POLYMERIC WAVEGUIDE WITH SINGLE DOPANT

Abstract

The present invention relates to the use of dopants for polymer optical fibres or polymer waveguides containing the dopants, sensors in the polymer optical fibres or polymer waveguides, which may be used in the biomedical industry for the measurement of different physiological and physical variables.

Claims

1. A polymer optical fibre comprising a single dopant material permitting both refractive index increase and photosensitivity enhancement for Fibre Bragg grating (FBG) fabrication, wherein the single dopant material comprises at least two phenyl rings linked by a disulfide bridge.

2.-3. (canceled)

4. A polymer optical fibre as claimed in claim 1, wherein the one or more of the phenyl rings may be functionalized with one or more groups selected from alkyl, substituted alkyl, alkenyl, substituted alkenyl, alkynyl, substituted alkynyl, phenyl, substituted phenyl, aryl, substituted aryl, heteroaryl, substituted heteroaryl, halo, hydroxyl, nitro, amino, alkoxy, substituted alkoxy groups, or carboxylic acid groups, or combinations of any two or more thereof.

5. A polymer optical fibre as claimed in claim 1, wherein the single dopant material is a compound selected from the group consisting of: ##STR00040## ##STR00041##

6. A polymer optical fibre as claimed in claim 5, wherein the single dopant material is diphenyl disulfide.

7. A polymer optical fibre as claimed in claim 1, comprising alternating integrated light source sections and sensing sections.

8. A polymer optical fibre as claimed in claim 7, wherein the integrated light source sections are doped with a laser dye and the sensing sections are doped with the single dopant material.

9. A polymer optical fibre as claimed in claim 1, wherein between about 0.1 to about 10 mol % of the single dopant material is employed.

10. A sensor system comprising a polymer optical fibre according to claim 1.

11. A sensor system as claimed in claim 10, wherein the sensor system is a medical sensor system.

12. A method of measuring one or more physical, physiological or biomedical variables, comprising using a sensor system according to claim 11.

13. A method of manufacturing a polymer optical fibre according to claim 1, comprising the steps of: doping a polymer optical fibre with a single dopant material, without UV irradiation wherein the single dopant material comprises at least two phenyl rings linked by a disulfide bridge.

14. A method according to claim 13, further comprising the step of: inserting the doped polymer optical fibre through an opening in a pre-made polymer preform cladding to form a preform

15. A method according to claim 14, further comprising the step of: drawing the preform into a photosensitive polymer optical fibre

16. A method as claimed in claim 15 further comprising the step of: exposing the photosensitive polymer optical fibre to UV radiation to fabricate a fibre Bragg grating.

17. A method according to claim 16, wherein the polymer optical fibre is exposed to UV radiation for less than 1 second to fabricate a fibre Bragg grating.

18. A method as claimed in claim 15, wherein the polymer optical fibre is a single mode polymer optical fibre.

Description

[0079] The invention will now be described further by way of example with reference to the following examples which are intended to be illustrative only and in no way limiting upon the scope of the invention.

[0080] FIG. 1a shows a complete sensor system in which a section of the POF is doped with laser dye and side pumped by one or more low-cost laser diodes;

[0081] FIG. 1b shows a 3-D optical preform;

[0082] FIG. 1c shows production of drawing 3-D polymer fibre that integrates light sources and grating sensors during the fibre drawing process;

[0083] FIGS. 2a-2c show reflection spectra recorded after FSGs were inscribed at with different irradiation times in a POF, with gratings recorded 10 s after inscription (FIG. 2a), 1 hour after inscription (FIG. 2b); and 2 weeks after inscription (FIG. 2c);

[0084] FIG. 3a shows the growth of FBGs written in 10 s, 1 s and 0.3 s, over a period of two weeks;

[0085] FIG. 3b shows the growth of FBGs written in 0.2 s, 50 ms and 7 ms, over a period of two weeks;

[0086] FIGS. 4a-4d show the fluctuations in wavelength and peak power during temperature cycling. Temperature profiles set from 20 to 50 C. with steps of 2 h and 10 cycles are presented for (a) a 10 s FBG, (b) a 0.2 s FBG, (c) a 50 ms FBG and (d) a 7 ms FBG. Only the first 2 and last 3 cycles were plotted in the figure with the dash line representing the missing cycles;

[0087] FIGS. 5a and 5b show the fibre chemical constituents and chemical degradation of (a) Diphenyl Disulfide and its UV-induced cleavage, and (b) the PMMA repeating unit and UV-induced degradation.

