Low Extinction Ratio Research Articles

Non‐Volatile Optical Switch Element Enabled by Low‐Loss Phase Change Material

AbstractMach–Zehnder interferometers (MZIs) integrated with phase‐change materials have attracted great interest due to their low power consumption and ultra‐compact size, which are favored for reconfigurable photonic processors. However, they suffer from a low optical extinction ratio and limited switching cycles due to high material loss and poor reversible repeatability caused by material degradation. Here a non‐volatile electrically reconfigurable 2 × 2 MZI integrated with a low‐loss phase‐change material Sb2Se3 encapsulated in Al2O3 layers is demonstrated. The phase change is electrically actuated by a forward‐biased silicon p‐i‐n diode. The switch extinction ratio is more than 20 dB due to the low‐loss Sb2Se3‐based phase shifter. By dividing the Sb2Se3 patch into small sub‐cells to restrict the material reflow, more than 10 000 reversible phase‐change cycles and 6‐bit multilevel switching states are achieved by programming the electrical pulses. Its non‐volatility, high endurance, and fine‐tuning capability makes the device promising in large‐scale low‐power reconfigurable photonic processors.

Low Loss, Broadband, and Non-Volatile ‘Directed Logic Operations’ Using Phase Change Materials in Silicon Photonics

In this paper, we present novel architectures for non-volatile directed logic circuits, that demonstrate low insertion losses, high extinction ratios, as well as multi-bit and broadband operations. The non-volatile switching operations are performed using a non-volatile phase change material Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Sb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Se <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> Te <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> (GSST). The phase of the GSST is controlled by electro-thermal switching (from amorphous to crystalline and vice versa) using graphene micro-heaters. We demonstrate all the fundamental logic operations (such as OR, AND, XOR, NOR, NAND, and XNOR), adder, comparator, and parity checker with multi-bit operation capability. Moreover, we demonstrate directed logic architectures that can perform multiple logic operations (which includes providing inverted outputs) using the same logic circuit. The designs of the logic circuits show low insertion losses (< 1 dB) and high extinction ratios (> 10 dB) with broadband operations (~ 100 nm) operating in the telecommunication C-band. To the best of our knowledge, this is the first demonstration of the broadband operation for photonic directed logic circuits. For cascading multiple logic operations, the proposed architectures are ideal in terms of logic delay, extinction ratios and the ability to drive the number of other circuits. This study provides a practical technique to design future high-speed nanoscale non-volatile photonic integrated circuits for logic computing applications.

Wavelength-shift-free racetrack resonator hybrided with phase change material for photonic in-memory computing.

The photonic in-memory computing architecture based on phase change materials (PCMs) is increasingly attracting widespread attention due to its high computational efficiency and low power consumption. However, PCM-based microring resonator photonic computing devices face challenges in terms of resonant wavelength shift (RWS) for large-scale photonic network. Here, we propose a PCM-slot-based 1 × 2 racetrack resonator with free wavelength shift for in-memory computing. The low-loss PCMs such as Sb2Se3 and Sb2S3 are utilized to fill the waveguide slot of the resonator for the low insertion (IL) and high extinction ratio (ER). The Sb2Se3-slot-based racetrack resonator has an IL of 1.3 (0.1) dB and an ER of 35.5 (8.6) dB at the drop (through) port. The corresponding IL of 0.84 (0.27) dB and ER of 18.6 (10.11) dB are obtained for the Sb2S3-slot-based device. The change in optical transmittance of the two devices at the resonant wavelength is more than 80%. No shift of the resonance wavelength can be achieved upon phase change among the multi-level states. Moreover, the device exhibits a high degree of fabrication tolerance. The proposed device demonstrates ultra-low RWS, high transmittance-tuning range, and low IL, which provides a new scheme for realizing an energy-efficient and large-scale in-memory computing network.

High sensitivity compact Mach Zehnder interferometer based on the cascade application of seven-core fiber and graded index multimode fiber

Mach Zehnder interferometric sensor fabricated by graded index multimode fiber has high sensitivity, but due to the defects of wide interference valley and low extinction ratio, it is difficult to be applied, and there are few studies about the MZI made of graded index fiber. In this paper, a Mach Zehnder interferometer cascaded with seven-core fiber and graded index multimode fiber was proposed and demonstrated. While maintaining the high sensitivity of the graded index multimode fiber interferometer, it overcomes its series of defects. The testing results indicate that the full width at half maxima (FWHM) of the interference valley can be less than 30 nm, and the valley depth is close to 10 dB when the length of the graded index multimode fiber is less than 2 dm. The maximum strain sensitivity of the sensor is 20.71 pm/με, and the temperature sensitivity is 261.14 pm/℃. The proposed sensor has compact structure and high strength, which will promote the practical process of graded index multimode fiber in the field of fiber interference sensing.

