Optical Single Sideband Signal Research Articles

Photonic generation of ASK microwave signals with SSB format.

Optical beating is the usual approach to generation of microwave signals. However, the highest frequency achievable for microwave signals is limited by the bandwidths of optoelectronic devices. To maximize the microwave frequency with a limited bandwidth of a photodetector (PD) and relieve the bandwidth bottleneck, we propose to generate microwave signals with the single sideband (SSB) format by beating a continuous wave (CW) light with an optical SSB signal. By simply adjusting the frequency difference between the CW light and the carrier of the optical SSB signal, the frequency of the generated microwave SSB signal is changed correspondingly. In the experiment, amplitude shift keying (ASK) microwave signals with the SSB format are successfully generated with different carrier frequencies and coding bit rates, and the recovered coding information agrees well with the original pseudo random binary sequence (PRBS) of 27 - 1 bits. The proposed approach can significantly relieve the bandwidth restriction set by optoelectronic devices in high-speed microwave communication systems.

Simultaneous RF Self-Interference Cancellation, Local Oscillator Generation, Frequency up- and down-Conversion in an Integrated In-Band Full-Duplex 5G RF Transceiver Front-End

Wideband radio-frequency (RF) self-interference cancellation (SIC), local oscillator (LO) generation, frequency up-, and down-conversion are all crucial for an in-band full-duplex (IBFD) 5G RF transceiver front-end. In this paper, a microwave photonic approach enabled by only one optical dual-polarization IQ modulator (DP-IQMZM) is proposed to simultaneously realize wideband RF SIC, LO generation, frequency up- and down-conversion. Note that, the LO signal with high spectral purity and low phase noise is produced by a dual-loop optoelectronic oscillator (OEO) using a self-polarization stabilization structure, and the generated LO signal is applied for frequency up- and down-conversion. Besides, the optical single-sideband signal is produced at the output of this 5G RF transceiver for uplink transmission from the remote antenna unit (RAU) to the baseband unit (BBU), so the power fading induced by fiber dispersion could be completely suppressed. A proof-of-concept experiment is performed, and the LO signal with the central frequency of 10 GHz, the phase noise performance of −108.97 dBc/Hz at a frequency offset of 10 kHz is achieved. A 5 × 20 MHz Long Term Evolution (LTE) 16/64-ary quadrature amplitude modulation-orthogonal frequency division multiplexing (16/64QAM-OFDM) signal with the central frequency of 12.6 GHz is down-converted to 2.6 GHz. Meanwhile, the cancellation ratio of 30.22 dB and 26.26 dB could be achieved when the bandwidth of the self-interference signal is 150 MHz and 300 MHz, respectively. Moreover, a 3 × 20 MHz LTE 64QAM-OFDM signal with a central frequency of 2.6 GHz is up-converted to 12.6 GHz, and the electrical signal-to-noise ratio of the generated RF signal can achieve 31.34 dB. The measured error vector magnitude performance shows that the generated down- and up-converted signal both meet the 3^rd Generation Partnership Project (3GPP) requirements for 5G systems.

QAM Vector mm-Wave Signal Generation Based on Optical Orthogonal Polarization SSB Scheme By a Single Modulator

