Showing papers on "Optical communication published in 1990"

Dense wavelength division multiplexing networks: principles and applications

Abstract: The very broad bandwidth of low-loss optical transmission in a single-mode fiber and the recent improvements in single-frequency tunable lasers have stimulated significant advances in dense wavelength division multiplexed optical networks This technology, including wavelength-sensitive optical switching and routing elements and passive optical elements, has made it possible to consider the use of wavelength as another dimension, in addition to time and space, in network and switch design The independence of optical signals at different wavelengths makes this a natural choice for multiple-access networks, for applications which benefit from shared transmission media, and for networks in which very large throughputs are required Recent progress in multiwavelength networks are reviewed, some of the limitations which affect the performance of such networks are discussed, and examples of several network and switch proposals based on these ideas are presented Discussed also are critical technologies that are essential to progress in this field >

Coherent ultrashort light pulse code-division multiple access communication systems

Abstract: A new technique for encoding and decoding of coherent ultrashort light pulses is analyzed. In particular, the temporal and statistical behavior of pseudonoise bursts generated by spectral phase coding of ultrashort optical pulses is discussed. the analysis is motivated by recent experiments that demonstrate high-resolution spectral phase coding of picosecond and femtosecond pulses and suggest the possibility of ultrahigh speed code-division multiple-access (CDMA) communications using this technique. The evolution of coherent ultrashort pulses into low intensity pseudonoise bursts as a function of the degree of phase coding is traced. The results are utilized to analyze the performance of a proposed CDMA optical communications system based upon encoding and decoding of ultrashort light pulses. The bit error rate (BER) is derived as a function of data rate, number of users, and receiver threshold, and the performance characteristics are discussed for a variety of system parameters. It is found that performance improves greatly with increasing code length. >

Semiconductor lasers for coherent optical fiber communications

Abstract: The current status of semiconductor lasers used in coherent optical fiber communications is reviewed for nonexperts in the field. The issues of spectral purity, tuning, modulation, and advanced fabrication methods for photonic integration are discussed, with examples drawn from current experimental devices. >

Performance of coherent optical receivers

Abstract: Coherent optical communications, an area of research that shows great promise for future high-bandwidth and long-haul applications, is reviewed. Coherent optical receivers, which add light to the received signal as part of the detection process, have numerous advantages over direct-detection receivers, most notably increased sensitivity and increased selectivity, at the cost of increased complexity. The performance of coherent optical receivers under shot-noise-limited conditions is reviewed for a variety of modulation and demodulation formats. In addition, laser phase noise is discussed, and its effect on receiver performance is analyzed. >

Dual parallel modulation schemes for low-distortion analog optical transmission

Abstract: The use of two integrated-optical modulators operated in parallel to attain low-distortion analog optical transmission is examined. This static feedforward approach achieves significantly improved linearity at the expense of small increases in the required optical power and moderate increases in the required drive voltage. A novel variation that uses the unique characteristics of an integrated-optical modulator to correct the dominant quadratic distortion of a directly modulated laser is proposed. >

Optical fiber link card

Abstract: An optical fiber link card communication module, and process for fabricating the module, where the module provides a parallel electrical interface to the user, facilitates high speed serial transmission of data over an optical data link, and contains a plurality of converters for performing conversions between both electrical and optical signals. A preferred embodiment of the invention contemplates fabricating the optical communication module on a single multilayer card with all the transmitter electrical components being located on one side of the card, all receiver electrical components being located on the other side of the card, and the transmitter and receiver components being separated by shielding layers in the card. By using two transmitter/receiver pairs (with the transmitters and receivers being located on respective sides of the card) an embodiment of the invention provides for double full duplex communications.

