Module 6-6

FILTERS AND BEAM SPLITTERS


© Copyright 1987 by The Center for Occupational Research and Development

All rights reserved. No part of this book may be reproduced in any form or by any means without permission in writing from the publisher.

The Center for Occupational Research and Development
601 C Lake Air Drive
Waco, Texas 76710

Printed in the U.S.A.

ISBN 1-55502-024-0


(1) A filter is a device that separates a substance trying to flow through it by allowing part of the substance to be transmitted while selectively inhibiting the transmission of the rest. Filters are used with liquids (such as gasoline and water) to remove solid impurities. Electrical filters restrict the frequency spectrum of current flowing in a circuit. Optical filters are used to transmit specific frequencies, uniformly reduce the intensity of a light beam or control the beam’s polarization.

(2) Common examples of colored filters are the filters on colored spotlights for lighting fountains, buildings and theatrical stages. Examples of attenuation filters are sunglasses and tinted windows. (These examples also may include polarization and color isolation.) The envelope of a heat lamp is used as a filter to pass infrared rays and reject the transmission of ultraviolet and most visible rays.

(3) Optical filters are used in many laser/electro-optical applications where accurate numerical measurements are needed. This requires that filter characteristics be accurately controlled and specified. You must know accurately the degree of attenuation reflection or polarization induced by the filters as a function of frequency.

(4) This module will familiarize you with the functional classification of optical filters, the various types of filters within each classification, the advantages and disadvantages of each type and how to specify and use them.

(5) Before you study filters and beam splitters, you must know basic laser eye safety. You should have studied elements and operation of a laser and laser safety.

objectiv.jpg (4761 bytes)

(6) When you complete this module, you should be able to do the following.

  1. Define the following terms as they are related to filters:

      Optical density

      Neutral region

      Bandwidth (FWHM)

      Spectrophotometric curve

      Central wavelength

      Peak transmittance

      Reflection ratio

  2. Describe the following types of filters and tell when they are normally applied:

      Neutral-density

      Gelatin

      Interference

      Glass absorbing

      Short-wave pass (high pass)

      Long-wave pass (low pass)

      Bandpass

      Narrow pass (narrow band)

  3. Predict the effect of several neutral-density filter combinations on the output power of a helium-neon laser. Measure the effects in the laboratory and compare the results to the predictions. Results should be within ± 5% of each other.

  4. Determine the transmittance of a narrow-pass filter at several angles of incidence as well as for normal incidence.

 

DISCUSSION

(7) Several of the applications of scientific optical filters are in photography, optical communication, radar systems, holography, and general laser and electro-optical laboratory experiments.

(8) Filters can be classified into three categories, according to the way they’re used:

  • Attenuation filters

  • Wavelength-selective filters (both transmissive and reflective)

  • Polarization filters

(9) The following discussion will be organized to describe filters and their uses according to these classifications.

 

Attenuation Filters

(10) Attenuation filters are used to reduce the intensity of a light beam. High-quality attenuation filters are said to have a "flat response." This means that they attenuate all wavelengths of light over their usable spectral range by the same amount.

(11) Attenuation filters are used over a photosensitive surface when the light signal received is too intense. This would prevent overdriving a photodetector or overexposing photosensitive film. You can use calibrated attenuation filters to determine if the photosensitive surface is reacting linearly to the exposure.

(12) An example of this application is a light signal being measured by a photodetector. If the photodetector is responding linearly, the insertion of a 50% attenuation filter in the light beam should cause a 50% reduction in the output electrical signal.

(13) Attenuation filters can be divided into two groups according to the mechanism used to reduce the beam power.

 

Geometrical Filters

(14) In the first group are those filters that physically block a fraction of an aperture through which the light beam passes. If you assume that the beam is uniform in intensity across the aperture, then the percentage of the aperture area that’s blocked is the percent reduction in beam power.

(15) One example of a geometrical filter is the iris diaphragm, shown in Figure 1a. The iris or "stop" is used to reduce the aperture of a light beam. Consequently it reduces the transmitted beam power.

Fig. 1
Geometrical filters

(16) Another example is the screen or mesh filter, shown in Figure 1b. This type of geometrical filter actually breaks the relatively large beam into many smaller ones, resulting in a nonuniform distribution across the large aperture in the near field immediately preceding the screen. However, as the beam travels farther away from the screen, it becomes more uniform. Because of the possible development of undesired interference patterns caused by this type of filter, it’s wise to use it only near the detector. When used properly, however, such filter screens can reduce the light level in front of imaging systems without significantly reducing the resolution of the system.

 

Neutral-density Filters

(17) Neutral-density filters are uniform, "grey" filters that absorb and/or reflect a fraction of the energy incident upon them. The term "neutral" is designated because the absorption and/or reflection characteristics of the filter are constant over a wide wavelength range.

