Polarisation beam-splitting and combining characteristics of fused-taper single-mode fibre couplers

Polarisation beam-splitting and combining characteristics of fused-taper single-mode fibre couplers have been investigated. It is found that the states of polarisation of the two output ports of the coupler are slightly different, and in the case that it is used as a monitor of polarisation matching in coherent optical communication systems, the polarisation compensation at the other output port of the coupler will be needed.     

Comment: Polarisation characteristics in long length v.a.d. single-mode fibres

An assumption stated in a previous letter, concerning states of polarisation of a single-mode fibre, is criticised. Experimental proof is required to support the assumption.     

Polarising fibre coupler with high extinction ratio

Fused polarising fibre couplers have been fabricated using single-polarisation single-mode fibres. When circularly polarised light is fed into the coupler, one of the two orthogonal polarisation modes suffers strong attenuation, and only one polarisation mode couples in the fused biconical taper region. The extinction ratio of the coupler is 41 dB at a wavelength ? = 1.3 ?m with a coupling ratio of 48:52.     

Acousto-optic frequency shifter for single-mode fibres

A fibre-optic frequency shifter has been constructed using a birefringent single-mode fibre positioned asymmetrically within two acoustic resonators driven 90° out of phase. The incident wave in one polarisation state is coupled to a frequency-shifted wave in the orthogonal state. Sideband suppression of better than 20 dB is achieved.     

New single-mode single-polarisation optical fibre

The theory for understanding fibres composed of `highly? birefringent material is presented. One of the two possible polarisation states of the fundamental mode can be made leaky, suggesting a novel single-mode single-polarisation fibre design. No leakage occurs in planar waveguides constructed similarly from the identical material.     

Single-mode fibre fractional wave devices and polarisation controllers

We describe new single-mode fibre optic devices which are completely equivalent to fractional wave plates of classical optics. They employ a very simple configuration which uses the stress birefringence induced by bending the fibre. Several devices can be combined to control the polarisation inside any single-mode fibre.     

Total integrated reflection loss in a laser diode to single-mode fibre coupling system

A geometrical optics approach is used to analyse the reflection of a divergent beam falling onto a concave or convex surface. The polarisation state of a ray in the incident beam depends on the position where the ray impinges upon the surface. The distribution of transmittance over the surface for different polarisation states is determined. An integration of the transmittance distribution determines the total reflection loss of a coupling lens used in a laser diode to single-mode fibre coupling system.     

Degree of polarisation in a birefringent single-mode optical fibre

We examine the evolution of the polarisation of a field propagating in a birefringent single-mode optical fibre. The explicit dependence of the degree of polarisation on pulse duration, coherence time of the source and chromatic delay of the fibre is derived.     

Active stabilisation of polarisation on a single-mode fibre

We report on a practical method for stabilising the state of polarisation on a single-mode fibre exhibiting a 100% degree of polarisation. The principle of operation is reviewed, as are the practical implementation aspects and experimental results for polarisation stabilisation with a 1.3 ?m-wavelength laser diode.     

Polarisation characteristics in long length v.a.d. single-mode fibres

Polarisation characteristics in long single-mode fibres are investigated. Single-mode fibres used in the experiment have been fabricated by the v.a.d. (vapour-phase axial deposition) method. It was shown from the measurements on a 21 km single-span v.a.d. fibre that the state of polarisation of the incident light beam can be maintained over a long length when the incident plane of polarisation coincides with the major or minor axis of the single-mode optical fibre.     

Vibration-induced polarisation modulation of light guided in single-mode optical fibres

The polarisation state of the HE11-mode in a single-mode optical fibre with a fused silica core was varied by mechanical vibrations of the fibre. The accompanying amplitude modulation is suppressed in the experimental arrangement. The influence of the plane of the incident polarisation on the signal modulation is discussed. The measurements were made with both reflecting and nonreflecting fibre endfaces.     

Simultaneous dual polarisation operation of a diode pumped femtosecond fibre laser

The behaviour of a diode pumped figure-of-eight ytterbium/erbium-doped fibre laser which generates subpicosecond pulses is described. This laser is observed to mode-lock simultaneously in two orthogonal polarisation states with distinct temporal and spectral characteristics.<>     

Endlessly rotatable fractional-wave devices for single-mode-fibre optics

Endlessly rotatable fractional-wave devices for polarisation control in single-mode fibres are described which utilise stress birefringence induced by fibre bending, and are free from fibre twist by rotating the fibre around its own axis. Experiments confirm that these devices rotate endlessly and have small hysteresis.     

Polarisation-maintaining single-mode fibre cable design

Guidelines for the design of a circular-polarisation-maintaining single-mode fibre cable are given. The use of circular polarisation allows use of low birefringence fibres and there is no need for orienting the fibres at splices. Quarter-wave plates at input and output of the fibres achieve the desired linear-circular polarisation conversion, and vice versa.     

Rocking filter formation in photosensitive high birefringence optical fibres

Parent et al. (1985) observed that the refractive index changes induced in optical fibres by Hill grating formation (at 488 and 514-5 nm) were birefringent. A wavelength filter, formed in a simple and novel way by exposing high birefringence fibre to linearly polarised 488 nm light, is reported. Narrow bandwidth operation and high conversion efficiencies are achieved experimentally between the two orthogonal polarisation states of the fibre.<>     

Polarisation-dependent gain spectrum of a 1.5 ?m travelling-wave optical amplifier

We present high-resolution gain spectra measurements of a 1.5 ?m near-travelling-wave amplifier for both TE and TM polarisation. We use 49km of single-mode fibre at the amplifier input to demonstrate a reduction in the effective bandwidth of the amplifier caused by a combination of dichroism in the amplifier and the polarisation dispersion present in long lengths of fibre.     

New measurement method for polarisation dispersion in single-mode fibres employing frequency-modulated optical signal

A new measurement method for polarisation dispersion in a single-mode fibre, utilising the coherence property of frequency-modulated laser diode light, is proposed. Polarisation dispersion is determined exactly by selecting the modulation condition which gives depolarisation light at the fibre end.     

Electro-optical polarisation control on single-mode optical fibres

The polarisation of the output light of a single-mode optical fibre was controlled and stabilised by an electro-optical feedback system consisting of two LiNbO3 crystals and voltage controllers. Using this system, polarisation mismatching loss was reduced to 0.2 dB.     

Light coupling from a semiconductor laser into a polarisation-maintaining single-mode fibre

For laser to polarisation-maintaining fibre coupling we propose the insertion of a short piece of normal single-mode fibre. As a result a power coupling loss of 2.8 dB is obtained, whereas additional crosspolarisation is very small compared with the unwanted polarisation excitation of the laser itself.     

Fabrication of single-polarisation single-mode-fibre couplers

Fused biconical taper fibre couplers have been fabricated from single-polarisation single-mode fibres. A CVD technique is applied to permit maintenance of parallel alignment of the fibres in the coupler fabrication process. The couplers can hold linear polarisation along the principal axis of the fibre branches, showing an extinction ratio of about ?15dB.