1. Introduction

Photoinduced Bragg gratings written in the optical fibers or waveguides have received much attention as key components in wavelength division multiplexing systems. These gratings have been used in many applications such as dispersion-control devices, add/drop modules, distributed feedback mirrors, and so on [1–6]. In particular, the channel waveguide gratings have much higher potential for integration with other functional components in a wafer than do fiber gratings. Photoinduced gratings are formed through periodic refractive index changes by exposure of an intense interference pattern to the photosensitive core using an ultraviolet (UV) laser. Ge doped SiO₂, which is the material of choice for the core of fibers or waveguides, is a promising candidate for a host material for photoinduced grating formation because of its photosensitivity, high transparency, and stability. However, the amount of the photoinduced refractive index change is known to not always be sufficiently large to fabricate the devices. Therefore, several research groups so far proposed the special photosensitizing processes. For example, J. Canning, et al., have reported hypersensitisation techniques, which can enhance the photosensitivity of fiber or waveguide cores permanently [7, 8]. Doping of B₂O₃ to Ge-SiO₂ reportedly enhances its photosensitivity [9, 10]. Such doping is an effective means to achieve large refractive index changes without photosensitizing treatments. In addition, B₂O₃ doping is also useful for realization of athermal optical devices because B₂O₃ has negative temperature dependence of optical path length [11]. However, the thermal stability of photoinduced gratings in Ge-B-SiO₂ fiber cores is reported to be less stable than that in Ge-SiO₂ cores. For example, photoinduced gratings in fibers with Ge-B-SiO₂ core started to decay by annealing at 150°C [12, 13]. Such low thermal stability strongly limits the further application of photoinduced gratings to more advanced optical devices. Although there are several methods to form stabilized photoinduced gratings, refractive index changes induced by these processes are relatively small.

We have reported that crystalline Ge nanoparticles can be precipitated space-selectively in Ge-B-SiO₂ thin glass films by combination of irradiation with a UV pulse laser and thermally induced phase changes of the glasses [14, 15]. Especially, the periodic structures consisting of Ge nanoparticles fabricated by this process serve as Bragg gratings with large refractive index modulation and high thermal stability in the visible wavelength region. Such gratings are termed thermally stabilized gratings (TGs). In this paper, for practical application as integrated optical components, this space-selective precipitation technique of Ge nanoparticles was applied to the formation of periodic structures in the buried channel structures. We then evaluated the optical properties as optical band-pass filters in the optical communication wavelength region.

2. Experimental

On silica glass substrates at 400°C, 15GeO₂-5B₂O₃-80SiO₂ (mol%) thin glass films of 4–5 μm thickness were fabricated using plasma-enhanced chemical vapor deposition. Then, 1-μm-thick SiO₂ clad layers were deposited on the Ge-B-SiO₂ films. Liquid sources of Si(OC₂H₅)₄, Ge(OCH₃)₄, and B(OC₂H₅)₃ were used, respectively, as raw materials for SiO₂, GeO₂, and B₂O₃. The thin films’ chemical compositions were analyzed using electron-probe microanalysis. 15GeO₂-5B₂O₃-80SiO₂ thin films exhibit phase change by annealing at 600°C. Here, the phase change means the phase separation and subsequent nanoparticle precipitation. Photoinduced gratings with a period of 530 nm were written by irradiation with a 248-nm-wavelength KrF excimer laser through a phase mask at room temperature without H₂ loading. The films used in this study have intense absorption below 210 nm wavelength. Therefore, it is difficult to form practical photoinduced gratings in the films by irradiation with 193-nm-wavelength ArF excimer laser. The laser fluence and pulse number for the grating formation were, respectively, 80 mJ/cm²/pulse and 12 000 pulses. The refractive index modulations of the gratings in the slab waveguides were estimated by measuring the first order diffraction efficiency using a He-Ne laser. Thermal annealing was performed in a tube furnace under a nitrogen atmosphere. The annealing time for investigation of thermal stability of TGs in the buried channel waveguides was 1 h at each temperature. The transmission spectra of the channel waveguides were measured using an optical spectrum analyzer with a wavelength-tunable laser diode as a light source. The incident beam was butt-coupled into the channel waveguides using a commercial single-mode optical fiber.

