Automation for the manufacturing of fiber Bragg grating arrays enables new applications

Transcription

1 Invited Paper Automation for the manufacturing of fiber Bragg grating arrays enables new applications P. Lefebvre, A. Vincelette, C. Beaulieu, P. Ficocelli * LxSix Photonics Inc., 52 McCaffrey, Montreal, Quebec, Canada, H4T-1N1 ABSTRACT The manufacturing process has a huge impact on the characteristics of the all optical fiber sensors array. By automating the manufacturing of fiber Bragg gratings, FBG arrays with much larger count of sensing points, stronger mechanical strength, tighter optical parameters tolerances and enhanced reliability are produced in a cost effective manner. Such fiber Bragg grating arrays are now commercially available with both acrylate or polyimide coating widening the range of applications for FBG sensors to larger scale of services for strain and temperature in a distributed configuration. Keywords: fiber Bragg gratings, sensors, sensors array, manufacturing process 1. INTRODUCTION Fiber Bragg gratings were discovered quite inadvertently at the Canadian Communication Research Center in 1978 by Ken Hill et al. [1] during experiments with argon laser light launched in a germanium-doped fiber. For the next decade, the subject remained in a small circle of researchers due to the difficulties of creating the Bragg gratings structure in optical fiber by pumping laser light into the ends of the fiber. The work of Gerry Meltz et al. [2] in 1989 resulted in a new fabrication method for fiber Bragg grating using a holographic UV laser exposition transversal on to the fiber. This new method constituted the breakthrough that enabled the spread of research and development in fiber Bragg gratings technology and manufacturing. Since most of this initial prototyping work was done in the 199s, which corresponded with the telecommunication bubble, it was mostly targeted towards fiber optics telecommunication network and not sensing. The other consequence was the instantaneous multiplication of the researcher s prototyping set-ups to address the rapid ramp-up in the volume demand. This method relied mostly on the skills of manual labor since it skipped the development of process controls that are common in industrial production lines. These highly manual processes limit the capacity in term of repeatability, mechanical strength and the number of Bragg gratings and length of the FBG array. The booming telecommunication industry of 199s could absorb the cost of the low yield, and since the usages of fiber Bragg gratings in a telecommunications environment are on a unitary base, the industry was satisfied with small fiber length containing only one fiber Bragg grating. The sensor industry is more cost sensitive, requires multiple sensing points and greater mechanical strength. The ability to fabricate an array of multiple fiber Bragg gratings at different locations along a same length of optical fiber permits the use of wavelength for the de-multiplexing and optimization of the optical power for multi-points sensing systems. This multiple sensing point capability enables the design of more complex sensing system at lower costs. But in order to fabricate fiber Bragg gratings sensor arrays at these acceptable cost, the manufacturing process must be well under controlled and very predictable since the global yield is the product of each of the individual FBG criterion and the number of sensing points. Therefore any aspect that the process can not control will exponentially impact the cost of the array by a factor that is directly proportional to the number of sensing points. Automation of each aspects of the manufacturing process combined with the use of industrial optimization techniques, such as Six Sigma can lead to sufficient process control, repeatability, reproducibility and predictability. This is the approach that LxSix Photonics developed for the automated reel to reel fiber Bragg grating arrays fabrication platform. * pficocelli@lxsix.com; phone (514) ext. 263; fax (514) ; lxsix.com Fiber Optic Sensor Technology and Applications IV edited by Michael A. Marcus, Brian Culshaw, John P. Dakin Proc. of SPIE Vol. 64, 648, (25) X/5/$15 doi: / Proc. of SPIE Vol

