JP3472095B2 - Optical element for spot shaping - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a spot-shaping optical element, and more particularly to a spot-shaping optical element useful for changing the shape of a light spot on a recording medium in an optical head device to improve reading and writing accuracy. It is a thing.

[0002]

2. Description of the Related Art For example, in order to increase the recording density in an optical head device, it is necessary to reduce a focused spot on a recording medium, and a super-resolution phenomenon is used as an effective means for that purpose. In this super-resolution phenomenon, the central part of the light beam is blocked by an opaque shielding plate, and the spatial distribution of the light intensity is changed to obtain a spot smaller than the diffraction limit. There is a drawback in that light amount loss occurs and the light from the light source cannot be used effectively.

In order to solve such a drawback, a technique has been proposed in which a phase filter is used to change the phase by ½ wavelength with respect to the central portion of a light beam to produce the same effect as that of a shielding plate. Even with this method, although the light amount loss can be solved, the following drawbacks associated with the super-resolution phenomenon cannot be solved.

That is, when the spot diameter is reduced by super-resolution, in addition to the central main spot, an intensity distribution called a side lobe appears remarkably around it. As a result, even if the center spot is reduced, the recording adjacent to the target recording is simultaneously read due to the generation of the side lobe, crosstalk and read signal jitter occur, and the accuracy of the signal is lowered.

Moreover, the peak intensity at the center of the spot decreases as the spot diameter decreases due to super-resolution, which is an important problem during recording and writing. That is, when writing a record on the recording medium, the recording medium 5 is locally made by the focused laser beam to change the recording surface regardless of the method such as the magneto-optical effect, the phase change, and the chemical change. A high light intensity is required at the center of the spot, but if a sufficient light intensity is not obtained, it will be a problem that stable recording is not performed and an obstacle to increasing the writing speed.

Further, a semiconductor laser generally used as a light source for an optical head emits laser light diverging from a junction surface of a p-type semiconductor and an n-type semiconductor, and light perpendicular to this surface is horizontal. It is emitted with a wider spread than light. When this emitted light is converted into a parallel light beam by a collimator lens without correction, the vertical direction becomes closer to uniform intensity than the horizontal direction due to the difference in the spread angle of the emitted light. If the intensity distribution of the parallel light beam cross section differs depending on the direction,
The shape of the focused spot also changes depending on the direction. When a light beam having an intensity distribution whose intensity is significantly reduced when it is separated from the center is condensed, the spot diameter becomes large. If the light intensity distributions are different in the directions orthogonal to each other, the spot becomes elliptical. Therefore, it is necessary to correct the parallel light flux in the same manner in any direction by correcting the circularity.

For this purpose, there is a method in which the light emitted from the semiconductor laser is converted into parallel light by a collimating optical system and then the beam diameter is expanded in only one direction by a prism optical system. However, this has the disadvantage of being expensive.

The present invention has been made to solve the above drawbacks, and provides a spot shaping optical element capable of simultaneously reducing a side lobe and a spot diameter while suppressing a decrease in center intensity as much as possible. With the goal.

[0009]

According to the present invention, the above object is to be arranged between a light source 1 for laser light and a condensing optical system,
A spot shaping optical element 2 for shaping a condensed spot of a condensing optical system, in which transparent materials are superposed, and a diameter D is 50% or more of an aperture diameter D0 on an aperture surface of an optical system, and an outer circumference of the aperture. A phase displacement region 3 having a phase displacement amount of ½ wavelength or less is formed on the contact interface between the materials by making the thickness different from the remaining region in the inner region, and passes through the region other than the phase displacement region 3. This is achieved by providing the spot-shaping optical element 2 that changes the phase of the light passing through the phase displacement region 3 without changing the phase of the light.

In the present invention, the phase displacement region 3 is formed by making the thickness different from the remaining region.
The light flux that has passed through the phase displacement region 3 having a thickness different from that of the remaining region changes its phase because the optical path length is different from that of the remaining region, and functions as a phase filter as a whole. In the present invention in which the phase filter is formed by changing the thickness, there is an advantage that the manufacturing becomes simpler than the case where the phase shift film is formed, for example. That is, when the refractive index of the material is n, the wavelength of light is λ, the thickness change is Δd, and the phase displacement is Δφ, Δφ = 2π (n−1) · Δd / λ ... ... The relationship of (1) is established.

