A ferrule-based method of direct fs FBG inscription through protective plastic coating is demonstrated. Fluctuations of fiber core position relative to the writing fs beam are compensated by the developed auto-alignment system. As a result, high-quality FBGs with length from 0.1 to 50 mm are fabricated in polyimide-coated fibers, whose spectra are well described by the theory. The fabricated FBGs have great potential in sensor applications at high temperature and harsh environments both point-action and distributed ones.
© 2016 Optical Society of America
The first demonstration of fiber Bragg grating (FBG) inscription  is one of significant events in fiber optics evolution, because after that fiber lasers have actively been developed. FBG is used as a selective wavelength reflector, either as point mirror in normal cavity or distributed one in case of DBR (distributed Bragg reflector) and DFB (distributed feedback) lasers. FBGs are also widely used in fiber sensing systems since any external impact (temperature change or deformation) results in the shift of FBG central wavelength. Besides mentioned applications, FBGs are widely used in optical communication systems, medicine, microwave photonics etc., see  for a review.
The progress in femtosecond (fs) laser modification of refractive index (RI) in transparent materials results in the inscription of FBGs with new features and extended technical capabilities. For these reasons, the FBG inscription with fs laser radiation is a promising way for both fiber laser and sensing systems development. To date, two main approaches for FBG inscription with fs laser radiation were demonstrated: phase mask technique  and point-by-point (PbP) inscription . The second approach is more attractive for sensor applications, where many FBGs with different center wavelengths are needed. Moreover, in the absence of H2-loading, it is difficult to write a highly reflective FBG through polyimide protective coating with phase mask and IR fs radiation . The grating period can be varied by changing the velocity of translation stage only, at the fixed pulse repetition rate. The lateral dimension of FBG pitch inscribed with PbP technique is several times smaller than the diameter of fiber core, so the value of overlap integral in this case is less than that for phase mask inscription technique. For this reason, the effective RI change is varied in the range from 10−6 to 10−3 for standard single mode fibers, that allows inscription of ultra-long FBGs for distributed sensors systems. However, PbP technique has some disadvantage concerning a necessity of radiation focusing with a high precision into the center of fiber core. The method of FBG inscription by drawing of a fiber through a transparent ferrule having flat polished side was proposed in , where standard ferrule with inner diameter of 126 μm used in this case enables only a coating-free fiber drawing. Thus the advantage of direct FBG inscription through protective coating  was lost.
In this paper, a ferrule-based method of direct fs FBG inscription through protective plastic coating is demonstrated for FBGs with overall length in the range from 0.1 to 50 mm. These FBGs could be used for development of point (with the high spatial resolution) fiber sensors as well as distributed fiber sensors systems with extended technical capabilities, resistant to aggressive chemicals and radioactive environments. As far as polyimide has higher adhesion and resistance to high temperature (short term up to 400 °C and continuous operation at 300 °C), polyimide coated fiber is attractive for structural health monitoring of composite materials. Since the thickness of plastic coating is fluctuating along the fiber, a free drawing of the coated fiber through ferrule requires some gap between the fiber and ferrule. The key feature of proposed method is the system of auto-alignment of fiber core relative to the writing beam that allows one to significantly improve spectral characteristics of FBGs (that are in good agreement with theoretical ones) without the loss of inscription performance.
