1. Introduction

Er³⁺-doped uniaxial orthovanadates YVO₄ and GdVO₄ have recently been shown to be among the most promising gain materials for eye-safe lasers operating in the 1.6 μm wavelength range [1–3]. Their major advantages over conventional Er³⁺-doped YAG are in their stronger and broader absorption lines. This feature makes resonant pumping by 15XX-nm InGaAsP/InP laser diodes easier and allows for higher laser efficiency with good beam quality in simple laser designs. Both Er³⁺:YVO₄ and Er³⁺:GdVO₄ have similar spectroscopic features and have recently demonstrated excellent, low quantum defect (QD) laser performance with slope efficiencies as high as 54-58% at room temperature [3, 4]. It is reasonable to expect that other orthovanadates can be equally, if not more, attractive for high power eye-safe laser development. Indeed, recent studies of Yb- and Nd-doped Lutetium Orthovanadate (LuVO₄), a newer member of the vanadate family, show that this laser host has significant potential for laser power scaling. On the other hand, while thermal properties and CW laser performance of Yb- and Nd-doped LuVO₄ have been sufficiently explored [5–8], Er³⁺-doped LuVO₄ has not been adequately studied. In fact, we are aware of only one publication, which had an emphasis strictly on spectroscopic features of this crystal [9]. To the best of our knowledge, the actual laser potential of the Er³⁺:LuVO₄ crystal has never been demonstrated.

This paper presents the results of a detailed spectroscopic characterization as well as the first laser performance of resonantly pumped Er³⁺:LuVO₄. Polarized output power of 4.6 W has been achieved with slope efficiencies of 62-64% at the 1609 or 1597.5 nm wavelength - the highest laser efficiency reported for Er-doped orthovanadate lasers at room temperature.

2. Spectroscopy

Er³⁺:LuVO₄ single crystals were grown by the Czochralski technique. The Er³⁺ doping concentration of 0.59 ± 0.03% (N_Er = 7.7 x 10¹⁹ cm⁻³) was determined by Galbraith Laboratories. Spectroscopic characterization was performed in the 1450 – 1650 nm wavelength range corresponding to the ⁴I_13/2 ↔ ⁴I_15/2 transitions of Er³⁺. The polarization resolved absorption spectra collected with a Cary 6000i spectrophotometer are shown in Figs. 1(a) and 1(b). The spectral resolution for these measurements was set to 0.02 nm.

 

Fig. 1 Absorption spectra of Er³⁺:LuVO₄ for π- (a) and σ-polarization (b). The insets show the long-wavelength wings of the spectra (extended scale).

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The emission spectra of Er:LuVO₄ were obtained by illuminating the crystal with a 970 nm diode laser and collecting the luminescence with an Optical Spectrum Analyzer (Yokogawa AQ6370C, spectral resolution of 0.1 nm). A polarization beam splitter was inserted between the sample and the collecting optics to separate π- and σ-polarized emission. The measured fluorescence of the ⁴I_13/2 manifold was found to be a single exponential with the lifetime of 2.98 ms – the same as reported by Lisiecki et al for a 1% Er:LuVO₄ sample [9]. The emission cross-sections calculated using the Fuchtbauer-Landenburg method [10], are depicted in Figs. 2(a) and 2(b).

 

Fig. 2 the π- and σ-polarized emission cross-sections at 77K (a) and 300K (b). The inset in (a) shows the measured temperature dependence of the ⁴I_13/2 lifetime.

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Generally, the σ-polarized absorption is stronger except for the π-polarized lines around 1490 nm. Absorption cross-sections for both polarizations in the 1529 nm band are approximately equal for both polarizations. Thus, this particular band is convenient for resonant pumping with unpolarized sources. Also, the absorption peaks are 3 – 4 times stronger than those in Er³⁺-doped YVO₄ and GdVO₄ [4]. Since higher absorption facilitates better spatial overlap between the pumped volume and the laser cavity mode over shorter crystal lengths, Er3+:LuVO4 has an advantage over other studied Er3+-doped vanadates for applications demanding compact and high efficiency lasers.

An energy level diagram of the ⁴I_13/2 and ⁴I_15/2 manifolds is shown in Fig. 3. It was derived from the collected polarization resolved absorption and emission spectra at 10, 77 and 300K. Our diagram differs from the previous one [9], in energies of the five lowest Stark energy levels Z_i and the upper Y₁ sub-level.

