Photonic Link Research Articles

One kilometer balanced analog photonic link based on a single multicore fiber

A one kilometer balanced microwave photonic link based on a single four core multicore optical fiber is demonstrated for the first time, to the best of our knowledge. The radio frequency gains and differential phase stabilities over temperature were measured and compared to a conventional balanced link based on two spans of single mode fiber. Both the multicore fiber link and the single mode fiber balanced link exhibited differential phase fluctuations of |Δφ|≤13° at 25 GHz (corresponding to a differential delay or skew, |Δt|, of ≤1.4ps) under identical periodic temperature oscillations of ±13°C over a 60 min period. In contrast, previous studies have shown that two single mode fiber spans should have exhibited up to an order of magnitude greater Δφs compared to the multicore fiber. Our different results can be attributed to how the fibers are arranged; the previous works kept the two single mode fiber spans separate, on different spools, whereas in this work the two single mode fiber spans were co-located as close as possible together by spooling them ‘side-by-side’ onto a single mandrel, more representative of a balanced link in application. In addition, the radio-frequency crosstalk between the multicore fiber’s cores were measured, exhibiting acceptable levels at least 60 dB below the signal for all core combinations. The crosstalk is shown to be dominated by occurrences at localized points, e.g. at the multiplexers (fan-in/fan-outs) and splices, rather than during propagation. Regardless of the single mode fiber link’s performance, the measurements here demonstrate that multicore fiber-based balanced links are a viable single fiber alternative to conventional dual single mode fiber-based balanced links.

A broadband integrated microwave photonic mixer based on balanced photodetection

An integrated microwave photonic mixer based on silicon photonic platforms is proposed, which consist of a dual-drive Mach–Zehnder modulator and a balanced photodetector. The modulated optical signals from microwave photonic links can be directly demodulated and down-converted to intermediate frequency (IF) signals by the photonic mixer. The converted signal is obtained by conducting off-chip subtraction of the outputs from the balanced photodetector, and subsequent filtering of the high frequency items by an electrical low-pass filter. Benefiting from balanced detection, the conversion gain of the IF signal is improved by 6 dB, and radio frequency leakage and common-mode noise are suppressed significantly. System-level simulations show that the frequency mixing system has a spurious-free dynamic range of 89 dB·Hz2/3, even with deteriorated linearity caused by the two cascaded modulators. The spur suppression ratio of the photonic mixer remains higher than 40 dB when the IF varies from 0.5 to 4 GHz. The electrical-electrical 3 dB bandwidth of frequency conversion is 11 GHz. The integrated frequency mixing approach is quite simple, requiring no extra optical filters or electrical 90° hybrid coupler, which makes the system more stable and with broader bandwidth so that it can meet the potential demand in practical applications.Graphical

Ultrahigh-linearity dual-drive scheme using a single silicon modulator.

Here, a high-linearity dual-drive scheme using a single silicon dual-drive Mach-Zehnder modulator is presented. The bias voltages and RF amplitudes of the two driving arms are adjusted such that the nonlinearity of the transfer function of the Mach-Zehnder interferometer cancels out the nonlinear response of the arms. Using the proposed scheme, the spurious-free dynamic range of the third-order intermodulation distortion is 123.4 dB Hz6/7, which is believed to be a record-breaking value for silicon modulators. In comparison, the result obtained using a conventional single-drive scheme is 102.6 dB·Hz2/3. The proposed scheme could simplify the design of modulators and promote high-performance microwave photonic links.

Photonic compressive sampling of wideband sparse radio frequency signals with 1-bit quantization.

Photonic compressive sampling (PCS) is an effective method to recover wideband sparse radio frequency (RF) signals. However, the noisy and high-loss photonic link leads to signal-to-noise ratio (SNR) degradation of the RF signal to be tested, which limits the recovery performance of the PCS system. In this paper, a random demodulator-based PCS system with 1-bit quantization is proposed. The system consists of a photonic mixer, a low-pass filter, a 1-bit analog-to-digital converter (ADC), and a digital signal processor (DSP). The 1-bit quantized result is used to recover the spectra of the wideband sparse RF signal with the binary iterative hard thresholding (BIHT) algorithm, which can alleviate the negative impact of the SNR degradation caused by the photonic link. A full theoretical framework of the PCS system with 1-bit quantization is given. Simulation results show that the PCS system with 1-bit quantization can provide better recovery performance than the traditional PCS system under low SNR and stringent bit budget.

