Electrical Dispersion Compensation Research Articles

Modified Gerchberg-Saxton Algorithm Based Electrical Dispersion Pre-Compensation for Intensity-modulation and Direct-detection Systems

In intensity-modulation and direct-detection (IM-DD) communication systems, electrical chromatic dispersion (CD) compensation cannot be directly applied in the transceivers due to the absence of phase information. Gerchberg-Saxton (GS) algorithm has been utilized to iteratively solve for real-valued CD pre-compensated signal in an IM-DD system. In this scheme, the amplitude information at the transmitter and phase information before the direct detection receiver are treated as the degree of freedoms (DoFs) and jointly optimized. In this paper, a modified GS algorithm is proposed and experimentally demonstrated for CD pre-compensation in a 28-GBaud PAM-4 system over 80-km single mode fiber (SMF) transmission. Compared with the basic GS electrical dispersion pre-compensation (pre-EDC) scheme, the amplitude information of received samples at the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T/2</i> position is regarded as DoFs as well, in order to reduce the error. Besides, to accelerate the convergence speed, two error reversing factors associated with the phase error at the transmitter and amplitude error at the receiver are introduced. The experimental results show that the bit error rate (BERs) of 56-Gbps PAM-4 signal over 80-km SMF transmission without CD compensation and with 10 iterations of basic GS pre-EDC scheme are 2.35×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> and 5.13×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−2</sup> , respectively, when a 21-taps feedforward equalizer (FFE) is equipped at the receiver side. While the BER can be reduced to below the hard-decision forward error correction (HD-FEC) threshold of 3.8×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3</sup> by the proposed scheme with the iterative number of 10. The results reveal that the proposed scheme is a promising solution for 50-Gbps class 80-km metro optical communication links based on the IM-DD structure.

Impact of Equalization-Enhanced Phase Noise on Digital Nonlinearity Compensation in High-Capacity Optical Communication Systems.

Equalization-enhanced phase noise (EEPN) can severely degrade the performance of long-haul optical fiber transmission systems. In this paper, the impact of EEPN in Nyquist-spaced dual-polarization quadrature phase shift keying (DP-QPSK), dual-polarization 16-ary quadrature amplitude modulation (DP-16QAM), and DP-64QAM optical transmission systems is investigated considering the use of electrical dispersion compensation (EDC) and multi-channel digital backpropagation (MC-DBP). Our results demonstrate that full-field DBP (FF-DBP) is more susceptible to EEPN compared to single-channel and partial-bandwidth DBP. EEPN-induced distortions become more significant with the increase of the local oscillator (LO) laser linewidth, and this results in degradations in bit-error-rates (BERs), achievable information rates (AIRs), and AIR-distance products in optical communication systems. Transmission systems using higher-order modulation formats can enhance information rates and spectral efficiencies, but will be more seriously degraded by EEPN. It is found that degradations on AIRs, for the investigated FF-DBP schemes, in the DP-QPSK, the DP-16QAM, and the DP-64QAM systems are 0.07 Tbit/s, 0.11 Tbit/s, and 0.57 Tbit/s, respectively, due to the EEPN with an LO laser linewidth of 1 MHz. It is also seen that the selection of a higher-quality LO laser can significantly reduce the bandwidth requirement and the computational complexity in the MC-DBP.

Dual-Drive Mach-Zehnder Modulator-Based Single Side-Band Modulation Direct Detection System Without Signal-to-Signal Beating Interference

Kramers-Kronig (KK) receiver based optical single sideband (SSB) direct detection (DD) systems have been extensively investigated to cope with signal-to-signal beating interference (SSBI) for long-reach optical interconnect and 400G ZR. In this paper, we analyze the principle of dual-drive Mach-Zehnder modulator (DDMZM)-based SSB modulation due to its low cost compared with IQ modulator. We find that SSBI does not exist in back-to-back (BTB) after square-law direct detection, and electrical dispersion compensation (EDC) cannot be realized at the transmitter due to the nonlinear modulation of DDMZM. In our proposed scheme, Cartesian to Polar conversion (CPC) is used to achieve the linear modulation of DDMZM, then pre-EDC is achievable. Signal construction operation is applied to generate a target signal avoiding SSBI in DDMZM-based SSB modulation with direct detection, thus KK algorithm is not necessary both in BTB and after fiber transmission with pre-EDC, and the computation complexity of the receiver can be effectively reduced. In the simulation results of 25 GHz SSB Discrete Fourier transform spread discrete multi-tone (DFT-S DMT), the BER of our proposed scheme and conventional DDMZM scheme with optical signal-to-noise ratio (OSNR) of 26 dB are 1.2 × 10−3 and 1.7 × 10−2, respectively, after 80 km standard single mode fiber (SSMF) transmission with pre-EDC. The simulation results show that our proposed scheme is better than conventional DDMZM scheme. Although it is still inferior to the conventional IQ scheme with KK algorithm, the computation complexity of CPC is much lower than KK algorithm as it saves the numbers of adders and multipliers by the factors of 119 and 62, respectively, and reduces the memory size by a factor of 4.3.