 

Fig. 3 Energy level diagram of Er³⁺ in LuVO₄ at 77K. Arrows represent major transitions pertinent to resonantly-pumped ~1.6 μm laser operation.

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Our spectroscopic analysis also shows that Er³⁺:LuVO₄ has the two most promising transitions for laser operation at room temperature with approximately equal cross-sections: at ~1609 nm (π-polarization) with the cross-section of σ ~0.5 × 10⁻²⁰ cm², and at ~1597.5 nm (σ - polarization) with the cross-section of σ ~0.45 × 10⁻²⁰ cm². Despite the unusual high absorption strength of Er³⁺ ions in LuVO₄, the strength of its emission transitions is about the same as that of similar transitions in Er³⁺:GdVO₄ and Er³⁺:YVO₄ [4].

3. Laser experiments

3.1. Experimental setup

Laser experiments were performed with anti-reflection (AR) coated Er³⁺:LuVO₄ slabs with transverse dimensions of 3.5 x 6 mm. The c-axis of the crystal was normal to the laser cavity axis. Thus, the Er³⁺:LuVO₄ could be longitudinally pumped in any chosen polarization. The slabs were wrapped in indium foil, and clamped between water-cooled copper plates. The water temperature was maintained at ~14° C. A simplified experimental laser setup is shown in Fig. 4.

 

Fig. 4 An optical layout of the end-pumped Er³⁺:LuVO₄ laser. The polarization beam splitter (PBS) is used to separate π- and π-polarized laser outputs.

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As in our previous laser experiments with Er³⁺-doped orthovanadates [1, 2], we used two different pump sources. A single-mode, narrow bandwidth (~0.3 nm), CW Er-fiber laser with collimated output was configured to pump into the (somewhat weaker) 1532 nm absorption band of Er³⁺:LuVO₄. The unpolarized pump beam was focused into the crystal by a spherical lens L2 (F₂ = 100 mm) through a flat dichroic mirror and formed a cylindrically-shaped pumped volume with a diameter of ~330 μm (at 1/e²) along the entire crystal length.

The second pump source was a spectrally narrowed, InGaAsP/InP, fiber-coupled laser diode module (FCLDM) emitting at ~1529 nm with the output spectral bandwidth of ~2 nm. The fiber core diameter was 105 μm with the NA of 0.15. The pump beam was collimated by a spherical lens L1 (F₁ = 60 mm) and focused into the crystal by another lens L2 (F₂ = 75 mm) forming a conically-shaped pumped volume with a waist diameter of ~250 μm.

The laser cavity was formed by a flat dichroic mirror (T > 90% at 1520-1540 nm, R > 99.5% at 1580-1650 nm) and a plano-concave output coupler (OC) with an optimized radius of curvature (RoC) of 100 mm. Spectral transmission of the OC in the 1580-1620 nm range was almost flat.

The pump focusing lens L₂ and the concave output coupler were spaced approximately F +RoC apart (with proper correction for laser material index at the pump wavelength). This optically conjugated configuration allowed for the double-passing of the pump beam through the crystal as well as for providing the best spatial overlap between the pumped volume and the cavity mode.

In both cases, when measuring the absorbed pump, we took into account the contribution of the residual pump reflected back into the crystal from the concave OC. Here we assumed that the absorption coefficient is the same for both pump directions. The ratio of the absorbed pump power to the incident pump power in the case of pumping with the Er-fiber laser was 0.5 – 0.55 (measured for three output couplers with reflectivity of 85, 90% and 95%). With the diode pumping the same ratio decreased by about 5%. It is worth mentioning that the contribution of the second-pass pumping in the FCLDM case was small.

3.2. Laser performance

Figure 5(a) shows the output power of the CW Er:LuVO₄ laser pumped by the Er-fiber laser at 1532 nm. During laser optimization the R_OC varied between 85% and 95% and the cavity length Z_CAV ~102 mm. The best slope efficiency of 64% by the absorbed pump power was achieved when the R_OC = 85%. To the best of our knowledge, this is the highest reported efficiency for Er-doped orthovanadate lasers at room temperature. The maximum achieved output power in this case was ~4.6 W.

