High efficiency, multi-Watt CW yellow emission from an intracavity-doubled self-Raman laser using Nd:GdVO4

Diode-side-pumped, intracavity Nd:YLF/KGW/LBO Raman laser at 573 nm for retinal photocoagulation

Wavelengths in the yellow-orange range are of significant interest for retinal photocoagulation and are especially important in the case of diabetic retinopathy, which can cause blindness and affects 3.3% of all working-age adults. This work presents a highly-efficient, compact, and cost-efficient side-pumped, intracavity Raman configuration to achieve this objective. A side-pumped Nd:YLF/KGW/LBO frequency-doubled Raman laser producing 11.7 W of output power at 1147 nm with 21% of slope efficiency and 6 W of output power at 573.5 nm with 12% slope efficiency is demonstrated.

Multifunctional Optical Crystals for All-Solid-State Raman Lasers

In the past few decades, the multifunctional optical crystals for all-solid-state Raman lasers have been widely studied by many scholars due to their compactness, convenience and excellent performance. In this review, we briefly show two kinds of multifunctional Raman crystals: self-Raman (laser and Raman effects) crystals and self-frequency-doubled Raman (frequency-doubling and Raman effects) crystals. We firstly introduce the properties of the self-Raman laser crystals, including vanadate, tungstate, molybdate and silicate doped with rare earth ions, as well as self-frequency-doubled Raman crystals, including KTiOAsO4 (KTA) and BaTeMo2O9 (BTM). Additionally, the domestic and international progress in research on multifunctional Raman crystals is summarized in the continuous wave, passively Q-switched, actively Q-switched and mode-locked regimes. Finally, we present the bottleneck in multifunctional Raman crystals and the outlook for future development. Through this review, we contribute to a general understanding of multifunctional Raman crystals.

A single-frequency intracavity Raman laser

A continuous-wave (CW) single-longitudinal-mode (SLM) intracavity Raman laser is demonstrated for the first time, by virtue of the spatial hole-burning free nature of stimulated Raman scattering (SRS) gain. By using a single etalon in the Nd:GdVO₄ fundamental laser cavity, the spectral linewidth of the multimode fundamental field is suppressed below the Raman linewidth of Raman crystal BaWO₄; hence power in all the longitudinal modes of fundamental field can be extracted by one single Stokes mode. Therefore, the hole-burning free SRS gain exhibits a spectral cleanup effect whereby a stable SLM Stokes field is derived from the multimode fundamental field within a simple standing-wave cavity arrangement. The low-threshold SLM Raman laser delivered 3.42 W SLM Stokes and 1.53 W SLM one-way yellow harmonic at the guide-star wavelength of 589.16 nm. The results here provide a new approach to SLM laser operation with good simplicity and power dynamic range. Further engineering for power scaling and better stability is also discussed.

10.3-W actively Q-switched Nd:YVO4/YVO4 folded coupled-cavity Raman laser at 1176 nm

We report herein an efficient actively Q-switched Nd:YVO₄/YVO₄ intracavity Raman laser operating at 1176 nm. Factors such as resonator geometry and pumping scheme are optimized to strengthen the power scalability and the conversion efficiency of the intracavity Raman laser. With a folded coupled cavity adopted to make full use of the high pump intensity on the Raman crystal, the first-order Stokes output of 10.32 W at 1176 nm is achieved under the incident pump power of 39 W and pulse repetition frequency of 160 kHz. The corresponding optical efficiency reaches 26.4%, and even higher efficiency of 27.8% is obtained at lower incident pump of 34.4 W.

Efficient self-stimulated Raman scattering with simultaneously self-mode-locking in a diode-pumped Nd:GdVO4 laser

We demonstrated an efficient diode-pumped picosecond self-Raman Nd:GdVO₄ laser with the simultaneous processes of stimulated Raman scattering and self-mode-locking in the same crystal. The design of self-mode-locked was theoretically analyzed, and a compact and feasible dual-concave cavity was adopted. The maximum output power of the first-Stokes Raman laser was 736 mW with the repetition rate of 1.51 GHz. In addition, the second-harmonic generation of a yellow laser at 586.5 nm is accomplished with an external LiB₃O₅ crystal.

