Virendra S. Shah

Efficient power coupling from a 980-nm, broad-area laser to a single-mode fiber using a wedge-shaped fiber endface

A theoretical and experimental study of an efficient technique for coupling a broad-area laser to a single-mode fiber without the use of bulk optical components was carried out. The technique uses a wedge-shaped fiber endface to approximate a cylindrical lens action to correct for the phase mismatch between the curved laser beam wavefront and the planar fiber beam. A theoretical formula for the coupling efficiency in the absence of both angular and transverse misalignments is derived. By optimizing the wedge angle and the axial separation between the laser facet and the fiber endface, a maximum coupling efficiency of 47%, compared to 20% for the square endface, achieved, resulting in 15.2 mW of power in the single-mode fiber. The fabrication process consists of a simple polishing procedure requiring a wedge-shaped polishing tool and is highly reproducible. >

Curvature dependence of the effective cutoff wavelength in single-mode fibers

The curvature dependence of the effective cutoff wavelength is investigated for both matched-cladding and depressed-cladding fibers. Contrary to the depressed-cladding fiber, the cutoff wavelength of the matched-cladding fiber is strongly curvature dependent. A formula is developed for the bending loss of the first higher-order mode group in a depressed-cladding fiber. By taking into account the effect of the stress-induced changes in the refractive index on the loss the first higher-order mode group, the effective cutoff wavelength as a function of curvature is calculated. The theoretical results show good agreement with measured values, which verify the curvature insensitivity of the effective cutoff wavelength of depressed cladding fibers.

Mode coupling effects of the cutoff wavelength characteristics of dispersion-shifted and dispersion-unshifted single-mode fibers

The authors present theoretical background and experimental results to clarify the nature of the hump in the cutoff region of different types of single-mode fibers and discuss its implications. A theoretical model in terms of the coupling between the whispering-gallery mode formed near the silica-cladding/primary-coating boundary and the first higher-order core mode is presented to explain the presence of the hump. An experimental verification of this theoretical model is presented for these fibers. In addition, the curvature sensitivity of the hump is studied for these three fiber types. The results of the study show that for both dispersion-shifted and matched-cladding, dispersion-unshifted fibers, the hump is curvature sensitive and occurs at intermediate (28 and 15 cm) bend diameters. At large (40 cm) and small (10 cm) bend diameters, the hump was not observed in the cutoff region of these two fiber types. However, in the case of the depressed-cladding fiber, the hump was found to be curvature insensitive and occurred at all bend diameters (40, 28, 15, and 10 cm) used in this study. >

Transmitted power variations in single-mode fiber joints with obliquely polished endfaces

An experimental and theoretical study of multiple-beam interference effects at oblique endface airgap joints, with endfaces oriented parallel to each other, is discussed. It was found that although excellent return loss performance can be obtained by using such joints, large variations in the transmitted power can result in small changes in endface separation. Compared to the case of nonoblique endfaces, endface separation between the successive fringes in oblique endfaces increases by a factor which depends on the oblique angle and the refractive index of the gap material. >

Cleaved oblique fiber end faces for high-performance mechanical splices

Optical reflections from index discontinuities in the optical path can adversely affect the performance of various lightwave communications systems1 and may cause detrimental changes in the characteristics of active components, such as optical amplifiers2 To minimize the detrimental effects of optical reflections, low-reflectance connectors and splices may be required.

Modal interference and performance characterization in stub fiber connectors

This paper presents both the theoretical and experimental results of the performance characterization in the emerging technology of stub fiber connectors. A stub fiber connector, consisting of two closely spaced joints, shows negligible modal noise. However, modal interference at such a connector joint results in a wavelength dependent oscillations in insertion loss. It is shown that this phenomenon is strongly affected by the cutoff wavelength of the stub fiber and the losses at both joints. Amplitude of the loss oscillations depends on each joint loss and on the polarization angle between the two interfering beams. A measurement method is also presented to characterize the insertion loss of such connectors.

Factors Affecting Splice Loss Measurements Using The OTDR

The Optical Time Domain Reflectometer (OTDR) has been used to measure splice loss in fiber optic cables for several years. The OTDR requires bidirectional measurements be made and values averaged to get an accurate result. One theory implies that knowing optical characteristics of the fibers being spliced could allow for a measurement to be made in one direction only. This paper investigates that theory, and our findings indicate that bidirectional measurements are required in order to achieve accurate results.