Sideband Signals Research Articles

Comparison of Optical Single Sideband Techniques for THz-Over-Fiber Systems

The use of single sideband (SSB) signals and envelope detection is a promising approach to enable the use of economic free-running lasers in photonic THz communications. To combat the signal–signal beat interference associated with envelope detection, broad guard bands (GBs) may be used given the large unregulated spectrum available at THz frequencies (100 GHz–10 THz). In this scenario, the conventional way of generating SSB signals through a digital SSB filter (here referred to as the CSSB scheme) would require quite high analog digital-to-analog converter (DAC) bandwidths (BW). Digital virtual SSB (DVSSB) and analog virtual SSB (AVSSB) have been proposed in direct-detection optical systems for relaxing the DAC BW requirements. In this paper, we compare the three techniques through simulations and implement them, for the first time, in a THz-over-fiber system operating at 250 GHz. For the transmission experiments, we employ 5 GBd 16-quadrature amplitude modulation signals with three different GBs (5.5, 4.75, and 3.5 GHz). The simulations show that the best performance is obtained with the AVSSB technique, while the worst is obtained with the DVSSB scheme, where the quality of the generated sideband degrades with carrier-to-sideband power ratio. In the experimental transmissions, where receiver noise was the main source of noise, similar behavior was found between the three techniques. At the 3.5 GHz GB, however, the DVSSB exhibited a penalty of 1 dB with respect to the other two. This is likely to be due to nonlinear distortions caused by the increase in the virtual tone power.

Switchable Microwave Photonic Filter Between Dual-Notch and Dual-Passband Responses

We present a microwave photonic filter (MPF) which is switchable between dual-notch and dual-passband responses. In the MPF, a dual-drive Mach–Zehnder modulator is used to generate a quasi-single-sideband or phase-modulated signal by controlling the bias of the modulator. The first-order sidebands of the modulated signal are processed by stimulated Brillouin scattering in optical fiber to build or destroy the amplitude balance of the sidebands. In this way, a dual-notch or dual-passband MPF is finally obtained. The center frequencies of the notches or passbands are symmetric about the Brillouin frequency shift. The frequency spacing between the two notches or two passbands is determined by the frequency spacing of the dual-wavelength pump. The experimental results show that the MPF can be freely switched between dual-notch and dual-passband response with wideband tunability.

A mobile crack detection system for plate-like structure based on non-linear ultrasonic fibre Bragg grating

Many researchers have highlighted the fact that non-linear ultrasonic technology is more suitable than linear ultrasonic technology for detecting micro-cracks. In most existing non-linear ultrasonic detection systems, piezoelectric patches are pasted directly onto specimens and used to detect ultrasonics. In this paper, a fibre Bragg grating (FBG) was adopted for sensing ultrasonics because FBGs possess many advantages over piezoelectric patches. Non-linear ultrasonic technology was combined with FBG sensing technology to detect cracks in thin aluminium plates. This study proposed a mobile system in which the excitation and monitoring points were easily moved on the surface of a specimen because piezoelectric patches and FBGs were glued to acrylic plates and placed in contact with the specimen surface using industrial ultrasonic couplants. The influence of acrylic plate thicknesses on the characteristics of mobile piezoelectric patches (MPPs) and FBGs was discussed. This mobile system was more flexible than traditional systems, in which piezoelectric patches and FBGs are fixed to the structures, and it did not destroy the surface of the specimen. Experimental results showed that the mobile system can detect non-linear sideband signals caused by cracks at every excitation and sensing point. The factors influencing sideband amplitude, including excitation amplitude and frequency, and the relative positions of two excitations and a FBG were studied.

