Significant reach enhancement for dispersion limited direct detection systems using offset carrier and linear electrical equalizers

A scheme utilizing an offset optical carrier transmission and adaptive linear equalizer in electrical domain is proposed to enhance the transmission reach of a direct detection optical transmission system. In the proposed scheme, the detection process becomes linear and thereby, a simple linear equalizer in electrical domain can undo the distortion due to linear dispersive effects.     

Chromatic dispersion induced distortion of modulated monochromatic light employing direct detection

The spectrum of an intensity modulated (IM) and a combined intensity-frequency modulated (IM-FM) monochromatic light source has been generated. The amplitudes of the modulated carrier and the first three pairs of sidebands are plotted showing the influence of IM on an FM signal. The effects of first order chromatic dispersion on the baseband amplitude response and harmonic distortion are determined. The manner in which modulation type and depth, modulating frequency, wavelength, and fiber length alter harmonic distortion is presented. Numerical examples giving the amplitude response of a single-mode fiber system as well as the magnitude of the second- and third-harmonic distortion caused by chromatic dispersion are presented. Based on this material, the limits placed on analog transmission due to chromatic dispersion may be assessed.     

Importance sampling for analysis of direct detection opticalcommunication systems

Analytical solutions of the performance of optical communication systems are difficult to obtain and often, Monte Carlo simulations are used to achieve realistic estimates of the performance of such systems. However, for high performance systems, this technique requires a large number of simulation trials for the estimates to be in a reasonable interval of confidence, with the number of trials increasing linearly with the performance of the system. We apply an importance sampling technique to estimate the performance of direct detection optical systems, where the “gain” of importance sampling over Monte Carlo simulations is shown to increase linearly with the system performance. Further, we use this technique to study the performance of optical communication systems employing avalanche photodetectors as well as fibre-optic code division multiple access systems (FO-CDMA). We also show that the quick simulation technique developed can be used for a wide variety of coding schemes, and for the first time, we present a comparative analysis of the performance of FO-CDMA systems employing optical orthogonal codes and prime sequences. In all cases, it is shown that importance sampling simulations require less than 50-100 trials for estimating error probabilities of 10^-10 and below     

Simplifying the error probability analysis in optical direct detection DPSK systems

This letter describes a rigorous formulation based on an eigenfunction expansion technique to estimate the performance of optical direct detection DSPK systems and discusses appropriate procedures to simplify the process without solving any eigenvalue problem. A great emphasis is put on an approach derived from the wide-band optical filtering assumption. The results show that this simplification is acceptable for ideal receivers with time-bandwidth products far larger than 1, but it is clearly inadequate for more realistic receivers.     

Performance of preamplified direct detection systems underinfluence of receiver noise

We present new results for optically preamplified direct detection systems where the receiver thermal noise is significant. The results obtained with this model is in good agreement with measured results from a system experiment at 10 Gbit/s. It is found that an optical gain of 30 dB is sufficient to keep the penalty due to thermal noise below 0.1 dB for a simple receiver consisting of a photodiode loaded with a broadband amplifier with 50-Ω input impedance     

Constraints on the bit rates in direct detection opticalcommunication systems using linear or soliton pulses

The effects of stimulated Raman scattering (SRS) and the self-frequency shift on soliton communication systems are analyzed. These effects impose stringent limitations on the maximum data rates that can be carried in such systems. The limitations arise as soliton amplitude or frequency fluctuations are translated into pulse position jitter after propagation along a fiber. Estimates of the maximum data rates in single-channel and multichannel soliton systems are obtained and are compared with data rates in linear direct detection systems. A single-channel soliton system can, in principle, carry data at a rate of between two and ten times faster than a simple, direct-detection, linear system. In a multichannel system, using data channels at different wavelengths, the advantages of using solitons are reduced     

新型光谱直接检测OCDMA系统分析和仿真

探讨采用增强双重码(EDWC)的新型光谱直接检测技术的光码分多址系统的性能.EDWC是一种增强的双重码(DWC),具有理想的互相关特性,其权重可以是任意大于1的奇数.文章对新型光谱直接检测技术和常规互补减法技术进行了对比分析和仿真.     

