Single-wall carbon nanotubes (SWNTs) are promising materials for saturable absorbers (SAs) in mode-locked lasers. However it has been widely recognized that the degradation of optical properties of film-type SWNTs used in femtosecond mode-locked lasers limits the achievable long-term stability of such lasers. In this paper, we study the degradation of optical properties of SWNT-SA fabricated as sandwich type using HiPCO SWNTs with an Er-doped all-fiber laser. The thresholds of laser pump power are examined to avoid the damage of the SWNT-SA. Based on the proposed analysis, it is shown that all-fiber laser pulses of 300 fs pulse width, 3.85 mW average output power, 211.7 MW/cm2 peak intensity and 69.9 MHz repetition rate can be reliably generated without any significant damage to the SWNT-SA film.
© 2012 OSA
Demands for research on carbon nanotubes (CNTs) have significantly increased due to their superior physical and chemical properties since the discovery in 1991 by Iijima . CNTs have been applied in nano-electronic components and reinforcements thanks to their excellent mechanical strength and electric conductivity. Recently, CNTs have drawn much attention for photonic applications for their nonlinear optical and electro-optical properties .
One of the most successful photonic applications of CNTs is saturable absorbers (SAs) for femtosecond mode-locked lasers. The key requirements for SAs are the fast response time, strong nonlinearity, broad wavelength range, low optical loss, high power capacity, low power consumption, and ease of integration into an optical system [3, 4]. Single wall carbon nanotubes (SWNTs) have several advantages over the semiconductor SA mirrors (SESAMs) such as low saturation intensity, faster recovery time, broad spectral range, large third-order nonlinear susceptibilities and easy fabrication . Cho et al. were successfully generated 115 fs pulse width, 202 mW average output power at 89 MHz with SWNT-SAs using bulk optics . Sun et al. realized a stretched-pulse fiber laser based on a nanotube SA, with pulse duration of 113 fs and spectral width of 33.5 nm . Independently 113 fs pulse duration, 42 MHz repetition rate fiber laser was successfully generated by Shohda et al. using a cascadable film-type SA with P3HT-incorporated SWNT coated on a polyamide . However, the output power of mode-locked all-fiber lasers with SWNT-SAs is limited to a low level (<5mW) compared to bulk femtosecond lasers due to damages of SWNT-SAs.
In using SWNT-SAs to mode lock an all-fiber laser, to obtain an even shorter pulse width requires a SWNT-SA with an even greater modulation depth, where it is inserted in the fiber, leading to a high absorption of light in a small area.
The SWNT mode lockers suffer from the optical power induced thermal damage such that the SWNTs are burned out with the optical power of less than 30 mW . To avoid it, Y. W, Song et al suggested a vertically aligned SWNT-SA for the evanescent field interaction of propagating light and achieved a 1.02 ps pulse with 250 mW output average power .It turns out that the structure of CNTs may change under certain laser conditions [11, 12]. For example, the KrF excimer laser irradiation in the air may destroy the wall structure of the low-temperature-grown multiwall carbon nanotubes (MWNTs) and decrease the smoothness of the edge of them . Laser-irradiated MWNTs film was studied by measuring the surface morphology before and after irradiation, and it was found that graphitization occurs on the samples when irradiated with a higher energy than the threshold energy, which in turn causes the suffice damage .
Although in the case of SWNT-SAs, where the optical properties are also expected to change by laser heating due to continuous exposure to high-energy pulses in the laser cavity, up to now, no study has been systematically performed on the changes in the output pulse characteristics and the SWNT-SAs damage.
In this paper, the long-term stability of a mode-locked Er-doped fiber laser is investigated using a sandwich type SWNT-SA fabricated with a polydimethylsiloxane (PDMS) and SWNTs film. We found that the power and spectral bandwidth of the output pulse vary when the SWNT-SA is exposed to high power pulses in the cavity. Characteristics of SWNT-SA also change over time when irradiated by a CW laser of ~4.5 mW power. Graphitization of SWNTs film is verified based on the changes of the optical micrographs, Raman spectra, and SEM images before and after the irradiation with a laser.
