1. Introduction

Pulse sources operating at a wavelength of around 2 μm, called “eye safe” wavelength region, offer more advantages for free space applications compared to those operating at shorter wavelengths [1]. This enables them to potentially use in the fields of light detection and ranging, remote gas sensing, and free space optical communication [2–4]. Another feature of high absorption in water makes these laser sources useful for medical applications such as cutting of biological tissue in laser surgery [5]. They are also useful for material processing of glasses and plastics due to their intrinsic absorption at this wavelength range [1]. Besides the above applications, laser systems with 2 μm wavelength are also attractive for military and security application and for pump source for nonlinear conversion to mid-infrared wavelength of few microns [6].

Passively mode-locked fiber lasers at 2 μm show the merits over solid-sate lasers, such as alignment-free operation, compact size and excellent spatial beam quality. Previously, natural saturable absorption elements such as semiconductor saturable absorber mirrors (SESAMs) [7] or single walled-carbon nanotubes (SWCNTs) [8] were employed in ultrafast Tm-doped fiber lasers for environmentally stable operation of the lasers. However, SESAMs require sophisticate fabrication process with limited spectral bandwidth [9]. Although SWCNT SAs exhibits large third-order nonlinearity, fast nonlinear recovery time, and ease of fabrication [10–14], they need precise control of nanotube diameter to match the absorption at the desired wavelength. Recently graphene, a two-dimensional atomic layer of carbon atoms, has been regarded as a promising material for SAs to overcome above drawbacks due to its unique features including ultra-broadband operation range and ultrafast nonlinear response with huge Kerr-nonlinearity.

Up to date, there have been several reports on ultrafast 2 μm fiber lasers using a SA with graphene-related materials. For example, graphene flake/polymer composite [15] or reduced graphene oxide (RGO) dispersion [16] were employed for passive mode-locking of fiber lasers. However, in these works, irregular structure of the graphene flake or RGO causes an inevitable scattering loss during graphene-light interaction, resulting in large non-saturable loss in SAs. In addition, the graphene/polymer composite usually contains graphene layers with different thickness. Thus, a SA with uniform graphene layer will be desirable to reduce the unwanted scattering loss. A SA fabricated with bi-layer graphene was proposed for Tm-doped fiber laser mode-locking [17], where a graphene layer grown via chemical vapor deposition (CVD) method was transferred to an optical fiber connector ferrule. Although this fiber ferrule-type graphene SA has merits, such as simple and precise fabrication process, and polarization independent properties, it generally exhibits low optical damage threshold and short nonlinear interaction length due to the thin thickness of the graphene layer. A SA based on evanescent field interaction with layered graphene was also reported [18]. But these previous works usually exhibited limited laser performance in pulse duration (~picosecond) or output power (few mW) [15–18]. Recently a Ho-doped soliton fiber laser with pulse energy of nJ-scale (~1.3 nJ) was demonstrated by J. Sotor et al. [19], which show possibility of delivering soliton pulse with nJ-scale pulse energy in a 2 μm mode-locked fiber laser oscillator.

In this letter, we report an all-fiber Tm-doped soliton laser with high pulse energy by using a uniform monolayer graphene SA. We transferred large area, uniform monolayer graphene synthesized by CVD method to surface of the side-polished fiber (SPF) to realize an evanescently interacting in-line graphene SA. To increase nonlinear interaction with graphene layer, we applied the over-cladding with a refractive index of 1.429 to the SPF, where significant light absorption occurs in a monolayer graphene with small scattering loss. We fabricated an all-fiber Tm-doped soliton laser with our in-line graphene SA where we observed self-starting operation of a Tm-doped mode-locked fiber laser that stably delivers the pulses with a pulse duration of 773 fs. The measured 3-dB spectral bandwidth was 5.14 nm at a central wavelength of 1910 nm. The average output power was measured to be 115 mW for an applied pump power of 1.3 W at 1560 nm. The estimated pulse energy, at the fundamental repetition rate of 19.31 MHz, was 6 nJ, which is the highest value among previously demonstrated all-fiber Tm-doped soliton lasers based on carbon nanomaterial SAs to our knowledge.

