Open Access
Add to BibSonomy
Abstract
The pulse energy and average power are two long-sought parameters of femtosecond lasers. In the fields of nonlinear-optics and strong-field physics, they respectively play the role to unlock the various nonlinear processes and provide enough photon fluxes. In this paper, a high-energy and high-power Yb:CALGO regenerative amplifier with 120 fs pulse width is reported. This high-performance regenerative amplifier can work with high stability in a large tuning range of repetition rates. Varying the repetition rate from 3 to 180 kHz, the maximum output power of 36 W and the pulse energy up to 4.3 mJ, corresponding to a peak power of more than 20 GW are demonstrated. The output beam is near diffraction limited with M2 = 1.09 and 1.14 on the horizontal and vertical directions, respectively. In addition, multi-plate compression is employed to achieve 30 fs output with 23 W average power which is attractive for applications such as high-harmonic generation.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The last two decades have witnessed the fast development of ultrafast lasers with extreme parameters. The pulse energy and average power are crucial for exploring new phenomena in the fields of nonlinear optics and strong-field physics. For example, for lasers with ultra-short pulses, a high pulse energy which means high peak power is demanded to enhance the efficiency for driving optical parametric amplifiers [1] and filamentation in air [2]. Advanced applications such as high-harmonic generation [3] and attosecond pulse generation [4] have recently raised more stringent demands on both the pulse energy (i.e., peak power) and average power so that the strong-field cut-off and decent photon fluxes for the subsequent measurement could be obtained simultaneously.
While titanium-doped sapphire (Ti:sapphire) lasers have been the workhorse in the fields of nonlinear optics and strong-field physics, ultrafast lasers based on Yb-doped crystals are exhibiting strong competitiveness coming from behind for the superior power scaling capability thanks to the small quantum defects and the availability of high-power laser diodes as the pump source [5–7]. Among a number of Yb-doped crystals, Yb:CaAlGdO4 (Yb:CALGO) has the broadest emission bandwidth (Δλe ∼80 nm) [8] and remarkable thermal conductivity (6.9 W·m−1·K−1 and 6.3 W·m−1·K−1 along the a- and c-axis, respectively) [9,10], therefore becomes an ideal candidate for the next generation femtosecond laser with high energy and high power. In 2013, Caracciolo et al. demonstrated 36 W output (before compression) with 217 fs pulse width at 500 kHz repetition rate in a Yb:CALGO regenerative amplifier [11]. Subsequently, by the same group the pulse energy was boosted to 1.13 mJ with a pulse width of 380 fs, which generates a peak power of 3 GW [12]. At a lower repetition rate of 1 kHz, the pulse energy was increased to 3 mJ through a dual-crystal design [13]. Besides the pulse energy and average power, the ultra-short pulse width is another crucial parameter to pursue. Sub-100 fs pulse width was demonstrated for the first time in a Yb:CALGO regenerative amplifier by the nonlinear amplification [14]. More recently, with a designed spectral shaper to compensate the gain narrowing, we reported a Yb:CALGO regenerative amplifier with 95 fs amplified pulse width and 12.5 W and 2.45 GW average and peak powers, respectively [15]. Despite the rapid development of the Yb:CALGO amplifier, a multi-mJ of pulse energy and multi-tens-W of average power still could not be achieved from the same laser system to unlock the various nonlinear processes and provide enough photon fluxes. In addition, multi-tens GW of peak power from Yb:CALGO regenerative amplifiers is highly demanded by applications such as driving parametric amplifiers and filamentation in air. To generate even shorter pulses, nonlinear compression through hollow-core fiber compressor [16,17] has been routinely used. Multi-pass cell is another emerging technique with high compression efficiency [18,19]. On the other hand, multi-plate compressor attracts special attention for its traits of high compression efficiency, alignment insensitivity and economical apparatus. Recently, 30 fs, 0.8 mJ pulses from a Ti:sapphire laser were compressed to 5.4 fs with a total efficiency of 85% [20]. For longer pulses from Yb-doped lasers, 22 fs pulse width is obtained from 170 fs pulses via multi-plate compression with 85% efficiency [21].
