Showing papers on "Optical Carrier transmission rates published in 2018"

Thin film lithium niobate electro-optic modulator with terahertz operating bandwidth.

Abstract: We present a thin film crystal ion sliced (CIS) LiNbO3 phase modulator that demonstrates an unprecedented measured electro-optic (EO) response up to 500 GHz. Shallow rib waveguides are utilized for guiding a single transverse electric (TE) optical mode, and Au coplanar waveguides (CPWs) support the modulating radio frequency (RF) mode. Precise index matching between the co-propagating RF and optical modes is responsible for the device’s broadband response, which is estimated to extend even beyond 500 GHz. Matching the velocities of these co-propagating RF and optical modes is realized by cladding the modulator’s interaction region in a thin UV15 polymer layer, which increases the RF modal index. The fabricated modulator possesses a tightly confined optical mode, which lends itself to a strong interaction between the modulating RF field and the guided optical carrier; resulting in a measured DC half-wave voltage of 3.8 V·cm−1. The design, fabrication, and characterization of our broadband modulator is presented in this work.

Chaotic optical communications over 100-km fiber transmission at 30-Gb/s bit rate.

Abstract: For the first time, to the best of our knowledge, we experimentally demonstrate a successful 30-Gb/s signal transmission of a duobinary message hidden in a chaotic optical carrier over 100-km fiber. Thanks to the duobinary modulation format with high spectral efficiency, the 30-Gb/s signal can be encrypted by a 10-GHz-wide chaotic carrier. A digital signal processing technique can be used to convert duobinary data into binary data on the receiver side. This proposal lowers the requirement for wideband chaos generation and synchronization in high-speed long-distance chaotic optical communications, and fiber dispersion compensation can also be simplified, which has potential to be used in high-speed long-distance secure optical communications.

Hybrid Silicon Photonic-Lithium Niobate Electro-Optic Mach-Zehnder Modulator Beyond 100 GHz

Abstract: Electro-optic modulation, the imprinting of a radio-frequency (RF) waveform on an optical carrier, is one of the most important photonics functions, being crucial for high-bandwidth signal generation, optical switching, waveform shaping, data communications, ultrafast measurements, sampling, timing and ranging, and RF photonics. Although silicon (Si) photonic electro-optic modulators (EOMs) can be fabricated using wafer-scale technology compatible with the semiconductor industry, such devices do not exceed an electrical 3-dB bandwidth of about 50 GHz, whereas many applications require higher RF frequencies. Bulk Lithium Niobate (LN) and etched LN modulators can scale to higher bandwidths, but are not integrated with the Si photonics fabrication process adopted widely over the last decade. As an alternative, an ultra-high-bandwidth Mach-Zehnder EOM based on Si photonics is shown, made using conventional lithography and wafer-scale fabrication, bonded to an unpatterned LN thin film. This hybrid LN-Si MZM achieves beyond 100 GHz 3-dB electrical bandwidth. Our design integrates silicon photonics light input/output and optical components, including directional couplers, low-radius bends, and path-length difference segments, realized in a foundry Si photonics process. The use of a simple low-temperature (200C) back-end integration process to bond a postage-stamp-sized piece of LN where desired, and achieving light routing into and out of LN to harness its electro-optic property without any etching or patterning of the LN film, may be broadly-useful strategies for advanced integrated opto-electronic microchips.

A Survey of Optical Carrier Generation Techniques for Terabit Capacity Elastic Optical Networks

Abstract: Elastic optical networks (EON) have been proposed to meet the network capacity and dynamicity challenges. Hardware and software resource optimization and re-configurability are key enablers for EONs. Recently, innovative multi-carrier transmission techniques have been extensively investigated to realize high capacity (Tb/s) flexible transceivers. In addition to standard telecommunication lasers, optical carrier generators based on optical frequency combs (OFC) have also been considered with expectations of reduced cost and inventory, improved spectral efficiency, and flexibility. A wide range of OFC generation techniques have been proposed in the literature over the past few years. It is imperative to summarize the state of the art, compare and assess these diverse techniques from a practical perspective. In this survey, we identify salient features of optical multicarrier generators, review and compare these techniques both from a physical and network layer perspective. OFC demultiplexing/filtering techniques have also been reviewed. In addition to transmission performance, the impact of such sources on the network performance and real-world deployment strategies with reference to cost, power consumption, and level of flexibility have also been discussed. Field trials, integrated solutions, and flexibility demonstrations are also reported. Finally, open issues and possible future directions that can lead to real network deployment are highlighted.

