Abstract:
A laser ignition device capable of achieving stable ignition, preventing deterioration of a semiconductor laser element is provided, by suppressing the intensity of oscillated light leakage leaking towards semiconductor laser side from the laser resonator with a simple configuration. A laser ignition device  7  includes an excitation light source  1  emitting coherent excitation light L PMP , an optical element  2  transmitting excitation light L PMP , a laser resonator  3  oscillating oscillated light having high energy density by being irradiated with excitation light L PMP , and condensing means  6  condensing the oscillated light L PLS  oscillated by the laser resonator  3 . Moreover, the laser ignition device  7  is provided with a light-transmissive-reflective film  5  disposed between the excitation light source  1  and the laser resonator  3 . The light-transmissive-reflective is film  5  permeating the excitation light L PMP  having short wavelength and reflecting oscillated light leakage L LEAK  having long wavelength.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a laser ignition device, in which a plurality of semiconductor laser devices are used as excitation light sources, transmitting a laser to a laser resonator via an optical element, thereby generating a pulse laser having high energy density, the pulse laser being condensed inside a combustion chamber of an internal combustion engine so as to ignite air-fuel mixture. 
       BACKGROUND ART 
       [0002]    Various types of laser ignition devices have been considered. These laser ignition devices are used for internal combustion engines having low ignitability such as a gas fuel engine for cogeneration, a lean-burn engine or the like. Each of the laser ignition devices uses a semiconductor laser element as an excitation light source, irradiating a Q-switch laser resonator with excitation light oscillated by the semiconductor laser element to oscillate a pulse laser having high energy density. The energy density is further enhanced by using a condensing means that condenses the oscillated pulse laser in an air-fuel mixture introduced into the combustion chamber, so as to ignite the air-fuel mixture. 
         [0003]    For example, PTL 1 discloses a laser ignition device performing ignition of an internal combustion engine, provided with a laser device including a passive Q-switch having a solid-state active laser. The solid state active laser includes a laser-active region and a laser-inactive region which are designed to have specific length. 
         [0004]    On the other hand, in the semiconductor laser device, when light emitted from the light source of the semiconductor laser element or the like enters various optical elements, optical fibers or the like, a part of the light is reflected or scattered on an incident surface of the optical element or the optical fiber, and the light may partially return to the light source. In the case where the returning light enters an active layer of the semiconductor laser element, wavelength of the oscillation may be disturbed or the output thereof may be varied. In the worst case scenario, it has been known that the semiconductor laser element may be broken. 
         [0005]    Therefore, according to a general semiconductor laser device, as a means for eliminating the returned light, an optical isolator is used. Specifically, a polarization dependent optical isolator which depends on a polarization state of the incident light, and a polarization independent optical isolator which does not depend on a polarization state of the incident light have been known (refer to PTLs 2 and 3). 
         [0006]    Likewise, for the laser ignition device, by using the optical isolator, it is considered that the semiconductor laser element can be prevented from being damaged by the returned light. 
       CITATION LIST 
     Patent Literature 
       [0007]    [PTL 1] JP-A-2009-141362 
         [0008]    [PTL 2] JP-A-H11-223797 
         [0009]    [PTL 3] JP-A-2008-268847 
       SUMMARY 
     Technical Problem 
       [0010]    However, in the case where a plurality of semiconductor laser devices are used as excitation light sources in the laser ignition device so as to increase the output energy, thereby performing ignition of an internal combustion engine having low ignitability, when an optical isolator is used, as in the conventional technique, to prevent the semiconductor laser element being broken due to returned light, following problems arise. 
         [0011]    Firstly, various optical components which compose the optical isolator are required so that manufacturing cost may increase. 
         [0012]    Secondary, it is necessary to adjust an optical axis of the excitation light oscillated by the plurality of semiconductor laser elements to be received by the optical isolator. Hence, a mechanism or a work for adjusting the optical axis is further required, which causes a further increase of manufacturing cost. 
         [0013]    In the laser ignition devices, a pulse laser having extremely large energy density has to be condensed. Accordingly, a semiconductor laser module composed of a plurality of semiconductor laser elements is used as an excitation light source, whereby loads of each of the semiconductor laser elements can be reduced. 
         [0014]    In this case, when the excitation light oscillated by the plurality of semiconductor laser elements is condensed by a condenser lens, excitation light having mutually different phases are present, and each of the optical axes are different from each other. Hence, in the case where a beam of the excitation light which passed through the optical isolator are recombined similar to the beam of the original excitation light, and the returned light is emitted to the light source side, it is very difficult to adjust the ordinary light and the extraordinary light separated by the plurality of semiconductor laser elements so that they do not enter the semiconductor laser elements. 