[0088] FIG. 6 shows a polymer optical FBG fabrication set up using a static beam.

[0089] FIG. 7 shows a POF being secured into a V-groove for FBG fabrication.

[0090] FIG. 8 shows two graphs depicting the measured refractive index profiles of several samples of DPdS core doped POFs and DPS+TS core doped fibre fabricated using the Teflon string method (the left-hand graph) and DPdS core doped fibres fabricated using the pull-through method (the right-hand graph).

[0091] FIG. 9 shows the pulling of the solid core through the cladding to form the preform.

[0092] FIG. 10 shows a DPdS-doped PMMA thin film being used in the fabrication of a planar photonic device.

[0093] FIG. 1a depicts the complete sensor system in which a section of the POF is doped with laser dye and side pumped by one or more low-cost laser diodes. The pumping technique overcomes the problem of photo-bleaching by extending the lifetime of the fluorescence output of dye-doped POFs. The sensing section of the POF should be highly photosensitive, allowing fibre gratings to be inscribed in it rapidly.

[0094] The 3-D optical preform shown in FIG. 1b could be fabricated with high-resolution 3-D printers. The centre of the preform that eventually forms the core of the POF consists of alternative sections of laser-dye doped materials separated by photosensitive materials for rapid inscription of grating sensors. The production of the 3-D POF would require a high-resolution 3-D printer for the cladding material, the laser-dye doped material, and the photosensitive material.

[0095] FIG. 1c shows the production of drawing a 3-D polymer fibre that integrates light sources and grating sensors during the fibre drawing process. The major obstacle of this novel approach is to develop a highly photosensitive polymer that allows rapid grating inscription times of less than 1 second.

[0096] Here, with this short inscription time, gratings can be produced during the fibre-drawing process. A potential mass production scheme for the gratings is depicted in FIG. 1c. The whole drawing-and-writing process can be automated by synchronising the winding speed of the take-up mechanism. Production of gratings with different Bragg wavelengths in one production line is also possible by applying tension or applying simple thermal treatment to the POF section where the FBG is to be inscribed (see the inset of FIG. 1). This is because of the large thermal expansion and heat transfer coefficients of POFs.

EXAMPLES

Method

Polymer Optical Fibre Fabrication

[0097] All the fibre and associated results presented herein are drawn from preforms fabricated using the pulled through method whereby the core fibre/rod (doped with a photosensitive material such as DPdS), is pulled through a hole in the centre of a pre-made polymer preform cladding. This method, although similar, has not to be mistaken with the Teflon string method, where the core monomer is poured in the cladding preform. Such method was responsible of a large diffusion of the core material during polymerization.

[0098] Two steps were employed to fabricate the polymer preform, namely a pre-polymerization stage and a polymerization stage. The pre-polymerization process takes place in a glove box while the polymerization process is made in an oven. Firstly, the suitable weight of chemicals should be prepared using an electronic balance with optimal ratio between monomer (MMA), initiator and chain transfer to prepare the cladding preform. Secondly, all the chemicals were mixed and placed in a glove box filled with argon gas at a relatively constant pressure, until the cleaning standard reached the ISO 5 (particles of 0.5 um<3250 in the ISO 14644-1 cleanroom) standard. Afterwards, the solution was stirred and heated in an oil bath inside an argon atmosphere, to avoid any explosive polymerization occurring.

