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We report a substantial increase in the heat resistance in a connector-type single-wall carbon nanotube (SWNT) saturable absorber by sealing SWNT/P3HT composite with siloxane. By applying the saturable absorber to a passively mode-locked Er fiber laser, we successfully demonstrated 280 fs, 31 mW pulse generation with a fivefold improvement in heat resistance.
©2012 Optical Society of America
1. Introduction
Passively mode-locked fiber lasers using a single-wall carbon nanotube (SWNT) saturable absorber have been receiving a lot of attention because of their potential for ultrashort pulse generation with a simple cavity structure. SWNTs have a number of attractive optical properties, including high nonlinearity, an ultrafast recovery time in the near-infrared region [1, 2], and absorption wavelength tunability depending on the nanotube diameter [3]. SWNT-based saturable absorbers have been realized with film, bulk, or waveguide structures. Specifically, SWNTs are sprayed or deposited on a substrate or a waveguide [4–6], dispersed into polymer [7–14], or coated on a film or a fiber connector end [15–18]. SWNT-incorporated polymers are particularly useful as saturable absorbers because passive mode-locking can be easily realized by installing them in the laser cavity or simply inserting them between fiber connectors. In addition, SWNT-polymer composite materials can be fabricated at low cost and are easy to handle. Until now, SWNTs have been uniformly distributed in various host polymer materials, such as polyvinyl alcohol (PVA) [7–9], polyimide [10, 11], polymethylmethacrylate (PMMA) [12], polystyrene [12], polycarbonate (PC) [13, 14], and poly-3-hexylthiophene (P3HT) [15, 18, 19]. Bulk-type SWNT-incorporated polymers have also been realized with a thickness of more than a few hundred micrometers, and they can be employed as optical waveguide devices.
However, one drawback of these SWNT-polymer composites is their relatively low tolerance to optical damage due to the low heat resistance of the polymers, in which SWNTs are burned by the high optical power. The highest laser output power yet reported is 250 mW, which was realized by employing the evanescent interaction between guided light and an SWNT, which allows them to interact over a certain distance but with reduced power [5]. An average power of 114 mW has been obtained with the direct irradiation of an SWNT, but the pulse width remained broader than 5 ps [20]. To increase the optical damage threshold, it has been proposed that SWNT burning be avoided by sealing the SWNTs in nitrogen gas, so that they do not suffer from any oxidation reaction [21]. Laser operation with long-term stability has been observed, but a pulse width shorter than 1 ps is still difficult to obtain while maintaining a high output power.
In this paper, we demonstrate high-power, femtosecond pulse generation using a novel fiber-connector-type SWNT saturable absorber with high heat resistance by coating the SWNT/P3HT composite with siloxane. The siloxane sealing is very useful for preventing an oxidation reaction with the SWNT and thus plays an important role in increasing the optical damage threshold. This method enables us to realize a simple and compact configuration that can be easily installed in the cavity simply by inserting it between fiber connectors as with the conventional SWNT saturable absorber. A 31 mW, 280 fs soliton pulse was successfully obtained at 1.56 μm with a repetition rate of 22 MHz, in which the SWNT/P3HT composite could tolerate an incident power of 50 mW and higher.
2. Fabrication of SWNT/siloxane saturable absorber
Figure 1 shows the molecular formula of the siloxane. It is characterized by a stable and strong siloxane bond, Si-O-Si, with a bond energy as high as 106 kcal/mol, which is much higher than that of a carbon bond (85 kcal/mol). Because of this feature, siloxane has heat resistance even at temperatures above 250 °C. In addition, siloxane is a kind of silicone polymer and thus has a high optical transparency over a wide wavelength.
We fabricated a fiber-connector-type SWNT/P3HT saturable absorber sealed with siloxane in the following way. First we dispersed SWNTs in chloroform to prepare a suspension by employing ultrasonification for 30 minutes. Here we used SWNTs with a diameter of 1.2 nm, which corresponds to a band gap in the 1.5 μm band [3] produced by the high-pressure carbon monoxide (HiPCO) method. We then added P3HT to the solvent, and continued the ultrasonification for another 30 minutes. P3HT, which is known as a conductive polymer, enables SWNTs to be uniformly dispersed, in which the surface of the dispersed SWNT is covered by P3HT so that the reaggregation of SWNT is prevented [15, 18, 19].
After dipping the fiber connector end into the obtained SWNT/P3HT complex and drying it, we sealed it with siloxane. Specifically, we coated the SWNT/P3HT with a siloxane precursor solution (organopolysiloxane) and heated it with a hot plate at a temperature of 200 °C. The siloxane polymer was obtained from the solution via a dehydration-condensation reaction. Figure 2 shows an overview of the fabricated connector-type SWNT/P3HT saturable absorber sealed with siloxane. The optical transparency in the 1.5 μm band was approximately 15.8%, where the transmittance values of SWNT/P3HT and siloxane were 30.4 and 52%, respectively. The modulation depth and saturation intensity of the saturable absorber were 6% and 10.8 MW/cm2, respectively.
3. Femtosecond fiber laser with SWNT/siloxane saturable absorber
We developed a 1.5 μm passively mode-locked erbium fiber laser with the fabricated fiber-connector-type SWNT/siloxane saturable absorber installed in the cavity. Figure 3 shows the configuration of the laser cavity. A 2 m-long erbium-doped fiber (EDF) was used as the gain medium with an Er3+ concentration of 7100 ppm. The EDF was pumped by a 980 nm laser diode (LD) via a WDM coupler, and the maximum pump power was 400 mW. A polarization controller was included in the fiber laser cavity to maintain a fixed polarization state for stable laser operation.