The effect of four-wave mixing in fibers on optical frequency-division multiplexed systems

Abstract: The limiting effects of four-wave mixing on optical frequency-division multiplexing (OFDM) systems are described. The optical nonlinearity in a single-mode fiber imposes a fundamental limitation on the capacity of optical frequency-division multiplexed systems. In particular, four-wave mixing (FWM) crosstalk may severely degrade the system performance when the fiber input powers are large and/or the channel spacing is too small. Theoretical and experimental results of the effects of FWM in OFDM systems are presented. The theoretical results demonstrate the dependence of FWM on various system parameters. An analysis of FWM in both undirectional and bidirectional transmission systems is included. The receiver sensitivity degradation from FWM crosstalk is measured in a 16-channel coherent system. >

A 100-channel optical FDM transmission/distribution at 622 Mb/s over 50 km

Abstract: A 100-channel optical frequency-division multiplexing (FDM) transmission/distribution experiment at 622 Mb/s is demonstrated for a fiber length of 50 km. The feasibility of a polarization-insensitive waveguide frequency selection switch for 10-GHz intervals and a frequency-shift-keying (FSK) direct-detection scheme employing a Mach-Zehnder filter is verified. The demodulation circuit employs a Mach-Zehnder filter and a balanced receiver, which utilizes optical power more efficiently than the Fabry-Perot filter. No receiver sensitivity degradation is observed due to interchannel crosstalk of the 128-channel tunable waveguide frequency selection switch (FS-SW) or fiber four-wave mixing for transmissions over a 50-km-long nondispersion-shifted (NDS) fiber and a 26-km-long dispersion-shifted (DS) fiber. >

Optical communication system for the subscriber area

Abstract: A group of subscribers serviced from a center has a common remote distribution unit located in its vicinity. The subscribers are connected to this remote distribution unit by individual lines, preferably optical waveguides. A single optical waveguide connects the center to the remote distribution unit. Subscriber-assigned signals with subscriber-assigned wavelengths and signals to be distributed to all subscribers in the direction from the center to the subscribers are transmitted as a multiplexed optical signal, using wavelength-division multiplexing, to the remote distribution unit. The optical signal containing the signals to be distributed is separated out at the remote distribution unit and is distributed by optical means to the subscribers. The subscriber-assigned signals are also separated in the remote distribution unit into individual signals and are transmitted to the subscribers over individual lines. In the opposite direction from the subscribers to the center, the signals to be transmitted from the subscribers to the center are also transmitted over the individual lines with subscriber-assigned wavelength to the remote distribution unit where they are grouped into an optical multiplexed signal and are transmitted to the center over an optical waveguide. A suitable selection of the wavelengths makes possible a simple multiplexed formation and multiplexed resolution of the various signals.

Optical amplifier and optical communication system provided with the optical amplifier

Abstract: An optical communication system transmits information with a pumping light beam acting as the carrier, and with a signal light beam. This optical communication system includes an optical fiber amplifier adapted to amplify the signal light beam by having the signal light beam and the pumping light beam propagate through a rare-earth-doped fiber doped with a rare-earth element. A modulating circuit modulates the pumping light beam with a high-frequency modulating signal having a period shorter than the life span of fluorescence resulting from an excited state. When the signal light beam and the pumping light beam are propagated in the same direction through the rare-earth-doped fiber, the transmission of information with the pumping light beam acting as the carrier, can provide transmission of a supervisory signal for an optical repeater. When the signal light beam and the pumping light beam are propagated in opposite directions through the rare-earth-doped fiber, a two-way transmission can be achieved by the transmission of information with the signal light beam and the transmission of information with the pumping light beam.

Amplifier induced crosstalk in multichannel optical networks

Abstract: The effect of crosstalk introduced due to gain saturation in an optical amplifier when it is used for amplifying multiple channels in a wavelength division multiplexed (WDM) network employing ON-OFF keying with direct detection is studied. The system power penalty is quantified as a function of the amplifier input power, the number of channels, and the extinction ratio. >

