Modulated Optical Signal Research Articles

Simulation of optical triode based on PT symmetric structure under electro-optic modulation

In modern microwave and optical communication systems, it is urgent to develop optical components with the same performance as electronic components. In this paper, a PT-symmetric structure including electro-optical medium is designed to construct optical triode. It is found that the structure has a pole effect of amplification under certain parameters. The frequency of the incident wave at the pole is taken as the carrier frequency, and the electrical signal is modulated into an optical signal. By setting the bias electric field, the modulated optical signal can be amplified in phase and out phase, and even by doubling frequency, and a new optical triode model is obtained. The designed model will play an important role in the design of optical integrated devices.

Amplitude compensation using homodyne detection for inverse synthetic aperture LADAR.

Inverse synthetic aperture LADAR (ISAL), based on an electro-optic in-phase and quadrature (I/Q) modulator, and homodyne detection, has high pulse repetition frequency, simple structure, and minimum intra-pulse phase errors. Homodyne detection can implement the de-chirp operation in the optical domain, while the output amplitude is susceptible to light intensity. However, the modulated optical signal amplitude fading, induced by radio frequency devices and the I/Q modulator, is quite tricky to measure and be compensated for. In this work, the effect of amplitude distortion on the ISAL system is analyzed. A simple and effective compensation method (based on homodyne detection) is proposed, followed by some experiments. The improvement of homodyne signal power to the compensated signal compared with non-compensated signal was about 1 dB (26%), and the image quality of our ISAL system along range direction was also improved by using the proposed method. Moreover, the image of an extended target was acquired via our ISAL system with 500 mW transmitting power at a distance of 1.1 km.

Performance Analysis Of Optical Modulation Techniques Based On Reduction Of Second Order Harmonics And Chromatic Dispersion on OFDM Signals

ABSTRACT This work explains the theoretical and mathematical investigations of the transmission performance of an optical Single Sideband (SSB) modulation technique generated by a Mach Zehnder Modulator (MZM) with a 90° and 120° phase shift using hybrid coupler (Modified SSB) in addition to the Vestigial Sideband technique (VSB). The problem addressed in this paper is the effect of non-linearity of the transmitter system and the chromatic dispersion in single mode fiber due to the presence of second- and higher order harmonics. The non-linearity of the transmitter generates second and higher order harmonics, which affects the system performance leading to reduction in the data rate and increased distortion. The transmission performance of SSB, M-SSB and VSB is evaluated based on the extinction ratio of the MZM, different transmission lengths of the fiber, optical signal modulation index and the receiver sensitivity. A mathematical model has been developed for all the systems representing the modulation techniques till the second order. The baseband modulation consists of an RF signal of frequency 18 GHz and an Orthogonal Frequency Division Multiplexing (OFDM) signal with 4-QAM modulation technique converted to optical signal, using the MZM and transmitted over a SMF of distance of 75 Km. Our results show that the M-SSB technique proves to be more immune toward transmitter nonlinearity and chromatic dispersion as compared to its counterparts. In addition to this, it improves the receiver sensitivity by 6.56 dBm. The results generated by software simulation are in tandem with the mathematical model.

A High Spectral Efficiency Radio Over Fiber Link Based on Coherent Detection and Digital Phase Noise Cancellation

An approach to transmitting two independent microwave vector signals on a single optical carrier with one polarization state based on coherent detection and digital phase noise cancellation is proposed and experimentally demonstrated. At the transmitter, two independent microwave vector signals are modulated on an optical carrier via a dual-drive Mach-Zehnder modulator (DD-MZM). The modulated optical signals are transmitted over a single-mode fiber (SMF) and sent to a coherent receiver. At the receiver, the optical signals are detected where a local oscillator (LO) optical wave generated by a second free-running laser source is also applied. To recover the two microwave vector signals, a novel digital signal processing (DSP) algorithm is developed and applied to eliminate the joint phase noise terms from the transmitter and the LO laser sources as well as the unstable offset frequency between the two laser sources. An experiment is performed. The transmission of two independent 16 quadrature amplitude modulation (16-QAM) microwave vector signals at 4 GHz with a symbol rate of 1 GSymb/s over a 9-km SMF is demonstrated. The transmission performance in terms of error vector magnitudes (EVMs) and bit error rates (BERs) is also evaluated.

Photonic generation of flexible and adjustable microwave waveforms based on a dual-polarization Mach-Zehnder modulator.

