Light source device

Abstract

To provide a light source device that can improve illumination performance. [Solution] The light source device 1A includes a lamp 2 having a sealed container 21 filled with discharge gas G and a pair of electrodes 22 facing each other inside the sealed container 21, an emission unit 3 that emits laser light L, and an optical system 4 that focuses the laser light L emitted from the emission unit 3 so that the focusing region R of the laser light L is located between the pair of electrodes 22. The optical system 4 focuses the laser light L so that the shape of the focusing region R in a cross section perpendicular to the optical axis A of the laser light L is elongated.

Description

Technical Field

[0003]

[0001] The present invention relates to a light source device.

Background Art

[0002] There is known a light source device called a so-called laser-excited plasma light source, which includes a lamp having a pair of electrodes, an emission unit that emits laser light, and an optical system that condenses the laser light emitted from the emission unit between the pair of electrodes (see, for example, Patent Document 1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the light source device as described above, lighting may be started by causing a trigger discharge between a pair of electrodes in a state where laser light is condensed between the pair of electrodes, or by condensing laser light between the pair of electrodes in a state where a trigger discharge is generated between the pair of electrodes. In such a case, due to the irregular change of the discharge path of the trigger discharge between the pair of electrodes, the condensing region of the laser light and the discharge path of the trigger discharge do not properly intersect, resulting in a decrease in lighting performance (lighting probability).

[0005] An object of the present invention is to provide a light source device capable of improving lighting performance.

Means for Solving the Problems

[0006] The light source device of the present invention is [1] "a light source device comprising a sealed container filled with discharge gas, a light-emitting seal having a pair of electrodes facing each other inside the sealed container, an emission unit that emits laser light, and an optical system that focuses the laser light emitted from the emission unit such that the focusing region of the laser light is located between the pair of electrodes, wherein the optical system focuses the laser light such that the shape of the focusing region in a cross section perpendicular to the optical axis of the laser light is elongated."

[0007] In the above light source device, the optical system focuses the laser beam so that the laser beam's focusing region is located between a pair of electrodes in the light-emitting seal, and the shape of the laser beam's focusing region in a cross-section perpendicular to the optical axis of the laser beam is elongated. As a result, when starting the illumination using the laser beam's focusing region and a trigger discharge between the pair of electrodes, even if the discharge path of the trigger discharge between the pair of electrodes changes irregularly, the discharge path of the trigger discharge is more likely to intersect with the focusing region compared to a configuration where the shape of the laser beam's focusing region in a cross-section perpendicular to the optical axis of the laser beam is circular. Therefore, the above light source device can improve illumination performance.

[0008] The light source device of the present invention may also be [2] "the light source device according to [1] above, wherein the optical system focuses the laser light such that the longitudinal direction of the elongated shape is perpendicular to the direction in which the pair of electrodes face each other." With this light source device, the range of the laser light focusing region in which the discharge paths of the trigger discharge can intersect is expanded in a direction perpendicular to the direction in which the pair of electrodes face each other, thereby improving the ignition performance.

[0009] The light source device of the present invention may also be [3] "the light source device according to [1] above, wherein the optical system focuses the laser light such that the longitudinal direction of the elongated shape is parallel to the direction in which the pair of electrodes face each other." With this light source device, the range of the laser light focusing region in which the discharge paths of the trigger discharge can intersect is expanded in the direction in which the pair of electrodes face each other, thereby improving the ignition performance.

[0010] The light source device of the present invention may also be [4] "the light source device according to any one of [1] to [3] above, wherein the emission unit includes a laser diode." When an emission unit including a laser diode is used, the emission unit that emits laser light can be arranged inside the light source device without using a light guide member such as a fiber to guide the laser light from an emission unit outside the light source device. As a result, the light source device can be miniaturized and malfunctions caused by light guide members can be suppressed.

[0011] The light source device of the present invention may also be [5] "the light source device according to [4] above, wherein the optical system focuses the laser light such that the focal point of the longitudinally divergent light emitted from the laser diode as the laser light and the focal point of the transversely divergent light emitted from the laser diode as the laser light are offset from each other in a direction parallel to the optical axis of the laser light." With this light source device, the range of the laser light focusing region in which the discharge paths of the trigger discharge can intersect is expanded in a direction parallel to the optical axis of the laser light, thereby improving the ignition performance.

