A photoelectric measurement system for measuring the beat length of birefringence fiber is set up including a set of
rotating-wave-plate polarimeter using single photodiode. And two improved cutback methods suitable for measuring
beat-length within millimeter range of high birefringence fiber are proposed through data processing technique. The cut
length needs not to be restricted shorter than one centimeter so that the auto-cleaving machine is freely used, and no need
to carefully operate the manually cleaving blade with low efficiency and poor success. The first method adopts the
parameter-fitting to a saw-tooth function of tried beat length by the criterion of minimum square deviations, without
special limitation on the cut length. The second method adopts linear-fitting in the divided length ranges, only restrict
condition is the increment between different cut lengths less than one beat-length. For a section of holey
high-birefringence fiber, we do experiments respectively by the two methods. The detecting error of beat-length is
discussed and the advantage is compared.
Based on the analysis on the variable coupled-mode equation, we proposed an optimal profile of variably spun rate Q(z) = 0.5tan{arcsin[(z/L)sin(arctan2Q(L)]} to get better performance for all-fiber polarization transformer (AFPT), where Q(z) is normalized spun rate by beat length LB along the fiber length L. Then, in accordance with the intrinsic structure of AFPT, and in consideration of considering the phase-difference effect, we calculated the Jones vector from point to point along fiber to simulate the evolution of state of polarization (SOP) in AFPT with this optimal spun rate profile. For comparison, calculations are also made with other two kinds of spun rate profile (linear, cosine) often used in the AFPT fabricating process. The Influence of spun rate variation profile on the polarization transforming performance of AFPT is discussed and compared in detail. It is found that both in the case of linear-in/circular-out where the extinction ratio of output light is required as small as possible and in the case of circular-out/linear-in where the extinction ratio of output light is required as large as possible, the AFPT with optimal spun-rate profile presents best performance, and the linear spun-rate profile is the worst. The analysis and calculation results have referential values to the design, fabrication and evaluation of AFPT samples.
Examined is the influence by input state of polarization and analyzing scheme on sensitivity in the measurement system of beat length that relies on magneto-optic modulation technique in which the magnetic gap and intensity of magnetic field have been given. Through theoretic analysis and experimental study, it is found that there are three experiment schemes getting maximum sensitivity. First, the polarization direction of the input linearly polarized light is at θ = 00 or 900 to the birefringence fast axis of e fiber, at the same time, the two analyzing axes of Wollaston prism are at γ = 450 to the birefringence fast or slow axis of fiber. Second, the polarization direction of input linearly polarized light is at θ = 450, and γ = 00 or 900. Third, the input light beam is circular polarized light, at the same time, γ = 00 or 900. The sensitivity under three cases is approximately equal, but the third scheme needn't pinpoint the direction of birefringence axis of fiber, which makes the experiment more convenient and avoid the measurement error caused by the adjustment of angle.
An all-fiber polarization transformer made of variably twisted holey birefringence fiber is proposed to decrease the
temperature instability. After reviewing the beat length (or modal birefringence) variation of holey birefringence fiber
with temperature and wavelength, the model for simulating the polarization transforming ability of the variably spun or
twisted birefringence fiber is set up based on the mode-couple equation with variable coupled coefficients. Compared
with the conventional polarization transformer made of variably spun stress birefringence fiber, the temperature
instability from -20°C to +80°C and wavelength sensitivity from 1200nm to 1600nm of their polarization transforming
performance are simulated and discussed.
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