Patent Publication Number: US-2022231475-A1

Title: Laser radar device performing multi-stage amplification

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority under 35 U.S.C. § 119 to Korean Patent Application No. 10-2021-0008217, filed on Jan. 20, 2021, in the Korean Intellectual Property Office, the disclosure of which is incorporated by reference herein in its entirety. 
     BACKGROUND 
     1. Field of the Invention 
     Embodiments of the present disclosure described herein relate to a laser radar device for acquiring three-dimensional images and video, and more particularly, relate to a laser radar device for performing multi-stage amplification by using a single optical coupler. 
     2. Description of Related Art 
     A laser radar device is an image sensor device that secures three-dimensional images, and is used in many fields such as unmanned autonomous robots and automobiles, structural change status check, landslide check, and military robots. 
     The laser radar device is not a method of constructing an image from external light, but a method of measuring the received light by shooting a light source, and may be used regardless of a surrounding environment. The laser radar device may acquire distance information to an object by shooting a laser light source to an object and measuring the returned light source. A pulse light source and a continuous wave (CW) light source may be used as the light source. The pulse light source is more widely used than the CW light source because it is relatively advantageous in far-field measurement and a resolution is improved to cm-level with the development of technology. 
     To increase an output of the laser radar device using the pulse light source, the number of amplification stages for amplifying a seed light source (laser) may be increased. In this case, when the number of pump light sources (lasers) increases as much as the number of amplification stages, a price of the laser radar device increases, and the number of pump-signal optical couplers to be used together increases. In addition, an area of a driving board of the pump light source should be increased for independent control, and an amount of input current is also increased. 
     SUMMARY 
     Embodiments of the present disclosure provide a laser radar device that efficiently performs multi-stage amplification using a single signal light source (seed light source) and a single pump light source. 
     According to an embodiment of the present disclosure, a laser radar device includes a signal light source that outputs a first signal light, a pump light source that outputs a pump light, a pump optical fiber that transfers the pump light, a first signal optical fiber that transfers the first signal light, a first amplifier that receives and amplifies the first signal light from the first signal optical fiber, a second signal optical fiber that receives and transfers a second signal light from the first amplifier, the second signal light being obtained by amplifying the first signal light, a second amplifier that receives and amplifies the second signal light from the second signal optical fiber, and an optical coupler connected to the first signal optical fiber, the second signal optical fiber, and the pump optical fiber, and that distributes the pump light to the first signal optical fiber and the second signal optical fiber. 
     According to an embodiment, the optical coupler may be configured such that the first signal optical fiber and the second signal optical fiber are tapered and fused in parallel to each other, and the optical coupler may be configured such that the pump optical fiber is tapered and fused in parallel to the second signal optical fiber and the pump optical fiber is spaced apart from the first signal optical fiber. 
     According to an embodiment, the pump optical fiber may be heated and elongated in a direction parallel to the second signal optical fiber and may be tapered and fused to the second signal optical fiber. The first signal optical fiber may include a first core and a first clad, the second signal optical fiber may include a second core and a second clad, the second core may receive the second signal light from the first amplifier, and the second clad may receive the pump light from the pump optical fiber in the optical coupler. 
     According to an embodiment, the first core may receive the first signal light from the signal light source, and the first clad may receive a first portion of the pump light from the second clad in the optical coupler. 
     According to an embodiment, the first amplifier may include a first gain medium that absorbs a first portion of the pump light and amplifies the first signal light. 
     According to an embodiment, the second amplifier may include a second gain medium that absorbs a second portion of the pump light and amplifies the second signal light. 
     According to an embodiment, a power of the second portion may be greater than a power of the first portion. 
     According to an embodiment, the first amplifier may further include an optical isolator that enters the second signal light in a traveling direction, and a filter that removes noise of the second signal light. 
     According to an embodiment, the first signal optical fiber and the second signal optical fiber may include a double clad optical fiber. 
     According to an embodiment of the present disclosure, a laser radar device includes a signal light source that outputs a first signal light, a pump light source that outputs a pump light, a pump optical fiber that transfers the pump light, a first signal optical fiber that transfers the first signal light, a first amplifier that receives and amplifies the first signal light from the first signal optical fiber, a second signal optical fiber that receives and transfers a second signal light from the first amplifier, the second signal light being obtained by amplifying the first signal light, a second amplifier that receives and amplifies the second signal light from the second signal optical fiber, a third signal optical fiber that receives and transfers a third signal light from the second amplifier, the third signal light being obtained by amplifying the second signal light, a third amplifier that receives and amplifies the third signal light from the third signal optical fiber, and an optical coupler connected to the first signal optical fiber, the second signal optical fiber, the third signal optical fiber, and the pump optical fiber, and that distributes the pump light to the first signal optical fiber, the second signal optical fiber, and the third signal optical fiber. 
