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^-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     

Channel switching between receivers of FDM coherent optical star network

The authors present channel switching results obtained between receivers of an FDM optical coherent fibre star network. The system consists of six optical channels FSK-modulated at 200 Mbit/s and spaced by 2.2 GHz. The channels are multiplexed by a 16*16 optical star coupler. Two of the multichannel signals exiting this device are demultiplexed by their respective random access digitally tuned heterodyne receivers. Simultaneous switching between pairs of channels is obtained by command of a personal computer controlling both receivers.<>     

Densely spaced WDM coherent optical star network

An optical WDM star network consisting of three lasers transmitting at about 234 000 GHz, spaced by 300 MHz, has been used to demonstrate dense packing of WDM signals. The three optical signals, FSK-modulated at 45Mbit/s, are multiplexed by a 4 × 4 star coupler and demultiplexed by a balanced heterodyned receiver. Receiver sensitivity is -61dBm for a BER of 10?9, or 113 photons/bit, which is 4.5dB from the shot noise limit and represents the best sensitivity yet reported for FSK modulation. The results indicate that 100 000 users in a 10km radius could be interconnected with such a system.     

High-performance dense FDM coherent optical network

The results obtained with a densely spaced frequency division multiplexing (FDM) optical star network providing random channel selection by a digitally tuned high-sensitivity heterodyne receiver are presented. The experimental system consists of six 200-Mb/s frequency-shift keyed (FSK) modulated channels, spaced by 2.2 GHz and multiplexed by a star coupler. Channel selection is performed by a computer-controlled random-access heterodyne receiver having a sensitivity of 74 photons/b at a BER of 10^-9, which is 1.7 dB from the quantum limit. The digital-tuning random-access capability (which depends on the frequency-tuning current relationship of the local oscillator (LO) laser) is protected against frequency drifts of the LO laser. The receiver can also detect the absence of a selected channel. In this case, the receiver locks to the next available channel and displays which channel is received instead. The results obtained indicate that this system has potential throughput of 2000 Gb/s     

Densely spaced FDM optical coherent system with near quantum-limited sensitivity and computer-controlled random access channel selection

An FDM coherent optical network consisting of six 200 Mbit/s-FSK channels spaced by 2.2 GHz, is reported. Receiver sensitivity of 74 photons/bit for a BER of 10/sup -9/ is obtained. The channels are randomly accessed by computer control. The system has the potential to provide a through-put of 2000 Gbit/s.<>     

Optical FSK signal detection in a heterodyne system using semiconductor lasers

A 10?8 bit error rate is achieved for 100 Mbit/s signals in an FSK heterodyne frequency-discrimination detection system, in which AlGaAs double-heterostructure lasers are used as both the FSK transmitter and the local oscillator. Receiving signal power level to achieve a low bit error rate is experimentally obtained, which confirms the principle of optical heterodyne detection. AM quantum noise in the laser local oscillator and FM quantum noise are important factors which determine the system performance.     

Optical heterodyne receiver for selecting densely frequencydivision multiplexed signals

An optical heterodyne receiver that can select any channel out of 2.5 Gbit/s frequency division multiplexed (FDM) signals with spacing as narrow as 5 GHz has been developed. Sharp filtering characteristics of the heterodyne receiver at the intermediate frequency stage enable FDM signals with such narrow channel spacing to be demultiplexed     

Coherent optical receiver for 680 Mbit/s using phase diversity

A coherent optical receiver using a multiport optical coupler to provide phase diversity is described. By this means, only homodyne bandwidth is required in the signal processing, but with tolerances on the frequency tracking typical of heterodyne systems. Measured sensitivities with limited local-oscillator power are ?47.5 dBm at 320 Mbit/s and ?42 dBm at 680 Mbit/s. The results at the higher bit rate are strongly influenced by limited receiver module bandwidth.     

