## 4 Discussion and Conclusions  
A self-seeded hybrid CW and pulsed Ti:sapphire laser system has been developed. Compared to other pulsed self-seeded or pre-lasing approaches the present system allows precise and mode-hop free control of the laser frequency using only minor modifications to standard techniques typically used in many CW laser systems. The pulsed gain modulation of the cavity caused significant noise in the error signal for regulation, however for a PRR below 10 kHz this was found to be not prohibitive for stable operation. Use of an electrooptic etalon [45] to achieve higher dither frequencies may help in improving the SN ratios to allow operation at higher PRR. Analysis of the spectral linewidth and monitoring via a commercial wavemeter provides evidence that the system should be suitable for high resolution spectroscopy, such as required for studies of hyperfine structure and isotope shifts. The linewidth was found to be about 11 MHz, which is comparable to the natural linewidth of many optical dipole transitions in atoms and molecules and is only slightly larger than the Fourier transform limit expected for the measured pulse duration of 73(5) ns. The present laser cavity exhibited fairly high intracavity losses and has a small output-coupling ratio of 2%, which limited efficient energy extraction. High intracavity-losses are typical for diode pumping of Ti:Sapphire lasers, as the high crystal doping concentrations required are correlated with a poor FOM value [46, 47] indicating high self-absorption at the lasing wavelength. If a narrower wavelength tuning range is sufficient a higher output coupling ratio can be used, which should significantly improve efficiency. Furthermore, no efforts to optimize the mode-matching of the pulsed pump laser to the cavity mode were made, so additional performance gains are anticipated here. Specifically, the Nd:YAG mode diameter in front of the pump lens was about 4 mm, resulting in a Rayleigh range of only about 0.15 mm based on calculations, which is much shorter than the crystal length. Due to the fairly small Ti:sapphire mode diameter in the laser crystal, scaling to much higher pulse energies is not feasible with the present design because of gain saturation and optical damage thresholds. This could be alleviated using a two-crystal cavity, where one highly doped crystal is used for CW pumping and another weakly-doped, high FOM crystal is placed at a different cavity location for pulsed pumping. This also would effectively eliminate concerns of damaging the CW diodes with the pulsed pump laser as their optical paths would be separated. In this case nothing impedes the use of additional green CW diode pumping anymore, increasing the maximum CW gain. Thus, the cavity could be optimized for either wider tuning range or increased extraction efficiency. Alternatively, steep-edge dichroic filters may be used to separate the green diode wavelengths (520-525 nm) from the Nd:YAG wavelength (532 nm). Figure 9: Slow frequency scan of the Ti:sapphire laser showing mode-hop free operation in CW and pulsed mode. A different option to achieve higher performance would the inclusion of intracavity second harmonic generation [48; 49; 50]. This would allow the use of a HR mirror for the fundamental wavelength instead of the OC, resulting in a lower CW lasing threshold, but at the same time enabling higher nonlinear output coupling during the pulse peak via the SHG process. Based on the output peak power of 0.2 kW the circulating intracavity peak power should approach up to 10 kW, potentially yielding very high conversion efficiency. A secondary focus in the cavity for the SHG crystal may be advantageous in this case. In conclusion, our system provides narrow linewidth at medium power levels, sufficient for many applications. The system balances between the complexity and cost of typical injection-locked Ti:sapphire laser systems and pure pulsed systems, which usually exhibit poorer spectral purity and linewidth.