1. Introduction

Much attention is being paid to the study of permanent photorefractive effects in transparent optical materials. The aim is to fabricate waveguiding structures and diffractive elements for optically integrated devices by exposure from a light source. The advantages of direct writing, especially from the technological point of view, are evident, since the conventional technologies require more costly and slower fabrication steps, such as photolithography and chemical or physical etching. Among the materials considered, silica-based-glasses are attractive thanks to their greater compatibility with the silica-optical-fiber networking systems: SiO₂-GeO₂ glass, in particular, is certainly one of the most promising candidates for a high-functionality material platform for integrated optics [1–7]. Its use was fostered by the demonstration of the UV photo-induced Bragg gratings in Ge-doped fiber cores. For the purposes of integrated optics, however, achievability of large index contrasts (Δn>10^-3) is a more stringent and critical requirement than for fibers, since the feasibility of photo-induced channel waveguides and Bragg gratings is closely related to the entity of the refractive index changes.

Despite the substantial experimental effort in the characterization of the photorefractive effect, the origin of the UV laser-induced refractive index change has not been fully understood. Even if particular treatments such as H₂ loading, boron co-doping and OH flooding can increase considerably the photorefractive proprieties of the glass, large UV-induce refractive index changes have been measured also in sputter deposited [5,6] and plasma-enhanced-chemical-vapor deposited [2,7] thin films, without the need of particular sensitization procedures. The present films represent an excellent opportunity for the investigation of the UV-induced phenomena of the SiO₂-GeO₂ system, since the experimental evidences of the photo-induced structural modifications are large enough to allow us measurements with good accuracy.

Here we present the characterization of highly photorefractive planar waveguides, single-mode at 1550 nm, prepared by conventional RFMS technique. The positive refractive index and the absorption changes have been explored as a function of the laser irradiated energy. The entity of the index changes was found to be sufficient for the fabrication of photo-induced channel waveguides. Direct measurements of UV-induced volume densification were performed and their contribution to the index change evaluated. A highly efficient phase grating was also demonstrated.

2. Experimental

Thin films with 75SiO₂-25GeO₂ (molar %) composition, co-doped with 0.27 mol% each of Er₂O₃ and Yb₂O₃, were deposited onto silica substrates by radio-frequency-magnetron-sputtering technique, in order to produce planar waveguides single-mode at 1550 nm. Sputtering deposition of the film was performed by using a 4” silica target on which pieces of GeO₂, metallic ytterbium and metallic erbium pieces were placed. The residual pressure, before deposition, was about 2×10^-5 Pa. During the deposition process the substrates were not heated. The sputtering was carried out with an Ar gas at a pressure of 0.7 Pa and an applied RF power of 150 W, with a reflected power of 18 W. The deposition time was 4 h 15 min, producing a film thickness of about 3.35±0.2 µm. The as-deposited films were acting as waveguides, but they were showing rather high propagation losses; thus, following a procedure similar to the one that had demonstrated quite effective with silica-titania sputtered waveguides [8], the deposited films were annealed in air at 600 °C for 6 h, with a heating/cooling rate of 6 °C/minute.

Subsequent irradiations of the samples were performed by using a Lambda Physik Compex KrF excimer laser source, with emission at λ=248 nm. Single pulse fluency was 36 mJ/cm² and repetition rate was set at 10 Hz. Transmission spectroscopy in the UV region was performed by using a Perkin-Elmer λ19 spectrophotometer.

3. Results and discussion

The guiding properties of the deposited films were tested by using an m-line apparatus developed in house, based on the prism coupling technique. The annealed films had a refractive index at 632.8 nm around 1.495, less than 1% smaller than that of the corresponding as-deposited films. Refractive index variations were less than 0.2% in a same sample and smaller than 1% from sample to sample. The value of 1.495 fits quite well the expected refractive index, calculated by the Lorentz-Lorenz equation, of a bulk glass having the same stoichiometric ratio between silica and germania. The optical quality of the annealed films was good, as testified by the propagation losses of the TE₀ mode, measured at 633 nm (0.8±0.2 dB/cm) and at 1300nm (<0.3 dB/cm) by the photometric detection of the light intensity scattered out of the waveguide plane. The photoluminescence properties of these films are quite good as well, and will be reported elsewhere.

Figure 1 illustrates the UV absorption band of the annealed film, for different values of cumulative exposure dose. The pristine film (curve a) presents an intense absorption peak centered at 5.16 eV (α_5.16eV=4.75·10³ cm^-1). The first exposure, with energy density 1.08 kJ/cm² (curve b), has strongly modified the absorption spectrum: part of the 5 eV band was bleached and the absorption increased for higher photon energies. Subsequent irradiations, with 2.16 kJ/cm² (curve c) and 3.25 kJ/cm² (curve d) cumulative exposure doses, produced an additional increase of the absorption above 5.3 eV. The spectral changes in the range between 4.0 and 6.2 eV saturated for higher irradiated energies. Spectroscopic features in this UV region are similar to those reported by Nishii et al. [9–11] for SiO₂-GeO₂ sputter-deposited thin films. The UV-light absorption has been recognized to be produced by the presence of germanium-oxygen-defect-centers (GODC’s) [12–15] that are formed, depending on the sintering conditions, during the glass matrix growth. It appears that, due to the reduced pressure and pure oxygen-free argon atmosphere, sputtering favors the generation of the oxygen vacancies, so increasing the photosensitive proprieties of the film.

