JP2009109669A - Polarized light switching element, luminous flux splitting element, optical scanner and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a polarized light switching element which has a large tolerance to the shift of the wavelength of incident light and an installing error of the element, and can contribute to the improvement of productivity of an optical scanner. <P>SOLUTION: The polarized light switching element 10 is composed of: a λ/4 plate 11; and a liquid crystal element 12 which switches the alignment direction of liquid crystal molecules into a predetermined direction by switching the direction of electric field, both arranged in this order from the incident light side. The light which is made incident to the polarized light switching element 10 is vertically polarized as shown in Figure. The incident light which was linear polarized light P1 becomes clockwise circular polarized light P2 after passing through the λ/4 plate 11. The light further becomes horizontally polarized light P3 by being added with a phase shift of λ/4 when the liquid crystal molecules of the liquid crystal element 12 are oriented in 45° direction. Further, the clockwise circular polarized light P2 after passing through the λ/4 plate 11 becomes linear polarized light P5 which vertically oscillates when the direction of the electric field applied on the liquid crystal is switched and the alignment of the liquid crystal molecules is directed to -45° as shown in (b) of the Figure. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a polarization switching element that emits the polarization of incident light as it is or after being rotated, a light beam splitting element having the polarization switching element, and an optical scanning device (hereinafter referred to as “optical writing device”) using the light beam splitting element. The present invention also relates to an image forming apparatus such as a copying machine, a printer, a facsimile machine, a plotter, and a multifunction machine including at least one of them.

Patent Document 1 discloses an optical scanning device including a scanning optical element having four light sources, a rotating polygon mirror, and an anamorphic surface. When used in an image forming apparatus, one laser is required for writing on one photoconductor with one laser beam, so four laser light sources are required for four photoconductors. Discloses a one-light source multiple-drum recording method using beam splitting. As a means for dividing the number of beams from the light source by a factor of 2, a prism using a half mirror as a dividing surface is disclosed. This beam splitting element always splits incident light into two beams. With this configuration, the tandem image forming apparatus enables high-speed image output while reducing the number of light sources.

JP 200470110 A JP 2005-92129 A

In the passive type described in Patent Document 2, half of the LD light is always lost.
An active beam splitting element based on a FLC having a tilt angle of 22.5 ° (cone angle of 45 °) (surface stabilization) and a polarization-dependent optical path separation element (PBS or polarization-dependent hologram) has a light use efficiency. Although improved, it was sensitive to the wavelength and incident angle, and the allowable assembly width was small.
The light emitted from the LD light source is almost linearly polarized light, but its vibration plane varies. Rotation deviation of the LD chip installed in the LD case and rotation deviation when the LD case is assembled to the apparatus (optical writing apparatus) occur. Further, in an optical writing apparatus using an LD unit composed of two LDs as a light source, the interval between two LD lights is finely adjusted by adjusting the rotation of the LD unit.

An optical writing device in which a multi-surface reflecting mirror (polygon mirror) uses a two-stage configuration, and a method is proposed in which light emitted from an LD is divided into two light beams by a light beam dividing element so as to correspond to the two stages of the multi-surface reflecting mirror. .
In addition, a method using a diffractive optical element (hologram) as the light beam splitting element has been proposed. As a light beam splitting element using this diffractive optical element, there is a system in which a polarization-dependent type with high diffraction efficiency is used for the first diffractive optical element. In this method, the vibration surface of the LD can be rotated by 45 ° with a λ / 2 plate or the like, and the intensity ratio of the diffracted light and transmitted light of the first diffractive optical element can be made uniform. However, when the rotational displacement of the LD vibration surface occurs as described above, the vibration surface converted by the λ / 2 plate deviates from 45 °. For this reason, the two luminous flux intensity ratios deviate from 1: 1.

As a specific example, when the LD vibration surface is perpendicular to the plane in which light is deflected by the polyhedral reflector, it is assumed that there is no rotational deviation, and the light beam splitting ratio by the first diffractive optical element is assumed to be 1 in this state.
If a 5 ° rotation shift occurs due to the LD installation shift, the beam splitting ratio of 1 can be maintained if the λ / 2 plate is further rotated 2.5 °. However, in the case of an LD unit composed of two LDs, the vibration surfaces of the two LD lights are often non-parallel. Assuming that the angle difference between the slow axis of the λ / 2 plate and each LD vibration surface is shifted from 22.5 ° to 5 °, the light beam splitting ratio is 1.19 (or its inverse 0.84).
Further, there is a problem that the light intensity ratio of the light beam splitting is converted by the wavelength shift due to the incident angle shift to the light beam splitting element or the wavelength variation of the incident light.

