We have demonstrated a high-energy Q-switched double-clad thulium-doped fiber laser (TDFL) using a graphene-oxide-deposited tapered fiber (GODTF) device as a saturable absorber operating at a wavelength of 2 μm for the first time. Because of the side-interaction of the graphene-oxide with the evanescent field on the taper waist, the GODTF devices have potential for offering high laser damage threshold. Using a 788 nm laser diode as the pump source, the TDFL generated stable single transverse mode Q-switched pulses with a single pulse energy of 6.71 μJ (corresponding to an average power of 302 mW) at a wavelength of 2032 nm. This is significantly higher than the highest pulse energy/average power from any rare-earth-doped fiber lasers employing a graphene or graphene-oxide based Q-switch so far. The demonstrated TDFL in this paper represents an encouraging step towards the practical applications of graphene or graphene-oxide based Q-switched 2 μm TDFLs.
© 2013 OSA
Q-switched thulium-doped fiber lasers (TDFLs) operating at eye safe wavelength of 2 μm have attracted considerable research interest in recent years owing to their wide range of potential applications in fields such as medicine, lidar, range finding and remote sensing, etc. Passively Q-switched fiber lasers have many advantages over actively Q-switched ones, including simplicity, low cost and flexibility. Several saturable absorbers (SAs) have been used at the 2 μm wavelength, including, crystals such as Cr2+:ZnSe , semiconductor saturable absorber mirrors (SESAMs)  and single-wall carbon nanotubes (SWNTs) . However, crystal SAs require additional components to couple light into the fiber, which compromising the key benefit of compactness for fiber lasers. SESAMs are considered as expensive and complex-fabrication devices for 2 μm Q-switching. With SWNTs, it often needs to control the diameter or chirality of nanotubes for obtaining saturable absorption in the desired wavelength. The unique characteristics of saturable absorption of graphene have recently been recognized [4, 5], and a number of research groups worldwide have used graphene or graphene-oxide for passive Q-switching or mode-locking in fiber lasers [6–14]. While most of graphene-based Q-switched fiber lasers were obtained at 1.06 μm (ytterbium-doped) or 1.5 μm (erbium-doped), only two papers on graphene-based 2 μm Q-switched TDFLs have just been published [15, 16]. In 2012, Wang, et al.  demonstrated 2 μm Q-switching using graphene in a single-mode TDFL with a pulse energy of 70 nJ and average power of less than 2 mW. Most recently, Liu, et al.  have further generated 2 μm Q-switched pulses of 85 nJ pulse energy and 4.5 mW average power. The pulse energies and average powers obtained from both of the TDFLs are too low for many practical applications, such as medical surgeries and rang finding, etc.
Besides the resonant cavity loss, another critical factor limiting pulse energy and average power of those graphene-based Q-switched TDFLs is the low threshold damage power of graphene-based Q-switch. It has been reported that a Q-switch using the side-interaction of graphene with the evanescent field of a D-shaped fiber  or tapered fiber  has potential for offering high laser damage threshold.
In this letter, in order to obtain significantly high pulse energy/average power from a TDFL, we use a graphene-oxide-deposited tapered fiber (GODTF) device as the 2 μm-wavelength passive Q-switch for the first time. Graphene-oxide was selected as the SA because the fabrication of graphene-oxide is simpler and more cost-effective than that of graphene. In addition, it has been demonstrated that graphene-oxide also has ultrafast characteristics and strong saturable absorption, which is comparable to that of graphene [14, 19, 20]. Moreover, a single mode double-clad Tm3+-doped fiber with a larger core (10 μm vs. 6 μm in ) is used so that the mature cladding pumped technique can be used to achieve higher average power and higher pulse energy. Therefore, a high energy, high average power GODTF-based passively Q-switched double-clad TDFL operating at a wavelength of 2032 nm is thus demonstrated. The output pulse energy/average power is significantly higher than the highest pulse energy/average power measured from any rare-earth-doped fiber lasers employing a graphene or graphene-oxide based Q-switch [16, 21–23].
2. Experimental setup
The GODTF device used in our experiment is schematically shown in Fig. 1(a) . The fiber taper was fabricated by the flame brushing technique. A single-mode fiber (SMF, Corning SMF-28) was heated by a flame and properly stretched until the waist was down to 6.4 μm. The stretching length was about 25 mm with an insertion loss of about 0.5 dB. The graphene-oxide used in the experiment was prepared by the liquid-phase exfoliation of graphite oxide, as described in our previous papers . Then, a droplet of graphene oxide solution with a concentration of 10 mg/mL was dripped onto the taper waist region, and a 60 mW 974 nm laser was injected into the tapered fiber. The deposition-induced loss of the GODTF was monitored in real time by an optical power meter. Approximately 120 seconds later, the deposition of the graphene-oxide commenced because of the heat convention effect and the optical tweezer effect . When the total insertion loss became 3.5 dB, we turned off the LD and shifted the tapered fiber from the droplet. The GODTF device, as a high-power-compatible SA, was used in our proposed Q-switched double-clad TDFL, as schematically depicted in Fig. 1(b). It has a linear cavity with a total cavity length of about 40 m. A 10 m long double-clad thulium-doped single mode fiber (SM-TDF-10P/130-HE, Nufern) was used as the gain fiber. The diameters of core/inner-cladding are 10/130 μm, and the core/inner-cladding NAs are 0.15/0.46. The absorption at 793 nm is 3 dB/m. The pump laser from a 12 W/788 nm LD was coupled into the thulium-doped fiber through a (2 + 1) × 1 signal-pump combiner (from ITF Labs). The signal-input and output fibers of the combiner are un-doped double clad fibers. The diameters of core/inner-cladding are 10/125 μm, and the core/inner-cladding NAs are 0.15/0.46. A single-mode fiber polarization controller (PC) was employed in the cavity to optimize the Q-switched laser operation because GODTF is polarization-dependent . The GOTDF was connected to a 25 m long SMF-28 fiber, the other end of which was vertical cleaved and butted to a mirror (M1). The transmission spectrum of M1 is showed in Fig. 1(c). The reflection of M1 is above 99.9% at 1.8~2.1 μm and the transmission is above 80% at 700~850 nm. The vertical cleaved endface of Tm3+-doped fiber with the Fresnel reflection of 4% was used as the other cavity mirror (M2). The laser output pulses were detected by a 15 MHz photodetector (Thorlabs, PDA10D-EC) together with a 100 MHz digital storage oscilloscope. The output optical spectrum was monitored by an optical spectrum analyzer (OSA, Ocean Optics, NIRQuest512-2.5).
