L. Curtis

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. >

Multiwavelength DFB laser array transmitters for ONTC reconfigurable optical network testbed

We discuss the design, fabrication, and performance of experimental multiwavelength laser array transmitters that have been used in the reconfigurable optical network testbed for the Optical Network Technology Consortium (ONTC). The experimental four-node multiwavelength network testbed is SONET/ATM compatible. It has employed multiwavelength DFB laser arrays with 4 nm wavelength spacing for the first time. The testbed has demonstrated that multiwavelength DFB laser arrays are indeed practical and reproducible. For the DFB laser arrays used in such a network the precise wavelength spacing in the array and the absolute wavelength control are the most challenging tasks. We have obtained wavelength accuracy better than /spl plusmn/0.35 nm for all lasers, with some registered to better than /spl plusmn/0.2 nm. We have also studied the array yield of our devices and used wavelength redundancy to improve the array yield. Coupling efficiencies between -2.1 to -4.5 dB for each wavelength channel have been obtained. It is achieved by using specially designed lensed fiber arrays placed on a silicon V-grooved substrate to exactly match the laser spacing. The transmitter consisted of a multichip module containing a DFB laser array, an eight-channel driver array based on GaAs ICs, and associated RF circuitry.

All-fiber 90° optical hybrid for coherent communications

A 90 degrees optical hybrid is a four-port optical device with two inputs and two outputs. The two output signals are proportional to E(1) + E(2) and E(1) + E(2) exp(j90 degrees ), respectively, where E(1) and E(2) are the complex amplitudes of the two input signals. The 90 degrees hybrids are needed in many applications, including homodyne optical receivers in both phase-locked and phase-diversity configurations. In this paper, the principle of operation of an all-fiber 90 degrees hybrid is described, and an experimental unit using connectorized in-line single-mode fiber components is reported.

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.

Cutoff characteristics of dispersion-shifted fibers

Dispersion-shifted (DS) single-mode fibers are designed to have low dispersion and low attenuation in the 1550-nm wavelength region. To achieve this, these fibers require complex index profiles and may, therefore, exhibit characteristics that are different from those observed in unshifted fibers with generally simple index profiles.1 Although DS fibers are optimized for operation at 1550 nm, it is generally considered useful to have the 1300-nm wavelength region also available for transmission. We, therefore, studied the cutoff wavelength behavior of a DS fiber with triangular-index core and a higher-index ring surrounding the core.2 The results of the study show that for the bend diameter of 28 cm, used for specifying the cutoff wavelength of unshifted fibers as recommended by CCITT,3 the cutoff region of the DS fiber investigated exhibits an interesting hump which may impact on the definition of the cutoff wavelength of this fiber.

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.