1. Introduction

The use of polycrystalline ceramics offers several advantages compared to conventional single crystals used as gain media in solid-state lasers [1]. They can be produced in arbitrary shapes or sizes, with more uniform optical properties and reduced manufacturing costs compared to single crystals grown by the Czochralski method. Neodymium-doped yttrium aluminium garnet (Nd:YAG) ceramics have been shown to have laser performances equivalent or superior to single crystals [2–4]. Typically, higher concentrations of dopants can be accommodated in Nd:YAG ceramics although it turns out that the fluorescence lifetime is dramatically reduced at doping concentrations higher than 2 at.% because of quenching effect and therefore the laser efficiency is degraded [5–7]. In this context, neodymium-doped mixed scandium garnet (Nd:YSAG) ceramics have been demonstrated to have significantly prolonged fluorescence lifetime and hence potentially better laser performances [8–10]. In Nd:YSAG, some of the aluminum ions are randomly replaced by scandium ions whose ionic radius is larger, thus expanding the lattice and moderating the effect of quenching [8]. Random substitution of scandium ions results in inhomogenous broadening of the fluorescence spectrum and therefore YSAG ceramics are potentially more attractive for ultrashort pulse generation than YAG single crystals. Besides, Nd:YSAG ceramic lasers have been demonstrated to operate at the two wavelengths of 1061 nm and 1064 nm simultaneously which are potentially useful for teraherz generation [10, 11]. Recently, it was suggested that the relative intensity of the two wavelengths is determined by the amount of heat accumulated by pumping in Nd:YSAG ceramics [12]. Typically when the thermal load of the ceramics is small – as this is the case when the pump power is weak or when the ceramics are efficiently cooled – the laser operates on the single wavelength of 1061 nm. But another laser wavelength gradually emerges at 1064 nm as the pump power is increased, whereas the 1061 nm laser component eventually vanishes as the pump power is further raised. This phenomenon can be interpreted as a consequence of the Boltzmann effect leading to significant modifications of the fluorescence spectrum of Nd:YSAG ceramics. In particular, it has been shown that the ratio of intensity between the two main peaks of the fluorescence spectrum which correspond to the laser transitions of 1061 nm and 1064 nm is depending on temperature [12]. Because of the natural evolution of the Boltzmann statistics, it was observed that the rate of spontaneous emission at the wavelength of 1061 nm is larger than that at 1064 nm when the temperature is low, whereas it is typically superior at 1064 nm at higher temperatures, a balance between the two wavelengths occuring when the temperature is in the order of a hundred of Celsius. It is worth noticing that a similar evolution of the rate of spontaneous emission at 1064 nm and 1061 nm has been observed in Nd:YAG ceramics in [13] although the balance occurs then at a cryogenic temperature and therefore the 1064-nm laser wavelength dominates at the ambiant temperature and above.

In this paper, we demonstrate a dual-wavelength Nd:YSAG ceramic laser in which the gain is structurated because of nonuniform distribution of the temperature inside the gain medium. The next section of this paper is organized as follow: in the first subsection, we briefly describe the elaboration process of the 2 at. % Nd:YSAG ceramic used further in our experiments; in the second subsection, we present the experimental setup of the 2 at. % Nd:YSAG ceramic laser; in the third subsection, we discuss the evolution of the laser spectra obtained by varying the size of the pump beam; in the fourth subsection, we investigate the effect of varying the length of the cavity on the relative intensity of the two laser wavelengths; finally in the fifth subsection, we analyse the laser modes both in space and frequency associated with the two laser wavelengths respectively.

2. Experiments

2.1. Nd:YSAG ceramics

The Nd:YSAG ceramic was elaborated using the solid-state reaction method. High-purity Y₂O₃, Sc₂O₃, and Al₂O₃ powders were weighed with Nd₂O₃ in stoichiometric proportions so as to achieve the Y_2.94Nd_0.06ScAl₄O₁₂ composition after thermal treatment. After ball milling in water with an organic binder, the resulting powder was shaped by cold uniaxial pressing into cylindrical pellets of 20 mm in diameter and 5 mm in height. Debinding under air was carried out to remove organic residues. The pellets were then placed in an alumina crucible and sintered in a tungsten mesh-heated furnace at 1700°C under vacuum to achieve fully dense material. Both sides of the ceramic sample were polished. Figure 1 shows the resulting 2-mm thick transparent 2 at.% Nd:YSAG ceramic which is further used experimentally.

