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Writing waveguides and gratings in silica and related materials by a femtosecond laser
Journal of Non-Crystalline Solids 239 (1998) 91±95
Section 2. Photo-induced and non-linear optical eects in glasses
Writing waveguides and gratings in silica and related materials by a femtosecond laser K. Hirao b
a,b,*
, K. Miura
b
a Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Sakyo-ku, Kyoto 606-01, Japan Hirao Active Glass Project, ERATO, Japan Science and Technology Corporation, 15 Morimoto-cho Shimogamo Sakyo-ku, Kyoto 606, Japan
Abstract With the goal of creating various optical glass devices for the telecommunications industry, the eects of 810 nm, femtosecond laser radiation on various glasses were investigated. By focusing the laser beam via a microscope objective, transparent but visible, round-elliptical damage lines were successfully written inside high silica, borate, soda-limesilicate, ¯uoride and chalcogenide glasses. Microscopic ellipsometric measurements of the damaged region in pure and Ge-doped silica glasses showed refractive index increases of 0.01 to 0.035. The formation of several types of defects, including Si E0 or Ge E0 centers, non-bridging oxygen hole centers, and peroxy radicals, was also detected in addition to the identi®cation. These results suggest that multi-photon interactions occurs in the glasses and that it is possible to write three-dimensional optical circuits in bulk glasses via such a focused laser beam technique. Ó 1998 Elsevier Science B.V. All rights reserved.
1. Introduction The recent development of femtosecond lasers has stimulated interest in the ®eld of optical devices. Among them, we have found that Bi2 O3 containing glass can be used for ultrafast ( 100 fs) optical switching using the third-order nonlinearity of the glasses [1]. Furthermore, it has been found that femtosecond infrared laser irradiation results in an increase in the refractive index (0.04) at the focal point inside the glasses [2]. This phenomenon is attractive, since it gives the potential to direct-write three-dimensional optical waveguide structures inside substrate materials.
* Corresponding author. Tel.: +81-75 753 5521; fax: +81-75 751 6640; e-mail: [email protected].
So far, investigations [3] of the eects of ultraviolet (UV) radiation eects in glasses have been performed with one of the main motivators being the desire to produce optical devices such as Bragg gratings on the glass surface. On the other hand, laser radiation eects via infrared (IR) laser light has received little attention due to the photon energy at these wavelengths. The development of higher energy density, femtosecond pulsed lasers, however has prompted us to investigate the unexplored potential for inducing multi-photon photochemical reactions which cause larger refractive index changes in glass than with UV lasers. Here, we present results which show that stable, visible laser damage lines based on photo-induced refractive index changes can be achieved using an IR femtosecond laser. Moreover, we demonstrate the possibility that this laser
0022-3093/98/$ ± see front matter Ó 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 8 ) 0 0 7 5 5 - 8
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K. Hirao, K. Miura / Journal of Non-Crystalline Solids 239 (1998) 91±95
damage line works as an optical waveguide in glasses. 2. Experimental A regeneratively ampli®ed, 810 nm, Ti:Sapphire laser which emits 120 fs, 200 kHz, mode locked pulses and delivers an average power of 975 m W was utilized for our study. The 5 mm diameter beam was focused via 5 to 20 X microscope objectives and injected into polished plates of various glasses, as shown in Fig. 1. The average power of the laser beam at the sample location was controlled between approximately 40 and 800 mW via neutral density ®lters, which were inserted between the laser and microscope objective. With the help of an XYZ stage the samples were translated at rates of 100 to 10 000 lm/s either parallel or perpendicular to the incident laser beam, thus, creating damage lines inside of the glasses. Fig. 2. Waveguides written in various kinds of glasses.
3. Results For all translation speeds and directions, structural changes were induced along the path transversed by the focal point of the laser, and colorless, transparent, linear damage marks were produced inside glasses such as silica, ¯uoride, SFG(lead silicate glass, Hoya) and chalcogenide glasses. These damage lines and their cross-sections are clearly visible using transmitted light optical microscopy, as shown in Fig. 2, and they
Fig. 1. Experimental setup for writing waveguides.
