**2. Design of high-average power Yb:KGW laser system**

The laser was realized as a chirped-pulse amplification (CPA) system [10]. The system shown in **Figure 1** consists of a femtosecond master oscillator (MO) based on Yb:KGW crystal, a

**Figure 1.** Schematic layout of the femtosecond laser system [15]. FM is a high reflective flat mirror; CM1 is a curved mirror with ROC = 400 mm; CM2 is a curved mirror with ROC = 600 mm; DM is a flat dichroic mirror; FL is a focusing lens; CL is a collimating lens; C1 and C2 are Yb:KYW crystals; TFP is a thin film polarizer; λ/4 is a quarter wave plate; λ/2 is a half wave plate.

common module of stretcher and compressor based on single diffraction grating, a spectral shaper based on Lyot filter, and a dual-slab regenerative amplifier (RA) with combined gain spectra. The system also includes two Faraday isolators: one for isolation of MO against leaking amplified pulses and the other for extraction of laser pulses from the system after RA. To increase the contrast ratio of output pulses according to pre-pulses and post-pulses, we used a pulse picker based on a second Pockels cell and a few thin film polarizers (TFP) with high quality.

### **2.1. Mode-locked Yb:KYW oscillator**

Femtosecond laser sources based on Yb-doped laser materials became promising tool for various technological and industrial applications including femtosecond lasers. Among Ybmedia ytterbium tungstates (Yb:KYW/Yb:KGW), crystals Yb:KY(WO4)2 (Yb:KYW) or Yb:KGd(WO4)2 (Yb:KGW) exhibits an attractive set of parameters that makes it as one of the best choices for high-power fs lasers operating around 1 μm [4]. Bandwidth of Yb:KGW/ Yb:KYW is sufficient for amplification of sub-200 fs pulses, but typical pulse length on the output of Yb:KYW amplifier system is limited on the level 300–400 fs [5, 6] primarily due to gain narrowing [7, 8]. A promising method to reduce the effect of gain narrowing and to increase the effective gain bandwidth is to combine laser media with separated gain maxima and to overlap broadband gain. Using Yb:KYW crystals with different orientation of crystal‐ lographic axes, e.g. Ng- and Np-cut orientation, this approach has been realized in [9]. Another way for increasing gain bandwidth is using special spectral filters introducing controlled losses

In this review chapter we present combination of those approaches to a double-slab regener‐ ative amplifier (RA). Each slab is pumped separately, which enables additional possibility to control gain. Furthermore as a seed source, we used high-power master oscillator (MO) based on Np-cut Yb:KYW crystal with output pulse length ~100 fs and central wavelength agreed well with spectral gain profile of regenerative amplifier. A highly efficient stretcher and compressor based on single transmitted diffraction grating are used for stretching and

The laser was realized as a chirped-pulse amplification (CPA) system [10]. The system shown in **Figure 1** consists of a femtosecond master oscillator (MO) based on Yb:KGW crystal, a

**Figure 1.** Schematic layout of the femtosecond laser system [15]. FM is a high reflective flat mirror; CM1 is a curved mirror with ROC = 400 mm; CM2 is a curved mirror with ROC = 600 mm; DM is a flat dichroic mirror; FL is a focusing lens; CL is a collimating lens; C1 and C2 are Yb:KYW crystals; TFP is a thin film polarizer; λ/4 is a quarter wave plate;

at maximum gain spectrum [7].

34 High Energy and Short Pulse Lasers

λ/2 is a half wave plate.

recompressing initial pulses after master oscillator.

**2. Design of high-average power Yb:KGW laser system**

A schematic design of the Yb:KYW oscillator is shown in **Figure 2**. As a gain material, we used a 3×3×3 mm at 5% Yb3+-doped KGW crystal with antireflection-coated ends both for the pump radiation and for the lasing radiation. The crystal was used in an Ng-cut (Np-cut) geometry, such that the laser polarization was aligned along the Np axis (Nm axis), respectively. The pump beam from the 100 μm core diameter fiber output was collimated by an achromatic doublet (CL) with focal length *F* = 60 mm. Then the beam was focused into the laser crystal (LC) by another achromatic doublet (FL) with focal length *F* = 60 mm through special dichroic coated mirror (DM1) with transmittance of 95%. By using three chirped mirrors with group-velocity dispersion −1350 fs<sup>2</sup> per single pass, the dispersion in the laser cavity was compensated. The details of the optical layout of the laser are described in Ref. [11]. The concave mirror in the second arm of the cavity was replaced by dichroic coated mirror (DM2) with radius of curvature *R* = 100 mm and a dumper (d) is mounted additionally. Therefore, the pump beam can transmit through the dichroic mirror DM2 and laser operation can be more stabilized without heating optical mounts of mirror M3.

