WO1985005465A1

WO1985005465A1 - Method and device for deflecting a light beam for the writing and reading in matrix form of a document

Info

Publication number: WO1985005465A1
Application number: PCT/CH1985/000084
Authority: WO
Inventors: Thomas Sidler
Original assignee: Fondation Suisse Pour La Recherche En Microtechniq
Priority date: 1984-05-23
Filing date: 1985-05-21
Publication date: 1985-12-05
Also published as: EP0181893A1; FR2564992A1

Abstract

The device comprises a holographic structure including a plurality of holograms (4) carried by the facets of a polygonal pyramid (1). A coherent light source emits a collimated beam (8) through a shaping optics (7) to the holograms (4) rotatingly driven about the axis (2) of the pyramid. The reconstructed beam projects a focal spot (11) on a screen (12). To correct the geometrical curvature of the line described by the focal spot, the device comprises an oscillating lens (16) mounted on a mechanical, electromechanical or piezoelectric homogeneous transducer (17) interposed in the beam (8).

Description

METHOD AND DEVICE FOR DEFLECTING A BEAM OF LIGHT FOR WRITING AND READING IN MATRIX FORM OF A DOCUMENT

The present invention relates to a method of deflecting a beam of light for the electrophotographic writing in matrix form of a document and / or for the sequential opto-electronic reading of a document or of an image, in which it is illuminated. at least one holographic structure using a coherent beam of light, this holographic structure being carried by a support driven in rotation around its axis, and this rotation producing a linear scanning of a plane or cylindrical receiver by a spot focal length reconstructed from the holographic structure, in which a correction is made to the geometric curvature to which the line described by the focal spot is subjected when the holographic structure is rotated, so as to reproduce a straight line located exactly in the tangent plane or the plane of the receiver.

It also relates to a device for implementing this method, this device comprising a rotary support of at least one holographic structure working in transmission or in reflection, drive members for driving this support in rotation about its axis, a coherent light source to generate a collimated beam illuminating the holographic structure and a planar or cylindrical receiver on which is reconstructed from the holographic structure a focal spot describing a straight line, comprising at least one member for correcting the geometric curvature of the line described by the focal spot.

The principle of using holograms for the deflection and scanning of a laser light beam is well known, and a first industrial application of this principle has been described by LD Dickson, GT Sincerbox, A. Wolfheimer, in the article entitled "Holography in the IBM 3684 Supermarket Scanner published in 1982 in IBM J. Res. develop. No. 26 page 228 to 234.

Different other holographic deflectors using components conventional optics such as flat field lenses, spherical, parabolic or elliptical mirrors are described in the literature.

The US patent Mo. 4,239,326 relates to a system for scanning a holographic structure mounted on a rotating disc, illuminated by a parallel laser beam and producing a point focal spot on a screen.

Among the different methods of deflecting a light beam from a laser in order to develop an electrophotographic matrix writing process, holographic elements are of some interest. Indeed, they make it possible to combine in a single space-saving and light element the deflection or scanning and the focusing of the light. These elements are in principle made up of a regular diffraction grating (for deflection) combined with a holographic lens (for focusing). Regardless of the particular holographic configuration, however giving a stigmatic image, the focal point formed by the holographic lens generally describes an arc of a circle centered on the point of impact of the reading beam on the hologram and situated in a plane perpendicular to the axis of rotation of this holographic element during the rotation thereof. The diameter and the homogeneity of the focal spot in a plane tangent to the aforementioned circle depend essentially on the depth of field of the beam coming from the holographic element. For a Gaussian beam from a laser, the only parameters describing the depth of field are the wavelength λ and the size of the beam w _o .

However, the use of a hologram for scanning and focusing simultaneously and with a minimum of additional optical components comes up against the restrictions imposed by holographic recording and reconstruction without image aberrations as well as the geometric curvature. of the image plane inherent in the scanning by rotation of a regular network, which is also encountered in the case of deflection by polygonal mirrors. International patent application WO 82/02955 describes a device for deflecting a light beam for the electrophotographic writing in a matrix form of a document, comprising two cylindrical lenses or a cylindrical mirror and a cylindrical lens, and a support in disc shape with a holographic structure. This device does not provide for any correction of the geometric curvature to which the line described by the focal spot is subjected.

US Patent No. 4,274,101 describes a similar device in which the curvature correction is ensured by a bimorph piezoelectric vibrator.

The present invention aims to overcome the drawbacks of known systems, by providing a simple device, economical and very effective.

For this purpose, the method according to the invention is characterized in that the geometric curvature correction is carried out by interposing on the lighting beam of the holographic structure an oscillating lens, working at the resonant frequency of the system.

According to a particularly advantageous embodiment, the lens oscillations are generated using a mechanical device.

