Arpit:

Whether an optic is damaged or not depends on several things. Depending on conditions, the mirror may do better than the lens. The reverse could also be true in a different context. Peak power density (watts per unit area) plays a major roll. But other characteristics like if the laser is CW or pulsed, pulse width, repetition rate, average power and wavelength also play significant rolls. This is because the mechanism of damage varies depending on these parameters.

For example, I work with an Nd:YAG laser with 25ps pulse width and 20Hz PRF. The average power is quite low, but the peak power is nearly 1GW. Because of this, it's quite easy to damage optics if the beam is compressed too tightly.

And, of course the optic itself is important, as you have observed. Anti-reflective coated optics at first look like they would have a higher laser induced damage threshold (LIDT), but that's generally not true. Why? Applied coatings tend to degrade rapidly, leading to catastrophic failure. Generally, uncoated optics have higher LIDTs. Again, that is not always true either.

You might also think that dielectric mirrors would have high LIDTs. But that's not really what happens. True, more energy is reflected due to higher reflectivity, but simple energy absorption is not the only mechanism of damage. If the power density is high enough, the electric fields can be so high as to damage the molecules in the coating (breaking chemical bonds), even though no significant energy is deposited in the film. You might say that one needs UV light to break chemical bonds, but that's not true. Indeed, UV light can break bonds, but even longer wavelengths can break bonds too if the intensity (electric fields) is high enough. This illustrates two of several different damage mechanisms that can be present.

In general though, if your 10kW laser is focused on a spot such that the energy density is greater than about 10MW/cm^2, you're going to have problems. Hence, the key to manipulating high power lasers is to keep the power densities as low as practical. This can have several benefits. First, you won't damage your optics. Second, a larger beam is easier to collimate and will show lower diffraction limited divergence. Third, the lower energy density in the beam will induce less atmospheric heating and be less sensitive to air currents. Compress the beam only at the very last stage in your optical train. Even so, you might need to simply accept that damage will occur in these later few stages and be prepared to replace those optics periodically.

Sometimes, you can keep the entire system in a 'low energy density' configuration and let the last optic be focusing (mirror or lens). I use this strategy in several of the instruments I work with. That way the high energy density region is entirely outside your optical system. This might not be possible or practical in the system you are considering though.

Arpit, I hope this helps!

Dear Arpit,

The lens or mirror will both be damaged basically because of too much power absorption. In this manner, the mirror usually withstand higher power than the lens because for it to work the laser beam just go through a thin layer of coated film (metal, dielectric) => less material absorption.

When you purchase a lens or mirror, consider the available data from the manufacturer website which contain the damage threshold for each component (in J/cm^2 for pulsed laser, or in W/cm^2 for CW laser) .

https://inis.iaea.org/collection/NCLCollectionStore/_Public/25/028/25028722.pdf

https://pdfs.semanticscholar.org/27e7/d27c242a518d2ba8661af90d0f07d1550352.pdf

https://link.springer.com/article/10.1007%2FBF00326492

Arpit:

Whether an optic is damaged or not depends on several things. Depending on conditions, the mirror may do better than the lens. The reverse could also be true in a different context. Peak power density (watts per unit area) plays a major roll. But other characteristics like if the laser is CW or pulsed, pulse width, repetition rate, average power and wavelength also play significant rolls. This is because the mechanism of damage varies depending on these parameters.

For example, I work with an Nd:YAG laser with 25ps pulse width and 20Hz PRF. The average power is quite low, but the peak power is nearly 1GW. Because of this, it's quite easy to damage optics if the beam is compressed too tightly.

And, of course the optic itself is important, as you have observed. Anti-reflective coated optics at first look like they would have a higher laser induced damage threshold (LIDT), but that's generally not true. Why? Applied coatings tend to degrade rapidly, leading to catastrophic failure. Generally, uncoated optics have higher LIDTs. Again, that is not always true either.

You might also think that dielectric mirrors would have high LIDTs. But that's not really what happens. True, more energy is reflected due to higher reflectivity, but simple energy absorption is not the only mechanism of damage. If the power density is high enough, the electric fields can be so high as to damage the molecules in the coating (breaking chemical bonds), even though no significant energy is deposited in the film. You might say that one needs UV light to break chemical bonds, but that's not true. Indeed, UV light can break bonds, but even longer wavelengths can break bonds too if the intensity (electric fields) is high enough. This illustrates two of several different damage mechanisms that can be present.

