JPH0695020A - Optical scanning device - Google Patents

Abstract

PURPOSE:To obtain an optical scanning device having a light source by which printing speed is made higher and which emits the light of plural wavelengths for gradation display. CONSTITUTION:A luminous flux from a light source means (wavelength variable semiconductor laser) 21 is condensed on a body to be irradiated (photosensitive drum) 26 through lens systems 22, 24 and 25 so as to perform optical scanning. The light source means 21 can emit the luminous fluxes of plural wavelengths, and the lens system includes a wavelength dispersion element 24 or an element having axial chromatic aberration. The luminous fluxes emitted from the means 21 and including different wavelengths are condensed on the body to be irradiated 26 so that the direction of wavelength dispersion may be at a certain angle with an optical scanning direction or the area of the beam waste surface of the luminous flux on the body 26 is changed to perform area gradation display.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is used in an apparatus for exposing and scanning an image bearing member such as a laser beam printer and a laser beam copying apparatus to form an image. Related to the device.

[0002]

2. Description of the Related Art Conventionally, in a laser beam printer or a laser beam copying machine which forms a high density pixel by optical scanning of a modulated laser beam and records and outputs an image by an electrophotographic process, the image carrier is optically scanned. In order to increase the speed, there is a first method of increasing the optical scanning speed and the image carrier rotation speed. Other methods include a second method using a plurality of semiconductor laser light sources and a third method using a monolithically formed array type laser.

Further, conventionally, in a laser beam printer or a laser beam copying machine which forms a high-density pixel by optical scanning of a modulated laser beam and records and outputs an image by an electrophotographic process, a light beam to an image carrier is formed. The recording of is a binary display of 0 and 1, and therefore, the number of scanning of the light flux is increased to correspond to the representation of the light and shade of the image.

[0004]

[Problems to be Solved by the Invention]
In the conventional example, the optical scanning speed depends on the mechanical speed because it is necessary to increase the rotation speed of the polygon mirror in order to increase the optical scanning speed, and it is also necessary to increase the rotation speed of the image carrier. is doing. On the other hand, the characteristics of the semiconductor laser device as the light source means are progressive, the modulation speed exceeds 1 GHz, the threshold current is decreasing, and the maximum optical output is increasing.

That is, in order to increase the speed of the laser beam copying apparatus or the like by the conventional means, the mechanical rotation speed is important, and since it has a limit, the increase of the optical scanning speed is at a ceiling.

In the second conventional example, it is necessary to combine the light fluxes emitted from a plurality of laser light sources through a complicated lens system and to accurately form light spots on the image carrier at appropriate intervals. Therefore, there is a drawback that the optical adjustment is very difficult.

Further, in the third conventional example, the adjustment of the lens optical system is easier than in the second conventional example, but since the emission points of the laser light are different, a plurality of images are formed on the image carrier. It is not easy to connect the light spots in the right place.

In the last conventional example described above, 2 of 0 and 1 is used.
It had the drawback that it could only display values.

Therefore, a first object of the present invention is to provide an optical scanning device capable of increasing the printing speed of a laser beam copying device or the like by scanning two light beams at the same time in order to solve the above problems. To provide.

A second object of the present invention is to provide an optical scanning device capable of easily performing high-speed gradation display in order to solve the above problems.

[0011]

According to the first aspect of the present invention, a multi-wavelength tunable semiconductor laser or the like is used as a light source means, and a plurality of light fluxes having a plurality of wavelengths are passed through a wavelength dispersion element to thereby obtain a plurality of converging points. As a result, it is possible to perform exposure scanning on the image carrier, and it is possible to simultaneously write a plurality of rows of data points by one scanning, so that it is possible to dramatically increase the printing speed.

In this case, the multi-wavelength semiconductor laser as the light source means needs to carry different signals for each of the plurality of wavelengths. That is, in the case of a semiconductor laser capable of emitting two wavelengths, a light beam having two wavelengths can be condensed on an image carrier such as a photoconductor drum as two converging points by passing through a wavelength dispersion element such as a prism. Yes, two scans of data points can be written in one scan.

