1. Introduction

In 2003, Set et al. first demonstrated the potential of using carbon nanotubes (CNTs) as saturable absorbers (SA) for the passive modelocking of fiber lasers [1]. Since that first demonstration we have seen a steady rise in the presence of CNT-SA both in research and in commercial applications. The initial interest in such CNT-SAs could be understood based on the excellent optical, mechanical and electrical properties of CNTs. These materials show a sub picosecond optical response, broadband operation covering the spectrum from 1µm to the mid-infrared, small foot-print, easy integration and compatibility with optical fibers. As a result, CNTs have been extensively considered not only for standard fiber laser applications where their cost-effectiveness and the simplicity of use was attractive, but also for applications where other saturable absorber techniques, such as the semiconductor saturable absorbers (SESAM) and nonlinear polarization evolution (NPE), were challenged. In sum, CNT-SAs are nowadays widely employed when fabricating novel fiber laser devices [2–8]. There are, however, remaining challenges that need to be address before CNT-SAs become the mainstream passive modelocking technique such as for example the relatively low modulation depth and high linear losses of CNT-SAs. In this paper, we will discuss another factor that remains under scrutiny in particular considering the potential commercialization of these devices and their application to novel, high pulse energy laser operations. This is their long term stability when exposed to high intensity pulses. In this paper, we first study the degradation of CNTs under intense continuous wave (CW) radiation and then propose a method to increase the damage threshold of CNT-SAs by sealing the device in a nitrogen rich environment that can prevent the oxidation of the CNTs.

The issue of the long term stability of CNTs under intense light radiation has been under discussion from the very early days of CNT-SAs and has intensify with the arrival of dissipative soliton fiber lasers capable of operating at increasingly high pulse energies [9]. Song et al. first indicated that CNT-SA in a ring cavity fiber laser emitting soliton-like pulses could start to degrade with intracavity powers exceeding 10dBm. They went on to propose a method where the CNTs were deposited on a D-shape fiber, in this way only the evanescent optical field of the pulse propagating outside of the core interacts with the nanotubes allowing operation at significantly higher optical powers [10]. Since that first demonstration numerous approaches have been considered for this purpose employing tapered fibers [11, 12] and micro-channeled fiber devices [13]. All these approaches involve evanescent coupling, where the CNTs are placed adjacent to the optical waveguide. Devices that operate using the evanescent field interaction are desirable for numerous applications but also have some inherent limitations based on their geometry. For instance, using a D-shape fiber introduces a highly birefringent component to the laser system and both D-shape fibers and taper fibers are hardly suitable when working with polarization maintaining fibers or to be implemented in the miniature cavities required for multi-gigahertz laser operation [8]. Thus, it is still necessary to understand and quantify the damage threshold of CNT-SA thin films under intense optical light and to find ways to increase their robustness and long term stability.

In this work, we first study the degradation of CNT-SA under high intensity light in air, and we demonstrate that it is possible to significantly increase the damage threshold by sealing the CNT-SA in a Nitrogen gas. Then, we demonstrate enhanced stability of fiber laser operation when the CNT-SA is sealed in a pressurized Nitrogen environment, such enhancement has been previously reported in literature [14, 15], here a systematic study of the deterioration in air by oxygenation and the enhancement by nitrogen sealing is carried out. The SAs under study here consist of free standing CNTs deposited in a fiber-end. The CNTs were fabricated by the high pressure carbon monoxide (HiPCO) method and a mean diameter of 1.2nm. However, the method of Nitrogen sealing can be used to ensure the stability of CNTs fabricated by other methods as well as graphene. This results can however not be directly extrapolated to SAs where the CNTs are embedded into a host material such as CNT-polymer composites for example [15–18].

2. Experimental set up and results

In this work, we deposited the CNTs directly in fiber ends by using two different techniques that have been described elsewhere; the spray method [8, 19] and the optical deposition method [20]. The total insertion losses of the CNT-SAs used in this study ranged from 1dB to 1.5dB. The CNT-SAs degraded in similar fashion regardless of the technique used for deposition.

