Bernard Glance

A precompetitive consortium on wide-band all-optical networks

The technical core of a precompetitive consortium formed by AT&T, DEC, and MIT to study the technology, architecture and applications of wideband all-optical networks of local to national (or international) extent is described. A general introduction to all-optical networks is given, and some proposed applications are discussed. The architecture, technology and testbed portions of this effort are described. >

Polarization independent coherent optical receiver

This paper describes an optical heterodyne receiver for DPSK signals which can receive an optical signal having an arbitrary polarization state. This is achieved by splitting the received signal between two orthogonal polarization axes and processing the resulting two signals as in a conventional DPSK heterodyne receiver. The sum of the two demodulated signals provides a baseband signal independent of the polarization state of the received optical signal. When the receiver noise is dominated by the shot noise of the photodetectors, the receiver provides a BER of 10-9for an average number of 22 photon/bit. In comparison, a conventional optical heterodyne receiver requires under the same noise condition 20 photon/bit to achieve the same BER for a received optical signal polarized along the polarization axis of the local optical signal.

Frequency-Selective Fading Effects in Digital Mobile Radio with Diversity Combining

we analyze the effects of frequency-selective fading in a cellular mobile radio system that uses 1) phase-shift keying (PSK) with cosine rolloff pulses, and 2) space diversity with maximal-radio combining. The distorting phenomena with which we deal are multipath fading (which produces the frequency selectivity), shadow fading, and cochannel interference. The relevant quality measure is defined to be the bit error rate averaged over the multipath fading, denoted by (BER). The relevant system performance characteristic is defined to be the probability distribution for (BER), taken over the ensemble of shadow fadings and locations of the desired and interfering mobiles. To obtain numerical results, we use a combination of analysis and Monte Carlo simulation, invoke widely accepted models for the multipath and shadow fadings, and assume a cellular system with seven channel sets and centrally located base stations. The outcome is a set of performance curves that reveal the influences of various system and channel parameters. These include: the number of modulation levels (two or four), the diversity order, the shape of the multipath delay spectrum, and the standard deviation (or delay spread, τ 0 ) of the multipath delay spectrum. Practical factors accounted for in these assessments include fading- and interference-related timing recovery errors and combiner imperfections. Our results highlight the importance of the ratio \tau_{0}/T , where T is the digital symbol period. They show that the delay spectrum shape is of no importance for \tau_{0}/T \leq 0.2 , but can have a profound influence for \tau_{0}/T \geq 0.3 . We also find that using 4-PSK leads to better detection performance, in certain cases, than using 2-PSK.

Applications of the integrated waveguide grating router

Discretely tunable filter, receiver, and laser devices based on the integration of wavelength router/multiplexers with active elements are proposed. Each of these functions has been demonstrated experimentally in integrated form in InGaAsP, and will be published elsewhere. Such devices are key elements in WDM networks. >

Wavelength conversion at 10 Gb/s using a semiconductor optical amplifier

Data at 10 Gb/s has been translated from an input signal wavelength to another wavelength, either longer or shorter, using gain compression in a 1.5- mu m semiconductor optical amplifier for wavelength conversion. To achieve operation at such high bit rates, the probe (shifted) input must be intense enough to compress the gain of the amplifier significantly. This reduces the gain recovery time of the amplifier because of probe stimulated emission. A consequence of the intense probe is an extinction ratio deduction. Using moderate input powers, wavelength conversion is achieved over a 17-nm (2-THz) range, with 0.7-3-dB power penalties.<<ETX>>

Optically restorable WDM ring network using simple add/drop circuitry

An optically restorable wavelength division multiplexing (WDM) ring network carrying ATM traffic is presented. It uses a new architecture with simple add/drop circuitry allowing traffic restoration in case of a ring breakage by independent local actions at the affected nodes. Multisession video traffic at 155 Mb/s was demonstrated between stations distributed along the ring. No visible video degradation was observed during the 50 ms restoration time caused by a ring breakage.

Tunable add/drop optical filter providing arbitrary channel arrangements

A novel add/drop optical filter providing arbitrary channel arrangements between two input wavelength division multiplexing (WDM) signals and two output WDM signals is presented. This device can have useful applications in WDM optical communication systems.<<ETX>>

Densely spaced FDM coherent star network with optical signals confined to equally spaced frequencies

The results obtained with a fiber-optical star network using densely spaced frequency-division-multiplexing (FDM) and heterodyne detection techniques are discussed. The system consists of three optical sources transmitting around 1.28 mu m, frequency-shift keying (FSK) modulated at 45 Mb/s and spaced by 300 MHz. A 4*4 optical coupler combines the three optical signals. The FDM signals, received from one of the four outputs of the coupler, are demultiplexed by a heterodyne FM receiver. The minimum received optical power needed to obtain a bit error rate (BER) of 10/sup -9/ is -61 dBm or 113 photons/bit, which is 4.5 dB from the shot noise limit. Cochannel interference is negligible for the above channel spacing and modulation rate. The results indicate that such a system has a potential throughput of 4500 Gb/s. The results obtained with two frequency stabilization circuits used to confine these three FDM optical signals to a comb of equally spaced frequencies are also presented. >

Large-capacity local access network

A large-capacity local access network providing users with a dedicated two-way link to a central office is presented. The network is built along a single optical fiber ring. Connection to the fiber is obtained by combining time-division-multiplexing (TDM) and frequency-division-multiplexing (FDM) techniques and by using a novel optical channel add-drop filter. Optical signals are generated at the central office. The users have a simple device integrated with the channel dropping filter to receive and transmit data without the use of an optical source. Such a system can support 210 users transmitting at 100 Mb/s.<<ETX>>

New advances on optical components needed for FDM optical networks

Two critical components, needed for optical networks based on frequency division multiplexing (FDM) and frequency reuse technologies, are presented. One is a fast tunable laser, yielding 24 discrete frequencies regularly spaced by 40 GHz around 1.53 mu m. The laser can be switched randomly in less than 8 ns among these frequencies. Frequency routing of a 3-Gb/s bit stream in 10-ns packets between ten different destinations has been demonstrated with this laser. The second component is a broadband wavelength shifter capable of switching multigigabit data between optical frequencies in the 1.5- mu m region. The device is tunable and cascadable, provides conversion gain, and is nearly polarization insensitive. Data degradation after wavelength shifting is negligible. Such a device provides the means of reusing the limited set of frequencies given by the lasers and thus of realizing large size networks. >