Efficient continuous-wave laser emission at 1856 nm from a Tm,Mg:LiNbO3 crystal slab with high Tm3+ doping concentration is reported. A maximum output power of 2.62 W is realized with a slope efficiency of 19.6% and the beam quality factor M2 of 1.7 at room temperature. We believe that this is the first demonstration of watt-level laser operation in Tm,Mg:LiNbO3 crystal and the output power is four orders of magnitude higher than that reported previously in Tm-doped LiNbO3 crystal. Performance degradation due to the photorefractive effect under high intensity 1856 nm laser is not observed thanks to the co-doping of magnesium ions. Quantitative analysis about the long-term photorefractive effect is also provided. Multi-wavelength laser operation is realized by using different narrow-band output couplers. This demonstration opens up a viable pathway towards 2-μm integrated optic devices for achieving laser oscillation, electro-optic and nonlinear optical effects within just one sample simultaneously.
© 2013 OSA
Lithium niobate (LiNbO3) crystal has been studied for many decades, as it has very excellent nonlinear optical properties and electro-optic effects [1, 2]. The basic properties of LiNbO3 are fundamental and crucial to its wide applications in nonlinear optics and integrated optics . Moreover, periodically poled LiNbO3 structures have enabled new features of frequency conversion by quasi-phase matching for the integrated devices . The further study of LiNbO3 as laser host for rare-earth ions has been extensively investigated [5, 6]. It is bringing forth new integration, in which laser oscillation, electro-optic and nonlinear optics effects can be integrated within just one crystal. However, the photorefractive effect in LiNbO3 crystal is the main bottleneck of high power laser operation, even for the 2-μm wavelength lasers. Up to now, the maximum output power is only 0.8 mW generated in Tm-doped LiNbO3 waveguide lasers .
An effective emission of Tm-doped LiNbO3 crystal is around 1.85 μm wavelength. This short 2-μm wavelength (1.7-1.85 μm) lasers have many potential applications. It can be applied in tomographic imaging for early detecting gastrointestinal inflammation and neoplasia, because on the one hand, it can lead to a deep penetration depth in compared with 1-μm wavelength lasers, and on the other hand, have a lower absorption of water than that of long 2-μm wavelength lasers (1.9-2.1 μm) [8, 9]. This advantage of 1.85-μm wavelength laser also makes it attractive for laser radar systems, since the emission around 1.85 μm gives much lower absorption of vapor while keeping the advantage of longer wavelength . Furthermore, it is an excellent pumping for Zinc selenide doped with divalent chromium ions (Cr2+:ZnSe), whose absorption spectrum spans over 1.5-2.2 μm with a maxima at around 1.8 μm . At present, lasers around 1.85μm wavelength are seldom reported because of the absence of suitable Tm3+-doped laser materials. As listed in Table 1, Tm-doped optical fiber laser, Tm:LiYF4, Tm:LuLiF4, Tm:YSGG, Tm,Yb:KY(WO4)2 and Tm:YAG crystal lasers can generate high output power but all operated at wavelength longer than 1.90 μm [12–17]. Though laser operation at 1.76 μm in Tm3+:LiNbO3 Zn-diffused channel waveguides is also reported, the laser output power is low .
In this paper, Tm,Mg:LiNbO3 crystal with high thulium doping concentration of 2.0 wt.% was grown with high quality. Pumped by a laser diode, we archived watt-level continuous-wave (CW) laser output at 1856 nm wavelength. The maximum output power as high as 2.62 W with good beam quality of M2 = 1.7 was realized at room temperature. The fitted slope efficiency was 19.6%. The output power represented an improvement of four orders of magnitude higher in compared with previously records in Tm-doped LiNbO3 crystals. Co-doping of MgO increased the threshold of photorefractive effect obviously and performance degradation under high intensity 1856 nm laser was not observed. Moreover, the laser was tuned from 1856 to 1936 nm by using different narrow-band output couplers. This demonstration offers great opportunities for 2-μm wavelength multi-function device and efficient short 2-μm wavelength lasers.