         [0015]    Thirdly, in the case where the optical isolator is used, intensity of the excitation light being irradiated to the laser medium may be decreased, because of a reflection loss when the excitation light enters the optical isolator and a transmission loss of optical components which compose the optical isolator. 
         [0016]    Other than the optical isolator, since transmission loss occurs when passing through the optical components, which is not avoidable, a simple configuration is preferably used where the number of optical components is reduced as much as possible to increase an energy efficiency of the laser ignition device. 
         [0017]    Fourthly, since the optical isolator produces Faraday effects by using a permanent magnet which applies a strong magnetic field to a Faraday rotator, a mechanism that cuts off magnetic field is required in the vicinity thereof, causing an increase in size of the laser ignition device. Therefore, recent requirements of shrinking laser ignition device may not be satisfied. 
         [0018]    Fifthly, conventionally, it has been considered that almost no laser light leakage exists from an edge surface of excitation side of the laser resonator. However, in the case where the laser ignition requires a laser for generating a megawatt order or more of giant pulse, it has been found that light leakage from the edge surface of the excitation side cannot be ignored. 
         [0019]    Specifically, the excitation edge surface of the microchip laser serves as a totally reflecting mirror of the resonator as well. Hence, conventionally, a transmittance of 0.1% is considered sufficient for the oscillation light, as a transmittance of the high reflection coating. 
         [0020]    However, considering that the optical power in the resonator of the laser oscillation is 10 MW or more, even when the transmittance of the high reflection coating is 0.1%, the optical power of light leakage transmitted from the edge surface reaches 10 KW. 
         [0021]    On the other hand, the optical power required for the excitation light of such a microchip laser is approximately 100 W, which is only 1/100 of the optical power of the light leakage of the oscillation light, even when 100% amount of the excitation light is returned. 
         [0022]    Specifically, the light leakage of the oscillated light from the resonator, as a conventional problem of the semiconductor laser, is much larger than the returned light due to a reflection from the excitation light. Hence, there is a concern that the light leakage of the oscillated light from the resonator may directly hit the semiconductor laser device used for an excitation light source. 
         [0023]    The semiconductor laser element is irradiated by the oscillated light leakage for extremely short period of time such as several nanoseconds. However, it was found that long-term reliability is degraded. Note that the duration where the excitation reflection light irradiates the semiconductor laser element is extremely larger than the duration where the oscillation light leakage from the laser resonator side irradiates the semiconductor laser element. In this respect, an amount of energy of the excitation reflection light is larger than that of the oscillation light leakage. However, since the electric power the excitation reflection light is small, the laser diode does not suffer any damage. However, the oscillation light leakage of which the electric power is extremely high causes a damage to the laser diode even in a very short period of time. 
         [0024]    It should be noted that damage to the semiconductor laser element is caused by thermal reasons or light intensity (power/area) reasons. The present invention is to solve a failure caused by an increase in the power of the oscillated light leakage, the oscillated light leakage being among the light returning to the excitation light source side. 
         [0025]    The present invention has been achieved in light of the above-mentioned circumstances and aims to provide a laser ignition device capable of preventing semiconductor laser element being deteriorated so as to accomplish stable ignition operation, by suppressing the intensity of light leakage towards the semiconductor laser device from the laser resonator with a simple configuration. 
       Solution to Problem 
       [0026]    A laser ignition device ( 7 ,  7   a ,  7   b ,  7   c ,  7   d ,  7   e ,  7   f ,  7   g ) according to one aspect of the present disclosure is configured to ignite an air-fuel mixture introduced into a combustion chamber ( 80 ) of an internal combustion engine ( 8 ) by condensing oscillated light (L FCS ) having high energy density. The laser ignition device is characterized in that the device includes: an excitation light source ( 1 ) that emits coherent excitation light (L PMP ); an optical element ( 2 ,  2   a ,  2   b ,  2   c ,  2   d ,  2   e ,  2   f ,  2   g ) that transmits the excitation light emitted from the excitation light source; a laser resonator ( 3 ,  3   a ) that oscillates oscillated light (L PLS ) having high energy density by being irradiated with the excitation light transmitted via the optical element; and a condensing means ( 6 ) that condenses the oscillated light oscillated by the laser resonator, in which a light-transmissive-reflective film ( 5 ,  5   a ,  5   b ,  5   c ,  5   d ,  5   e ,  5   f ,  5   g ) is provided between the excitation light source and the laser resonator, the light-transmissive-reflective film permeating the excitation light having short wavelength and reflecting oscillated light leakage (L LEAK ) leaked from the laser resonator to an excitation light source side, the oscillated light leakage being a part of the oscillated light having long wavelength. 
