Laser device and internal combustion engine

Laser devices include a light source that emits a laser beam, an optical system that concentrates the laser beam emitted from the light source, an optical window through which the laser beam exited from the optical system passes, a housing that accommodates the optical system, and an optical window holding member fixed to the housing. The optical window holding member holds the optical window. In the first laser device, the optical window has a face or a protruding face through which the laser beam passes. When the optical window has the face, the face is flush with an edge of the optical window holding member and a film is formed on the face. When the optical window has the protruding face, the protruding face protrudes with reference to the edge of the optical window holding member and a film is formed on the protruding face.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is based on and claims priority pursuant to 35 U.S.C. § 119(a) to Japanese Patent Application Nos. 2017-019616, 2017-051947, and 2017-238788, filed on Feb. 6, 2017, Mar. 16, 2017, and Dec. 13, 2017, respectively, in the Japan Patent Office, the entire disclosures of which are hereby incorporated by reference herein.

BACKGROUND

Technical Field

Embodiments of the present invention relate to a laser device and an internal combustion engine.

Background Art

Laser devices that adopt a semiconductor laser as a pump source are expected to be applied to various kinds of fields including, for example, ignition systems, laser beam machines, and medical equipment. In particular, methods have been studied in which such laser devices are used as an ignition system in internal combustion engines of cars or the like.

In such an ignition system, a Q-switched laser resonator is irradiated with the laser beams (pump light) that are emitted from a semiconductor laser to emit pulsed laser beams of high energy density. The ignition system is provided with a condenser lens inside the cylinder head and a transparent window (optical window) to which light is incident, and the emitted pulsed laser beams are concentrated into the mixture of gases inside the combustion chamber through the condenser lens and the transparent window of the combustion chamber. As a result, plasma is generated inside the combustion chamber, and the fuel that is injected into the combustion chamber is ignited (see, for example, JP-2016-109128-A).

In the known ignition systems, the both edges of an optical window are clamped by a protective glass holder, and the protective glass holder is fixed to the housing. Moreover, an antireflection film is arranged on the incident plane side of the optical window to prevent the pulsed laser beams that are emitted from a laser resonator from being reflected by the incident plane of the optical window.

SUMMARY

Embodiments of the present disclosure described herein provide two types of laser devices. These laser devices include a light source that emits a laser beam, an optical system that concentrates the laser beam emitted from the light source, an optical window through which the laser beam exited from the optical system passes, a housing that accommodates the optical system, and an optical window holding member fixed to the housing. The optical window holding member holds the optical window. In the first laser device, the optical window has a face or a protruding face through which the laser beam passes. When the optical window has the face, the face is flush with an edge of the optical window holding member and a film is formed on the face of the optical window. When the optical window has the protruding face, the protruding face protrudes with reference to the edge of the optical window holding member and a film is formed on the protruding face of the optical window. In the second laser device, the optical window has a depressed face through which the laser beam passes and the depressed face is depressed with reference to an edge of the optical window holding member, the optical window holding member has a diameter-enlarged portion whose internal diameter is enlarged from a depressed position of the depressed face of the optical window towards an outside of the optical window holding member, and a film is formed on the depressed face that is at the depressed position.

DETAILED DESCRIPTION

In describing example embodiments shown in the drawings, specific terminology is employed for the sake of clarity. However, the present disclosure is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that have the same structure, operate in a similar manner, and achieve a similar result.

Some embodiments of the present disclosure are described below in detail with reference to the accompanying drawings.

First Embodiment

An embodiment in which a laser device according to a first embodiment of the present disclosure is used for as an internal combustion engine is described with reference to the drawings. In the present embodiment, an engine is used as the internal combustion engine.

FIG. 1is a schematic diagram of elements of an internal combustion engine for which a laser device according to the first embodiment the present disclosure is provided.

As illustrated inFIG. 1, an engine10includes, for example, a laser device11, a fuel injector12, an exhauster13, a combustion chamber14, and a piston15.

The operation of the engine10is briefly described. The fuel injector12injects the inflammable fuel-air mixture into the combustion chamber14(aspiration). Then, the piston15moves upward, the inflammable fuel-air mixture is compressed (compression). The laser device11concentrates the laser beams into the compressed mixture of gases in the combustion chamber14, in order to generate plasma. Then, the fuel in the mixture of gases is ignited by the generated plasma (ignition). As the mixture of gases burns (explodes) due to the ignition, the inflammable gas expands inside the combustion chamber14. As a result, the piston15moves downward (combustion). After that, the exhauster13exhausts the inflammable gas to the outside of the combustion chamber14(exhaust).

As described above, a series of processes including aspiration, compression, ignition, combustion, and exhaust are repeated in the engine10. Then, the piston15moves upward and downward according to the changes in the volume of the gas in the combustion chamber14, and kinetic energy is produced. As fuel, for example, natural gas and gasoline are used.

Note that the laser device11is electrically connected to a driver16that is arranged outside the engine10, and the driver16controls the laser beams emitted from the laser device11based on instructions given from an engine controller17.

FIG. 2is a diagram illustrating a configuration of the laser device11, according to the present embodiment.

As illustrated inFIG. 2, the laser device11includes a surface emitting laser (light source)21, a first condensing optical system22, an optical fiber (transmission member)23, a second condensing optical system24, a laser resonator25, a third condensing optical system26, a window27A, and a housing28. InFIG. 2, the laser beams are indicated by the two-dot chain line. In the present disclosure, a three-dimensional rectangular coordinate system where the X-axis direction, Y-axis direction, and the Z-axis direction make up the triaxial directions is used. It is assumed that the direction in which the surface-emitting laser21emits light is in the +Z-direction, and that the two directions orthogonal to each other on a plane perpendicular to the optical axis of the laser beams are the X-axis direction and the Y-axis direction.

The surface-emitting laser21is a pump source, and includes a plurality of light-emitting units. Each of the light-emitting units is a vertical cavity-surface emitting laser (VCSEL). When the surface-emitting laser21emits laser beams, the multiple light-emitting units emit laser beams at the same time. On the other hand, when the surface-emitting laser21does not emit laser beams, the multiple light-emitting units are turned off at the same time. Moreover, the wavelength of the laser beams that are emitted from the surface-emitting laser21is, for example, about 808 nanometers (nm).

The surface-emitting laser21is electrically connected to the driver16, and the driver16drives the surface-emitting laser21based on instructions given from the engine controller17. Accordingly, the laser beams are emitted from the surface-emitting laser21.

It is to be noted that an end-surface emitting laser is known as a semiconductor laser. However, the wavelength of the laser beams that are emitted from such an end-surface emitting laser tends to fluctuate widely depending on the temperature. The laser device11is used under high-temperature environments around the engine10. For this reason, when an end-surface emitting laser is used as a pump source, a high-precision temperature control unit that maintains the temperature of the end-surface emitting laser at a constant level needs to be provided. This leads to an increase in the cost of manufacturing the laser device11and an increase in the size of the laser device11.

By contrast, changes in the wavelength of the laser beams that are emitted from the surface-emitting laser21is about one-tenths of the changes in the wavelength of the laser beams that are emitted from the end-surface emitting laser. The laser device11uses the surface-emitting laser21as a pump source. Accordingly, a high-precision temperature control unit is not necessary. This leads to a reduction in the cost of manufacturing the laser device11and a reduction in the size of the laser device11. Moreover, the light-emitting area of the surface-emitting laser21is arranged inside the semiconductor. Accordingly, the surface-emitting laser21can emit laser beams in a stable manner with no concern about the damage at the end surface.

Note also that the surface-emitting laser21has very little temperature-driven wavelength displacement in the emitted laser beams. For this reason, the surface-emitting laser21is a light source advantageous in increasing the energy density of the laser beams in a Q-switched laser resonator whose characteristics vary widely due to the wavelength displacement. Accordingly, when the surface-emitting laser21is used as a pump source, the temperature control of the environment becomes easier.

The first condensing optical system22concentrates the laser beams that are emitted from the surface-emitting laser21into the core diameter of the optical fiber23on the −Z side lateral edge face. The first condensing optical system22includes at least one condenser lens. In the present embodiment, the first condensing optical system22includes a microlens221and a condenser lens system222.

The microlens221is disposed in the optical path of the laser beams emitted from the surface-emitting laser21. The microlens221includes a plurality of lenses that correspond to the multiple light-emitting units of the surface-emitting laser21. The lenses of the microlens221approximately collimates the laser beams emitted from the corresponding light-emitting units of the surface-emitting laser21. In other words, the microlens221collimates the laser beams emitted from the surface-emitting laser21.

The distance between the surface-emitting laser21and the microlens221in the Z-axis direction is determined according to the focal length of the microlens221.

The condenser lens system222concentrates the laser beams that have passed through the microlens221.

The condenser lens system222is appropriately selected according to the cross-sectional area of the laser beams that have passed through the microlens221and the core diameter and numerical aperture (NA) of the optical fiber23. The condenser lens system222may include a plurality of optical elements.

The first condensing optical system22is satisfactory as long as it includes at least one condenser lens, and may include a plurality of optical elements.

The optical fiber23is disposed such that the laser beams exited from the first condensing optical system22is condensed at the center of the −Z side lateral edge face of the core. In the present embodiment, for example, an optical fiber where the core diameter is 1.5 mm is used as the optical fiber23.

The laser beams incident on the optical fiber23propagate through the core, and exit from the +Z side lateral edge face of the core.

Due to the provision of the optical fiber23, the surface-emitting laser21may be disposed at a position distant from the laser resonator25. Accordingly, the degree of flexibility in the arrangement of the surface-emitting laser21or the first condensing optical system22increases. Moreover, the surface-emitting laser21can be disposed at a position away from high-temperature regions around the engine10. Accordingly, the engine10can be cooled using a variety of methods. Further, the surface-emitting laser21can be disposed at a position away from the engine10that is a vibration source. Accordingly, the deflection of the laser beams that are emitted from the surface-emitting laser21can be prevented.

The second condensing optical system24is disposed in the optical path of the laser beams emitted from the optical fiber23, and concentrates the light emitted from the optical fiber23. The laser beams that are concentrated by the second condensing optical system24enters the laser resonator25. In the present embodiment, the second condensing optical system24includes, for example, a first lens241and a second lens242.