2. Experimental and results
Compared to singly Tm3+ doped LiNbO3, addition of Mg2+ in LiNbO3 greatly reduces the photorefractive damage and increases the electrical conductivity [19, 20]. However, up to now, there has been no report on laser properties of Tm, Mg co-doped LiNbO3 crystal. The Tm,Mg:LiNbO3 used in our experiments was grown by pulling along the <001> direction using traditional Czochralski method in an inductively heated iridium crucible (Li/Nb = 0.942). In the starting materials, the concentration of Tm3+ and Mg2+ was 2.0 wt.% and 5.0 wt.%, respectively. High thulium doping concentration in LiNbO3 will improve laser efficiency due to the “two-for-one” cross relaxation , but the Tm,Mg:LiNbO3 crystallizes difficultly and cracks frequently during the growth of crystal due to the large atomic radius of Tm3+ ion. To reduce the strain inside the crystal, we carefully performed annealing at 1150 °C with a heating rate of 50 °C/h in air atmosphere. With a specific current density applied after heating for 6 hours at 1150 °C. The crystal was then cooled at a slower cooling rate to pass the Curie temperature. The Tm,Mg:LiNbO3 crystal was polarized along the c-axis to produce a single-domain crystal for reducing the scattering loss inside the crystal. Compared with the Tm:LiNbO3 crystal fabricated by Tm-diffusion effect, Tm,Mg:LiNbO3 crystal grown by the Czochralski method has advantages of uniform doping concentration and excellent optical properties. Co-doping of MgO into LiNbO3 affects optical qualities of Tm3+ ions slightly, except for a general broadening of the emission spectrum. Oscillator strengths and Judd-Ofelt parameters are not affected by MgO co-doping. The measured scattering loss is about 0.3% cm−1, it illustrates that the high thulium doping concentration neither induces lattice distortion nor affects the optical properties of Tm,Mg:LiNbO3 crystal.
The absorption cross section and the emission cross section of Tm:LiNbO3 depend strongly on the polarization of the pumping laser [5, 22]. Compared to π-polarization, both absorption cross section and the emission cross section of Tm:LiNbO3 under σ-polarization are higher. Therefore, the lasing properties of Tm,Mg:LiNbO3 crystal under σ-polarization were investigated. The σ-polarized absorption coefficient of Tm,Mg:LiNbO3 at room temperature is shown in Fig. 1. The absorption coefficient around 794 nm corresponding to 3H6→3H4 transition is 3.36 cm−1 and the FWHM of the absorption peak is 6 nm, which is benefit for commercial GaAlAs laser diodes being used as the pumping sources. This fluorescence spectrum at room temperature is acquired by a TRIAX550 spectrophotometer with a 793 nm laser diode as a pump source under σ-polarized. The measured fluorescence lifetime of Tm,Mg:LiNbO3 crystal is 4.2 ms. The emission cross sections are subsequently calculated by the Füchtbauer-Ladenburg equation . The result was shown in Fig. 2. It is obvious that the maximum emission cross section is about 0.552 × 10−20 cm2 around 1756 nm. This emission band is corresponding to the 3F4→3H6 transition at 1626-2018 nm, covering the strong absorption band of Cr2+:ZnSe laser. The broad emission band makes it possible to develop efficient tunable solid-state lasers in the short 2-μm wavelength range. Furthermore, this wide emission band suggests the potentiality for ultrashort pulse generation in the Tm,Mg:LiNbO3 crystal.