**Figure 2.** Schematic diagram of the Yb:KYW laser pumped by laser diode (LD) [25]. λ/2 – half wave plate; CL – colli‐ mating lens; FL – focusing lens; LC – Yb:KYW laser crystal; DM1, DM2 – spherical dichroic mirror; M1–M6 – cavity mirrors; OC – output coupler; SAM – semiconductor saturable-absorber mirror; d – dumper for blocking and cooling.

As is well known, the transverse size of the pump zone should match with the size of the cavity mode to achieve maximum output power. **Figure 3(a)** shows the schematic drawing to assist in understanding the mode matching between the cavity mode and pump. The pump beam is focused onto the laser crystal as shown in **Figure 3(a)** with black lines and the cavity mode is depicted as pink color. The position of laser crystal can be defined by the displacement *x* from the center of the caustic of the beam in the cavity. When the crystal is located in the caustic center of the cavity mode (*x* = 0 mm), the pump size is larger than the size of cavity mode. Optimal matching of cavity mode and pumped zone in the active media is important for an efficient laser operation. But it is not easy to match the pump size to cavity mode by changing the specification of pump module, because the conventional optics such as fiber and lens are limited in core size and focal length. Instead, mode matching can be easily achieved by displacing the crystal longitudinally along the optical axis. The comparison of the beam profile of pump and cavity mode for both cases of displacement *x* = 0 mm and 3.5 mm clearly shows this phenomenon as shown in **Figure 3(c)** and **(d)**. The pump profile was obtained by using ZEMAX optical system design software. The cavity mode is assumed to have Gaussian profile and the relevant factors are obtained from LASCAD simulation [12].

**Figure 3.** (a) Schematic diagram for the definition of displacement *x* from the center of the caustic of the beam in the cavity and for the understanding of mode matching. (b) CW output power versus position of laser crystal in the cavity for Ng-cut crystal with OC = 4% of experiment (red) and simulation (black). Comparison of beam profile between pump and cavity mode is shown in the condition of (c) *x* = 0 mm and (d) *x* = 3.5 mm [26].

To optimize the position of the crystal, we calculated the CW output power of the resonator as a function of the displacement *x* as depicted in **Figure 3(b)** with black dots and line by using the LASCAD software. In LASCAD simulation we approximate the top-hat pump spot in focal plane of FL as super-Gaussian distribution in *xy* plane with diameter 100 μm (1/e2 level). Calculated cavity mode diameter between two curved mirrors DM1 and DM2 significantly changes from diameter 36 μm in a waist to 1.5 mm on the mirror. **Figure 3(b)** shows that the optimal position of the crystal is ~3.5 mm apart from the center of the beam caustic. In order to check the mode matching simulation, we performed the experiment with Ng-cut crystal oscillator, which was developed previously and in a same condition with simulation [11]. As shown in **Figure 3(b)** with red dots and line, the CW output power is highly dependent on the position of laser crystal and the experimental result coincides well with simulation result. As a note, multimode operation occurred in experiment when the size of the pump beam is larger than TEM00 cavity mode and it causes higher CW output power in the displacement region below 3.5 mm.

depicted as pink color. The position of laser crystal can be defined by the displacement *x* from the center of the caustic of the beam in the cavity. When the crystal is located in the caustic center of the cavity mode (*x* = 0 mm), the pump size is larger than the size of cavity mode. Optimal matching of cavity mode and pumped zone in the active media is important for an efficient laser operation. But it is not easy to match the pump size to cavity mode by changing the specification of pump module, because the conventional optics such as fiber and lens are limited in core size and focal length. Instead, mode matching can be easily achieved by displacing the crystal longitudinally along the optical axis. The comparison of the beam profile of pump and cavity mode for both cases of displacement *x* = 0 mm and 3.5 mm clearly shows this phenomenon as shown in **Figure 3(c)** and **(d)**. The pump profile was obtained by using ZEMAX optical system design software. The cavity mode is assumed to have Gaussian profile

**Figure 3.** (a) Schematic diagram for the definition of displacement *x* from the center of the caustic of the beam in the cavity and for the understanding of mode matching. (b) CW output power versus position of laser crystal in the cavity for Ng-cut crystal with OC = 4% of experiment (red) and simulation (black). Comparison of beam profile between

