Modulated Optical Signal Research Articles

Silicon Optical Modulators Based on Tunable Plasmonic Directional Couplers

For the development of Si-based next generation electronic-photonic integrated circuits, a silicon optical modulator is designed based on tunable plasmonic directional couplers. The characteristics are investigated using numerical simulations with a full-vectorial beam propagation method. The center waveguide in the three-core coupler is a MOS-type hybrid plasmonic waveguide that consists of a Si-ITO-SiO2-Si structure. By electrically tuning the ITO's refractive index, the coupling efficiency of the directional coupler is switchable and, hence, we can obtain modulated optical signals at the outer waveguide, which is configured with a vertical offset. The extinction ratios are 6.7 and 2.3 dB at 1.31- and 1.55-μm wavelength, respectively. The optical modulator can be transformed into a plasmonic absorption modulator based on a two-core directional coupler at the cost of a long coupling length. The proposed silicon optical modulator has the potential to play a key role in complementary metal-oxide-semiconductor-compatible 3-D silicon photonic integrated circuits.

Calculation of Bit Error Rates in Optical Systems With Silicon Photonic Wires

A theoretical approach to calculate the bit error rate (BER) in optical systems containing silicon photonic wires (Si-PhWs) is presented. Specifically, the optical link consists of a single-mode silicon-on-insulator strip waveguide followed by a direct-detection optical receiver containing an optical filter, an ideal square-law photodetector, and an electrical filter. We assume that the optical input consists of a superposition of a nonreturn-to-zero ON–OFF keying modulated optical signal and an additive white Gaussian noise, the BER of the transmitted optical signal being calculated using the time domain Karhunen–Loève expansion method. The propagation of the optical signal in the Si-PhW is described by employing both a rigorous theoretical model that incorporates all relevant linear and nonlinear optical effects and the mutual interaction between the free carriers and the optical field, as well as a linearized model valid in the low-noise power regime. These analytical and computational tools are then used to comprehensively investigate the influence of the parameters characterizing the waveguide and optical signal on the transmission BER.

Design of Vanadium Dioxide-Based Plasmonic Modulator for Both TE and TM Modes

Based on vanadium dioxide (VO2) insulator-metal phase transition, a compact plasmonic modulator is designed for both transverse-electric (TE) and transverse-magnetic (TM) polarization modes. The device consists of a Si waveguide and a metal–oxide–semiconductor-type Si−SiO2–VO2–Cu hybrid plasmonic waveguide that is inserted into a Si waveguide. By electrically changing the phase of VO2 from an insulator to a metal, the guidance of the TE and TM modes along the device is greatly suppressed and, hence, we can obtain modulated optical signals. For a 500-nm-long plasmonic modulator operating at a 1.55- $\mu $ m wavelength, simulation shows an on/off extinction ratio of 3.9 dB for TE and of 8.7 dB for TM mode.

Enhanced Blind Equalization for Optical DP-QAM in Finite Precision Hardware

In this letter, we apply the inverse QR decomposition-constant modulus algorithm (IQRD-CMA), blind equalization technique, for impaired 14 GBd DP-16 quadratic-amplitude modulation optical modulated signal. The IQRD-CMA is considered here to mitigate the effect of residual chromatic dispersion (CD) and polarization mode dispersion (PMD). We evaluate the performance of the proposed inverse QR decomposition in terms of convergence rate, steady-state/residual mean square error (MSE), and bit error rate (BER), in comparison with standard blind constant modulus algorithm (CMA) and recursive least squares (RLS-CMA). Assuming infinite precision, the IQRD-CMA and RLS-CMA achieve similar performance and induce the same computational complexity; however, both largely outperform standard CMA for wide ranges of CD and PMD. However, for finite precision hardware, compared with RLS-CMA, the proposed blind IQRD-CMA clearly achieves one order of BER magnitude at 8- and 10-b resolutions, and further reduces the steady-state MSE by 3 dB. Moreover, results show that IQRD-CMA maintains the system stability at acceptable number of bits resolutions. In addition, this letter addresses the tradeoff between the complexity and the performance of all these equalizers for several precision settings.

Design, Simulation and Testing of the OOK NRZ Modulation Format for Free Space Optic Communication in a Simulation Box

This article deals with the construction of a modulator and demodulator for Free-Space Optical (FSO) communication. In FSO optics, the modulated optical signal is propagated in the constantly changing environment (atmosphere). The optical signal is strongly influenced by the actual composition of air, which is directly linked to the change in refractive index in the turbulent cells. This article examines primarily the appropriate modulation format for FSO. For this purpose, one type of an OOK-NRZ modulator and one type of a demodulator were designed. This article also describes the construction of two types of photo detectors (the high impedance and the transimpedance ones). All electronic constructions were tested in the MicroCap simulator and they were experimentally measured as well. For the OOK NRZ modulation the, maximum transmission speed achieved the value of 160~Mbps. Measuring the quality of the modulation formats was carried out under mechanical and thermal turbulences. The last part of this work gives the results of the measurements of fog influences.

