High-gain Er-doped fiber amplifier generating eye-safe MW peak-power, mj-energy pulses

High-gain Er-doped fiber amplifier generating eye-safe MW peak-power, mj-energy pulses Sebastien Desmoulins and Fabio Di Teodoro 1,* Aculight Corporation, 22121 2 th Avenue S.E., Bothell, WA 921 1 Currently with Northrop Grumman Space Technology, Redondo Beach, CA 927 * Corresponding author:fabio.diteodoro@ngc.com Abstract: We report on a large-core, Er-doped fiber amplifier that generates pulses of ~1.1ns duration and maximum pulse energy/peak power ~1.4 mj/1.2 MW, at 157nm wavelength, while concurrently providing optical gain in excess of 25 db, in a multi-mode output beam (M 2 ~.5). 2 Optical Society of America OCIS codes: (14.351) Lasers, fiber; (14.44) Optical amplifiers; (.241) Fibers, erbium. References and links 1. C. D. Brooks and F. Di Teodoro, 1-mJ energy, 1-MW peak-power, 1-W average-power, spectrally narrow, diffraction-limited pulses from a photonic-crystal fiber amplifier, Opt. Express 13, 999-92 (25), http://www.opticsinfobase.org/abstract.cfm?uri=oe-13-22-999. 2. R. L. Farrow, D. A. V. Kliner, P. E. Schrader, A. A. Hoops, S. W. Moore, G. R. Hadley and R. L. Schmitt, High-peak-power (>1.2MW) pulsed fiber amplifier, Proc. SPIE 12, 12L (2). 3. W. E. Torruellas, Y. Chen, B. McIntosh, J. Farroni, K. Tankala, S. Webster, D. J. Hagan, and M. J. Soileau, High peak power ytterbium doped fiber amplifiers, Proc. SPIE 12, 12N (2). 4. M.-Y. Cheng, Y-C. Chang, A. Galvanauskas, P. Mamidipudi, R. Changkakoti, and P. Gatchell, Highenergy and high-peak-power nanosecond pulse generation with beam quality control in 2-µm core highly multimode Yb-doped fiberamplifiers," Opt. Lett. 3, 35-3 (25). 5. C. D. Brooks and F. Di Teodoro, Multi-megawatt peak-power, single-transverse-mode operation of a 1μm core diameter, Yb-doped rod-like photonic crystal fiber amplifier, Appl. Phys. Lett. 9, 111119-111122 (2).. F. Di Teodoro, M. Savage-Leuchs, and M. Norsen, High-power pulsed fiber source at 157nm, Electron. Lett. 4, 1525-152 (24). 7. B. Desthieux, R. I. Laming, and D. N. Payne, 111 kw (.5 mj) pulse amplification at 1.5 μm using a gated cascade of three erbium-doped fiber amplifiers, Appl. Phys. Lett. 3, 5-5 (1993).. M. Savage-Leuchs, E. Eisenberg, A. Liu, J. Henrie, M. Bowers, and A. J. W. Brown, High pulse energy extraction with high peak power from short-pulse, eye-safe all-fiber laser system, Proc. SPIE 12, 127 (2). 9. B. C. Dickinson, S. D. Jackson, T. A. King, 1mJ total output from a gain-switched Tm-doped fiber laser, Opt. Commun. 12, 199-23 (2). 1. V. N. Philippov, J. K. Sahu, C. A. Codemard, W. A. Clarkson, J.-N. Jang, J. Nilsson, and G. N. Pearson, All-fiber 1.15-mJ pulsed eye-safe optical source, Proc. SPIE 5335, 1 (24). 11. Y. Zhang, B. -Q. Yao, Y. -L. Ju, and Y. -Z. Wang, "Gain-switched Tm3+-doped double-clad silica fiber laser," Opt. Express 13, 15-19 (25), http://www.opticsinfobase.org/abstract.cfm?uri=oe-13-4-15. 12. Y. Jeong, J. K. Sahu, D. B. S. Soh, C. A. Codemard, and J. Nilsson, High-power tunable single-frequency single-mode erbium: ytterbium codoped large-core fiber master-oscillator power amplifier source, Opt. Lett. 3, 2997-2999 (25). 13. D. Taverner, D. J. Richardson, L. Dong, J. E. Caplen, K. Williams, and R. V. Penty, "15-µJ pulses from a single-transverse-mode, large-mode-area erbium-doped fiber amplifier," Opt. Lett. 22, 37-3 (1997). 14. J. Minelly, F. Di Teodoro, M. Savage-Leuchs, D. Alterman, S. Desmoulins, C. Brooks, and E Eisenberg, High peak power and high energy fiber amplifiers, Proc. SPIE 453, 4532-1(27). 15. S. Desmoulins and F. Di Teodoro, Watt-level, high-repetition-rate, mid-infrared pulses generated by wavelength conversion of an eye-safe fiber source, Opt. Lett. 32, 5-5 (27). 1. G. P. Agrawal, Nonlinear Fiber Optics (Academic Press, 21, Third Edition), Chap. 4.1.4. 17. J.C. Jasapara, M. J. Andrejco, A. D. Yablon, J. W. Nicholson, C. Headley, and D. DiGiovanni, Picosecond pulse amplification in a core-pumped large-mode-area erbium fiber, Opt. Lett. 32, 2429-2431(27). (C) 2 OSA 1 February 2 / Vol. 1, No. 4 / OPTICS EXPRESS 2431

