1. Introduction

The ability of ultra-short laser pulses to induce permanent changes on the refractive index of solid state laser media is nowadays attracting a great attention from both the fundamental and applied point of view. One of the most promising and challenging applications of this technique is the integrated laser active optical circuit fabrication. When femtosecond (fs) pulses are focused inside the bulk media, permanent micro-modifications around the focus region are induced which can lead to the creation of buried channel waveguides [1–5]. On the other hand, if the fs pulses are focused at surface, then ablation (material removal) can take place, which could lead to the appearance of a surface channel waveguide close to the ablated volume [6, 7]. Although ultra-short laser writing has rapidly become a valid technique for the fabrication of optical channel waveguides, a complete knowledge of the physical mechanisms involving the local positive variations of the refractive index is still lacking.

Among the different solid state laser media, Neodymium doped YAG ceramics are emerging as one of the most promising systems for low threshold-high power laser sources. The laser performance of Nd:YAG ceramics is comparable, or even superior, to that corresponding to Nd:YAG crystals [8–11]. The main advantages of Nd:YAG ceramics over traditional Nd:YAG crystals are their lower manufacturing costs, the possibility of high doping levels without any deterioration in the optical quality and the large size laser gain media achievable. Despite the interest in the possible fabrication of efficient laser channel waveguides in Nd:YAG ceramics by fs laser pulses, this is still an unexplored possibility.

In this work we report on the formation of surface channel waveguides in the surroundings of fs laser written ablation grooves fabricated in Nd:YAG ceramics. By a combination of fiber-coupling and micro-luminescence experiments we have studied the induced modifications in the spectroscopic properties of the Neodymium ions associated with the waveguide formation process. Based on these experiments we have discussed the possible mechanisms underlying the origin of the surface waveguide formation.

2. Experimental results

The Neodymium doped YAG ceramics used in this work were provided by Baikowski Ltd (Japan). The Neodymium concentration was 2% and the average grain size (determined by a combination of chemical etching and SEM experiments) was found to be 1.5 μm. The Nd:YAG ceramic sample was a 4×4×10 mm³ prism with all its faces polished up to optical quality (λ/4). For fs laser writing we have used a CPA Ti:Sapphire laser system providing 0.9 mJ, 120 fs laser pulses at a central wavelength of 796 nm and at a low repetition rate of 1 KHz. Pulse energy was controlled by using a variable neutral density filter. The Nd:YAG ceramic sample was mounted on a XYZ motorized stage with a spatial resolution of 0.8 μm. The fs laser beam was focused at the surface of the Nd:YAG ceramic sample by using a 10x microscope objective (N.A.≈0.3). In order to improve the width and depth control of the writing beam a 3 mm pinhole aperture was positioned just before the microscope objective. The use of the pinhole gives a reduced effective numerical aperture of 0.1, this giving an effective Rayleigh length of 20 μm. The single-pass speed used for the creation of the ablation grooves was set at 25 μm/s. Figure 1(a) shows an optical micrograph of the cross section of the ablation groove obtained with a peak laser fluence of 4 J/cm². An ablation depth of 3.6 μm was measured for this dose. From similar pictures of the ablation grooves obtained for different peak laser fluences, an ablation threshold of 1.8 J/cm² was deduced.

 

Fig. 1. (a). Optical micrograph of the cross section of the ablation groove created in the Nd:YAG ceramic with a peak laser fluence of 4 J/cm² and (b) output modal intensity distribution of the surface waveguide created in the Nd:YAG ceramic with a peak laser fluence of 4 J/cm².

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The possible presence of a channel waveguide below the ablation grooves was checked by fiber coupling a 660 nm diode laser into the Nd:YAG ceramic sample. The output modal intensity distribution was then captured by a CCD camera together with a 50x microscope objective and a polarizing filter. The existence of channel waveguides was only observed for low pulse energies (0.91 μJ) corresponding to peak laser fluences near 4 J/cm². For higher doses (i.e. higher ablation depths) large modification channels were observed along the fs pulse propagation direction (along Y axis). These filament like channels have been previously studied under the same writing conditions in Nd:YAG crystals and consist of a high defect density medium with strong scattering losses [12]. Figure 1(b) shows the output modal intensity distribution for a waveguide made with a peak laser fluence of 4 J/cm². From this figure it is clear that the channel waveguide has been created at around 7 μm below the ablation edge. The modal intensity profile has been found to be highly symmetric and with dimensions of about 7 × 8 μm², which perfectly match the mode size of standard optical monomode fibers for low coupling losses. In order to judge the waveguide quality, the propagation losses have been calculated from the analysis of the spatial transverse dependence of the scattered intensity. Propagation losses close to 1 dB/cm were measured for this waveguide, this value being quite similar to that previously reported by the same authors in LiNbO₃ crystals under the same writing conditions [6]. The optical losses strongly depend on the laser parameters and focusing conditions [13–16]. As it has been mentioned previously, waveguide formation was only observed for intermediate laser fluences around 4J/cm². This probably reflects the fact that low laser fluences lead to low small changes in the refractive index, whereas high laser fluences produce high optical losses as it occurs in the case of femtosecond laser written waveguides in chalcogenide glasses [15]. Other writing conditions such as translation speed, pulse duration, laser polarization and NA of the focusing objectives could have a strong influence in the waveguide losses. For example, we expect that further increments in the translation speed could lead to the appearance of inhomogeneities, increasing in this way the optical losses of the waveguide. Nevertheless, the optimization of the waveguide looses requires an extended and systematic investigation that will be the subject of a future work.

