JP2010014030A - Laser ignition device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a laser ignition device capable of achieving at least one of reduction of a number of lenses for collecting a leaser beam or reduction of the length in an optical axis direction in the laser ignition device compared to a laser ignition device of which surface on a side for taking out the laser beam in an output mirror is plane. <P>SOLUTION: The laser ignition device for igniting fuel in a combustion chamber of an internal combustion engine is provided with a Q switch solid laser oscillator wherein the output mirror is one of a biconcave lens, a plano-concave lens and a meniscus concave lens, and the surface on the side for taking out the laser beam is a concave surface; and an output optical system for collecting the laser beam output from the Q switch solid laser oscillator in the combustion chamber. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a laser ignition device, and in particular, the number of lenses for condensing laser light is smaller than that of a laser ignition device having a flat surface on the side from which laser light is extracted in an output mirror, or laser ignition. The present invention relates to a laser ignition device that can achieve at least one of the lengths in the optical axis direction of the device being short.

  A spark plug is generally used as an ignition device for an internal combustion engine. A spark plug is a device that ignites an air-fuel mixture by applying a high voltage between electrodes to generate a discharge between the electrodes. Since the spark plug has a relatively simple structure and has practically sufficient durability, it has long been used as an ignition device in an internal combustion engine.

  In recent years, higher compression ratios have been desired for internal combustion engines in order to reduce fuel consumption and harmful emissions. In general, the lower the pressure, the easier the discharge, so a high compression ratio is a disadvantageous condition for ignition by a spark plug. In order to discharge reliably even at a high compression ratio, it is necessary to change the electrode shape and increase the applied voltage. However, these may cause a decrease in durability, a decrease in ignitability, a dielectric breakdown, or an increase in cost.

  Further, ignition position control and ignition from a plurality of locations are being studied in order to end combustion in a shorter time and at a lower temperature. However, since the spark plug is ignited in the vicinity of the inner wall surface of the combustion chamber, it is disadvantageous for the requirement of controlling the ignition point.

  Laser ignition devices have attracted attention as ignition means that meet these requirements. The laser ignition device condenses a pulse laser with a lens, and ignites the plasma generated at the condensing point as a starting point. In contrast to the spark plug, the laser ignition device improves the ignitability as the compression ratio increases. Further, by adjusting the condensing point position, there is an advantage that ignition from a single or a plurality of arbitrary ignition points in the combustion chamber is possible. In addition, there are advantages such as no generation of electrical noise, no short circuit due to attached dirt, and no protrusions that obstruct flame propagation in the combustion chamber.

  Although the concept of laser ignition devices has been around for a long time, many problems remain to be put into practical use. However, due to the ever-increasing demand for advanced combustion control of internal combustion engines and the high output and low price of laser diodes, diode-pumped solid-state lasers can be made smaller and cheaper than before. In recent years, it has been in the spotlight again.

In order to ignite the air-fuel mixture with a laser, it is necessary to generate a pulse having a very large energy of several mJ to several tens of mJ and a pulse width of about ns. Among several methods for generating such giant pulses, laser generators that generate high energy pulses by Q-switch oscillation of a resonator are widely used. The Q switch includes an active Q switch and a passive Q switch. As a typical example of an active Q switch, the direction of a mirror or a prism inserted in the resonator is mechanically controlled, and similarly, an oscillation wavelength of an electro-optic element, an acousto-optic element, or the like inserted in the resonator. There is a method of changing the transmittance by switching from the outside. A typical example of the passive Q switch is a method of inserting a saturable absorber such as Cr ⁴⁺ : YAG into the resonator.

  In passive Q switching using a saturable absorber, there is little change in pulse energy due to fluctuations in the intensity of the excitation light source, and a saturable absorber is inserted into the laser optical system without adding electrical or mechanical elements. The passive Q switch is known as a simple pulse generation method because it has a feature that a pulse can be obtained only by the above method.

  Among pulse laser oscillators with a passive Q switch, a typical example of a pulse laser oscillator is an end face excitation type pulse laser oscillator in which excitation light is introduced along the optical axis of a Fabry-Perot resonator. .

  In particular, a dielectric multilayer coating that transmits excitation light and totally reflects oscillation light is formed on one surface of a plate-shaped solid amplification medium, and this dielectric multilayer coating surface is formed on one end surface of a Fabry-Perot resonator. A plano-concave lens is arranged on the solid amplification medium side with respect to the coating surface, and a dielectric multilayer coating that reflects part of the oscillation light is formed on the concave surface, and this dielectric multilayer coating surface is Fabry-Perot. The Fabry-Perot resonator configured by using the other end face of the type resonator is widely used due to the simplicity of the configuration, the ease of manufacture of individual optical elements, and the ease of adjustment.

