Basic principles of holography:
Three
easy steps
The first step is to produce interference between the reference beam and
the object beam. Figure 8a shows how the output light of a laser is divided between
reference beam and a beam illuminating an object (mouse). The light scattered by the object is
combined with the reference at the holographic recording plate. The intensity
picture contains the spatial interference pattern between the object and reference
beams. Note that in order for the interference pattern to be recorded, the object and
the reference need to
be present simultaneously. The next step is to remove the object and to
illuminate the hologram plate with the reference beam alone (Figure 8b). The
reference read beam diffracts off the grating in the holographic plate. The beam produced
by diffraction creates a replica of the image of the mouse. An observer (cat)
sees this image exactly as if the original object were present. This is called holographic
reconstruction of the object. The third, and final step is to realize that the
reconstructed object is, in fact, an exact replica of the original. Figure 8c shows the cat
jumping trying to catch the mouse only to find out that the real mouse isn't
present at all!
Interference
in
frequency-selective material
Spectral hole
burning has very special properties, which are not present in conventional
light-sensitive materials. First of all, in SHB materials interference can
take place in frequency domain. Figure 9 shows the frequency domain amplitude of an object
pulse (b) and a reference pulse (a). When these two are combined in
a frequency-selective media,
then the resulting intensity will contain a frequency domain interference pattern
like that shown in the Figure 9c.
Such frequency domain gratings can diffract light in
a similar way as the gratings in conventional spatial holograms. The special property is,
however, that the diffracted beam reproduces not only the spatial image, but also the
frequency domain amplitude of the object. By virtue of Fourier transformation, this allows
to record and play back optical signals in time-domain. The SHB material records the frequency domain
interference pattern between the reference and the object pulse. Shaded area in Figure
9c shows the
inhomogeneously broadened absorption spectrum of a typical organic spectral hole burning
material that has been altered by such writing exposure. The imprinted structure is
proportional to the interference pattern in the lower part of Figure 9c.
Figure 10 shows how to write
a time-and-space-domain hologram. A plane wave reference pulse is incident on a
frequency-selective recording material together with an object pulse. The two signals are
applied with a certain time delay between them. The structure in frequency
domain contains information about the temporal shape of the object pulse and about the
relative delay between the object and the reference. The frequency domain interference pattern can be
different for different spatial locations of the recording media. The spatial structure
contains information about the spatial structure of the object pulse.
Diffraction by frequency-space
gratings
The read-out procedure is similar to conventional
holograms. The object beam is terminated and the hologram is illuminated with a
replica of the reference beam. If the recording media is a thin plate, then two
complementary diffraction orders (diffraction directions) appear. The distribution of the
hologram signal between the diffraction orders depends on the temporal ordering of the
object and the reference pulse during the writing process.
Figure
11 shows that there are three basic
possibilities to read out a time-and-space-domain hologram, depending on the relative
delay between the writing beams.
(a) - If, in the writing process, the reference
precedes the object, then the hologram will diffract light only in the +1 direction. The
signal is a replica of the temporal and the spatial structure of the original object wave
pulse.
(b) - If the writing reference pulse comes after the
object, then the hologram diffracts light in the -1 direction and the signal reproduces a
time-inverted replica of the original object wave. The spatial wave front is also
conjugated.
(c) - If the writing reference pulse is timed to
overlap with the middle of the object pulse, then the hologram diffracts light in both
directions. The signal in +1 order reproduces the part of the object after the reference,
while the signal in -1 order reproduces time-inverted replica of the object before the
reference. These unusual asymmetric diffraction properties are a consequence of the
principle of causality, which prohibits any response signal at a time that is earlier than
the cause.
Femtosecond time-space holograms
SHB with very broad inhomogeneous absorption bands,
such as some dye-doped polymers can be used to record holograms by illumination with
ultrashort laser pulses. Full resolution image holograms (Figure 12) can be recorded even
if the light pulses have a really negligible coherence time of a few femtoseconds (1
femtosecond = 10-15 s). The recording of holographic images is in this case
possible only because the interference is achieved not through the coherence of the
pulses, but rather through the intrinsic coherence provided by the special recording
media. The effective coherence time of a frequency-selective material is given by the
inverse value of the homogeneous line width of the zero phonon line. The corresponding
numerical value ranges from a fraction of a nanosecond to milliseconds. Intrinsic
coherence time is independent of the duration of the illuminating beams. Direct
time-resolved measurements, such as presented in Figures 12 show that SHB
hologram reproduces not only the spatial image, but also the full time domain structure of
the object.
Figure
13 shows how the temporal ordering of the object and reference
pulses can change the direction of time.
(a) - During the writing procedure the reference is applied
after the object;
(b) - Readout with a short reference pulse (dashed line)
recalls a time-inverted replica of the object.
We
have also shown the reproduction of a very high-speed optical data pulse
train. The data rate here is on the order of 1012-1013 bits per
second. Each pulse in this data train has duration of about 300 femtoseconds. The shortest duration of one data pulse is limited by the inverse value of
the inhomogeneous line width of the recording material. Some
organic dye-doped materials can provide time resolution of hundreds to few tens of
femtoseconds. This experiment shows the reproduction of a
70-femtosecond data pulse by a single spot of a time-and-space-domain hologram. Thousands
and tens of thousands of data pulses this short can be recorded and played back from a
single spatial spot in a time less than one nanosecond. It is interesting to compare this
rate to the conventional PC, which currently requires 1 to 5 nanoseconds to complete one
cycle. In principle, this new technology can handle data rates of several terahertz per
one single optical channel. Spatial channel multiplexing can further tremendously increase
the data throughput. Future advanced technology will surely try to profit from these
unique possibilities offered by SHB materials.
Spectral
Hole Burning
Frequency-Selective
Storage
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