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Abstract
The polarized spectra associated with 1.55 µm lasing in an Er:Yb:Ca3NbGa3Si2O14 crystal were investigated at room temperature. The crystal had a large absorption cross section of 3.60 × 10−20 cm2 at 978 nm, long fluorescence lifetime of 5.81 ms for the 4I13/2 multiplet of Er3+, and broad emission band at 1.55 µm with full width at half-maximum of 60 nm. End-pumped by a 975 nm diode laser, a 1555 nm continuous-wave microlaser with a maximum output power of 0.4 W and a slope efficiency of 10.6% was demonstrated in a c-cut, 2.5-mm-thick crystal. Combining with natures of the long fluorescence lifetime and the broad emission band, the Er:Yb:Ca3NbGa3Si2O14 crystal with a good thermal performance may be a promising gain medium for high-energy pulse and broadly tunable lasers around 1.55 µm.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Er3+/Yb3+ co-doped phosphate glass has been widely used as a commercial gain medium for eye-safe 1.55 µm lasers [1,2], which can be applied in many fields, such as lidar, laser ranging, three-dimensional imaging, and remote sensing. However, owing to the low thermal conductivity (approximately 0.8 W m−1 K−1) of phosphate glass, the average output power of 1.55 µm lasers realized in Er:Yb:phosphate glass is limited [1,2]. By exploiting their high Yb3+ → Er3+ energy transfer efficiency and weak upconversion loss, Er3+/Yb3+ co-doped borate crystals with high thermal conductivity, such as RAl3(BO3)4 (RAB, R = Y, Gd, and Lu) and LaMgB5O10, have been demonstrated as efficient 1.55 µm laser gain media to obtain high continuous-wave (cw) output power [3–7]. However, the high phonon energy (approximately 1400 cm−1) of the borate crystal increases the multiphonon relaxation rate from the 4I13/2 multiplet to the 4I15/2 multiplet of Er3+, and thus reduces the fluorescence lifetime of the 4I13/2 upper laser level. For example, the fluorescence lifetimes of the 4I13/2 multiplet in Er:Yb:YAl3(BO3)4 (Er:Yb:YAB) and Er:Yb:LaMgB5O10 crystals are only approximately 0.33 and 0.54 ms, respectively [8,9], which are far shorter than that of Er:Yb:phosphate glass (approximately 7 ms) [1,2]. It is well-known that a gain medium with a short fluorescence lifetime of the upper laser level will yield a higher laser threshold and lower pulse energy.
Compared with the borate crystal, the silicate crystal has a lower phonon energy (approximately 1000–1100 cm−1). Thus, the fluorescence lifetimes of the 4I13/2 upper laser level of Er3+ in silicate crystals are much longer than those in borate crystals, for example, 8.68 ms for Er:Yb:Lu2Si2O7 (Er:Yb:LPS) and 8.0 ms for Er:Yb:Ce:Ca2Al2SiO7 (Er:Yb:Ce:CAS) crystals [10,11]. Therefore, Er3+/Yb3+ co-doped silicate crystals may be more suitable gain media for low-threshold and high-pulse-energy 1.55 µm lasers. However, the slope efficiencies realized in these crystals may be limited by the inefficient Yb3+ → Er3+ energy transfer and strong upconversion loss. A 1564 nm cw laser with an output power of 0.94 W and a slope efficiency of 20% has been demonstrated in an Er:Yb:LPS crystal [12]. By using an Er:Yb:Ce:CAS crystal, a 1555 nm cw laser with an output power of 0.275 W and a slope efficiency of 9.8%, as well as an acousto-optic Q-switched pulse laser with an energy of 1.5 mJ and a duration of 40 ns, have also been realized [11,13].
