JPS6011816A - Optical beam scanner - Google Patents

Abstract

PURPOSE:To obtain a titled scanner having a high accuracy by providing a dispersing prism for deflecting a laser light in the direction for off-setting a variation of a scanning position by a wavelength variation of the laser light, between a laser and a hologram. CONSTITUTION:When a wavelength of a laser light emitted from a semiconductor laser 6 is of a reference value lambda=780nm, the laser light which has been made incident on a prism 7 at an incident angle PSI1=40 deg. is refracted by the prism 7, made incident at an incident angle thetai=0 deg. to a prescribed position B on a hologram disk 2, diffracted at an angle of diffraction thetad=30 deg., and converged to a point C on a screen 3. In this case, if the wavelength becomes longer by DELTAlambda, an angle of deviation by the prism 7 becomes smaller by DELTAthetai, therefore, said light is made incident on a position G on the hologram disk 2, and its diffracted light is coverged to the point C on the screen 3. On the contrary, even in case when the wavelength becomes shorter by DELTAlambda, the light is converged to the same point C. In this way, by utilizing a variation of the angle of deviation of the prism 7 by a wavelength variation, a variation of the angle of diffraction thetad by a hologram is offset, and a shift of a scanning position on the screen 3 can be eliminated.

Description

DETAILED DESCRIPTION OF THE INVENTION (1) Technical Field of the Invention The present invention relates to a device for performing optical scanning by irradiating a hologram with laser light, and particularly to a light beam scanning device using a semiconductor laser.

(2) Background of the Technology Scanners that scan laser beams in various patterns are widely used in barcode leagues for POS (paint-of-sales) terminals, laser printers, and the like.

To scan the laser beam in various patterns,
It is necessary to deflect and focus the laser beam. For this purpose, a method using a polygon mirror and an fθ lens is usually used.

Recently, methods using holograms have also been developed. However, in any case, it is important to accurately focus the laser beam on a desired position.

Especially in linear scanning devices such as laser printers.

In order to hit dots with an accuracy of about ±25 .mu.m, it is necessary to extremely accurately locate the laser beam convergence position.

(3) Prior art and problems Linear scanning scanners used in laser printers and the like generate linear scanning patterns by combining three usually polygon mirrors and an fθ lens. In this case, since the scanning pattern is a simple straight line, the pattern can be generated using a relatively simple optical system. However, in order to accurately converge the light beam at the desired position, the polygon mirror and f-theta lens must be processed with high precision and aligned to 3 meters, making it expensive for a scanner with a simple structure. It had become.

Therefore, hologram line scanners that perform optical scanning using holograms have been attracting attention recently.

Holograms have the functions of both a deflector and a converging lens, and can be easily mass-produced using photographic technology.

A conventional example of a line scanner using a hologram having such advantages will be explained with reference to the drawings.

FIG. 1 is a schematic perspective view showing the configuration of a conventional hologram line scanner. Laser light 4 emitted from a laser 1 in the figure is diffracted by a hologram recorded on a hologram disk 2, and the diffracted light 5 is converged on a screen 3. Since the hologram disk 2 is rotating at a predetermined speed, the diffraction angle of the diffracted light 5 can be changed by appropriately designing the pitch of the hologram, and linear scanning on the screen 3 can be realized.

The hologram line scanner has the above-described configuration, but recently there has been a tendency to use a semiconductor laser as the laser 1 shown in FIG. This trend follows the demand for lower prices and more compact laser printers, and semiconductor lasers, which are smaller than gas lasers and can be directly modulated, will greatly contribute to realizing this demand. It's obvious.

However, when a semiconductor laser is used as a laser light source in a hologram line scanner, a major problem has arisen regarding the accuracy and stability of the light beam convergence position described above.

That is, since the oscillation wavelength of a semiconductor laser (hereinafter referred to as LD) easily changes due to factors such as temperature changes, the diffraction angle changes depending on the wavelength change, and as a result, the scanning position on the screen 3 changes. The problem is that the image is misaligned.

This problem will be specifically explained with reference to FIG. FIG. 2 is a schematic side view for explaining the shift in the scanning position on the screen due to the wavelength change of the LD. However, the same members as in FIG. 1 are given the same numbers.

The laser beam emitted from the LD 6 is incident at a position at a distance ro = 5Qrnm from the center point O of the hologram disk 2 with an incident angle θi = 0', and is deflected once at a diffraction angle θd = 30°, and the diffracted light is It converges on the screen 3 located at L=290mm.

