JPH1026747A - Optical modulation device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To perform intensity modulations with one optical modulation piece on the same incident condition in a 1st modulation and also in a 2nd modulation by intensity-modulating a laser beam at a prescribed ratio while converging it on an optical modulation element and converging the intensity-modulated laser beam on the optical modulation element again. SOLUTION: The laser beam 2 of a linearly polarized light emitted from a laser beam source 1 is made incident on a polarizing beam splitter 3. The laser beam transmitting the plane of polarization of the splitter 3 becomes a circularly polarized light via a quater-wave plate 4 to be made incident on a condenser lens 5. The laser beam made incident on the lens 5 is converged by its condensing action to form a beam waist at the focal position of a side opposite to the beam source of the condenser lens 5. Consequently, the laser beam is made incident on the acousto-optical element 6 positioned at the focal position opposite to the beam source of the condenser lens 5 in a state in which a beam diameter is smallest. Therefore, an intensity modulation is performed according to a prescribed extinction ratio with a prescribed high diffraction efficiency in the acoustic optical element 6.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an optical modulation device,
In particular, the present invention relates to a light modulation device that performs intensity modulation of a laser beam.

[0002]

2. Description of the Related Art FIG. 3 is a diagram schematically showing a configuration of a conventional light modulation device using an acousto-optic device as a light modulation device. In the light modulation device of FIG. 3, a linearly polarized laser beam 2 emitted from a laser light source 1 becomes circularly polarized light via a polarization beam splitter 3 and a 板 wavelength plate 4 and enters a condenser lens 5.

The laser beam passing through the condenser lens 5 is condensed on an acousto-optic device 6 positioned at a focal position on the opposite side of the light source of the condenser lens 5. The laser beam intensity-modulated via the acousto-optic device 6 is reflected by the plane reflecting mirror 9 and then enters the acousto-optic device 6 again. The laser beam intensity-modulated again via the acousto-optic device 6 is separated out of the optical path via the condenser lens 5, the quarter-wave plate 4, and the polarization beam splitter 3.

When one acousto-optic element is used, its extinction ratio is usually about 1000: 1. That is, the intensity of the incident laser beam is reduced to about 1/1000 times and the laser beam is emitted. Therefore, as shown in FIG.
When performing temporal intensity modulation, the extinction ratio is about 1,000,000: 1, and a large extinction ratio can be obtained with a relatively simple configuration.

[0005]

By the way, in the above-mentioned conventional light modulation device, the laser beam passing through the condenser lens 5 emits the laser beam through the acousto-optic device 6 with the smallest beam diameter.
Incident on. However, since the laser beam that has passed through the acousto-optic element 6 diverges, the beam diameter at the time of being reflected by the plane reflecting mirror 9 and re-entering the acousto-optic element 6 is large.

As described above, in the conventional light modulation device, even if the diameter of the laser beam first entering the acousto-optic element 6 is reduced, the diameter of the laser beam entering the acousto-optic element 6 is increased again. . As a result, a high diffraction efficiency cannot be obtained when the light passes through the acousto-optic element 6 for the second time, and the required extinction ratio in design cannot be obtained.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and provides an optical modulation device capable of performing intensity modulation under substantially the same incident conditions for the first and second times in one optical modulation element. With the goal.

[0008]

According to the present invention, there is provided an optical modulator for modulating the intensity of a laser beam emitted from a laser light source. A light modulating element for modulating, a first condensing means for converging a laser beam from the laser light source on the light modulating element, and a laser beam intensity-modulated through the light modulating element. And a second light condensing means for condensing light again on the light modulation element.

According to a preferred aspect of the present invention, the second
The condensing means is a reflecting means having a concave reflecting surface facing the light modulation element, or a phase conjugate mirror for inverting the wavefront of the laser beam passing through the light modulation element to generate a phase conjugate wave. The second condensing means includes a relay lens for converting the laser beam through the light modulation element into a parallel light beam, and a plane for reflecting the parallel light beam through the relay lens along an incident direction. It is preferable to have reflection means.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, a laser beam from a laser light source is condensed on a light modulation element such as an acousto-optic element, and the laser beam once intensity-modulated through the light modulation element is also subjected to light modulation. It is configured to focus light again on the element. Therefore, the diameter of the laser beam that first enters the light modulation element becomes the smallest, and the diameter of the laser beam that again enters the light modulation element also becomes the smallest.
As a result, the first incidence condition and the second incidence condition with respect to the light modulation element become substantially the same, and a required extinction ratio in design can be secured with high accuracy.

