700 MW peak power of a 380 fs regenerative amplifier with Tm:YAP ∗
Andreas Wienke,1, Dieter Wandt,1 Uwe Morgner,1,2 J¨org Neumann,1 and Dietmar Kracht1 1 Laser
Zentrum Hannover e.V., Laser Development Department, Ultrafast Photonics Group, Hollerithallee 8, 30419 Hannover, Germany 2 Institut f¨ ur Quantenoptik, Leibniz Universit¨at Hannover, Welfengarten 1, 30167 Hannover, Germany ∗ [email protected]
Abstract: We report on a high power ultrashort pulse regenerative amplifier system, entirely based on thulium-doped laser materials operating around 1.94 µm. At a repetition rate of 1 kHz the Tm:YAP regenerative amplifier emits pulse energies > 700 µJ, only limited by the damage threshold of the Tm:YAP crystal. The pulses can be compressed to 380 fs at an efficiency of 50 %. Purging of the regenerative amplifier cavity with nitrogen is necessary due to atmospheric absorptions causing long ps pedestals in the autocorrelation. © 2015 Optical Society of America OCIS codes: (140.3280) Laser amplifiers; (140.3510) Lasers, fiber; (140.3538) Lasers, pulsed; (140.3580) Lasers, solid-state; (140.4050) Mode-locked lasers; (320.7090) Ultrafast lasers.
References and links 1. K. Scholle, S. Lamrini, P. Koopmann, and P. Fuhrberg, “2 µm laser sources and their possible applications,” in Frontiers in Guided Wave Optics and Optoelectronics, B. Pal, ed. (InTech, 2010). 2. J. L. Krause, K. J. Schafer, and K. C. Kulander, “High-order harmonic generation from atoms and ions in the high intensity regime,” Phys. Rev. Lett. 68, 3535–3538 (1992). 3. P. Kadwani, R. Sims, J. Chia, F. Altat, L. Shah, and M. Richardson, “Atmospheric propagation testing using broadband Thulium fiber systems,” in Advances in Optical Materials, OSA Technical Digest (CD) (Optical Society of America, 2011), paper FWB3. 4. N. Leindecker, A. Marandi, R. L. Byer, K. L. Vodopyanov, J. Jiang, I. Hartl, M. Fermann, and P. G. Schunemann, “Octave-spanning ultrafast OPO with 2.6-6.1 µm instantaneous bandwidth pumped by femtosecond Tm-fiber laser,” Opt. Express 20, 7046–7053 (2012). 5. C. P. Hauri, R. B. Lopez-Martens, C. I. Blaga, K. D. Schultz, J. Cryan, R. Chirla, P. Colosimo, G. Doumy, A. M. March, C. Roedig, E. Sistrunk, J. Tate, J. Wheeler, L. F. DiMauro, and E. P. Power, “Intense self-compressed, self-phase-stabilized few-cycle pulses at 2 µm from an optical filament,” Opt. Lett. 32, 868–870 (2007). 6. X. Gu, G. Marcus, Y. Deng, T. Metzger, C. Teisset, N. Ishii, T. Fuji, A. Baltuˇska, R. Butkus, V. Pervak, H. Ishizuki, T. Taira, T. Kobayashi, R. Kienberger, and F. Krausz, “Generation of carrier-envelope-phase-stable 2cycle 740-µJ pulses at 2.1-µm carrier wavelength,” Opt. Express 17, 62–69 (2009). 7. F. Stutzki, C. Gaida, M. Gebhardt, F. Jansen, C. Jauregui, J. Limpert, and A. T¨unnermann, ”Tm-based fiber-laser system with more than 200 MW peak power,” Opt. Lett. 40, 9–12 (2015). 8. P. Malevich, G. Andriukaitis, T. Fl¨ory, A. J. Verhoef, A. Fern´andez, S. Aliˇsauskas, A. Pugˇzlys, A. Baltuˇska, L. H. Tan, C. F. Chua, and P. B. Phua, “High energy and average power femtosecond laser for driving mid-infrared optical parametric amplifiers,” Opt. Lett. 38, 2746–2748 (2013). 9. A. Wienke, F. Haxsen, D. Wandt, U. Morgner, J. Neumann, and D. Kracht, “Ultrafast, stretched-pulse thuliumdoped fiber laser with a fiber-based dispersion management,” Opt. Lett. 37, 2466–2468 (2012). 10. O. E. Martinez, J. P. Gordon, and R. L. Fork, “Negative group-velocity dispersion using refraction,” J. Opt. Soc. Am. A 1, 1003–1006 (1984). 11. L. S. Rothman, I. E. Gordon, A. Barbe, D. C. Benner, P. F. Bernath, M. Birk, V. Boudon, L. R. Brown, A. Campargue, J.-P. Champion, K. Chance, L. H. Coudert, V. Dana, V. M. Devi, S. Fally, J.-M. Flaud, R. R. Gamache, A.
#238295 (C) 2015 OSA
Received 17 Apr 2015; revised 22 May 2015; accepted 22 May 2015; published 18 Jun 2015 29 Jun 2015 | Vol. 23, No. 13 | DOI:10.1364/OE.23.016884 | OPTICS EXPRESS 16884
Goldman, D. Jacquemart, I. Kleiner, N. Lacome, W. J. Lafferty, J.-Y. Mandin, S. T. Massie, S. N. Mikhailenko, C. E. Miller, N. Moazzen-Ahmadi, O. V. Naumenko, A. V. Nikitin, J. Orphal, V. I. Perevalov, A. Perrin, A. ˇ Predoi-Cross, C. P. Rinsland, M. Rotger, M. Simeˇ ckov´a, M. A. H. Smith, K. Sung, S. A. Tashkun, J. Tennyson, R. A. Toth, A. C. Vandaele, and J. Vander Auwera, “The HITRAN 2008 molecular spectroscopic database,” J. Quant. Spectrosc. Radiat. Transf. 110, 533–572 (2009). 12. L. M. Frantz and J. S. Nodvik, “Theory of pulse propagation in a laser amplifier,” J. Appl. Phys. 34, 2346–2349 (1963). 13. R. Adair, L. L. Chase, and S. A. Payne, “Nonlinear refractive index of optical crystals,” Phys. Rev. B 39, 3337– 3350 (1989). 14. J. D¨orring, A. Killi, U. Morgner, A. Lang, M. Lederer, and D. Kopf, “Period doubling and deterministic chaos in continuously pumped regenerative amplifiers,” Opt. Express 12, 1759–1768 (2004).