Fig. 2
Idealized spectrophotometric curve for a neutral-density filter

(18) It’s useful here to define "optical density" as it applies to filters. Optical density is the degree of opacity of a translucent medium. It is given by the equation OD = –log10T where T is the transmittance of the filter.

(19) Table 1 lists the values of optical density for transmission filters. Note, for example, that if the transmission through the filter is 10%, the transmittance is 0.1 and the corresponding optical density is

A filter with 100% transmittance quite properly has an optical density of 0.

(20) When light travels through a filter, a part of the beam is reflected at the two air-glass interfaces (plastic-air interface of gelatin filters) and another part of the beam is absorbed. Assuming that scattering and fluorescence are negligible, the remainder of the irradiance is transmitted through the filter. Accordingly, the following equation is applicable:

EO = ER + EA + ET
Equation 1

where:
EO = Irradiance of incident light

ER = Irradiance of reflected light

EA = Irradiance of absorbed light

ET = Irradiance of transmitted light

 

Table 1. Optical Densities of Transmission Filters

% Transmission

Transmittance

Optical Density

0.01

0.0001

4

0.1

0.001

3

1.0

0.01

2

10

0.1

1

20

0.2

0.7

40

0.4

0.4

70

0.7

0.15

90

0.9

0.05

100

1.0

0

 

(21) Neutral-density (ND) filters usually are rated in optical density (OD) numbers. If two or more ND filters are used together, the density of the composite of the filters is the sum of their individual filters. As an example, if three filters of ND 0.1, 0.3, and 0.4 are stacked together they comprise an effective 0.8 ND filter.

 

Plastic or Gelatin Neutral-density Filters

(22) Several types of ND filters are available. The most inexpensive (and commonly used) type of ND filter is the plastic or gelatin filter. These often are called No. 96 Wratten filters by the Eastman Kodak Company. These filters are made of colloidal carbon dispersed in gelatin. They can be purchased in two-, three-, and four-inch square sheets, 0.1 ± 0.01 mm thick. The calibration on neutrality of these filters is valid for the visible wavelengths only. M-type carbon, gelatin filters are also available. They can be used in the near ultraviolet, visible and near infrared. However, the M-type filters use larger carbon particles (~1m ) and tend to slightly scatter a light beam. If you must use ND filters on an image-forming beam, it is recommended that you use the No. 96 type filters.

(23) You can get gelatin filters easily in densities from 0.1 – 4.0. You can specially order them with densities as high as 6.0. Although the gelatin film filter is protected by a thin lacquer coating, you should handle it only by the edges and extreme corners. Keep it flat and stored in a dark, dry place. Continued stress can deform it permanently, and moisture tends to cloud it. If you must cut the filter, place it between two pieces of clean, fairly stiff paper and cut with a sharp pair of scissors.

(24) To maintain the stability or calibration of the filter we recommend that you not subject it to temperatures above 120°F. The output of high-power lasers (pulsed lasers exceeding roughly 0.1 joule and CW lasers exceeding 0.1 watt, depending on beam diameter) should not be used directly on gelatin film ND filters. Such laser outputs will permanently damage the filter.

(25) You also can get gelatin film filters cemented between glass plates. This keeps the surface from being easily scratched. Cemented filters are lacquered on the edges to resist the entry of water, which will cause the gelatin to swell and separate from the protective cover.

 

Absorbing Glass, Neutral-density Filters

(26) Neutral-density filters made of absorbing glass are also available. A typical example of this type is the Schott "NG series" glass. (See Figure 3.) The neutrality of these filters is valid to a wavelength of ~ 2.8 mm. These filters are more rugged than the gelatin type. They are less susceptible to scratching and offer more resistance to damage from high-power laser radiation. These filters are also much less susceptible to damage from high temperature than the gelatin film filters. They can be cleaned similarly to other optical glass.

(27) The most likely possibility of a defect in these filters is inhomogeneity of the glass. You can detect inhomogeneities by looking through the filter toward intense light and inspecting for streaks and bubbles in the glass. Corning Glass Company manufactures similar types of colored and neutral-density glass filters.

 

Metallic Film on Glass

(28) Metallic-film ND filters have the flattest response over a wide range of all the classes of filters. These filters are composed of a vacuum-deposited metal film on a glass or quartz substrate. (Quartz is used if we want to operate the filter near UV wavelengths.) Most manufacturers use the alloy inconel (nickel-chromium-aluminum) for the deposited films.

Fig. 3
Transmission curves for actual absorbing glass, neutral-density filters.
Note that the "flat" region becomes more restricted for the high OD filters.
These filters are constructed from Schott colored glass.
Courtesy of Newport Corporation, Fountain Valley, CA.

(29) Attenuation is by both reflection and absorption. You must consider the reflected beam energy in any application if there’s any chance of it being reflected in an undesirable direction.

