white paper Connector Return Loss Source of Return Loss Fiber optic connectors are produced by polishing their endfaces to create a smooth domed shape. If the shape or endface geometry is precisely controlled, the result will be physical contact between fiber cores when two connectors are mated together. The polishing process creates a very small damaged layer at the tip of the fiber that has a higher index of refraction (index layer) than the glass before the damaged layer. Since reflections in optical fiber are caused by changes in index of refraction, it is the index layer that causes the return loss associated with a mated pair of connectors. Factory Return Loss Measurement to a Master After polishing, insertion loss and return loss are measured on each connector by mating to a master or reference connector. The master connector is selected by characterizing the offset of the fiber core from the center of the ferrule to be sufficiently small so that the insertion loss will be the same regardless of the orientation of the offset on the mating connector. The master connector is also subjected to a special polishing process that removes or reduces the thickness of the index layer. The reflection from the master connector fiber endface is then measured by dipping it in index matching fluid to verify that the reflection is very low relative to the reflection level of the connectors that will be mated to it during testing. The return loss of the connector under test will include the reflections due to both the change in index of refraction as the light enters the index layer and the change in index of refraction as the light leaves the index layer (Figure 1). The light reflected from each of these interfaces has traveled different distances and, when recombined, will interfere with each other either constructively or destructively depending upon the wavelength and the thickness of the index layer. The equation (EQ. 1) which describes the return loss that can be expected is: RL=-10log[2R(1-cos((4πnlayer/wavelength)x))] Where: R=(nlayer-nfiber/nlayer+nfiber) 2 x=index layer thickness To a Master vs. Intramated Figure 1 Reflections at Index Changes Connector to Master EQ. 1 Index Layer Index of Refraction n2 Master Connector Index of Refraction n Connector Under Test Index of Refraction n 2 Intramated Return Loss Measurement When return loss is measured without using a master connector, the two connectors each have an index layer. When measuring to a master, the connector under test has an index layer and the master may be assumed to have none. If we examine the simple case where two connectors that measured 57 dB to a master are mated together, and if we assume that the index of refraction of the index layer for each connector is the same, we find that we have the situation shown in figure 2. In this simple case the same equation (EQ. 1) is used but the index layer thickness is doubled. The resulting return loss for these two connectors that were at 57 dB to a master becomes approximately 51 dB, not 54 dB as would be the case if just adding together 57 dB from each connector. In a more detailed analysis of an intramated connection the situation shown in Figure 3 could occur. The situation shown here complicates things by adding a third index of refraction and two different index layers, each with their own thickness. Figure 2 Reflections at Index Changes Intramated Identical Connectors Figure 3 Reflections at Index Changes Intramated Connectors Index Layer Index of Refraction n2 Intramated Connector A Index of Refraction n Intramated Connector B Index of Refraction n Index Layer Index of Refraction n1 n2 Intramated Connector A Index of Refraction n Intramated Connector B Index of Refraction n Layer 2 Thickness Layer 1 Thickness ADC Telecommunications, Inc., P.O. Box 1101, Minneapolis, Minnesota USA 55440-1101 Specifications published here are current as of the date of publication of this document. Because we are continuously improving our products, ADC reserves the right to change specifications without prior notice. At any time, you may verify product specifications by contacting our headquarters office in Minneapolis. ADC Telecommunications, Inc. views its patent portfolio as an important corporate asset and vigorously enforces its patents. Products or features contained herein may be covered by one or more U.S. or foreign patents. An Equal Opportunity Employer 1238223 7/02 Original © 2002 ADC Telecommunications, Inc. All Rights Reserved Web Site: www.adc.com From North America, Call Toll Free: 1-800-366-3891 • Outside of North America: +1-952-938-8080 Fax: +1-952-917-3237 For a complete listing of ADC’s global sales office locations, please refer to our web site. References Kazunori Kanayama, Yasuhiro Ando, Ryo Nagase and Shin’ichi Iwano, “Evaluation Method and Performance of Advanced Low-Reflection Optical Connectors,” International Wire & Cable Symposium Proceedings 1992 Presentation by William C Young of Telcordia, OFC 1995 . the return loss associated with a mated pair of connectors. Factory Return Loss Measurement to a Master After polishing, insertion loss and return loss. white paper Connector Return Loss Source of Return Loss Fiber optic connectors are produced by polishing their endfaces