Abstract:
A highly efficient laser ignition device is provided. The highly efficient laser ignition device fundamentally includes: a pumping light source adopting a multi-chip single emitter-packaged optical fiber output laser diode; a laser medium to which ytterbium is added; and a saturated absorber as a passive Q-switch medium, wherein a pulse of 100-999 ps as the passive Q-switch laser output can be obtained. According to the disclosed, the problems of high cost/low efficiency/low reliance/non-uniformity, which are disadvantages for replacing an ignition device using an electric spark with a laser ignition device, can be solved.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to PCT Application No. PCT/KR2016/000263 having a filing date of Jan. 12, 2016, based off of KR 10-2015-0009263 having a filing date of Jan. 20, 2015, the entire contents of which are hereby incorporated by reference. 
     
    
     FIELD OF TECHNOLOGY 
       [0002]    The following relates to a laser ignition device used for igniting an internal combustion engine of a vehicle or the like, and more particularly, to a laser ignition device in which a pumping light source, a laser medium, a saturable absorber, an emission cross section, and the like are improved to realize a low cost, miniaturization, high efficiency thereof. 
       BACKGROUND 
       [0003]    Engine ignition devices used for the longest time are electric spark ignition devices in which positive and negative electrodes are installed to maintain suitable intervals and high voltage electric discharge pulses are generated to ignite fuel in engines. Since such a spark ignition electrode device has a disadvantage in that its design makes it difficult to uniformly ignite to the utmost in an entire internal space of an engine, the spark ignition electrode device has a slightly lower fuel efficiency in the range of 16 to 18%. Since the 1990s, various types of laser ignition devices have been suggested to realize uniform discharge and to improve an ignition efficiency which is a problem of the spark ignition device. Although British and Japanese research teams suggested a laser ignition system and demonstrated that an engine efficiency in the range of 22 to 24%, which is improved by about 6% compared to an electric discharge method, can be realized, the laser ignition system still has low efficiency or lacks the specificity and economic feasibility needed to be actually applied to a vehicle engine, and thus commercialization has not yet been achieved. The main reason for this is that high cost pumping laser diodes are used, and thus actual application was almost impossible. As a specific example, for a conventional laser ignition device, research and development has been progressed in a method in which a laser medium to which mainly neodymium (Nd) is added is used, but a Nd-based laser medium, which is core laser technology, has a disadvantage in that an optimum pumping time thereof is in the range of 200 to 300 μs or less, and thus only an energy of a pumping laser diode pulse corresponding to a maximum of 300 μs per pulse can be used. In this case, the cost of a pumping laser diode, in which a usable pumping wavelength band is 800 nm and a required average output is 100 W or more, is four to five thousand dollars or more, and thus pumping laser diode is an optical component with a cost that is almost impossible to lower. Meanwhile, in 2012, a Japanese research team suggested a new laser ignition system in which laser pulse output light having a 1064 nm wavelength and generated by pulse-pumping the Nd-based laser medium with a laser diode having an 808 nm pumping wavelength is focused at high intensity on a very small diameter by a lens to generate light emission pulses to ignite fuel. However, the commercialization of such a system at low cost is estimated to be almost impossible even when a vertical-cavity surface emitting laser diode (VCSEL), which is a mass production type, is used. 
         [0004]    Since a laser discharge system using such an Nd-based laser has an Nd ion having a short upper-state lifetime of 170 μs, a pumping pulse width of a pumping laser diode having a similar pulse width also has to be maintained in the range of about 200 to 300 μs, and even when a high cost 120 W pumping laser diode is used, only a pumping pulse energy in the range of 24 to 36 mJ per pulse can be input to the laser medium. As described above, the laser discharge system using an Nd laser has problems in that (1) a high cost 808 nm pumping laser diode has to be used, (2) since an upper-state lifetime of an Nd ion is 170 μs, an efficiency thereof is maximized when a pumping pulse width is about 300 μs at maximum, and thus a maximally usable pumping pulse energy is small and inefficient, (3) since the peak power of the pumping laser diode is high, a high current is used as an input power, and thus a driving circuit is complex, (4) accordingly, the entire system becomes expensive, and thus there is a problem in that actual application becomes almost impossible. (5) In addition, since there is a high possibility that combustion by-products, which are a problem when the system is actually applied and are generated during the combustion of fuel, adhere to a laser beam emission window, and an automatic cleaning function for a window inside a combustion chamber is not included, actual application is almost impossible. (6) Although there is a design in which an optical component having a high refractive index is applied for multi-focal combustion, when oscillation directions of output laser beams of multiple laser resonators are finely changed per pulse or beam divergence angles are changed, there is a high possibility that an output energy of the laser beams transmitted to a focal point in the combustion chamber through a focal lens is changed, and thus there is a possibility that incomplete ignition or ignition failure occurs. 
