There are several different lasers which are used in the production process for holograms. The most common lasers used in holography are Helium-neon (He-Ne), Helium-cadmium (He-Cd), Argon-ion (Ar+) and Krypton-ion (Kr+) lasers.38 Many types of CW lasers can also be operated in a pulsed mode, though so far none of them seems to be suitable for holography. Monocrystalline aluminium oxide doped with lanthanide elements such as yttrium ( yttrium aluminium garnet, or YAG crystal ) can be used to change the wavelength of a laser.
A semiconductor laser is a special kind of light-emitting diode.
It produces a beam of light in the near infrared with a divergence
of about 15o, but the cone of emitted light is elliptical rather
than circular, so that the beam appears to have originated from
a line rather than a point. If the astigmatism of this beam is
corrected by means of aspherical optics, a spatially-coherent
beam can be obtained, and this has been used experimentally for
making holograms. The main attraction of semiconductor lasers
is that they are cheap and very small. They also operate at comparatively
low voltages and art similar power range to that of He-Ne lasers.
There are a number of things to be considered in the choosing of a laser. A laser used to produce holograms needs good stability, and must be free from vibrations.37 The laser beam must be as plane as possible. A laser beam with multi modes is useless for making holograms. We want that the laser should have a circular beam diameter without any noise. The beam diameter is the important parameter in the calculation of the pinhole of the spatial filter.
The coherence length of the laser should be as large as possible. If the coherence length is small, the requirements of the path difference between the object and reference beam become harder to meet. This means that the path difference between these beams must be nearly zero. The number of modes in the laser is also an important parameter. In holography we prefer a laser with as few as possible modes. If we use a multi mode laser, we have problems with low visibility and the contrast in the hologram will be low.
Figure 4-1 The basic element of the laser
Three basic interaction process of light with matter are important
for the laser.40 These are absorption, stimulated emission,
and spontaneous emission. We assume that two states, of energies
E1 and E2, take part in the interaction.7
Absorption is when a photon of energy hv strikes an atom of the laser medium in the state E1 and disappears, exciting the atom to the higher state E2. The photon can only be absorbed, if the absorption energy is hv E2 E1. When no suitable energy level is available, no absorption takes place, and the medium is transparent for photons of this energy.
We have stimulated emission when the atomic system has absorbed the energy hv and thus the upper level is occupied, a second photon of energy hv may cause this energy to be emitted as a photon.33 Then two photons having identical properties leave the atom. Upon absorption, the atomic system starts from the state of lower energy, upon stimulated emission it starts from the state of higher energy. The transmission probability is equal for both processes.
In spontaneous emission the atomic system in the state of higher energy, E2, decays into a state of lower energy, E1, by the emission of a photon. The word spontaneous indicates that the transition take place with the randomness that is characteristic for quantum processes.
Where the frequency is given by
(4.1)
E1 Energy level 1, also called ground level.
E2 Energy level 2, also called excited level.
h Planck constant
v frequency
The helium-neon laser, usually abbreviated to He-Ne, is the most common type of gas laser. The tube contains helium gas at a pressure of about 1 torr and neon pressure of about 0.1 torr. (a torr is a unit of pressure equivalent to 1/760 of an atmosphere). The main purpose of the helium is to act as a continuos reservoir of energy (supplied with electrical discharge) for the neon. This laser is the one that is best suited to general-purpose holography.
Figure 4-2 Internal design of a modern helium-neon laser
He-Ne lasers as used for holography operate at a wavelength of 632.8 nm, with a power ranging from 0.5 mW to 100 mW. The randomly-polarised type are unsuitable for serious holography, as the direction of polarisation27 is an important factor for obtaining optimum image quality. A laser with Brewster angle windows has a somewhat lower output than its randomly-polarised equivalent, but it has a completely stable plane of polarisation. In this thesis work has the choice of laser fell on red He-Ne lasers. In the beginning of the experimental work there was used a 12 mW red He-Ne laser. During the experimental work this laser was changed to a new and more powerful red He-Ne laser with an output power of 24 mW. The reason for the choice of this type of laser is the He-Ne laser's advantage in laser beam stability, laser modes, beam diameter, coherence length, output power and price. Another reason is that most of the literature recommends the use of He-Ne laser in the production of holograms.