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

Abstract: 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 10–3, 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.

Solid core PCF-based mode selector for MDM-Ro-FSO transmission systems

Abstract: Radio-over-free-space (Ro-FSO) technology is a combination of free-space optics (FSO) and radio over fiber. It plays a significant role in radio-frequency signal transmission in mobile network communication through high-speed optical carrier without any licensing and costly cables. Photonic crystal fibers also play a significant role to deliver data at faster rate for short haul communication. This paper, for the first time to the author’s best knowledge, utilizes mode division multiplexing in conjunction with solid core PCFs to transmit $$2 \times 2.5$$ Gbps–5 GHz data over 2.5 km free-space link. The results are reported in terms of bit error rate, spatial profiles of received modes, mode spectrum of modes and eye diagrams. Furthermore, proposed PCF-MDM-Ro-FSO transmission system is also reported under the impact of atmospheric turbulences.

Chip-based Brillouin processing for carrier recovery in self-coherent optical communications

Abstract: Modern fiber-optic coherent communications employ advanced, spectrally efficient modulation formats that require sophisticated narrow-linewidth local oscillators (LOs) and complex digital signal processing (DSP). Self-coherent optical orthogonal frequency-division multiplexing (self-CO-OFDM) is a modern technology that retrieves the frequency and phase information from the extracted carrier without employing a LO or additional DSP. However, a wide carrier guard is typically required to easily filter out the optical carrier at the receiver, thus discarding many OFDM middle subcarriers that limit the system data rate. Here, we establish an optical technique for carrier recovery, harnessing large-gain stimulated Brillouin scattering (SBS) on a photonic chip for up to 116.82 Gbit·s−1 self-CO-OFDM signals, without requiring a separate LO. The narrow SBS linewidth allows for a record-breaking small carrier guard band of ∼265 MHz in self-CO-OFDM, resulting in higher capacity than benchmark self-coherent multi-carrier schemes. Chip-based SBS-self-coherent technology reveals comparable performance to state-of-the-art coherent optical receivers while relaxing the requirements of the DSP. In contrast to on-fiber SBS processing, our solution provides phase and polarization stability. Our demonstration develops a low-noise and frequency-tracking filter that synchronously regenerates a low-power narrowband optical tone, which could relax the requirements on very-high-order modulation signaling for future communication networks. The proposed hybrid carrier filtering-and-regeneration technique could be useful in long-baseline interferometry for precision optical timing or reconstructing a reference tone for quantum-state measurements.

180-GBaud All-ETDM Single-Carrier Polarization Multiplexed QPSK Transmission over 4480 km

Abstract: We demonstrate PDM-QPSK transmission over 4480 km at a record all-electronically multiplexed symbol rate of 180 GBaud, providing a line rate of 720 Gb/s on a single optical carrier enabled by high-speed InP-DHBT selectors.

A Photonically Enabled Compact 0.5–28.5 GHz RF Scanning Receiver

Abstract: An innovative and effective photonics-based radio frequency (RF) spectrum scanner is presented and demonstrated, enabling the detection in the 0.5–28.5 GHz range. The proposed receiver scans the whole frequency range at discrete steps, down-converting at baseband single portions of the detected spectrum, where they can be precisely acquired, without using RF bandpass filters nor optical filters. The wideband scansion is achieved by simply tuning an optical carrier only, avoiding the use of an RF synthesizer or of multiple parallel local oscillators. In more detail, the optical carrier is modulated by the detected RF signal through an electro-optical modulator, and photomixed with an optical local oscillator to down-convert the modulation sideband to baseband. The optical carrier also feeds an optical frequency comb generator—driven by a fixed RF oscillator at 1 GHz—whose output injects the local oscillator. This way, the optical carrier and oscillator are phase locked and can be tuned at steps of 1 GHz. The developed demonstrator achieves an input range up to 28.5 GHz, and a simulative analysis suggests the capability of exceeding the 0.5–40 GHz range employing commercially available devices. Moreover, it shows a linear dynamic range of 160 dB/Hz and a spuriousfree dynamic range of 109 dB/Hz2/3 , aligned with the state-of-the-art broadband RF receivers. The proposed solution appears to fulfill the performance requirements of the demanding electronic support measures applications, with the potentials for largely reducing the receiver size.