PMD penalties in long nonsoliton systems and the effect of inline filtering

We consider the polarization-mode dispersion (PMD) tolerance of long nonsoliton systems. We find that the effect of PMD in such systems is to derail the nonlinear evolution of the waveform from its PMD-free trajectory at an early stage of transmission, leading to extremely high penalties at the receiver end. It is shown that PMD affects the propagating bandwidth in a way that is strongly correlated with the received penalty. Use of inline optical filters is shown to be extremely efficient in reducing the penalties caused by PMD.     

Performance evaluation of very long span direct detection intensityand polarization modulated systems

In this paper long distances very high capacity NRZ optical transmission systems adopting direct detection are considered in links with a low average chromatic dispersion. Particular attention is devoted to polarization modulated (PM-DD) systems. Polarization modulated systems results to he more degraded with respect to intensity modulated (IM-DD) systems by the light depolarization induced by the interplay among the Kerr effect, the ASE noise of the optical amplifiers and the polarization mode dispersion. The light depolarization is particularly strong in conditions of large spectral broadening that are met when the chromatic dispersion value is maintained very low along the link. On the other hand the use of a fluctuating chromatic dispersion with a mean value equal to zero, whereas the local dispersion is different from zero, shows the double advantage to reduce the chromatic dispersion impairments and to limit the spectral broadening. The advantages offered by this dispersion management technique have been already shown in several experiments for IM-DD systems: in this work we show that this technique is very important also for PM-DD systems since the limitation in the spectral broadening reduces the light depolarization. We show that adopting a suitable dispersion management and an opportune preamplifier optical filter transmissions at 5 Gb/s can be attained in transoceanic links by means of FM-DD systems     

Impact of chromatic and polarization-mode dispersions on DPSK systems using interferometric demodulation and direct detection

We study the impact of chromatic dispersion (CD) and first-order polarization-mode dispersion (PMD) on systems using binary differential phase-shift keying (2-DPSK) or quaternary DPSK (4-DPSK) with nonreturn-to-zero (NRZ) or return-to-zero (RZ) formats. These signals are received using optical preamplification, interferometric demodulation, and direct detection. We consider the linear propagation regime and compute optical power penalties at fixed bit-error ratio (BER). In order to evaluate the BER precisely taking account amplifier noise, arbitrary pulse shapes, arbitrary optical and electrical filtering, CD, and PMD, we introduce a novel model for DPSK systems and compute the BER using a method recently proposed by Forestieri for on-off keying (OOK) systems. We show that when properly applied, the method yields highly accurate results for DPSK systems. We have found that when either the NRZ or RZ format is used, 2-DPSK exhibits lower power penalties than OOK in the presence of CD and first-order PMD. RZ-2-DPSK, as compared with NRZ-2-DPSK, incurs smaller penalties due to PMD, but offers no advantage in terms of CD. 4-DPSK, as it has twice the symbol duration of OOK or 2-DPSK for a given bit rate, incurs much lower CD and PMD power penalties than either of these techniques. RZ-4-DPSK is especially promising, as it offers CD and PMD penalties significantly smaller than all other techniques, including NRZ-4-DPSK.     

Novel multi-band DFT-spread OFDM-PON systems based on intensity modulation and direct detection for cloud computing

Passive optical network (PON) has become a preferable access technique for cloud computing due to its elastic bandwidth capacity and transmission stability. In particular, the orthogonal frequency division multiplexing PON based on intensity modulation and direct detection (IM/DD OFDM-PON) has gained extensive attention since it is a cost- and spectral-efficient system, while for the traditional IM/DD OFDM-PON, the use of OFDM could lead to the high peak-to-average power ratio (PAPR), and it is impossible to satisfy the different QoS degrees required by ONUs under a cloud environment. Thus in this paper, we design a novel multi-band discrete Fourier transform (DFT)-spread IM/DD OFDM-PON. The DFT-spread is utilized to reduce the PAPR; meanwhile, a multi-band power allocation and bit loading are achieved to satisfy the different degrees of QoS requirement owned by ONUs. The simulation results show that our system has the better performance of PAPR reduction compared with the traditional IM/DD OFDM-PON; meanwhile, the different QoS degrees of all ONUs are guaranteed.     

