Modulated Optical Signal Research Articles

NEMS-Based Infrared Metamaterial via Tuning Nanocantilevers Within Complementary Split Ring Resonators

Dynamic control of the electromagnetic properties of metamaterials requires wide modulation bandwidth. Tunable metamaterials with large tunability and fast speed are thus highly desirable. Due to the small dimensions, subwavelength meta-atoms or resonant elements that constitute a metamaterial in the mid-to-near-infrared (IR) wavelength range are often not easy to be tuned at a high rate of several tens of megahertz (MHz). Here, we report on a nanoelectromechanical systems (NEMS)-based tunable IR metamaterial realized by unique embedding of nanocantilevers into complementary split ring resonators (c-SRRs) suspended over individual wells. The optical field confined in the air gap of the c-SRR is strongly influenced by electrostatically induced mechanical deflection of the nanocantilever, thus modulating the reflection spectrum of the metamaterial. With the easy-to-implement tunable meta-atom design, the IR metamaterial with 800-nm-long cantilevers provides an ultrahigh mechanical modulation frequency of 32.26 MHz for optical signal modulation at a wavelength of 2.1 $\mu \text{m}$ , and is rather easy to manufacture and operate. We envision a compact, efficient, and high-speed electrooptic modulation platform in the IR region using this NEMS tunable metamaterial technology. [2017–0012]

Microwave Photonic Frequency Tripler Based on an Integrated Dual-Parallel Modulator Structure

A new technique for tripling the frequency of a microwave signal is presented. It is based on an integrated dual-parallel Mach–Zehnder modulator, which generates two orthogonally polarised RF modulated optical signals with different modulation indexes, and a polarisation-dependent optical attenuator. The photodetector output fundamental RF components generated by the two orthogonally polarised RF modulated optical signals have the same amplitude but an opposite phase, which are cancelled, leaving the frequency tripled microwave signal and the unwanted fifth order harmonic. The new frequency tripler has a simple structure, a wide bandwidth, and a high electrical spurious suppression ratio. Experimental results are presented demonstrating three times frequency multiplication of different-frequency microwave signals with electrical spurious suppression ratio of over 20 dB. The phase noise and the stability performance of the frequency tripler are also investigated experimentally.

Microwave signal delay line based on multicore optical fiber

A fiber-optic delay line based on the successive transmission of a modulated optical signal through all seven cores of an optical fiber has been fabricated. The total loss of the signal after the transmission through all cores is less than 12 dB. The group delay in the range of modulation frequencies of 6 to 12GHz is found to be 5.62 μs for a fiber as short as 160 m.

III–V-on-Silicon C-Band High-Speed Electro-Absorption-Modulated DFB Laser

Externally modulated lasers have proven to be key components for optical communication because of their compactness, low-power consumption, and speed. In this paper, we present unique heterogeneously integrated InP-on-Si electro-absorption-modulated DFB lasers, which were used to implement two optical modulation schemes. The first scheme uses a double-sided externally modulated DFB laser. Two taper sections on each side of the single DFB laser are fabricated with an identical epitaxial structure as the laser and perform two roles: coupling the light to the underlying Si waveguide and acting as modulators. These taper sections are electrically isolated from the DFB laser cavity. Each section can independently be driven with a 56-Gb/s non-return-to-zero (NRZ) on–off-keying signal resulting in 112–Gb/s aggregate data transmission from the device over 2-km non-zero dispersion-shifted single-mode fiber. The second scheme is an original method to generate an optical pulse amplitude modulation (PAM) signal using a similar device structure. By simultaneously directly modulating the DFB laser and one of the tapers (operating as an electro-absorption modulator) with two independent NRZ signals, we demonstrate the generation of a PAM-4 signal. In this way, the PAM-4 signal generation can be shifted from the electrical to the optical domain in a rather simple and power efficient way. We demonstrate the transmission of 25-Gbaud PAM-4 over 2-km non-zero dispersion-shifted single-mode fiber.

