1. Introduction

Hypersensitisation, a technique by which photosensitivity is enhanced in optical fibres and waveguides, has been demonstrated both photolytically and thermally [1–7]. It is underpinned by the need to reduce the undesirable component of photosensitivity associated with differences in the photosensitive response between core and core/cladding regions [1,2]. The basic two-step mechanism of the core index change involves a complex set of catalytic hydrogen reactions, some of which give additional changes at the core-cladding interface [1,4]. As well as photo-hypersensitisation, thermal hypersensitisation is possible [1,5]. It is characterised by the treatment of optical fibres at low temperatures during hydrogen loading or grating writing. However, to date most photolytic processing has involved laser exposure with UV wavelengths (e.g., from 157nm to 355nm). Multi-photon access of the same pathways is possible using longer wavelengths. The significantly improved properties of gratings prepared this way and the ability to hypersensitise through the polymer coating [7] has seen growing interest. Further, the possibility of low energy hypersensitisation at longer wavelengths suggests that very low power and relatively cheap UV lamps can be employed to carry out this step. In this paper we demonstrate hypersensitisation with extremely cheap low power UV lamps used to sterilise medical equipment. Although the cumulative fluence requires long exposure times, UV lamps of considerably more power are available for relative low costs - the need for high intensity pulsed exciplex lamps of the type previously suggested [1,2], such as KrF, ArF, XeCl, F₂ and others, can be avoided.

2. Experiment

UV lamp hypersensitisation experiments were carried out with readily available gas filled fluorescent tubes (Philips G15 T8) with just 15W power spread over a relatively wide range of wavelengths, the bulk of the light covering 200–400nm. The broadband intensity at the surface of these lamps is estimated to be just ~0.054J/cm². These particular lamp sources are commercially available low-cost germicidal lamps commonly used for sterilisation of instruments, mainly within medical and food industries. These lamps are thus designed to have a spectral peak at approximately 254nm, the most effective wavelength for germicidal treatment. For more optimal performance more specific lamps with higher power and relative costs (both purchase and running) still well below that of a UV laser are readily available.

The experiments were carried out with boron co-doped germanosilicate fibre (12 mol% GeO₂) as was used in the 355nm hypersensitisation experiments previously reported [7]. A number of fibres were hydrogen-loaded at 373K at 100 atm pressure for a period of 24 hours, then stripped of the polymer coating and placed in a UV steriliser oven containing the 15W lamp source. For maximum knowledge of the sensitisation fluence, the fibres were placed at the surface of the lamp. If lamps centred at longer wavelengths are used it is not necessary to strip the coating. The fibres were allowed to remain in the oven for a 5 day exposure (total cumulative fluence ~22kJ/cm²) - this exposure time can be reduced substantially by employing high power UV lamps and appropriate optics to collect and focus the light. For the purposes of this paper we demonstrate hypersensitisation is possible even at very low intensities. Further, in this case the fibres have been over-sensitised to ensure that sensitisation did take place. Despite out-diffusion of a substantial amount of the hydrogen over the exposure period the process remains effective indicating that not much hydrogen is required in the catalysed two-step hypersensitisation process.

Gratings were written conventionally through an optical phase mask once all the hydrogen is allowed to out-diffuse using 244nm from a frequency-doubled Ar⁺ laser. A writing speed of 0.5mm/min and 244nm fluence intensity of 1.3kJcm^-2 over the hypersensitised region was maintained for all the fibres with the grating length kept at a constant 10mm. Gratings were also prepared under identical conditions in 355nm laser hypersensitised optical fibres for direct comparison. The characteristic growth curves shown in Fig. 1 were derived from the reflection and transmission profiles recorded in between each grating writing pass.

 

Fig. 1. (a) Natural log plots of grating growth for standard boron co-doped germanosilicate fibres hypersensitised with a UV lamp and 355nm laser and pristine fibre. (b) Fringe contrast plots for lamp and laser hypersensitised and pristine fibres.

