WO2005090450A1 - Solution doping of polymer optical fibres - Google Patents

Abstract

The present invention relates to the manufacture of polymer optical fibres, and in particular, microstructured polymer optical fibres. The invention provides a process of forming a microstructured polymer optical fibre wherein one or more dopants are introduced through the microstructure of the fibre after polymerisation. Preferably, controlled diffusion is used to disperse the dopant uniformly across the fibre core, and the final concentration is systematically varied by appropriate choice of conditions. Preferably, the dopant is dissolved in a material that is a non-solvent for the polymer, and more preferably, is dissolved in a volatile material that can be subsequently removed from the polymer.

Description

SOLUTION DOPING OF POLYMER OPTICAL FIBRES

FIELD OF THE INVENTION The present invention relates to the manufacture of polymer optical fibres, and in particular, microstructured polymer optical fibres.

BACKGROUND TO THE INVENTION Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field. The use of polymers to make microstructured optical fibres has made it possible to make a variety of microstructures that cannot be easily made by the capillary stacking technique used for similar silica fibres. However the fabrication techniques generally used to produce microstructured polymer optical fibres have themselves made it problematic to produce doped fibres, because of their use of a monolithic preform. It is therefore an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.

SUMMARY OF THE INVENTION To this end, the present invention provides a process of forming a microstructured polymer optical fibre wherein one or more dopants are introduced through the microstructure of the fibre after polymerisation. Preferably, controlled diffusion is used to disperse the dopant uniformly across the fibre core, and the final concentration can be systematically varied by appropriate choice of conditions. Preferably, the dopant is dissolved in a material that is a non-solvent for the polymer, and more preferably, is dissolved in a volatile material that can be subsequently removed from the polymer. In instances where the dopant is dissolved in a material that has solvent characteristics for the polymer, contact time with the polymer needs to be minimised. The method may potentially be used for any dopant, both organic and inorganic, provided a suitable solvent can be found. The process of the present invention has been tested in the context of microstructured polymer optical fibres ("MPOFs") at the intermediate preform stage, but may also be used in other contexts, including possibly to fibres themselves after drawing. In this case it may be possible to isolate the dopant to a particular section of the fibre. Advantageously the process of the present invention allows doped microstructured polymer optical fibres to be made relatively easily, using commercially available low-loss polymer, and without any significant chemical resources. It is particularly applicable to MPOFs because only the central area need be doped - the holes allowing access directly and the large surface area enhances the uptake of material. Unless the context clearly requires otherwise, throughout the description and the claims, the words 'comprise', 'comprising', and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of "including, but not limited to".

BRIEF DESCRIPTION OF DRAWINGS A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Fig. 1 a is a cross-section of a preform during doping; Fig. lb is a graph illustrating the position of the dye and solvent fronts as a function of time; Fig. 2a illustrates a cross-section of a microstructured polymer optical fibre which has been doped according to the present invention; Fig. 2b is a graph of fluorescence intensity as a function of radius for the fibre of

Fig. 2a, illustrating uniform doping in the core region of the fibre; Fig. 3a illustrates distribution of dye in three preforms each removed from the solution after being exposed to it for different amounts of time; Fig. 3b illustrates the same preforms after annealing and showing a uniform distribution in the core; Fig. 4 is a graph illustrating loss versus wavelength for a doped fibre in comparison to an undoped fibre drawn from the same polymer preform; Fig. 5 a illustrates a mPOF doped with Rhodamine 6G; Fig. 5b is a graph of intensity across the cross-section of the mPOF of Fig. 4a; and Fig. 6a is an optical microscope image of a typical mPOF; Fig. 6b is a scanning-electron microscope image of the 18-μ-diameter core region; Fig. 7 illustrates an experimental setup used to test a fibre as a fibre amplifier, wherein: BSs denotes beam splitters; M's denotes mirrors; A denotes attenuator; L denotes lens; OL denotes objective lens; PM denotes photomultiplier; PD denotes photodiode. The photo-diode was used to trigger the oscilloscope; Fig. 8 is a graph of gain measured at λ = 574nm as a function of the launched pump energy; Fig. 9a is a graph of the spectrum of the output of the mPOF laser; Fig. 9b is a graph of the spectrum near 632 run on a linear scale; Fig. 10a is a graph of normalized power temporal profiles for the pump and the mPOF laser; Fig. 10b is a graph of output pulse energy from the dye-doped mPOF laser at 632 nm as a function of the launched pump energy at 532 nm; and Fig. 11 is a graph of pulse energy of the mPOF laser as a function of the number of shots for two launched pump energies.

