JP6269710B2 - Light modulation device - Google Patents

Description

  The present invention relates to a light modulation device that modulates light incident from one optical fiber by a light modulation element and emits the light from another optical fiber. In particular, the present invention relates to a light modulation device formed on a separate substrate or arranged on a single substrate. Integrated light having a plurality of formed light modulation elements, and combining the two modulated linearly polarized lights output from the plurality of light modulation elements, respectively, and outputting them from a single optical fiber. The present invention relates to a modulation device.

In high-speed / large-capacity optical fiber communication systems, an optical modulator incorporating a waveguide type optical modulation element is often used. In particular, a light modulation element using LiNbO ₃ (hereinafter also referred to as LN) having an electro-optic effect as a substrate can realize a light modulation characteristic with a small amount of light loss and a wide band, so that a high-speed / large capacity optical fiber Widely used in communication systems.

  In this light modulation element using LN, for example, a Mach-Zehnder type optical waveguide is formed on an LN substrate, and a high frequency signal is applied to an electrode formed in the vicinity of the optical waveguide, whereby a modulation signal corresponding to the high frequency signal is obtained. Light (hereinafter referred to as modulated light) is output. When such a light modulation element is used in an optical transmission device, a housing that houses the light modulation element, an incident optical fiber that makes light from the light source incident on the light modulation element, and an output from the light modulation element An optical modulation device composed of an outgoing optical fiber that guides the emitted light to the outside of the housing is used.

  As a modulation method in an optical fiber communication system, in response to a recent increase in transmission capacity, two linearly polarized lights whose polarization directions are orthogonal to each other are phase-shifted or orthogonally amplitude-modulated, respectively. Transmission formats incorporating polarization multiplexing such as DP-QPSK (Dual Polarization-Quadrature Phase Shift Keying) and DP-QAM (Dual Polarization-Quadrature Amplitude Modulation) are becoming mainstream.

  In an optical modulation device that performs such DP-QPSK modulation or DP-QAM modulation, linearly polarized light output from one light source is incident on the optical modulation element, and the linearly polarized light that is incident on the optical modulation element is input to the optical modulation element. The light is split into two lights and modulated using two independent high-frequency signals, and the two linearly polarized modulated lights thus modulated are combined by combining into one optical fiber for output.

  On the other hand, in order to further increase the transmission capacity of the optical transmission system, for example, after performing DP-QPSK modulation and DP-QAM modulation on a plurality of lights having different wavelengths, a plurality of light having different modulated wavelengths is obtained. A wavelength multiplexing system is conceivable in which the light beams are combined into a single light beam by a wavelength synthesizer and transmitted through a single optical fiber. In such an optical transmission device that modulates a plurality of lights and transmits the light through a single optical fiber, a plurality of light modulation elements (or a plurality of light beams) are provided in a single housing from the viewpoint of miniaturization of the device. It is desirable to provide an integrated light modulation device that includes an integrated light modulation element in which a modulation element is formed on one LN substrate and modulates a plurality of input lights and outputs a plurality of modulated lights.

  In this case, a polarization beam combiner for combining two light beams (linearly polarized light beams) emitted from each of a plurality of light modulation elements, or a beam emitted from the polarization beam combiner is coupled to an optical fiber. Since there is a need to secure a space for providing an optical component such as a lens, the distance between two linearly polarized light beams emitted from one light modulation element and two linearly polarized light beams emitted from another light modulation element is It needs to be expanded.

  Conventionally, as such an integrated light modulation device, two light modulation elements are provided, and two linearly polarized lights output from one light modulation element and two linear polarization lights output from another light modulation element. After expanding the distance between the wave light by two optical path shift prisms (prisms for translating the optical path), each of the two linearly polarized lights is polarized by a polarization synthesizing prism, etc. There is known an integrated light modulation device that outputs to the outside of a housing with a single optical fiber (Patent Document 1).

