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We report an efficient fiber-laser-based source of high-power, continuous-wave (cw), linearly-polarized radiation at 970 nm in a simple, compact, and practical design. Using direct single-pass second-harmonic-generation (SP-SHG) of a cw thulium fiber laser at 1940 nm in a 40-mm-long periodically-poled LiNbO3 (PPLN) crystal, we have generated 13.1 W of output power at 970 nm for a fundamental power of 40 W at a conversion efficiency as high as 32.7%. The generated second-harmonic output exhibits a passive power stability better than 1.4% (1σ value) over 1 hour, has a linewidth better than 0.3 nm, and a TEM00 spatial beam profile with M2<1.6. Relevant theoretical calculations for the characterization of SP-SHG in the crystal have also been performed.
©2011 Optical Society of America
1. Introduction
High-power, continuous-wave (cw) laser sources at 970 nm are of considerable interest for a variety of scientific and technological applications including pumping of solid-state and fiber lasers, spectroscopic detection of ozone, water-vapor, and CO2 in the atmosphere, and various medical applications [1–4]. In particular, a narrow-linewidth source at 970 nm offers potential impact for bio-sensing applications [4]. Such sources could also be deployed in combination with nonlinear frequency up-conversion schemes such as second-harmonic-generation (SHG) to provide high-power cw blue radiation to replace bulky water-cooled argon-ion lasers, which inherently operate in multiple longitudinal modes [5]. For many such applications, a high spatial beam quality in addition to high output power is imperative. To date, the possible sources of 970 nm radiation are high-power diode lasers, but their beam quality still remains an important limitation [6,7]. The Ti:sapphire laser can provide output radiation at 970 nm, but the low gain far from the emission peak results in very low output power [8]. Optically-pumped semiconductor lasers are another alternative source of 970 nm radiation, but limited progress has been achieved in this direction so far [9]. High-power cw ytterbium-doped photonic crystal fiber lasers have been recently demonstrated near 980 nm, but it has been shown that the beam quality factor degrades with the increase in output power [10]. Although these sources provide high conversion efficiencies and have proved highly effective [7,9,10], the attainment of good beam quality with good M2 factor and narrow linewidth together with high-power at 970 nm, and in a simple design, still remains a challenge. It is, thus, desirable to explore alternative techniques for the generation of cw radiation at 970 nm in a simple, compact and cost-effective design to provide a practical high-power source with good spatial beam quality, mandatory requirements for many applications.
Advances in cw fiber lasers together with the development of quasi-phase-matched (QPM) ferroelectric materials provide a unique opportunity for the realization of high-power cw sources in new spectral regions using nonlinear frequency conversion techniques. In a particularly simple, compact and practical scheme, we recently reported direct single-pass (SP) SHG of cw ytterbium fiber lasers in MgO-doped periodically-poled stoichiometric LiTaO3 (MgO:sPPLT), providing 13 W of green radiation at an unprecedented conversion efficiency as high as 56% [11,12]. With progress in cw thulium (Tm) fiber laser technology near 2 μm, such techniques can now be effectively exploited for the generation of high-power cw radiation near 1 μm at high efficiency and in good beam quality. In the pulsed regime, Frith et al. demonstrated frequency-doubling of gain-switched Tm-doped fiber laser at 1908 nm in periodically-poled LiNbO3 (PPLN), generating ~1 W of average power at 954 nm with 60% conversion efficiency [13]. Here, we demonstrate the generation of multiwatt cw radiation at 970 nm by exploiting SP-SHG of a high-power cw Tm fiber laser in combination with PPLN as the doubling crystal. Using a 40-mm crystal, we generate an output power of 13.1 W at 970 nm with a SP-SHG conversion efficiency of 32.7%. The output at 970 nm is in a TEM00 spatial mode with M2<1.6 and exhibits narrow linewidth with passive power stability better than 1.4% (1σ value) over 1 hour. We believe this is the first report on SHG of cw Tm fiber laser, which provides a compact, practical, and efficient source of high-power radiation at 970 nm in high beam quality and simplified design, and with competitive performance over alternative cw sources in this wavelength range.
