Multichannel FSK experiment using an optical amplifier repeater and wavelength tunable direct detection receiver

With a continuously tunable free space optical delay interferometer and direct detection D(Q)PSK signals can be received at varying symbol rates or under optimized conditions. The interferometer's phase is stabilized and its polarization dependence mitigated.

In this paper we introduce a nonlinear equalizer using the radial basis function (RBF) network with decision feedback equalizer (DFE) for electronic dispersion compensation in optical communication systems with on-off-keying and a direct detection receiver. The RBF method introduces a non-linear equalization technique suitable for optical communication direct detection systems that include nonlinear transformation at the photodetector. A bit error rate performance comparison shows that the RBF with DFE out performs the RBF without DFE and achieves similar results provided by maximum likelihood sequence estimator.

We investigate polarization mode dispersion (PMD) and polarization dependent loss (PDL) impairments in polarization division multiplexing (PDM) signals with optical polarization demultiplexing and direct detection. We find that the time alignment between the bits in the two polarizations has a significant impact on the PMD impairments, and PMD impairments also depend on the bandwidth of PDM signals, whereas PDL impairments have little dependence on the relative time alignment between the two polarizations and the signal bandwidth. We show that with a proper configuration of the polarization demultiplexing, the PDL-induced crosstalk between the two polarizations can be completely eliminated. The combined effects of PMD and PDL are also studied, and we find that, in the presence of concatenated PMD and PDL, the impairment from one effect does not enhance that from the other.

DELTAS<SUB>NRC</SUB><SUP>TM</SUP> is a laser radar signature modeling and simulation code that was developed to evaluate potential LADAR systems for defense applications. DELTAS<SUB>NRC</SUB><SUP>TM</SUP> strength is in its flexibility. The user can create scenarios with realistic targets (the target model is based on constructive solid geometry) and real materials, and model coherent or direct detection LADAR systems at any wavelength (limited only to availability of material reflectance information). This paper focuses on recent improvements to the direct and coherent detection imaging capabilities of the code. These improvements were mandated by the BMDO Discriminating Interceptor Technology Program (DITP) which requires LADAR system models for the next generation of LADAR imaging systems on an interceptor platform. We introduce upgrades to the range-Doppler imaging algorithms that improve the fidelity of the laser cross section calculation and the range-Doppler image. We describe a model that introduces ambiguity in direct detection angle- angle-range images as a result of pulse duration and detector bandwidth. A direct detection receiver model has also been implemented. This model introduces optical modulation transfer functions and receiver noise to the direct detection images. The result of these improvements is an end-to-end LADAR simulation which can be used as a stand alone code or as part of a suite of sensor models capable of generating signatures which can be used for discrimination algorithm development, system analysis, etc. Demonstrations of these improvements as applied to DITP are presented, and a discussion of current applications of DELTAS<SUB>NRC</SUB><SUP>TM</SUP> in simulations such as Synthetic Scene Generation Model is included.

In this work, an improved receiving technique for Optical Wireless Intensity Modulation and Direct Detection (OW IM-DD) communication system, with Asymmetrically-Clipped Optical OFDM (ACO-OFDM) modulations, is proposed. The (socalled in the literature) "clipping noise", which carries 50% of the transmitted signal power is customarily discarded. Instead we show that the clipping noise carries information related to the original unclipped signal. Hence it is used in the proposed receiver scheme to generate an improved estimator for the transmitted signal. It is shown both theoretically and numerically, that the new approach improves the receiver performance of the system by up to 3dB without a change to the modulated signal.

The main objective of this paper is to achieve high normalized spectral efficiency (in bits per OCDMA chip) in optical CDMA network transmission. We design a time-wavelength M-ary transmission scheme that uses error-control coding and trellis coded modulation (TCM), optical M-ary intensity modulation and direct detection at the receiver. We use a probabilistic wavelength-time hopping approach, in order to transmit error-control coded and TCM modulated M-ary data directly through the optical channel without using optical spreading sequences. We present simulation results showing that the proposed OCDMA scheme is robust to Poisson and thermal noise. Hundreds of active users can be supported at target BER of 10-9 and achieved spectral efficiency (1.37 bits per OCDMA chip) is almost twice larger that the best result previously reported.

