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Mode profile modification of H+ ion beam irradiated waveguides using UV processing
Journal of Non-Crystalline Solids 239 (1998) 121±125
Mode pro®le modi®cation of H ion beam irradiated waveguides using UV processing M.L. von Bibra b
a,*,1 ,
J. Canning b, A. Roberts
a
a School of Physics, The University of Melbourne, Parkville, VIC 3052, Australia Australian Photonics Cooperative Research Centre, Optical Fibre Technology Centre, University of Sydney, 101 National Innovation Centre, Eveleigh, Sydney, NSW 1430, Australia
Abstract Buried channel waveguides made by a direct-write focussed hydrogen ion beam in high quality fused silica were investigated for mode and spectral changes under 193 nm ultra violet irradiation. The guides exhibited a negative index change of approximately 10% of the core cladding dierence after a dose of 1 kJ cmÿ2 , and an increase in attenuation at infrared wavelengths. This eect could have application in tailoring the shape of mode ®eld pro®les for ecient coupling between waveguides. Ó 1998 Elsevier Science B.V. All rights reserved.
1. Introduction In planar guide studies, the evidence [1] seems to indicate that the degree of photosensitivity is closely related to defect-induced losses such as increased Rayleigh scattering. For example, guides made using ¯ame hydrolysis deposition (FDM) techniques have very low propagation losses 0.1 dB cmÿ1 indicating a high level of glass purity [2]. Consequently, to achieve sucient photosensitivity to ultra violet (UV) irradiation, a glass has to be sensitised with hydrogen to achieve an adequate UV-induced index change [1]. On the other hand, Bragg gratings have been written in plasma enhanced chemical vapour deposition (PECVD) based glass [3] with no hydrogen assisted sensitisation. However, those gratings have larger propagation losses.
* Corresponding author. Tel.: +61-3 9344 5465; fax: +61-3 9347 4783; e-mail: [email protected]. 1 Supported by the Australian Research Council.
A more impressive example of photosensitivity has been the demonstration of direct writing waveguides with a mercury lamp source in organically derived glass made using sol±gel deposition techniques [4]. The technique of incorporating organic components into a silica matrix achieves an index change through polymerisation of the organic ends [4] which is a substantially dierent mechanism to that relying on oxygen de®cient centre absorptions. The result is a material with a larger photosensitivity and with 0.1±0.3 dB cmÿ1 propagation losses [4] at 1.55 lm. UV-photosensitivity has also been observed in ion implanted material, implanted with silicon and germanium ions [5,6]. Ion implantation induced index increases >0.01 have been reported with UVinduced refractive index photobleaching of about ÿ0.002 [5]. A similar eect has been seen with low energy (300 keV) hydrogen ions implanted into germanium doped glasses at 800 C [7]. In this paper we examine the properties, under 193 nm UV processing, of optical waveguides made with a direct-write focussed H ion microbeam
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 2 7 - 3
122
M.L. von Bibra et al. / Journal of Non-Crystalline Solids 239 (1998) 121±125
irradiated fused silica [8]. No germanium was added to the material. The rationale for previous ion implantation work [6] in fused silica (for UV processing) was to establish a large number of defects through bond breaking by heavy ions as they penetrate the material, creating oxygen de®cient centres. This defect production increases the UV absorption [9] which generates, through further bond breaking or rearrangement, new defect species [9] that should give rise to large negative index changes via the Kramers±Kr onig relations [6]. Defect formation by electronic excitation with 1.5 MeV H ions in silica has also been reported [10], although the primary mechanism for index increases under ion irradiation is understood to be nuclear interactions at the end of range of the ions [8]. Despite large observed index changes, no Bragg gratings have been reported raising the possibility that the index changes may not be localised to the irradiated area (on the scale of a micron). If correct, this delocalisation would have implications for the proposed mechanism. 2. Experiment To investigate the requirements for UV-photosensitivity in ion-implanted material, we decided to examine the changes upon irradiation, if any, to the mode pro®le and spectral properties of waveguides formed by implanting hydrogen ions into silica glass. High (2.0 MeV) energy H ions cause impact damage at the end of the range in the form of oxygen and silicon vacancies and causes glass compaction which increases the refractive index for waveguiding [8]. The number and density of defects produced depends on the ion mass, energy and dosage [11]. Some defects may be capable of undergoing UV photolysis leading to similar index changes as reported previously [5]. The magnitude of these changes may be less [5] since the substrate is free of oxygen-de®cient centres and the H bombardment will produce 102 fewer vacancies per ion than for heavier species at the same energy [11] because of the dierence in particle momentum. Further, the absence of germanium in the glass should result in less photosensitivity [5]. In practice
Fig. 1. Schematic diagram of the ion implanted silica waveguide geometry. The waveguides dier in their vertical (y) and horizontal (x) dimensions. The ion beam penetrates the substrate from above the surface.
