- Email: [email protected]
Photosensitive and athermal glasses for optical channel waveguides
Journal of Non-Crystalline Solids 326&327 (2003) 464–471 www.elsevier.com/locate/jnoncrysol
Photosensitive and athermal glasses for optical channel waveguides Junji Nishii a
a,*
, Kenji Kintaka a, Hiroaki Nishiyama b, Masahide Takahashi
c
Photonics Research Institute, AIST Kansai, 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan b Department of Manufacturing Science, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan c Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan
Abstract Ge–SiO2 thin glass films photosensitive to excimer laser irradiation were fabricated by plasma enhanced chemical vapor deposition. A channel waveguide was successfully fabricated by irradiation with only an excimer laser through a Cr mask pattern, which was previously coated on the slab-waveguide by the sputtering method. Bragg gratings with high diffraction efficiency were also fabricated in the waveguide by another KrF laser irradiation through the phase mask without hydrogen loading. Channel waveguides with Bragg gratings, on the other hand, were fabricated on the glass ceramic substrates with negative thermal expansion coefficient. Doping of the B2 O3 to Ge–SiO2 glass film effectively suppressed the temperature drift of the stop band (dk=dT ) of the gratings. The dk=dT attained in this study was 4 pm/C, which was less than one-third of those reported for commercially available waveguide gratings. Although the grating printed in the Ge–B–SiO2 film was almost erased by annealing at a temperature lower than 500 C, a new type of grating with much higher diffraction efficiency than that before annealing was formed after annealing at 600 C. The diffraction efficiency of the new grating was unchanged after repeated heating between room temperature and 600 C. 2003 Elsevier B.V. All rights reserved. PACS: 42.65.H; 78.40.F; 42.70.C; 42.88
1. Introduction Access networks require low cost and highly reliable optical devices. Formation of Bragg gratings in the waveguide is especially important technology for the development of optical devices applicable to wavelength division multiplexing
*
Corresponding author. Tel.: +81-727 51 9543; fax: +81-727 51 9637. E-mail address: [email protected] (J. Nishii).
(WDM) optical networks. This paper reports our recent three works on the fabrication of optical waveguide devices using highly photosensitive thin glass films deposited by the plasma enhanced chemical vapor deposition (PECVD) method. The first objective of our study is the fabrication of channel waveguides by irradiation only with excimer laser. So far, a complex photolithography process is required to obtain the channel waveguide structures. Thus the development of highly photosensitive thin glass films enables us to write a channel waveguide, leading to an effective cost
0022-3093/$ - see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0022-3093(03)00454-X
J. Nishii et al. / Journal of Non-Crystalline Solids 326&327 (2003) 464–471
reduction. Boron, lead, or tin ions were reported as effective sensitizers for magnifying the photon induced refractive index changes of the core materials such as Ge–SiO2 glass [1–3]. A high pressure H2 (or D2 ) treatment of the Ge–SiO2 glasses, the so-called H2 - (or D2 -) loading, is an effective method to increase its photosensitivity [4]. A refractive index change of 103 order was confirmed by the H2 loading process followed by irradiation with KrF excimer laser. Actually, the direct writing of the channel waveguide was demonstrated using a Ge–SiO2 thin film treated under an ultra-high pressure D2 atmosphere. A large amount of hydroxyl groups was, however, produced in the glass by irradiation with excimer laser, resulting in a large increase of optical attenuation at around the 1.4-lm wavelength. Therefore, a highly photosensitive glass, which does not require H2 -(D2 -) loading, must be developed in order to realize a simple fabrication of channel waveguides. Recently, it was reported that the Ge2þ center, which is a dominant oxygen-deficient center in Ge–SiO2 glasses, is the most active species among the defects in the glass produced by ultraviolet (UV) laser irradiation [5]. In this study the PECVD process was chosen to deposit Ge–SiO2 thin glass films containing sufficient amount of Ge2þ centers, and the channel waveguide was successfully fabricated by irradiation only with excimer laser through a Cr mask pattern, which was previously coated on the slabwaveguide by the sputtering method. Optical characteristics of Bragg gratings formed in waveguides strongly depend on temperature fluctuation because of the thermal refractive index change and the thermal expansion of the devices [6]. Therefore, the use of an athermalization technique is usually required for their practical use, which dose not seriously affect the long-term reliability and the cost of the devices. It was reported that polymer cladding could reduce the temperature dependence of the effective refractive index [7,8]. This technique, however, is rather complicated and less reliable because of the poor thermal stability of the polymer. Thus the second objective in this study is how to minimize the temperature drift of the diffraction wavelength of the grating without using polymer or electricity. In this study we fabricated a channel waveguide based on
465
Ge–B–SiO2 glasses on a crystallized glass substrate with negative thermal expansion coefficient in order to compensate the positive expansion of the waveguide layer. The positive temperature dependence of the refractive index (dn=dT ) of the Ge– SiO2 glass could be effectively reduced by the co-doping with boron, because B2 O3 has the negative derivative dn=dT [9,10]. In the final section, a unique thermo-optical property of Ge–B–SiO2 thin glass film is described. The Bragg grating printed by excimer laser irradiation in the film was not erased after the annealing at 600 C, which is a completely new phenomenon, because the usual gratings written in oxide glasses by irradiation with UV laser do disappear after annealing at around 500 C. The origin of the formation of such thermally stabilized grating is discussed.
2. Experimental Photosensitive thin glass films were deposited by a PECVD system (Samco International, Inc., PD-10C), which is a conventional system with parallel-plate electrodes. The raw materials used were Si(OC2 H5 )4 , Ge(OCH3 )4 , and B(OC2 H5 )3 for SiO2 , GeO2 , and B2 O3 , respectively, which were heated at 80 C. The vapor pressures of these materials were similar at this temperature. The vapors of raw materials were introduced into the chamber by its own vapor pressure and decomposed to oxides in an O2 plasma enhanced at a radio frequency of 13.56 MHz and 250 W. The operating pressure and substrate temperature were 0.4 Torr and 400 C, respectively. The composition of the films, which was controlled precisely by changing the flow ratio of raw materials, was analyzed by an electron probe microanalysis.