Multi-wavelength Yb:YAG/Nd3+:YVO4 continuous-wave microchip Raman laser

Multi-wavelength continuous-wave (CW) Raman lasers in a laser diode pumped Yb:YAG/Nd³⁺:YVO₄ microchip Raman laser have been demonstrated for the first time to our best knowledge. The multi-wavelength laser of the first Stokes radiation around 1.08 μm has been achieved with a Raman shift of 261  cm^-1 for a-cut Nd:YVO₄ crystal corresponding to the fundamental wavelength at 1.05 μm. Multi-wavelength laser operation simultaneously around 1.05 and 1.08 μm has been achieved under the incident pump power between 1.5 and 1.7 W. Multi-wavelength Raman laser with frequency separation of 1 THz around 1.08 μm has been obtained when the incident pump power is higher than 1.7 W. The maximum Raman laser output power of 260 mW at 1.08 μm is obtained and the corresponding optical-to-optical conversion efficiency is 4.2%. Elliptically polarized fundamental laser and linearly polarized Raman laser were observed in an Yb:YAG/Nd:YVO₄ CW microchip Raman laser. The experimental results of linearly polarized, multi-wavelength Yb:YAG/Nd:YVO₄ CW microchip Raman laser with adjustable frequency separation provide a novel approach for developing potential compact laser sources for Terahertz generation.

Four-wavelength laser based on intracavity BaWO₄ Raman conversions of a dual-wavelength Q-switched Nd:YLF laser

By using diode-end-pumped acousto-optically Q-switched intracavity Raman laser configurations, we demonstrate a four-wavelength laser emitting at 1047.0, 1053.0, 1159.4 and 1166.8 nm. Two Nd:YLiF₄ crystals are employed to generate 1047.0-nm and 1053.0-nm laser radiations. These two lasers are then frequency converted by a BaWO₄ Raman crystal to generate 1159.4-nm and 1166.8-nm first-Stokes waves. With pulse synchronization realized, we obtain the maximum output powers of 427, 418, 423 and 332 mW for 1047.0-nm, 1053.0-nm, 1159.4-nm and 1166.8-nm lasers, respectively. The total optical-to-optical conversion efficiency is 15.1%.

Extracavity pumped BaWO4 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.

Efficient continuous-wave self-Raman Yb:KGW laser with a shift of 89 cm⁻¹

We demonstrated a continuous-wave (CW) self-Raman laser with high conversion efficiency by using Yb:KGW as the Raman crystal. The first Stokes line of wavelength centered at 1095.2 nm with spectral bandwidth of 8 nm and the cascaded Raman conversion wavelength at 1109.5 nm with spectral bandwidth of 3.4 nm were observed with a Raman shift of 89 cm⁻¹ with respect to the fundamental laser wavelength at 1085.0 nm with spectral bandwidth of 10 nm. The CW Raman output power of 1.7 W was attained under the diode pump power of 7.8 W which corresponds to the slope efficiency and the diode-to-Stokes optical conversion efficiency of 26.6% and 21.8%, respectively.

Self-frequency-doubled BaTeMo2O9 Raman laser emitting at 589 nm

In this paper, the spontaneous Raman spectra and second harmonic generation (SHG) properties at 589 nm of a novel Raman crystal BaTeMo(2)O(9) (BTM) were investigated. The BTM crystal was cut along the type-II SHG phase-matching direction for the first-order Raman shift at 1178 nm to realize the SRS and SHG simultaneously. Pumped by a nanosecond 1064 nm laser source, a self-frequency-doubled BTM Raman laser operating at 589 nm has been demonstrated for the first time. At the pump pulse energy of 48 mJ, the maximum yellow laser output pulse energy of 5.6 mJ was obtained with an optical-to-optical conversion efficiency of 11.7%. Our results show that BTM crystal is one of the promising candidate Raman materials to generate yellow laser radiation.