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3. Results and discussion

Figure 1(a) shows the natural log plot of the average index evolution versus 244nm grating writing fluence for a UV-lamp hypersensitised fibre, a 355nm hypersensitised fibre, and, finally, a pristine fibre included for comparison. The cumulative hypersensitising fluence of 8kJcm^-2 at 355nm was determined to be the optimum 355nm hypersensitising fluence [7]. This is related to the linearity of its characteristic curve. The lamp hypersensitised fibre, although closely following the 355nm curve, is less linear suggesting that the optimum hypersensitising fluence has not been reached. The maximum grating strength reached for a 5 day lamp hypersensitisation is ~22.5dB whilst a 355nm hypersensitised fibre yielded a maximum of 32dB. The saturation of the grating strength in the lamp hypersensitised fibre occurred for much lower writing fluences (12.7kJcm^-2) than the laser hypersensitised fibre (18kJcm^-2). This indicates that the optimum hypersensitisation fluence has been exceeded and the exposure time needs to be reduced considerably. In comparison, the total cumulative fluence used is approximately 3–5 times that used for a single wavelength 355nm laser source [7]. This may seem somewhat surprisingly low given the optical power is distributed over a much large spectral window outside peak absorption windows. However, this particular germicidal lamp has greater access to the 240nm absorption band, which requires less fluence than at 355nm [1,2]. Also, the low intensity of the process indicates that very little OH is expected - no growth of an absorption peak at 1400nm was observed during hypersensitisation. The boron co-doped silica glass possesses a strong absorption band at 242nm and an accompanying weaker band at 330nm and both are accessible to the lamp. The overall index changes are observed to be very similar for all three fibres, indicating that the hypersensitised fibres have the wavelength stability characteristic of the pristine fibre (as has been observed with CW 244nm hypersensitisation). The fringe contrast plot shown in Figure 1(b) suggests that the lamp hypersensitised case, despite saturation, has a slightly better fringe visibility to the laser hypersensitised case. This is an interesting observation since the much lower energy of 355nm also appears to lead to improved fringe contrast over that of shorter UV wavelengths [7]. Further work is necessary before any conclusions can be drawn.

Figure 2 shows the transmission profile of the grating fabricated in a lamp hypersensitised fibre. The grating profile is uniform and symmetric about the Bragg wavelength. Further, the total insertion loss remain <0.1dB, consistent with similar gratings produced using conventional CW 244nm grating writing. It can be concluded that the lamp exposure was therefore also highly uniform and the photosensitive response identical in all parts of the grating. Thus it is likely that hypersensitisation with this UV lamp excites both these bands simultaneously with photolytic creation of a strong photosensitivity response in the fibre. By using such low power lamps the role of direct background heating may be considered negligible since such lamps produce very little heat. The low intensities also suggest that local heating through absorption is not large.

 

Fig. 2. Transmission profile of a grating fabricated using a 244nm frequency tripled argon ion laser in UV tube lamp hypersensitised fibre.

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4. Conclusions

Lamp hypersensitisation of hydrogen loaded optical fibres has been shown to work as effectively as laser hypersensitisation with relatively strong gratings fabricated by conventional means in exposed fibres. The longer hypersensitisation time required can be overcome using readily available stronger lamps (powers in excess of 1000W are available) at a fraction of the cost of a laser or even a pulsed halide-based exciplex lamp. By choosing more appropriate lamps (such as mercury lamps) centred at longer wavelengths we expect that practical hypersensitisation of fibre through the polymer coating is achievable without the requirement of special fibre coatings. Further, the minimal maintenance costs of such lasers also favour them as the preferred way to hypersensitise fibres. In this work the use of optics was avoided in order to demonstrate the simplest sensitisation scenario possible. However, they can also help to increase the intensity of the lamps significantly and therefore reduce the exposure time. It is also conceivable that a combination of thermal hypersensitisation and lamp hypersensitisation can further reduce the exposure time and costs involved. However, the ability to bulk hypersensitise fibres without any form of optics with lamps costing a fraction of lasers is a significant advantage. This work again confirms that the hypersensitisation process does not require a large concentration of hydrogen in the core (as in previous work low cumulative fluence overall, out-diffusion required and no OH formation characteristic of excess hydrogen during normal grating writing was observed).