(30) These filters generally are not susceptible to changes brought about by temperature or aging. They may be used for extended periods at temperatures from –50°C to 120°C. Damage will occur if the temperature reaches approximately 160°C. The filters operate over a wavelength range from 0.275 mm to 2.5 mm.

 

Filter Wheel

(31) For applications such as holography, it’s often convenient to have a way to continuously vary the intensity of a light beam. One way to do this is by use of a "filter wheel" as shown in Figure 4.

(32) This particular variable ND filter is produced with a variable-density evaporated aluminum coating deposited on 270° of the disc. The remaining 90° is clear. The aluminized surface of the wheel is protected with an Si0 overcoating. The opposite side is AR coated (reflectance less than 0.75%, 450 nm to 700 nm). Because much of the light is reflected rather than absorbed by the metallic coating, these wheels can be used as variable beam splitters as well as filters. The substrate material is either BK7A crown glass or OPTOSIL-1 quartz.

Fig. 4
Variable ND filter/beam splitter.
This 5-inch-diameter wheel varies from OD 0.05 to 1.0 (± 0.15 at 633 nm) over
270° range. The coating is of the variable-density evaporated aluminum type.
Courtesy of Newport Corporation, Fountain Valley, CA.


Wavelength-selective Filters (Color Filters)

(33) The most complete description of the optical behavior of a filter is given by its spectrophotometric curve (also called transmittance): that is, a plot of wavelength versus transmittance. An example of a family of rather idealized spectrophotometric curves for a specific absorption filter is shown in Figure 5.

Fig. 5
Spectrophotometric curve for a family of various thickness absorption filters

(34) For absorbing filters, the transmittance is an exponential function of thickness,

Equation 2

where: ti = Transmission of front surface of filter
t2 = Transmission of back surface of filter
al = Filter absorption coefficient (cm–1) at wavelength l
x = Filter thickness (cm)

(35) The product t1t2 sometimes is called the "filter correction factor." For light incident normally on the filter surface, it’s given by,

Equation 3

where: n = index of refraction of the filter glass (relative to air).

(36) The top curve in Figure 5 is for a filter two millimeters thick. The second curve is for a filter four millimeters thick. It has 63.2% less transmittance. The top curve in Figure 5 transmits light at 500 nm very well. But is would transmit very little light at a wavelength less than 440 nm or greater than 560 nm.

Fig. 6
Actual transmission curves for two blue-green transmitting colored glass filters.
Filter dimensions are 50 ´ 50 ´ 3.0 (± 0.25) mm. Designations BG 18 and BG 38
refer to standard types of Scholl colored filter glass. These curves show internal
transmission of the filter only. They don’t include loss due to surface reflections.
Courtesy of Melles Griot, Irvine, CA.

(37) This type of filter is much different in its transmission characteristics from that of the neutral-density filter shown in Figures 2 and 3. The idealized ND filter in Figure 2 has a "flat" response from 300 nm to 900 nm. This flat region is called the neutral region.

(38) Wavelength-selective filters—such as those shown in Figure 6—are used to produce or select specific color or band of color from a white light source, to isolate a specific wavelength, or to reject a specific wavelength or band of wavelengths. We will study three general classes of wavelength-selective filters according to their usefulness:

  1. (38.1.1) Cut-off filters—The first class consists of cut-off filters that have an abrupt division between the regions of high and low transmission. If a filter transmits the shorter wavelengths and rejects the longer wavelengths, it’s called a short-wave-pass filter. If it transmits the longer wavelengths and rejects the shorter wavelengths, it’s called a long-wave-pass filter. Note that some books use annotation derived from electronics, where frequency rather than wavelength is signified. In this notation a long-wave-pass filter is called a "low-frequency-pass filter," and a short-wave-pass filter becomes a "high-frequency-pass filter." Examples of short-wave-pass and long-wave-pass filters are shown in Figures 7 and 8.

    Fig. 7
    Idealized short-wave-pass filter (high-
    frequency-pass filter)

     

    Fig. 8
    Idealized long-wave-pass filter (low-
    frequency-pass filter)

    (38.1.2) The most important characteristic of a cut-off filter is the wavelength describing the position of the sharp cut-off. Different manufacturers define this position to be the wavelength at 3, 5, 30 or 37% transmission (0.37 transmittance). This definition is quite arbitrary, but we recommend that you use the 37% point.

    (38.1.3) It’s also important to describe the sharpness of the cut-off or the steepness of the curve at the transition between low and high transmission. This has been set arbitrarily as the wavelength difference between 5 and 70% transmission points. It is called the slope of cut-off.

  2. (38.2.1) Bandpass filters—The second class of filters are those called "bandpass." An example of a bandpass filter is shown in Figure 9.

