Electron <-> photon conversion

The curious case of conversion

Here’s a riddle: what gets lost every time you gain? The answer? Electrons and photons – when you’re converting electricity, that is.


Did you know that Formula 1 racing developed out of an experiment to improve on downforce? Of course, the drivers found that as soon as they increased downforce to go around corners more quickly, they experienced an increase in drag as well. This conundrum was eventually solved with the discovery of ground effect, but it illustrates an important point in engineering: there is always a price to pay for any gain.


This is especially true when we talk about the conversion of electrons to photons, or vice versa. We all know that this can be done – but there are always issues around conversion efficiency. In other words, it’s never a straight swap, one proton for one electron. A certain number of protons will always be lost to another characteristic like, say, heat. 


This has important implications for designing devices like lidars, where the big question is: what characteristics are most important? How much does conversion efficiency really matter?


Look at it this way: if you’re designing a LiDAR, you’re looking to generate a bright flash of light. It doesn’t really matter how many electrons had to be sacrificed in order to generate that flash; all that matters is that it was, indeed, produced. 


Conversion efficiency is the reason why solar panels don’t generate as much electricity as we might expect. After all, the electrons are all ready and waiting to transform light energy into electricity – but because the conversion rate is never 100%, there’s a limit to just how effective these panels can be. 


Of course, that’s with a constant stream of sunlight providing a source of energy, so imagine how much more complex matters become when designing a LiDAR. This is why your choice of detector is critical. A standard detector – one you could easily purchase from your local electronics store – may actually have a high conversion efficiency – until you factor in macro phenomena like capacitance, electron mobility, resistance and inductance around current carrying wires, which can have a massive impact. That’s why it may, sometimes, be better to use an insensitive detector that’s very fast, and gives a more accurate signal. It’s that old issue around balancing costs and gains.


That’s why we favour avalanche photo diodes, which neatly sidestep the challenges posed by pin photo diodes (good on paper, but with an output that’s very noisy) and photo multiplier tubes which require a high voltage power supply system and are difficult to operate. The avalanche photo diode, on the other hand, behaves like a photo multiplier tube, with an amplification that can multiply the electrons generated to the power of 100. It’s highly repeatable and predictable – but, of course, there is always a price to pay, and in this case, it’s the variability between diodes across every characteristic, which needs to be accounted and calibrated for. This is tremendously challenging, and remains a pervasive challenge in designing LiDAR systems.


It may not be perfect – but, since one-to-one conversions never are, it’s an excellent compromise.


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