2. Experimental setup
The schematic of the experimental setup is shown in Fig. 1 . The fundamental source is a cw Tm fiber laser (IPG Photonics, TLR-50-1940-LP), delivering up to 44 W of output power at 1940 nm in a linearly-polarized beam with a quality factor of M 2=1.2 and a measured linewidth of Δλ~0.5 nm. A Faraday isolator (FI) with an isolation of ~23 dB is used at the output end of the fiber to prevent back-reflection into the laser. The frequency stability of the fiber laser at the maximum power is recorded to be better than 3.3 GHz over two hours. The nonlinear crystal is 40-mm-long, 11.5-mm-wide, 1-mm-thick, undoped PPLN sample with multiple gratings, ranging in period from Λ=25.5 to Λ=28.7 μm. The calculated spectral acceptance bandwidth of the PPLN crystal for SHG at 1940 nm is 3.52 nm.cm, resulting in 0.88 nm for the 40 mm interaction length used in this experiment [14]. This value is larger than the Tm laser fundamental linewidth, ensuring efficient SHG to 970 nm. The reflectivity of the crystal input face at 1940 nm is experimentally measured to be R<6%, resulting in a small input loss at the fundamental. The crystal is housed in an oven with a stability of ±0.1 °C, which can be tuned from room temperature to 200 °C. The fundamental beam is focused at the center of the crystal using a CaF2 focusing lens (L). Although a fundamental power of up to 44 W is available at the output of the fiber laser, due to non-optimized optics used to deliver the pump beam, the maximum power at the input of the crystal is 40 W. We used the Λ=28.2 μm grating period only, as it is quasi-phase-matched for SHG at 970 nm at a temperature of 93.8 °C. A dichroic mirror, M (R>99.8%@1940 nm and T>86%@970 nm), is used to separate the unconverted fundamental from the second-harmonic output beam.
3. Results and discussion
For the attainment of the highest SHG efficiency, we studied different spatial mode-matching conditions. Using lenses of different focal lengths, we focused the fundamental beam to waist radii of wο~55.5, 45, 37.5, and 30.5 μm at the center of the crystal, corresponding to confocal focusing parameters of ξ=1.88, 2.86, 4.12 and 6.23, respectively [15]. Figure 2(a) shows the maximum SHG efficiency achieved, together with the phase-matching temperature (TQPM), at the maximum input fundamental power of 40 W, as a function of ξ. As evident, the SHG efficiency increases with the confocal focusing parameter, reaching as high as 32.7% at ξ~2.86. Further increase in ξ value results in the drop in SHG efficiency. It is interesting to note that at ξ~2.86, the maximum efficiency is achieved, which is almost equal to optimum focusing parameter, ξ~2.84, predicted by Boyd and Kleinman in the cw limit [15]. It is also clear from Fig. 2(a) that the phase-matching temperature decreases with increase in ξ and has a steep fall towards tighter focusing. This may be attributed to the thermal effects arising from the tighter focusing of pump beam in the crystal, resulting in increase of thermal load, hence requiring a decrease in the required external heat applied to the crystal to achieve phase-matching. Another possible reason could be the increase in angular divergence of the beam with tighter focusing, which results in the propagation of the divergent components of the fundamental beam over longer grating periods, resulting in a shift in the peak phase-matching temperature. We calculated the propagation length of the divergent components, and the corresponding change in phase-matching temperature, for a propagation distance corresponding to grating period of Λ=28.2 μm along the crystal, as a function of beam waist radius. The results are shown in Fig. 2(b). It is found that under loose focusing (wο~55.5 μm), the most divergent components of the fundamental beam propagate over an increased grating period of 28.20175 μm, while for tight focusing (wο~30.5 μm) the increased grating period for the most divergent components is 28.20575 μm. Therefore, in going from wο~55.5 μm to wο~30.5 μm, the grating period for the most divergent components increases by 4 nm, resulting in an average decrease in phase-matching temperature of the crystal with increased divergence and tighter focusing, as observed in Fig. 2(a). Using a wavemeter (Bristol, 721B-IR, absolute accuracy of 1 ppm), we measured the central wavelength (λω) of the fundamental beam at 40 W to be λω = 1941.5 nm. The inset of Fig. 2(b) shows the variation of theoretical predicted values of phase-matching temperature with focusing parameter ξ, calculated for a single-frequency fundamental, using relevant Sellmeiers equation [16]. The theoretical TQPM curve shows the same behavior as that of the experimental curve of Fig. 2(a).