2 2. FIBER BRAGG GRATING AS SENSOR Fiber Bragg gratings are constituted by a series of periodical fringes along the wave-guide path, characterized by small refractive index changes in the core s material that reflects back portion of specific wavelengths, see figure 1. For fiber Bragg gratings of constant period, the reflected wavelengths follow a Gaussian distribution in intensity around a central Bragg wavelength (λ Bragg ): λ Bragg = 2 n eff Λ (1) Where Λ is the period of the fringes and n eff is the effective refractive index of the wave-guide. The refractive index is a function of its geometry and of the refractive indexes of the medium and its immediate surrounding. From this equation (1), it is obvious that if the fiber optic is stretched, changing Λ, the Bragg wavelength reflected will also changes. The other parameter having direct influence on the Bragg wavelength is the value of n eff, which vary linearly with temperature. In fact, the variation of the Bragg wavelength can be written in the general form: λ Bragg = A T + B ε (2) Where T is the variation of temperature, ε is the variation of axial strain and A and B are two constants, or gauge factors. Typical values of fiber Bragg grating sensors in the 155 nm window are around 1 pm/ o C for A and 1.2 pm/µε for B. Multiple fiber Bragg gratings can be interrogated simultaneously either through wavelength-domain or time-domain multiplexing. In the wavelength-domain multiplexing (WDM), each fiber Bragg grating reflects a specific Bragg wavelength (λ i ) which is interrogated using a light source and a reflected power meter covering the full range of all the λ i,. The differentiation is done by analyzing the full spectrum composed of the response of the individual Bragg grating s reflection. In the time-domain multiplexing (TDM), a pulse of light is sent down the fiber and each fiber Bragg grating reflects the specific Bragg wavelength. The differentiation is done by reading the reflected power after a delay corresponding to each specific travel time between the source and the specific Bragg grating. In both configurations, an interrogator can monitor a series of sensors at different locations. Reflected Signals nith navelengihs? and -V Fiber Bragg Gratings Optical fiber with gratings will reflect back the spifie wavelengths that the grating were designed tn reflect. Figure 1: Fiber Bragg grating sensors array Fiber Bragg grating sensors can be single point, one per fiber length, multi-points, several per fiber length, or distributed by juxtaposing several Bragg gratings along a fiber segment, see figure 2. Another sensing configuration uses the variation in the phase of the reflected light between two identical fiber Bragg gratings along a same fiber to Proc. of SPIE Vol

3 monitor changes occurring in the fiber between the two gratings. Slanted fiber Bragg gratings and long period gratings can be used to send light into the cladding where the wavelength modes will be function of n eff. In this case wavelengths will vary with change in the index of the medium surrounding the cladding. By selecting proper coating material that reacts to the selected substances in the environment that changes the refractive index of the coating will affect the shape of the light in the cladding thus providing a signal that the desired substance has been detected J Single-point sensor Fiber Multi-point (quasi-distributed) sensor Sensing element Distributed sensor re- r Multiple sensing points Continuous sensing element Figure 2: Fiber Bragg grating sensors configurations In addition to these intrinsic sensitivities to temperature and strain, the fiber Bragg gratings can be use to measure other parameters by using transducers that transform the measurand into either strain or temperature. The transducer can be a mechanical device that would transfer the measurand, such as force, pressure, torque, into a strain. Transducers can also be developed in much smaller package like a selective coating reacting to the environment, as described in the paragraph above. Of course, many variations are possible and proper compromises must be found between sensitivity, selectivity and the required ruggedness necessary to insure long-term reliability. Glass is an inert material that melts at high temperature and is insensitive to electro-magnetic fields making fiber optic sensors particularly adapted to harsh environments. In addition to the usual advantages of optical fiber sensors, fiber Bragg grating arrays provide multi-point values at precise relative positions and are particularly adapted to form a monitoring grid to fit a finite element model, enabling the design of sensing systems with complex data interpretation and provide a greater level of actionable information. 3. LxSix ARRAY S CAPABILITIES LxSix Photonics is able to write a multitude of fiber Bragg gratings with varied characteristics at any desired locations in any optical fiber. To do so, LxSix Photonics has developed a patented (USPTO application # 1/276,91) reel to reel fully automated platform to manufacture fiber Bragg gratings in which the entrant is a standard reel of optical fiber and the output is a reel of the same standard optical fiber having the same properties but containing a desired amount of fiber Bragg gratings at the desired locations along that optical fiber. The platform is presently compatible with standard single-mode or polarization maintaining optical fiber of 8 and 125 microns diameter of glass, coated with acrylate or polyimide, in broadband windows from 98 to 165 nm. Proc. of SPIE Vol