Chromatic dispersion compensation in coherent optical communications

Abstract: Chromatic dispersion compensation techniques in coherent transmission systems are reviewed and discussed for potential feasibility. The key compensation device is the wideband delay equalizer. It is shown that stripline-type delay equalizers have the potential for compensating distortion up to 10 Gb/s using a conventional 1.3- mu m zero-dispersion single-mode fiber at 1.5 mu m. Chromatic dispersion is successfully compensated with a stripline delay for CPFSK transmission at 4 and 6 Gb/s over 200 km of 1.3- mu m zero-dispersion single-mode fiber at 1.55 mu m. The bandwidth requirement of the compensation techniques for heterodyne detection is more than 10 GHz. However, it is difficult to realize such broadband receivers. Therefore, phase diversity detection with dispersion compensation is a promising scheme. >

Multilevel coherent optical system based on Stokes parameters modulation

Abstract: A multilevel transmission system that allows efficient exploitation of a single-channel bandwidth in coherent optical communication systems is proposed. It is based on Stokes parameter modulation and a decision on the Poincare sphere. The block scheme of the transmitter, which is able to provide a generic output state of polarization (SOP) starting from a linearly polarized optical field, is shown and described. The propagation along the fiber is described in order to derive the expression of the optical field at the receiver input. The spectral characteristics of the transmitted and the received optical signals are analyzed. >

Multichannel wavelength-switched transmitters and receivers-new concepts for broadband networks and distributed switching systems

Abstract: Three component concepts for use in high-density wavelength-division-multiplexed optical systems are described. The applications envisaged for these components include broadband overlay for subscriber access networks and ultrahigh-capacity packet switching for telecommunications or parallel processing computer applications. The components each use a dispersive optical grating and integrated array of optoelectronic transducers to form a component with precisely defined multiple operating wavelength channels. Multichannel laser transmitter and direct detection receiver components suitable for up to about 50 independent channels separated by 1-2 nm in the 1300- to 1600-nm-wavelength range are described. >

FSK with direct detection in optical multiple-access FDM networks

Abstract: High-performance optical data networks of the future may interconnect hundreds of users at gigabit rates for a reasonable cost. A novel approach is described that utilizes a passive star coupler for distribution, a multiplicity of frequency-shift keyed (FSK) tunable lasers to provide a frequency division multiplexed (FDM) network, and a tunable optical fiber Fabry-Perot filter to select the channels and convert the FSK signal to ASK (amplitude-shift keying) for direct detection. The design of high-speed FDM-FSK direct detection networks and the performance of critical devices, such as tunable FM lasers, tunable filters, integrated star couplers, and optical amplifiers, are reviewed and discussed. >

Novel optical frequency comb synthesis using optical feedback

Abstract: An optical frequency synthesiser configured as a loop comprising a frequency translator and an amplifier, is proposed and demonstrated for the first time. Generated comb spectra show that the number of teeth observed increases dramatically as the loop losses approach zero.

Optical equalization to combat the effects of laser chirp and fiber dispersion

Abstract: Optical equalizers capable of combating the effects of laser chirp and fiber chromatic dispersion on high-speed long-haul fiber-optic communication links at 1.5 mu m are described. They consist of both reflective and transmissive cavity structures. The equalizers are adaptive in the sense that the position of their periodic frequency responses are optimally adjusted in real time. The equalizers are evaluated by using computer simulation routines based on the rate equations for intensity-modulated lasers. It is shown that, by using equalizers of the type proposed, the receiver data eyes remain open well beyond where they would close without equalization. >

High speed low loss optical switch for optical communication systems

Abstract: A Fabry-Perot etalon is utilized as an optical switch for controllably switching an input optical signal on an input optical conduit between first and second output optical conduits. The etalon is comprised of a cavity formed by plane parallel periodic multilayer reflective surfaces with a variable index of refraction semiconductor medium sandwiched therebetween. The medium and periodic multilayer structures comprise, for example, Aluminum Gallium Arsenide. An optical pump injects an optical control beam into the medium to vary the index of refraction of the medium so as to cause the input optical signal to either be reflected from the cavity into the first output conduit or transmitted through the cavity to the second output conduit. Alternatively, electric fields, thermal fields, or injected carriers can be applied to the medium to vary the index of refraction thereof, thereby effecting the optical switching action.

