# Characterization of Chromatic Dispersion and
## 3 Experimental Set-Up  
The various fibers analyzed in this work are commercially-available polymer optical fibers developed by Toray Industries Inc. and by Mitsubishi International PolymerTrade Corporation [21; 22]. From each manufacturer, three different fiber lengths were prepared, namely, 25, 50, and 75 m. The fiber end-faces were carefully polished by hand using polishing papers. All the fibers employed are SI ones with high-purity PMMA cores and fluorinated-polymer claddings. Table 1 summarizes the main mechanical and optical characteristics of the fiber samples employed in this work. Figure 1 sketches the experimental set-up employed to carry out our measurements. It includes a tunable ultrafast femtosecond laser (Spectra Physics Mai-Tai HP, Newport, Santa Clara, CA, USA) with a repetition rate of 80 MHz and a spectral emission range between 690 and 1040 nm. The selection of the desired emission wavelength is easily carried out by means of the specific software provided by the manufacturer. The laser emits pulsed beams of around 100 fs, 1.2 mm in diameter, and with a Gaussian-spatial distribution. The spectral width of the pulses is around 15 nm. For our measurements, the repetition rate of this laser was reduced to 8 MHz by using a pulse picker (Spectra Physics 3986, Newport, Santa Clara, CA, USA), which is based on an acoustic-optic modulator. In order to obtain excitation in the visible range from 400 to 520 nm, a second-harmonic generator (Inspire Blue, Radiantis, Barcelona, Spain) was employed. The impinging power is controlled by manually adjusting the variable attenuator placed before the POF sample. For all the wavelengths considered, the fibers were excited with an average light power of 9 nW, which is well below the limit that would produce damage in the fiber and nonlinear effects. The measured fiber samples were held by an _xy_-micropositioner in order to focus the incident laser beam just on the fiber-end face. The transmitted pulses were acquired by means of a combination of a monochromator (Acton SP2300, Princeton instruments, Trenton, NJ, USA) \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Toray raytela** & **Mitsubishi eska** \\ \cline{2-3}  & **PGU-CD1001-22-E** & **GH4001** \\ \hline Core material & PMMA & PMMA \\ Cladding material & Fluorinated polymer & Fluorinated polymer \\ Refractive-index profile & SI & SI \\ Core refractive index & 1.49 & 1.49 \\ Numerical aperture & 0.5 & 0.5 \\ Fiber diameter (mm) & 1 & 1 \\ Jacket diameter (mm) & 2.2 & 2.2 \\ Attenuation at 650 nm (dB/m) & \(\leq\)0.15 & \(\leq\)0.19 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Main mechanical and optical characteristics of the fiber samples. PMMA = poly(methyl methacrylate); SI = step-index. and an ultrafast Streak Camera (C10627, Hamamatsu, Hamamatsu City, Japan). This combination of both devices gives the option to resolve the acquired signals both spectrally and temporally, which means that the whole spectrum is obtained at the same instant of time. The maximum possible acquisition-wavelength range extends from 0 to 1400 nm. In order to properly synchronize both the excitation and the acquisition systems, a small amount of the laser signal is diverted from the laser output and directed to a photodetector, whose electric signal is delayed by means of a delay unit. A precise use of this delay unit guarantees the visualization of the transmitted pulses on the corresponding temporal window of the Streak Camera. The acquisition system is controlled by a 2.66 GHz Intel personal computer (PC) with 2 GB of random access memory (RAM). Both parts of the acquisition system and the measurements are controlled by the Hamamatsu HPDTA software (High-Performance Digital Temporal Analyzer, Hamamatsu, Hamamatsu City, Japan), which allows real-time data acquisition. Once the acquisition was performed, the data analysis was carried out by employing an ad-hoc Matlab program developed for this purpose.