JPH0553068A - Optical system for multibeam scan - Google Patents

Abstract

PURPOSE:To improve image reproducibility in a multibeam scan optical system using plural laser beams. CONSTITUTION:This system is provided with a collimator lens 10 which converts the plural laser beams L1, L2 with wavelength lambda to collimate light at spread angle theta1 outputted from plural laser diodes LD1, LD2 whose oscillation positions are arranged being separated by distance r1 and a first scan optical system M1 which converges the laser beams converted to the collimate light on the mirror surface of a rotary polygonal mirror 14, and an f-theta lens 15 arranged between the rotary polygonal mirror 14 and a photosensitive material surface 18a and a second scan optical system M2 which converges the laser beams emitted from the f-theta lens 15 at a position separated by a prescribed jump scan cycle (i) in the subscan direction of the photosensitive material surface 18a, and an aperture 11 is arranged at a position where the plural laser beams converted to the collimate light cross with the optical axis of the first scan optical system M1, and it is defined so that those parameters r1, lambda, theta1, and (i) can satisfy equation I.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser beam scanning optical system for writing a light beam on a digital copying machine, a laser beam printer, an optical disk or the like, and more particularly to writing simultaneously by a plurality of laser beams. The present invention relates to a multi-beam scanning optical system.

[0002]

2. Description of the Related Art FIGS. 3A and 3B are explanatory views of an example of a conventional laser scanning device used in a writing device of a digital copying machine, and FIG. 3A shows two lines of two laser beams simultaneously on a photosensitive member. FIG. 3B is a plan view of a multi-beam scanning optical system for writing, and FIG. 3B is a side view of the same. This multi-beam scanning optical system includes a multi-beam diode laser array 01 (see FIGS. 3A, 3B and 4) as a semiconductor laser light source that simultaneously emits two laser beams.

As shown in FIG. 4, the multi-beam laser diode array 01 has an electrode substrate 02 on the lower surface and an LD chip (laser diode chip) 0 mounted thereon.
3, the LD chip 03 includes laser diodes LD ₁ and LD ₂ having two oscillation regions 06 and 07 separated by an insulating layer 05 on the chip substrate 04. The distance between the laser diodes LD ₁ and LD ₂ (the distance between the oscillation regions 06 and 07) is r ₁ .
When a drive current is supplied from the connection terminals Ta and Tb connected to an LD drive circuit (not shown) via the electrodes 06a and 07a, the laser diodes LD ₁ and LD
₂ forwards the _first and second laser beams L ₁ and L ₂ , and
Back beams L ₁ ', L _2' is adapted to emit backwards. In general, the laser beams L ₁ and L ₂ have a diameter at the time of emission of 1 to 4 μm in a direction parallel to the hetero interface.
Since it is difficult to determine as small as m, the size as the light source is represented by the spread angle θ ₁ .

Further, the laser array 01 is provided with a photodiode 08, and the photodiode 08 receives the back beams L ₁ 'and L ₂ ' and its light amount signal is connected to a light amount control circuit (not shown). It is arranged so as to output to the connection terminal Tc.

As shown in FIGS. 3A and 3B, in the optical paths of the laser beams L ₁ and L ₂ emitted from the oscillation regions 06 and 07, a collimating lens 010 having a focal length f ₁ and optical power only in the sub-scanning direction are provided. A cylindrical lens 012 having a focal length f ₂ , a mirror 013, and a rotary polygon mirror 01.
4, f-θ lens 015, and cylindrical lens 0
A multi-beam scanning optical system M consisting of 16 is provided, and the surface of the photosensitive drum 018 (that is, the surface of the photoconductor) as a scanning surface rotatably provided around the central axis 0
18a is scanned. Then, the collimating lens 010 and the cylindrical lens 012
The horizontal magnification in the sub-scanning direction of the first scanning optical system M ₁ is composed of m ₁ , and the lateral magnification in the sub-scanning direction of the second scanning optical system M ₂ composed of the f-θ lens 015 and the cylindrical lens 016. Magnification is m ₂ . The photosensitive drum 01
An optical sensor 019 is provided at the end of 8 and the beam position detection signal SOS (Sta (Sta)
rt of scan (see FIG. 3A), the output timing of the image signal to each of the laser diodes LD ₁ and LD ₂ is set.

5 and 6 show the multi-beam scanning optical system M.
Surface 018a of the photoconductor when writing scanning is performed using
Upper laser beam L₁, L₂It is a figure which shows the spot.
Here, two adjacent laser beams L₁, L₂Scan with
R between scanning lines ₃Scan line width (scan
) The natural number i divided by p is defined as the "interlaced scanning period".
Mean

FIG. 5 shows a case where the number n of laser beams is 2 and the interlaced scanning period i is 1. The spots a and b of the first and second laser beams L ₁ and L _{2 on} the photosensitive member surface 018a are equal in the sub-scanning direction with a diameter d ₃ . In this specification, the diameter of the laser beam and its spot is such that the light intensity is (1 / e)
² means the diameter of the part of 0.135. And
a ₁ and b ₁ are positions at the time of the first scanning of the spots a and b (displayed as scanning number (1) in the figure),
a ₂ , b ₂ are positions at the time of the second scanning, ..., At, bt are the t-th (t =
(1,2,3 ...) Indicates the scanning position.