A High Spectral Efficiency Coherent Microwave Photonic Link Employing Both Amplitude and Phase Modulation With Digital Phase Noise Cancellation

A high spectral efficiency coherent microwave photonic link (MPL) supporting amplitude and phase modulation incorporating a digital phase noise cancellation is proposed and experimentally demonstrated. At the transmitter, a continuous-wave light wave is amplitude- and phase-modulated by two microwave vector signals carried by a microwave carrier at an identical frequency. The modulated optical signal is polarization multiplexed with an unmodulated optical carrier and transmitted over a length of a single-mode fiber (SMF). At the receiver, the optical signal is detected coherently by a coherent receiver to which a local oscillator (LO) laser source is also applied. Through advanced digital signal processing, the microwave vector signals are recovered, and the phase noise introduced by both the transmitter laser source and LO laser source is cancelled. An experiment is performed. The transmission of a 2.5-Gb/s 16-QAM and a 1.25-Gb/s QPSK microwave vector signals both at 2.5 GHz over a 25-km SMF is implemented. The total bit rate of the MPL is 3.75 Gb/s. The transmission performance of the MPL in terms of error vector magnitudes and bit error rates is evaluated.

교란대기 지상 광무선 통신에서 변조방식에 따른 패킷 오류율 비교

지상 광무선 통신에서 대기의 교란 및 대기 입자에 의한 광의 감쇄가 있을 때, 대기교란의 강도 및 가시도가 수신 광 강도에 미치는 영향을 해석하였다. 광신호는 On-Off Keying(OOK), Pulse Position Modulation(PPM) 및 Digital Pulse Interval Modulation(DPIM)의 방식으로 변조하였다. 교란대기 하에서 각 변조방식에 대해 광 전송 거리에 따른 광수신 전력을 계산하고 이를 이용하여 전송거리에 따른 패킷 오류율(packet error rate: PER)을 계산하였다. 광신호를 전송하기 위한 광원의 파장은 850nm, 1310nm 및 1550nm을 선택하였다. 대기는 약 교란 상태로 가정하여 대기 굴절률 구조상수 <TEX>$Cn2{\approx}10-14m-2/3$</TEX>, 대기의 가시도 V=2km로 하였다. 약교란 대기 상태에서 세가지 변조방식 중 DPIM 방식이 우수하며, 광 신호의 전송을 위한 파장으로는 1550nm가 PER 성능이 우수함을 알 수 있었다. In the terrestrial optical wireless communication(OWC), the performance is affected by atmospheric turbulence and particles in the air. The received signal power loss mainly is caused by turbulence and scattering. To minimize the adverse atmospheric effects, the OWC used optical signal modulation, such as OOK, PPM and DPIM. In this paper, the packet error rate(PER) was analyzed above three modulation methods to ground optical link in atmospheric turbulence, scattering and link distance. The OWC system used three wavelengths which are 850nm, 1310nm and 1550nm. I assumed the atmospheric turbulence intensity is weak, so the refractive index is <TEX>$Cn2{\approx}10-14m-2/3$</TEX> and the visibility is 2km. The numerical results shown that the L-DPIM scheme and the wavelength 1550nm are better than other modulation methods and wavelengths.

Ultrasound modulated optical tomography contrast enhancement with non-linear oscillation of microbubbles.

Ultrasound modulated optical tomography (USMOT) is an imaging technique used to provide optical functional information inside highly scattering biological tissue. One of the challenges facing this technique is the low image contrast. A contrast enhancement imaging technique based on the non-linear oscillation of microbubbles is demonstrated to improve image contrast. The ultrasound modulated signal was detected using a laser pulse based speckle contrast detection system. Better understanding of the effects of microbubbles on the optical signals was achieved through simultaneous measurement of the ultrasound scattered by the microbubbles. The length of the laser pulse was found to affect the system response of the speckle contrast method with shorter pulses suppressing the fundamental ultrasound modulated optical signal. Using this property, image contrast can be enhanced by detection of the higher harmonic ultrasound modulated optical signals due to nonlinear oscillation and destruction of the microbubbles. Experimental investigations were carried out to demonstrate a doubling in contrast by imaging a scattering phantom containing an embedded silicone tube with microbubbles flowing through it. The contrast enhancement in USMOT resulting from the use of ultrasound microbubbles has been demonstrated. Destruction of the microbubbles was shown to be the dominant effect leading to contrast improvement as shown by simultaneously detecting the ultrasound and speckle contrast signals. Line scans of a microbubble filled silicone tube embedded in a scattering phantom demonstrated experimentally the significant image contrast improvement that can be achieved using microbubbles and demonstrates the potential as a future clinical imaging tool.

