5442
OPTICS LETTERS / Vol. 38, No. 24 / December 15, 2013
1.1 kW average output power from a thin-disk multipass amplifier for ultrashort laser pulses Jan-Philipp Negel,1,* Andreas Voss,1 Marwan Abdou Ahmed,1 Dominik Bauer,2 Dirk Sutter,2 Alexander Killi,2 and Thomas Graf1 1
Institut für Strahlwerkzeuge (IFSW), University of Stuttgart, Pfaffenwaldring 43, 70569 Stuttgart, Germany 2
TRUMPF Laser GmbH+Co. KG, Aichhalder Straße 39, 78713 Schramberg, Germany *Corresponding author: Jan‑[email protected]‑stuttgart.de
Received October 3, 2013; revised November 8, 2013; accepted November 11, 2013; posted November 15, 2013 (Doc. ID 198816); published December 13, 2013 We report on a thin-disk multipass amplifier for ultrashort laser pulses delivering an average output power of 1105 W. The amplifier was seeded by a Trumpf TruMicro5050 laser with a power of 80 W at a wavelength of 1030 nm, pulse duration of 6.5 ps, and repetition rate of 800 kHz. The energy of the amplified pulses is 1.38 mJ with a duration of 7.3 ps. The amplifier exhibits an optical efficiency of 44% and a slope efficiency of 46%. The beam quality was measured to be better than M 2 1.25. © 2013 Optical Society of America OCIS codes: (140.3280) Laser amplifiers; (140.3580) Lasers, solid-state; (140.3615) Lasers, ytterbium; (060.2370) Fiber optics sensors. http://dx.doi.org/10.1364/OL.38.005442
Material processing with ultrashort laser pulses has been a rapidly growing field in recent years. In order to achieve higher processing speeds and thus higher productivity, there is a growing interest in developing sources with high average power. At the same time, pulse durations and energies have to be suitable for the intended processes. In recent years, various concepts were developed to increase the average output power of lasers with ultrashort pulses. The first approach is to scale the power directly available from an oscillator. Most commonly this was performed using passively mode-locked thin-disk lasers, which allow minimizing the thermal effects in the laser crystal. Following this approach, output power of up to 275 W [1] and pulse energies exceeding 40 μJ [2] were demonstrated with different setups. Intense research is devoted to the use of new active materials for thin-disk lasers to achieve pulse durations in the order of 200 fs and below while maintaining good performance [3,4]. However, due to the high intracavity intensities, further scaling into the range of kilowatt average power remains challenging. Another concept to scale up the average output power is the use of amplifiers. 640 fs pulses with an average output power of up to 830 W at a repetition rate of 78 MHz corresponding to 10.6 μJ of pulse energy were reported based on a photonic crystal-fiber amplifier [5]. Since the onset of nonlinear effects limits the peak power densities directly available from an active fiber, chirped-pulse amplification has to be employed for ultrashort pulses with pulse energies beyond a few nanojoules. This approach adds complexity, optical losses, size, and costs to the system. Using slab amplifiers, 615 fs pulses with an average output power of 1.1 kW at a repetition rate of 20 MHz (55 μJ of pulse energy) were demonstrated with two amplification stages [6]. Due to the strong asymmetric thermal lens in the slab crystal, the preservation of the beam quality in the multi-kilowatt power range is challenging. The longer optical path inside the laser crystal also imposes limitations on the pulse energy by nonlinear effects. 0146-9592/13/245442-04$15.00/0
Thin-disk lasers exhibit favorable thermal management within the laser crystal and are thus well suited for high power even in fundamental-mode operation; for example, by using deformable mirrors [7] or zerophonon line pumping [8], which most recently led to 4 kW of output power [9]. Furthermore, thin-disks show low nonlinearities when used in pulsed operation. Regenerative amplifiers for ultrashort pulses offer a high gain, and such devices based on the thin-disk technology currently deliver average output powers exceeding 300 W [10]. The critical component limiting the average output power as well as the repetition rate achievable from such systems is the Pockels cell used as optical switch. Thin-disk multipass amplifiers avoid these limitations by geometrically folding the amplified beam many times onto the pumped thin-disk crystal. Therefore, they are a very promising approach to further increase the average output power and pulse energy of ultrafast lasers. Due to the limited number of passes through the amplifying crystal, geometrical multipass amplifiers typically exhibit lower gain than regenerative amplifiers. Nevertheless, they can provide high efficiencies when seeded with a sufficiently powerful source. So far, multipass thin-disk laser amplifiers were reported for operation with triggered nanosecond pulses [11] or for generating 800 μs long bursts of 820 fs pulses at a repetition rate of 100 kHz with an average power of 4.45 kW during these bursts, but at a much lower average power [12]. Here we report on a thin-disk multipass amplifier for ps pulses with an average output power of currently 1105 W, which—to the best of our knowledge—is the highest average power for an ultrafast thin-disk laser reported on so far. The concept of the presented multipass amplifier is a development of the IFSW that was originally conceived as a high-energy pump for spectroscopic investigations of the lamb-shift in muonic hydrogen [11,13]. During the measurement campaigns to determine the radius of the proton [13] the thin-disk multipass amplifier proved to work extremely reliably and led to further developments to generate
