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Abstract
We report on a high-power picosecond all-fiber Tm-doped fiber amplifier (TDFA) seeded by a gain-switched laser diode (LD) in the 2 µm spectral range. A total average output power of 409 W (304 ps) has been generated at 320 MHz of repetition rate with 10 dB bandwidth of ~48 nm centered at 1970 nm. Over 140 W of spectrally flat supercontinuum (SC) output has been produced at 40 MHz of repetition rate with optimized fiber length. The 10 dB spectral bandwidth was 615 nm, ranging from 1965 to 2580 nm. The prospects for further power scaling of LD seeded ~2 µm picosecond all-fiber sources are discussed.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
High-power picosecond lasers have attracted extensive attention in applications such as material processing [1,2], nonlinear frequency conversion [3,4], and SC generation [5]. Based on these demands, many studies have used the Yb [6], Er [7], and Tm-doped [8] fiber lasers at 1 μm, 1.5 μm, and 2 μm, respectively. Compared with the Yb and/or Er-doped fibers, Tm-fiber, which offers a broader emission band [9], larger mode field diameter, and a higher mode instability threshold [10], is a promising candidate for high power ultrafast lasers in the 2 μm spectral range. High power picosecond TDFAs have been reported with ~200 W of average output power using mode-locked Tm:fiber lasers as the seed [8,11]. Mode-locked fiber sources are sometimes sensitive to environmental disturbance, e.g., mechanical vibration. Seed instability will inevitably lead to output fluctuation and, in some cases, device damage of the MOPA system.
Fiber amplifiers seeded by gain-switched LDs have recently been reported and show advantages over the traditional amplifiers in compactness and reliability [12–16]. Driven by a periodic electronic pulse, the LD can deliver an arbitrary shape pulse in the nanosecond and picosecond regions with tunable repetition rates up to gigahertz [16], which is suitable for high average power operation. So far, over 600 W of average output power at ~1 μm has been reported from Yb:fiber amplifiers seeded by a gain-switched picosecond LD. Further improving the output power was limited by the mode instability [13]. In the 2 μm spectral range, over 18 W of average output power has been generated from a TDFA seeded with a picosecond diode laser [14] and up to 295 kW of peak power has been demonstrated at 1 MHz of repetition rate [15].
In this paper, we report on the high power operation of an all-fiber TDFA system directly seeded by a gain-switched picosecond LD delivering 409 W of average output power at 320 MHz repetition rate, corresponding to a slope efficiency of 61% with respect to the launched pump power. The output pulse width was 304 ps and the 10 dB spectral linewidth was ~48 nm centered at 1970 nm. At a reduced repetition rate of 40 MHz and by properly optimizing the fiber length in the main-amplifier, significant spectral broadening was achieved and a 142 W high-flatness SC was generated with a 10 dB spectral bandwidth covering from 1965 to 2580 nm. The optical-to-optical conversion efficiency was 27.7% with respect to the launched pump power at 793 nm. To the best of our knowledge, these results represent the highest output power of LD directly seeded picosecond all-fiber TDFA and SC source in the 2 µm spectral range.
2. Experimental setup
The experimental setup of the all-fiber TDFA system is shown in Fig. 1. The seed source is a gain-switched picosecond laser diode (ps LD) with a 3 dB pulse width of ~200 ps at 1961 nm (PicoQuant Inc.). The average output power of the LD is 340 μW with a tunable repetition frequency up to 80 MHz. It has a broad spectral width ranging from 1850 nm to 2030 nm, which is too broad to suppress the amplified spontaneous emission (ASE) in the following amplifiers. The LD spectrum was first filtered by a 1970 nm band-pass filter (3 dB pass bandwidth of ~6 nm). Multi-stage pre-amplifiers and band-pass filters were constructed to supply enough signal power and suppress the ASE. The first two stage amplifiers were single-mode TDFAs, i.e., 0.5 m single-mode TDF (SM-TDF) with a core diameter of 10 μm (0.11 numerical aperture [NA]) and cladding diameter of 130 μm (0.46 NA) (Nufern Inc.), core-pumped by a 1550 nm Er, Yb-codoped fiber laser with an absorption coefficient of ~90 dB/m. The third stage of the TDFA was a 4 m SM-TDF (cladding absorption coefficient of 3 dB/m at 793 nm), which was cladding-pumped by a 793 nm LD through a (2 + 1) × 1 combiner. Cascaded isolators (ISOs) and band-pass filters between the two amplifiers were used to avoid parasitic lasing and remove the excess ASE.
