JPH01131515A - Optical scanning method - Google Patents

Abstract

PURPOSE:To satisfactorily correct curvature of field so that good optical scanning is enabled by using a single lens which is well corrected in ftheta characteristic as an ftheta lens and subjecting either of a collimating lens or laser light source whichever is movable to simple oscillation in an optical axis direction in synchronization with the optical scanning. CONSTITUTION:The ftheta lens which is well corrected in the ftheta characteristic and curvature of field tends to have intricate lens constitution but the ftheta lens is easily realized even with a single lens if the correction of only the ftheta characteristic is involved. The single lens which is well corrected in the ftheta characteristic is, thereupon, used as the ftheta lens 4 and the curvature of field is corrected by the simple oscillation of the collimating lens 2 or the laser light source 1. The curvature of field is thereby satisfactorily corrected and the good optical scanning is enabled.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an optical scanning method that performs optical scanning using a beam of light from a laser light source.

(Prior art) A light beam from a laser light source is incident on a rotating deflection means through a collimating lens, is deflected once by the deflection means to form a deflected light beam, and this deflected light beam is irradiated onto a scanning surface via an fO lens. The optical scanning method for performing optical scanning is conventionally well known.

The rotational deflection means is a rotating polygon mirror or a pyramidal mirror, but since the deflection of the deflected light beam by such a rotation deflection means is at a constant angular velocity, in order to perform light scanning on the scanning surface at a constant angular velocity, An fθ lens is required.

Also the fθ lens. It also has the function of forming a spot image of the deflected light beam on the scanning surface. Therefore, a good f.theta. lens is one in which both the so-called f.theta. characteristics and field curvature are well corrected. If field curvature is not sufficiently corrected, the size of the spot that scans the scanning plane will vary greatly depending on the scanning position. For example, when writing optical information, the resolution of the recorded image will not be constant on the same image. It won't happen.

(Problem to be Solved by the Invention) However, it is not necessarily easy to satisfactorily correct both fθ characteristics and field curvature with an fθ lens, and fθ lenses that satisfy such conditions often have a complicated lens configuration. There are problems such as high cost and high cost.

The present invention has been made in view of the above-mentioned circumstances, and an object of the present invention is to use an fθ lens with a simple lens configuration, to satisfactorily correct field curvature, and to achieve good optical scanning. The object of the present invention is to provide a novel optical scanning method that makes it possible.

(Means for Solving the Problems) The optical scanning method of the present invention includes making a light beam from a laser light source enter a rotary deflection means via a collimating lens, deflecting it by a five-rotation deflection means to form a deflected light beam; In this method, light is irradiated onto the scanning surface through an fθ lens to perform optical scanning.

As the rotational deflection means, a rotating polygon mirror or a pyramidal mirror can be used.

A single lens is used as the fθ lens.

This f.theta. lens uses one whose f.theta. characteristic and field curvature are well corrected.

The curvature of field is corrected by causing the collimating lens or the laser light source to undergo simple vibration and displacing the imaging position by the fθ lens.

That is, when the collimating lens is caused to perform a simple harmonic motion, the collimating lens is caused to perform a simple harmonic motion in the optical axis direction in synchronization with optical scanning.

Further, when the laser light source is caused to undergo simple vibration, the laser light source is caused to undergo simple vibration in the optical axis direction of the collimating lens in synchronization with the optical scanning.

(Function) An fO lens that has well corrected both fθ characteristics and field curvature tends to have a complicated lens configuration, but if only the fθ characteristics are corrected, even a single fθ lens can be used. This can be easily realized, and furthermore, extremely good correction is possible.

Therefore, in the present invention, taking advantage of this fact, a single lens with well-corrected fθ characteristics is used as the fθ lens.

The field curvature is corrected using a collimating lens or simple harmonic vibration of the laser light source.

A detailed description will be given below with reference to the drawings.

FIG. 1 is a diagram schematically showing only the parts necessary for explanation of an example of an apparatus for carrying out the present invention. In the figure.

Reference numeral 1 indicates an LD (laser diode) as a light source. The light from the LDI enters a rotating polygon mirror 3 as a rotational deflection means via a collimating lens 2, and when reflected, enters a single fθ lens 4, and after passing through this, it enters a scanning surface 5. do.