Now, the refractive index of the material is 1.5 and the wavelength is 68.
With a thickness of 5 nm, a thickness change of about 270 nm is required to generate a phase shift of 0.2 wavelength.
Such a thickness change is caused by partial thin film deposition, etching,
It can be created relatively easily by lithography or the like.

Further, by making the phases different in the circular belt-shaped region 4 excluding the center, it is possible to reduce the peak value of the focused spot and the side lobes in the focusing optical system as much as possible.

Further, the spot shaping optical element 2 may be formed by stacking two transparent bodies having a refractive index difference at a contact interface smaller than a refractive index difference between respective materials and air. It is possible and the phase displacement region 3 is formed at the contact interface of these transparent bodies.

Here, forming a phase change region at the contact interface of a transparent body means forming the above-mentioned thickness change section on any one of the transparent bodies, and engaging the other transparent body with this thickness change section. In this state, the phase difference occurs due to the unevenness at the boundary position.

When two transparent bodies having different refractive indices n1 and n2 are superposed, the relation given by the above equation (1) is Δφ = 2π (n1-n2) · Δd / λ ... ·····························································································. can do.

For example, polystyrene (nC = 1.578)
And glass material BaF3 (nC = 1.57893) is used as a transparent body, the difference in refractive index (n1-n2) is 0.00.
1 and the thickness change required to generate a phase difference of 0.2 wavelength expands from 270 nm to 140 μm. That is, the amount of phase displacement that occurs with respect to the deviation of the boundary position during manufacturing of the phase filter becomes insensitive, and the processing accuracy can be reduced. In addition, since this degree of accuracy can be achieved even by using a general molding technique for mass production of lenses, mass production and cost reduction can be easily realized.

In this case, the phase displacement amount can be set to a half wavelength or a value obtained by adding a displacement amount of an integral multiple of the wavelength to the half wavelength. By suppressing the amount of phase displacement to be smaller than 1/2 wavelength, preferably about 1/5 wavelength, it is possible to suppress the decrease in center intensity and the increase in side lobes.

It is desirable that the shape of the phase displacement region 3 in a front view is a circular band shape having a width W of approximately 5% of the opening diameter D0. Further, in the phase displacement region 3, each width W has an opening diameter D0. It is also possible to dispose different phase shift amounts in each of the circular band-shaped regions 4 by disposing a plurality of adjacent circular band-shaped regions 4, 4 ... Here, the circular band shape includes an ellipse and the like in addition to a perfect circle. Further, when a plurality of circular belt-shaped regions 4, 4, ... Are formed,
The degree of freedom of spot shape control is improved. When the phase displacement area 3 is composed of a plurality of circular band areas 4 having different phase displacement amounts, the phase displacement area 3 is arranged so that the signs of the phase displacement amounts in the adjacent circular band areas 4 are reversed. It is desirable to design

It is desirable that the phase displacement region 3 be formed in a concentric circular band shape having a diameter D of about 60% of the opening diameter D0. With the center at the optical axis, the diameter D is about 60% of the opening diameter D0,
A thin circular band (ring shape) with a width W of about 5% of the opening diameter D0
Unlike the conventional super-resolution filters, not only the spot diameter can be reduced, but also the side lobe can be reduced by forming the phase displacement region 3 and generating the phase displacement in the above range of the light flux. The diameter D here means the diameter at the center of the concentric circle band.

The phase displacement region 3 can be formed in an elliptical band as in the invention according to claim 2 other than the concentric circular band. The spot-shaping optical element 2 having the elliptical band phase displacement region 3 is effective for the intensity distribution of the light beam having the direction dependence, and applies the phase displacement in the elliptical band shape to the intensity distribution of the light beam having the direction dependence. , Correct the elliptical spot shape to a perfect circle. In this case, the direction in which the emission angle of the light source 1 is maximum and the direction in which the emission angle is minimum are set so as to coincide with the long axis and the short axis.

Further, the roundness correction of the spot is performed by claim 3.
The present invention can also be achieved by generating different phase displacements in directions orthogonal to each other on a plane having the optical axis of the laser light as a normal line. In this case, the emission of the laser light from the light source 1 can be achieved. Different phase displacements are generated in different angles. To generate different phase displacements in different directions, it is only necessary to change the thickness in that direction.

