Additive-pulse mode locking (APM) of a Nd:YLF laser was studied theoretically and experimentally. The Nd:YLF laser cavity was coupled with and 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.
A modified formula for the threshold condition of APM was obtained including the effect of dynamic gain saturation. This modified model was compared with the experiment. The threshold power of the laser was measured varying the nonlinearity of the external cavity by controlling the coupling of the cavities with the quarter waveplate in the external cavity. It is shown in this experiment that the effect of the dynamic gain saturation plays a resistive role in the self-starting APM. The first beat note of the power spectrum of the free-running laser was observed. The FWHM of the beat note was measured varying the intracavity power. It is observed that the FWHM has weak power dependency in the arc-lamp pumped laser.
The output pulses of APM mode locked laser is nearly transform-limited and the linewidth is broad comparing that of the conventional mode locked laser, being expected as a suitable seed pulse for high power laser systems. It is also possible for the APM pulses to be converted to its second harmonics or higher harmonics very efficiently for the purpose of the pump source for the dye laser in the ultrafast spectroscopy and for the high resolution lithography in the semiconductor process.
High power laser system consisting of Nd:YLF master oscillator-Nd:phosphate glass laser amplifier is being developed in this laboratory. Single pulse was selected from the cw mode locked Q-switched Nd:YLF laser output using a Pockels cell, a synchronization unit, and a delay generator. The energy of the selected pulse was 270μJ and its energy stability was less than 5%. The extraction efficiency by the Pockels cell was better than 90%.
For maximum pump efficiency and the most uniform pumping optimum designing of the quadruple elliptical reflectors for the first two stage rod amplifires, RA1 and RA2, was performed using the ray tracing method. The lengths of the major axes are 61 mm and 60 mm, and their ellipticities are 0.623 and 0.667 for RA1 and RA2, respectively. The pulse forming network driving the flashlamp was designed and constructed, and the FWHM of the flashlamp radiation was 250μsec. Using these reflectors and pulse forming network the performance of the high power laser system was investigated. The energy of the laser amplifier had maximum value when the delay time between the flashlamp triggering signal and the single pulse triggering signal was 480μsec. The energy gains of RA1 and RA2 were 55 and 12 at the capacitor voltage of 5 kV, respectively.
The spatial intensity distribution of the laser beam was measured using a CCD camera and a frame grabber. The beam quality was improved by the spatial filters.