AN AUTOMATIC MACHINE USING THERMAL PULSES FOR FABRICATION OF PERIODIC STRUCTURES IN POLYMER OPTICAL FIBRES (POFs) A.V. da Silva, R.M. Ribeiro and A.P.L. Barbero Laboratório de Comunicações Ópticas, Departamento de Engenharia de Telecomunicações Universidade Federal Fluminense, Niterói, RJ, Brasil, 24.210-240 Corresponding author:
[email protected]
Abstract: This paper describes an automatic machine for fabrication of periodic structures on perfluorinated GIPOFs. It is based on a "recorder head" (RH) comprising a ring of very thin NiCr wire that can be heated by current pulses with controlled amplitude and time-width. The POF is introduced through the RH. After a single or stream of thermal pulses, the RH is precisely repositioned to accomplish a new recording. Key words: Gratings, Passive Devices, Device Fabrication, Periodic Structures, Polymer Optical Fibres.
1. Introduction Telecommunications and sensing devices based on POFs have been developed. PMMA POFs show many wellknown and interesting characteristics when compared with the (conventional) Glass Optical Fibres (GOFs) [1]. The thermal expansion and thermo-optical coefficient of PMMA POFs are ~ 100 and ~12 times higher than of the silica fibres, respectively [2]. Furthermore, the Young modulus of PMMA POFs is 3.2 GPa that is smaller than 72 GPa of GOFs [3]. PMMA may be chemically modified on their surface [4] and is biocompatible thus being a suitable material for applications on biological and chemical sensors [5]. Therefore, many PMMA POF based sensors have been developed in the last years [6,7]. Usually, it is accepted that the main disadvantage of PMMA POFs when compared with GOFs is the higher attenuation of the former. However, it is well known that PMMA POFs are suitable for small links and networks, e.g. in cars and homes [1]. Nevertheless, the development of low loss (850-1300 nm) per-fluorinated (PF) polymers as the CYTOP (Cyclic Transparent Optical Polymer) from 1990’s by Asahi Glass and Keio University created a new scenario for POF technology. Because PF-POFs feature C-C, C-F and C-O molecules instead of CH, it presents more chemical resistance against organic solvents. Therefore, PF-POFs may be an alternative of PMMA-POFs to build sensors for chemically severe environments. Furthermore, passive devices of PF-POFs are less common than those made from PMMA-POFs, especially for communication applications. In this paper, it is described an automatic machine developed to record periodic structures, i.e. modulations of the fibre taper (or waist), that are morphologically similar to Long Period Gratings (LPGs) well known in GOFs [8]. One of the technique used to record LPGs on GOFs, is the high-voltage electrical arc generated from fusionsplice machines for silica fibres [9]. However, this paper describe a novel technique and a related machine able to record LPG-like structures on PF GI-POFs using controlled “heat pulses” that is not suitable for GOFs because the fusion temperature of the latter is much higher than of the polymers.
2. The thermal record technique Optical polymers usually present low glass transition temperatures (Tg), i.e. of ~ 80 oC for PF and ~105 oC for PMMA POFs. Such typically lows Tg may be used to produce a diameter shrinkage or dilatation of POFs by using ~1 A current pulses to generate thermal irradiation able to soften PMMA or CYTOP material, thus producing POF tapers [10-12]. Therefore, the developed machine is based on such thermal properties of POFs. Thermal pulses of typically sub-millisecond time-width are generated around precise axial position on the POFs (i.e., radially applied). The POF under thermal recording is simultaneously stretched. The axial profile of the tapers generated along the POFs may be varied as a function of the amplitude and time-width of the current pulses flowing through the RH as well the length of POF stretched and the speed of the traction. The PF POFs recorded using this technique with and without PMMA reinforcement cladding originally present 450-500 µm and 85-230 µm diameters, respectively.
3. The machine prototype for record of periodic structures in POFs The machine prototype for record of periodic structures in POFs comprises three modules: the fibre stretching, the fibre positioning systems, and the recorder head (RH). Many record parameters may be set on the machine by means of the own developed software. Such parameters are: strength and speed of fibre stretch, the periodicity and number of thermal records, and the amplitude and time-width of the electrical current pulses ultimately required to control the recorder machine. 3.1. The stretching and positioning systems for the RH The stretching systems comprise two stretch-modules placed on the same rail and are shifted on opposite directions. High-precision stepper motors were used to drive the two stretching modules and the single positioning systems. In the present prototype, three endless screws were used instead of gear belts. The latter were connected to the stepper motor axis thus converting rotations on translations movements. Bipolar stepper motors working in half-step mode with 400 steps/turn, i.e. 0.9o/step resolution, were used on both fibre stretching and fibre positioning systems of RH. The stretching and positioning system use 0.5 mm/turn and 0.8 mm/turn endless screws, leading resolution of 1.25 µm and 2.00 µm, respectively. The endless screws were coupled to the stepper motor axis by means of suitable sleeves. The other ends of the screws were connected to the respective mobile rails by means of a nut-bolt system thus allowing the translations of the rail as the endless screws rotates. Figures 1 and 2 schematically show the drawing of the stretching and the positioning systems of the RH, respectively. The POFs were hold to the stretching systems by means of suitable and well-known fibre-holders as is shown in Figure 1.
