CN112198665B

CN112198665B - Array waveguide near-to-eye display device

Info

Publication number: CN112198665B
Application number: CN202011175559.XA
Authority: CN
Inventors: 程德文; 王其为; 汪希萌; 倪栋伟
Original assignee: Beijing Ned+ Ar Display Technology Co ltd
Current assignee: Beijing Ned+ Ar Display Technology Co ltd
Priority date: 2020-10-27
Filing date: 2020-10-27
Publication date: 2022-10-18
Anticipated expiration: 2040-10-27
Also published as: CN112198665A

Abstract

The embodiment of the invention provides an array waveguide near-to-eye display device, which comprises an optical projection part and a tabular waveguide sheet, wherein the waveguide sheet is internally provided with an array light splitting sheet which forms a preset angle with the plane of the waveguide sheet, and image light coupled into the waveguide sheet is transmitted in a preset direction and is emitted to human eyes; the optical projection part comprises a polarization state control light path and a rear group compensating mirror comprising at least one lens, wherein the rear group compensating mirror is positioned between the first lens and the second lens of the polarization state control light path or positioned on one side of the polarization state control light path close to the emitting side of the image source. The near-to-eye display device provided by the invention is short in length, small in size and easy to wear.

Description

Array waveguide near-to-eye display device

Technical Field

The invention relates to the field of optics, in particular to an array waveguide near-to-eye display device.

Background

Since the concepts of Virtual Reality (VR) and Augmented Reality (AR) have been proposed, head-mounted image display devices based on VR or AR modes have been developed, and consumer-grade products such as Samsung gear VR, sony PSVR, epson BT300, microsoft holllens and the like have appeared in recent years, and since these head-mounted display devices need to be worn on the head of an observer when in use, compactness and lightness have been constantly pursued in the industry to reduce the load on the observer and improve usability.

The Lumus company of israel produces a light and thin array optical waveguide, which is advocated as a typical light and thin scheme, incident image light can be dispersed to a preset field angle through an array by a plurality of beam splitting surfaces in a waveguide sheet, and simultaneously, the expansion of an exit pupil is realized by utilizing the reflection and emergence of the plurality of beam splitting surfaces which are parallel to each other, so as to obtain an AR optical element with the thickness of less than 2 mm.

The one-dimensional array type waveguide element can only realize the expansion of the exit pupil of light rays in the waveguide transmission direction, the exit pupil perpendicular to the waveguide transmission direction is gradually reduced along with the transmission of the light rays in the waveguide, and in order to ensure the exit pupil diameter at the position worn by human eyes, the exit pupil of a projection amplification system in front of the waveguide element perpendicular to the waveguide transmission direction needs to be large enough.

As shown in fig. 1, which is an optical projection scheme commonly adopted in an optical projection system of a conventional waveguide near-eye display system, light emitted by an LED lamp panel 16 is split by a splitting surface 141 of a PBS prism 14 and then enters an LCoS 15, the light is reflected by the LCoS and then enters an arc reflecting surface 121 of a lens 12 through a PBS prism 13, then is reflected by a half mirror 131 and then is coupled and enters a waveguide 11, and finally is coupled and exited by a waveguide array beam splitter 111 and enters human eyes. The traditional refraction type coaxial optical projection system has the problems of long overall length and large volume because the light path is not folded.

Disclosure of Invention

In view of the above problems, an embodiment of the present invention provides an arrayed waveguide near-eye display device with a short length and a small volume, including an optical projection unit and a planar waveguide sheet, where the optical projection unit is configured to receive image light and project the image light toward a direction of the waveguide sheet to be coupled into the waveguide sheet, and the waveguide sheet is internally provided with an arrayed light splitting sheet forming a predetermined angle with a plane of the waveguide sheet, and transmits the coupled image light in a predetermined direction to emit the image light to human eyes; the optical projection part comprises a polarization state control light path realized by a first lens and a second lens, and a rear group compensating mirror comprising at least one lens, wherein the rear group compensating mirror is positioned between the first lens and the second lens or between the polarization state control light path and the micro display.

Further, the polarization state control optical path couples the image light into the waveguide sheet after 2 times of refraction and reflection.

