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by mmmBacon
2776 days ago
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I don’t think 850nm is a good choice for LIDAR for several reasons. Firstly, the article talks about absorption but that’s not the only phenomenon that they need to be concerned about. The other is scattering and 850nm will scatter from particulate matter in the air (like water vapor in fog) and suffer reduced range. The 2nd reason why 850nm sources are not a good choice are VCSELs themselves. VCSELs are low power devices and always will be due to the narrow current aperture in their construction, you simply can’t send much current through it because even at low currents the current density is already very high. The 850nm wavelength is also limited in how much output power you can get and still have an eye safe device. Additionally, VCSELs do not work well at high temperatures. Their LIV curve bends over meaning that the combination of reduced slope efficiency and self heating of the junction cause the output power to decrease as you increase current beyond a certain point. Lastly, VCSELs are relatively noisy devices. VCSELs are not single spatial mode devices and there is mode partition noise caused by the sloshing around of power between the various modes. When combined with RIN, this places an Upper bound on the SNR of the system. Finally, if you are already doing photon counting, you’re pretty much at the limit of this tech already. To my mind, longer wavelength tech is more promising because it can use higher powers and still be eye safe, has better immunity to scattering in various atmospheric conditions which gives it longer range. Longer wavelengths can leverage technologies like silicon photonics to produce more advanced detectors (coherent detectors which will be more sensitive and improve range). Generally the lasers at these wavelengths are less noisy as they’ve been optimized for decades for use in long distance communication. |
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