Carrier-suppressed Single-sideband Research Articles

Generation of GHz line-spacing tunable optical frequency combs using Talbot effects.

In this paper, the generation of GHz line-spacing tunable optical frequency combs (OFCs) was demonstrated using an electro-optical (EO) Talbot laser and a phase modulator. In the EO Talbot laser, the frequency shifting was realized with a dual-parallel Mach-Zehnder modulator (DPMZM) working for carrier-suppressed single-sideband modulation. The PM was employed to achieve the spectral Talbot effect and compensate the phase introduced by the temporal Talbot effect in the laser loop. Arbitrary control of OFC line-spacing was realized using temporal and spectral Talbot effects. The principle of this OFC generator was theoretically modeled. In the experiments, the 2GHz line spacing of an OFC was multiplied to be 4GHz, 6GHz, 8GHz, and 10GHz. The frequency spacing of the OFC can also be multiplied with a fractional factor of 3/4, 7/2, 8/5, and 10/7, which was confirmed by simulations.

Photonic Generation of Background-Free Phase-Coded Microwave Pulses with Elimination of Power Fading

We report a novel photonic scheme to generate background-free phase-coded microwave pulses with elimination of power fading by cascading a dual-polarization dual-parallel Mach–Zehnder modulator (DP-DPMZM) and a polarization modulator (PolM). The DP-DPMZM is driven by a radio frequency (RF) signal to generate two first-order optical sidebands with an orthogonal polarization state, while the PolM is driven by a three-level electrical coding signal. By properly adjusting the polarization state, a series of background-free frequency-doubled phase-coded microwave pulses can be generated after optical-to-electrical conversion. Benefiting from the carrier-suppressed single-sideband (CS-SSB) modulation, the proposed signal generator can suppress the chromatic-dispersion-induced power-fading effect, which has excellent potential for long-distance fiber transmission. In addition, the system can directly generate phase-coded microwave signals in pulse mode by truncating continuous wave (CW) microwave signals. Moreover, the microwave signal generator has wideband tunability since no optical filter is involved in our scheme. The proposed method was theoretically analyzed and experimentally verified. Phase-coded microwave pulses centered at 14 GHz and 19.2 GHz with a bit rate of 0.5 Gb/s were successfully generated.

Photonic-based radar for distance and velocity measurement with multiformat waveforms.

A radar with multiformat waveforms is proposed to measure distance and velocity. A dual-polarization, dual-parallel Mach-Zehnder modulator (DP-DPMZM) is used to realize carrier-suppressed single-sideband (CS-SSB) modulation, so the switchable down-, up-, and dual-chirp signals, and the amplitude shift keying (ASK) signal, can be generated. The central frequency and bandwidth of the signals are flexible. A Mach-Zehnder modulator (MZM) is added for an optical switch to realize the generation of a pulsed signal and suppress interference in the dechirping process. A dual-drive MZM (DDMZM) is used for the dechirping process. The key component of this radar is that there are no filters used, which can significantly extend the operation bandwidth. In our test, the interference frequency caused by the CW can be suppressed. The error of the distance measurement is less than 2cm, and the velocity error is less than 0.180m/s. In addition, the direction of the target can be distinguished.

Filter-free photonics-assisted frequency translator with a tunable phase shift and amplitude based on an integrated DP-QPSK modulator.

In this paper, we propose and demonstrate a novel, to the best of knowledge, filter-free photonics-assisted microwave frequency translator with a tunable phase shift and amplitude. The pivotal component of the proposed scheme is an integrated dual-polarization quadrature phase shift keying (DP-QPSK) modulator, which is applied to generate a polarization orthogonal carrier-suppressed single sideband modulation signal and frequency shifted optical carrier signal. The polarization-multiplexed optical signal outputs from the DP-QPSK modulator is then sent to a photodetector (PD) via a polarization controller (PC) and a polarizer to implement photoelectric conversion. The electrical signal output from the PD is the desired frequency translated microwave signal, and the amount of frequency shift is determined by the frequency of the sawtooth wave applied to the DP-QPSK modulator. In addition, since the PC can be used to adjust the polarization angle and introduce a phase difference between the two orthogonally polarized optical signals, the phase shift and amplitude of the obtained translated signal can also be easily tuned. A theoretical analysis and simulation experiment are carried out to verify the feasibility of the proposed scheme. The simulation results show that the novel scheme can realize frequency translation with a 360° continuously tunable phase shift and adjustable amplitude for both a single-tone signal and linearly frequency modulated signal with a 50MHz bandwidth. The spurious suppression ratios of the single-tone signal and LFM signal after frequency translation are larger than 48 and 30dB, respectively.