In FIG. 5, after the first and second scanning lines are simultaneously scanned by the _first and second laser beams L ₁ and L ₂ in the first scanning, that is, the scanning number (1), the scanning number
In (2), scanning of the 3rd and 4th scanning lines by the _1st and 2nd laser beams L ₁ and L ₂ is performed simultaneously. In the same manner, scanning of two lines is repeated in the order of scanning numbers (3), (4), (5), ....

Further, as shown in FIG. 6, when the interlaced scanning period is 5, that is, when the interval r ₃ between the _two laser beams L ₁ and L ₂ is set to 5p (p is a scanning line pitch), the scan number (1), the second laser beam L ₂ is the second scanning line (2), after the scanning of the fourth scan line, the second laser beam L ₂ in scan number (3) or later sixth ，
At the same time as scanning the scanning lines 8, 10, ..., the first laser beam L ₁ simultaneously scans the scanning lines 1, 3, 5, ... scanning lines 5 lines before the second laser beam L _2. ..

The outline of the conventional multi-beam scanning device has been described above with reference to FIGS.
In such interlaced scanning with multi-beams, generally, if the number n of the laser beams and the interlaced scanning period i are relatively prime, that is, the number of laser beams and the interlaced scanning period are divisible by a common positive integer other than 1. It is possible if there is not a laser beam, and a multi-beam scanning device using three or more laser beams is conventionally known.

Next, the conditions for establishing interlaced scanning will be described with reference to the case of four laser beams shown in FIG.
As shown in FIG. 8, the scanning line (line) interval p is i (i =
3) n (n = 4) spots sj (j = 1 to n) are lined up at intervals (that is, at the interlaced scanning period i), which is q (q = 4) times the scanning line interval p. Sub-scanning interval q
Consider a case in which scanning is performed at p and one cycle is obtained by the number of scans t = to (to = 3), that is, the number of scans to in one cycle is 3. In FIG. 8, the scan number is indicated by t, the first scan is t = 1, and the second scan is t =.
The second and third scans are indicated by t = 3, fourth ... Then, the scan of the scan number (number of times) t = 3 completes the scan of the first cycle, and the scan of t = 4 to 6 completes the scan of the second cycle. The scan lines in the first period are L ₁ , _{0 to} L ₁ ,
Indicated by _11, the scanning lines of the second period is indicated by L _{2, 0} ~L _{2, 11.}

Since the position of the spot sj (j = 1 to 4) needs to be on the scanning line, i and q are natural numbers. The following two conditions (a) and (b) are necessary for interlaced scanning to be established. (B) All scanning lines must be scanned. (B) Do not scan the same scan line twice.

As shown in FIG. 8, in the scanning of t = 1 (that is, the first time), four lines L ₁ , ₀ , L ₁ , ₃ , L ₁ , ₆ ,
L ₁ , ₉ are scanned. Since one cycle is completed by the scanning of t = 1, 2, 3, the first scanning of the next cycle is started at t = 4. Therefore, at this time,
In order to satisfy the conditions of (a) and (b), the position of the spot S _{1 at} the time of the t = 4th scanning needs to be at the position shown by the spot S ₅ shown by the dotted line in FIG. That is, to
(= 3) times, that is, the moving distance to × qp of the spot when scanning is performed only once in one cycle and the length n × ip of the spot row for one cycle must be equal. Therefore, toq = ni …………………………………… (a) Moreover, each spot interval ip is to (= 3) during one cycle.
Since it is filled by scanning once, ip = top
And i = to ……………………………………………… (b). From the formulas (a) and (b), q = n ……………………………………………… (c)

As can be seen from FIG. 8, the position (line) of the spot S _{1 at} the _first scan of the second cycle, that is, at t = 4 is the spot at the first scan of the previous cycle, that is, at t = 1. Ni from the position (line) of S ₁
It is in a line advanced position. The movement amount of one cycle of the predetermined spot Sj as well as the spot S ₁ is the common multiple in of the interlaced scanning cycle i and the number of spots n.

By the way, the condition (b) is that the respective spots do not scan the same scanning line in the same period. For that purpose, n and i are relatively prime. is necessary. Next, it will be described with reference to FIG.
Referring to FIG. 9, when the number of the scanning line is represented by L (t, k) using the number of beams n and the interlace period i, L (t, k) = (n · t + 1) + i (k−1) ………………… (d) where t = -i + 1, ...- 1, 0,1, ..., i-1; scan number is an integer k = 1, 2, ..., n; spot number is a natural number In FIG. 9 and the equation (d), the scanning line number scanned by the spot Sk (k = 1, 2, ...) At the scanning number t is represented by L (t, k), and t = 0, The scanning line number L (t, k) = 1 when k = 1.

In order not to scan the same scanning line, it is only necessary that the numbers of scanning lines scanned by two arbitrarily selected spots do not match within the spot length n · i. Where t ₁ and t ₂ are arbitrary spot numbers and k ₁ and k ₂ are different spot numbers, the following equation may be established from the equation (d). L (t ₁ , k ₁ ) = (n · t ₁ +1) + i (k ₁ −1) ≠ (n · t ₂ +1) + i (k ₂ −1) = L (t ₂ , k ₂ ) ∴n · (t ₁ −t ₂ ) ≠ i (−k ₁ + k ₂ ) ……………………………… (e) where (t ₁ −t ₂ ) and (−k ₁ + k ₂ ) are arbitrary. In particular, −n + 1 ≦ (−k ₁ + k ₂ ) ≦ n−1 −k ₁ + k ₂ ≠ 0 ………………………………………………………… (f )

When the signs of (t ₁ -t ₂ ) and (-k ₁ + k ₂ ) are the same and are positive integers, if i and n are coprime, the least common multiple is in, From (f), i (−k ₁ + k ₂ ) <i · n on the right side of expression (e). Therefore, the number obtained by multiplying n by an arbitrary integer (t ₁ −t ₂ ) is not equal to i (−k ₁ + k ₂ ) which is an integer smaller than the least common multiple i · n of i and n. Therefore, the equation (e) is established. Similarly,
It also holds when the sign is a negative integer. On the other hand, when the signs are different, since n · i is a positive integer, the formula (e) is clearly established.

