One thing I don't get

[Geek Tripp]

Hello everyone, I've been quite interested in lasers lately, and I've been reading up on how they work, but there's just one thing I don't understand:

I get that atoms require a certain wavelength to stimulate photons, but what I don't understand is why it wouldn't work the same with a regular light in mirrors, but just take longer and why monochromatic is so important.

If a regular power source caused enough photons to be created, when they bounce off the mirrors, there should be enough of the wavelengths (and even multiple wavelengths) that are capable of stimulating the other atoms. I would think it only requires one photon of the right wavelength to cause a cascading effect of stimulated photons. So you would essentially have a laser of a few different colors (the colors that are capable of stimulating atoms to give off photons) instead of one.

Clearly I'm missing something, but I just don't see it. Please help.

 

[diachi]

Hello everyone, I've been quite interested in lasers lately, and I've been reading up on how they work, but there's just one thing I don't understand:

I get that atoms require a certain wavelength to stimulate photons, but what I don't understand is why it wouldn't work the same with a regular light in mirrors, but just take longer and why monochromatic is so important.

If a regular power source caused enough photons to be created, when they bounce off the mirrors, there should be enough of the wavelengths (and even multiple wavelengths) that are capable of stimulating the other atoms. I would think it only requires one photon of the right wavelength to cause a cascading effect of stimulated photons. So you would essentially have a laser of a few different colors (the colors that are capable of stimulating atoms to give off photons) instead of one.

Clearly I'm missing something, but I just don't see it. Please help.

You can have lasers that produce multiple colours simultaneously - many gas lasers are capable of this for example. See this image, all wavelengths produced from one laser:

All down to the optics/gain medium.

Helium-Selenium metal vapour lasers are capable of up to 24 different lines between UV and red - many, if not all of them, simultaneously. Depends on the optics/tube pressures.

 

[Geek Tripp]

You can have lasers that produce multiple colours simultaneously - many gas lasers are capable of this for example. See this image, all wavelengths produced from one laser:

All down to the optics/gain medium.

Helium-Selenium metal vapour lasers are capable of up to 24 different lines between UV and red - many, if not all of them, simultaneously. Depends on the optics/tube pressures.

Oh okay. So does it require a particular gas because certain materials can only stimulate photons at certain frequencies, but that gas can stimulate photons at multiple frequencies?

 

[LEDbeam]

Beyond this, there's something that I don't get either! Why can't a regular LED be used as a laser diode? LED's for flashlights put out a single color, so how are they different from a laser diode?

 

[diachi]

Oh okay. So does it require a particular gas because certain materials can only stimulate photons at certain frequencies, but that gas can stimulate photons at multiple frequencies?

Yes, only certain gain mediums (Argon/Krypton gas mix in this case) are capable of it. There are others - HeNe (Helium-Neon) can manage it, although it's uncommon. And as I mentioned HeSe (Helium Selenium, with Selenium in the gas phase). There are others.

Beyond this, there's something that I don't get either! Why can't a regular LED be used as a laser diode? LED's for flashlights put out a single color, so how are they different from a laser diode?

For starters LEDs don't have the optics required to be a laser, you need a High Reflector and an Output Coupler for lasers to work (In most cases anyway). The other issue is the semiconductor itself, that also needs to be suitable for lasing action to occur.

Keep in mind LEDs - even single colour LEDs - aren't quite monochromatic like a laser diode, they usually have a fairly wide "band" of light that they emit.

Emission spectra of various light sources. Top to bottom: CFL, CFL blacklight, red LED, amber LED, green LED, green LED again with blooming, blue LED, blue LED shining through piece of fluorescent orange material, white LED 1, white LED 2. Notice how the LEDs are more of a "smudge" than a single band like you'd see from a laser. Some of that "smudginess" may be down to the equipment too, but not all of it.

Here's another one with a laser for comparison.

 

[Cyparagon]

^ "Green laser 484nm" made me giggle ^

Why can't a regular LED be used as a laser diode?

Look up what LASER stands for, and the answer should be obvious.

LED's for flashlights put out a single color

Maybe most of them output one "color", but they are FAR from monochromatic in the single wavelength sense.

I would think it only requires one photon of the right wavelength to cause a cascading effect of stimulated photons.

Right. The assumption you're overlooking is the presence of a population inversion. Lasing cannot happen without one.

 

[diachi]

^ "Green laser 484nm" made me giggle ^



Look up what LASER stands for, and the answer should be obvious.



Maybe most of them output one "color", but they are FAR from monochromatic in the single wavelength sense.



Right. The assumption you're overlooking is the presence of a population inversion. Lasing cannot happen without one.

Yeah ... not sure where they got that number from - perhaps from a spectroscope that wasn't quite accurate, or maybe 484nm (What even has a line at 484?) looks green to them?

Sounds like a fundamental understanding of how lasers actually work is perhaps missing here. You may need a bit of basic physics knowledge to grasp it fully so some other reading may be required.

The Wiki article may be of some use: https://en.wikipedia.org/wiki/Laser

There are likely simpler explanations out there if you do some googling.

Understanding the meaning behind the different parts of the acronym LASER is helpful. Especially the A, S and E parts. (Cyparagon already mentioned this, just expanding on his point).

Light Amplification by Stimulated Emission of Radiation.

 

[H2Oxide]

What even lases on 484nm? Maybe a doubled Ytterbium Line?

Anyway, these guys have done a pretty good job of explaining it, though one thing I'd like to point out is that you actually can have lasing without a population inversion if you can find a way to induce quantum coherence in the atoms in the gain medium. Though it's highly impractical and for all normal lasers lasing can't be achieved without it.

 

[Geek Tripp]

Right. The assumption you're overlooking is the presence of a population inversion. Lasing cannot happen without one.

