# Characterization of Chromatic Dispersion and
## 4 Measuring Technique and Results  
As an example of how the measurements were carried out, Figure 2a shows a Streak image obtained from the acquisition system. It corresponds to an excitation pulse centered at 520 nm that was launched into a 50 m Toray fiber. The spectral features of the analyzed image along the horizontal axis are limited by the monochromator, which includes three interchangeable gratings with grooves of different sizes, whereas the temporal features (vertical axis) are provided by the Streak Camera located after the monochromator. As shown in Figure 2a, the observed spectral span is 35 nm, which is given  
Figure 1: Experimental set-up employed to carry out the measurements. Legend: AT: variable attenuator, PD: photodetector, DU: delay unit, PP: pulse picker, SHG: second-harmonic generator, _xy_-POS: _xy_-micprositioner, POF: polymer optical fiber, M: monochromator, SC: Streak Camera. by the characteristics of the grating. In all of our measurements, a grating with 300 grooves/mm was employed. As for the temporal span, this was 5 ns, which is wide enough to capture the whole signal even if the longest fiber length is employed, namely, the 75 m fiber. Figure 2b,c represent, respectively, the time-integrated and the wavelength-integrated images of Figure 2a. The spectral shape of the output pulse is almost symmetrical (a Gaussian-type curve), whereas its temporal shape is not. This asymmetry, which worsens when the fiber length is increased, is a consequence of the behavior of the modal dispersion, because the arrival of the fastest modes (the lowest-order ones) causes a rapid increase in the output intensity, but the intensity decreases gradually as the slowest modes (the highest-order ones) reach the output fiber end. This effect can constitute a serious issue when high-bit-rate communications links are needed. Since one of our goals is to characterize the chromatic dispersion of POFs, it is enough to accurately determine the instants of maximum pulse intensity, without having to directly measure the temporal and spectral widths of the pulses. In this respect, the wavelength-integrated image obtained from Figure 2a can be employed to accurately calculate the instant of maximum intensity at the fiber output (see Figure 2c). Specifically, this value turns out to be 0.85 \(\pm\) 0.01 ns in the figure. The process was repeated for different excitation wavelengths, i.e., the same fiber sample was excited from 400 to 520 nm, with steps of 15 nm. The relative time delays were calculated in two steps: first, the instant of the maximum was localized in the frame of the delay window employed to visualize the pulse, and then the relative delay of that frame window with respect to the previous one was taken into account, by means of the delay unit of the Streak Camera (see Figure 1). Due to the high degree of automatization in the excitation and in the measuring systems, the whole set of measurements was performed in just a few minutes. Once the relative time delay between different pulses was obtained, it was necessary to determine the absolute time delay when they propagated through the optical fiber  
Figure 2: (**a**): Pseudo-3D Streak image obtained when a 50 m Toray fiber is excited at 520 nm with 9 nW of average power; (**b,c**): Respectively, spectral (time-integrated) and temporal (wavelength-integrated) images. In order to do so, a 50 cm fiber was used as a reference to measure the time taken by the 520 nm pulse to reach the fiber output. By comparing the time delays in our reference fiber and in the sample by means of the delay unit of the camera, we determined the absolute arrival time of a pulse at the considered wavelength of 520 nm. Specifically, that value was 254.4 ns in the 50 m fiber. The delay of the rest of the pulses at other wavelengths were transformed into absolute values by taking into account the measured relative delays with respect to the pulse of 520 nm.