Direct detection system based on a single photodiode receiving the independent quadruple-SSB signal.

We propose and verify a direct detection (DD) system based on a single photodiode (PD) receiving the independent quadruple-single-sideband (quadruple-SSB) signal. At the transmitter side, an I/Q modulator is utilized to modulate the independent quadruple-SSB signal, the signal is received via one PD without optical bandpass filters (OBPFs). Then, the independent quadruple-SSB signal is separated into four sideband signals by subsequent digital signal processing (DSP). In the scheme of back-to-back (BTB), 1-km and 5-km standard single-mode fiber (SSMF) transmission, the four sideband signals are extensively studied and analyzed. The simulation results show that the bit error rate (BER) of 1Gbaud, 2Gbaud and 4Gbaud independent quadruple-SSB signal can reach the 7% hard-decision forward error correction (HD-FEC) threshold of 3.8 × 10-3 when the received optical power (ROP) is -21, -20 and -17.2 dBm in 5-km SSMF transmission. Meanwhile, as the frequency interval gets wider, the crosstalk in the sideband signal reception can be mitigated and the BER decreases. This scheme for the first time demonstrates that the independent quadruple-SSB signal can further expand the system transmission capacity and enhance the spectrum efficiency. Our simplified independent quadruple-SSB signal direct detection system has a simple structure and high spectral efficiency, which will have a promising future in high-speed optical communication.

Simplified independent triple-sideband signal generation and transmission scheme based on one I/Q modulator at 0.3-THz.

To meet the ultra-bandwidth high-capacity communication, improve spectral efficiency and reduce the complexity of system structure, we have proposed the independent triple-sideband signal transmission system based on photonics-aided terahertz-wave (THz-wave). In this paper, we demonstrate up to 16-Gbaud independent triple-sideband 16-ary quadrature amplitude modulation (16QAM) signal transmission over 20 km standard single mode fiber (SSMF) at 0.3 THz. At the transmitter, independent triple-sideband 16QAM signals are modulated by an in-phase/quadrature (I/Q) modulator. Carrying independent triple-sideband signals optical carrier coupled with another laser to generate independent triple-sideband terahertz optical signals with a carrier frequency interval of 0.3THz. While at the receiver side, enabled by a photodetector (PD) conversion, we successfully obtain independent triple-sideband terahertz signals with a frequency of 0.3THz. Then we employ a local oscillator (LO) to drive mixer to generate intermediate frequency (IF) signal, and only one ADC is used to sample independent triple-sideband signals, digital signal processing (DSP) is finally performed to obtain independent triple-sideband signals. In this scheme, independent triple-sideband 16QAM signals is delivered over 20 km SSMF under the bit error ratio (BER) of 7% hard-decision forward-error-correction (HD-FEC) threshold of 3.8 × 10-3. Our simulation results show that the independent triple-sideband signal can further improve THz system transmission capacity and spectral efficiency. Our simplified independent triple-sideband THz system has a simple structure, high spectral efficiency, and reduced bandwidth requirements for DAC and ADC, which is a promising solution for future high-speed optical communications.

Direct detection system for independent triplet-sideband signals based on a single photodiode.

This paper proposes a novel, to the best of our knowledge, independent triplet-single-sideband (triplet-SSB) transmission system scheme aimed at increasing channel capacity and improving spectrum efficiency. The conventional independent multiband transmission systems are limited by their complexity and high computational requirements, which hinder the improvement of spectrum efficiency and channel capacity. To address these challenges, this scheme uses three independent signals, modulated by an in-phase/quadrature (I/Q) modulator, and transmits them over a 5-km standard single-mode fiber (SSMF). At the receiver end, a single photodiode (PD) is used for signal reception, and the signals are separated using digital signal processing (DSP) algorithms. Through simulation and verification, the feasibility and reliability of the system are demonstrated, with the bit error rates (BERs) of all three signals below the hard decision forward error correction (HD-FEC) threshold value of 3.8 × 10-3. This independent triplet-SSB transmission system scheme effectively improves spectrum efficiency and channel capacity, providing a valuable solution for meeting the growing demands of data transmission.

Synthesis of a GS-16QAM signal for a simplified optical ISB system and direct detection with the K-means clustering algorithm.

