The history, fabrication, theory, numerical modeling, optical properties, guidance mechanisms, and applications of photonic-crystal fibers are reviewed

Photonic-Crystal Fibers

Polarization-maintaining fibers and their applications are reviewed. The classification of high-birefringent fibers and low-birefringent fibers and their fabrication methods and characteristics are discussed in Section II. Analytical methods and numerical methods for fiber design on the birefringence are presented in Section III. Degradation factors of polarization maintenance expressed as crosstalk or mode-coupling parameters caused by internal origins such as structural imperfections, wavelength, and nonlinear effects, and by external origins such as temperature fluctuations, mechanical perturbations, and electromagnetic effects, are discussed in Section IV. Characterization methods on beat length, mode coupling, stress distribution, and mechanical strength are presented in Section V. Applications to the fiber devices and nonlinear effects, and splicing methods for the polarization-maintaining fibers are described in Sections VI and VII.

Polarization-maintaining fibers and their applications

Origins and control of polarization effects in single-mode fibers (A)

The polarization state of light in single-mode fibers is very sensitive to any perturbation which is not symmetric about the fiber axis. While this is a source of noise, drift, or signal fading in some applications, it can also be exploited in novel guided-wave devices. The basic birefringences that couple the two modes and change the polarization state along the fiber are reviewed. The three cases of uniform, phase-matched, and random coupling are considered. Polarization preservation in both low- and high-birefringence fibers is achieved by reducing this coupling. In addition to polarization-state changes, bireftingent fibers can quickly reduce the polarization degree of nonmonochromatic light if both modes are excited, a characteristic that greatly simplifies evaluation of the degree of polarization preservation in these fibers. Current evaluations of the birefringence and the polarization-holding ability of state-of-the-art fibers are discussed, and it is concluded that fibers with good polarization-holding properties are becoming available.

Origins and control of polarization effects in single-mode fibers

A low-loss and highly birefringent polarization maintaining photonic crystal fiber has been fabricated. The fiber loss and modal birefringence at 1550 nm were 1.3 dB/km and 1.4×10-3, respectively.

Optical properties of a low-loss polarization-maintaining photonic crystal fiber

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.

/pdf/fabrication-of-polarisation-maintaining-fibres-using-gas-26mrsljyj4.pdf

Fabrication of polarisation-maintaining fibres using gas-phase etching

Experimental results show that bow-tie fibres with high levels of stress-induced birefringence can be operated such that they support only a single linearly polarised mode. Under these conditions the loss of the fibre is 5 dB/km for the guided mode and 55 dB/km for the suppressed mode For short-length operation as a polariser, extinction ratios as high as 50 dB/m have been obtained.

/pdf/single-polarisation-operation-of-highly-birefringent-bow-tie-si50w098e4.pdf

Single-polarisation operation of highly birefringent bow-tie optical fibres

Polarization-maintaining optical fibers are usually made by inducing a large anisotropic thermal stress in the core so that it appears highly birefringent. A simple analytic solution has been found for the birefringence in terms of the cross-sectional distribution of the high-expansion material used to create the thermal stress. The analysis is able to predict optimal structures which efficiently utilize the available stress and thus maximize the birefringence. It is shown that the optimum structure has a cross-sectional geometry resembling a bow-tie. Design rules are given whereby the dimensions may be chosen and these are verified in a simple experiment.

/pdf/analytic-solution-for-the-birefringence-produced-by-thermal-2yysx0bq5c.pdf

Analytic solution for the birefringence produced by thermal stress in polarization-maintaining optical fibers

Temperature cycling of highly birefringent optical fibers and preforms has been used to investigate the thermal properties of bow-tie and elliptically clad structures. The thermal hysteresis of the birefringence is shown to be a direct consequence of the thermal history of the fiber or preform and has been related to volume changes in the stress-producing borosilicate sections. Annealing increases the axial stress as well as the stress anisotropy and hence the birefringence. Increases of up to a factor of 2 in the birefringence on suitable thermal treatment indicate a new method for further improvement of high birefringence fibers. The implications of the results in the design, fabrication, and use of such fibers are discussed.

/pdf/thermal-properties-of-highly-birefringent-optical-fibers-and-33t3hpdram.pdf

Thermal properties of highly birefringent optical fibers and preforms.

A technique for fabricating helical-core circularly birefringent fibres is reported. Extremely high levels of circular birefringence are achieved (B = 2.1 × 10-4 at 633 nm). Because of its unique structure, this fibre remains single-mode up to V values of 25.

/pdf/fabrication-and-characterisation-of-circularly-birefringent-3ace23eb1e.pdf

R.D. Birch

Papers

Fabrication of polarisation-maintaining fibres using gas-phase etching

Single-polarisation operation of highly birefringent bow-tie optical fibres

Analytic solution for the birefringence produced by thermal stress in polarization-maintaining optical fibers

Thermal properties of highly birefringent optical fibers and preforms.

Fabrication and characterisation of circularly birefringent helical fibres