1. Introduction

The Nd-based lasers are traditionally pumped into the ⁴F_5/2 level and by this mean a quantum defect between the pump- and lasing-wavelength of ~900 cm^-1 is introduced. This defect influences the emission characteristics, such as the laser threshold and slope efficiency, and has a major contribution to the heat generation in the laser material. Thus, the reduction of the quantum defect is an important issue in diminution of heat, and for Nd laser materials this can be accomplished by pumping directly into the ⁴F_3/2 emitting level. The direct pumping into the ⁴F_3/2 level, at 869 nm, was used, in fact, in the first demonstration of diode laser pumped Nd:YAG lasers both in transverse [1] and end-pumped [2] configurations. However, in spite of a quite high absorption coefficient (~3.5 cm^-1 in a 1.0-at.% Nd:YAG), this absorption line has a FWHM under 1 nm and thus is less suitable for diode pumping. The direct pumping into ⁴F_3/2 level was then replaced by pumping at 809 nm, in the strongly absorbing ⁴F_5/2 level.

Very promising for pumping of Nd:YAG into the ⁴F_3/2 level is the double-peaked absorption band centered around 885 nm that collects the thermally activated lines Z₂→R₁ (885.7 nm) and Z₃→R₂ (884.3 nm). The absorption coefficients of the two peaks are almost equal, ~1.7 cm^-1 for 1.0-at.% Nd and 14–15 cm^-1 for 9.0-at.% Nd, with an ~15% lower deep between the peaks [3]. Continuous wave (CW) laser operation under 885-nm direct pumping was recently investigated for diluted Nd:YAG single-crystals [4] and for diluted and concentrated Nd:YAG single-crystals and ceramics [5–8]. Laser emission at 1.06 μm and 946 nm with slope efficiency of 0.79 and 0.68, respectively, was obtained in a 1.0-at.% Nd:YAG single-crystal under pumping by Ti:Sapphire laser. The most concentrated Nd:YAG medium to lase at 1.06 μm was a 6.8-at.% Nd:YAG ceramic [5], and the influence of Nd-doping concentration (C_Nd ) on the performances at 946 and 1064 nm in Nd:YAG was discussed [8].

The recent development of diode lasers covering the spectral range around 885 nm can revitalize the problem of direct pumping of Nd:YAG. Thus, 14 W of CW output power at 1.06 μm with 0.63 slope efficiency was demonstrated by diode-laser direct pumping of a low-doped 1.1-at.% Nd:YAG [9]. Efficient utilization of the 885-nm pumping can be obtained by using highly-concentrated Nd:YAG: CW 2.7 W power at 1.06 μm with 0.67 slope efficiency was obtained from a 2.5-at.% Nd:YAG miniature laser [10].

In this work we report the first demonstration of 1.3-μm emission in Nd:YAG under direct pumping into the ⁴F_3/2 emitting level, to the best of our knowledge. CW 3.8 W power at 1.34 μm with overall optical-to-optical efficiency of 0.26 is obtain from a highly-doped 2.5-at.% Nd:YAG single-crystal that was pumped by diode laser at 885 nm. A discussion on the heat generated into the crystal, in lasing and non-lasing regimes, function on C_Nd is presented. It is shown that the known behavior of the 1.06-μm emission in Nd:YAG, i.e. reducing of thermal effects under efficient lasing compared with non-lasing for all the Nd concentrations of interest for lasers, differs for lasing on the ⁴F_3/2→⁴I_13/2 transition. There is a value of C_Nd ~ 1.14 at.% Nd under which the heat generation in absence of lasing is smaller than in condition of lasing, but above it the behavior becomes similar to the 1.06-μm emission. A highly-doped Nd:YAG material pumped into the emitting level is demonstrated to be a good solution for efficient laser emission and power scaling at 1.34 μm.

2. Results and discussion

A 3-mm thick, 1.0-at.% Nd:YAG single crystal and a highly-doped 2.5-at.% Nd:YAG single crystal (thickness of 5 mm) were used in experiments. Both crystal surfaces were antireflection (AR) coated at the lasing wavelengths of 1.06 and 1.34 μm and high-transmission (HT) coated for the pumping wavelengths of 809 and 885 nm. The crystals were wrapped in indium foil and clamped in a copper holder, whose temperature was kept at 18°C with a thermoelectric cooler. The optical pumping at 809 nm was made with a 400-μm diameter, 0.22-NA fiber-coupled diode (HLU32F400, LIMO Co., Germany) that outputs a maximum power of 32 W in a FWHM spectrum width Δλ~ 2.5-nm. The pumping into the emitting level was performed with an 885-nm diode laser (Hamamatsu K.K., Japan) with the same fiber characteristics as the 809-nm diode laser. This diode emits ~20 W at the fiber end in a spectrum with a Δλ~ 2.5 nm: compared with the diode we used in Ref. [10] this represents an ~3 times increase of the pumping brightness. A 1:1 achromatic optical system was used in order to imagine the fiber end into the laser crystal.