The Ca3NbGa3Si2O14 (CNGS) crystal has trigonal structure with space group P321 and is optically uniaxial [14]. The crystal can be grown by the Czochralski method, and the melting point is approximately 1350°C [14], which is lower than those of LPS (approximately 1900°C) and CAS (approximately 1580°C) crystals [15,16]. As a result, it is much less expensive to grow CNGS crystal. Furthermore, although its room-temperature thermal conductivity is only approximately 1.82 W m−1 K−1, the linear thermal expansion coefficients of the three crystallographic axes of the crystal are almost the same (5.49–5.92 × 10−6 K−1). Further, the crystal also exhibits a high specific heat (0.57 and 0.83 J g−1 K−1 at 20 and 300°C, respectively) [14,17]. Therefore, the CNGS crystal has been considered as a good laser host. Efficient cw laser operations with multiwatt output power have been demonstrated in Nd3+, Yb3+, and Tm3+ singly doped CNGS crystals, respectively [14,17–19]. Recently, a 1556 nm cw laser with a maximum output power of 0.2 W and a slope efficiency of 11.4% has also been realized in a linear plano-concave cavity using an Er3+/Yb3+/Ce3+ tri-doped CNGS crystal [20]. In this work, the polarized spectra and 1.55 µm microlaser of an Er3+/Yb3+ co-doped CNGS crystal are thoroughly investigated. By comparing the spectroscopic and laser performances of Er:Yb:CNGS, Er:Yb:Ce:CNGS, and Er:Yb:phosphate glass, the potential of the Er:Yb:CNGS crystal as a characteristic gain medium for 1.55 µm lasers is evaluated.
2. Spectroscopic properties
A CNGS crystal containing 0.75 at.% (7.9 × 1019 ions/cm3) Er3+ and 1.70 at.% (1.80 × 1020 ions/cm3) Yb3+ was grown by the Czochralski method. Previous experimental results reported for the Er:Yb:RAB lasers have shown that the crystals doped with the Er3+ concentration close to 8 × 1019 ions/cm3 and Yb3+ concentration higher than 6 × 1020 ions/cm3 may be more favorable for realizing the efficient 1.55 µm laser operation [3–6]. In the Er:Yb:CNGS crystal, a similar Er3+ concentration and a lower Yb3+ concentration were adopted, because the optical quality of the crystal with a higher Yb3+ concentration became worse in present growth condition. Figure 1 shows polarized absorption spectra of the crystal at 875–1050 nm recorded at room temperature by a UV–VIS–NIR spectrophotometer (Lambda-950, PerkinElmer). The peak absorption cross sections at 978 nm for σ and π polarizations are 3.60 × 10−20 and 1.87 × 10−20 cm2, respectively, which are close to those of the Er:Yb:Ce:CNGS crystal (3.28 × 10−20 and 2.08 × 10−20 cm2 for σ and π polarizations, respectively [20]) and larger than that of Er:Yb:phosphate glass (approximately 1.0 × 10−20 cm2 [2]). The discrepancy between the absorption cross section at 978 nm in this work and that of a previously reported Yb:CNGS crystal (1.3 × 10−20 cm2 for σ polarization [14]) may result from the different Yb3+ concentrations used in the calculation. In the Yb:CNGS crystal [14], the segregation coefficient KYb of Yb3+ was assumed to be 1.0, whereas the KYb value of the Er:Yb:CNGS crystal was measured to be 0.38. The full widths at half-maximum (FWHMs) of this absorption band are 4.6 nm for both polarizations. Furthermore, there is another broad and smooth absorption band peaked at 938 nm for the Er:Yb:CNGS crystal, which is consistent with the emission wavelength of the 940 nm laser diode (LD) that is frequently used for pumping the Er:Yb:YAG crystal and Er:Yb:phosphate glass [21–23]. The FWHMs of the absorption band at 938 nm are approximately 50 nm for both polarizations, which is favorable for realizing stable laser operation when the emission bandwidth and temperature-dependent wavelength shift of the LD are taken into account. The peak absorption cross sections at 938 nm of the Er:Yb:CNGS crystal are 0.85 × 10−20 and 0.37 × 10−20 cm2 for σ and π polarizations, respectively, which are also larger than that (approximately 0.3 × 10−20 cm2) of Er:Yb:phosphate glass [2].
Fig. 1. Room-temperature polarized absorption cross-section spectra of Er:Yb:CNGS crystal at 875–1050 nm.
