JPS5942855B2 - Multiple beam simultaneous scanning device - Google Patents

Description

[Detailed description of the invention] The present invention relates to a simultaneous multiple beam scanning device.

A laser beam is incident on the ΛO modulation element, and this AO
A plurality of sets of signals are simultaneously applied to the modulation element, the AO modulation element emits a plurality of first-order diffracted light beams whose intensity is modulated by the above signals, and these are simultaneously and periodically deflected to produce a photosensitive material. 2. Description of the Related Art A scanning method for simultaneously writing and scanning multiple lines on a recording medium has been proposed for use in computer output devices, facsimile receiving and recording devices, copying devices, and the like. The plurality of sets of signals are placed on a plurality of high frequency carriers having different frequencies, respectively, and are applied to the AO modulation element.

The exit direction of the plurality of l-order diffracted light beams exiting from the AO modulation element is determined by the frequency of the corresponding high-frequency carrier. Adoption of such a scanning method has the following merits. That is, since beam splitting and intensity modulation of each split beam are performed simultaneously by a single AO modulation element, there is no need to provide a separate high-speed optical modulator, and the structure of the multiple beam simultaneous scanning device is simplified. Because it uses multiple beams to simultaneously write and scan multiple lines, even if the deflection speed of polarizing means such as a rotating polygon mirror or galvano mirror is slowed down, there is no need to reduce the scanning speed. The deflection accuracy can be improved by slowing down the deflection speed, and since the beam is divided and modulated, the clock frequency for extracting the image signal is lowered, making signal processing easier. .

On the other hand, the width △f of the fixed wave number band of the high frequency carrier applied to the AO modulation element for beam splitting, the so-called modulation frequency band, is determined by the applicable bandwidth △F determined as a special characteristic of the AO modulation element itself. In addition to being limited by the propagation velocity V of the ultrasonic wave propagating through the AO modulation element and the beam diameter ψo of the laser beam.

When Δf becomes smaller, the modulation speed of the divided first-order diffracted light beam becomes slower. Therefore, in order to perform high-speed write scanning, it is necessary to ensure the above-mentioned Δf as wide as possible. For this reason, the laser beam incident on the AO modulation element is usually focused by a focusing lens system, and the AO modulation element is placed at the focal point to reduce the beam diameter ψo and increase Δf. ing.

This situation is shown in FIG.

In FIG. 4, a laser beam L. emitted from a laser light source 1. B is focused by the focusing lens system 4 toward the point PO, and is incident on the AO modulation element 5 disposed at the focusing point PO. It goes without saying that in the figure, numerals 2 and 3 indicate plane mirrors. A plurality of sets (four sets in this example) of image signals are applied to the transducer 5a of the AO modulation element 5. for example,
High frequency gear COS (2πF3
t+δ3), COS(2πF4t+δ4), and the information signals for AM modulating these are a1(t), A2(t),
If A3(t) and A4(t), the image signal is Σ A
i(TXOS(2πFlt+δi)) is applied to the transducer 5a.

Then, the first-order diffracted light beam LBi (1=1
-4) are emitted, and these l-order diffracted light beams LBi are intensity-modulated by the corresponding information signals Ai(t) (11-4).

The O-order light beam LBO, which is emitted from the AO modulation element 5 without being diffracted, is cut off by a stopper 6.

The first-order diffracted light beams LB1 to LB4 are incident on an imaging lens system 7, and are focused onto a recording medium 8 surrounded by a photosensitive area by the imaging action of the lens.

Of course, a deflection means not shown in the figure is interposed between the AO modulation element 5 and the recording medium 8, and a deflection means is provided between the AO modulation element 5 and the recording medium 8 so that scanning is performed on the recording medium 8 in a direction perpendicular to the figure. It deflects the next diffracted light beam. However, as shown in FIG. When placed at the focal point PO of B, the focal point PO also becomes a branching point for beam splitting.

In other words, the first-order diffracted light beam LBi is split using the point PO as a common source, and each beam shares the PO point as the position of its light source. Then, the imaging lens system 7 converts the first-order diffracted light beam LBi into
(1-1 to 4) on the recording medium 8 is P
Since this is nothing but forming an image of the generated light existing at point O on the recording medium, the first-order diffracted light beam LBl,
As shown in FIG. 1, LB2, LB3, and LB4 converge at one point q on the recording medium 8, which makes it meaningless to have taken the trouble to divide the l-order diffracted light beam into multiple beams. .

