Optical Signal Generation Research Articles

A general model for taper coupling of multiple modes of whispering gallery resonators and application to analysis of coupling-induced Fano interference in a single cavity.

In this paper, we give a general model for analysis of multimode Whispering Gallery Mode (WGM) resonators coupled to multimode tapered fibers based on the coupled-mode theory. Such formulation takes into account the asymmetry of the taper-resonator coupling. Simulations for a microsphere show that the tapered fiber coupling mechanism induces cross-coupling between coherent orthonormal WGMs. We show that the degree of such cross-coupling depends basically on the fiber diameter, air-gap between the taper and resonator, intrinsic losses and eccentricity. The WGM cross-coupling affects the total transmission and spectral line-shape of the internal powers resulting in a controllable transformation of the line-shape to non-Lorentzian spectra. This analysis can be utilized to precisely determine the output and intra-cavity intensity of multimode microresonators, which is important in lasers, nonlinear optical signal generation and realization of optical delays.

Wavelength-Tunable Chaotic Signal Generation With On-Chip O/E Conversion

A wavelength-tunable chaotic optical signal generation and on-chip O/E conversion scheme based on an integrated laser-photodiode chip subject to filtered optical feedback is proposed and experimentally demonstrated. The integrated chip consists of a Fabry–Perot (FP) gain section and a photodiode, which was fabricated on an n-InP substrate with the same active material. By applying a filtered feedback signal to the FP gain section, wavelength-tunable chaotic carrier ranging from 1576 to 1587 nm was achieved. On-chip microwave chaotic signal generation with a standard bandwidth over 5.5 GHz was obtained using the integrated photonic chip.

Experimental demonstration of optical single sideband signal generation based on multi-frequency phase modulation in optical fibers

We theoretically analyzed and experimentally demonstrated the feasibility of optical single sideband (OSSB) signal generation based on multi-frequency phase modulation. The numerically calculated results show that OSSB signal can be efficiently generated with the proposed method. Optical carrier suppression ratio (OCSR), optical sideband suppression ratio (OSSR) and energy efficiency (EF) of the modulated optical signal increase with the quantity of harmonic waves in the modulation signal. In a principle verifying experiment, 6 frequency phase modulation was carried out with a narrow line-width fiber laser and a LiNbO3 electro-optic phase modulator. The fundamental frequency of the modulation signal was controlled to 30 MHz. OCSR of 18.9 dB, OSSR of 14.1 dB and EF of 78% were obtained, respectively. The experimental results agree well with the numerically calculated results. The proposed scheme provides an effective way to generate an OSSB signal for optical communication or optical sensing systems.

Optical millimeter wave generation utilizing stimulated brillouin scattering for radio over fiber system

<span>Optical millimeter-wave signal generation in Radio over Fiber (RoF) based on external modulators using different modulation schemes such as Single-Sideband (SSB), Double-Sideband (DSB) are experimentally compared. The major fallout for external modulation scheme system performance to downgrade is due to dispersion and the fading effect in optical signal generation while generating high Bit Error Rate (BER) when involving long distance delivery. In this work, we proposed mm-wave signal utilize Stimulated Brillouin Scattering (SBS) scheme which indicates the best performance amongst the scheme based on simulation that it achieves to propagate longest of distance over fiber, highest receiver sensitivity, the lowest BER and coherence of Mach-Zehnder Modulator (MZM) extinction ratio. Mm-wave signal of 40 Ghz and 10 Gb/s data rate was generated with the plotting of eye diagram.</span>

Generation of high-intensity pulse incoherent visual optical signals in 0.35–2.5 μm spectral range

Subject of research. The paper considers the approach to generation of pulse optical jamming signals in broad range of wavelengths aimed to suppress visual optical and electro-optical channels of surveillance equipment operating in different spectral bands. Method. The high-intensity emission optical source was created utilizing compact cermet case short-arc high-pressure xenon lamps with internal reflectors. Optical emitter with multiplication of lamps was used in order to attain the values of energy density above the specified criterion levels at the distances of suppression. Main results. The emitter was constructed for visual-optical jamming in the spectral range of 0.35-2.5 μm with maximum light intensity of 95∙105 cd and irradiance of 120 W/m2 at 10 meters. The low-frequency modulation varies from 5 to 15 Hz while the high-frequency modulation of low-frequency pulses is kept constant at 5 kHz with 60 % modulation depth. The beam width can be mechanically switched from 6 to 15°. The emitter is embodied in all-climate type of enclosure and can be operated from –40 to +50 °C and air humidity up to 98 %. Practical relevance. Implementation of cermet case short-arc high-pressure xenon lamps with fixed integrated reflector provides an opportunity to construct compact high-power emitters of pulsed optical signals. The lamps are explosion-proof and not prone to devitrification and depressurization. The emitters utilizing the lamps with internal reflectors do not require in-service adjustment, have high reliability and durability. The emitter can be configured with the lamps of variant optical power in order to satisfy the electrical power availability of delivery vehicle of all kinds.

