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Lightwave Technology, Journal of

Issue 3 • Date Mar 1990

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Displaying Results 1 - 19 of 19
  • Modulation and demodulation techniques in optical heterodyne PSK transmission systems

    Page(s): 309 - 322
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    Modulation and demodulation techniques are described for an optical PSK heterodyne transmission system operating at 560 Mb/s and 1.2 Gb/s. Performance limitations affecting the receiver sensitivity in a 1.2-Gb/s DPSK system, such as laser phase noise, phase modulation depth, IF center frequency deviation, and local laser power, are studied. High receiver sensitivities for PSK systems were achieved. The applicability of the Mach-Zehnder modulator as a phase modulator for 1.2-Gb/s DPSK is also demonstrated. A 1.2-Gb/s DPSK transmission of over 100 km, using polarization diversity with novel polarization-insensitive automatic frequency control in an attempt to overcome signal fading caused by polarization fluctuation in the transmitting fiber, is also described. A receiver sensitivity of less than -42.8 dBm and varying within 1.4 dB for all states of polarization was achieved. A multichannel high-definition TV (HDTV) transmission experiment using a DPSK polarization-diversity tunable receiver is described View full abstract»

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  • Impact of phase noise in weakly coherent systems: a new and accurate approach

    Page(s): 329 - 337
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    Coherent optical systems for future broadband local loops may use lasers with significant phase noise, manifest as broad linewidths. This phase noise can be accommodated if the receiver is correctly designed, i.e. if nonsynchronous (envelope or square-law) IF demodulation is used and sufficient IF bandwidth is provided. It is difficult to analyze the performance of a coherent optical receiver when the signals are corrupted by phase noise. The central theoretical problem arising from filtering a signal with phase noise is defined in a particular form which permits the derivation of the forward or Fokker-Planck partial differential equation for probability density of the output voltage of the receiver. The results are used to discuss the IF bandwidth required for optical heterodyne receivers for amplitude-shift-keying (ASK) signals View full abstract»

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  • ASK and FSK coherent lightwave systems: a simplified approximate analysis

    Page(s): 338 - 352
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    A simplified approximate analysis of amplitude shift keying (ASK) and frequency shift keying (FSK) coherent optical communication systems is presented. The analysis accounts for the phase noise of the transmitter and the local oscillator lasers and for the additive Gaussian noise stemming from the shot noise and thermal noise. The analysis yields a closed-form estimate of the bit error rate (BER) and allows an immediate physical insight and appreciation of the impact of the IF filter bandwidth, laser linewidth, and bit rate on the system performance. The theory also yields reasonably accurate estimates of the optimum IF bandwidths and of the sensitivity penalty stemming from the phase noise View full abstract»

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  • Performance analysis of optical heterodyne PSK receivers in the presence of phase noise and adjacent channel interference

    Page(s): 353 - 366
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    A phase-shift-keying (PSK) optical heterodyne receiver using synchronous detection by means of a Costas phase-locked loop (PLL) is investigated. Taking into account the laser phase noise and adjacent channel interference (ACI), an expression of the phase error variance is derived and error probability calculation is performed. Plots of the error probability versus the number of photons per bit are presented as a function of the optical domain channel spacing (D) and for several linewidth-to-bit-rate ratios (δf/Rb ). Relative sensitivity penalties, based on the performance with and without ACI, are evaluated for several combinations of D and δf/Rb. It is shown that, if lasers with larger linewidths are used, the frequency separation between optical carriers has to be increased in order to allow the same relative sensitivity penalty View full abstract»

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  • Penalty free biphase linecoding for pattern independent FSK coherent transmission systems

    Page(s): 323 - 328
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    Biphase coding is used to eliminate the unwanted influence of nonuniform laser characteristics in frequency-shift-keying (FSK) transmission systems. Compared to other (ternary) linecoding schemes, biphase has the advantage that it does not give a 3-dB sensitivity penalty when properly decoded in the receiver. This has been demonstrated in a 140-Mb/s 280-Mbaud transmission experiment. The receiver features a new type of reliable unambiguous clock extraction. Due to the compact IF spectrum, as compared with ternary linecoding schemes, biphase equipped FSK systems are especially suited for efficiently spaced coherent multichannel (CMC) systems. FSK heterodyne transmission experiments show a sensitivity ηP of -56.7 dBm for 210-1 pseudorandom patterns, only 4.8 dB from the shot-noise limit View full abstract»

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  • Coherent ultrashort light pulse code-division multiple access communication systems

    Page(s): 478 - 491
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    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 View full abstract»

