OTDR Aided Assessment of Fiber-Induced WDM Spectrum Unflatness
The imperatively excellent performance of wavelength-division multiplexing (WDM) transmission over a fiber optic link, demands flat, i.e. not wavelength-selective transfer function. This implies that, mostly during installation and commissioning of a WDM-aimed fiber link, it is of interest to measure its frequency response, by complex stimulus-response tests using a tunable laser source coupled with an optical spectrum analyzer. On the contrary, a simple and practically costless alternative that we propose here, is testing in time domain by means of the ubiquitous optical time-domain reflectometer (OTDR), considering its distinctly reflective trace pattern as approximation of the fiber channel two-way power-delay profile, whose rms delay spread is the straightforward indicator of the fiber attenuation vs. wavelength characteristics’ unflatness qualifying the fiber as either appropriate for WDM transport, or requiring piecewise “flattening” of the transfer function by applying coherent optical orthogonal frequency-division multiplexing (CO-OFDM). Specifically, the proposed OTDR – aided WDM suitability fiber test model applied on the exemplar traces, showed significant peak-to-peak DWDM spectrum unflatness, i.e. the pronounced frequency selectivity strongly indicating the need for introducing CO-OFDM on top of WDM. This was found to be monotonically tracked by the WDM transmission performance – the bit-error rate (BER) values in particular, measured with and without the CO-OFDM applied.