  In this light modulation device, the distances from the two light modulation elements to the two optical path shift prisms are made different from each other, so that damage to the optical components due to the two optical path shift prisms coming into contact with each other can be prevented.

  However, when configuring an optical modulation device, the viewpoint of improving the optical coupling efficiency between the optical modulation element and the outgoing optical fiber, the viewpoint of stabilization of temperature fluctuation and secular change of the optical coupling efficiency, and the device size From the viewpoint of miniaturization and reduction in device cost, it is desirable to reduce the number of optical components inserted into the optical path as much as possible.

  That is, the conventional integrated light modulation device still has room for improvement from the viewpoints of improvement and stabilization of optical characteristics, miniaturization, cost reduction, and the like.

JP2015-172630A

  Based on the above background, two modulated linearly polarized light components each having a plurality of light modulation elements formed on a separate substrate or arranged side by side on a single substrate, each output from the plurality of light modulation elements. In an integrated optical modulation device that synthesizes and outputs the signals from a single optical fiber, the optical characteristics can be improved and stabilized, and further improvements can be made from the viewpoints of miniaturization and cost reduction. Realization of a configuration that can be achieved is desired.

One aspect of the present invention receives a first light modulation element and a second light modulation element that respectively emit two output lights, and each of the four output lights emitted from the two light modulation elements. Polarization rotation that rotates the polarization of four lenses, one of the two output lights from the first light modulation element, and one of the two output lights from the second light modulation element An element, a first polarization beam combining element that combines the two output lights from the first light modulation element into one beam, and outputs the two output lights from the second light modulation element And a second polarization beam combining element that combines and outputs a single beam, and the light emitted from each of the four lenses passes through the optical path shift prism without passing through the polarization path. Wave rotator and / or first and second polarization synthesis Child, is configured to be incident directly.
According to another aspect of the present invention, the polarization rotation element includes a region through which one of the two output lights from the first light modulation element passes, and two of the two light output from the second light modulation element. And a region through which one of the output light passes.
According to another aspect of the present invention, there are provided first and second optical path shift elements for moving the optical paths of the beams output from the first and second polarization beam combining elements in directions away from each other.
According to another aspect of the present invention, the first light modulation element and the second light modulation element are arranged to emit the output light side by side, and the direction of the output to be emitted side by side The first polarization beam combining element and the second polarization beam combining element are arranged at line symmetrical positions with respect to the line segment. Yes.
According to another aspect of the present invention, an optical component composed of a parallel plate made of an optical medium between the four lenses and the polarization rotation element and / or the first and second polarization combining elements. Is arranged.
According to another aspect of the invention, the first and second light modulation elements are light modulation elements that perform phase shift keying or quadrature amplitude modulation.
According to another aspect of the present invention, the first and second light modulation elements are formed on different substrates or arranged side by side on the same substrate.
According to another aspect of the invention, the four exit lenses are an integrally formed microlens array.

It is a figure which shows the structure of the light modulation device which concerns on one Embodiment of this invention. FIG. 2 is a partial detail view around a microlens array in the light modulation device shown in FIG. 1.

Embodiments of the present invention will be described below with reference to the drawings.
FIG. 1 is a diagram showing a configuration of an optical modulation device according to an embodiment of the present invention. The optical modulation device 100 includes an optical modulator 102, incident optical fibers 104 a and 104 b that are optical fibers that input light from a light source (not shown) to the optical modulator 102, a microlens array 106, and a half-wave plate. , Polarization combining prisms 110a and 110b, optical path shift prisms 112a and 112b, coupling lenses 114a and 114b, outgoing optical fibers 116a and 116b, and a housing 118.

  Each of the incident optical fibers 104 a and 104 b enters, for example, linearly polarized light having two different wavelengths from two light sources (not shown) into the optical modulator 102.

  The light modulator 102 has two light modulation elements 120a and 120b formed of an optical waveguide formed on a single LN substrate. These light modulation elements 120a and 120b are light modulation elements that perform, for example, DP-QPSK modulation or DP-QAM modulation.