The first lens241is a collimator lens that approximately collimates the laser beams emitted from the optical fiber23.

The second lens242is a condenser lens that approximately concentrates the laser beams that are approximately collimated by the first lens241.

The second condensing optical system24is satisfactory as long as it includes at least one condenser lens. The second condensing optical system24may consist of one optical element, or may include three or more lenses.

The laser resonator25is a Q-switched laser resonator. In the present embodiment, the laser resonator25includes a laser medium251and a saturable absorber252. In the laser resonator25, the energy density of the incident laser beams is increased, and the laser beams whose wavelengths are, for example, about 1064 nm are emitted with short pulse widths.

The laser medium251is an approximately cuboid-shaped neodymium (Nd): yttrium aluminum garnet (YAG) crystal, where 1.1 percent Nd is doped.

The saturable absorber252is an approximately cuboid-shaped chromium (Cr): YAG crystal. The optical transmittance of the saturable absorber252changes depending on the amount of absorption of laser beams, and the initial transmittance is about 0.50 (50%). When the amount of absorption of laser beams is small, the saturable absorber252serves as an absorber, and when the amount of absorption of laser beams is saturated, the saturable absorber252becomes transparent. As the saturable absorber252becomes transparent, Q-switch oscillation occurs.

The Nd: YAG crystal and the Cr: YAG crystal are both ceramic. The production cost of ceramics is lower than that of single crystal and inexpensive. In the present embodiment, the Nd: YAG crystal and the Cr: YAG crystal are bonded together to form a so-called composite crystal. Accordingly, the boundary between the Nd: YAG crystal and the Cr: YAG crystal is not detached, and the properties and characteristics equivalent to those of single crystal can be achieved in the laser resonator25.

The surface (incident plane251a) of laser medium251on the incident side (−Z side) and the surface (exit plane252b) of the saturable absorber252on the light-exiting side (+Z side) are optically polished, and each of the surfaces serves as a mirror.

Further, dielectric layers are formed on the incident plane251aand the exit plane252baccording to the wavelength of the light that is emitted from the surface-emitting laser21(e.g., 808 nm) and the wavelength of the laser beams that exit from the laser resonator25(e.g., 1064 nm). For example, a dielectric layer that indicates sufficiently high transmittance to the laser beams having a wavelength of 808 nm and indicates sufficiently high reflectance to the laser beams having a wavelength of 1064 nm are formed on the incident plane251a. For example, a dielectric layer that indicates reflectance of about 50 percent to the laser beams having the wavelength of 1064 nm is formed on the exit plane252b.

The laser beams that are concentrated by the second condensing optical system24enters the laser resonator25. Then, the laser beams are resonated and amplified inside the laser resonator25. Moreover, the laser medium251is optically pumped by the laser beams that are incident on the laser medium251. Note that the wavelength of the laser beams that are emitted from the surface-emitting laser21(e.g., 808 nm in the present embodiment) is a wavelength where the absorption efficiency is the highest in the YAG crystal. The laser beams that are emitted from the surface-emitting laser21, and then pass through the first condensing optical system22and the optical fiber23and become incident on the laser medium251may be referred to as pump light.

As the laser beams are resonated and amplified inside the laser resonator25, the energy density of the laser beams is increased. When the amount of absorption of laser beams is saturated in the saturable absorber252, Q-switch oscillation occurs in the saturable absorber252. Accordingly, the laser beams of high energy density are emitted from the laser resonator25with a short pulse width and concentrated energy. The laser beams that are emitted from the laser resonator25may be referred to as a pulsed laser beam. The wavelength of such a pulsed laser beam is, for example, about 1064 nm.

The laser beams that are amplified by the laser resonator25are incident on the third condensing optical system26.

The third condensing optical system26is disposed in the optical path of the laser beams that are emitted from the laser resonator25. The third condensing optical system26concentrates the laser beams that are emitted from the laser resonator25to obtain a high energy density at a focal point. When the energy density of the concentrated laser beams exceeds a certain degree, the molecules that make up the gas included in the inflammable fuel-air mixture in the combustion chamber14are ionized, and are separated into positive ions and electrons.

In other words, the molecules are broken down into plasma.

In the present embodiment, the third condensing optical system26consists of a third lens261, a fourth lens262, and a fifth lens263.

The third lens261is an optical element that increases the divergence angle of the laser beams that are emitted from the laser resonator25, and a concave lens is used as the third lens261in the present embodiment.

The fourth lens262is an optical element that collimates the light diverging from the third lens261, and a collimator lens is used as the fourth lens262in the present embodiment.

The fifth lens263is an optical element that concentrates the laser beams emitted from the fourth lens262, and a condenser lens is used as the fifth lens263in the present embodiment.

As the laser beams are concentrated by the fifth lens263, a high energy density can be obtained at a focal point. When the energy density of the concentrated laser beams exceeds a certain degree, the molecules that make up the gas included in the inflammable fuel-air mixture in the combustion chamber14are ionized, and plasma is generated.

The third condensing optical system26can adjust the focal point of the light that is emitted from the laser device11in the Z-axis direction by adjusting the positions of the lenses of the third condensing optical system26in the optical-axis direction of the lenses or by changing the combination of the lenses of the third condensing optical system26.

The third condensing optical system26according to the present embodiment consists of three lenses. However, third condensing optical system26is satisfactory as long as it includes at least one condenser lens, and may include only one optical element or a plurality of optical elements.

Next, before describing a configuration of the window27A, the housing28is described. As illustrated inFIG. 2, the housing28accommodates the second condensing optical system24, the laser resonator25, the third condensing optical system26, and an optical window271of the window27A. In the present embodiment, the housing28consists of the first housing28-1and the second housing28-2. The first housing28-1accommodates the second condensing optical system24and the laser resonator25, and

The second housing28-2accommodates the third condensing optical system26and the optical window271.

The housing28is made of, for example, a heat-resistant metallic material such as iron (Fe), Ni—Fe alloy, Ni—Cr—Fe alloy, Ni—Co—Fe alloy, and stainless steel. The Ni—Cr—Fe alloy may be, for example, Inconel, and

The Ni—Co—Fe alloy may be, for example, Kovar.

Next, the configuration of the window27A according to the present embodiment is described.

FIG. 3is a diagram illustrating an example configuration of the window27A, according to the present embodiment.

As illustrated inFIG. 3, the window27A includes an optical window (main unit of window)271, an optical window holding member272, an antireflection (AR) film273, and a catalyst layer274. In the present embodiment, the antireflection film273is arranged as a film formed on the incident plane271aof the optical window271through which the laser beams pass, and the catalyst layer274is arranged as a film formed on the exit plane271bof the optical window271through which the laser beams pass.

The optical window271is disposed in the optical path of the laser beams that are emitted from the third condensing optical system26. The optical window271is made of a transparent or semitransparent material, and includes an incident plane271aon which the laser beams are incident and an exit plane271bfrom which the laser beams exit. The incident plane271ais the surface of the optical window271on the third condensing optical system26side, and the laser beams pass through the incident plane271a. The incident plane271ais on the same plane as an incident-side end surface272athat is an edge of the optical window holding member272on the incident-side of the laser beams. The exit plane271bis the surface of the optical window271on the combustion chamber14side, and the laser beams pass through the exit plane271b. The exit plane271bis on the same plane as an exit-side end surface272bthat is an edge of the optical window holding member272on the exit-side of the laser beams. In the present embodiment, it is assumed that the incident-side end surface272aand the exit-side end surface272bthat are the edges of the optical window holding member272are planar. However, the incident-side end surface272aand the exit-side end surface272bmay be, for example, curved or convex.

The optical window271is fixed on the inside surfaces of the optical window holding member272, by a brazing member29that is formed by using a brazing material (binder). The optical window271is disposed at an opening that is formed on the surface of the housing28on the combustion chamber14side. The laser beams that are emitted from the third condensing optical system26pass through the optical window271and are concentrated inside the combustion chamber14.

Note also that the shape of the optical window271in a planar view is not limited, and may be, for example, rectangular, circular, ellipsoidal, rectangular, or polygonal.

For example, an optical glass, a heat-resistant glass, a quartz glass, and a sapphire glass may be used as a material of the optical window271. In particular, the optical window271needs sufficient pressure resistance to protect, for example, the optical elements inside the housing28, from the firing pressure produced inside the combustion chamber14. In order to achieve such sufficient pressure resistance, the thickness of the optical window271may be increased. However, if the thickness of the optical window271is increased, some of the laser beams that are incident on the exit plane of the optical window271tends to be reflected and concentrated inside the optical window271. In order to prevent the concentration of light inside the optical window271, the focal length of the third condensing optical system26needs to be extended.

When the focal length of the third condensing optical system26is extended, the numerical aperture (NA) of the lenses of the third condensing optical system26becomes small. Accordingly, the light-gathering power decreases, and the ignition quality decreases. For this reason, it is desired that the thickness of the optical window271be as thin as possible. In order to handle such a situation, it is desired that sapphire glass be used as the material of the optical window271. The sapphire glass exhibits good durability under high-temperature and high-pressure environments.

The optical window holding member272is attached around the opening that is formed on the surface of the housing28on the combustion chamber14side, so as to cover the housing28. The optical window holding member272is fixed to the housing28through a welded portion30that is formed, for example, by laser welding. In the present embodiment, the optical window holding member272is fixed to the housing28by welding such as laser welding. However, no limitation is indicated thereby, and the optical window holding member272may be fixed to the housing28by, for example, fastening screw, shrinkage fit, and adhesion.

An exit-side end surface272bof the optical window holding member272and an end face28bof a second housing28-2are approximately on the same geometric plane. Due to this configuration, when the welded portion30is formed, for example, by laser welding, it is easy to concentrate the laser beams onto the welded portion30. As a result, the welded portion30can evenly be disposed between the optical window holding member272and the second housing28-2, with high stability and reliability. Accordingly, the optical window holding member272can be fixed onto the second housing28-2with high stability.

The optical window271is fixed on inside surface (inner wall)272cof the optical window holding member272, by the brazing member29. In the present embodiment, a brazing material is used as the binder. However, no limitation is indicated thereby, and other kinds of heat-resistant binder may be employed. The optical window271may be fixed to the optical window holding member272by means of fastening screw, shrinkage fit, or the like, without using binder.

