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Abstract
Spectroscopic properties of erbium (Er3+)-, holmium (Ho3+)-, dysprosium (Dy3+)-doped and Er3+/Ho3+, Er3+/Dy3+ co-doped ZBLAN (ZrF4-BaF2-LaF3-AlF3-NaF) glasses were studied. The experimental results show that efficient energy transfer from Er3+ to Ho3+ and Dy3+ occurs in the Er3+/Ho3+ and Er3+/Dy3+ co-doped ZBLAN glasses, respectively. This valuable discovery enables us to design and develop high power Ho3+-doped and Dy3+-doped ZBLAN fiber lasers in the 3 µm wavelength region that can be pumped with readily available high-efficiency, high-power diode laser pumps at 980 nm.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Lasers in the mid-infrared (mid-IR) spectral region are of great interest for a wide range of scientific and technological applications including spectroscopy, medical surgery, free space communication, remote sensing, and material processing [1–5]. Compared to other laser platforms, optical fiber lasers have many well-known advantages such as excellent beam quality, high power scalability, outstanding heat dissipation capability, simplicity and compactness. ZBLAN (ZrF4-BaF2-LaF3-AlF3-NaF) fibers have been widely used as the host media for rare-earth ions for mid-IR fiber lasers and the nonlinear media for high-efficiency ultra-broad band supercontinuum generation due to their low intrinsic loss, wide transparency window, and small phonon energy [6]. Rare-earth (Er3+, Ho3+, and Dy3+) doped ZBLAN fiber lasers operating in the 3 µm wavelength region have attracted considerable interest because their emissions cover the fundamental rovibrational absorption lines of molecules containing C-H, N-H, and O-H chemical bonds, and they can be used for a lot of practical applications, such as medical diagnosis and surgery, remote sensing for gas and vapor, and material processing [7–10].
The first demonstration of rare-earth-doped ZBLAN fiber laser at 3 µm can be dated back to 1988 [6] and thereafter considerable investigations on Er3+-doped ZBLAN fiber lasers have been completed due to the readily available diode lasers at the 790 nm and 980 nm absorption peaks of Er3+[11]. Several watt-level Er3+-doped ZBLAN fiber lasers have been reported in late 1990s [11–13]. In 2007, Zhu and Jain reported the first 10-W-level 3 µm fiber laser, which was demonstrated with a 4-m 6 mol% Er3+-doped double-clad ZBLAN fiber and a high power diode pump laser at 976 nm [14]. A slope efficiency of 21.3% was obtained by taking the advantage of energy transfer up-conversion process between Er3+ and Er3+ ions [14]. Since then, several 10-watt-level Er3+-doped ZBLAN fiber lasers with higher output powers have been reported [15–16]. In 2015, a 30-W Er3+-doped all-fiber laser operating at 2938 nm was demonstrated by using ZBLAN fiber Bragg gratings to form the all-fiber laser cavity and combining several high power laser diodes near 980 nm to provide a total pump power of 188 W [17]. The stokes limit has been exceeded in a 2.8 µm cascaded laser with 50% efficiency in 2017, which paves the way to 100-W-level mid-infrared fiber laser [18]. In 2018, a 2824 nm passive cooled Er3+-doped fluoride fiber approaches an average output power of 41.6 W, which is the highest output average power achieved with the mid-infrared fiber laser [19].
