Novel fibre optic gyroscope with a configuration combining Sagnac interferometer with fibre ring resonator

A novel fibre optic gyroscope, with a structure making use of the combination of a Sagnac interferometer with an active fibre ring-resonator, is proposed and analysed. The proposed gyroscope can be viewed as a generalised structure of the conventional Sagnac interferometric type fibre optical gyroscope. It has the advantages of both long fibre loop length and multipath interferences. By inserting a fibre amplifier in the loop to provide an optical gain of several dB, the relative maximum sensitivity is enhanced to approximately 30 times more than that of conventional Sagnac interferometric fibre optic gyroscopes.<>     

Measurement of fibre-optic chromatic dispersion from RIN spectra of a tunable fibre laser

The chromatic dispersion of optical fibre is determined from the wavelength dependence of the mode spacing of the fibre laser with and without the intracavity test fibre. This approach is particularly useful for characterising the dispersion of modelocked fibre lasers, where control of the total laser dispersion is required.<>     

Multiwavelength erbium-doped fibre laser based on ahigh-birefringence fibre loop mirror

A novel multiwavelength fibre laser is proposed, which is based on a fibre loop mirror formed by a high-birefringence (HiBi) fibre, a polarisation controller, and a directional coupler. By setting the polarisation controller properly, the HiBi fibre loop mirror acts as a polarisation-independent wavelength filter. With this configuration, the wavelength spacing of the laser can be varied accurately by adjusting the length of the HiBi fibre. Experimentally, by cooling the erbium-doped fibre with liquid nitrogen to reduce its homogeneous linewidth, stable emission at 16 wavelengths with a spacing of 0.8 nm and an amplitude variation of <5 dB was achieved     

Compensation of fibre chromatic dispersion in optical heterodynedetection

Compensation of fibre chromatic dispersion in coherent optical fibre transmission is demonstrated. The chromatic dispersion of a 70 km single-mode fibre with 1.3 μm zero dispersion wavelength is compensated for using a microstrip line equaliser in the intermediate frequency band. Amplitude distortion due to fibre chromatic dispersion at 1.55 μm wave-length is reduced to below 2.5 % with the equaliser     

Graded-index-type crystalline fibre for IR light transmission

A new method for making a graded-index-type crystalline fibre is proposed. A fibre for IR light transmission with crystalline claddings was desired for improving the reliability of the crystallinc fibre and for avoiding any excess loss at the core surface. A fibre with the core-cladding structure was made by complex extrusion with KRS-5 as the core and KRS-6 as the cladding. Obtaining a smooth core-cladding interface was very difficult. This problem was overcome by a diffusion of composite ions by annealing after the extrusion. The fibre has a concentration distribution that can be accounted for by a GI fibre. The transmission loss of the CO2 laser is 0.2 dB/m with a launching NA of 0.05.     

Fibre-polarimeter based on grating taps

The fabrication of novel tilted fibre Bragg gratings coupling light out of the fibre core into two different directions is demonstrated. These gratings in combination with a fibre based quarter waveplate are used to form a high-speed fibre polarimeter for real-time polarisation monitoring.     

Optical-fibre waveguide with a single-crystal core

Some properties of a new single-crystal optical fibre are described. The fibre is constructed with the nonlinear optical crystal meta-nitroaniline as its core. In its present configuration, the fibre acts as an effective polariser. Alternative configurations may yield a number of electro-optic fibre devices.     

Mitigating the risk of high power damage to the BT network

Optical powers used in optical telecommunications networks have increased over time with the advent of technologies such as erbium doped fibre amplifiers, dense WDM, and fibre Raman amplifiers. Erbium fibre and Raman amplifiers with output powers up to 1 W and 2 W respectively are now available for deployment giving optical power densities of ≈14 000 MW/m² per Watt launched into a single mode fibre. This compares with 74 MW/m² for the surface of the sun. It is thus essential to ensure that the risk of optical damage to the fibre is understood and steps are taken to mitigate it where necessary. Research carried out by BT over the last few years has shown that fibres carrying relatively modest optical powers, as low as 200 mW for the most sensitive fibres, can be catastrophically damaged at tight fibre bends. Damage is caused by some of the optical power lost at the bend being absorbed by the fibre coating that may over time lead to a sudden increase in temperature to over 700 °C causing either the silica to kink mimicking a fibre break, or the coating to burn off at the bend leading to fibre failure on subsequent handling. This paper describes factors that increase the risk of short-term catastrophic damage at fibre bends, presents experimental results to show the variation in sensitivity between different fibres, and discusses new important theoretical results modelling the temperature rise in the fibre.     

High-extinction-ratio low-loss tightly bent PANDA fibre

The realisation of tightly bent PANDA polarisation maintaining fibre with minimal reduction of extinction ratio and throughput power is described for the miniaturisation of polarisation maintaining fibre circuits. After mechanically side-polishing the bare PANDA fibre, bend diameters as small as 1 mm have been achieved that preserve the fibre s extinction ratio ( 35 dB) with minimal loss ( 0.1 dB).     