1. Introduction Pulsed optical sources operating at eye-safe wavelengths are in high demand for applications such as remote sensing/imaging and materials processing, where light scattering off targets can pose a hazard. Er-doped fiber sources offer the benefit of direct eye-safe operation (wavelength > 1.5μm) and, thanks to the large investments of the telecommunication industry, can leverage a wide array of qualified off-the-shelf components, which provides affordability, reliability, and form/fit/function flexibility for smooth integration in deployable platforms or established industrial processes. However, the power performance of these sources has lagged considerably behind Yb-doped pulsed lasers and amplifiers, which have already reached peak power of ~1 MW [1-5] or higher [3-5] with concurrent pulse energy > 1mJ [3-5]. In fact, the highest reported peak powers directly emitted by eye-safe fiber sources do not exceed 17 kw [], to our knowledge. In addition, the pulse energies are limited to <.5mJ for pulse durations of a few nanoseconds [-], while exceeding the mj level only in longer-pulse operation [9-11]. To date, many realizations of high-peak-power eye-safe fiber sources have been based on Er/Yb-codoped fibers [, ], which offer the advantage of efficient ~97nm-wavelength pump absorption over short lengths, ensured by Yb ions. However, these fibers require careful composition control to optimize the non-radiative energy transfer between Yb and Er ions and are, therefore, more difficult to fabricate and less reproducible than fibers based on directly pumped rare earths. Moreover, they may present energy storage limitations due to the finite Yb Er energy transfer rate, which leads to bottlenecking effects at high power and concurrent Yb-ion amplified spontaneous emission (ASE) at ~1μm wavelength [12]. These issues can be circumvented by using fibers doped with erbium only [13]. In particular, using Er-doped fibers to amplify a long-wavelength seed (e.g. in the 15-11nm range, a.k.a. L band) is expected to improve energy storage due to the near four-level nature of the gain medium in this wavelength region and ensuing higher saturation energy compared to sources operating near the Er gain peak (~153nm) [14]. SEEDER EDF EDF IBPF SOA IBPF IP FBG IBPF BPF EYDF DM EDF Pulsed current DM OI LC-EDF Output POWER AMP Fig. 1. Schematic view of the eye-safe fiber-based source architecture described in this article. FBG: Fiber Bragg grating; IP: In-line (fiber-coupled) polarizer; : ~9nm pump diode; EDF: Erbium doped fiber; : In-line optical isolator; IBPF: In-line band-pass filter; SOA: Fiber coupled semiconductor optical amplifier; EYDF: Er/Yb-co-doped fiber; DM: Dichroic mirror; BPF: Bulk band-pass filter; OI: Bulk optical isolator; LC-EDF: Large-core Er-doped fiber. In this article, we report on a 5μm core-diameter Er-doped fiber amplifier that generates 157nm-wavelength, ~1ns pulses of energy > 1 mj and peak power > 1 MW, at multi-khz, continuously variable pulse repetition frequency (PRF), while concurrently supplying optical (C) 2 OSA 1 February 2 / Vol. 1, No. 4 / OPTICS EXPRESS 2432

