JP2016072617A - Laser apparatus, ignition apparatus and internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a laser apparatus capable of achieving high excitation efficiency.SOLUTION: A laser apparatus 200 includes a surface-emitting laser array 201, a first light converging optical system 203, an optical fiber 204, a second light converging optical system 205, and a laser resonator 206. The surface-emitting laser array 201 includes a plurality of light-emitting portions. The first light converging optical system 203 includes a microlens array 203a and a light converging lens system 203b. The microlens array 203a includes a plurality of lens portions corresponding to the plurality of light-emitting portions. Each lens portion collimates light emitted from its corresponding light-emitting portion. In such a case, the light converting lens system 203b converges light so that optical energy is converged into the vicinity of a center of the optical fiber 204 and a Mvalue of light made incident on the laser resonator 206 is reduced.SELECTED DRAWING: Figure 5

Description

  The present invention relates to a laser device, an ignition device, and an internal combustion engine, and more particularly to a laser device having a surface emitting laser, an ignition device having the laser device, and an internal combustion engine including the ignition device.

  A laser device having a laser crystal that oscillates by optical excitation is expected to be applied to various fields such as an ignition device, a laser processing machine, and a medical device.

  For example, Patent Literature 1 discloses a laser ignition device including a laser device and a pump light source having a plurality of surface emitting lasers.

  Further, in Patent Document 2, the laser device includes at least one refracting device, the refracting device refracts at least a part of the pumping light, and is formed integrally with the laser device. An ignition device is disclosed.

  However, it has been difficult to achieve high excitation efficiency with conventional laser devices.

  The present invention includes a surface emitting laser, a lens system disposed on an optical path of light emitted from the surface emitting laser, and a laser resonator into which light is incident through the lens system, and the lens system Is a laser device including a collimating lens on which light emitted from the surface emitting laser is incident, and a condensing lens for condensing the light via the collimating lens.

  According to the laser device of the present invention, high excitation efficiency can be realized.

It is a figure for demonstrating the outline of the engine 300 which concerns on one Embodiment of this invention. It is a figure for demonstrating the ignition device. 6 is a diagram for explaining a laser resonator 206. FIG. It is a figure for demonstrating the surface emitting laser array 201. FIG. It is a figure for demonstrating the 1st condensing optical system. It is a figure for demonstrating the laser apparatus 400 of a comparative example. FIG. 7A is a diagram for explaining a spatial energy distribution of light incident on the optical fiber 204 in the laser device 200, and FIG. 7B is incident on the optical fiber 204 in the laser device 400. It is a figure for demonstrating the spatial energy distribution of the light to do. FIG. 8A is a diagram for explaining light incident on the laser resonator 206 in the laser device 200, and FIG. 8B illustrates light incident on the laser resonator 206 in the laser device 400. It is a figure for demonstrating. It is a figure for demonstrating an example of the experimental result of Q switch laser output when changing the magnitude | size of a light emission part area | region, a shape, the core diameter of an optical fiber, etc. FIG.

"Overview"
Hereinafter, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 schematically shows a main part of an engine 300 as an internal combustion engine according to an embodiment.

  The engine 300 includes an ignition device 301, a fuel ejection mechanism 302, an exhaust mechanism 303, a combustion chamber 304, a piston 305, and the like.

The operation of engine 300 will be briefly described.
(1) The fuel ejection mechanism 302 ejects a combustible mixture of fuel and air into the combustion chamber 304 (intake).
(2) The piston 305 rises and compresses the combustible air-fuel mixture (compression).
(3) The ignition device 301 emits laser light into the combustion chamber 304. Thereby, the fuel is ignited (ignition).
(4) Combustion gas is generated and the piston 305 descends (combustion).
(5) The exhaust mechanism 303 exhausts the combustion gas to the outside of the combustion chamber 304 (exhaust).

