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Abstract
We present a narrow-bandwidth (167 pm) few-nanosecond fiber laser based on tapered fiber amplification and phase modulation to mitigate stimulated Brillouin scattering (SBS). We compare the use of chirped diode seeding and standard sinusoidal phase modulation. Below SBS appearance threshold, output temporal profiles can be accurately tailored. Typical results for flat-top profiles include 170 kW peak-power (for 3 ns duration) and 1.25 mJ pulse energy (for 10 ns duration).
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
High peak-power fiber lasers, with narrow-linewidth nanosecond pulses and single-mode linearly-polarized beams are valuable sources for a large variety of applications such as coherent lidar systems [1], remote sensing [2], frequency conversion [3], fibered optical parametric chirped pulse amplification pumping [4] or inertial confinement fusion high-power laser facilities seeding [5–7]. As it allows fine temporal tailoring, precise synchronization, excellent optical signal-to-noise ratio and efficiency enhancement, master oscillator power amplifier (MOPA) scheme is the preeminent architecture for state-of-the-art narrow-linewidth nanosecond sources. In such sources, the temporal pulse shaping capability is obtained by using a Mach-Zehnder modulator driven by an arbitrary waveform generator and seeded by a continuous-wave fiber laser (CW). Then, the output is amplified by successive fiber amplification stages. Temporal tailoring allows gain saturation pre-compensation arising in the amplifiers chain that works in highly saturated regime. In case of narrow-linewidth operation, the peak power rising is primary limited by Stimulated Brillouin Scattering (SBS) which causes pulses instabilities, back reflection pulses and catastrophic damages effects of optical components. Several approaches for SBS mitigation during amplification have been proposed including strain or temperature gradient [8], gain-tailored core fiber [9], very short fiber [10] and many others. In the few nanosecond range, thanks to sinusoidal phase modulation technique SBS mitigation and pulse temporal shaping, 1.02 mJ energy and 5 ns duration (200 kW peak power) pulses have been obtained with a rod-type fiber in the last-stage amplifier [11]. By phase modulating the seeder with a pseudo-random noise signal, Di Teodoro et al achieved 2 mJ in a 1.55 ns pulse with two rod-type fibers in final stage [12]. By using a flexible single-mode fiber, Lago et al. achieved 0.5 mJ in a flat-top shaped 10 ns pulse and an energy record of 1.5 mJ but then at the expense of SBS appearance and significant degradation of the pulse shape [13].
As it consists to enlarge the core size along the fiber length, fiber tapering is also particularly attractive because it not only leads to a shift in the SBS spectrum [14] but it also allows efficient replacement of complex mode-field-adapter used to seed large-mode-area fiber in monolithic systems [15]. Indeed, tapered fiber design allows a single-mode end for signal input and a large-mode-area end for signal output. Adiabatic transition from the single-mode side to the slightly multimode side thus leads to selective, efficient and stable excitation of the fundamental mode, which ensures excellent beam quality. Thanks to such tapered fiber amplification, Huang et al. obtained a 300 W output in the CW regime [16] and Bobkov et al. amplified 8 ps pulse to a peak power of up to 760 kW [17]. In the nanosecond regime, Patokoski et al. recently achieved 288 µJ and 524 µJ energies in respectively single and both polarization states excitation with narrow bandwidth 130 ns pulses [18].
In this work, our goal is to realize a versatile all-fiber front-end able to seed Nd-doped phosphate-glass large-scale laser facilities dedicated to the process of inertial confinement fusion. We combine phase modulation technique and tapered fiber amplification to obtain 1.25 mJ in a 10 ns flat-top pulse with narrow bandwidth (167 pm) and very good beam quality. Up to 170 kW peak power (510 µJ) is also obtained for 3 ns pulses. These values for flexible fiber (i.e. that can be bent in contrast to rod-type fiber) amplification represent significant improvement of previous results, mainly limited by SBS appearance [13].
