Infrared interference beat - Page 1
Forum section: electronics / beginners

author: Maranto
author_rank: Contributor
timestamp: January 22, 2025, 06:32:06 pm
content: Hi, I'm doing a lot of research but I can't find a solution. then considering infrared (leds or laser diodes) how can I obtain beat interference and that is create a third wave equal to the difference of the two original ones with a difference equal to 10 hz?? the problem is the high frequency of infrared, how should I proceed, with frequency modulation with side bands or something else?

author: Terry Bites
author_rank: Super Contributor
timestamp: January 23, 2025, 05:03:04 pm
content: LEDs are easy to drive with a simple current limited MOSFET or BIPOLAR transistor switch.To get a controlled offset at these drive frequencies use a DDS generator. You can get low cost DDS modules from Aliexpress or Ebay etc.Program from Arduino etc. You'll need to use a buffer at the output of the DDS, that can just be line driver. For low LED currents you may not need a transistor to switch the LED.For higher powers I'd be inclined to use a MOSFET and a dedicated MOSFET driver (does the buffering job too) eg NCP5901. For middling currents you could use this as a stand alone LED driver.https://www.aliexpress.com/item/1005005610558062.html?spm=a2g0o.productlist.main.101.584a31b4JDxzld&algo_pvid=ec7709d2-bddb-40fa-a1c6-7ed81f84ac94&algo_exp_id=ec7709d2-bddb-40fa-a1c6-7ed81f84ac94-51&pdp_npi=4%40dis%21GBP%216.26%215.88%21%21%217.48%217.03%21%40210385db17376510107805066e3ad0%2112000033741915240%21sea%21UK%213100120370%21X&curPageLogUid=CkHdO7n4nsro&utparam-url=scene%3Asearch%7Cquery_from%3A

author: Maranto
author_rank: Contributor
timestamp: January 25, 2025, 12:59:55 pm
content: Quote from: Terry Bites on January 23, 2025, 05:03:04 pmwow thanks terry for your answer, it's excellent. a question, but in your opinion, extracting by beat, a frequency of 10 hz, from such a high frequency as infrared, do you think it can work, that is, there will be the beat, or the infrared frequency is too high for 10 hz can they be "heard"?

author: SteveThackery
author_rank: Super Contributor
timestamp: January 25, 2025, 01:39:03 pm
content: Just to be clear, are you saying that you want two infrared lasers, just 10Hz apart in frequency?Assuming a laser wavelength of 800nm, that equates to a frequency of 375THz. Separating them by 10Hz will be challenging, to say the least.Or are you talking about modulating them with carriers which are 10Hz apart?Perhaps you could give a more complete description of what you are hoping to achieve. Right now you haven't been clear enough for us to help.

author: Maranto
author_rank: Contributor
timestamp: January 28, 2025, 04:57:12 pm
content: Quote from: SteveThackery on January 25, 2025, 01:39:03 pmhere I am, yes, in order for there to be a beat, the frequencies must differ by 10 Hz in frequency, I am considering frequency modulation, and therefore I could have the infrared carrier and the modulating signal at 10 Hz, but this is an example, obviously if there are better ways they are welcome, the important thing is that the frequency of 10 hz occurs

author: TimFox
author_rank: Super Contributor
timestamp: January 28, 2025, 05:30:38 pm
content: The NBS experiments that led to the re-definition of the speed of light used a very narrow bandwidth dye-stabilized laser, which went through an electro-optical modulator (I think a Kerr cell) that was driven by a frequency in the microwave region to generate an AM waveform: two sidebands plus original carrier frequency.Getting an interferometer to lock up on all three frequencies allowed them to measure the ratio of the light frequency to the microwave frequency; since the latter could be measured directly, this was a measurement of the light frequency.The interferometer also gave the wavelength of the light beam, thus making a simultaneous measurement of wavelength and frequency, hence the velocity.Seehttps://tf.nist.gov/general/pdf/307.pdffor details.Even with extreme methods to narrow the frequency bandwidth of the laser, it was still well above 10 Hz.A quick survey of the literature indicate that 10 kHz line width is really good, 1 kHz with great effort, for stabilized lasers.