Optical frequency division using an optical parametric oscillator.

Abstract: A novel method of frequency division based on optical parametric oscillation is proposed. This scheme converts with high efficiency an input signal into two intense, coherent subharmonic outputs whose frequencies are tunable and whose linewidths are essentially limited by the input pump linewidth. By locking their difference frequency to a microwave, a millimeter-wave, or an infrared reference source, the output frequencies are precisely determined. The proposed frequency dividers can be operated in series or in parallel to measure, compare, and synthesize frequencies from optical to microwave. A line-narrowing effect for the generation of ultrastable radiation is discussed.

Spectral broadening due to fibre amplifier phase noise

Abstract: Erbium-doped fibre amplifiers (EDFA) operating in the 1.5µm wavelength region will play an important role in future optical communication systems. Such devices offer large, polarisation-independent gain, low noise and high efficiency. Advanced coherent communication systems employing optical amplifiers in combination with phase or frequency modulation will allow for greatly increased system capacity and are therefore of considerable interest. However, coherent systems may suffer from spectral broadening due to phase noise introduced by the optical amplifiers and this may limit the number of amplifiers which can be concatenated in an optical link. To quantify the effect, we have measured for the first time spectral broadening due to an Er3+-doped fibre amplifier operating at 1.535µm, and have found the amount of broadening to be less than 20kHz for an amplifier operating with 17dB gain.

Receiver for coherent optical communication

Abstract: A receiver for coherent optical communication in a phase diversity system. An optical hybrid circuit receives a signal beam and receives a local-oscillator beam which has a predetermined phase shift. The optical hybrid circuit branches each beam into pairs, each pair having one signal beam and one local-oscillator beam. These beam pairs are output to first and second photodetectors which each apply a photoelectric conversion, thus producing first and second output signals. The output signals are output to an electrical 90° hybrid circuit which shifts the first signal by 90° and adds it to the second signal to produce a first output signal, and shifts the second signal by 90° and adds it to the first signal to produce a second output signal. The first and second output signals are applied to first and second demodulators, respectively, which delay the signals by a delay corresponding to their frequency. The demodulators output the delayed signals to an adder which adds the two signals to produce an output signal. As a result, the receiver is capable of compensating the wavelength dispension is an optical fiber to secure an enhanced receiving sensitivity.

Simultaneous bidirectional optical interconnect

Abstract: An optical interconnect system facilitates the simultaneous bi-directional transfer of data between facing integrated circuits. The laser simultaneously emits two oppositely directed continuous beams of light. Holographic elements placed in the paths of the beams of light split each into multiple light beams, creating independent optical communication channels. Reflective modulators are positioned on integrated circuits in the path of each communication channel to selectively reflect the respective light beams. The light beams are modulated in response to data input to the reflective modulators from the integrated circuits. Opto-electronic receivers are positioned on the integrated circuits in the path of the light beams reflected by the reflective modulators. Holographic elements focus each reflected light beams onto an opto-electronic receiver. Data is extracted from the light beams by the opto-electronic receivers and output to the integrated circuits. Each integrated circuit can thus simultaneously transmit and receive data.

An optical bit-rate flexible transmission system with 5-Tb/s-km capacity employing multiple in-line erbium-doped fiber amplifiers

Abstract: An optical bit-rate flexible transmission system using erbium-doped fiber amplifiers (EDFAs) is proposed, and the system design is discussed. An optical bit-rate flexible system using multiple in-line erbium-doped fiber amplifiers has produced a regenerative repeater spacing of 505 km at 10 Gb/s and 523 km at 5 Gb/s for direct-detection systems. This system proves that an optical bit-rate flexible system with a transmission capacity of 5.05 Tb/s-km can be feasibly constructed. System capacity is clarified both theoretically and experimentally. The power penalties involved are discussed. The related optical and electrical circuits proved operational above 10 Gb/s. The 523 km at 5 Gb/s and 505 km at 10 Gb/s transmission experiments successfully demonstrated that EDFAs effectively enhance a system's transmission capacity. >

