JPH11337867A - Method and device for light coherence reduction, and method and device for illumination - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a coherent light, which is uniform in a two-dimensional plane and has a coherence sufficiently reduced, by an optical device having a simple constitution. SOLUTION: Laser beam A and laser beam B are made incident on a prism 1 from different incidence faces and are propagated in different optical paths. With respect to each laser beam, it is branched into first transmission laser beam and first reflected laser beam by a partial reflection face 3, and the first reflected laser beam is reflected by a total reflection face 2 and is led to the partial reflection face 3 again and is branched into second transmission laser beam and second reflected laser beam, and first transmission laser beam and second transmission laser beam are emitted from the partial reflection face 3 with a certain optical path difference between them.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for reducing coherence of light, which reduce the coherence of a light beam, and an illumination method and an apparatus for using a light beam having reduced coherence as a light beam for illumination. Things.

[0002]

2. Description of the Related Art As one of application fields of laser light, there is a light source for an image display device or an illumination light source for a semiconductor exposure device. In these examples, the characteristics of laser or coherent light, such as high intensity, directivity, and monochromaticity, are used as advantages, but on the other hand, a problem specific to coherent light called speckle (speckle noise) occurs. , Which hinders its application.

Next, the problem of speckle will be described.

In general, as shown in FIG. 12, when a laser beam has a single-peak power spectrum, the degree of coherence g (τ) of this light source is given by | g (τ) by the Fourier transform G (τ) of the power spectrum. τ) | = | G (τ) | / | G (0) | and is shown as in FIG. 12 (in this way,
A laser that oscillates at a single wavelength or a single frequency is hereinafter referred to as a longitudinal single mode laser).

That is, _assuming that the full width at half maximum (spectrum width) of the monomodal power spectrum in FIG. 12 is ν _s , the full width at half maximum τ of the coherence degree is τ as shown in FIG.
_{Although s} depends on the function form of the power spectrum, _s is approximately given by τ _s ≒ 1 / ν _s .

Here, τ _s and L _c (L _c = _c τ _s : c
Is the speed of light in the medium through which the light propagates) are called the coherence time τ _s and the coherence length L _c , respectively. For example, splits the light beam emitted from the coherent light source into two, whereas the optical path length longer than the other of the optical path length by L going through the, at this time, the optical path length difference L Do coherence length L _c as much, or If it is smaller than these, these two split light beams have a large degree of coherence and strongly interfere with each other. On the other hand, if the optical path length difference L is equal to or larger than the coherence length L _C, it is regarded as non-coherent, and the interference between these two light beams is small.

When such a laser light source is used, for example, as an illumination light source of an image display device, contributions from an object surface, for example, each point and each area of a screen, are collected on an image plane, for example, a retina of an observer. To form an image.

At this time, since it is natural that the object surface (for example, a screen) has irregularities with a depth of about the wavelength or more, light beams having complicated phase relations overlap on the image surface, and these light beams are overlapped. If they are coherent with each other, they will interfere with each other, resulting in a complex light and dark pattern. This is speckle (or speckle noise), and in the case of a display device, it significantly causes image damage.

Therefore, in order to reduce the speckle, it is important that the coherent light beams emitted from the coherent light source be incoherent with each other.

[0010]

A first method for achieving this is to sufficiently widen the spectrum width of the light source.
That is, it is conceivable to make the coherent length Lc sufficiently small. However, this method is generally not preferred because it sacrifices the inherent property of laser light, ie, high monochromaticity.

The second method is a method in which coherent light emitted from a light source having a certain coherence length is split into a large number of light beams, and after giving an optical path length difference of about the coherence length or more, they are merged again. . In this case, non-coherence occurs between the respective light beams. Therefore, the greater the number of light beam branches, the more the degree of spatial coherence of the combined laser light can be reduced.

As an apparatus for realizing the second method, for example, an optical element (a bundle fiber or an optical fiber bundle) in which optical fibers are bundled is known. In this bundle fiber, a plurality of optical fibers are bundled,
An optical path length difference longer than the coherence length of the light beam incident on the length of each optical fiber is given. When both ends of the bundle fiber are aligned and laser light enters from one end, the emitted laser light from each optical fiber becomes non-coherent at the other end, and the spatial coherence of the emitted laser light is reduced as a whole. I do.
Therefore, when this is used as a light source of an illumination light beam, a high-quality image with reduced speckle noise can be obtained.

However, in order to reduce the speckle of the single mode laser light emitted from the laser light source which oscillates a single-peak power spectrum as shown in FIG. is there.

For example, if 31 optical fibers are bundled and the difference in length between them is 10 cm, the difference in length between the shortest optical fiber and the longest optical fiber is 3 m.
That is, in consideration of the fact that both ends of the optical fiber are aligned and bundled, the volume as a whole must be actually large.

Japanese Patent Application Laid-Open No. 4-250455 discloses an optical device having a structure in which three mirrors are arranged facing each other as means for generating an optical path length difference greater than the coherence length. This is because a light beam is made incident on one side of the mirror and the light circulates between the mirrors.One of the mirrors is translucent, and a part of the light beam circling next is radiated sequentially from the translucent mirror to the outside. You. Therefore, a certain optical path length difference is generated in the emitted light beam, and the spatial coherence is reduced as a whole.

However, in this case, since the light beam incident surface and the light beam reflecting surface are the same surface, the light use efficiency is low. For example, if the transmittance of the translucent mirror is increased in order to efficiently guide the incident light beam to the circulating optical path, the reflectance becomes low when viewed from the circulating light, and the light beam cannot be circulated many times.

Further, Japanese Patent Application No. 7-345566 discloses an optical element comprising a transparent prism as another means for generating an optical path length difference. Here, since the incident surface and the reflecting surface are separate, if a high transmittance is given to the incident surface and a high reflectance is given to the reflecting surface, high-order circulating light can be generated efficiently.

However, in this case, since the emitted light beams are arranged one-dimensionally on the emission surface while being attenuated, the two-dimensional in-plane distribution of the light intensity is non-uniform. That is, a uniform light beam cannot be obtained in a plane, and it is difficult to use the light source as a light source for illumination. Furthermore, although an example is shown in which the intensity of the emitted light beam is made one-dimensionally uniform, a special device must be provided for the coating of the emission surface for that purpose, and its production is troublesome.

