Chao-Yung Yeh

Stable mode-locked fiber laser based on CVD fabricated graphene saturable absorber.

A stable mode-locked fiber laser (MLFL) employing multi-layer graphene as saturable absorber (SA) is presented. The multi-layer graphene were grown by chemical vapor deposition (CVD) on Ni close to A-A stacking. Linear absorbance spectrum of multi-layer graphene was observed without absorption peak from 400 to 2000 nm. Optical nonlinearities of different atomic-layers (7-, 11-, 14-, and 21- layers) graphene based SA are investigated and compared. The results found that the thicker 21-layer graphene based SA exhibited a smaller modulation depth (MD) value of 2.93% due to more available density of states in the band structure of multi-layer graphene and favored SA nonlinearity. A stable MLFL of 21-layer graphene based SA showed a pulsewidth of 432.47 fs, a bandwidth of 6.16 nm, and a time-bandwidth product (TBP) of 0.323 at fundamental soliton-like operation. This study demonstrates that the atomic-layer structure of graphene from CVD process may provide a reliable graphene based SA for stable soliton-like pulse formation of the MLFL.

Concentration effect of carbon nanotube based saturable absorber on stabilizing and shortening mode-locked pulse

We comprehensively investigated the concentration effect of dispersed single-walled carbon nanotubes (SWCNTs) in polymer films for being a saturable absorber (SA) to stabilize the mode locking performance of the erbium-doped fiber laser (EDFL) pulse through the diagnosis of its nonlinear properties of SA. The measured modulation depth was from 1 to 4.5% as the thickness increased 18 to 265 microm. The full-width half-maximum (FWHM) of the stable mode-locked EDFL (MLEDFL) pulse decreased from 3.43 to 2.02 ps as the concentrations of SWCNTs SA increased 0.125 to 0.5 wt%. At constant concentration of 0.125 wt%, the similar pulse shortening effect of the MLEDFL was also observed when the FWHM decreased from 3.43 to 1.85 ps as the thickness of SWCNTs SA increased 8 to 100 microm. With an erbium-doped fiber length of 80 cm, the shortest pulse width of 1.85 ps were achieved at 1.56 microm with a repetition rate of 11.1 MHz and 0.2 mW of the output power under an output coupling ratio of 5%. An in-depth study on the stable mode-locked pulse formation employing SWCNTs SA, it is possible to fabricate the SWCNT films for use in high performance MLEDFL and utilization of many other low-cost nanodevices.

Pulse shortening mode-locked fiber laser by thickness and concentration product of carbon nanotube based saturable absorber.

The dependence of thickness and concentration product (TCP) of single-wall carbon nanotubes saturable absorber (SWCNTs SA) on stabilizing and shortening pulsewidth in mode-locked fiber lasers (MLFLs) was investigated. We found that an optimized TCP for pulse energy and nonlinear self-phase modulation (SPM) enabled to determine the shorter pulsewidth and broader 3-dB spectral linewidth of the MLFLs. The shortest MLFL pulsewidth of 418 fs and broad spectral linewidth of 6 nm were obtained as the optimized TCP was 70.93 (μm•wt%), which was in good agreement with the area theorem prediction. This significant effect of TCP on pulse energy, SPM, pulsewidth, and spectral linewidth of MLFLs suggests that the TCP represents the total amount of SWCNTs in SA, which can be used as one of important and key parameters for characterizing the passive MLFL pulsewidth.

Passively mode-locked lasers using saturable absorber incorporating dispersed single-wall carbon nanotubes

Passively mode-locked lasers using saturable absorber incorporating dispersed single-wall carbon nanotubes (SWCNTs) is demonstrated. The peak absorption wavelength of saturable absorber can be engineered within the gain band-width of erbium-doped fiber (EDF) centered at 1550 nm. The mean diameter of SWCNTs and the linear optical absorption of SWCNTs-polyvinyl alcohol (PVA) film are verified by Raman spectroscopy and UV-Visible-NIR spectrophotometer. By integrating the SWCNTs-PVA film into EDF ring laser (EDFL) centered at 1550 nm, we observed three pulse mode operations, Q-switching, mode-locking, and 5th-order harmonic mode-locking. The measured pulsewidths of the mode-locking and 5th harmonic mode-locking EDFL are 4.2 ps and 2.7 ps, respectively

High Electromagnetic Shielding of a 2.5-Gbps Plastic Transceiver Module Using Dispersive Multiwall Carbon Nanotubes

A novel polyimide film, consisting of finely dispersed multiwall carbon nanotubes (MWCNTs) in an ionic liquid (IL), is demonstrated to be high shielding effectiveness (SE) for use in packaging a 2.5-Gbps plastic transceiver module. The IL-dispersed MWCNT composite exhibits a high SE of 40 ~ 46 dB. By comparison, the composite fabricated by nondispersive process requires a higher loading of MWCNTs at 50 wt- % than the IL-dispersed process at only 30 wt-%. The electromagnetic susceptibility (EMS) performance is experimentally evaluated by the eye diagram and bit-error-rate for a 2.5-Gbps lightwave transmission system. The package housing fabricated by the dispersive MWCNT composites shows an enhanced EMS performance, an improved mask margin, and a lower-power penalty. These results indicate that the IL-dispersed MWCNT composites are suitable for packaging low-cost and high-performance optical transceiver modules used in the fiber to the home lightwave transmission systems.