       Advantageous Effects of Invention 
       [0027]    Even when a part of the excitation light oscillated by resonant amplification in the laser resonator is incident as a light leakage on the excitation light source side, the light-transmissive-reflective film reflects it so that the energy of the excitation light becomes extremely small before reaching the excitation light source. Hence, the excitation light source is prevented from being damaged by the oscillated light leakage. 
         [0028]    Further, since the light-transmissive-reflective film is provided as an extremely thin multi-layered film disposed on an end surface of the optical element, unlike the case where a conventional optical isolator is provided, increasing device size can be avoided. 
         [0029]    Furthermore, since the light-transmissive-reflective film is formed on existing optical elements, additional optical elements are not required so that an increase of transmission energy loss can be minimized. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0030]      FIG. 1A  is cross sectional view taken along a line A-A shown in  FIG. 1B , showing an overall configuration of the major portion of a laser ignition device according to the first embodiment. 
           [0031]      FIG. 1B  is a partial cross sectional view taken along a line B-B shown in  FIG. 1A . 
           [0032]      FIG. 1C  is a schematic view showing effects of a light-transmissive-reflective film. 
           [0033]      FIG. 2  is a cross sectional view showing a verification method for confirming effects of the present embodiment. 
           [0034]      FIG. 3A  is a characteristics diagram showing effects of suppression of the oscillated light leakage according to the present embodiment together with a comparative example. 
           [0035]      FIG. 3B  is a characteristics diagram showing conditions of a durability test. 
           [0036]      FIG. 3C  is a characteristics diagram showing effects of improvement of durability according to the present embodiment together with a comparative example. 
           [0037]      FIG. 4A  is a picture in place of a drawing, showing a state of a light-emission emitter after the durability test is applied in the embodiment. 
           [0038]      FIG. 4B  is a picture in place of a drawing, showing a state of damage of a light-emission emitter after the durability test is applied in a comparative example. 
           [0039]      FIG. 4C  is a characteristics diagram showing an energy density in a laser resonator. 
           [0040]      FIG. 4D  is an overlay diagram in which an area having a specific energy density in a conventional laser resonator, and locations where the light-emission emitters are often damaged, are overlaid. 
           [0041]      FIG. 5A  is a cross sectional view showing a major portion of a laser ignition device according to a second embodiment. 
           [0042]      FIG. 5B  is a cross sectional view showing a major portion of a laser ignition device according to a third embodiment. 
           [0043]      FIG. 5C  is a cross sectional view showing a major portion of a laser ignition device according to a fourth embodiment. 
           [0044]      FIG. 5D  is a cross sectional view showing a major portion of a laser ignition device according to a fifth embodiment. 
           [0045]      FIG. 5E  is a cross sectional view showing a major portion of a laser ignition device according to a sixth embodiment. 
           [0046]      FIG. 5F  is a cross sectional view showing a major portion of a laser ignition device according to a seventh embodiment. 
           [0047]      FIG. 5G  is a cross sectional view showing a major portion of a laser ignition device according to an eighth embodiment. 
           [0048]      FIG. 6  is a diagram showing an overall configuration of the laser ignition device. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0049]    In the present disclosure, an excitation light source  1  is provided that emits coherent excitation light L PMP , an optical element  2  that transmits excitation light emitted from the excitation light source  1 , a laser resonator  3  that produces oscillated light L PLS  having high energy density by being irradiated with an excitation light L PMP  transmitted via the optical element  2 , and a condensing means  6  that condenses the oscillated light L PLS  oscillated by the laser resonator  3 . The present disclosure relates to a laser ignition device  7  in which oscillated light L FCS  having high energy density is condensed into the air-fuel mixture introduced inside the combustion chamber  80  of an internal combustion engine  8 . In the present disclosure, a forward direction may be defined as a direction extending along an optical path which extends to the internal combustion engine  8  from the light excitation light source  1 , and extending to the internal combustion engine  8 , and a backward direction may be defined as a direction extending along an optical path which extends to the internal combustion engine  8  from the excitation light source  1 , and extending to the excitation light source  1 . An “incident side” refers to a side on which light proceeding towards the forward direction is incident, and an “emission side” refers to a side from which light proceeds to the forward direction. 
