JP6810650B2 - Video projection device - Google Patents

Description

The present invention relates to an image projection device using a light source that combines and emits visible light such as R (red light), G (green light), and B (blue light) in the same polarized state.

In recent years, the development of a small RGB light source using a laser diode (LD) that outputs light of the three primary colors of R (red light), G (green light), and B (blue light) has been developed for eyeglass terminals and pico projectors. It is done. Since the LD has higher straightness than the LED, it is possible to realize a focus-free projector. In addition, LD has high luminous efficiency, low power consumption, and high color reproducibility, and has been attracting attention in recent years.

FIG. 1 shows a schematic view of an image projection device using a liquid crystal device. FIG. 1 shows an image projection device including the LD1 and the space modulation optical element 10. The space modulation optical element 10 includes an input side lens 2, a polarization beam splitter (PBS) 3, a polarizer 4, an LCOS (Liquid Crystal on Silicon) 5, and an output side lens 6. FIG. 1 shows an example of an image projection device using LCOS as a liquid crystal device.

In the image projection device as shown in FIG. 1, an image is projected by controlling the intensity of light for each pixel by changing the birefringence index using the liquid crystal orientation of LCOS5. The mainstream of image projection using a liquid crystal device is a method in which an RGB color filter is provided for each pixel, or a method in which RGB light is time-divided and irradiated on a liquid crystal, and each image is superimposed as afterimage light. In either case, the light incident on the liquid crystal device must be polarized.

A. Nakao, R. Morimoto, Y. Kato, Y. Kakinoki, K. Ogawa and T. Katsuyama, “Integrated waveguide-type red-green-blue beam combiners for compact projection-type displays”, Optics Communications 330 (2014) 45-48 Y. Hibino, “Arrayed-Waveguide-Grating Multi / Demultiplexers for Photonic Networks,” IEEE CIRCUITS & DEVICES, Nov., 2000, pp.21-27 A. Himeno, et al., “Silica-Based Planar Lightwave Circuits,” J. Sel. Top. Q.E., vol. 4, 1998, pp.913-924 J. Sakamoto et al. “High-efficiency multiple-light-source red-green-blue power combiner with optical waveguide mode coupling technique,” Proc. Of SPIE Vol. 10126 101260M-2

In LD, since the output of R and G is weaker than that of B, an RGB light source in which two R and G are polarized and multiplexed may be used in order to increase the output. In such an RGB light source, usually, when light of the same color is combined, it is combined by using polarization multiplexing, so that the output light includes both vertically polarized light and horizontally polarized light.

However, a spatial optical element having a polarization dependence such as a liquid crystal device is usually used in an image projection device, and either the vertically polarized or horizontally polarized polarization component of the output light is cut. Therefore, when a video projection device is configured by using an RGB light source that performs such polarization multiplexing and a spatial optical element having polarization dependence such as a liquid crystal, there is a problem that the output cannot be increased. did.

On the other hand, an RGB light source that performs such polarization multiplexing may be used in a video projection device using a digital mirror device (DMD) or the like that operates independently of polarization, but the DMD is a liquid crystal. On the other hand, it is expensive and has problems such as a color breaking phenomenon.

The present invention aims to increase the light output by using a visible light coupler that combines light of the same color and the same polarization in an image projection device using a spatially modulated optical element having polarization dependence. The purpose is to provide a realized image projection device.

The image projection device according to one aspect of the present invention is a plurality of visible light sources that output light of the same polarization, and includes at least one set of visible light sources that output light of the same color. A visible light coupler using a plane light wave circuit that combines the light output from the plurality of visible light sources and outputs the combined wave light while maintaining the polarization state, and the combination output from the visible light coupler. It is characterized by including a spatially modulated optical element having polarization dependence, which receives wave light, spatially modulates the combined wave light, and outputs it as a video output.

According to the present invention, since it is possible to combine and emit light of the same color with the same polarization by using PLC, a space-modulation optical element having polarization dependence such as a liquid crystal device is used. Even in this case, it is possible to achieve high output.

Further, according to the present invention, interference is avoided without affecting the image quality by controlling the LD drive timing so that the light emission timings of the LDs that output the same color of light are different when performing the combined wave. , It is possible to prevent signal deterioration.