For example, a heat-resistant metallic material such as iron (Fe), nickel (Ni), Ni—Fe alloy, Ni—Cr—Fe alloy, Ni—Co—Fe alloy, and stainless steel may be used as a material that forms the optical window holding member272. The Ni—Cr—Fe alloy may be, for example, Inconel, and the Ni—Co—Fe alloy may be, for example, Kovar (trademark of CRS Holdings, inc., Delaware). Above all, it is most strongly desired that the optical window271be made of sapphire in the present embodiment. For this reason, it is desired that a material that forms the optical window holding member272be Kovar whose coefficient of thermal expansion is close to that of sapphire.

In the present embodiment, it is desired that the optical window holding member272be formed of a material having the same coefficient of thermal expansion as the second housing28-2on which the optical window holding member272is fixed. In the present embodiment, the optical window holding member272and the second housing28-2are formed of the same material. However, the optical window holding member272and the second housing28-2may be made of different materials as long as these materials have the same coefficient of thermal expansion. As the optical window holding member272and the second housing28-2are formed of a material having the same coefficient of thermal expansion, the compressive stress that could be caused by the difference in coefficient of thermal expansion can be prevented from being applied to the brazing material and the welded portion30. Note that the expression “the same coefficient of thermal expansion” indicates not only the precisely identical value, but also allows an error of about several percent as stress difference that does not affect the brazing material or the welded portion30.

The reasons why it is desired that the optical window holding member272and the second housing28-2be formed of a material having the same coefficient of thermal expansion are described. As the optical window holding member272and the second housing28-2is exposed to the combustion chamber14, the optical window holding member272and the second housing28-2are susceptible to the temperature of the combustion chamber14. For this reason, when the mixture of gases are burning in the combustion chamber14, the temperature of the optical window holding member272and the second housing28-2increases, for example, to about several hundred degrees Celsius to about 1000 degrees Celsius.

Here, it is assumed that the optical window holding member272and the second housing28-2are formed of two or more materials having different coefficients of thermal expansion. In such cases, when the inside of the combustion chamber14is at high temperature, compressive stress is caused due to the difference in coefficient of thermal expansion between the optical window holding member272and the second housing28-2. Due to such compressive stress, the load on the brazing member29or the welded portion30may increase as, for example, the brazing member29or the welded portion30may be stretched, or cracks may appear on the brazing member29or the welded portion30. As a result, the brazing member29or the welded portion30may deteriorate.

By contrast, as long as the optical window holding member272and the second housing28-2are formed of a material having the same coefficient of thermal expansion, even if the temperature of the optical window holding member272and the second housing28-2increases, for example, to high temperature of about several hundreds of degrees Celsius to about 1000 degrees Celsius as affected by the temperature of the combustion chamber14, the compressive stress that could be caused due to the difference in coefficient of thermal expansion is not applied to the brazing member29and the welded portion30. For this reason, the brazing member29or the welded portion30is not stretched or cracked, for example, due to the stress caused between the optical window holding member272and the second housing28-2. In other words, the load on the brazing member29and the welded portion30can be reduced. Accordingly, the optical window271can be fixed to the optical window holding member272with high stability.

In the present embodiment, it is desired that the optical window holding member272and the second housing28-2be made of a material such as Kovar. Among all sorts of metal, Kovar has a low coefficient of thermal expansion near normal temperature, and the coefficient of thermal expansion of Kovar is close to that of a hard glass such as sapphire glass. For example, as Kovar is adopted as the material of the optical window holding member272and the second housing28-2, when the optical window271is made from sapphire, thermal strain is caused in a similar manner between the optical window271, and the optical window holding member272and the second housing28-2. Accordingly, the stress that is caused between the optical window271, and the optical window holding member272and the second housing28-2, due to the difference in coefficient of thermal expansion, can be reduced.

Due to this configuration, the stress that is caused by the difference in coefficient of thermal expansion can be prevented from being applied to the brazing member29that is disposed between the optical window271and the optical window holding member272. Moreover, the stress can also be prevented from being applied to the welded portion30that is disposed between the optical window holding member272and the second housing28-2. Accordingly, the load on the brazing member29and the welded portion30can further be reduced.

Due to this configuration, when the optical window holding member272and the second housing28-2are formed of a material having the same coefficient of thermal expansion in the laser device11, a reduction in the light quantity of laser beams that enter the combustion chamber14can further be attenuated with increased stability. Moreover, in the laser device11, the optical window271can be fixed to the optical window holding member272with high stability. Due to this configuration, even when the inside of the combustion chamber14is under high temperature environments due to combustion, the laser device11can be used with high stability on a long-term basis. Accordingly, a highly-reliable laser device is achieved.

In the present embodiment, the second housing28-2on which the optical window holding member272is fixed is formed of a material having the same coefficient of thermal expansion as the optical window holding member272. However, no limitation is intended thereby. For example, the first housing28-1and the second housing28-2may be formed of a material having the same coefficient of thermal expansion as the optical window holding member272.

The antireflection film273is a film that is formed on the incident plane271aof the optical window271to prevent the laser beams from being reflected. In the present embodiment, the antireflection film273has a high optical transmittance for the laser beams with wavelengths of 1064 nm.

The antireflection film273may be made of, for example, a material whose main component is one of silicon (Si), sodium (Na), aluminum (Al), calcium (Ca), magnesium (Mg), boron (B), carbon (C), titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), gallium (Ga), strontium (Sr), zirconium (Zr), niobium (Nb), ruthenium (Ru), palladium (Pd), silver (Ag), indium (In), tin (Sn), hafnium (Hf), tantalum (Ta), tungsten (W), osmium (Os), gold (Au), and bismuth (Bi), or a material that includes at least one of a nitride, oxide, carbide, and a fluoride of the above main component.

In particular, for example, magnesium fluoride (MgF2), silicon nitride (Si3N4), and silicon dioxide (SiO2) may be used. The less difference in refractive index between the antireflection film273and the optical window271, the more the antireflective properties and characteristics improves.

As a method of forming the antireflection film273on the optical window271, for example, vapor deposition, sputtering, thermal spraying (flame plating), coating, or sol-gel processes may be used.

It is desired that the antireflection film273be formed on the incident plane271aof the optical window271, for example, in a state where the optical window271is fixed to the optical window holding member272by brazing or the like. When the optical window271is fixed to the optical window holding member272, the optical window holding member272is heated to a high temperature, and thus the optical window271also tends to be heated to a high temperature. For example, when the optical window271is fixed to the optical window holding member272using a brazing material, the optical window271is heated to a high temperature (for example, about 1000° C.). In so doing, the heat resistance of the antireflection film273may be not as high as the above temperature. The heat resistance of the antireflection film273is, for example, about 400° C. For this reason, it is desired that the antireflection film273be formed on the incident plane271aof the optical window271in a state where the optical window271is fixed to the optical window holding member272.

In the present embodiment, the antireflection film273is designed to have thickness enabling a high optical transmittance for the laser beams with wavelengths of 1064 nm. For example, when the optical window271is made from sapphire glass, the refractive index of the sapphire is about 1.74. Accordingly, it is desired that the thickness of the antireflection film273be about 202 nm and the refractive index of the antireflection film273be about 1.32.

In the present embodiment, the thickness of the antireflection film273indicates the length of the antireflection film273in the vertical direction with reference to the contact surface with the optical window271. For example, the thickness of the antireflection film273is obtained by measuring any desired position of the cross section of the antireflection film273.

As illustrated inFIG. 3, in the present embodiment, the incident plane271aof the optical window271and the incident-side end surface272aof the optical window holding member272are approximately on the same geometric plane. Due to this configuration, the incident plane271aof the optical window271is not recessed towards the light exiting side with reference to the incident-side end surface272aof the optical window holding member272, but the incident plane271aof the optical window271and the incident-side end surface272aof the optical window holding member272are substantially on the same plane. Accordingly, the antireflection film273where the variations in average thickness are small can be formed with stability on the incident plane271aof the optical window271and the incident-side end surface272aof the optical window holding member272.

When the variations in thickness are small in the antireflection film273, the reflection of some of the laser beams that are incident on the incident plane273aof the antireflection film273can further be reduced, and the reduction in the transmittance of the laser beams can further be controlled. As a result, a reduction in the light quantity of laser beams that enter the combustion chamber14can be controlled. Moreover, the reflection of some of the laser beams that are incident on the antireflection film273and the concentration of the reflected laser beams onto the third condensing optical system26or the like can further be controlled. Due to this configuration, damage to an optical element such as the third condensing optical system26can be prevented.

In the present embodiment, it is desired that the surface roughness Ra of the antireflection film273be equal to or less than 100 nm. It is more desirable if the surface roughness Ra of the antireflection film273is equal to or less than 50 nm, and it is even more desirable if the surface roughness Ra of the antireflection film273is equal to or less than 10 nm. Although it is undesired, some of the laser beams that are incident on the incident plane273aof the antireflection film273are reflected on the incident plane273aof the antireflection film273. However, when the surface roughness Ra of the antireflection film273is equal to or less than 100 nm, the laser beams that are incident on the incident plane273aof the antireflection film273can further be prevented from being reflected on the entirety of the incident plane273aof the antireflection film273.

In the present embodiment, the surface roughness Ra of the antireflection film273is the average of the concave and convex states on the surface of the antireflection film273on the incident-side of the laser beams. The surface roughness Ra is arithmetic average roughness, and indicates a value measured in accordance with Japanese Industrial Standards (JIS) B 0601 (2013). The surface roughness Ra may be measured, for example, by a known surface-roughness measuring instrument.

In the present embodiment, the antireflection film273is formed across the incident plane271aof the optical window271and the incident-side end surface272aof the optical window holding member272. Alternatively, when the antireflection film273is to be formed only on the incident plane271aof the optical window271, for example, a mask needs to be disposed at portions where the antireflection film273is not to be formed. Due to such a step where a mask is disposed, the positions at which the antireflection film273is to be formed may be misaligned. Due to the arrangement in which the antireflection film273is formed across the incident plane271aand the incident-side end surface272a, the antireflection film273is formed with reliability on the plane through which the laser beams pass, and the cost required to form the antireflection film273can be reduced.