 

Fig. 1 The transparent 2-at.% Nd:YSAG ceramic sample used in the laser experiments.

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2.2. Laser setup

The experimental setup is schematically depicted in Fig. 2. The laser resonator consists of two end mirrors whose reflexion coefficients at the wavelength of 1064 nm are >99% and ∼95% respectively. The Nd:YSAG ceramic is placed in the resonator in such a way that one side is bonded to the first mirror with a thin layer of index-matching liguid in-between while a glass plate coated with antireflexion coating at the wavelength of 1064 nm is placed on the other side. The laser is pumped with a fiber-coupled laser diode emitting at the wavelength of 808 nm. The pump beam is focussed using a doublet of lenses of large numerical aperture placed before the first mirror. Two additional lenses are placed inside the resonator to compensate for diffraction of the light beam.

 

Fig. 2 Experimental setup.

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2.3. Laser spectra

The size of the pump beam in the gain medium is varied by moving the doublet of lenses located before the first mirror (see Fig. 2). We recorded the laser spectra represented in Fig. 3 for increasing sizes of the pump beam while the pump power was fixed to 5 W. As can be seen in Fig. 3, the laser produces monochromatic light when the pump beam diameter is relatively large and thus the power density per unit of surface is relatively weak. Typically a single laser wavelength is produced at ∼1061 nm at 10.6 kW/cm² or less. But stronger focussing of the pump beam yields the emergence of another wavelength component at ∼1064 nm as it can be observed in Fig. 3. Further decreasing the size of the pump beam, the intensity of the 1064 nm wavelength component grows and then saturates above 30 kW/cm² while the intensity of the 1061-nm wavelength component decreases and eventually vanishes above 43 kW/cm².

 

Fig. 3 Laser spectra.

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This result can be understood in the following way. Weak pump focussing yields a reduction of the maximum temperature in the center of the pump volume, that in turns reduces the amount of spontaneous emission experienced at 1064 nm and thus laser oscillation at 1061 nm is more efficient. This is a direct consequence of the evolution of the fluorescence spectrum of Nd:YSAG against temperature [12] as typically represented in Fig. 4. In contrast, when the pump focussing is strong, the amount of spontaneous emission at 1064 nm is increased that eventually yields the emergence of the 1064 nm laser wavelength and vanishing of the 1061 nm laser wavelength. In that experiment, no modification of the laser resonator is made which ensures stable intracavity loss conditions during the spectral switching between 1061 nm and 1064 nm.

 

Fig. 4 Fluorescence spectra of Nd:YSAG ceramic measured for the different temperatures of 30, 111, and 166°C, respectively.

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2.4. Laser stability

The output mirror was moved longitudinally, thus gradually modifying the resonator length l from l = 37.5 cm to l = 42.5 cm. Figure 5 represents the evolution of the intensity of the two laser components at 1061 nm and 1064 nm as a function of l. Stable laser oscillations occur when the resonator length is between l = 38.3 cm and l = 41.5 cm. The laser emits monochromatic light at the wavelength of 1061 nm between l = 38.3 cm and l = 39.1 cm whereas dual wavelength components at 1061 nm and 1064 nm are generated between l = 39.1 and l = 41.5 cm.

 

Fig. 5 Evolution of the intensity of the two laser components at 1061 nm and 1064 nm as a function of the resontor length from l = 37.5 cm to l = 42.5 cm.

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In the present configuration of the laser, an increase of the cavity length translates into a reduction of the size of the laser modes and thus a reduction of the overlap between the laser modes inside the YSAG ceramic and the surrounding part of the pump beam. In other words, when the cavity is relatively short as typically when its length ranges between 38.3 cm and 39.1 cm, laser oscillations overlap with the whole pump volume. In this situation, the overall rate of spontaneous emission is larger at 1061 nm and only the 1061-nm laser wavelength is emitted, the gain at 1064 nm being not sufficient to reach the laser threshold. As the cavity length is further increased above 39.1 cm, it turns out however that the cavity loss is reduced and the 1064-nm eventually develops as well. But the diameter of the transverse modes in the Nd:YSAG ceramic is decreased when the cavity is made longer and thus the overlap is reduced between the laser modes and the surrounding region of the pump volume where the spontaneous emission at 1061 nm is favoured. Therefore, the intensity level of the 1064 nm laser wavelengths tends to catch up that of the 1061 nm as the cavity length is increased up to l = 40.5 cm. When the cavity length exceeds 40.5 cm, the losses increase again thus decreasing both the levels of the 1061 nm and 1064 nm wavelengths. Hence, it is clear from these observations that the gain is not equally distributed for both laser wavelengths but acts as gain filtering.

