Tellurite glass has emerged as a promising ~2.0 μm laser material owing to its excellent features of supporting efficient mid-infrared emission and easy fiber-pulling over the commonly investigated glasses. In this paper, we report an efficient ~2.0 μm laser output from a new-type of Nd3+/Ho3+ co-doped tungsten tellurite fiber laser by using an ultra-short length fiber of 5 cm upon a 795 nm laser diode. The tellurite fibers have been fabricated by a suction technique with the core and cladding diameters of 32 and 125 μm, respectively. The propagation loss of the fiber is measured to be 4.44 dB/m at 1310 nm by using the cutback method. The large energy transfer coefficient from Nd3+ to Ho3+ (3.8 × 10−40 cm6·s–1) and high gain coefficient of Ho3+: 5I7 → 5I8 transition (1.2 cm–1) have confirmed the success of using Nd3+ as sensitizer to realize the ~2.0 μm laser emission of Ho3+. The maximum output power of the laser is obtained to be 12 mW at a center wavelength of 2052 nm with a slope efficiency of 11.2%. The laser threshold is as low as 38 mW, which is around one order of magnitude lower than the previously reported Tm3+/Yb3+ doped tellurite fiber lasers with the similar pump scheme and fiber geometry. The results indicate that the Nd3+/Ho3+ co-doped tungsten tellurite glass fiber laser is a promising laser medium candidate for achieving the ultra-compact and efficient ~2.0 μm laser.
© 2016 Optical Society of America
In recent years, fiber lasers operating at the eye-safe spectral region of ~2.0 μm have been extensively investigated due to their numerous applications in coherent laser radar systems, laser imaging, remote chemical sensing, biomedical systems, pump sources as mid-infrared (MIR) lasers, optical parametric oscillators, and so on [1–3]. Tellurium oxide (TeO2) based glasses have attracted a great deal of interest as they possess some important advantages over other commonly investigated fiber host glasses such as silicates, fluorides and germanates. Firstly, tellurite glass has a phonon energy of ~750 cm–1, which is much lower than that of silicate (~1,100 cm–1) and germanate (~900 cm–1), extends the infrared transparency range to ~5 μm, resulting in high radiative rates and long lifetimes of the lasing levels of rare earth (RE) ions that are doped into the glass . The lower phonon energy of tellurite glass also leads to a theoretically low background loss than silica at longer wavelengths, which helps to reduce the laser thresholds. Secondly, tellurite glass has much higher solubility of RE ion than silicate glass, enabling the development of ultra-compact fiber lasers by greatly increasing the RE ion concentration . Thirdly, this material has a high refractive index (~2.05) and large absorption and emission cross sections . Besides, tellurite glass is more chemically and environmentally stable than fluoride glass and has advantages in fabrication techniques as shown in oxide glass . These merits make the tellurite glass to be an efficient laser medium for the mid-infrared fiber laser emission.
Among RE ions, Tm3+ and Ho3+ ions are capable of leading to the generation of ~2.0 μm laser because of Tm3+: 3F4 → 3H6 and Ho3+: 5I7 → 5I8 transitions. Considering the larger emission cross section (σe) and longer lifetime the lasing state, Ho3+ is made more suitable for the ~2.0 μm laser output, particularly for reducing the laser threshold [7,8]. However, Ho3+ can’t be pumped directly by a readily available 800 or 980 nm commercial laser diode (LD) due to the lack of appropriate ground state absorption bands. In order to achieve efficient excitation of Ho3+, RE ions with a strong absorption band at around 800 or 980 nm are considered necessary to be used as sensitizers, such as Yb3+, Tm3+ and Er3+ [9–12], and the lasing output at ~2.0 μm have been achieved in Ho3+-doped tellurite fibers by using these ions as sensitizers [13–16].
Besides the above mentioned sensitizer ions, Nd3+ has an intensive absorption band at 800 nm corresponding to the laser wavelength of high performance and low cost commercial LD. Recently, we have found two kinds of novel approaches for obtaining intense ~2.0 μm emission of Ho3+ by using Nd3+ as sensitizer in Nd3+/Ho3+ co-doped and Nd3+/Yb3+/Ho3+ triply doped tungsten tellurite glasses [17,18]. Figure 1 shows the simplified pump scheme and energy level diagram of the Nd3+-Ho3+ co-doped system. Under 795 nm excitation, Nd3+ ions are excited to the (4F5/2, 2H9/2) states from the ground state via the ground state absorption, leading to the population of 4F3/2 followed by rapid nonradiative (NR) multi-phonon decay. This allows the energy transfer from Nd3+ to Ho3+ (ET: Nd3+: 4F3/2 + Ho3+: 5I8 → Nd3+: 4I9/2 + Ho3+: 5I5), and subsequently populates the lasing level of Ho3+: 5I7 through nonradiative decay or cross relaxation (CR: Nd3+: 4I9/2 + Ho3+: 5I6 → Nd3+: 4I13/2 + Ho3+: 5I7) process , resulting in efficient ~2.0 μm emission. Under 795 nm excitation, three intense emission bands of Nd3+ peaking at around 905, 1064, and 1339 nm can be observed due to the Nd3+: 4F3/2 → 4I9/2, 4F3/2 → 4I11/2, and 4F3/2 → 4I13/2 transitions, respectively. Here it should be noted that no upconversion luminescence due to excited state absorption (ESA) is observed for Nd3+.