Further, a 1/4 phase retarder and a reflection-type polarization splitting plate are attached to the front surface of the first lens of the polarization state control light path from inside to outside in sequence.

Furthermore, a light splitting film is attached to the rear surface of the second lens of the polarization state control light path.

Furthermore, a polarizer is arranged in or outside the micro-display, and a 1/4 phase delay plate is pasted on the polarizer.

Furthermore, the surface types of the lenses in the polarization state control optical path and the rear group of compensation lens group are free-form surfaces, spherical surfaces or aspheric surfaces.

Further, the microdisplay is a self-emitting display.

Further, when the microdisplay is DLP or Lcos, the arrayed waveguide near-to-eye display device further includes a PBS splitter at a predetermined angle with respect to the optical axis of the optical projection unit, for reflecting polarized light emitted from the backlight source located below the PBS splitter to the microdisplay and transmitting image light emitted from the microdisplay and modulated by the polarized light; the backlight source is arranged at a preset angle with the horizontal plane and used for emitting polarized light.

Further, when the microdisplay is a DLP or Lcos, the arrayed waveguide near-eye display device further includes: the PBS beam splitter prism is close to the image light emitting side of the micro display, the beam splitting surface of the PBS beam splitter prism faces the backlight source, and the PBS beam splitter prism is used for reflecting the received linearly polarized light emitted by the backlight source to the micro display and projecting the image light which is emitted by the micro display and modulated by the linearly polarized light; and the backlight source is horizontally arranged below the PBS beam splitter prism and is used for emitting linearly polarized light.

Further, the polarization state control light path, the rear group compensating mirror and the micro display are coaxial.

By adopting the array waveguide near-to-eye display device, the polarization state of light is changed through the refraction and reflection of image light in the designed optical projection part, the length of the optical projection part is shortened, and the volume of the array waveguide near-to-eye display device is reduced.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, and it is obvious that the drawings in the following description are some embodiments of the present invention, and those skilled in the art can also obtain other drawings according to the drawings without creative efforts.

FIG. 1 is a schematic diagram of a prior art near-to-eye display device with arrayed waveguides;

fig. 2 and fig. 3 are schematic structural diagrams of the arrayed waveguide near-eye display device according to the present invention;

FIG. 4 is a YOZ plane optical path diagram of an optical projection portion of the near-eye display device of the present invention;

FIG. 5 is a XOZ plane optical path diagram of an optical projection portion of a near-eye display device according to the present invention;

FIG. 6 is a schematic diagram illustrating the polarization state change of image light according to the present invention;

FIG. 7 is a schematic structural diagram illustrating an optical projection section according to still another design parameter of the first embodiment of the present invention;

FIG. 8 is a schematic structural diagram of a second embodiment of an optical projection unit according to the present invention;

FIG. 9 is a schematic diagram of a near-to-eye display device in the form of eyeglasses;

fig. 10 and 11 are schematic wearing diagrams of the near-eye display device;

FIG. 12 is a schematic view of the simultaneous wearing of corrective eyeglasses and a near-eye display device;

FIG. 13 is a schematic structural diagram of a modification of the optical projection unit according to the present invention;

FIG. 14 is a schematic structural diagram of a third embodiment of an optical projection unit according to the invention.

Detailed Description

In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are some, but not all, embodiments of the present invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

First embodiment

Fig. 2 and 3 are schematic diagrams illustrating an arrayed waveguide near-eye display device according to the present invention, which correspond to the left and right eyes of a human respectively, and the left and right eyes are symmetrical in structure. The arrayed waveguide near-to-eye display device comprises a waveguide sheet 1 and an optical projection part 2, wherein the waveguide sheet 1 is provided with an array beam splitter which is parallel to the plane of the waveguide sheet and forms a preset angle with the plane of the waveguide sheet, the structure of the optical projection part 2 is shown in fig. 4 and 5, wherein fig. 4 is a YOZ plane optical path diagram of the optical projection part 2, fig. 5 is an XOZ plane optical path diagram of the optical projection part 2, as shown in the figure, the optical projection part 2 comprises a polarization state control optical path and a rear group compensating mirror which are coaxially arranged with the waveguide sheet 1 from near to far, wherein the polarization state control optical path is used for converting the polarization state and the direction of image light, the polarization state control optical path is realized by a first lens 21 close to the waveguide sheet 1 and a second lens 22 far from the waveguide sheet 1, the rear group compensating mirror is used for correcting aberration and controlling the focal length and comprises at least one lens, in the embodiment of the invention, the rear group compensating mirror comprises a third lens 23 and a fourth lens 24, wherein the third lens 23 is close to the second lens 21, the second lens 22 and the rear group compensating mirror can be a free surface or a free surface type spherical surface. The arrayed waveguide near-to-eye display device of the present embodiment emits image light from a self-emitting microdisplay, such as an OLED type microdisplay, and those skilled in the art can understand that the same backlight type LCD and LED type microdisplay have the same image display surface as the OLED type microdisplay, and are also applicable to the present embodiment.