[0099] A particle counter was used to record particles with sizes of 0.3, 0.5, 1, 2 and 5 m, and the temperature and humidity was recorded with a commercial detector. This process has allowed the inventors to fabricate preforms with lower dust particles and humidity, giving the fibres better properties than those made with a conventional process. After the pre-polymerization, the low viscosity solution was poured into a test tube in which a Teflon string has been fixed along the central axis. The Teflon string was pulled out of the preform, leaving the cladding part with a hole along the centre. The core mixture was prepared separately and poured into a glass tube. After curing in the oven, the core preform was drawn using a POF drawing tower into a rod. The rod was pulled through the central hole of the cladding preform. The assembled preform was then drawn into fibre. The final fibre diameter was 120 m with 3% tolerance. This pulling of the rod, i.e. solid core through the cladding to form the preform is illustrated in FIG. 9.

Fibre Bragg Grating Inscription

[0100] With reference to FIGS. 6 and 7, a KIMMON HeCd 325 nm laser 10 was used to fabricate the FBGs. Four mirrors 12, 14, 16, 18 were employed to guide the laser beam 20 from the laser to the POF, allowing the beam to reach a 150 mm plano-convex lens 22 placed right after the last mirror 18 to focus the beam 20 on the fibre. A beam shutter 24 was inserted in the optical path after the second mirror 14 and a 10 beam expander 26 was placed after the third mirror 16 to expand the beam 20 from 1.2 to 12 mm.

[0101] The fibre was secured into a Newport 125 m V-groove 28 using tape 30 and 2 layers of regular tape were placed on each side of the V-groove 28, on which was placed the Ibsen phase mask 32 for FBG fabrication. The phase mask pitch was 1046.3 nm. The tape thickness was around 60 m per layer, allowing a short distance between the fibre and the phase mask 32 without damaging it. In FIG. 8, one can see the measured refractive index profiles of several samples of DPdS core doped POFs and DPS+TS core doped fibre fabricated using the Teflon string method (the left-hand graph) and DPdS core doped fibres fabricated using the pull through method (the right-hand graph).

[0102] The POFs used were 120 m thick, and the core diameter was 5.5 m, consisting of 96 mol % PMMA and 4 mol % DPdS. The Reflection spectrum was investigated using a SMF-28 fibre with the end cut with an angle of 8 degrees (to annihilate the Fresnel reflection) and a Micron Optics SM 125 interrogator which range from 1510 nm to 1590 nm and resolution of 5 pm. All the FBGs were 10 mm long. FBGs of much shorter length (<1 mm) were also successfully inscribed in the POF in less than 1 second.

[0103] The refractive-index profiles of samples of POFs, fabricated with two different methods, were measured using a fibre index profiler (IFA100 from Interfibre Analysis), with the refractive index measurements compared between two DPdS core doped POF and one DPS+TS core doped fibre fabricated using the Teflon string method.

[0104] In the case of the DPS+TS core doped fibre, serious dopants diffusion from core to cladding can be observed which occurred during the core polymerization process, making it difficult to produce single-mode fibre. In the case of the DPdS core doped POF, virtually no dopant diffusion was observed. This results in an observed excellent abrupt change in the refractive index between the core and cladding.

[0105] The averaged transmission loss of the DPdS POF was measured by the multiple cutback method and the fibres were cut by hand using a hot plate and razor blade. Attenuation measurements were carried out at 870 nm and 1550 nm.

[0106] Tables 1 and 2 show attenuation data comparing for multimodes fibres comprising different dopant materials: DPS+TS, DPS alone, or DPdS alone, using a wavelength of 870 nm and 1550 nm, respectively.

TABLE-US-00001 TABLE 1 Core Dopant Attenuation (dB/m) DPS + TS 39.27 DPS 24.12 DPdS 18.17

TABLE-US-00002 TABLE 2 Core Dopant Attenuation (dB/m) DPS + TS 135.66 DPS 91.74 DPdS 96.16

[0107] It can be seen that for multimode fibres operating at a wavelength of 870 nm and 1550 nm respectively, the attenuation reduces significantly. The reduction is by as much as 39% at 870 nm and 32% at 1550 nm, when using DPS alone as the dopant material, in comparison with DPS+TS. This improvement is even more marked at 870 nm when DPdS is used as the sole dopant material, with a reduction in attenuation of 52% being observed. A reduction in attenuation of 29% at 1550 nm can also be achieved when DPdS is used as the sole dopant material.

