Tag Archives: Photonics

Mode Converters and Spot Size Converters

 Spot size converters are important for photonic integrated circuits where a coupling is done between two different waveguide sizes or shapes. The most obvious place to find a spot size converter is between a waveguide of a PIC and a fiber coupling lens.

 Spot size converters feature tapered layers on top of a ridge waveguide for instance, to gradually change the mode while preventing coupling loss.

The below RSoft example shows how an optical path is converted from a more narrow path (such as a waveguide) to a wider path (which could be for a fiber).

While the following simulation is designed in Silicon, similar structures are realized in other platforms such as InP or GaAs/AlGaAs.

RSoft Beamprop simulation, demonstrating conversion between two mode sizes. Optical power loss is calculated in the simulation for the structure.

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 This is the 3D structure. Notice the red section present carries the more narrower optical path and this section is tapered to a wider path.

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 The material layers are shown:

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Structure profile:

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Arrayed Waveguide Grating for Wavelength Division Multiplexing

Arrayed Waveguide Grating (or AWG) is a method for wavelength division multiplexing or demultiplexing. The approach for multiplexing is to use unequal path lengths to generate a phase delay and constructive interference for each wavelength at an output port of the AWG. Demultiplexing is done with the same process, but reversed.

Arrayed Waveguide Gratings are commonly used in photonic integrated circuits. While Ring Resonators are also used for WDM, ring resonators see other uses, such tunable or static filters. Further, a ring resonator selects a single wavelength to be removed from the input. In the case of AWGs, light is separated according to wavelength. For many applications, this is a more superior WDM, as it offers great capability for encoding and modulating a large amount of information according to a wavelength.

Both the design of the star coupler and the path length difference according to the designed wavelength division make up the significant amount of complexity of this component. RSoft by Synopsys includes an AWG Utility for designing arrayed waveguide gratings.

RSoft AWG Utility Guide

Using this utility, a star coupler is created below:

Star Coupler for AWG designed in RSoft using AWG Utility

Ring Resonators for Wavelength Division Multiplexing

The ring resonator is a rather simple passive photonic component, however the uses of it are quite broad.

The basic concept of the ring resonator is that for a certain resonance frequency, those frequencies entering port 1 on the diagram below will be trapped in the ring of the ring resonator and exit out of port 3. Frequencies that are not of the resonance frequency will pass through to port 2.

ringres

Ring resonators can be used for Wavelength Division Multiplexing (WDM). WDM allows for the transmission of information allocated to different wavelengths simultaneously without interference. There are other methods for WDM, such as an Asymmetric Mach Zehnder Modulator.

Here I present one scheme that will utilize four ring resonators to perform wavelength division multiplexing. The fifth output will transmit the remaining wavelengths after removing the chosen wavelengths dependent on the resonating frequency (and actually, the radius) of the ring resonators.

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Introduction to Electro-Optic Modulators

Electro-optics is a branch or topic in photonics that deals with the modulation, switching and redirection of optical signals. These functions are produced through the application of an electric field, which alters the optical properties of a material, such as the refractive index. The refractive index refers to the speed of light propagation in a medium relative to the speed of light in a vacuum.

 

Modulators vs. Switches

In a number of situations, the same device may function as both a modulator and a switch. One dependent factor on whether the device would be suitable or not for a switch as opposed to a modulator would be the strength of the effect that an electric field may have on the device. If the device’s primary role is to impress information onto a light wave signal through temporary varying of the signal, then it is referred to as a modulator. A switch on the other hand either changes the direction or spatial position of light or turns it off completely.

phase-modulators

 

Theory of Operation

Electro-optic Effect

The electro-optic effect presumes the dependence of the refractive index on the the applied electric field. The change in refractive index, although small allows for various applications. For instance, a lens may be applied an electric field and depending on the material and the applied field, the focal length of the lens can change. Other optical instruments that utilize this effect may also see use, such as a prism. A very small adjustment to the refractive index may still produce a delay in the signal, still large enough to detect and, if information was implied by the delay that was produced on the signal, the delay can be phase demodulated at the receiving end.

 

Electroabsorption

Electroabsorption is also another effect that is used to modify the optical properties of a material by the application of an electric field. An applied electrical field may increase the bandgap of the optical semiconductor material, turning the material from optically transparent to optically opaque. This process is useful for making modulators and switches.

