Module 6-4

WINDOWS

(6) Before you begin this module, you should have studied an introduction to lasers, especially laser safety. You also should understand geometric optics, types and properties of optical materials, limitations and testing of optical materials, and polarization.

OBJECTIVES

1. Describe Fresnel reflection, and state the conditions under which it occurs. Using a sketch, show the incident and reflected rays, angles of incidence, reflection and refraction, and the interacting surface. Give the approximate values (in %) of the ratios of reflected to incident, and transmitted to incident light, for Fresnel reflection of a material with a refractive index of 1.5.

2. Explain, and illustrate with a sketch, Brewster’s angle. Give its relationship to the refractive index of the material, and its effect on the polarization of the reflected portion of the incident light. Include in the sketch the reflecting interface, the incident and reflected rays, Brewster’s angle, and the polarization of the reflected ray.

3. List five defects found within optical materials referred to in the Discussion.

4. List two defects found on optical component surfaces.

5. List five important characteristics of an optical window.

6. Using an intense, coherent light beam, visually examine optical surfaces of seven different materials. Evaluate their finish by observing scattered light.

7. Use a shop microscope and grazing incidence illumination to examine two surfaces to determine the nature and severity of surface defects.

8. Use an expanding beam of coherent light to test five windows for their effect on the intensity pattern of that beam.

9. Evaluate the surface flatness of five windows, using interference fringes produced between them and a master flat.

10. Determine which of two lenses has an antireflective coating.

Calculate the pressure differential that can be applied to a window before significant distortion occurs.

 

DISCUSSION

(9) One common example of windows used in a laser is shown in Figure 1. Here, the Brewster windows are the ends of a plasma tube for a helium-neon laser. The intracavity laser beam must pass through each of these Brewster windows twice during each round trip through the laser cavity. This is a very low-gain laser. That means that, if each Brewster window caused as much as one percent loss per pass to the laser beam, the laser would not operate.

Fig. 1
Brewster Windows on plasma tube of helium-neon laser

(10) Another application of windows in an electro-optical system is shown in Figure 2. Here, the window protects the output optics of a laser metal-cutting instrument from splattered, molten metal. The window is cleaned or replaced periodically.

Fig. 2
Window used to protect output optics in a laser metal-cutting operation

(12) The discussion which follows will describe how to get these desirable characteristics for optical windows by proper material selection, fabrication, and component use.

(15) The curves in Figures 4a, b, c and d show percent transmissions through various optical windows in the UV, visible, and IR regions. Figure 4a shows internal transmittance curves for five glass materials over a range from 0.4 mm out to 2.1 mm. The materials labelled SFS, SF11, and K5 are certain types of glasses marketed by Edmond Industrial Optics. Those labelled BK7 and LaSFN9 are common to many glass manufacturers. Note for instance that these two are all also mentined in Table 1, which comes frm glasses marketed by Melles Griot. Internal transmittance refers to the transmission through bulk material without concern for reflective lenses at the front and near surfaces of the windows. Note that the five glasses shown in Figure 4a have excellent transmission in the visible region (0.4–0.7 mm) as well as values between 96% and 100% in the near infrared region (1.4–2.1 mm).

(16) The curves in Figures 4b, c, d show percent external transmittance (taking surface reflection into account) for various optical materials in the range 1–8 (Fig. 4b), 1–10 mm (Fig. 4c) and 1–50 mm (Fig. 4d.) The percent transmission, in accordance with Equation 1 is thickness dependent, so each of the curves shown in these figures is for the thickness specified in the legend of the Figure.

(17) Figure 4e shows how the reflection loses from the surfaces of BK7 windows are reduced from around 4-5% down to 1-2% after the BK7 is coated with a thin l/4 layer of magnesium fluoride (MgF2). All optics–windows, lenses, beamsplitter, prisims, etc.–are coated to reduce reflection losses. Mirrors, on the other hand, where high reflection is desired, are coated so as to enhance the reflectivity.

Fig. 4a
Internal transmittance for five glass materials over a wavelength range from 0.3-2.1 mm.

Fig. 4b
Various infrared window materials.

Fig. 4c
Various medum IR window materials.

Fig. 4d
UV visible far-IR window materials.

