Carbon Nanotube Mode-Locked Thulium Fiber Laser With 200 nm Tuning Range

www.nature.com/scientificreports OPEN received: 11 November 2016 accepted: 17 February 2017 Published: 21 March 2017 Carbon Nanotube Mode-Locked Thulium Fiber Laser With 200 nm Tuning Range Yafei Meng, Yao Li, Yongbing Xu & Fengqiu Wang We demonstrated a mode-locked thulium/holmium (Tm/Ho) fiber laser continuously tunable across 200 nm (from 1860 nm to 2060 nm), which to the best of our knowledge represents the widest tuning range ever achieved for a passively mode-locked fiber laser oscillator. The combined use of a broadband carbon nanotube (CNT) saturable absorber and a diffraction grating mirror ensures ultra-broad tuning range, superb stability and repeatability, and makes the demonstrated laser a highly practical source for spectroscopy, imaging and optical communications. The laser emits <5 ps pulses with an optical spectral bandwidth of ∼3 nm across the full tuning range. Our results indicate that carbon nanotubes can be an excellent saturable absorber for achieving gain-bandwidth-limited tunable operation for 2 μm thulium fiber lasers. Mode-locked fiber lasers operating in the 2 μm region have attracted considerable attention due to various applications in medical surgery, free space optical communications, light detection and ranging (LIDAR), nonlinear frequency conversion and transparent material processing1–6. Thulium and holmium ions based active fibers show a broad amplification bandwidth typically from 1.85–2.1 μm and are therefore good candidates for ultrashort pulse generation and wideband wavelength tuning. For example, utilizing the full gain bandwidth (>200 nm) of thulium-doped fiber would allow for the generation of sub 30 fs pulses directly from a mode-locked oscillator7. Several techniques have been developed for generating ultrafast pulses including semiconductor saturable absorber mirrors (SESAMs)8, nonlinear polarization evolution (NPE)9,10 and nonlinear amplifying loop mirror (NALM)11. In recent years, low-dimensional carbon nanomaterials such as carbon nanotubes (CNTs)12–14 and graphene15,16 have been intensively investigated for ultrashort pulse generation at 2 μm. This has further led to the investigations of other forms of novel materials such as topological insulators17,18, transition-metal dichalcogenides19,20 and black phosphorus21,22. Despite significant progress for 2 μm mode-locked lasers, most of the prior works focused on operation at a fixed wavelength which may limit their application potential in scenarios where wavelength tuning is desirable, such as frequency comb generation, molecular absorption line detection, time resolved nonlinear photon spectroscopy and optical parametric oscillation (OPO)23–28. Thus far, only a few works reported wavelength tunable operations. For example, through mechanically bending of a multimode interference filter (MMIF), a 95 nm tuning range is achieved for a mode-locked thulium fiber laser29. A tapered fiber device was used to modulate the peak insertion loss and combined with a CNT absorber, a wavelength tuning range of 50 nm is reported30. A graphene saturable absorber deposited on micro-fiber exhibits not only saturable absorption but also a strong polarization-dependent band-pass filtering effect. By controlling the intra-cavity polarization state, a wavelength tuning range of 60 nm is achieved31. Moreover, for a fiber laser mode-locked by NPE, it is demonstrated that the wavelength dependent insertion loss induced by the linear and nonlinear phase can be harnessed to achieve a 104 nm tuning range32. By utilizing a bidirectional pumping scheme, this is further optimized to show a 136 nm tuning range33. To access the entire gain-bandwidth offered by thulium dopants (in excess of 200 nm) is of significant practical importance. Such system would provide a robust, low-cost and field-deployable source that are not currently being met by the more bulky solutions based on conventional OPOs. To this end, a broadband saturable absorber and wavelength tuning mechanism are required. Both CNTs and graphene are good candidates for broadband saturable absorbers. For carbon nanotubes, the mixing of different diameters and chiralities facilitates broadband operations with enhanced modulation depth34,35. Graphene possesses ultra-broadband nonlinear absorption. School of Electronic Science and Engineering and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing, 210023, China. Correspondence and requests for materials should be addressed to Y.L. (email: ) or F.W. (email: ) Scientific Reports | 7:45109 | DOI: 10.1038/srep45109 1 www.nature.com/scientificreports/ Reference Key method Tuning Range Year Filter based on NPE 1798–1902 nm (104 nm) 1995 Fang et al.30 Fiber-taper-filter & CNT absorber 1866.3–1916.4 nm (50.1 nm) 2010 Yang et al.31 Microfiber filter & graphene absorber 1880–1940 nm (60 nm) 2016 Li et al.29 Multimode interference filter (MMIT) & CNT absorber 1919.6–2014.9 nm (95.3 nm) 2016 Yan et al.33 Filter based on NPE & bidirectional pumping 1842–1978 nm (136 nm) 2016 This work Diffraction grating with littrow configuration & CNT- absorber 1860–2060 nm (200 nm) 2017 Nelson et al.32 Table 1. Summary of broadly tunable mode-locked fiber lasers around 2 μm. Figure 1. Optical characterization of the composite film. (a)Absorption spectrum of the CNT-SA film. The red dash lines mark the spectral gain region of Tm3+-doped fiber. (b) Normalized absorption of CNT-SA as a function of pump pulse peak intensity. The black bubbles are the experiment date and the red line is the fitting result. However, its modulation depth (in absolute terms) is rather low (<2.3%). While this may not be a big issue for solid-state lasers, e.g. graphene is recently used to mode-lock a Cr2+: ZnSe laser with ~300 nm tunability36, it may indeed lead to difficulty of self-started operation for fiber lasers. In this letter, by combining a CNT-SA with a diffraction grating based mirror, we demonstrated a passively mode-locked Tm/Ho fiber laser with an ultra-broadband tuning range of 200 nm (from 1860 nm to 2060 nm). Such a wavelength versatile source would be instrumental in studying nonlinear optical phenomena, including supercontinuum generation and fiber based OPO system. In addition, due to the inherently simple geometry, the demonstrated laser exhibits excellent stability and repeatability, making it a highly practical source for field-deployable instrumentation for spectroscopy, imaging, and optical communications. Table 1 compares the results in this work with previous wavelength tunable mode-locked fiber lasers operating at 2 μm. The CNT-SA used in this work plays a crucial role for achieving wide-band mode-locking. To ensure operation in the 2 μm band, we used commercially available arc-discharge singe-wall nanotubes (Carbon solutions Scientific Reports | 7:45109 | DOI: 10.1038/srep45109 2 www.nature.com/scientificreports/ Figure 2. Schematic setup o (...truncated)