  Thus, a series of processes consisting of intake, compression, ignition, combustion, and exhaust are repeated. Then, the piston 305 moves in response to a change in the volume of the gas in the combustion chamber 304 to generate kinetic energy. For example, natural gas or gasoline is used as the fuel.

  Engine 300 performs the above operation based on an instruction of an engine control device that is provided outside engine 300 and is electrically connected to engine 300.

  As shown in FIG. 2 as an example, the ignition device 301 includes a laser device 200, an emission optical system 210, a protection member 212, and the like.

  The emission optical system 210 condenses the light emitted from the laser device 200. Thereby, a high energy density can be obtained at the condensing point.

  The protection member 212 is a transparent window provided facing the combustion chamber 304. Here, as an example, sapphire glass is used as the material of the protection member 212.

  The laser device 200 includes a surface emitting laser array 201, a first condensing optical system 203, an optical fiber 204, a second condensing optical system 205, and a laser resonator 206. In the present specification, description will be made using the XYZ three-dimensional orthogonal coordinate system and assuming that the light emission direction from the surface emitting laser array 201 is the + Z direction.

  The surface emitting laser array 201 is an excitation light source and has a plurality of light emitting units. Each of the light emitting units is a vertical cavity surface emitting laser (VCSEL: Vertical Cavity Surface Emitting Laser).

  The surface-emitting laser array is a light source that is advantageous for exciting a Q-switched laser whose characteristics change greatly due to a shift in excitation wavelength because the wavelength shift of emitted light due to temperature is very small. Therefore, when a surface emitting laser array is used as an excitation light source, there is an advantage that environmental temperature control can be simplified.

  The first condensing optical system 203 condenses the light emitted from the surface emitting laser array 201.

  The optical fiber 204 is arranged so that the center of the end face on the −Z side of the core is located at a position where the light is collected by the first condensing optical system 203.

  By providing the optical fiber 204, the surface emitting laser array 201 can be placed at a position away from the laser resonator 206. Thereby, the freedom degree of arrangement design can be increased. Further, when the laser device 200 is used as an ignition device, the surface emitting laser array 201 can be moved away from the heat source, so that the range of methods for cooling the engine 300 can be increased.

  The light incident on the optical fiber 204 propagates through the core and is emitted from the + Z side end face of the core.

  The second condensing optical system 205 is disposed on the optical path of the light emitted from the optical fiber 204 and condenses the light. The light condensed by the second condensing optical system 205 enters the laser resonator 206.

  The laser resonator 206 is a Q-switched laser and includes a laser medium 206a and a saturable absorber 206b as shown in FIG. 3 as an example.

  The laser medium 206a is a rectangular parallelepiped Nd: YAG crystal having a resonator length of 8 mm. The saturable absorber 206b is a rectangular parallelepiped Cr: YAG crystal having a length of 2 mm.

  Here, the Nd: YAG crystal and the Cr: YAG crystal are joined to form a so-called composite crystal. Both the Nd: YAG crystal and the Cr: YAG crystal are ceramics.

  The light from the second condensing optical system 205 is incident on the laser medium 206a. That is, the laser medium 206a is excited by the light from the second condensing optical system 205. Note that the wavelength of light emitted from the surface emitting laser array 201 is desirably a wavelength of 808 nm, which has the highest absorption efficiency in the YAG crystal. The saturable absorber 206b operates as a Q switch.

  The surface on the incident side (−Z side) of the laser medium 206a and the surface on the exit side (+ Z side) of the saturable absorber 206b are subjected to an optical polishing process and serve as a mirror. Hereinafter, for convenience, the incident-side surface of the laser medium 206a is also referred to as a “first surface”, and the exit-side surface of the saturable absorber 206b is also referred to as a “second surface” (see FIG. 3).

  The first surface and the second surface are coated with a dielectric film corresponding to the wavelength of light emitted from the surface emitting laser array 201 and the wavelength of light emitted from the laser resonator 206. .

