Efficient high-power continuous-wave Nd:YVO4 visible lasers at versatile wavelengths of 532 (green), 559 (lime), and 588 nm (yellow) are demonstrated to be achieved by using the identical cavity mirrors and gain medium. A dichroic coating is deposited on one end surface of the gain medium to gather the backward green-yellow emission. The green, lime, and yellow outputs are individually optimized by using different phase-matched lithium triborate (LBO) crystals for second harmonic generation (SHG) of the fundamental field, sum frequency generation (SFG) of the fundamental and the stimulated Raman fields, and SHG of the stimulated Raman field, respectively. At a pump power of 31.6 W, the output powers at 532, 559, and 588 nm can be up to 6.8, 5.4, and 3.1 W. The high efficient and compact Nd:YVO4 lasers at green-lime-yellow wavelengths can be potentially beneficial to future applications in retinal photocoagulation.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
The continuing improvements in visible solid-state lasers have led to many and new applications in medical therapies, such as transpupillary thermotherapy, photodynamic therapy, and retinal photocoagulation [1–4]. The continuous-wave (CW) green laser has been regarded as a standard light source for pan-retinal photocoagulation because it can be highly absorbed by both hemoglobin and melanosomes. However, since the macula is full of xanthophyll pigments which have significant absorptions for green lights, high-power CW lime-yellow lasers are usually used in the treatment of macula. Therefore, the development of high-power green-lime-yellow lasers is highly desirable and indispensable in the ophthalmology for retinal photocoagulation [3,4].
The high-power lime-yellow laser can be efficiently achieved by combining the stimulated Raman scattering (SRS) in a crystalline material with second harmonic or sum frequency generation (SHG/SFG) in a nonlinear crystal [5–11]. An intriguing subgroup of the SRS process is the self-Raman laser in which the laser gain medium concurrently plays as a Raman active medium. It has been confirmed that YVO4 and GdVO4 crystals possess a rather high Raman gain of approximately 4.5 cm/GW in the V-O stretching mode with the frequency shift at 890 cm−1 and 882 cm−1, respectively [12–15]. The fundamental wavelength at 1064 nm and the Stokes wavelength at 1176 nm can be designed to be simultaneously resonate in a high-Q Nd:YVO4 laser cavity. By using this dual-wavelength cavity, a non-critically phase-matched (NCPM, θ = 90°, ϕ = 0°) LBO crystal was previously exploited to generate the 559-nm lime output when the crystal temperature was tuned at the phase matching for SFG of the fundamental and Stokes fields or the 588-nm yellow output when the temperature was tuned for the SHG of the Stokes fields .
The most critical design issue for achieving efficient lime-yellow output from self-Raman lasers is to minimize cavity losses. Since the cavity losses generally increase with increasing the number of the cavity components, the two-mirror linear resonator is often used to minimize the losses. However, Nd-doped vanadate crystals have a significant absorption in the green-lime-yellow spectral region. In previous studies, an intracavity mirror with the dichroic coating was usually used to reflect the backward green-yellow emission for avoiding the absorption in the gain medium [17–20].
In this work, we develop a compact cavity design that can efficiently generate high-power CW Nd:YVO4 visible lasers at wavelengths of 532, 559, and 588 nm. With the same cavity mirrors and laser gain medium, the output wavelengths can be individually selected and the output powers can be simply optimized, just by using different lithium triborate (LBO) crystals for critical phase-matched SHG or SFG. To save the usage of an intracavity mirror, the dichroic coating for reflecting the backward green-yellow emission is directly deposited on one end surface of the Nd:YVO4 crystal. The effectiveness of the dichroic coating is evaluated by making a comparison with the results obtained by using a gain medium with conventional anti-reflection coating. At a pump power of 31.6 W, the output power for 532-nm laser is found to increase from 3.4 W obtained with an anti-reflection coated gain medium to 6.7 W obtained with a dichroic coated crystal. At the same pump power, the output power at 559 nm is found to increase from 4.0 W to 5.4 W. The output power at 588 nm is enhanced from 2.4 W to 3.1 W. It is believed that the high efficient and compact Nd:YVO4 lasers at green-lime-yellow wavelengths can be potentially beneficial to future applications in retinal photocoagulation.
