MP-5: Laser Optics and Absorption’s Dominant Role

The industry's lowest-absorption CO2 laser lens.Since its beginning in 1971, II-VI has played a key role in developing optical materials and coatings that enabled the CO2 laser to emerge into a leading technology for materials processing, and for applications in fields as diverse as laser surgery, laser imaging, target acquisition, and surveillance.CO2 laser technology advancements allowed lasers -- with power levels exceeding 1 kW -- to develop in the early 1970s. The corresponding need in understanding optical materials and optical coatings was evident.High-power infrared lasers performance, including high-energy density waveguide lasers, depends heavily upon the absorption control levels in optical substrates, their thin-film coatings, and interfaces. II-VI is the leader in infrared laser optics technology.

Absorption in Laser Optics

Contamination due to foreign materials on the optic’s surface includes dust, oil, grease, fingerprints, and hydrocarbons. These contaminants, if deposited on the optic’s surface, may lead to absorption and shorten optic lifespans and efficiency.Localized heating, caused by contamination, can lead to “thermal runaway” in high-power laser optics. High temperatures create an increase in free carriers within the bulk material which increases absorption. This process reaches an avalanche state, and thermal runaway commences at > 50° C for Ge, and > 200° C for ZnSe and GaAs.Surface imperfections also cause absorption and can include:

  • Scratches
  • Pits or digs
  • Imbedded polishing abrasives
  • Pinholes in coatings
  • Inclusions in coatings

These surface defects act as damage sites which suffer degradation due to intense perturbations in the electric field surrounding the sites.

Absorption Effects in CO2 Lasers

The CO2 wavelength absorption level coupled with the optic thermal conduction characteristics and its mount are important in determining the laser system’s performance and optic’s lifespan.While the source and control of factors contributing to absorption are complex, the results are clear and include:

  • Decreased output power
  • Fluctuations in output power
  • Mode instability
  • Focal point drifting
  • Coating failures
  • External cavity optics failures (due to output coupler thermal lensing or beam delivery system contamination)

All these failure mechanisms are the result of thermal lensing (the actual change of an optic’s physical characteristics due to absorption).The thermal lensing effect on the beam mode is increased further by a change in the material’s refractive index due to temperature. This latter and more significant effect induces additional optical distortion in the transmitted beam.

Testing to Ensure Low Absorptivity

II-VI was the first IR optics manufacturer to establish a laser vacuum calorimetry test facility for measuring absorption in commercial CO2 laser optics.In laser calorimetry, optic samples are mounted in a vacuum for thermal isolation. The sample is then irradiated with a CO2 laser beam, while thermocouples monitor the sample temperature rise. The laser beam is then turned off and the sample is cooled. By precisely measuring the sample mass, the laser beam incident power, and the heating and cooling slopes generated during the test, the total sample absorption (as a percentage of incident laser power) is determined.To maintain the leadership in quality and low-absorption coatings, the laser calorimetry system regularly undergoes calibration testing and refinement by II-VI’s technical staff.This test facility was the first of a number of industry leading innovations which keeps II-VI at the forefront of CO2 laser optics technology.

Factors Affecting Absorption

  • Substrate bulk absorption
  • Coating absorption
  • Surface contamination
  • Surface deterioration