The UV-induced refractive index change of these films was estimated by measuring, before and after the irradiations, the effective index of the single mode supported at 1550 nm by the planar waveguides. Much care was taken in order to repeat the measurement in a same position inside the waveguide, to avoid errors due to film inhomogeneities. Figure 2 shows the effective index changes Δn obtained for increasing values of cumulative irradiated energy. For doses less than 5 kJ/cm² the refractive index of the film increases linearly with irradiated energy. The saturation value of the index change, of the order of 3·10^-3, is reached at a dose of about 20 kJ/cm² (see inset of Fig. 2). The results shown were obtained in two samples and indicate a very good repeatability and reliability of measurements. Such a positive value of the index change is high enough to achieve a good lateral confinement of the radiation: channel waveguides’ direct writing on similar films is therefore now under way in our laboratory.

 

Fig. 1. UV absorption spectra of the film (a) after deposition and annealing; (b) irradiated with 1.08 kJ/cm²; (c) irradiated with 2.16 kJ/cm² ; and (d) irradiated with 3.25 kJ/cm².

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Fig. 2. UV laser-induced changes of the effective index at 1550 nm of a single mode waveguide for increasing values of the cumulative exposure dose. The inset shows the saturation behavior of the index change measured in another, nominally equal, waveguide.

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In order to test the contribution of the UV photo-induced volume densification [16,17] to the index change, we placed a thin metallic wire of diameter 150 µm, as a simple exposure mask, onto the sample surface. The sample was then irradiated by the excimer laser; after exposure we removed the wire and scanned the sample surface with a Tencor P-10 profilometer. Fig. 3 shows the profile produced by the instrument, and the inset shows a 3D map of a 550 µm×180 µm area. The step in the profile corresponds to the position of the wire during the exposure, namely to the not-irradiated area. The spike is an artifact, likely due to a point defect or a debris grain. For 16.2 kJ/cm² of irradiated energy we measured a decrease ΔT=-16±2 nm of the film thickness T.

 

Fig. 3. Profilometer scan of the sample surface after the UV-exposure, using a metal wire as a simple mask. The step reveals the position assumed by the wire during the irradiation. The inset shows a 3D image of a 550 µm×180 µm area, where the masking effect of the wire appears evident.

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The contribution of the photo-induced volume densification to Δn can be evaluated quantitatively by differentiating the Lorentz-Lorenz formula [18]:

where n is the refractive index, N is the number of molecules per unit volume and α’ is the mean polarizability. After the irradiation with 16.2 kJ/cm² energy density we measured a change of the effective index

${〈\frac{\Delta n}{n}〉}_{\mathit{EXP}}=1.7\pm 0.2.{10}^{-3}.$

On the other hand, from Eq. (1), by considering only the volume contribution and introducing the measured value of thickness decrease, we obtain:

${〈\frac{\Delta n}{n}〉}_{\mathit{CALC}}=\frac{\left(n^2+2\right)·\left(n^2-1\right)}{6·n^2}·\frac{\Delta N}{N}=\frac{\left(n^2+2\right)·\left(n^2-1\right)}{6·n^2}·\frac{\left(-\Delta T\right)}{T}=1.8\pm 0.3.{10}^{-3}.$

We can therefore conclude that, by taking into account the errors, the UV-induced glass compaction accounts for a very large part, if not all, of the positive refractive index change. Additional measurements, carried out onto similar samples, confirmed this indication.

Figure 4 shows the operation of a high-efficiency waveguide Bragg grating, that was photo-induced by using the phase mask method [19]. The sample was placed in contact with a silica phase mask (central period Λ=1070 nm) and exposed to the excimer laser source, using 36 mJ/cm² pulse fluency and 10 Hz pulse repetition rate; the total irradiated energy density was 2.5 kJ/cm². In order to make evident the effect of the grating, we prism-coupled a He-Ne laser beam (λ=633 nm) into the TE₁ guided mode. When the angle θ between the direction of the coupled light z and the grating vector k satisfies the Bragg condition cos (θ)=λ/n_e Λ, where ne is the modal effective index, the guided light is deflected symmetrically from the fringe planes. Spatial displacement, with a near 100% efficiency is achieved in a small distance (≈1 mm) along the propagation direction.

 

Fig. 4. Deflection of the guided light at 633 nm produced by a highly efficient photo-induced Bragg grating.

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4. Conclusions

The silica-germania films we deposited and characterized exhibited interesting properties. Low propagation losses and good index matching with fibers are two of the fulfilled requirements for the development of integrated optical circuits. The rare-earth-doping increases the material’s functionality. Large positive refractive index changes were produced and easily controlled by the exposure from a KrF excimer laser source. It was demonstrated that the photo-induced volume densification accounted for a very large part of the photorefractive index change. The amounts of the index changes were sufficient for the fabrication of highly efficient sub-micrometric phase gratings, and photo-induced channel waveguides are now being written and characterized.