the output coupler at the first Stokes was figured out numerically to achieve the maximum conversion efficiency of the first Stokes, and found to be closely related to the pump pulse duration, peak intensity of the pump pulse, and Raman crystal length. With the resonator mirrors highly reflective at the first Stokes, the highest conversion efficiency of the second Stokes was obtained when the input mirror was highly reflective at second Stokes, whereas the output coupler was highly transmissive at the second Stokes. It was found that too high intracavity intensity of the second Stokes would impede the efficient energy extraction from the pump pulse to the first Stokes, and consequently, limit the conversion efficiency of the second Stokes.

Highly Efficient Raman Frequency Converter With Strontium Tungstate Crystal

A highly efficient extracavity Raman laser pumped by the infrared nanosecond laser pulses is presented utilizing the recently recommended Raman medium-strontium tungstate crystal (SrWO/sub 4/). The maximum conversion efficiency of the first and second Stokes pulses both reached about 50%, and the maximum total conversion efficiency of the first and second Stokes was obtained to be 70% in the experiment. The conversion efficiency dependence on the polarization and the temporal characteristics of the Stokes and pump pulses were also studied. A theoretical model for the solid-state extracavity Raman laser was established based on the radiation transfer equations describing stimulated Raman scattering processes, and solved numerically. This model can accurately predict the energy transfer dynamics observed in extracavity Raman lasers.

Theoretical and experimental research on the multi-frequency Raman converter with KGd(WO4)2 crystal

An efficient multi-frequency extracavity Raman laser for nanosecond pulses was realized by taking advantage of the anisotropic optical property of the KGd(WO4)2 crystal. The conversion efficiencies of the converter were investigated versus the pump pulse energy, pump polarization, and output coupling rate experimentally and theoretically. Based on the coupled radiation transfer equations, a theoretical model was deduced to predict the performance of solid-state extracavity Raman lasers. This model was solved numerically to analyze the operation of the extracavity Raman laser with the KGd(WO4)2 crystal, and the numerical results had a good agreement with the experimental ones.