 

Fig. 2. Room temperature ⁴F_3/2 → ⁴I_11/2 micro-luminescence spectra obtained in the Nd:YAG ceramic bulk and in the fs written channel waveguide (at positions denoted by B and A in Fig. 1(a), respectively).

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In order to analyze the suitability of the obtained optical waveguides as integrated laser sources it is necessary to know how the spectroscopic properties of the active Nd³⁺ ions are affected by the waveguide fabrication procedure. For this purpose, we have carried out time resolved micro-luminescence experiments. As excitation source we used a pulsed 808 nm fiber coupled diode. The 808 nm radiation was focused into the Nd:YAG ceramic sample by using a 100X microscope objective (N.A.=0.9). The Nd³⁺ ions were excited towards the ⁴I_9/2 → ⁴F_5/2 absorption channel. The subsequent ⁴F_3/2 → ⁴I_11/2 emission was collected by the same microscope objective and, after passing through a confocal aperture, the infrared emission was coupled into a 50 μm core fiber connected either to a high resolution spectrometer (Ocean Optics HR4000) or to a cooled photomultiplier. The sample was mounted on a motorized stage with a spatial uncertainty of 0.2 μm. The spatial zero origin was set in all cases at the apex of the ablation cone. Figure 2 shows the room temperature micro-luminescence spectra obtained in the Nd:YAG ceramic bulk and in the fs written channel waveguide (at positions denoted by B and A in Fig. 1(a), respectively). Inset in Fig. 2 shows the Nd³⁺ fluorescence decay time curves corresponding to the bulk and waveguide positions (τ_bulk= 143.1 μs and τ_waveguide= 144.4 μs, with a relative error of ± 0.1 μs). The very similar spectra and decay times obtained indicate that the spectroscopic properties of the ⁴F_3/2 metastable state of Nd³⁺ ions are almost not changed by the waveguide fabrication procedure.

Nevertheless, a detailed analysis of the obtained emission spectra in the channel waveguide (Fig. 2) reveals the existence of a slight red shift of the Nd³⁺ emission bands. This red shift comes also accompanied by an increase in the FWHM (Full Width at Half Maximum) of the emission peaks within the ⁴F_3/2 → ⁴I_11/2 transition. To gain information on these modifications we have analyzed both the position and bandwidth of the most isolated emission peak (λ=1.052 μm). The results obtained along Scan 1 and Scan 2 are included in Figures 3(a) and 3(b), respectively. Colored bands inside Fig. 3 indicate the spatial extension of the waveguide mode as obtained from Fig. 1(b). It is clear that waveguide formation is accompanied by changes in both peak position and bandwidth of the Nd³⁺ emission lines.

 

Fig. 3. Peak position and FWHM of the most isolated emission peak within the ⁴F_3/2 → ⁴I_11/2 band (λ=1.052 μm). The results obtained along Scan 1 and Scan 2 in Fig. 1(a) correspond to Figures 3(a) and 3(b), respectively. Colored bands inside Fig. 3 indicate the spatial extension of the surface channel waveguide modal intensity distribution as seen from Fig. 1(b).

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Let us now discuss on the information provided by the experimental data included in Fig. 3. According to previous works a red shift of the fluorescence peaks of Nd³⁺ ions in the Nd:YAG system can be correlated with a shift of the sub-stark levels caused by a local lattice densification [17]. Consequently, data of Fig. 3 suggests that lattice densification has been induced at the surroundings of the ablated volume. Furthermore, the spatial extension of this local densification well matches the waveguide intensity distribution location. From the observed net spectral shift (≈ 0.8 cm^-1) and by taking into account previous works on the pressure dependence of the Nd³⁺ luminescence, it is reasonable to estimate a permanent induced stress at the waveguide location of few GPa [7, 17–19]. Figure 3 also reveals that the creation of the waveguide involves an increase of the FWHM of the Nd³⁺ emission peaks. This increase denotes a small increment in local disorder which, in turn, can be also associated with the observed local lattice compression at the waveguide mode position. The fact that this crystalline region of higher refraction index is found 7 μm apart from the ablated volume lends supporting evidence that it is not the laser focus itself creating the observed refraction index increase, but rather some other collateral mechanism.

Essentially two possible mechanisms exist that could explain the observed changes in the spectroscopic properties of the Nd³⁺ ions in the channel waveguide. Previous works have pointed out that local densifications around the fs laser affected volume are caused by the propagation of shock-waves [3, 4, 20]. These shock-waves are formed due to the large pressure difference between the laser induced plasma and the surrounding crystal. If the shock-waves reach the elastic limits of the lattice then a residual stress can be induced in the lattice which could account for the increments in local disorder and lattice dilatation observed at the waveguide location [7]. This induced stress could then be responsible for the observed local increase in the refractive index around the ablation volume. It should be also pointed out that during the fs irradiation process high free electron densities (FEDs) are produced not only at the ablation volume but also in its surroundings [21]. Because our writing procedure corresponds to a multipulse configuration (in which hundreds of laser pulses overlap within the spot size), cumulative effects arising from the generated FEDs could be also taking important part at the waveguide generation process together with shock-wave propagation.

3. Conclusion

In summary, we have demonstrated fs laser writing of surface channel waveguides in Nd:YAG ceramics. Characterization by means of confocal time-resolved micro-Luminescence of the laser induced changes at the waveguide channel has been performed. From this, we have found that a slight lattice densification together with some small increment in local disorder has been induced along the fs pulse propagation direction and at the waveguide mode location. Single mode operation at 660 nm has been demonstrated with propagation losses of ~1 dB/cm. Further investigation needs to be done in order to improve this value. Nevertheless, the good laser properties of Nd:YAG ceramics make them good candidates for ultra-short pulse laser written active channel waveguides and we are working experimentally along this direction.