All About Optics

Why are low-absorption coatings crucial in high-power laser applications?

Absorption plays a crucial role in optical manufacturing. Different materials and wavelengths absorb radiation to varying degrees, depending on the amount of absorbing material present and its absorption capacity at that specific wavelength.

When atoms or molecules that absorb interact with an electric field, it causes an oscillating dipole interaction, leading to photon absorption and exciting the atoms/molecules to an excited state. However, this process only occurs at resonant wavelengths. In solid or liquid absorbers, the excitation energy is converted to heat through particles' vibrations. This makes filters that rely mainly on absorption unsuitable for high-power laser applications due to the intense local heating that can cause structural damage.

Our Photothermal Common-Path Interferometers are highly sensitive instruments that detect low absorption levels down to 0.1% or less. We currently measure absorption at 266, 532, 976, 1030, and 1070 nm. These interferometers are designed to measure the sample's thermal response to laser radiation, enabling them to detect even the slightest change in the sample's refractive index due to absorption.

In summary, absorption is a critical factor in optical manufacturing, and low-absorption coating technologies are desirable because they minimize the amount of absorbed radiation. Our Photothermal Common-Path Interferometers are excellent tools for measuring the absorption of materials at different wavelengths, making them helpful in developing low-absorption coatings.

What is the definition of Laser Damage Threshold and why is it significant in optical manufacturing?

The laser damage threshold (LDT) or laser-induced damage threshold (LIDT) refers to the maximum amount of energy per area, power per area, and wavelength that a material or optic can withstand before being damaged by a laser. This value is vital for both reflective and transmissive optical elements and in situations where the desired outcome is the modification or destruction of a material through the use of a laser.

For every damage-threshold specification, the provided information includes the highest fluence (energy per square centimeter), pulse width, peak irradiance (power per square centimeter), and the test wavelength. The peak fluence refers to the total energy delivered per pulse, while pulse width refers to the pulse’s full width at half maximum (FWHM). The test wavelength is the laser wavelength used to induce the damage. Peak irradiance, on the other hand, is the energy of each pulse divided by the effective pulse length, which is approximately 12.5 to 25 percent longer than the pulse FWHM. All tests are repeated at a frequency of 20Hz for 10 seconds at each test point. It is important to note that longer durations can result in damage at lower fluence levels, even at the same repetition rate.

The damage resistance of a coating depends on the substrate, wavelength, pulse duration, and environmental conditions. Improper handling and cleaning can also reduce a coating's damage resistance.

When selecting a coating based on its ability to handle power, some simple guidelines can make the decision process more manageable. Firstly, the substrate material is of utmost importance. Fused silica is preferable to BK7 as it can achieve higher damage thresholds. Secondly, it is important to consider the type of coating. Metal coatings have the lowest damage thresholds, while broadband dielectric coatings are better. For even better results, choose single-wavelength or laser-line coatings like the V coatings. If even higher thresholds are required, then higher energy laser coatings are necessary.