Continuous-Wave Self-Raman Vanadate Lasers Generating Versatile Visible Wavelengths

Abstract

In this review, the developments of efficient high-power CW orange-lime-green lasers by using intracavity stimulated Raman Scattering (SRS) in Nd-doped vanadate lasers are systematically discussed. The overall properties of the spontaneous Raman spectra in Nd:YVO₄ and Nd:GdVO₄ crystals are overviewed. The critical phase matchings of using the lithium triborate (LBO) crystals for sum frequency generation (SFG) and second harmonic generation (SHG) are thoroughly reviewed. We make a detailed review for achieving the individual green-lime-orange emissions from the self-Raman Nd:YVO₄ and Nd:GdVO₄ lasers with LBO crystals. The following is to review the dual-wavelength operations of the lime-green and orange-green lasers. Finally, the procedure for generating the triple-wavelength operation of orange-lime-green simultaneous emissions is completely described. The present review is expected to be useful for developing compact, efficient, high-power CW visible lasers for applications including medical treatment, biology, spectroscopy, and remote sensing.

1. Introduction

Intracavity second harmonic generation (SHG) of the fundamental field in neodymium (Nd) doped lasers is the most used method to generate high-power continuous-wave (CW) green lasers. In addition to the green laser, there is a growing demand for multi-watt CW lime-yellow laser sources for retinal photocoagulation, especially in the treatment of macula. This demand comes from the fact that the macula contains abundant xanthophyll pigments that have a small amount of absorption for green light and almost no absorption in the lime-yellow spectral region. Since the treatment laser absorbed by xanthophyll pigments may cause collateral damage to the eye, high-power lime-yellow light sources become requisite for retinal photocoagulation near the macula [1,2,3,4].

Stimulated Raman scattering (SRS) is a practical and useful approach to widen the wavelength range from ultraviolet to mid-infrared. The SRS phenomenon was originally observed by Woodbury and Ng in 1962 from the exploration of Q-switched ruby lasers with nitrobenzene materials [5]. Nowadays, intracavity SRS by using solid-state crystals is an efficient and reliable method to extend the spectral range of solid-state lasers [6,7,8,9,10]. Nd-doped solid-state lasers have been successfully demonstrated to realize compact high-power CW lime-yellow-orange light sources by combining intracavity SRS in a Raman gain medium with sum frequency generation (SFG) or SHG in a nonlinear crystal [11,12,13,14,15,16].

Nowadays, the most popular gain media for intracavity SRS are tungstate crystals [17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35] such as BaWO₄, SrWO₄, KGd(WO₄)₂, and vanadate crystals such as GdVO₄ [36,37,38,39,40,41,42,43,44,45,46,47,48], YVO₄ [49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91], and LuVO₄ [92,93]. In particular, the Nd-doped vanadate laser materials are often categorized as self-Raman materials since they can simultaneously generate the fundamental laser field and the Raman Stokes field. Using the V-O stretching mode of Nd:GdVO₄ crystal with the Raman frequency shift of 882 cm⁻¹, the wavelength of the Stokes field is approximately 1173 nm for the 1063-nm fundamental field. Consequently, the SHG of the Stokes field and the SFG of the Stokes field with the fundamental field can yield the lasers at 586 nm (orange) and 558 nm (lime), respectively. On the other hand, the wavelength of the Stokes field in the Nd:YVO₄ crystal is around 1176 nm for the 1064-nm fundamental field with the Raman frequency shift of 890 cm⁻¹. Therefore, the lime and orange wavelengths generated using the Nd:YVO₄ crystal are 559 nm and 588 nm, respectively. So far, both Nd:GdVO₄ [41,42,43,44,45] and Nd:YVO₄ [86,87,88,89] crystals have been widely used to generate multi-watt CW lime and orange lasers. The linear resonator is usually used to achieve the criterion of ultralow losses for efficiency in the CW intracavity SRS process. Since Nd-doped vanadate materials possess a high absorption in the green-lime-orange wavelengths, an intracavity dichroic coating was often exploited in the two-mirror linear resonator to reflect the backward SHG/SFG for eliminating the absorption [86,87,88,89].

