1. Introduction

Ever since Hill has reported the first ultraviolet (UV) light induced photosensitivity in germanium-doped optical fibre in 1978 [1], an intense level of research activity has been stimulated with a special focus on its potential applications in optical communications and sensor networks. A great deal of progress has been made over the past 25 years both in the understanding of the irradiation induced photosensitivity phenomena and also towards the development of the photosensitive waveguide-materials and thereby devices fabrication technology [2–4]. Unfortunately, however there are some difficulties such as insufficient photosensitivity efficiency, temperature-dependent devices performance and the lack of high efficiency and low-cost deposition technique, have so far been hindering the Ge-doped SiO₂ planar waveguides from their full commercialisation [5–7]. These problems might be tackled by a couple of approaches: (1) developing silica-based planar lightwave circuits (PLCs) on a pure silica substrate (silica-on-silica PLCs) and designing waveguide-cores with significantly improved optical properties using doping, and (2) developing the deposition technology of silica-based waveguides with a better performance. Silica-on-silica PLCs have recently attracted more attention due to their excellent properties such as high reliability and high temperature stability, good compatibility with optical fibre, low-cost and low-loss. It is well known that many physical properties of materials would be varying significantly with an appropriate change in the chemical compositions either through doping or substitution of certain desired impurities, although the changes in the physical properties could be due to a completely different mechanism and the doping level would therefore be varying substantially from material to material. We have applied this approach in silica-on-silica PLCs, so as to enhance their UV photosensitivity and thus improving temperature-dependent devices performance of the silica-on-silica planar waveguides with the boron doping. On the other hand, the most widely used methods of fabricating of Ge-doped SiO₂ planar waveguides, so far, are flame hydrolysis deposition (FHD) and plasma-enhanced chemical vapor deposition (PECVD) [8–10]. However, these are generally equipment dependent and those require a complex process and a relatively high deposition temperature. In contrast, the newly developed low-temperature and low-pressure inductively coupled plasma (ICP) enhanced CVD technique offers several advantages as a versatile one, more flexible and a cost effective method and hence it has been found useful in many areas [7,11,12].

In this paper, we present our latest work on the fabrication of silica-on-silica Mach-Zehnder interferometer-like (MZI) PLCs by using a combination of reactive ion etching (RIE), ICP-CVD and photolithography. Low-loss 8GeO₂-xB₂O₃-(92-x)SiO₂ with x in the range from 1 to 5, have been deposited by low-temperature and low-pressure ICP-CVD as waveguide-cores. The partial substitution of SiO₂ by B₂O₃ up to 5 mol.% has resulted in an appreciably enhanced UV photosensitivity and significantly lowered temperature-dependent performance of germanosilicate planar waveguides.

2. Experimental details

 

Fig. 1. Schematic diagram of ICP-CVD experimental setup.

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A schematic diagram of the ICP-CVD experimental setup (Samco PD-160ip) has been shown in Fig. 1. The ICP reactor consists of a stainless steel cylinder with a helical coil antenna located above a quartz window. Up to 1 KW of RF power at 13.56 MHz was supplied to the coil via an impedance matching unit. Tetraethoxysilane (TEOS), tetramethoxygermane (TMOG), and triethoxyborane (TiOB) were used as the starting materials for the growth of SiO₂, GeO₂, and B₂O₃, respectively. The mixture of TEOS, TMOG, TiOB and a small amount of oxygen as the source gases were introduced into the chamber through the lower inlet (Gas inlet 2) near the substrate holder, and a larger amount of oxygen was introduced through the upper inlet (Gas inlet 1) below the powered coil. The proposal of the oxygen-rich mixture used in this work was to prevent CH radicals incorporated into the deposited films. The mixture of raw materials was decomposed in the O₂ plasma under the pressure of 20 mTorr at 300 °C during deposition. The plasma source and deposition chamber were pre-evacuated to 5×10^-5 Torr by a turbo molecular pump and backfilled with the necessary gases. A mirror-polished pure silica substrate was fixed mechanically on a substrate electrode with a resistive heater. Samples were annealed at temperature of 1000 °C in atmosphere for 2 hours and were then loaded in a pure hydrogen gas at a pressure of 1600 psi for two weeks.

Photobleaching experiments were carried out by using a KrF excimer laser with λ=248 nm at a UV laser fluency of 350 mJ/cm² per pulse, UV laser operating at a 10 Hz repetition rate with 25 ns pulse duration. The refractive index (RI) of the waveguide films was measured for both transverse electric and transverse magnetic polarization on a Metricon 2010 by means of the prism coupling technique. A 0.8 mW He-Ne laser tube with standard silicon detectors and a 2 mW diode laser tube with germanium detectors were employed operating at 0.633 μm and 1.55 μm respectively. In order to evaluate temperature-dependence of silica-based PLCs performance, a simple 2×2 MZI-like optical filters with 8GeO₂-xB₂O₃-(92-x)SiO₂ (x = 1, 2, 4 and 5 in mol.%) waveguide-cores were fabricated on a pure silica substrate by using a combination of RIE (Samco RIE-2000), ICP-CVD (Samco PD-160ip) and photolithography. The propagation loss of the Ge-doped SiO₂ waveguides was measured by using actual MZI devices. A light beam from a single-mode light source was coupled to the front facets of the waveguide with a butt coupling.