Photonic-assisted frequency downconverter with self-interference cancellation and fiber dispersion elimination based on stimulated Brillouin scattering.

A photonic-assisted frequency downconverter with self-interference cancellation and fiber dispersion elimination is proposed for in-band full-duplex (IBFD) radio-over-fiber (ROF) systems based on stimulated Brillouin scattering. In this work, a dual-polarization Mach-Zehnder modulator (DPol-MZM) is employed to cancel the self-interference signal in the optical domain, thus the proposed system has a large operation bandwidth without the limitation of the electrical bottleneck. Meanwhile, a widely tunable microwave photonic filter (MPF) based on stimulated Brillouin scattering (SBS) is constructed to perform carrier-suppressed single-sideband modulation of LO. Therefore, the proposed photonic frequency downconverter is naturally free from fiber dispersion. Furthermore, thanks to the characteristics of the SBS-based MPF, such as large out-of-band rejection ratio, high resolution and wideband tunability, the proposed system has a high flexibility and downconversion efficiency, which is of great significance for IBFD ROF systems.

Generation of Single Sideband-Suppressed carrier (SSB-SC) Signal Based on Stimulated Brillouin Scattering

A novel cost-effective wavelength shifter is suggested in this paper. This paper shows how to shift signals in the mm-wave area using a frequency shifter. Due to stimulated Brillouin scattering in optical fibre, the proposed approach produces a single sideband signal without the carrier signal. When compared to SSB-SC transmissions, the bandwidth efficiency of double sideband signals is lower. Because it employs an optical fibre with a performance degrading condition such as SBS as an application, the frequency shifter is entirely optical. The theory of Gain and Loss Spectrum of SBS within optical fibre is utilised to minimise the upshifted frequency component and boost the low shifter frequency component in order to generate improved carrier power and single sideband. The SBS phenomenon produces a single sideband, which is advantageous in advanced modulation systems and consumes less bandwidth. It’s worth noting that the SSB-SC signal may be generated using any optical approach, and the proposed work eliminates the need for non-optical components like 90-degree hybrid couplers. SSB-SC signals are capable of combating pulse broadening difficulties and have the ability to operate at terahertz frequencies. Transmission of an SSB-SC signal is also shown to validate the performance of the produced signal. Furthermore, the Q factor of SSB-SC, SSB with carrier, DSB-SC, and DSB with carrier signals is studied at various connection lengths.

Single Sideband Suppressed Carrier Modulation With Spatiotemporal Metasurfaces at Near-Infrared Spectral Regime