In this review, we make a thorough discussion of the recent development of efficient high-power CW orange-lime-green lasers based on intracavity SRS in Nd-doped vanadate lasers with SFG and SHG in lithium triborate (LBO) crystals. In Section 2, the characterizations of the spontaneous Raman spectra in Nd:YVO₄ and Nd:GdVO₄ crystals are completely reviewed. In Section 3, the critical phase matchings of LBO crystal for visible emissions from SFG and SHG are discussed. In Section 4, we review the results of single wavelength operations for individual green-lime-orange emissions [86]. Then, the dual-wavelength operations of the lime-green [87] and orange-green [88] lasers are completely reviewed in Section 5. Finally, we review the results for the triple-wavelength operation of orange-lime-green simultaneous emissions [89]. It is believed that the present review for compact, efficient, high-power CW visible lasers will be of practical usefulness for applications such as medical treatment, biology, spectroscopy, and remote sensing. In addition to CW operation, an effective and reliable method for developing dual-wavelength passive Q-switched self-Raman lasers with controllable polarization has also progressed recently [49].

2. Spontaneous Raman Spectrum of Vanadate Crystals

First of all, the spontaneous Raman spectra of vanadate crystals are briefly reviewed. The configuration of the Raman scattering measurement is usually indicated by the Porto notation [94] that specifies the orientation of the crystal with respect to the polarization of the laser in both the excitation and analyzing directions for Raman scattering processes. The Porto notation consists of four letters: A(BC)D, where A is the direction of the propagation of the incident light (k_i), B is the direction of the polarization of the incident light (E_i), C is the direction of the polarization of the scattered light (E_s), and D is the direction of the propagation of the scattered light (k_s).

Figure 1a,b shows the experimental data of the spontaneous Raman scattering spectra of YVO₄ crystal with the Porto notations of $\mathrm{X}\left(\mathrm{ZZ}\right){\displaystyle \overline{\mathrm{X}}}$ and $\mathrm{X}\left(\mathrm{YY}\right){\displaystyle \overline{\mathrm{X}}}$. Both Raman configurations in the YVO₄ crystal can be observed to have different active vibration modes. Furthermore, the spectra reveal several conspicuous Raman peaks associated with the internal modes of the VO₄³⁻ group as well as the external modes of VO₄³⁻ tetrahedra and Y³⁺ ions in the YVO₄ unit cell. The external mode at 157 cm⁻¹ (B_1g(1)), related to the O–Y–O bending vibration, can be found in both Raman configurations. Additionally, the internal modes, which can be attributed to the VO₄ bending and stretching vibrations, situated at higher wave numbers: 259 (B_2g), 376 (A_1g(1)), 487 (B_1g(3)), 816 (B_1g(4)), 838 (E_g(5)), and 890 cm⁻¹ (A_1g(2)) can be entirely observed in the $\mathrm{X}\left(\mathrm{YY}\right){\displaystyle \overline{\mathrm{X}}}$ configuration. However, the vibrations at 487 and 816 cm⁻¹ are not found in the $\mathrm{X}\left(\mathrm{ZZ}\right){\displaystyle \overline{\mathrm{X}}}$ configuration. The strongest vibrational Raman peak can be confirmed to be located at 890 cm⁻¹ with a Raman line width of 3.0 cm⁻¹. The Raman gain coefficient was estimated to be approximately 5 cm/GW [95]. The Raman gain coefficients of two weak Raman shifts, 838 and 376 cm⁻¹, in the $\mathrm{X}\left(\mathrm{ZZ}\right){\displaystyle \overline{\mathrm{X}}}$ configuration are estimated to be 1.3 and 2.0 cm/GW, respectively. On the other hand, the Raman gain coefficients of two weak Raman shifts, 838 and 816 cm⁻¹, in the $\mathrm{X}\left(\mathrm{YY}\right){\displaystyle \overline{\mathrm{X}}}$ configuration are estimated to be 2.5 and 1.5 cm/GW, respectively. Figure 2a,b shows the experimental data of the spontaneous Raman scattering spectra of GdVO₄ crystal with the Porto notations of $\mathrm{X}\left(\mathrm{ZZ}\right){\displaystyle \overline{\mathrm{X}}}$ and $\mathrm{X}\left(\mathrm{YY}\right){\displaystyle \overline{\mathrm{X}}}$. The overall characteristics of the spontaneous Raman scattering spectra can be clearly seen to be similar to the results shown in Figure 1 for the YVO₄ crystal.

