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October, 2004 EEEL Researchers Develop New Technique for High-Resolution Characterization of Optical Modulator Chirp |
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Chirp is a parasitic phase modulation that can occur when an optical carrier is intensity modulated. When combined with the chromatic dispersion of an optical fiber it can cause signal distortion in transmission systems. However, chirp can also be used advantageously to pre-distort a modulated signal to compensate for chromatic dispersion. Whether modulator chirp needs to be minimized or used advantageously, its characterization is important. However, chirp characterization in optical modulators is difficult to achieve, since it involves indirect measurements and high modulation frequencies. Tasshi Dennis of the EEEL Optoelectronics Division has demonstrated a new technique for high-resolution measurement of optical modulator chirp. This technique is based on measuring the distortion in measured group delay when a chirped signal with a wavelength near a gas absorption line is transmitted through a cell containing the gas. This technique can measure the chirp parameter with a resolution that surpasses the resolution of conventional techniques. Additionally, this new method allows the measurement to be made without the conventional requirement of high electrical bandwidths, or that the signal propagate in long (~ 100 km) lengths of optical fiber that must be accurately characterized for chromatic dispersion. This new technique also allows the measured chirp to be resolved as a function of modulation frequency.
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