 

Fig. 5 Output of the Er³⁺:LuVO₄ laser, pumped by the Er-fiber laser (a) and the FCLDM (b).

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Figure 5(b) shows the output power of a similar Er:LuVO₄ laser pumped by the FCLDM. The best slope efficiency of 62% with respect to the absorbed pump power was obtained when the R_OC = 90%. The maximum achieved output power was almost the same ~4.5 W. It should be noted that the available Er:LuVO₄ crystal possessed a relatively high passive loss. We measured it at ~0.04-0.05 cm⁻¹ (at 1618 nm wavelength where ground-state absorption does not affect attenuation of the probe beam). Thus, even higher laser efficiency can be achieved with better quality crystals.

The observed laser output was single transverse mode, which is consistent with the cavity geometry described above and the small diameter of the pumped volume.

Our Er:LuVO₄ laser output consisted of two spectral components: π-polarized 1608.9 nm and σ-polarized 1597.5 nm, see Fig. 6(a). We had to run the laser in the QCW mode in order to accumulate spectral data with the same Yokogawa OSA. Time-resolved spectral studies revealed that in most cases lasing was π-polarized, with a full bandwidth of 0.4 - 1.76 nm (depending on the pump power). Typical wavelength switching between these two closely-spaced spectral components is shown in Fig. 6(b). However, by manipulating the parameters of the cavity and pump power, it was possible to observe lasing only at a σ-polarized wavelength of 1597.5 nm with the bandwidth of 0.56 – 1.4 nm.

 

Fig. 6 An integrated spectrum of the QCW Er:LuVO₄ laser (a); Time traces of lasing π- and σ-components of the laser output (b). Unabsorbed pump was intentionally allowed to leak into the 1609 nm recording channel. Er-fiber pump: average power - 3 W, pulsewidth - 12.5 msec, repetition rate - 20Hz. Laser cavity: R_OC = 98%, RoC = 100 mm, Z_CAV = 102 mm.

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As can be seen from the emission spectra of Er:LuVO₄ (see Fig. 2), two major laser transitions form relatively wide (~10 nm FWHM) emission bands centered at the 1609 nm and the 1597.5 nm wavelengths. In addition, their peak cross-sections are approximately equal. Both bands originate from the tightly spaced Stark sublevels Y₁ and Y₂ of the ⁴I_13/2 manifold with nearly equal energies of 6543 and 6547 cm⁻¹, see Fig. 3. Two lasing bands terminate at slightly different upper Stark sublevels Z₇ and Z₈ of the ground state ⁴I_15/2 manifold (energies 286 and 332 cm⁻¹, respectively). At room temperature the latter levels substantially overlap (kT ~207 cm⁻¹ whereas the energy gap between them is only ~46 cm⁻¹). Thus, any variation in wavelength- or polarization-dependent losses in the cavity significantly impacts the laser output spectrum. This situation could cause major spectral competition. An example of such time-resolved spectral behavior of the Er³⁺:LuVO₄ laser in the QCW regime is shown in Fig. 6(b).

A more detailed investigation of these peculiarities of the Er:LuVO₄ laser spectrum would be desirable but it was outside the scope of this paper.

4. Conclusions

We report what is believed to be the first laser operation based on Er³⁺:LuVO₄ single crystal. Absorption and emission spectra of Er³⁺:LuVO₄ were investigated to yield a corrected energy level diagram of ⁴I_15/2 and ⁴I_13/2 manifolds. A highly efficient resonantly (in-band) pumped laser emitting at 1609 nm (π-polarization) or 1597.5 nm (σ-polarization) has been demonstrated. With pumping by a single-mode, narrow bandwidth (~0.3 nm FWHM), CW Er-fiber laser at 1532 nm, the maximum output power of 4.6 W at 1609 nm was achieved with a maximum slope efficiency of 64% – the highest efficiency among the lasers based on Er³⁺-doped orthovanadates at room temperature. With pumping by a spectrally-narrowed InGaAsP/InP fiber-coupled laser diode module emitting at ~1529 nm the maximum CW output power of 4.5 W was obtained with the slope efficiency of 62%. Such high efficiencies were possible to achieve due to the much stronger absorption of Er in Lutetium Orthovanadate comparatively to the other known Er-doped vanadates.