An independent sideband (ISB) is a promising scheme for short-reach and metro applications because of its high spectral efficiency, low complexity, and tolerance to chromatic dispersion. Here, we develop a signal synthesis scheme to further reduce the complexity of ISB direct-detection (DD) systems. Two lower-order quadrature amplitude modulation (QAM) sideband signals are generated digitally, then the left sideband (LSB) and right sideband (RSB) are modulated with regular quadrature phase shift keying (QPSK) and geometrically shaped shifted QPSK (GS-S-QPSK), respectively. Then, the two independent sideband signals are received in a single photodiode to synthesize a GS-16QAM signal. The LSB and RSB signals can be separated and demodulated by a digital signal process (DSP) instead of using two optical bandpass filters. The proposed scheme significantly reduces the complexity of the ISB-DD receiver, thus saving system cost. Three different GS-S-QPSK signals are evaluated, with the square GS-S-QPSK achieving the best bit error rate (BER) with its optimal shaping factor. Considering signal-signal beating interference, a sub-blind K-means clustering algorithm is used to improve the BER performance, and the results indicate that it can achieve a large received optical power (ROP) gain at a threshold of 3.8 × 10-3.

Simplified twin-single-sideband direct detection system separating left and right sideband signals using a digital signal processing algorithm instead of optical bandpass filters.

Twin-single-sideband (twin-SSB) signals can be generated based on an in-phase- quadrature (I/Q) modulator, and two independent left sideband (LSB) and right sideband (RSB) signals carry individual data to effectively harvest the advantage of twin-SSB modulation, which achieves higher spectral efficiency. However, the conventional twin-SSB scheme employs two optical bandpass filters (OBPFs) and two photodetectors (PDs) for complete separation and detection at the receiver side. To mitigate the crosstalk between RSB and LSB signals and reduce the complexity and cost of the twin-SSB system, we propose a new scheme to realize twin-SSB without OBPFs separating LSB and RSB signals by a single-ended PD to improve system performance. According to the beating characteristics of the LSB and RSB, we can separate two independent sideband signals using a digital signal processing (DSP) algorithm added to the receiver end. Our simulation results demonstrate that our proposed scheme can obtain good bit error ratio (BER) performance of LSB and RSB signals. We designed a twin-SSB system with different modulation formats in the two sidebands, adopting geometric shaping 3PSK (GS-3PSK) modulation for the LSB and quadrature phase shift keying (QPSK) modulation for the RSB. The BER of the LSB GS-3PSK and RSB QPSK signal can reach hard-decision forward error correction (HD-FEC) when the received optical power (ROP) was > -17.5 and > -16 dBm, respectively, at different baud rates of 1-, 2-, and 4-Gbaud with a carrier frequency of 12-GHz over 10-km standard single-mode fiber (SSMF) transmission. For an 8-Gbaud baud rate with a carrier frequency of 12-GHz over 5-km SSMF transmission, the BER of the two sideband signals can still be below the HD-FEC threshold of 3.8 × 10-3 when the ROP was > -17 and > -16 dBm, respectively.

Independent dual-single sideband lower-IF RF receiver based on a single photodetector.

The two sidebands of the independent dual-single-sideband (dual-SSB) signal can carry different information, which achieves higher spectral efficiency and system capacity. However, the receiver divides the signal into two channels, so a pair of optical bandpass filters (OBPFs) and photodetectors (PDs) are generally required to select and detect an independent dual-SSB signal. To reduce the complexity and cost of the receiver, we propose an independent dual-SSB lower intermediate frequency (IF) radio frequency (RF) signal detection scheme based on a single PD combined with conventional digital signal processing (DSP). A theoretical analysis is presented and validated by a simulation. The left sideband (LSB) and the right sideband (RSB) are quadrature phase-shift keying (QPSK) modulated. After 50-km standard single-mode fiber (SSMF) transmission, a 10-Gbps 16QAM signal is synthesized from 2 × 5-Gbps dual-SSB QPSK and the independent dual-SSB is extracted by DSP. The bit error rate (BER) of the dual-SSB (LSB and RSB) is below the hard-decision forward-error-correction (HD-FEC) threshold of 3.8 × 10-3.

Optical Polarization Division Multiplexing Transmission System Based on Simplified Twin-SSB Modulation.

