Modulated Optical Signal Research Articles

Double rectangular-grooves metasurface for highly efficient electric modulation

With the rapid development of optical communication, how to achieve efficient modulation (fast response speed and high modulation depth) of optical signals has attracted more and more attention from researchers. Among all electro-optical modulator (EOM) designs, the electro-optical metasurface is undoubtedly a competitive solution for optical signal modulation in free space. Although current research on electro-optical metasurfaces has realized improving response speed owing to the Pockels effect, there are still difficulties in achieving high modulation depth under CMOS-compatible voltage and developing rational designs of metasurfaces to achieve voltage application that trigger electro-optical effects. In this work, an ultrahigh-Q factor BaTiO3 (BTO) electro-optical metasurface, which consists of a periodic array of rectangular grooves, was designed to provide a feasible solution to address these shortcomings. Based on bound states in the continuum (BIC) theory, ultrahigh-Q factor (2.87 × 105) quasi-BIC (Q-BIC) was obtained around 1550 nm by breaking the in-plane symmetry of the two rectangular grooves in a unit cell, which could significantly deepen the modulation depth. The concave and continuous structure of rectangular grooves made the application of voltage more efficient. The simulation results show that an optical signal modulation in free space with a modulation depth of 100% could be achieved. Multipole decomposition indicated that toroidal dipole (TD) was dominant in this Q-BIC. Our work may further promote the development of electro-optical modulation towards faster and deeper modulation.

Impressive nonlinear optical response of π-conjugated, photoluminescent soft nanoparticles in polymer matrix

Nonlinear optical (NLO) materials are useful for applications varying from modulation of optical signals to cancer therapy. Besides inorganic crystals and organic molecules with defined structures, carbonized products have emerged as a new generation of materials showing good NLO properties. Due to the small sizes, the NLO performance of such materials mainly originates from nonlinear absorption. Obtaining a product with a high content of π-conjugated moiety is the key to improving the NLO performance. By using aldehydes bearing π units as the precursors, in this work we synthesized three polymeric nanoparticles (PNPs) through one-step pyrolysis, which also have large contents of π-conjugated structures. It was found that a larger size of π unit in the precursor led to a higher content as well as the size of the π-conjugated structure, which induced a bathochromic shift in the emission and an improved performance in the NLO response. When embedded in polymethylmethacrylate films, the products showed reverse saturable absorption (RSA) behavior with β up to 4.20 × 10−6 m W−1 and optical limit threshold down to 0.0012 J cm−2, which are comparable to those of metal-doped counterparts. This work gives deep insight on the structure-property relationship of the pyrolyzed products from π-conjugated small molecules, which is also useful for the preparation of task-specific carbon dots (CDs) or PNPs in future.

Manipulating photonic spin Hall split based on nonlinear effect and surface plasmon resonance

In this work, we present a surface plasmon resonance (SPR) structure containing AuAl2 and a nonlinear Kerr medium. Based on the modulation of the refractive index of the Kerr medium by pumped light, we analyze the effects of the SPR structure and pumped light on the photonic spin Hall effect (PSHE). The numerical results indicate that the thickness of the AuAl2 exerts a profound impact on the SPR, while the intensity of the pump light (Ip) exerts a significant influence on the resonance angle. The transverse shift (TS) is very sensitive to the refractive index (RI) of the sensing layer after increasing through the SPR. Furthermore, it is observed that the sign of the TS of the PSHE can be modulated according to Ip within a specific angle of incidence. In light of these findings, we put forth a novel optical signal modulation and RI sensing scheme based on PSHE. Our study provides a new mechanism for practical applications of PSHE in optical communication and precision measurement.

Novel Optical Modulator Photonic Device Based on TiN/Ti3C2 Heterojunction.

Due to the ability of optical modulators to achieve rapid modulation of optical signals, meeting the demands of high-speed data transmission, modulators based on different novel nanomaterials have become one of the research hotspots over the past dacade. Recently, TiN/Ti3C2 heterojunction exhibits highly efficient thermo-optic performance and extremely strong stability. Therefore, we have demonstrated an all-optical modulator based on the principle of Michelson interference and the thermo-optic effect in this paper. The modulator employs a TiN/Ti3C2 heterojunction-coated microfiber (THM) and further demonstrates its ability to generate phase shifts through an ASE light source. The modulator, with a phase shift slope of 0.025π/mW, can also convert the phase shifts of signal light into amplitude modulation through Michelson interference. The fixed signal light wavelength is 1552.09 nm, and the modulation depth is stable at about 26.4 dB within a wavelength detuning range of -10 to 6 nm; The waveforms of signal light at modulation rates of 500 Hz, 1000 Hz, 2000 Hz, and 3000 Hz were tested, and a 3 dB modulation bandwidth of 2 kHz was measured. The all-optical modulator based on THM has the advantages of high efficiency and stability and has broad application prospects in the fields of all-optical signal processing and high-speed optical communication.

