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Abstract
Efficient diode-pumped passively Q-switched Nd:YVO4 yellow and orange lasers are developed with the pulse pumping scheme and the intracavity stimulated Raman scattering (SRS) and second harmonic generation (SHG). A Np-cut KGW is exploited in the SRS process to generate the yellow 579 nm laser or the orange 589 nm laser in a selectable way. The high efficiency is achieved by designing a compact resonator to include a coupled cavity for intracavity SRS and SHG and to provide a focused beam waist on the saturable absorber for reaching an excellent passive Q-switching. The output pulse energy and peak power can reach 0.08 mJ and 50 kW for the orange laser at 589 nm. On the other hand, the output pulse energy and peak power can be up to 0.10 mJ and 80 kW for the yellow laser at 579 nm.
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1. Introduction
High-peak-power yellow-orange lasers between 570 and 590 nm are of practical importance for plentiful applications such as dermatology [1], ophthalmology [2], flow cytometry [3], optical-resolution photoacoustic microscopy [4], and stimulated emission depletion microscopy [5]. Owing to the match with the peak absorption of oxygenated hemoglobin, yellow lasers near 577±3 nm are frequently used in medical treatment [6,7]. On the other hand, orange lasers at 589 nm are especially useful for atmospheric lidar measurement [8] and sodium guide star [9,10]. Numerous technologies have been developed for generating yellow-orange lasers including semiconductor lasers [11], optically pumped semiconductor lasers [12], and fiber lasers [13].
Currently, the most efficient technology for generating high-peak-power yellow-orange lasers is to exploit the diode-pumped Q-switched solid-state laser with the intracavity stimulated Raman scattering (SRS) and second harmonic generation (SHG) [14–17]. In comparison with the active Q-switching, the diode-pumped solid-state laser passively Q-switched by a saturable absorber has the benefits of compactness, simplicity, and ruggedness [18,19]. Nevertheless, the optical-to-optical conversion efficiency attained by passively Q-switched yellow-orange lasers was usually less than 10% [20,21]. Besides, passively Q-switched lasers were usually disadvantaged by the time jitter arising from the certain fluctuation of the pump and emission [22]. It has been recently confirmed that the timing jitter could be substantially reduced with the pulse pumping by using a shorter pump duration and a higher pump power [23,24].
In this work, we originally employ the pulse pumping scheme and the intracavity SRS and SHG to accomplish efficient diode-pumped passively Q-switched Nd:YVO4 yellow and orange lasers with a near-concentric cavity. By using a Np-cut potassium gadolinium tungstate (KGW) as the Raman gain medium, the yellow laser at 579 nm or the orange laser at 589 nm are generated in a selectable way by orienting the polarization of the fundamental wave along the Ng or Nm axis of the KGW crystal. In addition to satisfying the criterion of an efficient passive Q-switching, the developed resonator consists of a coupled cavity for the intracavity SRS and SHG. The output characterizations for both operations of yellow and orange lasers are thoroughly explored by considering the pump-to-mode size ratio. For the operation of yellow laser at 579 nm, the highest pulse energy and peak power are 0.10 mJ and 80 kW, respectively. To the best of our knowledge, this is the maximum peak power at 579 nm obtained in a passively Q-switched Raman laser. On the other hand, the highest pulse energy and peak power for the operation of orange laser at 589 nm are 0.08 mJ and 50 kW, respectively. The optimal optical-to-optical conversion efficiencies for the yellow and orange lasers are 11.5% and 11.0%, respectively.