From the above description, the following two conditions must be satisfied in order for interlaced scanning to be established. When the number of laser beams is n and the scanning line interval is p, (1) The amount of parallel movement in the sub-scanning direction is n for each main scanning.
must be p. (2) When a natural number obtained by dividing the interval r ₃ between adjacent laser spots by p is interlaced scanning period i, i must be a natural number that is relatively prime to n (the greatest common divisor is 1). FIG. 10 shows the interlace period i and the number of laser beams n satisfying the above two conditions. As shown in FIG. 10, when the number n of laser beams is determined, a possible interlace period i is discretely given.

Generally, the factors that determine the quality of a reproduced image are the spot diameter of the laser beam applied to the surface of the photoconductor, the width of one scanning line on the photoconductor surface (that is, the scanning pitch), and the laser. There is power etc. For example, an image reproduced when gradation display by halftone dots is performed by background exposure, and if the diameter of the spot in the sub-scanning direction is too large, the colored portion becomes small in the highlight portion and becomes an image. Will not be reproduced,
In the shadow area, whiteout areas are crushed. On the other hand, if the diameter is too small, the colored portion becomes too large in the highlight portion. That is, the quality of the reproduced image depends on the spot diameter of the laser beam on the surface of the photoconductor.

5 and 6, the diameter of the spot of the laser beam applied to the surface of the photosensitive member in the sub-scanning direction is d ₃ , and the width of one scanning line on the surface of the photosensitive member (that is, the scanning pitch). ) Is p, K = (d ₃ / p)
It is known that good gradation reproducibility can be obtained when the value of is set in the range of 1.4 to 1.8 (Tanaka: The Optical Society of Japan,
The 6th Color Optics Conference Proceedings, P77, 198
See 9 years).

[0021]

By the way, in a laser beam scanning optical system using a multi-beam, a plurality of laser beams L ₁ and L ₂ pass through the same scanning optical system. Therefore, the distance r _{3 in} the sub-scanning direction between the spots a and b of the plurality of laser beams L ₁ and L _{2 on} the photosensitive member surface 018a and the diameter d ₃ of the spots a and b in the sub-scanning direction are light sources, respectively. If the distance in the sub-scanning direction between the laser diodes LD ₁ and LD ₂ of the laser array 1 and the diameter of the oscillation region of the laser diode in the sub-scanning direction (beam diameter at the time of emission) can be accurately determined, The interval r ₃ between the spots a and b of the plurality of laser beams L ₁ and L ₂ is set to an integral multiple of the scanning pitch p, and at the same time, K = (d
It is easy to set the value of ( ₃ / p) in the range of 1.4 to 1.8.

However, it is difficult to accurately determine the beam diameter at the time of emission because the diameter is small, and the diameter d ₃ of the spots a and b in the sub-scanning direction is the laser diode L.
It is obtained from the divergence angle θ ₁ of the laser beams L ₁ and L ₂ emitted from D ₁ and LD ₂ . Further, while the laser beam passes through the scanning optical system, the outer peripheral portion of the laser beam may be removed by the components of the optical system, such as the rotating polygon mirror (so-called laser beam vignetting may occur). The interval r ₃ between the spots a and b of the plurality of laser beams L ₁ and L ₂ is set to be an interlaced scanning period times the scanning pitch p, and at the same time, the value of K = (d ₃ / p) is set to 1.
It was difficult to set in the range of 4 to 1.8. Therefore, conventionally, it has not been easy to improve gradation reproducibility and line image reproducibility in a multi-beam scanning optical system.

Therefore, the present inventor considered as follows.
That is, the interval r ₁ between the laser diodes of the semiconductor laser array that emits multiple beams, the divergence angle θ ₁ and the wavelength λ of each laser beam emitted from each of the laser diodes, and the components of the scanning optical system. The distance r ₃ between the spots a and b and the diameter d _{3 in the} sub-scanning direction should be determined by the value of the parameter such as the focal length. Therefore, if the relational expression between those parameters is obtained,
By setting the above parameters so as to satisfy the relational expression, the interval r ₃ between the spots a and b of the plurality of laser beams L ₁ and L ₂ is set to the interlacing period times the scanning pitch p, and at the same time The value of K = (d ₃ / p) is 1.4
It becomes easy to set in the range of to 1.8. Then, the gradation reproducibility of the multi-beam scanning optical system should be easily improved.