2. Description of experimental set-up
The experimental set-up is depicted in Fig. 1. It is composed of a seeder, an ytterbium doped fiber preamplifier and an ytterbium doped tapered fiber amplifier. In order to optimize the SBS mitigation in the fiber amplifier chain, we have compared two seeders with different phase modulation (PM). The first one, shown in Fig. 2(a), is based on pure sinusoidal PM. First, a distributed feedback (DFB) fiber laser delivers a CW wave with very narrow spectral linewidth (<10 kHz). After a first ytterbium-doped fiber amplifier (YDFA), an acousto-optic modulator (AOM) gates this wave (150 ns pulses) at 5 kHz. The optical spectrum is then broadened up to 32 GHz thanks to a phase modulator driven by single tone signal at 2 GHz. Figure 2(b) shows the optical spectrum composed of uniformly separated spaced Bessel peaks [19]. The pulses are then amplified by a second YDFA and spectrally filtered to remove most of the generated amplified stimulated emission (ASE). Finally, an electro-optic modulator (EOM), driven with an arbitrary waveform generator (Kentech) allows arbitrarily temporal shaping with a 12-bit amplitude resolution, 100 ps steps over a pulse envelope of several nanoseconds and 200 ps rising time. Figure 2(c) shows typical 10 ns waveform defined to pre-compensate the gain saturation arising in downstream amplifiers [20] and obtain a flat-top output temporal profile which is useful for materials processing and in the efficiency maximization of nonlinear wavelength conversion processes.
Fig. 2. (a) Sinusoidal phase modulation (PM) seeder set-up. CW DFB : continuous-wave distributed feedback laser, YDFA : ytterbium doped fiber amplifier, AOM : acousto-optic modulator, PM : phase modulator, RF : radiofrequency (2 GHz) driver, EOM : electro-optic modulator, AWG : arbitrary waveform generator (b) optical spectrum after the sinusoidal phase modulation (c) Typical pulse temporal profile at the seeder output
Instead of sinusoidal PM, the second seeder makes use on frequency chirping associated with modulation of a laser diode current [21]. Figure 3(a) depicts the corresponding set-up, similar to the sinusoidal PM seeder but a 1053 nm DFB semiconductor laser (provided by QD Laser) replaces the DFB fiber laser, the first YDFA, the AOM and the PM. Driving the DFB diode by 150 ns width square current pulses (700 mA) at 5 kHz allows chirped pulse delivering. The diode is heated at 23.8°C to center its central wavelength at 1053 nm. The pulse chirp and central wavelength are determined by mixing the DFB diode output to a very narrow (<10 kHz) linewidth 1053 nm DFB fiber laser (the one of the sinusoidal PM seeder). Figure 3(b) shows the induced beating measured after EOM temporal shaping at 10 ns with a 70 GHz oscilloscope and 55 GHz photodiode. Beating frequency evolution along the pulse corresponds to the two lasers wavelength shift. Therefore, beating cancellation at the beginning of the pulse points out 1053 nm wavelength. Figure 3(c) shows the spectrogram (in logarithmic scale) calculated from the beating measurement. We can see that a quasi-linear frequency chirp (quadratic PM) at 4.5 GHz/ns is obtained. Accordingly, the output of the seeder is then composed of pulses whose spectral broadening depends on the exact pulse duration.
Fig. 3. (a) Chirped diode seeder set-up (b) Frequency beating obtained by mixing the chirped diode seeder output with a narrow-bandwidth 1053 nm DFB fiber laser (c) corresponding spectrogram
Both seeders deliver output pulses in the few nJ energy range. These pulses are then preamplified by a 3.8 m long Yb-doped, polarization-maintaining, single-mode and double-clad fiber (Nufern PLMA-YDF-10/125-M) forward pumped by a CW diode at 976 nm. A 99/1 tap coupler is used to monitor the output and control the pulse shaping. Typically, 5 µJ energy can be obtained at the output of the preamplifier whatever the temporal shape of the seeder. The quantity of residual ASE is strongly reduced by spectral filtering (1 nm spectral width). The power amplifier is composed of an Yb-doped tapered fiber (provided by FORC) backward pumped by a 50 W CW diode at 976 nm. To lower the intensity at the output facet, a beam expanding end-cap was achieved by fusing 1.5 mm of a 400 µm pure silica multimode fiber at the fiber end facet. End-cap is angle cleaved at 8° to avoid parasitic lasing in the amplifying fiber.
