Broadband tunable continuous-wave laser action and double-clad fiber amplifier of Yb:GSO, Yb:GYSO and Yb:LYSO crystals have been demonstrated. Under 940 nm diode pump, the free-running Yb:GSO, Yb:GYSO and Yb:LYSO lasers output power are 3.1 W at 1091 nm, 3.0 W at 1083 nm and 1.1 W at 1058 nm, with the corresponding optical-to-optical conversion efficiency of 57%, 42% and 13%, respectively. By using intra-cavity dispersion prism and a slit to restrict the spatial mode structure, continuously tunable range extends from 1002 to 1120 nm, covering some specific wavelengths as required for metrological or optical pumping applications. By using an ytterbium-doped double-clad fiber amplifier, several watts were obtained at tunable wavelengths. Particularly, the output power was 6.5 W for Yb:GSO, 12 W for Yb:GYSO, and 4 W for Yb:LYSO at 1083 nm, which matched the atomic transition wavelength useful for optical pumping of helium.
© 2007 Optical Society of America
It is of great importance to attain high-power laser outputs at some desired wavelengths in metrological and spectroscopic applications [1, 2], especially in optical pumping which may provide wide potential applications in neutron spin filters and polarized electron sources. Ytterbium-doped materials have been recognized in recent years as very attractive active media for tunable high-power lasers around 1 µm under direct pump by commercial high-power InGaAs diodes. In particular, high-power continuous-wave (cw) and pulsed lasers of high optical-to-optical conversion efficiencies have been demonstrated with some recently-developed Yb-doped oxyorthosilicates, such as Yb:Y2SiO5 (Yb:YSO), Yb:Lu2SiO5 (Yb:LSO) [3, 4], Yb:Gd2SiO5 (Yb:GSO) , Yb:GdYSiO5 (Yb:GYSO)  and Yb:LuYSiO5 (Yb:LYSO) . It is of particular interest that those lasers can be readily tuned around 1083 nm, which coincides with the well-known He 23S1→23PJ (J=0, 1, 2) triplet lines, and thus serve as competitive candidates to replace solid-state lasers at 1083 nm previously realized with neodymium-doped crystals such as Nd:LaMgAl11O19, Nd:LuAlO3 and Nd:GdVO4 [8–10]. In contrast with small fundamental manifold splitting (typically a few hundreds of cm-1 comparable to the thermal energy at room temperature) in most of the ytterbium-doped crystals, Yb:GSO, Yb:GYSO and Yb:LYSO have larger ground-state splittings up to 1067 cm-1, 995 cm-1, and 993 cm-1, respectively. Such large ground-state splittings result in small thermal populating of the terminal laser level and very weak re-absorption at the emission wavelengths. Therefore, Yb-doped oxyorthosilicates lasers at the longest-wavelength emission band (corresponding to the band for the 1083 nm laser) encounters the smallest re-absorption losses and becomes efficient with low threshold and high optical-to-optical conversion efficiency under a direct diode pump. In addition, Yb-doped oxyorthosilicates lasers can be tuned to generate other specific laser lines desired in many metrological and spectroscopic measurements. For instance, laser at 1014 nm can be frequency-quadrupled to reach the resonant transition of Hg0 (6s 1S0→ 6p 3P1) at 253.7 nm  and laser at the 1003 nm can be frequency-doubled to excite I2 transitions at 501.7 nm as an ultra-stable optical frequency standard .
The tunable Yb-doped lasers can be further boosted to high power by using Yb-doped double-clad fiber (YDCF) amplifiers. Compared with conventional bulk lasers, the main advantages of YDCF are the special fabrication and the excellent heat dissipation result from the large surface to volume ratio. Its special structure with one more cladding than common fibers provides an effective way to transfer the energy from diode lasers into fiber core in which the signal wave propagates. The inner cladding with a refractive index smaller than the core facilitates a non-absorption pass for the high-power pump and thus enables high-power diode pumping through the entire fiber core. The large mode area of the inner cladding allows multimode and relatively inexpensive pump sources to be adopted and avoids unwanted nonlinear effects or even damage to the gain region. In order to increase the pump absorption, kinds of pump technology and several double-clad fiber geometries have been developed [13,14]. By using an YDCF amplifier, several hundreds watts were obtained for cw lasers and femtosecond lasers in recent years [15, 16].
In this work, tunable cw Yb:GSO, Yb:GYSO and Yb:LYSO lasers were demonstrated under a high-power 940-nm diode laser. We obtained efficient laser operation at different wavelengths and demonstrated what is to our knowledge the shortest output wavelength down to 1002 nm and the broadest tunable range up to 118 nm in a cw Yb:GSO laser, which covers some specific laser lines as desired for metrological and spectroscopic measurements or laser-pumped helium magnetometers [1, 2]. The tunable ytterbium-doped oxyorthosilicates lasers were further amplified by using YDCF to obtain high-power cw lasers.