As a result of the examination based on the above consideration, FIG.
In the apparatus as shown in A and 3B, with respect to the multi-beam scanning optical system in which the plurality of laser beams (L ₁ , L ₂ ) which are the collimated light beams are provided with the apertures at the positions intersecting the optical axis of the scanning optical system. , Succeeded in obtaining the following relational expression (1). However, [pi ... pi r ₁ ... laser diode LD _1, the spacing between the LD _2, θ ₁ ... laser diode LD _1, the sub-scanning direction spread angle of the laser beam L _1, L ₂ output from the LD ₂ lambda The wavelengths of the laser beams L ₁ and L ₂ output from the laser diodes LD ₁ and LD ₂ , i ... The interlaced scanning period (the spots a and b of the laser beams L ₁ and L ₂ on the photosensitive member surface 018a (FIG. 5, 6)) is a natural number obtained by dividing the interval r ₃ by the scanning pitch p, α is a value depending on the diameter of the aperture in the sub-scanning direction,
The focal length of the cylindrical lens 12 is f ₂ , the diameter of the collimated light or the approximately collimated laser beams L ₁ and L ₂ incident on the aperture in the sub-scanning direction is D, and the reflection of the rotary polygon mirror (14) A value that satisfies d ₂ = (4α / π) (λf ₂ / D), where the diameter of the laser beam (L ₁ , L ₂ ) that converges on the surface in the sub-scanning direction is d ₂ , K ... Laser beam L A value obtained by dividing the diameter d ₃ of the spots a and b on the photoreceptor surface 018a of ₁ and L ₂ in the sub-scanning direction by the width (scanning pitch) p of the scanning line, that is, K = d ₃ / p,

Next, the basis of the above equation (1) will be described. 7A and 7B are views showing a scanning optical system in which an aperture 011 is added to the multi-beam scanning optical system M similar to those in FIGS. 3A and 3B. The aperture 011 is arranged at a position where the laser beams L ₁ and L ₂ that have passed through the collimator lens 010 intersect the optical axis. 7A and 7B, if the lateral magnification of the multi-beam scanning optical system M composed of the optical members denoted by reference numerals 010 to 016 is m, then (2)
Expression is established r ₃ = mr ₁ = m ₁ m ₂ r ₁ …………………… (2) Further, if the interlaced scanning cycle is i, the following expression (3) is established. r ₃ = ip = mr ₁ (3) If the focal length of the collimator lens 010 is f ₁ and the diameter of the laser beam emitted from the collimator lens 010 is D, then (4) The formula holds.

[Equation 1]

Since the laser beams L ₁ and L ₂ intersect with the optical axis at the position where the apertures are arranged, they are focused on the mirror surface of the rotary polygon mirror 014 by the cylindrical lens 012 having the focal length f ₂ , so that the rotary polygon mirror. The diameter d ₂ of the spots of the laser beams L ₁ and L ₂ formed on the mirror surface of 014 in the sub-scanning direction
Can be expressed by Eq. (5) from the Fresnel Kirchhoff diffraction integral. d ₂ = (4α / π) (λf ₂ / D) …………………………………… (5)

The value of α in the equation (5) is a constant determined by the diameter of the aperture 011 in the sub-scanning direction. For example, the diameter da of the aperture 011 is the same as the diameter of a portion having a half intensity of the highest intensity portion (beam central portion) of the collimated laser beam having the outer diameter D (hereinafter, referred to as A diameter of a certain size is expressed as “1/2 intensity diameter.” This 1/2 intensity diameter is D / 1.70 = 0.0.0 for a beam having a Gaussian intensity distribution.
59D. ), The value of α is α = 1.94 as shown in FIG. Similarly, the inner diameter of the aperture 011 is 1 / e ² intensity diameter (that is, 1 / e ² of the highest intensity portion of the laser beam =
(The diameter is the same as the diameter of the part with 0.135 strength)
Then, the value of α is α = 1.28. If the inner diameter of the aperture 011 is much larger than the diameter D of the collimated laser beam or if the aperture 011 is not used, the value of α is α = 1.

Further, the collimating lens 0 having the focal length f ₁
10 and a cylindrical lens 012 having a focal length f ₂
The lateral magnification m ₁ of the first scanning optical system M ₁ composed of can be expressed by equation (6). m ₁ = f ₂ / f ₁ ─────────── (6) From the equations (4), (5) and (6), the following equation (7) is obtained. d ₂ = (4α / π) (λf ₂ / D) = (4α / π) (λf ₂ / f ₁ θ ₁ ) = (4α / π) (λm ₁ / θ ₁ ) …………………… (7) If the diaphragm of the second scanning optical system M ₂ between the rotary polygon mirror 014 and the photosensitive member surface 018a is set to be very large with respect to the laser beam diameter, the formula (8) is established. d ₃ = m ₂ d ₂ …………………………………… (8) From equations (7) and (8), d ₃ = (4α / π) λm ₁ m ₂ / θ ₁ = (4α / π) λm / θ ₁ ……… (9)

As mentioned above, since K = d ₃ / p,
From this and equation (9), equation (10) is obtained. Kp = (4α / π) λm / θ ₁ (10) From the above equations (3) and (10), the following equation (11) is obtained. K / i = (4α / π) λ / r ₁ θ ₁ (11) The formula (1) is obtained by modifying the formula (11). In the equation (1), by appropriately setting the value of K within the range of 1.4 to 1.8, it becomes possible to easily set the values of other parameters.

In view of the above circumstances and research results, it is an object of the present invention to improve the gradation reproducibility and the line image reproducibility of a halftone image in a multi-beam scanning optical system using a plurality of laser beams. To do.