2. Fabrication of in-line monolayer graphene SA

Graphene monolayer films were grown on copper foils by chemical vapor deposition (CVD) method [20]. As a supporting layer, a poly(methyl methacrylate) (PMMA) was spin coated on the as-grown graphene, and the coper foil was etched using ammonium persulphate aqueous solution. Before transferring the graphene film to a SPF, we transferred it first to a quartz substrate for the characterization of the graphene film. The graphene/PMMA film was moved to deionized water bath to rinse off remaining etchant. We then scooped the floating graphene/PMMA film from the bath with a quartz substrate in order to directly transfer it onto the substrate. Then we removed the PMMA layer with acetone. Figure 1(a) shows measured linear transmission of the graphene film where nearly uniform absorption of 2.3% was observed at the wavelength ranging from 900 nm to 2400 nm. The measured Raman spectrum of monolayer graphene is shown in the inset of Fig. 1(a). Here relative intensity ratio between the 2D peak and G peak (I_2D/I_G) was measured to be 4.06 with well suppressed defect peak over the large area graphene film, indicating uniform growth of graphene monolayer.

 

Fig. 1 (a) Linear transmission of monolayer graphene (inset: Raman spectreoscopy results of monolayer graphene and (b) schematic view of monolayer graphene SA with over-cladding structure (inset: cross-sectional image of SA)

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Figure 1(b) and its inset depicted the fabricated graphene SA on a SPF. A commercial single mode fiber (SMF-28e, Corning) was side polished until the oil drop loss became – 45 dB for applied index-matched oil. The minimum distance between the polished surface and the fiber core boundary was estimated to be about 1 μm considering the oil-drop test result and the polished length. The interaction length of the SPF was estimated to be 5 mm. The measured insertion loss of the SPF with air over-cladding was – 0.1 dB. The PMMA-coated graphene film with size of 7x5 mm² was then transferred onto the SPF with similar manner used for graphene film transfer on the quartz substrate, and the PMMA film was removed by acetone. The SPF covered with monolayer graphene shows slightly increased insertion loss of – 0.3 dB with polarization dependence loss (PDL) of 0.1 dB. Finally, index oil with refractive index of 1.429 at 2 μm was applied on the surface of the graphene transferred SPF to increase the graphene-evanescent field interaction [21, 22]. After applying the index oil as an over-cladding, the measured PDL increased up to be 5.5 dB while maintaining similar insertion loss (– 0.3 dB) of the device. Thus the maximum linear transmission loss of the SA was estimated to be 73.7% including scattering loss at 2 μm. Although we did not directly measure the nonlinear transmission properties of our SA due to our limited light source at 2 μm, we expect that the modulation depth will be more than 10% considering the measurement at 1550 nm and previous works [21, 22].

3. Fiber laser experiment and discussion

Figure 2 depicts the configuration of all-fiber Tm-doped soliton laser oscillator with our fabricated monolayer graphene SA on the SPF. A heavily Tm-doped fiber (SM-TSF-5/125, Nufern, β₂ ~– 12 ps²/km, peak absorption 340 dB/m at 1560 nm) with a length of 0.2 m was pumped by amplified 1.56 μm source via a wavelength division multiplexing (WDM) coupler. The pump source was amplified via Erbium-Ytterbium co-doped fiber amplifier (EYDFA) (KPS-BT2-C-33-PB-FA, Keopsys) where the maximum pump power reached up to 32.5 dBm. A polarization controller (PC) and an isolator were inserted in the intra-cavity to adjust the polarization state of intra-cavity and to ensure unidirectional operation of laser, respectively. A 3-dB coupler was used as a laser output coupler. The fabricated monolayer graphene SA was then integrated into the laser cavity. Total length of the laser cavity including a SMF-28e and SM1950 (Nufern) was estimated to be approximately 10.8 m where the estimated net cavity group velocity dispersion (GVD) was – 0.71 ps². Finally we attached metal block to the gain medium as a heat sink to suppress the thermal issues in Tm-doped fibers. We observed that the slope efficiency of continuous wave (CW) mode increased from 6% to 24% by applying the metal block.

 

Fig. 2 Configuration of the fabricated Tm-doped all-fiber soliton laser oscillator with monolayer graphene SA on the SPF

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The laser output characteristics of Tm-doped all-fiber soliton laser using monolayer graphene SA are shown in Fig. 3. The spectral bandwidth of the laser was measured to be 5.14 nm at the central wavelength of 1910 nm where spectral shape was a typical spectral shape of soliton as shown in Fig. 3(a). The repetition rate of the generated soliton pulse train shown in the Fig. 3(b) was 19.31 MHz, which was corresponding to a given cavity length. The pulse duration of the laser output was directly measured by an intensity auto-correlator (FR-103HP, Femtochrome), which was 773 fs when fitted by sech²-shape as shown in Fig. 3 (c). The estimated value of time bandwidth product (TBP) was 0.33, which indicates that the generated pulse was close to the transform limited pulse. Figure 3(d) shows a radio frequency (RF) spectrum of the laser output where the background noise was well suppressed by 61 dB from the signal of fundamental repetition rate of 19.31 MHz, indicating stable passive mode-locking of the fiber laser.