Optical Behavior Analysis of Negative Wavelength Detuning in SMFP-LD and Its Effect on Multi-RF Generation

Injection locking with positive wavelength detuning has been used for a long time for optical signal processing, signal generation, and computing. Recently a new technique, “injection locking with negative wavelength detuning,” has been introduced and has shown its potential on microwave generation. In this paper, we analyze optical behavior of negative injection locking and its impact on multi-RF generation using single-mode Fabry-Perot laser diode (SMFP-LD). SMFP-LD is composed of a commercial multi-mode FP-LD with a built-in external cavity. As compared to conventionally used positive detuning optical injection, the negative detuning optical injection (NOI) shows advantageous differences on the suppression of the dominant mode, side modes, bistability property, and the hysteresis width. The laws of free carrier motion in the laser cavity is leveraged to explain these phenomena. Based on the analysis, the NOI in the SMFP-LD has a distinct advantage for multi-RF generation and switching RF signals. In addition, the output performances of the generated RF signal such as linewidth, power-dependent frequency shifting, and signal-to-noise ratio (SNR) are investigated.

Research and development of high-speed on-chip photodetectors based on AIIIBV heterostructures

This paper is aimed at the solution of the fundamental scientific and technical problem of research and development of high-performance optoelectronic devices designed for on- and inter-chip optical interconnecting in integrated circuits. Previously, we developed a laser with a double AIIIBV nanoheterostructure and a functionally integrated optical modulator. The device is based on the principle of controlled spatial relocation of charge carrier density peaks within quantum regions and provides the generation of optical signals with high modulation frequencies. The detection of short laser pulses generated by the lasermodulator requires a technologically compatible on-chip photodetector with subpicosecond response time. To meet the given requirements, we propose a novel design of a high-speed photodetector that employs the same relocation principle as the laser-modulator. The photodetector contains a traditional p-i-n photosensitive structure and an orthogonally oriented control heterostructure. During the back edge of a laser pulse, the control heterostructure displaces the peaks of electron and hole densities into special low-temperature-grown regions with short lifetimes and low carrier mobilities. We developed the quantum mechanical numerical model of the photodetector with controlled relocation of carrier density peaks and estimated the duration of the photocurrent back edge.

Quadruple-frequency optical two-tone signal generation using a DP-QPSK modulator

Quadruple-frequency optical two-tone signal generation using a DP-QPSK modulator

Generation of optical signal and terahertz idler photons by spontaneous parametric down-conversion

Frequency-angular characteristics of signal and idler photons generated under spontaneous parametric down-conversion are studied in a strongly frequency nondegenerate regime, without paraxial approximation, accounting for possible inherent absorption of idler waves in a nonlinear crystal, classical thermal field fluctuations, and the multimode character of parametric interaction induced by transverse spatial limitation of the pump beam. Spatial limitation is shown to lead to a huge increase in angular divergence of the idler photons generated at terahertz frequencies. General expressions are obtained for the frequency-angular sensitivity function of the nonlinear-optical terahertz wave detector and for power densities of the signal and idler photon fluxes. The absorption-induced difference in the parametric conversion coefficients for the noise and externally incident radiation of the idler frequency is shown to be described by approximately the same loss factor for all active spatial idler modes. Two different parametric contributions of the internal thermal noise to the number of output idler photons were revealed with different dependence on the idler-wave absorption. Expressions for the loss factors, which describe absorption-induced effects in signal and idler channels, are obtained and shown to be turning into one another by changing the sign of the absorption coefficient. Relative contribution of thermal and quantum field fluctuations into the intrinsic radiation of a nonlinear crystal at signal and idler frequencies is analyzed accounting for the crystal absorption properties.