[0012] The light source device of the present invention may also be [6] "a light source device according to any one of [1] to [5] above, further comprising a control unit that controls at least one of the light-emitting seal, the light-emitting unit, and the optical system so that the state of the light-gathering region is in a predetermined state." With this light source device, the state of the light-gathering region can be made to a more appropriate state depending on various circumstances.

[0013] The light source device of the present invention may also be the light source device described in [6] above, wherein the control unit controls at least one of the light-emitting seal, the emission unit, and the optical system so that the shape of the light-gathering area becomes the elongated shape when the light is started and the shape of the light-gathering area becomes a predetermined shape when the light is maintained. With this light source device, the shape of the light-gathering area can be made into a more appropriate shape according to the conditions at the start of lighting and when the light is maintained.

[0014] The light source device of the present invention may be "[8] the light source device according to [7] above, wherein the predetermined shape is a shape in which the difference between the maximum width of the shape of the condensing region in the longitudinal direction of the elongated shape and the maximum width of the shape of the condensing region in a direction perpendicular to the longitudinal direction is adjusted to be small". According to this light source device, the shape of the light output from the light-emitting enclosure can be made into a more versatile shape.

Advantages of the Invention

[0015] According to the present invention, it is possible to provide a light source device capable of improving lighting performance.

Brief Description of the Drawings

[0016] [Figure 1] It is a longitudinal sectional view of a light source device of an example. [Figure 2] It is a sectional view of the light source device along the line II-II shown in FIG. 1. [Figure 3] It is a sectional view of the lamp along the line III-III shown in FIG. 2. [Figure 4] It is a schematic view showing the light-emitting point of the laser diode shown in FIG. 1. [Figure 5] It is a schematic view showing the condensing state of the longitudinal and lateral divergent lights emitted from the laser diode shown in FIG. 1. [Figure 6] It is a longitudinal sectional view of a light source device of a modified example. [Figure 7] It is a sectional view of the light source device along the line VII-VII shown in FIG. 6. [Figure 8] It is a sectional view of the lamp along the line VIII-VIII shown in FIG. 7. [Figure 9] It is a longitudinal sectional view of a light source device of a modified example.

Embodiments for Carrying Out the Invention

[0017] Hereinafter, an example of the present invention will be described in detail with reference to the drawings. In each figure, the same or corresponding parts are denoted by the same reference numerals, and redundant descriptions are omitted.

[0018] As shown in FIGS. 1 and 2, the light source device 1A includes a lamp (light-emitting envelope) 2, an emission unit 3, and an optical system 4. The lamp 2 has a sealed container 21 and a pair of electrodes 22. In the light source device 1A, the lamp 2, the emission unit 3, and the optical system 4 are housed in a housing not shown, and the optical system 4 between the lamp 2 and the emission unit 3 does not include a light guide member such as an optical fiber. The sealed container 21 is filled with a discharge gas G. The pair of electrodes 22 face each other inside the sealed container 21. In the light source device 1A, a trigger discharge is generated between the pair of electrodes 22 in a state where the laser beam L emitted from the emission unit 3 is condensed between the pair of electrodes 22, or the laser beam L is condensed between the pair of electrodes 22 in a state where a trigger discharge is generated between the pair of electrodes 22, thereby generating plasma in the sealed container 21 (that is, starting lighting by plasma emission), and maintaining the plasma in the sealed container 21 (that is, maintaining lighting by plasma emission) by maintaining the condensation of the laser beam L between the pair of electrodes 22. That is, the light source device 1A is a laser-excited plasma light source. Hereinafter, the direction in which the pair of electrodes 22 face each other is referred to as the X-axis direction, a direction perpendicular to the X-axis direction is referred to as the Y-axis direction, and a direction perpendicular to both the X-axis direction and the Y-axis direction is referred to as the Z-axis direction.

[0019] In the lamp 2, each electrode 22 penetrates the sealed container 21 airtightly. The sealed container 21 is, for example, a bulb formed of glass into a hollow spherical shape. The discharge gas G is filled in the sealed container 21 at a high pressure. The discharge gas G is, for example, xenon gas. Each electrode 22 is formed in a rod shape of a high melting point metal such as tungsten. The tip portion of each electrode 22 inside the sealed container 21 has a tapered shape toward the center side of the sealed container 21. The tip portions of the electrodes 22 face each other inside the sealed container 21 at a predetermined interval. Each electrode 22 is electrically connected to a power supply unit (not shown), whereby a high voltage for generating a trigger discharge is applied between the pair of electrodes 22.

