Practical Receiver Systems

The authors have achieved a 2.488 Gb/s, 318 km repeaterless transmission without any fiber dispersion penalty through a nondispersion-shifted fiber in a direct detection system. The system was loss limited with a T-R power budget of 57 dB. Three key components enabled the authors to achieve this result: (1) a Ti:LiNbO/sub 3/ external amplitude modulator enabling a dispersion-free transmission, (2) erbium-doped fiber amplifiers increasing the transmitting power to +16 dBm, and (3) an erbium-doped fiber preamplifier enabling a high-receiver sensitivity of -4.1 dBm for 10/sup -9/ BER. To the author's knowledge, this result is the longest repeaterless transmission span length ever reported for direct detection at this bit rate. From the experimental results and a theoretical model, the authors identified the sources of the receiver sensitivity degradation from the quantum limit (-48.6 dBm) and estimated the practically achievable receiver sensitivity of approximately -44 dBm ( approximately -124 photons/bit) for 2.5 Gb/s optical preamplifier detection.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The use of fiber amplifiers in future lightwave systems now appears certain. Many experiments have been published demonstrating both the long-distance and multi-channel capability of fiber amplifier-based systems. Most experiments have used conventional direct detection systems [1,2]; but some experiments, including a record transmission distance experiment, have used coherent detection systems [3,4]; and models of both systems have been developed. This paper will compare the two systems under the same conditions using both experiments and models. We will emphasize a practical comparison by using a direct detection regenerator from AT&amp;T’s commercial, 1.7 Gb/s system and a shelf-mounted, coherent detection regenerator from a recent AT&amp;T field trial [5]. We will also emphasize a direct comparison by using transmitters of the same wavelength and identical optical paths including erbium-doped fiber amplifiers.

Accurate physical layer modeling for optical fiber communication systems is one of the key aspects of implementing an intelligent optical network. In this paper, we propose and study a data-driven model enabled by digital signal processing (DSP) methods for predicting the electrical and optical behavior of an intensity modulation direct detection (IM/DD) system based on an electro-absorption modulated laser (EML). This model captures both the linear and nonlinear characteristics of an IM/DD system with an adaptive finite impulse response (FIR) filter and a look-up table (LUT). Similar to conventional supervised machine learning problems, the proposed model is measurement-informed by iterating through a PAM-M signal training dataset. Additionally, we demonstrate that the proposed model parameters can be generalized by applying 2D piecewise bilinear interpolation across different driving voltages and received optical powers. To highlight the model advantages, we benchmark the proposed model against an existing equivalent circuit model and a deep neural network (DNN) model using multiple performance metrics, including signal root mean square error (RMSE), bit error rate (BER), training time, and model complexity. We demonstrate that the proposed FIR-LUT model outperforms the conventional equivalent circuit model using the PAM-2 dataset by achieving 0.021 sample RMSE and 0.018 symbol RMSE. Similarly, a 0.020 sample RMSE and 0.015 symbol RMSE are achieved with the PAM-4 dataset. Compared with the DNN alternative, the proposed FIR-LUT model delivers comparable accuracy but significantly reduced model complexity. The FIR-LUT model achieves <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$34\times$</tex-math></inline-formula> PAM-2 and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$24\times$</tex-math></inline-formula> PAM-4 training phase speed-up and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$486\times$</tex-math></inline-formula> PAM-2 and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$283\times$</tex-math></inline-formula> PAM-4 inference phase speed-up when compared with the DNN model.

Over the past several years, non-repeated transmission distances using coherent detection have been longer than those using direct detection. It is noticed that minimum detectable photon numbers of the direct detection schemes are no more than those of the coherent schemes, under shot-noise limited condition. Optical amplification techniques play important roles to improve the receiver sensitivity in the direct detection system [1] and to increase the fiber launched power.

Feasibility study of β-ray detection system for small leakage from reactor coolant system

In molecular communications, the direct detection of signaling molecules may be challenging due to a lack of suitable sensors and interference from co-existing substances in the environment. Motivated by research in molecular biology, we investigate an indirect detection mechanism using chemical reactions between the signaling molecules and a molecular probe to produce an easy-to-measure product at the receiver. We consider two implementations of the proposed detection mechanism, i.e., unrestricted probe movement and probes restricted to a volume around the receiver. In general, the resulting reaction-diffusion equations that describe the concentrations of the reactant and product molecules in the system are non-linear and coupled, and cannot be solved in closed form. To evaluate these molecule concentrations, we develop an efficient iterative algorithm by discretizing the time variable and solving for the space variables of the concentration equations in each time step. In the special case when the concentration of the unrestricted probes is high and not significantly changed by the chemical reaction, we obtain insightful closed-form solutions. Our results show that the concentrations of the product molecules and the signalling molecules have a similar characteristic over time, i.e., a single peak and a long tail. We highlight that by carefully choosing the molecular probe and optimizing the decision threshold, the BER can be improved significantly such that a direct detection system is outperformed. Moreover, when the molecular probes are kept in a small volume around the receiver, fewer resources are needed to achieve a lower BER and/or a higher data rate compared to the case of unrestricted molecular probes.

