JPH01145621A - Multiwavelength light beam optical system - Google Patents

Abstract

PURPOSE:To obtain high energy efficiency and to make a device compact by compensating wavelength dispersion generated among acoustooptic elements of a composite system by a dispersion optical system. CONSTITUTION:The wavelength dispersion generated between an acoustooptic modulator 3 and an acoustooptic deflector 6 is compensated as the whole composite system by one dispersion optical system by arranging a prism 5 between both, and placing the acoustooptic modulator 3 and prism 5 in conjugate relation by an incidence-side relay lens system 41 and the acoustooptic deflector 5 and prism 5 in conjugate relation by a projection-side relay lens system 42. Then the wavelength dispersion of the acoustooptic elements is compensated as the composite system by the dispersion optical system, so the number of dispersion optical members for compensation can be decreased. Consequently, the number of components of the optical system is decreased and the optical path lens is also shortened, so the energy loss of the optical system is reduced and the whole optical system can be made compact.

Description

Detailed Description of the Invention [Industrial Field of Application] The present invention is a high-speed photoreceptor that directly forms an image on a low-sensitivity photoreceptor by scanning a light beam such as a laser that includes a plurality of wavelength components on the same optical axis. This invention relates to a multi-wavelength light beam optical system used in drawing devices and the like.

[Conventional technology]

Conventionally, when high-speed image formation is required despite the extremely low sensitivity of photoreceptors, laser beams containing multiple wavelength components simultaneously are placed on the same optical axis in order to increase the energy density. A multi-wavelength light beam optical system is used that guides and scans the light beam.

For example, an Ar'' laser is known as a laser light source that can emit laser beams containing multiple wavelength components on the same optical axis.
It can oscillate laser light with multiple wavelengths including nm,
With the same size and power consumption, it is possible to obtain laser light with several times the output of a single wavelength.

However, an acousto-optic element used to modulate a light beam in an optical system that forms an image is generally an element that utilizes the light diffraction phenomenon caused by the traveling wavefront of an ultrasonic wave in an optical crystal. If it contains a wavelength component of
The diffraction angle differs depending on each wavelength, and the emitted laser beam spreads in the direction of diffraction. This will be called chromatic dispersion.

This will be explained using a mathematical formula. An acousto-optic device such as an acousto-optic modulator or an acousto-optic deflector has a configuration as shown in FIG. Ultrasonic waves (ultrasonic wavefront (14)) are incident.

When the speed of sound in the medium is V and the frequency of the ultrasonic wave is F, a traveling wave of wavelength V/F is generated in the medium, and its wavefront diffracts the incident light beam (LB) like a diffraction grating. The diffraction angle θ is expressed by equation (1), where λ is the wavelength of the incident light beam.

For example, if the incident light beam (LB) contains two wavelength components, as is clear from equation (1), each of them will be emitted at a different diffraction angle, so as shown in Figure 6 (LBI),
The light beams are separated as shown in (1B2).

Generally, when an incident light beam has a wavelength width of Δλ, it will be emitted with a spread of diffraction angle expressed by equation (2), according to equation (1).

For this reason, even if the chromatic aberrations of other optical systems are well corrected, the wavelength dispersion caused by the acousto-optic element remains, significantly deteriorating the quality of the beam spot finally obtained.

Conventionally, as a method of compensating for the wavelength dispersion of an acousto-optic element represented by equation (2), a method as disclosed in Japanese Patent Application Laid-open No. 193130/1983 has been known.

In this conventional method, as shown in FIG. 4, a dispersive optical system (prism (5), relay lens system (42)) that produces wavelength dispersion corresponding to the wavelength dispersion caused by the acousto-optic element (11) is used. By arranging at least one for one acousto-optic element and making them in a conjugate relationship by the relay lens system (42), the acousto-optic element (11)
A dispersive optical system (prism (5)) that handles wavelength dispersion due to
, relay lens system (42)).

That is, the deflection angle ψ due to the prism (5) is the apex angle α,
Letting the refractive index of the glass be n, (p#(n-1)α...(3) To the wavelength width of the incident light, let the refractive index difference of the glass with respect to λ be △n, the incident light is received by the prism (5). The amount of wavelength dispersion can be written as in equation (4).

