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Femtosecond laser written channel optical waveguide in Nd:YAG crystal
Optics & Laser Technology 58 (2014) 89–93
Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Femtosecond laser written channel optical waveguide in Nd:YAG crystal Shi-Ling Li a,b,n, Yong-Kai Ye c, Ming-Wei Wang d a
College of Physics and Engineering, Shandong Provincial Key Laboratory of Laser and Information Technology, Qufu Normal University, Qufu 273165, China Department of Opto-electronics and Information Engineering, School of Precision Instruments and Opto-electronics Engineering, Tianjin University, Tianjin 300072, China c College of Computer Science, Qufu Normal University, Qufu 273165, China d Institute of Modern Optics, Key Laboratory of Optoelectronic Information Science and Technology, MEC, Nankai University, Tianjin 300071, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 31 August 2013 Received in revised form 3 November 2013 Accepted 5 November 2013 Available online 25 November 2013
Low-repetition-rate (1 kHz) femtosecond laser inscription is used to fabricate channel waveguides in Nd: YAG crystal. Guiding occurs in the surroundings of the focal spot. End-to-end coupling measurements reveal the near-field intensity distribution of waveguides. The index profile reconstruction, mode analysis performed by the beam propagation method of these waveguides are presented. The waveguide written with 3.0 μJ pulse energy and 100 μm/s scan velocity shows strong guidance at 632.8 nm, and a propagation loss of about 0.5 dB/mm. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Channel waveguides Ultrafast lasers Laser materials
1. Introduction The ultra-short laser direct writing has rapidly become a powerful technique for the fabrication of photonic waveguide components in dielectric materials [1,2]. Permanent structural and refractive-index modifications which can lead to the creation of buried channel waveguides are created by focusing the intense ultrafast laser pulses into the dielectric materials where multiphoton absorption and/or avalanche effects take place. The extent of such modification can be controlled by the writing conditions such as focusing conditions, pulse energy, pulse duration, repetition rate and polarization, as well as writing velocity and direction [3–5]. Compared with other fabrication techniques, ultrafast laser direct writing technique has outstanding advantages such as truly three-dimensional fabrication and short processing times. Using this technique optical devices like directional couplers [6], beamsplitters [7], amplifiers [8], and lasers [1,9] were realized. The nonlinear interaction processes that are strongest at the focus of the writing laser can yield a region of local refractive index change. There are two different cases for the refractive index change, one case is that there is direct refractive-index increase at the laser focus [4,10]; another case is that there is refractive index decrease at the
n
Corresponding author at: College of Physics and Engineering, Shandong Provincial Key Laboratory of Laser and Information Technology, Qufu Normal University, Qufu 273165, China. Tel.: þ 86 053 744 56806. E-mail address: [email protected] (S.-L. Li). 0030-3992/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.optlastec.2013.11.006
written track, while indirectly increases on its surroundings. In the second case, usually several parallel tracks being written around an unmodified region, waveguiding in this region is possible. The two parallel tracks approach was applied in Refs. [11–13], and the center between the two tracks was waveguide region. Nd:YAG crystal is one of the favorite gain media for solid-state lasers owing to its outstanding fluorescence, thermal and mechanical properties. In this paper, we report the fabrication of channel waveguides under single track and the double track configuration in Nd:YAG crystals using a low-repetition-rate (1 kHz) laser with 35 fs pulses of 800 nm light. The scan speeds are 50–400 m/s which are 10– 40 fold faster than those used in Ref. [1], leading to a much faster fabrication time. We have characterized these laser-written structures, demonstrated waveguiding of 632.8 and 1550 nm light. The change in refractive index, the polarized propagation property and the propagation losses of the obtained waveguides were measured at 632.8 nm wavelength. From our investigations strong confinement of both TE and TM modes was observed in single-track and double-track waveguides, which shows the presence of a degree of birefringence. 2. Experimental An amplified Ti:sapphire laser system (35 fs, 800 nm, and 1 kHz repetition rate) was used to inscribe tracks in Nd:YAG crystals (a cut with size of 10 8 2.6 mm3 along b, c and a axes, doped by 1 at% Nd3 þ ions). Fig.1(a) shows the schematic plot of femtosecond laser writing setup. The femtosecond laser was incident on the bc face and
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S.-L. Li et al. / Optics & Laser Technology 58 (2014) 89–93
was focused into the samples approximately 200 μm below the surface by a microscope objective (N.A.¼0.4). Pulse energies ranging from 1.5 to 4.0 mJ were used in the formation of optical waveguides. The sample was moved perpendicular to the laser-beam axis by a computer-controlled positioning system with a velocity of 50– 400 μm/s during the writing process. The end-face coupling method was used to characterize the transmission properties of channel waveguide. Fig. 1(b) shows the experimental setup. The polarized light beam at the wavelength of 632.8 or 1550 nm was coupled into the waveguide by a 25 microscope objective lens to excite the guide modes. And at the
output facet of the sample, the output light was collected using another 25 microscope objective lens, then imaged onto a CCD camera. Therefore, we obtained the near-field intensity distributions of waveguide structures. We reconstructed the refractive index profiles of the waveguides according to the procedure described in Refs. [14–16]. Based on the reconstructed refractive index distribution, the finite-difference beam propagation method (FD-BPM) [17] was applied to simulate the guided modes.
