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Large UV-induced negative index changes in germanium-free nitrogen-doped planar SiO2 waveguides D. Wiesmann, J. Hübner, R. Germann, I. Massarek, H.W.M. Salemink, G.L. Bona, M. Kristensen and H. Jäckel Negative refractive index changes of up to 1.7 × 10–3 have been induced by 248nm excimer laser light in planar germanium-free SiON waveguides. Highly temperature-stable 40dB Bragg gratings with a length of 8mm have been written. A novel mechanism of photosensitisation was used to create the high-index changes.

Introduction: Silicon oxynitride (SiON) offers a higher degree of design freedom in the fabrication of planar optical waveguides, because the refractive index can be varied from 1.45 (SiO2) up to 2.00 (Si3N4). A refractive index contrast of 0.05 allows the fabrication of waveguides with a curvature radius of 1–2mm, which is a factor of 10 smaller than that of conventional Ge-doped SiO2 waveguides [1]. Compact planar optical devices, and hence high device densities on a wafer, can be achieved in this way. So far, in contrast to germaniumdoped SiO2 waveguides, photosensitivity in planar SiON waveguides has not been shown. In this Letter we report, for the first time to our knowledge, strong negative refractive-index changes in germaniumfree planar SiON waveguides induced by 248nm light emitted from a KrF excimer laser. The resulting refractive index modulation of up to 5 × 10–4 is about one order of magnitude larger than that in germanium-free SiON fibres induced by a comparable wavelength of 244nm [2]. Changing the refractive index of SiON planar waveguides by UV light introduces the possibility of combining high-index step SiON waveguides with short, strong, directly written Bragg gratings in order to make very compact, integrated, optical filters. Experiment: The 2 µm thick SiON waveguide structures are grown by plasma-enhanced chemical vapour deposition (PECVD) onto an 8 µm thick cladding layer of thermal oxide on a silicon substrate. To reduce the hydrogen content the films were annealed at 1145 °C in a nitrogen atmosphere. After defining the channel waveguides by reactive ion etching (RIE), an upper SiO2 cladding was grown by PECVD [1]. The channel waveguides exhibit a propagation loss of ~0.4dB/cm. A KrF excimer laser emitting at 248nm was used for the UV exposures [3]. The applied UV fluences per pulse varied from 300 to 700mJ/cm2. To create periodic refractive index changes (gratings) the waveguides were exposed through a phase mask. We measured the reflection and transmission of the UV-induced gratings with an erbium-broadband source, and an optical spectrum analyser (OSA) with a resolution of 0.05nm.

Fig. 1 Transmission and reflection spectra of 8mm long grating written with an average fluence of 600mJ/cm2 Inset: reflection spectrum

In the course of our experiments we exposed both unloaded and deuterium-loaded samples. For the deuterium loading, the samples were stored in a pressure chamber for at least three days at room temperature under a deuterium pressure of 200 bar. The exposure of

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an unloaded and a loaded sample to ~2 × 104 pulses with a fluence of 560mJ/cm2/pulse induced a refractive-index modulation of 5 × 10–5 and 8 × 10–5, respectively. These values are in agreement with results for SiON fibres [2]. Deuterium loading is therefore not sufficient to significantly increase the photosensitivity of SiON to 248nm light. To enhance the photosensitivity the samples were pre-exposed to UV light of low fluence upon removal from the deuterium chamber. After pre-exposure, Bragg gratings with transmission dips of up to 40dB were written into the SiON waveguides. Changing the time period between the pre-exposure and the main exposure from a few seconds to several days did not result in a change of photosensitivity. Results: Fig. 1 shows the transmission spectrum of a waveguide that was exposed to a UV fluence of 600mJ/cm2/pulse and 2.2 × 104 pulses of 248nm light. The exposure resulted in a uniform 8mm long grating with a transmission dip of ~40dB. The inset shows the reflection spectrum from the Bragg grating. The mismatch between the modes of the small waveguide core and the fibre causes the high background reflection.