Time-frequency (TF) filtering of analog signals has played a crucial role in the development of radio-frequency communications and is currently being recognized as an essential capability for communications, both classical and quantum, in the optical frequency domain. How best to design optical time-frequency (TF) filters to pass a targeted temporal mode (TM), and to reject background (noise) photons in the TF detection window? The solution for 'coherent' TF filtering is known-the quantum pulse gate-whereas the conventional, more common method is implemented by a sequence of incoherent spectral filtering and temporal gating operations. To compare these two methods, we derive a general formalism for two-stage incoherent time-frequency filtering, finding expressions for signal pulse transmission efficiency, and for the ability to discriminate TMs, which allows the blocking of unwanted background light. We derive the tradeoff between efficiency and TM discrimination ability, and find a remarkably concise relation between these two quantities and the time-bandwidth product of the combined filters. We apply the formalism to two examples-rectangular filters or Gaussian filters-both of which have known orthogonal-function decompositions. The formalism can be applied to any state of light occupying the input temporal mode, e.g., 'classical' coherent-state signals or pulsed single-photon states of light. In contrast to the radio-frequency domain, where coherent detection is standard and one can use coherent matched filtering to reject noise, in the optical domain direct detection is optimal in a number of scenarios where the signal flux is extremely small. Our analysis shows how the insertion loss and SNR change when one uses incoherent optical filters to reject background noise, followed by direct detection, e.g. photon counting. We point out implications in classical and quantum optical communications. As an example, we study quantum key distribution, wherein strong rejection of background noise is necessary to maintain a high quality of entanglement, while high signal transmission is needed to ensure a useful key generation rate.

We present high-speed WDM-PON systems utilizing reflective semiconductor optical amplifiers and direct detection. We employ the polar RZ format and optical/electrical equalization techniques to maximize the performance of the highly band-limited devices.

The cutoff rate is derived for a digital communication system employing an optical carrier and direct detection. The coordinated design of the encoder, optical modulator, and demodulator is then studied using the cutoff rate as a performance measure rather than the more commonly employed error probability. Modulator design is studied when transmitted optical signals are subject simultaneously to average-energy and peak-value constraints. Pulse-position modulation is shown to maximize the cutoff rate when the average-energy constraint predominates, and the best signals when the peak-value constraint predominates are identified in terms of Hadamard matrices. A time-sharing of these signals maximizes the cutoff rate when neither constraint dominates the other. Problems of efficient energy utilization, choice of input and output alphabet dimension, and the effect of random detector gain are addressed.

Spectral efficient frequency division multiplexing (SEFDM) can offer a higher spectral efficiency (SE) than orthogonal frequency division multiplexing (OFDM). In this work, we propose a diversity technique based on SEFDM for beyond 100-Gb/s optical intensity modulation and direct detection (IM/DD) long reach (LR) applications. We mathematically demonstrate that the self-created inter-carrier interference of SEFDM signals can be reused to achieve a diversity gain on each sub-carrier and, in turn, improve the tolerance to power fading induced by chromatic dispersion (CD) in IM/DD LR links. Based on the proposed diversity technique, we further demonstrated a 112-Gb/s SEFDM transmission over 80-km standard single-mode fiber, using only 28-GHz bandwidth and modulation format of up to 16-QAM. Experimental results show that SEFDM with the proposed diversity technique performs robust against CD effects and outperforms the conventional OFDM with adaptive bit and power loading of the same bandwidth and data rate, which validates the superiority of the proposed SEFDM in optical IM/DD LR transmissions.

Optical amplifiers are quite attractive for use as optical repeaters because of their simplicity and bit rate transparency for both intensity-modulated optical signals1–3 and frequency/phase-modulated optical signals4. Also, recent non-regenerative 904km optical transmission with 12 cascaded Er-doped fiber amplifiers5 has shown the promising future of optical amplifier repeater systems. For further investigations, experimental study on the effect of a large amount of chromatic dispersion is of great importance. This is a report on 2.4Gbit/s intensity-modulated optical signal transmission over 710km of non-dispersion-shifted optical fiber with a total chromatic dispersion of 12300ps/nm. In addition, the effect of transmitter phase noise associated with large chromatic dispersion was experimentally investigated.

A novel phased array receive antenna steering system is introduced. The core of this system is an optical ring resonator-hased broadband, continuously tunable optical beam forming network (OBFN). In the proposed system architecture, filter-based optical single-sideband suppressed-carrier modulation and balanced coherent optical detection are used. Such architecture has significant advantages over a straightforward architecture using optical double-sideband modulation and direct optical detection, namely relaxed bandwidth requirements on the optical modulators and detectors, reduced complexity of the OBFN chip, and enhanced dynamic range. Initial measurements on an actual 1 times 8 OBFN chip and an optical sideband filter chip arc presented. Both are realized in CMOS-compatible planar optical waveguide technology.

We review the optical direct detection subsystems based on the Stokes vector receiver (SV-R), which is fit for a variety of modulation formats in Stokes space. SV-R accommodates 2-dimension (2-D) or 3-D detection, greatly increasing the spectrum efficiency.

Optical diffusion and polarization characteristics of ice accreting in dynamic conditions using a backscattering fibre optic technique

Signal and Quantum Noise in Optical Communications and Cryptography