An optical head device using the above-mentioned spot shaping optical element 2 is provided. The optical head device has a light source 1 for generating a laser beam, is configured to collect the light reflected on the recording surface of the recording medium 5 by an optical element, and to collect the light reflected from the recording surface. The spot shaping optical element 2 is arranged between the medium 5 and the medium 5.

In this case, when the hologram diffraction grating 6 is formed in the spot shaping optical element 2, the function of splitting the light in the spot shaping optical element and the phase displacement 2 are generated.
Since it can have two functions, an increase in the number of parts and the assembly process can be prevented.

Further, when using the spot shaping optical element which is capable of correcting the perfect circle as described above for the intensity distribution of the light beam having the direction dependence, the focused spot on the recording medium 5 is changed. The recording and reading accuracy is improved by arbitrarily changing the scanning direction.

[0025]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows an optical head device using the spot shaping optical element of the present invention. Optical head device,
The laser light is focused on the recording surface of the recording medium 5, and the light reflected from the recording surface is extracted and guided to the detection optical system 70. A collimating optical element such as a collimating lens 8 that converts the light emitted from the light source 1 into a parallel light flux, and a condensing optical element such as a condenser lens 81 that condenses the parallel light flux on the recording medium 5. Further, the reflected light from the recording medium 5 is converted into a parallel light flux by a condensing optical element such as a condensing lens 81 and then guided to a detection optical system 70 by a beam splitter 82, and the detection optical system 70 reads out recording. , And the position shift of the focused spot and the focus shift are detected.

A spot shaping optical element formed of a transparent glass material in a substantially low-profile cylindrical shape is arranged between the collimator lens 80 and the beam splitter 82 on the light source 1 side, and the parallel light flux converted by the collimator lens 80 is formed. In order to change the phase into a concentric band shape, a concentric band phase displacement region 3 is provided. As shown in FIG. 2, the phase displacement region 3 is formed by making the thickness of the glass material different from other regions.

The phase displacement region 3 has a diameter D (diameter at the center position of the circular band) of 50% to 70% of the opening diameter D0.
Desirably, the phase displacement amount in the phase displacement region 3 is ½ wavelength or less, preferably 1 at approximately 60% of the aperture diameter D0.
It is set to be about / 5 wavelength. The required thickness change ΔH for generating a predetermined amount of phase shift is given by the above-mentioned equation (1), and the phase shift region 3 is formed by partial thin film deposition, etching, and lithography.

FIGS. 3 to 5 show the results of calculations performed with the width W of the circular band set to 5% of the opening diameter D0 in order to confirm the influence of the phase displacement region 3 described above. First, the graph shown in FIG. 3 shows the change of the side lobe with respect to the diameter D of the circle band (the diameter at the center position of the circle band), and the circle when the aperture diameter D0 is 1 on the horizontal axis with the phase displacement amount as a parameter. The diameter D of the band is taken, and the vertical axis shows the peak intensity of the side lobe when the central intensity is 100%. Further, a solid straight line parallel to the horizontal axis drawn with a sidelobe intensity of 1.75 is obtained when the spot shaping optical element 2 according to the present invention is not used (hereinafter, referred to as “when not in use”). The measured value is shown.
Although not plotted on the graph, the sidelobe intensity measured using a conventional phase filter that causes a phase shift at the center of the aperture was 10%.

From the graph shown in the figure, the larger the amount of phase displacement, the greater the increase / decrease in side lobes, but the tendency of change is the same regardless of the amount of phase displacement, and when the diameter D of the circular band is 55% or more, it becomes unclear. It can be seen that the side lobes are lower than when used. On the contrary, when the circular band is near the center of the aperture, the side lobe increases, and when the amount of phase displacement is large, it becomes more than double that when not in use, and the influence on the signal reading accuracy becomes remarkable. In the graph shown in the figure, the side lobe intensity twice as high as when not in use is shown by a broken line parallel to the horizontal axis. From the above results, the diameter D of the circular band affects the side lobe strength, and in order to suppress the adverse effect of the side lobe, the diameter D of the circular band needs to be about 50% or more of the opening diameter D0.