2. Experimental setup
The scheme of experimental setup for PbP FBG inscription is shown in Fig. 1. Femtosecond laser PHAROS 6W (Light Conversion Ltd) with a central wavelength of 1026 nm and pulse duration of about 230 fs was used. The repetition rate was set at 1 kHz. The laser beam was focused on a sample by a microscope objective with NA = 0.7. The transparent ferrule with inner diameter of 152 μm and one polished side (see inset in Fig. 1) was placed on a 3D piezopositioning stage with travel range of 20 μm. The gap between inner side of ferrule and coated fiber was filled with an immersion oil. Fibercore SM1500(9/125)P fiber with polyimide coating and 143 μm outer diameter was used for FBG inscription. The fiber was clamped on the high-precision linear stage Aerotech ABL1000, which is used for drawing the fiber through the ferrule. The position of focal spot inside the fiber core is fluctuating during the inscription process and the shift can reach 9 μm that is comparable with the fiber core diameter. To eliminate these shifts, the feedback system has been developed that allows accurate positioning of induced RI modifications inside the fiber core at the entire FBG length. The system operation consists of the following main steps. First, the fiber is manually aligned and the CCD camera (40 fps) captures the image of fiber core area (see Fig. 2(a)). Next, the image is filtered to make visible the fiber boundaries (see Fig. 2(b)), then intensity profile transverse to the fiber axis is plotted for boundaries recognition (see Fig. 2(c)). For each set of points corresponding to the individual peak the coordinates are averaged thus giving two initial coordinates of the fiber core boundaries (Yleft (T = 0) and Yright (T = 0)). When the boundaries shiftto Yleft (T = t) and Yright (T = t) during the fiber drawing, the fiber is moved in the opposite direction to initial coordinates by the piezopositioning stage (with actuation time of ~1 μs) by value of ∆Y(t) = (Yright (0) −Yleft (0))/2 − (Yright (t) −Yleft (t))/2, thus keeping the optimal core alignment. For the 143-μm coated fiber drawn through the 152-μm ferrule, the maximal deviation of 9 μm between the centers was reduced to 0.7 μm in the auto-alignment mode.
The FBG reflection spectrum was measured with super-luminescent laser diode and optical spectrum analyzer (OSA) Yokogawa AQ6370. In case of long FBGs with the reflection peak narrower than OSA resolution (0.02 nm) a scheme shown in Fig. 3 was used. The radiation from a tunable DFB fiber laser with spectral linewidth of 10 kHz was directed onto the FBG. Then the power of reflected light was measured with a photodiode. The wavelength tuning of DFB laser was performed by stretching the laser resonator with piezoelement thus providing variable wavelength shift up to 23 pm. Piezoelement was driven by the triangular signal from a high-voltage generator and the same signal is sent to channel 1 of oscilloscope for synchronization. Using the signal from photodiode at channel 2, it is possible to plot the graph of reflected power versus DFB laser wavelength, i.e. FBG spectrum.
3. Experimental results
3.1. Short FBG inscription
FBGs with a short length (less than 0.5 mm) are applied as point sensors allowing temperature/strain measurements with high spatial resolution, which is determined by the FBG length, as well as for highly sensitive acoustic sensing . When FBG is inscribed using a phase mask, its total length cannot be controlled precisely due to the diffraction of the writing beam at the narrow slit, which in this case defines overall length of a grating . One of the key advantages of fs PbP inscription technique is the ability to create FBGs with a strictly fixed physical length (with accuracy equal to one pitch, i.e. FBG period). So, the minimal FBG length in this case is limited only by a required reflection coefficient that is defined by the product of length and refractive index modulation.
Figure 4(a) shows the reflection spectra of 100 and 200 μm long FBGs inscribed with 535 nm period at fs pulse energies of 180 and 160 nJ, respectively. The 100 μm long grating has the reflectance of 1% and the spectral width of 7.3 nm, the 200 μm long one has 2% and 3.6 nm respectively. The effective RI modulation ∆n = 5×10−4 for the 100 μm long FBG and ∆n = 3.5×10−4 for the 200 μm long FBGs is derived from the analytical approximation for uniform grating with sinusoidal RI modulation . It should be noted that the actual value of the RI change in the fs pulse modified region can be an order of magnitude higher than the effective RI modulation of a grating. As the dynamic range of modern interrogators is more than 50 dB, such FBGs can be easily detected. Possible ways to improve the reflectance of the short FBGs is to increase the overlap integral by inscribing FBGs with continuous fiber core scanning  or to use fibers with higher concentration of germanium or hydrogenated fibers .