  As shown in FIG. 1, the light modulation elements 120a and 120b are arranged so that output light is emitted side by side. That is, in FIG. 1, the light modulation elements 120 a and 120 b emit all output light from the light modulation elements 120 a and 120 b side by side from the left end face 170 of the light modulator 102 in the left direction in the figure and in the vertical direction in the figure. As is done. Also. In the present embodiment, the light modulation elements 120a and 120b are arranged in line-symmetric positions with respect to the line segment 180 parallel to the direction of the output light emitted side by side.

  In this embodiment, the light modulation elements 120a and 120b are arranged so that all output light emitted from the light modulation elements 120a and 120b is emitted in a straight line in the vertical direction in FIG. However, the present invention is not limited to this, and the light emitted from the light modulation elements 120a and 120b may be arranged so as to have an arbitrary positional relationship as long as they are emitted “in line”. For example, the light modulation elements 120a and 120b are arranged such that the light emission end faces (left end faces in the figure in FIG. 1) of the light modulation elements 120a and 120b are shifted from each other by a predetermined distance in the horizontal direction in FIG. Also good. Further, for example, in the light modulation elements 120a and 120b, the light emission points from the light modulation elements 120a and 120b are in the substrate thickness direction of the light modulation elements 120a and 120b (direction perpendicular to the paper surface of FIG. 1). ) May be configured to be in different positions from each other.

  The light modulation element 120a is a first light modulation element, and the linearly polarized light incident from the incident optical fiber 104a is branched into two lights, modulated by different electrical signals, respectively, and then output waveguides 130a, Output from 132a. The light modulation element 120b is a second light modulation element, and the linearly polarized light incident from the incident optical fiber 104b is branched into two lights, modulated by different signals, respectively, and then output waveguides 130b. , 132b.

  A microlens that is four emission lenses is provided on the substrate end surface 170 on the light emission side of the optical modulator 102 (the substrate end surface on the side where the emission waveguides 130a, 132a, 130b, and 132b are formed (that is, on the left side in the drawing)). A microlens array 106 in which 140a, 142a, 140b, and 142b are integrally formed is disposed.

  Light output from the output waveguides 130a and 132a of the light modulation element 120a enters the microlenses 140a and 142a, and light output from the output waveguides 130b and 132b of the light modulation element 120b enters the microlenses 140b and 142b. To do. The light incident on the microlenses 140a, 142a, 140b, and 142b is collimated, for example, and output as parallel light (collimated light).

  Then, the light output from the output waveguide 132a, which is one output light output from the light modulation element 120a, and the light output from the output waveguide 132b, which is one output light output from the light modulation element 120b. Are respectively incident on the half-wave plate 108 after passing through the microlenses 142a and 142b.

  The half-wave plate 108 is a polarization rotation element. When the output light, which is the two linearly polarized lights incident on the half-wave plate 108, passes through the half-wave plate 108, each of the polarization waves is 90%. Rotated degrees. In the present description, one half-wave plate 108 is used so as to be shared by two output lights. However, one half-wave plate 108 may be arranged individually for each of the two output lights. However, it is possible to reduce the number of parts, reduce the number of assembly steps, and improve the reliability by using one half-wave plate 108 so as to be shared by two output lights.

  As a result, the light output from the output waveguide 132a, which is one output light output from the light modulation element 120a, and the light output from the output waveguide 130a, which is the other output light, have a polarization direction of each other. It becomes orthogonally polarized light orthogonal to the polarization combining prism 110a. Similarly, the light output from the output waveguide 132b, which is one output light output from the light modulation element 120b, and the light output from the output waveguide 130b, which is the other output light, have the polarization directions of each other. It becomes orthogonally polarized light orthogonal to the polarization combining prism 110b.

