A Look at the Latest in Optics

The study of ‘optics’ investigates the source and propagation of light, what effects it creates as it interacts with matter, its diverse range of phenomena, how it changes, and the instruments used to examine it.

Optics also includes the study of sight – how the human eye sees things. While all light is electromagnetic radiation, only some wavelengths are visible to the human eye, other wavelengths such as infrared light remain invisible to a human’s visual range.

The history of optics extends its reach all the way back to the ancient Egyptians and Mesopotamians, who created the first range of lenses. Progress in the theories and science of optics was later advanced by Greco-Roman philosophers and scientists, and further improved by Islamic study throughout the middle ages. Subsequently, in early modern Europe, specifically through the work of Johannes Kepler in the early 17th century, optics and the surrounding technology developed at a rapid pace.1

Throughout the 20th and early 21st centuries, the science of optics has come to integrate wave optics and quantum optics into the field. From holograms and lasers to contact lenses and electron microscopes, some of the most important technologies in everyday use have come into being due to some aspect and development in optical science. As new research and innovations continue to enhance knowledge and broaden the applications of optics; this article focuses on a few of the significant breakthroughs.

Optics applications include telecommunication fiber optics cables (left), and the 36-inch refracting telescope of Lick Observatory in Mt. Hamilton, California (right).

Optics applications include telecommunication fiber optics cables (left), and the 36-inch refracting telescope of Lick Observatory in Mt. Hamilton, California (right). (Image: Source)

The Optical Frequency Comb

In recent decades, one of the most important innovations in optics is the ‘frequency comb,’ a type of laser-based spectroscopy that can distinguish the color of light atoms and molecules with astonishing precision.2 Enabled by the advances in ultra-fast laser technology, optical frequency combs depend on the relation between time and frequency: the number of oscillations per unit of time.

Initially developed to improve the precision of atomic clocks, frequency combs have become a tool that has been frequently used for detecting and measuring different frequencies of light with ultra-fine precision. “Unlike conventional lasers, which emit a single frequency, these lasers emit multiple frequencies in lockstep, evenly spaced to resemble the teeth of a comb. Today, they are used in everything from environmental monitoring and chemical sensing to the search for exoplanets, optical communications, and high- precision metrology and timing.”3

The National Institute of Standards and Technology (NIST) outlines how optical frequency combs are created indicating, “NIST scientists start with lasers that emit a continuous train of very brief, closely spaced pulses of light containing a million different colors. The properties of the light over time are converted to frequency numbers to make what looks like a comb. Time and frequency are inversely related; that is, smaller units of time (or faster oscillations of light waves) result in larger frequency numbers.”4

This image is greatly simplified to show how different colors of light oscillate over time, demonstrating that the blue waves oscillate fastest and red waves oscillate slowest (units/specifics are unimportant, in reality units would be tiny fractions of seconds).

This image is greatly simplified to show how different colors of light oscillate over time, demonstrating that the blue waves oscillate fastest and red waves oscillate slowest (units/specifics are unimportant, in reality units would be tiny fractions of seconds). (Image source: NIST)

This image (also greatly simplified) shows the frequency comb corresponding to the graph above. “Each ‘tooth’ of the comb is a different color, arranged according to how fast the light wave oscillates in time. The waves that oscillate slowly (red) are on the left and the waves that oscillate faster (blue) are on the right. Frequency is measured in hertz, or cycles per second. An actual optical comb does not begin at zero on left, but at a very high number, 300 trillion hertz.”5

This image (also greatly simplified) shows the frequency comb corresponding to the graph above. “Each ‘tooth’ of the comb is a different color, arranged according to how fast the light wave oscillates in time. The waves that oscillate slowly (red) are on the left and the waves that oscillate faster (blue) are on the right. Frequency is measured in hertz, or cycles per second. An actual optical comb does not begin at zero on left, but at a very high number, 300 trillion hertz.”5 (Image source: NIST)

NIST further describes the process stating, “A real optical frequency comb spans the entire visible spectrum of light, and has very fine, evenly spaced teeth. The teeth can be used like a ruler to measure the light emitted by lasers, atoms, stars, or other objects with extraordinarily high precision.”6

The award for the Nobel Prize in Physics 2005 was assigned, in part, to John L. Hall and Theodor W. Hänsch "for their contributions to the development of laser-based precision spectroscopy, including the optical frequency comb technique."7 These days, the technique is mostly utilized for applications that include chemical sensing and environmental monitoring as well as optical communications, the search for exoplanets, and ultra-precise metrology and timing.

