In this paper, the effects of gridless spectrum allocation in Wavelength Division Multiplexed (WDM) optical networks are examined. The advanced modulation formats and multi-rate transmissions of the signals, which are key parameters in the optical system project, are taken into account. The consumed spectrum, as well as the impact of linear and nonlinear impairments on the signal transmission, are compared to WDM network adopting standard grid and gridless ITU. To analyze the influence of these physical effects, some key network design parameters are monitored and evaluated, such as the guard band size, the signal occupied bandwidth, the laser power and the quality of channels. The applied signal modulation formats were On/Off Keying (OOK), Quadrature Phase Shift keying (QPSK), and Dual Polarization State Phase Modulation (DP-QPSK), whereas the transmission rate per wavelength was varied from 10 Gb/s to 100Ghz. The guard band, signal band, and laser power were swept and the resulted Bit Error Rate (BER) was estimated from the eye-diagram. Analytical calculations and simulations are conducted in order to evaluate the impact of the gridless spectrum allocation on both the spectral consumption and the signal quality of transmission (QoT). Results reveal that a gridless transmission system reduces the spectral consumption while offering an acceptable QoT. This work was carried out with both analytical modeling and numerical calculation using the Optisystem as well as Matlab.
In this paper, a multiband, multi-wavelength, all-fibre source array consisting of an 810nm pump laser diode, thretwo fiber splitters and three segments of Er-, Tm- and Nd-doped fiber is proposed for PON applications. In the set-up, cascaded pairs of standard fiber gratings are used for extracting the required multiple wavelengths within their corresponding bands. A thorough design parameter description, optical array details and full simulation results, such as: full multi-wavelength spectrum, peak and average powers for each generated wavelength, linewidth at FWHM for each generated signal, and individual and overall conversion efficiency, will be included in the manuscript.
The optical transfer function of a pulsed local oscillator coherent receiver is derived. We point out that the transfer function is the amplitude of the spectral envelope of the locked optical modes of the pulsed local oscillator source. Using quantum theory of homodyne detection, and assuming an additive Gaussian circular optical noise, with a spectral density hν/2, at each optical input port of the optical mixer, a general discussion on the fundamental limit of the Signal-to-Noise Ratio of a pulsed coherent receiver is provided. It includes the influence of the time jitter of the sampling pulses. Results are numerically discussed in the context of time equivalent under sampling technique, but may be obviously extended to any real time sampling of optical signals.
The use of rare-earth-doped fiber section working in amplified spontaneous emission regime for different emission wavelengths is analyzed theoretically. From simulation results, the design of all-fiber superluminescent sources employing different rare earths as dopants for new optical windows and different applications is proposed. Results on different pump and signal powers in forward and backward propagation direction with respect to fiber length are presented.
We describe a homodyne optical Costas loop receiver intended to detect weak coherent states with diffused phase and suppressed carrier phase modulation. In order to get the information contained in the quadrature components of the optical field, we implement an 8-port receiver operating at 1550 nm, based on the manipulation of the state of polarization of both the local oscillator and the data signal. Employing binary phase-shift keying, we make measurements in the time and frequency domain of the quantum noise and bit error rate using an optimum loop filter, and compare the performance of our receiver against the standard quantum limit for the simultaneous quadrature detection, considering both ideal conditions and the overall efficiency of our set up.
Weak coherent states (WCS) are being extensively employed in quantum communications and cryptography at
telecommunications wavelengths. For these low-photon-number applications, simultaneous field quadrature
measurements are frequently required, such as in the detection of multilevel modulations in the communications scenario
or in cryptographic applications employing continuous variables. For this task multiport balanced homodyne detection
(BHD) structures are employed, based on the splitting of the received field into its (non-commutating) in-phase (I) and
quadrature (Q) components and their separate beating with a local oscillator (LO) in two BHD. This allows the
simultaneous measurements of the 2 quadratures at the price of an additional noise due to the vacuum fields that leak via
the unused ports. These schemes require the proper optical phase synchronization between the LO and the incoming
field, which constitutes a challenge for WCS reception, especially for suppressed carrier modulations that are required
for power economy. For this task, a Costas loop is implemented for low photon number WCS, with the design of an
optimum feedback scheme considering the phase diffusion of WCS generated by semiconductor lasers. We
implemented an optical Costas loop at 1550 nm based on polarization splitting of the laser field to detect I and Q
quadratures simultaneously. We present results on the performance in phase error and bit error rate and compare with
corresponding quantum limit.
Optical direct detection usually operates far above the quantum limit, due to the high thermal noise level of PIN
photodiodes. For signal energy at the quantum level, the thermal effects in photon counters are also a strong limitation.
The optical amplification or the heterodyne detection of the 2 quadratures of the field, widely used in high bit rate and
long haul optical systems, overcome this limitation at the expense of a minimum 3db noise figure. By allowing a noise
free mixing gain, as well as single quadrature measurements, the balanced homodyne receiver is allowed to reach
quantum noise limited operation.
