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Abstract
High-power efficient continuous-wave Nd:YVO4/KGW Raman lasers at 555 and 559 nm are achieved by using a double-sided dichroic coating output coupler to improve the resonance quality factor. The Np-cut potassium gadolinium tungstate (KGW) is used to generate the Stokes waves at 1159 and 1177 nm by placing the polarization of the1064 nm fundamental wave parallel to the Ng and Nm axes, respectively. The lithium triborate (LBO) crystal with the cut angle in the XY plane for the type-I phase matching is used to perform the intracavity sum frequency generation for yielding the green light at 555 nm and the lime light at 559 nm at the optimal phase matching temperature. Experimental results were systematically accomplished to comprehend the optimal cavity length for the conversion efficiency. Under the optimal cavity length, the output powers can reach 6.6 and 6.3 W at a pump power of 22 W for the wavelengths of 555 and 559 nm, respectively. The conversion efficiencies can be up to 30% and 28.6% for 555 and 559 nm, respectively.
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1. Introduction
Compact efficient laser sources within 550-560 nm can be used in many applications, such as medical treatment, display, communications, illumination, and atomic physics [1–4]. In particular, the property of the optical absorption difference around 555 nm between hemoglobin and carboxyhemoglobin can be practically used to determine the carboxyhemoglobin content by measuring the absorptance at 555 nm [4–6]. The process of carbon monoxide poisoning leads to the generation of carboxyhemoglobin and makes the hemoglobin irreducible in blood. Therefore, the fast and accurate measurement of the carboxyhemoglobin content is highly desirable for detecting carbon monoxide poisoning. As a consequence, light sources within 550-560 nm are not only suitable for biomedical science but also practically important for determining carbon monoxide poisoning. Three main laser technologies, including the sum frequency generation (SFG) of Nd-doped vanadate at 1342 nm and Nd-doped garnet at 946 nm lasers [7,8], the second harmonic generation (SHG) of Nd-doped garnet crystal lasers within 1110-1112 nm [9–12], and the intracavity stimulated Raman scattering (SRS) with SFG in solid-state lasers [13–18], have been developed to operate for the spectral region around 550-560 nm. So far, the optical-to-optical efficiencies for generating 550-560 nm lasers seldom exceeded 10% except for solid-state Raman lasers [19]. Recently, the output efficiencies of solid-state Raman lasers in the yellow-orange spectral region were considerably raised to be up 26% by using an output coupler (OC) with double-sided dichroic coating to reduce the cavity losses [20]. Therefore, it is greatly worthwhile to explore the feasibility of upgrading the output efficiencies in the green-lime range of 550-560 nm.
In this work, we originally design the double-sided dichroic coating OC to explore the output performance of the continuous-wave (CW) Nd:YVO4 Raman lasers at 555 and 559 nm. The potassium gadolinium tungstate (KGW) is used to achieve the SRS process. The KGW crystal has been confirmed to be a very promising Raman material with high thermal conductivity, low temperature coefficient of refractive index, and high damage threshold [21–25]. For a Np-cut KGW crystal, the strongest peaks of the spontaneous Raman spectrum for the polarization parallel to the Ng and Nm axis are approximately the wavenumbers of 768 and 901 cm−1, respectively. Consequently, the wavelengths of the Stokes fields at the Raman shifts 768 and 901 cm−1 for the fundamental wave of 1064 nm are 1159 and 1177 nm, respectively. By using a LBO crystal, the SFG of the fundamental and Stokes waves is performed to generate the green light at 555 nm and the lime light at 559 nm in a selectable way. We calculate the mode sizes of the fundamental and Stokes waves in the KGW and LBO crystals as functions of the cavity length to evaluate the mode area matching for SRS and SFG. We make thorough experiments to explore the connection between the mode area matching and the output performance. Under the optimal cavity length at a pump power of 22 W, the output powers for 555 and 559 nm can be up to 6.6 and 6.3 W, respectively. The overall conversion efficiency is close to 30%. To the best of our knowledge, the conversion efficiencies are the highest values obtained in the CW solid-state lasers at 555 and 559 nm.
2. Material preparation and laser setup
Figure 1 depicts the resonator configuration for exploring the output performance of high-power CW solid-state Nd:YVO4/KGW Raman lasers at 555 and 559 nm by using a LBO crystal for intracavity SFG and a double-sided dichroic coating OC for reducing cavity losses. We utilized a 25 W fiber-coupled laser diode with the central wavelength at 808 nm as a pump source. The coupling fiber for the pump laser diode had the specification of a core diameter of 200 mm and a numerical aperture of 0.22. We used a pair of coupling lens to focus the pump beam with the average radius of 200 µm in the laser crystal. We employed a 3 × 3 × 10 mm3 a-cut Nd:YVO4 material with the Nd-dopant concentration of 0.3 at.% to be the laser crystal. The input facet of the laser crystal was coated to form a resonator mirror that was highly reflective (HR, reflectance > 99.9%) for the spectral region within 1060-1260 nm as well as highly transmissive (HT, transmittance > 95%) at 808 nm. The other facet of the laser crystal was designed to be anti-reflective (AR, reflectance < 0.2%) within 1060-1260 nm.