We first designed a system to directly observe and measure the optical damage threshold of the thin film of CNTs in air. A continuous wave (cw) laser light source emitting at 1550nm amplified by an erbium-doped fiber amplifier and a coupler were used to launch 90% of the light into the fiber with the CNT film, 10% of the light is used for control as shown in Fig. 1 . Using a microscope, we could observe in real-time the combustion of the CNTs when exposed to high intensity cw light producing a spark. This indicates that the damage process is due to oxidation of the CNTs. In Fig. 1(a) to Fig. 1(d) we can see the optically deposited CNTs before damage Fig. 1(a) at the precise moment when it is burned out Fig. 1(b) and after the CNTs had been burned Fig. 1(c). Finally, we remove the CNTs and observe that not only the CNTs were damaged but the fiber core was itself damaged in the combustion process, Fig. 1(d).

 

Fig. 1 (a)Experimental set-up. (b) Fiber-end with CNT optically-deposited in the core area, (c) combustion of the CNT when subjected to high intensity light. (d) Fiber-end after CNT damage. (e) After removing the CNTs, damage in the fiber due to CNT combustion can be observed (Media 1).

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We then modified the experimental set-up shown in Fig. 1 to measure the optical damage threshold directly by adding a circulator and a second power meter, we can monitor both the reflected and transmitted light from the CNTs. Degradation of the samples comes with a drastic drop in reflectivity of the CNT thin film. Such drop in reflectivity took place at cw powers ranging from 20mW to 40mW depending on the samples CNT concentration. Since the apparent catalyst for this combustion is the oxidation of the CNTs under intense light in the oxygen-rich atmosphere, we sealed the CNTs in a nitrogen gas container. In this case, damage was only observed at optical powers exceeding 500mW, thus increasing the damage threshold by over 1 order of magnitude.

Finally, in order to compare the long term stability of Nitrogen-sealed CNT-SA with the CNT-SAs in air, we incorporated the CNT-SAs into a standard ring cavity fiber laser as shown in Fig. 2 . The optically deposited CNT-SAs were sandwiched between two fiber ferrules, and sealed in a gas chamber with pressurized nitrogen. A 10 meter-long erbium doped fiber (EDF) was used as the laser gain medium pumped by a 980nm laser diode through a 980/1550nm wavelength division multiplexing (WDM) coupler. A 10 meter-long standard single mode fiber (SMF) was added to ensure average anomalous dispersion and soliton operation in the laser cavity. An isolator was used to ensure unidirectional lasing. 10% of the intra-cavity power was coupled out through a fiber coupler. Without the nitrogen sealing, stable self-start soliton mode locking was achieved at the pump power of 15mW at the fundamental repetition rate of 5.85MHz and a pulse duration of approximately 600fs.

 

Fig. 2 Fiber laser set-up using a CNT-SA sealed in a pressurized Nitrogen-gas chamber.

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After stable soliton operation was achieved, we monitored the reflectivity of the CNT-SA via a circulator and the optical spectrum of the laser output throughout 24 hour periods at increasingly high pump powers first in air and then in a chamber with a continuous nitrogen flow with a pressure of 20kPa. The degradation of the CNT-SA was signaled by a drop in its reflectivity.

In Fig. 3 , the performance of the fiber laser with the CNT-SA operating in air Figs. 3(a)-3(c) and in nitrogen Figs. 3(d)-3(f) are shown. In air, the laser was pump by a laser diode (LD) at 980nm with pump powers of 17.5mW, 57.5mW, 97.8mW, 131mW and 278.5mW corresponding to intracavity power at 1560nm of 5mW, 26mW, 48mW, 65mW, 140mW respectively. Figure 3(a) shows the reflected light over 24 hours at the indicated pump powers. At 17.5mW and 57.5mW only minor fluctuations in the reflected powers were observed and stable mode-locked laser operation was maintained throughout the 24hours as seen in Fig. 3(b). From this, we concluded that no degradation of the CNT film took place at those powers over the studied period. At pump powers of 97.8mW, 131mW and 278.5mW some degradation was observed and the reflectivity dropped over the 24hours by 11%, 14% and 30% respectively. Stable modelocked operation was maintained at the powers of 97.8mW as can be seen in Fig. 3(b). However, at powers of 131mW and above the degradation of the CNT lead to the presence of a strong cw peak in the spectrum after the 24hours of laser operation, seen in Fig. 3(c).