Fig. 2 (a) SHG efficiency and phase-matching temperature at maximum fundamental power, as a function of focusing parameter, ξ. Solid lines are guide to eye. (b) Theoretically calculated propagation length for divergent components and the corresponding change in phase-matching temperature, as a function of beam waist radius. Inset: theoretically calculated phase-matching temperature, as a function of focusing parameter, ξ.
To characterize the crystal under different focusing conditions, we measured the temperature acceptance bandwidths and the corresponding phase-matching temperatures for different beam waists. The measurements were performed at low fundamental power, in order to avoid any thermal contributions. Figure 3(a) shows the result for optimum focusing (wο~45 μm), together with the sinc2 fit, at a fundamental power of ~1 W. The measured full-width-half-maximum (FWHM) temperature acceptance bandwidth (ΔT) is found to be 5.9 °C at TQPM = 113 °C. This is larger than the theoretical value of ΔT=3.8 °C at TQPM=93.8 °C for the 40-mm crystal, which has been calculated for a single-frequency fundamental in plane-wave approximation without focusing, using the relevant Sellmeier equations [16]. The discrepancy between the experimental and theoretical value of ΔT could be in part due to the finite bandwidth of the fundamental (Δλ~0.5 nm), while the discrepancy in the value of TQPM could be due to possible non-uniformity in the grating period along the crystal length.
Fig. 3 (a) Temperature phase-matching curve for SHG at wο~45 μm, and (b) Temperature acceptance bandwidth and the corresponding phase-matching temperature under different focusing conditions. Pump power is ~1 W. Solid lines are guide to eye.
The difference between the experimental and theoretical values of ΔT and TQPM could also in a large part be attributed to the focusing of the fundamental beam. As noted above, this can give rise to an average increase in the grating period for all spatial components, which results in a decrease in the peak phase-matching temperature TQPM as well as a broadening of the phase-matching bandwidth towards lower temperatures, resulting in an increase in the experimental value of ΔT. We measured the variation of the FWHM temperature acceptance bandwidth and the corresponding phase-matching temperature as a function of fundamental beam waist radius at an input power of ~1 W. The results are shown in Fig. 3(b), where it is observed that at low power, where thermal effects are negligible, the phase-matching temperature increases with loose focusing, as also seen for high fundamental power (Fig. 2(a)). With negligible contribution from thermal effects, the change in TQPM is attributed mainly to the fundamental beam divergence in the crystal. It is also clear from Fig. 3(b) that for tighter pump focusing, the temperature acceptance bandwidth increases, while the phase-matching temperature decreases, as predicted. On the other hand, we also theoretically calculated the temperature acceptance bandwidth of the 40-mm-long PPLN crystal as a function of phase-matching temperature, in the plane-wave limit in the absence of focusing, and confirmed an increase in ΔT with the decrease in phase-matching temperature [14]. Therefore, the inherent variation of ΔT and TQPM in the plane-wave limit combined with the focusing effects lead to the observed behavior in Fig. 3(b).
The variation of SHG power and the corresponding efficiency up to the maximum available fundamental power for ξ~2.86 is shown in Fig. 4(a) . We obtained 13.1 W of SHGpower at 970 nm for 40 W of input power to the crystal, representing a single-pass efficiency of 32.7%. As seen in Fig. 4(a), the SHG power has a quadratic dependence on the fundamental power up to 18.5 W, beyond which it increases linearly, implying saturation of SHG efficiency at higher power. This can be attributed to pump depletion, back-conversion and thermal de-phasing effects in the PPLN crystal. However, we confirmed the role of thermal effects to be negligible, by performing power scaling measurements with tight focusing at wο~37.5 μm (ξ=4.12) under quasi-cw condition. This measurement was performed by chopping the beam at 530 Hz with a duty cycle of 5%, and then comparing the results with that under cw condition. In both the cases, we found the SHG efficiency saturation at higher power, with a maximum efficiency of ~26%. Using the slope of the generated SHG power versus square of the fundamental power curve, we also calculated the effective nonlinear coefficient for our PPLN crystal at 1940 nm wavelength [17], which we found to be d eff~13.6 pm/V. Similar d eff value were obtained from measurements at different beam waists and under quasi-cw condition. Figure 4(b) shows the phase-matching temperature as a function of fundamental power for ωο~45 μm (ξ~2.86), where we observe a rise in phase-matching temperature with the increase in fundamental power. We obtain TQPM~112 °C at 0.8 W and 119.5 °C at 40 W, corresponding to a difference in phase-matching temperature of 7.5 °C. We measured the central wavelength of the fundamental output at different power levels using the wavemeter and found that the peak wavelength shifts towards the longer value with the rise in fundamental power. For example, λω=1939.8 nm at ~1 W, and 1941.5 nm at ~40 W of fundamental power, which corresponds to a change of 1.7 nm in central wavelength. We theoretically calculated the phase-matching temperature as a function of fundamental wavelength considering negligible thermal effects [16]. As seen in the inset of Fig. 4(b), an increase in the fundamental wavelength by 1.7 nm corresponds to an increase of 7.4 °C in phase-matching temperature, thus confirming the observed rise in the phase-matching temperature with the increase in fundamental power.