1.
Introduction
High pulse energy lasers with ultrashort pulse duration operating in the wavelength region around 2 µm have gained an increased interest in past years because of their wide field of application. Besides many industrial applications like material processing of plastics and polymers [1], also high harmonic generation to extend the cutoff energy [2], atmospheric sensing [3], and frequency conversion to longer wavelengths to access the molecular fingerprint region [4] are attractive topics in nowadays research. Until now, only few systems have been published at 2 µm wavelength, which produce energies in the µJ region with sub-ps pulse durations. On the face of it, there are two approaches to generate ultrashort pulses at 2 µm with µJ pulse energy: firstly, a high power Titanium:Sapphire chirped pulse amplification (CPA) system, which use nonlinear difference frequency and optical parametric processes to generate multi-µJ, few-cycle pulses. But these systems lack efficiency, costs, and compactness [5, 6]. Anyway, further scaling of the output pulse energy is mainly limited by the available pump powers for the nonlinear processes. The second approach is the direct use of thulium- and holmium-doped fibers and crystals which have broad emission bandwidths at 2.0 µm and 2.1µm, respectively. Just recently, a fiber-based thulium (Tm) sub-ps CPA system has been demonstrated with up to 120 µJ compressed pulse energy and 200 MW peak power [7]. This has been achieved by the use of rod-type, large-pitch fibers and highly efficient dielectric gratings. Further power scaling with the used fiber is quite challenging as nonlinearities have a high impact at this peak power level and the high average power leads to unwanted thermal degradation of the beam profile. A better approach are ultrafast regenerative amplifiers (RA), which can boost nJ-pulses directly up to the multi-µJ or even mJ level. This was demonstrated in 2013 for the first time at 2.1 µm wavelength with a holmium RA, producing pulse energies up to 990 µJ at 530 fs pulse durations [8]. But again, this system has the drawback of a bulky, cost-intensive and inefficient nonlinear parametric seed source, producing the demanded pulses at 2.1 µm. Here, we demonstrate an ultrashort pulse RA system based completely on Tm-doped materials, which operates at 1937 nm. The system consists of a compact and straight-forward solution: an ultrashort pulse fiber-based seed oscillator, a fiber-preamplifier and a regenerative amplifier based on a Thulium-doped YAlO3 (Tm:YAP) crystal. At a repetition rate of 1 kHz, the RA delivers an uncompressed pulse energy of > 700 µJ, only limited by the damage threshold of the laser crystal. The pulses can be compressed to a pulse duration of 380 fs. As atmospheric absorptions take place at this wavelength, purging of the RA cavity is necessary. 2.
Experimental setup
The setup depicted in Fig. 1 consists of an ultrashort pulse seed oscillator, fiber stretcher and preamplifier, the RA cavity, and a grating compressor. The passively mode-locked Tm-doped fiber oscillator, which is comparable to the one described in [9], emits pulses with 145 pJ pulse energy and 120 fs pulse duration. 100 m of polarization maintaining (PM) passive fiber
#238295 (C) 2015 OSA
Received 17 Apr 2015; revised 22 May 2015; accepted 22 May 2015; published 18 Jun 2015 29 Jun 2015 | Vol. 23, No. 13 | DOI:10.1364/OE.23.016884 | OPTICS EXPRESS 16885
(PM1950) are necessary to stretch the fs-oscillator pulses to 90 ps pulse duration. The negatively stretched pulses are amplified in a following single-stage double-cladding pumped Tmdoped PM-fiber amplifier up to 50 nJ pulse energy. The resulting seed pulses have a central wavelength of 1940 nm, a full width at half maximum (FWHM) of the optical spectrum of 21.7 nm and could still be compressed to 250 fs pulse duration. A Pockels cell (PC) consisting of two rubidium titanyl phosphate (RTP) crystals picks the pulses to 1 kHz repetition rate before they are injected into the RA cavity. Furthermore, a telescope is introduced into the seed beam path to ensure a good mode matching between seed beam and resonator mode, resulting in a high seeding efficiency. A thin film polarizer (TFP) and a Faraday rotator (FR) separate the amplified pulses from the seed pulses. Pulse Picker
Fiberstretcher
Preamplifier
l/2 l/4
l/4 l/2
PC
TFP
FR
PC
l/2
LD
Isolator
l/2
Seedoscillator
Isolator l/2
FR
TFP
TFP TFP
WDG
l/2
GR
CCM2 CCM
Autocorrelator
CCM1
Diagnostic
Tm:YAP
Regenerative Amplifier
Grating Compressor
Fig. 1. Experimental setup of the system. PC: Pockels cell, TFP: thin film polarizer, FR: Faraday rotator, CCM1: Concave mirror with 600 mm ROC, CCM2: Concave mirror with 300 mm ROC, WDG: Wedge, GR: Grating.
The RA consists of a 2.2 m long, standing wave, three mirror cavity with two concave and one plane end mirror. The concave mirrors are placed 35.4 cm (CCM1) and 29.5 cm (CCM2) apart from the crystal. A second RTP PC is used in combination with a TFP and a quarter wave plate (QWP) as an optical switch. The resonator mode diameters are 320 µm inside the laser crystal (5 x 5 mm2 aperture) and 1.7 mm inside the PC, which has an aperture of 3.2 mm. The mode diameters are calculated with a simulated thermal lens inside the laser crystal of 80 mm, that has been experimentally determined at an absorbed pump power of 9.1 W. The simulation shows that for a 10 % decreased thermal lens, the mode diameters inside the crystal and at the position of the PC increases by less than 5 %. The 4 mm long Tm:YAP crystal is cut for c-axis orientation and has a doping concentration of 4 at. %. It is wrapped in indium foil and placed in a copper mount, which is electrically cooled down to 17◦ C. The pump diode is a multi-mode fiber-coupled laser diode (LD) with 35 W pump power at a central wavelength of 793 nm. The output fiber of the pump diode has a core diameter of 105 µm and a numerical aperture of 0.22. At this pump wavelength, 60 % of the pump light is absorbed inside the crystal. The collimated pump light is focused with a 100 mm lens into the Tm:YAP crystal through one of the dichroic end mirrors, which is located close to the crystal, producing a pump spot diameter of 500 µm. The RA cavity is enclosed to purge the cavity with inert gas (argon, nitrogen etc). After amplification, the pulses are compressed with a Martinez-type compressor [10], which consists of a single gold coated grating (600 grooves per mm, Blaze angle: 34◦ ), one concave mirror (ROC = 1000 mm) and two plane mirrors. The compressor efficiency of 50 % is mainly limited by the grating efficiency and the FR. 3.
