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Abstract
A stable, efficient, and powerful 1314 nm Nd:YLF laser inband-pumped by a wavelength-locked narrowband 880 nm laser diode is demonstrated. The influence of mode-to-pump ratio on the performance of the diode-end-pumped Nd:YLF laser has been systematically investigated by taking into account the thermal effect and the energy transfer upconversion effect. For the optimum mode-to-pump ratio of 0.84, the maximum continuous wave output power of 21.9 W was extracted under the pump power of 70 W, which corresponded to the optical power efficiency of 31.3% and the beam quality of M2 ≈ 1.6. The resultant output power stability was determined to be 0.059% (RMS) within 1 h. In addition, by increasing the mode-to-pump ratio to 1.0, the near-diffraction-limited beam (M2 ≈ 1.3) was achieved with the output power of 17.0 W and the optical power efficiency of 24.3%.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Lasers emitting in the 1.3 µm spectral region have received significant attention owing to a growing number of promising applications such as remote sensing, timing systems, cosmetic and dermatologic procedures, and strong-field physics [1–3]. Additionally, the 1.3 µm laser output can be first-order Raman shifted to the eye-safe 1.5 µm region, which is of great interest for lidar and range-finding [4], and further cascaded Raman shifted to the 1.7 µm waveband for imaging deeper brain issue [5]. Furthermore, this wavelength can be frequency-doubled to the red spectral region which can be utilized for laser display and used as an efficient pump source for the Kerr-lens mode-locked Cr3+-doped (typically Cr3+:LiSAF) lasers [6]. Especially, the deep-ultraviolet laser sources below 170 nm can be produced by the eighth harmonic generation of the 1.3 µm lasers, which have great significance in angle-resolved photoemission spectroscopy (ARPES) [7]. Generally, all these applications mentioned above could immensely benefit from the 1.3 µm lasers with excellent performance in power, efficiency, beam quality, stability, and some commercial characteristics, such as low cost and reduced complexity.
To date, the 4F3/2 to 4I13/2 transition of neodymium (Nd)-doped mediums, such as YAG, YAP, YVO4, GdVO4, and KGW [8–10], has become an established approach for the 1.3 µm laser emission. Among these laser mediums, Nd:YLF is a promising candidate for generating high pulse energies during Q-switched operation because of its long upper-laser-level lifetime of ∼500 µs [11], thus allowing efficient energy storage and extraction. Furthermore, the σ-polarized emission at 1314 nm of Nd:YLF characterized by the very weak thermal lens is attractive for excellent beam quality and robust laser output [12]. Specifically, this unique emission line at 1314 nm of Nd:YLF can be conveniently frequency-doubled and frequency-quadrupled to 657 and 328 nm, respectively, which can be used for the interrogation of the clock transition in calcium and the optical trapping of silver atoms [1,13]. However, power scaling of 1.3 µm Nd:YLF laser has been a pretty challenging topic for direct restrictions of the small stimulated emission cross sections of ∼2–2.5×10−20 cm2 for both polarizations (about 1/10 of Nd:YVO4) and low thermal fracture limit (about 1/6 of Nd:YAG) [14]. Such a low-gain and fragile active medium is quite sensitive to high pump power intensity, in which the consequential high excitation densities within short absorption length will lead to over-concentrated heat generation and serious energy transfer upconversion (ETU), thereby decreasing the optical power efficiency and the capability of receiving pump power, and increasing the laser threshold [15].
