High-Power and High-Energy Short Pulse Lasers

Over the last decade, optical parametric chirped-pulse amplification (OPCPA) [1] has been developed to directly generate few-cycle high-energy pulses as an alternative method to chirped-pulse amplification (CPA) based on a conventional laser amplification that is limited by the gain bandwidth of the laser material. In OPCPA a femtosecond pulse comprising only a few cycles of light is stretched to picosecond or even nanosecond durations for high-energy amplification and then recompressed to the original pulse duration, which enables the generation of high-intensity laser pulses for high-field physics experiments. Parametric amplification has several distinct advantages over conventional laser amplification: ultrabroadband phase matching can be achieved with reduced gain narrowing, it is wavelength agile, and it leads to low accumulated nonlinear phase and high contrast ratio. For these reasons, OPCPA has been intensively investigated over the past 10 years.

Our group in collaboration with the group of Giulio Cerullo at Politecnico Milano has demonstrated the generation of three cycle optical pulses at 800 nm and 2 μm wavelengths using OPCPA [2-4]. Most recently, we have coherently combined the output of these two OPCPAs when seeded with CEP-controlled pulses to generate sub-cycle optical waveforms later to be used for high-order harmonic generation and attosecond pulse generation [5]. Figure 1 shows the schematic layout of the high-energy optical waveform synthesizer.

Fig. 1. Two CEP- stabilized, few-cycle OPCPAs centered at different wavelengths are combined based on the concept of coherent wavelength multiplexing to produce a fully controlled non-sinusoidal optical waveform with 15-μJ pulse energy at 1-kHz repetition rate. Full control over the optical phase allows for any optical waveform given the amplified spectrum. YDFA: Ytterbium-doped fiber amplifier; BPF: bandpass filter.

The OPCPAs are seeded from a common octave-spanning, carrier-envelope phase-controlled, 5-fs Ti:sapphire laser. The combined spectrum is shown in Fig. 2 (A) and spans over 1.8 octaves, and has the capability of producing 2.5-fs, 0.6-cycle pulses. The phases of each spectral component is controlled by the AOPDFs, such that any waveform supported by the spectrum can be generated. A feedback loop based on a balanced optical cross-correlator (BOC) [5] is implemented to synchronize the two pulses, allowing attosecond-precision relative timing stability. With the feedback control on over a bandwidth of 30 Hz, the relative timing drift between the two output pulses is reduced to 250 as, less than 5% of the oscillation period of the SWIR-OPCPA (7.2 fs). Figure 2B and 2C show the group delay over each pulse measured by two dimensional spectral shearing (2DSI) [6]. The 2DSI measurement shows that the two OPCPA pulses are temporally overlapped and each is well compressed to within 10% of its transform-limited pulse duration. Figures 2D and 2E demonstrate the CEP stability of the two individual pulses, with r.m.s. fluctuations as low as 135 mrad and 127 mrad, respectively. Figure 2F plots a synthesized electric-field waveform and intensity profile assuming the CEPs (φ₁=650mrad, φ₂=-750mrad) optimal for achieving the shortest high-field transient, which lasts only 0.8 cycles (amplitude FWHM) of the carrier (centroid) frequency (λ_c = 1.26 μm). The lower inset of Fig. 2F clearly shows that the synthesized electric-field waveform is non-sinusoidal and the main feature lasts less than an optical cycle. Once the output of each of the OPCPAs is scaled to mJ energy, the combined pulse can be used for isolated attosecond pulse generation in the EUV range.

Fig. 2 (A) Spectra of combined 800-nm and 2-micron OPCPA pulses, indicating power spectral density, and group delay, extracted from the measured two-dimensional spectral interferometry traces shown in (B) and (C). (D) 1f-2f and (E) 1f-3f interferograms indicating 130-mrad and 150-mrad CEP fluctuations for 800-nm and 2-micron OPCPA systems, respectively. (F) synthesized sub-cycle waveform with intensity shown on upper right inset and fit to the carrier wave, lower left inset.

Energy scaling of this and other sources critically depends on robust, high-power pump lasers. However, the requirements on pump sources for OPCPA are much more delicate than that of CPA because parametric amplification with high efficiency while maintaining good beam quality is only possible with good spatio-temporal characteristics of the pump beam. The development of a high-average-power picosecond pump source that can be synchronized with seed beams at either high or low repetition rate is therefore one of the most important challenges for future OPCPA technologies and their applications. Besides the OPCPA applications, the large-average-power picosecond lasers can be widely used for driving other nonlinear optical processes such as frequency conversion.

Over the last years, high-power high-repetition-rate picosecond laser technologies have been developed both on the basis of fiber and bulk amplifiers [7-9]. Cryogenically-cooled Yb:YAG lasers have proven to be an excellent high-power and also high-energy laser technology for average power scaling because of its good thermo-optic properties, small quantum defect, and low saturation fluence. At cryogenic temperatures (70 K), Yb:YAG has an emission bandwidth of 1.5 nm, suitable for picosecond pulse amplification. A high-power cw Yb:YAG laser with output powers up to >450 W [10] and a picosecond amplifier at tens of kHz with 24 W of average power [6] have been demonstrated.