2. Material preparation and laser setup
Figure 1 depicts the cavity setup for generating the yellow and orange lasers by using intracavity SRS and SHG in a diode-pumped passively Q-switched Nd:YVO4 laser. The pump source was a fiber-coupled diode laser at 808 nm with the maximum output power of 40 W. The fiber of the pump source had a core diameter of 0.2 mm and a numerical aperture of 0.22. The output power of the pump diode was precisely controlled at a pump duration of 40 µs and a pump frequency of 10 kHz. We used an a-cut Nd:YVO4 crystal with a Nd3+ concentration of 0.2 at.% as the gain medium. The size of the Nd:YVO4 crystal was 3×3×20 mm3. Both end surfaces of the Nd:YVO4 crystal had an anti-reflection (AR) coating at the wavelengths of covering 808, 1064, and 1160-1180 nm (reflectance < 0.2%). The rear mirror of the laser cavity was a concave mirror with a radius of curvature of 85 mm. The first surface of the input mirror toward the pump source had an AR coating at 808 nm (reflectance < 0.2%). The concave surface of the input mirror had a high reflection (HR) coating in the region of 1000 to 1200 nm (reflectance > 99.9%) and high transmission (HT) coating at 808 nm (transmittance > 95%). We utilized a Cr4+:YAG crystal with initial transmission of To = 60% as a saturable absorber for the passive Q-switching. Both end surfaces of the Cr4+:YAG crystal had an AR coating with the range of 1060 to 1180 nm (reflectance < 0.2%). We employed a Np-cut KGW crystal with the dimension of 3×3×20 mm3 as the Raman gain medium for the intracavity SRS. As shown in Fig. 2 for the spontaneous Raman spectrum of the KGW crystal, the yellow laser at 579 nm could be generated by setting the polarization along the Ng axis for the highest Raman shift at 768 cm-1. On the other hand, the orange laser at 589 nm was obtained by setting the polarization along the Nm axis for the highest Raman shift at 901 cm-1 [25–28]. The first surface of the KGW crystal toward the input mirror had a coating with HR at 1150-1180 nm (reflectance > 99.9%) and HT at 1064 nm (transmittance > 99%). The other surface of the KGW crystal toward the output coupler had a coating with HT (transmittance > 99%) at the wavelengths covering 1064 nm and 1150-1180 nm and HR at 570-590 nm (reflectance > 99%) to reflect the backward SHG for the yellow-orange light. We used indium foils to wrapped all the crystals of Nd:YVO4, Cr4+:YAG, and KGW. The wrapped crystals were installed in copper holders that were actively cooled at 20 °C. We exploited a LBO crystal with the length of 8 mm and the cut angle of θ = 90° and ϕ = 3.9° for the intracavity SHG of the Stokes wave. The temperature of the LBO crystal could be specifically tuned by using a thermoelectric cooler for the critical phase matching. Both end surfaces of the LBO crystal had an AR coating at the wavelengths of covering 570-590, 1064, and 1150-1180 nm (reflectance < 0.2%). We utilized a concave mirror with a radius of curvature of 85 mm as the output coupler. The concave surface of the output coupler had a coating with HR at 1050-1180 nm (reflectance > 99.9%) and HT at 570-590 nm (transmittance > 95%). The other surface of the output coupler had an AR coating for yellow-orange output (reflectance < 0.2%).
The geometric length of the resonator for the fundamental wave was adjusted to be near 187 nm. Accordingly, the optical length of the fundamental cavity could be found to be approximately 161 mm. The separation between the laser crystal and the input mirror was approximately 1.0 mm. The average mode radius for the fundamental wave in the laser crystal could then be computed to be around ωc = 0.32 mm. The quality of a passive Q-switching is principally associated with the parameter $\alpha = A{\sigma _{gs}}/(\gamma {A_s}\sigma \textrm{)}$, where σ is the stimulated emission cross section of the laser crystal, γ is the inversion reduction factor, and σgs is the absorption cross sections of the ground state of the saturable absorber, A and As are the mode areas for the fundamental wave in the laser crystal and saturable absorber, respectively [29–31]. The theoretical analysis reveals that a higher value of the parameter α leads to a better performance for the passive Q-switching. For satisfying the criterion of a good passive Q-switching, the saturable absorber was positioned at the location of the beam waist, namely in the center of the cavity. The average mode radius for the fundamental wave in the saturable absorber could be computed to be around ωs=0.08 mm. From $A/{A_s} = \omega _c^2/\omega _s^2$, σ = 15.6×10−19 cm2, σgs= 8.7×10−19 cm2, and γ = 1, we could confirm that the value of the parameter α was approximately 8.9. Note that as long as the value of α is greater than 5.0, there is no much room for enhancing the performance of the passive Q-switching [29–31]. The cavity for the Stokes wave was constituted by the first surface of the KGW crystal and the output coupler. The optical length for the Raman cavity was adjusted to be around 80 mm for forming a nearly hemispherical resonator to enhance the SRS process. The Raman crystal was located close to the saturable absorber. Similarly, the separation between the LBO and KGW crystals was rather small. The distances among the crystals of Cr4+:YAG, KGW, and LBO were all around 1.0 mm.
Fig. 1. Experimental setup for generating the yellow and orange lasers by using intracavity SRS and SHG in a diode-pumped passively Q-switched Nd:YVO4/KGW Raman laser.
