JPS6145234B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Technical Field of the Invention The present invention relates to a method for evaluating characteristics of a hologram in an optical scanning device.

(b) Technical Background Recently, methods that use holograms have been adopted for barcode reading in POS (point of sale) systems or for non-impact printing in printers.

Holograms used for the above purposes were generally applied to glass plates. For example, it is formed by irradiating a recording material such as an emulsion photosensitive layer with two coherent beams (A) and (B) as shown in FIG. In the figure, 1 is a recording material.

The interference fringes 2 formed in this case are caused by the wavefronts and wavelengths of the light beams (A) and (B).
The distribution within one plane of the recording material is determined.
Furthermore, the interference fringes 2 are formed to spread in the thickness direction of the recording material 1, and the direction of this spread coincides with the direction of the bisector of the angle formed by the light beams (A) and (B).

When reproducing the hologram formed as described above, i.e., performing optical scanning using the hologram, the incident direction of the laser beam for reproduction is optimal for the interference fringes of the hologram (Bragg angle condition).
When , the diffraction efficiency becomes maximum and the scanning light intensity becomes the strongest.

However, the distribution and the inclination in the thickness direction of the interference fringes 2 formed as shown in FIG. It doesn't necessarily mean that

This is because, as shown in FIG. 1, the recording material 1 swells or contracts during a process such as development or baking that is performed after hologram formation, causing a slight deviation in the interference fringes 2 from their initial positions. It's for a reason.

Therefore, it is important to be able to easily and accurately obtain the Bragg angle conditions and other characteristic evaluation data for each hologram during the production process or at the completed stage.

(C) Prior Art and Problems By the way, as mentioned above, the diffraction efficiency of a hologram is maximum under the Bragg angle condition, and gradually decreases as the angle deviates from the Bragg angle. This situation is shown in FIG. In the figure, the horizontal axis is the incident angle (β) of the reproduction laser beam, and the vertical axis is the diffraction efficiency (%).

In the following, the angle of incidence of the laser beam on the hologram surface when the Bragg angle condition is satisfied will be defined and used as the Bragg angle (α).

The shape of the curve shown in Figure 2 varies depending on the pitch (spatial frequency) of the interference fringes of the hologram, the thickness of the hologram material, etc., but the half-width of this curve is approximately 10 to 20 degrees in angle. .

Conventionally, the Bragg angle (α) has been determined as shown in FIG.

That is, first, one laser beam 4 is made incident on the hologram 3, and on the other hand, a photodetector 5 is installed in the optical path of the diffracted light of the laser beam 4 by the hologram 3. Next, the hologram 3 is rotated in the direction of the arrow, the diffraction efficiency distribution characteristics as shown in FIG. 2 are measured, and the Bragg angle (α) is determined.

The above conventional method has the following drawbacks.

(i) At least, the shape of the diffraction efficiency distribution characteristic curve near the Bragg angle (α) must be in a state that can be measured.

(ii) Since the rate of change in diffraction efficiency is small near the Bragg angle (α), the measurement error is large.

(iii) Measurements must be made by cut-and-try, and fluctuations in laser light output during the measurement period must be constantly monitored and corrections must be made accordingly.

(d) Purpose of the Invention The present invention provides a novel hologram characteristic evaluation method that eliminates the drawbacks of the above-mentioned conventional methods and enables accurate and easy measurement of the Bragg angle of a hologram, as well as measurement of diffraction efficiency at the same time. The purpose is to provide

(e) Structure of the Invention The present invention involves making a plurality of laser beams intersect with each other on the hologram surface, making them incident on the hologram surface, and diffracting the plurality of laser beams of the same specific order. A photodetector is provided near the optical path of the light, and a Bragg angle condition for the hologram is determined from the incident angle of each laser beam with respect to the hologram surface and the intensity ratio of the diffracted light detected by the photodetector. It is characterized by

(f) Embodiments of the invention Examples of the invention will be described below with reference to the drawings.

In the following drawings, the same parts as in the previous figures are given the same reference numerals.

FIG. 4 is a diagram showing the principle of the present invention, and shows a case in which two laser beams L1 and L2 are made incident on the hologram 3 so as to intersect at an intersection angle γ. In the same figure, in order to detect the diffracted lights (for example, +1st order diffracted lights) D1 and D2 of the laser beams L1 and L2 by the hologram 3, a photodetector S1 is installed near these optical paths.
and S2 are provided. In this case, the angle that the photodetectors S1 and S2 make with the point O is the intersection angle γ
be approximately equal to

Furthermore, the sensitivity of the photodetectors S1 and S2 is set so that they give the same output value when the same amount of light is incident thereon.

In the above configuration, when the diffracted lights D1 and D2 are detected while rotating the hologram 3 about the point O in the direction of the arrow, there is a condition that the output values of the photodetectors S1 and S2 become equal.

In this case, the angle of incidence from the direction of the bisector of the intersection angle of the laser beams L1 and L2 becomes the Bragg angle (α). Therefore, by measuring the rotation angle from the surface normal direction of the hologram 3 at this time, the Bragg angle (α) can be determined.

This will be explained with reference to FIG.