  A passive Q-switched laser using such a Fabry-Perot resonator and a laser ignition device using the passive Q-switched laser are well-known technologies, and are described in many documents such as Patent Document 1 and Patent Document 2. .

  By the way, when such a laser ignition device is used for an internal combustion engine, the pulse laser is condensed at a position where it is desired to be ignited, and further the diameter of the condensed light is made sufficiently small so that the electric field strength at the same position is extremely reduced. Need to be high. The condensing diameter is generally said to be 0.05 mm, preferably 0.02 mm or less. In order to obtain a pulse laser having a desired condensing diameter, the diameter of the pulse laser is enlarged after being enlarged using at least two lenses, or the diameter of the pulse laser is sufficiently enlarged. A method of condensing the enlarged pulse laser with a lens at a distance is conceivable. However, since the former needs to add the number of lenses, there is a problem that the cost increases with the increase in the number of parts and the trouble of adjustment. The latter has a problem that the distance from the laser output to the condensing point becomes long, leading to an increase in the length of the passive Q-switch pulse laser resonator.

JP-A-01-112788 JP 2006-329186 A

  The problem of the present invention is that the number of lenses for condensing the laser light is smaller than that of a laser ignition device in which the surface on the laser beam extraction side in the output mirror is a flat surface, or the optical axis in the laser ignition device. It is an object of the present invention to provide a laser ignition device capable of achieving at least one of the short direction lengths.

As means for solving the problems,
Claim 1
A laser ignition device for igniting fuel in a combustion chamber of an internal combustion engine, wherein an output mirror is any one of a biconcave lens, a planoconcave lens, and a meniscus concave lens, and a surface on the side from which laser light is extracted is concave A Q-switched solid-state laser oscillator and an output optical system for condensing the laser light output from the Q-switched solid-state laser oscillator in the combustion chamber,
Claim 2
2. The laser ignition device according to claim 1, wherein the Q-switched solid-state laser oscillator includes an amplification medium and a saturable absorber between a reflecting mirror and an output mirror.
Claim 3
3. The laser according to claim 1, wherein the output mirror is formed of a biconcave lens, and a radius of curvature of a surface on the side from which laser light is extracted is smaller than a radius of curvature of a surface on the side facing the reflecting mirror. An ignition device,
Claim 4
The laser ignition device according to any one of claims 1 to 3, further comprising an excitation optical system for condensing excitation light on the amplification medium.
Claim 5
Furthermore, it has an excitation light source which supplies the excitation light which excites the said amplification medium, It is a laser ignition apparatus as described in any one of Claims 1-4 characterized by the above-mentioned.

  According to the laser ignition device of the present invention, the output mirror in the Q-switch solid-state laser oscillator is any one of a biconcave lens, a plano-concave lens, and a meniscus concave lens. Since the downstream surface (hereinafter also referred to as the emission side surface) is a concave surface with the direction in which the laser beam is output from the solid-state laser oscillator as the forward direction, the laser beam in the output mirror on the side from which the laser beam is extracted. Compared to an output mirror having a flat surface, the spread angle of the laser light output from the Q-switched solid-state laser oscillator is increased. Therefore, it is not necessary to provide a lens for increasing the diameter of the laser beam, that is, the radius of the cross section perpendicular to the optical axis direction of the laser beam in the output optical system. In addition, since the surface on the laser output side of the output mirror is concave, the diameter of the laser light output from the Q-switched solid-state laser oscillator is sufficiently large, and the diameter of the laser light is further reduced by the condenser lens. be able to. As a result, the length of the output optical system in the optical axis direction can be shortened. Therefore, the number of lenses for condensing the laser light is smaller than that of the conventional output mirror in which the laser light extraction surface is a flat surface, or in the optical axis direction of the laser ignition device. A laser ignition device capable of achieving at least one of the short lengths can be provided.

  When the output mirror is a biconcave lens, if the radius of curvature of the surface from which the laser light is extracted is smaller than the radius of curvature of the surface facing the reflecting mirror, the laser light output from the Q-switched solid-state laser oscillator is further increased. Therefore, the length of the excitation optical system in the optical axis direction can be further shortened, and as a result, a laser ignition device with a short overall length can be provided.

  Furthermore, when the amplification medium has an excitation optical system that condenses the excitation light, the incident diameter of the excitation light incident on the surface of the amplification medium can be set as desired. As a result, the pulse energy can be set as desired.

