Showing papers on "Optical communication published in 2007"

Electronic compensation of chromatic dispersion using a digital coherent receiver.

Abstract: Digital signal processing (DSP) combined with a phase and polarization diverse coherent receiver is a promising technology for future optical networks. Not only can the DSP be used to remove the need for dynamic polarization control, but also it may be utilized to compensate for nonlinear and linear transmission impairments. In this paper we present results of a 42.8Gbit/s nonlinear transmission experiment, using polarization multiplexed QPSK data at 10.7GBaud, with 4 bits per symbol. The digital coherent receiver allows 107,424 ps/nm of chromatic dispersion to be compensated digitally after transmission over 6400km of standard single mode fiber.

Optical Communication Using Subcarrier PSK Intensity Modulation Through Atmospheric Turbulence Channels

Abstract: This paper studies optical communications using subcarrier phase-shift keying (PSK) intensity modulation through atmospheric turbulence channels. The bit error rate (BER) is derived for optical communication systems employing with 00K or subcarrier PSK intensity modulation. It is shown that a ${\rm BER}=10^{-6}$ and a scintillion level $\sigma=0.1$ , an optical communication system employing subcarrier BPSK is 3 dB better than a comparable system using fixed threshold 00K. When $\sigma=0.2$ , an optical communication system employing subcarrier BPSK achieves a ${\rm BER}=10^{-6}$ at SNR 13.7 dB, while the BER of a comparable system employing 00K can never be less than ${\hbox{10}}^{-4}$ . Convolutional codes are discussed for optical communication through atmospheric turbulence channels. Interleaving is employed to overcome memory effect in atmospheric turbulence channels. An upper bound on BER is derived for optical communication systems employing convolution codes and subcarrier BPSK modulation.

Dual band radio frequency (RF) and optical communications antenna and terminal design methodology and implementation

Abstract: A dual-band antenna is provided that combines two normally disparate communications modes into a single compact aperture minimizing overall mass and volume, while maintaining high performance efficiency and reciprocity of each individual mode. The antenna is compatible with both optical (near-IR/visible) and RF (microwave/millimeter-wave) transceiver subsystems for high bandwidth communications, applicable primarily to long- to extremely long-range (space-to-ground) link distances. The optical link provides high bandwidth while the RF provides a lower data-rate weather backup, accommodation for traditional navigation techniques, and assistance in cueing the extremely tight optical beam by matching the RF beamwidth to an optical fine-steering mechanism field-of-regard. The configuration is built around a near-diffraction-limited high performance primary mirror shared by both a direct-fed RF antenna design and a Cassegrain optical telescope. Material properties are exploited to combine the optical secondary mirror with the RF feed structure, providing a collimated optical beam interface at the antenna vertex.

Fiber nonlinearity pre- and post-compensation for long-haul optical links using OFDM.

Abstract: The nonlinear power limit of optical links using optical Orthogonal Frequency Division Multiplexing (OFDM) for dispersion compensation can be significantly improved using an optimum combination of nonlinearity precompensation and postcompensation. The compensation is implemented at the transmitter and at the receiver as computationally-efficient power-dependent phase shifts with a single tuning parameter. The system is robust against the exact details of the fiber plant’s dispersion and power levels. Using an optimum combination of pre and post compensation allows a 2-dB increase in launch power for 2000-km standard single-mode fiber (S-SMF) systems and 5-dB when 6 ps/nm/km fibers are used. Using pre or post compensation alone approximately halves these values.

Methods and systems for simultaneous real-time monitoring of optical signals from multiple sources

Abstract: Methods and systems for real-time monitoring of optical signals from arrays of signal sources, and particularly optical signal sources that have spectrally different signal components. Systems include signal source arrays in optical communication with optical trains that direct excitation radiation to and emitted signals from such arrays and image the signals onto detector arrays, from which such signals may be subjected to additional processing.

Mode-locked silicon evanescent lasers.

Abstract: We demonstrate electrically pumped lasers on silicon that produce pulses at repetition rates up to 40 GHz. The mode locked lasers generate 4 ps pulses with low jitter and extinction ratios above 18 dB, making them suitable for data and telecommunication transmitters and for clock generation and distribution. Results of both passive and hybrid mode locking are discussed. This type of device could enable new silicon based integrated technologies, such as optical time division multiplexing (OTDM), wavelength division multiplexing (WDM), and optical code division multiple access (OCDMA).