In many applications, especially as a mirror of a high-power fiber laser, FBG should provide high reflectivity. For this purpose FBG with a length of 2 mm and period of 535 nm was inscribed (Fig. 4(b)). The energy of fs pulses was 170 nJ. The FBG has the reflectance of 90% and the spectral width of 0.68 nm, its effective RI modulation is ∆n = 4.5×10−4.
3.2. Long FBGs inscription
Creation of high-quality long FBGs has a great importance for various photonics applications [12, 13]. In particular, long FBGs make it possible to perform distributed objects monitoring with high spatial resolution at relatively long base . The main results on such FBGs have been obtained with UV fabrication technology using a phase mask and photosensitive optical fibers, in which the FBG length is determined by the beam width or length of the phase mask . But using electro-optic phase-modulation (EOPM) based interferometer with a high precision 1-meter long translation stage and phase mask, it is possible to produce ultra-long (up to 1 m) fiber Bragg gratings with UV radiation .
To test the proposed stabilizing system, two FBGs with a total length of 50 mm and period of 535 nm were inscribed in the free drawing (Fig. 5(a)) and auto-alignment (Fig. 5(b)) modes. The energy of fs laser pulses was 105 nJ and 85 nJ respectively. As seen in Fig. 5(a), the spectrum of FBG created in the free-drawing mode consists of a set of peaks with the integral spectral width of 93 pm. The spectrum of long FBG fabricated in auto-alignment mode is much narrower and amounts to 16.5 pm. The good agreement between experimental and theoretical reflection spectra indicates high accuracy of the FBG fabrication with the proposed method of inscription. The interesting question arises about the minimal possible value of ∆n which can be reached by the fs laser modification technique, since this value defines the maximal possible length of weak FBGs. Effective value of RI modulation derived from the analytical formula is ∆n = 4.1×10−6. For FBGs with reflectance of <90% and length about 1 m the modulation depth should be ∆n ~ 10−7. In the PbP inscription method this value can be achieved by grating offset relative to the center of fiber core . Another factor determining the minimal value of the RI change is the wavelength of laser radiation. In  it was shown that the absorption efficiency at the wavelength of Yb laser is lower as compared with its second harmonic and obviously is lower as compared with the short-wavelength radiation of Ti:Sa laser (λ = 800 nm), which is commonly used for fs modification of materials.
Errors occurring during the FBG inscription in free-drawing mode require a more detailed analysis. In  it is shown that longitudinal modulation of Δn by harmonic signal with a frequency γ ≪ 2π/Λ leads to the additional resonant peaks in FBG spectrum at wavelengths detuned by . Thus, for a grating with Λ = 0.535 μm and perturbation frequency γ = 0.1 μm−1 the detuning of additional peaks is ∆λ± ≈ 13 nm for neff = 1.45. For lower frequencies (0.0075 μm−1 or less) the detuning value is less than 1 nm and additional peaks may superimpose with the main resonance that results in the distortion of FBG spectrum. Another factor leading to “collapse” of the spectral shape of uniform FBG concerns errors in the period . However, this contribution is not considered here, since in the presence of auto-alignment the spectra distortions associated with period errors were not observed.
Thus, we have demonstrated the method of PbP inscription of FBGs through the polyimide protective coating based on the fiber drawing through the ferrule with auto-alignment of the fiber core position relative to the focus position of writing beam, which allows one to create FBGs of different length and reflectance. The technique offers fabrication of high-quality gratings with characteristics corresponding to calculated ones in a broad range of FBG lengths. Created ultra-short (0.1 mm) FBGs can be used for point measurements with high spatial resolution, whereas long (50 mm) FBGs are applicable for distributed sensor systems with advanced performance parameters in respect of maximum temperature (400 °C).
The authors acknowledge financial support of Russian Science Foundation (grant 14-22-00118). The work of A.V.D is supported by Russian Foundation for Basic Research (grant 16-32-00777) and grant of the President of the Russian Federation (MK-6657.2016.2).
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