Ultra-fast solid state electro-optical modulator based on liquid crystal polymer and liquid crystal composites

A different generation of polymer-dispersed liquid crystals (PDLCs) based on a liquid crystalline polymer host is reported wherein the fluid behavior of the reactive mesogenic monomer is an enabler to concentration windows (liquid crystal polymer/liquid crystal) (and subsequent morphologies) not previously explored. These liquid crystal (LC) polymer/LC composites, LCPDLCs, exhibit excellent optical and electro-optical properties with negligible scattering losses in both the ON and OFF states. These systems thus have application in systems where fast phase modulation of optical signal instead of amplitude control is needed. Polarized optical microscopy and high resolution scanning electron microscopy confirm a bicontinuous morphology composed of aligned LC polymer coexisting with a phase separated LC fluid. Operating voltages, switching times, and spectra of LCPDLCs compare favourably to conventional PDLC films. The LCPDLCs exhibit a low switching voltage (4–5 V/μm), symmetric and submillisecond (200 μs) on/off response times, and high transmission in both the as formed and switched state in a phase modulation geometry.

Broadband Microwave Photonic Splitter With Arbitrary Amplitude Ratio and Phase Shift

We report a broadband $1\times N$ microwave photonic splitter (MPS) with arbitrary amplitude ratio and phase shift. The key devices used in the MPS are a polarization modulator (PolM), an optical bandpass filter (OBPF), polarization controllers (PCs), and polarizers (Pols). The cascaded PolM and OBPF generate an orthogonally polarized single-sideband modulated optical signal. The phase difference between the two orthogonal components and the state of polarization of the optical signal can be adjusted by the PC independently. After the polarization-to-intensity modulation conversion by the Pol, the microwave is recovered in the PD. The amplitude and phase of the microwave signal can be independently tuned by adjusting the three plates of the PC. The proposed MPS is theoretically analyzed and experimentally verified. Broadband MPS with bandwidth from 8 to 40 GHz is successfully achieved. In addition, we apply the proposed MPS to a two-tap microwave photonic filter (MPF). The notch position and depth of the MPF are tunable by adjusting the phase shift and amplitude ratio between the two optical branches, respectively.

On the applicability boundaries of the restoration method for the temporal shape of modulated optical signals with a frequency higher than the boundary frequency of a photoresistor

The applicability boundaries of the restoration method for the temporal shape of an optical signal having an intensity varying with frequency ω larger than the inverse lifetime of majority carriers are studied. Theoretical studies of distortions emerging upon the detection of modulated optical signals are performed. Parameters characteristic of a photodetector fabricated on the basis of Si with In recombination centers are used in the calculations. An increase in ω decreases the nonlinear, frequency, and phase distortions. The conclusions can be used to study periodic processes of another nature.

Optimal Design of High-Speed Optical Fiber Communication System Spectral Efficiency of New Modulation Formats

As human society to the information in the process of moving and growing demand for bandwidth communications capacity, the optical of new modulation formats increasingly attention and quickly play an important role in optical communications. How can the system bit error rate within a certain degree of stability while still maintaining high-speed long-distance dispersal system, has been a popular issue is the optical communications industry. Starting from the optical modulation format herein, the generation process of the system introduced various optical signal modulation format, the optical signal through the optical fiber was studied and the performance of the simulation, on the basis of the design of advanced optical modulation formats in an optical fiber communication system .

Dual-lock-in sensor IC with integrated PIN photodetectors

Within this work a monolithically integrated low-power dual-lock-in amplifier (LIA) with integrated photodiodes is presented. This structure as a whole defines an opto-sensitive single-pixel sensor, being fabricated in a 0.35 µm 1P4M CMOS process with integrated PIN photodiodes. Since two correlators are exploited, the sensor is able to detect the modulated optical signal in the frequency domain, as well as its phase, which can be used in numerous applications. Among the others, the time-of-flight and interferometry based distance measurement system can benefit from this structure. The single pixel occupies an active area of 120 × 100 µm2 at 58 % fill factor. The measurement results show that a dynamic range of 27 dB could be achieved at a modulation frequency of 10 MHz. While the bandwidth of the LIA is adjustable, a signal can be detected up to a modulation frequency of 35 MHz and beyond dependent on the received optical power.

Demonstration of 6.25 Gbaud advanced modulation formats with subcarrier multiplexed technique on silicon Mach-Zehnder modulator.

Silicon Mach-Zehnder modulators (Si MZMs) with good linearity are designed and fabricated. 6.25 Gbaud Nyquist 16, 32 and 64-Quadrature Amplitude Modulation (QAM) optical signals were successfully generated by intensity modulation from the Si MZM, and the effective data rates are 22.61 Gb/s, 28.26 Gb/s and 33.91 Gb/s respectively. The subcarrier multiplexed technique and direct detection scheme were employed in this experiment. After 53.1 km transmission, the BERs of 16-QAM and 32-QAM are both below the 7% hard-decision forward error correction limit, while the back-to-back BER of 64-QAM is well below the 20% soft-decision forward error correction limit. These results demonstrated that the Si MZM can be used in the high-capacity low-cost short-haul intensity modulation and direct detection system.