We have proposed a photonic waveform generation scheme based on a dual-polarization Mach-Zehnder modulator (DPol-MZM). In this scheme, a dual-frequency optical signal generated by RF optical carrier suppression modulation via a Mach-Zehnder modulator (MZM) is divided into two beams; one is further modulated by the RF signal via a DPol-MZM to generate ±1st and ±3rd-order sidebands, and the other is further modulated by the RF signal via a second MZM to generate ±2nd-order sidebands. After two modulated optical signals are detected by a balanced photodiode, the second, fourth, and sixth-order harmonics without cross-beating terms are generated in the differential photocurrent. Since the harmonic amplitudes can be controlled independently by the optical power of the two optical beams and the DC biases of the DPol-MZM, the waveforms can be tuned conveniently. Based on the simulation, frequency-doubled triangular, rectangular, and sawtooth waveforms with a full-duty cycle at a repetition rate of 20 GHz are successfully generated with their harmonic amplitude ratio in the electric spectrum close to the ideal amplitude ratio, and the symmetry of the triangular waveform from 20% to 80% is tuned.

Fully fiber-optic intensity modulator for large-core, high-NA multi-mode fibers

Intensity modulation of broadband optical signals transmitted through large-core fiber-optic cables often resorts to opto-mechanical choppers, usually in the form of bulky rotating wheels. We demonstrate an alternative fiber-optic intensity modulator with a compact footprint of 55.6 × 12.8 × 4.6 mm3 that can be integrated in-line in an optical system. The design comprises two optical fiber cantilevers separated by a small air gap. The optical modulation is achieved by controlling the transverse offset between the cores of the input and output fibers using a piezoelectric actuator. The demonstrated implementation can be used as a quasi-static attenuator down to 0 Hz or an arbitrary waveform modulator up to 400 Hz, providing 30-dB modulation contrast across the complete fiber transmission range. The feasibility of this device is demonstrated in a fiber-based remote pyrometry sensor using lock-in detection in the 2 to 2.5-μm infrared band, where the thermal radiation collected by a multi-mode optical fiber with a 360 μm core is modulated at 390 Hz. The resulting noise-equivalent temperature difference is demonstrated to be better than 0.6°C for object temperatures above 35°C.

Spin Laser Local Oscillators for Homodyne Detection in Coherent Optical Communications.

We numerically investigate spin-controlled vertical-cavity surface-emitting lasers (spin-VCSELs) for local oscillators, which are based on an injection locking technique used in coherent optical communications. Under the spin polarization modulation of an injection-locked spin-VCSEL, frequency-shifted and phase-correlated optical sidebands are generated with an orthogonal polarization against the injection light, and one of the sidebands is resonantly enhanced due to the linear birefringence in the spin-VCSEL. We determine that the peak strength and peak frequency in the spin polarization modulation sensitivity of the injection-locked spin-VCSEL depend on detuning frequency and injection ratio conditions. As a proof of concept, 25-Gbaud and 16-ary quadrature amplitude modulation optical data signals and a pilot tone are generated, and the pilot tone is used for the injection locking of a spin-VCSEL. An orthogonally-polarized modulation sideband generated from the injection-locked spin-VCSEL is used as a frequency-shifted local oscillator (LO). We verify that the frequency-shifted LO can be used for the homodyne detection of optical data signals with no degradation. Our findings suggest a novel application of spin-VCSELs for coherent optical communications.

Optoelectronic mixing with high-frequency graphene transistors

Graphene is ideally suited for optoelectronics. It offers absorption at telecom wavelengths, high-frequency operation and CMOS-compatibility. We show how high speed optoelectronic mixing can be achieved with high frequency (~20 GHz bandwidth) graphene field effect transistors (GFETs). These devices mix an electrical signal injected into the GFET gate and a modulated optical signal onto a single layer graphene (SLG) channel. The photodetection mechanism and the resulting photocurrent sign depend on the SLG Fermi level (EF). At low EF (<130 meV), a positive photocurrent is generated, while at large EF (>130 meV), a negative photobolometric current appears. This allows our devices to operate up to at least 67 GHz. Our results pave the way for GFETs optoelectronic mixers for mm-wave applications, such as telecommunications and radio/light detection and ranging (RADAR/LIDARs.)

