Toward high-power nonlinear fiber amplifier Download: 611次
1 Introduction
The output power of single chain fiber laser has been growing in recent years due to the fast development of pump laser diode (LD), active fiber, advanced heat management method, and so on. There has been a long time when stimulated Raman scattering (SRS) effect is considered to be one of the main obstacles for power scaling in general-type fiber lasers[1–3]. The generation of SRS in fiber laser system might cause serious effects due to the backscattered Stokes light[4]. Therefore, SRS in high-power fiber laser system has been under intensive investigation and lots of technical solutions to suppress SRS have been proposed and validated, such as employing large-mode-area fiber, and decreasing the length of active fiber by using highly doped fiber[4–10]. Nevertheless, these are not intrinsic solutions and sometimes they would bring in side effects. For example, increasing the mode area would degrade the beam quality apart from mode control techniques. Using highly doped gain fiber may increase the thermal load per length and enhance the photon-darkening effect. However, from the other point of view, SRS can be used to help lasing, which is often called Raman fiber laser. Raman fiber laser has unique properties such as the broad gain spectrum and the wavelength versatility, which have been demonstrated in a large variety of wavelength bands[11–21].
Up to now, most of the reported Raman fiber laser is achieved by core-pumping single-mode (SM) fiber, where the output power is determined directly by the pump laser. The maximal output power of the conventional core-pumped Raman fiber laser is about several-hundred-watt level[13–16]. In addition, the brightness can not be increased in the core-pumping configuration. Cladding pumped Raman fiber laser by using double-clad (DC) fiber has the potential for power scaling because of the significant increase in pump power[22–26]. In order to increase the conversion efficiency, specialized fiber is often required in cladding pumped fiber lasers, by which hundred-watt level output power has been achieved[24]. It is to be noted that recently LD pumped Raman fiber laser based on multimode (MM) fiber has also drawn intensive attention[27–30] because of its potential in brightness enhancement due to the self-beam cleanup effect in MM fiber[31, 32]. Up to 140 watts level output power has been reported[30]. Generally, by now, pure Raman fiber lasers based on various kinds of configurations, including core-pumping SM fiber, cladding pumping DC fiber, and LD pumped MM fiber, have been successfully demonstrated. Nevertheless, the maximal output power is hundreds of watts, which is much lower than that of fiber lasers based on active fiber.
Recently, several independent groups have proposed a novel system setup[33–40] named Yb-Raman fiber amplifier or nonlinear fiber amplifier (NFA, we call it nonlinear fiber amplifier in this paper), where laser gain provided by both SRS and active fiber is employed. In this new setup, the signal laser is firstly amplified via doped ions in the fiber and then the laser power is transferred into Stokes wave via SRS effect. Cladding pumping technique is employed in the system to ensure sufficient pumping power. Raman gain is employed designedly to induct forward scattered Stokes light and reduce the unwanted backscattered Stokes light. In addition, the system setup is compatible with standard fiber laser configuration that is usually based on master oscillator power amplifier (MOPA). Thus COTS active fiber and fiber components can be used without special design. Based on the aforementioned properties, NFA has achieved significant results in recent 5 years and has become a new route for high-power fiber lasers. Near 4 kW level output power based on Yb gain and Raman gain has been realized[37], which is comparable with standard fiber laser/amplifier. And this concept has been extended to obtain high-power lasing at band based on both Tm gain and Raman gain[40].
In the present paper, we provide a general and detailed study on high-power NFA. In Section
2 Modeling of nonlinear fiber amplifier
The general system setup of an NFA is plotted in Figure
The emission spectrum of Yb-doped fiber (from 970 nm to 1200 nm) can be divided into discrete spectral channels with width of . The subscript represents the th channel but , and represent the pump and two signal waves specially. The superscript corresponds to positive and negative directions, respectively. is the Yb ions concentration distribution along the fiber, for passive fiber ; is the excited state population; represents the signal power; is the power of laser ; and are Yb absorption and emission cross sections, respectively[41]. is the overlapping factor between the pump (ASE signal) and the fiber doped area. is the Raman gain coefficient; is the loss coefficient. is the Raman noise and the factor 2 corresponds to two polarization states. is the Planck constant; is frequency; is bandwidth of the signal. For spontaneous Raman noise we assume the bandwidth equals the gain bandwidth that is about40 THz.
