CHAPTER-2
T18B OPTICAL FIBRE HARDWARE CONNECTORS, COUPLERS AND SPLICES
INTRODUCTION: Connectors, couplers and splices are vital elements in the fiber optics technology. Connectors can be defined as a rematable means for arranging transfer of optical energy from one fiber optic component to another in an optical fiber system. These components include fibers, couplers, Multiplex devices and opto-electronic devices. Couplers are multi-port devices, which permit the transfer of optical power from one port to all others simultaneously, or vice-versa. Splices are permanent joints between two fibers. These devices introduce losses at the inter-connect point, which can be broadly categorized as intrinsic losses and extrinsic losses. Intrinsic losses are those inherent in the fiber-manufacturing process and occur due to fiber core variation, the numerical aperture variation, core eccentricity etc. The overall intrinsic losses can be 5.7 dB in the worst case. The extrinsic losses result primarily from the connector design such as Fresnel Reflection loss, lateral, axial displacement, tilting angle loss etc., and can be controlled. CONNECTORS: The basic function required of connectors is to allow the transfer of optical power from one fiber component to another with minimum loss and to make for that possibility of removal for disconnection and remating any number of times with consistent minimum insertion loss. With practical connectors, the attenuation introduced has been found to be less than 1 dB. The connector must also provide protection to the fiber so that it does not break during handling. A connector consists fundamentally of two parts, a plug and an adapter. For fiber-to-fiber connections, the fibers are terminated in individual plugs and mated in the adopter. For fiber to devices connection, the devices may be housed in the adapter part and the fiber in the plug part. The fixing of the fiber in the plug may be achieved directly or by using sleeves commonly known as ferrules. The basis elements in the connectors are fiber fixing mechanism and the alignment mechanism. The alignment mechanism permits optical fibers to be brought in a face-to-face butt position allowing transfer of power. Some of the alignment techniques are:i) 3-Rod Connector. ii) 3-Sphere Connector. iii) V-Groove Connector. The types of practical connectors are:
1) FC, D3 & D4 : Precision plug connectors developed in Japan. The fibers are fixed in the plugs. The pin in the plug and notch in the adopter body prevents twist. The standard connector fitted to all measuring instruments is FC. 2) Biconic, diamond: The Biconic or Strates connector is based on the biconical principle and the diamond on the precision ferrule principle. COUPLERS: Optical couplers are designed to either branch one optical signal into two parts or couple two optical signals into one port. Practical optical couplers may use a beam splitter or a hybrid for combining optical fibers. It can be used: a) For power level measurement b) As an optical monitor in conjunction with an oscilloscope. c) For measurement of reflected light. d) For bi-directional communication using a single wave length band e) Noise insertion for BER measurement etc. SPLICERS: Splices can be defined as a permanent connection between two fibers and the process evolves cutting of the edges of the two fibers to be spliced and fixing them permanently in a butt position. The two commonly used methods are:1) By using Mechanical Splice. 2) By using Fusion Splice. Mechanical Splicing: Optical fibers can be spliced mechanically by inserting the fibers into a centering sleeve, fitted with a jelly that matches refractive indices. The jelly is necessary to prevent the cut surfaces of the fibers from acting as mirrors and reflecting part of the transmitted light. This type of splice is available in many variants such as V-groove, clamping, centering rod etc. Mechanical splicing technique is used when repairing cable edge, caused as a temporary measure to re-instate communication quickly when no fusion splicer is available. The disadvantages of mechanical splicing are that there is a high level of reflection and the splicing loss is around 0.2 to 1 dB. Fusion Splicer: The fiber-to-fiber splicing using fusion splicer is carried out in the following steps. A) Removal of secondary/primary coats. B) Fiber edge preparation C) Fusion splice D) Re-enforcement Prior to fusion, the fibers are cut at right angle, with the flattest surface. Controlled melting, in an arc created between two electrodes then splices them. The process involves fixing the cleared fibers on micro positioners on the fusion-splicing machine. The fibers are then aligned to the requisite position manually while viewing through microscope, which cover alignment in both the X and Y planes. The process is automated, controlled by microprocessors. The operation that takes place involves with drawing of the fiber to a specified distance, pre-heating of the fiber ends by electric arc bringing the fiber ends close together in a butt position and welding by high temperature arc fusion. Thereafter a tension is applied momentarily to check mechanical strength of the splice. The fiber is removed from the machine and re-enforced. Splice loss is around 0.05-0.1 dB on SM fibers. The splice machines are generally battery operated for field working.