To overcome these limitations, one strategy based on the conventional side-pumped setup has generated 21.6 W of continuous wave (CW) output power but with a limited beam quality of M2 = 17.8 and an optical power efficiency of 12.0% [16]. Although another more complex side-pumped configuration, based on the double beam mode-controlling (DBMC) technique, can deliver a diffraction-limited laser beam while the peak output power was restricted to 14.9 W even for quasi-CW pumping with 5% duty cycle [17]. Whereas, the end-pumped scheme allows for good overlap between the inverted region and the laser beam, thus favoring high efficiency and good beam quality, and it has great potential for efficient operation on low-gain transition. To alleviate the more serious problems associated with heat generation in end-pumped 1314 nm Nd:YLF laser, the large pump beam size and low doping concentration are expediently utilized. Remarkably, the highest CW output power of 26.5 W generated from the 806 nm diode-end-pumped two-crystal Nd:YLF laser was demonstrated with the pump beam diameter of 1 mm and the linear doping gradient ranging from 0.30% to 0.52%, which gave rise to the optical power efficiency of 21.2% and the average beam quality of M2 = 2.3 [14]. Nevertheless, its rapid development was restrained by the moderate optical power efficiency. In contrast, pumping the Nd3+ ions directly into the 4F3/2 upper level can reduce ∼15% quantum defect for 1.3 µm laser radiation, hence helping to lower heat deposition and improve laser efficiency. Several efforts have been focused on the broadband laser diode at 880 nm [18,19], which are capable of producing ten-watts-level CW output power at 1314 nm. However, the stability of output laser suffers from the thermal wavelength shift and line width fluctuation of the broadband laser diode, and further power scaling with high overall optical efficiency is limited by the unavailable trade-offs among the pump absorption efficiency, the mode-to-pump overlap efficiency and the thermal fracture pump limit.
In this work, we demonstrate a high-performance diode-end-pumped 1314 nm Nd:YLF laser by employing a fiber Bragg grating (FBG) stabilized narrowband 880 nm laser diode and optimizing the mode-to-pump ratio. The dependence of the laser characteristics on the mode-to-pump ratio was analyzed by considering the thermally induced diffraction loss and the ETU effect. Benefiting from the comparatively smoothed longitudinal thermal load along with high absorption efficiency as well as the high power and high spectral stabilities of the FBG-locked pump source, the maximum CW output power of up to 21.9 W was acquired with the optimal mode-to-pump ratio of 0.84, corresponding to an optical power efficiency of 31.3% and an outstanding power stability of 0.059% RMS. Additionally, by adjusting the resonator length to achieve another mode-to-pump ratio of 1.0, the average beam quality was improved from M2 ≈ 1.6 to M2 ≈ 1.3 while maintaining a relatively high output power of 17.0 W. To our knowledge, these results represent not only the most powerful 1.3 µm output generated from the end-pumped single-crystal Nd:YLF lasers, but also the highest optical power efficiency among any above-ten-watts Nd:YLF laser architectures operating near 1.3 µm.
2. Experimental setup
The schematic diagram of the CW 1314 nm Nd:YLF laser system is illustrated in Fig. 1(a). The pump source was the commercially available FBG-locked narrowband fiber-coupled laser diode (HAN’S TCS M880-150-DK) with a numerical aperture of 0.22 and a core diameter of 200 µm. Over the entire range of pump power, the center wavelength was stabilized at 879.9 nm with the narrow spectral bandwidth of ∼0.2 nm (FWHM) to overlap Nd:YLF’s weak absorption peak at 880 nm, corresponding to the absorption cross sections of ∼6.2×10−21 cm2 for π polarization and ∼2.5×10−21 cm2 for σ polarization [20]. The non-polarized pump beam was re-imaged by a pair of convex lenses (F1 and F2, 1:5 magnification) with anti-reflection (AR) coated at 880 nm, delivering a waist spot diameter of ∼1 mm at the position of ∼12 mm away from the entrance surface of gain medium. Such a specially designed waist position can not only guarantee the good overlap efficiency for all the mode-to-pump ratios we tested later [21], but also possess the relatively high thermal fracture pump limit. An available a-cut 1.0 at.% Nd:YLF crystal with a cross section of 3 × 3 mm2 and a length of 40 mm was selected as the gain medium, which was coated for high transmission (HT) at 880 and 1047–1321 nm on the entrance surface, and HT at 1047–1321 nm and partial reflectivity (PR) at 880 nm (R ≈ 60%) on the rear surface. The c-axis direction of the a-cut Nd:YLF crystal was aligned parallel to the horizontal direction. The pump absorption efficiency was as high as ∼93% under non-lasing situations, but the absorption coefficient was only ∼0.48 cm−1, contributing to spreading out the inversion density and thermal load along the longitudinal direction of Nd:YLF crystal. During the experiments, the laser crystal was wrapped with indium foil and mounted into a water-cooled copper block operating at a constant temperature of 16 °C. The linear resonator, composed of a plano-concave mirror M1 with a radius of curvature R = 500 mm and a plane output coupler M2, was chosen because it is insensitive to fluctuations of the pump-induced lens [22]. The input mirror M1 was coated to be HT at 880 and 1047–1053 nm and high reflection (HR) at 1314–1321 nm, while the plane mirror M2 coated for PR at 1314 nm (T ≈ 5%) was preferred as the output coupler. Based on the ABCD matrix theory, the distance between M1 and Nd:YLF was set to be ∼20 mm, and the desired mode-to-pump ratio can be realized by adjusting the resonator length. The output power was measured by an optical power meter (Physcience Opto-Electronics, LP-3C), and a longpass filter (Thorlabs, FELH0950) was utilized to block the residual pump power for all measurements.