4 The manufacturing platform developed by LxSix Photonics is based on a modular approach segmented by functional unitary operations, see figure 3. These major functional unitary operations include ; (1) unwinding the fiber, (2) locating the desired position for the fiber Bragg grating, (3) opening a window in the fiber coating (stripping), (4)creating the Bragg grating by UV pattern exposition, (5)thermodynamically stabilizing the Bragg grating for the desired lifetime in the specific environment, (6) closing the coating window by re-applying and re-curing the coating material (recoating), and (7) marking the Bragg grating position (alternatively marking an unique serial number) on the fiber and winding the fiber back on a standard reel. Most of the modules for individual functional unitary operations have the same footprint and have been developed in several versions to address specific types of fiber and/or characteristics of the desired final products; for example, the modules for stripping and recoating acrylate or polyamide coatings on a 8 or 125 microns diameter fiber can be interchanged in less than five minutes. The fiber unwinding and winding operations are made without moving the spools permitting fixed connections to optical measurement units at each fiber ends, and so, enabling constant in situ monitoring and use of feed-back loop during the manufacturing of fiber Bragg gratings. Figure 3: Automated reel to reel fiber Bragg grating arrays manufacturing platform Fiber Bragg gratings are made by exposing a section of the core of the fiber to a transversal series of sub-micronic UV fringes. Since traditional photo-lithographic methods do not have the required resolution, the UV fringe patterns are generated by creating a stationary interference pattern between two coherent wave-fronts of UV laser light. The incident relative angle between the laser rays determines the period of the stationary interference fringes. Once this pattern is formed, the core of the fiber is placed perpendicular to the laser light to begin the formation of the Bragg grating. One of the core competencies of LxSix Photonics is its patent pending (USPTO application # 1/296,79) UV exposition technology that combines the far-field and secondary noise orders trimming capability of traditional holographic approach (3) and the ruggedness and repeatability of the traditional direct phase mask approach (4). Using obscurators, lens, optical prisms and diffractive elements, LxSix Photonics has developed a series of optical modules able to project uniformed, apodised and/or chirped interference pattern with automatically adjustable length, period and intensity profile. Combined with the feedback loops ability of the platform and the LxSix Photonics patented (USPTO application # 1/276,9) fiber core auto-alignment technology, the manufacturing line has the required flexibility to fabricate splice-less fiber Bragg grating arrays that can be easily de-multiplex and configured for multi-functional characteristics with our on the fly programmable pattern parameters adjustments. The automated approach enables the use of the full potential of statistical industrial techniques to optimize the process, and the modular segmentation permits to individually optimize each aspects of the process, as well as, separate the tolerance on each of the final products parameters. With no manual intervention, it also increases considerably the repeatability and the reproducibility of the process, and enables more accurate long-term reliability characterizations. This make the manufacturing platform particularly fit for volume production. Figure 4 and 5 present s typical distributions of the main parameters of manufactured fiber Bragg grating in a volume production of unitary gratings configuration for a telecommunication application. Figure 4 presents the strength distribution of fiber Bragg gratings manufactured for a 25 years life span with a service stress under 35 kpsi which require an initial strength over 15 kpsi. Even though the distribution predicts a 233 ppm chance of failure is small, LxSix Photonics screen all of its productions by a proof test approach to further insure maximum product life span. Figure 5 presents the distributions of the three main optical parameters for LxSix manufactured Bragg gratings, in (a) the Proc. of SPIE Vol