Until now, there has no report about the ~2.0 μm laser in Nd3+/Ho3+ co-doped tungsten tellurite glass fibers. In this work, we report an efficient ~2.0 μm laser output in this material under the excitation of the 795 nm LD. The energy transfer, optical properties of the glass fiber and fiber laser are discussed.
The molar composition of the core was 60TeO2–30WO3–3ZnO–6La2O3–0.5Ho2O3–0.5Nd2O3 (TWHN–0.5), which was demonstrated to be the best composition for the ~2.0 μm lasing output in our previous study . A few content of Na2O was added into the cladding composition in order to adjust and slightly reduce its refractive index, and the best cladding composition was designed to be 59TeO2–30WO3–3ZnO–7La2O3–1Na2O. The fiber was fabricated by a convenient suction technique and the detailed fabricating process was described in our previous work . The core and cladding diameters of the active fiber are 32 and 125 μm, respectively, with the numerical aperture (NA) of 0.28.
The absorption spectra of the glass samples were performed on a Perkin-Elmer Lambda 900 UV-VIS-NIR spectrophotometer (Waltham, MA) with a resolution of 1 nm. The fluorescence spectra were measured by a computer controlled Triax 320 spectrofluorimeter (Jobin-Yvon Corp., HORIBA, Ltd.,) with a SR510 lock-in amplifier (Stanford Research Systems, Sunnyvale, CA), a C-995 Optical chopper (Terahertz Technologies Inc., USA), and a LE-SP-DInAs-3.4A optical detector (LEO Photonics Co., Ltd., China) for measuring the mid-infrared emission. The propagation loss of the Nd3+/Ho3+ co-doped tungsten tellurite fiber was measured at 1310 nm using the cutback technique. The output power was tested by a Nova II Display ROHS optical power meter (Ophir Optronics Solutions Ltd., Israel). The wavelength of the pump LD (LE-LS-795-700TFCF, LEO Photonics Co., Ltd., China) was centered at 795 ± 5 nm with a maximum output power of 1500 mW, spot diameter of 30 μm, and NA of 0.154. The characteristic temperatures of glass transition, onset crystallization peak, and top crystallization were analyzed using a Netzsch STA 449 C Jupiter Different scanning calorimetery (DSC, GER) under Ar atmosphere at a heating rate of 10 °C/min from 25 °C to 700 °C. The lifetime of Ho3+: 5I7 level was obtained from the first e-folding time of the emission intensity in the decay curve tested with a TDS3052B digital oscilloscope (Tektronix INC., USA). All the measurements were carried out at room temperature.
3. Results and discussions
3.1 Spectroscopic characterization
Figure 2 shows the absorption spectra of Nd3+, Ho3+, and Nd3+/Ho3+ co-doped tellurite glasses from 770 to 2200 nm. The characteristic absorption bands corresponding to transitions from the ground state to the excited states are labeled in the spectra. An intense absorption band near 800 nm was observed in Nd3+ and Nd3+/Ho3+ co-doped glasses due to the Nd3+: 4I9/2 → 4F5/2, 2H9/2 transition, but it could not be observed in the Ho3+-doped glass. In addition, the absorption band of Ho3+ nearby 900 nm (inset of Fig. 2) is very close to the emission band of Nd3+ at around 905 nm result from the Nd3+: 4F3/2 → 4I11/2 transition (as shown in Fig. 1), indicating that Ho3+ can absorb the energy from Nd3+ efficiently (e.g., Nd3+: 4F3/2 → Ho3+: 5I5).
The emission spectra of Ho3+ and Nd3+/Ho3+ co-doped tellurite glasses under the excitation using a 795 nm LD is presented in Fig. 3. In the spectra, no fluorescence emission was observed when only Ho3+ was doped into the glass because there was no ground absorption band at 795 nm. However, an intense broadband fluorescence emission at ~2.0 μm was obtained in the Nd3+/Ho3+ co-doped glass, further confirming that Nd3+ can act as an efficient sensitizer of Ho3+.
