Extinction Ratio Of Modulator Research Articles

Free-Space QKD with Modulating Retroreflectors Based on the B92 Protocol.

Free-space quantum key distribution (QKD) has attracted considerable attention due to its lower channel loss and link flexibility. It allows two participants share theoretical unconditional secure keys, and can potentially be applied to air-to-ground quantum communication to establish a global quantum network. Free-space QKD using modulating retro-reflectors (MRR-QKD) significantly reduces the pointing requirement and simplifies the structure of the mobile terminal, therefore making it suitable for lightweight aircraft such as unmanned aerial vehicle and Cubesat, etc. Based on intensity modulation of two non-orthogonal states and the B92 protocol, we proposed a scheme to improve the previous work (Optics Express 2018, 26, 11331). Our scheme simplifies the optical structure and shows more robustness in equipment imperfection. The analysis and simulation show that the number of multiple quantum well modulators needed in our scheme decreases from eight to three with similar performance. Additionally, while the previous scheme cannot work due to low modulator extinction ratio or high optical misalignment, our scheme can still operate.

Single-polarization single-mode hollow-core negative curvature fiber with nested U-type cladding elements

Hollow-core negative curvature fibers (HC-NCFs) have become one of the research hotspots in the field of optical fiber because of their potential applications in the data and energy transmissions. In this work, a new kind of single-polarization single-mode HC-NCF with nested U-type cladding elements is proposed. To achieve the single-polarization single-mode transmission, we use two different silica tubes in thickness, which satisfy the resonance and anti-resonance conditions on the U-type cladding elements and the cladding tubes, respectively. Besides, the elliptical elements are introduced to achieve good single-mode performance. By studying the influences of the structure parameters on the propagation characteristics, the optimized structure parameters are obtained. The simulation results show that when the wavelength is fixed at 1550 nm, the single-polarization single-mode transmission is achieved, with the polarization extinction ratio of 25749 and minimum high-order mode extinction ratio of 174. Furthermore, the confinement loss is only 0.0015 dB/m.

A survey of applied machine learning techniques for optical orthogonal frequency division multiplexing based networks

AbstractIn this survey, we analyze the newest machine learning (ML) techniques applied in modern optical orthogonal frequency division multiplexing (O‐OFDM) systems for access, core networks, and multi‐channel transmission. From rudimentary to more advanced approaches, ML is proven to be a gold standard technique for signal quality improvement when low transmitter modulation extinction ratio dominates in coherent O‐OFDM, and when stochastic‐induced nonlinearities are present such as parametric noise amplification in long‐haul transmission and the interplay between polarization‐mode dispersion and the Kerr‐induced nonlinearity. In addition, ML algorithms can effectively tackle determinist nonlinear distortions in O‐OFDM networks, as well as inter‐subcarrier nonlinear effects (ie, inter‐subcarrier four‐wave mixing and cross‐phase modulation). In essence, ML techniques could be potentially beneficial for any multi‐carrier approach (eg, filter bank modulation). The survey illustrates an extensive ML taxonomy for O‐OFDM based networks, covering supervised, reinforcement learning and unsupervised ML categories. The transmission performance of various ML‐assisted O‐OFDM systems is presented taking into account the ML computational complexity toward real‐time implementation. We also highlight the strict operating conditions for such systems under which a ML algorithm should perform classification, regression or clustering. Finally, the survey opens research issues and future directions toward ML implementation in radio‐over‐fiber (RoF) and 5G new radio (NR) systems.

Ultrawide bandwidth single-mode polarization beam splitter based on dual-hollow-core antiresonant fiber.

An ultrawide bandwidth and single-mode polarization beam splitter (PBS) based on an air-gap type of dual-hollow-core antiresonant fiber (DHC-ARF) is proposed. Nested tubes are introduced into two cladding tubes between two cores to weaken the wavelength dependence of coupling length in DHC-ARF for obtaining ultrawide bandwidth. By tuning the cladding tube sizes, higher-order core modes with the lowest loss can be coupled with cladding tube modes, and thus, effectively, single-mode operation is achieved. Numerical results demonstrate that an 8.15 cm long DHC-ARF can be used to develop a PBS with an operating bandwidth of 370 nm ranging from 1.28 to 1.65 µm, where a polarization extinction ratio is below -20dB and a high-order mode extinction ratio exceeds 100.

