Optical Applications of Diamond - An Overview of Current Applications
Diamond, whether natural or synthetic, offers an extraordinary combination of optical, thermal and mechanical properties that, when combined, make it a material that has potential in many advanced optical applications. Today, a wide range of synthetic and natural diamond products and grades exist, covering medical, industrial, research, telecommunications, data storage and military applications. Many products are already on the market and more are at the research and development stage, being prepared to emerge over the next few years.
The ability to manufacture and process diamond in a variety of shapes allows the material to be tailored fo specific optical applicationsProperties of Diamond
Diamond, in all its forms, transmits from the ultraviolet through the visible and infrared regions. Its characteristics are such that it is the only material that offers a wide far-infrared (8-14 um) transmission range combined with high strength and excellent resistance to thermal shock, scratching and erosion. Coupled with this, diamond offers good performance as a window for microwave, visible and ultraviolet radiation.
It can be heated or cooled at rates that would cause other optical materials to fail due to thermal shock. The extremely low thermal expansion coefficient and the high thermal conductivity combine to give diamond with the lowest thermo-optic distortion of any infrared optical material1. Unlike other thermal shock resistant materials, such as sapphire or gallium phosphide, it is also transparent in the far infrared.
The properties of diamond arise from its crystal structure. It is an extremely stiff three dimensional structure in which each atom is strongly bonded to its four nearest neighbours in a tetrahedral arrangement. 1 cm3 of diamond consists of 1.76 x 1023 atoms, the highest of any known material, and therefore diamond has the highest bond density for any material, giving rise to its stiffness and strength. When compared to other hard materials, such as silicon carbide and sapphire, natural diamond is up to four times stronger and 2 to 3 times stiffer.
Diamond is the only material that offers a wide far-infrared (8-14 um) range transmission combined with high strength and excellent resistance to thermal shock, scratching and erosion
The advent of chemical vapour deposition (CVD) processes for the synthesis of diamond has made it possible for large area diamond windows to be manufactured so removing the limitations of size and selection imposed by using natural diamond. This manufacturing process also allows the properties of the diamond material to be optimised for the particular end-use. The potential uses of diamond cover a significant portion of the electromagnetic spectrum, including microwaves, infrared, visible, ultraviolet and X-rays.
The ability to process large size transparent windoews in diamond with consistant quaility is key to its application in advanced optics
Diamond is part of the worldwide research collaboration to produce energy by fusion. Diamond microwave transmission windows are a key component of existing fusion experiments in Germany and Japan, and of ITER (International Thermonuclear Experimental Reactor), construction of which has just begun in France. Working with leading fusion research organisations around the world, Element Six has developed diamond microwave transmission windows that can handle in excess of 1 MW of microwave power and more than twice as much as any other material.
Fusion reproduces the reaction at the heart of stars like our own sun by transforming isotopes of hydrogen into helium. To do this, a huge amount of energy is required to force two hydrogen nuclei to fuse together. However, once fusion is achieved, creating a heavier helium nucleus and a free neutron, it releases vastly more energy than it took to force the nuclei together. This could lead to self-sustaining reactions that could give a new source of power on earth.
In this application, diamond windows are used for the transmission of high power microwave radiation, separating the microwave sources, a special type of vacuum tube called a "gyrotron", each capable of producing about 1 MW of microwave power, from the cavity of the reactor where the fusion reaction actually occurs. The low absorption of microwave energy, high thermal conductivity and relatively low dielectric constant (giving low reflection losses) of diamond make it the main contender in this application.
Moving along the spectrum, we reach into the infrared band which is a region where diamond has a broad range of uses. Element Six has developed a particular grade of polycrystalline CVD diamond, "IR-tran", specifically for infrared applications. This includes high power lasers, infrared imaging applications in the range 8-12 um, process control and analysis, and in analytical research applications such as in Fourier Transform Infrared, FTIR, instruments.
High Power Laser Exit Windows
The use of CVD diamond optics in high power laser applications overcomes the common problem of thermal distortion of the beam by the window material.