3. The Critical Phase Matching of LBO Crystal for Visible Emissions

The LBO crystal [96], belonging to an orthorhombic system with point group symmetry of mm², is the most widely used nonlinear material for generating visible lights from SHG and SFG of self-Raman vanadate lasers. The high-quality growth and the nonlinear characterizations of the LBO crystal were originally and mainly explored by the Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences. The LBO material is a negative biaxial crystal with a high damage threshold that makes it superior in the high-power SHG and SFG. Moreover, the LBO crystal can achieve high conversion efficiencies for a wide-ranging spectrum.

The unit cell dimensions of the LBO crystal are given by a = 8.4473 Å, b = 7.3788 Å, c = 5.1395 Å. The crystallographic axes a, c, and b are one-to-one parallel to the principal axes X, Y, and Z (n_z > n_y > n_x). For the LBO crystal, the Sellmeier equations of n_x, n_y, and n_z with the temperature dependence can be expressed as [96]

$$n_x(\lambda ,T)={\left(2.4542+\frac{0.01125}{{\lambda }^2-0.01135}-0.01388{\lambda }^2\right)}^{1/2}+\frac{d{n}_x}{dT}\left(T-T_{\mathrm{o}}\right),$$

(1)

$$n_y(\lambda ,T)={\left(2.5390+\frac{0.01277}{{\lambda }^2-0.01189}-0.01848{\lambda }^2\right)}^{1/2}+\frac{d{n}_y}{dT}\left(T-T_{\mathrm{o}}\right),$$

(2)

$$n_z(\lambda ,T)={\left(2.5865+\frac{0.01310}{{\lambda }^2-0.01223}-0.01861{\lambda }^2\right)}^{1/2}+\frac{d{n}_z}{dT}\left(T-T_{\mathrm{o}}\right),$$

(3)

where λ is the wavelength with the units of micrometer, T is the temperature with the units of degrees Celsius, and T_o = 20 °C. Velsko et al. [97] earlier published the thermo-optical coefficients that were simply expressed as

$$\frac{d{n}_x}{dT}=-1.8\times {10}^{-6},$$

(4)

$$\frac{d{n}_y}{dT}=-13.6\times {10}^{-6},$$

(5)

$$\frac{d{n}_z}{dT}=-(6.3+2.1\lambda )\times {10}^{-6}.$$

(6)

Tang et al. [98] later studied the temperature-dependent thermo-optical coefficients and modified them as

$$\frac{d{n}_x}{dT}=2.0342\times {10}^{-7}-1.9697\times {10}^{-8}T-1.4415\times {10}^{-11}{T}^2,$$

(7)

$$\frac{d{n}_y}{dT}=-1.0748\times {10}^{-5}-7.1034\times {10}^{-8}T-5.7387\times {10}^{-11}{T}^2,$$

(8)

$$\frac{d{n}_z}{dT}=-8.5998\times {10}^{-7}-1.5476\times {10}^{-7}T+9.4675\times {10}^{-10}{T}^2-2.2375\times {10}^{-12}{T}^3.$$

(9)

The type-I phase matching in the XY plane of the LBO crystal is similar to that in the negative uniaxial crystal. The condition for the phase matching of SFG can be expressed as

$$\frac{n_z({\lambda }_1,T)}{{\lambda }_1}+\frac{n_z({\lambda }_2,T)}{{\lambda }_2}=\frac{n_{xy}^e({\lambda }_3,T,\varphi )}{{\lambda }_3},$$

(10)

where ${\lambda }_1^{-1}+{\lambda }_2^{-1}={\lambda }_3^{-1}$ and the effective refractive index $n_{xy}^e(\lambda ,T,\varphi )$ is given by

$$n_{xy}^e(\lambda ,T,\varphi )={\left[\frac{{\sin }^2\varphi }{n_x^2(\lambda ,T)}+\frac{{\cos }^2\varphi }{n_y^2(\lambda ,T)}\right]}^{-1/2},$$

(11)

