Quantum memory scheme based on optical fibers and cavities

An optical quantum memory scheme using two narrow-linewidth cavities and some optical fibers is proposed. The cavities are connected via an optical fiber, and the gap of each cavity can be adjusted to allow photons with a certain bandwidth to transmit through or reflect back. Hence, each cavity acts as a shutter and the photons can be stored in the optical fiber between the cavities at will. We investigate the feasibility of using this device in storing a single photon. We estimate that with current technology storage of a photon qubit for up to 50 clock cycles (round trips) could be achieved with a probability of success of 85%. We discuss how this figure could be improved.

Passive nonlinear pulse shaping in normally dispersive fiber

We propose a simple method for passive nonlinear optical pulse shaping that utilizes pulse prechirping and nonlinear propagation in a normally dispersive nonlinear fiber to generate various temporal waveforms of practical interest from conventional laser pulses.

A comparison of the sensing characteristics of long period gratings written in three different types of fiber

This paper compares the environmental sensing behaviour of long period gratings written in three fibers with different refractive index profiles: step, W and a progressive three layered fiber. The measurands considered are temperature, refractive index, axial strain and bending, and the spectral behaviour of individual attenuation bands were observed and, where possible, compared to theoretical predictions. Significant differences in the behaviour of the three fiber types were found.

Simultaneous clock recovery and data regeneration using a nonlinear optical loop mirror as an all-optical mixer

High-speed optical clock recovery, demultiplexing and data regeneration will be integral parts of any future photonic network based on high bit-rate OTDM. Much research has been conducted on devices that perform these functions, however to date each process has been demonstrated independently. A very promising method of all-optical switching is that of a semiconductor optical amplifier-based nonlinear optical loop mirror (SOA-NOLM). This has various advantages compared with the standard fiber NOLM, most notably low switching power, compact size and stability. We use the SOA-NOLM as an all-optical mixer in a classical phase-locked loop arrangement to achieve optical clock recovery, while at the same time achieving data regeneration in a single compact device

Ultralong Raman fibre lasers as virtually lossless optical media

By transforming the optical fiber span into an ultralong cavity laser, we experimentally demonstrate quasilossless transmission over long (up to 75 km) distances and virtually zero signal power variation over shorter (up to 20 km) spans, opening the way for the practical implementation of integrable nonlinear systems in optical fiber. As a by-product of our technique, the longest ever laser (to the best of our knowledge) has been implemented, with a cavity length of 75 km. A simple theory of the lossless fiber span, in excellent agreement with the observed results, is presented.

Virtual Gires-Tournois etalons realized with phase-modulated wideband chirped fiber gratings

We propose and demonstrate novel virtual Gires–Tournois (GT) etalons based on fiber gratings. By introducing an additional phase modulation in wideband linearly chirped fiber Bragg gratings, we have successfully generated GT resonance with only one grating. This technique can simplify the fabrication procedure while retaining the normal advantages of distributed etalons, including their full compatibility with optical fiber, low insertion loss, and low cost. Such etalons can be used as dispersion compensation devices in optical transmission systems.

A refractometer based on a micro-slot in a fiber Bragg grating formed by chemically assisted femtosecond laser processing

A liquid core waveguide as a refractometer is proposed. Microtunnels were created in standard optical fiber using tightly focused femtoscond laser inscription and chemical etching. A 1.2(h)x125(d) x500(l) µm micro-slot engraved along a fiber Bragg grating (FBG) was used to construct liquid core waveguide by filling the slot with index matching oils. The device was used to measure refractive index and sensitivity up to 10- 6/pm was obtained.

Fibre gratings in novel optical fibres for applications in sensing

This thesis presents the fabrication of fibre gratings in novel optical fibres for sensing applications. Long period gratings have been inscribed into photonic crystal fibre using the electric-arc technique. The resulting sensing characteristics were found to depend on the air-hole geometry of the particular fibre. This provides the potential of designing a fibre to have enhanced sensitivity to a particular measure and whilst removing unwanted cross sensitivities. Fibre Bragg gratings have been fabricated in a variety of polymer optical fibres, including microstructured polymer optical fibre, using a continuous wave helium cadmium laser. The thermal response of the gratings have been characterised and found to have enhanced sensitivity compared to fibre Bragg gratings in silica optical fibre. The increased sensitivity has been harnessed to achieve a grating based device in single mode step index polymer optical fibre by fabricating an electrically tunable fibre Bragg grating. This was accomplished by coating the grating region in a thin layer of copper, which upon application of a direct current, causes a temperature induced Bragg wavelength shift.

Simultaneous temperature and strain measurement with enhanced resolution up to 20 times by using a compact hybrid optical waveguide Bragg grating

A hybrid waveguide Bragg grating in optical fiber was fabricated and characterized, showing thermal responsivity of 211pm/°C. Proposed being used in fiber sensor, it demonstrates enhanced resolution by 20x and 2x for temperature and strain.

High frequency sensing interrogation system using fiber Bragg grating based microwave photonic filtering

A high frequency sensing interrogation system by using fiber Bragg grating based microwave photonic filtering is proposed, in which the wavelength measurement sensitivity is proportional to the RF modulation frequency applied to the optical signal.

Transversal loading sensor based on tunable beat frequency of a dual-wavelength fiber laser

Microwave signal generation by using the photonic beating from a phase-shift fiber Bragg grating (PS-FBG)-based dual-wavelength laser is proposed and experimentally demonstrated. The dual-wavelength laser is formed by a linear cavity, in which a PS-FBG is used as a dual-wavelength selective component. Transversal loading on the PS-FBG enhances the birefringence of the optical fiber and consequently makes the transmission peak of the PS-FBG splitting into two sharp transmission peaks of orthogonal polarizations. The wavelength spacing between the two transmission peaks increases with the transversal loading on the PS-FBG, thus making the polarization beating frequency increase. This property is exploited in a transversal loading sensor.

Simultaneous temperature and strain measurement with enhanced resolution up to 20 times by using a compact hybrid optical waveguide Bragg grating

A hybrid waveguide Bragg grating in optical fiber was fabricated and characterized, showing thermal responsivity of 211pm/°C. Proposed being used in fiber sensor, it demonstrates enhanced resolution by 20x and 2x for temperature and strain.

Thermal insensitive optical liquid level sensor based on excessively tilted fibre Bragg grating

We demonstrate a liquid level sensor based on the surrounding medium refractive index (SRI) sensing using of an excessively tilted fibre Bragg grating (ETFBG). The sensor has low thermal cross sensitivity and high SRI responsivity.

Two-axis temperature-insensitive accelerometer based on multicore fiber Bragg gratings

We report a compact two-dimensional accelerometer based upon a simple fiber cantilever constructed from a short length of multicore optical fiber. Two-axis measurement is demonstrated up to 3 kHz. Differential measurement between fiber Bragg gratings written in the multicore fiber provides temperature- insensitive measurements.

Research activities arising from the University of Kent

In this paper I describe research activities in the field of optical fiber sensing undertaken by me after leaving the Applied Optics Group at the University of Kent. The main topics covered are long period gratings, neural network based signal processing, plasmonic sensors, and polymer fiber gratings. I also give a summary of my two periods of research at the University of Kent, covering 1985–1988 and 1991–2001.