Structural and optical characterisation of holey fibres usingscanning probe microscopy

An investigation of a holey fibre using scanning probe microscopy techniques, which the authors believe is the first such investigation, is presented. Atomic force microscopy images provide knowledge of the fibre structure without the artefacts associated with scanning electron microscopy imaging. A tapered fibre scanning near-field optical microscopy probe has been used to investigate holey fibre modes at 785 nm     

Implicit analytical solution for erbium doped fibre amplifier

An implicit analytical solution for the gain and output signal power of a realistic erbium doped fibre amplifier is given as a function of pump power, fibre length, input signal power and various fibre and materials parameters. This solution is shown to accurately predict the gain variation with fibre length and pump power, and the saturation behaviour with increasing signal power. It is a useful block for accurately modelling multiamplifier systems while minimising computation time.<>     

Mitigating the risk of high power damage to the BT network

Optical powers used in optical telecommunications networks have increased over time with the advent of technologies such as erbium-doped fibre amplifiers, dense WDM, and fibre Raman amplifiers. Erbium fibre and Raman amplifiers with output powers up to 1 W and 2 W respectively are now available for deployment giving optical power densities of ≈ 14 000 MW/m² per Watt launched into a single mode fibre. This compares with 74 MW/m² for the surface of the sun. It is thus essential to ensure that the risk of optical damage to the fibre is understood and steps are taken to mitigate it where necessary. Research carried out by BT over the last few years has shown that fibres carrying relatively modest optical powers, as low as 200 mW for the most sensitive fibres, can be catastrophically damaged at tight fibre bends. Damage is caused by some of the optical power lost at the bend being absorbed by the fibre coating that may over time lead to a sudden increase in temperature to over 700 °C causing either the silica to kink mimicking a fibre break, or the coating to burn off at the bend leading to fibre failure on subsequent handling. This paper describes factors that increase the risk of short-term catastrophic damage at fibre bends, presents experimental results to show the variation in sensitivity between different fibres, and discusses new important theoretical results modelling the temperature rise in the fibre.     

Bidirectional optical transmission using polarised light waves

Bidirectional optical transmission using polarised light waves is demonstrated for the first time. Transmission of 100 Mbit/s signals over 13.6 km of polarisation-maintaining fibre occurs simultaneously in both directions at 1.55/?m. LD transmitters with polarisation-maintaining fibre pigtails, polarisation-maintaining fibre couplers, fibre polarisers and Ge-APD receivers are used.     

Birefringent fibre frequency-modulated continuous-wave Sagnac gyroscope

A birefringent fibre Sagnac gyroscope is described, which is based on the principle of optical frequency-modulated continuous-wave interference and primarily consists of a single-longitudinal-mode frequency-modulated laser diode and a birefringent fibre coil. The rotation velocity of the fibre coil can be determined simultaneously by detecting the phase shift of the beat signal produced by two counter propagating laser beams in the birefringent fibre coil. A resolution of 0.02/spl deg//s with a 100 m fibre coil has been observed. The advantages and limitation of the gyroscope are discussed.     

Coherent combining of two pulsed fibre lasers in phase modulated mutually coupled fibre laser array

A scalable architecture for coherent combining of pulsed fibre lasers is reported. Stable synchronous and phase-locked pulsed fibre laser beams are generated in a phase modulated mutually coupled fibre laser array. The mechanism for generating a pulsed laser is different from previous Q-switching, modelocking or pump modulation methods. Two pulsed fibre lasers are coherently combined with 0.48 J pulse energy at the repetition rate of 115 kHz. The repetition rate can be controlled by adjusting the pump current. The combining efficiency of the two pulsed fibre lasers reaches 90 and the fringe contrast is larger than 40 .     

Switchable dual-wavelength Raman erbium-doped fibre laser

A fibre Bragg grating feedback fibre laser with both Raman and erbium-doped fibre pumps is proposed. Dual-wavelength switching is achieved by controlling the power of the Raman pump. The characteristics of the dual-wavelength switching are studied experimentally, and the mechanism is explained physically.     

Multi-coupler side-pumped Yb-doped double-clad fibre laser and pump light leakage at coupler

A multi-coupler side-pumped Yb-doped double-clad fibre laser with output power of 1.38 W and slope efficiency of 48.9% by the fibre angle-polished method is demonstrated. Its pump light leakage at the fibre coupler is also investigated experimentally and theoretically.     

Poling of single electrode fibre with polyimide conductive coating and investigation of poling mechanism

Long lengths of fibre with a single internal electrode and electrically conductive coating incorporated during the fibre draw have been previously reported. Now, thermal poling of this fibre creating a permanent electro-optic effect is reported. Also, the fabrication methods employed for this fibre provide a unique opportunity to investigate further the mechanism of thermal poling.     

Location of imperfections in optical glass-fibre waveguides

An echo-pulse technique for the location of fibre imperfections is described. The technique has been successfully applied to a plastics-coated Selfoc fibre and a plastics-coated silica-based fibre. Both showed a break. In the Selfoc fibre, an additional fibre defect was located. The same method may be used to measure rapidly the fibre length.     

Suppression of SBS by discontinuous Brillouin frequency shifted fibre in CPFSK coherent lightwave system with booster amplifier

The impact of a compound fibre exhibiting discontinuous Brillouin shifts on stimulated Brillouin scattering (SBS) in a 2.5 Gbit/s CPFSK transmission system consisting of an optical booster amplifier and a nondispersion shifted transmission fibre is described. The power penalty due to SBS is almost completely eliminated by the novel compound fibre which is constructed with four short lengths of nondispersion shifted fibre joined end to end.<>