Corrections to “Photonic Sub-Terahertz IM Links: Comparison Between Double and Single Carrier Modulation” [Sep 22 6064-6070

In this manuscript we report four mistakes in [1] and their respective corrections. Two mistakes are notation typos with no implications, whereas the other two are mistakes regarding the simulations. The correction of these two mistakes changes slightly the technical analysis presented in [1], however, they do not affect the main conclusions reported in such a paper.

Linearization of a Dual-Parallel Mach-Zehnder Modulator Using Optical Carrier Band Processing

The linearization of a microwave photonic link based on a dual-parallel Mach-Zehnder modulator is theoretically described and experimentally demonstrated. Up to four different radio frequency tones are considered in the study, which allow us to provide a complete mathematical description of all third-order distortion terms that arise at the photodetector. Simulations show that a complete linearization is obtained by properly tuning the DC bias voltages and processing the optical carrier band. As a result, a suppression of 17 dB is experimentally obtained for the third-order distortion terms, as well as a SDFR improvement of 3 dB. The proposed linearization method enables the simultaneous modulation of four different signals without the need of additional radio frequency components, which is desirable to its implementation in integrated optics and makes it suitable for several applications in microwave photonics.

A Power-Efficient 20–35-GHz MZM Driver With Programmable Linearizer for Analog Photonic Links in 28-nm CMOS

Analog photonic links require linear drive of electrooptic modulators at high frequency to support emerging millimeter wave (mm-wave) communication standards. However, this is challenging due to the nonlinear distortion of commonly used Mach–Zehnder modulators (MZMs). This article presents the first CMOS MZM driver capable of operating over a wide 20–35-GHz frequency range with a programmable linearizer. A broadband design is achieved with a magnetically coupled resonator (MCR) technique that provides wideband impedance matching for the interstage and output-stage matching networks. Linearization is achieved with a topology consisting of inverter-based amplifier segments that provide programmable predistortion gains over the input signal regions. Fabricated in 28-nm CMOS, the driver delivers 2.5-Vpp swing to an external MZM. Operating the majority of the circuitry at 0.9 V, except for the 1.6-V output stage, allows for a power consumption of only 180 mW. The proposed programmable linearizer is able to compensate the amplitude-to-amplitude modulation (AM–AM) compression of the amplifier stages and the external MZM, extending output power 1-dB compression point ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text {OP}_{1~\text {dB}}$ </tex-math></inline-formula> ) of the whole radio-over-fiber (RoF) link by 3 dB and achieving 4.8-dBm third-order input intercept point (IIP3) at 25 GHz.

Beyond CPO: A Motivation and Approach for Bringing Optics Onto the Silicon Interposer

Co-packaged optics (CPO) technology is well positioned to break through the bottlenecks that impede efficient bandwidth scaling in key near-term commercial integrated circuits. We begin by providing some historical context for this important sea change in the optical communications industry. Then, motivated by GPU-based accelerated computing requirements, we investigate the next pain points that are poised to constrain bandwidth and efficiency in future CPO-based systems. We identify 2.5D integrated optics (i.e., bringing optics onto the interposer) as a promising solution that can enable continued scaling for these systems due to the dense wiring available which facilitates more efficient slow-and-wide electrical interfaces. We explore the benefits, challenges, and requirements associated with such tight coupling of the processors and optical engines by considering high-level photonic link design, technology, and packaging. We demonstrate the viability of a control loop which can adequately regulate temperature within the aggressive thermal environment. Then, we introduce a custom simulation framework that allows quantified comparisons of detailed design decisions; the simulations validate the feasibility of the general approach while also providing key guidance to designers on best directions to pursue for efficient optimization.

