Scaling of broadband Ho:CALGO regenerative amplifier to multi-mJ pulse energy

We report on energy scaling of a 2.08-μm wavelength regenerative amplifier (RA) system based on the broadband gain material Ho:CaAlGdO4 (CALGO) to multi-mJ pulse energy at kHz repetition rates. Compared to previous reports, energy scaling was enabled thanks to an upgraded seed laser with a higher fluence and better spectral overlap to the gain spectrum of Ho:CALGO, which increased amplification efficiency. Bifurcation-free energy extraction was investigated experimentally and numerically for various repetition rates. A stable output was obtained at 10 W average power for repetition rates of 30 kHz and above. In addition, stable 3.4-mJ energy extraction was achieved at a 1-kHz repetition rate. We discuss the further scaling potential of pulse energy and pulse duration.

💡 Research Summary

The paper presents a comprehensive study on scaling the pulse energy of a 2.08 µm regenerative amplifier (RA) based on the broadband gain medium Ho:CaAlGdO₄ (CALGO). The authors address the limitations of previous implementations, which were constrained by a low‑fluence seed source and sub‑optimal spectral overlap with the Ho:CALGO gain spectrum, by introducing an upgraded seed laser and a detailed analysis of bifurcation‑free operation regimes.

Seed laser improvement
The previous seed, a Tm,Ho‑codoped CLNGG laser at 2093 nm delivering only 1.3 nJ, was replaced with a Tm:Lu₂O₃ ceramic oscillator operating at 2085 nm. This new seed provides 3.2 nJ per pulse, a 300 fs duration, and a 15 nm full‑width‑half‑maximum (FWHM) spectrum that aligns closely with the peak gain of Ho:CALGO (≈2077 nm). The higher fluence reduces the required number of round‑trips (RTs) in the RA from 28 to 21, thereby lowering the accumulated B‑integral and raising the bifurcation threshold.

CPA configuration
The amplified system follows a classic chirped‑pulse‑amplification (CPA) architecture. Pulses are stretched to ~500 ps using a Treacy grating pair (800 lines mm⁻¹) providing –72 ps² group‑delay dispersion. The gain medium is a Brewster‑cut 24.5 mm Ho(1 at.%):CALGO crystal pumped by a 1908 nm continuous‑wave Tm‑doped fiber laser (up to 100 W). After amplification, a Martinez compressor with identical gratings recompresses the pulses.

Numerical modeling of bifurcation
Using a spectrally resolved Frantz‑Nodvik model, the authors map the RA’s operating regimes as a function of pump intensity and repetition rate. Four regimes are identified: (1) bifurcation‑free, (2) early‑bifurcation with a high‑energy stable window, (3) pervasive bifurcation limiting operation to low energies, and (4) high‑energy extraction with bifurcation appearing only after the optimal RT count. Simulations predict that for pump powers up to ~90 W, bifurcation should be absent at repetition rates ≥30 kHz and also at low repetition rates (≈1 kHz) when the RT count is minimized.

Experimental results – high‑repetition‑rate operation
At 30 kHz, 40 kHz, and 50 kHz the RA delivers 10 W average power after 26 RTs, corresponding to pulse energies of 333 µJ, 250 µJ, and 200 µJ, respectively. The output spectra are centered at 2085 nm with a 7.9 nm bandwidth, and compressed pulse durations are 970 fs (30 kHz, 40 kHz) and 950 fs (50 kHz). Pulse‑to‑pulse energy fluctuations are measured at ~2 %, confirming stable operation without observable bifurcation. The power scaling curve begins to saturate above 85 W pump power, likely due to approaching the damage threshold of the crystal.

Experimental results – kilohertz operation
Operating at 1 kHz, the model suggests an optimal RT count of 14. With this configuration the system reaches 3.4 W average power at 57 W pump power, yielding 3.4 mJ per pulse. The compressed pulse duration, measured by SH‑FROG, is 1.23 ps (transform‑limited 1.05 ps), and the compressor transmission is 75 % (limited by grating efficiency). Attempts to exceed 3.4 mJ resulted in crystal damage, indicating that further scaling will require improved thermal management or additional amplification stages.

Discussion and outlook
The work demonstrates that Ho:CALGO, with its intrinsically broad and flat gain spectrum and high thermal conductivity, can support both high‑average‑power (10 W) and high‑pulse‑energy (multi‑mJ) operation in the 2‑µm region. Critical to this achievement are (i) a seed source whose wavelength and fluence are matched to the gain peak, (ii) careful control of the number of RTs to balance gain extraction against accumulated nonlinear phase, and (iii) operation within the bifurcation‑free or high‑energy stable regimes identified by the numerical model. Future improvements could involve cryogenic cooling of the Ho:CALGO crystal, implementation of additional booster amplifiers, or scaling the pump power beyond 100 W, potentially enabling >10 mJ pulse energies and sub‑picosecond durations suitable for advanced applications such as soft‑X‑ray high‑harmonic generation, high‑field THz generation, and precision bulk processing of wide‑bandgap semiconductors.