In the present embodiment, the antireflection film273has single-layer structure. However, no limitation is intended thereby, and the antireflection film273may have multilayer structure.

The catalyst layer274is formed on the exit plane271bof the optical window271, and enhances the oxidation-reduction reaction.

The catalyst layer274may be formed of an aggregate of particulate photocatalysts. For this reason, a photocatalyst may be made from titanium dioxide (TiO2), tungsten trioxide (WO3), ferric oxide (Fe2O3), molybdenum disulfide (MoS2), silicon (Si), cuprous oxide (Cu2O), roquesite (CuInS2), TaON, carbon nitride (C3N4), silicon carbide (SiC), strontium titanate (SrTiO3), gallium phosphide (GaP), gallium arsenide (GaAs), cadmium selenide (CdSe), cadmium sulfide (CdS), or zinc oxide (ZnO). These materials may be used independently or in combination. In particular, it is desired that TiO2be used as the material of a photocatalyst.

When the photocatalyst is TiO2, it is desired that TiO2has crystal structure of anatase type, rutile type, or brookite type. As TiO2whose crystal structure is of anatase type has the highest reactivity among the above three types of crystal structure, it is desired that TiO2of anatase type be used.

The functionality of photocatalyst varies depending on the material of that photocatalyst. In order for the photocatalyst to function as a photocatalyst, the photocatalyst needs to be irradiated with light with photon energy (wavelength) corresponding to the band gap of the photocatalyst or light with even greater photon energy (wavelength). When the photocatalyst is irradiated with light with wavelength having a value equal to or shorter than a predetermined value, the photocatalyst absorbs light, and electrons and electron holes are generated. These generated electron holes directly contribute to oxidation reaction, and organic matters are decomposed. Moreover, the electron holes react with the moisture or hydroxyl (—OH) on the surfaces of crystal, and hydroxyl radical (OH) is generated. “OH” contributes to oxidation reaction and is strongly oxidative. Accordingly, organic matters are oxidized and decomposed. The generated electrons deoxidize the oxygen existing on the surfaces of the photocatalyst to generate a superoxide ion (O2−). O2−reacts with the moisture on the surfaces of crystal, and hydrogen peroxide or “OH” is generated. Hydrogen peroxide or “OH” contribute to oxidation reaction and is strongly oxidative. Accordingly, organic matters are oxidized and decomposed.

When the photocatalyst in the catalyst layer274manifests itself and functions as a photocatalyst, the light that is generated as the laser beams are concentrated inside the combustion chamber14and become plasmic, the air that is supplied to the combustion chamber14, and the molecules of water that exists inside the combustion chamber14are used. Due to this configuration, the photocatalyst in the catalyst layer274manifests itself and functions as a photocatalyst, and for example, organic matters are decomposed.

As known in the art, in the engine10, engine oil is used in order to reduce, for example, the friction in the piston15, and oil mist is floating inside the combustion chamber14. Accordingly, in the known windows, for example, oil mist sticks to a plane of the window on the combustion chamber side. Moreover, when, for example, soot is accumulated and soot deposits are formed, the light quantity of laser beams that enter the combustion chamber14may decrease, and the ignition stability may deteriorate.

In the present embodiment, the catalyst layer274is formed on the exit plane271bof the optical window271. Due to this configuration, even when, for example, the organic matters and the soot generated inside the combustion chamber14stick to the surface of the catalyst layer274, such organic matters and soot can be decomposed. For this reason, stains or the like can be prevented from sticking to the surfaces of the catalyst layer274. Accordingly, the catalyst layer274can maintain the transmittance of the laser beams that are emitted from the third condensing optical system26, and thus the light quantity of laser beams that enter the combustion chamber14can be maintained.

The exit plane271bof the optical window271and the exit-side end surface272bof the optical window holding member272are arranged approximately on the same geometric plane. The exit plane271bof the optical window271may protrude to the combustion chamber14side, which is on the light exiting side with reference to the exit-side end surface272bof the optical window holding member272(seeFIG. 5A). As will be described later, if the exit plane271bof the optical window271is disposed on the light entering side with reference to the exit-side end surface272bof the optical window holding member272, the materials that form the catalyst layer274may contact the optical window holding member272or the brazing member29when the catalyst layer274is formed on the optical window271. Due to this configuration, the variations in the thickness of the catalyst layer274or the thickness of the catalyst layer274tend to increase. Moreover, the transmittance may also deteriorate.

In the present embodiment, the exit plane271bof the optical window271and the exit-side end surface272bof the optical window holding member272are approximately on the same geometric plane. Due to this configuration, the catalyst layer274can be formed on the exit plane271bof the optical window271without being affected by the optical window holding member272and the brazing member29. the catalyst layer274where the variations in thickness are small and the thickness is small can easily be formed.

In the present embodiment, it is desired that the thickness of the catalyst layer274be equal to or smaller than 1 micrometer (μm). When the thickness of the catalyst layer274exceeds 1 μm, there is a possibility that the optical transmittance of the catalyst layer274decreases and the laser beams cannot be maintained with high transmittance. As the reduction in transmittance degrades the intensity of the laser beams that are emitted from the laser resonator25, the laser beams may become plasmatic and the light intensity may deteriorate. For this reason, the combustion efficiency of the engine10may deteriorate, or in some cases, the fire could be lost.

Note also that as long as the catalyst layer274has a high optical transmittance and can be formed on the optical window271with stability, the lower limit of the thickness of the catalyst layer274is not limited to any specific range of values. In view of the optical transmittance of the catalyst layer274, it is desired that the thickness of the catalyst layer274be thin. However, the surface of the optical window271is highly water-repellent in many cases. For this reason, if the catalyst layer274is too thin and when the catalyst layer274is directly formed onto the optical window271, there is a possibility that the catalyst layer274cannot be formed on the optical window271with stability. In particular, when the optical window271is made of sapphire that is highly water-repellent, the catalyst layer274cannot easily be formed on the optical window271in many cases. For the above reasons, the lower limit of the thickness of the catalyst layer274is adjusted as appropriate according to the material of the optical window271.

Moreover, it is desired that, in order to prevent the reflection of the laser beams, the catalyst layer274be further adjusted to have appropriate thickness in view of the incident light and the refractive index of the catalyst layer274. For example, when the catalyst layer274is a single layer, nAR1that denotes the refractive index of the catalyst layer274is expressed in Equation 1 given below, and dAR1that denotes the thickness of the catalyst layer274is expressed in Equation 2 given below. Note also that nAR1and dAR1are calculated so as to minimize the reflectance of the catalyst layer274. In these equations, no denotes the refractive index of the air (n0=1), and nmdenotes the refractive index of the optical window271. Further, λ denotes the wavelengths of the laser beams (1064 nm).
nAR1=√{square root over ( )}(n0×nm)  [Equation 1]
dAR1=λ/(4×nAR1)  [Equation 2]

For example, when the material of the optical window271is sapphire, the refractive index is 1.74. In such a configuration, nAR1becomes 1.32, and dAR1, i.e., the thickness of the catalyst layer274, is set to 201.5 nm. By so doing, the single layer of the catalyst layer274can achieve high antireflective functionality. Due to this configuration, when the catalyst layer274is a single layer, it is desired that the thickness of the catalyst layer274and the refractive index of the catalyst layer274be adjusted to become close to the values that are obtained from the above Equations 1 and 2.

In the present embodiment, the thickness of the catalyst layer274can be defined in a similar manner to the thickness of the antireflection film273, and the thickness of the catalyst layer274can be measured in a similar manner to the antireflection film273.

It is desired that the average diameter of the particles of photocatalyst that form the catalyst layer274be equal to or shorter than 100 nm. In order for the photocatalyst to function as a photocatalyst, it is desired that a number of electrons and holes be formed on the surface of the photocatalyst. In order for the electrons and holes to appear on the surface of the photocatalyst, it is desired that the particles of the photocatalyst be smaller. By contrast, when the particles of the photocatalyst is large, the transparency of the catalyst layer274may decrease. For the above reasons, it is desired that the average diameter of the particles of photocatalyst that form the catalyst layer274be equal to or shorter than 100 nm. Due to such a configuration, the transmittance of the laser beams in the catalyst layer274can be maintained, while the catalyst layer274functioning as a photocatalyst.

Note that the average diameter of the particles (average particle diameter) of the photocatalyst is obtained as follows. Firstly, a desired number of photocatalysts (for example, a hundred photocatalysts) are observed using a transmission electron microscope (TEM), and its projected area is measured. Then, the circle-equivalent diameter of the obtained area is calculated, and the particle diameters are obtained. Finally, the average of the obtained particle diameters is obtained as the average particle diameter.

The catalyst layer274may be formed on the optical window271using a method known in the art. For example, vapor deposition, sputtering, thermal spraying (flame plating), coating, or sol-gel processes may be used to form the catalyst layer274on the optical window271.

It is desired that the catalyst layer274be formed on the exit plane271bof the optical window271in a state where the optical window271is fixed to the optical window holding member272and the antireflection film273is formed on the incident plane271aof the optical window271. When the optical window271is fixed to the optical window holding member272or when the antireflection film273is fixed to the optical window271, the optical window271and the antireflection film273are heated at high temperature (for example, about 1000° C.). Accordingly, the optical window271also tends to be heated to a high temperature. For example, when the optical window271is fixed to the optical window holding member272using a brazing material, the optical window271is heated to a high temperature (for example, about 1000° C.). In so doing, in a similar manner to the antireflection film273, the heat resistance of the catalyst layer274may be not as high as the above temperature. Depending on the materials that form the catalyst layer274, the crystal structure may be changed due to the heating temperature. For this reason, it is desired that the catalyst layer274be formed on the exit plane271bof the optical window271in a state where the optical window271is fixed to the optical window holding member272and the antireflection film273is formed on the incident plane271aof the optical window271. For example, when the catalyst layer274is made from TiO2, it is desired that the catalyst layer274be heated at temperature equal to or lower than, for example, 650° C. When the heating temperature exceeds 650° C., the crystalline state of TiO2ends up changing from anatase type to rutile type whose reactivity is lower than the anatase type.