Designing an Optical Router Based on a Multimode-Interference Silicon-On-Insulator Coupler with Tunable Power Transmittance

The demand on fast and high-bandwidth data transmission is in continuous increase. These demands are highly dependent on optical signal manipulation, including switching, modulation, and routing. We demonstrate a two-port silicon optical router based on the multimode interferometer (MMI) configuration. The same MMI structure was used for both inward and backward waveguiding to reduce the total length of the device. A phase shifter consisting of two ring-like waveguides made of silicon p-n junctions was used to introduce the phase shift needed for optical routing upon voltage application. Two designs for the MMI optical router were studied: Firstly, a conventional MMI with a crosstalk ratio of 15.1 dB was investigated. Finally, an angled MMI reaching a crosstalk ratio of 18.2 dB at a wavelength of 1.55 μm was investigated.

Micro-opto-mechanical glass interferometer for megahertz modulation of optical signals

Waveguide-based interferometric circuits are widely employed in optical communications, sensing, and computing applications. In particular, glass-based devices are appealing due to the transparency and bio-compatibility of this substrate, or where low-loss interfacing with fiber networks is required. However, fast electro-optic phase modulation is hard to achieve in glass materials. Here, we demonstrate an optical phase and intensity modulator in glass, working in the megahertz range. This modulator exploits the elasto-optic effect inside a mechanical microstructure, brought to oscillation at resonance, and is entirely realized by femtosecond laser micromachining. In detail, we demonstrate 23-dB optical intensity modulation at 1.17 MHz, with an internal optical loss of the phase-modulator component as low as 0.04 dB.

16QAM optical signal generation scheme based on weighted optimal Euclidean distance.

A two-dimensional signal constellation scheme for binary uniform memoryless source transmission in optical fiber channels is studied in this paper. In geometric shaping (GS), optimization algorithms are usually used to change the overall position of constellation points while maintaining the probability of constellation points unchanged. Different optimization functions are used to allocate the position of constellation symbols, thereby improving constellation performance. A 16 quadrature amplitude modulation (QAM) optical signal generation scheme based on weighted optimal Euclidean distance is proposed in this paper. In order to obtain the best constellation diagram and increase the shaping gain, the weighted optimal Euclidean distance that can minimize the bit error rate (BER) over multiple iterative optimizations is used as the objective function. On the one hand, the proposed 16QAM optical signal generation scheme based on weighted optimal Euclidean distance always outperforms the uniform square 16QAM and the uniform circle 16QAM schemes in the back to back (BTB) transmission. On the other hand, after analyzing the simulation demonstration in a 50GBaud coherent optical communication system over 3000km, results demonstrate that the optical signal to noise ratio (OSNR) performance of this system is better than that of the uniform square 16QAM and the uniform circle 16QAM, which is improved by 0.52dB and 0.85dB, respectively. In addition, the proposed 16QAM system increases the transmission distance by 989km and 741km, respectively, compared to the other two systems. The performance confirms that the proposed novel 16QAM scheme, to the best of our knowledge, can effectively improve the reliability and transmission distance. Therefore, the proposed scheme has a certain development prospect in the future long-distance transmission of high-speed optical fiber communication.

Giant Ultrafast All-Optical Modulation Based on Exceptional Points in Exciton-Polariton Perovskite Metasurfaces.

Ultrafast all-optical modulation with optically resonant nanostructures is an essential technology for high-speed signal processing on a compact optical chip. Key challenges that exist in this field are relatively low and slow modulations in the visible range as well as the use of expensive materials. Here we develop an ultrafast all-optical modulator based on MAPbBr3 perovskite metasurface supporting exciton-polariton states with exceptional points. The additional angular and spectral filtering of the modulated light transmitted through the designed metasurface allows us to achieve 2500% optical signal modulation with the shortest modulation time of 440 fs at the pump fluence of ∼40 μJ/cm2. Such a value of the modulation depth is record-high among the existing modulators in the visible range, while the main physical effect behind it is polariton condensation. Scalable and cheap metasurface fabrication via nanoimprint lithography along with the simplicity of perovskite synthesis and deposition make the developed approach promising for real-life applications.