Single sideband suppressed carrier (SSB-SC) modulation enables high efficiency and dispersion-tolerant shifting of light that is of great interest for many optical systems and applications. Herein, the design procedure of a spatiotemporal reflective metasurface is proposed to realize SSB-SC modulation in near-infrared spectral regime. The metasurface consists of a periodic array of plasmonic nanostrips integrated with indium-tin-oxide (ITO) in metal-insulator-metal configuration, wherein two sets of time-varying biasing signals are independently applied to the metasurface for modulating the permittivity of ITO in space and time. The spatiotemporal metasurface features a sawtooth phase profile with linear span over <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$2\pi$</tex-math></inline-formula> and a constant amplitude in time domain, that can be obtained by judicious adjustment of the biasing waveforms. It is established that such spatiotemporal metasurface allows for spurious-free frequency conversion by transforming the incident signal into the first-order up-modulated sideband and suppressing all the undesired mixing products. The normalized conversion efficiency of more than 99% is achieved for the metasurface, while the peak levels of the largest undesired mixing product and the fundamental frequency are reduced down to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$-39$</tex-math></inline-formula> dB and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$-44$</tex-math></inline-formula> dB, respectively. The proposed spatiotemporal metasurface is exploited for multi-channel and multi-beam scanning via pixelated control over the modulation phase delays assigned to the constituent elements of the spatially-interleaved sub-arrays, rendering a shared-aperture metasurface in space-time. In this case, the crosstalk between the channels with identical operating frequencies is substantially reduced. In addition, spatiotemporal redirection of light based on frequency gradient metasurface is demonstrated, which enables ultrafast, all-angle, and continuous beam-scanning as progressing in time.

Photonics-assisted joint communication-radar system based on a QPSK-sliced linearly frequency-modulated signal.

A photonics-assisted joint communication-radar system is proposed by introducing a quadrature phase-shift keying (QPSK)-sliced linearly frequency-modulated (LFM) signal. An LFM signal is carrier-suppressed single-sideband modulated onto the optical carrier in one dual-parallel Mach-Zehnder modulator (DPMZM) of a dual-polarization dual-parallel Mach-Zehnder modulator (DPol-DPMZM). The other DPMZM is biased as an IQ modulator to implement QPSK modulation on the optical carrier. The polarization orthogonal optical signals from the DPol-DPMZM are further combined and detected in a photodetector to generate the QPSK-sliced LFM signal. The QPSK-sliced LFM signal is used to realize efficient data transmission and high-performance radar functions including ranging and imaging. An experiment is performed. Radar range detection with an error of less than 4 cm, inverse synthetic aperture radar imaging with a resolution of 14.99cm×3.25cm, and communication with a data rate of 105.26 or 210.52 Mbit/s are successfully verified.

Transmission of dual-chirp microwave signal over fiber with suppression chromatic-dispersion-induced power-fading based on stimulated Brillouin scattering

We report a microwave photonic link to transmit dual-chirp microwave signals over fiber based on stimulated Brillouin scattering (SBS) using a dual-polarization Mach–Zehnder modulator (DPol-MZM). The DPol-MZM is driven by a radio frequency (RF) carrier and a baseband single-chirp waveform to realize carrier-suppressed single-sideband (CS-SSB) modulation based on SBS. The dual-chirp waveforms centered at 12.4 GHz and 15.5 GHz with bandwidths of 0.4 GHz have been successfully generated. In our link, the CS-SSB modulation based on SBS is performed to overcome fiber chromatic-dispersion-induced power fading (CDIP). The proposed scheme is theoretically analyzed and experimentally verified.

Photonic-based analog and digital RF self-interference cancellation with high spectral efficiency.

A photonic-assisted analog and digital radio frequency (RF) self-interference cancellation (SIC) approach with high spectral efficiency is reported for base stations in in-band full-duplex radio-over-fiber systems on the basis of our previous research. One dual-polarization quadrature phase-shift keying (DP-QPSK) modulator is used as the canceller for one base station. The two dual-parallel Mach-Zehnder modulators of the DP-QPSK modulator are both biased as carrier-suppressed single-sideband modulators and driven by the received signal and reference signal, respectively, to achieve high spectral efficiency while implementing the SIC in the optical domain. The baseband optical signal after SIC is further transmitted to the central station, where the electrical signal is recovered, sampled, and processed to further suppress the residual self-interference in the digital domain by using the recursive least-square (RLS) algorithm. An experiment is then performed. The proposed system is demonstrated by employing two independent channels. The analog cancellation depths of the 200, 500, and 800 Mbaud QPSK-modulated self-interferences are around 24, 20, and 20 dB, respectively; the total cancellation depths are around 29, 28, and 25 dB, respectively, when the analog cancellation and the RLS algorithm digital cancellation are applied. Meanwhile, the fiber distribution has no significant influence on SIC performance.