In the present invention, the surface of the lens in the optical projection portion away from the waveguide is the rear surface, the surface close to the waveguide is the front surface, the surface of the lens in the optical projection portion is the front surface, the first circularly polarized light reaches the 1/4 phase retardation plate on the front surface of the first lens 21 and is converted into the second circularly polarized light after reaching the first lens 21, the second circularly polarized light is reflected by the reflective type polarization plate and reaches the 1/4 phase retardation plate again, the second circularly polarized light is converted into the second circularly polarized light after reaching the first lens 21 through the first lens, the second circularly polarized light is converted into the third circularly polarized light after passing through the second lens, and the third circularly polarized light is reflected by the reflective type polarization plate and is reflected by the second polarization plate, the third circularly polarized light is reflected by the second polarization plate, and is converted into the third circularly polarized light after passing through the first circularly polarized light, and the third circularly polarized light is reflected by the first circularly polarized light after passing through the second polarization plate, and is reflected by the reflective type polarization plate, and then is converted into the third circularly polarized light after passing through the first circularly polarized light after passing through the second polarization plate, and the second circularly polarized light is reflected by the reflective type polarization plate, and is converted into the third circularly polarized light.

The image light is coupled and emitted to human eyes through the array light splitting sheet which is arranged in the waveguide sheet 1 and forms a certain angle with the plane of the approximately flat plate, the array light splitting sheet comprises at least two light splitting sheets, the angle of the array light splitting sheet is set according to the requirement, and the image light can be reflected out of the waveguide through total reflection. The optical projection part of the invention utilizes the catadioptric transmission of light rays among the on-axis lenses, and shortens the length and the volume of the optical projection part without introducing extra off-axis aberration. In addition to the relatively thin waveguide elements, the overall near-to-eye display device generally takes on the appearance of ordinary eyeglasses.

In this embodiment, the surface parameters of the optical lenses are shown in table 1, the

surface marks

211 and 212 are the front and back surfaces of the first lens 21, the

surface marks

221 and 222 are the front and back surfaces of the second lens 22, and so on, and the surface mark 25 is the front surface of the microdisplay, and with this parameter design, as can be seen from fig. 2 and 3, the optical projection portion is located on one side of the array waveguide, in this embodiment, the total length of the array waveguide near-to-eye display device is 28mm, the size is greatly reduced, the total length refers to the length of the optical projection portion, and is determined by the polarization state control optical path, the rear group of compensation mirrors, and the distance between the two.

Table 1:

surface marking	Surface type	Radius of the pipe	Thickness of	Refractive index	Abbe number
							211	Spherical surface	Infinite number of elements	3.6	1.52	48.5
212	Spherical surface	-19.2	0.5
						221	Spherical surface	-17.5	1.5	1.79	44.5
222	Spherical surface	-58.8	2.3
						231	Spherical surface	-55.2	4.0	1.49	75.6
232	Spherical surface	-18.6	1.0
						241	Spherical surface	16.3	6.6	1.49	75.6
242	Spherical surface	-80.9	8
						Image plane (25)	Spherical surface	Unlimited in size

The second set of parameters for each optical lens of this embodiment is shown in table 2, and with this set of parameters, the overall length of the optical projection portion is 16mm, and the structure is shown in fig. 7, and it can be seen that the overall length is greatly reduced by optimizing the power distribution.