Simultaneous bidirectional message transmission in a chaos-based communication scheme.

Abstract: We introduce a chaos-based communication scheme allowing for bidirectional exchange of information. By coupling two semiconductor lasers through a partially transparent optical mirror, delay dynamics is induced in both lasers. We numerically demonstrate that this dynamics can be identically synchronized, and moreover, information introduced on both ends of the link can be simultaneously transmitted. This scheme allows one to negotiate a key through a public channel.

Laser communication transmitter and receiver design

Abstract: Free-space laser communication systems have the potential to provide flexible, high-speed connectivity suitable for long-haul intersatellite and deep-space links. For these applications, power-efficient transmitter and receiver designs are essential for cost-effective implementation. State-of-the-art designs can leverage many of the recent advances in optical communication technologies that have led to global wide-band fiber-optic networks with multiple Tbit/s capacities. While spectral efficiency has long been a key design parameter in the telecommunications industry, the many THz of excess channel bandwidth in the optical regime can be used to improve receiver sensitivities where photon efficiency is a design driver. Furthermore, the combination of excess bandwidth and average-power-limited optical transmitters has led to a new paradigm in transmitter and receiver design that can extend optimized performance of a single receiver to accommodate multiple data rates.

Multi-wavelength source using low drive-voltage amplitude modulators for optical communications.

Abstract: A simple and cost-effective technique for generating a flat, square-shaped multi-wavelength optical comb with 42.6 GHz line spacing and over 0.5 THz of total bandwidth is presented. A detailed theoretical analysis is presented, showing that using two concatenated modulators driven with voltages of 3.5 Vp are necessary to generate 11 comb lines with a flatness below 2dB. This performance is experimentally demonstrated using two cascaded Versawave 40 Gbit/s low drive voltage electro-optic polarisation modulators, where an 11 channel optical comb with a flatness of 1.9 dB and a side-mode-suppression ratio (SMSR) of 12.6 dB was obtained.

Integrated all-optical logic and arithmetic operations with the help of a TOAD-based interferometer device--alternative approach

Abstract: Interferometric devices have drawn a great interest in all-optical signal processing for their high-speed photonic activity. The nonlinear optical loop mirror provides a major support to optical switching based all-optical logic and algebraic operations. The gate based on the terahertz optical asymmetric demultiplexer (TOAD) has added new momentum in this field. Optical tree architecture (OTA) plays a significant role in the optical interconnecting network. We have tried to exploit the advantages of both OTA- and TOAD-based switches. We have proposed a TOAD-based tree architecture, a new and alternative scheme, for integrated all-optical logic and arithmetic operations.

Spatial correlation and irradiance statistics in a multiple-beam terrestrial free-space optical communication link

Abstract: By means of numerical simulations we analyze the statistical properties of the power fluctuations induced by the incoherent superposition of multiple transmitted laser beams in a terrestrial free-space optical communication link. The measured signals arising from different transmitted optical beams are found to be statistically correlated. This channel correlation increases with receiver aperture and propagation distance. We find a simple scaling rule for the spatial correlation coefficient in terms of the propagation distance and we are able to predict the scintillation reduction in previously reported experiments with good accuracy. We propose an approximation to the probability density function of the received power of a spatially correlated multiple-beam system in terms of the parameters of the single-channel gamma-gamma function. A bit-error-rate evaluation is also presented to demonstrate the improvement of a multibeam system over its single-beam counterpart.

Intermodulation Distortion Improvement for Fiber–Radio Applications Incorporating OSSB+C Modulation in an Optical Integrated-Access Environment

Abstract: In this paper, we investigate the reduction of intermodulation distortion (IMD) in fiber-radio systems incorporating a dispersion-tolerant optical single sideband with carrier modulation. We present a systematic analysis and quantification of the third-order IMD generated due to optical components in the nonlinear optical front-end. Our proposed technique to improve the optical front-end linearity is by the removal of the optical components that contribute most to the third-order IMD in the RF domain. We experimentally demonstrated the proposed technique with two- and three-tone tests and showed more than 9-dB improvement in the overall carrier-to-IMD ratio. The proposed technique was also investigated via simulation analysis for a larger number of radio channels and showed an IMD suppression of >10 dB. In addition, the proposed technique is not only able to improve the carrier-to-interference of the radio signals but also to enable simultaneous baseband transmission, thereby facilitating the merging of millimeter-wave fiber-radio systems with other wired-access infrastructure. We present a detailed investigation and characterization of this technique.