Efficient optical modulation in ring structure based on Silicon-ITO heterojunction with low voltage and high extinction ratio

We numerically propose a low voltage, high extinction ratio optical modulator with a tunable group delay in a resonance enhanced ring resonator based on Silicon (Si) - Indium Tin Oxide (ITO) heterojunction. This is accomplished by incorporating a thin ITO layer into a Si-ring resonator. By electrically inducing carrier changes in ITO, the epsilon-near-zero (ENZ) state of ITO is attained, resulting in a sudden change in optical absorption. The resonance condition shifts because of the ENZ state, and the transmission at the ring resonator’s through port changes dramatically. The extinction ratio (ER) of 11 dB at a low forward bias of 1 V is reported with a low energy consumption of 2.6 fJ. An optical network has a non-trivial demand for a high extinction ratio optical modulator that operates at a low driving voltage and consumes minimal energy. The proposed ring modulator with a high extinction ratio at low driving voltage outperforms conventional waveguide-based modulators in terms of energy efficiency. In addition, we report around 25 psec of electrical tuning in the group delay The current concept has favorable results in terms of exploring tunable delay lines in conjunction with optical modulation with a high extinction ratio.

High Extinction Ratio and Low Temperature Cross-Sensitivity Strain Sensor Based on Vernier Effect Through Hybrid Cascaded Sagnac Loop

Low extinction ratio (ER) and high temperature cross-sensitivity are serious but common problems for most strain sensors based on Vernier effect. In our work, we propose a strain sensor based on Vernier effect. It consists of hybrid cascaded Sagnac loop, one part is a microfiber coupler Sagnac loop (MFC–SL), and the other is a Sagnac loop connected to a Section of polarization-maintaining photonic crystal fiber (PMPCF–SL). Different from the traditional Vernier effect, in the proposed sensor, the spectra of two interferometers shift to opposite directions when the strain changes, which belongs to the enhanced Vernier effect. When ambient temperature varies, the spectra of the two interferometers shift to the same direction, which belongs to the reduced Vernier effect. Thus, its strain sensitivity is improved and its temperature cross sensitivity is reduced. In addition, the MFC–SL and PMPCF–SL exhibit a high fringe ER due to their good transmission of two orthogonal modes. Experimental show that the sensitivity of strain is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$- 124.17\,\,\text {pm}/\mu \varepsilon $ </tex-math></inline-formula> , while the temperature sensitivity is only <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$6.13~\text {pm}/^{\circ }\text{C}$ </tex-math></inline-formula> , and the ER of envelope is about 9 dB. Its robustness can be enhanced by the method of packaging with silicone adhesive. It will have a potential application in the field of strain sensing.

High-sensitivity and high extinction ratio fiber strain sensor with temperature insensitivity by cascaded MZI and FPI.

Low extinction ratio (ER) and high temperature cross-sensitivity are serious but common problems for most strain sensors based on Vernier effect. In this study, a hybrid cascade strain sensor of a Mach-Zehnder interferometer (MZI) and a Fabry-Perot interferometer (FPI) with high sensitivity and high ER based on Vernier effect is proposed. The two interferometers are separated by a long single-mode fiber (SMF). The MZI is used as the reference arm, which can be flexibly embedded in the SMF. The FPI is used as the sensing arm and the hollow-core fiber (HCF) as the FP cavity to reduce optical loss. Simulation and experiments have proven that this method can significantly increase ER. At the same time, the second reflective face of the FP cavity is indirectly spliced to increase the active length to improve the strain sensitivity. Through the amplification of Vernier effect, the maximum strain sensitivity is -649.18p m/μ ε, and the temperature sensitivity is only 5.76pm/∘C. The magnetic field was measured by combining the sensor with a Terfenol-D (magneto-strictive material) slab to verify the strain performance, and the magnetic field sensitivity is -7.53nm/mT. The sensor has many advantages and has potential applications in the field of strain sensing.