Optical twin-single sideband (Twin-SSB) modulation, due to the left sideband (LSB) and right sideband (RSB) signal carrying individual data, has become an attractive technique in fiber transmission because it satisfies the demand of the explosive increase in data traffic. This paper focuses on reducing the complexity of Twin-SSB system and further enhancing the spectral efficiency by proposing a polarization division multiplexing (PDM) Twin-SSB modulation scheme. LSB and RSB signals are extracted using de-mapping algorithm instead of optical bandpass filters (OBPFs) to reduce system complexity. To further improve spectral efficiency, PDM is employed to meet the polarization multiplexing transmission and achieve a higher transmission capacity. Based on the PDM Twin-SSB system, the LSB is 3-arr phase-shift-keying (3PSK) modulated, while RSB is quadrature phase-shift keying (QPSK) modulated. We simulated that the bit error ratio (BER) performance of LSB and RSB of X-polarization (X-Pol) and Y-polarization (Y-Pol) at 8-Gbaud, 10-Gbaud, 12-Gbaud, 14-Gbaud, and 16-Gbaud in the case of back-to-back (BTB) and 2 km standard single-mode fiber (SSMF) transmission. The simulation results verify the effectiveness and practical feasibility of the proposed PDM Twin-SSB scheme for future short-distance transmission owing to low cost, simplified structure, low algorithm complexity, and high data transmission capacity.

Interleaved superposed-64QAM-constellation design for spatial multiplexing visible light communication systems.

Superposed constellation combined with spatial multiplexing multiple-input multiple-output (MIMO) techniques have been increasingly utilized in visible light communication (VLC) systems, as multiplexing gains can be achieved regardless of the correlation extent of the VLC channel. Herein, we propose a novel superposed 64-quadrature amplitude modulation (QAM) constellation scheme for 2 × 2 MIMO VLC systems. Considering the nonlinearity of light-emitting diodes (LEDs), two geometrically shaped 8QAM constellations are introduced to reduce the peak-to-average power ratio at the transmitter. Because the two 8QAM constellations are superposed in an interleaved manner, the required power ratio between two transmitted signals is 1, which further reduces the risk of nonlinear distortion and avoids signal-to-noise ratio deterioration induced by power competition. The proposed superposed 64QAM constellation scheme is experimentally investigated comprehensively, where the system performance is evaluated under different transmitted powers, direct current bias currents, and driving peak-to-peak voltages (Vpps). The experimental results show that the proposed scheme achieves better bit error rate (BER) performance than the traditional superposed 64QAM constellation schemes. Below the 7% pre-forward error correction BER threshold of 3.8 × 10-3, the dynamic range of the driving Vpps of the two LEDs improves from 0.06 to 0.4 V and 0.23 to 0.4 V when the optimal power ratio is achieved.

Intra-symbol frequency-domain averaging for turbulence mitigation in optical orbital angular momentum multiplexing.

Optical vortex beams (VBs) possessing helical phase-front have attracted considerable attention in multiplexing communication for their orthogonal orbital angular momentum (OAM) modes. However, the mode-crosstalk and signal jitter caused by turbulence fluctuation are two main challenges in OAM multiplexing communication. Here, we introduce an intra-symbol frequency-domain averaging technology (ISFA) for turbulence mitigation. By equalizing the distorted multiplexing signals, ISFA mitigates the amplitude and phase jitter of received signals without adding system complexity and information redundancy. The experimental results show that VBs are successfully demultiplexed, and the transmission rate reaches 48 Gbit/s. After ISFA, the bit-error-rate of QPSK-OFDM signals is reduced from 1.10 × 10-3 to 6.31 × 10-4, and the error-vector-magnitude (EVM) is reduced from 31.69% to 26.29% under the turbulence strength of Cn2 = 1×10-13m-2/3 and equivalent transmission distance of 200 m. By combining ISFA with MIMO diversity gain, the EVM can be further reduced from 46.70% to 26.70%. These indicate that ISFA is available for turbulence mitigation and compatible with MIMO technology, which may have perspective potential in improving the performance of OAM multiplexing communication.

Cylindrical vector beam multiplexing for radio-over-fiber communication with dielectric metasurfaces.

Radio-over-fiber (ROF) technology, loading microwave signal on light beams, has attracted considerable attention in wireless access network for its superiority in processing high-frequency microwave signals. Multiplexing for achieving high-capacity density, however, remains elusive in ROF communication because the optical microwave occupies large bandwidth. Here, we introduce a cylindrical vector beam (CVB) multiplexing for ROF communication with dielectric Pancharatnam-Berry phase-based metasurfaces (PBMs). CVBs, a structured light beam possessing spatially nonuniform polarization distribution and carrying vector mode, provide an additional multiplexing dimension for optical communication with the advantages of weak scintillation in free-space and low mode injure in few-mode-fiber. Exploiting the spin-orbit interaction of the PB phase, we construct PBMs to manipulate CVBs, which show broadband working wavelengths ranging from C- to L-band. After 3 m free-space propagation, two multiplexed CVBs carrying 100 GHz microwave are successfully demultiplexed, and the 100 GHz ROF communication with 12 Gbit/s QPSK-OFDM signals is realized. The crosstalk of the multiplexed CVBs is less than -15.13 dB, and the bit-error-rates (BERs) are below 3.26 × 10-5. With 5 km few-mode-fiber transmission, the CVBs are also demultiplexed with the BERs of 6.51 × 10-5. These results indicate that CVB is not only capable of free-space transmission but also available for few-mode-fiber transmission, which might pave new avenues for the multiplexing of ROF communications.