The aim of this paper is to review the different quantum receiver implementations and to compare the minimum signal
energy required to achieve a given bit error rate, or a given bit erasure rate, in high bit rate communications and quantum
communications. Application to quantum cryptography will be also addressed.
KEYWORDS: Modulation, Single sideband modulation, Modulators, Radio over Fiber, Radio optics, Signal to noise ratio, Networks, Optical networks, Single mode fibers, Optical fibers
We present the first demonstration of the generation of single sideband (SSB) modulation using a monolithic integrated
electro-absorption modulated laser (EML) device. Suppression of upper or lower sidebands is observed under
synchronous dual analog narrow band driving of the laser and the modulator sections. The 10 GHz carrier can transport
digital format data for a wide variety of Radio-over-Fiber (RoF) transmission systems. A 50 MBaud/s transmission of a
16-QAM signal has been achieved over 150 km of Standard Single Mode (SMF) fiber. Constellations, eye diagrams and
error vector magnitude (EVM) measurements are presented, all of which are temperature independent up to 45°C. This
demonstration of SSB modulation capability, which allows for signal transmission with a high spectral efficiency, free of
side-band beating and with a uniform signal power over the entire length of the optical fiber, makes the device ideal for
use in both optical metropolitan and optical access networks. Our experimental results establish the dual-EML SSB
transmitter as a serious candidate for optical-wireless network convergence and future OFDM systems.
We present an application of coherent homodyne detection to the problem of low photon number communications and
cryptography. As the coherent demodulation of an optical field requires the measurement of its (non commutating) inphase
and quadrature components, we present the structure and operation of an 8-port optical hybrid comprising 2
balanced homodyne detection structures, for the simultaneous measurement of the 2 quadratures. We analyze this
receiver operating with a strong local oscillator field, when the received field is in weak coherent states, with digital
phase modulation: we obtain the homodyne statistics and the uncertainty product in the presence of vacuum noises from
the input signal port and unused ports and discuss the increase in uncertainty due to the simultaneous measurements of
the quadratures. We obtain the signal to noise ratio as well of the bit error rate performance for binary phase shift keying
and discuss the departure from the standard quantum limit.
This paper presents the Direct Sequence Optical Code Division Multiple Access (DS-OCDMA) system using two
configurations of the optical source. To encode/decode transmitted data, we used Superstructured Fiber Bragg Grating
(S-FBG) as encoders/decoders. We consider three unipolar family codes which are the Prime Sequence (PS), Quadratic
Congruence (QC) and the Extended Quadratic Codes (EQC) codes. In order to evaluate the performances of our system
in term of Bit Error Rate (BER), we implement the Importance Sampling (IS) technique, which is a variant of the wellknown
Monte-Carlo (MC) method. Our simulation results depict that EQC codes outperform QC and PS codes for the
DS-OCDMA system using either coherent or incoherent source. We show also that using EQC codes with increasing the
optical bandwidth and maintaining the electrical filter leads to improve the performance of incoherent system.
KEYWORDS: Picosecond phenomena, Monte Carlo methods, Error analysis, Telecommunications, Fiber Bragg gratings, Code division multiplexing, Computer simulations, Interference (communication), Statistical analysis, Signal to noise ratio
In this paper, we compare the performances of a coherent versus incoherent Direct Sequence Optical Code Division
Multiple Access (DS-OCDMA) system. Superstructured Fiber Bragg Grating (S-FBG) encoders/decoders are used to
implement unipolar codes such as Prime Sequence (PS) and Extended Quadratic Codes (EQC) codes. We implement the
Importance Sampling (IS) technique, which is a variant of the well-known Monte-Carlo (MC) method, to evaluate the
Bit Error Rate (BER) performances of the system. Our simulation results depict that coherent system outperforms the
incoherent one. The last system can be used but a BER floor is demonstrated due to the beat noise of the incoherent
source. We show that increasing bit rate leads to a deterioration of the BER behavior and requiring an increase of the
optical bandwidth of the signal.
A new architecture of multiple private networks independent of optical line terminal (OLT) over Ethernet passive optical
network (EPON) using ring topology is proposed. This architecture integrates the multiple private networks (PNs) with
downstream/upstream EPON. Self determining private communications between optical network units (ONUs) are
established using code-drop units. The use of optical code division multiple access (OCDMA) technique results in secure
and reconfigurable PNs in the ring. Each ONU is assigned an appropriate codeword for private network communication
and also the standard equipments for the up/downstream standard EPON communication. As an example, by using
quadratic congruent (QC) codes with (p=5) leads to four optical private networks in the ring. To demonstrate the
integration feasibility of multiple 622 Mb/s PNs over 1.25 Gb/s EPON using QC code-drop units, we analyze the
network architecture by evaluating power budget, network dimensioning and BER performances.
