header image
sub header image
header divider
build holography system header
Introduction

Overview

Optical Table

Environment

Laser

Beamsplitter

Mirrors/Lenses

Table Mounts

Optic Mounts

Plate Holder

Objects/Scenes

Hardcopy

Resources

Laser

You must use a laser, as shown in Figures 3a, to make a high quality hologram. 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).

laser image
Figure 3a: Five milliwatt Helium-Neon laser.

laser mounting image
Figure 3b: Table mounts for laser.

There are four 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 3a if you're starting with a 4 inch x 5 inch recording 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 recording 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 recording 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.

Additional Information

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. I know. 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 50 mW He-Ne laser). This web site and its author are in no way liable for your use of any laser.

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 zone. Manufacturers usually do not specify the coherence length of He-Ne lasers. This property determines how deep your object scene can be before the illumination of the object scene in the recorded hologram starts to fall off to zero. If the depth 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 will not record these areas. That is why when you measure your optical path lengths, to be discussed later, you should always measure to the center of your object scene. Imagine the depth-of-field zone as a circle with a diameter of 10 inches with the circle's center located at the center of your object scene. Any part of your scene outside this diameter will not be recorded.

As an example, a 5 mW He-Ne laser's coherence length is around 10 inches. So if your object scene is 10 inches wide and 20 inches deep and you've measured to the center of the object scene, then 5 inches in front of, and behind, that center point will be recorded in the hologram. The rest of the object scene will be dark and not visible.

 

Revised 5/2/2017