Millimeter-wave (mm-wave) has become an attractive technique in the fifth generation (5G) because it meets the demand of explosive increase in data traffic. In this study, we propose and demonstrate the generation of quadrature amplitude modulated (QAM) vector mm-wave signal employing optical orthogonal polarization single sideband (SSB) modulation, which employs a single polarization-division multiplexing MachZehnder modulator (PDM-MZM). Based on our scheme, there is no inter-band crosstalk and inter-polarization signal-signal beat interference (SSBI). In our scheme, the two sub-modulators in PDM-MZM both work within their linear modulation range. After PDM-MZM, the two optical SSB signals are orthogonally polarized. By using an interleaver (IL), the low sideband (LSB) and upper sideband (USB) optical signals in the two polarization directions are extracted respectively. In addition, IL also suppresses the unwanted components of the opposite orthogonally polarized signal. We demonstrate the generation of two independent 28 GHz 12 Gbaud quadrature-phase-shift-keying (QPSK) and 6 Gbaud 16-ary quadrature amplitude modulation (16-QAM) signals. 65 km single-mode fiber (SMF) transmission is achieved with the bit-error-rate (BER) below 20% forward-error-correction (FEC) soft-decision threshold of 2.4 102 without dispersion compensation. We also demonstrate the generation of two independent 28GHz 1 Gbaud 64-QAM signals and transmission SMF over 10-km with the BER below 20% FEC soft-decision threshold of 2.4 102. Since the LSB and USB signals are on opposite polarization sides, the inter-band crosstalk and the inter-polarization SSBI can be significantly reduced. Because SSB modulation scheme is deployed, our proposed QAM vector signal generation system is robust to fiber dispersion.

Generation of Broadband Optical SSB Signal Using Dual Modulation of DML and EAM

The dual modulation transmitter, where both the directly modulated laser (DML) and electro-absorption modulator (EAM) are modulated, has attracted considerable attention due to its monolithic integration and high output power. This transmitter is capable of manipulating the optical intensity and phase independently since the DML and EAM serve mainly as phase and intensity modulators, respectively. Thus, the dual modulation scheme has been used to generate optical single sideband (SSB) signals. However, due to a lack of workable model applicable to high modulation frequency (e.g., >10 GHz), the data rates of optical SSB signals reported previously were all limited to ≤40 Gb/s. In this paper, we generate a 120-Gb/s optical SSB orthogonal frequency-division multiplexed (OFDM) signal using the dual modulation scheme. By using the time-harmonic model of dual modulation we have recently reported, we establish a generation procedure for broadband optical SSB signals, where the modulation conditions for DML and EAM are provided. In order to satisfy these conditions over a broad frequency range, we also propose (1) utilizing the frequency-modulation discriminator to measure the DML's chirp characteristics and (2) pre-compensating for the frequency responses of DML and EAM. In our experimental verification, we generate the 120-Gb/s SSB OFDM signal having an optical sideband suppression ratio of 19 dB. We also successfully transmit this signal over 80 km long standard single-mode fiber.

A Robust Sparse RLS-Volterra Nonlinear Equalizer Using ℓ₀-Regularization for 4 × 150 Gbit/s IMDD-Based Optical Interconnect

Volterra equalizer (VE) is a well-known and effective algorithm to deal with the linear and nonlinear distortions in optical interconnect, but the high computational complexity hinders its practical application. Generally, sparse VEs based on ℓ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> - or ℓ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> -regularization are good ways to reduce the complexity by discarding some inessential taps. However, a tap threshold needs to be chosen in these sparse VEs like the threshold-based pruned retraining VE (TR-VE) to decide the discarded taps. And this tap threshold should be adjusted fine to balance the reduced complexity and equalization performance, especially when the testing environments alter. Thus, the reduced complexity in these sparse VEs may fluctuate. To address this issue, a robust and stable complexity reduced sparse VE using ℓ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> -regularization (ℓ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> -SR-VE) is proposed in this paper. The recursive least square (RLS) algorithm is used to replace the least mean square algorithm for the faster convergence speed and better equalization performance. The complexity of this equalizer depends on its parameters but not the tap threshold. Once the equalizer parameters are determined, the complexity would not change with the system characteristics, contributing to higher practicability. In our experiment, a 150 Gbit/s PAM8 signal transmission system based on intensity modulation and direct detection (IMDD) is achieved, and a dual-drive Mach-Zehnder modulator for optical single-sideband signal generation is used to mitigate the power fading effect. The experimental results show that with the help of ℓ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> -SR-VE, the reduced complexity percentage is stable at 66.18% compared with the RLS-based VE, even after 75 km standard single-mode fiber (SSMF) transmission. By using this equalizer, 4×150 Gbit/s PAM8 signals have also been successfully transmitted over 30 km SSMF at C-band. The reduced complexity variation of ℓ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> -TR-VE is >20%, but the reduced complexity of the proposed ℓ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> -SR-VE is stable at 60% even after 30 km SSMF for all four lanes.