3. Results and discussion
Fig. 1. (a) Schematic of femtosecond laser writing setup and (b) a sketch of the end-face coupling experimental setup.
Microscope image in end view and near-field intensity distributions of waveguides are shown in Fig. 2. As can be seen in Fig. 2(a) and (c), a guided mode occurs in the sides of the written single tracks (white dashed area in Fig. 2(c)). Fig. 2(b) and (d) shows that waveguide was also observed in the center between two adjacent tracks (white dashed area). The guided mode in the center of the pair structure has a nearly Gaussian profile. Fig. 3(a)–(c) depicts the microscope images of cross section perpendicular to the inscribed tracks. With increasing pulse energy both height and width of the tracks increase. The track corresponding to a pulse energy of 2.5 μJ is 50 μm in height and 5 μm in width. End-face coupling experiments at 632.8 nm show that singletrack waveguides could be formed by using pulse energies of 1.5– 4.0 mJ and scan velocity of 50–400 μm/s. In this case, the guiding region occurred at the sides of laser modified region where the refractive index increased. Guiding mode was not observed for pulse energy below 1.5 mJ, perhaps due to insufficient index change.
Fig. 2. Microscopy image of end view and near-field mode profile at 632.8 nm: (a) and (c) single-track structure for a pulse energy of 3.0 μJ and scan velocity of 50 μm/s; (b) and (d) double-track structure for a pulse energy of 3.0 μJ and scan velocity of 100 μm/s, respectively (the track marked as white dashed area).
S.-L. Li et al. / Optics & Laser Technology 58 (2014) 89–93
The mechanisms that give rise to index modification can be divided into two distinct parts, local damage is produced at focus, and that this damage is accompanied by a lattice compression in its surroundings [13]. The first effect (damage) causes a refractive index reduction whereas the second effect (compression) leads to a local refractive index enhancement. The writing parameters and geometry strongly determine the dominant effect. We can roughly estimate the change of refractive index in the waveguide region by measuring the maximum incident angle θm at which no change of the transmitted power is occurring, and using the formula 2
Δn ¼ sin θm =2n
ð1Þ
where n is the refractive index of the unstructured sample [1]. For the waveguide written with pulse energy of 3.0 mJ at a speed of 50 mm/s, the estimated refractive index change of the waveguides is in the order of ΔnE3 10 4. Due to uncertainties of measuring the maximum incident angle, there is an error of about 35% for the refractive index change. According to the measured near-field intensity profile as displayed in Fig. 2(c), we have reconstructed the 2D refractive index change profile of the waveguide. Fig. 4 (a) shows the reconstructed refractive index profile of the waveguide, which consists of reduced refractive index ( 2.7 10 3) at the writing track and stress induced refractive index increment ( 3 10 4) on both sides of the writing line. Based on this refractive index distribution we calculated the modal profile of waveguide by using FD-BPM as shown in Fig. 4(b). By comparing Figs. 4(b) and 2(c), one can conclude that the simulated distribution
91
is generally in agreement with the measured modal profile. Double-track waveguides were realized by using pulse energies of 1.5–4.0 mJ and two tracks separation of 20 μm. Fig. 5(b) shows the refractive index profiles of the waveguide reconstructed according to the measured near-field mode profiles depicted in Fig. 5(a). It can be observed from Fig. 5(b) that there is a refractive-index increment between two filaments of the order of 2 10 4, and accompanied by a refractive index reduction of the order of 1.7 10 3 at the two filaments which constitute two low index barriers that provide a strong light confinement in the horizontal direction. Fig. 5(c) shows the calculated modal profile based on this reconstructed index distribution. Comparing Fig. 5(a) and (c), one can conclude that the simulated distribution is in agreement with the measured modal profile. The “double filament” approach, which uses both mechanisms (stress and damage) in a complementary way. The effect of the propagating light polarization on the guiding properties of single-track and double-track waveguides was investigated by placing a linear polarizer after 632.8 nm He–Ne laser. From our investigations, strong confinement of both TM and TE modes was observed in the waveguides as shown in Figs. 2(d), 4 (a) and 5(a) and (d). End-face coupling experiments at 1550 nm show that only double-track waveguides were realized by using pulse energies of 4.0 mJ and separation of 20 μm. While guiding mode was not observed for single-track structure. Fig. 6 shows the near-field intensity distribution of the guiding mode at 1550 nm and 632.8 nm. According to formula (1), the estimated refractive index increase in the waveguide is 5 10 4.