△ψ=△n・α...(4) When the relay lens system (42) is arranged so that the prism (5) and the acousto-optic element (11) are conjugated, the conditions for compensating for chromatic dispersion are (2) From equation (4), equation (5) appears.

Δθ＋△ψ=　0 The angle is taken into account with the counterclockwise direction being positive and the sign taken into account.

Incidentally, in actual drawing apparatuses of this type, scanning of the light beam is often performed using a rotating polygon mirror. Each of the reflecting surfaces of this rotating polygon mirror is not necessarily perpendicular to the plane of rotation, and usually includes some degree of surface inclination error between the surfaces.

Therefore, in the optical system of this type of device, one more acousto-optic element is required to compensate for the surface tilt error, and a chromatic dispersion compensation system is also required for this.

FIG. 5 is a schematic diagram of the configuration of a multi-wavelength light beam scanning optical system in which the wavelength dispersion of two optical elements arranged on the same optical axis is compensated by the method disclosed in Japanese Patent Application Laid-Open No. 193130/1982. It is.

In the figure, for example, a prism (5A) and a relay lens system (42A) are arranged to satisfy equation (5) with respect to an acousto-optic modulator (3) for performing modulation using image data. (3) compensates for the wavelength dispersion that occurs.

Similarly, for example, the acousto-optic deflector (
6) Another prism (
5B) and a relay lens system (42B) are arranged.

Further, (4) is a relay lens system for shaping the laser beam into an appropriate beam system.

[Problems to be solved by the invention] In the conventional multi-wavelength light beam optical system as described above,
To compensate for the wavelength dispersion of individual acousto-optic elements, at least one compensating dispersion optical system is required for one acousto-optic element, and in a composite system including multiple acousto-optic elements, a prism, etc. This method has a problem in that it has a complicated configuration with a large number of parts, such as a dispersive optical member and a relay lens system for establishing a conjugate relationship between each acousto-optic element and the dispersive optical member.

Here, an increase in the number of parts not only means that maintenance is time-consuming, but also leads to an increase in energy loss due to an increase in the lens transmission surface, especially when forming images at high speed on a low-sensitivity photoreceptor. etc., this is a serious drawback.

In addition, as it is desired to make the entire device more compact, in this type of optical system, it is necessary to compare the focal length of the relay lens due to the limitations of chromatic aberration correction and the resolution of diffracted light from the acousto-optic element. Because you have to stay on target for a long time,
Frequent use of relay lens systems causes a problem in that the optical path length becomes extremely long.

This invention was made in view of these points, and by compensating the wavelength dispersion of multiple acousto-optic elements with a simpler configuration, it is possible to produce multi-wavelength light with high energy efficiency and the possibility of making the device more compact. Its purpose is to provide a beam optical system.

Means for Solving Problem C] In the present invention, the above problem is solved by disposing a dispersion optical system that compensates for wavelength dispersion occurring in each of a plurality of acousto-optic elements as a composite system.

(Function) In this invention, the wavelength dispersion of a plurality of acousto-optic elements is compensated by a dispersive optical system as a composite system, so the number of compensating dispersive optical members can be reduced.

Furthermore, since the dispersive optical system is shared, the number of relay lens systems for maintaining a conjugate relationship between each acousto-optic element and the dispersive optical member in the dispersive optical system can also be reduced.

In this way, the number of parts in the optical system is reduced and the optical path length is also shortened, so that energy loss in the optical system is reduced and the entire optical system can be made very compact.

〔Example〕

Next, embodiments of the present invention will be described in detail based on the drawings.

Embodiment 1 FIG. 1 is a schematic diagram of the configuration of a first embodiment of the present invention, and is an example in which a prism is used as the dispersive optical member.

In this example, the dispersive optical system includes a single prism (5) and relay lens systems (41
), (42).

