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Study of UV-written channels in lead silicate glasses
Journal of Non-Crystalline Solids 291 (2001) 113±120
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Study of UV-written channels in lead silicate glasses C. Contardi *, E.R. Taylor, A. Fu Optoelectronics Research Centre, University of Southampton, SO17 1BJ, UK Received 4 December 2000
Abstract Light-induced refractive index change was investigated in two lead-silicate (SF57 and F2) glasses by irradiation using a pulsed 248 nm KrF excimer laser and a CW frequency doubled argon ion laser at 244 nm. UV±Vis spectra were recorded after each exposure to the excimer laser to study the change in absorption. The exposures were cumulative. The results showed that, with increasing number of pulses, there was an increase in the attenuation of the samples. Large photosensitivity was observed from direct 244 nm UV-written channels without the use of a phase mask. The refractive index change measured at 633 nm for SF57 glass was to 2:9 10 2 and a channel waveguide loss of 4.8 dB/cm and for F2 glass was to 7 10 3 and a loss of 5 dB/cm. The large refractive index changes measured here are achieved at the expense of large induced losses. The channel waveguides were thermally stable up to 300 °C. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 42.70.Ce; 61.43.Fs; 78.20.)e
1. Introduction Photosensitivity is the permanent change in refractive index following exposure to optical radiation. It is exploited in the fabrication of gratings in waveguides and therefore in the many applications of gratings in the opto-electronics industry. It has been investigated intensively in germanium (Ge) ion doped glasses [1,2] and recently in tin (Sn) ion doped glasses [3,4]. It has been shown that Sndoping has enhanced photosensitivity compared to Ge-doping. Together with Ge and Sn, lead (Pb) belongs to, and is the last element in, group IV-A
* Corresponding author. Present address: Optical Technologies Center, Via Reiss Romoli 274, 10148 Torino, Italy. Tel.: +39-011 2292 422; fax: +39-011 2292 434. E-mail address: [email protected] (C. Contardi).
of the periodic table. Pb ions are also added in several glasses to raise the refractive index of the glass, for example, it is added to the core glass composition in the fabrication of multicomponent glass ®bres [5] like Ge ions are in silica-based telecommunication ®bres. An investigation of the photosensitivity of lead is thus of interest in the light of the commercial need for stronger photosensitivity. Lead oxide in silicate glass has a strong absorption in the ultraviolet centred around 235 nm [6]. The UV band is broad and has a very large extinction coecient and unless materials can be prepared as thin samples, measurements made on bulk glasses record only absorption edges. As lead oxide increases, the absorption edge shifts towards longer wavelength [7]. Large photosensitivity has been reported in lead silicate glasses. Long and Brueck [8] studied the eect of the composition of lead in lead silicate
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glasses to see the dependence of the refractive index change in UV-written surface relief gratings. They also reported formation of permanent gratings written through a phase-mask by irradiation with a pulsed KrF excimer laser [9] and Mailis et al. [10] wrote gratings by exposure to 244 nm light through a phase mask on lead germanate glasses. In this report large photosensitivity in lead±silicate glasses has been observed in direct UV-written channels without the use of a phase mask. Schott optical glasses, F2 and SF57, have been investigated in this work representing low (PbO 43±47% wt) and high (PbO 72±76% wt) lead oxide content, respectively. The high PbO content SF57 glass represents the region of structural transformation from silicate glass forming network to the lead±oxygen one [7]. Both glasses were exposed to UV irradiation operating at 248 nm for eventual absorption measurement and at 244 nm for direct UV-writing of channels.