Electronic Dispersion Precompensation of Direct-Detected NRZ Using Analog Filtering

We demonstrate (in real-time) electrical dispersion compensation in direct detection links using analog transmit side filtering techniques. By this means, we extend the fiber reach using a low complexity solution while avoiding digital preprocessing and digital-to-analog converters (DACs) which are commonly used nowadays. Modulation is done using an IQ Mach-Zehnder modulator (MZM) which allows straightforward compensation of the complex impulse response caused by chromatic dispersion in the fiber. A SiGe BiCMOS 5-tap analog complex finite impulse response (FIR) filter chip and/or a delay between both driving signals of the MZMs is proposed for the filter implementation. Several link experiments are conducted in C-band where transmission up to 60 km of standard single-mode fiber (SSMF) of direct detected 28 Gb/s NRZ/OOK is demonstrated. The presented technique can be used in applications where low power consumption is critical.

1.6 Tb/s Virtual-Carrier Assisted WDM Direct Detection Transmission Over 1200 km

Single-sideband (SSB) self-coherent detection (SCD) has attracted a lot of attention recently as a promising technology for short and medium reach optical transmission systems, such as data center interconnect (DCI) and metro applications, due to the low cost, the compact module size, and the possibility of electrical dispersion compensation. In addition, it has been shown that the SSB SCD can be effectively realized using a “virtual carrier,” which is generated digitally together with the information-bearing signal to significantly simplify the hardware implementation. In this paper, we present an experimental demonstration of an 8 × 256 Gb/s WDM SSB SCD transmission with virtual carriers over a record distance of 1200 km with Corning TXF fiber. To achieve this performance, two digital linearization techniques have been considered and compared, namely the Kramers–Kronig (KK) field reconstruction and iterative signal-signal beat interference cancellation. Furthermore, we introduce a novel scheme called dc-offset KK to minimize the performance penalty associated with filtering and resampling process of the conventional KK scheme.

IM/DD Transmission Techniques for Emerging 5G Fronthaul, DCI, and Metro Applications

For short range optical transmissions, the intensity modulation and direct detection (IM/DD) offers the advantages of low cost and low complexity. In this paper, we will review our recent investigation results about IM/DD schemes such as polybinary and discrete multitone (DMT). Polybinary four-level pulse amplitude modulation (PAM-4) system shows better performance than the conventional PAM-4 system for both multimode and single-mode fibers. If the electrical dispersion compensation (EDC) is conducted with DDMZM, the reachable distance can be extended to 80 km which is suitable for the metro access scenario. Single sideband (SSB) polybinary-PAM-4 is also feasible to greatly increase the spectral efficiency (SE) and system capacity. Also, some DMT architectures for metro applications without using dispersion compensating modules will be introduced. SSB-DMT is very simple, while chromatic dispersion (CD) calculation and pre-compensation are not necessary. EDC double sideband (DSB) DMT owns 3-dB better performance than the SSB-DMT. CD can be calculated according to the power fading and the result is sent back to the transmitter for pre-compensation. Twin-SSB-DMT has the highest SE. More than 200-Gb/s data have been successfully generated and transmitted over 80 km single mode fiber (SMF).