Generalized model of optical single sideband generation using dual modulation of DML and EAM.

The dual modulation of a directly modulated laser (DML) and an electro-absorption modulator (EAM) is known to be an effective way to generate an optical single sideband (SSB) signal in highly compact and cost-effective fashions. Two theoretical models were developed for this dual modulation scheme under the assumptions of negligible intensity modulation of DML and/or zero chirp of EAM. In this paper, we develop a generalized model of dual modulation scheme without those assumptions. We show theoretically that the previous models can be unified as special cases of our generalized model. We evaluate the validity of our model using numerical simulations and experiments. The results show that our model estimates the modulation conditions for optical SSB generation accurately over wide ranges of modulation frequency and EAM's chirp. We also show experimentally that we can achieve an optical sideband suppression ratio higher than 40 dB by using our generalized model. This is >25 dB higher than the ratios obtained from the previous models.

Millimeter wave switching for single carrier and aggregated filter bank multi-carrier signals in radio over fiber networks

The 5G wireless communication systems promise to utilize the RF spectrum more efficiently and use advanced modulation and access techniques to provide ultra-high data rate speeds. Millimeter wave (MMW) signals and MMW based systems are playing a fundamental role in achieving these requirements. The modules and devices, proposed for these systems, are intended to use the robust capabilities of photonic technologies. In this paper, we consider MMW switching and wavelength conversion using a semiconductor optical amplifier (SOA) as a technique to support MMW transmission over fiber-based networks. In particular, we exploit the nonlinear special effects in SOA to generate a switched optical single-sideband signal that carries aggregated filter bank multicarrier (FBMC) 5G signals. First, we optimize the SOA for single FBMC band switching. Then, the switching of six FBMC aggregated signals is considered. The intention is to investigate the performance of such aggregated FBMC signals after applying either single or cascaded wavelength conversion. The results include analyzing the performance in terms of bit error rate, optical signal-to-noise ratio, and error vector magnitude.

A Silicon Optical Single Sideband Modulator With Ultra-High Sideband Suppression Ratio

Optical single sideband (OSSB) modulation is widely used in radio-over-fiber (RoF) system, nanophotonic phase noise filter, frequency-modulated continuous wave (FMCW) lidar and so on. We demonstrate a silicon dual-parallel Mach-Zehnder modulator (DP-MZM) with tunable optical power splitting ratio to generate OSSB signal with ultra-high sideband suppression ratio (SSR). To mitigate the SSR deterioration caused by the limited extinction ratios (ER) of the two sub-MZMs due to the process tolerance and the imbalance in their RF driving powers in practical RF links, we adjust the distribution of the incident optical power between the two sub-MZMs with a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1\times 2$ </tex-math></inline-formula> switch. With this technique, the SSR measured at 20 GHz is better than 39.1 dB. A theoretical analysis is also provided to explain how the unwanted sideband is suppressed by adjusting the optical power splitting ratio.

Photonic filterless scheme to generate V-band OFDM vector mm-wave signal without precoding

A novel and relatively simple scheme for the generation of high-quality V-band (50–75 GHz) OFDM vector millimeter-wave (mm-wave) is presented, which is only enabled by a dual-parallel Mach–Zehnder modulator (DP-MZM). In this scheme, precoding technique and optical filter are not needed. At the transmitter, electrical dual single sideband (SSB) signals are generated by a digital signal processing (DSP) module, in which the frequency of modulated signals could be adjusted flexibly. The two push–pull sub-MZMs are biased at quadrature transmission points to generate dual optical single sideband (OSSB) signals. The DP-MZM is biased at null transmission point to suppress the center optical carrier and an asymmetrical OSSB signal can be obtained. Then, a higher frequency OFDM vector mm-wave signal is generated after the beating of the asymmetrical OSSB signal at the photodiode. Theoretical analysis is given out in the paper, and V-band OFDM vector mm-wave at the frequency of 65 GHz is generated in the simulation. The results show that the bit-error ratio (BER) of the transmission system is smaller than forward-error correction (FEC) threshold of 3.8×10−3, when the optical power into photodiode (PD) of 10/20/30 Gbaud cases are not less than −19.13 dB, −17.81 dB, −16.50 dB respectively after 50km single model fiber transmission.

