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Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 15, Iss. 20 — Oct. 1, 2007
  • pp: 13266–13271

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Femtosecond laser written surface waveguides fabricated in Nd:YAG ceramics

Gustavo A. Torchia, Pablo F. Meilán, Airan Rodenas, Daniel Jaque, Cruz Mendez, and Luis Roso  »View Author Affiliations


Optics Express, Vol. 15, Issue 20, pp. 13266-13271 (2007)
http://dx.doi.org/10.1364/OE.15.013266


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Abstract

Near surface channel waveguides have been fabricated in Neodymium doped YAG ceramics by using IR femtosecond laser irradiation at the low frequency regime. Single mode guidance has been demonstrated with propagation losses of ~1 dB/cm. Time resolved confocal micro-luminescence experiments have been used to determine the spectroscopic properties of the Nd3+ laser ions in the channel waveguide as well as to elucidate the waveguide formation processes.

© 2007 Optical Society of America

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

K. M. Davis, K. Miura, N. Sugimoto, and K. Hirao, “Writing waveguides in glass with a femtosecond laser.” Opt. Lett. 21, 1729–1731 (1996). [CrossRef] [PubMed]

]. 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

C. Méndez, G. A. Torchia, D. Delgado, I. Arias, and L. Roso, Fibres and Optical Passive Components, Proceedings of 2005 IEEE/LEOS Workshop. 1462113, 131–134 (2005). [CrossRef]

, 7

A. Ròdenas, J. A. Sanz García, D. Jaque, G. A. Torchia, C. Mendez, I. Arias, L. Roso, and F. Agullò-Rueda, “Optical investigations of femtosecond laser induced microstress in neodymium doped lithium Niobate crystals.” J. Appl. Phys. 100, 033521 (2006). [CrossRef]

]. 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

J. Lu, M. Prabhu, J. Xu, K. Ueda, H. Yagi, T. Yanagitani, and A. A. Kaminskii, “Highly efficient 2% Nd:yttrium aluminium garnet ceramic laser.” Appl. Phys. Lett. 77, 3707–3709 (2000). [CrossRef]

]. 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.

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

J. Lamela, A. Rodenas, D. Jaque, and F. Jaque, “Near-field optical and micro-luminescence investigations of femtosecond laser micro-structured Nd:YAG crystals.” Opt. Express 15, 3285–3290 (2007). [CrossRef] [PubMed]

]. 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

C. Méndez, G. A. Torchia, D. Delgado, I. Arias, and L. Roso, Fibres and Optical Passive Components, Proceedings of 2005 IEEE/LEOS Workshop. 1462113, 131–134 (2005). [CrossRef]

]. The optical losses strongly depend on the laser parameters and focusing conditions [13–16

R. Osellame, N. Chiodo, G. D. Valle, G. Cerullo, R. Ramponi, P. Laporta, A. Killi, U. Morgner, and O. Svelto, “Waveguide Lasers in the C-Band Fabricated by Laser Inscription With a Compact Femtosecond Oscillator.” IEEE J. Sel. Top. Quantum Electron 12, 277–285 (2006). [CrossRef]

]. 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

M. Hughes, W. Yang, and D. Hewak, “Fabrication and characterization of femtosecond laser written waveguides in chalcogenide glass.” Appl. Phys. Lett. 90, 131113 (2007). [CrossRef]

]. 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 4F3/2 → 4I11/2 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).

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 4I9/2 → 4F5/2 absorption channel. The subsequent 4F3/2 → 4I11/2 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 (τbulk= 143.1 μs and τwaveguide= 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 4F3/2 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 4F3/2 → 4I11/2 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 4F3/2 → 4I11/2 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).

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

S. Kobyakov, A. Kaminska, A. Suchocki, D. Galanciak, and M. Malinowski, “Nd3+-doped yttrium aluminium garnet crystal as a near-infrared pressure sensor for diamond anvil cells.” Appl. Phys. Lett. 88, 234102 (2006). [CrossRef]

]. 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

A. Ròdenas, J. A. Sanz García, D. Jaque, G. A. Torchia, C. Mendez, I. Arias, L. Roso, and F. Agullò-Rueda, “Optical investigations of femtosecond laser induced microstress in neodymium doped lithium Niobate crystals.” J. Appl. Phys. 100, 033521 (2006). [CrossRef]

, 17–19

S. Kobyakov, A. Kaminska, A. Suchocki, D. Galanciak, and M. Malinowski, “Nd3+-doped yttrium aluminium garnet crystal as a near-infrared pressure sensor for diamond anvil cells.” Appl. Phys. Lett. 88, 234102 (2006). [CrossRef]