Experimental results
The following experimental results are achieved with a seed pulse energy of 24.8 nJ at a repetition rate of 1 kHz and with 34 roundtrips unless stated otherwise. At an absorbed pump power of 11.1 W a pulse energy of 700 µJ can be achieved (s. Fig. 2(a), red curve). As the damage #238295 (C) 2015 OSA
Received 17 Apr 2015; revised 22 May 2015; accepted 22 May 2015; published 18 Jun 2015 29 Jun 2015 | Vol. 23, No. 13 | DOI:10.1364/OE.23.016884 | OPTICS EXPRESS 16886
J l s e u P
l i f i e p m A
0 4
5
6
7
8
9
1 0
1 1
A b s o r b e d p u m p p o w e r in W
u n p u rg e d
0 .8
In te n s ity in a r b . u .
1 5 0
µ
1 5 0
i n
3 0 0
y
3 0 0
r g
i n r e w o p
4 5 0
d
4 5 0
e
6 0 0
1 .0 0 .6
n
6 0 0
(b )
7 5 0
N itr o g e n p u r g e d u n p u rg e d
e
7 5 0
m
W
(a )
0
0 .4 0 .2
0 .0 1 .0
N itr o g e n p u r g e d
0 .8 0 .6
∆λ= 8 . 4 n m
0 .4 0 .2
0 .0 1 9 1 0
1 9 2 0
1 9 3 0
1 9 4 0
W a v e le n g th in n m
1 9 5 0
1 9 6 0
Fig. 2. Purged (black curve) vs. unpurged cavity (red curve): (a) Uncompressed amplified power / pulse energy vs. absorbed pump power. Inset: Output beam profile of the RA at 700 µJ output pulse energy. (b) Optical spectrum at 700 µJ output pulse energy.
threshold of the laser crystal was found during earlier experiments between a power density of 2.0 - 2.1 J / cm2 we limited the achievable pulse energy below 1.77 J / cm2 in all following experiments. Figure 2(b) (red line) shows the optical spectrum of the amplified pulses at a maximum pulse energy of 700 µJ. It is centered around 1937 nm and has a bandwidth of 19 nm at -10 dB. Furthermore, it is highly structured with sharp dips, which have already been presented in previous publications at this wavelength range [9] and could be identified by the HITRAN database [11] as atmospheric absorption lines. In consequence, the cavity of the RA is purged with nitrogen gas. After purging the RA to a relative humidity of 2 - 3 %, the absorption lines vanish at least partly and the optical spectrum remains highly structured with a FWHM of 8.4 nm (18.5 nm at -10 dB, respectively). Additionally, the output pulse energy increases about 20 % on average, so that an uncompressed pulse energy of 709 µJ at an absorbed pump power of 10.7 W is achieved (s. Fig 2(a), black curve). The output beam profile (example shown in inset of Fig. 2(a)) is perfectly circular in all measurements, but the diameter varies from 2.44 mm to 1.98 mm with increasing pump power due to the high thermal lensing. A measurement taken at 11.1 W absorbed pump power reveals a beam propagation parameter M2 in direction parallel to the optical table of 1.61 ± 0.04 and 1.57 ± 0.06 perpendicular to the optical table. (a )
1 .0
( b
N itr o g e n p u r g e d u n p u rg e d
1
. 0
0
. 8
∆τ ∆τ ∆τ
u r b
. 6
a
0
=
3
4
1
5
. 2
3
5
3
µ
J
7
0
9
µ
µ
J
J
f s
. 2
3
5
3
7
0
9
=
3
8
3
f s
=
4
1
0
f s
s i t y
i n
0 .4
0
. 4
0
. 2
0
. 0
t e
n
0 .2 0 .0
5
.
0 .6
I n
In te n s ity in a r b . u .
.
0 .8
)
-2 0
-1 0
0
D e la y in p s
1 0
2 0
- 7
. 5
- 5
. 0
- 2
. 5
0
D
e
l a
y
. 0
2
i n
p
. 5
5
. 0
7
. 5
s
Fig. 3. Compressed AC traces: (a) At 700 µJ output pulse energy (black curve: purged, red curve: unpurged). (b) At output pulse energies 5 µJ (blue), 353 µJ (red) and 709 µJ (black).
Figure 3(a) shows the autocorrelation (AC) trace in a 50 ps AC range after pulse compression at an output pulse energy of 700 µJ. The effect of the atmospheric absorption is clearly visible in the broadband ps-pedestal (red curve), which vanishes nearly completely after purging with nitrogen (black curve). The only remain is a satellite pulse with a relative intensity of 6.2 % in a temporal distance of 3.2 ps, which corresponds to an optical path length of 0.96 mm. Furthermore, a second satellite pulse appears in twice the temporal distance to the main peak. #238295 (C) 2015 OSA
Received 17 Apr 2015; revised 22 May 2015; accepted 22 May 2015; published 18 Jun 2015 29 Jun 2015 | Vol. 23, No. 13 | DOI:10.1364/OE.23.016884 | OPTICS EXPRESS 16887
During the experiment, the distances of the satellite pulses to the main peak does not change within measurement inaccuracy, but the relative intensity decreases from 19.5 % to 6.2 % with increasing pump power / pulse energy (s. Fig. 3(b)). A possible explanation for this counterintuitive behaviour can be found in the different amplification levels of main and satellite pulse: if the satellite pulse is a post-pulse following the main pulse in time, then it is amplified by the residual fraction of gain left by the main pulse. The time between main and satellite pulse leaves no time for the active material to recover although continuous wave pumping. In this case, the pulses are amplified differently and the relative intensity of the satellite pulse decreases. When the PC is replaced by another RTP cell from a different vendor with same crystal lengths and measured at similar output energies, more copies of the satellite pulse appear, the temporal distance from the first satellite pulse to the main pulse shortens to 2.9 ps and the relative intensity highly increases in the AC. Therefore, we can attribute the generation of satellite pulses at least partly to the PC. One possible explanation for the generation of the satellite pulse could be uncompensated birefringence of the two 90 ◦ turned RTP crystals. By various experiments with additional wave plates and thick birefringent quartz plates, it was not possible to reduce the satellite pulses. However, etalon effects can be excluded as the two 10 mm long RTP crystals are separated 5 mm from each other. Tilting the PC to achieve a static quarter wave retardation and removing the QWP of the RA shows no change in the AC trace. Anyway, an AC trace after a single pass through the resonator (4 passes through the PC) without regenerative amplification shows no signs of satellite pulses. This indicates a resonant effect during multiple roundtrips caused by technical imperfection of the PC. Assuming a squared hyperbolic secant temporal profile, the pulses can be compressed to pulse durations ranging from 341 fs to 410 fs with 76 % in the main peak at 709 µJ output energy. For this performance (34 roundtrips, 709 µJ uncompressed output energy, 1 kHz repetition rate), a B-Integral of 7.23 is estimated by the use of Frantz-Nodvick equations [12]. A B-Integral of π is reached at an output pulse energy of 413 µJ. The highest influence on the B-Integral is attributed to the Tm:YAP crystal and the RTP crystals of the PC, of which the nonlinear refractive index n2 is not available in literature. Therefore, we choose the nonlinear refractive index of KTP (2.76 cm2 / W, from [13]) for our estimations because KTP is the closest isomorph to RTP in terms of optical and chemical properties of which data are available. (a )
(b )
7 5 0 6 0 0
In te n s ity in a r b . u .
