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We fabricated a fiber-connector-type saturable absorber in which SWNTs and P3HT (poly-3-hexylthiophene) were coated on the fiber connector end. This saturable absorber allowed us to realize a short laser cavity length. We used a soliton cavity configuration to generate the shortest pulse (147 fs) at the highest repetition rate (51 MHz) yet obtained with carbon nanotubes (CNT) related saturable absorbers.
©2008 Optical Society of America
1. Introduction
Passively mode-locked fiber lasers operating in the 1.5 µm band enable an ultra-short pulse train to be emitted with a simple cavity structure. These lasers have many industrial applications such as all-optical switching and optical metrology. However, the saturable absorbers employed in passively mode-locked fiber lasers such as semiconductor saturable absorber mirrors (SESAM) have several shortcomings including a high cost and a low tolerance to optical damage. Recently it has been found that single-wall carbon nanotubes (SWNTs) have ultrahigh-speed third-order optical nonlinearity, and the recovery time of the saturable absorption effect is less than 1 ps [1–2]. In addition, SWNTs are inexpensive and have a high tolerance to high-intensity light, compared with other saturable absorber materials. Therefore, from a practical point of view, SWNTs are expected to be used as saturable absorbers.
Until now, SWNT-based saturable absorbers have been realized by spraying or directly synthesizing carbon nanotubes on a substrate [3], or by dispersing them in polyvinyl alcohol (PVA) [4–5]. However, when SWNTs are sprayed or directly synthesized, they form bundles, and this causes Rayleigh scattering. In addition, the structure is limited to thin film. When SWNTs are dispersed in PVA, they suffer from OH absorption loss because water must be used as a solvent to disperse them. It may not be a serious disadvantage for a film device whose thickness is 20~30 µm [6], however this becomes disadvantageous for a thick film or a waveguide with a large optical path length. SWNTs have also been dispersed in polyimide, which has good heat resistance [7–8], and a 165 fs, 23.2 MHz mode-locked fiber laser has been realized with this film-type saturable absorber [7]. By contrast, we have already reported a thick SWNT polymer material that employs polymethylmethacrylate (PMMA), polystyrene (PS), and polycarbonate (PC) [9–10]. These saturable absorbers were applied to a passively mode-locked erbium fiber laser, and a 171 fs Gaussian pulse and a 115 fs sech soliton pulse were successfully generated.
In this paper, we report a fiber-connector-type SWNT saturable absorber, which enables us to reduce the insertion loss and realize a high repetition rate because of the short cavity length due to a simple configuration. The insertion loss of the fiber-connector-type saturable absorber is negligible compared with that of the SWNT module previously used to house the SWNT polymer wafer, because collimator lenses are not required. A fiber-connector-type SWNT saturable absorber in which SWNT is dispersed in dimethylformamide (DMF) has already been reported, and a 400 fs pulse was obtained [11]. In this work, we introduce a new technique for the uniform dispersion of SWNT using P3HT (poly-3-hexylthiophene) [12]. Using this new saturable absorber, we generated a 147 fs pulse, which is the shortest yet reported, with an average power of 4.1 mW at a repetition rate as high as 51 MHz.
2. Fiber-connector-type SWNT/P3HT saturable absorber
The insolubility of SWNTs in solvents and polymers is a serious problem when employing SWNTs as optical devices. This is because the SWNTs tend to bundle and this hinders the effectiveness of the nonlinear optical effect. Therefore, the uniform dispersion of SWNTs is very important as regards separating the bundles. In this work, we used a conductive polymer P3HT. The structural formula of P3HT is shown in Fig. 1. It has been found that P3HT molecules interact sufficiently with SWNTs to penetrate the SWNT bundles, thus reducing the van der Waals interaction between the SWNTs [12]. Therefore P3HT is very useful for separating bundles of SWNTs and facilitating the optimum saturable absorption effect.
The fabrication process of a fiber connector-type SWNT/P3HT saturable absorber is summarized in Fig. 2. We first dispersed SWNTs in chloroform to prepare a suspension, and followed this with 30 minutes of ultrasonification. Here we used SWNTs produced by the high-pressure carbon monoxide (HiPCO) method. It is important to note that an SWNT diameter of 1.2 nm corresponds to a band gap in the 1.5 µm band [13]. We then added P3HT to the solvent during the ultrasonification. We continued the ultrasonification for another 30 minutes and finally obtained SWNT/P3HT complex. The SWNT concentration of the resulting solution was 0.05 mg/ml. We coated this solution on a fiber connector end and dried it. We repeated this procedure 12 times and finally formed the SWNT/P3HT film on a fiber connector end as shown in Fig. 3. The deposited P3HT is not dissolved in chloroform, so repeating the procedure enables us to increase the SWNT/P3HT film thickness. The linear transmission of the SWNT/P3HT film at 1.5 µm was about 44.8 % (including 0.1~0.2 dB connector loss) and its thickness was 100 to 200 nm.
