Wave interference - If I shine 2 lasers on a spot,will there be actual blinking? - Page 1
Forum section: electronics / rf-microwave

author: daqq
author_rank: Super Contributor
timestamp: January 20, 2016, 09:56:08 am
content: Hi guys,Assuming I have 2 lasers, their spectral peaks some small frequency (say, 2Hz?) apart from one another, and I shine these two lasers onto the same area. Would there be actual visible blinking on the area?Thanks,David

author: nfmax
author_rank: Super Contributor
timestamp: January 20, 2016, 10:30:38 am
content: Yes. Though good luck finding visible-light lasers only 2Hz apart from each other (at least for a time period of more than a few milliseconds) I forgot to say that you will only see blinking if the two lasers share the same polarization (or have at least some part of their output identically polarised)

author: m98
author_rank: Frequent Contributor
timestamp: January 20, 2016, 02:28:48 pm
content: Found some interesting topic about that:https://www.researchgate.net/post/The_interference_between_two_different_lasers_at_the_same_wavelength

author: krivx
author_rank: Frequent Contributor
timestamp: January 20, 2016, 03:25:15 pm
content: Quote from: nfmax on January 20, 2016, 10:30:38 amAlso requires that the laser spots have little spatial variation as well temporal.To put it in context, a 2Hz variation is about 1 part per 10^14

author: tesla500
author_rank: Regular Contributor
timestamp: January 20, 2016, 07:56:03 pm
content: I recently did a teardown of an HP laser head that did something similar to this. A special laser tube produces two wavelengths simultaneously, separated by about 2MHz. The "blinking" is easily picked up with a photo diode. This is used for an interferometer for measuring distances, and can detect movements in the nanometer range.

author: awallin
author_rank: Frequent Contributor
timestamp: January 20, 2016, 08:08:18 pm
content: This would be fairly easy to do in an optics lab: take a HeNe laser, split it in two with a 50/50 splitter, use two AOM:s at say 50 Mhz and 50 MHz+2 Hz to shift the frequencies of the beams, and combine again with a 50/50 splitter.

author: sarepairman2
author_rank: Frequent Contributor
timestamp: January 21, 2016, 12:08:36 am
content: yes, this is how they generate low power high frequency signals at times (like terahertz i believe)

author: daqq
author_rank: Super Contributor
timestamp: January 21, 2016, 07:35:17 am
content: Thanks guys!Tesla500: Nice device and video, I'll look at it.

author: coppice
author_rank: Super Contributor
timestamp: January 21, 2016, 04:05:19 pm
content: Quote from: awallin on January 20, 2016, 08:08:18 pmA simpler test with most HeNe lasers it to shine them onto a photosensor attached to a spectrum analyser. They generally aren't true monochromatic sources, and output fairly strong spectral lines at n, n+1, n+2 etc wavelengths in the cavity. These are typically frequencies a few hundred MHz apart, and you will see that spacing as lines in the spectrum coming out of the photoreceptor.

author: helius
author_rank: Super Contributor
timestamp: January 21, 2016, 04:59:28 pm
content: No, because two separate lasers are not coherent with each other. The way to build a Michelson-Morley interferometer is to split a single laser beam along two paths and then converge them together.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: January 21, 2016, 05:24:54 pm
content: It all boils down to the details. Not only would the two lasers have to be 2 Hz away, but the line widths would have to be much less than 2 Hz. Lasers aren't actually monochromatic, just very narrow band. I haven't run the numbers, but you would have to operate said lasers very cold to get thermal noise that low, and thermal is just one of the sources. Think of things like doppler on the end mirrors from vibration and piezo effects. That one part in ten to the 14th is tough. It may be possible theoretically (not even sure of that), but in practice it just can't be done.

author: awallin
author_rank: Frequent Contributor
timestamp: January 21, 2016, 05:51:24 pm
content: Quote from: CatalinaWOW on January 21, 2016, 05:24:54 pm<=1 Hz linewidth lasers are built quite routinely now in many many groups around the world working on optical clocks or other precision experiments. Most of these lasers are stabilized to a Fabry-Perot cavity made from a low thermal expansion material (Zerodur, ULE glass, single crystal silicon). The cavity is put in a vacuum chamber with thermal shields and good thermal control. Thermal time-constant of days or weeks. Choose a material with parabolic thermal expansion and operate it where the slope is zero. When all other technical noise sources are suppressed what remains is thermal noise, mostly from the multilayer mirror coatings. The latest development is crystalline mirrors (instead of conventional multilayer dielectric mirrors) which have low thermal noise.Here is a fun video, from the Hänsch (2005 Nobel prize) lab AFAIK: