Patent Description:
In recent years, many three-dimensional (3D) movies are appearing, and thus many techniques related to 3D image display apparatuses are being studied. Since the 3D image display apparatuses display 3D images based on binocular parallax, 3D image display apparatuses which are being currently commercialized use binocular parallax of two eyes of a viewer. As a left-eye image and a right-eye image having different viewpoints are provided to the left and right eyes of a viewer, the viewer may perceive a stereoscopic effect. Such 3D image display apparatuses may include glasses type 3D image display apparatuses for which special glasses are necessary and non-glasses type 3D image display apparatuses for which glasses are not necessary.

However, when the viewer watches a 3D image displayed in a binocular parallax fashion, the viewer's eyes feel very tired, and 3D image display apparatuses configured to provide only two viewpoints, i.e., a left-eye image and a right-eye image, do not reflect a viewpoint change according to the movement of the viewer. As such, there is a limitation in providing a natural stereoscopic effect to the viewer.

In order to display more natural stereoscopic images, holographic display apparatuses are being studied. Light may be considered to be waves having amplitude information and phase information. A holography technique controls a phase and an amplitude of light to display an image. Accordingly, in the holographic display apparatuses, a device capable of controlling an amplitude (intensity) and a phase of light is necessary.

German patent application <CIT> teaches a holographic display using an optical beam combiner for combining two pixels of a spatial light modulator to generate complex pixels. US patent application <CIT> teaches a three-dimensional light modulator, of which the pixels are combined to form modulation elements wherein each modulation element can be coded with a preset discrete value such that three-dimensionally arranged object points can be holographically reconstructed. International patent application <CIT> teaches a lighting device having a planer optical fiber and at least one light source device for illuminating a controllable spatial light modulator wherein the optical fiber comprises a light-conducting core and a cover coating, and the light modulator comprises a pixel matrix, the light source device is disposed on the side of the optical fiber, and the light emitted by at least one light source of the light source device propagates laminarly in the optical fiber.

In a display apparatus adapted for switching between a holographic and two-dimensional display mode, device capable of switching between the holographic display mode and the two-dimensional display mode is necessary.

Provided is a beam combining/splitting modulator capable of selectively combining or splitting light according to one of claims <NUM>-<NUM>.

Provided is a display apparatus, according to claim <NUM>, configured to display an image by using the beam combining/splitting modulator.

Provided is a spatial light modulation method for modulating a phase and an intensity of light by using the beam combining/splitting modulator according to one of claims <NUM> and <NUM>.

Additional aspects will be set forth in part in the description which follows and, in part, will be apparent from the description.

According to the claimed invention, a beam combining/splitting modulator is provided according to claim <NUM>.

Preferred embodiments are subject of the dependent claims.

According to another aspect of the claimed invention, a beam combining/splitting modulator is provided according to claim <NUM>.

According to still another aspect of the claimed invention, a display apparatus is provided according to claim <NUM>.

According to still another aspect of the claimed invention, a spatial light modulation method is provided according to claim <NUM>.

Hereinafter, exemplary embodiments of the claimed invention will be described in detail with reference to the accompanying drawings. In the following drawings, the same reference numerals indicate the same components, and in the drawings, sizes of elements may be exaggerated for clarity and convenience of the description.

<FIG> schematically shows a beam combining/splitting modulator <NUM> according to an embodiment of the claimed invention.

A beam combining/splitting modulator <NUM> of the present embodiment includes a light modulator <NUM>, a polarization converter <NUM>, a birefringence modulator <NUM>, and a polarizer <NUM>.

