LIDAR, optical distance & time of flight sensors
Fully integrated dToF modules and iToF VCSEL illuminators for short range applications. Laser sources for long range LIDAR systems.
Optical distance sensors
A range of approaches exist to directly measure distance as the length of the reflected optical path from laser, to a target where it is reflected, and back to a sensor. These are commonly known by various names, including LIDAR and time of flight sensors, although there is actually overlap in principles between them.
The main system types are summarized in the table below. They are principally defined by the following key parameters:
- Optical distance measurement principle: This is the method by which optical depth in the z dimension is measured. Main approaches are indirect Time of Flight (iToF), direct Time of Flight (dToF), and Frequency Modulated Continuous Wave (FMCW).
- Scanning architecture: This is how the system measures multiple depth points across the x and y dimensions to create a 3D depth map. Main approaches here are either a single emitter with a sensor array, emitter array plus sensor array, and scanning mirror systems with only a single source/detector.
- Optical aperture/power: There is a trade-off between optical power and aperture size, and achievable range. There are two key categories: Compact, low power short range systems, for example integrated modules with wafer level optics for consumer electronics; and larger longer-range systems built using discrete components, more powerful sources and larger aperture bulk optics.
All typically operate in the infrared spectrum. This enables interference from ambient light to be minimized by using a matching infrared bandpass filter at the receiver, and for the system to appear largely invisible to users.
Optical distance measurement principles
Direct Time of Flight (dToF)
The laser source is pulsed, and the time taken for each pulses to reflect and return to the sensor is measured. This time is then converted to distance using the speed of light. dToF systems enable robust and low power distance measurements. However, the receiver is typically implemented with a Single Photon Avalanche detector and timing circuit. Practical limits to the size of array that can be achieved for this limits the resolution for solid state systems to typically <100 depth points.
Indirect Time of Flight (iToF)
The laser source is amplitude modulated. The phase difference between the transmitted light, and the light reflected to the sensor, is measured. This phase difference is converted to time, and then to distance using the speed of light. The receiver can be implemented as part of a specialized image sensor, enabling high resolutions with no moving parts. However, iToF is vulnerable to crosstalk and multipath interference, making it less robust than dToF systems. It is typically only used in short range, high resolution, systems.
Frequency Modulated Continuous Wave (FMCW)
The laser source is CW and frequency modulated with a sawtooth waveform (“Chirp”). The reflected signal is optically mixed with a reference from the source. The resulting signal then contains a “beat” frequency corresponding to distance, that is extracted by spectral analysis. FMCW has robustness and performance advantages compared to dToF, including long range, low emitter power, high immunity to ambient light and the ability to directly measure radial velocity. However, for reasons of optical complexity, it is typically only deployed for more specialist systems at present.
Scanning architectures
Single emitter + detector array
The most common approach for solid state iToF and dToF systems uses a single flood illuminator, and a detector array. In the case of short range modules the emitter is a VCSEL with diffuser optics to achieve the required field of view, and matching imaging optics on the detector. Longer range “flash LIDAR” systems use the dToF approach with a higher power VCSEL array or Edge Emitting laser source, plus larger apertures on each side.
Emitter array + detector array
The performance of dToF / flash LIDAR systems is limited by several practical factors. In order to remain eye safe, there is a limit to the maximum optical power that can be transmitted, and this impacts range. Furthermore, a time detection circuit is required for each depth point, limiting the practically achievable resolution. True Solid State (TSS) Lidar systems address these issues by sequentially illuminating different parts of the scene using a pixelated emitter array. This allows the available optical power to be more focused for each pulse, and allows pixel TDCs to be shared. However, it comes at the expense of additional transmit complexity.
Scanning mirrors
The longest-range LIDAR systems use a single source and detector, and scan it across the scene. This approach enables range to be optimized by use of a focused source and high fidelity time detection. In addition, with just a single channel, the more complex but high performance approach of FMCW can also be deployed. Various scanning configurations are possible, including two dimensional scanning with a MEMs mirror, and one dimensional scanning using a rotating polygon mirror.
Direct Time of Flight sensor modules
ams OSRAM offers fully integrated direct Time of Flight sensor modules. These compact and low power devices integrate a 940nm VCSEL (laser), a SPAD (Single Photon Avalanche Photodiode) pixel array, Time-to-Digital Converters (TDCs), and all the necessary signal processing to give a direct readout of distance over I2C.
Single and multi zone devices up to 8x8 are available in package sizes down to 2.2x3.6x1.0mm, with operating ranges and field of view up to 5m and 63 degrees.
Applications include autofocus for cameras and projectors, obstacle detection for robotics and drones, low power wakeup for camera systems, touchless controls and hand gesture sensing.
For more Information, see our white paper: understanding time of flight sensing.
VCSELs and VCSEL Modules for Indirect Time of Flight sensing
ams OSRAM offers a wide range of infrared VCSELs and VCSEL modules at 850nm and 940nm for iToF systems. For example, our
- BIDOS™ P2433 VCSEL modules deliver up to 6.5W in a 2.4 x 3.3 x 1.2 mm package with integrated photodiode and fields of view of 60°x45° and 72°x58°.
Full iToF system reference designs are available from our partners:
- KEA Time of Flight Camera Development Kit from Chronoptics (pictured) features our BIDOS VCSEL illuminators at 940nm, and comprises a complete solution to accelerate and simplify the integration of 3D depth sensing into products.
- Melexis EVK75027 iToF evaluation kit features our automotive qualified TARA-2000-AUT-SAFE VCSEL illuminator. It demonstrates a complete VGA depth camera system for in cabin monitoring and gesture sensing. Further details of the eye safe illumination system implementation can be found in this white paper.
VCSELs and EELs for long range LIDAR
ams OSRAM offers both VCSEL and edge emitting Lasers (EEL) for use in pulsed operation mode in LIDAR systems. Both types of lasers are provided in different configurations, like VCSEL arrays or single to multichannel edge emitting lasers, as well as several power levels for a variety of system and optical design approaches. Thanks to our proprietary wavelength stabilization technology for edge emitting lasers, these emitters now provide a low temperature-dependent wavelength shift on a similar level as VCSEL.
Our products include:
- SPL S4L90A_3 The flagship of ams OSRAM’s LiDAR portfolio: 4 Channel SMT Laser in QFN package, wavelength stabilized, 905 nm, 220 µm, AEC-Q102
- SPL S1L90A_3 1 Channel SMT laser in QFN package, 905 nm, 125 W 220 µm
- SPL DP90_3 Nanostack pulsed laser diode, 905 nm, 65W, 110 µm, AEC-Q102