Two different tapered fibers have been exploited in this work. Their longitudinal outer profiles are shown in Fig. 4(a). They present similar characteristics (pump absorption of 23 dB/m at 976 nm and 2.6 m length) and their narrow ends are comparable: around 70 µm cladding diameter, 10 µm core diameter, 8.8 µm mode field diameter (MFD). In contrast, their wide ends are quite different: 323 µm cladding diameter, 43 µm core diameter, 25 µm MFD for the first one [blue line in Fig. 4(a)] and 423 µm cladding diameter, 59 µm core diameter, 32 µm MFD for the second one [red line in Fig. 4(a)]. We measured the modal content at the output of both tapered fibers (without pumping the fibers) using a spatially and a spectrally resolved imaging technique (S2) [22]. Following the experimental setup described by Nguyen et al, we used a tunable narrow-bandwidth laser and an infrared camera [23]. For each camera pixel, an interference spectrum originating from the beating between guided modes can be recorded and for each high order modes (HOM) supported by the fiber a peak appears in the Fourier transforms of these spectra. Numerical treatment gives the intensity and phase profiles of the modes supported by the fiber, their relative power and their relative group delay. Figure 4(b) shows the summation of all the spectra’s Fourier transforms for the two tapered fibers. Both have been coiled down to a 20 cm radius with a 15 cm straight section at the input. However, different output straight section lengths have been tested: 40 cm with the 25 µm MFD fiber (thin black line), 40 cm (thick blue line) and 75 cm with the 32 µm MFD fiber (thick red line). No peak is observed with the 25 µm MFD tapered fiber, clearly showing that tapering multimode fiber is a very efficient and robust way to excite only the fundamental mode. On the contrary, because of wider mode area, the 32 µm MFD tapered fiber exhibits a peak corresponding to interferences between the fundamental mode and the first HOM for a 2 ps group delay. The power of this first HOM relative to the fundamental mode depends of the length of the output straight section (respectively −22 dB and −32 dB for the 40 cm and 75 cm lengths). Longer straight section clearly decreases the HOM excitation by minimizing the bending constraints.
Fig. 4. (a) Longitudinal profiles of 25 µm output MFD (blue line) and 32 µm output MFD (red line) tapered fibers. (b) Results of the S2 measurements : sum of all Fourier transforms over all the pixels for the 25 µm MFD tapered fiber with 40 cm long output straight section (thin black line) and for the 32 µm MFD tapered fiber with 40 cm (thick red line) and 75 cm (thick blue line) output straight sections. Inset: Image of the first HOM retrieved by S2 algorithm (amplitude + phase)
3. Results and discussions
The system is capable to deliver very high-energy pulses (typically few mJ but precise results depend on the taper and the seeder) without any damages. However, such high peak powers (∼MW) are associated with SBS appearance giving pulse temporal deformation and energy instabilities. Therefore, we decide to restrict ourselves to stable system performances clearly connected to the SBS appearance threshold. We determine this threshold by parasite pulse appearance monitoring on the backward port of the tap coupler placed between the preamplifier and the tapered fiber amplifier. Output pulses energies are measured with a pyroelectric joulemeter insensitive to CW (Ophir PE9-F). Accordingly, Fig. 5(a) and Fig. 5(b) show respectively the energy and corresponding peak power at the SBS threshold in function of the output pulse duration for the two seeders and the two tapered fibers. One has to point out that pre-compensated temporal shaping is defined to obtain square output temporal profile for each measurement point. As we can see, linear chirp allows higher pulse energy than the sinusoidal PM. Continuous spectral broadening is clearly more efficient than peak structured spectrum broadening even when its spectral width is narrower (spectral width of the chirped diode seeder varying from 9 GHz to 45 GHz compared to constant 32 GHz spectral width for the sinusoidal PM seeder). However, below 2 ns, the inverse behavior is expected because of insufficient spectral width. Moreover, as we can see for the 25 µm taper, the SBS threshold peak power is close to a 1/τ behavior for the sinusoidal PM seeder as expected for a transient Brillouin scattering regime [24]. This comportment is less obvious for the chirped diode seeder because of the spectral width temporal dependence. Besides, thanks to higher output MFD, the 32 µm MFD tapered fiber allow to deliver higher output energy than the 25 µm MFD tapered fiber. Then, the highest pulse energy for a 10 ns pulse duration is 1.25 mJ obtained for the 32 µm tapered fiber with the chirped diode seeder. It corresponds to a 125 kW peak power and 6.25 W output average power.
Fig. 5. (a) Output pulse energies and (b) corresponding peak powers at the SBS threshold in function of output pulse duration for the 25 µm MFD (blue), and the 32 µm MFD (red) tapers and for the chirped diode (dots) and the sinusoidal PM (triangles) seeders.