are stable at room temperature. However, the damage thresholds dier among these samples. This might be related to the case of the defect generation in the glasses, which is discussed later. It should be noted that these glasses do not have absorption bands at the 810 nm wavelength employed in this study. Therefore, the peak power of the laser beam at the focal point is concentrated up to 1014 W/cm2 which is enough to bring about the non-linear multiphoton absorption. Using a microscopic ellipsometer, refractive index pro®les were measured across the cross-sections of the damage lines formed perpendicular to the laser beam. After 10 passes of the laser, the refractive index at the center of the damage was approximately 0.035 larger than the surrounding glass in the case of Ge-doped silica, as shown in Fig. 3. The refractive indices of the as-received glasses vary by less than 0.0005 across the probed region, according to the manufacturer's speci®cations, and the error in the ellipsometer measurements is less than 0.01. We also examined the eects of average power, pulse width and number of scans on the refractive
K. Hirao, K. Miura / Journal of Non-Crystalline Solids 239 (1998) 91±95
93
Fig. 3. Distribution of the refractive index for Ge-doped silica glass (scanning speed is 20 lm/s).
index variation and the core size of optical wave guides. The core diameters increased with increasing average laser power as shown in Fig. 4. The core diameters remain unchanged despite the number of scans, though the refractive index of the core increased with repetition of laser scabs in the same area. The refractive index change increased with decreasing pulse width, that is, the refractive index change increases with increasing peak power (average power/pulse width). The results suggest that of the refractive index dierence and core diameter can be controlled by adjusting the writing conditions. Electron spin resonance (ESR) spectroscopic measurements of the damage spots in pure silica glass, silica glass with 200 ppm OH, and silica glass with 5 mol% GeO2 , showed an increase in the concentrations of Si and Ge E'centers with respect to the untreated samples, as shown in Fig. 5. The formation of peroxy-radicals (POR) and nonbridging oxygen hole centers (NBOHC) was also observed in both glasses. The spectra of the laserinduced optical absorption shown in Fig. 6 are in
Fig. 4. Dependence of the average laser power on core diameter using pulse width of 120 fs.
Fig. 5. ESR spectra of high silica glasses.
agreement with these assignments, but also show a possible decrease in the concentration of neutral oxygen monovacancies (NOMV) in the Ge-doped
Fig. 6. Change in the UV spectra of the high silica glasses due to laser damage.
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K. Hirao, K. Miura / Journal of Non-Crystalline Solids 239 (1998) 91±95
glass. In addition unidenti®ed broad absorption bands (320±700 nm) are formed in both glasses. Fifteen millimeter long-waveguides were formed by the femtosecond laser, and the near®eld patterns at 800 nm were observed by a CCD camera, as shown in Fig. 7. The loss of these waveguides is approximately the same (0.1 dB/ cm) as the commercial Ge-doped silica waveguides by ®ber bat coupling measurement. The core diameter of the waveguide was controlled by changing the average power of the writing laser beam. At the core diameter of 8 lm, it is possible to propagate only LP01 of the fundamental mode, which is consistent with the fundamental theory of ®ber optics. The higher order modes (LP11 or LP22 ) can also be observed to propagate with an increase of core diameter.
Fig. 7. Near-®eld patterns on wave guides of 15 mm length at 800 nm.
4. Discussion At present, we cannot propose a mechanism for the refractive index variation. However, the silica glass was examined in the laser irradiation region by an atomic force microscope (AFM). Fig. 8 shows the AFM image of the surface of a core end on silica glass. The shrinkage of the surface gradually increases from outside the core towards the center and it reaches a maximum at 45 nm. We suggest that densi®cation of glass occurs in the laser irradiated region. The increases in refractive index can be related to local densi®cation which ®nally occurred inside the glass, though the initial process of optical waveguide formation may be accompanied by various phenomena such as the formation of color centers or lattice defects, or melting. The creation of defects in the high silica glasses via 810 nm radiation suggests that the damage mechanism involves a multi-photon process. To our knowledge, damage at this wavelength has not been previously reported, and it is suspected that the high pulse energy of the femtosecond laser is responsible for this eect. We conclude that the photo-induced refractive index increases in the glasses is due to both densi®cation and defect generation.
Fig. 8. AFM pro®le of the damage spot.
K. Hirao, K. Miura / Journal of Non-Crystalline Solids 239 (1998) 91±95
95
5. Conclusion
References
Optical waveguides were successfully formed in various glasses by ultra-fast pulses from a femtosecond laser. In addition, it was con®rmed that single mode waveguides could be written with femtosecond laser pulses. These facts open new possibilities in the ®eld of integrated optics and three-dimensional optical circuits, especially for compact, all-solid-state lasers, ampli®ers and optical switches.
[1] N. Sugimoto, H. Kanbara, S. Fujiwara, K. Tanaka, K. Hirao, Opt. Lett. 21 (1996) 1637. [2] K.H. Davis, K. Miura, N. Sugimoto, K. Hirao, Opt. Lett. 21 (1996) 1729. [3] K. O Hill, Y. Fuji, D.C. Johnson, B.S. Kawasaki, Appl. Phys. Lett. 32 (1978) 647.
