Stuart D. Walker

Strong exciton-photon coupling in an organic semiconductor microcavity

The modification and control of exciton–photon interactions in semiconductors is of both fundamental and practical interest, being of direct relevance to the design of improved light-emitting diodes, photodetectors and lasers. In a semiconductor microcavity, the confined electromagnetic field modifies the optical transitions of the material. Two distinct types of interaction are possible: weak and strong coupling. In the former perturbative regime, the spectral and spatial distribution of the emission is modified but exciton dynamics are little altered. In the latter case, however, mixing of exciton and photon states occurs leading to strongly modified dynamics. Both types of effect have been observed in planar microcavity structures in inorganic semiconductor quantum wells and bulk layers. But organic semiconductor microcavities have been studied only in the weak-coupling regime. Here we report an organic semiconductor microcavity that operates in the strong-coupling regime. We see characteristic mixing of the exciton and photon modes (anti-crossing), and a room-temperature vacuum Rabi splitting (an indicator of interaction strength) that is an order of magnitude larger than the previously reported highest values for inorganic semiconductors. Our results may lead to new structures and device concepts incorporating hybrid states of organic and inorganic excitons, and suggest that polariton lasing may be possible.

Hybrid WDM/TDM PON using the AWG FSR and featuring centralized light generation and dynamic bandwidth allocation

A novel time/space/wavelength division multiplexing (TDM/WDM) architecture using the free spectral range (FSR) periodicity of the arrayed waveguide grating (AWG) is presented. A shared tunable laser and a photoreceiver stack featuring dynamic bandwidth allocation (DBA) and remote modulation are used for transmission and reception. Transmission tests show correct operation at 2.5 Gb/s to a 30-km reach, and network performance calculations using queue modeling demonstrate that a high-bandwidth-demanding application could be deployed on this network.

Strong exciton–photon coupling in a low-Q all-metal mirror microcavity

We report the experimental observation of strong exciton–photon coupling in a planar microcavity composed of an organic semiconductor positioned between two metallic (silver) mirrors. Via transmission and reflectivity measurements, we observe a very large, room temperature Rabi splitting in excess of 300 meV. We show that the Rabi-splitting is enhanced in all-metal microcavities by a factor of more than 2 compared to an organic film positioned between a silver mirror and a dielectric mirror. This enhancement results from the significantly larger optical fields that are confined within all-metal microcavities.

4-Gbps Uncompressed Video Transmission over a 60-GHz Orbital Angular Momentum Wireless Channel

We demonstrate successful transmission of 4-Gbps uncompressed video over a 60-GHz orbital angular momentum (OAM) wireless channel. Matlab simulation was employed to support the experimental work and to generate the holographic masks used. Matlab coding is a unique approach which can produce any desired shape on copper or dielectric plates by mean of a commercial routing tool. We believe this is the first reported transmission of 4-Gbps uncompressed video over the 60-GHz OAM wireless channel. Good agreement was achieved between the simulated and measured results. Practical opportunities for multi-gigabit future wireless communications are available.

Analysis of optical crosstalk effects in multi-wavelength switched networks

Two optical crosstalk mechanisms, homo- and hetero-wavelength crosstalk, are identified in multi-wavelength switched networks. Homo-wavelength crosstalk does not arise in point-point systems and is a significant factor in multi-wavelength network design. A new signal dependent statistical model was used to evaluate the crosstalk effects due to non-ideal filtering in an established multi-wavelength transport network architecture. The authors find that the optical filters used in this network must have an adjacent channel rejection of /spl ges/20 dB in order to achieve the system design target of 622 Mbit/s channel base rate, at 10/sup /spl minus/10/ BER with 1 dB crosstalk penalty through 10 nodes.<<ETX>>

Design of arrayed-waveguide gratings using hybrid Fourier-Fresnel transform techniques

We describe how transform techniques may be applied to the design of arrayed-waveguide gratings (AWGs). A hybrid Fourier-Fresnel transform model indicates that a flat AWG router passband over 30% of the device free-spectral range (FSR) is feasible. Minimized passband ripple is achieved by applying a subparabolic phase-profile (phase-exponent P=1.82) to the Fourier plane of the AWG. Simulations of a complex-apodized AWG with passband 3 dB width increased from 1 to 12 nm, with a near-trapezoidal amplitude response, and band-edge group delay of 70 ps reduced to 10 ps are presented for a device with a FSR of 40 nm.

