1. Introduction

Most ultrafast fiber lasers employ a passive mode-locking technique, whereby a saturable absorber (SA) initiates and stabilizes the ultrashort optical pulses. An ideal SA for ultrafast fiber lasers must meet the following key requirements: high nonlinearity, broad bandwidth, low cost, simplicity of fabrication and integration into fiber systems, and high damage threshold [1]. Conventional semiconductor SA mirrors (SESAMs), however, have several drawbacks, including a narrow operational wavelength range and a complex fabrication process similar to molecular beam epitaxy [2,3]. Although the SAs based on single-walled carbon nanotubes (SWNTs) achieve low cost and easy integration into fiber cavities [4–9], they require complex tuning procedures to operate at a particular wavelength, such as band gap engineering and chirality/diameter optimization, because resonant absorption occurs only in semiconducting SWNTs whose diameter corresponds to the relevant photon energy [1,10].

Following the first demonstration of monolayer graphene as an SA in 2009 [11], graphene-based saturable absorbers (GSAs) received attention as alternatives that could overcome the limitations of SESAMs and SWNTs SA because of their ultrafast recovery time, strong nonlinearity, compatibility with optical fibers, and wideband operation without band gap engineering [12–20].

Monolayer and few-layer graphene SAs, however, exhibit an insufficient modulation depth for achieving and maintaining mode locking in ultrafast fiber lasers [21,22] because graphene flakes absorb only ~2.3% of light per layer. The simplest method of increasing the nonlinear absorption of GSAs is to apply a graphene/polymer composite, which is fabricated by dispersing graphene flakes into a host polymer, as a GSA [10,23]. Typical GSAs using graphene/polymer composites are in the form of a free-standing thin composite film, which is normally inserted into a laser cavity by being sandwiched between two fiber ferrules. The GSAs present a simple fabrication process and allow easy integration; however, they can be mechanically damaged by unnecessary physical contact between the two ferrules during the integration process. Moreover, optically induced thermal damage, which is attributed to direct interaction with high-power light, can occur. Thermal damage causes long-term instability and pulse breaking and eventually limits high-power operation.

There are two approaches to overcoming thermal damage: i) increasing the heat resistance of the graphene/polymer film by sealing it to suppress oxidation and ii) preventing the absorption of high-power light within a small volume by extending the interaction length through the evanescent wave interaction to achieve efficient heat dissipation. In the first approach, it is difficult to significantly improve the damage threshold because of the inevitable high confinement of light along the short interaction length of a few tens of micrometers. The second approach is a substantially more promising alternative because the graphene layers only interact with the evanescent wave of the pulse propagating outside of the fiber (a small fraction of the optical power) with a relatively long interaction length. To use the evanescent wave interaction of the graphene layer, GSAs based on a side-polished fiber, in which the graphene layer is transferred onto the polished surface, have been proposed [16,24]. They provide higher thermal damage thresholds for high-power operation; however, they are subject to unavoidable limitations, such as polarization sensitivity due to their geometry, which causes non-symmetrical interaction with the guided light mode. Since polarization states of the fiber laser systems are susceptible to environmental disturbance such as stress, bending, and temperature variation, polarization insensitivity of SA is required to maintain stable operation of the mode-locked fiber laser. In addition, the polarization-independent SAs are also important for generating vector solitons.

In this paper, we demonstrate a mode-locking fiber laser with a graphene/PDMS composite saturable absorber based on evanescent wave interaction, therein achieving highly stable 216 fs pulses with an 80 dB signal-to-noise ratio. Specifically, we present a process of fabricating the GSA considering the strength of the evanescent wave interaction, which is normally calculated via mode analysis of the propagating light within the etched region. The analysis results were used to determine the amount of etching. The nonlinear optical properties of the fabricated SA were also characterized at different polarization states to evaluate the polarization dependence of the SA.

2. Experimental setup

The refractive index (RI) of the host polymer and the etched diameter of the fiber are important design parameters in obtaining efficient evanescent wave interactions between the graphene/polymer composite and the propagating light in the fiber. Because the concentration of graphene in the graphene/polymer composite is relatively low, the RI of the composite is approximately equal to the RI of the polymer. To maintain total internal reflection and form an evanescent wave between the fiber surface and the coating material at the etched region to achieve low radiation loss, the RI of the coating material should be lower than that of the fiber surface. In addition, an RI close to the RI of the fiber surface enhances the evanescent wave interaction by increasing the power fraction outside of the fiber. A weak interaction results in a large saturation fluence, which prevents pulse mode locking.

In this work, polydimethylsiloxane (PDMS), which has an RI of ~1.404 and no significant absorption peak near the 1550 nm wavelength range, was selected as the coating material. The strength of the evanescent wave interaction was numerically estimated using mode analysis based on the simulation using the finite element method. Figure 1 shows the calculation results of the fraction of the optical power outside of the etched fiber as a function of the waist diameter. When the etched diameter is a few micrometers and graphene/PDMS composites surround the etched region, the strength of the evanescent wave interaction can be approximately one order lower than the direct interaction, whereas the interaction is very weak, even if the fiber is etched mostly under air-fiber surface contact conditions without coating.

 

Fig. 1 Calculation result of the power fraction outside the etched fiber as a function of the refractive index of the coating material and the waist diameter of the etched region (inset: guide light mode profile at the etched region).

Download Full Size | PDF

We employed commercially available graphene flakes (Graphene Supermarket) with an average thickness of 7 nm to fabricate the graphene/PDMS composite. The flakes and PDMS were dispersed in chloroform by ultra-sonication for one hour. After the evaporation of chloroform in a vacuum oven at a temperature of over 61.2 °C, which is the boiling point of chloroform, the graphene/PDMS composite was prepared.