  The present invention has a large tolerance with respect to a wavelength shift of incident light and an installation error of an element, and can contribute to improvement of productivity of an optical scanning device, a light beam splitting element having the polarization switching element, and the light beam splitting element. The main object of the present invention is to provide an optical scanning device having the optical scanning device and an image forming apparatus having the optical scanning device.

In order to achieve the above object, the invention according to claim 1 comprises a liquid crystal element capable of switching between clockwise circularly polarized light and counterclockwise circularly polarized light according to the polarity of the applied voltage, and a λ / 4 plate. The incident light is switched to a different polarization direction.
According to a second aspect of the present invention, in the polarization switching element according to the first aspect, the liquid crystal element has a pair of transparent substrates, an alignment film provided on at least one of the transparent substrates, and homogeneous alignment by the alignment film. It comprises a liquid crystal layer comprising a chiral smectic C phase and an electric field generating means for generating an electric field direction in the normal direction of the transparent substrate, and the liquid crystal layer has a tilt angle of 45 ° (cone angle of 90 °). It is characterized by.
In particular, the purpose is to increase the switching speed.

According to a third aspect of the present invention, in the polarization switching element according to the first aspect, a vertical alignment type liquid crystal element using a nematic liquid crystal is arranged in a two-stage configuration, and the liquid crystal molecules in the first and second stages are arranged. Azimuth θ1, θ2 projected onto the translucent substrate of
θ1 = 0 °, θ2 = 45 °
When,
θ1 = 45 °, θ2 = 0 °
Are sequentially repeated.

The invention according to claim 4 is characterized in that the light beam splitting element comprises the polarization switching element according to any one of claims 1 to 3 and polarization separation means.
According to a fifth aspect of the present invention, in the light beam splitting element according to the fourth aspect, the polarization separation means includes a polarization beam splitter and a deflection prism.

According to a sixth aspect of the present invention, in the light beam splitting element according to the fifth aspect, the polarization beam splitter is a polarization-dependent hologram.
According to a seventh aspect of the present invention, in the optical scanning device, the light beam splitting element according to any one of the fourth to sixth aspects, a multistage rotary polygon mirror, and an imaging optical system using at least an fθ lens. It is characterized by becoming.
In particular, the object here is to obtain a good white balance.
According to an eighth aspect of the present invention, the image forming apparatus includes the optical scanning device according to the seventh aspect.

According to the first aspect of the present invention, a combination of a liquid crystal element that gives a phase difference of ± λ / 4 and a λ / 4 plate realizes a polarization switching element that has a wide tolerance for wavelength shift of incident light and element shift. This can contribute to the improvement of the productivity of the optical scanning device.
According to the second aspect of the invention, in addition to the above effect, a ferroelectric liquid crystal is used, so that a polarization switching element with a high response speed can be realized.
According to the invention described in claim 3, it is possible to facilitate the production of the liquid crystal element by using the nematic liquid crystal.
According to the inventions of the fourth, fifth and sixth aspects, by using the polarization switching element and the polarization separation means, it is possible to realize a light beam splitting element having a wide tolerance with respect to a wavelength shift of incident light and an installation shift of the element, This can contribute to the improvement of the productivity of the optical scanning device.
According to the seventh aspect of the present invention, by mounting the light beam splitting element, it is possible to realize an optical scanning device having a wide tolerance with respect to a wavelength shift of incident light and a shift of the element installation and a good white balance.

Embodiments of the present invention will be described below with reference to the drawings.
First, a first embodiment will be described with reference to FIGS. FIG. 1 is a diagram for explaining the function of the polarization switching element 10 according to this embodiment. The polarization switching element 10 changes the alignment direction of the liquid crystal molecules to a predetermined direction by switching the λ / 4 plate 11 from the incident light side, the electric field generating means (not shown), and the polarity of the applied voltage (electric field direction) by the electric field generating means. It is comprised with the liquid crystal element 12 which can be switched.
The λ / 4 plate 11 is arranged such that the slow axis is inclined 45 ° clockwise as viewed from the exit side of the polarization switching element 10. The liquid crystal element 12 is controlled so that a liquid crystal having uniaxial anisotropy (for example, nematic liquid crystal) is oriented at 45 ° (FIG. 5A) or −45 ° (FIG. 5B) by an applied electric field.
Further, the product of the difference Δn between the ordinary light refractive index and the extraordinary light refractive index of the liquid crystal and the liquid crystal layer thickness d so that the retardation of the liquid crystal layer is λ / 4,
Δnd = λ / 4 (λ is the wavelength of incident light)
And