To optimize the position of the crystal, we calculated the CW output power of the resonator as a function of the displacement *x* as depicted in **Figure 3(b)** with black dots and line by using the LASCAD software. In LASCAD simulation we approximate the top-hat pump spot in focal plane of FL as super-Gaussian distribution in *xy* plane with diameter 100 μm (1/e2 level). Calculated cavity mode diameter between two curved mirrors DM1 and DM2 significantly changes from diameter 36 μm in a waist to 1.5 mm on the mirror. **Figure 3(b)** shows that the optimal position of the crystal is ~3.5 mm apart from the center of the beam caustic. In order to check the mode matching simulation, we performed the experiment with Ng-cut crystal

pump and cavity mode is shown in the condition of (c) *x* = 0 mm and (d) *x* = 3.5 mm [26].

and the relevant factors are obtained from LASCAD simulation [12].

36 High Energy and Short Pulse Lasers

Not only the position of crystal in the cavity but also the amplification axis of crystal affects the laser operation greatly, because the absorption and emission cross sections of the crystal are quite different according to the orientation of the crystal. In previous work [11], we chose a lasing with *E*//Np for antireflection-coated slab crystal because of its broader emission spectrum [6]. We obtained the generator's average power exceeding 1 W at the central wavelength of 1043 nm with a pulse about 90 fs wide. But, according to the literature [13], laser can operate in the shorter wavelength range than 1043 nm by using the different amplification axis of the crystal such as *E*//Nm. For providing effective amplification as a seeding source of CPA femtosecond laser system, the oscillator with shorter wavelength such as 1030 nm is more suitable than the one with 1043 nm [14, 15].

Before going into experiments, we compare the CW output power of the oscillator between Np- and Nm-polarization with respect to the transmission of output coupler with LASCAD simulation. The corresponding output powers for Np- and Nm-polarization crystals are shown in **Figure 4(a)** as black and red dots and lines, respectively. From the simulated results, we can conclude that Nm-polarized crystal with 6% output coupler can be used as a candidate for the purpose of seeding source, since only the center wavelength would change with similar output power when we replace the crystal from Np-polarization (Ng-cut) to Nm-polarization (Np-cut). And the transmission of 6% of output coupler is chosen due to the output power saturation near 6%.

**Figure 4.** (a) CW output power versus transmission of output coupler for both the Np- and Nm-polarized crystal. (b) CW output power versus incident pump power for Np- (OC = 4%) and Nm-polarized crystal (OC = 6%) [26].

**Figure 4(b)** shows the simulated results according to the replacement from Np-polarized crystal with 4% output coupler to Nm-polarized crystal with 6% output coupler, in point of the dependence of the average output power on the incident pump power. The slope efficiency with respect to the incident radiation power is 41% and 35% for each condition. From the simulation results, it is expected to get high output power and high slope efficiency with more suitable center wavelength in the new oscillator.

Based on the simulation results, we conducted the experiment for femtosecond laser based on Nm-polarization Yb:KYW with 6% output mirror and compared the result with the previously reported oscillator based on Np-polarization Yb:KYW with 4% output mirror in Ref. [11]. An experimental setup of the laser is presented in **Figure 2** as mentioned above. An 8 W highbrightness InGaAs semiconductor injection laser module coupled into optical fiber with a 105 μm core diameter (0.11 NA) and 10 cm length was employed for optical pumping. By using the thermoelectric element (a Peltier cell), the temperature of the laser diode was adjusted for displacing its wavelength to the absorption maximum in the laser crystal. The crystal had a slab design to facilitate efficient heat removal and placed on the optimized position, 3.5 mm apart from the center of the caustic of the beam in the cavity in the longitudinal direction, for size matching of the pump beam and the cavity modes as mentioned. All the laser elements were placed in a compact one-body housing case with 424×214×84-mm size for stable laser operation.

**Figure 5.** Experimentally observed CW output power versus incident pump power for (a) Np-polarization (OC = 4%) and (b) Nm-polarization (OC = 6%). Gray lines indicate the calculated values from the LASCAD simulation for compar‐ ing with the experimental data [26].