Further, when a light beam oscillating at a single wavelength, particularly a single mode laser, is used as a light source for any of the above optical devices (or optical elements), the coherence length is generally sufficiently long. There is a limit to reducing the spatial coherence by using a method such as a bundle (bundle fiber), a facing mirror, and a prism.

For example, if the light source is a single mode helium (He) -neon (Ne) laser, its coherence length is generally 300 m, and it is not practical to produce such a long optical path length difference. Further, for example, even when a single-mode semiconductor laser is used as a light source, a typical coherence length is about 1 m. It is actually difficult to generate such an optical path length difference because the device becomes large.

As described above, in view of the prior art, there is an optical device having a simple configuration capable of sufficiently reducing the spatial coherence of a coherent laser light source and obtaining a nearly uniform light beam in a two-dimensional plane. I haven't.

An object of the present invention is to provide a method and an apparatus for reducing coherence of light, whereby substantially uniform coherent light can be obtained in a two-dimensional plane and coherent light with sufficiently reduced coherence can be obtained. Is to provide.

Still another object of the present invention is to provide a method of reducing coherence of light, which is substantially uniform in a two-dimensional plane and has sufficiently reduced coherence in accordance with the above-described method and apparatus for reducing coherence of light. An object of the present invention is to provide a lighting method and a device for use as a light beam for lighting.

[0024]

That is, according to the present invention, a plurality of coherent light beams incident at different positions are respectively split into a first transmitted light beam and a first reflected light beam at a partial reflection surface, and the partial light The first reflected light beam from the reflecting surface is reflected, guided again to the partial reflecting surface, branched into a second transmitted light beam and a second reflected light beam, and the first transmitted light beam and the second transmitted light are reflected. A light and a plurality of outgoing coherent light beams in which a constant optical path length difference is generated from the partial reflection surface in a state where a constant optical path length difference is generated therebetween. (Less than,
This is referred to as the coherence reduction method of the present invention. ).

According to the coherence reduction method of the present invention,
With respect to the plurality of coherent lights incident at different positions, the plurality of the first transmitted light beams and the second transmitted light beams in each of the coherent lights are combined in a state where a constant optical path length difference is generated therebetween. Since the light is emitted from the partial reflection surface as the outgoing coherent light of
A substantially coherently reduced light beam is obtained that is substantially uniform in a two-dimensional plane.

According to the present invention, as a device for implementing the coherence reducing method of the present invention with good reproducibility, a plurality of coherent lights incident at different positions are respectively transmitted by a first reflected light beam and a first reflected light beam at a partial reflection surface. And the first reflected light beam from the partial reflection surface is reflected, guided to the partial reflection surface again, and branched into a second transmitted light beam and a second reflected light beam. In the state where the first transmitted light beam and the second transmitted light beam have a constant optical path length difference between them, the partial reflection surface is used as a plurality of outgoing coherent lights in which the constant optical path length difference is generated. And a light coherence reduction device (hereinafter, referred to as a coherence reduction device of the present invention) respectively emitted from.

Further, according to the present invention, the light beams emitted from the light source are incident as a plurality of coherent lights at different positions, and these coherent lights are respectively transmitted to the first reflected light beam and the first reflected light beam at the partially reflecting surface. And the first reflected light beam from the partial reflection surface is reflected, guided to the partial reflection surface again, and branched into a second transmitted light beam and a second reflected light beam. Light beam and the second
The transmitted light beam is emitted from the partial reflection surface as a plurality of outgoing coherent lights in which the certain optical path length difference is generated in a state where a certain optical path length difference is generated therebetween, and the output coherent light is emitted. The present invention relates to an illumination method that uses light as a light beam for illumination (hereinafter, referred to as an illumination method of the present invention).

According to the illumination method of the present invention, for the plurality of coherent lights incident at different positions, the first transmitted light beam and the second transmitted light beam in each coherent light are fixed between them. In the state where the optical path length difference is generated, the light is emitted from the partial reflection surface as a plurality of outgoing coherent lights, and the outgoing coherent light is used as an illumination light beam. Thus, a light beam with sufficiently reduced coherence can be obtained, and the light beam with reduced speckle noise can be used as a light beam for illumination.

According to the present invention, as an apparatus for implementing the illumination method of the present invention with good reproducibility, a light source for emitting a light beam, and the light beam emitted from the light source are incident as a plurality of coherent lights at different positions. The plurality of coherent lights are respectively transmitted by the first transmitted light beam and the first transmitted light beam.
The first reflected light beam is re-directed to the partially reflected surface where the first reflected light beam is branched into the second transmitted light beam and the second reflected light beam. The first transmitted light beam and the second transmitted light beam, wherein a constant optical path length difference is generated between the first transmitted light beam and the second transmitted light beam. Are respectively emitted from the partial reflection surface as a plurality of emission coherent lights in which an optical path length difference has occurred,
An illumination device in which the emitted coherent light is used as an illumination light beam (hereinafter, referred to as an illumination device of the present invention).
Is provided.

[0030]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First, a coherence reduction method and apparatus according to the present invention will be described.

In the method and the apparatus for reducing coherence according to the present invention, the plurality of coherent lights are made incident on an optical element having the partial reflection surface and a total reflection surface for reflecting a light beam reflected from the partial reflection surface. It is preferable that the plurality of coherent lights propagating in the optical element through different optical paths are sequentially emitted from the partial reflection surface.

It is preferable that the partial reflection surface (or light emission surface) and the total reflection surface are planes common to the plurality of coherent lights. On the common partial reflection surface, the intensities of the plurality of coherent lights incident on the optical element are superimposed, and the two-dimensionally uniform outgoing coherent light is emitted. Also,
The optical paths of the plurality of coherent lights propagating in the optical element are preferably different from each other, but may be partially overlapped.

That is, in the coherence reduction method and apparatus according to the present invention, the plurality of coherent lights are made incident on the optical element, and a two-dimensionally uniform light flux is emitted from the partial reflection surface as the outgoing coherent light. be able to.