High-Discrimination Circular Polarization Detection Based on Dielectric-Metal-Hybrid Chiral Metamirror Integrated Quantum Well Infrared Photodetectors.

Circular polarization detection enables a wide range of applications. With the miniaturization of optoelectronic systems, integrated circular polarization detectors with native sensitivity to the spin state of light have become highly sought after. The key issues with this type of device are its low circular polarization extinction ratios (CPERs) and reduced responsivities. Metallic two-dimensional chiral metamaterials have been integrated with detection materials for filterless circular polarization detection. However, the CPERs of such devices are typically below five, and the light absorption in the detection materials is hardly enhanced and is even sometimes reduced. Here, we propose to sandwich multiple quantum wells between a dielectric two-dimensional chiral metamaterial and a metal grating to obtain both a high CPER and a photoresponse enhancement. The dielectric-metal-hybrid chiral metamirror integrated quantum well infrared photodetector (QWIP) exhibits a CPER as high as 100 in the long wave infrared range, exceeding all reported CPERs for integrated circular polarization detectors. The absorption efficiency of this device reaches 54%, which is 17 times higher than that of a standard 45° edge facet coupled device. The circular polarization discrimination is attributed to the interference between the principle-polarization radiation and the cross-polarization radiation of the chiral structure during multiple reflections and the structure-material double polarization selection. The enhanced absorption efficiency is due to the excitation of a surface plasmon polariton wave. The dielectric-metal-hybrid chiral mirror structure is compatible with QWIP focal plane arrays.

Error analysis and correction of a photoelastic method based on a pixelated polarization camera

The utilization of pixelated polarization cameras has brought a technical breakthrough to traditional photoelasticity method. In this paper, the principles of the new method are introduced. Then a few of systematic error factors, especially, the non-ideal polarization state of background light (it mainly refers to elliptically polarized incidence, but the stress birefringence of imaging lens was also counted in) and the low extinction ratio of the pixelated polarization camera are analyzed and evaluated. Finally, the corresponding algorithms for error removal are proposed. For the non-ideal polarization state of background light, we first determine its Stokes vector field, and then we obtain the phase retardance and polarization azimuth of the specimen by solving the equations describing relationship between the measured Stokes vector and the theoretical ones of the emergent light field of specimen. For the low extinction ratio of internal polarizer in the camera, we analyze the mechanism of how extinction ratio affects the measurement accuracy, and remove the measurement error by applying the degree of linear polarization (DoLP) of a standard quarter wave plate as the correction coefficient to be divided by the measured DoLP of the specimen. The effectiveness of correction algorithm is validated by some typical experiments, while reasons for the phenomenon that the errors were not completely eliminated are also remarked.

Ultra-compact and broadband all-silicon TM-pass power splitter using subwavelength holey-structured metamaterial waveguides.

Power splitters with polarization management features are highly desired to construct high-density silicon photonic integrated circuits. However, few attempts have been made to design a single device that can act as both a power splitter and a TE- or TM-pass polarizer. In this paper, for the first time, we experimentally demonstrate an ultra-compact and broadband all-silicon TM-pass power splitter, where a triple-guide directional coupler (TGDC) composed of three parallel subwavelength holey-structured metamaterial waveguides (SHMWs) is located at central coupling region and three regular strip waveguides are connected at the input/output ports. Such a SHMW can enhance the reflection to realize a wide stop-band for the undesired TE polarized light, while achieving the low loss transmission for the TM polarized light. Besides, the TM dispersion can be significantly flattened by the designed SHMWs, leading to a broadband power splitting for TM polarization. Simulated results show that an ultra-compact device of 1.7 × 4 µm2 in size is obtained with an insertion loss (IL) of 0.34 dB and an extinction ratio (ER) of 36 dB at 1550 nm, and its working bandwidth can be extended to ∼240 nm by keeping IL < 0.9 dB and ER > 16 dB. The measurements of the fabricated devices show low IL (<1 dB) and high ER (>15 dB) over the measured wavelength range of 1460 to 1580 nm, which is consistent with the simulation results.

