High-peak-power pulse amplification in higher-order mode fibers

Fiber lasers have commercial applications in materials processing, machining, and remote sensing. As these applications can benefit from high peak powers and pulse energies, there is a corresponding need to reduce nonlinearities that are caused by high peak intensities. Consequently, a number of approaches have been used to try and increase the effective area of the fibers and thus achieve large-mode-area (LMA) high-power fiber lasers.¹ In all these approaches, however, the fibers are operated in the fundamental mode, which causes a significant reduction in mode area when the fiber is coiled.² This practical consideration means that it is difficult to scale the effective area of the fundamental mode beyond 1000μm² for manageable package sizes. Peak powers are therefore limited, for example, to a few hundred kilowatts. Furthermore, many of these approaches are difficult to fusion splice. The advantages of high-power fiber lasers (i.e., their robust and compact packaging) are therefore often lost when LMA fiber designs are used.

In previous work, we developed an alternate method in which we intentionally operated a specifically designed multi-mode fiber in a single, large-effective-area, higher-order mode (HOM).^{3, 4} These HOMs are—somewhat counter-intuitively—less susceptible to bend-induced area reduction than the fundamental mode.⁵ For example, an HOM with a 6000μm² effective area only reduces to 4500μm² with a coil diameter of 30cm. A fiber with a 6000μm² fundamental mode, however, can be reduced to 1700μm². In HOM amplifiers, long-period gratings (LPGs) can be used to provide phase-matched coupling with high modal purity. The fundamental mode can thus be converted to the desired HOM (LP_{0, N}, where N is the mode order). This works well at the input to the amplifier, i.e., where peak powers are low. An LPG at the output of the amplifier, however, is problematic. Peak powers in high-pulse-energy amplifiers can easily exceed many hundreds of kilowatts. This is enough to cause nonlinearity-induced changes to mode coupling in an output LPG, which is a situation that should be avoided.

To overcome this problem, we have thus developed a bulk-optic mode converter for the output of HOM fiber amplifiers. This converter is based on an axicon. Axicons are specifically designed conical lenses that are known to generate Bessel beams. We operate the axicon in the reverse of its usual direction. In this way, we are able to convert a Bessel-like fiber mode to a lower divergence, Gaussian-like beam. In addition—because we use bulk optics in our technique—we achieve nonlinear thresholds that are significantly higher than with fiber-based approaches.

An illustration of an HOM fiber amplifier system that includes our axicon mode convertor is shown in Figure 1. In this setup, the pump and signal are combined into a single fiber, i.e., the signal is launched into the fundamental mode of the HOM fiber. An LPG is used to convert the fundamental mode to the HOM, in which amplification takes place within an ultra-large area so that low nonlinearity is achieved. A specifically selected combination of lenses, as well as the axicon, are used at the output of the fiber to reconvert the HOM back to a low-divergence beam. In the final step, we use an aperture to strip away any high-divergence light that has not been reconverted. Crucially, the fiber portion of our system is fully fusion spliced, and it is coiled so that it has a 25cm diameter.

Figure 1. Schematic diagram of a higher-order mode (HOM) amplifier used in conjunction with an axicon to achieve mode conversion.

Beam profiles from an HOM amplifier—with and without our axicon mode converter—are shown in Figure 2. The beam profile that we measured when only the lenses are used (i.e., without the axicon) is illustrated in Figure 2(a). In this case, we are unable to achieve a tightly focused beam. However, when we place the axicon in the system—see Figure 2(b)—we are able to obtain a beam that is tightly focused. With the axicon, we measured a nearly diffraction-limited beam quality (M²) of 1.2 for 80% conversion efficiency and of 1.1 for 70% conversion efficiency.

Figure 2. Beam profiles measured from an HOM amplifier (as shown in Figure 1). (a) Beam profile of the amplifier when the axicon is not used in the system. (b) Beam profile of the amplifier when the axicon is used in the system.

The most important feature of our axicon mode converter is the high peak powers that can be achieved. For instance, we can obtain a peak power as high as 700kW from an erbium-doped HOM fiber amplifier, without adding any nonlinearity to the system.⁶ This is due to our output mode conversion approach. We have also conducted a femtosecond chirp-pulse amplifier experiment. In this test, an erbium-doped HOM amplifier showed a sevenfold increase in pulse energy over a fundamental mode erbium-doped amplifier that had a 1000μm² effective area.⁷

HOM amplifiers provide ultra-large effective area fibers that can be coiled and fusion spliced. We have developed a new approach, in which we use axicons combined with specific lenses to achieve high peak powers without introducing nonlinearities to the system. With this method we can thus fully use the large effective area of HOM fibers by producing nonlinearity-free conversion to beams that are almost at the diffraction limit after they have been amplified to the high-peak-power pulses. Our future work will be aimed at improving the mode conversion efficiency that can be achieved with the axicons. We will also develop fiber-integrated versions of the axicons to eliminate the necessity for bulk-optic alignment of the mode convertor.