The catalyst layer274is formed across the exit plane271bof the optical window271and the exit-side end surface272bof the optical window holding member272. Alternatively, when the catalyst layer274is to be formed only on the exit plane271bof the optical window271, for example, a mask needs to be disposed at portions where the catalyst layer274is not to be formed. Due to such a step where a mask is disposed, the positions at which the catalyst layer274is to be formed may be misaligned. Due to the arrangement in which the catalyst layer274is formed across the exit plane271band the exit-side end surface272b, the catalyst layer274is formed with reliability on the plane through which the laser beams pass, and the cost required to form the catalyst layer274can be reduced.

In the present embodiment, the catalyst layer274has single-layer structure. However, no limitation is intended thereby, and the catalyst layer274may have multilayer structure.

As described above, the laser device11is provided with the antireflection film273, where the variations in thickness are small, on the incident plane271aof the optical window271that is on the same plane as the incident-side end surface272aof the optical window holding member272. Due to this configuration, the laser beams that are emitted from the laser resonator25can further be prevented from being reflected on the antireflection film273. Accordingly, a reduction in the light quantity of the laser beams that enter the combustion chamber14can further be attenuated. Due to this configuration, the laser device11can maintain the light quantity of laser beams that enter the combustion chamber14.

Moreover, the laser device11is provided with the catalyst layer274on the exit plane271bof the optical window271that is on the same plane as the exit-side end surface272bof the optical window holding member272. Due to this configuration, oil mist inside the combustion chamber or stain such as soot can be prevented from sticking to the exit plane271b. As a result, the transmittance of the pulsed laser beams in the catalyst layer274can be maintained, and thus the light quantity of laser beams that enter the combustion chamber14can be maintained.

As described above, in the laser device11, is provided with the antireflection film273that is formed on the incident plane271aof the optical window271, and is provided with the catalyst layer274that is formed on the exit plane271bof the optical window271. Due to this configuration, plasma can be produced with stability in the combustion chamber14. Accordingly, ignition can be performed with high stability in the combustion chamber14.

In the present embodiment, the antireflection film273is formed across the incident plane271aof the optical window271and the incident-side end surface272aof the optical window holding member272, and the catalyst layer274is formed across the exit plane271bof the optical window271and the exit-side end surface272bof the optical window holding member272. Due to this configuration, the antireflection film273and the catalyst layer274can reliably be formed on the plane through which the laser beams pass, and the cost required to form the antireflection film273and the catalyst layer274can be reduced.

Furthermore, in the present embodiment, the surface roughness Ra of the antireflection film273is equal to or less than 100 nm. Due to this configuration, some of the laser beams that are incident on the incident plane273aof the antireflection film273can further be prevented from being reflected. As a result, a reduction in the light quantity of laser beams that enter the combustion chamber14can be attenuated, and plasma can be produced with increased stability in the combustion chamber14.

In the present embodiment, the thickness of the catalyst layer274is configured to be equal to or shorter than 1 μm. Due to this configuration, the reduction in the transmittance of the laser beams in the catalyst layer274can be controlled. Accordingly, the laser beams can be maintained with high transmittance.

In the present embodiment, the average diameter of the particles of photocatalyst that form the catalyst layer274is configured to be equal to or less than 100 nm. Due to this configuration, while the catalyst layer274functioning as a photocatalyst, the transmittance of the laser beams in the catalyst layer274can be maintained.

As the engine10(seeFIG. 1) is provided with the laser device11, the combustion efficiency can be maintained with stability. Accordingly, the performance of the engine10(seeFIG. 1) can be stabilized.

In the present embodiment, the optical window271is provided with both the antireflection film273and the catalyst layer274. However, no limitation is indicated thereby, and the optical window271may be provided with only either one of the antireflection film273and the catalyst layer274. For example, in the present embodiment, the antireflection film273is formed on the incident plane271aof the optical window271. However, no limitation is indicated thereby, and when no disadvantage expected by the reflected light or the like, the antireflection film273may be omitted.

As illustrated inFIG. 3, in the present embodiment, the incident plane271aof the optical window271and the incident-side end surface272aof the optical window holding member272are approximately on the same geometric plane. However, no limitation is intended thereby. For example, as illustrated inFIG. 4A, the incident plane271amay protrude to the third condensing optical system26side, which is on the incident side (in the −Z-axis direction) with reference to the incident-side end surface272aof the optical window holding member272. Even in this case, the antireflection film273where the surface roughness Ra is small can be formed on the incident plane271aof the optical window271. Due to this configuration, a reduction in the transmittance of the laser beams that are incident on the antireflection film273can be controlled all over the incident plane273aof the antireflection film273. As a result, a reduction in the light quantity of laser beams that enter the combustion chamber14can further be attenuated with increased stability. Moreover, the reflection of some of the laser beams that are incident on the optical window271and the concentration of the reflected laser beams onto the third condensing optical system26or the like can be controlled. Due to this configuration, damage to an optical element such as the third condensing optical system26can be prevented.

As illustrated inFIG. 4B, the incident plane271amay be disposed at a depressed position with reference to the incident-side end surface272a. When the incident plane271aor the exit plane271bof the optical window271are disposed at a depressed position, the incident plane271aor the exit plane271bof the optical window271is disposed on an inside surface272cof the optical window holding member272so as to be recessed towards the inner side of the optical window holding member272with reference to the edges of the optical window holding member272(i.e., the incident-side end surface272aor the exit-side end surface272b). InFIG. 4B, the incident plane271ais disposed on the inside surface272cof the optical window holding member272so as to be recessed towards the light-exiting side in the +Z-axis direction with reference to the incident-side end surface272aof the optical window holding member272. In such a configuration, the inside surface272cof the optical window holding member272needs to be formed such that the diameter of the inside surface272cis enlarged (for example, in a tapered shape) from the position of the incident plane271aof the optical window271towards the outside in the incident direction (i.e., in the −Z-axis direction). The angle that the incident plane271aof the optical window271forms with a diameter-enlarged portion of the optical window holding member272, which is formed from the position of the incident plane271atowards the incident side, is wider than 90°. For example, when the optical system is tapered as illustrated inFIG. 4B, an angle θ1that the incident plane271aof the optical window271forms with a diameter-enlarged portion272c1of the inside surface272cof the optical window holding member272, which is formed from the position of the incident plane271atowards the incident side, needs to be wider than 90°, and preferably 1000 or wider. The angle θ1may be equal to or greater than 180°. When the angle θ1is greater than 90°, the variations in thickness in the antireflection film273can be reduced on the entirety of the antireflection film273that is formed on the incident plane271a. The diameter-enlarged portion272c1is a part of the inside surface272cof the optical window holding member272where the internal diameter is enlarged from the position of the incident plane271aof the optical window271towards the outside. When the optical window holding member272is viewed in the direction towards the light exiting side of the laser beams (in the +Z-axis direction), the shape of the diameter-enlarged portion272c1may be, for example, circular or rectangular.

In the present modification, for example, it is assumed that the antireflection film273is formed on the incident plane271ain a state where the optical window271is fixed to the optical window holding member272such that the incident plane271ais positioned on the light exiting side (in the +Z-axis direction) with reference to the incident-side end surface272a. In such a configuration, points of the incident plane271aof the optical window271, which are close to the optical window holding member272, tend to be in the shade of edges of the incident-side end surface272aof the optical window holding member272. For this reason, it is difficult to form the antireflection film273where the variations in thickness are small, on the incident plane271aof the optical window271. Due to such a configuration, it is likely that some of the laser beams that are incident on the incident plane273aof the antireflection film273are reflected, and in some cases, the antireflection film273does not satisfactorily serve as an antireflection (AR) film. As a result, the transmittance of the laser beams decreases, and the light quantity of laser beams that enter the combustion chamber14may also decrease.

As illustrated inFIG. 3, in the present embodiment, the exit plane271bof the optical window271and the exit-side end surface272bof the optical window holding member272are approximately on the same geometric plane. However, no limitation is intended thereby. For example, as illustrated inFIG. 5A, the exit plane271bof the optical window271may protrude to the combustion chamber14side, which is on the light exiting side (in the +Z-axis direction) with reference to the exit-side end surface272bof the optical window holding member272. Also in such a configuration as above, the catalyst layer274may be formed on the exit plane271bof the optical window271without being affected by the optical window holding member272and the brazing member29. Moreover, the catalyst layer274where the variations in thickness are small and the thickness is small can easily be formed.

In a similar manner to the alternative case of the incident plane271aas described above, as illustrated inFIG. 5B, the exit plane271bmay be disposed at a depressed position with reference to the exit-side end surface272b. In other words, the exit plane271bis disposed on the inside surface272cof the optical window holding member272so as to be recessed towards the light incident side in the −Z-axis direction with reference to the exit-side end surface272bof the optical window holding member272, and the exit plane271bmay be disposed at a depressed position towards the inner side of the optical window holding member272. In such a configuration, the inside surface272cof the optical window holding member272needs to be formed such that the diameter of the inside surface272cis enlarged from the position of the exit plane271bof the optical window271towards the outside in the light exit direction (i.e., in the +Z-axis direction). Moreover, an angle θ2that the exit plane271bof the optical window271forms with a diameter-enlarged portion272c2of the inside surface272cof the optical window holding member272, which is formed from the position of the exit plane271btowards the light exiting side needs to be wider than 90°, and preferably 100° or wider, in a similar manner to the angle θ1. The angle θ2may be equal to or greater than 180°. When the angle θ2is greater than 90°, the variations in thickness can be reduced in the catalyst layer274that is formed on the exit plane271b. In a similar manner to the diameter-enlarged portion272c1as above, the diameter-enlarged portion272c2is a part of the inside surface272cof the optical window holding member272where the internal diameter is enlarged from the position of the exit plane271bof the optical window271towards the outside. in a similar manner to the diameter-enlarged portion272c1as above,

When the optical window holding member272is viewed in the direction towards the light incident side of the laser beams (in the −Z-axis direction), the shape of the diameter-enlarged portion272c1may be, for example, circular or rectangular.

FIG. 5Cis a diagram illustrating a case in which the angle θ2is 180°, according to an alternative embodiment of the present disclosure.