Performance of Graphene Mediated Saturable Absorber on Stable Mode-Locked Fiber Lasers Employing Different Nano-Dispersants

High-performance stable mode-locked fiber lasers (MLFLs) employing nano-composite polymer-graphene saturable absorber (SA) are demonstrated. The graphene layers were dispersed by two different dispersants including fluorinated MICA clay (MICA) and poly(oxyethylene)-segmented imide (POEM). Using the SA made by graphene dispersed in MICA with thickness and concentration product (TCP) of 36 (μ m*wt%), the MLFLs exhibited pulsewidth, 3-dB spectral bandwidth, and modulation depth (MD) of 393±14 fs, 6.6 nm, and 2.57%, respectively. By contrast, the graphene dispersed in POEM provides a TCP of 38 (μm*wt%) to make the MLFLs deliver pulsewidth, 3-dB spectral bandwidth, and MD of 442±32 fs, 5.9 nm, and 1.70%, respectively. In comparison, the graphene SA dispersed by MICA performs a better MLFL pulse quality and uniformity than that dispersed by POEM. Both MLFLs with SAs fabricated by graphene/MICA and graphene/POEM reveal shortened pulsewidth and enhanced MD as the TCP increases. This observation indicates that the TCP and MD serve as key parameters for characterizing the MLFL pulsewidth. In this study, the nano-MICA dispersant is used for the first time to fabricate the graphene-polymer based SA film and has demonstrated highly stable and uniform ML pulse laser output. Judicious selection of dispersants for dispersing graphene in a homogeneous state is essential for enhancing the MLFL performance. This stable and uniform mode-locked pulse formation by employing the novel graphene-mediated SA has proven the high performance MLFLs that potentially applicable for a myriad of low-cost nano-devices.

Investigation of Saturable and Reverse Saturable Absorptions for Graphene by Z-Scan Technique

The optical nonlinear absorption (NLA) property of multilayer graphene was investigated by Z-scan technique with picosecond laser pulse of 532-nm wavelength. Graphene samples were fabricated by chemical vapor deposition (CVD) with different number of layers, which were 1-, 8-, and 16-layer. The dependence of NLA coefficient α(I) on number of layers was investigated under the laser intensity of 7-80 GW/cm2. The results showed that α(I) increased as the number of layer increased, and decreased as the laser intensity increased. Nonlinearities of graphene were mainly caused by saturable absorption (SA) effect. The 1-, 8-, and 16-layer graphene exhibited SA nonlinearity. However, reversed SA (RSA) nonlinearity was observed for 16-layer graphene only. The RSA was resulted from two-photon absorption due to layer stacking-induced bandgap opening, confirmed by the α(I) measurement. The RSA may reduce the stability of mode-locking and therefore, it suggests that the lower layer stacking in CVD fabricated graphene is preferable to use as saturable absorbers in laser mode-locking.

Electromagnetic Shielding Performance for a 2.5 Gb/s Plastic Transceiver Module Using Dispersive Multiwall Carbon Nanotubes

Electromagnetic (EM) shielding performance for a plastic composite by employing polyimide-based dispersive multiwall carbon nanotubes (MWCNTs) is presented. A well-dispersed MWCNT composite offers a higher electrical conductivity with a lower weight percentage of MWCNTs. The dispersive MWCNT composites with their high shielding effectiveness (SE) are suitable for packaging low-cost optical transceiver modules used in fiber to the home (FTTH) lightwave transmission systems.

Few-Layer Graphene-Based Saturable Absorbers Employing Mica Dispersant for Fiber Lasers

Few-layer graphene-based saturable absorbers (SAs) employing the novel fluorinated mica clay (mica) dispersant for stable mode-locked fiber lasers (MLFLs) are developed. Optical linear absorbance spectrum of graphene-in-polymer film is observed without absorption peak from 400 to 2000 nm. Optical nonlinearities of 1, 2, and 3 wt% graphene-based SAs are initially screened and optimized to be 3 wt% as the highest modulation depth value of 2.46%. By utilizing the mica dispersant for graphene, the developed SA exhibits a stable MLFL showing a pulse width of 382 fs, a bandwidth of 6.80 nm, and a time-bandwidth product of 0.322 at fundamental soliton-like operation. The use of plate-like mica to uniformly disperse graphene-based SAs is crucial for producing stable, ultrafast, and high-performance MLFLs.

Optimization of stacking graphene layers as a saturable absorber for mode-locked lasers

The optimization of stacking graphene layers as a saturable absorber (SA) for achieving both wider dynamic range and shorter pulsewidth in mode-locked lasers (MLLs) is demonstrated. This study clearly shows an important conclusion that the lower stacking graphene layers, such as the 5-layer is preferable to use as SA for wider dynamic range and hence better pulse tunability range of 5.1-μW, whereas the higher stacking graphene layers, such as the 17-layer is preferable to use as a SA for providing stable and shorter pulsewidth of 523-fs.