Analog Optical Links Research Articles

Micro-Ring Modulator Linearity Enhancement for Analog and Digital Optical Links

Micro-Ring Modulator Linearity Enhancement for Analog and Digital Optical Links

The Optimal Operating Point for Linearizing an Integrated Optical Lithium Niobate Directional Coupler Modulator

The influence of an operating point on the linearity of an integrated optical lithium niobate directional coupler modulator was studied. It was found that the optimal setting for the position of an operating point for suppressing the third-order intermodulation distortion depended on the power of the high-frequency modulation signal. Thus, despite the simple design of the device, the directional coupler modulator requires a complex algorithm for setting an operating point to achieve a high linearity of operation. An active system for setting an operating point based on the low-frequency pilot signal and zeroing its third harmonic was used to demonstrate the possibility of linearization when the amplitude of the modulation signal changes. The use of an operating point control system became possible after limiting the drift of the operating point by etching the dielectric buffer layer in the interelectrode gap. The results obtained look promising for high-performance analog optical links.

A TO-CAN DFB Laser with Transmission Line Impedance Transformer for Analog Optical Link

A TO-CAN DFB Laser with Transmission Line Impedance Transformer for Analog Optical Link

Optical printed circuit boards with multimode polymer waveguides and pluggable connectors for high-speed optical interconnects.

We demonstrate the development of optical printed circuit boards (OPCBs) containing multimode polymer waveguides and pluggable optical connectors. The basic optical characteristics of the PCB-embedded waveguides, waveguide connectors, and high-speed performance were comprehensively evaluated. The fabricated OPCB comprises eight electrical layers and one optical layer. Waveguides are terminated at both ends with MT/MPO connectors. The optical channels comprising 10 cm-long waveguides embedded in OPCBs with two connectors show an average insertion loss of 6.42 dB. The resulting coupling loss is 0.77 dB per interface, which is very low and to our knowledge is among the lowest reported to date for waveguides embedded in rigid PCBs. 30 Gbps per channel NRZ data transmission was demonstrated with a measured waveguide bandwidth of 23 GHz × m, which gives a possible data traffic of 720 Gb/s for such 24-channel parallel optical link. Our efforts lay the foundation for the further development of OPCBs with higher performance.

Multispectral sampling sequence formation in an analog optical link: the possibility of automatization using digital feedback

Multispectral sampling sequence formation in an analog optical link: the possibility of automatization using digital feedback

Optical link as an alternative for MRI receive coils: toward a passive approach.

The objective of this work was to propose an alternative solution to NMR signal transmission by replacing the coaxial cables of the receiver radiofrequency (RF) coil in the context of MRI so as to improve safety. Starting from the analysis of previous studies and reports on the topic, the difficulty of supplying power wirelessly to an RF coil was identified. To avoid this difficult task, the development of a passive analog optical link was studied. In order to quantify the requirements for achieving an analog conversion, the performance of the link was evaluated based on the input NMR signal amplitude and the optical power and compared with that of a galvanic link. Acquisitions were performed on a 7-T preclinical MRI system with a doped saline solution as phantom. A passive and MRI-compatible polarization-state custom-made modulator was tested as well as a commercial Mach-Zehnder interferometer. The conversion was not sensitive enough to keep similar SNRs, but the main source of noise was identified along with parameters for improvement. Optical power emitted by the laser, insertion loss, and full-phase inversion voltage of the modulators were found to be crucial characteristics for the application. These data indicate that custom application devices are required since the frequency, bandwidth, and amplitude of NMR signals are quite different to usual telecommunication signals. An electro-optic modulation and a transmission channel were successfully conceived and tested. Images were reconstructed with some significant SNR drawbacks that are expected to be compensated with an appropriate modulator. While technical challenges remain, our approach to a two-decades-long problem could solve a major issue of MRI safety by removing the need for supplying on-coil electrical current.

Testing High-Resolution Optical Spectrum Analyzers for Characterization of Analog Optical Links with External Modulation

Testing High-Resolution Optical Spectrum Analyzers for Characterization of Analog Optical Links with External Modulation

Design and Implementation of High-Linearity Wavelength Division Multiplexing Analog Optical Link

Design and Implementation of High-Linearity Wavelength Division Multiplexing Analog Optical Link

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.

Linearity-Enhanced Dual-Parallel Mach–Zehnder Modulators Based on a Thin-Film Lithium Niobate Platform

In this work, we report a linearity-enhanced dual-parallel Mach–Zehnder modulator (MZM) on a thin-film lithium niobate platform. By setting the optical and electrical splitting ratios at a specific condition, the third-order intermodulation distortions (IMD3) of the child MZMs cancel with each other, whereas the first-order harmonics (FH) reach the maximum. Passive devices instead of thermo-optical switches are used to control the optical power and phase of the child MZMs, which greatly improve the device stability and simplify the operation complexity. To the best of our knowledge, the experimental results show a record-high spurious-free dynamic range on a thin-film lithium niobate platform (110.7 dB·Hz2/3 at 1 GHz). The E-O response decayed about 1.9 dB from 10 MHz to 40 GHz, and the extrapolated E-O 3 dB bandwidth is expected to be 70 GHz. A half-wave voltage of 2.8 V was also achieved. The proposed modulator provides a promising solution for high-bandwidth and low-voltage analog optical links.