Table 2:

surface marking	Surface type	Radius of	Thickness of	Refractive index	Abbe number
							211	Spherical surface	Infinite number of elements	4.0	1.49	75.6
212	Spherical surface	-31.4	1.8
						221	Spherical surface	-17.1	1.5	1.51	79.6
222	Spherical surface	-37.3	0.6
						231	Spherical surface	-29.2	2.0	1.49	75.6
232	Spherical surface	-18.9	1.0
						241	Spherical surface	9.1	4.0	1.49	75.6
242	Spherical surface	-69.5	0.2
						Image plane (25)	Spherical surface	Infinite number of elements

Second embodiment

Unlike the first embodiment, the microdisplay in the optical projection portion in the second embodiment of the present invention is a DLP display or Lcos display, and it is not a self-luminous display, and an illumination system 26 in front of the image emission surface is needed, as shown in fig. 8, the optical projection portion in this embodiment includes a polarization control optical path, a rear compensation mirror, and an illumination system, which are arranged in sequence. The polarization control light path is used for converting the polarization state and direction of image light, and comprises a first lens 21 with a front surface sequentially coated with a 1/4 phase retarder and a polarization beam splitter from inside to outside, and a second lens 22 with a rear surface coated with a beam splitting film, wherein the rear group of compensation lenses comprise at least one lens, in this example, a third lens 23 and a fourth lens 24, and the surface types of the first lens 21, the second lens 22, the third lens 23 and the fourth lens 24 can be free curved surfaces, preferably spherical surfaces or aspherical surface types. The microdisplay is a DLP or Lcos display, a polarizer is arranged inside or outside, a 1/4 phase delay plate is attached to the polarizer, the illumination system 26 is located between the microdisplay and the rear group compensating mirror, and comprises a PBS splitter 261 and a backlight 262, the backlight 262 and the horizontal plane form a predetermined angle, a typical exemplary predetermined angle value is set to 45 degrees, a polarizing plate is attached to the polarizer, the emitted linearly polarized light is reflected by the PBS splitter 261 and then irradiates the microdisplay, the PBS splitter 261 and the optical axis of the optical projection part form a predetermined angle, the linearly polarized light is modulated by image information after passing through the microdisplay to form image light with different light intensity distributions, the image light enters the rear group compensating mirror after passing through the PBS splitter again, passes through the rear group compensating mirror, is reflected for 2 times by the polarization state control light path and then is coupled into the waveguide through the first lens, wherein the refraction and reflection of the image light and the polarization state control light path are the same as those of the first embodiment.

By adopting the way of matching the optical projection part and the waveguide piece, the whole length and the volume of the optical projection part are smaller because the waveguide element is thinner, and the light near-to-eye display device with the appearance of common glasses can be realized by matching the optical projection part and the waveguide piece, as shown in fig. 9, the near-to-eye display device is in the form of glasses and comprises the array waveguide, and fig. 10 and 11 are wearing schematic diagrams, so that the appearance of the array waveguide near-to-eye display device is not obviously different from that of the normal glasses, the shielding of the waveguide optical projection part on the side visual field of a user is reduced by about 1/2, more gaps are formed in the side area, the user can wear the array waveguide near-to-eye display device after wearing vision correction glasses, as shown in fig. 12, the problem of vision correction is solved, and the requirements of more users are met.

Modification example

A modification of the optical projection section according to the second embodiment of the present invention is shown in fig. 13, and includes a polarization state control optical path, a rear group compensating mirror and a microdisplay, wherein the rear group compensating mirror includes one lens, and a fifth lens 27, and the optical projection section further includes an illumination system, and the illumination system includes a PBS beam splitter 263 and a backlight 262. The micro-display is internally or externally provided with a polarizer, a 1/4 phase retarder is pasted on the polarizer, the front surface of a first lens 21 in a polarization state control light path is sequentially attached with the 1/4 phase retarder and a reflection type polarization beam splitter from inside to outside, and the rear surface of a second lens 22 is attached with a beam splitting film. The first lens 21, the second lens 22 and the fifth lens 27 in the rear compensation mirror may be a free-form surface, preferably, a spherical or aspherical surface. The PBS beam splitter prism 263 is located between the microdisplay and the third lens 23, and the backlight is placed horizontally below the PBS beam splitter prism 263. The microdisplay makes an angle with the splitting plane in the PBS splitting prism 263 less than 90 degrees.

