Ketan M. Patel

Enhanced multimode fiber link performance using a spatially resolved receiver

In this letter, we demonstrate a new spatially resolved photodetection and equalization technique that compensates for differential modal delay in multimode fiber (MMF) links. We experimentally demonstrate in excess of a two-fold bandwidth increase using a simple two-segment photodetector that retains all of the advantages associated with multimode fiber including alignment tolerance. Furthermore, we characterize the MMF output in the far-field demonstrating that the temporal response depends upon spatial coordinate and allowing numerical optimization of the technique.

Spatially resolved equalization and forward error correction for multimode fiber links

Multimode fiber links suffer from intermodal dispersion, which, at high data rates, produces intersymbol interference in the received optical signal. Without signal processing or equalization, this limits the achievable data rates. We consider the combination of a spatially resolved equalizer (SRE) (in the optical domain) and forward error correction (FEC) to increase the achievable data rates and reliability on these links. Our performance results obtained with actual measured impulse responses show that SRE and FEC on a multimode fiber link enable a significant increase (about 2.5 times) in data rate and/or a reduction in bit error rate (BER) at relatively low cost. Furthermore, we show that improvements in data rate and reliability are generally not possible using FEC alone, making the SRE/FEC combination a cost-effective, attractive approach.

Spatially resolved detection and equalization of modal dispersion limited multimode fiber links

The authors present theoretical and experimental results of an optoelectronic-equalization technique that mitigates intersymbol interference caused by differential modal delay in multimode fiber. By exploiting the spatial diversity of the transverse optical-fiber modes, the authors are able to provide a sufficient additional information in the form of mode-dependent photocurrents to enhance total signal integrity. A fabricated two-segment photodetector is demonstrated with a routinely achievable two-times improvement in bandwidth-distance product. They also show the robustness of the technique to the expected variations in graded-index profiles.

160-Gb/s optically time-division multiplexed link with all-optical demultiplexing

An ultrafast single-wavelength optically time-division multiplexed (OTDM) link is described. The link exploits a unique integrated, all-optical serial-to-parallel (S/P) converter based on second-harmonic generation that demultiplexes multiple high-speed optical channels with a single operation. The link is composed of five major components: (1) a high-repetition-rate picosecond-pulse source; (2) a planar waveguide multiplexer that incorporates electroabsorption modulators with integral spot-size converters (SSCs); (3) a dispersion-managed (DM) short-pulse fiber channel; (4) a quasi-phase-matched, resonant-cavity-enhanced AlGaAs waveguide designed for surface-emitted second-harmonic generation (SESHG); and (5) a 775-nm receiver optimized for return-to-zero (RZ) operation. We describe our recent advances with resonant cavity enhancement of the all-optical demultiplexer and the first bit error rate (BER) measurements for this demultiplexing scheme.

Exploiting diversity in multimode fiber communications links via multisegment detectors and equalization

we propose diversity combination via optical multisegment detectors and electrical equalization techniques to mitigate the effects of intermodal dispersion in multimode fiber (MMF). With no equalization in MMF links, intermodal dispersion produces intersymbol interference (ISI) in the received optical signal that severely limits the achievable data transmission rates and fiber link lengths. Our quasi-simulated performance results (obtained with measured impulse responses) demonstrate that multisegment detection and equalization provide a low-cost and efficient solution to combat ISI and hence, to enhance the performance of MMF communications links.

Spectral method for the simultaneous determination of uncorrelated and correlated amplitude and timing jitter

We present a technique for simultaneous determination of the uncorrelated and correlated timing and amplitude jitter of a pulse train from its radio-frequency spectrum. First, we present a robust analysis that separately identifies both correlated and uncorrelated jitter. Second, we show that all four noise components can be uniquely determined by combining the traditional integrating calculation with a nonintegrating computation. We present simulation results that confirm the accuracy of this technique.

Reduction of power fluctuations in ultrafast optically time-division-multiplexed pulse trains by use of a nonlinear amplifying loop mirror

The development of optical-pulse sources for ultrafast optically time-division-multiplexed data links is limited by coherent interaction among neighboring pulses, which results in intensity fluctuations. We describe a source of 2.5-GHz 0.9-ps 1550-nm pulse trains with peak-to-pedestal ratio /spl ges/32 dB. When multiplexed to 160 GHz, the root-mean-square power fluctuations are 2.1%. To achieve this performance, a nonlinear amplifying loop mirror is employed to reduce the pulse width by 49%, increase the peak-to-pedestal ratio by /spl ges/14.1 dB and decrease the fluctuations by 7.8 dB. These results are shown to agree with a derived calculation.

Intelligent receivers for multimode fiber: optical and electronic equalization of differential modal delay

We demonstrate an opto-electronic differential modal delay (DMD) compensation technique called spatially resolved equalization (SRE) that exploits low intermodal coupling and the resultant transverse spatial diversity in the output. Furthermore, we combine SRE with post detection electronic signal processing techniques, including decision feedback equalization, to conceptually demonstrate a simple dynamic DMD compensating receiver.

Improved multimode link bandwidth using spatial diversity in signal reception

Summary form only given. The demands placed on LANs and the use of multimode fiber (MMF) in gigabit Ethernet has motivated development of differential mode delay (DMD) compensation. An adopted method of compensation uses a restricted mode launch, limiting the number of excited modes. This method can realize significant improvement in MMF links; however, it requires an optical transmitter optimized for single-mode fiber. Unfortunately, this precludes the use of multimode VCSELs, which can achieve the low cost of ownership required of LANs. In this paper, we propose and demonstrate spatially diverse detection, where multiple photodetectors are used to detect separate groups of modes. The weighted signals are recombined to compensate for DMD. Consequently, this method does not require strict control of the launch condition, i.e. it is compatible with VCSELs, and retains the alignment tolerance of MMF.

Multimode fiber link equalization by mode filtering via a multisegment photodetector

For short-haul optical networks, multimode fiber is ideal in regards to ease of use and cost-effectiveness. Unfortunately, modal dispersion in the fiber, can lead to severe intersymbol interference. A multisegment photodetector is used to perform spatially resolved equalization of the channel response. We demonstrate, through measured impulse response and bit error rate, robust performance to variations in channel condition.