Because the magnetic dipole contributes significantly to the transition between the 4I15/2 and 4I13/2 multiplets of Er3+ ions, the absorption spectra of the crystal at 1410–1650 nm for three typical polarizations, i.e., α (E⊥c, H⊥c), σ (E⊥c, H||c), and π (E||c, H⊥c) [20,24], were also recorded at room temperature and are shown in Fig. 2(a). Here, E, H, and c are the electric field, magnetic field, and optical axis of the crystal, respectively. Then, the stimulated emission cross-section spectra at 1450–1650 nm were calculated using the reciprocity method [20,24] and are shown in Fig. 2(b). The peak emission cross sections at 1533 nm are 0.88 × 10−20, 0.84 × 10−20, and 0.90 × 10−20 cm2 for α, σ, and π polarizations, respectively, which are close to those of the Er:Yb:Ce:CNGS crystal (0.79 × 10−20, 0.76 × 10−20, and 0.67 × 10−20 cm2 for α, σ, and π polarizations, respectively [20]) and that of Er:Yb:phosphate glass (0.8 × 10−20 cm2 [2]). The FWHMs of this emission band are 60, 60, and 30 nm for α, σ, and π polarizations, respectively. Broad emission bands have also been observed in Nd3+, Yb3+, and Tm3+ singly doped CNGS crystals [14,18,19]. The different valences of the doping rare earth ions (Er3+ and Yb3+) and the substituted alkaline earth cations (Ca2+) in the host may produce clusters of rare earth ions during doping process, as having been demonstrated in the well-known Yb:CaF2 crystal [25]. Then, owing to the structural disorder inside and between the clusters, the spectra of the Er:Yb:CNGS crystal are inhomogeneously broadened.
Fig. 2. Room-temperature polarized spectra of Er:Yb:CNGS crystal around 1.55 µm. (a) Absorption cross-section spectra at 1410–1650 nm. (b) Emission cross-section spectra at 1450–1650 nm.
Using the absorption cross-section spectrum σabs and emission cross-section spectrum σem of the Er:Yb:CNGS crystal around 1.55 µm, the gain cross-section spectrum σg can be calculated as ${\sigma _g}(\lambda )= \beta {\sigma _{em}}(\lambda )- ({1 - \beta } ){\sigma _{abs}}(\lambda )$ [8,24]. Here, the inversion parameter β is the ratio of the number of Er3+ ions in the upper laser level to the total number of Er3+ ions. Figures 3(a) and 3(b) show the α-polarized gain cross-section spectra for different β values and the gain cross-section spectra for different polarizations at β = 0.6, respectively. When β is increased from 0.3 to 0.6, the wavelength of the maximum gain cross section is blue-shifted from approximately 1600 to 1555 nm. The gain band of the crystal is smooth and broad, with an FWHM of 44 nm at β = 0.5, which is larger than that of Er:Yb:phosphate glass (approximately 30 nm [26]). Therefore, the Er:Yb:CNGS crystal may be useful as a gain medium for broadband optical amplifiers and ultrashort-pulse lasers around 1.55 µm. The fluorescence lifetimes of the 4I11/2 and 4I13/2 multiplets of Er3+ in the crystal were measured by using a spectrometer (FLS980, Edinburgh Instruments) in conjunction with an NIR photomultiplier tube (R955, Hamamatsu) and a microsecond flash lamp (µF900, Edinburgh Instruments) for excitation at 976 nm. The lifetime of the 4I11/2 multiplet was measured to be 29.7 µs, which is much longer than those of Er:Yb:phosphate glass (2-3 µs [1]) and Er:Yb:YAB crystal (80 ns [3]). Then, the upconversion loss from the 4I11/2 multiplet and the reverse energy transfer from Er3+ to Yb3+ may be more serious in the Er:Yb:CNGS crystal and become the bottleneck that limits the laser efficiency. The lifetime of the 4I13/2 multiplet was measured to be 5.81 ms, which is shorter than that of Er:Yb:phosphate glass (approximately 7 ms [1,2]), but much longer than those of Er3+/Yb3+ co-doped borate crystals (several hundreds of microseconds) [8,9]. Therefore, the Er:Yb:CNGS crystal has a large energy storage capacity and is favorable for realizing a high-energy pulse laser. The fluorescence lifetimes τ0 and τf of the 2F5/2 multiplet of Yb3+ in the (1.70 at.%) Yb:CNGS and Er:Yb:CNGS crystals were measured to be 690 and 190 µs, respectively. Then, the Yb3+ → Er3+ energy transfer efficiency ηET of the Er:Yb:CNGS crystal was estimated to be 72% according to the formula ηET = 1 − τf/τ0. This value is slightly lower than that of the Er:Yb:Ce:CNGS crystal (79% [20]), in which the phonon-assisted cross-relaxation process 4I11/2(Er3+) + 2F5/2(Ce3+) → I13/2(Er3+) + 2F7/2(Ce3+) results in more efficient energy transfer owing to Ce3+ doping of the crystal [11,20].
