Speed of light using a pulsed red laser diode  - Page 1
Forum section: electronics / projects

author: Physikfan
author_rank: Regular Contributor
timestamp: September 01, 2017, 06:57:02 pm
content: HiI have measured the speed of light using a HP8082A as a pulse source, a red laser diode as a light source,a thin normal glass plate as a beam splitter and a corner cube as a reflector.A 6ns broad electric pulse created a 1ns laser pulse.The detector was a commercial 2GHz silicon photo diode with about 150 ps rise and fall times.The figure shows the complete experimental setup:The distance between the beam splitter and the corner cube was about 0.6 m, therfore the total path for the light pulses was about 1.2 m corresponding to a running time of about 4 ns.RegardsPhysikfan

author: SKHILLBILLYSlures
author_rank: Regular Contributor
timestamp: September 01, 2017, 09:01:47 pm
content: Are you sure you're laser driver doesn't consist of capacitors so that they don't introduce time you have to compensate for? And even if you did their capacitance is highly variable (around the rated capacitance it will vary by a percentage) and nonconstant . This leads to inaccurate calculations because your time variable is not a constant , therefore throwing off your numbers when calculating the intergral

author: StillTrying
author_rank: Super Contributor
timestamp: September 01, 2017, 10:30:09 pm
content: SKHILLBILLY the very same pulse of light is detected twice, half-reflected into the detector, and then the other half detected again after traveling a further 1.2m, so only the time between the 2 detections of the same pulse is measured.

author: raptor1956
author_rank: Frequent Contributor
timestamp: September 02, 2017, 02:18:46 am
content: Quote from: StillTrying on September 01, 2017, 10:30:09 pmYes, to rule out timing issues I'd have to believe you have to measure the pulse twice and then calculate the time difference and with knowledge of the distance to then calculate the speed of C.Brian

author: colorado.rob
author_rank: Frequent Contributor
timestamp: September 02, 2017, 02:39:11 am
content: Nicely done!

author: Physikfan
author_rank: Regular Contributor
timestamp: September 02, 2017, 06:04:11 am
content: Here is the figure of the experimental setup:RegardsPhysikfan

author: raptor1956
author_rank: Frequent Contributor
timestamp: September 02, 2017, 08:31:44 pm
content: Quote from: Physikfan on September 02, 2017, 06:04:11 amAs a ballpark measure of C this is fine, but to get greater precision out to 5+ digits how could you develop the experiment to achieve that? If the distance were increased to, say, 1km each way that would help but then you'd need some way to verify the distance to 5+ digits. Similarly, you'd need to make sure the timing device was accurate to 5+ digits. Any thoughts on how you might extend the experiment to achieve a precision of 5+ digits?It has always amazed me how precise they were able to calculate things at the Cavendish with tools that were hardly more advanced than what some of us have in our garage.Brian

author: StillTrying
author_rank: Super Contributor
timestamp: September 02, 2017, 09:07:58 pm
content: "After centuries of increasingly precise measurements, in 1975 the speed of light was known to be 299792458 m/s (186,282 mi /s) with a measurement uncertainty of 4 parts per billion. In 1983, the metre was redefined in the International System of Units (SI) as the distance travelled by light in vacuum in 1/299792458 of a second. As a result, the numerical value of c in metres per second is now fixed exactly by the definition of the metre.""the speed of light in air is about 299,700 kilometres (186,200 mi) /s"https://en.wikipedia.org/wiki/Speed_of_lightI think measuring the speed of light is too easy these days, let's try to measure the speed of gravity instead.

author: BradC
author_rank: Super Contributor
timestamp: September 03, 2017, 12:05:12 am
content: Quote from: StillTrying on September 02, 2017, 09:07:58 pmThat's too easy. It's how fast your new expensive widget falls off the bench. It's inversely related to device cost and directly correlated with fragility.

