Optical waveguides were fabricated in neodymium-doped silicate glass by using a low-repetition-rate (1 kHz) femtosecond laser inscription. Two different types of waveguide structure are fabricated. In the first, guiding occurs in the focal spot. In the second, guiding occurs in the region between the two filaments. The near-field intensity distribution, propagation loss, index profile reconstruction, and calculation of the modal intensity distribution by the beam propagation method of these waveguides are presented. On the basis of near-field intensity distribution of the light guided through the waveguides and the propagation loss measurement, the optimum writing conditions such as the pulse energy and scan velocity were determined. The waveguide written with 2.2 µJ pulse energy and 50 µm/s scan velocity shows strong guidance at 632.8 nm, with an index contrast of 7 × 10−4 and a propagation loss of ~0.8 dB/cm.
©2011 Optical Society of America
Nd3+-doped silicate glass is one of the excellent laser materials and has been extensively used for the laser out put at 1064 nm. Nd: silicate glass offers a wider gain bandwidth than crystalline hosts, therefore it is better suited for ultra short pulse mode-locking applications. J. Aus der Au et al. have demonstrated 60-fs pulse generation with an average output power of 84 mW from a diode-pumped Nd: glass laser . And also, recently A. Agnesi et al. reported a Nd: silicate glass laser, yielding 80-fs pulses pumped by a single-mode 200-mW diode . The optical waveguide is one of the most important components in integrated optics. The fabrication of low-loss waveguiding structures is of particular interest for the production of passive and active optical devices, such as amplifiers and lasers [3–6]. For lasers applications, channel waveguides can be used to tightly confine the pump and laser modes and achieve a high spatial overlap. Therefore, waveguide confinement can facilitate lower pump thresholds and higher efficiencies than are obtainable in bulk laser counterparts .
Different techniques are available for the fabrication of active optical waveguides, for instance, diffusion of metal ions , ion exchange [8,9], ion implantation [3, 10, 11], and ultrafast laser inscription [5, 6, 12]. As one of the important techniques for material-property modification, ultrafast laser inscription has been proved to be a powerful technique for the fabrication of photonic waveguide components in transparent optical materials. When intense ultrafast laser pulses are tightly focused inside transparent materials, nonlinear absorption in the focal volume will take place, where permanent structural and refractive-index modifications can be induced. The extent of such modification is a complex interplay of factors like laser energy, pulse duration repetition rate, polarization, and the scan velocity [13–15]. With respect to other fabrication techniques, ultrafast laser inscription shows several advantages such as truly three-dimensional fabrication, one-step process, and short processing times. In recent years, many groups succeeded in inscribing waveguides in different glasses and crystals by producing a permanent change of the refractive index [16–19]. Ultrafast laser inscription techniques for writing waveguides can be divided into those relying on the direct refractive-index increase at the laser focus [14, 16], and those in which the refractive index decreases at the laser focus while indirectly increases on its surroundings. In the latter case, usually, two parallel lines in close separation, which form low-refractive-index barriers, were written, and the center between the two lines was waveguide region [17–19].
In this paper, we report the fabrication of channel waveguides in Nd: silicate glasses using femtosecond laser inscription under the one line configuration and the “double-line” configuration. The basic propagation properties of the obtained waveguides as a function of the writing conditions, such as pulse energy, scanning speed, and number of scans, are reported and discussed. The refractive index profiles of the buried channel waveguide were also constructed. According to the reconstructed index profile, mode intensity distribution was also numerically calculated by the beam propagation method, which shows a reasonable agreement with the experimental results. In addition, the polarized propagation property of the obtained waveguides was analyzed at 632.8 nm wavelength.
The Nd: silicate glass sample, which was doped with 2 at. % neodymium has a refractive index of 1.5065. The channel waveguide was fabricated by using an amplified Ti: sapphire laser system providing 35 fs pulses linearly polarized at 800 nm with 1 kHz repetition rate. The laser was focused into the sample approximately 200 μm below the surface with a 25 × microscope objective (N.A. = 0.4). In order to produce waveguiding structures the sample was moved perpendicular to the laser-beam axis by a computer-controlled positioning system with a velocity of 50 – 400 μm/s. Single and double filaments were inscribed by increasing pulse energies from 0.6 to 4 µJ. And in the second case, the separation between two filaments was 27 µm. Figure 1 shows a schematic diagram of the waveguide fabrication experimental setup.
