How are VCSELs used in biomedical sensors?

Vertical-cavity surface-emitting lasers (VCSELs) are used in biomedical sensors because they emit highly focused, coherent light at specific wavelengths, enabling precise detection and analysis of biomolecules. Applications include blood oxygen level monitoring, optical coherence tomography (OCT) for imaging tissue structures, fluorescence microscopy, and analysis of live biomolecules in lab-on-a-chip systems.

VCSELs offer a third option compared with conventional light emitting diodes (LEDS) and edge-emitting laser diodes (EELs). Each technology has a different structure, leading to different emission patterns and other features and supporting different applications.

LEDs emit light from the side and top of the die. VCSELs emit light from the top surface. EELs emit light from the side of the chip. The beam shape is an important differentiator. LEDs emit relatively wide circular beams. EELs emit oblong beams, and VCSELs emit focused circular beams (Figure 1).

Figure 1. VCSELs provide a tighter beam pattern compared with LEDs and EELs. (Image: Guangmai Technology)

VCSEL performance

In addition to a low-divergence beam, VCSELs offer several performance advantages for three-dimensional (3D) sensing and imaging applications. VCSELs have a small cavity wavelength standard deviation under 2 nm that supports the production of arrays for imaging and beam steering applications.

EELs are difficult to integrate into arrays but can produce higher power outputs. LEDs are best suited for general illumination and low-bandwidth communications.

Another advantage of VCSELs is better thermal dissipation, especially compared to EELs, which require thermally inefficient mounting for optimal optical performance. The ability of VCSELs to operate at temperatures up to 80°C also simplifies thermal management.

VCSEL thermal management

VCSELs are commonly made using indium phosphide (InP) and gallium arsenide (GaAs). Thermal management needs depend on the material used and the application. InP VCSELs are often used in laser communication applications that benefit from active thermal management.

GaAs VCSELs are used in a wide variety of applications, from consumer devices to medical imaging. VCSELs used in applications like OCT and fluorescent imaging can benefit from active cooling. Precise temperature control helps maintain wavelength stability and optimize the quality of the images.

Micro thermoelectric coolers (micro-TECs) have been developed for use with VCSELs. Application-specific packaging is used that mounts a VCSEL on top of a micro-TEC to provide improved thermal management and temperature stability (Figure 2).

Figure 2. Example of a VCSEL mounted on top of a micro-TEC for high power applications. (Image: Laird Thermal Systems)

Biomedical sensing examples

The beam characteristics of VCSELs make them useful in a range of biomedical sensing applications. OCT imaging uses the depth-resolved backscattered light from a sample to produce a high-resolution image. The near-infrared (NIR) spectrum of VCSELs is used to measure blood oxygen saturation based on changes in light absorption.

VCSELs are also used for the detection and quantification of biomolecules. They can excite fluorescent dyes attached to specific molecules, enabling the detection and quantification of the molecules. They also support real-time label-free detection of certain molecules based on changes in the refractive index

VCSELs in OCT

OCT uses a beam of NIR light to scan a tissue, measuring the reflected light from different depths. That enables the creation of detailed cross-sectional images. VCSEL characteristics like single-mode emissions and high modulation speeds enable fast, high-resolution OCT imaging. Examples of VCSEL-based OCT include:

Figure 3. Resolution comparison of common medical imaging technologies. (Image: Edmund Optics)

Ophthalmology: detailed imaging of the retina
Dermatology: analyzing skin layers
Cardiology: imaging of coronary arteries
Gastroenterology: examining the lining of the gastrointestinal tract

OCT and adaptive optics OCT (AO-OCT) provide superior axial and lateral resolution compared with other imaging technologies like magnetic resonance imaging (MRI), computed tomography (CT) imaging, also called CAT scan, confocal scanning laser ophthalmoscopy (cSLO), and adaptive optics scanning laser ophthalmoscopy (AOSLO). Reflectance confocal microscopy (RCM) can support higher lateral resolutions but only provides shallow image depth, while OCT and AO-OCT can penetrate deeper into the tissue and provide a more detailed image (Figure 3).

Summary

Compared with LEDs and EELs, VCSELs have a narrower beam pattern. They also deliver high levels of coherence, moderate power, and can operate up to 80 °C. That makes them suitable for biomedical sensors in applications like OCT, blood oxygen level monitoring, fluorescence microscopy, and similar applications.