High-Resolution Optical Vector Analysis Based on Symmetric Double-Sideband Modulation

Abstract: An optical vector analyzer (OVA) based on symmetric optical double-sideband (ODSB) modulation using a phase modulator (PM) and an intensity modulator (IM) is proposed and demonstrated. In the symmetric-ODSB-based OVA, frequency response measurements are implemented by transmitting the phase-modulated and intensity-modulated ODSB signals through an optical device-under-test (DUT), respectively. Then, removing the responses of the two electro-optic modulators and processing the measured responses, accurate frequency responses of the DUT on both sides of the optical carrier are obtained. Comparing with the conventional OVA based on optical single-sideband (OSSB) modulation, the proposed ODSB-based OVA has the doubled measurement range and the simple wavelength-independent configuration. Moreover, the measurement system inherently has large dynamic range and high accuracy, which is difficult to achieve for the OSSB-based OVA due to the restriction of the limited sideband suppression ratio. An experiment for measuring the magnitude and the phase responses of a programmable optical filter is carried out. The responses in a range of 100 GHz are measured with a resolution of 10 MHz by using 50-GHz components.

Digital Domain Power Division Multiplexed Dual Polarization Coherent Optical OFDM Transmission.

Abstract: Capacity is the eternal pursuit for communication systems due to the overwhelming demand of bandwidth hungry applications. As the backbone infrastructure of modern communication networks, the optical fiber transmission system undergoes a significant capacity growth over decades by exploiting available physical dimensions (time, frequency, quadrature, polarization and space) of the optical carrier for multiplexing. For each dimension, stringent orthogonality must be guaranteed for perfect separation of independent multiplexed signals. To catch up with the ever-increasing capacity requirement, it is therefore interesting and important to develop new multiplexing methodologies relaxing the orthogonal constraint thus achieving better spectral efficiency and more flexibility of frequency reuse. Inspired by the idea of non-orthogonal multiple access (NOMA) scheme, here we propose a digital domain power division multiplexed (PDM) transmission technology which is fully compatible with current dual polarization (DP) coherent optical communication system. The coherent optical orthogonal frequency division multiplexing (CO-OFDM) modulation has been employed owing to its great superiority on high spectral efficiency, flexible coding, ease of channel estimation and robustness against fiber dispersion. And a PDM-DP-CO-OFDM system has been theoretically and experimentally demonstrated with 100 Gb/s wavelength division multiplexing (WDM) transmission over 1440 km standard single mode fibers (SSMFs).

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

Abstract: 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.

A Multichannel Phase Tunable Microwave Photonic Mixer With High Conversion Gain and Elimination of Dispersion-Induced Power Fading

Abstract: A microwave photonic system that can realize frequency up- and down-conversion, multichannel phase shift, high conversion gain, and elimination of dispersion-induced power fading is proposed and experimentally demonstrated. The scheme is based on an integrated dual-polarization quadrature phase shift keying modulator that contains two dual parallel Mach–Zehnder modulators (X-DPMZM and Y-DPMZM). The X-DPMZM implements dual side band carrier suppression (DSB-CS) modulation of radio frequency signal, and the Y-DPMZM implements frequency shift of an optical carrier. They are combined in orthogonal polarizations to implement frequency up- and down-conversion. The polarization multiplexed signal will go through polarization controllers and polarizers to implement multichannel phase shift. In the experiment, the phase shift can be tuned independently over 360° in each channel. By suppressing the optical carrier, the conversion gain and LO isolation are improved by 20.5 dB and 51.26 dB, respectively, compared with conventional dual side band modulation scheme. In addition, the proposed scheme can achieve a spurious-free dynamic range (SFDR) of 103.6 dB·Hz2/3.