]. 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

V. Apostolopoulos, L. Laversenne, T. Colomb, C. Depeursinge, R. P. Slathé, M. Pollnau, R. Osellame, G. Cerullo, and P. Laporta, “Femtosecond-irradiation-induced refractive-index changes and channel waveguiding in bulk Ti3+:Sapphire.” Appl. Phys. Lett. 85, 1122–1124 (2004). [CrossRef]

, 4

J. Siegel, J. M. Fernández-Navarro, A. García-Navarro, V. Díez-Blanco, O. Sanz, J. Solis, F. Vega, and J. Armengol, “Waveguide structures in heavy metal oxide glass written with femtosecond laser pulses above the critical self-focusing threshold.” Appl. Phys. Lett. 86, 121109 (2005). [CrossRef]

, 20

S. Juodkasis, K. Nishimura, S. Tanaka, H. Misawa, E. G. Gamaly, B. Luther-Davies, L Hallo, P. Nicolai, and V. T. Tikhonchuk, “Laser-induced Microexplosion Confined in the Bulk of a Sapphire Crystal: Evidence of Multimegabar Pressures.” Phys. Rev. Lett. 96, 166101 (2006). [CrossRef]

]. 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

A. Ròdenas, J. A. Sanz García, D. Jaque, G. A. Torchia, C. Mendez, I. Arias, L. Roso, and F. Agullò-Rueda, “Optical investigations of femtosecond laser induced microstress in neodymium doped lithium Niobate crystals.” J. Appl. Phys. 100, 033521 (2006). [CrossRef]

]. 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

J. R. Vázquez de Aldana, C. Méndez, and L Roso, “Saturation of ablation channels micro-machined in fused silica with many femtosecond laser pulses.” Opt. Express. 14, 1329–1328 (2006). [CrossRef]

]. 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.

Acknowledgments

This work has been supported by the Spanish Ministerio de Educaciòn y Ciencia (MAT2004-03347, TEC2004-05260-C02-02 and MAT2005-05950) by FEDER founds (FIS2005-01351), by the Universidad Autònoma de Madrid and Comunidad Autonoma de Madrid (project CCG06-UAM/MAT-0347) and by the Junta de Castilla y Leòn (Grant No. SA026A05). G. A. T. wishes to thank to the Spanish Ministerio de Educacion y Ciencia (Project # FIS2006-04151), to the Agencia de Promocion Cientifica y Tecnologica de Argentina (Project # PICT 15210) and to the Conicet for the financial support received.

References and links

1.

K. M. Davis, K. Miura, N. Sugimoto, and K. Hirao, “Writing waveguides in glass with a femtosecond laser.” Opt. Lett. 21, 1729–1731 (1996). [CrossRef] [PubMed]

2.

J. W. Chan, T. R. Huser, S. H. Risbud, J. S. Hayden, and D. M. Krol, “Waveguide fabrication in phosphate glasses using femtosecond laser pulses.” Appl. Phys. Lett. 82, 2371–2373 (2003). [CrossRef]

3.

V. Apostolopoulos, L. Laversenne, T. Colomb, C. Depeursinge, R. P. Slathé, M. Pollnau, R. Osellame, G. Cerullo, and P. Laporta, “Femtosecond-irradiation-induced refractive-index changes and channel waveguiding in bulk Ti3+:Sapphire.” Appl. Phys. Lett. 85, 1122–1124 (2004). [CrossRef]

4.

J. Siegel, J. M. Fernández-Navarro, A. García-Navarro, V. Díez-Blanco, O. Sanz, J. Solis, F. Vega, and J. Armengol, “Waveguide structures in heavy metal oxide glass written with femtosecond laser pulses above the critical self-focusing threshold.” Appl. Phys. Lett. 86, 121109 (2005). [CrossRef]

5.

R. Osellame, S. Taccheo, M. Marangoni, R. Ramponi, P. Laporta, D. Polli, S. De Silvestri, and G. Cerullo, “Femtosecond writing of active optical waveguides with astigmatically shaped beams.” J. Opt. Soc. Am. B 20, 1559–1567 (2003). [CrossRef]

6.

C. Méndez, G. A. Torchia, D. Delgado, I. Arias, and L. Roso, Fibres and Optical Passive Components, Proceedings of 2005 IEEE/LEOS Workshop. 1462113, 131–134 (2005). [CrossRef]

7.

A. Ròdenas, J. A. Sanz García, D. Jaque, G. A. Torchia, C. Mendez, I. Arias, L. Roso, and F. Agullò-Rueda, “Optical investigations of femtosecond laser induced microstress in neodymium doped lithium Niobate crystals.” J. Appl. Phys. 100, 033521 (2006). [CrossRef]

8.

