Lens-effect tracing technique aligns and splices polarization-maintaining fibers

An advanced lens-effect tracing method allows passive azimuthal alignment and fusion splicing of polarization- maintaining fibers and accurate estimation of extinction ratio

wenxin zheng, Ola hulten and mats bengtsson

ericsson cables ab

To overcome the shortcomings of existing polarization-maintaining fiber alignment and splicing technologies, a novel azimuth and transverse alignment method has been incorporated into an automatic fusion splicer that can also estimate the splice`s extinction ratio. This method is based on image-processing techniques.

When the circular cross section of a fiber is light-illuminated from one side, an image is formed on the other side because the fiber itself functions as a cylindrical lens and provides its own focus point during illumination. Moving an observation plane onto the focus point results in an image profile with maximum contrast in light intensity at the fiber`s center position without interference from other fiber design parameters.

When the polarization-maintaining fiber is rotated, the height of the light-intensity image changes at the focus point, because the refractive indices of these fibers are rotationally asymmetric. However, the shape of the light-intensity profile remains similar and independent of the azimuthal position.

By tracing the maximum contrast at the focus point while rotating the fiber 360 degrees, an image profile of maximum contrast can be obtained in relation to rotation angle. This polarization observation by lens profile is obtained using a light source, an optical lens, a charge-coupled camera and special data-processing software algorithms. All of these parts have been incorporated into an automatic-operating fusion splicer.

With the lens-effect tracing method, the azimuthal position of a polarization-maintaining fiber is recognized only after fiber rotation. Before rotation, a single image profile can be used to measure the maximum contrast value but it does not aid in determining the azimuthal position. Note that a common singlemode fiber with normal cladding circularity displays a horizontal line for its light-intensity profile and yields no information on its azimuthal position.

Angular offset

The polarization observation with lens profiles enables the determination of the polarization angle offset of two polarization-maintaining fibers before splicing them. They constitute the basis for polarization-maintaining fiber azimuthal alignment and assist in estimating the prospective splice`s extinction ratio.

To accomplish the alignment and extinction analyses, two automated rotators driven by precision step motors and custom software are installed in the automatic fusion splicer equipped with a digital image processing system. This splicer can rotate the fibers to a specific position and at a desired speed.

Rotating the two fibers to be spliced 360 degrees and taking continuous image intensity samples produce two polarization observation by lens profiles. By calculating the correlation between these two profiles for different angle deviations, a correlation coefficient profile can be established that shows how the two profiles vary with the shifted angle. By simulating the sampled profile with the help of an interpolation function, the angular offset between the two fibers can be accurately determined.

When the angular offset between the fibers and the azimuthal orientation of each fiber are known, the fiber can be rotated to any position with the controllable rotators to obtain the desired angular offset and orientation. For example, if stress-induced birefringence polarization-maintaining fibers are to be spliced, the angular offset would be set to zero degrees. If a stress-induced birefringence fiber is to be spliced to a geometrical birefringence polarization-maintaining fiber, the angular offset would be set to 90 degrees. If depolarizers are to be made, then the angular offset would be set to 45 degrees.

Although the angular offset analysis and the rotational alignment can be accomplished with high accuracy (theoretically to approximately 0.1 degree) during a normal splicing procedure, the accurately aligned angular offset cannot be maintained without changes because of the imperfect cleaving angles of the fiber ends. When the two fibers touch during the splicing procedure, the imperfect cleaving angles generate a torsion that twists the fibers. This torsion produces an undesired deviation in the angular offset. In practice, the larger the cleaving angle, the greater the possibility of having an angular offset deviation after splicing.

To avoid this system inaccuracy resulting from angular offset deviation, two factors must be evaluated. First, the cleaving angle must be checked before splicing. Second, the angular offset must be measured again after splicing to confirm the estimated extinction ratio.

After splicing, the angular offset can be determined by first re-rotating the fibers 360 degrees and measuring polarization observation by lens values. Then, direct or indirect correlation methods can be used to calculate the angular offset. When the angular offset is established, the extinction ratio is calculated by a derived formula.

Accurate splicing

The light-intensity profiles and the splice-measurement procedures incorporated into the automatic fusion splicer system have been extensively tested. Operational studies of the fusion splicer yield a mean extinction ratio of 32.2 decibels with a 2.73-dB standard deviation, a mean difference between the measured and estimated extinction ratio of 0.18 dB, a splice loss of less than 0.1 dB and a splice strength of approximately 300 kpsi.

The correlation between the measured and estimated extinction ratios is precise for values less than 35 dB. When the extinction ratio is higher, the accuracy of the measurement setup degrades because the extinction ratio for the system polarizers is approximately 38 dB.

The total measurement time of the fusion splicer from loading the polarization-maintaining fibers to completing the splice takes almost 3 minutes, without including the estimation step for the extinction ratio, and 4 minutes when the estimation is included.

The rotational alignment technique is not limited to polarization-maintaining fibers, however. It can also serve as a flexible platform for developing more sophisticated optical fiber networks with D-shaped, V-shaped, twin-core and multicore fibers, as well as with common singlemode, multimode and erbium-doped optical fibers.

The polarization-maintaining properties of optical fibers can be achieved by introducing an azimuthal asymmetry into the circularly symmetrical fiber structure, thereby suppressing the cross-coupling of power between the two perpendicular polarized modes--magnetic and electric fields--in a transmitted lightwave. This asymmetry can be incurred by changing the shape of the fiber structure, thereby giving the fiber different propagation properties depending on the polarization of the transmitted light.

One approach is to construct a fiber with an elliptically-shaped core that has a higher refractive index than that of the surrounding cladding. This approach generates a geometrical birefringence effect.

Another approach involves applying an internal physical stress on the fiber core. This stress-induced birefringence can be imparted by physically emplacing stress applying parts--which are made of glass possessing different thermal coefficients of expansion--around the fiber core. These highly doped parts can be made in different shapes, depending on the manufacturing process, and cause a residual stress during the cooling phase of the fiber drawing process. Common shapes include bow-tie fiber, panda fiber and elliptical cladding fiber.

The installation of a polarization-maintaining fiber system requires a method for splicing these specialty fibers. Polarization-maintaining fibers, which maintain the polarized state of the propagating light, are widely used in coherent communications systems and interferometric sensors.

Although extensive research has been performed on fusion splicing techniques for standard types of optical fibers, the splicing of polarization-maintaining fibers demands extra effort to overcome the difficulty of correct rotational alignment as well as the measurement of the splice`s extinction ratio. These requirements are in addition to alignments in the x, y and¥directions, which are the only criteria for splicing standard fibers.

Other available methods for azimuthal alignment include the active azimuthal method, the passive profile alignment and end-view image processing. The active azimuthal method is impractical in the field because it mandates the use of bulk optics. The conventional passive profile alignment is useful for only a few polarization-maintaining fiber types. The end-view system is useful only before performing fusion splicing. u

Wenxin Zheng is senior specialist, Ola Hulten is technical manager and Mats Bengtsson is research engineer at Ericsson Cables AB, Sundbyberg, Sweden.