Dispersion and anisotropy of thermo-optic coefficients in tetragonal GdVO4 and YVO4 laser host crystals

A detailed experimental study of dispersion and anisotropy of thermo-optic coefficients dn/dT and thermal coefficients of the optical path W=dn/dT+(n-1)α is performed for tetragonal YVO(4) and GdVO(4) laser host crystals by a laser beam deviation method. It is supported by theoretical description of thermo-optic effects, taking into account the volumetric thermal expansion effect and the temperature dependence of electronic bandgap E(g). Linear thermal expansion coefficients α were also determined by a dilatometric technique. Thermo-optic dispersion formulas describing temperature variation of the refractive index and the thermal lens effect are derived for a wide spectral range of 0.4-2 μm. It allows us to estimate the values of E(g) and dE(g)/dT for the studied crystals. The influence of materials parameters and pumping conditions on thermal lens properties is discussed, revealing the significant impact of anisotropic and temperature-dependent thermal conductivity.

1st-Stokes and 2nd-Stokes dual-wavelength operation and mode-locking modulation in diode-side-pumped Nd:YAG/BaWO4 Raman laser

1st-Stokes and 2nd-Stokes dual-wavelength operation within a diode-side-pumped Q-switched Nd:YAG/BaWO(4) intracavity Raman laser was realized. Using an output coupler of transmission of 3.9% at 1180 nm and transmission of 60.08% at 1325 nm, the maximum output power of 8.30 W and 2.84 W at a pulse repetition rate of 15 kHz for the 1st Stokes and the 2nd Stokes laser were obtained, respectively. The corresponding optical conversion efficiency from diode laser to the 1st Stokes and 2nd Stokes laser are 5.0% and 1.4%, respectively. With the pump power of 209 W and a pulse repetition rate of 15 kHz, the 1st Stokes and the 2nd Stokes pulse widths were 20.5 ns and 5.8 ns, respectively. The stable simultaneous Q-switching and mode locking of the 2nd Stokes laser without mode locking component was obtained at the pump power of about 29~82 W. The estimated mode-locked pulse width was approximately 31 ps at the pump power of 50 W and a pulse repetition rate of 15 kHz.

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.

Continuous-wave VECSEL Raman laser with tunable lime-yellow-orange output

We report a compact CW KGW Raman laser with intracavity nonlinear mixing, pumped by the intracavity field of a VECSEL. By temperature tuning an intracavity LBO crystal, we obtained two separate tunable emissions bands, namely 548.5 - 566 nm for sum-frequency-generation (SFG) of the fundamental and Stokes wavelengths, and 577.5 - 596 nm for second-harmonic-generation (SHG) of the Stokes wavelength. The maximum output powers for SFG and SHG were 0.8 W @ 560 nm and 0.52 W @ 592.5 nm, with corresponding diode-to-visible optical conversion efficiencies of 4.2% and 2.9%. These preliminary results show strong potential for expanding the spectral coverage of VECSEL lasers.

Efficient, miniature, cw yellow source based on an intracavity frequency-doubled Nd:YVO₄ self-Raman laser

We report cw yellow emission from a miniature self-Raman laser using highly doped Nd:YVO₄ crystals combined with intracavity frequency doubling. Pump-limited 587.8 nm output of 220 mW was obtained from an 18 mm long resonator, pumped by a 3.8 W diode laser.

Potential sodium D2 resonance radiation generated by intra-cavity SHG of a c-cut Nd:YVO4 self-Raman laser

Intra-cavity frequency doubling with 589 nm emission from a compact c-cut Nd:YVO4 crystal self-Raman laser was investigated. A 15-cm-length LBO with non-critical phase-matching cut (θ = 90°, ϕ = 0°) was used for efficient second-harmonic generation. At a pump power of 16.2 W and a pulse repetition frequency of 40 kHz, output power up to 2.15 W was achieved with a pulse width of 16 ns and a conversion efficiency of 13.3% with respect to the diode pump power. The center wavelength was measured to be 589.17 nm with a Half-Maximum-Full-Width of 0.2 nm, which was well in accordance with the sodium D2 resonance radiation.

A continuous wave SrMoO4 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.

Near-infrared and orange-red emission from a continuous-wave, second-Stokes self-Raman Nd:GdVO4 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%.

A wavelength-versatile, continuous-wave, self-Raman solid-state laser operating in the visible

We report a continuous-wave Nd:GdVO(4) self-Raman laser generating wavelength-selectable output in the green-yellow spectral region. The laser combines stimulated Raman scattering (SRS) with intracavity second harmonic and sum-frequency generation (SHG/SFG) to enable "on-demand" operation at any one of three wavelengths, 532 nm, 559 nm and 586 nm, each with output power greater than 3.5 W. Using experimental and numerical modelling results, we show how the highly-interactive, simultaneous non-linear processes of SRS and SHG/SFG may be balanced to achieve stable, selectable multi-wavelength visible operation.

