C. Hönninger

Semiconductor saturable absorber mirrors (SESAM's) for femtosecond to nanosecond pulse generation in solid-state lasers

Intracavity semiconductor saturable absorber mirrors (SESAMs) offer unique and exciting possibilities for passively pulsed solid-state laser systems, extending from Q-switched pulses in the nanosecond and picosecond regime to mode-locked pulses from 10s of picoseconds to sub-10 fs. This paper reviews the design requirements of SESAMs for stable pulse generation in both the mode-locked and Q-switched regime. The combination of device structure and material parameters for SESAMs provide sufficient design freedom to choose key parameters such as recovery time, saturation intensity, and saturation fluence, in a compact structure with low insertion loss. We have been able to demonstrate, for example, passive modelocking (with no Q-switching) using an intracavity saturable absorber in solid-state lasers with long upper state lifetimes (e.g., 1-/spl mu/m neodymium transitions), Kerr lens modelocking assisted with pulsewidths as short as 6.5 fs from a Ti:sapphire laser-the shortest pulses ever produced directly out of a laser without any external pulse compression, and passive Q-switching with pulses as short as 56 ps-the shortest pulses ever produced directly from a Q-switched solid-state laser. Diode-pumping of such lasers is leading to practical, real-world ultrafast sources, and we will review results on diode-pumped Cr:LiSAF, Nd:glass, Yb:YAG, Nd:YAG, Nd:YLF, Nd:LSB, and Nd:YVO/sub 4/.

Q-switching stability limits of continuous-wave passive mode locking

The use of a saturable absorber as a passive mode locker in a solid-state laser can introduce a tendency for Q-switched mode-locked operation. We have investigated the transition between the regimes of cw mode locking and Q-switched mode locking. Experimental data from Nd:YLF lasers in the picosecond domain and soliton mode-locked Nd:glass lasers in the femtosecond domain, both passively mode locked with semiconductor saturable absorber mirrors, were compared with predictions from an analytical model. The observed stability limits for the picosecond lasers agree well with a previously described model, while for soliton mode-locked femtosecond lasers we have developed an extended theory that takes into account nonlinear soliton-shaping effects and gain filtering.

60-W average power in 810-fs pulses from a thin-disk Yb:YAG laser.

We demonstrate a passively mode-locked diode-pumped thin-disk Yb:YAG laser generating 810-fs pulses at 1030 nm with as much as 60 W of average output power (without using an amplifier). At a pulse repetition rate of 34.3 MHz, the pulse energy is 1.75 microJ and the peak power is as high as 1.9 MW. The beam quality is close to the diffraction limit, with M2 < 1.1.

Femtosecond Yb:YAG laser using semiconductor saturable absorbers

We demonstrate a passively mode-locked femtosecond Yb:YAG laser using different semiconductor saturable absorber devices, a high-finesse and a low-finesse antiresonant Fabry–Perot saturable absorber. We achieved pulses as short as 540 fs with dispersion compensation and 1.7-ps pulses without dispersion compensation. We also mode locked the laser at either 1.03 or 1.05 μm by adjusting the band gap and antiresonance wavelength design of the antiresonant Fabry–Perot saturable absorber.

Directly diode-pumped millijoule subpicosecond Yb:glass regenerative amplifier

An Yb:glass regenerative amplifier directly side pumped by four 20-W diodes is demonstrated. By use of a novel pumping scheme and introduction of cylindrical optics into the cavity, a free-running average output power as great as 4 W with a TEM(00) -like mode was achieved from the bare cavity, with a 0.56 pump duty cycle. When the regenerative amplifier injected, 1-mJ 200-fs FWHM pulses were obtained following compression by use of 2-ms pump pulses and up to a 150-Hz repetition rate.

HIGH-POWER DIODE-PUMPED PASSIVELY MODE-LOCKED YB:YAG LASERS

We obtained 74-kW peak power and 3.5-W average output power in 1-ps pulses from a diode-pumped Yb:YAG laser at 1030 nm that was passively mode locked with a semiconductor saturable-absorber mirror. Another laser produced 57-kW peak power and as much as 8.1-W average output power in 2.2-ps pulses, split into two nearly diffraction-limited beams (M(2)<1.2) . To our knowledge, these are by far the highest reported peak and average output powers from a diode-pumped mode-locked laser in this pulse-duration regime.

Efficient intracavity frequency doubling of a passively mode-locked diode-pumped neodymium lanthanum scandium borate laser

We passively mode locked a diode-pumped neodymium lanthanum scandium borate laser with an antiresonant Fabry-Perot saturable absorber and achieved 2.8-ps pulses at an average output power of 400 mW and a pump power of 1.2 W. With Ti:sapphire laser pumping we obtained pulses as short as 1.6 ps. Intracavity doubling produced 190 mW of 531-nm light for a diode-pump power of 1.2 W, resulting in a conversion efficiency of 48% with respect to the fundamental and 16% with respect to the diode-pump power. Noise characterization of the laser demonstrates a trade-off between high conversion efficiency and low intensity noise.

60-W average power in 810-fs pulses from a thin-disk Yb:YAG laser

We demonstrate a passively mode-locked diode-pumped thin-disk Yb:YAG laser generating 810-fs pulses at 1030 nm with as much as 60 W of average output power (without using an amplifier). At a pulse repetition rate of 34.3 MHz, the pulse energy is 1.75 µJ and the peak power is as high as 1.9 MW. The beam quality is close to the diffraction limit, with M 2 < 1.1.

Efficient high-power diode-pumped passively modelocked Nd:YLF laser

We have passively modelocked a diode-pumped Nd:YLF laser at 1.047 µm using semiconductor saturable absorber mirrors and achieved 1.3 W of average output power at 3.3 W pump power, which results in an optical efficiency of 39%. The pulse repetition rate was 50 MHz and pulse widths as short as 3.8 ps were obtained. We cavity-dumped single pulses from the circulating cw modelocked pulse train using a BBO Pockels cell and achieved 270 nJ pulses. The repetition rate was limited to below 2 kHz due to the Pockels cell driver electronics.

Diode-pumped high-power soliton modelocked Yb:YAG laser

We present a soliton modelocked Yb:YAG laser, which produces 1 ps pulses at 45 MHz repetition rate and 1.5 W average output power at a center wavelength of 1030 nm. The Yb:YAG crystal is pumped with a 40-W, 1-cm, high-power, low-brightness diode bar at 940 nm. Passive modelocking is initiated by a semiconductor saturable absorber mirror. We suppressed Q-switching instabilities by choosing the cavity and absorber parameters according to a previously developed theory.