An embodiment of the present invention will be described below with reference to the accompanying drawings. FIG. 1 is a diagram schematically showing a configuration of an optical modulation device according to a first embodiment of the present invention. In the light modulation device shown in FIG. 1, a linearly polarized laser beam 2 emitted from a laser light source 1 is incident on a polarization beam splitter 3.
The laser beam transmitted through the polarization plane of the polarization beam splitter 3 becomes circularly polarized light via the １ ／ wavelength plate 4 and enters the condenser lens 5.

The laser beam incident on the condenser lens 5 is condensed by its condensing action, and a beam waist is formed at a focal position of the condenser lens 5 opposite to the light source. Thus, the laser beam enters the acousto-optic element 6 positioned at the focal point on the opposite side of the light source of the condenser lens 5 with the smallest beam diameter. Therefore, in the acousto-optic device 6, intensity modulation is performed at a predetermined high diffraction efficiency and at a predetermined extinction ratio.

The laser beam transmitted through the acousto-optic element 6 enters the spherical reflecting mirror 7 while diverging. The spherical reflecting mirror 7 has a reflecting spherical surface having a radius of curvature R with the concave surface facing the acousto-optic element 6 side, and is arranged at a position separated by a predetermined distance R from the beam waist position. Therefore, the laser beam reflected by the spherical reflecting mirror 7 is condensed by the focusing action, and a beam waist is formed again at the focal position of the focusing lens 5 on the spherical reflecting mirror side.

In this manner, the laser beam is incident on the acousto-optic element 6 in the second time, similarly to the first time, with the beam diameter being the smallest. Therefore, in the acousto-optic device 6, intensity modulation is performed again at a predetermined high diffraction efficiency according to a predetermined extinction ratio. The laser beam transmitted through the acousto-optic element 6 and intensity-modulated again becomes a parallel light beam via the condenser lens 5 and enters the quarter-wave plate 4. The laser beam passing through the １ ／ wavelength plate 4 becomes linearly polarized light whose polarization plane is rotated by 90 ° with respect to the linearly polarized light emitted from the laser light source 1, and after being reflected by the polarization plane of the polarization beam splitter 3, Separated out of the optical path.

As described above, in the first embodiment, the laser beam once intensity-modulated through the acousto-optic device 6 can be focused again on the acousto-optic device 6 by the action of the spherical reflecting mirror 7. . In other words, the diameter of the laser beam incident on the acousto-optic element 6 becomes the smallest both in the first time and in the second time. As a result, the first incidence condition and the second incidence condition with respect to the acousto-optic element 6 become substantially the same, and the required extinction ratio in design can be secured with high accuracy.

FIG. 2 is a diagram schematically showing the configuration of an optical modulator according to a second embodiment of the present invention. The second embodiment has a configuration similar to that of the first embodiment. However, the only difference is that the spherical reflecting mirror 7 in the first embodiment is replaced by the relay lens 8 and the plane reflecting mirror 9 in the second embodiment. Therefore, in FIG. 2, elements having the same functions as those of the first embodiment are denoted by the same reference numerals as in FIG. Hereinafter, the second embodiment will be described focusing on the differences from the first embodiment.

In the light modulation device shown in FIG. 2, a linearly polarized laser beam 2 emitted from a laser light source 1 is incident on a condenser lens 5 via a polarization beam splitter 3 and a quarter-wave plate 4. The laser beam having passed through the condenser lens 5 is condensed on the acousto-optic device 6, and the acousto-optic device 6 performs the first intensity modulation. The laser beam transmitted through the acousto-optic element 6 enters the relay lens 8 while diverging,
It becomes a parallel light flux. The laser beam having passed through the relay lens 8 is reflected by the plane reflecting mirror 9 along the incident direction, and then enters the relay lens 8 again.

The laser beam passing through the relay lens 8 is
The light is focused again on the acousto-optic device 6. Therefore, in the acousto-optic device 6, the intensity modulation is performed for a second time at a predetermined high diffraction efficiency according to a predetermined extinction ratio. The laser beam transmitted through the acousto-optic device 6 is extracted out of the optical path via the condenser lens 5, the quarter-wave plate 4, and the polarization beam splitter 3.

As described above, also in the second embodiment, the combination of the relay lens 8 and the plane reflecting mirror 9 causes
The laser beam whose intensity has been modulated once via the acousto-optic element 6 can be focused again on the acousto-optic element 6.
As a result, the first incidence condition and the second incidence condition with respect to the acousto-optic element 6 become substantially the same, and the required extinction ratio in design can be secured with high accuracy.