The light modulator <NUM> includes a first modulation region <NUM> and a second modulation region <NUM> which independently modulate light. The first modulation region <NUM> and the second modulation region <NUM> independently modulate a phase of light. Light passing through the light modulator <NUM> has a predetermined polarized state. For example, light emitted from the light modulator <NUM> may be linearly polarized light. A polarization direction of light emitted from the light modulator <NUM> may be considered as, for example, an s-polarized wave in relation to the birefringence modulator <NUM> which will be described later. The light modulator <NUM> may be a known transmissive phase spatial light modulator. As an example, the light modulator <NUM> may be a light modulator for a parallel-aligned nematic-liquid-crystal (PAL) panel. The PAL panel may be configured so that rubbing directions (that is, orientation directions of liquid crystal) of upper and lower plates are parallel to each other (at <NUM>°). When liquid crystal is oriented by performing rubbing in directions in which the upper and lower plates are anti-parallel a positive type liquid crystal may be used. As such, when a voltage is applied, the liquid crystal may stand at a tilt angle perpendicular to the upper and lower plates without twisting. Therefore, phase modulation and control of a birefringence state may be possible without changing amplitude. As another example, the light modulator <NUM> may be a light modulator for a vertically aligned (VA) LC (liquid-crystal) panel. Even though the VA LC panel has an initial orientation of a liquid crystal perpendicular to the upper and lower plates, a negative type liquid crystal may be used. As such, when a voltage is applied, the liquid crystal may be reoriented in a direction perpendicular to an electric field. Therefore, even in this case, phase modulation and control of a birefringence state may be possible without twisting the liquid crystal.

The polarization converter <NUM> is disposed at a side of an emitting surface of the light modulator <NUM>. The polarization converter <NUM> includes a first transmissive region <NUM> and a second transmissive region <NUM>. The first transmissive region <NUM> is a region corresponding to the first modulation region <NUM> of the light modulator <NUM>, and polarizes and converts light (hereinafter, referred to as first light) L1 incident from the first modulation region <NUM> of the light modulator <NUM> so that the light L1 have a first polarization (for example, s-polarization). The second transmissive region <NUM> is a region corresponding to the second modulation region <NUM> of the light modulator <NUM>, and polarizes and converts light (hereinafter, referred to as second light) L2 incident from the second modulation region <NUM> of the light modulator <NUM> so that the light L2 have a second polarization (for example, p-polarization). The meaning of polarizing and converting light may be broadly considered to include maintaining polarization of incident light. As an example, the first transmissive region <NUM> of the polarization converter <NUM> may have no retardation of phase, and the second transmissive region <NUM> of the polarization converter <NUM> may be a region of a half-wave plate for retarding a half-wave phase.

The birefringence modulator <NUM> is disposed at the side of the emitting surface of the polarization converter <NUM>. The birefringence modulator <NUM> is an active modulator which switches between a first state in which birefringence occurs and a second state in which birefringence does not occur. The birefringence modulator <NUM> includes a liquid crystal layer <NUM> in which birefringence occurs according to a voltage applied thereto from a drive control unit <NUM>, and electrode layers <NUM> and <NUM> which apply a voltage to both ends of the liquid crystal layer <NUM>. Since the liquid crystal layer <NUM> of the birefringence modulator <NUM> functions to branch light or couple lights when the liquid crystal layer <NUM> is in birefringent state as will be described later, a thickness (T1 of <FIG>) of the liquid crystal layer <NUM> is designed to sufficiently branch or couple a light path changed by birefringence of the liquid crystal layer <NUM> as will be described later.

The polarizer <NUM> is disposed at the side of the emitting surface of the birefringence modulator <NUM>. A polarization axis of the polarizer <NUM> is inclined with respect to the first polarization and the second polarization. In other words, the polarizer <NUM> is installed so that light passing through the polarizer <NUM> include both a projection component with respect to the polarization axis of the first polarization and a projection component with respect to the polarization axis of the second polarization. As an example when the first polarization is s-polarization, and the second polarization is p-polarization, the polarization axis of the polarizer <NUM> is installed so as to be inclined with respect to both an s-polarization direction and a p-polarization direction.

On the other hand, the light modulator <NUM>, the polarization converter <NUM>, the birefringence modulator <NUM>, and the polarizer <NUM> may all form a flat panel structure. The beam combining/splitting modulator <NUM> may be integrated into one flat panel using a known liquid crystal panel manufacturing process.

Next, a spatial light modulation method for use wih the beam combining/splitting modulator <NUM> will be described with reference to <FIG> and <FIG>. <FIG> shows the birefringence modulator <NUM> in a first state in which birefringence occurs, and <FIG> shows when the birefringence modulator <NUM> is in a second state in which birefringence does not occur.

First, referring to <FIG>, first light L1 incident on the first modulation region <NUM> of the light modulator <NUM> is incident on the first transmissive region <NUM> of the polarization converter <NUM> in a modulated state. Similarly, second light L2 incident on the second modulation region <NUM> of the light modulator <NUM> is incident on the second transmissive region <NUM> of the polarization converter <NUM> in a modulated state. The light modulator <NUM> independently modulates phases of the first light L1 and the second light L2. Each of the first light L1 and the second light L2 emitted from the light modulator <NUM> includes a first polarization (for example, s-polarization).