Digital fronthaul link based on optimised digital radio over fibre system

The high optical bandwidth requirement in the mobile fronthaul link of the next generation cloud radio access network represents one of the major design constraints. In this study, a digital fronthaul link based on digital radio over fibre system that integrates the duobinary coding scheme with the optical single sideband (OSSB) transmission is proposed. This system reduces the spectral occupancy of the transmitted signals as well as the chromatic dispersion effects induced by fibre in both the electrical and optical domains. The OSSB signal is created by driving two cascaded optical modulators with a combination of a baseband digital signal and the Hilbert transform of that signal. The transmission performance of a digitised 16-quadrature amplitude modulation 16-QAM radio frequency signal with a bit rate of 1.25 Gb/s over a standard single mode fibre is investigated. The results show that the fronthaul transmission length can be extended by 75 and 16.6% relative to the case of transmitting the digitised signal using double sideband and single sideband (SSB) transmission formats, respectively. The power budget is improved by 10 dB compared to the SSB case with respect to the measured optical signal-to-noise ratio at 70 km fibre distance, while maintaining the EVM below the standardised values of the 3GPP.

SFDR Enhancement of 120° Phase Angle-Based RoF Link by Using Linear Polarizers

A new scheme to improve spurious free dynamic range (SFDR) of high-performance radio over fiber (RoF) link based on 120° phase angle has been proposed and investigated. The proposed link consists of a dual electrode Mach–Zehender modulator (DE-MZM), linear polarizers, and optical filter. The performance is improved by generating an optical single sideband (OSSB) signal using 120° phase angle in DE-MZM. The suppression of third-order intermodulation (IM3) and other harmonics are significantly controlled by adjusting the state of polarization of both the polarizers. The resulting SFDR shows an improvement of 13.4 dB for the proposed linearized link when compared with conventional 90° phase angle-based DE-MZM link.

Experimental demonstration of optical single sideband signal generation based on multi-frequency phase modulation in optical fibers

We theoretically analyzed and experimentally demonstrated the feasibility of optical single sideband (OSSB) signal generation based on multi-frequency phase modulation. The numerically calculated results show that OSSB signal can be efficiently generated with the proposed method. Optical carrier suppression ratio (OCSR), optical sideband suppression ratio (OSSR) and energy efficiency (EF) of the modulated optical signal increase with the quantity of harmonic waves in the modulation signal. In a principle verifying experiment, 6 frequency phase modulation was carried out with a narrow line-width fiber laser and a LiNbO3 electro-optic phase modulator. The fundamental frequency of the modulation signal was controlled to 30 MHz. OCSR of 18.9 dB, OSSR of 14.1 dB and EF of 78% were obtained, respectively. The experimental results agree well with the numerically calculated results. The proposed scheme provides an effective way to generate an OSSB signal for optical communication or optical sensing systems.

Analysis of a Multibeam Optical Beamforming Network Based on Blass Matrix Architecture