2. Material preparation and laser setup
We employed a-cut Nd:YVO4 crystals with 0.3 at.% Nd3+ concentrations as the laser gain media. The pump power required to reach the Raman threshold is given by Eq. (1), the threshold pump power for SRS is inversely proportional to the crystal length. Although a 8-mm Nd:YVO4 crystal is sufficiently long for absorbing the pump power at 808 nm, a 20-mm gain medium was used for reducing the SRS threshold.
A sample with broad-band anti-reflection coating in the visible spectrum was used to measure the single-pass absorption in the range of 530-590 nm. As shown in Fig. 1, the single-pass absorption in the range of 530-590 nm is generally greater than 60%. In other words, the green-yellow light back propagating into the gain medium will be mostly absorbed. In the previous work, an intracavity mirror with the dichroic coating was usually used to reflect the backward green-yellow emission for avoiding the absorption in the gain medium [5,6]. Here the dichroic coating was directly deposited on one end surface of the Nd:YVO4 crystal to replace the usage of an intracavity mirror for reflecting the backward green-yellow emission. Since the increase of the cavity losses will lead to a significant raise in threshold for the self-Raman operation, the specification of the dichroic coating on the end surface of the gain medium must be very rigid. Figure 2 shows the measured reflectance of the dichroic coating on the end surface of the gain medium. The reflectance between wavelengths of 530 and 590 nm is generally higher than 95%; on the other hand, the values of the reflectance at the wavelengths of 1064 nm and 1176 nm are as low as 0.12% and 0.3%, respectively.
The experimental setup for the cavity configuration of diode-end-pumped Nd:YVO4 laser with intracavity nonlinear conversion is shown in Fig. 3. Two types of surface coatings on Nd:YVO4 crystals were employed to investigate the performance difference. One type was the conventional anti-reflection coating (reflectance < 0.2%) at 808 and 1060-1190 nm, as shown in Fig. 3(a). The second type was the sample with the same anti-reflection coating on the surface toward the input mirror and the dichroic coating on the other surface, as shown in Fig. 3(b). The dimensions for both types of Nd:YVO4 crystals were 3 × 3 × 20 mm3. The laser crystals were wrapped with indium foil and mounted in a water-cooled copper holder at a temperature of 20 °C. The pump source was an 808-nm fiber-coupler laser diode with a core diameter of 600 μm, a numerical aperture of 0.16, and a maximum power of 32 W. The pump light was focused into the gain medium by using a pair of plano-convex coupling lenses each with a focal length of 50 mm. The waist radius of the pump beam was approximately 300 μm. The input mirror was a flat mirror that had an antireflection coating at 808 nm (reflectance < 0.2%) on the entrance face and a high-reflection coating at 1060-1180 nm (reflectance > 99.9%) and 530-590 nm (reflectance > 99.5%) as well as a high-transmission coating at 808 nm (transmittance > 95%) on the second surface. A concave mirror with a radius of curvature of 100 mm was used as the output coupler that 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 antireflection coating at 530-590 nm (reflectance < 0.2%) on the other face. The total cavity length was set up to be approximately 50 mm.
By using the same cavity mirrors and gain medium, the 532-nm green, 559-nm lime, and 588-nm yellow lasers can be individually generated by inserting different LBO crystals with cutting angles for the type-I phase-matching. In the principal XY plane of LBO crystal, the type I phase matching is determined by Figure 4 shows the theoretical results of the phase-matching cutting angles for the lasing wavelength located at 532, 559, and 588 nm. In our experiment, the 8-mm-long type-I LBO crystals were cut at θ = 90°; ϕ = 11.4°, 8.0°, and 3.9° to generate the lasers at wavelengths of 532, 559, and 588 nm, respectively. Both end surfaces were anti-reflection coated at the wavelength of fundamental and visible outputs. Thermo-electric coolers were utilized to maintain the temperatures of the LBO crystals at 24 °C.
3. Experimental results and discussion
First of all, the quality of Nd:YVO4 crystal was confirmed by using an output coupler with a reflectivity of 92% at 1064 nm to evaluate the output performance. The radius of curvature of the output coupler was 100 mm and the cavity length was 50 mm. The threshold pump power for 1064-nm output was found to approximately 0.7 W. At a pump power of 31.6 W, the output power could reach 17.6 W, corresponding to the conversion efficiency of 55.7%. The high output performances confirmed the good choice of the Nd:YVO4 and of the laser design.