3. Results and discussion 3.1. Direct formation of channel waveguide by excimer laser irradiation The composition of the film used was 30GeO2 – 70SiO2 in mol%. Metricon model 2010 prism
J. Nishii et al. / Journal of Non-Crystalline Solids 326&327 (2003) 464–471
25
(b)x1000 Refractive index change(x10-3)
Absorption Coefficient(103 cm-1)
466
20
(a) 15 10 5 0 4.0
4.5
5.0
5 .5
(b) 1.0
0.5
(a)
6.0
Photon Energy (eV) Fig. 1. Absorption spectra of (a) 30GeO2 –70SiO2 film deposited by PECVD and (b) 15GeO2 –85SiO2 film prepared by VAD.
coupler was used for the measurement of refractive index, film thickness, and transmission loss of the film. UV light sources were XeF (k ¼ 351 nm, E ¼ 3:5 eV) and KrF (k ¼ 248 nm, E ¼ 5:0 eV) excimer lasers (Lambda Physik COMPex100). The irradiation power was set to 50 mJ/cm2 per pulse at 10 Hz for both excimer lasers. Irradiation was carried out at room temperature without H2 loading. Fig. 1 shows the optical absorption spectra of the film and an optical fiber preform prepared by the axial vapor deposition method (Shinetsu Chemical). The chemical composition was 15GeO2 –85SiO2 in mol% at the core region, which is a one-half of GeO2 content in the film. The intense absorption band at the photon energy of 5 eV is attributed to the oxygen-deficient defects related to Ge ions [11]. Two possible distinct oxygen-deficient defects yielded the 5-eV band: a neutral oxygen monovacancy (BGe–Ge(or Si)B) and a neutral oxygen divacancy (Ge2þ ) associated with Ge ions [12]. It was reported that the optical absorption of Ge2þ defects is observed at 5.16 eV, while the peak due to BGe–Ge(or Si)B is located at 5.07 eV. The absorption peak for the thin film in the figure is observed at 5.16 eV. Thus, the majority of the defects in the film is Ge2þ . It is apparent that the film exhibits an absorption coefficient at the 5-eV absorption band of three
0
0
2
4
6
8
10
Cumulative dose(kJ/cm2) Fig. 2. Induced refractive index changes in 30GeO2 –70SiO2 thin films after irradiation with (a) XeF and (b) KrF lasers.
orders of magnitude larger than that of the VAD fiber preform, which means the concentration of Ge2þ is approximately 103 times higher than that in the fiber preform. Fig. 2 shows the photorefractive index change of the 30GeO2 –70SiO2 film on the silica substrate after irradiation with XeF or KrF laser. Here the original refractive index and thickness of the film used were 1.5098 at 633 nm and 8.7 lm, respectively. The positive index changes of 0.3 · 103 and 1.1 · 103 were induced by irradiation with XeF and KrF lasers, respectively, which were generally one or two orders of magnitude higher than that of a commercially available fiber [13]. The maximum value of the index change was not depending on the Ge content but on the intensity of 5-eV absorption band. The intensity of photoluminescence caused by Ge2þ defects under the 248 nm excitation was reduced to 10% of the initial intensity after irradiation with KrF laser of 3 kJ/cm2 in total dose, which indicates that 90% of Ge2þ was bleached by the irradiation. Therefore the existence of Ge2þ in the glass matrix is closely related to the photo-induced refractive index change [14]. Our highly photosensitive films exhibited the optical attenuation loss of 0.2 dB/cm at 1553 nm by the prism coupling method, which is considered to be an acceptable loss level for the channel waveguide. The channel waveguide was formed
J. Nishii et al. / Journal of Non-Crystalline Solids 326&327 (2003) 464–471
only by irradiation with KrF laser through a Cr mask pattern, which was previously coated on the slab-waveguide by the sputtering method. The fabrication procedure is shown in Fig. 3. The dimension of the core was designed to be a square of 8 · 8 lm2 . The optical characterization of the waveguides obtained was carried out at around 1550 nm wavelength using a super luminescent diode (SLD) or a tunable laser diode (LD). The incident light was butt-coupled into the waveguide by using a commercially available single-mode fiber with a numerical aperture of 0.3. Fig. 4 shows the near-field pattern of the channel waveguide
Slab waveguide of Photosensitive thin film
467
Fig. 4. Near-field pattern of output beam at 1550 nm for channel waveguide with patterned Cr thin film.
obtained after irradiation with KrF laser of 50 mJ/ cm2 and 6000 shots. The numerical apertures (NA) parallel and perpendicular to the substrate were estimated to be 0.14 and 0.38, respectively. The side peaks of the intensity profile of the outgoing beam were considered to be the leak mode of incident light in the cladding layer. The intensity of such mode was relatively high around the next neighbor core. Under best coupling conditions, the side peaks can be eliminated. Furthermore, a Bragg grating was successfully fabricated in the core of this waveguide by irradiation with KrF laser through the phase mask. Fig. 5 shows the
Cr & resist coating
Resist patterning & Cr etching excimer laser
Transmission loss(dB)
0
10
20
1542
Irradiation with excimer laser Fig. 3. Fabrication process of channel waveguide with the overcoating of Cr thin film.
1544 1546 Wavelength (nm)
1548
Fig. 5. Transmission spectrum of channel waveguide with Bragg grating fabricated by irradiation only with KrF laser.