OPTICS LETTERS / Vol. 38, No. 24 / December 15, 2013
1.1 kW average output power from a thin-disk multipass amplifier for ultrashort laser pulses Jan-Philipp Negel,1,* Andreas Voss,1 Marwan Abdou Ahmed,1 Dominik Bauer,2 Dirk Sutter,2 Alexander Killi,2 and Thomas Graf1 1
Institut für Strahlwerkzeuge (IFSW), University of Stuttgart, Pfaffenwaldring 43, 70569 Stuttgart, Germany 2
TRUMPF Laser GmbH+Co. KG, Aichhalder Straße 39, 78713 Schramberg, Germany *Corresponding author: Jan‑[email protected]‑stuttgart.de
Received October 3, 2013; revised November 8, 2013; accepted November 11, 2013; posted November 15, 2013 (Doc. ID 198816); published December 13, 2013 We report on a thin-disk multipass amplifier for ultrashort laser pulses delivering an average output power of 1105 W. The amplifier was seeded by a Trumpf TruMicro5050 laser with a power of 80 W at a wavelength of 1030 nm, pulse duration of 6.5 ps, and repetition rate of 800 kHz. The energy of the amplified pulses is 1.38 mJ with a duration of 7.3 ps. The amplifier exhibits an optical efficiency of 44% and a slope efficiency of 46%. The beam quality was measured to be better than M 2 1.25. © 2013 Optical Society of America OCIS codes: (140.3280) Laser amplifiers; (140.3580) Lasers, solid-state; (140.3615) Lasers, ytterbium; (060.2370) Fiber optics sensors. http://dx.doi.org/10.1364/OL.38.005442
Material processing with ultrashort laser pulses has been a rapidly growing field in recent years. In order to achieve higher processing speeds and thus higher productivity, there is a growing interest in developing sources with high average power. At the same time, pulse durations and energies have to be suitable for the intended processes. In recent years, various concepts were developed to increase the average output power of lasers with ultrashort pulses. The first approach is to scale the power directly available from an oscillator. Most commonly this was performed using passively mode-locked thin-disk lasers, which allow minimizing the thermal effects in the laser crystal. Following this approach, output power of up to 275 W [1] and pulse energies exceeding 40 μJ [2] were demonstrated with different setups. Intense research is devoted to the use of new active materials for thin-disk lasers to achieve pulse durations in the order of 200 fs and below while maintaining good performance [3,4]. However, due to the high intracavity intensities, further scaling into the range of kilowatt average power remains challenging. Another concept to scale up the average output power is the use of amplifiers. 640 fs pulses with an average output power of up to 830 W at a repetition rate of 78 MHz corresponding to 10.6 μJ of pulse energy were reported based on a photonic crystal-fiber amplifier [5]. Since the onset of nonlinear effects limits the peak power densities directly available from an active fiber, chirped-pulse amplification has to be employed for ultrashort pulses with pulse energies beyond a few nanojoules. This approach adds complexity, optical losses, size, and costs to the system. Using slab amplifiers, 615 fs pulses with an average output power of 1.1 kW at a repetition rate of 20 MHz (55 μJ of pulse energy) were demonstrated with two amplification stages [6]. Due to the strong asymmetric thermal lens in the slab crystal, the preservation of the beam quality in the multi-kilowatt power range is challenging. The longer optical path inside the laser crystal also imposes limitations on the pulse energy by nonlinear effects. 0146-9592/13/245442-04$15.00/0
Thin-disk lasers exhibit favorable thermal management within the laser crystal and are thus well suited for high power even in fundamental-mode operation; for example, by using deformable mirrors [7] or zerophonon line pumping [8], which most recently led to 4 kW of output power [9]. Furthermore, thin-disks show low nonlinearities when used in pulsed operation. Regenerative amplifiers for ultrashort pulses offer a high gain, and such devices based on the thin-disk technology currently deliver average output powers exceeding 300 W [10]. The critical component limiting the average output power as well as the repetition rate achievable from such systems is the Pockels cell used as optical switch. Thin-disk multipass amplifiers avoid these limitations by geometrically folding the amplified beam many times onto the pumped thin-disk crystal. Therefore, they are a very promising approach to further increase the average output power and pulse energy of ultrafast lasers. Due to the limited number of passes through the amplifying crystal, geometrical multipass amplifiers typically exhibit lower gain than regenerative amplifiers. Nevertheless, they can provide high efficiencies when seeded with a sufficiently powerful source. So far, multipass thin-disk laser amplifiers were reported for operation with triggered nanosecond pulses [11] or for generating 800 μs long bursts of 820 fs pulses at a repetition rate of 100 kHz with an average power of 4.45 kW during these bursts, but at a much lower average power [12]. Here we report on a thin-disk multipass amplifier for ps pulses with an average output power of currently 1105 W, which—to the best of our knowledge—is the highest average power for an ultrafast thin-disk laser reported on so far. The concept of the presented multipass amplifier is a development of the IFSW that was originally conceived as a high-energy pump for spectroscopic investigations of the lamb-shift in muonic hydrogen [11,13]. During the measurement campaigns to determine the radius of the proton [13] the thin-disk multipass amplifier proved to work extremely reliably and led to further developments to generate
