To produce phase cancellation both frequencies must be present at the same time,and be the same wavelength.. and blinking a light off and on will not shift the frequency.

yes, I obtain the same frequency of the two beams (arms) using a single source, in my case the LED, only that my initial reasoning told me that since the LED is incoherent, when the two beams interfere they do not produce any beat of the intensity modulation (and not optical), but Danko instead confirmed to me that even with the LED you can obtain the beat of the intensity modulation (turn on and off between 40 and 50 Hz), remaining unchanged the optical frequency of 333 THz, so what do you think??

 

Interesting: Checking the movements of ion concentrations in a tank.That would seem to be a higher level graduate school sort of project.
What sort of physical properties vary with ion concentration?? And how will you detect the variation with 900NM illumination???
I am thinking that mechanically scanned laser beams will be quite a bit simpler.

 

Dc_kid Thanks for your reply

I am not all up on my physics on this specific subject, but I guess you could just AM modulate a 904nm LED to get the beat. What that specific AM modulation looks like is TBD. Feeding a 10Hz sine into a AM mixer, etc. LED is the carrier, current control via AM gets you a beat, but this is not the same as the laser examples.

 

Interesting: Checking the movements of ion concentrations in a tank.That would seem to be a higher level graduate school sort of project.
What sort of physical properties vary with ion concentration?? And how will you detect the variation with 900NM illumination???
I am thinking that mechanically scanned laser beams will be quite a bit simpler.

I have already tried laser beams but visible light does not penetrate the tank and neither does UV, and microwaves are difficult to direct, the only choice falls on near infrared, but having a limited budget I cannot purchase DFB lasers and arrays, so I was wondering if I could use infrared LEDs which are much cheaper than infrared laser diodes, also considering the part of the optical lenses to collimate and diverge the beam, but after having said this, I repeat my question, a LED diode in the configuration I had described before, that is, splitting it into two beams and then pulsing each single beam at different frequencies, produces beats (not of the optical frequency, but of the beat frequency?

 

I am not all up on my physics on this specific subject, but I guess you could just AM modulate a 904nm LED to get the beat. What that specific AM modulation looks like is TBD. Feeding a 10Hz sine into a AM mixer, etc. LED is the carrier, current control via AM gets you a beat, but this is not the same as the laser examples.

AM modulation has a less steep rise curve than PWM, and in practice this gives me less peak power, a wider spectrum (LED), and more difficult-to-find hardware. I've done a lot of research online, and it confirms that using a single LED diode as a source and keeping the two arms the same distance apart, I can get the beats, because the beams, even if they come from an LED source, have the same coherence between them, even if the beat (50 Hz - 40 Hz) will not be as clean as with lasers.

 

Now it is puzzling me as to how a string of optical nulls is going to sense whatever property a variable ion concentration in a water tank. A possible alternative could be a switched string of IR LEDs with a sensor to detect the variations. I am still wondering about the optical effect of a variable ion concentration. Or is that the secret part of this research project?? I am thinking that none of the optical items mentioned are at all cheap. Not from EDMOND at least.

 

Now it is puzzling me as to how a string of optical nulls is going to sense whatever property a variable ion concentration in a water tank. A possible alternative could be a switched string of IR LEDs with a sensor to detect the variations. I am still wondering about the optical effect of a variable ion concentration. Or is that the secret part of this research project?? I am thinking that none of the optical items mentioned are at all cheap. Not from EDMOND at least.

I'm Italian, and in Italy we often say a phrase, "You asked yourself the question and you gave yourself the answer." It's not an offensive phrase, on the contrary, it means you're very intelligent. In fact, if I were to use a series of switched IR LEDs with a detector, I would be using a technology already used and known by everyone. Instead, I'm studying a new technology. The 10 Hz is arbitrary. I could use a different intensity pulse frequency, but I have to do the experiments. That's all, mine is a study I'm conducting to improve the lives of many farmers. If I succeed, it will be a success, and if I fail, it will have been an experience. So, after all this, in the end, can you tell me if the single IR LED, in which the beam is divided into two arms and each arm is pulsed at different frequencies, the first at 40 Hz and the second at 50 Hz, when they interfere, produce an intensity modulation pulse?

 

Here is a serious question: WHAT is the benefit of using an alternative technology that seems to me will require some quite costly components??? If there is already a low cost way to do the evaluation?? My guess is that expensive optical components will also need calibration.

 

I am a bit confused on the 40Hz 50Hz "arms" of a coherent source. The Michelson interferometer (interfere-o-meter) is based on superposition of monochromatic waves. If you have a LED at 900nm, or a laser at Xnm, the source is of single frequency.