[0031]

In order to solve the above-mentioned problems, the multi-beam scanning optical system of the first invention of the present application is characterized by having the following constitution. That is, a laser array having a plurality of laser diodes each driven by an independently modulated LD drive signal and having oscillating positions separated by a distance r _1; and a laser array in the sub-scanning direction output by the plurality of laser diodes. A collimating lens for collimating a plurality of laser beams having a divergence angle θ ₁ and a wavelength λ, and a first sub-scan having optical power in a sub-scanning direction for converging the laser beams converted into the collimated light on a mirror surface of a rotating polygon mirror. A first scanning optical system composed of a directional power optical member, an f-θ lens disposed between the rotary polygon mirror and the surface of the photoconductor, and a laser beam emitted from the f-θ lens on the subsurface of the photoconductor. Second sub-scanning direction power light having optical power in the sub-scanning direction Y that is converged at a position separated by a predetermined interlaced scanning period i in the scanning direction Y. A multi-beam scanning optical system including a second scanning optical system including a member, and an aperture is disposed at a position where the plurality of laser beams converted into the collimated light approximately intersect with the optical axis of the first scanning optical system. , A multi-beam scanning optical system in which the respective parameters r ₁ , λ, θ ₁ and i satisfy the following expressions. K = (4α / π) (iλ / r ₁ θ ₁ ) However, 1.4 ≦ K ≦ 1.8, and α is a value depending on the diameter of the aperture in the sub-scanning direction Y, and the first sub The focal length of the power optical member in the scanning direction is f ₂ , the diameter of the collimated laser beam incident on the aperture in the sub-scanning direction is D, and the diameter of the laser beam converging on the mirror surface of the rotating polygon mirror in the sub-scanning direction. the when and _{d 2, d 2 = (4α} /
π) λf ₂ / D. In addition, the second of the present application
The multi-beam scanning optical system of the invention is characterized in that in the multi-beam scanning optical system of the first invention, 1.28 ≦ α ≦ 1.94. The multi-beam scanning optical system of the second invention of the present application is the multi-beam scanning optical system of the first or second invention, wherein the number of the plurality of laser beams (L ₁ , L ₂ ) is n. The interlaced scanning period i and the laser beam number n are relatively prime.

[0032]

In the first and second inventions of the present application having the above-described structure,
Therefore, (4α / π) iλ / r ₁θ₁Value of 1.4 to 1.8
So that each of the parameters α,
i, λ, r₁, Θ₁By setting the value of
Of the laser beam spot in the sub-scanning direction
The value K divided by the width of the scan (scan pitch) is between 1.4 and 1.8.
Will be set to an appropriate value. And K is this
Multi-beam scanning optical system set to such a value reproduces gradation
And line image reproducibility are good.

[0033]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1A is a schematic plan view of the scanning optical system of the embodiment,
FIG. 1B is a schematic view of the same side view. In FIGS. 1A and 1B, the components corresponding to the components of FIGS. 7A and 7B are denoted by the reference numerals other than the most significant digit 0 of the reference numerals of FIGS. 7A and 7B, and the duplicated detailed description is omitted. To do. This Figure 1A,
The component 16 of 1B is the second sub-scanning direction power optical member, and is the cylindrical lens 016 of FIGS. 7A and 7B.
Although a cylindrical mirror is used instead of, the other constituent elements are the same as those in FIGS. 7A and 7B.

1A and 1B, reference numerals 10 to 13
The first scanning optical system M by the optical member shown₁Is configured
The lateral magnification in the sub-scanning direction is m₁Is. Figure 1B
, The focal length of the collimating lens 10 is f₁, Shi
Of the cylindrical lens (first sub-scanning direction optical member) 12
Focal length f₂, F-θ lens 15 focal length f₃, Shi
The focal length of the cylindrical mirror 16 is f_FourAnd That
Is arranged between the laser array 1 and the rotary polygon mirror 14.
Also, collimator lens 10, aperture 11, cylinder
First composed of rectal lens 12 and mirror-13
Scanning optical system M₁Lateral magnification in the sub-scanning direction Y of m₁, And then times
F- disposed between the polygonal mirror 14 and the surface 18a of the photoconductor
Consists of a θ lens 15 and a cylindrical mirror 16.
Second scanning optical system M₂Lateral magnification in the sub-scanning direction of m₂When
And these lateral magnification m ₁, M₂Mar, which is a combination of both optical systems
The lateral magnification of the beam scanning optical system M is m.

Further, the laser diodes LD ₁ and LD ₂
Interval r ₁ of the respective laser beam emitting position, the spread angle of each of the sub-scanning direction of the laser beams L ₁ and L ₂ emitted from the laser diode LD ₁ and LD ₂ theta of
_Set to ₁ . Then, these laser beams L ₁ and L ₂
Passes through the first scanning optical system M ₁ having the lateral magnification m ₁ and once forms a diameter d ₂ in the sub-scanning direction on the rotary polygon mirror 14 to converge. Then, the laser beams L ₁ and L ₂ reflected by the rotary polygon mirror 14 pass through the second scanning optical system M ₂ having the lateral magnification m ₂ and are spotted a on the surface 18a of the photosensitive member with a diameter d _{3 in} the sub scanning direction Y, b is formed at a position separated by a distance r ₃ in the sub-scanning direction Y.