a negative index change of about 10% of the corecladding dierence was observed, similar to the results reported for Si ions in silica [5]. The substrate chosen for this work was Suprasil I (Heraeus) glass which has negligible absorption in the UV 2. Waveguides were made using the method outlined in Ref. [8] by irradiating the sample with a direct-write H ion beam. The implanted ion dose was 1 1016 ions cmÿ2 which produces a measured index change of +0.003 [12]. The geometry of the waveguides is shown in Fig. 1. The guides under investigation are 2 mm long, approximately 5 lm thick in the vertical y direction, and 7 lm wide in the horizontal x direction. Irradiation with 2.5 MeV H ions causes the buried guides to be formed 50 lm below the substrate surface [8]. An erbium doped ®bre ampli®er was used as an unpolarised infrared source. Transverse electric (TE) and transverse magnetic (TM) polarisation states were selected using an in-line ®bre polariser providing greater than 25 dB contrast. The waveguides were irradiated with 193 nm light from an ArF laser whilst the near ®eld image was monitored. The TE and TM modes at a wavelength
2 Manufacturer's speci®cations, Heraeus Quarzglaz GmbH, `Quartz glass for optics'.
M.L. von Bibra et al. / Journal of Non-Crystalline Solids 239 (1998) 121±125
1.5 lm, were imaged with an infrared charge coupled device (CCD) camera. The mode ®eld measurments were repeatable for individual camera exposures and the apparatus was not disturbed during UV exposure. 3. Results The TE mode images of the waveguide taken before and after a cumulative ¯uence of about 1 kJ cmÿ2 are shown in Fig. 2. The near-circular pro-
Fig. 2. Contour mode images of the TE mode taken before (top) and after (bottom) UV irradiation.
123
®les indicate single mode propagation and there was no observable dierence between polarisation states despite the asymmetry of the waveguide cross-section. Fig. 3 shows a comparison of the TE mode cross-sections before and after UV treatment. The increase in mode pro®le width after UV irradiation was observed in all of the waveguides examined. Since there was an apparent increase in mode size, white light spectra were taken of a similar sample during UV irradiation. Fig. 4 shows the dierential spectra of the sample with increasing cumultive ¯uence (0.02±3.0 kJ cmÿ2 ). The induced attenuation measured is the dierence in attenuation between the unprocessed guide and subsequent processing. The small (5.0 nm) spectral resolution is due to the insertion loss of these particular samples.
Fig. 3. Horizontal and vertical TE mode pro®les taken before (solid line) and after (dotted line) UV irradiation.
124
M.L. von Bibra et al. / Journal of Non-Crystalline Solids 239 (1998) 121±125
pro®le observed in the sample. The mode ®eld intensity is of the form: 1 I ; g cos h 2x=h where
1 p g 1 m2 ÿ 1; 2 p m kh 2ns Dn;
Fig. 4. UV-induced dierential attenuation spectra of H ion irradiated silica waveguide after a UV dose of 0.2 kJ cmÿ2 (solid line) and 3.0 kJ cmÿ2 (dotted line). The attenuation increases signi®cantly in the UV.
