Mode-locked Yb-doped fiber laser based on mode coupler

The spatially structured optical field has special characteristics of spatial phase and polarization distribution, which has important research value in the super-resolution microscopy system, particle manipulation, nonlinear optics, and large-capacity optical communication system. The vector optical fields generated by spatial light modulators (SLM), vortex phase plate and q plates are always limited by the power threshold and thermal damage thresholds and light energy utilization. In addition, there are several problems including complex light path and vulnerability to the environment. Moreover, the average power, pulse energy and peak power of the new type optical field based on the field manipulation scheme have met the bottleneck.

Ultrashort laser micro-machining has been widely implemented in the industry. When compared with that of Gaussian beams, the hollow intensity distribution of the circular cylindrical vector beams enables the applications in some special laser cutting scenarios, such as the commercial ring lasers provided by IPG and FEIBO Laser. In the optical fiber laser system, the optical field manipulation and mode coherent control based on the fiber eigenmodes provide new ideas for the new type optical field manipulation. The different high-order vector mode fields in the optical fiber are strict fiber eigenmodes. They can be transmitted stably in the optical fiber and  are interference resistant. Meanwhile, they have the advantages of compact all-fiber structure, easy fabrication method, low cost, and low loss. At present, fiber mode converters are mainly based on the generation and transmission of high-order modes in few-mode fibers. The methods include displaced phase excitation, grating mode coupling, fiber evanescent field coupling, and acousto-optic mode coupling.

To solve the above problems, the research group of Prof. Zeng Xianglong from Shanghai University has proposed the mode-locking schemes based on fiber mode devices (mode selective couplers, acoustic grating, and broadband long-period grating) and corresponding laser technology, which enables high-order transverse mode from visible to near-infrared regime by intracavity high-order transverse participation in laser oscillation. the. The research group has achieved fiber mode selective couplers in 1.5 μm,and 1.0 μm laser wavebands and visible light wavebands, which are utilized to generate ring optical field and vortex optical field by taking advantages of high-order mode vector characteristics of the fiber.

Fig. 1 (a) Schematic of mode-locked Ytterbium-doped fiber laser based on cascaded MSC. (b) Mode field distribution of linearly polarized LP11 mode, ring-shaped vortex light and its interference fringes. (c) Mode conversion of cascade MSC.

Fig. 2 Mode-locked spectral and pulse sequence of high-order mode oscillation output

Recently they have demonstrated a mode-locked Yb-doped fiber laser (YDFL) able to achieve HOM intracavity laser oscillation[1]. A pair of cascaded mode selective couplers (MSCs) acts as an efficient mode convertor to generate the intracavity oscillating HOMs in 1.0 μm laser wavebands. This method can achieve mode-locked HOM pulses in an Yb-doped few-mode fiber cavity. The results show this approach is promising for directly generating pure and efficient HOMs in all-FMF ultrafast Yb-doped fiber lasers. Additionally, MSCs are also applicable at visibale wavebands (400-800 nm).

The ring-shaped beams  in STED application are usually generated by free-space optical devices, which is with high-cost, unstable and requires strict alignments between the depleted and excitation beams along propagation in space transmission, not to mention the complicated collimation of the optical path. While the mechanisms of the all-fiber  structures have significant advantages including compact structures, high stability, and low cost, which play important roles in the research of ring-shaped excitation

The research group has also fabricated all-fiber MSC at the visible wavelength, which was used to generate ring-shaped beam and as multiplexer of Gaussian beam at arbitrary wavelength. The two beams enter the two input ports of the MSC respectively, which can also be multiplexed. The PSF of donut beam measured by confocal system shows the 10-dB high extinction ratio. The MSC can be employed to the STED microscopy system. All-Fiber MSC has the characteristics of strict beam space overlap, which can be applied to all-fiber STED microscopy light source system [2]. Therefore, all-fiber HOMs solution in the visible band becomes of great significance.

Fig. 3 (a) The mechanism of visible MSCs and (a) wave-length multiplexing of donut-shaped and Gaussian beams (c) The PSF of donut-shaped beam

Visible MSCs can convert fundamental mode into a depleted ring-shaped beam at the output port of FMF. By optimizing the parameters of the MSCs, the wavelength-dependent donut beams are produced and multiplexed by Gaussian beams with different wavelengths, which are output from a common FMF and concentrically distributed. Figure 3 shows the optical field distribution of the donut-like beam, Gaussian beam and wavelength multiplexing of these two beams. The point spread function of the donut-shaped beam is experimentally measured to be 182 nm.

The research group will focus on the optical field manipulation based on the optical fiber mode system next, which has high flexibility and stability compared with the spatial optical path system. The group will continue to study the applications of high-power laser spatial structure optical field and optical fiber STED donut beam generation and transmission.

The research work was funded by the National Natural Science Foundation of China, Shanghai Science and Technology Commission Project.

Reference:
1.Yao Han, Shi Fan, et al. Mode-locked Yb-doped fiber laser based on high-order mode oscillation [J]. Opto-Electronic Engineering, 2020, 47(11): 200040.
2.Han Yao, Fan Shi, Zhaoyang Wu, Xinzhu Xu, Teng Wang, Xiaomin Liu, Peng Xi, Fufei Pang, and Xianglong Zeng. "A mode generator and multiplexer at visible wavelength based on all-fiber mode selective coupler." Nanophotonics 9, no. 4: 973-981 (2020).

About The Group

Xianglong Zeng received the Ph.D. degree from Shanghai Jiao Tong University in 2004. He currently holds a research Professor in the key laboratory of Specialty Fiber Optics and Optical Access Networks, Shanghai University. His research mainly focuses on the nonlinear optics, special fiber-based devices, the exploration of novel materials, and laser techniques in fiber laser applications. His research experience includes a JSPS Post-Doctoral Fellowship (Tokyo University) from 2004-2007 and a Marie Curie Fellowship (Technical University of Denmark) from 2011-2013. He has received the professorship of special appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning in 2013 and 2019. Recent years, he has published more than 60 papers in international academic journals such as Nanoscale, Optics Letters, Photonics Research, Nanophotonics, Applied Physics Letters, and Optics Express.

Article

Yao Han, Shi Fan, Huang Yiping, et al. Mode-locked Yb-doped fiber laser based on mode coupler[J]. Opto-Electronic Engineering[J]. Opto-Electronic Engineering, 2020, 47(11): 200040.

DOI: 10.12086/oee.2020. 200040