The performance of the laser in the CW regime is shown in **Figure 5** for two cases of (a) the Np-polarization crystal with 4% output coupler and (b) the Nm-polarization crystal with 6% output coupler. The high slope efficiency of 54% with respect to the incident pump power was achieved with Nm-polarization and the maximum output power exceeded 1.5 W at incident pump power of 6 W. For comparison, simulated CW output powers using LASCAD are also shown in **Figure 5** as grey lines. Computational simulations coincide well with experimental results in general, especially on high incident pump power. The discrepancy in CW output power between experiment and simulation for the low incident pump power can be explained by the characteristics of laser diodes [16]. The center wavelength of diode lasers typically depends on the injection current, operating temperature, and composition. In our case, the operating temperature of the laser diode was adjusted to 36.5°C in high injection current, corresponding to around 5 W incident pump power in **Figure 5(a)** and **(b)**, for the maximum absorption in the laser crystal. As the injected electric current of laser diode decreases, the pump light shifts toward shorter wavelengths, which results in the decreasing of pump absorption efficiency and low output power of oscillator.

with respect to the incident radiation power is 41% and 35% for each condition. From the simulation results, it is expected to get high output power and high slope efficiency with more

Based on the simulation results, we conducted the experiment for femtosecond laser based on Nm-polarization Yb:KYW with 6% output mirror and compared the result with the previously reported oscillator based on Np-polarization Yb:KYW with 4% output mirror in Ref. [11]. An experimental setup of the laser is presented in **Figure 2** as mentioned above. An 8 W highbrightness InGaAs semiconductor injection laser module coupled into optical fiber with a 105 μm core diameter (0.11 NA) and 10 cm length was employed for optical pumping. By using the thermoelectric element (a Peltier cell), the temperature of the laser diode was adjusted for displacing its wavelength to the absorption maximum in the laser crystal. The crystal had a slab design to facilitate efficient heat removal and placed on the optimized position, 3.5 mm apart from the center of the caustic of the beam in the cavity in the longitudinal direction, for size matching of the pump beam and the cavity modes as mentioned. All the laser elements were placed in a compact one-body housing case with 424×214×84-mm size for stable laser

**Figure 5.** Experimentally observed CW output power versus incident pump power for (a) Np-polarization (OC = 4%) and (b) Nm-polarization (OC = 6%). Gray lines indicate the calculated values from the LASCAD simulation for compar‐

The performance of the laser in the CW regime is shown in **Figure 5** for two cases of (a) the Np-polarization crystal with 4% output coupler and (b) the Nm-polarization crystal with 6% output coupler. The high slope efficiency of 54% with respect to the incident pump power was achieved with Nm-polarization and the maximum output power exceeded 1.5 W at incident pump power of 6 W. For comparison, simulated CW output powers using LASCAD are also shown in **Figure 5** as grey lines. Computational simulations coincide well with experimental results in general, especially on high incident pump power. The discrepancy in CW output power between experiment and simulation for the low incident pump power can be explained by the characteristics of laser diodes [16]. The center wavelength of diode lasers typically depends on the injection current, operating temperature, and composition. In our case, the operating temperature of the laser diode was adjusted to 36.5°C in high injection current,

suitable center wavelength in the new oscillator.

38 High Energy and Short Pulse Lasers

operation.

ing with the experimental data [26].

We shall note that for quasi three-level laser media it is difficult to estimate the absorbed pump power experimentally because of the effect of absorption saturation appearing in such media. Therefore we considered the power of launching pump radiation for an estimation of laser efficiency instead of the absorbed power.

**Figure 6.** (a) Optical spectra and (b) autocorrelation traces of the 96 fs pulses with center wavelength 1043 nm for Nppolarization (blue lines) and the 111 fs pulses with center wavelength 1035 nm for Nm-polarization (red lines). (c) Typi‐ cal spatial pattern of the beam and the corresponding intensity profiles. (d) Measured average output power and its temporal stability [26].

The passively mode-locked regime was realized by replacing a cavity mirror M6 to semicon‐ ductor saturable-absorber mirror (SAM) at one end of the resonator in the focal region of a concave mirror (M5) with a radius of curvature *R* = 300 mm. In **Figure 6(a)** and **(b)**, the measured emission spectra and autocorrelation traces with the fit assuming a sech2 -pulse shape are shown for the Np- and Nm-polarization as blue and red line, respectively. For Nppolarization (Ng-cut), 96 fs pulses were generated with the emission spectra centered at 1043 nm. Using Nm-polarization (Np-cut) crystal, the center wavelength could be changed to 1035 nm with only somewhat longer pulse durations of 111 fs. The time-bandwidth-product of 0.308 is almost close to the Fourier-transform-limited pulse with an intensity profile described by the sech2 function, and it indicates that dispersion is well compensated and there is no frequency modulation.

Typical spatial pattern of the beam at a distance of 1 m from the output coupler for the new oscillator based on Nm-polarization (Np-cut) crystal is shown in **Figure 5(c)** with the vertical and horizontal intensity profiles. The cross section is nearly Gaussian with horizontal width 1.02 mm and vertical width 0.92 mm. The *M*<sup>2</sup> values of the beam are 1.10 in the horizontal plane and 1.07 in the vertical plane, which are fairly close to *M*<sup>2</sup> = 1 for an ideal Gaussian beam.

