1. Introduction

Lasers in wavelength range of 1.5x μm are of great interest because of their eye-safe feature [1,2]. By virtue of the reduced risk of hurting human-eye, the 1.5x-μm lasers are widely used in remote sensing and communication. Stimulated Raman scattering (SRS) is one of the mature approaches to generate lasers in this region. By exploiting the Raman frequency shifting of 800-1100 cm⁻¹, the 1.3x-μm emission of Nd-doped lasers could be converted to 1.5x μm efficiently [3–8]. Though optical parametric oscillators and Er-, Er/Yb-doped fiber lasers/amplifiers could generate tens to hundreds watts of output in this wavelength range efficiently [9,10], Raman lasers are still a promising method for moderate power applications for their merits of compact structure, high efficiency and economical cost. During the past decade, YVO₄ and GdVO₄, two commonly used laser host materials, were proved to be reliable Raman-active mediums [11–13]. Eye-safe Raman lasers based on self-SRS conversion in the Nd-doped vanadates and diffusion-bonded composite crystals have enhanced these advantages and therefore attracted increasing interests [14,15].

Despite the advantages mentioned above, eye-safe self-Raman lasers suffer from serious thermal effects. The large quantum defect when laser operates at 1.3 μm results in heavy thermal load, which would be further exacerbated by the heat generated during SRS process. Thermal load blocks the output power of self-Raman lasers mainly on the following two aspects. The thermal lens induced cavity instability limits the maximum pump power allowed and thus hinders the power scaling. Meanwhile, high crystal temperature would decay the Raman gain coefficient, and therefore harm the SRS efficiency. To overcome this drawback, variety of methods, e. g. diffusion-bonded crystals and in-band pumping, are used to relieve the influence of thermal effects. In 2009, K. W. Su and Y. F. Chen et al. reported an 1525-nm self-Raman laser with a double-end diffusion-bonded Nd:YVO₄ crystal as the gain medium, which obtained the highest average output power reported to date of 2.23 W [16]. In 2011, 880-nm in-band pumping is introduced to 1.5-μm self-Raman lasers by X. H. Chen and X. Y. Zhang et al. The maximum conversion efficiency with respect to absorbed pumping reached 19.2% [17]. However, the relatively low pump absorption would limit the optical efficiency of the laser to some extent, especially for self-Raman lasers that only low doping concentration is appropriate for the reason of Raman gain [14]. In this paper, we report an efficient Q-switched eye-safe laser based on SRS in Nd:YVO₄-YVO₄ crystals. A wavelength-locked laser diode array (LDA) at 878.6 nm is used to in-band pump the laser to minimize the thermal load and meanwhile ensure the pump absorption. A maximum 1525-nm average output power of 5.2 W is obtained under the incident diode power of 30.6 W with a high pulse repetition frequency (PRF) of 140 kHz. The optical efficiency is 17.0% and the conversion efficiency with respect to absorbed pump power reaches 19.8%. The output power and efficiencies are the highest within this kind of compact eye-safe lasers ever reported.

2. Experimental arrangement

The experimental arrangement is shown in Fig. 1. The output of a fiber-coupled LDA, which is used as the pump source, is reimaged into an a-cut Nd:YVO₄ crystal by a multi-lens coupler. The core diameter and the NA of the fiber are 400 µm and 0.22, respectively. The 0.3-at.% doped, 3 × 3 × 10 mm³ crystal provides part of the Raman gain as well as the 1342-nm laser gain. Considering the Raman gain is proportional to the length of Raman gain medium, an a-cut pure YVO₄ crystal with dimensions of 3 × 3 × 20 mm³ is inserted into the cavity for the purpose of sufficient Raman gain instead of using one long and monoblock but relatively expensive composite crystal. The facets of both crystals are anti-reflectively (AR) coated at 800-1600 nm (R<3%@880 nm, R<0.1%@1342&1525 nm). The two crystals are both wrapped in indium foil and clamped in aluminium holders which are cooled by refrigerant water at the temperature of 15°C. The flat mirror M1 is anti-reflectively (AR) coated at 800-1200 nm on both facets (R<5%@880 nm) and high-reflectively (HR) coated at 1300-1600 nm on one facet (R>99.9%@1342&1525 nm). It makes the resonator with a concave mirror M2 of 100-mm radius of curvature which is coated for AR at 1064 nm (R<20%), HR at 1342 nm (R>99.8%) and T = 7.2% at 1525 nm. The 20-mm-long, 80-MHz acousto-optic Q-switch driven by 15.0 W of RF power has AR coatings over fundamental laser and first Stokes wavelength range (R<0.2%) on both faces. The high ultrasonic frequency of 80 MHz is used to ensure the diffraction efficiency.