A frequency multiplier formed by cascaded two-stage fiber Mahzedel interferometers was constructed to raise the repetition rate 4 times, i.e., 320 MHz. After that, the signal power was injected to a fourth pre-amplifier, which employed a 4 m SM-TDF and was cladding-pumped by a 12 W pigtailed LD through a (2 + 1) × 1 combiner. Then, the signal power was injected to the main-amplifier through an ISO and a home-made mode field adaptor (MFA). In the main-amplifier, co-pumped and counter-pumped amplifier structures were employed. A piece of 4 m large-mode-area double-clad Tm-doped fiber (LMA-TDF, core diameter of 25 μm [0.09 NA] and inner cladding diameter of 400 μm [0.46 NA]) with a cladding absorption coefficient of 4 dB/m at 793 nm was pumped by six 793 nm LDs through a (6 + 1) × 1 combiner. The available pump power launched into the cladding of the TDF was measured to be ~683 W. A cladding pump stripper (CPS) made by a matched passive double-clad fiber (FUD-3793, Nufern Inc.) was used to strip the residual pump light and some high-order modes in the inner cladding of the TDF. The active fiber was coiled in a radius of ~10 cm and embedded in a water basin directly, and the water was maintained at ~10 °C to increase the laser conversion efficiency. Finally, an endcap was spliced to the end of the amplifier.
3. Results and discussions
3.1 High-power picosecond pulse generation
The repetition frequency of the ps LD seed source was set to be 80 MHz. Figure 2 shows the laser spectra during the pre-amplification. The optical spectra were measured by an optical spectrum analyzer (0.5 nm resolution). The ps LD has a broadband ASE-like spectrum. This kind of spectra results from the carrier density of the LD changing with the current pulse under high-speed current modulation, leading to the variation in the refractive index of the active channel of the LD. Thus, many side modes are produced consequently [16]. After passing through the filter, the spectral width of the seed laser was narrowed to ~7 nm and the power was reduced to 11 μW. After the first TDFA, the signal to ASE ratio was only 17 dB due to the generation of ASE in the TDFA (see Fig. 2). The signal was further filtered to reduce the ASE reaching 3.6 mW with a ratio of 45 dB. After the second TDFA, the ASE was suppressed by a factor of 38 dB. The signal to ASE ratio was increased to 50 dB after a third filter and the power was 37 mW. The signal power was further amplified by the third TDFA to reach 410 mW. No obvious ASE was observed from the optical spectrum. Single-pulse profiles were monitored using a 12.5 GHz InGaAs photodetector (818-BB-51, Newport Inc.) with a rising time of 28 ps, and recorded by a 62 GHz real-time digital oscilloscope (DSA-X 96204Q, Agilent Inc.). The corresponding results are shown in Fig. 3(a). Driven by the electronic pulse, the ps LD seed produces strongly negatively chirped pulses [17,18] with a 3 dB pulse width of ~191 ps. Consequently, the pulse width was broadened slowly from 191 ps to 232 ps in the following amplifiers, which were constructed by fibers with negative group velocity dispersion. After the three pre-TDFAs, the pulse energy and peak power were amplified to 5.1 nJ and 22 W, respectively.
Fig. 3 Pulse properties during the amplification. (a) single-pulse profiles detected by a fast photodetector; (b) Pulse train before and after the frequency multiplier.
In the frequency multiplier, each interferometer was constructed by a 2 × 2 coupler with a power ratio of ~50:50. The fiber delay lines in the first and second interferometers were ~129 cm and ~64 cm, respectively. Before the frequency multiplier, the pulse-to-pulse amplitude fluctuation was ~3%, as shown in Fig. 3(b). The power ratio deviation of the couplers and the power loss induced by the fiber delay lines increased the pulse-to-pulse amplitude fluctuation to ~5%. After the frequency multiplier, the available signal power was 189 mW from one arm of the coupler. Then, on the fourth stage, the TDFA boosted the signal power to 3.8 W after the ISO. There was no significant change in pulse width (238 ps), as shown in Fig. 3(a). The pulse energy and peak power were 11.9 nJ and 50 W respectively, resulting in slight spectral broadening, as shown in Fig. 2(b).
In the main-amplifier, laser performances of the co-pump and counter-pump structures were firstly compared. Figure 4(a) shows the average output power of the co-pumped and counter-pumped amplifiers. For the co-pumped amplifier, an output power of 409 W was achieved when the launched pump power was 683 W, corresponding to a slope efficiency (η) of 61%. Further power scaling was limited by the available pump power. On the other hand, for the counter-pumped structure, the laser produced an output power of 223 W for the launched pump power of 395 W, corresponding to a slope efficiency of 57%. Considering the ~90% signal transfer efficiency of the (6 + 1) × 1 combiner, about ~25 W laser power was loaded inside the combiner. Power scaling was limited by the potential damage of the combiner. Figure 4(b) shows the output laser spectra around the 220 W average output power of the two configurations with a resolution of 2 nm. Both of the spectral widths of the co-pumped and counter-pumped amplifiers were broadened due to nonlinear effects, such as self-phase modulation and modulation instability [19]. However, the spectral broadening in the co-pumped structure was greater than that of the counter-pumped structure due to a relatively longer nonlinear optical length. Figure 4(c) shows a typical single-pulse profile in the co-pumped amplifier at the maximum output power. The pulse width was broadened to be 304 ps, and the corresponding pulse energy and peak power were 1.3 μJ and 4.2 kW, respectively. The output spectra were recorded at different output power, as shown in Fig. 4(d). With the increase of the power, the 3 dB spectral width remained unchanged at 1970 nm, while the 10 dB spectral width broadened from 12 to 48 nm. Under high repetition frequency, the peak power was reduced so that the spectral broadening was controlled effectively.