         [0050]    With reference to  FIGS. 1A, 1B and 1C , a major portion of the laser ignition device  7  according to the first embodiment will be described. Note that the overall configuration of the laser ignition device  7  will be described later with reference to  FIG. 6 . 
         [0051]    The excitation light source  1  according to the present embodiment includes a light-emission emitter composed of a semiconductor laser element, and emits coherent excitation light L PMP  by being energized. In the present embodiment, as excitation light L PMP , an infrared laser having a peak wavelength λ PMP  of 808 nm is used, and it is exemplified that oscillated light L PLS  having a peak wavelength λ PLS  of 1064 nm is emitted from the laser resonator  3  by being irradiated with the excitation light L PMP . The wavelength λ PMP  of the excitation light L PMP  emitted from the excitation light source  1  and the wavelength λ PLS  of the oscillated light LDLs can be approximately selected. 
         [0052]    The excitation light source  1  is provided with cylindrical lenses  20  and  21  as an optical element, which collimate the excitation light L PMP , configuring a semiconductor laser module  10 . For the cylindrical lenses  20  and  21 , known optical material is used including an optical glass, a heat-resistant glass, quartz glass, and a sapphire glass. A plurality of semiconductor laser modules  10  are arranged in tiers (step) on a semiconductor laser fixing base  13 . 
         [0053]    According to the present embodiment, it is exemplified that semiconductor laser modules  10  are arranged in 2 rows, each row having 8 semiconductor laser modules  10  arrayed therein. However, the number of semiconductor laser modules is not limited. 
         [0054]    Excitation light LPMP, which is emitted from the plurality of semiconductor laser modules  10  and collimated, is condensed by condensing lenses  22  and  23  provided at a tip end as an optical element. For the condensing lenses  22  and  23 , known optical material is used including an optical glass, a heat-resistant glass, a quartz glass, and a sapphire glass. 
         [0055]    Each surface of the condensing lenses  22  and  23  may be covered with known antireflection coating. 
         [0056]    Each end surface in the incident side of the condensing lenses  22  and  23  is processed to be a plane shape, and the emission surface thereof is formed as an aspherical lens. 
         [0057]    Convergent light L CND  focused by the condensing lenses  22  and  23  is coupled to an optical fiber  25  via a coupling element  24 . 
         [0058]    For the coupling element  24 , an optical ferrule or a hollow sleeve or the like can be used. The optical ferrule is composed of known optical material such as crystallized glass, and the hollow sleeve holds an end portion of the optical fiber  25 . For the optical fiber  25 , a known optical fiber can be used having a numerical aperture 0.22 or less and a core diameter φ of 600 μm or less. A beam diameter of the excitation light L PMP  irradiating the laser resonator  3  is set as φ=1200 μm. 
         [0059]    The convergent light L C ND transmitted via the optical fiber  25  is collimated by the collimating lens  26  to produce parallel light L CMT  which is incident on the laser resonator  3 . 
         [0060]    For the collimating lens  26 , known optical material is used including an optical glass, a heat-resistant glass, a quartz glass, and a sapphire glass. 
         [0061]    Each end surface in the incident side of the collimating lens  26  according to the present embodiment is processed to be a plane shape, and the emission surface thereof is formed as an aspherical lens. 
         [0062]    The convergent light L CND , in which excitation light L PMP  is focused, is modulated to the parallel light L CMT . 
         [0063]    The collimating lens  26  has an antireflection coating  4  formed on the surface thereof. Further, a light-transmissive-reflective film  5  as a major portion of the present embodiment is formed at least on the end surface in the incident side, which is formed in a plate shape. In the light-transmissive-reflective film  5  according to the present embodiment, a high refractive index film  50  made of Ta 2 O 5  having high refractive index (n H =2.16) and a low refractive index film  51  made of SiO 2  having low refractive index (n L =1.41) are laminated alternately to form a 19-layered film. 
         [0064]    The light-transmissive-reflective film  5  permeates 99.8% of the excitation light L PMP  having short wavelength (e.g., λ PMP =808 nm), and reflects 99.6% of the light leakage L LEAK  of the oscillated light L PLS  having wavelength longer than that of the excitation light L PMP  (e.g., λ LEAK =λ PLS =1064 nm). 
         [0065]    Further, as a translucent film  4 , the high refractive index film  50  made of Ta 2 O 5  having high refractive index (n H =2.16) and the low refractive index film  51  made of SiO 2  having low refractive index (n L =1.41) are laminated alternately on an emission surface of the collimating lens  26  to form a 4-layered film. 99.8% of the excitation light L PMP  passes through the translucent film  4  to be emitted as the parallel light L CMT . 