To develop this technology for OPCPA pumping and other applications, we have teamed up with the group of T.Y. Fan from MIT Lincoln Laboratory. Using a cryogenically-cooled cw Yb:YAG amplifier developed at MIT Lincoln Laboratory, we demonstrated the amplification of 5.5-ps pulses at a repetition rate of 78 MHz to 287 W of average output power, which is one of the highest average power picoseconds pulse sources at MHz repetition rates ever demonstrated [11]. More recently we developed a cryogenically-cooled regenerative amplifier and power amplifier combination, that can generate 40mJ pulses at 2kHz repetition rate [12]. The system is shown in Figure 3.

Fig. 3 Layout of a high-energy picosecond laser system at kHz PRF: (a) Fiber seed source composed of a Yb-fiber laser, CFBG stretcher, and Yb-fiber pre-amplifier, (b) >5-mJ kHz cryogenic Yb:YAG RGA, (c) 40-mJ multipass cryogenic Yb:YAG amplifier, and (d) high-energy high-average-power pulse compressor based on MLD gratings. The path (1) represents the direct compression of the RGA output, while the paths (2) and (3) show double-pass and 4-pass amplification, respectively. PBS, polarization beamsplitter; λ/4, quarter waveplate; λ/2, half waveplate; F1029, FI, Faraday isolator; CFBG, chirped fiber Bragg grating; TFP, thin-film polarizer; PC, Pockels cell; L1-L4, lens; LD, fiber-coupled pump laser diode; DM, dichroic mirror; G, MLD diffraction grating; unspecified mirrors are high reflectors at given angles of incidence.

The amplification result is shown in Fig. 4. A maximum power of 80 W (40 mJ) at 2-kHz PRF is obtained at 9-W seed power from the RGA with a slope efficiency of 30% and ~320-W pump power. Currently, the output power is limited by the damage of dewar windows at ~85 W. The cw amplification results using 12-W and 6-W cw seed power (blue dotted and red dashed lines in Fig. 4) clearly indicate that the achievable output power is only limited by the damage threshold of dewar window and other amplifier optics.

Figure 2— Average power versus pump power in the double-pass amplifier. The slope efficiency is 30%. Optical damage is observed for output powers at ~85 W. The dotted blue and dashed red lines show the output power for 12-W and 6-W cw-seeds for comparison.

This laser system is a promising pump source for high-average-power OPCPA applications and work is currently underway to use it in pumping of the frequency synthesizer discussed above.

References

[1] Dubietis, R. Butkus, and A. Piskarskas, “Trends in chirped pulse optical parametric amplification,” IEEE JST QE 12, 163 (2006).

[2] J. Moses, S.-W. Huang, K.-H. Hong, O. D. Mücke, E. L. Falcao-Filho, A. Benedick, F. Ö. Ilday, A. Dergachev, J. A. Bolger, B. J. Eggleton, and F. X. Kärtner, “Highly stable ultrabroadband mid-infrared optical parametric chirped pulse amplifier optimized for superfluorescence suppression,” Opt. Lett. 34, 1639-1641 (2009).

[3] J. Moses, C. Manzoni, S-W Huang, G.Cerullo, and F. X. Kärtner, “Temporal Optimization of Ultrabroadband High-Energy OPCPA,“ Opt. Express 17:(7), pp 5540-5555 (2009).

[4] A. Siddiqui, G. Cirmi, D. Brida, F. X. Kärtner and G. Cerullo, “Generation of <7fs pulses at 800nm from a blue-pumped optical parametric amplifier at degeneraty,” Opt. Lett. 34:(22) pp.3592-3594 (2009).

[5] S-W. Huang, G. Cirmi, J. Moses, K-H. Hong, S. Bhardwaj, J. R. Birge, L-J. Chen, E. Li, B. J. Eggleton, G. Cerullo, and F. X. Kärtner, “ High-energy pulse synthesis with sub-cycle waveform control for strong-field physics,” Nat. Photonics 5, 477, (2011).

[6] J. R. Birge, R. Ell and F. X. Kärtner, “Two-dimensional spectral shearing interferometry for few-cycle pulse characterization” Optics Letters, 31, 2063 (2006).

[7] J. Limpert et al., Opt. Lett. 26(23), 1849 (2001); J. Limpert et al., Opt. Lett. 30(7), 714 (2005).

[8] P. Dupriez et al., IEEE Photon. Tech. Lett. 18(9), 1013 (2006).

[9] G. J. Spühler et al., Appl. Phys. B 71, 19 (2000); F. Brunner et al., Opt. Lett. 29(16), 1921 (2004); L. McDonagh et al., Opt. Lett. 32(10), 1259 (2007).

[10] T. Y. Fan et al., IEEE JST-QE 13(3), 448 (2007).

[11] K.-H. Hong, A. Siddiqui, J. Moses, J. Gopinath, J. Hybl, F. Ö. Ilday, T. Y. Fan, and F. X. Kärtner, “Generation of 287-W 5.5-ps pulses at 78-MHz repetition rate from a cryogenically-cooled Yb:YAG amplifier seeded by a fiber chirped-pulse amplification system,” Optics Letters 33:(21), pp. 2473-2475 (2008).

[12] K.-H. Hong, J. Gopinath, D. Rand, A. Siddiqui, S-W. Huang, E. Li, B. Eggleton, J. Hybl, T. Y. Fan, and F. X. Kärtner, “High-energy, kHz-repetition-rate, picosecond cryogenic Yb:YAG chirped-pulse amplifier,” Opt. Lett. 35:(11) pp.1752-1754 (2010).