Diamond windows have come into their own for high power CO2 laser applications. Operating in the far infrared at a wavelength of 10.6 um, CO2 lasers are increasingly used for industrial cutting, machining and welding applications. By using diamond exit windows, the output power of these lasers has been increased by several kilowatts, enabling faster and deeper cutting and welding. Element Six is working with the world's leading industrial laser companies to develop diamond output couplers and beam splitters as well as exit windows.
Using CVD diamond optics in a high power laser overcomes the common problem of thermal distortion of the beam by the window material. Beam distortion is caused by thermally-induced refractive index gradients as the window is heated by the transmitted beam. With CVD diamond optics, beam distortion is minimized because of diamond's extremely high thermal conductivity (which minimises temperature gradients), its low absorption coefficient and its low value of the temperature coefficient of the refractive index.
Thermal imaging applications have used CVD diamond windows for more than a decade4. Thermal imaging relies on the fact that every object in the universe emits infrared radiation above a temperature of absolute zero. The hotter the object, the shorter the peak wavelength of the radiation; objects at around room temperature have a peak emission in the far infrared (8-12 um) band. Night vision cameras and IR detectors for heat-seeking missiles are typical applications of thermal imaging systems.
In the 8-12 um waveband, research into polycrystalline CVD diamond has demonstrated that it is suitable for very high speed flight applications because it is extremely robust and can cope with harsh environmental conditions while having good, consistent optical properties. Hemispherical domes of CVD diamond have been developed for missile applications. Windows and domes used in infrared applications have two main functions: they provide protection against the environment for the relatively delicate sensing and electronics components and they allow the radiation to pass through to reach the sensor. So, on a heat seeking missile the infrared-transparent dome at the front needs to remain transparent, allow good image resolution up to the maximum operational temperature, as well as surviving environmental impacts from sand and rain. It also has to deal with the extremes of thermal shock and pressure loading when launched. Diamond's exceptional properties meet all of these requirements.
In hot environments, such as in military or medical applications, the window should not emit light that would obscure the view being scanned. This means that transmission, emission and the related properties of reflection, refraction and scatter are all important. Diamond windows are also suitable for process control applications in the chemical, pharmaceutical and food industries. For process control applications, one of diamond's key properties is that it is an extremely inert material and is not attacked by a wide range of otherwise aggressive chemicals.
Research IR Applications
Fourier transform infrared (FTIR) spectroscopy is a research tool where diamond's optical properties combined with the fact that it is scratch proof and bio-compatible are exploited. The extended lifetime of the optical parts leads to the benefit of less maintenance and down-time, providing valuable cost savings. FTIR spectroscopy is a widely used method for characterising materials. When infrared radiation is passed through a sample, some of the infrared radiation is absorbed by the sample and some of it passes through. The resulting spectrum of absorption and transmission features provides a unique fingerprint of the molecules making up the material under investigation. FTIR spectroscopy is a very useful investigative tool because materials can be identified from their spectra, the constituents of mixtures can be quantified and the quality or consistency of samples assessed.
Diamond windows made by Element Six, amongst other applications, have been incorporated into ground-breaking portable FTIR analysis equipment which is used routinely to identify environmental contaminants in the field.
Moving into the visible spectrum, diamond still holds great promise in a number of potential applications. As computers demand ever more data storage, diamond optics are being considered as part of the technology that will deliver future generations of compact disks that could store in excess of 150 GB each. As consumers and industrial users build archives of data ranging from photographs and documents to video and graphics, the need for a compact, high capacity data storage medium is increasingly evident. The coming generation of disks known as "Blu-Ray" will deliver 25 GB storage per disk, but hardware manufacturers are looking further into the future and see diamond lenses as key components of systems with vastly increased storage densities.
In medical applications, diamond's chemical inertness, biological compatibility, scratch resistance and transparency make it an ideal window for endoscopes and other medical diagnostic equipment.
