Introduction to Planos
Elements whose surfaces have been finished to flat specifications are known generically as plano components. Windows and flats are the most familiar members of this class. They are used in applications where a beam or cone of light must be manipulated without altering its basic optical properties. Deviations in the flatness of a plano element can be reduced to less than one-twentieth of a wavelength of light. In more common units of linear measure one-twentieth of a wavelength of visible light corresponds to about .025 microns or one millionth of an inch!
Special manufacturing tools have been developed to produce plano elements. One type is known as a planetary polisher. Thin glass sections, which have been ground to approximate flatness, float in a slurry of polishing compound between two large, horizontal polishing plates. The upper and lower plates rotate in opposite directions. Each plano element is trapped by a mask that rotates like a planetary gear as it orbits around the hubs of the two polishing plates. It is the mask and the motion imparted to each element that give the “planetary” polisher its name. High-precision flatness and parallelism can be obtained over large apertures because the floating elements are not subjected to warping stress during the polishing process.
Figure 1
Applications for Plano Elements
Applications for plano elements can be broadly categorized as “tools” (optical flats) or as “components” (windows, mirrors, plano filters and substrates).
Tools or Optical Flats - As tools plano elements are called reference flats, optical flats or test plates and are found in every optical shop. The flat surface of a lens element or mirror is checked against a reference flat during the process of manufacturing by bringing the piece under test into direct contact with the flat. A lens maker observes Newton’s Rings created by microscopic gaps between the surfaces to evaluate the quality of his or her work.
In an interferometer, an optical flat can be considered either a tool or a component. Although a flat in an interferometer does not come into direct physical contact with the surface under test, the principle of operation that explains its function as a component in the interferometer is the same principle that explains its function as a separate, hand-held tool.
Components - As a component, a plano can serve as a window, a substrate or an unpolished disc.
Windows. Many opto-electronic devices require windows because they must be housed in controlled environments. For example, laser diodes must be protected from dust and moisture so manufacturers hermetically seal them in metal housings and use a window to allow the laser beam to propagate. Because the windows must not degrade the quality of the laser beam, engineers specify precision plano elements for the task.
Another large category for the application of windows involves special environmental chambers. Windows provide optical access to a sample or device that must operate inside a chamber that is usually made of thick aluminum or steel. Feed-through ports allow a user to “feed” electrical transmission lines, mechanical rods or coolant lines through the walls of the chamber. A window can be called an “optical feedthrough” because it allows an engineer to “feed” a beam of light through the chamber’s wall. A window can change the location of an image and serve as an optical micrometer in one of two ways. First, a window will longitudinally displace the focal point of an uncollimated beam by about one-third its own thickness. This property can be used to bring different object planes of a lens into simultaneous focus. Second, a window will displace the centerline of a beam of light transversely as a function of its tilt. The beam’s outgoing trajectory will be parallel to its original, incoming trajectory.
Substrates and Discs. Unpolished plano elements, or discs, are used as the starting point for the manufacture of ground-glass screens and almost every kind of optical component. Polished plano elements, or substrates, serve as the foundation for the deposition of mirror and filter coatings.
Please Contact Us to discuss your substrate and disc requirements. We provide substrates in various sizes and stages of completion, including diamond saw cut, core drilled, generated, ground and polished.
Optical Flats
Quality control has been a respected and essential part of the manufacture of precision optics for more than 100 years. Optical surfaces must be so close to perfection, within a few millionths of an inch, such that changes in temperature, humidity or the acidity of the water used in grinding and polishing will affect the manufacturing process.
The hand-held optical flat is the traditional tool for monitoring the quality of plano surfaces. It is typically flat to one-twentieth of the wavelength of visible light or one millionth of an inch. In the past 20 years, laser-based interferometers have become important measuring tools in the optical shop and precision optical flats play a central role in the operation of these instruments.
As a hand-held reference tool, the optical flat is brought into contact with the surface whose flatness is to be measured, known as the “surface under test.” The lens maker’s goal is to polish the surface under test so that its flatness approximates or equals the flatness of the reference flat. If the surface under test is not flat, then some regions of the two surfaces will not touch each other. Since deviations from flatness of only a few millionths of an inch can cause air gaps between the surface of the reference flat and the surface under test, the lens maker uses an indirect method to evaluate how extensively the two surfaces actually touch each other. It requires a monochromatic source of light to create well-defined Newton’s Rings.