It is the schematic of the image projection apparatus using a liquid crystal device. It is a figure which illustrates the image projection apparatus which concerns on Example 1 of this invention. It is a figure which illustrates the image projection apparatus which concerns on Example 2 of this invention. The relationship between the effective refractive index of light in the 0th-order, 1st-order, and 2nd-order modes and the width of the waveguide for each wavelength in the RGB coupler of the image projection apparatus according to the second embodiment of the present invention is shown. It is a figure which illustrates the image projection apparatus which concerns on Example 3 of this invention.

(Example 1)
FIG. 2 illustrates an image projection device according to a first embodiment of the present invention. In FIG. 3, a quartz-based plane lightwave circuit (PLC) type optically connected to the first to fifth visible light sources 101 to 105 and the first to fifth visible light sources 101 to 105 is shown. An image projection device 100 including an RGB coupler 110 and a spatially modulated optical element 120 having polarization dependence, which spatially modulates the output light from the RGB coupler 110 and outputs it as an image output, is shown. The spatial modulation optical element 120 includes an input side lens 121 optically connected to the RGB coupler 110, a PBS 122, a polarizer 123, an LCOS 124, and an output side lens 125.

The RGB coupler 110 includes first to fifth input waveguides 111 to 115 optically connected to the first to fifth visible light sources 101 to 105, and first to fifth input waveguides 111 to 115, respectively. Includes a combiner 116 that combines and outputs the light propagating in the light, and an output waveguide 117 that outputs the light combined by the combiner 116 to the input side lens 121. The output waveguide 117 is composed of a multimode waveguide.

The first to fifth visible light sources 101 to 105 output, for example, green light G2 and G1, blue light B, and red light R1 and R2 in one polarized state of horizontally polarized light and vertically polarized light, respectively. Can be done. Lights R1, G1, B, R2 and G2 are lights having the same polarization state. Further, the light B, R1 and G1 are light having wavelength ranges of 400 to 495 nm, 495 to 570 nm and 620 to 750 nm, respectively, and G2 and R2 are light having wavelengths of 495 to 570 + β nm and 620 to 750 + γ nm, respectively. Β and γ were set to minute values of 0 or more and line width or less, and the line width was set to approximately 1 nm or less depending on the light source of the incident light. The matters described above for the light R1, G1, B, R2 and G2 are similarly applicable to the following examples.

The lights G2, G1, B, R1 and R2 output from the first to fifth visible light sources 101 to 105 enter the RGB coupler 110 and pass through the first to fifth input waveguides 111 to 115, respectively. It is combined by the wave combining unit 116 and emitted from the output waveguide 117. The emitted combined wave light is collimated by the input side lens 121 and incidents on the LCOS 124 via the PBS 122 and the polarizer 123, is modulated and reflected by the LCOS 124, and is incident on the output side lens 125 via the polarizer 123 and the PBS 122. The image is output by collimating.

In the present invention, the light from each LD is combined by using the PLC type RGB coupler 110. In recent years, an RGB coupler module using a PLC has attracted attention (see, for example, Non-Patent Document 1). PLC creates an optical waveguide on a flat substrate such as Si by patterning by photolithography and reactive ion etching processing, and multiple basic optical circuits (for example, directional coupler, Mach-Zehnder interferometer, etc.) ) Can be combined to realize various functions (see, for example, Non-Patent Documents 2 and 3).

PLC has birefringence because the substrate is warped due to the stress difference due to different materials because the cladding layer and the core layer are deposited on the substrate such as Si to form a waveguide. Therefore, the horizontally polarized or vertically polarized light propagating in the PLC propagates in the waveguide while maintaining the polarized state by the same principle as the polarization holding fiber (such as sandwiching a wave plate in the waveguide or sandwiching the wavelength plate in the waveguide). Polarization conversion is possible by creating a waveguide such as a twisted waveguide). Therefore, when light is incident on the PLC and combined with one of the horizontally polarized light and the vertically polarized light, the light combined with the PLC is emitted while maintaining the polarized state. Since this combined wave light is emitted with one of horizontally polarized light and vertically polarized light, it can be handled even when a liquid crystal device is used.

