Broadband Electro-optic Modulator Research Articles

Electronically preresonant stimulated Raman scattering microscopy in the visible

We report an experimental scheme for stimulated Raman scattering (SRS) microscopy with excitation in the visible spectral region. This allows electronically preresonant (epr) SRS microscopy of a broad range of chromophores with sensitivities as low as 1 μM. Our experiment is based on two synchronously near-infrared pumped optical parametric oscillators (OPO). One of the outputs is modulated at a fourth of the repetition rate with a novel broadband electro-optical modulator. Using a combination of spectral focusing and tuning of the OPO, we show the recording of epr-SRS spectra over the whole range of molecular vibrations at a speed up to 20 times faster than classical wavelength tuning. The imaging capabilities of this setup are demonstrated with material scientific and cellular samples.

Application of Dual-Frequency Self-Injection Locked DFB Laser for Brillouin Optical Time Domain Analysis.

Self-injection locking to an external fiber cavity is an efficient technique enabling drastic linewidth narrowing of semiconductor lasers. Recently, we constructed a simple dual-frequency laser source that employs self-injection locking of a DFB laser in the external ring fiber cavity and Brillouin lasing in the same cavity. The laser performance characteristics are on the level of the laser modules commonly used with BOTDA. The use of a laser source operating two frequencies strongly locked through the Brillouin resonance simplifies the BOTDA system, avoiding the use of a broadband electrooptical modulator (EOM) and high-frequency electronics. Here, in a direct comparison with the commercial BOTDA, we explore the capacity of our low-cost solution for BOTDA sensing, demonstrating distributed measurements of the Brillouin frequency shift in a 10 km sensing fiber with a 1.5 m spatial resolution.

Investigation of the Characteristics of a Photodetector with a High Photocurrent when Transmitting Microwave Radio Signals Through an Optical Fiber

Introduction. At present, an optical transmission of a microwave signal is of great scientific and practical interest. Moreover, this transmission line can also be used to create microwave photonic devices. Microwave signal losses decrease with an increase of laser power. Commercial photodetectors withstand radiation with a power of several tens of milliwatts. Using a photodetector with a high photocurrent can improve characteristics of photonic transmission lines; in particular, it can reduce microwave signal losses. Aim. Investigation of characteristics of a photodetector with a high photocurrent when transmitting microwave radio signals through optical fiber. Research of microwave signal losses as a function of optical power. Materials and Methods. Experimental studies were carried out on created experimental schemes for studying the characteristics of the photodetector with modulated and unmodulated optical radiation. Theoretical studies were carried out by mathematical modeling of optical path transfer characteristics from the laser power at various powers of an input microwave signal. Results. The dependencies of photocurrent and photovoltage of the photodetector versus laser power were obtained. The experimental amplitude-frequency characteristics of the photonic transmission line were measured at different optical powers. A frequency dependence of the photodetector sensitivity in the range of 0...12 GHz was obtained. Modeling of amplitude-frequency characteristics of the optical path in the range of 0...12 GHz was performed. An approximate frequency dependence of the photodetector sensitivity was obtained. Conclusion. Due to the use of a photodetector with a high photocurrent value and with increasing laser power, microwave losses were reduced to about 10 dB. It was shown that for improving the transmission characteristics of an optical transmission line, it is necessary to use a broadband electro-optical modulator.

Plasmonic monolithic lithium niobate directional coupler switches

Lithium niobate (LN) has been the material of choice for electro-optic modulators owing to its excellent physical properties. While conventional LN electro-optic modulators continue to be the workhorse of the modern optoelectronics, they are becoming progressively too bulky, expensive, and power-hungry to fully serve the needs of this industry. Here, we demonstrate plasmonic electro-optic directional coupler switches consisting of two closely spaced nm-thin gold nanostripes on LN substrates that guide both coupled electromagnetic modes and electrical signals that control their coupling, thereby enabling ultra-compact switching and modulation functionalities. Extreme confinement and good spatial overlap of both slow-plasmon modes and electrostatic fields created by the nanostripes allow us to achieve a 90% modulation depth with 20-μm-long switches characterized by a broadband electro-optic modulation efficiency of 0.3 V cm. Our monolithic LN plasmonic platform enables a wide range of cost-effective optical communication applications that demand μm-scale footprints, ultrafast operation and high environmental stability.