Newton’s Rings are bands of light and dark that cross the region under test. They form a contour map of the microscopic gaps between the surfaces. A lens maker interprets the rings to assess the quality of the fit between the surface under test and the optical flat. In an interferometer designed for testing flat surfaces, a single beam of light is divided into two beams. One beam serves as a reference by illuminating an optical flat and the second is used to illuminate the surface under test. The wavefronts in each beam take on the shape of the surfaces off which they reflect. When the two beams are recombined the wavefronts add in a way that shows how the shape of the surface under test deviates from the shape of the reference surface.
Other industries use optical flats, for example, a metalworking shop might purchase an optical flat. Since certain glasses such as Pyrex® have very small coefficients of thermal expansion, a Pyrex optical flat can maintain its thickness and its precise flatness over a larger temperature range than metal. Therefore, an optical flat can serve as a very stable source of calibration under variable environmental conditions for precision measuring tools such as coordinate-measuring machines.
Windows
In laser diodes, a window hermetically seals the electronic package to protect it from the environment and to allow its beam of light to propagate. The window in a photocopier or a facsimile machine serves as the transparent shelf for the original document. In vacuum chambers, windows provide access to optical components located inside the vacuum. Sometimes a window simply separates a dirty environment from a clean one.
Since windows, like lenses, transmit an optical signal their bulk properties and their surface properties must meet stringent requirements. Several kinds of inhomogeneities within the window’s material can degrade its performance: embedded bubbles can scatter light; variations in the index of refraction across the window’s aperture can cause a beam to distort; and birefringence changes the polarization of a beam. Additionally, the two transmitting surfaces of a window must be extremely flat and parallel, otherwise, the window will behave as a lens or prism.
Even when windows are flawless, they cause aberrations in many circumstances by distorting converging or diverging cones of light. Sometimes an engineer can use a window’s aberrations to his or her advantage; at other times, the optical system design must be modified to remove the effects of the window.
Displacement of an Oblique Ray
A window will alter the trajectory of an oblique ray of light. At the first surface of the window the light ray will be refracted according to Snell’s Law of Refraction. At the second surface, when the light exits the window, the ray will experience another refraction that restores its trajectory to its original direction of propagation. The total effect of the window upon the ray of light is to displace it without changing its direction of travel (See Figure F2.) The magnitude of its displacement depends upon the angle of incidence of the ray and the refractive index and thickness of the window.
Figure 2
Displacement of Collimated Beams
A window will not impart any aberrations to a collimated beam of light. All of the rays experience the same displacement because they enter the window with the same angle of incidence and traverse the same thickness. Some optical micrometers use a window to provide variable displacement of a collimated beam. A well-designed pivot can tilt a window precisely and produce extremely small translations of the beam’s centerline. The displaced output beam will travel parallel to the original input beam.
Figure 3
For all diverging or converging cones of light, or finite f-numbers, a window will induce aberrations. If the angle of the cone is small (large f-number) and the field of view is small, then the aberrations will also be small. However, if the cone angle or the field of view is large, then serious aberrations can result.
Back Focus
The back focus of a lens can be adjusted with a window because of its ability to displace rays of light.
Figure 3
As the light from a point in the object passes through a lens and comes to focus its individual rays converge like a cone. The peak of the cone is at the focus of the lens. A window will reduce the angle of the cone’s convergence as it passes through the glass. When the rays emerge they resume their original angle of convergence, so the effect of the window is to displace the peak of the cone away from the lens and its original location. If a window is placed in front of a lens, it will alter the diverging cone of light that emanates from each point on the object. Inside the glass the rays of light will not diverge as quickly, so the peak of the cone will appear to be displaced toward the lens.
For narrow fields of view the only significant aberration induced by the window is spherical aberration. Its origin lies in the fact that rays at the edge of the cone enter the window at angles of incidence different from those of the central rays. Sometimes, automated inspection stations must simultaneously focus the image of two objects that are at different distances from the camera. A camera lens is not designed to focus two different object planes at the same time. A window placed between the more distant object and the lens will displace the object’s apparent position closer to the lens; then, its image may be focused with the closer object. For moderate and large f-numbers, the window thickness and small field of view do not seriously affect image quality.
Field Curvature and Astigmatism
All cones of light encountering the window do not experience the same refraction. Off-axis cones are more strongly displaced and they suffer from asymmetric distortions that cause aberrations such as astigmatism. Astigmatism is particularly severe for large field angles or thick windows. The focus in the tangential meridian of a cone of light experiences a displacement away from the lens when the window is placed between the lens and its image. The sagittal focus is displaced one-third as much.
