Zhi-Dong Shi

Theoretic analysis of mode-couple equation in all-fiber polarization transformer

The variably coupled-mode equation in all-fiber polarization transformer (AFPT) is theoretically analyzed and an optimal profile of variably spun rate to get better performance for AFPT samples is proposed as Q(z) = 0.5tan{arcsin[(z/L)sin(arctan2Q(L))]} where Q(z) is the normalized spun rate by beat length LB along the spinning section of AFPT with total length L. Then, in accordance with this specific intrinsic structure of AFPT, we found that there exists a set of strictly analytic solution for the variably coupled-mode equations of AFPT. It is also found that the linear polarized exact eigen-modes of AFPT at the un-spun end are not parallel but with a slight angle relative to the birefringence axes of the un-spun fiber. The difference of phase factor and amplitude between the exact analytic solution and the asymptotic solution is compared and discussed. The analysis has referential values to the design, fabrication and evaluation of AFPT samples.

Analysis and simulation of an all-fiber polarization transformer

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.

Quarter wave plate made by cutting straight holey birefringent fiber

Compact samples of quarter wave plate have been fabricated respectively by cutting holey birefringent fiber and conventional birefringent fiber into different length of short sections. The temperature instability and wavelength sensitivity of these samples are analyzed and measured. The influence of fiber length and fiber prototype on the retardation variation is compared. The retardation deviation almost linearly increases with the fiber length of samples. Compared with the sample made of conventional birefringent fiber, the temperature stability of the sample made of holey birefringent fiber is greatly improved by one order, but the wavelength bandwidth needs to be further expanded by optimizing the holey cladding structure.

Structural design for birefringent holey fiber with a beat length insensitive to wavelength

By combination of two defect structures with positive and negative birefringence, we design a holey fiber with a beat length that is less sensitive to wavelength. The influence of different structural parameters on birefringence of holey fiber is calculated by the finite-difference beam propagation method. A stable beat length can be achieved at some given wavelength window by adjusting the parameters. An almost uniform beat length with a greater than 180 nm bandwidth at 1310 and 1550 nm wavelength windows is obtained, which is useful for the design and fabrication of fiber-optic wave plates with a wide band.

Optimal cladding structure of holey fiber designed for broadband fiber-optic wave plate

Two asymmetric structures are combined together in holey cladding of microstructure fiber. One is the rectangular lattice of holes elongated in y-axis. The other is a pair of holes with enlarged diameters near the fiber core oppositely along xaxis. The influence of structural parameters on relation between birefringence with wavelength is calculated. Their interaction at different wavelength can make the beat length insensitive to wavelength. By optimizing the lattice pitches and the holes diameter, 10mm stable beat length with relative bandwidth 12% are obtained in 1310nm and 1550nm bands. This holey fiber is suitable for fabricating wideband fiber-optic wave-plate.

Stable Quarter Wave Plate Made by Holey Birefringent Fiber

Compact samples of quarter wave plate have been fabricated by cutting holey birefringent fiber into short sections. The temperature instability and wavelength sensitivity of these samples are analyzed and measured. The temperature stability is greatly improved, but the wavelength bandwidth needs further optimization.

Influence of extension lead and input polarization state on performance of quarter wave plate made by increscent-spinning birefringent fiber

The influence of extension lead at both sides and input polarization state on polarization transform performance of increscent-spinning birefringent fibre is calculated and examined. In the extension lead, the output polarization state is fluctuating with period relevant with the max-spin-rate or the beat length of birefringent fiber. It is found, the optimal input polarization state for the best output linear polarization is not the ideal circular polarization but a elliptical polarization related with the maximum spin rate, the optimal input linear polarization orientation is not in line but has a slight divergence angle with the birefringent axes of fiber, and if the input linear polarization is not perfect linear polarization, the output circular polarization would be nonlinearly degraded. These results have advisable meaning for the design, test and evaluation of the quarter wave plate made by the increscent-spinning birefringent fibre.

All fiber polarization transformer of holey birefringence fiber

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.

Improved cutback method measuring beat-length for high-birefringence optical fiber by fitting data of photoelectric signal

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.

Experimental observation of spectral transmittance of spun optical fibers

Spun optical fibers with different structure and different spinning rates are fabricated and their spectral transmittance is measured. It is found that the transmittance of spun stress-type fiber drops down in a certain long wavelength band as the spinning rate is increased, while this phenomenon is not found in the spun conventional fiber or un-spun stress-type fiber. We suggest it due to the micro-bending-loss induced by the spinning eccentricity of the fiber core.