DPSS process

[artix]

I have wondered what kind of crystals are in Red DPSS lasers and violet DPSS lasers Does any body know how these lasers work? :-?

 

[steve001]

Legions of people know how these work. You can too by reading SAMS LASER FAQ. It's one of the best sources for info. There are other sites too, just Google

 

[Murudai]

I don't think he's talking about the general DPSS process, I think he's more asking about how Red and Violet DPSS specifically works.

 

[FrothyChimp]

671nm red DPSS is a SHG of 1342nm minor lasing line of Nd:YAG or Nd:YVO4.

 

[artix]

671nm red DPSS is a SHG of 1342nm minor lasing line of Nd:YAG or Nd:YVO4.

AWESOME!(?) But isn't it better to just use a diode?
Why is the 435nM violet laser only 1mW? (and expensive!)
(last question(for this post)) What are the advantages of having a DPSS laser versus a diode laser?

 

[Murudai]

DPSS gives much better beam specs than diode lasers do.

If you've got a greenie (which is DPSS) and a red (which is almost always a diode) then put them together and compare the beam specs. The greenie should have a tighter beam and better divergence.

 

[artix]

DPSS gives much better beam specs than diode lasers do.

If you've got a greenie (which is DPSS) and a red (which is almost always a diode) then put them together and compare the beam specs. The greenie should have a tighter beam and better divergence.

At about 100m (meters) my pulsar has a smaller dot then my core...

 

[FrothyChimp]

for a hobbyist a diode is fine but what is the beam quality (M[sup]2[/sup]) value of a 671 DPSS versus a red diode? 1.2 versus about 20. Beam quality generally sucks using a diode.

435nm is very inefficient (a new SHG type so very low power). They are really novelty items at this point.

 

[Murudai]

[quote author=Murudai link=1219620570/0#5 date=1219640151]DPSS gives much better beam specs than diode lasers do.

If you've got a greenie (which is DPSS) and a red (which is almost always a diode) then put them together and compare the beam specs. The greenie should have a tighter beam and better divergence.

At about 100m (meters) my pulsar has a smaller dot then my core...[/quote]

Haven't you seen the Wicked Site? The Pulsar has negative divergence, it actually focuses to a distant point before diverging again.

And also, that's just a matter of focusing it. If you focus it to infinity, you can get tiny divergence. Plus you can get nice divergence on some diode lasers because the beam is so fat.

Like Frothy said, the beam quality is way better on DPSS systems than diode systems.

 

[Murudai]

Just found a good example!

Two 500mW red systems from Dragon Lasers. One's a diode system, other is DPSS:

http://www.dragonlasers.com/catalog/655nm-Diode-Laser-500mW-p-16328.html
http://www.dragonlasers.com/catalog/671nm-Red-Laser-500mW-p-16306.html

You can even see from the pictures that the diode one has much worse beam specs.

 

[Cyparagon]

Not only does the pulsar have negative divergence, it has an initial 4mm beam while the core has a 1mm initial beam.

 

[Foobario]

But isn't it better to just use a diode?

Diodes can't make very many colors. They are really only good at making infrared and red.

When you pour the right kind of energy into some materials, electrons in certain shells will absorb that energy and jump up to a new level. When they fall back down to their previous level, they release a photon whose energy is dependent on the specific electron transition. Since the energy of a photon is related to its wavelength, each of these transitions produces a specific characteristic color.

There are a couple of problems with this when it comes to making visible-light lasers: first, to make light 'coherent' you need to be able to trigger the electrons so they all fall at the same time (this is the 'stimulated emission' part of the acronym LASER). Second, and more importantly for your question, there are only so many different ways an electron can switch states, so there are a finite number of wavelengths that can be produced using this property. Making matters worse, only a small portion of those wavelengths are detectable by the human eye.

Frequency and energy are related; it stands to reason that the easiest transitions to make will be the lowest energy transitions. And, indeed, infrared and red lasers have been the most common lasers for a long time. (Note that despite the cultural convention that red is 'hot' and blue is 'cold', red is the lower energy state. You can prove this to yourself by heating something up - what's the first color it turns? Red. And before it emits red, it emits invisible infrared.) Making higher-energy transitions gets more complicated, because an electron that has jumped up multiple levels may choose to jump down in different-sized steps, each of which will produce a different mix of wavelengths. This will prove useful in a minute, when we talk about filtering wavelengths.

So making lasers with diodes, we are limited by the types of transitions that occur, by our ability to stimulate and filter the emitted wavelengths, and by our ability to see certain frequencies. Add these all together and we end up with a situation where it is easy to generate infrared and red, but much harder to generate other colors. Generating other colors requires using a different process than the one occurring in the diode. This is where 'DPSS', diode-pumped solid-state lasers come in.

