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Tm3+-doped 0.1Tm2O3-1Al2O3-98.9SiO2 (mol%) silica glass with good uniformity was prepared by sol-gel method combining with high temperature sintering. The core glass sized Φ3.2 × 50 mm with Δn of 5 × 10−4 was obtained after gelation, heat treatment, melting and polishing. Its spectroscopic properties were evaluated according to the detected absorption and fluorescence spectra. The maximum emission cross-section of Tm3+ ion in this glass is 6.2 × 10−21 cm2 and tested fluorescence lifetime is 836 μs at 1806 nm. Large core double cladding fiber with core NA of 0.102 was prepared by rod-in-tube and high temperature drawing. Its tested optical loss is 1.1 dB/m at 1333 nm. 1.11 W fiber laser output centered at 1969 nm with M2 factor of 1.99 was obtained from a 140 cm length double-cladding fiber with core diameter of 38 μm. The quasi-single mode laser with M2 factor of 1.33 was achieved in the fiber with core diameter of 19 μm.
© 2014 Optical Society of America
1. Introduction
Thulium based large core fiber provides an efficient method to realize high power laser ranging from 1.8~2.2 μm. Notable milestones include high-power single-frequency output, high-peak-power generation, and 1 kW average power [1–4].
With the increase of pump power, higher thermal load will be generated in the core materials and might cause thermal destruction in high power fiber laser. Therefore, the large core fiber is in need to decrease the energy density and suppress the nonlinear effect in active fiber. Y. Jeong from University of Southampton achieved 1.36 kW continuous-wave output power from a Yb3+-doped large-core fiber prepared by the modified chemical vapor deposition (MCVD) method [5]. But MCVD method has difficulty to make large core fibers because the homogeneity of perform is deteriorated in multi-layer deposition processing. In recent years some other techniques have been developed to make large core glass for large mode area rare earth ion doped silica fibers, such as the sol-gel method [6], solution doping combined with granulation technique [7–9]. Starting from sol-gel method different components in precursor solution can be mixed in molecular level. Therefore, the homogeneous doping of rare earth oxide and other oxides, such as Al2O3, P2O5, in silica can be realized. Large core fiber with the refractive index fluctuation ~5 × 10−4 can be prepared through sol-gel method combining with high temperature sintering and fiber drawing [10].
By now reports on rare-earth doped optical fibers starting from sol-gel method are mainly focused on the ytterbium doped silica fibers [6,10–12]. Baz [6] reported 225 mw output laser at 1034 nm with 73% slope efficiency was achieved from Yb3+ doped silica fiber from sol-gel method. The maximum 81W laser from Yb3+ doped silica fiber prepared by sol-gel method was reported by S. K. Wang, and the slope efficiency is 70.8% [10]. Z. L. Li reported for the first time watt level laser output of Tm3+ doped silica fiber with core glass prepared by sol-gel method [13]. But unfortunately, the optical quality of laser beam was not given.
In this work, using sol-gel method combining with high temperature sintering large size Tm3+ doped silica glass was obtained. Double cladding fibers with large core diameters were prepared through rod in tube method and high temperature drawing. We present our researches on the properties of Tm3+ doped core glass and the laser behaviors of double cladding fibers prepared from this core glass.
2. Experiments
Using TmCl3·6H2O, AlCl3·6H2O, TEOS as precursors of Tm2O3, Al2O3, SiO2, HCl as catalyst, deionized water and C2H5OH as solvents, the mixed solution was kept stirring at 50 °C for 20 h to form hydrolyzed and homogeneous sol. All reactants were analytically pure. The mole ratio of TmCl3·6H2O: AlCl3·6H2O: TEOS was set to be 0.2:2:98.9 according to the required glass composition (0.1Tm2O3·1Al2O3·98.9SiO2 in mol%). Then the sol was kept at room temperature for one week to form transparent gel. The gel was heated from 80 °C to 1100 °C to make powder in which hydroxyl and organics were fully decomposed. The powder was then melted at 1750 °C for 2 hours and transformed into bulk glass, named TAS glass in the following. A core glass rod sized Φ3.2 × 50 mm and slices with 0.5 mm and 2 mm thickness were prepared after mechanical cutting and optical polishing from the original glass blank.
The 2 mm thick glass slice was used to study the spectroscopic properties. Absorption spectrum ranging from 200 nm to 2200 nm was recorded with a Perkin-Elmer-Lambda 900UV/VIS/NIR spectrometer. Fluorescence spectrum was measured with FLSP920 spectrophotometer (Edinburgh Co., UK) using a 808 nm laser diode as excitation source. The fluorescence lifetime was measured via pulsed 808 nm LD excitation by the FLSP920 (Edinburgh Co., UK). In order to access the absorption and emission cross sections, the glass density was tested by Archimedes liquid-immersion method, and the concentration of Tm3+ ion in glass was tested by ICP-AES device. The 0.5 mm thick glass slice was used to test the refractive index by waveguide prism coupling method at 1552 nm (Metricon, Model 2010/M, Prism Coupler).