Fig. 3. Microscope images of cross section perpendicular to the inscribed tracks from 1.5 to 4.0 μJ pulse energy in Nd:YAG (a)–(c). The scanning velocities were all 50 μm/s. (a) 1.5 μJ, (b) 2.5 μJ and (c) 4.0 μJ.
Fig. 4. (a) The reconstructed refractive index distribution in the waveguide fabricated with a pulse energy of 3.0 μJ and a scan velocity of 50 μm/s, (b) the calculated modal profile, and (c) the measured TE mode.
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Fig. 5. (a) The measured near-field intensity profile of TM mode fabricated with 2.0 μJ pulse energy and 50 μm/s scan velocity, (b) the reconstructed refractive index distribution, (c) the corresponding calculated modal profile, and (d) the measured TE mode.
Fig. 6. The measured near-field intensity profile of waveguide mode fabricated with 4.0 μJ pulse energy and 400 μm/s scan velocity at wavelength of (a) 1550 nm and (b) 632.8 nm.
The propagation loss of the waveguide was estimated by using the formula [18] L ¼ 10 log ðP out =P in Þ dB
ð2Þ
where Pin was the power measured before coupling into the waveguide, and Pout was the out-coupled laser power. It should be noted that Fresnel reflections at the end facets, the transmission of the outcoupling objective and additional coupling losses, such as those resulting from an imperfect overlap between the pump and waveguide mode, are all included in L. Therefore, the propagation losses for the 8 mm long channel waveguide are lower than L. We found that the loss of double-track waveguide is lower than that of single-track
waveguide fabricated with the same energy and scan velocity. For the waveguide as shown in Fig. 2(d), the waveguide losses was L¼4.1 dB ( 0.5 dB/mm) at 632.8 nm determined by formula (2). And the propagation loss of the double-track waveguides with fixed scan velocity of 100 μm/s was 0.9 dB/mm for 1.5 μJ, 0.7 dB/mm for 2.0 μJ, and 0.6 dB/mm for 4.0 μJ.
4. Conclusions In this paper we have shown that optical waveguiding was observed in the surrounding region of the tracks and between two tracks. The mode guided between two tracks has a nearly Gaussian
S.-L. Li et al. / Optics & Laser Technology 58 (2014) 89–93
intensity profile. The refractive index changes in the region of waveguide and the track of these waveguides were estimated to be in the order of 10 4 and 10 3, respectively. 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. The propagation loss of the channel waveguide fabricated with 3.0 mJ pulse energy and 100 mm/s scan velocity was lower than 4.1 dB.
Acknowledgments This work is supported by the National Natural Science Foundation of China (Grant nos. 61205055 and 11005070), Shandong Provincial Natural Science Foundation, China (Grant no. ZR2011AQ026), Educational Commission of Shandong Province, China (Grant no. J11LA12). References [1] Siebenmorgen J, Petermann K, Huber G, Rademaker K, Nolte S, Tünnermann A. Femtosecond laser written stress-induced Nd:Y3Al5O12 (Nd:YAG) channel waveguide laser. Appl Phys B 2009;97:251–5. [2] Thomson RR, Campbell S, Blewett IJ, Kar AK, Reid DT. Optical waveguide fabrication in z-cut lithium niobate (LiNbO3) using femtosecond pulses in the low repetition rate regime. Appl Phys Lett 2006;88:111109. [3] Yamada K, Watanabe W, Toma T, Itoh K, Nishii J. In situ observation of photo induced refractive-index changes in filaments formed in glasses by femtosecond laser pulses. Opt Lett 2001;26:19–21. [4] Ams M, Marshall GD, Withford MJ. Study of the influence of femtosecond laser polarisation on direct writing of waveguides. Opt Express 2006;14:13158–63. [5] Burghoff J, Hartung H, Nolte S, Tunnermann A. Structural properties of femtosecond laser-induced modifications in LiNbO3. Appl Phys A 2007;86:165–70.