Ultrawide bandwidth dual sakura hollow-core antiresonant fiber polarization beam splitter

A dual sakura hollow-core antiresonant fiber (DSHC-ARF) is proposed for developing an ultrawide-bandwidth polarization beam splitter (PBS). An air gap is introduced into the fiber between dual sakura hollow cores acting as a mode-coupling channel, which determines the implementation of a DSHC-ARF-based PBS. We demonstrate the wavelength dependence of coupling length can be reduced by introducing nested tubes and investigate their impact on single-mode operation further. Through optimizing the fiber, a high-performance PBS based on a 4.42 cm long DSHC-ARF can be obtained with an ultrawide bandwidth of 460 nm ranging from 1400 to 1860 nm, good single-mode operation with a higher-order mode extinction ratio above 100, and a low fundamental mode loss below 0.543 dB.

Asymmetric all-dielectric active metasurface for efficient dual reflection modulation

Active metasurface provides an efficient way to achieve optical response in the subwavelength range, dielectric metasurface has attracted much attention due to its low-loss mode and excitable electric/magnetic resonance mode, resulting in a far wider range of applications. However, the resonance spectrum is relatively broad due to the strong radiative loss of the symmetric dielectric metasurface, which limits its application in large modulation extinction ratio. Hence, an active metasurface integrated with the phase change material Ge2Sb2Te5 (GST) is proposed. The active metasurface can support the symmetry-protected quasi bound states in the continuum (QBIC) and excite resonance modes with extremely high quality-factors. As an active medium, the GST layer undergoes a transition from the amorphous state to the crystalline state when the temperature increases, leading to a change in the amplitude/phase of the reflection spectrum. It is demonstrated that dual reflection modulation and the figure-of-merit (FoM) can reach up to 92.0% at the wavelength of 1.45 μm and 92.5% at the wavelength of 1.52 μm as the GST layer is in the middle of nanodisks. An extremely high FoM of 98.5% is also realized when the surface of silicon nanodisks is coated with the GST layer. In addition, the modulation mechanism of the optical response of the active metasurface has been investigated, which is of great significance to the design of the active metasurface. The proposed active all-dielectric asymmetric metasurface has a huge potential in tunable nonlinear optical devices.

Design of high extinction ratio, low loss, and compact optical modulators and switches based on GST phase change material

In this paper, an electrically driven optical modulator and switch based on micro-ring resonators using the phase change material (PCM) of G e 2 S b 2 T e 5 (GST) is designed and their optical properties are investigated. The change in refractive index between two states in PCMs is orders of magnitude larger than conventional electro-optic and free-carrier dispersion effects. These extraordinary variations make PCMs excellent candidates for optical active devices such as modulators and switches. In our design, GST is used as the active material because it is far more stable and results in lower insertion loss compared to other PCMs such as vanadium dioxide ( V O 2 ) (at λ = 1.55 µ m , the imaginary part of the refractive index is 1.49 for GST and 2.53 for V O 2 is their metallic phases). By optimizing the GST dimensions, the device figure of merit is maximized. Three-dimensional finite-difference time-domain simulations show a very high extinction ratio of 37.3 dB with a low insertion loss of 0.427 dB for the proposed modulator and high extinction ratios of 19 dB and 10 dB in off and on states and insertion losses of 1.52 dB and 2.02 dB in off and on states, respectively, of the proposed 1 × 2 switch.

Design of an all-fiber mode power splitter for mode-division-multiplexing transmission.