The transmission properties of the channel waveguides were investigated by end-face coupling method. During the measurement, the polarized light beam at the wavelength of 632.8 nm was focused into the channel waveguide to excite the guide modes, which was mounted on a 6D optical stage, by a microscope objective lens (25 × ), and another microscope objective lens (25 × ) was used to collect the light from the output facet of the sample. And the output facet was imaged onto a CCD camera. Therefore, the near-field intensity distributions of channel waveguide were presented. The refractive index profiles of the waveguide were reconstructed according to the procedure described in Ref [20, 21]. To calculate the guide modes at the wavelength of 632.8 nm of channel waveguide, we performed a numerical simulation using the FD-BPM . Propagation loss of the waveguide mode was measured with a Fabry-Perot resonator method at wavelength of 632.8 nm .
3. Results and discussion
Depending on the writing parameters like pulse energy, the distance between two filaments, two types of structure can be distinguished. Figure 2 shows examples of microscope image in end view and near-field intensity distributions of light in guide modes for each type. We will refer to the first type of modification and the second type of modification as type I and type II, respectively, in the following.
3.1 Type I waveguide
End-face coupling experiments at 632.8 nm show that type I waveguides were realized using pulse energies of 1.2 – 4.0 µJ and scan velocity of 50 – 400 μm/s. In this case, the guiding region occurred at laser modified region where the refractive index increased. Guiding mode was not observed for pulse energy below 1.2 µJ, perhaps due to insufficient index change. The mechanisms that give rise to index modification can be divided into two distinct parts, the absorption of laser energy by the sample and, then, dissipation of the absorbed energy within the irradiated material . Reordering of chemical bonds caused by electronic and thermal effects may also be a factor. The increase of the refractive index in the filaments may result from structural rearrangements inside the optical material.
In Figs. 3(a) and 3(b) measured near-field intensity profiles (fundamental mode TE00) and the calculated refractive index change profile are shown for a waveguide written with pulse energy of 2.2 µJ at a speed of 50 µm/s. The change of refractive index in the waveguide could be roughly estimated by measuring the maximum incident angle Θm at which no change of the transmitted power is occurring, and using the formula6,24]. The resulting refractive index change of the waveguides is in the order of Δn ≈7 × 10−4. Based on this reconstructed index distribution, we calculated the modal profile [Fig. 3(c)] of waveguide by using FD-BPM. By comparing Figs. 3(a) and 3(c), one can conclude that the simulated distribution is in good agreement with the measured modal profile.
Figure 4 shows the near-field intensity distribution of TM modes in the waveguides single written and overwritten six times with 2.2 µJ pulse energy and scan velocity of 400 µm/s. As can be seen from Fig. 4 (b), the mode profiles of multi-scan waveguide showed highly symmetric. The asymmetry waveguide mode can be seen by noting the near-field profiles shown in Fig. 4(a), where there are elongated structures which are a direct result of self-focusing of the writing beam due to Kerr lensing at high pulse energies . Comparing Figs. 4 (a) and 4(b), we can get that the mode profile asymmetry caused by self-focusing effects can be compensated for by employing multi-pass scanning.
The effect of scan velocity was investigated at pulse energy of 2.2 µJ. The maximum refractive index change was determined in the way mentioned above. The refractive index change was found to increase with decreasing writing speed as a result of a higher incident total energy deposited at a particular point of the waveguide. Only single mode propagation was noted at scan velocity of 100 µm/s and above. At 50 µm/s, higher order mode (TE10) was also noted as shown in Fig. 3(e).
We measured the loss value of fundamental mode only, because the single-mode waveguide has more applications in practical devices design, and there has many difficult in detecting the loss of the higher order mode. All the studies showed that the fabrication parameters that yielded the minimum losses are 2.2 μJ pulse energy, 50 μm/s scanning velocity. The loss of the TE00 mode is ~0.8 dB/cm. No deterioration of high confinement guiding at 632.8 nm was observed after more than one month.
3.2 Type II waveguide
End-face coupling experiments at 632.8 nm show that type II waveguides were realized by using pulse energies of 0.6 – 2.2µJ, scan velocity of 400 μm/s and separation of 27 μm. According to the measured near-field mode profiles [Fig. 1(d)], we reconstructed refractive index profiles of the waveguide as shown in Fig. 5(a) . It can be seen from Fig. 5(a) that the index profile consists of an enhanced index (ΔnWG = 3 × 10−4) in the waveguide region and two tracks (ΔnF = −2 × 10−3) which constitute two low index barriers that provide a strong light confinement in the horizontal (y) direction. Based on this reconstructed index distribution, we calculated the modal profile as shown in Fig. 5(b). By comparing Fig. 1(d) and Fig. 5(b), one can conclude that the simulated distribution is in good agreement with the measured modal profile.