When the rotary polygon mirror 3 is rotated, the light beam reflected by the single-rotation pot mirror becomes a deflected light beam and optically scans the scanning surface 5. Although fθ lens 4 is a single lens, it has well corrected fθ characteristics, so
The uniformity of optical scanning is good. However, on the other hand, a large arc-shaped curvature of field 6 occurs.

This field curvature 6 is caused by a spot-shaped condensing point of the deflected light beam when the rotating polygon mirror 3 is rotated while the light beam from the LDI is parallelized by the Cori-Two lens 2 and incident on the fθ lens 4. It is a trajectory. In this way, the position of the focal point of the deflected light beam shifts from the scanning plane with the deflection angle θ, so
When optical scanning is realized using such an optical system, the diameter of the spot gradually increases at both ends of the scanning area, and the resolution of optical scanning decreases. Therefore, this field curvature is corrected by moving the collimating lens 2.

FIG. 2(A) shows the optical path from the LDI, which is the light source, to the scanning surface 5, extending in a straight line.

In addition, in this figure 2 (A), the deflection angle θ (see figure 1) is 0.
It is said that

The light beam emitted from the LDI is collimated by the collimator lens 2, and when it enters the fO lens 4, the light beam forms a spot image on the scanning surface 5 as shown by the solid line. In other words, at this time.

The collimating lens 2 is located at the position indicated by the solid line, and the distance between the collimating lens 2 and the LDI is equal to the focal length f1 of the collimating lens 2, and the distance between the fθ lens 4 and the scanning surface 5 is equal to the focal length f1 of the collimating lens 2.
The distance between them is equal to the focal length f2 of the fθ lens 4.

However, when the collimating lens 2 is displaced by Δ on the optical axis to the position shown by the broken line on the optical axis as shown in FIG. It becomes diverging, and the imaging plane 5A formed by the fθ lens 4 is shifted by Z to the rear of the scanning plane 5, and the light beam forms a spot-like image on this imaging plane 5A. By displacing the collimating lens 2 on the optical axis in this manner, the position of the image forming point is shifted, and this fact is utilized to correct the curvature of field.

If the distance from the image-side focus of the collimating lens 2 to the object-side focus of the fθ lens 4, which is located at the position indicated by the solid line in FIG. 1)
Expressed by the formula.

Also, when f□〉〉Δ, equation (1) approximately becomes Z=
(f2/f□)2・Δ (1
-1) can be written. When equation (1) is solved for Δ, the following equation (2) is obtained.

Δ=(1/2)(((f2'/Z)-d)-F7 stone-a
>-F'r (2) In FIG. 1, the intersection of the optical axis of the collimating lens 2 and the deflection reflection surface of the rotating polygon mirror 3 is called a deflection point. The fO lens 4 is arranged so that its optical axis substantially coincides with a perpendicular line drawn from the deflection point to the scanning surface 5.

If we express the field curvature 6 as a function of the deflection angle θ as D=D(θ), when such field curvature D(θ) occurs,
Let us consider correcting field curvature by moving the collimating lens 2 in the optical axis direction.

That is, by moving the collimating lens 2, the fθ lens 4
The image forming position is displaced, and the displacement amount offsets the deviation of the image forming spot from the scanning plane due to field curvature. The amount of displacement Δ' of the collimating lens 2 necessary for this correction is given by the following equation (3) with respect to the deflection angle θ and the curvature of field D(θ).

Δ' = (1/2) (((f2″/D(θ))+L
, ) cos θ - L2 Here, the distance from the object side focus of the fθ lens 4 to the deflection point is defined as L2, and the distance from the image side focus of the collimating lens 2 located at the position indicated by the solid line in Fig. 2 to the deflection point is L2. . Regarding the deflection angle θ, as is clear from Fig. 1, the deflected light beam is fθ along the optical axis of the fθ lens.
The time when the light is incident on the θ lens 4 is defined as O. Incidentally, (3
) in equation (1).

d3L2−Licosθ, (L”/Z) (fz”10
(θ))cOsθ, respectively.

To make the explanation concrete, f1=15. f2=2
95, L□=150. The value of Δ' for the curvature of field when L2=50 is calculated using the deflection angle θ as a parameter, as shown in FIG. 3(A).

