DE2323712C3 - Phase shift plate and process for its manufacture - Google Patents

Description

d, = - ■ [m r 1.

¹ An \ HI /

20th

wherein

25 ώ, d - the strength of the first and second thin, respectively

transparent film (17 or 18),
/ I = refractive index of the same,
λ = wavelength of the incident light and
m = positive integer.

2. phase shifter plate according to claim 1, characterized in that the first and the second transparent film (17,18) have the same thickness.

3. Phase shifter plate according to claim 1, characterized in that the thin transparent films (17, 18) consist of ZnS and m = 5

4. phase shifter plate according to claim 1, characterized in that the thin transparent films are made of CeF and m = 7

5. A method for manufacturing an optical phase shift plate according to any one of claims 1 to 4, characterized in that a first thin transparent one on a transparent base Film of uniform thickness is formed over the first thin transparent film second thin transparent film is applied and that the second thin transparent film through a mask with a predetermined pattern is etched through.

55

The invention relates to a phase shifting rat of the type described in the preamble of claim 1: and to a method for its creation, for the formation of holographic analog pattern information such as aligned hole formations, patterns, arrangements, 2pits, etc.
In holography, better quality of a dlogram means that the reconstructed image must contain less penetration. The diffraction efficiency must be high and the deviation of the information must be kept as small as possible.

One of the most widely used holographic methods of recording information is Fourier transform holography, d, e is performed through a lens.

As shown in Fig. 1 of the drawing, a coherent or laser light beam 2 passing through a lens 1 onto a photosensitive recording medium 4 focused, through an element that carries or mediates the information 3. He interferes with a reference light beam 5 incident on the recording medium 4 from another light source is focused, so that the information on the recording medium 4 in the form of an interference pattern 6 is recorded.

The greatest difficulty with the holographic process of the type described consists in the intensity distribution of the light through the recording medium 4. As shown in Fig. 2, the light intensity increases the distance from the optical axis, which leads to the deterioration of a focused image To overcome this problem, a defocusing method has hitherto been used in which the recording medium is used 4 is shifted out of the focal plane of lens 1. However, this method has the Lack that the radius of the hologram is increased, resulting in a noticeable decrease in the recording or information density leads

In Fourier transform holography, to solve the above problem according to FIG. 3 of the drawing, a phase shifter plate 7 with a single or two-dimensional «! non-uniform pattern in close contact with the information carrier element 3 brought so that the phase of the light passing through can be changed arbitrarily. Examples of the Phase shifter plate 7 are shown in cross section in FIGS. 4a and 4b according to FIG. 4a is made by vacuum deposition through a mask on a base 8 a desired two-dimensional pattern of transparent substances 9 is applied. Alternatively A thin transparent film can be formed on the base 8 and through a mask to conventional Way to be etched. The phase shifter plate 7 of the type shown in Fig.4b is produced by Vapor deposition substances 11 are applied through a mask, which in a very small distance from the base 10 is arranged so that the deposit on the lower surface of the Base 10 can be prevented. As in Fig.4b too see, the deposited part 11 has a flat surface 11 'and sloping sides 11 ". In use of the phase shifting plates 7 described, the phase of the information carrying Elements 3 passing light arbitrarily changed and the undesirable concentration of light on the recording medium 4 can be prevented. As a result, the concentration of light can also increase the recording medium 4 can be avoided, so that the quality of the hologram compared with a Hologram can be significantly improved after one working without a phase shift plate Process was established.

However, the holographic process that works with one of the phase shifting plates described above has a defect, which is to be explained with reference to Fig.4. The discrete thin films 9 or 11 are formed directly on the base 8 or 10 so that that penetrates through the thin film area

ydjt due to the multiple occurring therein Refractions suffer a refraction loss under yJK & tet differs in intensity from the part that is not coated with the thin RIm-φη ydit. as a result, it becomes two-dimensional jjgnneFümniuster on the phase shifter plate in the form ^ Differences in light intensity in one Reflectivity off

y ^ i in the light intensity in one

tefcoestructured image reproduced, whereby the Quality of the reconstructed image is deteriorated. This phenomenon is particularly pronounced, “Hen a hologram with lots of low-frequency ones Components is reconstructed.

The invention is based on the object of creating an optical phase shifter plate through which the Intensity of the light passing through can be evenly distributed.

Furthermore, an optical phase shift plate for use in a holographic device is to be created through which the light passing through has arbitrary phases and the intensity of the light passing through Light can be evenly distributed.

This object is achieved according to the invention by the phase shifting plate described in claim 1 solved The reflectivity at the different points of the phase shifting plate can with this Arrangement are substantially evened out, so that the intensity of the light passing through is uniform is distributed.

Preferred developments and configurations of the phase shifting plate according to the invention and that A method for their production and its refinements are the subject matter of claims 2 to 5.

On the basis of the preferred embodiment shown in the drawing, the invention explained in more detail It shows (the figures already mentioned are shown again for the sake of completeness listed)

F i g. 1 is a schematic view for explaining the Fourier transform hologram;

F i g. 2 is a diagram showing the intensity distribution of light on a photosensitive recording medium, as it results in the conventional production of a Fourier transform hologram

Fig. 3 is a view for explaining the Fourier transform hologram when using a phase shift plate,

F i g. 4a and 4b cross sections of known phase shifting plates,

5 shows a partial section on an enlarged scale a phase shift plate with a transparent base and one on top by vapor deposition deposited thin film, which is applied when multiple reflection losses occur,

F i g. 6 is a diagram for explaining the relationship between reflectivity and strength a thin film formed by vapor deposition and

7 shows the cross section of an inventive Phase shifting plate.