The thermal lens in the laser crystal did not affect the quality of output beam due to weakness of thermo-optical effects. Our calculations with LASCAD show that the focal lengths of thermal lens are 1273 and 1063 mm for horizontal and vertical directions, respectively, that are much larger than the focal length of focusing mirrors.

**Figure 5(d)** shows the average output power and its stability of the Nm-polarization oscillator after it was warmed up. We have achieved a maximum average power of 1.27 W at a pulse repetition rate of 87.8 MHz, and it gives a single femtosecond pulse energy greater than 14 nJ. The stability of the output power is to be 0.14% by using the definition such as twice the standard deviation of the measured output power divided by the mean output power.

As a summary, a comparison between the laser performances of the Np- and Nm-polarization (Ng- and Np-cut) Yb:KYW materials is given in **Table 1**. Both the relative beam angular stability and relative beam positional stability were measured by using the factors related with angular movement and beam positional movement in accordance with ISO 11670 [17]. The achieved parameters show that the new oscillator with Nm-polarization (Np-cut) crystal is more stable and powerful with well-defined center wavelength as a seeding source.


**Table 1.** Comparison between Np- and Nm-polarization Yb:KYW mode-locked oscillators with 4% and 6% output coupler, respectively.

#### **2.2. Stretcher-compressor**

Optical pulse stretcher and compressor are a key subsystem in CPA laser systems [10]. In those systems, the stretcher is used to lengthen optical pulses before the amplification, and the compressor is used to restore original short pulses. In this way, the peak power inside the amplifier cavity can be kept low enough to avoid any damage to the optical element and to suppress nonlinear distortions on the pulse temporal shape and beam spatial profile due to the self-focusing effect.

1043 nm. Using Nm-polarization (Np-cut) crystal, the center wavelength could be changed to 1035 nm with only somewhat longer pulse durations of 111 fs. The time-bandwidth-product of 0.308 is almost close to the Fourier-transform-limited pulse with an intensity profile

Typical spatial pattern of the beam at a distance of 1 m from the output coupler for the new oscillator based on Nm-polarization (Np-cut) crystal is shown in **Figure 5(c)** with the vertical and horizontal intensity profiles. The cross section is nearly Gaussian with horizontal width

The thermal lens in the laser crystal did not affect the quality of output beam due to weakness of thermo-optical effects. Our calculations with LASCAD show that the focal lengths of thermal lens are 1273 and 1063 mm for horizontal and vertical directions, respectively, that are much

**Figure 5(d)** shows the average output power and its stability of the Nm-polarization oscillator after it was warmed up. We have achieved a maximum average power of 1.27 W at a pulse repetition rate of 87.8 MHz, and it gives a single femtosecond pulse energy greater than 14 nJ. The stability of the output power is to be 0.14% by using the definition such as twice the standard deviation of the measured output power divided by the mean output power.

As a summary, a comparison between the laser performances of the Np- and Nm-polarization (Ng- and Np-cut) Yb:KYW materials is given in **Table 1**. Both the relative beam angular stability and relative beam positional stability were measured by using the factors related with angular movement and beam positional movement in accordance with ISO 11670 [17]. The achieved parameters show that the new oscillator with Nm-polarization (Np-cut) crystal is more stable

and powerful with well-defined center wavelength as a seeding source.

1.25 (*y*)

1.07 (*y*)

*M***<sup>2</sup> Power stability (%)**

function, and it indicates that dispersion is well compensated and there

values of the beam are 1.10 in the horizontal plane

**Positional stability**

**Angular stability**

**(%)**

0.3 (*x*) 1.9 (*y*)

0.2 (*x*) 0.3 (*y*)

**(%)**

3.2 (*y*)

0.15 (*y*)

0.30 0.5 (*x*)

0.14 0.1 (*x*)

**Table 1.** Comparison between Np- and Nm-polarization Yb:KYW mode-locked oscillators with 4% and 6% output

Optical pulse stretcher and compressor are a key subsystem in CPA laser systems [10]. In those systems, the stretcher is used to lengthen optical pulses before the amplification, and the compressor is used to restore original short pulses. In this way, the peak power inside the amplifier cavity can be kept low enough to avoid any damage to the optical element and to

= 1 for an ideal Gaussian beam.

described by the sech2

40 High Energy and Short Pulse Lasers

*λ***center (nm)** 

coupler, respectively.

