Fused Silica for Analytical Instruments

Applications

Optical Laboratory Equipment

Laboratory

Many analytical instruments rely on optical detection methods. Yet, optical laboratory equipment by itself is a very diverse field. Depending on the employed analytical methods, different optical equipment and corresponding optical elements with different performance levels are required. As the equipment design depends on the individual supplier, we discuss some basic components of optical lab equipment and spare parts or auxiliary equipment, that could make a difference in performance.

The performance of optical components of fused silica as view ports, windows or lenses depends foremost on the optical finishing and potentially on the coating. However, choosing the right fused silica can make a difference as well. Absorption in the ppm / cm range can be significant for sensitive measurements.

Want to find out which material is suitable for your application? Please use our  Material Selection Guide .

Optical Metrology

Optical metrology is the technology to measure with light. The properties can be dimensions and shape as well as temperatures or other properties that interact with light. Dimensional measurements with light are contact free and often very precise, as they are mostly only limited by the wavelength of the light used.

Industrial Metrology

In many industrial processes light is used to measure the progress of production. The positioning on a conveyer is typically done with simple cameras. Cooling of heated parts, UV inspection of the cleanliness or laser-based positioning are a few examples of optical metrology in industrial applications.

As the applications are numerous, so are the challenges. Challenges that optical components made of fused silica can help to address are:

  • Sensitivity (lower absorption of the bulk glass, high transmission)
  • Accuracy (low wavefront distortion which leads to high optical homogeneity, excellent surface finish and coating, low absorption)
  • Lifetime (even in hostile environments, low to no aging under UV irradiation)
  • Usable over a broad wavelength range (UV to NIR)

Semiconductor Metrology

In chip production optical metrology is a very common technology. The positioning of the wafer in many processes, as well as the lithography steps are monitored by optical systems. As the node sizes on computer chips get smaller and smaller, the precision of the metrology needs to improve. Fused silica is the material of choice due to the broad transmission window, the very low thermal expansion and, for the highest resolution, the very good optical homogeneity of the refractive index.

 More information on semiconductor manufacturing

Medical Metrology

Medical metrology is key for efficient treatment of diseases today. Tissue analysis before cancer treatment or optical remote sensing and analyzing of the stone composition of kidney or bladder stones or calcification in the arteries is a success factor in choosing the right surgical method or laser source for the treatment.

Optical fibers are used to guide the light from the laser source to the point of use and back to the analyzing unit. For example, a fiber optic bundle can be used to transmit a picture to the surgeon and in parallel act as a beam splitter to guide a part of the light from the tissue which needs to be analyzed to an analyzing unit.

Synthetic fused silica has a broad spectral range for transmitting light. Nevertheless, depending on the wavelength there are differences in transmission performance and therefore in sensitivity. Heraeus offers materials tailored for all wavelength ranges from ultraviolet, visible, to infrared applications.

 More information on specialty fiber preforms
 Brochure: Specialty Fiber Preforms for Medical Applications
 Data Sheet: Specialty Fiber Preforms

Spectroscopy

Another important optical analytical method is spectroscopy. Spectroscopy is a wide field of optical analytics focused on the study of matter via electromagnetic (EM) radiation.

Fused silica optics and fiber optic bundles are used if the wavelength region ranges from UV-VIS-NIR (from 180nm to 3500nm). It can be the direct detection of absorption bands or include indirect detection schemes, like nuclear particle detection from blue emitted Cherenkov light or detection of emission from irradiation of other EM sources on scintillating or fluorescing materials.

Optics and optical fibers used in spectroscopy follow the same design considerations as commercial optical components (mirrors, windows, lenses, and substrates) with particular attention to zero fluorescence and minimal absorption. As it is very difficult to differentiate if absorption is due to the measurement apparatus or the substance investigated, the presence of either would complicate the measurement or even make it impossible.

For example, if certain areas of the EM spectrum are absorbed by the optic, it is difficult or impossible to observe any potential absorption by the investigated substance. A similar effect occurs if the optical material shows fluorescence. In this case, the optic may cover up any absorption of the investigated substance.

Despite its good chemical stability, fused silica may age or deteriorate under some hostile conditions. Some fused silica grades perform better or are more resistant to these conditions than others.

For the reasons given above, it is important to choose the substrate, an optical fiber, or a cuvette for spectroscopic investigations with care.

Cuvettes

For analyzing liquids in optical spectroscopy, the cuvette used should be chemically stable and absorb as little as possible. Fused silica is ideal for this kind of application. A very broad transmission spectrum from the UV to the NIR means a large part of the optical spectrum can be investigated with the same cuvette. To reduce illumination by back scattering, a highly absorbing fused silica grade (  Nerasil® ) can be used. Some cuvettes can partly consist of diffusing material to optimize the illumination.

Spectroscopy Lamps

Analysis and measurement of gases, liquids and solid matters must be as precise as possible to obtain exact results. Typical processes are absorption, reflection, transmission, fluorescence and ionization.

In high-quality analytical instruments for these processes, the light sources needs to fit the instrument and the measured part of the optical spectrum. Different special lamps are in use for this broad portfolio of spectroscopy lamps. However, one component applies for all analytics applications: the quartz glass for the lamp. It must be of the highest purity, homogenous and visually perfect. The Heraeus lamp tubing process is predestined for the high requested quality, independent of the fusion method or quartz glass grade. Wavelength and intensity determine whether a natural quartz glass such as  HLQ 200 or even a fully synthetic material as  Suprasil 130 is the best choice.

Fiber Optic Bundles for Remote Sensing

When the matter of interest cannot be placed in the spectrometer, because of the size or shape or for in-process control of matter in harsh environments, fiber optic bundles enable flexible optical remote sensing of materials. They can act as beam splitter or beam combiner or even deliver the light and collect the signal within the same fiber optic bundle. The optical fibers within a fiber optic bundle can also be used to convert the beam cross section for example from a line profile into a round spot.

Synthetic fused silica is of highest purity and the fibers show best transmission performance in optical spectroscopy. Nevertheless, there are differences in the material grades. High power UV light can damage the material and cause the formation of color centers and decrease performance over time. A similar effect can be caused by nuclear radiation in power plants. On the other hand, these effects can be used to detect radiation over longer distances for a longer period.

Materials with a higher resistance against UV radiation can show a weaker performance in the infrared range which should be considered when selecting the material for an optical fiber. Other factors influencing the performance are fiber design characteristics, beside the material grade, are size and geometry of the optical fiber core and the cladding material. Heraeus has a broad product portfolio with tailored solutions for all wavelength ranges and can support you to determine the best design for your application.

 More information on specialty fiber preforms
 Data Sheet: Specialty Fiber Preforms

Further Information and Contact

Downloads

Downloads

Product Information

Product Information

Talk to our Experts

Talk to our Experts

Our experts are here to help you find the best material for your application.