         [0005]    Meanwhile, heat having a high temperature of about 2,000° C. is generated around an ignition point during an explosion stroke (expansion stroke) in an engine. Accordingly, there is a concern that the heat, which adversely affects the laser ignition device, can be transferred to the laser ignition device due to an emission spectrum, and thus there is a problem of insufficient preparation therefor in an ignition device of the conventional technology. 
       SUMMARY 
       [0006]    An aspect relates to a highly efficient laser ignition device in which low costs and small type packaging are realized, which has been difficult up to now. 
         [0007]    In addition, embodiments of the present invention is also directed to providing a highly efficient laser ignition device in which the maximum usable pumping pulse energy is increased, a low current is applicable as an input power, and a driving circuit is simplified. 
         [0008]    Embodiments of the invention are also directed to providing a highly efficient laser ignition device capable of preventing by-products, which are generated during the combustion of fuel, from adhering to a laser beam emission window. 
         [0009]    Embodiments of the invention are also directed to providing a highly efficient laser ignition device capable of being stably operated by preventing the occurrence of incomplete ignition or ignition failure even when performing multi-focal focusing for multi-focal combustion required for combustion in a high capacity combustion chamber. 
         [0010]    Embodiments of the invention are also directed to providing a highly efficient laser ignition device capable of blocking high temperature heat which is generated around an ignition point during an explosion stroke (expansion stroke) in an engine, and reversely moves toward the laser ignition device. 
       Technical Solution 
       [0011]    One aspect of the embodiments of the present invention provides a highly efficient laser ignition device including: (a) a pumping light source including a multi-chip single emitter-packaged optical fiber output laser diode; (b) a laser medium to which ytterbium (Yb) is added; and (c) a saturable absorber used as a passive Q-switch medium, wherein a 100 to 999 picosecond pulse is obtained as a passive Q-switch laser output. 
         [0012]    Here, the laser diode may have a wavelength band of 900 to 990 nm, and the saturable absorber may be a chromium-doped yttrium aluminium garnet (Cr:YAG) saturable absorber. 
         [0013]    In addition, the highly efficient laser ignition device may also include a plurality of sets each including the components (a) to (c) and may further include a multi-focused ignition point generation unit at a next stage of the saturable absorber of each set. 
         [0014]    Moreover, the above-described highly efficient laser ignition device may further preferably include a reverse flowing heat blocking unit. 
       Advantageous Effects 
       [0015]    According to embodiments of the the present invention, there are effects as follows. 
         [0016]    (1) A high efficiency laser, which is capable of outputting a several hundred picosecond pulse and passively Q-switched, is implemented by including a layer diode having a multichip single emitter packaging optical fiber output type, which is the latest technology, as a pumping light source and an ytterbium (Yb) added laser medium (for example, Yb-doped yttrium aluminium garnet (Yb:YAG)) and a saturable absorber, and is applied to a laser ignition device for engine ignition of an internal combustion engine and the like, and thus a low cost, small, and high efficient laser ignition system can be implemented. 
         [0017]    (2) At this time, since a pulse repeat rate, which is two or three times the repeat rate required for ignition as a laser output, is used, pulses between ignition pulses can be used to automatically remove by-products which can adhere to a laser beam emission window, and thus a high efficient laser ignition system can be actually used. 