In the figure, multiple wavelengths λ1. A laser beam (LB) containing λ2 (λ,≠λ2) is incident on the acousto-optic modulator (3). The sound speed in the medium of the acousto-optic modulator (3) is ■1,
Letting the ultrasonic frequency be Fl, the wavelength λ+(i
= t, z...) The diffraction angle of the laser beam is corrected counterclockwise with respect to the direction of the incident laser beam (LB), and is applied to the acousto-optic modulator (3) as shown in Figure 1. The incident laser beam (F8) has a wavelength λ1.according to equation (6). λ
2, each is injected separately as (LBI) (LB2).

Let the magnification of the relay lens system (41) be ml. The relay lens system (41) emits a parallel beam with an incident diameter of Φ as a parallel beam with a diameter of 1 m + Φ1, and is arranged to make the acousto-optic modulator (3) and prism (5) conjugate, so that the laser beam (LBI) (LB2) reaches the same position again on the prism (5). At this time, the laser beam angle is based on the direction of the incident beam (LB), and if the refractive index of the glass constituting the prism for the wavelength λ1 is nl (i = 1.2...) and the apex angle of the prism is α, the incident light is given a deflection angle δ1 expressed by equation (8) and is ejected.

δ+ = (n+-1)α (8) Therefore, the laser beam after being emitted from the prism is emitted in the direction of δ++90+ with respect to the direction of the incident beam (B).

If the magnification is m2, the relay lens system (42) emits a parallel beam with an incident diameter Φ as a parallel beam with a diameter of 1 m 2 Φ1, as well as the relay lens system (41), and has a prism (5) and an acousto-optic deflection. They are arranged so as to make the vessel (6) conjugate.

The angle of the laser beam after exiting the relay lens system (42) is determined based on the direction of the incident beam (LB) through the prism (5).
), again the wavelength λ1 (LBI) and the wavelength λ2 (LB
2) The separated laser beam is passed through a relay lens system (42
) to reach the same position on the ultrasound deflector (6) again.

The diffraction angle ε1 given by the acousto-optic deflector (6) to the incident beam is determined by the sound velocity in the medium being F2 and the ultrasonic frequency being F2.
2, considering the direction of the diffracted light so that the direction of diffraction is opposite to that of the acousto-optic modulator (3), the laser beam after exiting the acousto-optic deflector (6) is based on the direction of the incident beam (LD). Then, (9) From equation (10), (11)
It is expressed as the formula.

χ1=ε1+τ1...(11) If we rearrange the above equations (6) to (11), the wavelength λm(
The laser beam with i = 1.2...) is deflected by the angle given by equation (12) with respect to the direction of the incident laser beam (LB) and is emitted from the acousto-optic deflector (6).

The difference in the deflection angle for the wavelengths λ1 and λ2 is calculated from equation (12) as (
If we set Δχ=0 in equation 13), at least the wavelength λ0. After exiting the optical system of FIG. 1, the laser beam of λ2 overlaps on the optical axis.

Among the parameters in equation (13), F r , V l
.

F2. Since F2 is a parameter that is related to the material and structure of the acousto-optic element and also affects other performances of the element, it is difficult to select an arbitrary value from a design standpoint.

Further, nl+02 is a constant of the glass of the prism, and it is difficult to change it freely.

Therefore, the condition that Δχ=0 is obtained mainly by adjusting the value of α1ml1°ml. Solving for α, the conditions for wavelength compensation are as follows: It is difficult to control α, which is the apex angle of the prism, to an arbitrary value, so in design, the magnifications m, , m3 of the relay lens system must be adjusted so that the apex angle α is Set to a value that is reasonable for processing.

According to equation (14), the term (λ2 - λI)/(n, -02) can be expressed as ( Equation (14) no longer depends on wavelength, and Equation (14) holds true in a wide wavelength range other than λ1λ2. In other words, this shows that chromatic dispersion can be completely compensated. In addition, if the apex angle of the prism is not made as specified due to manufacturing errors and does not satisfy the condition of equation (14), the ultrasonic frequency of the sound optical deflector (6) should be adjusted at the final adjustment stage. F,
It is also possible to satisfy equation (14) by changing a small amount.

However, since the refractive index of glass is generally non-linear with respect to wavelength as expressed by a dispersion formula, λ0. Some residual dispersion occurs for laser beams with wavelengths other than λ2. The amount is determined by the wavelength λ1 according to equations (12) and (14).
(ii, 2...), the wavelength λ1. whose chromatic dispersion should be compensated for. λ2 is another wavelength λ1 (t=3.4...)
It may be determined by considering the value of equation (15) for .

