A diode-pumped, ultrafast Yb:KYW laser system utilizing chirped-pulse amplification in a dual-slab regenerative amplifier with spectral shaping of seeding pulse from a master oscillator has been developed. A train of compressed pulses with pulse length of 181 fs, repetition rate up to 200 kHz, and average power exceeding 8 W after compression and pulse picker was achieved.
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Compact and reliable femtosecond laser systems with high peak and average power are essential laser sources for a wide field of applications in science and industry including micro machining, biomedicine, XUV-ray generation and so on [1, 2]. Some of applications usually require sub-300-fs pulses with energies exceeding ablation threshold (>10 μJ). Certain of material processing applications often require pulse energies of some hundreds of μJ.
Recently, great progress has been achieved in the development of high energy directly diode pumped femtosecond master oscillator power amplifier (MOPA) systems [3–9]. Ytterbium-doped monoclinic double tungstates, such as Yb:KYW and Yb:KGW are a good choice for high energy and high average power fs lasers because of their good laser and thermo-mechanical properties . 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 primarily due to gain narrowing [11, 12]. 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 crystallographic axes (so called Ng and Np-cut orientation) this approach can be realized [8,9]. Another way for increasing gain bandwidth is using special spectral filters introducing controlled losses at maximum of gain spectrum .
In this paper we present of those approaches to a double-slab regenerative amplifier. Each slab is pumped separately, which enables additional possibility to control gain. Furthermore, we used high-power master oscillator 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 recompressing initial pulses after master oscillator.
2. Experimental setup
The setup was realized as a chirped-pulse amplification (CPA) system . The schematic layout of the laser system is shown in Fig. 1 . The system consists of a femtosecond master oscillator (MO) based on Yb:KYW crystal, a unified module of stretcher and compressor based on single diffraction grating, a spectral shaper based on Lyot filter  and a dual-slab regenerative amplifier with combined gain spectra [8, 9]. The system also includes two Faraday isolators: one for isolation of master oscillator against leaking amplified pulses and the other for withdrawal of laser pulses from the system after amplification. 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. Master oscillator
The seed pulses for the amplifier were generated from a femtosecond Yb:KYW laser oscillator using chirped mirrors to control the dispersion. Detailed optical layout of the oscillator is shown in Ref . An Yb:KYW slab crystal with size 3x3x2 mm was used as the active medium. The crystal contained 5 at. % Yb3+ ions. The polarization vector of the pumping was parallel to the Nm axis to maximize the absorption. The laser radiation was also polarized along Nm axis due to the highest value of the stimulated emission cross section. In this research we have used a master oscillator with Np-cut crystal to provide a different central wavelength for better matching with gain spectrum of dual-slab regenerative amplifier instead of an oscillator with Ng-cut crystal.
Figure 2 shows measured intensity autocorrelation traces (a, c) and optical spectra (b, d) of femtosecond laser pulses for oscillators with Ng-cut (a, b) and Np-cut (c, d) crystals. As can be seen, the lasing for Np-cut crystal occurred at the central wavelength of 1035 nm, and the wavelength band width was 9 nm at the half-intensity level (FWHM). The product of the spectrum width and the pulse length (0.347) is about a factor of 1.1 greater than the limit for pulses with an intensity proðle described by the established sech2 function. The pulse length for Ng-cut crystal was slightly shorter and spectrum of lasing is centered at 1043 nm. Table 1 summarizes parameters of the oscillators based on the Ng-cut and Np-cut crystals .
Optical pulse stretcher and compressor are a key subsystem in CPA laser systems . 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.
In the stretcher-compressor module a single transmission diffraction grating was used to improve the efficiency and to make it compact . We made two modules with different transmission gratings with grooves densities: N=1700 mm−1 and N=1500 mm−1. These gratings give the calculated duration of chirped pulse 90 ps and 50 ps, respectively. Since the beam diameter on the doublet was smaller for lower density grating the spherical aberration for this case should be also smaller that can affect on duration of recompressed pulse.