In other words, an angle θ3that the exit plane271bforms with a diameter-enlarged portion272c3of the optical window holding member272is 180°. Also in this configuration, the variations in thickness can be reduced in the catalyst layer274that is formed on the exit plane271b.

As illustrated inFIG. 6, in the optical window271according to the present embodiment, a tapered portion271cmay be formed at the periphery of the incident plane271aand the exit plane271bof the optical window271. Due to this configuration, when the optical window271and the optical window holding member272are joining together, a brazing material can easily get into the gap therebetween. Note also that the tapered portion271cis formed in an area through which no laser beam passes. Alternatively, the tapered portion271cmay only be formed on either one of the incident plane271aor the exit plane271b.

When the tapered portion271cis formed at the periphery of the incident plane271aand the exit plane271bof the optical window271, as illustrated inFIG. 7, the exit plane271bof the optical window271may be recessed towards the light incident side in the −Z-axis direction with reference to the exit-side end surface272bof the optical window holding member272. In such a configuration, the diameter-enlarged portion272c2needs to be formed on the inside surface272cof the optical window holding member272. The diameter-enlarged portion272c2, which is a part of the inside surface272cof the optical window holding member272that contacts the exit plane271b, forms the widest angle with the optical axes of the laser beams. Here, note that the angle θ2that the diameter-enlarged portion272c2forms with the exit plane271bof the optical window271needs to be wider than 90°, and preferably 100° or wider. When the angle θ2is greater than 90°, the variations in thickness can be reduced in the catalyst layer274that is formed on the exit plane271b. When the angle θ2is equal to or wider than 1000, the catalyst layer274can evenly be formed on the exit plane271b.

When the exit plane271bof the optical window271is recessed towards the light incident side in the −Z-axis direction with reference to the exit-side end surface272bof the optical window holding member272and the diameter-enlarged portion272c2is not formed on the inside surface272cof the optical window holding member272, as illustrated inFIG. 8, the angle θ2becomes 90°. If the angle θ2is equal to or narrower than 900, the optical window holding member272disturbs when the catalyst layer274is formed on the optical window271, and the catalyst layer274cannot evenly be formed. Even if the tapered portion271cis formed on the periphery of the optical window271, the catalyst layer274that is formed near the inside surface272cof the optical window holding member272tends to be bowed outward at the center due to the configuration in which the exit plane271bis recessed towards the light incident side in the −Z-axis direction with reference to the exit-side end surface272bof the optical window holding member272. As a result, the evenness of the catalyst layer274that is formed on the exit plane271bdecreases.

InFIG. 6andFIG. 7, the tapered portion271cis formed on the periphery of both the incident plane271aand the exit plane271bof the optical window271through which the laser beams enter and exit, respectively. However, the tapered portion271cmay be formed only on the periphery of either one of the incident plane271aand the exit plane271bof the optical window271.

In the present embodiment, the housing28accommodates the second condensing optical system24, the laser resonator25, the third condensing optical system26, and the optical window271. However, the housing28may further accommodate the first condensing optical system22and the optical fiber23.

In the present embodiment, the first housing28-1accommodates the second condensing optical system24and the laser resonator25, and the second housing28-2accommodates the third condensing optical system26and the window27. However, no limitation is intended thereby. For example, the first housing28-1may accommodate only the second condensing optical system24, and the second housing28-2may further accommodate the laser resonator25. Alternatively, the first housing28-1may further accommodate the third condensing optical system26in addition to the second condensing optical system24and the laser resonator25, and the second housing28-2may accommodate only the window27.

In the present embodiment as described above, cases in which the surface-emitting laser21is used as a pump source are described. However, no limitation is intended thereby, and other kinds of light source may be used.

When it is not necessary to arrange the surface-emitting laser21at a position distant from the laser resonator25in the present embodiment, the provision of the optical fiber23may be omitted.

In the present embodiment, cases in which the laser device11is used as an ignition system for the engine10that serves as the internal combustion engine and moves upward and downward a piston with flammable gas are described. However, no limitation is intended thereby. The laser device11may be used for an engine that burns fuel to produce flammable gas. For example, the laser device11may be used for a rotary engine, a gas turbine engine, and a jet engine. Moreover, the laser device11may be used for cogeneration that is a system in which exhaust heat is reused to increase the comprehensive energy efficiency. The exhaust heat in cogeneration is used for obtaining motive power, heating energy, or cooling energy.

Furthermore, the laser device11may be used as a window for, for example, image forming apparatuses such as a laser copier and a laser printer, image projection devices such as a projector, laser beam machines, laser peening devices, or terahertz (THz) generators.

Second Embodiment

Next, a laser device according to a second embodiment of the present disclosure is described with reference toFIG. 9. Note that like reference signs are given to like elements similar to those described as above in the first embodiment, and their detailed description is omitted. Compared with the laser device according to the first embodiment, the configurations of the present embodiment are similar to those of the first embodiment except the window27A of the laser device11, as illustrated inFIG. 2andFIG. 3. Accordingly, only the configuration of the window according to the present embodiment is described.

FIG. 9is a diagram illustrating a configuration of a window of the laser device, according to the second embodiment of the present disclosure.

As illustrated inFIG. 9, the window27B is provided with a hydrophilic layer275that is formed between the optical window271and the catalyst layer274.

The hydrophilic layer275is formed of a material that has an affinity for water. Such a material that has an affinity for water may be, for example, oxides, nitrides, carbides, or fluoride of for example, Si, Al, Ca, Ti, Zr, Ta, and Bi. Among these, it is desired that silicon dioxide (SiO2) that is an oxide of Si be adopted. The hydrophilic layer275may be made of one of these materials, or may be made of two or more of these materials in combination.

When the hydrophilic layer275is formed on the exit plane271bof the optical window271, the maximum value of the thickness of the catalyst layer274can further be reduced to a value smaller than 1 m. When the thickness of the catalyst layer274is small, as described above, even when the catalyst layer274is directly formed on the surface of the optical window271, the catalyst layer274tends to be separated from the optical window271. In the present embodiment, the hydrophilic layer275is formed on the on the exit plane271bof the optical window271in advance. Accordingly, the catalyst layer274can be formed on a plane that has a high affinity for water. Due to this configuration, even if the thickness of the catalyst layer274is made even thinner, the catalyst layer274can stably be formed on the surface of the catalyst layer274on the light exiting side.

Note also that the thickness of the hydrophilic layer275is not limited to any particular thickness, and it is satisfactory as long as the hydrophilic layer275has thickness that enables a high optical transmittance for the laser beams with wavelengths of 1064 nm and maintains the adhesive force with the optical window271.

As described above, the thickness of the catalyst layer274can further be reduced by forming the hydrophilic layer275on the exit plane271bof the optical window271. Due to this configuration, the transparency of the catalyst layer274can further be improved. Accordingly, the transmittance of the laser beams can further be improved, and stains can be prevented from sticking to the optical window271.

In order to prevent the reflection of the laser beams, it is desired that the two layers composed of the catalyst layer274and the hydrophilic layer275be adjusted to have appropriate thickness, respectively, in view of the refractive indexes of the catalyst layer274and the hydrophilic layer275. When the two layers that are composed of the catalyst layer274and the hydrophilic layer275are formed on the optical window271on the light exiting side, the reflectance R of the two layers of the catalyst layer274and the hydrophilic layer275is expressed in Equation 3 given below. Moreover, dAR1that denotes the thickness of the hydrophilic layer275is expressed in Equation 4 given below, and dAR2that denotes the thickness of the catalyst layer274is expressed in Equation 5 given below. In these equations, no denotes the refractive index of the air (n0=1), and nm denotes the refractive index of the optical window271. Moreover, nAR1denotes the refractive index of the hydrophilic layer275, and nAR2denotes the refractive index of the catalyst layer274. Further, λ denotes the wavelengths of the laser beams (1064 nm).
R=[(n0×(nAR2)2−nm×(nAR1)2/(nAR2)2+nm×(nAR1)2]2[Equation 3]
dAR1=λ/(4×nAR1)  [Equation 4]
dAR2=λ/(4×nAR2)  [Equation 5]

As understood from the above Equation 3, the difference in refractive index between the catalyst layer274and the hydrophilic layer275needs to be increased in order to reduce the reflectance R of the two layers composed of the catalyst layer274and the hydrophilic layer275and achieve high antireflective functionality. For this reason, it is desired that the materials of the catalyst layer274and hydrophilic layer275be chosen so as to have a large difference in refractive index. In the present embodiment, each of the catalyst layer274and the hydrophilic layer275is adjusted to have the thickness calculated from the above equations. By so doing, the two layers of the catalyst layer274and the hydrophilic layer275can achieve high antireflective functionality.

The hydrophilic layer275may be formed on the exit plane271bof the optical window271, for example, after the optical window271is fixed on the inside surfaces of the optical window holding member272by brazing and the antireflection film273is formed on the plane of the optical window271on the incident side. As a method of forming the hydrophilic layer275on the optical window271, for example, vapor deposition, sputtering, thermal spraying (flame plating), coating, or sol-gel processes may be used.

In the present embodiment, the hydrophilic layer275has single-layer structure. However, no limitation is intended thereby, and the hydrophilic layer275may have multilayer structure.

Third Embodiment

Next, a laser device according to a third embodiment of the present disclosure is described with reference toFIG. 10toFIG. 26. Note that like reference signs are given to like elements similar to those described as above in the first embodiment, and their detailed description is omitted. Compared with the laser device according to the first embodiment, the configurations of the present embodiment are similar to those of the first embodiment except the window27A of the laser device11, as illustrated inFIG. 2andFIG. 3. Accordingly, only the configuration of the window according to the present embodiment is described.

FIG. 10is a diagram illustrating a configuration of a window of a laser device, according to the third embodiment of the present disclosure.

FIG. 11is a partially magnified view of the window illustrated inFIG. 10.

As illustrated inFIG. 10andFIG. 11, the window27C has a microstructure division31on the exit plane271bof the optical window271, and the catalyst layer274is formed on the surfaces of the microstructure division31. The microstructure division31includes a plurality of convex micro parts (hereinafter, such a plurality of convex micro parts will be referred to as “micro-convex parts”)31a.