2. Experimental Table 1 lists the compositions of SF57 and F2. Samples of rectangular shapes of 13±18 mm in length were cut with a steel wheel. These were ground successively with alumina slurries of sizes 9 and 3 lm on a cast iron plate to provide the initial shaping process and also to obtain a thin sample. To remove the scratches, the samples were polished with Syton (0:125 lm silica particles) on a polyurethane plate and at the end they were thoroughly washed with solvents. Thin samples of thickness 160 lm were prepared for excimer KrF laser radiation at 248 nm and a corresponding thicker sample, with end faces polished, was irradiated using a CW frequency doubled argon ion laser at 244 nm. The absorption between 190 and 700 nm was recorded on a double-beam UV±Vis spectrophoTable 1 Composition of F2 and SF57 in wt% SF57 F2
SiO2
PbO
Na2 O
K2 O
As2 O3
22±26 44±48
72±76 43±47
0.5±1.5 3±4
0.5±1.5 4±6
Traces Traces
tometer before and after 248 nm KrF excimer irradiation. The samples were exposed to the UV excimer laser for 10, 100, 1000 and 10 000 pulses, at a ¯uence of 120 mJ=cm2 per pulse with repetition rate 10 Hz. The exposure from 10 to 10 000 pulses was cumulative. The spectra reported in dB/ cm versus wavelength plots are corrected for Fresnel re¯ection. The argon laser operating at 244 nm was used to write channel waveguides. The samples were positioned on a vacuum chuck connected to a computer controlled translation stage, which shifted perpendicularly to the incident UV laser beam at dierent speeds. Various speeds and ¯uences were used to evaluate the behaviour of the glass at the dierent conditions. Three series of exposures at powers of 20, 50 and 100 mW, were made with scan rates 3, 30, 200, 300 and 3000 mm/min. The beam focus incident on the glass surface was 10 3 lm. After direct UV-writing, the samples were observed under an optical microscope and alpha step pro®ling was performed to verify the observations obtained from the optical microscope. The channel waveguides were characterised for change in refractive index and loss. Refractive index changes in the channel waveguides were measured using a He±Ne laser light focused onto a standard monomode Telecom ®ber (N.A. 0.11 ± core/cladding 9=125 lm) via a 10 objective lens. An optical microscope was used to check the coupling of the ®ber with the waveguide. The refractive index was deduced from numerical aperture measurements using near-®eld and far-®eld imaging. The waveguide transmission losses were measured by launching the light of the He±Ne laser focused via 6 objective lens into the channel waveguides. A power meter measured the power coming out of the 6 objective lens, placed before the sample, and the light coming out of the channel. The loss measured included coupling loss. The spot size of the waveguides at 633 nm was measured by guiding the light of the He±Ne laser focused via 10 objective into the channel waveguide and collecting the output light in a CCD camera, which was connected to a computer to record the experimental results.
C. Contardi et al. / Journal of Non-Crystalline Solids 291 (2001) 113±120
3. Results 3.1. SF57 The absorption of SF57 in the range 190±700 nm is shown in Fig. 1. The results show that with increasing number of pulses there is an increase in the attenuation of the sample. After 1000 pulses an instrumental saturation in the signal was observed. This corresponds to large attenuation in Fig. 1, too large for accurate detection. Inspection of the sample revealed a brown spot that was visible in the region exposed. Another piece of glass was used to write channel waveguides with the CW 244 nm laser. A combination of powers and speeds were investigated. After direct UV-writing, the channels were checked under the optical microscope. Working at 20 mW, no visible change to the glass was created and this was con®rmed by alpha step pro®ling.
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When a HeNe laser was launched into the region exposed it was not possible to see any waveguide. With higher writing powers of 50 and 100 mW, channels were observed. A picture of four channel waveguides written at 50 mW is shown in Fig. 2. The corresponding alpha step pro®le is shown in Fig. 3. There are four peaks with dierent intensities. The presence of these peaks suggests a melting process occurred followed by an expansion process. The fast quenching of the melt as the laser traverses on preserves the expanded region (photothermal expansion). For a given power, the photothermal expansion is inversely proportional to the speed of writing; the slower the speed the larger the photo-thermal expansion. For very slow speeds, there is surface damage as evident in Fig. 3(a). Working at this low speed a white powder at the edge of the channels was also observed. In a separate experiment, where the glass was heated to melting, we observed that evolution of material
Fig. 1. Loss spectra (dB/cm) versus wavelength (nm) of SF 57 after KrF excimer laser exposures at 0, 10, 100, 1000 pulses, 10 Hz repetition rate and ¯uence 120 mJ=cm2 per pulse. Thickness sample is 186 lm.
