Multichannel system design using optical preamplifiers andaccounting for the effects of phase noise, amplifier noise, and receivernoise

A rigorous and accurate model for multichannel direct detection systems where optical preamplification is used is presented. The model accounts for the influence of an optical bandlimiting filter as well as of a polarization filter. Effects of laser phase noise, of spontaneous emission noise from the amplifier, and of shot noise and thermal noise in the receiver front end are accounted for. The predicted phase noise penalty for a single channel configuration is found. For multichannel system design, it is found that a system with many optical channels and no phase noise should have a channel separation of 9.4 bit rates to have less than 1-dB worst-case crosstalk penalty. For a linewidth of 27% of the bit rate in each channel the required spacing increases to 25 bit rates or more     

法布里-珀罗腔游标式级联可调谐光滤波器在DWDM系统中的串扰分析

研究了法布里 -珀罗 (F -P)型级联可调谐光滤波器在密集波分复用系统中引入的信道间串扰对系统的影响 ,并分别讨论激光线宽、光波导损耗、信道间距对串扰的影响。对设计的器件 ,在信道间隔为 2 5GHz,激光线宽为 5GHz时 ,串扰可达 -17dB左右     

Sixteen-channel coherent broadcast network at 155 Mbit/s

An experimental 155.52 Mb/s coherent broadcast network with 16 channels using a conventional distributed feedback (DFB) laser operating at 1540 nm is described. It includes absolute laser frequency stabilization, a fiber span of 74.4 km, and a balanced polarization-diversity single-filter frequency-shift keying (FSK) heterodyne receiver. At a channel spacing of 8.5 GHz a power penalty of 0.3 dB is incurred due to adjacent channel crosstalk     

Design and performance of FSK-direct detection scheme for opticalFDM systems

The authors describe the design and performance of an optical frequency-division-multiplexing (FDM) system that uses a frequency-shift-keying (FSK) direct detection scheme. This system utilizes a Mach-Zehnder type periodic filter as a channel selective filter and optical frequency discriminator. Requirement conditions for laser diode linewidth, frequency deviation, channel frequency spacing, detuning of optical filters, and signal power variance are discussed. These characteristics are experimentally confirmed using planar waveguide type filters     

Influence of adjacent channel interference on thefrequency-modulated WDM optical communication system

The effect of adjacent channel interference on the frequency modulated wavelength division multiplexing (FM-WDM) optical communication system is analyzed in this paper. This system uses injection-locked optical discriminator at the output for the demodulation of FM channels. Adjacent channel interference results in crosstalk in the FM-WDM communication system which has been calculated. Dependence of crosstalk on the channel spacing, received signal power in a particular channel, and linewidth enhancement factor of the slave laser incorporated in the optical discriminator has been numerically investigated. The injection-locked discriminator has a typical output crosstalk level of -50 dB for a channel spacing of 100 GHz with optical injection level of -30 dB into the slave laser and is a suitable subsystem to be used in FM-WDM communication system     

M-ary FSK Performance for Coherent Optical Communication Systems Using Semiconductor Lasers

This paper describes the design and performance of anMary frequency shift keyed (FSK) signaling and demodulation scheme for an optical communication system using semiconductor lasers and heterodyne detection. Frequency or phase noise in semiconductor lasers causes spectral spreading, producing a nonzero linewidth laser signal. This degrades communication performance when compared to a system using an ideal laser with zero linewidth. We present estimates of the bit error rate (BER) performance ofM-ary frequency shift keying (FSK) with noncoherent demodulation in the presence of white Gaussian frequency noise and additive channel noise. This is typical of an optical system using semiconductor lasers and heterodyne detection. Estimates use the union-Chernoff bound with a simplified channel model to predict the effects of frequency noise. Two effects of frequency noise are identified: signal attenuation or suppression, and crosstalk. These cause an offset in the BER curve from the BER in the absence of frequency noise, and an error rate floor, respectively. The error rate floor is lower than previously predicted. When performance is not crosstalk limited,M-ary FSK is found to perform better than binary FSK with the same system bandwidth constraints, as would be predicted if ideal lasers are used. Theoretical results are compared with Monte Carlo simulations of the system.     

DFB active filter/detector for multichannel FSK optical transmission

In a DFB laser amplifier, used simultaneously as narrow band optical filter and detector in O-FDM systems the increase of the input optical power induces distortion of the filter transfer function. The effect of such a distortion on the receiver sensitivity and dynamic range has been investigated in a 155 Mbit/s FSK direct detection experiment. A two-channel experiment is also reported for the first time. Owing to the effect of interference between the two channels, it has been found that a minimum channel spacing of about 15 GHz is required to maintain the bit error rate below 10/sup -9/.<>     