  First, a laser ignition device which is an embodiment of a laser ignition device according to the present invention will be described with reference to FIG. FIG. 1 is an explanatory diagram of a configuration of a laser ignition device according to an embodiment of the present invention. The laser ignition device 1 of the present embodiment condenses the excitation light 2 on a semiconductor laser 3 that is an example of an excitation light source that outputs the excitation light 2, an optical fiber 4 that transmits the excitation light 2, and an amplification medium 5. A Q-switched solid-state laser oscillator 10 having an amplifying medium 5 and a saturable absorber 9 between an excitation optical system 6 to be performed, a reflecting mirror 7 and an output mirror 8, and a laser output from the Q-switched solid-state laser oscillator 10 And an output optical system 12 for condensing the light 11 in the combustion chamber of the internal combustion engine.

  The semiconductor laser 3, which is an example of an excitation light source, has desired energy from the Q-switched solid-state laser oscillator 10 by irradiating the amplification medium 5 in the Q-switched solid-state laser oscillator 10 with the excitation light 2 and exciting the amplification medium 5. As long as a pulse laser can be output, a known semiconductor laser can be employed.

  The optical fiber 4 transmits the excitation light 2 output from the semiconductor laser 3 which is an example of the excitation light source, and irradiates the amplification medium 5 in the Q-switch solid-state laser oscillator 10 with the excitation light 2 to excite the amplification medium 5. As long as the pulse laser 11 having desired energy can be output from the Q-switched solid-state laser oscillator 10, a known optical fiber can be used. The pumping light 2 output from the semiconductor laser 3 may be condensed by the pumping optical system 6 without being passed through the optical fiber 4 and irradiated to the amplification medium 5. It is preferable to make the excitation light 2 incident on the excitation optical system 6 because the semiconductor laser 3 can be placed away from the internal combustion engine and is less susceptible to the heat of the internal combustion engine.

The excitation optical system 6 includes an input unit 13 to which the excitation light 2 is supplied and at least one lens 14 and 15, and condenses the excitation light 2 output from the semiconductor laser 3 which is an example of the excitation light source. As long as it can enter the amplification medium 5 with a desired incident diameter, a lens having an arbitrary shape, for example, a biconcave lens, a biconvex lens, a planoconcave lens, a planoconvex lens, a meniscus concave lens, a meniscus convex lens, or the like can be disposed. . In the present embodiment, the excitation optical system 6 includes a plano-convex lens 14 having a convex portion on the downstream side of the input unit 13 and the excitation light 2 and a plano-convex lens 15 having a convex portion on the upstream side of the excitation light from the side irradiated with the excitation light. The pumping light 2 arranged in this order and input from the input unit 13 is collected after collimation using the two lenses 14 and 15, passes through the reflecting mirror 7 in the Q-switch solid-state laser oscillator 10, and is amplified. Is incident. The incident diameter is a value obtained by approximating a region that is 1 / e ² (e represents a natural logarithm) of the peak light intensity with a circle when considering the light intensity distribution in a cross section orthogonal to the incident light. Is the diameter.

  The pumping light 2 only needs to be able to irradiate the pumping light 2 to a region including the optical axis of the Q-switched solid-state laser oscillator 10 in the amplification medium 5. In the laser ignition device 1 according to this embodiment, the Q-switching solid-state is used. An excitation optical system 6 is arranged on an extension line of the optical axis of the laser oscillator 10, and the excitation light 2 collected by the excitation optical system 6 passes through the reflecting mirror 7 and reaches the surface of the plate-like amplification medium 5. Irradiated vertically. In addition, for example, the excitation optical system 6 may be arranged at a position where the excitation light 2 is incident obliquely with respect to the surface of the amplification medium 5 on which the excitation light 2 is incident.

  In the Q-switched solid-state laser oscillator 10 of this embodiment, the reflecting mirror 7 and the output mirror 8 are arranged to face each other, and the amplification medium 5 and the saturable absorber 9 are disposed between the reflecting mirror 7 and the output mirror 8. Are arranged to form a passive Q-switched solid state laser oscillator. The Q-switched solid-state laser oscillator only needs to be able to output a pulse laser having a desired energy at a desired timing, which can ignite the fuel in the combustion chamber of the internal combustion engine, and instead of the saturable absorber, An active Q-switched solid-state laser oscillator such as an acousto-optic device, an electro-optic device, or a switch method using ultrasonic waves or a method using a rotating mirror can also be employed. The method of oscillating a pulsed laser by arranging a saturable absorber is such that there is little fluctuation in pulse energy due to fluctuations in the intensity of pumping light output from the pumping light source, and reflection without adding electrical or mechanical elements. Since a pulse laser can be obtained simply by inserting a saturable absorber between the mirror and the output mirror, it is convenient and preferable.

