How 3D Sensing Applications Rely on VCSEL Accuracy and Performance

Theodor Maiman built the first laser device in 1960 at Hughes Research Laboratories in Malibu, CA, from a cylinder of synthetic rubber and photographic flash lamps.1

Using his theory of stimulated emission, Albert Einstein described the concept in 1917 but it took decades and the development of microwave spectroscopy in the 1950s before a detailed proof of concept could be produced by Arthur Schawlow and Charles Townes.2 Their 1958 paper resulted in a patent for the laser, or “light amplification by stimulated emission of radiation” device.

One characteristic of laser light is that wavelengths emitted by a laser diode are all in phase in space and time, meaning it is coherent. This usually makes lasers quite powerful as they can deposit a large amount of energy on a very small area, this can also make them more hazardous.

Ubiquitous in the marketplace and in our daily lives, today lasers are employed for everything from corrective eye surgery to fiber-optic communications, to supermarket scanners.

The Logitech MX3 is one of the latest models of computer mice that use near-infrared laser sensing for operation.

The Logitech MX3 is one of the latest models of computer mice that use near-infrared laser sensing for operation. Image Credit: Radiant Vision Systems

Vertical-Cavity Surface-Emitting Lasers (VCSELs)

The first vertical-cavity surface-emitting laser, or VCSEL, was created by Kenichi Iga in 1977. Over the next decade or so, the technology was further developed at Bell Labs by Jack Jewell and others.

VCSELs are now frequently found in many commercial, industrial and consumer device applications, especially for 3D sensing which heavily relies on near-infrared (NIR) light.

VSCELs are made up of layers of semiconductor materials which are grown on an epitaxial substrate as individual wafers. Each VCSEL diode produces light between two layers of Bragg mirrors (distributed Bragg reflectors, or DBRs) which are parallel to the surface of the wafer, forming a resonator region, or cavity, with numerous quantum wells.

As opposed to edge-emitting laser diodes (EEL), where the light exits at the edge of the chip, this monolithic laser resonator radiates the light vertically to the surface of the semiconductor chip.

Even though this vertical emission architecture can lead to a lower power light, the beam quality of VCSELs is usually higher than that of EELs or of LEDs, which are also used for NIR sensing applications - a trade-off for higher reliability and performance.

Comparison of VCSEL, EEL, and LED light sources in terms of beam direction and spatial light distribution.

Comparison of VCSEL, EEL, and LED light sources in terms of beam direction and spatial light distribution. Image Credit: Source

There are numerous advantages of the vertical design. VCSELs provide coherent light with high power density, direct emission and simple packaging.

Compared to an EEL, the VCSEL structure is easier to assemble than edge emitters. VCSELs’ simple beam structure considerably decreases the cost and complexity of the coupling/beam-shaping optics and optimizes the efficiency.3

Structure of a typical VCSEL: two distributed Bragg reflector (DBR) layers with quantum wells in between, emitting light from the top of the unit.

Structure of a typical VCSEL: two distributed Bragg reflector (DBR) layers with quantum wells in between, emitting light from the top of the unit. Image Credit: Osram

VCSELS can be utilized in numerous different ways, with some of the most common applications being in the fields of high-speed communications and precision sensing.

In fact, VCSELs have found a place in: 4

  • 3D sensing
  • Automated driving and manufacturing
  • Lidar
  • Optical mice
  • Smartphones
  • Biomedical and gas sensing
  • High-speed data communication
  • Laser printers
  • Computing

This versatility is driving exponential growth in the VCSEL market, which is estimated to nearly triple in size by 2025, from revenues of $1.1 billion in 2020 to $2.9 billion (a CAGR of 23.7% over the time period)5 and to be over $5 billion by 2027.6 

A VCSEL wafer in production.

A VCSEL wafer in production. Image Credit: Photonics Media

VCSEL 3D Sensing Applications

VCSEL-based sensing has become commonplace for driver monitoring and occupant monitoring systems (DMS and OMS), eye tracking and iris detection, gesture and facial recognition, lidar and autonomous vehicle sensing and multi-modal sensing for motion and gesture control. 

Their utilization for time-of-flight (ToF) measurement is one reason VCSELs have become the technology of choice for 3D sensing. ToF emits a beam of NIR light, measuring the time gap between when the signal is emitted from a source (like VCSEL or LED) to when the signal returns after being reflected off an object. 

An NIR camera is used to capture the return signal. The distance of the object is shown by the time difference from emission and return, or the signal’s “flight”. This depth measurement permits the 3D measurement of objects like an obstruction in the road (for automotive lidar), or a human face (for facial recognition applications). 

Representation of Time-of-Flight measurement using a near-infrared emitter such as a VCSEL.