The laser emission performances were investigated in a plane-concave resonator of 45-mm length and an output mirror of 100-mm radius. The active component was placed close of the plane mirror, which was high-reflectivity coated at the investigated laser wavelength and HT coated for the pumping wavelengths.

 

Fig. 1. Output power vs. absorbed pump power for the 1.0-at.% Nd:YAG emitting at (a) 1.06 μm and (b) 1.34 μm.

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The output power versus absorbed pump power for the 1.0-at.% Nd:YAG is presented in Fig. 1. The best performances at 1.06 μm were obtained with an output mirror with 0.05 transmission (T). The slope efficiency in absorbed power, η_s,a under 809-nm pumping was 0.55, the threshold in absorbed power, P_th was 0.39 W, and 5.2 W of infrared power were obtained for 10.6 W of absorbed power, P_abs . Saturation of the output power was observed for P_abs in excess of ~11 W, this phenomenon being attributed to the thermal effects induced by pumping into the active component. When the pumping was made at 885 nm, η_s,a increased to 0.61, P_th decreased to 0.36 W and 2.1 W at 1.06 μm were obtained for P_abs = 3.8 W.

The best results for the 1.34-μm emission were obtained with a T= 0.03 output mirror: 3.4 W output power for P_abs =11.9 W at 809 nm, with η_s,a = 0.37 and P_th = 0.62 W. Saturation of output power was observed again, this time for P_abs in excess of ~10 W, as presented in Fig. 1(b), showing stronger thermal effects in Nd:YAG at this lasing wavelength than those under lasing at 1.06 μm. A maximum output power of 1.3 W (P_abs = 3.5 W) was obtained under 885-nm pumping; P_th was 0.60 W and η_s,a ~0.45.

Compared with pumping at 809 nm, the 885-nm pumping into the ⁴F_3/2 level improves the laser parameters in absorbed power. However, since the absorption efficiency, η_a at this wavelength is smaller (η_a ~0.27, compared with ~0.76 at 809 nm) the output characteristics in input power are modest. This problem can be overcome by using highly doped components, which assure an increased η_a and thus an efficient utilization of the 885-nm pump power.

The output results obtained with the 2.5-at.% Nd:YAG crystal are presented in Fig. 2. Under 809-nm pumping the laser operated at 1.06 μm with slope η_s,a = 0.49. The maximum output power was 6.6 W at P_abs = 16.7 W (η_a ~ 99%). Saturation of the output power was observed for P_abs in excess of ~13.5 W. The slope η_s,a increased at 0.56 when the pumping was made at 885 nm and 5.5 W power at 1.06 μm was measured for P_abs = 10.2 W (η_a = 0.70).

 

Fig. 2. Output power vs. absorbed pump power for the 2.5-at.% Nd:YAG emitting at (a) 1.06 μm and (b) 1.34 μm.

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The maximum power at 1.34-μm obtained from the 2.5-at.% Nd:YAG under pumping at 809 nm was 2.5 W at P_abs ~10.8 W. The output power decreased beyond this pumping level, as shown in Fig. 2(b), suggesting strong thermal effects in the Nd:YAG crystal. This behavior was not observed under 885-nm pumping: the laser operated with η_s,a = 0.39 and 3.8 W output power was measured for P_abs = 10.3 W. In spite of a lower η_a the optical efficiency expressed vs. the incident power was 0.26, compared with 0.23 obtained for the 809-nm pumping.

The influence of lasing wavelength and C_Nd on the thermal effects induced by optical pumping into the laser medium will be now discussed. In a CW four-level laser the fractional thermal loading η_h (the fraction of the pump power that is dissipated as heat) is given by [11],

where η_p is the pump level efficiency, η_ℓ represent the laser extraction efficiency, and η_qd denotes the quantum defect ratio λ_p /λ_em of the pump and emission wavelengths, with superscripts f and ℓ referring to fluorescence and lasing emission, respectively. The fraction of the ions from the ⁴F_3/2 metastable state that de-excite radiatively in the absence of laser emission is described by η_qe , the emission quantum efficiency. Under non-lasing condition the product between η_qe and the quantum defect ratio ${\eta }_{\mathit{\text{qd}}}^f$ = λ_p /${\lambda }_{\mathit{\text{em}}}^f$ governs η_h , whereas for efficient lasing emission η_h is determined by ${\eta }_{\mathit{\text{qd}}}^{\mathit{\ell }}$ = λ_p /${\lambda }_{\mathit{\text{em}}}^{\mathit{\ell }}$ .

 

Fig. 3. η_h vs. C_Nd under 885-nm pumping, non-lasing and efficient lasing at 1.06 and 1.34 μm.