The boundary conditions can be described by the following equations:
3 Numerical analysis of nonlinear fiber amplifier
3.1 Power scaling potential of NFA
SRS is a main restriction for the power scaling of wide bandwidth YDFA. There is still not a straightforward technique that can suppress SRS effectively without introducing any drawback in YDFA. In this section we would like to show the advantage of the NFA in the potential of suppressing SRS by using a numerical example. The parameters used in the calculation are shown in Table
Fig. 2. The power distribution of pump, signal, and Raman waves in forward and backward propagating directions. It can be found that Raman Stokes wave (1120 nm) has arisen in both directions, and the power is 642 W and 387 W for forward and backward directions, respectively.
Table 1. The parameters of the calculated fiber amplifier.
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Firstly, we calculate a traditional case of only the 1070 nm laser in the seed. The seed power is 200 W. The power distribution along the fiber is shown in Figure
Fig. 3. (a) The power distribution of the pump, signal, and first-order Stokes waves; (b) the forward and (c) backward output spectra of the NFA. The seed is consisted of 200 W 1070 nm laser and 10 W 1120 nm laser, respectively.
Figure
3.2 Effect of suppressing backscattered Stokes light
From the numerical example mentioned in Section
Fig. 4. The dependence of backward output power on the power of 1120 nm seed laser. Insets are the output spectra in backward direction at different power.
Table 2. Parameters used to calculate the thermal distribution.
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3.3 Heat analysis of NFA
Fiber laser has the advantage on thermal conduction due to its special geometry. But for high-power fiber laser system, fibers are not completely immune from thermal effects, so reducing the thermal burden is one of the most important things to guaranty the system running safety. Theoretically, the quantum defect of the nonlinear fiber amplifier is larger than that of conventional fiber amplifier because of the using of longer signal wavelength (e.g., 1120 nm compared with 1070 nm). In this section, we would compare the temperature distribution of these two kinds of amplifiers. The center temperature of the core in the YDF can be described by Equations (
Fig. 5. The center temperature of the core for NFA and conventional amplifier in (a) forward pumping scheme and (b) bi-direction pumping scheme. In the calculation, all the parameters of the bi-direction pumping scheme are the same as of the forward pumping scheme, excepting the pump power is divided equally into two directions.
We calculated the temperature distribution of the core center for the example proposed in Section
In the forward pumping NFA, the energy extraction can roundly divided into two parts. The first part is the amplification of the short signal wavelength laser (1070 nm), in which the ytterbium gain contributes more. And the second part is the nonlinear amplification, which requires the power of short signal wavelength laser to reach the nonlinear threshold. Consequently, the nonlinear effect induced energy transfer happens in the latter half of the fiber amplifier. But in the backward pumping or bi-direction pumping configurations, the nonlinearity related energy extraction and the Yb ions related amplification would be overlapped in the same piece of fiber, which will result in more thermal burden generation compared with conventional YDFA. Figure
4 Experimental study on nonlinear fiber amplifier
4.1 Laser diode pumped high-power NFA
In recent years, laser diode pumped high-power NFA has been demonstrated by several independent groups. In this sub-section, we will generally review the high-power experimental result achieved in our group.
As a proof-of-concept demonstration, we built an NFA system as shown in Figure
After that we aim at more powerful NFA. The system shown in Figure
It is to be noted that the concept of NFA can be extended to polarization-maintained fiber amplification straightforwardly, and we have achieved an output power of 1181 W with polarization-extinction ratio (PER) of 18.2 dB[38].
4.2 Tandem-pumped high-power NFA
As indicated in previous discussions, NFA has the potential to break the power limitation induced by SRS during the power scaling process, and then the next power limitation might be the brightness of pump diode. In most of present high-power fiber laser systems, laser diodes are used as pump source. Recently, tandem pumping technique, which uses ‘fiber laser’ to pump active fiber, has been under intensive research, where the brightness of the pump laser is significantly increased[3, 46–49]. In the present sub-section, we demonstrate the NFA in tandem pumping scheme for the first time.