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Fig. 2. Optical microscope picture of channel waveguides written at 50 mW and speeds 3 mm/min (a), 30 mm/min (b), 300 mm/min (c), 3000 mm/min (d) on SF57. Sample thickness is 5mm, sample length is 13 mm, distance between channels is 100 lm.
had an N.A. value of 0.10 at 633 nm, corresponding to a refractive index change of 2:9 10 2 . The waveguide loss measured with the HeNe laser was 4.8 dB/cm, the bulk loss of the glass at this wavelength is 0.5 dB/cm. The high loss measured can be partly attributed to induced losses in the glass on UV exposure, Fig. 1. The temperature stability of the channel waveguide in SF57 was investigated. A cumulative annealing process was carried out always on the same piece of glass to observe if heating will aect the waveguide. A sample of 18 mm length with four channels written under 50 mW and 300 mm/min conditions was heat treated. The sample was heated in steps at 100 °C then 200 °C, then 300 °C and 420 °C for 1 h at each temperature. After each treatment the change in the refractive index was measured, there was no change in the refractive index up to 300 °C. After the treatment at 420 °C, corresponding to the glass transition temperature, the channels disappeared and in its place the surface of the glass was visibly guiding (Fig. 5). No guiding was observed on the parallel unexposed face of the rectangular sample. At 420 °C, there appears to have been a redistribution of the optically photosensitive ion at dierent sites in the surface of the UV exposed glass. 3.2. F2
Fig. 3. Alpha-step showing four peaks obtained working at 50 mW and (a) 3 mm/min, (b) 30 mm/min, (c) 300 mm/min and (d) 3000 mm/min.
commenced at 700 °C and this material condensed to a white powder possibly PbO. This suggests that during the UV direct writing process at slow rates of 3 mm/min, the glass is heated to at least 700 °C. For high writing rates of 300 mm/min, the sample was free of visible cracks or white residue at the edge of the channels. The direct UV-writing experiments were repeated several times. The channel waveguides were characterised for change in refractive index and loss. The best result was obtained under the following condition: 50 mW and 300 mm/min, channel c of Fig. 2. Intensity pro®le measurements of the output from this channel showed a Gaussian pro®le in both x and y directions (Fig. 4). The waveguide
The absorption of F2 is shown in Fig. 6 for the bulk glass and for the same glass after exposure to the UV excimer laser at dierent pulses. In this case it was possible to observe that after 10 000 pulses there was no instrumental saturation even if a dark spot was visible due to exposure. Fig. 7 shows the induced losses (D loss) after 100 pulses for SF57 and for F2. There is no evident discolouration in either sample after exposure. The induced absorption is higher for SF57 than for F2, e.g. 46 and 5 dB/cm, respectively, at 600 nm. Direct UV-writing of a waveguide was not possible in F2 glass at 20 mW and 50 mW. However, at 100 mW of power, the same observations as Figs. 2 and 3 of SF57 were also true for F2. For this glass the best result obtained was working at 100 mW and 200 mm/min. For the F2 sample a N.A. value of 0.15 at 633 nm was measured
C. Contardi et al. / Journal of Non-Crystalline Solids 291 (2001) 113±120
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Fig. 4. Output of a channel waveguide written in SF57 glass with the following write conditions: source is Fred CW 244 nm, power is 50 mW focused to a 6 lm at the surface and speed is 300 mm/min.
Fig. 5. Surface of the glass after treatment at 420 °C. The channels have disappeared and the surface of the glass is visibly guiding.
corresponding to a refractive index change of 7 10 3 . The waveguide loss measured with the HeNe laser was 5 dB/cm and the bulk loss of the glass is 0.5 dB/cm. This loss again can be attributed to excess loss on UV irradiation. 4. Discussion Photosensitivity is observed by the change in permanent index on irradiation by UV or other sources and is described by the Lorentz±Lorenz
[11] relationship where the refractive index change results from the dierence between the change in electron polarisability and the change in volume expansion. In compound glasses, the change arising from volume expansion can be large due to a large coecient of linear expansion and the refractive index change is often negative. To ensure a positive index change, allowing directly written waveguides, and to control volume expansion, which introduces stress, it is important to use glass compositions with strong photosensitivity so that the change in electron polarisability exceeds the
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Fig. 6. Loss spectra (dB/cm) versus wavelength (nm) of F2 after KrF excimer laser exposures at 0, 10, 100, 1000, 10 000 pulses, 10 Hz repetition rate and ¯uence 120 mJ=cm2 per pulse.