For example, when two coherent lights are incident on the optical element as the plurality of coherent lights,
The two coherent lights propagate in different optical paths, and the emission positions of the sequentially emitted coherent lights are two-dimensionally arranged on the partial reflection surface. Therefore, the output coherent light is emitted as a two-dimensionally uniform light flux from the partial reflection surface common to the two coherent lights.

It is desirable that the plurality of coherent lights which are repeatedly reflected in the optical element travel at a reflection angle so as not to take the same optical path.

The coherent light guided into the optical element travels while repeating reflection between the total reflection surface and the partial reflection surface. It is preferable that the reflected light path is different between the forward path and the return path, and the reflected light path in the plane direction of the total reflection surface and the partial reflection surface is different between the forward path and the return path.

That is, the plurality of coherent lights repeat reflections in the optical element and travel while attenuating. For example, two coherent lights propagating through different optical paths are made to complement each other's attenuated portions. By emitting the light from the partial reflection surface (that is, the light beam emission surface) in the form,
A two-dimensionally uniform light beam can be emitted. That is,
By arranging the rows of emitted light to face each other, a more uniform light intensity distribution can be obtained on the emission surface (that is, the partial reflection surface).

Further, the plurality of coherent lights incident on the optical element can be incident from an incident surface provided perpendicular to the partial reflection surface and the total reflection surface. In particular, it is preferable that the incident surface is provided with a non-reflection coating in order to suppress reflection of the incident coherent light and efficiently introduce the plurality of coherent lights into the optical element. However, the plurality of coherent lights entering the optical element may form a light incident portion on a part of the partial reflection surface or the total reflection surface, and may be incident from the light incidence portion.

It is desirable that the plurality of coherent lights propagating in the optical element be reflected and propagate also in the plane directions of the partial reflection surface and the total reflection surface. By reflecting light also in the plane directions of the total reflection surface and the partial reflection surface, a light beam having a more uniform two-dimensional spread can be emitted.

It is preferable that the optical element is a prism having the total reflection surface, the partial reflection surface (semi-transparent surface), and the incident surface. However, in the coherence reduction method and the apparatus of the present invention, the optical element is not limited to a prism, for example, a total reflection mirror as the total reflection surface, a light beam splitting film as the partial reflection surface, The optical element may include an anti-reflection film as the incident surface.

On the total reflection surface of the prism,
Desirably, a total reflection coating is applied, and similarly, it is preferable that the incident surface is provided with a non-reflection coating. The partial reflection surface is a light beam splitting film (beam splitter) that splits (divides) an incident light beam into a transmitted light beam and a reflected light beam at a predetermined ratio.
It may be. Further, the partially reflecting surface may be a polarizing beam splitter that splits a polarizing light beam.

In the prism having the above-described structure, the light beam incident surface is different from the reflection surface (mirror surface) for generating the predetermined optical path difference. And it is possible to efficiently obtain sequentially emitted coherent light.

Further, the constant optical path length difference may be equal to or more than の of the coherent distance of the coherent light. If the constant optical path length difference is equal to or more than の of the coherent distance (coherent length) of the coherent light, the spatial coherence of general coherent light can be sufficiently reduced. Further, in the optical element, the constant optical path length difference can be arbitrarily set by appropriately adjusting a distance between the total reflection surface and the partial reflection surface, and a reflection angle therebetween. is there.

Preferably, the coherent light is a light beam having a plurality of different oscillation wavelengths. That is, the coherent light is desirably a multi-mode laser light.

The degree of coherence of a plurality of light beams (particularly, multi-mode laser light) oscillating at different wavelengths (or frequencies) at fixed intervals has a periodically maximum value as a function of distance. If the constant optical path length difference does not match the maximum value of the coherence degree, the spatial coherence can be sufficiently reduced even if it is not always equal to or longer than the coherence length. That is, even if the optical path length difference is shorter than the coherence length, the coherence can be sufficiently reduced.

In this case, the coherent light is a laser light having a periodicity in the maximum value of the coherence degree,
It is preferable that the constant optical path length difference is an optical path length difference that does not coincide with the maximum value of the coherence degree of the laser light.

More specifically, the degree of coherence of the coherent light is expressed as a function of time, the full width at half maximum of the first maximum waveform appearing as the degree of coherence is τ _t , the first maximum waveform, and the first maximum waveform. The second appearing next to the waveform
When the center-to-center distance with the maximum waveform of is τ _d , it is desirable to set the constant optical path length difference L so as to satisfy the following equation 1. speed of light c _{[(n-1) τ d +} τ t / 2 ] ≦ L ≦ c [nτ _d -τ _t / 2] Equation 1 (where in the formula 1, c is in a medium light propagates , N represents an arbitrary natural number.)

[0048] Generally, as shown in FIG. 10, the spectrum modes of the multimode laser, when the resonator length of the laser and _{L 1, ν d = c 0} / 2nL 1 ( where, _{c 0} is the vacuum The speed of light, n indicates the index of refraction of the laser medium, and are arranged at a constant interval ν _d (however, in FIG. 10, ν _s indicates the full width at half maximum of the power spectrum of the multimode laser, ν _t indicates the full width at half maximum of the envelope of the power spectrum of the multimode laser).

Accordingly, the coherence degree g (τ) of such a multi-mode laser is schematically represented as shown in FIG. 11 (where, in FIG. 11, τ _t is a half value of the maximum waveform of the coherence degree). Full width). That is, as shown in FIG. 11, in the case of the multi-mode laser, the maximum value of the coherence degree appears periodically, and the full width at half maximum τ _t of each of the maximum value waveforms is approximately τ _t ≒ 1. / V _t .

Further, the full width at half maximum τ _t of the maximum value waveform is the coherence time τ _s (τ _s ≒ 1) when the longitudinal mode having the full width at half maximum ν _t of the envelope of the power spectrum stands alone.
/ V _s ). Also, in general, the coherence time τ _c is

(Equation 1) Therefore, it can be said that the full width at half maximum τ _t of the local maximum waveform is generally smaller than the coherence time τ _c .

