Optical On-off Keying Research Articles

Optimized equalization based on MAP detection for C-band bandwidth-limited IM/DD systems.

Entangled dynamic and deterministic inter-symbol interferences (ISIs) induced by complicated channel impairments, limit the transmission capacity of intensity modulation and direct detection (IM/DD) systems. This Letter proposes a colored noise-suppressed channel shortening filter (CNS-CSF)-enabled maximum a posteriori (MAP) estimation (CNS-CSF-MAP) scheme to disentangle and mitigate deterministic and dynamic ISIs, where the CNS-CSF is deployed to perform the optimized dynamic ISI equalization with equalization-enhanced noise suppression, and the subsequent MAP algorithm is used to eliminate the residual deterministic ISI. The performance of the CNS-CSF-MAP scheme is evaluated and demonstrated in a C-band 61-Gb/s 100-km optical on-off keying (OOK) IM/DD system. The experimental results show that the proposed CNS-CSF-MAP scheme reaches the 20% and 7% forward error correction (FEC) thresholds at received optical powers (ROPs) of -6.6 dBm and -4 dBm, achieving 0.5- and 1.5-dB gains over a conventional post-filter-enabled MAP (PF-MAP) scheme.

Integrated Optical Sensing and Communication (IOSAC) System Based on Hybrid‐Waveguide Structures

AbstractAdvanced silicon photonic technologies enable integrated optical sensing and communication (IOSAC) in real time for the emerging application requirements of simultaneous sensing and communication for next‐generation networks. Herein, an IOSAC system is proposed and demonstrated on the silicon nitride (Si3N4) photonics platform. The IOSAC devices leveraging microring resonators excel in real‐time monitoring of analyte fluctuations and efficiently transmit this data to the terminal, positioning them as viable alternatives to bulk optics in applications that demand both high precision and speed. By directly integrating Si3N4 ring resonators with optical communication networks, simultaneous sensing and optical communication are demonstrated by an optical signal transmission experimental system using especially filtering amplified spontaneous emission spectra. The refractive index (RI) sensing ring with a sensitivity of 172 nm RIU−1, a figure of merit (FOM) of 1220, and a detection limit (DL) of 8.2 × 10−6 RIU is demonstrated. Simultaneously, the 1.25 Gbps optical on‐off‐keying (OOK) signal is transmitted at the concentration of different NaCl solutions, indicating the bit‐error‐ratio (BER) decreases with the increase in concentration. The novel IOSAC technology shows the potential to realize high‐performance simultaneous biosensing and communication in real time and further accelerate the development of IoT and 6G networks.

Experimental demonstration: Differential detection of 10 Gbps optical OOK signal over turbulent channel

In this paper, we experimentally demonstrate the bit error rate (BER) performance of fixed threshold differential detection for both 5 Gbps and 10 Gbps On-Off keying (OOK) signals over a turbulent channel, which induces slowly varying intensity fluctuation on the signals. The experimental results are also validated using numerical simulations. This study is significant for the terrestrial free-space optical (FSO) communication system in which the channel induces similar intensity fluctuation due to atmospheric turbulence. To show the efficiency of the differential detection scheme for the OOK signal two proof-of-principle systems are realized. In the first system, the effect of channel-induced intensity variation on the data is realized by passing a differentially coded OOK signal through a Mach–Zehnder modulator (MZM), which is driven by a low frequency sinusoidal electrical signal emulating slowly varying channel fluctuation. The amplitude of the sinusoid has been varied to create different levels of intensity fluctuation, mimicking different turbulent conditions in the optical link. In the second setup, we measure the BER performance of differential detection assisted OOK transmission over a short-range (10)m FSO channel, which is affected by turbulence-induced intensity fluctuations. The different turbulences are created by different heating levels of a blower, which injects hot air across the FSO channel. Experimental results show that in the presence of intensity fluctuation, differential detection of OOK signal using a fixed threshold offers a better BER performance at higher optical to signal ratio (OSNR) values compared to conventional direct detection (DD) scheme. Results obtained from numerical simulation of OOK transmission over turbulent FSO channels also confirm the similar behavior. Both simulation and experimental results show that differential detection can offer a BER of 10−3 at 5 Gbps and 10 Gbps data rates with slow varying channel-induced intensity fluctuation when the DD scheme fails to produce the same BER values.