As shown in Figure
When the pump power is injected, the total output power increases linearly with the change of the power ratio of the two waves. It should be noted that when the pump power is higher than 800 W, 1150 nm laser starts to increase fast while 1090 nm laser ceases rising. The 1150 nm laser gain mainly comes from the Raman amplification that becomes prominent as the power increases or waves propagate. Finally, the total output power is 1530 W with 1090 nm laser power of 703 W and 1150 nm laser power of 827 W. The power of 1150 nm laser is the highest one at this wavelength as we know. The output spectrum at full power is shown in Figure
For comparison, we also conduct the experiment where only 1090 nm laser is seeded. It is a conventional tandem-pumped Yb-doped fiber amplifier, in which the 56 W seed could be linearly boosted to 1530 W, shown in Figure
It is to be noted that most of the published literatures for YDFL focus on the common wavelength band such as 1070–1080 nm, whose operating power can achieve kilowatt level without too much difficulty. Because of the much smaller relative net gain and the significant amplified spontaneous emission at common band, lasing at 1120–1200 nm band (which also locates in the emission region of Yb-doped silica fiber) is much more challenging[53–58]. Nevertheless, lasing at 1120–1200 nm band could find tremendous applications including biomedicine, new-style pump sources and remote sensing. Most of the demonstrated NFAs work at those wavelength bands, which provides a coincident solution for high-power laser source.
Fig. 10. The output property of the tandem-pumped NFA: (a) output power; (b) spectrum at full power.
Fig. 11. The measured 1090 nm laser power in the case of only 1090 nm seed with power of 56 W.
5 Discussion
In this section, we briefly discussed several new topics in high-power NFAs.
The first one is the FWM. In practical high-power NFA seeded by multi-wavelength laser, FWM might happen because the phase-mismatch could be compensated by the Yb gain[59]. In this case, the modeling of high-power NFA might be modified by taking FWM into consideration, which could be re-written as follows.
The superscript corresponds to positive and negative directions, respectively. The subscript represents the pump, signal, first-order and second-order Stokes waves, respectively. The complex electric-field envelope is related to power by . The terms on the right-hand side of Equations (
Fig. 12. The power distribution of the NFA example proposed in Section 3.1 calculated by the FWM model.
The boundary conditions can be described by the following equations.
In order to evaluate the influence of FWM in the NFA, we calculate again the example proposed in Section
Figure
The second one is MI. We have to note that MI has become a serious challenge for further scaling the output power[60–67]. Generally, MI could be attributed to the power coupling between fundamental mode and high-order mode supported by the fiber waveguide. Theoretical and experimental results have proved that the MI threshold would be higher if the core diameter of the active fiber is smaller[60, 65]. In this case, tandem-pumped NFA, where larger-mode-area fiber is preferred to ensure sufficient absorption, would encounter complex nonlinear effect, i.e., the MI effect trends to transfer power to high-order mode, while the Raman gain provided by the NFA simultaneously has a self-cleaning effect that is apt to lower mode operation. MI in pure cladding pumped Raman fiber laser has been investigated[66], and thus the coupling between MI and Yb/Raman might have plenty nonlinear dynamics that should be further investigated.
The third one is PD effect. Since SRS effect suppression is no longer required, highly doped fiber, which is often used to shorten the fiber length, is also not required in high-power NFA. Moderately or lowly doped active fiber could be employed and thus PD effect suppression could be expected in NFA. However, as shown in Section
Last but not the least, NFA in different wavelength bands, for example, band, has also been achieved[40]. We have to note that up to now there are no theoretical predictions. It is known that lasing at is more difficult for Tm-doped fiber than for Ho-doped fiber[69], however, Tm-doped fiber is relatively more mature than its Ho-doped counterpart[70–72]. In this case, it is worthwhile to study the feasibility of obtaining high-power laser at band based on Tm/Raman NFA. The potential in power scaling of mid-infrared fiber laser[73] based on rare-earth ion and Raman gain is another interesting topic.
6 Conclusion
Through theoretical investigation and experimental validation, it can be seen that NFA fully explores the power scaling potential of both rare-earth ions and SRS in cladding pumped fibers, while simultaneously settled the power limitation and hazard induced by SRS in classical high-power fiber laser system, and it opens a new physically straightforward and technically feasible solution for obtaining ultra-high-power fiber lasers. Tremendous study might be illuminated by this concept to develop high-power fiber laser system. In addition, the plenty new physical insight within the amplifier that includes several nonlinear optical effects coupled with each other also might be interesting for theoretical studies.
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Article Outline
Hanwei Zhang, Pu Zhou, Hu Xiao, Jinyong Leng, Rumao Tao, Xiaolin Wang, Jiangming Xu, Xiaojun Xu, Zejin Liu. Toward high-power nonlinear fiber amplifier[J]. High Power Laser Science and Engineering, 2018, 6(3): 03000e51.