         [0018]    (3) When a multi-focal focusing is performed for a multi-focal combustion required for combustion in a high capacity combustion chamber, since a laser pulse energy is transmitted using an optical fiber of which an emission cross section has an inclined surface instead of a simple high refractive index optical component, the laser pulse energy can be stably transmitted to an ignition point even when output directions of laser output pulses are finely changed and beam divergence angles are changed. 
         [0019]    (4) Since high temperature heat, which is generated around an ignition point during an explosion stroke (expansion stroke) in an engine and reversely moves toward a laser ignition device, can be blocked, a highly efficient laser ignition device can be reliably operated. 
     
    
     
       BRIEF DESCRIPTION 
         [0020]    Some of the embodiments will be described in detail, with reference to the following figures, wherein like designations denote like members, wherein: 
           [0021]      FIG. 1  is a schematic configuration diagram illustrating a pumping light source used in a highly efficient laser ignition device according to embodiments of the the present invention used for ignition in an internal combustion engine of a vehicle or the like; 
           [0022]      FIG. 2  is a schematic configuration diagram illustrating a highly efficient laser ignition device according to a first embodiment of the present invention; and 
           [0023]      FIG. 3  is a schematic configuration diagram illustrating a highly efficient laser ignition device according to a second embodiment of the present invention. 
       
    
    
     REFERENCE NUMERALS 
       [0000]    
       
           10 : MULTICHIP SINGLE EMITTER 
           12 : FIRST FOCUSING LENS 
           14 : PUMP FIBER INPUT 
           16 : PUMP FIBER DELIVERY 
           18 ,  18 - 1 ,  18 - 2 : PUMP FIBER OUTPUT 
           20 : LASER DIODE OUTPUT LIGHT 
           22 ,  22 - 1 ,  22 - 2 : SECOND FOCUSING LENS 
           24 ,  24 - 1 ,  24 - 2 : YTTERBIUM ADDED LASER MEDIUM 
           24 A: TOTAL REFLECTION COATING OF LASER MEDIUM 
           24 B: ANTI-REFLECTIVE COATING OF LASER MEDIUM EMISSION SURFACE 
           26 ,  26 - 1 ,  26 - 2 : SATURABLE ABSORBER 
           26 A: ANTI-REFLECTIVE COATING OF SATURABLE ABSORBER 
           26 B: OUTPUT GLASS COATING OF SATURABLE ABSORBER 
           28 ,  28 - 1 ,  28 - 2 : THIRD FOCUSING LENS 
           30 ,  30 - 1 ,  30 - 2 : INCIDENT WINDOW 
           30 B: TOTAL REFLECTION COATING OF INCIDENT WINDOW FOR PREVENTING REVERSE MOVEMENT. 
           34 : HEAT BLOCKING BLOCK 
           36 ,  36 - 1 ,  36 - 2 : FOCUSED IGNITION POINT 
           42 - 1 ,  42 - 2 : SIGNAL TRANSMISSION OPTICAL FIBER 
       
     
       DETAILED DESCRIPTION 
       [0043]      FIG. 1  is a schematic configuration diagram illustrating a pumping light source  100  used in a highly efficient laser ignition device according to embodiments of the the present invention used for ignition in an internal combustion engine of a vehicle or the like. Referring to  FIG. 1 , light emitted by a multichip single emitter  10  is focused by a first focusing lens  12  and input to a pump fiber input  14  in the pumping light source  100 , and then the light is moved through a pump fiber delivery  16  and output through a pump fiber output  18  as laser diode output light  20 . Although one lens is used as the first focusing lens  12  in  FIG. 1 , two or more lenses may be used thereas according to necessity. An suitable example of the pumping light source  100  illustrated in  FIG. 1  is a multi-chip single emitter-packaged optical fiber output laser diode having a wavelength band of 900 to 990 nm, and has an advantage in that an entire highly efficient laser ignition device can be manufactured at low cost by realizing lower costs and smaller packaging compared to an 808 nm pumping laser diode of the conventional technology. The reason for such a pumping light source  100  being employed is that, since an upper-state lifetime of a Yb ion is 1,000 μs which is about six times longer than that of an Nd ion, a pumping pulse width may be as long as 1,200 to 2,000 μs when a ytterbium (Yb)-based laser medium rather than a neodymium (Nd)-based laser medium is used in the highly efficient laser ignition device, a pumping pulse energy in the range of 24 to 40 mJ per pulse may be input to a laser medium even when a 20 W pumping laser diode is used, and this will be described in detail below. 