The first light L1 incident on the first transmissive region <NUM> of the polarization converter <NUM> is emitted in a state in which the first polarization (for example, s-polarization) is maintained. The second light L2 incident on the second transmissive region <NUM> of the polarization converter <NUM> is light undergoing retardation to include second polarization (for example, p-polarization).

The birefringence modulator <NUM> may be in the first state (that is, a state in which birefringence occurs) according to a voltage applied thereto.

In general, light passing through a birefringent body may have different velocities according to a polarization direction thereof. A birefringent medium may be of a uniaxial type or a biaxial type. The liquid crystal layer <NUM> of the birefringence modulator <NUM> of the present invention is not limited to a particular type of birefringence. In the present embodiment, a case in which the liquid crystal layer <NUM> is uniaxially birefringent will be described as an example.

When the birefringence modulator <NUM> is in a first state (birefringent), an ordinary ray having a polarization direction perpendicular to an optical axis that is an axis of rotational symmetry of the liquid crystal layer <NUM> and an extraordinary ray having a polarization direction that is not perpendicular to the optical axis of the liquid crystal layer <NUM> have different velocities. Therefore, when the birefringence modulator <NUM> is in the first state (birefringent), the first light L1 is incident on the birefringence modulator <NUM> so that the first polarization of the first light L1 is perpendicular to a birefringent optical axis of the birefringence modulator <NUM>, and the second light L2 is incident on the birefringence modulator <NUM> in a state in which the second polarization of the second light L2 is inclinded with respect to the birefringent optical axis of the birefringence modulator <NUM>. The first light L1 and the second light L2 have different velocities in the liquid crystal layer <NUM> of the birefringence modulator <NUM> and thus have different light paths. When the first light L1 is incident perpendicular to the birefringence modulator <NUM> in a state in which the first light L1 becomes a first polarization (s-polarization), and the second light L2 is incident perpendicular to the birefringence modulator <NUM> in a state in which the second light L2 becomes a second polarization (p-polarization), the first light L1 undergoes normal refraction and thus travel in the liquid crystal layer <NUM> in a straight line without being refracted, and then is emitted from the liquid crystal layer <NUM>.

However, the second light L2 is refracted by subnormal refraction in the liquid crystal layer <NUM> due to birefringence of the birefringence modulator <NUM>, and then is emitted from the liquid crystal layer <NUM>. Therefore, a pitch interval P1 between the first modulation region <NUM> of the light modulator <NUM> (and the first transmissive region <NUM> of the polarization converter <NUM>) and the second modulation region of the light modulator <NUM> (and the second transmissive region <NUM> of the polarization converter <NUM>), and a thickness (T1) of the liquid crystal layer <NUM> of the birefringence modulator <NUM> are appropriately designed, the second light L2 is emitted from the birefringence modulator <NUM> in a state in which the second light L2 is refracted to be combined with the first light L1.

Accordingly, as shown in the drawings, when the birefringence modulator <NUM> is in the first state (birefringent), the first and second light L1 and L2 passing through the birefringence modulator <NUM> are emitted from the birefringence modulator <NUM> in a state in which the first and second light L1 and L2 are combined into one light that is incident on the polarizer <NUM>.

Since the polarization axis of the polarizer <NUM> is inclined with respect to the first polarization and the second polarization, the light passing through the polarizer <NUM> is provided as a component of a polarization direction of the polarizer <NUM> of the first light L1 and the second light L2. In order words, a phase of the light passing through the polarizer <NUM> is provided as a function of a modulated phase of the first light L1 and a modulated phase of the second light L2. In addition, a magnitude of the light passing through the polarizer <NUM> is provided as a sum of a projection component of a first polarization direction (for example, an s direction) of the first light L1 and a projection component of a second polarization direction (for example, a p direction) of the second light L2, and this magnitude is also provided as a function of the modulated phase of the first light L1 and the modulated phase of the second light L2. Therefore, a phase and an intensity of emitted light are independently controlled by phase modulation of the first modulation region <NUM> and the second modulation region <NUM> of the light modulator <NUM>.

