• Question: Hi Ashleigh! If you wanted to improve the range of a laser, how would you go about doing this? Also, how would you improve the accuracy of a laser?

    Asked by Ella G to Ashleigh on 17 Jul 2020.
    • Photo: Ashleigh Barron

      Ashleigh Barron answered on 17 Jul 2020:

      Hi Ella,
      The range of the laser range finder is determined by how much of the laser light you can hit of the target and return to the detector that is strong enough to still see if over the noise level. So to improve the range you have the option of either getting more light onto the target or making the detector more sensitive to be able to pick up smaller signals.
      If we start with option 1, we can get more light onto the target by either increasing the energy coming out of the laser or making the laser spot on the target smaller so more light hits it and is reflected back. Both of these have limits so you tend to use a trade off, increasing the energy there is a limit to when the laser becomes too high that it will damage someones eye so you tend to want to be below that limit. To get the spot on the target smaller you can make the laser divergence smaller, this is how much the laser light spreads out the further the distance it travels. To do this you can improve the beam profile (how the energy is distributed across the beam), the ideal energy distribution is known as a Gaussian beam and/or you can also make the laser beam smaller to begin with. If you do these the limit lies in how likely laser damage is to occur inside the laser, too high an energy in a smaller area can burn holes in the coating on the optics which then stops them working. Coating technology is improving all the time though so the energy per area is can handle is getting bigger.
      For option 2 increasing the sensitivity of the receiver, you can buy a more sensitive diode, diodes are used in a lot of different applications so the sensitivity is improving all the time. If you improve the sensitivity to increase the light it can see you have to be careful it doesn’t have an negative effect at shorter distances. If the receiver is too sensitive at shorter distances it can saturate and when this happens the accuracy gets worse and sometimes the receiver is blind for a period of time after that. In rangefinders you need to measure the time the laser pulse leaves and when it comes back, if you do this using the same detector and the pulse at the start is too big it saturates the receiver, you will have a less accurate rangefinder and the minimum range it can see will also be longer. You can do things to improve this though, by either using a different detector to see the outgoing pulse (though this is normally less accurate due to time delays), or using something known as time variable gain where you make the receiver less sensitive at shorter distances (lower time) and more sensitive at higher distances (longer time). Light travels at 299792458m/s so to get a range of 10km the beam travels 20km (there and back) in a time of 67us (a 1/15000th of a second) so any changes you do in the electronics have to be fast to see it.
      Improving accuracy in a rangefinder is generally improving how quickly you can measure the laser beam at the start and at the end this is mostly to do with the receiver and how well it can detect signals of different sizes. When a pulse saturates the detector what happens is it seems wider, in terms of time a 1m accuracy is 6.7ns so if the pulse (which is normally 5-20ns depending on the laser) gets fatter this can effect the accuracy. The decisions on whether the diode “see” the pulse are not made by the diode but by electronics designed around it, the longer the path the signals travel to the chip can also change accuracy by adding in noise etc. Electronics have gotten smaller over the years so it can be closer to the diode which means less noise and higher accuracy. Accuracy can also be improved with the same sort of methods as improving the sensitivity of the receiver.