Due to the lack of suitable laser beam analyzer, the beam quality factor was measured by the scanning knife-edge method [23]. The corresponding experimental arrangement is illustrated in Fig. 1(b). After passing through an attenuator, the output beam was transmitted through a convex lens F3 with a focal length of f = 200 mm, and periodically obscured by the rotating chopper with a knife-edge. Then, the chopped laser beam was focused by a lens F4 with a focal length of f = 200 mm to be completely received by a large area photodiode connected to a digital oscilloscope. The measurement of beam radii at different positions was realized by moving the chopper along the optical path. Moreover, the far-field spatial distribution of laser beam was recorded through an infrared sensor (LUMITEK Q-32-R), which was placed ∼15 cm away from the beam waist [see Fig. 1(b)].
3. Results and discussions
Striving for efficient, powerful, and near-diffraction-limited 1314 nm laser output, pre-design of the mode-to-pump ratio β was carried on. To begin with, on the basis of Ref. [21], the thermally induced diffraction loss with respect to the mode-to-pump ratio was calculated under the prearranged pump power of 50 W, as depicted in Fig. 2(a). The results demonstrate that the thermally induced diffraction loss increases rapidly with the mode-to-pump ratio for β > 0.84. Afterwards, the TEM00 mode output power as a function of the mode-to-pump ratio was numerically simulated by considering the thermally induced diffraction loss and the upconversion rate γ into the space-dependent rate equation analysis [24], as displayed in Fig. 2(b). According to Ref. [25], the upconversion rate γ can be reasonably set to 1.7 × 10−16 cm3/s. For comparison, the simulated results with either the ETU effect (green dotted curve) or the thermally induced diffraction loss (blue dashed curve) are also plotted in Fig. 2(b). It has been shown analytically that the ETU effect can be negligible due to the low upper state population density $\Delta N$, which is mainly attributable to the low pump absorption coefficient and the large pump beam size in current pumping scheme. The ETU term of rate equation is proportional to the upconversion rate γ and the square of the upper-state population density $\Delta {N^2}$, so it is more vulnerable to the upper-state population density. Another noticeable aspect is that the influence of thermally induced diffraction loss on the output power is very significant for β > 0.84, which is in fairly good agreement with the theoretical prediction in Fig. 2(a). Consequently, the optimum mode-to-pump ratio is approximately 0.84, which is a consequence of the compromise between the thermally induced diffraction loss and the overlap efficiency. In order to verify our theoretical analysis, the output powers for different mode-to-pump ratios were experimentally presented. It can be seen from the black curve and the red dots that the theoretical results including the thermal and ETU effects agree very well with the experimental data for β ≥ 0.6. However, there is a certain degree of deviation between the calculated value and experimental data for β = 0.5.
Fig. 2. Under the pump power of 50 W, (a) simulated thermally induced diffraction loss with respect to the mode-to-pump ratio β; (b) simulated and experimental output power versus the mode-to-pump ratio β.