All-solid-state cw sodium D2 resonance radiation based on intracavity frequency-doubled self-Raman laser operation in double-end diffusion-bonded Nd3+:LuVO4 crystal

We report for the first time (to our knowledge) cw orange-yellow emission at 589 nm from a compact double-end diffusion-bonded Nd(3+):LuVO(4) self-Raman laser with intracavity frequency doubling in LiB(3)O(5), pumped by an 880 nm diode laser. A 3.5 W cw orange-yellow emission with an overall diode-to-visible conversion efficiency of 13.3% is achieved through the use of an 18-mm-long double-ended diffusion-bonded Nd(3+):LuVO(4) crystal. The M(2) factors are 1.35 and 1.74 in both the horizontal and vertical dimensions, respectively.

Continuous-wave diamond Raman laser

Continuous-wave operation of a diamond Raman laser is demonstrated. Low-birefringence synthetic single-crystal diamond is used and is intracavity pumped by a Nd:YVO(4) laser. A cw output power of 200 mW is achieved at the Raman wavelength (1240 nm), and 1.6 W of on-time output power is obtained in quasi-cw mode. Losses in the diamond (approximately 1% per pass) and thermal effects in the Nd:YVO(4) limit the efficiency.

Theoretical and experimental study on the Nd:YAG/BaWO4/KTP yellow laser generating 8.3 W output power

A diode-side-pumped actively Q-switched intracavity frequency-doubled Nd:YAG/BaWO(4)/KTP Raman laser is studied experimentally and theoretically. Rate equations are used to analyze the Q-switched yellow laser by considering the transversal distributions of the intracavity photon density and the inversion population density. An 8.3 W 590 nm laser is obtained with a 125.8 W 808 nm pump power and a 15 kHz pulse repetition frequency. The corresponding optical conversion efficiency from diode laser to yellow laser is 6.57%, much higher than that of the former reported side-pumped yellow laser. The output powers with respect to the incident pump power are in agreement with the theoretical results on the whole.

Study of relaxation oscillations in continuous-wave intracavity Raman lasers

We study the relaxation oscillations in a continuous-wave intracavity Raman laser both theoretically and experimentally. Analytic expressions for the relaxation oscillation frequency are derived from the rate-equations and are validated by experiments. We show that some important experimental parameters such as the effective Raman gain coefficient and intracavity Stokes loss can be determined simply by measuring the relaxation oscillation frequency versus pump power.

An intracavity, frequency-doubled BaWO(4) Raman laser generating multi-watt continuous-wave, yellow emission

We report the generation of multi-watt continuous-wave (CW) yellow laser emission from an intracavity diode-pumped Nd:GdVO(4) /BaWO(4) Raman laser utilising a high-Q resonator (for fundamental and first-Stokes wavelengths) and intracavity frequency-doubling in LBO. CW output power of 2.9 W is achieved with a high overall diode-to-yellow conversion efficiency of 11% and with good beam quality (M(2) approximately 2.5).

Efficient 5.3 W cw laser at 559 nm by intracavity frequency summation of fundamental and first-Stokes wavelengths in a self-Raman Nd:GdVO4 laser

We report the generation of 5.3 W cw laser emission at 559 nm by sum-frequency mixing of the fundamental and first-Stokes fields generated within an Nd:GdVO(4) self-Raman laser. A high overall diode-to-visible conversion efficiency of 21% is demonstrated. We discuss the balance that must be maintained between the two nonlinear processes of Raman generation and sum-frequency mixing in order to obtain maximum emission at 559 nm.