  Here, since the wavelengths of light incident from the incident optical fibers 104a and 104b are different from each other, the wavelength of the light output from the output waveguide 132a of the light modulation element 120a and the light output from the output waveguide 132b of the light modulation element 120b. If the wavelength of the output light is different from each other (and it is necessary to do so), the light output from the output waveguide 132a of the light modulation element 120a in the half-wave plate 108. The optical thickness of the region through which the light passes and the optical thickness of the region through which the light output from the output waveguide 132b of the light modulation element 120b passes may have different thicknesses according to their wavelengths.

  The half-wave plate 108 is, for example, a region through which light output from the output waveguide 132a of the light modulation element 120a constituting the half-wave plate 108 passes, and light output from the output waveguide 132b of the light modulation element 120b. The passing region is arranged to be line symmetric with respect to the line segment 180. The half-wave plate 108 having each region may be composed of a single half-wave plate. Moreover, it is good also as a structure which each produces the half-wave plate which has each area | region, and arranges them individually, and is good also as a structure which combined them.

  The polarization combining prism 110a is a first polarization combining element that combines two linearly polarized lights that are emitted from the light modulation element 120a and whose polarization directions are orthogonal to each other into one beam and output. To do. The polarization combining prism 110b is a second polarization combining element, and combines two linearly polarized lights that are emitted from the light modulation element 120b and whose polarization directions are orthogonal to each other into one light beam. And output.

  Further, the polarization beam combining prism 110 is arranged so that the polarization beam combining prisms 110a and 110b are line symmetric with respect to the line segment 180, for example.

  The optical path shift prisms 112a and 112b are first and second optical path shift elements, respectively. The optical paths of the light beams output from the polarization beam combining prisms 110a and 110b are separated from each other (in the embodiment shown in FIG. 1). Is shifted in a direction away from the vertical direction in the figure.

  The light output from the optical path shift prism 112a enters the outgoing optical fiber 116a through the coupling lens 114a and is guided to the outside of the housing 118. Similarly, the light output from the optical path shift prism 112b enters the output optical fiber 116b through the coupling lens 114b and is guided to the outside of the housing 118.

  As a result, the light incident from the incident optical fiber 104a is modulated by the light modulation element 120a, and then is combined by the half-wave plate 108 and the polarization combining prism 110a to be output from the output optical fiber 116a. . Similarly, the light incident from the incident optical fiber 104b is modulated by the light modulation element 120b, and then is combined by the half-wave plate 108 and the polarization combining prism 110b to be output from the output optical fiber 116b. It becomes.

  The optical path shift prisms 112a and 112b, the coupling lenses 114a and 114b, and the outgoing optical fibers 116a and 116b are arranged so as to be symmetrical with respect to the line segment 180, for example.

  In particular, in the light modulation device 100 according to the present embodiment, two linearly polarized lights respectively output from the light modulation elements 120a and 120b pass immediately after passing through the microlenses 140a, 142a, 140b, and 142b (that is, the optical path). Other optical components such as shift prisms that greatly extend the optical distance (or optical path length) between the microlenses 140a, 142a, 140b, 142b and the half-wave plate 108 and / or the polarization combining prisms 110a, 110b. First, the light passes through the half-wave plate 108 and / or the polarization combining prisms 110a and 110b to be combined into one light beam. For this reason, even when the focal lengths of the microlenses 140a, 142a, 140b, and 142b are short and the divergence angles of the four light beams emitted from the microlenses 140a, 142a, 140b, and 142b are respectively Gaussian beams are large. Before the two light beams propagate and begin to overlap each other, polarization combining can be reliably performed to generate two beams (that is, polarization combined beams).

  In general, light emitted from the light modulation element is collimated by a lens (becomes parallel light) and output. The parallel light is a Gaussian beam (Gaussian beam) having a constant beam diameter, and can ideally propagate far while maintaining a constant beam diameter. However, the parallel light usually has a portion (beam waist) where the beam diameter becomes the smallest. That is, the beam diameter of the parallel light output from the lens gradually decreases, becomes minimum at the beam waist, and thereafter gradually increases (diverges). This is because the light output from the light modulation element is a point light source having a certain area and the linearly polarized light is diffracted.