FIG. 4 shows the change of the spot diameter with respect to the diameter D of the circular band. The phase displacement amount is used as a parameter, the horizontal axis shows the diameter D of the circular band with the opening diameter D0 being 100%, and the vertical axis shows the spot diameter. , Standardized when not used. From the graph shown in the figure, the diameter D ratio of the circular band is 67 regardless of the phase displacement amount.
It can be confirmed that when the value is small, the value shrinks when it is small, and when it is large, it expands. From the above results, the diameter D of the circular band affects the spot diameter, and in order to reduce the spot diameter, the diameter D of the circular band needs to be 70% or less of the opening diameter D0.

FIG. 5 shows the change in the central strength with respect to the phase displacement when the diameter D of the circular band is fixed to 60% of the opening diameter D0. From this graph, the central strength (vertical axis) decreases as the amount of phase displacement (horizontal axis) increases, and the central strength when not in use is 1
When set to 00%, in the range of 1.5 (radian) ≈ 0.25 wavelength or less, it is possible to set a minimum of 80%, that is, a reduction of 20% or less that does not hinder reading and writing in practical use. I know there is.

A phase-shifted region 3 is a circular band region having a diameter D of 60% of the aperture diameter D0 and a width W of 5% of the aperture diameter D0, and a phase shift amount of 1/5 wavelength is generated in the phase-shifted region 3. FIG. 6 shows an example of a change in spot diameter and a decrease in side lobes when the spot shaping optical element 2 designed to be used is used. In the figure, the solid line indicates that the spot-shaping optical element 2 is used, the broken line indicates that it is not used, and the horizontal axis indicates the distance from the spot center. From this figure, it can be seen that the side lobe intensity ratio (vertical axis) to the central intensity is reduced by about 19.7%. In addition, when the spot diameter and the center strength of the spot were calculated under the same conditions, the spot diameter was 1.3% lower and the center strength was 19.6% lower than when not in use. It was confirmed that the diameter reduction and the side lobe reduction were realized at the same time.

FIG. 7 shows a second embodiment of the present invention.
In this embodiment, the phase displacement regions 3 are adjacent to each other.
It is composed of two concentric circular zone regions 4, 4 and 4 and causes different phase displacements in each concentric circular zone region 4. The width W of each concentric perfect circular zone region 4 is 5% or less of the opening diameter D0, and the diameter D is set to 55% to 70% of the opening diameter D0. Further, the directions of the phase displacements of the three circular zone regions are such that the opposite phase displacements are generated in the adjacent circular zone regions, and the side lobe reduction and the spot diameter reduction are realized at the same time.

Since the phase has periodicity and returns to the original phase when it shifts by one wavelength, the phase difference shown in FIG.
Alternatively, by adding a phase difference that is an integral multiple of the wavelength, a structure equivalent to this can be obtained as shown in FIG. In this case, since it is not necessary to generate a positive or negative phase difference, it is not necessary to form a protrusion having a minute width W on the surface, and the processing becomes easy. Note that FIG. 7A shows a cross-sectional shape when the generated phase difference is plotted on the vertical axis.
(B) shows an actual design dimension when the opening diameter D0 is 3 mm and the refractive index is 1.5.

FIG. 8 shows the result of reducing the condensing spot side lobes by using the spot shaping optical element 2 shown in FIG. The width W of each circular band region is 2.2% with respect to the opening diameter D0. In FIG. 8, the vertical axis represents the ratio of the side lobe intensity to the central intensity, and the spot shape when not in use is indicated by a broken line. From this, it can be confirmed that the side lobe is 43% lower than when not in use. In addition, at this time, the reduction rate of the central strength was suppressed to 15%, and it was confirmed that the spot diameter was also reduced as compared with when not used. This is an example of the effect of the present invention,
If the side lobe reduction rate is set gently, the reduction rate of the spot diameter can be increased.

FIG. 9 is a schematic view of a cross section of the spot shaping optical element 2 in which the amount of phase displacement continuously changes. This is Figure 7
This corresponds to the case where the number of circular zones in is increased infinitely and the phase is finely changed, and the degree of freedom of spot control can be increased by adding a complicated phase displacement.