The increase of the refractive index surrounded the filaments can be explained by the occurrence of micro-explosions at the focal spot, which push material outwards under local pressures . Therefore, there was decreased refractive index in the focal spot caused by volume expansion, and then the filaments would form two strong barriers for the guided mode. This is may be the reason why the pulse energy used to form Type II waveguide is lower than that in Type I waveguide.
The propagation loss of the waveguide (shown in Fig. 1 (d)) was measured to be about 1.9 dB/cm at 632.8 nm. The experimental investigations about the effect of other parameters, such as scanning speed and separation between two filaments in type II waveguide are in progress.
The effect of the propagating light polarization on the guiding properties of the two types of waveguides was investigated by placing a linear polarizer after 632.8 nm He-Ne laser. From our investigations, strong confinement of both TE and TM modes was observed in the two types of waveguides as shown in Figs. 3(a), 3(d) and Fig. 6 .
In this paper we have shown that two types of channel waveguides can be fabricated in Nd: silicate glass using a low-repetition-rate femtosecond laser writing. The fabrication parameters are found to affect significantly the guiding properties of these waveguides. Type I waveguide is fabricated using both single and multi-scan fabrication technique. Refractive index modifications of up to 1 × 10−3 are observed in the waveguide fabricated with a pulse energy of 4.0 µJ and scan velocity of 50 µm/s. The optimized fabrication parameters for the sample are 2.2 µJ pulse energy and 50 µm/s scan velocity, and the propagation loss of the channel waveguide was about 0.8 dB/cm. Based on the constructed refractive index distribution of the channel waveguide cross section, we calculated the mode profile of the waveguide mode, which showed a reasonable agreement with the experimental results.
This work is supported by Shandong Provincial Natural Science Foundation, China (Grant No. ZR2011AQ026) and Youth Foundation of Qufu Normal University (Grant No. XJ201011). And the authors would like to thank Chao Zhang from Shandong University for helpful discussions.
References and links
3. F. Chen, X.-L. Wang, and K.-M. Wang, “Development of ion-implanted optical waveguides in optical materials: A review,” Opt. Mater. 29(11), 1523–1542 (2007). [CrossRef]
4. Y. L. Lee, T. J. Eom, W. Shin, B.-A. Yu, D.-K. Ko, W.-K. Kim, and H.-Y. Lee, “Characteristics of a multi-mode interference device based on Ti:LiNbO3 channel waveguide,” Opt. Express 17(13), 10718–10724 (2009). [CrossRef] [PubMed]
5. T. T. Fernandez, S. M. Eaton, G. Della Valle, R. M. Vazquez, M. Irannejad, G. Jose, A. Jha, G. Cerullo, R. Osellame, and P. Laporta, “Femtosecond laser written optical waveguide amplifier in phospho-tellurite glass,” Opt. Express 18(19), 20289–20297 (2010). [CrossRef] [PubMed]
6. J. Siebenmorgen, K. Petermann, G. Huber, K. Rademaker, S. Nolte, and A. Tünnermann, “Femtosecond laser written stress-induced Nd:Y3Al5O12 (Nd:YAG) channel waveguide laser,” Appl. Phys. B 97(2), 251–255 (2009). [CrossRef]
7. Y. Tan, A. Rodenas, F. Chen, R. R. Thomson, A. K. Kar, D. Jaque, and Q. Lu, “70% slope efficiency from an ultrafast laser-written Nd:GdVO4 channel waveguide laser,” Opt. Express 18(24), 24994–24999 (2010). [CrossRef] [PubMed]
8. L. P. Shi, T. C. Chong, Z. Zhuo, W. X. Hou, and P. F. Hu, “Properties of ion exchanged planar and channel optical waveguides fabricated in Cu doped KTiOPO4 substrates,” Appl. Phys. Lett. 71(19), 2737–2739 (1997). [CrossRef]
9. S.-L. Li, “Optical waveguides in LiNbO3 and stoichiometric LiNbO3 crystals by proton exchange,” Sci. China Ser. G 51(10), 1479–1488 (2008). [CrossRef]