An all-fiber mode power splitter (MPS) for mode-division multiplexing (MDM) transmission is proposed and numerically investigated. The MPS is based on the configuration of the symmetric few-mode fiber (FMF) coupler, with the operation of four modes (LP01, LP11, LP21, and LP02). By comprehensively optimizing the FMF coupling parameters, including both the coupling area distance D and the coupling length L, MPSs with various power splitting ratios from 90%:10% to 50%:50% under the condition of different single guided mode injection are obtained. Then, the MPS with a 90%:10% power splitting ratio under the simultaneous injection of LP01, LP11, LP21, and LP02 modes is reported, which can be used as the mode tap for the optical performance monitoring. Moreover, based on the proposed MPS structure, selective mode power strippers with more than 25 dB mode extinction ratio for LP01, LP11, LP21, and LP02 modes are numerically verified. The arbitrary single guided mode arising in the FMF can be selectively coupled into another FMF, together with an insertion loss of less than 1 dB. Owing to the flexible setting of the mode power splitting ratio, the proposed MPS may become a potential solution to realizing system performance monitoring and mode power routing in MDM systems.

Single-polarization single-mode hollow-core photonic-bandgap fiber with thin slab waveguide.

A novel single-polarization single-mode hollow-core photonic bandgap fiber with thin slab waveguide (TSW) was designed and simulated. Single-polarization guidance is achieved by the high loss of a polarized fundamental mode (FM) induced by mode coupling with a higher-order TE/TM mode of TSW and low loss of another polarized FM. We achieve a polarization loss ratio ∼ 46.9 dB, birefringence Δn ∼ 2.4 × 10-4, loss ∼ 35.9 dB/km and minimum higher-order mode extinction ratio > 15 dB. Moreover, the performance could be maintained when the guidance wavelength λ = 1.44 ∼ 1.56 µm and bending radius rc > 9 mm. The proposed model will be suitable for application as resonator sensing paths of miniaturized resonator fiber optic gyroscopes, high-performance interferometers, fiber lasers, frequency metrology, quantum communications, and laser-based gas sensing, etc.

Low-loss single-mode modified conjoined tube hollow-core fiber.

We explain the effects of cladding geometries on conjoined tube hollow-core negative curvature fibers and offer a modified conjoined tube negative curvature fiber with appropriate positioning of an additional negative curvature D-shaped layer joining the flat bar to reveal attractive performances over existing recent related fibers. The proposed fiber ensures the least loss of 0.003 dB/km at 1.43 µm, a ∼0.04dB/km loss covering the wide bandwidth of approximately 300 nm, the lowest surface scattering loss of ∼0.02dB/km, and the lowest microbending loss of ∼0.04dB/km, thus providing a propagation loss of 0.10 dB/km at the 1.55 µm wavelength and also offering excellent bend loss performance (∼0.015dB/km loss at a 7 cm bend radius). The fiber, with a core diameter of 30.50 µm, also shows a higher-order mode extinction ratio of ∼1600 and maintains greater than 100 over most of the telecom bands; hence, it effectively provides single-mode operation. We show the potential of conjoined tube hollow-core negative curvature fibers in optical communications systems.

Understanding bending-induced loss and bending-enhanced higher-order mode suppression in negative curvature fibers

We present a numerical analysis on bending-induced loss and bending-enhanced higher-order mode suppression in negative curvature fibers. We provide underlying mechanisms on how geometrical parameters affect the bending properties. We find that fiber parameters influence the bending performance by altering the resonant coupling conditions, as well as light leakage through inter-tube gaps. We identify regions in the parameter space that exhibit excellent bending properties and offer general guidelines for designing negative curvature fibers that are less sensitive to bending. Moreover, we explore the possibility of enhancing higher-order core mode suppression through mechanical bending. We find that up to nine-fold increase in the higher-order mode extinction ratio can be achieved by bending the fiber.