  Specifically, the first surface is coated with a sufficiently high transmittance for light having a wavelength of 808 nm and a sufficiently high reflectance for light having a wavelength of 1064 nm. In addition, the second surface is coated with a reflectance that is selected so that a desired threshold is obtained for light having a wavelength of 1064 nm.

  As a result, the light resonates and is amplified in the laser resonator 206. Here, the resonator length of the laser resonator 206 is 10 (= 8 + 2) mm.

  Returning to FIG. 2, the driving device 220 drives the surface emitting laser array 201 based on an instruction from the engine control device 222. That is, drive device 220 drives surface emitting laser array 201 so that light is emitted from ignition device 301 at the timing of ignition in the operation of engine 300. The plurality of light emitting units in the surface emitting laser array 201 are turned on and off simultaneously.

  In the above embodiment, when it is not necessary to place the surface emitting laser array 201 at a position away from the laser resonator 206, the optical fiber 204 may not be provided.

  Here, the case of an engine (piston engine) in which a piston is moved by combustion gas as an internal combustion engine has been described. However, the present invention is not limited to this. For example, a rotary engine, a gas turbine engine, or a jet engine may be used. In short, what is necessary is just to burn the fuel and generate the combustion gas.

  In addition, the ignition device 301 may be used for cogeneration, which is a system that uses exhaust heat to extract power, heat, and cold to improve energy efficiency comprehensively.

  Although the case where the ignition device 301 is used in an internal combustion engine has been described here, the present invention is not limited to this.

  Although the case where the laser device 200 is used in an ignition device has been described here, the present invention is not limited to this. For example, it can be used for a laser processing machine, a laser peening apparatus, a terahertz generator, and the like.

"Details"
A laser device having a laser crystal that oscillates by optical excitation is expected to be applied to various fields such as an ignition device, a laser processing machine, and a medical device. In addition, a Q-switched laser provided with a saturable absorber at the end of the laser crystal is very promising for applications requiring a high energy density because it can output a laser with a very high peak value. .

  Incidentally, a surface emitting laser (VCSEL) is a semiconductor laser that performs laser oscillation in a direction perpendicular to a substrate. This surface-emitting laser has characteristics that the wavelength stability with respect to temperature is very high, and that high output can be easily achieved by using a two-dimensional array. From these features, the surface emitting laser is expected to be used as a light source (excitation light source) for exciting a laser crystal.

  However, when the light emitted from the surface emitting laser is directly condensed by the condenser lens, the condensed light has a spatially relatively uniform intensity distribution in the radial direction.

  Since light with a spatially uniform intensity distribution has poor beam quality, if it is focused on the laser crystal and incident, the beam diameter changes greatly before and after the beam waist, and the laser crystal cannot be excited efficiently. there were.

  Here, the plurality of light emitting portions in the surface emitting laser array 201 are arranged in a hexagonal manner so that the interval between the light emitting portions is 50 μm in a region having a diameter of 9.0 mm (see FIG. 4). When light is emitted from the surface emitting laser array 201, the plurality of light emitting units emit light simultaneously. The wavelength of light emitted from the surface emitting laser array 201 is 808 nm.

  As shown in FIG. 5 as an example, the first condensing optical system 203 includes a microlens array 203a and a condensing lens system 203b.

  The microlens array 203a is disposed on the optical path of the light emitted from the surface emitting laser array 201. The microlens array 203a has a plurality of lens units corresponding to the plurality of light emitting units of the surface emitting laser array 201. And each lens part makes the light inject | emitted from the corresponding light emission part into substantially parallel light. That is, the microlens array 203a collimates the light emitted from the surface emitting laser array 201.

  The distance in the Z-axis direction between the surface emitting laser array 201 and the microlens array 203a is determined according to the focal length of the microlens array 203a.

  The condensing lens system 203b condenses the light via the microlens array 203a. An appropriate condensing lens system 203b is selected according to the cross-sectional area of the light passing through the microlens array 203a, the core diameter of the optical fiber 204, and the NA. Here, a condensing lens having a focal length of 8 mm and an effective diameter of 10 mm is used as the condensing lens system 203b. The condensing lens system 203b may be composed of a plurality of optical elements.