According to the second aspect of the present invention, a wavelength tunable semiconductor laser or the like is used as the light source means, and the wavelength of the wavelength tunable semiconductor laser light source is changed by passing the light beam through a lens system having axial chromatic aberration. As a result, it is possible to change the size of the irradiation spot on the image carrier, and it is possible to express gradation.

For example, if the wavelength tunable semiconductor laser as the light source means can change the oscillation wavelength to λ ₁ and λ ₂ (λ ₁ > λ ₂ ), in the light flux passing through the lens system having axial chromatic aberration, , When the optical system is adjusted so that the beam waist at the wavelength of λ ₁ connects the irradiation spot on the image carrier, the beam waist at the wavelength of λ ₂ is formed on the front side or the rear side of the image carrier, The irradiation spot on the image carrier of No. ₂ becomes large. Therefore, by changing the wavelength of λ _{1 and} the wavelength of λ ₂ , the area of the irradiation spot can be changed, so that gradation display is possible.

[0015]

FIG. 1 shows a first embodiment of the present invention. In addition, in order to make the explanation easy to understand, in the following, there are two types of wavelengths. A two-wavelength semiconductor LD 21 is used as a light source, and a two-wavelength light beam from the LD 21 is collimated through a collimator lens 22 and reflected by a rotating polygon mirror 23 to scan the two-wavelength light in a horizontal direction. The scanning light passes through the prism 24 and is decomposed with respect to wavelength in the vertical direction (direction perpendicular to the scanning surface formed by the scanning light).
The light is focused on the photosensitive drum 26 via the θ lens 25 at two different points for each wavelength.

FIG. 2 schematically shows wavelength decomposition by the dispersive element 24. When the two-wavelength light beam scanned by the polygon mirror 23 passes through the prism 24, it is divided into two light beams in a direction perpendicular to the scanning direction, and the two light beams are scanned on the photosensitive drum 26 in a direction perpendicular to the paper surface of FIG. become. Therefore, two rows of light beams are scanned by one scan, and the printing speed of the apparatus can be increased.

The two-wavelength semiconductor laser 21 used in this embodiment will be described here. First, FIG. 3 is a schematic energy band diagram near the active layer of the semiconductor laser.
An example will be described using (a). In the figure, 10a
Is a P-Al _Xs Ga _1-Xs As light, electron confinement layer (Se
parate Confinement, short referred to as SC layer), 10b are _{n-Al Xs 'Ga 1-} Xs' AsSC
Layer, 11a is Al _Xa Ga _1-Xa As light emitting layer, 11b is Al
_Xb Ga _1-Xb As light emitting layer, 12 is P ⁺ -Al _XB Ga _1-XB A
s barrier layer. These layers form the optical waveguide structure 4.

In this example, the short wavelength (λ ₂ ) light emitting layer 11b is provided on the n-type cladding layer 3 side and the long wavelength (λ ₁ ) light emitting layer 11a is provided on the p-type cladding layer 5 side. It is difficult for holes injected from the layer 5 side to reach the light emitting layer 11b having a short wavelength (λ ₂ ) (as compared with the case of moving in the opposite direction). Therefore, the barrier layer 12 is doped with a high concentration of p-type to supply holes in advance.

As described above, if holes are sufficiently supplied in advance,
When no current is applied, holes are appropriately distributed in both light emitting layers 11a and 11b. In such a case, in order to discuss the laser oscillation, it suffices to consider only the electron distribution. The following description of the operation will be given for this case, but it can be easily inferred in other cases (such as when p and n in this embodiment are exchanged). An example in which p and n are interchanged will be described later.