Precise Optical Modulation and Its Application to Optoelectronic Device Measurement

Optoelectronic devices which play important roles in high-speed optical fiber networks can offer effective measurement methods for optoelectronic devices including optical modulators and photodetectors. Precise optical signal modulation is required for measurement applications. This paper focuses on high-speed and precise optical modulation devices and their application to device measurement. Optical modulators using electro-optic effect offers precise control of lightwaves for wideband signals. As examples, this paper describes frequency response measurement of photodetectors using high-precision amplitude modulation and wavelength domain measurement of optical filters using fast optical frequency sweep. Precise and high-speed modulation can be achieved by active trimming which compensates device structure imbalance due to fabrication error, where preciseness can be described by on-off extinction ratio. A Mach-Zehnder modulator with sub Mach-Zehnder interferometors can offer high extinction-ratio optical intensity modulation, which can be used for precise optoelectronic frequency response measurement. Precise modulation would be also useful for multi-level modulation schemes. To investigate impact of finite extinction ratio on optical modulation, duobinary modulation with small signal operation was demonstrated. For optical frequency domain analysis, single sideband modulation, which shifts optical frequency, can be used for generation of stimulus signals. Rapid measurement of optical filters was performed by using an optical sweeper consisting of an integrated Mach-Zehnder modulator for optical frequency control and an arbitrary waveform generator for generation of a source frequency chirp signal.

Photo thermal effect graphene detector featuring 105 Gbit s\u22121 NRZ and 120 Gbit s\u22121 PAM4 direct detection

One of the main challenges of next generation optical communication is to increase the available bandwidth while reducing the size, cost and power consumption of photonic integrated circuits. Graphene has been recently proposed to be integrated with silicon photonics to meet these goals because of its high mobility, fast carrier dynamics and ultra-broadband optical properties. We focus on graphene photodetectors for high speed datacom and telecom applications based on the photo-thermo-electric effect, allowing for direct optical power to voltage conversion, zero dark current, and ultra-fast operation. We report on a chemical vapour deposition graphene photodetector based on the photo-thermoelectric effect, integrated on a silicon waveguide, providing frequency response >65 GHz and optimized to be interfaced to a 50 Ω voltage amplifier for direct voltage amplification. We demonstrate a system test leading to direct detection of 105 Gbit s−1 non-return to zero and 120 Gbit s−1 4-level pulse amplitude modulation optical signals.

Fundus Autofluorescence in Neovascular Age-Related Macular Degeneration: A Clinicopathologic Correlation Relevant to Macular Atrophy

Macular atrophy (MA) of retinal pigment epithelium (RPE) and photoreceptors leads to vision loss in neovascular age-related macular degeneration (nAMD) despite successful treatment with antiangiogenic agents. To enhance understanding of MA, fortify the cellular basis of fundus autofluorescence (FAF) imaging, and inform management of nAMD, we performed histologic analysis of an eye with multimodal clinical imaging and apparent prior exudation due to nAMD. Case study and clinicopathologic correlation. A White woman in whom age-related macular degeneration (AMD) findings of inactive subretinal fibrosis (right eye) were followed for 9 years using FAF and OCT, with no detectable subretinal fluid or other recurrent exudation and no intravitreal injections before her death at age 90 years. The right eye was preserved 6.25 hours after death, postfixed in osmium tannic acid paraphenylenediamine, and prepared for submicrometer epoxy resin sections (n = 115), with 19 matched to clinical OCT B-scans. Light microscopic morphology of a hyperautofluorescent (hyperFAF) area due to prior exudation ("floodplain" hyperFAF), hypoautofluorescent (hypoFAF) spots of MA, and areas of unremarkable FAF. Floodplain hyperFAF was visible throughout the 9 years of follow-up, with several hypoFAF atrophic spots expanding within it over time. The hyperFAF pattern corresponded to outer retinal atrophy (ORA) on OCT and photoreceptor loss over dysmorphic yet continuous RPE in histology. The hypoFAF spots inside the floodplain corresponded to complete RPE and outer retinal atrophy (cRORA) on OCT and loss of both photoreceptors and RPE in histology. In contrast, areas of unremarkable FAF showed continuous RPE accompanied by full-length photoreceptors and a thick outer nuclear layer. This direct clinicopathologic correlation for FAF imaging is the first for nAMD. Fundus autofluorescence is a projection image that involves optical signal modulation by photoreceptors as well as emission signal sources in RPE. Hyperautofluorescence due to an exudative floodplain signifies loss of photoreceptors over continuous RPE. Hypoautofluorescence in MA signifies loss of both cell layers. For maximal value, fundus autofluorescence imaging should be interpreted with the multilayer perspective provided by OCT. Prevention of exudation in nAMD may preserve photoreceptors.