P
u
l s e
e
n
e
r g
y
i n
µ
J
0 .8
4 5 0 3 0 0 1 5 0 0
1 .0
2 0
2 4
2 8
3 2
3 6
4 0
N u m b e r s o f r o u n d tr ip s
4 4
4 8
4 4 R T 3 9 R T 3 4 R T
∆τ4 4 = 4 2 2 f s ∆τ3 9 = 4 0 4 f s ∆τ3 4 = 4 1 0 f s
0 .6 0 .4 0 .2 0 .0
-7 .5
-5 .0
-2 .5
0 .0
D e la y in p s
2 .5
5 .0
7 .5
Fig. 4. (a) Output pulse energy in relation to number of roundtrips at 8.5 W absorbed pump power. (b) Compressed AC traces at 34, 39 and 44 roundtrips at an output energy of 700 µJ.
The linear relation of output energy to cavity roundtrips is shown in Fig. 4(a), starting from 24 roundtrips with an output energy of 47 µJ at an absorbed pump power of 8.5 W. The maximum pulse energy limited by the damage threshold of the laser crystal is achieved at a roundtrip number of 44. Figure 4(b) compares the AC traces after compression at an output pulse energy of ca. 700 µJ at 34, 39 and 44 roundtrips. At this fixed pulse energy, the pulses can still be compressed to 404 fs to 422 fs. Due to the longer roundtrips in the RA, the amplitude of the #238295 (C) 2015 OSA
Received 17 Apr 2015; revised 22 May 2015; accepted 22 May 2015; published 18 Jun 2015 29 Jun 2015 | Vol. 23, No. 13 | DOI:10.1364/OE.23.016884 | OPTICS EXPRESS 16888
satellite peak increases from 6.2 % to 15.8 % and shifts slightly from 3.2 ps to 3.6 ps. This can be either introduced by the longer roundtrip time (more absorptions, more complex phase profile) or higher nonlinearities (B-Integral of 12.2 at 44 roundtrips) inside the RA cavity or even both. ( b
) 7
5
0
5
0
0
6
7
5
4
5
0
6
0
0
4
0
0
5
2
5
3
5
0
4
5
0
3
0
0
t i o r a u d u p
n
d
e l s e
r e p
0 .5
1 .0
1 .5
2 .0
R e p e titio n r a te in k H z
2 .5
3 .0
m
0 .0
o
0
u
1 5 0
l s e
y r g e
3 0 0
s s e
J µ i n
4 5 0
P
P
u
l s e
e
n
e
r g
y
i n
µ
J
n
i n
6 0 0
f s
7 5 0
1
0
2
S
0
e
3
e
d
p
u
l s e
0
e
4
n
e
r g
y
i n
0
n
5
0
C
(a )
J
Fig. 5. Parameter variation at 34 roundtrips: (a) Output pulse energy vs. repetition rate at 7.5 W absorbed pump power. (b) Output pulse energy and compressed pulse duration vs. seed pulse energy at 9.8 W absorbed pump power.
The pulse energy can be further increased by choosing an appropriate repetition rate, which relation is depicted in Fig. 5(a) at a fixed number of 34 roundtrips and an absorbed pump power of 7.5 W. Starting from a repetition rate of 3 kHz with an output energy of 111 µJ, the output energy increases to the limited maximum of 702 µJ at a repetition rate of 100 Hz. Figure 5(b) shows the behaviour of the RA output while varying the seed pulse energy. The output pulse energy (black curve) shows a linear increase from 491 µJ at a seed energy of 12.3 nJ to a maximum pulse energy of 700 µJ at a seed energy of 48.7 nJ. Additionally, the compressed pulse duration (red curve) stays nearly constant between 380 fs and 402 fs for the different seed pulse energies and is shortest at the highest output power. Taking the compressor efficiency of 50 % into account, these values result in a peak power of 700 MW with 76 % of the energy confined in the main peak. In all measurements, no bifurcation was observed [14] which can occur at repetition rates close to the inverse of the upper state lifetime. 4.
Conclusion
In conclusion, we present an ultrafast RA system, completely based on Tm-doped materials (fiber oscillator, fiber preamplifier, and bulk regenerative amplifier). The system delivers > 700 µJ output pulse energy, only limited by the damage threshold of the laser crystal. The compressed AC traces show a pedestal in the unpurged case caused by atmospheric absorptions. This pedestal can be reduced by purging the cavity with nitrogen, whereas satellite pulses remain beside the main pulse. These satellite pulses are at least partly generated by the Pockels cell and can not be removed. Dechirping the pulses with a Martinez grating compressor results in a compressed pulse energy of 350 µJ and minimum pulse duration of 380 fs, which calculates to a peak power of 700 MW. By variation of the roundtrip numbers, repetition rate and seed energy, possible ways to increase the overall output pulse energy and efficiency are pointed out. Using more appropriate gratings with higher efficiencies of >90 % will allow for even higher output energies and peak power. This system can be used as input for a multi-pass amplifier with Tm:YAP to further scale the pulse energy to the multi-mJ level. Acknowledgment This work has been funded by the German Federal Ministry of Education and Research (BMBF) under contract 13N12079 “NEXUS”. #238295 (C) 2015 OSA
Received 17 Apr 2015; revised 22 May 2015; accepted 22 May 2015; published 18 Jun 2015 29 Jun 2015 | Vol. 23, No. 13 | DOI:10.1364/OE.23.016884 | OPTICS EXPRESS 16889
Andreas Wienke,1, Dieter Wandt,1 Uwe Morgner,1,2 J¨org Neumann,1 and Dietmar Kracht1 1 Laser
Zentrum Hannover e.V., Laser Development Department, Ultrafast Photonics Group, Hollerithallee 8, 30419 Hannover, Germany 2 Institut f¨ ur Quantenoptik, Leibniz Universit¨at Hannover, Welfengarten 1, 30167 Hannover, Germany ∗ [email protected]
Abstract: We report on a high power ultrashort pulse regenerative amplifier system, entirely based on thulium-doped laser materials operating around 1.94 µm. At a repetition rate of 1 kHz the Tm:YAP regenerative amplifier emits pulse energies > 700 µJ, only limited by the damage threshold of the Tm:YAP crystal. The pulses can be compressed to 380 fs at an efficiency of 50 %. Purging of the regenerative amplifier cavity with nitrogen is necessary due to atmospheric absorptions causing long ps pedestals in the autocorrelation. © 2015 Optical Society of America OCIS codes: (140.3280) Laser amplifiers; (140.3510) Lasers, fiber; (140.3538) Lasers, pulsed; (140.3580) Lasers, solid-state; (140.4050) Mode-locked lasers; (320.7090) Ultrafast lasers.