Six-Port Direct Modulator With Carrier Suppression Technology for High-Speed High-Frequency Wireless Communications

In this letter, a six-port direct modulator with carrier leakage suppression is proposed and experimentally demonstrated for downlinks in the future high-speed high-carrier-frequency wireless communication systems. To handle high carrier frequencies and provide good performance, GaAs metal–semiconductor field-effect transistor RF switches are used to complete the modulation. Up to 500-MBd quadrature phase shift keying signals at 24 GHz are generated and evaluated in our system. Moreover, the proposed six-port modulator is easy to integrate with the optical link and modulated signals at 24 GHz are evaluated after transmission over up to 20-km standard single mode fiber.

Theoretical analysis of a method for extracting the phase of a phase-amplitude modulated signal generated by a direct-modulated optical injection-locked semiconductor laser

The phase modulation (PM) and amplitude modulation (AM) of optical signals can be achieved using a direct-modulated (DM) optical injection-locked (OIL) semiconductor laser. We propose and theoretically analyze a simple method to extract the phase component of a PM signal produced by a DM-OIL semiconductor laser. The pure AM component of the combined PM–AM signal can be isolated by square-law detection in a photodetector and can then be used to compensate for the PM–AM signal based on an optical homodyne method. Using the AM compensation technique, we successfully developed a simple and cost-effective phase extraction method applicable to the PM–AM optical signal of a DM-OIL semiconductor laser.

A flexible millimeter-wave radio-over-fiber system for various transmission bit rate

In this paper, we propose a flexible scheme to generate millimeter-waves (MMWs) for millimeter-wave radio-over-fiber (MMW-RoF) system. The proposed scheme has two configurations to adapt to various transmission bit rate by slightly rearranging the allocation of the system. The main structure of the system is cascaded LiNbO3 Mach-Zehnder modulators (LiNbO3-MZMs). The first configuration is cost effective method, where we utilize a wavelength division multiplexing de-multiplexer (WDM Demux) and an optical splitter (OS) to generate two unmodulated optical signals and a modulated optical signal. Each unmodulated optical signal and the modulated signal will form an optical single sideband (OSSB) modulation and therefore we simultaneously achieve a Q-band (45GHz) and a V-band (60GHz) MMWs in the receiver. The bit-error-ratio (BER) curves show that the power penalties of 45GHz and 60GHz are 0.6dB and 1.1dB at a BER of 1×10−9 with 2.5Gb/s transmission data rate and 50km fiber transmission distances. The second configuration can accommodate high bit rate, where few optical devices is rearranged to only generate a 60GHzMMW with 40Gb/s 4 quadrature amplitude modulation orthogonal frequency division multiplexing (4-QAM OFDM) signals. The power penalty is 1.1dB at a BER of 1×10−9 with 50km fiber transmission distances. Therefore, the proposed scheme can flexibly generate either two Q&V-band MMWs (45GHz and 60GHz) for the applications of the low data-rate transmission systems or one V-band MMW (60GHz) for the applications of the high data-rate transmission systems.

Low-loss Electrically Controllable Vertical Directional Couplers

We propose a nearly lossless, compact, electrically modulated vertical directional coupler, which is based on the controllable evanescent coupling in a previously proposed graphene-assisted total internal reflection (GA-FTIR) scheme. In the proposed device, two single-mode waveguides are separate by graphene-SiO2-graphene layers. By changing the chemical potential of the graphene layers with a gate voltage, the coupling strength between the waveguides, and hence the coupling length of the directional coupler, is controlled. Therefore, for a properly chosen, fixed device length, when an input wave is launched into one of the waveguides, the ratio of their output powers can be controlled electrically. The operation of the proposed device is analyzed, with the dispersion relations calculated using a model of a one-dimensional slab waveguide. The supermodes in the coupled waveguide are calculated using the finite-element method to estimate the coupling length, realistic devices are designed, and their performance was confirmed using the finite-difference time-domain method. The designed 3 μm by 1 μm device achieves an insertion loss of less than 0.11 dB, and a 24-dB extinction ratio between bar and cross states. The proposed low-loss device could enable integrated modulation of a strong optical signal, without thermal buildup.

Study of material properties important for an optical property modulation-based radiation detection method for positron emission tomography.

We compare the performance of two detector materials, cadmium telluride (CdTe) and bismuth silicon oxide (BSO), for optical property modulation-based radiation detection method for positron emission tomography (PET), which is a potential new direction to dramatically improve the annihilation photon pair coincidence time resolution. We have shown that the induced current flow in the detector crystal resulting from ionizing radiation determines the strength of optical modulation signal. A larger resistivity is favorable for reducing the dark current (noise) in the detector crystal, and thus the higher resistivity BSO crystal has a lower (50% lower on average) noise level than CdTe. The CdTe and BSO crystals can achieve the same sensitivity under laser diode illumination at the same crystal bias voltage condition while the BSO crystal is not as sensitive to 511-keV photons as the CdTe crystal under the same crystal bias voltage. The amplitude of the modulation signal induced by 511-keV photons in BSO crystal is around 30% of that induced in CdTe crystal under the same bias condition. In addition, we have found that the optical modulation strength increases linearly with crystal bias voltage before saturation. The modulation signal with CdTe tends to saturate at bias voltages higher than 1500V due to its lower resistivity (thus larger dark current) while the modulation signal strength with BSO still increases after 3500V. Further increasing the bias voltage for BSO could potentially further enhance the modulation strength and thus, the sensitivity.

Electrical tunability due to coalescence of exceptional points in parity-time symmetric waveguides.

We demonstrate theoretically the electric tunability due to the coalescence of exceptional points in PT-symmetric waveguides bounded by imperfect conductive layers. Owing to the competition effect of multimode interaction, multiple exceptional points and PT phase transitions could be attained in such a simple system, and their occurrences are strongly dependent on the boundary conductive layers. When the conductive layers become very thin, it is found that the oblique transmittance and reflectance of the same system can be tuned between zero and one by a small change in the carrier density. The results may provide an effective method for fast tuning and modulation of optical signals through electrical gating.

WDM Transmission of Single-Carrier 120-GBd ETDM PDM-16QAM Signals Over 1200-km Terrestrial Fiber Links

We demonstrate the generation and wavelength-division multiplexing (WDM) transmission of single-carrier 120-GBd electronic time-division multiplexing (ETDM) polarization division multiplexed 16-ary quadrature amplitude modulation (16 QAM) optical signals. Both transmitter-side optical preemphasis and receiver-side maximum likelihood sequence estimation are utilized to mitigate the impairments caused by bandwidth limitation of the optoelectronic components. Thanks to the preemphasis and post equalization, we have successfully transmitted 12 WDM channels on 150-GHz grid, each loaded with 960-Gb/s (800 Gb/s data signals + 20% FEC emulated overhead bits) over 1200-km link based on 100 km/span TeraWave terrestrial optical fiber. The achieved data rate interface ensured spectral efficiency of 5.33 b/s/Hz. To the best of our knowledge, this is the first experiment with the highest symbol rate reported for ETDM-based 16 QAM signals.

Peptide Optical waveguides.

Small-scale optical devices, designed and fabricated onto one dielectric substrate, create integrated optical chip like their microelectronic analogues. These photonic circuits, based on diverse physical phenomena such as light-matter interaction, propagation of electromagnetic waves in a thin dielectric material, nonlinear and electro-optical effects, allow transmission, distribution, modulation, and processing of optical signals in optical communication systems, chemical and biological sensors, and more. The key component of these optical circuits providing both optical processing and photonic interconnections is light waveguides. Optical confinement and transmitting of the optical waves inside the waveguide material are possible due to the higher refractive index of the waveguides in comparison with their surroundings. In this work, we propose a novel field of bionanophotonics based on a new concept of optical waveguiding in synthetic elongated peptide nanostructures composed of ordered peptide dipole biomolecules. New technology of controllable deposition of peptide optical waveguiding structures by nanofountain pen technique is developed. Experimental studies of refractive index, optical transparency, and linear and nonlinear waveguiding in out-of-plane and in-plane diphenylalanine peptide nanotubes have been conducted. Optical waveguiding phenomena in peptide structures are simulated by the finite difference time domain method. The advantages of this new class of bio-optical waveguides are high refractive index contrast, wide spectral range of optical transparency, large optical nonlinearity, and electro-optical effect, making them promising for new applications in integrated multifunctional photonic circuits. Copyright © 2016 European Peptide Society and John Wiley & Sons, Ltd.