**2.2. Stretcher-compressor**

E//Np, OC = 4%

E//Nm, OC = 6% **Δ***λ* **(nm)** **Poutput (W)**

1043 12.5 0.64 96 1.10 (*x*)

1035 9.0 1.27 111 1.10 (*x*)

**τp (fs)**

is no frequency modulation.

1.02 mm and vertical width 0.92 mm. The *M*<sup>2</sup>

larger than the focal length of focusing mirrors.

and 1.07 in the vertical plane, which are fairly close to *M*<sup>2</sup>

Design of stretcher and compressor module in a commercial laser system must provide maximum stretched pulse duration and good output spatial and temporal beam quality in minimum module size. To improve total efficiency and make design of stretcher and com‐ pressor more compact, we used in our stretcher-compressor module common transmission diffraction grating and folding mirrors (**Figure 7**).

**Figure 7.** Optical scheme of the stretcher-compressor module based on the transmission diffraction grating (G), spheri‐ cal lens (SL), right-angle prisms (Pr), and folding mirrors (FM).

In the stretcher and compressor, the beam enters and passes through transmission grating four times. In the stretcher, the beam after first and third passes through diffraction grating is focused and collimated by the spherical lens (SL). To change chirped-pulse duration, we used diffraction grating with two groove densities: *N* = 1700 mm−1 and *N* = 1500 mm−1 and two different spherical lenses with focal length *F* = 100 mm and *F* = 200 mm with NA = 0.25 and NA = 0.12, respectively. Using this stretcher-compressor module, we can supply chirped-pulse duration from 50 ps to 300 ps. The compressed pulse duration varies from 230 to 270 fs depending on the optical elements used. Increasing the chirped-pulse duration up to 300 ps using optical elements, the same aperture results in decreasing of spectral pass bandwidth of stretcher up to Δ*λ* = 14 nm. Spectral clipping introduced by the aperture of optical elements in stretcher affects a recompression pulse duration and its time-bandwidth product.

Increasing the optics aperture is connected with the proportional increase of linear distances in stretcher (spherical aberration introduced by the spherical lens increase with increasing the NA number). Using the diffraction grating with higher groove density to provide more compact design is limited by increasing third-order dispersion.

To check stretcher-compressor module, we used our master oscillator as seed source. We measured initial pulse length and compressed pulse length after the beam passed through the stretcher and compressor. We also measured quality of a beam and total efficiency of the stretcher and compressor. The stretcher-compressor based on 1500 mm−1 grating operating with 50 ps chirped-pulse duration provides better degree of recompression and more short output pulse. It is connected with aberration avoiding due to smaller NA number. The measurements made by CCD camera shows that spatial quality of beam did not change after it passed through stretcher and compressor.

Amplification with different pulse durations shows limitation of maximum output pulse energy by Raman scattering excitation and was measured 150 μJ for chirped-pulse duration 50 ps, 300 μJ for 130 ps, and we expected more than 0.5 mJ for 300 ps. Further increase of the pulse energy needs a longer chirped-pulse duration and an increase of the stretcher-compres‐ sor module size.

**Figure 8.** Second harmonic generator output power versus time using thermo stabilized compressor module (a) and nonstabilized one (b).

One of the most important parameters of stretcher-compressor is a temporal stability of compressed pulse duration. Because there is no 100% diffraction grating efficiency, approxi‐ mately 30% of amplified power is dissipated inside stretcher-compressor module and causes misalignment of compressor due to thermo expansion of mechanical parts. The requirement for thermomechanical stability of stretcher-compressor elements increases with a decrease in compressed pulse duration. The thermal stabilization of the stretcher-compressor module can significantly improve temporal stability of the compressed pulse. Most dramatically, the effect of temporal detuning of compressor can by observed by the subsequent second harmonic generation. **Figure 8** shows temporal dependence of second harmonic power with water thermo stabilized stretcher-compressor module (a) and without stabilization (b).

In case of no thermo stabilized stretcher-compressor module, fundamental pulse duration increases up to two times with the gradual heating of the module resulting in significant drop of the second harmonic generation efficiency. Thermal stabilization by water cooling the module improves fundamental pulse duration stability and as a result stability of the second harmonic output power.