Timing Synchronization and Channel Estimation in Free-Space Optical OOK Communication Systems

Fast and reliable synchronization in free-space optical (FSO) communications is a crucial task that has received little attention so far. Since in these applications the data rate is much higher than in traditional radio-frequency (RF) systems, novel technological constraints may arise in the design of the synchronization algorithms, as for example the need to operate at symbol rate instead with an oversampled data stream. In this work, we consider an FSO link and investigate the problem of channel estimation, symbol timing recovery and frame detection using a known synch pattern. The modulation format is on-off keying (OOK) and the received signal is plagued by a mixture of thermal and shot noise. By applying the least-squares criterion, we derive a novel synchronization scheme that can jointly retrieve all the unknown parameters using symbol-spaced samples. Although designed without taking the noise statistics into account, the estimator performance is assessed in a realistic scenario where shot noise is present. Comparisons are made with the relevant Cramér-Rao bound for the joint estimation of the synchronization parameters and signal-dependent noise variances. Numerical simulations and complexity analysis indicate that the resulting scheme performs satisfactorily with an affordable processing load. Hence, it represents a promising solution for fast synchronization in high-speed FSO communications.

Burst-Error-Propagation Suppression for Decision-Feedback Equalizer in Field-Trial Submarine Fiber-Optic Communications

In this paper, we present a field-trial C-band 72Gbit/s optical on-off keying (OOK) system over 18.8km dispersion-uncompensated submarine optical cable in the South China Sea. Chromatic dispersion (CD) of 18.8km submarine optical cable causes four spectral nulls on the 36GHz bandwidth of 72Gbit/s OOK signal, which is the main obstacle for achieving an acceptable bit-error-rate (BER) performance. Decision feedback equalizer (DFE) is effective to compensate for the spectral nulls. However, DFE has a serious defect of burst-error propagation when the burst errors emerge due to the unstable submarine environment. Weighted DFE (WDFE) can be used to mitigate the burst-error propagation, but it cannot fully compensate for the spectral nulls because only a part of feedback symbols is directly decided. Fortunately, maximum likelihood sequence estimation (MLSE) can be added after the WDFE to simultaneously eliminate the resisting spectral distortions and implement optimal detection. Compared to the joint DFE and MLSE algorithm, the joint WDFE and MLSE algorithm can effectively suppress the burst-error propagation to obtain a maximum 2.9dB improvement of $\boldsymbol{Q}$ factor and eliminate the phenomenon of BER floor. In conclusion, the joint WDFE and MLSE algorithm can solve the burst-error propagation for the field-trial fiber-optic communications.

Weakly-Coupled MDM-WDM Amplification and Transmission Based on Compact FM-EDFA

Recently optical amplification technologies for mode-division multiplexing (MDM) transmission systems and networks have attracted great interest. In this article, we first propose a compact structure of few-mode Erbium-doped fiber amplifier (FM-EDFA) for weakly-coupled MDM amplification, which is characterized by weakly-coupled few-mode Erbium-doped fiber (EDF), cascaded mode-selective couplers (MSCs) for spatial mode control of pump lights, and few-mode wavelength multiplexer (FM-WMUX) for pump/signal coupling. Compared to previous studies, the proposed FM-EDFA has a similar structure with commercial single-mode EDFA (SM-EDFA) and can support weakly-coupled MDM transmission without inducing large modal crosstalk. Then related components are designed and fabricated for a 2-mode proposed FM-EDFA. The mode-field mismatching between transmission FMF and EDF under central alignment is investigated by simulation. Thanks to the effective suppression of modal crosstalk for the optical components, the transmission FMF, the EDF and their coupling, we experimentally demonstrate the first weakly-coupled MDM-WDM amplification and transmission over 15-km 2-mode FMF with 10-Gb/s optical on-off keying (OOK) modulation and direct detection. The proposed FM-EDFA can be extended to MDM transmission systems and networks supporting more modes.

Adaptive Channel-Matched Detection for C-Band 64-Gbit/s Optical OOK System Over 100-km Dispersion-Uncompensated Link

In this paper, we propose adaptive channel-matched detection (ACMD) for C-band 64-Gbit/s intensity-modulation and direct-detection (IM/DD) optical on-off keying (OOK) system over a 100-km dispersion-uncompensated link. The proposed ACMD can adaptively compensate most of the link distortions based on channel and noise characteristics, which includes a polynomial nonlinear equalizer (PNLE), a decision feedback equalizer (DFE) and maximum likelihood sequence estimation (MLSE). Based on the channel characteristics, PNLE eliminates the linear and nonlinear distortions, while the followed DFE compensates the spectral nulls caused by chromatic dispersion. Finally, based on the noise characteristics, a post filter can whiten the noise for implementing optimal signal detection using MLSE. To the best of our knowledge, we present a record C-band 64-Gbit/s IM/DD optical OOK system over a 100 km dispersion-uncompensated link achieving 7\% hard-decision forward error correction limit using only the proposed ACMD at the receiver side. In conclusion, ACMD-based C-band 64-Gbit/s optical OOK system shows great potential for future optical interconnects.