The parameters of the embodiment of the multi-beam scanning optical system shown in FIGS. 1A and 1B are set as follows. λ = 0.78 μm r ₁ = 10 μm θ ₁ = 13.5 ° = 0.235 (radian) f ₁ = 25 mm f ₂ = 419.43 mm m ₁ = f ₂ / f ₁ = 16.78 α = 1.28 m ₂ = 0.5588 m = m ₁ · m ₂ = 9.377 p = 31.25 μm

When the above parameters are set as described above, the values of the diameter d ₃ of the spots a and b on the photosensitive member surface 18a in the sub scanning direction and the value of the interval r ₃ between the spots a and b are as follows. Try to calculate. r ₃ = r ₁ · m = 93.77 μm ∴i = r ₃ /p=93.77/31.25=3 Also, D = f ₁ · θ ₁ = 25 × 0.235 = 5.9 (mm) ∴d ₂ = (4α / π) λf ₂ / D = 90 μm ∴d ₃ = d ₂ · m ₂ = 90 × 0.5588 = 50 μm ∴K = d ₃ / p = 50 μm / 31.25 μm = 1.6

Therefore, in the multi-beam scanning optical system M shown in FIGS. 1A and 1B in which the respective parameters are set as described above, the spots a of the laser beams L ₁ and L ₂ shown in FIG. b is formed. In FIG. 2, the interlaced scanning period i = 3, and the spots a and b of the _two laser beams L ₁ and L ₂ on the surface 18a of the photoconductor are shown.
Is r ₃ = ip = 3p.

[0039] In this Figure 2, after the scan number (1) and the second laser beam L ₂ was scanned in the second scan line, scan number (2) a second laser in the subsequent beam L ₂
Scans the 4th, 6th, ...
The laser beam L ₁ simultaneously scans the _first , third, fifth, ... In a scanning optical system with such a scanning pitch p = 31.25 μm, 1 m
An image can be written on the surface 18a of the photoconductor with a resolution of m / 31.25 μm = 32 dots / mm. In the scanning optical system shown in FIGS. 1A and 1B in which the values of the parameters are set as described above, the value of K (= d ₃ /p=1.6) is in the range of 1.4 to 1.8. Therefore, it is possible to obtain good image reproduction.

Next, the resolution described with reference to FIGS. 1A, 1B and 2 is 32 dots / mm, and the value of K (= d ₃ / p) is in the range of 1.4 to 1.8. A method of setting parameters of the multi-beam scanning optical system will be described. Here, the multi-beam as a laser array 1 of the scanning optical system, the laser diode LD _1, the spread angle theta ₁ of the laser beam L ₁ emitted from the LD _2, L ₂ in the sub-scanning direction (heterointerface direction parallel to) = 13.5 °, wavelength λ = 0.78
The one with μm is used. The value of the distance r ₁ between the laser diodes LD ₁ and LD ₂ will be determined later.

When the resolution is 32 dots / mm, the scanning pitch p is p = 1000/32 = 31.25 (μm). Further, as described above, K = which is suitable for image reproduction
The value of d ₃ / p is 1.4 ≦ K (= d ₃ /p)≦1.8. Therefore, if d ₃ /p=1.6, then d ₃ = 50 μm.

Here, an appropriate value is adopted as the lateral magnification m ₂ of the second scanning optical system M ₂ arranged between the rotary polygon mirror 14 and the photosensitive member surface 18a, for example, m ₂ = 0.5588. Then, the diameter d ₂ of the spots of the laser beams L ₁ and L ₂ formed on the rotary polygon mirror 14 in the sub-scanning direction Y is d ₂ = d ₃ / m ₂ = 89 μm. In the scanning optical system M ₂ with the lateral magnification m ₂ = 0.5588, the parameters of its constituent elements are, for example,
It is obtained by defining the following (a) to (e). That is, (a) the focal length of the f-θ lens 15 f ₃ = 358.75 mm, (b) the focal length of the cylindrical mirror 16 f ₄ = 10.
9.88 mm, (c) distance from the surface of the rotary polygon mirror 14 to the principal point of the f-θ lens 15 133.01 mm, (d) distance from the principal point of the cylindrical mirror 16 to the photoconductor surface 18a 147.79 mm, (E) Distance between principal points of the f-θ lens 15 and the cylindrical mirror 16 is 210.96 mm,

Next, an appropriate value, for example, f ₂ = 419.43 mm, is adopted as the focal length f ₂ of the cylindrical lens 12, and the intensity of 1 / e ² of the peak intensity of the collimated light beam incident on this cylindrical lens 12 is adopted. The diameter in the sub-scanning direction Y of the portion having
And an aperture having an inner diameter equal to the diameter of the portion having an intensity of 1 / e ² of the peak intensity (aperture with α = 1.28) is used, d ₂ = (from the Fresnelkirchhoff diffraction integral equation 4 × 1.28 / π) × (λf ₂ / D). Here, since the oscillation wavelength λ of the laser diodes LD ₁ and LD ₂ is λ = 0.78 μm, D = 5.9 mm. Further, since D = f ₁ · θ ₁ and θ ₁ = 13.5 ° (= 0.235 radian) as described above, f ₁ = 25 mm.

From the values of f ₁ and f ₂ described above, m1 = f ₂ / f ₁ = 16.78. Therefore, the lateral magnification m of the entire multi-beam scanning optical system M is m = m ₁ · m ₂ = 16.78 · 0.5588 = 9.377. By the way, in the case shown in FIG. 2, since the interlaced scanning period i = 3 and the scanning pitch p = 31.25 μm, r ₃ = i · p = 93.75 μm. Therefore, the distance r ₁ between the laser diodes LD ₁ and LD _{2 of} the laser array 1 is r ₁ = r ₃ /m=93.75/9.377 = 10 μm, that is, each component of the multi-beam scanning optical system M. By setting the parameters as described above, it is possible to obtain a laser scanning device having good image reproducibility.