gain in excess of 25 db. To our knowledge, the result amounts to the highest peak power generated in a rare-earth-doped fiber operating at eye-safe wavelength and, at the same time, highest optical gain obtained in a fiber amplifier producing mj-level pulse energy. Although the output beam was multimode (M 2 ~.5), the demonstrated power performance and high gain indicate that large-core Er-doped fibers are good candidates for single-stage, simple, and practical high-energy eye-safe amplifiers. 2. Experimental layout The architecture of our fiber source, schematically shown in Fig. 1, consists of a pulseadjustable seeder followed by a large core Er-doped fiber power amplifier. The seeder is similar to that described in Ref. 15 and features a piece of Er-doped, singlemode (SM), polarization-maintaining (PM) fiber core-pumped (through a fiber multiplexer) by a 9nm-wavelength SM fiber-coupled diode laser and operated as a double-pass CW ASE source. The double-pass design permits to select as the signal a linearly polarized spectral slice (of defined central wavelength and linewidth) from the broadband ASE. Pulsed operation is then obtained by transmitting this signal through a pair of semiconductor optical amplifiers equipped with SM-PM fiber pigtails and driven by a pulsed current. The seeder is completed by two single-mode, core-pumped PM Er-doped fiber amplifiers and a final, large mode area (~15μm core-diameter), double-clad PM Er/Yb-co-doped, booster fiber amplifier backwardpumped using free-space optics. Throughout the seeder architecture, inter-stage isolators and band-pass filters are used to prevent feedback and reject ~1535nm ASE, respectively. From the seeder, we obtain linearly polarized pulses of ~1.2ns duration, continuously adjustable PRF (which we varied from 7.5 to 2 khz for this work, see below), energy in excess of 4 μj, 157nm wavelength, and spectral width <.4 nm. Pulse energy (mj) 1.2 1....4.2 Data Fit 2 4 Incident pump power (W) Fig. 2. Pulse energy (directly measured with a pyroelectric joulemeter) and corresponding average power (calculated as pulse energy PRF) emitted by the 5μm core-diameter Erdoped fiber amplifier as a function of pump power incident on its output facet. Blue dots: data; Red line: linear fit. The seeder output is injected into the power amplifier, which consists of a 5μm corediameter,.1 core-numerical-aperture, Er-doped multimode fiber (~32μm pump-cladding diameter, ~9.5m length, ~. db/m cladding absorption at 9nm). The fiber is backward pumped (using free-space optics) by a diode bar (~9nm wavelength, >9W max output power) equipped with a 2μm core-diameter,.22 NA delivery fiber. The output end of the Er-doped fiber was fusion-spliced to an -angle-polished, ~μm-diameter, ~1mm-long silica endcap to ensure adequate beam expansion and avoid facet damages. 12 1 4 2 Pulse average power (W) (C) 2 OSA 1 February 2 / Vol. 1, No. 4 / OPTICS EXPRESS 2433

3. Results and discussions Figure 2 shows the pulse energy and corresponding pulse average power (pulse energy PRF) vs. pump power incident on the large-core Er-doped fiber with the seeder operating at a PRF = 1 khz. To unambiguously discriminate power in the pulse from background ASE, the pulse energy was measured directly using a high-prf pyroelectric joulemeter (Coherent/Molectron) insensitive to cw radiation. Pulse energy (mj) 1.5 1..5 5 1 15 2 25 Pulse repetition frequency (khz) Fig. 3. Pulse energy (blue) and corresponding average power (red) emitted by the 5μm corediameter Er-doped fiber amplifier as a function of pulse repetition frequency. The largest pulse energy obtained at PRF = 1 khz was 1.2 mj, corresponding to pulse average power of 12 W. Since the incident seed pulse average power was ~34mW, the Erdoped fiber amplifier gain exceeds 25 db. The amplifier slope efficiency with respect to pump power incident on the fiber end facet is greater than 2%, as extracted from a linear fit to the data. The total power exiting the fiber amplifier was also measured using a thermopile detector and found to be ~12.15 W. This finding indicates that approximately 15mW of copropagating cw ASE is emitted from the output end, which corresponds to overall pulse contrast (defined as pulse energy PRF/ total average power) in excess of 19dB. 14 12 1 4 2 Pulse average power (W) Pulse power (MW) 1.2..4 PRF (khz) 7.5 1 2 (a) 1.2 Peak power (MW)..4 (b) 2 4 Time (ns) 5 1 15 2 Pulse repetition frequency (khz) Fig. 4. (a) Temporal profiles of the pulses emitted by the Er-doped power fiber amplifier at maximum pulse energy for three distinct PRF values (7.5, 1, and 2 khz). The vertical scale has been calibrated in power units by equating the integral under the recorded profiles to the directly measured pulse energy. (b) Maximum peak power vs. PRF. In Fig. 3, output pulse energy and average power are plotted against the PRF in the 7.5-to- 2kHz range. All data points were recorded at constant pump power. Maximum pulse energy of 1.4mJ was obtained at 7.5 khz. The pulse average power was as high as ~ 12.W at (C) 2 OSA 1 February 2 / Vol. 1, No. 4 / OPTICS EXPRESS 2434