 

Fig. 3 Laser output characteristics of all-fiber Tm-doped soliton laser oscillator with our graphene SA. (a) Optical spectrum, (b) Pulse train, (c) Pulse duration measured by intensity auto-correlator, and (d) RF spectrum.

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Figure 4 shows the average laser output power of the fabricated soliton laser as a function of pump power. CW lasing operation of the Tm-doped fiber laser was started from the applied pump power of 770 mW. Our Tm-doped fiber laser exhibits a little higher lasing threshold than normal Er-doped fiber lasers due to the relatively large insertion loss of the laser components, including an isolator, a PC and a WDM coupler at 2 μm. When we applied the pump power of 797 mW, we observed self-starting operation of passive mode-locking of our Tm-doped fiber laser. The soliton laser operation was stably maintained for the applied the pump powers up to 1.3 W. The maximum average output power of our soliton fiber laser operating with fundamental repetition rate was 115 mW at the applied pump power of 1.3 W. When higher pump power than 1.3 W was applied to the laser, we observed that the laser output delivers unstable split pulses or bunched pulses. Since our SPF-based SA exhibits polarization-dependent behavior, it can support several mode-locked conditions depending on the polarization state of the laser intra-cavity [23]. We further explore the laser performance by adjusting the PC included in the fiber laser. We observed that our soliton laser stably delivered multiple soliton pulses by passive harmonic mode-locking [24–26] at specific polarization condition where the repetition rates could be controlled from 19.31 MHz to 77.24 MHz by adjusting the pump power.

 

Fig. 4 Measured laser output power of the Tm-doped soliton laser as a function of the applied pump power at 1560 nm.

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According to soliton area theorem [27, 28], pulse energy of fundamental soliton can be expressed by following equation.

Where A₀ is the peak amplitude, τ is the pulse duration, of soliton pulses, and β_2,ave is the average dispersion, and γ_ave is the average nonlinearity of a laser cavity. The fundamental guided mode in an optical fiber has larger effective area at longer wavelength, resulting in reduced nonlinearity. In addition, silica material possesses larger anomalous dispersion value at 2 μm, compared to that at 1.55 μm. Hence, by combined effect of these reduced nonlinearity and enhanced dispersion, the mode-locked fiber lasers at 2 μm can usually support a higher energy soliton pulse than 1.55 μm fiber laser. The use of the output coupler with higher coupling ratio and proper position of the SA in the laser cavity can also be important factors to increase the soliton pulse energy. The maximum pulse energy in our work is estimated to be about 6 nJ at the average output power of 115 mW, which is the highest value reported in all-fiber Tm-doped soliton fiber oscillator with carbon nanomaterial-based SA. We expect that our SA exhibiting small scattering loss, high optical damage threshold, and enhanced modulation depth plays an important role in high power operation of the laser.

We also characterize the long term stability of our Tm-doped soliton fiber laser. Figure 5(a) and (b) show measured optical spectrum and pulse train over 6 hours. We observed that the laser mode-locking was stably maintained during the measurement where power fluctuation of the laser output was less than 5%. Currently we are investigating the application of low refractive index polymers to secure structural stability of the SA. Realization of all-fiber Tm-doped dissipative soliton laser is also in progress by employing a fiber with large normal dispersion to achieve higher pulse energy mode-locked fiber laser 2 μm.

 

Fig. 5 Measured laser output characteristics over 6 hours (a) Optical spectrum and (b) Pulse train

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4. Conclusion

In summary, we demonstrated an all-fiber Tm-doped soliton laser with high power using a monolayer graphene in-line SA. We achieved strongly enhanced evanescent field interaction of monolayer graphene with light at 2 μm in the platform of a SPF by applying over-cladding structure. The fabricated all-fiber Tm-doped mode-locked laser including our SA stably delivered the soliton pulses with 3-dB spectral bandwidth width of 5.14 nm at a central wavelength of 1910 nm. The pulse duration was measured to be 773 fs, resulting in estimated TBP of 0.33. The maximum average power of the soliton fiber laser was measured to be 115 mW. The corresponding pulse energy, at the repetition rate of 19.31 MHz, was 6 nJ, which is the highest value ever reported in all-fiber Tm-doped fiber laser using a carbon-material based SA.