Electrically packaged silicon-organic hybrid (SOH) I/Q-modulator for 64 GBd operation.

Silicon-organic hybrid (SOH) electro-optic (EO) modulators combine small footprint with low operating voltage and hence low power dissipation, thus lending themselves to on-chip integration of large-scale device arrays. Here we demonstrate an electrical packaging concept that enables high-density radio-frequency (RF) interfaces between on-chip SOH devices and external circuits. The concept combines high-resolution Al2O3 printed-circuit boards with technically simple metal wire bonds and is amenable to packaging of device arrays with small on-chip bond pad pitches. In a set of experiments, we characterize the performance of the underlying RF building blocks and we demonstrate the viability of the overall concept by generation of high-speed optical communication signals. Achieving line rates (symbols rates) of 128 Gbit/s (64 GBd) using quadrature-phase-shift-keying (QPSK) modulation and of 160 Gbit/s (40 GBd) using 16-state quadrature-amplitude-modulation (16QAM), we believe that our demonstration represents an important step in bringing SOH modulators from proof-of-concept experiments to deployment in commercial environments.

Colloidal Nanoparticles for Signal Enhancement in Optical Diagnostic Assays.

The use of nanotechnologies for the development of highly sensitive and affordable diagnostic assays has significantly improved the ability to detect and characterize multiple types of biomarkers. Semiconductor and metal nanoparticles with unique optical properties have been successfully integrated within biomarker detection schemes for the generation and enhancement of optical signals in label-based and label-free assays. Highly sensitive label-based diagnostics has been realized particularly via using quantum dots (QDs) as labeling probes. Similarly, many label-free techniques that are emerging as potential complements to label-based approaches benefit from signal enhancement strategies using e.g., metal nanoparticles. This review presents a concise overview of recent advances in diagnostic assays that utilize nanoparticles for the generation and enhancement of optical signals in fluorescence- and surface plasmon resonance-based techniques. Advanced diagnostic assays that utilize nanoparticles provide major improvements in detection sensitivity, which can potentially meet the challenging requirements of clinical diagnostics.

FORMATION OF A LOW-FREQUENCY COMPONENT OF AN OPTICAL SIGNAL BY PULSES OF A GAUSSIAN FORM

Background. Increasing the spectral efficiency of fiber-optic transmission systems (FOTS) is a topical problem. Currently used single-wave FOTS on the basis of equipment type STM allow the transfer of data at rates up to 40 Gbit/s. Given the available optical bandwidth of approximately from 175 THz till 375 THz, the bandwidth of the possible transmission band is about 200 THz. The value of the spectral efficiency index is about 0.005 bits/Hz. Usage of DWDM systems with widely spaced carriers allows increasing the spectral efficiency approximately an order of magnitude, which in principle does not solve the initial problem. Experimental developments are known in which the transmission rate is reached up to 160 Tbit / s. A special optical fiber with 348 transparency windows is used. The spectral efficiency of such systems reaches unity. Theproblem with the widespread use of such systems is the need to replace an already mounted optical cable. For comparison, in industrial mobile communication systems, the spectral efficiency is of the order of 0.5 till 5 bits / Hz.Objective. Development of theoretical ideas on the formation methods of optical signals with a given structure that allows increasing the bandwidth of FOTS taking into account the dispersion factors and the dependence of the attenuation coefficient on the frequency. These theoretical studies have the ultimate goal of increasing the spectral efficiency of the FOTS using the existing cable infrastructure.Methods. The formation of a low-frequency component of an optical signal with a given shape under conditions when the basic optical pulses are formed by a pulsed laser in the state of locking mode. In this case, the spectral composition of the optical signal can be formed in such a way as to balance the effects of dispersion and distortion of the spectrum due to the dependence of the damping coefficient on the frequency.Results. The theoretical substantiation of the principle possibility of approximating processes of a certain form by a sequence of Gaussian pulses is given. Convergence estimates of the corresponding functional series are obtained. The results are valid for narrowband transmission channels with a width of the order of 100 GHz and for the length of the regeneration sections of the order of 100-300 km.Conclusions. There is a fundamental possibility of short optical signal generation with a given shape using Gaussian impulses. Pulses of approximately Gaussian shape give the laser in the state of locking mode.Keywords: Fourier transform; Gaussian pulse; state of locking mode; convergence; Jon Tikhiy.

