B.L. Kasper

Multigigabit-per-second avalanche photodiode lightwave receivers

High-speed avalanche photodiodes and high-sensitivity receivers are vital components for future multigigabit-per-second lightwave transmission systems. We review theoretical and experimental performance of high-speed III-V avalanche photodiodes, and also that of multigigabit-per-second lightwave receivers using FET and bi-polar amplifiers. Particular attention is given to APD gain-bandwidth product, and to its effect on high-speed receiver sensitivity. Comparisons between measured receiver sensitivities and calculated performance are presented for bit rates up to 8 Gbit/s.

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. >

8-Gbit/s transmission experiment over 68 km of optical fiber using a Ti:LiNbO 3 external modulator

We report a single-channel optical fiber data link with the largest bit-rate<tex>x</tex>repeater spacing product achieved to date- ∼ 0.5 Tb . km/s. The multi-gigabit per second system is the first to incorporate a Ti:LiNbO<inf>3</inf>external modulator as the data encoder.

Observation of coherent Rayleigh noise in single-source bidirectional optical fiber systems

The authors report noise in a directional optical fiber system that they believe arises from the interference of the signal light with the Rayleigh backscattered light. This noise, called coherent Rayleigh noise (CRN), is the dominant noise source in this system. The interference provides a mechanism for translation of laser phase fluctuations into receiver photocurrent fluctuations. A number of noise reduction schemes are proposed. The authors demonstrate that CRN can be reduced, but not eliminated, by providing a small drive current modulation signal to the source laser to broaden its linewidth. Systems using this single-source-bidirectional architecture must take this noise source into account. >

Wide-band lightwave distribution system using subcarrier multiplexing

A description is given of a bidirectional lightwave distribution system based on subcarrier multiplexing. Eight independent wideband channels (180 Mb/s/channel) are transmitted from the head-end on one 1.3- mu m wavelength laser as frequency-shift-keyed (FSK) subcarriers, between 2.6 and 4.7 GHz. Each receiving node uses a parallel-resonant p-i-n FET receiver, a microwave bandpass filter and a limiter-discriminator demodulator to select and demodulate the appropriate channel. A 45-Mb/s return channel is provided from each node to the head-end on a separate subcarrier frequency. Error-free bidirectional transmission is obtained without penalties from laser nonlinearities, optical reflections or interference, or interchannel interference from closely spaced subcarrier channels. >

WDM coherent optical star network

The results obtained with a fiber-optical star network using densely-spaced wavelength division multiplexing (WDM) and heterodyne detection techniques are reported. The system consists of three lasers transmitting at optical frequencies around 234000 GHz, spaced at a frequency interval of 300 MHz. The lasers are frequency-shift-key (FSK) modulated at 45 Mb/s. A 4*4 optical star coupler combines the three optical signals. The WDM signals received from one of the four outputs of the star coupler are demultiplexed by a heterodyne receiver. The minimum received optical power needed to obtain a bit-error rate of 10/sup -9/ is -61 dBm or 113 photon/bit, which is 4.5 dB from the shot noise limit. The degradation caused by co-channel interference was measured and found to be negligible when the channels, modulated at 45 Mb/s, are spaced by more than 130 MHz in the IF domain. These results indicate that a WDM coherent optical star network of this type has a potential throughput of 4500 Gb/s. >

An APD/FET optical receiver operating at 8 Gbit/s

A high-sensitivity optical receiver has been designed for a bit rate of 8 Gbit/s and wavelengths of 1.3-1.55\mu m. The receiver uses a 60-GHz gain-bandwidth-product InGaAs/InGaAsP/InP avalanche photodiode followed by a high-impedance hybrid GaAs MESFET preamplifier. A bandwidth of 6.9 GHz was measured, with flat frequency response ±2 dB being obtained through the use of a 3-tap transversal equalizer. A sensitivity \bar{P} as high as -25.8 dBm was measured for 10-9bit-error rate.

An optical-feedback transimpedance receiver for high sensitivity and wide dynamic range at low bit rates

A novel transimpedance optical receiver using optically coupled feedback rather than a conventional feedback resistor is described. The optically coupled feedback has a number of advantages, including: (1) elimination of feedback-resistor Johnson noise for higher sensitivity; (2) elimination of feedback capacitance for higher bandwidth; and (3) the capability of large feedback current with low output voltage for wide dynamic range. A theoretical analysis is presented, along with experimental results for a long-wavelength optical-feedback receiver at a bit rate of 1.5 Mb/s. The experimental receiver uses InGaAs p-i-n photodiodes and a silicon JFET preamplifier and obtains a maximum sensitivity of -63.8 dBm for an error rate of 1*10/sup -7/. When the receiver is optimized for high level signals, an optical dynamic range of 40 dB is attained with no preamplifier automatic gain control (AGC). The results of a transmission experiment over a length of 303 km of single-mode optical fiber at a wavelength of 1.55 mu m are presented. >

High‐speed junction‐depleted Ga0.47In0.53As photoconductive detectors

A high‐speed junction‐depleted Ga0.47In0.53As photoconductive detector without making use of radiation damage to the sample is reported. The detector has a light absorbing region in the n−layer of an unbiased p‐n junction. As a result of the built‐in field associated with the p‐n junction, the carriers generated far from the surface were removed, leading to a picosecond response time. When tested by 100‐ps, 1.55‐μm light pulses, the detector showed a fall time of 90 ps with an external gain >1.3 (no antireflection coating). Receiver sensitivity at 1 Gb/s was −25.3 dBm at 1.55 μm and an error rate of 10−9.

Densely-spaced FDM coherent star network with optical signals frequency-locked to comb of equally-spaced frequencies

Results obtained using a fiber optical star network using densely-spaced frequency-division multiplexing (FDM) and heterodyne detection techniques are presented. The system consists of three optical sources frequency-shift keyed at 45 Mb/s and spaced by 300 MHz. Frequency selection of the desired channel is done by a heterodyne FM receiver at a bit-error rate of 10/sup -9/ is -61 dBm or 113 photons/b, which is 4.5 dB from the shot-noise limit. Cochannel interference is found negligible for this channel spacing and modulation rate. Frequency stabilization of the FDM signals to a comb of equally-spaced frequencies, imperative in such a densely-spaced FDM system, is also demonstrated. The results indicate that this system can provide a throughput of 4500 Gb/s.<<ETX>>