Next, referring to <FIG>, when the birefringence modulator <NUM> is in the second state (that is, the state in which birefringence does not occur), the first light L1 and the second light L2 is emitted from the birefringence modulator <NUM> without any change of a light path in the birefringence modulator <NUM>. Therefore, the first light L1 and the second light L2 are emitted through the polarizer <NUM> in a state in which the first light L1 and the second light L2 are separated from each other.

As described above, the beam combining/spiltting modulator <NUM> of the present embodiment selectively controls the birefringence modulator <NUM> in the first state (birefringent) and the second state (non-birefringent). In this case, when the birefringence modulator <NUM> is controlled in the first state, the first light L1 and the second light L2 incident on the beam combining/splitting modulator <NUM> are combined and emitted, and an intensity and a phase of emitted light are independently modulated. When the birefringence modulator <NUM> is controlled in the second state, the first light L1 and the second light L2 incident on the beam combining/splitting modulator <NUM> are individually emitted.

<FIG> schematically shows a beam combining/splitting modulator <NUM> according to another embodiment of the claimed invention.

A beam combining/splitting modulator <NUM> of the present embodiment includes a polarizer <NUM>, a birefringence modulator <NUM>, and a light modulator <NUM>. The polarizer <NUM> is a member configured to transmit light polarized in a predetermined direction (for example, linearly polarized light). The birefringence modulator <NUM> is disposed at a rear side of the polarizer <NUM>. In this case, a side on which light is incident from the outside or a light source is referred to as a front side of the polarizer <NUM>. As will be described later, incident light is emitted again into the front of the polarizer <NUM> via the birefringence modulator <NUM> and the light modulator <NUM>.

The birefringence modulator <NUM> is switched between the first state in which birefringence occurs and the second state in which birefringence does not occure. The birefringence modulator <NUM> includes a liquid crystal layer <NUM> in which birefringence occurs according to a voltage applied thereto by a drive control unit <NUM>, and electrode layers <NUM> and <NUM> which apply a voltage to both ends of the liquid crystal layer <NUM>. As will be described later, the liquid crystal layer <NUM> of the birefringence modulator <NUM> functions to branch a light path or combine light paths when the liquid crystal layer <NUM> is birefringent. A thickness (T2 of <FIG>) of the liquid crystal layer <NUM> of the birefringence modulator <NUM> and a pitch interval (P2 of <FIG>) of a first modulation region <NUM> and a second modulation region <NUM> of the light modulator <NUM> are designed to sufficiently branch or couple a light path changed by birefringence of the liquid crystal layer <NUM> as will be described later.

The light modulator <NUM> is disposed at a rear side of the birefringence modulator <NUM>. The light modulator <NUM> includes the first modulation region <NUM> and the second modulation region <NUM> which indenpendently modulate light. The first modulation region <NUM> and the second modulation region <NUM> are regions on which the first light L1 and the second light L2, into which the light path is separated when the birefringence modulator <NUM> is in the first state as will be described later, are incident. The first modulation region <NUM> and the second modulation region <NUM> reflect light while independently modulating a phase of the light. The light modulator <NUM> is a known reflective phase spatial light modulator. Light incident on or reflected from the light modulator <NUM> remains in a polarized state as it is.

On the other hand, the polarizer <NUM>, the birefringence modulator <NUM>, the light modulator <NUM> may all form a flat panel structure, and the beam combining/splitting modulator <NUM> may be integrated in one flat panel using a known liquid crystal panel manufacturing process.

Next, an operation of the beam combining/splitting modulator <NUM> of the present embodiment will be described with reference to <FIG> shows when the birefringence modulator <NUM> is in a first state in which birefringence occurs, and <FIG> shows when the birefringence modulator <NUM> is in a second state in which birefringence does not occur.

First, referring to <FIG>, light passing through the polarizer <NUM> is light polarized in a predetermined direction. As an example, the light may be linearly polarized light having s- polarization or p-polarization components. In this case, the s-polarization and the p-polarization may be determined based on a birefringent optical axis of the birefringence modulator <NUM>.