References and links 1. K. Scholle, S. Lamrini, P. Koopmann, and P. Fuhrberg, “2 µm laser sources and their possible applications,” in Frontiers in Guided Wave Optics and Optoelectronics, B. Pal, ed. (InTech, 2010). 2. J. L. Krause, K. J. Schafer, and K. C. Kulander, “High-order harmonic generation from atoms and ions in the high intensity regime,” Phys. Rev. Lett. 68, 3535–3538 (1992). 3. P. Kadwani, R. Sims, J. Chia, F. Altat, L. Shah, and M. Richardson, “Atmospheric propagation testing using broadband Thulium fiber systems,” in Advances in Optical Materials, OSA Technical Digest (CD) (Optical Society of America, 2011), paper FWB3. 4. N. Leindecker, A. Marandi, R. L. Byer, K. L. Vodopyanov, J. Jiang, I. Hartl, M. Fermann, and P. G. Schunemann, “Octave-spanning ultrafast OPO with 2.6-6.1 µm instantaneous bandwidth pumped by femtosecond Tm-fiber laser,” Opt. Express 20, 7046–7053 (2012). 5. C. P. Hauri, R. B. Lopez-Martens, C. I. Blaga, K. D. Schultz, J. Cryan, R. Chirla, P. Colosimo, G. Doumy, A. M. March, C. Roedig, E. Sistrunk, J. Tate, J. Wheeler, L. F. DiMauro, and E. P. Power, “Intense self-compressed, self-phase-stabilized few-cycle pulses at 2 µm from an optical filament,” Opt. Lett. 32, 868–870 (2007). 6. X. Gu, G. Marcus, Y. Deng, T. Metzger, C. Teisset, N. Ishii, T. Fuji, A. Baltuˇska, R. Butkus, V. Pervak, H. Ishizuki, T. Taira, T. Kobayashi, R. Kienberger, and F. Krausz, “Generation of carrier-envelope-phase-stable 2cycle 740-µJ pulses at 2.1-µm carrier wavelength,” Opt. Express 17, 62–69 (2009). 7. F. Stutzki, C. Gaida, M. Gebhardt, F. Jansen, C. Jauregui, J. Limpert, and A. T¨unnermann, ”Tm-based fiber-laser system with more than 200 MW peak power,” Opt. Lett. 40, 9–12 (2015). 8. P. Malevich, G. Andriukaitis, T. Fl¨ory, A. J. Verhoef, A. Fern´andez, S. Aliˇsauskas, A. Pugˇzlys, A. Baltuˇska, L. H. Tan, C. F. Chua, and P. B. Phua, “High energy and average power femtosecond laser for driving mid-infrared optical parametric amplifiers,” Opt. Lett. 38, 2746–2748 (2013). 9. A. Wienke, F. Haxsen, D. Wandt, U. Morgner, J. Neumann, and D. Kracht, “Ultrafast, stretched-pulse thuliumdoped fiber laser with a fiber-based dispersion management,” Opt. Lett. 37, 2466–2468 (2012). 10. O. E. Martinez, J. P. Gordon, and R. L. Fork, “Negative group-velocity dispersion using refraction,” J. Opt. Soc. Am. A 1, 1003–1006 (1984). 11. L. S. Rothman, I. E. Gordon, A. Barbe, D. C. Benner, P. F. Bernath, M. Birk, V. Boudon, L. R. Brown, A. Campargue, J.-P. Champion, K. Chance, L. H. Coudert, V. Dana, V. M. Devi, S. Fally, J.-M. Flaud, R. R. Gamache, A.
#238295 (C) 2015 OSA
Received 17 Apr 2015; revised 22 May 2015; accepted 22 May 2015; published 18 Jun 2015 29 Jun 2015 | Vol. 23, No. 13 | DOI:10.1364/OE.23.016884 | OPTICS EXPRESS 16884
Goldman, D. Jacquemart, I. Kleiner, N. Lacome, W. J. Lafferty, J.-Y. Mandin, S. T. Massie, S. N. Mikhailenko, C. E. Miller, N. Moazzen-Ahmadi, O. V. Naumenko, A. V. Nikitin, J. Orphal, V. I. Perevalov, A. Perrin, A. ˇ Predoi-Cross, C. P. Rinsland, M. Rotger, M. Simeˇ ckov´a, M. A. H. Smith, K. Sung, S. A. Tashkun, J. Tennyson, R. A. Toth, A. C. Vandaele, and J. Vander Auwera, “The HITRAN 2008 molecular spectroscopic database,” J. Quant. Spectrosc. Radiat. Transf. 110, 533–572 (2009). 12. L. M. Frantz and J. S. Nodvik, “Theory of pulse propagation in a laser amplifier,” J. Appl. Phys. 34, 2346–2349 (1963). 13. R. Adair, L. L. Chase, and S. A. Payne, “Nonlinear refractive index of optical crystals,” Phys. Rev. B 39, 3337– 3350 (1989). 14. J. D¨orring, A. Killi, U. Morgner, A. Lang, M. Lederer, and D. Kopf, “Period doubling and deterministic chaos in continuously pumped regenerative amplifiers,” Opt. Express 12, 1759–1768 (2004).
1.