  Here, a condensing lens having a focal length of 6 mm is used as the second condensing optical system 205. A plurality of optical elements may be used as the second condensing optical system 205.

  FIG. 6 shows a laser device 400 as a comparative example of the laser device 200. The laser device 400 has a configuration in which the microlens array 203a is removed from the laser device 200.

  FIG. 7A shows the spatial energy distribution of light incident on the optical fiber 204 in the laser device 200, and FIG. 7B shows the light incident on the optical fiber 204 in the laser device 400. Spatial energy distribution is shown.

  In the laser device 200, light incident on the condenser lens system 203b is made substantially parallel light by the microlens array 203a. Therefore, the condensing lens system 203b of the laser device 200 performs condensing so that the light energy is more concentrated near the center of the optical fiber 204 than the laser device 400.

If the spatial energy distribution of the light incident on the optical fiber 204 is different, the beam quality (M ² value) of the light emitted from the optical fiber 204 is different. Specifically, the M ² value is smaller in the laser device 200 than in the laser device 400. That is, the beam quality is improved. By the way, when the light emitted from the surface emitting laser array is directly condensed by the condenser lens, the condensed light has a spatially relatively uniform intensity distribution in the radial direction. When light having such an intensity distribution is incident on the optical fiber, the beam quality of the light emitted from the opposite side of the optical fiber is poor (the M ² value is large).

  FIG. 8A shows a schematic diagram of light incident on the laser resonator 206 in the laser device 200, and FIG. 8B shows the light incident on the laser resonator 206 in the laser device 400. A schematic diagram is shown. Hereinafter, the light incident on the laser resonator 206 is also referred to as “excitation light”.

The M ² value is a parameter representing the beam quality of light, and indicates the spread of light before and after the beam waist w ₀ . In the laser apparatus 400, since the M ² value is large, the beam diameter is greatly expanded before and after the beam waist w ₀ as shown in FIG. 8B.

By the way, the region excited most strongly in the laser resonator 206 is a region within the beam waist w ₀ (inside of the broken line in the figure), and only this region contributes to the oscillation of the Q-switched laser. All the light incident on the outside of the beam waist is lost and converted into heat.

  For this reason, in the laser apparatus 400, the excitation efficiency of the laser resonator 206 is very low as compared with the laser apparatus 200, and an excitation light source with a very large output is required to obtain desired Q-switched laser characteristics. . Further, the laser device 400 has the disadvantage that the characteristics of the laser resonator 206 fluctuate due to the influence of heat, and it is difficult to stably obtain desired Q-switched laser characteristics.

On the other hand, in the laser apparatus 200, since the M ² value is small, the beam diameter spread before and after the beam waist w ₀ is small as shown in FIG. For this reason, in the laser apparatus 200, the ratio of the light which becomes a loss becomes small, and excitation efficiency becomes very high. Furthermore, since the heat generation is small, the characteristics of the laser resonator 206 are stable, and as a result, desired Q-switch laser characteristics can be stably obtained.

In the laser apparatus 200, the output of the light emitted from the optical fiber 204 (fiber-out output) was about 180 W (watts), and the M ² value indicating the beam quality was about 1200. A desired Q-switch output could be obtained from the laser resonator 206.

By the way, in order to obtain a desired Q-switched laser output, it is necessary to efficiently excite the laser resonator 206. This requires (1) a high pumping light output and (2) a good beam quality, that is, a small M ² value.

  Therefore, as means for achieving both of these two characteristics, the inventors of the surface emitting laser array 201 have a shape of a region where a plurality of light emitting portions are formed, the presence / absence of the microlens array 203a, and the optical fiber 204. We paid attention to the combination with the core diameter. In the following, in order to avoid complication, a region where a plurality of light emitting portions in the surface emitting laser array 201 are formed is abbreviated as “light emitting portion region”.