change in volume expansion. This is the case for multicomponent germanosilicate glasses where the change in refractive index on UV exposure is positive [12]. This present work shows that with a high lead concentration, directly written UV channels can be achieved with low power and fast writing speed resulting in a large change in refractive index. This is in good agreement with Long and Brueck's work [8], in which they presented the dependence of the refractive index change as a function of the lead concentration. In decreasing the quantity of PbO, the authors observed a decrease in the refractive index change. Long and Brueck wrote surface relief gratings utilising the volume expansion on UV exposure. On polishing down the relief, they observed a two-order of magnitude change in the diraction eciency from 10 2 to 10 4 . In Fig. 2, we show that volume expansion also occurs during direct UV-writing. Furthermore, we observed that
if no thermal expansion occurs, no waveguiding is observed. In a multicomponent germanosilicate glass [12], we observed a positive guide with or without photo-thermal expansion. It appears that in the case of lead oxide in lead silicate glasses, absorption of the UV and subsequent heating of the glass to melting, where chemical bonds are made weaker, contributes to formation of the photosensitive centres. It is speculated that the breaking of bonds and an associated activation energy for the process creates the active centres responsible for the permanent photosensitivity. Attempts to polish down the photo-thermal expansion in the channel resulted in cracking and so we have no data on the position of the channel from the surface. However, heat treatment at the glass transition temperature, Fig. 5, con®rms that active photosensitive centres have been created by UV exposure. These new centres introduce an induced excess loss to the guide and for the lead
C. Contardi et al. / Journal of Non-Crystalline Solids 291 (2001) 113±120
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Fig. 7. Induced losses (D loss) after 100 pulses for SF57 and for F2 (broken line).
concentrations in F2 and SF57 glasses, the excess loss can be large, and increases with the lead concentration. Using the dispersion relation for lead silicate glass SF59 from [8], the estimated refractive index change at 1300 nm for SF57 is about 2:3 10 2 (0.80 of the value at 633 nm). Using the Kramers± Kronig relation [13,14] and assuming only a single absorption centre in the UV with a central wavelength of 235 nm [6] and an amplitude of 3535 cm 1 (75 wt% PbO) [15,16] and a width of 530 10 7 cm (derived from the induced loss at 633 nm) then the estimated refractive index change at 1300 nm is 2:6 10 2 . Precise measurements of the UV absorption of PbO in glasses is needed to apply Kramers±Kronig with certainty.
5. Conclusion Photosensitivity in materials allows us to write gratings and waveguides directly into substrates. A study of photosensitivity in compound glasses will open up new opportunities for extending the
functionalities now available in silica glass and ®bres into new materials for which silica is not an appropriate host glass. We have shown that commercial lead silicate glasses are photosensitive to UV irradiation. The photosensitivity is thermally stable up to the glass transition temperature. The large refractive index changes measured here are achieved at the expense of large induced losses. When lead ions are added to the core composition of a multicomponent glass ®bre to raise its refractive index, normally about 5 mol% of lead ion is added [5]. The induced loss will therefore not be as large as is reported here. Higher irradiation powers will however be needed. An approach to overcome this is to add a co-dopant with the lead ion such as cerium ion which has a strong UV absorption and which will aid in the local heating of the irradiated regions [17]. This work is underway. Acknowledgements The authors would like to acknowledge D.W.J. Harwood for his help with the waveguide measurements.
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References [1] J. Nishii, K. Fukumi, H. Yamanaka, K.I. Kawamura, H. Hosono, H. Kawazoe, Phys. Rev. B 52 (1995) 1661. [2] R.M. Atkins, V. Mizrahi, T. Erdogan, Electron. Lett. 29 (4) (1993) 385. [3] G. Brambilla, V. Pruneri, L. Reekie, Appl. Phys. Lett. 76 (2000) 807. [4] S. Radic, R.J. Essiambre, R. Boyd, P.A. Thick, N. Borrelli, Opt. Lett. 23 (22) (1998) 1730. [5] P.W. France, Fluoride Glass Optical Fibres Blackie, London, 1990, p. 108. [6] J.A. Duy, J. Non-Cryst. Solids 76 (1985) 391. [7] I.A. Vainshtein, A.F. Zatsepin, V.S. Kortov, V. Shchapova, Phys. Solid State 42 (2) (2000) 230. [8] X.-C. Long, S.R.J. Brueck, Opt. Lett. 24 (16) (1999) 1136. [9] X.-C. Long, S.R.J. Brueck, Appl. Phys. Lett. 74 (15) (1999) 2110.
