The authors experimentally demonstrate that the spectral flatness of chaotic light from an external-cavity feedback semiconductor laser is significantly improved using a fibre Bragg grating. The effects of detuning between the Bragg frequency and chaotic frequency on the bandwidth and spectral flatness are both investigated. The flatness of the power spectrum is enhanced to ±1.4 dB whilst maintaining the spectral bandwidth at around 8 GHz. The experimental results are in accordance with theoretical predictions which also have shown that the spectral flatness decreases initially then increases with increasing the 3 dB bandwidth of grating. A flattened spectrum with a low-pass energy distribution increases the utilisation efficiency of chaotic light in chaotic radar and reflectometry.

The authors demonstrate distributed temperature sensing based on slope-assisted Brillouin optical correlation-domain reflectometry (BOCDR) with a long measurement range of >10 km. They find that to achieve such a long-range measurement, a delay line in a reference path needs to be at least four times longer than the sensing fibre. In addition, they show that the use of such a long delay line induces forward-propagating Brillouin-scattered light in the reference path, which deteriorates the signal-to-noise ratio (SNR) of the system and should be suppressed. Finally, by exploiting a beyond-nominal-resolution effect of the slope-assisted BOCDR, they detect a 3-m-long heated section in a 13-km-long silica fibre in a distributed manner; the reason for its low SNR is also discussed.

Fibre Bragg grating inscription in the crystalline silicon (Si)-core fibre is described using visible light femtosecond laser pulses. The femtosecond pulses at 517 nm propagate through the transparent silica glass cladding and are fully absorbed by the Si core generating locally high temperatures and stress fields in forming the grating periods. The Si FBGs were characterised in reflection, calibrated for a first time as strain and temperature sensors and compared with that of FBGs inscribed in standard silica fibres.

This study presents the external modulation and direct detection of duobinary signal in optical access networks at 25 and 50 Gb/s with the use of first-order optical equaliser at the transmitter output (pre-equalisation) to compensate the waveform distortion induced by fibre dispersion. The first-order optical equaliser is then replaced by second-order optical equaliser to further improve the bit error rate (BER) performance and extend the maximum transmission distance by compensating the residual dispersion for the first time. The authors explain the enhancement in the BER by analysing the eye diagrams and optical spectra. The authors also investigate non-return-to-zero signal with similar setup parameters for reference.

The work investigates a magnetic fluid filled photonic crystal fibre (MFPCF) at 1557 nm under applied magnetic field. They computed the refractive index change of the MF due to the magnetic field and used it in simulations to find the effective refractive indices of the core and the first cladding modes of the MFPCF. Then, they analytically computed the effective mode area and the confinement loss. The effective refraction index of the core mode increasing by 0.0341% under the applied field of 0–500 Oe, increases the effective mode area by 128% and the confinement loss by 866%. Thus, a rather small change in the effective refractive index gives rise to a rather large change in confinement loss. However, this effect saturates after about 500 Oe because the MF nanoparticles no longer change their magnetic orientation. It is also noteworthy that the confinement loss changes almost linearly with the effective area as the magnetic field change. These results suggest that MFPCF are suitable for a tunable attenuator and a field sensor.

The work reports on the exploitation of semi-sequential deep reactive ion etching (RIE) processes for realisation of deep vertically etched Si structures on Si substrate applicable in fibre-optic sensing systems. These processes employ different mixtures of gases including hexaflourosulphide, hydrogen and oxygen in a RIE system with a programmed passivation and etching sub-cycles. In the so-called semi-sequential processes, the intermediate purging steps are eliminated, which results in remaining little trace of reactant gases from the previous sub-cycle in the RIE machine's chamber, which can participate in the process of current sub-cycle. For the sake of minimum optical losses, which are accomplished by highly smooth vertical sidewalls, several etching processes were applied. In these processes, by controlling the etching parameters such as the flow of gases, plasma power and timing of each subsequence, the process can be controlled for minimum under-etch and surface roughness and maximum verticality of sidewalls. However, time and cost considerations should also be noted in the optimum fabrication process.

We propose an all-optical switch based on a microfibre Mach–Zehnder interferometer (MMZI) in which the MMZI is considered with a microfibre loop resonator (MLR) in one arm of it. The authors numerically demonstrate that the enhanced phase shift of the MLR near its resonance wavelengths can be used for constructive/destructive interference and switching functionality of the MMZI. The calculated switching threshold power of the switch is about 2 mW, which can be further reduced by the use of an MLR with more finesse, and the rising and falling times are computed with 21 and 91 ps, respectively. Natural connection to silica single mode fibres, easy configuration and high speed make it as a suitable choice for all-optical signal processing and communications applications.

Refractive index sensing in single-mode optical fibers is demonstrated using stimulating Brillouin scattering. In particular, we show that the Brillouin frequency shift (BFS) can be made sensitive to the surrounding refractive index (SRI), by letting the evanescent fields of the interacting optical fields to penetrate into the external medium. The experiments, carried out using a high spatial resolution Brillouin Optical Frequency-Domain Analysis (BOFDA) apparatus and side-polished fiber (SPF) sensors, revealed a BFS sensitivity to SRI of ~293 MHz/RIU in the sensing region, when the SRI was 1.40. Furthermore, we show that, using a more efficient sensor structure such as an adiabatic taper, the BFS sensitivity to the SRI could be strongly enhanced, increasing up to ~910 MHz/RIU around an SRI of 1.40 in a tapered fiber with 4-μm diameter.

This paper is aimed to realize simple optical fiber devices involving metal-dielectric colloidal crystal (MDCC) structures as enhanced platform for sensing application. The MDCC structures find large interest in last years since they permit to combine localized surface plasmonic resonance (LSPR) of noble metallic nanoparticles and photonic bandgap (PBG) of colloidal type photonic crystals PhCs. While most investigations have been provided onto planar substrate and investigated as surface enhanced Raman-scattering (SERS) probe, very few works aimed to integrate the MDCC with optical fiber technology. The present paper reports recent results of metal-dielectric colloidal crystals (MDCCs) achieved by successive deposition of Au-NPs on PS-based CCs. The gold of this work is to enable the direct fabrication of the MDCC directly onto fiber optic tip surface to produce miniaturized fiber optic devices.