CMOS-Photonics Codesign of an Integrated DAC-Less PAM-4 Silicon Photonic Transmitter

Codesign and integration of optical modulators and CMOS drivers is crucial for high-speed silicon photonic (SiP) transmitters to reach their full potential for low-cost, low-power electronic-photonic integrated systems. We present a CMOS-driven SiP multi-level optical transmitter implemented using a commercially available lateral p-n junction process. It uses a Mach-Zehnder modulator (MZM) segmented to increase speed and to lower the required power on a per segment basis to a level achievable with CMOS. A multi-channel driver is designed and implemented in 130 nm RF CMOS, providing a swing of 4 V in a push-pull configuration at 20 Gbaud. Binary data at the CMOS input is manipulated via digital logic to produce the proper per-segment drive signals to generate a four-level pulse-amplitude modulation optical signal. Multi-level modulation is achieved using only binary signals as input (DAC-less). Cosimulation of the optical and electrical circuits shows good agreement with experiment. Reliable transmission is achieved without post-compensation at 28 Gb/s, and at 38 Gb/s when using post-compensation.

Nanomechanical electro-optical modulator based on atomic heterostructures.

Two-dimensional atomic heterostructures combined with metallic nanostructures allow one to realize strong light–matter interactions. Metallic nanostructures possess plasmonic resonances that can be modulated by graphene gating. In particular, spectrally narrow plasmon resonances potentially allow for very high graphene-enabled modulation depth. However, the modulation depths achieved with this approach have so far been low and the modulation wavelength range limited. Here we demonstrate a device in which a graphene/hexagonal boron nitride heterostructure is suspended over a gold nanostripe array. A gate voltage across these devices alters the location of the two-dimensional crystals, creating strong optical modulation of its reflection spectra at multiple wavelengths: in ultraviolet Fabry–Perot resonances, in visible and near-infrared diffraction-coupled plasmonic resonances and in the mid-infrared range of hexagonal boron nitride's upper Reststrahlen band. Devices can be extremely subwavelength in thickness and exhibit compact and truly broadband modulation of optical signals using heterostructures of two-dimensional materials.

Tuning of near‐ and far‐field properties of all‐dielectric dimer nanoantennas via ultrafast electron‐hole plasma photoexcitation

AbstractAll‐optical ultrafast signal modulation and routing by low‐loss nanodevices is a crucial step towards an ultracompact optical chip with high performance. Here, we propose a specifically designed silicon dimer nanoantenna, which is tunable via photoexcitation of dense electron‐hole plasma with ultrafast relaxation rate. On the basis of this concept, we demonstrate the effect of beam steering by up to 20 degrees through simple variation of the intensity of incident light. The effect, which is suitable for ultrafast light routing in an optical chip, is demonstrated both in the visible and near‐IR spectral regions for silicon‐ and germanium‐based nanoantennas. We also reveal the effect of electron‐hole plasma photoexcitation on the local density of states (LDOS) in the dimer gap and find that the orientation averaged LDOS can be altered by 50%, whereas modification of the projected LDOS can be even more dramatic, almost five‐fold for transverse dipole orientation. Moreover, our analytical model sheds light on the transient dynamics of the studied nonlinear nanoantennas, yielding all temporal characteristics of the suggested ultrafast nanodevice. The proposed concept paves the way to the creation of low‐loss, ultrafast, and compact devices for optical signal modulation and routing. image