Figure 5
The effect of astigmatism on tangential and sagittal field curvature can produce a backward-curving focal surface. The astigmatism induced by a window depends on the squares of sines and cosines of the field angle and increases significantly with larger field angles. A focal surface that is flat without a window will become curved when a window is inserted; the shape of the curvature is roughly that of a parabola. The axial intersection of this backward-curving focal surface will also be displaced backward from the originally flat focal plane by the ability of the window to change the back focus of a lens.
The astigmatic power of a window can be used to flatten the field of a lens artificially. Examples of this application can be found in some laser scanning systems. Some scan lenses exhibit residual inward field curvature. A window can be used to induce backward curvature and thereby flatten the inward-curving field.
Other Aberrations
Other aberrations caused by windows include coma and longitudinal color. Coma in off-axis cones is caused by asymmetrical distortions in the tangential meridian. Longitudinal color arises from the dependence of a window’s effect upon its refractive index, which is different for every color. Although windows introduce aberrations in many circumstances, they are important components in optical systems. Designers modify lens and mirror prescriptions to nullify the aberrations of windows when a large field of view or small f-number justifies the extra effort.
Choosing a Window
JML stocks both square and round windows in a variety of sizes and materials. Your first consideration when choosing a window should be the material. JML supplies “off-the-shelf ” windows in five different materials:
1. BK7 glass
2. Fused Silica
3. Sapphire
4. Germanium
5. Zinc Selenide
Size and shape of the window you choose will be determined by your specific physical constraints and needs. Thickness of the window should be considered if strength is an issue, for example, using the window as a porthole with one side under vacuum.
Material Considerations and Advantages
BK7 is the most commonly used optical material for windows. This is due to its good optical properties, economical cost and availability. It is a hard material which will resist scratching and it transmits well from 350 nm to 2500 nm.
Fused Silica is a very hard material which also resists scratching. It transmits well over a wider range than BK7, from the UV to the IR regions. It also has a low coefficient of thermal expansion and therefore resists thermal shock.
Sapphire may be considered the superior material of choice for windows. It is extremely hard and durable and is very resistant to scratching. It also is chemically inert. It transmits from 200 nm to 6 microns, which is a very broad spectral range. Sapphire has a high index of refraction and because of its high thermal conductivity, it can be effectively heated and cooled. Due to the strength of sapphire, sapphire windows can be much thinner than windows made of other materials.
Germanium is one of the most common materials used to transmit in the infrared region. It transmits well from 2 to 11 microns. Its index of refraction at 10.6 microns is 4.00475. Germanium is inert, hard and has a low coefficient of thermal expansion and good thermal conductivity.
Zinc Selenide is another material used to transmit in the infrared. Its index of refraction at 10.6 microns is 2.40. It is more transmissive than germanium in the IR and it transmits down to as low as 580 nm. Zinc Selenide is softer and more expensive than germanium, but it has a low absorption coefficient and high damage threshold.
Substrates and Discs
Lens elements appear to be simple products, but appearances can be deceiving. Their surfaces are so smoothly polished that deviations from perfection are measured in millionths of an inch. The manufacture of a low-volume lens or of a colored filter starts with roughly cut discs of glass and ends with precision polishing. Mirrors and thin-film filters require additional processing.
Engineers use the term disc to refer to a roughly plane-parallel slice of glass that will be ground and polished. The term substrate refers to a polished piece of glass that will receive a coating. The manufacture of lenses, mirrors and filters involves several steps. The first step is the creation of the basic material, such as glass sold in blocks or “slabs.” Manufacturers reduce the blocks to smaller rods or rectangular sections using diamond core drills and diamond bandsaws. Craftsmen slice the rods or sections into discs that are roughly the size of the final component. The roughly plane-parallel surfaces of the discs are ground with coarse diamond slurry to bring the optical surfaces into approximate conformance with the final specification. Lens makers use precision grinding compounds to form the proper surface curvatures and then use a polishing rouge to create a precision polish. At this point the surfaces are smooth to a few millionths of an inch and a lens or colored filter can function as an optical device.
Mirrors and other components, such as thin-film filters, often require a coating to function properly. Polished components serve as substrates for these coatings. Usually, glass substrates and discs are purchased by companies that manufacture optical components. Nevertheless, important applications for optical materials exist in other branches of engineering. For example, its rigidity (small Poisson’s Ratio) and low coefficient of thermal expansion make Pyrex® and other specialty glass ideal for certain critical parts of mechanical assemblies. Substrates and discs can provide the starting points for the manufacture of these mechanical components. JML can supply special shapes and various sizes of substrates or discs. The surfaces can be diamond saw cut, ground or precision polished.
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