FIG. 10 shows a third embodiment of the present invention. In this embodiment, the spot-shaping optical element 2 is for superimposing two transparent bodies 90 and 91 having refractive indices close to each other and changing the position of the boundary to generate a phase difference. In this embodiment, as described above, since the generation of the phase displacement amount with respect to the deviation of the boundary position during manufacturing becomes insensitive, the processing accuracy can be reduced.

FIG. 11 shows a fourth embodiment of the present invention. In this embodiment, the phase displacement region 3 is formed in an elliptical strip shape. As is clear from FIG. 4, the magnitude of the super-resolution effect when the phase is displaced in a circular band shape depends on the diameter D of the circular band, that is, the distance from the center of the beam. Therefore, in the present embodiment in which the phase displacement region 3 is formed in an elliptical band shape, the reduction ratio of the spot diameter becomes high in the minor axis direction of the elliptical band, and the reduction ratio of the spot diameter decreases in the major axis direction, or Expanding.

On the other hand, the emitted laser light of a semiconductor laser generally used as the light source 1 of the optical head has a directional dependence on the light intensity distribution, and if it is focused without correction, the spot becomes an ellipse. If the spot diameter deviates from the perfect circle, the reading and writing of data will be adversely affected. According to the present embodiment, the spot shape can be corrected to a perfect circle by changing the reduction ratio of the spot diameter depending on the direction without using a generally used perfect circle correcting prism.

Now, assuming that the spot diameters when not in use differ by 5% depending on the direction, consider a case where the spot shaping optical element 2 according to the present embodiment corrects the spot shape. The amount of phase displacement is fixed at 0.2 wavelength. In the direction in which the spot is small, that is, in the direction in which the width W of the light beam is wide, the diameter D of the elliptical band is set to the aperture diameter D0 so that the spot diameter is enlarged.
To 85% of the length of the ellipse. In the direction in which the spot is large, that is, in the direction in which the width W of the light beam is narrow, the diameter D of the elliptical band is set to 40% of the opening diameter D0 so as to reduce the spot diameter, and the minor axis of the ellipse is set. From FIG. 4, the spot diameter increases by about 3% in the small spot direction, and the spot diameter decreases by about 2% in the large spot direction.
As a result, the spot shape can be corrected to a circle,
The recording and reading accuracy can be improved.

FIG. 12 shows a fifth embodiment of the present invention. In this embodiment, the phase displacement region 3 is formed so that the phase displacement amount changes depending on the direction. Figure 4
As can be seen from the graph, the magnitude of the super-resolution effect when the phase is displaced in the shape of a circular band increases with the amount of phase displacement, and becomes maximum at 1/2 wavelength. Therefore, the spot diameter becomes small in the direction in which the phase displacement amount is large, and the spot diameter is hardly reduced in the small direction. According to the present embodiment, even if the light intensity distribution of the light source 1 of the optical head has a direction dependency, the spot shape can be corrected to a perfect circle without using a commonly used perfect circle correction prism. it can.

Now, assuming that the spot diameters when not in use differ by 5% depending on the direction, consider a case where the spot shaping optical element 2 according to the present embodiment corrects the spot shape. In this case, the phase difference is not generated in the direction in which the spot diameter is small, and the diameter D5 is in the direction in which the spot diameter is large.
The phase shift region 3 is designed so that a phase difference of 0.5 wavelength is generated at 5%. As a result, the spot diameter is reduced by about 5% in the direction in which the spot is larger than the figure, and the spot shape can be corrected to a circular shape.

FIG. 13 shows a sixth embodiment of the present invention. This embodiment is a modification that is particularly effective when used as an optical head, and a hologram diffraction grating 6 that divides the reflected light beam from the recording medium 5 into a plurality of light beams is formed on the transparent body. The hologram diffraction grating 6 has a grating structure formed on a flat substrate with a period of several times the wavelength, and the phase displacement region 3 is formed on the surface without the diffraction grating. Therefore, in this embodiment, since the spot shaping optical element 2 also has a function of splitting the light reflected from the recording medium 5 and guiding it to the detection optical system 70, an increase in the number of parts and the assembly process can be prevented. Further, since the hologram diffraction grating 6 and the phase shift region 3 can be formed by a common method such as etching or lithography, the element manufacturing process can be simplified.