Compared to Er3+-doped ZBLAN fiber lasers, Ho3+- and Dy3+-doped ZBLAN fiber lasers can operate at a wavelength beyond 3 µm and even at 3.380 µm [20], where compact diode-pumped high power laser sources are in great demand for most of the applications mentioned above. However, high efficiency high power (10s-watt or even > 100 watts) diode lasers at the near-infrared (near-IR) absorption peaks of Ho3+ (885 nm and 1150 nm) and Dy3+ (1090 nm, 1300 nm, and 1700 nm) are still not available. So far, the maximum output power of a Ho3+-doped ZBLAN fiber laser at 3 µm was 2.5 W, which was pumped with a 1100 nm Yb3+-doped silica fiber laser [21]. The maximum output power of a Dy3+-doped ZBLAN fiber laser pumped at near-IR was 180 mW [22]. Most recently, in-band pumping of the Dy3+-doped ZBLAN fiber lasers with a Er3+-doped ZBLAN fiber laser at 2.8 µm was proposed and an efficiency as high as 73% was obtained [23]. A 10-W Dy3+-doped ZBLAN fiber laser in all-fiber configuration was also demonstrated recently by in-band pumping with an Er3+-doped all-fiber laser at 2.83 µm [24]. Optical transitions in synthesized samples such as Er3+/Yb3+ and Er3+/Nd3+ have already been studied [25,26]. All these demonstrations motivate us to design and fabricate Er3+ synthesized Ho3+- and Dy3+-doped ZBLAN fibers that can be used to develop compact all-fiber lasers above 3 µm directly pumped with readily available high power high efficiency diode lasers near 980 nm. In this paper, we present the spectroscopic studies of the Er3+, Ho3+, Dy3+, and their synthesized ZBLAN glasses and investigate the energy transfer processes between these ions. Our experimental results confirm that efficient energy transfer from Er3+ to Ho3+ and Dy3+ can occur in Er3+/Ho3+ and Er3+/Dy3+ co-doped ZBLAN glasses. The energy transfer coefficients were also obtained by solving the rate equations and fitting the measured fluorescence.
2. Glass preparation and experimental setup
Rare-earth doped fluoride glasses with a composition of ZrF4-BaF2-LaF3-AlF3-NaF as the host network were fabricated by FiberLabs Inc. using the conventional melting-quenching technique. 2 mol% Er3+-doped, 1 mol% Ho3+-doped, 1 mol% Dy3+-doped, 2 mol% Er3+/1 mol% Ho3+ co-doped, and 2 mol% Er3+/1 mol% Dy3+ co-doped ZBLAN glasses were prepared. The singly doped ZBLAN glass samples with a thickness of about 1 cm were cut and polished for the transmission spectrum measurement. The singly doped and co-doped glass samples with a thickness of 1 mm were cut and their two large-area surfaces and one small-area surface were polished to optical quality for the fluorescence and lifetime measurements.
In our experiment, the transmission spectra of the glass samples from 300-3300 nm were measured with a Cary 5000 spectrometer. The fluorescence emissions of the glass samples and the lifetimes of excited states of the Er3+, Ho3+, and Dy3+ were measured with conventional measurement techniques with the experimental setups depicted in Figs. 1(a) and 1(b), respectively. The pump light was launched onto the glass sample at a small region close to the edge of the polished small-area surface. The fluorescence emitting from the glass sample was collected by a black diamond aspheric lens (Thorlab C036TME-E) from the polished small-area surface at 90° to the direction of the pump light. For the fluorescence measurement, near-IR continuous-wave (CW) laser sources at their absorption peaks were used as the pumps to excite Er3+, Ho3+, and Dy3+. The CW fluorescence was modulated by a mechanical chopper and focused into a monochromator (ORIEL Instruments, Model 77702) by a CaF2 plano-convex lens (Thorlab LA5370). To filter the scattered pump light and the fluorescence of no interest, long-pass filters with cut-on wavelengths of 2500 nm (Spectrogon LP-2500nm) and 1250 nm (Thorlab FEL1250) were used for the fluorescence measurement in the 3 µm and near-IR wavelength regions, respectively. The fluorescence intensity was measured with lock-in detection technique by using a lock-in amplifier (Princeton Applied Research, Model 5209) to detect the modulated signal received by the detector of the monochromator. A Labview software was used to control the monochromator and record the fluorescence spectrum.
For the lifetime measurement, a nanosecond pulse optical parametric oscillator (OPO) laser with a pulse duration of 10 ns at a repetition rate of 10 Hz (Continuum Surelite) was used as the pump source. The fluorescence emitting from the glass sample was collected and focused onto a detector (Thorlab PDA 20H) by two aspheric black diamond lenses (Thorlab C036TME-E). The fluorescence decay curve was recorded by an oscilloscope (Tektronics TDS 1012). A set of filters were used to obtain the fluorescence only corresponding to the transition of interest from a specific energy level. The lifetime of an energy level was achieved by fitting the fluorescence decay curve with an exponential decay function.