Lower bound for the communication volume required for an optically interconnected array of points

Abstract: The information-carrying capacity of optical fields is usually stated in terms of an area density as being related to communication through a surface. We render these well-understood results in a form such that they can be interpreted as a volume-density limit, applicable to an arbitrary array of points communicating with one another. An important example of such a situation is an optically interconnected computing system. We show that regardless of their actual spread or mutual overlap, optical communication links may be viewed as solid wires of minimum cross section λ2/2π for the purpose of calculating bounds on volumes and cross sections. Thus the results of area–volume complexity theory for solid wires are also applicable to optically communicating systems. The maximum number of binary pulses that may be in transit in an optical communication network occupying volume V is found to be ρ2πV/λ3, ρ denoting the modulation bandwidth normalized by the carrier frequency. Previously suggested optical-interconnection schemes are discussed in this context.

Fiber optics communication modules

Abstract: The present invention provides modules for interfacing optical fibers with very low light loss and with provision for monitoring of the optical signal. The modules according to the present invention are characterized by the precise tolerances required in high capacity optical communication systems and yet may be mass produced at reasonable costs. A device according to the present invention comprises an imaging element having a curved reflective surface at one end and prealigned fiber insertion holes at the other end. Various embodiments of the invention are disclosed.

Feasibility study on QPSK optical-heterodyne detection system

Abstract: The feasibility of a QPSK (quadrature-phase-shift-keying) optical heterodyne detection system for a high-speed and long-span coherent transmission is investigated. The key factors, which are the modulation method and the spectral linewidth requirement for optical light sources, are discussed. Through theoretical analysis and the simulation experiment, it is confirmed that the required beat spectral linewidth for QPSK coherent detection is less than 7.5*10/sup -5/*bit rate. It is also shown that the required spectral linewidth for QPSK differentially coherent detection is less than 2.5*10/sup -5/*bit rate. These results show that the gigabits-per-second range QPSK optical heterodyne detection can be realized with conventional external cavity laser diodes. >

904 km, 1.2 Gbit/s non-regenerative optical fibre transmission experiment using 12 Er-doped fibre amplifiers

Abstract: A non-regenerative optical fibre transmission experiment over 904 km is reported. With a total net optical gain of 203 dB obtained from 12 cascaded Er-doped fibre amplifiers, 1.2 Gbit/s directly intensity modulated optical signals were transmitted without any optical bandpass filters and without an error floor.

A coherent photonic wavelength-division switching system for broad-band networks

Abstract: A coherent photonic wavelength-division (WD) switching system, utilizing a coherent wavelength switch ( lambda switch), is proposed. In the proposed coherent lambda switch, the tunable wavelength filter function is accomplished using coherent optical detection with a wavelength tunable local oscillator. The coherent photonic WD switching system has the following features; (1) low crosstalk switching for dense WDM signal, and (2) large line capacity capability. Design considerations show that 32 wavelength division channels can be available with a coherent lambda switch. It is also shown that a broadband metropolitan-area-network with over 1000 line capacity is possible, using a multistage connection in the coherent lambda switches. The switching function of the coherent lambda switch is demonstrated in a two-channel wavelength-synchronized switching experiment, using 8-GHz-spaced, 280-Mb/s optical FSK signals. >

Optical communications systems

Abstract: A system for distributing interactive HDTV signals includes a head-end station (1) and a number of customer stations (3) The head-end station (1) includes at least one optical source A bi-directional transmissive broadband passive branched optical network (2) distributes HDTV signals form the head-end station (1) to the customer stations (3) Each customer station (3) includes an optical coupler (6) arranged to couple the subscriber station (3) to a respective arm of the optical network (2), a coherent receiver including a local oscillator laser (7) and a transmitter arranged to modulate light from the local oscillator laser (7) with return data for transmission via the coupler (6) onto the network The head-end station (1) also includes a broadband optical receiver arranged to receive and demodulate the return data received via the optical network (2)