Long-reach 60-GHz MMWoF link with free-running laser diodes beating

Abstract: With the remote beating of two mutually incoherent laser carriers, the local-oscillator-free long-reach millimeter-wave over fiber (MMWoF) link at 60-GHz band is demonstrated The unique schemes of the proposed MMWoF are the wavelength-locked colorless laser diode (CLD) modulator, the mutually incoherent optical carrier for heterodyne MMW generation, and the square-law power envelope detection at receiving end By directly encoding the single-mode with the CLD modulator, the single-carrier modulated QAM-OFDM data is achieved to release the RF power fading after fiber transmission The mutually incoherent laser beating enables the optical heterodyne MMW generation with two independent optical carriers, which provides the advantages of local-oscillator-free operation and rules out the requirement of dual-mode optical carrier delivery from central office At the wireless receiving end, the received QAM-OFDM data is self-down-converted to the baseband by employing the square-law power envelope detection This eliminates the requirement of local oscillator and rules out the influence of the MMW carrier frequency fluctuation between two mutually incoherent lasers (used at central office and remote node), which effectively provides the MMW carrier immunity against the down-conversion instability caused by clock jitter or carrier incoherence This architecture ensures the transmission of 165-Gbit/s 32-QAM OFDM data over 50 km in SMF and 3 m in free-space with the FEC certificated error vector magnitude of 12%, signal-to-noise ratio (SNR) of 184 dB, and bit error rate of 38 × 10−3 For multi-channel DWDM-PON applications, the proposed local-oscillator-free MMWoF link can successfully perform 11 DWDM channels of 32-QAM OFDM data access at 165 Gbit/s per channel via the wavelength controlling of the CLD modulation stage and the detuning of the beating carrier at remote node

Tunable RF and microwave photonic sideband generator based on cascaded 49GHz and 200GHz integrated ring resonators

Abstract: We demonstrate a continuously RF tunable orthogonally polarized optical single sideband (OP-OSSB) generator based on dual cascaded micro-ring resonators. 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 Flexible Two-Way PM-Based Fiber-FSO Convergence System

Abstract: This paper aims to propose and conduct a practical demonstration of a flexible two-way phase modulation (PM)-based fiber-free-space optical (FSO) convergence system employing one optical carrier transmission scheme and vertical cavity surface emitting laser (VCSEL)-based tunable optical band-pass filter (TOBPF). One optical carrier is delivered to efficaciously reduce the dispersion effect caused by a 40-km single-mode fiber (SMF) transportation and the distortion caused by beating among multiple optical sidebands. Using a VCSEL-based TOBPF at the receiving site, the selected optical signal will be acquired flexibly from one of the multiple injected optical signals. This is the first time that an optical carrier transmission scheme and a VCSEL-based TOBPF are used in a two-way PM-based fiber-FSO convergence system. The downstream light is flexibly phase-remodulated with microwave/millimeter-wave data signal for uplink delivery. In-depth observation shows that bit error rate performs brilliantly via a 40-km SMF transportation with a 100-m FSO link. Such a flexible two-way PM-based fiber-FSO convergence system would be very attractive for the integration of fiber backbone and indoor networks to provide data signal flexibly.

Double-efficiency photonic channelization enabling optical carrier power suppression

Abstract: A novel double-efficiency photonic channelization scheme with optical carrier power suppression (OCS) is proposed for the first time, to the best of our knowledge. In this scheme, a tunable optical frequency comb generator is used to efficiently generate radio-frequency (RF) carriers, and a Fabry–Perot filter (FPF) is introduced to split a broadband signal into multiple narrowband signals. With the well-designed frequency of RF carriers, the wavelength spacing of optical carriers, and the free spectrum range of the FPF, double-channelized efficiency can be obtained. A proof-of-concept system is demonstrated to verify the feasibility of this double-efficiency channelization scheme, in which a 5 GHz baseband signal is channelized into five 1 GHz sub-channels, and about 35 dB OCS is obtained. Moreover, the performance of the double-efficiency channelization scheme is analyzed based on a 5 Gbit/s baseband signal with an on-off keying (OOK) format in which the influence of the 3rd order term interference is discussed.