Spectral efficiency of optical FDM systems impaired by phase noise

The required frequency spacings between channels in an optical frequency division multiplexing (FDM) network are considered. The minimum permissible spacings consistent with meeting bit error rate (BER) objectives are derived. The assumed transmission uses on-off keying (OOK), at a data rate 1/T (in bits per second), via external modulation of a laser source having linewidth β (in hertz). The assumed receiver consists of an optical channel selection filter followed by a p-i-n photodiode and a postdetection integrate-and-dump circuit. The analysis estimates the adjacent channel interference (ACI)-induced floor on BER for the middle of three FDM channels, as a function of frequency spacing and linewidth-to-bit rate ratio (βT). For BER=10^-9 and βT ranging from 0.32 to 5.12, the required channel spacing ranges from 5.2 to 27.5 bit rates. The multiplying factors associated with using (wide-deviation) frequency shift keying (FSK), coherent (heterodyne) detection, and infinitely many FDM channels, respectively, are estimated to be 2.0, at most 3.0, and at most 1.37     

Frequency stabilization of semiconductor lasers for applications in coherent communication systems

The stabilization of the mean frequency of a single-mode semiconductor laser used under modulation in a FSK heterodyne transmission system is achieved by a novel method. The frequency stabilization scheme locks the laser to one of the resonance frequencies of a Fabry-Perot interferometer (FPI). The laser is weakly modulated at a frequency higher than any signal frequencies. Light reflected from the FPI is detected and synchronously demodulated for feedback and current control of the laser. A laser modulated by a pseudorandom sequence with a rate of 140 Mbit/s and an optical frequency deviation of 280 MHz is stabilized with this method against variations in operating temperature and bias current. The mean optical frequency is maintained within ± 10 MHz for hours. Analysis and experiments confirm that the laser frequency remains stable even when the laser linewidth changes under weak optical feedback.     

Theoretical analysis of heterodyne optical receivers for transmission systems using (semiconductor) lasers with nonnegligible linewidth

We present a general theoretical model of receivers for coherent optical communication systems where transmitters and local oscillators having nonzero linewidth are used. Key issues in the model are the concept of single realization measurements of a stochastic intermediate frequency, and development of the probability density function for this stochastic process. Analytical results are derived for heterodyne ASK and dual filter FSK receivers and include the shot-noise limit, the asymptotic error-probability limits in ASK and FSK receivers, the influence of the IF on receiver noise, and the effective local oscillator strength. Detailed numerical results for typical p-i-n-FET wide-band receivers illustrate the influence on receiver sensitivity of IF filter bandwidth and relative threshold setting in ASK systems and of modulation index and IF filter bandwidth in FSK systems. A receiver sensitivity penalty for nonzero linewidth is found to be, for IF linewidths of 0.1 to 0.3 of the bit-rate, 3 to 9 dB in optimum ASK receivers, and 2 to 8 dB in optimum FSK receivers. Thus DFB lasers of linewidth 5 to 20 MHz could be used without external cavities in simple systems with near-ideal performance, which could find application wherever the great multiplexing advantage of coherent systems is a prime advantage. We present some guidelines for system design based on the results of this work.     

The use of optically coherent detection techniques for true-timedelay phased array and systems

We analyze a true-time-delay, optically controlled phased array antenna system whereby beam forming is accomplished using a large number of antenna elements that can receive any of several different microwave true-time-delays via the use of coherent optical carriers transmitted through a single fiber. The transmitter and receiver architectures are described in detail. We present calculations for the signal-to-noise ratio (SNR), the signal-to-interchannel interference ratio (SIR) and dynamic range (DR) of one channel for various modulation-demodulation schemes (i.e., AM-heterodyne, FM, PM). We show for an example system that can accommodate 128 antenna elements and provide 128 different delays (7 bits), assuming AM modulation with a bandwidth of 1 MHz, a transmitter power of -5 dBm and a laser linewidth of 10 MHz, we can obtain DR≃50 dB for one channel using a channel separation twelve times of that of the microwave frequency (f_m). For the FM scheme, DR≃60 dB can be obtained for one channel with a laser linewidth of 1 MHz, a FM modulation index of one, and a channel spacing of about 13f_m. With a local oscillator laser tuning range of 100 Å, the system can provide seven-bit resolution for a 128 element antenna array. It is found that coherent optically controlled phased array antennas provide improved controllability over direct detection method, and can have the SNR, SIR and DR that meet the stringent requirements of modern high resolution microwave antenna systems     

Optical 4-ary FSK modulation and demodulation experiment

Experimental results of the new 4-ary FSK modulation and demodulation systems are reported. Two binary signals are directly injected into different electrodes of a 3-electrode DFB-LD, producing an optical 4-ary FSK signal with 1 GHz frequency spacing. For demodulation, two differential detectors convert the 4-ary FSK signal into two binary signals     

FDMA-FSK star network with a tunable optical filter demultiplexer

An optical frequency-division-multiple-access (FDMA) star network is analyzed and demonstrated experimentally using two 45-Mb/s frequency-shift-keyed (FSK) laser channels at 1.5 μm. A tunable fiber Fabry-Perot (FFP) filter is used to select channels and convert FSK to intensity modulation for direct detection. The analysis predicts and experiment supports a minimum channel spacing of about six times bit rate B for a single FFP. These constraints are similar to those for more complex heterodyne demultiplexing. Estimates show that a network with 1000 users, independent of bit rate, is feasible with a tandem FFP. For B=1 Gb/s per channel the network capacity would be 1 Tb/s     