author: SteveThackery
author_rank: Super Contributor
timestamp: January 28, 2025, 07:06:29 pm
content: Quote from: Maranto on January 28, 2025, 04:57:12 pmI'm sorry, but I still cannot make sense of what you are saying. What do you mean by "the infrared carrier"? There is no convenient way of frequency modulating the infrared itself because it's at 375THz. You could amplitude modulate the laser by varying the power to it, but why would you do that?In your original post you said this:"how can I obtain beat interference and that is create a third wave equal to the difference of the two original ones with a difference equal to 10 hz??"What are the "two original ones" you talk about? Two lasers? But there is no way you can separate two 375THz lasers by just 10Hz.Perhaps it might be easier to start from a different place. You say you want a 10Hz signal? Do you mean electromagnetic radiation at 10Hz? If so, that is in the ELF radio band and you wouldn't use lasers to produce it. Or do you want to modulate an infrared laser beam at 10Hz? If so, to what end?What, exactly, do you need this 10Hz signal for? Maybe if we understood that we could get a better understanding of your objective.

author: CaptDon
author_rank: Super Contributor
timestamp: January 28, 2025, 10:42:39 pm
content: It would seem that the laser light source would have a frequency instability of far greater than 10Hz to begin with.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 29, 2025, 10:37:02 am
content: One could creat a frequency shift from doppler effect. So start with only 1 source and split the light. One path goes via a movable mirror. It does not need much speed to get 10 Hz frequency shift.

author: SteveThackery
author_rank: Super Contributor
timestamp: January 29, 2025, 11:07:48 am
content: Quote from: Kleinstein on January 29, 2025, 10:37:02 amAssuming the mirror has to oscillate back and forth, the speed affects the depth of the modulation, not the frequency. But I don't think this is what the OP wants. He seems to be talking about doing it the other way round: two infrared sources 10Hz apart going into an optical mixer to produce a 10Hz beat frequency. As far as I know, that isn't practically possible.

author: mikerj
author_rank: Super Contributor
timestamp: January 29, 2025, 01:41:54 pm
content: Quote from: CaptDon on January 28, 2025, 10:42:39 pmAs per TimFox's post just the spectral width of a laser will be 2-3 orders of magnitude higher than this, so it's a non-starter. The frequency drift will likely be higher again, even with a frequency stabilised source.

author: coppercone2
author_rank: Super Contributor
timestamp: January 29, 2025, 01:46:33 pm
content: this is how you get some THz-ish oscillators with laser beams. But you see that the practical application is orders of magnitude off in frequency.

author: Maranto
author_rank: Contributor
timestamp: January 29, 2025, 05:17:51 pm
content: Quote from: TimFox on January 28, 2025, 05:30:38 pmHi Steve, yes I also read about that experiment, but I think that using that equipment is very expensive, then you also said that in the experiment they weren't able to reach 10 Hz. Are there other ways to obtain that frequency through infrared, i.e. with modulation or other techniques?

author: Maranto
author_rank: Contributor
timestamp: January 29, 2025, 05:23:20 pm
content: Quote from: SteveThackery on January 28, 2025, 07:06:29 pmHi Steve, by original frequencies I meant the two initials that interfere and create the beat effect and therefore the 10 Hz. yes, I would like to modulate an infrared signal to be able to extract the 10 Hz, and frequency modulation or beats are the two ways that immediately come to mind to do it, but you were saying that it is difficult since you cannot modulate a signal infrared carrier, with 10 hz. maybe you know a different way or frequency modulation would be fine?

author: Maranto
author_rank: Contributor
timestamp: January 29, 2025, 05:32:19 pm
content: Kleinstein, Mikery, Coppercone2, Capitandon hi guys I'm replying to you with this post because your answer concerns optical modulators, yes I had also thought about using Kerr cells and the pockels effect but I think the costs are high, what I would like to do is get 10 hz , but since they are too low in frequency, I would like to use infrared beams (LED or laser) to send them. the beats or the frequency modulation are 2 methods that I hypothesized that could work to extract the 10 Hz frequency from the infrared frequency, so I ask you is it possible to do it with the beats and modulation, or the infrared frequencies are too high? with frequency modulation, I could have a carrier with an infrared signal, and the modulating signal would have been the frequency of 10 Hz, could this be possible?