Channel Capacity of IM/DD Optical Communication Systems and of ACO-OFDM

Abstract: In this paper we investigate the channel capacity of intensity modulated direct detection (IM/DD) wireless optical communication systems for an AWGN channel with a limit on the average transmitted optical power. It has recently been shown that asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) is more efficient in terms of optical power than conventional optical modulation techniques such as pulse position modulation (PPM). When ACO-OFDM is used, the transmitted signal has a clipped Gaussian probability distribution. We calculate the channel capacity for systems using transmitted signals with exponential and clipped Gaussian distributions, and for an ACO-OFDM system. For practical signal to noise ratios, ACO-OFDM has a slightly lower capacity than the other distributions, due to the correlation between samples caused by the ACO-OFDM modulation process. ACO-OFDM has many practical advantages including its tolerance to multipath distortion. This paper shows that it also makes efficient use of the available power and bandwidth.

Ultrawideband monocycle generation using cross-phase modulation in a semiconductor optical amplifier

Abstract: We propose a novel scheme to generate ultrawideband (UWB) monocycle pulses based on cross-phase modulation (XPM) of a semiconductor optical amplifier (SOA). The proposed system consists of a SOA and an optical bandpass filter (OBF). Due to the XPM, a continuous wave (CW) probe signal is phase modulated by another optical Gauss pulse in the SOA. The OBF will convert the phase modulation to intensity modulation. A pair of polarity-reversed UWB monocycle pulses is achieved by locating the probe carrier at the positive and negative linear slopes of the OBF. Both cases conform to the UWB definition of the Federal Communications Commission.

An Ultralow Phase Noise Coupled Optoelectronic Oscillator

Abstract: Ultralow jitter optical pulse sources are much needed in optical communications, optical sampling, and metrology applications. We report a coupled optoelectronic oscillator employing nonpolarization-maintaining components and an erbium-doped fiber amplifier that generates 9.4-GHz microwave signals with -150-dBc/Hz phase noise at 10- to 100-kHz offset, and 2-fs jitter (integrated in 100-Hz to 100-kHz range) optical pulses. To our knowledge, this is the lowest phase noise and time jitter reported in the same frequency range for such mode-locked laser-based systems

High-speed WDM-PON using CW injection-locked Fabry-Pérot laser diodes.

Abstract: We propose a new WDM-PON architecture using Fabry-Perot laser diodes (FP-LDs) that are injection-locked by continuous wave (CW) seed light. The modulation characteristics of the CW light injection-locked FP-LD are first investigated. Both uplink and downlink transmissions at 10 Gb/s are experimentally demonstrated using the proposed CW injection-locked FP-LDs. It is shown that up to 16 laser cavity modes can be selectively injection-locked with side mode suppression ratio larger than 30dB. The effects of the location of FP-LD cavity modes, transmission distance, and injection wavelength detuning on the overall transmission performance are investigated. The possibility of eliminating polarization dependence of the proposed CW injection scheme is also experimentally demonstrated by properly configuring a depolarizer. The deployment cost for the proposed WDM PON is potentially low from the fact that the CW laser sources located at the central office can be shared by many WDM-PONs and low-cost FP-LDs are used as light sources for data rates as high as 10 Gb/s.

Brightness Control Methods for Illumination and Visible-Light Communication Systems

Abstract: We have developed two brightness control methods for illumination and visible-light communication systems. One method is pulse width modulation (PWM), the other is changing modulation depth. Using these proposed methods, we can achieve both dimmer i.e. brightness control and wireless communication at the same time. We discussed the relationship between PWM frequency, LED brightness, and communication performance. The results show that we can control brightness from 0% to 87.5% when we use PWM and data transmission possible when PWM frequency is high enough. Brightness can be controlled from 0% to 100% and communication performance is better than that of PWM when we change modulation depth.

A reconfigurable architecture for continuously variable optical slow-wave delay lines.