In this letter, we demonstrate a multi-mJ and multi-tens-W Yb:CALGO regenerative amplifier with 120 fs pulse width. This regenerative amplifier works with high stability in a large tuning range of repetition rates from 3 to 180 kHz. Specifically, 36 W average power (before compression) is obtained at 72 kHz repetition rate, while a pulse energy of 4.3 mJ corresponding to a peak power of 20 GW is realized by reducing the repetition rate to 3 kHz. It is worth mentioning that both the pulse energy and average power are the highest in the Yb:CALGO lasers, to the best of our knowledge, and the peak power is boosted by more than 6 times compared to the previous record of the regenerative amplifier based on Yb:CALGO [12]. The output beam is near diffraction-limited with M2 = 1.09 and 1.14 on the horizontal and vertical directions, respectively. The output pulses are further compressed to 30 fs via multi-plate nonlinear compression. 23 W output power after the compression stage with nearly no spatial chirp is obtained. In our opinion, the demonstrated high-energy and high-power Yb:CALGO laser system with ∼30 fs pulse width provides an alternative technique of Ti:Sapphire and Yb:YAG lasers for the nonlinear optics and strong-field physics applications.
2. High power Yb:CALGO regenerative amplifier
As shown in Fig. 1, the Yb:CALGO regenerative amplifier is seeded by pulses from a customized broadband fiber oscillator (Yacto-FL-Ultra), which has a full width at half-maximum (FWHM) spectral bandwidth of 35 nm. The oscillator delivers constant 1 nJ pulses with a tunable repetition rate. Before injecting into the amplifier, the seed pulses are stretched by a Martinez stretcher to ∼300 ps with an efficiency of ∼80%. The strong gain narrowing in the regenerative amplifier is compensated by a specially designed notched spectral shaper. The reflectivity of the dielectric spectral shaper has a dip of 20% centered at 1040 nm, and 80% of attenuation is achieved in the spectral center after seven-bounces reflection. As the optimal pump power varies with the repetition rate, a symmetric cavity is designed to desensitize the amplifier to the variation of thermal lensing under different pump power. The thermal lensing is determined by comparing the simulated cavity parameters (with rezonator-2.0.5) using an estimated thermal lensing with the measured cavity parameters. Thus, the laser can maintain a good beam shape and stable output power under different pump power.
Fig. 1. Schematic of the high power laser system includes laser front, Yb:CALGO regenerative amplifier, Treacy compressor and multiple-plate nonlinear compressor. The laser front consists of a broadband fiber oscillator, a Martinez stretcher. PBS, polarization beam splitter; FR, Faraday rotator; HWP, half-wave plate; PC, Pockels cell; CM, 500 mm concave mirror, the tilt angle of concave mirrors is 7°, QWP, quarter-wave plate; DM, dichroic mirror; LD, laser diode.
The gain medium of the regenerative amplifier is a 5×5×8 mm3 antireflection (AR)-coated Yb:CALGO crystal with 1.5% doping concentration and a-cut orientation, which is mounted on a water-cooled copper holder soldered with indium foil. A Pockels cell with double beta-barium borate (BBO) crystals (overall length of 40 mm) and a quarter-wave plate are used as a polarization switch to trap and extract the pulses. The distances between the concave mirrors to laser crystal and end mirrors are 680 mm and 750 mm, respectively. The total cavity length is designed to be 2.5 m, giving a round trip time of 17 ns which is greater than the response time of the Pockels cell (8 ns). In the pump side, the laser diode (Coherent, Inc.) provides up to 250 W pump power at 980 nm out of a 200 µm fiber, and the pump beam is split into two beamlines before focusing into the crystal. Double-end pumping could reduce the possibility of crystal damage and inhibit the gain narrowing effect during amplification [22]. The beam diameters of the pump and signal on the Yb:CALGO crystal is ∼600 µm and ∼500 µm respectively, which are designed to ensure a good gain performance and mitigate the risk of damage at high pulse energy. To decrease the accumulated nonlinear phase shift during amplification the beam diameter is enlarged to ∼2 mm on the Pockels cell. The amplification roundtrips are varying from 100 to 120 depending on the repetition rate. The amplified laser output is characterized by an optical spectral analyzer (Yokogawa AQ6370D), a power meter (Ophir FL250A-BB-50), and a beam profiler (Dataray WinCamD). The temporal profiles of the compressed pulses are characterized by a commercial second-harmonic generation frequency-resolved optical gating (SHG-FROG) setup (Mesa Photonics).
Figure 2 presents the output characteristics of the regenerative amplifier. As shown in Fig. 2(a), varying the repetition rate from 3 to 180 kHz, the maximum pulse energy up to 4.3 mJ before compression is obtained at 3 kHz. The output power tends to saturate at 36 W when the repetition rate is greater than 70 kHz. It is worth mentioning that when the pulse energy reaches 4 mJ, the accumulated B-integral in the laser cavity is calculated to be 3.5, and modulations to the amplified spectrum are observed. Thus, further stretching the pulses to ∼1 ns by using larger gratings could relieve the nonlinear phase accumulation. In principle, there is no technical obstacle to raise the output pulse energy to ∼10 mJ in the demonstrated Yb:CALGO regenerative amplifier. The amplified output power as a function of the absorbed pump power under three different repetition rates are shown in Fig. 2(b). 13 W, 25 W and 36 W output power are obtained at 3 kHz, 10 kHz and 100 kHz repetition rate, respectively, which verifies the insensitivity of the designed symmetric cavity to the variation of thermal loads. As the repetition rate increases, the laser efficiency raises from 7.2% to 16%, and tends to saturate when the repetition rate exceeds 70 kHz.
