Introduction
This technical note will describe the application in optical systems – and the distinct advantages – of advanced, carbon fiber reinforced polymers (CFRP) used in optical mounts and breadboards.
Carbon fiber reinforced polymers (CFRP) have many advantages over conventional materials, including a high stiffness, low weight, and correspondingly a high strength-to-weight ratio. In addition to this, CFRP products have a low coefficient of thermal expansion and are corrosion resistant. This combination of properties has proven to be incredibly beneficial within the aerospace, automotive, and medical sectors; indeed, a growing number of CFRP components have been used in these markets over the past few decades.
Optical breadboards are used to align optical setups onto a portable, easily positioned surface, with the aim of minimising any external disturbances to the arrangement. The major external influences are vibrations in the ground due to traffic and nearby footfall, and disturbances due to electrical devices such as air conditioning or fan units that not only cause movement in the air but also utilise motors operating at a set frequency, typically 60 Hz. When considering longer term stability, the expansion of materials due to thermal effects becomes important as well, especially when disturbances on the micron level can significantly affect optical processes.
Given the properties of carbon fiber composites it makes sense then that CFRP materials are also well suited for use in the manufacture of optical breadboards, especially in applications where such properties are highly sought after.
Reduced Weight and Heightened Stiffness
High modulus carbon fibers are lightweight and yet retain strength, resulting in a superior strength to weight ratio when compared with steel or aluminium. The stiffness of CFRP can also be tuned depending on the recipe used. This can be anywhere from 65 GPa (equivalent to Aluminium) all the way to 350 GPa (150% that of Steel), although this does add weight as the stiffness increases. Being able to reach a higher stiffness than Steel allows for very high payloads to be supported.
Conversely, using a recipe catered to saving weight can open up CFRP breadboards to use in an extended range of applications where standard boards may not be suitable, such as for precisely mounting optics in space qualified payloads, use for in-the-field requirements for medical or defense functions, or in highly complex laboratory setups where multiple breadboards need to be stacked together.F
Superior Thermal Stability
The typical coefficient of thermal expansion for CFRP is 1.5 x 10-6 / K which is 15x better than that of Aluminium (~2.3 x 10-5 / K) and 10x that of Stainless Steel (~1.5 x 10-5 / K). CFRP can be further enhanced by optimising both the selection of fibers and resins, and their orientation, resulting in composites with coefficients of thermal expansion down to the level of 3.0 x 10-7 / K, which is superior to that of Invar (~1.2 x 10-6 / K). This property obviously makes CFRP boards ideal for applications with varying thermal environments, or conditions where the localised temperature will not be uniform (such as when combined with very powerful lasers, integrated into machinery with high temperature fluctuations, or used in space).
Optical Breadboard Material | Characteristics | Density (kg/m³) | CTE (10-6/K) |
Stainless Steel 430 | Stiff but heavy | 7750 | 10.4 |
Aluminium | Lightweight but with a large thermal expansion | 2670 – 2730 | 22.2 – 23.4 |
Invar | Low thermal expansion but are very expensive | 8100 – 8200 | 0.5 – 2 |
Parallel Orientation CFRP | Stiff, lightweight and offers low thermal expansion at reasonable costs | 1300 – 1800 | 1.5 |
Minimised Vibrations
As carbon fiber is a cylindrically orthotropic material, the physical orientation of the carbon fibers can facilitate damping coefficients of up to 60 % higher than steel. Even more than this, by using additional adaptive elements within the makeup of the composite, natural frequencies can be modulated to suit varying needs, allowing the product to be tailored specifically towards an application.
In practice, this has the most benefit for use “in-the-field” where external vibrations are non-constant and can vary in magnitude and in aerospace applications where turbulent environments such as at take-off may adversely affect optical setups.
CarbonVision
The „MICADO Relay Optics Bench“ as supplied to the Max Planck Institute for Astronomy. © CarbonVision/MPIA
Dimensions 4.7m x 2.2m x 0.2m
At Pro-Lite we work with German company CarbonVision , who combine the beneficial material properties of CFRP with modern manufacturing technology in order to produce high-end carbon fiber breadboards. The boards are available with flat working surfaces or alternatively with protruding head inserts, which allow a high degree of flatness by grinding of insert faces. Four different standard designs cover a vast array of potential requirements, and in addition, boards can be designed specifically for use in demanding applications where high precision and minimised vibrations and thermal creep are of the upmost importance.
In addition to breadboards, CarbonVision are also bringing the properties of carbon fiber to photonics applications through a variety of CFRP components on demand, such as optic holders, calibration arms, low-weight instrument housing and more.