Abstract: A novel reconfigurable architecture based on slow-wave propagation in integrated optical ring resonators is proposed for the realization of variable optical delay lines. A continuously variable delay is achieved by combining a coarse discrete (digital) delay, provided by a coupled resonator slow-wave structure, with a fine continuous (analog) delay given by a cascaded ring-resonator phase-shifter. The reflective configuration of the structure enables a simple, accurate and robust tuning of the delay and provides a footprint reduction by a factor 2 with respect to conventional coupled resonator optical waveguides. Proof-of-concept devices realized in 4.4% silicon oxynitride waveguides and activated by a thermal control are discussed. Experimental results demonstrate, in both spectral and time domain, a continuously variable delay, from zero to 800 ps (2 bit fractional delay), on a 2.5 Gbit/s NRZ signal, with less than 8 dB insertion loss and less than 5 mm2 device footprint.

Optimized multiemitter beams for free-space optical communications through turbulent atmosphere

Abstract: Using laser beams with less than perfect spatial coherence is an effective way of reducing scintillations in free-space optical communication links. We report a proof-of-principle experiment that quantifies this concept for a particular type of a partially coherent beam. In our scaled model of a free-space optical communication link, the beam is composed of several partially overlapping fundamental Gaussian beams that are mutually incoherent. The turbulent atmosphere is simulated by a random phase screen imprinted with Kolmogorov turbulence. Our experiments show that for both weak-to-intermediate and strong turbulence an optimum separation between the constituent beams exists such that the scintillation index of the optical signal at the detector is minimized. At the minimum, the scintillation reduction factor compared with the case of a single Gaussian beam is substantial, and it is found to grow with the number of constituent beams. For weak-to-intermediate turbulence, our experimental results are in reasonable agreement with calculations based on the Rytov approximation.

Systems and methods for underwater optical communication

Abstract: The systems and methods of the invention provide for improved underwater communication systems In particular, the systems and methods of the invention provide for improved underwater optical modems including optical transmitters and optical receivers that allow omni-directional transmission and reception of optical signals underwater and having a range of about 100 m and allowing data rates greater than 1 Mbit/s The systems and methods of the invention also provide for underwater communication networks having a plurality of optical modems communicating with each other

High-speed all-optical differentiator based on a semiconductor optical amplifier and an optical filter.

Abstract: We demonstrate a novel all-optical differentiator that carries out the first-order temporal derivation of optical intensity variation at high speed. It consists of a semiconductor optical amplifier (SOA) and an optical filter (OF) serving as an optical phase modulator and a frequency discriminator, respectively. A polarity-reversed derivative pair with a certain bias can be obtained by locating the probe wavelength at the positive or negative slope of the OF. Differentiations of super-Gaussian and Gaussian signals are obtained at various data rates. Defined as the mean absolute deviation of the measured result from the ideal result, total average errors of less than 0.12 are observed in all cases. Input power dynamics as well as control wavelength dependence are investigated and show that the cross-gain modulation in the SOA is detrimental to the differentiation performance.

Spectral Line-by-Line Pulse Shaping on an Optical Frequency Comb Generator

Abstract: We demonstrate optical processing based on spectral line-by-line pulse shaping of a frequency comb generated by an optical frequency comb generator (OFCG) The OFCG is able to generate a smooth, broad, stable, known-phase frequency comb, which is ideal for recently developed spectral line-by-line pulse shaping technology applications We demonstrate line-by-line pulse shaping on 64 lines at 10-GHz line spacing, generate transform-limited 16-ps short pulses at 10 GHz by combining two pulses in each period directly from the OFCG, and show various examples for optical arbitrary waveform generation Further, we demonstrate that these pulse sources are of sufficient quality to support optical fiber communication applications as confirmed by bit error rate measurements

Comparison of microwave and light wave communication systems in space applications

Abstract: The performances of optical and radio frequency communication systems are compared for long distance applications, such as deep space communications, where the signal-to-noise ratio (SNR) is crucial. We compare an optical communication system operating at 0.8 µm using intensity modulation and direct detection with an avalanche photodiode, an optical communication system operating at 1.5 µm using on-off keying and an optical preamplifier, and a radio frequency communication system operating in the X band. Assuming typical system parameters for the link budget analysis, we find that for distances between the transmitting and receiving antennas (R) of 107 km, the SNR for the optical systems is proportional to R?4, while for the radio frequency system it is always proportional to R?2. The maximum data rate achievable with the radio frequency system is higher than that with the optical systems for distances beyond 108 km. For near-Earth communication links, an optical system with optical preamplification is preferable when the data rate is higher than several gigabits per second. Clearly our results are based on specific system parameters. However, the equations involved and the method of comparison will be applicable for a wide range of system parameters.