Therefore, when a multi-mode laser is used as the light source, it is not necessary to make the optical path length difference large up to about the coherence time τ _c , and an optical path length difference that does not coincide with the maximum value of the coherence degree is generated. It will be good. That is, utilizing the periodicity of the degree of coherence, the optical path length difference L, c _{[(n-1) τ d +} τ t / 2 ] ≦ L ≦ c [nτ _d -τ _t / 2] ... to satisfy the formula 1 , The spatial coherence can be effectively reduced.

Next, the lighting method and the lighting device of the present invention will be described.

In the illumination method and the illumination device according to the present invention, the plurality of coherent lights are incident on an optical element having the partial reflection surface and a total reflection surface for reflecting the first reflected light beam, and It is preferable that the plurality of coherent lights propagating in different optical paths in the inside are sequentially emitted from the partial reflection surface as the illumination light beam.

It is preferable that the partial reflection surface (or light emission surface) and the total reflection surface are planes common to the plurality of coherent lights. On the common partial reflection surface, the intensities of a plurality of coherent lights incident on the optical element can be superimposed, and the high-intensity illumination light beam can be obtained as a two-dimensionally uniform light flux. In particular, if the coherent light is laser light,
Illumination light that fully utilizes the characteristics of laser light, such as high intensity, directivity, and monochromaticity, can be obtained.

That is, in the present invention, the plurality of coherent lights are made incident on the optical element, and a two-dimensionally uniform light beam is emitted from the partial reflection surface as the outgoing coherent light, and this light is used as the illumination light. It can be used as a beam.

For example, when, for example, two coherent lights are incident on the optical element as the plurality of coherent lights, coherence is reduced if the two coherent lights are emitted as the outgoing coherent light from the partial reflection surface common to the two coherent lights. In addition, it can be applied to various illumination light applications as a two-dimensionally uniform light flux.

Further, it is desirable that the plurality of coherent lights, which are repeatedly reflected in the optical element, travel at a reflection angle that does not take the same optical path.

The coherent light guided into the optical element travels while repeating reflection between the total reflection surface and the partial reflection surface. It is preferable that the reflected light path is different between the forward path and the return path, and the reflected light path in the plane direction of the total reflection surface and the partial reflection surface is different between the forward path and the return path.

That is, the plurality of coherent lights repeat reflection in the optical element and travel while being attenuated. For example, two coherent lights propagating in different optical paths are made to complement each other's attenuated portions. By emitting the light from the partial reflection surface (that is, the light beam emission surface) in the form,
A two-dimensionally uniform illumination light beam can be emitted. That is, by arranging the outgoing light trains to face each other, a more uniform light intensity distribution can be obtained on the outgoing surface (ie, the partial reflection surface).

Further, it is desirable that the plurality of coherent lights incident on the optical element be incident from an incident surface provided perpendicular to the partial reflection surface and the total reflection surface. In particular, it is preferable that the incident surface is provided with a non-reflection coating in order to suppress reflection of the incident coherent light and efficiently introduce the plurality of coherent lights into the optical element. However, the plurality of coherent lights entering the optical element may form a light incident portion on a part of the partial reflection surface or the total reflection surface, and may be incident from the light incidence portion.

It is preferable that the plurality of coherent lights propagating in the optical element be reflected and propagate also in the plane directions of the partial reflection surface and the total reflection surface. By reflecting light in the plane directions of the total reflection surface and the partial reflection surface, a light beam having a two-dimensionally more uniform spread can be emitted as an illumination light beam.

It is preferable that the optical element is a prism having the total reflection surface, the partial reflection surface (semi-transparent surface), and the incident surface. However, in the coherence reduction method and the apparatus of the present invention, the optical element is not limited to a prism, for example, a total reflection mirror as the total reflection surface, a light beam splitting film as the partial reflection surface, An optical element consisting of

On the total reflection surface of the prism,
Desirably, a total reflection coating is applied, and similarly, it is preferable that the incident surface is provided with a non-reflection coating. The partial reflection surface is a light beam splitting film (beam splitter) that splits (divides) an incident light beam into a transmitted light beam and a reflected light beam at a predetermined ratio.
It may be. Further, the partially reflecting surface may be a polarizing beam splitter that splits a polarizing light beam.

In the prism having the above-described configuration, the light beam incident surface is different from the reflection surface (mirror surface) for generating the predetermined optical path difference. And it is possible to efficiently obtain sequentially emitted coherent light.

The constant optical path length difference may be equal to or more than １ ／ of the coherent distance of the coherent light. If the constant optical path length difference is equal to or more than の of the coherent distance (coherent length) of the coherent light, the spatial coherence of general coherent light can be sufficiently reduced. Further, in the optical element, the constant optical path length difference can be arbitrarily set by appropriately adjusting a distance between the total reflection surface and the partial reflection surface and a reflection angle therebetween.

It is preferable that the coherent light is a light beam having a plurality of different oscillation wavelengths. That is, the coherent light is desirably a multi-mode laser light.

The degree of coherence of a light beam oscillating at a plurality of different wavelengths (or frequencies) has a local maximum periodically as a function of distance, and the constant optical path length difference indicates the maximum value of the coherence degree. If they do not coincide with each other, the spatial coherence can be sufficiently reduced even if the length is not longer than the coherence length, and the speckle noise can be sufficiently reduced. That is, even if the optical path length difference is shorter than the coherence length, it is possible to sufficiently reduce coherence and further reduce speckle noise.

In this case, the coherent light is a laser light having a periodicity in the maximum value of the degree of coherence,
It is preferable that the constant optical path length difference is an optical path length difference such that the maximum values of the coherence degrees of the laser light do not coincide with each other.

More specifically, the degree of coherence of the coherent light is represented as a function of time, the full width at half maximum of the first maximum waveform appearing as the degree of coherence is τ _t , the first maximum waveform, and the first maximum waveform. The second appearing next to the waveform
When the center-to-center distance with the local maximum waveform is τ _d , the constant optical path length difference L is set to satisfy the following equation 1 for the same reason as described in the coherent reduction method of the present invention described above. It is desirable. speed of light c _{[(n-1) τ d +} τ t / 2 ] ≦ L ≦ c [nτ _d -τ _t / 2] Equation 1 (where in the formula 1, c is in a medium light propagates , N represents an arbitrary natural number.)

