Photonic radar for contactless vital signs detection

Vital signs are a set of clinical measurements that reflect basic human functions and are used as diagnostic parameters to monitor medical and health conditions. Therefore, vital sign detection has been widely used in many clinical scenarios, such as intensive care units for monitoring critically ill patients, elderly care institutions for day and night health monitoring to prevent medical emergencies for unattended patients, and in vehicles to determine whether the driver is drowsy.

Traditional vital sign detection relies on contact devices, such as pulse oximeters, which use electrodes to detect weak electrical changes caused by heart contractions, and smart watches, which use changes in the intensity of infrared detection light caused by changes in blood flow and volume (photoplethysmography). Such applications are already very widespread, but contact solutions still cause user discomfort during 24/7 monitoring. Although solutions such as wearable sensors embedded in wristbands or clothing have greatly improved the user experience, they are still not suitable for patients with burns or skin irritation, nor for infants with insufficient attachment area.

The industry has explored non-contact detection solutions based on optical sensors, for example, using cameras to track certain target areas of the human body. However, camera-based systems (including infrared cameras and traditional visible light cameras) are sensitive to skin color and lighting conditions. These systems usually require complex computing algorithms, and the thermal imaging video generated by infrared cameras usually has limited resolution. In addition, some systems based on high-resolution cameras may cause privacy issues, especially when it comes to invasive monitoring and potential security risks to cloud computing and data storage infrastructure.

Radars using radio frequency (RF) waves can capture the target's vital signs at a long distance to overcome the shortcomings of contact sensors. This vital sign information is generated based on RF sensing rather than camera photography, so it naturally provides the required privacy protection. In recent years, there have been explorations into the use of single-tone frequency-modulated wave electronic radars for vital sign detection. Single-tone radars based on the Doppler principle can obtain vital signs through the phase information of the reflected signal from a moving object.

However, this technology lacks the ability to detect round-trip time and therefore cannot obtain target range information. They cannot use range information to separate closely located targets and isolate targets from surrounding clutter, which limits their performance and usefulness in real-world applications. In contrast, FM radars can extract range information to overcome this problem. More importantly, the range resolution and accuracy of FM radars can be improved by widening the bandwidth of the sensing signal.

Unfortunately, traditional electronic radar systems typically have limited sub-gigahertz bandwidth, resulting in a resolution of only tens of centimeters, which is not enough to accurately detect weak vital signs (such as human breathing, where chest displacement is only about 1 cm). This resolution makes it difficult to separate vital signs from body motion and cannot track multiple targets. In addition, emerging applications often require distributed sensing across multiple frequency bands and deployment locations, which is challenging for traditional electronic devices without complex parallel hardware architectures.

Microwave photonic radar technology has been developed due to its numerous advantages over traditional electronic radar sensing schemes. Photonic radar systems have demonstrated their ability to produce ultra-wideband signals, which can achieve excellent range resolution. Dispersion-based technologies can provide bandwidths up to 40 GHz and achieve resolutions as low as 3.9 mm.

The swept-frequency light source based on external light injection achieves a large time-bandwidth product of more than 1.2 × 10⁵, and the longer pulse time improves the signal-to-noise ratio and overall performance in noisy environments. The photonic frequency multiplier and digital-to-analog converter provide sufficient bandwidth and high time-frequency linearity for accurate radar sensing. Photonic radar can generate different types of radar signals, including linear frequency modulation (LFM) and stepped frequency (SF) signals.

In addition, it can operate in multiple frequency bands in the millimeter wave region, optimizing performance according to operating conditions. These features overcome the limitations of electronic radars and make them very suitable for vital sign detection. However, despite its potential, photonic radars for vital sign detection have not yet been explored in practical applications.

According to MEMS Consulting, researchers from the Institute of Photonics and Optical Science at the University of Sydney, Australia, recently published an article titled "Photonic radar for contactless vital sign detection" in the journal Nature Photonics. In this article, the researchers demonstrated a photonic radar for vital sign detection, using a human breathing simulator and a live animal (cane toad) instead of humans for experiments. The radar can generate a 10 GHz wide SF RF signal in the Ka band (26.5–40 GHz) to detect the simulator's breathing activity, achieving a distance resolution of 13.7 mm and micron-level detection accuracy. Even though the animal radar cross-section is small, this high resolution and accuracy are critical for detecting subtle vital signs of cane toads.

The researchers demonstrated bandwidth scalability up to 30 GHz, unconstrained by RF antennas and amplifiers. The researchers also demonstrated a LiDAR vital sign detection system based on the same microwave photon source, demonstrating the potential of the system to complement the capabilities of radar and LiDAR. The researchers envision the application of this high-performance distributed radar system in a range of healthcare scenarios, such as 24/7 vital sign monitoring in elderly care facilities, hospitals, and regulatory agencies.

For example, a distributed photonic radar sensing network with multiple radar optical RF access points can use RF waves to detect human vital signs (as shown in Figure a below). Unlike deploying a single radar access point, this approach can continuously track freely moving back or side targets. The optical radar signal source supporting multiple optical RF access points can cover different viewing angles to monitor one or more targets using low-loss optical fiber.

Photonic radar system for contactless vital sign detection

Results of multi-target vital sign detection based on breathing simulator

Experiments using cane toads to detect vital signs instead of humans

In summary, the researchers demonstrated a photonic vital sign detection system with millimeter-level resolution and micrometer-level accuracy, capable of multi-target detection without causing comfort and privacy issues. The experiment demonstrated its ability and effectiveness in detecting subtle breathing anomalies and accurately extracting cane toad oral movements. More importantly, it has a simplified system structure and provides improved bandwidth and flexibility, which is not achievable with the current state-of-the-art electronic radars for vital signs.

This photonic scheme provides a new path to realize high-resolution, fast-response, and cost-effective hybrid radar-lidar modules for distributed, contactless vital sign sensing.