From Equation (10), the condition of the phase matching can be manifested by using the function of the wave number difference given by [96]

$$\mathrm{\Delta }k=\frac{n_z({\lambda }_1,T)}{{\lambda }_1}+\frac{n_z({\lambda }_2,T)}{{\lambda }_2}-\frac{n_{xy}^e({\lambda }_3,T,\varphi )}{{\lambda }_3},$$

(12)

For attaining high conversion efficiency, the phase-matching condition Δk = 0 needs to be satisfied. In the self-Raman Nd:YVO₄ laser cavity, the fundamental wavelength at 1064 nm and the Stokes wavelength at 1176 nm can be used for the green (532 nm), lime (559 nm), and orange (588 nm) lasers from the SHG of 1064 nm, the SFG of 1064 and 1176 nm, and the SHG of 1176 nm, respectively. One way to satisfy the phase-matching condition is to employ a non-critically phase-matched (NCPM, θ = 90°, ϕ = 0°) LBO crystal with tunning the crystal temperature [41,42,43,44,45]. Alternatively, the green (532 nm), lime (559 nm), and orange (588 nm) lasers can be selectively generated by exploiting the different LBO crystals with different cutting angles to reach the type-I phase-matching at room temperature of 25 °C. Figure 3 shows the numerical calculations for the phase-matching condition Δk as a function of the cut angle for the LBO crystal in the XY plane for generating 532, 559, and 588 nm from the fundamental and Stokes waves of 1064 and 1176 nm at room temperature. The optimal cut angles for generating the wavelengths of 532, 559, and 588 nm can be found to be approximately ϕ = 11.6°, 8.1°, and 4.1°, respectively. As usual, the length of the LBO crystal used in the self-Raman lasers was 8 mm long with anti-reflection coating on both end surfaces at wavelengths of the fundamental and Stokes waves as well as the green-lime-orange outputs.

4. Single Wavelength Operations for Individual Green-Lime-Orange Emissions

Here, we review employing the self-Raman crystal in the same two-mirror linear cavity to generate three wavelengths for the green, lime, and orange lasers by using three different LBO crystals for intracavity SHG or SFG with the critical phase matching near room temperature [86]. Figure 4 depicts the cavity configuration for generating high-power CW Nd:YVO₄ (Nd:GdVO₄) visible lasers at three different wavelengths of 532 (531.5), 559 (558), and 588 (586) nm. Two kinds of laser crystals, a-cut Nd:GdVO₄ and Nd:YVO₄, were individually employed to play the dual roles of lasing and SRS gain medium. A detailed comparison of the output efficiency between Nd:YVO₄ and Nd:GdVO₄ crystals was systematically performed. For both gain media, the Nd³⁺ dopant concentrations and the crystal sizes were the same, 0.3-at.% and 3 × 3 × 20 mm³, respectively. Although the effective crystal length for the laser crystal to absorb the pump power at 808 nm was only approximately 8 mm, a length of up to 20 mm was exploited to enhance the gain of SRS. The surface of the laser crystal toward the input mirror had an anti-reflection coating at 808 and 1060–1190 nm (reflectance < 0.2%). The other surface had a dichroic coating, the measured reflectance of which is shown in Figure 5. For both gain media, the reflectance can be seen to be generally higher than 97% in the range of 530–590 nm. Furthermore, the values of the reflectance at the wavelengths around 1064 nm and 1176 nm are as low as 0.11% and 0.28%, respectively. In the experiment, the laser materials were covered with indium foil and mounted in a copper holder with active conduction cooling at a temperature of 20 °C.

The excitation light was a fiber-coupled laser diode that had a central wavelength of 808 nm, core diameter of 600 μm, and numerical aperture of 0.16. The pump source was focused into the laser crystal with a waist radius of approximately 300 μm. The entrance face of the input flat mirror had an anti-reflection coating at 808 nm (reflectance < 0.2%), and the second surface had a high-reflection coating at 1060–1180 nm (reflectance > 99.9%) and a high-transmission coating at 808 nm (transmittance > 95%). The concave output coupler with a radius of curvature of 100 mm had a high-reflection coating at 1060–1180 nm (reflectance > 99.9%) and a high-transmission coating (transmittance > 95%) at 530–590 nm on the concave surface and an anti-reflection coating at 530–590 nm (reflectance < 0.2%) on the other surface. By using three LBO crystals with different cutting angles for the type-I phase-matching, the green, lime, and orange lasers can be individually generated from the same cavity mirrors and gain medium. A thermo-electric cooler was utilized to tune the temperatures of the LBO crystal for optimal operation. The overall cavity length was around 50 mm.

