Photonics

Relationship to other fields

Classical optics

Photonics is closely related to optics. Classical optics long preceded the discovery that light is quantized, when Albert Einstein famously explained the photoelectric effect in 1905. Optics tools include the refracting lens, the reflecting mirror, and various optical components and instruments developed throughout the 15th to 19th centuries. Key tenets of classical optics, such as Huygens Principle, developed in the 17th century, Maxwell's Equations and the wave equations, developed in the 19th century, do not depend on quantum properties of light.

Modern optics

Photonics is related to quantum optics, optomechanics, electro-optics, optoelectronics and quantum electronics. However, each area has slightly different connotations by scientific and government communities and in the marketplace. Quantum optics often connotes fundamental research, whereas photonics is used to connote applied research and development.

The term photonics more specifically connotes:

• The particle properties of light,
• The potential of creating signal processing device technologies using photons,
• The practical application of optics, and
• An analogy to electronics.

The term optoelectronics connotes devices or circuits that comprise both electrical and optical functions, i.e., a thin-film semiconductor device. The term electro-optics came into earlier use and specifically encompasses nonlinear electrical-optical interactions applied, e.g., as bulk crystal modulators such as the Pockels cell, but also includes advanced imaging sensors.

An important aspect in the modern definition of Photonics is that there is not necessarily a widespread agreement in the perception of the field boundaries. Following a source on optics.org, the response of a query from the publisher of Journal of Optics: A Pure and Applied Physics to the editorial board regarding streamlining the name of the journal reported significant differences in the way the terms "optics" and "photonics" describe the subject area, with some description proposing that "photonics embraces optics". In practice, as the field evolves, evidence that "modern optics" and Photonics are often used interchangeably is very diffused and absorbed in the scientific jargon.

Emerging fields

Photonics also relates to the emerging science of quantum information and quantum optics. Other emerging fields include:

• Nanophotonics, also known as nano-optics, which is the study of light in the nanoscale
• Optoacoustics or photoacoustic imaging where laser energy delivered into biological tissues will be absorbed and converted into heat, leading to ultrasonic emission.
• Optomechanics, which involves the study of the interaction between light and mechanical vibrations of mesoscopic or macroscopic objects;
• Optomics, in which devices integrate both photonic and atomic devices for applications such as precision timekeeping, navigation, and metrology;
• Plasmonics, which studies the interaction between light and plasmons in dielectric and metallic structures. Plasmons are the quantizations of plasma oscillations; when coupled to an electromagnetic wave, they manifest as surface plasmon polaritons or localized surface plasmons. In turn,magnetoplasmonics is an interdisciplinary field that merge magnetism and plasmonics.
• Polaritonics, which differs from photonics in that the fundamental information carrier is a polariton. Polaritons are a mixture of photons and phonons, and operate in the range of frequencies from 300 gigahertz to approximately 10 terahertz.
• Programmable photonics, which studies the development of photonic circuits that can be reprogrammed to implement different functions in the same fashion as an electronic FPGA
• Topological photonics, which studies the manipulation of the flow of light by applying the theory of topology in material physics.

Applications

Photonics has applications in many fields including light detection, telecommunications, information processing, photovoltaics, photonic computing, lighting, metrology, spectroscopy, holography, medicine (surgery, vision correction, endoscopy, health monitoring), biophotonics, military technology, laser material processing, art diagnostics (involving infrared reflectography, X-rays, ultraviolet fluorescence, XRF), agriculture, and robotics.

Just as applications of electronics have expanded dramatically since the first transistor was invented in 1948, the unique applications of photonics continue to emerge. Economically important applications for semiconductor photonic devices include optical data recording, fiber optic telecommunications, laser printing (based on xerography), displays, and optical pumping of high-power lasers. The potential applications of photonics are virtually unlimited and include chemical synthesis, medical diagnostics, on-chip data communication, sensors, laser defense, and fusion energy, to name several interesting additional examples.

• Consumer equipment: barcode scanner, printer, CD/DVD/Blu-ray devices, remote control devices
• Telecommunications: fiber-optic communications, optical down converter to microwave
• Renewable Energy: Solar power systems
• Medicine: correction of poor eyesight, laser surgery, surgical endoscopy, tattoo removal
• Industrial manufacturing: the use of lasers for welding, drilling, cutting, and various methods of surface modification
• Construction: laser leveling, laser rangefinding, smart structures
• Aviation: photonic gyroscopes lacking mobile parts
• Military: IR sensors, command and control, navigation, search and rescue, mine laying and detection
• Entertainment: laser shows, beam effects, holographic art
• Information processing
• Passive daytime radiative cooling
• Sensors: LIDAR, sensors for consumer electronics
• Metrology: time and frequency measurements, rangefinding
• Photonic computing: clock distribution and communication between computers, printed circuit boards, or within optoelectronic integrated circuits; in the future: quantum computing

Microphotonics and nanophotonics usually include photonic crystals and solid state devices.