A novel mode-division (de)multiplexer with degenerate modes output for MIMO-FREE applications

In this study, we propose a novel three-dimensional architecture mode (de)multiplexer with degenerate modes output using a pure silica FMF ring core transmission channel, which solves the problem caused by random mode rotation and can be used in multiple-input multiple-output free (MIMO-FREE) applications such as data center application in the future. By using the pure silica FMF ring core transmission channel and larger effective refractive index difference, the performance with low loss, high extinction ratio (ER) and low crosstalk is achieved. The main channel with a few-mode fiber (FMF) ring-core structure supports the modes LP01, LP11, and LP21, and the large effective refractive index difference between each mode in the core ensures low crosstalk characteristics between the modes. Using the pure silica core channel can effectively reduce propagation attenuation and fusion loss. Our proposed MUX/DEMUX with degenerate modes output is achieved when the degenerate modes LP11a/LP11b and LP21a/LP21b are transmitted as two independent mode signals, which can be used in MIMO-FREE applications. The extinction ratios (ERs) of the degenerate modes LP11 and LP21 are kept above 31.66 dB and 24.43 dB, respectively, and the ER of mode LP01 is kept above 38.72 dB in the C band. The coupling efficiency of mode LP01 is approximately 0 dB, which is almost unchanged with the increase of the wavelength. The coupling efficiency of LP11 is higher than −3.49 dB and that of LP21 is higher than −7.24 dB in the whole C-band. At 1550 nm, the coupling efficiencies of modes LP01, LP11, and LP21 are −0.002 dB, −0.052 dB, and −0.178 dB, respectively. The coupling efficiency and ER of LP01 mode are the best, and those of the degenerate mode LP11 are always better than those of mode LP21. Our proposed MUX/DEMUX achieves low crosstalk and high ER performance and solves the problem caused by the degenerate modes rotations during transmission.

Ultra-Compact Digital Metasurface Polarization Beam Splitter via Physics-Constrained Inverse Design

Inverse design effectively promotes the miniaturization of integrated photonic devices through the modulation of subwavelength structures. Utilizing a theoretical prior based inverse design, we propose an ultra-compact integrated polarizing beam splitter consisting of a standard silicon-on-insulator (SOI) substrate and a tunable air–silicon column two-dimensional code metasurface, with a footprint of 5 × 2.7 μm2. The effective refractive index of the waveguide is modulated by adjusting the two-dimensional code morphology in the additional layer to achieve efficient polarization beam splitting. The simulation results demonstrate high performance, with a low insertion loss (&lt;0.87 dB) and high extinction ratio (&gt;10.76 dB) in a bandwidth of 80 nm covering the C-band. The device can withstand manufacturing errors up to ±20 nm and is robust to process defects, such as the outer proximity effect, and thus is suitable for ultra-compact on-chip optical interconnects.

High-performance silicon TE-pass polarizer assisted by anisotropic metamaterials.

The polarizer is a key component for integrated photonics to deal with the strong waveguide birefringence, especially for silicon photonics. A high-performance silicon TE-pass polarizer covering all optical communication bands with low insertion loss (IL) and high polarization extinction ratio (PER) is proposed here. This polarizer is based on anisotropic subwavelength grating (SWG) metamaterials, which maintain the fundamental TE mode as a guided mode but make the fundamental TM mode leaky. Furthermore, based on this working mechanism, the proposed polarizer can work well for any upper cladding material, including air and silicon dioxide (SiO2). The numerical results show that our proposed TE-pass polarizer has a remarkable performance with IL < 0.34 dB over 420 nm (PER > 23.5 dB) or 380 nm (PER > 30 dB) for the air cladding, and IL < 0.3 dB over 420 nm (PER > 25 dB) or 320 nm (PER > 30 dB) for the SiO2 cladding. The fabricated polarizer shows IL < 0.8 dB and PER > 23 dB for the bandwidths of 1.26-1.36 µm and 1.52-1.58 µm (other bandwidths were not measured due to the limited instrument in our research center, but it still covers the most important O-band and C-band).

Twisted Bands with Degenerate Points of Photonic Hypercrystals in Infrared Region.

Photonic hypercrystals (PHCs) are materials combining hyperbolic metamaterials (HMMs) with widely used photonic crystals. We found that finite-sized Type-I HMMs can support unique electromagnetic modes, which could be utilized in two-dimensional photonic crystals to achieve PHCs with twisted bands in the infrared region. Numerical investigation of the PHCs showed that the twisted bands have degenerate points that can support all-angle self-collimation effects. The behaviors of light beams change dramatically in such bands, which provides an effective method in controlling light propagation and can be applied as switching. The effect of the filling factor and the permittivity of the dielectric medium of the HMM on the twisted bands were studied. Furthermore, by considering the nonlinear effect of the dielectric layers, an all-optical switch working on the PHC twisted bands is proposed, which has low switching power and high extinction ratio (19.75 dB), superior to conventional HMM switches that require type transformation of metamaterial.

