1. Introduction

Single-walled carbon nanotubes (SWCNTs) have been extensively studied and investigated for future photonic and optoelectronic applications due to their excellent electric, optical and mechanical properties [1]. Among their outstanding features, huge third-order optical nonlinearities reported in suspension or polymer composite of SWCNT have been received recent attention because it can be immediately used as optical limiters, ultrafast all-optical switching devices and nonlinear saturable absorbers (SAs) for mode-locked lasers [2–5]. SAs for passively mode-locked laser applications generating femtosecond (fs) optical pulses are of particular interest since it can be found numerous applications both in fundamental researches and industrial fields. When compared with the widespread SAs based on multiple quantum-well heterostructures, i.e. semiconductor saturable absorber mirrors (SESAMs) requiring relatively complex and expensive fabrication process, SWCNT-SAs exhibit several superior features such as fast recovery times in the range of 1 ps, low saturation intensity, easy of fabrications, wide operation range and relatively high optical damage threshold, which lead to numerous recent investigations both in bulk solid-state and fiber lasers for mode-locking at different spectral ranges [5–12].

The SWCNT/polymer composite film spin-coated on dielectric mirrors or quartz substrates are mostly used method as SAs in solid-state lasers systems [6–8]. However to directly apply this scheme to fiber laser, additional alignments are required during extracting or re-coupling the light from or to the fiber which may cause instability of the laser operation. Alternatively, carbon nanotube (CNT) SAs by spray-coating or directly depositing the CNT film on the fiber ferrule can be compact solution in fiber laser systems [5,12]. However this approach is subject to the physical damage by direct physical contact between fiber ferrules or thermal damage due to the interaction of high energy pulse with CNTs within the limited interaction length. To overcome these problems, several novel schemes exhibiting compatibility with all-fiber format are proposed such as CNT deposited on the side-polished fiber [13] or tapered fiber embedded in SWCNT/polymer composite [14]. Both approaches are exploiting the evanescent field interaction of propagating light with the CNTs. Thus it provides relatively long and stable nonlinear interaction between the light and CNTs, even though there are still some drawbacks remaining. For example side-polished fiber shows polarization-dependent behavior due to its geometrical nature, and tapered fiber might show strong nonlinearity and large group velocity dispersion at the waist region of tapers in the laser cavity. Careful fabrication process might be additionally required to obtain or control the long nonlinear interaction.

In the present work we propose the novel all-fiber SA using a hollow optical fiber (HOF) containing the SWCNT/polymer composite. The SWCNT-filled HOF has a ring-core structure with a central air-hole including SWCNT dispersion where the evanescent field of the guided mode interacts with the SWCNT composites distributed over whole length of the HOF. Low-concentration and well-dispersive SWCNT composites previously employed in solid-state mode-locked laser system [8] are applied in this work to take the advantage of low scattering loss. This approach provides very long and robust nonlinear interaction of the guided mode with low concentration CNT composite. Thus this will be potentially beneficial to increase the damage threshold of the SA for high power pulse formation in laser cavity. Efficient heat dissipation over the distributed length of the fiber is expected during the interaction as well. The interaction length can be easily controlled by selecting the proper length of CNT-filled HOF and splicing it with normal single-mode fiber (SMF). A passively mode-locked fs fiber ring laser are successfully demonstrated as a proof of our SA concept, exhibiting 3-dB spectral bandwidth of 5.5 nm with the pulse duration of 490 fs resulting in time-bandwidth product of 0.33. It is interesting to compare that the previously reported in-fiber micro-channel device [15] uses the CNT-filled air-hole normal to the propagating direction of the guided mode; meanwhile our one is designed to be along the waveguide direction. Thus our concept can be similarly extended to other kind of holey fibers such as photonic crystal fibers [16] or microstructured fibers [17]. Other nonlinear photonic device applications are also expected based on highly nonlinear CNTs in combination with various kinds of holey fibers capable of engineered dispersion and nonlinear properties.

2. Fabrication of the SWCNT SA

The SWCNTs (Unidym Inc.) synthesized by high-pressure CO conversion (HiPCO) technique is used in the experiment where the purity more than 90% is verified based on thermo-gravimetric analysis. Figure 1 describes the manufacturing process of SWCNT/polymer composite. The dried SWCNTs are dispersed in dichlorobenzene (DCB) via ultrasonic agitation and mixed subsequently with the poly(methyl methacrylate) (PMMA) solution. We first deposit the SWCNT/PMMA mixture on quartz substrates by spin-coating to characterize the absorption behavior. The recorded linear transmission of the SWCNT/PMMA film shows quite broad absorption around 1.5 μm. The absorption resonance will originate from E₁₁ transition of semiconduting HiPCO SWCNTs. This feature of broadband absorption is crucial to implement the ultrafast lasers with wide operating range including 1.55 μm.

 

Fig. 1 Fabrication process of SWCNT/PMMA film.

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As-prepared SWCNT/PMMA mixture is then injected into the HOF to fabricate the in-line SA. The hollow fiber is composed of central air hole, GeO₂-SiO₂ ring-core and silica cladding as described in Fig. 2(a) . It has been originally investigated for applications of various optical devices including mode converters for higher-order dispersion compensation and tunable optical filters [18]. The annulus guided mode of the HOF can be adiabatically coupled to the fundamental mode of the conventional SMF with proper splicing condition. This annulus mode easily interacts with any materials with liquid or gas form by filling these into the air-hole region, and in our case the SWCNT/polymer composite is selected.

 

Fig. 2 (a) Interaction scheme of the SWCNT with guided mode in the HOF. The cross-section of the HOF is also described in the figure. (b) Calculated group velocity dispersion (GVD) of the fundamental mode in the ring core fiber of the HOF. Inset figure shows the calculated mode-field distribution of the fundamental mode.

