Abstract
Introduction
Visible laser sources around 670 nm are in great demand in biomedical fields, such as photocoagulation
In general, laser operations around 670 nm are generated by the conventional frequency-doubled Nd:YVO4 lasers
Experimental setup
As shown in Fig. 1, a commercial InGaN laser diode (LD) array emitting at ~444 nm with the maximum output power of 24 W was applied as the pump source. To reduce the spherical aberration and make the focusing spot of the pump beam smaller, an aspherical plane-convex lens with a focal length of 75 mm was used to focus the pump light. A simple end-pumped plane-concave laser cavity with the insertion of one/two etalons (i.e., Etalon-1 and/or Etalon-2) was constructed to obtain the wavelength-switchable laser operation. The input coupler (IM) was based on a coated plane mirror, which has a high reflectivity (>99.9%) at 665−700 nm and high transmissions at 600−639 nm. A piece of 100 mm radius-of-curvature plane-concave mirror with transmissivities of 0.3% at 670 nm, 0.5% at 675 nm, and 0.9% at 679 nm was applied as the output coupler (OC). It is worth pointing out that the transmission of the OC at 698 nm is over 60%, and can effectively prevent the available optical gain from lasing. The etalons are made of optical glass BK7. The thickness of Etalon-1 is 100 µm, and it was inserted at the Brewster angle to suppress the π-polarized emissions. Another three pieces of Etalon-2 with different thicknesses of 100 µm, 150 µm, and 200 µm were vertically inserted into the cavity and tilted to achieve wavelength-switchable laser oscillations.
Figure 1.
A commercial fabricated a-cut Pr3+:YLF crystal employed for the following experiments has a low Pr3+ doping ratio of 0.12 at. %, the length of 15 mm, and 3 mm × 3 mm polished facets without anti-reflection coating. To protect the crystal from thermally induced fragmentation, we wrapped the crystal with indium foil and placed it in a water-cooled copper crystal holder. The temperature of the cooling water was set to 13 °C. A wide ~2.2 nm of full width at half maximum (FWHM) of the pump laser spectrum leads to a relatively low absorption efficiency ~51% of the Pr3+:YLF crystal. Here ~10% loss of pump laser power was introduced by the plane-convex lens and the plane mirror (i.e., IM). The physical cavity length was optimized to be 53 mm. It should be noted that the performance of the single-wavelength 670.4 nm laser was optimized with a concave-plane cavity (to reduce thermal effects) with the same cavity parameters, rather than the above-described plane-concave cavity.
Results and discussion
Visible single-wavelength laser operation
Figure 2 presented the experimental results of the single-wavelength laser operation around 670 nm. The maximum output powers of 2.60 W, 1.26 W, and 0.21 W with the maximum slope efficiencies of 34.7%, 27.3%, and 12.3% were achieved at 670.4 nm, 674.2 nm, and 678.9 nm, respectively. Firstly, laser oscillation at π-polarized 670.4 nm was obtained by the concave-plane cavity above-mentioned without etalons. To obtain single-wavelength operations at two σ-polarized wavelengths, Etalon-1 was inserted into the cavity at the Brewster angle to suppress emission at π-polarized 670.4 nm. Meanwhile, two pieces of Etalon-2 with thicknesses of 100 µm and 150 µm were vertically inserted into the cavity to obtain lasing at 674.2 and 678.9 nm, respectively. The laser spectra were measured by a spectrometer (Ocean Optics, HR4000+). Due to the relatively low resolution (0.3 nm), the linewidths appeared to be a bit large. Since the YLF crystal generally exhibits negative thermal lensing effects
Figure 2.
Figure 3.
To further understand the high-power and high-efficiency performance under high-power pumping conditions, simulations of the 670-nm laser with different beam radius (affected by the thermal lensing effects) were carried out. The input-output power characteristics can be expressed by
where
where
where
Figure 4.
T(%) | |||||||
0.0476 | 0.173 | 0.3 | 0.002 | 1694.2 | 6.7×10-4 | 0.391 | 15 |
Table 1.
