We successfully fabricated a cascadable film-type single-wall carbon nanotube (SWNT) saturable absorber coated on aromatic polyamide film, in which the saturable absorption effect can be controlled with the number of films. A conductive polymer P3HT (poly-3-hexylthiophene) was adopted to obtain a uniform SWNT solution. We applied saturable absorber films to a passively mode-locked fiber laser and successfully generated a 113 fs, 42 MHz pulse by inserting two film layers between fiber connectors in the cavity.
©2010 Optical Society of America
There is a growing demand for femtosecond pulse lasers in various industrial fields including optical metrology, high-resolution microscopy, and biophotonics, as well as ultrahigh-speed communication and all-optical signal processing. Passively mode-locked fiber lasers have been widely used to generate such ultrashort optical pulses because they can be easily achieved simply by employing a saturable absorber in the laser cavity. Recently, single-wall carbon nanotubes (SWNTs) have attracted a lot of attention as ultrahigh-speed saturable absorbers in passively mode-locked fiber lasers because of their high nonlinearity and ultrafast recovery time (~1 ps) in the near-infrared region [1,2]. There are other saturable absorbers including semiconductor-based multi-quantum-well (MQW)  or fiber-based nonlinear devices [4–7]. Of these, SWNTs offer absorption wavelength selectivity through control of the tube diameter and a high tolerance to optical damage. In addition, SWNT optical devices can be fabricated at low cost and are easy to handle.
SWNT-based saturable absorbers have been realized with several configurations. In the first demonstration by Set et al., SWNTs were sprayed on a substrate . However, they suffered from Rayleigh scattering induced by the SWNTs bundles, and long-term stability may not be very high since the SWNTs are exposed externally. For these reasons, various schemes have been proposed for dispersing SWNTs uniformly in a solvent to realize low-loss and stable saturable absorbers. Many kinds of film or bulk type SWNT-incorporated polymers have already been fabricated. The film-type SWNT-incorporated polymers include composites with polyvinyl alcohol, polyimide or polycarbonate [9–14]. These materials have been applied to saturable absorber devices by spin-coating the film onto a mirror or by inserting the film between the two fiber connectors in the laser cavity. The bulk-type SWNT-incorporated polymers have been realized using polymethylmethacrylate, polystyrene or polycarbonate, which feature a thickness of more than a few hundred micrometers, which makes them applicable to optical waveguide devices [15,16]. Inclusion of SWNT in a hollow core fiber  and doping of SWNT in a polymer optical fiber  have also been demonstrated recently. In addition to these schemes, fiber-connector-type saturable absorbers have been proposed, in which SWNTs are directly deposited onto a fiber connector or a fiber endface [19–21]. This makes it possible to realize both a simple fabrication process and a compact saturable absorber with a low insertion loss, but it is not easy to control the SWNT layer thickness.
In this paper, we demonstrate a new type of SWNT-based cascadable saturable absorber where SWNTs that are dispersed uniformly with a conductive polymer P3HT (poly-3-hexylthiophene) are coated on a polymer film with high transparency. As the base film, we newly adopted an aromatic polyamide, which has higher heat resistance than polyvinyl alcohol [9–11] and polycarbonate . In this scheme, a uniformly dispersed SWNT solution is directly coated on a polymer film without dispersing the SWNTs into the host polymer, which greatly simplifies the fabrication process. Furthermore, the saturable absorption effect can be easily controlled by changing the number of films inserted between the fiber connectors in the cavity. The saturable absorption characteristics are determined by the product of SWNT concentration and the thickness of the material. The tunability of material thickness by simply changing the number of films offers large flexibility in the optimization of mode-locked laser operation.
In this work, by optimizing the number of films and designing the laser cavity as a soliton laser, we generated a 113 fs pulse with an average power of 5.0 mW at a repetition rate as high as 42 MHz, which is the shortest pulse yet reported in a fiber laser with an SWNT saturable absorber at 1.5 μm.