The light incident on the polarization switching element 10 is polarized in the vertical direction in the figure. Incident light that has been linearly polarized light P1 passes through the λ / 4 plate 11 and becomes clockwise circularly polarized light P2. When the liquid crystal molecules of the liquid crystal element 12 are oriented in the 45 ° azimuth, a phase difference of λ / 4 is further added to become the horizontally polarized light P3.
Next, when the direction of the electric field applied to the liquid crystal is switched and the orientation of the liquid crystal molecules is oriented at −45 ° as shown in FIG. 5B, the clockwise circular polarized light P2 transmitted through the λ / 4 plate 11 is in the vertical direction. The linearly polarized light P5 oscillates at the same time.
Therefore, the incident light can be rotated by 90 ° by the electric field applied to the liquid crystal element 12 or can be emitted while maintaining the polarization (without rotating).

The effect of this embodiment will be described in detail below.
FIG. 2 shows a polarization switching element by the liquid crystal element 21 giving a phase difference λ / 2 for comparison. The Δnd of the liquid crystal is λ / 2. For example, the wavelength used is 650 nm, and a liquid crystal ZLI-1840 manufactured by Merck is used. Since Δn at 650 nm is 0.14, the thickness of the liquid crystal layer is preferably 2.32 μm.
The XYZ coordinate system was taken as shown in FIG. 2, and the effects of wavelength shift and incident angle shift from the normal direction were examined. Consider a case where incident light is incident on a polarization switching element at an incident angle of 0.3 ° and an azimuth angle of 0 ° (or 45 °).
The wavelength is assumed to be 655 nm assuming a case where the wavelength is shifted by 5 nm from the design value. The azimuth angle is 0 ° in the X-axis direction on the XY plane and positive in the clockwise direction with respect to the z-axis. In the case of the present embodiment (FIG. 1) and the comparative example (FIG. 2), when the emitted light is desired to be in the horizontal direction, the x-direction light intensity Ix in (a), and (b) y in the case of switching in the vertical direction. The directional light intensity Iy was calculated, and the result is shown in Table 1. In addition, although the incident light intensity was set to 1.0 and the glass substrate for sandwiching the liquid crystal layer was ignored in the calculation, Fresnel loss due to the difference in refractive index between the liquid crystal layer and air (light loss due to reflection) was considered.

From Table 1, regardless of the incident light azimuth angle, the intensity of the emitted light when the polarization is switched is smaller in this example (this embodiment) than in the comparative example. That is, the present invention shows that the wavelength variation and the element installation allowance are wider than those of the prior art (polarization switching element that gives a phase difference λ / 2).
In FIG. 1, the order of the λ / 4 plate 11 and the liquid crystal element 12 may be reversed, that is, the incident light may be installed so as to pass through the liquid crystal element 12 and the λ / 4 plate 11 in this order.
Further, the liquid crystal is not limited to nematic liquid crystal, and may be a liquid crystal composed of a chiral smectic C phase having homogeneous orientation and having a tilt angle of 45 ° (that is, a cone angle of 90 °). In this case, since the response speed can be increased as compared with the nematic liquid crystal, the polarization switching speed can be improved.

A liquid crystal composed of a chiral smectic C phase having a cone angle of 90 ° is shown below.
FIG. 3 is a schematic diagram for explaining the switching of the ferroelectric liquid crystal. In general, a ferroelectric liquid crystal layer composed of a chiral smectic C phase has a helical structure. However, when sandwiched between cell gaps d thinner than the helical pitch, the helical structure is unwound and a surface-stabilized ferroelectric liquid crystal layer (SSFLC). )
As shown in FIG. 4, the SSFLC has an alignment state in which liquid crystal molecules are inclined and stabilized with respect to the smectic layer normal by an inclination angle −θ (here, θ = 45 °), and is stabilized by being inclined by θ in the opposite direction. A state in which the alignment state is mixed can be realized.
In FIG. 3, W is the layer normal of the smectic phase, n is the major axis direction of the liquid crystal molecules (director), the black circle symbol in the white circle, and the + symbol in the white circle indicate the direction of spontaneous polarization. By applying a voltage so that the direction of the electric field is generated in a direction perpendicular to the paper surface, the directions of the liquid crystal molecules and the spontaneous polarization can be made uniform, and the state can be maintained. And switching between two states can be performed by switching the polarity of the voltage to apply.