    Fig. 9
    Description of a bandpass filter

    (38.2.2) These filters are characterized by the following parameters:

    • The peak transmittance (Tmax) and its corresponding wavelength (lp).

    • The bandwidth, the wavelength interval between the two points on the transmittance curve where the transmittance is 50% of Tmax. This is called the full width at half maximum (FWHM).

    • The central wavelength (lc), which is midway between the halfwidth points on the curve.

    • The passband, which normally is defined as the wavelength interval between the values on the curve where the transmittance is 5% of the peak transmittance. However, for narrowband filters, the passband is defined arbitrarily 1.5 times the FWHM.

    • The rejection ratio for narrowband filters is the transmittance outside the passband divided by the peak transmittance. Rejection ratios can be as low as 10–5.

    (38.2.3) Bandpass filters can be produced that transmit only a very narrow wavelength range. Such filters are called by many equivalent names. "Narrowpass filters," "spike filters," "notch filters," are some of the more common. One important application of such filters in electro-optics is the isolation of individual laser lines. Interference filters are available "off the shelf" with a bandwidth of 1 ± 0.2 nm (FWHM) for use with common visible laser lines. The narrow bandwidth is achieved by multilayer dielectric coatings. The negative result of the multi-layer coatings is high cost and relatively poor peak transmission (usually less than 50%).

  3. (38.3.1) Compensating filters—In this class are filters that have gradually sloping spectral curves. Compensating filters have many widely varying applications, but there are no distinct quantitative terms to describe them. However, one group that we can classify is the color-temperature correcting filters used in color film photography. They are used to compensate for incorrect lighting such as when indoor film is used outdoors or when outdoor film is used indoors (under tungsten lamps). These filters are of the amber type that lower color temperature, and the blue type that raise color temperature.

  4. (38.4.1) Other special-purpose filters—Many other filters are used in the optics and electro-optics industries for specialized purposes. A common example is the so-called "hot mirror" that reflects strongly in the wavelength range of approximately 750 to 1200 nm while transmitting in the visible and near UV. Such a mirror can be used as a filter to IR radiation from, for example, reaching the film plane in a high-intensity projector. Conversely "cold mirrors" are really filters that transmit in the near IR but block radiation in the near UV and visible range. Both of these filters or "mirrors" are really examples of pass filters.

    (38.4.2) Rather than reflecting certain wavelengths, some specialized filters absorb particular wavelength ranges. As an example, heat-controlling filters often are made out of common Schott glasses, such as glass types KG1, KG2, KG3, and KG4. Such filters can get very hot if placed close to a bright source of IR radiation. Design of an optical system using heat-absorbing filters must take into account the heat-dissipation problem.

    (38.4.3) In pulsed solid lasers, such as Nd:YAG, it’s now common practice to surround the Flash lamp with a flow tube that is coated to block UV light. Such a flow tube is really a specialized example of a pass filter. UV light from a xenon or krypton filled flashlamp has at least three deleterious effects on a solid laser: (1) it often causes "solarization" of the rod material, (2) it can increase thermal loading of the rod, and (3) it can accelerate deterioration of the interior of the pump chamber by interacting with contaminants in the cooling system. Inclusion of the UV reflecting coating on the flow tube decreases all of the deleterious effects while actually increasing the efficiency of the laser at high pump levels. The increase in efficiency is in great part due to the reflection of the UV pump light back into the lamp where it is reemitted at longer wavelengths and more efficiently absorbed by the laser rod. For example, a small Nd:YAG laser with a 20 joule input may increase its output energy/pulse by 15–20% when a UV-blocking flow tube is included in the laser head design.

(39) We have introduced wavelength-selective filters according to the characteristics of their spectrophotometric (or transmittance) curves. These filters can be physically described by grouping them according to the way they alter the light, as follows:

  • Absorption

  • Reflection

  • Combination of Absorption and Reflection

 

Interference Filters

(40) Interference filters for wavelength selection are used when you need sharp cut-off or very narrow bandpass. They often are called "spike filters" or "narrow-pass" filters. They generally are made by depositing alternating layers (thin coatings) of dielectric materials on a glass or quartz window. Selection of materials and thickness of the coatings are chosen to provide reflection or transmission at the desired wavelengths. When the number of layers is increased, the cut-off or the passband becomes sharper, but the peak transmittance also decreases. An interference filter may have as many as 30 layers of coatings. These multilayer coating techniques are the same as are used to produce high-reflective laser mirrors.

(41) Absorption and scattering from these surfaces is less than 1%. Consequently, when these coatings are used as filters, practically all of the light is either transmitted or reflected. The reflective properties constitute the filter. Since the transmitted beam shows negligible distortion from the interference coating, these filters can be used in imaging systems.