         [0066]    For the low refractive index film  51 , a dielectric substance selected from SiO 2  and MgF 2  can be used. For the high refractive index film  50 , a dielectric substance selected from TiO 2  and Ta 2 O 3  can be used. Multi layered film can be formed by known thin film forming methods such as vapor deposition and ion plating. 
         [0067]    The laser resonator  3  is disposed at the front side of the collimating lens  26  in the forward direction. 
         [0068]    For the laser resonator  3 , a known passive Q-switch laser resonator can be used. 
         [0069]    The laser resonator  3  is configured of a laser medium  30 , an antireflection coating  31  provided in the incident side thereof, a totally reflecting mirror  32 , a saturable absorber  33  provided in the emission side thereof, and an emission mirror  34  composed of a partial reflection film, which are accommodated in a housing  35  having cylindrical shape. 
         [0070]    For the laser medium  30 , a known laser medium is used, for example Nd:YAG, where Nd is doped to single crystal YAG. 
         [0071]    The totally reflecting mirror  32  is formed such that the excitation light L PMP  having short wavelength is permeated and the oscillated light L PLS  having long wavelength is totally reflected. 
         [0072]    For the saturable absorber  33 , Cr:YAG in which Cr 4+  is doped into single crystal YAG or the like is used. 
         [0073]    For the laser resonator  3 , Nd in the laser medium  30  is excited by the excitation light L PMP  introduced in the resonator to emit light having a wavelength of 1064 nm, and the emitted light is accumulated in the laser medium  30 . 
         [0074]    The oscillated light L PLS  oscillates from an output mirror  34  when an energy level in the laser medium  30  reaches a prescribed level. 
         [0075]    At this moment, from the incident surface of the totally reflecting mirror  32  in the back surface side, approximately 0.4% of the intensity of the oscillated light L PLS  is inevitably propagated to the excitation light source  1  side as the oscillated light leakage L LEAK . 
         [0076]    According to the present embodiment, when the oscillated light leakage L LEAK  having approximately 0.4% of the intensity of the oscillated light L PLS  reaches the end surface in the incident surface side of the collimating lens  26 , the light-transmissive-reflective film  5  formed on the surface thereof reflects 99.8% of the oscillated light leakage L LEAK , the reflected light leakage L LEAK  returns to the laser resonator  3  side, and 0.2% of the oscillated light leakage L LEAK  is permeated to the excitation light source side. Also, on the end surface in the emission side of the collimating lens  26 , 0.2% of the oscillated light leakage L LEAK , which is reflected at the light-transmissive-reflective film  5 , is reflected to the excitation light source side. 
         [0077]    While the oscillated light leakage L LEAK  is reflected and permeated multiple times (e.g., approximately 3 ns duration) between the incident surface of the laser resonator  3  and the light-transmissive-reflective film  5  as a major portion of the present embodiment, the oscillated light leakage L LEAK  disappears. 
         [0078]    As a result, even if the 0.4% of the intensity of the oscillated light L PLS  is leaked to the excitation light source side from the laser resonator  3 , 99.6% of the leaked oscillated light is cut off by the light-transmissive-reflective film  5 . Therefore, 0.4% of the oscillated light leakage L LEAK , i.e., up to 0.0016% of the intensity of the oscillated light L PLS , leaks, and so the intensity of the oscillated leakage light L LEAK  transmitted to the excitation light source  1  side can be suppressed. 
         [0079]    Thus, even if the oscillation light L PLS  having power extremely larger than that of the excitation light L PMP  is partially leaked, the power of the oscillated light leakage L LEAK  is reduced to an amount of power similar to that of the reflected light of the excitation light L PMP . Accordingly, in the case where the light leakage L LEAK  reaches the excitation light source  1 , the semiconductor laser element is not damaged. 
         [0080]    As a specific example,  FIG. 1C  illustrates an example where the excitation light L PMP  having wavelength λ PMP  of 808 nm is introduced to the laser resonator  3  at 100 watt of power, and an oscillated light leakage transmittance T LEAK  is calculated when the oscillated light L PLS  having 1064 nm of wavelength λ PLS  is emitted at 10 MW of power. It should be noted that the power of the oscillated light L PLS  is not limited to the above-mentioned value in the laser ignition device according to the present invention. 