Electrical spectrum synthesis technique using digital pre‐processing and ultra‐broadband electrical bandwidth doubler for high‐speed optical transmitter

Proposed is an electrical spectrum synthesis technique that converts low-speed signals to a high-speed signal using transmitter-side digital signal processing (DSP) and an ultra-broadband electrical bandwidth doubler. The transmitter-side DSP converts a high-speed signal to low-speed upper and lower sideband signals, down- and up-converts the low-speed signals into baseband signals, and adds and subtracts the in-phase and the quadrature components of the baseband signals. The digital pre-processed low-speed signals output from digital-to-analogue converters are multiplexed to the high-speed signal by the bandwidth doubler, which consists of analogue multiplexers designed and fabricated using in-house indium phosphide heterojunction bipolar transistor technologies. A 120 GBaud quadrature phase shift keying signal has been successfully generated by using the proposed technique as a demonstration of a high-speed optical transmitter.

Photonic generation of binary phase-coded microwave waveforms and phase-coded pulses with ultrawide frequency tunable range

A scheme that can produce a continuous wave (CW) phase-coded signal and phase-coded pulses is proposed. The proposed scheme is based on a dual-polarization Mach–Zehnder modulator (DPol-MZM), a polarization modulator (PolM) and a balanced photodetector (BPD) without the help of an electrical phase shifter or an electrical power divider. In DPol-MZM, a carrier-suppressed double sideband (CS-DSB) signal and an optical carrier are, respectively, produced in the upper and lower arms, which are in two orthogonal polarization states. The output is then sent to a PolM. The polarization multiplexed CS-DSB and optical carrier are phase modulated by driving the RF port of the PolM with a coding signal. After beating between the modulated CS-DSB and optical carrier in a BPD, a stable CW phase-coded signal or phase-coded pulses can be obtained. The proposed scheme is experimentally verified. Phase-coded microwave waveforms at 6 and 11 GHz are generated and phase-coded pulses at 11 GHz are also produced.

Continuously tunable orthogonally polarized RF optical single sideband generator based on micro-ring resonators

We demonstrate a continuously RF tunable orthogonally polarized optical single sideband (OP-OSSB) generator based on dual cascaded micro-ring resonators (MRRs). By splitting the input double sideband signal into an orthogonally polarized carrier and lower sideband via TE- and TM-polarized MRRs, an OP-OSSB signal is generated. A large tuning range of the optical carrier to sideband ratio of up to 57.3 dB is achieved by adjusting the polarization angle of the input light. The operation RF frequency of the OP-OSSB generator can be continuously tuned with a 21.4 GHz range via independent thermal control of the two MRRs. Our device represents a competitive approach towards OP-OSSB generation with wideband tunable RF operation, and is promising for photonic RF signal transmission and processing in radar and communication systems.

A K-Band Low-Noise and High-Gain Down-Conversion Active Mixer Using 0.18-μm CMOS Technology

This paper fabricates a K-Band 24 GHz high-gain, low-power down-conversion mixer using a standard TSMC 0.18-μm CMOS technology. The architecture that is used is based on that of a Gilbert cell mixer. Transformer coupling technology is used at the node between the transconductance stage and the local oscillator switches stage. The conversion gain, the noise figure and the size of the chip area performance are significantly better, so this mixer with specific RF parameters gives excellent performance. The simulation (post-sim) results for the proposed mixer show a 15–19 dB power conversion gain, a − 8.3 dBm input third-order intercept point (IIP3) at 24 GHz, a 10–12.3 dB signal side band (SSB) noise figure and an RF bandwidth of 20–26 GHz. The core power consumption for the mixer is 3.096 mW and the output buffer power consumption is 3.122 mW. The total dc power consumption for this mixer, including the output buffers, is 6.22 mW. The total chip size for the K-Band mixer is 0.95 × 1.14 mm2.

