Infrared Sensing with ROHM’s VCSELED Technology: Combining the Benefits of VCSELs and LEDs

Abstract

Traditional infrared (IR) light sources such as light-emitting diodes (LEDs) and vertical-cavity surface-emitting lasers are not optimized for the latest infrared sensing applications. ROHM VCSELED^™ technology overcomes this challenge by incorporating a VCSEL element with an optical diffuser made of resin material, combining the properties of LED and VCSELs into a compact package. This paper covers basic principles and applications of infrared sensing, challenges with LEDs and VCSELs, and introduces VCSELED technology from ROHM for accurate and reliable infrared sensing.

Infrared Sensing: Principles, Benefits, and Limitations

Infrared light is invisible to human eyes and occupies a part of the electromagnetic spectrum between visible light and microwaves. This longer wavelength enables infrared radiation to penetrate materials such as smoke, fog, and plastic, making it suitable for use in challenging or low-visibility environments.

When infrared light illuminates an object, three key processes occur — reflection, absorption, and transmission and the relative proportions of these processes depend on the properties of the object, such as surface texture, composition, and temperature. For example, reflective materials, such as metals and certain plastics, reflect a significant portion of incident infrared light back towards the source. On the other hand, absorptive materials, e.g., darker-colored objects, absorb much of the infrared energy, converting it into heat.

Transmissive materials, like glass and certain polymers, allow infrared light to pass through with minimal attenuation. Infrared sensing systems use these interactions to detect objects in their field of view. They can even measure distances using time of flight (ToF) techniques, where the infrared sensing system measures the round-trip time of a pulse of light from the source to the object and back to the detector.

Compared to visible light sensors, infrared sensing is less impacted by ambient conditions, making them useful in low-light environments. On the downside, the atmosphere tends to absorb and scatter infrared light at longer wavelengths, impacting the range and sensitivity of such systems. Moreover, heat from external sources and sunlight can introduce background noise, which may cause false detections.

Limitations of Standard Infrared Light Sources

Light Emitting Diodes (LEDs)

A potential disadvantage of some LEDs for infrared sensing is their relatively wide emission spectrum. This broad spectral width can lower the sensitivity and signal-to-noise ratio (SNR) of the sensor system, as the emitted light will be spread over various wavelengths, and only a fraction of the total optical power will fall within the bandwidth of the detector. This reduces the effective signal strength and can make it challenging to detect low reflective objects or long-distance targets.

Another drawback of using LEDs in infrared sensing is their temperature dependence, which affects both the emission wavelength and optical power. In principle, as the temperature of a LED increases, the bandgap energy of the semiconductor material decreases, causing a redshift in the peak emission wavelength that can be as high as 0.3 nm/°C. Similarly, the optical power of LEDs decreases with increasing temperature, leading to fluctuations in signal strength and signal-to-noise ratio over the LED’s operating temperature range.

LEDs also have a limited modulation bandwidth that restricts their use in high-speed and time-of-flight (ToF) sensing applications. Laser diodes, such as VCSEL, have faster response times (compared with LEDs) because of their smaller intrinsic regions and the way the light is produced. Laser diodes produce coherent light through stimulated emission, whereas LEDs produce incoherent light through spontaneous emission.

Lastly, many LEDs work using a Lambertian emission pattern, in which the intensity of emitted light is proportional to the cosine of the angle from the surface normal. Irregular emissions can result in reduced irradiance on distant targets compared to more focused light sources.

Vertical Cavity Surface Emitting Lasers (VCSELs)

Vertical cavity surface emitting lasers (VCSELs) are a variant of semiconductor laser in which light is emitted perpendicular to the surface of the wafer, in contrast to edge-emitting lasers where light is emitted parallel to the surface. VCSELs offer several key benefits compared to LEDs for infrared sensing, such as narrow emission spectrum, high modulation bandwidth, and a smaller form factor.

Despite their advantages, VCSELs have few limitations that can affect their performance in infrared sensing applications. One drawback is the narrow emission beam angle of  less than 30-degree angle  full-width at half-maximum (FWHM). This narrow beam angle is due to small emission apertures and the high beam quality of VCSELs. Although a narrow beam angle is useful for long-range or high-resolution sensing, it can limit the field of view and coverage area of the infrared sensing system.

Another downside of VCSELs is the high energy density of the emitted light, which can raise eye safety concerns in human detection or facial-monitoring system applications. The small emission aperture and the high beam quality may result in a high irradiance (power per unit area) on the target that exceeds Maximum Permissible Exposure (MPE) limits for eye safety at close distances or with prolonged exposure.