[10] S. Mailis, A.A. Anderson, S.J. Barrington, W.S. Brocklesby, R. Greef, H.N. Rutt, R.W. Eason, N.A. Vainos, C. Grivas, Opt. Lett. 22 (23) (1998) 1751. [11] M.V. Bazylenko, D. Moss, J. Canning, Opt. Lett. 23 (9) (1998) 697. [12] D. Milanese, L.N. Ng, A. Fu, E R Taylor, C. Contardi, M. Ferraris, BGPP99 Stuart Florida, 23±25 September 1999, ThE31. [13] D.P. Hand, P. Russell, Opt. Lett. 15 (1990) 102. [14] V. Sulimov, V. Sokolov, in: Proceedings of the Summer School in Photosensitivity, Switzerland, July 1998. [15] D. Ehrt, M. Carl, T. Kittel, M. Muller, W. Seeber, J. NonCryst. Solids 177 (1994) 405. [16] D. Ehrt, J. Non-Cryst. Solids 196 (1996) 304. [17] D.W.J. Harwood, A. Fu, E.R. Taylor, R.C. Moore, Y.D. West, D.N. Payne, ECOC 2000 Munich, vol. 2, September 5 2000, pp. 191±192.
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Study of UV-written channels in lead silicate glasses C. Contardi *, E.R. Taylor, A. Fu Optoelectronics Research Centre, University of Southampton, SO17 1BJ, UK Received 4 December 2000
Abstract Light-induced refractive index change was investigated in two lead-silicate (SF57 and F2) glasses by irradiation using a pulsed 248 nm KrF excimer laser and a CW frequency doubled argon ion laser at 244 nm. UV±Vis spectra were recorded after each exposure to the excimer laser to study the change in absorption. The exposures were cumulative. The results showed that, with increasing number of pulses, there was an increase in the attenuation of the samples. Large photosensitivity was observed from direct 244 nm UV-written channels without the use of a phase mask. The refractive index change measured at 633 nm for SF57 glass was to 2:9 10 2 and a channel waveguide loss of 4.8 dB/cm and for F2 glass was to 7 10 3 and a loss of 5 dB/cm. The large refractive index changes measured here are achieved at the expense of large induced losses. The channel waveguides were thermally stable up to 300 °C. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 42.70.Ce; 61.43.Fs; 78.20.)e
1. Introduction Photosensitivity is the permanent change in refractive index following exposure to optical radiation. It is exploited in the fabrication of gratings in waveguides and therefore in the many applications of gratings in the opto-electronics industry. It has been investigated intensively in germanium (Ge) ion doped glasses [1,2] and recently in tin (Sn) ion doped glasses [3,4]. It has been shown that Sndoping has enhanced photosensitivity compared to Ge-doping. Together with Ge and Sn, lead (Pb) belongs to, and is the last element in, group IV-A
* Corresponding author. Present address: Optical Technologies Center, Via Reiss Romoli 274, 10148 Torino, Italy. Tel.: +39-011 2292 422; fax: +39-011 2292 434. E-mail address: [email protected] (C. Contardi).
of the periodic table. Pb ions are also added in several glasses to raise the refractive index of the glass, for example, it is added to the core glass composition in the fabrication of multicomponent glass ®bres [5] like Ge ions are in silica-based telecommunication ®bres. An investigation of the photosensitivity of lead is thus of interest in the light of the commercial need for stronger photosensitivity. Lead oxide in silicate glass has a strong absorption in the ultraviolet centred around 235 nm [6]. The UV band is broad and has a very large extinction coecient and unless materials can be prepared as thin samples, measurements made on bulk glasses record only absorption edges. As lead oxide increases, the absorption edge shifts towards longer wavelength [7]. Large photosensitivity has been reported in lead silicate glasses. Long and Brueck [8] studied the eect of the composition of lead in lead silicate
0022-3093/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 0 7 9 6 - 7
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glasses to see the dependence of the refractive index change in UV-written surface relief gratings. They also reported formation of permanent gratings written through a phase-mask by irradiation with a pulsed KrF excimer laser [9] and Mailis et al. [10] wrote gratings by exposure to 244 nm light through a phase mask on lead germanate glasses. In this report large photosensitivity in lead±silicate glasses has been observed in direct UV-written channels without the use of a phase mask. Schott optical glasses, F2 and SF57, have been investigated in this work representing low (PbO 43±47% wt) and high (PbO 72±76% wt) lead oxide content, respectively. The high PbO content SF57 glass represents the region of structural transformation from silicate glass forming network to the lead±oxygen one [7]. Both glasses were exposed to UV irradiation operating at 248 nm for eventual absorption measurement and at 244 nm for direct UV-writing of channels.
