Fiber lasers currently attract great interest. High-power continuous-wave Q
-switched and mode-locked fiber lasers at wavelengths around 1 and 1.5 µm have been developed [1
]. High efficiency combined with the possibility of direct diode pumping make fiber lasers attractive for many applications.
Recent progress in thulium (Tm)- and holmium (Ho)-doped fiber lasers has enabled the demonstration of high-power fiber lasers in the midinfrared region. Average powers up to 100 W and 68% efficiency have been demonstrated in Tm- and Ho-doped fiber lasers [5
]. The possibility of achieving quantum efficiency greater than one due to cross-relaxation energy transfer processes make this class of fiber lasers very attractive. Furthermore, for many applications in nonlinear optics, medicine, and sensing, integrated and robust laser sources around 2-µm wavelength are needed.
Tm-doped fiber is known to have a broad and smooth fluorescence spectrum, which is suitable for generating ultrashort pulses. However, only a few mode-locked oscillators based on Tm fiber have been reported. Nonlinear polarization evolution (NPE) was used by Nelson et al.
to demonstrate a mode-locked thulium fiber laser that generated 500-fs pulses [8
]. Sharp et al.
used a semiconductor saturable absorber mirror (SESAM) in a Tm fiber laser to achieve 190-fs pulses [9
]. Recently, Engelbrecht et al.
reported a laser with grating-based dispersion compensation and double-clad thulium-doped fiber. The laser operates in the stretched pulse regime [10
] with pulse energy as high as 4.3 nJ and dechirped pulse duration around 300 fs. NPE was used to initiate and stabilize the mode-locked pulses in this system, which offers excellent performance but sacrifices some of the benefits of fiber by its use of bulk optics. Regarding femtosecond amplification in Tm fiber, Imeshev et al.
demonstrated a watt-level source operating around 2 µm, using a Raman-shifted Er-doped femtosecond laser as the seed [11
]. Recently, Kivistö et al.
also reported a tunable Raman soliton source using a Tm-Ho codoped seed laser [12
In ultrafast fiber lasers, NPE and SESAMs are widely used to provide amplitude modulation. However, these two techniques have drawbacks. NPE requries additional elements in the cavity, including a polarizer and polarization controllers (PCs). Furthermore, fiber lasers mode-locked with NPE will generally not be environmentally stable. SESAMs have recently become readily available, but tend to damage in fiber lasers, perhaps owing to the large modulation depth that is needed. Recently, a new type of saturable absorber (SA) based on single-walled carbon nanotubes (SWCNTs) has been investigated [13
]. SWCNTs possess subpicosecond recovery times, and broad absorption spectra. Solid-state and fiber lasers operating at 1- [16
], 1.3- [17
], and 1.5-µm [18
] wavelengths have been mode-locked with SWCNT SAs. During the preparation of this manuscript, a report by Solodyankin et al.
of a mode-locked Tm fiber laser that uses SWCNTs as the SA appeared [19
]. The SWCNTs were deposited as a thin film on the end of a fiber segment. This laser generated only low-energy picosecond pulses due to the cavity design (a ring cavity without an isolator) and damage of the thin-film absorber. It is desirable to reach the excellent performance of the laser reported in [10
], but in an all-fiber integrated format.
In this letter, we report a step toward that goal, by demonstrating a mode-locked Tm fiber laser that employs only fiber-format components, including a SA based on SWCNTs. In contrast to the approach reported in [19
], the SA that we use is based on a fiber taper that is embedded in an SWCNT/polymer composite, following the design in [18
]. In this geometry, the absorption is distributed, and so is the generated heat, which allows reliable operation at much higher power. The laser produces 750-fs solitons with 0.5-nJ pulse energy and 25-mW average power. The laser has been operated for many hours with no sign of degradation of the SA.