ROHM’s VCSELED Technology for Infrared Sensing

ROHM has developed a new technology called VCSELED^™ that combines the benefits of VCSELs and LEDs in a single, compact package. VCSELED stands for “Vertical Cavity Surface Emitting Laser with Enhanced Diffusion,” a new class of infrared light sources for higher performance, reliability, and flexibility. ROHM’s VCSELED uses a high-performance VCSEL element as a light-emitting device that is well-established in near-infrared emission with peak wavelength at 940nm.

Comparing the structures of different LED package types: LED vs. VCSELED vs. VCSEL with a diffuser.

To overcome the limitations of the narrow emission beam angle and the high energy density in standard VCSELs, ROHM has developed a proprietary optically diffused packaging technology, monolithically integrated with the VCSEL element in a single package. The optical diffuser is a resin-based material dispensed and molded onto the surface of the VCSEL, forming a dome-shaped structure that covers the aperture. The resin material is formulated to provide high optical transmittance, low scattering loss, and controlled diffusion properties — all critical for efficient and uniform light distribution.

Key Features and Benefits

One of the main advantages of VCSELED is its narrow emission spectrum, with a spectral width of only 4 nm — seven times narrower than LEDs. This performance improves imaging resolution while eliminating the red glow irradiance often associated with broadband LEDs. Another benefit is lower wavelength temperature dependence, with a typical wavelength shift of 0.072 nm/°C, approximately four times lower than standard LEDs. This property ensures reliable optical performance over a wide operating temperature range (-40°C to 105°C), without the need for active temperature control or compensation techniques. This feature is particularly important for automotive and industrial applications, where sensors are often exposed to harsh environmental conditions.

ROHM VCSELED also has response times as low as 2 ns, 7.5 times faster than standard LED. This fast response time is ideal for high-speed modulation and TOF sensing, with modulation bandwidths up to 1 GHz and depth resolutions of a few centimeters. These features are highly useful in applications like gesture recognition and 3D imaging.

ROHM’s new integrated diffused resin packaging technology eliminates the risk of laminated diffuser removal often associated with conventional VCSEL solutions. This risk mainly centers on eye safety because of the nature of high-power emitting laser in VCSEL.

Comparison of emission intensity vs. wavelength for VCSELED and standard LED (left) and wavelength change with increasing temperature for VCSELED and standard LED (right).

VCSELED is designed for high reliability, robustness, and ease of assembly. It is also built to efficiently dissipate heat, with a low thermal resistance path from VCSEL active area to the package substrate. The compact and low-profile form factor of the package, at a height of 0.55 mm, makes the VCSELED ideal for space-constrained applications such as smartphones, wearables, and IoT devices.

Key Applications

In automotive applications, driver monitoring systems (DMS) can use ROHM’s VCSELED solutions to detect driver fatigue and distraction by tracking eye movements, and blink rates. In-cabin monitoring systems can also use these solutions to monitor the interior of vehicles to detect the presence of humans, position, and activity to trigger functions, such as airbag deployment, child detection, etc. By tracking the driver’s hand and finger movements using infrared and illumination, drivers can easily navigate menus and adjust the vehicle’s settings without taking their eyes off the road or their hands off the wheel.

In industrial applications, VCSELED solutions can facilitate processes like machine vision, process control, quality inspection, and 3D scanning. For example, factories can use them to detect objects on production lines. Infrared thermography, or thermal imaging is another application where facility operators can leverage ROHM’s VCSELED technology to detect hot or cold spots as well as thermal gradients for fault detection and predictive maintenance.

Augmented reality and virtual reality systems can use VCSELED technology to track user movements and interactions, offering more immersive and realistic experiences with precise spatial mapping and object occlusion. VCSELED-based infrared sensing allows engineers to design intuitive user interfaces, such as hand tracking and gesture recognition, to control virtual objects and navigate menus. Gaming controllers can also incorporate VCSELED to capture player movements and gestures, for example, in virtual sports and fitness games.

For medical and healthcare applications, VCSELED technology can facilitate non-invasive monitoring, imaging, or diagnostics using infrared. For example, infrared thermography with VCSELED solutions can be used to detect changes in patients’ skin temperature, indicating underlying medical conditions. Similarly, in wound care, VCSELED-enabled sensing can capture the thermal profile of wounds and surrounding tissues, providing wound care professionals with quantitative information on wound depth and healing progress.

Conclusion

VCSELED technology achieves a remarkable balance of the useful properties of VCSELs and LEDs. This combination addresses the limitations of typical infrared light sources, such as the broad emission spectrum and thermal instability of LEDs, and the narrow beam angle and eye safety risks of VCSELs. The result is a compact and high-performance infrared light source for accurate and reliable infrared sensing in a wide range of applications.

For more information about ROHM’s VCSELED™ solutions or application-specific inquiries, please contact ROHM online.