Generation of (3, 1) vector signals based on optical carrier suppression without pre-coding.

We experimentally and numerically demonstrated the generation of a (3, 1) vector signal by a single Mach-Zehnder modulator (MZM) without pre-coding. The MZM is driven by the (3, 1) modulated signal after photoelectric conversion by the "square law" of a photodetector. Although the phase changes, the corresponding constellation distribution is consistent with that of the regular signal. Our proposed scheme effectively avoids the pre-coding process with a simple architecture. The bit-error-ratio (BER) results indicate that the (3, 1) signal has a better BER performance than the pre-coded quadrature phase shift keying vector signal, and both are below ${3.8}\times {{10}^{ - 3}}$3.8×10-3 after 25 km optical fiber transmission.

W-band OFDM photonic vector signal generation employing a single Mach-Zehnder modulator and precoding.

We present a simple radio-over-fiber (RoF) link architecture for millimeter-wave orthogonal frequency division multiplexing (OFDM) transmission using only one Mach-Zehnder modulator (MZM) and precoding technique. In the transmission system, the amplitudes and the phase of the driving radio-frequency (RF) OFDM signal on each sub-carrier are precoded, to ensure that the OFDM signal after photodetector (PD) can be restored to original OFDM signal. The experimental results show that the bit-error ratios (BERs) of the transmission system are less than the forward-error-correction (FEC) threshold of 3.8 × 10(-3), which demonstrates that the generation of OFDM vector signal based on our proposed scheme can be employed in our system architecture.

40-Gb/s PDM-QPSK signal transmission over 160-m wireless distance at W-band.

We experimentally demonstrate a W-band optical-wireless transmission system over 160-m wireless distance with a bit rate up to 40 Gb/s. The optical-wireless transmission system adopts optical polarization-division-multiplexing (PDM), multiple-input multiple-output (MIMO) reception and antenna polarization diversity. Using this system, we experimentally demonstrate the 2×2 MIMO wireless delivery of 20- and 40-Gb/s PDM quadrature-phase-shift-keying (PDM-QPSK) signals over 640- and 160-m wireless links, respectively. The bit-error ratios (BERs) of these transmission systems are both less than the forward-error-correction (FEC) threshold of 3.8×10-3.

Heterodyne detection and transmission of 60-Gbaud PDM-QPSK signal with SE of 4b/s/Hz.

We experimentally demonstrate 8 × 240-Gb/s super-Nyquist wavelength-division-multiplexing (WDM) polarization-division-multiplexing quadrature-phase-shift-keying (PDM-QPSK) signal transmission on a 50-GHz grid with a net spectral efficiency (SE) of 4b/s/Hz adopting hardware-efficient simplified heterodyne detection. 9-ary quadrature-amplitude-modulation-like (9QAM-like) processing based on multi-modulus blind equalization (MMBE) is adopted to reduce analog-to-digital converter (ADC) bandwidth requirement and improve receiver sensitivity. The transmission distance at the soft-decision forward-error-correction (SD-FEC) threshold of 2 × 10(-2) is 2 × 420 km based on digital post filtering while largely extended to over 5 × 420 km based on 9QAM-like processing, which well illustrates 9QAM-like processing is more efficient for heterodyne coherent WDM system. Moreover, only two ADC channels are needed for simplified heterodyne detection of one 60-Gbaud PDM-QPSK WDM channel, and thus only one commercial oscilloscope (OSC) with two input ports can work well for each WDM channel.

Full-duplex bidirectional transmission of 10-Gb/s millimeter-wave QPSK signal in E-band optical wireless link.

We experimentally demonstrated full-duplex bidirectional transmission of 10-Gb/s millimeter-wave (mm-wave) quadrature phase shift keying (QPSK) signal in E-band (71-76 GHz and 81-86 GHz) optical wireless link. Single-mode fibers (SMF) are connected at both sides of the antenna for uplink and downlink which realize 40-km SMF and 2-m wireless link for bidirectional transmission simultaneously. We utilized multi-level modulation format and coherent detection in such E-band optical wireless link for the first time. Mm-wave QPSK signal is generated by photonic technique to increase spectrum efficiency and received signal is coherently detected to improve receiver sensitivity. After the coherent detection, digital signal processing is utilized to compensate impairments of devices and transmission link.