With the simulation analysis results, we chemically etched several strands of standard single-mode fibers using hydrofluoric acid (HF). The conventional acid-etch techniques, through which bare fibers are immersed into acid baths, caused high insertion losses of the etched fibers due to the rapid fiber diameter transition and the surface corrugation in the etched region. To minimize the insertion loss, we utilized a method based on a micro-droplet of HF and surface-tension-driven flows of the acid, namely, the so-called flow etching method [25]. The primary coats of the fibers were partially stripped and cleansed with isopropyl alcohol. The bare fibers were tightly fixed close to the surface of an HF-resistant petri dish, and the etching was initiated by injecting 100 μl of 49 wt. % HF solution onto the dish to immerse the stripped part of the fibers. During the etching process, the HF flowed from the droplet and traversed the fiber by surface tension as shown in Fig. 2(a), which leads to the graded diameter profiles for adiabatic transition and prevents surface corrugation on the etched area.

 

Fig. 2 (a) Experimental setup for the hydrofluoric acid (HF) flow etching of a standard single mode fiber (b) Schematic of the graphene-based saturable absorber using an etched fiber coated with graphene/polymer composite (inset: microscope image of the transition region of the etched fiber).

Download Full Size | PDF

The optical microscopy image of the etched region shows the adiabatic transition of the fabricated fiber with smooth surfaces, which contributes to the low insertion loss. By controlling the etching time and HF volume, various etched fibers can be fabricated with different waist thicknesses and lengths. We used 8-mm-long etched fibers with waist diameters in the 4~6 μm range. The etched fibers were fixed into a groove, and prepared graphene/PDMS composites were poured into the groove to cover the entire etched region. After curing the graphene/PDMS composites at room temperature for 2 days, the etched fiber-based GSA was obtained, as illustrated in Fig. 2(b).

3. Results and discussion

To characterize the nonlinear optical properties of the fabricated GSA, the power-dependent absorption was measured with a laboratory-built femtosecond oscillator delivering ~300 fs pulses at a central wavelength of 1550 nm according to the experimental setup depicted in Fig. 3(a). The nonlinear transmittance increases from ~20% to ~28%. The measurements were repeated several times with different polarization states managed by a polarization controller to identify the polarization dependence of the nonlinear absorption. Before the polarization dependence of the GSA was measured, we identified that the polarization dependence of the fiber coupler used in the experimental setup is negligible. Figure 3(b) shows that there is no significant difference in the various polarization states. Note that the results are different from those using GSAs based on side-polished fibers, which introduce an almost ~4 dB polarization-dependent loss [22,24].

 

Fig. 3 (a) Experimental setup for power-dependent absorption measurements. (VA: variable attenuator, OC: optical coupler). (b) Nonlinear absorption properties of the fabricated GSA as a function of polarization state.

Download Full Size | PDF

The fabricated GSA was integrated in a fiber ring cavity to achieve mode locking, as shown in Fig. 4. A 0.6 m highly doped Er + 3 fiber (Liekki, Er 80-4/125) with a dispersion of + 33.6 ps²/km at 1550 nm was forward pumped using a 980 nm pump laser diode through a wavelength division multiplexing (WDM) coupler. A fiber optic isolator was added to ensure unidirectional operation. A fused 90/10 optical coupler was placed just after the isolator, and a 10% port of the optical coupler was used at the fiber laser output. A polarization controller (PC) was included in the fiber laser cavity for mode-locking optimization. The total cavity length was ~4 m, and the total intracavity GVD was ~-0.048 ps².

 

Fig. 4 Demonstrated fiber laser setup using the graphene-based saturable absorber (LD: laser diode, WDM: wavelength division multiplexer, EDF: erbium-doped fiber, ISO: isolator, OC: optical coupler, PC: polarization controller).

Download Full Size | PDF

At 128 mW pump power, self-starting single-pulse mode locking is introduced. The typical output power is 1.3 mW for 135 mW pump power. The relatively high pump power threshold for mode-locking can be attributed to the high losses per round trip due to large linear loss of GSA and small pulse energy due to high repetition rate. Figure 5(a) shows the optical spectrum of the fiber laser, which exhibits a typical soliton pulse spectrum shape with a central wavelength of 1557 nm. The full-width half-maximum (FWHM) bandwidth is 12.5 nm. The pulse duration was measured using an intensity autocorrelator, as shown in Fig. 5(b). Assuming a sech² pulse shape, the inferred FWHM pulse duration was 216 fs. The time-bandwidth product is 0.333, indicating that the obtained pulse is nearly transform-limited. We also measured the radio frequency (RF) spectrum around the fundamental repetition rate (f₁ = 48.14 MHz) to investigate the stability of operation as shown in Fig. 5(c). The signal-to-noise ratio was ~80 dB. The continuous long-term sable operation was confirmed over few tens of hours, which is attributed to the high damage threshold and polarization insensitivity of the SA.

 

Fig. 5 Pulse output properties of the laser: (a) optical spectrum (b) autocorrelation trace, (c) RF signal spectrum at the fundamental repetition rate with 10 Hz resolution, (d) pulse train.

Download Full Size | PDF

4. Conclusion

In conclusion, we introduced a GSA based on a chemically etched fiber coated by graphene/PDMS composites. The GSA presents ~8% modulation depth with polarization insensitivity. The ultrafast fiber laser applying the GSA generated pulses with 216 fs pulse duration, 12.5 nm spectral bandwidth, and ~48 MHz repetition rate. The experimental results show that the etched fiber for evanescent wave interaction offers notable advantages such as polarization insensitivity, all-fiber configuration, and high damage threshold with a long interaction length. We expect that graphene/polymer composites could be applied to various optical components as saturable absorbers, such as planar waveguides and on-chip waveguides as well as fiber optics for wideband operation.