Corrigendum: <>

MINGHENG YUAN ^1,2* Yuan Sui ^1,2 Zhenao Bai ¹ Zhongwei Fan ^1,2

• ¹ Aerospace Information Research Institute, Chinese Academy of Sciences (CAS), Beijing, Beijing, China
• ² School of Optoelectronics, University of the Chinese Academy of Sciences, Beijing, Beijing, China

The rise in output power from different kinds of laser sources over the past decades has gradually made it possible to realize (or closer to) the directional energy, high-performance material processing, laser propulsion, etc., [1–8] Fiber laser has a distinguished position amongst high-power lasers due to the advantages of high efficiency, high beam quality and strong environmental adaptability, therefore, has become one of the key breakthrough directions in laser community. In recent years, the development momentum of fiber laser has been extremely rapid, and its output power has been greatly improved [9, 10].In the past few years, people have devoted themselves to the power scaling of narrow linewidth (<100 GHz) output in all polarization-maintaining fiber lasers. In 2017, IPG Photonics developed a 1.5 kW polarization-maintaining narrow linewidth laser and a 2 kW non-polarization-maintaining narrow linewidth laser [11]. In 2018, they further increased the output power of the polarization-maintaining fiber laser to 2 kW, and the output power of the non-polarization-maintaining fiber laser to 2.5 kW [12]. In 2019, Huang et al. from the Department of Precision Instruments of Tsinghua University built a few-longitudinal-mode fiber oscillator using a narrow-linewidth fiber Bragg grating [13]. After amplification, a 2.19 kW laser output was achieved, with an output efficiency of 78.3%. In 2020, Wang et al. from the Institute of Applied Electronics, China Academy of Engineering Physics demonstrated a 3 kW-class narrow linewidth polarization-maintaining fiber laser [14]. The output power is up to 3.08 kW with the beam quality factor (M2) of 1.4 and the polarization extinction ratio of 94%. However, as the output power of the fiber laser increases, nonlinear effects such as stimulated Raman scattering (SRS), stimulated Brillouin scattering (SBS) and transverse mode instability (TMI) effects in the fiber gradually become apparent, leading to the reduction of output power and the degradation of beam quality [15–19]. These nonlinear effects limit the further improvement of fiber laser output power. In order to solve such problems, researchers have turned their attention to improving the structure and materials of optical fibers, hoping to design fibers that meet high peak power output. At present, commonly used fibers include large mode area fiber (LMA) and photonic crystal fiber (PCF). LMA fiber can also be used to achieve high power output from fiber lasers. But the large mode field area optical fiber transmission process easily leads to the appearance of high-order modes, and it is necessary to adopt the correct and reasonable mode control methods such as bending and coiling to realize single-mode transmission. Moreover, once the core diameter of a fiber with a large mode field area is larger than 25 μm, it is difficult to stably control the transmission mode of the fiber. Although PCF fiber can achieve single-mode output, it will cause great mode loss. It is not conducive to the integration of the system.In response to the above problems, in 2007, the Ultrafast Optics Research Center of the University of Michigan [20] proposed a new optical fiber structure named chirally-coupled core (CCC) fiber. It can break through the limitation of the normalized cut-off frequency of V = 2.405 of traditional single-mode fiber, and achieve stable single-mode output in the case of large core size (greater than 30 μm) without any mode control technology. In addition, CCC fibers offer the benefits of modal distortion-free splices and compact coils (coil radius less than 15 cm), matching optics fabricated using standard fiber splicing and processing techniques [21]. With the advantage of high integration and high TMI threshold, CCC fiber provides a new approach to realizing high peak power and high energy lasing, therefore has become a promising research area in the laser community.In this paper, the recent advances in CCC fiber are reviewed along with the introduction of its basic structure and related parameters. Meanwhile, the special functions of the CCC structure are expounded from three aspects, and the coupling between the higher-order modes in the fiber is discussed from the quasi-phase matching condition.2 Concept and Development of Chirally Coupled Core Fiber2.1 Basic Structure of Chirally Coupled Core FiberChirality refers to the fact that the object cannot coincide with the mirror image after any spatial operation such as translation and rotation. The macroscopic continuous media formed by such chirality analysis are called chiral media. As early as 1989, Engheta and Pelet [22] proposed the concept of chiral waveguides, namely waveguide structures containing chiral media.Different from the ordinary optical fibers, chiral coupling fibers are composed of two chiral waveguide fibers. There is a central straight core on the shaft with a large core diameter, which can be over 55 μm. An additional helix-side core deviates from the central one, which forms the CCC around the central straight core. Figure 1A shows the 3D geometry of this structure and Figure 1B shows the cross-section view. This chiral coupling fiber [22] structure can be formed in the fiber prefabricated rod using a conventional fiber prefabricated bar. The central straight core is used for signal light transmission [23]. The main function of the helix-side core [24, 25] is to control the mode of the central straight core, coupling the high order mode into the side core and producing high loss to it. The base modes [26, 27] in the central core can be transmitted almost without loss. In this way, the CCC fiber [28–33] does not rely on any mode control technology to maintain a single-mode transmission while achieving a large mode field area. And the above problems are well solved.