Introduction
High pulse energy lasers with ultrashort pulse duration operating in the wavelength region around 2 µm have gained an increased interest in past years because of their wide field of application. Besides many industrial applications like material processing of plastics and polymers [1], also high harmonic generation to extend the cutoff energy [2], atmospheric sensing [3], and frequency conversion to longer wavelengths to access the molecular fingerprint region [4] are attractive topics in nowadays research. Until now, only few systems have been published at 2 µm wavelength, which produce energies in the µJ region with sub-ps pulse durations. On the face of it, there are two approaches to generate ultrashort pulses at 2 µm with µJ pulse energy: firstly, a high power Titanium:Sapphire chirped pulse amplification (CPA) system, which use nonlinear difference frequency and optical parametric processes to generate multi-µJ, few-cycle pulses. But these systems lack efficiency, costs, and compactness [5, 6]. Anyway, further scaling of the output pulse energy is mainly limited by the available pump powers for the nonlinear processes. The second approach is the direct use of thulium- and holmium-doped fibers and crystals which have broad emission bandwidths at 2.0 µm and 2.1µm, respectively. Just recently, a fiber-based thulium (Tm) sub-ps CPA system has been demonstrated with up to 120 µJ compressed pulse energy and 200 MW peak power [7]. This has been achieved by the use of rod-type, large-pitch fibers and highly efficient dielectric gratings. Further power scaling with the used fiber is quite challenging as nonlinearities have a high impact at this peak power level and the high average power leads to unwanted thermal degradation of the beam profile. A better approach are ultrafast regenerative amplifiers (RA), which can boost nJ-pulses directly up to the multi-µJ or even mJ level. This was demonstrated in 2013 for the first time at 2.1 µm wavelength with a holmium RA, producing pulse energies up to 990 µJ at 530 fs pulse durations [8]. But again, this system has the drawback of a bulky, cost-intensive and inefficient nonlinear parametric seed source, producing the demanded pulses at 2.1 µm. Here, we demonstrate an ultrashort pulse RA system based completely on Tm-doped materials, which operates at 1937 nm. The system consists of a compact and straight-forward solution: an ultrashort pulse fiber-based seed oscillator, a fiber-preamplifier and a regenerative amplifier based on a Thulium-doped YAlO3 (Tm:YAP) crystal. At a repetition rate of 1 kHz, the RA delivers an uncompressed pulse energy of > 700 µJ, only limited by the damage threshold of the laser crystal. The pulses can be compressed to a pulse duration of 380 fs. As atmospheric absorptions take place at this wavelength, purging of the RA cavity is necessary. 2.
Experimental setup
The setup depicted in Fig. 1 consists of an ultrashort pulse seed oscillator, fiber stretcher and preamplifier, the RA cavity, and a grating compressor. The passively mode-locked Tm-doped fiber oscillator, which is comparable to the one described in [9], emits pulses with 145 pJ pulse energy and 120 fs pulse duration. 100 m of polarization maintaining (PM) passive fiber
#238295 (C) 2015 OSA
Received 17 Apr 2015; revised 22 May 2015; accepted 22 May 2015; published 18 Jun 2015 29 Jun 2015 | Vol. 23, No. 13 | DOI:10.1364/OE.23.016884 | OPTICS EXPRESS 16885
(PM1950) are necessary to stretch the fs-oscillator pulses to 90 ps pulse duration. The negatively stretched pulses are amplified in a following single-stage double-cladding pumped Tmdoped PM-fiber amplifier up to 50 nJ pulse energy. The resulting seed pulses have a central wavelength of 1940 nm, a full width at half maximum (FWHM) of the optical spectrum of 21.7 nm and could still be compressed to 250 fs pulse duration. A Pockels cell (PC) consisting of two rubidium titanyl phosphate (RTP) crystals picks the pulses to 1 kHz repetition rate before they are injected into the RA cavity. Furthermore, a telescope is introduced into the seed beam path to ensure a good mode matching between seed beam and resonator mode, resulting in a high seeding efficiency. A thin film polarizer (TFP) and a Faraday rotator (FR) separate the amplified pulses from the seed pulses. Pulse Picker
Fiberstretcher
Preamplifier
l/2 l/4
l/4 l/2
PC
TFP
FR
PC
l/2
LD
Isolator
l/2
Seedoscillator
Isolator l/2
FR
TFP
TFP TFP
WDG
l/2
GR
CCM2 CCM
Autocorrelator
CCM1
Diagnostic
Tm:YAP
Regenerative Amplifier
Grating Compressor
Fig. 1. Experimental setup of the system. PC: Pockels cell, TFP: thin film polarizer, FR: Faraday rotator, CCM1: Concave mirror with 600 mm ROC, CCM2: Concave mirror with 300 mm ROC, WDG: Wedge, GR: Grating.
The RA consists of a 2.2 m long, standing wave, three mirror cavity with two concave and one plane end mirror. The concave mirrors are placed 35.4 cm (CCM1) and 29.5 cm (CCM2) apart from the crystal. A second RTP PC is used in combination with a TFP and a quarter wave plate (QWP) as an optical switch. The resonator mode diameters are 320 µm inside the laser crystal (5 x 5 mm2 aperture) and 1.7 mm inside the PC, which has an aperture of 3.2 mm. The mode diameters are calculated with a simulated thermal lens inside the laser crystal of 80 mm, that has been experimentally determined at an absorbed pump power of 9.1 W. The simulation shows that for a 10 % decreased thermal lens, the mode diameters inside the crystal and at the position of the PC increases by less than 5 %. The 4 mm long Tm:YAP crystal is cut for c-axis orientation and has a doping concentration of 4 at. %. It is wrapped in indium foil and placed in a copper mount, which is electrically cooled down to 17◦ C. The pump diode is a multi-mode fiber-coupled laser diode (LD) with 35 W pump power at a central wavelength of 793 nm. The output fiber of the pump diode has a core diameter of 105 µm and a numerical aperture of 0.22. At this pump wavelength, 60 % of the pump light is absorbed inside the crystal. The collimated pump light is focused with a 100 mm lens into the Tm:YAP crystal through one of the dichroic end mirrors, which is located close to the crystal, producing a pump spot diameter of 500 µm. The RA cavity is enclosed to purge the cavity with inert gas (argon, nitrogen etc). After amplification, the pulses are compressed with a Martinez-type compressor [10], which consists of a single gold coated grating (600 grooves per mm, Blaze angle: 34◦ ), one concave mirror (ROC = 1000 mm) and two plane mirrors. The compressor efficiency of 50 % is mainly limited by the grating efficiency and the FR. 3.
