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We present a 1.07 μm all-fiber femtosecond soliton laser employing a film-type saturable absorber with a P3HT (poly-3-hexylthiophene) incorporated SWNT coated on polyimide film. We optimized the laser cavity as a dispersion-managed soliton laser with photonic crystal fiber (PCF) as an anomalous dispersion fiber at 1.07 μm. As a result, a 131 fs, 33 MHz pulse was successfully generated with a simple laser configuration.
©2010 Optical Society of America
1. Introduction
Ultrafast passively mode-locked fiber lasers with single-wall carbon nanotube (SWNT) saturable absorbers have received a lot of attention because of their many attractive features including a simple laser cavity configuration, low-cost and the ultrahigh-speed saturable absorption effect of SWNTs [1,2]. Until now, SWNT saturable absorbers have been realized in various ways, such as the spray method [3], dispersion into polymers [4–11] and the deposition method [12–14]. There have been a number of demonstrations of femtosecond optical pulse generation at repetition rates exceeding several tens of MHz with SWNT-based fiber lasers at 1.5 μm. Such lasers are expected to be applicable in various industrial fields, such as optical metrology, biophotonics and medicine.
It is important to note that SWNTs allow us to select the saturable absorption wavelength by adjusting the tube diameter [15]. This property recently enabled SWNTs to be applied to a wavelength of 1 μm [13,16–18]. At this wavelength, ytterbium-doped fiber (YDF) is available as a gain medium. YDF is attractive for ultrashort and high-energy pulse generation because of its broad gain spectrum and high quantum efficiency. However, it must be remembered that fibers typically exhibit normal dispersion at such a short wavelength, which is disadvantageous for realizing a soliton laser. Goh et al. realized the first mode-locked fiber laser with an SWNT saturable absorber at 1.03 μm, in which a 180 fs, 23 MHz pulse was successfully generated [16]. However the average output power (0.4 mW) was small, and the use of a diffraction grating and a pair of prisms for dispersion compensation makes it difficult to realize a simple cavity configuration. Nicholson et al. have also generated a 137 fs, 20 MHz pulse at 1.07 μm [13], by using higher-order mode (HOM) fiber as an anomalous dispersion medium, but the direct average output power from the laser was 0.1 mW, and an external optical amplifier was needed to measure the pulse waveform. An average output power of 10 mW has recently been reported by Kivistö et al., but the pulse width (0.67 ps) was in the sub-picosecond region [17]. With a bulk solid-state laser using a Yb:KYW/KYW crystal and SWNT which was spin-coated onto a dielectric mirror, 83 fs pulse generation has recently been reported in the 1 μm region [18].
In this paper, we demonstrate an all-fiber soliton laser at 1.07 μm with a film-type SWNT saturable absorber, in which the laser cavity is optimized as a dispersion-managed soliton laser by using a photonic crystal fiber (PCF) with an anomalous dispersion. SWNTs were coated on a polyimide film with high heat resistance and high transparency at 1.07 μm to form a base film. In fabricating this sample, we coated the SWNT solution by using a conductive polymer P3HT (poly-3-hexylthiophene), which enabled us to realize a very simple fabrication process. With the PCF and the new SWNT saturable absorber, we realized a simple all-fiber laser cavity configuration and successfully generated a stable optical pulse as short as 131 fs with a repetition rate of 33 MHz, which is to our knowledge the best performance yet reported for fiber lasers with SWNT saturable absorbers at 1 μm.
2. Configuration of a 1.07 μm passively mode-locked femtosecond fiber laser with film-type SWNT saturable absorber
Figure 1 shows the configuration of a passively mode-locked femtosecond fiber laser at 1.07 μm, using SWNT/P3HT coated on a polyimide film as a saturable absorber. A 1.2 m long YDF was used as the gain medium and was pumped with a 980 nm laser diode (LD). The cavity length was 6.1 m, corresponding to a repetition rate of 33 MHz. A polarization controller was inserted in the fiber laser cavity to maintain a fixed polarization state. We employed a 70:30 coupler and the 30% port was used as an output from the laser cavity.
Fig. 1 Configuration of a passively mode-locked fiber laser with a film-type SWNT/P3HT saturable absorber.
We optimized the fiber laser cavity as a soliton laser. Figure 2 shows the dispersion map of the fiber laser cavity. At 1.07 μm, anomalous dispersion is difficult to realize with a conventional step-index fiber. Therefore, we employed a PCF with an anomalous dispersion of + 79 ps/nm/km at 1.07 μm to make the average dispersion anomalous. This PCF had an air-hole diameter of 1.1 μm, an air-hole spacing of 1.8 μm, an effective area of 3.3 μm2 and a nonlinear coefficient of 50 W−1km−1. By inserting a 2.4 m PCF into the cavity, the average dispersion of the fiber cavity became anomalous ( + 6.5 ps/nm/km). The local variation in the fiber dispersion was within tens of ps/nm/km around the average dispersion, which constitutes relatively strong dispersion management. In this case, the laser operated as a dispersion-managed soliton laser and the pulse waveform was closer to Gaussian [19,20].
