Our anti–reflective coatings are optimized for a certain wavelength or wavelength ranges. They allow a high transmittance of the laser light and less absorption of energy in the lens for specific wavelengths. Low–absorption coatings are recommended for lasers with a high peak power, as they minimize thermal effects. These coatings are only available for fused silica lenses. Beside our standard coatings we also offer customized coatings. In the following table the LIDT (laser induced damage threshold) can be found for our coatings and are measured at 355nm, 532nm or 1064nm respectively. The used Laser had a pulse duration of 1ns at a pulse frequency of 50Hz. For detailed information about damage thresholds, plaese refer to our extra lexicon chapter about LIDT.

Coating | Type | Specification |
---|---|---|

/008 | anti-reflective | 1500 - 1600 nm, R < 0.25% |

/065 | anti-reflective + broadband | 400 - 900 nm, R < 0,5% avg. |

/075 | anti-reflective | 355 nm, R < 0.2% |

/081 | anti-reflective + Duo | 532 nm, R < 0.25% + 1064nm, R < 0.25% |

/094 | anti-reflective | 800 - 980 nm, R < 0.25% |

/121 | anti-reflective | 532 nm, R < 0.2% |

/123 | anti-reflective | 633 nm, R < 0.2% |

/126 | anti-reflective | 1064 nm, R < 0.2% |

/159 | anti-reflective | 1850 - 1980 nm, R < 0.25% |

/173 | anti-reflective | 400 - 410 nm, R < 0.2% |

/199 | anti-reflective | 255 - 266 nm, R < 0.2% |

/292 | anti-reflective + low-absorption | 515 - 545 nm, R < 0.2% |

/328 | anti-reflective + low-absorption | 1030 - 1090 nm, R < 0.2% |

/449 | anti-reflective + low-absorption | 900 - 1070 nm, R < 0.25% |

/574 | anti-reflective + low-absorption | 343 - 355 nm, R < 0.2% |

The following coating curves show measured reflections of our typical coatings per surface. If the transmission through a complete lens is of interest, the reflection value at the specific wavelength has to be multiplied with twice the number of lens elements (each element has two surfaces) and then subtracted from 100%. The number of lens elements can be found on the datasheets.

For ƒ-theta lenses which are not only made of fused silica this value will increase dramatically.

When estimating the spot size of a diffraction limited lens, an additional apodization factor (APO) is needed. Its dependency on the truncation ratio T is shown in the following graph. The APO factor includes a dependency on the intensity distribution at the confining edges, where diffraction effects take place. Assuming a Gaussian beam, which is much smaller that the clear aperture, very few parts of the beam experience diffractive effects. By comparison, a Gaussian beam vignetted at the 1/e² beam diameter has larger portion of the beam influenced by diffraction.

The minimal adjustable focal spot size is calculated by the wavelength of the laser multiplied with the focal length of the scan lens, the APO factor and the diffraction parameter M² of the laser divided by the 1/e² beam diameter dL.

*d*_{F} = focal spot diameter*d*_{EP} = entrance pupil diameter*d*_{L} = beam diameter (1/e²)*f'* = focal length

In this example, the focal spot size will be calculated for a Gaussian beam with dL=6.0mm and dL=10.0mm. We assume the use of a f-theta lens S4LFT4010/292 with a frequency doubled Nd:YAG laser at 532nm and a diffraction value M²=1.2. The lens has an effective focal length of f’=100.0mm. Another very important value to determine in addition to the truncation ratio T, is the clear aperture or entrance pupil. This is not the clear aperture of the f-theta lens (Ø35mm), but typically the limiting factor is the beam entrance diameter or aperture of the scan system. Assume a very common value of dEP =10mm in this case.

Example 1

f’=100mm, λ=532nm, *d*_{EP} =10mm, M²=1.2, *d*_{L}=6.0mm

Example 2

f’=100mm, λ=532nm, *d*_{EP} =10mm, M²=1.2, *d*_{L}=10.0mm

In optics and especially laser science, the Rayleigh length or Rayleigh range is the distance along the propagation direction of a beam from the waist to the place where the area of the cross section is doubled.

The Rayleigh length is calculated by the focus area multiplied by a factor (depending on the APO-factor) divided by the wavelength and the diffraction value M² of the laser.

The depth of focus of the scan lens can be estimated by a doubled Rayleigh length. Be aware, that this is just a rough estimation and in many modern applications this value can be too large to still fulfill needed spot diameter requirements.

Sill Optics GmbH & Co. KG

Johann-Höllfritsch-Str. 13

DE-90530 Wendelstein

Tel. +49 (0) 9129 / 90 23 - 0

info@silloptics.de

Johann-Höllfritsch-Str. 13

DE-90530 Wendelstein

Tel. +49 (0) 9129 / 90 23 - 0

info@silloptics.de