(42) Compared to metallic films, many interference coatings are quite resistant to damage from high-power lasers. However, they are vulnerable to mechanical and chemical damage. Because they are easily scratched, they should not be wiped with lens tissue. You can remove dust by blowing clean, dry air across them or by gently wiping with a soft, camel’s hair brush. Do not wash them in water but rather clean them by pouring clean, dry ethyl alcohol, (preferably "nano grade," one part in 109 contamination) over the surface and allowing it to drain and evaporate. It’s best to store them in a dry container. Poor-quality coatings will develop cracks with age. Recent improvements have been realized in developing "hard" coatings that won’t scratch or wipe off as easily. A good, hard coating should not be damaged when a piece of masking tape is applied to the surface and removed (a standard "MIL spec" test).

(43) Normally, an interference filter is designed to be used at 0° angle of incidence. The spectrophotometric curve of these filters can be shifted to longer or shorter wavelengths by varying the temperature and/or the angle of incidence at which they are used. To a lesser extent, the shape of the curve also is altered by these variations.

(44) The curve of a multilayer interference filter shifts toward shorter wavelengths as the angle of incidence increases. The magnitude of this shift depends on the type of filter (bandpass, long-wave-pass, short-wave-pass, etc.), the filter design, and the refractive indices of the coating materials used. The relationship between "percent shift to shorter wavelength" and "angle of incidence" is shown graphically in Figure 10. The crosshatched area represents the variation in this relationship: Figures 11, 12 and 13 show the measured incident angle effects on three typical infrared filters. For incident angles up to about 40° , the shape of the curve is not appreciably distorted except for a slight decrease in transmittance. (See Figures 11 and 12.) However, when the incidence of angle becomes greater than 40°, changes begin to occur due to polarization effects and increased mismatch in the optical thickness of adjacent layers. (See Figure 13.)

Fig. 10
Wavelength shift as a function of angle of incidence

Fig. 11
Wavelength shift of passband filter curve as a function of angle of incidence

Fig. 12
Long-wave-pass filter slope shift as a function of incidence angle

 

Fig. 13
Passband filter curve distortion as a function of large changes in incidence angle

(45) The wavelength shift of a multilayer interference filter transmittance curve is dependent on film temperature in as much as the materials that form the various layers have indices of refraction that change with temperature. Generally interference filter curves will shift to longer wavelengths when the film temperature is increased, and to shorter wavelengths when the film temperature is decreased. An interference filter is designed for its transmittance curve to be valid at one temperature, usually 20°C. The relationship between temperature variations above and below 20° and the shift of the transmittance curve are shown in Figure 14. The crosshatched area shows the variation in this relationship for a number of tested filters.

 

Fig. 14
Wavelength shift of an interference filter as a function of temperature

(46) Generally, interference filter curves shift but don’t change their transmittance and shape characteristics over a wide range of temperatures from –190°C to 40°C. (See Figures 15 and 16.) However, significant transmittance losses may occur at filter temperature ranges exceeding 50°C. (See Figure 17.) Typical examples of applications where filter temperatures may be extreme are cryogenically cooled infrared systems (cold), and spaceborne optical systems that are exposed to the sun (hot).

Fig. 15
Effect of temperature on a long-wave-pass IR filter

 

Fig. 16
Effect of temperature on a bandpass filter

 

Fig. 17
Distortion of transmittance curve due to temperature rise
greater than 50°C above design temperature


Combination Absorption/Interference Filters

(47) In the following typical application, it’s desirable to use a very narrow passband fiIter with a low rejection ratio outside the passband.

  • To isolate the 488-nm line of an argon laser output while rejecting the 514.5-nm line.

  • To filter out as much background light as possible from a neodymium laser radar receiver photodetector. This will provide a relatively high receiver signal to noise ratio.

(48) Very narrow transmittance passbands can be achieved in interference filters by increasing the number of layers of dielectric coatings. An example of such a filter is shown in Figure  18. It is used for isolating the 488-nm argon laser line from the 514.5-nm line. Note that there are four other transmission lines also in this filter that don’t affect this line isolation. However, if you want to isolate this line from a broad, continuous light source such as the sun, then it’s desirable to:

 

Fig. 18
Interference filter deposited on a glass substrate, with no blocking filters.
The passband of interest is at 489.4 nm.

  • Eliminate the other four transmittance lines.

  • Reduce the passband of the 488-nm transmittance line, thus lowering the rejection ratio.

(49) You can accomplish these two task by combining a glass absorption filter with the interference filter. The glass filter has a broader transmittance line about 488 nm but reduces its transmittance everywhere outside this region to very low values. The passband of the glass filter and a portion of the interference filter are shown in Figure 19. When these filters are combined, a composite filter curve is achieved similar to that shown in Figure 20.

 

Fig. 19
Transmittance of interference filter (unblocked) and
absorption filter used as a narrow-passband filter

 

Fig. 20
Transmittance of blocked, narrow-passband fllter
composed from the two filters shown in Figure 19.

