1. Field of the Invention
The present invention relates in general to making a hologram, and more particularly to a method and an apparatus for making a hologram wherein interference patterns to be recorded on a hologram plate are prevented from travelling to be efficiently modulated, thereby generating a high quality hologram.
2. Description of the Prior Art
Referring to FIG. 1, there is shown an arrangement of a conventional apparatus for making a transmissive hologram. As shown in this drawing, the conventional apparatus comprises a laser 11 as a light source, a shutter 12 for blocking or passing a laser beam L being emitted from the laser 11, a beam splitter 13 for splitting the laser beam L passed through the shutter 12 into two beams L1 and L2, reflection mirrors 14 and 15 for reflecting the beams L1 and L2 split by the beam splitter 13 at desired angles, respectively, a beam expander 16 and a lens 17 for producing an object beam Lo by expanding a diameter of the reflected beam L1 to allow the reflected beam L1 to light a desired area, a beam expander 18 and a lens 19 cooperative with each other for producing a reference beam Lr by expanding a diameter of the reflected beam L2 to allow the reflected beam L2 to light the same area as that of the reflected beam L1, and a hologram plate 20 for forming interference patterns 21 thereon based on an interference phenomenon of the object beam Lo and the reference beam Lr incident thereon, to record the hologram thereon.
The operation of the conventional transmissive hologram making apparatus with the above mentioned construction will hereinafter be described.
The laser beam L emitted from the laser 11 as a light source is blocked or passed according to an operating state of the shutter 12. Namely, when the shutter 12 is turned on, the laser beam L emitted from the laser 11 is applied through the turned-on shutter 12 to the beam splitter 13, which splits the laser beam L into the two beams L1 and L2 which are then reflected at their desired angles by the reflection mirrors 14 and 15, respectively.
The reflected beams L1 and L2 are altered, respectively, into the object beam Lo and the reference beam Lr, with the diameters thereof being expanded by the beam expanders 16 and 18 and the lenses 17 and 19, in order to light desired areas of the hologram plate 20. The object beam Lo and the reference beam Lr are incident on the hologram plate 20 at their desired angles.
In the above structure, if desired, the lenses 17 and 19 may be omitted and an object, an image of which is to be recorded, may be used instead such that a beam transmitted to or dispersed by the object is used as the object beam.
The object beam Lo and the reference beam Lr incident on the hologram plate 20 form a series of black and dark portions, or the optical interference patterns 21 on the hologram plate 20 based on an interference phenomenon. The formed interference patterns 21 are recorded on the hologram plate 20.
The process of recording the interference patterns 21 on the hologram plate 20 will hereinafter be described.
As the laser beam L from the laser 11 is passed through the shutter 12 in the turned-on state, a point P on the hologram plate 20 is exposed to the object beam Lo and the reference beam Lr. Present at the point P on the hologram plate 20 is an intensity I(P) resulting from the interference phenomenon of the object beam Lo and the reference beam Lr. Assuming that the time for which the laser 11 is at its ON state is T, energy at the point P on the hologram plate 20 can be expressed by the following equation: EQU E=I(P).multidot.T (1)
From the above equation (1), it can be seen that the point P on the hologram plate 20 is subject to the energy as much as E=I(P).multidot.T. As a latent image resulting from the energy is developed, it is recorded as the interference patterns 21 based on a variation in an optical nature on the hologram plate 20.
It is impossible to make a direct observation on the interference patterns 21 on the hologram plate 20 since adjacent ones of the interference patterns 21 are arranged to have, typically, an interval of 1 .mu.m with respect to each other.
In making the hologram as mentioned above, assuming that dl1 and dl2 are, respectively, paths along which the laser beam L from the laser 11 is split into the two beams L1 and L2 by the beam splitter 13 and then incident on the point P on the hologram plate 20, .lambda. and is a wavelength of the laser beam L, a path difference .delta. and a phase difference .DELTA..phi. between the two beams L1 and L2 can be expressed by the following equations: EQU .delta.=dl1-dl2 EQU .DELTA..phi.=2.pi..multidot.[(dl1-dl2)/] (2)
The intensity of the beams being, in a moment, incident on the point P on the hologram plate 20 is determined based on the phase difference .DELTA..phi. in the above equation (2). EQU That is, if .DELTA..phi.=2n.pi., then I(P)=Imax, and EQU if .DELTA..phi.=(2n+1).pi., then I(P)=Imin=0
For this reason, in order to accurately record the hologram on the hologram plate 20, the intensity of the beams incident on the hologram plate 20 must be maintained accurately and constantly for the exposure time T of the shutter 12 required to record the hologram. However, the paths dl1 and dl2 along which the laser beam L from the laser 11 is split into the two beams L1 and L2, altered into the object beam Lo and the reference beam Lr and then incident on the point P on the hologram plate 20 are liable to be varied for the exposure time T of the shutter 12, because of air flow and fine moving of the reflection mirrors 14 and 15, the beam expanders 16 and 18 and the lenses 17 and 19. As the beam incident paths dl1 and dl2 are varied, the phase difference .DELTA..phi. between the two beams L1 and L2 is varied, thereby causing the beam intensity I(P) at the point P on the hologram plate 20 to be varied.
The variation of the beam intensity I(P) at the point P on the hologram plate 20 for the exposure time T of the shutter 12 signifies travelling of the interference patterns 21 to be recorded on the hologram plate 21 for that time. The travelling of the interference patterns results in a degradation of the hologram to be finally obtained. For the purpose of preventing such degradation, it may be required to minimize the exposure time T of the shutter 12 to remove an external disturbance for that time. However, the exposure time may run into several seconds to several minutes if desired. As a result, the degradation problem cannot be solved without an accurate control for the positions of the optical elements through which the laser beam is passed.