According to the image projection device 100 according to the first embodiment of the present invention, since the light is combined by using the PLC type RGB coupler 110, the light of the same color is combined and emitted with the same polarization. This makes it possible to achieve high output even when a spatially modulated optical element having polarization dependence such as a liquid crystal device is used.

In this embodiment, the red light and the green light are combined to enhance the light intensity, but the present invention is not limited to this, and the blue light may be combined to be combined. .. Further, in this embodiment, an example using five visible light sources is shown, but the present invention is not limited to this, and includes at least one set that outputs light of the same color among red light, blue light, and green light. As described above, a plurality of visible light sources may be used. The same applies to the following examples.

(Example 2)
FIG. 3 illustrates an image projection device according to a second embodiment of the present invention. In FIG. 3, the first to fifth visible light sources 201 to 205, the PLC type RGB coupler 210 optically connected to the first to fifth visible light sources 201 to 205, and the outputs from the RGB coupler 210 are shown. An image projection device 200 including a space modulation optical element 220 having polarization dependence, which spatially modulates light and outputs it as an image output, is shown. The spatial modulation optical element 220 includes an input side lens 221 optically connected to the RGB coupler 210, a PBS 222, a polarizer 223, an LCOS 224, and an output side lens 225. As the first to fifth visible light sources 201 to 205, for example, LD can be used.

The RGB coupler 210 includes first to fifth input waveguides 211 to 215, first and second directional couplers 231 and 232 having the same waveguide width, and third and fourth directional couplers having different waveguide widths. Includes directional couplers 233 and 234 of.

As shown in FIG. 3, the third input waveguide 213 is configured with a tapered waveguide structure so as to have _first and _second portions 213 ₁ and 213 ₂ having different waveguide widths. Further, the first, second, fourth and fifth input waveguides 211, 212, 214 and 215 are single-mode waveguides, and the third input waveguide 213 is a multi-mode waveguide.

Green light G2 and G1, blue light B, and red light R1 and R2 output from the first to fifth visible light sources 201 to 205 are incident on the first to fifth input waveguides 211 to 215, respectively. Will be done. R1, G1, B, R2 and G2 are all 0th-order mode lights having the same polarization state.

The first directional coupler 231 couples the light G1 incident from the second input waveguide 212 to the third input waveguide 213, and the light B incident from the third input waveguide 213 is the second. The waveguide length, waveguide width, and gaps between the waveguides are designed so as to couple to the input waveguide 212 of 2 and recouple to the third input waveguide 213.

The second directional coupler 232 couples the light R1 incident from the fourth input waveguide 214 to the third input waveguide 213, and the third input waveguide in the first directional coupler 231. The waveguide length, waveguide width, and gap between the waveguides are designed so as to transmit the light G1 and B coupled to the 213.

The third directional coupler 233 is arranged near the first portion 213 _{1 of} the third input waveguide 213. The fourth directional coupler 234 is located near the second portion 213 _{2 of} the third input waveguide 213.

The third and fourth directional couplers 233 and 234 are mode couplers composed of asymmetric directional couplers having different waveguide widths (see, for example, Non-Patent Document 4). In the mode coupler, the effective refractive index of the 0th-order mode light in the narrow waveguide with the narrower waveguide width and the effective refractive index of the higher-order mode light in the wider waveguide with the wider waveguide match. At this time, the light of the 0th order mode propagating in the narrow waveguide can be 100% coupled to the adjacent wide waveguide while being converted into the light of the higher order mode. On the other hand, the effective refractive index of the 0th-order mode light propagating in the wide waveguide does not match the effective refractive index of the 0th-order mode light in the narrow waveguide, so that the light is coupled to the adjacent narrow waveguide. It passes through the mode coupler without doing anything.

To function as a mode coupler, the third directional coupler 233 has an effective index of refraction of light G2 with respect to the first input waveguide 211 in the 0th order mode and a first portion 213 of the third input waveguide 213. The effective refractive index of the light G1, B, R1 in the higher-order mode with respect to the first portion 213 ₁ of the third input waveguide 213 so that the effective refractive index of the light G2 with respect to ₁ is equal to that in the higher-order mode. The width of the first input waveguide 211 and the width of the first portion 213 ₁ are set so that the effective refractive index of the light G1, B, and R1 with respect to the first input waveguide 211 in the 0th-order mode is not equal to each other. Has been done.