     According to an embodiment, the optical coupler may be configured such that the first signal optical fiber, the second signal optical fiber, and the third signal optical fiber are tapered and fused in parallel to one another, and the optical coupler may be configured such that the pump optical fiber is fused in parallel to the third signal optical fiber and the pump optical fiber is spaced apart from the first signal optical fiber and the second signal optical fiber. 
     According to an embodiment, the pump light may be transferred from the pump optical fiber to the third signal optical fiber through the optical coupler. 
     According to an embodiment, a first portion of the pump light may be transferred from the third signal optical fiber to the first signal optical fiber through the optical coupler, and a second portion of the pump light may be transferred from the third signal optical fiber to the second signal optical fiber through the optical coupler. 
     According to an embodiment, the first amplifier may include a first gain medium that absorbs the first portion of the pump light and amplifies the first signal light, and the second signal light amplified by the first gain medium may be input to the second signal optical fiber. 
     According to an embodiment, the second amplifier may include a second gain medium that absorbs the second portion of the pump light and amplifies the second signal light, and the third signal light amplified by the second gain medium may be input to the third signal optical fiber. 
     According to an embodiment, the third amplifier may include a third gain medium that absorbs a third portion of the pump light and amplifies the second signal light, and a signal light amplified by the third gain medium may be output to an end cap. 
     According to an embodiment, an output of the third portion may be greater than a sum of an output of the first portion and an output of the second portion. 
     According to an embodiment of the present disclosure, a pump-to-signal optical coupler includes a first signal optical fiber that receives a first signal light from a signal light source, a pump optical fiber that receives a pump light from a pump light source, and a second signal optical fiber that receives the pump light from the pump optical fiber and to transfer an second signal light. The first signal optical fiber includes a first core through which the first signal light is transferred and a first clad surrounding the first core, the second signal optical fiber includes a second core through which the second signal light is transferred and a second clad surrounding the second core, the pump optical fiber is fused to the second clad in parallel to transfer the pump light to the second clad, and the second clad is in contact with the first clad in parallel to couple a first portion of the pump light to the first clad. 
     According to an embodiment, the first clad may transfer the first portion of the pump light to the first core through a first gain medium, and the first core may amplify the first signal light based on the first portion to generate the second signal light, and may transfer the second signal light which is obtained by amplifying the first signal light to the second core. 
     According to an embodiment, the second clad may transfer a second portion of the pump light to the second core through a second gain medium, the second core may amplify and output the second signal light based on the second portion, and a power of the second portion may be greater than a power of the first portion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects and features of the present disclosure will become apparent by describing in detail embodiments thereof with reference to the accompanying drawings. 
         FIG. 1  is a block diagram describing a laser radar device that performs multi-stage amplification through a plurality of pump light sources. 
         FIG. 2  is a block diagram of a laser radar device according to an embodiment of the present disclosure. 
         FIG. 3  is a block diagram of a laser radar device according to an embodiment of the present disclosure. 
         FIGS. 4A to 4C  are diagrams illustrating an optical coupler of  FIG. 3 . 
         FIG. 5  is a flowchart illustrating an operation method of the laser radar device of  FIG. 3 . 
         FIG. 6  is a block diagram of a laser radar device according to an embodiment of the present disclosure. 
         FIGS. 7A to 7C  are diagrams illustrating an optical coupler of  FIG. 6 . 
         FIG. 8  is a flowchart illustrating an operation method of the laser radar device of  FIG. 6 . 
         FIG. 9  is a flowchart illustrating a method of manufacturing an optical coupler according to an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, embodiments of the present disclosure will be described clearly and in detail such that those skilled in the art may easily carry out the present disclosure. 
       FIG. 1  is a block diagram describing a typical laser radar device that performs multi-stage amplification through a plurality of pump light sources. Referring to  FIG. 1 , a laser radar device  1  may include a signal light source  10 , a plurality of pump light sources  21 ,  22 ,  23 , and  24 , a plurality of optical couplers  31 ,  32 , and  33 , and a plurality of gain media  41 ,  42 , and  43 , a plurality of optical isolators  51  and  52 , a plurality of filters  53  and  54 , and an end cap  60 . 
     The signal light source  10  may output a first signal light to a core of an optical fiber. The first pump light source  21  may output a first pump light to a clad of the optical fiber through the first optical coupler  31 . The first gain medium  41  may transfer the first pump light to the first signal light to amplify the first signal light and output a second signal light. The first optical isolator  51  may transfer the second signal light without loss in a traveling direction and may remove noise or reflected signals incident in the opposite direction to the traveling direction. The first filter  53  may remove noise generated after amplification and may transfer a clean second signal light to the second optical coupler  32 . 
     The second pump light source  22  may output the second pump light to the clad of the optical fiber through the second optical coupler  32 . The second gain medium  42  may transfer the second pump light to the second signal light to amplify the second signal light and output a third signal light. Since the second optical isolator  52  and the second filter  54  are similar to the first optical isolator  51  and the first filter  53 , additional description thereof will be omitted to avoid redundancy. 