  In order to obtain large output excitation light, it is conceivable to enlarge the light emitting region. In order to transmit light from this large light emitting region to the laser resonator 206, it is necessary to increase the core diameter of the optical fiber 204.

However, M ² value of the light emitted from the optical fiber 204 is determined by the core diameter of the optical fiber 204, M ² value of the light core diameter of the optical fiber 204 is emitted larger increases. If the M ² value is excessively large, it is difficult to efficiently excite the laser resonator 206, and a desired Q-switched laser output may not be obtained even if the pumping light output is large.

  Therefore, the inventors conducted various experiments while changing the size and shape of the light emitting region, the core diameter of the optical fiber, and the like, and obtained the relationship between them and the Q-switched laser output. An example of the result is shown in FIG. In the evaluation of the Q-switch laser output, the case where a desired Q-switch laser output is obtained is “◯”, and the case where the desired Q-switch laser output is not obtained is “x”.

From the above various experimental results, in the present embodiment, (1) the light emitting portion region is a circular shape having a diameter of 7.0 mm or more, or a regular polygon having six or more corners, and (2) the optical fiber 204 It has been found that when the core diameter is 1.0 mm or more and less than 2.0 mm, the above two characteristics can be made compatible. Further, it has been found that it is preferable to have the microlens array 203a, the fiber-out output is 120 W (watts) or more, and the M ² value of the light emitted from the optical fiber 204 is 1200 or less.

  As is clear from the above description, in the laser device 200 according to this embodiment, the microlens array 203a and the condensing lens system 203b cause “on the optical path of the light emitted from the surface emitting laser” in the laser device of the present invention. An optical system arranged in the above is configured. The first optical element is configured by the microlens array 203a, and the second optical element is configured by the condenser lens system 203b.

  Further, the second condensing optical system 205 constitutes “at least one optical element disposed on the optical path between the transmission member and the laser resonator” in the laser apparatus of the present invention, and the optical fiber 204 transmits the light. A member is configured.

  Further, in the ignition device 301 according to the present embodiment, the optical system in the ignition device of the present invention is configured by the emission optical system 210.

  As described above, the laser apparatus 200 according to this embodiment includes the surface emitting laser array 201, the first condensing optical system 203, the optical fiber 204, the second condensing optical system 205, and the laser resonator 206. Yes.

  The surface emitting laser array 201 has a plurality of light emitting units. The first condensing optical system 203 includes a microlens array 203a and a condensing lens system 203b. The microlens array 203a has a plurality of lens units corresponding to a plurality of light emitting units. Each lens unit collimates the light emitted from the corresponding light emitting unit.

  The laser resonator 206 is a composite crystal of a laser medium 206a and a saturable absorber 206b.

In this case, the condensing lens system 203 b performs condensing so that the light energy is concentrated near the center of the optical fiber 204. The light incident on the laser resonator 206 has a small M ² value. For this reason, in the laser resonator 206, the ratio of light that is lost becomes small, and the excitation efficiency becomes very high. That is, according to the laser apparatus 200 according to the present embodiment, high excitation efficiency can be realized. Further, the characteristics of the laser resonator 206 are stabilized, and as a result, desired Q-switch laser characteristics can be stably obtained.

  In this embodiment, the light emitted from the surface emitting laser array 201 is transmitted to the laser resonator 206 through the optical fiber 204. In this case, since the surface emitting laser array 201 can be installed in a place away from the combustion chamber 304, it is possible to suppress the reliability of the surface emitting laser array 201 from being impaired.

  Further, the light emitted from the optical fiber 204 is condensed by the second condensing optical system 205 and enters the laser resonator 206. In this case, even if the sectional area of the laser resonator 206 is small, the light emitted from the optical fiber 204 can be efficiently incident on the laser resonator 206.

