Recent Research

High nonlinearity extruded single-mode holey optical fibers The development of high effective nonlin- earity fibers represents a promising new application for holey fibers, which offer a combination of wavelength-scale features and design flexibility. Holey fibers have typically been made by stacking capillaries to form a microstructured cladding re- gion. The Southampton team reported for the first time that extrusion can be used to produce the microstructured fiber pre- form. They also demonstrated single- mode guidance and nonlinearity enhance- ment in a non-silica holey fiber. Measured effective nonlinearity was 550 W –1 km –1, which they said was more than 500 times larger than that of standard silica fiber. The researchers found that extrusion of- fered a controlled and reproducible method for fabricating complex, struc- tured preforms with good surface quality. Since fewer interfaces are used than in the stacking approach, extrusion could ulti- mately offer lower losses than existing techniques. They reported that by com- bining the tight mode confinement of an effectively air-suspended core with the high intrinsic nonlinearity of glass materi- al, a practical, compound glass optical fiber—with an extremely large nonlinear- ity per unit length of 550 W –1 km –1 at 1550 nm—was produced. T.M. Monro, K.M. Kiang, J.H. Lee, K. Frampton, Z.Yu- soff, R. Moore, J.Tucknott, D.W. Hewak, H.N. Rutt, and D.J. Richardson, University of Southampton, U.K. Active semiconductor microdisk switching devices using gain and electroabsorption effects The USC researchers previously reported an approach for fabrication of passive semiconductor microdisk devices relying on thermal wafer-to-wafer bonding tech- niques for building vertical-coupled struc- tures. Here they report on active switching semiconductor microdisk devices with gain and electroabsorption active regions. They found that the devices exhibited high-Q operation (Q > 5,000) and high fi- nesse (F ~ 40). They also demonstrated low current and voltage switches. The epi-structure of the devices con- sisted of two vertically stacked waveguides with a common cladding/coupling layer 0.8 m thick. Growth started in a buffer layer followed by two etch-stop layers and p-cladding layers with decreased doping close to the disk core layer. The researchers said the gain and electroabsorption trim- ming of transmission characteristics em- ployed here mean the devices could be useful as building blocks for future pho- tonic integrated circuits. Among the possi- ble applications they foresee are resonant detectors, modulators, laser sources, tun- able filters, switches, and routers. Kostadin Djordjev, Seung-June Choi, Sang-Jun Choi, and P.D.Dapkus,University of Southern California,Los Angeles. Insertion loss reduction by optimization of waveguide perturbations Current approaches to reducing wave- guide losses in planar lightwave circuit- based devices include minimizing the ex- tent of each perturbation and adjusting the waveguide core dimensions and core/cladding index contrast to reduce light sensitivity to the perturbations. The team took a different approach to the waveguide loss problem by discounting the assumption that the loss through a se- ries of waveguide perturbations accumu- lates in a linear combination of losses that act independently at each break. Instead, they considered several breaks in series, showing that the total loss depends not only on intrinsic loss at each break, but also on the spacing of the breaks. By opti- mizing dielectric perturbations, their method reduced insertion loss in wave- guide-based devices, including optical cross connects, by as much as 3 dB in 32 x 32 switches. They concluded that the new method could be used to improve device performance in existing optical switches and allow for larger port-count switches. Shalini Venkatesh and Marshall DePue, Agilent Labs, Palo Alto, California, and Hiroaki Okana and Hisato Uetsuka, Hitachi Cable Ltd., Japan. Fast, digitally variable differential group delay module using polarization switching Differential group delay (DGD) refers to the relative delay time between two or- thogonal polarization states. Tunable DGDs have been found to improve per- formance in a number of applications in- cluding microwave signal processing and phased array beam forming. Polarization mode dispersion (PMD) has also become a critical challenge for high data rate sys- tems. The investigators report on the first compact, programmable DGD module based on a novel polarization-switching approach. By use of a binary tuning mech- anism, the device was able to generate any DGD value from -45 ps to +45 ps in < 1 ms with a resolution of 1.36 ps (6-bit). They showed that the repeatable DGD generation capability can be used to gen- erate any DGD distribution with tunable average values for first-order PMD emulation. The researchers reported that the mod- ule exhibited negligible transient-effect- induced power penalty (< 0.2 dB) in a 10-Gbit/s non-return-to-zero transmis- sion link. Along with good static per- formance, including low insertion loss (< 11.4 dB), low polarization-dependent loss (< 0.2 dB) and small high-order PMD (< 85 ps2), the device showed negligible transient effects on system performance. For this reason, they concluded that it is a candidate for PMD compensation. Negli- gible power penalties because of fast po- 52 Optics & Photonics News  June 2002 Recent Research Summarized by George Leopold These post-deadline papers were prepared for the Optical Fiber Communication Conference (OFC), March 17-22, 2002, Anaheim, California. RECENT RESEARCH June 2002  Optics & Photonics News 53 larization switching, or jogging, were also obtained. They believe that this feature could enable applications of the DGD ele- ment in PMD emulation and compensa- tion of microwave photonic networks. L.