The micro-convex parts31aare cone-shaped (tapered) where the cross-sectional areas of the micro-convex parts31agradually decrease in the +Z-direction. More specifically, the cross-sectional areas of the micro-convex parts31agradually decrease from the light entering side (incident side) to the light exiting side of the window27C. Due to this configuration, the changes in refractive index on the interface between the window27C and the combustion chamber14can be minimized, and thus the reflection of the laser beams on the interface can be controlled.

The multiple micro-convex parts31aare arranged with a pitch P shorter than the wavelength of incident laser beams (incident light) (for example, 1064 nanometers (nm)). More specifically, the vertices of the micro-convex parts31aarranged with a pitch shorter than the wavelength of incident laser beams. In the present embodiment, it is assumed that the spaces between the multiple micro-convex parts31aare the spaces between the vertices of a neighboring pair of the micro-convex parts31a. However, no limitation is indicated thereby, and the spaces between the multiple micro-convex parts31amay be the distance between the centers of the bases of the neighboring pair of the micro-convex parts31a.

The diameters of the micro-convex parts31aand the pitches P of the micro-convex parts31aare satisfactory as long as they are shorter than the wavelengths of the laser beams in order to reduce the reflectance of the laser beams. It is desired that the diameters of the micro-convex parts31abe equal to or shorter than the half of the wavelengths of the laser beams, and it is more desirable if the diameters of the micro-convex parts31aare within range of 5 nanometers (nm) to 1000 nm. As long as the diameters of the micro-convex parts31aare within range of 5 nm to 1000 nm, the reflectance of the laser beams can sufficiently be reduced.

It is desired that the pitch P of the multiple micro-convex parts31abe equal to or shorter than the half of the wavelengths of the laser beams, and it is more desirable if the pitch P of the multiple micro-convex parts31aare within range of 10 nm to 2000 nm. As long as the space of the multiple micro-convex parts31aare within range of 10 nm to 1000 nm, the reflectance of the laser beams can sufficiently be reduced.

Moreover, it is desired that a flat surface33be formed between a neighboring pair of the micro-convex parts31a. More specifically, the areas of the flat surfaces33are expected to be, for example, 30% to 60% of the entire area of the microstructure division31.

Next, examples of the tapered micro-convex parts31aare described with reference toFIG. 12,FIG. 13, andFIG. 14. As illustrated inFIG. 12, in a microstructure division31A, cone-shaped micro-convex parts31a-1whose heights are about 300 nm and the diameters of the bases are about 300 nm are arranged in a staggered manner at intervals of about 300 nm. In other words, the micro-convex parts31a-1are arranged such that the spaces between the vertices or the distances between the centers of the bottom are about 300 nm.

As illustrated inFIG. 13, in a microstructure division31B, cone-shaped micro-convex parts31a-2whose heights are about 500 nm and the diameters of the bases are about 300 nm are arranged in a staggered manner at intervals of about 300 nm. Compared with the micro-convex parts31a-1, the micro-convex parts31a-2has a high aspect ratio where the ratio of the height to the maximum diameter is high. Due to this configuration, the micro-convex parts31a-2can make the changes in the refractive index of the laser beams gentler. Accordingly, the reflectance of the laser beams can further be reduced.

As illustrated inFIG. 14, in a microstructure division31C, micro-convex parts31a-3each of which is shaped like a quadrangular pyramid having a square base are arranged in a grid pattern. Moreover, the heights of the micro-convex parts31a-3are 500 nm and the sides of the bottoms of the micro-convex parts31a-3are 300 nm. In the microstructure division31C, there is no space between the multiple micro-convex parts31a-3, and the changes in refractive index (n2->nk(>n1)) can reliably be reduced. For this reason, the reflectance of the incident light on the interface can be decreased to a degree less than that of the microstructure division31A and the microstructure division31B as above.

As illustrated inFIG. 15, the micro-convex parts31amay be shaped like a hanging bell. Alternatively, as illustrated inFIG. 16, the spaces between a neighboring pair of the micro-convex parts31amay be irregular. For the sake of explanatory convenience, the microstructure division and the micro-convex parts illustrated inFIG. 15are referred to as a microstructure division31D and micro-convex parts31a-4, respectively. In a similar manner, the microstructure division and the micro-convex parts illustrated inFIG. 16are referred to as a microstructure division31E and micro-convex parts31a-5, respectively.

Even when the spaces between a neighboring pair of the micro-convex parts31aare irregular, it is desired that the average space between a neighboring pair of the micro-convex parts31ain the microstructure division31be equal to or shorter than the half of the wavelengths of the incident light. Moreover, it is desired that the space between a neighboring pair of the micro-convex parts31ain the microstructure division31be in accordance with the Gaussian distribution. When the space is in accordance with the Gaussian distribution, as illustrated inFIG. 17, the value of 2 σ in the Gaussian distribution is equal to or smaller than the wavelength multiplied by 1/√{square root over ( )}2.

The microstructure divisions31A to31C are often manufactured by the electron-beam lithography and etching, and the microstructure division31D and the microstructure division31E are often manufactured by etching processes where metallic nanoparticles are used as a mask. Alternatively, the microstructure division31D and the microstructure division31E may be manufactured using, for example, X-ray lithography and photolithography.

Next, the area of the microstructure division31(the area of the microstructure division) that is arranged on the exit plane271bof the window27C is described.

As illustrated inFIG. 18, the first area of the microstructure division is the area inside the planar area shaped like a rectangular frame joined to the housing28on the exit plane271b, and includes the beam passing area of the exit plane271b. Due to such a configuration, all the laser beams that pass through the exit plane271bare incident on the first area of the microstructure division, and the reflectance of all the laser beams is reduced. Note also that even when the positions of the exit plane271bthrough which the laser beams pass are slightly misaligned, the reflectance of all the laser beams is still reduced.

As illustrated inFIG. 19, the second area of the microstructure division matches the beam passing area of the exit plane271b. Due to such a configuration, all the laser beams that pass through the exit plane271bare incident on the second area of the microstructure division, and the reflectance of all the laser beams is reduced. In this configuration, the microstructure division31is formed only to the area of the exit plane271bthrough which the laser beams pass, and thus the area to be processed can be narrowed.

As illustrated inFIG. 20, the third area of the microstructure division is included within the beam passing area of the exit plane271b, and includes an area on the exit plane271bwhere the effective beam diameter is the maximum diameter. The term “effective beam diameter” refers to a beam diameter having a relative intensity of 1/e2(13.5%) relative to the maximum intensity (100%) in the Gaussian distribution of light intensity as illustrated inFIG. 21.

In such a configuration, the laser beams pass through the exit plane271b, and all the laser beams within the area where the effective beam diameter, which has a substantial impact on changes in the intensity of the laser beams that are emitted from the surface-emitting laser21, is the maximum diameter are incident on the third area of the microstructure division. Accordingly, the reflectance of all the laser beams is reduced. Note also that even when the positions of the exit plane271bthrough which the laser beams pass are slightly misaligned, the reflectance of all the laser beams is reduced within the area where the effective beam diameter is the maximum diameter. Moreover, the microstructure division31is formed only to the area of the exit plane271bthrough which the laser beams, which has a substantial impact on changes in the intensity of the laser beams, pass, and thus the area to be processed can further be narrowed.

As illustrated inFIG. 22, the fourth area of the microstructure division matches the area on the exit plane271bwhere the effective beam diameter is the maximum diameter. In such a configuration, the laser beams pass through the exit plane271b, and all the laser beams within the effective beam diameter, which has a substantial impact on changes in the intensity of the laser beams that are emitted from the surface-emitting laser21, are incident on the fourth area of the microstructure division. Accordingly, the reflectance of all the laser beams is reduced. Moreover, it is satisfactory as long as the micro-convex parts31a-4are formed only within the area where the effective beam diameter is the maximum diameter. Accordingly, the area to be processed can further be narrowed.

As illustrated inFIG. 23, the fifth area of the microstructure division is included in the area on the exit plane271bwhere the effective beam diameter is the maximum diameter, and includes the center of the area on the exit plane271bwhere the effective beam diameter is the maximum diameter. In such a configuration, when the laser beams pass through the exit plane271b, some of the laser beams pass through the area where the effective beam diameter is the maximum diameter. Such some of the laser beams are laser beams with high intensities and includes the laser beams with the maximum intensity, and the intensities tend to fluctuate widely depending on the surface-emitting laser21. Some of the laser beams are incident on the fifth area of the microstructure division, and the reflectance of such some of the laser beams is reduced. Moreover, it is satisfactory as long as the micro-convex parts31a-5are formed only within an area smaller than the area where the effective beam diameter is the maximum diameter. Accordingly, the area to be processed can further be narrowed.

In the present embodiment, the area of the microstructure division is configured to be one of the above first to fifth areas of the microstructure division.

As described above, the window27C according to the present embodiment has a microstructure division31on the exit plane271bof the optical window271, and thus the reflection of the laser beams on the exit plane271bof the window27C can be controlled. Further, the adhesion of stain can also be prevented.

Next, advantageous effects of the microstructure division31are described below in detail.

As illustrated inFIG. 24, in known windows where the microstructure division31is not formed on the optical window271, the changes in refractive index (n2->n1) is abrupt on the interface between the optical window271and the atmosphere inside the combustion chamber14. InFIG. 24, n2denotes the refractive index of the optical window271, and n1denotes the refractive index of the atmosphere of the combustion chamber14. Due to such a configuration as above, the reflectance of the incident laser beams on the interface increases.

Accordingly, in the known windows, some of the laser beams that are incident on the above interface may be reflected and concentrated within the window. Due to this configuration, the optical window271may deteriorate, or soot or the like generated in the combustion chamber14may stick to the window. As some of the laser beams that are incident on the optical window271are reflected and return to the laser medium251or the surface-emitting laser21, the intensity of the laser beams that are emitted from the laser device11may fluctuate. Moreover, soot or the like generated in the combustion chamber14may stick to the optical window271, and the light quantity of laser beams that enter the combustion chamber14may also decrease.

In the known windows, an antireflection film is formed on the window in order to reduce the reflection of some of the laser beams that are incident on the window. However, the window is arranged facing towards the combustion chamber in the ignition systems. For this reason, the antireflection film may deteriorate as exposed to high temperature (for example, about 600° C.). Accordingly, the functionality of the exit plane271bas an antireflection film may substantially be lost.