However, the oscillation wavelength λ of the LD6 is different from the diffraction angle θ due to longitudinal mode hopping of 1 to 3 modes due to temperature changes.
d changes by Δθd. If this mode hopping occurs during scanning, the scanning line will shift by ΔY on the screen due to the change in the diffraction angle θd, as shown in FIG. In the conventional example shown in FIG.
The oscillation wavelength λ is approximately 0.3n by popping only one mode.
When the scanning position is increased by m, the deviation ΔY of the scanning position on the screen 3 reaches about several hundred μm. Scanning position deviation△
The maximum value of Y is ±25 μm, and if the scanning position is shifted by several hundred μm due to mode hopping, it will not be practical as a linear scanning device.

(4) Purpose of the invention The present invention solves the above-mentioned conventional drawbacks.

It is an object of the present invention to provide a highly accurate and stable optical beam scanning device that prevents misalignment of a scanning line even if a semiconductor laser causes wavelength fluctuation due to mode hopping.

(5) Structure of the Invention According to the present invention, in an optical scanning device that irradiates a hologram with a laser beam and uses the emitted light as a scanning beam, the scanning position is changed between the laser and the hologram due to a change in the wavelength of the laser beam. This is achieved by providing a light beam scanning device characterized in that it is provided with a dispersive prism that deflects the laser beam in a direction that cancels out the laser beam.

(6) Examples of the invention Hereinafter, two embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 4 is a schematic side view for explaining the principle of the optical beam vertical scanning device according to the present invention.

A light ray AB in the figure represents a laser beam of wavelength λ emitted from an LD (not shown).

This light ray AB enters position B on the hologram disk 2 at an incident angle θi=o° and is diffracted at a diffraction angle θd=30°,
It converges on point C on screen 3. However, position B is a distance ro from the center 0 of hologram disk 2, distance a represents the distance between A3, and distance mtt represents the vertical distance from point C on screen 3 to hologram disk 2. ing.

Now, the LD is causing mode hopping and the oscillation wavelength is λ
Suppose that it changes from λ+△λ. In this case, in the conventional technique shown in FIG. 2, the scanning position on the screen shifts, but in the light beam scanning device according to the present invention,
In order to avoid shifting the scanning position, that is, the position of point C, a method is adopted in which the incident angle θi is changed by Δθi to offset the influence of the change Δθd in the diffraction angle θd. In other words, the incident angle is set so that the position where the shift in the scanning position due to the change Δθd in the second diffraction angle θd is offset, that is, the incident position of the laser beam with wavelength λ+Δλ is shifted from B by Δγ. All you have to do is determine the amount of change Δθi.

Hereinafter, the relationship between the wavelength variation Δλ of the LD and the incident angle variation Δθi will be explained using FIG. 4. However, in this embodiment, the incident angle θi=Q°.

For convenience of explanation, it will be written as θi.

In the figure, the relationship between the incident angle θi and the diffraction angle θd of the laser beam with the wavelength λ, and the incident angle θ of the laser beam with the wavelength λ+△λ
The relationship between i+Δθi and the diffraction angle θd+Δθd is expressed by the following equations (1) and +21, respectively.

sinθd=-sinθi+λf (r o) ・1l
l sin (θd + △θd) = -sin (θi + △θk>
+(λ+△λ)f(ro+△r)・(2) However, f(r
o) is the spatial frequency at a distance ro from the center 0 of the hologram disk 2.

In this example, ra=5Qmm and f(ro)=662 [lines/mm].

Here, assuming that Δθd, Δθi, and Δr are sufficiently small, the following equation (3) is obtained from equations 5 (1) and (2).

△θd-cosθa=-Δθ1-cosθi-(λ+△
λ)・Δr−f′(ro) 1Δλ・f(ro) (3) However, f′(ro) expresses the differential of the spatial frequency with respect to ra.

On the other hand, in order for the 9-diffracted light to converge on point C even if the wavelength changes by Δλ, the following relationships are required from FIG.

In △ABD a (tan (θi+△θi)−, tanθi)=△
r...(4) 12 in △CBD (tan (θd+△θd)-tanθd)=△
r...(5) Again, if we perform approximate calculations assuming that Δθi and Δθd are sufficiently small, we obtain the following equations (61, (7) from Equations 1 (4) and (5).
).

...(6) By substituting equation (61, (7) into equation (3), △θi
The following equation (8) expressing the relationship between and Δλ is obtained.