         [0081]    A simulation can be applied for the light-transmissive-reflective film  5  using Snell&#39;s law, Fresnel&#39;s formula and Maxwell equation to appropriately obtain combinations of conditions which accomplish high transmittance T PMP  of the excitation light LPMP and high reflectance R LEAK  of the light leakage L LEAK . The conditions include a refractive index n H  of the high refractive index film  50  and the film thickness d H  thereof, a refractive index n L  of the low refractive index film  51  and the film thickness d L  thereof, wavelength λ PMP  of the excitation light L PMP  and wavelength λ PLS  of the oscillated light L PLS , i.e., wavelength λ LEAK  of the oscillated light leakage L LEAK . 
         [0082]    Theoretically, conditions where 100% amount of the excitation light L PMP  is permeated and 100% of the oscillated light leakage L LEAK  is reflected can be calculated. However, practically, as described in the embodiment, the transmittance T PMP  of the excitation light L PMP  is approximately 99.8% and the reflectance R LEAK  of the oscillated light leakage L LEAK  is approximately 99.6%, i.e., the transmittance T LEAK  of the oscillated light leakage L LEAK  is approximately 0.4%. 
         [0083]    Hence, as in the present embodiment, the light-transmissive-reflective film  5  is provided with a totally reflecting film  32  of the laser resonator  3  to be overlapped from each other, whereby the oscillated light leakage L LEAK  as a part of the oscillated light L PLS  is reciprocally transmitted between the light-transmissive-reflective film  5  and the laser resonator  3  so as to cutoff 99.5% amount of light leakage L LEAK  propagating to the excitation light source  1  side from a partial reflection film  31 , the oscillated light leakage L LEAK  corresponding to 0.4% amount of the intensity of the oscillated light L PLS . As a result, the oscillated light leakage L LEAK  can be approximately 0.0016% amount of the oscillated light leakage L PLS . 
         [0084]    With reference to  FIG. 2 , hereinafter a method of verification will be described, the verification being applied to effects of suppression of the oscillated light leakage according to the present invention. 
         [0085]    The above-described semiconductor laser module  100  and the laser resonator  3  are connected, and a beam splitter  90  which totally reflects light having wavelength of 1064 nm is disposed in the middle of the optical fiber  25  which transmits the excitation light L PMP . A photodetector  92  detects intensity of the oscillated light leakage L LEAK  via the optical fiber  91 . 
         [0086]    A comparison example was provided in which the light-transmissive-reflective film  5  according to the present embodiment is not formed. The effects of the present embodiment were verified by comparing the comparison example with an example  1  in which the light-transmissive-reflective film  5  is formed on a plane part of the collimating lens  26 . 
         [0087]    As a result, as shown in  FIG. 3A , the intensity of the oscillated light leakage L LEAK  detected in the example 1 was 0.5% or less of the intensity of the returned light having 1064 nm detected in the comparison example. Specifically, it was confirmed that the light-transmissive-reflective film  5  reflects 99.5% of the oscillated light leakage L LEAK , and thus the oscillated light leakage L LEAK  to the excitation light source  1  was suppressed. 
         [0088]    Further, 10.5 amps of current (corresponding to 81 mJ of light energy) was supplied in pulse form to the semiconductor laser module  100  with a condition shown in  FIG. 3B  so as to drive the semiconductor laser module  100 . Then, a durability test was performed for the example 1 and the comparative example. 
         [0089]    The result is shown in  FIG. 3C . In the comparative example, the output power of the semiconductor laser module  100  is decreased to 30% within several hours. 
         [0090]    On the other hand, according to the example 1, despite continuous driving for several tens of hours, the output power is not decreased at all. 
         [0091]    After the durability test, the semiconductor laser modules  100  used for the example 1 and the comparative example were checked. As shown in  FIG. 4A , according to the example 1, every light-emission emitter operated properly. However, as shown in  FIG. 4B , according to the comparative example, damage was confirmed on light-emission emitters at specific locations. 
         [0092]    As shown in  FIG. 4C , an energy distribution of the laser resonator  3  shows Gaussian like distribution, and having two peaks. It was found that high energy distribution was observed in a specific range. 
         [0093]    As shown in  FIG. 4D , with respect to the energy distribution, when a region exceeding a specific energy in the cross-sectional direction is projected on an area of the light-emission emitters of the semiconductor laser module  100 , it was found that an area where the energy density exceeding a prescribed value corresponds to locations of the light-emission emitters, which are likely to be damaged. 