J. Nishii et al. / Journal of Non-Crystalline Solids 326&327 (2003) 464–471
transmission spectra of the waveguide with a Bragg grating. A stop band deeper than 25 dB in depth and 0.2 nm in half width was confirmed. Although slight divergence of propagated beam was observed, the channel waveguide still remained after the Cr mask was removed. 3.2. Athermalization of channel waveguide Channel waveguides with Bragg gratings with athermal characteristics were fabricated by PECVD and a conventional photolithography technique. A Ge–B–SiO2 system was used for the waveguide. The core size was about 5 lm · 5 lm, and the refractive index difference between the core and the cladding layer was about 1%. The overcladding and undercladding layers were each 15lm thick. The waveguide length was about 20 mm. The substrates for the waveguide were Si, silica, and crystallized multicomponent glass with thermal expansion coefficients of 4.15 · 106 , 0.4 · 106 , and )2.0 · 106 (C1 ), respectively. The thickness of these substrates was approximately 1 mm. The propagation loss of the waveguide was measured by the Fabry–Perot method [15] using a tunable LD and an optical spectrum analyzer, which was estimated to be 1–2 dB/cm from the power variations of the output light in the wavelength. The Bragg grating was formed by the method described in the previous section. The number of irradiated pulses and the optical energy density were 3000 shots and 60 mJ/cm2 , respectively. The transmission spectra of the waveguide with the Bragg grating were measured for the TE-like mode. The diffraction peak was observed at around 1530 nm. The temperature dependence of the Bragg wavelength (dk=dT ) was measured at temperatures from 10 to 70 C by placing the waveguide on a Peltier module. Fig. 6 shows the peak shifts of the Bragg wavelengths against temperature. For comparison, the temperature dependence of the Bragg wavelength for a sample with a 10GeO2 –90SiO2 core, SiO2 cladding layer, and Si substrate, which was a conventional-type waveguide with the Bragg grating, is also shown in the figure. The dk=dT in this temperature range was estimated from the slope of the lines. The
0.8 (b)
Wavelength shift(nm)
468
0.6
0.4 (a)
0.2
0 0
20
40
60
80
Temperature (°C) Fig. 6. Bragg wavelength shift against temperature for TE-like mode of (a) 6GeO2 –13B2 O3 –81SiO2 core and 12B2 O3 –88SiO2 cladding waveguide on crystallized glass substrate and (b) 10GeO2 –90SiO2 core and SiO2 cladding waveguide on Si substrate.
dk=dT of the conventional-type waveguide Bragg grating was 11 pm/C. In contrast, the dk=dT of the grating formed in the waveguide with a 6GeO2 –13B2 O3 –81SiO2 core and 12B2 O3 –88SiO2 cladding was 4 pm/C, which is, to our knowledge, the smallest value ever reported for silica-based waveguide Bragg gratings. The experimental results for the temperature dependence of the diffraction wavelength were compared with the theoretical temperature dependence of the diffraction wavelength. The Bragg wavelength k is expressed as k ¼ 2Neff K, where Neff is the effective refractive index of the waveguide and K the grating period. The temperature dependence of the Bragg wavelength (dk=dT ) is given by dk=dT ¼ 2KðdNeff =dT þ Neff aÞ, where a is the thermal expansion coefficient. As for a of the waveguide, we used the value of the substrate but not that of the waveguide because the thickness of the waveguide is much thinner than the substrate. In the estimation, we assumed the values of a, n, and dn=dT of the waveguide layers were additively contributed by each glass component. In addition, the dn=dT of the core was regarded as the dNeff =dT of the waveguide, because the value of dn=dT of the core was comparable with that of the cladding layer in the fabricated devices. Table 1 lists the estimated and measured values of dk=dT in waveguides with several compositions and
J. Nishii et al. / Journal of Non-Crystalline Solids 326&327 (2003) 464–471
469
Table 1 Estimated and measured values of temperature dependence of Bragg wavelength Composition of core
Refractive index at 588 nm
Index change against temperature (·106 )
Expansion coefficient (·106 /C)
Wavelength shift of diffraction peak (pm/C) Estimated
Measured
10GeO2 –90SiO2 10GeO2 –90SiO2 12GeO2 –6B2 O3 –82SiO2 14GeO2 –12B2 O3 –74SiO2 6GeO2 –13B2 O3 –81SiO2
1.462 1.462 1.467 1.470 1.450
11 11 9.3 7.7 6.8
4.15 (Si) )2.0 (crystallized glass) 0.4 (SiO2 ) )2.0 (crystallized glass) )2.0 (crystallized glass)
18 8 10 5 4
11 7 10 5 4
3.3. Thermally stabilized Bragg gratings formed in Ge–B–SiO2 by irradiation with KrF laser Bragg gratings formed in Ge–B–SiO2 thin films exhibited an interesting phenomenon during the thermal annealing [17]. The gratings almost disappeared upon annealing at around 500 C, but the diffraction efficiency of the grating drastically enhanced again after the annealing at 600 C. Consequently the extremely stable grating against heat-treatment at a temperature up to 600 C could be newly developed. Bragg gratings were formed in the film by irradiation with KrF excimer laser light of 248 nm wavelength through the phase mask with 1060 nm pitch. The optical energy density, shot number and repetition rate were 80 mJ/cm2 , 2.7 · 104 shots and 10 Hz, respectively. The first order diffraction efficiencies (g: ratio of diffracted power to incident power) of the gratings printed in the films were measured at a wavelength of 633 nm, which was 0.01%. Littrow mounting set up was used for the measurement. The grating pitch estimated from the incident and diffracted angles precisely agreed with the half of the mask pitch. Fig. 7 shows the changes in g of these gratings after the isochronal annealing for 1 h at each temperature in nitrogen atmosphere. The diffraction efficiencies decreased gradually after the annealing at a temperature up to 500 C, which
0.03
Diffraction Efficiency(%)
substrates. The experimental values of dk=dT agreed fairly well with the theoretically estimated values. Further reduction of the Bragg wavelength shift with temperature would be realized by optimizing waveguide compositions and using a crystallized glass substrate with a larger negative thermal expansion coefficient.