* Irtran is a registered trademark of the Eastman Kodak Company.

As one can see by examing Figures 4b, c, d, many optical materials are available to serve as windows (or lenses) in the near and far infrared regions of the electro magnetic spectrum. Knowing the specific application for a particular window (cost, transmission required, mechanical strength required, coefficient of expansion, fracture vulnerability) generally helps one select the optimum window material. For example, Tabled 1 would point to zinc selenide as a window material for use with CO² lasers emitting light of 10.6 mm. Wavelength ranges for several IR window materials are provided in Figure 5.

Fig. 5
Useful wavelength range of selected materials.
The numbers at the ends of the bars indicate the endpoint wavelengths.

Since transmission increases (absorption decreases) with decreasing thickness of a window (see Equation 2), it makes sense to select a window of minimum thickness for best transmission. That's a logical conclusion as long as the window is thick enough to be sufficiently rigid (mechanical property of the material) and maintain flatness and parallelism of its surfaces. In general, if there is no pressure differentiat on a window that is (the pressure is the same on both sides) the ration of window diameter D to window thickness t can be as high as 100:1 for a round window. Mechanical distortion of windows is discussed in greater detail later in this module.

Table 2: Characteristics of Selected Infrared Transmitting Materials

Low Reflection

(20) If a ray of light of any polarization strikes normal (perpendicular) to an interface (junction) between two optical materials of dissimilar indices of refraction (n₁ and n₂), a fraction of the incident ray will be reflected according to the relationship:

(21) This type of reflection sometimes is called "Fresnel reflection." (See Figure 6.)

Fig. 6
Diagram of Fresnel reflection

(23) Optical theory shows that, if multiple reflections within a window are taken into account—as well as the first-surface reflection—the total theoretical external transmittance is given by:

=		Equation 4
where:

(24) So the total reflectance R_tot is 1 minus T_tot, or

= 1 –		Equation 5
where:

Table 3: Reflective Losses for Surfaces from Selected IR Materials

1. (27.1.1) Surface reflections can be reduced to less than 1% by applying vacuum-deposited antirefiection coatings to each surface of the window. The coatings can be simply a thin film of a dielectric material (such as magnesium fluoride) selected because its index of refraction (n₃) is close to the square root of the product of the indices of refraction of the two materials (n₁ and n₂).

(27.1.2) In other words,

(27.1.3) You can determine by visual observation if a window is antireflection.(AR) coated. If you look at the reflection from a light source at a low grazing angle to the window, the light will appear to be somewhat blue-violet. Antireflection coatings are usually very "hard." They can be cleaned with alcohol and lens tissue without danger of scratching or otherwise damaging the coating.

2. (27.2.1) Brewster’s angle window—In the previous discussion, the angle between the incident light rays and the window was considered to be 90° (normal incidence). When the window is positioned at a different angle, the percentage of reflected light changes.

(27.2.2) Alignment of the window to the incident ray or optic axis usually is expressed by the angle, f . This angle is measured between the incident ray and a line normal to the plane of the window. (See Figure 7.) For normal incidence, the incidence angle is therefore f = 0°.

Fig. 7
Diagram of angle of incidence for a light ray striking a window

(27.2.3) A light beam (which is an electromagnetic wave) is said to be linearly polarized if the amplitude of the corresponding electric field (called the "E-vector") varies in only one plane along the optic axis. If a linearly polarized light beam is transmitted through an optical window, the percentage of the beam that’s reflected at each surface varies with the incidence angle, f , according to the curves shown in Figure 8. Note that there are two curves. One is designated E_^ , and the other one is designated E_ll. (See Figure 9.)

Fig. 8
Reflection of polarized light at an air/glass (n = 1.5) interface
as a function of angle of incidence.

Fig. 9
Reflection and refraction of light at Brewster’s angle

(27.2.4) This notation is the same as that introduced in Module 1-4, Course 1, on Properties of Light. To repeat,  E_^ refers to the E-component perpendicular to the plane of incidence (here the plane of the paper) and is shown by dots in Figure 9a. By contrast, E_ll refers to the E-component lying in the plane of incidence and is shown in Figure 9a by the short arrows attached to the light rays. (Note that in some texts, what we refer to as E_^ is called TM, and what we refer to as E_ll is called TE.)

