A simple heat imprinting method for producing stable longperiod gratings (LPGs) in microstructured polymer optical fibre (mPOF) is presented as well as the examination of their lifetime and the modelling results of these gratings. Writing LPGs in mPOF presents opportunities for sensors in fibre that can withstand greater bending and strain and are adaptable to specific applications through modification of the cladding structure.
© 2006 Optical Society of America
Microstructured polymer optical fibres (mPOF) , like their silica counterparts photonic crystal fibre (PCF)  guide light through the use of a pattern of tiny holes that run the full length of the fibre. MPOF allow a wide variety of hole patterns to be produced, which change the optical properties of the fibre, and the material properties of polymer (including a smaller Young’s modulus, greater elastic limit) also make them desirable for sensing applications, particularly for measuring strain and bending.
Long-period gratings (LPGs) have depressions in their transmission spectrum at wavelengths where a periodic variation in the fibre produces coupling between the core mode and a cladding mode. They have numerous applications such as filters, attenuators, gain-flattening and sensors . Of particular interest is the simultaneous measurement of a number of physical quantities. This is made possible by the multiple features of LPGs which can be independently effected by different physical changes . While LPGs are a well established technology in silica fibre, they are relatively new in polymer, with Li et. al. producing the first permanent LPG in polymer optical fibre (POF) in 2005  using a 275 µm amplitude mask and a mercury lamp. This resulted in a grating depth of 3 dB.
Microstructured fibres offer particular scope for exploiting LPGs because changes to the microstructure allow the effective index of the cladding modes of a fibre to be widely varied. LPGs have been successfully manufactured in PCF through a variety of methods including writing with a UV laser , a CO2 laser [7,8], arc discharge [9–11], acoustic waves  and temporary mechanical inducement . Prior to this work, temporary LPGs in mPOF were created using a grooved polymethylmethacrylate (PMMA) template pressed against the fibre . The difficulty producing LPGs through other methods stems from the importance of alignment noted by Dobb et. al. They successfully wrote fibre Bragg gratings in mPOF using a low power continuous wave UV source  but found this required a particular orientation of the microstructure relative to the writing beam. This is problematic for LPGs as the alignment must be maintained over a relatively long length of the fibre.
Here we present results of the first method of creating LPGs in mPOF which could be considered permanent due to the demonstrated long lifetime and stability of the gratings. The gratings are produced by heating the fibre under mechanical stress, which has a number of advantages over other methods that have been used to date. Firstly there is no need for a high powered laser or amplitude masks to write the grating. Secondly gratings of long length can be written quickly and easily without the need to shift and align the fibre accurately to ensure that the written areas are in phase. Finally, there is no need for dopants to make the core photosensitive. This paper also presents the results of modelling the LPG resonances and examines the degree to which cladding structure impacts the resonant wavelengths.
2. Experimental setup
The experimental setup used to imprint gratings is the same as that reported in  with the inclusion of a heated base. White light is launched via a 10x objective lens into a short (approximately 1 m) section of single mode mPOF shown in Fig. 1 and the output is imaged by a 40x objective lens onto the detector of an optical spectrum analyzer. The section of fibre that is to be imprinted is placed across the heated base and fixed to stands on either side of the base so that the fibre is taut. A thin strip of magnetic material (not chosen for magnetic properties) is positioned between the fibre and the heated base to provide a softer contact for the fibre. Index matching oil was used to strip the cladding modes so that they may not be coupled back to the core mode. The grating template is then placed on the fibre. For these experiments two 15cm cylindrical PMMA templates with triangular grooves were made. The first with grooves of depth 0.29 mm and a period of 1 mm, the second of depth 0.125 mm and a period of 0.5 mm. The template is held in place by two metal rods and weight is added evenly across the grooved section. The grating is written by heating the base and progress monitored by observing the transmission spectrum.
3. Imprinting results
Figures 2 and 3 show the transmission spectra of the fibre with LPGs produced using this method after the weights and the PMMA template were removed from the fibre. The 1 mm period grating exhibits strong attenuation at 510 and 651 nm as well as numerous smaller features while the 0.5 mm grating has fewer and broader features with the strongest features at 510 and 683 nm.
The strength of these gratings is comparable to those currently produced in PCF. The PCF gratings described in [6–13] range in strength from 6 to 31.5 dB, with the majority around 11 dB. For LPGs in polymer fibre these gratings are considerably stronger than the 3dB obtained by Li et. al.  and of the same order as those reported in .
