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Laser 101

Direct Time of Flight (dTOF) Principle and core components

In the realm of distance measurement and object detection, direct Time-of-Flight (dTOF) technology stands out for its precision and efficiency. The principle behind dTOF, as introduced in the first chapter, is straightforward: it employs Time-Correlated Single Photon Counting (TCSPC) to directly measure the flight time of light. A specific dTOF system, illustrated below (referenced), alongside AMS's TOF Proximity Sensor Module, reveals the intricacies of its emitter, receiver, and module pins. By analyzing both the system's schematic and the actual module, we can delve into the fundamental principles, core components, and system parameters of dTOF.

Click here for Understanding Time of Flight Systems: TOF,iTOF,and dTOF 

dtof sensor working principle.jpg

Laser Driver

At the heart of the emitter circuit lies the Laser Driver, a pivotal component tasked with driving the laser. It controls the emission of digital pulse signals, typically modulating MOSFETs for laser emission.

Laser Emission

The laser, a critical component in dTOF, is chosen for its narrow spectrum, high energy intensity, fast modulation, and ease of integration. Consumer electronics predominantly utilize Vertical Cavity Surface Emitting Lasers (VCSELs) for their dTOF applications due to these benefits. However, applications in other platforms, especially in automotive LiDAR, might opt for higher power Edge Emitting Lasers (EEL) or Fiber Lasers. Typically, dTOF systems in electronics choose lasers with wavelengths of 850 nm or 940 nm, with the selection heavily influenced by their absorption peaks and noise considerations under sunlight.


 TOF Ranging pulsed fiber laser 

Transmission Optics

The emitter side employs a simple optical lens to direct the laser energy within the desired field of view. Some applications might use collimators plus Diffractive Optical Elements (DOEs) to divide the VCSEL's beam into multiple angles.

Reception Optics

On the receiver side, optics are crucial for collecting light within the target field of view. A lens with a smaller F-number and higher relative illumination is preferred, often treated with coatings to enhance transmissivity while using narrowband filters to eliminate out-of-band light interference.

SPAD Receiver

For detection, dTOF systems predominantly utilize Single-Photon Avalanche Diodes (SPADs) or Silicon Photomultipliers (SiPMs), with the key difference being that each SiPM pixel contains multiple SPAD elements. Unlike traditional CMOS sensors, SPADs can respond to single photons, triggering a significant avalanche current with each detection, effectively offering a binary output of light presence.

Time-to-Digital Converter (TDC)

The Time-to-Digital Converter (TDC) translates analog signals into time-represented digital signals, marking the pulse times relative to a start point. This data, processed into histograms, helps determine the position of objects by comparing peak histogram counts.

Detection Range and Precision

The theoretical detection range of dTOF is limited only by the system's ability to discern reflected photons from noise, influenced by the emitted wavelength's signal-to-noise ratio on the target surface. Factors like the laser modulation frequency impact the maximum unambiguous detection range, introducing considerations for pulse separation to avoid signal overlap and ambiguity.

Accuracy and Error Factors

dTOF's distance measurement precision inherently depends on the pulse width of the emitted laser, while its accuracy is affected by various uncertainties introduced throughout the signal's journey from emission to detection. These include the timing precision of the laser driver, SPAD's response time, and TDC's accuracy.

Noise and Interference

Addressing background noise and crosstalk is crucial for dTOF systems. Strategies include utilizing SPADs with varied attenuation levels to handle diverse lighting conditions and employing algorithms to mitigate reflection-induced crosstalk.

Spatial Resolution and Power Consumption

Despite the larger pixel size of SPADs compared to traditional CMOS sensors, advancements in back-illuminated SPAD technology by manufacturers like Sony are increasing photon absorption rates and filling factors, promising higher spatial resolutions for dTOF systems. Additionally, the pulsed nature of dTOF transmission ensures lower power consumption relative to continuous-wave systems.

CMOS sensor vs SPAD Sensor.jpgCMOS vs SPAD sensor.jpg

Technological and Manufacturing Challenges

The design and manufacture of dTOF systems encompass high-speed electronic circuits, laser emission control, and SPAD-related electronics, presenting significant challenges. The availability of SPAD providers is limited, and the high voltage requirements for SPAD operation add complexity to circuit design.

In summary, dTOF technology exemplifies a sophisticated blend of optical and electronic engineering, offering high precision and efficiency in distance measurement and object detection. However, the technological complexity and associated costs underscore the need for continued innovation and optimization in the field.

From the web page 02.02 TOF系统 第二章 dTOF系统 - 超光 Faster than light (faster-than-light.net)

by the author: Faster than light


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