Fabrication and development of polarisation maintaining fibres using gas phase etching

Polarisation-maintaining fibres are of considerable interest in the field of optical fibre sensors because of their ability to transmit either of the two orthogonal li early polarised modes over long distances - an extinction ratio of 20dB in 5km of PANDA fibre has been reported (Hosaka et a1 (I)). The fibres owe their polarisation holding performance to the high levels of birefringence designed into the structure. Although the form birefringence of an elliptical core can be used (Dyott et a1 (2)), the fibres are usually made birefringent by doping the silica on either side of the core with materials having different expansion coefficients. The resulting fibre has a birefringence proportional to the anisotropic stress across the core, whose magnitude depends upon the expansion coefficient mismatch and fibre geometry. In this paper, the optimum structure for a polarisation-maintaining fibre is first designed and then a process to make it is described together with typical performance figures. Finally, two techniques are described to enhance the already high levels of birefringence obtainable, and experimental results are given which show how short lengths of fibre can be used as a high extinction polariser, while long lengths of fibre can be used for the transmission of linearly polarised light aligned to one of the axes only - the other linearly polarised mode being suppressed

An all-fibre polarising beamsplitter

The first fibre equivalent of a polarising beamsplitter is reported. The device is based on a fused-taper fibre coupler. Separation of the orthogonally polarised components of the input light can be accomplished to better than 17 dB

An all-fibre polarising beam splitter and spectral filter

Monomode fibre couplers are key components in many sensor and communication applications, with uses ranging from general-purpose 3dB optical-power splitters to wavelength multiplexers. More specialised couplers for polarisation control can also be fabricated using birefringent fibres. Thus polarisation-maintaining couplers have been fabricated and couplers which transmit only one plane of polarisation have been fabrication from polarising fibre

Fundamental limits to the transmission of linearly-polarised light by birefringent optical fibres

Experimental results are presented which show that the fundamental mode in highly birefringent fibres is not plane-polarised, as is normally assumed, but has significant orthogonal field components. These components limit the maximum measurable polarisation intensity extinction ratio to ~40 dB. Implications for polarisation measurements and fibre gyros are outlined

Three-dimensional stress profiling of highly birefringent optical-fibre preforms

In the fabrication of highly-birefrinqent, e.g. "Bow-Tie", optical-fibre it is required to maximise the stress, and hence the birefringence, within the core of the preform. Conversely, the stresses in other regions of the preform should be minimised to prevent the preforms shattering. This is particularly true for the so-called 'Stress-Guide' preforms in which the refractive index difference in the core is obtained by using the stress-optic effect, rather than by the more usual method of incorporatinq a dopant. The ability to measure the stress distribution within such preforms would therefore be extremely useful

Fabrication of polarisation-maintaining fibres using gas-phase etching

A new fabrication technique for the production of high-birefringence fibres is described. The process is shown to produce fibres with a cross-sectional geometry which is close to the optimum predicted by stress analysis. As a result, fibres with extremely short beat lengths (0.55 mm at a wavelength of 633 nm) have been produced

Thermal stress measurements in optical fibre preforms using preform profiling techniques

An analysis is presented of the effect of thermal stress on transverse refractive-index profiling of optical-fibre preforms. The theory leads to a new measurement technique for axial stress profiling

Polarisation characteristics of fibres for coherent detection systems

Coherent transmission systems utilising the optical heterodyning principle require a stable polarisation state for both the local oscillator and incoming signal waves. For a fixed input state the output polarisation of a single-mode fibre in general varies according to the prevailing environmental conditions. Efficient heterodyning can be obtained only by stabilising the output state. This may be accomplished by either (i) designing the fibre polarisation properties to be intrinsically immune to the environment, or (ii) by actively controlling the output to maintain the required state of polarisation at the detector. In this paper the design, manufacture and properties of fibres suited to both these approaches will be considered

Helical-core circularly-birefringent fibres

A new type of circularly-birefringent fibre is demonstrated based on the optical rotation which occurs in a helical core fibre. The birefringence is an order of magnitude higher than that obtained with previous fibres

Crosstalk in polarisation-maintaining fibres

We show that internal geometric imperfections have a negligible effect on polarisation cross-talk in a birefringent fibre. This is contrary to popular opinion and has considerable implications for the way in which polarisation maintaining fibres are specified