Uncrewed Systems Technology 049 - April/May 2023

14 Platformone April/May 2023 | Uncrewed Systems Technology Researchers in Japan have developed a new type of laser diode that can boost the performance of solid-state Lidar sensors (writes Nick Flaherty). Photonic-Crystal Surface-Emitting Lasers (PCSELs) are built from two different types of materials with a large refractive index contrast, such as air and semiconductors. The PCSELs are different from 2D distributed feedback (DFB) lasers. DFB lasers have periodic structures with a smaller refractive index, and so can only couple the primary, or fundamental, lasing mode to produce the laser light. This same issue has an impact on the vertical-cavity surface-emitting laser diodes commonly used for Lidar systems nowadays. That means the brightness cannot be increased even by increasing the size of the device, as multilateral- mode oscillations occur. In PCSELs, however, the multilateral- mode oscillations can be kept very small, even though the size increases. That allows the brightness to be increased to up to 10 GW/cm 2 /sr (Watts per square centimetre steradian), which is comparable to high-performance fibre lasers. PCSELs also have a very narrow divergence and symmetric beam with a very narrow lasing spectra in a single mode, and their temperature dependence is much smaller than that of conventional broad area Fabry-Perot (FP) semiconductor lasers. Together, these features allow for lens- free and adjustment-free operation in Lidar sensors with a higher signal-to-noise ratio thanks to the higher brightness. This can be used to provide longer range andmore accuracy, or smaller sensors. The researchers have developed a double-lattice structure where the light waves diffracted by individual lattices have an optical path difference of a half- wavelength. That creates destructive interference at the edges of the lattice, which gives a higher beam quality and higher brightness. When a PCSEL is mounted upside down on a package, the output beam is emitted from the substrate side, with 10 W of output power and a very narrow beam divergence, of 0.1o. The PCSEL can also operate reliably across a wide temperature range, of -40 to 100 oC, and the temperature dependence of output power at a fixed current injection is -0.36%/oC on average, which is better than an FP laser. The temperature dependency of lasing wavelength is as small as 0.08 nm/oC, which is superior to FP lasers. By further expanding the concept of the double-lattice photonic crystal, the researchers have created a PCSEL with a large lasing diameter, of up to 10 mm, with a power output of 100 W to 1 kW. They have also developed a beam- scanning approach for compact solid- state Lidars. This ‘dual modulated’ PCSEL simultaneously modulates the lattice point sizes and positions to direct a beam. They have built a chip for electrical 2D beam scanning based on dual modulated photonic crystals by integrating 10 x 10 different dual-modulated PCSELs in a 2D matrix, where individual PCSELs can be driven independently. The researchers have now produced a solid-state Lidar using the PCSEL devices, it uses a dually modulated photonic crystal laser (DM-PCSEL) as its light source with electronically controlled beam scanning and flash illumination used in flash Lidar to acquire a full 3D image with a single flash of light using a time-of- flight (ToF) sensor. This provides both a flash source that can illuminate a 30 x 30o field of view and a beam-scanning source that provides spot illumination with 100 narrow laser beams. “Our DM-PCSEL system lets us range highly reflective and poorly reflective objects simultaneously, which is difficult for other flash Lidar system designs,” said researcher Susumu Noda, from Kyoto University. “The lasers, ToF camera and all the associated components required to operate the systemwere assembled in a compact manner, resulting in a total system footprint that is smaller than a business card.” This allows the designers to achieve both flash and scanning illumination without any moving parts or bulky external optical elements such as lenses and diffractive optical elements. Sensors Solid-state Lidars boost The PCSELs allow for lens-free and adjustment-free operation in Lidar sensors All the components have less footprint than a business card