10. F. Chen, L. Wang, Y. Jiang, X. L. Wang, K. M. Wang, G. Fu, Q. M. Lu, C. E. Ruter, and D. Kip, “Optical channel waveguides in Nd: YVO4 crystal produced by O+ ion implantation,” Appl. Phys. Lett. 88(7), 071123 (2006). [CrossRef]
11. S.-L. Li, K.-M. Wang, F. Chen, X.-L. Wang, G. Fu, D. Y. Shen, H. J. Ma, and R. Nie, “Monomode optical waveguide excited at 1540 nm in LiNbO(3) formed by MeV carbon ion implantation at low doses,” Opt. Express 12(5), 747–752 (2004). [CrossRef] [PubMed]
12. R. R. Thomson, S. Campbell, I. J. Blewett, A. K. Kar, and D. T. Reid, “Optical waveguide fabrication in z-cut lithium niobate (LiNbO3) using femtosecond pulses in the low repetition rate regime,” Appl. Phys. Lett. 88(11), 111109 (2006). [CrossRef]
13. K. Yamada, W. Watanabe, T. Toma, K. Itoh, and J. Nishii, “In situ observation of photoinduced refractive-index changes in filaments formed in glasses by femtosecond laser pulses,” Opt. Lett. 26(1), 19–21 (2001). [CrossRef] [PubMed]
14. M. Ams, G. D. Marshall, and M. J. Withford, “Study of the influence of femtosecond laser polarisation on direct writing of waveguides,” Opt. Express 14(26), 13158–13163 (2006). [CrossRef] [PubMed]
15. J. Burghoff, H. Hartung, S. Nolte, and A. Tunnermann, “Structural properties of femtosecond laser-induced modifications in LiNbO3,” Appl. Phys., A Mater. Sci. Process. 86(2), 165–170 (2005). [CrossRef]
16. M. Kumatoriya, M. Nakbayashi, M. Sakakura, Y. Shimotsuma, K. Miura, T. Fujii, and K. Hirao, “Optical Properties of a Waveguide Written Inside a LiTaO3 Crystal by Irradiation with Focused Femtosecond Laser Pulses,” Opt. Rev. 18(1), 166–170 (2011). [CrossRef]
17. W. F. Silva, C. Jacinto, A. Benayas, J. R. Vazquez de Aldana, G. A. Torchia, F. Chen, Y. Tan, and D. Jaque, “Femtosecond-laser-written, stress-induced Nd:YVO4 waveguides preserving fluorescence and Raman gain,” Opt. Lett. 35(7), 916–918 (2010). [CrossRef] [PubMed]
18. S. M. Eaton, C. A. Merchant, R. Iyer, A. J. Zilkie, A. S. Helmy, J. S. Aitchison, P. R. Herman, D. Kraemer, R. J. D. Miller, C. Hnatovsky, and R. S. Taylor, “Raman gain from waveguides inscribed in KGd(WO4)2 by high repetition rate femtosecond laser,” Appl. Phys. Lett. 92(8), 081105 (2008). [CrossRef]
19. A. Ródenas, G. A. Torchia, G. Lifante, E. Cantelar, J. Lamela, F. Jaque, L. Roso, and D. Jaque, “Refractive index change mechanisms in femtosecond laser written ceramic Nd:YAG waveguides: micro-spectroscopy experiments and beam propagation calculations,” Appl. Phys. B 95(1), 85–96 (2009). [CrossRef]
20. I. Mansour and F. Caccavale, “An improved procedure to calculate the refractive Index profile from the measured near-field intensity,” J. Lightwave Technol. 14(3), 423–428 (1996). [CrossRef]
21. X. Liu, S. Qu, Y. Tan, C. Zhang, and F. Chen, “Buried channel waveguides in neodymium-doped KGd(WO4)2 fabricated by low-repetition-rate femtosecond laser writing,” Appl. Phys. B 103(1), 145–149 (2011). [CrossRef]
22. D. Yevick and W. Bardyszewski, “Correspondence of variational finite-difference (relaxation) and imaginary-distance propagation methods for modal analysis,” Opt. Lett. 17(5), 329–330 (1992). [CrossRef] [PubMed]
23. F. Chen, L. Wang, Y. Jiang, X. L. Wang, K. M. Wang, G. Fu, Q. M. Lu, C. E. Ruter, and D. Kip, “Optical channel waveguides in Nd: YVO4 crystal produced by O+ ion implantation,” Appl. Phys. Lett. 88(7), 071123 (2006). [CrossRef]
24. J. A. Dharmadhikari, A. K. Dharmadhikari, A. Bhatnagar, A. Mallik, P. C. Singh, R. K. Dhaman, K. Chalapathi, and D. Mathur, “Writing low-loss waveguides in borosilicate (BK7) glass with a low-repetition-rate femtosecond laser,” Opt. Commun. 284(2), 630–634 (2011). [CrossRef]
25. E. N. Glezer and E. Mazur, “Ultrafast-laser driven micro-explosions in transparent materials,” Appl. Phys. Lett. 71(7), 882–884 (1997). [CrossRef]