A coherent optical FDM CATV distribution system

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     

Analysis of repeated unequally spaced channels for FDM lightwavesystems

In long-haul optical frequency-division-multiplexing (FDM) systems, transmission characteristics are degraded by four-wave mixing (FWM). To overcome this problem, repeated unequally spaced (RUS) channels have been recently proposed as a new frequency allocation. In this paper, frequency distribution and intensity of generated FWM lights, and a total bandwidth of signal lights of RUS channels are compared with those of already known equally spaced (ES) and unequally spaced (US) channels. It is found that intensities of generated FWM lights of RUS are less than those of ES when the number of channels and a total bandwidth of signals are common in both channels. It is also revealed that RUS has a narrower total bandwidth than US when the number of channels and the minimum channel spacing are common in both channels. Since RUS simultaneously satisfies a low FWM light intensity and a narrow signal bandwidth, it is considered that RUS is suitable for FDM lightwave transmission systems     

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     

Crosstalk degradation caused by optical amplification in amultichannel FSK heterodyne system

The crosstalk degradation caused by an optical amplifier in a four-channel FSK (frequency-shift-keyed) heterodyne communication system is measured. A bit error rate (BER) floor of 3×10^-4 is observed when the channels are spaced by 200 MHz, FSK modulation at 45 Mb/s, and when the optical input signal is large enough such that the gain is compressed by 2 dB relative to its small-signal value. The receiver is substantially improved by reducing the optical power amplifier input. However, the sensitivity increases only to a maximum value beyond which it degrades as the optical power of the demodulated channel becomes small relative to the noise of the optical amplifier. The combined effect of the crosstalk and the amplifier noise yields an optimum sensitivity of 250 photons/b for BER=10^-9. This result is 5 dB poorer than the sensitivity obtained in the absence of an optical amplifier     

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.     

1.244-Gb/s 32-channel transmission using a shelf-mountedcontinuous-phase FSK optical heterodyne system

A total capacity of 40 Gb/s is achieved using a shelf-mounted continuous-phase frequency-shift-keying (CPFSK) optical heterodyne frequency-division-multiplexing (FDM) transmission system with 32 optical channels and a bit-rate of 1.244-Gb/s per channel. For achieving a stable bit-error-rate (BER) characteristics with high-sensitivity, narrow-linewidth laser diodes, a channel-spacing stabilization circuit, and an optical tuner are developed. The obtained sensitivity at a BER of 10^-9 for fiber transmission over 121 km ranges from -45.1 to -44.2 dBm, which is 9.8-10.7 dB lower than the shot-noise-limited sensitivity. The crosstalk penalty is suppressed to within 0.1 dB. The developed system has feasibility achieving a distribution system which can distribute more than 250 HDTV (high definition television) signals or 1250 current-standard TV signals to about 8000 subscribers 10 km from the office, or a 40-Gb/s trunk-line system with a fiber span of more than 50 km     

Optical amplification in a multichannel FSK coherent system

Reports the crosstalk degradation caused by an optical amplifier in a densely spaced four-channel heterodyne FSK system. A maximum receiver sensitivity of 250 photons/bit is obtained for an optimum input signal level. This result is 5 dB poorer than the sensitivity obtained in the absence of an optical amplifier     

560 Mb/s optical PSK synchronous heterodyne experiment

A phase-locked optical heterodyne receiver constructed using a 1320-nm diode-pumped miniature Nd:YAG ring laser is discussed. Using this receiver and a transmitter based on another Nd:YAG laser, a 560-Mb/s phase-shift keying (PSK) synchronous heterodyne transmission was demonstrated over 78 km of single-mode fiber. With an optical phase-locked loop (PLL) natural frequency of 32 kHz and a damping factor of 1.46, the receiver sensitivity, measured at the output of the transmission link, was -48.7 dBm, or 159 photons/b. The corresponding detected sensitivity, measured on the surface of the p-i-n diode, was -51.8 dBm or 78 photons/b. This result suggests that the receiver sensitivity would have been about 82 photons/b if a balanced receiver with 0.2-dB excess coupler loss had been used. The impact of the finite intermediate frequency (IF) on heterodyne system performance was investigated; it was found that an IF of at least twice the bit rate is needed for a negligibly small penalty     