The method of defining the above parameters is summarized as follows. That is, when a scanning optical system having a resolution of 32 dots with good image reproducibility is obtained by using a laser array with θ ₁ = 13.5 °, first, the value of d ₃ is determined, and then an appropriate lateral magnification m ₂ is set. The 2nd scanning optical system which has is adopted. Then, the value of d ₂ is determined. Next, a cylindrical lens (first optical power member in the sub-scanning direction) 12 having an appropriate focal length f ₂ is appropriately selected, and an appropriate value of α determined by Fresnel Kirchhoff's diffraction integral, for example, α = 1 If .28 is selected, the diameter D of the collimated light in the sub-scanning direction is determined from equation (5). Then, the focal length f ₁ of the collimating lens can be determined from the diameter D of the collimated light and θ ₁ . When f ₁ and f ₂ are determined, m ₁ is determined, and m is determined from m ₁ and m ₂ . Then, if m is determined, r
₁ is set. In this way, the laser array 1 to be used
Specifications are determined.

By the way, as described above, α = 1.28 ≦ α
≦ 1.94 means that the inner diameter of the aperture 11 in the sub-scanning direction is equal to or greater than 1/2 intensity diameter of the collimated light in the sub-scanning direction (that is, D / 1.70 = 0.59D) and 1
/ E ² Intensity diameter (that is, D) or less, but if α is determined in this way, an extremely excellent scanning optical system can be obtained for the following reason. That is, since the light quantity distribution of the laser beam generally has a Gaussian distribution, the inner diameter of the aperture 11 is set to 1/2 of that of the collimated light.
When the intensity is made smaller, the amount of light blocked by the aperture 11 (amount of vignetting light) sharply increases. That is, the loss of light amount becomes large, which is not convenient.

The laser light passing through the aperture 11 includes light that directly passes through the inner diameter of the aperture 11 and light that diffracts and passes therethrough. The diameter of the spot of the laser beam on the surface 18a of the photoconductor is 2 as described above. It is defined by the superposition of two lights, the Fresnel Kirchhoff diffraction integral. Therefore, if the inner diameter of the aperture 11 is made larger than ½, the influence of diffraction is reduced, so that the diameter of the spot easily changes as shown in FIG. It is difficult to adjust the diameter of the. Range of 0.58 ≦ T ≦ 1, ie 1.28
If the diameter of the aperture is set within the range of ≤α≤1.94, the variation of the spot diameter on the surface of the photosensitive material due to the variation of the diameter of the beam incident on the aperture is small, and the design of the scanning optical system becomes easy.

The above is the divergence angle θ _{1 of} the laser beams L ₁ and L ₂ emitted from the laser diodes LD ₁ and LD ₂ = 13.
Set 5 ° and wavelength λ = 0.78 μm in advance,
This is a method of obtaining a multi-beam scanning optical system with good image reproducibility by determining r ₁ for setting K = d ₃ / p to a desired value later. R ₁ = 1 between the laser diodes LD ₁ and LD _{2 of}
A case will be described in which 0 μm and wavelength λ = 0.78 μm are set in advance and θ ₁ is determined later.

Similar to the above, when the resolution is 32 dots / mm,
The scanning pitch p is p = 1000/32 = 31.25 (μm). Further, as described above, K = which is suitable for gradation reproduction
The value of d3 / p is _{1.4 ≦ K (= d 3 /p)≦1.8} . Therefore, if d ₃ /p=1.6, then d ₃ = 50 μm. Then, r ₃ = i · p = 93.75 μm. Therefore, m = r ₃ / r ₁ = 93.75 / 10 = 9.375.

Next, in the same manner as described above, the rotary polygon mirror 1
4 and the lateral magnification m ₂ of the second scanning optical system M ₂ arranged between the photosensitive member surface 18a and m ₂ = 0.5588, d ₂ becomes d ₂ = d ₃ / m ₂ = 89 μm. Next, if a f ₂ = 419.43 mm lens is adopted as the cylindrical lens 12, then d ₂ = (1.6λf ₂ ) / D similarly to the above. Here, since d ₂ , λ and f ₂ are known, D = 5.9 mm. By the way, since m ₁ = m / m ₂ = f ₂ / f ₁ and m, m ₂ and f ₂ are known, f ₁ is determined. Further, since D = f ₁ · θ ₁ , θ ₁ is determined.

Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above embodiments, and various small modifications can be made without departing from the present invention described in the claims. It is possible to make design changes. For example, by using the above-described parameter setting method, K =
It is possible to easily obtain a multi-beam scanning optical system in which the value of d ₃ / p is in the range of 1.4 to 1.8. Further, the number of laser beams used can be an arbitrary number of 2 or more as described with reference to FIGS. Further, as the first sub-scanning direction power optical member 12, a cylindrical mirror may be used instead of the cylindrical lens,
As the second sub-scanning direction power optical member 16, it is possible to use a cylindrical lens, a hologram element, or the like instead of the cylindrical mirror. And mirror 1
3 can be omitted.

[0052]

The multi-beam scanning optical system of the present invention described above.
According to the above (4α / π) iλ / r ₁θ₁Value of 1.4
Each parameter should be set to an appropriate value between ~ 1.8.
Data α, i, λ, r₁, By setting the value of θ1,
Traverse the spot of the laser beam on the body surface in the sub-scanning direction.
The value divided by the width of the inspection line (scanning pitch) is 1.4 to 1.8
Can be set to an appropriate value between. Therefore,
According to the multi-beam scanning optical system of the present invention, a good image can be obtained.
Reproducibility can be obtained.