In general though, if your 10kW laser is focused on a spot such that the energy density is greater than about 10MW/cm^2, you're going to have problems. Hence, the key to manipulating high power lasers is to keep the power densities as low as practical. This can have several benefits. First, you won't damage your optics. Second, a larger beam is easier to collimate and will show lower diffraction limited divergence. Third, the lower energy density in the beam will induce less atmospheric heating and be less sensitive to air currents. Compress the beam only at the very last stage in your optical train. Even so, you might need to simply accept that damage will occur in these later few stages and be prepared to replace those optics periodically.

Sometimes, you can keep the entire system in a 'low energy density' configuration and let the last optic be focusing (mirror or lens). I use this strategy in several of the instruments I work with. That way the high energy density region is entirely outside your optical system. This might not be possible or practical in the system you are considering though.

Arpit, I hope this helps!

Dr. Patric has clarified the all the probabilities of laser damage threshold. However , some important points have to be remembered, that is energy per unit area (J/cm2) for mirror as well as lens should be within the damage threshold. Another thing is that you can minimize the probability of damage threshold but can not avoid totally. So after multiple shots you need to change the optics, which are being used for higher energy density laser.

I would check the damage threshold for the mirror coating (metal or dielectric), which should be on the manufacturer's website. For the lens, I think that the AR coating on it is mainly at risk. The lens' bulk material (typically, glass) might deform due to melting in the worst case scenario. The max temperature in the lens can be estimated knowing the material absorption and heat capacity of glass... without going into some heat transfer simulations.

Make the beam incident on your components as wide as possible.

I have always used dielectric coated mirrors to steer optical beams because they carry less beam distortion. But to be accurate I have never lost a transmission optical component but have burnt out good number of mirrors. My answers is to define what you require to do with the beam after it has been through or reflected off the component in question.

There has been many answers regarding damage power. I would only add that another important factor to consider might be the nonlinear effects, which start happening at lower powers than those causing damage. These nonlinear effects might be noticed in transmitting medium at lower powers than in reflective medium if the nonlinear refractive index n2 is considered. This could determine 'withstand' ability (depending on what we mean by this word), although no damage would occur in such power regimens.

Practical answer from more than TW laser power: a lens. Theoretical answer: a lens. Reason is that a mirror has hotspots that cannot be prevented.

Dear Gerck

I have gone through your answer to this question .

Please explain what is the cause of damaging a mirror or a lens with optical power .

What is meant by hotspots ?

Regards

Nityananda Das

Dear Arpit Arvind Khandelwal ,

The book "Laser Induced Damage of Optical Materials" written by Roger WOOD, presents a comprehensive overview of the damage processes that occur at high laser intensity levels and explains how these factors limit the energy handling capabilities of optical systems. (Old but gold)

Regards

Dear George's

I have heard of this book but can't have it.

Regards

Nityananda Das

Dear Nityananda Das ,

I have a hard-copy of this book. I recommend you to purchase it online.

Regards,

Georges

Dear Georges

Thank you.

But I will buy if affordable for me.

Regards

Nityananda Das

You seem to be talking about a CW laser. In this case the pulse damage is irrelevant.

The damage is caused by overheating. The thermal load is defined by residual absorption in the medium. In dielectric mirrors the amount of material that is crossed by the laser beam is very small. In this case mirrors are capable to withstand significantly more power than a lens.

We have done some tests at 940nm. For 10kW special lens Materials like SUPRASIL are needed. On the other hand Layertec specifies (or specified) the CW damage threshold of their coatings with 100MW/cm².

I think which is firstly damaged is the lens, because the lens transmit and absorb the incident light, while, the mirror is reflect most of the incident power. So, which is damaged first is the lens

I appreciate analysis of Patrick Bisson and agree with it. I can add that damage of optical surface (lens or mirror) start not from whole surface, but from some spots in which absorption of incident light is higher than in other parts of surface. Origin of such absorption could be different: surface contamination, defects of antireflection layers, even dust particles. So, choice between lens and mirror have include examination of surface quality and state of optical elements.