One way to avoid such inconvenience is to slightly shift the surface of the recording medium 8 to the right or left in the figure with respect to point q. However, in this case, the spot diameter of the beam applied onto the recording medium 8 becomes large, resulting in significantly poor resolution. On the other hand, if the above-mentioned disadvantages were to be solved optically, the imaging lens system 7 would be required to have special optical characteristics, resulting in the disadvantage that the manufacturing cost of the imaging lens 7 would be significantly high. There is. Now, as the above-mentioned deflection means, a device that utilizes rotation or swinging of a mirror surface, such as a rotating polygon mirror or a galvano mirror, is used, but the use of such a deflection means has the following problems. It accompanies it.

In other words, when mechanically rotating or oscillating a mirror surface, it is extremely difficult to maintain the correct direction of the rotation axis or oscillation axis, and the precession or sway of these axes causes the deflected beam to change. The direction is the axial direction (referred to as the longitudinal direction).
), there is a slight angle change, which causes unevenness in the pitch of the write scan. When the deflection means is a rotating polygon mirror, in addition to this, non-uniformity in parallelism of each mirror surface to the rotation axis causes the occurrence of the pitch unevenness. An object of the present invention is to provide a multiple beam simultaneous scanning device that can avoid the above-mentioned disadvantages.

Hereinafter, the present invention will be described by way of examples with reference to the drawings. FIG. 2 shows only the essential parts of one embodiment of the present invention.

It should be noted that in order to avoid complication, the same reference numerals as in FIG. 1 are used for items that are unlikely to be confused. In the figure, reference numeral 9 indicates a rotating polygon mirror as a deflecting means, and reference numeral 10 indicates a single drive motor for the rotating polygon. Further, reference numerals 71 and 73 indicate imaging lens systems, and reference numerals 72 and 74 indicate cylindrical lenses. The rotating polygon mirror 9 is a mirror surface of the solid surface of a regular polygonal prism, and is fixed to the rotating shaft 10a of the drive motor 10, and is rotated at high speed in the direction of the arrow around the axis of symmetry by the drive of the motor. move.

The imaging lens system 71 and the cylindrical lens 72 constitute a second optical system, and the imaging lens system 73 and the cylindrical lens 74 constitute a third optical system.

The imaging lens systems 71 and 73 each include a single lens or a plurality of lenses.

The cylindrical lenses 72 and 74 may also have a single structure or a composite structure. The present invention is characterized by the following two points.

That is, the first is the arrangement position of the AO modulation element 5, and the other is the optical characteristics of the second and third optical systems.

The first feature ensures mutual separation of a plurality of writing scanning lines on the recording medium, and the second feature prevents the occurrence of pitch unevenness in writing scanning due to axis vibration of the rotating polygon mirror. is prevented.

First, the first feature will be explained.In the explanation based on FIG. is caused by the focusing lens system 4, which is the first focusing optical system,
Incident laser beam L. This is because the focal point PO of B coincides with the branch point of beam splitting by the AO modulation element 5.

Therefore, in the present invention, the arrangement position of the AO modulation element 5 is shifted from the PO point, and the laser beam L. B's,
The focal point PO by the focusing lens system 4 and the beam splitting point are separated. Two methods are possible for shifting the placement position of the AO modulation element 5 from the point PO. That is, one of them connects the AO modulation element 5 to the laser beam L. One method is to shift it closer to the focusing lens system 4 on the optical path of B (see Figure 3), and the other is to shift it in a direction away from the focusing lens system 4 on the optical path (see Figure 4). . The distance to be shifted depends on other conditions, but by shifting the placement position of the AO modulation element 5 from the PO point, the beam diameter ψo described above increases, and Δf increases. It goes without saying that the shift distance should be as small as possible within the range that can achieve the purpose, considering that it will cause a reduction.

