Photochromic behaviour of thulium-doped silica optical fibres

The first observation of the photochromic effect in thulium-doped silica fibres has been presented. The fact that the particular transmissive state is permanently recorded at room temperature in these doped silica glass fibres may find useful memory applications. Also, the possibilities of making rearrangeable absorptive gratings in the bulk material or fibre look interesting     

Effect of annealing on creation of second-order nonlinearity in twin-hole optical fibre of pure silica

The effect of thermal pre-annealing on the creation of second-order nonlinearity in a pure twin-hole optical fibre thermally poled at 3.5 kV and 280degC for 30 min has been investigated. In unannealed as-drawn fibres, a SON of ~0.18 pm/V was induced at the core-cladding interface. The induced SON showed little polarisation dependence. In fibres pre-annealed at 400degC for 3 h, the induced SON was smaller, decreasing to ~0.09 pm/V in magnitude. In fibres pre-annealed at 800degC for 3 h, no detectable SON was found at the core-cladding interface. Electron-migration along connected existing defects at the core-cladding interface was postulated to be responsible for the frozen-in space-charge field that generated the effective SON observed. Thermal annealing is proposed to have caused defect-annihilation, which resulted in a smaller SON.     

Chlorine concentration profiles in silica fibres

Chlorine concentration profiles in silica fibres were studied by X-ray microanalysis and consideration of the chemical reactions concerned with chlorine in the fibre fabrication process. The chlorine content was proportional to the doped element contents. The Cl/Ge content ratio was larger than the Cl/Si or Cl/P content ratio.     

Long-period fibre grating formation with 264 nm femtosecondradiation

A new approach for producing long period fibre gratings by exposure of the optical fibre to high-intensity femosecond pulses λ = 264 nm is presented. Different types of fibres were investigated and it was found that strong attenuation peaks (16-28 dB) can be induced in H₂-loaded fibres     

Annealing of arc-induced gratings at high temperatures

Long period gratings arc-induced in different types of fibre were annealed at 1000degC for 24degh. During the annealing, the spectrum of the gratings written in Ge-doped fibres shifted towards longer wavelengths whereas an opposite shift was observed for gratings written in Ge-free fibres. A discussion on the mechanisms responsible for that behaviour is presented.     

Efficient grating writing through fibre coating with femtosecond IR radiation and phase mask

Bragg gratings have been written through the polymer coating into hydrogen loaded SMF-28 fibres with ultrafast 800 nm radiation and a phase mask. Index modulations of up to 3.5times10^-4 were induced with fibre strengths remaining at ~85% of the pristine fibre value     

Measurement of high photodarkening resistance in heavily yb31-doped phosphate fibres

The measurement of record-high photodarkening resistance in single- mode phosphate fibres doped with 12 and 6 wt.% Yb₂O₃ is reported. After pumping for 10 000 min at a high population inversion (~47%), the fibre exhibited no photodarkening at 660 nm. Silica fibres pumped to the same inversion did show photodarkening, indicating that the phosphate fibre can be doped with at least six times as much Yb and still show no photodarkening.     

40 Gbit/s return-to-zero transmission over 500 km of standard fibreusing chirped fibre Bragg gratings with small group delay ripples

A 40 Gbit/s return-to-zero transmission has been successfully achieved over 500 km using standard fibres (non-dispersion shifted fibre, singlemode fibre) and chirped fibre Bragg gratings (CFBG). CFBGs were fabricated with small group delay ripples, and error free transmission (BER <1×10^-12) was achieved using these CFBGs for dispersion compensation     

Pulse broadening in multimode optical fibres with large ?n/n: numerical results

The r.m.s. impulse response width of germania doped fibres having power-law profiles at the carrier wavelength is evaluated with the help of a numerical method and measured values of dn/d?. For ?n/n?0.02, our result exceeds by more than one order of magnitude that obtained from an analytical formula based on the assumption that n dn/d? varies linearly with n2. The optimum profiles are found to differ very significantly from power-law profiles. Our numerical technique is based on scalar-ray optics. It is applicable to any fibre having a large V-number and a smooth profile.     

