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Abstract
Er3+/Yb3+ co-doped crystal operates at an elevated temperature during 1.55 μm laser oscillation because a large amount of pump power is converted to heat. The σ-polarized absorption spectra around 976 nm, σ-polarized fluorescence spectra around 1.55 μm, and the fluorescence decay curves of the 4I13/2 multiplet of a c-cut Er:Yb:YAl3(BO3)4 crystal were measured and analyzed at temperatures between 300–800 K. When the temperature was increased from 300 to 800 K, the σ-polarized absorption cross-section at 976 nm, σ-polarized stimulated emission cross-section at 1523 nm, and fluorescence lifetime of the 4I13/2 multiplet decreased from 2.67×10−20 cm2, 1.51×10−20 cm2, and 330 μs to 1.26×10−20 cm2, 0.73×10−20 cm2, and 247 μs, respectively, while the full width at half the maximum of the absorption band around 976 nm increased from 18 to 35 nm. The results demonstrate that the 1.55 μm laser performance of the crystal may change strongly with the temperature in the crystal.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
A compact and low-cost 1.55 μm solid-state laser can be conveniently obtained based on an Er3+/Yb3+ co-doped material pumped by a 976 nm laser diode (LD). Eye-safe 1.55 μm laser has an excellent transparency in atmosphere and can be used in some applications, such as lidar, laser ranging, three-dimensional imaging and target recognition [1,2]. Compared with the widely investigated 808 nm-diode-pumped Nd3+ 1.06 μm laser and 976 nm-diode-pumped Yb3+ 1.0 μm laser with quantum defects of 24% and 8.5% [3], respectively, 976 nm-diode-pumped Er3+/Yb3+ 1.55 μm laser has a higher value of 37%. Then, a large amount of pump power is converted to heat in an Er3+/Yb3+ co-doped material and then the material will operate at elevated temperature during the 1.55 μm laser oscillation.
However, up to now, the spectroscopic properties of Er3+/Yb3+ co-doped materials have been generally investigated at temperature lower than 300 K [4–9]. Some important spectroscopic parameters, such as absorption and emission cross-sections, transition probability, fluorescence lifetime, etc. were analyzed and used as figures of merit to evaluate the laser potential of the materials. In order to evaluate and predict its 1.55 μm laser performance accurately, it is necessary to investigate the temperature dependence of the spectroscopic properties of Er3+/Yb3+ co-doped material at the high temperature. For example, by the investigation of the spectroscopic properties of the Er:Yb:YAG crystal at high temperature [10], B. Denker et al. have predicted that the crystal may realize the 1.54 μm laser at elevated temperature between 600–800°C, rather than the 1.6 μm laser reported previously [9,11].
Er:Yb:YAl3(BO3)4 (Er:Yb:YAB) has been considered as an excellent 1.55 μm laser crystal [6,12,13]. 1550 nm continuous-wave (cw) laser with output power of 2.05 W and slope efficiency of 39.8%, as well as 1522 nm passively Q-switched pulse laser with energy of about 10 μJ, repetition frequency of 77 kHz, and width of 7 ns were demonstrated [13]. Compared with other widely investigated Er3+/Yb3+ co-doped materials [4,5,8,9], such as phosphate glass and crystals of YAG and YVO4, the Er:Yb:YAB crystal has a lower fluorescence quantum efficiency of upper laser multiplet 4I13/2 (only about 7%) [14], due to a larger multi-phonon non-radiative relaxation rate caused by the higher phonon energy (about 1400 cm-1) of the borate crystal. Therefore, more pump-induced heat may generate in the crystal, which will operate at a higher temperature. Experimental result has shown that temperature inside the crystal could be up to 710 K at the pump power of 5.11 W when a 1.55 μm laser was operating [15]. In this work, spectra of the Er:Yb:YAB crystal are measured and analyzed at high temperature between 300–800 K.
2. Laser experimental arrangement
A home-made system was designed to measure the absorption and fluorescence spectra of the crystal at high temperature and the experimental setup is depicted in Fig. 1. Due to the larger absorption coefficient at pump wavelength of 976 nm for σ polarization and the isotropic thermal property in radial direction, the c-cut Er:Yb:YAB crystal has been widely used as a 1.55 μm laser gain medium [6,12,13], and then the evaluation of its laser performance is mainly rely on the σ-polarized spectroscopic parameters. Therefore, the c-cut Er:Yb:YAB crystal was chose and then its σ-polarized spectra were investigated in this work. A c-cut 5 × 5 mm2 Er(1.5 at.%):Yb(12 at%):YAB crystal with thickness of 1.9 mm was used to measure the absorption spectrum around 976 nm originated from the 2F7/2→2F5/2 transition of Yb3+. Furthermore, in order to improve the signal to noise ratio of the absorption spectrum around 1550 nm originated from the 4I15/2→4I13/2 transition of Er3+ for the crystal with low Er3+ doping concentration, another 3.6 mm-thick Er(1.5 at.%):Yb(12 at%):YAB crystal with the same cross-section was used. The crystal was placed on a closed heating sample stage with a temperature control range from 300 to 800 K and accuracy of ± 1.0 K. There is a hole with radius of 1.0 mm in the center of stage for passing the light beam. For the measurement of the absorption spectrum, a tungsten lamp was used as the exciting source. The signals of the transmitting beams when the crystal was on or off the stage were collected into a fiber bundle catheter with diameter of 5.0 mm and detected by a spectrometer (FLS980, Edinburgh). In order to ensure the least heat generated by the exciting source, the incident power density on the crystal was limited to 0.01 W/mm2. A cw fiber-coupled LD and a pulse optical-parametric oscillator (NT242-1 K, EKSPLA) at 976 nm were used as the exciting sources for the measurements of the fluorescence spectrum and decay curve around 1550 nm, respectively. All the signals were also collected into the fiber and then detected by the FLS980 spectrometer. All the spectra were recorded with a temperature interval of 50 K.