To test the stretcher-compressor modules we used 110 fs pulses from the master oscillator with Np-cut crystal as a seeding source. We measured the initial pulse length and recompressed pulse length after the beam passing through the stretcher and compressor in order to define the aberrations introduced by optical elements. We also measured the beam quality and efficiency of the stretcher and compressor separately. The results of these measurements are summarized in the Table 2 . Compressed pulse lengths for modules with gratings 1700 mm−1 and 1500 mm−1 were 237 fs and 160 fs, respectively. The stretcher-compressor module with 1500 mm−1 grating provides better compression ratio and more short output pulse. It is connected with aberration avoiding due to smaller number of numerical aperture. This stretcher-compressor has better efficiency due to better efficiency of diffraction grating. The measurements by CCD-camera showed that spatial beam quality did not change after it passed through the stretcher and compressor.
2.3. Spectral shaping
It is well known that regenerative amplification with high gain leads to the spectral narrowing of amplified pulses [11, 12]. To broaden the 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 [11, 14]. 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 of the 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. Regenerative amplifier
The schematic layout of the double-slab regenerative amplifier is shown in Fig. 1. The cavity contains two Yb:KYW crystals in order to scale up the output power. One Ng-cut and another Np-cut Yb:KYW crystals with length of 5 mm and doping concentration of 3 at.% were used to combine the gain spectra and make it wide. The cavity mode was polarized vertically along the Np-axis of the first and the Nm-axis of the second crystal in order to combine their different gain spectra. Relatively long Yb:KYW crystals with low doping concentration allow substantially to decrease the thermal lensing effect and improve the spatial quality of output beam at high output power. For example, our numerical calculations based on LASCAD code (Las-Cad GmbH) showed that the optical strength of thermal lens and thermo mechanical stresses in case of 5 mm crystal length with doping concentration of 3% are weaker by 1.5 times in comparison with those in case of 3 mm crystal length with doping concentration of 5%. Calculations also showed that the degrees of astigmatism of thermal lenses in Ng-cut and Np-cut crystals are similar in the strength but different in the orientation axis. The ratio of thermal focal length (fx/ fy) was 1.15 and 0.88 for the pump power of 36 W for each crystal, respectively. It means that the astigmatism of amplified beam can be partially compensated each other after sequential beam propagation though Ng- and Np-cut crystals.
The pump radiation at wavelength of 981 nm was provided by two fiber-coupled diode lasers with maximum output power of 50 W each. Short pump fibers (30 cm long, 200 μm diameter, NA=0.22) were coupled to minimize the depolarization and provide high absorption of pump light. The pump beams were collimated and focused by doublet lens to a pump spot with diameter approximately 320 μm. Under these conditions total pump loss in transmitting optics was 14% and the absorption of pumping light was 75-80%. The cavity of regenerative amplifier was designed for the spot size of laser beam inside Yb:KYW crystals to be close to the spot size of pumping beams. To minimize the loss all cavity mirrors were highly reflective for the laser wavelength.
Q-switched operation of dual-slab Yb:KYW regenerative amplifier was obtained using an optical switch. The switch consisted of a thin film polarizer (TFP), a quarter wave plate (λ/4) and BBO Pockels cell. It was driven by a Pockels cell driver which can operate with repetition rate up to 200 kHz.
When the regenerative amplifier was seeded by pulses from the stretcher and the master oscillator, the amplified output pulses were separated from the input beam by means of a Faraday rotator and directed to the pulse picker module based using a second Pockels cell and its driver. The pulse picker cleared the main pulse from pre-pulses and post-pulses that leaked from the polarizer. Then, chirped pulses are directed to the compressor. The pulse picker module also includes a few TFP’s more to enhance the contrast ratio. The ratio was measured by pin photodiode and oscilloscope to be more than 1000:1. This value was limited by method of measurement. The total gain of the regenerative amplification can be adjusted by changing the number of round trips (RT) in the cavity that was determined by time gate of Pockels cell.
3. Experimental results
At the first stage we optimized the operation of regenerative amplifier in CW mode for each single crystal of Ng-cut and Np-cut, and next for dual crystals in the cavity. Figure 3 shows CW output power as a function of the incident pump power on the crystals. It shows that the maximum output powers for single crystal configuration are 12 W and 9 W with slope efficiency of 47% and 37% for Ng- and Np-cut, respectively, at the incident pump power of 36 W. And it is 18 W for dual crystals at incident pump power of 72 W at the entrance of crystals. The slope efficiency of the dual-slab regenerative amplifier was about 35%.