2. Experiment setup and results
2.1 cw Yb-doped oxyorthosilicates lasers
Our experiment employed three 5 at.%-doped Yb:GSO, Yb:GYSO, Yb:LYSO crystals, respectively. Each crystal was fixed in a water-cooled copper heat sink to remove the heat generated by laser operation and was end-pumped by a cw fiber-coupled diode laser (centered at 940 nm) with a 100-µm core diameter and a numerical aperture (NA) of 0.22. The same laser cavity configuration was employed for Yb:GSO, Yb:GYSO, and Yb:LYSO tunable lasers as shown in Fig. 1(a), consisting of one flat mirror M1, one concave mirror M2 (radius of curvature: ROC=-300 mm), a prism and an output coupler (OC) mirror. The pumping mirror M1 is coated with anti-reflection (AR) at 940 nm and reflection (HR) from 990 to 1200 nm. The concave mirrors M2 is also AR-coated at 940 nm and HR-coated from 990 to 1200 nm. The transmission of the OC from 990 to 1200 nm is 2%.
Firstly, free-running lasers pumped by 940 nm diode laser were achieved by using the V-shaped cavity. Figure 2 shows the laser output power versus respective absorbed pump power for Yb:GSO, Yb:GYSO and Yb:LYSO lasers. The pump threshold was 0.24 W for the Yb:GSO laser at 1091 nm, much higher than 0.13 W with 50 µm spot size focused on gain media reported in Ref. 5. However, the power density for 940nm-diode pumping was 3.0 kW/cm2, which was less than half of the power density 6.5 kW/cm2 for the 980nm-diode pumping. For the purpose of comparison, free-running Yb:GYSO and Yb:LYSO lasers were also investigated. The pump threshold for Yb:GYSO and Yb:LYSO lasers were 0.7 W at 1083 nm, and 2.3 W at 1059 nm, respectively. The pump threshold became smaller at long wavelength, indicating less thermal population generation and smaller re-absorption losses at emission bands of long wavelength. In our experiments, the maximum output powers for Yb:GSO, Yb:GYSO and Yb:LYSO lasers were 3.1 W at 1091 nm, 3.0 W at 1083 nm, and 1.1 W at 1059 nm, and the corresponding slope efficiencies were 61%, 42% and 20%, respectively.
We tuned the laser wavelength by inserting either SF10 (with moisture protected coating) or silica (AR-coated from 1020 to 1120 nm) prism into the resonator between M2 and OC at the Brewster’s angle. Figure 3 illustrates the tunable range of Yb-doped lasers by using different prism. Inserting an SF10 dispersion prism, we obtained a nearly smooth tuning curve from 1009 to 1110 nm with TEM00 mode. With an absorbed pump power of 5.4 W, the output power was 93 mW at 1014 nm and 1.2 W at 1083 nm, respectively. In order to increase laser output power, we substituted an SF10 prism with a silica prism so that less intra-cavity loss was introduced. Under an absorbed pump power of 5.4 W, the maximum output power was about 2.2 W at 1091 nm, and the second maximum output power was 1.9 W around 1058 nm.
When the laser wavelength was tuned to 1030 nm, the peak spectrum transferred back to 1050 nm with TEM10 mode, indicating that the gain of the TEM10 laser at 1050 nm was stronger than that of the TEM00 laser at 1030 nm. Similarly, the transversal mode changed at long wavelengths between 1091 and 1110 nm during wavelength tuning. As a relatively weaker angular dispersion was introduced by the silica prism than that by the SF10 prism, the intra-cavity angularly dispersed emissions at adjacent wavelengths could still reach the gain region, resulting in a competitive laser oscillation. When the silica prism was adjusted at some specific angles for the purpose to tune around the emission wavelengths of small emission cross-sections, such as those around 1028, 1040 and 1071 nm, adjacent wavelengths of larger emission cross-sections got competitive gains and thereby laser occurred around those wavelengths with a different transversal mode. The wavelengths of smaller emission cross-sections were not completely suppressed, which concurred as a TEM00 transversal-mode output with TEM10 lasers at other wavelengths. As a consequence, tunable laser could still be operated around the emission valleys centered at 1028, 1040 and 1071 nm, respectively. For instance, the transversal mode changed as TEM00 at 1030 nm, which competed with TEM10 at 1050 nm. By introducing a slit near the output coupler to restrict the spatial mode structure to TEM00, we achieved a continuously tunable range from 1002 to 1120 nm. The laser output power became small when the laser was tuned around the emission valleys centered at 1028, 1040 and 1071 nm. Efficient laser in short wavelength indicates that Yb:GSO exhibits enough large emission cross-section and small re-absorption losses around 1005 nm. Actually, the shortest laser wavelength was extended down to 1002 nm. The re-absorption loss became dominant over the laser gain at wavelengths shorter than 1002 nm. The tuning curve is indeed continuous and broad for Yb:GSO laser, but discontinuous for Yb:LYSO and Yb:GYSO lasers, indicating that strong gain competitions in the corresponding intermediate emission bands. The tunable range extends from 1007 to 1085 nm for Yb:GYSO, and from 1008 to 1088 nm for Yb:LYSO, with some distinct gaps on the tuning carves as indicated by dashed lines in Figs. 3(b, c).