Fig. 1. Cavity configuration for exploring the output performance of high-power CW solid-state Nd:YVO4/KGW Raman lasers at 555 and 559 nm.
We employed a 3 × 3 × 20 mm3 Np-cut KGW material to be the Raman crystal for generating the intracavity SRS. Based on the SFG of the fundamental and Stokes waves, the green laser at 555 nm could be obtained by setting the polarization of the fundamental field parallel to the Ng axis of the KGW crystal for achieving the Raman shift at 768 cm−1. On the other hand, the lime laser at 559 nm could be generated by setting the polarization parallel to the Nm axis for accomplishing the Raman shift at 901 cm−1. The first facet of the KGW crystal toward the Nd:YVO4 crystal was coated to be HR within 1090-1180 nm (reflectance > 99.9%) and HT at 1064 nm (transmittance > 99%). The other facet of the KGW crystal was coated to be HT within 1060-1260 nm (transmittance > 98%) and HR within 550-630 nm (reflectance > 98%). The HR coating of the KGW crystal for for the green-yellow light was used to reflect the backward SFG. We utilized indium foils to cover the Nd:YVO4 and KGW crystals and installed them in copper holders with active conduction cooling at 20 °C. We used a 3 × 3 × 8 mm3 LBO crystal with the cut angle at θ =90° and ϕ=8.1° to attain the intracavity SFG of the fundamental and Stokes waves. We exploited a thermo-electric cooler (TEC) to control the LBO temperature for obtaining the optimal phase matching. Both facets of the LBO crystal had the AR coating for the spectral range of 550-560 nm and 1060-1260 nm (reflectance < 0.2%). We used an optical spectrum analyzer (Advantest Q8381A) to measure the output spectrum with a resolution of 0.1 nm.
The OC was a concave mirror with a radius of curvature of 70 mm. The dichroic coating specification on the concave facet of the OC was HR within 1060-1260 nm (reflectance > 99.9%) and HT within 550-560 nm (transmittance > 98%). The same dichroic coating was applied to the other facet of the OC to increase the cavity quality factor via reducing the losses of the fundamental and Stokes waves. It has been demonstrated [20] that the double-sided dichroic coating on the OC can possess a relatively high reflectivity for the spectral range of fundamental and Stokes wave in comparison with the conventional one-sided coating. We made a more detailed comparison between the conventional single-sided and the present double-sided dichroic coating mirrors for the output performance. Experimental results revealed that the total leakage power of the fundamental and Stokes waves was considerably reduced from 1.8 W to 0.25 W at the pump power of 20 W by changing the OC from the single-sided to doubled-sided coating.
3. Analysis of mode area matching
First of all, we calculate the mode area matchings for the fundamental and Stokes waves in the KGW and LBO crystals as functions of the cavity length. From ABCD matrix analysis, the effective length for propagation through a medium with the length $\ell$ and the refractive index n is given by the reduced distance $\ell /n$. Accordingly, the optical cavity length $L_{CF}^\ast $ for fundamental wave is given by
The mode radius of the Stokes wave in the longitudinal center of KGW crystal is then given by
Fig. 2. Calculated results the mode area matching ${A_S}/{A_F}$ for the SRS and SFG processes for ${\lambda _S} = 1159$ and 1177 nm as functions of the geometric cavity length ${L_{CF}}$.
4. Analysis of phase matching temperature
Next, we make a numerical analysis for the temperature-dependent phase matching with the LBO crystal for the generation of 555 and 559 nm via SFG. The LBO material is the negative biaxial crystal with the unit cell dimensions of a = 8.4473 Å, b = 7.3788 Å, and c = 5.1395 Å. The crystallographic axes a, c, and b correspond to the principal axes X, Y, and Z (nz > ny > nx), respectively. The wavelength and temperature dependent Sellmeier equations for the nx, ny, and nz of the LBO crystal are analytically modelled as [26]
The conversion efficiency for the SFG is dependent on the phase matching function that is generally given by $F(\Delta k) = \,|{{\sin (\Delta k\ell )} / {\Delta k\ell }}|$, where $\ell$ is the crystal length and $\Delta k$ represents the phase velocity mismatch between the driving polarization and the generated optical wave. For the type-I SFG in the XY plane of the LBO crystal, the phase velocity mismatch $\Delta k$ is given by
Figure 3 shows the calculated results for $F(\Delta k)$ as a function of the LBO temperature for the present cut angle of ϕ=8.1° and the cases of ${\lambda _{SFG}} = 555$ and 559 nm. From numerical calculation, the optimal LBO temperatures can be seen to be approximately 32 and 22 °C for the cases of ${\lambda _{SFG}} = 555$ and 559 nm.
Fig. 3. Calculated results for $F(\Delta k)$ as a function of the LBO temperature for the present cut angle of ϕ=8.1° and the cases of ${\lambda _{SFG}} = 555$ and 559 nm.
