1. Introduction

Single-wall carbon nanotubes (SWNTs) are useful saturable absorbers for passively mode-locked ultrashort-pulse fiber lasers. Both transparent and reflective saturable absorbers can be demonstrated using SWNTs. Several types of SWNT devices have been investigated, especially as the mode-lockers of fiber lasers [1–16]. So far, the highest pulse energy of SWNT fiber lasers has been limited by the damage threshold of SWNT devices and the single-pulse operation limit of fiber lasers.

Dispersion management of the fiber laser cavity enables high-power single-pulse operation. Stretched pulse, similariton, and dissipative soliton regimes have been investigated for high-power operation, mainly in nonlinear polarization rotation schemes [17–19].

For SWNT fiber lasers, evanescent-type devices have been used for high-power operation [6,8,9,16]. Since only the evanescent field interacts with the SWNTs, these lasers have a high damage threshold compared with other devices in which the beam directly irradiates the SWNTs. For single-pulse operation, 55.6 mW average power and 2.35 nJ pulse energy were reported in the dissipative soliton regime in the normal dispersion region [16].

In this work, we demonstrated a high-power passively mode-locked SWNT fiber laser employing a dispersion management technique. As the SWNT device, we used a polyimide film containing dispersed SWNTs synthesized by a laser ablation method. We examined the dependence on the net cavity dispersion and output coupling ratio. By optimizing the cavity, we achieved 114 mW average power and 3 nJ pulse energy in the dissipative soliton regime. To the best of our knowledge, these are the highest reported values of output power and pulse energy from an SWNT fiber laser oscillator with single-pulse operation.

2. Ultrashort pulse fiber laser with SWNT film

Figure 1 shows the configuration of the developed laser. The laser consisted of non-polarization-maintaining single-mode fiber devices with an emission wavelength of 1.55 μm. As the Er-doped fiber (EDF), we used 0.8 m of highly doped commercially available fiber (LIEKKI 110/4) whose second-order dispersion was +15 ps²/km at 1.55 μm. The EDF was pumped by two high-power laser diodes with an emission wavelength of 980 nm through a polarization beam combiner (PBC) and a wavelength division multiplexed (WDM) coupler made of HI1060 fiber. The output of the EDF was spliced with an isolator, which was connected to a variable output coupler, an inline polarization controller, and a wavelength filter, which were made of SMF28 fiber. Since the output coupler was set after the EDF and isolator, the large output power can be obtained and the nonlinear effect and irradiation power into SWNT film can be reduced. The wavelength filter, whose bandwidth was 25 nm, was used to suppress the oscillation of the 1530 nm component. A normal-dispersion fiber (NDF) was used to control the net dispersion of the cavity. The mode field diameter (MFD) was 3.0 μm, and the second-order dispersion was +162 ps²/km. The length of the NDF was varied from 40 to 120 cm to achieve a net cavity dispersion, D_T, of –0.032 to 0.130 ps². The total cavity length was 5.1 to 6.1 m.

 

Fig. 1 Configuration of passively mode-locked, Er-doped ultrashort-pulse fiber laser with SWNT polyimide film. WDM, wavelength-division-multiplexed coupler; EDF, Er-doped fiber; PBC, polarization beam combiner; NDF, normal-dispersion fiber.

Download Full Size | PDF

A polyimide film containing dispersed SWNTs was inserted between the angled-polished FC/APC fiber connectors. The SWNTs were synthesized with the laser ablation method. SWNTs with a diameter of 1.2 nm were selectively synthesized, and they showed an intense absorption peak around 1.55 μm [13]. Figure 2 shows the saturable absorption property of the SWNT polyimide film used in this work. A 680 fs passively mode-locked Er-doped ultrashort-pulse fiber laser with a repetition rate of 50 MHz and a center wavelength of 1.55 μm was used as the pulse source for the measurement. The film had a linear absorption of 61%. A large modulation depth of ~20% was observed for an irradiation power up to 5 mW. The 10% saturation power density was 6 MW/cm². Owing to the low saturation power and large modulation depth, we expected that we could achieve passive mode-locking effectively with a low irradiation power on the SWNT film. Reducing the irradiation power is also beneficial for minimizing optical and thermal damage.

 

Fig. 2 Saturable absorption property of SWNT polyimide film used in this work.

Download Full Size | PDF

3. Dependence of laser performance on net cavity dispersion

First, we examined the dependence of the output performance on the net dispersion of the cavity. We varied the length of the NDF and observed the resulting variation of the output performance. The output coupling ratio, η, was fixed at 90%.

Figure 3 shows typical pulse spectra, and Fig. 4 shows the characteristics of the output pulses as a function of the net cavity dispersion, D_T. For the anomalous dispersion region (D_T < 0), the laser was operated in the dispersion-managed soliton mode-locking regime. An almost-chirp-free sech²-shaped ultrashort soliton pulse was generated. When D_T was –0.032 ps², the temporal width of the output pulse was 249 fs full-width at half-maximum (FWHM), and the maximum output power was 2.91 mW for single-pulse operation. The repetition rate was 39.4 MHz.

 

Fig. 3 Spectra and spectral phase of output pulses when the net cavity dispersion, D_T, was (a) –0.032 ps², (b) +0.017 ps², and (c) +0.098 ps².

