Fun with laser and PMT tube, or how much is the speed of light here? - Page 1
Forum section: electronics / projects

author: muvideo
author_rank: Frequent Contributor
timestamp: June 21, 2012, 09:27:25 pm
content: When I trying to build my first avalanche pulse generator,someone explained me that I was independently measuringthe dielectric constant of the RG58 cable I used as charge line.This triggered the idea to measure the time of flight of a light pulse.I was also curious if normal leds and laser diodes could be usedas very high speeds. Probably it's a basic excercise in universitylab but I've never had the luck to see the experiment live.The requirement was to use only equipment I had around,the pulser was working ok, I had some smd leds to testbut the problem was what to use as detector.The detector has to be fast, sensitive and be coupled to a scope input.Characteristics not easy to have from common components,but I had right in the “lab” (let me use this word) the righdevice for the task, this :https://www.eevblog.com/forum/product-reviews-photos-and-discussion/photomultiplier-tube-teardown-and-speed-test/A photomultiplier tube, boxed and almost plug and play.Once connected to a suitable power supply (rigorously highvoltage, as per vacuum devices tradition) this glass tubeis so nice that spits out a pulse ready to be coupled withan oscilloscope. Well, almost ready, but close enough.Ant it's sensitive and fast, at least the risetime is fast,the pulse width isn't that fast but it's enough for the task.Let's do the math: the light travels 30cm per nanosecond,so with the 6m path I have (two times bench to wall)I have 20nS flight time.The pulse generator risetime is less than 1.5nS, and narrow:The photomultiplier has an output pulse width of 10nS,this is the time it takes to discharge after being hit bylight pulse,so I should have enough time to generate two pulseswell spaced apart.The biasing network on the tube had to be slightly modified.The test shows that the leds are fast enough, at leastone of the orange leds I had around, a red one wasmuch worse.This is a led I used for testing the pmt, the capacitorand inductor are used to maintain the led near conduction:This is the final setup, I used a laser instead of a led,because it easy to collimate and be reflected backfrom distance.Under the rechargeable cell there is the avalanchepulser that feeds the laser head, one of these:https://www.eevblog.com/forum/product-reviews-photos-and-discussion/low-cost-ebay-red-lasers-teardown/The small glass piece in front of the laser reflectspart of the beam to the big mirror on the side of thephotomultiplier. The rest of the beam travelsstraight to a mirror 3m away that reflects itback into the photomultiplier.This beam is late by the 6meters it travels so the PMTreceiver will see two pulses 20nS apart.And this is the result:The nice thing is that the laser pulse is shorterthan the distance it travels, so I imagine it likea light string probably less than 1m longthat travels the 6m from the laser head and receiverNow I can answer to my curiosity:seem that also in my “lab” the lightbounces around at the usual speedFabio.

author: Time
author_rank: Frequent Contributor
timestamp: June 21, 2012, 09:44:30 pm
content: Neat. Thanks for sharing. I also love fast pulses!

author: caroper
author_rank: Regular Contributor
timestamp: June 21, 2012, 10:35:50 pm
content: Now you know the distance from your bench to the wall to 5 decimal places

author: muvideo
author_rank: Frequent Contributor
timestamp: June 22, 2012, 11:42:59 am
content: Quote from: caroper on June 21, 2012, 10:35:50 pmI prefer thinking I measured the speed of light to 1 decimal place

author: ejeffrey
author_rank: Super Contributor
timestamp: June 22, 2012, 12:57:38 pm
content: Nice work. I used to do this demo for an undergraduate physics class. Somehow measuring the speed of light in air is always more impressive than doing the same in coax or optical fiberI am actually a little bit surprised that the LEDs work so well, I generally think of LEDs as being pretty slow compared to laser diodes, but then I never tried. With the laser diode you also don't really need a PMT -- a standard biased 1 mm photodiode can generally be relied upon to have 1 ns rise time into a 50 ohm load, and with a laser you can often collect enough light to see directly on your scope without any amplification.

author: muvideo
author_rank: Frequent Contributor
timestamp: June 22, 2012, 01:30:01 pm
content: Quote from: ejeffrey on June 22, 2012, 12:57:38 pmThe first I tested was a disappointing 0603 red led,decent risetime but bad extinction, the second one was theorange led that was very good, the pulse shape was limited by the pmt, and even a little better thanthe laser, probably because the laser intensity was a little high for the pmt.Infact the best pulse for the led was obtained with pmt biased around 900V, whilethe laser gave excellent results at 700-750V, the nominal biasing is 1kV.I have also some smd white leds, it would be interesting to test these and compare toplain blue ones, anyone has an idea of how fast are white led's phosphors?Quote from: ejeffrey on June 22, 2012, 12:57:38 pmThis is interesting, can you explain, or point to some online source?I have are some IR detectors from remotes, but probably in the junk box I can find something better.edit: i found a pair of hamamatsu s1223-01, i think they are too slow, I should have some sfh203 somewhere,but also these are specified for 5nS risetime...Fabio.

author: SeanB
author_rank: Super Contributor
timestamp: June 22, 2012, 02:36:15 pm
content: The white phosphors are pretty slow, you can expect millisecond to second response times for the various component phosphors.

author: muvideo
author_rank: Frequent Contributor
timestamp: June 22, 2012, 03:23:53 pm
content: Quote from: SeanB on June 22, 2012, 02:36:15 pmWow, I thought that Ce:YAG were much faster.

author: SeanB
author_rank: Super Contributor
timestamp: June 22, 2012, 03:36:37 pm
content: Pretty much the same phosphors are used on CFL lamps, and a slow response reduces flicker. Remember white LED's are really designed for a human eye, and the response is just required to be as fast as that.

author: muvideo
author_rank: Frequent Contributor
timestamp: June 22, 2012, 10:14:05 pm
content: Quote from: SeanB on June 22, 2012, 03:36:37 pmAs far as I know CFL and LED use different phosphors. Now I'm curious,as soon as I can I'll mount one of the white leds and report back the results,I'll also try to use a photodiode to see if I can obtain a faster detector.Fabio.

author: ejeffrey
author_rank: Super Contributor
timestamp: June 23, 2012, 08:20:53 pm
content: Quote from: muvideo on June 22, 2012, 01:30:01 pmBasically, the thing limiting the speed of most photodiodes is their capacitance. The N and P doped regions are separated by a depletion region, and form a parallel plate capacitor. That capacitor plus the characteristic impedance of your load (50 ohm if you use standard lab coax and a 50 ohm scope) gives you a low pass filter. In reality it is considerably more complicated, but that is a good first approximation. The capacitance depends on the area of the photodiode and also on the reverse bias voltage. A reverse bias voltage increases the thickness of the depletion region and makes the 'plates' farther apart, decreasing the capacitance and improving the speed.QuoteYeah, those are 3.6x3.6 mm diodes and they have too high a capacitance (20 pF) to be used at high speed. If you look at the S5972 it has a diameter of 0.8 mm and only a capacitance of 3 pF.IR receivers for remote controls are photo transistors which are much, much slower. They are cheap, simple, and give a lot of gain, but you wouldn't use them in a high performance application.