SNR Enhancement of up to 9.5 Db Utilizing Four-Wave Mixing for Angle-Modulated Analog Optical Links

Angle modulation, such as phase and frequency modulation (FM/PM), has recently attracted attention for improving the fidelity of analog optical links. This is because angle modulation can enhance the signal-to-noise ratio (SNR) of the original analog message simply by increasing the modulation index or the bandwidth. However, direct generation of a broadband optical angle-modulated signal is not necessarily easy. On the other hand, there is an indirect generation method, where a narrowband angle-modulated signal is first generated and then converted to a broadband signal with the help of nonlinear devices. This method is often used in FM radio systems, but few reports are available on the optical implementation of this indirect method. In this paper, therefore, we propose the optical implementation by using four-wave mixing (FWM) in an optical fiber. Since an optical fiber naturally has a third-order nonlinearity (i.e., the Kerr effect), it should be possible to use fiber nonlinearity for the optical implementation of the indirect method to increase the bandwidth of an optical angle-modulated signal. We propose two types of FWM-based methods. In the first type, an angle-modulated signal is injected into a dispersion-shifted fiber (DSF) with an unmodulated pump wave. Consequently, one of the idlers generated via the FWM has a double modulation index, and the SNR of the original signal can be enhanced by 6 dB. In the second type, the input becomes a conjugate pair of an angle-modulated signal. Consequently, the idler has a threefold modulation index, and therefore, this type enhances the SNR by 9.5 dB. We experimentally confirmed the concept. The results showed that the modulation index can actually be increased almost threefold, and the idler can enhance the SNR of the original signal by approximately 8.8 dB.

Testing high-resolution optical spectrum analyzers for characterization analog optical links with external modulation

Testing high-resolution optical spectrum analyzers for characterization analog optical links with external modulation

Electro-optic polymer ring resonator modulators [Invited

Electro-optic (EO) ring resonator modulators have a number of communications and scientific applications, including analog optical links, optical signal processing, and frequency comb generation. Among the EO materials used to fabricate ring modulators, the EO polymer has many promising characteristics, including a high EO coefficient of 100–200 pm/V (3–7 times larger than that of LiNbO3), an ultrafast EO response time (<10 fs), a low dielectric constant (3 to 4) with very little dispersion up to at least 250 GHz, and a straightforward spin-coating fabrication process. These inherent characteristics will be able to combine excellent EO properties with simple processing in achieving exceptional performance in a variety of high-speed optical modulation and sensing devices. This review focuses on the research and recent development of ring resonator modulators based on EO polymers. The first part describes the operation principle of EO ring resonator modulators, such as modulation mechanism, EO tunability, and 3 dB bandwidth. Subsequently, the emphasis is placed on the discussion of the ring modulators with EO polymers as the waveguide core and the improvement of EO modulation by using an EO polymer/titanium dioxide hybrid core. At the end, a series of EO polymers on silicon platforms including slot modulators, etching-free modulators, and athermal modulators are reviewed.

Machine Learning Techniques in Radio-over-Fiber Systems and Networks

The radio-over-fiber (RoF) technology has been widely studied during the past decades to extend the wireless communication coverage by leveraging the low-loss and broad bandwidth advantages of the optical fiber. With the increasing need for wireless communications, using millimeter-waves (mm-wave) in wireless communications has become the recent trend and many attempts have been made to build high-throughput and robust mm-wave RoF systems during the past a few years. Whilst the RoF technology provides many benefits, it suffers from several fundamental limitations due to the analog optical link, including the fiber chromatic dispersion and nonlinear impairments. Various approaches have been proposed to address these limitations. In particular, machine learning (ML) algorithms have attracted intensive research attention as a promising candidate for handling the complicated physical layer impairments in RoF systems, especially the nonlinearity during signal modulation, transmission and detection. In this paper, we review recent advancements in ML techniques for RoF systems, especially those which utilize ML models as physical layer signal processors to mitigate various types of impairments and to improve the system performance. In addition, ML algorithms have also been widely adopted for highly efficient RoF network management and resource allocation, such as the dynamic bandwidth allocation and network fault detection. In this paper, we also review the recent works in these research domains. Finally, several key open questions that need to be addressed in the future and possible solutions of these questions are also discussed.

Enhanced analog-optical link performance with noiseless phase-sensitive fiber optical parametric amplifiers.

