Why Focusing?

Hi All,

Could you please explain why the laser torches have focusing, given that a laser is a source of coherent collimated radiation? Just to control the spot size at a given distance? To how small a spot can you usually focus?

And one more question. A red laser radiates a longer wave and therefore lower energy beam per photon than a violet one. So when you take a 200mW red and a 200mW violet laser, they both emit the same optic power, which means that the red laser draws more energy from its power supply and, given the target absorbs/reflects in a similar fashion for both wavelenghts, the burning capability would be the same for both red and violet laser. Is that the case?

Thanks for explanations!

kJoule said:

Hi All,

Could you please explain why the laser torches have focusing, given that a laser is a source of coherent collimated radiation? Just to control the spot size at a given distance? To how small a spot can you usually focus?

And one more question. A red laser radiates a longer wave and therefore lower energy beam per photon than a violet one. So when you take a 200mW red and a 200mW violet laser, they both emit the same optic power, which means that the red laser draws more energy from its power supply and, given the target absorbs/reflects in a similar fashion for both wavelenghts, the burning capability would be the same for both red and violet laser. Is that the case?

Thanks for explanations!

Click to expand...

With my 405-G-1 (408nm coated lens) I can focus my GGW down to literal pin point (0.5mm or less). As for your power comparision between red/bluray. You are correct about the wavelength differences although 200mW=200mW. At the same output power, the lasers will have equal strength. The only advantage of a 405nm laser is that it is absorbed into surfaces at a much quicker/easyier rate over 635/660nm. Hope this helps.

-Mike

cust11 said:

With my 405-G-1 (408nm coated lens) I can focus my GGW down to literal pin point (0.5mm or less). As for your power comparision between red/bluray. You are correct about the wavelength differences although 200mW=200mW. At the same output power, the lasers will have equal strength. The only advantage of a 405nm laser is that it is absorbed into surfaces at a much quicker/easyier rate over 635/660nm. Hope this helps.

-Mike

Click to expand...

Thanks! But wouldn't the rate of absorption depend on the properties of the target? I guess for burning black surfaces the wavelength doesn't matter that much anyway.
I'm just trying to find a nice affordable model. 200mW power range seems the way to go. A brief review shows that higher-power lasers are damn expensive and don't even always develop their rated power!

kJoule;

Focusing ability lets you have a collimated parallel beam ....

or all the power focused to a point.

Good optical lenses keep the collimated beam small,
and focus to a very fine point.

LarryDFW

To put it yet another way: yes, a laser does have a collimated beam. However, that beam will be a couple of mm wide. With a lens, you can focus this down to a tenth millimeter or less. Generally, the shorter the focal length of your focusing lens, the smaller your spot can be.

Due to the wave nature of light, you can't get a beam that will stay the same diameter - it always diverges for large distances (becomes wider). The degree to which this happens depends on the initial diameter (and the wavelength). With typical lasers having a beam diameter after the collimating lens of a few mm, this divergence will be on the order of 1mrad, meaning it will get wider by 1 mm for every m travelled.

With focusing, you can "offset" this tendency for some distance (at the price of even larger divergence beyond the focusing point).

Comparing 200mW red with 200mW violet, both have the same optical power. Assuming the same laser diode efficiency, they will also consume the same input power. The current will be less for the violet, because it operates at a higher voltage. This is (in part) due to the fact that the photons it emits have a higher energy - a red one has around 1.9eV, a violet one around 3.1eV.

At the same optical power, a bluray will emit only 62% as many photons as the red one.

And yes, burning power depends on the absorption characteristics of the target. A 250mW unfocused red cannot be felt on skin, while a 100mW unfocused bluray does cause a slight stinging sensation after a couple of seconds.

Also, in a perfect world, a laser could possibly be made with zero divergence. This is not a perfect world, so lasers always have divergence. Lenses, particularly adjustable ones, accommodate this imperfection by allowing you to adjust the beam to where you want in.

dr-ebert said:

At the same optical power, a bluray will emit only 62% as many photons as the red one.

And yes, burning power depends on the absorption characteristics of the target. A 250mW unfocused red cannot be felt on skin, while a 100mW unfocused bluray does cause a slight stinging sensation after a couple of seconds.

Click to expand...

A good point. As the energy per photon depends solely on the wavelength, hence there are fewer photons for the same optic power.

As to burning, the human skin then better absorbs violet wave than red? And the eye retina is supposed to be more sensitive to the green light?

Would the violet beam also diverge less with distance?

How about humidity? Would the violet beam would be attenuated much more significantly in humid air, which is supposed to absorb better at higher frequencies. But would there be nuch difference between 680 and 400nm?

Sorry for barrage of questions and thanks.

150mw of Violet, stings a LOT more than my 300mw Red. Just ask some of the forum members with 300mw 8x Blu ray diodes!

I cannot answer for you about humidity though.. sorry.

As to efficiency. Assuming the green diodes are 10% efficient. Then just measuring drawn power would give a good indication if you hold a fake 200mW or not, for which you would expect a 2W consumption.

green diodes aren't commercially available. so to acheive green you need an 808nm ir diode to be pumped through 2 crystals to get frequency doubled and then wave length halved to get 532nm (green) output. so using green for your examples isn't very good.

Even in a perfect world, you'd get divergence. It is caused by the wave nature of light, not by technical imperfections. Because of its lower wavelength, a violet beam would have less divergence than a red one, all other things being equal.

Yes, skin is partially transparent to red light. That's why you see a reddish glow if you hold a (white) flashlight behind your fingers.

The visual sensitivity of the retina is highest at 555nm, i.e. green. However, it can be thermally damaged by all wavelengths, so a red laser would not be safer than a green one, even if it looks much dimmer at the same power.

Shorter wavelengths get scattered more in air, so a violet beam would get attenuated more than a red one.

Efficiency of laser diodes was discussed here. However, for greens, I guess no fixed value can be given, as the actual output depends a lot on the quality of the crystals and their alignment, so only limited conclusions can be drawn from the input current (which will only tell you something about the output of the IR pump diode).