 

Fig. 3 Reflected power from the CNT-SA over a 24 hours period with (a) the CNT in air and (d) in a nitrogen-sealed during mode-locked operation at the indicated pump powers. Optical spectrum of the fiber laser output when the fiber laser is pumped at the indicated powers before (solid, color) and after (dashed, gray) the study in air (b) and (c) and nitrogen (e) and (f).

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In order to purge the oxygen out of the CNT-SA environment and hence prevent oxidation, we inserted the device in a chamber, as shown in the inset of Fig. 2 and filled it with a nitrogen gas. In this way, we were able to operate the laser at pump powers as high as 580mW without observing degradation in the CNT-SAs. In Fig. 3(d), the reflected light from the CNT-SA at pump powers at 980nm of 97.5mW, 131mW, 278.5mW, 379mW, 479.5mW and 580mW, corresponding to intracavity powers at 1560nm of 48mW, 65mW, 90mW, 188mW, 247mW and 316mW over a period of 24hours is shown. While the CNT-SAs were in the nitrogen environment, no damage was observed. At all pump powers except 580mW stable modelocked operation over 24 hours was maintained, at the pump power of 580mW, the laser was modelocked throughout the 24hours, yet the optical spectrum profile was unstable, changing with time. Nevertheless, even at that power, the CNT-SA was not damaged and after reducing the pump power laser stability could be regained. In Fig. 3(e) and Fig. 3(f), the spectral profiles before and after the 24 hours operation at the pump powers indicated are shown.

In this paper, we studied the damage of CNT under intense irradiation and demonstrated a drastic increase in the damage threshold by sealing the CNT-SA in nitrogen. For a detailed study on damage thresholds, intensity rather than power must be considered. (i.e. we must also consider the beam diameter). In this work, however, we are working with standard single-mode fibers with a well-known mode-field diameter so we are giving all data in power rather than intensity units for simplicity. In this work, we did studies using pulsed and cw laser operation and damage in air took place at similar power levels, however, in order to fully differentiate between the roles of peak and average intensities in the damage threshold further studies using lasers operating at different pulse durations will be required.

From a more practical point of view, these results indicate that Nitrogen-sealing is a viable solution to increase the lifetime and stability of CNT-SAs under high optical intensities. This is useful yet not vital for most soliton-like fiber lasers since these lasers generally operate at pump powers in the order of tens of mW and at those powers the CNT-SAs exhibit long term stability without Nitrogen-sealing. This method, however, will be beneficial for the long term stability of laser configurations where much higher pump powers (in the order of 100s of mW) are required, such as; the miniature cavity, multigigahertz fiber lasers [8] and in some cases for dissipative and wave-breaking free pulse evolutions where the pulse energies and required pump powers are significantly higher than for standard solitons.

4. Conclusions

The damage of CNT-SAs under intense light irradiation at 1560nm was investigated first using a cw laser source and then by incorporating the CNT-SA into a mode-locked ring cavity fiber laser operating in the soliton regime. We demonstrated that by sealing the CNTs in a nitrogen chamber the damage threshold can be drastically increased. This work was carried out using optically deposited and sprayed CNT-SAs fabricated using CNTs synthesized by the high pressure carbon monoxide (HiPCO) method. However, the method of nitrogen-sealing can be employed to increase the damage threshold of free standing CNTs fabricated by any other method; likewise, we expect that nitrogen sealing would decrease the deterioration of graphene-based SA in a similar fashion.