2. Fabrication of SWNT saturable absorber film and its characteristics
In applications of SWNTs to optical devices, the insolubility and aggregability of SWNTs in solvents and polymers are serious problems because they make it difficult to obtain an appropriate nonlinear optical effect. It has been found that the conductive polymer P3HT allows SWNTs to be dispersed uniformly . The structural formula of P3HT is shown in Fig. 1 . P3HT molecules interact sufficiently with SWNTs to penetrate the SWNT bundles, thus reducing the van der Waals interaction between the SWNTs. Therefore, we used P3HT to separate bundles of SWNTs and facilitate the optimum saturable absorption effect efficiently. Here the glass transition temperature (Tg), which is a measure of the heat resistance of the polymer material, is around 67 °C . P3HT plays an important role for uniform dispersion of SWNT but at the expense of somewhat lower heat resistance due to the low Tg of P3HT.
The aromatic polyamide (aramid) film that we used as a new SWNT saturable absorber has both high heat resistance and transparency. The glass transition temperature (Tg), which is a measure of the heat resistance of the polymer material, is as high as 300 °C, whereas it is 100 and 150 °C for polymethylmethacrylate (PMMA) and polycarbonate (PC), respectively. In addition, it is important to note that using the polyamide film as a base enables us to realize a self-standing SWNT/P3HT thin film. Figure 2 shows the linear transmission of the aromatic polyamide film measured with Fourier transform infrared spectroscopy (FTIR). The optical transparency in the 1.5 μm band is approximately 90%. The ripples of the fringe pattern seen in the Fig. 2 correspond to the free spectral range (FSR) with the Fabry-Perot cavity due to the film thickness (10 μm). The refractive index of the film is 1.64.
To coat the polyamide film with SWNTs, we prepared an SWNT/P3HT solution, in which the SWNT and P3HT were dispersed in chloroform by employing ultrasonification for 30 minutes. It should be noted that polyamide film has durability against chloroform, which enables us to coat it with this solution. We used SWNTs produced by the high-pressure carbon monoxide (HiPCO) method. As is well known, the SWNT band gap is in inverse proportion to the nanotube diameter . Specifically, a diameter of 1.2 nm corresponds to a band gap in the 1.5 μm band. The SWNT concentration of the resulting solution was 0.1 mg/ml. We coated this solution on a polyamide film and dried it, and repeated this procedure a few tens of times. A 2 x 2 cm SWNT/P3HT-based polyamide film with an SWNT/P3HT layer thickness of 100~200 nm is finally obtained as shown in Fig. 3(a) . The film is black because of the SWNT layer formation. Using this scheme, many saturable absorbers (2 x 2 mm) with the same properties can be obtained from one large film (2 x 2 cm). The deposited P3HT is not dissolved in chloroform quickly, so repeating the procedure enables us to increase the SWNT/P3HT film thickness.
An SWNT saturable absorber can be realized by installing the film between two fiber connectors. An important feature of this saturable absorber is that the magnitude of the saturable absorption effect can be controlled simply by changing the number of sandwiched films. Figure 3(b) shows the linear transmittance of the film-type saturable absorber for various numbers of inserted films. The transmittance at 1.5 μm can be varied from 21.5% to 0.2% by increasing the number of films from one to four. This feature greatly facilitates the control of the saturable absorption effect. The saturation fluence of the SWNT was measured to be 1.4 μJ/cm2 with the z-scan method, and the absorption was found to be reduced by 25% at the maximum intensity.
3. Passively mode-locked femtosecond fiber laser with a film-type SWNT saturable absorber
We constructed a 1.5 μm passively mode-locked femtosecond fiber laser in which the fabricated film-type SWNT saturable absorber was installed in the cavity. Figure 4 shows the laser configuration. An erbium-doped fiber amplifier (EDFA) was used as the gain medium. Here we used a 2 m-long EDF with an Er3+ concentration of 7100 ppm. This allowed us to reduce the cavity length to 4.7 m, corresponding to a repetition rate of 42 MHz. We inserted a polarization controller in the fiber laser cavity to maintain a fixed polarization state. The films were cut into 2 x 2 mm pieces and sandwiched between the fiber connectors.