120 Gbaud PAM-4 transmission over 80-km SSMF using optical band interleaving and Kramers-Kronig detection.

We demonstrate generation and detection of 120-Gbaud PAM-4 signals using an I/Q modulator based on optical band interleaving (OBI) technique. The spectral components of target PAM signals are split and pre-processed before being sent to two digital-to-analog convertors (sub-DACs) whose outputs are imprinted to an optical carrier by an optical I/Q modulator forming a carrier-suppressed tandem single side-band (SSB) signal. The PAM signals can be recovered after photo-detection provided that an optical beating tone is added at the edge of the signal spectrum along with the modulator output. The proposed method requires only half of the Nyquist bandwidth of the target PAM signal for the transmitter and has the advantage of a simple implementation. Using Kramers-Kronig (K-K) detection, a 120 Gbaud PAM-4 transmission over 80-km standard single mode fiber (SSMF) is successfully demonstrated. The proposed scheme entails a simple implementation and a much lower bandwidth requirement at the transmitter compared with conventional all-electronic high baud rate signal generation schemes.

Simulation of Third-Order Intermodulation Distortion Suppression Incorporating Dual-Parallel Mach–Zehnder Modulator in a CR-SSB Modulation

ABSTRACTA novel scheme is proposed to enlarge the spurious free dynamic range (SFDR) of microwave photonic (MWP) link by using dual-parallel Mach–Zehnder modulator, which modifies the single sideband signal and replaces the optical carrier with an unmodulated one to remove the optical main sources of third-order intermodulation distortion (IMD3). Simulation results show that the IMD3 and fifth-order intermodulation distortion can be suppressed greatly even when the modulation depth increases to a high value, and the link is limited by seventh-order intermodulation distortion and the SFDR is improved by 34.2 dB Hz in a 1-Hz bandwidth. The error vector magnitude of the MWP link with the proposed scheme is improved significantly for the transmitted 1-Gbit/s 16- quadrature amplitude modulation (QAM) signal.

Full-duplex hybrid FTTH and RoF transport system based on polarization modulators

This work demonstrates a full-duplex hybrid fiber-to-the-home (FTTH) and radio-over-fiber (RoF) transport system to transmit one wired signal and two wireless signals based on polarization modulation techniques. Here, the generated odd-order and even-order sidebands of polarization modulated signals can be colorlessly separated with the help of polarization beam splitters. This system has less interference among the wired signal, microwave and millimeter-wave signals. The uplink design is done by reusing the optical carrier and transmitting it through a single mode fiber (SMF) to the central office. The system is simulated using Orthogonal Frequency Division Multiplexing (OFDM) with 4-Quadrature Amplitude Modulation (QAM) as input. Frequency doubling and quadrupling is also used, such that optical microwave at 7.5 GHz and millimeter-wave at 15 GHz are generated successfully using polarization modulators by directly applying 3.75 GHz signal. This system is proved to have error-free transmissions, open eye diagrams, and clear constellation diagrams.

Research on photonic generation of quadrupling triangular-shaped waveform using external modulation

Abstract We propose an improved approach to generate frequency-quadrupled triangular-shaped waveform signals based on two cascaded modulators. A dual-parallel Mach-Zehnder modulator (DP-MZM) is employed to provide the quadrupling RF modulation, after which two primary optical sidebands (±2nd) are generated in spectrum. Then a dual-drive Mach-Zehnder modulator (DD-MZM) is followed to perform the optical double sideband (ODSB) modulation and signal with four sidebands (±2nd and ±6th) is obtained. By adjusting modulation index carefully, the power ratio of ±2nd and ±6th sidebands can be tuned to 9.5 dB. A piece of single mode fiber (SMF) is applied to remove the undesired harmonic. The expression of optical intensity is approximately equal to the Fourier expansion of ideal triangular-shaped waveform. The principle is illustrated by theory, simulation, and experiment. Finally, triangular-shaped waveform signals with repetition rate of 8 GHz, 12 GHz, 16 GHz and 20 GHz are generated.

