1. Introduction

In the last decade renewable energies, including solar energy, attract great attention to solve the issues of greenhouse effect as well as the shortage of fossil fuels. However, the broadband spectrum ranging from ultraviolet to infrared and the low energy density of the sunlight prevent achieving efficient solar-energy conversion. Solar-pumped lasers (SPLs) are novel approaches to convert the sunlight into monochromatic light of extremely high density, which in turn can be converted efficiently to other energy forms. Yabe et al. have proposed the magnesium-based energy cycle in which solar energy is stored by reducing magnesium oxide using SPL irradiation [1]. We have proposed another novel approach for efficient solar-energy conversion: combination of SPLs with photovoltaic cells to generate electricity [2]. Since the 1.06 μm Nd³⁺ laser wavelength of SPLs fits silicon cells, conversion efficiency of over 60% from laser to electricity is feasible.

The first oscillation of SPLs was reported in 1966 [3] using Nd-doped Y₃Al₅O₁₂ (YAG) crystal and Nd-doped glass rods; the former is currently one of the most widely used laser host materials. Although the laser-active Nd³⁺ ions absorb the solar energy in the two ranges of 730–760 nm and 790–820 nm, only a small portion of the dominant visible range of the sunlight is available to contribute to the laser oscillation. Cr³⁺ ions are suitable sensitizers coupled with Nd³⁺ ions, because two absorption bands (⁴T₁ and ⁴T₂) of Cr³⁺ in YAG extend from 400 nm to 700 nm. Although it is difficult to prevent Cr⁴⁺ formation in bulk single crystals, being extremely detrimental to the laser oscillation [4–6], a transparent ceramics technology has improved the issue of Cr⁴⁺, as well as sufficiently suppressed scattering loss from pores and boundaries in the ceramics [7–10]. Thus, Nd/Cr co-doped YAG transparent ceramics has been spotlighted as the key material for the efficient SPLs.

We have succeeded in stable laser oscillation under natural sunlight using small-sized Nd/Cr:YAG transparent ceramics rods with no active cooling [9,10]. This is contrasting to the fact that most of large-scaled SPLs need water cooling systems consuming extra electricity, and hence essential for efficient solar-energy conversion.

The solar energy absorbed by Cr³⁺ must efficiently transfer to Nd³⁺ for sufficient sensitization. The energy transfer efficiency defined as a ratio of assistant photons for the Nd³⁺ spontaneous emission to Cr³⁺ absorbed photons was previously reported [10–13]. However, there are no reports for the Nd³⁺ stimulated emission. In this study, we have examined the energy transfer phenomena from Cr³⁺ to Nd³⁺ in Nd/Cr:YAG transparent ceramics in the laser oscillation states. To evaluate the energy transfer efficiency precisely, we have adopted two laser pumping sources; 561 nm and 808 nm lasers directly excite Cr³⁺ and Nd³⁺ ions, respectively. From the difference in the laser outputs with or without the 561 nm pumping, we have evaluated the energy transfer efficiency from Cr³⁺ to Nd³⁺ in the laser oscillation states.

2. Experiment

We prepared Nd 1.0 at.%/Cr 0.1 at.% and Nd 1.0 at.%/Cr 0.4 at.% co-doped YAG transparent ceramics rods of 1 mm × 1 mm × 5 mm in size. Figure 1 illustrates the experimental set up to evaluate the energy transfer efficiency from Cr³⁺ to Nd³⁺. A laser cavity consists of the input coupler, being one of the ends of the rod with high reflection (reflectance R_hr = 100% at 1064 nm), and the output coupler (OC, reflectance R_OC) of a concave mirror (curvature radius 100mm) arranged at the other side. To couple the excitation beam, long working distance objective lens (focal length of 10mm) was used.

 

Fig. 1 Experimental set-up of the Nd/Cr:YAG pumped by 808nm and 561nm lasers.

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An 808 nm laser diode (LD) was used for direct pumping of Nd³⁺. The other 561nm laser is coincident with the wavelength in the Cr³⁺ absorption band, which does not overlap the Nd³⁺ absorption lines (see the insert in Fig. 2).

 

Fig. 2 Absorption spectra of the Nd/Cr co-deoped YAG transparent ceramics.

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Figure 2 shows the absorption spectrum of the Nd/Cr:YAG rod samples measured using a UV-3600 (Shimazu) spectrometer, in comparison with that for Nd:YAG (no Cr). The Cr³⁺ absorption of the 0.4 at.% sample is more intense than that of the 0.1 at.% sample by around 4 times, whereas there is no difference in the Nd³⁺ absorption lines obtained from both samples.

3. Results and discussion

At first we evaluated the quality of the Nd/Cr:YAG rod as a laser medium using three kinds of OCs with different R_OC ( = 90, 95 and 99%) under the 808nm single pumping. The relationships of the 1064 nm output laser power vs. 808 nm pumping laser power are depicted in Fig. 3(a). The slope efficiency η_slope and threshold power P_th for the laser oscillation are summarized in Table 1.

 

Fig. 3 Laser output power pumped by 808nm.

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Table 1. Laser output properties

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The output laser power P_laser and η_slope are described as follows,

Next, laser oscillation was performed using the 808 nm and 561 nm double pumping, and the OC R_OC = 95%. Evidence of the energy transfer from Cr³⁺ and Nd³⁺ was clearly observed. P_laser increases at a certain 808 nm power according to the assistance of the 561 nm pumping, and P_th lowers in increasing the 561 nm power, as shown in Figs. 4 and 5, respectively. The threshold value was defined by getting the cross point of the horizontal axis and the straight line derived by choosing the measurement points where the laser slope became linear.