3. Results and discussion

Figure 1 shows the fabrication processes of the channel waveguides with TGs, schematically. First, photoinduced gratings were written using laser irradiation. Then, the samples were annealed at 600°C for 20 min to induce TGs. Here, the refractive index modulation was estimated as 1.6 × 10⁻³ at 632.8 nm wavelength. The channel structures were subsequently fabricated using photolithography and dry etching processes. The respective width and height of the core were 7.0 and 5.3 μm. Finally, the upper layers of Ge-SiO₂ of 12 μm thickness were deposited to form the single-mode waveguides. The grating length was 5 mm. We observed the surfaces of the as-fabricated channel structure, namely, before deposition of the Ge-SiO₂ upper layer, after wet etching with 6% HF solution. For this observation, the SiO₂ cladding layers were removed by dry etching processing prior to the fabrication of channel structures. Figures 2(a) and 2(b) respectively show scanning electron microscope (SEM) images of the channel structure before and after HF etching for 2 min. The images of the structures from above and with an enlarged view are shown respectively in Figs. 2(c) and 2(d). Figure 2(a) shows that surfaces of the channel structure were rather smooth in spite of the precipitation of Ge nanoparticles. In particular, the sidewall surface roughness was almost not observed. It is readily apparent from Figs. 2(b) and 2(c) that periodic relief patterns appeared on the channel surfaces after HF etching. The pitch of these periodic patterns was 530 nm, which is identical to that of the photoinduced gratings formed in the film before annealing. In Fig. 2(d), nanoparticles are visible in the convex regions. We reported previously that the etching rate of the irradiated regions of TG became higher than that of the unirradiated ones because of the predominant formation of B-rich phase in the irradiated regions after annealing; furthermore, Ge nanoparticles were precipitated mainly in the unirradiated regions [14]. Therefore, we consider that the origin of these periodic relief patterns is TG induced in the channel. The periodic relief patterns on the sidewall indicate that TG formation occurred not only near the film surface, but also across films. The non-uniform interference relief patterns on the sidewall might result from the transmitted zero-order laser light, which was not suppressed completely by the phase mask in the writing process of photoinduced gratings [16]. Figures 3(a) and 3(b) show the photoluminescence (PL) spectra of the films originating from the Ge²⁺ before and after annealing, which were taken under the excitation of 248-nm light from the monocromated Xe lamp. By laser irradiation, the PL intensity was markedly decreased. Ge²⁺ is a dominant species responding to dense UV photon, and the structure of Ge²⁺ changes to other species during laser irradiation [17]. Therefore, it is considered that this decrease in intensity resulted from the photoinduced structural changes of Ge²⁺. By annealing up to 500°C, the PL intensity increased to become almost the same intensity before irradiation. It is notable that the PL intensity drastically decreased after annealing at 600°C. On the other hand, Ge nanoparticles, which were never observed after annealing up to 500°C, were precipitated after annealing at this temperature. These results strongly suggest that Ge nanoparticles originated from Ge²⁺. As for the conventional photoinduced gratings, refractive index changes are induced directly by the structural changes of defects such as Ge²⁺ during laser irradiation. Unlike in the case of these gratings, as for TGs, such photoinduced changes of defects are almost erased after annealing up to 500°C [10], and, instead of the photoinduced structural changes of defects, the conversion of Ge²⁺ to Ge atom with high refractive index is considered to have occurred during annealing at 600°C, yielding the large refractive index modulation.

 

Fig. 1. Schematic illustration of the fabrication processes of channel waveguides with TGs.

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Fig. 2. Scanning electron microscope images of the channel structure (a) before and (b) after HF etching for 2 min, and images (c) from above and (d) of the enlarged one. Periodic relief patterns appeared on the channel surfaces after HF etching. Nanoparticles were observed in the convex region of the channels. The SiO₂ upper layers were removed before HF etching for observation.

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Fig. 3. Photoluminescence spectra of (a) the films before and after irradiation and the films after annealing at 500°C and (b) the films after annealing 500°C and 600°C, which were taken under the excitation of 248-nm light.