 

Fig. 1 Schematic of the eye-safe Nd:YVO₄- YVO₄ Raman laser.

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3. Results and discussion

We first use an 880-nm LDA as the pump source and a commercial 1:1 coupler to refocus it, which results in a pump spot radius of 200 µm in the Nd:YVO₄ crystal. ~77% of the incident non-polarized pump could be absorbed by the 0.3-at.% doped, 3 × 3 × 10 mm³ Nd:YVO₄ crystal. The total cavity length is 82 mm, with the distances between the five cavity elements of 2, 15, 10 and 5 mm, respectively, from left to right. The beam spot radius of 1342-nm fundamental laser in the Nd:YVO₄ crystal is ~230 µm, which matches the 200-µm pump spot well, when the thermal focal length in the Nd:YVO₄ crystal reaches its shortest value allowed of ~60 mm. Figure 2 shows the 1525-nm Stokes average output power as functions of incident LD power at different PRFs. The curves are obtained by decreasing the pump power gradually after optimizing the cavity alignment under the maximum pump power. A maximum average output power of 2.76 W is obtained at the PRF of 140 kHz under the incident pump power of 16.7 W, corresponding to an optical efficiency of 16.5%. The conversion efficiency with respect to absorbed pump power reaches 21.4%, higher than other vanadate-based eye-safe self-Raman lasers reported before. The maximum output under the PRFs of 100 and 160 kHz are 2.52 and 2.57 W, respectively. The SRS threshold is 2.1 W at 100-kHz PRF and rises to 2.8 W at 140 and 160 kHz. Since the cavity is aligned under the maximum pump power, the actual thresholds should be lower if realign the cavity at pump level near them. The 1525-nm output power rolls over when we further increase the pump beyond 16.7 W. We find that thepattern on the IR detection card decays obviously by then. The inset of Fig. 2 shows a beam profile recorded by the Ophir Pyrocam III under incident pump power of 17.6 W and PRF of 140 kHz (average output power of 2.03 W). It can be seen that higher order transverse mode rather than the fundamental mode is oscillating, which indicates that the thermal focal length in the Nd:YVO₄ crystal is as short as below 60 mm. Such serious thermal effect is mainly due to the large quantum defect when laser operating at 1342 nm and the extra heat generation from SRS. In our former work, an 1176-nm self-Raman laser with the same crystal doping and cavity arrangement could still operate stably under 26.8-W incident 880-nm pump power. Thermal-lens-limited pump power is the major obstacle to high eye-safe output, though 880-nm pumping with less thermal load than 808-nm pumping is adopted. To further increase the eye-safe output power, we must circumvent the limitation of the thermal effects. Thereupon, following efforts are made.

 

Fig. 2 The average output power of 1525-nm Stokes as functions of incident/absorbed pump power at different PRFs. The inset is the beam profile when output power rolls over.

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First, we redesign the holders to place the elements as close as possible (2, 10, 4, 2 mm, respectively, from left to right) and thus shorten the cavity length to 68 mm, which results in a minimum thermal focal length allowed of ~45 mm (the thermal focal length of the pure YVO₄ crystal induced by SRS is estimated to be larger than 500 mm under the maximum power by using the Eq. (30) in ref [4].). In addition to that, two focal lenses with focal lengths of 25 and 50 mm, respectively, are used to reimage the pump beam instead of the commercial 1:1 coupler. The larger pump spot radius of ~400 μm could help relieve the thermal lens effect as well as avoid the thermal fracture under high pump power, and thus increase the upper limit of pump power. Furthermore, to improve the optical efficiency, a wavelength-locked LD array emits at 878.6 nm with spectral linewidth of less than 0.3 nm is used as the pump source instead of the common LD at 880 nm [18]. Because the narrow-linewidth pump matches accurately to the in-band pumping absorption peak of Nd:YVO₄ crystal, the pump absorption fraction of the 0.3-at.%-doped, 10-mm-long crystal increases from 77% to 86%. Though the wavelength-locked LDs are a little more expensive than the common ones, the better pump absorption helps improve the efficiency or allows the shorter crystal length and lower doping concentration without impeding the conversion efficiency, which offsets the cost to some extent and reduces the risk of crystal damage.