3. Experimental results and discussion
The partial energy-level diagrams of Er3+, Ho3+, and Dy3+, and the transitions related to the ground state absorptions and mid-IR emissions are shown in Fig. 2. Er3+ ions in the ground state can be excited to the excited level 4I11/2 (4I15/2→4I11/2) by absorbing near-IR light at 976 nm. The radiative transition from level 4I11/2 to level 4I13/2 generates the light in the 3 µm wavelength region and that from level 4I13/2 to the ground level generates the light in the 1.5 µm wavelength region. Ho3+ ions has a near-IR absorption band at 1150 nm and the ground state absorption (5I8→5I6) at this wavelength can populate the upper laser level 5I6 for the 3 µm emission (5I6 → 5I7). The radiative transition from level 5I7 to the ground level 5I8 generates the light in the 2 µm wavelength region. Dy3+ ions have three near-IR absorption bands at 1090 nm, 1300 nm, and 1700 nm. The 3 µm emission can be produced by the radiative transition from level 6H13/2 to level 6H15/2. The transmission spectra of 2 mol% Er3+-doped, 1 mol% Ho3+-doped, and 1 mol% Dy3+-doped ZBLAN glass samples with thickness of about 1 cm were measured and are shown in Fig. 3. Because the absorption bands of Ho3+ and Dy3+ at 1150 nm and 1090 nm are close to the absorption band of Er3+ at 976 nm and the energy levels 5I6 and 6F9/2 are lower than that of the exited level 4I11/2 of Er3+, energy transfer from level 4I11/2 of Er3+ to level 5I6 of Ho3+ (ET3-6) and level 6F9/2 of Dy3+ (ET3-9) as shown in Fig. 2 could happen when they are co-doped in a ZBLAN glass. It should be noted that, the excited state absorption of 980 nm pump light was not considered in the simulation model because the pump power intensity is very low and the effect of excited state absorption on the measurement results is negligible.
Fig. 2. Partial energy-level diagrams of Er3+, Ho3+, and Dy3+ and the transitions and energy transfer processes related to the emissions in the mid-IR.
Fig. 3. Measured transmission spectra of (a) 0.75 cm-thick 1 mol% Ho3+-doped, (b) 1.1 cm-thick 2 mol% Er3+-doped, and (c) 1 cm-thick 1 mol% Dy3+-doped ZBLAN glass samples.
The fluorescence spectra of singly Er3+-, Ho3+-, and Dy3+-doped ZBLAN glass in the 3 µm wavelength region were measured and are shown in Fig. 4. Clearly, the Er3+-doped ZBLAN has a fluorescence emission covering from 2500 nm to 3000 nm with a peak at 2740 nm. There are small dips on the fluorescence spectrum around 2740 nm that are due to the absorption of the water vapor in the measurement setup. The fluorescence of the Ho3+-doped ZBLAN has a peak at 2850 nm and the long wavelength emission extends to 3100 nm. The Dy3+-doped ZBLAN has a very broad fluorescence covering from 2600 nm to 3400 nm, which is of great interest for developing ultra-wide wavelength tunable laser source and ultrashort pulse laser source in the 3 µm wavelength region. For example, a wavelength-tunable Dy3+-doped ZBLAN fiber laser with a tuning range of 2.8-3.4 µm was reported [20] and a mode-locked Dy3+-doped fiber laser with a tuning range of 2.9-3.3 µm was demonstrated [27]. The lifetimes of level 4I11/2, level 5I6, and level 6H13/2 of singly Er3+-, Ho3+-, and Dy3+-doped ZBLAN glass in the 3 µm wavelength region were measured to be 6.9 ms, 3.76 ms, and 0.512 ms, respectively, as shown in Fig. 5. Because the lifetimes of level 5I6 and level 6H13/2 are smaller than that of level 4I11/2, efficient energy transfer from Er3+ to Ho3+ and Dy3+ can occurs in ZBLAN.