Also, when equation (3) is solved for D(θ), the following (4) is obtained.
It becomes like the expression.

D(θ)=(fz"cosθ)/((f,"/Δ'
) +L, -L1 cos θ + Δ') (4) This formula shows that by displacing the collimating lens 2 by Δ' toward the light source, the image plane is shifted by D(θ) at the deflection angle θ.
The closer you get, the more you get.

In addition, when Δ′〈f, the above (3) applies.

Equations (4) can be approximately written as follows.

Δ'=(ft/fz)"(D(θ)/cosθ)
(3-1) D(θ)=(f2/f,)"Δ'
cos (4-2) As mentioned above, the curvature of field can be completely corrected by displacing the collimating lens 2 according to equation (3) according to the deflection angle θ. It is difficult to displace the collimating lens. Therefore, in the present invention, the collimating lens 2 is made to perform a simple harmonic motion, and the displacement of the collimating lens 2 due to this simple harmonic motion approximates the movement of equation (3), thereby effectively correcting the curvature of field.

Since the deflection of the deflected light beam by the rotating polygon mirror 3 is at a constant angular velocity, if the scanning frequency of the deflected light beam, that is, the number of optical scans per unit time is ν, then the deflection angle θ is such that the time t is n-1.
The following (5
) can be written as the formula.

θ=2 (vt-n)θll1ax/E
(5) Here, θl1ax is the maximum deflection angle (-
〇ll1aS≦θ≦θmax), E is a quantity expressed as E=ντ, where τ is the time required for θ to change from −θmax to θmax, and the deflected light beam effectively scans the light. Indicates the percentage of time spent

The position of the collimating lens 2 is set so that when the deflection angle θ is θ=0 and θ=±on+ax, the imaging position by the fθ lens 4, that is, the spot-shaped light condensing position coincides with the scanning plane 5, and θ=O Using the position at the time of , as a reference position, and assuming that displacement occurs on the light source side from this reference position, the collimating lens is caused to perform simple harmonic vibration at the frequency, that is, at the scanning frequency, in synchronization with optical scanning.

Originally, the field curvature 6 (D(θ)) due to the fθ lens 4 does not have a very complicated shape, so θ=O1θ=±θwa
It is possible to satisfactorily correct field curvature not only at x but also overall.

This simple harmonic motion has an amplitude A of A = Xo/(1-cos(πE)), where X, =
(1/2) [((fz2/D, +L,) cosθ
As maX Lz, X=A (1-cos (2tc v t))
It can be expressed as (7). In equation (6), Do is the field curvature when θ=θmax, that is, D0
=D(θwax), and xo represents the movement of the collimating lens necessary to align the imaging spot with the scanning plane when θ=θll1ax.

Figure 4 shows this simple harmonic motion. Using equation (5), we rewrite the variable in equation (7) from time t to deflection angle θ.

X=X(θ)=A (1-cos(z E ・θ/θ
max)) (8) This equation (8) is shown in FIG.

When the collimating lens 2 is caused to perform the simple harmonic motion expressed by the equation (7), the imaging point of the deflected light beam is calculated according to the equation (4).

G(θ)=(f,"cosθ)/((f,"/x(θ
))+L! −L1 cosθ+X(θ))
(9) moves toward the scanning surface side. Therefore, G(θ
)teeth.

It represents the amount of field curvature correction due to simple harmonic motion of the collimating lens 2. In other words, the field curvature, which was -D(θ) before correction, becomes D'(θ)=-D(θ)+G after correction.
(θ) (10).

Specifically, the above-mentioned numerical value f as fl, f, HL 2; 15, f = 295, Lz = 150, L,
=50, the correction amount G(θ) is E=0.5. Table 1 for E=0.7 respectively.
, as shown in Table 2.

Table 1 (E=0.5) Table 2
(E=0.7) In order to cause the collimating lens to undergo simple harmonic vibration.

Conventionally known precision drive mechanisms related to voice coils, piezoelectric semiconductors, objective lens control of optical pickups, etc. can be used.

In the above explanation, due to the simple harmonic motion of the collimating lens, fθ
This is an explanation of the case of correcting the field curvature of a lens.

Below, a case will be described in which the above-mentioned curvature of field is corrected by simple harmonic motion of the laser light source.