author: SteveThackery
author_rank: Super Contributor
timestamp: January 29, 2025, 07:06:42 pm
content: Quote from: Maranto on January 29, 2025, 05:32:19 pmThis is where the confusion comes in. Frequency modulation doesn'textractthe 10Hz from the infrared frequency, itinserts10Hzintothe infrared frequency.Can you see how confusing your language is?I still cannot visualise what you are trying to achieve. These are my concerns:1/ Generating a 10Hz beat frequency by mixing two infrared sources is not possible because no infrared source is sufficiently stable, accurate, or narrow enough in bandwidth.2/ It isn't practically possible to frequency modulate a 375THz infrared source. (Except by wobbling the laser.)3/ You haven't explained what your device is supposed to do - if you did that I'm sure you would get immediate help.Quote from: Maranto on January 29, 2025, 05:32:19 pmIs it possible to dowhat"with the beats and modulation" ?Could you please provide a diagram of what your system should do. That way we don't have to rely on imprecise language.

author: Maranto
author_rank: Contributor
timestamp: January 30, 2025, 04:40:10 pm
content: Quote from: SteveThackery on January 29, 2025, 07:06:42 pmhi steve, thanks as always for your answers, unfortunately I use the translator and many times it gets confused, I know very well that in frequency modulation no frequency is extracted, but the modulating frequency in this case 10hz oscillates between the central frequency (carrier). my main goal is to create this frequency of 10 hz, and so I was looking for methods to do it, unfortunately the infrared frequency is very high, I thought of modulating two infrared sources, with the same carrier frequency (400 thz) and then use two modulating frequencies, for each source, so that the difference was 10hz, in this way interfering in a linear system the beat was created, but you are explaining to me that this is not possible since the frequencies are too far apart, right? in an article on arxiv it was stated that through the use of electro-optical modulators this was possible, but the problem is the high cost of the components, that's why I tried to use a simple electronic modulator with varicap. so what i ask you is, how can i get the frequency of 10 hz? should i continue with the standard electronic modulators, or is it better to use the electro optic modulators?

author: TimFox
author_rank: Super Contributor
timestamp: January 30, 2025, 04:45:34 pm
content: What line width (bandwidth in Hz) of the light emitted from your IR laser do you expect before it is modified or modulated? 10 Hz is a very, very small value compared with the frequency of IR light.

author: SteveThackery
author_rank: Super Contributor
timestamp: January 30, 2025, 06:12:40 pm
content: Would amplitude modulating the laser at 10Hz meet your needs? I've never done it but I suspect it would be relatively simple to modulate the drive current with a 10Hz sine wave.Quote from: Maranto on January 30, 2025, 04:40:10 pmI'm really sorry,@Maranto, but my brain cannot interpret the output of the translator. Rather than wasting your time further, I will withdraw from the thread at this point. Hopefully someone who thinks more clearly than me will be along soon. My apologies again, and good luck with your project.

author: Maranto
author_rank: Contributor
timestamp: January 31, 2025, 05:42:25 pm
content: Quote from: TimFox on January 30, 2025, 04:45:34 pmGood morning Steve and Tim I will give you a single answer so as not to create problems with the translator. So yes the line width of a laser diode "starts" from about 50 MHz, so here I was wondering if it is possible to have two modulating signals one from 50,000,000 hz and the other from 50,000,010 hz here if you can add 10 hz to 50 MHz, but if it is not possible it does not matter, maybe I will move on to acousto optical or electro optical modulators hoping that the prices are low. Steve I thank you very much for your great kindness and commitment to me, so if the hypotheses that I wrote above (adding 10 hz to 50 mhz and optical modulators) are not feasible then I will choose the amplitude modulation

author: Maranto
author_rank: Contributor
timestamp: January 31, 2025, 05:43:00 pm
content: Quote from: SteveThackery on January 30, 2025, 06:12:40 pmGood morning Steve and Tim I will give you a single answer so as not to create problems with the translator. So yes the line width of a laser diode "starts" from about 50 MHz, so here I was wondering if it is possible to have two modulating signals one from 50,000,000 hz and the other from 50,000,010 hz here if you can add 10 hz to 50 MHz, but if it is not possible it does not matter, maybe I will move on to acousto optical or electro optical modulators hoping that the prices are low. Steve I thank you very much for your great kindness and commitment to me, so if the hypotheses that I wrote above (adding 10 hz to 50 mhz and optical modulators) are not feasible then I will choose the amplitude modulation