Fig. 4. Measured fluorescence spectra of the 2 mol% Er3+-, 1 mol% Ho3+-, and 1 mol% Dy3+-doped ZBLAN glasses in the 3 µm wavelength region.
Fig. 5. Measured 3 µm fluorescence decay curves and fitting curves of (a) 2 mol% Er3+-doped, (b) 1 mol% Ho3+-doped, and (c) 1 mol% Dy3+-doped ZBLAN glasses.
As mentioned above, a lot of investigations on Er3+-doped ZBLAN fiber lasers have been conducted and several ten-watt-level fiber lasers at 2.8 µm have been demonstrated during the last decade due to the readily available high power pump diodes at 976 nm. Therefore, similar progress on Ho3+- and Dy3+-doped ZBLAN fiber lasers could be achieved if they can be pumped with high power high efficiency diode lasers. However, high power diode lasers at the absorption peaks of Ho3+ and Dy3+ are still not available. Therefore, using energy transfer from Er3+ to Ho3+ and Dy3+ could be a promising solution to this problem.
The fluorescence spectra of the 2 mol% Er3+, 2 mol% Er3+/1 mol% Dy3+, and 2 mol% Er3+/1 mol% Ho3+ doped ZBLAN glass samples pumped with a 976 nm diode laser at a power level of 316 mW were measured at ranges of 1400-2200 nm and 2500-3500 nm and are shown in Fig. 6. Figures 6(a) and 6(b) show the fluorescence spectra of the singly Er3+-doped ZBLAN glass with peaks at 1540 nm and 2740 nm, corresponding to the transitions of 4I13/2→4I15/2 and 4I11/2→4I13/2, respectively. The fluorescence spectra of the Er3+/Ho3+ co-doped ZBLAN glass in the near-IR and the 3 µm wavelength regions are shown in Figs. 6(c) and 6(d), respectively. Besides the fluorescence of Er3+ at the 1.5 µm wavelength region, the Er3+/Ho3+ co-doped ZBLAN glass has fluorescence emission with a peak at 1950 nm, corresponding to the transition from 5I7 to 5I8 of Ho3+. Moreover, the fluorescence of the Er3+/Ho3+ co-doped ZBLAN glass in the 3 µm wavelength region exhibits the combined features of the fluorescence of singly Er3+- and Ho3+-doped ZBLAN samples. Because Ho3+ ions don’t have absorption at 976 nm, these results clearly prove that energy transfer from level 4I13/2 of Er3+ to level 5I6 of Ho3+ occurs in ZBLAN as illustrated by the ET3-6 process in Fig. 2. The fluorescence spectra of the Er3+/Dy3+ co-doped ZBLAN glass in the 1.5 µm and 3 µm wavelength regions are shown in Figs. 6(e) and 6(f), respectively. The fluorescence spectrum of the Er3+/Dy3+ co-doped ZBLAN at 3 µm is almost the same as that of the singly Dy3+-doped ZBLAN, showing that very efficient energy transfer from level 4I11/2 of Er3+ to level 6F9/2 of Dy3+ occurs as illustrated by the ET3-9 process in Fig. 2. The fluorescence of the Er3+/Dy3+ co-doped ZBLAN at 1.54 µm, however, is much smaller than that of the singly Er3+ doped ZBLAN, also indicating most energy is transferred from Er3+ to Dy3+.
Fig. 6. Fluorescence spectra of the 2 mol% Er3+-doped, 2 mol% Er3+/1 mol% Ho3+ co-doped, and 2 mol% Er3+/1 mol% Dy3+ co-doped ZBLAN glass samples measured at 1400-2200 nm and 2500-3500 nm when they were pumped at 976 nm.