I guess I'm a little confused now. I thought I was correct in my assumption. I understand a population inversion is required. That's why I mentioned that different wave lengths can cause this, but I didn't know most atoms only shed 2 protons with one particular wavelength. I knew that certain wavelengths were required, but I thought each atom could use each one. For example, I know the blue, red and green waves are able to cause the excited atoms to shed 2 photons, so I figured you could have a laser that could essentially have a light that shines both blue, red and green. But I see now that only certain gases are capable of that.

 

[diachi]

I guess I'm a little confused now. I thought I was correct in my assumption. I understand a population inversion is required. That's why I mentioned that different wave lengths can cause this, but I didn't know most atoms only shed 2 protons with one particular wavelength. I knew that certain wavelengths were required, but I thought each atom could use each one. For example, I know the blue, red and green waves are able to cause the excited atoms to shed 2 protons, so I figured you could have a laser that could essentially have a light that shines both blue, red and green. But I see now that only certain gases are capable of that.

Protons?

If the atoms were "shedding protons" they'd be come different elements.

The atoms don't shed anything ... well, I guess in gas ion they do, because it's gas ion... Anyway, the photon is emitted by an electron jumping up an energy level and then jumping back down to it's original state, releasing a photon with energy equal to the energy lost by the electron in the process. Simply shedding an electron isn't enough - the electron has to revert to it's original energy state to emit a photon.

 

[Geek Tripp]

Also, you could use an LED to cause the atoms to go into an excited state, right? I think that's what he was asking. I don't really know why you couldn't. It would just be used instead of say electricity. Now, the LED would have to be pretty powerful to do that of course.

 

[Geek Tripp]

Protons?

The atoms don't shed anything ... well, I guess in gas ion they do, because it's gas ions... Anyway, the photon is emitted by an electron jumping up an energy level and then jumping back down to it's original state, releasing a photon with energy equal to the energy lost by the electron in the process.

I meant photon. My mistake. Shed photons, and I also didn't really mean shed, but I couldn't think of the word.

 

[Geek Tripp]

Basically, what I understand is that when the atoms are excited and then lower into the ground state, they "shed", for lack of a better word excess energy as a photon. Then, as the photons bounce back and forth through the mirrors, this makes the excited atoms that haven't gone back to the ground state to release two of the same photons in the same pattern instead of at random times.

My confusion was that I know they only do this with certain wavelengths, but I thought each atom could do it with any of the certain wavelengths. For example, I know the red, green and blue wavelengths can cause an atom to shed, but from what I understand, each atom that requires red, can't shed for green, so that makes more sense now if I understand correctly. That's essentially why it requires monochromatic.

Though there are exceptions to the rule in the gases you mentioned.

 

[diachi]

Basically, what I understand is that when the atoms are excited and then lower into the ground state, they "shed", for lack of a better word excess energy as a photon. Then, as the photons bounce back and forth through the mirrors, this makes the excited atoms that haven't gone back to the ground state to release two of the same photons in the same pattern instead of at random times.

My confusion was that I know they only do this with certain wavelengths, but I thought each atom could do it with any of the certain wavelengths. For example, I know the red, green and blue wavelengths can cause an atom to shed, but from what I understand, each atom that requires red, can't shed for green, so that makes more sense now if I understand correctly. That's essentially why it requires monochromatic.

Though there are exceptions to the rule in the gases you mentioned.

Ahh yes, you got it, mostly anyway.

When an excited atom encounters a photon it drops back down to its original energy state and releases another photon with the same energy(Wavelength and energy are proportionate), phase and direction as the photon that it encountered. This only happens on one axis (probably a better word than axis that I could use...) as any photons that aren't bouncing between the mirrors of the resonator either are absorbed or leave the resonator. Of course, some of the light passes through the output coupler but it's a small enough amount that enough photons remain within the resonator to maintain population inversion.

Simple-ish explanation... Could probably have explained that better but I've had a few whiskys!

 

[H2Oxide]

Basically, what I understand is that when the atoms are excited and then lower into the ground state, they "shed", for lack of a better word excess energy as a photon. Then, as the photons bounce back and forth through the mirrors, this makes the excited atoms that haven't gone back to the ground state to release two of the same photons in the same pattern instead of at random times.

My confusion was that I know they only do this with certain wavelengths, but I thought each atom could do it with any of the certain wavelengths. For example, I know the red, green and blue wavelengths can cause an atom to shed, but from what I understand, each atom that requires red, can't shed for green, so that makes more sense now if I understand correctly. That's essentially why it requires monochromatic.

Though there are exceptions to the rule in the gases you mentioned.

It all has to do with the absorption and emission spectra of the atoms in the gain medium. Almost every laser is capable of lasing on different wavelengths, though not always simultaneously. The same Nd:YVO4 in a typical green laser pointer could theoretically also create the primary harmonic for red and blue lasers.

For example, here's the energy level structure for Nd:YAG:

As you can see, it's not limited to just one emission line, but 4 of them.

For example, the 589nm yellow handhelds from CNI that are fairly common around here use an Nd:YAG crystal lasing on the 1064nm and 1319nm line simultaneously.

 

[Cyparagon]

Also, you could use an LED to cause the atoms to go into an excited state, right?

You're talking about optical pumping. The decay rate (which results in spontaneous emission, not stimulated emission)of a given lasing medium is very short, which means you need a LOT of optical energy in a small area to achieve population inversion. That is why only ultra-high intensity light sources are used for pumping. Flash lamps, arc lamps, or other lasers are common devices for this purpose.

Also, even if an LED could somehow be modified for ultra-high power, it would not necessarily have the emission spectra to align properly with the lasing medium's absorption bands, resulting in huge amounts of wasted energy.