[0020] The emission unit 3 emits laser light L. The emission unit 3 includes a laser diode 30. In the light source device 1A, the laser diode 30 is positioned such that, with its emission end face 30a facing the lamp 2, the optical axis A of the laser light L emitted from the emission end face 30a of the laser diode 30 is parallel to the Z-axis direction, the fast axis direction is parallel to the X-axis direction, and the slow axis direction is parallel to the Y-axis direction. In the light source device 1A, the optical axis A of the laser light L passes through the center of the sealed container 21, that is, between the pair of electrodes 22. The emission unit 3 may continuously oscillate the laser light L or pulse the laser light L. The wavelength of the laser light L is the wavelength corresponding to the absorption spectrum of the discharge gas G, and if the discharge gas G is xenon gas, it is preferably about 980 nm, for example.

[0021] The optical system 4 focuses the laser beam L emitted from the laser diode 30 such that the focusing region R of the laser beam L is located between the pair of electrodes 22. The focusing region R of the laser beam L is a region containing the focusing point (focusing spot) of the laser beam L, and is a region having an energy density in which illumination can occur (i.e., plasma can be generated) when the paths of the trigger discharge generated between the pair of electrodes 22 intersect. In the light source device 1A, the optical system 4 includes a cylindrical lens 41 and a focusing lens 42. The cylindrical lens 41 is positioned near the emission end face 30a of the laser diode 30 on the optical axis A of the laser beam L. The cylindrical lens 41 reduces the divergence angle of the laser beam L only in the fast axis direction parallel to the X axis. The focusing lens 42 is positioned between the cylindrical lens 41 and the sealed container 21 on the optical axis A of the laser beam L. The focusing lens 42 focuses the laser beam L in all directions centered on the optical axis A of the laser beam L.

[0022] With the configuration of the optical system 4 described above, the laser light L is focused such that the shape of the focusing region R (hereinafter simply referred to as "cross-sectional shape of the focusing region R") in a cross section perpendicular to the optical axis A of the laser light L incident on the sealed container 21 is elongated. In the light source device 1A, with the configuration of the optical system 4 described above, the laser light L is focused such that the longitudinal direction of the elongated shape is perpendicular to the X-axis direction in which the pair of electrodes 22 face each other, as shown in Figure 3. Here, elongated shape means a shape in which the maximum width in one direction is greater than the maximum width in the direction perpendicular to that direction (for example, elliptical, oblong, rectangular, etc.), and longitudinal direction means that one direction. In the light source device 1A, the maximum width α of the cross-sectional shape of the focusing region R at the center position between the pair of electrodes 22 is 5 times or more (preferably 10 times or more, more preferably 30 times or more) of the maximum width β of the cross-sectional shape in the X-axis direction. In the light source device 1A, the angle between the longitudinal direction of the cross-sectional shape of the light-gathering region R and the X-axis direction is approximately 90 degrees (for example, an angle in the range of 90 ± 5 degrees).

[0023] As shown in Figures 4(a) and 4(b), the laser light L emitted from the laser diode 30 includes longitudinal divergent light L1 and transverse divergent light L2. The longitudinal divergent light L1 is light emitted from the light-emitting point of the laser diode 30 (not shown) and its direction of vibration is along the fast axis direction of the laser diode 30 (the X axis direction in the light source device 1A). The transverse divergent light L2 is light emitted from the light-emitting point of the laser diode 30 (not shown) and its direction of vibration is along the slow axis direction of the laser diode 30 (the Y axis direction in the light source device 1A). The divergence angle of the longitudinal divergent light L1 emitted from the exit end face 30a (the divergence angle in a plane perpendicular to the Y axis direction) is greater than the divergence angle of the transverse divergent light L2 emitted from the exit end face 30a (the divergence angle in a plane perpendicular to the X axis direction).