We fabricated a new film-type saturable absorber with a P3HT-incorporated SWNT coated on a polyimide film for use at 1.1 μm. The conductive polymer P3HT, whose structural formula is shown in Fig. 3(a) , was employed to realize a uniform dispersion of SWNTs because the P3HT molecules interact sufficiently strongly with SWNTs to penetrate the SWNT bundles, thus reducing the van der Waals interaction between the SWNTs [21]. In addition, polyimide film with both high heat resistance and transparency was employed as a base film. The decomposition temperature, which is a measure of heat resistance, is as high as 560 °C, indicating that the polymer has high heat resistance. This is expected to enhance the tolerance of SWNT saturable absorber against optical damage. Additionally, we can realize a self-standing SWNT/P3HT thin film by using the polyimide film as a base. Figure 4 shows the linear transmission of the polyimide film measured with Fourier transform infrared spectroscopy (FTIR). The optical transparency in the 1.07 μm band was 84%. The fringe pattern seen in Fig. 4 corresponds to the free spectral range (FSR) with the Fabry-Perot cavity, whose cavity length is given by the thickness of the film (25 μm). The refractive index of the film was 1.78.
We first fabricated an SWNT/P3HT solution, in which SWNTs and P3HT were dispersed in chloroform for 30 minutes by using an ultrasonic homogenizer. We finally obtained the SWNT/P3HT complex shown in Fig. 3(b). We used SWNTs produced by the CoMoCAT (Co-Mo catalyst) method whose mean diameter was 0.8 nm. Here a diameter of 0.8 nm corresponds to a band gap in the 1.1 μm band, according to the Kataura plot, which shows the relationship between the tube diameter and energy band gap [15]. We filtered the resulting solution by using a membrane filter to eliminate the aggregated SWNT. It is important to note that a polyimide film has durability against chloroform, which enables us to coat the obtained solution on a polyimide film. The solution was coated on a polyimide film and dried. We repeated this procedure a few tens of times. An SWNT/P3HT-based polyamide film was finally obtained as shown in Fig. 5(a) . Its color was black because of the SWNT layer formation. By cutting this film into small pieces, it is possible to obtain many saturable absorber films having the same characteristics from one large film. The deposited P3HT did not dissolve in chloroform, and so repeating the procedure enabled us to increase the SWNT/P3HT film thickness. Figure 5(b) shows the linear transmittance of the obtained film. The thickness of the SWNT/P3HT layer was 100~200 nm. The transmittance at 1.07 μm was 44%. The saturation fluence of the SWNT was about 14 μJ/cm2, and the transmission was increased by 9% at the maximum intensity. We inserted the film into the laser cavity by sandwiching it between the two fiber connectors.
Fig. 5 An overview of the fabricated SWNT/P3HT coated on a polyimide film (a) and its transmission characteristic of film (b).
3. Experimental results of pulsed oscillation
Figure 6 shows the relationship between laser output power and pump power. As the pump power increased, first continuous-wave (CW) oscillation was obtained above a threshold power of 136 mW and then stable pulsed oscillation was achieved above 181 mW. When the pump power was increased to more than 200 mW, a CW peak appeared in the optical spectrum and this made the pulsed oscillation unstable. Figure 7 shows the autocorrelation waveform and optical spectrum with a pump power of 187 mW. At a repetition rate of 33 MHz, a pulse width of 921 fs (assuming a Gaussian pulse) was obtained with an average power of 1.8 mW. This is highly chirped, considering that the optical spectral width is as broad as 15.2 nm. We therefore compensated for the chirp by using an anomalous-dispersion PCF at the output and obtained a shorter pulse with a pulse width of 131 fs with an average power of 0.72 mW considering the splice loss of PCF (2 dB per splice). The time-bandwidth product was 0.52, indicating that the obtained pulse was close to Gaussian and that the laser operated as a dispersion-managed soliton laser.
Fig. 7 Laser output characteristics at a pump power of 187 mW: (a) autocorrelation waveform, (b) optical spectrum.
When a stable dispersion-managed soliton pulse propagates in the cavity, the peak power PDM becomes sufficiently larger than that required for a fundamental sech soliton Psoliton.
where 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. With the present laser configuration, Psoliton was calculated at 32 W, which is sufficiently smaller than the 970 W peak power of the pulse circulating in the cavity, thus satisfying the condition given by Eq. (1). This result confirms the propagation of the dispersion-managed soliton in the cavity.4. Conclusion
We demonstrated a passively mode-locked femtosecond fiber laser with a simple all-fiber cavity configuration at 1.1 μm by using a PCF with anomalous dispersion at 1.07 μm and a film-type saturable absorber with P3HT incorporating SWNTs coated on polyimide film. A 131 fs pulse with an average power of 1.8 mW (0.72 mW after chirp compensation) was successfully generated at a repetition rate of 33 MHz.
References and links
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