Figure 4-3 Energy levels of He and Ne involved in the He-Ne laser.
For measuring the power stability of the 24 mW He-Ne laser the following set-up was arranged on the optical table.
Figure 4-4 Optical set-up for measuring of laser power stability.
The neutral density filter was used to reduce the laser's output
power with 50 %, to a readable value for the laser power meter.
To detect the power of the laser, the laser power meter reads
the data continuously. This data is then logged in the PC 28
by the data logging software program PICO ADC-1236.
The data is logged for two different sampling rates and time lags.
The values from ADC-12 are then converted to LOTUS 1-2-3
to make it possible to present the data in a suitable way.
The first measurement is a short time logging made with sample pr. 100 ms in 10 seconds. The other measurement is a long time logging made with 1 sample pr. second in 30 minutes. The idea behind two different measures is to see how the laser works during holographic recordings (short time) and how stable the lasers output power is over time.
Figure 4-5 Laser beam stability for 24 mW He-Ne laser with sample each 100 ms in 10 seconds.
Laser output power data from sample rate at 100 ms in 10 seconds
(short time).
Average value : 680.6
Standard deviation : 13.1
The laser output power stability for this measurement is about
1.9 %.
From Melles Griot product catalog34 the laser output power stability is given by 2.5 %.
Figure 4-6 Laser beam stability for 24 mW He-Ne laser with
sample each second in 30 minutes.
Laser output data from sample rate at 1 second in 30 minutes (long
time).
Average value : 673
Standard deviation : 2.8
The output power stability for this measurement is about 0.4 %
During the production of a holographic transmission multi-stereogram, where 70 different part holograms are exposed onto the film, each exposure is about 10 seconds and the entire recording process takes about 30 minutes. From figure 4-5 can we see that the laser power stability for one part exposure of the film is good, and the measurement agrees with the data from the manufacturer, Melles Griot34. In practice, the spikes measured in the short time of measurement should not reduce the hologram's visibility.
From figure 4-6 can we see that the output power from the laser
is quite stable over the whole recording process of 30 minutes.
This means, that each of the part holograms on the multi-stereogram
are evenly exposed on the film. The possibility of getting good
results in the holographic multi-stereogram production with the
use of this laser is good.
The laser was turned on at least 3 hours before the measurement was taken. It is very important that the laser is heated and becomes stable before the recording of holography is started.
Transverse modes are classified according to the number of noughts that appear across the beam cross section in two directions. The lowest-order, or fundamental mode, where intensity peaks at the centre, is known as TEM00. The mode with a single nought along one axis and no nought in the perpendicular direction is TEM01 or TEM10, depending on orientation. A sampling of these modes, which is produced by stable resonators, is shown in figure 4-7.50
Figure 4-7 Lower-order laser modes that can be produced by a stable resonator.
For most applications for example like holography, the TEM00 mode is considered most desirable, but multi-mode beams can often deliver more power in a poorer-quality beam, and thus are acceptable for some uses.
The multiple longitudinal mode structure gives rise to a power
fluctuation phenomenon termed mode sweeping. All unstabilized
helium neon lasers exhibit this effect, which is due to thermal
instability causing variation in the cavity length. As the cavity
length changes, there is a small change in mode spacing which
is typically 10 kHz or less under normal conditions.
However, the absolute wavelength of each cavity mode is also changed by variation in tube length. This is typically 2.5 10-3 nm/C; i.e., 2103 MHz/C, depending on the glass type used for the tube. In effect, the comb of longitudinal modes drifts with respect to the Doppler broadened line centre, repeating its initial relative position in less than 1K. Because of the non-flat, Gaussian profile of the gain curve, the overall power output changes. If the mode spacing is very small, as with a long laser tube, these changes may be very small. On the other hand, a short laser tube may have only one or two cavity modes under the Doppler profile, and the sum of their position on the Gaussisan gain curve.
This effect is almost identical for all unstabilized commercial TEM00 tubes and is a function of cavity length. The overall amplitude fluctuations are typical a few percent.