Spectrally Fragmented Electrical Dispersion Compensation for High-Speed Microstrip Traces in Data Centers Connections

The frequency-dependent dispersion impairment of high-speed printed analog traces in next-generation pluggable modules is studied. This frequency dependence is considered for the first time in optical fiber communication systems. Transmission of ultra-broadband electrical signals over such traces dramatically enhances the frequency-dependent dispersion effect. Here, for the first time, an extended model of the ultrabroadband signal transmission that includes this frequency-dependent dispersion effect of the printed analog traces is proposed. Additionally, a novel approach to compensate for this frequency-dependent dispersion impairment is introduced. Inclusive analysis reveals that a significant performance improvement of up to 4 dB is achieved by using the proposed symbol-spaced sampling and reduced-complexity digital signal processing technique.

Analogue‐to‐digital converter phase insensitive dispersion search method with large search step size for low computational cost

Accumulated chromatic dispersion (CD) estimation is crucial for frequency-domain equalisation in an optical coherent digital receiver. To solve the problems of small search step size and analogue-to-digital converter (ADC) sampling phase sensitivity from conventional CD searching algorithm, a CD searching method with modified shift parameters and waveforms is proposed in this study. The modified CD searching method allows a much larger search step size and can achieve faster CD estimation with low computational complexity. The performance of the modified CD searching method is also insensitive to ADC sampling phase. On the basis of simulation results, a very low search error probability is confirmed with a search step size of 50 km. About 1 K symbols are found to be enough for dispersion estimation. After the frequency-domain equaliser (FDE) with the estimated dispersion value, the residual dispersion length <30 km can be well compensated with a 9-tap adaptive time-domain equaliser (TDE) with fast convergence. The overall error rate performance of the tunable electrical dispersion compensation is also studied and compared with lumped and distributed optical dispersion compensation for dynamically switched optical networks.

Effect of Optical Filtering for Error-free Transmission in Optical Networks

This paper presents a new approach to maximize the dispersion-limited distance of vertical cavity surface emitting lasers (VCSELs) operating at 1550.127 nm with 10 Gb/s bit rate. Optical non return to zero (NRZ) signal is generated by using an optical band pass filter (OBPF) along with MachZehnder interferometer (MZI) at the output of VCSEL. A third order super-Gaussian and fourth order Butterworth filters are used as optical filters. The dependence of bit error rate (BER), the eye opening and transmission distance on variations in the optical filter and MZI parameters is investigated. The numerical results signify that with the proposed scheme one can enhance the single mode fiber (SMF) link-length to 127 km in the absence of electrical or optical dispersion compensation methods. Results are compared between the proposed method and transmitters implemented with VCSEL diode. General Terms Optical communications.

A DOP feedback controlling multi-stage electrical PMD compensator in digital coherent receiver

We proposed a degree of polarization (DOP) controlling multi-stage electrical polarization mode dispersion (PMD) compensator in digital coherent receiver. The compensator is modulation format independent and can mitigate both first order and higher order PMD. We evaluated this PMD compensator in both 100-Gb/s 16-QAM and QPSK signal transmission systems with 15ps and 20ps average differential group delay (DGD) respectively. The results show that, for both two cases, less than 0.2dB optical signal to noise ratio (OSNR) penalty at 1e−3 symbol error rate (SER) can be achieved after 4-stage PMD compensation.

A Comparison between Electrical and Optical Chromatic Dispersion Compensation in Wavelength Divison Multiplexing Network Regarding to Electrical Pulse Shapes

Problem statement: Besides of some impairment that has been inherited form single channel, specialists confronted with new obstacles in WDM implementation which bared them for achieving desired performance. Although Chromatic Dispersion (CD) exists in single channel too, it can worsens new nonlinearities which are occurs just in WDM systems. So CD compensation in WDM is even more vital than single channel one. Approach: A wide range of optical and electrical CD compensation techniques have been represented. In this study we evaluate the efficiency of feed Forward-Decision Feedback Equalizer (FFE-DFE) as one type of electrical compensation methods and Dispersion Compensation Fiber as one type of optical compensator. Also we will look how electrical pulse shapes induced by pulse generator in transmitter, can impact on the performance of the either optical or electrical compensations. Results: After implementation, it was revealed that overall optical compensation with DCF gives us better performance than electrical equalizer and NRZ is more vulnerable than two other pulse shapes. Conclusion: Chromatic compensation was implemented with different pulse shapes and RZ pulse shape with optical compensation showed the best performance.