[0024] As shown in Figures 5(a) and (b), the optical system 4 focuses the laser beam L such that the focal point C1 of the longitudinal divergent light L1 and the focal point C2 of the transverse divergent light L2 are offset from each other in a direction parallel to the optical axis A of the laser beam L incident on the sealed container 21. In the light source device 1A, the focal point C1 of the longitudinal divergent light L1 is located on the side of the focusing lens 42 relative to the focal point C2 of the transverse divergent light L2 in the optical path of the laser beam L. In other words, the focal point C2 of the transverse divergent light L2 is located on the opposite side of the focusing lens 42 relative to the focal point C1 of the longitudinal divergent light L1 in the optical path of the laser beam L. Since the focusing region R of the laser beam L includes the focusing point C1, which is the focusing spot of the longitudinal divergent light L1, and the focusing point C2, which is the focusing spot of the transverse divergent light L2, in the light source device 1A, the range of the focusing region R of the laser beam L expands in a direction parallel to the optical axis A of the laser beam L incident on the sealed container 21.

[0025] In the light source device 1A configured as described above, as shown in Figure 3, when laser light L is emitted from the emission unit 3 and the focusing region R of the laser light L is located between the pair of electrodes 22, a high voltage is applied between the pair of electrodes 22, causing a trigger discharge between the pair of electrodes 22. At this time, when the discharge path P of the trigger discharge, which changes irregularly between the pair of electrodes 22, intersects with the focusing region R of the laser light L, plasma is generated inside the sealed container 21 (i.e., illumination by plasma emission begins). Then, as the focusing of the laser light L between the pair of electrodes 22 is maintained, the plasma is maintained inside the sealed container 21 (i.e., illumination by plasma emission is maintained).

[0026] As explained above, in the light source device 1A, the optical system 4 focuses the laser beam L such that the focusing region R of the laser beam L is located between the pair of electrodes 22 of the lamp 2 and the cross-sectional shape of the focusing region R is elongated. As a result, when starting the lamp using the focusing region R of the laser beam L and the trigger discharge between the pair of electrodes 22, even if the discharge path P of the trigger discharge between the pair of electrodes 22 changes irregularly, the discharge path P of the trigger discharge is more likely to intersect with the focusing region R compared to a configuration where the cross-sectional shape of the focusing region R is circular. Therefore, the light source device 1A can improve the lamp's ignition performance.

[0027] In the light source device 1A, the optical system 4 focuses the laser beam L such that the longitudinal direction of the elongated shape of the focusing region R is perpendicular to the X-axis direction in which the pair of electrodes 22 face each other. As a result, the range of the focusing region R of the laser beam L, in which the discharge paths P of the trigger discharge can intersect, expands in the Y-axis direction perpendicular to the X-axis direction in which the pair of electrodes 22 face each other. Therefore, even if the discharge paths P of the trigger discharge are scattered along multiple paths in the Y-axis direction, the probability of one of the discharge paths P intersecting with the focusing region R of the laser beam L increases. Thus, the ignition reliability of the light source device 1A can be improved.

[0028] In the light source device 1A, the emission unit 3 includes a laser diode 30. When using an emission unit 3 that includes a laser diode 30, the emission unit 3 that emits the laser beam L can be placed inside the light source device 1A without using a light guide member such as a fiber to guide the laser beam L from an external emission unit to the light source device 1A. Therefore, the light source device 1A can be miniaturized, and malfunctions caused by light guide members can be suppressed. In addition, the laser diode 30 makes it easy to focus the laser beam L such that the focal point C1 of the longitudinal divergent light L1 and the focal point C2 of the transverse divergent light L2 are offset from each other in the Z-axis direction parallel to the optical axis A of the laser beam L. Therefore, by focusing the laser beam L in this way in the light source device 1A, the focused area R of the laser beam L where the discharge path P of the trigger discharge can intersect expands in the Z-axis direction parallel to the optical axis A of the laser beam L, thus improving the ignition performance.

[0029] In the light source device 1A, the optical system 4 focuses the laser beam L such that the focal point C1 of the longitudinal divergent light L1 and the focal point C2 of the transverse divergent light L2 are offset from each other in the Z-axis direction parallel to the optical axis A of the laser beam L. As a result, the range of the focused area R of the laser beam L, where the discharge path P of the trigger discharge can intersect, is expanded in the Z-axis direction parallel to the optical axis A of the laser beam, thereby improving the ignition performance.