We present a 100GHz millimeter-wave communication enabled by a photonics-assisted intensity modulation and direct detection system. The frequency-resolved discrete multi-tone (DMT) modulation is used to adapt the nonlinear response of the system and increase the rate of transmission compared to conventional pulse amplitude modulation. By loading different number of bits to the subcarriers based on the channel frequency response, 20 Gbps DMT signals is transmitted over 0.25 m free space distance, and the measured bit error rate (BER) is below the 7% FEC threshold without any equalization. This work is expected to provide a cost-effective solution to achieve high-speed millimeter-wave transmission for future beyond 5G and 6G mobile communications.

The Digital Imaging and Remote Sensing Image Generation (DIRSIG) model is a synthetic imagery generation model developed at the Center for Imaging Science (CIS) at the Rochester Institute of Technology (RIT). It is a quantitative first principle based model that calculates the sensor reaching radiance from the visible through to the long wave infrared on a spectral basis. DIRSIG generates a very accurate representation of what a sensor would see by modeling all the processes involved in the imaging chain. Currently, DIRSIG only models passive sources such as the sun and blackbody radiation due to the temperature of an object. Active systems have the benefit of the user being able to control the illumination source and tailor it for specific applications. Remote sensing Laser Detection and Ranging (LADAR) systems that utilize a laser as the active source have been in existence for over 30 years. Recent advances in tunable lasers and infrared detectors have allowed much more sophisticated and accurate work to be done, but a comprehensive spectral LADAR model has yet to be developed. In order to provide a tool to assist in LADAR development, this research incorporates a first principle based elastic LADAR model into DIRSIG. It calculates the irradiance onto the focal plane on a spectral basis for both the atmospheric and topographic return, based on the system characteristics and the assumed atmosphere. The geometrical form factor, a measure of the overlap between the sensor and receiver field-of-view, is carefully accounted for in both the monostatic and bistatic cases. The model includes the effect of multiple bounces from topographical targets. Currently, only direct detection systems will be modeled. Several sources of noise are extensively modeled, such as speckle from rough surfaces. Additionally, atmospheric turbulence effects including scintillation, beam effects, and image effects are accounted for. To allow for future growth, the model and coding are modular and anticipate the inclusion of advanced sensor modules and inelastic scattering.

The detection of road area in the surroundings of the ego-vehicle is a key issue for modern ADAS. Camera-based direct detection systems are able to reliably accomplish this task only within a limited spatial range or in simple environments, due to hardware limitations and unfavorable situations, like shadows or occlusions. In complex environments, like inner city, traffic is a real issue, since the mere presence of other cars can significantly restrict the field of view of the ego-vehicle. In order to extend the spatial range of road detection, indirect detection systems are a viable resource. They can complement state-of-the-art direct detection systems and help motion control systems to plan smooth and stable trajectories.

The dual functionality of assuring ‘illumination’ and “communication” contemporaneously by relying on sustainable and energy-efficient opto-electronic devices like light emitting diodes (LEDs) has enabled visible light communication to garner the attraction of research era. This paper proposes a discrete sine transform (DST)-based optical orthogonal frequency division multiplexing (OFDM) system bestowing as the simplest and alternative multicarrier scheme complying the requirements of intensity modulated and direct detection systems. Moreover, this paper addresses the most vital issue like peak to average power ratio (PAPR) that emerges principally due to the limited dynamic range of LEDs when exploiting DC-biased optical OFDM (DCO-OFDM). Therefore, this paper imparts DST spreading to DST-based DCO-OFDM (DST-S-DCO-OFDM), and consequently, the PAPR performance is compared with other PAPR reduction schemes for the same system. Furthermore, in order to serve multiple subscribers with high-data rates as well as to significantly reduce the amount of PAPR of the multicarrier system, this work investigates a DST-based single carrier frequency division multiple access (DST-SC-FDMA) and derives the mathematical analysis of the time-domain signal formats for different subcarrier mapping strategies like DST-based optical interleaved frequency division multiple access (DST-OIFDMA) and optical localized frequency division multiple access (DST-OLFDMA). The simulated results evidences that the reduction in PAPR for DST-S-DCO-OFDM is more predominant when compared with other PAPR reduction schemes and among the multiple access schemes, the DST-OIFDMA is more power efficient upon comparison with DST-OLFDMA. However, both the schemes exhibit a significant reduction in PAPR when compared with DST-based optical OFDM (DST-OOFDMA).