Improvement of linearity in intensity-modulated analog photonic link based on all-optical wave shaping

In this paper, a novel all-optical architecture for multi-order nonlinear distortions (NDs) suppression in the intensity-modulation direct-detection (IM-DD) analog photonic link (APL) is proposed. A systematic analysis of the relationship between the NDs and all optical sidebands (OSBs) is summarized. Numerical simulation results show that, by optimizing the amplitude and phase of OSB, the 3rd-order intermodulation distortion (IMD3) can be significantly suppressed. Consequently, the spurious-free dynamic range (SFDR) of 107.1 dBHz8/9 is obtained, which is improved by 16.17 dB compared with a single quadrature biased Mach–Zehnder modulator (MZM) used in APL.

High-Performance Microwave Photonic Transmission Enabled by an Adapter for Fundamental Mode in MMFs

Microwave photonic links (MPLs) have long been considered as an excellent way for radio frequency (RF) transmission due to their advantages such as light weight, high bandwidth, low cost and large spurious-free dynamic range (SFDR). However, the effective mode-field area (Aeff) of the single-mode fiber (SMF) used in the traditional MPL is not large, so the MPL based on SMF have relatively strong nonlinearity, which limits the processing power of SMFs to a level of few milliwatts. Few-mode fibers (FMFs) have been applied in MPL as an alternative due to the larger Aeff, and photonic lanterns are used simultaneously to excite the high-order mode of FMFs for RF signal transmission. However, the photonic lantern could bring additional insertion loss, and the production cost of FMFs is high, so we propose an MPL based on multimode fibers (MMFs) with mode field adapters (MFAs). Since MMFs have larger Aeff, the nonlinearity of the link can be greatly reduced. And matched MFAs realized by reverse tapering, to excite only the fundamental mode in MMFs to reduce the crosstalk, which are very stable. As a result, the stimulated Brillouin scattering threshold and SFDR are improved by 5 dB and 14.5 dB, respectively.

A ‘DOMZM–Filter–BPD’-Based Microwave Photonic Link for Eliminating Even Harmonics Distortion

The ‘Mach-Zehnder modulator (MZM)–wavelength division demultiplexer (WDM)–photodetector (PD)’ structure in photonic analog-to-digital converters (PADCs) may result in residual even harmonics after photodetection. In this paper, we build the model for the ‘Dual-output MZM (DOMZM)–filter–balanced PD (BPD)’-based microwave photonic link, where the amplitude and the phase responses of the filter can be arbitrary. It is proved that this link can eliminate the aforementioned even harmonics. Theoretical analysis is verified by the simulation and experimental results. By using this structure, the spur-free dynamic range (SFDR) limited by the even harmonics can be improved and the sampling frequency range can be expanded for the PADCs. As the spectrum contains only odd harmonics, the inverse cosine transform for calibrating the nonlinearity of the MZM can be applied to further improve the SFDR.

Ku-band analog photonic down-conversion link with coherent I/Q image rejection and digital linearization.

Analog photonic down-conversion links have been widely used in radar, electronic warfare, and satellite communication systems. Aiming at the optimization demands of the link performance, we demonstrate and experimentally verify a Ku-band photonic down-conversion link based on coherent in-phase/quadrature (I/Q) image rejection and digital nonlinear compensation. The image-rejection ratio at 17.5 GHz is measured to be 47 dB. After digital processing, the image intermodulation distortions (MMD) and the intermodulation distortions (IMD3) are suppressed by 18.1 dB and 10.9 dB, respectively. The corresponding spurious-free dynamic range (SFDR) reaches 108.83 dB·Hz2/3.

Observation of Non-Abelian Charged Nodes Linking Nonadjacent Gaps.

Nodal links are special configurations of band degeneracies in the momentum space, where nodal line branches encircle each other. In PT symmetric systems, nodal lines can be topologically characterized using the eigenvector frame rotations along an encircling loop and the linking structure can be described with non-Abelian frame charges involving adjacent bands. While the commutation rules between the frame charges are well established, the underlying relationship between distant band gap closing nodes remains to be explored. In this Letter, we present a photonic multiple nodal links system, where the nodal lines of nonadjacent bands are investigated with symmetry constraints on frame charges. Through an orthogonal nodal chain, the nodal line from the lower two bands predicts the existence of nodal lines formed between the higher bands. We designed and fabricated a metamaterial, with which the multiple nodal links and the topological connection between nonadjacent nodal lines are experimentally demonstrated.