PRF=2kHz, remained to within 5% from this value in the 1-2 khz PRF range, and dropped to 1.W at PRF = 7.5 khz due to a more pronounced ASE buildup. Figure 4(a) shows temporal profiles of amplified pulses emitted by the 5μm-core Erdoped fiber amplifier at maximum pulse energy, for three PRF values (7.5, 1, and 2 khz). Peak-normalized spectrum (db) -1-2 -3-4.4 nm -1-5 5 1 λ λ (nm) 15 152 154 15 15 1 Wavelength (nm) 12 14 Fig. 5. Peak-normalized spectrum (logarithmic scale) of the Er-doped power fiber amplifier output, recorded at PRF = 7.5kHz, maximum energy (1.4 mj). Inset: Amplified output (black) and seeder (red) spectrum (linear scale) plotted vs. distance from the pulses central wavelength (λ ). The profiles were recorded using a fast photodiode and a real-time digital oscilloscope (overall temporal resolution < 2 ps). The measured pulse duration was approximately 1.1ns. The peak power was determined by calculating the integral under the pulse profile and equating it to the pulse energy (directly measured with the pyroelectric joulemeter). The highest peak power, obtained for PRF = 7.5 khz, is approximately 1.2 MW, the highest value ever obtained in an eye-safe fiber source, to our knowledge. As shown in Fig. 4(b), the peak power was still > 1 MW at PRF = 1 khz and exceeded kw at PRF = 2 khz. Figure 5 shows the output pulse spectrum recorded at PRF = 7.5 khz and pulse energy/peak power ~1.4 mj/1.2 MW. Although the pulse spectral full-width at half maximum (FWHM) remained approximately equal to.4 nm, hence virtually unchanged with respect to the seed pulses, significant broadening is observed in the pulse tails as captured by the logarithmic-scale plot. This broadening is ascribed to self phase modulation [1] and possibly four wave mixing, with some contribution from background ASE. In particular, ASE is deemed responsible for the asymmetry between the noise levels at the right and left of the pulse feature, due to the greater in-fiber absorption for ASE generated near the Er gain peak. To provide a more quantitative assessment of pulse spectral brightness than the mere FWHM, we determined (by integrating under the recorded spectra) the width, Δν, of the spectral windows that contain 7,, and 9% of the total pulse power. In Fig., these Δν values are plotted vs. emitted pulse energy, for the example case of PRF = 1 khz. As can be seen, the spectral window containing 9% of the pulse energy exhibits a width in excess of 14nm at maximum pulse energy (=1.2 mj). (C) 2 OSA 1 February 2 / Vol. 1, No. 4 / OPTICS EXPRESS 2435

Spectral width, Δν (nm) 14 12 1 4 2 Fraction of pulse energy in Δν 9% % 7%.2.4.. 1. 1.2 Pulse energy (mj) Fig.. Spectral linewidth vs. pulse energy emitted by the Er-doped power fiber amplifier (PRF = 1kHz). Three types of linewidth are considered, each defined by the fraction of the total pulse energy contained in it: 9% (black solid circles), % (red hollow circles), and 7% (green solid triangles). 1/e 2 beam radius (mm)...4.2 R x data Fit (M 2 x =.3) R y data Fit (M 2 y =.7) -2-1 1 2 Distance from beam waist (mm) Fig. 7. Solid circles and hollow squares: Beam radius measured along perpendicular directions (R x and R y, respectively) as a function of distance from the waist location. Solid lines: Hyperbolic fits to the data. The amplifier emitted a spatially multi-mode, smooth, near-gaussian beam, which appeared relatively insensitive to mechanical perturbation of the fiber. The beam quality was measured by using a calibrated moving knife-edge apparatus and results are illustrated in Fig. 7. The M 2 values determined from the fit were.3 and.7 (obtained along orthogonal transverse directions), corresponding to a beam parameter product (radius NA) of approximately 4 mm mrad. We stress that the seed light was coupled into the Er-doped fiber amplifier using free-space optics and no particular optimization of the beam launching conditions was carried out to improve the beam quality. (C) 2 OSA 1 February 2 / Vol. 1, No. 4 / OPTICS EXPRESS 243

5. Conclusions We report on the performance of a large-core (5μm diameter) double-clad Er-doped fiber in the amplification of ~1ns-duration pulses to concurrently obtain high pulse energy and peak power at an actively adjustable, multi-khz PRF. From this amplifier, we obtained maximum peak power of 1.2MW and concomitant pulse energy of 1.4mJ at PRF=7.5 khz. Pulse average power in excess of 12 W was also obtained in the 1-2 khz PRF range. Although the beam exiting the fiber is multimode (M 2 ~.5), the present results are significant in that they show, for the first time to our knowledge, the potential of fibers doped with Er only and operated near or within the L band for generating very high peak power (> 1 MW, a record for eye-safe fiber sources), while affording ample energy storage and large gain (25dB). The beam quality can be improved by optimizing the launch conditions into the Erdoped fiber amplifier [12] through input mode-field matching. Moreover, Er-doped fibers of lower core NA (hence fewer guided modes) can be used. Such fibers are usually more straightforward to fabricate compared to Er/Yb-codoped ones, which feature in-core phosphorous doping resulting in higher core refractive index [17]. Acknowledgments This work was funded by the Missile Defense Agency under the SBIR contract number W9113M--C-7. (C) 2 OSA 1 February 2 / Vol. 1, No. 4 / OPTICS EXPRESS 2437