### **2.3. Spectral shaping**

To check stretcher-compressor module, we used our master oscillator as seed source. We measured initial pulse length and compressed pulse length after the beam passed through the stretcher and compressor. We also measured quality of a beam and total efficiency of the stretcher and compressor. The stretcher-compressor based on 1500 mm−1 grating operating with 50 ps chirped-pulse duration provides better degree of recompression and more short output pulse. It is connected with aberration avoiding due to smaller NA number. The measurements made by CCD camera shows that spatial quality of beam did not change after

Amplification with different pulse durations shows limitation of maximum output pulse energy by Raman scattering excitation and was measured 150 μJ for chirped-pulse duration 50 ps, 300 μJ for 130 ps, and we expected more than 0.5 mJ for 300 ps. Further increase of the pulse energy needs a longer chirped-pulse duration and an increase of the stretcher-compres‐

**Figure 8.** Second harmonic generator output power versus time using thermo stabilized compressor module (a) and

One of the most important parameters of stretcher-compressor is a temporal stability of compressed pulse duration. Because there is no 100% diffraction grating efficiency, approxi‐ mately 30% of amplified power is dissipated inside stretcher-compressor module and causes

it passed through stretcher and compressor.

sor module size.

42 High Energy and Short Pulse Lasers

nonstabilized one (b).

It is well known that regenerative amplification with high gain leads to the spectral narrowing of amplified pulses [7, 8]. To broaden spectrum in the regenerative amplifier, we tried both extra-cavity and intra-cavity spectral shaping based on the polarization-interference filter (Lyot filter). This technique is well known but it has been used generally for Ti:sapphire lasers [7]. The spectral shaper consists of a birefringent quartz plate placed between two polarizers. To realize optimal spectral shaping, a transmission minimum of the birefringent filter should coincide with maximum gain spectrum and their widths should be close. To fulfill these conditions, a quartz plate with thickness of 8 mm was cut along the optical axis and mounted to the rotation stage to rotate it in two planes for adjusting modulation depth and position of transmission minimum.

### **2.4. Double-slab laser and amplifier**

The maximum average power of solid-state lasers is limited either by the thermal destruction of the active medium or by thermo-optical aberrations [16]. Therefore, one of a few methods of increasing this power in the case of bulk laser media is to increase the active medium length or to use several active elements in the cavity. To increase the average laser power and simultaneously to retain the high spatial quality of the output beam, we used a scheme with two active elements (see also [9, 14, 15]). The optical scheme of a laser amplifier with two active elements made of Yb:KGW crystals with different orientations of optical axes is shown in **Figure 1**. The use of Np- and Ng-cut crystals, which have different spectral-luminescent characteristics at a corresponding orientation of the polarization vector of generated or amplified beams, makes it possible to widen the gain band and thus generate or amplify more broadband chirped or femtosecond pulses [10]. The crystals were 5 mm long and contained 3% of ytterbium ions, which ensured a good (above 70%) absorption of the pump radiation [18]. Similar to the case of the master oscillator, the pump beam and cavity mode sizes were matched by a longitudinal shift of the active elements. To eliminate possible modulation of the spectrum, the faces of the active elements were antireflection coated and inclined at an angle of ~30'. As pump sources, we used two fiber-coupled laser diode arrays with a maximal output power of 70 W each. The depolarization of the pump radiation was minimized using a short (30 cm) optical fiber 200 μm in diameter with the numerical aperture NA = 0.22. The pump beams from the fiber output were collimated and focused through dichroic mirrors into the active crystal to a spot with a diameter of about 320 μm, which is close to the diameter of the TEM00 cavity mode. This mode was calculated by the well-known ABCD matrix method taking into account the astigmatic thermal lenses induced in the active elements by the pump radiation. Using the LASCAD software, the focal length of these lenses was calculated to be 480–550 mm for the Np-cut crystal at an incident power of 30 W. These values are in adequate agreement with the literature data [19, 20] and are confirmed by our measurements [18].

We used this laser as a standalone oscillator and as a regenerative amplifier. As an output mirror in this oscillator, we used a thin-film polarizer in combination with a quarter-wave plate, which was rotated to obtain the maximum output power at a given pump power. For electro-optic Q-switching, we used a Pockels cell based on a β-BaB2O4 (BBO) crystal and controlled by a special driver. The pulse repetition rate was smoothly changeable from single pulses to 1000 kHz.

The output laser power in the CW regime in the cases of using single active element and two elements simultaneously is shown in **Figure 9** versus the pump power incident on the crystals. The maximum output power was 12.5 and 9 W at a slope efficiency of 47% and 37% for Ngand Np-cut single crystals, respectively. Note that the output powers for the crystal in which the polarization vector of the laser beam is parallel to the Np axis (E||Np) are higher than for the crystal with E||Nm despite a smaller stimulated emission cross section [9]. This behavior of power can be explained, in our opinion, by different spectral losses in the cavities with crystals emitting at different wavelengths. The maximum output power in the case of two crystals in the cavity was 18.5 W at a pump power of 72 W, which yields the total and slope efficiencies of 25% and 35%, respectively. The laser radiation spectrum was centered at a wavelength of 1035 nm, and its width did not exceed 1 nm.

