Investigations on birefringence effects in polymer optical fiber Bragg gratings

Step-index polymer optical fiber Bragg gratings (POFBGs) and microstructured polymer optical fiber Bragg gratings (mPOFBGs) present several attractive features, especially for sensing purposes. In comparison to FBGs written in silica fibers, they are more sensitive to temperature and pressure because of the larger thermo-optic coefficient and smaller Young's modulus of polymer materials. (M)POFBGs are most often photowritten in poly(methylmethacrylate) (PMMA) materials using a continuous-wave 325 nm HeCd laser. For the first time to the best of our knowledge, we study photoinduced birefringence effects in (m)POFBGs. To achieve this, highly reflective gratings were inscribed with the phase mask technique. They were then monitored in transmission with polarized light. For this, (m)POF sections a few cm in length containing the gratings were glued to angled silica fibers. Polarization dependent loss (PDL) and differential group delay (DGD) were computed from the Jones matrix eigenanalysis using an optical vector analyser. Maximum values exceeding several dB and a few picoseconds were obtained for the PDL and DGD, respectively. The response to lateral force was finally investigated. As it induces birefringence in addition to the photo-induced one, an increase of the PDL and DGD values were noticed. © 2014 Copyright SPIE

Near-infrared grating-assisted SPR optical fiber sensors : design rules for ultimate refractometric sensitivity

Plasmonic optical fiber sensors are continuously developed for (bio)chemical sensing purposes. Recently, surface plasmon resonance (SPR) generation was achieved in gold-coated tilted fiber Bragg gratings (TFBGs). These sensors probe the surrounding medium with near-infrared narrowband resonances, which enhances both the penetration depth of the evanescent field in the external medium and the wavelength resolution of the interrogation. They constitute a unique configuration to probe all the fiber cladding modes individually. We use them to analyze the modal distribution of gold-coated telecommunication-grade optical fibers immersed in aqueous solutions. Theoretical investigations with a finite-difference complex mode solver are confirmed by experimental data obtained on TFBGs. We show that the refractometric sensitivity varies with the mode order and that the global SPR envelope shift in response to surrounding refractive index (SRI) changes higher than 1e-2 RIU (refractive index unit) can be ~25% bigger than the local SPR mode shift arising from SRI changes limited to 1e-4 RIU. We bring clear evidence that the optimum gold thickness for SPR generation lies in the range between 50 and 70 nm while a cladding diameter decrease from 125 µm to 80 µm enhances the refractometric sensitivity by ~20%. Finally, we demonstrate that the ultimate refractometric sensitivity of cladding modes is ~550 nm/RIU when they are probed by gold-coated TFBGs

All-fiber tunable optical delay line

We present a tunable optical delay line based on the use of a single chirped fiber Bragg grating written into a standard single mode optical fiber. In the proposed scheme, the delay is induced through the Bragg grating differential group delay curve. This is achieved by launching orthogonally polarized optical pulses in both directions into the Bragg grating and by controlling its local birefringence. This bidirectional propagation allows to compensate the second-order dispersion. The setup is suitable to delay pulses with a spectral width just less than the grating reflection bandwidth, which is particularly useful in the context of forthcoming wavelength division multiplexing ultra-high bit rate lightwave systems. In this work, the performances of the setup are investigated using a pulsed laser delivering 6.3 ps Fourier transform limited pulses at 1548 nm. A maximum delay of 120 ps (about 20 times the pulse width) is reported experimentally

The sensitivity characteristics of tilted fibre Bragg grating sensors with different cladding thicknesses

In this paper, we present the sensitivity characteristics of core and cladding modes in weakly tilted fibre Bragg grating sensors with different cladding thicknesses. Both experimental and analysis results are presented. The results show that the strain sensitivity of cladding mode resonances in tilted fibre Bragg gratings does not depend on the cladding diameter but on the wavelength separation from the core resonance over the range of the cladding diameters studied

Interrogation technique for TFBG-SPR refractometers based on differential orthogonal light states

The generation of near-IR surface plasmon resonance in gold-coated tilted fiber Bragg gratings is strongly dependent on both the polarization state of the transmission light and the property of confining materials (including the coating materials and surrounding media). These dependencies can be advantageously used to demodulate the amplitude spectrum and retrieve the surrounding refractive index. In this paper, we present an automated demodulation technique that measures the surrounding refractive index by comparing the differential amplitude of resonance peaks near the plasmon attenuation for two orthogonal amplitude spectra recorded in the same operating conditions. A mean sensitivity of more than 500 nm per refractive index unit is reported. This new refractive index measurement method is shown to be accurate to 5×10−5 over a full range of 0.01 in water solutions

Self-optimized metal coatings for fiber plasmonics by electroless deposition

We present a novel method to prepare optimized metal coatings for infrared Surface Plasmon Resonance (SPR) sensors by electroless plating. We show that Tilted Fiber Bragg grating sensors can be used to monitor in real-time the growth of gold nano-films up to 70 nm in thickness and to stop the deposition of the gold at a thickness that maximizes the SPR (near 55 nm for sensors operating in the near infrared at wavelengths around 1550 nm). The deposited films are highly uniform around the fiber circumference and in spite of some nanoscale roughness (RMS surface roughness of 5.17 nm) the underlying gratings show high quality SPR responses in water

Quasi-distributed refractometer using tilted Bragg gratings and time domain reflectometry

Tilted fiber Bragg gratings (TFBGs) have been demonstrated to be accurate refractometers as they couple light from the fiber core to the cladding. Because they require spectral measurements on several tens of nanometers, demodulation techniques reported so far are not suited for quasi-distributed refractive index sensing using TFBGs cascaded along a single optical fiber. We demonstrate here that a commercial Optical Time Domain Reflectometer (OTDR) can be used to multiplex identical TFBGs refractometers written in the same optical fiber. Our solution is simple, relatively fast, cost-effective and is particularly interesting for the monitoring of long structures