Photonic Generation and Transmission of Triangular and Square Waveforms With a Large Repetition Rate Tunable Range

Abstract: A photonic approach to generate and transmit triangular and square-shaped microwave waveforms with a large repetition rate tunable range is proposed and experimentally demonstrated using a dual-polarization quadrature phase-shift keying (DP-QPSK) modulator. The upper dual-parallel Mach-Zehnder modulator (DP-MZM) integrated in the DP-QPSK modulator, which is driven by a microwave signal, is properly biased to serve as an optical carrier-suppressed single-sideband modulator to generate a first-order optical sideband and an opposite third-order optical sideband. The lower DP-MZM in the DP-QPSK modulator, driven by dc voltage, functions as an optical phase shifter to tune the phase of the optical carrier. By properly controlling the phase of the optical carrier and the amplitude of the optical sidebands, a triangular or a square microwave waveform can be generated. In addition, the influence of the fiber dispersion on the shape of the waveform can be overcome by properly adjusting the phase of the optical carrier, so the generated microwave waveforms can be transmitted over an optical fiber. An experiment is performed. Triangular waveforms with a repetition rate from 3 to 13 GHz and square waveforms with a repetition rate from 3 to 7 GHz are generated. The compensation of the dispersion-induced distortion is also evaluated in the experiment.

Silicon-Photonic Electro-Optic Phase Modulators Integrating Transparent Conducting Oxides

Abstract: Higher-order digital modulation formats are demonstrated by electrically inducing free-carrier concentration changes in thin films of transparent conducting oxides, integrated into well-established silicon-photonic waveguiding architectures. The proposed near-infrared modulators employ as physical platforms the silicon-rib and silicon-slot waveguides, exploiting the highly dispersive and carrier-dependent epsilon-near-zero behavior of transparent conducting oxides to modulate the optical carrier. Advancing the existing studies on conventional amplitude modulation, phase-shift keying formats are investigated in this paper, using a rigorous and physically consistent modeling framework that seamlessly combines solid-state physics with Maxwell wave theory through carrier-dependent material models. The designed in-line modulators achieve $V_\pi L$ products in the order of 0.1 Vmm, two orders of magnitude lower than their respective all-silicon or lithium niobate counterparts, accompanied by an insertion loss of about $3~\mathrm {dB/\pi }$ . Switching speeds in the order of 50 GHz are feasible along with a potential for sub-pJ/symbol energy consumption, meeting the demands for on-chip optical modulation.

Photonic generation of binary and quaternary phase-coded microwave signals by utilizing a dual-polarization dual-parallel Mach-Zehnder modulator

Abstract: A photonic method to generate binary and quaternary phase-coded microwave signals using a dual-polarization dual-parallel Mach-Zehnder modulator (DP-DPMZM) is proposed and experimentally demonstrated. The upper DPMZM driven by a radio frequency (RF) signal acts as an optical wavelength shifter, while the lower DPMZM is used to generate a binary phase shift key (BPSK) or quadrature phase shift key (QPSK) signal. By combining the wavelength-shifted optical sideband and phase-modulated optical carrier, both binary and quaternary phase-coded microwave signals can be generated. Such signals with the carrier frequency of 10 GHz and 15 GHz are demonstrated. The pulse compression performance is also investigated.

A Photonic Pre-Distortion Technique for RF Self-Interference Cancellation

Abstract: We propose a photonic pre-distortion technique to perform self-interference cancellation for radio-frequency signals. The intensity-modulation direct-detection system is modified by adding a Mach-Zehnder modulator and an optical delay line. With a prior knowledge of the interferer, the optical carrier is pre-distorted before modulated by the received corrupted signal. The delay and amplitude adjustments in our system are optically implemented. Experimental results show that the system achieves 32.6 dB of cancellation across 100-MHz bandwidth and 57 dB of cancellation for 10-KHz narrowband interference.

Ultrahigh-resolution and wideband optical vector analysis for arbitrary responses.

Abstract: An ultrahigh-resolution and wideband optical vector analyzer (OVA) with the simplest architecture, to the best of our knowledge, is proposed and demonstrated based on chirped optical double-sideband (ODSB) modulation in a single-drive Mach-Zehnder modulator (MZM) To distinguish the magnitude and phase information carried by the two sidebands in the ODSB signal, a two-step measurement, in which biasing, respectively, the MZM at two different points is applied Because no optical filtering is required in the scheme, the optical carrier can be located at any wavelength that is suitable for accurate measurement, eg, close to the notch of a notch response or within the passband of a bandpass response, so the proposed OVA has the capability to measure an arbitrary response An experiment is carried out, which achieves the magnitude and phase responses of a programmable optical processor with bandpass, notch, or falling-edge responses The measurement bandwidth is 134 GHz, and the measurement resolution is 112 MHz

