High-energy spectrally combined diode-pumped long-cavity Ti:sapphire laser and its relative intensity noise characterization

Abstract

We demonstrate a high-energy, spectrally combined diode-pumped long-cavity Ti:sapphire femtosecond laser designed for high-energy pulse generation and stable ultrafast operation. To overcome the pulse energy limitations of standard high-repetition-rate oscillators, a Herriott-type multipass cell was employed to extend the cavity length, achieving a reduced repetition rate of 25.5 MHz. The laser is pumped by three spectrally combined visible laser diodes operating at 465, 490, and 520 nm, providing a total pump power of 8 W. This multi-wavelength pumping configuration was carefully optimized to match the absorption profile of the Ti:sapphire gain medium while minimizing residual absorption losses at shorter wavelengths. The laser delivers 134-fs, 12-nJ pulses at a total pump power of 8 W, corresponding to an optical-to-optical efficiency of 3.8%. Stable mode-locking is maintained using a semiconductor saturable absorber mirror with a 0.8% modulation depth, resulting in a low long-term power fluctuation of 0.65% root mean square over 48 hours. Furthermore, a quantitative relative intensity noise analysis was performed to correlate the noise characteristics of individual pump diodes with the mode-locked output. The measured integrated relative intensity noise of 0.0537% suggests that the intrinsic gain-filtering effect of the Ti:sapphire medium effectively suppresses high-frequency pump-induced fluctuations. These results provide an efficient and scalable framework for developing high-energy diode-pumped ultrafast light sources.