Next, the SC layer, which is a feature of this example, will be described. Originally, the SC layers 10a and 10b effectively capture the injected electrons and injected holes in the active layers 11a and 11b, and serve to enhance the recombination efficiency of carriers. In this example, the SC layers 10a and 10b that sandwich the plurality of quantum well layers that are the active layers 11a and 11b from above and below are set asymmetrically in the vertical direction, so that the band gap of the plurality of quantum well layers 11a and 11b is small. It is possible to increase the carrier density injected into the quantum well layer 11b closer to the SC layer 10b. As a result, the SC with the smaller bandgap
The gain of the quantum well layer 11b on the layer 10b side can be increased.

In FIG. 3A, since the light emitting layer 11b has a larger energy gap than the light emitting layer 11a, the injected carriers are the light emitting layer 1 in the active layer structure of the conventional semiconductor laser.
Since the light-emitting layers 11a and 11b spread and are distributed in a thermal equilibrium state, the gain of the light-emitting layer 11a having a smaller energy gap becomes too large, and the gain of the other light-emitting layer 11b does not become too large.

Therefore, in this example, as described above, the barrier layer 1
2 and a part of the SC layer are P-type doped so that sufficient holes are present in the light emitting layer. Since the injected electrons are injected from the n-clad 3, the injected electrons pass through the SC layer 10b having the smaller band gap and drop into the light emitting layer 11b. At this time, since the bandgap of the barrier layer 12 is large, most of the electrons cannot jump over the barrier layer 12, further increasing the electron concentration of the light emitting layer 11b.

At this time, the light emitting layer 1 having a small band gap
The thickness of 1a is set to be equal to or less than the carrier scattering length (for example, in LO (longitudinal optical) phonon scattering, since the 37 meV energy decreases, the scattering length is about 100 Å or less).
Only a small amount of injected carriers are captured by the light emitting layer 11a,
A large amount of carriers are captured by the light emitting layer 11b.

Further, the thickness of the barrier layer 12 and the height (depth) of the potential are set to be sufficiently large, and when a current close to the threshold value of laser oscillation is passed, each light emitting layer or well layer The carrier distributions of 11a and 11b are as shown in FIG. 3A (that is, the carrier concentration of the light emitting layer 11b having the larger band gap is larger).

When the barrier layer 12 is too thin or too low, the uniform carrier distribution is the same as when the barrier layer 12 is not provided, but in the example of FIG. 3 (a), a shorter wavelength ( The barrier layer 12 is set so that electrons are distributed to the well layer 11b of λ 2) at a higher rate. However, if the barrier layer 12 is made too thick and / or too high, long wavelength (λ1
Since electrons do not come to the well layer 11a), it is necessary to optimize the setting of the barrier layer 12.

FIGS. 3B and 3C respectively show the gain spectra in the well layers 11a and 11b immediately before the threshold current value. FIG. 3D shows these spectra in consideration of the optical confinement coefficient. The added gain spectrum is shown.

As can be seen from FIG. 3D, each wavelength λ
_Since the gains of ₁ and λ ₂ depend on the gains of the well layers 11a and 11b, the gains of λ ₁ and λ ₂ can be efficiently increased.

In the example of FIG. 3, the SC layers 10a and 10b are
Uses a GRIN (Graded Index) composition, but an energy gap, that is, a step index in which the refractive index changes stepwise (Step Index)
It may be of a type or a composition that changes linearly. In short, it suffices to sandwich a plurality of light emitting layers and set up SC layers with different band gaps above and below.

In the example of FIG. 3, the bandgap of the barrier layer 12 is equal to the bandgap of the composition of the SC layer 10a adjacent to the light emitting layer 11a.
It suffices that the band gap is set to be larger than the band gap of the composition of the SC layer 10b adjacent to the light emitting layer 11b and smaller than that of the cladding layers 3 and 5.