The energy transfer from Er3+ to Ho3+ and Dy3+ was also confirmed by measuring the 3 µm fluorescence decay from level 4I11/2 of Er3+ in the codoped glass samples using the lifetime measurement setup shown in Fig. 1(b). The glass samples were pumped with 10 ns second pulse laser at 976 nm. The decaying curves of the 3 µm fluorescence of Er3+/Ho3+ and Er3+/Dy3+ codoped ZBLAN samples were measured by using filters to remove the light below 2.5 µm and are shown in Figs. 7(a) and 7(b), respectively. The decay time of the 3 µm fluorescence of the 2 mol% Er3+/1 mol% Ho3+ co-doped ZBLAN sample is 6.15 ms, which is smaller than that of the singly Er3+-doped ZBLAN, indicating that the energy transfer from Er3+ to Ho3+ occurs. The decay time of the 3 µm fluorescence of the 2 mol% Er3+/ 1 mol% Dy3+ co-doped ZBLAN sample is only 0.84 ms, which is significantly reduced due to the efficient energy transfer from Er3+ to Dy3+. It is worth noting that the fluorescence decay times are consistent with the measured fluorescence spectra. In the Er3+/Ho3+ co-doped ZBLAN, the energy transfer from Er3+ to Ho3+ is not significant, so the fluorescence decay time is close to the lifetime of level 4I11/2 of Er3+ and the fluorescence spectrum exhibits the combined features of the fluorescence of Er3+ and Ho3+. The energy transfer from Er3+ to Dy3+ is very efficient in the Er3+/Dy3+ co-doped ZBLAN, so the fluorescence decay time is close to the lifetime of level 6H13/2 of Dy3+ and the fluorescence spectrum is almost the same as that of the singly Dy3+-doped ZBLAN.
Fig. 7. Measured 3 µm fluorescence decay curves and fitting curves of (a) 2 mol% Er3+/1 mol% Ho3+ co-doped and (b) 2 mol% Er3+/1 mol% Dy3+ co-doped ZBLAN glasses in the 3 µm wavelength region.
In addition to the energy transfer processes from level 4I11/2 of Er3+ to Ho3+ and Dy3+(ET3-6 and ET3-9 processes shown in Fig. 2), the energy transfer processes from level 4I13/2 of Er3+ to level 5I7 of Ho3+ (ET2-5 process shown in Fig. 2) and level 6H11/2 of Dy3+ (ET2-8 process shown in Fig. 2) also occur in the co-doped ZBLANs and were confirmed by the measured fluorescence spectra and the reduced lifetime of level 4I13/2 of Er3+ when they were pumped at 1480 nm. The fluorescence of Er3+/Ho3+ co-doped ZBLAN sample pumped by a 1480 nm diode laser was measured and is shown in Fig. 8(a). Besides the fluorescence of Er3+ at the 1.55 µm wavelength region, the fluorescence of Ho3+ with a peak at 1950 nm was also measured although Ho3+ ions don’t have any absorption at 1480 nm, indicating that energy transfer from 4I13/2 of Er3+ to level 5I7 of Ho3+ occurs. Figure 8(b) shows the fluorescence of Er3+/Dy3+ co-doped ZBLAN sample pumped by a 1480 nm diode laser. Besides the fluorescence of Er3+ at the 1.55 µm wavelength region, the fluorescence of Dy3+ with a peak at 2850 nm was also measured, confirming the energy transfer from level 4I13/2 of Er3+ to level 6H11/2 of Dy3+. The 1.55 µm fluorescence decay curves of the three ZBLAN glass samples pumped at 1480 nm were also measured and are shown in Fig. 9. The lifetime of level 4I13/2 of the Er3+/Ho3+ co-doped ZBLAN was calculated to be 3.18 ms and that of the Er3+/Dy3+ co-doped ZBLAN was calculated to 0.462 ms. Both lifetimes are much smaller than that of the singly Er3+-doped ZBLAN (10.92 ms), again proving the efficient energy transfer from level 4I13/2 of Er3+ to level 5I7 of Ho3+ and level 6H11/2 of Dy3+, respectively.
Fig. 8. Measured fluorescence spectra of (a) Er3+/Ho3+ and (b) Er3+/Dy3+ co-doped ZBLAN samples pumped at 1480 nm.
Fig. 9. Measured 1.55 µm fluorescence decay curves and fitting curves of (a) 2 mol% Er3+-doped, (b) 2 mol% Er3+/1 mol% Ho3+ co-doped, and (c) 2 mol% Er3+/1 mol% Dy3+ co-doped ZBLAN glasses pumped at 1480 nm.