5. Experimental results and discussion
The OC is fixed on a translation stage to investigate the dependence of the output performance on the cavity length ${L_{CF}}$. In experiment, the cavity length ${L_{CF}}$ can be varied within 50-85 mm. To begin with, the optimal LBO temperatures for the generation of 555 and 559 nm are explored by setting ${L_{CF}}$= 50 mm. Figure 4 shows the experimental results for the output powers versus the LBO temperature for the generation of 555 and 559 nm at a pump power of 22 W. It can be seen that the optimal temperatures are approximately 32 and 22 °C for the generation of 555 and 559 nm. The experimental results are in good agreement with numerical analysis shown in Fig. 3.
Fig. 4. Experimental results for the output powers versus the LBO temperature for the generation of 555 and 559 nm at a pump power of 22 W. Solid lines for eye guidance with linear connections.
On the whole, the dependences of the output performances on the cavity length ${L_{CF}}$ are quite similar for the two cases of 555 and 559 nm. For convenience, the experimental results for the case of 555 nm are used in the following discussion. Figure 5 shows the experimental results for the threshold pump power versus the cavity length ${L_{CF}}$ under the optimal LBO temperature. The threshold pump power can be found to increase from 1.9 W to 3.6 W for the cavity length ${L_{CF}}$ varying from 50 to 85 mm. The increase of the threshold pump power is mainly due to the increase of the cavity mode size. Figure 6 shows the experimental results for the output powers versus the cavity length ${L_{CF}}$ under the optimal LBO temperature at three different pump powers of 5.5, 12.5, and 20 W. It can be seen that the output power weakly depends the cavity length for the cavity length ${L_{CF}}$ shorter than 75 mm. In contrast, the output power exhibits a significant decrease for ${L_{CF}}$ longer than 75 mm. The conspicuous drop of the output power for ${L_{CF}}$ longer than 75 mm can be comprehended from the analysis of mode area matching shown in Fig. 2.
Fig. 5. Experimental results for the threshold pump power versus the cavity length ${L_{CF}}$ under the optimal LBO temperature. Solid lines by best fitting for eye guidance.
Fig. 6. Experimental results for the output powers versus the cavity length ${L_{CF}}$ under the optimal LBO temperature at three different pump powers of 5.5, 12.5, and 20 W. Solid lines by best fitting for eye guidance.
Experimental results reveal that the output powers for 555 and 559 nm are found to display the phenomenon of thermal rollover for the pump power higher than 22 W. The cavity length ${L_{CF}}$ for generating the maximum output power is found to be approximately 70 mm. Figure 7(a) shows the output powers versus the incident pump power for the generation of 555 and 559 nm at the optimal LBO temperatures with ${L_{CF}}$=70 mm. At a pump power of 22 W, the output powers for 555 and 559 nm can be seen to reach 6.6 and 6.3 W, respectively. The optical-to-optical conversion efficiencies from diode to green and lime lights are up to 30% and 29.5%, which are the highest efficiencies achieved in the CW solid-state Raman lasers at 555 and 559 nm, to the best of our knowledge. The lasing spectra for the cases of 555 and 559 nm at a pump power of 22 W are shown in Fig. 7(b) and 7(c), respectively.
Fig. 7. (a) Output powers versus the incident pump power for the generation of 555 and 559 nm at the optimal LBO temperatures with ${L_{CF}}$=70 mm, solid lines by best fitting for eye guidance. (b), (c) Lasing spectra for the cases of 555 and 559 nm at a pump power of 22 W.
The beam quality factor M2 was measured from the focused spot size and beam divergence angle. The knife-edge technique was used to measure the variation of the beam size along the propagation for the laser beam focused by a lens [29]. The measured results revealed that due to the Raman beam clean-up effect, the measured beam quality factors are found to be better than 2.5 at a pump power of 22 W for both cases of 555 and 559 nm.
6. Conclusion
In summary, we have achieved compact efficient CW Nd:YVO4 Raman lasers at 555 and 559 nm by using a KGW crystal as Raman gain medium and using an double-sided dichroic coating OC to enhance the resonance quality factor. We used the Np-cut KGW crystal to generate the Stokes waves with wavelengths at 1159 and 1177 nm by placing the polarization of the fundamental wave at 1064 nm to be parallel to the Ng and Nm axes, respectively. We exploited the LBO crystal for accomplishing the SFG to generate the green light at 555 nm and the lime light at 559 nm in a selectable way. We have analyzed the mode area matching for SRS and SFG as functions of the cavity length. Experimental results were thoroughly performed to comprehend the interplay between the mode area matching and the conversion efficiency. Under the optimal cavity length, the output powers 6.6 and 6.3 W could be generated at a pump power of 22 W for the wavelengths of 555 and 559 nm, respectively. Consequently, the overall conversion efficiencies were found to be up to 30% and 28.6% for 555 and 559 nm, respectively.
Funding
National Science and Technology Council (112-2112-M-A49-022-MY3).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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