The illumination light beam can be used as illumination light for various optical devices such as a display device, a measuring device, a microscope, and an exposure device.

Next, a preferred embodiment of the present invention will be described with reference to the drawings.

FIG. 1 is a schematic perspective view schematically showing a prism according to the present embodiment. FIG. 2 is a view of the prism of FIG. 1 viewed from another angle, and FIG.
(B) is a side view from the + x direction and the + y direction, respectively. FIG. 4 is a view of the prism viewed from above, and FIG. 5 is a schematic diagram schematically showing a propagation path of one light beam.

First, the first laser light A emitted from the laser light source 8a is collimated by the lens 9a, and then enters the prism 1 as the optical element from one end of the side surface (non-reflective surface) 4. It is desirable that a non-reflection coating is applied to at least a part of the side surface 4 where the laser beam A is incident.

The incident laser light is thereafter reflected by the total reflection surface 6a provided with the total reflection coating. FIG. 3 (A)
And (B), the total reflection surface 6a is inclined from the x-axis direction and the y-axis direction with respect to the total reflection surface (bottom surface) 2 at an angle γ and an angle (θ / 2 + π / 4), respectively. Therefore, after reflection, the light enters the partial reflection surface (upper surface) 3 of the prism 1 at angles of γ and θ from its perpendicular. The upper surface 3 of the prism 1 is coated so as to be partially transmissive.

As shown in FIG. 5, for example, the partial reflection surface 3
Assuming that 10% transmission and 90% reflection are set for the light beam from the inside of the laser beam and the light amount of the laser beam A 'is 100%, the first transmitted light beam 12a
10% of the laser light is emitted, and 90% of the laser light is reflected as the first reflected light beam 13a. Then, the first reflected light beam 13a having a light amount of 90% is reflected by the total reflection surface 2 and guided to the partial reflection surface 3, where the laser light of 9% light amount is transmitted through the second reflection surface 3 in the second reflection surface 3. The laser beam is emitted as a light beam 12b, and a laser beam with an amount of 81% is reflected as a second reflected light beam 13b. Then, the laser light repeats such an operation in the prism 1 and travels in the -y direction and the -x direction in the figure while attenuating at a constant rate.

Thereafter, the laser beam travels while repeating reflection between the upper surface 3 and the bottom surface 2 and reaches the total reflection surface 7a. The total reflection surface 7a is a surface that totally reflects the light beam, and is inclined at an angle θ with respect to the total reflection surface (bottom surface) 2. The light beam traveling while repeating reflection is totally reflected by the total reflection surface 7a, and similarly, while being reflected in a zigzag manner between the partial reflection surface (top surface) 3 and the total reflection surface (bottom surface) 2. , In the + y direction and + x direction in the figure.
At the upper surface 3, a part, for example, 10%, of the light passes through each time of reflection, so that a gradually attenuated light beam is emitted from the upper surface 3 regularly.

Similarly, the second laser beam B incident on the prism 1 via the laser light source 8b and the lens 9b
But the non-reflective surface 5 so as to face the laser beam A.
Incident from one end of The non-reflective surface 5 is similar to the non-reflective surface 4,
It is desirable that at least the light beam incident portion is coated with non-reflection. Laser light B incident on prism 1
Behaves in the same manner as the laser beam A described above, travels while repeating reflection between the common total reflection surface (bottom surface) 2 and the common partial reflection surface (top surface) 3, and furthermore, the total reflection surface 6b
At a predetermined angle, and travels on an optical path different from the outward path.

The laser beam B travels so as to face the laser beam A, as schematically shown in FIG. Therefore, the laser light emitted from the partial reflection surface (upper surface) 3 is the sum of two attenuated light beams, and the power distribution on the surface is more uniform than that of a single incident laser light. I do. Further, since these emitted laser lights are distributed in a two-dimensional plane, they can be easily used as an illumination light source. In addition, it is desirable that the transmitted light emitting portion 12 of the transmitted light beam emitted from the partial reflection surface 3 be located at different positions for the laser light A and the laser light B, but may be partially overlapped.

Further, the laser light A and the laser light B traveling while zigzag-reflecting are shared by the common upper surface 3 and lower surface 2.
When the light source is a single mode laser, the optical distance passed during one reciprocation between the light source and the light source is approximately equal to or longer than the coherence length of the laser light of the light source. It is desirable that it is made to be long.

Therefore, the transmitted light beams by the respective reciprocating laser beams on the upper surface 3 have low coherence with each other, so that the spatial coherence on the emission surface (upper surface) 3 can be reduced as the entire emitted light beam. For example, when this is used as an illumination light source, generation of speckle noise can be suppressed.

In the present embodiment, since the two incident laser beams A and B are laser beams emitted from independent light sources, they are non-interfering with each other, and The spatial coherence on the emission surface 3 can be further reduced as compared with the case where the incident laser light is used.

The above-described example is an example in which a single mode laser is used as a light source. Next, an example in which a multi-mode laser is used will be described.

A design example of a prism in the case where a semiconductor laser is used as a light source of an incident light beam and a high-frequency signal is superimposed on its input current will be described below.

In general, it is known that a semiconductor laser oscillating at a single wavelength oscillates at a plurality of frequencies as shown in FIG. 8 when a high-frequency signal is superimposed on its input current. I have. Here, the coherence degree as shown in FIG. 9 can be derived from the power spectrum shown in FIG. 8 by performing the Fourier transform similarly to the case of the above-mentioned single mode laser. FIG. 9 shows the degree of coherence multiplied by the light flux c and expressed as a function of distance.

As shown in FIG. 9, in this example, when the optical path length difference is about 0.5 mm, the coherence between the light beams is reduced,
It is almost zero until the optical path length difference becomes about 4.5 mm again. However, when the optical path length difference becomes 5.0 mm, the degree of coherence reaches the maximum value again, and thereafter, it becomes 10 mm, 15 mm
And so, ..., periodically maxima appear at intervals L _d (however, in FIG. 9, L _s is the full width at half maximum of the degree of coherence envelope).