When an 8-mm-long LBO material with the cutting angle at θ = 90° and ϕ = 4.1° was used in the cavity, the orange laser could be generated via the SHG of the Raman Stokes field. Figure 6 shows the average output powers versus the incident pump power for 586 and 588 nm obtained with the Nd:GdVO₄ and Nd:YVO₄ crystals, respectively. The lasing thresholds for 586 and 588 nm outputs are just the thresholds for the SRS process and can be seen to be approximately 3.3 and 4.3 W, respectively. Since the sole difference between these two performances lies in the laser crystal, the overall Raman gain of the GdVO₄ crystal can be, therefore, deduced to be approximately 1.3 times higher than that of the YVO₄ crystal. Due to the higher Raman gain, the output power of 3.4 W generated from the Nd:GdVO₄ crystal is evidently greater than 2.6 W from the Nd:YVO₄ crystal at a pump power of 26 W. The beam quality factors for the overall outputs were found to be around 3.5–4.0.

When an 8-mm-long LBO material with the cutting angle at θ = 90° and ϕ = 8.1° was employed in the cavity, the lime laser could be generated through the SFG of the fundamental wave and Raman Stokes field. Figure 7 shows the average output powers versus the incident pump power for 558 and 559 nm generated from the Nd:GdVO₄ and Nd:YVO₄ crystals, respectively. The lasing thresholds for the lime operation can be seen to be nearly the same as the results obtained in the orange operation, as shown in Figure 6. In other words, the thresholds for the yellow and lime performances are wholly determined by the SRS process. At a pump power of 26 W, the output power of 4.8 W generated from the Nd:GdVO₄ crystal for the 558-nm emission is clearly greater than 4.0 W from the Nd:YVO₄ crystal for the 559-nm emission.

When an LBO crystal (θ = 90°, ϕ = 11.6°) for the SHG phase matching of the fundamental field was exploited in the cavity, the green emission could be generated from the SHG of the fundamental wave. Figure 8 shows the average output powers versus the incident pump power for 531.5 and 532 nm generated from the Nd:GdVO₄ and Nd:YVO₄ crystals, respectively. Two pump thresholds are found to be almost the same, as low as 0.45 W. The output efficiency based on the Nd:GdVO₄ crystal is slightly lower than that based on Nd:YVO₄ crystal. The inferiority of the present Nd:GdVO₄ cavity for the SHG at 531.5 nm may come from the losses of the fundamental wave caused by the SRS process. In contrast, the influence of the SRS process on the SHG of the Nd:YVO₄ laser at 532 nm is relatively smaller due to a lower Raman gain.

5. Dual-Wavelength Operations for Lime-Green and Orange-Green Emissions

5.1. Lime-Green Emission

Based on the Nd:YVO₄ self-Raman resonator for the 559 nm laser output, an additional LBO with the cutting angle at θ = 90° and ϕ = 11.6° was positioned behind the first LBO crystal for generating 532-nm output via the intracavity SHG of the fundamental wave [87]. Under this circumstance, the lime-green dual-wavelength operation can be efficiently achieved. Figure 9a shows the cavity setup for the lime-green dual-wavelength emission. The temperature of the first LBO crystal was kept at 25 °C for the optimal phase-matching of the lime emission, whereas the temperature of the second LBO crystal was flexibly changed within 14–29 °C for controlling the output power of the 532 nm emission. The SHG efficiency can be expressed as [96]

$$\hspace{0.33em}\eta =\frac{8{\pi }^2}{{{\lambda }_1}^2\hspace{0.17em}}\hspace{0.33em}\frac{1}{n_1^2{n}_2}\frac{L^2}{{\epsilon }_{o}c}{d}_{eff}^2\hspace{0.33em}\hspace{0.33em}{\left[\frac{\sin (\mathrm{\Delta }kL/2)}{(\mathrm{\Delta }kL/2)}\right]}^2,$$