According to another embodiment, the oscillations of the lens are generated using an electromechanical transducer.

According to a preferred embodiment, the oscillations of the oscillating lens are generated using a homogeneous piezoelectric transducer.

The device for implementing this method is characterized in that the oscillating lens is mounted on a homogeneous piezoelectric transducer.

According to a particularly advantageous embodiment, the lens oscillating is mounted on a mechanical device.

According to another embodiment, the oscillating lens is mounted on an electromechanical transducer.

According to a preferred embodiment, the oscillating lens is mounted on a homogeneous piezoelectric transducer.

The support of the holographic structure preferably comprises a polygonal pyramid, each of the facets of which carries a hologram. This construction is particularly simple, precise and relatively economical.

The present invention will be better understood with reference to the description of an exemplary embodiment and the attached drawing in which:

FIG. 1 is a perspective view schematically representing an experimental arrangement illustrating the method according to the invention,

FIGS. 2 and 3 respectively illustrate the principle of recording and reproducing a holographic lens,

FIG. 4 represents a pyramid support for holographic structures, usable for implementing the method according to the invention,

FIG. 5 is a schematic view illustrating the defect in the geometric curvature of the image in traditional systems,

FIG. 6 illustrates a third mode of correction of this defect by the use of an oscillating lens,

FIG. 7 represents the defocusing for a deflection angle,

FIG. 8 represents the value of the defocus as a function of the deflection angle, and

Figure 9 shows the movement of the oscillating lens. The experimental setup shown in fig. 1 comprises a polygonal pyramid 1 fixed to the end of an axis 2 driven in rotation by a motor 3. This pyramid 1 has six facets, serves to support six identical holographic structures, hereinafter called holograms 4, which produce at in turn six consecutive scanning lines projected on the receiver 12. A coherent light generator 5 generates a beam 6 shaped by an appropriate optical system 7 producing a collimated beam 8 which illuminates the holograms 4. The beam reconstructed by the holograms 4 is successively reflected by the return mirrors 9 and 10 and forms a focal spot which scans the receiver 12 along a straight line 11 during the rotation of the holograms. The holograms 4 are arranged on the faces of the polygonal pyramid 1, inclined at 45 degrees relative to the axis of rotation of this pyramid. This pyramid could also be replaced by a cone whose half-angle in the center would be equal to 45 degrees. In practical terms, however, the faceted pyramid is easier and cheaper to build than the cone. The holograms 4 are illuminated by the collimated beam 8 whose incidence is parallel to the axis of the pyramid and whose directions of incidence and reconstruction are symmetrical with respect to the holographic surface. This configuration gives rise to a reconstruction free from astigmatism, which is the most important aberration in the case of holographic lenses, on a rotating disc.

Fig. 2 shows a hologram 20 which is the recording of the interference pattern between an object beam 21 and a reference beam 22. FIG. 3 shows the beam 23 restored by the hologram 20 when it is illuminated by the reference beam 22.

Fig. 4 illustrates the reconstruction of the focal spot from a hologram 4, recorded on a facet of the polygonal pyramid 1. The holograms are lit in a direction parallel to the axis of rotation of the pyramid, which gives rise, as mentioned above, to a reconstruction without astigmatism, schematically represented by the beam 13 forming an image 11 on the screen. Since the angle formed by the facets with respect to the axis of rotation of the pyramid is 45 degrees, the scanning is done linearly and the focal spot describes an arc.

Fig. 5 schematically illustrates a particularly compact electrophotographic reproduction device, comprising a minimum of optical components. It is obvious that the greater the distance d from the hologram 4 carried by one of the facets of the polygonal pyramid 1 to the focal point F, the more the image there in the form of an arc of a circle is comparable to its projection on the screen. 12. When we tend towards an increasingly compact system, the approximation becomes more and more difficult and it is then necessary to carry out a correction of the geometric curvature. This correction is the object of the embodiment illustrated by the following figure.

In the example illustrated in FIG. 6, the geometric curvature of the image is corrected by an oscillating lens 16 mounted on a mechanical transducer comprising for example a homogeneous cam or eccentric (not shown), electromechanical or piezoelectric 17, for example tubular (not shown). This lens is arranged to perform a sinusoidal movement along the optical axis of the lighting beam 8 and thus very slightly modifies the divergence of the incident beam on the hologram 4, so as to maintain the focal spot on a localized straight line. exactly on the flat surface of the screen 12.

This embodiment makes it possible to design a compact, low-volume deflection unit comprising the coherent light source, the beam shaping optics 17, the oscillating lens 16 and its support 17 and the polygonal pyramid 1 whose facets bear holograms 4.

