An Accurate Method of Determining Single-Mode Fiber Cutoff Wavelength.

In this memorandum an accurate method of determining the effective cutoff wavelength of single-mode optical fibers is presented. The standard EIA procedure (RS-455-80) uses linear interpolation to estimate the effective cutoff wavelength of a fiber based on measured data values which are typically spaced 10 - 20 nm apart. This estimated cutoff wavelength tends to be higher than the actual fiber cutoff wavelength because linear interpolation does not adequately fit the actual loss curve. The new method which involves fitting an empirical model to the measured data determines the cutoff wavelength more accurately. The model is achieved by deriving and linearizing the loop loss equations with some assumptions. With two or more measured data points, linear regression analysis is performed on the simplified linear equation and the cutoff wavelength is determined from the result of the linear regression analysis. To determine the "traditional" cutoff wavelength (the wavelength at which the loop loss is 0.1 dB), two data points, one above and below the 0.1 dB, are used to fit the model. For a cutoff wavelength which is more representative of system applications, additional data points below the cutoff wavelength are included in the fit. The model works well on AT&T Depressed Cladding fibers and on Matched Cladding fibers. This new method will probably save some of the fibers which are rejected for having high cutoff wavelength because of over estimation by the linear interpolation method. Since fewer data points are needed to achieve a better resolution, measurement time is also saved resulting in corresponding savings in cost.