At an angle of incidence equal to Brewster’s angle f B, the reflected ray contains no E_ll vector, only E_^. If the incident beam were totally linearly polarized with E_ll only, at Brewster’s angle of incidence, there would be no reflected light. All of the E_^vibration would be refracted at the first surface and pass on through, as shown in Figure 9b. The transmitted beam is then totally E_ll polarized.

(27.2.7) Note that the E_ll parallel polarization curve in Figure 7 approaches zero reflection for an incidence angle of approximately 57°. This is called "Brewster’s angle." Many laser outputs are linearly polarized. So Brewster-angle windows often are used to eliminate losses due to air-to-glass surface reflections. Brewster angle is not always 57°. It will vary according to the ratio of the index of refraction of the window material to the index of refraction of the surrounding medium according to

(27.2.8) If the window is used in air, f B simply equals tan^–1 n_window.

 

Reducing Distortion Due to Refractive Effects

(31) There’s also a distortion that can be introduced by a change in refractive index with temperature. This distortion can cause similar problems.

Window Distortion Due to Imperfections in
Optical Material or Surface Finish

1. Scratches—any marking or tearing of the optical surface. Scratch types are identified as the following:

• Block reek—chain-like scratch produced in polishing.

• Runner cut—curved scratch caused by grinding.

• Sleek—hairline scratch.

• Crush or rub—surface scratch or a series of small scratches generally caused by mishandling.

The scratch number mentioned above refers to the width of the scratch in ten thousands of a millimeter. Thus a scratch indicator of 60 represents a scratch equivalent to a standard 6 mm wide scratch. The determination is generally subjective since it is made by visual inspection. Since the appearance of a scratch can depend on factors such as the glare material and coatings, a 60 standard scratch may not necessarily be 6 mm wide.

2. Digs—small rough spots on the polished surface similar to pits in appearance, generally caused by mishandling.

The dig number mentioned above refers to the diameter of small pits or craters on a polished optical surface. This number represents the actual size of the dig in hundreths of a millimeter. For non-circularly shaped digs, the diameter is taken to be one half of the length plus width of the dig. With this definition, the dig specifications given below follow:

10 dig = 0.1 mm diameter
20 dig = 0.2 mm diameter
30 dig = 0.3 mm diameter
40 dig = 0.4 mm diameter
50 dig = 0.5 mm diameter, and so on.

In terms of scratch/dig defects, 8050 represents a commonly accepted cosmetic standard surface; 60/40, as mentioned above, a surface finish acceptable for research applications; and 10-5 represents a precise (and costly) standard for demanding laser applications.

• MIL-0-13830A—General Specifications Covering the Manufacture, Assembly and Inspection of Optical Components.

• MIL G-174 - Optical Glass.

 

Mechanical Distortion of Windows*

————
* This discussion is based on an analysis presented in Catalog OP 101, Harshaw/Filtrone, Solon, Ohio.

See Idea Bank

(36) A pressure differential applied to an optical window may cause distortion that is unacceptable in many applications. In extreme cases, the stress even could cause catastrophic failure. The maximum stress in a uniformly loaded window is given by,

(37) To avoid plastic deformation of the window, which in turn could cause significant optical distortion, we must keep the maximum load on the window below a threshold number called the "flexural apparent elastic limit" F_a. This limit is defined as the load point on the load versus deformation curve at which the material will deform at a rate 1.50 times the initial rate. So we can write

(38) For additional assurance that our window will not significantly deflect under an applied pressure differential, we can introduce an additional safety factor, SF, such that,

(39) Substituting into Equation 8 gives us a design equation based on the dimensionless ratio of window thickness to diameter,

(41) Typical values of the elastic limit F_a are given in Table 4. A summary of the equations for the deformation of a circular window and a rectangular window is given in Table 5.

Fig. 10
Two methods of supporting a window

Table 4: Elastic Limit F_a for Some
Common Window Materials

Table 5: Design Equations for Circular and Rectangular Windows

Maintenance and Durability of Optical Windows

1. Thermal—Extreme heat or cold, or rapid changes in temperature, can cause temporary or permanent changes in the physical characteristics of the window. The critical material characteristics are melting temperature, coefficient of thermal expansion, and variation in thermal expansion along two orthogonal coordinates in the material.