Fig. 2. (a) The amplified output power and pulse energy in the Yb:CALGO regenerative amplifier at different repetition rates. The corresponding pump power varies from 180 W to 240 W when the repetition rate increases from 3 kHz to 180 kHz. The inset images show the beam profiles under different repetition rates. (b) The amplified output power as a function of the absorbed pump power at 3 typical repetition rates. (c) The retrieved spectral intensity and phase of the compressed pulses, compared to the spectrum independently measured using a spectral analyzer. (d) The 120 fs retrieved and the 100 fs transform-limited (TL) temporal profiles from the FROG measurement.
After amplification, the beam diameter is enlarged to 3.5 mm so as not to damage the gratings in compressor. The amplified pulses are then sent to a Treacy compressor with an overall efficiency of 80%. The measured and retrieved spectra have a good agreement as presented in Fig. 2(c). The temporal profile of the output pulses at 5 kHz with 2.9 mJ pulse energy after compression is characterized by SHG-FROG as shown in Fig. 2(d). The compressed pulse width is 120 fs which is slightly longer than the 100 fs transform-limited (TL) pulse width. Unsuppressed pedestals hold ∼16% of the total energy from the measured pulses, originating from the uncompensated third-order dispersion from the regenerative amplifier. Notably, 20 GW peak power is obtained which is more than 6-fold enhancement compared to the previous record of Yb:CALGO regenerative amplifier [12].
We have typically operated the regenerative amplifier for half an hour at 80 kHz and 10 kHz, which generates an average power (a pulse energy) of 36 W (0.5 mJ) and 18 W (3.6 mJ) before compression, respectively. Figure 3(a) shows the power stability under two different repetition rates. Within half an hour, the root-mean-square (RMS) of the ∼27,000 consecutive points is ∼0.2% and 0.1% for 36 W and 18 W output, respectively. With the symmetric cavity design, the beam ellipticity distortion at high output power is significantly mitigated. The M-square (M2) factor of the output beam with 36 W average power is characterized as shown in Fig. 3(b), revealing M2 = 1.09 and M2 = 1.14 on the horizontal and vertical axes, respectively. A good Gaussian far field beam as the typical output beam profile is displayed in the inset of Fig. 3(b).
Fig. 3. (a) The measured output power over 30 min of continuous operation at 80 kHz (36 W) and 10 kHz (18 W). (b) M2 measurement of the amplified beam with an output power of 36 W at 80 kHz repetition rate in the horizontal and vertical axes. The inset image shows the beam profile at focus.
3. Multi-plate nonlinear compression
Many advanced applications such as high-harmonic generation require even shorter pulses from high power regenerative amplifier. Here multi-plate nonlinear compressor is designed for a four-fold pulse compression. In this experiment, the regenerative amplifier is operated at a moderate repetition rate of 50 kHz with an average power of 28 W. The nonlinear compression setup is shown in Fig. 4. It consists of six periods of fused silica plates with a varying thickness from 0.1 mm to 0.4 mm for the spectral broadening. The fused silica plates are placed at the Brewster angle to suppress the reflection loss. The location of each plate is optimized for achieving the broadest spectrum and at the same time avoiding the filamentation phenomenon. The beam from the regenerative amplifier is loosely focused by a lens with 1-m focal length. Without the silica plates, the beam diameter at the focus is measured as 500 µm. The first plate is put 150 mm before the linear focus, and the beam diameter on the first plate is measured as 0.8 mm which results in an intensity density of 0.92 TW/cm2. The distances between each two adjacent plates are 60 mm, 55 mm, 50 mm, 50 mm, and 40 mm, respectively. The thickness of the first plate is chosen as 100 µm. While in the process of spectral broadening, the peak power decreases due to the accumulation of dispersion, which limits the spectral broadening in subsequent plates. As the compensation of the declined peak power, the thickness of the silica plates is gradually increased as shown in Fig. 4. After the six fused silica plates, the output beam is subsequently collimated and sent to chirped mirror pairs which supplies a total negative dispersion of -1200 fs2 over the spectral range of 850-1200 nm.