3. Experimental results and discussions
In the experiment, the passively Q-switched TDFL started continuous-wave operation at the pump power of 4.8 W. When the pump power increased to 5.1 W, the laser transited to Q-switched mode, and it became robust when the pump power reached to 5.2 W. The threshold pump power for the Q-switched TDFL in our experiment is quite high, resulting from the high loss of laser cavity. There are probably three key factors responsible for the high cavity loss. The first is to use the fiber endface as one of the cavity mirrors, the reflection of which is only about 4%. The second is the connection loss between the double-clad fiber of the combiner and single cladding standard fiber of PC. The third is the coupling loss between the mirror M1 and the fiber endface. The output laser pulses are single-transverse-mode, resulting from the use of standard single mode fiber SMF-28 and single mode TDF.
A stable Q-switching instead of mode-locking operation was achieved in our laser configuration because the GODTF works as a bandpass filter . When a passive SA is used in a laser cavity, a stable mode-locking operation needs the phase locking among a large number of longitudinal modes, whilst a stable Q-switching usually requires a narrowband lasing optical spectrum for suppressing the self-mode-locking effect.
When the 25 m SMF-28 single mode fiber was removed from the cavity, Q-switch operation became unstable. We have repeated the observation, but we are not sure the exact reasons at the moment. The possible reason might be that because the Q-switched laser is operating just above the threshold pump power (5.1 W), a stable Q-switched operation would be easier to establish when the pulse width is longer with larger pulse energy due to a longer cavity.
Figure 2(a) shows the typical Q-switched pulse train at pump power of 6.27 W. The pulse repetition frequency (PRF) is 45 kHz. The Q-switched laser has been operated with the same pump power for more than 40 minutes, and the pulse train remained stable. The pulse width is about 3.8 μs as shown in Fig. 2(b). The microsecond-level pulse width is comparable to those of a number of graphene based Q-switched fiber lasers recently reported [9, 10, 18]. The pulse width can possibly be reduced by: 1) shortening the cavity length, and 2) improving the performance of GODTF.
For the same pump power, the measured spectrum of Q-switched laser pulses using the OSA is showed in Fig. 2(c). The peak wavelength of the laser is 2032 nm. Because of the low resolution of the OSA (6.3 nm), the 3 dB spectral width of the laser has not been properly measured.
In Fig. 3(a) , the pulse width and the PRF are plotted as a function of pump power, respectively. The PRF increases almost linearly from 20 kHz to 45 kHz and the pulse width decreases from 9 μs to 3.8 μs when the pump power increases from 5.1 W to 6.27 W. A larger pump power leads to a higher PRF and narrower pulses. As increasing the pump power, the high gain will be established within a shorter time, leading to faster bleaching of the graphene-oxide SA.
In Fig. 3(b), the average output power and pulse energy are plotted as a function of the pump power. When the pump power exceeded the threshold, the average output power and single pulse energy increased almost monotonously with the pump power. For a pump power of 6.27 W, the average power of 302 mW and the corresponding pulse energy of 6.71 μJ were achieved. The slope efficiency is ~21.8%. The high average power and pulse energy in our experiment result from the uses of the GODTF and the cladding pumped technique. When the launched pump power was higher than 6.4 W, we observed that the Q-switched operation became unstable. Further investigation of GODTF devices is required to improve their performance.
In Table 1 , we compared the output average power and pulse energy of our Q-switched TDFL with the highest average power and pulse energy recorded for different rare-earth-doped fiber lasers employing a graphene/graphene-oxide Q-switch [16, 21–23]. The measured average power of 302 mW and pulse energy of 6.71 μJ from our Q-switched TDFL are 25 and 36 times higher than those recorded highest average power and pulse energy, respectively. By optimizing the performance of the GOTDF and reducing the laser cavity loss, the pulse energy/average power could be further scaled up.
We have demonstrated a high-energy single-transverse-mode GOTDF-based Q-switched TDFL operating at a wavelength of 2032 nm. For the first time, a GOTDF has been used as a SA at the mid-infrared wavelength to achieve Q-switched TDFL. The single pulse energy of 6.71 μJ and average power of 302 mW were achieved when a pump power of 6.27 W at 788 nm was launched. This is significantly higher than the highest pulse energy/average power measured from any rare-earth-doped fiber lasers employing a graphene or graphene-oxide based Q-switch, and is an encouraging step towards the practical applications of graphene or graphene-oxide based Q-switched 2 μm TDFLs.
The authors acknowledge the National Natural Science Foundation of China (61177044), the Fundamental Research Funds for the Central Universities (2010121057, 201112G019) and the Natural Science Foundation of Fujian Province of China (2011J01370).
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