Spatio-spectral control of coherent nanophotonics

Abstract Fast modulation of optical signals that carry multidimensional information in the form of wavelength, phase or polarization has fueled an explosion of interest in integrated photonics. This interest however masks a significant challenge which is that independent modulation of multi-wavelength carrier signals in a single waveguide is not trivial. Such challenge is attributed to the longitudinal direction of guided-mode propagation, limiting the spatial separation and modulation of electric-field. Here, we overcome this using a single photonic element that utilizes active coherent (near) perfect absorption. We make use of standing wave patterns to exploit the spatial-degrees-of-freedom of in-plane modes and individually address elements according to their mode number. By combining the concept of coherent absorption in spatio-spectral domain with active phase-change nanoantennas, we engineer and test an integrated, reconfigurable and multi-spectral modulator operating within a single element. Our approach demonstrates for the first time, a non-volatile, wavelength-addressable element, providing a pathway for exploring the tunable capabilities in both spatial and spectral domains of coherent nanophotonics.

Eliminating Dispersion Power Fading in Radio over Fiber Communication through Unbalanced Sideband Modulation

In Radio Over Fiber (RoF) systems, a commonly used approach to reduce the Dispersion Power Fading (DPF) effects is to convert the spectrum of modulated optical signals into Optical Single Sideband (OSSB). This process can be done by modulating the optical carrier using two RF signals that differ in phase by 90. The optical modulator used is the Dual-Drive Mach Zehnder Modulator (DD-MZM). However, implementing this modulation scheme requires a complex transmitter setup. In this study, a novel technique is proposed to address DPF, called as Unbalanced Sideband Modulation (USM). USM is generated by varying the optical signal carrier according to the RF signal using a Single-Drive Mach Zehnder Modulator (SD-MZM). The output of the SD-MZM is subsequently combined with an optical signal of a frequency equivalent to the sum of the optical carrier frequency and the Radio Frequency (RF) signal frequency. The result outcomes indicate that at a modulation index (m) of 1, the DPF level in an RoF link employing conventional SDMZM as an Electro-Optic (E/O) converter is approximately 8.2. In contrast, when using the OSSB produced by the DDMZM, the DPF level of a RoF link is around 0.9. Remarkably, the DPF level significantly drops to 0.2 when the USM scheme is adopted. This result shows the high efficacy of the USM scheme in eliminating DPF compared to both the conventional SD-MZM modulation scheme and the OSSB scheme generated by DD-MZM. Both model calculations and numerical simulations conclusively demonstrate that the USM method effectively mitigates DPF, offering a promising solution in optical communication systems.

Polaronic Nonlinear Optical Response and All-Optical Switching Based on an Ionic Metal Oxide.

It has been well-established that light-matter interactions, as manifested by diverse linear and nonlinear optical (NLO) processes, are mediated by real and virtual particles, such as electrons, phonons, and excitons. Polarons, often regarded as electrons dressed by phonons, are known to contribute to exotic behaviors of solids, from superconductivity to photocatalysis, while their role in materials' NLO response remains largely unexplored. Here, the NLO response mediated by polarons supported by a model ionic metal oxide, TiO2, is examined. It is observed that the formation of polaronic states within the bandgap results in a dramatic enhancement of NLO absorption coefficient by over 130 times for photon energies in the sub-bandgap regions, characterized by a 100fs scale ultrafast response that is typical for thermalized electrons in metals. The ultrafast polaronic NLO response is then exploited for the development of all-optical switches for ultrafast pulse generation in near-infrared (NIR) fiber lasers and modulation of optical signal in the telecommunication band based on evanescent interaction on a planar waveguide chip. These results suggest that the polarons supported by dielectric ionic oxides can fill the gaps left by dielectric and metallic materials and serve as a novel platform for nonlinear photonic applications.

Transmitter dispersion eye closure quaternary assessment based on linearCNN with 1 × 1 convolutional kernel.