0.02
0.01
(b)
(a)
0
0
200
400
600
Annealing Temperature (°C) Fig. 7. Changes in diffraction efficiencies of gratings formed in (a) Ge–SiO2 and (b) Ge–B–SiO2 thin films by annealing in a nitrogen atmosphere. The annealing was carried out for 1 h at each temperature.
was similar in tendency to the previously reported result [16]. However there was a drastic increase in g after the annealing at 600 C. This precipitous increase in g is extremely exceptional, compared with the current photo-induced gratings in Gedoped silica-based glasses. The grating could not be erased unless the thin film was heat-treated repeatedly between room temperature and 600 C. Therefore, this thermally stabilized grating should be applicable to several optical devices that require reliable thermal stability. It is generally understood that the trigger responsible for the photo-induced refractive index change in Ge–SiO2 and Ge–B–SiO2 is the absorption of UV light by the oxygen-deficient defects in the glass matrix. The absorption coefficient of Ge– B–SiO2 at 248 nm wavelength was approximately
470
J. Nishii et al. / Journal of Non-Crystalline Solids 326&327 (2003) 464–471
1.5 times larger than that of Ge–SiO2 . Thus the laser energy was absorbed effectively, resulting in the formation of color centers with intense absorption coefficient and the densification of glass matrix, which should be closely related to the initial formation of gratings by irradiation with laser. However the remarkable increase in g could not be explained using such an earlier model. Then the isochronal annealing was carried out after the irradiation with KrF excimer laser without phase mask. The sample was irradiated with homogeneous laser pulses consisting of 2.7 · 104 shots and energy density of 48 mJ/cm2 . The annealing condition was identical to that used for the grating preparation. Fig. 8 shows the changes in refractive indices of irradiated and unirradiated areas of Ge– B–SiO2 after the annealing. The indices of both areas decreased monotonically after annealing up to 500 C, and then increased suddenly upon annealing at 600 C. Furthermore the absolute value of the refractive index of the unirradiated area was higher than that of the irradiated area after the annealing at 600 C. Thus the laser irradiation prior to annealing suppressed the increase in the thermally induced refractive index. After confirming the reproducibility of the data, we conclude that the reversed phenomenon of the refractive index caused by the annealing at 600 C should be related to the drastic increase in the g of the grating.
Refractive index at 633nm
1.484
1.483
(a) 1.482
1.481
(b) 1.480
1.479
0
200
400
4. Conclusion (1) Ge–SiO2 thin films deposited by the PECVD method exhibited the index change larger than 103 order of magnitude after irradiation with a KrF excimer laser. A channel waveguide was successfully fabricated only by irradiation through the Cr mask, which was previously coated and patterned on the slab-waveguide. Furthermore, the Bragg grating with a stop band above 25 dB was formed in the core of the waveguide by irradiation with KrF laser through the phase mask. It is, therefore, proposed that the photosensitive glass is useful for the preparation of waveguide and grating devices without using the conventional photolithography process. (2) Ge–B–SiO2 waveguides with Bragg gratings were fabricated on the crystallized glass substrates in order to reduce the temperature drift of the diffraction peak. The shift of the diffraction peak against temperature (dk=dT ) was minimized to 4 pm/C when the waveguide was constructed using a 6GeO2 –13B2 O3 – 81SiO2 core and 12B2 O3 –88SiO2 cladding layer on a crystallized glass substrate with a thermal expansion coefficient of )2.0 · 106 . The attained dk=dT was one-third of that of the grating formed in a conventional Ge–SiO2 waveguide on the Si substrate. (3) Bragg gratings printed in Ge–B–SiO2 thin films by irradiation with a KrF excimer laser through the phase mask erased after the annealing at temperatures up to 500 C, but another type of grating was formed after the annealing at 600 C. This grating could not be erased by the repeated heat-treatment between room temperature and 600 C. We expected that the refractive index modulation related to the grating might be reversed during the annealing at high temperature.
600
Annealing Temperature (°C) Fig. 8. Changes in refractive indices of (a) irradiated and (b) unirradiated areas of the Ge–B–SiO2 thin film.
References [1] D.L. Williams, B.J. Ainslie, J.R. Armitage, R. Kashyap, R. Campbell, Electron. Lett. 29 (1993) 45.
J. Nishii et al. / Journal of Non-Crystalline Solids 326&327 (2003) 464–471 [2] K. Imamura, T. Nakai, K. Moriura, Y. Sudo, Y. Imada, Electron. Lett. 34 (1998) 1016. [3] Y. Wang, L. Phillips, C. Yelleswarapu, T. Geroge, A. Sharma, S. Burgett, P. Ruffin, Opt. Commun. 163 (1999) 185. [4] P.J. Lemaire, R.M. Atkins, V. Mizrahi, K.L. Walker, K.S. Kranz, W.A. Reed, Electron. Lett. 29 (1993) 1191. [5] M. Takahashi, H. Shigemura, Y. Yakamoto, J. Nishii, T. Yoko, J. Non-Cryst. Solids 259 (1999) 149. [6] Y. Inoue, A. Kaneko, F. Hanawa, H. Takahashi, K. Hattori, S. Sumida, Electron. Lett. 33 (1997) 1945. [7] D. Bosc, B. Loisel, M. Moisan, N. Devoldere, F. Legall, A. Rolland, Electron. Lett. 33 (1997) 134. [8] Y. Kokubun, S. Yoneda, S. Matsuura, Electron. Lett. 34 (1998) 36. [9] A. Sakamoto, T. Matano, H. Takeuchi, IEICE Trans. Electron. E83-C (2000) 1441.