Fig. 1. Experimental setup for measuring the spectra of the Er:Yb:YAB crystal at temperature between 300–800 K.
3. Results and discussion
The absorption cross-section σabs(λ, T) can be calculated as following [2]:
Fig. 2. (a) σ-polarized absorption cross-section spectra in 900–1040 nm of the Er:Yb:YAB crystal at temperature between 300–800 K. The inset shows the spectra at 300 K recorded by a Lambda 950 spectrophotometer and the setup used in this work, respectively. (b) The peak absorption cross-sections at 976 nm and the FWHMs of the absorption band around 976 nm of the crystal at different temperatures.
The σ-polarized absorption cross-section and fluorescence spectra in 1450–1650 nm of the Er:Yb:YAB crystal at temperature between 300–800 K are shown in Figs. 3(a) and (b), respectively. Similarly, the absorption cross-sections at some main absorption peaks, such as 1485, 1498, 1523 and 1532 nm, decrease gradually with the increment of temperature. Furthermore, when the temperature is increased, the intensities at all fluorescence peaks are also reduced and the fluorescence spectrum becomes smoother. Figure 4(a) shows the σ-polarized stimulated emission cross-section spectra σem(λ, T) in 1450–1650 nm at different temperatures T, which can be calculated by the reciprocity method from the measured σ-polarized absorption cross-section spectra σabs(λ, T) shown in Fig. 3(a) [6]:
Fig. 3. (a) σ-polarized absorption cross-section spectra in 1450–1650 nm of the Er:Yb:YAB crystal at temperature between 300–800 K. (b) σ-polarized fluorescence spectra in 1450–1650 nm of the Er:Yb:YAB crystal at temperature between 300–800 K.
Fig. 4. (a) σ-polarized emission cross-section spectra in 1450–1650 nm of the Er:Yb:YAB crystal at temperature between 300–800 K. (b) σ-polarized gain cross-section spectra in 1500–1620 nm of the Er:Yb:YAB crystal at temperature between 300–800 K when the inversion parameter β is 0.5.
Table 1. Values of Zl and Zu at different temperatures T
The σ-polarized gain cross-section spectra σg(λ, T) at different temperatures T, which were calculated by σg(λ, T)=βσem(λ, T)-(1-β)σabs(λ, T) [16], are shown in Fig. 4(b) when the inversion parameter β is 0.5. It can be seen that when the temperature is increased from 300 to 800 K, the wavelength with the maximum gain cross-section of the crystal blue-shifts from 1603 to 1554 nm. This trend is consistent with the experimental results reported previously [12,13], in which the output laser wavelength blue-shifted when pump power was increased and then more heat was generated. Furthermore, gain spectrum of the crystal becomes more flat at high temperature, especially around 1550 nm with FWHM of 34 nm at 800 K. Therefore, a wide tunable range can be expected in the Er:Yb:YAB crystal at high temperature.
Based on the calculated emission cross-section spectra σem(λ, T) shown in Fig. 4(a), the σ-polarized spontaneous emission probabilities A(T) of the 4I13/2→4I15/2 transition of Er3+ at different temperatures can be calculated as following [17]:
where n is the refractive index 1.75 of the Er:Yb:YAB crystal around 1550 nm [18]. The calculated results are shown in Fig. 5(a). The value at 300 K was calculated to be 272 s-1, which is close to that (255 s-1) calculated by the Judd-Ofelt theory [14]. When the temperature is higher than 400 K, the spontaneous emission probability decreases almost linearly with the increment of the temperature. This phenomenon may be originated from higher crystal field symmetry around Er3+ sites caused by the expansion of the crystal lattice with the increment of the temperature [19]. Fluorescence lifetimes of the 4I13/2 multiplet of Er3+ of the crystal at temperature between 300–800 K, which were fitted from the measured fluorescence decay curves, are shown in Fig. 5(b). When the temperature is increased from 300 to 800 K, the fluorescence lifetime reduces from 330 to 247 μs, which is caused by the increment of the multi-phonon non-radiative relaxation [20].Fig. 5. (a) σ-polarized spontaneous emission probabilities Aσ of the 4I13/2→4I15/2 transition of the Er:Yb:YAB crystal at different temperatures. (b) Fluorescence lifetimes of the 4I13/2 multiplet of the Er:Yb:YAB crystal at temperature between 300–800 K. The inset shows the fluorescence decay curve at 1550 nm of the crystal at 300 K.
4. Conclusion
The spectra of the Er:Yb:YAB crystal were measured and investigated at temperature between 300–800 K. The spectroscopic properties of the crystal change obviously with the temperature, which may strongly affect its laser performance. The investigation of the spectroscopic properties at high temperature of the Er3+/Yb3+ co-doped material is useful for accurately evaluating and predicting its 1.55 μm laser performance.
Funding
Ministry of Science and Technology of the People's Republic of China (MOST) (2016YFB0701002); Chinese Academy of Sciences (CAS) (2014113, 2018T3005, XDB20000000).
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