Transmitter dispersion eye closure quaternary (TDECQ) is a vital metric to characterize the quality of four-level pulse amplitude modulation (PAM-4) optical signals. However, the traditional TDECQ assessment scheme is complex and time consuming, with heavy iterative operations. Therefore, accelerating the TDECQ assessment has great significance for photonic data-center interconnection (DCI) applications. Here, we propose and experimentally demonstrate a TDECQ assessment based on linear-convolutional neural network (L-CNN) with the 1 × 1 convolutional kernel to reduce the implementation complexity. Our experimental results verify that the lightweight L-CNN can realize the accurate TDECQ assessment, without the involvement of nonlinear activation functions (NAFs). The mean absolute error (MAE) of 26.5625 and 53.125 GBaud PAM-4 signals are 0.16 dB and 0.18 dB, respectively, over a TDECQ range from 1.5 to 4.0 dB. Meanwhile, in comparison with existing CNN-based schemes, the L-CNN based TDECQ assessment scheme only needs 2048 multiplications, which have been reduced by five orders of magnitude.

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

Chaotic Internal Dynamics of Dissipative Optical Soliton Molecules

AbstractWhen a laser cavity supports the propagation of several ultrashort pulses, these pulses can form compact bound states called soliton molecules. Soliton molecules are fascinating objects of nonlinear science, presenting striking analogies with their matter molecule counterparts. The relative timing and phase between the copropagating pulses are the most salient internal degrees of freedom of the soliton molecule. The soliton pair, composed of two identical pulses, constitutes the chief soliton molecule of fundamental interest. Its two major internal degrees of freedom allow self‐oscillating soliton molecules, which have been recurrently observed. However, despite theoretical predictions, the low‐dimensional chaotic dynamics of a soliton‐pair molecule remain elusive. This article reports the observation of chaotic soliton‐pair molecules within an ultrafast fiber laser by means of a direct measurement of the relative optical pulse separation with sub‐femtosecond precision in real time. Moreover, it demonstrates an all‐optical control of the chaotic dynamics followed by the soliton molecule by injection of a modulated optical signal that resynchronizes the internal periodic vibration of the soliton molecule. The fast error‐free switching between ordered and chaotic soliton molecules enabled by pump current sweeping and external injection highlights the potential prospects of all‐optical logic gates and chaotic communication using soliton molecules.

Integration of Raspberry Pi and antennas for multiplexing digital signals over a fiber optical communication system

A Fiber Optical Communication System (FOCS) is used to experimentally demonstrate the transmission of multi-channel information of two High Definition (HD) video signals and 18 digital TV channels. These signals are supplied using commercial Raspberry Pi boards and digital TV Antennas, respectively. Both digital signals and lightwaves of a couple of Distributed Feedback (DFB) lasers with wavelengths located at 1547.5 and 1552.5 nm interact with two Mach-Zehnder Intensity Modulators (MZ-IM). The resulting modulated optical signals from each modulator are combined by using an optical coupler. The combined optical field is amplified and transmitted through 30 km of single-mode optical fiber. To demultiplex the combined optical signals, a bandpass optical filter is used, then a PIN-photodetector is connected to recover HD video signals and the 18 digital TV channels. The measured electrical spectrum of the received signals shows that the transmission is performed without interference and the multi-channel information is successfully received in an HD display. The use of wavelengths with linewidths of 10 MHz and an optical fiber with a chromatic dispersion of 17 ps/nm-km allows for a maximum bit rate of approximately 11 Gbps to be achieved numerically. This result guarantees that HD video signals can be experimentally transmitted successfully with Bit Error Rate (BER) values between 10-6 and 10-15 when the optical power of lasers is tuned between 7 and 9 dBm.

Quantitative optical imaging method for surface acoustic waves using optical path modulation

A Rayleigh-type surface acoustic wave (SAW) is used in various fields as classical and quantum information carriers because of its surface localization, high electrical controllability, and low propagation loss. Coupling and hybridization between the SAW and other physical systems such as magnetization, electron charge, and electron spin are the recent focuses in phononics and spintronics. A precise measurement of the surface wave amplitude is often necessary to discuss the coupling strengths. However, there are only a few such measurement techniques and they generally require a rather complex analysis. Here we develop and demonstrate a straightforward measurement technique that can quantitatively characterize the SAW. The technique optically detects the surface waving due to the coherently driven SAW by the optical path modulation. Furthermore, when the measurement system operates in the shot-noise-limited regime, the surface slope and displacement at the optical spot can be deduced from the optical path modulation signal. Our demonstrated technique will be an important tool for SAW-related research.