  As the optical components such as the reflecting mirror 7, the output mirror 8, the amplification medium 5, and the saturable absorber 9, known optical components can be adopted as long as a pulse laser having desired energy can be output. For example, as shown in FIG. 1, the Q-switched solid-state laser oscillator 10 is formed such that the reflecting mirror 7, the amplification medium 5, and the saturable absorber 9 are in close contact with each other. A Fabry-Perot resonator is formed between them. In addition, any resonator configuration can be adopted as long as the pulse laser can be oscillated. For example, a ring resonator and a composite resonator can be used.

  The reflecting mirror 7 preferably reflects the oscillation light 16 oscillated from the amplification medium 5 with a reflectance greater than 99% (a reflectance of approximately 100%) and transmits the excitation light. As shown in FIG. 1, in the laser ignition device 1 of the present embodiment, the reflecting mirror 7 is formed by coating a dielectric multilayer film on the surface of the plate-like amplification medium 5 on the side where the excitation light 2 is incident. Although formed, the reflecting mirror 7 may be arranged as an independent component. The output mirror 8 preferably reflects the oscillation light 16 oscillated from the amplification medium 5 with a reflectance of 10 to 98%, particularly preferably with a reflectance of 60 to 90%, and the remaining oscillation light 16 is transparent. As shown in FIG. 1, in the laser ignition device 1 of the present embodiment, a dielectric multilayer film is coated on the surface facing the reflecting mirror 7 in the biconcave lens having the same curvature radius on both sides. Although the output mirror 8 is formed, the output mirror 8 does not reflect a part of the oscillation light 16 by coating the biconcave lens with the dielectric multilayer film, but the biconcave lens reflects a part of the oscillation light 16. It may be made of a reflective material. The reflecting mirror 7 can have an arbitrary shape as long as the light stimulated and emitted from the amplification medium 5 reciprocates between the reflecting mirror 7 and the output mirror 8 and can oscillate the laser light 11. A shape such as a flat lens, a biconcave lens, a biconvex lens, a planoconcave lens, a planoconvex lens, a meniscus concave lens, and a meniscus convex lens can be adopted. The output mirror 8 can oscillate the laser light 11 between the reflecting mirror 7 and the output mirror 8 so that the light stimulated and emitted from the amplification medium 5 can reciprocate, and can increase the spread angle of the laser light. As long as possible, it can have an arbitrary shape. For example, a biconcave lens, a plano-concave lens, a meniscus concave lens, or the like can be employed. At this time, the surface on the side from which laser light is extracted is always a concave surface.

  The reflecting mirror 7, the amplifying medium 5, the saturable absorber 9, and the output mirror 8 are arranged in this order from the side irradiated with the normal excitation light 2. However, when the saturable absorber 9 is a material that does not absorb the excitation light 2, the order of the amplification medium 5 and the saturable absorber 9 can be switched. Further, the reflecting mirror 7, the amplifying medium 5, the saturable absorber 9, and the output mirror 8 may be formed so that all the optical components are in close contact, or at least two of these optical components are in close contact. The remaining optical components may be formed independently, or all the optical components may be formed independently.

  The amplification medium 5 is a material that is excited to emit light by being irradiated with the excitation light 2 output from the semiconductor laser 3 that is an example of the excitation light source. For example, Nd: YAG, Yb: YAG, and the like. Single crystals and ceramics are preferably used. The shape of the amplifying medium 5 is plate-like in the example shown in FIG. 1, but the shape of the amplifying medium 5 is such that light reciprocates inside the Q-switched solid-state laser oscillator 10, and the Q-switched solid-state laser oscillator 10 The laser beam 11 is not particularly limited as long as it can be output, and may have a convex lens shape, a concave lens shape, or the like. The size of the amplifying medium 5 is not particularly limited as long as the light reciprocating inside the Q-switched solid-state laser oscillator 10 is generated and the laser beam 11 can be output from the Q-switched solid-state laser oscillator 10. What is necessary is just to adjust suitably according to the performance requirement of the oscillator 10.

The saturable absorber 9 can change the light transmittance by absorbing light stimulated and emitted from the amplification medium 5, and is not particularly limited as long as it has a function as a Q switch. For example, Cr ⁴⁺ : YAG is preferably used.

  The output optical system 12 has at least one lens and can focus the pulse laser 11 output from the Q-switched solid-state laser oscillator 10 to a desired position in the combustion chamber of the internal combustion engine with a desired condensing diameter. As long as possible, a lens having an arbitrary shape, for example, a biconcave lens, a biconvex lens, a planoconcave lens, a planoconvex lens, a meniscus concave lens, and a meniscus convex lens can be arranged. In the present embodiment, one biconvex lens 17 is disposed as a condenser lens. In the present embodiment, the surface on the side from which the laser beam 11 is extracted in the output mirror 8 is formed in a concave shape, so that the divergence angle of the laser beam 11 is increased and the diameter of the laser beam 11 is rapidly expanded. Therefore, as compared with the conventional laser ignition device in which the surface on the side from which the laser beam 11 is extracted in the output mirror 8 is flat, the laser beam is condensed by only one biconvex lens 17. The distance to the point 18 can be shortened.