C-band 56 Gbit/s on/off keying system over a 100 km dispersion-uncompensated link using only receiver-side digital signal processing.

In this Letter, we present a record C-band 56 Gbit/s intensity-modulation/direct-detection optical on/off keying (OOK) system over a 100 km dispersion-uncompensated link using only the receiver-side digital signal processing (DSP). The proposed DSP mainly includes an adaptive moment estimation (Adam)-based polynomial nonlinear equalizer (PNLE), autoregression (AR)-based post filter, and maximum likelihood sequence estimation (MLSE). Due to square-law detection, chromatic dispersion induces 11 nulls on the 28 GHz spectrum of the 56 Gbit/s OOK signal after the 100 km standard-single-mode-fiber transmission. Adam-based PNLE eliminates major linear and nonlinear distortions, but it cannot compensate the nulls. After the Adam-based PNLE, the AR-based post filter and MLSE further deal with the inter-symbol interference caused by the nulls to improve the system performance. The proposed C-band 56 Gbit/s OOK system shows great potential for future metro networks and data center networks.

A 6-LP-mode ultralow-modal-crosstalk double-ring-core FMF for weakly-coupled MDM transmission

Weakly-coupled mode division multiplexing (MDM) technique based on few mode fiber (FMF) with low linear-polarized (LP) modal crosstalk is promising for capacity enhancement of short-reach transmission. In this paper, we propose a double-ring-core FMF for simultaneous 6-LP-mode weakly-coupled MDM transmission. FMF with ring-core structure is theoretically analyzed by deriving from a step-index circular-core FMF with perturbed ring areas. Then simplified method is proposed to define boundaries of ring areas in FMF and increase the minimum effective refractive index difference (min|Δneff|) among all the modes. Fabricated by commercial fabrication technique, the FMF achieves a min|Δneff| up to 1.49×10−3 while its maximum Δn between core and cladding is only 0.828%, which greatly reduces the difficulty of fiber fabrication. Thanks to the ultralow modal crosstalk of the FMF, we successfully demonstrate simultaneous 6-LP-mode (LP01, LP11, LP21, LP02, LP31 and LP12) 4-wavelength 71-km weakly-coupled MDM transmission using 10 Gb/s optical on-off-keying (OOK) intensity modulation and direct detection (IM/DD) without any digital signal processing (DSP). The design method can be extended to weakly-coupled FMFs with more ring areas.

An Interference Suppression for Transporting Radio Frequency Signals with 10 Gbps Optical On-Off Keying

An Interference Suppression for Transporting Radio Frequency Signals with 10 Gbps Optical On-Off Keying

All-optical XOR and OR by Mach-Zehnder Interferometer engineered photonic crystal fibers

In this paper, we present a numerical investigation of all-optical logic gates based on a Mach-Zehnder Interferometer (MZI) configuration of micro structured fibers, also known as photonic crystal fibers (PCF) under optical OOK (on-off keying). We determine several relevant quantities to characterize the system performance, such as transmission and extinction ratio as a function of the nonlinearity profile (β) added to one of the Mach-Zehnder Interferometer (MZI) arms. High-order effects such as third-order dispersion, intrapulse Raman scattering and self-steepening were included in the generalized nonlinear Schrödinger equation describing the pulse propagation. For this study, we used an optical pulse with a shape of hyperbolic secant of 100fs (femtosecond). A pumped laser with three power values was simulated. With critical power Pc=103.5kW (P=Pc), below the critical power P=90kW (P<Pc) and above the critical power P=110kW (P>Pc). We added a nonlinearity profile in one of the Mach-Zehnder Interferometer (MZI) arms. Nonlinearity profiles were expressed in terms of the parameter β. We studied three different profiles: constant, increasing and decreasing. Our results show that the proposed device can be used to obtain all-optical OR and XOR logic gates as well as logic functions A and A.B¯. The OR logic gate was the one which achieved the greatest FOMELG (Figure of Merit of Logic Gates) when using increasing profile with critical power (P0=103.5kW) for β=1.05 and FOMELG=15.68dB. The XOR logic gate presented the greatest FOMELG=7.75dB when using the decreasing profile with power below the critical (P0=90kW) for β=1.44. These results provide support for applications in all-optical networks.