3. Result and discussion

Figure 2(a) shows the optical absorption spectra of the as-deposited and annealed 8GeO₂-5B₂O₃-87SiO₂ (8G5B87S) samples. A strong UV absorption at the short wavelength of ~190 nm along with an intense absorption centered at ~240 nm (with the full width at half maximum of around 24 nm) were observed in the as-deposited sample. The absorption at ~240 nm has been found to be related to the germanium concentration in the sample with an approximately linear relationship between ~240 nm peak and the germanium concentration as reported previously [11]. The ~240 nm band in the as-deposited sample comprises two components: the neutral oxygen monovacancies (NOV) defects (245 nm) and the Ge lone-pair center (GLPC) (240 nm) [3,4,13]. There is an agreement over the assignment of the absorption band at around ~200 nm to the Si E’ and Ge E’ centers [3,4]. A remarkable decrease has been found both in the case of ~190 nm and ~240 nm absorption bands with an increase in the annealing temperature up to 1000 °C in air as illustrated in Fig. 2(a), indicating both the NOV and GPLC defects have disappear and more stoichimetric Ge-O-Ge bonds have been formed with high annealing temperatures in air.

 

Fig. 2. UV absorption spectra of 8G5B87S films as function of annealing temperature (a), and UV laser irradiation times (b).

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Figure 2(b) shows the UV optical absorption spectra of 8G5B87S films as function of UV laser irradiation (UVLI) times. Clearly, it is noticed that there is a proportional relationship between the ~240 nm absorption and UV laser irradiation times as shown in Fig. 2(b). Intensity of absorption band centered at ~240 nm is found decreased while intensity of absorption above ~200 nm increases with UV laser irradiation duration. This is obviously because that the absorption band at ~240 nm is bleached and absorption above ~200 nm is induced.

Figure 3 shows optical absorption spectra of the as-deposited and UV laser illuminated (UVLI) samples with boron doping 1 mol.% and 5 mol.%: 8GeO₂-1B₂O₃-91SiO₂ (8G1B91S) and 8G5B87S. It is noticed that intensity of absorption band centered at ~240 nm has been found be decreased while the intensity of the absorption above ~190 nm increases with UVLI durations for both the as-deposited samples. The 1000 °C-annealed samples could exhibit very limited absorption at ~240 nm as shown in Fig. 3(b), indicating that the materials are almost intrinsically defects free and therefore those are not photosensitive, however, it is interesting to note that a large absorption region at short wavelength ~190 nm with a shoulder at ~240 nm are identified for the samples with hydrogen loading (HL) as shown in Fig. 3(b) indicating the formation of oxygen deficient related defects [14,15]. UVLI results in a decrease in the absorption at ~240 nm together with an increase in the absorption centred at ~190 nm for both the as-deposited and HL samples as shown in Fig. 4(b). Sample with 5 mol.% boron codoping exhibits a more intensity with an increase of the NOV defects-dependent ~240 nm absorption upon HL, and a significant changes of ~240 and ~190 nm absorptions after UVLI. Clearly an enhanced photosensitive response with increasing boron doping is demonstrated.

 

Fig. 3. (a) UV absorption spectra of as-deposited 8G1B91S (dot line) and 8G5B87S (solid line) films upon various UVLI durations. (b) UV absorption spectra of 1000 °C-annealed films unpon HL and UVLI.

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The RI changes are observed with the annealing temperatures, HL and UVLI and results are shown in Figs. 4 and 5 and Table 1. RI of the 8G5B87S films decrease initially from 1.4699 to 1.4639 at 632.8 nm, and from 1.4560 to 1.4533 at 1550 nm with the increase of the annealing temperature from 300 °C to 1000 °C. No more change of RI could be observed with any further increase of annealing temperature beyond 1000 °C. This is accompanying with the decrease of ~240 nm absorption band as shown in Fig. 2(a), due to the reduction of NOV and GLPC defects. It is noticed that there are almost no changes in RI for the 1000 °C-annealed sample irradiated by UV laser without any further treatment. However, it should be mentioned that the RI of 8G5B87S films have slightly increased to 1.4643 at 632.8 nm and 1.4535 at 1550 nm after HL, and these are found decreased to 1.4636 at 632.8 nm and 1.4533 at 1550 nm with UVLI. This phenomenon accompanies with the increase of the ~240 nm absorption after HL, and the decrease of the ~240 nm absorption upon UVLI, as shown in Fig. 2(b), which might be due to the formation and reduction of NOV defects. The RI changes of all the 8GeO₂-xB₂O₃-(92-x)SiO₂ (x = 1, 2, 4 and 5 in mol.%) films exhibit similar trends as those of 8G5B87S films.