DESCRIPTION OF PREFERRED EMBODIMENT The present invention will be further described by way of reference to the following experimental trials which have been performed to date. Microstructured polymer optical fibres are generally fabricated by drilling the desired pattern of holes into a cylindrical preform, made of commercially available polymethylmethacrylate (PMMA), and drawing that to form an intermediate preform that is in turn sleeved and drawn to fibre. To produce doped MPOF, it is not necessary to dope the polymer prior to polymerization. Instead, the holes of the intermediate preform are filled with a solution of the dye, allowing the solvent and dye to enter the PMMA matrix. Subsequent heating allows the dye to diffuse evenly throughout the core region and removes the solvent, thus locking the dye in place. The dyes are generally large molecules and so are mobile in the polymer only in the presence of a solvent. Example 1 In one example, microstructured polymer optical fibres ("MPOFs") were made using a two-stage drawing technique. Doping was carried out after the first draw, at the intermediate preform stage when the holes in the preform were about 250 microns in diameter- sufficiently large to allow solutions to pass through them. The preforms were annealed before doping to alleviate any residual stress, which could cause cracking when the solution was introduced, and also to ensure that prior thermal history did not play a role in determining the uptake of dopant. The mPOF preforms were made using atactic polymethylmethacrylate (PMMA) supplied by NINK EXPORT, Belgium. Rhodamine 6G was obtained from Aldrich and used as supplied. The Rhodamine 6G was dissolved in methanol, a good solvent for Rhodamine, but non-solvent for the Polymethyl methacrylate ("PMMA") from which the preform was made. Methanol is also quite volatile, allowing it to be largely removed from the polymer below its glass transition temperature. Other possible non-solvents for PMMA include ethanol, hexane, cyclo hexane, water, isopropyl ether. A similarly appropriate choice of solvent would allow a wide variety of dopants to be introduced into PMMA. The transport of methanol in PMMA has been widely studied, including methanol/Rhodamine mixtures. At ambient temperatures it exhibits Case II diffusion, characterised by a sharply defined diffusion front that moves with uniform velocity, with the uniform concentration behind the front. The dye diffuses behind the methanol front, also with a uniform velocity. The dopant (dye/solvent) plasticises the polymer and the penetration is proportional to the concentration at the front. Varying the dye concentration in the solution therefore allows us to vary the concentration in the polymer. Another route to varying the dopant concentration is to vary the temperature. At higher temperatures the diffusion becomes increasingly Fickian, and the equilibrium dopant fraction increases. Fig. la shows the cross -section of a preform during the doping, showing two holes adjacent to the core with the dye and solvent fronts diffusing in from the holes. Fig. lb is a plot of the position of the dye and solvents fronts as a function of time. The linear dependence indicates Case II diffusion. The dotted line indicates the position of the core radius. The preforms were left in a solution of the dye/methanol until the diffusion fronts had met at the centre of the core region, a process that took up to three days at room temperature, depending on the preform design. They were dried to remove the methanol from the PMMA, and drawn to fibre. The removal of the solvent from the polymer dramatically reduces the diffusion of the dye. After its removal there is no measurable change to dye distribution, even when the preform is maintained at elevated temperatures for extended periods. The concentration of dye in the polymer can be varied by varying the dye concentration in the solution, the solvent system or the temperature. At higher temperatures the diffusion becomes increasingly Fickian, and the equilibrium dopant fraction increases. Using these techniques uniformly doped samples with dopant concentrations ranging from lμmol/L to lmmol/L were produced. Fig. 2a illustrates the cross-section of a resulting MPOF formed according to the present invention. Fig. 2b illustrates the fluorescence intensity as a function of radius for the fibre of Fig. 2a, where it is to be noted that there is relatively uniform fluorescence intensity in the core region of the fibre. The concentration of dopant in the fibre can be controlled using several methods.