         [0044]      FIG. 2  is a schematic configuration diagram illustrating a highly efficient laser ignition device  200  according to a first embodiment of the present invention. For convenience of illustration, in  FIG. 2 , only a pump fiber output  18  in the pumping light source  100  in  FIG. 1  from which pumped light is output is illustrated, and the other components thereof are omitted. Referring to  FIG. 2 , the highly efficient laser ignition device  200  of embodiments of the the present invention employs a Yb added laser medium  24 , such as Yb-doped yttrium aluminium garnet (Yb:YAG), as a component of a laser resonator. A short pulse of several hundred picoseconds is needed to obtain a high intensity laser beam required for ignition. In the highly efficient laser ignition device  200  of embodiments of the the present invention, a laser resonator includes a Yb added laser medium  24 , in which a total reflection coating  24   a  for a 1030 nm wavelength band is applied on a pumping light incident surface side (pumping light is positioned on the left and proceeds from left to right) and an anti-reflective coating  24   b  for a 1030 nm wavelength band is applied on an emission surface side thereof, and a saturable absorber  26 , which is a passive Q-switch medium, next to the Yb added laser medium  24 . For example, a chromium-doped YAG (Cr:YAG) saturable absorber (for a 1030 nm wavelength band, the anti-reflective coating  26   a  is applied on a side of the laser medium and an output glass coating  26   b  is applied on the other side thereof) is disposed as an integrated type or separate types, and thus a several hundred picosecond pulse is obtained as the passive Q-switch laser output. At this time, when a length of the resonator needs to be adjusted, the output glass coating  26   b  may also be implemented by a separate output glass mirror. In  FIG. 2 , reference numerals  22  and  28  respectively denote a second focusing lens and a third focusing lens for focusing light, a reference numeral  30  denotes an incident window through which light output by the laser ignition device is transmitted into an engine, and a reference numeral  36  denotes an ignition point to which the light is focused by a third lens. Meanwhile, since there is a concern that high temperature heat generated around the ignition point during an explosion stroke (expansion stroke) in an engine reversely moves toward the laser ignition device  200  due to an emission spectrum, the following two reverse flowing heat blocking units are applied to prevent such a thermal transfer. 
         [0045]    (1) A total reflection coating  30   b  for a wavelength in the range of 300 to 900 nm configured to prevent a reverse movement of an emission spectrum of an ignition fuel is applied on a right side surface of the incident window  30 , through which light output by the last laser ignition device is transmitted into an engine, to block heat due to the emission spectrum of the ignition fuel or radiant heat. 
         [0046]    (2) A heat blocking block  34 , in which a zirconia-based ceramic having excellent thermal resistance, low thermal conductivity, and a thermal expansion rate similar to that of a metal at a high temperature or a similar ceramic thereto is used to prevent radiated heat from being transferred through a wall of an engine cylinder  40 , is installed between a coupler positioned at a side of an engine cylinder of the laser ignition device and the engine cylinder, or is installed at a coupler positioned at the side of the engine cylinder of the laser ignition device in a single or separate type. In  FIG. 2 , the heat blocking block  34  is illustrated as blocking a light path because of being two-dimensionally illustrated, and since the heat blocking block  34  is manufactured and installed in a cylindrical type of which central portion is removed to prevent the light path from being interfered therewith, the heat blocking block  34  does not interfere with the light path. 