Figure 3 gives the eye-safe average output power as functions of incident pump after the series of optimizing. The maximum output power of 5.2 W is also obtained with the PRF of140 kHz, under the incident pump power of 30.6 W. The corresponding optical efficiency of 17.0% is higher than that of 16.5% above because of the better pump absorption. However, the conversion efficiency with respect to absorbed pump power of 19.8% is a little lower than that with the 200-μm pump radius. We blame this on the poor pump/laser mode matching in the Nd:YVO₄ crystal. The thermal focal length under the maximum pump power is calculated to be 80-90 mm with the 400-μm pump radius, which leads to a fundamental mode beam radius of ~150 μm in the crystal. Large difference between the pump and laser mode size decreases the conversion efficiency. 4.91 and 5.03 W of average output power are obtained at the PRFs of 120 and 160 kHz, respectively, under the same pump power. The 1.5-μm self-Raman lasers reported before are mostly operated with lower PRFs of tens of kilohertz for the reason of SRS efficiency which is related to peak power of fundamental wave. The results here reveal that compact vanadate self-Raman lasers are capable of generating multi-watts of output efficiently with high PRFs of over 100 kHz. The output powers do not exhibit any saturation under this level of pump. Higher pump power is not applied for the risk of crystal damage. The SRS thresholds with the 800-μm pump spot are ~5 W, higher than those with the smaller pump spot above. The M² factor in x and y directions are 3.53 and 3.67, respectively, at the maximum power of 5.2 W.

 

Fig. 3 1525-nm average output power versus incident/absorbed pump power and optical efficiency after optimizing the laser cavity, pump spot size and pump linewidth.

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It is worth notice that there are obvious dips on the average output power and efficiency curves under the incident pump power of ~20 W and the average output power fluctuate seriously by then, as shown in Fig. 3. The good pulse-to-pulse stability recorded by oscilloscope also becomes worse by then. The error bars and efficiencies at the PRFs of 120 and 160 kHz, which exhibit the same trends with those at 140 kHz, were not drawn in the figure for conciseness. In Fig. 2 when pump radius is 200 μm, the dips occur under the pump power of ~7-10 W. Similar dips have also been observed by other researchers with frequency-doubled intracavity Raman lasers and were attributed to cavity instability and mode size changing caused by thermal lens [19,20]. It is observed in our experiment that the output power could recover to a reasonable level after realigning the cavity at the dip. The beam profiles of the 1525-nm output at different pump level are shown in Fig. 4. It can be seen that the beam profile is elliptical when the dip occurs (Fig. 4b). After realigning, it becomes better but still not very good (Fig. 4c). Figure 5 shows the pulse duration of 1525-nm output as functions of incident/absorbed pumping at 878.6 nm. Under the same maximum incident pump power of 30.6 W, the pulse durations at the PRFs of 120, 140, and 160 kHz are 10.3, 10.9, and 12.2 ns, which are longer than those in other researchers’ work because of the high PRF and large pump spot here [16,17]. The pulse energy and peak power with the maximum average output power of 5.2 W are 37.1 μJ and 3.4 kW, respectively.

 

Fig. 4 Beam profiles of 1525-nm Stokes output at different pump/output powers with PRF of 140 kHz. (a) just above threshold; (b) dip occurs; (c) output power recovered after realigning cavity at the dip; (d) maximum output power of 5.2 W.

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Fig. 5 Pulse duration of 1525-nm Stokes as functions of incident pump power at different PRFs.

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It should be mentioned that we do not use a diffusion-bonded crystal as a monoblock self-Raman gain medium for the reason of cost and the SRS conversion takes place in the two separate crystals. Therefore, the experiment setup here is not a strict self-Raman laser. However, the pure YVO₄ crystal’s effect of providing additional Raman gain here is the same with those diffusion bonded to the rear facet of Nd:YVO₄ pieces [16,17,21]. The only difference is that diffusion-bonded crystals could further reduce the influence of thermal effects and the insertion loss, so that even higher output power and conversion efficiency could be expected.

4. Conclusion

In conclusion, efficient eye-safe Raman laser based on Nd:YVO₄-YVO₄ has been demonstrated. Using 878.6-nm wavelength-locked LDA as pump source thus to relieve the thermal load and meanwhile ensure the pump absorption, 5.2-W average output power at 1525 nm is obtained under the incident pump power of 30.6 W at a pulse repetition rate of 140 kHz, with corresponding optical efficiency being 17.0%. The power and efficiencies are both the highest among end-pumped self-Raman lasers reported. The results also reveal that the vanadate-based self-Raman lasers are capable of operating efficiently with high repetition rate and low threshold.