2. Experimental Table 1 lists the compositions of SF57 and F2. Samples of rectangular shapes of 13±18 mm in length were cut with a steel wheel. These were ground successively with alumina slurries of sizes 9 and 3 lm on a cast iron plate to provide the initial shaping process and also to obtain a thin sample. To remove the scratches, the samples were polished with Syton (0:125 lm silica particles) on a polyurethane plate and at the end they were thoroughly washed with solvents. Thin samples of thickness 160 lm were prepared for excimer KrF laser radiation at 248 nm and a corresponding thicker sample, with end faces polished, was irradiated using a CW frequency doubled argon ion laser at 244 nm. The absorption between 190 and 700 nm was recorded on a double-beam UV±Vis spectrophoTable 1 Composition of F2 and SF57 in wt% SF57 F2
SiO2
PbO
Na2 O
K2 O
As2 O3
22±26 44±48
72±76 43±47
0.5±1.5 3±4
0.5±1.5 4±6
Traces Traces
tometer before and after 248 nm KrF excimer irradiation. The samples were exposed to the UV excimer laser for 10, 100, 1000 and 10 000 pulses, at a ¯uence of 120 mJ=cm2 per pulse with repetition rate 10 Hz. The exposure from 10 to 10 000 pulses was cumulative. The spectra reported in dB/ cm versus wavelength plots are corrected for Fresnel re¯ection. The argon laser operating at 244 nm was used to write channel waveguides. The samples were positioned on a vacuum chuck connected to a computer controlled translation stage, which shifted perpendicularly to the incident UV laser beam at dierent speeds. Various speeds and ¯uences were used to evaluate the behaviour of the glass at the dierent conditions. Three series of exposures at powers of 20, 50 and 100 mW, were made with scan rates 3, 30, 200, 300 and 3000 mm/min. The beam focus incident on the glass surface was 10 3 lm. After direct UV-writing, the samples were observed under an optical microscope and alpha step pro®ling was performed to verify the observations obtained from the optical microscope. The channel waveguides were characterised for change in refractive index and loss. Refractive index changes in the channel waveguides were measured using a He±Ne laser light focused onto a standard monomode Telecom ®ber (N.A. 0.11 ± core/cladding 9=125 lm) via a 10 objective lens. An optical microscope was used to check the coupling of the ®ber with the waveguide. The refractive index was deduced from numerical aperture measurements using near-®eld and far-®eld imaging. The waveguide transmission losses were measured by launching the light of the He±Ne laser focused via 6 objective lens into the channel waveguides. A power meter measured the power coming out of the 6 objective lens, placed before the sample, and the light coming out of the channel. The loss measured included coupling loss. The spot size of the waveguides at 633 nm was measured by guiding the light of the He±Ne laser focused via 10 objective into the channel waveguide and collecting the output light in a CCD camera, which was connected to a computer to record the experimental results.
C. Contardi et al. / Journal of Non-Crystalline Solids 291 (2001) 113±120
3. Results 3.1. SF57 The absorption of SF57 in the range 190±700 nm is shown in Fig. 1. The results show that with increasing number of pulses there is an increase in the attenuation of the sample. After 1000 pulses an instrumental saturation in the signal was observed. This corresponds to large attenuation in Fig. 1, too large for accurate detection. Inspection of the sample revealed a brown spot that was visible in the region exposed. Another piece of glass was used to write channel waveguides with the CW 244 nm laser. A combination of powers and speeds were investigated. After direct UV-writing, the channels were checked under the optical microscope. Working at 20 mW, no visible change to the glass was created and this was con®rmed by alpha step pro®ling.