[0030] The present invention is not limited to the above example. For example, as shown in the light source device 1B in Figures 6, 7, and 8, the laser diode 30 may be arranged such that, with the output end face 30a of the laser diode 30 facing the lamp 2 side, the optical axis A of the laser beam L emitted from the output end face 30a of the laser diode 30 is parallel to the Z-axis direction, the fast axis direction is parallel to the Y-axis direction, and the slow axis direction is parallel to the X-axis direction. In the light source device 1B, the cylindrical lens 41 is arranged to reduce the divergence angle of the laser beam L only in the fast axis direction parallel to the Y-axis direction. In the light source device 1B, the laser beam L is focused by the optical system 4 such that the longitudinal direction of the elongated shape of the pair of electrodes 22 facing each other is parallel to the X-axis direction. In the light source device 1B, the angle between the longitudinal direction of the cross-sectional shape of the focusing region R and the X-axis direction is approximately 0 degrees (for example, an angle in the range of 0 ± 5 degrees). According to the light source device 1B, the range of the laser beam focusing region R, where the discharge path P of the trigger discharge may intersect, extends in the X-axis direction where the pair of electrodes 22 face each other. Therefore, the range in which the discharge path P of the trigger discharge extends in the X-axis direction and the focusing region R of the laser beam L are more likely to overlap. Thus, the ignition performance of the light source device 1B can be improved.

[0031] The emission unit 3 is not limited to one including a laser diode 30, but can be any light source capable of emitting laser light L. For example, the emission unit 3 may be placed outside the housing containing the lamp 2 and optical system 4, and the laser light L may be introduced into the housing using a light guide member such as an optical fiber. In addition, in each light source device 1A, 1B, the optical path of the laser light L from the emission unit 3 to the pair of electrodes 22 extended linearly along the Z-axis direction, but the optical path may be bent by a mirror or the like, as long as the optical system 4 can focus the laser light L so that the cross-sectional shape of the focusing region R is elongated. Furthermore, the configuration of the optical system 4 is not limited to one having a cylindrical lens 41 and a focusing lens 42, but can be any configuration that can focus the laser light L so that the cross-sectional shape of the focusing region R is elongated. Also, the angle between the longitudinal direction of the cross-sectional shape of the focusing region R and the direction in which the pair of electrodes 22 face each other is arbitrary, and may intersect diagonally, for example, at 45 degrees.

[0032] In lamp 2, the sealed container 21 may be made of an insulating material such as ceramic, a conductive material such as metal, or a mixture thereof. The discharge gas G may be argon, neon, krypton, or helium, or a mixture thereof including xenon. The material of each electrode 22 is not limited to a single high-melting-point metal, but may be a material such as a high-melting-point metal containing an electron-emitting substance. Furthermore, the positional relationship between the focal point C1 of the longitudinal divergent light L1 and the focal point C2 of the transverse divergent light L2 of the laser diode 30 on the optical path of the laser beam L may be reversed from the examples described above.

[0033] As shown in Figure 9, the light source device 1A may further include a control unit 5. The control unit 5 controls at least one of the lamp 2, the emission unit 3, and the optical system 4 so that the state of the focusing region R is in a predetermined state. For example, the control unit 5 controls at least one of the lamp 2, the emission unit 3, and the optical system 4 so that the cross-sectional shape of the focusing region R is elongated at the start of lighting and the cross-sectional shape of the focusing region R is a predetermined shape when lighting is maintained. For example, the predetermined shape is a shape (for example, a roughly circular shape) in which the difference between the maximum width of the cross-sectional shape of the focusing region R in the Y-axis direction (maximum width of the shape of the focusing region in the longitudinal direction of the elongated shape) α and the maximum width of the cross-sectional shape of the focusing region R in the X-axis direction (maximum width of the shape of the focusing region in the direction perpendicular to the longitudinal direction of the elongated shape) β is adjusted to be smaller than at the start of lighting. Note that the start of lighting means the time period that overlaps with at least a part of the time when a trigger discharge occurs between the pair of electrodes 22. Also, the maintenance of lighting means the time period after the start of lighting, during which lighting by plasma emission is maintained.