Next, as mentioned above, the acousto-optic deflector (6) changes the ultrasonic frequency F2 to compensate for the surface tilt error of the rotating polygon mirror and deflects the laser beam. 14) Slight wavelength dispersion occurs without satisfying the formula. If the modulation width is ΔF2, then the acousto-optic deflector (
The ultrasonic frequency in 6) can be written as F2±ΔF2, which deviates from the condition of equation (14) and causes chromatic dispersion. From equation (13), the amount is determined by the wavelength λ1. When the wavelength dispersion is compensated for λ2, the amount of residual dispersion for the wavelength λ, (i=1.2...) is as follows, assuming that the laser beam is deflected by changing the acousto-optic deflector by F2±ΔF2. Equation (15), (
From equation 16), for example, Ar + laser (wavelength 457.9, 476
．． 5°488.496.5,514.5 nm-) consider the system shown in FIG. F as an acousto-optic modulator =
100MHz.

V , = 3000 m/sec, F2 = 40 MHz as an acousto-optic deflector, V 2 = 600 m/s
Set the ec conditions and set the wavelength λ, =488 nm,
Compensation according to equation (14) is performed for λ2=514.5 nm.

The refractive index of the prism is n H = 1.52224, n
When 2 = 1.52049, ml = -10, ml
= -0.5, which is 6 degrees and 11.6 degrees.

At this time, the amount of residual dispersion expressed by equation (17) is, for example, λ
, = prism refractive index n3=496.5nm
1.52165, ΔF2=±5 MHz, then Δz 3 =-5, IX 10-'rad 7. I
X 1O-5rad, which can be made very small.

Furthermore, as a special solution to equation (14), That is, In this case, it is mathematically possible to omit the prism (5).

Embodiment 2 Next, a case will be described in which a diffraction grating is used in place of the prism (5) in FIG. 1. Figure 3 shows the prism shown in Figure 1 (
The case where a diffraction grating (5c) is inserted at position 5) is shown.

Considering the grating pitch of the diffraction grating as d and the diffraction direction (8
) instead of δI press −λ+/d...(8゛)Otherwise (6
) to (11) are exactly the same as in the case of FIG. 1 described above.

Therefore, the deflection angle of the laser beam exiting the acousto-optic deflector (6) with respect to the incident laser beam (LB) is the condition that Δχ = 0 at the same time as equation (13), similar to equation (14), as follows. It becomes like this.

Since the equation (14°) does not depend on wavelength, it is possible to compensate for chromatic dispersion over a wide wavelength band. Therefore, the remaining chromatic dispersion equation (17) can also be written as follows.

The difference between the configuration in Figure 1 and the configuration in Figure 3 is whether a prism or a diffraction grating is used as the dispersive optical element. It is characterized by a high compensation effect.

Embodiment 3 FIG. 2 shows an application of the multi-wavelength light beam scanning optical system according to the present invention shown in the embodiment of FIG. 1 to a laser scanning drawing apparatus.

In the figure, (1) is a laser light source that simultaneously sends out multiple wavelength components on the same optical axis, (3) is an acousto-optic modulator that turns the laser beam on/off according to image data, and (8) is a laser light source that simultaneously sends out multiple wavelength components on the same optical axis. ) is a rotating polygon mirror that scans the laser beam, (9
) focuses the laser beam scanned by the rotating polygon mirror (8),
and an f-theta lens designed to scan at constant speed, (
10) is a stage on which a photoreceptor (not shown) is placed and moves at a constant speed in a direction substantially perpendicular to the scanning direction of the laser beam.

Here, the reflecting surface of the rotating polygon mirror (8) is not properly perpendicular to the rotating surface, and the laser beam reflected due to the surface tilt error between each surface deviates from the rotating plane angle, causing each reflecting surface of the rotating polygon mirror to The pitch of the scanning lines created on the drawing surface by the laser beam scanned by the laser beam cannot be maintained at equal intervals, resulting in so-called pitch unevenness.