(13)

where Δk is the wave vector mismatch, λ₁ is the fundamental wavelength, n₁ is the fundamental refraction index, n₂ is the refraction index of the second harmonic wave, d_eff is the effective nonlinearity, L is the nonlinear crystal length, and ε₀ is the permittivity of free space. From Equation (13), the conversion efficiency of the SHG process achieves maximal for ∆k = 0, whereas the efficiency will considerably decrease for ΔkL ≠ 0. The conversion efficiency is reduced to 1/2 of the maximum value for ΔkL = 2.784. Therefore, the temperature bandwidth can be calculated with ΔT_BW = 2.784/γ_TL, where γ_T is given by

$$\hspace{0.33em}{\gamma }_T=\frac{4\pi }{{\lambda }_1}{\left(\frac{\partial n_1}{\partial T}-\frac{\partial n_2}{\partial T}\right)}_{T=T_{\mathrm{pm}}},$$

(14)

where T_pm is the temperature for the phase matching condition. From the LBO crystal length of L = 8 mm, the temperature bandwidth ΔT_BW could be calculated to be around 5.9 °C. For convenience, the temperature of the second LBO crystal is denoted as T₅₃₂. Figure 9b shows the 532 and 559 nm output powers and the total output power versus the temperature of the second LBO crystal T₅₃₂ at the pump power of 31.6 W. The temperature bandwidth of the SHG at 532 nm was estimated to be around 6 °C from the experimental range of 17–23 °C. The experimental result is in good agreement with the theoretical calculation. When the temperature T₅₃₂ was lowered from 29 °C to 23 °C, the 532 nm output power increased from 0.4 W to 4.3 W. Under this condition, the output power at 559 nm was found to be almost unchanged, with an average output power of approximately 4.2 W. In the dual-wavelength operation, the total output power can significantly increase from 4.8 W to 8.5 W for the temperature T_532, decreasing from 29 °C to 23 °C. Consequently, the overall conversion efficiency increases from 15.2% to 26.9% in the dual-wavelength operation. The green output could be up to the highest power of 7.5 W for the temperature T_532, decreasing from 23 °C to 20 °C. Under this circumstance, the lime output power was reduced from 4.2 W to 2.0 W. The dropping of the lime output power means that the generation of the 532 nm laser consumes too much of the fundamental wave, thereby affecting the conversion efficiency of the Raman laser. In other words, although the power of the Raman laser increases linearly with the 808 nm pump power, the multiple SHG and SFG processes may interfere with each other. Even so, the total conversion efficiency for the dual-wavelength emissions could reach 30.1% at T₅₃₂ = 20 °C. The green output power displayed decreasing for the temperature T_532, decreasing from 20 °C to a lower value. The total output for the dual-wavelength emissions could achieve the maximal power of 10 W at T₅₃₂ = 18 °C. Consequently, the conversion efficiency was up to 31.6%, and the green and lime output powers were 7.1 W and 2.9 W, respectively. When the temperature T₅₃₂ was lower than 15 °C, the lime output power could return to 4.2 W.

As shown in Figure 9b, the green and lime output powers are well balanced at T₅₃₂ = 23 °C or 15.5 °C. For some applications of the dual-wavelength lasers, the balanced emissions are desirable and useful. Figure 10 shows the green and lime output powers and the total output power versus the pump power at T₅₃₂ = 23 °C. The ratio of green and lime power was found to be within 1.1 ± 0.05 for the pump power in the range of 15.0–31.6 W. Experimental results revealed that the competition between the SFG and SHG was not conspicuous. The overall output variations for both wavelengths were generally smaller than ±5%. Note that the order of the LBO crystals is critically important. Since the intensity of the Stokes wave is usually weaker than that of the fundamental wave, the SFG for the lime output should be performed in advance.

In the experiment, the output spectra were measured with an optical spectrum analyzer (Advantest Q8381A) with a resolution of 0.1 nm. Figure 11 depicts the optical spectrum for the dual-wavelength emission at a pump power of 31.6 W at T₅₃₂ = 23 °C. The central wavelengths are around 532.5 and 559.1 nm. The spectral widths are approximately 0.4 and 0.2 nm for the green and lime outputs, respectively. The transverse patterns are also shown in the inset of Figure 11. The beam quality factors for overall results were found to be around 3.5–4.0.