In FIG. 5, the vertical axis is the diffracted light intensity, and the horizontal axis is the rotation angle (β) of the hologram.

Curves C1 and C2 are diffracted light intensity distribution curves of the laser beams L1 and L2, respectively,
Angle β1 and β2 corresponding to each peak
corresponds to the incident angle at which the laser beams L1 and L2 meet the Bragg angle condition with respect to the hologram 3, and the difference between the angles β1 and β2 is equal to the intersection angle γ. Therefore, the angles β1 and β2 exist at positions separated by γ/2 on both sides of the angle (β0) where both curves intersect, so the intersection angle γ and the hologram rotation angle β0 are given by If so, the Bragg angle (α) can be found.

FIG. 6 is a diagram showing a method of forming two laser beams having an intersection angle γ from one laser.

The laser beam 4 emitted from the laser 6 is split by a half mirror 7, and a portion is transmitted as transmitted light 41.
The other part enters the mirror 8 as reflected light 42, where it is reflected so as to intersect with the transmitted light 41 at an angle γ.

FIG. 7 is a diagram showing another embodiment of the present invention, in which the laser beam 4 emitted from the laser 6 is
Three light beams 41, 421, 42 are generated by two half mirrors 71 and 72 and mirrors 81 and 82.
It is divided into 2. Here, the luminous fluxes 41, 421,
422 are on the same plane, and the light beams 41 and 42
1 and the light beams 41 and 422 are arranged to intersect at the same angle of intersection, for example γ/2.

In the above configuration, the light beams 41 and 4 are directed to the hologram provided with its rotation center at the point O.
21 and 422 are made incident, the hologram is rotated in the same manner as in FIG. 4, and the rotation angle of the hologram is measured when the intensities of the diffracted lights of the light beams 421 and 422 become equal.

As mentioned above, since the direction of the bisector of the angle formed by the light beams 421 and 422 at this time becomes the Bragg angle incident condition for the hologram, the photodetector provided in the optical path of the diffracted light of the light beam 41 This means that the maximum value of diffraction efficiency is detected.

Therefore, by correcting the optical loss caused by the half mirrors 71 and 72 with respect to the above measurement value, the diffraction efficiency at the Bragg angle (α) can be directly determined.

In the above embodiment, a case was explained in which two or three laser beams were used and the hologram was rotated so that the outputs of the two photodetectors were the same. A laser beam and a photodetector are provided corresponding to each of these, and by finding two photodetectors with the same output among the photodetectors, the laser beam corresponding to the two photodetectors is determined. The intersection angle can be known and, as a result, the Bragg angle (α) can be determined without rotating the hologram.

Further, if the sensitivities of the photodetectors are not set equally, processing for standardizing the output values may be performed.

It goes without saying that the present invention is applicable not only to holograms used in optical scanning devices but also to evaluation of characteristics of general holograms.

(g) Effects of the Invention According to the present invention, the Bragg angle condition of a hologram can be easily and accurately measured, and at the same time, the diffraction efficiency under the Bragg angle condition can be directly determined, and the characteristics of the hologram can be improved at the production process stage. This has the effect of making it possible to easily and accurately set conditions at the evaluation or implementation stage.

[Brief explanation of the drawing]

Figure 1 is a diagram for explaining the formation of a hologram using a two-beam laser beam, Figure 2 is a diagram showing the diffraction efficiency of a hologram, and Figure 3 is a diagram for explaining the conventional method for determining the Bragg angle of a hologram. 4 and 5 are diagrams illustrating the principle of the method according to the present invention for measuring the Bragg angle of a hologram, and FIG.
The figure shows a method for obtaining two laser beams from one laser, and FIG. 7 shows a method according to the present invention for simultaneously measuring the Bragg angle and diffraction efficiency of a hologram using three laser beams. . In the figure, 1 is a recording material, 2 is an interference fringe, 3 is a hologram, 4, L1 and L2 are laser beams, 5, S1 and S2 are photodetectors, 6 is a laser, 7 and 71
and 72 are half mirrors, 8, 81, and 82, 41, 421, and 422 are light beams, and D1 and D2 are diffracted lights.

Claims (1)

[Scope of Claims] 1. A plurality of laser beams are made to cross each other on the hologram surface and are incident on the hologram surface, and the optical path of the diffracted light of the same specific order of the plurality of laser beams is A photodetector is provided in the vicinity of each laser beam, and a Bragg angle condition for the hologram is determined from the incident angle of each laser beam with respect to the hologram surface and the intensity ratio of the diffracted light detected by the photodetector. A method for evaluating the characteristics of holograms. 2. A hologram characteristic evaluation method according to claim 1, characterized in that a plurality of laser beams are formed using the same laser. 3. A hologram characteristic evaluation method according to claim 1, characterized in that the sensitivities of a plurality of photodetectors are set to be the same. 4. In the method for evaluating characteristics of a hologram according to claim 1, the optical paths of at least three laser beams are arranged on the same plane with a predetermined angular difference, and at least three optical detection By providing a device near the optical path of each diffracted light, one of which is associated with one of the diffracted lights of the three laser beams,
A hologram characteristic evaluation method characterized by determining diffraction efficiency under Bragg angle conditions.