We present an extensive analysis of an optical Blass-matrix architecture as a beamforming network with potential for multibeam operation in wireless systems. Its design relies on the use of phase shifters and Mach-Zehnder Interferometers (MZIs) inside an $M\times N$ matrix, and enables the generation of M beams by N-element antenna arrays. We start our analysis from an optical signal with amplitude modulation by discrete microwave tones, and confirm the possibility to translate its optical phase shifts inside the matrix into equivalent phase shifts in the microwave domain. We show this is possible when the input is an optical single-side band signal and the optical carrier is reinserted before photodetection. We extend the conclusions to the case of an optical signal carrying a microwave with quadrature amplitude modulation (QAM) and the case of simultaneous inputs at the M input ports. Based on this analysis, we propose a Blass-matrix configuration algorithm taking into account the properties of the MZIs. Through simulations, we validate the potential for multibeam operation, and evaluate the beamforming performance at 28.5 GHz with respect to the QAM order, symbol rate, and pulse shaping parameters. In all cases with rate up to 3 Gbaud, the bit-error rate remains lower than 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> , showing that the beam squinting effect, which is present in our design, can be tolerated. Finally, we study the frequency dependence of the beamforming performance due to inevitable asymmetries of the MZIs and length variations of the waveguides, and evaluate the impact of the imperfections in the couplers inside the MZIs and the phase shifters. We show that in all cases the performance degradation is negligible for realistic fabrication and operation conditions.

Experimental characterization of the radio over fiber aided twin-antenna spatial modulation downlink.

In this paper, we present the design and the experimental demonstration of a radio over fiber (RoF) network relying on state-of-the-art spatial modulation (SM), that activates one out of multiple antennas. We propose a novel RoF-aided SM encoding scheme, where the optical single side-band signal generated by a Mach-Zehnder modulator (MZM) is used for both the antenna selection and for the classic modulated symbol selection. The SM encoding is optically processed in a centralized fashion, aiming for the reduction of power consumption and for enabling cost-effective maintenance and management, which can be employed in the context of a cloud radio access network (C-RAN) and a small-cell front-haul. Furthermore, an experimental demonstration of the proposed system is discussed and analyzed, where a 20 km standard single mode fiber (SSMF) is used for transmission. In this experiment, a 2 Gbps transmission relying on two transmit and two receive antennas is achieved with less than 1 dB SNR degradation compared to those operating without RoF.

Optical single side-band Nyquist PAM-4 transmission using dual-drive MZM modulation and direct detection.

We present the design and optimization of the optical single side-band (SSB) Nyquist four-level pulse amplitude modulation (PAM-4) transmission using dual-drive Mach-Zehnder modulator (DDMZM)modulation and direct detection (DD), aiming at the C-band cost-effective, high-speed and long-distance transmission. At the transmitter, the laser line width should be small to avoid the phase noise to amplitude noise conversion and equalization-enhanced phase noise due to the large chromatic dispersion (CD). The optical SSB signal is generated after optimizing the optical modulation index (OMI) and hence the minimum phase condition which is required by the Kramers-Kronig (KK) receiver can also be satisfied. At the receiver, a simple AC-coupled photodiode (PD) is used and a virtual carrier is added for the KK operation to alleviate the signal-to-signal beating interference (SSBI).A Volterra filter (VF) is cascaded for remaining nonlinearities mitigation. When the fiber nonlinearity becomes significant, we elect to use an optical band-pass filter with offset filtering. It can suppress the simulated Brillouin scattering and the conjugated distortion by filtering out the imaging frequency components. With our design and optimization, we achieve single-channel, single polarization 102.4-Gb/s Nyquist PAM-4 over 800-km standard single-mode fiber (SSMF).

Radar signal transmission and switching over optical networks

In this paper, we experimentally demonstrate a radar signal distribution over optical networks. The use of fiber enables us to distribute radar signals to distant sites with a low power loss. Moreover, fiber networks can reduce the radar system cost, by sharing precise and expensive radar signal generation and processing equipment. In order to overcome the bandwidth challenges in electrical switches, a semiconductor optical amplifier (SOA) is used as an all-optical device for wavelength conversion to the desired port (or channel) of a wavelength division multiplexing (WDM) network. Moreover, the effect of chromatic dispersion in double sideband (DSB) signals is combated by generating optical single sideband (OSSB) signals. The optimal values of the SOA device parameters required to generate an OSSB with a high sideband suppression ratio (SSR) are determined. We considered various parameters such as injection current, pump power, and probe power. In addition, the effect of signal wavelength conversion and transmission over fiber are studied in terms of signal dynamic range.