To further explain the underlying reasons, we analyzed the transverse-mode behavior of the diode-end-pumped 1314 nm Nd:YLF laser. Based on the rate equation of the multimode laser [26–28], the dependence of output power on the pump power was modeled for β = 0.5, 0.6, and 1.0, as visualized in Fig. 3. One may recognize from Fig. 3(a) that the laser starts to oscillate in the multimode (TEM00 and TEM01) as long as it exceeds the laser threshold, and then the TEM01 mode will gradually exceed the TEM00 mode with the increase of pump power. Meanwhile, we find that the power curve of the TEM02 mode almost coincides with that of the TEM00 mode, indicating that the TEM02 mode will oscillate if β continues to decrease. Therefore, the simulative results using the TEM00 mode cannot be applicable for β ≤ 0.5 under the pump power of 50 W, as shown in Fig. 2(b). For β = 0.6 [see Fig. 3(b)], the laser starts to oscillate in the TEM00 mode, and then becomes multimode (TEM00 and TEM01) at the pump power of approximately 35 W. Even though, the output power of the TEM01 mode is basically the same as that of the TEM00 mode for β = 0.6 under the incident pump power of 50 W, so the theoretical result is still in good agreement with the experimental data for β = 0.6, as depicted in Fig. 2(b). Further simulations show that the TEM00 mode will gradually dominate with the increment of β, and the difference between the TEM00 mode and the higher-order mode will be growing, which manifests that the better beam quality can be anticipated with the larger β. As seen from Fig. 3(c), TEM01 joins TEM00 oscillation at a much higher incident pump power of approximately 50 W.
Fig. 3. Simulated input-output power curves of multi-order transverse modes for β = (a) 0.5, (b) 0.6, and (c) 1.0.
Based on the above-mentioned analysis, we selected two special cavities for higher power laser output: one with β = 0.84 was used to produce the highest output power while maintaining relatively good beam quality, and the other with β = 1.0 was suitable for generating the excellent beam quality combined with relatively high output power. Notably, despite the optimum mode-to-pump ratio being a decreasing function of the incident pump power in a strict sense [21], it remains nearly constant when the incident pump power increases from 50 to 70 W in the present experimental configuration, according to our theoretical prediction. Here, the resonator length was tuned from 240 to 330 mm for the target β of 0.84 and 1.0. Typically, taking β = 0.84 as an example to illustrate the thermal stability of resonator, the TEM00 mode beam radii in gain medium at different pump powers were calculated based on the estimated thermal lens focal length given in Ref. [12], as shown in Fig. 4(b). Clearly seen is that the variation of beam waist inside gain medium is only 1.2% over the whole pump power range, which is beneficial for the highly stable laser output.
Fig. 4. (a) Output power for β = 0.84 and 1.0, (b) TEM00 mode beam radius and thermal lens focal length for β = 0.84 with respect to the pump power; (c) power stability at the full output power of 21.9 W over 1 h.
The output performance of the diode-end-pumped Nd:YLF lasers for β = 0.84 and 1.0 were systematically studied thereafter. Figure 4(a) shows the evolution of the CW output power with the pump power. The oscillator had the pump thresholds of 5.6 and 5.1 W for β = 0.84 and 1.0, respectively. With β = 0.84, the maximum output power amounted to 21.9 W under the pump power of 70 W, resulting in an optical power efficiency of 31.3% and a slope efficiency of 34.5%. As far as we know, the output power can be comparable to the highest value previously reported by the end-pumped two-crystal Nd:YLF laser [14], and the resultant optical power efficiency is the highest reported value to date for any 1.3 µm Nd:YLF laser systems with the output power larger than 10 W [14,16–19]. Whereas, the slope efficiency obtained here is well below those of the directly pumped 1.3 µm lasers of Nd3+ in oxides, including Nd:YAG and Nd:YVO4 [29–31]. It could be mainly attributed to the small stimulated emission cross section at 1.3 µm for Nd:YLF, which is a factor of three less than that of Nd:YAG and an order of magnitude less than that of Nd:YVO4 [32]. Meanwhile, the β = 1.0 resonator delivered a maximum output power of 17.0 W under the full pump power, representing an optical power efficiency of 24.3% and a slope efficiency of 26.2%. Besides, the power stability was monitored under the full output power of 21.9 W, as displayed in Fig. 4(c), and the fluctuation of output power was found to be 0.059% RMS over 1 h. This outstanding power stability could be mainly ascribed to the excellent power and spectral stabilities of the FBG-locked pump source and the thermal insensitive resonator design.