(50) The absorbing filter is called a "blocking filter" because it blocks out the other four transmittance bands and it reduces the width of the passband. Blocking filters can reduce the reflection ratio of the filter to 10–5.

(51) Narrow-passband filters often are called "spike" filters because of the shape of their transmittance curve. As previously discussed, these filters are available with a half width of 0.1% or less of lc. As an example, a spike filter with a central transmittance wavelength of 1.06 m (output wavelength of Nd:YAG laser) could have a 2.0 nm FWHM or less. In these very narrow spike filters, the peak transmission is reduced to approximately 40 to 50% instead of the usual 70 to 80% for those filters with FWHM that are 2.0% of the central wavelength.

 

Polarization Filters

(52) We can make the following brief statements about the phenomenon of polarization and the use of optical elements as polarizers:

  • Polarizing prisms (such as Wollaston, Glan and Nicol) act as polarizing filters by separating the two polarization components of a light beam into two beams leaving the prism at different angles in space.

  • Polarizing films on glass substrates or glass stacks act as polarizing filters by reflecting one polarization component and transmitting the other.

  • Polarizing film is a thin, plastic sheet that acts as a polarizing filter because it absorbs one polarization while transmitting the other.

 

Beam Splitters

(53) A beam splitter is a device that’s used to divide an optical beam into two or more components according to given criteria. Beam splitters can be grouped into the following four classifications. These classes include those beam splitters that divide the beam size according to beam intensity, wavelength, polarization, or physical size.

(54) The most common beam splitter is a partially reflective mirror. This mirror allows a predetermined percentage of the light to be transmitted while reflecting the remaining portion. The actual percentage of the beam reflected and transmitted depends on the coatings applied during the manufacturing process. Coatings commonly available include inconel and achromatic dielectric. Beam splitters with these coatings can be made to give a transmitted to reflected beam ratio of 30% – 70%, 40% – 60% and 50% – 50%. Variable-density beam splitters wherein reflectivity of the coating is varied across the surface of the beam splitter are also available. The variable density beam splitter allows you to change the beam ratio by using different locations on the beam splitter.

(55) Partially reflective beam splitters have serious limitations. They provide a high loss factor, multiple reflections and distortions. To understand these limitations see Figure 21. Multiple reflections cause both the reflected and transmitted output beams to look similar to the cross-sectional views shown in Figure 22. Areas where the beams overlap produce interference patterns when the incident light is monochromatic, coherent laser light. If the light is not monochromatic, coherent light, ghost images can be produced.

 

Fig. 21
Multiple reflections produced by partially reflective beam splitters

 

Fig. 22
Cross-sectional views of output beams from a partially-reflective beam splitter

(56) Distortion also can be introduced by a beam splitter that’s not made of the highest-quality material or by one that is not perfectly flat.

(57) A pellicle is a precise optical component that works like the partially reflective glass beam splitter. It is made of a plastic membrane stretched over an optically flat metal frame. The membrane has a thickness of approximately 7 microns. Because of their thinness and flatness, pellicle beam splitters demonstrate several advantages over glass beam splitters. For example, they produce almost no change in the optical path length of a light ray that occurs when thick glass beam splitters are used. The thin plastic membrane provides very low absorption, and multiple reflections are no longer a problem. A pellicle beam splitter is, however, more easily damaged and more costly than a glass beam splitter.

(58) Dichroic cube beamsplitters have been in use for a long time. They are constructed by cementing together two right-angle prisms with an appropriate interference coating deposited on the hypotenuse surface. The absorption in the coating is very low so that the reflected portion and transmitted portions approach 50% each. The prism materials are generally made of BK7 glass with a quarter-wave MgF2 coating on the outside surfaces of the beamsplitter. They are available from most optical vendors in cube sizes varying from 5 × 5 × 5 mm up to 50 × 50 × 50 mm. Figure 23 shows a sketch of a cube beamsplitter with the coated hypotenuse face identified, and with the incident (I), reflected (R) and transmitted (T) beams shown for optical transmission of the letter K.

Fig. 23
Dichroic cube beamsplitter indicating a 50-50 split.

(59) The dichroic cube beamsplitter is also available as a polarizing cube beamsplitter, transmitting 90% of one linear polarization and essentially 0% of the other, over a wavelength range from 625 nm to 725 nm. Available cube sizes vary from 15 × 15 × 15 mm up to 25 × 25 × 25 mm.

(60) A recently-developed beamsplitter, the so-called polka-dot beam splitter offers a 50-50 split over a large spectral range, covering the UV to mid-1R wavelength regions. Polka-dot beam splitters are not polarization sensitive orincident-angle sensitive, working well with incident light angled at up to 45° from the normal. In addition, since they are only 1.5 mm thick, there is little energy loss. The polka-dot appearance comes from a rectangular array of coated-to-uncoated square surface areas—as shown enlarged in Figure 24—with enhanced aluminum as the coating over half of the square-sized apertures. The rectangular array has a UV-fused silica base. The input beam is split evenly, with 50% transmitted through the uncoated-fused-silica glass.