Therefore, assuming that the optical path difference for one round trip in the prism is 4 mm, the light beam emitted from the upper surface 3 and the round trip in the prism 1 from one round trip to four round trips, that is,
4.0mm, 8.0mm, 12.0mm, 16.0mm
Is almost zero. Note that the coherence between the light beams having an optical path length difference of 50.0 mm, that is, 20.0 mm, corresponds to a local maximum. However, as shown in the example shown in FIG. After a small optical path length difference, the degree of coherence is sufficiently small and often does not matter.

This means that the full width at half maximum of each mode of the power spectrum of the laser subjected to the high frequency superposition is larger than the full width at half maximum of the power spectrum when the single frequency oscillates without superimposing the high frequency. It is due to having. The coherence length of a single mode oscillation semiconductor laser without high frequency superposition is generally 3 m.
Considering the degree, the volume of the optical device (prism) can be reduced by applying the high frequency superposition, which is extremely advantageous when used as an illumination light source.

As described above, according to the optical element of the present embodiment, the following excellent effects can be obtained. (1) Since a plurality of incident surfaces are provided, and the output light trains formed by the respective incident coherent lights are arranged to face each other so as to cancel the attenuation, a more uniform two-dimensional intensity distribution is obtained on the output surface. Can be (2) Since the incident surface and the opposing reflecting surface for generating the optical path length difference are different, optimal reflectance can be given to each of them, and the sequentially emitted light in the prism can be generated more efficiently. . (3) Since the difference in the optical path length of the sequentially emitted light within the prism is set to be equal to or longer than the coherent length of the incident laser light, the spatial coherence on the emission surface can be reduced. (4) If the laser light source is a multi-mode laser,
The spatial coherence can be more effectively reduced with the optical path difference shorter than the coherence length by optimizing the optical path difference of the sequentially emitted light from inside the prism using the periodic coherence degree.

Although the present invention has been described with reference to the preferred embodiments, the present invention is not limited to the above-described embodiments.

For example, as shown in FIG.
A plurality of laser beams are vertically incident on the incident surface 15 and the incident surface 16, respectively, and the optical paths of the laser beam from the incident surface 15 and the laser beam from the incident surface 16 are alternately set. Returning to the direction of the laser beam,
If it is configured to proceed while repeating reflection between the upper surface 3 and the lower surface, a larger number of laser beams can be used,
A light beam with a higher intensity can be emitted. In addition, a configuration as shown in FIG. 7 is also possible. In the case of (A), as shown in (A-2), the light is incident opposite to the step. As shown in (A-3), such a prism can be manufactured from two types and three parts. These are bonded together with index matching. (B) is a similar example, but in this case, one type and two parts can be bonded together.

There are various options for the light source other than the superposition of the high-frequency signal on the injection current of the semiconductor laser as described above. For example, a so-called longitudinal multi-mode laser which originally oscillates at a plurality of frequencies may be used, and a so-called self-excited oscillation semiconductor laser also behaves as a longitudinal multi-mode laser, so that the same effect can be obtained. In the case where laser light is used as an illumination light source, generally, there is almost no problem even if the vertical mode is a multi-mode.

The optical element according to the present invention is not limited to the above-described prism. For example, a partial reflection mirror (half mirror) having the same operation as the partial reflection surface 3, and the same as the total reflection surface 2 are used. Total reflection mirror with action,
It may be an optical element constituted by a total reflection mirror or the like having the same action as the total reflection surfaces 6a and 6b.

Further, the optical element according to the embodiment can be used as a light source for illumination of a display device (display), for example, a measuring device, a microscope (such as a fluorescence microscope), and an exposure device (particularly, a semiconductor device). Various applications can be considered as a light source for illumination such as an exposure apparatus.

[0094]

According to the coherence reducing method of the present invention, a plurality of coherent lights incident at different positions are respectively split into a first transmitted light beam and a first reflected light beam at a partial reflection surface, and then, The first reflected light beam from the partial reflection surface is reflected, guided again to the partial reflection surface, and split into a second transmitted light beam and a second reflected light beam. With the second transmitted light beam, with a certain optical path length difference between them,
Since the light exits from the partial reflection surface as a plurality of emission coherent lights in which the constant optical path length difference has occurred,
A light beam that is substantially uniform in a two-dimensional plane and has sufficiently reduced coherence can be obtained.

According to the coherence reducing device of the present invention,
Each of the plurality of coherent lights incident at different positions is split into a first transmitted light beam and a first reflected light beam at a partial reflection surface, and then the first reflected light beam from the partial reflection surface is reflected. Is guided again to the partial reflection surface, and is split into a second transmitted light beam and a second reflected light beam. Further, the first transmitted light beam and the second transmitted light beam are fixed at a certain distance therebetween. In the state in which the optical path length difference is generated, the plurality of outgoing coherent lights in which the constant optical path length difference is generated are respectively emitted from the partial reflection surface, so that the coherence reduction method of the present invention can be performed with good reproducibility.

According to the illumination method of the present invention, the light beams emitted from the light source enter at different positions as a plurality of coherent lights, and these coherent lights are respectively transmitted to the first transmitted light beam and the Branching into one reflected light beam, and then reflecting the first reflected light beam from the partially reflecting surface, guiding the reflected light beam to the partially reflecting surface again, and branching into a second transmitted light beam and a second reflected light beam. Further, a plurality of outgoing coherent lights in which the constant optical path length difference is generated in a state where the first transmitted light beam and the second transmitted light beam have a constant optical path length difference therebetween. The light is emitted from the partial reflection surfaces, and the emitted coherent light is used as a light beam for illumination. Therefore, a light beam that is substantially uniform in a two-dimensional plane and has sufficiently reduced coherence is obtained. Is This light beam can be effectively utilized as a reduced illuminating light beam of the speckle noise.

