Ultrafast lasers have many applications mainly due to its two properties, the ultrashort pulse width and the ultrahigh intensity. Because the former is the main cause of the latter, it is very important to exactly measure the pulse width of the ultrafast laser. Currently, there are several different kinds of experimental methods to measure the ultrashort pulse width. Among those systems for this measurement, the autocorrelator using the second harmonic generation (SHG) is by far the most simple and basic method. This type of autocorrelators usually uses inorganic crystals, such as BBO, as the SHG medium. The thinner medium is necessary for analyzing the shorter laser pulses. However, the polishing process which is necessary for obtaining the optically good surfaces makes it difficult to reduce the thickness of medium as desired. We present an autocorrelator system which overcomes these shortcomings. Our system is based on the SHG using organic polymer. Polymers can be easily prepared in the form of thin film on the strong substrate through the process of spin casting. Thickness less than 1 m can be obtained without difficulties. Furthermore, due to its high nonlinearity, thin film of polymer can produce the bright second harmonic light. Polyurea was used as the second harmonic generation material of the autocorrelator because it has the pretty good transparency. An autocorrelator system based on the 397nm-thick poled polyurea thin film has been developed and used to measure the pulse width of a home-made Ti:sapphire laser oscillator. Then, the system was compared with that based on a 100 μm-thick BBO crystal, which is widely used. The pulse width of laser beam was measured to be 9.8 fs with the former. The value is believed to be more accurate than that of 7.2 fs measured with the latter.
We have demonstrated bandwidth control and reshaping of second harmonic (SH) curve in a periodically poled Ti:LiNbO<sub>3</sub>
(Ti:PPLN) waveguide ( period=16.6 um) by using a temperature-gradient-control technique and a local-temperature-control technique.
We have achieved more than 13 nm second harmonic phase-matching bandwidth and several useful shapes of SH curve such as almost ideal sinc function, and double peaks in a 74 mm long Ti:PPLN waveguide that has pre-chirped SH curve in room temperature.
We have demonstrated all-optical wavelength-selective single- and dual-channel dropping and wavelength conversion in a periodically poled Ti:LiNbO<sub>3</sub> waveguide which has two second-harmonic phase-matching peaks by cascaded sum and difference frequency generation (cSFG/DFG). Less than -17 dB of channel dropping extinction ratio was observed with coupled pump power of 325 mW and the wavelength conversion efficiency was measured to be -7 dB with coupled pump power of 233 mW.