Fig. 4. Schematic of the experimental setup of the multi-plate nonlinear compressor. The focusing lens (L1) has the focal length of f = 1000 mm. The chirped mirrors are from PC1611, Ultrafast Innovations.
Figure 5(a) shows the spectra after each plate in the multi-plate nonlinear compressor. After the sixth plate, a spectrum covering 975 to 1100 nm at -20 dB, supporting a 25 fs TL pulse is manifested. The temporal profile after chirped mirror pairs is characterized again by SHG-FROG as shown in Figs. 5(b)-(d). The pulse width is 30 fs which is very close to the TL pulse width. The main peak of the pulses contains 88% of the total energy. The total transmission efficiency is ∼83%, which includes ∼15% loss from fused silica plates and ∼2% loss from chirped mirrors. After the multi-plate compressor, an average power of 23 W is obtained. The spatial chirp of the output beam after nonlinear compression is characterized, as shown in Fig. 6(a). The spectral shape remains nearly identical, when scanning the beam across both axes, which indicates there is almost no spatial chirp resulting from the multi-plate compressor. The beam quality factor is measured to be M2 = 1.15 × 1.38 which is shown in Fig. 6(b).
Fig. 5. (a) The measured spectra after different numbers of fused silica plates. (b) Reconstructed temporal intensity (red curve) and transform limited pulse (black curve). The TL pulse width is 25 fs while the measured pulse width is 30 fs. The pedestal holds about 12% of the total energy. The measured (c) and retrieved (d) FROG traces of the 30 fs pulse. The FROG error is 0.9%.
Fig. 6. (a) The measured spectra at different spatial positions which are indicated by the five stars across the beam as shown in the inset. (b) Beam quality measurement after nonlinear compression.
4. Conclusion
In conclusion, we demonstrated a high-energy high-power Yb:CALGO regenerative amplifier with 120 fs pulse width and peak power exceeding 20 GW. By designing a symmetric cavity, the regenerative amplifier could maintain a stable output in a broad repetition rate tuning range. Varying the repetition rate from 3 to 180 kHz, the maximum output power and pulse energy of 36 W and 4.3 mJ is demonstrated, respectively. Further scaling up of the pulse energy and peak power to 10 mJ and 50 GW, respectively is straightforward by stretching the pulses to ∼1 ns with larger gratings. In addition, multi-plate nonlinear compression based on fused silica plates is employed to further compress the output pulses to 30 fs. 23 W output power at 50 kHz repetition rate is obtained with 83% compression efficiency, which could serve as a new driving source for high-harmonic generation with high photon fluxes.
Funding
National Natural Science Foundation of China (62075144); Engineering Featured Team Fund of Sichuan University (2020SCUNG105).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. G. Cerullo and S. De Silvestri, “Ultrafast optical parametric amplifiers,” Rev. Sci. Instrum. 74(1), 1–18 (2003). [CrossRef]
2. A. Couairon and A. mysyrowicz, “Femtosecond filamentation in transparent media,” Phys. Rep. 441(2-4), 47–189 (2007). [CrossRef]
3. R. Klas, A. Kirsche, M. Gebhardt, J. Buldt, H. Stark, S. Hädrich, J. Rothhardt, and J. Limpert, “Ultra-short-pulse high-average-power megahertz-repetition-rate coherent extreme-ultraviolet light source,” PhotoniX 2(1), 4 (2021). [CrossRef]
4. J. Li, J. Lu, A. Chew, S. Han, J. Li, Y. Wu, H. Wang, S. Ghimire, and Z. Chang, “Attosecond science based on high harmonic generation from gases and solids,” Nat. Commun. 11(1), 2748 (2020). [CrossRef]
5. Y. Wang, H. Chi, C. Baumgarten, K. Dehne, A. R. Meadows, A. Davenport, G. Murray, B. A. Reagan, C. S. Menoni, and J. J. Rocca, “1.1 J Yb:YAG picosecond laser at 1 kHz repetition rate,” Opt. Lett. 45(24), 6615 (2020). [CrossRef]
