Choosing Optics for Medical Lasers
- Thursday, 01 October 2009
Designers of medical equipment using lasers must be careful in their selection of optical components in order to ensure safe and reliable operation. The high power density and narrow wavelength range of lasers create problems not found in other medical optics such as microscopes and cameras. Proper selection of materials, manufacturing specifications, and coatings becomes essential to avoid such problems.
The medical laser system designer does well to design for optics with high damage thresholds from the beginning of the optical design process, starting with material selection. One typical high-power substrate for lenses and windows is calcium-fluoride (CaF2), which provides high transmission from the UV through the IR. Because of the material’s low index of refraction, it can often be used without the need for additional coatings. Magnesium fluoride (MgF2) has a similarly wide transmission region, but is transparent deeper into the UV and is also harder (and thus more durable) than CaF2. Both are used in mid-wave IR thermal imaging applications and in DUV lithography with excimer lasers. Materials commonly used for CO2 laser applications (at 10.6μm) include germanium and zinc selenide. Both of these materials, however, have high indices of refraction and thus typically require anti-reflection (AR) coatings to maximize transmission of the laser energy.
The next factor to consider is specification of manufacturing parameters. One key parameter is surface quality because small imperfections scatter light, which can lead to potentially dangerous effects. The optics for eye surgery, for instance, must precisely administer the appropriate amount of energy without damaging the patient’s eye. Scattered laser energy due to a lens’ poor surface quality both decreases the equipment’s reliability in energy delivery and could injure the patient by directing energy in unintended directions.
Optical surfaces can never be perfect, however. Polishing an optical surface inevitably creates small defects such as scratches (marks or tears along the surface) and digs (pits or divots). Thus, it is important for designers to determine a scratch-dig specification that their system can tolerate.
The scratch-dig specification defines a standard surface quality as an amount of acceptable variation on the optic surface. The specification combines two numbers: a scratch number followed by a dig number, such as 20-10. Lower numbers indicate higher surface quality. These numbers come from a visual comparison to a set of standard surfaces in accordance with MIL-PRF-13830B, a U.S. military specification for inspecting optical components.
It is important to note that these numbers do not directly correspond to the number of defects on the surface. The scratch specification includes both the number and the total length of allowable scratches, although as a common reference the scratch number relates to the “apparent” width of an acceptable scratch. Dig numbers, however, do relate to a specific value. For example, a dig number of 10 relates to a 0.10 mm, or 100-μm, diameter pit.
The second key manufacturing factor for laser optics, especially the filters and windows that comprise most of a laser systemís optical elements, is parallelism. This specification indicates how parallel the first surface is to the second and is typically given as an angular measurement such as arcsec or arcmin. In most cases with laser systems, however, the surfaces of a flat optical element will not be parallel but will form a deliberate wedge. This wedge shape helps prevent the formation of interference patterns due to partial reflections off of each surface. The parallelism specification in these cases measures how well-controlled the angle between the surfaces is maintained.