A promising new mechanism of ionizing radiation detection for positron emission tomography: modulation of optical properties

Using conventional scintillation detection, the fundamental limit in positron emission tomography (PET) time resolution is strongly dependent on the inherent temporal variances generated during the scintillation process, yielding an intrinsic physical limit for the coincidence time resolution of around 100 ps. On the other hand, modulation mechanisms of the optical properties of a material exploited in the optical telecommunications industry can be orders of magnitude faster. In this paper we borrow from the concept of optics pump-probe measurement to for the first time study whether ionizing radiation can produce modulations of optical properties, which can be utilized as a novel method for radiation detection. We show that a refractive index modulation of approximately is induced by interactions in a cadmium telluride (CdTe) crystal from a 511 keV photon source. Furthermore, using additional radionuclide sources, we show that the amplitude of the optical modulation signal varies linearly with both the detected event rate and average photon energy of the radiation source.

A Local Nanofiber-Optic Ear

The development of acoustic sensors that are compact, have simple read-out mechanisms, and have geometries that enable them to be inserted/embedded deep in materials is of great interest for acoustic-based imaging technologies and novel analytical instruments. Fiber-optic-based detectors are the most logical choice to satisfy these demands, but scaling down the size to sub-micrometer dimensions and uncovering transduction mechanisms that can be more robust than interferometric techniques in dynamic environments has been challenging. In this work, we demonstrate a non-interference-based acoustic ear that utilizes the movement of plasmonic nanoparticles embedded in the near field of a nanofiber optic. The modulated optical signal induced by sound waves can be read-out through transmission through the nanofiber or by tracking the scattering of the nanoparticles in the far field. By utilizing a thin, compressible cladding on the nanofibers, acoustic intensities of <10–8 W/m2 can be detected by the devices ove...

Directly Phase-Modulated Light Source

The art of imparting information onto a light wave by optical signal modulation is fundamental to all forms of optical communication. Among many schemes, direct modulation of laser diodes stands out as a simple, robust, and cost effective method. However, the simultaneous changes in intensity, frequency and phase are a drawback which has prevented its application in the field of secure quantum communication. Here, we propose and experimentally demonstrate a directly phase-modulated light source which overcomes the main disadvantages associated with direct modulation and is suitable for diverse applications such as coherent communications and quantum cryptography. The source separates the tasks of phase preparation and pulse generation between a pair of semiconductor lasers leading to very pure phase states. Moreover, the cavity enhanced electro-optic effect enables the first example of sub-volt halfwave phase modulation at high signal rates. The source is compact, stable and versatile, and we show its potential to become the standard transmitter for future quantum communication networks.

Photonic-assisted microwave frequency measurement system based on a silicon ORR

A photonic-assisted instantaneous microwave frequency measurement (IFM) system is demonstrated with add-drop optical ring resonators (ORRs) on silicon-on-insulator (SOI) platform. By launching a double-sideband suppressed carrier modulated optical signal into the ring, a monotonous amplitude comparison function (ACF) irrespective of the amplitudes of both optical and RF signals is established to translate the RF frequency to the power ratio between the through and drop ports of the ring. Two experiments have been set up with two rings which have different Q values. Two 25µm radius ORRs with Q values of 3974 and 25833 are used to offer different measurement ranges and accuracies. In the experiments the ORR with low Q value has a large measurement range of 0.5–35GHz, and the other one with high Q value exhibits a high accuracy of 0.1GHz in the frequency range of 0.1–5GHz.

High power and high SFDR frequency conversion using sum frequency generation in KTP waveguides.

We characterize the intermodulation distortion of high power and efficient frequency conversion of modulated optical signals based on sum frequency generation (SFG) in a periodically poled potassium titanyl phosphate (KTP) waveguide. Unwanted frequency two-tone spurs are generated near the converted signal via a three-step cascaded three-wave mixing process. Computer simulations describing the process are presented along with the experimental measurements. High-conversion efficiencies and large spur-free dynamic range of the converted optical signal are demonstrated.