4. Discussion The largest change in the mode pro®les of Fig. 3 is in the vertical direction. This dierence is a result of the waveguide geometry, since the vertical thickness of the guide is smaller than the horizontal dimension. The guide is closer to cuto in the vertical direction, and therefore the changes in the mode pro®le are larger in this direction. With UV ¯uence of this magnitude, this eect can be used to control the mode shape of the waveguide mode ®eld and produce a more circular shape to improve its coupling eciency to ®bres. It is evident in Fig. 4 that the attenuation in the infrared range above the cuto wavelength (which determines the cuto between single and multimode propagation) has increased. The more tightly con®ned multi-mode wavelengths below cuto show no change within measurement errors, although there is an induced absorption band at 780 nm. At this stage the origin of this band is unknown. Formation of OH or H species was not detected in the spectra despite the ion doses used in the fabrication step. From the changes in the mode cross-section it is possible to make estimates of the induced negative index change. An analytic planar waveguide model [13] with a 1= cos h2 2x=h refractive index pro®le, was selected since it most closely matches the mode
ns is the cladding index, h the guide width and x the position along the axis being analysed. Fitting the model to the two mode pro®les before and after UV treatment yielded a dierence in refractive index of approximately 10% of the core-cladding dierence. Given index changes exceeding 10% one application of this technique is to enhance mode matching between imperfect waveguides. An attempt was made to write Bragg gratings through a phase mask using a setup similar to that reported in Ref. [3] where gratings exceeding 35 dB were easily produced in PECVD glass. There was no evidence in the white light spectra to indicate a grating had been formed using this technique in spite of the fact that a negative index change occurred. From this null result we infer that the induced index changes occur on a macroscopic scale, triggered by localised UV-induced defects. To date, the ability to achieve a localised negative index change on the micron scale required for strong Bragg grating formation has only been reported for ®bre gratings where a so-called `type IIa' grating phenomena is observed [14]. Consequently, further work needs to be done to establish the reason we have been unable to produce such gratings in our material. 5. Conclusions In conclusion, we have demonstrated negative index changes of approximately 10% of the corecladding dierence in H implanted buried channel waveguides formed within UV-transparent glass containing no germanate species and with few defects present initially. These changes allow control of the waveguide mode shape for applications such as improved mode coupling.
M.L. von Bibra et al. / Journal of Non-Crystalline Solids 239 (1998) 121±125
References [1] G.D. Maxwell, B.J. Ainslie, Electron. Lett. 31 (1995) 95. [2] G.D. Maxwell, R. Kashyap, B.J. Ainslie, D.L. Williams, J.R. Armitage, Electron. Lett. 28 (1992) 2106. [3] J. Canning, D.J. Moss, M. Faith, P. Leech, P. Kemeny, C.V. Poulsen, O. Leistiko, Electron. Lett. 32 (1996) 1479. [4] P. Coudray, J. Chisam, M. Andrews, S. Naja®, Opt. Eng. 36 (1997) 1234. [5] J. Albert, B. Malo, K.O. Hill, D.C. Johnson, J.L. Brebner, R. Leonelli, Opt. Lett. 17 (1992) 1652. [6] M. Verhaegen, L.B. Allard, J.L. Brebner, M. Essid, S. Roorda, J. Albert, Nucl. Instr. and Meth. B 106 (1995) 438. [7] D.J. Kyle, B.L. Weiss, G.D. Maxwell, J. Appl. Phys. 77 (1995) 1207.
125
[8] A. Roberts, M.L. von Bibra, J. Lightwave Tech. LT±14 (1996) 2554. [9] M. Verhaegen, J.L. Brebner, L.B. Allard, J. Albert, Appl. Phys. Lett. 68 (1996) 3084. [10] H. Hosono, N. Ueda, H. Kawazoe, N. Matsunami, J. NonCryst. Solids 182 (1995) 109. [11] J.P. Biersack, L.G. Haggmark, Nucl. Instr. and Meth. 174 (1980) 257. [12] M.L. von Bibra, A. Roberts, J. Lightwave Tech. 15 (1997) 1695. [13] H. Kogelnik, in: T. Tamir (Ed.), Integrated Optics, Springer, Berlin, 1975, pp. 55±57. [14] W.X. Xie, P. Niay, P. Bernage, M. Douay, J.F. Bayon, T. Georges, M. Monerie, Opt. Commun. 104 (1993) 185.