By contrast, the window27C according to the present embodiment has the microstructure division31that are provided with the multiple micro-convex parts31aon the exit plane271bof the window27C at intervals shorter than the wavelength of incident laser beams (incident light). More specifically, the micro-convex parts31aare tapered where the cross-sectional areas of the micro-convex parts31agradually decrease, and the cross-sectional areas of the micro-convex parts31agradually decrease from the light entering side (incident side) to the light exiting side of the window27C. Due to this configuration, as illustrated inFIG. 25andFIG. 26, the refractive index can continuously be changed in such a manner that the changes in refractive index on the interface between the window27C and the combustion chamber14(n2->nk(>n1)) are gentle. Accordingly, the reflectance of the laser beams on the interface can be reduced, and thus the reflectance of the laser beams on the interface can be decreased. When the length of the micro-convex parts31ain the convex direction is longer than the maximum diameter, the changes in refractive index can be made even gentler, and the reflectance of the laser beams can further be reduced.

Due to this configuration of the present embodiment, the transmittance can be prevented from decreasing as the laser beams are concentrated inside the window27C and the window27C deteriorates, or the laser beams can be prevented from returning to the laser medium251or the surface-emitting laser21. Due to this configuration, the intensity of the laser beams can be prevented from fluctuating.

Due to the microstructure division31, soot or the like generated in the combustion chamber14does not easily get into the spaces between the multiple micro-convex parts31a. Due to this configuration, the adherence of soot or the like to the multiple micro-convex parts31acan be weakened. Accordingly, the adherence of soot or the like to the microstructure division31can be controlled. In the window27C, while controlling the adherence of soot or the like due to the microstructure division31, the soot or the like, which is stuck to the surface of the catalyst layer274arranged on the light exiting side of the microstructure division31, can be decomposed. As a result, the window27C can further efficiently be prevented from being contaminated. As a result, the transmittance of the laser beams that are emitted from the third condensing optical system26can be maintained with high stability, and variations in intensity of the laser beams can further be prevented or reduced. Accordingly, the light quantity of laser beams that enter the combustion chamber14can be maintained with further improved stability.

In the present embodiment, the optical window271is formed of a material with high heat resistance and high pressure resistance. For this reason, even if the microstructure division31is disposed at a position where the microstructure division31is exposed to the combustion chamber14, the microstructure division31does not deform. For this reason, the reflection of the laser beams on the exit plane271bcan be controlled with high stability in the window27C according to the present embodiment, compared with when the microstructure division31is not formed.

Moreover, it is desired that in the window27C the spaces between the center points of the multiple micro-convex parts31abe approximately even. Due to this configuration, in the window27C, the reflectance can nearly uniformly be reduced throughout the entire area where the micro-convex parts31aare formed.

As described above, the laser device11according to the present embodiment is provided with the window27C. Due to this configuration, the reflection of the laser beams on the antireflection film273and the optical window271can be controlled. For this reason, with the laser device11according to the present embodiment, a reduction in the light quantity of laser beams that enter the combustion chamber14can further be controlled with increased stability.

As long as the micro-convex parts31athat are formed on the window27C has convex structure where the micro-convex parts31a(for example, the vertices of the micro-convex parts31a) are arranged at intervals shorter than the wavelengths of incident light, the shape, size, and the layout of the micro-convex parts31ais not limited.

In the present embodiment, cases in which the micro-convex parts31ais cone-shaped are described. However, no limitation is intended thereby. For example, the micro-convex parts31amay be tapered and cone-shaped like an elliptical cone, a circular truncated cone, an elliptical truncated cone, a polygonal truncated cone, or tapered and pyramid-shaped like any polygonal pyramid other than a square pyramid. Alternatively, the micro-convex parts31amay be shaped like, for example, a circular cylinder, an elliptical cylinder, and a polygonal cylinder.

In some embodiments, the vertical cross sections of the micro-convex parts31a(i.e., the cross section parallel to the light exit direction) may be curved on the sides.

In the present embodiment, the micro-convex parts31aare tapered, and the cross-sectional areas of the micro-convex parts31agradually decrease in the light exit direction. However, no limitation is intended thereby, and the micro-convex parts31amay have a shape where the cross-sectional area decreases step-by-step in the light exit direction.

In the present embodiment, the multiple micro-convex parts31ahave the same shape and the same size. However, no limitation is intended thereby, and in some embodiments, the shapes or sizes of the multiple micro-convex parts31amay be different from each other.

In the present embodiment, the multiple micro-convex parts31aof the microstructure division31are regularly arranged. However, as long as the spaces between the multiple micro-convex parts31aare shorter than the wavelengths of incident light, the spaces between the multiple micro-convex parts31aare not necessarily regularly arranged.

Next, some modifications of the microstructure division31according to the above embodiments of the present disclosure are described.

FIG. 27is a diagram illustrating a first modification of the window according to the above embodiments of the present disclosure.

A window27C-1according to the first modification is illustrated inFIG. 27, and as in the window27C-1, a plurality of concave micro parts (hereinafter, such a plurality of concave micro parts will be referred to as “micro-concave parts”)31bmay be formed on the exit plane271b. InFIG. 27, “h” denotes the combined depth of the multiple micro-concave parts31band the catalyst layer274, and “P” denotes the pitches of the combination of the multiple micro-concave parts31band the catalyst layer274. The multiple micro-concave parts31bis obtained by etching a plurality of portions of one side of the window27C-1to form the multiple micro-concave parts31b. Also in the configuration according to the first modification, advantageous effects similar to those of the embodiments of the present disclosure as described above can be achieved.

FIG. 28is a diagram illustrating a second modification of the window according to the above embodiments of the present disclosure.

A window27C-2according to the second modification is illustrated inFIG. 28, and the microstructure division31may be formed on the optical input end (i.e., the surface on the incident side). InFIG. 28, “h” and “P” denote the depths and pitches of the microstructure division31, respectively. In the second modification of the embodiments of the present disclosure, the changes in refractive index on the interface between the atmosphere inside the housing28and the optical input end of the window27C-2can be made even gentler than “n3−>n2” (n3->ni(>n2)). Accordingly, the reflection of the laser beams on the optical input end can be controlled, and the laser beams can be prevented from returning to the laser medium251or the surface-emitting laser21. As a result, variations in intensity of the laser beams can be prevented or reduced.

FIG. 29is a diagram illustrating a third modification of the window according to the above embodiments of the present disclosure.

A window27C-3according to the third modification is illustrated inFIG. 29, and multiple micro-concave parts31bmay be formed on the optical input end. InFIG. 29, “h” and “P” denote the depths and pitches of the multiple micro-concave parts31b, respectively. More specifically, the multiple micro-concave parts31bmay be formed by etching a plurality of portions of one side of the optical window271. Also in the third modification of the embodiments of the present disclosure, the changes in refractive index on the interface between the atmosphere inside the housing28and the optical input end of the optical window271can be made gentler than “n3->n2” (n3->ni(>n2)). Accordingly, the reflection of the laser beams on the optical input end can be controlled, and thus the laser beams can be prevented from returning to the laser medium251or the surface-emitting laser21. As a result, variations in intensity of the laser beams can be prevented or reduced.

FIG. 30is a diagram illustrating a fourth modification of the window according to the above embodiments of the present disclosure.

A window27C-4according to the fourth modification is illustrated inFIG. 30, and like the window27C-4, the multiple micro-convex parts31amay be formed on the exit plane271band the optical input end. On the exit plane271bside ofFIG. 30, “h” denotes the combined depth of the multiple micro-concave parts31band the catalyst layer274, and “P” denotes the pitches of the combination of the multiple micro-concave parts31band the catalyst layer274. On the optical input end side, the multiple micro-concave parts31bmay be formed with the depth “h” and the pitch “P.”

In the fourth modification, the reflection of the laser beams on the exit plane271band the optical input end can be controlled. Due to this configuration, the window can be prevented from degrading, and the laser beams can be prevented from returning to the laser medium251or to the surface-emitting laser21. Accordingly, variations in intensity of the laser beams can further be prevented or reduced.

FIG. 31is a diagram illustrating a fifth modification of the window according to the above embodiments of the present disclosure.

A window27C-5according to the fifth modification is illustrated inFIG. 31, and like the window27C-5, the shape of the micro-convex parts31amay be columnar. Also in this configuration, the laser beams to be reflected can be reduced compared with the known configurations. Accordingly, both improvement in heat resistance and reduction in laser beams to be reflected can be achieved. Note also that the cylindrical micro-convex parts31amay be, for example, cylindrical, elliptically cylindrical, quadrangularly cylindrical, polygonally cylindrical.

FIG. 32is a diagram illustrating a sixth modification of the window according to the above embodiments of the present disclosure.

A window27C-6according to the sixth modification is illustrated inFIG. 32, and like the window27C-6, the gap between a neighboring pair of the micro-convex parts31amay substantially be zero. In other words, the flat surfaces33as in the above embodiments of the present disclosure may be omitted. Also in this configuration, the laser beams to be reflected can be reduced compared with the known configurations. As a result, both improvement in heat resistance and reduction in laser beams to be reflected can be achieved.

In the first to sixth modifications of the above embodiments of the present disclosure, the area of the exit plane271bwhere a micro-convex structure and a micro-concave structure are formed may be configured in a similar manner to the first area to fifth area of the microstructure division as described above.

In the fourth modification as described above, the concave parts or the convex parts of the micro-convex structure and the micro-concave structure are in line with each other. However, no limitation is indicated thereby, and the concave parts or the convex parts of the micro-convex structure and the micro-concave structure may be misaligned or displaced from each other.

Note also that the area of the microstructure division is not limited to a circular shape, and may be in other shapes such as an ellipse and a polygon.

In the above embodiments of the present disclosure, a micro-convex structure and a micro-concave structure are formed on the window27C. However, no limitation is intended thereby, and a micro-convex structure and a micro-concave structure may be formed on the catalyst layer274.

In the above embodiments of the present disclosure, the catalyst layer274is formed on the surfaces of the microstructure division31. However, no limitation is intended thereby, and the hydrophilic layer275, which is described as above with reference toFIG. 9in the second embodiment, may be formed between the microstructure division31and the catalyst layer274.