Experimental results
The following experimental results are achieved with a seed pulse energy of 24.8 nJ at a repetition rate of 1 kHz and with 34 roundtrips unless stated otherwise. At an absorbed pump power of 11.1 W a pulse energy of 700 µJ can be achieved (s. Fig. 2(a), red curve). As the damage #238295 (C) 2015 OSA
Received 17 Apr 2015; revised 22 May 2015; accepted 22 May 2015; published 18 Jun 2015 29 Jun 2015 | Vol. 23, No. 13 | DOI:10.1364/OE.23.016884 | OPTICS EXPRESS 16886
J l s e u P
l i f i e p m A
0 4
5
6
7
8
9
1 0
1 1
A b s o r b e d p u m p p o w e r in W
u n p u rg e d
0 .8
In te n s ity in a r b . u .
1 5 0
µ
1 5 0
i n
3 0 0
y
3 0 0
r g
i n r e w o p
4 5 0
d
4 5 0
e
6 0 0
1 .0 0 .6
n
6 0 0
(b )
7 5 0
N itr o g e n p u r g e d u n p u rg e d
e
7 5 0
m
W
(a )
0
0 .4 0 .2
0 .0 1 .0
N itr o g e n p u r g e d
0 .8 0 .6
∆λ= 8 . 4 n m
0 .4 0 .2
0 .0 1 9 1 0
1 9 2 0
1 9 3 0
1 9 4 0
W a v e le n g th in n m
1 9 5 0
1 9 6 0
Fig. 2. Purged (black curve) vs. unpurged cavity (red curve): (a) Uncompressed amplified power / pulse energy vs. absorbed pump power. Inset: Output beam profile of the RA at 700 µJ output pulse energy. (b) Optical spectrum at 700 µJ output pulse energy.
threshold of the laser crystal was found during earlier experiments between a power density of 2.0 - 2.1 J / cm2 we limited the achievable pulse energy below 1.77 J / cm2 in all following experiments. Figure 2(b) (red line) shows the optical spectrum of the amplified pulses at a maximum pulse energy of 700 µJ. It is centered around 1937 nm and has a bandwidth of 19 nm at -10 dB. Furthermore, it is highly structured with sharp dips, which have already been presented in previous publications at this wavelength range [9] and could be identified by the HITRAN database [11] as atmospheric absorption lines. In consequence, the cavity of the RA is purged with nitrogen gas. After purging the RA to a relative humidity of 2 - 3 %, the absorption lines vanish at least partly and the optical spectrum remains highly structured with a FWHM of 8.4 nm (18.5 nm at -10 dB, respectively). Additionally, the output pulse energy increases about 20 % on average, so that an uncompressed pulse energy of 709 µJ at an absorbed pump power of 10.7 W is achieved (s. Fig 2(a), black curve). The output beam profile (example shown in inset of Fig. 2(a)) is perfectly circular in all measurements, but the diameter varies from 2.44 mm to 1.98 mm with increasing pump power due to the high thermal lensing. A measurement taken at 11.1 W absorbed pump power reveals a beam propagation parameter M2 in direction parallel to the optical table of 1.61 ± 0.04 and 1.57 ± 0.06 perpendicular to the optical table. (a )
1 .0
( b
N itr o g e n p u r g e d u n p u rg e d
1
. 0
0
. 8
∆τ ∆τ ∆τ
u r b
. 6
a
0
=
3
4
1
5
. 2
3
5
3
µ
J
7
0
9
µ
µ
J
J
f s
. 2
3
5
3
7
0
9
=
3
8
3
f s
=
4
1
0
f s
s i t y
i n
0 .4
0
. 4
0
. 2
0
. 0
t e
n
0 .2 0 .0
5
.
0 .6
I n
In te n s ity in a r b . u .
.
0 .8
)
-2 0
-1 0
0
D e la y in p s
1 0
2 0
- 7
. 5
- 5
. 0
- 2
. 5
0
D
e
l a
y
. 0
2
i n
p
. 5
5
. 0
7
. 5
s
Fig. 3. Compressed AC traces: (a) At 700 µJ output pulse energy (black curve: purged, red curve: unpurged). (b) At output pulse energies 5 µJ (blue), 353 µJ (red) and 709 µJ (black).
Figure 3(a) shows the autocorrelation (AC) trace in a 50 ps AC range after pulse compression at an output pulse energy of 700 µJ. The effect of the atmospheric absorption is clearly visible in the broadband ps-pedestal (red curve), which vanishes nearly completely after purging with nitrogen (black curve). The only remain is a satellite pulse with a relative intensity of 6.2 % in a temporal distance of 3.2 ps, which corresponds to an optical path length of 0.96 mm. Furthermore, a second satellite pulse appears in twice the temporal distance to the main peak. #238295 (C) 2015 OSA
Received 17 Apr 2015; revised 22 May 2015; accepted 22 May 2015; published 18 Jun 2015 29 Jun 2015 | Vol. 23, No. 13 | DOI:10.1364/OE.23.016884 | OPTICS EXPRESS 16887
During the experiment, the distances of the satellite pulses to the main peak does not change within measurement inaccuracy, but the relative intensity decreases from 19.5 % to 6.2 % with increasing pump power / pulse energy (s. Fig. 3(b)). A possible explanation for this counterintuitive behaviour can be found in the different amplification levels of main and satellite pulse: if the satellite pulse is a post-pulse following the main pulse in time, then it is amplified by the residual fraction of gain left by the main pulse. The time between main and satellite pulse leaves no time for the active material to recover although continuous wave pumping. In this case, the pulses are amplified differently and the relative intensity of the satellite pulse decreases. When the PC is replaced by another RTP cell from a different vendor with same crystal lengths and measured at similar output energies, more copies of the satellite pulse appear, the temporal distance from the first satellite pulse to the main pulse shortens to 2.9 ps and the relative intensity highly increases in the AC. Therefore, we can attribute the generation of satellite pulses at least partly to the PC. One possible explanation for the generation of the satellite pulse could be uncompensated birefringence of the two 90 ◦ turned RTP crystals. By various experiments with additional wave plates and thick birefringent quartz plates, it was not possible to reduce the satellite pulses. However, etalon effects can be excluded as the two 10 mm long RTP crystals are separated 5 mm from each other. Tilting the PC to achieve a static quarter wave retardation and removing the QWP of the RA shows no change in the AC trace. Anyway, an AC trace after a single pass through the resonator (4 passes through the PC) without regenerative amplification shows no signs of satellite pulses. This indicates a resonant effect during multiple roundtrips caused by technical imperfection of the PC. Assuming a squared hyperbolic secant temporal profile, the pulses can be compressed to pulse durations ranging from 341 fs to 410 fs with 76 % in the main peak at 709 µJ output energy. For this performance (34 roundtrips, 709 µJ uncompressed output energy, 1 kHz repetition rate), a B-Integral of 7.23 is estimated by the use of Frantz-Nodvick equations [12]. A B-Integral of π is reached at an output pulse energy of 413 µJ. The highest influence on the B-Integral is attributed to the Tm:YAP crystal and the RTP crystals of the PC, of which the nonlinear refractive index n2 is not available in literature. Therefore, we choose the nonlinear refractive index of KTP (2.76 cm2 / W, from [13]) for our estimations because KTP is the closest isomorph to RTP in terms of optical and chemical properties of which data are available. (a )
(b )
7 5 0 6 0 0
In te n s ity in a r b . u .