[0108] Tables 3 and 4 show attenuation data comparing for single mode fibres comprising different dopant materials: DPS+TS, DPS alone, or DPdS alone, using a wavelength of 870 nm and 1550 nm, respectively.

TABLE-US-00003 TABLE 3 Core Dopant Attenuation (dB/m) DPS + TS 36.54 DPS 31.31 DPdS 26.67

TABLE-US-00004 TABLE 4 Core Dopant Attenuation (dB/m) DPS + TS 117.09 DPS 98.89 DPdS 87.12

[0109] It can be seen that for single mode fibres operating at a wavelength of 870 nm and 1550 nm respectively, the attenuation reduces significantly. The attenuation reduces by as much as 14% at 870 nm and 16% at 1550 nm, when using DPS alone as the dopant material, in comparison with DPS+TS. This improvement is even more marked when DPdS is used as the sole dopant material, with a reduction in attenuation of 27% being observed at 870 nm and a reduction of 26% at 1550 nm.

[0110] In general, fibres with higher concentration of core dopant exhibits higher losses, thus there is a trade-off between photosensitivity and transmission loss. Whilst the single dopant materials demonstrated much better reduction in attenuation when compared with DPS+TS, it is envisaged that the attenuation can be reduced significantly further by conducting the preform fabrication in an all-closed environment.

[0111] The grating inscription setup involves a beam shutter being placed in the optical path and controlled with a shutter controller, allowing a minimum irradiation time of 7 ms. A beam expander to expand the beam is mounted between the third and fourth mirror giving a 12 mm long elliptical beam on the V-groove. During FBG fabrication, the POF is secured in the V-groove with adhesive tape that limits the FBG length to 10 mm. The optical power of the UV beam after the plano-convex lens was measured to be about 25.5 mW (using Thorlabs S120VC). The same optical output power was used to inscribe all the FBGs presented in this work. This inscription method offers two main advantages: 1) shorten the FBG inscription time; and 2) irradiating the whole phase mask at once. This irradiation scheme uses low power density to illuminate the fibre and was demonstrated to be sufficient to write good quality FBG in just 7 ms. FIG. 3 shows the spectra recorded 10 s, 1 h and 14 days after UV irradiation for FBGs written in 10 s, 1 s, 0.3 s, 0.2 s, 50 ms and 7 ms.

[0112] The interesting behaviour observed from FIG. 2a is that all the FBGs appear nearly immediately after UV irradiation. Obviously, the most important information in FIG. 2a is the fabrication of the millisecond FBGs, where 50 ms and 7 ms FBGs exhibited SNR of respectively 12 dB and 7 dB, 10 s after UV irradiation. In FIGS. 2b and 2c, significant growth of all the FBGs was observed. The SNR of 50 ms and 7 ms FBGs were recorded to grow to 18.5 dB and 14.8 dB, 14 days after the UV irradiation process. Moreover, the FBG written with a UV irradiation time of 0.3 s or longer exhibited a noise level higher than FBGs inscribed in less than 0.3 s. This phenomenon is due to the effect of the Norrish type II photochemical reaction, where crosslinking of the PMMA side chains was dominant, which caused irregularities along the irradiated regions along the FBGs.

[0113] It is also to be noted that the reflectivity of the FBGs written with a longer UV irradiation time stabilised faster. Furthermore, the FWHM (Full Width Half Maximum) of the FBGs increases with the irradiation time except for FBG written in 0.3 s or less where the FWHM was measured to be 80 pm, 14 days after the inscription. Furthermore, the low side lobes of the millisecond FBGs (especially 7 ms and 50 ms), produce a high SNR, which is desirable for sensing application.

Stability of the Gratings

[0114] Gratings written in polymer fibres can be unstable, and their peak wavelength and power can fluctuate over time. Stress in POFs is induced during the fibre drawing process. The stress can be released slowly by thermal annealing at about 80 C. for many hours. Without thermal annealing after FBG fabrication, the fluctuations of the reflection peak wavelength and peak power of the FBGs were recorded for gratings left in an air-conditioned laboratory. FIG. 3 shows the result for gratings left in the environment without strict control of temperature or humidity for up to two week. For all the gratings, the data referred as Day 0 were taken 30 s after the end of irradiation.