= f(ro) ・△λ...(8) As already mentioned, the wavelength change △λ when the LD causes one mode popping is about 0.3 nm, and 2 usually 1~
Wavelength change △λ caused by 3-mode homping is ±l
It is about nm. At this level of Δλ, the relationship between Δλ and Δθi can be considered to be approximately linear according to equation (8). Therefore, a dispersive prism that satisfies the relationship between Δλ and Δθi is placed at the position of point A in FIG.

FIG. 5 is a side view of the prism used in this example. The angles of the prism 7 are α=30° and β=75°, and the point A in the prism 7 corresponds to the point A in FIG. 4,
Δθi in the prism 7 corresponds to the wavelength change compensation angle Δθi in FIG. Generally, the angle of deflection in a prism becomes smaller as the wavelength becomes longer, so the light ray E in FIG. 5 has a longer wavelength, and the light ray F has a shorter wavelength.

In this embodiment, the material of the prism 7 is arsenic trisulfide (AS).
2S3) △λ and △θi in equation (8) using glass
I created a prism that satisfies the relationship.

The refractive index of arsenic trisulfide glass is 2. when λ=800 nm.
5209. At λ-780nm, it changes to 2.52756. If the change in refractive index near λ=780 nm is linearly approximated and the deviation in the emission angle Δθi is calculated by assuming φ1−40°, it becomes 0.8 minutes per 1 nm of wavelength change.

Therefore, according to equation (8), △θi=o, s min/n m
The position where the prism 7 should be placed when , that is, the distance a
Subtracting the value of a=124mm.

From the above, one embodiment of the light beam scanning device according to the present invention using the prism 7 is shown in FIG.

In the figure, first, when the wavelength of the laser beam emitted from the LD 6 is the reference value λ-780 nm, the prism 7 has an incident angle ψ
The laser beam incident at +=40' is refracted by the prism 7 and reaches a predetermined position B on the hologram disk 2 at an incident angle θi=0.
Incident at °. Then, it is diffracted at a diffraction angle θd-30° and converged on a point C on the screen 3.

Here, if the wavelength becomes longer by △λ, the prism 7
The deflection angle is △ which corresponds to △λ than when the wavelength is λ.
The diffraction light becomes smaller by θi, and therefore enters the position G on the hologram disk 2, and the diffracted light converges on the point C on the screen 3.

Moreover, when the wavelength becomes shorter by Δλ, the deflection angle by the prism 7 becomes larger by Δθi corresponding to the wavelength change Δλ, and as a result, the beam enters the position H on the hologram disk 2, The diffracted light converges at the same point C as described above.

In this way, by utilizing the change in the angle of deflection of the prism 7 due to the change in wavelength, it is possible to cancel out the change in the diffraction angle θd in the hologram and eliminate the shift in the scanning position on the screen 3.

Further, since such a correction means is a prism, the loss of light amount can be kept extremely small.

Note that the prism 9 is not limited to being a triangular prism made of arsenic trisulfide glass. A desired change in deflection angle can also be obtained by using an amine prism and adjusting the distance a. Also, the applicable target is not limited to holograms, line scanners, but 2
Applicable to various hologram scanners.

(7) Effects of the Invention As explained in detail above, one aspect of the present invention is to provide a dispersive prism between the semiconductor laser and the hologram disk to eliminate changes in the scanning position due to wavelength fluctuations of the semiconductor laser. This has a great effect in that it is possible to perform highly accurate light beam scanning.

[Brief explanation of the drawing]

FIG. 1 is a schematic perspective view showing the configuration of a conventional hologram line scanner, and FIG. 2 is a schematic side view to explain the shift in scanning position due to changes in the wavelength of laser light in a conventional hologram scanner. , FIG. 3 is a front view of the screen showing the deviation of the scanning position on the screen shown in FIG. 2, FIG. 4 is a schematic side view of the optical scanning device for explaining the present invention in detail, and FIG. This figure is a side view of a dispersive prism used in this embodiment, and FIG. 6 is a schematic side view of one embodiment of a light beam scanning device according to the present invention. 2... Hologram disk, 3... Screen, 6... Semiconductor laser. 7... Prism Figure 1 Figure 3 Figure 4 Figure 5

Claims (2)

[Claims] (1) In an optical scanning device that irradiates a vologram with a laser beam and uses the emitted light as a scanning beam, dispersion is used to deflect the laser beam between the laser and the hologram in a direction that cancels out fluctuations in the scanning position due to changes in the wavelength of the laser beam. A light beam scanning device characterized by being provided with a polar prism. (2) The light beam scanning device according to claim 1, wherein the laser is a semiconductor laser.