The gain coefficient of Ho3+ can be calculated by the following Eq. :Eqs. (2) and (3). The calculated gain coefficients as a function of wavelength with different p values are shown in the inset of Fig. 3. When the population inversion levels equal to 0.4, the gain coefficient becomes positive, which demonstrate that a low pump threshold can be achieved. Additionally, the maximum gain coefficient at ~2.0 μm reaches 1.2 cm–1, which is higher than that of germanate glass (0.47 cm–1)  and fluorophosphate glass (0.66 cm–1) , indicating excellent gain performance of the present host glass.
3.2 Basic parameters3], N is the concentration of the corresponding RE ions, and l is the sample thickness. The σa of Nd3+ at 795 nm is calculated to be 2.64 × 10−20 cm2, which is larger than that of Tm3+ (0.98 × 10−20 cm2 nearby 808 nm)  and Yb3+ (1.89 × 10−20 cm2 nearby 976 nm) . Large σa of Nd3+ at 795 nm is benefit for increasing the absorptivity of Nd3+, absorbing more pump power and improving the pump efficiency [17,18], which is in favor of indirect energy transfer from Nd3+ to Ho3+. And the σa of Ho3+ at 900 nm is calculated to be 0.573 × 10−20 cm2.
The stimulated emission cross section σe(λ) is obtained from the fluorescence spectra using the Fuchtbauer-Ladenburg theory :17]. And the σe of Ho3+ at 2052 nm is 5.66 × 10−21 cm2, which is larger than that in fluoride glass (5.3 × 10−21 cm2) and germanate glass (4.0 × 10−21 cm2) . All these advantages are benefit for the ~2.0 μm laser output.
Energy transfer coefficient (CD-A) from Nd3+ to Ho3+ was achieved from the overlapped absorption and emission cross section by the following Eq. [20,25]:Eqs. (2) and (3), the CD-A was calculated to be 3.8 × 10−40 cm6·s–1. Meanwhile, the energy transfer efficiency from Nd3+ to Ho3+ can be determined from the lifetime value by using the following formula :
The Judd-Oflet (J-O) intensity parameters Ωi (i = 2, 4, 6) of the Ho3+ doped tungsten tellurite glass are presented in Table 1. Generally, the spectroscopic quality factor χ (Ω4/Ω6) is an important parameter to predict the stimulated emission in a laser-active host and a large χ is beneficial to obtain intense laser output . The χ of the Ho3+ doped tungsten tellurite glass is the highest when compared with other glass hosts such as fluoride, fluorophosphate, phosphate glasses etc .
Table 2 lists the basic parameters of the Nd3+/Ho3+ co-doped tellurite glass. The value of transition temperature Tg (453 °C) is much higher than the classical TeO2–ZnO–Na2O (TZN) glass (299 °C) , so it can effectively increase the damage threshold of the glass fiber. The ΔT of this glass is as high as 122 °C, suggesting a wide range of operating temperature and outstanding glass stability against crystal nucleation or growth during the fiber drawing process. The measured lifetime of Ho3+ at 5I7 level (τm) is about 2.99 ms, extremely longer than that of fluorophosphate glass (0.95 ms), germanate glass (0.36 ms) , and silicate glass (0.32 ms) . In short, higher values of ΔT, σa, σe, and τm demonstrate that the present TWHN–0.5 glass is a promising host material to obtain efficient 2.0 μm laser.
3.3 Fiber performance
The background loss () of the Nd3+/Ho3+ co-doped tungsten tellurite fiber is one of the important parameters to evaluate the fiber properties, and it is closely related with the pump threshold and output power. The fiber background loss can be determined by Eq. (6):Fig. 4. By linear fitting to specific output power values of different points, the background loss was calculated to be 4.44 dB/m. The value is higher than the results reported by Richards et al. (1.0 dB/m at 1400 nm)  and Li et al. (2.86 dB/m at 1310 nm) . The high background loss is mainly due to the chemical impurities and tiny defects introduced during the fiber preform fabrication process, and there was no atmosphere protection in the fiber drawing process, which results in secondary pollution of the fiber preform in the heating process. In order to reduce the fiber background loss, the purity of the raw material, the technology of fabricating the fiber preform and the drawing process need to be optimized in our future research. The inset in Fig. 4 shows the photomicrograph of the Nd3+/Ho3+ co-doped multimode fiber cross section.
3.4 Fiber laser
The one-end pumped experimental configuration is shown in Fig. 5. The output power from the 795 ± 5 nm LD with spot diameter of 30 μm was collimated and focused into the tellurite fiber. The optical cavity was made up of a pair of dichroic mirrors and a 5 cm-long Nd3+/Ho3+ co-doped tungsten tellurite fiber. The dichroic mirror near the pump end is highly reflective (HR: 99.7%) at ~2.0 μm and highly transparent (HT: 91.2%) at the pump wavelength, which is to ensure vast majority of the ~2.0 μm laser being reflected back to the cavity and most of the pump power being launched into the active fiber. The other dichroic mirror near the output end is partially reflective (PR: 86.8%) at ~2.0 μm and highly reflective (HR: 89.7%) at the pump wavelength in order to reflect most of the unabsorbed pump light back into the fiber. Meanwhile, it was demonstrated that highly reflective is benefit for decreasing the threshold pump power by using the theoretical modeling method with Matlab software . An optical power meter was used to measure the output power of the 2052 nm laser after processed by a band-pass filter.
