Meniscus lens

A positive meniscus lens features two spherical surfaces. The term is derived from the Greek word μηνίσκος, which means a small crescent moon.

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This convex-concave lens has a thickness that is more substantial at the center compared to its edges.

For example, many basic cameras, such as the Kodak Instamatic 100, utilize a single fixed meniscus lens.

This type of lens is employed to reduce spherical aberration. When used in conjunction with another lens, it can shorten the focal length, enhance the numerical aperture (NA) of the system, thus improving image resolution and reducing distortion.

The specifications provided are the measured data for Thorlabs' AR-coated N-BK7 singlet lenses. The damage threshold specifications remain constant for any given coating type, irrespective of the lens size or focal length.

Understanding Laser Induced Damage Threshold (LIDT)

This section offers an overview of how laser induced damage thresholds are measured and how the data can be used to determine the suitability of an optic for a particular application. It is essential to comprehend the LIDT of the optics in use, which significantly depends on the type of laser. Continuous wave (CW) lasers typically damage the optic through thermal effects, while pulsed lasers may strip electrons from the optical lattice structure before causing thermal damage. Note that these guidelines are for optics at room temperature and in new condition. Contaminants like dust can lower the damage threshold, so it’s crucial to maintain cleanliness.

Methodology for LIDT Testing

Thorlabs conducts LIDT testing according to ISO/DIS 11254 and ISO 21254 standards. Initially, a low-power beam is directed towards the optic. The optic is then exposed to this beam for 30 seconds at 10 locations, and examined under a microscope for any visible damage. The process is repeated with varying power levels until damage occurs, determining the highest power level the optic can withstand without damage.

According to the experiment, the damage threshold of the mirror was found to be 2.00 J/cm² at 532 nm.

Applications with Continuous Wave and Long-Pulse Lasers

Damage from CW lasers usually results from surface melting or coating damage. Pulsed lasers longer than 1 μs are treated similarly to CW lasers. However, lasers with pulse lengths between 1 ns to 1 μs can cause dielectric breakdown or absorption-related damage, meaning both CW and pulsed LIDT values must be considered. Factors like absorption and scattering in the cement or metal coating contribute to the lower damage thresholds of some optics.

Pulsed lasers with high pulse repetition frequencies (PRF) may mimic CW beams, though thermal effects depend on various factors. For high PRF beams, consider both average and peak powers against the CW power equivalent. Optical products should meet the specified CW damage threshold to ensure durability under laser exposure.

Analyzing Beam Power Density

Thorlabs presents LIDT for CW lasers as linear power density in W/cm². This can be applied across different beam diameters without adjustment. For Gaussian beams, the maximum power density is typically double that of a uniform beam. To check the appropriateness of the CW damage threshold for your optic, compare the maximum power density with specified LIDT values, accounting for wavelength adjustments where necessary.

For detailed test results or certificates, contact Tech Support. Note that testing may incur additional costs or lead times.

Considerations for Pulsed Lasers

Pulsed lasers typically damage optics via strong electric fields, leading to dielectric breakdown. Various pulsed laser regimes exist, mainly governed by pulse length. For pulses shorter than 10-9 s, mechanisms like multiphoton-avalanche ionization predominate, whereas pulses between 10-7 s and 10-4 s may cause either dielectric breakdown or thermal effects. Both CW and pulsed thresholds must be cross-referenced to ensure optic suitability.

Energy density (J/cm²) calculations should account for non-uniform intensity profiles to derive maximum energy density, comparing it with specified LIDT adjusted for wavelength differences.

Practical Calculations for LIDT

For pulses between 1 - 100 ns, adjust LIDT for pulse length using the provided formulas. Compare the calculated maximum energy density to this adjusted LIDT to evaluate optic suitability. Testing certificates and individual test information are available on request.

[1] R. M. Wood, Optics and Laser Tech. 29, 517 (1998).
[2] Roger M. Wood, Laser-Induced Damage of Optical Materials (Institute of Physics Publishing, Philadelphia, PA, 2003).
[3] C. W. Carr et al., Phys. Rev. Lett. 91, 127402 (2003).
[4] N. Bloembergen, Appl. Opt. 12, 661 (1973).

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