Maximum average output power obtained in Q-switch mode was 16 W at time gate of 800 ns and repetition rate of 200 kHz. Such a small reduction of output power is connected with the losses by inserting an optical switch to the cavity. The length of output pulses was measured to be about 20 ns and spectral width was about 16 nm. The spectrum showed characteristic “M-shape” with two maxima at 1035 nm and 1043 nm corresponding to the gain peaks for each crystal.
When the regenerative amplifier was seeded by stretched pulses we investigated the output power, spectrum and pulse shape after compression under different conditions. The dependences of output power as a function of time gate of the Pockels cell and pump power at the repetition rate of 200 kHz are shown in Fig. 4 . As shown in Fig. 4(a) the output power increased linearly up to a value of 8.7 W when the time gate was 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 round-trips. It means that the gain is balanced by losses. Figure 4(b) shows that the output power is 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. Average output power was 8.3 W when the incident pump power was 67 W and time gate was 400 ns. It was 11.6 W before compressor, and 14 W before Faraday rotator and pulse picker. It means that maximum average output power of the CPA Yb:KYW laser system would be around 10 W without pulse picker.
The laser system operated in the range of repetition rate of 50-200 KHz. The average power of output pulses slightly decreased down to about 8 W at repetition rate of 50 KHz. The single pulse energy was measured to be 160 μJ and 43 μJ at repetition rates of 50 kHz and 200 kHz correspondently that is important for micro-processing applications. Maximum pulse energy at 50 kHz was limited by Raman scattering excitation in Yb:KYW crystal that was observed in the experiment.
The shape of the output spectrum for equal pump power in both arms of pumping at the repetition rate of 200 kHz is shown in Fig. 5(a) . The gain narrowing effect is noticeable. FWHM spectral width is narrower by about 1.5 times in comparison with the spectrum of master oscillator pulses. Pulse compression provides 265 fs output pulses under this condition as shown in Fig. 5(c).
This gain narrowing effect can be suppressed, for example, by making the pump power of Yb:KYW crystals not to be equal . We changed the pump power launched on the Np-cut and Ng-cut crystals to the ratio of 3:2 [8, 10]. 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 9 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 at the expense of pulse width. For example, the output power dropped by 37% in our experimental conditions.
Another way to suppress the spectrum narrowing is to use preliminary spectrum shaping as it was discussed in Sec. 2.3. The example of output spectrum for extra-cavity spectrum shaping by filter Lyot is shown in Fig. 5(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 Fig. 5(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 Fig. 5(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 approximately 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.
Output beam profile 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 7 W resulted in gradual distortion of beam profile and transformation of beam cross section into the slightly elliptical one as shown in the inset of Fig. 5(b). However the beam quality parameter M2 was measured to be below 1.2 even at the laser output power of 8 W.
4. Conclusion and outlook
A diode pumped femtosecond laser system based on CPA-MOPA configuration has been developed and demonstrated. The combined gain of dual slab active elements in the regenerative amplifier and spectral shaping before the amplifier allowed the amplification of sub 200 fs pulses. Besides of combined gain another advantage of dual-slab concept is a reduced thermal loading and reduced thermo-optical aberrations. To seed the regenerative amplifier by 110 fs pulses the master oscillator based on Np-cut crystal was developed. At repetition rate of 200 kHz the average output powers of 8.7 W corresponding to pulse energy of 43.5 μJ and 8.3 W corresponding to pulse energy of 41.5 μJ were achieved without and with spectral shaping, respectively, after compression and pulse picker. Reducing the repetition rate down to 50 kHz led to pulse energy of 160 μJ with the same pulse duration approximately. The beam quality M2 of output beam was below 1.2 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 micro-processing applications.
Power scaling of multi-slab configuration of regenerative amplifier is possible by adding another slabs aligned seriously in the cavity. We estimate that the laser system with two sets of dual-slab regenerative amplifier could produce the average output power in excess of 30 W.
This work was funded by the Seoul metropolitan government, Korea under contract of R&BD Program WR100001.
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