2.2 Yb-doped double-clad laser amplifier
The tunable cw lasers could be further amplified by using YDCF amplifiers. The amplifier configuration is shown in Fig. 1(b). HR1, HR2, and HR3 are HR-coated from 1010 to 1100 nm at 45° incident angle. HWP is a half wave-plate, and OI is an optical isolator. The pumping source of the YDCF amplifier was a fiber-coupled diode laser with the diameter of 400 µm and numerical aperture of 0.22 at 976 nm. Focused by two aspheric lenses, the pump laser beam diameter was a little more than 400 µm on the end of the YDCF. We chose, 3 m-long YDCF with a diameter of 38 µm and a NA of 0.1 for the active core, and a diameter of 650 µm and a NA of 0.46 for the inner cladding. The Yb-concentration of the YDCF was 6000 ppm. A large-mode-area fiber for both the gain region and the pump energy passage facilitated expedient couplings of the signal and the pump into respective regions. The fiber ends were polished at an angle of 8° to suppress parasitic lasing. The main loss of pump power was introduced by the mirror M3, due to the difficulty in HR-coating from 1020 to 1090 nm and AR-coating at 976 nm at 45° incident angle. A D-shaped inner cladding was adopted to provide high transmission efficiency for the pump energy. In our experiments, approximately 60% of the pump power was coupled into inner cladding and 50% of signal power into the fiber core. Figures 4(a1, a2, a3) show the output powers of the YDCF amplifiers for different signal wavelengths of the three Yb-doped lasers. The output power dropped obviously when signal wavelength deviated from 1050 or 1090 nm. YDCF amplifiers are unavoidably accompanied with amplified spontaneous emission (ASE), which may create unwanted noises of the amplified signal and prevent lasing at some specific wavelengths. ASE could be restrained by increasing signal power. There existed a minimum seed power required to efficiently suppress ASE, which was estimated as 100 mW, corresponding to a lower-limit signal power of 200 mW before coupling into YDCF amplifier in our experiments. The output power decreased to 200 mW at 1020 nm for the tunable Yb:GSO laser, while it decreased to 200 mW around 1035 nm for the tunable Yb:GYSO and Yb:LYSO lasers, respectively. It means that, under our experimental condition, the YDCF amplifier cut-off wavelength was 1020 nm for Yb:GSO laser and 1035 nm for other two. Amplification was also limited in the long wavelength bands by the gain bandwidth. ASE was dependent on the signal wavelength, and it dramatically increased by tuning the signal wavelength to the wings of the gain bandwidth of YDCF. As shown in Fig. 4(a1), the lower-limit power of the signal input is about 500 mW as the signal wavelength exceeds 1090 nm.
We achieved 6.5 W for the YDCF of Yb:GSO laser at 1083 nm with a signal input of 100 mW and absorbed pump power of 22 W, corresponding to an optical-to-optical conversion efficiency of 30% and a slope efficiency of 40% in the YDCF amplifier (shown in Fig. 4(b1)). The emission spectra of output in contrast with that of signal were shown in the insets of Fig. 4(b1). Figure 4(b3) shows the output on the absorbed pump power with emission spectra for both signal (Yb:LYSO) and amplified lasers. The input signal power was also 100 mW and the amplified slope efficiency was approximately 22%. With 1.3 W Yb:GYSO cw laser at 1083 nm coupled into the YDCF core, 12 W output power was obtained with 20 W absorbed pump power. The corresponding slope efficiency (57%) and emission spectra for the Yb:GYSO laser and amplified laser are illustrated in Fig. 4(b2). The slope efficiency decreased as the absorbed pump power was larger than 20 W, due to wavelength shift of pumping diode laser from 974 to 982 nm under high-power operation. Multi-mode amplification occurred as a result of the large YDCF core diameter of 38 µm. Coiling the fiber around a cylindrical mandrel is the most efficient way to get single-mode operation. Unfortunately, it is not feasible to coil the fiber with a large outer coating fiber diameter of ~1 mm. The unyielding fiber could be coiled to a minimum circle diameter of 15 cm, which produced no effects on restriction of high-order modes. High-order modes still took up a large power proportion. It still requires further technique improvement in fabricating YDCF of large outer coating diameters to get single-mode amplification.
In summary, Yb:GSO, Yb:GYSO and Yb:LYSO lasers of broad tunable ranges were realized under diode pumping at 940 nm. A pump threshold power density as low as 3.0 kW/cm2 was demonstrated for Yb:GSO laser. A maximum tunable range of 118 nm was obtained by inserting a silica prism into the laser cavity. Evidently, Yb-doped oxyorthosilicates are the promising materials in generating high output power and tunable cw lasers. By using an Yb-doped double-clad fiber, higher power tunable laser was achieved from 1020 to 1100 nm. Moreover, 12 W output powers were achieved at 1083 nm desired for many applications, which will promote studies of atomic and molecular spectral measurements, development of the ultra-stable standard of optical frequency, and laser-pumped helium magnetometers.
This work was partly supported by National Natural Science Fund, National Basic Research Program of China, and Program for Changjiang Scholars and Innovative Research Team. The authors are indebted to Dr. Xu Jun’s group for having provided us Yb-doped crystals for experiments.
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