The preform with TAS core glass (Φ3.2 × 50 mm) and an octagonal shaped pure silica glass as inner cladding was prepared by rod in tube method. The diameter ratio between inner cladding and core rod is 8:1. The refractive index profile of the preform was tested with the PK 2600 instrument.
The double cladding fibers with two different core sizes of 19 μm and 38 μm (named as TAS-F1 and TAS-F2, respectively) with a layer of ultraviolet curing adhesive were prepared at drawing temperature of 2000 °C from fiber drawing tower. Laser experiment will be discussed in the following sections. All the tests were done at room temperature.
3. Results and discussion
3.1. Spectroscopic properties
According to the Fuchbauer-Ladenburg theory and [14,15], the absorption cross-section σabs and the emission cross-section σemi of Tm3+ ion are expressed as Eq. (1) and (2):
Where λ is the wavelength. OD(λ) is the optical density. N is Tm3+ ion concentration. l is the sample thickness. Arad is the spontaneous transition probability. I(λ) is the emission intensity, n is the refractive index of glass sample, and c is the light speed in vacuum.
Figure 1 shows normalized absorption and emission spectra of Tm3+ ranging from 1400~2200 nm. Both transitions of Tm3+: 3F4→3H6(~1800 nm) and 3H4→3F4(~1470 nm) can be observed from the emission spectrum, the luminescence intensity ratio between 1800 nm and 1470 nm peaks is 36:1. The absorption peak centered at 1658 nm is due to Tm3+: 3H6→3F4 transition, the different center wavelength between absorption (1658 nm) and emission (1800 nm) spectra is caused by the 3F4 energy level splitting. Figure 2 show the absorption and emission cross-sections of the TAS glass ranging from 1400~2200 nm. The maximum emission cross-section is 6.2 × 10−21 cm2, and the fluorescence lifetime is measured to be 0.836 ms at 1806 nm. σemi × τ is an important parameter to evaluate the laser gain [16, 17]. The σemi × τ of TAS glass at 1806 nm was 5.18 × 10−21 cm2·ms. Table 1 lists the spectroscopic parameters of the TAS glass compared with data of other works [18, 19], which are typical of Tm: silica glass published elsewhere [20–22]. It is found that TAS glass has larger σemi × τ value than that of other Tm: silica glass. It is ascribed to the longer fluorescence lifetime of TAS glass. The long fluorescence lifetime of TAS glass indicates that high population inversion of 3F4 level can be realized in TAS glass. This will benefit its laser property.
Fig. 1 The normalized absorption and emission spectra (stimulated by 808 nm laser) of TAS glass ranging from 1400 ~2200 nm
Table 1. The spectroscopic parameters of TAS glass compared with those of Tm3+ doped silica glasses from MCVD
The numerical apertures (NA) of core and cladding are calculated by Eqs. (3) and (4), and the V parameter of TAS fibers is determined by Eq. (5). All these data together with tested refractive index data of TAS glass and SiO2 inner glass as well as outer cladding at 1552 nm are given in Table 2.These TAS fibers have small core NA of 0.102, which will be helpful to improve the beam quality in large core fiber.
Table 2. The basic optical parameters and sizes of TAS double cladding fibers
where d is the core diameter of the fiber, and λ is the laser wavelength.
Figure 3 shows the refractive index profile of the preform consisting of Tm3+ doped core glass and pure SiO2 glass cladding. It is shown that the refractive index fluctuation in core region is within 5 × 10−4. It indicates that good homogeneity in core glass can be achieved via sol-gel method combining with high temperature sintering.
Fig. 3 The refractive index profile of preform made from Tm3+ doped TAS core glass and pure silica cladding, laser with wavelength of 632 nm was used.
3.2. Laser properties of large core double cladding fibers
The laser behaviors of Tm3+ doped double cladding silica fibers with core diameters of 38 μm (TAS-F1) and 19 μm (TAS-F2) were tested. The core numerical aperture (NA) and inner cladding NA of are 0.102 and 0.366, respectively. Tm3+ ion concentration in core glass is tested to be 4.33 × 1019 cm−3 corresponding to 0.1 mol% Tm2O3 doping level in TAS glass. The optical attenuation spectrum measured by OSA (Yokogawa, AQ 6370C) using the cutback method is shown in Fig. 4.The optical loss at 1333 nm is ~1.1 dB/m. It is higher than that of Tm3+ doped fiber prepared by MCVD. This may be caused by the existence of scattering sources observed in core glass. The absorption peak at 1385 nm is mainly ascribed to existence of small amounts of hydroxyl groups. It is confirmed that only 1-5ppm OH- exists in the rare earth ion doped core silica glass after high temperature sintering starting from sol-gel method [23]. So the hydroxyl groups in fiber may be from attacking of moisture in atmosphere during the fiber drawing process.