That is, when an electric field of −E is formed in FIG. 3, an electric field of + E is formed in the orientation state 1 inclined by −θ from the layer normal direction W of the smectic phase, and from the layer normal direction W of the smectic phase. It is possible to stabilize the alignment state 2 tilted by θ. That is, when θ = 45 °, the alignment state 2 tilted by 90 ° from the alignment state 1 can be stabilized.
The thickness (cell gap) d of the liquid crystal layer is determined by the wavelength λ of incident light (for example, 650 nm or 780 nm) and the refractive index anisotropy Δn of the liquid crystal material at 650 nm or 780 nm, and satisfies Δn × d = λ / 4. decide.
Here, the incident polarization direction needs to be adjusted and arranged so as to be a bisector of the two alignment states of the liquid crystal molecules in the liquid crystal layer. As an adjustment method, it is possible to adjust the polarization direction by arranging the phase plate. It is also possible to set the initial alignment of the liquid crystal molecules or to adjust the rotation of the liquid crystal element itself by an alignment process such as rubbing.

With this configuration, as shown in FIG. 3, when an electric field of −E is formed between the transparent electrodes, the liquid crystal molecules are aligned in an orientation state (alignment state) inclined by −θ from the layer normal direction W of the smectic phase. 1), the incident polarized light is emitted while maintaining the polarization direction as it is. On the other hand, when a + E electric field is formed on the transparent electrode, the liquid crystal molecules take an alignment state (alignment state 2) inclined by + θ from the layer normal direction W of the smectic phase.
In this case, since θ = 45 °, the liquid crystal molecule major axis direction (director) is tilted by 2θ = 90 ° with respect to the incident polarized light. As a result, the λ / 4 plate condition is satisfied, and the incident linearly polarized light becomes clockwise circularly polarized light or counterclockwise circularly polarized light.
That is, polarization switching can be realized by electric field control, and the ferroelectric liquid crystal is used. Therefore, the response speed for polarization switching is a high-speed response of several tens to several hundreds of μsec.

Here, the fabrication and operation check of the polarization switching means (polarization switching element) using the ferroelectric liquid crystal element will be described.
An ITO electrode (thickness: 1500 mm) was formed on a non-alkali glass substrate having a thickness of 1.1 mm. An alignment film (AL3046-R31 manufactured by JSR) was formed on the surface of the substrate electrode by spin coating to a thickness of about 800 mm, and the substrate surface was subjected to alignment treatment by rubbing.
Two glass substrates described above were used, and the electrode surfaces were opposed to each other, and bonded together with an adhesive mixed with beads so that the distance between the substrates was about 1 μm. A ferroelectric liquid crystal (Clariant R5002 Δn = 0.17, 2θ = 90 degrees) is injected as a liquid crystal layer between the two substrates in a state where the substrate is heated to 90 degrees by a capillary method. Was sealed after cooling in a state where a DC voltage of 10 V / μm was applied, and the alignment state was observed with an optical microscope. As a result, a substantially uniform alignment state was confirmed. A square wave signal with a frequency of 100 Hz and ± 10 V / μm was input to this liquid crystal element, and the switching speed of light and dark under crossed Nicols was measured (liquid crystal element characteristic evaluation device, manufactured by Otsuka Electronics). The response speed is about 1 msec, which is one order of magnitude faster than a general liquid crystal element.

Further, here, regarding the polarization switching operation by electric field control, the polarization direction of emission when linearly polarized light was incident was evaluated using the manufactured liquid crystal element and the λ / 4 plate.
A red LD (wavelength 650 nm) was used as a light source. First, an electric field of −10 V / μm was applied between the device electrodes, and the λ / 4 plate (corresponding to 11 in FIG. 1) was rotated and adjusted so that incident polarized light (vertical direction) was rotated 90 ° and output.
Next, when an electric field of −10 V / μm having different polarities was applied between the element electrodes, the polarization directions at the time of incidence and at the time of emission became substantially parallel. That is, the retardation of the liquid crystal element could be switched between + λ / 4 and −λ / 4 by controlling the electric field application.

