Nanotube mode-locked, wavelength and pulsewidth tunable thulium fiber laser

: Mode-locked oscillators with highly tunable output characteristics are desirable for a range of applications. Here, with a custom-made tunable filter, we demonstrate a carbon nanotube (CNT) mode-locked thulium fiber laser with widely tunable wavelength, spectral bandwidth, and pulse duration. The demonstrated laser’s wavelength tuning range reached 300 nm (from 1733 nm to 2033 nm), which is the widest-ever that was reported for rare-earth ion doped fiber oscillators in the near-infrared. At each wavelength, the pulse duration can be regulated by changing the filter’s bandwidth. For example, at ~1902 nm, the pulse duration can be adjusted from 0.9 ps to 6.4 ps (the corresponding output spectral bandwidth from 4.3 nm to 0.6 nm). Furthermore, we experimentally and numerically study the spectral evolution of the mode-locked laser in presence of a tunable filter, a topic that has not been thoroughly investigated for thulium-doped fiber lasers. The detailed dynamical change of the mode-locked spectra is presented and we observed gradual suppression of the Kelly sidebands as the filter’s bandwidth is reduced. Further, using the polarization-maintaiing (PM) cavity ensures that the laser is stable and the output laser’s polarization extinction ratio is measured to exceed 20 dB.


Introduction
For a wide range of applications, including optical parametric oscillation (OPO), frequency comb and nonlinear photon spectroscopy [1][2][3][4][5], ultrafast lasers with a fixed wavelength and pulse duration are increasingly unable to meet emerging technical requirements. As a result, tunable ultrafast lasers have received wide attention over the past decades. A number of techniques have been proven to be effective for realizing wavelength and pulsewidth tunable operation, but most are achieved for 1.5 μm cavity [6][7][8][9][10]. For the 2 micron band, where thulium fiber provides a desirably broad emission bandwidth spaning 1.7-2.1 μm and emerging applications for sensing and communications abound, there are still very few reports. Thusfar, most wavelength tunable 2 μm mode locked oscillators are based on birefringence induced filters [11][12][13]. By the use of a curvature multimode interference filter (MMIF), a mode-locked laser with an output wavelength tuning range of 95 nm was achieved [11]. Moreover, a 60 nm tunable mode-locked Tm 3+ doped fiber laser was demonstrated with a graphene saturable absorber on microfiber [12]. By NPE, a widely tunable mode locked thulium-doped fiber laser with a tuning range from 1842 nm to 1978 nm was reported [13]. However, the fiber birefringence is very sensitive to external perturbations [14], which limits the stability and repeatability of wavelength tuning operation. By contrast, it is simpler and more stable to insert Vol. 27,No. 3 | 4 Feb 2019 | OPTICS EXPRESS 3518 a tunable bandpass filter (TBF) in the cavity to achieve wavelength tuning [15][16][17]. By inserting a tunable filter in the cavity, a 120 nm tuning range can be obtained in a Tm-doped fiber laser mode locked with SESAM [16]. With the help of a diffraction grating, a carbon nanotube mode-locked fiber laser tunable from 1860 nm to 2060 nm was reported, corresponding to a 200 nm tuning range [17]. Recently, Woodward et al. have demonstrated a 330 nm tuning range dysprosium doped fiber laser at 3 μm mode-locked by frequency shifted feedback (FSF) [18]. However, all of the works mentioned above demonstrate only wavelength tuning. No work has been reported to achieve a wide pulsewidth tuning in the 2 μm oscillator, except by extra-cavity techniques [19,20].
In addition, with respect to mode-locked Tm-fiber laser, Kelly sidebands are generally considered to be an intrinsic feature of conventional solitons, whose formation is widely attributed to the constructive interference between the soliton pulse and the associated dispersive wave [21,22]. While many aspects of Kelly sidebands are known and can be quantitatively inferred by the total dispersion, cavity length, and pulse duration [23], detailed experimental studies of influence of a bandwidth-tunable filter on mode-locked laser are still rare and incomplete, and such an investigation is best carried out with a broad gain spectrum laser, such as provided by thulium-doped fiber [24].
In this paper, we demonstrate a CNT mode-locked Thulium-doped fiber laser with broadly tunable wavelength and pulse duration. Although there are many novel saturable absorbers (SAs), such as graphene, transition-metal dichalcogenides (TMDs), black phosphorus and etc [25][26][27][28][29], can be used for mode-locking, the CNT thin film SA is choosed in our work because of its easy preparation, stability and broadband nonlinear absorption (due to the distribution of different diameters) [30][31][32]. The tuning of wavelength and pulse duration is realized through our homemade grating based filter. The maximum tuning range for wavelength is 300 nm (from 1733 nm to 2033 nm). At 1901.8 nm, the pulse duration can be adjusted from 0.9 ps to 6.4 ps (the corresponding spectral bandwidth is from 4.26 nm to 0.64 nm). To the best of our knowledge, it's the first time a 2 μm mode-locked oscilator with both widely tunable wavelength and pulse duration is obtained, and also the tuning range marks the widest ever reported for a mode-locked fiber oscillator in the near-infrared. At the same time, all polarization maintaining structure of the laser ensures strong stability and good self-starting. Moreover, the influence of the filter's bandwidth on mode-locking dynamics was investigated in detail, from both the experimental and numerical perspectives. Our results provide valuable reference for better understanding the interplay of various cavity parameters on mode-locking dynamics.