Graphene photonic crystal fibre with strong and tunable light–matter interaction

The integration of photonic crystal fibre (PCF) with various functional materials has greatly expanded the application regimes of optical fibre1–12. The emergence of graphene (Gr) has stimulated new opportunities when combined with PCF, allowing for electrical tunability, a broadband optical response and all-fibre integration ability13–18. However, previous demonstrations have typically been limited to micrometre-sized samples, far behind the requirements of real applications at the metre-scale level. Here, we demonstrate a new hybrid material, Gr–PCF, with length up to half a metre, produced using a chemical vapour deposition method. The Gr–PCF shows a strong light–matter interaction with ~8 dB cm−1 attenuation. In addition, the Gr–PCF-based electro-optic modulator demonstrates a broadband response (1,150–1,600 nm) and large modulation depth (~20 dB cm−1 at 1,550 nm) under a low gate voltage of ~2 V. Our results could enable industrial-level graphene applications based on this Gr–PCF and suggest an attractive platform for two-dimensional material-PCF. A hybrid material based on uniform graphene on both the outer surface and inner hole walls of a photonic crystal fibre offers a strong, tunable light–matter interaction and good broadband electro-optic modulation performance under low gate voltage.

500 GHz plasmonic Mach-Zehnder modulator enabling sub-THz microwave photonics

Broadband electro-optic intensity modulators are essential to convert electrical signals to the optical domain. The growing interest in terahertz wireless applications demands modulators with frequency responses to the sub-terahertz range, high power handling, and very low nonlinear distortions, simultaneously. However, a modulator with all those characteristics has not been demonstrated to date. Here, we experimentally demonstrate that plasmonic modulators do not trade-off any performance parameter, featuring—at the same time—a short length of tens of micrometers, record-high flat frequency response beyond 500 GHz, high power handling, and high linearity, and we use them to create a sub-terahertz radio-over-fiber analog optical link. These devices have the potential to become a new tool in the general field of microwave photonics, making the sub-terahertz range accessible to, e.g., 5G wireless communications, antenna remoting, Internet of Things, sensing, and more.

Large-Dynamic Frequency Response Measurement for Broadband Electro-Optic Phase Modulators

A novel method for characterizing the frequency responses of broadband electro-optic phase modulators (PMs) by employing stimulated Brillouin scattering (SBS) is proposed and experimentally demonstrated. In the proposed method, the SBS amplifies the +1st-order sideband and attenuates the −1st-order sideband of the phase-modulated signal from a PM under test. Thereby, the phase-modulated signal is converted into an optical single-sideband modulation signal. After square-law photodetection, the overall frequency responses are obtained. Removing the frequency responses of the SBS gain and the photodetector from the overall responses, the responses of the PM are thus achieved. Benefitting from the amplification of the SBS, the proposed method possesses a large dynamic range. Sub-Hz resolution and broadband measurement range are also achievable. A commercial broadband PM is experimentally characterized from 10 MHz to 50 GHz with a resolution of 5 MHz. According to the amplification of the SBS, the dynamic range is enhanced by 17.74 dB. The measurement result is verified by the conventional method utilizing optical spectrum analysis.

Impact of the mobile terminal scheme on millimeter-wave radio over fiber systems based on photonic heterodyning techniques

The use of communication networks relying on millimeter-wave (mm-wave) wireless links promises a great capacity enhancement as well as improved security. However, given the high-directivity of mm-wave links, coverage requirements are difficult to meet unless the network is assisted by an infrastructure. Given its low-cost, power-efficiency, and high capacity, radio over fiber has emerged as a strong candidate for the implementation of such infrastructure. Among the different generation techniques, photonic heterodyning has attracted considerable attention due to its capacity to generate radio frequency (RF) signals in the entire microwave/mm-wave range without requiring broadband electro-optical modulator. However, the RF signals generated using these techniques suffer from significant phase noise, a major impairment that degrades the system performance. In this paper we study two approaches to overcome this limitation: (1) the use of optical sideband injection locking (OSBIL) to generate tones with highly correlated phase noise and (2) heterodyning independent lasers in combination with a mobile terminal (MT) that is insensitive to the phase of the RF signal. A qualitative comparison between the two techniques in terms of MT sensitivity to the RF phase noise, the power sensitivity, and base station power efficiency leads to the conclusion that OSBIL is more suitable for networks with medium-range node separation, whereas heterodyning of independent laser with phase-insensitive MT is a cost-efficient solution for networks where nodes are closer to each other.

