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Single-exposure holographic multiplexing
Volume
7, number
OPTICS COMMUNICATIONS
4
SINGLE-EXPOSURE
HOLOGRAPHIC
April 1973
MULTIPLEXING
P.C. MEHTA and Mahipal SINGH Instruments
Research
and Development
Received
Establishment,
26 February
A single-exposure method using a convex lens and a diffuser reconstructed images have been obtained without cross-talk.
Multiplex holograms have normally been prepared by either changing the angle between the reference and the object beams [I], or by using complementary masks [2]. Both of these techniques require separate exposures for each hologram, thus decreasing the diffraction efficiency. The present communication describes a single-exposure method by which satisfactory image reconstruction is possible without cross-talk. In the first method either the reference beam is kept constant or both the reference and the object beams are changed between the multiple exposures. The main drawback of this technique is that the total power that can be diffracted into anyone of the N reconstructed images is approximately l/N* of the power that can be diffracted by storing only one information on the same photographic plate and that the signal-to-noise ratio of one image varies as l/N2. In the second method there are two approaches. The recording plate is divided into N separate regions andN different holograms are recorded into each region. In this case the resolution becomes less as the aperture size for each hologram is proportional to l/N’/‘. The other approach is to record each image on a spatially distinct area of the photographic plate, the distinct area occupies, in parts, the full area of the plate. Thus, each information is recorded in a large number of small areas distributed over the full area of the plate. For recording other information complementary areas are used by using complementary masks. In this technique the image quality is inversely proportional toN, since as N increases the total area available for each information decreases. Moreover, the reconstruc394
Debra Dun, India
1973
is described
for holographic
multiplexing,
by which
tion is difficult, as it requires the perfect alignment of the mask with the hologram. We describe here a single-exposure method in which hologram multiplexing has been achieved by using a convex lens. This technique requires only a single exposure and at the same time entire area of the plate is exposed to each of the information to be recorded. Therefore, the diffraction efficiency and hence the quality of each reconstructed image remains good. The schematic drawing of the recording system is shown in fig.1. The beam was cleaned by a spatial filter and the angle between the reference beam and the object beam was about 15”. Two object transparencies were placed in the front and back focal planes of a convex lens and the third object transparency was placed so as to achieve its real image by the lens. Thus the photographic plate receives the information from all of the three object transparencies. The focal length of the lens was 12.5 cm, the explosure time was 15 seconds with an Agfa Scientia 10 E 75 photographic plate for the laser of 2mW power. The film was developed by Kodak D-19 developer for 5 minutes. In reconstruction with a convergent beam, all the three images are focussed on a screen placed at different distances from the hologram, without overlapping. The quality of each image was comparable to that of an ordinary hologram of the corresponding object. The reconstructed images are shown in fig.2. Although this method has been applied for only N=3, it can be extended to more objects by placing them at suitable positions with respect to the lens. The previously known methods can be combined with the
Volume
7, number
Fig.1. Schematic PH-photographic
diagram plate.
April 1973
OPTICS COMMUNICATIONS
4
of the recording
system.
S-laser, SF-spatial
filter, BS-beam
splitter,
M-mirror,
D-diffuser,
O-object,
L-lens,
(a)
(b) Fig.2. Reconstructed
images.
395
Volume
7, number
4
OPTICS COMMUNICATIONS
April 1973
PH
lig.3.
Schematic
diagram
fur an cxtenslon
of the present
present method to increase the number of informations that can be stored. One possible extension of the present method is shown in fig.2. A detailed theory of the method will be published elsewhere. The authors are grateful to Dr. P.K. Katti for help-
396
method.
(Symbols
as used in fig.1.)
ful discussions and suggestions.
References [ 11 P.J. van Heerden, Appl. Opt. 2 (1963) 387. [2] H.J. Caulfield, Appl. Opt. 9 (1970) 1218.
7, number
OPTICS COMMUNICATIONS
4
SINGLE-EXPOSURE
HOLOGRAPHIC
April 1973
MULTIPLEXING
P.C. MEHTA and Mahipal SINGH Instruments
Research
and Development
Received
Establishment,
26 February
A single-exposure method using a convex lens and a diffuser reconstructed images have been obtained without cross-talk.
Multiplex holograms have normally been prepared by either changing the angle between the reference and the object beams [I], or by using complementary masks [2]. Both of these techniques require separate exposures for each hologram, thus decreasing the diffraction efficiency. The present communication describes a single-exposure method by which satisfactory image reconstruction is possible without cross-talk. In the first method either the reference beam is kept constant or both the reference and the object beams are changed between the multiple exposures. The main drawback of this technique is that the total power that can be diffracted into anyone of the N reconstructed images is approximately l/N* of the power that can be diffracted by storing only one information on the same photographic plate and that the signal-to-noise ratio of one image varies as l/N2. In the second method there are two approaches. The recording plate is divided into N separate regions andN different holograms are recorded into each region. In this case the resolution becomes less as the aperture size for each hologram is proportional to l/N’/‘. The other approach is to record each image on a spatially distinct area of the photographic plate, the distinct area occupies, in parts, the full area of the plate. Thus, each information is recorded in a large number of small areas distributed over the full area of the plate. For recording other information complementary areas are used by using complementary masks. In this technique the image quality is inversely proportional toN, since as N increases the total area available for each information decreases. Moreover, the reconstruc394
Debra Dun, India
1973
is described
for holographic
multiplexing,
by which
tion is difficult, as it requires the perfect alignment of the mask with the hologram. We describe here a single-exposure method in which hologram multiplexing has been achieved by using a convex lens. This technique requires only a single exposure and at the same time entire area of the plate is exposed to each of the information to be recorded. Therefore, the diffraction efficiency and hence the quality of each reconstructed image remains good. The schematic drawing of the recording system is shown in fig.1. The beam was cleaned by a spatial filter and the angle between the reference beam and the object beam was about 15”. Two object transparencies were placed in the front and back focal planes of a convex lens and the third object transparency was placed so as to achieve its real image by the lens. Thus the photographic plate receives the information from all of the three object transparencies. The focal length of the lens was 12.5 cm, the explosure time was 15 seconds with an Agfa Scientia 10 E 75 photographic plate for the laser of 2mW power. The film was developed by Kodak D-19 developer for 5 minutes. In reconstruction with a convergent beam, all the three images are focussed on a screen placed at different distances from the hologram, without overlapping. The quality of each image was comparable to that of an ordinary hologram of the corresponding object. The reconstructed images are shown in fig.2. Although this method has been applied for only N=3, it can be extended to more objects by placing them at suitable positions with respect to the lens. The previously known methods can be combined with the
Volume
7, number
Fig.1. Schematic PH-photographic
diagram plate.
April 1973
OPTICS COMMUNICATIONS
4
of the recording
system.
S-laser, SF-spatial
filter, BS-beam
splitter,
M-mirror,
D-diffuser,
O-object,
L-lens,
(a)
(b) Fig.2. Reconstructed
images.
395
Volume
7, number
4
OPTICS COMMUNICATIONS
April 1973
PH
lig.3.
Schematic
diagram
fur an cxtenslon
of the present
present method to increase the number of informations that can be stored. One possible extension of the present method is shown in fig.2. A detailed theory of the method will be published elsewhere. The authors are grateful to Dr. P.K. Katti for help-
396
method.
(Symbols
as used in fig.1.)
ful discussions and suggestions.
References [ 11 P.J. van Heerden, Appl. Opt. 2 (1963) 387. [2] H.J. Caulfield, Appl. Opt. 9 (1970) 1218.