All-fiber optical mode switching based on cascaded mode selective couplers for short-reach MDM networks

We propose and experimentally demonstrate an all-fiber optical mode switching structure supporting independent switching, exchanging, adding, and dropping functionalities in which each mode can be switched individually. The mode switching structure consists of cascaded mode selective couplers (MSCs) capable of exciting and selecting specific higher order modes in few-mode fibers with high efficiency and one multiport optical switch routing the independent spatial modes to their destinations. The data carried on three different spatial modes can be switched, exchanged, added, and dropped through this all-fiber structure. For this experimental demonstration, optical on-off-keying (OOK) signals at 10-Gb/s carried on three spatial modes are successfully processed with open and clear eye diagrams. The mode switch exhibits power penalties of less than 3.1 dB after through operation, less than 2.7 dB after exchange operation, less than 2.8 dB after switching operation, and less than 1.6 dB after mode adding and dropping operations at the bit-error rate (BER) of 10−3, while all three channels carried on three spatial modes are simultaneously routed. The proposed structure, compatible with current optical switching networks based on single-mode fibers, can potentially be used to expand the switching scalability in advanced and flexible short-reach mode-division multiplexing-based networks.

Three-mode mode-division-multiplexing passive optical network over 12-km low mode-crosstalk FMF using all-fiber mode MUX/DEMUX

We propose three-mode mode-division-multiplexing passive optical network (MDM-PON) based on low mode-crosstalk few-mode fiber (FMF) and all-fiber mode multiplexer/demultiplexer (MUX/DEMUX). The FMF with step-index profile is designed and fabricated for effectively three-independent-spatial-mode transmission and low mode-crosstalk for MDM-PON transmission. The all-fiber mode MUX/DEMUX are composed of cascaded mode selective couplers (MSCs), which simultaneously multiplex or demultiplex multiple modes. Based on the low mode-crosstalk of the FMF and all-fiber mode MUX/DEMUX, each optical network unit (ONU) communicates with the optical line terminal (OLT) independently utilizing a different optical linearly polarized (LP) spatial mode in MDM-PON system. We experimentally demonstrate MDM-PON transmission of three independent-spatial-modes over 12-km FMF with 10-Gb/s optical on-off keying (OOK) signal and direct detection.

Cascaded Mode-Division-Multiplexing and Time-Division-Multiplexing Passive Optical Network Based on Low Mode-Crosstalk FMF and Mode MUX/DEMUX

We propose cascaded mode-division-multiplexing and time-division-multiplexing passive optical network (MDM-TDM-PON) based on low mode-crosstalk few-mode fiber (FMF) and all-fiber mode multiplexer/demultiplexer (MUX/DEMUX), in which optical network units communicate with the optical line terminal utilizing different time slots and specific optical linearly polarized spatial modes. An MDM optical distribution network (ODN) is cascaded with multiple conventional TDM ODNs to effectively extend a larger scale of current commercial PON systems based on TDM. The upgrade from TDM-PON to the cascaded MDM-TDM-PON is simple and cost-effective. No multiple-input–multiple-output (MIMO) digital signal processing is required to eliminate the mode crosstalk. The all-fiber mode MUX/DEMUXs are composed of mode selective couplers, which simultaneously multiplex or demultiplex multiple modes. We experimentally demonstrate MDM-PON transmission over 10 and 55 km two-mode FMFs and cascaded MDM-TDM-PON transmission over a 10-km two-mode FMF and a 10-km standard single-mode fiber with 10-Gb/s optical on–off keying (OOK) signal and direct detection.

On the Decision-Feedback Equalizer in Optically Amplified Direct-Detection Systems

This paper investigates the combined feed-forward and decision-feedback equalizer (DFE) in a lightwave system with optical amplifiers and a direct-detection receiver. Based on a nonlinear channel model, the paper provides a modification of the classical minimum mean square error theory of the DFE. Furthermore, an analytical method that is capable of accurate bit error rate (BER) evaluation is used to optimize the DFE for minimum BER. The paper evaluates the DFE performance for both optical ON-OFF keying and duobinary modulation formats in the presence of chromatic dispersion as well as the DFE performance for the mitigation of higher order polarization mode dispersion in first-order compensated systems. The paper shows that the DFE can compensate for the BER degradation due to narrow-band receiver-side optical filtering and can significantly improve the spectral efficiency of dense wavelength-division multiplexed systems.