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When a HeNe laser was launched into the region exposed it was not possible to see any waveguide. With higher writing powers of 50 and 100 mW, channels were observed. A picture of four channel waveguides written at 50 mW is shown in Fig. 2. The corresponding alpha step pro®le is shown in Fig. 3. There are four peaks with dierent intensities. The presence of these peaks suggests a melting process occurred followed by an expansion process. The fast quenching of the melt as the laser traverses on preserves the expanded region (photothermal expansion). For a given power, the photothermal expansion is inversely proportional to the speed of writing; the slower the speed the larger the photo-thermal expansion. For very slow speeds, there is surface damage as evident in Fig. 3(a). Working at this low speed a white powder at the edge of the channels was also observed. In a separate experiment, where the glass was heated to melting, we observed that evolution of material
Fig. 1. Loss spectra (dB/cm) versus wavelength (nm) of SF 57 after KrF excimer laser exposures at 0, 10, 100, 1000 pulses, 10 Hz repetition rate and ¯uence 120 mJ=cm2 per pulse. Thickness sample is 186 lm.
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Fig. 2. Optical microscope picture of channel waveguides written at 50 mW and speeds 3 mm/min (a), 30 mm/min (b), 300 mm/min (c), 3000 mm/min (d) on SF57. Sample thickness is 5mm, sample length is 13 mm, distance between channels is 100 lm.
had an N.A. value of 0.10 at 633 nm, corresponding to a refractive index change of 2:9 10 2 . The waveguide loss measured with the HeNe laser was 4.8 dB/cm, the bulk loss of the glass at this wavelength is 0.5 dB/cm. The high loss measured can be partly attributed to induced losses in the glass on UV exposure, Fig. 1. The temperature stability of the channel waveguide in SF57 was investigated. A cumulative annealing process was carried out always on the same piece of glass to observe if heating will aect the waveguide. A sample of 18 mm length with four channels written under 50 mW and 300 mm/min conditions was heat treated. The sample was heated in steps at 100 °C then 200 °C, then 300 °C and 420 °C for 1 h at each temperature. After each treatment the change in the refractive index was measured, there was no change in the refractive index up to 300 °C. After the treatment at 420 °C, corresponding to the glass transition temperature, the channels disappeared and in its place the surface of the glass was visibly guiding (Fig. 5). No guiding was observed on the parallel unexposed face of the rectangular sample. At 420 °C, there appears to have been a redistribution of the optically photosensitive ion at dierent sites in the surface of the UV exposed glass. 3.2. F2
Fig. 3. Alpha-step showing four peaks obtained working at 50 mW and (a) 3 mm/min, (b) 30 mm/min, (c) 300 mm/min and (d) 3000 mm/min.
commenced at 700 °C and this material condensed to a white powder possibly PbO. This suggests that during the UV direct writing process at slow rates of 3 mm/min, the glass is heated to at least 700 °C. For high writing rates of 300 mm/min, the sample was free of visible cracks or white residue at the edge of the channels. The direct UV-writing experiments were repeated several times. The channel waveguides were characterised for change in refractive index and loss. The best result was obtained under the following condition: 50 mW and 300 mm/min, channel c of Fig. 2. Intensity pro®le measurements of the output from this channel showed a Gaussian pro®le in both x and y directions (Fig. 4). The waveguide
The absorption of F2 is shown in Fig. 6 for the bulk glass and for the same glass after exposure to the UV excimer laser at dierent pulses. In this case it was possible to observe that after 10 000 pulses there was no instrumental saturation even if a dark spot was visible due to exposure. Fig. 7 shows the induced losses (D loss) after 100 pulses for SF57 and for F2. There is no evident discolouration in either sample after exposure. The induced absorption is higher for SF57 than for F2, e.g. 46 and 5 dB/cm, respectively, at 600 nm. Direct UV-writing of a waveguide was not possible in F2 glass at 20 mW and 50 mW. However, at 100 mW of power, the same observations as Figs. 2 and 3 of SF57 were also true for F2. For this glass the best result obtained was working at 100 mW and 200 mm/min. For the F2 sample a N.A. value of 0.15 at 633 nm was measured
C. Contardi et al. / Journal of Non-Crystalline Solids 291 (2001) 113±120
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Fig. 4. Output of a channel waveguide written in SF57 glass with the following write conditions: source is Fred CW 244 nm, power is 50 mW focused to a 6 lm at the surface and speed is 300 mm/min.