If the core is allowed to saturate (ie. all the dye fronts have met in the core) the amount of dopant can be controlled by the temperature and concentration of dopant in the solution. The amount of dopant can also be controlled through the time for which the preform is exposed to the solution, i.e. removing the preform before the dye fronts have met. The preforms can also be removed from the solution prior to the dye fronts meeting in the core, at which stage a smaller amount of dopant has entered the polymer. Further diffusion during the drying stage results in a uniform dopant concentration. The resulting dopant concentration is lower than if the solvent fronts had been allowed to meet. This offers an alternative route to controlling the concentration, in addition to control of the solution concentration and temperature. In either case, the annealing stage that follows will allow the dopant, however distributed originally, to diffuse evenly in the core resulting in a uniform dopant concentration in the core (see Figs 3 a, 3b). The holes that surround the core block outward diffusion and prevent the dye from leaving the core. At the same time the solvent is being removed, meaning that once the uniform concentration is achieved, it is locked in place, as the dopant is not mobile in the polymer in the absence of the solvent (the conditions need to ensure that the uniform concentration is achieved prior to the removal of the solvent, which is generally possible). Several preforms were kept at a temperature of 90 degrees Celsius over a period of two months and the distribution of dye was observed to remain unchanged, indicating good stability at such a high temperature. This technique also allows for doped unstructured regions to be formed. Absorption of the solution causes the polymer to swell behind the solvent front. The conditions can be such (eg. smaller holes and higher temperatures than what one would use above) that the swelling closes the holes completely. The remaining distribution of dye was also found to be stable upon heating to 90 degrees Celsius for two months. The amount of dye in this case can be controlled as above, but given that a minimum amount of time is needed cause sufficient swelling, the swelling may be initiated by soaking in pure solvent first, followed by dopant solution. The removal of the solvent is important. Thermo-gravimetric analysis was carried out at the intermediate preform stage to determine whether solvent remained within the polymer matrix after the drying stage. The sample was heated at a rate of 2°C/minute, and was then held isothermally at 250 °C for 10 minutes. There was no evidence of enhanced weight loss at or around the boiling point of methanol, indicating that it had been successfully removed. Rather there was a small continuous weight loss of 1.49%, over the course of the thermo-gravimetric analysis run. This was probably due to a small amount of depolymerisation. Optical measurements also indicated that the process did not increase the attenuation of the resulting fibre, other than through the expected absorptions due the dye. Fig. 4 illustrates optical loss measurements of a doped and undoped fibre, drawn from the same perform, indicating that the doping does not introduce losses other than those due to the absorption of the dye (the sharp increase at the short wavelength region of the graph). In an alternative technique, an acetone/Rhodamine solution was prepared and flushed through the preform, leaving a residual layer of dye, which subsequently migrated through the preform will annealing. This process is faster but more difficult to control than the methanol method. As acetone is a solvent for PMMA, care must be taken to avoid damage to the microstructure. Different dye-solvent mixtures can result in very different concentrations in the final polymer, even when the concentrations in the starting solutions are similar. Other solvents in this category include, for example, isoproyl alcohol, tolune, chloroform. It is advantageous if the solvent is a poor solvent for the dopant, as this allows the dopant to easily come out of solution and be deposited on the inner surface of the holes, from where it can migrate into the core. If the dopant remains well in solution, it will be washed out with the solvent, and will not enter the polymer. The present invention makes possible the use of organic dopants in MPOF. The addition of highly conjugated chromophores to the polymer system may significantly affect the rheology of the draw, so that defining a suitable grafted or co-polymer system may be a lengthy process. In a further development, electrodes are incorporated into the fibre during drawing. This allows an electric field to be applied across the core region to produce poling. The