Grating-Assisted Directional Coupler in Lithium Niobate for Tunable Mode Filtering

We propose a long-period grating assisted directional coupler in lithium niobate (LN) for the realization of electro-optic tunable mode filtering functionality. Our typical device, fabricated with the annealed proton-exchange process and designed for filtering out the fundamental mode in a two-mode LN waveguide, achieves a maximum mode extinction ratio of 29 dB at 1551.7 nm wavelength and a -20 dB bandwidth of ~2.5 nm with an electrical wavelength tuning sensitivity of 0.182 nm/V.

Significance of modulator extinction ratio in long distance coherent optical time domain reflectometry

In coherent optical time-domain reflectometry, external modulation is used to maintain the coherence of laser probe pulses launched into optical fibers. However, the residual continuous wave (CW) component produced by modulation may considerably degrade the system sensitivity. The backscattered signal from the pulse must be dominant compared to the CW signal. We discuss the effects of the finite extinction ratio (ER) on the instrument’s sensing range. A model analyzing the impact of the CW component on the backscattered signal as a function of the ER, fiber length, and pulse widths is proposed. It is also shown that acousto-optic modulation is more suitable than electro-optic modulation in optical fiber for longer distances. The results are confirmed experimentally in a 31–km-long fiber. A 1-Hz vibration was applied at 25.5 km, and the resulting signal-to-noise ratio of ∼13 dB was measured using 75-ns pulses. This result is in agreement with the performance predicted by the model.

Low-loss single-mode hybrid-lattice hollow-core photonic-crystal fibre

Remarkable recent demonstrations of ultra-low-loss inhibited-coupling (IC) hollow-core photonic-crystal fibres (HCPCFs) established them as serious candidates for next-generation long-haul fibre optics systems. A hindrance to this prospect and also to short-haul applications such as micromachining, where stable and high-quality beam delivery is needed, is the difficulty in designing and fabricating an IC-guiding fibre that combines ultra-low loss, truly robust single-modeness, and polarisation-maintaining operation. The design solutions proposed to date require a trade-off between low loss and truly single-modeness. Here, we propose a novel IC-HCPCF for achieving low-loss and effective single-mode operation. The fibre is endowed with a hybrid cladding composed of a Kagome-tubular lattice (HKT). This new concept of a microstructured cladding allows us to significantly reduce the confinement loss and, at the same time, preserve truly robust single-mode operation. Experimental results show an HKT-IC-HCPCF with a minimum loss of 1.6 dB/km at 1050 nm and a higher-order mode extinction ratio as high as 47.0 dB for a 10 m long fibre. The robustness of the fibre single-modeness is tested by moving the fibre and varying the coupling conditions. The design proposed herein opens a new route for the development of HCPCFs that combine robust ultra-low-loss transmission and single-mode beam delivery and provides new insight into IC guidance.

Filterless frequency 16-tupling millimeter-wave signal generation with cascaded Mach–Zehnder modulators

A simple scheme is proposed for generating frequency 16-tupling millimeter-wave (MMW) signal based on cascaded Mach–Zehnder modulators (MZMs) and an optical phase shifter (PS) without optical filter. Theoretical analysis is given to generate the high-quality 16-tupling MMW signal by eliminating the undesired optical carrier and sidebands. Simulations show that the generated 16-tupling optical MMW signal has the optical sideband suppression ratio higher than 29.85 dB and radio frequency (RF) spurious sideband suppression ratio higher than 23.9 dB. The impact of non-ideal modulation index, extinction ratio, and phase difference of optical PS- and RF-driven signal applied to the cascaded MZMs are discussed and analyzed. After different lengths of fiber transmission, the generated optical MMW signal shows good performance. Furthermore, the influence of different laser linewidths on the transmission performance of the generated MMW signal is also performed and analyzed.

