Development of Loss-evaluation Tool for Efficient Characterization of Optical Fiber Cables

3.1 Conventional method for evaluating transmission loss

Figure 2 shows the method for evaluating optical fiber transmission loss during conventional maintenance. The evaluator places markers (red lines) on the waveform measured with an OTDR and reads their intensity values to evaluate the change in loss and slope between markers. However, the values often vary according to the evaluator in question, and a certain level of experience and skill is required of the evaluator. Moreover, when a large number of optical fiber cables are measured with multiple wavelengths, the amount of data to be evaluated becomes enormous, and handling such a large amount of data is time-consuming and labor-intensive.

Fig. 2. Conventional evaluation method.

3.2 Overview of optical fiber cable loss-evaluation tool

Figure 3 shows the optical fiber cable loss-evaluation tool developed by NTT EAST Technical Assistance and Support Center (TASC). The tool has the following four key features:

Fig. 3. Features of optical fiber cable loss-evaluation tool.

Each feature is briefly described in the following sub-sections.

3.2.1 Batch reading of waveform data

To check the loss status of an optical fiber cable, testing at multiple wavelengths is effective. Accordingly, our tool was designed on the basis of multiple OTDR measurement data obtained with three or four wavelengths.

When saving the multiple OTDR measurement data as the file name “(4-digit fiber number)_(wavelengths)_(free text).SOR” (SOR: Standard OTDR Record), the tool can read the multiple measurement data at once and sort them by using the first four digits of the file name as a key and recognizing the wavelength information from the second part of the name.

3.2.2 Setting analysis conditions and normal/abnormal judgment

Figure 4 shows an example of the condition-setting screen of the tool. Seven items can be set: (1) threshold, (2) inspection-section width, (3) analysis range, (4) loss-value range, (5) start point of analysis range, (6) display of color map, and (7) output of analysis-result file. Arbitrarily adjusting these seven items makes it possible to evaluate losses in accordance with the purpose.

Fig. 4. Example of condition-setting screen.

3.2.3 Display of color map of loss distribution

Figure 5 shows a color map of the loss distribution obtained by analyzing the OTDR measurement data.

Fig. 5. Display of loss distribution by color map.

In Fig. 5(a), the horizontal axis represents the distance along the optical fiber cable, and the vertical axis represents the wavelength. In certain sections, namely, between splice points A and B and between C and D, transmission loss exceeds the threshold.

The change in the color map against the loss value is shown in Fig. 5(b). When the threshold of the slope of the OTDR waveform is set to 1 dB/km, loss values below 0.4 dB/km are indicated in dark blue to light blue. When the loss values are in 0.4 to 0.8 dB/km, the corresponding sections are indicated in yellow-green to yellow. Sections with loss exceeding 0.8 dB/km are indicated in red, so the loss along the length of the optical fiber cable can be checked at a glance.

3.2.4 Display of loss spectra

Figure 6 shows an example of displaying wavelength dependence of loss (loss spectrum) in a certain section of an optical fiber cable. The horizontal axis shows the wavelength of the test light, and the vertical axis shows loss.

Fig. 6. Example display of loss spectra.

For glass optical fibers, loss per distance is lowest at the wavelength of 1550 nm and increases when measured at shorter or longer wavelengths. As shown in Fig. 6(a), analyzing the OTDR measurement data reveals that loss at 1550 nm is minimum for a normal cable. As shown in Fig. 6(b), however, when the optical fiber is subjected to bending, the loss increases with increasing wavelength. Therefore, it is possible to estimate the state in which loss occurs by checking the status of the loss spectrum.