[0115] Significant fluctuations of the peak power and wavelength were observed during the first few days after inscription. All FBGs stabilised after about 1 week, and the exhibited growth is quite pronounced even for the 7 ms grating. FIGS. 3a and 3b demonstrate that there are two types of growth behaviour which are actually related to the irradiation time and involved two different chemical processes. For longer UV irradiation time (1 s and 10 s), an overall peak power growth is noticeable within one week although a sharp decay is observed within the first few days after fabrication (FIG. 3a), due to damage in the PMMA main chain. For FBGs fabricated with a shorter irradiation time, such as 0.2 s, 50 ms and 7 ms (FIG. 3b), the gratings growth follow a similar trend and the stabilisation is inversely proportional to the UV fluence.

[0116] The inset in the upper right corner of FIG. 3b shows the refractive index profile of DPdS-doped fibre both with and without 4 s of UV-irradiation.

[0117] Further evidence of these different growth behaviours due to the two different chemical processes is the stabilised peak power after 2 weeks, where 1 s and 10 s FBGs stabilised at greater than 32 dBm whereas the 0.2 s, 50 ms and 7 ms FBGs stabilised around 45 dBm involving a stabilisation of a UV-induced chemical process in the fibre core. An FBG written in 0.3 s (FIG. 3a) exhibited a behaviour produced from the combination of the two chemical processes mentioned above. A sharp growth was observed within the first day, followed by a stable state with a peak power recorded at around 35 dBm. The inset of FIG. 3b shows the measured positive UV induced refractive index change in the DPdS-core fibre doped which was irradiated for 4 s.

[0118] To further characterise the stability of the FBGs, thermal tests were conducted using an environmental chamber where the FBGs were cycled with temperature profile over the range of 20 to 50 C. for 10 cycles of 8 hours each, at a constant humidity of 20%. FIGS. 4a, 4b, 4c and 4d show the results of the stabilized FBGs inscribed in 10 s, 0.3 s, 50 ms and 7 ms, respectively.

[0119] The grating wavelengths shifted to shorter wavelength with temperature which is consistent with previous findings. Interestingly, all the FBGs exhibit a pellucid stability. The observed wavelength drop for all the FBGs within the first 2 hours is due to the decrease of humidity from ambient air to 20% while temperature was stabilised at 20 C. Furthermore, the FBGs exhibited excellent thermal response and their average sensitivity during the three last cycles (8th, 9th and 10th) were 55 pm/ C. for 0.3 s and 50 ms, and 40 pm/ C. for 10 s and 7 ms FBG. The difference in sensitivity or temperature coefficient is mainly due to the non-uniform doping concentration of DPdS in the fibre core. The test results demonstrated that the use of DPdS as a single material dopant in POF not only permit rapid fabrication of FBGs but also possess excellent properties for potential application as single-use medical sensors.

[0120] It can therefore be seen that the use of a single dopant material, exemplified here with DPdS, enables the increase of the refractive-index of polymer optical fibres and their photosensitivity, allowing rapid FBG inscription in just 7 ms, which is more than 60,000 times faster than has previously been reported result in the writing of FBGs in any kinds of POFs at the 325 nm wavelength. These POFs are able to be used in the production of 3D polymer optical fibres with an integrated light source and sensor, with high quality gratings in POFs to be produced during the fibre drawing process.

[0121] With reference to FIG. 10, it is envisaged that the use of a single dopant material, exemplified here with DPdS, enables rapid waveguide fabrication of planar photonic devices, in addition to POFs. The use of a single dopant such as DPS or DPdS permits the writing of photonic devices using a UV laser beam on a thin-layer of DPS or DPdS film supported by a substrate of PMMA without any chemical post-processing.

[0122] It is of course to be understood that the present invention is not intended to be restricted to the foregoing examples which are described by way of example only.