Fig. 2. The fiber-connector-type SWNT/P3HT saturable absorber fabrication process: 1) SWNTs and P3HT were dispersed in chloroform with ultrasonification and SWNT/P3HT complex was obtained. 2) The dispersion solution was coated on a fiber connector end and dried twelve times.
Fig. 3. The end of a fiber connector: (a) Before coating with the SWNT/P3HT solution. (b) After coating with the SWNT/P3HT solvent. (c) Expanded view of SWNT/P3HT saturable absorber obtained with an optical microscope. (×100)
3. Passively mode-locked femtosecond fiber laser with fiber-connector type SWNT/P3HT saturable absorber
Figure 4 shows the configuration of a passively mode-locked fiber laser that employs a fiber connector type SWNT/P3HT saturable absorber. As a gain medium, we used an erbium-doped fiber amplifier (EDFA) with an Er3+ concentration of 7100 ppm, which is much higher than that of the previously used EDF (1000 ppm) [9]. Because a highly-concentrated EDF is used and no SWNT module is required, the cavity length is reduced from 26.8 to 3.9 m, resulting in an increase in the repetition rate from 7.6 to 51 MHz compared with the previous result [9]. The dispersion map of the fiber laser cavity is shown in Fig. 5, in which the average dispersion of the laser is anomalous (+4.2 ps/nm/km).
Fig. 4. Configuration of a passively mode-locked soliton fiber laser with a fiber-connector-type SWNT/P3HT saturable absorber.
We introduced a soliton effect into the fiber laser cavity. A soliton is a stable optical pulse that can propagate over long distances without any change in its waveform by balancing fiber dispersion and nonlinearity. A soliton can propagate in a fiber when the average dispersion of the laser is anomalous. A sech soliton can propagate in an average sense even when the laser cavity is composed of both anomalous and normal dispersion fiber, as long as the average dispersion is anomalous. In addition, certain conditions must be satisfied if we are to generate a stable fundamental soliton pulse in a laser cavity [14].
Here Pp is the peak power of the pulse circulating in the cavity and Psoliton is the peak power required for a fundamental soliton. Z0 is the soliton period, and L is the cavity length. Psoliton and Z0 are given by
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.
Figure 6 shows the laser output characteristics under pulsed oscillation. Figure 6 (a) shows the laser output power against pump power, and Fig. 6 (b) shows the time-bandwidth product and pulse width against pump power. Pulsed oscillation was obtained immediately above a threshold power of 111 mW. Figure 7 shows the autocorrelation waveform and optical spectrum of the fiber laser under the minimum pulse width condition. Stable passive mode-locking was achieved for a pump power of 212 mW at 51.0 MHz and a pulse width of 147 fs with an average output power of 4.1 mW was obtained. The time-bandwidth product was 0.35, indicating that a nearly transform-limited sech pulse was generated. In order to check the soliton condition, here we evaluate the peak power required for a soliton, Psoliton, and the soliton period, Z0. Psoliton was calculated to be 523 W, which was close to the peak power of the pulse circulating in the cavity, Pp=546 W. Thus Pp and Psoliton satisfied the condition given by Eq. (1). The soliton period, Z0, was calculated to be 2.0 m, which was shorter than the cavity length of 3.9 m, and satisfied the condition given by Eq. (2). These results indicate that the output pulse was a stable femtosecond soliton pulse.
Fig. 6. Output characteristics of pulsed oscillation. (a) is laser output power vs pump power. (b) is the time-bandwidth product and pulse width vs pump power.
4. Conclusion
We demonstrated a fiber-connector-type compact SWNT saturable absorber by using a new technique for uniform SWNT dispersion that employed the conductive polymer P3HT. By installing this saturable absorber, we realized a passively mode-locked fiber laser in a simple cavity configuration. A 147 fs soliton pulse with an average power of 4.1 mW was successfully generated at a high repetition rate of 51 MHz using this compact SWNT saturable absorber.
References and links
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