Development of a Methane-Detection System Using a Distributed Feedback Laser Diode and Hollow-Core Photonic Crystal Fiber

A highly integrated methane-detection system was experimentally established by using a distributed feedback laser diode and hollow-core photonic crystal fiber. The self-developed circuits with a laser diode and essential optical devices were integrated into an instrument that generated a modulated optical signal in a fiber-coupled gas cell that contained the hollow-core photonic crystal fiber. The instrument could also process the return optical signal that contained the gas concentration information. The experiments demonstrated the good performance of the developed system. In the spectrum tests, the center wavelength of the laser diode could be tuned linearly by controlling the laser’s working temperature and driving current. The second harmonic signal could be extracted in order to reflect the gas concentration. According to the Allan deviation method, the low limit of detection of the system was determined to be 29.52 ppm. In addition, a long-term stability test demonstrated that the system has a good stable performance. The proposed system can be further optimized in order to be applied in paddy fields to detect and monitor the methane concentration in a large area by using the optical fibers.

Toward augmenting tip-enhanced nanoscopy with optically resolved scanning probe tips

A thorough understanding of biological species and emerging nanomaterials requires, among other efforts, their in-depth characterization through optical techniques capable of nanoresolution. Nanoscopy techniques based on tip-enhanced optical effects have gained tremendous interest over the past years, given their potential to obtain optical information with resolutions limited only by the size of a sharp probe interacting with focused light, irrespective of the illumination wavelength. Although their popularity and number of applications is rising, tip-enhanced nanoscopy (TEN) techniques still largely rely on probes that are not specifically developed for such applications, but for atomic force microscopy. This limits their potential in many regards, e.g., in terms of signal-to-noise ratio, attainable image quality, or extent of applications. We take the first steps toward next-generation TEN by demonstrating the fabrication and modeling of specialized TEN probes with known optical properties. The proposed framework is highly flexible and can be easily adjusted to be used with diverse TEN techniques, building on various concepts and phenomena, significantly augmenting their function. Probes with known optical properties could potentially enable faster and more accurate imaging via different routes, such as direct signal enhancement or facile and ultrafast optical signal modulation. We consider that the reported development can pave the way for a vast number of novel TEN imaging protocols and applications, given the many advantages that it offers.

Optical Signal Modulation in Photonic Waveguiding Heteroarchitectures with Continuously Variable Visible‐To‐Near‐Infrared Emission Color (Adv. Mater. 45/2022)

Photonic Logic Gates In article number 2204839, Chi-Ming Che and co-workers describe a photonic logic gate device based on supramolecular heterostructures made by phosphorescent Au, Cu, Pt, and Pd complexes. The photonic logic gate operation is realized by an optical waveguiding effect and energy transfer among the different building blocks. These photonic logic gates have high emission anisotropy, low optical waveguiding loss, and continuously varied emission colors ranging from blue to near-infrared spectral regions.

Ultra-narrow bandwidth and large tuning range single-passband microwave photonic filter based on Brillouin fiber laser

A scheme to perform ultra-narrow filter bandwidth and high frequency selectivity microwave photonic filter (MPF) with wide tuning range is proposed and experimentally demonstrated. The ultra-narrow bandwidth of the MPF is implemented by a Brillouin laser resonator, which is composed of a cascaded ring Fabry–Pérot (CR-FP) resonator formed by a main cavity with a cavity length of 100 m and a secondary cavity with a cavity length of 10 m. A Brillouin laser resonator is formed by the main cavity to obtain an extremely narrow comb-shaped Brillouin gain spectrum. The Vernier effect of the double ring cavity can effectively suppress the side modes, thereby realizing the narrow linewidth Brillouin laser filtering. Besides, two identical tunable lasers provide the stimulated Brillouin scattering (SBS) pump light and optical carrier signal for this device respectively. After the interaction between the Brillouin gain spectrum and the optical modulation signal, the filter passband can be stably tuned by simply changing the wavelength of SBS pump light. The experimental results show that the microwave photonic filter can be stably tuned in the frequency range of 0–20 GHz, the out-of-band rejection ratio is about 20 dB, and the minimum 3 dB bandwidth is about 114 Hz.