**3. Performance of the Yb:KGW femtosecond laser system**

When the regenerative amplifier was seeded by stretched pulses, we investigated the output power, spectrum, and pulse shape after compression under different conditions. Our meas‐ urements showed that the output power after regenerative amplifier increased up to a value of 21 W when the time gate was increased up to 400 ns and the number of round trips was 24. And then the output power saturated at same value as the time gate increased up to 29 roundtrips. It means that the gain is balanced by losses. **Figure 12** shows that the output power is practically linearly dependent on the incident pump power. It means the absence of such parasitic effects restricted gain as amplified spontaneous emission (ASE) and parasitic oscillations. We measured the output power with spectral shaping in addition. There was a power reduction of about 5% when the spectral shaping using a Lyot filter is applied outside the cavity. The compressor and the picker are making a loss, which reduces output power by 25% (15 W).

High-Power Diode-Pumped Short Pulse Lasers Based on Yb:KGW Crystals for Industrial Applications http://dx.doi.org/10.5772/64571 47

**Figure 12.** Average output power after RA as a function of incident pump power.

profile and transformation of beam cross section into the slightly elliptical one as shown in **Figure 5(b)**. However the beam quality parameter *M*<sup>2</sup> was measured to be below 1.5 even at

of the laser output for different dual-crystal configurations.

the laser output power of 22 W as shown in **Figure 11**.

46 High Energy and Short Pulse Lasers

**Figure 11.** Near-field images and beam qualities *M*<sup>2</sup>

25% (15 W).

**3. Performance of the Yb:KGW femtosecond laser system**

When the regenerative amplifier was seeded by stretched pulses, we investigated the output power, spectrum, and pulse shape after compression under different conditions. Our meas‐ urements showed that the output power after regenerative amplifier increased up to a value of 21 W when the time gate was increased up to 400 ns and the number of round trips was 24. And then the output power saturated at same value as the time gate increased up to 29 roundtrips. It means that the gain is balanced by losses. **Figure 12** shows that the output power is practically linearly dependent on the incident pump power. It means the absence of such parasitic effects restricted gain as amplified spontaneous emission (ASE) and parasitic oscillations. We measured the output power with spectral shaping in addition. There was a power reduction of about 5% when the spectral shaping using a Lyot filter is applied outside the cavity. The compressor and the picker are making a loss, which reduces output power by The laser system can operate in the repetition rate range of 50–500 kHz. In this range, the output power is almost not dependent on repetition rate. The single pulse energy was measured to be 300 and 30 μJ at repetition rates of 50 and 500 kHz, correspondently, that is important for microprocessing applications. Maximum pulse energy at 50 kHz was limited by Raman scattering excitation in Yb:KYW crystal that was observed in the experiment [23, 24]. Pulse shape at this repetition rate is distorted phase-modulation of the pulse at the Kerr nonlinearity [24].

**Figure 13.** (a, b) Spectra and (c, d) intensity autocorrelation traces (black-experimental data, red-fitting) of output puls‐ es at incident pump power of 67 W and repetition rate of 200 kHz without spectral shaping (a, c) and with spectral shaping (b, d). Insets show the output beam profile (b) and autocorrelation trace in the range of 5 ps (d) [15].

The shape of the output spectrum for equal pump power in both arms of pumping is shown in **Figure 13(a)**. The gain narrowing effect is noticeably well—FWHM spectral width is ~1.5 times narrower compared with the spectrum of master oscillator pulses. Compression provides 265 fs output pulses under this condition as shown in **Figure 13(c)**.

This gain narrowing effect can be suppressed, for example, by making the pump power of Yb:KYW crystals not to be equal [9]. We changed the pump power launched on the Np-cut and Ng-cut crystals to the ratio of 3:2 [15]. The experimental measurements showed that the spectral width became broader and its shape was modified considerably. In this case, the spectral width was measured to be 11 nm and the pulse length was measured to be 210 fs for assuming sech2 profile. This method has a drawback that the restriction of pumping power on one crystal results in the restriction of total output power in expense of pulse width. For example the output power dropped 37% in our experimental conditions.

Another way to suppress the spectrum narrowing is to use preliminary spectrum shaping [11] as it was discussed earlier. The example of output spectrum for extra-cavity spectrum shaping by filter Lyot is shown in **Figure 13(b)**. The optical spectrum showed a characteristic "bell" shape with a spectral FWHM bandwidth of 11 nm. Such a bandwidth provides smooth output pulse with width of intensity autocorrelation trace 305 fs that gives the pulse length of τFWHM = 182 fs for sech2 pulse profile as shown in **Figure 13(d)**. This pulse length is close to the pulse length of 160 fs defined by aberrations in the stretcher-compressor module. To measure the ultrashort pulse width, we used a PulseCheck autocorrelator (APE GmbH). The inset of **Figure 13(d)** shows that there is no noticeable peak beyond the range of 1.5 ps.

Inserting a spectrum shaper inside the cavity of regenerative amplifier, we obtained approx‐ imately the same spectral width but less output power of about 20%. It is connected with accumulated effect of intra-cavity losses by Lyot filter inside the cavity. Thus combination of Lyot filter outside the cavity as a spectral shaper and identical pump power for two slabs in the dual-slab regenerative amplifier provides optimal condition of output power and pulse length.

Beam quality *M*<sup>2</sup> of output beam was below 1.2 at output power <12 W that allows the beam focusing to small spot size of 5–10 μm. High average output power, with more than tens of μJ, and beam quality are important for industrial microprocessing applications.