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[10] K. Shima, K. Himeno, T. Sasaki, S. Okude, A. Wada, R. Yamauchi, in: Tech. Dig. of Optical Fiber Communication Conference, 1997, p. 347. [11] J. Nishii, K. Kintaka, H. Hosono, H. Kawazoe, M. Kato, K. Muta, Phys. Rev. B 60 (1999) 7166. [12] H. Hosono, Y. Abe, D.L. Kinser, R.A. Weeks, K. Muta, H. Kawazoe, Phys. Rev. B 46 (1992) 11445. [13] R. Kashyap, in: P.L. Kelly, I. Kaminow, G. Agrawal (Eds.), Fiber Bragg Gratings, Academic Press, 1998, p. 40. [14] M. Takahashi, K. Ichii, Y. Tokuda, T. Uchino, T. Yoko, J. Nishii, T. Fujiwara, J. Appl. Phys. 92 (2002) 3442. [15] R. Regener, W. Sohler, Appl. Phys. B 36 (1985) 143. [16] S.R. Baker, H.N. Rourke, V. Baker, D. Goodchild, J. Lightwave Technol. 15 (1997) 1470. [17] J. Nishii, K. Kintaka, H. Nishiyama, T. Sano, E. Ohmura, I. Miyamoto, Appl. Phys. Lett. 81 (2002) 2346.
Photosensitive and athermal glasses for optical channel waveguides Junji Nishii a
a,*
, Kenji Kintaka a, Hiroaki Nishiyama b, Masahide Takahashi
c
Photonics Research Institute, AIST Kansai, 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan b Department of Manufacturing Science, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan c Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan
Abstract Ge–SiO2 thin glass films photosensitive to excimer laser irradiation were fabricated by plasma enhanced chemical vapor deposition. A channel waveguide was successfully fabricated by irradiation with only an excimer laser through a Cr mask pattern, which was previously coated on the slab-waveguide by the sputtering method. Bragg gratings with high diffraction efficiency were also fabricated in the waveguide by another KrF laser irradiation through the phase mask without hydrogen loading. Channel waveguides with Bragg gratings, on the other hand, were fabricated on the glass ceramic substrates with negative thermal expansion coefficient. Doping of the B2 O3 to Ge–SiO2 glass film effectively suppressed the temperature drift of the stop band (dk=dT ) of the gratings. The dk=dT attained in this study was 4 pm/C, which was less than one-third of those reported for commercially available waveguide gratings. Although the grating printed in the Ge–B–SiO2 film was almost erased by annealing at a temperature lower than 500 C, a new type of grating with much higher diffraction efficiency than that before annealing was formed after annealing at 600 C. The diffraction efficiency of the new grating was unchanged after repeated heating between room temperature and 600 C. 2003 Elsevier B.V. All rights reserved. PACS: 42.65.H; 78.40.F; 42.70.C; 42.88
1. Introduction Access networks require low cost and highly reliable optical devices. Formation of Bragg gratings in the waveguide is especially important technology for the development of optical devices applicable to wavelength division multiplexing
*
Corresponding author. Tel.: +81-727 51 9543; fax: +81-727 51 9637. E-mail address: [email protected] (J. Nishii).
(WDM) optical networks. This paper reports our recent three works on the fabrication of optical waveguide devices using highly photosensitive thin glass films deposited by the plasma enhanced chemical vapor deposition (PECVD) method. The first objective of our study is the fabrication of channel waveguides by irradiation only with excimer laser. So far, a complex photolithography process is required to obtain the channel waveguide structures. Thus the development of highly photosensitive thin glass films enables us to write a channel waveguide, leading to an effective cost
0022-3093/$ - see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0022-3093(03)00454-X
J. Nishii et al. / Journal of Non-Crystalline Solids 326&327 (2003) 464–471
reduction. Boron, lead, or tin ions were reported as effective sensitizers for magnifying the photon induced refractive index changes of the core materials such as Ge–SiO2 glass [1–3]. A high pressure H2 (or D2 ) treatment of the Ge–SiO2 glasses, the so-called H2 - (or D2 -) loading, is an effective method to increase its photosensitivity [4]. A refractive index change of 103 order was confirmed by the H2 loading process followed by irradiation with KrF excimer laser. Actually, the direct writing of the channel waveguide was demonstrated using a Ge–SiO2 thin film treated under an ultra-high pressure D2 atmosphere. A large amount of hydroxyl groups was, however, produced in the glass by irradiation with excimer laser, resulting in a large increase of optical attenuation at around the 1.4-lm wavelength. Therefore, a highly photosensitive glass, which does not require H2 -(D2 -) loading, must be developed in order to realize a simple fabrication of channel waveguides. Recently, it was reported that the Ge2þ center, which is a dominant oxygen-deficient center in Ge–SiO2 glasses, is the most active species among the defects in the glass produced by ultraviolet (UV) laser irradiation [5]. In this study the PECVD process was chosen to deposit Ge–SiO2 thin glass films containing sufficient amount of Ge2þ centers, and the channel waveguide was successfully fabricated by irradiation only with excimer laser through a Cr mask pattern, which was previously coated on the slabwaveguide by the sputtering method. Optical characteristics of Bragg gratings formed in waveguides strongly depend on temperature fluctuation because of the thermal refractive index change and the thermal expansion of the devices [6]. Therefore, the use of an athermalization technique is usually required for their practical use, which dose not seriously affect the long-term reliability and the cost of the devices. It was reported that polymer cladding could reduce the temperature dependence of the effective refractive index [7,8]. This technique, however, is rather complicated and less reliable because of the poor thermal stability of the polymer. Thus the second objective in this study is how to minimize the temperature drift of the diffraction wavelength of the grating without using polymer or electricity. In this study we fabricated a channel waveguide based on
465
Ge–B–SiO2 glasses on a crystallized glass substrate with negative thermal expansion coefficient in order to compensate the positive expansion of the waveguide layer. The positive temperature dependence of the refractive index (dn=dT ) of the Ge– SiO2 glass could be effectively reduced by the co-doping with boron, because B2 O3 has the negative derivative dn=dT [9,10]. In the final section, a unique thermo-optical property of Ge–B–SiO2 thin glass film is described. The Bragg grating printed by excimer laser irradiation in the film was not erased after the annealing at 600 C, which is a completely new phenomenon, because the usual gratings written in oxide glasses by irradiation with UV laser do disappear after annealing at around 500 C. The origin of the formation of such thermally stabilized grating is discussed.