Due to the reduction in Δf described above, implementation of the present invention involves a decrease in the modulation speed per divided beam, but considering that the recording speed is originally sufficiently fast in simultaneous scanning of multiple beams, the above modulation speed It will be easily understood that the effect of the decrease in the recording speed on the recording speed is not so great as to pose a practical problem. Now, when the AO modulation element 5 is arranged in a positional relationship as shown in FIG. It converges at points such as P4 and spreads further from there.

Therefore, the points Pl, P2, P3, P4, etc. serve as light sources for generating first-order diffracted light beams LBl, LB2, LB3, LB4, etc. These points Pl, P2,
Since points P3, P4, etc. are on the same plane SO along with point PO and are separated from each other, even if the first-order diffracted light beam LBi is focused on the recording medium by a normal focusing optical system, the individual points P3, P4, etc. The focal points of the beams should be separated from each other. When the AO modulation element 5 is arranged in the state shown in FIG. 4 with respect to the point PO, the first-order diffracted light beam LBi
, respectively, emit points Pll, Pl2, Pl3, Pl4, etc., which are located on the plane S together with the point PO and are separated from each other, as virtual light sources.

Therefore, by using a normal focusing lens system, 1
Even if the second-order diffracted light beam LBl is focused, the focused points are obtained separately from each other. The above-mentioned surfaces SO, S are hereinafter referred to as the focusing surfaces of the first-order diffracted light beam LBi.

Now, in order to simultaneously scan multiple lines with multiple beams, the convergence points of the first-order diffracted light beams LBi focused on the recording medium should be kept at line intervals,
Must be equally spaced.

For this purpose, points Pl, P2, P3, P4, points Pll,
Pl2, Pl3, Pl4, etc. must be spaced at five equal intervals on each focusing surface.

To do this, it is sufficient to correlate the frequencies of the high-frequency carriers under certain conditions. That is, as is well known, the polarization angle θi (see FIG. 5) of the l-order diffracted light beam LBi emitted from the AO modulation element 5 is the polarization angle θi (see FIG. 5) of the laser beam L. B wavelength λ, AO modulation element 5
It depends on the propagation velocity V of the ultrasonic wave transmitted through the , and the frequency fl of the high frequency carrier, and is given by the following equation according to the so-called black condition.

Since λFi is small compared to V, this formula becomes λf
Approximately multiply by θ1Z-.

2V V is a constant specific to the material of the AO modulation element 5, and λ is a constant specific to the material of the laser beam L. Since it is a constant specific to B, in practical devices, the deflection angle θi depends only on the frequency f1 of the high-frequency carrier.

Therefore, when applying multiple sets of information to the AO modulation element 5, the frequency f1 of the high frequency carrier that conveys these information is set as an arithmetic series, and f1+1 - fl - δf (δf × Fi)
Then, the first-order diffracted light beams are mutually △θAOI2(θ
It is emitted with an angular deviation V of i+1-θi)-1.

However, the unit of the above angle is radian. The beam splitting point in the AO modulator 5 and the laser beam L.
If the distance between B and the focal point PO is D, then the focal plane SO. S
Above, the points Pl, P2, P3, P4 and Pll, Pl2, Pl
The interval ΔP of 3, P, 4, etc. is as follows, which is an equal interval.

As a specific example, a HeNe laser is used as the laser light source 1, and PbM is used as the crystal for the modulation part of the AO modulation element 5.
Assuming the case where OO4 is used, it becomes.

175.5MHz as f1, 8.5MHz as δf
If , ΔP becomes 60 μm. Next, the focusing surface S
The beam diameter DF of the first-order diffracted light beam on O will be briefly described. The above beam diameter is determined by the laser beam L.
B beam DO, focal length F of the focusing lens system 4, laser beam L. The wavelength λ of B is given by the following equation.
Here, k is a constant, and its theoretical value is k. π The optical characteristics of the second and third optical systems, which is the second feature of the present invention, will be explained below.

In the following description, it is assumed that the AO modulation element 5 is arranged as shown in FIG. 6 and 7 are redrawn explanatory diagrams of the arrangement of the second and third optical systems in order to explain the above-mentioned optical characteristics. FIG. 6 is an explanatory side view, and FIG. 7 is an explanatory plan view. In the figure, reference numeral 91 indicates a mirror surface of the rotating polygon mirror 9 that is involved in beam deflection. Here, the vertical direction and the horizontal direction are defined as follows.

