Abstract
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[
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[
Recently, several independent groups have proposed a novel system setup[
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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[
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
Parameter | Value | Unit | Parameter | Value | Unit |
---|---|---|---|---|---|
976 | nm | 1070 | nm | ||
1120 | nm | 1180 | nm | ||
20 | 400 | ||||
8000 | W | 0.84 | ms | ||
200 | W | 0.8 | — | ||
17 | m | ||||
0.3 | THz | 0.3 | THz | ||
40 | THz | ||||
Table 1. The parameters of the calculated fiber amplifier.
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
Figure
3.2 Effect of suppressing backscattered Stokes light
From the numerical example mentioned in Section
Parameter | Value | Unit | Parameter | Value | Unit |
---|---|---|---|---|---|
1.38 | 10 | ||||
1.38 | 200 | ||||
250 | |||||
1000 | 293 | K |
Table 2. Parameters used to calculate the thermal distribution.
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 (
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[
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[
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[
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[
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 (
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[
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[
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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