Mode pro®le modi®cation of H ion beam irradiated waveguides using UV processing M.L. von Bibra b
a,*,1 ,
J. Canning b, A. Roberts
a
a School of Physics, The University of Melbourne, Parkville, VIC 3052, Australia Australian Photonics Cooperative Research Centre, Optical Fibre Technology Centre, University of Sydney, 101 National Innovation Centre, Eveleigh, Sydney, NSW 1430, Australia
Abstract Buried channel waveguides made by a direct-write focussed hydrogen ion beam in high quality fused silica were investigated for mode and spectral changes under 193 nm ultra violet irradiation. The guides exhibited a negative index change of approximately 10% of the core cladding dierence after a dose of 1 kJ cmÿ2 , and an increase in attenuation at infrared wavelengths. This eect could have application in tailoring the shape of mode ®eld pro®les for ecient coupling between waveguides. Ó 1998 Elsevier Science B.V. All rights reserved.
1. Introduction In planar guide studies, the evidence [1] seems to indicate that the degree of photosensitivity is closely related to defect-induced losses such as increased Rayleigh scattering. For example, guides made using ¯ame hydrolysis deposition (FDM) techniques have very low propagation losses 0.1 dB cmÿ1 indicating a high level of glass purity [2]. Consequently, to achieve sucient photosensitivity to ultra violet (UV) irradiation, a glass has to be sensitised with hydrogen to achieve an adequate UV-induced index change [1]. On the other hand, Bragg gratings have been written in plasma enhanced chemical vapour deposition (PECVD) based glass [3] with no hydrogen assisted sensitisation. However, those gratings have larger propagation losses.
* Corresponding author. Tel.: +61-3 9344 5465; fax: +61-3 9347 4783; e-mail: [email protected]. 1 Supported by the Australian Research Council.
A more impressive example of photosensitivity has been the demonstration of direct writing waveguides with a mercury lamp source in organically derived glass made using sol±gel deposition techniques [4]. The technique of incorporating organic components into a silica matrix achieves an index change through polymerisation of the organic ends [4] which is a substantially dierent mechanism to that relying on oxygen de®cient centre absorptions. The result is a material with a larger photosensitivity and with 0.1±0.3 dB cmÿ1 propagation losses [4] at 1.55 lm. UV-photosensitivity has also been observed in ion implanted material, implanted with silicon and germanium ions [5,6]. Ion implantation induced index increases >0.01 have been reported with UVinduced refractive index photobleaching of about ÿ0.002 [5]. A similar eect has been seen with low energy (300 keV) hydrogen ions implanted into germanium doped glasses at 800 C [7]. In this paper we examine the properties, under 193 nm UV processing, of optical waveguides made with a direct-write focussed H ion microbeam
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 2 7 - 3
122
M.L. von Bibra et al. / Journal of Non-Crystalline Solids 239 (1998) 121±125
irradiated fused silica [8]. No germanium was added to the material. The rationale for previous ion implantation work [6] in fused silica (for UV processing) was to establish a large number of defects through bond breaking by heavy ions as they penetrate the material, creating oxygen de®cient centres. This defect production increases the UV absorption [9] which generates, through further bond breaking or rearrangement, new defect species [9] that should give rise to large negative index changes via the Kramers±Kr onig relations [6]. Defect formation by electronic excitation with 1.5 MeV H ions in silica has also been reported [10], although the primary mechanism for index increases under ion irradiation is understood to be nuclear interactions at the end of range of the ions [8]. Despite large observed index changes, no Bragg gratings have been reported raising the possibility that the index changes may not be localised to the irradiated area (on the scale of a micron). If correct, this delocalisation would have implications for the proposed mechanism. 2. Experiment To investigate the requirements for UV-photosensitivity in ion-implanted material, we decided to examine the changes upon irradiation, if any, to the mode pro®le and spectral properties of waveguides formed by implanting hydrogen ions into silica glass. High (2.0 MeV) energy H ions cause impact damage at the end of the range in the form of oxygen and silicon vacancies and causes glass compaction which increases the refractive index for waveguiding [8]. The number and density of defects produced depends on the ion mass, energy and dosage [11]. Some defects may be capable of undergoing UV photolysis leading to similar index changes as reported previously [5]. The magnitude of these changes may be less [5] since the substrate is free of oxygen-de®cient centres and the H bombardment will produce 102 fewer vacancies per ion than for heavier species at the same energy [11] because of the dierence in particle momentum. Further, the absence of germanium in the glass should result in less photosensitivity [5]. In practice
Fig. 1. Schematic diagram of the ion implanted silica waveguide geometry. The waveguides dier in their vertical (y) and horizontal (x) dimensions. The ion beam penetrates the substrate from above the surface.