Section 2. Photo-induced and non-linear optical eects in glasses
Writing waveguides and gratings in silica and related materials by a femtosecond laser K. Hirao b
a,b,*
, K. Miura
b
a Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Sakyo-ku, Kyoto 606-01, Japan Hirao Active Glass Project, ERATO, Japan Science and Technology Corporation, 15 Morimoto-cho Shimogamo Sakyo-ku, Kyoto 606, Japan
Abstract With the goal of creating various optical glass devices for the telecommunications industry, the eects of 810 nm, femtosecond laser radiation on various glasses were investigated. By focusing the laser beam via a microscope objective, transparent but visible, round-elliptical damage lines were successfully written inside high silica, borate, soda-limesilicate, ¯uoride and chalcogenide glasses. Microscopic ellipsometric measurements of the damaged region in pure and Ge-doped silica glasses showed refractive index increases of 0.01 to 0.035. The formation of several types of defects, including Si E0 or Ge E0 centers, non-bridging oxygen hole centers, and peroxy radicals, was also detected in addition to the identi®cation. These results suggest that multi-photon interactions occurs in the glasses and that it is possible to write three-dimensional optical circuits in bulk glasses via such a focused laser beam technique. Ó 1998 Elsevier Science B.V. All rights reserved.
1. Introduction The recent development of femtosecond lasers has stimulated interest in the ®eld of optical devices. Among them, we have found that Bi2 O3 containing glass can be used for ultrafast ( 100 fs) optical switching using the third-order nonlinearity of the glasses [1]. Furthermore, it has been found that femtosecond infrared laser irradiation results in an increase in the refractive index (0.04) at the focal point inside the glasses [2]. This phenomenon is attractive, since it gives the potential to direct-write three-dimensional optical waveguide structures inside substrate materials.
* Corresponding author. Tel.: +81-75 753 5521; fax: +81-75 751 6640; e-mail: [email protected].
So far, investigations [3] of the eects of ultraviolet (UV) radiation eects in glasses have been performed with one of the main motivators being the desire to produce optical devices such as Bragg gratings on the glass surface. On the other hand, laser radiation eects via infrared (IR) laser light has received little attention due to the photon energy at these wavelengths. The development of higher energy density, femtosecond pulsed lasers, however has prompted us to investigate the unexplored potential for inducing multi-photon photochemical reactions which cause larger refractive index changes in glass than with UV lasers. Here, we present results which show that stable, visible laser damage lines based on photo-induced refractive index changes can be achieved using an IR femtosecond laser. Moreover, we demonstrate the possibility that this laser
0022-3093/98/$ ± see front matter Ó 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 8 ) 0 0 7 5 5 - 8
92
K. Hirao, K. Miura / Journal of Non-Crystalline Solids 239 (1998) 91±95
damage line works as an optical waveguide in glasses. 2. Experimental A regeneratively ampli®ed, 810 nm, Ti:Sapphire laser which emits 120 fs, 200 kHz, mode locked pulses and delivers an average power of 975 m W was utilized for our study. The 5 mm diameter beam was focused via 5 to 20 X microscope objectives and injected into polished plates of various glasses, as shown in Fig. 1. The average power of the laser beam at the sample location was controlled between approximately 40 and 800 mW via neutral density ®lters, which were inserted between the laser and microscope objective. With the help of an XYZ stage the samples were translated at rates of 100 to 10 000 lm/s either parallel or perpendicular to the incident laser beam, thus, creating damage lines inside of the glasses. Fig. 2. Waveguides written in various kinds of glasses.
3. Results For all translation speeds and directions, structural changes were induced along the path transversed by the focal point of the laser, and colorless, transparent, linear damage marks were produced inside glasses such as silica, ¯uoride, SFG(lead silicate glass, Hoya) and chalcogenide glasses. These damage lines and their cross-sections are clearly visible using transmitted light optical microscopy, as shown in Fig. 2, and they
Fig. 1. Experimental setup for writing waveguides.
are stable at room temperature. However, the damage thresholds dier among these samples. This might be related to the case of the defect generation in the glasses, which is discussed later. It should be noted that these glasses do not have absorption bands at the 810 nm wavelength employed in this study. Therefore, the peak power of the laser beam at the focal point is concentrated up to 1014 W/cm2 which is enough to bring about the non-linear multiphoton absorption. Using a microscopic ellipsometer, refractive index pro®les were measured across the cross-sections of the damage lines formed perpendicular to the laser beam. After 10 passes of the laser, the refractive index at the center of the damage was approximately 0.035 larger than the surrounding glass in the case of Ge-doped silica, as shown in Fig. 3. The refractive indices of the as-received glasses vary by less than 0.0005 across the probed region, according to the manufacturer's speci®cations, and the error in the ellipsometer measurements is less than 0.01. We also examined the eects of average power, pulse width and number of scans on the refractive
K. Hirao, K. Miura / Journal of Non-Crystalline Solids 239 (1998) 91±95
93
Fig. 3. Distribution of the refractive index for Ge-doped silica glass (scanning speed is 20 lm/s).