According to the illumination device of the present invention, a light source for emitting a light beam and the light beam emitted from the light source are incident at different positions as a plurality of coherent lights. A partially reflecting surface branched into a first transmitted light beam and a first reflected light beam;
An optical element having a reflecting surface for re-introducing the first reflected light beam to the partially reflecting surface where the first reflected light beam is split into a second transmitted light beam and a second reflected light beam. A plurality of outgoing coherent light beams in which the constant optical path length difference is generated in a state where the first transmitted light beam and the second transmitted light beam have a constant optical path length difference therebetween. And the emitted coherent light is used as an illumination light beam, so that the illumination method of the present invention can be implemented with good reproducibility.

[Brief description of the drawings]

FIG. 1 is a schematic perspective view of an optical element according to an embodiment of the present invention.

FIG. 2 is another schematic perspective view of the optical element.

FIG. 3 is a plan view (A) of the optical element viewed from the side,
FIG. 4B is another plan view of the optical element as viewed from the side.

FIG. 4 is a plan view of the same optical element as viewed from above.

FIG. 5 is a schematic side view for explaining the operation of a light beam in the optical element.

FIG. 6 is a schematic plan view of another optical element according to the present invention.

FIG. 7 is a schematic perspective view or a front view of still another optical element according to the present invention.

FIG. 8 is a schematic diagram showing a power spectrum of a multi-mode laser for explaining the operation of the present invention.

FIG. 9 is a schematic diagram showing a coherence degree of the multi-mode laser.

FIG. 10 is a schematic diagram for explaining a power spectrum of the multimode laser.

FIG. 11 is a schematic diagram for explaining a coherence degree of the multi-mode laser.

FIG. 12 is a schematic diagram showing a power spectrum of a single mode laser.

FIG. 13 is a schematic diagram showing the degree of coherence of a single mode laser.

[Description of Signs] 1 ... optical element (prism), 2 ... total reflection surface, 3 ... partial reflection surface (beam split surface), 4, 5 ... non-reflection surface (light incident surface), 6a, 6b, 7a, 7b ... total reflection surface (total reflection part), 8a, 8b ... laser light source, 9a, 9b ... lens, 10, 11, ... side surface, 12 ... transmitted light emission part, 12a,
12b: transmitted light beam, 13a, 13b: reflected light beam, 14: prism, 15, 16: light incident surface

Claims (34)