Referring to FIG. 2(B), this figure corresponds to the above-mentioned FIG. 2(B).
Similar to A), the optical path from the LDI, which is the light source, to the scanning surface 5 is drawn as a straight line.The light beam emitted from the LDI is collimated by the collimating lens 2, and then the fθ lens 4 When the light beam is incident on the scanning surface 5, the light beam forms a spot image on the scanning surface 5.

In other words, at this time, the LDI is at the position indicated by ■ in the figure, the distance between the collimating lens 2 and the LDI is equal to the focal length f1 of the collimating lens 2, and the distance between the fθ lens 4 and the scanning surface 5 is The distance between is equal to the focal plane @f of the fθ lens 4.

However, as shown in FIG. 2(B), when the LDI is displaced by δ on the optical axis of the collimating lens 2 toward the collimating lens 2 side to the position indicated by ■, the light flux that has passed through the collimating lens 2 at this time is indicated by the broken line. As shown, it becomes slightly divergent, and the imaging plane 5B formed by the fθ lens 4 is shifted by Z' to the rear of the scanning plane 5, and the light beam is focused on a spot on this imaging plane 5B. By displacing the LDI on the optical axis of the collimating lens 2 in this way.

Since the position of the image forming point is shifted, this fact is utilized to correct the field curvature 6.

In FIG. 2(B), from the image side focal point of the collimating lens 2, f
Assuming that the distance to the object side focal point of the θ lens 4 is d as described above (it is positive when the image side focal point of the collimating lens 2 is on the light source side, and negative when it is on the imaging plane side), Image plane 5A
The displacement amount Z' is given by the following equation (11).

When equation (11) is solved for δ, it becomes linear equation (12).

As mentioned above, the field curvature 6 is expressed as a function of the deflection angle θ by D=D(
θ), and such field curvature D(θ
) occurs, the LDI is moved in the optical axis direction to correct the field curvature. That is, L.D.
By moving I, the imaging position of the fθ lens 4 is displaced,
The amount of displacement δ' of LDl required for correction is the following: It is given by equation (13).

Here, as in the above description, the distance from the object side focus of the fθ lens 4 to the deflection point is L, and the distance from the image side focus of the collimating lens 2 to the deflection point is L2. Regarding the deflection angle θ, as is clear from FIG. 1, O is defined as the time when the deflected light beam enters the fθ lens 4 along the optical axis of the fθ lens. Incidentally, in equation (12), d is L2-LiCO3θ and (fI/Z') is (
f2''/D(θ)) cos θ.

In order to make the explanation more concrete, as in the case of the previous example in which the collimating lens 2 is made to undergo simple harmonic vibration, f: 15°f, = 29
5. L = 150. The value of δ' with respect to the curvature of field when L, :50 is calculated using the deflection angle θ as a parameter, as shown in FIG. 3(B).

Also, when equation (]3) is solved for D(θ), the following (1
4) It becomes like the formula.

This equation (14) shows that LDl is δ to the collimating lens 2 side.
'', at the deflection angle θ, D(
6) shows that it is possible to move the image plane closer to the scanning plane 5.

Note that when δ'<<f, the above equation (13), (1
4) Equations can be approximately written as follows.

δ' ”(L/fz)2(D(0)/cosθ)
(13-1) D(θ) = (f2/f)2δ'
cosθ (14-2) these (13-1
), (14-2) are the above equations (3-1) and (4-2)
corresponds to

As mentioned above, the curvature of field can be completely corrected by displacing the LDL according to equation (13) according to the deflection angle θ, but in reality, displacing the LD in this way is difficult. Have difficulty. Therefore, in the present invention, the LDI is caused to vibrate +li, and the displacement of LDl due to this simple harmonic motion is expressed as (13)
This approximates the movement of the equation and effectively corrects the curvature of field 6.

A voice coil motor is shown in FIG. 6 as an example of a device for performing simple harmonic motion of the LDI. That is, the coil 7 is wound around the movable part to which the LDI is attached, and the movable part is connected to the flexure 8 having a small spring constant only in the axial direction.
By generating a force in the direction of the optical axis between the current flowing through the coil 7 and the magnetic field, a single displacement is applied to the LDI, so that the LDI is caused to vibrate in a simple manner without being deviated from the optical axis. It is something. Note that 9 indicates a magnet.