index variation and the core size of optical wave guides. The core diameters increased with increasing average laser power as shown in Fig. 4. The core diameters remain unchanged despite the number of scans, though the refractive index of the core increased with repetition of laser scabs in the same area. The refractive index change increased with decreasing pulse width, that is, the refractive index change increases with increasing peak power (average power/pulse width). The results suggest that of the refractive index dierence and core diameter can be controlled by adjusting the writing conditions. Electron spin resonance (ESR) spectroscopic measurements of the damage spots in pure silica glass, silica glass with 200 ppm OH, and silica glass with 5 mol% GeO2 , showed an increase in the concentrations of Si and Ge E'centers with respect to the untreated samples, as shown in Fig. 5. The formation of peroxy-radicals (POR) and nonbridging oxygen hole centers (NBOHC) was also observed in both glasses. The spectra of the laserinduced optical absorption shown in Fig. 6 are in
Fig. 4. Dependence of the average laser power on core diameter using pulse width of 120 fs.
Fig. 5. ESR spectra of high silica glasses.
agreement with these assignments, but also show a possible decrease in the concentration of neutral oxygen monovacancies (NOMV) in the Ge-doped
Fig. 6. Change in the UV spectra of the high silica glasses due to laser damage.
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K. Hirao, K. Miura / Journal of Non-Crystalline Solids 239 (1998) 91±95
glass. In addition unidenti®ed broad absorption bands (320±700 nm) are formed in both glasses. Fifteen millimeter long-waveguides were formed by the femtosecond laser, and the near®eld patterns at 800 nm were observed by a CCD camera, as shown in Fig. 7. The loss of these waveguides is approximately the same (0.1 dB/ cm) as the commercial Ge-doped silica waveguides by ®ber bat coupling measurement. The core diameter of the waveguide was controlled by changing the average power of the writing laser beam. At the core diameter of 8 lm, it is possible to propagate only LP01 of the fundamental mode, which is consistent with the fundamental theory of ®ber optics. The higher order modes (LP11 or LP22 ) can also be observed to propagate with an increase of core diameter.
Fig. 7. Near-®eld patterns on wave guides of 15 mm length at 800 nm.
4. Discussion At present, we cannot propose a mechanism for the refractive index variation. However, the silica glass was examined in the laser irradiation region by an atomic force microscope (AFM). Fig. 8 shows the AFM image of the surface of a core end on silica glass. The shrinkage of the surface gradually increases from outside the core towards the center and it reaches a maximum at 45 nm. We suggest that densi®cation of glass occurs in the laser irradiated region. The increases in refractive index can be related to local densi®cation which ®nally occurred inside the glass, though the initial process of optical waveguide formation may be accompanied by various phenomena such as the formation of color centers or lattice defects, or melting. The creation of defects in the high silica glasses via 810 nm radiation suggests that the damage mechanism involves a multi-photon process. To our knowledge, damage at this wavelength has not been previously reported, and it is suspected that the high pulse energy of the femtosecond laser is responsible for this eect. We conclude that the photo-induced refractive index increases in the glasses is due to both densi®cation and defect generation.
Fig. 8. AFM pro®le of the damage spot.
K. Hirao, K. Miura / Journal of Non-Crystalline Solids 239 (1998) 91±95
95
5. Conclusion
References
Optical waveguides were successfully formed in various glasses by ultra-fast pulses from a femtosecond laser. In addition, it was con®rmed that single mode waveguides could be written with femtosecond laser pulses. These facts open new possibilities in the ®eld of integrated optics and three-dimensional optical circuits, especially for compact, all-solid-state lasers, ampli®ers and optical switches.
[1] N. Sugimoto, H. Kanbara, S. Fujiwara, K. Tanaka, K. Hirao, Opt. Lett. 21 (1996) 1637. [2] K.H. Davis, K. Miura, N. Sugimoto, K. Hirao, Opt. Lett. 21 (1996) 1729. [3] K. O Hill, Y. Fuji, D.C. Johnson, B.S. Kawasaki, Appl. Phys. Lett. 32 (1978) 647.