Similarly, in order to function as a mode coupler, the fourth directional coupler 234 has an effective index of refraction of light R2 with respect to the fifth input waveguide 215 in the 0th order mode and a second of the third input waveguide 213. In the higher-order mode of the light G1, B, R1 with respect to the second part 213 ₂ of the third input waveguide 213, so that the effective refractive index of the light R2 with respect to the portion 213 ₂ is equal to that of the third input waveguide 213. The fifth input waveguide 215 and the second portion 213 ₂ waveguide so that the effective index of refraction and the effective refractive index of the light G1, B, R1 with respect to the fifth input waveguide 215 in the 0th-order mode are not equal. The width is set.

The operation of the RGB coupler 210 shown in FIG. 3 will be described. The 0th-order mode light G1 incident from the second input waveguide 212 is coupled to the third input waveguide 213 in the first directional coupler 231 having the same waveguide width, and the third input guide is provided. The 0th-order mode light B incident from the waveguide 213 is coupled to the second input waveguide 212 in the first directional coupler 231 and recoupled to the third input waveguide 213.

The 0th-order mode light R1 incident from the fourth input waveguide 214 is coupled to the third input waveguide 213 in the second directional coupler 232 having the same waveguide width, and is coupled to the third input waveguide. Lights G1 and B propagating through the waveguide 213 are transmitted through the second directional coupler 232. As a result, the lights G1, B, and R1 incident on the first to third input waveguides 211 to 213 are combined via the first and second directional couplers 231 and 232 to form a third. It is input to the first portion 213 ₁ of the input waveguide 213.

On the other hand, the light G2 in the 0th-order mode propagating in the first input waveguide 211 is converted into a higher-order mode in the third directional coupler 233 that functions as a mode coupler, and the waveguide mode is converted to the higher-order mode. It couples to the first portion 213 ₁ of the input waveguide 213. The coupled higher-order mode light G2 is combined with the 0th-order mode light G1, B, R1 propagating in the first portion 213 ₁ of the third input waveguide 213 to guide the third input. It is input to the second part 213 ₂ of the waveguide 213.

Further, the light R2 in the 0th order mode propagating through the 5th input waveguide 215 is converted into a higher order mode in the 4th directional coupler 234 functioning as a mode coupler, and the waveguide mode is converted to the higher order mode. It couples to the second portion 213 ₂ of the input waveguide 213. The coupled higher-order mode light R2 is further combined with the combined light propagating in the second portion 213 ₂ of the third input waveguide 213. As a result, the combined light of the light G2, G1, B, R1 and R2 incident on the first to fifth input waveguides 211 to 215 is transmitted from the output port of the third input waveguide 213 to the space modulation optical element 220. It is output.

As described above, high-intensity combined light can be output by combining light of the same color having different waveguide modes.

FIG. 4 shows the effective refractive index and waveguide width of light in the 0th-order, 1st-order, and 2nd-order modes for each wavelength λ1 = 450 nm, λ2 = 520 nm, and λ3 = 650 nm in the RGB coupler 210 according to the second embodiment. The relationship is shown. The film thickness of the waveguide was 3.6 μm, and the specific refractive index difference Δ was 0.45%.

For example, the gap between waveguides in the first directional coupler 231 is 2 μm, the gap between waveguides in the second directional coupler 232 is 4.7 μm, the first input waveguide 211 and the first portion 213 ₁ The gap between the waveguides in the fifth input waveguide 215 and the second portion 213 ₂ is 2.5 μm, and the bond lengths in the first and second directional couplers 231 and 232 are 702 μm, respectively. The bond length of the directional coupler 233 is 2380 μm, the bond length of the fourth directional coupler 234 is 900 μm, the width of the first and fifth input waveguides 211 and 215 is 1.5 μm, and the second It is assumed that the waveguide widths of the fourth input waveguides 212 and 214 are 2 μm. In this case, when the width of the waveguide having the same effective refractive index for each wavelength λ1 to λ3 is examined based on FIG. 4, the width of the waveguide of the first portion 213 ₁ is 7.15 μm, and the width of the waveguide of the second portion 213 ₂ is 7.15 μm. It can be seen that the RGB coupler 210 according to the second embodiment can be realized by setting the width of the waveguide to 8.00 μm.

