Charalambos Koutsides

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.

Point-by-point fiber Bragg grating inscription in free-standing step-index and photonic crystal fibers using near-IR femtosecond laser

We report what we believe to be the first highly symmetric first-order IR femtosecond laser fiber Bragg gratings within the telecommunications C band in free-standing optical fiber, fabricated with a relatively low NA lens and without use of oil immersion techniques. This grating features the smallest dimensions for a point-by-point fiber grating reported so far (to our knowledge). This achievement paves the way to rapid mass manufacturing of highly efficient and stable Bragg gratings using ultrafast lasers in any type of fiber. Mastering this femtosecond grating inscription technique also allowed the fabrication of the first Bragg gratings with direct near-IR femtosecond inscription in photonic crystal fibers, and without the use of techniques that rely on the compensation of the holey structure.

Characterisation and performance of a Terfenol-D coated femtosecond laser inscribed optical fibre Bragg sensor with a laser ablated microslot for the detection of static magnetic fields

We present a novel device for the characterisation of static magnetic fields through monitoring wavelength shifts of femtosecond inscribed fibre Bragg grating and micromachined slot, coated with Terfenol-D. The device was sensitive to static magnetic fields and can be used as a vectoral sensor for the detection of magnetic fields as low as 0.046 mT with a resolution of ± 0.3mT in transmission and ± 0.7mT in reflection. The use of a femtosecond laser to both inscribe the FBGs and micromachine the slot in a single stage prior to coating the device significantly simplifies the fabrication.

Characterizing femtosecond laser inscribed Bragg grating spectra

We present numerical modeling based on a combination of the Bidirectional Beam Propagation Method and Finite Element Method that completely describes the wavelength spectra of point by point femtosecond laser inscribed fiber Bragg gratings, showing excellent agreement with experiment. We have investigated the dependence of different spectral parameters such as insertion loss, all dominant cladding and ghost modes and their shape relative to the position of the fiber Bragg grating in the core of the fiber. Our model is validated by comparing model predictions with experimental data and allows for predictive modeling of the gratings. We expand our analysis to more complicated structures, where we introduce symmetry breaking; this highlights the importance of centered gratings and how maintaining symmetry contributes to the overall spectral quality of the inscribed Bragg gratings. Finally, the numerical modeling is applied to superstructure gratings and a comparison with experimental results reveals a capability for dealing with complex grating structures that can be designed with particular wavelength characteristics.

Femtosecond Laser Inscription of Fiber Bragg Gratings with Low Insertion Loss and Minor Polarization Dependence

The inscription of low insertion loss and negligibly polarization dependent fiber Bragg gratings inscribed using a femtosecond laser system is reported. Insertion losses were <0.4dB/20mm and polarization wavelength shift of <5pm, with transmission changes <0.1dB.

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.

Point-by-point Bragg grating inscription in single-mode microstructure fibre using NIR femtosecond laser

We report on the first inscription of fibre Bragg gratings in microstructure optical fibre using a NIR femtosecond laser system, inscribed using the point-by-point method, and without the use of any oil immersion techniques to remove the effects of fibre curvature or the role of the holey microstructure. The Bragg gratings are second order and recorded in a microstructure optical fibre that is highly birefringent, single-mode at 1550nm, photosensitive and compatible with FBG inscription technology due to the minimized number of air holes surrounding the fibre core. Both axial strain and temperature sensitivity are measured.

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.

Designing High-Performance, Power-Efficient NoCs With Embedded Silicon-in-Silica Nanophotonics

On-chip electrical links exhibit large energy-to-bandwidth costs, whereas on-chip nanophotonics, which attain high throughput, yet energy-efficient communication, have emerged as an alternative interconnect in multicore chips. Here we consider silicon nanophotonic components that are embedded completely within the silica (SiO2) substrate as opposed to existing die on-surface silicon nanophotonics. As nanophotonic components now reside subsurface, within the silica substrate, non-obstructive interconnect geometries offering higher network throughput can be implemented. First, we show using detailed simulations based on commercial tools that such Silicon-in-Silica (SiS) structures are feasible, and then demonstrate our proof of concept by utilizing a SiS-based mesh-interconnected topology with augmented diagonal optical channels that provides both higher effective throughput and throughput-to-power ratio versus prior-art.

Design of high-performance, power-efficient optical NoCs using Silica-embedded silicon nanophotonics

With on-chip electrical interconnects being marred by high energy-to-bandwidth costs, threatening multicore scalability, on-chip nanophotonics, which offer high throughput, yet energy-efficient communication, form an alternative attractive counterpart. In this paper we consider silicon nanophotonic components that are embedded completely within the silica (SiO2) substrate as opposed to prior-art that utilizes die on-surface silicon nanophotonics. As nanophotonic components now reside in the silica substrates subsurface non-obstructive interconnect geometries offering higher network throughput can be implemented. First, we show using detailed simulations based on commercial optical tools that such Silicon-In-Silica (SiS) structures are feasible, derive their geometry characteristics and design parameters, and then demonstrate our proof of concept by utilizing a hybrid SiS-based photonic mesh-diagonal links network-on-chip topology. In pushing the performance envelope even more, we next develop (1) an associated contention-aware photonic adaptive routing function, and (2) a parallelized photonic channel allocation scheme, that in tandem further reduce message delivery latency. An extensive experimental evaluation, including utilizing traffic benchmarks gathered from full-system chip multiprocessor simulations, shows that our methodology boosts network throughput by up to 30.8%, reduces communication latency by up to 22.5%, and improves the throughput-to-power ratio by up to 23.7% when compared to prior-art.