We introduced the soliton effect into the fiber laser cavity. The dispersion map of the fiber laser cavity is shown in Fig. 5 . Since the laser cavity contains a 2 m EDF with normal dispersion, we inserted a 1.5 m SMF to make the average dispersion of the laser cavity anomalous for average soliton propagation. The resulting anomalous average dispersion ( + 4.9 ps/nm/km) enabled us to achieve soliton laser operation. To generate a stable soliton pulse, the following two conditions must be satisfied [25,26].
Figure 6 shows the output characteristics of the laser under pulsed oscillation. Figure 6(a) shows the relationship between laser output power and pump power, and Fig. 6(b) shows the pulse width versus pump power. Pulsed oscillation was realized when one or two films were inserted, but with three films no pulse was obtained because there was insufficient gain since the transparency was reduced to only 0.9%. Pulsed oscillation was obtained above a pump power of 103 mW and 137 mW with one and two films, respectively. The minimum pulse width of 127 fs with an average output power of 4.4 mW was obtained for a pump power of 260 mW with a single film. Figure 7 shows the corresponding autocorrelation waveform and optical spectrum. As the pump power was increased, the average output power was also increased and the maximum average output power of 8.0 mW was obtained with a pump power of 413 mW. On the other hand, the pulse width was reduced to 113 fs with two films for a pump power of 321 mW, as shown in Fig. 8 . The average power was 5.0 mW, and the time-bandwidth product was 0.39. This is not an exact sech soliton pulse (0.32) as the cavity is dispersion-managed. The peak power of the pulse, Pp, was 1059 W, which is almost the same as the power of 999 W required for a fundamental soliton, Psoliton, thus satisfying the condition given by Eq. (1). The soliton period, Z0, was calculated to be 1.0 m, which is shorter than the cavity length L = 4.7 m, thus satisfying the condition given by Eq. (2). These conditions indicate that the requirements for soliton generation are satisfied, and therefore a stable femtosecond soliton pulse was successfully generated. According to Fig. 6(b), the pulse width with two films was shorter than that with a single film at any pump power. This indicates that the optimum saturable absorption was obtained with two films inserted in the cavity.
We performed a numerical analysis of the transient waveform evolution and steady-state pulse propagation in the laser cavity using the nonlinear Schrödinger equation:Eq. (8) are given by the values for two films obtained from the measurement as presented in Section 2. The recovery time of saturable absorption in SWNT is found to be less than 1 ps , but in the present case the effect of relaxation is not included for simplicity. We employed the split-step Fourier method  to evaluate the transient pulse evolution using Eq. (5) from ASE noise as an initial condition. Figure 9 shows the dispersion map of the laser cavity and the calculated result of the change of the pulse width under steady state propagation. The variation in the fiber dispersion was within approximately 10 ps/nm/km against the average dispersion ( + 4.9 ps/nm/km), which is relatively weak dispersion management. In addition, the pulse width variation inside the cavity was not very large. Figure 10 shows the calculated pulse waveform and the optical spectrum with a pump power Pp = 321 mW. The time bandwidth product was 0.25, indicating that the stationary pulse was close to a sech waveform and that the laser cavity operated as an average soliton laser, which confirmed the experimental result.
We also evaluated the laser output pulse width for various values of pump power with the numerical simulation. The calculated result is shown in Fig. 11 . The shortest pulse width of 110 fs was obtained at Pp = 320 mW, which agrees well with the experimental result. The pulse is broadened for either lower or higher pump power, because of the insufficient spectral broadening due to SPM in the cavity and the weaker saturable absorption effect due to larger input power into SWNT film, respectively.
We have successfully fabricated a film-type SWNT/P3HT cascadable saturable absorber coated on polyamide with both high heat resistance and transparency. This allows us to optimally adjust the saturable absorption magnitude by changing the number of inserted films. A 113 fs soliton pulse with an average power of 5.0 mW was successfully generated at a repetition rate of 42 MHz by using two films as a saturable absorber.
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