Challenges in Optics

Fused silica for Optical Applications and Photonics

The selection of materials and technologies used in optical systems depends on the wavelength of the light used in the system.

Fused silica can be particularly advantageous in the ultraviolet (>= 193 nm) and in the far infrared (1-3.5 micron) or where very high optical homogeneity is required. In each case, the optimum fused silica material grade must be selected. For instance, depending on the raw material source (quartz containing mineral or synthetic chemicals) and the manufacturing method which leads to different levels of OH, each grade of silica glass can be optimized for UV transmission, IR transmission, broadband transmission, and different levels of homogeneity in one dimension, two dimensions or three dimensions. For even shorter wavelengths of light (EUV), it may be impossible to use transmissive optics, then reflective systems (plano or curved mirrors) may be used.

The number of optical concepts, processing and machining is practically endless. In the following, we will cover a few applications where fused silica is the material of choice.

Aerospace

Rosetta and Philae Astronomy Projects were succesful with the help of fused silica
Rosetta and Philae mission

Many applications in the field of aerospace require the use of high-end materials. They should endure a hostile environment, need to be shock resistant, low weight and most importantly reliable with a long lifetime.

Many aircrafts and spacecrafts are equipped with numerous sensors that use optics to detect, track or identify countless things. In general, many applications are a smaller remote version of a laboratory setup on the ground. Many of these applications include remote sensing. From the larger distance, light is analyzed using a spectrometer. This light may need to be diffused to achieve a homogenized illumination or remove any angular dependence. Many materials can serve as a diffusor; these include plastic, ground glass or opaque glass.

The sensors may need a simple transparent cover that allows UV to NIR radiation to pass through, or require some optical components (e.g. lenses or prisms). Here it is important to understand what grade of fused silica offers what transmission performance for a specific wavelength region. However, not only is the absolute value of transmission important, it may also be of interest to know what to expect in terms of bubble or inclusion size and density. This is important to judge if any scattering defects or obscurations will be in the clear aperture of the optic.
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Because the sensors are in the air (or in space), it is difficult or impractical for a technician to perform maintenance during flight. Therefore, it is paramount to use materials that can sustain the working conditions for at least the duration of the flight. For space applications, this may be several years or more than a decade. Particularly in space, the materials have to sustain a dose of ionizing radiation without significant aging or degeneration of properties. Knowing how high intensity light and radiation can damage fused silica is very valuable to select the optimum materials.
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Fused silica can be surprisingly shock resistant, if it is handled correctly and some general design rules are observed.

Heraeus materials have been involved in several Aerospace projects over the years:

  • Gravity Probe B:
    measure spacetime curvature near Earth, and thereby the stress–energy tensor in and near Earth
  • Euclid-NISP:
    space telescope with the primary goal to analyze the distribution of dark matter and to study the properties of dark energy
  • Civa camera system on Philae, Rosetta mission:
    camera system for panorama view on comet Churyumov-Gerasimenko
  • GAIA:
    mission is to make the largest, most precise three-dimensional map of our Galaxy
  • EnMAP:
    monitoring and characterising the Earth’s environment on a global scale
  • GALILEO:
    global navigation satellite system

…. And several others where high performance optics are required.

Astronomy

VISTA telescope
VISTA telescope (Credit: G. Hüdepohl (atacamaphoto.com)/ESO)

Deals with the observation and study of celestial objects (such as galaxies, stars, planets, moons, asteroids and comets) and processes (such as supernovas, explosions, gamma ray bursts and cosmic microwave background radiation). Scientists employ devices on earth and aboard satellites for their research.

The most commonly known means for astronomical studies are telescopes. Depending on the wavelength they operate in, they use reflecting optics (mirrors) or trans-missive optics (lenses / beam-splitters). Some key components in telescopes are made of fused silica. Especially if the telescope operates from the visual into the near infrared wavelength region.

The larger the telescope, the better the resolution. Therefore, even arrays of telescopes are built. This means that individual telescopes that are spaced meters or km apart can work together to generate high-resolution images. In this case, it is important to synchronize the image generation. This is typically done by employing fiber optic communication.

Scientists not only use telescopes for their research but special detectors to detect particles or phenomena that originate in space. Another example is the gravitational observatory that measures gravitational waves by very precise interferometry.

Heraeus has been involved in several Astronomy projects over the years.