2. Experimental Photosensitive thin glass films were deposited by a PECVD system (Samco International, Inc., PD-10C), which is a conventional system with parallel-plate electrodes. The raw materials used were Si(OC2 H5 )4 , Ge(OCH3 )4 , and B(OC2 H5 )3 for SiO2 , GeO2 , and B2 O3 , respectively, which were heated at 80 C. The vapor pressures of these materials were similar at this temperature. The vapors of raw materials were introduced into the chamber by its own vapor pressure and decomposed to oxides in an O2 plasma enhanced at a radio frequency of 13.56 MHz and 250 W. The operating pressure and substrate temperature were 0.4 Torr and 400 C, respectively. The composition of the films, which was controlled precisely by changing the flow ratio of raw materials, was analyzed by an electron probe microanalysis.
3. Results and discussion 3.1. Direct formation of channel waveguide by excimer laser irradiation The composition of the film used was 30GeO2 – 70SiO2 in mol%. Metricon model 2010 prism
J. Nishii et al. / Journal of Non-Crystalline Solids 326&327 (2003) 464–471
25
(b)x1000 Refractive index change(x10-3)
Absorption Coefficient(103 cm-1)
466
20
(a) 15 10 5 0 4.0
4.5
5.0
5 .5
(b) 1.0
0.5
(a)
6.0
Photon Energy (eV) Fig. 1. Absorption spectra of (a) 30GeO2 –70SiO2 film deposited by PECVD and (b) 15GeO2 –85SiO2 film prepared by VAD.
coupler was used for the measurement of refractive index, film thickness, and transmission loss of the film. UV light sources were XeF (k ¼ 351 nm, E ¼ 3:5 eV) and KrF (k ¼ 248 nm, E ¼ 5:0 eV) excimer lasers (Lambda Physik COMPex100). The irradiation power was set to 50 mJ/cm2 per pulse at 10 Hz for both excimer lasers. Irradiation was carried out at room temperature without H2 loading. Fig. 1 shows the optical absorption spectra of the film and an optical fiber preform prepared by the axial vapor deposition method (Shinetsu Chemical). The chemical composition was 15GeO2 –85SiO2 in mol% at the core region, which is a one-half of GeO2 content in the film. The intense absorption band at the photon energy of 5 eV is attributed to the oxygen-deficient defects related to Ge ions [11]. Two possible distinct oxygen-deficient defects yielded the 5-eV band: a neutral oxygen monovacancy (BGe–Ge(or Si)B) and a neutral oxygen divacancy (Ge2þ ) associated with Ge ions [12]. It was reported that the optical absorption of Ge2þ defects is observed at 5.16 eV, while the peak due to BGe–Ge(or Si)B is located at 5.07 eV. The absorption peak for the thin film in the figure is observed at 5.16 eV. Thus, the majority of the defects in the film is Ge2þ . It is apparent that the film exhibits an absorption coefficient at the 5-eV absorption band of three
0
0
2
4
6
8
10
Cumulative dose(kJ/cm2) Fig. 2. Induced refractive index changes in 30GeO2 –70SiO2 thin films after irradiation with (a) XeF and (b) KrF lasers.
orders of magnitude larger than that of the VAD fiber preform, which means the concentration of Ge2þ is approximately 103 times higher than that in the fiber preform. Fig. 2 shows the photorefractive index change of the 30GeO2 –70SiO2 film on the silica substrate after irradiation with XeF or KrF laser. Here the original refractive index and thickness of the film used were 1.5098 at 633 nm and 8.7 lm, respectively. The positive index changes of 0.3 · 103 and 1.1 · 103 were induced by irradiation with XeF and KrF lasers, respectively, which were generally one or two orders of magnitude higher than that of a commercially available fiber [13]. The maximum value of the index change was not depending on the Ge content but on the intensity of 5-eV absorption band. The intensity of photoluminescence caused by Ge2þ defects under the 248 nm excitation was reduced to 10% of the initial intensity after irradiation with KrF laser of 3 kJ/cm2 in total dose, which indicates that 90% of Ge2þ was bleached by the irradiation. Therefore the existence of Ge2þ in the glass matrix is closely related to the photo-induced refractive index change [14]. Our highly photosensitive films exhibited the optical attenuation loss of 0.2 dB/cm at 1553 nm by the prism coupling method, which is considered to be an acceptable loss level for the channel waveguide. The channel waveguide was formed
J. Nishii et al. / Journal of Non-Crystalline Solids 326&327 (2003) 464–471
only by irradiation with KrF laser through a Cr mask pattern, which was previously coated on the slab-waveguide by the sputtering method. The fabrication procedure is shown in Fig. 3. The dimension of the core was designed to be a square of 8 · 8 lm2 . The optical characterization of the waveguides obtained was carried out at around 1550 nm wavelength using a super luminescent diode (SLD) or a tunable laser diode (LD). The incident light was butt-coupled into the waveguide by using a commercially available single-mode fiber with a numerical aperture of 0.3. Fig. 4 shows the near-field pattern of the channel waveguide
Slab waveguide of Photosensitive thin film
467
Fig. 4. Near-field pattern of output beam at 1550 nm for channel waveguide with patterned Cr thin film.
