VCSELs Break New Ground In LiDAR Design

A LiDAR system's integrated lasers emit light which is then reflected off objects, returned, recorded, and applied for autonomous vehicle control. While autonomous vehicles and smart city traffic controls demonstrate key arenas for LiDAR applications, the technology is highly applicable in other uses. Robots moving or picking goods in industrial warehouses or unmanned aerial vehicles used for mapping topologies need to move quickly and safely without human intervention; drones used to deliver packages must be able to avoid buildings, people, power lines, and other drones. These systems analyse road condition information in real time, outputting multi-channel data based on the point in travel and target objects in the surrounding environment.

Within all these applications are highly mobile machines. They experience significant vibration and require stable and reliable data communication methods transmitted between the time of flight (TOF) sensor and the computing brain of the LiDAR [typically based on FPGAs and microcontrollers (MCUs)].

While fiber optic communications are extremely popular in fixed applications such as offices, residential buildings, or data centers, they bring some notable limitations and challenges to mobile LiDAR applications due to vibration, mechanical stress, and continuous sensor 360 degrees rotation. In contrast, wireless optical modules, coupled with VCSEL transmitters - or vertical cavity surfaceemitting lasers - and high-speed photo detectors, help solve reliable data communication challenges in highly mobile LiDAR applications. Although this approach may be a relatively new concept in LiDAR designs, these types of small and highly reliable VCSEL emitters have been produced and proven in industrial applications for more than 30 years. Originally developed for IBM optical punch-card reading and used to replace their less reliable mechanical counterparts, VCSEL technologies are finding a new home in modern data communication and LiDAR applications.

How it works: VCSELS in action

VCSELs are compact, high powered light-emitting diodes (LEDs) that are designed into a 'pill package,' or small, hermetically-sealed system, that can easily pass received signals to the detecting sensor that is contained in its own pill package. Miniaturisation is critical because the system must be light and energy efficient as well as unobtrusive to the design of the autonomous vehicle or drone. In a VCSELbased system, data communication moves through an optical air gap (without optical fiber) where an emitter-air-receiver module provides high bandwidth, bidirectional optical data transfer. For high-reliability LiDAR applications, the transmitter/emitter mustbe capable of delivering high data rates of up to 2.5Gbps and more, while receiver data rates must be no less than 100Mbps.

The design allows maximum flexibility via 360-degree rotation of the module, ensuring reliable, undisturbed communication between the sensor and the MCU. VCSELs fuel reliable, flexible design options. VCSEL pill packages are available in ultrasmall footprints, just 1.57 mm in diameter and under 3.45 mm in length. The same package is also used to encapsulate a PN silicon photodiode in a small, self-contained footprint ideal for direct mounting to a PC board. These enable linear response to irradiance, fast switching speed with 100ns rise/ fall time, narrow receiving angle of 36 degrees, and are spectrally optimised for 850nm light. Additional options integrate a PN silicon photodiode in a T-1 clear epoxy package with 5ns of rise/fall time - these provide even faster switching and linear response but may be a more cost-effective receiver for spacelimited LiDAR applications.

VCSELs with microbead lenses are designed for high-speed data communication and offer data rates up to 2.5Gbps, high thermal stability, narrow and concentric beam angle, and are processed with burn-in for a high level of reliability. Those equipped with power monitor diodes offer precise control of optical power, while others utilising flat lenses are designed for air transmission of data. Many of these VCSELs are intended for applications where low current (for long battery life) is required with high on-axis optical power for maximum optical coupling with the receiver.

In other options, VCSELs are offered in dome lens plastic T-1 package. The dome lens design creates a narrow 4 degree beam angle from the device - this benefits long-distance applications as secondary optics can be eliminated, reducing total cost of the system. To ensure optimal performance, the VCSEL must be optically and spectrally compatible with its accompanying phototransistors, photodarlingtons and photodiodes. Photodiodes offer the fastest switching speed, while photodarlingtons provide the highest signal amplification.

Practical vision system design with LiDAR Optical data communication utilising VCSEL transmitters and photodiode receivers is a design approach superior to fiber optic options in vibration-prone mobile LiDAR applications. VCSEL transmitters offer fast data transfer rates up to 2.5Gbps and beyond, low drive current (for long battery life) with high optical output power, in a compact package for space-limited applications. Coupled with photodiode receivers, designers have access to fast switching speed with 5ns rise/fall time and linear response spectrally matched to 850nm VCSEL IR emitters. Both VCSEL and their compatible photodiodes are available in hermetically-sealed pill packages optimised for either direct PC board mounting or in low-cost epoxy packages well suited for space-limited LiDAR modules.

Autonomous driving, drone, robotics, and other automated machines represent one of the fastest growing segments of electronics design. Even at this early stage of design and development, LiDAR offers real promise - blending TOF sensors, data communications, and connectivity to deliver a safe, high-performance vision solution in a very small footprint. Robust, yet compact and energy efficient, LiDAR, utilising VCSEL communication portals, is also cost-effective - a crucial factor in EV design and development, and one that is breaking new ground in LIDAR designs.

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