In the production of holographic multi-stereograms, where the recording process can be long, it is very important that the laser is thermal stable. If there is thermal instability and the output power is changing, the hologram can be unevenly exposed.
Ordinary light is disorganised, not capable of producing interference. Such light is called incoherent. Light from a laser is highly organised, and easily produces interference. Such light is called coherent.
Some electromagnetic radiation such as microwaves, radio waves as well as sound waves, water waves and other mechanical waves can be generated as an infinite number of waves, one after another. Light wave cannot, because light waves always come in wave trains. The wave trains are of finite length, and each train containing only a limited number of waves. The length of a wavetrain is called the coherence length.
Figure 4-8 Wavetrain from a laser
Coherence length can be expressed as the product of the number
of waves, N, contained in the train and their wave length, .
The formula for coherence length is then given by
s = N
(4.2)
Since the velocity is the distance travelled per unit of time,
it takes a wave train of length s a certain length of time, t,
to pass a given point and we get therefore
(4.3)
where c is the velocity of light, and the length of time t is
called the coherence time.
In holography it is important that the path difference between the reference and object-beam is zero, or very small.
If the path different between these waves is too long, as long as the coherence length, the contrast of the image will be very weak and it is impossible to see the image.
That can be done with help of Michelson interferometer,7 41 and plotting the visibility as a function of the path difference between these waves.
With the knowledge of the coherence length and the visibility
plot shown in figure 4-10 it is possible to find the difference
of the laser beam distance between the reference and object beam,
which reduces the holograms contrast.
Figure 4-9 The Michelson interferometer.
The light from the laser is divided into two beams by the cube splitter ( 50 : 50 splitting ratio). One beam is reflected back onto itself by a fixed mirror, the other one is also reflected back by a mirror, but one that can be shifted along the beam. Both reflected beams are divided again into two by the beam splitter, whereby one beam from each mirror propagates to a screen. On
this screen the light intensity is measured by a laser power meter.
When the position of the adjustable mirror is changed, the interference
fringes on the screen also change. The light intensity from the
laser is measured for several different positions of the adjustable
mirror.
The light intensity from the laser is measured for 30 different
path lengths of the laser- interferometer arm. The adjustable
mirror on Michelson interferometer is changed from zero path difference
to a total of 150 cm path difference, at a step rate of 5 cm.
The data from the measurements is logged with the help of a software
program called Picolo.36 The data was
logged for one sample for every 100 ms, and a total of 2000 samples.
From this data it is possible to find the coherence length of
the laser.
The visibility of the fringes is defined as
(4.4)
Because of the light from the background, this must be corrected.
(4.5)
The theoretical visibility for a laser with 3 modes is given by
(4.6)
The visibility data from the Michelson interferometer visibility
measurement and the theoretical visibility is plotted in figure
4-10.
Figure 4-10 Visibility plot for theoretical
and measured data.
From figure 4-10 can we see that the experimental data correspond
quite well with the theoretical visibility of a 3-mode laser.
This means that the 24 mW He-Ne laser used in this thesis has
3 modes. The practical definition of coherence length is the distance
travelled by the laser beam where the visibility is reduced to
1/e2, measured with Michelson interferometer.
The plot of measured data shows the coherence length is around 30 cm.
The visibility maximum occur when the path difference is 0 and
115 cm. It means that (2L=115cm).
From the technical data for the laser, the longitudinal mode spacing
is given as 257 MHz.
The formula for the distance between two longitudinal modes is
given by
, and 
.
(4.7)
The measured value for the distance between two longitudinal modes
fits the value for the fabrication data of the laser.
The coherence length for a 3-mode laser is given by
Lk
0.596 L (4.8)
From the measured data of visibility plot we know that 2L =
115 cm.
Thus the He-Ne laser has a coherence length of 34 cm.
In holography this is an important value because the visibility plot gives us an idea of the contrast of the hologram. From the visibility plot of the laser, we can see that the contrast will
fall to 0.6 if the different between the object and reference beam is 10 cm. The best result is obtained when the difference between the beams is 0 or 115 cm, when the visibility (contrast) is maximum.