FIG. 4 shows the structure of a more specific embodiment of the semiconductor laser device 21 of the present invention. FIG. 4 (a) is a side sectional view and FIG. 4 (b) is a front sectional view. Such an element can be produced by using a molecular beam epitaxy (MBE) method, a metal organic chemical vapor deposition (MOCVD) method, etc.
Since the process is the same as that for producing a normal semiconductor laser, detailed description thereof will be omitted.

In the figure, 1 is an n ⁺ -GaAs substrate, and 2 is an n ⁺ -.
A GaAs buffer layer, 3 is an n-Al _Xc Ga _1-Xc As clad layer, 4 is an optical waveguide structure having the above-mentioned configuration, and 5 is p-
Al _Xc Ga _1-Xc As clad layer, 6 p ⁺ -GaAs cap layer, 7 Au / Cr electrode, 8 Au-Ge / Au
It is an electrode.

As shown in FIG. 4B, a ridge-type waveguide is formed by a method such as etching with a reactive ion beam in order to confine the current and light in the stripe region in the lateral direction. After forming the Si ₃ N ₄ film 9 by the plasma CVD method, only the upper portion of the ridge is removed by etching, and the electrode 7 is vapor-deposited.

As a means for adjusting the wavelength, in this example, as shown in FIG. 4 (a), the electrode 7 is divided into two parts, and currents flow independently of each other. These two electrodes 7
By injecting currents with different current densities J ₁ and J ₂ and changing their ratio and magnitude, the wavelength dispersion of the effective gain of the entire laser is slightly changed to change the oscillation wavelength.

As a method of changing the oscillation wavelength, there is also a method of controlling the wavelength by the magnitude of the current without dividing the electrode (that is, with a single electrode). In this case, it was confirmed that light with a long wavelength λ ₁ oscillates _first and then light with a short wavelength λ ₂ also oscillates as the current increases. When the current is further increased, there are cases where (i) the light of long wavelength λ ₁ stops oscillating, and (ii) the oscillation continues, in order to obtain the operation of (i), the injection in the light emitting layer 11a is performed. The gain g _{1 at} the wavelength λ ₂ when the electron concentration is n ₁ and the injection hole concentration is p ₁ is g ₁ (λ ₂ , n ₁ , p ₁ )> 0 (see FIG. 3 (b)). 11a, 11b
And the barrier layer 12 may be set. In this case,
The gain (> O) at λ ₂ as shown in (b) contributes to the oscillation, the gain at λ ₁ is consumed, and the light of long wavelength λ ₁ stops oscillating. In (ii), the light emitting layers 11a and 11b and the barrier layer 12 may be set such that the gain g ₁ is g ₁ (λ ₂ , n ₁ , p ₁ ) ≦ O ₂ .

Particularly, in the case of (i), the wavelength can be completely switched, so that the application is wide.

By the way, the mobility of GaAs holes at room temperature is 400 cm ² / V · s, and the mobility of electrons is 88.
Considering that it is smaller than 00 cm ² / V · s,
Since it can be said that non-uniform injection into the holes is easier,
An example in which p and n in (a) are exchanged as shown in FIG. 5 is preferable.

The operation and the like are substantially the same as in the case of FIG. 3 except that the roles of holes and electrons are exchanged.

That is, in this modification, the light emitting layer 11
The holes a are controlled by filling a and 11b with electrons. In this case, at least a part of the barrier layer 12 or the SC layer 10a is n-type doped,
The SC layer 10b having the smaller band gap is arranged on the p-cladding layer 3 side, and the light emitting layer 11b having the larger band gap is p-typed among the plurality of light emitting layers 11a and 11b.
It may be arranged closer to the clad layer 3.

In the above example of the semiconductor LD, the case where Al _X Ga _{1 -X} As is used as the semiconductor has been described for the sake of convenience of description, but if it is a semiconductor material capable of forming a heterostructure,
Obviously anything is fine. Also. As the structure for confining the light and the current, the case of using the ridge type waveguide has been described, but this may be any method used for a normal semiconductor laser. These methods and manufacturing methods are described in, for example, Applied Physics Let.
ters and IEEE Journal of Quan
The description will be omitted because it can be easily understood by referring to the recent 15 years of tumElectronics.