author: CD4007UB
author_rank: Contributor
timestamp: September 03, 2017, 12:46:44 am
content: Well done, Physikfan, in producing a nice demo. If you've not done so, it's a good idea to measure the round-trip time as a function of the distance to the reflector. You then get the speed of light from the slope of a graph (distance v. time), without having to worry about any fixed delays in the system.Our students do this as an experiment using Leybold equipmenthttps://www.leybold-shop.co.uk/vp5-6-2-1.html. A recent, and much cheaper, alternative is to use a VL53LOX chip (available from AliExpress), which allows you to measure the speed of light using an Arduino (or your MCU of choice).Of course the speed of light (or gravity, if you like) is fixed by international agreement, so one can argue that the expt is just calibrating the length and/or time measurement. However, for our students, it's a fun challenge for them to get an accurate result.

author: Physikfan
author_rank: Regular Contributor
timestamp: September 03, 2017, 02:27:24 pm
content: Hi CD4007UB and raptor1956The main differences to Leybold`s setup:"The light velocity measuring instrument emits extremely shortpulses of red light with a pulse width of about 20 ns via ahigh-performance LED."1. I am using 1 ns broad optical pulses and a laser diode, my electronic devices are much faster."A measuring distance of 10 m corresponds to a transit time ofthe light pulse of around 60 ns for the outgoing and returndistances."2. My measuring distance is 2 x 0.6 m, time delay between the two pulses is 4 ns.The alignment in my experiment can be done by one person on a desk.The 10 m distance in Leybold´s experiment is more a task for two persons.The picture of my first post was taken in a Physics lecture hall in front of 40 students ( age 17) of a high school, alignment time before starting the demonstration was 20 minutes.Ad raptor1956:This setup is for demonstration purposes, the accuracy may be around 1 %, if you use a sampling oscilloscope like the Tek 11801 instead of this Tek 11402.Using this setup one of my students will try to demonstrate the index of refraction of water and possibly with green and blue laser diodes also the optical dispersion of the index of refraction.

author: CD4007UB
author_rank: Contributor
timestamp: September 03, 2017, 10:55:59 pm
content: You're quite right that your electronics is much faster than the Leybold expt - probably much cheaper too. The Leybold kit is intended to be used with an oscilloscope for timing the pulses - slow and not very accurate. Although that works (and our lab is >10m long, so it's not a problem), the accuracy is, understandably, not great.I don't want our students to leave our lab thinking that a scope is the best instrument for accurately timing pulses. So, we also get the students to try using a good-quality Hameg counter-timer instead (in A-B time-interval mode with averaging). You can then resolve changes in time delay of <<1ns - we mounted the kit on a 1m linear movement, which has a resolution <0.1mm.We hope to get some students to try using the VL53LOX as a project this year - that could make a nice plug-and-play lecture demo that would be very quick to setup. (We only have a few minutes between lectures for this.)

author: Physikfan
author_rank: Regular Contributor
timestamp: September 04, 2017, 05:51:24 pm
content: My setup is really cheap:Fast photo diode 150$Used items:HP8082A 250 $Tek 11402 400 $Corner cube 50$

author: StillTrying
author_rank: Super Contributor
timestamp: September 04, 2017, 05:59:22 pm
content: Quote from: Physikfan on September 04, 2017, 05:51:24 pmDepends on your definition of cheap.I don't see how a corner cube is needed, surely just a mirror would be better.

author: Physikfan
author_rank: Regular Contributor
timestamp: September 04, 2017, 06:13:43 pm
content: Hi StillTryingUnfortunately I could not buy a used very fast photodiode.This item is new and has a band width of 2 GHz and 150 ps rise and fall times.Because of the very special reflecting property of a corner cube the alignment becomes much faster and also the complete setup is optically much more stable.

author: raptor1956
author_rank: Frequent Contributor
timestamp: September 04, 2017, 10:00:28 pm
content: I wonder how they measure the distance with 9 digits precision? Defining the meter is one thing -- actually using it with 9 digits of precision is another. The time base would appear to be the easier of the two to get that level of precision. Also, if the light path is reflected, say, 90 degrees, then the mirror or prism could introduce a potential error based on where, exactly, the beam hit the mirror/prism as that could change the calculated path length even if just a few microns.Brian

author: bson
author_rank: Unknown
timestamp: September 05, 2017, 06:46:42 am
content: Quote from: Physikfan on September 03, 2017, 02:27:24 pmThe wavelength of red light is ~700nm. IR is even longer. How can you emit a light pulse 1/700 of a wave length? You may pulse your LED, but I strongly suspect it's going to emit a full wavelength (probably many actually) whether you want it to or not.