Optical millimeter wave signal generation with frequency 16-tupling using cascaded MZMs and no optical filtering for radio over fiber system

BackgroundAn optical Millimeter Wave (MM-Wave) generation with high frequency multiplication factor is very attractive solution to avoid the use of high frequency local oscillators at the central station. MethodsIn this paper, a frequency 16 tupling technique is demonstrated with the use of four cascaded MZMs. With the proper adjustment of DC bias of the Mach-Zehnder Modulators (MZM), amplitude and phases of the RF Local oscillators, MM-Wave with a frequency 16 times the input RF LO is demonstrated. ResultsThe cascaded structure has resulted very low modulation index of 2.827 and 54 dB and 42 dB of OSSR and RFSSR respectively. Analysis of both optical and RF sided band suppression ratios under non-ideal cases are also simulated and presented. Further, 10 Gbps NRZ data transmission performance over 50 km Single Mode Fiber (SMF) is also evaluated. ConclusionsThe proposed structure can tolerate up to few degrees of non-ideal parameter variation and is very simple to realize.

Controlled fluorescence quenching by antibody-conjugated graphene oxide to measure tau protein.

We report an ultrasensitive immunoassay for tau protein—a key marker of Alzheimer's disease. This sensing platform relies on graphene oxide (GO) surfaces conjugated with anti-human tau antibody to provide quantitative binding sites for the tau protein. The GO quenches standard fluorescein isothiocyanate labelled tau (tau-FITC) when tau protein and tau-FITC are both present and compete for the binding sites. This change in fluorescence signal can be used to quantitate tau protein. In contrast with traditional enzyme-linked immunosorbent assay (ELISA), our method does not require enzyme-linked secondary antibodies for protein recognition nor does it require an enzyme substrate for optical signal generation. This requires fewer reagents and has less systematic error than the antigen–antibody recognition steps in ELISA. Our method has a tau protein detection limit of 0.14 pmol ml−1 in buffer. This approach could be developed into a promising biosensor for the detection of tau protein and may be useful in the clinical diagnosis of tau-induced neurodegeneration syndromes.

Generation and evaluation of an artificial optical signal based on X-ray measurements for bubble characterization in fluidized beds with vertical internals

The performance of fluidized bed reactors strongly depends on the bubble behavior, for which reason knowledge concerning the bubble properties is important for modeling and reactor optimization. X-ray measurements can be used to characterize bubbles within the cross-section of a fluidized bed on a laboratory scale, but cannot easily be extended to hot, pressurized large scale plants. For future measurements at hot conditions in a fluidized bed methanation reactor, we have developed an optical probing system that can be employed under these conditions. However, optical sensors are only able to investigate the local fluidization patterns at a defined position in the bed. The objective of this study is to characterize differences in bubble properties between local optical measurements and an X-ray tomography method that is able to detect bubbles over the entire cross-section. Therefore, an artificial optical signal is created out of existing hydrodynamic X-ray measurement data obtained at a cold flow model of a pilot scale methanation reactor. The determined bubble properties of both methods (i.e. evaluation of the derived artificial optical probe signal and image reconstruction based on the evaluation of original X-ray tomographic data) are compared with regard to the bubble rise velocity and the bubble size (for the X-ray method) or pierced chord length (for the optical evaluation method), respectively. The comparison shows that for the evaluation of the optical probe data, statistical effects have to be considered carefully. The detected mean chord length of the optical method does not immediately correspond to the mean bubble size determined by the X-ray method. Moreover, also differences regarding the bubble rise velocity were detected for some fluidization states. The reason for the discrepancies between both methods could be identified and corrected, amongst others by means of a Monte Carlo simulation in which rising bubbles in a fluidized bed were simulated and characterized by a local virtual optical sensor.

Radio-over-fiber system with octuple frequency optical millimeter-wave signal generation using dual-parallel Mach–Zehnder modulator based on four-wave mixing in semiconductor optical amplifier

We have proposed a scheme of radio-over-fiber (RoF) system employing a dual-parallel Mach–Zehnder modulator (DP-MZM) based on four-wave mixing (FWM) in a semiconductor optical amplifier (SOA). In this scheme, the pump and the signal are generated by properly adjusting the direct current bias, modulation index of the DP-MZM, and the phase difference between the sub-MZMs. Because of the pump and the signal deriving from the same optical wave, the polarization states of the two lightwaves are copolarized. The single-pump FWM is polarization insensitive. After FWM and optical filtering, the optical millimeter-wave with octuple frequency is generated. About 40-GHz RoF system with a 2.5-Gbit / s signal is implemented by numerical simulation; the result shows that it has a good performance after the signal is transmitted over 40-km single-mode fiber. Then, the effects of the SOA’s injection current and the carrier-to-sideband ratio on the system performance are discussed by simulation, and the optimum value for the system is obtained.