The second embodiment will be described with reference to FIGS.
The polarization switching element 30 according to this embodiment includes a first liquid crystal element 31, a second liquid crystal element 32, and a λ / 4 plate 11.
Both the liquid crystal elements 31 and 32 can be vertically aligned liquid crystal elements. The liquid crystal element 31 has a structure in which a nematic liquid crystal is sandwiched between two glass substrates with an ITO film. There is an alignment film between the liquid crystal layer and the glass substrate, and the liquid crystal is aligned in a predetermined direction by rubbing treatment.
The rubbing orientation is 45 ° in the first liquid crystal element 31 and −45 ° in the second liquid crystal element 32. When the voltage difference between the two ITOs is low, the liquid crystal molecules are inclined from the normal direction of the glass substrate and are directed in a predetermined direction by rubbing treatment.
When the voltage difference is increased, the liquid crystal molecules are oriented in the normal direction. As shown in FIG. 6A, in the first state T1, the first liquid crystal 31 decreases the voltage difference (VL), and the second liquid crystal 32 increases the voltage difference (VH).

When linearly polarized light is incident on the first liquid crystal element 31, it becomes clockwise circularly polarized light. In the second liquid crystal 32, no phase difference is generated, so that it is emitted as clockwise circularly polarized light. The next λ / 4 plate 11 converts it into linearly polarized light in the horizontal direction.
On the other hand, as shown in FIG. 6B, in the second state T2, the method of applying the voltage difference is reversed. For this reason, incident light maintains linearly polarized light in the first liquid crystal 31 and becomes counterclockwise circularly polarized light in the second liquid crystal 32. The next λ / 4 plate 11 returns to the original linearly polarized light.
In the present embodiment, the liquid crystal used for the liquid crystal light valve can be used, and the degree of difficulty in aligning the nematic liquid crystal is lower than that of the ferroelectric liquid crystal, so that the productivity is excellent.

A third embodiment will be described with reference to FIG.
FIG. 7 is a diagram for explaining a light beam splitting element 50 using the above-described polarization switching element.
As shown in FIG. 7A, the light beam splitting element 50A includes the above-described polarization switching element 10 or 30, the polarization separating means 51, and the deflecting prism 52.
As the polarization separation means 51, a polarization beam splitter can be used. If the polarization of the light emitted from the polarization switching element is parallel to the paper surface, the beam passes through the polarization beam splitter 51.
On the other hand, if the polarization of the light emitted from the polarization switching element is perpendicular to the paper surface, the beam is reflected by the polarization beam splitter 51 and totally reflected by the deflecting prism 52 and emitted. In this embodiment, a deflection prism 52 is used to switch the light beam up and down.

As shown in FIG. 7B, the same effect can be obtained with a light beam splitting element 50 </ b> B in which the deflecting prism 52 is a polarizing beam splitter 53.
Further, as shown in FIG. 7C, a light beam splitting element 50C using polarization-dependent holograms 55 and 56 as the polarization separating means and the deflecting prism may be used.
As the polarization-dependent hologram, a blazed hologram, a deep groove surface relief hologram, or a holographic PDLC (polymer dispersed liquid crystal) can be used.

A fourth embodiment will be described with reference to FIG.
FIG. 8 is a diagram for explaining an optical scanning device 60 using the above-described polarization switching element and light beam splitting element.
The optical scanning device 60 can be constructed by arranging the light source 61, the light beam splitter 50, the cylindrical lens 62, the two-stage polygon mirror (multistage rotary polygon mirror) 63, and the imaging optical systems 65a and 65b.
The light source 61 is an LD unit including two LDs, and the light beam splitting element 50 uses the above-described element 50A, 50B, or 50C. The imaging optical system 65 uses at least the fθ lens 64. In FIG. 8, in addition to the fθ lens 64, a correction lens and a folding mirror are arranged.
The two beams can be optically scanned to the line of the optical scan 66a in a certain time zone, and to the line of the optical scan 66b in the remaining time zone.
Although not shown, two sets of each of the optical system in the front stage from the LD unit to the cylindrical lens 62 after the light beam splitting element 50 and the imaging optical system 65 including the fθ lens 64 are used, and the two-stage polygon mirror 63 is used. Thus, it is possible to construct an optical writing device for writing to four scanned surfaces with two beams.
The four scanned surfaces are photoconductors corresponding to four colors (cyan, yellow, magenta, and black), and a color printer using the present invention can be constructed.