The birefringence modulator <NUM> may be in the first state (that is, a state in which birefringence occurs) according to a voltage applied thereto. When the birefringence modulator <NUM> is in the first state (birefringent), first light L1 having a polarization direction (s-polarization) perpendicular to an optical axis that is an axis of rotational symmetry of a liquid crystal layer <NUM> travels in a straight line without being refracted in the liquid crystal layer <NUM>, but second light L2 having a polarization direction (p-polarization) that is not perpendicular to the optical axis of the liquid crystal layer <NUM> is refracted in the liquid crystal layer <NUM>. As a result, the first light L1 and the second light L2 are emitted from the birefringence modulator <NUM> in a state in which the first light L1 and the second light L2 are separated from each other. The first light L1 and the second light L2 emitted from the birefringence modulator <NUM> are incident on a first modulation region <NUM> and a second modulation region <NUM> of the light modulator <NUM>. The first light L1 and the second light L2 incident on the first modulation region <NUM> and the second modulation region <NUM> of the light modulator <NUM> independently undergo phase modulation, are reflected, and are incident again on the birefringence modulator <NUM>. The first light L1 and the second light L2 that are incident again on a rear side of the birefringence modulator <NUM> may travel in an opposite direction along the light path before separation, and are emitted from a front side of the birefringence modulator <NUM> in a state in which the first light L1 and the second light L2 are combined with each other. The first and second light L1 and L2 passing through the birefringence modulator <NUM> are emitted in a state in which the first and second light L1 and L2 are combined with each other and are incident again on the polarizer <NUM>. Since a polarization axis of the polarizer <NUM> is installed to be inclined with respect to first polarization of the first light L1 and second polarization of the second light L2, the light passing through the polarizer <NUM> is provided as components of polarization directions of the polarizer <NUM> of the first light L1 and the second light L2. That is, a phase of the light passing through the polarizer <NUM> is provided as a function of a modulated phase of the first light L1 and a modulated phase of the second light L2.

Next, referring to <FIG>, when the birefringence modulator <NUM> is in the second state (that is, a state in which birefringence does not occur), the light incident from the front side of the polarizer <NUM> is incident on the light modulator <NUM> without any change of a light path in the birefringence modulator <NUM>. The light incident on the first modulation region <NUM> and the second modulation region <NUM> of the light modulator <NUM> is emitted again via the polarizer <NUM> without any change of the light path in the birefringence modulator <NUM>.

As described above, according to the beam combining/splitting modulator <NUM> of the present embodiment, an intensity and a phase of the light incident from the front side of the polarizer <NUM> are understood to be independently modulated while the light is emitted again in front of the polarizer <NUM>.

According to the beam combining/splitting modulators <NUM> and <NUM> of the above-mentioned embodiments, since two incident lights may be emitted in a state in which the lights are combined or separated, the beam combining/splitting modulators <NUM> and <NUM> operate as an active polarization beam combiner/splitter.

<FIG> schematically shows a display panel <NUM> according to still another embodiment of the claimed invention, and <FIG> shows an example of a pixel array of the display panel <NUM>.

Referring to <FIG>, a display panel <NUM> of the present embodiment is an application of the beam combining/splitting modulators according to the above-mentioned embodiments. For example, the display panel <NUM> includes a pixel array having a plurality of pixels arranged in a two-dimensional fashion, and each pixel of the pixel array includes a first and/or second modulation region of the beam combining/splitting modulator (<NUM> of <FIG>) of the above-mentioned embodiment. In addition, the display panel <NUM> of the present embodiment may further include a color filter <NUM>. When the color filter <NUM> is further provided, red (R), green (G) and blue (B) subpixels or just some of these may form a pixel. According to the red (R), the green (G), and the blue (B) subpixels, the light modulator <NUM> described above includes first and second modulation regions <NUM> and <NUM>, first and second transmissive regions <NUM> and <NUM> of a polarization converter <NUM>, a birefringence modulator <NUM>, and a polarizer <NUM>. The birefringence modulator <NUM> may be driven according to the red(R), the green (G), and the blue (B) subpixels or integrally. It may also be possible to display a color in a time division scheme performed by sequentially driving red, green, and blue light sources, instead of a spatial division scheme using the color filter <NUM>.