Figure 6(a) shows the 3 ns width pulse temporal profiles measured with a 55 GHz photodiode and a 70 GHz oscilloscope at the output of the tapered fibers for both seeders. As we can see, by limiting the output energy to the SBS threshold, it is possible to shape the output temporal profiles despite very high peak power (up to 170 kW). In fact, it is possible to raise the energy beyond the SBS threshold but we can no longer control the temporal shape. To evaluate temporal modulation in the top of the pulse, we use the distortion criterion $\alpha = 2[{({{P_{max}} - {P_{min}}} )/({{P_{max}} + {P_{min}}} )} ]$ where Pmax and Pmin represent respectively the maximum and minimum of pulse peak power [19]. It is equal to twice the temporal contrast and it ranges from 0% (no intensity variation) to 200%. Depending on the seeder and the taper, we find α=16% (black curve), α=13% (green curve), α=13% (blue curve), α=17% (red curve). Others pulse durations give similar results (not shown here) and using a feedback algorithm could achieved better values.
Fig. 6. (a) Output 3 ns pulse temporal profiles for sinusoidal PM seeder and 25 µm taper (black), sinusoidal PM seeder and 32 µm taper (green), chirped diode seeder and 25 µm taper (blue) and chirped diode seeder and 32 µm taper (red). (b) Power spectral density of the electrical signal measured in (a) for the sinusoidal PM seeder and 25 µm taper (black) and the chirped diode seeder and 32 µm taper (red).
In order to evaluate the phase or frequency modulation (FM) to amplitude modulation (AM) conversion [19], we have plotted in Fig. 6(b) the power spectral density of the electrical signal (“electrical spectrum”). Although we obtain distinct 2 GHz spaced peaks for the sinusoidal PM seeder (black), they are quite low (<40 dB) and do not lead to disqualifying power modulations. Discarding regularly spaced peaks at 12.5 GHz, 25 GHz, 37.5 GHz and 50 GHz that are intrinsic to the measurement system, we cannot see (as expected) any particular modulation frequency for the chirped diode seeder.
Figure 7(a) presents optical spectrum analyzer (OSA) recordings for the 3 ns temporal width showing narrow bandwidth (167 pm at FWHM) amplified signal and the ASE. We observe no stimulated Raman scattering. We estimate from the measured spectra that the relative ASE power compared to the one of the signal is about 6% (black curve) and 4% (green curve) for the PM sinusoidal seeder and it is raised to 8% (blue curve) and 12% (red curve) for the chirped diode seeder. Keeping in mind that the OSA integrates both CW ASE and very low duty cycle pulsed signal, we find that ASE induced optical signal-to-noise ratio is better than 50 dB even for the worst ASE ratio.
Fig. 7. (a) Optical spectra for the 3 ns pulse temporal profiles in logarithmic scale and in linear scale (inset) for sinusoidal PM seeder and 25 µm taper (black), sinusoidal PM seeder and 32 µm taper (green), chirped diode seeder and 25 µm taper (blue) and chirped diode seeder and 32 µm taper (red). (b) Spatial profile of the beam at the output of the 32 µm taper for 1.25 mJ and 10 ns pulse train
Figure 7(b) shows the spatial profile at the output of the 32 µm tapering fiber at 1.25 mJ pulse energy. The M2 value is characterized to be 1.1. Finally, the polarization extinction ratio measured at maximum energy with a polarimeter (Thorlabs) is about 25 dB at the output of the 25 µm tapered fiber and 15 dB at the output of the 32 µm tapered fiber.
4. Conclusion
To conclude, we have described a narrow-bandwidth (167 pm) controlled flat-top profile few-nanosecond (<10 ns) fiber laser with very good beam quality (M2=1.1) based on tapered flexible fiber amplification and phase modulation. 170 kW peak-power (for 3 ns duration) and 1.25 mJ pulse energy (for 10 ns duration) are obtained using chirped diode seeding instead of standard sinusoidal phase modulation. Such results should constitute an important step toward the integration of cumbersome high-power laser facility bulky front-ends.
Funding
Agence Nationale de la Recherche (ANR-11-EQPX-0017, ANR-11-LABX-0007); Région Hauts-de-France (CPER Photonics for Society P4S); Conseil Régional Aquitaine (CPER #16004205, FEDER #2663710).
Acknowledgement
This work has been supported in part by IRCICA, USR 3380, CNRS-Univ, F-59000 Lille, France (https://ircica.univ-lille.fr).
Disclosures
The authors declare no conflicts of interest.
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