Fig. 4 Optical attenuation spectrum of Tm3+ doped silica fiber ranging from 850 to 1450 nm measured by the cutback method.
The laser experiment setup is shown in Fig. 5. A multimode laser diode of 793 nm was used as pumping source, which has a pigtailed fiber with core diameter of 400 μm and NA of 0.22. It was collimated and focused into the octagonally shaped inner cladding of the TAS fiber by a 10 × objective lens. The pump light launch efficiency into active fiber TAS-F1 and TAS-F2 is 55% and 17%, respectively. The TAS double cladding fiber was placed on a testing table without cooling, and the coating layer near the pumping end was stripped in case it burns due to the direct absorption of the pumping laser. Both ends of the fiber were cleaved perpendicularly. A dichroic mirror with high reflectivity(>99.9%) at ~1900 nm and high transmission (>93%) at ~800 nm was glued to the front-end of TAS double cladding fiber, and broadband reflective mirror with 80% reflectivity at 1969 nm was glued to the back-end. A filter was put before the power meter to filter pump light. The beam quality factor (M2) was measured by Thorlabs BP109-IR2 beam profiler.
Fig. 5 Schematic diagram of the fiber laser experiment setup, pump laser with wavelength of 793 nm was used
Figure 6 shows the laser spectrum of the TAS fiber, the inset is the octagonally shaped cross-section of the fiber, laser ranging from 1958~1978 nm was obtained, and the center lasing wavelength is 1969 nm.
Figure 7 presents the absorbed pumping power versus output power curves of TAS-F1 and TAS-F2 double cladding fibers. In Fig. 7(a), Maximum 1.11 W continuous laser centered at 1969 nm was obtained from a piece of 140 cm length TAS-F1 double-cladding fiber with core diameter of 38 μm. The laser threshold is about 1.6 W. The output power scaling is limited by the available power of LD pumping source and low coupling efficiency from pumping source to active fiber. Slope efficiency is 4.5% in TAS-F1 fiber. In Fig. 7(b), the output power scaling of TASF2 fiber with core diameter of 19 μm was limited due to the poor coupling efficiency of 17%, but similar slope efficiency with TASF1 fiber was achieved due to the similar pumping power intensity, which can be estimated by the absorbed pump power and the core diameter. No saturation of output power was observed in the measurement. Low slope efficiency may be ascribed to the low Tm3+ doping level and relative high background loss in TAS fiber. As well known, the laser efficiency of Tm3+ doped fiber is greatly affected by the Tm3+ ion concentration [24]. High efficiency can be achieved at relative high Tm3+ concentration due to the contribution of cross relaxation among Tm3+ ions. The low Tm3+ doping level in TAS glass leads to small cross relaxation rate (CR) among Tm3+ ions in TAS fiber. Low coupling efficiency is due to the large pump laser spot and the stripping of the outer cladding near the pumping end as previously mentioned.
Fig. 7 Laser output power versus absorbed pump power curves of TAS-F1 and TAS-F2 double cladding fibers with core diameters (a) 38 μm and (b) 19 μm.
The definition of beam quality (M2 factor) is shown in Eq. (6), where R and θ are the waist radius and divergence angle of the practical laser beam, R0 and θ0 are the waist radius and divergence angle of the gauss beam. The inset of Fig. 7 shows the beam quality profiles of TAS-F1 and TAS-F2 fibers. In Fig. 7(a), the M2 factor of TAS-F1 fiber is 1.99. In Fig. 7(b), M2 factor of 1.33 (V = 3.1) indicates quasi-single mode laser was achieved in TAS-F2 fiber in which the core diameter is reduced to 19 μm.
Further work will focus on reducing the optical loss to improve the laser efficiency of large mode Tm3+ doped sol-gel silica fiber.
4. Conclusion
In summary, Tm3+-doped 0.1Tm2O3-1Al2O3-98.9SiO2 (in mol%) core glass sized Φ3.2 × 50 mm was prepared via sol-gel method combining high temperature sintering. The refractive index fluctuation Δn in core area is within 5 × 10−4. Its maximum emission cross-section is 6.2 × 10−21 cm2 and fluorescence lifetime is 836 μs at 1806 nm.
Double cladding fibers with 19 and 38 μm core diameters were prepared by rod-in-tube method and high temperature drawing. Their core NA and inner cladding NA values are 0.102 and 0.366, respectively. Pumped with 793 nm LD, watt level and good beam quality 2 μm laser is first reported in the Tm3+ doped large mode silica fiber with core glass prepared by sol-gel method.
Acknowledgments
This research is financially supported by the Chinese National Natural Science Foundation (Grant No. 61308084) and Natural Science Foundation of Shanghai, China (Grant No. 12ZR1451600).
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