(6) is an acousto-optic deflector installed to correct the pitch unevenness, and it adjusts the deflection according to the surface tilt error so that the reflected light from each surface of the rotating polygon mirror (8) is kept within the rotation plane. It is used to tilt the incident laser beam by a small angle.

(2) is a relay lens system that reduces the laser beam emitted from the light source to an appropriate diameter, and (7) is an f-theta lens (9) that expands the beam diameter in advance so that it is focused at a predetermined beam diameter. This is an expander optical system for

In this embodiment, in the multi-wavelength light beam scanning optical system as described above, a prism (5) is arranged between the acousto-optic modulator (3) and the acousto-optic deflector (6) to reduce wavelength dispersion occurring in the acousto-optic modulator (3) and the acousto-optic deflector (6). The relay lens system (41) on the incident side connects the acousto-optic modulator (3) and the prism (5).
) in a conjugate relationship, and the output side relay lens system (4
By setting the acousto-optic deflector (6) and the prism (5) in a conjugate relationship according to 2), one dispersive optical system compensates for wavelength dispersion as a whole in the synthesis system.

In addition, the relay lens systems (41) and (42) not only create a conjugate relationship between the acoustic optical element and the dispersive optical system, but also each have an appropriate magnification and compensate for wavelength dispersion, so the prism (5) The material, apex angle, etc. can be set within a reasonable range.

In the drawing apparatus of this embodiment, the angular spread of the laser beam due to the wavelength dispersion of the plurality of acousto-optic elements is kept very small without requiring a complicated configuration.
A high-quality beam spot can be formed on the stage (10), and energy efficiency can be extremely high.

If such a drawing device is used, it is possible to directly form a minute image on a resist material with low photosensitivity at high speed, and it is extremely useful, for example, in the manufacturing process of printed circuit boards, where finer patterns are becoming more and more advanced.

〔Effect of the invention〕

In this invention, in a synthesis system including a plurality of acousto-optic elements, the wavelength dispersion occurring in each acousto-optic element is compensated for by the dispersion optical system as a synthesis system, so the number of components of the dispersion optical member and relay lens system is reduced. This has the extremely excellent effect of not only making it possible to simplify the configuration of the multi-wavelength light beam optical system and making the optical system smaller, but also to further reduce energy loss in the optical system. ing.

If such a multi-wavelength light beam optical system is used in the scanning optical system of a writing device, extremely high energy density can be obtained with a small laser, so high quality images can be formed at high speed even on photoreceptors with low photosensitivity. It is something that can be done.

Furthermore, since the optical path length is shortened, the entire drawing device can be made more compact, reducing running costs and making maintenance management easier.

[BRIEF DESCRIPTION OF THE DRAWINGS] FIG. 1 is a schematic diagram of the configuration of a first embodiment of the present invention, FIG. 2 is a perspective view of the main parts of a drawing device using the embodiment shown in FIG. 1, and FIG. 3 is a schematic diagram of the configuration of the second embodiment of the present invention, FIG. 4 is a schematic diagram of the configuration of a conventionally known wavelength dispersion compensation method, and FIG. 5 is a schematic diagram of the configuration of a conventional multi-wavelength light beam optical system. FIG. 6 is a structural diagram of an acousto-optic element. 1... Laser device, 4, 2, 41.42° 42A, 4
2B... Relay lens system, 3... Acousto-optic modulator,
5, 5A, 5B...prism, 5C...diffraction grating,
6... Acousto-optic deflector, 7... Expander optical system, 8... Rotating polygon mirror, 9... f-theta lens, 10
...stage, 11...acousto-optic element, 12...
Ultrasonic medium, 13... Transducer, 14...
Ultrasonic wavefront, LB...laser beam, LBI...laser beam with wave 6λ1, LB2...laser beam with wavelength λ2. Agent Patent Attorney Masatoshi Sato

Claims (1)

[Claims] In a multi-wavelength light beam optical system that simultaneously sends out a light beam containing multiple wavelength components on the same optical axis via multiple acousto-optic elements, the chromatic dispersion that occurs in each of the multiple acousto-optic elements is compensated as a synthesis system. A multi-wavelength light beam optical system characterized by being equipped with a dispersion optical system.