         [0094]    Accordingly, even if the light-transmissive-reflective film  5  is disposed exclusively at a location having high energy density of the returned light leak L LEAK , effects thereof can be obtained. 
         [0095]    With reference to  FIGS. 5A, 5B, 5C, 5D, 5E, 5F and 5G , other embodiments will be described. 
         [0096]    In the following embodiments, the basic configuration is the same as the one of the above-described first embodiment. However, as the major portions of the present embodiments, only the dispositions of the light-transmissive-reflective films ( 5   a - 5   g ) are modified. 
         [0097]    Therefore, since the same reference symbols are added to the same configurations as the above-described embodiment, and branch numbers of alphabet characters a to g are added to characteristic portions in each of the embodiments, explanations for common parts are omitted, but only the characteristic portions will be described. 
         [0098]    Moreover, any two embodiments among the first embodiment to the eighth embodiment can be combined to implement them. 
         [0099]    In the laser ignition device  7   a  according to a second embodiment, the light-transmissive-reflective film  5  is not provided for the collimating lens  26   a , but provided at the end surface in the incident side of the laser resonator  3   a.    
         [0100]    Even with this configuration, oscillated light leakage L LEAK  leaked from the laser resonator  3   a  corresponding to 0.4% of intensity of the oscillated light L PLS , and further 0.4% of this light leakage L LEAK , i.e., only 0.0016% of the intensity of the oscillated light L PLS  is propagated to the excitation light source side. Therefore, similar effects of the above-described embodiments can be obtained. 
         [0101]    It should be noted that the light-transmissive-reflective film  5  may be provided on the collimating lens  26  similar to the above-described first embodiment, and a light-transmissive-reflective film  5   a  may be further provided on the end surface of the incident side of the laser resonator  3   a , the light-transmissive-reflective film  5  and the light-transmissive-reflective film  5   a  being overlapped from each other. 
         [0102]    However, when the light-transmissive-reflective film  5  and the light-transmissive-reflective film  5   a  are provided being overlapped from each other, the intensity of the oscillated light leakage L LEAK  reaching the excitation light source  1  becomes substantially 0. Hence, assuming the light-transmissive-reflective films  5  are provided to be further overlapped with each other, the transmittance T PMP  of the excitation light L PMP  will be rather decreased. Accordingly, additional light-transmissive-reflective films  5  are not necessary. 
         [0103]    In the laser ignition device  7   b  according to a third embodiment, a light-transmissive-reflective film  5   b  is formed on the emission surface of the optical fiber  25   b.    
         [0104]    According to the present embodiment, the oscillated light leakage L LEAK  is reciprocally transmitted between the light-transmissive-reflective film  5   b  and the laser resonator  3 . Therefore, similar effects as the above-described embodiments can be obtained. 
         [0105]    In the laser ignition device  7   c  according to a fourth embodiment, a light-transmissive-reflective film  5   c  is formed on the end surface in the incident surface side of the optical fiber  25   c.    
         [0106]    According to the present embodiment, the light leakage L LEAK  is reciprocally transmitted between the light-transmissive-reflective film  5   c  and the laser resonator  3 . Therefore, similar effects of the above-described embodiments can be obtained. 
         [0107]    Further, in the present embodiment and the following embodiments, since the oscillated light leakage L LEAK  leaked from the resonator  3  is transmitted to the optical fiber  25  and  25   c , by providing an oscillated light leakage detection unit  9  shown in  FIG. 2 , the intensity of the oscillated light leakage L LEAK  is detected, and the detected result can be used for detecting a combustion failure or the like in the internal combustion engine. 
         [0108]    In the laser ignition device  7 d according to a fifth embodiment, a light-transmissive-reflective film  5   d  is formed on the end surface in the incident surface side of a condensing lens  23   d.    
         [0109]    According to the present embodiment, the oscillated light leakage L LEAK  is reciprocally transmitted between the light-transmissive-reflective film  5   d  and the laser resonator  3 . Therefore, similar effects to the above-described embodiments can be obtained. 
         [0110]    In the laser ignition device  7   e  according to a sixth embodiment, a light-transmissive-reflective film  5   e  is formed on the end surface in the incident surface side of a condensing lens  22   e.    
         [0111]    According to the present embodiment, the oscillated light leakage L LEAK  is reciprocally transmitted between the light-transmissive-reflective film  5   e  and the laser resonator  3 . Therefore, similar effects to the above-described embodiments can be obtained. 
         [0112]    In the laser ignition device  7   f  according to a seventh embodiment, a light-transmissive-reflective film  5   f  is formed on the end surface in the incident surface side of a collimating lens  21   f.    
