## 1 Introduction
Furthermore, the additional intracavity losses will limit the efficiency of the system, while the high pulse intensity may cause optical damage on the tuning elements. Techniques such as self-seeding, self-injection as well as prelasing of pulsed lasers have been attempted to achieve these goals with reduced losses. In self-seeded lasers [25; 26; 27] either a portion of the laser is re-injected into the cavity by an external reflector or a coupled dual-cavity configuration with either a single or multiple gain media is used. One cavity features strong frequency selection, but low gain and power to generate a narrow linewidth seed. This seed couples via a partially reflective surface or via electro-optic switching [28] to a lower loss and less selective cavity. In this way, portions of the same cavity or even the same gain medium can be used both for generation of the seed as well as for amplification. With a grazing incidence grating configuration single longitudinal mode operation has been achieved [29] even enabling wavelength scanning and control using a scheme based on monitoring angular deviations of the laser output [30]. Drawbacks of these approaches are the typically low efficiency and high lasing threshold of grazing incidence cavities as well as a fairly complex and slow feedback scheme using either CCD sensors or segmented photodiodes, resulting in a relatively large pulse-to-pulse frequency jitter of 40 MHz [31]. Nevertheless, this is a promising technique for moderate-resolution spectroscopy. In a similar manner, prelasing [32; 33; 34] uses a scheme where the cavity is at first kept at low round-trip gain, until the linewidth has sufficiently decreased, at which point the pump intensity is increased or the loss is reduced to generate a strong output pulse. The method and system described in this paper is most comparable with pre-lasing, however as the pre-lasing here is performed without interruptions, a continuous error signal for wavelength stabilization and control can be generated and mode-hop free frequency tuning is made feasible. In addition, our system combines CW semiconductor diode laser pumping with a pulsed high power diode pumped solid state (DPSS) pump, taking advantage of recent advances in high power blue diode laser manufacturing and easy beam combination via dichroic mirrors. There has been rapid progress in the development of diode pumped Tisapphire lasers with availability of low-priced high power diodes. First commercial Ti:sapphire systems based on this technology have been introduced. Much of the recent research focus has been on the development of diode-pumped mode-locked oscillators [35; 36; 37; 38], with a recent review [39] summarizing many of the developments. Though some work has been done in characterizing CW operation [40; 41; 42], we are only aware of our groups own system featuring single-frequency CW operation [43; 44] of a diode-pumped Ti:sapphire laser. The beam quality of high power visible diode lasers in the blue-green wavelength region is fairly poor. However, a tight focusing geometry combined with a short, highly-doped Ti:sapphire crystal can reach sufficient gain to enable continuous wave Ti:sapphire laser operation with several intracavity elements as we have demonstrated in our previous prototype. Though there are some changes in the geometry of the laser cavity presented here, the general operating principle is similar. Compared to the four pump diodes (two blue, two green) used in the earlier prototype, only two blue diodes were used here, limiting the wavelength tuning range to about 730-855 nm.