In the above, if the scanning frequency of the light beam deflected by the one-rotation polygon mirror 3, that is, the number of optical scans per unit time, is Y, then the deflection angle θ is determined by the time t being n-(1/2) <vt
< n + (1/2) (n is an integer) as described above (
5) It is expressed as in the equation. This equation (5) is now −
It is written as follows.

θ=2(vt-n)θwax/E (
5) Here, θmax is the maximum deflection angle (-〇IaaX≦θ
≦θwax)E is the amount expressed as E=vt, where the time required for θ to change from 10max to emax is E=vt, and it is the proportion of time during which the deflected light beam is effectively scanning the light. As mentioned above,

The position of the LDI is set so that the deflection angle θ is θ=0 and θ=±θma.
x, the imaging position by the fθ lens 4, that is, the spot-shaped light condensing position is determined to coincide with the scanning plane 5,
The position when θ=0 is used as a reference, and assuming that displacement occurs on the light source side than this reference position, the LD is set at a frequency ν, that is,
When a simple harmonic motion is generated in synchronization with optical scanning at the scanning frequency, the curvature of field 6 (D(θ)) due to the fθ lens 4 does not have a very complicated shape, so when θ=0 and θ=±θmax, Not only this, but also the field curvature 6 can be favorably corrected overall.

This simple harmonic motion has the amplitude A as A' = X, / (1-cos (z E)), where IO, and then
) (16). (
In Equation 15), Do represents the amount of movement of LDl required to align the imaging spot with the scanning plane when θ=θll1ax.

Figure 7 shows this simple harmonic motion. Using equation (5) to rewrite the variable in equation (16) from time t to deflection angle θ, we get X'=X'(θ)=A' (1-cos (πE ・
θ/θwax)) (17). This equation (17) is shown in FIG.

When LDI is made to perform the simple harmonic motion shown by equation (16),
The imaging point of the deflected light beam moves toward the scanning plane 5 by an amount as calculated according to equation (14). Therefore, G'(θ) is
It represents the field curvature correction amount due to simple harmonic motion of LDl. In other words, the field curvature, which was 10(θ) before correction, becomes D'(θ) knee D(ill)+G'(θ) after correction.
(19).

Specifically, the above-mentioned numerical value f==15. f2=295. L = 150.
When L2=50 is used, the correction JitG'(θ) is set with E as a parameter and E=0.5. For E=0.7, the results are shown in Table 3.4.

Table 3 (E==0.5) Table 4
(E=0.7) (Example) Eight specific examples are listed below. In each example, f
1=15. f2=295. It is set as θmax=21', and the shape of the fO lens, the refractive index, and the fO
Distance L□ from the object side focal point of the lens to the deflection point, distance f'FI L z from the image side focal point of the collimating lens 2 when it is in the position to collimate the light beam from LD1.
, and the curvature of field when θ=θmax.

=D(θ1naX) * Movement amount X of the collimating lens required to align the imaging spot with the scanning plane when θ=θmax. Alternatively, each embodiment is expressed in terms of the amount of movement of the LD. r□+r2 represents the radius of curvature of the object-side and image-side lens surfaces of the fθ lens, Lo represents the lens thickness, and n represents the lens refractive index.

Examples 1 to 4 shown in Table 5 use collimating lenses.
5. Aberration diagrams according to each of the above embodiments are shown below in FIG. 9. 9th
The figures to FIG. 11 relate to Example 1,
FIG. 9 shows the fθ characteristic, FIG. 10 shows the curvature of field without correction by simple harmonic motion of the collimating lens (referred to as before correction), and FIG. 11 shows the curvature of field after correction.

12 to 14 relate to Example 2, in which FIG. 12 shows the fθ characteristic, FIG. 13 shows the curvature of field before correction, and FIG. 14 shows the curvature of field after correction.

15 to 17 relate to Example 3, in which FIG. 15 shows the fθ characteristic, FIG. 16 shows the curvature of field before correction, and FIG. 17 shows the curvature of field after correction.

18 to 20 relate to Example 4, in which FIG. 18 shows the fθ characteristic, FIG. 19 shows the curvature of field before correction, and FIG. 20 shows the curvature of field after correction.

Table 6 shows four examples in which the LD is caused to undergo simple harmonic vibration.