Fig. 5. Surface of the glass after treatment at 420 °C. The channels have disappeared and the surface of the glass is visibly guiding.
corresponding to a refractive index change of 7 10 3 . The waveguide loss measured with the HeNe laser was 5 dB/cm and the bulk loss of the glass is 0.5 dB/cm. This loss again can be attributed to excess loss on UV irradiation. 4. Discussion Photosensitivity is observed by the change in permanent index on irradiation by UV or other sources and is described by the Lorentz±Lorenz
[11] relationship where the refractive index change results from the dierence between the change in electron polarisability and the change in volume expansion. In compound glasses, the change arising from volume expansion can be large due to a large coecient of linear expansion and the refractive index change is often negative. To ensure a positive index change, allowing directly written waveguides, and to control volume expansion, which introduces stress, it is important to use glass compositions with strong photosensitivity so that the change in electron polarisability exceeds the
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Fig. 6. Loss spectra (dB/cm) versus wavelength (nm) of F2 after KrF excimer laser exposures at 0, 10, 100, 1000, 10 000 pulses, 10 Hz repetition rate and ¯uence 120 mJ=cm2 per pulse.
change in volume expansion. This is the case for multicomponent germanosilicate glasses where the change in refractive index on UV exposure is positive [12]. This present work shows that with a high lead concentration, directly written UV channels can be achieved with low power and fast writing speed resulting in a large change in refractive index. This is in good agreement with Long and Brueck's work [8], in which they presented the dependence of the refractive index change as a function of the lead concentration. In decreasing the quantity of PbO, the authors observed a decrease in the refractive index change. Long and Brueck wrote surface relief gratings utilising the volume expansion on UV exposure. On polishing down the relief, they observed a two-order of magnitude change in the diraction eciency from 10 2 to 10 4 . In Fig. 2, we show that volume expansion also occurs during direct UV-writing. Furthermore, we observed that
if no thermal expansion occurs, no waveguiding is observed. In a multicomponent germanosilicate glass [12], we observed a positive guide with or without photo-thermal expansion. It appears that in the case of lead oxide in lead silicate glasses, absorption of the UV and subsequent heating of the glass to melting, where chemical bonds are made weaker, contributes to formation of the photosensitive centres. It is speculated that the breaking of bonds and an associated activation energy for the process creates the active centres responsible for the permanent photosensitivity. Attempts to polish down the photo-thermal expansion in the channel resulted in cracking and so we have no data on the position of the channel from the surface. However, heat treatment at the glass transition temperature, Fig. 5, con®rms that active photosensitive centres have been created by UV exposure. These new centres introduce an induced excess loss to the guide and for the lead
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Fig. 7. Induced losses (D loss) after 100 pulses for SF57 and for F2 (broken line).
concentrations in F2 and SF57 glasses, the excess loss can be large, and increases with the lead concentration. Using the dispersion relation for lead silicate glass SF59 from [8], the estimated refractive index change at 1300 nm for SF57 is about 2:3 10 2 (0.80 of the value at 633 nm). Using the Kramers± Kronig relation [13,14] and assuming only a single absorption centre in the UV with a central wavelength of 235 nm [6] and an amplitude of 3535 cm 1 (75 wt% PbO) [15,16] and a width of 530 10 7 cm (derived from the induced loss at 633 nm) then the estimated refractive index change at 1300 nm is 2:6 10 2 . Precise measurements of the UV absorption of PbO in glasses is needed to apply Kramers±Kronig with certainty.
5. Conclusion Photosensitivity in materials allows us to write gratings and waveguides directly into substrates. A study of photosensitivity in compound glasses will open up new opportunities for extending the
functionalities now available in silica glass and ®bres into new materials for which silica is not an appropriate host glass. We have shown that commercial lead silicate glasses are photosensitive to UV irradiation. The photosensitivity is thermally stable up to the glass transition temperature. The large refractive index changes measured here are achieved at the expense of large induced losses. When lead ions are added to the core composition of a multicomponent glass ®bre to raise its refractive index, normally about 5 mol% of lead ion is added [5]. The induced loss will therefore not be as large as is reported here. Higher irradiation powers will however be needed. An approach to overcome this is to add a co-dopant with the lead ion such as cerium ion which has a strong UV absorption and which will aid in the local heating of the irradiated regions [17]. This work is underway. Acknowledgements The authors would like to acknowledge D.W.J. Harwood for his help with the waveguide measurements.
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