Fig. 24
Enlarged section of a polka-dot beamsplitter with rectangular array of equal size coated/uncoated squares.

(61) Holography and interferometry are very important applications for partially reflective beam splitters. Figure 25 shows a typical arrangement of optical components for exposing a hologram. Note the position and function of the beam splitter.

Fig. 25
Beam splitter used in a typical holographic arrangement

(62) Certain types of dielectric filters can be considered to be beam splitters because they divide a beam according to its wavelength. The dielectric coating permits wavelengths longer than a specified value to be transmitted. It reflects wavelengths shorter than that value. Figure 26 illustrates this type of beam splitter. Notice that the graphs in the figure show the relationship of the intensity of the respective beams as a function of the wavelength of that beam.

Fig. 26
Intensity versus wavelength of the incident, reflected, and transmitted
beams for the dielectric filter beam splitter

(63) Certain optical solids known as birefringent crystals separate a randomly polarized light beam into two plane polarized components. The crystal presents a different index of refraction to each polarized component, causing the components to be refracted at different angles. As a result, the polarized components exit the crystal in separate directions. One of these two polarized components is called the "extraordinary ray" (E-ray). It is plane polarized perpendicular to the other ray, which is called the "ordinary ray" (O-ray). Figure 27 illustrates a polarizing beam splitter.

Fig. 27
Beam splitting by double-refracting crystals

(64) The final beam splitter to be discussed in this module is one that divides a beam according to its position or geometry. For instance when a mirror is placed in a portion of a light beam it obviously will reflect the part of the beam that strikes it while leaving the remaining part of the beam undiverted. Figure 28 illustrates this beam splitter more fully.

 

Fig. 28
Beam splitting by position of a mirror

(65) The beam splitting mirror arrangement shown in Figure 29 sometimes is used as the output coupler on a CO2 laser.

 

Fig. 29
Beam splitting by geometry of a mirror

  1. Define the following terms as they are related to filters so that the meaning is the same as that conveyed in the text.

    1. Optical density

    2. Neutral region

    3. Bandwidth (FWHH)

    4. Spectrophotometric curve

    5. Central wavelength

    6. Peak transmittance

    7. Rejection ratio

  2. Describe the following types of filters and tell when they are normally applied.

    1. Neutral density

    2. Gelatin

    3. Interference

    4. Glass absorbing

    5. Short-wave-pass

    6. Long-wave-pass

    7. Bandpass

  3. Using the following ND filters (0.1, 0.2, 0.3, 0.4, and 0.5):

    1. Calculate the transmittance of each filter separately.

    2. Calculate the transmittance of a combination of the (0.1, 0.3 and 0.4) filters.

    3. Calculate the transmittance of a combination of all five filters.

    4. Show mathematically the validity of adding the OD values to find an equivalent value for the combination (easy!).

  4. What conclusions can you draw from your experimental results for the transmittance of a spike filter at various angles of incidence? (Answer only if you completed part 3 of the Laboratory Procedure)

  5. Compare the price and quality of filters from two different suppliers. For this comparison, use a one-inch-diameter laser line filter for 6328 Å with a passband of 3 nm (FWHM) or better. From the specifications given in the supplier’s catalog, which of the two would you recommend for purchase?

  6. A colored filter with a thickness of 5.00 mm has an absorption coefficient of 1.25 cm–1 at 6328 Å. For a red HeNe laser beam, what is the equivalent optical density number of this filter? Ignore reflections from the surface of the filter.

  7. A glass filter with an absorption coefficient of 1.55 cm–1 at 488 nm is required to have the same effect as a neutral-density filter of OD = 2.50 when placed in the argon laser beam. Neglecting surface reflections from the filter glass, how thick should the filter be?

  8. Consider the absorption curve for Schott type-BG38 glass given in Figure 6. Assume that the filter we required is to be used for attenuating light from the 5145-Årgon laser line. At that wavelength, the index of refraction of the glass is approximately 1.53. Taking into account reflections at the surface of the glass filter (used at normal incidence), how thick should the filter be to allow 50.0% transmission?

  9. What is the relative index of refraction (index of the glass divided by that of the surrounding medium) of a glass filter that has a correction factor (tt2) of 0.900?

  10. Obtain quotations from three vendors for a Pyrex flow tube that will surround a 4.0"-long xenon linear flashlamp with an outer lamp diameter of 9.0 mm. Use your judgment regarding the dimensions of the flow tube. The laser head is designed for an Nd:YAG rod and will be pumped with up to 200 joules of energy. The flow tube must be coated to minimize "solarization" and other deleterious effects. Which vendor would your recommend?