Enhanced performance of 400 Gb/s DML-based CAP systems using optical filtering technique for short reach communication.

A parallel transmission approach is more likely to realize 400 Gb/s and above short reach transmission as it helps to reduce the cost of both electrical and optical device largely. Directly modulated lasers (DML) are more attractive in 400 Gb/s approach, because it requires relatively small amount of driving power and has low insertion loss, thus lowering its cost. However, the intrinsic chirp will degrade the transmission performance. In this paper, an optical filtering technique is introduced for 400 Gb/s high-speed DML-based carrierless amplitude and phase (CAP) modulation short reach systems for the first time. Owing to the additional optical filter, 1 dB and 3.6 dB sensitivity improvement at BER of 3.8 x 10(-3) is obtained for the back-to-back and 15 km fiber link transmission for single lane at the bitrate of 28 Gb/s. Then a 16-lane CAP16 system with a total bit rate of 413 Gb/s is demonstrated experimentally using low-cost 10 GHz-class DML using optical filtering technique.

A 30 Gb/s full-duplex bi-directional transmission optical wireless-over fiber integration system at W-band.

We propose and experimentally demonstrate a full-duplex bi-directional transmission optical wireless-over fiber integration system at W-band (75-100 GHz) with the speed up to 15 Gb/s for both 95.4 GHz link and 88.6 GHz link for the first time. The generation of millimeter-wave (mm-wave) wireless signal is based on the photonic technique by heterodyne mixing of an optical quadrature-phase-shift-keying (QPSK) signal with a free-running light at different wavelength. After 20 km fiber transmission, up to 30 Gb/s mm-wave signal is delivered over 2 m wireless link, and then converted to the optical signal for another 20 km fiber transmission. At the wireless receiver, coherent detection and advanced digital signal processing (DSP) are introduced to improve receiver sensitivity and system performance. With the OSNR of 15 dB, the bit error ratios (BERs) for 10 Gb/s signal transmission at 95.4 GHz and 88.6 GHz are below the forward-error-correction (FEC) threshold of 3.8 × 10(-3) whether post filter is used or not, while the BER for 15 Gb/s QPSK signal employing post filter in the link of 95.4 GHz is 2.9 × 10(-3).

Antenna polarization diversity for high-speed polarization multiplexing wireless signal delivery at W-band.

We propose and experimentally demonstrate a novel architecture for a W-band integrated optical wireless system, which adopts a 2×2 multiple-input multiple-output (MIMO) wireless link based on antenna polarization diversity, and can realize 80 km single-mode fiber-28 transmission and 2 m wireless delivery for up to 39 Gbaud polarization-division-multiplexing quadrature-phase-shift-keying (PDM-QPSK) signal at 100 GHz. Classic constant-modulus-algorithm (CMA) equalization is adopted at the receiver to implement polarization demultiplexing. The 2×2 MIMO wireless link adopts one pair of horizontal-polarization (H-polarization) horn antennas (HAs) and one pair of vertical-polarization (V-polarization) HAs. Because the two pairs of HAs are fully isolated, the wireless cross talk can be effectively avoided. Thus, compared to the 2×2 MIMO wireless link at the same antenna polarization, the adoption of antenna polarization diversity cannot only make the HA adjustment easier but can also reduce the required CMA tap number. After removing 20% forward-error-correction overhead, the 39 Gbaud baud rate corresponds to a net bit rate of 130 Gb/s, which, to our best knowledge, is the highest bit rate per PDM channel demonstrated for wireless signal delivery up to now.

Ultrahigh-capacity access network architecture for mobile data backhaul using integrated W-band wireless and free-space optical links with OAM multiplexing.

In this Letter, we propose and experimentally demonstrate a novel access network architecture using hybrid integrated W-band wireless and free-space optical (FSO) links with orbital angular momentum (OAM) multiplexing. The transmission of a 20 GBd quadrature phase-shift keying signal modulated over 10 OAM modes has been demonstrated over a 0.6 m FSO link and a 0.4 m W-band wireless link at 100 GHz. The experimental results show that the architecture can support future ultrahigh-capacity, converged optical-wireless access networks that require extra bandwidth and system flexibility in mobile data networks.