Laser Resonant Cavity Optics Application Note

Highly and partially reflective mirrors are the key optical components in the laser resonant cavity. The partially reflective mirror is the output coupler of the cavity while the highly reflective mirror usually has a reflectance of usually larger than 99.5%.

Reflective Mirror

The high reflector and output coupler mirrors usually have a certain curvature to play the role of oscillating laser beam, such as in the YAG laser cavity. In some laser designs, the reflective mirror is also used to reduce the length of the laser tube. The reflective mirror generally uses molybdenum and silicon as the substrate. Molybdenum mirrors can work in harsh environments and have the advantages such as a long lifetime, withstanding high power, no surface coating needed, and wiping resistance. However, its reflectivity is low. Monocrystalline silicon is a cost-effective substrate material with good optical thermal properties.

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Figure 1. The schematic diagram of Nd:YAG
solid-state laser optical cavity.

The reflective mirrors have the following key parameters to be considered: surface flatness, surface finish, roughness, absorption, laser damage threshold, group delay dispersion (GDD), and the reflection/transmission ratio, etc.

  1. Ultra-high reflectivity mirror
    • Reflectivity: 99.9%-99.99%
    • Surface Finish: better than 20/10
    • Surface flatness: better than 1/8 λ
    • Roughness: better than 3A (Ra)
  2. Partial reflective mirror
    • Reflectivity: 1%-99%
    • Surface Finish: better than 20/10
    • Surface flatness: better than 1/8 wavelength
    • Roughness: better than 3A (Ra)
  3. Chirped Reflective mirror
    • Rs & Rp>99.8%
    • GDD from -50 to -1000 fs2
    • High damage threshold
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Figure 2. AG Cavity Optics

Wavelength Opto-Electronic (WOE) can provide a variety of reflective mirrors to meet different laser cavity requirements.

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Figure 3. IBS Coating Equipment
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Figure 4. IBS Coating Curve

Thin Film

Optical thin film is an important part of the laser system. In the resonant cavity of high power laser, the limited reflectivity prevents further increase of the laser output power. Therefore, it is critical to improve the reflectivity of the high-reflectivity (HR) film on the mirror. WOE has the production and testing capability to provide high-quality Ion Beam Sputtering (IBS) vacuum coating. IBS accelerates and focuses the ions from the source into a beam and sputters the target material surface to form a layer of the coating film.

The advantages of IBS vacuum coating:

  1. Better process control
  2. Coating design with wider choices
  3. Improved surface quality and less scattering
  4. Reduced spectral drift
  5. Thicker thickness of coating in a single cycle
  6. Reflectivity can reach 99.9%-99.99%
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Figure 5. Laser Line Mirrors

For an output coupling mirror, the damage threshold of the reflective coating becomes critical when the output power is high such as chemical oxygen iodine (COIL) laser. We have designed a new highly reflective film system, which greatly reduces the thickness of the high refractive index layer compared with the traditional design film system, making it possible for the damage threshold of the film system to be greatly improved.

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