 

Fig. 4. The refractive index of 8G5B87S films as function of annealing temperature.

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Fig. 5. RI of 8G5B87S films upon different treatments comprising annealing temperatures, HL and UVLI. The solid triangles correspond to the RI of the 1000 °C-annealed samples after HL and UVLI respectively.

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Table 1. Optical properties of 8GeO₂-xB₂O₃-(92-x)SiO₂ (x = 1~5 mol.%) planar waveguides (1000 °C annealed) and the temperature-dependent MZI wavelength.

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Figsures 6(a) and 6(b) show the transmission response of 8GeO₂-xB₂O₃-(92-x)SiO₂ (x = 1 and 5 in mol.%) MZI thermally tuned from -20 to 80 °C and temperature-dependent MZI wavelength, respectively. The designed MZI has double-directional 3-dB couplers at around 1.55 mm. The waveguide core dimension is 7.5 mm, and the corresponding refractive indices of the cores are summarized in Table 1. The two light paths have a geometric length difference of ΔL≈ 1 mm, resulting in a passband pitch of 200 GHz (1.6 nm). The optical-propagation-loss of the Ge-doped SiO₂ waveguides was measured by using actual MZI devices. At a wavelength of 1.55 μm, the single mode 8G1B91S and 8G5B87S waveguides have shown typical losses of 0.18 and 0.11 dB/cm, respectively. Obviously, the present trench types planar waveguides are exhibiting low-propagation-loss for integrated optics. The relatively high propagation-loss values of the waveguides with 2 mol.% and 4 mol.% boron doping presented in Table 1 might be caused by particle contaminations in the waveguides during the fabrication process, which could be reduced by optimized the fabrication process and sample cleaning technology.

The temperature dependence of the central wavelength, dλ/dT, of the MZ filters was measured at 1.55 μm by changing the wavelength of a tunable laser diode (Agilent 81689 A) from 1.525 μm to 1.575 μm upon varying the temperature from -20 to 80 °C. The accuracy of the measurement is 0.01 nm. It was noticed that GeO₂ and B₂O₃ could not only effectively adjust the RI, but also the temperature dependence of the central wavelength of the Ge-B-SiO₂ waveguides. The RI decreases from 1.4537 to 1.4533 at 1.55 μm while the dλ/dT decreases from 9.4 to 8.1 pm/°C with the B₂O₃ concentration increases from 1 to 5 mol.% by keeping the GeO₂ concentration at 8 mol.%. The measured temperature dependence of the central wavelength of MZI was 8.1 pm/°C with 5 mol.% boron doping. Since the temperature coefficient of the conventional optical filter is larger than 0.01 nm/°C for silica waveguides, we have thus successfully reduced the dλ/dT to ~80% and lower.

 

Fig.6. (a) Temperature-dependent MZI wavelengths. Inset shows the schematic structure of MZI optical filter. (b) Transmission response of 8G5B87S MZI thermally tuned from -20 to 80 °C.

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It is quite interesting to report here that 8GeO₂-xB₂O₃-(92-x)SiO₂ optical waveguides containing up to 5 mol.% boron doping that are fabricated through a low-temperature and low-pressure ICPCVD has exhibited a propagation-loss as low as 0.11 dB/cm, with an enhanced UV photosensitivity and significant lowered temperature-dependent performance of 8.1 pm/°C (~80% of that of a conventional optical filter), and thus we have realized its potentiality for promising applications in optics communication networks. The photosensitivity of the Ge-B-SiO₂ planar waveguides studied here possibly changes the output response of a MZI when one arm is exposed to UV laser, and it encourages us to get a deserved shift in the central wavelength of a reflection spectrum of MZI optical filter. Further study on photosensitive PLCs devices are in good progress now.

4. Conclusions

In summary, we have systematically investigated on the development and optical properties of Ge-B-SiO₂ planar waveguides by means of a low-temperature and low-pressure ICP-CVD technique. Low optical-propagation-loss of 0.11 dB/cm of 8G5B87S planar waveguides at 1.55 μm has been achieved. The partial substitution of SiO₂ by B₂O₃ up to 5 mol.% has resulted in appreciably encouraging trends not only with an enhanced UV photosensitivity but also with a significantly lowered temperature-dependent performance of silica-based planar waveguides. These low-loss planar waveguide materials are more promising candidates for their use as the waveguide-cores of planar lightwave circuits such as distributed feedback filters, arrayed waveguide grating filters and Er-doped waveguide amplifiers, etc.