It will be apparent that the number and type of light emitting layers are not limited to two as described above, and may be three or more.

Next, a method of driving the semiconductor laser light source 21 used in this embodiment will be described. FIG. 6 shows the current-light output characteristics of this semiconductor laser. With increasing injection current, the threshold current density J _th (lambda ₁₎ oscillates over the lambda ₁ wavelengths of light is a, lambda ₁ but increases the light output, the injected current density J _th of the (lambda ₂₎ When it exceeds, the light of wavelength λ ₂ starts to oscillate, and the optical output of λ ₁ decreases as the optical output of λ ₂ increases,
When the current density reaches J _th ′ (λ ₁ ), the optical output of λ ₁ becomes zero, and only λ ₂ oscillates. That is, the optical output is
It can be seen that the wavelength of λ ₁ is switched to the wavelength of λ ₂ as the injection current increases.

Therefore, the set current is changed by four values of I ₀ , I ₁ , I ₂ , and I ₃ , as shown in FIG. Current is I
_In the case of 0, the optical output is 0 for both λ ₁ and λ ₂ . Current is I ₁
In this case, the light output of λ ₁ is P ₁ and the light output of λ ₂ is 0.
When the current is I ₂ , the optical output of λ ₁ is P ₁ and the optical output of λ ₂ is P
_{Is 1} . When the current is I ₃ , the light output of λ ₁ is 0 and the light output of λ ₂ is P ₂ .

Therefore, only by setting the injection current, λ ₁
It is possible to independently generate ON and OFF states for two wavelengths of λ ₂ and λ ₂ .

Therefore, the two-wavelength laser device 21 is used as the light source of this embodiment, and the two-wavelength lights λ ₁ and λ ₂ are different on the irradiation object 26 by way of the wavelength dispersion element 24.
The light is focused in columns, and by modulating the two-wavelength laser 21 with four values as described above, independent data of λ ₁ and λ ₂ can be written in two columns by one light beam scanning. .

As a modification of the first embodiment, the position of the dispersive element such as a prism in the optical path may be before or after the polygon mirror 23, or just before the irradiated object 26. In this case, the optical system needs to be set so that the beam waist of the multi-wavelength laser light comes on the irradiation target 26.

FIG. 7 shows a second embodiment of the present invention. Again, there are two types of wavelengths for the sake of clarity.

A two-wavelength semiconductor LD 121 is used as a light source, and a two-wavelength light beam from the LD121 is collimated through a collimating lens 122 and reflected by a rotating polygon mirror 123, so that the two-wavelength light is horizontally directed. To be scanned. The scanning light is a lens system 12 having an appropriate axial chromatic aberration.
4 through the fθ lens 125 to the photosensitive drum 12
The light is focused on the irradiation spot area 6 different for each wavelength.

FIG. 8 shows a lens system 124 having axial chromatic aberration.
FIG. 6 is a diagram for explaining a change in the irradiation spot diameter on the photosensitive drum 126 when the wavelength is changed due to the passage of a light beam through the beam. To simplify the explanation, the polygon mirror 123 and the fθ lens 125 are not shown.

The light flux emitted from the wavelength tunable semiconductor laser 121 becomes parallel light by the collimator lens 122,
The light having a wavelength of λ ₁ enters the optical lens system 124 having axial chromatic aberration and forms an image on the irradiated surface 126 at the beam waist position of the light beam. Therefore, the irradiation spot with the wavelength of λ ₁ becomes smaller. On the other hand, if the wavelength tunable semiconductor laser 121 emits a wavelength of lambda _2, the beam waist of the lambda ₂ light will to come in front of the irradiation object 126, the irradiated body 126
The upper illuminated spot is larger than that of λ ₁ .

Therefore, since the irradiation spot diameter can be changed by changing the wavelength of the light emitted from the wavelength tunable semiconductor laser 121, gradation display is facilitated.