The backlight 262 emits linearly polarized light, which is reflected to the microdisplay through the splitting surface of the PBS splitting prism 263, and modulated by the microdisplay to form image light with different light intensity distributions, and the image light reaches the rear compensating mirror through the PBS splitting prism 263, and then is coupled into the waveguide after 2-fold reflection of the polarization state control light path. Here, the effect of the polarization state control optical path and the rear group compensation mirror on the image light is the same as the foregoing embodiment.

The parameters of the optical surfaces in this embodiment are shown in table 3, the

surface markers

211 and 212 are the front and back surfaces of the first lens 21, the

surface markers

221 and 222 are the front and back surfaces of the second lens 22, and so on \8230;, the

surface markers

2611 and 2622 are the front and back surfaces of the PBS beam splitter prism, the surface marker 25 is the front surface of the microdisplay, and the overall length of the optical projection part is 27mm by adopting the parameter design described in this embodiment.

Table 3:

surface marking	Surface type	Radius of the pipe	Thickness of	Refractive index	Abbe number
							211	Spherical surface	Unlimited in size	4.0	1.49	75.6
212	Spherical surface	-25.2	0.85
						221	Spherical surface	-21.1	4.0	1.51	62.6
222	Spherical surface	-65.9	1.3
						271	Spherical surface	37.8	4.0	1.65	45.6
272	Spherical surface	-40.2	0.1
						2611	Spherical surface	Unlimited in size	12	1.51	64.1
2622	Spherical surface	Infinite number of elements	0.2
						Image plane (25)	Spherical surface	Infinite number of elements

Third embodiment

The optical projection unit according to the third embodiment of the present invention is different from the first two embodiments and the modifications, and includes a polarization state control optical path, a rear group compensation mirror and a microdisplay, as shown in fig. 14, wherein the polarization state control optical path includes a first lens 21 with a front surface sequentially coated with a 1/4 phase retarder and a polarization beam splitter from inside to outside, and a second lens 22 with a rear surface coated with a beam splitting film, the rear group compensation mirror includes a lens, a fifth lens 27, the microdisplay is a self-emission or transmission microdisplay, and a built-in or external polarizer, and the polarizer is attached with a 1/4 phase retarder. The rear group of compensation mirrors is located between the first lens 21 and the second lens 22 of the polarization state control optical path. The image light emitted by the micro display is first linearly polarized light, passes through the 1/4 phase delay plate, the second lens, the rear group compensating mirror and the first lens on the polarizer, then reaches the second lens again after being refracted and reflected by the first lens, is transmitted by the first lens after being reflected by the second lens, and is coupled into the waveguide plate. Then, the total reflection is carried out through the array beam splitter in the waveguide chip, and finally, the image light is transmitted to enter human eyes. The state change of the image light is also the same as that of the first embodiment, and is not described herein again.

The first lens, the second lens and the third lens are free-form surfaces, and preferably, the surfaces can be spherical surfaces or aspherical surfaces.

In the present embodiment, the optical surface parameters are shown in table 4, the

surface markers

211 and 212 are the front and rear surfaces of the first lens 21, the

surface markers

221 and 222 are the front and rear surfaces of the second lens 22, and so on \8230;, the surface marker 25 is the front surface of the microdisplay, and the overall length of the optical projection portion is 15mm using the parameters shown in the following table.

TABLE 4

Surface marking	Surface type	Radius of	Thickness of	Refractive index	Abbe number
							211	Spherical surface	Unlimited in size	4.5	1.71	55.3
212	Spherical surface	-65.1	0.1
						221	Spherical surface	46.3	4.5	1.51	62.6
222	Spherical surface	-42.00	1.8
						271	Spherical surface	-20.1	1.5	1.80	39.6
272	Spherical surface	-65.1	1.75
						Image plane (25)	Spherical surface	Unlimited in size

In addition, environmental data can be acquired through a built-in sensor, an inertia measurement unit and the like, and the environmental data are displayed in front of human eyes after being processed by a controller, so that the functions of the array waveguide near-eye display device are enriched and expanded.