Group velocity dispersion monitoring for QPSK signals using direct detection

ABSTRACTIn this letter, we propose a technique to monitor the group velocity dispersion analyzing a quadrature phase‐shift keying modulated optical signal. The method is based on the analysis of the direct detection electrical signal power, measured at the clock frequency, being able to return the dispersion magnitude value and sign. The proposed method was tested for 20 Gb/s signal in a dispersion range of [−600,600] ps/nm. © 2014 Wiley Periodicals, Inc. Microwave Opt Technol Lett 56:2078–2080, 2014

Self-calibrating measurement of high-speed electro-optic phase modulators based on two-tone modulation.

We propose a self-calibrating method for high-frequency response measurement of electro-optic phase modulators based on two-tone modulation. The method utilizes the electrical domain measurement of heterodyning spectrum between the two-tone modulation optical signal and the frequency-shifted optical carrier, and eliminates the need to correct the responsivity fluctuation in the photodetection. High-frequency modulation depth and half-wave voltages are measured and compared to those with the traditional optical spectrum analysis method in the experimental demonstration. The proposed method enables calibration-free and accurate frequency response measurement of electro-optic phase modulators by using high-resolution electrical spectrum analysis.

Chaotic dynamics in coupled resonator sequences.

Optically induced thermal and free carrier nonlinearities in silicon micro-ring resonator influence their behavior. They can be either deleterious by making them instable and by driving their resonances out of the designed wavelengths, or enabler of different applications. Among the most interesting one, there are optical bistability and self induced oscillations. These lead to all optical logic, signal modulation, optical memories and applications in neural networks. Here, we theoretically and experimentally demonstrate that when many resonators are coupled together, thermal and free carrier nonlinearities induce also chaos. The chaotic dynamics are deeply analyzed using experimentally reconstructed phase space trajectories and the tool of Lyapunov exponents.

Externally-Modulated Electro-Optically Coupled Detector Architecture for Nuclear Physics Instrumentation

A new laser-based externally-modulated electro-optically coupled detector (EOCD) architecture is being developed to enable high-density readout for radiation detectors with accurate analog radiation pulse shape and timing preservation. Unlike digital conversion before electro-optical modulation, the EOCD implements complete analog optical signal modulation and multiplexing in its detector front-end. The result is a compact, high performance detector readout that can be both radiation tolerant and immune to magnetic fields. In this work, the feasibility of EOCD was explored by constructing a two-wavelength laser-based externally-modulated EOCD, and testing analog pulse shape preservation and wavelength-division multiplexing (WDM) crosstalk. Comparisons were first made between the corresponding initial pulses and the electro-optically coupled analog pulses. This confirmed an excellent analog pulse preservation over ~ 29% of the modulator's switching voltage range. Optical spectrum analysis revealed less than -14 dB crosstalk with 1.2 nm WDM wavelength bandgap, and provided insight on experimental conditions that could lead to increased inter-wavelength crosstalk. Further discussions and previous research on the radiation tolerance and magnetic field immunity of the candidate materials were also given, and quantitative device testing is proposed in the future.

Impact of interferometer delay time on the performance of ODSB-SC RoF system with wavelength interleaving

In this paper, we investigate the impact of interferometer delay time in a 5Gb/s optical double sideband-suppressed carrier (ODSB-SC) RoF system transmitting two wavelength interleaved radio frequency (RF) signals at 10 and 15GHz over an optical fiber. Here, an optical Mach–Zehnder modulator is used for both optical carrier suppression and signal modulation. At the receiver, delay interferometer is used for the separation of RF frequency signals. We analyze the performance of the RoF system by varying the value of delay time of interferometer from 0.02 to 0.14ns. The result shows that the RoF system performance is optimum for the time delay of 0.1ns. Further, the optical spectrums, RF spectrums and eye diagrams of two interleaved RF signals have been compared.

Silicon technology compatible photonic molecules for compact optical signal processing

Photonic molecules (PMs) based on multiple inner coupled microring resonators allow to surpass the fundamental constraint between the total quality factor (QT), free spectral range (FSR), and resonator size. In this work, we use a PM that presents doublets and triplets resonance splitting, all with high QT. We demonstrate the use of the doublet splitting for 34.2 GHz signal extraction by filtering the sidebands of a modulated optical signal. We also demonstrate that very compact optical modulators operating 2.75 times beyond its resonator linewidth limit may be obtained using the PM triplet splitting, with separation of ∼55 GHz.