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[6] Suzuki K, Sharma V, Fujimoto JG, Ippen EP. Characterization of symmetric [3 3] directional couplers fabricated by direct writing with a femtosecond laser oscillator. Opt Express 2006;14:2335–43. [7] Choi J, Ramme M, Richardson M. Multiple beam splitter using volumetric multiplexed Fresnel zone plates fabricated by ultrafast laser-writing. Opt Lett 2012;37:3375–7. [8] Fletcher LB, Witcher JJ, Troy N, Brow RK, Krol DM. Single-pass waveguide amplifiers in Er–Yb doped zinc polyphosphate glass fabricated with femtosecond laser pulses. Opt Lett 2012;37:1148–50. [9] Tan Y, Jia Y, Chen F, de Aldana JRV, Jaque D. Simultaneous dual-wavelength lasers at 1064 and 1342 nm in femtosecond-laser-written Nd:YVO4 channel waveguides. J Opt Soc Am B 2011;28:1607–10. [10] Kumatoriya M, Nakbayashi M, Sakakura M, Shimotsuma Y, Miura K, Fujii T, et al. Optical properties of a waveguide written inside a LiTaO3 crystal by irradiation with focused femtosecond laser pulses. Opt Rev 2011;18:166–70. [11] Silva WF, Jacinto C, Benayas A, Vazquez de Aldana JR, Torchia GA, Chen F, et al. Femtosecond-laser-written, stress-induced Nd:YVO4 waveguides preserving fluorescence and Raman gain. Opt Lett 2010;35:916–8. [12] Eaton SM, Merchant CA, Iyer R, Zilkie AJ, Helmy AS, Aitchison JS, et al. Raman gain from waveguides inscribed in KGd(WO4)2 by high repetition rate femtosecond laser. Appl Phys Lett 2008;92:081105. [13] Ródenas A, Torchia GA, Lifante G, Cantelar E, Lamela J, Jaque F, et al. Refractive index change mechanisms in femtosecond laser written ceramic Nd:YAG waveguides: micro-spectroscopy experiments and beam propagation calculations. Appl Phys B 2009;95:85–96. [14] Mansour I, Caccavale F. An improved procedure to calculate the refractive index profile from the measured near-field intensity. J Lightwave Technol 1996;14:423–8. [15] Liu X, Qu S, Tan Y, Zhang C, Chen F. Buried channel waveguides in neodymium-doped KGd(WO4)2 fabricated by low-repetition-rate femtosecond laser writing. Appl Phys B 2011;103:145–9. [16] Li S-L, Han P, Shi M, Yao Y, Hu B, Wang M, et al. Low-loss channel optical waveguide fabrication in Nd3 þ -doped silicate glasses by femtosecond laser direct writing. Opt Express 2011;19:23958–64. [17] Yevick D, Bardyszewski W. Correspondence of variational finite-difference (relaxation) and imaginary-distance propagation methods for modal analysis. Opt Lett 1992;17:329–30. [18] Kahn A, Heinrich S, Kuhn H, Petermann K, Bradley JDB, Worhoff K, et al. Low threshold monocrystalline Nd:(Gd, Lu)2O3 channel waveguide laser. Opt Express 2009;17:4412–8.
Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Femtosecond laser written channel optical waveguide in Nd:YAG crystal Shi-Ling Li a,b,n, Yong-Kai Ye c, Ming-Wei Wang d a
College of Physics and Engineering, Shandong Provincial Key Laboratory of Laser and Information Technology, Qufu Normal University, Qufu 273165, China Department of Opto-electronics and Information Engineering, School of Precision Instruments and Opto-electronics Engineering, Tianjin University, Tianjin 300072, China c College of Computer Science, Qufu Normal University, Qufu 273165, China d Institute of Modern Optics, Key Laboratory of Optoelectronic Information Science and Technology, MEC, Nankai University, Tianjin 300071, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 31 August 2013 Received in revised form 3 November 2013 Accepted 5 November 2013 Available online 25 November 2013
Low-repetition-rate (1 kHz) femtosecond laser inscription is used to fabricate channel waveguides in Nd: YAG crystal. Guiding occurs in the surroundings of the focal spot. End-to-end coupling measurements reveal the near-field intensity distribution of waveguides. The index profile reconstruction, mode analysis performed by the beam propagation method of these waveguides are presented. The waveguide written with 3.0 μJ pulse energy and 100 μm/s scan velocity shows strong guidance at 632.8 nm, and a propagation loss of about 0.5 dB/mm. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Channel waveguides Ultrafast lasers Laser materials
1. Introduction The ultra-short laser direct writing has rapidly become a powerful technique for the fabrication of photonic waveguide components in dielectric materials [1,2]. Permanent structural and refractive-index modifications which can lead to the creation of buried channel waveguides are created by focusing the intense ultrafast laser pulses into the dielectric materials where multiphoton absorption and/or avalanche effects take place. The extent of such modification can be controlled by the writing conditions such as focusing conditions, pulse energy, pulse duration, repetition rate and polarization, as well as writing velocity and direction [3–5]. Compared with other fabrication techniques, ultrafast laser direct writing technique has outstanding advantages such as truly three-dimensional fabrication and short processing times. Using this technique optical devices like directional couplers [6], beamsplitters [7], amplifiers [8], and lasers [1,9] were realized. The nonlinear interaction processes that are strongest at the focus of the writing laser can yield a region of local refractive index change. There are two different cases for the refractive index change, one case is that there is direct refractive-index increase at the laser focus [4,10]; another case is that there is refractive index decrease at the
n
Corresponding author at: College of Physics and Engineering, Shandong Provincial Key Laboratory of Laser and Information Technology, Qufu Normal University, Qufu 273165, China. Tel.: þ 86 053 744 56806. E-mail address: [email protected] (S.-L. Li). 0030-3992/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.optlastec.2013.11.006
written track, while indirectly increases on its surroundings. In the second case, usually several parallel tracks being written around an unmodified region, waveguiding in this region is possible. The two parallel tracks approach was applied in Refs. [11–13], and the center between the two tracks was waveguide region. Nd:YAG crystal is one of the favorite gain media for solid-state lasers owing to its outstanding fluorescence, thermal and mechanical properties. In this paper, we report the fabrication of channel waveguides under single track and the double track configuration in Nd:YAG crystals using a low-repetition-rate (1 kHz) laser with 35 fs pulses of 800 nm light. The scan speeds are 50–400 m/s which are 10– 40 fold faster than those used in Ref. [1], leading to a much faster fabrication time. We have characterized these laser-written structures, demonstrated waveguiding of 632.8 and 1550 nm light. The change in refractive index, the polarized propagation property and the propagation losses of the obtained waveguides were measured at 632.8 nm wavelength. From our investigations strong confinement of both TE and TM modes was observed in single-track and double-track waveguides, which shows the presence of a degree of birefringence. 2. Experimental An amplified Ti:sapphire laser system (35 fs, 800 nm, and 1 kHz repetition rate) was used to inscribe tracks in Nd:YAG crystals (a cut with size of 10 8 2.6 mm3 along b, c and a axes, doped by 1 at% Nd3 þ ions). Fig.1(a) shows the schematic plot of femtosecond laser writing setup. The femtosecond laser was incident on the bc face and
90
S.-L. Li et al. / Optics & Laser Technology 58 (2014) 89–93
was focused into the samples approximately 200 μm below the surface by a microscope objective (N.A.¼0.4). Pulse energies ranging from 1.5 to 4.0 mJ were used in the formation of optical waveguides. The sample was moved perpendicular to the laser-beam axis by a computer-controlled positioning system with a velocity of 50– 400 μm/s during the writing process. The end-face coupling method was used to characterize the transmission properties of channel waveguide. Fig. 1(b) shows the experimental setup. The polarized light beam at the wavelength of 632.8 or 1550 nm was coupled into the waveguide by a 25 microscope objective lens to excite the guide modes. And at the
output facet of the sample, the output light was collected using another 25 microscope objective lens, then imaged onto a CCD camera. Therefore, we obtained the near-field intensity distributions of waveguide structures. We reconstructed the refractive index profiles of the waveguides according to the procedure described in Refs. [14–16]. Based on the reconstructed refractive index distribution, the finite-difference beam propagation method (FD-BPM) [17] was applied to simulate the guided modes.