Table 6 FIG. 21 and subsequent figures show aberration diagrams regarding each of the above-mentioned sides.

FIG. 21 shows the fθ characteristic in the case of Example 5, and the field curvature before correction (when no correction by simple harmonic motion of LDI is performed) is as shown in FIG. 22, but the field curvature after correction is The curvature becomes - as shown in FIGS. 23(A) and 23(B) ((A shows the case where E=0.5, (B) shows the case where E=0.7, and Example 6- 8).

Moreover, FIG. 24 shows the fθ characteristic in the case of the above-mentioned Example 6, and the field curvature before correction is as shown in FIG. 25, but the field curvature after correction is shown in FIG. 26 (A), ( As shown in B).

Furthermore, FIG. 27 shows the fθ characteristic in the case of Example 7, and the curvature of field before correction is as shown in FIG. 28, but the curvature of field after correction is as shown in FIG. As shown in B).

Furthermore, FIG. 30 shows the fO characteristics in the case of Example 8, and the field curvature before correction is as shown in FIG. 31, but the field curvature after correction is as shown in FIG. 32 (A), ( As shown in B).

In the above embodiment, a voice coil motor was used to cause the LDI to undergo simple harmonic vibration, but other conventionally known piezoelectric semiconductors or precision drives known in connection with objective lens control of optical pickups, etc. In addition, as the rotating deflection means, a pyramidal mirror can be used in addition to a rotating polygon mirror, and it goes without saying that various other deflections can be applied without departing from the gist of the present invention. .

(Effects of the Invention) As described above, in the present invention, f
Using a collimating lens or simple harmonic motion of the laser light source while properly correcting the θ characteristics.

Field curvature can be corrected well.

Furthermore, since a single fθ lens can be used, the optical scanning device can be made smaller and lower in price.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an example of the optical arrangement of an optical scanning device to which the present invention is applied, and FIG. Figure 2 (B). Figure 3 is a diagram for explaining the state in which the imaging point of the fθ lens is displaced due to the movement of the laser diode as a laser light source. Figures 4 and 5 are diagrams showing the amount by which the diode is displaced, Figures 4 and 5 are diagrams for explaining simple harmonic motion of the collimating lens, and Figure 6 is a diagram for explaining an example of a mechanism for causing the laser diode to move in simple harmonic motion. , FIG. 7, and FIG. 8 are diagrams for explaining simple harmonic motion of a laser diode, and FIGS. 9 to 11 are diagrams for Example 1.
The aberration diagrams shown in FIGS. 12 to 14 are for explaining Example 2.
The aberration diagrams shown in FIGS. 15 to 17 are for explaining Example 3.
The aberration diagrams shown in FIGS. 18 to 20 are for explaining Example 4.
21 to 23 are aberration diagrams for explaining the example 5.
The aberration diagrams shown in FIGS. 24 to 26 are for explaining Example 6.
The aberration diagrams shown in FIGS. 27 to 29 are for explaining Example 7.
The aberration diagrams shown in FIGS. 30 to 32 are for explaining Example 8.
It is an aberration diagram for explaining. 1... Laser diode as a light source, 2... Collimating lens, 3... 5-2 as rotational deflection means. -19-thing ν θtsu, -ru, outside θ弗1 placeJσ口杷qD圀! θSpecial〃5/C Japan Captivity IE Letter/)a @Fumeiaki tsu ℃ η )1N za)

Claims (1)

[Claims] 1. A light beam from a laser light source is made incident on a rotary deflection means through a collimating lens, is deflected by the rotary deflection means to form a deflected light beam, and this deflected light beam is passed through an fθ lens to a scanning surface. In this method, a collimating lens or a laser light source is movable in the optical axis direction of the collimating lens, and an fθ lens is used as an fθ lens.
Using a single lens with well-corrected θ characteristics, in synchronization with optical scanning, a movable one of the collimating lens and laser light source is caused to perform simple vibration in the optical axis direction, thereby adjusting the field curvature of the fθ lens. An optical scanning method characterized by correcting. 2. The optical scanning method according to claim 1, characterized in that the collimating lens is caused to vibrate in simple harmonic motion. 3. The optical scanning method according to claim 1, characterized in that the laser light source is caused to undergo simple harmonic vibration.