2. Fabrication and characterization of a SWNT-SA
The SWNT-SA, used as the mode-locker in an all-fiber laser, is manufactured in film-type and inserted in the fiber. Based on the manufacturing method the SWNT-SA is classified as either a composite type or a sandwich type. Sun et al. obtained 113 fs pulses at 1.56 μm using a SA fabricated with polyvinyl alcohol(PVA)-SWNT composite and Shohda et al. successfully fabricated a sandwich type SWNT-SA coated on aromatic polyamide film by spray method and with this they generated 113 fs pulses at 1.56 [7, 8].
The characteristics of a SWNTs film may change by photo oxidation when it is exposed to a high power pulse in the air . The sandwich type SWNT-SA was fabricated to prevent this as shown in Fig. 1(a) . Both sides of the SWNTs film made by a vacuum filtration method were wrapped by PDMS polymers.
SWNTs of 0.03wt% grown by a high-pressure carbon monoxide (HiPCO) method were mixed with sodium dodecyl sulfate (SDS) of 1wt% for functionalization. Bundled CNTs with SDS were initially dispersed by a tipsonicator at 140 W power for 10 minutes and at 540 W power for 30 minutes. Then it was centrifuged for 1.5 hours by an ultracentrifuge.
A vacuum filtration method was adopted to obtain the SWNTs film with a homogeneous absorbance over the entire area. Fabricated SWNTs film was naturally dried for 10 minutes and both sides were spin-coated by PDMS polymers. The characteristics and manufacturing conditions are summarized in Table 1 and a photograph of a SWNT-SA is shown in Fig. 1(b).
The SWNT-SA was characterized by absorption spectrophotometry as shown in Fig. 2(a) and power-dependent absorbance measurement shown in Fig. 2(b). The absorbance of the SWNTs film is 0.845 at a wavelength of 1550 nm and the modulation depth is 18.8%.
3. Laser setup and characterization
The stretched-pulse fiber laser is designed in a ring configuration with a SWNT-SA inserted between the FC/APC connectors as shown in Fig. 3 .
Self-starting of single-pulse mode-locking is observed at the pump power between 99.5 mW and 210 mW. The output power as well as the full width half maximum (FWHM) bandwidth of the optical spectrum increase along with the pump power. The maximum average output power and the FWHM bandwidth are 8.3 mW and 30 nm, respectively, at the pump power of 210 mW. Figure 4 shows the measured interferometric autocorrelation (IAC) and the optical spectrum. The measured FWHM pulse duration, FWHM optical bandwidth and center wavelength are 200 fs, 18.6 nm and 1552.9 nm, respectively, at a pump power of 150 mW, which results in the time-bandwidth product of 0.372. The repetition rate is 69.9 MHz.
4. Instability of an output pulse caused by damage of a SWNT-SA
When the SWNT-SA is used in the fiber laser, changes in the output power and FWHM optical bandwidth were simultaneously measured over >10 hours. Figure 5 shows the change in the output power and the FWHM optical bandwidth at 164.2 mW pump power condition. Both the output power (Fig. 5(a)) and the bandwidth (Fig. 5(b)) decreased in the initial 5 hours and then increased. From these results, we find that the resulting FWHM bandwidth is almost linearly proportional to the output power as shown in Fig. 5(c).
Changes in the output characteristics are mainly caused by the change of gain and loss in the cavity with time. A continuous wave (CW) output with a center wavelength of 1558.9 nm and FWHM bandwidth of 1.8 nm was generated by controlling the cavity length without the SWNT-SA. The output of the CW laser changed when the gain of an EDF was varied with time, but the output power was maintained at a certain value for a day. This implies that the instability of an output pulse is due to the change of the SWNT-SA over time.
In order to study the origin and power dependence of the degradation of SWNT-SAs, we measured the power-dependent absorbance as shown in Fig. 6 . The incident pulsewidth is 200 fs and repetition rate is 69.9 MHz. The normalized absorbance in the SWNT-SA placed between the FC/APC connectors was measured over several hours as a function of input power by adjusting the variable optical attenuator. We first investigated whether the SWNT-SA indeed causes the degradation by comparing the measured normalized absorption over several hours when no specimen is inserted between the FC/APC connectors, when only PDMS polymer film is inserted between the FC/APC connectors, and when the SWNT-SA is inserted between the FC/APC connectors. From the measurement results shown in Fig. 7 , we can observe that the normalized absorbance changes significantly (over 30%) for the SWNT-SA case whereas the other two cases (no specimen, PDMS film only) shows very little change (less than 1%).