In our experiment, the crystal was cut to a slab with a dimension of 2.5 mm × 2.5 mm in cross section and 10 mm in length. A fiber pigtailed laser diode at 793 nm with the maximum output power of 20 W was used as the pumping source. The pigtail fiber had a diameter of 200 μm and a numerical aperture of 0.22. A couple of aspheric lenses with the focal length of 50 mm were used to couple the pump laser into the Tm,Mg:LiNbO3 crystal slab. A schematic diagram of the experimental setup of the Tm,Mg:LiNbO3 laser is illustrated in Fig. 3. One flat mirror was used as the input mirror (IM) which is highly transmitting at the pump wavelength near 793 nm (T > 99.5%) and high reflective (R > 99.5%) at the laser wavelength between 1840 and 1950 nm for multi-wavelength laser experiment. The laser beam propagates parallel to the a-axis of the Tm,Mg:LiNbO3 crystal (the length direction). Both end surfaces of the crystal slab were polished and AR-coated at lasing wavelengths. The Tm,Mg:LiNbO3 slab was wrapped with an indium foil tightly and mounted in a water-cooled copper heat sink to dissipate waste heat effectively. The temperature of the cooling water was kept at 20 °C in the experiment. In addition, in order to prevent the unabsorbed 793 nm pump power, which left through the output coupler (OC), from impinging onto the detector, a dichroic mirror with high reflectivity at 793 nm and high transmission at lasing wavelengths was placed away from the output mirror. The laser spectrum was recorded by an optical spectrum analyzer with a resolution of 0.22 nm and a range from 1500 to 3000 nm (SIR-5000, SANDHOUSE).
The fiber-coupled laser diode can provide a total of 20 W CW pump power with a spot diameter of 180 μm to the center of the Tm,Mg:LiNbO3 crystal slab; and about 82% of the incident pump light was absorbed by the crystal slab. To obtain maximum output power, the cavity length was initially set to approximately 15 mm. Figure 4 shows the CW laser output power as a function of the absorbed pump power with two different output couplings. The central wavelength of high reflective coating was 1860 nm and ∆λ at FWHM was 1855-1865 nm. We got 2.62 W and 2.01 W CW output power with 1.5% and 3% output couplings, respectively. The slope efficiency using 1.5% output coupling was 19.6%, compared with 16.3% for 3% output coupling. The pump threshold was about 3.42 W (incident pump power) with the 1.5% transmission coupler. It can be improved by employing high brightness pump sources or thin waveguide structures.
In both cases, no gain saturation phenomenon and optical damage appeared in experiment, the maximum laser output power of the Tm,Mg:LiNbO3 crystal laser was limited only by the available pump power. As shown in Fig. 4, the laser output power increased linearly, rolling-over of the output power was not appeared. It implied that the photorefractive effect did not occur under laser operating. The highest power density inside Tm,Mg:LiNbO3 slab was estimated to be 0.6 MW/cm2, which was much higher than the photorefractive threshold of LiNbO3 crystal. The inset of Fig. 4 shows the laser spectrum of Tm,Mg:LiNbO3 crystal laser at wavelength 1856 nm by employing the output coupler of T = 1.5% when the output power was 260 mW. In Fig. 2, the stimulated emission of Tm,Mg:LiNbO3 shows peak at 1756 nm, the reabsorption coefficient is also high around 1756 nm, as shown in Fig. 1. Therefore, the output laser has lasing wavelength at 1856 nm.
The degrees of polarizations of the output beams were also measured under different output powers. For s-polarization, the degree of polarization of the output beam was around 90% and almost unchanged, indicating the photorefractive effect not occurring inside the Tm,Mg:LiNbO3 slab. Probably because the single-domain poled Tm,Mg:LiNbO3 crystal was incomplete, the degree of polarization was low. The beam quality of the laser light was measured by focusing the output with an f = 80 mm convex lens and using the 90.0/10.0 scanning-knife-edge method. The output coupler of T = 1.5% was adopted. The beam radius as a function of distance from the waist location is shown in Fig. 5. The M2 factor of the laser beam at output power of 2.62 W was best-fitted to be 1.7 ± 0.1.