 

Fig. 4 Laser output power property by two-wavelength pumping with R_OC = 95%.

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Fig. 5 Laser oscillation threshold vs. the 561nm pumping power with R_OC = 95%.

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The energy transfer efficiency from Cr³⁺ and Nd³⁺ η_Cr,Nd can be estimated from the results of Figs. 4 and 5. Before the estimation, we have to consider both the intensity variation and beam profile of the 561 nm pumping laser along the propagation direction in the Nd/Cr:YAG rod. We assume that (i) both the 1064 nm output beam and 561 nm pumping beam profiles are Gaussian, (ii) the two beams are perfectly coaxial, and (iii) the fundamental mode is the most stable for the 1064nm output. The energy transfer efficiency η_Cr,Nd was obtained in the optimized overlapping between the 561nm pumping and the 1064nm laser mode. When the mode overlapping of the 561nm and the 1064nm is calculated, the conversion efficiency from 561nm to 808nm is given in Eq. (3) and the efficiency is independent of the slope efficiency and the threshold. The mode profiles (1/e² intensity) over the 5 mm-long Nd/Cr:YAG rod calculated using LASCAD and the actual values based on the measured pumping beam profile are illustrated in Fig. 6(a). The mode overlap efficiency η_OVR,abs,561 between the 1064 nm and the 561 nm beams shown in Fig. 6(b) is approximated to be unity when the 561 nm beam is inside the 1064 nm beam, and proportional to the overlap integral for larger 561 nm beam. The normalized intensity of the 561 nm beam η_αbs = exp(-α z), where α is the absorption coefficient, z the propagation distance from the end of the rod, is also depicted in Fig. 6(b). The absorption efficiency η_OVR,abs,561 is obtained by integrating the product of the overlap between the laser beam and the 561nm beam and η_αbs over the Nd/Cr:YAG rod length (5 mm) to be 42.3%.

 

Fig. 6 (a) Mode profiles of the 1064 nm output beam and 561 nm pumping beam, (b) mode overlap efficiency between the 1064 nm output beam and 561nm pumping beam (red), and relative intensity of the 561 nm beam (green).

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The power conversion efficiency from 561 nm to 808 nm, η_561,808 is expressed by the product of the efficiency of each process,

The input/output power conversion efficiency from 561 nm to 1064 nm η_561,1064 is represented by the ratio of the change of P_laser output power induced by the 561 nm pumping to the 561 nm power, is represented as follows,

In consideration of η_slope = 35.7% at R_OC = 95.1%, η_Cr,Nd at each 808 nm power higher enough than P_th was 60-70% when the 561 nm power was 50 mW, as shown in Fig. 4. When the pumping power is close to P_th, η_Cr,Nd was lower. We speculate the reason is as follows. At around P_th, Nd emission rate is low relative to the 561 nm pumping rate due to the instability of the Nd laser oscillation. Therefore the energy transfer saturates and the energy dissipates from Cr.

From the lowering in P_th as a function of the 561 nm power in Fig. 5, η_Cr,Nd was estimated to be 65.3% using Eq. (3) and parameters of ${\eta }_{561,808}$ = 0.192 ${\eta }_{561pump}\times {\eta }_{OVR,\alpha ,561}$, = 0.423, ${\eta }_{561/808}$ = 0.694, which should be an average of 60-70% depending on the 808 nm power. These values obtained here are close to the previously reported ones for spontaneous emission of Nd³⁺ determined from time-resolved photoluminescence measurements [12]. This agreement suggests that the energy transfer from Cr³⁺ to Nd³⁺ is independent of lasing except for at around the threshold power as described above.

Another energy transfer phenomena was reported on the base of small-signal gain measurements in a Nd/Cr:YAG amplifier under white light pumping [14]. However, the contributions of the Nd³⁺ direct pumping and the Cr³⁺ sensitization to the small-signal gain were not distinguished under such pumping. Therefore, this paper is, in our knowledge, the first report of the energy transfer efficiency from Cr³⁺ to Nd³⁺ in the laser oscillation states.

The energy conversion efficiency from the sunlight energy to the laser energy using an ideal SPL has been estimated to be around 46-47% assuming perfect absorption for the wavelengths shorter than the output laser wavelength in the AM1.5 spectrum and 100% energy transfer efficiency. Taking accounts of the obtained result of η_Cr,Nd, a practical modeling of SPLs using Nd/Cr:YAG should be constructed, as well as efforts to improve η_Cr,Nd. In addition, our result also contributes to the research field of Xe lamp-excited Nd/Cr:YAG lasers that are more cost-effective than LD excited ones.

4. Conclusion

The energy transfer efficiency from Cr³⁺ to Nd³⁺, η_Cr,Nd in a Nd 1.0 at.%/Cr 0.4 at.% co-doped YAG rod in the laser oscillation states has been estimated to be 65% using double pumping sources of 808 and 561 nm LDs, which excite directly Nd³⁺ ions and assist the Nd³⁺ excitation, respectively. This value is very close to that reported previously for the Nd³⁺ spontaneous emission. It is very important to consider the laser mode analysis of overlapping between the 561 nm pumping laser beam and the 1064nm output laser beam, the relation between the output power and the 561 nm pumping power and the reduction of the laser oscillation threshold of 808 nm pumping power through adding the 561 nm pumping power for the design of the SPL system.