         [0113]    According to the present embodiment, the oscillated light leakage L LEAK  is reciprocally transmitted between the light-transmissive-reflective film  5   f  and the laser resonator  3 . Therefore, similar effects to the above-described embodiments can be obtained. 
         [0114]    In the laser ignition device  7   g  according to an eighth embodiment, a light-transmissive-reflective film  5   g  is formed on the end surface in the incident surface side of a collimating lens  20   g.    
         [0115]    According to the present embodiment, the oscillated light leakage L LEAK  is reciprocally transmitted between the light-transmissive-reflective film  5   g  and the laser resonator  3 . Therefore, similar effects to the above-described embodiments can be obtained. 
         [0116]    With reference to  FIG. 6 , an overall configuration of the laser ignition devices  7 ,  7   a - 7   g  will be described. 
         [0117]    The laser ignition devices  7 ,  7   a - 7   g  are provided for respective cylinders of the internal combustion engine  8 , including the condensing means  6  fixed to an engine head  81 , and the excitation light source  1 , the optical elements  2  ( 20 - 26 ),  2   a  ( 20 - 26   a )- 2   g  ( 20   g - 26 ) and the laser resonators  3  and  3   a  which are described in the above-described first to eighth embodiments. 
         [0118]    The condensing means  6  is configured of an oscillated light expansion lens  60  that emits expansion light L EXP  in which the oscillated light L PLS  oscillated by the laser resonators  3  and  3   a  is expanded, a condenser lens  61  that condenses the expansion light L EXP  and emits the condensed light L FCS  to be condensed to a predetermined focused point FP in the combustion chamber  80 , a protective glass  62  that protects the condensing lens  61  from a pressure and temperature or the like of the combustion chamber  80 , an oscillated light expansion lens  60 , and a housing  63  that fixes the condensing lens  61  and the protective glass  62  to the engine head  81 . 
         [0119]    The oscillated light L PLS  oscillated by the laser resonators  3  and  3   a  is once expanded by the condensing means  6  and condensed to the predetermined focused point again, whereby the energy density can be extremely high so as to ignite the air-fuel mixture introduced in the combustion chamber  80 . 
         [0120]    According to the present embodiment, the intensity of the oscillated light leakage L LEAK  transmitted to the excitation light source  1  can be significantly reduced between the laser resonators  3  and  3   a , and the light-transmissive-reflective films  5 ,  5   a  to  5   g . Accordingly, the excitation light source  1  is prevented from being damaged by the oscillated light leakage L LEAK , and the laser ignition devices  7 ,  7   a  to  7   g  achieves stable ignition operation. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           1 : excitation light source 
           10 : semiconductor laser module 
           11 : semiconductor laser fixing member 
           100 : semiconductor laser array 
           2 : optical element 
           20 ,  21 : cylindrical lens 
           22 ,  23 : condensing lens 
           24 : excitation light coupling element 
           25 : optical fiber 
           26 : excitation light collimating lens 
           3 : laser resonator 
           30 : laser medium 
           31 : antireflection coating 
           32 : totally reflection mirror 
           33 : saturable absorber 
           34 : partial reflection film 
           35 : resonator accommodating housing 
           5 : light-transmissive-reflective film 
           50 : high refractive index film 
           51 : low refractive index film 
           6 : oscillated light condensing means 
           60 : oscillated light expansion element (beam expander) 
           61 : condenser lens 
           62 : protective glass 
           7 : laser ignition device 
           8 : internal combustion engine 
           80 : combustion chamber 
           81 : engine head 
           9 : oscillated light leakage detection unit 
           90 : beam splitter for sampling oscillated light leakage 
           91 : optical fiber 
           92 : light detection unit (photo detector) 
         L PMP : excitation light 
         L CND : convergent light 
         L CLM : parallel light 
         L PLS : excitation light 
         L LEAK : oscillated light leakage 
         L EXP : expansion light 
         L FCS : condensed light 
         FP: focused point 
         λ PMP : excitation light wavelength 
         λ PLS : oscillated light wavelength 
         R PMP : excitation light reflectance 
         T PMP : excitation light transmittance 
         R LEAK : oscillated light leakage reflectance 
         T LEAK : oscillated light leakage transmittance 
         I PLS : oscillated light intensity 
         I LEAK : oscillated light leakage intensity 
         n H : refractive index of high refractive index film 
         n L : refractive index of low refractive index film 
         n 0 : refractive index of air 
         n M : refractive index of optical element