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Unveiling the World of LiDAR: A Deep Dive into FMCW vs. ToF

May 22,2024

LiDAR, a technological marvel, plays a pivotal role in the advancements of self-driving cars, robotics, and various other applications. It essentially acts like a superpowered eye, using lasers to measure distance with exceptional precision. But within the realm of LiDAR, two distinct methodologies reign supreme: Time-of-Flight (ToF) and Frequency Modulated Continuous Wave (FMCW). Understanding their strengths and weaknesses is crucial for navigating the complexities of this technology.

ToF LiDAR: The Established Veteran
Imagine a laser gun firing a quick burst of light. That's the core principle behind ToF LiDAR. It emits a short laser pulse and meticulously measures the time it takes for the light to travel to an object and bounce back. Knowing the speed of light, a simple calculation reveals the distance between the LiDAR and the object. The distance measurement principle of TOF (Time of Flight) is to measure the distance by multiplying the flight time of the light pulse between the target and the laser radar by the speed of light. TOF laser radar uses pulse amplitude modulation technology (AM), so it is also called AM laser radar.

While established and well-understood, ToF LiDAR does have some limitations:
Vulnerability to Interference: Multiple ToF LiDARs operating in close proximity can create confusion. Their similar light pulses can overlap, leading to inaccurate readings. Imagine two people shouting the same word simultaneously – the message gets muddled.

Limited Speed Measurement: Determining an object's speed with ToF LiDAR requires capturing multiple distance measurements over time. This introduces a delay, which can be critical in fast-paced scenarios like autonomous vehicles navigating busy streets.

Lower Signal-to-Noise Ratio: ToF LiDAR's received signal can be easily affected by ambient light or other LiDARs operating nearby. This creates a noisy environment, making it challenging to distinguish the desired signal from the background clutter.

FMCW LiDAR: The Rising Star
FMCW LiDAR takes a different approach. It utilizes a continuous laser beam with a constantly changing frequency. Imagine a musical note that smoothly glides from high to low. The reflected light interacts with the original signal, and the resulting difference in frequencies tells the story of distance. FMCW mainly sends and receives continuous laser beams, interferes with the return light and local light, and uses mixing detection technology to measure the frequency difference between sending and receiving, and then calculates the distance of the target object through the frequency difference.

Here's why FMCW LiDAR is gaining significant attention:
Superior Interference Resistance: FMCW LiDAR focuses on a very narrow frequency band. Think of it as tuning a radio to a specific station. This narrow focus makes it much less susceptible to interference from other LiDARs or sunlight, leading to cleaner and more accurate measurements.

Direct Speed Measurement: FMCW LiDAR leverages the Doppler effect, a well-known scientific principle. As objects move relative to the LiDAR, the reflected light's frequency exhibits a slight shift. By analyzing this shift, FMCW LiDAR can directly measure an object's speed in real-time – a crucial capability for autonomous vehicles to make quick decisions.

High Signal-to-Noise Ratio: FMCW LiDAR employs a sophisticated technique called coherent detection. This significantly amplifies the desired signal compared to the background noise. Imagine turning up the volume on your favorite song while quieting down the surrounding chatter. This results in highly reliable measurements, even in challenging environments.

Beyond Ranging: The Allure of FMCW LiDAR
The advantages of FMCW LiDAR extend beyond just measuring distance:
Chip-based Potential: FMCW LiDAR holds immense promise for miniaturization. Leveraging silicon photonics technology, various components like the laser, detector, and scanning module can be integrated onto a single chip. This not only reduces size but also paves the way for potentially lower costs in the future.

Long-Distance Potential: While long-range detection with FMCW LiDAR is still under development, it has the potential to provide valuable velocity information even with limited point cloud data (a 3D representation of the surroundings). Imagine a car navigating through fog. Even if the detailed picture is obscured, FMCW LiDAR can still provide crucial information about the speed of approaching objects.

The Achilles' Heel of FMCW LiDAR
Despite its impressive capabilities, FMCW LiDAR currently faces one significant challenge:
Higher Cost: As a relatively new technology, FMCW LiDAR components like lasers and electronics are still expensive compared to ToF LiDAR. However, as the technology matures and economies of scale kick in, these costs are expected to decrease.

Choosing the Right LiDAR: A Balancing Act
The optimal LiDAR choice hinges on the specific application. ToF LiDAR remains a mature and cost-effective solution for shorter-range applications with less demanding interference environments.

There is another obvious difference between FMCW and TOF: in order to reduce the interference of ambient light, TOF focuses on filtering, that is, blocking the light outside the working wavelength from the radar receiver; while FMCW only interferes with the laser emitted by itself and is not interfered by light from other light sources. Simply put, TOF is "rejecting dissidents" while FMCW is "attracting similar ones."

The core technologies of FMCW laser radar mostly come from the field of optical communications. The modulation and demodulation algorithms of FMCW laser radar's transceiver signals are very similar to the optical modules of optical communication products. It can be understood as putting the transmitting and receiving ends of optical communication products together, and then folding the optical fiber to create an FMCW laser radar.

A large number of silicon photonic technologies are used in the field of optical communications, and silicon photonic chips are also needed in the reception and scanning of FMCW laser radars. The so-called silicon photonic chips are chips that perform a lot of optical path control on CMOS wafers, including active control, modulation and demodulation. Simply put, compared with ordinary silicon chips, silicon photonic chips can conduct both electricity and light.

Of course, optical communication products were not based on silicon photonics technology from the beginning. Early optical communication products also used a lot of discrete devices, which were typically large in size and high in cost. As silicon photonics technology matured and was introduced, optical communication products began to develop in the direction of integration, and the scale of application also began to grow significantly.

Given that FMCW lidar is highly dependent on the silicon photonics industry chain, the growth pace of FCMW lidar companies is also largely constrained by the maturity of the silicon photonics industry chain.

However, for applications like autonomous vehicles that demand real-time, high-fidelity data and superior performance in challenging scenarios, FMCW LiDAR shines. As the technology matures and costs become more competitive, FMCW LiDAR has the potential to become the dominant LiDAR method, revolutionizing the way we perceive and interact with the world around us.