Laser pulse compression has been performed. A single mode optical fiber and a grating pair were used for frequency chirping and pulse compression. The output pulse of the CW mode-locked Nd:YLF laser was launched into the single mode optical fiber 500 m long to generate a self-phase modulated pulse and then the chirped pulse was compressed again by a diffraction grating pair. A theoretical analysis of the self-phase modulation inside the optical fiber and compression by using a grating pair were studied in this research.
The theoretical value of a compressed pulse width at optimum condition was calculated to be 0.8 psec. To measure the pulse widths of the frequecy chirped and compressed pulses, an optical autocorrelator consisted of a KDP crystal, a rotating glass block, and prism reflectors. The KDP was used to generate the SHG by noncollinear phase matching to increase S/N ratio.
The bandwidth of input laser pulse is $\sim 8 \mbox{\AA}$, and after passing the fiber of 500 m long the bandwidths became $\sim 30 \mbox{\AA}$. The pulse width of the compressed pulse by a grating pair was reduced by a factor of 32(1.4 psec), when the Nd:YLF laser output pulse was 42 psec at 320 W. These measured values are comparable to the theoretical values. As a result, the compressed short pulse is accompanied with a low intensity, long duration pulse wings and satellite pulse. In recent year, active studies have been made in generating ultrashort pulses by coupling the nonlinear external cavity to the main cavity in solid state lasers with large gain bandwidth. Since this technology can be described by coherent addition of the feedback pulses from the external cavity to the main cavity, it is called additive-pulse mode-locking(APM). Since the process of APM is purely passive, the APM technique has many merits such as its simplicity in the apparatus and its capability of generating nearly transform limited pulses, whose pulse widths are much shorter than those obtained by conventional active mode-locking. In this study, APM was performed with a $1.053 \mu m$ CW Nd:YLF laser by coupling it to an external cavity with a single mode optical fiber as a nonlinear medium. The APM pulse width was 4.5 psec and the average output power was 1.5 W. Since the output power of the APM is sufficiently large and the energy wings of the pulse is small, a much shorter pulse can be obtained by pulse copression. This APM pulse was compressed using a fiber-grating pair for various grating separations. For optimum grating separation, we obtained a pulse as short as 210 fsec. Additive-pulse mode locking (APM) of a Nd:YLF laser was studied theoretically and experimentally. The Nd:YLF laser cavity was coupled with an external cavity containing an optical fiber. The length of the external cavity was adjusted to be twice the length of the main cavity. The 100 MHz APM mode locked train produced an autocorrelation trace of 4.9 psec pulsewidth by assuming ${sech}^2$ pulse shape.