 

Kerr Effect and Pockels Effect

The Pockels Effect and the Kerr Effect both account for the change in refractive index through the application of an electric field. The Kerr Effect states that this effect is nonlinear, while the Pockels Effect states that the effect is linear. Although the Pockels Effect is more pronounced in Electro-optical modulator design, both are applied in many situations. The linear electro-optic effect exists only in crystals without inversion symmetry. The design of electro-optic modulators or switches requires special attention to the waveguide material and how the electric field reacts with the material. Common materials (also maintaining large Pockels coefficients) are GaAs, GaP, LiNbO3, LiTaO3 and quartz. The Kerr Effect is relatively weak in commonly used waveguide materials.

 

Properties of the Electro-Optic Modulator

Modulation Depth

Important for both modulators and switches is the modulation depth, also known as the modulation index. Modulation depth has applications for the several types of optical modulators, such as intensity modulators, phase modulators and interference modulators. The modulation depth may be conceptually understood as the ratio of effect that is applied to the signal. In other words, is the modulation very noticeable? Is it a strong modulation or is it a weak modulation?

 

Bandwidth

The bandwidth of the modulator is critically important as it determines what range of signal frequencies may be modulated onto the optical signal. Switching time or switching speed may be equally applied to an optical switch.

 

Insertion Loss

Insertion loss of optical modulators and switches is a form of optical power loss and is expressed in dB. However, the result of insertion loss often results in the system requiring more electrical power and would not explicitly reduce performance of the modulation or switching function of the device.

 

Power Consumption

In distinction from the electric field, a modulator or switch also needs a power supply for itself. The amount of power required increases with modulation frequency. A common figure of merit is the drive power per unit bandwidth, typically expressed in milliwatts per megahertz.

 

References: [1], [4], [6]

Optics, Optoelectronics, Electro-Optics and Photonics

Optics vs. Photonics

What is the difference between Optics and Photonics? These words are sometimes used interchangeably. A distinction may be made however. Optics, on one hand is a very old subject, whereas Photonics is a term that has only recently been used. Photonics is a word which refers to devices that primarily involve the flow of photons as opposed to electronics, which deals with the flow of electrons. The main inventions that lead to the use of the word Photonics are the laser, fabrication of low-loss optical fibers and semiconductor optical devices. Other terms that are often used to refer to these inventions and their various applications are electro-optics, optoelectronics, quantum electronics, quantum optics and lightwave technology. Many of these terms may be used interchangeably, although some of them refer to specific technologies.

The first figure below may be viewed as an optical system, while the second figure may be referred to as a photonic system. The first figure features a light beam that is modulated, reflected and reflacted through a medium. The second figure is of a photonic integrated circuit device.

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photonics1

 

Electro-Optics is term for devices that incorporate both an optical and electrical properties, however are primarily optical devices. Examples of electro-optical devices are lasers and electro-optic modulators and switches.

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Optoelectronics refers on the other hand to devices that are primarily electronic, but involve light, such as light-emitting diodes, photodetectors or liquid-crystal display devices.

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Quantum Optics refers to the study of the quantum mechanical and coherence properties of light. Lightwave Technology typically is used to describe optical communications and optical signal processing devices and systems. Quantum Electronics is the study of technology concerned with the interaction of light and matter, such as lasers, optical amplifiers and optical wave mixing devices.

Fiber Optics (Introduction)

RF/Photonics Lab at UMASS Dartmouth
November 2019
Michael Benker

Fiber Optics

When the frequency of a signal is increased, so does the transfer rate. On the electromagnetic spectrum, light waves occupy frequency ranges of several hundred Terahertz. Fiber optics and photonics take advantage of the speed of light waves to allow for a different approach to data communications. When using light waves instead of electrical charges, this drastically alters the normal characteristics of electrical information transfer. A light wave being sent through glass in a fiber optic wire is no longer restricted to Ohm’s law for example, since a light wave will move through a resistor without any loss. Although light waves are susceptible to quantum noise, they are immune to noise caused by heat (in many cases, this means they are virtually noise-less). Fiber optics, due to their high data rates, flexibility and immunity to noise offer an extraordinary opportunity for scientific and engineering progress.

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