         [0047]      FIG. 3  is a schematic configuration diagram illustrating a highly efficient laser ignition device  202  according to a second embodiment of the present invention. A feature of the second embodiment compared with the first embodiment includes individual sets which are pump fiber outputs  18 - 1  and  18 - 2  included in a pumping light source  100  including a multi-chip single emitter-packaged optical fiber output laser diode, Yb added laser mediums  24 - 1  and  24 - 2 , saturable absorbers  26 - 1  and  26 - 2  as passive Q-switch mediums, second focusing lenses  22 - 1  and  22 - 2  according thereto, and third focusing lenses  28 - 1  and  28 - 2 . Since each of the sets generates one focused ignition point in an engine, a plurality of sets finally generate multi-focused ignition points  36 - 1  and  36 - 2 , and thus simultaneous multi-focal and multi-ignition which is an advantageous in high capacity combustion can be performed. Another feature of the second embodiment is that signal transmission optical fibers  42 - 1  and  42 - 2  of which incident cross sections have angles of 90° to the direction of incident light and emission cross sections are inclined are installed between the saturable absorbers  26 - 1  and  26 - 2  and the third focusing lenses  28 - 1  and  28 - 2  as multi-focused ignition point generation units instead of a simple high refractive index optical component, and thus laser pulse energy may be stably transmitted to ignition points even when output directions of laser output pulses are finely changed or beam divergence angles are changed. For convenience of illustration, a total reflection coating layer, an anti-reflective coating layer, and the like, which are included in components, are not illustrated in  FIG. 3 , but coating layers are added thereto similarly as in  FIG. 2 , and thus a highly efficient laser ignition device is suitably operated. The same components in the first embodiment and the second embodiment serve the same roles, one of the laser mediums  24 - 1  and  24 - 2  to which Yb is added is used in the first embodiment, and both of the laser mediums  24 - 1  and  24 - 2  are used in the second embodiment, but one of the laser mediums  24 - 1  and  24 - 2  may also be commonly used in the second embodiment as long as there is no problem with laser output. In addition, design and manufacture thereof can be expanded to three or four sets, and it may also have more advantages when implemented. 
         [0048]    As described above, the highly efficient laser ignition device according to the embodiment of the present invention has operational features and advantages as follows. 
         [0049]    (1) Low cost 900 to 990 nm pumping laser diodes can be used. (2) Since an upper-state lifetime of Yb is 1000 μs, a pumping pulse width is as long as about 2000 μs at maximum, and thus the maximum usable pumping pulse energy is increased and the highly efficient laser ignition device is efficient. (3) Since the peak power of a pumping laser diode is low, a low current is used as input power, and thus a driving circuit becomes simple. (4) Accordingly, there is an advantage in that a total system is inexpensive and can be actually applied. (5) In addition, since a repeat rate, which is two or three times a repeat rate required as a laser output for ignition, is used, pulses between ignition pulses are used to automatically remove combustion by-products which may adhere to a laser beam emission window, the highly efficient laser ignition device has an advantage in that actual application becomes possible. When an amount of adsorbed by-product is increased, laser beams are absorbed by the by-products, laser energy transmitted to a focal point, which is an ignition point, is decreased, ignition may become unstable or impossible, and thus such automatic removal is particularly effective. (6) In addition, when multi-focal focusing is performed for multi-focal combustion which is required to be used for combustion of a high capacity combustion chamber, an optical fiber having an inclined emission cross section is used to transmit light instead of a simple high refractive index optical component, and thus there is an advantage in that even when output directions of laser output pulses are finely changed and beam divergence angles are changed, laser pulse energy can be transmitted to an ignition point. (7) In addition, since a window coating to which an emission spectrum total reflection coating is applied and a heat blocking block designed to prevent interference with a light path are used, there is an advantage in than reverse movement of radiant heat due to an ignition fuel emission spectrum and thermal transfer though an inner wall of an engine cylinder are prevented. 
       INDUSTRIAL APPLICABILITY 
       [0050]    Since embodiments of the present invention solves the problems of high cost/low efficiency/low reliability/non-uniformity which are disadvantages arising from the replacement of an ignition device using an electric spark with a laser ignition device, embodiments of the the present invention has high industrial applicability in the field of ignition devices.