-S.Yan,C.Yeh,G.Yang,L.Lin,Z.Chen,Y.Q.Shi, and X. Steve Yao, General Photonics Corp., Chino, California. Ultrawideband tellurite-silica fiber Raman amplifier and supercontinuum lightwave source for 124-nm seamless bandwidth DWDM Ultrawideband technology is emerging in the U.S. following recent government ap- proval of limited use of the technology. Once interference concerns are worked out, the technology could become perva- sive. One hurdle is the development of opti- cal amplifiers and optical sources capable of using the intrinsic ultrawideband trans- mission window of optical fibers. The NTT researchers report on a series of 120-nm optical elements, including hy- brid tellurite-silica fiber Raman ampli- fiers, a supercontinuum lightwave source, and an interleave filter. Their 120-nm- gain-bandwidth post and inline amplifiers consist of laser-diode-pumped, gain- flattened hybrid Raman amplifiers and a 200-nm-bandwidth supercontinuum lightwave source. The source is seen as a promising technology for generating 100- plus-channel optical carriers for wave- length-division-multiplexed photonic net- works. The researchers also reported de- tails of a 140-nm bandwidth interleave fil- ter with loss under 2.2 dB. Together, these ultrawideband optical elements produced a 124-nm seamless bandwidth transmission of 313 x 10Gbit/s signals over 160 km. The ultrawideband transmission could help simplify trans- mission lines, as well as eliminate gaps be- tween bands and excess loss of demulti- plexers multiplexers. H.Takara,H.Masuda,K.Mori,K.Kohtoku,Y.Miyamoto, T. Morioka, and S. Kawanishi, NTT Corp., Japan. Third-order cascaded Raman amplification Previous reports on cascaded Raman am- plification involved second-order pump- ing in a bidirectional pumping configura- tion to add Raman gain. The utility of this scheme had been limited to relatively short spans (~80 to 90 km). The re- searchers report on the implementation of third-order cascaded Raman amplifica- tion and a comparison of the resulting performance with both first- and second- order pumping. They also demonstrated single- and dual-wavelength third-order pumping schemes requiring a single active pump source. The amplification scheme provided a 2.5 dB improvement in optical signal-to-noise ratio (OSNR) and receiver sensitivity compared to conventional first- order distributed Raman pre-amplifica- tion. Further, third-order pumping was shown to provide a broader and flatter gain profile than conventional first-order Raman pumping. The OSNR advantage for third- versus first-order pumping was found to be a function of the on/off Ra- man gain. The researchers found that as gain de- creased, the extra OSNR improvement provided by third-order pumping gradu- ally increased. For gains of ~10 dB (typical of gain levels applied in links with re- peaters), it reached a value of ~2.8 dB. S.B. Papernyi,V.I. Karpov, and W.R.L. Clements, MPB Communications Inc., Point Claire, Qu├ębec, Canada. Wavelength agile, photonic integrated circuits using a novel, quantum-well intermixing process The quantum-well intermixing (QWI) technique developed by the investigators was reported to provide the design flexi- bility required to manufacture wave- length-agile InP-based photonic integrat- ed circuits. The QWI process also allows for the simultaneous optimization of ac- tive region design for high modal gain while providing good tuning efficiencies in QWI material. Using a simple processing technique based on QWI, they report the technique was integrated to fabricate a widely tun- able multisection laser for the first time. They also integrated a backside absorber to provide multiple active sections and multiple tuning sections. Higher modal gain was achieved using an optimized ac- tive region structure to increase the con- finement factor by 50%, while the QWI material was used for tuning via current injection. Hence, they report the first tun- ing characteristics using current injection in QWI material, providing more than a 1.5% change in the modal group index, a 37-nm tuning range and a 14-mW output power at 100 mA. The re- searchers said the QWI process is suited to general application and can be applied to a variety of optoelectronic components. Along with the design flexibility need- ed to improve sampled-grating DBR laser characteristics, they said the technique also clears the way for further integration of photonic ICs. Erik J. Skogen, Jonathon S. Barton, Steven P. DenBaars and Larry A. Coldren, University of California-Santa Barbara. Record highest sensitivity of -28.0 dBm at 10 Gbit/s achieved by a newly developed, extreme- compact, superlattice-APD module with TIA-IC Large data transmission requirements are driving both speed and capacity require- ments for emerging metropolitan area networks. Ten Gbit/s/channel dense-wave- length-division multiplexing (DWDM) is expected to be the mainstream network- ing technology to handle data rates meas- ured in Tbit/s. The photodetector modules used in these networks must be compact and highly sensitive as 10 Gbit/s transpon- ders move to smaller size and higher per- formance. With that in mind, the NEC team developed a compact and highly sen- sitive APD optical receiver module with an integrated transimpedance-amplifier (TIA) for 10 Gbit/s/channel DWDM ap- plications. The highest receiver sensitivity of -28 dBm was achieved by a combination of a superlattice APD with a large-gain-band- width product (110 GHz) and a high-gain HJFET TIA chip. More than -26.5 dBm sensitivity was also demon- strated over the wide wavelength range covering the C and L bands by use of an extremely small module measuring 9.8 x 7.5 x 4 mm. The researchers said their optical re- ceiver module offers features suitable for use in 10 Gbit/s transponders as those components of metropolitan area net- works move to smaller size and higher performance, including sufficient sensitiv- ity over wide wavelengths (1500 to 1607 nm). K. Sato,T. Hosoda,Y.Watanabe, S.Wada,Y. Iriguchi, K. Makita,A.Shono, J.Shimuzi,K.Sakamoto, I.Watanabe, K.Mitamura and M.Yamaguchi,NEC Corp.,Kanagawa, Japan.

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