  In more detail, an advantage of the concave surface of the output mirror on the side from which laser light is extracted will be described. In order to ignite the fuel in the combustion chamber of an internal combustion engine with a laser beam, a pulse laser having a high energy of several mJ to several tens of mJ and a pulse width of about ns is collected in a spot having a small diameter of 0.05 mm or less. It is necessary to obtain a high electric field strength by making it light. Among these, as a method of condensing the pulse laser to a spot having a small diameter, it is conceivable to sufficiently expand the laser light, increase the diameter of the laser light, and further condense it with a condensing lens. When the light is condensed by the condensing lens without increasing the diameter of the laser light, the laser light cannot be narrowed down and the condensing diameter cannot be made smaller than a certain value. Therefore, when it is desired to condense laser light so as to have a predetermined condensing diameter, it is necessary to increase the diameter of the laser light to a predetermined value. As a method of expanding the laser beam and increasing the laser beam diameter to a predetermined value, there is a method of arranging a condensing lens with a sufficient distance that the laser beam diameter can reach the predetermined value. . However, in this case, the distance between the output mirror and the condenser lens tends to be long. When the divergence angle of the laser beam is small, the distance between the output mirror and the condenser lens is particularly long, so that the distance in the optical axis direction of the excitation optical system is increased, resulting in an increase in the length of the laser ignition device. Become. As another method of expanding the laser beam and increasing the laser beam diameter to a predetermined value, a method of increasing the laser beam diameter by arranging a lens between the output mirror and the condenser lens is conceivable. . However, since it is necessary to add a lens for enlarging the laser light, this leads to an increase in cost as well as an increase in the number of lenses and adjustment work. As the condensing lens for condensing the expanded laser light, any lens can be adopted as long as the laser light can be condensed at a desired position in the combustion chamber. When a convex lens is used, As the convex lens having a shorter rear focal length is used, the distance between the condensing lens and the condensing point of the laser beam can be shortened. If the surface of the output mirror on the side from which the laser beam is extracted is formed in a concave shape as in the laser ignition device of this embodiment, the laser beam can be expanded quickly without causing the above-described problem, and the desired focused diameter can be increased. The laser beam can be condensed so as to have.

  FIG. 2 is a structural explanatory view showing another embodiment of the laser ignition device according to the present invention. The laser ignition device of the present embodiment is the same as the laser ignition device shown in FIG. 1 except that the radius of curvature of the surface on the side of the output mirror from which laser light is extracted is smaller than the radius of curvature of the surface on the side facing the reflecting mirror. The configuration is the same as that of the laser ignition device shown in FIG. As the radius of curvature of the output mirror 208 on the side from which the laser beam 211 is extracted is smaller, the divergence angle of the laser beam 211 output from the output mirror 208 increases and is quickly expanded. Therefore, the output mirror 208 and the biconvex lens 217 are enlarged. It is possible to shorten the distance from the condenser lens. Therefore, the light of the output optical system 212 is more than the laser ignition device of FIG. 1 showing the case where the radius of curvature of the surface from which the laser beam 11 is extracted in the output mirror 8 is equal to the radius of curvature of the surface facing the reflecting mirror 7. The axial length can be shortened. As a result, the length of the laser light 211 in the output direction of the laser ignition device 201 can be shortened.

  FIG. 3 is a structural explanatory view showing another embodiment of the laser ignition device according to the present invention. The laser ignition device of the present embodiment is the laser ignition device shown in FIG. 1, the surface of the output mirror facing the reflecting mirror is a plane, the reflecting mirror, the amplification medium, the saturable absorber, and the output mirror, The configuration is the same as that of the laser ignition device shown in FIG. 1 except that it is arranged in close contact with the excitation light in this order. In the laser ignition device of this embodiment, the reflecting mirror 307, the amplifying medium 305, the saturable absorber 309, and the output mirror 308 are arranged in close contact with each other, so that the length in the optical axis direction of the laser ignition device 301 is shortened. can do. In this configuration, since the length of the Q-switched solid-state laser oscillator 310 is short, a pulse having a relatively short time and a high peak power can be obtained, and the length of the Q-switched solid-state laser oscillator 310 is short and individual optical components can be obtained. Since they are in contact with each other, not only the internal loss of the Q-switched solid-state laser oscillator 310 is small, but there is almost no need for alignment as long as the parallelism of each component is good, and the laser can be easily oscillated.