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From an analysis of the emission decay in up to 8.2-at.% Nd:YAG single crystals and ceramics under low excitation intensity it was found [12] that η_qe decreases with increasing C_Nd owing to efficient cross-relaxation inside the system of Nd ions. Thus, η_h calculated in absence of lasing (η_h ≡${\eta }_h^{\mathit{\ell }}$ ) increases with C_Nd , as shown in Fig. 3 for the 885-nm pumping, since the increased concentration quenching transforms a larger part of the absorbed excitation into heat. At the same time, in condition of efficient laser emission with 885-nm pumping η_h ≡ ${\eta }_h^{\mathit{\ell }}$ is independent on C_Nd and amounts to ~0.17 and ~0.34 for the 1.06 and 1.34 μm lasers, respectively; under 809-nm pumping these values increase to ~0.24 and ~0.40. This indicates that ${\eta }_h^f$ is larger than ${\eta }_h^{\mathit{\ell }}$ in case of efficient 1.06-μm emission for all the C_Nd of interest for lasers. However, ${\eta }_h^{\mathit{f}}$ is lower than ${\eta }_h^{\mathit{\ell }}$ in case of efficient of 1.34-μm laser emission for C_Nd below ~1.14 at.% Nd, although it becomes larger above this concentration, similar to the case of 1.06 μm laser.

In order to check these conclusions, the temperature of the Nd:YAG output surface was measured directly with a Therma-Cam-S60 infrared camera (FLIR System, Sweden). The maximum temperature (T_max ) of the output surface of the 1.0-at.% Nd:YAG is shown in Fig. 4 for pumping at 885 nm. Under non-lasing a temperature of 33°C was recorded for P_abs ~ 3.9 W. This value increased to 36.5°C for lasing at 1.34 μm and decreased to 30°C for 1.06-μm emission. A similar behavior was observed for the 809-nm pumping: whereas 51.5°C were measured for P_abs = 10.5 W under non-lasing, T_max increased at 57°C and decreased at 39°C for lasing at 1.34 and 1.06 μm, respectively. Thus, an increase of thermal effects in Nd:YAG with low doping concentration (lower than 1.14 at.% Nd) that operate at 1.34 μm was proved. A similar effect was also observed in a high-power 1.32-μm emitting Nd:YAG (C_Nd = 0.6-at.%) laser pumped at 809 nm [13]. By using of a HeNe beam probe, it was found that the refractive power D of a side-pumped Nd:YAG rod increases under lasing, compared with non-lasing regime. Moreover, increase of thermal effects for lasing at 1.34 μm, by a factor of 2 compared with non-lasing regime, was recently reported in low-doped (0.3-at.%) Nd:YVO₄ and Nd:GdVO₄ crystals pumped at 808 nm [14].

 

Fig. 4. Maximum temperature of the 1.0-at.% Nd:YAG output surface vs. absorbed power at 885 nm. The recorded images for the maximum P_abs ~3.9 W are shown.

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Figure 5 presents the measured temperatures for the 2.5-at.% Nd:YAG single crystal pumped at 885 nm. Under non-lasing T_max of the medium output surface was 59.5°C at P_abs ~10.2 W and decreases to 53.5 and 37°C for lasing at 1.34 and 1.06 μm, respectively. When the pumping was made at 809 nm T_max under non-lasing was 46°C at P_abs ~13.8 W, whereas for lasing at 1.34 and 1.06 μm this temperature dropped to 44 and 37.5°C, respectively. This behavior agrees with Fig. 3 and a clear dependence between η_h in lasing and non-lasing regimes function of C_Nd is demonstrated.

 

Fig. 5. Maximum temperature of the 2.5-at.% Nd:YAG output surface vs. absorbed power at 885 nm. The recorded images for the maximum P_abs ~ 10.2 W are shown.

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We comment that the influence of various factors, such as the position of the focusing point inside the active media and the value of the laser-to-pump beam overlap efficiency, has to be considered for a quantitative description of the thermal effects induced by optical pumping into the active media. This is under investigation and the results will be published elsewhere.

3. Conclusions

In conclusion, this work reports 1.34-μm laser emission in Nd:YAG under direct pumping into the emitting level, at 885 nm, the first demonstration of such a system to the best of our knowledge. The reduction of quantum defect between the pump and laser radiation in case of direct pumping into the emitting laser level ⁴F_3/2 enhances the laser emission parameters and reduces the generation of heat by non-radiative processes compared with traditional 809-nm pumping. CW output power of 3.8 W at 1.34 μm with overall optical-to-optical efficiency of 0.26 is obtained from a 2.5-at.% Nd:YAG single crystal pumped into the ⁴F_3/2 level with a high-brightness 885-nm diode laser. The influence of the lasing wavelength and of Nd concentration on the thermal effects induced by optical pumping into the laser material is discussed. It is shown that laser emission at 1.3 μm increases the thermal effects in the laser material only under a certain value of C_Nd . This behaviour was checked by mapping the temperature of the output surface of the Nd:YAG crystals. These results demonstrate that diode laser pumping into the emitting level of concentrated Nd-based laser materials is a solution for construction of efficient micro-lasers and for scaling to high power.