Impact of chirping on polarization-mode dispersion compensated systems

Investigates the effect of chirp, resulting from either the transmitter, chromatic dispersion, or Kerr effect in the fiber, on polarization-mode dispersion (PMD) compensated systems. We demonstrate both experimentally and numerically the deleterious impact of chirping on systems with first-order PMD compensation. We present a way to optimize the PMD compensator performance by minimizing the total chirp of the signal at the receiver side.     

Effects of chromatic dispersion on optical coherent-detection systems

Chromatic dispersion induces to a phase-modulated optical signal a constand phase rotation that was never included in most previous studies. When the constant phase shift is removed by a phase-locked loop, the dispersion tolerance of a coherent-detection system is increased. The dispersion tolerance of phase-shift keying (PSK) signal is compared with differential PSK (DPSK) signals by numerical simulation and experimental measurement. Contrary to conventional belief, PSK signal has larger dispersion tolerance than DPSK signal.     

The effect of waveguide dispersion on VLF timing systems

It is shown that VLF timing and navigation systems, which use two or more frequencies to resolve cycle ambiguities, may be in error when the propagation mechanism is dispersive. The error arises if the difference between group and phase velocity in such cases is neglected. At 10 kHz the error on a path 10 000 km long, for a reasonable daytime model of the earth-ionosphere waveguide, may be as much as 400mus for the first-order mode. If the second-order mode is dominant as it may be at night the error will be greatly increased.     

Linear dispersion codes for MIMO systems based on frame theory

Multiple-input multiple-output (MIMO) wireless communication systems provide high capacity due to the plurality of modes available in the channel. Existing signaling techniques for MIMO systems have focused primarily on multiplexing for high data rate or diversity for high link reliability. In this paper, we present a new linear dispersion code design for MIMO Rayleigh fading channels. The proposed design bridges the gap between multiplexing and diversity and yields codes that typically perform well both in terms of ergodic capacity as well as error probability. This is important because, as we show, designs performing well from an ergodic capacity point of view do not necessarily perform well from an error probability point of view. Various techniques are presented for finding codes with good error probability performance. Monte Carlo simulations illustrate performance of some example code designs in terms of ergodic capacity, codeword error probability, and bit error probability.     

Temperature effects on dispersion and loss for high-bit-rate LED-based lightwave systems

Examination of the impact of wide temperature extremes on high-bit-rate LED-based systems indicates that the LED room-temperature wavelength should be selected below 1300 nm to avoid excess loss owing to the 1390 nm water peak in fibre.     

Calculation of the dispersion penalty for the route design ofsingle-mode systems

A system model for analyzing the effects of dispersion when multilongitudinal-mode laser diodes are used is presented. The analysis considers in a general manner the digital symbol statistics, the system pulse shaping, the laser diode spectrum, the laser diode extinction ratio, and mode partition noise. For mode partition noise, separate consideration is given to the statistics of the longitudinal-mode powers within a baud period and to the statistics between different baud periods. It is demonstrated that failure to separate the mode partition statistics in this manner can lead to underestimation of the dispersion penalty. By incorporating statistical models for the important system parameters, a statistical approach to route design is demonstrated     

Effects of polarization-mode dispersion on cross-phase modulation in dispersion-managed wavelength-division-multiplexed systems

This paper develops a vector theory of cross-phase modulation (XPM) in optical fibers and use it to investigate the impact of polarization-mode dispersion (PMD) on the crosstalk induced by XPM in wavelength-division multiplexed lightwave systems. Under certain reasonable approximations, the theory permits us to obtain an analytic expression for the amplitude of probe fluctuations induced by a copropagating pump channel through XPM. We use this expression to calculate the average level of XPM-induced crosstalk together with its variance for several dispersion maps. We show that PMD not only reduces the crosstalk on average, but also impacts the efficiency of a commonly used polarization-interleaving technique.