Goal: To provide the fundamental concepts pertaining to laser damage in optical coatings.
What is a laser?
Laser is an acronym that stands for Light Amplification through Stimulated Emission of Radiation.
Lasers use the phenomenon of constructive interference of light waves to amplify light energies to very high levels. Destructive interference is what happens in anti-reflection coatings, but that is a subject for another day. Constructive and destructive interference is illustrated in the graph below.
Stimulated Emission of Radiation
In the stimulated emission of radiation theory, laser atoms are kept in an excited state by the "pumping system" and some photons are inserted. The laser atoms go through a laser medium and bounce from the rear mirror (R=100%) to the front mirror (R=95%) where the front mirror is allowing 5% light to release as the output laser beam. This process is illustrated below.
There are three main components within stimulated emission of radiation:
- Spontaneous Emission
- Stimulated Emission
Absorption is known as the process of absorbing energy from photons. As energy increases, the electron absorbing the photon energy jumps from a lower energy state to a higher energy state (Shaik).
Spontaneous emission is where excited electrons emit photons while falling to the ground level or lower energy level (Shaik). Spontaneous emission occurs naturally so the photon emission also occurs naturally or spontaneously (Shaik).
Stimulated emission is where excited electrons are stimulated to emit photons while falling to the ground state or lower energy state (Shaik). Stimulated emission is an artificial process due to the electrons in the excited state not needing to wait for natural spontaneous emission to occur (Shaik).
There are various parameters that define a laser. Lasers are monochromatic, which means they have a single wavelength of light. Some common wavelengths are 532, 633, 1064, or 1540 nanometers (nm). Laser beams are collimated, which means they have narrow beams of light and they can be very small in diameter. Lasers can have either a pulsed or constant wave. Power is generally measured in Watts (W). Spot size is measured in centimeter squared (cm2). The repetition rate also known as the frequency of the pulse is measured in Hertz (Hz). Pulse duration can be measured from nanoseconds (ns) to femtoseconds (fs). 1 ns equals 1 x 10-9 seconds while 1 fs equals 1 x 10-15 seconds. 1 fs is also equaled to 1 x 10-6 ns. Both of these pulse durations are so quick that you cannot physically see the laser beam pulsing. Due to the significance of the pulse duration, the naked eye only sees a constant beam. Click the "Optical Manufacturing Tolerance Chart" button below to download our tolerance chart, which goes into detail on our tolerance limits and the coatings associated with the limits.
So, what is Laser Damage and why is it important?
If lasers can cut through steel, they can obviously damage optics and optical coatings. The main difference here is that optics and coatings transmit laser light while materials like steel absorb laser light and their energy.
If and when damage occurs, the entire optical/laser system is impaired and is no longer as efficient as designed.
How do we know our coatings measure up?
Laser testing is performed by Spica Technologies, Inc or Quantel Laser, the only two commercial testing facilities in the United States. This testing is performed using the ISO-11254 standard to ensure fair/equal testing of all samples.
There are two aspects of laser testing: threshold testing and certification.
This is more of a quantified approach to laser testing. The goal is to see how much power the coatings can handle without damage to the optics or coatings. In threshold testing, power is increased incrementally until damage occurs. LaCroix coatings have passed as high as 3GW/cm2 on anti-reflection (AR) coatings.
This is more of a qualified approach to laser testing. Coatings are tested to a predetermined power level. This ensures safe operation level. Routine testing level for LaCroix coatings is 500 MW/cm2, 20 ns, and 20 Hz (10 joules).
So, what determines at what level damage occurs?
Laser parameters that influence damage probability are:
- Shorter wavelength = higher frequency = higher energies/photons
- Absorption in materials increases with decreasing wavelength.
- Pulse Length (Duration, ns, ps, fs)
- CW is constant wave where temperature effects is most common
- Nanosecond (ns) range is mostly defects dominated (nodules, lint, voids, digs, or scratches)
- Shorter than ns (pico/fempto) damage can be electric field related
- Coating design evaluation and techniques for distribution of the electric field can help
- Absorption and scattering are related to the electric field component
- Shorter pulse with same peak power means more energy in shorter period of time
- Repetition Rate (Pulse per second, Hz)
- Constant Wave (CW): temperature effects such as heat building up which leads to no time for dissipation
- Higher repetition rates are closer to CW
- Lower repetitive rates mean less energy per unit of time if peak power remains constant
- Number of Pulses
- As number of pulses increases, so does the probability of damage
- This is due to the cumulative or "fatigue" effects
- Spot Size (area of beam, cm2)
- Smaller beam spot means more energy per unit area
- Larger spot with same energy means energy is distributed over larger area
- Power/Energy Density
- Constant wave is specified in power or intensity (Watts/cm2)
- Pulsed is peak power or fluence (Joules/cm2)
- Angle of Incidence: Make sure testing required matches spectral requirements
Optical parameters that influence damage probability are:
- Material (internal transmittance)
- Surface quality (scratch-dig)
- Surface roughness
- Surface cleanliness-contaminants
- Materials: Absorption and bandgap (energy required to move electrons)
- Defects, splatter, voids, or contaminants in or on coating
Nodular defects can be caused from: substrate imperfections, lint, dust (before or after loading in chamber), or spatter during coating process. Pete Kupinski and Michael Thomas define a nodule as "an over-coated particle, typically caused by either airborne contamination or spatter due to dielectric breakdown in precursor coating materials". The illustration below shows the effect a nodule seed can have on a multilayer mirror.
How do we identify laser optics/coatings?
- Laser damage notes
- Spectroscopic grade acetone notes
- In general when we see AR coatings (designed for single wavelength) or when we see Dielectric Mirror (DM) designed for single wavelength
In this blog we discussed various topics such as what is a laser; stimulated emission of radiation; laser parameters; what is laser damage and why is it important; how our coatings measure up; what parameters influence laser damage; nodular defects; and how we identify laser optics/coatings relating to Laser Induced Damage Threshold (LIDT). The goal of this blog was to give a basic overview of LIDT. Please feel free to contact us if you have any further questions pertaining to laser damage. Please feel free to check out our blog on interferometery if interested in the various forms of optical measurements we use to verify our optics are made to specification.
Kupinski, Pete, and Michael Thomas. “Optical Systems: Transmissive High-Energy Laser Optics: Manufacturing and Testing Considerations.” Laser Focus World, 8 Sept. 2014, www.laserfocusworld.com/articles/print/volume-50/issue-09/features/optical-systems-transmissive-high-energy-laser-optics-manufacturing-and-testing-considerations.html.
Shaik, Asif. “Absorption of Radiation, Spontaneous Emission and Stimulated Emission.” Types of Satellites Based on Purpose and Size, Physics and Radio-Electronics, www.physics-and-radio-electronics.com/blog/absorption-of-radiation-spontaneous-emission-and-stimulated-emission/.