Fig. 3. (a) Room-temperature α-polarized gain cross-section spectra of Er:Yb:CNGS crystal at 1500–1640 nm for different values of inversion parameter β. (b) Room-temperature polarized gain cross-section spectra of Er:Yb:CNGS crystal at 1510–1650 nm for β = 0.6.
3. CW laser performance
The experimental setup is shown in Fig. 4. A c-cut, 2.5-mm-thick Er:Yb:CNGS crystal with a cross section of 3 × 3 mm2 was used. As shown in Fig. 1, the absorption cross section around 978 nm is larger than that around 940 nm for the crystal. Therefore, in order to obtaining a higher absorption efficiency of the crystal with respect to the incident pump laser, a cw fiber-coupled LD at 975.4 nm with a core diameter of 100 µm and a numerical aperture of 0.15 was used as a pumping source. The emission wavelength of the LD was stabilized at 975.4 nm by the volume Bragg grating technique and FWHM of emission band was less than 1.0 nm. Then, the stable 1.55 µm laser operation can be realized in the crystal although its absorption band peaked at 978 nm is narrow. The absorption coefficient of the crystal at 975.4 nm is approximately 3.2 cm−1. Thus, approximately 55% of the incident pump power was absorbed by the crystal in a single pass. By using a telescopic lens system (TLS) consisting of two convex lenses, the pump beam was focused to a spot in the crystal with a waist radius of approximately 60 µm. The Er:Yb:CNGS crystal was placed between two sapphire crystals with dimensions of 3 × 3 × 1.2 mm3. Then, all the crystals were optically contacted and mounted in a copper holder cooled by water at approximately 20 °C. There was a hole with a radius of approximately 1 mm in the center of the holder to permit the laser beams to pass. An input mirror (IM) film with 90% transmission around 975 nm and 99.8% reflectivity at 1.5–1.6 µm was deposited directly on the input surface of the front sapphire crystal. An output mirror (OM) film was deposited directly on the output surface of the rear sapphire crystal. Three OM films with transmissions of 2.5%, 4%, and 6% at 1.5–1.6 µm, which are available in our lab at present, were used in the experiment. The cavity length was 4.9 mm.
Figure 5(a) shows the cw output power realized in the Er:Yb:CNGS crystal versus absorbed pump power for different OM transmissions T. When T was 4.0%, the maximum output power of 0.4 W was obtained at an absorbed pump power of 4.15 W, which is limited by the available output power of the LD used in our lab. Owing to the inefficient Yb3+ → Er3+ energy transfer (72%) and large upconversion loss in the crystal, the highest obtained slope efficiency η was 10.6%, which is lower than that realized in Er:Yb:phosphate glass (approximately 20–30% [23,26–28]). The slope efficiency of the Er:Yb:CNGS laser may be increased when the Yb3+ and Er3+ concentrations as well as the crystal growth technique are optimized in the follow-up experiment. The absorbed pump threshold of the Er:Yb:CNGS laser was approximately 0.41 W at T = 4.0%, which is smaller than that of the Er:Yb:YAB microlaser (approximately 1.0 W at T = 4.0%) under similar experimental condition [4], owing to the longer fluorescence lifetime of the 4I13/2 multiplet in the Er:Yb:CNGS crystal. By measuring the pump thresholds at different OM transmissions [29], optical loss of the Er:Yb:CNGS crystal, which originates from defects, impurities and reabsorption, etc, was estimated to be approximately 0.7%. The output laser beam was measured to be unpolarized. The laser spectrum was recorded using a monochromator (Triax 550, Jobin-Yvon) with a thermoelectrically cooled Ge detector. At different OM transmissions and pump powers, the oscillating wavelength of the Er:Yb:CNGS microlaser was unchanged and centered at approximately 1555 nm, which is consistent with the gain peak wavelength at β value of 0.6 shown in Fig. 3(a). For brevity, only the laser spectrum at an absorbed pump power of 4.15 W for T = 4.0% is shown in the inset of Fig. 5(a). The spatial profile of the laser beam was recorded with a Pyrocam III camera (Ophir Optronics Ltd.). A nearly circular output beam was observed, and the beam quality factor M2 of the output laser was fitted to approximately 5.1 at an absorbed pump power of 4.15 W for T = 4.0%, as shown in Fig. 5(b). When the pump power was reduced to 1.43 W, M2 improved to 2.5. The thermal focal length of the crystal at an absorbed pump power of 4.15 W was estimated to be approximately 18 mm, based on the measurement of the waist radius of output laser beam and ABCD law of Gaussian beam propagation in the thin lens approximation [29].