It's easy to make powerful infrared diodes. 808nm is particularly easy to make. Using 808nm infrared to pump exotic materials like neodymium-doped yttrium aluminum garnet (Nd:YAG) crystals, we can make those crystals emit photons with a wavelength of 1064nm, which is even farther away from the visible spectrum. But then those photons can be sent through a non-linear optical material like KTP (Potassium titanyl phosphate) which has the useful property of doubling certain frequencies. Doubling frequency means halving wavelength, so the KTP emits 532nm, which is a color of green very near to the 555nm peak sensitivity of our eyes.

I left something out of the preceding description, though... remember when I said that higher-energy transitions can take a number of different paths down, each of which will emit a different wavelength? In a green DPSS laser, the Nd:YAG is covered by a filter that reflects or absorbs everything except 1064nm (if it didn't do this, a lot of the 808nm would come through as well as the unwanted wavelengths generated in the crystal). But different filters can be used to pass different wavelengths - Nd:YAG also emits, for instance, 946nm. If we cover the Nd:YAG with a filter that only passes 946nm, the KTP will double that frequency, creating 473nm blue light. Yellow lasers use a similar (though slightly more complex) process, but you get the idea.

 

[pullbangdead]

Good description of DPSS, but the case for a diode laser is not exactly that. For a laser diode, it is no longer really about electron transitions between shells (as is the energy levels around individual atoms). It is about the bandgap of a material. With blackbody radiation, gas lasers, etc, it is about localized electrons changing their energy levels. But with diodes, it is about electrons and holes in the overall lattice, which are not localized to individual shells, and it is about recombining those holes and electrons. The light emitted will correspond to the energy of the bandgap of the material in the diode's active layer. It's not about what you can get the electrons to do to take different paths from high energy to low energy. With gas lasers and with the crystals in a DPSS, yes, you have electrons at high energies and they take multiple paths to ground states to give off multiple wavelengths. With a laser diode, you have 1 bandgap, and therefore one wavelength that the laser diode can give off, and that bandgap is still a function of energy levels of atoms, but it is not the same thing.

The difficulty comes with making a material of the wavelength that you want and the ability to give off all the bandgap energy in the form of a photon, as well as the ability to get electrons/holes into and out of the material. It was easier to make a material with a lower bandgap and the ability to efficiently emit light, for a variety of reasons. Suitable materials were found much sooner for longer wavelengths, because those materials are much easier to make. You have to be able to make the semiconductor conduct holes or electrons (when you want it to), and it is much easier to do with a lower bandgap material. Lower bandgap means less severity in making the material conduct. To go to a higher bandgap, like gallium nitride for violet/blue laser diodes, it is much more difficult to get all the properties you need out of the material. It proved difficult to make p-type gallium nitride, and VERY difficult to make p-type gallium nitride that could conduct those holes/electrons when you wanted it to.

Diodes now are capable of working on the long wavelength regime (red, IR), and also capable of working on the short wavelength regime (UV, violet, blue), but it is a function of the bandgap of the materials you are working with and engineering the bandgap to be what you want it to be, not a function of getting certain states to work or filtering the output.

With diode lasers, it is exactly as easy to make an 800nm laser as it is to make an 808nm laser as it is to make an 815nm laser. We now have the ability to change the bandgap of one material system and move the bandgap around within a certain range. The farther from the central value it is, the harder it is to make.

Gallium nitride, for example, has a bandgap that is right around near UV/violet. To make a laser in the visible range, we add indium to lower the bandgap, moving it into the violet range. To make blue, add more indium, and the bandgap gets lower still. Anywhere inbetween the central value of gallium nitride and the maximum amount of indium that you can put into the gallium nitride is a valid point, and a light emitting device can be made at that wavelength. The hope is that the limit to the amount of indium that can be put into gallium nitride is high enough to get green laser behavior out. but the father you get from pure gallium nitride, the harder it is to do, and the "less good" the laser will be.

Also, stimulated emission does not mean that they all fall at one time. It means that a photon triggers each electron-hole recombination, which will result in the new photon being in-phase/coherent with the photon that stimulated it.

 

[Foobario]

I was trying to leave words like 'bandgap' out of my explanation. For people who know what it is, no explanation is necessary... and for people who don't know what it is, a lot more explanation is necessary. But everyone knows about electrons and shells, and the dynamics still work in a hand-wavingly approximate manner.

There's no point in describing something at the Uni level when the original question was asked by someone who didn't think to Google the answer first.

 

[scopeguy20]

Great inf. pulbangdead!

 

[hydrogenman15]

for a hobbyist a diode is fine but what is the beam quality (M[sup]2[/sup]) value of a 671 DPSS versus a red diode? 1.2 versus about 20. Beam quality generally sucks using a diode.

435nm is very inefficient (a new SHG type so very low power). They are really novelty items at this point.

Yeah,but let just say the focus on my red is as perfect as it can get. I was under the impression that
red had BETTER divergence then DPSS's. :-?

--hydro15