5.2. Orange-Green Emission

In addition to the lime-green dual-wavelength emission, two LBO crystals could be exploited in the self-Raman laser cavity to generate the orange-green emission from the individual SHG of the Stokes and the fundamental waves [88]. Similar to the lime-green emission described above, the first LBO temperature was kept at the optimal phase-matching to obtain the maximum orange output power. Then, the second 8-mm-length LBO crystal with the cut angle at θ = 90° and ϕ = 11.6° was used to achieve the SHG of 1064 nm for green output power. As shown in Figure 9b, the optimal temperature of the second LBO crystal for the green generation was approximately 20 °C. The power ratio between the orange and green emissions could be flexibly adjusted by controlling the second LBO temperature to manipulate the SHG conversion efficiency for 532 nm. Figure 12 shows experimental results for the green, the orange, and the total output powers varying with the second LBO temperature at a pump power of 31.6 W. It can be seen that when the temperature changes from 28 °C to 23 °C, the green output power increases from 0.2 W to 4.0 W, and the orange output power can be almost maintained at approximately 4.0 W. In other words, the total output power in the orange-green dual-wavelength operation can increase from 4.0 W to 8.0 W for the temperature of the second LBO crystal changing from 28 °C to 23 °C, correspondingly, the overall conversion efficiency increasing from 12.7% to 25.4%. When the temperature of the second LBO crystal changes from 23 °C to 19 °C, the green output power can be continually boosted up to the maximal value of 6.1 W. Under this circumstance, the orange output power at 588 nm can be seen to be reduced from 4.0 W to 2.2 W. Nevertheless, the total conversion efficiency at the temperature of 19 °C in the dual-wavelength operation can achieve the highest value of 26.3% with the green and orange output powers to be 6.1 W and 2.2 W, respectively. When the temperature of the second LBO crystal starts to be lower than 19 °C, the green output power gradually decreases from the maximal value, and simultaneously, the orange emission returns to the power level of 4.0 W.

Figure 13 shows the optical spectra at a pump power of 31.6 W for the orange-green dual-wavelength operation at four different temperatures of 19, 23, 26, and 28 °C for the second LBO crystal. The various ratios of the orange-green powers can be clearly seen in the lasing spectra. The transverse patterns of the total intensities are also shown in the insets of Figure 13. The colors of the transverse patterns can be found to vary from green-rich to orange-rich for the temperature of the second LBO crystal changing from 19 °C to 28 °C. The beam quality factors of both wavelengths were found to be approximately 3.5 at the maximal pump power of 31.6 W. As seen in Figure 12, the green and orange output powers can be almost balanced for the temperature of the second LBO crystal to be controlled at 23 °C or 17 °C. Figure 14 shows the green and orange output powers and the total output power versus the incident pump power at the temperature of 23 °C. The ratio of green to orange output powers can be found to be maintained at 1.0 ± 0.1 for the pump power within 10.0–31.6 W.

6. Triple-Wavelength Operation of Orange-Lime-Green Simultaneous Emissions

As reviewed in the previous section, the dual-wavelength operations of orange-green and lime-green emissions have been successfully implemented by using Nd:YVO₄ self-Raman lasers [87,88]. Furthermore, a reliable method for achieving the triple-wavelength operation with emissions of 588, 559, and 532 nm was also developed [89]. The developed method can provide the output powers of 588, 559, and 532 nm with a good balance. The method consists of three steps by using three LBO materials to generate the 588 nm, 559 nm and 532 nm lasers in sequence. The first step to set up the cavity is to maximize the 588 nm orange output by using the first LBO crystal, as discussed in the previous section.