FPGA based Quantum Key Distribution Electronic System for Multipurpose Optical Setup and Protocols

The purpose of the FPGA electronic control system for Quantum Key Distribution (QKD) is to improve the analog electronic system in optoelectronic conversion. In the existence of the Mezzanine FPGA board, the main conversion of optical laser signal and optical detector signal can be improved. Mezzanine board can reduce the noise errors during the transmission and receiving a signal. Commonly, the optical signal needs to interface with an analog electronic system. This process requires the conversion process and interfacing with a digital signal to control the optical device and produce a good quality optical signal. The Mezzanine board can reduce the uncertainty during signal conversion and gives the digital capability to control the optical devices. The optical modulation signal for phase modulator control can achieve a full complete phase shift in the Mezzanine board. Plus, the voltage phase has capability up to 10 V with an accuracy of 2.4 µV. The time conversion for phase modulator can be extended in the Mezzanine board based on the digital control signal from FPGA. The system is capable to extend into multiple timing control to handle all the signal and to synchronize the signal in receiving, transmission or in between of the transmission. Thus, the system can handle one way or two-way QKD system accordingly.

Design of EM-artifact-free earphone based on the photoacoustic effect

Electromagnetic interactions between conventional earphones and the electroencephalography (EEG) electrodes used for analyzing brain waves give rise to efficiency problems in neurophysiological studies of auditory perception. Currently used speakers and headphones are electromagnetic devices based on strong magnets. In spite of intensive use of such systems, there has been no effective way to eliminate the electromagnetic artifacts produced by such audio transmitting devices to date. The ability for transferring audible sounds without the use of electromagnetic devices that can affect the EEG signal would open up many innovative possibilities in Audio Technologies. Audible sound transfer over long distances is possible by the photoacoustic effect. In such studies, the modulated optical signal can be converted into an audible signal arising from the absorption of the light energy of relevant molecules. In this study, we propose an earphone based on the photoacoustic effect, and calculated the dB SPL (Sound Pressure Level) values for a spherical cell filled with olive pomace. By the use of the method of Diebold and Westervelt, we theoretically calculated the sound pressure levels for our cell and determined a 60 dB SPL at a sound frequency of 1000 Hz for our preliminary earphone design.

Photonic generation of multiband and multi-format microwave signals based on a single modulator.

We report a photonic scheme to generate multiband and multi-format microwave signals based on a commercial dual-polarization Mach-Zehnder modulator (DP-MZM). The key novelty of this work is the compact signal generator benefitting from the single modulator and the flexible multi-format signals at the multiband. The upper Mach-Zehnder modulator (MZM) of the DP-MZM is used to generate an optical frequency comb, while the lower MZM is driven by baseband electrical coding or single-chirp signal to obtain the multi-format modulated optical signal. The proposed scheme is theoretically analyzed and experimentally demonstrated. A multiband phase-coded microwave signal with a bit rate of 2 Gb/s and complementary linearly chirped microwave waveform pairs with a time duration of 1 µs and a bandwidth of 2 GHz have been successfully generated. The performance of pulse compression of generated signals is also demonstrated.

Employing a Kramers–Kronig receiver with a directly-modulated laser

The Kramers–Kronig (KK) receiver has been used for dispersion compensation in direct detection systems employing external-modulator-based transmitters. In this paper, we investigate the use of the KK receiver for dispersion compensation in a direct detection system based on a directly-modulated laser. We show the validity of the KK receiver by considering a linear contribution to the optical phase arising from a numerical solution of the laser rate equations as a frequency shift of the optical spectrum. The modulated optical signal can then meet the minimum-phase condition required by the KK algorithm. A system model is presented based on the direct modulation of a 10 Gb/s distributed feedback laser with 16-ary quadrature amplitude modulation (16-QAM) subcarrier modulation. The modulated signal is optically filtered, and a 53.5 Gb/s vestigial sideband signal is transmitted over 65 km of standard single-mode fiber. Dispersion compensation is applied after direct detection with a KK receiver. In addition, the impact of the optical filter parameters and receiver sampling rate on system performance are investigated.

Universal generation of devil's staircases near Hopf bifurcations via modulated forcing of nonlinear systems

The discrete circle map is the archetypical example of a driven periodic system, showing a complex resonance structure under a change of the forcing frequency known as the devil's staircase. Adler's equation can be seen as the direct continuous equivalent of the circle map, describing locking effects in periodic systems with continuous forcing. This type of locking produces a single fundamental resonance tongue without higher-order resonances, and a devil's staircase is not observed. We show that, with harmonically modulated forcing, nonlinear oscillations close to a Hopf bifurcation generically reproduce the devil's staircase even in the continuous case. Experimental results on a semiconductor laser driven by a modulated optical signal show excellent agreement with our theoretical predictions. The locking appears as a modulation of the oscillation amplitude as well as the angular oscillation frequency. Our results show that by proper implementation of an external drive, additional regions of stable frequency locking can be introduced in systems which originally show only a single Adler-type resonance tongue. The induced resonances can be precisely controlled via the modulation parameters.