Further, as a third embodiment, as shown in FIG. 4, an element using the Faraday effect, that is, a Faraday rotation element 40 is used instead of the quarter-wave plate 4 in the second embodiment of FIG. You can also. The Faraday rotation element 40 is set so as to rotate linearly polarized light by 45 degrees in one way. Therefore, the light beam incident on the acousto-optic device 6 remains linearly polarized, and an acousto-optic device using a crystal having a different efficiency depending on the polarization characteristics of the incident light can be used. The laser beam whose intensity has been modulated once via the acousto-optic element 6 is reflected by the plane reflecting mirror 9 and enters the acousto-optic element 6.

The laser beam intensity-modulated again by the acousto-optic element 6 passes through the Faraday rotation element 40 again, and
It becomes linearly polarized light whose polarization plane is rotated by 90 degrees with respect to the original incident light, and enters the polarization beam splitter 3. Thus, the laser beam incident on the polarizing beam splitter 3 is reflected out of the optical path as in the second embodiment. In addition,
In the third embodiment, the linear polarization plane of the laser beam after passing through the Faraday rotator 40 and the incident polarization plane of the acousto-optic element 6 need to be adjusted to the direction with the highest diffraction efficiency.

In the first embodiment, the spherical reflecting mirror 7 is used.
Instead, a phase conjugate mirror can be used. The phase conjugate mirror is a reflecting mirror that reverses the wavefront time of the incident light and generates a phase conjugate wave with the wavefront inverted (for example, OplusE,
June 1995, pp. 68-72).

Further, in the above-described first to third embodiments, examples are shown in which an acousto-optic element is used as a light modulation element. However, it is clear that the effects of the present invention can be obtained using a light modulation element such as a magneto-optical element or an electro-optical element. Further, in the first to third embodiments described above, examples are shown in which the transmitted light of the acousto-optic element is used. However, the present invention can also be applied to a light modulator using reflected light of an acousto-optic element.

[0024]

As described above, according to the present invention, the diameter of the laser beam incident on the light modulation element for the first time and the diameter of the laser beam incident on the light modulation element for the second time can be minimized. . That is, the first incidence condition and the second incidence condition with respect to the light modulation element are substantially the same, and the required extinction ratio in design can be secured with high accuracy.

[Brief description of the drawings]

FIG. 1 is a diagram schematically showing a configuration of a light modulation device according to a first embodiment of the present invention.

FIG. 2 is a diagram schematically illustrating a configuration of a light modulation device according to a second embodiment of the present invention.

FIG. 3 is a diagram schematically showing a configuration of a conventional light modulation device using an acousto-optic element as a light modulation element.

FIG. 4 is a diagram schematically showing a configuration of a light modulation device according to a third embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Laser light source 2 Laser beam 3 Polarization beam splitter 4 1/4 wavelength plate 5 Condensing lens 6 Acousto-optic element 7 Spherical reflecting mirror 8 Relay lens 9 Planar reflecting mirror

Claims (6)

[Claims] An optical modulator for modulating the intensity of a laser beam emitted from a laser light source, comprising: a light modulation element for modulating the intensity of an incident laser beam at a predetermined ratio; First condensing means for condensing light on the light modulation element, and second light condensing means for condensing the laser beam intensity-modulated via the light modulation element again on the light modulation element. An optical modulation device comprising: 2. The light modulation device according to claim 1, wherein the second light collection means is a reflection means having a concave reflection surface facing the light modulation element. 3. The apparatus according to claim 1, wherein the second condensing means is a phase conjugate mirror for generating a phase conjugate wave by inverting the wavefront of the laser beam passing through the light modulation element. An optical modulator according to any one of the preceding claims. 4. A relay lens for converting a laser beam passing through the light modulation element into a parallel light beam, and the second light focusing unit reflects the parallel light beam passing through the relay lens along an incident direction. The light modulating device according to claim 1, further comprising a plane reflecting means for the light. 5. The first condensing means includes: a polarizing beam splitter for transmitting a laser beam having a predetermined polarization direction among laser beams from the laser light source; and a laser beam passing through the polarizing beam splitter. 9. A light-emitting device comprising: a quarter-wave plate for converting into circularly polarized light; and a condensing lens for condensing a laser beam passing through the quarter-wave plate on the light modulation element. 1 to 4
The light modulation device according to any one of the above items. 6. The first condensing means includes: a polarizing beam splitter for transmitting a laser beam having a predetermined polarization direction among laser beams from the laser light source; and a laser beam splitter for passing the laser beam through the polarizing beam splitter. 4. A device according to claim 1, further comprising: a Faraday rotator for rotating a polarization plane by 45 degrees; and a condenser lens for converging a laser beam passing through the Faraday rotator onto the light modulation element. 5. The light modulation device according to any one of 4.