Figure 6 shows the laser output power with respect to the absorbed pump power, and the inset presents the laser spectrum of the present fiber laser. The absorbed pump power in the Fig. 6 was estimated by the differences between the power of the front-end and the back-end surface of the active fiber, both values were measured by the optical power meter, while measuring the back-end power, a low pass filter (λ<1200 nm) was placed in front of the power meter to prevent the influence of the laser output power. According to Fig. 6, the output laser power rises linearly versus the absorbed pump power, yielding a maximum value of 12 mW with a slope efficiency of 11.2%, the output power shows no signs of saturation. The laser wavelength is centered at 2052 nm, which is 32 nm longer than the emission peak in the bulk glass.
Table 3 summarizes the active fiber length, pump and laser wavelengths, laser threshold, maximum laser output power, and slope efficiency of various RE ions doped tellurite fiber lasers. Compared with other RE ions doped tellurite glass fibers, laser output can be obtained from the 5 cm ultra-short Nd3+/Ho3+ co-doped fiber which confirms its high gain per unit length. Meanwhile, the laser threshold pump power is as low as 38 mW, which is around one order of magnitude lower than that of the Tm3+/Yb3+ doped tellurite fiber laser with similar pump scheme and fiber geometry . The shortest fiber length and lower threshold pump power make this material a promising candidate for developing the ultra compact MIR laser sources.
The output power and slope efficiency are limited by several factors. As shown in Fig. 1, three intense fluorescence peaks around 905 nm, 1064 nm, and 1339 nm were observed due to Nd3+: 4F3/2 → 4I9/2, 4F3/2 → 4I11/2, and 4F3/2 → 4I13/2 transitions, respectively, and the near infrared (NIR) fluorescence spectra have been measured in our previous report . However, among the three fluorescence peaks, the branching ratio for the 1064 nm transition is the largest , which restricts the energy transfer efficiency from Nd3+ to Ho3+ because the absorption peek of Ho3+ is nearby 900 nm. On the other hand, from Eq. (4), the energy transfer coefficient (CD-A) from Nd3+ to Ho3+ is proportional to the emission cross section of the donor and the absorption cross section of the acceptor , but the absorption cross section of Ho3+ at 900 nm (in Fig. 2) is relatively low, which result in lower energy transfer coefficient. Meanwhile, the mismatch of NA between the pump LD and the as-drawn tellurite glass fiber, Fresnel reflection loss from the fiber end surface, and tiny placement skewing would result in pump power wastage and reducing the coupling efficiency when collimating and focusing the experiment system. Other possible reasons responsible for the limited output power and slope efficiency include the high absorption coefficient of OH- and high fiber background loss in the present fiber etc.
To improve the energy transfer efficiency and laser output, RE ions, such as Yb3+, can be added into the present glass fiber through acting as an energy bridge ion for promoting the energy transfer efficiency from Nd3+ to Ho3+. Meanwhile, the coupling efficiency has to be improved by optimizing the match ratio of NA between the as-drawn tellurite glass fiber and the pump LD, improving the collimation and focus degree of the experiment system. More efforts are needed to further reduce the fiber background loss. Additionally, different fiber length and reflectivities of the dichroic mirrors will be chosen in the laser experiment, a pair of fiber Bragg gratings instead of the dichroic mirrors will be adopted to form the optical cavity. We believe significant improvements can be achieved by optimizing these factors in our further research.
In summary, we have demonstrated the laser emission at ~2.0 μm from a new-type of 5-cm-long Nd3+/Ho3+ co-doped multimode tungsten tellurite glass fiber by one-end pumped with a 795 nm LD. The laser wavelength is centered at 2052 nm with a maximum output power of 12 mW and a slope efficiency of 11.2%. The laser threshold (38 mW) is around one order of magnitude lower than that of a Tm3+/Yb3+ co-doped tellurite fiber lasers with similar pump scheme and fiber geometry. The shortest fiber length and the lower threshold pump power allow simple pumping with a single laser diode. Our results give a deeper understanding of the interactions involving RE ions, and more importantly, offer a promising candidate material and scheme design for the development of ultra-compact and efficient MIR ~2.0 μm fiber laser which has shown extensive applications in remote sensing, medicine, and lidar, etc.
This work is financially supported by the National Natural Science Foundation of China (NSFC) (Grant Nos. 51125005, 51472088 and 51302086), and the Fundamental Research Funds for the Central Universities, SCUT.
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