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Amplification of 10-ns laser pulses to an energy of 500 mJ at a 10-Hz repetition rate in a cryogenic multi-pass multi-total-reflection-active-mirror (multi-TRAM) amplifier was achieved. By using a multi-TRAM, which is a YAG ceramic composite with three Yb:YAG active layers, a maximum single-pass gain of 12 and a total storage energy of 1.5 J were obtained.
© 2014 Optical Society of America
1. Introduction
Transparent ceramics technology has opened a new possibility for solid-state lasers to deliver both high energy and a high repetition rate [1,2]. In particular, Yb:YAG ceramics have appropriate saturation fluence and good thermal properties at low temperatures, and can therefore be used as a suitable medium for the efficient amplification of picosecond to nanosecond pulses [3]. High-energy ceramic lasers in the >10-J class have been achieved by using power-scalable architectures, such as multi-slab [4] and active mirrors [5].
The total-reflection-active-mirror (TRAM) architecture has been proposed and demonstrated as an active-mirror design for high-average-power and high-pulse-energy lasers [6–11]. A TRAM is a monolithic ceramic undoped YAG prism with a thin Yb:YAG active layer. An input beam is refracted at a facet of the prism, and is then reflected by total internal reflection at the active layer; finally, the amplified beam exits from another facet. The prism provides some advantages over conventional active-mirror lasers. (1) In pulsed operation, a reduction in surface damage threshold as a result of spatial overlap of the input and output beams can be avoided. (2) By removing the highly reflective multilayer coating on the back of the active layer, heat transfer to the heat sink can be improved. (3) Deformation of the active layer is suppressed, and direct cooling with liquid nitrogen is possible. (4) To avoid reduction of the surface damage threshold, an anti-reflective coating can be omitted by using the Brewster angle as the angle of incidence.
As the size of the beam increases as a result of scaling to higher pulse energies, amplified spontaneous emission (ASE) and parasitic oscillations emerge and limit the output pulse energy. Therefore, a multi-TRAM has been proposed [12]. The multi-TRAM is a monolithic ceramic with one undoped YAG tetragonal prism and three thin Yb:YAG active layers. This allows lowering the gain in each of the active layers, thereby decreasing ASE and improving energy storage efficiency while keeping the overall gain constant. In this paper, we report the first demonstration of laser amplification of nanosecond pulses to sub-joule energy using the multi-TRAM architecture.
2. Experimental setup
Figure 1 shows a schematic of the multi-TRAM used for the experiment. It consists of an undoped YAG tetragonal prism and three Yb:YAG active layers. Yb dopant concentrations in the active layers Yb1, Yb2, and Yb3 are 2, 5, and 2 at.%, respectively. Each active layer has dimensions of 60 mm × 15 mm × 0.8 mm. The input beam is incident to an uncoated prism surface at an angle of 60°, which is near the Brewster angle for YAG. The clear aperture of the beam is 10.8 mm × 15 mm (width × height). The multi-TRAM was attached to a copper block to allow conductive cooling from the surfaces of the active layers. Indium sheets were used for thermal contact between the ceramic and the copper. The copper block was cooled to 77 K by liquid nitrogen in a vacuum chamber.
The laser system consisted of a continuous-wave (CW) fiber laser oscillator, an electro-optic modulator, a fiber amplifier, a cryogenic TRAM regenerative amplifier, and a cryogenic multi-TRAM multi-pass amplifier. The configuration is illustrated in Fig. 2.The CW fiber oscillator provides a single-frequency output that has power of 10 mW and is tunable between 1029 and 1030 nm. It was tuned to the wavelength with the highest gain in Yb:YAG cooled to 80 K (1029.4 nm). The output was modulated by the fiber-based electro-optic modulator that produced 10 ns pulses at a repetition rate of 100 kHz. These pulses were amplified in the fiber amplifier to a pulse energy of 1 nJ. After passing through Faraday isolators, the pulses were injected into the regenerative amplifier by a Pockels cell. The regenerative amplifier was operated at a repetition rate of 10 Hz and amplified pulses to 10 mJ. The output beam had a Gaussian profile with a diameter of 3.6 mm at 1/e2 of maximum intensity.
The beam propagated in free space and was injected into the four-pass amplifier, where the beam diameter increased to 5 mm. A schematic layout of the four-pass amplifier is shown in Fig. 3.The beam profile at the center of the multi-TRAM was relay-imaged to the next pass of the amplification by using two lenses with a focal length of 1000 mm. In the vertical plane, the optical paths to the TRAM for each pass have slightly different angular directions in order to obtain separation of the input and output paths of the beam, as shown in Fig. 4.The separation angle is about 0.5°. Two 5-kW fiber-coupled laser diode modules (Hamamatsu Photonics K.K.) were used for pumping the Yb:YAG active layer. They were operated in QCW mode with a pulse duration of 1 ms and a repetition rate of 10 Hz. The center wavelength was measured to be 938 nm at maximum power. The output fiber of the laser diode had a core diameter of 1 mm. The pump beam was collimated to be 7 mm in diameter at the first layer of the multi-TRAM.
Fig. 3 Schematic layout of the multi-TRAM multi-pass amplifier pumped by two fiber-coupled laser diode modules (LD1 and LD2); the components labeled DBS are dichroic beam splitters.
Fig. 4 Schematic side view of the beam propagation in the multi-TRAM multi-pass amplifier; the numbers indicate the sequence of passes.
3. Results
Firstly, the small-signal gain was measured for single-pass amplification. The beam from the fiber amplifier in CW mode was expanded to fill the pumped region. After one pass through the multi-TRAM, the beam was focused onto a photodiode by a lens. The results of the gain measurements are shown in Fig. 5(a).The graph shows no signs of saturation caused by ASE or parasitic oscillations. The maximum small-signal gain for single-pass amplification was measured to be approximately 12. The maximum storage energy was estimated to be 1.5 J, assuming a saturation fluence of about 1.5 J/cm2. The measurement was then repeated for four passes through the amplifier (Fig. 5(b)). The beam from the CW fiber amplifier was relay-imaged through the amplifier. The small-signal gain shows saturation at higher pump intensities. A likely reason for this is parasitic oscillations caused by reflections on the optics used for the relay imaging. Therefore, we limited the maximum pump power to 4 kW in the present experiment. For 4 kW pumping, the storage energy was estimated to be 1.3 J.
Fig. 5 Small-signal gain of the multi-TRAM after (a) a single pass and (b) four passes with relay imaging.
After these measurements, 10-ns pulses with a repetition rate of 10 Hz produced by the regenerative amplifier were amplified by the multi-TRAM. The input energy to the multi-TRAM amplifier was 10 mJ, and the output energy reached 500 mJ, as shown in Fig. 6. Figure 7 shows an output profile of a beam relay-imaged from the gain medium. The output beam had a Gaussian profile with a diameter of 4.9 mm at 1/e2 of maximum intensity. The energy extraction efficiency was limited to about 40% due to the small input beam size, which is constrained by the size of the clear aperture of the multi-pass system. The multi-pass amplifier supports beam sizes up to 7 mm.
4. Conclusion
We have demonstrated nanosecond-pulse amplification using a multi-TRAM. The monolithic multi-TRAM, which has three Yb:YAG active layers, provided a significant improvement in energy storage capability. The output pulse energy was limited in this experiment by the small size of the input beam, but could be increased to more than 1 J by enlarging the beam.
Acknowledgments
This research has been partially supported by Grants-in-aid for Scientific Research (No. 26287145) of MEXT of Japan.
References and links
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