Crosstalk and prefiltering in a two-channel ASK heterodynedetection system without the effect of laser phase noise

The authors present an experimental and theoretical study on the crosstalk in a two-channel amplitude-shift keying (ASK) heterodyne detection system in which the effect of laser phase noise is negligible. Three results are described: (1) the dependence of the crosstalk penalty on the ratio of channel separation to bit rate and on the optical power level of the image band (2) comparison of the measured crosstalk penalities with the ones obtained from a simple model, and (3) the effect of electrical prefiltering on the crosstalk penalty. It is concluded that the channel separation can be as low as four times the bit rate without incurring any crosstalk penalty as long as the optical power of the image band is comparable to the optical power of the desired channel. In addition, electrical prefiltering of the transmitted signals significantly reduces the crosstalk penalty in multichannel ASK heterodyne systems in which the effect of laser phase noise is negligible     

Calculation of dispersion and nonlinear effect limited maximum TDMand FDM bit rates of transform-limited pulses in single-mode opticalfibers

The dispersion-limited maximum time-division-multiplexed (TDM) bit rates and the optical nonlinear-effect-limited maximum frequency-division multiplexed (FDM) channel numbers in single-mode optical fibers have been calculated for transform-limited optical pulses. The total bit rate attainable with combinations of TDM and FDM on Gaussian-type transform-limited pulses is about 7 Tbt/s in the typical 15 THz wide low-loss region of single-mode fibers at each of 1.3 and 1.5 μm wavelength bands. The maximum total bit rate attainable with dispersion-shifted (DS) fibers in the Er-doped fiber amplifier (EDFA's) gain region of 1525-1565 nm is calculated to be about 2.3 Tbt/s, but reduces to 1.2 to 1.8 Tbt/s depending on fiber length for cases of a uniform TDM bit rate over the entire FDM channels. For DS fibers the four-wave mixing effect is a dominant effect limiting the channel power and the maximum FDM channel number, but for normal single-mode fibers the chromatic dispersion effect and cross-phase modulation (CPM) and stimulated Raman scattering (SRS) effects are dominant effects limiting the TDM bit rate and channel power, respectively     

Factors affecting the design of optical FDM informationdistribution systems

Major factors affecting optical frequency division multiplexing (FDM) distribution system design are described. In particular, multiplexing/distribution configuration and channel-selection methods are compared from the viewpoint of the number of channels, the number of subscribers, and the transmission distance. The applicability of optical fiber amplifiers to the optical FDM information-distribution network is also discussed. The experimental results of a 5 GHz spaced 16-channel FDM distribution/transmission at 622 Mb/s, using a waveguide frequency-selection switch (tunable filter) and a multicarrier frequency-stabilization technique, are also discussed     

WDM systems with unequally spaced channels

Crosstalk due to four-wave mixing (FWM) is the dominant nonlinear effect in long-haul multichannel optical communication systems employing dispersion-shifted fiber. A method is discussed to find non-uniform channel separations for which no four-wave mixing product is superimposed on any of the transmitted channels, therefore suppressing FWM crosstalk. The residual crosstalk, due to channel power depletion only, is analytically evaluated for intensity-modulated repeaterless wavelength-division-multiplexed (WDM) systems and compared to experimental results. The theory includes the effect of the channel depletion on the amplitude of each phase-matched FWM wave. The probability of error is evaluated including the statistics of the pattern dependent channel depletion. The BER curve computed for an 8-channel WDM system is found to be in good agreement with experimental results. In the experiment, repeaterless transmission of eight 10 Gb/s WDM channels over 137 km (11 Tb/s-km) of dispersion-shifted fiber was demonstrated and error-free operation was achieved over a wide range of input powers using unequally spaced channels. The same system with equally spaced channels could not achieve a probability of error lower than 10^-6. The use of unequal channel spacing allowed fiber input power to be increased by as much as 7 dB, which could be translated into a fivefold increase of the bit rate per channel (and therefore of the system capacity), or to an increase in the system length of about 30 km