All-optical demultiplexing of 640 to 40 Gbits/s using filtered chirp of a semiconductor optical amplifier.

Abstract: We present a high-capacity ultrafast all-optical time demultiplexer that can be employed to retrieve 40 gigabits/second (Gb/s) base-rate channels from a 640 Gb/s single-polarized signal. The demultiplexer utilizes ultrafast effects of filtered chirp of a semiconductor optical amplifier. Excellent demultiplexing performance is shown at very low switching powers: +8 dBm (640 Gb/s data) and -14 dBm (40 GHz clock). The demultiplexer has a simple structure and, in principle, allows monolithic integration.

Soliton based optical communication

Abstract: The group velocity dispersion (GVD) imposes severe limit on information carrying capacity of optical communication systems. By choosing appropriate pulse shape highly stable light pulses known as solitons are generated when effect of GVD is balanced by self-phase modulation (SPM). The application of solitons in communication systems opens the way to ultrahigh-speed information superhighways. Transmission speed of order of Tbit/s can be achieved if optical amplifiers are combined with WDM in soliton based communication systems.

Performance Evaluation of Next Generation Free-Space Optical Communication System

Abstract: Free-space optical communication systems can provide high-speed, improved capacity, cost effective and easy to deploy wireless networks. Experimental investigation on the next generation free-space optical (FSO) communication system utilizing seamless connection of free-space and optical fiber links is presented. A compact antenna which utilizes a miniature fine positioning mirror (FPM) for high-speed beam control and steering is described. The effect of atmospheric turbulence on the beam angle-of-arrival (AOA) fluctuations is shown. The FPM is able to mitigate the power fluctuations at the fiber coupling port caused by this beam angle-of-arrival fluctuations. Experimental results of the FSO system capable of offering stable performance in terms of measured bit-error-rate (BER) showing error free transmission at 2.5 Gbps over extended period of time and improved fiber received power are presented. Also presented are performance results showing stable operation when increasing the FSO communication system data rate from 2.5 Gbps to 10 Gbps as well as WDM experiments.

Monolithic 40-GHz Mode-Locked MQW DBR Lasers for High-Speed Optical Communication Systems

Abstract: The state-of-the-art of the development and fabrication of advanced 1.55-mum/40-GHz pulse laser modules with a monolithic InP-based mode-locked distributed Bragg reflector (DBR) MQW laser chip inside is reported, with respect to the future applications in high-speed optical communications. We emphasize the latest improvements for hybrid mode-locked devices, which integrate a saturable absorber or an electroabsorption modulator. We further illustrate different technical approaches that enable meeting predetermined frequencies and wavelengths, and which are at the same time promising with respect to high fabrication yields

Fiber Nonlinearity Mitigation in Optical Links That Use OFDM for Dispersion Compensation

Abstract: The nonlinear power limit of optical links using optical orthogonal frequency division multiplexing for dispersion compensation can be significantly improved using a simple and computationally efficient nonlinearity precompensation technique that requires no additional optical components. Simulations show that a 2-dB increase in transmission power is possible for 6 ps/nm/km fibers in a 4000-km system, and this increases to >4 dB for 2 ps/nm/km fibers. Alternatively, the bit error ratio can be substantially reduced with precompensation when the systems are operated at an optimum power. Only a single tuning parameter is used to represent the whole link, and the system is robust against variations in this parameter.

All-optical regeneration on a silicon chip

Abstract: We demonstrate optical 2R regeneration in an integrated silicon device consisting of an 8-mm-long nanowaveguide followed by a ring-resonator bandpass filter. The regeneration process is based on nonlinear spectral broadening in the waveguide and subsequent spectral filtering through the ring resonator. We measure the nonlinear power transfer function for the device and find an operating peak power of 6 W. Measurements indicate that the output pulse width is determined only by the bandwidth of the bandpass filter. Numerical modeling of the nonlinear process including free-carrier effects shows that this device can be used for all-optical regeneration at telecommunication data rates.