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Connector Loss & Reflection Coefficient Variations With Wavelength

There is a widespread misconception that a connector can be completely characterized by measuring its insertion loss and reflection properties at a single wavelength. This paper presents experimental evidence of large connector loss variations with wavelength and a theorectical model of this phenomena. Connectors which have an air gap separating two polished fiber end-faces (such as FC connectors) exhibit loss oscillations as high as 1.1 dB (peak-to-peak) as the wavelength is varied from 1300 nm to 1550 nm. In addition, the optical power reflected from such a connector also fluctuates considerably (from less than -30 dB to as high as -7 dB) over the same wavelength range. In a fiber span terminated with FC connectors at both ends it is possible, therefore, for the total connector loss and reflected power to increase by more than 2 dB and 20 dB, respectively, when operated at a wavelength of 1550 nm rather than 1300 nm. This would be of concern in high speed lightwave systems which have low system gains and whose lasers are very sensitive to reflected light. This effect can also lead to large errors in spectral attenuation measurements (both in the lab and in the field) unless the proper precautions are taken. We observed this phenomenon while constructing a spectral attenuation test set composed of multiple lasers in the 1300 nm and 1550 nm windows for a "window verification" project.