Toward terabit-per-second capacities over multimode fiber links using SCM/WDM techniques

This paper outlines the progress made on the use of subcarrier multiplexing (SCM) for high-speed datacommunications links over multimode fiber. Results include the demonstration of penalty-free transmission of 2.5 Gb/s data over worst case multimode fiber (MMF). A complete link demonstration of an SCM transmission system is reported, based on a quadrature phase-shift keying modulator and demodulator capable of a record 5.1 Gb/s per subcarrier data transmission. Superior performance compared with conventional baseband modulation techniques is shown. It is also demonstrated that when SCM is combined with dense wavelength-division multiplexing (WDM), aggregate data capacity of 200 Gb/s is feasible. Preliminary results demonstrate the possibility of 20-GHz WDM channel spacings, which if scaled show the potential for 1-Tb/s aggregate rates with a bandwidth-length product of 3 Tb/s/spl middot/km.

Experimental Demonstrations and Extensive Comparisons of End-to-End Real-Time Optical OFDM Transceivers With Adaptive Bit and/or Power Loading

Experimental demonstrations are reported for end-to-end real-time optical orthogonal frequency division multiplexing (OOFDM) transceivers incorporating three widely adopted adaptive loading techniques, namely, power loading (PL), bit loading (BL), and bit-and-power loading (BPL). In directly modulated distributed-feedback (DFB) laser-based, intensity-modulation, and direct-detection (IMDD) transmission systems consisting of up to 35-km single-mode fibers (SMFs), extensive experimental comparisons between these adaptive loading techniques are made in terms of maximum achievable signal bit rate, optical power budget, and digital signal processing (DSP) resource usage. It is shown that BPL is capable of supporting end-to-end real-time OOFDM transmission of 11.75 Gb/s over 25-km SMFs in the aforementioned systems at sampling speeds as low as 4 GS/s. In addition, experimental measurements also show that BPL (PL) offers the highest (lowest) signal bit rate, and their optical power budgets are similar. The observed signal bit rate difference between BPL and PL is almost independent of sampling speed and transmission distance. All the aforementioned key features agree very well with numerical simulations. On the other hand, BPL-consumed DSP resources are approximately three times higher than those required by PL. The results indicate that PL is a preferred choice for cost-effective OOFDM transceiver design.

Spectral properties of resonant-cavity, polyfluorene light-emitting diodes

We report on the fabrication and device properties of light-emitting resonant cavity conjugated-polymer diodes. The microcavity structures were constructed using a dielectric mirror, an indium–tin–oxide anode, a green light-emitting polyfluorene blend, and a metal (cathode) mirror. The best performance was obtained for a composite calcium–aluminum cathode, that combines high reflectivity with a low workfunction. This structure emitted electroluminescence with a linewidth of 12 nm and a maximum brightness of 300 cd m−2 at a bias of 35 V with an efficiency of 0.95 cd A−1.

Applications of active arrayed-waveguide gratings in dynamic WDM networking and routing

We describe how active arrayed-waveguide gratings (AWGs) may find a diverse range of applications in future dynamic wavelength division multiplexed (WDM) networking and routing. Our initial simulations indicate that these applications include dynamic signal power and erbium-doped fiber amplifier (EDFA) gain equalization with a dynamic range of 12 dB, and interchannel amplified spontaneous emission (ASE) suppression by more than 20 dB; optical add/drop multiplexing with passband-flattened channels and suppressions of 15 dB; and dynamic dispersion compensation of up to /spl plusmn/300 ps/nm.