Generating Loss Dependent Bifid Dips of Optical Carrier Based Microwave With Self-Feedback Anti-Symmetric Coupled Fiber Ring Resonator

A self-feedback anti-symmetric coupled fiber ring resonator (SACFRR) is proposed and demonstrated to generate the distinctive loss dependent bifid dips. The universal derivation of the ring resonators in both lightwave and optical carrier based microwave domains considering different conditions including attenuation, coupling and self-feedback is systematically elaborated, which is applicable to diverse ring resonator configurations. The lightwave and optical carrier based microwave experience different phase evolution during the cross-coupling procedure, thereby causing very different output. The anti-symmetric coupling strategy is creatively introduced into the ring resonator, which dominates the splitting of the original resonance dip for the optical carrier based microwave interferometry (OCMI). And OCMI shows the noticeable bifid dips within the lower envelope which receives less attention before. By performing the measurement of temperature and bend-induced loss, the bifid dips based demodulation of OCMI is employed to present the property of SACFRR and compared with the conventional upper envelope based demodulation, contributing to the utilization of lower envelope information. The temperature sensitivity is 521.4 kHz/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{\circ }$</tex-math></inline-formula> C at 3.05 GHz and the bend-induced loss sensitivity is 24.845 MHz/turn (diameter of 2.30 cm) at 10 turns where is adjacent to the critical point of bifid dips merging. The proposed scheme provides an extraordinary design avenue which can be extended to diverse ring resonators in both photonic integrated circuits and fiber links for exploring compelling functions.

110 GHz, 110 mW hybrid silicon-lithium niobate Mach-Zehnder modulator

High bandwidth, low voltage electro-optic modulators with high optical power handling capability are important for improving the performance of analog optical communications and RF photonic links. Here we designed and fabricated a thin-film lithium niobate (LN) Mach-Zehnder modulator (MZM) which can handle high optical power of 110 mW, while having 3-dB bandwidth greater than 110 GHz at 1550 nm. The design does not require etching of thin-film LN, and uses hybrid optical modes formed by bonding LN to planarized silicon photonic waveguide circuits. A high optical power handling capability in the MZM was achieved by carefully tapering the underlying Si waveguide to reduce the impact of optically-generated carriers, while retaining a high modulation efficiency. The MZM has a V_pi L product of 3.1 V.cm and an on-chip optical insertion loss of 1.8 dB.

Superior dark-state cooling via nonreciprocal couplings in trapped atoms

Cooling the trapped atoms toward their motional ground states is key to applications of quantum simulation and quantum computation. By utilizing nonreciprocal couplings between two atoms, we present an intriguing dark-state cooling scheme in Λ-type three-level structure, which is shown superior than the conventional electromagnetically-induced-transparency cooling in a single atom. The effective nonreciprocal couplings can be facilitated either by an atom–waveguide interface or a free-space photonic quantum link. By tailoring system parameters allowed in dark-state cooling, we identify the parameter regions of better cooling performance with an enhanced cooling rate. We further demonstrate a mapping to the dark-state sideband cooling under asymmetric laser driving fields, which shows a distinct heat transfer and promises an outperforming dark-state sideband cooling assisted by collective spin–exchange interactions.

Highly linear lithium niobate Michelson interferometer modulators assisted by spiral Bragg grating reflectors.

Highly linear electro-optic modulators are key components in analog microwave photonic links, offering on-chip direct mixing of optical and RF fields. In this work, we demonstrate a monolithic integrated Michelson interferometer modulator on thin-film lithium niobate (LN), that achieves linearized performance by modulating Bragg grating reflectors placed at the end of Michelson arms. The modulator utilizes spiral-shaped waveguide Bragg gratings on Z-cut LN with top and bottom electrodes to realize extensive reflectors, essential for linearized performance, in a highly integrated form. Optical waveguides are realized using rib etching of LN with precisely engineered bottom and top cladding layers made of silicon dioxide and SU-8 polymer, respectively. The compact design fits a 3 mm long grating in an 80 µm × 80 µm area, achieving a broad operating bandwidth up to 18 GHz. A spurious free dynamic range (SFDR) of 101.2 dB·Hz2/3 is demonstrated at 1 GHz, compared to 91.5 dB·Hz2/3 for a reference Mach-Zehnder modulator fabricated on the same chip. Further enhancement in SFDR could be achieved by reducing fiber-to-chip coupling loss. The proposed demonstration could significantly improve the linearity of analog modulator-based integrated optical links.

