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Introduction

Overview

Optical Table

Environment

Laser

Beamsplitter

Mirrors/Lenses

Table Mounts

Optic Mounts

Plate Holder

Objects/Scenes

Resources

Laser

You must use a laser to make a high quality hologram as shown in Figures 4 and 5. A laser is a source of coherent light necessary to produce a high quality hologram. Fully coherent light sources, such as lasers, are both spatially coherent and temporally coherent. A laser emits light in a very narrow beam and is considered a point source (spatially coherent), as opposed to an extended source (spatially incoherent) such as a frosted incandescent bulb or a fluorescent lamp. A laser also emits light of a single color or wavelength (temporally coherent) whereas an incandescent light bulb or fluorescent lamp emits light of many wavelengths (temporally incoherent).

Additionally, only one laser can be used in any optical setup. Two or more lasers cannot be used for various beams in an optical setup.

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Figure 4: Five milliwatt Helium-Neon laser.

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Figure 5: Table mounts for laser.

There are five properties of a laser you need to consider when purchasing a laser for making holograms:

Laser's Power Output

I recommend you start your holography experience using a helium-neon (He-Ne), 5.0 milliwatt (mW) gas laser as shown in Figure 4 if you're starting with a 4 inch x 5 inch photographic plate or smaller. You need to use at least a 5.0 mW laser to keep your exposure times short and reduce the potential of table vibrations, and/or component movements, destroying the image. The important message here is that the higher the power output of the laser, the shorter the exposure time. My exposures range from 10 seconds to 60 seconds, depending on the reflectivity and size of the object scene, the optical arrangement I'm using, and the photographic plate sensitivity.

If you want to create 8 inch x 10 inch or 12 inch by 16 inch, you will need at least a 20 mW or 35 mW laser, respectively. On the other hand, you could start with 2.5 inch x 2.5 inch photographic plates which will further reduce your exposure time and cost using a 5 mW laser.

Laser's Beam Polarization

The laser should be linearly polarized as opposed to randomly polarized. A linearly polarized beam means that the electric and magnetic fields of the beam spend all of their time orientated in one direction perpendicular to the propagating beam direction and the two fields are at right angles to each other. A randomly polarized beam means these fields are continually rotating perpendicular around the propagating beam direction. When the beam is linearly polarized and two of these beams (from the same laser) interfere at the hologram, the energy in the electric fields add together in a maximum way and you get the highest image brightness and contrast in the recorded interference pattern. With a randomly polarized beam, it's a hit and miss affair and maximum brightness and contrast in the recorded patterns is usually not obtained.

Laser's TEM Mode

The TEM mode (transverse electromagnetic mode) should be single mode as opposed to multimode. The single mode designation is TEM00. Multimode designations are TEM01, TEM02, etc. I'm not going to explain what TEM00 means since it would be too lengthy. You just need to made sure the laser manufacturer's specifications designate this mode.

Laser's Beam Wavelength

Most He-Ne lasers emit light at 632.8 nanometers (red) although they are available with infrared, yellow, orange, and green wavelengths. You should purchase a He-Ne with a 632.8 nanometers output because the recording plate sensitivity is set to this wavelength (red sensitive plates and films). Additionally, this red wavelength gives the highest power output from a He-Ne laser.

Regardless of what laser company you purchase from, their brochures (or web sites) will list the specifications for power output, polarization, TEM mode, and wavelength. He-Ne lasers are air cooled, operate on 110-120 VAC (volts alternating current), and have a life expectancy of greater than 20,000 hours.

Coherence Length

A fifth property of a laser is the coherence length of a laser beam. This is also known as the depth-of-field of the illumination zone. This property determines how deep and wide your object scene can be before the uniform brightness level of the illuminated object scene in the recorded hologram starts to gradually decline to zero. If the depth and/or width of your object scene is greater than the coherence length of the laser beam, those areas outside your depth-of-field zone in your object scene will look illuminated on the table, but the hologram image will gradually darken as the distance increases from the zone. That is why when you measure your optical path lengths, to be discussed later, you should always measure from the center of your object scene to the center of the photographic plate. Bottom line: for a 35 mW He-Ne laser at 632.8 nm, the coherence length is about 10 inches, and areas of the object scene greater than 5 inches in front of and behind the center of the scene, as well as areas greater than 5 inches to the right and left of the scene, will gradually fade out in the recorded image. He-Ne lasers having output powers greater or less may have different coherence lengths.

Manufacturers usually do not specify the coherence length of He-Ne lasers in their specifications. Call them and ask them about the coherence length of their laser. You might also want to ask them how they determine this coherence length so you can try the procedure yourself.

Laser Safety

Never let an undiverged He-Ne laser beam hit your eye. It may severely damage your retina and your sight. It could possibly blind you. It really hurts too, like nothing you've ever felt! I've experienced it once in fifty years. When I say "undiverged" beam, I mean the beam coming directly out of the laser, which is about 0.08 inches in diameter and at full power. Once the beam gets spread out using a diverging lens, then it's relatively safe (for a 5.0 mW to 35 mW He-Ne laser) at a distance of 6 feet or greater. Bottom line: I can't imagine any reason to look directly into any laser beam, even diverged! When reviewing a recorded hologram in a diverging laser beam, the reconstruction angle is such (56 degrees) that when you look at the plate image with the plate vertical, the reconstructing beam is angled upward or downward and your eyes are not in the beam's path. This web site and its author are in no way liable for your use of any laser.