[0034] When the control unit 5 controls the optical system 4, it is preferable that the control unit 5 controls the optical system 4 so that the cylindrical lens 41 moves along the optical axis A. Specifically, it is preferable that the control unit 5 controls the optical system 4 so that the cylindrical lens 41 moves towards the emission unit 3 along the optical axis A after the start of illumination. As another example, the control unit 5 may control the optical system 4 so that an optical shaping member such as a slit is positioned along the optical axis A after the start of illumination. Also, when the control unit 5 controls the emission unit 3, it is preferable that the control unit 5 controls the emission unit 3 so that the position of the emission unit 3 relative to the optical system 4 is changed. Also, when the control unit 5 controls the lamp 2, it is preferable that the position of the lamp 2 relative to the optical system 4 is changed.

[0035] The control unit 5 may control at least one of the lamp 2, the emission unit 3, and the optical system 4 so as to change not only the cross-sectional shape of the focusing region R, but also the size and / or orientation of the focusing region R. For example, when the lamp is kept lit, the focusing region R may be smaller than when it was first turned on, or the focusing region R may be rotated by a predetermined angle (e.g., 90 degrees) around the optical axis A of the laser beam L. When the focusing region R is rotated, it is preferable that the focusing region R is rotated so that the cross-sectional shape of the light output from the lamp 2 (i.e., the light output from the light source device 1A by plasma emission) becomes approximately circular.

[0036] As explained above, when the control unit 5 controls at least one of the lamp 2, the emitter 3, and the optical system 4 so that the state of the focusing region R is in a predetermined state, the state of the focusing region R can be made more appropriate depending on various circumstances. Also, when the control unit 5 controls at least one of the lamp 2, the emitter 3, and the optical system 4 so that the cross-sectional shape of the focusing region R is elongated when the light is turned on and becomes a predetermined shape when the light is maintained, the cross-sectional shape of the focusing region R can be made more appropriate depending on the conditions at the start of lighting and when the light is maintained. Furthermore, when the predetermined shape is one in which the difference between the maximum width α of the cross-sectional shape of the focusing region R in the Y-axis direction and the maximum width β of the cross-sectional shape of the focusing region R in the X-axis direction is adjusted to be small, the shape of the light output from the lamp 2 can be made into a more versatile shape. The above control unit may be provided in the light source device 1B. [Explanation of Symbols]

[0037] 1A, 1B...light source device, 2...lamp (light-emitting seal), 3...emission unit, 4...optics system, 5...control unit, 21...sealed container, 22...electrode, 30...laser diode, A...optical axis, C1, C2...focusing point, G...discharge gas, L...laser light, L1...longitudinal divergence, L2...transverse divergence, R...focusing region.

Claims

1. A sealed container containing a discharge gas, and a light-emitting seal having a pair of electrodes facing each other inside the sealed container, A laser emission unit that emits laser light, The system comprises an optical system that focuses the laser light emitted from the emission unit such that the focal region of the laser light is located between the pair of electrodes, The optical system is a light source device that focuses the laser light such that the shape of the focusing region in a cross section perpendicular to the optical axis of the laser light is elongated.

2. The light source device according to claim 1, wherein the optical system focuses the laser light such that the longitudinal direction of the elongated shape is perpendicular to the direction in which the pair of electrodes face each other.

3. The light source device according to claim 1, wherein the optical system focuses the laser light such that the longitudinal direction of the elongated shape is parallel to the direction in which the pair of electrodes face each other.

4. The light source device according to any one of claims 1 to 3, wherein the emission unit includes a laser diode.

5. The light source device according to claim 4, wherein the optical system focuses the laser light such that the focal point of the longitudinally divergent light emitted from the laser diode as the laser light and the focal point of the transversely divergent light emitted from the laser diode as the laser light are offset from each other in a direction parallel to the optical axis of the laser light.

6. The light source device according to claim 1, further comprising a control unit that controls at least one of the light-emitting seal, the emission unit, and the optical system so that the state of the light-collecting region is in a predetermined state.

7. The light source device according to claim 6, wherein the control unit controls at least one of the light-emitting seal, the emission unit, and the optical system such that the shape of the light-gathering area becomes the elongated shape when the light is started and the shape of the light-gathering area becomes a predetermined shape when the light is maintained.

8. The light source device according to claim 7, wherein the predetermined shape is a shape in which the difference between the maximum width of the light-gathering region in the longitudinal direction of the elongated shape and the maximum width of the light-gathering region in a direction perpendicular to the longitudinal direction is adjusted to be small.

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