Antreas Theodosiou

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.

POFBG-Embedded Cork Insole for Plantar Pressure Monitoring

We propose a novel polymer optical fiber (POF) sensing system based on fiber Bragg gratings (FBGs) to measure foot plantar pressure. The plantar pressure signals are detected by five FBGs, in the same piece of cyclic transparent optical polymer (CYTOP) fiber, which are embedded in a cork insole for the dynamic monitoring of gait. The calibration and measurements performed with the suggested system are presented, and the results obtained demonstrate the accuracy and reliability of the sensing platform to monitor the foot plantar pressure distribution during gait motion and the application of pressure. This architecture does not compromise the patient’s mobility nor interfere in their daily activities. The results using the CYTOP fiber showed a very good response when compared with solutions using silica optical fibers, resulting in a sensitivity almost twice as high, with excellent repeatability and ease of handling. The advantages of POF (e.g., high flexibility and robustness) proved that this is a viable solution for this type of application, since POF’s high fracture toughness enables its application in monitoring patients with higher body mass compared with similar systems based on silica fiber. This study has demonstrated the viability of the proposed system based on POF technology as a useful alternative for plantar pressure detection systems.

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.

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.

Direct writing of plane-by-plane tilted fiber Bragg gratings using a femtosecond laser

In this Letter, we report a flexible, plane-by-plane direct write inscription method for the development of tailored, tilted fiber Bragg gratings using a femtosecond laser. Compared to ultraviolet or femtosecond laser inscription based on the phase mask, interferometric, or point-by-point methods, the presented approach is far more flexible and offers several advantages. Laser inscription is made through the fiber coating, while the grating planes are controlled to minimize birefringence, with precise control over the wavelength location and strength of cladding modes. Tenth-order gratings were produced in the C+L bands so that higher-order gratings could be studied at shorter wavelengths. In particular, we show that the refractometric sensitivity depends on the grating order, ranging from ∼28  nm/refractive index unit (RIU) at ∼1510  nm to ∼13  nm/RIU at ∼1260  nm.

Accurate and Fast Demodulation Algorithm for Multipeak FBG Reflection Spectra Using a Combination of Cross Correlation and Hilbert Transformation

We demonstrate a combined cross correlation and Hilbert transform-based demodulation algorithm for tracking the wavelength shifts of fiber Bragg gratings (FBGs) having a multiple peak reflection spectrum. We show how the Hilbert transform can be employed to convert the task of locating the maximum of the wavelength profile to the one of finding the zero crossing. We observed higher accuracy and fast response compared to other well-known demodulation algorithms such as the centroid detection algorithm and the cross-correlation algorithm. In addition, we show that the multipeak reflection spectrum that occurs in multimode fibers does not greatly affect the algorithm results. Finally, we experimentally recover axial strain measurements using a multipeak reflection spectrum of an FBG inscribed in a multimode gradient index CYTOP fiber using the newly developed algorithm.

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.

Polymer Optical Fiber Sensors Approaches for Insole Instrumentation

Advantages like electromagnetic field immunity, fracture toughness, high strain limits, flexibility in bending and impact resistance of polymer optical fibers (POFs) are beneficial for applications that involve embedment in flexible structures. Since insoles are one of these flexible structures that may be used in different wearable applications, POFs can be applied and this paper proposes the application of POF sensors in insole instrumentation with two different approaches: intensity variation-based and polymer optical fiber Bragg gratings (POFBGs). Results show that both approaches present low errors with root mean squared errors (RMSEs) of 45.17 kPa for the plantar pressure monitoring with the POFBG-based insole and 5.30 N for the ground reaction force measurement with the intensity variation sensors. These results demonstrate the feasibility of POF sensors applications in flexible structures and in wearable applications such as insoles and soft robotics instrumentation.