[0044]

As is apparent from the above description, according to the present invention, it is possible to simultaneously realize the reduction of the side lobe and the reduction of the spot diameter while suppressing the reduction of the central strength as much as possible. Further, rounding and optimization of the spot shape can be realized without using another optical element.

Further, according to the optical head device of the present invention, since a spot having a small diameter and having no side lobe can be obtained, the read / write accuracy of the recording medium can be improved.

[Brief description of drawings]

FIG. 1 is a diagram showing a first embodiment of the present invention.

FIG. 2 is an explanatory view showing an optical element for spot shaping,
(A) is a perspective view, (b) is sectional drawing cut | disconnected by the line segment containing the center of (a).

FIG. 3 is a diagram showing a relationship between a diameter ratio of a circular band and side lobe strength.

FIG. 4 is a diagram showing a relationship between a diameter ratio of a circular band and a spot diameter.

FIG. 5 is a diagram showing the relationship between the amount of phase displacement and the intensity at the spot center.

FIG. 6 is a diagram showing a decrease in side lobes.

FIG. 7 is a diagram showing a second embodiment of the present invention, (a)
6A is a cross-sectional view, and FIG. 6B is a cross-sectional view showing a phase displacement region equivalent to FIG.

FIG. 8 is a diagram showing a decrease in side lobes.

9 is a cross-sectional view showing a modified example of FIG.

FIG. 10 is a sectional view showing a third embodiment of the present invention.

FIG. 11 is a diagram showing a fourth embodiment of the present invention.

FIG. 12 is a diagram showing a fifth embodiment of the present invention,
(A) is a perspective view, (b) is sectional drawing which cut | disconnected (a) along the circumference and developed.

FIG. 13 is a diagram showing a sixth embodiment of the present invention.

[Explanation of symbols]

1 light source 2 Spot shaping optical element 3 Phase displacement area 4 Circle band area 5 recording media 6 Hologram diffraction grating D0 opening diameter W width

Continuation of the front page (56) Reference JP-A-6-95038 (JP, A) JP-A-5-88108 (JP, A) JP-A-8-17068 (JP, A) JP-A-10-302300 (JP , A) JP-A-10-188322 (JP, A) JP-A-5-249414 (JP, A) JP-A-4-281231 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB) Name) G11B 7/12-7/135

Claims (6)

(57) [Claims] 1. A disposed between the light source and the focusing optical system of the laser beam, an optical element spot shaping for shaping the focused spot of the converging optical system, superposing a transparent material, diameter in more than 50% of the opening diameter on the optical system aperture surface, and, between the phase shift region in which the phase shift amount below half wavelength by varying the remaining region and a thickness on the inside of the area than the opening periphery material Contact
An optical element for spot shaping, which is formed on an interface and changes the phase of light passing through the phase shift region without changing the phase of light that has passed through the region other than the phase shift region. 2. The phase shift region is an elliptical band.
The optical element for spot shaping as described. 3. The phase shift region is formed by changing the thickness in directions orthogonal to each other on a plane whose normal line is the optical axis of the laser light, and causes different phase changes in different directions. 2. The optical element for spot shaping according to 2. 4. An optical head device having a light source for generating a laser beam, which collects light reflected by the recording surface on the recording medium by an optical element, and between the light source and the recording medium. , The phase displacement amount is set to 1 by making transparent materials overlap each other and having a diameter of 50% or more of the opening diameter on the opening surface of the optical system and having a thickness different from the remaining area inside the outer circumference of the opening. / Contact between materials in the phase displacement region below 2 wavelengths
An optical element for spot shaping formed at the interface is arranged, and recording is performed by changing the phase of light passing through the phase shift area without changing the phase of light passing through an area other than the phase shift area by the optical element for spot shaping. An optical head device that shapes the spot on the medium. 5. A phase displacement area of the spot shaping optical element is an ellipsoid, and a major axis and a minor axis of the phase displacement area are orthogonal to a scanning direction of a focused spot on a recording medium. The optical head device according to claim 4, wherein the optical head device is aligned in the same direction. 6. The phase displacement region of the spot shaping optical element generates different phase changes in different directions by changing thicknesses in directions orthogonal to each other on a plane having the optical axis of the laser light as a normal line. The optical head device according to claim 4, wherein the phase displacement differs between the scanning direction of the focused spot on the recording medium and the direction orthogonal to the scanning direction.