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Figure 4 shows the transmission spectra for the TE-like and TM-like modes when light of near-infrared wavelength was coupled to the channel waveguides with TGs fabricated by the processes shown in Fig. 1. The depths and positions of the diffraction peaks were 37.77 dB at 1536.2 nm and 38.72 dB at 1537.6 nm, respectively, for TE-like and TM-like modes. Only one peak was observed for each polarization mode, meaning that the structure functions as a single-mode waveguide. Such high diffraction efficiencies indicate that the refractive index modulation of TGs is large not only in the visible wavelength range, but also in the optical communication wavelength. Similar depth of the diffraction peaks between both polarization modes indicates that the precipitation of Ge nanoparticle occurred uniformly in the unirradiated regions across the channel core, which is consistent with the relief formation on the sidewalls in Fig. 2(b). The polarization dependence of the diffraction wavelength might result from the asymmetrical core structure and the birefringence because of the residual thermal stress in the waveguide. Although the annealing time used for TG formation was only 20 min in this study, the refractive index modulation of TGs increased with annealing time at 600°C [14]. Therefore, we can expect that much higher diffraction efficiencies of TGs are achieved after longer annealing time at step (b) in Fig. 1. Figures 5(a) and 5(b) show changes in the diffraction efficiencies and diffraction wavelengths for both polarization modes after additional annealing up to 600°C. The annealing time was 1 h at each temperature. The conventional photoinduced gratings in the fibers were reported to decay markedly, even after annealing at 150°C [12]. In contrast, it is noteworthy that the diffraction efficiencies of TG for both modes remain unchanged after annealing up to 500°C. Especially, no decays, either in diffraction efficiencies or diffraction wavelengths, were observed after annealing up to 400°C. The difference of the diffraction wavelengths decreased from 1.4 nm to 0.8 nm without decay in the diffraction efficiencies after annealing at 500°C. Such a decrease might be attributable to reduction of the residual thermal stress between the core and upper layer of Ge-SiO₂ films because the upper layer was deposited at temperatures lower than 500°C. It has been impossible to apply annealing, which reduces birefringence, to conventional waveguides or fibers with unstable photoinduced gratings. So far, several research groups have proposed the stabilized processes of photoinduced gratings such as hypersensitisation techniques and type IIA grating formation [18]. In particular, the diffraction efficiencies of the type IIA gratings remain unchanged even after annealing up to 600°C. These processes are useful to form the stabilized gratings. However, the resultant refractive index modulations are relatively small. Although TGs are slightly less stable than these stabilized gratings, our presented technique is effective to induce much larger refractive index modulations than them because our technique utilizes the space-selective precipitation of Ge nanoparticles with high refractive indices. In fact, we successfully formed TGs with the refractive index modulation of 6.8 × 10⁻³ at 632.8 nm wavelength without optimization of irradiation condition and glass composition [14]. The presented technique is effective for enhancement of both the refractive index modulation and thermal stability. In addition, the polarization dependence of the diffraction wavelengths can be reduced between modes. Waveguide gratings with higher diffraction efficiencies, thermal stability, and lower polarization dependence of diffraction wavelengths would be realized by optimization of irradiation and annealing conditions.

 

Fig. 4. Transmission spectra of the channel waveguides with TGs for TE-like and TM-like modes.

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Fig. 5. Changes in (a) the diffraction efficiencies and (b) diffraction wavelengths for TE- like and TM-like modes after further annealing up to 600°C. The annealing time was 1 h at each temperature. Note that no decay is apparent in the diffraction efficiencies after annealing up to 500°C.

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4. Conclusion

In conclusion, periodic structures consisting of Ge nanoparticles were induced in the buried channel waveguides by laser irradiation followed by thermally induced phase changes of the glasses. In addition, SEM observation of the channel waveguides after HF etching revealed that the periodic structures were formed not only near the film surfaces, but also across the films. Results demonstrated that the periodic structures served as band-pass filters with large refractive index modulation and high thermal stability in the optical communication wavelength. The diffraction efficiencies remain unchanged, even after further annealing at temperatures as high as 500°C. The polarization dependence of the diffraction wavelengths was reduced by annealing, with no associated decay of diffraction efficiencies. The waveguides with TGs are applicable to highly reliable optical filters and durable sensing devices operating under high-temperature conditions.