Directional coupling surface plasmon polariton electro-optic modulator for optical ring networks-on-chip

Electro-optic modulators are essential components of the optical network-on-chip. To resolve the large footprints, poor thermal stability, and low modulation rate of traditional optoelectronic devices such as micro-ring resonators, a directional coupled electro-optic modulator based on surface plasmon polaritons and coupled mode theory is designed. The modulator controls the change in carrier concentration of an indium-tin-oxide-activated material film by applying a voltage to realize electro-optic control. The modulator uses coupled-mode theory to couple the modulated optical signal into a ring waveguide. This type of modulator is suitable for an optical-on-chip network with an optical ring network-on-chip topology. The results show that the device operates at the 1550 nm wavelength with a coupling efficiency of more than 90%, insertion loss of 1.17 dB, extinction ratio of 15.4 dB, modulation rate of up to 0.75 Tbit/s, and size of only 3.8×3.2×1.2µm.

Scheme of coherent optical chaos communication

We propose and numerically demonstrate a scheme of coherent optical chaos communication using semiconductor lasers for secure transmission of optical quadrature amplitude modulation (QAM) signals. In this scheme, a laser intensity chaos and its delayed duplicate are used to amplitude-quadrature modulate a continuous-wave light to generate a chaotic carrier. High-quality chaotic carrier synchronization between the transmitter and receiver is guaranteed by laser intensity chaos synchronization, avoiding laser phase fluctuation. Decryption is implemented by a 90 deg optical hybrid using the synchronous chaotic carrier as local light. Secure transmission of an optical 40 Gb/s 16QAM signal is demonstrated by using a laser intensity chaos with a bandwidth of 11.7 GHz. The system performances are evaluated by analyzing a bit error ratio with different masking coefficients, signal rates, synchronization coefficients, parameter mismatches, and dispersion compensation. It is believed that this scheme can pave a way for high-speed optical chaos communication.

High-Order Electrical Spectrum Method for Measuring the Chirp of a Silicon Mach–Zehnder Modulator

The chirp parameter of a silicon optical modulator (SOM) is difficult to measure precisely because the plasma dispersion effect is due to electro-refraction and electro-absorption occurring simultaneously. A novel method for measuring the chirp parameter of a SOM is presented, which is adopted from the beat-frequency method. Firstly, based on the model of the differential driver silicon Mach-Zehnder modulator(DDSMZM), the chirp parameter of the DDSMZM is presented by mathematical derivations, and we prove that achieving the multi-order coefficients of the radio frequency phase shift is a key point for characterizing the chirp accurately. In the proposed method, a wavelength tunable laser, as a local source, is applied to mix with the chirped modulated optical signal of the DDSMZM. In this way, the multi-order electric signals of beat frequency are mapped from high-frequency to low-frequency. Meanwhile, a balanced detector with a low-speed trans-impedance amplifier(TIA) is employed for realizing photo-electric conversion, which can improve the signal-to-noise ratio of the measurement. Finally, we have examined the chirp parameter of the DDSMZM in the case of low-frequency and high-frequency modulation signal condition and the results show the beat-frequency method has a higher frequency resolution and measurement accuracy for measuring the chirp parameter of the DDSMZM compared with the optical spectrum analysis(OSA) method and the optical heterodyne method.

Photoluminescence of single gold nanorods following nonlinear excitation.

The photoluminescence (PL) mechanisms of gold nanorods following nonlinear excitation are described. Using single-particle nonlinear optical measurements, we compare PL signals resulting from both the plasmon-resonant and non-resonant excitations. In both cases, spectrally broad interband PL emission was observed. However, we also show that resonant excitation of the longitudinal plasmon mode leads to an increased photonic density of states at energies corresponding to the transverse plasmon resonance. This increased density of states is achieved by a multi-step mechanism, which is initiated by three-photon excitation and followed by an Auger relaxation process. Importantly, the results show that nonlinear excitation can lead to energy and polarization modulation of nanoparticle optical signals that are not observed using linear excitation. This work also demonstrates the effectiveness of single-nanoparticle PL studies for understanding how plasmon-resonant excitations can be used to modify hot carrier distributions.