3. Results and discussion
Fig. 1. (a) Schematic of femtosecond laser writing setup and (b) a sketch of the end-face coupling experimental setup.
Microscope image in end view and near-field intensity distributions of waveguides are shown in Fig. 2. As can be seen in Fig. 2(a) and (c), a guided mode occurs in the sides of the written single tracks (white dashed area in Fig. 2(c)). Fig. 2(b) and (d) shows that waveguide was also observed in the center between two adjacent tracks (white dashed area). The guided mode in the center of the pair structure has a nearly Gaussian profile. Fig. 3(a)–(c) depicts the microscope images of cross section perpendicular to the inscribed tracks. With increasing pulse energy both height and width of the tracks increase. The track corresponding to a pulse energy of 2.5 μJ is 50 μm in height and 5 μm in width. End-face coupling experiments at 632.8 nm show that singletrack waveguides could be formed by using pulse energies of 1.5– 4.0 mJ and scan velocity of 50–400 μm/s. In this case, the guiding region occurred at the sides of laser modified region where the refractive index increased. Guiding mode was not observed for pulse energy below 1.5 mJ, perhaps due to insufficient index change.
Fig. 2. Microscopy image of end view and near-field mode profile at 632.8 nm: (a) and (c) single-track structure for a pulse energy of 3.0 μJ and scan velocity of 50 μm/s; (b) and (d) double-track structure for a pulse energy of 3.0 μJ and scan velocity of 100 μm/s, respectively (the track marked as white dashed area).
S.-L. Li et al. / Optics & Laser Technology 58 (2014) 89–93
The mechanisms that give rise to index modification can be divided into two distinct parts, local damage is produced at focus, and that this damage is accompanied by a lattice compression in its surroundings [13]. The first effect (damage) causes a refractive index reduction whereas the second effect (compression) leads to a local refractive index enhancement. The writing parameters and geometry strongly determine the dominant effect. We can roughly estimate the change of refractive index in the waveguide region by measuring the maximum incident angle θm at which no change of the transmitted power is occurring, and using the formula 2
Δn ¼ sin θm =2n
ð1Þ
where n is the refractive index of the unstructured sample [1]. For the waveguide written with pulse energy of 3.0 mJ at a speed of 50 mm/s, the estimated refractive index change of the waveguides is in the order of ΔnE3 10 4. Due to uncertainties of measuring the maximum incident angle, there is an error of about 35% for the refractive index change. According to the measured near-field intensity profile as displayed in Fig. 2(c), we have reconstructed the 2D refractive index change profile of the waveguide. Fig. 4 (a) shows the reconstructed refractive index profile of the waveguide, which consists of reduced refractive index ( 2.7 10 3) at the writing track and stress induced refractive index increment ( 3 10 4) on both sides of the writing line. Based on this refractive index distribution we calculated the modal profile of waveguide by using FD-BPM as shown in Fig. 4(b). By comparing Figs. 4(b) and 2(c), one can conclude that the simulated distribution
91
is generally in agreement with the measured modal profile. Double-track waveguides were realized by using pulse energies of 1.5–4.0 mJ and two tracks separation of 20 μm. Fig. 5(b) shows the refractive index profiles of the waveguide reconstructed according to the measured near-field mode profiles depicted in Fig. 5(a). It can be observed from Fig. 5(b) that there is a refractive-index increment between two filaments of the order of 2 10 4, and accompanied by a refractive index reduction of the order of 1.7 10 3 at the two filaments which constitute two low index barriers that provide a strong light confinement in the horizontal direction. Fig. 5(c) shows the calculated modal profile based on this reconstructed index distribution. Comparing Fig. 5(a) and (c), one can conclude that the simulated distribution is in agreement with the measured modal profile. The “double filament” approach, which uses both mechanisms (stress and damage) in a complementary way. The effect of the propagating light polarization on the guiding properties of single-track and double-track waveguides was investigated by placing a linear polarizer after 632.8 nm He–Ne laser. From our investigations, strong confinement of both TM and TE modes was observed in the waveguides as shown in Figs. 2(d), 4 (a) and 5(a) and (d). End-face coupling experiments at 1550 nm show that only double-track waveguides were realized by using pulse energies of 4.0 mJ and separation of 20 μm. While guiding mode was not observed for single-track structure. Fig. 6 shows the near-field intensity distribution of the guiding mode at 1550 nm and 632.8 nm. According to formula (1), the estimated refractive index increase in the waveguide is 5 10 4.