As described above, since the beam combining/splitting modulator <NUM> may combine two incident lights to independently control the phase and intensity of the light, the phase and the intensity of the emitted light may be controlled according to each pixel (or subpixel). Therefore, the display panel <NUM> of the present embodiment is used as a hologram display panel displaying a computer generated hologram (CGH). Furthermore, the display panel <NUM> of the present embodiment may also function as a multi-way three dimensional (3D) display panel providing multi-views by appropriately controlling a phase of light. In addition, in the display panel <NUM> of the present embodiment, the beam combining/splitting modulator <NUM> may also display a two-dimensional (2D) image by controlling only an intensity of emitted light. When a 2D image is displayed (that is, when only an intensity of light is modulated), the first and second modulation regions <NUM> and <NUM> of the light modulator <NUM> may individually correspond to pixels (or subpixels). Thus, a resolution of the 2D image may be twice that of a 3D image.

In addition, in the display panel <NUM> of the above-mentioned embodiment, even though the beam combining/splitting modulator <NUM> described with reference to <FIG>, and <FIG> is described as an example, it is possible to apply the beam combining/splitting modulator <NUM> described with reference to <FIG>, <FIG> to a pixel (or a subpixel) of the display panel.

<FIG> schematically shows a display apparatus <NUM> according to still another embodiment of the present invention. The display apparatus <NUM> includes a display panel <NUM>. The display panel <NUM> is the display panel <NUM>.

As described above, since a beam combining/splitting modulator (<NUM> of <FIG>, and <NUM> of <FIG>) applied to the display panel <NUM> controls a phase and an intensity of emitted light, the CGH is displayed. Therefore, the display apparatus <NUM> also functions as a holographic image display apparatus. Furthermore, as described above, since the display panel <NUM> may also function as a multi-way 3D display panel providing a multi-view by appropriately controlling the phase of light, the display panel <NUM> may also function as a 3D display apparatus providing a multi-view. In addition, in a not claimed example, the beam combining/splitting modulators <NUM> and <NUM> may selectively control only an intensity of emitted light L1 and L2, the display apparatus <NUM>, in a not claimed example, may also function as a 2D/3D switchable stereoscopic image display apparatus.

In addition, since the beam combining/splitting modulator <NUM> or <NUM> may be implemented as a flat panel type, the display panel <NUM> using the beam combining/ splitting modulator <NUM> or <NUM> may also be a flat panel. Furthermore, the display apparatus <NUM> may also be a flat panel display apparatus.

As described above, according to the one or more of the above embodiments of the claimed invention, a beam combining/splitting modulator and a spatial light modulation method according to the embodiments of the claimed invention control a phase and an intensity of incident light.

A beam combining/splitting modulator and a spatial light modulation method according to embodiments of the claimed invention emit two incident light in a state in which the two incident lights are combined or splitted.

A beam combining/splitting modulator according to the embodiments of the claimed invention is also used as a complex spatial light modulator (a display panel) configured to display a holographic image.

Claim 1:
A beam combining/splitting modulator (<NUM>) comprising:
a transmissive phase spatial light modulator (<NUM>) including first and second modulation regions (<NUM>, <NUM>) for independently modulating a phase of incident light and configured to pass incident light as polarized light with a predetermined polarized state;
a polarization converter (<NUM>) disposed at a side of an emitting surface of the transmissive phase spatial light modulator (<NUM>), and including a first transmissive region (<NUM>) for polarizing and converting light incident from the first modulation region (<NUM>) to have a first polarization and a second transmissive region (<NUM>) for polarizing and converting light incident from the second modulation region (<NUM>) to have a second polarization;
a birefringence modulator (<NUM>) disposed at the side of an emitting surface of the polarization converter (<NUM>) and including a liquid crystal layer (<NUM>) in which birefringence occurs according to a voltage applied thereto; and
a polarizer (<NUM>) disposed at the side of an emitting surface of the birefringence modulator (<NUM>);
wherein a polarization axis of the polarizer (<NUM>) is inclined with respect to the first and second polarization;
characterized in that
the birefringence modulator (<NUM>) is configured to switch between a first state in which birefringence occurs and a second state in which birefringence does not occur;
wherein, when the birefringence modulator is in the first state, a light path of at least one light of the light incident from the first modulation region (<NUM>) and the light incident from the second modulation region (<NUM>) is changed in the birefringence modulator (<NUM>), the incident lights are combined, and the combined light is emitted from the birefringence modulator (<NUM>), and when the birefringence modulator is in the second state, the light incident from the first modulation region (<NUM>) and the light incident from the second modulation region (<NUM>) are emitted from the birefringence modulator (<NUM>) without any change of light paths thereof.