Optical Signal Processing Circuits Research Articles

Ultra-compact plasmonic unidirectional wavelength multiplexer/demultiplexer based on slot cavities

An ultra-compact plasmonic unidirectional wavelength multiplexer/demultiplexer based on slot cavities is proposed and numerically simulated. The structure consists of slot cavities which are etched on either sides of a metal–insulator–metal (MIM) bus waveguide. The cavities capture surface plasmon polariton (SPP) waves at their resonant wavelengths and unidirectionally couples them to the drop waveguide etched parallel to the bus. The structure basically relies on resonance and interference of SPP waves and its functioning is validated through finite element method (FEM) simulations. The simulation results show that the proposed structure functions as expected with a full width at half maximum (FWHM) bandwidth of less than 50 nm, extinction ratio (ER) more than 10 dB, and crosstalk (CT) of less than − 10 dB for the designed wavelengths. The proposed structure holds lot of potential to enhance the miniaturization of ultra-compact integrated photonic circuits for optical signal processing and other related applications.

Silicon Nitride in Silicon Photonics

The silicon nitride (Si3N4) planar waveguide platform has enabled a broad class of low-loss planar-integrated devices and chip-scale solutions that benefit from transparency over a wide wavelength range (400–2350 nm) and fabrication using wafer-scale processes. As a complimentary platform to silicon-on-insulator (SOI) and III–V photonics, Si3N4 waveguide technology opens up a new generation of system-on-chip applications not achievable with the other platforms alone. The availability of low-loss waveguides (<1 dB/m) that can handle high optical power can be engineered for linear and nonlinear optical functions, and that support a variety of passive and active building blocks opens new avenues for system-on-chip implementations. As signal bandwidth and data rates continue to increase, the optical circuit functions and complexity made possible with Si3N4 has expanded the practical application of optical signal processing functions that can reduce energy consumption, size and cost over today’s digital electronic solutions. Researchers have been able to push the performance photonic-integrated components beyond other integrated platforms, including ultrahigh Q resonators, optical filters, highly coherent lasers, optical signal processing circuits, nonlinear optical devices, frequency comb generators, and biophotonic system-on-chip. This review paper covers the history of low-loss Si3N4 waveguide technology and a survey of worldwide research in a variety of device and applications as well as the status of Si3N4 foundries.

Reversible orbital angular momentum photon–phonon conversion

Orbital angular momentum (OAM) has gained great interest due to its most attractive feature of high dimensionality, and several ground-breaking demonstrations in communication based on OAM multiplexing have been carried out. Accordingly, a rapid data-density growth from OAM multiplexing has posed a great challenge to the signal-processing layer. Meanwhile, in another area, optical signal-processing circuit based on photon-phonon conversion has received considerable attention and made rapid progress. Here, with an aim of finding the intersection between OAM multiplexing and photon-phonon conversion, we report on the observation of reversible OAM photon-phonon conversion. A specific OAM state can be flexibly and controllably interconverted between photonic and phononic domains via Brillouin photon-phonon coupling within the decay time of acoustic signal, in which OAM and spin angular momentum are independently conserved. Our result demonstrates the controllable OAM transfer between photons and phonons, shows the potential of using OAM multiplexing to extend the capacity of photon-phonon conversion based signal-processing scheme, and may trigger the development of OAM-multiplexed photon-phonon circuit.

Influence of Nose Radius on Surface Integrity in Ultra-precision Cylindrical Turning of Single-crystal Calcium Fluoride

Optical micro-resonators attract interests as essential components to develop optical signal processing circuits which can reduce energy loss. Single-crystal calcium fluoride (CaF2) could be the most suitable material from a viewpoint of optical property. To satisfy high form accuracy and surface roughness, ultra-precision cylindrical turning (UPCT) is a feasible fabrication process. In previous research, it was indicated that the tool with sharper nose radius could make UPCT more stable and reduce subsurface damage. In this study, the tool with much smaller nose radius 0.01mm was applied, then, the influence on surface integrity was investigated by TEM observation.