Fig. 2 UV-induced refractive-index modulation ∆nMOD and average refractive-index change ∆nAVE for a set of five simultaneously exposed waveguides UV intensity increased from waveguide at position 125µm to that at 625µm ■ ∆nMOD ● ∆nAVE

To investigate the intensity dependence of the grating formation, we made use of the Gaussian-like lateral intensity distribution of the excimer laser beam, and exposed a set of five parallel waveguides to different UV intensities at the same time. We induced 7.5mm long gratings with 2 × 104 pulses, with an average fluence per pulse of 600mJ/cm2. Fig. 2 shows the average index change and the index modulation that we calculated from the transmission spectra. The waveguides were aligned such that the UV intensity increased from the waveguide at position 125 µm to that at 625 µm. Both the absolute value of the average refractive-index change, and the refractiveindex modulation, increase accordingly. We therefore conclude a monotonic dependence of the average index change and the index modulation on the intensity of the UV light. The average refractiveindex change was found to be negative for all exposures, the observed maximum being ~–1.7 × 10–3. Generally we observe that the absolute value of the average refractive-index change is three to four times greater than the refractive-index modulation amplitude, depending on the exposure, although the visibility of the interference fringes behind the phase mask should be close to 100%. The result might be explained by a refractive-index altering process that is not sufficiently localised to the areas of UV light-interference maxima. To test the temperature stability of the gratings we have performed 2h isochronal annealing experiments. Fig. 3 shows the evolution of the grating strength for a very weak grating with a transmission dip of only 0.7dB, and a strong grating with an initial transmission dip of 28dB. Both gratings do not degrade up to a temperature of at least 500 °C. This is quite in contrast to gratings in conventional Gedoped silica waveguides, for which thermal decay occurs at much lower temperatures [4].

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low-fluence pre-exposure of the deuterium-loaded samples was necessary to photosensitise the samples. The written Bragg gratings are highly temperature-stable, which suggests that the induced refractive-index change is caused by glass-phase transitions rather than by the formation of colour centres. The benefits of UV-engineering of the refractive index can now be applied to SiON planar optical circuits. Acknowledgments: We thank B.J. Offrein (IBM Rüschlikon) and D. Erni (ETH Zürich) for helpful discussions. Financial support by the Commission for Technology and Innovation (CTI) of the Swiss government is gratefully acknowledged. © IEE 1998 Electronics Letters Online No: 19980288

8 December 1997

D. Wiesmann and H. Jäckel (Electronics Laboratory, ETH Zürich, Gloriastrasse 35, CH-8092 Zürich, Switzerland) Fig. 3 2h isochronal anneal of two Bragg gratings with different initial grating strengths

Discussion: The process leading to a refractive-index change in germanium-free SiON waveguides is not completely understood. Nevertheless we can draw the following conclusions from the experimental results. The photosensitivity is unchanged for several days upon pre-exposure of the deuterium-loaded samples. This suggests that under the low-fluence UV pre-exposure, the deuterium is incorporated into the glass matrix. The incorporated deuterium might lead to a higher absorption of the 248nm excimer light and therefore cause heating at the locations of UV light interference maxima behind the phase mask. The elevated temperatures in turn can induce structural changes in the glass or local annealing, causing hydrogen to leave the glass matrix. The high temperature stability of the gratings is more likely to be due to UV-induced glass-phase transitions in the SiON, than to the formation of colour centres. Conclusion: Negative refractive index changes as large as 1.7 × 10–3 have been induced in SiON planar waveguides by 248nm light. A

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J. Hübner and M. Kristensen (Mikroelektronik Centret, Technical University of Denmark, DK-2800 Lyngby, Denmark) R. Germann, I. Massarek, H.W.M. Salemink and G.L. Bona (IBM Research Division, Zurich Research Laboratory, CH-8803 Rüschlikon, Switzerland)

References 1

OFFREIN, B.J., MASSAREK, I.,

BONA, G.L., GERMANN, R., KROMMENDIJK, F., and SALEMINK, H.W.M.: ‘High contrast and low loss SiON optical waveguides by PECVD’. Proc. IEEE/LEOS Symposium Benelux Chapter, Enschede, NL, 1996, pp. 290–293 2 DIANOV, E.M., GOLANT, K.M., KHRAPKO, R.R., KURKOV, A.S., LECONTE, B., DOUAY, M., BERNAGE, P., and NIAY, P.: ‘Strong Bragg gratings formation in germanium-free nitrogen-doped silica fibers’. OFC, 1997, Paper PD5 3 HÜBNER, J., SVALGAARD, M., NIELSEN, L.G., and KRISTENSEN, M.: ‘Phenomenological model of UV-induced Bragg grating growth in germanosilicate fibers’. SPIE, 1997, Vol. 2998, pp. 11–21 4 MELTZ, G., and MOREY, W.W.: ‘Bragg grating formation and germanosilicate fiber photosensitivity’. SPIE, 1992, Vol. 1516, pp. 185–199

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