We studied the laser performance at different wavelengths of Tm,Mg:LiNbO3 crystal. Narrow-band output couplers were used to select different lasing wavelengths in the resonator. Four kinds of output couplings were employed, whose central wavelengths of high reflective coatings are 1860, 1880, 1900 and 1940 respectively. Transmission peak of the output couplings are 1.5%, and all the ∆λ at FWHM of the transmission spectra are 10 nm. Various wavelength emissions from the Tm,Mg:LiNbO3 laser were observed as shown in Fig. 6. With the same cavity length and the max incident pump power of 20 W, the different laser output powers were 2. 62 W at 1856 nm, 2.28 W at 1881 nm, 1.85 W at 1902 nm and 1.64 W at 1938 nm, respectively. By using different output couplers, the output wavelengths were not continuously tunable, but covering the range of broad emission spectral region. As shown in Fig. 6, laser emission spectra at different wavelengths were normalized respectively. For comparison, the absorption spectrum of water and Cr2+:ZnSe in a range of 1700-2000 nm were also drawn in Fig. 6, where the data of absorption spectrum was taken from  and . The laser would be absorbed much less by water and more by Cr2+:ZnSe when tuned to short wavelength. This result suggested the potentiality for laser radar system, optical remote sensing and pumping Cr2+:ZnSe to generate mid-IR emission effectively.
The laser output powers versus the absorbed pump power at different wavelengths were shown in Fig. 7 under the same cavity length of 20 mm. It showed that the laser output power decreased as the emission wavelength was shifted to long wavelength. In order to analyze the laser performance at different wavelengths, the gain cross section spectra of the Tm, Mg: LiNbO3 crystal were plotted at different inversion factors p, as shown in Fig. 8. At different inversion factors, the gain cross section at shorter wavelength was always higher than that at longer wavelength from 1850 nm to 1950 nm. Therefore, better laser operation could be obtained at short wavelength.
In order to convince there were no photorefractive effects observed in our experiment, we did quantitative analysis via a small scale experiment. An obvious characteristic of photorefractive effect is that the transmitted laser beam spot is elongated along c-axis and diffused light appears beside c-axis [20, 25]. A TEM00 mode operating laser at 532 nm was focused into the Tm,Mg:LiNbO3 crystal along the length direction (a-axis) by a lens of focal length 100 mm. The probe laser beam caustic inside the crystal was measured, resulting in a laser spot 1/e2 radius of 145 μm and a divergence half-angle of 9.6 μrad. We estimated that the probe beam intensity (maximum power level of 1.0 W) in the vicinity of the focus was 1500 kW/cm2. With this choice of focused laser beam size the Rayleigh length inside the Tm,Mg:LiNbO3 crystal was calculated to be 2.1 mm. We detected the transmitted 532 nm laser beam profiles under different power density for a long period of time by using beam diagnostics camera (LASERCAM HR, Coherent Co.), the ratio of radii along c-axis and b-axis to represent the photorefractive effect could be gotten. For accurately measurement of the radii, five sets of data were measured for every power densities, the average data were shown in Fig. 9. It was evident that the ratios of radii along c-axis and b-axis were almost unchanged under different laser power densities after long-time measurement. It illustrated that there were no long-term photorefractive effect occurred. For more convincing comparison, we presented the original 532 nm laser beam profile and transmitted laser beam profile after 600 s exposure of 1500 W/cm2 power density. As shown in Fig. 10, two beam profiles were all most the same.
In conclusion, we have demonstrated a CW laser operated at 1856 nm from Tm,Mg:LiNbO3 crystal with high Tm3+ doping concentration of ~2.0 wt.%, end-pumped by a 793 nm fiber-coupled laser diode. A maximum CW laser output power of 2.62 W at 1856 nm was achieved with a slope efficiency of 19.6% by using an output coupler of 1.5% transmission. To the best of our knowledge, this is the first time to achieve watt-level laser operation in Tm,Mg:LiNbO3 crystal and the output power is four orders of magnitude higher than that previously reported in Tm-doped LiNbO3 crystal. Furthermore, multi-wavelength laser operation was realized by using different narrow-band output couplers. Performance degradation due to the photorefractive effect under high intensity 1856 nm laser was not observed. Quantitative analysis about the long-term photorefractive effect is also provided by the crystal under exposure of high power density laser. The short 2-μm wavelength laser has an important meaning of application in tomographic imaging for detecting gastrointestinal inflammation, pumping Cr2+:ZnSe, laser radar system and optical remote sensing.
This work was partially supported by the National Natural Science Foundation of China (No. 61138006) and the State Key Program for Basic Research of China (Grant No. 2010CB630703).
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