[Brief description of drawings]

FIG. 1 is a schematic plan view and a schematic side view of an embodiment of a multi-beam scanning optical system according to the present invention.

FIG. 2 is a diagram showing a relationship between a laser beam diameter on the surface of the photoconductor and a scanning pitch in the same embodiment.

FIG. 3 is a plan view and a side view of a conventional example of a multi-beam scanning optical system.

FIG. 4 is an explanatory diagram of a laser array light source of a multi-beam scanning optical system of the conventional example.

FIG. 5 is a diagram showing a relationship between a laser beam (spot) on a surface of a photoconductor and a scanning interval in a conventional example.

FIG. 6 is a diagram showing a relationship between a laser beam (spot) on the surface of a photosensitive member of another conventional example and a scanning interval.

7 is an explanatory view of the conventional multi-beam scanning optical system shown in FIG. 3 in which an aperture is arranged.

FIG. 8 is a diagram showing the relationship between the number of laser beams (spots) on the surface of a photoconductor and the scanning interval in a conventional example.

FIG. 9 is an explanatory diagram of a condition for establishing an interlace period.

FIG. 10 is a diagram showing a combination of a laser beam number n and an interlace period i satisfying an interlaced scanning condition.

FIG. 11 is a diagram showing a relationship between a 1 / e ² intensity diameter D of a laser beam and α at a position where an aperture having a diameter da is provided.

FIG. 12 is a diagram showing a relationship between a change in 1 / e ² intensity diameter D of a laser beam and a laser spot diameter at a position where an aperture having a diameter da is provided.

[Explanation of symbols]

a, b ... Spot of laser beam on surface of photoconductor, D ... Diameter of collimated laser beam in sub-scanning direction, d
₂ ... Diameter of laser beam on sub-scanning direction of reflecting surface of rotating polygon mirror, d ₃ ... Diameter of laser beam on photoconductor surface in sub-scanning direction, f ₂ ... Focal length of first sub-scanning direction power optical member,
i ... Interlaced scanning period, K ... A value obtained by dividing the diameter d ₃ of the laser beam on the surface of the photoconductor in the sub-scanning direction by the scanning pitch p, L ₁ ,
L ₂ ... Laser beam, LD ₁ , LD ₂ ... Laser diode, M ₁ ... First scanning optical system, M ₂ ... Second scanning optical system, M ...
Multi-beam scanning optical system, p ... Scanning pitch, r ₁ ... Spacing between a plurality of laser diodes provided in the laser array, r ₃ ... Laser beam spot a on the photosensitive member surface,
b distance, α ... A value that depends on the diameter of the aperture and is determined by Kirchhoff's diffraction integration, θ ₁ ... 1 / e ² intensity divergence angle of the laser beam emitted from the laser diode, λ ... Laser beam wavelength, 1 ... Laser array, 10
... Collimating lens, 11 ... Aperture, 12 ... First sub-scanning direction power optical member (cylindrical lens), 1
4 ... Rotating polygon mirror, 15 ... f-.theta. Lens, 16 ... Second sub-scanning direction power optical member (cylindrical mirror), 18
a: surface of photoconductor,

Claims (3)

[Claims] 1. An LD drive signal that is independently modulated is used.
And the oscillation position is distance r₁Away
Laser diodes (LD₁, LD₂ )
A laser array (1) having a plurality of laser diodes (LD₁, LD₂ ) Output
In the sub-scanning direction (1 / e²) Strength spread angle θ₁At wavelength λ
Multiple laser beams (L₁, L₂ ) As collimated light
The collimating lens (10) and the collimated light are
Laser beam (L₁, L₂) Of the rotating polygon mirror (14)
The optical power in the sub-scanning direction that converges on the mirror surface
First scanning consisting of power optical member (12) in one sub-scanning direction
Optical system (M₁) Between the rotary polygon mirror (14) and the surface of the photoconductor (18a)
F-θ lens (15) and the f-θ lens
Laser beam (L₁, L ₂) Feeling
Predetermined jumping in the sub-scanning direction Y on the surface of the optical body (18a)
Light in the sub-scanning direction Y is converged at a position separated by the inspection cycle i.
Second sub-scanning direction power optical member having optical power (
16) and a second scanning optical system (M₂) And a multi-beam scanning optical system including a plurality of laser beams (L₁，
L₂) Is the first scanning optical system (M₁) At a position that intersects the optical axis of
A percha (11) is provided, and each of the parameters r₁, Λ, θ₁, And i satisfy the following equation
A multi-beam scanning optical system defined to do.However, 1.4 ≦ K ≦ 1.8, and α is the aperture (11).
The value depends on the diameter in the sub-scanning direction,
The focal length of the optical member (12) is f₂, The apert
Laser beam that is collimated light incident on the projector (11)
Mu (L₁, L₂) In the sub-scanning direction (1 / e²) Strength value diameter
D, a laser beam that converges on the mirror surface of the rotating polygon mirror (14)
Room (L₁, L₂), The diameter in the sub-scanning direction is d₂When
And d₂= (4α / π) × (λf₂/ D)
It 2. The multi-beam scanning optical system according to claim 1, wherein 1.28 ≦ α ≦ 1.94. 3. The interlaced scanning period i and the laser beam number n are relatively prime to each other, where n is the number of the plurality of laser beams (L ₁ , L ₂ ). Alternatively, the multi-beam scanning optical system described in 2.