Simulation of Resonant Cavity-Coupled Colloidal Quantum-Dot Detectors with Polarization Sensitivity

Infrared detectors with polarization sensitivity could extend the information dimension of the detected signals and improve target recognition ability. However, traditional infrared polarization detectors with epitaxial semiconductors usually suffer from low extinction ratio, complexity in structure and high cost. Here, we report a simulation study of colloidal quantum dot (CQD) infrared detectors with monolithically integrated metal wire-grid polarizer and optical cavity. The solution processibility of CQDs enables the direct integration of metallic wire-grid polarizers with CQD films. The polarization selectivity of HgTe CQDs with resonant cavity-enhanced wire-grid polarizers are studied in both short-wave and mid-wave infrared region. The extinction ratio in short-wave and mid-wave region can reach up to 40 and 60 dB, respectively. Besides high extinction ratio, the optical cavity enhanced wire-grid polarizer could also significantly improve light absorption at resonant wavelength by a factor of 1.5, which leads to higher quantum efficiency and better spectral selectivity. We believe that coupling CQD infrared detector with wire-grid polarizer and optical cavity can become a promising way to realize high-performance infrared optoelectronic devices.

Design of Ultra-High Extinction Ratio TM- and TE-Pass Polarizers Based on Si-Sc0.2Sb2Te3 Hybrid Waveguide.

The selective polarizers play an important role in silicon-based integrated circuits. The previous polarizers based on silicon waveguides have the defects of large scale and low extinction ratio. In this work, TM- and TE-pass polarizers only 10 μm long were developed based on phase-change material of Sc0.2Sb2Te3 (SST) hybrid silicon waveguide, where several SST bars with a varied distance was designed. Because of the excellent characteristics of the refractive index of the material, ultra-high extinction ratios (ERs) were achieved. A 3-D finite element simulation was carried out to optimize the structure of the polarizers, and the distribution of light field, as well as the transmission behavior of TE and TM modes in the two polarizers, was further demonstrated in detail. When the SST is crystalline, the unwanted mode can be attenuated, while the wanted mode can pass through with low loss. Compared with the GST-based polarizers, the proposed ones achieved high extinction ratios of ~43.12 dB (TM-pass one) and ~44.21 dB (TE-pass one), respectively; at the same time, ILs for the wanted modes could be negligible. The design of high-performance polarizers paves a new way for applications of all-optical integrated circuits.

CMOS compatible ultra-compact MMI based wavelength diplexer with 60 Gbit/s system demonstration.

We design and experimentally demonstrate an ultra-compact 1310/1550 nm wavelength diplexer based on a multimode interference (MMI) coupler. The proposed device is designed at the first imaging length for 1550 nm wavelength resulting in an MMI length of only 41 µm. In order to improve the extinction ratio, the output ports are made asymmetric in width. A low insertion loss (< 1dB) and high extinction ratio (> 20 dB) is measured at the two operating wavelengths. It also displays a wide 3-dB bandwidth of 100 nm centered around 1310 nm and 1550 nm wavelengths. Furthermore, an on-chip wavelength demultiplexing experiment carried out on the fabricated device, with a non-return-to-zero (NRZ) on-off keying (OOK) signal at 60 Gbit/s, shows clear eye diagrams for both the wavelengths.

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.

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.

A High Sensitivity Curvature Sensor Based on Microfiber Mach-Zehnder Interferometer With Tapered Seven-Core Fiber

A Microfiber Mach-Zehnder interferometer (MMZI) based on tapered seven-core fiber (SCF) for curvature measurement has been reported and demonstrated. The tapered SCF promoted the light coupling efficiency from the single-mode fiber (SMF) to the SCF, which results in the transmission spectrum with a low loss and high extinction ratio. In the process of curvature measurement, the sensitivity can reach up to 174.02957 nm/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> , which is the highest curvature sensitivity in the MZI with SCF as best as we have known. Furthermore, the maximum temperature sensitivity is 0.00956 nm/°C when the temperature changes from 30 °C to 140 °C. It shows that this sensor is naturally insensitive to temperature, which has a good performance in avoiding the cross-sensitivity effect between the temperature and curvature.