The Role of Electrical Pulse Shapes in Electrical and Optical Chromatic Dispersion Compensation

Problem statement: The performance of the optical network is highly dependent to Chromatic Dispersion (CD). CD has been a big issue when higher bitrate or longer distance is desired. Many methods have been represented to mitigate or eliminate this phenomenon. A wide range of optical and electrical compensation has been investigated. Approach: In this study we evaluate the efficiency of the electrical pulse shapes including (NRZ,RZ and Triangular form) in electrical compensation with Feed Forward-Decision Feedback Equalizer (FFE-DFE) technique and optical compensation; Dispersion Compensation Fiber. Results: It was compared the mentioned situation with Quality factor versus increasing the distance with different pulse shapes. It was observed that in Electrical compensation there is no considerable with different pulse shapes while in optical compensation NRZ is definitely more susceptible than others. Optical and electrical compensators were compared regarding to jitter and optical compensator showed much better performance. Conclusion: chromatic compensation was implemented with different pulse shapes and RZ pulse shape with optical compensation showed best performance.

应用于100Gb/s长距离PDM-CO-OFDM系统中的电域补偿算法的研究

We study an electronic compensator (EC) as a receiver for a 100-Gb/s polarization division multiplexing coherent optical orthogonal frequency division multiplexing (PDM-CO-OFDM) system without optical dispersion compensation. EC, including electrical dispersion compensation (EDC), least squares channel estimation and compensation (LSCEC), and phase compensation (PC), is used to compensate for chromatic dispersion (CD), phase noise, polarization mode dispersion (PMD), and channel impairments, respectively. Simulations show that EC is highly effective in compensating for those impairments and that the performance is close to the theoretical limitation of optical signal-to-noise rate (OSNR), CD, and PMD. Its robustness against those transmission impairments and fiber nonlinearity are also systematically studied.

A 1.6–3.2-GHz Sixth-Order ${+}$13.1-dBm OIP3 Linear Phase ${ g}_{ m}$-$C$ Filter for Fiber-Optic EDC Receivers

A sixth-order low-pass g m-C filter for fiber-optic electrical dispersion compensation receivers cascades three biquads permitting a reduction of group delay variation down to 10 ps. A cutoff frequency is tuned from 1.6 to 3.2 GHz by using two methods, namely, changing gm and switching a CMOS varactor's polarity. Output third-order intercept point reaches 13.1 dBm and total harmonic distortion is below -40 dB at 0.9 V pp-diff output. The continuous time filter is implemented in a 0.18-?m SiGe process, it occupies 0.17 mm2 on a 3.2 × 3.7 mm2 test chip and consumes 0.3 W from a 3.3-V supply.

Optical Coherent Receiver With a Switchable Electrical Dispersion Compensator for 10 Gb/s DPSK Transmission up to 300 km of SSMF in Metro Optical Networks

The purpose of this paper is to demonstrate the potential of an electrical signal processing technique to mitigate the effect of intersymbol interference in metro optical networks, using optical coherent detection. Intersymbol interference in fiber-optic systems can severely degrade the system performance and subsequently limit both the maximum transmission distance and data rate. The main sources of intersymbol interference include chromatic dispersion (CD) and polarization mode dispersion (PMD). The typical reach for 10 Gb/s metro optical systems is on the order of 300 km. To bridge this distance either with or without repeaters, we demonstrate the potential use of 10 Gb/s differential phase-shift keying (DPSK) modulation format in conjunction with an optical coherent receiver with a built-in switchable electrical dispersion compensator. The overall performance of the optical coherent receiver is compared with that of the corresponding direct detection (DD) receiver based on Mach-Zehnder delay interferometer. The optical signal-to-noise (OSNR) required to operate the optical coherent receiver at the enhanced forward error correction (EFEC) threshold of 2.1 X 10-3 is 6 dB/0.1 nm resolution bandwidth. The optical coherent receiver outperforms in absolute required OSNR by 0.5 dB at the EFEC threshold when compared to a DD receiver. A four position switchable CD compensator compensates up to ?5100 ps/nm of CD, using the method of heterodyne detection. The optical coherent receiver has a first-order PMD tolerance of 36.2 ps for 1 dB OSNR penalty. The utility of our optical coherent receiver is demonstrated by doing transmission experiments over 304 km of standard single-mode fiber (SSMF) in both repeatered (multiple spans) and repeaterless (single span) to measure the self-phase modulation (SPM) tolerance of 10 Gb/s nonreturn-to-zero (NRZ)-DPSK. The measured SPM tolerance of NRZ-DPSK over 304 km of SSMF in three spans of ~100 km each is +8.8 dBm fiber launch power for 1.5 dB OSNR penalty. In addition, the best Q factor of ~12 dB is obtained for the fiber launch power of +18 dBm over single repeaterless span of 304 km of SSMF, which gives ~3 dB of system margin at the second generation EFEC threshold of 9.1 dB.