Optical single sideband millimeter-wave signal generation and transmission using 120° hybrid coupler

We propose a novel 60 GHz optical single sideband (OSSB) millimeter-wave (mm-wave) signal generation scheme using 120° hybrid coupler based on external integrated Mach–Zehnder modulator (MZM). The proposed scheme shows that the bit error ratio (BER) performance is improved by suppressing the +2nd-order sideband. Meanwhile, the transmission distance is extended as only the optical +1st-order sideband is modulated by using 5 Gbit/s baseband signal while the carrier is blank, owing to the elimination of walk-off effect suffered from fiber dispersion. The simulation results demonstrated that the eye diagrams of the generated 60 GHz OSSB signal keep open and clear after 100 km standard single-mode fiber (SSMF). In addition, the proposed scheme can achieve 2 dB receiver sensitivity improvements than the conventional 90° hybrid coupler when transmitted over 100 km SSMF at a BER of 10−9.

Experimental demonstration of an efficient hybrid equalizer for short-reach optical SSB systems

We propose an efficient enhanced hybrid equalizer combining the feed forward equalization (FFE) with a modified Volterra filter to mitigate the linear and nonlinear interference for the short-reach optical single side-band (SSB) system. The optical SSB signal is generated by a relatively low-cost dual-drive Mach–Zehnder modulator (DDMZM). The two driving signals are a pair of Hilbert signals with Nyquist pulse-shaped four-level pulse amplitude modulation (NPAM-4). After the fiber transmission, the neighboring received symbols are strongly correlated due to the pulse spreading in time domain caused by the chromatic dispersion (CD). At the receiver equalization stage, the FFE followed by higher order terms of modified Volterra filter, which utilizes the forward and backward neighboring symbols to construct the kernels with strong correlation, are used as an enhanced hybrid equalizer to mitigate the inter symbol interference (ISI) and nonlinear distortion due to the interaction of the CD and the square-law detection. We experimentally demonstrate that the optical SSB NPAM-4 signal of 40 Gb/s transmitting over 80 km standard single mode fiber (SSMF) with a bit-error-rate (BER) of 7.59×10−4.

Hilbert superposition and modified signal-to-signal beating interference cancellation for single side-band optical NPAM-4 direct-detection system.

We propose a Hilbert superposition and modified signal-to-signal beating interference (SSBI) cancellation scheme in an optical single side-band (SSB) modulation and direct-detection system. The optical SSB signal is generated by a relatively low-cost dual-drive Mach-Zehnder modulator (DDMZM). The two driving signals are a pair of Hilbert signals with Nyquist pulse-shaped four-level pulse amplitude modulation (NPAM-4). In addition to the transmitted baseband signal, both its Hilbert transform and the SSBI can also be detected by direct-detection, which introduce the interference to the transmitted signal. We use the first-stage Hilbert superposition cancellation (HSC) to cancel the unwanted Hilbert transform signal and a modified single-stage linearization filter which contains the second-stage HSC to deduct the SSBI in the receiver. We experimentally demonstrate that 40 Gb/s optical SSB NPAM-4 signal transmission over 80 km standard single mode fiber (SSMF).

A novel method to realize optical single sideband modulation with tunable optical carrier to sideband ratio

AbstractAn optical single sideband (OSSB) modulation method with tunable optical carrier to sideband ratio (OCSR) is proposed and experimentally demonstrated based on a dual‐polarization Mach–Zehnder modulator (DPol‐MZM). An orthogonally polarized double sideband (DSB) signal output from the DPol‐MZM is filtered out the carrier and one sideband utilizing an optical bandpass filter (OBPF). Then the orthogonally polarized OSSB signal is transformed into a linear polarized OSSB signal by a polarizer (Pol). By changing the polarization direction of the Pol utilizing a polarization controller (PC), the tunable OCSR can be achieved. In the demonstrated experiments, an OSSB modulation is implemented over the wide microwave frequency range of 5–30 GHz. The OCSR is tuned from −30 dB to infinity. The proposed OSSB modulation method is advantageous in many respects as large tunable OCSR range, wide operation bandwidth, and simple structure.