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  • Performance analysis of polarization-insensitive phase diversity optical FSK receivers

    Page(s): 385 - 395
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    A polarization-insensitive phase-diversity optical frequency-shift-keying (FSK) receiver is proposed, and its performance is evaluated. The basic model and signal processing formulation are developed to describe the functions and features of this receiver. Two different approaches (one of which is an upper bound) are used to treat the mixed noise terms, and Gaussian approximation is employed to estimate the receiver performance in terms of the laser linewidth, the modulation index, and the filter bandwidth. The virtually polarization-insensitive property is also verified. As an example, numerical results are presented for a 150-Mb/s receiver, which show that the difference between the upper bound and the other approximation is about 2 to 3 dB. It is also shown that the receiver with small modulation index is less tolerant of the linewidth so that an error floor may appear. When the modulation index is large, the receiver can tolerate linewidths comparable with the bit rate View full abstract»

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  • Miniature Nd:YAG lasers: noise and modulation characteristics

    Page(s): 294 - 301
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    The frequency and intensity noise spectra, as well as the frequency modulation (FM) response, of 1320-nm laser-diode-pumped miniature Nd:YAG ring lasers have been measured. The frequency noise spectrum has a resonance peak at the relaxation oscillation frequency of the laser (between 123 and 150 kHz) and is flat beyond 200 kHz with a spectral density of 613 rad2-Hz, much smaller than that of semiconductor lasers; the corresponding laser linewidth is less than 49 Hz. The relative intensity noise is -140 dB/Hz at the valley and has a resonance peak at the relaxation oscillation frequency of the laser. The FM response is flat from DC to 110 kHz and is in the 0.65-3 MHz/V range; the modulation frequency is limited by the relaxation oscillation frequency of the laser View full abstract»

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  • Squeezed light for coherent communications

    Page(s): 466 - 477
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    Coherent lightwave communications systems are approaching a limit where the error rates and channel capacities are limited by the quantum properties of light. This is often referred to as the shot-noise limit. If ideal laser light is used in the system, there is no way to avoid this limit. However, new states of the light field called squeezed states have recently been developed that allow an improvement in error rates below the shot-noise limit. Squeezed light concepts and recent experiments are reviewed with emphasis on aspects important to coherent communications. It is shown that channel capacity can be improved using squeezed light by only a factor of two. Larger improvements are in principle possible for error rates, e.g. a factor of three reduction in the number of required photons per bit for a 10-9 bit error rate. An example of a recent high-performance system is described where optical losses and electronic noise reduce the improvement expected using squeezed light to the 10-20% level. It is concluded that squeezed light only offers significant improvement in bit error rates for very-high-efficiency systems View full abstract»

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  • Progress towards the field deployment of coherent optical fiber systems

    Page(s): 423 - 437
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    A description is given of the first demonstration of a coherent optical transmission system deployed in an operational network. The miniaturized external cavity laser and the automated endless polarization control scheme which made it possible to mount this demonstration are described in detail, together with specific system results. A discussion is presented of the possible field deployment of other coherent system options, based on FSK modulation of DFB and DBR lasers, and polarisation diversity reception. The considerable potential afforded by coherent techniques for meeting future network requirements is highlighted. The demonstrated advantages of greater power budget and wavelength selectivity, combined with the use of optical amplifier multiwavelength repeaters, make coherent techniques particularly relevant to the growing demand for greater transmission capacity, transparency, and network flexibility. An increasingly urgent need for robust coherent optical technology can thus be anticipated, for wideband transmission and passive routing, for both telecommunications and computer networks of the near future View full abstract»

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  • Structural design criteria for polarization insensitive semiconductor optical amplifiers

    Page(s): 302 - 308
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    A study is made of the possibility of realizing polarization independent semiconductor optical amplifiers by operating on waveguide parameters such as active- and cladding-layer thickness, stripe width, etc. The accurate design of antireflection coating has also been considered in a formulation that allows one to consider the stripe width. It is shown that in coated ridge waveguide structures, the carrier effects on the refractive index may allow equal TE and TM gain to be obtained for various values of the gain when the waveguide geometry is designed with accuracy. The performance of a polarization-independent structure is analyzed, showing that the cost of gain equalization is an increase of the excess coupling losses View full abstract»

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  • Double-stage phase-diversity optical receiver: analysis and experimental confirmation of the principle