Maximum-likelihood sequence detection with closed-form metrics in OOK optical systems impaired by GVD and PMD

This paper thoroughly investigates the maximum-likelihood sequence detection (MLSD) receiver for the optical ON-OFF keying (OOK) channel in the presence of both polarization mode dispersion and group velocity dispersion (GVD). A reliable method is provided for computing the relevant performance for any possible value of the system parameters, with no constraint on the sampling rate. With one sample per bit time, a practically exact expression of the statistics of the received samples is found, and therefore the performance of a synchronous MLSD receiver is evaluated and compared with that of other electronic techniques such as combined feedforward and decision-feedback equalizers (FFE and DFE). It is also shown that the ultimate performance of electronic processing can be obtained by sampling the received signal at twice the bit rate. An approximate accurate closed-form expression of the receiver metrics is also identified, allowing for the implementation of a practically optimal MLSD receiver

Sensitivity Performance of Indoor Optical Wireless OOK and PPM Systems

The sensitivity performance of both a Pulse Position Modulation (PPM) and an On-Off Keying (OOK) optical wireless communication system has been carried out in the presence of both ambient noise and multipath dispersion. The optical wireless (OW) channel has been modeled by means of experimental results and the carthesian co-ordinates of locations in a default test room. It is further shown that the sensitivity of a PPM scheme is more affected by multipath dispersion than a similar OOK format. Under the constraint of ambient noise, the findings were opposite.

A Performance Comparison of Optical Heterodyne OOK and FSK Communications Systems

An error probability analysis is performed for two types of optical communications systems employing spread-spectrum code-division multiple-access (CDMA) techniques. Spread-spectrum is specifically employed to overcome additive noise and increase system user capacity. The two types of systems considered are a coherent optical heterodyne on-off keying (OOK) system using optical orthogonal spreading code sequences (OOCs), and a coherent optical heterodyne frequency shift keying (FSK) system using randon spreading codes. The OOK system spreads the signal optically, and the FSK system spreads the signal electromagnetically before converting it to an optical signal for transmission. System performance is degraded by laser phase noise, additive Gaussian multiuser interference, and additive Gaussian receiver noise. Single user OOK system performance for different linewidth-to-bit rate ratios is analyzed over a range of both signal-to-noise-ratios (SNR) and normalized decision thresholds. This analysis is used to determine which noise source dominates system performance and the maximum acceptable linewidth-to-bit rate ratio. The limiting linewidth-to-bit rate ratio is then used in previously derived equations to obtain the performance of the coherent optical heterodyne OOK-CDMA system for various spreading code length and numbers of users. An error probability analysis is then performed for the coherent optical heterodyne FSK-CDMA system for various spreading code length and numbers of users. Finally, the performance of the two systems is compared, and it is shown that optical FSK-CDMA systems perform better than optical OOK-CDMA systems

Delay-and-multiply clock regeneration in APD-based direct-detection optical OOK communication systems

A clock recovery scheme for direct-detection optical on-off keying (OOK) communication systems with nonreturn-to-zero pulse shaping is proposed and investigated. In the suggested model, the optical field is detected with the aid of an avalanche photodiode (APD) photodetector, which is followed by a clock regeneration subsystem. The proposed clock recovery subsystem consists of a delay-and-multiply nonlinearity followed by a conventional phase-locked loop (PLL), tuned to the slot frequency of the desired optical OOK signal. Performance of the proposed system is obtained in terms of the signal-to-noise ratio (SNR/sub L/) of the linearized PLL device (or, equivalently, the inverse of phase, or timing, error variance) when background noise and receiver thermal noise are present. Numerical results are presented in order to explain the influence of noise processes on the performance of the proposed clock recovery subsystem. The performance of this system is also compared to that of an early-late gate and square-law symbol synchronizers. >

PN code acquisition for the spread-spectrum optical on-off-keying communication system

A model for spread-spectrum (SS) communication using a direct-detection optical on-off-keying (OOK) channel is proposed and investigated. In this model, the intensity of the transmitted optical field is modulated by a pseudonoise (PN) code and random binary data symbols. Two PN code acquisition models, namely the maximum-likelihood (ML) and the single dwell-time (SDT) models, are proposed. The performances of the ML and the SDT models are obtained in terms of the probability of missed acquisition, the probability of false alarm, and the probability of missed detection under the Gaussian approximation. The mean acquisition times are obtained and used to provide a comparison between the proposed ML and SDT models.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>