According to the image projection device 200 according to the second embodiment of the present invention, it is possible to combine and emit light of the same color with the same polarization, and a spatially modulated optical element having polarization dependence such as a liquid crystal device. It is possible to realize high output even when using.

(Example 3)
FIG. 5 shows an image projection device according to a third embodiment of the present invention. FIG. 5 shows the output from the first to fifth visible light sources 301 to 305, the PLC type RGB coupler 310 optically connected to the first to fifth visible light sources 301 to 305, and the RGB coupler 310. An image projection device 300 including a polarization-dependent spatial modulation optical element 320 that spatially modulates light and outputs it as an image output is shown. The spatial modulation optical element 320 includes an input side lens 321 optically connected to the RGB coupler 310, a PBS 322, a polarizer 323, an LCOS 324, and an output side lens 325.

The RGB coupler 110 includes first to fifth input waveguides 311 to 315 optically connected to the first to fifth visible light sources 301 to 305, respectively, and first to fifth input waveguides 311 to 315. Includes a multimode waveguide 316, which is optically connected to.

According to the image projection device 300 according to the third embodiment, a simple configuration using the multi-mode waveguide 310 makes it possible to combine and emit light of the same color with the same polarization, such as a liquid crystal device. It is possible to achieve high output even when a spatially modulated optical element having polarization dependence is used.

(Example 4)
In the fourth embodiment, the control method of the first to fifth visible light sources in the image projection apparatus according to the first to third embodiments will be described.

Normally, polarization multiplexing is used to combine light of the same color, so that interference does not occur when light of the same color is combined. On the other hand, in the image projection apparatus according to the first to third embodiments, since the light of the same polarization is combined, the respective lights interfere with each other even if the wavelengths of the lights of the same color are aligned at the line width level. Such interference causes deterioration of the image.

Normally, since the oscillation wavelength of LD varies from individual to individual, the wavelength of light of the same color often differs by several nm (it is also possible to make such a combination), but LD is affected by heat during driving. Since the central wavelength of the output light shifts by about 5 to 10 nm, the wavelengths of the output light may match under specific circumstances, and the light may interfere with each other.

Therefore, in order to avoid interference, the output light of the LD, which outputs light of the same color at a high frequency that cannot be recognized by the human eye, is time-divided and driven so that the light of the same color does not emit at the same time. Interference can be avoided.

Specifically, the frequency that can be recognized by the human eye is said to be about 20 Hz, and the modulation frequency of LCOS is generally about 100 Hz. Therefore, the first to fifth visible light sources of the image projection apparatus according to the present invention are driven by using a time-division modulation signal of about 1 kHz, which is sufficiently higher than 100 Hz, so that light of the same color is not emitted at the same time. Therefore, it is possible to avoid interference without affecting the video quality.

Claims (5)

A plurality of visible light sources that output light of the same polarization and include at least one set of visible light sources that output light of the same color.
A visible light coupler using a plane light wave circuit that combines the light output from the plurality of visible light sources and outputs the combined wave light while maintaining the polarized state .
A spatially modulated optical element having polarization dependence, in which the combined wave light output from the visible light coupler is incident, and the combined wave light is spatially modulated and output as a video output.
An image projection device characterized by being equipped with. The image projection device according to claim 1, wherein the plurality of visible light sources are laser diodes. The image projection device according to claim 1 or 2, wherein the visible light coupler combines the light output from the plurality of visible light sources by using a multimode waveguide. The image projection device according to claim 1 or 2, wherein the visible light coupler uses a mode coupler to combine light of the same color. The invention according to any one of claims 1 to 4, wherein the plurality of visible light sources are driven by using a time division modulation signal having a frequency of 1 kHz or more so that the set of visible light sources do not emit light at the same time. Image projection device.

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