**Figure 9.** CW output power of laser as a function of incident pump power on crystals in CW mode operation for each single- and dual-slab configuration [15].

Note that the output laser power was maximized in measurements for each pump power by optimal adjustment of the quarter-wave plate. As can be easily shown, this measurement method leads to an underestimation of the slope efficiency. It is this fact that can explain the considerable difference in the slope efficiencies of single- and two-crystal lasers at close total efficiencies (27–30% and 25%, respectively).

power of 70 W each. The depolarization of the pump radiation was minimized using a short (30 cm) optical fiber 200 μm in diameter with the numerical aperture NA = 0.22. The pump beams from the fiber output were collimated and focused through dichroic mirrors into the active crystal to a spot with a diameter of about 320 μm, which is close to the diameter of the TEM00 cavity mode. This mode was calculated by the well-known ABCD matrix method taking into account the astigmatic thermal lenses induced in the active elements by the pump radiation. Using the LASCAD software, the focal length of these lenses was calculated to be 480–550 mm for the Np-cut crystal at an incident power of 30 W. These values are in adequate agreement with the literature data [19, 20] and are confirmed by our measurements [18].

We used this laser as a standalone oscillator and as a regenerative amplifier. As an output mirror in this oscillator, we used a thin-film polarizer in combination with a quarter-wave plate, which was rotated to obtain the maximum output power at a given pump power. For electro-optic Q-switching, we used a Pockels cell based on a β-BaB2O4 (BBO) crystal and controlled by a special driver. The pulse repetition rate was smoothly changeable from single

The output laser power in the CW regime in the cases of using single active element and two elements simultaneously is shown in **Figure 9** versus the pump power incident on the crystals. The maximum output power was 12.5 and 9 W at a slope efficiency of 47% and 37% for Ngand Np-cut single crystals, respectively. Note that the output powers for the crystal in which the polarization vector of the laser beam is parallel to the Np axis (E||Np) are higher than for the crystal with E||Nm despite a smaller stimulated emission cross section [9]. This behavior of power can be explained, in our opinion, by different spectral losses in the cavities with crystals emitting at different wavelengths. The maximum output power in the case of two crystals in the cavity was 18.5 W at a pump power of 72 W, which yields the total and slope efficiencies of 25% and 35%, respectively. The laser radiation spectrum was centered at

**Figure 9.** CW output power of laser as a function of incident pump power on crystals in CW mode operation for each

a wavelength of 1035 nm, and its width did not exceed 1 nm.

pulses to 1000 kHz.

44 High Energy and Short Pulse Lasers

single- and dual-slab configuration [15].

The maximum output power obtained in the case of Q-switching reached 16 W at a pulse repetition rate of 100 kHz and 14 W at a repetition rate of 500 kHz (Q-switching time 800 ns). This decrease in the average power with respect to the CW regime can be explained by additional losses introduced by the electro-optic Q-switch. As the Q-switching time decreased to 400 ns, the output power decreased by three times, which obviously occurred because this time was too short for oscillation development. The output pulse duration, determined mainly by a rather long length of the cavity, was 20 ns. The width of the spectrum was 16 nm, which points to a pronouncedly multimode regime. The spectrum had two peaks, at wavelengths of 1035 and 1043 nm, which correspond to the spectral maxima of the gain coefficient for the two used crystals.

We optimized our laser by better alignment of the beam size of pump and laser modes in laser crystals at high pumping levels [21, 22]. This was achieved by moving the mirror CM1 along the optical axis. To suppress thermal losses, the thickness of the laser crystal was also reduced from 2 to 1.2 mm. The optimization results are shown in **Figure 10**. After optimization of the laser, output power increased by 32% from 18.5 to 24.5 W at incident pump power of 110 W.

**Figure 10.** Output power of Q-switched laser versus incident pump power at repetition rate of 200 kHz for different dual-crystal configurations.

Output beam profiles measured using a CCD camera at distance of 1 m from output of compressor was symmetric with beam diameter of 4 mm at e−2 intensity level and nearly Gaussian. Increasing of the output power above 15 W resulted in gradual distortion of beam 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 the laser output power of 22 W as shown in **Figure 11**.

**Figure 11.** Near-field images and beam qualities *M*<sup>2</sup> of the laser output for different dual-crystal configurations.