A Sub-ps Stability Time Transfer Method Based on Optical Modems

Abstract: Coherent optical fiber links recently demonstrate their ability to compare the most advanced optical clocks over a continental scale. The outstanding performances of the optical clocks are stimulating the community to build much more stable time scales, and to develop the means to compare them. Optical fiber link is one solution that needs to be explored. Here, we are investigating a new method to transfer time based on an optical demodulation of a phase step imprint onto the optical carrier. We show the implementation of a proof-of-principle experiment over 86-km urban fiber, and report time interval transfer stability of 1 pulse per second signal with sub-ps resolution from 10 s to one day of measurement time. Prospects for future development and implementation in active telecommunication networks, not only regarding performance but also compatibility, conclude this paper.

Full duplex radio over fiber system with frequency quadrupled millimeter-wave signal generation based on polarization multiplexing

Abstract: A full duplex radio over fiber system with frequency quadrupled millimeter-wave signal generation based on polarization multiplexing using a single-drive intensity modulator is proposed and experimentally demonstrated. At the central station, downstream signal modulated on the single-drive intensity modulator is polarization multiplexed with pure optical carrier by a polarization beam combiner (PBC) before transmitted over the single mode fiber to the base station. The single-drive intensity modulator is biased at the maximum transmission point to generate optical carrier and two second-order sidebands. At the base station, by simply adjusting the difference angle between the principle axis of polarizer and one principle axis of PBC to 135°, a frequency quadrupled millimeter-wave signal is generated. In addition, when the difference angle between the principle axis of polarizer and one principle axis of PBC is adjusted to 90°, the original pure optical carrier is recovered, which is wavelength reused to provide light source for the uplink to deliver upstream signal. A proof-of-concept experiment is performed. Pure optical carrier and 40 GHz millimeter-wave signal with 20 dB optical harmonic suppression ratio are obtained. The power penalties of the bidirectional links are less than 0.3 dB after transmitted over 10.5 km single mode fiber. The measured power fluctuation of the generated millimeter-wave signal is less than 1 dB in one hour, showing the proposed scheme is relatively stable for long-distance transmission system.

Optical Frequency Tuning for Coherent THz Wireless Signals

Abstract: THz wireless signals have become of interest for future broadband wireless communication. In a scenario where the wireless signals are distributed to many small remote antenna units, this will require systems which allow flexible frequency tuning of the generated THz carrier. In this paper, we demonstrate experimentally the implementation of two tuning methods using an optical frequency comb generator for coherent optical frequency tuning in THz wireless-over-fiber systems. The first method is based on using a photonic integrated circuit optical phase lock loop (OPLL) subsystem implemented as a high quality optical filter for single comb line selection and optical amplification. The OPLL generates an optical carrier, which is frequency and phase stabilized in reference to one of the optical comb lines with a frequency offset precisely selectable between 4 and 12 GHz. The second method is based on optical single sideband suppressed carrier (SSB-SC) modulation from the filtered comb line using an optical IQ modulator. With this technique, it is possible to suppress the other unwanted optical tones by more than 40 dB. This generated optical carrier is then heterodyned with another filtered optical comb line to generate a tuneable and stable THz carrier. The full system implementations for both methods are demonstrated by transmitting THz wireless signal over fiber with 20 Gb/s data in QPSK modulation. The system performance and the quality of the generated THz carrier are evaluated for both methods at different tuned THz carrier frequencies. The demonstrated methods confirm that a high quality tuneable THz carrier can easily be implemented in systems where dynamic frequency allocation is required.

4 × 128-Gb/s PDM-DMT signal transmission over 1440-km SSMF with high phase noise tolerance.