High extinction, broadband polarisers using long-period fibre gratings in few-mode fibres

Long-period fibre gratings in specially designed birefringent few-mode fibres are used to demonstrate spectrally flat polarisers with bandwidths of 10 nm and polarisation extinction levels of 20 dB. This is enabled by the special dispersive properties of few-mode fibres, yielding fibre polarisers with performance approaching that of bulk-optic devices.     

Fabrication and properties of large core, high NA, high Nd/sup 3+/ content multimode optical fibres for temperature sensor applications

The fabrication and properties of 0.28 NA, 100/140 mu m, multimode alumino-silicate fibre doped with 7.5 wt.% Nd/sup 3+/ is reported. These fibres show excellent potential for use as the sensing element in intrinsic optical fibre temperature sensors.<>     

High-temperature fibre Bragg grating thermometer

The use of gratings written in tin-doped silica fibres as a thermometer for high-temperature applications is described. Measurements up to ~800°C show a significant advantage over conventional fibre gratings. A considerable blue-shift has been observed at high temperatures. If corrections for blue-shift are taken into account, the errors are smaller than ~2°C     

Defects in optical fibres in regions of high stress gradients

Germanosilicate optical fibres produced by the modified chemical vapour deposition process are characterised by point defects which disrupt the tetrahedral network structure. It is shown that the GeE' defect profiles in germanosilicate preforms and fibres, as measured by cathodoluminescence, do not follow the Ge composition profile, but are most intense in regions of high stress gradients, at the core-cladding interface and the Ge depletion region. The results suggest that the GeE' centres arise from SiGe bond rupture during the draw of the fibre to reduce the thermoelastic stress.<>     

35 dB optical gain at 2.716 mu m in erbium doped ZBLAN fibre pumped at 0.642 mu m

An optical gain of 35 dB at 2.716 mu m signal wavelength has been measured in an erbium doped ZBLAN fibre. A pump-wavelength scanning determined the pump wavelength for maximum gain. This pump wavelength was found to be 0.642 mu m. The maximum absorption wavelength for this Er/sup 3+/ doped fibre is 0.649 mu m. It was deduced that the excited state absorption process improves the gain performance of such fibres at the signal wavelength considered.<>     

Low-loss fibres for wavelengths beyond 1 ?m

Graded-index optical fibres with germanium-boron and with germanium-phosphorus doped silica core have been fabricated. The dependence of the infrared absorption edge on the boron concentration was investigated. For the germanium-phosphorus doped fibre a broad transmission window with losses below 1.5 dB/km between 1500 and 1720 nm was found.     

Design parameters for dispersion-shifted triangular-profile single-mode fibres

The performance of dispersion-shifted triangular- and step-index-profile fibres is compared. It is predicted that operation near a V-value of half the cutoff value gives a fibre that has low splice loss, low cabling loss and low additional loss due to Rayleigh scattering from the doped core. At this V-value, triangular and step fibres have similar propagation properties. Thus, the lower fabrication loss resulting from the graded core/cladding boundary of the triangular profile fibre is achieved without detriment to any other parameter.     

Optical fibres coated with a UV-curable material at a speed of 12 m/s

The letter reports the successful achievement of coating silica fibre at a rate of greater than 10 m/s. We draw 125 ?m-diameter fibres from silica rods and preforms, and coated in-line with UV-curable material. All fibres were proof-tested at two different stress levels of 0.7 and 1.4 GN/m2 We also checked the transmission properties of the fibre. It is found that the optical properties of the fibre remain the same while the mechanical properties are comparable to those of the fibres coated at a low speed.     

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.     

Application of UV laser induced Ge-oxygen defect fluorescence for fibre characterisation

A new technique to monitor germanium oxygen deficiency centre defects (GeODC) and their relation to the refractive index profile in Ge-Si optical fibre have been developed. This method is based on UV excited luminescence of GeODC in silica based optical fibres. The GeODC defect profiles are unique to the fibre refractive index (profile of GeO₂) and method of fibre making.