3. Experimental results and discussion
The theoretical analysis [29] indicates that the output performance of the passive Q-switching is significantly related to the pump-to-mode size ratio ωp/ωc. To explore the dependence of the output performance on the ratio ωp/ωc, three different pairs of coupling lenses were prepared to focus the pump beam into the gain medium. Consequently, three different pump radii of ωp=0.21, 0.32, and 0.43 mm could be utilized in experiment. Experimental results revealed that the optimal temperatures of the LBO crystal were 37 and 24 °C for the operations of yellow and orange lasers at 579 nm and 589 nm, respectively. Figure 3 shows experimental threshold energy at 808 nm versus the pump-to-mode size ratio ωp/ωc for the operations of yellow and orange lasers. The pump threshold energy for the yellow generation is slightly higher than that for the orange generation because the stimulated Raman gain coefficient at 901 cm-1 is somewhat greater than that at 768 cm-1. For the ratio ωp/ωc rising from 0.66 to 1.34, the pump threshold energy grows from 0.58 to 0.86 mJ for the 579 nm yellow generation. On the other hand, the threshold energy increases from 0.50 to 0.70 mJ for the 589 nm orange generation. The overall growing rate for the pump threshold is proportional to the effective more area $\pi (\omega _p^2 + \omega _c^2)$, matching the theoretical analysis [29–31].
Fig. 3. Experimental threshold energy at 808 nm versus the pump-to-mode size ratio ωp/ωc for the operations of yellow and orange lasers. Solid lines for eye guidance.
Figure 4 shows experimental results for the output pulse energies of yellow and orange lasers versus the pump-to-mode size ratio ωp/ωc. Following the increase of the pump threshold energy, the output pulse energies of yellow and orange lasers reveal the same increasing tendency with increasing the ratio ωp/ωc. For the ratio ωp/ωc rising from 0.66 to 1.34, the output pulse energy increases from 0.072 to 0.10 mJ for the yellow laser at 579 nm and increases from 0.058 to 0.077 mJ for the orange laser at 589 nm. The optical-to-optical conversion efficiencies for both the yellow and orange lasers are all greater than 11.0% for three different pump-to-mode size ratios ωp/ωc.
Fig. 4. Experimental results for the output pulse energies of yellow and orange lasers versus the pump-to-mode size ratio ωp/ωc. Solid lines for eye guidance.
Fig. 5. Experimental results for the pulse widths of yellow and orange lasers versus the pump-to-mode size ratio ωp/ωc. Solid lines for eye guidance.
Figure 5 plots the experimental results for the pulse widths of yellow and orange lasers versus the pump-to-mode size ratio ωp/ωc. For the ratio ωp/ωc rising from 0.66 to 1.34, the pulse width decreases from 1.5 to 1.25 ns for the yellow output at 579 nm and decreases from 2.0 to 1.52 ns for the orange output at 589 nm. On the whole, the peak-to-peak fluctuation in the pulse train could be in the range of ±5.0% to ±7.0% for both yellow and orange lasers.
Figure 6 shows the typical pulse train and temporal profile in the yellow and orange outputs; both results are measured for the case of ωp/ωc =1.34. The temporal profile could be confirmed to have no satellite pulse in both operations of yellow and orange lasers. Furthermore, the beam quality factor M2 for both yellow and orange outputs were measured to be generally better than 1.6 due to the clean-up effect of the SRS process. Figure 7 shows the output peak powers of yellow and orange lasers versus the pump-to-mode size ratio ωp/ωc. The maximal output peak powers can be seen to be 80 and 50 kW for the yellow and orange lasers, respectively.
Fig. 6. Typical pulse train and temporal profile in the yellow output at 579 nm (upper) and orange output at 589 nm (lower) measured with ωp/ωc =1.34.
Fig. 7. Output peak powers of yellow and orange lasers versus the pump-to-mode size ratio ωp/ωc. Solid lines for eye guidance.
4. Conclusion
In summary, we have developed efficient diode-pumped passively Q-switched Nd:YVO4 yellow and orange lasers by using the pulse pumping scheme and the intracavity SRS and SHG. We employed a Np-cut KGW as the Raman crystal to flexibly generate the yellow 579 nm laser or the orange 589 nm laser just by placing the polarization of the fundamental wave along the Ng or Nm axis of the KGW crystal. The designed resonator not only included a nearly hemispherical cavity for the Raman cavity with intracavity SHG but also providing a focused beam waist for reaching an efficient passive Q-switching. By using an appropriate pump size, the highest pulse energy and peak power can be up to 0.10 mJ and 80 kW for the yellow laser at 579 nm, whereas for the orange laser at 589 nm the highest pulse energy and peak power can reach 0.08 mJ and 50 kW, respectively.
Funding
Ministry of Science and Technology, Taiwan (109-2119-M-009-015-MY3).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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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