  Therefore, the Gaussian beams that are output from the light modulation elements 120a and 120b and two collimated by the microlenses 140a, 142a, 140b, and 142b respectively diverge as described above and propagate at a desired distance. , Some of them will start to overlap each other.

  FIGS. 2A and 2B are partial detailed views around the microlens array 106 of the light modulation device 100 shown in FIG. Particularly, in FIG. 2A, the four lights emitted from the light modulation elements 120a and 120b of the light modulation device 100 shown in FIG. 1 are collimated by the four microlenses 140a, 142a, 140b, and 142b, respectively, and go straight. In this case, it is schematically shown that they overlap each other. In FIG. 2 (a), the divergence angle of the collimated light is shown to be larger than the actual angle for convenience of illustrating that the collimated light diverges.

  The collimated light beams 200a, 202a, 202b, and 200b emitted from the output waveguides 130a, 132a, 132b, and 130b of the light modulation elements 120a and 120b and collimated by the four microlenses 140a, 142a, 142b, and 140b are Gaussian. The light is emitted from the micro lenses 140a, 142a, 142b, and 140b while maintaining the shape.

  The collimated lights 200a, 202a, 202b, and 200b have beam waists at which the beam diameter becomes the minimum value at a position 210 that is emitted from the microlenses 140a, 142a, 142b, and 140b and propagates a certain distance. When the beam waist position 210 is exceeded, the collimated light 200a, 202a, 202b, 200b propagates to the left in the figure while expanding the beam diameter by the divergence angle θ, and a part of adjacent beams overlap at the position 212. start. In FIG. 2A, on the left side of the beam overlap start position 212, a region where the collimated lights 200a and 202a emitted from the microlenses 140a and 142a partially overlap each other is indicated by a hatched region denoted by reference numeral 220. ing. Further, an area where the collimated lights 202a and 202b emitted from the micro lenses 142a and 142b partially overlap each other is a hatched area denoted by reference numeral 222, and one of the collimated lights 202b and 200b emitted from the micro lenses 142b and 140b. A region where the portions overlap each other is indicated by a hatched region denoted by reference numeral 224.

  In general, a polarization beam combining prism includes a polarization beam combining film on one optical surface, and two orthogonally polarized light beams propagating independently of each other (without overlapping) are respectively transmitted to one surface of the polarization beam combining film and The light is incident from the other surface, and one linearly polarized light is transmitted through the polarization combining film, and the other linearly polarized light is reflected from the polarization combining film. (Combined beam).

  When a part of two linearly polarized light beams having polarization directions orthogonal to each other are overlapped, the overlapping part is incident from any one surface of the polarization beam combining film constituting the polarization beam combining prism. That is, linearly polarized light having a polarization direction unnecessary for polarization synthesis is incident on each surface of the polarization beam synthesis film. Linearly polarized light having a polarization direction that is not necessary for polarization synthesis is lost because it is not polarization synthesized in a desired direction (deviates from the optical axis of the polarization synthesized beam).

  In the light modulation device 100 according to the present embodiment, as shown in FIG. 2B, the optical path shift is such that the light emitted from each of the microlenses 140a, 142a, 140b, 142b greatly extends the optical path length of the light. First, the light is directly incident on the half-wave plate 108 and / or the polarization combining prisms 110a and 110b without passing through other optical components such as a prism. Here, the “optical path shift prism” refers to a prism that moves an optical path in a direction orthogonal to the optical path (that is, a polyhedron formed of a transparent medium such as glass having a higher refractive index than the surroundings).

