Question about wavelength and power

Okay, just a quick question that I've been wondering about. People say higher wavelengths burn better, because a single photon has more energy. Doesn't this mean the wattage of the laser also goes up? Then why does a 200mW 405nm burn better than a 200mW 650nm? The amount of power is exactly the same.

Yes, lower wavelength's tend to burn better due to more energy per photon, but that isn't the full story. It also depends on what you are trying to burn, see? Some material will easily absorb 650nm light whilst 405nm would be slightly reflected, leading to more power being required!

A good example of this is glass. A powerful 808nm diode would have a though time burning through it, while a CO2 laser with a wavelength of 10600nm(I believe it is? ) will easily cut through it. Sure the wavelength is far longer, but almost everything is absorbed by it

-Alex

output power is output power --- the shorter wavelength 405nm is better absorbed than longer 650 by some materials so people sasy it burns better.

A laser beam has no temperature - there is no inherent "temperature" to a laser beam. Heat is the random motion of matterparticles (atomic or molecular particles).
A laser beam itself is not made of matter but of ‘photons, which have no mass, thus a laser beam can have no temperature.
"Heat" is caused by a laser beams energy being absorbed by a material surface and turning light energy into heat energy.

doubleone44 said:

Okay, just a quick question that I've been wondering about. People say lower wavelength burn better, because a single photon has more energy. Doesn't this mean the wattage of the laser also goes up? Then why does a 200mW 405nm burn better than a 200mW 650nm? The amount of power is exactly the same.

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That's higher wavelength not lower, 405nm is higher frequency/shorter wavelength than 650nm, and yes it's true the photons have more energy as you move toward the shorter wavelengths (UV) and less energy as you move toward IR, yet it's easier to make super high power lasers that are IR, that's why military lasers are IR and we have CO2 lasers in far IR that are very powerful. A 405nm and a 650nm of the same power are not the same, the 405nm will be more powerful, but there is more to it than just wavelength. Power density is very important, a higher power density will burn much better, if you have two lasers of the same wavelength and one puts a dot on something that is 1mm wide, and the second one makes a dot that is 2mm wide, then the one that is 1mm wide will burn much better. This is partly why we are always concerned with divergence of the diode, some can put a small dot on something far away and another will be a spotlight at the same distance, this of course effects power density and the distance at which it can burn.

Alan

Optically 100mW of 405nm , 445nm , 650nm , 808nm is still 100mW regardless .

"powerful" or the term used on here is just how well the wavelength is absorbed and as you said the smaller the dot the more local the heating is thus the better it burns .

Pi R Squared said:

A 405nm and a 650nm of the same power are not the same, the 405nm will be more powerful,

Alan

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By what measure of powerful?

1W of 405nm and 1W of 650nm are the same in my book ---1W is 1W is 1W.

When you're looking at total power, individual photon energy doesn't come into it EXCEPT in determining how many photons you're getting out. Total power is total power, regardless of individual photon energy.

Encap said:

By what measure of powerful?

1W of 405nm and 1W of 650nm are the same in my book ---1W is 1W is 1W.

When you're looking at total power, individual photon energy doesn't come into it EXCEPT in determining how many photons you're getting out. Total power is total power, regardless of individual photon energy.

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Exactly this!

There are several reasons why 405nm lasers are typically better at burning.

1) Higher absorption - Lots of materials tend to absorb 405nm very well - compared to other visible wavelengths.
2) Smaller beam waist (i.e it focuses to a smaller spot than say 650nm would with the same optics.)
3) High powered 405nm diodes are single mode - helping with the smaller beam waist over other similarly powerful diodes at other wavelengths.

A violet laser of the same output power as a red laser will emit fewer photons per second. About 40% fewer.

Hap said:

Yes, lower wavelength's tend to burn better due to more energy per photon

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That's not true at all. It's because lower wavelengths (in the visible spectrum) tend to be absorbed by a broader range of materials.

Thanks for the replies guys! So if I had a theoretical 100% absorbing surface, and a 1W of 405nm and of 650nm with the same dot size, both will heat up (burn) at the same speed?

Pi R Squared said:

That's higher wavelength not lower, 405nm is higher frequency/shorter wavelength than 650nm,

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Edited (always mess that up)

Cyparagon said:

A violet laser of the same output power as a red laser will emit fewer photons per second. About 30% fewer

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Very interesting. What about an x-ray laser of 1W? Is there a cool formula I can use for this?

Well I may have been mistaken, I know I have read that with the shorter wavelengths the photons have more energy, but that may just be urban legend started by someone that doesn't know what they are talking about, not an unusual thing online. I'll try to look into it more when I have more time, but in any case I think the power density is the most important (size of beam/dot).

An X-Ray laser of 1W I suspect would be very dangerous but I really have no idea, I don't know how to build one and I am not sure if we even can if we want to. X-Ray tubes are a restricted item in the U.S. They are not sold to the general public unfortunately, it would be fun to experiment but you would have to know what you're doing or it could be very hazardous.

Alan

doubleone44 said:

Very interesting. What about an x-ray laser of 1W? Is there a cool formula I can use for this?

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Not sure what you are asking but 1W of X-Ray is the same power as 1W of any wavelength.

If you want to know the energy of 1 photon of any wavelength have a look here ---plug number into calculator: Energy of Photon | PVEducation

What is it you actually want to know and why?

Encap said:

Not sure what you are asking but 1W of X-Ray is the same power as 1W of any wavelength.

If you want to know the energy of 1 photon of any wavelength have a look here ---plug number into calculator: Energy of Photon | PVEducation

What is it you actually want to know and why?

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1W of X-Ray may still be only 1W, but its effect on things isn't the same as 1W of visible light, X-Rays are very dangerous ionizing radiation, they are not reflected like visible light, they can pass through anything.

