Glossary
Definitions of terms used in this course. Some of these terms...
Contents
- Adiabatic Coupler
- Combiner
- Directional Coupler
- Dispersion
- Effective index (neff)
- Electron-beam lithography (also EBeam Lithography, EBL)
- Free spectral range (FSR)
- GDS
- Grating couplers, Fibre grating coupler
- Group index (ng, group velocity refractive index)
- Interference, constructive/destructive interference
- Layout
- Mach-Zehnder Interferometer (MZI)
- Mode
- Mode competition
- Mode mismatch loss
- Modulation
- Optical path length difference
- Optical Confinement
- Polarization
- Propagation loss
- Splitters
- Taper
- Wave number
- Waveguide
- Waveguide Loss
- Adiabatic Coupler
- A splitter or combiner consisting of two waveguides that are gradually brought together and whose geometry is gradually changing. The result is a broadband 50/50 splitter. More info H. Yun, SPIE 2013.
- Combiner
- The opposite of a splitter. Combines light. See Splitters.
- Directional Coupler
- A splitter or combiner consisting of two (identical) waveguides that are brought together. The waveguide modes are coupled, and energy exchanges back and forth between the two waveguides. The result is a splitter whose splitting ratio can be adjusted by varying the parameters (length, separation of the waveguides, etc).
- Dispersion
- When a wave travels at a velocity that depends on the wavelength. This phenomenon that is responsible for the separation of colours in a prism, and the spreading of data communication pulses in an optical fibre. When there is dispersion, the group velocity is different than the phase velocity. In silicon photonics, the group index can be as much as 2X larger than the effective index. Wikipedia.
- Effective index (neff)
- Similar to the refractive index, n, which is related to the phase velocity of light (Wikipedia). Whereas the refractive index is defined for a single homogenous material, the effective index is defined for a waveguide (which consists of more than one material). It can be thought of as a weighted average of the refractive indices of the waveguide materials, where the weighting function is the optical mode. (RP Photonics)
- Electron-beam lithography (also EBeam Lithography, EBL)
- A technique to draw patterns on a surface using a focused beam of electrons. When discussing EBL in the context of silicon photonics, we also include the other fabrication steps – spinning a resist, the lithography itself, developing, etching, etc. Wikipedia.
- Free spectral range (FSR)
- In an interferometer, as a function of wavelength, the spacing between successive maxima or minima, in the transmitted optical intensity. Wikipedia.
- GDS, GDSII
- File format used to share a design Layout. Wikipedia.
- Grating couplers, Fibre grating coupler
- An optical device based on a diffraction grating (Wikipedia) which is used to change the direction of light propagation. Specifically, it converts a free-space beam incident on a silicon photonic chip, into a guided mode in a waveguide.
- Group index (ng, group velocity refractive index)
- Is related to the group velocity (Wikipedia) of a wave, which is the speed at which a pulse propagates, e.g., in a waveguide. The group index is similar to the refractive index, except the velocity is the group velocity and not the phase velocity. The group index is important in integrated optics since it determines the Free Spectral Range of an interferometer or resonator. Wikipedia.
- Interference, constructive/destructive interference
- Phenomenon where two waves superimpose, resulting in a higher or lower amplitude. This is a vector summation of two waves hence both amplitude and phase are important. It is coherent when the waves are at the same frequency (or wavelength). Wikipedia.
- Layout
- A top-view 2D representation of a physical design. Also called “Mask Layout” when used in manufacturing via optical lithography. Wikipedia.
- Mach-Zehnder Interferometer (MZI)
- An optical device where light is split into two paths, then recombined. The output consists of the interference of the two light beams. Wikipedia.
- Mode
- When light travels in a waveguide, we can have electromagnetic field pattern that does not change as a function of distance. This is called a mode. We use TE or TM modes in the silicon photonic waveguides. Wikipedia.
- Mode Competition
- In a semiconductor laser, mode competition is the phenomenon where different modes experience laser amplification in the same gain medium, leading to a competition between the modes. In some cases, one mode wins; in others, the modes are constantly fighting which leads to a mode partition noise in the laser. RP Photonics.
- Mode mismatch loss
- When light travels and experiences a discontinuity, for example if the waveguide abruptly changes shape or direction, there will be loss. Mode mismatch refers to the difference in the mode profiles in the two regions. Lumerical.
- Modulation
- In communications (electronics, optical), the process of putting information on a carrier (sinusoidal) signal. In silicon photonics, the carrier is light, at a frequency of e.g., 192 THz. And the information may be a digital signal of 1s and 0s. A silicon photonic modulator takes the 0s and 1s and modulates the optical beam, using one of many modulation formats (on-off, phase, etc). Wikipedia.
- Optical path length difference
- In an interferometer, the light travels in two different paths, each with a length. The difference between these lengths (multiplied by the effective index of the waveguide) is the “optical path length difference”. Wikipedia.
- Optical Confinement
- Optical confinement of a waveguide describes how much light is present inside the waveguide core (silicon in our case) versus how much is outside (silicon dioxide). It is expressed as a ratio of the field inside the core divided by the total field everywhere, and has a value between 0 to 1. This term is commonly used in semiconductor laser design, since only the light intensity inside the active semiconductor provides optical gain.
- Polarization
- Polarization of light refers to the direction in which the electric field is oscillating. In free space, a plane wave has the electric (E) and magnetic (H) fields oscillating transverse, or perpendicular, to the direction of propagation; this is called a transverse electromagnetic wave (TEM). Wikipedia. In waveguides, we define transverse as perpendicular to the direction of propagation, and parallel to the wafer surface. The transverse electric (TE) polarization is defined as having the electric field transverse to the direction of propagation, and parallel to the wafer surface. Similarly, the transverse magnetic (TM) polarization has the magnetic field oscillating parallel to the wafer surface.
- Propagation loss
- The loss in power as an optical wave travels in space. Wikipedia.
- Splitters
- A device that splits a beam of light into two (or more) parts. In free-space optics, this is known as a beam splitter (Wikipedia). In integrated optics, there are several types of splitters, including Y-Branches, Multi-Mode Interference (MMI) splitters/couplers. In the MZI, we use a splitter in the first stage, and a combiner in the second stage.
- Taper
- In photonic waveguides, a taper is a waveguide that gradually changes size. For example, it can be used to connect two different sized tapers using a gradual transition. Wikipedia.
- Wave number
- The spatial frequency of a wave. It is the inverse of wavelength of light. Wikipedia.
- Waveguide
- A structure that guides light. In this course, our waveguides are made of dielectric (silicon, surrounded by glass). Wikipedia.
- Waveguide loss
- The optical propagation loss in an optical waveguide. Typically expressed in dB/cm or in terms of the imaginary index of refraction, k (Lumerical).