Amedee Lacraz

Femtosecond Laser Inscribed Bragg Gratings in Low Loss CYTOP Polymer Optical Fiber

We report on the first inscription of fiber Bragg gratings (FBGs) in cyclic transparent optical polymer (CYTOP)-perfluorinated polymer optical fibers (POFs). We have used a direct write method with a femtosecond laser operating in the visible. The FBGs have a typical reflectivity of 70%, a bandwidth of 0.25 nm, a 3-mm length, and an index change of ~10-4. The FBGs operate in the C-band, where CYTOP offers key advantages over polymethyl methacrylate optical fibers, displaying significantly lower optical loss in the important near-infrared (NIR) optical communications window. In addition, we note that CYTOP has a far lower affinity for water absorption and a core-mode refractive index that coincides with the aqueous index regime. These properties offer several unique opportunities for POF sensing at NIR wavelengths, such as compatibility with existing optical networks, the potential for POF sensor multiplexing and suitability for biosensing. We demonstrate compatibility with a commercial Bragg grating demodulator.

Modified fs-Laser Inscribed FBG Array for Rapid Mode Shape Capture of Free-Free Vibrating Beams

We report on the development of a multiplexed sensor array of fiber Bragg gratings (FBGs), inscribed using a femtosecond laser, and its demonstration as a quasi-distributed sensor to capture the mode shapes, through surface displacement sampling, of a free-free vibrating beam. Our method is based on a plane-by-plane inscription approach, whereby a 2-D index change is written across the fiber core, controlling the width and depth of the modified region. This allows for the fast inscription of multiple wavelength FBGs in coated optical fibers, with the advantage of less stringent alignment requirements. The FBGs are multiplexed in the wavelength domain using a high-speed demodulator, using a fast, custom-made computational algorithm. We recover rapid and single-step wavelength- and time-dependent displacement information, extracting the first two mode shapes of a vibrating beam and their respective degrees of freedom resonance frequencies in <;0.4 s.

Pressure Dependence of Fiber Bragg Grating Inscribed in Perfluorinated Polymer Fiber

We investigate the hydrostatic pressure dependence of the Bragg wavelength of a fiber Bragg grating (FBG) inscribed in a perfluorinated graded-index (PFGI-) polymer optical fiber (POF) at 1550 nm. At 0.5 MPa, the Bragg wavelength increased with time and became almost constant ~150 min later. Such a long time constant probably originates from the unique structure of the PFGI-POF, which has a thick overcladding around its core and cladding. The pressure-dependence coefficient without considering the time constant was estimated to be 1.3 nm/MPa; this is over five times the values of other types of POF-FBGs. This indicates that by removing the overcladding of the PFGI-POF, fast-response high-sensitivity pressure sensing will be feasible. Once the Bragg wavelength became constant at 0.5 MPa, the pressure-dependence coefficient of the Bragg wavelength was measured to be −0.13 nm/MPa, the absolute value of which was comparable with those of other POF-FBGs, but with an opposite sign.

Femtosecond-Laser-Based Inscription Technique for Post-Fiber-Bragg Grating Inscription in an Extrinsic Fabry–Perot Interferometer Pressure Sensor

In this paper, a novel fiber Bragg grating inscription technique based on a femtosecond laser is presented. The grating was inscribed in close proximity to the tip of an extrinsic Fabry-Perot interferometer (EFPI)-based optical fiber pressure sensor. This therefore represents an optical fiber pressure and temperature sensor (OFPTS) for simultaneous pressure and temperature measurement for use in exactly the same physical location. The temperature measurement can also be used to compensate thermal drift in the EFPI sensor. The Bragg wavelength can be tailored precisely to any given wavelength in the optical spectrum and the degree of reflection can be adjusted to suit the FPI spectrum. The OFPTS has a diameter of 200 μm and is fully biocompatible. Furthermore, the sensor shows a high stability after grating inscription, of better than 0.5% in 20 min. The small size and high stability makes the sensor especially interesting for volume restricted areas, like blood vessels or the brain.

Novel FBG femtosecond laser inscription method for improved FPI sensors for medical applications

A novel fibre Bragg grating (FBG) post-inscription technique using a femto second laser (FSL), used to modify an optical fibre pressure sensor (OFPS) based on an extrinsic Fabry Perot Interferometer (EFPI) is presented. The resultant sensor is an optical fibre pressure and temperature sensor (OFPTS), able to measure temperature and pressure simultaneously in precisely the same location within the optical fibre. Hence the temperature measurement can be used to accurately compensate any thermal fluctuations in the pressure measurements, leading to an improved long term stability. The Bragg-wavelength can be tailored to coincide with any part of the Fabry-Perot Interferometer (FPI) spectrum (e.g. define the FBG at a valley of the FPI spectrum). We use a modified femtosecond laser, point-by-point inscription method for precise and controlled placement of the fibre Bragg grating. Our technique can be readily adapted to commercial production methods for optical fibre sensors as it greatly mitigates the alignment problems associated with femtosecond laser inscription of gratings in optical fibres. The sensor presented in this paper is entirely fabricated with quartz glass, which makes it fully bio-compatible and can be used for biomedical application. The sensors achieved a high sensitivity of 1.3 nm over kPa resulting in a resolution of ~ 1mmHg and a temperature sensitivity of ~ 10.7pm over K. After the inscription, the sensors still demonstrated a stability of better than 0.1% in 30min. The small diameter of only 200μm allows biomedical in-vivo application in volume restricted areas (e.g. blood vessels or the brain) for simultaneous temperature and pressure measurements.