If the need is just 10Hz of 900nm, then just use 10Hz PWM to drive the LED, you'll get 10-per-sec 900nm pulses of coherent waves. If you need more or less dwell time while keeping 10Hz, then just change % PWM.

Maybe another idea is to bounce 900nm off of a vibrating mirror? Which will technically make it FM of fixed amplitude, no interference.

I am curious, would you straighten LED waves (900nm) using fresnel lens, a sorta 900nm spotlight?

 

Now this whole venture, and not wanting to use the same scheme as others have done, evidently with success, causes me to guess that this is a grad school research project and we are being asked to do "the homework". Is that acceptable??
Post #27 seems to sort of verify that: "In fact, if I were to use a series of switched IR LEDs with a detector, I would be using a technology already used and known by everyone. Instead, I'm studying a new technology. "
Those words: "studying a new technology" tend to verify my guess.

 

@DC_Kid Good morning, I have already explained that if I use a single source without dividing it and therefore create a single beam, this propagates in a straight line for many centimetres and I cannot know how far it reaches and how much of it reaches, however if I use two beams, I can make them interfere in one point and therefore I know in which points there is the frequency that interests me

 

@MisterBill2 Good morning, yes, I would like to create a new technology that is cheaper and more precise. Obviously, the cost difference between LEDs and lasers is significant. For an LED system (with lenses), I could spend around $30 for an entry-level setup, while for the laser counterpart, I'm looking at close to $100. However, I didn't ask you to carry out my entire project, but only on this one question of mine that I can't resolve: whether a pulse of two pulsed beams from a single LED source produces a beat, that's all, and I'll take care of the rest myself.

 

The gating on and off of a signal does not alter the frequency, since the OFF/On modulation does not alter the phase. What would alter the phase relationship is the distance traveled between beams.
That effect applies to both sound waves and radio waves, as well as to light waves. It has been demonstrated with coherent laser beams, I believe. But generation of a phase shift in a coherent beam would only require a distance change of half a wavelength. Amplitude modulation does not alter that. The piezo effect changing the length of a transmission medium should be able to produce a phase shift.

 

The gating on and off of a signal does not alter the frequency, since the OFF/On modulation does not alter the phase. What would alter the phase relationship is the distance traveled between beams.
That effect applies to both sound waves and radio waves, as well as to light waves. It has been demonstrated with coherent laser beams, I believe. But generation of a phase shift in a coherent beam would only require a distance change of half a wavelength. Amplitude modulation does not alter that. The piezo effect changing the length of a transmission medium should be able to produce a phase shift.

I've reread your answer many times. Yes, to have a beat, there must be a phase shift, and in my case, the phase shift is due to the pulse frequency, not the optical frequency. In fact, the source is the same. In amplitude modulation, which is very similar to PWM modulation, there are points where the interfering waves are strengthened, and in other points or moments where the waves cancel each other out. So, if I understand correctly, in my case, the two waves cancel each other out when the pulse frequency coincides, and therefore there is coherence between the two waves. That is, in my case, it's not the optical frequency that needs to change, but the final pulse frequency, that is, reaching 10 Hz starting from the two pulses of 50 and 40 Hz.

 

There may be variations with cancellation and boosting of pulses but not of the light waves. In fact, with LEDs, which are non-coheret, the only effect will be between pulses.
There are probably photonics masters who can provide much better analysis than I can.

 

Just out of interest. Let's are 10 cents , have you tried it ?

 

Just out of interest. Let's are 10 cents , have you tried it ?

@Dr John: it seems that your autocorrect has not been ware of LEDs. AND, only the cheap ones are that cheap.

 

There may be variations with cancellation and boosting of pulses but not of the light waves. In fact, with LEDs, which are non-coheret, the only effect will be between pulses.
There are probably photonics masters who can provide much better analysis than I can.

Yes, I believe so too, the beat will be between the pulses (which is what interests me), and not between the optical frequency

 

You can use two LED's, one fixed and another that can move at fractions of 1/1000th inch, like 1/x X 1/1000.
This allows you to phase shift.

However, the desired effect cannot be achieved because the frequency of LED will be constantly changing. Even a small fraction of wavelength change impacts the desired effect.

 

You can use two LED's, one fixed and another that can move at fractions of 1/1000th inch, like 1/x X 1/1000.
This allows you to phase shift.

However, the desired effect cannot be achieved because the frequency of LED will be constantly changing. Even a small fraction of wavelength change impacts the desired effect.

The TS just wants to see the ~10Hz beat frequency of the flashing, not of the differences between the ~333THz light sources.