Wood-Hi Cheng

Chromaticity tailorable glass-based phosphor-converted white light-emitting diodes with high color rendering index

In this paper, Lu3Al5O12:Ce3+ and CaAlSiN3: Eu2+ co-doped glass are presented as color conversion materials for white light-emitting diodes (WLEDs). Through adjusting the thickness of the glass phosphors, the chromaticity and CCT of the WLEDs follows the Planckian locus well. The WLEDs show CCT ranging from ~4000K to ~7000K with high CRI ranging from 83 to 90 due to the wide emission spectrum from the proposed glass phosphors. The glass phosphors provide an effective way to achieve chromaticity-tailorable WLEDs with high color quality for indoor lighting applications.

A New Scheme of Oriented Hyperboloid Microlens for Passive Alignment Lasers to Polarization Maintaining Fibers

A new scheme of oriented-hyperboloid microlens (OHM) with passive alignment to achieve high polarized extinction ratio (PER) is proposed and demonstrated high efficiency coupling of high-power 980 nm pump lasers to polarization maintaining fibers (PMFs). Using an automatic grinding machine and a charge-coupled-device to precisely control and attain the required minor radius of curvature 4-4.8 μm, offset within 0.7 μm, and axis orientation accuracy ±1°, the OHM exhibited a high-average PER of 31.7 dB, and a high-average coupling efficiency of 83.4%. For a 30 dB PER, the angular misalignment tolerance of the OHM was measured to be ±2°. The unique advantage of the proposed OHM is passive alignment to achieve high PER by only aligning the OHM endface of the fast axis parallel to the axis of laser polarization. This newly developed OHM with unique passive alignment to achieve high polarization is beneficial for the applications of laser/PMF modules where mode polarization and high coupling efficiency are required for use in high precision fiber optic gyroscopes as well as many low-cost and high-performance lightwave interconnection applications.

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.

Comparison of single-/few-/multi-mode 850 nm VCSELs for optical OFDM transmission

For high-speed optical OFDM transmission applications, a comprehensive comparison of the homemade multi-/few-/single-transverse mode (MM/FM/SM) vertical cavity surface emitting laser (VCSEL) chips is performed. With microwave probe, the direct encoding of pre-leveled 16-QAM OFDM data and transmission over 100-m-long OM4 multi-mode-fiber (MMF) are demonstrated for intra-datacenter applications. The MM VCSEL chip with the largest emission aperture of 11 μm reveals the highest differential quantum efficiency which provides the highest optical power of 8.67 mW but exhibits the lowest encodable bandwidth of 21 GHz. In contrast, the SM VCSEL chip fabricated with the smallest emission aperture of only 3 μm provides the highest 3-dB encoding bandwidth up to 23 GHz at a cost of slight heat accumulation. After optimization, with the trade-off set between the receiving signal-to-noise ratio (SNR) and bandwidth, the FM VCSEL chip guarantees the highest optical OFDM transmission bit rate of 96 Gbit/s under back-to-back case with its strongest throughput. Among three VCSEL chips, the SM VCSEL chip with nearly modal-dispersion free feature is treated as the best candidate for carrying the pre-leveled 16-QAM OFDM data over 100-m OM4-MMF with same material structure but exhibits different oxide-layer confined gain cross-sections with one another at 80-Gbit/s with the smallest receiving power penalty of 1.77 dB.

Higher Gain of Single-Mode Cr-Doped Fibers Employing Optimized Molten-Zone Growth

Higher gain of single-mode Cr-doped crystalline core fibers (SMCDCCFs) with longer fiber length employing an optimized molten-zone in laser-heated pedestal growth (LHPG) technique is demonstrated. The optimized molten zone in the LHPG is determined by an empirical formula. The SMCDCCFs exhibit a length of 24-cm, a core diameter of 25-μm, and a V-value of 2.40 which confirms the LP01 single-mode operation. A 6.44-dB gross gain and a 4.44-dB net gain with 24-cm fiber length of the SMCDCCF at wavelength of 1.4-μm were obtained. Further studies on higher gain of more than 10-dB gross gain of the SMCDCCFs including a longer length of 30-cm, a lower loss fiber, and a suitable annealing technique are essential to develop the SMCDCCFs as broadband fiber amplifiers for practical use in the next-generation fiber transmission systems and are currently under investigation.

