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Introduction

Overview

Optical Table

Environment

Laser

Beamsplitter

Mirrors/Lenses

Table Mounts

Optic Mounts

Plate Holder

Objects/Scenes

Resources

Beamsplitter

Single-beam hologram optical setups do not require a beamsplitter, but multi-beam setups do and is a critical optical component of the system. A beamsplitter has three important functions:

There exists an important intensity relationship between the reference (R) and object (O) beams at the photographic plate in a multi-beam setup. This is called the beam intensity ratio (R/O). The intensity of the reference beam at the photographic plate must always be greater than the intensity of any reflected point from the object scene to the photographic plate. This intensity ratio directly affects the brightness of your recorded image. To achieve the brightest image, your reference to object beam ratio should be in the inclusive range of 2 to 1 (2/1) and 1.5 to 1 (1.5/1).

Just before you make a multi-beam hologram exposure in the section on Creating Transmission & Reflection Holograms, you will need to measure the beam intensities of the reference and object beams, individually, at the plate location by blocking one of the beams with a black cardboard while you measure the other beam. You then block the beam you just measured and measure the previously blocked beam. Taking intensity measurements at the plate is discussed in the sub-section Measuring and Adjusting the Beam Intensity Ratio in the sub-section Creating a Multi-Beam Transmission Hologram in the section Creating Transmission & Reflection Holograms. If you need to make an intensity adjustment to the beams, the type of beamsplitter you are using will determine how you do this. I will cover these adjustments as I discuss the type of beamsplitters you can use.

The ability to adjust the intensity between the two beams, without changing the direction of the reflected and transmitted beams from their original preset directions for the optical setup, can be accomplished by using either a fixed beam ratio beamsplitter or a circular variable gradient beamsplitter. There are many other types of beamsplitters on the market, but the ones I will mention here work best for holography. Fixed beam ratio beamsplitters are relatively inexpensive, but may require the additional use of a separate linear or circular variable neutral density filter or clear glass plate to adjust one of the beams' intensity. Circular variable beamsplitters are more expensive, do not require a separate neutral density filter or glass plate, and are easily used to adjust the two beams' intensity ratio. If you can afford the circular variable beamsplitter, I strongly recommend it but a fixed ratio beamsplitter will work fine. Check the Resources link to compare prices.

In the multi-beam optical setup sub-sections in Creating Transmission & Reflection Holograms, be aware that when I talk about the reflective and transmission sides of a beamsplitter and then talk about the object beam and reference beam, these are two different aspects of the optical setup. In the multi-beam transmission hologram optical setup, the object beam is the reflected beam off the beamsplitter and the reference beam is the transmitted beam through the beamsplitter. In the multi-beam reflection hologram optical setup, the reference beam becomes the reflected beam off the beamsplitter and the object beam is the transmitted beam through the beamsplitter.

Linear and circular "neutral density" filters (ND filters) will not work for us as a beamsplitter because they do not have an aluminum or silver coating on one side of the glass. These types of filters are used for attenuating (increasing or decreasing) a beam's intensity only. They can be used, though, to control the intensity of either the reference beam or object beam when attempting to achieve the correct beam ratio intensity between the two beams. I'll discuss this further on.

Fixed Beam Ratio Beamsplitter

Figure 6 shows an un-mounted fixed beam ratio beamsplitter.

fixed beam ratio beamsplitter image
                          Figure 6: Fixed beam ratio beamsplitter.

One side of the beamsplitter is coated with enhanced aluminum (or silver) for the reflected beam and the other side is uncoated for the transmitted beam. This beamsplitter should be placed in the optical setup so that the incident laser beam is on the coated side at a 45 degree angle (optimal angle for these types of beamsplitters). All multi-beam optical setups are laid out to use this 45 degree angle.

You can use the optic mount shown in Figure 7 to hold the beamsplitter. The optic mount with the beamsplitter is then attached to a table mount. Building optic mounts is covered under the sub-section Optic Mounts and building table mounts is covered under the sub-section Table Mounts, both in this section on Building a Holography System.

fixed beam ratio beamsplitter mount image
             Figure 7: Optic mount for fixed beam ratio beamsplitter.

I recommend starting with a 50/50 (R/T) beamsplitter (R means reflected and T means transmitted). For the multi-beam transmission hologram setup, this means that 50% of the reflected beam (O) off the beamsplitter used in the object beam will carry 50% of the laser's power (intensity) and 50% of the transmitted beam (R) through the beamsplitter used in the reference beam will carry 50% of the laser's power (intensity). If nothing else was happening (optically) between the object beam and reference beam as they reached the photographic plate, the beam ratio would be 1 to 1. But, of course, there are a number of optical things happening before the two beams hit the plate, as you can see from the diagrams on the multi-beam transmission hologram (Figures 82 and 85) and a multi-beam reflection hologram (Figures 86 and 89) in the section on Creating Transmission & Reflection Holograms.