  • LIGO - On February 11th, 2016 the detection of gravitational waves was announced by Advanced Ligo. These projects used Suprasil® 311, 3001 and 312. These materials have low absorption, high mechanical Q and are bubble and inclusion free.
  • VISTA:
    Visible and Infrared Surveillance Telescope for Astronomy is a 4 meter class wide field survey telescope for the southern hemisphere, equipped with a near infrared camera. This project used Infrasil® 302 due to low absorption in the NIR K band, 2.1µm.
  • Einstein Gravity Probe B:
    This project required a monolithic 500kg ingot for the telescope structural component. Herasil® 1 was selected for the homogeneity of its’ ultra-low coefficient of thermal expansion due to 2D refining at very low temperatures.
  • Lunar Laser Ranging:
    During the late 1960s aboard NASAs Apollo spacecraft, corner cube retro-reflector arrays were placed on the lunar surface. The CCRs were fabricated out of Suprasil® 1 which has excellent radiation resistance as well as optical homogeneity due to 3D refining. The next generation of corner cube retro-reflectors will be placed on the lunar surface in 2018 and once again Heraeus fused silica has been chosen

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Final optics inspection. Credit: Lawrence Livermore National Laboratory
Final optics inspection. Credit: Lawrence Livermore National Laboratory

Inertial confinement fusion (ICF)
The most demanding application for High Energy Lasers is the inertial confinement fusion research. The lasers used in this application operate with short pulses that last for nano seconds to femto seconds and pico seconds. Using multiple beams, these systems achieve a total delivered energy up to Mega joule levels in both near infrared (NIR) and ultra violet (UV) wavelength.

Today, these laser systems commonly operate at solid-state Nd-glass laser wavelength 1053nm and the frequency tripled UV wavelength 351nm. Optics used in fusion application require large (300-400mm +) fused silica blanks with low absorption, good optical index homogeneity in both overall index variation in P-V as well as limits to sub-aperture index gradients measured in RMS.

Defects within high energy laser optics are an important design consideration. Short UV pulses can create high-peak power densities that can damage or destroy optical components, especially at distortions within the fused silica. Therefore, these optics require fused silica with no bubbles or inclusions down to the micron range. Demand for so called “Inclusion-Free” material has pushed specifications for both bubbles & inclusions far beyond the common DIN and ISO specifications and led to the Heraeus development of special Automatic Bubble and Inclusion Inspection (ABII) process that can certify large optical blanks to be free of defects down to the 10 micron range.

Research and commercial applications
The ongoing development of smaller high-peak-power Ti-Sapphire lasers benefit from the experience generated in the fusion application. The fused silica needed has the same requirements:

  • Homogeneity of the refractive index
  • Low absorption
  • Defect free

Heraeus brands ideally suited for HE applications include Suprasil® 312 (lenses and windows) and Suprasil® 313 or Spectrosil® 2000 (debris shields).

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Laser Material Processing

The use of lasers in material processing includes cutting, welding or writing. How well a material absorbs a specific energy depends on the wavelength dependent absorption coefficient of the material. The wavelength of the laser should be chosen to be in the high absorption range of the material that is to be processed.
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In order to transport light from the laser source to the desired working point an optic path needs to be created. This path consist of fiber optics, lenses, prisms, mirrors, windows or a combination of these optical components. The material used depends on the wavelength of the laser. For instance, for CO2 Lasers operating at 10.6 µm in the infrared (IR) spectral range, zinc selenide is the material of choice for transmissive optics.
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Non-pure fused silica glasses (aka optical glasses) can be used in the visual spectral range from 400nm to 2,000nm. In higher power laser applications the absorption coefficient of optical glass is too high. The absorption leads to local heat generation, which in turn can distort or destroy the optical component. In these cases as well as for UV and deeper UV (DUV) applications, fused silica is the material of choice for its lower absorption, higher temperature stability and high damage resistance. Depending on the laser wavelength, the correct grade of fused silica must be selected for optimum results .
More information on fused silica properties
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Fiber laser system

Particularly high energy and high power lasers can induce defects in the optical components used to generate the focal point needed for processing. Certain grades of fused silica have the lowest laser induced damage properties of materials used, greatly increasing reliability and lifetime of the laser optic.

During processing, fumes and debris from processing can be emitted from the processing plane and potentially damage the laser optics. Many laser machining optics use AR (anti-reflection) coated windows as cover plates or debris shields. These small discs are a consumable, yet their properties influence the overall system performance and lifetime.
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Fused silica blanks are used in microlithography tools

Since the invention of integrated circuits (semiconductor chips) microlithography is the key process step of the manufacturing chain for electronics. In this step light is used to structure silicon or other semiconducting materials by imaging the tiny structures of a reticle (mask) onto the wafer that has been coated with a photo resist. After development this photo resist acts as a template for the subsequent processes, like doping and etching, needed to alter locally the electronic properties of the semiconductor. This functionalisation of the wafer is the base for the generation of all the electronic units (transistors, capacitors, …) on the chip.

The ongoing trend to miniaturise integrated circuits (Moore’s Law) requires extremely precise optical imaging of the mask onto the wafer with minimal aberrations close to theoretical limits. The tiniest structures of high-end chips have only a width of less than one tenth of the used wavelength! The optical design and the manufacturing of such projection optics modules (s. photo) are the most challenging in optics.

