激光损伤阈值测试

激光损伤阈值测试
Laser Damage Threshold Testing
Laser Damage Threshold (LDT) is one of the most important
specifications to consider when integrating an optical component into
a laser system. Using a laser in an application offers a variety of
benefits to a standard light source, including monochromaticity,
directionality, and coherence. Laser beams often contain high energies, though, and are capable of damaging sensitive optical components.
When integrating a laser into an optical system, it becomes crucial to
understand the effects of laser beams on optical surfaces and how laser damage threshold is quantified for optical components.
The degree of damage induced to an optical component by a laser beam is highly dependent on the type of laser being used. Thermally-induced damage occurs under Continuous Wave (CW) laser operation. During exposure to the CW laser, the optical material may not have sufficient time to thermally relax, and failure can occur due to thermal damage to the bulk material or the optical coating. Alternatively, the damage caused by a short, intense laser pulse is due to ionization: the breakdown of the molecular bond. The electric field generated by the laser beam at the optical surface stimulates electrons at the outer energy band, causing ionization. However, it is important to keep in mind that lasers with long pulse widths (<10s) or high repetition rates
(>10MHz) may also cause thermally induced damage. For these reasons, understanding laser damage threshold is crucial to designing and maintaining an optical system. -6
T esting Laser Damage Threshold
Laser-induced damage threshold testing is a good method for quantifying the amount of electromagnetic radiation an optical component can withstand. There are a variety of different LDT tests. For example, Edmund Optics follows the ISO-11254 procedures and methods, which is the industry standard for determining the laser damage threshold of an optical component. Utilizing the ISO-11254 standard enables the fair comparison between optical components from different manufacturers.
Edmund Optics' LDT testing is conducted by irradiating a number of test sites with a laser beam at
different energy densities for pulsed lasers, or different power densities for CW lasers. The energy density or power density is incrementally increased at a minimum of ten sites at each increment. The process is repeated until damage is observed in 100% of the irradiated sites. The LDT is the highest energy or power level at which no damage is observed in any of the irradiated sites. Inspection of the sites is done with a Nomarski-type Differential Interference Contrast (DIC) microscope with 100X - 150X magnification. Visible damage is observed and the results are recorded using pass/fail criteria. Figure 1 is a typical damage probability plot of exposure sites as a function of laser
pulse energy.
In addition to uncoated optical components, optical coatings are also subject to damage from the presence of absorption sites and plasma burn. Figure 2 is a real-world image of coating failure due to a coating defect. For additional information on the importance of LDT testing on coatings, view The Complexities of High-Power Optical Coatings.
Figure 1:
Exposure Histogram of Laser Damage Threshold Probability versus Exposure Site
Figure 2: Coating Failure from 73.3 J/cm3 Source due to Coating Defect
Defining Laser Damage Threshold
There are many variables that affect the Laser Damage Threshold (LDT) of an optical component. These variables can be separated into three categories: laser, substrate, and optical coating (Table 1).
LDT is typically quantified with units of power or energy densities for CW and pulsed lasers, respectively. Power density is the power per cross-sectional beam area. Similarly, energy density is the energy per cross-sectional beam area of a specific pulse duration. Lasers are available with a multitude of different wavelengths and pulse durations, therefore, it is important that the optical component's LDT is suitable for the laser's parameters. As a general rule of thumb, Newton's square root scaling factor can be used to determine whether a laser can be used with an optic that is not rated at the
same LDT pulse duration specification. Equation 1 calculates a new LDT for the different pulse duration.
(1)
The LDT(y) is the estimated LDT for laser Y, and LDT(x) is the specified LDT for laser x. τ is the pulse duration for laser y, and τ is the pulse durat ion for laser x. Additionally, since the energy of a photon is inversely proportional to its wavelength, then theoretically the LDT scales linearly as a function of wavelength, as expressed in Equation 2. yx
(2)
Where PD is the Power or Energy Density at the new wavelength, PD is the Power or Energy Density at the old wavelength, λ is the new wavelength, and λ is the old wavelength. A laser with a PD of 2 W/cm at 1064nm would have a power density of 1 W/cm at 532nm, 0.667 W/cm at 355nm, etc. (y)(x)yxCW
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There are some drawbacks to the scaling, as there are non-linear effects associated with the conversion. However, they are a good rule of thumb for estimating the LDT of an optic at varying wavelengths and pulse durations. Note: Optical manufacturers only guarantee the specified LDT, not scaled estimations. Laser Damage Threshold (LDT) testing is crucial when working with laser optics. Understanding how LDT is tested and defined helps choose the right optical components for the application. Laser optics that
are designed with an LDT that is suitable for a given laser ensure superior results and product lifetime, and help avoid additional expenses due to damaged components.。