6. B. E. Schmidt, A. Hage, T. Mans, F. Légaré, and H. J. Wörner, “Highly stable, 54 mJ Yb-InnoSlab laser platform at 0.5 kW average power,” Opt. Express 25(15), 17549 (2017). [CrossRef]
7. T. Nubbemeyer, M. Kaumanns, M. Ueffing, M. Gorjan, A. Alismail, H. Fattahi, J. Brons, O. Pronin, H. G. Barros, Z. Major, T. Metzger, D. Sutter, and F. Krausz, “1 kW, 200 mJ picosecond thin-disk laser system,” Opt. Lett. 42(7), 1381 (2017). [CrossRef]
8. H. Wang, J. Pan, Y. Meng, Q. Liu, and Y. Shen, “Advances of Yb:CALGO Laser Crystals,” Crystals 11(9), 1131 (2021). [CrossRef]
9. P. Loiko, F. Druon, P. Georges, B. Viana, and K. Yumashev, “Thermo-optic characterization of Yb:CaGdAlO4 laser crystal,” Opt. Mater. Express 4(11), 2241 (2014). [CrossRef]
10. Frédéric Druon, Mickaël Olivier, Anaël Jaffrès, Pascal Loiseau, Nicolas Aubry, Julien DidierJean, François Balembois, Bruno Viana, and Patrick Georges, “Magic mode switching in Yb:CaGdAlO4 laser under high pump power,” Opt. Lett. 38(20), 4138 (2013). [CrossRef]
11. E. Caracciolo, M. Kemnitzer, A. Guandalini, F. Pirzio, J. Aus der Au, and A. Agnesi, “28-W, 217 fs solid-state Yb:CAlGdO4 regenerative amplifiers,” Opt. Lett. 38(20), 4131 (2013). [CrossRef]
12. E. Caracciolo, M. Kemnitzer, A. Guandalini, F. Pirzio, A. Agnesi, and J. Aus Der Au, “High pulse energy multiwatt Yb:CaAlGdO4 and Yb:CaF2 regenerative amplifiers,” Opt. Express 22(17), 19912 (2014). [CrossRef]
13. A. Calendron, “Dual-crystal Yb:CALGO high power laser and regenerative amplifier,” Opt. Express 21(22), 26174 (2013). [CrossRef]
14. J. Pouysegur, M. Delaigue, Y. Zaouter, C. Hönninger, E. Mottay, A. Jaffrès, P. Loiseau, B. Viana, P. Georges, and F. Druon, “Sub-100-fs Yb:CALGO nonlinear regenerative amplifier,” Opt. Lett. 38(23), 5180 (2013). [CrossRef]
15. W. Wang, H. Wu, C. Liu, B. Sun, and H. Liang, “Multigigawatt 50 fs Yb:CALGO regenerative amplifier system with 11 W average power and mid-infrared generation,” Photonics Res. 9(8), 1439 (2021). [CrossRef]
16. T. Nagy, S. Hädrich, P. Simon, A. Blumenstein, N. Walther, R. Klas, J. Buldt, H. Stark, S. Breitkopf, P. Jójárt, I. Seres, Z. Várallyay, T. Eidam, and J. Limpert, “Generation of three-cycle multi-millijoule laser pulses at 318 W average power,” Optica 6(11), 1423 (2019). [CrossRef]
17. G. Fan, P. A. Carpeggiani, Z. Tao, G. Coccia, R. Safaei, E. Kaksis, A. Pugzlys, F. Légaré, B. E. Schmidt, and A. Baltuška, “70 mJ nonlinear compression and scaling route for an Yb amplifier using large-core hollow fibers,” Opt. Lett. 46(4), 896 (2021). [CrossRef]
18. P. Russbüldt, J. Weitenberg, J. Schulte, R. Meyer, C. Meinhardt, H. D. Hoffmann, and R. Poprawe, “Scalable 30 fs laser source with 530 W average power,” Opt. Lett. 44(21), 5222 (2019). [CrossRef]
19. C. Grebing, M. Müller, J. Buldt, H. Stark, and J. Limpert, “Kilowatt-average-power compression of millijoule pulses in a gas-filled multi-pass cell,” Opt. Lett. 45(22), 6250 (2020). [CrossRef]
20. P. He, Y. Liu, K. Zhao, H. Teng, X. He, P. Huang, H. Huang, S. Zhong, Y. Jiang, S. Fang, X. Hou, and Z. Wei, “High-efficiency supercontinuum generation in solid thin plates at 0.1 TW level,” Opt. Lett. 42(3), 474 (2017). [CrossRef]
21. S. Zhang, Z. Fu, B. Zhu, G. Fan, Y. Chen, S. Wang, Y. Liu, A. Baltuska, C. Jin, C. Tian, and Z. Tao, “Solitary beam propagation in periodic layered Kerr media enables high-efficiency pulse compression and mode self-cleaning,” Light: Sci. Appl. 10(1), 53 (2021). [CrossRef]
22. P. Raybaut, F. Balembois, F. Druon, and P. Georges, “Numerical and experimental study of gain narrowing in ytterbium-based regenerative amplifiers,” IEEE. J Quantum Electron. 41(3), 415–425 (2005). [CrossRef]