The backward energy transfer from Dy3+ and Ho3+ to Er3+ was also investigated in our experiment. The fluorescence spectrum of the Er3+/Ho3+ co-doped ZBLAN sample pumped at 1150 nm was measured at 1400-2200 nm and is shown in Fig. 10(a). In addition to the 2 µm emission from Ho3+, a very weak fluorescence at 1.55 µm from Er3+ was measured and is shown in Fig. 10(a), indicating that the backward energy transfer from Ho3+ to Er3+ occurs but the energy transfer probability is very low. The fluorescence spectrum of the Er3+/Dy3+ co-doped ZBLAN sample pumped at 1090 nm was measured at 1000-2000 nm and is shown in Fig. 10(b). The fluorescence at 1.55 µm from Er3+ was not measured, indicating that the backward energy transfer from Dy3+ to Er3+ is negligible.
Fig. 10. (a) Fluorescence spectrum of 2 mol% Er3+/1 mol% Ho3+ co-doped ZBLAN pumped at 1150 nm measured at 1400-2200 nm and (b) Fluorescence spectrum of 2 mol% Er3+/1 mol% Dy3+ co-doped ZBLAN pumped at 1090 nm measured at 1000-2000nm.
The parameters of energy transfer processes (k25, k36, k28, k39) can be obtained by solving the rate equations for Er3+/Dy3+ and Er3+/Ho3+ co-doped ZBLAN samples to calculate the populations on the corresponding energy levels and fitting the measured fluorescence spectra. The rate equations for Er3+/Ho3+ co-doped ZBLAN pumped at 976 nm can be written as follows.
Table 1. Transition rates and absorption cross-section of Er3+, Ho3+.
Figure 11(a) shows the measured 3 µm fluorescence spectra of the Er3+/Ho3+ co-doped ZBLANs pumped by the 976 nm diode laser at 120 mW and 316 mW and their fitting curves. The parameters k36 and k25 were calculated to be 5.46 ± 0.78×10−19 cm3/s and 1.24 ± 0.21×10−18 cm3/s, respectively, by fitting the fluorescence spectra of the Er3+/Ho3+ co-doped ZBLANs with the fluorescence spectra of the singly Er3+- and Ho3+-doped ZBLANs. Clearly, the energy transfer rate from level 4I11/2 of Er3+ to level 5I6 of Ho3+ is smaller than that from level 4I13/2 of Er3+ to level 5I7 of Ho3+. It should be noted that, the parameter k25 can be also obtained by fitting the fluorescence spectrum of the Er3+/Ho3+ co-doped ZBLAN pumped at 1480 nm that is shown in Fig. 8(a).
Fig. 11. Measured 3 µm fluorescence spectra and their fitting curves for (a) Er3+/Ho3+ co-doped ZBLAN and (b) Er3+/Dy3+ co-doped ZBLAN pumped with 120 mW and 316 mW 976 nm laser.
The rate equations for Er3+/Dy3+ co-doped ZBLAN pumped at 976 nm can be written as follows.
4. Conclusion
Spectroscopic properties of Er3+-, Ho3+-, Dy3+-, Er3+/Ho3+-, and Er3+/Dy3+-doped ZBLAN glasses were studied and energy transfer from Er3+ to Ho3+ and Dy3+ ions in ZBLAN were confirmed with the experimental results. The parameters for energy transfer processes from level 4I13/2 of Er3+ to level 5I7 of Ho3+ and level 6I13/2 of Dy3+ were estimated to be 5.46 ± 0.78×10−19 cm3/s and 2.89 ± 0.03×10−18 cm3/s, respectively. This discovery opens a new path to design and develop high power diode-pumped Ho3+- and Dy3+-doped fiber lasers at 3 µm.
Funding
University of Arizona (TRIF Photonics Initiative).
Acknowledgments
This work was supported by Technology Research Initiative Fund (TRIF) Photonics Initiative of University of Arizona.
Disclosures
The authors declare no conflicts of interest.
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