Based on FIG. 9, a tandem type color image forming apparatus (fifth embodiment) using the above-described optical scanning device 60 will be described.
The color image forming apparatus has four photoconductors 71Y, 71C, 71M, 71K juxtaposed along the moving direction of the transfer belt 70. Around the photoreceptor 71Y for yellow image formation, a charger 72Y, a developing device 73Y, a transfer unit 74Y, and a cleaning unit 75Y are sequentially arranged in the rotation direction indicated by the arrow. Other colors also have the same configuration, and are distinguished by adding European letters (C: cyan, M: magenta, K: black) for each color, and a description thereof is omitted.
The charger 72 is a charging member that constitutes a charging device for uniformly charging the surface of the photoreceptor. An optical latent image is formed on the photosensitive member 71 by irradiating the photosensitive member surface with a beam between the charger 72 and the developing unit 73 by the optical scanning device 60.
Based on the electrostatic latent image, a toner image is formed on the surface of the photoreceptor by the developing device 73. The transfer unit 74 sequentially transfers the transfer toner images of the respective colors onto the recording medium (transfer sheet) conveyed by the transfer belt 70, and finally the superimposed image is fixed on the transfer sheet by the fixing unit 76.

As described above, in the tandem image forming apparatus, high-speed image output is possible while reducing the number of light sources, and the use of the polarization switching element according to the present invention provides high light utilization efficiency and polarization. It is possible to reduce the intensity change when switching the.
That is, since there is no difference in the writing intensity to each photoconductor, an image with good color balance can be obtained.

It is a principal part perspective view of the polarization switching element which concerns on the 1st Embodiment of this invention. It is a principal part perspective view of the polarization switching element which concerns on a comparative example. It is a schematic diagram for demonstrating switching of a ferroelectric liquid crystal. It is a schematic diagram which shows operation | movement of the polarization switching element using the surface stabilization ferroelectric liquid crystal layer. It is a schematic block diagram of the polarization switching element which concerns on 2nd Embodiment. It is a figure which shows operation | movement of a polarization switching element. It is a schematic block diagram of the light beam splitting element which concerns on 3rd Embodiment. It is a general | schematic perspective view of the optical scanning device which concerns on 4th Embodiment. It is a schematic block diagram of the image forming apparatus which concerns on 5th Embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10, 30 Polarization switching element 11 λ / 4 plate 12 Liquid crystal element 50A, 50B, 50C Light beam splitting element 51, 53 Polarizing beam splitter 52 Deflection prism 55, 56 Polarization-dependent hologram 60 Optical scanning device 63 Rotating polygon mirror 64 fθ lens

Claims (8)

  Polarization switching characterized by comprising a liquid crystal element capable of switching between right-handed circularly polarized light and left-handed circularly polarized light according to the polarity of the applied voltage and a λ / 4 plate, and switching incident light to different polarization directions element. The polarization switching element according to claim 1,
The liquid crystal element includes a pair of transparent substrates, an alignment film provided on at least one transparent substrate, a liquid crystal layer composed of a chiral smectic C phase that is homogeneously aligned by the alignment film, and a normal direction of the transparent substrate. A polarization switching element comprising an electric field generating means for generating an electric field direction, wherein the tilt angle of the liquid crystal layer is 45 ° (cone angle is 90 °). The polarization switching element according to claim 1,
The liquid crystal element is a vertical alignment type liquid crystal element using nematic liquid crystal arranged in two stages, and the orientations θ1 and θ2 projected onto the light-transmitting substrates of the first and second liquid crystal molecules are in the first stage. For the incident polarization plane,
θ1 = 0 °, θ2 = 45 °
When,
θ1 = 45 °, θ2 = 0 °
The polarization switching element characterized by repeating the above.   A light beam splitting element comprising the polarization switching element according to any one of claims 1 to 3 and polarization separation means. The light beam splitting element according to claim 4,
The light beam splitting element, wherein the polarization separating means comprises a polarizing beam splitter and a deflecting prism. The light beam splitting element according to claim 5.
The light beam splitting element, wherein the polarization beam splitter is a polarization-dependent hologram.   An optical scanning device comprising the light beam splitter according to any one of claims 4 to 6, a multistage rotary polygon mirror, and an imaging optical system using at least an fθ lens.   An image forming apparatus comprising the optical scanning device according to claim 7.

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