Electronic sideband locking of a broadly tunable 318.6 nm ultraviolet laser to an ultra-stable optical cavity

We demonstrate frequency stabilization of a tunable 318.6 nm ultraviolet (UV) laser system using electronic sideband locking. By indirectly changing the frequency of a broadband electro-optic phase modulator, the laser can be continuously tuned over 4 GHz, while a 637.2 nm laser is directly stabilized to a high-finesse ultra-stable optical cavity. The doubling cavity also remains locked to the 637.2 nm light. We show that the tuning range depends mainly on the gain-flattening region of the modulator and the piezo-tunable range of the seed laser. The frequency-stabilized tunable UV laser system is able to compensate for the offset between reference and target frequencies, and has potential applications in precision spectroscopy of cold atoms.

Sensitivity improvement of broadband electro-optic polymer-based optical phase modulator using 1D and 2D photonic crystal structures

This Letter introduces the design and simulation of a microstrip-line-based electro-optic (EO) polymer optical phase modulator (PM) that is further enhanced by the addition of photonic crystal (PhC) structures that are in close proximity to the optical core. The slow-wave PhC structure is designed for two different material configurations and placed in the modulator as a superstrate to the optical core; simulation results are depicted for both 1D and 2D PhC structures. The PM characteristics are modeled using a combination of the finite element method and the optical beam propagation method in both the RF and optical domains, respectively. The phase-shift simulation results show a factor of 1.7 increase in an effective EO coefficient (120 pm/V) while maintaining a broadband bandwidth of 40 GHz.

Broadband electro-optical modulator based on transparent conducting oxide.

We report a broadband electro-optical (EO) modulator based on tunable plasmonic metamaterial. Transparent conducting oxides provide an excellent active plasmonic material for optoelectronic applications. By utilizing our indium-tin-oxide- (ITO) based multilayer structure, light absorption of the active ITO layer can be electrically modulated over a large spectrum range. Based on the attenuated total reflectance configuration, bias polarity-dependent modulation up to 37% has been experimentally demonstrated. This EO modulator has advantages of simple design, easy fabrication, compact size, broadband performance, large modulation depth, as well as compatibility with existing silicon photonics platforms.

40 GHz electro-optic modulation in hybrid silicon–organic slotted photonic crystal waveguides

In this Letter we demonstrate broadband electro-optic modulation with frequencies of up to 40 GHz in slotted photonic crystal waveguides based on silicon-on-insulator substrates covered and infiltrated with a nonlinear optical polymer. Two-dimensional photonic crystal waveguides in silicon enable integrated optical devices with an extremely small geometric footprint on the scale of micrometers. The slotted waveguide design optimizes the overlap of the optical and electric fields in the second-order nonlinear optical medium and, hence, the interaction of the optical and electric waves.

High speed optical coherence microscopy with autofocus adjustment and a miniaturized endoscopic imaging probe.

Optical coherence microscopy (OCM) is a promising technique for high resolution cellular imaging in human tissues. An OCM system for high-speed en face cellular resolution imaging was developed at 1060 nm wavelength at frame rates up to 5 Hz with resolutions of < 4 µm axial and < 2 µm transverse. The system utilized a novel polarization compensation method to combat wavelength dependent source polarization and achieve broadband electro-optic phase modulation compatible with ultrahigh axial resolution. In addition, the system incorporated an auto-focusing feature that enables precise, near real-time alignment of the confocal and coherence gates in tissue, allowing user-friendly optimization of image quality during the imaging procedure. Ex vivo cellular images of human esophagus, colon, and cervix as well as in vivo results from human skin are presented. Finally, the system design is demonstrated with a miniaturized piezoelectric fiber-scanning probe which can be adapted for laparoscopic and endoscopic imaging applications.

