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Lasers In Holography
Excerpted from Holography MarketPlace (Ross Books)
The three primary types of cw lasers used in holography are argon-ion, helium cadmium (HeCd), and helium neon (HeNe) lasers. Each has distinct advantages that are related to the holographer's needs. Typically, the recording medium, the size and depth of field of the hologram, and budget considerations determine which laser is best suited for the application.In embossed holography, photoresist is the primary medium used for recording images. The use of photoresist enables mass-produced holograms. Photoresist chemically etches the holographic image onto a glass plate. The optically engraved glass plate (called a master) is electroplated, producing a shim. The shim is placed on an embossing machine for mass stamping of embossed holograms.
Because photoresist is extremely sensitive to wavelengths between 420 nm and 450 nm. HeCd lasers (which lase at 442 nm) are ideally suited for this application.
Artistic holography is not constrained by the necessity to mass produce. This gives the holographer freedom to choose from a variety of emulsions when producing holograms. In such cases, most holographers prefer to use emulsions that are sensitive to the primary wavelength (514 nm) of an argon-ion laser. These lasers provide an attractive combination of high power and long coherence length, enabling holograms that are both large and visually striking.
In most forms of holography, the coherence length of the laser determines the size of the hologram. Generally, a laser with a 10-cm coherence length can produce holograms that are 10 x 10 cm. Seasoned holographers can shoot holograms as large as 15 x 15 cm with a laser whose coherence length is 10 cm.
Novice holographers typically cannot justify the expense of an argon-on laser or HeCd laser. For "week-end shooters" on a budget, the HeNe laser provides a cost-effective alternative.
Certain laser parameters are essential to all forms of cw holography. The beam must have a single transverse mode (i.e., a gaussian TEM00 mode). A polarized laser beam (at least a 100:1 extinction ration) is necessary in all forms of holography except dot matrix. The low exposure times of dot-matrix holography, typically five milliseconds per dot, reduce the need for polarization. The laser cannot generate excessive heat or vibration, two mortal enemies of the holographer.
With the exception of dot-matrix holography, high power is very desirable. The higher the power, the shorter the exposure times, and the less chance that something will go wrong.
Argon-Ion Lasers
Before the development of the HeCd laser, argon-ion lasers were the preferred choice for almost all forms of cw holography.
The argon laser provides a generous amount of power, polarization and coherence length--three of the more prized parameters
in holography.
Argon-ion lasers produce lasing at many wavelengths between 454 nm and 529 nm and can be equipped with a prism
wavelength selector in the cavity to allow the operator to select a specific wavelength. Of primary interest in holography
are the 514-, 488-, and 458-nm wavelengths. The 514-nm line is the most powerful, followed closely by the 488-nm line.
In embossed holography, the powerful 514-nm and 488-nm wavelengths of an argon laser have little effect on the photoresist
used to record the holograms. Because photoresist is extremely sensitive only to wavelengths between 420 nm and 450 nm,
only the 458-nm argon line can effectively expose it. Unfortunately, the 458-nm line is relatively weak in comparison
with the more powerful 514- and 488-nm lines -- less than 20% of the relative power
Helium Cadmium (HeCd) Lasers
For manufacturers of dot-matrix holograms, in which the exposure time is trivial, HeCd lasers have always been the
laser of choice. They cost less to buy and operate, are easier to use, have less maintenance, and last longer than
a comparable argon laser.
With the development of higher power systems, the HeCd laser has also come to dominate embossed holography. The 442-nm
line of a HeCd laser exposes the photoresist used in embossed holography ten times more effectively than the 458-nm line of
an argon laser. A typical HeCd laser can deliver more than 150 mW TEM00 and can have coherence lengths as high as 30 cm -- more
than enough for a 6-in. by 6-in. hologram. The effective exposure on photoresists for HeCd lasers meets or exceeds that of
large-frame argon-ion lasers, while saving the average holographer in excess of $800 per month per laser on electricity and
water bills. This improvement has created a profound change in the embossed holography marketplace. Today, nearly 90 percent
of the lasers purchased for commercial embossed holography are HeCds.
Helium Neon (HeNe) Lasers
The HeNe laser was the first gas laser to be commercially available, brought to market in 1961. Over thirty years later,
the HeNe laser is still the most commonly used gas laser. OEM manufacturers as well as high schools and universities find
their low price, ease-of-use, beam pointing stability, long tube life, and ability to operate from standard 115 Vac outlets
extremely attractive.
An average HeNe laser cost a few hundred dollars, making it an affordable tool for those who normally could not afford the
expense of an argon-ion or HeCd laser. HeNe lasers are low powered, typically delivering between 0.5 and 1 mW, TEM00 at
633 nm. More expensive models are available, delivering up to 35 mW, TEM00 at 633 nm. The beam is generally
polarized with a coherence length of 20 to 30 cm, however stabilized models are available with coherence lengths of several
kilometers.
The low-cost HeNe will always be the perfect laser for novice holographers. The combination of affordability,
high-reliability, and ease of use makes this laser perfect for the production of budget holograms.