  FIG. 4 is a schematic view showing an example of a mode in which the laser ignition device shown in FIG. 1 is attached to an internal combustion engine. As shown in FIG. 4, in the laser ignition device 401 of this embodiment, an excitation optical system 406, a Q-switched solid-state laser oscillator 410, and an output optical system 412 are coaxially arranged in this order and accommodated in a casing 419. ing. An optical fiber 404 is fitted into an input unit 413 in the excitation optical system 406, and a semiconductor laser 403, which is an example of an excitation light source, is coupled via the optical fiber 404. An output window 420 for isolating the inside of the casing 419 and the combustion chamber of the internal combustion engine is provided on the surface of the casing 419 on the side from which the laser beam 411 is extracted. Any material can be used for the output window 420 as long as there is no optical action. For example, sapphire and transparent YAG can be used.

  FIG. 5 is an explanatory diagram of a cylinder when the laser ignition device 401 of the present embodiment shown in FIG. 4 is mounted on a cylinder of an internal combustion engine. A cylinder 500 of the internal combustion engine is formed by a cylinder block 501, a cylinder head 502, and a piston 503. A space surrounded by the cylinder block 501, the cylinder head 502, and the piston 503 is a combustion chamber 504. A suction pipe 505 that supplies a mixture of fuel and air is connected to the cylinder head 502, and the supply of the mixture is adjusted by opening and closing the suction valve 506. Further, an exhaust pipe 507 for discharging the air-fuel mixture is connected to the cylinder head 502, and the exhaust of the air-fuel mixture is adjusted by opening and closing the exhaust valve 508. The laser ignition device 401 of the present embodiment can be installed at any position as long as the air-fuel mixture in the combustion chamber 504 can be ignited. For example, as shown in FIG. Between the exhaust valve 508, the laser ignition device of this embodiment can be installed. The pulse laser output from the laser ignition device causes breakdown in the combustion chamber 504, and the air-fuel mixture in the combustion chamber 504 is combusted starting from the condensing point 418 of the pulse laser. Although an example of a cylinder in an internal combustion engine has been described, the configuration of the cylinder is not limited to the above configuration.

  Next, the operation of the laser ignition device 401 will be described with reference to FIG. As shown in FIG. 4, the excitation light 402 output from the semiconductor laser 403 is transmitted through the optical fiber 404 and input to the excitation optical system 406 from the input unit 413. The input excitation light 402 is condensed by two plano-convex lenses 414 and 415 in the excitation optical system 406, passes through the reflection mirror 407 of the Q-switch solid-state laser oscillator 410, and enters the amplification medium 405. The incident excitation light 402 is absorbed by the amplifying medium 405, whereby the amplifying medium 405 is excited with time, the inversion distribution density is increased, and stimulated emission occurs. In the process in which the light oscillated from the amplification medium 405 reciprocates in the Q-switched solid-state laser oscillator 410 formed by the output mirror 408 and the reflecting mirror 407, the saturable absorber 409 reciprocates in the Q-switched solid-state laser oscillator 410. Since light is absorbed and the transmittance of the saturable absorber 409 increases, the Q value, which is the ratio between the energy stored in the Q-switched solid-state laser oscillator 410 and the lost energy, rapidly increases, and the Q switch The oscillation light 416 that reciprocates inside the solid-state laser oscillator 410 and the laser light 411 that passes through the output mirror 408 increase rapidly. As the output increases rapidly, the inversion distribution density of the amplifying medium 405 rapidly decreases. Therefore, the oscillation light 416 that reciprocates in the Q-switched solid-state laser oscillator 410 and the laser light 411 that passes through the output mirror 408 can be transmitted in a short time. The output is reduced to a giant pulse.

  When the excitation light 402 from the semiconductor laser 403 is continuously irradiated, the above process is repeated, so that the pulse laser 411 is output from the laser ignition device 401 of this embodiment at predetermined intervals. On the other hand, when the inversion distribution density of the amplification medium 405 reaches the threshold value, when the excitation light 402 output from the semiconductor laser 403 is quickly stopped, a single pulse laser 411 is obtained.

  Since the output mirror 408 is formed by a biconcave lens, the pulse laser 411 output from the output mirror 408 is quickly enlarged, and further condensed by a condensing lens composed of a biconvex lens 417 and isolated by an output window 420. A condensing point 418 having a desired condensing diameter is formed in the combustion chamber of the cylinder. When an electric field higher than a threshold value determined by various conditions is generated at the condensing point 418, breakdown occurs due to the electric field, and the air-fuel mixture in the combustion chamber burns starting from the condensing point 418.