Efficient second harmonic generation of double-end diffusion-bonded Nd:YVO4 self-Raman laser producing 7.9 W yellow light

A high power and efficient 588 nm yellow light is demonstrated through intracavity frequency doubling of an acousto-optic Q-switched self-frequency Raman laser. A 30-mm-length double-end diffusion-bonded Nd:YVO(4) crystal was utilized for efficient self-Raman laser operation by reducing the thermal effects and increasing the interaction length for the stimulated Raman scattering. A 15-mm-length LBO with non-critical phase matching (theta = 90 degrees, phi = 0 degrees) cut was adopted for efficient second-harmonic generation. The focus position of incident pump light and both the repetition rate and the duty cycle of the Q-switch have been optimized. At a repetition rate of 110 kHz and a duty cycle of 5%, the average power of 588 nm light is up to 7.93 W while the incident pump power is 26.5 W, corresponding to an overall diode-yellow conversion efficiency of 30% and a slope efficiency of 43%.

Yellow-light generation of 5.7 W by intracavity doubling self-Raman laser of YVO(4)/Nd:YVO(4) composite

A high-power 588 nm light produced by an intracavity frequency-doubling acousto-optic Q-switched self-frequency Raman laser is reported. A 20-mm-long YVO(4)/Nd:YVO(4) composite crystal and a 15-mm-long LiB(3)O(5) (LBO) with noncritical phase-matching (theta=90 degrees , phi=0 degrees ) cut were adopted for efficient self-Raman laser operation and second-harmonic generation, respectively. By the optimizing the focus position of incident pump light and Q-switch repetition rate, yellow light with 5.7 W average power was generated under the pump power of 23.5 W, corresponding to the overall diode-yellow conversion efficiency of 24.2% and slope efficiency of 32%. The pulse width is about 16 ns, and the pulse energy is up to 95 microJ.

Efficient diode-end-pumped actively Q-switched Nd:YAG/SrWO4/KTP yellow laser

An efficient intracavity frequency-doubled Raman laser was obtained by using an SrWO(4) Raman medium, an Nd:YAG ceramic gain medium, and a KTP frequency-doubling medium. Three laser cavities, including a two-mirror cavity, a three-mirror coupled cavity, and a folded cavity, were investigated. With the coupled cavity, a 2.93 W, 590 nm laser was obtained at an incident pump power of 16.2 W and a pulse repetition frequency of 20 kHz; the corresponding conversion efficiency was 18.1%. The highest conversion efficiency of 19.2% was obtained at an incident pump power of 14.1 W and a pulse repetition frequency of 15 kHz. The obtained maximum output power and conversion efficiency were much higher than the results previously obtained with intracavity frequency-doubled solid-state Raman lasers.

Self-frequency-doubled KTiOAsO4 Raman laser emitting at 573 nm

A self-frequency-doubled KTiOAsO4 (KTA) Raman laser is realized in a diode-end-pumped acousto-optically Q-switched intracavity Raman laser configuration. A 30-mm-long x-cut KTA crystal is used as the Raman medium, and its 671 cm(-1) Raman mode is employed to finish the conversion from 1064 nm fundamental laser to 1146 nm Raman laser. Self-frequency doubling of the Raman laser is accomplished in the same KTA crystal, and a 573 nm yellow laser is obtained. With an incident diode power of 10.9 W and a pulse repetition rate of 20.8 kHz, a yellow-laser power of 0.82 W is obtained. The conversion efficiency from diode power to yellow-laser power is 7.5%.

High-efficiency Q-switched dual-wavelength emission at 1176 and 559 nm with intracavity Raman and sum-frequency generation

An efficient Q-switched dual-wavelength laser with self-frequency Raman conversion in composite Nd:YVO(4) and intracavity sum-frequency generation in BBO is reported. With an input pump power of 17.5 W, average power of 0.53 W at the first-Stokes 1176 nm and average power of 1.67 W at the sum-frequency mixed 559 nm are simultaneously generated at a pulse repetition rate of 100 kHz, corresponding to a total conversion efficiency of 12.5%.

A diode side-pumped KTiOAsO4 Raman laser

A KTiOAsO(4) Raman laser is realized within a diode side-pumped acousto-optically Q-switched Nd:YAG laser. Efficient nanosecond first-Stokes generations at 1091.4 nm are obtained with three 30-mm-long KTA crystals. Under an incident diode power of 60.9 W and a pulse repetition rate of 4 kHz, a first-Stokes power of 4.55 W is obtained, corresponding to a diode-to-Stokes conversion efficiency of 7.5%. The single pulse energy is up to 1.14 mJ and the peak power is 18.0 kW.