Switchable and tunable multi-wavelength emissions in pulsed ytterbium fiber lasers with black phosphorus saturable absorbers and polarization-maintaining fiber Bragg gratings
Introduction
Multi-wavelength fiber lasers, which are versatile light sources capable of providing multiple discrete laser emission lines, have been found widespread applications in optical fiber communication, photonic generation of microwave radiation, optical fiber sensing, and spectrum analysis etc. [1], [2], [3], [4], [5], [6]. Various technical approaches have been proposed and demonstrated for multi-wavelength fiber lasers including utilization of comb filters [7], phase modulators [8], [9], specialty fibers [10], [11], nonlinear optical effects [12], [13], and hybrid-gain schemes [14]. One alternative in the comb filter approach is to use Fiber Bragg gratings (FBGs) written in polarization-maintaining fibers (PMFs) and achieve multi-wavelength emissions by control of the two orthogonal polarization states [15], [16], [17]. Considerable research attention has been dedicated recently for its simple and stable laser structure, along with its pragmatic switchability and tunability in convenience. Despite excellence in performing switchable multi-wavelength lasers with polarization-maintaining fiber Bragg gratings (PM-FBGs), majority of the research effort has been concentrated on the erbium-doped fiber lasers at the telecom wavelength of 1. [15], [16], [17]. In contrast, surprisingly little studies have been reported on pulsed ytterbium-doped fiber lasers emitting the other widely popular wavelength of in switchable multi-wavelength status with the PM-FBG scheme.
Pulsed fiber lasers, passively Q-switched and mode-locked in particular, are most commonly achieved with various saturable absorbers (SA) [18], [19], [20], [21]. Black phosphorus (BP), the most stable allotrope of phosphorus [20], is a promising SA candidate among recently developed two-dimensional (2-D) materials [22], [23]. BP-SAs possess fast response time and broader bandwidth beyond the reach of other non-carbon 2-D materials [24], [25], resulting in extensive studies on BP-SA-based pulsed fiber lasers recently [26], [27], [28], [29]. However, switchable multi-wavelength emissions in the band are still relatively absent to date. Therefore, we hereby intend to investigate the BP-SA-based pulsed ytterbium fiber lasers, in both Q-switched and mode-locked status, for multi-wavelength operations with switchability and tunability.
In this paper, we present a systematic study of switchable dual-wavelength pulsed fiber lasers emitting at the band with tunability. The all-fiber ring cavity was constructed mainly with a piece of ytterbium-doped fiber, a PM-FBG, and a fiber BP-SA for pulsation. Both Q-switched and mode-locked laser pulses were achieved by controlled BP layer thickness, with thicker (10 nm) BP flakes for Q switching and thinner (5 nm) BP flakes for mode locking. The repetition rates of Q-switched laser pulses were varying upon the pump level, while the repetition rate locked in the same 18.47 MHz for all mode-locked emissions. The PM-FGB for multi-wavelength operation had two reflection peaks. By adjusting the in-fiber polarization state, the lasers operated in three discrete states of wavelength selection: individual wavelength emission at 1063.8 & 1064.1 nm, and simultaneous emission of both. Finally, by applying stresses at the PM-FBG’s fast and slow axis, the wavelength spacing between the two emission peaks was tuned from 0.02 to 0.52 nm. To the best of our knowledge, this is the first experimental demonstration to incorporate the PM-FBGs into pulsed ytterbium fiber lasers and achieve switchable and tunable dual-wavelength emissions, which could potentially provide promising contributions to the future development of multi-wavelength pulsed fiber lasers in the 1-m region.
Section snippets
The experimental setup
The proposed scheme of switchable dual-wavelength ytterbium fiber laser is illustrated in Fig. 1. A 976-nm continuous-wave laser diode (LD) was the pump source with a maximum output of 400 mW. The pump light was combined into the ring cavity by a 980/1060-nm wavelength division multiplexer (WDM). A piece of 80-cm-long ytterbium-doped fiber (YDF, NUFERN SM-YSF-HI) was the gain medium. Its absorption coefficients were 250 dB/m at 976 nm and 0.91 dB/m at 1064 nm, respectively. After the YDF, an
The preparation and characterization of BP-SAs
Commercial BP quantum dots in ethyl alcohol solution (MKNANO Inc.) of a 0.1 mg/ml concentration was utilized for our BP-SA fabrication. In the BP solution, the single layer rate was 80% and single size of BP quantum dots was 1–10 nm, as illustrated by the transmission electron microscopic (TEM) image in Fig. 2(a) where the inset high-resolution TEM image indicated a lattice fringe of 0.2 nm approximately. To transfer BP quantum dots onto the fiber facets to form few-layer BP flakes, we
The performance of PM-FBG
In the experiment, a panda-type PMF, with its cross-section illustrated in Fig. 3(a), was used to fabricate the FBG. For this PM-FBG, the Bragg wavelengths for polarization states along the slow and fast axes are expressed as where is the refractive index of the slow axis, is the index of the fast axis, and is the grating period. The reflection spectrum of the fabricated PM-FBG, as shown in Fig. 3(b), was measured by an ASE source (VENUS, VASS-Yb-B) from 1035 nm to
Q-switched laser operation
We first investigated the Q-switching laser operation using a thicker BP-SA in the ring cavity. For the Q-switched laser pulses, three states of wavelength emission were observed by adjusting the PC and controlling the polarization state in the cavity: a single wavelength emission at 1063.8 nm, as shown in Fig. 4(a), corresponding to the polarization state along the fast axis; a single wavelength emission at 1064.1 nm, as shown in Fig. 4(b), corresponding to the polarization state along the
Conclusion
In summary, we demonstrated Q-switched and mode-locked ytterbium-doped fiber lasers operating in three different wavelength emission states with BP-SAs and PM-FBGs. The incorporated PM-FBGs enabled emissions of switchable and tunable laser pulses at the individual wavelength of 1063.8 nm and 1064.1 nm, respectively, as well as simultaneously both. The 2-D BP material was utilized to fabricate SAs for laser pulsation, with a thicker BP-SA for Q switching and a thinner sample for mode locking.
Acknowledgments
This work is supported by NSAF, China No. U1830123, the National Natural Science Foundation of China (No. 61627802), and the High-Level Educational Innovation Team Introduction Plan of Jiangsu, China .
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