beijing optics

Difficulties associated with UV and IR optics

Introduction

Design and fabrication using UV and IR optics requires consideration of optical materials, surface finish, mechanical and optical fabrication techniques, housing and mounting methods, alignment and test, and cost. These same considerations are addressed when designing and fabricating visible optics, but the magnitude of the problems differ in the different regions. Generally speaking, the UV goes from below 0.2 microns to about 0.4 microns. The visible region is taken to be from 0.4 microns to 0.7 microns, and the IR stretches from 0.7 microns to over 15 microns. The UV and IR regions can be further split up into subregions.

Optical materials

Availability Optical materials suitable for the visible and near-infrared regions tend to be glasses, and those suitable for the IR regions tend to be crystalline.  Fig. 1 shows a histogram of number of materials available vs. wavelength.  There are clearly fewer choices in the UV and long-wave IR regions and a large variety of choices in the visible and near-infrared regions [1].

Figure 1. Histogram of number of optical materials available as a function of wavelength. There are clearly fewer choices in the UV and long-wave IR regions and a large variety of choices in the visible and near-infrared regions [1].

Figure 1.  Histogram of number of optical materials available as a function of wavelength.  There are clearly fewer choices in the UV and long-wave IR regions and a large variety of choices in the visible and near-infrared regions [1].

Thus, one difficulty associated with working with UV and IR optics is the limited number of materials available to work with.

Common materials

Common material choices for the UV include fused silica, calcium fluoride, lithium fluoride, and magnesium fluoride.  Common material choices for the IR include germanium, silicon, sapphire, zinc selenide, and zinc sulfide [1].

Most UV optics are made from fused silica or calcium fluoride (CaF2). Fused silica is an easily fabricated, robust material, making it an economical substrate choice.  Fused silica comes in two major types based on UV transmission.  UV grade transmits down to about 200 nm, while standard-grade fused silica attenuates significantly below 260 nm.  UV grade fused silica components with diameters larger than 100 mm become expensive [2].

The second common material option, CaF2, is a crystalline material that transmits down to 170 nm. The anisotropy and brittleness inherent in CaF2 causes chipping and fracturing during polishing, and make it necessary to control the orientation of the crystal’s axis with respect to the polished face. Therefore, it is hard to meet tight surface-quality and flatness specifications simultaneously, and CaF2 components are typically more expensive than the equivalent fused silica optics [2].

Dispersion

UV materials exhibit much greater dispersion than do visible or infrared materials, as shown in Fig. 2 [1].

Figure 2.  Dispersion is shown (dn/d) as a function of wavelength for the materials used at each wavelength.  UV materials exhibit greater dispersion than either visible or IR materials.

Solarization

Solarization is a serious problem in the UV, and can also be a problem in the visible and IR regions, though in the IR, internal absorption is more likely to be the limiting factor for refractive optics in high-powered laser applications [1].

Dimensional

stability All wavelength regions can use the same structural materials.  However, the UV requires greater dimensional stability due to the shorter wavelength.  An athermal design based on the coefficient of thermal expansion (CTE) of the structural and optical materials, and a temperature coefficient of refractive index (TCRI, or dn/dT) of the optical materials is needed to meet the dimensional stability tolerances for UV and IR systems.  Active athermalization design is much easier in the IR than in the UV.  Due to the difficulty of this task over any significant temperature range in the UV, UV optical systems are often carefully temperature-controlled, making them more complex [1].

Lens designs

UV and IR optical systems tend to have all-reflective optics, catadiatropic optics, and fewer optical elements than visible systems.  IR systems tend to have large fields of view, and UV systems tend to have tight tolerances on field flatness and distortion.  Binary or diffractive optical surfaces are often used in visible and IR systems, but performance is not always adequate for UV systems due to material and fabrication limitations [1].

Coating designs

Almost any coating type can be designed and fabricated in the visible and infrared, but that is not the case for the UV.  AR coatings are difficult below 0.25 microns, but bandpass filters can still be designed and fabricated to operate in this region.  Reflective coatings are limited to wavelengths greater than 0.15 microns if they are to be broadband, though narrow bandwidth filters can be designed for lower wavelengths [1].

One of the difficulties associated with coating UV optics is that coatings are applied at high temperatures, so stress is induced upon cooling, which pulls the flatness out of specification.  This is not such a problem in the IR because more materials are available that reduce this stress problem [2].  Additionally, many coatings exhibit adequate surface quality in one humidity environment, but perform poorly in a different environment.  This is due to water absorption by thin film coatings.  In general, porous films fare better in differing environments [2].

Fabrication and test

Most optics are still fabricated using grinding and polishing techniques.  IR optics generally meet surface quality requirements if fabricated using only diamond-turning, though additional processing is required to meet scattering requirements for UV optics.  Fig. 3 shows predicted scattering from a surface as a function of surface roughness and wavelength.  Typical polishing levels are indicated by the vertical dashed lines.  This plot clearly shows that it is more difficult to reduce scattering in the UV than in the visible or IR regions [1].

Figure 3. Predicted light scattering as a function of surface roughness and wavelength. Dotted lines indicate typical polishing levels.

Figure 3. Predicted light scattering as a function of surface roughness and wavelength. Dotted lines indicate typical polishing levels.

For UV optics, conventional polishing techniques are used almost exclusively. Additionally, both surface and system testing is complicated by the fact that most interferometers test at a wavelength much greater than the operational wavelength of a UV system, so fringe sensitivity is reduces by the wavelength ratio. On the other hand, testing IR optics is much easier, also due to the wavelength ratio between the operational wavelength and the testing wavelength [1].

If UV optics are to be tested using UV light, testing will likely need to occur in a vacuum chamber to reduce the effects of atmospheric absorption.  Far more facilities are capable of testing visible and IR optical systems than UV systems.

Conclusions

Differences between UV, visible and IR optics were discussed.  In general, UV optics are more difficult to design and fabricate due to the tighter tolerances on light scattering and dimensional stability.  Testing and measurement is also likely to be more difficult for UV optics due to the stable temperature and vacuum environments needed.

References

  1. Johnson, B.R., 1992.  What’s Different about Ultraviolet and Infrared Optics?  SPIE Critical Review Vol. CR43, Optomechanical Design, ed. P.R. Yoder, p.61-75.
  2. Collier, D., Schuster, R., 2005.  How to buy UV optics, Opto-Laser Europe, http://optics.org/cws/article/research/21870. Additional references regarding UV optics found at: http://www.arocorp.com/pub.aspx
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