J. Lu, M. Prabhu, J. Xu, K. Ueda, H. Yagi, T. Yanagitani, and A. A. Kaminskii, “Highly efficient 2% Nd:yttrium aluminium garnet ceramic laser.” Appl. Phys. Lett. 77, 3707–3709 (2000). [CrossRef]

9.

J. Lu, T. Murai, K. Takaichi, T. Uematsu, K. Misawa, M. Prabhu, J. Xu, K. Ueda, H. Yagi, T. Yanagitani, A. A. Kaminskii, and A. Kudryashov, “72 W Nd:Y3Al5O12 ceramic laser.” Appl. Phys. Lett. 78, 3586–3588 (2001). [CrossRef]

10.

D. Kracht, M. Frede, R. Wilhelm, and C. Fallnich, ”Comparison of crystalline and ceramic composite Nd:YAG for high power diode end-pumping.” Opt. Express. 13, 6212–6216 (2005). [CrossRef] [PubMed]

11.

V. Lupei, A. Lupei, N. Pavel, T. Taira, I. Shoji, and A. Ikesue, “Laser emission under resonant pump in the emitting level of concentrated Nd:YAG ceramics.” Appl. Phys. Lett. 79, 590–592 (2001). [CrossRef]

12.

J. Lamela, A. Rodenas, D. Jaque, and F. Jaque, “Near-field optical and micro-luminescence investigations of femtosecond laser micro-structured Nd:YAG crystals.” Opt. Express 15, 3285–3290 (2007). [CrossRef] [PubMed]

13.

R. Osellame, N. Chiodo, G. D. Valle, G. Cerullo, R. Ramponi, P. Laporta, A. Killi, U. Morgner, and O. Svelto, “Waveguide Lasers in the C-Band Fabricated by Laser Inscription With a Compact Femtosecond Oscillator.” IEEE J. Sel. Top. Quantum Electron 12, 277–285 (2006). [CrossRef]

14.

M. Ams, G. D. Marshall, and M. J. Withford, “Study of the influence of femtosecond laser polarization on direct writing of waveguides.” Opt. Express. 14, 13158–13163 (2006). [CrossRef] [PubMed]

15.

M. Hughes, W. Yang, and D. Hewak, “Fabrication and characterization of femtosecond laser written waveguides in chalcogenide glass.” Appl. Phys. Lett. 90, 131113 (2007). [CrossRef]

16.

H. Zhang, S. M. Eaton, and P. R. Herman, “Low-loss Type II waveguide writing in fused silica with single picosecond laser pulses.” Opt. Express 14, 4826–4834 (2006). [CrossRef] [PubMed]

17.

S. Kobyakov, A. Kaminska, A. Suchocki, D. Galanciak, and M. Malinowski, “Nd3+-doped yttrium aluminium garnet crystal as a near-infrared pressure sensor for diamond anvil cells.” Appl. Phys. Lett. 88, 234102 (2006). [CrossRef]

18.

U. R. Rodríguez-Mendoza, A. Rodenas, D. Jaque, I. R. Martin, F. Lahoz, and V. Lavin, “High-pressure luminescence in Nd3+-doped MgO:LiNbO3.” High Press. Research. 26, 341–344 (2006). [CrossRef]

19.

F. J. Manjon, S. Jandl, G. Riou, B. Ferrand, and K. Syassen, “Effects of pressure on crystal-field transitions of Nd-doped YVO4.” Phys. Rev. B. 69, 165121 (2004). [CrossRef]

20.

S. Juodkasis, K. Nishimura, S. Tanaka, H. Misawa, E. G. Gamaly, B. Luther-Davies, L Hallo, P. Nicolai, and V. T. Tikhonchuk, “Laser-induced Microexplosion Confined in the Bulk of a Sapphire Crystal: Evidence of Multimegabar Pressures.” Phys. Rev. Lett. 96, 166101 (2006). [CrossRef]

21.

J. R. Vázquez de Aldana, C. Méndez, and L Roso, “Saturation of ablation channels micro-machined in fused silica with many femtosecond laser pulses.” Opt. Express. 14, 1329–1328 (2006). [CrossRef]

OCIS Codes
(230.7370) Optical devices : Waveguides
(320.2250) Ultrafast optics : Femtosecond phenomena
(350.3390) Other areas of optics : Laser materials processing

ToC Category:
Laser Micromachining

History
Original Manuscript: May 30, 2007
Revised Manuscript: June 27, 2007
Manuscript Accepted: June 27, 2007
Published: September 27, 2007

Citation
Gustavo A. Torchia, Pablo F. Meilán, Airan Rodenas, Daniel Jaque, Cruz Mendez, and Luis Roso, "Femtosecond laser written surface waveguides fabricated in Nd:YAG ceramics," Opt. Express 15, 13266-13271 (2007)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-20-13266