Experimental setup
The experimental setup of the laser is shown in Fig. 1. A 1550 nm laser diode (LD) is used as the pump source. The LD has an average output power of 14.2 mW and it can be amplified to 1.5 W by a commercial erbium-doped fiber amplifier (EDFA). Then the 1550 nm pump is coupled into the cavity through a polarization-maintaining 1550/2000 nm fiber wavelength division multiplexer (WDM). A segment of PM TDF (PM-TSF-9/125, Nufern) is used as the gain medium. A dielectric mirror with a reflectivity of 80% is used as one end of the cavity and the 20% transmission is as the output of the laser. Meanwhile, our homemade filter is used as another end mirror with wavelength and spectral bandwidth tunability by changing the horizontal position and slit width respectively. The mode-locking of the Tm-fiber is initiated by the carbon nanotube-carboxymethycellulose (CNT-CMC) polymer composite film sandwiched between two fiber connectors. A more detailed introduction of the preparation and the properties of the CNT-SA can be referred to our previous work [33].
The total laser cavity length is ~6.85 m including 1.87 m active fiber and 4.98 m passive fiber (PM Panda-type fiber). At 2 μm, the dispersion values for the active and passive PM fiber are −0.076 ps 2 /m and −0.068 ps 2 /m respectively. The total cavity dispersion value is −0.48 ps 2 . An optical spectrum analyzer (OSA, Yokogawa AQ6375) and a mid-infrared autocorrelator (Femtochrom laser. An osc FSV 30) conn pulse traces a (Thorlabs, S1 ratio of the ou Before ch performance o emission (AS the filter. We nm. The cente the bandwidth is from 1730 charaterize th tuned the filt changing cont the bandwidth loss. This is m with different mater of fact, 2(b)). Compa account the ef to be around 3 μm.

Waveleng
We first test Self-started m appropriately. period of the verify the stab the pulse train confirming a polarization s ensure linear output laser is   . 6(a)). The cen hen we put back but with a rela e spectrum will olution of the sp y sidebands beg -"c"). When t 36 nm), only tw dth, it can be f sidebands are s dth is reduced the process, th andwidth. The e waves in the r's bandwidth power as a f uced from 40 nm output spectral he generation hs. Therefore, t nipulating the (a) Pulse duration minimum 0.64 nm dwidth. ng spectrum in oes not apply e-locking with ntral waveleng k the slit and m atively large ba appear as befo pectrum. As th gan to decrease the filter's band wo Kelly sideb found that the slowly decreas to 2.85 nm, the he pump power e spectrum evo e cavity, so the is reduced. Fig  function of

Conclusio
We report an tunable in a w nm