Next, the power dependence in the absorbance of SWNT-SA was measured with a CW laser for 4 different output power levels (4.39 mW, 5.15 mW, 8.77 mW, and 11.03 mW) as shown in Fig. 8 . The center wavelength of a CW laser is 1558.9 nm. While the absorbance was maintained for 4.39 mW incident power, it started changing at incident power levels above 5.15 mW and at power density above 594.76 W/cm2. The absorbance consistently decreased when the SWNT-SA was exposed to the power levels above 8.77mW. More than 40% of absorbance drop happens for 11 mW case.
Irradiation of light (high power, up to a certain degree) onto the SWNT causes graphitization and structure variation, thereby changing the SWNT-SA absorbance, normally increasing it. As the absorbance increases, the SWNT-SA absorbs more light and the surface temperature of the SA goes up substantially. As a result, portions of the SWNT get burned, leading to a decrease in the overall SWNT concentration, which also means a decrease in the absorbance.
When an SWNT-SA is exposed to such high-energy light, graphitization and the burning out stages occur simultaneously, which alters the characteristics of the SWNT-SA. We can see that at when exposed to an output power 8.77 mW, the normalized absorption repeatedly increases and decreases. This is seen to occur from the graphitization of the SWNT leading to an increase in absorbance, and the burning out of the SWNT from excessive light exposure leading to a decrease in absorbance.
To investigate the physical mechanisms of such absorbance drop, the change in surface morphology before and after irradiation is studied. Figure 9 shows the optical microscope images of SWNT-SAs when 8.88 mW CW laser with a center wavelength of 1558.9 nm and FWHM bandwidth of 1.8 nm is applied to the samples. The results clearly show the damage on the area passed by the CW laser light (white circles in Fig. 9).
In case of the MWNTs films, it had been known that property changes such as graphitization might be caused by laser heating over the threshold energy of surface damage. In  and , the graphitization is investigated by observing the IG/ID ratio from the Raman spectra. The decrement of the IG/ID ratio after laser heating reflects the increment of the dangling bonds and crystal defects. Moreover, through the scanning electron microscopy (SEM) or transmission electron microscopy (TEM) images, it was observed that the wall structure is largely destroyed and the edge of the nanotube becomes irregular.
To verify the graphitization of SWNT-SA from laser heating, Raman spectra of the areas before and after the CW laser irradiation were obtained as shown in Fig. 10 . The IG/ID ratio remarkably reduces from 13.93 to 5.67 after irradiation.
Figure 11 shows the SEM images for the SWNT-SA after laser irradiation. Figure 11(b) and 11(c) demonstrate the views of the areas with and without the laser irradiation. It is apparent that the image of the laser-irradiated area is more blurry due to the smoothness loss of the tube edge and irregular wall structure.
To find out the threshold energy of the SWNT-SA in the cavity, the changes in the output power and optical spectra of the mode-locked pulse were measured for a long time as a function of the pump power. Threshold pump power of a fabricated SWNT-SA was 106 mW, and the characteristics of the output pulse were maintained over 300 hours with pulse duration of 300 fs, FWHM bandwidth of 12 nm, an output power of 3.85 mW, and peak intensity of 211.7 MW/cm2.
In this paper, a mode-locked Er-doped fiber laser was presented with a sandwich-type single wall carbon nanotubes (SWNTs) saturable absorber (SA). It was newly shown that the SWNT-SA can be graphitized by the laser-heating over the threshold power causing the film surface damage, which is consistent with the known fact of the multi-wall carbon nanotubes films. The graphitization of SWNT-SA was experimentally verified by measuring the surface morphology, Raman spectra, and SEM images. By deigning the laser pump power lower than the threshold power of 106 mW, the proposed system guaranteed the stably mode-locked output pulse with a 300 fs pulse duration, 12 nm FWHM bandwidth, 3.85 mW output power and 211.7 MW/cm2 peak intensity over 300 hours. The damage threshold peak intensity for the SWNT-SA in the cavity is the range of 220-240 MW/cm2. The graphitization occurs on the SWNT-SA after irradiation over threshold energy.
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government(MEST) (No. 20110016486) and BK 21.
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