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References

  1. K. M. Davis, K. Miura, N. Sugimoto and K. Hirao, "Writing waveguides in glass with a femtosecond laser," Opt. Lett. 21, 1729-1731 (1996). [CrossRef] [PubMed]
  2. J. W. Chan, T. R. Huser, S. H. Risbud, J. S. Hayden and D. M. Krol, "Waveguide fabrication in phosphate glasses using femtosecond laser pulses," Appl. Phys. Lett. 82, 2371-2373 (2003). [CrossRef]
  3. V. Apostolopoulos, L. Laversenne, T. Colomb, C. Depeursinge, R. P. Slathé, M. Pollnau, R. Osellame, G. Cerullo and P. Laporta, "Femtosecond-irradiation-induced refractive-index changes and channel waveguiding in bulk Ti3+:Sapphire," Appl. Phys. Lett. 85, 1122-1124 (2004). [CrossRef]
  4. J. Siegel, J. M. Fernández-Navarro, A. García-Navarro, V. Díez-Blanco, O. Sanz, J. Solis, F. Vega and J. Armengol, "Waveguide structures in heavy metal oxide glass written with femtosecond laser pulses above the critical self-focusing threshold," Appl. Phys. Lett. 86, 121109 (2005). [CrossRef]
  5. R. Osellame, S. Taccheo, M. Marangoni, R. Ramponi, P. Laporta, D. Polli, S. De Silvestri and G. Cerullo, "Femtosecond writing of active optical waveguides with astigmatically shaped beams," J. Opt. Soc. Am. B 20, 1559-1567 (2003). [CrossRef]
  6. C. Méndez, G. A. Torchia, D. Delgado, I. Arias and L. Roso, "Fibres and Optical Passive Components," Proceedings of 2005 IEEE/LEOS Workshop. 1462113, 131-134 (2005). [CrossRef]
  7. A. Ródenas, J. A. Sanz García, D. Jaque, G. A. Torchia, C. Mendez, I. Arias, L. Roso and F. Agulló-Rueda, "Optical investigations of femtosecond laser induced microstress in neodymium doped lithium Niobate crystals," J. Appl. Phys. 100, 033521 (2006). [CrossRef]
  8. J. Lu, M. Prabhu, J. Xu, K. Ueda, H. Yagi, T. Yanagitani and A. A. Kaminskii, "Highly efficient 2% Nd:yttrium aluminium garnet ceramic laser," Appl. Phys. Lett. 77, 3707-3709 (2000). [CrossRef]
  9. J. Lu, T. Murai, K. Takaichi, T. Uematsu, K. Misawa, M. Prabhu, J. Xu, K. Ueda, H. Yagi, T. Yanagitani, A. A. Kaminskii and A. Kudryashov, "72 W Nd:Y3Al5O12 ceramic laser," Appl. Phys. Lett. 78, 3586-3588 (2001). [CrossRef]
  10. D. Kracht, M. Frede, R. Wilhelm and C. Fallnich, "Comparison of crystalline and ceramic composite Nd:YAG for high power diode end-pumping," Opt. Express. 13, 6212-6216 (2005). [CrossRef] [PubMed]
  11. V. Lupei, A. Lupei, N. Pavel, T. Taira, I. Shoji and A. Ikesue, "Laser emission under resonant pump in the emitting level of concentrated Nd:YAG ceramics," Appl. Phys. Lett. 79, 590-592 (2001). [CrossRef]
  12. J. Lamela, A. Rodenas, D. Jaque and F. Jaque, "Near-field optical and micro-luminescence investigations of femtosecond laser micro-structured Nd:YAG crystals," Opt. Express 15, 3285-3290 (2007). [CrossRef] [PubMed]
  13. R. Osellame, N. Chiodo, G. D. Valle, G. Cerullo, R. Ramponi, P. Laporta, A. Killi, U. Morgner and O. Svelto, "Waveguide Lasers in the C-Band Fabricated by Laser Inscription With a Compact Femtosecond Oscillator," IEEE J. Sel. Top. Quantum Electron 12, 277-285 (2006). [CrossRef]
  14. M. Ams, G. D. Marshall and M. J. Withford, "Study of the influence of femtosecond laser polarization on direct writing of waveguides," Opt. Express. 14, 13158-13163 (2006). [CrossRef] [PubMed]
  15. M. Hughes, W. Yang and D. Hewak, "Fabrication and characterization of femtosecond laser written waveguides in chalcogenide glass," Appl. Phys. Lett. 90, 131113 (2007). [CrossRef]
  16. H. Zhang, S. M. Eaton and P. R. Herman, "Low-loss Type II waveguide writing in fused silica with single picosecond laser pulses," Opt. Express 14, 4826-4834 (2006). [CrossRef] [PubMed]
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