Besides the quality of the optics, the imaging wavelength plays a critical role. Because the minimum feature size of an imaged structure gets smaller with shorter wavelengths, modern semiconductor chips are produced using ArF excimer laser as a light source with a wavelength of 193 nm (deep UV: DUV)).

The optical material of choice for microlithography optics is synthetic fused silica, because it supports perfectly the above mentioned demands for an aberration free DUV optical system. Synthetic fused silica has a very high DUV transmissivity and low absorption, so that no image defects due to lens heating occur. It can be produced with excellent optical 3D homogeneity (low refractive index variations) and negligible stress birefringence. An additional requirement for the optical material is the durability against UV radiation. Although the pulse energy densities used in microlithography steppers are relatively low (< 1 mJ/cm²), only optimized fused silica types keep their excellent initial properties during the expected lifetime of approximately 10 years operation.
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Commercial demand can vary from small quantities for R&D to large quantities of components for OEM equipment. Overall manufacturing cost is paramount. Additional factors include the availability, reproducible quality and reliability of material.

Design challenges
Often, designer must balance performance (optical performance as well it’s contribution to systems performance) with overall cost. This can be a complex task, especially considering the various properties that are specified for different fused silica grades. To help with the selection of the optimal material, it has proven beneficial to perform the following steps:

  • Select the used wavelength region
  • Define the principal optical function (light transmission or reflection)
  • Consider additional requirements (absorption, inclusions, bubbles, fluorescence)

In order to simulate the optical performance, a designer needs to know many properties of the fused silica grade he considers. Heraeus measures and publishes many data, which can help in these simulations.
More information optical material grades

Example: Influence of homogeneity on system performance
The variation of the refractive index can be an important aspect of a specification, but for some applications, it is hardly of importance. This depends on the function of the optical component. In general, if the optic focuses (either light or a laser beam or an image) then, index variation and it’s contribution to transmitted wave front error (TWE) for Refractive optics or reflected wave front error (RWE) for Reflective optics is an important design consideration.

If optical power or energy is to be transmitted (e.g. power detector window cover) or reflected without specification on wave-front quality; then index variation may not be an important factor but possibly absorption may the most important factor.

As this short introduction has shown, no single fused silica grade serves all applications equally well. At Heraeus we offer many fused silica grades that have been optimized for many different requirements. You can find a first overview of the available grades and their prime features here. If you need assistance, please contact us.

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Disc saw for for fused silica

In his goal to deliver a good product as quickly as possible at the lowest cost an optics manufacturers face many choices:

  • What is the right material?
  • What shape does the material come in?
  • How fast can the material be obtained?

Choosing the right material:
On first sight fused silica seems to allow for any quality, but there a differences that can effect performance. It may involve transmission in a certain wavelength region, bubble or inclusion tolerance or low tolerance on distortions of wave fronts, to name just a few examples. And as usual, every grade comes at a different price. The technically best solution may not be price competitive. So finding the most suitable material for a given application is a choice that is done by experience. At Heraeus we have gathered a lot of experience, that you can draw upon to make the right choice.

Why the shape of the material is important:
Many companies producing optical components have all the tools in house to process any shape of material into anything else. They can cut, core drill, machine, grind and polish. The possibilities to come up with a production process are almost endless. In order to support a quick turnaround we have developed a material grade that is supplied in near net shape; usually in rod form. We believe this reduces processing time and cost at the optical finisher so their customer gets the product as soon as possible. However, it is also possible to get our materials:

  • Raw formed as a round or rectangular ingot,
  • Blanks as cut, ground or polished
  • Any unusual or special shape prepared by a C&C machine

Fast delivery:
Delivery time might be what sets you apart from competition. You may have a fast processing time, but not the right material on stock? Then drawing material form our shelves may be the solution to get material quickly.
Find your required material and geometry in our stock list.

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Spectroscopy for Optical Applications

Spectroscopy is a wide field of optical metrology focused on the study of matter via electromagnetic (EM) radiation.

Fused silica optics are employed if the wavelength region ranges from UV-VIS-NIR (from 180nm to 3500nm). This can include indirect detection schemes like nuclear particle detection from blue emitted Cherenkov light or emission form irradiation of other EM sources on scintillating or fluorescing materials.

Optics used in Spectroscopy, follow the same design considerations as commercial optics (mirrors, windows, lenses, and substrates) with particular attention to fluorescence and absorption. The reason behind it is that otherwise no one can differentiate between the substance and the optic.

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.

Spectroscopy is also a remote sensing method. Particularly in hostile environment, spectroscopic analyses can be very important. This may include the presence of nuclear radiation. As well as spectroscopy, that employs nuclear radiation as the energy source for the detected light.

Impurities within the fused silica or stoichiometric variations in the Si-O2 chemical make-up can be a source of absorption and radiation darkening. Exposure conditions & duration over life-time are important considerations.

We believe we can help you select the best fused silica grades for your application. Please contact us.

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