The second step is to employ another LBO material to accomplish the dual-wavelength operation of orange-lime emissions at 588 and 559 nm, as shown in Figure 15a. The second LBO material was 8 mm in length, and the cutting angle was at θ = 90° and ϕ = 8.1° for the lime emission from the SFG of the fundamental and Stokes fields. The temperature of the first LBO crystal was fixed at 23.5 °C for the optimal orange emission. Under this condition, the output power ratio between the orange and lime emissions could be varied in a wide range by adjusting the temperature of the second LBO material. Figure 15b shows the lime and orange output powers versus the temperature of the second LBO crystal at a pump power of 30 W. The lime and orange output powers can be found to be well balanced for the temperature of the second LBO material at 20 °C or 27.5 °C. As shown in Figure 15b, both lime and orange output powers at the balanced temperature of 27.5 °C are nearly close to 2.6 W at the incident pump power of 30 W. Under the dual-wavelength orange-lime operation, the conversion efficiency is approximately 17.3%. Once again, the order of the LBO crystals is critically important. Since the intensity of the Stokes wave is usually weaker than that of the fundamental wave, the SHG for the orange output should be performed in advance. Figure 16 shows the orange and lime output powers versus the incident pump power at the balanced temperature of 27.5 °C. It can be seen that the orange and lime output powers can be well balanced for the pump power within 10–30 W.

The final step for accomplishing the triple-wavelength operation is to exploit the third LBO material for generating the green output power in the laser cavity, as shown in Figure 17a. From the resonator setup shown in Figure 15a, the third LBO material was positioned behind the second LBO crystal. The length of the third LBO crystal was 8 mm and the cutting angle was at θ = 90° and ϕ = 11.6°. By maintaining the first and second LBO crystals at the temperatures for the equal orange and lime emissions, the power ratio between the orange-lime and green outputs could be varied by controlling the temperature of the third LBO material in the range of 14–36 °C. Figure 17b shows the experimental results for the orange, lime, and green output powers versus the third LBO temperature at a pump power of 30 W. Experimental results revealed that the equal emissions for the orange and lime lasers were not significantly affected by adding the third LBO material into the resonator. The green output power at 532 nm could increase from 0.3 W to 2.4 W by changing the third LBO temperature from 36 °C to 27 °C. Under this condition, both the orange and lime output powers were found to be approximately 2.5 W without significant variation. By decreasing the third LBO temperature from 27 °C to 20 °C, the green output could continuously increase up to the highest power of 4.6 W. Under this circumstance, both the orange and lime output powers were found to decrease from 2.5 W to 1.9 W. For the third LBO temperature at 20 °C, the total conversion efficiency in the triple-wavelength operation could reach 27% with the green, lime, and orange output powers to be 4.6 W, 1.9 W and 2.0 W, respectively. When the third LBO temperature was lower than 20 °C, the green output power turned to decrease. Under this condition, both the orange and lime output powers were progressively back to the power level of 2.5 W. For the third LBO temperature at 27 °C, the green, lime, and orange output powers could be nearly equal, as seen in Figure 17b. Figure 18 shows the green, lime, and orange output powers versus the pump power for the third LBO temperature at 27 °C. The power ratio of the three outputs could be approximately 1.0 ± 0.1 for the pump power within 10–30 W. To be brief, the equal output powers for the three wavelengths could be maintained for the pump power within 10–30 W.

Figure 19 shows the optical spectra and transverse patterns measured at the incident pump power of 30 W for the triple-wavelength operation for the third LBO crystal at four different temperatures of 20, 27, 29, and 36 °C. The various power ratios can be clearly seen from the optical spectra. The colors of the entire output beams exhibited the transformation from green-rich to orange-rich for the third LBO temperature varying from 20 °C to 36 °C. The overall beam quality factors for triple-wavelengths were measured to be around 3.0–4.0 at a pump power of 30 W.

7. Conclusions

We have thoroughly reviewed the recent development of compact high-power CW orange-lime-green lasers by combining intracavity SRS in Nd-doped vanadate lasers with SFG and SHG in LBO crystals. We have overviewed the properties of the spontaneous Raman spectra in Nd:YVO₄ and Nd:GdVO₄ crystals, as well as the critical phase matchings of LBO crystal for visible emissions from SFG and SHG. The self-Raman lasers with vanadate and LBO crystals to achieve the individual green-lime-orange emissions have been systematically reviewed. We have presented a detailed review of the dual-wavelength operations of the lime-green and orange-green lasers. Finally, the procedure for generating the triple-wavelength operation of orange-lime-green simultaneous emissions has been discussed. The present review is expected to be of practical usefulness for designing compact, efficient, high-power CW visible lasers.