5 Bragg wavelength, in (b) the percentage of light reflected at the Bragg wavelength, and in (c) the bandwidth, for a typical volume production where the minimal acceptable value is LSL and USL is the maximal acceptable value. The distributions predicts a sub-ppm chance of producing a fiber Bragg grating with unacceptable Bragg wavelength or bandwidth, and a.7% chance of producing a Bragg grating with unacceptable reflectivity. Since the reflectivity is controlled via a feedback loop during manufacturing, this.7% probability of being off tolerance can be significantly reduce by (1) increasing the exposition time, (2)lower UV laser intensity to allow more in situ measurements thus increasing the accuracy of the feedback loop. The ppm and sub-ppm level of these defects can be viewed as excessive for non FBG array production runs, but is necessary for the production of high channel count fiber Bragg grating arrays since the global yield is equal to the single FBG unit yield powered by the number of elements in the array. Achfl*: N Pfta Tk!. N 95; ; SbjD ; M ; Mi Fft t Sir! F fttt! Figure 4: Strength distribution example of the manufacturing platform 3 LSL Capability Histogram: TCW Within SD:.145; Cp: 11.49; Cpk: Overall SD:.174; Pp: 9.569; Ppk: LSL: 974.5; USL: *S +3.*S Nominal USL 12 Capability Histogram: REFL Within SD:.497; Cp: 3.356; Cpk: 3.23 Overall SD:.5; Pp: 3.335; Ppk: 3.4 LSL: 1.5; USL: 2.5 LSL -3.*S Nominal +3.*S USL LSL 3 Capability Histogram: TBW Within SD:.52; Cp: 12.9; Cpk: 12.8 Overall SD:.67; Pp: 9.952; Ppk: LSL:.2; USL:.6-3.*S +3.*S Nominal USL a) b) c) Figure 5: Optical parameters distributions example of the manufacturing platform Proc. of SPIE Vol

6 4. PERFORMANCE EXAMPLES The fiber Bragg grating arrays manufacturing platform developed by LxSix Photonics is particularly adapted for the fabrication of high channel count sensor arrays for both TDM (time-domain multiplexing) and WDM (wavelengthdomain multiplexing) interrogator system. The flexibility of the writing method to adjust on the fly the individual Bragg grating s parameters is demonstrated in Figure 6, which presents the spectral response of a splice-less 1 Bragg gratings array containing one sensor at each.5 nm wavelength in the nm window. The full spectral response in Figure 6 a) is opaque and we need to present a magnification of the response in Figure 6 b), to be able to evaluate the uniformity between the individual sensor spectral responses. On 1 fiber Bragg grating arrays, we obtained standard deviations of less than 1% for all the optical parameters including wavelength spacing, 43 pm for a 5 pm steps, demonstrating the level of accuracy of the on the fly Bragg wavelength tuning of the manufacturing platform Sensor array 'awna IIJ*d sn s iiii'i q.i Reflection spectrum Transmission spectrum Reflection loss (db) Wavelength (nm) Transmission loss (db) 543M l M l M l M l L a) b) Figure 6: 1 sensors fiber Bragg grating array The increased repeatability and reproducibility of the platform, as well as its completely automated nature, allows a better optimization and more long-term predictability. For instance, components for telecommunication equipments require an initial strength of at least 15 kpsi to insure a lifetime of at least 25 years at a service stress of 35 kpsi. On the other hand, a series of accelerate aging tests have been developed to insure a lifetime of at least 25 years for components used in an outside plant environment for optical telecommunication systems, these tests are described in the GR-129 and GR-1221 Telcordia norms. Figure 7 presents the measured strength of LxSix Photonics manufactured optical components for telecommunication after the completion of the above mentioned tests. Although the requirement for unused parts is of 15 kpsi, out of 173 samples tested, the lowest value obtained after an accelerated 25 years usage conditioning is 26 kpsi, and the average value for end of lifetime strength is over 5 kpsi. In addition,, fiber Bragg gratings kept in clean room conditions (CRC) have a similar distribution than those who have been through accelerated aging; the most stringent tests for recoat quality are the immersion (5.4.4), humid temperature cycles (5.4.3 & 6.2.8) and 2 hours at 85 o C and 85 %HR (6.2.5), but the units which have undergone these tests are not the most weaken. This clearly demonstrates that the LxSix Photonics automated platform does fabricate fiber with similar coating protection as the original optical fiber. The laser writing process slightly weakens the fiber during manufacturing but the recoat process insures similar long term degradation as virgin fiber. LxSix Photonics is equipped with a complete reliability laboratory to test for telecommunication norms and standards, and is in the process of expanding the test laboratory to characterized long-term performances of fiber Bragg grating sensors and to quantify various environmental affects on gauge factor values. Proc. of SPIE Vol

