High-repetition-rate, high-average-power, mode-locked Ti:sapphire laser with an intracavity continuous-wave amplification scheme (original) (raw)

1999, Applied Physics Letters

We have demonstrated a high-average-power, mode-locked Ti:sapphire laser with an intracavity continuous-wave amplification scheme. The laser generated 150 fs pulses with 3.4 W average power at a repetition rate of 79 MHz. This simple amplification scheme can be applied for the power scaling of other lasers. Some years ago, there was a breakthrough in modelocking techniques for solid-state lasers. 1,2 Applying the techniques utilizing Kerr-type nonlinearity, most solid-state lasers could be mode-locked down to the femtosecond region. Shortly after that, amplification of these ultrashort pulses to gigawatt peak power was demonstrated 3 using chirped pulse amplification. 4 However, this kind of amplification reduces the pulse repetition rate to the order of ϳ100 kHz, and there is often a loss of time resolution in the final pulse. A higher repetition rate results in much smaller pulse fluctuation and excellent experimental signal-to-noise ratios. Much progress has been made in extending the spectral range of high-repetition-rate femtosecond pulses throughout the ultraviolet, visible, and infrared ͑IR͒ regions by using frequency conversion in crystals. The ϳ80 MHz, 2-W-level femtosecond Ti:sapphire lasers have been used for fourth harmonic generation near 200 nm, 5 visible range, 6-8 and IR range 9 optical parametric oscillator and coherent THzradiation from semiconductors. 10 If there is some scheme to scale the power of high-repetition-rate femtosecond lasers, there will be interesting applications for the various wavelength conversion techniques mentioned above. The requirement for the average power has been one of the most important factors especially for these lasers used as pump sources of THz-radiation from InAs in a magnetic field, 10 because THz-radiation power is known to have a quadratic dependence on the excitation high-repetition-rate femtosecond laser average power. Also, the average power is important for the intracavity doubling of a femtosecond laser. 11 In contrast to continuous wave ͑cw͒ high-average-power lasers, the power scaling of the high-repetition-rate femtosecond modelocked lasers is difficult. This is partly due to the limitation of available power from the pumping source. The major problem was the difficulty of maintaining the beam quality good enough for mode locking and balancing the thermal lens effect and Kerr lens effect at the same time. With these limitations, the output average power of femtosecond modelocked lasers has been limited to the 2 W level. In this letter, we describe a high-repetition-rate ͑79 MHz͒, highaverage-power ͑3.4 W͒, mode-locked femtosecond Ti:sapphire laser realized by applying an intracavity cwamplification scheme.