new methodology allows poling to be rapidly tested in guest-host systems. These do not have the thermal stability to be useful in real devices, but may allow promising chromophores to be identified and proof-of-concept experiments to be carried out. The technique exploits the very large surface area of MPOFs, and the diffusive processes of glassy polymers. Flushing the intermediate preform or "cane" with a doped solution allows a layer of material to be deposited on the inside of the holes. Subsequent heating of the cane close to its glass transition temperature causes this material to diffuse through the polymer. Whilst care must be taken that the solvent does not damage the cane, either by causing cracking, or dissolving the material, it is possible to use the technique to produce a uniform concentration profile across the core (see Figs. 5a, 5b). One area where the technique of the present invention is useful is in producing poled doped fibres for the electro-optic effect. It is possible to incorporate electrodes into MPOF during the drawing process. Given the requirement for molecular order to be thermally stable in this case, a guest-host system is not ideal. However it does allow promising chromophores to be tried out. Advantageously, a further application of the present invention is in the production of doped microstructured polymer optical fibre amplifiers and fibre lasers. The main advantage is the simplicity of fabrication and operation, with a consistent wavelength and a narrow linewidth obtained independently of the various fibre parameters and with no additional mirrors required, despite the high intrinsic loss of the fibre. Organic laser dyes can form the basis of tunable lasers, that can be operated over the entire visible spectrum. These dyes exhibit larger absorption and emission cross- sections and tunability over a wider range than most atomic species used in laser sources. They are commonly used in laser dyes, both continuous wave and pulsed, offering advantages such as compactness as well as alleviating some disadvantages associated with the flammable and volatile organic solvents used in dye lasers. Using the post polymerisation doping method of the present invention, it is possible to produce a microstructured polymer optical fibre characterised as an optical amplifier and fibre laser. The method of fabrication is not dye specific so a wide variety of organic dyes can be used to adjust the properties of the amplifier or laser. Testing has shown that the lasing wavelength and linewidth are highly reproducible and insensitive to the dye concentration, the fibre length and the solvent used for doping (the latter is to be expected since the solvent is removed at the preform stage, and is not present in the fibre). The lasing efficiency, however, clearly reduced with decreasing dye concentration. Example 2 The following example illustrates the application of the present invention in producing a solid state dye laser using an acetone doping method. A microstructured polymer optical fibre was fabricated by drilling the desired pattern of holes (see Figs. 6a, 6b) into an 8 cm diameter preform made from commercially available poly(methyl methacrylate) (PMMA). The preform was drawn to form an intermediate preform that was in turn sleeved and drawn into a fibre. To produce doped mPOF, the holes of the intermediate preform were filled with a solution of the dye. The small solvent molecules enter the PMMA and plasticise it, allowing the larger dye molecules to enter the matrix. Solvents with low boiling points are preferred, as they can easily be removed by heating once the doping is complete. Heating also allows the dye to diffuse evenly throughout the core region and, by removing the solvent, locks the dye in place. The generally large dye molecules are mobile in the polymer only in the presence of a solvent. The intermediate preform was 6mm in diameter. It was exposed to a saturated solution of Rhodamine 6G dye (R6G) and acetone for a period of 30 seconds (by drawing the solution into the holes) and then heated at 90°C for 16 hours. After this treatment the position of the dye was fixed and was observed to remain unchanged even after two months of further heating, indicating the thermal stability of the dye distribution. The final concentration of the dye in the core was estimated to be 1 mmol/1. The preform was then sleeved in a 12 mm tube and drawn to fibre with a 600-μm outer diameter (Fig. 1), a core size of 18 μm, a hole diameter d of 3.5 μm, and a hole spacing A of 5.2 μm, thus giving d/A = 0.67. A N.A. of 0.19 and a launch efficiency of 26% were measured at λ = 532 nm by use of a 16X microscope objective. A large core was chosen to increase the launch efficiency. The fibre was tested as a fibre amplifier by