If anyone wants to experiment with X-Rays here is a tube you can buy:
https://unitednuclear.com/index.php?main_page=index&cPath=2_13

Alan

Pi R Squared said:

1W of X-Ray may still be only 1W, but its effect on things isn't the same as 1W of visible light, X-Rays are very dangerous ionizing radiation, they are not reflected like visible light, they can pass through anything.

Alan

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Yes---some X-ray photons carry enough energy to ionize atoms and disrupt molecular bonds. Most X-rays have a wavelength ranging from 0.01 to 10 nanometers. X-rays with photon energies above 5–10 keV (below 0.2–0.1 nm wavelength) are called hard X-rays, while those with lower energy are called soft X-rays. Soft X-rays, x-rays with E<1keV cannot penetrate a sheet of paper

X-rays don't penetrate everything---there is a whole range of x-ray wavelengths of different photon energies--the penetration depth varies with several orders of magnitude over the X-ray spectrum.
Xrays can be reflected --here is one way: X-ray Mirror and there is a whole field called x-ray optics see: https://en.wikipedia.org/wiki/X-ray_optics ---see also：CXRO X-Ray Interactions With Matter

and can actually be seen by the human eye, reportedly. See below
"While generally considered invisible to the human eye, in special circumstances X-rays can be visible. Brandes, in an experiment a short time after Röntgen's landmark 1895 paper, reported after dark adaptation and placing his eye close to an X-ray tube, seeing a faint "blue-gray" glow which seemed to originate within the eye itself. Upon hearing this, Röntgen reviewed his record books and found he too had seen the effect. When placing an X-ray tube on the opposite side of a wooden door Röntgen had noted the same blue glow, seeming to emanate from the eye itself, but thought his observations to be spurious because he only saw the effect when he used one type of tube. Later he realized that the tube which had created the effect was the only one powerful enough to make the glow plainly visible and the experiment was thereafter readily repeatable. The knowledge that X-rays are actually faintly visible to the dark-adapted naked eye has largely been forgotten today; this is probably due to the desire not to repeat what would now be seen as a recklessly dangerous and potentially harmful experiment with ionizing radiation. It is not known what exact mechanism in the eye produces the visibility: it could be due to conventional detection (excitation of rhodopsin molecules in the retina), direct excitation of retinal nerve cells, or secondary detection via, for instance, X-ray induction of phosphorescence in the eyeball with conventional retinal detection of the secondarily produced visible light." from https://en.wikipedia.org/wiki/X-ray

Ok...here we go. 50mW from a laser is a measure not of overall output, it is a measure of how much acting energy the photons have on a given surface/material. this is why Thermal meters are generally made of graphite, as it has a very even and high absorption of most all types of light. Hap is right, it depends on what you're burning that matters most. shorter wavelengths carry more energy per photon in general, and do tend to be absorbed more readily by most materials due to this trait, in fact, this is often resulting in florescence of some kind. But the difference between 405 and 650nm for example per milliwatt power for most materials is marginal at best for most everyday cases. In fact this is why in machining, depending on the cut you need, CO2 10.6um is used for most hrough cutting and processing while excimer UV lasers are used for precision cuts, as the UV pulses have so much more energy they cause the cuts by ablation rather than heat. but ultimately the absobtion of the material and your focus and beam quality are far more important.

ultimatekaiser said:

Ok...here we go. 50mW from a laser is a measure not of overall output, it is a measure of how much acting energy the photons have on a given surface/material. this is why Thermal meters are generally made of graphite, as it has a very even and high absorption of most all types of light. Hap is right, it depends on what you're burning that matters most. shorter wavelengths carry more energy per photon in general, and do tend to be absorbed more readily by most materials due to this trait,

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For any wavelength:
Power = Energy ÷ time
1 Watt = 1 Joule per second

Lower wavelengths burn better due more energy per photon given the same number of photons over a time of either 405nm or 650nm hitting a target that absorbs 405nm better than 650nm.
50mW of 405nm or 50mW of 650nm is still 50mW regardless of the number of photons of either wavelength required to equal 50mW. If 50mW of both wavelengths are absorbed equally well by the target, both wavelength beams having same energy density----they both would both be absorbed the same/"burn the same. No?

OK, so you are saying a 50mW laser is not a 50mW laser because there is no way to measure absolutely or be able to say what a lasers overall output power is? The output power is dependent upon/an artifact of, the object the laser beam hits be it a mirror or a piece of graphite? That sounds more like a limitation of laser power measurement technology argument to me.
Regardless of any overall output measurement device if you specify 1W it is 1W. It is 1W by definition whether you can measure it absolutely or not.

"Optical power and energy measurements are traditionally tied to SI units through electrical standards. This is accomplished by means of optical radiometers and calorimeters, designed to allow accurate comparison of absorbed optical power with dissipated electrical power." From: High-Accuracy Laser Power and Energy Meter Calibration Servicehttp://www.nist.gov/calibrations/upload/sp250-62.pdf

See:
Measuring Laser Power and Energy Output
http://www.coherent.com/downloads/aboutmeasuringlaserpowerndenergyoutputfinal.pdf


High Power Laser Calibrations at NIST
http://www.nist.gov/calibrations/upload/sp250-77.pdf

and

NIST Measurement Services:
cw Laser Power and Energy Calibrations at NIST
http://www.nist.gov/calibrations/upload/sp250_75.pdf

i'm not a pro at this but i remember shorter wavelength have higher frequency.(frequency is a rate which energy is transfer over the period time). Violet have frequency of 8 x10 to the power of 14 while Red have 4 x 10 to the power of 14 which means a 405nm laser will deliver twice as much photons compare to 660nm at any given time.