a negative index change of about 10% of the corecladding dierence was observed, similar to the results reported for Si ions in silica [5]. The substrate chosen for this work was Suprasil I (Heraeus) glass which has negligible absorption in the UV 2. Waveguides were made using the method outlined in Ref. [8] by irradiating the sample with a direct-write H ion beam. The implanted ion dose was 1 1016 ions cmÿ2 which produces a measured index change of +0.003 [12]. The geometry of the waveguides is shown in Fig. 1. The guides under investigation are 2 mm long, approximately 5 lm thick in the vertical y direction, and 7 lm wide in the horizontal x direction. Irradiation with 2.5 MeV H ions causes the buried guides to be formed 50 lm below the substrate surface [8]. An erbium doped ®bre ampli®er was used as an unpolarised infrared source. Transverse electric (TE) and transverse magnetic (TM) polarisation states were selected using an in-line ®bre polariser providing greater than 25 dB contrast. The waveguides were irradiated with 193 nm light from an ArF laser whilst the near ®eld image was monitored. The TE and TM modes at a wavelength
2 Manufacturer's speci®cations, Heraeus Quarzglaz GmbH, `Quartz glass for optics'.
M.L. von Bibra et al. / Journal of Non-Crystalline Solids 239 (1998) 121±125
1.5 lm, were imaged with an infrared charge coupled device (CCD) camera. The mode ®eld measurments were repeatable for individual camera exposures and the apparatus was not disturbed during UV exposure. 3. Results The TE mode images of the waveguide taken before and after a cumulative ¯uence of about 1 kJ cmÿ2 are shown in Fig. 2. The near-circular pro-
Fig. 2. Contour mode images of the TE mode taken before (top) and after (bottom) UV irradiation.
123
®les indicate single mode propagation and there was no observable dierence between polarisation states despite the asymmetry of the waveguide cross-section. Fig. 3 shows a comparison of the TE mode cross-sections before and after UV treatment. The increase in mode pro®le width after UV irradiation was observed in all of the waveguides examined. Since there was an apparent increase in mode size, white light spectra were taken of a similar sample during UV irradiation. Fig. 4 shows the dierential spectra of the sample with increasing cumultive ¯uence (0.02±3.0 kJ cmÿ2 ). The induced attenuation measured is the dierence in attenuation between the unprocessed guide and subsequent processing. The small (5.0 nm) spectral resolution is due to the insertion loss of these particular samples.
Fig. 3. Horizontal and vertical TE mode pro®les taken before (solid line) and after (dotted line) UV irradiation.
124
M.L. von Bibra et al. / Journal of Non-Crystalline Solids 239 (1998) 121±125
pro®le observed in the sample. The mode ®eld intensity is of the form: 1 I ; g cos h 2x=h where
1 p g 1 m2 ÿ 1; 2 p m kh 2ns Dn;
Fig. 4. UV-induced dierential attenuation spectra of H ion irradiated silica waveguide after a UV dose of 0.2 kJ cmÿ2 (solid line) and 3.0 kJ cmÿ2 (dotted line). The attenuation increases signi®cantly in the UV.