  The laser ignition device according to the present invention is not limited to the above embodiment, and various modifications can be made within the scope of the present invention.

Example 1
With respect to the optical system shown in FIG. 6, the state of beam propagation was examined using the ray matrix method. The laser wavelength was 1030 nm and the beam quality was calculated assuming M2 = 1, that is, an ideal Gaussian beam. In this optical system, the output mirror 608 is a biconcave lens. The beam diameter (diameter) 2w2 immediately before entering the biconvex lens 617 and the condensing point 618 when the distance L2 from the surface of the output mirror 608 from which the pulse laser 611 is extracted to the biconvex lens 617 are changed. The diameter (diameter) 2w3 was calculated. The results are shown in FIGS.

The reflecting mirror is formed by forming a Ta ₂ O ₅ / SiO ₂ dielectric multilayer coating on the excitation light incident surface of the amplification medium. The amplification medium 605 is Yb: YAG, and the saturable absorber 609 is Cr: YAG, output mirror 608 and biconvex lens 617 are BK7 having a refractive index of 1.51, and the output window is sapphire having a refractive index of 1.76. The dimensions and arrangement of these optical members are as follows.
The total thickness in the optical axis direction of the amplification medium 605 and the saturable absorber 609 is 2.0 mm,
The distance L1 between the surface of the saturable absorber 609 facing the output mirror 608 and the surface of the output mirror 608 facing the reflecting mirror 605 is 23.5 mm,
The maximum thickness in the optical axis direction of the output mirror 608 is 9.5 mm,
The thickness at the central axis of the output mirror 608 is 8.7 mm,
The length of the output mirror 608 in the direction orthogonal to the optical axis direction is 12.7 mm,
The curvature radius Rc1 of the surface of the output mirror 608 facing the reflecting mirror 607 and the curvature radius Rc2 on the exit side are 50 mm,
The thickness at the central axis of the biconvex lens 617 is 3.4 mm,
The focal length of the biconvex lens 617 is 40 mm,
The thickness of the output window 620 in the optical axis direction is 2.0 mm.
The incident diameter 2w1 of the excitation light to the reflecting mirror 607 is 0.4 mm.

(Example 2)
In the optical system shown in FIG. 6, the beam is the same as in Example 1 except that the radius of curvature Rc2 of the output side surface of the output mirror 608 is 25 mm and the thickness at the central axis of the output mirror 608 is 8.3 mm. The state of propagation was examined. The results are shown in FIGS.

(Comparative Example 1)
In the optical system shown in FIG. 6, the curvature radius Rc2 of the output side surface of the output mirror 608 is ∞, that is, the output side surface of the output mirror 608 is a flat surface, and the thickness at the central axis of the output mirror 608 is 9.1 mm. Except for this, the state of beam propagation was examined in the same manner as in Example 1. The results are shown in FIGS.

FIG. 7 shows the distance L2 from the surface on the side of taking out the pulse laser 611 in the output mirror 608 to the biconvex lens 617 and the beam diameter (diameter) 2w2 immediately before entering the biconvex lens 617 in Examples 1 and 2 and the comparative example. The relationship is shown.
As shown in FIG. 7, in any of Examples 1 and 2 and Comparative Example 1, the beam diameter 2w2 increases in proportion to the length of L2. Among these, Example 2 has a large inclination, and it can be seen that a large beam diameter 2w2 can be obtained even if the length of L2 is short.

FIG. 8 shows the distance L2 from the surface on the side from which the pulse laser 611 is extracted in the output mirror 608 to the biconvex lens 617 and the condensing diameter (diameter) 2w3 of the condensing point 618 in Examples 1 and 2 and the comparative example. Show the relationship.
As shown in FIG. 8, in any of Examples 1 and 2 and Comparative Example 1, the light collection diameter 2w3 becomes smaller as L2 becomes longer. In order to make the condensing diameter 0.02 mm, it is necessary to set L2 to 267 mm in Example 1, L2 to 197 mm in Example 2, and L2 to 434 mm in Comparative Example 1. In order of Example 2, Example 1, and Comparative Example, the length of L2 for obtaining the same light collection diameter becomes shorter.

FIG. 9 shows the condensing diameter (diameter) 2w3 of the condensing point 618 and the distance L4 from the surface of the reflecting mirror 607 facing the output mirror 608 to the condensing point 618 in Examples 1 and 2 and the comparative example. The relationship is shown.
As shown in FIG. 9, in any of Examples 1 and 2 and Comparative Example 1, the length of L4 increases as the light collection diameter 2w3 decreases. In order to make the condensing diameter 0.02 mm, it is necessary to set L4 to 352 mm in Example 1, L4 to 284 mm in Example 2, and L4 to 516 mm in Comparative Example 1. In order of Example 2, Example 1, and Comparative Example, the length of L4 for obtaining the same light collection diameter becomes shorter. Therefore, the optical system of Example 2 having a smaller radius of curvature on the exit side than the radius of curvature on the output side of the output mirror can minimize the length in the optical axis direction to obtain the same light collection diameter. It was.