3D Printed Effective Single-Mode Terahertz Antiresonant Hollow Core Fiber

An antiresonant hollow core effective single-mode terahertz fiber is proposed and successfully fabricated with a photosensitive resin (SomosEvoLVe 128) by a 3D printer. The single mode characteristics of the fiber are found to be related to the area of the semielliptical tubes in the cladding. By optimizing the cladding structure parameters, a Higher Order Mode Extinction Ratio (HOMER) of above 6 can be achieved. A 30 cm long sample with optimized structural parameters is obtained by cascading two 15 cm long fiber samples fabricated by a 3D printer. The transmission loss and the electric field distribution are measured by a terahertz time domain spectroscopy (TDS) system. The electric field distribution measured at the output end of the fiber sample has the Gaussian profile, which accords with single-mode field distribution. Experimental results show that the average loss is 0.048 cm−1 within the frequency range from 0.2 to 1 THz, and the minimum loss is obtained as low as 0.009 cm−1 at the frequency of 0.82 THz.

Photonic generation of frequency-quadrupled triangular waveform based on a DP-QPSK modulator with tunable modulation index

In this paper, a scheme for generating frequency-quadrupled triangular waveform based on a dual-polarization quadrature phase-shift-keying (DP-QPSK) modulator is proposed. The repetition rate of the generated triangular waveform is four times the frequency of radio frequency (RF) signal. In this scheme, the modulation index of modulator is no longer a fixed value, and it can be adjusted in a certain range, which increases the flexibility of the scheme. Compared with previous frequency-quadrupled triangular waveform generation schemes, the structure of this scheme is simple, and the scheme enables good repetition rate tunability. The effect of extinction ratio and DC bias drift of modulator on the scheme is analyzed by simulation.

Photonic 2-D angle-of-arrival estimation based on an L-shaped antenna array for an early radar warning receiver.

Early radar warning is a significant step to lessen the fine scanning range of a receiver. The small size two-dimension (2-D) angle-of-arrival (AOA) estimation part with moderate accuracy and sensitivity is important for an early radar warning receiver. In our method, we specially design an L-shaped antenna array (L-sAA) and connect it with dual-polarization binary phase shift keying modulator (DP-BPSKM). The dual-sideband (DSB) modulation is performed to transfer most of the optical power to electrical, so as to increase the sensitivity. It is also possible to map the AOA information of the incoming beam to photo-detected electrical power without a high extinction ratio modulator or optical filter. During the estimation, the 2-D AOA is firstly measured, whose measurement range is 18.22°∼90° and the measurement error is lower than 1°. Then, based on the 2-D AOA estimation results, the third one is mathematically calculated to construct 3-D location of the target. Noteworthy, the amplitude comparison function (ACF) is adopted in this method to make the system response irrelative to the received signal power, which endows the system with signal power fluctuation immunity. Experimental results show that this method is capable of measuring a single-tone signal and a bandwidth signal. This structure is very concise and meets the potential of on-chip integration.

Multi-Function Mode Processing Device for Mode Division Multiplexing Optical Networks

In this study, we propose a configurable multifunction device for the mode division multiplexing (MDM) optical networks. The device can simultaneously function as low- and high-order mode block filters and a mode exchanger by selectively tuning different working states. We fabricated the device using standard microfabrication technology and the experimental performance of the device adequately follows the simulated performance. Particularly, the device exhibits a measured mode extinction ratio of 22.20 dB and 19.46 dB as a low-order mode block filter at 1.550 μm wavelength. The proposed device has great potential for use in a fully flexible and advanced MDM optical system.

Probabilistically Shaped 4-PAM for Short-Reach IM/DD Links With a Peak Power Constraint

Probabilistic shaping for intensity modulation and direct detection (IM/DD) links is discussed and a peak power constraint determined by the limited modulation extinction ratio (ER) of optical modulators is introduced. The input distribution of 4-ary unipolar pulse amplitude modulation (PAM) symbols is optimized for short-reach transmission links without optical amplification nor in-line dispersion compensation. The resulting distribution is symmetric around its mean allowing to use probabilistic amplitude shaping (PAS) to generate symbols that are protected by forward error correction (FEC) and that have the optimal input distribution. The numerical analysis is confirmed experimentally for both an additive white Gaussian noise (AWGN) channel and a fiber channel, showing gains in transmission reach and transmission rate, as well as rate adaptability.