820 nm Colliding-pulse Modelocked Dye Laser

Abstract

The 620 nm CPM laser has become the most widespread laser for femtosecond pulse generation, with perhaps as many as several hundred units around the world in operation. While it is desirable to extend this technique to other wavelengths, each new wavelength requires a painful and lengthy materials research program, with no guarantee of success. A number of conditions have been investigated [1], but the 620 nm CPM is still the most common laser, and one wonders whether the R6G-DODCI pair is the most "magic pair" in existence. We have developed a new CPM laser which operates in single pulse trains of high stability and find performance comparable to 620 nm CPM lasers. A six mirror flat cavity with gain jet of LD-800 rhodamine dye and absorber of HITCI dye is folded with four SF-10 prisms. We use a krypton laser to pump the LD-800, since it couples better to the LD-800 absorption spectrum than the Ti-Sapphire laser [2] (Figure 1), and the active-feedback which is now available [3] represents a major advance in start-up and long-term stability. In a standard 12 ns cavity length, double pulse trains are observed which cannot be eliminated. In a 7 ns short cavity this instability vanishes but we find a different instability which results from the long lifetime of the HITCI (1.2 ns). We eliminated this strong instability by reducing the gain mirror radii, thus increasing the gain saturation. The strong gain saturation and lower S-parameter results in a complex behavior with respect to intracavity dispersion. Minimum pulsewidths are about 100 fs (Figure 2), with transform-limited pulses obtained in the positive-GVD limit. A strong variation in the mean wavelength and output power is obtained (Figure 3). Interestingly, at low absorber concentrations, operation at positive GVD is not obtained.