Grating-inscription technique eliminates need for fiber stripping and recoating

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 sources^4,5 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.