Reproducible optical noise-like signal generation subjected by digital sequences

We experimentally demonstrate a reproducible broadband optical noise-like signal generation scheme whereby dispersion induced phase modulation to intensity modulation conversion and an electro-optic nonlinear transform are utilized to post process a binary random sequence. A flat spectrum and a symmetrical distribution of the generated analog noise-like signal can be observed. Moreover, Gaussian-like probability distribution function can be obtained by using the central limit theorem. The influences of the digital input and the analog parameters on complexity performance of the generated analogue noise are discussed in detail. Finally, the impact of key parameters on the reproducibility of the system is discussed. The proposed scheme could be a more controllable way to generate broadband optical noise-like signal, and has potential to be applied in various real world applications.

On second-harmonic generation in nonuniformly magnetized media

This paper presents a theoretical study of new effects of double-frequency optical signal generation in magnetic media with a nonuniform magnetization distribution. The study is based on the hydrodynamic approximation of the motion of conduction electrons in the field of an electromagnetic wave and equations for the mean electron spin. Within this approach, the mechanisms of the toroidal moment effect, which was experimentally discovered earlier in a system of magnetic particles with a vertical magnetization distribution as well as in a multilayered magnetic system (which is a collinear nonuniform magnetic system), are demonstrated. A new effect that appears due to the presence of equilibrium spin currents in a nonuniform magnetic system is studied in detail. This effect was predicted from the symmetry and can occur only in noncollinearly magnetized media. It is shown that this effect has a resonance nature with a resonance at a pump frequency equal to the plasma frequency of conduction electrons. Estimates of the susceptibility at the double frequency for the parameters of nickel and the typical scale of variation in the magnetization, which is equal to 10 nm, show that the susceptibility near the frequency of the plasma resonance at the chosen parameters is on the order of 10–9 esu, which holds up a hope of the experimental detection of this effect.

Generation of an orthogonally polarized dual-wavelength optical signal with large wavelength-spacing tunability by using an integrated modulator and a Sagnac loop

A tunable orthogonally polarized dual-wavelength optical signal generator is proposed based on an integrated dual-polarization quadrature phase shift keying modulator, a polarizer and a Sagnac loop. By driving the integrated modulator with an RF signal, an optical signal with two orthogonal polarization states is generated. Leading the polarization multiplexing optical signal to a polarizer with a specific principal axis, the low-order optical components of the signal are cancelled each other, while two high-order sidebands with a wavelength spacing of eight, twelve or sixteen times the frequency of the RF signal reserved. After that, a Sagnac loop is used to make the two sidebands separated and combined with orthogonal polarization states. The proposed approach features large wavelength spacing and good tunability for the generated optical signal. An experiment is performed. The generation of an orthogonally polarized dual-wavelength optical signal with wavelength spacing at 32, 80 and 91.2 GHz is verified by using an RF signal with frequency at 4, 10 and 11.4 GHz.

Compact Brillouin devices through hybrid integration on silicon

A range of unique capabilities in optical and microwave signal processing and generation have been demonstrated using stimulated Brillouin scattering (SBS). The need to harness SBS in mass-manufacturable integrated circuits has led to a focus on silicon-based material platforms. Remarkable progress in silicon-based Brillouin waveguides has been made, but results have been hindered by nonlinear losses present at telecommunications wavelengths. Here, we report on a new approach to surpass this issue through the integration of a high Brillouin gain material, As2S3, onto a silicon-based chip. We fabricated a compact spiral device within a silicon circuit, achieving an order-of-magnitude improvement in Brillouin amplification. To establish the flexibility of this approach, we fabricated a ring resonator with free spectral range precisely matched to the Brillouin shift, enabling the first demonstration, to our knowledge, of Brillouin lasing in a planar integrated circuit. Combining active photonic components with the SBS devices shown here will enable the creation of compact, mass-manufacturable optical circuits with enhanced functionalities.