JPS62103619A - Optical modulator - Google Patents

Abstract

PURPOSE:To separate effectively a first-order diffracted light as a used modulating light from zero-order diffracted light of the reflected light to prevent the degradation in extinction ratio by changing the optical path of the reflected light in an ultrasonic optical modulator to such direction by an optical path changing member that this optical path is deviated from a reflecting mirror provided in the emitting part of a laser light source. CONSTITUTION:A reflected light B reflected on an AOM 3 is made incident on a peripheral non-lens part 21b of a condenser lens 21. The reflected light B incident on the condenser lens 21 goes toward a laser light source 1 as a light approximately parallel with or coaxial to the reflected light B. Consequently, the reflected light B goes off a reflecting mirror 1s unless the length between the condenser lens 21 and the laser light source 1 is made extremely short. Thus, the reflected light of the reflected light B is not made incident on the AOM 3 again in coaxial relations to the reflected light and a first-order light A1 an the zero-order light of the reflected light made incident on the AOM 3 again do not overlap. That is, the degradation in the extinction ratio is prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical modulation device whose extinction ratio does not deteriorate.

[Background of the invention]

As a recording device using a laser beam, there is a device configured as shown in FIG. In this figure, a beam emitted from a laser light source 1 is focused by a condensing lens 2 and enters an ultrasonic optical modulator (hereinafter referred to as AOM), where the intensity is adjusted according to an information electrical signal. A recording medium such as a photosensitive material is modulated, further scanned (swinged) in the horizontal direction by a rotating polygon mirror 6 via a condensing lens 4 and a mirror 5, and placed on an imaging plane 8 via an fθ lens 7. The light is incident on the recording medium and recording is performed on the recording medium.

By the way, the above-mentioned AOM 3 is arranged at a position separated from the condenser lens 2 by approximately its focal length, and the beam emitted from the condenser lens 2 forms a beam waist near the AOM 3.

The modulation band Δf of AOM3 is Δ f = 0.54 x v/d
-(11V: Ultrasonic propagation speed d: Expressed by incident beam diameter. In order to increase the modulation band Δf to some extent,
It is necessary to narrow down the diameter d of the beam incident on the AOM 3, which is why the condenser lens 2 is placed on the incident side of the AOM 3.

FIG. 7 is a diagram showing the vicinity of this AOM3 in more detail. The AOM 3 is arranged at an angle of Bragg angle θ5 from the right angle to the optical axis of the laser beam A from the laser light source 1, and in order to obtain the 1st-order diffracted light AI (the modulated light finally obtained) separated from the 0th-order diffracted light A0, A pinhole plate 31 is arranged on the output side of the AOM 3.

The separation angle between the 1st-order diffracted light AI and the 0th-order diffracted light A0 is 208, which is twice the Bragg angle θ, and 2θ, = f・λ/V
(2) f: Carrier frequency of sound wave λ: Represented by wavelength of laser beam.

However, reflection occurs at the incident side end face of A OM 3,
This reflected light B enters the condensing lens 2 in the reverse direction and becomes light substantially parallel to the optical axis of the beam A. This means that the condenser lens 2 is AO
This is because it is placed at a position approximately equal to the focal length of the condenser lens 2 from M3.

Most of the reflected light B that has become parallel in this way is reflected by the output mirror 1a of the laser light source 1, becomes light parallel to the optical axis of the beam A, and enters the condenser lens 2 again. The optical axis is bent by this condensing lens 2 to an angle of 2
It enters AOM3 at θ8.

As a result, the first-order diffracted light A+ that is emitted from the laser light source 1 and modulated by the AOM3, and the reflected light B that is once reflected by the AOM3, further reflected by the laser reflector "a," and returned to the AOM3 and passes through it. It becomes impossible to separate the light from the O-order diffraction light component within, and as a result, the extinction ratio of the AOM 3 deteriorates.

For example, if the reflection at AOM3 is 1%, then
Even though OM3 is turned off, the light intensity of 1% is 1
It will be superimposed on the next diffracted light A, and the extinction ratio will be 100.
: It becomes 1.

Furthermore, even when the AOM is turned on, 1% of the light intensity is superimposed on the first-order diffracted light A, especially when high-precision modulation is required such as recording gradation images. In this case, accurate gradation cannot be expressed.

Note that although the above explanation was about reflection at the incident side end face of the AOM, a similar phenomenon naturally occurs at the output side end face as well. However, since it is after being modulated, there is no problem in terms of the amount of light.

[Purpose of the invention]

The present invention has been made in view of the above points, and its purpose is to prevent the first-order diffracted light and the O-th diffracted light of the light that re-enters the AOM from the above-mentioned reflection path from overlapping, so that the AO
An object of the present invention is to provide an optical modulation device that does not deteriorate the extinction ratio of M.

[Structure of the invention]

For this purpose, in the present invention, an optical path changing member is provided outside the optical path of the beam emitted from the laser light source and directed toward the ultrasonic optical modulator, and the optical path changing member changes the optical path of the reflected light from the ultrasonic optical modulator to the laser light source. The beam is configured to be changed in a direction away from the reflecting mirror provided at the emission part of the beam.