Comparative study of multimode CYTOP graded index and single-mode silica fibre Bragg grating array for the mode shape capturing of a free-free metal beam

The work described in this paper involved two different material fibre Bragg grating (FBG) arrays, investigating their performance as quasi-distributed sensors by capturing the vibrating response of a free-free metal beam close to its resonance frequencies. A six meter length of low-loss, gradient-index, multimode CYTOP fibre and of SMF-28 were used for the inscription of multiple FBG sensors using a femtosecond laser inscription method. The FBG arrays were multiplexed in the wavelength domain using a high-speed commercial demodulator, from which we recovered wavelengthand time-dependent displacement information. We compared the vibration response of the two arrays and using a novel computation algorithm we extract the first mode shape of the free-free metal beam that was exited at its first resonance frequency using a vibrating force.

Strain sensing with femtosecond inscribed FBGs in perfluorinated polymer optical fibers

In this work, the potential of fiber Bragg gratings (FBGs) in low-loss perfluorinated polymer optical fibers (PF-POFs) is explored. The FBG is femtosecond-inscribed in a commercial multi-mode (MM) PF-POF based on Cytop polymer. Femtosecond inscription leads to creation of a highly saturated grating with a number of higher order reflection peaks visible throughout the visible and near-infrared spectral region. For 2 mm long FBG having a pitch of 2.2895 μm, a total of nine higher-order MM reflection bands are visible spanning from 1548 nm (4th order) to 520 nm (12th order). Strain sensitivity was measured for 6 peak bands in 500-900 nm region, where relatively low cost CCD based spectrometers and broadband LEDs are available. Strain sensitivity increases almost linearly with increasing initial peak wavelength, growing from 4.82 ± 0.02 nm/% measured for 12th order peak at 517 nm to 8.12 ± 0.04 nm/% measured for 7th order peak at 883 nm. These values correspond to roughly 20 % higher sensitivity than silica FBGs exhibit in this spectral range. The gratings in PF-POFs combine the higher strain sensitivity and low-loss operation while maintaining the mechanical advantages of polymer optical fibers. Therefore, they hold a high potential for considerable broadening of polymer optical fiber Bragg gratings application range.

Femtosecond laser micromachining of compound parabolic concentrator fiber tipped glucose sensors

Abstract. We report on highly accurate femtosecond (fs) laser micromachining of a compound parabolic concentrator (CPC) fiber tip on a polymer optical fiber (POF). The accuracy is reflected in an unprecedented correspondence between the numerically predicted and experimentally found improvement in fluorescence pickup efficiency of a Förster resonance energy transfer-based POF glucose sensor. A Zemax model of the CPC-tipped sensor predicts an optimal improvement of a factor of 3.96 compared to the sensor with a plane-cut fiber tip. The fs laser micromachined CPC tip showed an increase of a factor of 3.5, which is only 11.6% from the predicted value. Earlier state-of-the-art fabrication of the CPC-shaped tip by fiber tapering was of so poor quality that the actual improvement was 43% lower than the predicted improvement of the ideal CPC shape.

Beam-shaping via femtosecond laser-modified optical fibre end faces

We present the results of investigations regarding laser micro-structuring of single mode optical fibres by direct access of the fibre end face and compare this with inscription in planar samples. We combine a high numerical aperture objective and femtosecond laser radiation at visible wavelengths to examine the spatial limits of direct writing and structuring at the surface of the optical fibre. We realise a number of interesting devices from one- and two-dimensional grating structures, to Bessel, Airy and vortex beam generators. We show the versatility of this simple but effective inscription method, where we demonstrate classic multiple slit diffraction patterns and patterns for non-diffracting beams, confirming that the flexible direct write method using femtosecond lasers can be to produce binary masks that can lead to beam shaping using a method that is applicable to all types of planar samples and through fine control of laser parameters to multi-mode and singlemode optical fibres.

Femtosecond laser waveguide and FBG inscription in four-core optical fibre

We present research into the use of femtosecond lasers to develop optical waveguides inscribed in the cladding of singlemode, silica optical fibre (SMF28). The waveguides are inscribed near to the fibre core, coupling light into them evanescently and so behaving as traditional couplers. By carefully controlling the laser parameters we are able to inscribe cladding waveguides with no evidence of damage through ablation. We show that this flexible inscription method can be used as an enabling technology to couple light from single-core fibres to new multi-core optical fibres, and in this work specifically to 4-core fibre. The SMF28 fibre is fusion spliced to the multi-core fibre and using the femtosecond laser we inscribe bridging waveguides from the centrally located single mode fibre core to a selected offset core of the 4-core fibre. To demonstrate the efficiency of the method and the possibility of making new kinds of optical fibre sensors, we inscribe a fibre Bragg grating (FBG) in one of the four fibre cores. The light reflected from the FBG is coupled back to the SMF28 core via bridging waveguide and we recovered the reflection spectrum of the grating using a commercial high-resolution spectrometer.