Spectral response measurements on polarisation splitting ratio of fused-taper single-mode fibre couplers

Using a Ge PIN photodiode cooled to 77 K (?196°C) with liquid nitrogen and the lock-in amplification technique, an automatic polarisation spectral response measurement system with a measuring dynamic range of about 36 dB has been developed. It has been confirmed experimentally that the spectral response of polarisation beam-splitting ratios of fused-taper fibre couplers depend on the incident polarisation angle     

Polarisation behaviour fluctuations of a single-mode fibre

The linear retardance and its azimuth angle of a 111 cm single-mode fibre have been measured to fluctuate with time. The amplitude and frequency of these fluctuations depend on temperature. The circular retardance of the fibre has been found to remain constant.     

Wavelength combining fused-taper couplers with low sensitivity to polarisation for use with 1480 nm-pumped erbium-doped fibre amplifiers

Fused-taper wavelength division multiplexing (WDM) couplers for use in combining the 1480 nm pump wavelength with the 1540 nm signal wavelength of erbium-doped fibre amplifier systems are reported. Output port isolation in excess of 15 dB has been achieved at the centre of each channel with very low polarisation sensitivity of <0.2 dB over a 30 nm range about the channel centres.<>     

Perturbation analysis of fused tapered single-mode fibre couplers

By treating a fused tapered single-mode fibre coupler as a perturbation of the same coupler with the fibre cores ignored, simple analytical expressions are derived for the coupling coefficients of the coupler. These new expressions highlight the effects of the fibre cores on the characteristics of the coupler.     

Novel method for making single-mode optical fibre directional couplers

An optical single-mode directional fibre coupler was made from two lengths of fibre of approximately semicircular cross-section. These new fibres were drawn from a preform ground parallel to the core axis. The coupling factor was measured as a function of wavelength. It is shown that the coupling factor can be influenced by bending the coupler.     

Polarisation characteristics of a side-tunnel fibre

Modal birefringence and polarisation mode dispersion in a single-mode fibre having vacuum pits beside the core are measured in the wavelength region from 0.633 ?m to 1.52?m. Both quantities monotonically increase as the operation wavelength becomes large. The magnitudes of birefringence and modal dispersion are evaluated to be 1.0 × 10?4 at 1.52?m and 148 ps/km at 1.41 ?m, respectively. These values are much less than those predicted theoretically for side-tunnel-type optical fibres.     

Polarisation splitters and birefringent couplers

The theoretical principles for understanding polarisation splitters are presented along with design parameters for their practical realisation. Birefringent couplers, with nonparallel optical axes are also described by deriving the modes of the composite anisotropic fibre system.     

Polarisation referencing in refractometry using fibre to planarwaveguide couplers incorporating thin metal layers

The wavelength response of fibre to metal clad planar waveguide couplers has been investigated as a function of superstrate index. The TE resonance wavelengths remain fixed with superstrate index variation while the TM resonance position demonstrates good superstrate index sensitivity. Such a technique may be used for polarisation referencing and temperature compensation in refractometry     

Polarisation modulation in long couplers

A simple mathematical model of long tapered couplers shows that the modulation of the output power due to polarisation has a period which varies inversely with the square of wavelength. This prediction is in agreement with experimental measurements.     

Polarisation beat length measurement in a single-mode optical fibre by backward Rayleigh scattering

Polarisation fluctuation on the backward Rayleigh scattered signal measured by the POTDR (polarisation optical time domain reflectometer) has been investigated by means of the power spectrum. As a result, the spectrum is sharply peaked at the Fourier frequency of 8.3 × 106 Hz determined by the polarisation beat length of 12 m due to the difference in phase velocities between the orthogonal eHE11 and oHE11 modes. The value is in good agreement with the analysis.     

Polarisation conservation in single-mode fibres

There exist proper orthogonal elliptical polarisation states, which are maintained over long fibre length without converting. When one of the proper polarisation states is excited, orthogonal polarisation less than ?40 dB is confirmed after 5 km single-mode fibre transmission. It is also proposed that twisted single-mode fibres show a high degree of polarisation for any incident polarisation state. The polarisation characteristics are hardly affected by external perturbations.     

Wavelength tunability and polarization characteristics of twistedpolarization beamsplitting single-mode fiber couplers

The wavelength tunability and the polarization characteristics of twisted polarization beamsplitting fiber couplers are investigated experimentally and analyzed theoretically. The measured spectrum transmissions and polarization characteristics agree well with the theoretical results. The wavelength working as a polarization beamsplitter can be tuned in the 1.3-1.75 μm wavelength range without significant degradation. Splitting ratios for orthogonally polarized light are more than 20 dB and less than -20 dB     

Analysis of fiber-optic polarizing beam splitters consisting of fused-taper couplers

The characteristics of polarization splitting in fiber-optic couplers employing both nonbirefringent and birefringent fibers (PANDA fibers) are investigated. An analytical method that takes into account the cores, stress-applying parts, and stress birefringence in the coupling region is proposed. From comparison between the theory and experiments, the validity of the present analytical method is confirmed. It is shown that the length of the coupling region in birefringent-fiber polarizing beam splitters (BPBS) is several times shorter than that in nonbirefringent-fiber polarizing beam splitters (NPBS).     

Transmission loss characteristics of fluoride glass single-mode fibre

The transmission loss spectrum of a fluoride glass single-mode fibre as obtained with a new preform fabrication technique is shown. It is clearly confirmed on the loss spectrum that the obtained fibre is a single-mode waveguide in the minimum loss wavelength range (2?4 ?m). The minimum loss of this fibre is 8.5 dB/km at 2.2 ?m and the OH excess loss is 15dB/km at 2.85 ?m. This fabrication technique is effective in eliminating OH contamination.     

High-contrast measurement of refractive-index profile in fused single-mode fibre couplers with high resolution by deconvolution

The optical contrast of the refracted near-field measurement method for fibre refractive-index profiles has been improved by a factor of about five by using numerical apertures adapted to the coupler to be measured. A spatial resolution of 0.3 mu m was achieved by deconvolving with the effective scanning near field.<>     

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.     

1.3 ?m bidirectional optical transmission over 31 km of installed single-mode fibre using optical couplers

Results are reported of 1.3 ?m bidirectional optical transmission over 31 km of installed cabled fibre at transmission rates of 34?650 Mbit/s using fused biconical tapered couplers manufactured at BTRL. With the introduction of 6 dB extra path loss it is possible to double the system capacity incurring only a small additional penalty at 34 and 140 Mbit/s and no other penalty at 320 and 650 Mbit/s.     

Polarisation analysis of strongly fused and weakly fused tapered couplers

The polarisation properties of both strongly fused and weakly fused single-mode tapered fibre couplers have been analysed. A comparison with experimental results shows excellent agreement with our analysis.     

Novel method for characterisation of single-mode fibres and prediction of crossover wavelength and bandpass in nonidentical fibre directional couplers

We present here a novel and accurate method for the characterisation of single-mode fibres from a measurement of the wavelength dependence of the Petermann spot size. We also show its applicability in the prediction of the crossover wave length and bandpass in wavelength filters using nonidentical fibre directional couplers.