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 1567nm wavelength, while concurrently providing optical gain in excess of 25 dB, in a multi-mode output beam (M2 ~8.5).
©2008 Optical Society of America
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 170 kW , to our knowledge. In addition, the pulse energies are limited to <0.5mJ for pulse durations of a few nanoseconds [6–8], 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 [6, 8], which offer the advantage of efficient ~976nm-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 . These issues can be circumvented by using fibers doped with erbium only . In particular, using Er-doped fibers to amplify a long-wavelength seed (e.g. in the 1560–1610nm 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 (~1530nm) .
In this article, we report on a 65µm core-diameter Er-doped fiber amplifier that generates 1567nm-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 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 (M2 ~8.5), 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 pulse-adjustable 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, single-mode (SM), polarization-maintaining (PM) fiber core-pumped (through a fiber multiplexer) by a 980nm-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 backward-pumped 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 20 kHz for this work, see below), energy in excess of 4 µJ, 1567nm wavelength, and spectral width < 0.4 nm.
The seeder output is injected into the power amplifier, which consists of a 65µm core-diameter, 0.16 core-numerical-aperture, Er-doped multimode fiber (~362µm pump-cladding diameter, ~9.5m length, ~0.8 dB/m cladding absorption at 980nm). The fiber is backward pumped (using free-space optics) by a diode bar (~980nm wavelength, >90W max output power) equipped with a 200µm core-diameter, 0.22 NA delivery fiber. The output end of the Er-doped fiber was fusion-spliced to an 8°-angle-polished, ~600µm-diameter, ~1mm-long silica endcap to ensure adequate beam expansion and avoid facet damages.
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=10 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.
The largest pulse energy obtained at PRF=10 kHz was 1.2 mJ, corresponding to pulse average power of 12 W. Since the incident seed pulse average power was ~34mW, the Er-doped fiber amplifier gain exceeds 25 dB. The amplifier slope efficiency with respect to pump power incident on the fiber end facet is greater than 20%, 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 150mW of co-propagating 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.
In Fig. 3, output pulse energy and average power are plotted against the PRF in the 7.5-to-20kHz 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 ~12.6W at PRF=20kHz, remained to within 5% from this value in the 10–20 kHz PRF range, and dropped to 10.6W at PRF=7.5 kHz due to a more pronounced ASE buildup.
Figure 4(a) shows temporal profiles of amplified pulses emitted by the 65µm-core Er-doped fiber amplifier at maximum pulse energy, for three PRF values (7.5, 10, and 20 kHz).
The profiles were recorded using a fast photodiode and a real-time digital oscilloscope (overall temporal resolution < 200 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=10 kHz and exceeded 600 kW at PRF = 20 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 0.4 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  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 70, 80, and 90% of the total pulse power. In Fig. 6, these Δν values are plotted vs. emitted pulse energy, for the example case of PRF = 10 kHz. As can be seen, the spectral window containing 90% of the pulse energy exhibits a width in excess of 14nm at maximum pulse energy (=1.2 mJ).
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 M2 values determined from the fit were 8.3 and 8.7 (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.
We report on the performance of a large-core (65µm 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 10-20 kHz PRF range.
Although the beam exiting the fiber is multimode (M2 ~8.5), 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 Er-doped fiber amplifier  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 .
This work was funded by the Missile Defense Agency under the SBIR contract number W9113M-06-C-0076.
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