Wideband microwave Doppler frequency shift measurement based on acousto-optic frequency shift and DP-QPSK receiver

Microwave Doppler frequency shift (DFS) measurement is one of the backbone parts in modern radar systems. In this paper, a simple and novel optical approach to implement broadband microwave DFS measurement is proposed. Two carrier-suppressed single-sideband modulation (CS-SSB) modules, which are constituted by two Mach Zehnder modulators (MZMs) and two optical band pass filters (OBPFs) are employed for modulating both transmitted signal and echo signal. Due to the frequency shift by an acousto-optic modulator (AOM), the proposed method converts the DFS signals to the intermediate-frequency (IF) domain for simultaneous estimation of DFS value and direction, which helps to avoid low-frequency interference. Moreover, an integrated dual-polarization quadrature phase shift keying (DP-QPSK) receiver is used to obtain the in-phase and quadrature (I/Q) DFS terms and improve the system stability. In the experiment, the microwave DFS is estimated with a clear direction and a maximum measurement error of 4.9 Hz over an ultra-wide operation frequency from 10 to 40 GHz. The micro-Doppler frequency shift (mDFS) measurement is also carried out to demonstrate the advantage of low-frequency DFS measurement. Excellent system stability is also observed in the experiment, where the corresponding I/Q amplitude and phase imbalance are maintained lower than 2.5 dB and 4.8 degrees during 60 min.

Peak-to-Average Power Ratio Reduction of Carrier-Suppressed Optical SSB Modulation: Performance Comparison of Three Methods

We compare the performances of three previously proposed methods to reduce the peak-to-average power ratio (PAPR) of the carrier-suppressed optical single-sideband (OSSB-SC) signal. PAPR of OSSB-SC signal becomes high due to the peaky Hilbert-transformed signal which is used for spectral suppression. Nonlinear phase shifts induced by high PAPR degrade OSSB-SC signal during fiber transmission. Previously, we proposed peak folding, peak clipping, and high-pass Hilbert transform methods to reduce the PAPR of OSSB-SC modulation. In this study, we numerically compare the effectiveness of proposed methods in a 10 Gbit/s non-return-to-zero (NRZ)-coded 100-km single-channel transmission link. Due to the reduced PAPR, peak folding and peak clipping can increase the self-phase modulation (SPM) threshold of the studied system by 2.40 dB and 2.63 dB respectively. The high-pass Hilbert transform method improves the SPM threshold by more than 9 dB.

Rayleigh Backscattering Rejection in Single-Laser Homodyne Transceiver for UD-WDM Using λ-Shifting

With the aim of using a single laser per transceiver in single fiber splitter-based passive optical networks (PON) with coherent homodyne receivers, we apply wavelength shifting in an ultra-dense wavelength division multiplexing (UD-WDM) scheme. The studied architecture is based on a dual-parallel Mach-Zehnder Modulator (DP-MZM) operating in carrier-suppressed single-sideband (CS-SSB). In this configuration, a single laser feeds both the coherent homodyne receiver, and the transmitter, simplifying the wavelength management in the PON. The modulator carries out both data modulation and wavelength shifting. In this study, a bit rate of 4 Gb/s in QPSK is used to test several roll-off factors and channel spacing configurations. The results show that power budget gains of 18 dB are achievable with a channel spacing equal to only the symbol rate.

Generation of a microresonator soliton comb via current modulation of a DFB laser

Dissipative Kerr-microresonator soliton combs (hereafter called soliton combs) have been rapidly progressing as compact frequency combs. Comb mode scanning of the soliton combs with a large range and fast speed is of paramount importance for applications such as LiDAR and spectroscopy, requiring large and rapid frequency scanning of a pump continuous-wave (CW) laser as well as resonance frequency of a microresonator. Here, we demonstrate the generation of a soliton comb by a distributed feedback (DFB) laser toward comb mode scanning with a large range and fast speed. Compared with conventional pump CW lasers (i.e. external cavity diode lasers: ECDLs), DFB lasers can be frequency-scanned more largely and rapidly without mode-hopping. In addition, because of the fast scan speed of the DFB laser, a single soliton comb is generated simply by controlling the injection current of the DFB laser, greatly simplifying the system without having any additional optical modulators such as a carrier-suppressed single-sideband modulator (CS-SSB modulator), acousto-optic modulator (AOM), and auxiliary CW laser.