P
u
l s e
e
n
e
r g
y
i n
µ
J
0 .8
4 5 0 3 0 0 1 5 0 0
1 .0
2 0
2 4
2 8
3 2
3 6
4 0
N u m b e r s o f r o u n d tr ip s
4 4
4 8
4 4 R T 3 9 R T 3 4 R T
∆τ4 4 = 4 2 2 f s ∆τ3 9 = 4 0 4 f s ∆τ3 4 = 4 1 0 f s
0 .6 0 .4 0 .2 0 .0
-7 .5
-5 .0
-2 .5
0 .0
D e la y in p s
2 .5
5 .0
7 .5
Fig. 4. (a) Output pulse energy in relation to number of roundtrips at 8.5 W absorbed pump power. (b) Compressed AC traces at 34, 39 and 44 roundtrips at an output energy of 700 µJ.
The linear relation of output energy to cavity roundtrips is shown in Fig. 4(a), starting from 24 roundtrips with an output energy of 47 µJ at an absorbed pump power of 8.5 W. The maximum pulse energy limited by the damage threshold of the laser crystal is achieved at a roundtrip number of 44. Figure 4(b) compares the AC traces after compression at an output pulse energy of ca. 700 µJ at 34, 39 and 44 roundtrips. At this fixed pulse energy, the pulses can still be compressed to 404 fs to 422 fs. Due to the longer roundtrips in the RA, the amplitude of the #238295 (C) 2015 OSA
Received 17 Apr 2015; revised 22 May 2015; accepted 22 May 2015; published 18 Jun 2015 29 Jun 2015 | Vol. 23, No. 13 | DOI:10.1364/OE.23.016884 | OPTICS EXPRESS 16888
satellite peak increases from 6.2 % to 15.8 % and shifts slightly from 3.2 ps to 3.6 ps. This can be either introduced by the longer roundtrip time (more absorptions, more complex phase profile) or higher nonlinearities (B-Integral of 12.2 at 44 roundtrips) inside the RA cavity or even both. ( b
) 7
5
0
5
0
0
6
7
5
4
5
0
6
0
0
4
0
0
5
2
5
3
5
0
4
5
0
3
0
0
t i o r a u d u p
n
d
e l s e
r e p
0 .5
1 .0
1 .5
2 .0
R e p e titio n r a te in k H z
2 .5
3 .0
m
0 .0
o
0
u
1 5 0
l s e
y r g e
3 0 0
s s e
J µ i n
4 5 0
P
P
u
l s e
e
n
e
r g
y
i n
µ
J
n
i n
6 0 0
f s
7 5 0
1
0
2
S
0
e
3
e
d
p
u
l s e
0
e
4
n
e
r g
y
i n
0
n
5
0
C
(a )
J
Fig. 5. Parameter variation at 34 roundtrips: (a) Output pulse energy vs. repetition rate at 7.5 W absorbed pump power. (b) Output pulse energy and compressed pulse duration vs. seed pulse energy at 9.8 W absorbed pump power.
The pulse energy can be further increased by choosing an appropriate repetition rate, which relation is depicted in Fig. 5(a) at a fixed number of 34 roundtrips and an absorbed pump power of 7.5 W. Starting from a repetition rate of 3 kHz with an output energy of 111 µJ, the output energy increases to the limited maximum of 702 µJ at a repetition rate of 100 Hz. Figure 5(b) shows the behaviour of the RA output while varying the seed pulse energy. The output pulse energy (black curve) shows a linear increase from 491 µJ at a seed energy of 12.3 nJ to a maximum pulse energy of 700 µJ at a seed energy of 48.7 nJ. Additionally, the compressed pulse duration (red curve) stays nearly constant between 380 fs and 402 fs for the different seed pulse energies and is shortest at the highest output power. Taking the compressor efficiency of 50 % into account, these values result in a peak power of 700 MW with 76 % of the energy confined in the main peak. In all measurements, no bifurcation was observed [14] which can occur at repetition rates close to the inverse of the upper state lifetime. 4.
Conclusion
In conclusion, we present an ultrafast RA system, completely based on Tm-doped materials (fiber oscillator, fiber preamplifier, and bulk regenerative amplifier). The system delivers > 700 µJ output pulse energy, only limited by the damage threshold of the laser crystal. The compressed AC traces show a pedestal in the unpurged case caused by atmospheric absorptions. This pedestal can be reduced by purging the cavity with nitrogen, whereas satellite pulses remain beside the main pulse. These satellite pulses are at least partly generated by the Pockels cell and can not be removed. Dechirping the pulses with a Martinez grating compressor results in a compressed pulse energy of 350 µJ and minimum pulse duration of 380 fs, which calculates to a peak power of 700 MW. By variation of the roundtrip numbers, repetition rate and seed energy, possible ways to increase the overall output pulse energy and efficiency are pointed out. Using more appropriate gratings with higher efficiencies of >90 % will allow for even higher output energies and peak power. This system can be used as input for a multi-pass amplifier with Tm:YAP to further scale the pulse energy to the multi-mJ level. Acknowledgment This work has been funded by the German Federal Ministry of Education and Research (BMBF) under contract 13N12079 “NEXUS”. #238295 (C) 2015 OSA
Received 17 Apr 2015; revised 22 May 2015; accepted 22 May 2015; published 18 Jun 2015 29 Jun 2015 | Vol. 23, No. 13 | DOI:10.1364/OE.23.016884 | OPTICS EXPRESS 16889
