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Optoelectronics & Communications

Grating-inscription technique eliminates need for fiber stripping and recoating

Bragg gratings written directly through the polymer coatings of optical fibers with femtosecond IR lasers and a phase mask result in devices with improved mechanical reliability and strength.
16 February 2009, SPIE Newsroom. DOI: 10.1117/2.1200901.1502

For more than a decade, fiber Bragg gratings (FBGs) have been cost-effective and versatile components in telecommunication applications and optical-fiber-based sensing systems. Today, a key challenge in manufacturing FBG devices is the time-consuming inscription process, which requires fiber stripping and recoating, often negatively impacting the mechanical integrity of the fiber. FBGs are inscribed, or ‘written’, by photo-imprinting a periodic variation in the refractive index of the optical-fiber core using a UV laser. Typically, these variations are created through interference of two coherent UV beams or by passing a single UV beam through a silica transmission-diffraction grating known as a phase mask.1 The FBG spectral-response variations can be correlated with the strain and temperature applied to the grating structure, thus making the FBG an effective sensing element.2 However, FBG devices and sensing elements in general need to be mechanically robust and — for applications in harsh environments — functional at elevated temperatures.

Figure 1. (a) and (b) Transmission spectra of fiber Bragg gratings (FBGs) written through the polyimide jacket of H2-loaded high-numerical-aperture fibers from Optical Fiber Solutions and FiberLogix. Overlaid: model spectrum, shown as white dots. Isochronal (c) and isothermal annealing study (d) of a FBG written through the polyimide coating of the FiberLogix fiber. Index modulation and temperature are denoted by black and gray traces, respectively.

FBG inscription requires the use of high-power UV lasers to create the photoinduced index-change modulation required for Bragg gratings. Unfortunately, the protective polymer coatings of standard optical fibers, while transparent to visible and near-IR light, highly absorb UV radiation. Therefore, UV-laser-based FBG inscription requires removal of the polymer coating from the fiber by mechanical or chemical means and subsequent fiber recoating or packaging. As an alternative to optical-fiber stripping, specialized UV-transmissive polymer coatings allow for UV-laser FBG inscription without coating removal.3 However, the nonstandard coatings are not stable at elevated temperatures and exotic fiber coatings are expensive. FBG inscription through the acrylate-polymer coating of standard optical fibers using femtosecond-pulse (fs) IR-radiation laser sources4,5 was recently demonstrated without high-photosensitivity fibers or specialized coatings. This process eliminates the steps of stripping and replacing of the acrylate coating, which thus improves device reliability and fiber strength.

We employed a phase-mask technique with a fs IR-laser source that resulted in reasonably high reflecting FBGs using standard telecom fiber that was photosensitized using a hydrogen-loading process.5 Index modulations of up to 3.5×10−4 were achieved, which — for a 6mm-long grating — resulted in a FBG reflectivity of >80%. The irradiated fiber maintained up to 85% of the pristine fiber strength. When low-bendloss fibers with high germanium content were exposed (such as those used in fiber-optic hydrophones), strong grating reflectivities with index modulations of 7×10−4 were achieved through the acrylate coating. Higher index modulations of 1.4×10−3 can be obtained by writing in the high-germanium-content fiber when it is hydrogen-loaded.6

For high-temperature applications, optical fibers are typically coated with polyimide (a thermally stable polymer), which maintains its mechanical integrity up to 400°C. Traditional UV-laser-written FBG sensors are difficult to implement in polyimide-coated fibers because the polymer exhibits higher absorbance in the UV than acrylate. Polyimide is also resistant to chemical attack. Therefore, FBG inscription into polyimide-coated fibers requires invasive and hazardous techniques, such as hot sulfuric-acid stripping for polymer removal, and a subsequent recoating with polyimide.

Again using the phase-mask approach with fs IR-laser radiation and polyimide-coated high-numerical-aperature (NA) hydrogen-loaded optical fiber, we successfully inscribed Bragg gratings through the polyimide coating, with index modulations of up to 1.4×10−4. These gratings were thermally stable, while maintaining index modulations of 7×10−5 above the rated temperature of the polyimide coating (400°C).7 Spectra and annealing studies of these gratings are shown in Figure 1. Preliminary reliability tests of the through-the-jacket grating inscription in the polyimide-coated high-NA fiber indicate that the fibers maintained approximately 50% of the pristine fiber strength. Such strengths correspond to the capability of measuring strains of >20,000με. With these index modulations, 25mm-long gratings produce devices with 85% reflectivity and 100pm bandwidths (FWHM) that are ideally suited for fiber-grating sensor arrays.

Although fiber integrity and reliability are paramount in sensing applications, improved reliability is also important for telecom-related applications of fiber Bragg gratings as well. The through-the-jacket inscription process using fs IR lasers and a phase mask improves the mechanical strength and reliability of FBGs. In addition, neither specialized optical fibers nor coatings are required. From a manufacturing perspective, this technique delivers significant advantages for both telecommunications and sensing applications by eliminating the production processes of fiber stripping and recoating and using standard materials, resulting in lower-cost devices with higher reliability. We are currently investigating techniques to increase the grating reflectivity and reduce bandwidth to optimize spectral responses for fiber-grating sensor arrays.

Stephen Mihailov, Dan Grobnic, Christopher Smelser, Robert Walker 
Optical Communications and Electrophotonics
Communications Research Centre Canada
Ottawa, Canada

Stephen Mihailov has worked in the field of fiber-Bragg-grating technology development for more than 15 years. He has written more than 140 journal and conference publications and is co-inventor of ten US patents. He is a research program manager.