obtained after irradiation with KrF laser of 50 mJ/ cm2 and 6000 shots. The numerical apertures (NA) parallel and perpendicular to the substrate were estimated to be 0.14 and 0.38, respectively. The side peaks of the intensity profile of the outgoing beam were considered to be the leak mode of incident light in the cladding layer. The intensity of such mode was relatively high around the next neighbor core. Under best coupling conditions, the side peaks can be eliminated. Furthermore, a Bragg grating was successfully fabricated in the core of this waveguide by irradiation with KrF laser through the phase mask. Fig. 5 shows the
Cr & resist coating
Resist patterning & Cr etching excimer laser
Transmission loss(dB)
0
10
20
1542
Irradiation with excimer laser Fig. 3. Fabrication process of channel waveguide with the overcoating of Cr thin film.
1544 1546 Wavelength (nm)
1548
Fig. 5. Transmission spectrum of channel waveguide with Bragg grating fabricated by irradiation only with KrF laser.
J. Nishii et al. / Journal of Non-Crystalline Solids 326&327 (2003) 464–471
transmission spectra of the waveguide with a Bragg grating. A stop band deeper than 25 dB in depth and 0.2 nm in half width was confirmed. Although slight divergence of propagated beam was observed, the channel waveguide still remained after the Cr mask was removed. 3.2. Athermalization of channel waveguide Channel waveguides with Bragg gratings with athermal characteristics were fabricated by PECVD and a conventional photolithography technique. A Ge–B–SiO2 system was used for the waveguide. The core size was about 5 lm · 5 lm, and the refractive index difference between the core and the cladding layer was about 1%. The overcladding and undercladding layers were each 15lm thick. The waveguide length was about 20 mm. The substrates for the waveguide were Si, silica, and crystallized multicomponent glass with thermal expansion coefficients of 4.15 · 106 , 0.4 · 106 , and )2.0 · 106 (C1 ), respectively. The thickness of these substrates was approximately 1 mm. The propagation loss of the waveguide was measured by the Fabry–Perot method [15] using a tunable LD and an optical spectrum analyzer, which was estimated to be 1–2 dB/cm from the power variations of the output light in the wavelength. The Bragg grating was formed by the method described in the previous section. The number of irradiated pulses and the optical energy density were 3000 shots and 60 mJ/cm2 , respectively. The transmission spectra of the waveguide with the Bragg grating were measured for the TE-like mode. The diffraction peak was observed at around 1530 nm. The temperature dependence of the Bragg wavelength (dk=dT ) was measured at temperatures from 10 to 70 C by placing the waveguide on a Peltier module. Fig. 6 shows the peak shifts of the Bragg wavelengths against temperature. For comparison, the temperature dependence of the Bragg wavelength for a sample with a 10GeO2 –90SiO2 core, SiO2 cladding layer, and Si substrate, which was a conventional-type waveguide with the Bragg grating, is also shown in the figure. The dk=dT in this temperature range was estimated from the slope of the lines. The
0.8 (b)
Wavelength shift(nm)
468
0.6
0.4 (a)
0.2
0 0
20
40
60
80
Temperature (°C) Fig. 6. Bragg wavelength shift against temperature for TE-like mode of (a) 6GeO2 –13B2 O3 –81SiO2 core and 12B2 O3 –88SiO2 cladding waveguide on crystallized glass substrate and (b) 10GeO2 –90SiO2 core and SiO2 cladding waveguide on Si substrate.
dk=dT of the conventional-type waveguide Bragg grating was 11 pm/C. In contrast, the dk=dT of the grating formed in the waveguide with a 6GeO2 –13B2 O3 –81SiO2 core and 12B2 O3 –88SiO2 cladding was 4 pm/C, which is, to our knowledge, the smallest value ever reported for silica-based waveguide Bragg gratings. The experimental results for the temperature dependence of the diffraction wavelength were compared with the theoretical temperature dependence of the diffraction wavelength. The Bragg wavelength k is expressed as k ¼ 2Neff K, where Neff is the effective refractive index of the waveguide and K the grating period. The temperature dependence of the Bragg wavelength (dk=dT ) is given by dk=dT ¼ 2KðdNeff =dT þ Neff aÞ, where a is the thermal expansion coefficient. As for a of the waveguide, we used the value of the substrate but not that of the waveguide because the thickness of the waveguide is much thinner than the substrate. In the estimation, we assumed the values of a, n, and dn=dT of the waveguide layers were additively contributed by each glass component. In addition, the dn=dT of the core was regarded as the dNeff =dT of the waveguide, because the value of dn=dT of the core was comparable with that of the cladding layer in the fabricated devices. Table 1 lists the estimated and measured values of dk=dT in waveguides with several compositions and
J. Nishii et al. / Journal of Non-Crystalline Solids 326&327 (2003) 464–471
469
Table 1 Estimated and measured values of temperature dependence of Bragg wavelength Composition of core
Refractive index at 588 nm
Index change against temperature (·106 )
Expansion coefficient (·106 /C)
Wavelength shift of diffraction peak (pm/C) Estimated
Measured
10GeO2 –90SiO2 10GeO2 –90SiO2 12GeO2 –6B2 O3 –82SiO2 14GeO2 –12B2 O3 –74SiO2 6GeO2 –13B2 O3 –81SiO2
1.462 1.462 1.467 1.470 1.450
11 11 9.3 7.7 6.8
4.15 (Si) )2.0 (crystallized glass) 0.4 (SiO2 ) )2.0 (crystallized glass) )2.0 (crystallized glass)
18 8 10 5 4
11 7 10 5 4
3.3. Thermally stabilized Bragg gratings formed in Ge–B–SiO2 by irradiation with KrF laser Bragg gratings formed in Ge–B–SiO2 thin films exhibited an interesting phenomenon during the thermal annealing [17]. The gratings almost disappeared upon annealing at around 500 C, but the diffraction efficiency of the grating drastically enhanced again after the annealing at 600 C. Consequently the extremely stable grating against heat-treatment at a temperature up to 600 C could be newly developed. Bragg gratings were formed in the film by irradiation with KrF excimer laser light of 248 nm wavelength through the phase mask with 1060 nm pitch. The optical energy density, shot number and repetition rate were 80 mJ/cm2 , 2.7 · 104 shots and 10 Hz, respectively. The first order diffraction efficiencies (g: ratio of diffracted power to incident power) of the gratings printed in the films were measured at a wavelength of 633 nm, which was 0.01%. Littrow mounting set up was used for the measurement. The grating pitch estimated from the incident and diffracted angles precisely agreed with the half of the mask pitch. Fig. 7 shows the changes in g of these gratings after the isochronal annealing for 1 h at each temperature in nitrogen atmosphere. The diffraction efficiencies decreased gradually after the annealing at a temperature up to 500 C, which
0.03
Diffraction Efficiency(%)
substrates. The experimental values of dk=dT agreed fairly well with the theoretically estimated values. Further reduction of the Bragg wavelength shift with temperature would be realized by optimizing waveguide compositions and using a crystallized glass substrate with a larger negative thermal expansion coefficient.