Overview of photonics research

Light sources

Photonics commonly uses semiconductor-based light sources, such as light-emitting diodes (LEDs), superluminescent diodes, and lasers. Other light sources include single photon sources, fluorescent lamps, cathode-ray tubes (CRTs), and plasma screens. Note that while CRTs, plasma screens, and organic light-emitting diode displays generate their own light, liquid crystal displays (LCDs) like TFT screens require a backlight of either cold cathode fluorescent lamps or, more often today, LEDs.

Characteristic for research on semiconductor light sources is the frequent use of III-V semiconductors instead of the classical semiconductors like silicon and germanium. This is due to the special properties of III-V semiconductors that allow for the implementation of light emitting devices. Examples for material systems used are gallium arsenide (GaAs) and aluminium gallium arsenide (AlGaAs) or other compound semiconductors. They are also used in conjunction with silicon to produce hybrid silicon lasers.

Transmission media

Light can be transmitted through any transparent medium. Glass fiber or plastic optical fiber can be used to guide the light along a desired path. In optical communications optical fibers allow for transmission distances of more than 100 km without amplification depending on the bit rate and modulation format used for transmission. A very advanced research topic within photonics is the investigation and fabrication of special structures and "materials" with engineered optical properties. These include photonic crystals, photonic crystal fibers and metamaterials.

Amplifiers

Optical amplifiers are used to amplify an optical signal. Optical amplifiers used in optical communications are erbium-doped fiber amplifiers, semiconductor optical amplifiers, Raman amplifiers and optical parametric amplifiers. A very advanced research topic on optical amplifiers is the research on quantum dot semiconductor optical amplifiers.

Detection

Photodetectors detect light. Photodetectors range from very fast photodiodes for communications applications over medium speed charge coupled devices (CCDs) for digital cameras to very slow solar cells that are used for energy harvesting from sunlight. There are also many other photodetectors based on thermal, chemical, quantum, photoelectric and other effects.

Modulation

Modulation of a light source is used to encode information on a light source. Modulation can be achieved by the light source directly. One of the simplest examples is to use a flashlight to send Morse code. Another method is to take the light from a light source and modulate it in an external optical modulator.

An additional topic covered by modulation research is the modulation format. On-off keying has been the commonly used modulation format in optical communications. In recent years, more advanced modulation formats like phase-shift keying or even orthogonal frequency-division multiplexing have been investigated to counteract effects like dispersion that degrade the quality of the transmitted signal.

Photonic systems

Photonics also includes research on photonic systems. This term is often used for optical communication systems. This area of research focuses on the implementation of photonic systems like high speed photonic networks. This also includes research on Optical communications repeaters, which improve optical signal quality.

Photonic integrated circuits

Photonic integrated circuits (PICs) are optically active integrated semiconductor photonic devices. The leading commercial application of PICs is optical transceivers for data center optical networks. PICs fabricated on III-V indium phosphide semiconductor wafer substrates were the first to achieve commercial success; PICs based on silicon wafer substrates are now also a commercialized technology.

Key Applications for Integrated Photonics include:

• Data Center Interconnects: Data centers continue to grow in scale as companies and institutions store and process more information in the cloud. With the increase in data center compute, the demands on data center networks correspondingly increase. Optical cables can support greater lane bandwidth at longer transmission distances than copper cables. For short-reach distances and up to 40 Gbit/s data transmission rates, non-integrated approaches such as vertical-cavity surface-emitting lasers can be used for optical transceivers on multi-mode optical fiber networks. Beyond this range and bandwidth, photonic integrated circuits are key to enabling high-performance, low-cost optical transceivers.
• Analog RF Signal Applications: Using the GHz precision signal processing of photonic integrated circuits, radiofrequency (RF) signals can be manipulated with high fidelity to add or drop multiple channels of radio, spread across an ultra-broadband frequency range. In addition, photonic integrated circuits can remove background noise from an RF signal with unprecedented precision, which will increase the signal to noise performance and make possible new benchmarks in low power performance. Taken together, this high precision processing enables us to now pack large amounts of information into ultra-long-distance radio communications.
• Sensors: Photons can also be used to detect and differentiate the optical properties of materials. They can identify chemical or biochemical gases from air pollution, organic produce, and contaminants in the water. They can also be used to detect abnormalities in the blood, such as low glucose levels, and measure biometrics such as pulse rate.
• LIDAR and other phased array imaging: Arrays of PICs can take advantage of phase delays in the light reflected from objects with three-dimensional shapes to reconstruct 3D images, and Light Imaging, Detection and Ranging (LIDAR) with laser light can offer a complement to radar by providing precision imaging (with 3D information) at close distances. This new form of machine vision has an immediate application in driverless cars to reduce collisions, and in biomedical imaging. Phased arrays can also be used for free-space communications and novel display technologies. Current versions of LIDAR predominantly rely on moving parts, making them large, slow, low resolution, costly, and prone to mechanical vibration and premature failure. Integrated photonics can realize LIDAR within a footprint the size of a postage stamp, scan without moving parts, and be produced in high volume at low cost.

Biophotonics

Biophotonics employs tools from the field of photonics in the study of biology. Biophotonics mainly focuses on improving medical diagnostic abilities (for example for cancer or infectious diseases) but can also be used for environmental or other applications. The main advantages of this approach are speed of analysis, non-invasive diagnostics, and the ability to work in-situ.