Abstract: In this paper, we propose an intensity modulation and coherent detection scheme with high phase noise tolerance for polarization division multiplexing (PDM) discrete multi-tone (DMT) signal transmission by employing Kramers-Kronig (KK) detection and digital carrier regeneration (DCR). At the transmitter side, DMT signal is modulated by a Mach-Zehnder modulator (MZM) setting bias around the null point and transmitted with the suppressed optical carrier. At the receiver side, a directly modulated lasers (DMLs) locating at the edge of DMT signal is used as the local oscillator (LO) for coherent detection. For signal recovery, KK detection is first used to reduce the signal to signal beating noise. Digital optical carrier is then regenerated by the DCR scheme and the DMT signal could be recovered by enveloping detection with the regenerated digital carrier, which can achieve high laser line-width tolerance and mitigate the residual phase noise caused by KK detection. To verify the effectiveness of the proposed scheme, we compare the KK and DCR based receiver digital signal processing (DSP) with conventional receiver DSP by using both DMLs with larger linewidth (~10 MHz) or external cavity lasers (ECLs) with smaller linewidth (less than 100 kHz) as optical carrier and LO. The results show that KK and DCR can mutually improve the system performance with ECLs as optical carrier and LO. Moreover, it is shown that the signal using DMLs cannot be recovered without the DCR method due to the high laser line-width of DML. Finally, we successfully demonstrate 4 × 128-Gb/s KK and DCR based PDM-DMT signal transmission over 1440-km SSMF by employing DMLs as optical carrier and LO.

Bidirectional long-reach PON using Kramers-Kronig-based receiver for Rayleigh Backscattering noise and SSBI interference elimination

Abstract: In this paper, a novel bidirectional long-reach PON is proposed and demonstrated by using Kramers-Kronig (KK)-based receiver and 7-core fiber to simultaneously cope with the induced signal-signal beating interference (SSBI) and Rayleigh backscattering (RB) noises. A low-cost self-homodyne detection using only one PD is used at both OLT and ONU, and the middle core of 7-core fiber is used to deliver the seed light to ONUs for colorless upstream transmission, and the upstream and downstream signals are transmitted simultaneously over the same outer core for each ONU. By this means, the signals and local oscillators for upstream and downstream transmission all originate from the same laser which is located at OLT. With the help of the KK-based receiver, SSBI could be effectively eliminated and the fiber dispersion can also be digitally compensated due to the reconstruction of the complex field of the received signal. Moreover, by using single sideband Nyquist-shaped subcarrier modulation with 16-ary quadrature amplitude modulation (SSB-Nyquist-16QAM) technique, the upstream and downstream signals are allocated to occupy the left and right sideband of the optical carrier respectively, and thus the RB noise can be easily removed by a simple optical filter in the receiver. In our experiment, the carrier-to-signal power ratio (CSPR) and the frequency gap between the upstream and downstream signals are investigated. Furthermore, bidirectional transmission of 60 Gbps SSB-Nyquist-16QAM signals over 50 km 7-core fiber are successfully achieved, and the frequency gap between the upstream and downstream signals is only 3 GHz.

Simple Stabilized Radio-Frequency Transfer with Optical Phase Actuation

Abstract: We describe and experimentally evaluate a stabilized radio-frequency transfer technique that employs optical phase sensing and optical phase actuation. This technique is achieved by modifying existing optical frequency transfer equipment and also exhibits advantages over previous stabilized radiofrequency transfer techniques in terms of size and complexity. Acousto-optic modulators (AOMs) are used to modulate an optical carrier. Stabilization of frequency fluctuations in the link is achieved by steering the frequency of one of the AOMs. We demonstrate the stabilized transfer of a 160-MHz signal over a 166-km fiber optical link, achieving an Allan deviation of 9.7 × 10 -12 at 1 s of integration, and 6.4 × 10 -15 at 10 4 s. This technique was considered for application to the Square Kilometre Array SKA1-low radio telescope.

Photonic Generation of Triangular-shaped Waveform Based on External Modulation

Abstract: We present a novel approach for triangular-shaped waveform generation by applying optical single sideband (OSSB) modulation and optical carrier suppression (OCS) modulation to an input signal. Firstly, an OSSB modulated signal consists of an optical carrier and +1st-order sideband, is initially generated with a dual-drive Mach-Zehnder modulator (DD-MZM1) driven by a quadruple frequency RF signal. By tuning the amplitude of the RF signal and the bias of the DD-MZM1, the power ratio between the carrier and the +1st-order sideband is controlled as 19 dB. These two components are then transmitted to a DD-MZM2 which is driven by a fundamental frequency. After OCS modulation, four sidebands are existed in optical spectra of the modulated signal. By utilizing a fiber Bragg grating (FBG) to remove the undesired sideband, an output signal that features a triangular-shaped waveform is finally achieved.