Download Full Size | PDF

 

Fig. 4 Dependence of output pulses on the net cavity dispersion, D_T.

Download Full Size | PDF

In the zero dispersion region, although the multiple pulse mode-locking was observed, it was difficult to achieve stable single-pulse mode-locking operation, as was also reported in Ref. 20. It is considered that if we would be able to optimize the modulation depth, we would be able to achieve the stable single pulse mode-locking operation around the zero dispersion region. In the normal dispersion region, single-pulse mode-locking operation was confirmed for the region +0.017 < D_T < +0.122 ps². In this region, the temporal width and output power increased as the magnitude of the dispersion increased . The maximum output power was almost saturated at ~108 mW in the region D_T > +0.082 ps². In this region, the pulse spectrum had sharp edges and almost linear chirping, which are typical characteristics of a dissipative soliton [21,22]. Since the cavity consisted of both normal- and anomalous-dispersion fibers, the operating mode was considered to be the dissipative dispersion-managed soliton mode-locking regime [23,24]. The spectral width increased as the magnitude of the net dispersion increased, taking a maximum at D_T = +0.082 ps², and then gradually decreased. The peak power of the generated ultrashort pulse after dispersion compensation is expected to be highest at D_T ~ +0.082 ps².

The pulse energy shows the similar behavior to that of the average power in Fig. 4. The highest peak power at the laser output was estimated to be 0.97 kW at D_T = 0.082 ps² without dispersion compensation.

4. High-power operation in normal cavity dispersion region

Next, we examined the strong normal dispersion region D_T = +0.082 to +0.13 ps², where both the widest spectral width and maximum output power were obtained.

Figure 5 shows the variation of output power and spectral width as a function of the output coupling ratio, η. As η was increased, the maximum output power increased. The maximum output power was obtained when η = 95%. For η > 95%, the maximum output power slightly decreased as η was increased. Thanks to the high efficiency of the fiber laser, we achieved passive mode-locking even when η was as high as 98%. The spectral width wasalmost unchanged for 50% < η < 90%. For η ≥ 90%, the spectral width gradually decreased as η was increased. As shown in Fig. 4, the spectral width increased as the magnitude of D_T was decreased in this strong normal dispersion region.

 

Fig. 5 Dependence on output coupling ration for (a) maximum output power and (b) spectral width.

Download Full Size | PDF

Figure 6 shows the output characteristics of the developed SWNT laser when η was 95% and D_T was +0.098 ps². This is considered to be one of the best conditions where we can achieve both high power and wide spectral width. The laser showed the self-starting operation. As the pump power was increased, the laser operation changed from cw oscillation, to self-Q switching, and then to single-pulse mode-locking. Using the polarization controller, we could slightly optimize the output power, oscillation wavelength, and pulse shape. Figure 6(b) shows the spectral shape, exhibiting sharp edges and linear chirping, which are the typical characteristics of a dissipative soliton. High-power single-pulse operation at ~112 mW was achieved for single-pulse mode-locking. The spectral width was 14 nm, the repetition rate was 33.8 MHz, and the estimated pulse energy was 3.3 nJ. From SHG-FROG measurements, the temporal shape was Gaussian-like with a temporal width of 4.9 ps and an almost linear chirping characteristic. The small bumps between the pulse train in Fig. 6(c) were the artifacts.

 

Fig. 6 Output performance for η = 95% and D_T = +0.098 ps². (a) Output power and operation mode as a function of pump power, (b) spectral shape and spectral phase, (c) pulse train, and (d) temporal shape and temporal phase.

Download Full Size | PDF

The highest power of 114 mW was achieved when η was 95% and D_T was +0.13 ps². The corresponding pulse energy was 3.5 nJ. To the best of our knowledge, these are the largest values of output power and pulse energy yet reported for an ultrashort-pulse SWNT fiber laser under single-pulse operation. The maximum output power was limited by the obtainable pump power. If the pump power could be increased, the maximum output power would be increased.

Figure 7 shows the observed RF spectra of the output pulse train. Clean spectra were observed, indicating stable mode-locking operation. The intense peaks around 80 MHz in Fig. 7(a) correspond to the FM radio signals. The fundamental RF spectra had an 80 dB extinction ratio from the noise level.

 

Fig. 7 Observed RF spectra for η = 95% and D_T = +0.098 ps² for (a) 0–500 MHz region and (b) fundamental frequency component of 33.8 MHz.

Download Full Size | PDF

6. Conclusion

We demonstrated a dispersion-managed, ultrashort-pulse, Er-doped fiber laser using a polyimide film containing dispersed single-wall carbon nanotubes (SWNTs). The SWNTs were synthesized by the laser ablation method. The dependence of the laser performance on the net cavity dispersion and output coupling ratio was examined experimentally. Generally, the larger the net cavity dispersion was, the larger the output power was for single-pulse operation in the normal dispersion region. The mode-locking at a large output coupling ratio enabled us to achieve high output power and small irradiation power on the SWNT film. In the strong normal dispersion region, we achieved a pulse energy of 3.5 nJ and an output power of 114 mW, the highest values yet reported, for an SWNT fiber laser under single-pulse operation. The dispersion compensation of the output pulse and the numerical analysis of the SWNT fiber laser are under investigation.