7 Strength after Telcordia environmental tests Failure Probability (% Tensile Strenght (kpsi) Pristine Aged CRC & Figure 7: Strength fiber Bragg gratings after Telcordia tests The modular approach to develop and optimize specific functional operation can be illustrated by the polyimide recoater. Polyimide coating is used for two purposes, to increase the temperature range compared to acrylate coating, and to transmit strain from the environment to the glass fiber since this type of coating has a very good interface adhesion to glass and a thin and very rigid layer. On the other hand, polyimide solutions are diluted in around 85% solvent that evaporate during the polymerization process. If the evaporating and curing rates are not well synchronized and uniformed then the shrink cured polyimide layer will be of variable thickness and will strain the glass fiber un-uniformly thus deforming the spectral response of the fiber Bragg grating underneath. Figure 8 presents typical spectral responses of an LxSix Photonics fiber Bragg grating before, a), and after, b), the polyimide recoating layer. The polyimide coating induces a variation of less than 5 pm of the Bragg wavelength and there is no noticeable change in the spectral response due to the high uniformity of the recoat layer. j-ijj JU 4p JU_A lill r F F F I RItt F F F F a) b) Figure 8: Effect of polyimide recoat on spectral response of Bragg grating Proc. of SPIE Vol

8 5. CURRENT AND FUTURE APPLICATIONS LxSix Photonics presently manufactures fiber Bragg grating arrays to be used with various interrogators from the leading manufacturers. Current standard products are; (1) arrays of 5, 1 and 2 apodized Bragg gratings of high reflectivity at different wavelengths compatible with standard WDM interrogators;(2) arrays of 1, 25 and 5 apodized Bragg gratings of low reflectivity at the same wavelength for TDM interrogators, and (3) arrays of 5, 1 and 2 Bragg gratings based Fabri-Perot cavities for interferometric interrogators. All of those arrays are available on single-mode or polarization maintaining fibers with 8 or 125 microns diameter and coated with acrylate or polyimide. These arrays can be covered with a loose buffer jacket and/or metallic armored layer, and the fiber s end can be connectorized. LxSix Photonics also fabricates on demand custom fiber Bragg grating arrays. Future development involves manufacturing sensing cables for different types of harsh environments. The objective of LxSix Photonics is to supply fully characterized rugged sensing cables with the proper user s procedures and software to calculate the compensated gauge factors for each sensing points. In order to do so, characterization programs have begun, as well as the development of UV exposition modules enabling the fabrication of more complex grating s structures that will be used to calibrate in situ different effects on the gage factors of the sensors. 6. CONCLUSION LxSix Photonics has developed a platform to manufacture fiber Bragg grating arrays. Its capacity to fabricate spliceless high channel count sensor arrays is not only due to its reel to reel configuration, but also to an automated modular approach that enables the use of statistical industrial optimization methods to achieve the necessary repeatability, reproducibility and predictability to obtain reasonable manufacturing yields. By also developing innovative rugged functional modules, LxSix Photonics was able to insert fiber Bragg gratings on a wide variety of optical fibers: single-mode or polarization maintaining, 8 or 125 diameter microns, acrylate or polyimide coated. Its innovative UV exposition technology enables on the fly changes of the projected interference pattern to fabricate arrays of fiber Bragg gratings with different characteristics for both de-multiplexing purpose and for multiparameters sensing. In addition to developing new modules to fabricate more complex grating s structures, LxSix Photonics is developing modules to address different type of coatings and cabling in order to be able to supply rugged sensing cables for different types of harsh environments. Characterization programs have started to be able to predict and calculate gauge factors of those cables while considering the effects on those gauge factors of different environments and through the different layers of the cables. REFERENCES 1. Hill K. O., Fujii Y., Johnson D. C., and Kawasaki B. S. Photosensitivity in optical waveguides: Application to reflection filter fabrication, Appl. Phys. Lett. 32(1), 647 (1978). 2. Meltz G., Morey W. W., and Glenn W. H. Formation of Bragg gratings in optical fibres by transverse holographic method, Opt. Lett. 14(15), 823 (1989). 3. Morey W. W., Meltz G., and Glenn W. H. Holographically generated gratings in optical fibers, Optics & Photonics News 1(7), 8 (1994). 4. Hill K. O., Malo B., Bilodeau F., Johnson D.C., and Albert J. Bragg gratings fabricated in monomode photosensitive optical fiber by UV exposure thorough a phase-mask, Applied Physics Letters, 62, (1993) Proc. of SPIE Vol