〔Example〕

Examples of the present invention will be described below. FIG. 1 shows one embodiment of the present invention, and the same components as those shown in FIGS. 6 and 7 described above are given the same reference numerals. In this embodiment, a lens 21 has a central lens part 21a and a light-transmissive peripheral non-lens 21b integrally formed around the central lens part 21a, as a condensing lens disposed between the laser light source 1 and the AOM 3. and center lens part 2.
1a achieves the beam waist of beam A at AOM3. The reflected light B reflected by the AOM 3 is made to enter the peripheral non-lens portion 21b of the condenser lens 21.

Therefore, as shown in FIG. 1, the reflected light B that has entered the condenser lens 21 is directed toward the laser light source 1 as substantially parallel or coaxial light.

Therefore, unless the distance between the condensing lens 21 and the laser light source 1 is extremely shortened, the reflected light B will be directed away from the reflecting mirror 1a.

Therefore, the re-reflected light of this reflected light B will not re-enter the AOM3 in a coaxial relationship with the reflected light B, and the -th order light A1 will not overlap with the 0th-order light of the reflected light that enters the AOM3 again. do not have. In other words, deterioration of the extinction ratio is prevented.

FIG. 2 shows a modification of the embodiment shown in FIG. In this embodiment, the condensing lens disposed between the laser light source 1 and the AOM 3 includes a central lens portion 22a for making the beam emitted from the laser beam #l into a beam waist at the AOM 3, and a central lens portion 22a for making the beam emitted from the laser beam #l into a beam waist. The condenser lens 22 is configured with a peripheral lens part 22b integrally formed around the periphery, and the refraction direction of the peripheral lens part 22b is the central lens part 22a.
The direction of refraction is opposite to the direction of refraction. Then, the reflected light B is made to enter this peripheral lens portion 22b.

Therefore, as shown in FIG.
Since the reflected light B incident on b is largely bent there, it can be directed in a direction largely away from the reflecting mirror 1a of the laser light source 1.

FIG. 3 shows another embodiment, in which a similar condensing lens 2 is arranged at the same position as in the conventional case, and a beam A that deviates from the beam A is placed between the condensing lens 2 and the laser light source 1. An optical path changing member 23 is arranged at the position, and the reflected light B, which is incident on the condenser lens 2 from the AOM 3 and becomes parallel to the beam A, is changed from the reflecting mirror 1a of the laser light source 1 by the optical path changing member 23. It is designed to point in a direction that is far off.

FIG. 4 shows a modification of the embodiment shown in FIG. 3, in which a convex lens 24 is placed in place of the optical path changing member 23 at the same position.

FIG. 5 also shows a modified example of the embodiment shown in FIG. By making the reflected light B incident on the laser light source I enter at a smaller angle than the angle shown in FIG. It was designed to point in the direction.

〔Effect of the invention〕

As described above, the present invention provides a condensing lens that is placed between the laser light source and the ultrasonic modulator outside the optical path of the beam emitted from the laser light source and directed toward the ultrasonic optical modulator, either integrally or separately. An optical path changing member is provided, and the optical path changing member is configured to change the optical path of the reflected light from the ultrasonic optical modulator in a direction away from the reflecting mirror provided at the emission part of the laser light source. It does not occur that the light is reflected by the reflector of the light source and passes through the same optical path again to enter the ultrasonic modulator, so the 1st-order diffracted light as the modulated light to be used is effectively separated from the 0th-order diffracted light of the reflected light. This makes it possible to prevent deterioration of the extinction ratio.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram of an optical system near A0M for explaining an embodiment of the present invention, FIG. 2 is an explanatory diagram showing a modification of the condenser lens portion of the embodiment of FIG. 1, and FIG. is an explanatory diagram showing the vicinity of the condenser lens showing another embodiment, FIGS. 4 and 5 are explanatory diagrams showing a modified example of the embodiment of FIG. 3, and FIG. 6 is an explanatory diagram showing a conventional general recording device. FIG. 7 is an explanatory diagram of an optical system near a conventional AOM. 1... Laser light source, 2... Condensing lens, 3... A
OM (ultrasonic optical modulator), 21.22... Condensing lens, 231, 25... Optical path changing member, 24... Convex lens. Agent Patent Attorney Tsuneaki Nagao Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 θB

Claims (2)

[Claims] (1) A laser light source, an ultrasonic light modulator for modulating the laser beam emitted from the laser light source, and a condenser lens for focusing the laser beam incident on the ultrasonic light modulator. In the light modulation device, an optical path changing member is provided outside the optical path of the beam emitted from the laser light source and directed toward the ultrasonic optical modulator, and the optical path changing member changes the optical path of the reflected light from the ultrasonic optical modulator to the laser light source. 1. A light modulation device, characterized in that the light modulation device is configured to change the direction away from a reflecting mirror provided at an output portion of the light modulation device. (2) The light modulation device according to claim 1, wherein the optical path changing member is provided integrally with or separately from the condenser lens.