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Registered Member #60240
Joined: Mon May 16 2016, 07:01PM
Location:
Posts: 304
Hi
I 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 2GHz silicon photo diode with about 150 ps rise and fall times.
In the figure the first pulse is the electric pulse for the laser diode, the small little pulse is the reflected lightpulse by the beam splitter. The last big needle is the laser light pulse coming back from the corner cube. The distance between the beam splitter and the corner cube was about 1m, therfore the total path for the light pulses was about 2m corresponding to running time of about 6.6 ns.
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
Welcome to the forum and great experiment :)
It is not easy to see, but it seems that the scope is set to 5 ns/div and to me it looks more like 15ns from rising edge of laser diode and rising edge of detector, even from falling edge of laser diode it is still 10ns. So how did you get 6.6ns for the total distance?
Registered Member #60240
Joined: Mon May 16 2016, 07:01PM
Location:
Posts: 304
Hi Mads Barnkob
In the figure of my first posting the first pulse of the trace with almost no noise corresponds to the electrical signal applied to the laser diode.
The second trace with 1mV/division (much noise) is the signal of the silicon photo diode. The very small little peak is the laser pulse directly reflected by the beam splitter. The rather big needle is the is the laser pulse going the long way from the beam splitter to the corner cube and back.
Here is a screen shot from a different experiment with a 10ns pulse generator with more power (not HP) and a longer light path where you can see the two detector peaks more clearly:
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
Thank you, that cleared it up, I had it understood as the 1 meter was between laser and corner, now it makes more sense why you mentioned the small bump as reflection in the splitter :)
Registered Member #60240
Joined: Mon May 16 2016, 07:01PM
Location:
Posts: 304
Hi Mads Barnkob and Proud Mary
More experimental details: You can see from the left to the right 1. pulsed red laser diode 2. Silicon photo diode, 2GHz band width, rise- and fall time around 150 ps 3. Beam splitter (5% reflectivity, 95% transmission)
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
@Physikfan - while the forum page appreciates you posting a small image, and small is mandatory in the avatar area, you can add large images to a post. When you upload a photo to the forum, it automatically creates and displays a thumbnail, while providing a link to the full size version. I for one would love to see a bit more of the detail in your setup.
Unfortunately editing/replacing images in an existing post is slightly more tedious than when first creating the post. You post the new image to the attachments forum, edit *that* post to copy the link, then finally edit your target post to replace the link. Tedious, but straightforward. You can link to remote images, but then you need to keep the size reasonable yourself.
Registered Member #60240
Joined: Mon May 16 2016, 07:01PM
Location:
Posts: 304
Hi Dr. Slack
Thank you for your advice uploading pictures in higher resolution. "I for one would love to see a bit more of the detail in your setup." In which details are you interested? I could upload the corresponding pictures in high resolution in my next post.
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Light speed measurement by pulsed red laser diodes