A Microwave Photonic Link With Quadrupled Capacity Based on Coherent Detection and Digital Phase Noise Cancellation

A microwave photonic link (MPL) with quadrupled capacity based on coherent detection and digital phase noise cancellation is proposed and experimentally demonstrated. At the transmitter, a continuous-wave (CW) light wave is intensity modulated by four independent microwave vector signals with two having an identical center microwave frequency at a dual-parallel Mach-Zehnder modulator (DPMZM) consisting of two dual-drive MZMs (DEMZMs) with the sub-DEMEMs biased at the quadrature transmission point. Four intensity-modulated optical signals are generated and transmitted over a single-mode fiber (SMF) to a coherent receiver. To perform coherent detection, a second CW laser source as a local oscillator (LO) is also applied to the coherent receiver. To recover the microwave vector signals, a novel digital phase noise cancellation algorithm is developed and applied to eliminate the joint phase noise from the transmitter laser source and the LO laser source as well as the unstable offset frequency between the two laser sources. A theoretical analysis is performed to show the recovery of the microwave vector signals which is verified by an experiment. For four independent 16 quadrature amplitude modulation (16-QAM) microwave vector signals with a symbol rate of 0.5 GSymbol/s, error-free transmission over a 9-km SMF is achieved when the received optical power at the coherent receiver is higher than −18 dBm with forward error correction (FEC).

Frequency Response Tailoring of a Phase Modulated Link With Stimulated Brillouin Scattering

Phase-modulated analog photonic links with Mach Zehnder interferometers are used for signal transport and to reduce transmitter complexity. However, the frequency response of these links suffers from dispersion-induced transmission nulls, severely limiting the operational bandwidth. To overcome this challenge, we propose a method to compensate for the transmission dips of phase-modulated links. The operating bandwidth of an analog photonic link formed by a phase modulator and a Mach Zehnder interferometer is increased by superimposing a stimulated Brillouin scattering gain response over the transmission null, resulting in compensation of the null response. The proposed approach is simulated and experimentally verified for different delays of the interferometer and multiple null compensation is also demonstrated. Tunability of the null compensation is also shown for the L, S, C, X, Ku, and Ka bands. Furthermore, link gain improvement is demonstrated through balanced detection.

Photonic Sub-Terahertz IM Links: Comparison Between Double and Single Carrier Modulation

Two different modulation schemes have been employed in photonic sub-THz systems to transmit intensity modulated signals: the dual-carrier (D-C) and single-carrier (S-C) schemes. Although it has been stated in the literature that D-C modulation performs better than S-C modulation, no demonstration of such claim has been provided so far. In this letter, we show that the superiority of one scheme or the other depends on the normalization factor that is used for comparison. Through mathematical analysis we show that, when 2-pulse amplitude modulation (2-PAM) signals are considered and the average photocurrent is taken as the normalization factor, the D-C scheme exhibits a 3-dB gain over the S-C scheme in the peak voltage of the recovered signal. However, the analysis also reveals that equal performance should be obtained when these schemes are compared in terms of transmitted sub-THz energy and that the S-C scheme should achieve better results when the comparison is made in terms of the output power of the lasers. We also run simulations to examine the impact of the non-linear transfer curve of a Mach-Zehnder modulator (MZM) on higher-order IM formats such as 4- and 8-PAM. The results of these simulations show that the penalty of the S-C scheme under photocurrent normalization progressively vanishes as the modulation order increases, and for 8-PAM signals, the S-C achieve better results than the D-C scheme. We attribute the deterioration of the D-C scheme with higher modulation orders to the signal-signal beat interference (SSBI).