  As a result, a position 212 at which light emitted as collimated light from the microlenses 140a, 142a, 142b, and 140b starts to overlap each other due to the divergence angle of the collimated light, and the four microlenses 140a, 142a, 142b, The half-wave plate 108 and the polarization beam combining prisms 110a and 110b can be disposed between the position where 140b is disposed.

  For this reason, in the light modulation device 100 of the present embodiment, even when the divergence angle of the collimated light output from the microlenses 140a, 142a, 140b, and 142b is large, the polarization is small without causing overlap between the beams. Wave synthesis can be performed to reduce light loss from the incident optical fibers 104a, 104b to the outgoing optical fibers 116a, 116b.

  In the present embodiment, other optical components such as an optical path shift prism are arranged in the space between the microlenses 140a, 142a, 140b, 142b and the half-wave plate 108 and / or the polarization combining prisms 110a, 110b. However, the present invention is not limited to this, and other than the optical path shift prism, for example, optical plates other than the optical path shift prism, for example, parallel plates of an optical medium such as glass (that is, the front and back surfaces) An optical component composed of plates that are parallel to each other may be inserted into the space. An optical component composed of such parallel plates is provided with, for example, a dielectric multilayer film (non-reflective coating, filter film (for example, low-pass filter, high-pass filter, band-pass filter)) on the surface of the parallel plate. Further, it may be an optical path length adjusting element or a wavelength filter element.

  Further, in the light modulation device 100 of the present embodiment, the output lights emitted adjacent to each other from the microlenses 142a and 142b are incident on the half-wave plate 108 before the distance between them is increased by an optical path shift prism or the like. Therefore, the wavelength of two lights can be rotated using the half-wave plate 108 as one optical element. For this reason, the number of optical elements can be reduced compared to a configuration in which a half-wave plate is provided for each output light, and the stability of the optical system is improved (for example, temperature fluctuations are reduced) and the number of assembly steps is reduced. Can do.

  In addition, since the optical paths of the light beams emitted from the polarization beam combining prisms 110a and 110b are shifted away from each other by the optical path shift prisms 112a and 112b, the focal lengths of the microlenses 140a, 142a, 140b, and 142b are small, and collimated light. Even when the divergence angle is increased and the beam diameter of light reaching the coupling lenses 114a and 114b is increased, a space for arranging the coupling lenses 114a and 114b having a large aperture area (or light receiving area) corresponding to the beam diameter is ensured. It is possible to increase design freedom.

  Furthermore, in the light modulation device 100 of the present embodiment, the light modulation element 120a and the light modulation element 120b are arranged at positions symmetrical with respect to the line segment 180 parallel to the direction of the light emitted from the light modulation elements 120a and 120b. In addition, the polarization beam combining prisms 110 a and 110 b are also arranged at positions symmetrical with respect to the line segment 180.

  For this reason, for example, the polarization beam combining prisms 110a and 110b can be configured as one optical element having a line-symmetric shape. In this case, the number of optical elements used in the housing 118 can be further reduced to increase the stability of the optical system and further reduce the number of assembly steps.

  In the light modulation device 100 of the present embodiment, the optical path shift prisms 112a and 112b, the coupling lenses 114a and 114b, and the outgoing optical fibers 116a and 116b are also arranged at positions symmetrical to each other with respect to the line segment 180.

  Thereby, the optical system from the incident optical fiber 104a to the outgoing optical fiber 116a and the optical system from the incident optical fiber 104b to the outgoing optical fiber 116b are symmetrical with respect to the line segment 180.

  In general, since a rectangular housing such as the housing 118 shown in FIG. 1 has a geometrically symmetrical distortion generated when the environmental temperature fluctuates, as described above, from the incident optical fiber 104a to the outgoing optical fiber 116a. And the optical system from the incident optical fiber 104b to the outgoing optical fiber 116b are arranged symmetrically with respect to the line segment 180, so that the positional deviation amount of the optical element in each optical system when the environmental temperature varies In addition, characteristic changes such as a change in refractive index accompanying distortion of each optical component that occurs when the environmental temperature fluctuates and a shift of the operating point of the light modulation device can be made similar to each other.