[Claims] 1. A plurality of coherent lights incident at different positions are respectively split into a first transmitted light beam and a first reflected light beam at a partial reflection surface, and the first reflected light beam from the partial reflection surface Is reflected,
The light is again guided to the partial reflection surface and split into a second transmitted light beam and a second reflected light beam. The first transmitted light beam and the second transmitted light beam are separated by a certain optical path length difference therebetween. A method of reducing light coherence, wherein the light is emitted from the partial reflection surface as a plurality of outgoing coherent lights in which the constant optical path length difference is caused in the generated state. 2. The plurality of coherent lights are incident on an optical element having the partial reflection surface and a total reflection surface that reflects a light beam reflected from the partial reflection surface, and the optical elements pass through different optical paths in the optical element. The light coherence reduction method according to claim 1, wherein the plurality of coherent lights propagating are sequentially emitted from the partial reflection surface. 3. The method of reducing coherence of light according to claim 2, wherein the plurality of coherent lights are incident on the optical element, and a two-dimensionally uniform light flux is emitted from the partial reflection surface as the outgoing coherent light. . 4. The method for reducing coherence of light according to claim 2, wherein the plurality of coherent lights propagating in the optical element are reflected and propagated also in the plane directions of the partial reflection surface and the total reflection surface. 5. The optical element is a prism having the total reflection surface, the partial reflection surface, and the entrance surface.
The method for reducing light coherence according to claim 2. 6. The method for reducing coherence of light according to claim 1, wherein the coherent light is a light beam having a plurality of oscillation wavelengths different from each other. 7. The coherent light is a laser light having a periodicity in a maximum value of the degree of coherence, and the constant optical path length difference is set to an optical path length difference that does not coincide with the maximum value of the degree of coherence of the laser light. 7. The method of reducing light coherence according to claim 6, wherein: 8. The coherence degree of the coherent light is represented as a function of time, the full width at half maximum of the first maximum waveform appearing as the coherence degree is τ _t , and the first maximum waveform is adjacent to the first maximum waveform. 7. The light coherence reduction method according to claim 6, wherein the constant optical path length difference L is set so as to satisfy Expression 1 below, where τ _d is the center-to-center distance with the second maximum waveform that appears. speed of light c _{[(n-1) τ d +} τ t / 2 ] ≦ L ≦ c [nτ _d -τ _t / 2] Equation 1 (where in the formula 1, c is in a medium light propagates , N represents an arbitrary natural number.) 9. A plurality of coherent lights incident at different positions are respectively split into a first transmitted light beam and a first reflected light beam at a partial reflection surface, and the first reflected light beam from the partial reflection surface is provided. Is reflected, guided again to the partial reflection surface, and branched into a second transmitted light beam and a second reflected light beam, and the first transmitted light beam and the second transmitted light beam are fixed between them. The light coherence reduction device, wherein the plurality of outgoing coherent lights in which the constant optical path length difference has occurred are emitted from the partial reflection surface in a state where the optical path length difference has occurred. 10. The plurality of coherent lights are incident on an optical element having the partial reflection surface and a total reflection surface that reflects a light beam reflected from the partial reflection surface, and the optical elements have different optical paths through the optical element. The light coherence reduction device according to claim 9, wherein the plurality of coherent lights that propagate are sequentially emitted from the partial reflection surface. 11. The coherence reduction of light according to claim 10, wherein the plurality of coherent lights are incident on the optical element, and a two-dimensionally uniform light flux is emitted as the outgoing coherent light from the partial reflection surface. apparatus. 12. The light coherence reducing apparatus according to claim 10, wherein the plurality of coherent lights propagating in the optical element are reflected and travel also in the surface directions of the partial reflection surface and the total reflection surface. 13. The light coherence reduction device according to claim 10, wherein the optical element is a prism having the total reflection surface, the partial reflection surface, and the incident surface. 14. The light coherence reducing device according to claim 9, wherein the coherent light is a light beam having a plurality of different oscillation wavelengths. 15. The coherent light is a laser light having a periodicity in the maximum value of the degree of coherence, and the constant optical path length difference is such that the difference in optical path length does not coincide with the maximum value of the degree of coherence of the laser light. 15. The light coherence reducing device according to claim 14, wherein: 16. The coherence degree of the coherent light is represented as a function of time, the full width at half maximum of the first maximum waveform appearing as the coherence degree is τ _t , and the first maximum waveform is adjacent to the first maximum waveform. 15. The coherence reduction of light according to claim 14, wherein the constant optical path length difference L is set so as to satisfy the following equation 1, where τ _d is the center-to-center distance with the second maximum waveform appearing as follows. apparatus. speed of light c _{[(n-1) τ d +} τ t / 2 ] ≦ L ≦ c [nτ _d -τ _t / 2] Equation 1 (where in the formula 1, c is in a medium light propagates , N represents an arbitrary natural number.) 17. A light beam emitted from a light source is incident as a plurality of coherent lights at different positions, and these coherent lights are respectively split into a first transmitted light beam and a first reflected light beam at a partially reflecting surface. Reflecting the first reflected light beam from the partially reflecting surface;
The light is again guided to the partial reflection surface and split into a second transmitted light beam and a second reflected light beam. The first transmitted light beam and the second transmitted light beam are separated by a certain optical path length difference therebetween. An illumination method, wherein the emitted coherent light is emitted from the partial reflection surface as a plurality of emitted coherent lights in which the constant optical path length difference is generated, and the emitted coherent light is used as an illumination light beam. 18. The plurality of coherent lights are incident on an optical element having the partial reflection surface and a total reflection surface that reflects a light beam reflected from the partial reflection surface, and the optical elements have different optical paths in the optical element. The illumination method according to claim 17, wherein the plurality of coherent lights that propagate are sequentially emitted from the partial reflection surface. 19. The light source according to claim 1, wherein the plurality of coherent lights are incident on the optical element, and a two-dimensionally uniform light flux is emitted from the partial reflection surface as the emitted coherent light.
8. The lighting method described in 8. 20. The illumination method according to claim 18, wherein the plurality of coherent lights propagating in the optical element are also reflected and made to travel in the plane directions of the partial reflection surface and the total reflection surface. 21. The illumination method according to claim 18, wherein the optical element is a prism having the total reflection surface, the partial reflection surface, and the incident surface. 22. The illumination method according to claim 17, wherein the coherent light is a light beam having a plurality of different oscillation wavelengths. 23. The coherent light is a laser light having a periodicity in the maximum value of the degree of coherence, and the constant optical path length difference is defined as an optical path length difference that does not match the maximum value of the coherence degree of the laser light. 23. The lighting method according to claim 22, wherein the lighting method is performed. 24. The coherence degree of the coherent light is represented as a function of time, the full width at half maximum of the first maximum waveform appearing as the coherence degree is τ _t , and the first maximum waveform is adjacent to the first maximum waveform. 23. The illumination method according to claim 22, wherein the constant optical path length difference L is set so as to satisfy Equation 1 below, where τ _d is the center-to-center distance with the second maximum waveform that appears. speed of light c _{[(n-1) τ d +} τ t / 2 ] ≦ L ≦ c [nτ _d -τ _t / 2] Equation 1 (where in the formula 1, c is in a medium light propagates , N represents an arbitrary natural number.) 25. The illumination method according to claim 17, wherein the illumination light beam is used as illumination light for a display device, a measurement device, a microscope, or an exposure device. 26. A light source for emitting a light beam, and the light beam emitted from the light source is incident as a plurality of coherent lights at different positions, and the plurality of coherent lights are respectively a first transmitted light beam and a second transmitted light beam. Applying the first reflected light beam again to the partially reflected surface where the first reflected light beam is split into a second transmitted light beam and a second reflected light beam; An optical element having a reflecting surface for guiding the light, wherein the first transmitted light beam and the second transmitted light beam have a constant optical path length difference between them, and An illuminating device, wherein a plurality of outgoing coherent lights having a constant optical path length difference are emitted from the partial reflection surface, and the outgoing coherent light is used as an illumination light beam. 27. The plurality of coherent lights are incident on an optical element having the partial reflection surface and a total reflection surface that reflects a light beam reflected from the partial reflection surface, and the optical elements pass through different optical paths in the optical element. 27. The plurality of coherent lights that propagate are sequentially emitted from the partial reflection surface.
The lighting device described in 1. 28. The plurality of coherent lights are incident on the optical element, and a two-dimensionally uniform light beam is emitted from the partial reflection surface as the emitted coherent light.
8. The lighting device according to 7. 29. The lighting device according to claim 27, wherein the plurality of coherent lights propagating in the optical element are also reflected and travel in the surface directions of the partial reflection surface and the total reflection surface. 30. The lighting device according to claim 27, wherein the optical element is a prism having the total reflection surface, the partial reflection surface, and the incident surface. 31. The lighting device according to claim 26, wherein the coherent light is a light beam having a plurality of different oscillation wavelengths. 32. The coherent light is a laser light having a periodicity in the maximum value of the degree of coherence, and the constant optical path length difference is such that the optical path length difference does not coincide with the maximum value of the coherence degree of the laser light. The lighting device according to claim 31, wherein the lighting device is provided. 33. The degree of coherence of the coherent light is represented as a function of time, the full width at half maximum of the first local maximum waveform appearing as the degree of coherence is τ _t , and the first local maximum waveform is adjacent to the first local maximum waveform. 32. The lighting device according to claim 31, wherein the constant optical path length difference L is set so as to satisfy Expression 1 below, where τ _d is the center-to-center distance with the second maximum waveform that appears. speed of light c _{[(n-1) τ d +} τ t / 2 ] ≦ L ≦ c [nτ _d -τ _t / 2] Equation 1 (where in the formula 1, c is in a medium light propagates , N represents an arbitrary natural number.) 34. The illumination device according to claim 26, wherein the illumination light beam is used as illumination light for a display device, a measurement device, a microscope, or an exposure device.