4. Discussion The largest change in the mode pro®les of Fig. 3 is in the vertical direction. This dierence is a result of the waveguide geometry, since the vertical thickness of the guide is smaller than the horizontal dimension. The guide is closer to cuto in the vertical direction, and therefore the changes in the mode pro®le are larger in this direction. With UV ¯uence of this magnitude, this eect can be used to control the mode shape of the waveguide mode ®eld and produce a more circular shape to improve its coupling eciency to ®bres. It is evident in Fig. 4 that the attenuation in the infrared range above the cuto wavelength (which determines the cuto between single and multimode propagation) has increased. The more tightly con®ned multi-mode wavelengths below cuto show no change within measurement errors, although there is an induced absorption band at 780 nm. At this stage the origin of this band is unknown. Formation of OH or H species was not detected in the spectra despite the ion doses used in the fabrication step. From the changes in the mode cross-section it is possible to make estimates of the induced negative index change. An analytic planar waveguide model [13] with a 1= cos h2 2x=h refractive index pro®le, was selected since it most closely matches the mode
ns is the cladding index, h the guide width and x the position along the axis being analysed. Fitting the model to the two mode pro®les before and after UV treatment yielded a dierence in refractive index of approximately 10% of the core-cladding dierence. Given index changes exceeding 10% one application of this technique is to enhance mode matching between imperfect waveguides. An attempt was made to write Bragg gratings through a phase mask using a setup similar to that reported in Ref. [3] where gratings exceeding 35 dB were easily produced in PECVD glass. There was no evidence in the white light spectra to indicate a grating had been formed using this technique in spite of the fact that a negative index change occurred. From this null result we infer that the induced index changes occur on a macroscopic scale, triggered by localised UV-induced defects. To date, the ability to achieve a localised negative index change on the micron scale required for strong Bragg grating formation has only been reported for ®bre gratings where a so-called `type IIa' grating phenomena is observed [14]. Consequently, further work needs to be done to establish the reason we have been unable to produce such gratings in our material. 5. Conclusions In conclusion, we have demonstrated negative index changes of approximately 10% of the corecladding dierence in H implanted buried channel waveguides formed within UV-transparent glass containing no germanate species and with few defects present initially. These changes allow control of the waveguide mode shape for applications such as improved mode coupling.
M.L. von Bibra et al. / Journal of Non-Crystalline Solids 239 (1998) 121±125
References [1] G.D. Maxwell, B.J. Ainslie, Electron. Lett. 31 (1995) 95. [2] G.D. Maxwell, R. Kashyap, B.J. Ainslie, D.L. Williams, J.R. Armitage, Electron. Lett. 28 (1992) 2106. [3] J. Canning, D.J. Moss, M. Faith, P. Leech, P. Kemeny, C.V. Poulsen, O. Leistiko, Electron. Lett. 32 (1996) 1479. [4] P. Coudray, J. Chisam, M. Andrews, S. Naja®, Opt. Eng. 36 (1997) 1234. [5] J. Albert, B. Malo, K.O. Hill, D.C. Johnson, J.L. Brebner, R. Leonelli, Opt. Lett. 17 (1992) 1652. [6] M. Verhaegen, L.B. Allard, J.L. Brebner, M. Essid, S. Roorda, J. Albert, Nucl. Instr. and Meth. B 106 (1995) 438. [7] D.J. Kyle, B.L. Weiss, G.D. Maxwell, J. Appl. Phys. 77 (1995) 1207.
125
[8] A. Roberts, M.L. von Bibra, J. Lightwave Tech. LT±14 (1996) 2554. [9] M. Verhaegen, J.L. Brebner, L.B. Allard, J. Albert, Appl. Phys. Lett. 68 (1996) 3084. [10] H. Hosono, N. Ueda, H. Kawazoe, N. Matsunami, J. NonCryst. Solids 182 (1995) 109. [11] J.P. Biersack, L.G. Haggmark, Nucl. Instr. and Meth. 174 (1980) 257. [12] M.L. von Bibra, A. Roberts, J. Lightwave Tech. 15 (1997) 1695. [13] H. Kogelnik, in: T. Tamir (Ed.), Integrated Optics, Springer, Berlin, 1975, pp. 55±57. [14] W.X. Xie, P. Niay, P. Bernage, M. Douay, J.F. Bayon, T. Georges, M. Monerie, Opt. Commun. 104 (1993) 185.