Fundamental noise limitations of distributed quantum amplifiers are discussed. For Raman amplifier pumps to signals
noise transfer, Rayleigh backscattering and polarization fluctuations of the pump are additional noise sources, which are
discussed including their impact on system performances.
We have demonstrated error-free operations of slow-light via stimulated Brillouin scattering (SBS) in optical fiber for
10-Gb/s signals with different modulation formats, including
non-return-to-zero (NRZ), phase-shaped binary
transmission (PSBT) and differential phase-shift-keying (DPSK). By directly modulating the pump laser diode (LD)
using current noise source, the SBS gain bandwidth and profile can be simply controlled by the peak-to-peak value and
power density distribution of the current noise. Super-Gaussian noise modulation of the Brillouin pump LD allows a
flat-top and sharp-edge SBS gain spectrum, which can reduce
slow-light induced distortion in case of 10-Gb/s NRZ and
PSBT signals. For the 10-Gb/s NRZ signal, the error-free slow-light operation has been achieved for the fist time and the
corresponding maximal delay-time with error-free operation is 35 ps. Then we propose the PSBT format to minimize
distortions resulting from SBS filtering effect and dispersion accompanied with slow light owing to its high spectral
efficiency and strong dispersion tolerance. The maximal delay of 51 ps with error-free operation has been achieved.
Furthermore, the DPSK format is directly demodulated through a Gaussian-shaped SBS gain, which is achieved using
Gaussian-noise modulation of the Brillouin pump. The maximal
error-free time delay after demodulation of a 10-Gb/s
DPSK signal is as high as 81.5 ps, which is the best demonstrated result for 10-Gb/s slow-light.
The effect of weak optical injection on the threshold gain and resonant frequency of index-coupled DFB semiconductor
lasers is theoretically analyzed. The locking bandwidth modification due to injection is discussed for structures having
different grating characteristics. Simple formulas are derived which allow comparison of the sensitivity to optical
injection for DFB lasers and Fabry-Perot lasers. A symmetrical locking bandwidth is verified and a stability analysis for
weak injection is also carried out, imposing a maximum limit of injection rate.
Clock recovery using self pulsating semiconductor laser structure is a sophisticated signal processing inside the device,
involving optical injection locking, gain and index modulation, self pulsation, radiofrequency oscillation
synchronization, and phase noise filtering. The mutual injection of the longitudinal mode and the passive mode locking
leading to self pulsation is analyzed by using multimode rate equations. Self pulsation is easily achievable and is
characterized by a large reduction of the RF spectral line width, as compared to those of the optical modes involved in
the beating process. When they are injected by a power modulated optical signal, the self pulsating laser structures act as
all-optical clock recovery devices. The self pulsation line width is found to be determinant to fulfill the high bit-rate
optical communications and optical signal processing clock recovery property requirement.
This paper presents the principles and experimental results of an optical fiber QKD system operating at 1550 nm, and
using the BB84 protocol with QPSK signals. Our experimental setup consists of a time-multiplexed super-homodyne
configuration using P.I.N detectors in a differential scheme as an alternative to avalanche photon counting. Transmission
over 11km of optical fiber has been done using this detection scheme and major relevant characteristics such as noise,
quantum efficiency and bit error rate (BER) are reported.
Theoretical results concerning injection locking in complex cavity semiconductor lasers are reported. A general equation for the complex temporal envelope of the optical field is derived. The injection direction influence is taken into account and the difference between front and rear phase noise is pointed out and demonstrated. Numerical results are given concerning the feed-in rate of different DFB lasers.
This paper presents the experimental demonstration of a 1 Gbps Direct Sequence Optical Code Division Multiple Access (DS-OCDMA) system using pulsed coherent source. Encoding and decoding using Prime Sequence codes are achieved by Sampled Fiber Bragg Gratings (S-FBGs). The encoders/decoders have been designed with OptiGrating software and
realized with Phase Mask Process. BER measurements have been performed in the asynchronous configuration when an interferer is delayed from the desired signal. A 2 dB penalty due to Multiple Access Interference effect (MAI) has been observed in the synchronous case. S-FBG technological limitations and optical interferences due to the source coherence time have been observed and will be discussed.
All-optical regeneration at 40 Gbit/s and beyond is a crucial element for future transparent networks. One solution to achieve the regeneration is an all-optical clock recovery element combined with a
Mach-Zehnder interferometer. Among the different approaches investigated so far to accomplish the clock recovery function, a scheme based on a single self-pulsating distributed Bragg reflector laser is of particular interest from practical and cost viewpoints. In this structure at least two longitudinal modes beat together, generating power oscillation even though the laser is DC biased. The oscillation frequency is given by the free spectral range of the structure. In order to optimize the clock recovery performance of such a laser, a model based on four-wave-mixing has been developed. It takes into account the evolution of the amplitude and the phase of the complex electricfield of each longitudinal mode. From this model, a stability analysis is derived through the adiabatic approximation. The spectral density of the correlated phases of these modes is calculated and compared to the uncorrelated spectral density of each mode.