Representation of Time-of-Flight measurement using a near-infrared emitter such as a VCSEL. Image Credit: Radiant Vision Systems

VCSELs provide the advantages of long operating distances, fast scanning, high efficiency and excellent resistance to ambient-light interference in ToF applications. In 3D depth detection modules, VCSELs enable wavelength, size, brightness and beam-angle selection to suit the application.7 

The VCSEL PV88m from Lextar, which is used for 3D depth detection, tunable for the appropriate wavelength, brightness, dimension and beam angle from 45°-100°.

The VCSEL PV88m from Lextar, which is used for 3D depth detection, tunable for the appropriate wavelength, brightness, dimension and beam angle from 45°-100°. Image Credit: © Lextar

The Need to Measure VCSEL Emitters for Accuracy and Safety

VSCELs from 810 nm - 940 nm are typically utilized for 3D sensing applications. Yet, to the human eye these NIR wavelengths are invisible, with the potential to cause damage over time from exposure. So, to ensure the laser’s emissions are within the intended performance parameters, special care is needed. 

Test equipment must be utilized during production for manufacturers of VCSEL-based devices to ensure accurate intensity and distribution of NIR light. Radiant provides an NIR measurement solution specially designed for employment in the lab and on the production line to guarantee the quality and consistency of these emitters. 

VSCEL Measurement Solution

The Radiant Vision Systems Near-Infrared (NIR) Intensity Lens system is an integrated camera/lens solution which measures the radiant intensity and angular distribution of NIR emitters, both VCSELs and LEDs. 

To capture a full cone of data in a single measurement to ±70 degrees, the system utilizes Fourier optics, supplying extremely fast, accurate results which are ideal for in-line quality control. 

Radiant’s solution for NIR VCSEL and LED measurement: the NIR Intensity Lens integrated with a ProMetric® Y-16 Radiometer.

Radiant’s solution for NIR VCSEL and LED measurement: the NIR Intensity Lens integrated with a ProMetric® Y-16 Radiometer. Image Credit: Radiant Vision Systems

Manufacturers of 3D sensing technology can use the NIR Intensity Lens solution for direct angular measurement of NIR lasers, LEDs and structured light patterns generated by Diffractive Optical Elements (DOE). 

The lens features ProMetric or TrueTest™ Software for intuitive system setup and customizable automated measurement sequences. It is directly integrated with a Radiant Vision Systems ProMetric® Y16 Imaging Radiometer. 

The TT-NIRI™ Software test suite contains further tests specific to NIR emission measurement. Users are able to sequence tests for the fast assessment of all relevant display characteristics in a matter of seconds, applying numerous selected tests to a single image captured by the camera.

Unique tests for NIR analysis include:

  • Max Power
  • Pixel Solid Angle
  • Total Flux (mW or W)
  • Flood Source Analysis
  • Points of Interest
  • POI Total Power
  • Image Export
  • Dot Source Analysis 

The angular distribution of an NIR emitter, as captured in a single image by the NIR Intensity Lens and shown in false color (heat map) polar plot generated in TT-NIRI analysis software.

The angular distribution of an NIR emitter, as captured in a single image by the NIR Intensity Lens and shown in false color (heat map) polar plot generated in TT-NIRI analysis software. Image Credit: Radiant Vision Systems

To comply with industry standards, applications like facial recognition, eye tracking and automotive lidar which are utilized on and around humans require rigorous testing. Radiant’s measurement solutions characterize the output of NIR sources, providing manufacturers with data which could be helpful when testing to these standards. 

To ensure performance and accuracy, the new generations of devices which utilize NIR laser emitters for 3D sensing demand new approaches to product quality testing. The Radiant NIR Intensity Lens solution provides the benefit of size, speed and software for precise measurement of 3D sensing systems. 

References

  1. A History of the VCSEL”, Photonics Media, May 18, 2020.
  2. This Month in Physics History: December 1958: Invention of the Laser”, American Physical Society (APS) News, Vol. 12, No.11, December 2003.
  3. Emilio, M., “Vertical-Cavity Surface-Emitting Lasers Hit the Target for Depth Detection,” EETimes Europe, January 1, 2020.
  4. Ahmed, F., “Low-Powered VCSELs Find a Wide Variety of Uses, Photonics Spectra, December, 2018.
  5. VCSEL Market by Type (Single-mode, Multimode), Material….and Geography – Forecast to 2025”, Markets and Markets Report, April 2020.
  6. Wadhwani, P., and Yadav, S., “VCSEL Market Size Worth Over $5bn by 2027,” Global Market Insights, February 1, 2021.
  7. Emilio, M., “Vertical-Cavity Surface-Emitting Lasers Hit the Target for Depth Detection,” EETimes Europe, January 1, 2020. 

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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