Laser Damage

Sponsor
Office of Naval Research
Joint Technology Office
Collaborators
Penn State Electro Optic Center
ATFilms
Anasys Instruments
University of Notre Dame
NSWC Port Hueneme Division
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Optical Materials and Laser Damage

With recent advances in solid state and fiber laser technology, high power continuous-wave (CW) lasers are becoming integrated into new roles in scientific, medical, military, and manufacturing areas. Necessary for these high power laser systems are optical coatings capable of withstanding very high irradiances while maintaining extremely low absorption and precise optical qualities. While such high performance coatings exist in laboratory conditions, the presence of environmental contamination greatly reduces their damage thresholds, causing seemingly intact optics to fail with little to no warning. Our research is dedicated to understanding this physical phenomena in order to develop new optics and systems that are resistant to laser induced damage even in difficult environmental conditions.

Typical damage after testing Total failure with heavy substrate and mount damage

Optical Materials and Band Gap

The choice of materials for coatings is especially important when optics are to be used with high power laser sources. Conventional thinking has been to use materials with as low absorption as possible to minimize laser heating. While this holds true for pristine optics, when surface contamination is present laser heating is all but inevitable and a new approach must be found. Materials must be chosen on their ability to survive the harsh conditions rather than finding the absolute lowest absorption. Our research has found higher band gap materials to be far more resistant to damage when contamination is present.

Laser damage of materials with band gap increasing from 3eV for titania (a) up to 9eV for silica (e)

Laser Conditioning

In addition to proper coating materials, one means of preventing laser damage is to expose critical optics to a lower irradiance prior to starting the laser system. This laser conditioning has been shown in our testing to increase the damage thresholds of contaminated dielectric mirrors by over an order of magnitude, enabling them to survive otherwise destructive power levels. Absorption testing using photo thermal common-path interferometry (PCI) has shown laser conditioning to be capable of reducing the absorption of contaminated optics by over 90% in less than 300 ms, making it a feasible option for real world laser systems.

PCI absorption measurement system Laser spot showing reduced absorption due to conditioning

Quantitative Phase Measurement

Quantitative optical phase is useful for precise measurement of physical dimensions. We developed a system to measure the small changes to glass during laser absorption. Glass used in high-power laser systems may absorb considerable radiation during operation. The measurement setup allows quantification of the amount of energy as heat absorbed by the system.

Video showing laser irradiation on a glass sample. The laser was a 1064 nm Q-switched Nd:YAG with a temporal pulse width of 100 ns, a beam waist of 100 microns and pulse energy of 10 mJ Video showing an electrical current heating the air around the wire carrying it. It was measured with the same phase measurement system
The high-speed phase measurement system. Our system utilized a HeNe laser, a Mach-Zender interferometer and a high-speed camera. The Mach-Zender interferometer was composed of mirrors M1 and M2 and non-polarizing beam-splitters BS1 and BS2. The temperature of a yttria-coated glass slide (a) was imaged with an objective asphere lens (b) and camera lens (e). The image was compared with a reference beam traversing an identical lens (d). The angle between beams was such that the temperature distribution was due to energy absorbed by a high-power Q-switched Nd:YAG laser coupled-in via a dichroic mirror (c) that reflected 1064 nm light and transmitted 632.8 nm light. Since the two lasers shared an objective lens, a dispersion-compensating lens (f) was necessary. The interferograms captured with the high-speed camera were transferred to a computer for conversion.
The high-speed phase measurement system. Our system utilized a HeNe laser, a Mach-Zender interferometer and a high-speed camera. The Mach-Zender interferometer was composed of mirrors M1 and M2 and non-polarizing beam-splitters BS1 and BS2. The temperature of a yttria-coated glass slide (a) was imaged with an objective asphere lens (b) and camera lens (e). The image was compared with a reference beam traversing an identical lens (d). The angle between beams was such that the temperature distribution was due to energy absorbed by a high-power Q-switched Nd:YAG laser coupled-in via a dichroic mirror (c) that reflected 1064 nm light and transmitted 632.8 nm light. Since the two lasers shared an objective lens, a dispersion-compensating lens (f) was necessary. The interferograms captured with the high-speed camera were transferred to a computer for conversion.

Phase retrieval was accomplished by two methods: a Hilbert-transform-based method and a sub-sampling method. Both techniques produced good results for interferographic videos recorded with our system. The Hilbert-transform technique was robust to drift and supported by literature. The sub-sampling technique was very fast and had better noise performance.

Airborne Contamination Behaviour

Video of high power laser testing has shown airborne particles to be accelerated by laser beams towards optical surfaces. This has the potential to contaminate optics and must be avoided in laser systems. To understand this behaviour we are dynamically characterizing particles as they are optically propelled using high speed photography. We believe that a combination of radiation pressure forces, photophoretic forces arising due to uneven heating on the particles, thermal accommodation forces due to irregular shapes of the particles, and reactive forces contribute to the high velocities of the particles that have been recorded. Particle evaporation due to the very high beam intensities has also been observed.

Optical Coating Design

Preventing laser damage is not the only challenge facing optical coatings in high power laser systems. Precise mirror curvature and other optical properties have to be maintained during operation despite heating and cooling cycles. To achieve thermal stability, we have created stress compensated film structures that are carefully designed to eliminate warpage caused by heating and cooling.

Thermally compensated dielectric mirror structure Interferometric image of a 100 micron x 100 micron micromirror with a thermally invariant dielectric coating. Curvature from intrinsic stress has been specifically designed into the structure for eventual use in an optical cavity

Other Research

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Optical Materials and Laser Damage

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Ice

Optical Characterization of Glacial Ice

Thermoluminescence

Thermoluminescent Extreme-Environment Temperature Sensors

Quantum Tuning

Tuning the Semiconductor Bandgap: Nanomechanical Control of Electron States

About (TL;DR)

We are the ECE research group of Professor Joey Talghader at the University of Minnesota.

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