Signal and Quantum Noise in Optical Communications and Cryptography

Photonic computing and signal processing are rapidly regaining attention, capitalizing on the saturation of digital electronics capabilities, owed to the ending of the Moore's law era. Optical communications are among the fields in which digital signal processing (DSP) struggles to offer a viable long-term solution, as its power consumption rises significantly in the latest generation optical links. For example, the traditional intensity modulation and direct detection (IM/DD) systems suffer from challenging problems such as the chromatic dispersion induced power fading effect, which necessitates the use of powerful digital equalizers in the 800G and 1.6T optical transceivers. Here, we experimentally validate our previous numerical works in recurrent optical spectrum slicing (ROSS) accelerators for dispersion compensation in IM/DD links. For this, we utilize recurrent filters implemented both through a waveshaper and by exploiting novel programmable photonic platforms. The ROSS accelerators exploit frequency processing in order to mitigate the power fading effect, which hinders the scalability of IM/DD systems. By equalizing even 80 km of 64 Gb/s PAM-4 transmission in C-band, we prove that our system can offer an appealing solution in highly dispersive channels. We employ the simplest digital equalization in the form of a feed-forward equalizer (FFE), avoiding throughput, latency, and complexity restrictions other digital equalizers impose. We achieve to reduce the bit error rate (BER) from 0.11 to less than 1 × 10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−2</sup> with the use of only two filter nodes, whereas BER approaches 10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> when three nodes are incorporated. These results correspond to almost two orders of magnitude BER gain.

For their capability of electronic dispersion compensation, transmission systems based on direct detection of single-sideband (SSB) signals are attractive candidates as energy-efficient and cost-effective alternative solutions to intradyne digital coherent systems for interdata center and metro applications. The Kramers–Kronig (KK) receiver scheme has been shown to provide superior performance compared to other schemes in signal-to-signal beat interference (SSBI) cancelation in these direct-detection systems. In this paper, we propose a low-complexity and cost-effective scheme of generating an optical superchannel comprising multiple SSB channels, based on a single quantum-dot mode-locked laser source. The proposed system does not require additional photonic or RF components at the transmitter to generate the required SSB signal with a continuous wave (CW) carrier. It also preserves the full digital-to-analog converters’ bit resolution for data modulation, in contrast to other methods based on digital generation of the CW component. Simulations of system performance with KK receiver, based on measured laser output field, show that the proposed system can achieve bit-error ratio below the hard-decision forward error correction threshold for 16-QAM Nyquist SSB signals after transmission through three amplified spans of single-mode fiber in a 240-km link. Using 8 KK channels at 23 GBaud each, the proposed scheme will be able to achieve a transmission rate of 736 Gb/s with noncoded spectral efficiency of 2.45 b/s/Hz. The impacts of carrier-to-signal power ratio, per channel launch power into the fiber, and component frequency drifting on transmission system performance are also discussed.

Single-polarization, single-photodiode-based direct-detection (DD) transceivers offer advantages for metro networks due to their simple and low-cost optical hardware structure. Single-sideband Nyquist-pulse-shaped subcarrier modulation (SSB Nyquist-SCM) is a promising signal format to achieve high information spectral density (ISD) in such DD systems. In this paper, we present theoretical and experimental evaluations of a variety of direct-detection SSB Nyquist-SCM system designs, operating at 100 Gb/s per wavelength in wavelength division multiplexing (WDM) metro network scenarios. Through simulations, several receiver-based digital linearization techniques to overcome the effect of signal-signal beat interference were investigated, and their performance compared with alternative approaches, including optical beat-interference cancellation receivers (BICRx) and coherent receivers (both heterodyne and homodyne). Subsequently, experimental assessments of the digitally-linearized DD receivers were carried out. Spectrally efficient (net ISD exceeding 3 b/s/Hz) 4 × 112 Gb/s WDM DD single-sideband 16-QAM Nyquist-SCM transmission over distances of up to 240 km (multi-span links) and 160 km (single-span links without mid-span amplification) were shown to be possible using uncompensated standard single-mode fiber.

In this paper we present an experimental analysis of several modulation formats (pulse amplitude modulation (PAM-2), quaternary pulse amplitude modulation (PAM-4) and electrical duobinary (EDB)) for passive optical network (PON) applications at 25 Gbps bit rate in a C-band 10G-class directly modulated lasers (DML) and avalanche photodiode (APD) intensity modulation and direct detection (IM-DD) system over a single mode fiber (SMF) of up to 25 km, optimizing DML operations and demonstrating that PAM-2 is a promising choice. We also theoretically and experimentally analyzed the channel frequency response of DML and SMF affected by DML chirp and SMF chromatic dispersion.