using the experimental setup shown in Fig. 7, by pumping the core at 532 nm with a frequency-doubled β-switched Nd:YAG laser operated at 10 Hz. A small signal (~1 nJ/pulse) was provided by a conventional dye laser with R6G dissolved in methanol that could be tuned from approximately 560 to 585 nm. The pulses from the two lasers arrived simultaneously at the fibre; the pulses were 10 and 8 ns long for the Nd:YAG and the dye lasers, respectively. The spectral output of the mPOF was measured with a 0.5-m spectrometer with a photomultiplier mounted onto the output slit. For a 2 m length of fibre it was determined that, within the signal's (dye lasers) tuning range, the gain was maximum near 574 nm. Fig. 8 shows the gain measured at 574 nm as the pump power was increased. The maximum gain observed was 30.3 dB (a factor of 1072), requiring a launched pump energy of 325 / /shot (peak power of 32.5 kW). The gain saturates near that value, and the reason for the low efficiency of 0.3% is that a significant amount of amplified spontaneous emission is produced, caused by the large emission cross section of the dye, the high peak pump power, and, to a lesser extent, the N.A. of the fibre. A higher signal power is expected to increase the efficiency, as it would allow the signal, rather than the spontaneous emission, to dominate the stimulated emission. Blocking the signal and adjusting the launch conditions caused the output of the fibre to change colour, from yellow (fluorescence) to red. A broader spectrum of the output was taken (Figs. 9a, 9b), and an intense and very narrow peak was observed near λ = 632 nm, superimposed upon a much weaker background fluorescence. Interference among the various modes of the fibre was observed for the red output, indicating that the red light emitted from the fibre was coherent. The coherence and the sharp, narrow peak are consistent with the fibre lasing. Further fibre samples ranging in length from 2.0 m to 5 cm were also observed to lase. No additional mirrors were required with the cavity being formed between the cleaved ends of the fibre. No special care was taken to polish the ends or to cool the fibre. The laser line appears at λ = 631.9 ± 0.3 nm with a full width at half-maximum of 0.5 ± 0.1 nm, which is significantly narrower than the 6-nm linewidths reported for other PMMA - R6G fibre lasers. The errors given here arise from statistical variations from fibre to fibre. The temporal profile of the output is presented in Figs. 10a, 10b, showing a full width at half-maximum of 8 ns. The performance of a 1.5 m length of fibre as the pump energy is increased is shown in Fig. 5. A threshold of 20 and a slope efficiency of 18% were observed when the fibre was pumped at 10 Hz. This result is quite satisfactory, given that the intrinsic loss of the fibre is 3 dB/m at both the pump and the lasing wavelengths (with an additional loss of at least 20 dB/m at 532 nm that is due to the dye absorption). The maximum copropagating (with respect to the pump) output observed was 16 /pulse, giving a peak power of 2 kW; this power was limited by the damage threshold of the polymer, which was estimated to be 13 GW/cm². The counterpropagating output was found to be small in comparison, ie, -1% of the copropagating value. This smaller output is believed to be a result of combining a long cavity with a short, high-power pump pulse and a gain medium with a short fluorescence lifetime (4.8 ns). The high emission cross section of the dye and the high-power pump pulse will result in most of the gain being depleted in the first pass by the laser pulse. Combined with the weak reflections off the fibre ends and the high loss of the fibre, this effect will result in little feedback to which little gain will be available, resulting in the large imbalance between copropagating and counterpropagating outputs. As in all solid-state dye lasers, the output was observed to diminish with the number of shots because of photodegradation of the dye. An extrapolated half-life of 80,000 shots was measured with a pump energy of 185 j/shot, which increased to 130,000 shots when the pump energy was reduced to 88 /shot, as shown in Fig. 11. The effects on the properties on the mPOF laser of the dye concentration and the solvent used for the doping were also investigated. Three more fibres were fabricated, with different concentrations of R6G, and a fourth fibre, which used methanol solvent. Methanol is a better solvent for R6G than acetone and is a non-solvent for PMMA, allowing for longer exposure and higher concentrations to be obtained. The concentration of dopant for these fibres was estimated to range from 0.6 to 1.3 mmol/1, as determined by examination of the wavelength of maximum fluorescence, which depends on concentration, and by