Variations of the heat imprinting method were used to determine the best parameters for inscribing the gratings. This included varying the temperature of the heated base, the heating conditions (cyclic or continuous heating) and the weight upon the grooved PMMA template. It was expected that the temperature required to write a stable grating would be above the glass transition point for PMMA (115°C). It was found however that lower temperatures were optimal, though further work is being undertaken to improve the high temperature writing. The best gratings were written by keeping the temperature at around 60–80°C. At approximately 60°C a deepening of the attenuation peaks was observed, at which point the heated base was turned off. The heating was resumed when the spectrum was stable and the process was repeated until the desired attenuation was reached or the features developed no further. At this point the weight and grating were removed and it was confirmed that the LPG features remained. There was an immediate reduction in the strength of the grating when the template was removed. The complete writing process took approximately 10–20 minutes. The low inscribing temperature used to write the gratings suggested that the gratings produced were at least partially due to stress effects rather than permanent physical deformation which is supported by the findings from the lifetime testing. The best results were obtained with approximately 1–3 kg distributed along the template (approximately 60–200 g/cm).
Gratings imprinted by this method were repeatable, with similar writing conditions resulting in features with consistent locations. The main variation experienced was the depth of certain features. Shifting the position of the weight along the grating or the changing the temperature at which the grating was written was able to enhance or suppress certain features.
4. Lifetime of gratings
Accelerated aging tests were used to assess the long-term stability of the gratings. Under raised temperature conditions the gratings experienced a drop in grating strength over a time period in the order of minutes. After this decrease the grating strength was found to be quite stable. We attribute these results to the grating having both a stress component, and a component due to physical deformation. Examination under a microscope confirmed physical deformation at intervals consistent with the period of the grating. It was noted that low temperature and high weights, which would produce the strongest stress component, also produced the least stable grating. Annealing is well known to remove stress in polymers.
Gratings produced with lower weights and higher temperatures produce shallower gratings initially however they result in a deeper grating remaining after being stored at higher temperatures or the passing of many days. This is evident in Fig. 4 which shows the depth of a feature in two gratings stored at 60°C, one inscribed using 3 kg at 75°C (low weight high temperature) and the other with 7 kg at 60°C (high weight low temperature). After 2 days stored at 60°C both gratings were still evident with the depth of the features reduced to approximately 8.4 dB and 2.8 dB respectively. In addition to this a grating written under similar conditions to that labeled LWHT was stored at 60°C for a period of 2 weeks and maintained a grating strength of approximately 8.7 dB.
In LPGs coupling occurs between the core and a cladding mode when the condition below is met:
where n co and n cl are the effective indices of the core mode and cladding mode, λ is the wavelength, ΛLPG is the grating pitch and m is the order of the coupling. The resonances of the LPGs were predicted by calculating the effective index of the core mode and the first three cladding modes using the adjustable boundary condition method . These were then used to calculate the necessary grating period to result in a resonance at the given wavelength.
The sensitivity of LPGs to changes to a small number of holes in the cladding structure is quite significant as demonstrated by Dobb et. al. . To better understand sensitivity of the LPG resonances to changes in microstructured cladding we modelled the effect of systematically varying the microstructure. The results are shown in Fig. 6. Increasing the hole size of all holes in the microstructure by 10% causes a large shift in the predicted resonance, in this case a shift of approximately 265 nm for a grating period of 1 mm.
This modelling is unable to precisely predict the observed features of the LPGs as it is based upon an ideal fibre structure. The fibre used in these experiments differs from the ideal structure in that two holes are partially collapsed (Fig. 1) making it birefringent. Modelling this asymmetry confirmed birefringent splitting in the core and cladding modes, which is also suggested by the experimental results where the larger features in Fig. 2 are accompanied by a smaller feature at a wavelength 20 nm shorter. Qualitatively the two smaller holes in the cladding structure will increase the value of n cl more than n co, resulting in a shifting of the resonances to longer wavelengths. Thus, while the modelling does not predict the exact location of the resonances, it does give approximate positions for the features and confirms general trends such as the shifting of the resonances to shorter wavelengths and closer spacing of the features with longer grating periods in agreement with the experimental results.
We have presented a simple, inexpensive and repeatable method of inscribing long-period gratings in mPOF. Gratings produced by this method have shown features with attenuation of up to 18 dB or have retained features with attenuation greater than 8 dB after being stored at 60°C for 2 weeks. Further work will involve attempting the imprinting of LPGs in photonic bandgap mPOF following Steinvurzel et. al. . We will also investigate the application of the current LPGs to sensing, an area for which these gratings are highly suited given their flexibility and ability to withstand strain as a result of being written in polymer fibre. This is combined with the freedom of fibre designs possible in mPOF creating LPGs with considerable potential for future applications.
The authors thank their colleagues B. Reed and R. Lwin for the fabrication of the grooved templates and fibre respectively. The ARC is acknowledged for the funding of this work.
References and links
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