Single Sideband Signals for Phase Noise Mitigation in Wireless THz-Over-Fibre Systems

In future photonic-based THz backhaul links, integrating the optical local oscillator (LO) in a remote antenna unit can be beneficial in terms of optical bandwidth efficiency and higher compatibility with passive optical networks. In such a scenario, two approaches can be used to reduce the high phase noise associated with free-running lasers: 1) baseband (BB) signals & carrier recovery, and 2) single sideband (SSB) signals & envelope detection. In this paper, we compare the performance of the two approaches for various optical LO linewidths using 5 GBd 16-QAM signals. We find that, for a total linewidth wider than 0.55 MHz, the SSB approach yields better results. The superior performance, however, comes at the expense of reducing the net information spectral density (ISD) of the SSB signal by 39% compared to that of the BB signal. However, using signal–signal beat interference-mitigation algorithms, an ISD only 15% lower than the BB signal ISD was sufficient to meet the FEC requirement. Given these results, we believe that the envelope detection of SSB signals is a promising solution to mitigate the phase noise problem of THz links based on free-running lasers, without excessively compromising the spectral efficiency of the system.

Direct detection of polarization multiplexed single sideband signals with orthogonal offset carriers.

We propose a novel direct detection (DD) scheme for polarization division multiplexed (PDM) single sideband (SSB) signals with two orthogonal carriers located at the opposite sides. Polarization diversity is realized with a pair of optical filters that are used to suppress the unwanted orthogonal carrier component. A PDM-SSB DD receiver is thus constructed without polarization de-rotation. The intra-polarization signal-signal beat interference (SSBI) can be mitigated by Kramers-Kronig detection or iterative SSBI cancellation. For inter-polarization SSBI mitigation, we propose a joint iterative SSBI cancellation method. The proposed PDM-SSB DD scheme is validated with a principle experiment of 40Gbaud PDM-SSB 16-ary quadrature amplitude modulation (16-QAM) signals. After 80km standard single-mode fiber (SSMF) transmission, the bit-error rates (BERs) achieve 20% hard-decision forward error correction (HD-FEC) threshold of 1.5 × 10-2. The performance of iterative SSBI cancellation, Kramers-Kronig detection, and joint iterative SSBI cancellation are evaluated for PDM-SSB signals with different carrier-to-signal ratios (CSPRs) through numerical simulations. Moreover, a multi-input-multi-output (MIMO) equalization scheme is proposed and validated with numerical simulation, which can suppress the linear inter-polarization crosstalk and relax the sharpness requirement of optical filter edges.

Photonic Generation of Microwave Frequency Shift Keying Signal Using a Polarization Maintaining FBG

A photonic generation of microwave frequency shift keying signal (FSK) with large frequency tunability is proposed and experimentally demonstrated. A polarization modulator combined with a polarization maintaining fiber Bragg grating is used to transform a carrier-suppressed double sideband signal to a wavelength-shift-keying one, which is subsequently mixed with a tunable optical source to achieve frequency modulation. A proof-of-concept experiment is demonstrated to obtain an FSK signal at 9.5/12.5 GHz. The carrier frequency has been tuned from 7.2 to 14.8 GHz by adjusting the laser wavelength. Moreover, a radio over fiber transmission link is established to investigate the transmission performance of the generated FSK signal. The bit-error-rate curve of the received FSK signal at 9.5/12.5 GHz for a 5-km single mode fiber and 2-m wireless transmission is presented.

Mitigation of RB noise in bidirectional fiber transmission systems based on different OFDM SSB techniques

The interferometric beat noise due to the reflections (RE) from passive devices and the Rayleigh backscattering (RB) in wavelength reuse bidirectional fiber optic transmission systems leads to degradation of the system if not employed with a suitable RB mitigation technique. These noises can be reduced by implementation of the modulation schemes which reduce the spectral overlap between the data signal and RB-RE signals. In this paper, a bidirectional architecture to reduce RB-RE noise in wavelength reused bidirectional Radio over Fiber (RoF) is proposed. Optical Orthogonal Frequency Division Multiplexing (OFDM) Single Sideband (SSB) techniques are used for upstream and downstream transmission. Two SSB millimeter wave signals viz. conventional SSB and modified SSB are generated by the MZM employing a hybrid coupler to generate different phase shifts (90° & 120°) of the RF signal applied to its electrodes. In our architecture, the spectral overlap between the data signal and RB-RE signals is reduced by transmitting the downstream and upstream signals on the higher and lower first order optical sidebands of the SSB signal respectively. A mathematical model is developed to analyze the transmission performance of the two SSB techniques based on RB-RE interferences. An m-QAM optical OFDM baseband modulation scheme is used to transmit 10 Gbps data over 75 km fiber using the two SSB modulation schemes. Our results show that the modified SSB eliminates 2nd order RB-RE harmonics reduces RB-RE crosstalk against the conventional SSB.