FIG. 1 is an explanatory diagram of a configuration of a laser ignition device according to an embodiment of the present invention. FIG. 2 is a configuration explanatory view of a laser ignition device according to another embodiment of the present invention. FIG. 3 is a diagram illustrating the construction of a laser ignition device according to another embodiment of the present invention. FIG. 4 is a schematic view showing an example of a mode in which the laser ignition device shown in FIG. 1 is attached to an internal combustion engine. FIG. 5 is an explanatory diagram of a cylinder when a laser ignition device according to an embodiment of the present invention is mounted on a cylinder of an internal combustion engine. FIG. 6 is a diagram illustrating the configuration of the optical system according to the first embodiment. FIG. 7 shows the distance L2 from the surface on the side of taking out the pulse laser 611 in the output mirror 608 to the biconvex lens 617 and the beam diameter (diameter) 2w2 immediately before entering the biconvex lens 617 in Examples 1 and 2 and the comparative example. It is a figure which shows the relationship. FIG. 8 shows the distance L2 from the surface on the side from which the pulse laser 611 is extracted in the output mirror 608 to the condensing lens 617 and the condensing diameter (diameter) 2w3 of the condensing point 618 in Examples 1 and 2 and the comparative example. It is a figure which shows the relationship. FIG. 9 shows the condensing diameter (diameter) 2w3 of the condensing point 618 and the distance L4 from the surface of the reflecting mirror 607 facing the output mirror 608 to the condensing point 618 in Examples 1 and 2 and the comparative example. It is a figure which shows the relationship.

Explanation of symbols

1, 201, 301, 401 Laser ignition device 2, 202, 302, 402 Excitation light 3, 203, 303, 403 Semiconductor laser 4, 204, 304, 404 Optical fiber 5, 205, 305, 405, 605, Amplifying medium 6 , 206, 306, 406 Excitation optical system 7, 207, 307, 407, 607, reflector 8, 208, 308, 408, 608, output mirror 9, 209, 309, 409, 609 saturable absorber 10, 210, 310, 410 Q-switched solid state laser oscillator 11, 211, 311, 411, 611 Pulse laser 12, 212, 312, 412 Output optical system 13, 213, 313, 413 Input unit 14, 214, 314, 414, 15, 215, 315, 415 Plano-convex lens 16, 216, 316, 416, 616 Oscillating light 17, 17, 317, 417, 617 Biconvex lens 18, 218, 318, 418, 618, condensing point 419 Case 420, 620, output window 500 cylinder 501 cylinder block 502 cylinder head 503 piston 504 combustion chamber 505 suction pipe 506 suction valve 507 Exhaust pipe 508 Exhaust valve L1 Distance between the surface of the saturable absorber facing the output mirror and the surface of the output mirror facing the reflecting mirror L2 The condensing lens from the surface of the output mirror from which the laser light is extracted L3 Distance from the condensing lens to the condensing point L4 Distance from the surface of the reflecting mirror facing the output mirror to the condensing point 2w1 Incident diameter of the excitation light to the reflecting mirror 2w2 Incident into the condensing lens Previous beam diameter 2w3 Condensing diameter at condensing point Rc1, Rc2 Curvature radius of output mirror

Claims (5)

  A laser ignition device for igniting fuel in a combustion chamber of an internal combustion engine, wherein an output mirror is any one of a biconcave lens, a planoconcave lens, and a meniscus concave lens, and a surface on the side from which laser light is extracted is a concave surface A laser ignition device comprising: a Q-switched solid-state laser oscillator, and an output optical system for condensing laser light output from the Q-switched solid-state laser oscillator in the combustion chamber.   2. The laser ignition device according to claim 1, wherein the Q-switched solid-state laser oscillator includes an amplification medium and a saturable absorber between a reflecting mirror and an output mirror.   3. The laser according to claim 1, wherein the output mirror is formed of a biconcave lens, and a radius of curvature of a surface from which laser light is extracted is smaller than a radius of curvature of a surface opposite to the reflecting mirror. Ignition device.   The laser ignition device according to any one of claims 1 to 3, further comprising an excitation optical system for condensing excitation light on the amplification medium.   The laser ignition device according to claim 1, further comprising an excitation light source that supplies excitation light for exciting the amplification medium.

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