Broadband electro-optic polymer modulators with high electro-optic activity and low poling induced optical loss

A screening method is reported for electro-optic (EO) polymers with an emphasis on broadband device suitability. The method is based on comparing the signal from second harmonic generation (SHG) and the leak through current (LTC) during poling. Plots of the SHG and LTC are used to define high EO activity-low current poling windows that are critical for low driving voltage, broadband devices with relatively low optical loss. The screening procedure was used to select an EO polymer core that resulted in a 40Gbits∕s broadband modulator with a driving voltage of 1.3V and an insertion loss of &amp;lt;8dB.

SiGeC/Si Electrooptic Modulators

The addition of carbon to silicon-germanium alloys provides the ability to lattice match thick layers with high germanium composition to silicon substrates. Thick strain-free silicon-germanium-carbon (SiGeC) layers on silicon allow the design of optical waveguides that have large optical mode overlap with the waveguide core. In addition, SiGeC/Si heterostructures enable strong confinement of large electron and hole concentrations. The combination of tightly confined carriers and photons can be used to realize high-performance broadband electrooptic modulators based on carrier density-induced refractive index changes. We show that modulators with lengths around 30 mum and turn-on times below 0.2 ns are possible with optimized designs

HIGH PERFORMANCE SIGEC/SI NEAR-IR ELECTROOPTIC MODULATORS AND PHOTODETECTORS

Numerical simulations are performed for SiGeC/Si electrooptic modulators and photodetectors operating at near-IR wavelengths. The addition of carbon provides the ability to lattice match layers with high germanium composition to silicon, which is shown to allow structures with a substantial increase in the optical confinement factor. In addition, SiGeC/Si heterostructures provide strong confinement of large electron and hole concentrations. The large optical confinement factor and strong carrier confinement enable broadband electrooptic modulators with sub-100 μm lengths and switching times below 0.5 ns with 25 mA current as well as photodetectors with quantum efficiencies as high as 90% for 300 μm length.

Travelling-wave electro-optic modulators with arbitrary frequency response utilising non-periodic polarisation reversal

A new design to realise travelling-wave electro-optic modulators, which utilise a non-periodically polarisation-reversed structure to produce an arbitrary frequency response, is proposed. A flat and broadband electro-optic modulator compensated for both velocity mismatch and degradation by microwave loss is presented.

Phase reversal electrode optimisation in broad-band electro-optic modulators by genetic algorithm

Phase reversal electrode optimisation by using a genetic algorithm for obtaining wide, flat frequency response in broadband electro-optic phase modulators is demonstrated. The genetic algorithm utilises a novel coding, crossover and mutation method directly based on electrode figures. The simulated calculations show that this method provides a rather flat frequency response of &gt; 100 GHz with an electrode length of 1 cm.

Design of modified phase reversal electrode in broad-band electrooptic modulators at 100 GHz

An analysis is given on the modified phase reversal structure of electro-optic modulators. It is shown that the bandwidth to half-wave voltage ratio (BVR) increases with the number of phase reversal sections. Under the assumption that the number of electrode sections is M, a set of M-elements second-order nonlinear equations has been derived and solved by Newton's iteration method. The calculated results provide the optimum overlap integral for each section of a phase reversal modulator in order to "flatten" the frequency response of the device.

Full-wave analysis of traveling-wave electrodes with finite thickness for electro-optic modulators by the method of lines

In order to optimize broadband electrooptic modulators with traveling-wave electrodes, detailed information about the propagation constant, the field distribution, and the wave impedance of the modulating field is needed. To get accurate results for these characteristic values, the numerically efficient method of lines (MoL) is used. The MoL takes advantage of the planar structure by discretizing the wave equation in the directions parallel to the metallization plane, whereas an analytical formulation is retained for the transverse direction. Thus, not only the hybrid character of the modulating field but also the finite conductor thickness and anisotropy of the substrate are taken into account. Results for asymmetric coplanar stripline electrodes with typical dimensions on SiO/sub 2/ buffered LiNbO/sub 3/ are presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>