Recently, the newest development regarding optical frequency combs was conducted by researchers at Harvard’s John Paulsen School of Engineering — harnessing turbulence in light waves. These researchers demonstrated the ability to transmit wireless signals from laser frequency combs to produce the first laser radio transmitter. This discovery has potential applications in telecommunications and portable sensing, optical spectroscopy, microresonators, and chemical sensing. 

Additional Recent Breakthroughs in Optics

Seeing Around Corners

A team from Princeton University’s School of Engineering has created a radar that can see around corners, which can be incorporated easily into today’s automobiles to enhance vehicle safety and prevent collisions. The system utilizes Doppler radar — already commonplace when tracking fastball pitches in baseball — to bounce radio waves off various surfaces such as parked cars and buildings.

The radar signal reflects back from stationary objects and rebounds at an angle, then hitting objects that are unseen or around a corner. Then, some of the return signal is received by receptors on the car. The system can make use of this information to see objects around corners and establish if they are stationary (such as stopped traffic) or in motion (such as a walking pedestrian).

An illustration of Princeton

An illustration of Princeton's new radar detection system that can "see" around corners. (Image ©Princeton University, Source)

Doing Cartwheels

Researchers at Rice University have discovered an unconventional kind of polarized light that does “cartwheels” – rotating end over end as it propagates from a point of origin. The effect, referred to as trochoidal dichroism, is an interaction between polarized light and matter which until recently was previously unknown. These types of “freely propagating circularly polarized light interactions” are crucial to many technology applications, including types of 3D glasses that discern between opposite light polarizations.

"A model by Rice University scientists shows how two positively charged spheres attached to springs are attracted to the electric field of light. Due to the motion of the spheres, the spring system scatters light at different energies when irradiated with clockwise and anticlockwise trochoidal waves.”

"A model by Rice University scientists shows how two positively charged spheres attached to springs are attracted to the electric field of light. Due to the motion of the spheres, the spring system scatters light at different energies when irradiated with clockwise and anticlockwise trochoidal waves.” Credit: Link Research Group/Rice University (Image Source).

Counting Photons

Researchers in Switzerland have developed a new type of camera that can identify single photons of light traveling at high-speed. This faculty of such as camera will assist the progress of advanced applications including augmented reality and LiDAR systems that depend on the rapid acquisition of 3D images for real-time processing. The foundations of the megapixel photon-counting camera is predicated on next-generation image sensor technology that utilizes single-photon avalanche diodes (SPADs).

Measuring Light and Color

If you have a need to measure the visible or near-infrared (NIR) spectrum of light and color, Radiant’s ProMetric® Imaging Photometers and Colorimeters offer high-resolution, low-noise measurement for an extensive variety of optical metrology applications. By matching the human eye’s sensitivity to light, Radiant’s imaging systems allow manufacturers of displays and lighting products to measure spatial values for radiant intensity, chromaticity, luminance, and more. Radiant has assisted companies in the advancement of optical applications with regard to lighting and displays in the R&D lab and on the production line for over 25 years. Radiant has many solutions for a large scope of applications.

Scientific-grade ProMetric imaging systems from Radiant measure and evaluate light in a way that simulates human photopic response, based on color-matching functions described by the CIE technical report, CIE 015:2018 Colorimetry.8

Scientific-grade ProMetric imaging systems from Radiant measure and evaluate light in a way that simulates human photopic response, based on color-matching functions described by the CIE technical report, CIE 015:2018 Colorimetry.8 Image Credit: Radiant Vision Systems

References

  1. Thompson, B. and Kingslake, R., “Optics”, Encyclopaedia Britannica. (Retrieved June 22, 2020).
  2. “Laser-based precision spectroscopy and optical frequency comb techniques.” in Advanced information on the Nobel Prize in Physics 2005, The Royal Swedish Academy of Sciences, October 4, 2005 
  3. Piccardo, M., Schwarz, B., et al. “Frequency combs induced by phase turbulence.” Nature, 2020; 582 (7812): 360 DOI: 10.1038/s41586-020-2386-6
  4. Optical Frequency Combs, NIST. (Retrieved July 2, 2020)
  5. Ibid.
  6. Ibid.
  7. “Laser-based precision spectroscopy and optical frequency comb techniques.” in Advanced information on the Nobel Prize in Physics 2005, The Royal Swedish Academy of Sciences, October 4, 2005 
  8. Carter, E., et al., Eds. CIE 015:2018 Colorimetry, 4th Editiondoi: 10.25039/tr.015.2018

Acknowledgments

Produced from materials originally authored by Anne Corning from Radiant Vision Systems.

This information has been sourced, reviewed and adapted from materials provided by Radiant Vision Systems.

For more information on this source, please visit Radiant Vision Systems.

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