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摘要:
本文提出了一种可以实现光纤高阶模式(HOM)在激光腔内振荡的锁模掺镱光纤激光器。通过使用一对级联的模式选择耦合器(MSC)作为有效的模式转换器，获得光纤锁模激光腔内HOM产生。其中，制备的MSC中心波长为1064 nm，可实现80 nm的模式转换带宽和94%的高阶模式纯度。通过搭建掺镱锁模光纤激光器，实验获得了3 dB谱宽7.4 nm、脉冲重复频率10.9 MHz、射频信噪比55 dB的锁模脉冲激光，输出功率的斜率效率为2.3%。实验证明，这种方法可在激光器内部通过模式级联转换，且能参与腔内锁模过程获得脉冲HOM激光。
Abstract:We demonstrate a mode-locked Yb-doped fiber laser (YDFL) that enables fiber high-order mode (HOM) oscillation inside the ring cavity, by using a pair of mode selective couplers (MSCs) as an effective mode converter, the optical fiber HOM is obtained. The central wavelength of MSC is located at 1064 nm, which can achieve 80 nm mode conversion bandwidth and 94% high-order mode purity. A mode-locked pulsed fiber laser with a 3 dB spectral width of 7.4 nm, a pulse repetition frequency of 10.9 MHz, and a radio frequency signal-to-noise ratio of 55 dB is obtained, and the slope efficiency of the output power is 2.3%. These results show that the HOM can be directly oscillated by the cascaded MSCs in the fiber laser and participated in the mode-locking process to obtain a pulsed HOM laser.
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Key words:
- mode-selective couplers /
- high-order mode /
- Yb-doped fibers /
- mode-locking fiber lasers
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Overview: High-order modes (HOMs), surpassing the capacity barrier in the traditional single-mode fiber (SMF) communication system, have attracted extensive attention and been widely applied in the fiber laser, optical communication, particle trapping, remote sensing technology, and so on. The HOMs include linear polarization LP11, LP21, LP02, LP31, and even higher-order modes, which can be generated by using free-space and fiber-based mode conversion devices. LP11 mode is one of the most important HOMs, which has four vector eigenmodes. These eigenmodes are called as cylindrical vector beams (CVBs) with the axially symmetric polarization and circular intensity distribution. A polarization controller (PC) added on the fiber can effectively eliminate the degeneracy of LP11 mode to excite individual vector modes in different polarization states. Additionally, the orbital angular momentum (OAM) characterized by helical wavefront can be generated by superimposing two orthogonal vector modes. Recently, ultrafast fiber lasers combined by HOMs have been reported owing to their outstanding characteristics, such as compactness, high peak power, narrow pulse width, and low cost. However, the HOMs were converted outside the laser, and the fundamental mode (LP01) was still transmitted in the laser cavity.
In this paper, a HOM directly oscillating in a mode-locked Yb-doped fiber laser (YDFL) is demonstrated. Two PCs and a polarization-dependent isolator are used to achieve the mode-locked mechanism of nonlinear polarization rotation. A pair of home-made mode selective couplers (MSCs) connecting through their few-mode fiber (FMF) ports, acts as an efficient mode convertor to generate and oscillate HOMs in the FMF section of the YDFL. A MSC is composed of a SMF and a FMF, which are fused by using hydrogen oxygen flame technology to keep two fiber cores close to each other. The claddings of two fibers are partly fused to form a coupling region. If the phase matching condition is satisfied, the LP01 mode is transferred to the LP11 mode in the coupling region. The MSC has a central wavelength of 1064 nm, a mode conversion bandwidth of 80 nm, and a HOM purity of 94%. Meanwhile, according to the reversibility of MSCs, the LP11 mode can be lunched in the FMF port and output the LP01 mode in the SMF port. The pulsed laser with a 3 dB spectral width of 7.4 nm, a pulse repetition frequency of 10.9 MHz, and a signal-to-noise ratio of radio frequency of 55 dB is obtained, and the slope efficiency of the pump and output power is 2.3%. The pulse LP11 mode, CVB, and first-order OAM are obtained from the YDFL. These results demonstrate that the HOM can be generated by the MSC and be directly oscillated in the YDFL, and this approach is promising for directly generating pure and efficient HOMs in all-FMF ultrafast Yb-doped fiber lasers.
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图 1 (a) 两个MSC级联的结构图；(b)单个MSC的传输谱；(c)级联一对MSC的传输谱
Figure 1. (a) Schematic of a pair of cascading MSCs through FMF ports; (b) The transmission spectrum of a MSC; (c) The transmission spectrum of a pair of cascading MSCs
图 2 基于模式选择耦合器的锁模掺镱光纤激光器示意图。插图：激光器内传输的模式
Figure 2. Experimental setup of the mode-locked Yb-doped fiber laser based on mode couplers
图 3 Output1端口监测锁模激光器的输出特性。(a)光谱；(b)脉冲序列；(c)中心频率为10.9 MHz的频谱图，插图：频谱范围为300 MHz的频谱图；(d)输出功率和泵浦功率的关系
Figure 3. Output1 characteristics of YDFL. (a) Optical spectrum; (b) Pulse trains; (c) The radio frequency (RF) spectrum with the central frequency of 10.9 MHz, inset: the RF spectrum with a 300 MHz span; (d) The YDFL output power and repetition rate variation along with elevating pump power
图 4 Output2端口监测模式的模场分布图。(a)测量和重构光场的线偏振LP11模式的模场分布，以及LP11模式的纯度；(b)柱矢量光束TM01、TE01、HE21even和HE21odd的模场分布；(c)线偏振LP11模、环状涡旋光及其干涉条纹的模场分布
Figure 4. Mode field distributions of output2. (a) Measured and reconstructed mode fields, and the LP11 mode purity; (b) Near-filed intensity distribution of TM01, TE01, HE21even and HE21odd modes; the first column: vector modes with donut-shape intensity profiles; the later four columns: distributions of each vector mode with a polarizer placed in front of CCD; (c) Intensity distributions of lobe-shaped LP11 mode, donut-shaped OAMs and their spiral interferences
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