  As a result, when, for example, two lights constituting two wavelength channels of the wavelength division multiplexing transmission system are modulated using the optical modulation device 100, an optical loss (passage) from the incident optical fiber 104a to the outgoing optical fiber 116a is obtained. Loss or insertion loss (the same applies hereinafter)) and the optical loss from the incident optical fiber 104b to the outgoing optical fiber 116b are assumed to have the same variation due to the environmental temperature variation, and between the wavelength channels due to the environmental temperature variation. Occurrence or increase in the transmission loss difference between the channels in the wavelength division multiplexing system (that is, generation or increase in the transmission light level difference between the wavelength channels). It is possible to prevent the increase.

  In the above-described embodiment, the single light modulator 102 in which the two light modulation elements 120a and 120b are formed on one substrate is used as the light modulator. However, the present invention is not limited to this. Two light modulators composed of one light modulation element formed on the substrate may be used.

  In the above-described embodiment, polarization combining is performed using the polarization combining prisms 110a and 110b. However, the present invention is not limited to this, as long as two linearly polarized lights polarized in the same direction can be combined. For example, it is possible to perform polarization synthesis using an arbitrary configuration such as using a birefringent crystal instead of the polarization synthesis prism.

  DESCRIPTION OF SYMBOLS 100 ... Light modulation device, 102 ... Light modulator, 104a, 104b ... Incident optical fiber, 106 ... Micro lens array, 108 ... Half-wave plate, 110a, 110b ... Polarization Synthetic prism, 112a, 112b ... optical path shift prism, 114a, 114b ... coupling lens, 116a, 116b ... outgoing optical fiber, 118 ... housing, 120a, 120b ... light modulation element, 130a, 132a, 130b, 132b... Exit waveguide, 140a, 142a, 140b, 142b... Micro lens, 170.

Claims (8)

A first light modulation element and a second light modulation element, each of which is composed of an optical waveguide formed on a substrate and emits two output lights,
Four lenses for receiving each of the four output lights emitted from the two light modulation elements;
A polarization rotation element that rotates the polarization of one of the two output lights from the first light modulation element and one of the two output lights from the second light modulation element;
A first polarization beam combining element that combines the two output lights from the first light modulation element into one beam and outputs the beam;
A second polarization beam combining element that combines the two output lights from the second light modulation element into one beam and outputs the beam;
With
The light respectively emitted from the four lenses is configured to be directly incident on the polarization rotation element and / or the first and second polarization beam combining elements without passing through an optical path shift prism. ing,
Light modulation device. The polarization rotation element includes a region through which one of the two output lights from the first light modulation element passes, and a region through which one of the two output lights from the second light modulation element passes. Are configured as a single optical element including
The light modulation device according to claim 1. Comprising first and second optical path shift elements for moving the optical paths of the beams output from the first and second polarization beam combining elements in directions away from each other, respectively.
The light modulation device according to claim 1. The first light modulation element and the second light modulation element are arranged so as to emit the output light side by side, and are line symmetric with respect to a line segment parallel to the direction of the output emitted side by side. In the position, and
The first polarization combining element and the second polarization combining element are arranged at positions symmetrical with respect to the line segment;
The light modulation device according to claim 1. Between the four lenses and the polarization rotation element and / or the first and second polarization combining elements, an optical component composed of a parallel plate made of an optical medium is disposed.
The light modulation device according to claim 1. The first and second light modulation elements are light modulation elements that perform phase shift keying or quadrature amplitude modulation.
The light modulation device according to any one of claims 1 to 5. The first and second light modulation elements are formed on different substrates, or are formed side by side on the same substrate.
Optical modulation device according to any one of claims 1 to 6. The four exit lenses are an integrally formed microlens array.
The light modulation device according to claim 1.

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