That is, the vertical direction is the axial direction of the light deflection movement of the mirror surface 91. That is, in this example, it is the direction of the rotation axis of the rotating polygon mirror 9. When the light deflection means is a galvanometer mirror, it goes without saying that the direction of the pivot axis of the mirror surface should be in the vertical direction. Note that the above-mentioned rotational axis direction and swinging axis direction refer to the axial direction in an ideal situation where there is no shaft wobbling. The horizontal direction is a direction perpendicular to both the optical axis of the optical system and the vertical direction.

Therefore, in reality, the lateral directions of the mirror surface 91 are different from each other in the front and back with respect to the optical path direction. Now, as mentioned above, the second optical system is composed of the imaging lens system 71 and the cylindrical lens 72, but this second optical system has different optical characteristics in the vertical and horizontal directions. are doing.

First, optical characteristics in the vertical direction will be explained. That is, in the vertical direction, the second optical system has a conjugate surface of the focusing surface SO at the position of the mirror surface 91. In other words,
The second optical system focuses the first-order diffracted light beam LBi, whose light sources are points Pl, P2, P3, etc. on the focusing surface SO, onto the conjugate plane only in the vertical direction (sixth
(see figure). Next, the vertical optical characteristics of the third optical system will be explained. As described above, the third optical system is composed of the imaging lens system 73 and the cylindrical lens 74, and its longitudinal optical characteristics are determined by the conjugate surface and the solid surface of the recording medium 8. The object of the present invention is to make the surfaces to be scanned that coincide with each other conjugate with each other.

That is, the first-order diffracted light beam LBi that is focused on the conjugate surface in the vertical direction by the second optical system is focused again on the scanned surface due to the optical characteristics of the third optical system in the vertical direction. Yes (see Figure 6). To summarize the above, it can be said that the second optical system and the third optical system function as mutually independent focusing optical systems as far as the vertical direction is concerned. On the other hand, in the lateral direction, these second and third optical systems have a completely different optical envelope than in the longitudinal direction. That is, in the lateral direction, the optical properties of the second and third optical systems cooperate with each other, allowing the second and third optical systems to function as a set of composite optical systems.

That is, the second and third optical systems cooperate with each other in the lateral direction to make the focusing surface SO and the surface to be scanned mutually conjugated. In other words, point P on SO
1st-order diffracted light beam L whose generation light source is 1, P2, P3, etc.
As shown in FIG. 7, Bi is focused on the surface to be scanned in the lateral direction due to the cooperative optical characteristics of the second and third optical systems. In this way, the first-order diffracted light beams LBi are focused into spots on the surface to be scanned at equal intervals.

Therefore, by rotating the rotating polygon mirror 9 in the direction of the arrow while applying an image signal to the AO modulation element, the first-order diffracted light beam L
Bl is periodically deflected, and a plurality of lines of write scanning are performed on the recording medium 8.

In this example, the recording medium 8 is a photoreceptor in the form of a photoconductive column, and by rotating it in the direction of the arrow, its circumferential surface is
When it is moved, an electrostatic latent image is formed on the circumferential surface according to the image signal. If this electrostatic latent image is processed according to a normal electrophotographic process, an image can be recorded on a suitable recording sheet. By the way, suppose that the mirror surface 91 is tilted as indicated by the broken line 91a in FIG. 6 due to the axis vibration of the rotating polygon mirror 9 during the write start scan. Then, accordingly,
The optical path of the first-order diffracted light beam LBi also changes in the vertical direction as shown by the broken line, but the action of the third optical system is that the generated light source Pl, which is imaged on the conjugate plane in the vertical direction, , P2, P3, etc.
1O, P2O, P3O images are formed on the scanned surface, and on the conjugate surface, a new light source Pl is formed.
As long as the positions of O, P2O, and P3O do not move, the mirror surface 91
However, even if the angle changes by a small amount due to shaft vibration, the focal point of the first-order diffracted light beam LBi on the surface to be scanned does not move in the vertical direction, and in this way, the above-mentioned writing scan This prevents the occurrence of uneven pitch. As the imaging lens system 73, a so-called f-theta lens system is generally used in order to keep the scanning speed constant in the writing scanning direction. By changing the timing of application of pixel information according to COs2α according to the deflection angle α of the first-order diffracted light beam, it is possible to use an ordinary imaging lens system in place of the above f-θ lens system. I can do it. Finally, we will briefly touch on the spot diameter of the beam spot on the surface to be scanned and the linear density of the writing line.