This paper shows the use of the Rice representation to obtain the correlation functions of the phase and power fluctuations of the optical noise of a laser oscillator when the Henry's factor is taken into account.
KEYWORDS: Interference (communication), Optical amplifiers, Signal attenuation, Signal to noise ratio, Integrated optics, Radio optics, Fiber amplifiers, Signal detection, Beryllium, Absorption
A classical phase-amplitude description of the optical field is proposed to discuss the optical amplifier noise generation. This description allows pointing out the contributions to output noise of the vacuum fluctuation input noise and of the intrinsic amplification and attenuation noises mechanisms. Comparison is done with the standard Amplified Spontaneous Emission (ASE) and associated beat noise approach. The mode is applied to the theoretical noise figure discussion for distributed amplifiers, as a function of built in internal loss and values of the net gain achieved, and to the derivation of the equivalent lumped amplifier noise figure.
KEYWORDS: Interference (communication), Optical amplifiers, Raman spectroscopy, Signal to noise ratio, Signal attenuation, Amplifiers, Photons, Solids, Signal generators, Backscatter
Using the semiclassical approach the gain and noise powers, associated to signal and, generated in optical Raman amplifiers, are estimated for forward and backward configurations. Because it combines a corpuscular approach to a phase-amplitude description of the optical field and of the associated noise, this classical formalism allows to identify, to distinguish and to evaluate the respective noise contributions linked, first, to incident fields, such as input zero-point fluctuations, and relative intensity noise associated to the pump, and to the amplifier itself due to the electron momentum fluctuations at the optical frequency. The contribution of Rayleigh backscattering and pump depletion effects are taken into account. For both configurations, the effects of gain and pump power distribution on noise generation are underlined. The determination of the origin and of the amount of intensity noise at the output constitutes a first step toward the amplification of signal with reduced noise amount in Raman amplifiers.
KEYWORDS: Optical amplifiers, Interference (communication), Signal to noise ratio, Photons, Signal attenuation, Raman spectroscopy, Signal processing, Beryllium, Polarization, Singular optics
Adopting a new classical approach, the additive noise power generated in optical amplifiers is calculated in terms of power spectral density. The classical formalism used combines a coruscular approach to a phase-amplitude description of the optical field. The noise contributions of the input field fluctuations, including zero-point fluctuations, and of the electron momentum fluctuations at optical frequency linked to the amplifier itself, are clearly identified. The excess of noise associated to coupling or built-in losses is determinated. The well-known result of the Amplified Spontaneous Emission (ASE) is obtained for the laser amplifiers. This description is applied to linear phase-insensitive amplifiers and to inhomogeneous, nonlinear phase-insensitive Raman amplifier, pointing out the effects of gain compression and gain distribution. This new approach makes possible the treatment of the squeezed-state of light and the quadrature reduced noise amplifications.
Laser phase noise deteriorates the high sensitivity of heterodyne optical receivers. To reduce phase noise influence, the intermediate frequency signal resulting from the coherent detection is filtered by a narrow bandpass filter (BPF). The phase noise at the input of the BPF generates an amplitude and phase noise at the output of the BPF. The joint probability density function of these noises is evaluated in the case of a first order filter by numerical resolution of a Fokker-Planck equation. A finite difference operator splitting scheme is used. The accuracy of the numerical solution is checked comparing numerically and analytically calculated moments. In addition, a new very efficient method for the analytical calculation of moments is developed. Contour plots of the probability density for both a finite time integrator and a first order filter are compared in order to show the impact of different filter types on phase noise filtering. The marginal pdf of the amplitude and phase noise at the output of the above filters are also calculated.
This paper proposes an accurate computer model of the nonuniform FM response of semiconductor lasers, to be used in the computer- aided design of coherent optical communication systems. The model is communications engineer oriented and does not involve the physical insight of the device. The main idea of this approach is that the FM response of the laser can be approximated by a recursive digital filter based directly on measurements of the FM response. The procedure is divided into two steps: First, measurements of the FM response are fitted by a rational interpolant using the theory of multi-point Pade approximants. Then, the impulse invariant transformation is used to calculate digital filter coefficients. The procedure is applied in the case of a conventional single-electrode distributed-feedback laser. The calculated digital filter is used to study the influence of the nonuniform FM response on the performance of a coherent heterodyne CPFSK system with differential receiver operating at 1 Gb/s. The sensitivity penalty is given as a function of SNR, phase noise, and sequence length by a semianalytical technique. Theoretical and experimental results are in excellent agreement.
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