Next, we explored the output performance at 532 nm by using a LBO crystal (θ = 90°, ϕ = 11.4°) with the phase matching for SHG of the fundamental field. Figure 5 shows the output power at 532 nm as a function of incident pump power. The pump thresholds are nearly the same and approximately 0.5 W for both kinds of Nd:YVO4 crystals. This result confirms that the dichroic coating does not lead to an obvious increase for the cavity loss at 1064 nm. At a pump power of 31.6 W, the output power can be seen to increase from 3.4 W obtained with an anti-reflection coated gain medium to 6.7 W obtained with a dichroic coated crystal. To be brief, the green output power is nearly doubled by gathering the backward SHG emission.
To optimize the output performance at 559 nm, a LBO crystal with the cutting angle of θ = 90° and ϕ = 8.0° was used in the cavity for the phase matched SFG of the fundamental and Stokes fields. Figure 6 shows the output power at 559 nm as a function of incident pump power. The pump thresholds were found to be 2.5 and 5.0 W for the gain media to have the conventional anti-reflection and the dichroic coatings, respectively. A considerable increase in the lasing threshold came from the additional intracavity loss caused by the dichroic coating on the gain medium. As shown in Fig. 2, the reflectance of the dichroic coating at the Stokes wavelength is approximately 0.3% that is higher than the value of the anti-reflection coating, typically 0.1%. From Eq. (1) it can be confirmed that even only a 0.3% cavity loss can lead to a significant increase in the intracavity SRS threshold. The higher threshold leads the output power obtained with the dichroic coating to be inferior to that obtained with the anti-reflection coating at the pump power lower than 14 W. Nevertheless, since the backward SFG can be effectively reflected to enhance the output efficiency, the output power obtained with the dichroic coating is superior to that obtained with the anti-reflection coating at the pump power greater than 15 W. As shown in Fig. 6, the output power at 559 nm increases from 4.0 W to 5.4 W at a pump power of 31.6 W. According to the results, when the Nd:YVO4 crystal with high-reflection coating was utilized, the average output power rose by 35% and the slope efficiency was improved from 13.6% to 21%. Although the high-reflection coating made the lasing thresholds at 559 nm become higher, the slope efficiencies and the output powers were both improved significantly.
Finally, a LBO crystal with the cutting angle of θ = 90° and ϕ = 3.9° was exploited to optimize the output performance at 588 nm. Figure 7 shows the output power at 588 nm as a function of incident pump power. The pump thresholds for generating 588-nm laser can be seen to be nearly the same as the results shown in Fig. 6 for the 559-nm case. The similar pump thresholds for 559-nm and 588-nm outputs come from the fact that the Stokes field is the necessary component for generating both outputs. As a result, the comparison between the dichroic coating and the anti-reflection coating for the 588-nm output is almost the same as that for 559-nm case shown in Fig. 6. The only difference between the 588-nm and 559-nm outputs is the conversion efficiency. At a pump power of 31.6 W, the output powers at 588 nm can be seen to increase from 2.4 to 3.1 W by using the gain medium with the dichroic coating. Based on the present results, the further improvement on the dichroic coating to reduce the SRS threshold can be expected to enhance the output efficiency considerably.
In conclusion, we have achieved efficient high-power continuous-wave Nd:YVO4 visible lasers at versatile wavelengths of 532 (green), 559 (lime), and 588 nm (yellow) by using the same cavity mirrors and gain medium. A dichroic coating was directly deposited on the end surface of the gain medium to reflect and gather the backward green-yellow generation. The green, lime and yellow outputs are individually optimized by using different phase-matched LBO crystals for SHG of the fundamental field, SFG of the fundamental and the Stokes fields, and SHG of the Stokes field, respectively. Experimental results reveal that the dichroic coating does not lead to an obvious increase for the cavity loss at 1064 nm and the green output power can be nearly doubled by gathering the backward SHG emission. On the other hand, for the 559-nm and 588-nm operations the dichroic coating on the gain medium leads to a considerable increase in the lasing threshold. Even so, since the backward component can be effectively gathered to enhance the output efficiency, the output power obtained with the dichroic coating can be higher than that obtained with the anti-reflection coating at the pump power greater than 15 W. The output powers at 532, 559, and 588 nm can be up to 6.8, 5.4, and 3.1 W at a pump power of 31.6 W.
Ministry of Science and Technology of Taiwan (MOST) (107-2119-M-009-015).
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