The analysis of high-order sideband signals in optomechanical system

The analysis of high-order sideband signals in optomechanical system

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.

Reconfigurable photonic dual‐frequency phase‐coded signals generator by using optical carrier phase shifting and polarisation selection

A novel photonic dual-frequency phase-coded signals generator based on the optical carrier phase shifting and polarisation selection techniques is proposed by using an integrated dual-polarisation quadrature phase shift keying (DP-QPSK) modulator. The key feature of the scheme is the capability to generate dual-frequency phase-coded signals with reconfigurable frequency multiplication factors. In the upper QPSK modulator, an optical binary phase shift keying (BPSK) signal and a carrier-suppressed double-sideband optical signal are generated, while in the bottom QPSK modulator, another carrier-suppressed double-sideband optical signal is generated. Thanks to the dual-polarisation property of the integrated modulator, the optical BPSK signal and different optical sidebands can be selected by using polarisation selection. As a result, two phase-coded signals with different frequencies can be achieved simultaneously. By adjusting the bias phases of the submodulators, dual-frequency phase-coded signals with frequencies of one and three, or two and four times the frequency of the RF carrier can be generated. The generation of 2.5 Gbps dual-frequency phase-coded signals with frequencies of 20 and 40 GHz, or 10 and 30 GHz are verified. The proposed scheme is simple, compact, and promising to find applications in future multi-band and multi-functional radars.

Broadband Photonic Microwave Signal Processor With Frequency Up/Down Conversion and Phase Shifting Capability

A new dual-function photonic microwave signal processing structure that has the ability to realize both frequency up/down conversion and RF/IF phase shifting, is presented. In the proposed signal processor, a dual-polarization dual-parallel Mach Zehnder modulator (DP-DPMZM) is used to generate an orthogonally polarized single RF/IF signal and local oscillator (LO) modulation sideband without an optical carrier. The optical phase difference between the two sidebands can be controlled by controlling a DC voltage applied to a LiNbO3 electro-optic phase modulator that is connected after the DP-DPMZM. Beating between the two sidebands at a photodetector generates an RF/IF signal with a phase equal to the optical phase difference between the IF/RF signal and LO sidebands. The dual-function photonic microwave signal processor has a wide bandwidth and does not require a precise control of the laser wavelength. Experimental results demonstrate that a flat >−3 dB down/up conversion efficiency is achieved for an RF signal from 3.5 to 26.5 GHz and for an IF signal from 0.5 to 3 GHz, and a full 360° continuous phase shift of the output IF/RF signal.

Understanding photon sideband statistics and correlation for determining phonon coherence

Generating and detection coherent high-frequency heat-carrying phonons has been a great topic of interest in recent years. While there have been successful attempts in generating and observing coherent phonons, rigorous techniques to characterize and detect these phonon coherence in a crystalline material have been lagging compared to what has been achieved for photons. One main challenge is a lack of detailed understanding of how detection signals for phonons can be related to coherence. The quantum theory of photoelectric detection has greatly advanced the ability to characterize photon coherence in the last century and a similar theory for phonon detection is necessary. Here, we re-examine the optical sideband fluorescence technique that has been used detect high frequency phonons in materials with optically active defects. We apply the quantum theory of photodetection to the sideband technique and propose signatures in sideband photon-counting statistics and second-order correlation measurement of sideband signals that indicates the degree of phonon coherence. Our theory can be implemented in recently performed experiments to bridge the gap of determining phonon coherence to be on par with that of photons.