Fig. 4 Output characteristics of the main-amplifier. (a) Average output powers of the co-pumped and counter-pumped amplifiers versus the launched pump power; (b) output laser spectra of the co-pumped and counter-pumped amplifiers around 220 W output power; (c) single-pulse profile of the co-pumped amplifier at the maximum output power; (d) output laser spectra of the co-pumped amplifier at different output power.
3.2 Flat SC generation in TDFA
Typically, the SC in the mid-infrared region is produced by soft glass fibers such as ZBLAN [5] or InF3 [20] that can provide the advantages of low intrinsic losses and strong nonlinear effects. However, the melting points of the soft glass fibers are relatively low, resulting in limited output power. Silica based fiber has an extremely high damage threshold, which enables it to produce high power SC at 2-3 μm. In the TDFA, the nonlinear effects such as self-phase modulation, stimulated Raman scattering, and modulation instability can lead to spectral broadening. In addition, the transition from 3H4 to 3H5 in the Tm energy system can generate the 2.2–2.5 μm lasing [21]. Flat SC under the two physical mechanisms can be generated from the TDFA. From Fig. 4(d), it can be observed that the spectra has been broadened at high output power in the TDFA. Further, the frequency multiplier was removed from the TDFA chain and the repetition frequency of the seed was set to be 40 MHz. The purpose was to increase the pulse peak power to enhance the nonlinear effects in the amplifiers, thus broadening the laser spectrum. Three LMA-TDFs with different fiber length of 4 m, 5 m, and 6 m followed by 1 m matched passive fiber were employed in the co-pumped main-amplifier to optimize the spectral flatness. When the Tm fiber length was 6 m, flat SC was generated with a 10 dB spectral bandwidth of 615 nm, i.e., from 1965 nm to 2580 nm (see Fig. 5(a)). Under high peak power, Raman soliton pulses can generate induced by the modulation instability in the anomalous dispersion region of the fiber. Therefore, the spectrum shifted to the long wavelength by the soliton self-frequency shift [22]. A total output power of 142 W was obtained when the launched pump power was 512 W with an optical-to-optical conversion of 27.7%, as shown in Fig. 5(b). Extending the passive fiber to 2 m has no significant contribution to the spectral flatness when the Tm fiber length was 6 m. On the contrary, the maximum output power decreased to 106 W due to the huge intrinsic loss [23]. Furthermore, increasing the Tm fiber length can promote the spectral flatness obviously compared with the passive fiber, if the total fiber length was constant in our experiment. In the main-amplifier, the amplification of the pulse signal and the spectral broadening occur at the same time. The generation of long-wavelength regions increases the quantum defect; in addition, when these power components propagate in the silica fiber, they will face a big loss. Consequently, the conversion efficiency decreases with the increase of output power, thus generating a large amount of heat in the optical fiber. However, from the experiment it can be observed that there is no obvious saturation in output power as a function of the launched pump power, meaning that the output power still can be scaled if the heat dissipation is managed properly.
Fig. 5 Output performances of the SC in the TDFA. (a) Final spectra for different fiber lengths; (b) average output powers for different fiber lengths.
4. Conclusion
In conclusion, we have demonstrated a high power all-fiber TDFA system at ~2 μm spectral range seeded by a picosecond LD. The TDFA delivers 409 W of average output power with 304 ps of pulse duration and 320 MHz of repetition rate, corresponding to a slope efficiency of 61% with respect to the launched pump power. At 40 MHz repetition rate, the TDFA yields 142 W of high flatness SC with a 10 dB spectral bandwidth ranging from 1965 to 2580 nm. In both cases, there is no obvious saturation in output power as a function of launched pump power, indicating that there is considerable scope for further power scaling of LD directly seeded high power all-fiber picosecond TDFA. Compact and reliable high-power picosecond laser sources in the 2 μm spectral range should find a wide range of applications in special material processing and mid-infrared generation.
Funding
National Key R&D Program of China (2017YFB1104400).
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