The vibrating lens system is used to compensate for the geometric curvature resulting from scanning with a focusing holographic deflector. The size of this curvature depends on the distance between the holographic element and the reception plane. For mounting a compact deflector, this distance is necessarily small and, consequently, the large geometric curvature. It is therefore essential to correct the geometric curvature in a compact high-resolution focusing holographic deflection system.

The vibrating lens system has the characteristic that the lens performs a sinusoidal movement generated for example by a mechanical, electromechanical or piezoelectric system oscillating at the mechanical resonance frequency of the system. This process has the advantage of providing a large mechanical amplitude for a low excitation power (multiplication factor due to the quality factor of the resonant mechanical system). In addition, the mechanical movement at resonance has a well-defined behavior, and the amplitude of vibration is large compared to the bandwidth of the mechanical system.

The defocusing due to the geometric curvature applies to the embodiment of Figure 7

z = d ₁ (1 / cosΨ-1), where z represents the defocusing, d ₁ the distance from the holographic element to the reception plane and Ψ the deflection angle-.

For angles ψ <25º, this can be considered equivalent to

z ≈ d ₁ ψ 2/2

If the vibrating lens is imparted a sinusoidal movement, the variation in focal distance thus produced can be expressed

p = A (1-cosα)

For small angies p ≈ A α 2/2

It is then possible to determine the amplitude A to be printed on the vibrating lens in order to optimally correct the defocusing due to the geometric curvature, one can do this optiraalisation with the exact formulas numerically according to the typical example below. The vibration frequency of the corrective lens is equal to the line frequency of the scanning system. The maximum deflection angle of the scanning system, as well as its dead time (time during which the reading beam passes from one facet to the other) determines the angular range in which it is necessary to correct the curvature.

Figure 8 shows the value of the uncorrected and corrected defocus for a 6-facet system, with a distance d ₁ of 300 mm and a dead time of 40%. Curve A illustrates the defocus in the absence of correction as a function of the deflection angle. Curve B illustrates the defocus after correction.

Figure 9 shows the movement of the vibrating lens. Part C represents the area where the correction is made. Part M represents the domain of dead time.

In the example cited above, the correction results in a focusing error of less than 0.4% over the entire area used, against an error of 2.8% for the uncorrected system. We also see that the distribution of the residual error is much more homogeneous than in the uncorrected case. By appropriately choosing the resonant frequency and damping, respectively the quality factor of the oscillating system, a very simple mechanism, either mechanical, electromechanical, or piezoelectric (preferably - homogeneous or tubular) is sufficient to operate this vibrating lens system correctly.

Claims

claims

1. A method of deflecting a light beam for the electrophotographic writing in matrix form of a document and / or for the sequential opto-electronic reading of a document or of an image, in which at least one is illuminated holographic structure using a coherent beam of light, this holographic structure being carried by a support driven in rotation around its axis, and this rotation producing a linear scanning of a plane or cylindrical receiver by a focal spot reconstructed at starting from the holographic structure, in which a correction is made of the geometric curvature to which the line described by the focal spot (11) is subjected during the rotation of the holographic structure (4), so as to reproduce a line straight line located exactly in the tangent plane or the plane of the receiver (12), characterized in that the geometric curvature correction is carried out by interposing on the beam lighting of the holographic structure a slow> oscillating island (16), working at the resonant frequency of the system.

2. Method according to claim 1, characterized in that one generates the oscillations of the oscillating lens (16) using a mechanical device.

3- Method according to claim 1, characterized in that one generates the oscillations of the oscillating lens (16) using an electromechanical transducer.

4. Method according to claim 1, characterized in that one generates oscillations of the oscillating lens (16) using a homogeneous piezoelectric transducer (17).

5. Device for implementing the reproduction method according to claim 1, comprising a rotary support of at least one holographic structure working in transmission, drive members for driving this support in rotation about its axis, a source of coherent light to generate a collimated beam illuminating the holographic structure and a plane or cylindrical receiver that on which is reconstructed from the holographic structure a focal spot describing a straight line, comprising at least one member for correcting (14, 15, 16) the geometric curvature of the line described by the focal spot (11), characterized in that the member for correcting the geometrical curvature of the line described by the focal spot comprises an oscillating lens (16) interposed on the lighting beam of the holographic structure, this lens working at the resonant frequency of the system.

6. Device according to claim 5, characterized in that the oscillating lens (16) is mounted on a mechanical device.

7. Device according to claim 5, characterized in that the oscillating lens (16) is mounted on an electromechanical transducer.

8. Device according to ia claim 5, characterized in that the oscillating lens (16) is mounted on a homogeneous piezoelectric transducer (17).

9. Device according to ia claim 5, characterized in that the support of the holographic structure (4) comprises a polygonal pyramid (1) of which each of the facets carries a hologram.