Fig. 3 Configuration of passively mode-locked fiber laser with fiber-type SWNT/P3HT saturable absorber (SA).
The dispersion variation inside the cavity is shown in Fig. 4 . The dispersion-shifted fiber (DSF) in Fig. 4 corresponds to the pigtail of the polarization controller in the cavity. We designed the dispersion profile to be that of an average soliton laser. The average dispersion of the cavity was set at + 10.2 ps/nm/km by inserting a 6.5-m long SMF ( + 17 ps/nm/km). We chose this relatively large average dispersion so that a stable soliton pulse could be generated in a higher power regime by taking advantage of the higher damage threshold of the present saturable absorber. With a lower average dispersion, such a high power oscillation leads to instability through higher-order soliton oscillation, since the power required to excite a higher-order soliton is reduced under a low dispersion [22]. The cavity length was 9.1 m, which corresponds to a repetition rate of 22 MHz.
We first evaluated the optical damage threshold by increasing the incident power to the SWNT/siloxane saturable absorber. To deliver as high as possible an optical power directly to the saturable absorber, here we used a 90:10 optical output coupler where 90% of the circulating pulse remained inside the cavity and 10% was coupled to the laser output. Figures 5(a) and 5(b), respectively, show the incident power to the saturable absorber and the spectral width of the laser output as a function of the pump power. The red and blue curves, respectively, show the results with SWNT/P3HT coated with siloxane and without siloxane (coated on polyimide). With siloxane coating, even when the pump power was increased to 392 mW, corresponding to a launched power of 50 mW, the pulse oscillation was maintained with an almost identical spectral width and without any indication of optical damage. The spectral width and pulse duration at a pump power of 392 mW were 11.2 nm and 237 fs, respectively. On the other hand, without siloxane the saturable absorber was damaged at pump powers above 150 mW, or at incident powers above 15 mW. Typically, conventional SWNT/P3HT polymers are damaged with an incident power of 10 mW, and therefore this indicates the realization of fivefold greater heat resistance by virtue of the siloxane sealing.
Fig. 5 Incident power to SWNT saturable absorber (a) and spectral width of laser output (b) for various pump powers. The red and blue curves correspond to the results obtained with SWNT/P3HT coated with siloxane and without siloxane (coated on polyimide), respectively.
We then optimized the output power ratio of the optical coupler to obtain higher output power from the laser. We found that when the coupling ratio was 30:70, where 70% is coupled to the laser output port, the highest output power could be obtained for any pump power. The laser output characteristics are shown in Fig. 6 . Figure 6(a) shows the laser output power versus the pump power. The threshold power was approximately 50 mW, and pulsed oscillation was achieved immediately above the threshold power. Figures 6(b) and 6(c), respectively, show the autocorrelation waveform and optical spectrum of the laser output for a pump power of 392 mW. The output power reached as high as 31 mW, and a 280 fs pulse was generated with a spectral width of 9.5 nm, which corresponds to a time-bandwidth product of 0.33. This indicates that a nearly transform-limited sech soliton pulse was generated. This is the highest output power yet realized with an SWNT fiber laser in the femtosecond regime. Figure 7 shows the RF spectrum of the signal around a clock frequency of 22 MHz. The clock spectrum has a sufficiently large S/N even when the pump power is increased, which indicates that a neatly repetitive oscillation is maintained in the high power regime. At present the output power is limited only by the maximum pump power (~400 mW) available with the currently used pump LD. A higher output power is expected with a higher pump power.
Fig. 6 Laser output characteristics: (a) laser output power vs. pump power, (b) autocorrelation waveform and (c) optical spectrum at a pump power of 392 mW.
To confirm stable laser operation as a soliton laser, we evaluated the peak power required for a fundamental soliton (Psoliton) and the soliton period (z0) in the present cavity, which are defined as
Here, λ is the wavelength, c is the velocity of light, γ is the nonlinear coefficient, Dave is the average dispersion of the laser cavity, and τFWHM is the full width at half maximum of the pulse. These values are compared with the peak power of the pulse circulating in the cavity (Pp) and the cavity length (L). At a pump power of 392 mW, Pp was 2.15 kW, which was well above Psoliton = 348 W. The relationship Pp > Psoliton indicates the effect of dispersion management employed because of the positive and negative dispersion variation shown in Fig. 4. The soliton period was calculated and found to be z0 = 3.0 m, whereas the cavity length (L = 9.1 m) was much longer. These results imply that the pulse oscillation was in the average soliton regime, which resulted in stable sech pulse generation. Stable soliton oscillation with a higher output power will be possible by employing stretched-pulse or dispersion-managed soliton laser operation through the adoption of strong dispersion management.4. Conclusion
We have successfully demonstrated a substantial increase in the optical damage threshold in a SWNT saturable absorber, in which SWNT/P3HT coated on a fiber connector end was sealed with siloxane. The siloxane sealing plays an important role in preventing SWNT from burning as a result of an oxidation reaction. By installing this saturable absorber in a passively mode-locked erbium fiber laser, a 31 mW, 280 fs soliton pulse was generated at 1.56 μm, which is to the best of our knowledge is the highest power femtosecond pulse obtained with an SWNT fiber laser.
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