Finally, it should be noted that: the above examples are only intended to illustrate the technical solution of the present invention, but not to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some technical features may be equivalently replaced; and such modifications or substitutions do not depart from the spirit and scope of the corresponding technical solutions of the embodiments of the present invention.

Claims

1. An array waveguide near-eye display device comprises an optical projection part and a planar waveguide sheet, wherein the waveguide sheet is internally provided with a plurality of array light splitting sheets which form preset angles with the plane of the waveguide sheet, and image light coupled into the waveguide sheet is transmitted in a preset direction and is emitted to human eyes; the optical projection part is used for receiving the image light and projecting the image light to the direction of the waveguide sheet so as to be coupled into the waveguide sheet;

it is characterized in that the preparation method is characterized in that,

the optical projection part comprises a polarization state control light path realized by a first lens and a second lens, and a rear group compensating mirror consisting of one or two lenses; the first lens is close to the waveguide, the first lens is a plano-convex lens protruding to the image source side, the second lens is far away from the waveguide, the second lens is a negative meniscus lens protruding to the image source side, the polarization state control light path is used for converting the polarization state and direction of image light, and the polarization state control light path couples the image light into the waveguide after 2 times of folding and reflecting between two surfaces of the first lens and the second lens which are mutually deviated; the rear group of compensation mirrors are positioned between the first lens and the second lens or positioned on one side of the polarization state control light path close to the image light emitting side.

2. The arrayed waveguide near-to-eye display device of claim 1, wherein a 1/4 phase retarder and a reflective polarization splitting plate are attached to a front surface of the first lens of the polarization state control optical path, the front surface being close to the waveguide plate, in this order from inside to outside.

3. The arrayed waveguide near-to-eye display device of claim 2, wherein a light splitting film is attached to a rear surface of the second lens in the polarization state control light path away from the waveguide sheet.

4. The arrayed waveguide near-to-eye display device of claim 1, wherein the surface types of the lenses in the polarization state control optical path and the rear group compensating lens group are free-form surfaces, spherical surfaces or aspherical surfaces.

5. The arrayed waveguide near-to-eye display device of claim 1, wherein the rear group of compensation mirrors is located on a side of the polarization control optical path close to the image light emitting side, and the rear group of compensation mirrors comprises a third lens and a fourth lens, wherein the third lens is a positive meniscus lens and is convex toward the fourth lens; the fourth lens is close to the image source and is a biconvex lens, and the radius of the surface facing the image source is larger than that of the surface facing the waveguide sheet.

6. The arrayed waveguide near-to-eye display device of claim 1, wherein the rear set of compensation mirrors is one in number, the rear set of compensation mirrors being lenticular lenses.

7. The arrayed waveguide near-to-eye display device of any one of claims 2-6, further comprising a microdisplay to provide image light, the microdisplay having an internal or external polarizer with a 1/4 phase retarder applied thereto.

8. The arrayed waveguide near-eye display device of claim 7, wherein the microdisplay is a self-emitting light display.

9. The arrayed waveguide near-eye display device of claim 7, wherein the microdisplay is a DLP or Lcos, the arrayed waveguide near-eye display device further comprises,

the PBS beam splitter which forms a preset angle with the optical axis of the optical projection part is used for reflecting the polarized light emitted by the backlight source positioned below the PBS beam splitter to the micro display and transmitting the image light which is emitted by the micro display and modulated by the polarized light;

the backlight source is arranged at a preset angle with the horizontal plane and used for emitting polarized light.

10. The arrayed waveguide near-eye display device of claim 7, wherein the microdisplay is a DLP or Lcos, the arrayed waveguide near-eye display device further comprising:

the PBS beam splitter prism is close to the image light emitting side of the micro display, the beam splitting surface of the PBS beam splitter prism faces the backlight source, and the PBS beam splitter prism is used for reflecting the received linearly polarized light emitted by the backlight source to the micro display and projecting the image light emitted by the micro display and modulated by the linearly polarized light;

and the backlight source is horizontally arranged below the PBS beam splitting prism and is used for emitting linearly polarized light.

11. The arrayed waveguide near-to-eye display device of claim 7, wherein the polarization state control optical path, the back group compensator mirror, and a microdisplay are coaxial.