1. Introduction

Within thin-disk laser technology the regenerative amplifier is one of the most suitable amplifier concepts. Especially for thin-disk amplifiers with their inherent low gain per reflection at the thin disk, regenerative amplification allows for compensating this relatively low gain by appropriately increasing the number of round trips. Most of the published regenerative thin-disk amplifiers run with pulse energy below or in the order of 10 mJ and kHz repetition rate. Even larger systems are operated at a maximum pulse energy of a few 10 milli-Joules [1,2]. For higher pulse energy or high CW output power multipass systems or single pass arrangements with a series of several amplifier heads have been reported [3–5]. Only very recently a regenerative thin-disk laser amplifier with output pulse energy of 220 mJ and 1 kHz repetition rate was demonstrated [6].

Most of these regenerative amplifiers have a relatively short resonator length and a small mode diameter. Consequently, the laser-induced damage threshold of the optical surfaces often limits the achievable pulse energy. To allow for higher pulse energy, the mode diameter has to be increased to such a degree that the peak power density is still below the damage threshold at all optical surfaces. We have enlarged the mode diameter by (a) increasing the resonator length and (b) using an appropriate combination of convex and concave mirrors. The diameter of the pump spot at the thin disk was increased accordingly. This resulted in an output energy in the range of 300 mJ.

2. Setup of the regenerative amplifier

Figure 1 shows the setup of the regenerative amplifier [7]. The seed pulse is coupled into the amplifier by means of a polarizer and a quarter-wave plate. A Pockels cell that can compensate the polarization change of the quarter-wave plate is used to close the resonator during the pulse gets amplified. To separate the input seed pulse from the amplified output pulse, the combination of a half-wave plate and a Faraday rotator is used. The Yb:YAG thin-disk serves as one of the end-mirrors. Due to the large radius of curvature of the amplifier disk (15 – 20 m), the beam shows only a low divergence in front of the disk. Therefore, the beam diameter keeps nearly constant in the part where the Pockels cell is positioned and where the in- and out-coupling of the laser pulse takes place.

 

Fig. 1 Setup of the thin-disk regenerative amplifier. For in- and out-coupling the polarizing elements (waveplates, polarizers, Pockels cell, Faraday rotator) are used. The Yb:YAG thin-disk serves as one end-mirror. The other end-mirror is motorized for remote controlled alignment. The combination of convex (M1, M4) and concave mirrors (M3, end-mirror) is used to achieve a relatively large beam diameter on the Yb:YAG thin-disk.

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The amplifier head with a 500 µm thick Yb:YAG (7%) thin-disk of 17 mm in diameter is a commercial component from Trumpf Laser GmbH. It is pumped by a fiber coupled laser diode module developed at the Ferdinand Braun Institute, Berlin [8,9]. The pump diode delivers up to 1.75 kW peak power (1.7 J in a 970 µs pulse) at 940 nm wavelength from a 1.2 mm fiber. The maximum duty cycle is 20%. The fiber exit is magnified onto the amplifier disk to a pump spot diameter of 5 mm, ensuring a pump power density above 8 kW/cm². In order to achieve a relatively large mode diameter, a combination of convex (M1, M4) and concave mirrors (M3, end mirror) in conjunction with a long beam path of 6.5 m was setup to form a stable resonator. This results in a diameter of the TEM-00 mode of 4 mm on the disk.

The amplifier is housed completely in a Plexiglas box that protects the beam path from dust particles and air turbulences. For remote-controlled realignment, it is sufficient to adjust the motorized end mirror (lower left corner of Fig. 1) in most cases.

3. Results

The regenerative amplifier is seeded with a chirped ~2 ns long laser pulse with about 0.3 nJ pulse energy from an homebuilt Yb:KGW oscillator. The oscillator delivers a 50 MHz pulse train. The pulse is stretched in a grating stretcher with a dispersion of about 1ns/nm. A pulse picker operating at 100 Hz selects the pulses to be amplified.