The Etalon Effect
Optical windows with well-polished parallel surfaces can have a "wavy" transmission spectrum instead of a constant flat transmission. This results from constructive and destructive interference of light waves bouncing back and forth between the two surfaces. Known as the "etalon effect", it finds use in optical fibre telecommunications as part of "dense wavelength division multiplexing" (DWDM). DWDM is a method of increasing the transmission capacity of an optical fibre, thereby supporting consumers' demands for more bandwidth to deliver services, such as video-on-demand, at even higher speeds without having to install more fibre. Etalons in this context are very accurate high-end filters for the lasers that generate the signals sent down the fibres. Etalons can filter a very narrow wavelength of light and are used to increase bandwidth particularly over fibre optic backbones.
Element Six has developed synthetic single crystal diamond etalons with consistent material properties that can operate in the C and L bands. As well as being robust, almost unbreakable in normal use and scratch resistant, diamond etalons, because of diamond's very high thermal conductivity, are not affected by temperature variations. The compactness of diamond etalons, compared with a conventional etalon, also allows components to be miniaturised.
Moving beyond the visible band into X-rays, diamond products have a role to play in synchrotrons as windows and monochromators to deliver targeted X-rays of different wavelengths. Again it is the material's extreme mechanical and thermal properties combined with its radiation hardness and transparency over a large part of the electromagnetic spectrum that make diamond suitable for this application. Synchrotron radiation is used to study the structure of materials and is increasingly at the centre of areas of research such as drug development, advanced computer chip design and the development of new materials from plastics to proteins.
Synchrotrons are circular particle accelerators, huge machines that accelerate electrons to almost the speed of light. They produce "synchrotron radiation", an amazing form of intense, monochromatic and coherent light that is shone on atoms, molecules, crystals and innovative new materials in order to understand their structure and behaviour. Synchrotron radiation extends over a broad range, from the infrared to very high energy ("hard") X-rays. Different parts of this broad spectrum can be used for a wide variety research purposes.
The intensity of light produced by a synchrotron is a million times brighter than sunlight and X-ray beams a billion times more intense than the radiation from a typical laboratory X-ray source are possible. The emerging beams can be extremely fine - just a few thousandths of a millimetre across - and are emitted in very short pulses. The beams emerge from the synchrotron along "beam lines" through which they are focused onto the target material.
In X-ray applications, diamond monochromators act as very precise filters, ensuring that the X-ray beam consists of a single wavelength, enabling the essential structure of a material sample to be studied in great detail. To function, the monochromator must be as crystallographically perfect as possible, with every atom sitting on its correct site and as few defects as possible disrupting the perfect atomic array - any defects in the material cause the X-ray beam to loose its coherence. Making diamond X-ray monochromators is a real technical challenge - Element Six has developed an advanced processing technique for the diamond material, which it manufactures by a high-pressure, high-temperature process, to ensure that the X-ray beam is undistorted by its path through the diamond monochromator.
Currently there are some 50 synchrotrons around the world and many are now evolving into so-called "third generation" designs. For example, the European Synchrotron Radiation Facility, ESRF, in Grenoble, France is undergoing a major upgrade programme that will take a number of years to complete. According to ESRF, "The upgrade will include investment in highly specialised nano-focus beamlines, with even brighter "hard" X-ray beams, and the renewal of beam line components such as detectors, optics, sample environment and sample positioning."
Element Six is involved with this upgrade programme and in late 2006 started delivery of the first batch of diamond optics, x-ray monochromators, to help create beams with finer dimensions and separate out the X-rays so that these brighter harder (shorter wavelength) beams can be produced.
These are just a flavour of the broad range of optical applications of diamond and reflect the growing importance of diamond as an advanced engineering material.
1. Infrared Properties of Chemical-Vapour Deposition Polycrystalline Diamond Windows, Paolo Dore et al. Applied Optics, Vol 37.No.24, 1998.
2. Properties of Free-Standing CVD Diamond Optical Components, Window and Dome Technologies and Materials V, SPIE, 1997.
3. Optical Applications of CVD Diamond, CSJ Pickles, TD Madgwick, RS Sussmann and CE Hall, Industrial Diamond Review, 2000.
4. Materials for Infrared Windows and Domes, Properties and Performance, Daniel C Harris, SPIE Optical Engineering Press, USA, 1999.