2. Surface abrasion—The surface of the window can be altered by external abrasions in the form of scratches from other materials (even cleaning cloths), or from particles in the air (sand, grit, dust). The critical material characteristic in determining a window’s vulnerability to surface abrasion is hardness.

3. Chemical damage—The damage to window surfaces from a chemical stems primarily from the chemical’s ability to partially dissolve the window material. Examples are acetone on plastic windows, and water on infrared "salt" windows.

4. Laser damage—Very high-power lasers can damage windows either at the surface or internally. The subject of laser damage to optical components is beyond the scope of this module. It’s enough here for you to know that this damage mechanism exists and to seek additional information on the subject if the possibility of the condition exists.

5. Contamination—The surface of a window can become contaminated from its exposure to polluted atmospheres. This contamination can be in the form of dust, grease, chemical films, or moisture. You usually detect it by visual observations. Proper operation of an optical system depends on the technician recognizing conditions of contamination and performing periodic maintenance in the form of surface cleaning.

 

Flats

EXERCISES

1. Describe Fresnel reflection. State the conditions under which it occurs. Use a sketch to show the incident and reflected rays, angles of incidence and refraction, and the reflection surface. State the approximate values (in %) of the ratios of reflected to incident, and transmitted to incident light, for Fresnel reflection of a material with a refractive index of 1.5.

2. Explain Brewster’s angle and illustrate it with a sketch. State its relationship to the refractive index of the reflecting material and its effect on the polarization of the reflected portion of the incident light. Include in the sketch the reflecting interface, the incident and reflected rays, Brewster’s angle, and the polarization of the reflected ray.

3. List five defects found within optical materials referred to in the discussion.

4. List two defects found on optical component surfaces.

5. List five important characteristics of an optical window.

6. The following is an interference pattern obtained by comparing a sheet of glass to a master flat. What is your interpretation of the flatness of the glass tested?

7. Describe the visual difference between a lens with an antireftective coating and a lens without a coating.

8. A flat glass plate has an index of refraction of 1.52. If light is to strike the plate at normal incidence, what percent of the light will transmit through the plate?

9. Consider the same plate as in Exercise 8. Assume now that we will apply a single-layer AR coating to one surface. Ideally, what will be the index of refraction of the coating? If we’re going to illuminate the plate with a HeNe laser, how thick should the single-layer AR coating be?

10. Find Brewster’s angle for TM polarized light striking an interface from air into water. (n_water = 1.33.)

11. A 10.0-cm-diameter window, fabricated from LiF, is designed to withstand a 15.0 atmosphere pressure differential without suffering significant optical distortion. Assume that the window is clamped around its circumference and that we will allow a safety factor of 4.0. How thick should we make the window?

12. Repeat Exercise 11, but this time allow a safety factor of 2.50 and allow free movement of the window’s edges.

13. A rectangular window, 4.00 “ 5.00 cm on a side, is fabricated from Polytran KCI. The window is held with flexible rubber O-rings along its perimeter, and has a uniform thickness of 6.00 mm. Assuming a safety factor of 2.00, what pressure differential can be applied to the window before it begins to suffer significant optical distortion? Express your answer in atmospheres of pressure.

 

MATERIALS

Optical windows

Borosilicate crown glass

Quartz (fused)

Sapphire

Plexiglass or similar plastic

Ordinary glass (window pane, microscope slide, etc.)

Irtran

Germanium

HeNe laser, 1 mW

Optical flat, l /10 or better

Mercury vapor lamp, high intensity

Green filter (gelatine or glass)

Diffuser (ground glass, translucent paper, etc.)

Shop microscope

Diverging lens, focal length –2 to –4 cm

Ordinary plate glass, 1/8" to 1/4" thick, about 3" square

Supports and mounts for lens, windows, filter and glass plates

Lens, antireflection coated, any size or focal length

Lens, without coating, any size or focal length

 

PROCEDURES

Surface Condition of Windows

1. Clean all surfaces that are contaminated. Use high-purity ethanol, methanol or Kodak lens cleaning fluid, with clean lens tissue. Set cleaned windows on a fresh piece of lens tissue. Cover them with another piece to avoid further contamination.