thermogravimetric analysis. Various lengths of these fibres that ranged from 0.9 to 2.0 m were tested, and all were found to lase at the same wavelength as the original fibre and with the same linewidth. This result shows that the lasing wavelength and linewidth are highly reproducible and insensitive to the dye concentration and to the fibre length for the range of values used here. They were also insensitive to the solvent used for doping, as expected, because the solvent is removed at the preform stage. The lasing efficiency was found to decrease for lower dye concentrations. The fluorescence maximum and lasing wavelengths observed here are shifted to the red by 30 nm. It has been observed that one can achieve wavelength shifts to the red by increasing the concentration of R6G in PMMA, mostly by reabsorption of the emitted radiation, which affects the blue end of the emission spectrum. The same effect is produced by the fibre geometry used here, in which the laser light travels through relatively long lengths of doped material, significantly increasing its interaction with the dye. Because no dependence of the lasing wavelength on concentration or length was observed here (within the ranges tested), it was concluded that in all cases the concentration-length products were too large and the reabsorption effects were saturated. The postpolymerisation doping method and the absence of additives in the PMMA may also change the local environment of the dye molecules, which will also affect the lasing wavelength. Another system in which molecular order is not important is that of chiral materials which offer the possibility of circularly birefringent or optically active fibres. Circular polarization is associated with important physical phenomena, including Faraday rotation, in which linearly polarized light is rotated by the application of a magnetic field. Many biologically important molecules are optically active. This means that for a variety of important applications relating to sensing circular birefringence offers the most appropriate optical route. For example optical electrical current sensors employing the Faraday effect may use interferometric approaches based on circularly polarized light. Circular birefringence offers a route to making optical fibres that are polarization maintaining. Circularly polarizing fibres (which allow only one handedness of light to be transmitted) requires the other handedness to be lost. This may either happen because of circular dichroism, or by the appropriate choice of fibre design. The inventors have begun investigating approaches to making optically active fibres, both by doping, and also by developing chiral polymers and copolymers of cholesteryl methacrylate. It is increasingly clear that the ability to dope mPOF will allow a large number of new applications to be developed, particularly in the sensing, and nonlinear optical areas. Advantageously, the present invention enables doped mPOFs to be made easily, using commercially available low-loss polymer and opens up a large range of new applications of mPOFs. These include some applications that have been previously explored with conventional polymer fibres, such as fibre lasers and non-linear optically active fibres, electro-optic fibres, where the additional optical control available with the microstructure may be advantageous. The polymer walls between the holes of the microstructure in mPOF can be made less than a half a micron thick. Such structures are sufficiently thin that they can be considered dense membranes. The possibility of doping such membranes opens the possibility of new applications such as biological sensing, where efficient optical detection would provide an attractive non-contact approach. Other possible applications for the present invention include the incorporation of chiral materials to produce optically active fibres, incorporation of cross-linking agents after the fibre has been drawn to allow improved thermal stability, and the incorporation of photo sensitive material to allow gratings to be formed more effectively. Although the invention has been described with reference to specific examples, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms.

Claims

1. A process of forming a microstructured polymer optical fibre wherein one or more dopants are introduced through the microstructure of the fibre after polymerisation.

2. The process as claimed in claim 1 wherein controlled diffusion is used to disperse the dopant uniformly across the fibre core.

3. The process as claimed in claim 1 or 2 wherein the dopant is dissolved in a material that is a non-solvent for the polymer.

4. The process as claimed in claim 3 wherein said dopant is dissolved in a volatile material that is subsequently removed from the polymer.