The principle on which the invention is based will first be explained with reference to FIG. The in F i g. 5 phase shift plate shown in partial section contains a first thin film 12 with a refractive index m, a second thin film 13 with the thickness ώ and the refractive index πι and an air layer 14 with the refractive index no. If a laser light beam 15 with the wavelength λ falls below a right angle to the Hun film 12, that's how it is calculated

25th

30th

35

40

45 An ₀ - n] ■ n ₂

"ι (" i + "o) ² - (" I - «5.) (« δ - "ΐ)« η ² ~ γ

wherein

With constant wavelength and constant refractive indices, the relationship between δ and Ißl ² corresponds to the diagram in FIG. 6. It can be seen from Fig. 6 that, for a given reflectivity R ^2, multiple values of ό are obtained so that the reflectivity can be made constant although the thickness of the thin film is different when, therefore, a thin transparent film of uniform thickness on a basis is applied and another thin film having the desired pattern is applied on the first thin film, the reflectivities at all points of the phase shift plate can be made equal to each other.

7 contains a transparent base 16 with the refractive index / n, a first thin transparent film 17 with the refractive index m and the uniform thickness en, a second thin transparent film 18 with the desired pattern with the refractive index m and the Thickness d and an air layer with the refractive index no.

The thickness e / of the second thin film 18 results from the following equation, so that the phase difference between the light beam falling through the thin films 17 and 18 and the light beam falling only through the thin film 17 is equal to π radians, i.e. equal to half a wavelength (λ = Wavelength of the laser light falling on the phase shift plate 19):

d = λ / 2: (ro-1).

If the first and second films 17 and 18 have the same refractive index, namely m = η , then from equation (1) for the reflectivity of a thin film with the thickness ώ ~ d \ + d results from equation (1)

5 ° \ R \ ² = I -

- n \) (nl- n ² ) sin ² ^ -

55 in which

4.-TH ₁ J ₂

60

65 In the same way, the reflectivity in the thin film 17 with the strength ώ results

+ , I ₀ ) ² - (n ² - H ² Mn ² , -

4 rr H ₁ rf,

"1 Λ 2nd-7th (4) When Ir ₁ P = Ia ₂ I ² , then the following must apply: "i - sin ² ( A ₁ N. ₂ / <W and -1 ,, ⁴ π ii, d ₂ " ^! (A I d \ h -I (5) /.

If one replaces in equation (5) d ₂ = d, + rf and

rf =

then by substituting it in equation (4) we get sin ² ^ - = sin ² (^ - + ~ ^ j \ ■ (6)

The following applies:

(7)

sin ^ -

IO

20th

For the equality of | rJ ² and | R ₂ | ^{2 the} following condition applies:

40

i = -L ( _m - -OkJ) _{1 m =} 1.2, ...

¹ 4 «, \ n, -iy

(8th)

45

Thus, if a thin film having a uniform thickness ώ is formed on the entire surface of a base and a second thin film having a desired pattern is formed thereon with a thickness d , the reflectivity can be made the same at every point on the phase shift plate.

In the phase shifter plate 7 of the type shown in Figure 4b, the reflectivity of the through the inclined side surface ti "of the light passing through is not the same as that passing through other parts because the thickness of the inclined side surface 11 "changes continuously. The area of the flat parts 11 'is substantially larger than that of the inclined surfaces 11 "so that the intensity of the light passing through the phase shifting plate is substantially uniform can be done.

Preferably, the value is to facilitate the vacuum deposition m chosen so that d \ = d can be an ideal value of m be selected to 5 when zinc sulfide (ZnS) is selected with a refractive index m = 2.3, or 7 when Ceriumfluorid ( CeF) with the refractive index m = 1.67 is used.

According to the invention, a thin transparent film is first applied to a transparent base. A second thin transparent film with a desired pattern is then applied using a mask first thin transparent film applied, as shown in Fi g. 4a or 4b is shown As mentioned, this can Reflectivities at any point on the phase shifting plate can be made equal by the strength of the first and second films deposited on the transparent base is selected so that the Intensity of the light passing through the phase shift plate can be evenly distributed. This allows the change in light intensity of a reconstructed image to be eliminated and its quality the same to be improved. The phase shifting plate according to the invention is particularly advantageous when when used in a holographic device to record analog information. Also serves the phase shifting plate according to the invention for suppressing the speckle noise of the Laser light due to its coherence. That doesn't matter if a conventional phase shift plate with an irregular pattern in contact with a transparent one two-dimensional information transmission link is brought about, the intensity of the through the same light passing through not evenly distributed. This difficulty can also be overcome with the inventive Phase shift plate are overcome.

The invention is not limited to a phase shifting plate with irregular patterns that are used for Use in holographic devices is provided. Rather, the phase shifting plate according to the invention can also be used with various other optical devices Facilities and equipment used when an even distribution of the intensity of the light passing through is desirable

For this purpose 3 sheets of drawings

Claims (1)

Patent claims: / 1. Phase shifting plate with one transparent base and one formed on the base transparent film, characterized in that on the with uniform thickness formed first FHm (17) a second transparent FUm (18) with a one- or two-dimensional, irregularly distributed pattern is applied, that the two frames have the same refractive index and that the strengths of the first and second film (17:18) are so determined that they satisfy the following relationships: 15th