Visible multi-wavelength laser operation
Multi-wavelength laser oscillations around 670 nm were achieved by tilting the crystal and inserting the etalons with different thicknesses. As shown in Fig. 5, laser performances of the multi-wavelength operation were characterized. By slightly tilting the YLF crystal to adjust the intracavity losses, the maximum output power of 2.52 W was achieved at dual-wavelength 670.1/674.8 nm (see Fig. 5(a)). But notably, such a dual-wavelength operation only appeared under high-power pumping (i.e., over 10 W of absorbed pump power). The dual-wavelength laser at 675.0/679.4 with the maximum output power of 1.80 W and the maximum slope efficiency of 34.1% was achieved by inserting the etalon-1 with 100 µm thickness at the Brewster angle. And the dual-wavelength laser at 670.1/679.1 nm with the maximum output power of 0.36 W was achieved by vertically inserting the Etalon-2 with 200-µm thickness and then tilting it finely.
Figure 5.(
Figure 5(b) shows the dual-wavelength laser spectra at 670.1/674.8, 670.1/679.1, and 675.0/679.4, respectively. The intensities of these pairwise lasers in the dual-wavelength operations were comparable, which implied the potential of obtaining a higher power laser at ~679 nm with suitable etalons to match the emission peak. Then, as seen in Fig. 5(c), two types of triple-wavelength laser operations were also achieved. The triple-wavelength laser at 670.4/674.8/679.4 nm with output power of 1.78 W was obtained by tilting the crystal, and it only appeared at the available highest pumping. The triple-wavelength laser at 672.2/674.2/678.6 nm with the maximum output power of 0.84 W also only appeared under the available highest pumping, but it was achieved by inserting both Etalon-1 with 100-µm thickness and Etalon-2 with 150-µm thickness. Power stabilities of these multi-wavelength lasers are presented in Figs. 5(d) and 5(e). Due to the mode competition, the stabilities of the multi-wavelength lasers are generally worse than the single-wavelength lasers. Wavelength drift was not observed during the output power stability measuring.
The power transmission of the etalon can be expressed by
where
Visible vortex laser operation
A visible vortex laser at 670.4 nm was obtained by using an off-axis pumping technique. The laser cavity was designed as the concave-plane one above-mentioned. By carefully rotating the plane mirror in x and y directions, two diagonal Hermite-Gaussian (HG) 01 modes could be obtained because the threshold of the fundamental mode could exceed the HG01 mode under off-axis conditions. Then, two diagonal HG01 modes at 3π/4 and π/4 angles (relative to the horizontal direction) could form the Laguerre-Gaussian (LG) mode with an introduced Gouy phase of π/2
Figure 6.(
Conclusions
A novel wavelength switching of CW visible Pr3+:YLF laser was demonstrated around 670 nm. The maximum output power of 2.60 W and the maximum slope efficiency of 34.7% obtained for a single-wavelength laser at 670.4 nm are the highest values so far. Single-wavelength laser operations at 674.2 nm and 678.9 nm were demonstrated for the first time. Investigated good beam qualities with M2 below 1.6 contribute to the practical applications. Multi-wavelength laser operations are characterized by the dual-wavelength lasings (i.e., 670.1/674.8 nm, 670.1/679.1 nm, and 675.0/679.4 nm, respectively) and the triple-wavelength lasings (i.e., 672.2/674.2/678.6 nm and 670.4/674.8/679.4 nm, respectively). Moreover, the visible vortex laser at 670.4 nm was also realized for the first time. Such a novel wavelength-switchable visible laser and vortex laser around 670 nm could open up new horizons for the practical applications in biophotonics fields. Though the high-power lasers around 670 nm were obtained, OCs with different transmissions at this wavelength region were not yet explored since there are no available mirrors in our lab. Other tuning methods are worth trying to obtain broader and continuous tuning since the free spectrum range of the etalon is quite narrow. Besides, the reason for the unsustainable vortex laser under high-power pumping should be studied in details in the future to propose more rational schemes to obtain higher output power.
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