Ultrashort Pulse Lasers and Amplifiers Based on Nd:YVO4 and Yb:YAG Bulk Crystals

Abstract

Abstract. This Chapter focuses on the generation of ultrashort pulses and their amplification to high energies in Nd:YVO4 and Yb:YAG bulk crystals. Particularly with regard to applications such as high-precision micromachining, we briefly describe why these materials can be a promising alternative for current systems mostly based on Ti:sapphire, and how the actual setup of a laser would benefit from the possible simplifications. Following this, recent progress in the development of highpower diode-pumped picosecond Nd:YVO4 and femtosecond Yb:YAG oscillators is briefly outlined. Mode locking of the lasers presented here is achieved with either semiconductor saturable absorber mirrors (SESAMs) or by employing phase selfadjusted mode locking (PSM). The setup and the characterization of a Nd:YVO4 regenerative amplifier is subsequently explained in detail and experimental results are presented. The output of the amplifier is also frequency converted to the visible spectral range by second-harmonic generation (SHG) in LBO and to the UV by fourth-harmonic generation (FHG) in BBO. For higher average output powers in the IR and to demonstrate the potential of a further power scaling, an additional single-pass amplifier is applied to the system. The last few years have seen a rapid development in the field of high-power ultrashort lasers and amplifiers, both with regard to systems based on the bulk material, as well as advanced concepts such as fiber lasers and the thindisk scheme. Much progress has been made to achieve the output powers and the pulse energies needed for numerous high-precision applications, as outlined in a later part of the book. Particularly with regard to the simplicity of the laser systems and their suitability for applications in an industrial environment, promising results with systems based on bulk crystals have been obtained over the last couple of years, and this Chapter will give an overview on these recent developments.The vast majority of industrial lasers for micromachining today is still based on bulk laser materials, mostly Ti:sapphire, which has become the standard material for femtosecond pulse generation and amplification. For this reason, some general remarks on the properties of this laser material and its application for the generation and amplification of ultrashort laser pulses will be given here. Compared to other classical active materials supporting femtosecond pulse generation, such as laser dyes, Ti:sapphire exhibits far superior thermal and laser properties. It offers an upper-state lifetime of

Passive mode-locking of a diode-pumped Nd:YVO_4 laser by intracavity SHG in PPKTP

“…Moreover, this technique could be interesting also for Yb-lasers, which emerge as an alternative to Nd-lasers in the 1-µm spectral range, with a huge potential for power scaling due to the smaller quantum defect, especially in combination with the thin disk concept. Although such systems, based on Yb:YAG, have already produced tens of Watts of output power at sub-picosecond pulse durations [5,24], one of the disadvantages of three-level Yb-based materials related to their longer lifetimes is that in the mode-locking regime they are more prone to Q-switching instabilities.…”

“…With few exceptions, e.g. the additive mode-locking [5], multi-Watt operation of picosecond diode-pumped Nd oscillators has been demonstrated mainly by two passive modelocking methods, one based on semiconductor saturable absorber mirrors (SESAMs) [6][7][8] and the other -on intracavity frequency doubling. The highest average powers demonstrated so far with SESAM technology and Nd:YVO 4 or Nd:YAG are in excess of 20 W [7][8][9], reaching 56 W with pumping at 888 nm [10].…”

8.2 mJ, 324 MW, 5 kHz picosecond MOPA system based on Nd:YAG slab amplifiers

“…The corresponding pulse duration was 25.3 ps in FWHM and resulted in a peak power of 324 MW, which was 69% higher than the former report [2]. The broadening of the amplified pulse duration is due to gain narrowing in the amplifier [23].…”

“…It can be seen from figure 7 that the central wavelength is located at 1064.32 nm with a FWHM of 0.14 nm, corresponding to a spectral width Δν = 37.08 GHz, which is broader than 18.51 GHz of the ps mode-locked oscillator. The spectrum broadening results from the selfphase modulation are accumulated in each pass through the laser crystals and the PC [23]. So the time-bandwidth product κ is 0.938, which is larger than the ideal transform limited time-bandwidth product of 0.315 for a sech 2 -shaped pulse.…”

High Energy, High Peak Power 1342-nm Picosecond Nd:YVO4Regenerative Amplifier

“…The measurement error in the autocorrelation trace was less than ±5 %. The RA pulse width was a little broader than the pulse width of the seed laser, which may be attributed to the gain narrowing effect in the gain medium [17].…”