400Gb/s (4 × 100Gb/s) orthogonal PDM-RZ-QPSK DWDM Signal Transmission over 1040km SMF-28

We have generated 4 x 100-Gb/s orthogonal WDM optical signal by employing polarization-division-multiplexed (PDM) return-to-zero (RZ) QPSK modulation format and tight optical filtering technique. The required optical signal-to-noise ratio (OSNR) at bit error ratio (BER) of 2 x 10(-3) for the 400 Gb/s orthogonal DWDM signal is measured to be approximately 22.8 dB/0.1 nm. After transmission over 1040-km standard single mode fiber (EDFA-only amplification, 80-km amplifier span and fully receiver-side electrical dispersion compensation), the measured BER for all the four orthogonal subchannels are smaller than 2 x 10(-3).

A Comparison of Electrical and Optical Dispersion Compensation for 111-Gb/s POLMUX–RZ–DQPSK

In this paper, we experimentally investigate the tolerance of 111-Gb/s polarization multiplexed return-to-zero differential quadrature phase-shift keying (POLMUX-RZ-DQPSK) signals with respect to nonlinear impairments in a link with either (inline) optical or (receiver-side) electrical dispersion compensation. After transmission of more than 2000 km of LEAF, a comparable nonlinear tolerance is observed for both dispersion maps. However, we observe that in the presence of differential group delay (DGD) the nonlinear tolerance of the transmission link with in-line optical dispersion compensation is reduced significantly, whereas the nonlinear tolerance of the configuration with receiver-side dispersion compensation is virtually unaffected.

Wiener solution of electrical equalizer coefficients in lightwave systems

Electrical dispersion compensation equalizer is a key and cost-effective element in optical communication on-off-keying systems in the presence of chromatic dispersion. Here, for the first time, to the best of the author's knowledge, an analytical solution is established for the electrical equalizer coefficients in an optical communication system. The solution is based on minimum mean-square error criterion. The analytical results show a perfect match with computer simulation. In addition BER performance comparison with the adaptive least mean square (LMS) method reveals that the analytical solution performs better due to LMS excess mean-square error.

Electrical Dispersion Compensation for 40-Gb/s DQPSK Signal Utilizing MIMO DFEs

We investigate electrical dispersion compensation for 40-Gb/s differential quadrature phase-shift keying modulation signals utilizing multi-input-multi-output (MIMO) decision-feedback equalizers (DFEs). In our scheme, all branch signals after balanced-photodiode detection or single-photodiode detection are analogous to antennae in a MIMO wireless transmission systems. Chromatic dispersion and polarization-mode dispersion tolerances for various MIMO DFE configurations are compared. The flexible scheme can be easily extended to <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</i> -ary PSK modulation formats.

Use of return-to-zero formats in electrical dispersion compensated optical single sideband transmission systems

The use of return-to-zero (RZ) formats is proposed to enhance the dispersion tolerance of intensity-modulated carrier-unsuppressed (IM-CUs) optical single sideband (oSSB) systems. The performance of these systems, employing electrical dispersion compensation (EDC), is accessed using unipolar and polar RZ signalling formats, and compared with common non-return-to-zero (NRZ). Simulation results at 10.7 Gb/s reveal that oSSB systems with EDC using both NRZ and unipolar RZ signalling formats are limited to low extinction ratio (ER) values. However, the use of polar RZ allows increased ER and transmission distances, because of higher inter-symbol interference tolerance and the absence of discrete spectral tones. Considering a minimum ER of 9 dB and resort to forward error correction schemes, polar RZ allows single-channel error free transmission over 1400 km of standard single-mode fibre, substantially improving the 980 km obtained for NRZ with ER of 6 dB. Dense wavelength-division multiplexing transmission was also assessed, retrieving improved resilience of IM-CUs oSSB polar RZ.