As an example, in the multi-beam transmission hologram setup shown in Figure 82, the reference beam will have 3 mirrors and a diverging lens in its path before reaching the plate. This beam losses about 5% of its intensity at each mirror plus the beam is being spread out wider by the DL1 lens, decreasing the beam intensity further. The object beam will have at least 4 mirrors and a diverging lens before illuminating the object scene and depending on the size, color, and the distance the object scene is from the plate, its beam intensity will also be diminished. Additionally, the light reflected from the object scene is usually diffused and you'll find that the reference beam is always brighter at the plate than the object scene reflected light at the plate. When you set up your multi-beam transmission hologram setup to your liking, following the instructions in the multi-beam transmission hologram sub-section in the section Creating Transmission & Reflection Holograms, you will measure each beam's intensity at the plate separately. If you are not within the beam ratios I've suggested, it's relatively easy to achieve the beam ratios you need.

Let's take a few examples in adjusting different beams' intensities. Let's say you want to achieve around a 2 to 1 beam ratio. If the reference beam's intensity is to high, you can use a higher magnification microscope objective or shorter focal length plano-concave lens in the beam to decrease its intensity at the plate.

Another approach is to use a linear or circular variable neutral density filter in the reference beam to attenuate (decrease) the reference beam's intensity, placed between mirror M2 and diverging lens DL1 in Figure 82. The filter should be perpendicular to the incident reference beam and slightly angled towards the edge of the optical table so any reflected light off the filter can be masked. By sliding this filter through the beam, you can control the beam's transmission intensity through the filter. You should make intensity measurements at the plate each time you slightly move the filter until you get the intensity you need. This intensity measuring technique is explained in the sub-section Measuring and Adjusting the Beam Intensity Ratio in the sub-section Creating a Multi-Beam Transmission Hologram in the section Creating Transmission & Reflection Holograms.

A third approach is to use a small piece of clear glass plating, such as picture framing glass (mounted like the fixed ratio beamsplitter), placed in the reference beam between mirror M2 and diverging lens DL1 to deflect part of the reference beam. Start with placing the glass plate perpendicular to the reference beam, then gradually rotate the glass, away from the table, to deflect part of the beam. As the angle of the glass plate moves further away from its perpendicular position, more of the beam intensity will be deflected away from the recording path. You should make intensity measurements at the plate each time you slightly move the glass plate until you get the intensity you need.

One more aspect you need to be aware of is when the reference beam passes through the neutral density filter or deflecting glass plate, it will be slightly refracted and may change the direction of the reference beam after passing through the filter or glass plate. To correct this, just slightly adjust the angle of mirror M2 located prior to the filter or glass plate until the plate holder is uniformly illuminated by the reference beam. Then re-check the reference beam intensity at the plate. If the intensity is not to your liking, you'll have to play with the filter or glass plate and mirror until you get what you need.

If it's the object beam that's too high, you can use the same approaches you used for the reference beam. But I've never found this to be the case. Other fixed beam ratio beamsplitters are available having ratios of 20R/80T, 30R/70T, and 40R/60T. You might want to replacing the 50R/50T beamsplitter with a 40R/60T beamsplitter to see if you still need to use a linear or circular variable neutral density filter or deflecting glass plate.

Circular Variable Gradient Beamsplitter

The best type of beamsplitter, and the easiest to use, is called a circular variable gradient beamsplitter shown in Figure 8. This type of beamsplitter has a wheel with an aluminum reflectivity gradient coating on one side of the wheel to control the amount of laser light reflected and transmitted (the incoming laser beam should impinge on the gradient side of the beamsplitter at a 45 degree angle). As the wheel is rotated from 0 to 360 degrees, the intensity of the reflected beam is decreased while the intensity of the transmitted beam is increased. There are also circular variable gradient neutral density filters available, but you don't want to use these. They only change the intensity of the transmitted beam, not the reflected beam. Although the laser and circular variable gradient beamsplitter are expensive components of your setups, this type of beamsplitter will make your experiences producing multi-beam holograms much more enjoyable. I strongly recommend this type of beamsplitter.

circular variable gradient beamsplitter image
                   Figure 8: Circular variable gradient beamsplitter.

Other Types of Beamsplitters

There are many other types of beamsplitters such as cube, prism, and pellicle beamsplitters just to mention a few. These three types only allow you to change the intensity ratio by changing the incident angle of the beam entering the beamsplitter. This in turn changes both the reflected and transmitted beams' directions causing you to realign all components downstream of the beamsplitter. I don't recommend using any of these beamsplitters. I just wanted you to know about them and not use them.