Optical SSB modulator/frequency shifter with ultralow spurious sidebands.

In this paper, we report and theoretically investigate an optical single-sideband suppressed-carrier (OSSB-SC) modulator/frequency shifter with ultralow spurious sidebands. The novel, to the best of our knowledge, structure is based on three Sagnac interferometers (SIs) and two bidirectional phase modulators (PMs). The two bidirectional PMs are driven by four radio frequency (RF) signals with equal powers and appropriate phases. An OSSB-SC signal with only (4n+1)th-order sidebands, where n is an integer number, is generated. Besides, by applying appropriate RF phase shifts, an OSSB with carrier (OSSB+C) modulator can be obtained whose optical carrier-to-sideband ratio (OCSR) can be tuned by controlling the PMs modulation indices. Also, by using the proposed OSSB-SC modulator in another SI, its OCSR can be tuned by controlling the state of polarization in addition to the modulation indices of the PMs. Since the proposed structure is based on SI and no direct current (DC) bias is required for the PMs, it is robust against environmental perturbations and decreases problems associated with DC bias-drift. In addition, performance of the proposed structure under the power and phase imbalances due to the external non-ideal RF hybrid couplers and the phase imbalance of practical non-ideal nonreciprocal optical phase shifters are investigated. Simulation results using commercial software are also presented which are in good agreement with analytical results.

Photonics-Assisted Frequency Up/Down Conversion With Tunable OEO and Phase Shift

A novel photonics-assisted method for frequency up/down-conversion based on an integrated dual-polarization dual-parallel Mach–Zehnder modulator (DP-DPMZM) with tunable optoelectronic oscillator (OEO) and phase shift is presented. The local oscillator (LO) signal with high spectral purity and low phase noise is generated by an OEO loop in one of the sub-DPMZMs. The frequency can be adjusted by an electrical bandpass filter (EBPF). The radio frequency (RF) signal is injected into another sub-DPMZM. The carrier-suppressed single-sideband (CS-SSB) modulation of both the LO and RF signals is achieved. The phase of the converted signal can be tuned by an integrated dual-channel phase modulator (DPM). Frequency up/down-conversion can be switched by tuning a single bias voltage of the DP-DPMZM. Multiple functions, including tunable LO signal generation, up/down-conversion and phase shifting, can be realized. Moreover, dispersion-induced power fading is well compensated due to CS-SSB modulation. In the experiments, the tunable LO signal from 5 to 18 GHz, with a sidemode suppression ratio as high as 46.1 dB and a phase noise level of −104.5 dBc/Hz @ 10 kHz at 10 GHz, is generated successfully. The spur suppression ratios of the up/down-converted signals are 32.6 dB and 35.6 dB, respectively. The up/down conversion performance for RF carrying 16 quadrature amplitude modulation (16-QAM) signal is also investigated. The phase shifting of dual-channel converted signals can be varied continuously and independently in the range of 0–360°, and the phase deviation is ±1.8°. The proposed compact link is characterized by function multiplexing and inherent dispersion compensation.