2.5. Laser modes

The laser light was spatially separated using a dispersive grating (1832 lines/mm) then pictured with a CCD camera to visualize the laser modes. The position of the output mirror is adjusted in order to achieve simultaneous emission at both wavelengths so that the resonator length is fixed to 40 cm. The pump power is fixed as well to 5 W. Figures 6(a)–6(c) show pictures observed on the screen obtained for slightly different orientations of the output mirror respectively. Figure 6(a) shows the patterns of the TEM₀₀ and TEM₀₁ modes on the left-hand side and on the right-hand side of the figure associated to the wavelengths of ∼1061 nm and ∼1064 nm, respectively. It can also be seen from Fig. 6(a) that these two radiations are distributed among several longitudinal modes around 1061 nm and 1064 nm which were not resolved before in the spectra represented in Fig. 3. The fact that the laser oscillations at the wavelengths of 1061 nm and 1064 nm are confined in two different transverse modes cannot be interpreted as resulting only from chromatic dispersion which is obviously negligible here given the small detuning between the two wavelengths. Hence the existence of different transverse modes at 1061 nm and 1064 nm can only be attributed to the effect of gain filtering which necessarily affects the mode shape at the two wavelengths in different ways.

 

Fig. 6 Output modes.

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By comparing the evolution between Fig. 6(a) and 6(b) for which the orientation of the output mirror was slightly modified, one can observe that the intensity of the TEM₀₁ mode associated with the radiation at 1064 nm is more or less constant while the intensity of the TEM₀₀ mode associated with the radiation at 1061 nm is reduced in Fig. 3(b) and eventually vanishes when the orientation is rotated further [see Fig. 6(c)]. In other words, by slightly adjusting the orientation of the output mirror it is possible to switch from one laser wavelength to another. The reason is that a modification in the orientation of the ouput mirror affects the way the two modes overlap with the two different regions of the pump volume described previously, thus favouring one wavelength or the other.

3. Discussion

It is well known that the temperature distribution inside the gain medium is not uniform in general but rather lorentzian-shaped when longitudinal diode pumping is used, thus suggesting that the amount of fluorescence at the wavelengths of 1061 nm and 1064 nm in Nd:YSAG ceramics should not be homogeneous but depend on local temperature. In other words, the rate of spontaneous emission at the wavelength of 1064 nm is likely to be higher in the central part of the pump beam where the temperature is higher, whereas the rate of spontaneous emission at 1061 nm is superior in the surrounding part where the temperature is lower. In practice, this effect has a significant impact on the laser operation because the pump volume is therefore structurated into two separated gain regions, namely one in the central part of the pump beam which tends to provide gain for laser oscillations at 1064 nm and another one surrounding which provides gain for oscillations at 1064 nm. Hence, the relative intensity of the two laser wavelengths depends on the way the transverse modes of the cavity overlap with each gain regions in the volume of pump beam.

4. Conclusion

In this paper, we have demonstrated a dual-wavelength longitutinally diode-pumped Nd:YSAG ceramic laser in which the gain volume is structurated into two different regions providing gain at the wavelength of 1061 nm and 1064 nm respectively. We have discussed the role of the nonuniform distribution of the temperature in structurating the gain region via the Boltzmann effect. We have shown that because of this gain structuration the two laser wavelengths can be switched by adjusting the size of the pump beam or by slightly modifying the geometrical parameters of the laser cavity, either the length of the cavity or the orientation of a mirror for instance. Additionnally, we have shown that the structuration of the gain region is responsible for an effect of gain filtering that tailors the shape of the transverse modes at the two laser wavelengths respectively.