Fig. 1: The schematic drawing of the stretching system of the RH.
Fig. 2: The schematic drawing of the positioning system of the RH.
3.2. The RH for PF GI-POFs Figure 3a shows the picture of RH suitable for PF GI-POFs. Another RH suitable for GOFs was also developed but it is not shown here. Figure 3b shows a schematic drawing of the RH “core”. The latter is a 0.5 mm diameter metallic ring built from 80 µm diameter NiCr thin wire where the current pulses may flow. In this way, the NiCr small ring works as a “pulsed” heater element. Figure 3b also shows that the PF GI-POF (with < 0.5 mm diameter) to be thermally recorded is concentrically positioned and traverses the wire ring.
Fig. 3: (a) The picture of the RH suitable for PF GI-POFs and (b) the schematic drawing of the thin NiCr wire ring (“core” of the RH) and the POF to be thermally recorded concentrically traversing it.
In order to provide mechanical support, the wire ring was placed between two insulation plates. Small holes with diameters slightly less than 0.5 mm were machined in both plates as is shown in Figure 4a. Figure 4b shows the “U-shaped” insulation support for the RH. It should be observed in the Figure 4b that exist three aligned small holes (~0.5 mm diameters) allowing the precise positioning of POFs that traverse the wire ring through it geometric middle. In this way, it is expected to thermally radiate the POF around it, i.e. a radially uniform heating.
Fig. 4: The schematic drawing of the (a) the insulation plates that sandwiches the NiCr wire ring and the (b) Üshaped” insulated support for the RH.
After each thermal cycle by means of the flowing of a single or multiple current pulses through the RH, the latter could be cooled using a fan also driven by the control software. Such procedure allows the fastening the recording processes and also to control the longitudinal deformation of the POF in each waist recording.
Fig.5: The picture of the complete automatic machine prototype able to record periodic structures in POFs.
4. Results and discussions PF GI-POFs made from CYTOP polymers with or without the PMMA reinforcement cladding were recorded with periodic structures by using the automatic machine shown in Figure 5. Figure 6a show a micro-picture of a Lucina PF GI-POF with PMMA reinforcement cladding before the thermal recording, marked as “1” (500 µm size or diameter). Figure 6a also shows a marked “2” of 160 µm size as to be the core diameter of the PF-POF. After 300 µm of POF stretching and simultaneously by launching of 0.5 A amplitude and 3s time-width current pulses through the RH, a periodic structure with 10 tapers (or waists) and ~1 mm periodicity was achieved. A shrunken diameter of 440 µm is achieved for the taper as is shown in Figure 6b and marked with “3” (size = 440 µm). Figure 6a and 6c shows that the core diameter could be shrunken from size “2” (120 µm) to size “4” (105 µm) and the core/cladding diameter ratio was kept unchanged.
Fig.6: The micro-pictures of a Lucina PF GI-POF with PMMA reinforcement cladding (a) before the thermal recording, (b) and (c) after a single recorded taper or waist in the same PF-POF (10 tapers or waists were recorded in such POF).
The next step was the etching of the PMMA reinforcement cladding of the PF GI-POF (Chromis Fiber of 62.5 µm core diameter) using dichloromethane. The pure per-fluorinated polymer POF thus achieved was recorded with thermal pulses with 0.5 A amplitude and 120 µs time-width forming 3 tapers as is shown in Figure 7 (middle).
Fig. 7: The micro-picture of the periodic tapering structures writing in a PF GI-POF (62.5 µm core diameter) from Chromis Fiber without reinforcement cladding. Sizes: 1: 93µm, 2: 75µm, 3: 180µm and 4: 200µm.
5. Conclusions A new technique and related automatic machine were developed to record periodic taper structures on PF GIPOFs with or without the PMMA reinforcement cladding by using the amplitude and time-width controlled thermal pulses. The shape of the recorded periodic tapers depends on the parameters of the thermal pulses and the stretch on the fibre during the manufacture process. The periodic taper structures produced are morphologically similar to LPGs as those can be recorded on GOFs by using high-voltage arc fusion or CO2 laser. Investigations are under way to characterise such structures as multimode LPGs [13], mode-scramblers [14] and mode-filters. Furthermore, the automatic machine has also been used to fabricate single POF tapers [10] and POF-terminations the latter potentially useful as a micro-lens.
Acknowledgements The authors would like to thank CNPq/MCT and Faperj for the financial support of this research.
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