    Page(s): 376 - 384
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    The double-stage phase-diversity (DSPD) scheme features phase-diversity-type optical homodyne detection and phase-diversity-type frequency up-conversion, producing a heterodynelike second intermediate frequency (IF) signal. It has the advantages of both heterodyning and homodyning and is applicable not only to optical, but also to all the electrical and radio communications ranging from VLF to millimeter-wave frequencies. The theoretical analysis of the receiver performance and the experimental confirmation of the principle of the DSPD scheme are described View full abstract»

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  • Multichannel coherent FSK experiments using subcarrier multiplexing techniques

    Page(s): 406 - 415
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    The results for coherent digital subcarrier multiplexed (SCM) lightwave system experiments are described. A total of 20 frequency-shift-keyed (FSK) channels at 100 Mb/s each were transmitted on one optical carrier using microwave subcarriers in a multioctave configuration. A complete description of the system performance, including carrier-to-noise ratio, crosstalk, intermodulation distortion, and receiver sensitivity, is given. With a phase modulation index of β=0.13, a receiver sensitivity of -27 dBm was obtained, representing a 14-dB improvement over a 20-channel direct detection SCM system. Crosstalk due to adjacent channels is negligible with a channel spacing of twice the data rate (200 MHz). Theoretical and experimental results are compared, and conditions for optimal performance as a function of channel number and phase modulation index are discussed View full abstract»

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  • Polarization control for coherent communications

    Page(s): 438 - 458
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    Two practical implementations of endless polarization control are described. The first approach uses polarization maintaining fiber transducer, while the second uses an integrated optic lithium niobate device. In order to characterize the lithium niobate device in detail, a technique for analyzing the birefringence as a function of the applied voltages was developed, and the measured performance of the devices is presented. Fractional-wave controllers for both static and endless control applications are considered, and design rules for maximum wavelength-window and minimum loss fiber-loop polarization adjusters are given. Endless control using various configurations of the three types of transducers is discussed, and it is shown how apparently different schemes are closely related View full abstract»

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  • Semiconductor lasers for coherent optical fiber communications

    Page(s): 274 - 293
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    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 View full abstract»

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  • A coherent optical FDM CATV distribution system

    Page(s): 396 - 405
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    A coherent optical frequency-division-multiplexing (FDM) experimental system for an optical CATV distribution service has been developed. This system employs a channel frequency spacing locked optical FDM transmitter and a random access optical heterodyne receiver. In the transmitter, ten 1.54-μm wavelength tunable distributed-Bragg-reflector laser-diode (DBR LD) modules were FSK modulated with a 400-Mb/s PN pattern. A reference pulse method is used for channel space control. Individual channel spacings for ten LDs are stabilized to 8 GHz. The random access optical heterodyne receiver is realized with a wavelength tunable local DBR LD, polarization diversity reception technique, and random access automatic frequency controller. A current address method realizes the random access function. The results of a ten-channel FDM transmission experiment carried out to evaluate these techniques are presented. It is estimated that over 80 channel high-definition TV signals can be distributed to 2000 subscribers with 500-GHz frequency tunable DBR LD. The feasibility of expanding the subscriber number to over 10000 was confirmed by an experiment with a traveling-wave optical amplifier View full abstract»

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  • Polarization control for coherent fiber-optic systems using nematic liquid crystals

    Page(s): 459 - 465
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    One of the obstacles to coherent fiber-optic communications is the unpredictable polarization drift which necessitates the use of an active polarization controlling system to match the polarizations of the signal and local oscillator. The polarization match must be maintained during the reset of any of the finite range components to prevent loss of data. A novel and practical system which uses three liquid-crystal devices for the polarization matching process is presented, and the required reset control algorithm and its derivation are described in detail View full abstract»

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  • Chromatic dispersion compensation in coherent optical communications

    Page(s): 367 - 375
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    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-μm zero-dispersion single-mode fiber at 1.5 μm. Chromatic dispersion is successfully compensated with a stripline delay for CPFSK transmission at 4 and 6 Gb/s over 200 km of 1.3-μm zero-dispersion single-mode fiber at 1.55 μm. 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 View full abstract»

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

    Page(s): 416 - 422
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    A coherent photonic wavelength-division (WD) switching system, utilizing a coherent wavelength switch (λ switch), is proposed. In the proposed coherent λ 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 λ 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 λ switches. The switching function of the coherent λ switch is demonstrated in a two-channel wavelength-synchronized switching experiment, using 8-GHz-spaced, 280-Mb/s optical FSK signals View full abstract»

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Aims & Scope

The Journal of Lightwave Technology contains articles on current research, applications and methods used in lightwave technology and fiber optics.

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Meet Our Editors

Editor-in-Chief
Peter J. Winzer
Alcatel-Lucent Bell Labs