In this paper, we investigate the enhancement of analog optical link performance with noiseless phase-sensitive fiber optical parametric amplifiers. The influence of different noise sources in the link impacts the quality of analog optical signals, especially with low optical signal power, which has not been investigated before. Theoretically, the increase in signal-to-noise ratio and spurious-free dynamic range can be up to ∼6 dB and ∼4 dB, respectively, if the noise figure of optical pre-amplifier drops 3 dB when the received optical power is less than -65 dBm. In addition, experiments based on a 1.3 dB-noise-figure phase-sensitive fiber optical parametric amplifier and conventional optical pre-amplifiers are implemented, and the measured results agree with the theoretical expectations. This illustrates that noiseless phase-sensitive optical amplification may pave the way to long-haul distribution of analog optical signals and find applications in microwave-photonic systems.

Nonlinear Characteristics of Uni-Traveling Carrier Photodiode With InGaAs/GaAsSb Type-II Multiple Quantum Wells Absorber

The linearity of high power photodetectors is the limiting factor to the dynamic range performance of analog optical links and microwave photonics applications. In this work, we characterized the third order intermodulation distortion (IMD3) of a uni-traveling carrier photodiode with InGaAs/GaAsSb type-II multiple quantum wells absorber. The 10 μm diameter photodiode shows a third order output intercept point of 36.6 dBm at 200 MHz and 12.9 dBm at 25 GHz, under reverse bias of −5 V and photo current of 15 mA. Combining with the measured device parameters, an equivalent circuit model based simulation is carried out, and the general trend of the simulated IMD3 is consistent with the measurement results. By separately involving the nonlinear mechanisms, the source of the nonlinearity is studied in detail.

Mitigating weak dispersion in affordable RF-over-fiber channels.

A dispersion-compensating filter for next-generation low-cost analog and digital optical links is developed. In these high-frequency optical channels, even weak dispersion can be detrimental to data encoding (e.g., radar detection). Dispersion-compensating filters must be affordable for these low-cost channels, and they must be reliable for weak dispersion channels. In this paper, we design a dispersion-compensating filter based on three properties of the analog channel: weak modulation depth, a spectrally bounded signal, and a low-dispersion channel. We calculate the fundamental performance limits of the channel with and without the filter and quantify the improvement. Furthermore, numerical simulations are taken to estimate the filter's performances for both pulse amplitude modulation digital channels and for analog ones. The simulations agree with the analytical derivation, and, in both cases, the filter's integration improves the channels' performances considerably.

Kerr Combs for Stimulated Brillouin Scattering Mitigation in Long-Haul Analog Optical Links

By limiting the optical input launch power, stimulated Brillouin scattering imposes detrimental effects on long haul analog optical links. Utilizing Kerr combs generated from an integrated Silicon Nitride microring resonator, we mitigate Brillouin scattering in a 25 Km sampled analog optical link. Such combs offer reduced footprint, high repetition rates and low power consumption, rendering them attractive for next generation integrated analog photonic links. The distribution of the optical carrier power over multiple spectral lines allows launching higher total average powers, significantly improving the link performance. Operating the link in an externally intensity-modulated direct-detection architecture, we compare link metrics using a soliton or a dark pulse as the sampling frequency comb source. An advantageous aspect of the dark pulse is its high pump conversion efficiency. We find that the Kerr comb pump conversion efficiency has a direct effect on the relative intensity noise and the link noise figure. We show that the spurious free dynamic range using Kerr combs can match that of the well-established electro-optic combs.

Reconfiguration in Source-Synchronous Receivers for Short-Reach Parallel Optical Links

This paper presents a source-synchronous receiver architecture for use in parallel optical links. The proposed system is reconfigurable, allowing any channel to be used as a clock or data lane. The architecture is designed for mode-division multiplexed (MDM) optical links with forwarded clocks and allows the sensitive clock signal to be placed in the lane with the least amount of optical crosstalk for a given photonic interconnect. This configurability, which accounts for variation in integrated optics by leveraging the more robust electronic chip, optimizes the performance in electronic/optic codesigned solutions and may improve the yield. The architecture contains a dynamic clock distribution network, able to send a reference clock signal from the chosen clock receiver to any other data-configured receiver. The proposed architecture has been implemented on an experimental chip consisting of two receivers designed in the 65-nm CMOS technology. Electrical measurements at 8 Gb/s were done, and bit error rate curves are presented. They demonstrate the ability to swap and repurpose the clock and data inputs between the receivers, with similar sensitivity upon reconfiguration as a proof of concept.

500 GHz plasmonic Mach-Zehnder modulator enabling sub-THz microwave photonics

Broadband electro-optic intensity modulators are essential to convert electrical signals to the optical domain. The growing interest in terahertz wireless applications demands modulators with frequency responses to the sub-terahertz range, high power handling, and very low nonlinear distortions, simultaneously. However, a modulator with all those characteristics has not been demonstrated to date. Here, we experimentally demonstrate that plasmonic modulators do not trade-off any performance parameter, featuring—at the same time—a short length of tens of micrometers, record-high flat frequency response beyond 500 GHz, high power handling, and high linearity, and we use them to create a sub-terahertz radio-over-fiber analog optical link. These devices have the potential to become a new tool in the general field of microwave photonics, making the sub-terahertz range accessible to, e.g., 5G wireless communications, antenna remoting, Internet of Things, sensing, and more.