Fig. 5. (a) CW output power realized in c-cut Er:Yb:CNGS crystal as a function of absorbed pump power for different OM transmissions T. The inset shows the laser spectrum at an absorbed pump power of 4.15 W for 4.0% OM transmission. (b) Squared beam radius ω2 of output laser as a function of the distance Z from the focusing lens at absorbed pump powers of 4.15 and 1.43 W for 4.0% OM transmission. The insets show two-dimensional and three-dimensional images of the output beam transversal profile.
For T = 4.0%, the cw laser performances of 2.5-mm-thick a-cut (0.75 at.%)Er:(1.7 at.%)Yb:CNGS and c-cut (0.73 at.%)Er:(1.29 at.%)Yb:(3.04 at.%)Ce:CNGS crystals were also investigated under the same experimental condition, as shown in Fig. 6. The oscillating wavelengths of both lasers were also centered at approximately 1555 nm. For the a-cut crystal, the maximum output power of 0.36 W was obtained at an absorbed pump power of 3.62 W, and the slope efficiency η was 11%. The output laser beam quality and thermal focal length of the a-cut crystal were both close to those of the c-cut crystal. Therefore, the a-cut and c-cut Er:Yb:CNGS crystals exhibited the similar laser performances, which may be mainly caused by the similar thermal lens of both the crystals originated from the nearly same linear thermal expansion coefficients of the three crystallographic axes in the crystal. The lower output power obtained in the a-cut crystal resulted from its lower absorption efficiency with respect to the incident pump laser. However, owing to the larger σ-polarized gain cross section around 1555 nm for β = 0.6, as shown in Fig. 3(b), the output laser beam realized in the a-cut crystal was measured to be σ-polarized. Consequently, it may be more suitable for some applications, such as actively Q-switched pulsing, quantum physics, and nonlinear frequency conversion. The obtained slope efficiency of the c-cut Er:Yb:Ce:CNGS crystal was 12.3%, which is slightly higher than that realized in the Er:Yb:CNGS crystal, possibly because of the higher Yb3+ → Er3+ energy transfer efficiency (79%) in the Er:Yb:Ce:CNGS crystal resulting from doping with Ce3+ [20]. The maximum output power obtained in the Er:Yb:Ce:CNGS crystal was 0.34 W at an absorbed pump power of 3.4 W, and the output power decreased as the pump power was increased further. It is well-known that the thermal conductivity of the host crystal can be reduced by doping with impurity ions [30]. Therefore, the saturation of the output power observed in the Er:Yb:Ce:CNGS crystal may be caused mainly by the decrease in the thermal conductivity of the crystal due to doping with Ce3+ at a concentration of 3.04 at.%.
Fig. 6. CW output power versus absorbed pump power at an OM transmission of 4.0% when c-cut Er:Yb:CNGS, a-cut Er:Yb:CNGS, and c-cut Er:Yb:Ce:CNGS crystals were used as gain media under identical experimental conditions, respectively.
4. Conclusion
The spectroscopic properties of the Er:Yb:CNGS crystal are comparable to those of commercial Er:Yb:phosphate glass except for the Yb3+ → Er3+ energy transfer efficiency, which can be improved by optimizing the Yb3+ and Er3+ concentrations in the crystal. End-pumped by a 975 nm LD, a 1555 nm cw microlaser with a maximum output power of 0.4 W and a slope efficiency of 10.6% was realized in the crystal. Combining with the large FWHM (60 nm) of the emission band around 1.55 µm and the long fluorescence lifetime (5.81 ms) of the 4I13/2 multiplet of Er3+, the Er:Yb:CNGS crystal may be an excellent gain medium for broadband optical amplifiers as well as ultrashort-pulse and high-energy pulse lasers around 1.55 µm.
Funding
Ministry of Science and Technology of the People's Republic of China (2016YFB0701002); Chinese Academy of Sciences (XDB20000000); Chinese Academy of Sciences (KFJ–STS–QYZX–069).
Disclosures
The authors declare no conflicts of interest.
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