Fig. 3. Microscope images of cross section perpendicular to the inscribed tracks from 1.5 to 4.0 μJ pulse energy in Nd:YAG (a)–(c). The scanning velocities were all 50 μm/s. (a) 1.5 μJ, (b) 2.5 μJ and (c) 4.0 μJ.
Fig. 4. (a) The reconstructed refractive index distribution in the waveguide fabricated with a pulse energy of 3.0 μJ and a scan velocity of 50 μm/s, (b) the calculated modal profile, and (c) the measured TE mode.
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Fig. 5. (a) The measured near-field intensity profile of TM mode fabricated with 2.0 μJ pulse energy and 50 μm/s scan velocity, (b) the reconstructed refractive index distribution, (c) the corresponding calculated modal profile, and (d) the measured TE mode.
Fig. 6. The measured near-field intensity profile of waveguide mode fabricated with 4.0 μJ pulse energy and 400 μm/s scan velocity at wavelength of (a) 1550 nm and (b) 632.8 nm.
The propagation loss of the waveguide was estimated by using the formula [18] L ¼ 10 log ðP out =P in Þ dB
ð2Þ
where Pin was the power measured before coupling into the waveguide, and Pout was the out-coupled laser power. It should be noted that Fresnel reflections at the end facets, the transmission of the outcoupling objective and additional coupling losses, such as those resulting from an imperfect overlap between the pump and waveguide mode, are all included in L. Therefore, the propagation losses for the 8 mm long channel waveguide are lower than L. We found that the loss of double-track waveguide is lower than that of single-track
waveguide fabricated with the same energy and scan velocity. For the waveguide as shown in Fig. 2(d), the waveguide losses was L¼4.1 dB ( 0.5 dB/mm) at 632.8 nm determined by formula (2). And the propagation loss of the double-track waveguides with fixed scan velocity of 100 μm/s was 0.9 dB/mm for 1.5 μJ, 0.7 dB/mm for 2.0 μJ, and 0.6 dB/mm for 4.0 μJ.
4. Conclusions In this paper we have shown that optical waveguiding was observed in the surrounding region of the tracks and between two tracks. The mode guided between two tracks has a nearly Gaussian
S.-L. Li et al. / Optics & Laser Technology 58 (2014) 89–93
intensity profile. The refractive index changes in the region of waveguide and the track of these waveguides were estimated to be in the order of 10 4 and 10 3, respectively. 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. The propagation loss of the channel waveguide fabricated with 3.0 mJ pulse energy and 100 mm/s scan velocity was lower than 4.1 dB.
Acknowledgments This work is supported by the National Natural Science Foundation of China (Grant nos. 61205055 and 11005070), Shandong Provincial Natural Science Foundation, China (Grant no. ZR2011AQ026), Educational Commission of Shandong Province, China (Grant no. J11LA12). References [1] Siebenmorgen J, Petermann K, Huber G, Rademaker K, Nolte S, Tünnermann A. Femtosecond laser written stress-induced Nd:Y3Al5O12 (Nd:YAG) channel waveguide laser. Appl Phys B 2009;97:251–5. [2] Thomson RR, Campbell S, Blewett IJ, Kar AK, Reid DT. Optical waveguide fabrication in z-cut lithium niobate (LiNbO3) using femtosecond pulses in the low repetition rate regime. Appl Phys Lett 2006;88:111109. [3] Yamada K, Watanabe W, Toma T, Itoh K, Nishii J. In situ observation of photo induced refractive-index changes in filaments formed in glasses by femtosecond laser pulses. Opt Lett 2001;26:19–21. [4] Ams M, Marshall GD, Withford MJ. Study of the influence of femtosecond laser polarisation on direct writing of waveguides. Opt Express 2006;14:13158–63. [5] Burghoff J, Hartung H, Nolte S, Tunnermann A. Structural properties of femtosecond laser-induced modifications in LiNbO3. Appl Phys A 2007;86:165–70.
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