Next, a method of driving the above-mentioned semiconductor laser light source used in this embodiment will be described. A method of driving the present semiconductor laser light source will be described with reference to FIG. FIG. 9 shows the current-light output characteristics of this semiconductor laser. With increasing injection current threshold current density J _th (λ ₁₎ oscillates over the lambda ₁ wavelengths of light is a, lambda ₁ but increases the output, the injected current density exceeds λ _th (λ ₂₎ Light with a wavelength of λ ₂ starts oscillating, and the optical output of λ ₁ decreases as the optical output of λ ₂ increases,
When the current density reaches J _th (λ ₁ ) ′, the optical output of λ ₁ becomes zero and only λ ₂ oscillates. That is, the optical output is
It can be seen that the wavelength of λ ₁ is switched to the wavelength of λ ₂ as the injection current increases.

Therefore, the set current is changed in three values of I ₀ , I ₁ , and I ₂ as shown in FIG. When the current is I _0, the optical output is 0 for both λ ₁ and λ ₂ . When the current is I ₁ , the light output of λ ₁ is P ₁ and the light output of λ ₂ is 0. When the current is I ₂ , the light output of λ ₁ is 0 and the light output of λ ₂ is P ₂ .

Therefore, by setting the injection current, λ ₁ and λ ₂
It is possible to emit one of the two wavelengths (or both may be included). That is, the two-wavelength laser device 121 is used as the light source of this embodiment,
By passing through the axial chromatic aberration element 124, the two-wavelength light λ ₁ and λ ₂ are focused on different spot areas on the irradiated object 126, and gradation display is possible.

As a modified example of the second embodiment, the light source may be an ordinary wavelength tunable DFB laser or wavelength tunable DBR laser. In this case, since the wavelength can be continuously changed, according to this configuration, gradation display can be continuously changed. Regarding the light output of the light source, when the photosensitivity of the irradiated object is almost constant with respect to the wavelength, the beam waist of that wavelength is adjusted so that the light density at the irradiation spot becomes constant even if the wavelength is changed. It is better to control so that the light output of the wavelength becomes stronger as the distance from the surface of the irradiated object increases (the spot area increases) (for example,
Increase the duty ratio). Alternatively, gradation display may be performed by positively utilizing the fact that the photosensitivity of the irradiated body differs with respect to the wavelength.

Regarding the adjustment of the optical system, the beam waist of the light having the longest wavelength or the shortest wavelength of the wavelengths used is set so as to be on the irradiated body.

[0056]

As described above, by using a multi-wavelength semiconductor laser or the like as a light source and arranging a wavelength dispersion element in the optical path, a plurality of (typically 2) light beams can be scanned by one time.
Since the wavelength light can be separated into a plurality of (2) rows and scanned on the irradiation target, and the modulation of each multi-wavelength light can be controlled substantially independently by appropriately driving the multi-wavelength semiconductor laser or the like, There is an effect that the printing speed can be increased.

The area of the irradiation spot on the object to be irradiated can be changed by changing the wavelength by using a wavelength tunable semiconductor laser or the like as a light source and passing through an optical system lens in which axial chromatic aberration is appropriately set. Therefore, there is an effect that gradation display can be easily performed at high speed.

[Brief description of drawings]

FIG. 1 is a diagram showing the configuration of a first embodiment of an optical scanning device embodying the present invention.

FIG. 2 is a partial detailed view of FIG.

FIG. 3 is a diagram showing a band structure and a gain spectrum of a semiconductor laser device used in the present invention.

FIG. 4 is a sectional view of the ridge waveguide semiconductor laser device described in FIG.

FIG. 5 is a diagram showing a band structure of another semiconductor laser device.

FIG. 6 is a diagram showing a driving method of the semiconductor laser of the first embodiment.

FIG. 7 is a diagram showing a configuration of a second embodiment of an optical scanning device embodying the present invention.

