Choosing Optics for Medical Lasers
- Thursday, 01 October 2009
Optical components often need coatings to make them reflecting, transmitting, polarizing, or beamsplitting. Specifying the right coating can greatly improve the behavior of a system. A well designed anti-reflection (AR) coating, for instance, can increase the delivered power of a laser system by cutting reflections to less than 0.25% per surface at the laser’s wavelength (uncoated optics typically reflect up to 4% per surface).
Filter coatings are especially important as they are essential to eliminating unwanted wavelengths in the optical path and separating different wavelengths into different optical paths. These are vital attributes in many different medical applications ranging from aesthetic laser treatments to microscopy. In fluorescence microscopy, for example, the excitation channel contains a filter to eliminate all wavelengths other than that of the excitation source while the emission channel contains a filter that transmits only the emission range of the fluorescent dye. A beamsplitter between the two channels selectively transmits or reflects each channel (Figure 1) so that the viewer is protected from the laser energy.
Filter coatings are specified by describing their wavelength-dependent transmission properties and blocking properties. These filter-coating-transmission curves should be combined with material transmission curves to fully evaluate the optic.
Avoid Optics Damage
For all types of coatings, laser system designers must consider the source’s power level and specify coating damage thresholds. Ignoring this specification greatly enhances the risk that the laser will damage the optics and possibly the entire system. Unless specified, however, most vendors provide coated optics without doing any damage testing.
The optical coating is generally the source of failure in a high-power laser system. Most fail because of the presence of absorption sites within the coating, at the coating’s interface with the substrate, or at the interface with the air. Such sites absorb the laser energy, heat up, and either melt or fracture the optic, usually causing catastrophic system failure (Figure 2).
There are also noncatastrophic failures, such as plasma burn. These are typically the result of unoxidized 1- to 5μm metallic nodules - small defect areas inherent to the coating material. Some manufacturers intentionally subject their coated elements to powers sufficient to trigger plasma burns to remove the defect nodules.
For high-power applications, coating designers choose materials with intrinsically low absorption at the relevant wavelengths. But the customer needs to be aware that the choice of coatings for high power is limited. Further, coatings for use with high-power ultraviolet (UV) lasers are made of different materials than those for use in the visible and near-infrared (IR). Materials for use in mid- and far-IR coatings are a third group.
The core structure of high-reflection coatings is typically a repeating stack of high- and low-index layers, each a quarter-wavelength thick. Silicon dioxide (SiO2) is the generally accepted and ubiquitous choice for low-index layers. Choosing a material for high-index layers is not as straightforward, although dielectric metal oxides in general are preferred materials for UV, visible, and near-IR laser applications. Oxides of titanium, tantalum, zirconium, hafnium, scandium, and niobium are all popular high-index materials.
The design and fabrication of the coating can significantly alter the damage threshold. Simply adding a half-wave of low-index material (normally silicon dioxide) as the final layer, for instance, can result in measurably higher damage thresholds. The use of sputtering to apply dense coating layers rather than using ion-assisted evaporation has an even greater impact — up to a ten-fold improvement — by eliminating the inherent porous micro-structure present in evaporated filters. Sputtering also makes the outer surface less susceptible to damage from handling and cleaning.
With proper selection, then, medical laser system designers can obtain optical elements that maximize performance, safety, and reliability. The key is keeping in mind the high power density of lasers and their impact on the optical elements as well as the possibilities of scattering and interference that can affect delivery of power to the target.