author: AndyC_772
author_rank: Super Contributor
timestamp: September 05, 2017, 08:41:45 am
content: Is a corner cube a good choice, in an experiment where the path length must be known accurately?What if the cube is slightly offset from the beam path, so it still reflects back toward the source, but having covered additional distance reflecting off the three mirrors?

author: IRFP460
author_rank: Contributor
timestamp: September 05, 2017, 09:03:08 am
content: Quote from: bson on September 05, 2017, 06:46:42 am1 ns * c = ~30 cm

author: ale500
author_rank: Frequent Contributor
timestamp: September 05, 2017, 11:32:15 am
content: Quote from: BradC on September 03, 2017, 12:05:12 amI think BradC nailed itWhat were we talking about ? ahhh, light. Speed of light. And how accessible such equipement has become, amazing.

author: CD4007UB
author_rank: Contributor
timestamp: September 05, 2017, 12:26:16 pm
content: Quote from: Physikfan on September 04, 2017, 05:51:24 pmThat is, indeed, much cheaper than the Leybold experiment!However, I think it's hard to beat the VL53L0X by SThttp://www.st.com/en/imaging-and-photonics-solutions/vl53l0x.html. It contains a 930nm IR laser diode and a fast avalanche photdiode. One can buy it and an Arduino Nano for about $10 from AliExpress. A high-quality corner cube is not required: almost any reflective surface is OK - after all, the chip can be used to sense people's hands for automatic soap dispensers, etc.We also spent some money buying a linear digital scale similar tohttps://www.machine-dro.co.uk/horizontal-linear-digital-scale-1500mm-60-inch.html. The digital scale allows the students to plot an accurate graph of delay time v. reflector position, from which they can determine c. (Or, if you prefer, given that c is defined, they can check the linear scale against the counter-timer.)Of course, the digital scale isn't required for a lecture demo, but for the lab experiment we aim to teach the students to make more accurate measurements. What you won't be able to see with the VL53L0X is the actual pulses for transmission and detection, as they are handled by the chip's internal electronics.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: September 05, 2017, 12:43:34 pm
content: I might try this...http://www.planet-science.com/categories/over-11s/physics-is-fun!/2012/01/measure-the-speed-of-light-using-chocolate.aspx

author: CD4007UB
author_rank: Contributor
timestamp: September 05, 2017, 12:59:05 pm
content: Yes, that certainly looks like fun. As I'm over 11, I may give it a try.The diagram of the wave is not quite right - it should show a standing wave, with half a wavelength between antinodes. (I understand that some microwave ovens contain a metal fan that 'stirs' the microwaves to prevent hot/cold spots. That would spoil the experiment.)

author: Physikfan
author_rank: Regular Contributor
timestamp: September 05, 2017, 07:29:11 pm
content: Hi bson and IRFP460"The wavelength of red light is ~700nm. IR is even longer. How can you emit a light pulse 1/700 of a wave length? You may pulse your LED, but I strongly suspect it's going to emit a full wavelength (probably many actually) whether you want it to or not."As IRFP460 correctly mentioned, a pulse of 1 ns duration is about 30 cm long.For green light, 500 nm, it is very easy to calculate without using a pocket calculator.The number of waves in green light pulse of 1 cm length is 20 000, corresponds to a pulse duration of 33 ps!

author: Physikfan
author_rank: Regular Contributor
timestamp: September 05, 2017, 07:58:22 pm
content: Hi AndyC_772"Is a corner cube a good choice, in an experiment where the path length must be known accurately?What if the cube is slightly offset from the beam path, so it still reflects back toward the source, but having covered additional distance reflecting off the three mirrors?"Your statement is absolutely correct.To increase the precision of this experiment one could use an optical bench of 2 m and change the distance between the beam splitter and the corner cube by steps of about 30 cm each. The corresponding change of the time delay between the two pulses could be measured by a calibrated sampling oscilloscope.More experimental details:You can see from the left to the right1. the pulsed red laser diode2. Silicon photo diode, 2GHz band width, rise- and fall time around 150 ps3. Beam splitter (5% reflectivity, 95% transmission)