Due to the relatively large beam diameter on the amplifier disk the regenerative amplifier allows the amplification to a pulse energy of more than 300 mJ limited by the damage threshold of mirror M4 where the beam has its smallest diameter of ~1.2 mm. The total amplification amounts to ~10⁸ in this case. Gain narrowing during the amplification process reduces the pulse bandwidth to <1.1 nm. This effect shortens the duration of the chirped pulse to approximately 1 ns (FWHM). The final pulse energy is achieved at the maximum available pump power of 1.75 kW after 58 round trips. Figure 2 (inset) shows the dependence of the output energy on the pump power of the amplifier. At the maximum pulse energy the optical-to-optical efficiency amounts to 18%. Due to the limited switching contrast of the Pockels cell, there are a number of very small pre-pulses and one larger post-pulse separated by a multiple of the round-trip time (42 ns). The amount of energy in these pulses sums up to less than 2.5% of the measured pulse energy.

 

Fig. 2 Output pulse energy measured over a period of 2.5 h. The mean pulse energy is 306.6 mJ +/− 0.8 mJ (rms). The inset shows the output pulse energy depending on the applied pump power.

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To demonstrate the high stability of the amplifier, Fig. 2 shows the pulse energy measured over a period of 2.5 h (approximately 900000 individual laser pulses). The mean pulse energy is 306.6 mJ +/− 0.8 mJ (rms). This is a pulse energy fluctuation of only 0.25% (rms). The stability was obtained in a temperature-stabilized environment (+/− 0.1°C temperature stability) and clean room conditions (class 1000).

The beam quality measurement at this pulse energy showed a nearly diffraction limited beam with M² = 1.04 with a beam waist diameter of 3 mm. The focus of the beam generated by a f = 300 mm lens had a diameter of about 120 µm (4 σ). Figure 3 shows the full measurement of the beam caustic. The measurement was performed with a Spiricon M2-200s measurement system.

 

Fig. 3 The beam quality was measured to be M² = 1.04.

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In addition to the excellent beam quality also the pointing showed a high stability. The measurement was performed by focusing the laser beam with a diameter of ~4 mm (4 σ) on a camera using a f = 1 m lens. During a period of about half an hour the centroid position of the focus showed a fluctuation of only 2 µm (rms) corresponding to a beam pointing instability of 2 µrad (s. Fig. 4). The measurement was evaluated using the Spiricon BeamGage software. The camera was operated in free-running mode with an exposure time of 10 ms, corresponding to the temporal laser pulse separation at 100 Hz repetition rate.

 

Fig. 4 Beam pointing measurement at the focus position of a f = 1 m lens over a time interval of 27 min. The centroid position of the focus fluctuates by +/− 2 µm (rms).

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The laser pulses of the regenerative amplifier have been compressed to 1.8 ps (FWHM), close to the bandwidth limited pulse duration of 1.5 ps. Figure 5 shows the autocorrelation trace together with the spectral bandwidth of the laser pulses at maximum pulse energy. At this pulse energy, the compressor shows a transmission of more than 80% corresponding to more than 240 mJ pulse energy in the compressed pulse. The measurement shows no pre-pulses or pedestals even at an increased scan range of 150 ps. The fit for a Gaussian pulse shape results in a pulse duration of 1.8 ps (FWHM). The measurement was performed with an autocorrelator of type pulseCheck SM from A.P.E.

 

Fig. 5 Autocorrelation trace of the compressed laser pulse. The pulse duration of 1.8 ps (FWHM) is close to the Fourier-transform limit of 1.5 ps. The corresponding spectrum is shown in the inset.

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4. Summary

We have optimized a regenerative amplifier to amplify chirped pulses of 1 ns duration to more than 300 mJ pulse energy. Here the main effort was to setup a stable resonator with sufficient large mode diameter to prevent damage on the optical surfaces. The regenerative amplifier is based on Yb:YAG thin-disk technology. We have demonstrated its energy stability over a period of 2.5 h. During this time the amplifier shows a stable output of 306 mJ +/− 0.25% (rms). At this pulse energy a beam quality with M² = 1.04 was measured. The beam pointing stability amounts to 2 µrad (rms). The laser pulses can be compressed to 1.8 ps.

This regenerative amplifier presently serves as the pre-amplifier of a larger thin disk laser system. This system contains an additional ring amplifier that reaches a total pulse energy of >1 J [10].