If the vertical imaging magnification by the second optical system is M1, the vertical imaging magnification by the third optical system is M2, and the horizontal magnification by the cooperation of the second and third optical systems is M3. , using the aforementioned DF (-kλF/DO), the diameter of the beam spot is MlM2dF in the vertical direction and M3dF in the horizontal direction, and the shape of the beam spot is generally an ellipse.

When the conditions MIM2-M3 are satisfied, the shape of the beam spot becomes circular. On the other hand, on the focusing surface, the spacing between the points Pl, P2, etc., which are light sources, is as described above, so the beam spot spacing, that is, the line spacing on the scanned surface is ΔP. It is given by MlM2ZλDM, M2δf, and the writing line density UR is given by V.

In other words, the spot diameter can be adjusted by controlling the incident beam diameter DO to the focusing lens system 4, the focusing distance of the lens system, and the magnification of the second and third optical systems, and the linear density is The above magnification, beam splitting point, and laser beam L. This can be adjusted by controlling the distance from B to the focal point PO and the frequency interval of the high frequency carrier.

As described above, according to the present invention, it is possible to provide a multi-beam simultaneous scanning device that can effectively prevent the occurrence of pitch unevenness in writing scan and can perform simultaneous writing scan of multiple lines without using an expensive optical system.

In addition, in the present invention, the configuration and arrangement of the optical system are as follows:
It goes without saying that the present invention is not limited to the examples. That is, for example, in the optical system shown in the embodiment, the imaging lens 7
1 and the cylindrical lens 72, and the imaging lens 73 and the cylindrical lens 74 may be replaced with each other, or the imaging lens 71 may be omitted. Also, a focusing lens system 4 is used as a first focusing optical system.
Instead, a concave mirror optically equivalent to this can be used, and instead of the second and third optical systems, curved mirrors optically equivalent to these can be used.

[Brief explanation of drawings]

FIG. 1 is a diagram for explaining a conventionally known beam splitting method and problems when applying it to a multiple beam simultaneous scanning method, and FIG. 2 shows an example of an embodiment of the present invention. The perspective views showing only the parts and FIGS. 3 to 7 are diagrams for explaining the principle of the present invention.

Claims (1)

[Claims] 1 A laser beam is incident on the AO modulation element, and the above A
A plurality of sets of signals are simultaneously applied to the O modulation element by a plurality of high-frequency carriers having mutually different frequencies, and a plurality of first-order diffracted light beams whose intensity is modulated according to the signals are emitted in mutually different directions. In a recording method that simultaneously and periodically polarizes the next-order diffracted light beam and simultaneously writes and scans multiple lines on a photosensitive recording medium, the laser beam incident on the AO modulation element is focused toward one point. a first focusing optical system, and an AO modulator disposed at a position away from the focusing point so that the focusing point of the laser beam by the first focusing optical system and the beam splitting point are separated. an element, a deflection means for simultaneously and periodically deflecting the plurality of first-order diffracted light beams split by the AO modulation element by means of a rotating or swinging mirror surface, and a deflection means that is involved in the deflection of the deflection means. Mirror surface and A above
a third optical system interposed between a converging surface of the plurality of first-order diffracted light beams emitted from the O modulation element; a third optical system interposed between the mirror surface and the recording medium; and the second
The optical characteristics of the optical system are determined such that the conjugate plane of the focusing surface coincides with the position of the mirror surface only in the axial direction of the optical deflection movement of the mirror surface (hereinafter referred to as the vertical direction), and the optical characteristics of the third optical system are The optical characteristics of the system are set such that the conjugate plane and the scanned surface are mutually conjugate only in the vertical direction, and in the horizontal direction perpendicular to the optical axis and the vertical direction, the second optical system A simultaneous multiple beam scanning device characterized in that the focusing surface and the scanning surface are determined to be mutually conjugate in cooperation with the above.