Design consideration on a DBR-laser transmitter for fastfrequency-switching in an optical FDM cross-connect system

The design of a laser transmitter for fast frequency switching in an optical frequency-division-multiplexing (FDM) cross-connect system that uses tunable laser diodes and passive frequency-channel selection filters is considered. Small-signal analysis of the laser rate equation is used to estimate laser switching time. The results show that the frequency switching time depends on the allowable frequency detuning as well as the frequency spacing and the carrier lifetime. Frequency switching time is obtained from the calculation of power penalty due to the frequency detuning at the receiver section with ASK and FSK direct detection schemes. To achieve frequency switching without bit loss in the cross-connect module, switching times of 4-200 ns are required to change frequencies of 500 GHz. This suggests that a duplex transmitter configuration using two laser diodes must be employed to perform fast frequency switching and bit-error-free operation in a cross-connect system. The thermally induced frequency drift is experimentally evaluated. The feasibility of the duplex transmitter configuration is experimentally verified with a 156 Mb/s cross-connect unit     

Theoretical analysis of phase diversity optical homodyne receiverwith FSK demodulation

A detailed analysis of optical coherent phase diversity single-filtered and dual-filtered frequency-shift keying (FSK) receivers corrupted by shot and phase noise is presented. With delay and cross-product detection, the effect of various parameters, including bandwidths of filters, delay time, frequency deviation and noises, are investigated. The tolerance of phase noise is quite large, when small time delay in the demodulator and appropriately large frequency deviation is selected. It is also shown that there exists an optimal bandwidth for the first filter in the dual-filtered FSK receiver. For a total linewidth equal to half of the bit rate, the power penalty incurred (at BER=10^-9) is 1.92 dB when the modulation index is 4, provided that an optimal filter bandwidth and a frequency deviation-delay product of 0.25 are used     

Analytical analysis of IF-penalties for different FSK and CPFSKdetection schemes

Due to the nonnegligible laser linewidth in coherent optical systems, IF bandwidths larger than the Nyquist signaling rate are required. This causes additional sensitivity penalties, which are evaluated for three types of coherent optical FSK (frequency-shift-keying) detection; two-filter-, delay-line discriminator-, and phase-diversity detection. Sensitivity as a function of the relative IF-bandwidth is given in a fully analytical form and confirmed by experimental measurements     

Performance of homodyne detection of binary PSK optical signals

This study evaluates the performance of an optical receiver for binary phase shift keyed (BPSK) signals in the presence of noise originating from the photodetectors and the phase fluctuations of the optical sources. Analysis of the homodyne detection process shows that the performance is degraded by two effects: One due to the phase error fluctuations of the recovered carrier and the other due to reduction of the energy per bit available for data recovery. The resulting power penalty can be minimized by dividing in an optimal way the received optical signal between the carrier recovery and the data recovery circuits of the receiver. The minimum penalty thus obtained depends on the 3-dB linewidth and on the transmission rate. For example, a penalty of 0.5 dB, relative to the quantum limit of 9 photon bit needed to achieve a BER of 10^-9, imposes a minimum transmission rate of about 180 Gbit/s when the optical source has a 3-dB linewidth of 20 MHz.     

Linewidth, tunability, and VHF-millimeter wave frequency synthesis of vertical-cavity GaAs quantum-well surface-emitting laser diode arrays

The authors report the measurement of the laser linewidth, wavelength tunability, and generation of microwave frequencies between individually addressable elements of a vertical-cavity GaAs quantum-well surface-emitting laser diode array (lasing in the wavelength range 850-865 nm). Using heterodyne techniques, the authors obtain a deconvolved 65 MHz laser linewidth from the 109 MHz beat signal. The laser linewidth corresponds to a semiconductor laser linewidth enhancement factor alpha =5.7, which is in excellent agreement with that obtained independently from optical gain measurements and corresponding calculated refractive index changes. The authors measured heterodyne beat frequencies of 2-20 GHz. The bandwidth was limited by the microwave amplifiers. A simple calculation shows that a tuning range of 65 MHz to 3 THz can be achieved.<>     

Brillouin fibre laser enhanced by Raman amplification

A multiple-wavelength Brillouin fibre laser is demonstrated, with channel spacings of /spl sim/10.5 GHz, using Raman amplification in dispersion-shifted optical fibre. More than 20 frequency lines with /spl sim/2 kHz linewidths are obtained using a 14.5 km length of fibre and a 1.3 MHz linewidth Brillouin pump. Gain depletion due to amplified spontaneous emission limits the total number of cascaded Brillouin lines. At high Raman pump powers, the laser produces a pulse train at the repetition rate corresponding to the cavity round-trip time.     

Crosstalk penalty in a two-channel ASK heterodyne detection system with non-negligible laser line width

Crosstalk penalties as a function of channel spacing were measured in a two-channel ASK heterodyne system using 1·55?m DFB lasers. For an IF linewidth/bit rate ratio of 0.5, the channel spacing in the IF domain must be greater than 13 times the bit rate for negligible crosstalk penalty.