High Average Power Generation of Single-Transverse Mode MW-peak Power Pulses using 80-µm Core Yb-doped LMA

Fibers

Ming-Yuan Cheng, Kai-Chung Hou, and Almantas Galvanauskas EECS Department, University of Michigan, 1301 Beal Avenue, Ann Arbor, MI 48109-2122

Phone:(734)515-7166; Fax:(734)763-4876; email: myc@umich.edu Doruk Engin, Rupak Changkakoti, and Pri Mamidipudi Optical Air Data System, 10531 Terminal Road, Manassas, Virginia 20110

Abstract: Diffraction-limited nanosecond pulse generation with MW-peak powers and multi-mJ-energies has been explored using 80-µm core Yb-doped fibers, demonstrating 1-MW peak power pulses with 85-W of average power at 100-kHz and 1.56-MW peak at 50-Hz. 2006 Optical Society of America OCIS codes: (140.3510) Fiber lasers (140.3070)Infrared and far-infrared lasers

Fiber laser robustness and power scalability provides a highly preferable technological platform for a variety of demonstrated nanosecond-pulse applications, including laser-plasma produced EUV generation and remote sensing. These applications require high peak powers in ~1-MW range, 1 – 10 ns pulse durations and high average powers in preferably diffraction-limited beams. Megawatt-peak power in ~1-ns pulses have been previously demonstrated with a diffraction-limited output from a fiber laser [1]. Recently similar peak powers in 1-ns pulses have been repeated for narrow-linewidth (~10GHz) amplified signals [2]. Even higher peak powers of >2.4-MW in 4-ns pulses have been achieved using very large-core fibers with multimode output beams (M2 ~ 6.5) [3]. However, all these results have been achieved at low average powers of less than 10 Watts, and single-mode operation has been demonstrated with LMA fibers of less than 50-µm core diameter (mode-field diameters of less than 35-µm) with corresponding peak-power and energy limits.

In this paper, we demonstrate a systematic study of high energy and high peak power generation with near-diffraction-limited beam quality using 80-µm core Yb-doped fiber amplifiers. This fiber constitutes the largest demonstrated mode area (2750-µm2) that maintains single-transverse-mode operation with output beam M2 = 1.2. We report achieving 1.56-MW peak power in 0.7-ns pulses at 50-Hz repetition rate, constituting the highest peak power achieved so far for a diffraction-limited output. We also demonstrated 1-MW peak power pulses at 100-kHz repetition rate in 85-W average power output beam, which is an order of magnitude improvement over the highest previously reported average power of high-energy pulsed fiber laser [2].

The experimental set-up is a three-stage all-fiber system. The first stage is a single-mode core-pumped Yb-doped fiber pre-amplifier stage, seeded with an electric-pulse-driven diode laser emitting at 1064-nm and pumped with telecom-grade 980nm single-mode diodes. Such a seeding scheme enables controls of pulse duration, pulse shape and repetition rate. The second stage is a 30-µm core double-clad 3.5m long Yb-doped fiber amplifier. Diffraction-limited output is achieved by coiling the fiber to a radius of 4.25cm. The final power amplifier stage is a large core double-clad 3m long Yb-doped fiber amplifier with 80-µm diameter and 0.06NA core, and 400-µm diameter and 0.46NA inner pump cladding. This large-mode-area fiber amplifier was specially designed for extracting millijoule pulse energies with megawatt peak powers with single-transverse mode output beam. Beam quality of M2=1.2 has been achieved by coiling fiber to the radius of 5cm. This fiber was pumped with up to 146-W of coupled power at 915-nm.

Figure 1 shows the achieved peak powers (solid circles) and pulse energies (empty circles) for various amplified pulse durations. At the shortest – 0.7-ns duration we have achieved the highest 1.56-MW peak power (corresponding to 1.1-mJ pulse energy – peak powers were accurately determined using measured pulse shape and measured pulse energy). We can see that achieved peak powers are lower at longer pulse durations. Solid line marks inverse-square-root fit to the experimental points. Pulse energy is generally increasing with pulse duration, reaching more than 3-mJ for pulses longer than 2-ns. Some apparent discrepancy between peak powers and energies is simply related to the different shapes (different amount of energy in pulse “tail”) of the different-duration pulses. Energies of greater than 7-mJ have been extracted with ~100-ns pulses (not shown in this plot). All data points in this plot have bean measured at 50-Hz repetition rate. Note that no damage has been observed in the last stage at any of the achieved peak power levels (end of fiber is end-cap protected from surface damage). Pulse shortening was observed for pulse energies exceeding saturation-energy of 2-mJ for this fiber. By tailoring the input pulse shape, for example, a step-shape pulse or a triangle pulse, square temporal pulses were achieved at the output of the last stage.

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Initial average power testing has been performed at 100-kHz repetition rate and sub-nanosecond pulse durations. So far we reached 85-W average power (shown in Fig. 2) and, simultaneously, 1-MW peak power, which is the highest average power achieved with a megawatt peak power pulsed fiber laser.

In summary we have explored simultaneous high average- and peak-power generation with nanosecond fiber lasers. Specially designed 80-µm diameter core fiber enables extraction of MW-peak and mJ-energy pulses in a single-transverse mode beam. We estimate that demonstrated power and energy characteristics do not constitute a limit and further scaling is anticipated. Demonstrated power scalability is particularly important for laser-plasma related applications such as generation of high-power EUV radiation. References 1. A. Galvanauskas, “Mode-scalable fiber-based chirped pulse amplification systems”, IEEE J. Sel. Topics Quantum Electron., 7, 504 (2001) 2. F. D. Teodoro and C. D. Brooks, “1.1MW peak-power, 7W average-power, high-spectral-brightness, diffraction-limited pulses from a photonic crystal fiber amplifier”, Optics Letters 30, 2694-2696(2005) 3. M.-Y. Cheng, Y.-C. Chang, P. Mamidipudi, R. Changkakoti, P. Gatchell, and A. Galvanauskas, High energy and high peak power nanosecond pulse generation with beam quality control in 200-µm core highly multimode Yb-doped fiber amplifiers, Optics Letters 30, p. 358 – 360 (2005)

Fig. 1. Measured amplified pulse peak power as a function of the pulse duration

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slope efficiency 60%

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Fig. 2. Measured amplified average power as a function of the pump power.

a2713_1.pdf CThAA3.pdf