FIG. 8 is a partial detailed view of FIG.

FIG. 9 is a diagram showing a method for driving a semiconductor laser according to a second embodiment.

[Explanation of symbols]

 21, 121 Multi-wavelength semiconductor laser 22, 122 Collimate lens 23, 123 Polygon mirror 24 Prism 25, 125 fθ lens 26, 126 Irradiation object 124 Lens with axial chromatic aberration

Claims (19)

[Claims] 1. An optical scanning device for condensing a light beam from a light source means onto an irradiation target through a lens system and optically scanning the light source means, wherein the light source means can emit light beams of a plurality of wavelengths, and the lens system Includes a wavelength dispersion element, and the light flux emitted from the light source means and having different wavelengths is condensed on the irradiation target with the direction of the wavelength dispersion being at an angle to the optical scanning direction. Optical scanning device. 2. The optical scanning device according to claim 1, wherein the light source means is a variable wavelength semiconductor laser device. 3. The optical scanning device according to claim 2, wherein the light source means oscillates a wavelength corresponding to a plurality of energy gaps having different laminated structures. 4. The optical scanning device according to claim 3, wherein the light source means is a multi-wavelength semiconductor laser device having a plurality of different quantum well layers as light emitting layers. 5. The optical scanning device according to claim 3, wherein the light source means is a multi-wavelength semiconductor laser device that oscillates a wavelength corresponding to a ground level and a higher level of the quantum well layer. 6. The optical scanning device according to claim 1, wherein the wavelength dispersion element comprises an optical prism. 7. The optical scanning device according to claim 1, wherein the wavelength dispersion element is a diffraction grating. 8. The light according to claim 4, wherein in the multi-wavelength semiconductor laser device as the light source means, an injection current is appropriately set to oscillate an optical output of a desired wavelength among a plurality of different wavelengths. Scanning device. 9. In a multi-wavelength semiconductor laser device as the light source means, by appropriately setting an injection current, four combinations of zero, one or both of two different wavelengths can be obtained. 6. The optical scanning device according to claim 4, wherein one set of optical outputs is oscillated. 10. An optical scanning device for condensing a light beam from a light source means onto an irradiation target through a lens system and optically scanning the light source means, wherein the light source means can emit light beams of a plurality of wavelengths, and the lens system Has a device having axial chromatic aberration, and the light source means can change the wavelength of the emitted light beam, and the area gray scale display is performed by changing the area of the beam waist surface of the light beam on the irradiated body. An optical scanning device characterized by the above. 11. The optical scanning device according to claim 10, wherein the light source unit is a variable wavelength semiconductor laser device. 12. The optical scanning device according to claim 11, wherein the light source means oscillates a wavelength corresponding to a plurality of energy gaps having different laminated structures. 13. The optical scanning device according to claim 12, wherein the light source means is a multi-wavelength semiconductor laser device having a plurality of different quantum well layers as light emitting layers. 14. The optical scanning device according to claim 12, wherein the light source means is a multi-wavelength semiconductor laser device that oscillates a wavelength corresponding to a ground level and a higher level of the quantum well layer. 15. The optical scanning device according to claim 10, wherein the element having the axial chromatic aberration is a diffraction grating type lens. 16. The optical scanning device according to claim 10, wherein the element having the axial chromatic aberration is a Fresnel lens. 17. The optical scanning device according to claim 13, wherein the oscillation wavelength is changed by changing the injection current in the wavelength tunable semiconductor laser device as the light source means. 18. A wavelength tunable semiconductor laser device as the light source means, one of four combinations of zero, one or both of two different wavelengths by changing the injection current. 15. The optical scanning device according to claim 13, which oscillates the optical output of. 19. A wavelength tunable semiconductor laser device as the light source means, wherein one of three combinations of zero and one of two different wavelengths by changing the injection current is used. 15. The optical scanning device according to claim 13, which oscillates.