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27 Apr 2022
US developer of laser diodes says its latest devices enable long-range FMCW lidar with coherent detection.
by Mike Hatcher in Munich
SemiNex, the Massachusetts-based company specializing in long-wavelength laser diodes, is expanding its range of options for automotive lidar applications with new semiconductor optical amplifiers (SOAs) suited to frequency modulated continuous-wave (FMCW) technology.
The high-gain SOAs, available in the 1250-1650 nm wavelength range, are said to emit a high saturation output power of 500 mW, thus enabling long-range FMCW performance with coherent detection.
“SemiNex can work with you on custom designs of individual amplifiers or arrays to meet the requirements in your integrated systems,” announced the firm at the LASER World of Photonics event in Munich.
Triple-junction advantages
The move to accommodate FMCW designs, which are more complex than conventional time-flight lidars but can measure both the position and velocity of other objects on the road, follows SemiNex’s launch of triple-junction 1550 nm diodes for pulsed systems last year.
Outlining the technology behind the triple-junction devices in a presentation on the LASER trade show floor, SemiNex VP of marketing and business development Daniel Chu said the firm’s global patent on the approach meant it was the only company in the world able to produce the devices.
Because of eye safety regulations that limit the amount of optical power that can be deployed in the 905 nm region, longer-wavelength lidars offer a much greater “photon budget” - enabling the kind of long-range performance demanded for fast-moving autonomous vehicles.
Since the longer-wavelength emitters are fabricated using an indium phosphide material, they are typically much more expensive than 905 nm devices. However, SemiNex indicated that, if produced in large volumes, the triple-junction chips would be compatible with the kind of price points targeted by the automotive industry.
Efficiency gains
Chu detailed the “game-changing” characteristics of a triple-junction emitter versus a single 905 nm chip, with the 1550 nm device offering far higher pulse energy, longer target range, and superior signal-to-noise.
SemiNex’s epitaxial design is based around three active regions and two tunnel junctions, producing up to a 100 W output from a 350 µm aperture, and a significantly higher wallplug efficiency than a single-junction device.
While the use of three junctions produces three times the optical power of a single-junction design, the device's voltage only increases by 50 per cent - meaning that the electrical efficiency is doubled.
“For time-of-flight [lidar], 1550 nm outperforms 905 nm in every category,” Chu said, adding that an array of the triple-junction emitters could deliver peak optical output power in excess of a kilowatt for flash lidar systems if required.
Chu and SemiNex’s VP of sales Ed McIntyre indicated that multiple customers were currently trialing the triple-junction diodes and high-gain SOAs, with samples of a new SOA slated to become available this summer.
The pair added that they did not expect the lidar industry to converge on a “single solution” for automotive applications, suggesting that there would continue to be a place for time-of-flight, flash, and FMCW approaches, as auto manufacturers work to figure out how best to combine lidar with other sensors such as radar and cameras in sensor fusion systems for different types of vehicles.
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