Fig. 4 (a) SHG power and the corresponding conversion efficiency as a function of incident fundamental power at ωο~45 μm, and (b) Phase-matching temperature versus incident fundamental power. Solid lines are guide to eye. Inset: theoretically calculated change in phase-matching temperature versus fundamental wavelength for Λ=28.2 μm.
We measured the power stability near the maximum SHG power at the fundamental power of 40 W. The result is shown in Fig. 5(a) , where we obtained a passive stability better than 1.4% (1σ value) over 1 hour, without any precautions. The SHG power fluctuation is mainly attributed to the changes in the laboratory environment and also to some extent the fluctuations in the input pump power with 0.9% (1σ value) over one hour, as seen in Fig. 5(b). Improved thermal and mechanical isolation of the system with proper temperature control and increased pump power stability can further enhance the SHG power stability.
Fig. 5 Time trace at maximum power of (a) generated SHG output, and (b) input fundamental over 1 hour.
We also recorded the spectrum of the generated SHG output at maximum power and found the linewidth (Δλ2ω) better than 0.3 nm, as shown in Fig. 6(a) , limited by the resolution of our spectrum analyzer. The generated second-harmonic linewidth is thus narrower than that of the fundamental. The far-field energy distribution of the SHG beam at maximum power together with the intensity profile is shown in Fig. 6(b). The slight asymmetry seen in the beam profile is due to the small tilt angle of the lens and separation mirror from the normal incidence. Using a f = 100 mm focal length lens and a scanning beam profiler, the beam quality factor of the SHG beam at maximum power was measured to be Mx2~1.43 and My2~1.6, confirming the TEM00 spatial mode. We did not observe any photorefractive damage in the PPLN crystal up to the maximum SH power, as evident from the output power stability and beam quality.
Fig. 6 (a) Spectrum of SHG output at maximum power, and (b) Far-field TEM00 energy distribution of the generated SHG beam. The thin curves are the intensity profiles along the two orthogonal axes.
4. Conclusions
In conclusion, we have demonstrated, for the first time, efficient SP-SHG of a cw Tm fiber laser, providing multiwatt cw radiation at 970 nm at high efficiency, in narrow linewidth and high beam quality, and with good stability. By deploying a 40-mm-long PPLN crystal, we have generated 13.1 W of cw output power at 970 nm at single-pass conversion efficiency as high as 32.7%. The SHG output has a linewidth better than 0.3 nm with a power stability better than 1.4% (1σ value) over 1 hour, and has a TEM00 beam profile with M2<1.6. The absence of thermal effects suggest the possibility of further increase in cw SP-SHG output power and efficiency by using longer crystals, higher pump powers, and narrower fundamental linewidths. The exploitation of multicrystal scheme using several PPLN crystals in a cascade [12] could also lead to further increases in SP-SHG efficiency to >50%, providing >20 W of cw output power at 970 nm. The demonstrated fiber-based device offers a viable, compact and practical source of high-power, linearly-polarized, narrow-linewidth, and high-beam-quality cw radiation at 970 nm, with competitive performance over alternative sources in this wavelength range, and offering promise for a variety of applications.
Acknowledgement
This research was supported by the Ministry of Science and Innovation, Spain, through grants TEC2009-07991 and the Consolider project, SAUUL (CSD2007-00013).
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