Using an optical spectrum analyzer (Yokogawa, AQ6374), we found that this laser only oscillated at 1314 nm on the σ polarization when the pump power was above 20 W because the oscillator associated with the π polarization became unstable due to its strong negative thermal lens. It is worth noting that only very weak yellow fluorescence, mostly attributing to the ETU process from 4F1/2 to 2G9/2 and 4G7/2 levels in Nd:YLF [33], was observed even at the full pump power, which further confirmed our theoretical analysis on the ETU effect. During the laser characterizations, rollover of output power was not observed thanks to the dynamic stable resonator, and higher output power was constrained by the crystal fracture, occurring at an attempt to increase the pump power more than 70 W. By solving the thermo-mechanical coupling equation based on ABAQUS software, the maximum principal stress appeared at the entrance surface of Nd:YLF with a simulated value of ∼30 MPa, which was slightly lower than the reported thermal fracture limit (∼33 MPa) [34]. This discrepancy could be introduced by three reasons: (i) the Nd:YLF crystal we used may has some inner defects; (ii) the propagation direction of pump beam deviates from the center of gain medium to some extent; (iii) a higher-than-expected heat load was generated on/near the pump-face if the pump-end of the crystal had a higher than specified doping.
The variation of M2 values with the pump power for β = 0.84 and 1.0 is shown in Fig. 5. The M2 values in orthogonal directions increased monotonously with boosting the pump power to 50 W, and then tended to saturation for a larger pump power. The degradation of beam quality at high power levels can be explained by the fact that the inversion is not depleted in the wings of the pumped region, leading to oscillation on higher-order transverse modes [35]. We can also see that the resonator with β = 1.0 provides better beam quality than that with β = 0.84 because of the effective suppression of energy extraction at higher-order transverse modes. Under the full output power, the M2 values along the x and y axis were determined to be $M_x^2 = 1.55$ and $M_y^2 = 1.71$ for β = 0.84, leading to the average value of ∼1.6, whilst the near-diffraction-limited propagation with $M_x^2 = 1.29$ and $M_y^2 = 1.37$ was observed for β = 1.0, representing the average value of ∼1.3. The difference in beam qualities in orthogonal directions can be attributed to the YLF’s astigmatic thermal lensing [12,14]. Furthermore, the far-field intensity profiles at several output powers are plotted in the insets of Fig. 5. Clearly, the absence of distortion in the laser beam profile even for the highest output power is a strong evidence of the robustness of the diode-end-pumped cavity configuration.
Fig. 5. Variation of the beam quality factor with the pump power for β = (a) 0.84 and (b) 1.0. The insets show the far-field beam profiles.
4. Conclusion
In summary, we have accomplished a high-performance 1.3 µm Nd:YLF laser source end-pumped by the narrow bandwidth FBG equipped 880 nm laser diode. The mode-to-pump ratio has been optimized by considering the thermal and ETU effects, in which the ETU effect was proven to be insignificant. For the optimum mode-to-pump ratio of 0.84, the highest output power of 21.9 W was attained with high optical power efficiency of 31.3% and good beam quality of M2 ≈ 1.6. Taking the advantages of the highly stable narrow-band laser diode and the dynamic stable resonator design, the superior power stability was achieved with 0.059% RMS. Subsequently, by moving the coupler to achieve another mode-to-pump ratio of 1.0, a near-diffraction-limited laser beam with M2 ≈ 1.3 was generated with the output power of 17.0 W. Future upgrades aiming for reaching higher output power will involve a gradient-doped Nd:YLF crystal and a double-end pumping scheme.
Funding
National Natural Science Foundation of China (62175093, 61935010, 62175091); Guangdong Province Basic and Applied Basic Research Fund (Joint Fund) (2020A1515110001); Research and Development Program in Key Areas of Guangdong Province (2020B090922006); Guangzhou Science and Technology Project (201904010294, 202102020949).
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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