Photonics-Assisted Bandwidth-Doubling Dual-Chirp Microwave Signal Generation With Freely-Tunable Central Frequency

A photonics-assisted dual-chirp microwave signal generation scheme is proposed based on electro-optic modulation and heterodyne detection. The dual-chirp microwave signal is generated by heterodyne beating between a dual-chirp optical waveform and a frequency-shifted optical carrier, where the dual-chirp optical waveform is obtained by applying a baseband symmetric-triangle linear frequency modulated signal to a Mach-Zehnder modulator biased at the minimum transmission point, and the frequency-shifted optical carrier is obtained by carrier-suppressed single-sideband modulation in a dual-parallel Mach-Zehnder modulator (DPMZM) with the assistance of an electronic 90° hybrid. The bandwidth of the generated dual-chirp microwave signal is twice of that of the input baseband signal, and the central frequency can be tuned by varying the frequency of the single-tone microwave signal applied to the DPMZM. Both numerical simulation and experiment are carried out to demonstrate the proposed scheme. In the simulation, a dual-chirp microwave signal with a center frequency of 15 GHz and a bandwidth of 6 GHz is generated. In the proof-of-concept experiment, dual-chirp microwave signals centered at 1.5 GHz and with bandwidth of 100 MHz and 200 MHz are generated, which verifies the feasibility of the proposed scheme.

A Novel Decoupling Bias Control Technique for Dual Parallel Mach-Zehnder Modulator

A Carrier Suppressed Single Sideband(CS-SSB) modulation decoupling control technique based on Dual Parallel Mach-Zehnder Modulator (DPMZM) is proposed to solve the problem that the bias point of the DPMZM leads to the degradation of the modulation signal under the influence of ambient temperature, fiber coupling loss or other factors. This technique introduces three non-disturbing dithering voltages at the bias end of the modulator and detects the power ratio of the optical signal at the output end to realize CS-SSB modulation. Experimental results show that the proposed technique can effectively stabilize Optical Sideband Suppression Ratio(OSSR) above 24dB. This letter also proves that this technique is feasible for other modulators and has great practical value in the microwave photonic system.

Photonic-assisted image rejection mixer based on Hilbert transform in the optical domain using a phase-shifted fiber Bragg grating

A photonic-assisted image rejection mixer is proposed based on Hilbert transform in the optical domain using a phase-shifted fiber Bragg grating (PS-FBG). The image signal and the desired RF signal are converted to the optical domain by carrier-suppressed single-sideband modulation, which is then split into two parts. One part is Hilbert transformed by a specially designed PS-FBG and a 90-deg optical phase shifter and then combined with the other part to directly reject the image signal in the optical domain. The image-free optical signal is downconverted to an intermediate frequency signal by combining it with an optical local oscillator signal and then detecting them in a photodetector. The image rejection capability of the system is analyzed, and an image rejection ratio of 68.0 or 58.6 dB is achieved when the bandwidth of the image signal is 20 or 60 MHz, respectively, while having an acceptable influence on the desired RF signal. Quadrature phase-shift keying (QPSK) signals, 8 phase-shift keying (8PSK) signals, and 16 quadrature amplitude modulation (16-QAM) signals are used to evaluate the image rejection performance. When the bandwidth of the image signal is 18.75 MHz, the error vector magnitudes of the downconverted QPSK, 8PSK, and 16-QAM signals with the same power, modulation format, and bandwidth are 2.5%, 2.5%, and 2.6%, respectively.

Photonic-Assisted Filter-Free Microwave Doppler Frequency Shift Measurement Using a Fixed Low-Frequency Reference Signal

A filter-free photonic approach for measuring microwave Doppler frequency shift (DFS) based on two cascaded electro-optic modulators (EOMs) and a fixed low-frequency reference signal is proposed. An optical carrier is carrier-suppressed single-sideband modulated by the transmitted signal and the reference signal at EOM-I. The output from EOM-I is then single-sideband modulated by the echo signal at EOM-II. Subsequently, the output from EOM-II is injected into a photodetector to implement optical-to-electrical conversion. Both the value and direction of the DFS can be obtained by analyzing the frequency of the generated low-frequency electrical signal. The proposed approach has a compact structure and no optical filtering is needed. An experiment is performed to verify the proposed approach. For different microwave frequencies from 7 to 16 GHz, DFS from −100 to 100 kHz is measured, with a measurement error of less than ±0.05 Hz. The stability of the system is also verified. Within 30 minutes, the change of measurement error is less than ±0.05 Hz.