0.02
0.01
(b)
(a)
0
0
200
400
600
Annealing Temperature (°C) Fig. 7. Changes in diffraction efficiencies of gratings formed in (a) Ge–SiO2 and (b) Ge–B–SiO2 thin films by annealing in a nitrogen atmosphere. The annealing was carried out for 1 h at each temperature.
was similar in tendency to the previously reported result [16]. However there was a drastic increase in g after the annealing at 600 C. This precipitous increase in g is extremely exceptional, compared with the current photo-induced gratings in Gedoped silica-based glasses. The grating could not be erased unless the thin film was heat-treated repeatedly between room temperature and 600 C. Therefore, this thermally stabilized grating should be applicable to several optical devices that require reliable thermal stability. It is generally understood that the trigger responsible for the photo-induced refractive index change in Ge–SiO2 and Ge–B–SiO2 is the absorption of UV light by the oxygen-deficient defects in the glass matrix. The absorption coefficient of Ge– B–SiO2 at 248 nm wavelength was approximately
470
J. Nishii et al. / Journal of Non-Crystalline Solids 326&327 (2003) 464–471
1.5 times larger than that of Ge–SiO2 . Thus the laser energy was absorbed effectively, resulting in the formation of color centers with intense absorption coefficient and the densification of glass matrix, which should be closely related to the initial formation of gratings by irradiation with laser. However the remarkable increase in g could not be explained using such an earlier model. Then the isochronal annealing was carried out after the irradiation with KrF excimer laser without phase mask. The sample was irradiated with homogeneous laser pulses consisting of 2.7 · 104 shots and energy density of 48 mJ/cm2 . The annealing condition was identical to that used for the grating preparation. Fig. 8 shows the changes in refractive indices of irradiated and unirradiated areas of Ge– B–SiO2 after the annealing. The indices of both areas decreased monotonically after annealing up to 500 C, and then increased suddenly upon annealing at 600 C. Furthermore the absolute value of the refractive index of the unirradiated area was higher than that of the irradiated area after the annealing at 600 C. Thus the laser irradiation prior to annealing suppressed the increase in the thermally induced refractive index. After confirming the reproducibility of the data, we conclude that the reversed phenomenon of the refractive index caused by the annealing at 600 C should be related to the drastic increase in the g of the grating.
Refractive index at 633nm
1.484
1.483
(a) 1.482
1.481
(b) 1.480
1.479
0
200
400
4. Conclusion (1) Ge–SiO2 thin films deposited by the PECVD method exhibited the index change larger than 103 order of magnitude after irradiation with a KrF excimer laser. A channel waveguide was successfully fabricated only by irradiation through the Cr mask, which was previously coated and patterned on the slab-waveguide. Furthermore, the Bragg grating with a stop band above 25 dB was formed in the core of the waveguide by irradiation with KrF laser through the phase mask. It is, therefore, proposed that the photosensitive glass is useful for the preparation of waveguide and grating devices without using the conventional photolithography process. (2) Ge–B–SiO2 waveguides with Bragg gratings were fabricated on the crystallized glass substrates in order to reduce the temperature drift of the diffraction peak. The shift of the diffraction peak against temperature (dk=dT ) was minimized to 4 pm/C when the waveguide was constructed using a 6GeO2 –13B2 O3 – 81SiO2 core and 12B2 O3 –88SiO2 cladding layer on a crystallized glass substrate with a thermal expansion coefficient of )2.0 · 106 . The attained dk=dT was one-third of that of the grating formed in a conventional Ge–SiO2 waveguide on the Si substrate. (3) Bragg gratings printed in Ge–B–SiO2 thin films by irradiation with a KrF excimer laser through the phase mask erased after the annealing at temperatures up to 500 C, but another type of grating was formed after the annealing at 600 C. This grating could not be erased by the repeated heat-treatment between room temperature and 600 C. We expected that the refractive index modulation related to the grating might be reversed during the annealing at high temperature.
600
Annealing Temperature (°C) Fig. 8. Changes in refractive indices of (a) irradiated and (b) unirradiated areas of the Ge–B–SiO2 thin film.
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