Ultrafast laser pulses are important to a variety of research fields, such as optical communication, optical signal sensing, and biomedical imaging.1–4 A popular way to generate such pulses is passive mode-locking, in which the phase difference between resonant modes of the laser cavity is fixed with no external signal required. To achieve this effect, various gain or loss modulation methods have been developed over recent decades, such as saturable absorption and others.5, 6 A saturable absorber (SA) transmits high-intensity light but absorbs low-intensity light. Using a semiconductor SA for modulating the cavity loss has the advantage that the mode-locker can also play a key role in pulse shaping and stabilization.7 An SA with fast recovery time and intense modulation depth is typically required to shorten the net gain window of the cavity and narrow the pulsewidth.
Graphene is a single-atomic-layer crystal with a 2D aligned-honeycomb structure with potential as a fast SA.8 It has excellent photonic characteristics, with an ultrafast carrier dynamic and broadband absorption. SAs incorporating graphene in forms such as graphene-polymer, chemical vapor deposition-grown graphene nanosheets, and graphene solution have been proposed for generating ultrafast laser pulses.9–11 Various forms of graphene have been employed to passively mode-lock an erbium-doped fiber laser (EDFL),12 and other solid-state lasers.13, 14 However, making a monolayer graphene SA requires complicated and expensive processes, including synthesizing and imprinting procedures. Our approach is simply to use nanoscale graphite particles.12 Freestanding graphite nanoparticles at the interface between two fiber connectors serve as an SA for a passively mode-locked ring-type EDFL. The nanoscale graphite particles are directly imprinted on the single-mode fiber (SMF-28, Corning) end-face without any polymer host. The EDFL successfully produces a picosecond nearly transform-limited pulsewidth (i.e., nearly as short as possible).
We mechanically polished graphite foil to obtain the particles. Microscopy and scanning electron microscopy show the nanoscale graphite particles imprinted on the fiber end-face. A scanning-electron microscope image confirms that the average size of the nanoscale graphite particles is 600±200nm: see Figure 1(a). We used a 300fs pulsed fiber laser with its average power increasing from 1mW to 30mW to measure nonlinear transmission. We saw an increase in the nonlinear transmittance versus input optical power of the nanoscale graphite particle saturable absorber from 0.51 to 0.69 followed by saturation: see Figure 1(b).
Figure 1. (a) Schematic of nanoscale graphite particles imprinted on fiber end-face. The microscope image clearly shows the distribution of nanoscale graphite particles and the scanning electron microscope (SEM) image confirms that the size of the nanoscale graphite particles is 600±200nm. (b) The nonlinear transmittance of nanoscale graphite particles with transmittance change, ΔT, of 0.18.
The configuration of the passively mode-locking ring-type EDFL loop is illustrated in Figure 2. The gain medium is an erbium-doped fiber amplifier bi-directionally pumped by two laser diodes with a central wavelength of 980nm. Inside the EDFL cavity, 90 % feedback ratio and 10 % output ratio are provided by a 1×2 optical coupler.15 The connected SMF-28 patchcords with directly imprinted graphite-nanoparticle-based saturable absorber are inserted in the EDFL loop. A polarization controller (PC) set before the saturable absorber detunes the intra-cavity light polarization. The light circulation of the EDFL cavity is decided by the optical isolator located after the graphite saturable absorber.
Figure 2. The configuration of the passively mode-locking erbium-doped fiber laser (EDFL) loop. EDF: Erbium-doped fiber. FC/APC: types of fiber connector.
In the optical spectrum of the passively mode-locking EDFL, the central wavelength is located at 1566nm with a full-width at half maximum of 1.3nm: see Figure 3(a). The shape of the autocorrelation trace of the passively mode-locking EDFL is well fitted by a sech2 profile: see Figure 3(b). The passively mode-locked EDFL pulsewidth is 1.9ps after deconvolution with a factor of 0.65, providing a nearly transform-limited time-bandwidth product of around 0.31. The highly stabilized pulse-train can be maintained for a long time due to the high optical damage threshold of graphite.16 The nanoscale graphite particles induce a relatively large insertion loss, and so soliton mode-locking does not occur. It was necessary to increase the power gain or to decrease the insertion loss of the EDFL cavity to enter a soliton mode-locking regime. We also found it was necessary to increase the coverage ratio and shrink the size of the nanoscale graphite particles on the fiber core area to reduce the insertion loss and the mode-locking threshold.
Figure 3. Optical spectrum of the passively mode-locking EDFL (a). Autocorrelation trace and sech 2 fitting profile of the passively mode-locking EDFL (b).
In conclusion, we employed graphite nanoparticles as a novel saturable absorber to passively mode-lock the EDFL for picosecond transform-limited pulse generation. It is convenient and simple to imprint the nanoparticles directly on the core end-face between connected single-mode fiber patchcords for the fast saturable absorber. Nonlinear transmission by the graphite nanoparticles reveal saturation after a transmittance change of ΔT=0:18. Such a passively mode-locked EDFL generates a long-term stabilized pulse-train with nearly transform-limited pulsewidth and linewidth of 1.9ps and 1.3nm. Our next step is to further shorten the pulsewidth for commercial applications by using smaller nanoscale graphite particles to reduce the cavity's insertion loss. Such a simpler and cheaper saturable absorber for obtaining optical pulses from fiber lasers is essential for ultrafast optical and optoelectronic applications, such as material and device characterization, optical communication, and biomedical imaging.
Gong-Ru Lin, Yung-Hsiang Lin
Graduate Institute of Photonics and Optoelectronics
National Taiwan University
Gong-Ru Lin leads the Laboratory of Fiber Laser Communications and Silicon Nanophotonics. He received an MS and PhD in electro-optical engineering from National Chiao Tung University, Taiwan, in 1990 and 1996, respectively. He has co-authored more than 180 journal papers and over 300 papers in international conferences. He has given 20 invited talks in international conferences, acted as associate editor for Journal of Nanomaterials, Current Nanoscience, and Journal of Lightwave Technology, and also serves on conference steering committees. He is a Fellow of SPIE, the Institution of Engineering and Technology, and the Institute of Physics, a senior member of IEEE and the Optical Society of America, and in 2011 received the Distinguished Research Award from the National Science Council of Taiwan.
Yung-Hsiang Lin is a PhD student.
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