AUTOMOTIVE/TRANSPORT
FRONTIERS PHOTONICS
Advances in the optic and photonic components that enable lidar have been instrumental in the safety boost it promises
is similar to that used by smartphone facial recognition software, but much higher powers are needed to send light up to 20 metres into the environment. The innovative aspect here is the five-junction emission structure on the VCSEL die. VCSELs typically generate light from a
single layer, but stacking multiple layers allows VCSELs to better amplify optical power, increasing gain. This is because fewer distributed Bragg reflector layers need to be deposited on the VCSEL to trap light and enable laser gain. Fewer layers lowers the internal resistance, boosting efficiency. This, in turn, reduces the electrical power needed to attain a particular optical power. Velodyne uses the Osram 5J multijunction
VCSEL 905nm, which uses dies from Osram subsidiary Vixar, the first company to commercialise multi-junction VCSEL technology.
The possibility to create much more
powerful illumination profiles at a reduced size opens the doors for lidar to be used in other applications where size, weight and cost are important. This technology is analysed in a teardown
report, Velodyne Short and Long Range Lidar with VCSEL and EEL (April 2023) by Yole SystemPlus, part of Yole Group.
Silicon photonic gains To reduce size and cost further, a small minority of lidar companies are banking on silicon photonics, an approach that puts lidar on microchips. New-York based firm Voyant is one such company, which has secured $15.4m in funding to commercialise its frequency-modulated continuous- wave (FMCW) technology. FMCW lends itself better to chip integration and is the technique of choice for chip-scale lidar companies. Intel is also looking at silicon photonic
lidar, saying it wants to put autonomous cars on the roads by 2025. But not everyone thinks it’s time to switch. Yole Group estimates that of the 80 or so lidar companies operating, about 80% are still using time-of-flight. One challenge for FMCW is that it
The Velarray H800
requires a very stable laser. In time-of-flight, increasing the laser power increases the sensing range, but because FMCW uses the phase and frequency of photons to measure
“Not everyone thinks it’s time to switch [to silicon photonics]. Yole Group estimates that of the 80 or so lidar companies operating, about 80% are still using time-of-flight”
distance, the coherence length of the laser sets a maximum sensing distance. Single- frequency operation can be achieved using gratings, which select a single laser mode, though this adds complexity and cost to the lidar compared with pulsed operation. Balance photodetectors, which mix the local and returning signal and extract the phase and frequency, also entail additional hardware. And while the peak power might be lower than in time-of-flight, the laser still needs an average power of a few hundred milliwatts to overcome losses in the silicon waveguides.
In addition, while silicon chips are cheaper than discrete FMCW components, photons are trickier to manipulate on a tiny scale than electrons. But FMCW offers many advantages, such as high precision, low cost and scalability. How will photonics and innovation in
integration enable further cost, weight and size improvements in lidar systems in the coming years? Watch this space… l
Photonics Frontiers 2023 29
Velodyne
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