Optical tilt sensor with direct intensity-modulated scheme

An optical tilt sensor based on a phase-sensitive diffraction grating with direct intensity-modulated scheme is proposed in this paper. The tilt sensor consists of a coherent light source, an integrated grating, a mechanical sensing part, and an optical signal processing circuit. The mechanical sensing part consists of a bulk proof mass and two cantilevers that can be fabricated with a two-mask process on a silicon-on-insulator substrate. A phase-sensitive diffraction grating, which was formed with the integrated grating and the upper surface of the proof mass, acts as the sensing element for the tilt measurement. Experimental results show this tilt sensor provides the optimal tilt sensitivity of 1.15 V/° and resolution of about 0.0046°, and a measurement range of ±20° in a rotational plane that is perpendicular to the direction of the earth's gravity.

Wavelength-selective directional couplers as ultrafast optical differentiators

Wavelength-selective directional couplers with dissimilar waveguides are designed for ultrafast optical differentiation within the femtosecond regime (corresponding to processing bandwidths > 10 THz). The theoretically proposed coupler-based differentiators can be produced by wavelength matching of the propagation constants of two different waveguides in the coupler at the center wavelength. A single directional coupler can be designed to achieve either a 2nd-order differentiator or a 1st-order differentiator by properly fixing the product of coupling coefficient and coupling length of the coupler. We evaluated the differentiation errors (~2%) and energetic efficiency (~11% for 1st order differentiation) of the designed optical differentiators through numerical simulations. The proposed design has a strong potential to provide a feasible solution as an integrated differentiation unit device for ultrafast optical signal processing circuits.

Energy consumption limits in high-speed optical and electronic signal processing

The energy consumption of high-speed optical signal processing circuits using optical and electronic components is compared. Integrated nonlinear optical circuits consume more energy than CMOS ICs with O/E and E/O converters in all but the very simplest of circuits. It is concluded that future generations of high-performance electronics will be the technology of choice for optical signal processing.

Narrow-beam aluminum-mirrored fiber optical-taps with controllable tapped power

A fiber optical-tap has been fabricated and tested. Two separate fiber tips were polished with an angle of 45/spl deg/, and one of the tips was coated with aluminum inside a vacuum deposition unit. Then, the coated and uncoated polished tips were spliced together using a fusion splicer to form a fiber optical-tap. The power of the tapped light is controllable by controlling the thickness of the aluminum coating during vacuum deposition. As the aluminum coating thickness increases, the amount of tapped light increases and consequently the power transmitted after the fiber optical-tap decreases. For the same aluminum coating thickness, the tapped beam power from a single-mode fiber tap is higher than that from a multimode fiber tap. In addition to the feasibility of tapping a narrow light beam and controlling its power, the fabricated optical tap has a short length, which means that it can be integrated easily in optical signal-processing circuits. The mechanical strength of the tap has been tested in the laboratory and found to be capable of supporting between 75 and 180 g before breaking.

Self-heterodyning optical waveguide beam forming and steering network integrated on Lithium Niobate substrate

A novel self-heterodyning optical waveguide beam forming and steering network (BFN) integrated on a Lithium Niobate (LN) substrate is introduced to realize optically controlled active phased array antennas. This integrated LN-BFN can simultaneously control eight array elements and is demonstrated for the first time. The key component in the self-heterodyning system, an optical frequency shifter (OFS), is composed of phase-and amplitude-balanced four parallel optical phase modulators. An optical signal processing circuit equipped with a weighted electrode structure simplifies the beam steering operation. A fully integrated LN-BFN connected by optical fibers can steer beam direction with the sensitivity of 1.5 degrees per volt. >

Integrated optical devices for high-speed signal processing

In recent years, the remarkable progress has been made in the optical guided-wave technology along with the developments of advanced fine structure fabrication technologies. It is of the great interests in the applications like optical communications, signal processing, and sensors. The main purpose of the present report is to give an outline of the recent progress in the guided-wave technologies for the application to the signal processing. One of the key elements for realizing high-speed optical signal processing circuit is the fast light modulator. First, the present status of high-speed guided-wave modulator is summarized, and then, a few examples of the integration of these modulators are presented with the accent on application to the signal processing.