1 helium-neon laser, 1 mW

1 power meter, range 1 mW to 1 mW

1 set of Kodak Wratten 96 neutral-density filters—ND 3, 2, 1, 0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, 0.2, 0.1

1 filter holder and stand

1 helium-neon spike filter, lc = 633 nm

1 rotary mount for spike filter, with scale calibrated in degrees in plane of rotation

1 lab jack

  1. Determine the transmittances of three combinations of neutral-density filters, and compare the experimental and calculated values of each.

    1. Set up the filter and helium-neon laser, about one foot apart, so that the beam is incident perpendicular to the filter.

      Fig. 30
      Experimental arrangement

      Adjust the spot reflected from the filter’s first surface onto the laser’s exit aperture, to obtain good alignment.

    2. Reduce room illumination or cancel ambient light with the power meter suppression control. Measure the beam power at a point (A) before the filter. Record power (Pin), making sure the entire cross section of the beam is collected by the detector.

    3. Measure power transmitted through the filter with the detector at (B). (If you can’t reduce or suppress ambient light, use a cylindrical shield of black paper mounted on the detector to provide extra shielding.) Record the transmitted power (Pout) in Data Table 1.

    4. Repeat Steps a through c for each filter combination.

    5. Calculate the transmittance of each set by Tmeas. = Pout/Pin, and compare (find the percentage of error) with the values obtained by:

      (OD1 + OD2 + OD3) = log10T,

      which can be rearranged to:

      where the combined optical density is the sum of the individual optical densities (OD1 + OD2 + OD3 . . .).

      Fig. 31
      Experimental arrangement

  2. Determine the transmittance of the spike at normal incidence.

    1. Mount the filter on the calibrated rotary holder. Orient it so the laser beam is normally incident on the first surface of the filter, with the degree scale at zero.

    2. Taking precautions to eliminate effects of ambient light, measure Pin and Pout as in Part 1. Record data in Data Table 2. If you know the transmittance of the filter, calculate and record the percentage error between the measured transmittance and the known value of T.

    (Idea Bank)

  3. Determine the effect of the angle of incidence on the transmittance of a spike filter.

    1. Using the same setup as in Part 2, measure and record Pin.

    2. Measure Pout at 89.5°, 89°, 88.5°, 88°, and successive 0.5° intervals of angle of incident, down to 80°. (If the degree scale is too small, use 1° intervals.) Record each value in the Data Table 3.

    3. Ensure that the detector reading is maximized for each angle of incidence, since a slight beam displacement may occur as you rotate the filter.

    4. Repeat Steps b and c for an equal number of angles of incidence on the other side of the normal. Record Pout in Data Table 3.

    5. Make a plot of the transmittance versus angle of incidence for this filter.

Data Table 1. Neutral-density Filters

 

Filters

Pin

Pout

Tmeasured

Tcalculated

% error

Combination #1

 
 
 
 
 
 

Combination #2

 
 
 
 
 
 

Combination #3

 
 
 
 
 
 

Data Table 2. Transmittance at Normal Incidence

FILTER  
Pin  
Pout  
Tmeasured  
Tknown  
% error  

  

Data Table 3. Effect of the Angle of Incidence on the Transmittance


Filter _____

Angle of Incidence

Pout
Left of
Normal

Pout
Right of
Normal

T
Left
T
Right
Pin _____          
           
           
           
           
           
           
           
           
           
           
           
           
           
           
           
           
           
           
           

 

 

Corning Glass Filter Catalog, Corning, NY.

CVI Laser Corporation Laser Products Catalog. CVI Laser Corp., 200 Dorado Place, SE, Albuquerque, NM 87192,(505)296-9541.

"Ealing Opties Catalog," Current Year, Cambridge, MA.

Kingslake, Rudolf. Applied Optics and Optical Engineering, Vol. I. NY: Academic Press, 1965.

The Newport Corporation Catalog No. 100. Newport Corporation, 18235 Mt. Baldy Circle, Fountain Valley, CA 92728-8020, (714) 963-9811.

Optical Coating Laboratory, technical report entitled "Effects of the Variation of Angle of Incidence and Temperature on Infrared Filters," Santa Rosa, CA, November 1962.

"Optical Filters Designers Guide and Catalog," Waltham, MA: Infrared Industries, 1973.

Optical Filter Catalog. Bedford, MA: Baird Atomic, 1972.

Optics Guide 3. Melles Griot, 1770 Kettering Street, Irvine, CA 92714, (714) 261-5600

Oriel Corporation Catalog Vol. III. Oriel Corporation, 250 Long Beach Blvd., Stratford, CT 06497-0872, (203) 377-8282.

Schott Glass Catalog, West Germany.

Wood, R. W. Physical Optics. 3rd Edition, NY: Dover Publications, 1961.