  Since the ignition device 301 according to the present embodiment includes the laser device 200, as a result, efficient and stable ignition is possible.

  Furthermore, since the engine 300 according to the present embodiment includes the ignition device 301, as a result, the operation can be stabilized efficiently.

  In the above-described embodiment, the case where the light emitting portion region has a circular shape with a diameter of 9.0 mm and the core diameter of the optical fiber 204 is 1.5 mm has been described. However, the present invention is not limited to this. In short, the light emitting region is a circular shape having a diameter of 7.0 mm or more, or a regular polygon having 6 or more corners, and the core diameter of the optical fiber 204 is 1.0 mm or more and less than 2.0 mm. I just need it.

In the above embodiment, the case where the output of the light emitted from the optical fiber 204 (fiber-out output) is about 180 W (watts) and the M ² value indicating the beam quality is about 1200 has been described. It is not limited to this. The fiber-out output is preferably 120 W (watts) or more, and the M ² value is preferably 1200 or less.

  In the laser device 200, a surface emitting laser having one light emitting unit may be used instead of the surface emitting laser array 201 depending on the application.

  DESCRIPTION OF SYMBOLS 200 ... Laser apparatus, 201 ... Surface emitting laser array (surface emitting laser), 203 ... 1st condensing optical system, 203a ... Micro lens array (1st optical element), 203b ... Condensing lens system (2nd optical element) 204 ... Optical fiber (transmission member), 205 ... Second condensing optical system (at least one optical element arranged on the optical path between the transmission member and the laser resonator), 206 ... Laser resonator, 206a ... Laser medium, 206b ... saturable absorber, 210 ... emission optical system (optical system for condensing light from the laser device), 300 ... engine (internal combustion engine), 301 ... ignition device, 302 ... fuel ejection mechanism, 303 ... Exhaust mechanism, 304 ... combustion chamber, 305 ... piston.

Special table 2013-545280 gazette Special table 2009-538402

Claims (16)

A surface emitting laser;
An optical system disposed on an optical path of light emitted from the surface emitting laser;
A laser resonator on which light through the optical system is incident,
The optical system includes a first optical element that collimates light emitted from the surface-emitting laser, and a second optical element that condenses light via the first optical element. The surface emitting laser has a plurality of light emitting units,
2. The laser device according to claim 1, wherein the first optical element is a microlens array having a plurality of lens units corresponding to the plurality of light emitting units.   The laser device according to claim 1, wherein the laser resonator is a Q-switched laser.   The laser device according to claim 3, wherein the laser resonator includes a laser medium and a saturable absorber.   5. The laser device according to claim 4, wherein the laser medium is a YAG crystal doped with Nd, and the saturable absorber is a YAG crystal doped with Cr.   The laser device according to claim 4, wherein the laser resonator is a composite crystal.   The laser device according to claim 1, wherein the laser resonator is ceramic.   The laser apparatus according to claim 1, further comprising a transmission member that transmits light from the second optical element to the laser resonator.   The laser apparatus according to claim 8, further comprising at least one optical element disposed on an optical path between the transmission member and the laser resonator.   The laser device according to claim 8, wherein the transmission member is an optical fiber.   The laser device according to claim 10, wherein a diameter of a core in the optical fiber is 1.0 mm or more and less than 2.0 mm.   The laser apparatus according to claim 10 or 11, wherein a fiber-out output of the optical fiber is 120 watts or more. 13. The laser device according to claim 10, wherein an M ² value of light emitted from the optical fiber is 1200 or less. The surface emitting laser has a plurality of light emitting units,
The laser according to any one of claims 1 to 13, wherein a shape of the region where the plurality of light emitting portions are formed is a circular shape or a regular polygon having six or more corners. apparatus. A laser device according to any one of claims 1 to 14,
And an optical system that collects light from the laser device. In an internal combustion engine that generates combustion gas by burning fuel,
An internal combustion engine comprising the ignition device according to claim 15 for igniting the fuel.

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