2. Set up the HeNe laser, with a diverging lens in the beam. Examine each surface using this source of illumination.

Use Extreme Care. Avoid directing the reflected beam into your own or others’ eyes.

Rotate the test piece as shown, to use various angles of incidence. The intense beam will readily reveal dirt and scratches, by scattering light. More intensity is available in the narrower part of the beam. Moving the test piece closer to the diverging lens provides this, and makes the test more revealing. As a final check, remove the diverging lens and inspect the piece with the full intensity of the narrow beam. Hold the piece nearly perpendicular to the beam path. Again, be careful of the direction of the reflected beam!

Fig. 11
Inspecting surface with divergent beam.

3. Record your observations of each surface. Note scratches, hazy appearance, and any other defects. Identify the piece with the best surface polish.

4. Use the microscope to examine one or two of the worst surfaces. Use illumination at a large angle of incidence, as shown in Figure 12.

Fig. 12
Microscopic examination of surface.

Internal Defects in Window Materials

1. Mount window and diverging lens to project a spot several inches in diameter onto a sheet of white paper, as shown in Figure 13.

Fig. 13
Testing the quality of a window by examining the expanded transmitted beam

2. Carefully examine the projected spot for any local variations in brightness. To confirm that any variations are not due to either the laser itself or the diverging lens:

a. Remove the window and compare results.

b. Rotate the lens, while observing spot variations. Those caused by the lens will follow lens movement.

 

Interference Test for Optical Flats

1. Very gently lay a test piece onto the master flat. Use care not to slide it across the flat.

2. While you observe the pattern as shown above, gently slide the window around over the flat until 5 to 10 fringes are visible. A clean toothpick, tongue depressor, or a pointed kitchen match makes a good tool to manipulate the test piece. This will allow most of the air between test piece and flat to escape, broadening and reducing the number of fringes. (If this doesn’t happen, carefully remove the piece and reclean both it and the flat.)

Fig. 14
Viewing arrangement for interference test.

3. When only two or three fringes are visible, compare one of them to a straightedge held over the center of each end of one fringe. See Figure 15.

Fig. 15
Examples of fringe patterns in interference test.

Since two successive fringes mean that a variation of the optical thickness between the surfaces of the master flat and test piece equal to a half wavelength has occurred, the curvature of the fringe shown in Figure 15c indicates that the test piece varies by approximately l /2 from whatever flatness the master plate possesses. To determine if it’s concave or convex with respect to the flat, simply lower your eye toward the piece. If convex, the fringes will appear to spread out. If concave, they will move closer together.

4. Test and record the deviation of both surfaces of each window.

 

Identification of Antireflection Coating

Examine the two lenses and determine which has the antireflection coating.

REFERENCES

Catalog OP 101, Crystal Optics, Harshaw/Filtron, Solon, Ohio, 44139, (216) 248-7400.

Harshaw Optical Crystals, Harshaw Chemical Co., Crystal & Electronics Products Department, Cleveland, Ohio.

Kodak Irtran Infrared Optical Materials. Eastman Kodak Publication U-72, 1971.

Kruse, P. W., and L. D. McQuistan. Elements of Infrared Technology. Chapter 4, John Wiley, 1962.

Mauro, J. A. Optical Engineering Handbook, 1966, General Electric Company.

MIL-G-174 Glass, Optical, US Government Printing Office.

MIL-0-13830A, Optical Components for Fire Control Instruments, General Specification Governing the Manufacture, Assembly and Inspection of, 11 September 1963, US Government Printing Office.

SPIE, Basic Optics and Optical Instruments, Vol. 1, 1965, Nortronics Division of the Northrop Corporation.

Walker, Bruce. "Anti-Reflection Coatings," Optical Spectra, October 1972.

Waugh, John. "Component Testing by Interferometry," Optical Spectra, June 1973.

Wolfe, William L., editor. Handbook of Military Infrared Technology. Office of Naval Research, 1965. Department of Documents, US Government Printing Office, Washington, DC.

Zanoni, Carl A. "Specifying and Measuring Optical Glass Homogeneity," Electro-Optical Systems Design, February 1973.