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In order to apply the HOF to the fiber soliton laser cavity, the group velocity dispersion needs to be considered. Figure 2(b) shows the calculated group velocity dispersion of the fundamental mode in the ring core of the HOF based on the information of the fiber used in the experiment. The inset also represents the calculated mode-field distribution of fundamental mode. The calculation shows that the fundamental mode in the HOF exhibits the anomalous dispersion from 2.5 to 6 ps/nm/km over our operational range from 1500 nm to 1600 nm. The small dispersion value is not expected to significantly affect the total laser cavity dispersion during soliton propagation. This small value can be compared with the large one which might exhibit in tapered fiber structure depending on the tapered diameter [19].

Figure 3(a) shows the cross-section of the hollow fiber used in the experiment where the air-hole and ring-core outer diameters are measured to be 5.9 and 9.8 μm, respectively. We filled the SWCNT composite into the central air-hole of the HOF using a pump. The microscopic image of lateral cross-section of the SWCNT-filled HOF is shown in the Fig. 3(b) where the SWCNT dispersion is clearly observed. Before connecting with the conventional SMF, we dried the SWCNT/PMMA mixture in the vacuum oven over several hours to evaporate the DCB from the mixture. The propagation loss or connection loss with the SMF could be substantially decreased through this process. The SWCNT-filled HOF is then connected with conventional SMF where the total insertion loss for the 27-mm-long HOF connection is measured to be −2 dB. Considering the typical insertion loss of −1.7 dB of the HOF without the SWCNT dispersion in the present work, we expect the loss originated from the interaction of CNT dispersion is about 7%. We currently investigate the linear and nonlinear contribution of the loss to optimize the HOF length and CNT concentration rate.

 

Fig. 3 (a) Cross-sectional image of the HOF used in the experiment (b) Microscopic image of the lateral cross-section of the HOF filled with SWCNT/PMMA solution.

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3. Fiber pulse laser experiment

We built a fiber ring laser using the SWCNT-filled HOF where the basic configuration is similar to that of typical ring cavity fiber laser as depicted in Fig. 4 . Er-doped fiber with a length of 2-m is used as a laser gain medium and a ~980-nm laser diode (LD) is used as a pump via 980/1550 fiber WDM coupler. A polarization controller (PC) is employed to match the state of polarization of the propagating light in each round trip. The 2:8-output coupler is inserted to extract the laser light of 20% from the cavity and an optical isolator is used to guarantee the unidirectional operation of the ring laser. The fabricated SWCNT SA using the HOF is finally included in the laser cavity. The inset in Fig. 4 shows the microscopic image of the spliced connection section between the HOF and normal SMF. The adiabatic mode transformation occurs through gradual collapse of the central air-hole of the ring-core in the HOF which efficiently couples the light to the normal SMF.

 

Fig. 4 Schematic of the fs fiber laser using the SWCNT-filled HOF. Inset figure shows the spliced image between the normal SMF and HOF where adiabatic mode transition occurs.

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We observe stable and self-starting mode-locking of the fiber laser at the pump power around 115 mW. Figure 5(a) represents the measured pulse train of the laser at a repetition rate of 18.5 MHz which agree well with the laser cavity length about 11 m. Stable single pulse generation is observed per round trip time of 54 ns. At this pump level, the average output power is measured to be about 2 mW. Figure 5(b) shows the output spectrum of the mode-locked laser where the central wavelength is 1567 nm with a 3-dB spectral bandwidth of 5.5 nm. We tapped the laser output in order to measure the pulse duration while monitoring the laser output spectrum simultaneously. As shown the inset of the Fig. 5(b), full-width half maximum of the pulsed duration (T_FWHM) is measured to be about 670 fs based on intensity autocorrelation where 2.7m-length of SMF28e^® Corning fiber is additionally used for the polarization control before the autocorrelator.

 

Fig. 5 (a) Output pulse train of the laser shows the repetition rate of 18.5 MHz. (b) Measured optical spectrum and pulse duration (inset) of the mode-locked fiber laser

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When we consider the pulse broadening due to the group velocity dispersion by the additional extension of the SMF where nonlinear contribution can be negligible at this power level of the tapped fiber, the T_FWHM of the fiber laser output is estimated to be ~490 fs corresponding to the time-bandwidth product of 0.33 assuming the sech² pulse shape close to the transform-limited value. Single pulse generation is maintained up to the output power of 4.5 mW and pulse splitting is observed over that output power. When we exchanged the output coupler port from 20% to 80%, we could observed single pulse generation with high output power up to 25 mW by proper adjustment of polarization state at high level of pump power around 250 mW where any degradation of the SWCNT SA is not detected. The optimization of the SWCNT SA fabrication for obtaining large modulation depth with low scattering loss and novel cavity design capable of high power soliton propagation are under investigation for the generation of transform-limited soliton-like pulses with higher output powers.

4. Conclusion

In summary, we propose the new nonlinear interaction scheme of SWCNTs with laser light by using the HOF, which offers the advantages of robust, efficient and long interaction of guided light with SWCNTs with simple fabrication process. The HOF filled with low concentration SWCNT/polymer composite exhibiting broadband absorption is prepared as an in-line SA. As a proof of our concept, fs mode-locked fiber laser including the SWCNT-filled HOF is demonstrated exhibiting 5.5 nm 3-dB spectral bandwidth at the repetition rate of 18.5 MHz. The optimization of the SA manufacturing process is in progress for high power operation of the fiber laser. We expect that our concept of HOF-based nonlinear interaction with SWCNT can also be extended to other kind of holey fibers including photonic crystal fibers and photonic bandgap fibers for more robust and efficient nonlinear interaction as well as manipulation of waveguide properties such as nonlinearity and dispersion.