Micro-hyperboloid lensed optical fibers for laser chip coupling

This study develops a novel approach for producing hyperboloid microlens structure directly on a single-mode optical fiber for high performance diode laser coupling. The hyperboloid shape lensed tip is for matching with the rectangular output of the semiconductor laser diode. The hyperboloid lensed fiber is produced a three-step process including a precision mechanical grinding, a spin-on-glass (SOG) coating and an electrostatic pulling process. A flat-end single mode fiber with the core diameter of 6.6 μm is aligned, fixed and grinded into trapezoid shape. Trace amount of spin-on-glass is applied on the grinded tip and then an electrostatic pulling is used to tune the radius of curvature of around 4.5 μm for the grinded tip. A high coupling efficiency around 80% is obtained while using the produced hyperboloid fibers, which is about double compared to the coupling efficiency of the flat end fiber. The measured coupling stability for 5 individual hyperboloid fibers is 0.116±0.044%, indicating the good coupling stability for the produced hyperboloid microlensed fibers. The developed hyperboloid microlensed fibers provides a solution for direct light coupling between the single mold fiber and the semiconductor diode laser.

Gain Enhancement of Single-Mode Cr-Doped Core Fibers by Online Growth System

Gain enhancement of single-mode Cr-doped crystalline core fiber (SMCDCCF) with longer fiber length fabricated by online laser-heated pedestal growth (LHPG) technique is demonstrated. In comparison with the SMCDCCF fabricated without online growth, the online technique enables real-time monitoring and controlling small molten zone in the LHPG to achieve longer length with better uniformity and smaller core diameter of the SMCDCCFs. The SMCDCCF exhibits a length of 10.6 cm, a core diameter of 25 μm, and a V-value of 2.40, which confirms the LP01 single-mode operation by the far-field pattern measurement. A 3.9-dB gross gain and a 1.9-dB net gain of the SMCDCCF at the wavelength of 1400 nm were obtained. The gross and net gains are the highest yet reported of the SMCDCCFs. Further development on higher gain of the SMCDCCF may be functioned the SMCDCCF as a broadband fiber amplifier for use in the next-generation fiber transmission systems.

An Oriented-Dependence-Microlens Visual Alignment and Packaging for Lasers Coupling to PMFs

An oriented-dependence-microlens (ODM) visual alignment and packaging for lasers coupling to polarization-maintaining-fibers (PMFs) is proposed and demonstrated. The ODM is fabricated by automatic grinding and accurate positioning techniques. The position accuracy of the grinding machine is ±1°, which is less than the aligned and fabricated angular tolerance of ±2° for the ODM necessary to achieve both high coupling of 80% and high polarized extinction ratio of 30 dB. This indicates that the ODM endface of the fast axis can be visually observed and then surface feature aligned to the axis of the laser polarization direction. A practical package of laser/PMF with compact size and a few components employing the ODM is also demonstrated. The proposed ODM with visually precise surface feature alignment to achieve both high coupling and polarization is beneficial for the application of laser/PMF modules for use in high-precision fiber optic gyroscopes and many high-performance and low-cost lightwave interconnections.

Mode Matching and Coupling of Lensed and Cleaved Fibers Employing Near-Field Technique

An experimental coupling mechanism is presented that demonstrates efficient coupling between a high-power 980-nm laser diode and a lensed fiber by employing mode (spot size and wavefront) matching in the near-field regime. The results show that the beam spot pattern of the lensed fiber of a horizontally elliptical shape is spot-size matched to the laser. The wavefront of the lensed fiber with a minor radius of curvature ranging from -9 to 10 μm exhibits a curve wave that is also phase matched to the laser. This indicates that the similarity in the mode patterns of both the spot size and the wavefront between lensed fiber and laser is responsible for mode-matched beam coupling, leading to a coupling efficiency of more than 80%. In contrast to the lensed fiber, the spot-size of the cleaved fiber is round, and the wavefront is a plane wave. The different mode distributions between the laser and the cleaved fiber lead to a mode mismatch, resulting in a coupling efficiency of lower than 40%. This experimental coupling mechanism investigation in the near-field regime provides useful information for practical micro-optic design with better mode matching to achieve efficient coupling from active to passive components for interconnect applications.

High Electromagnetic Shielding of Plastic Transceiver Packaging Using Dispersed Multiwall Carbon Nanotubes

A novel polymer-based multiwall carbon nanotube (MWCNT) composite with high shielding effectiveness (SE) and effective electromagnetic susceptibility (EMS) performance is proposed for use in packaging a high-speed 2.5 Gbps plastic transceiver module. Both polymer-based dispersed and non-dispersed MWCNT composites are fabricated and then the SE performances are compared. The results showed that the ionic liquid (IL)-dispersed MWCNT composites with 30% weight percentage MWCNTs exhibited high SE of 40–46 dB. By comparison, the MWCNT composites fabricated by a nondispersive process required a higher weight percentage (50%) of MWCNTs. Furthermore, the package housing developed, fabricated by IL-dispersed MWCNT composites, clearly improved EMS performance, mask margin, and power penalty for a 2.5 Gbps lightwave transmission system. This significantly improved result has marked the achievement of using the dispersive MWCNT composites for the high SE and suitability for packaging low-cost and high-performance optical transceiver modules used in the fiber-to-the-home (FTTH) lightwave transmission systems.