     The third pump light source  23  and the fourth pump light source  24  may output the third pump light and the fourth pump light to the clad of the optical fiber through the third optical coupler  33 . The third gain medium  43  may transfer the third pump light and the fourth pump light to the third signal light to amplify the third signal light and output a fourth signal light. The end cap  60  may output the fourth signal light to an external target of the laser radar device  1 . 
     As described above, the laser radar device  1  may constitute a three-stage amplification stages. A first-stage amplification stage may include the first pump light source  21 , the first optical coupler  31 , the first gain medium  41 , the first optical isolator  51 , and the first filter  53 . A second-stage amplification stage may include the second pump light source  22 , the second optical coupler  32 , the second gain medium  42 , the second optical isolator  52 , and the second filter  54 . A third-stage amplification stage may include the third pump light source  23 , the fourth pump light source  24 , the third optical coupler  33 , and the third gain medium  43 . 
     In detail, to configure the three-stage amplification stages, each amplification stage may receive the pump light from one or more pump light sources. However, this type of multi-stage amplification stage configuration has several problems. 
     First, the pump light source may output the pump light only when a driving current is equal to or greater than a threshold current, and thus separate driving power is required. Therefore, as the number of pump light sources increases or a pump light source having a high output is used, the driving power for generating the threshold current may be great. Next, the minimum power of the plurality of pump light sources  21 ,  22 ,  23 , and  24  is commercialized and manufactured based on 10 W. However, the power required to configure the first and second amplification stages may be less than 5 W even when the first and second amplification stages are combined. That is, for the configuration of the first amplification stage and the second amplification stage, it is inefficient because a pump light source of 10 W should be individually connected. Finally, as the multi-stage amplification stage as illustrated in  FIG. 1  is composed of the plurality of pump light sources  21 ,  22 ,  23 , and  24  and the plurality of optical elements  31 ,  32 ,  33 ,  51 ,  52 ,  53 , and  54 , the design is complex. Since the driving board also needs to independently control the plurality of pump light sources  21 ,  22 ,  23 , and  24 , an area of the driving board may be increased. 
     The present disclosure proposes a laser radar device capable of performing multi-stage amplification using a single signal light source and a single pump light source to reduce the size, cost, and power of the laser radar device. 
       FIG. 2  is a block diagram of a laser radar device according to an embodiment of the present disclosure. Referring to  FIG. 2 , a laser radar device  100  may include a single signal light source  110 , a single pump light source  120 , first to third optical couplers  131 ,  132 , and  133 , and first to third gain media  141 ,  142 , and  143 , first and second optical isolators  151  and  152 , first and second filters  153  and  154 , and an end cap  160 . 
     Since the signal light source  110 , the first to third gain media  141 ,  142 , and  143 , the first and second optical isolators  151  and  152 , the first and second filters  153  and  154 , and the end cap  160  of  FIG. 2  are similar to the signal light source  10 , the first to third gain media  41 ,  42 , and  43 , and the first and second optical isolators  51  and  52 , the first and second filters  53  and  54 , and the end cap  60  of  FIG. 1 , and thus additional description thereof will be omitted to avoid redundancy. 
     A first optical fiber OF 1  is an optical fiber through which the signal light travels, and it means an optical fiber connected from the signal light source  110  to the end cap  160 , and a second optical fiber OF 2  is an optical fiber through which the pump light travels and may be an optical fiber connected from the pump light source  120  to the third optical coupler  133 . 
     The signal light source  110  may output the first signal light to a core of the first optical fiber OF 1 . Although not illustrated, the first optical fiber OF 1  may be a double clad fiber including a first core, a first clad surrounding an outer circumferential surface of the first core, and a second clad surrounding an outer circumferential surface of the first clad. 
     The pump light source  120  may output the pump light to a core of the second optical fiber OF 2 . The second optical fiber OF 2  may be a multi-mode fiber including a second core of a multi-mode. The second core may be made of the same material as the first clad of the first optical fiber OF 1 . According to an embodiment, the second optical fiber OF 2  may include a third clad surrounding an outer circumferential surface of the second core. Alternatively, the second optical fiber OF 2  may include a silica optical fiber without a core cladding structure. 
     The first optical coupler  131  may couple a portion of the pump light to the first clad of the first optical fiber OF 1 . A portion of the pump light may be transferred from the second optical fiber OF 2  to the first optical fiber OF 1  through the first optical coupler  131 . In detail, the portion of the pump light may be transferred from the core of the second optical fiber OF 2  to the first clad of the first optical fiber OF 1 . 
     For example, when the output of the pump light is 27 W, 1 W, which is the portion of the pump light, may be coupled to the first signal light through the first optical coupler  131 . The output of the remaining 26 W may remain in the second optical fiber OF 2 . To this end, the first optical coupler  131  may be manufactured by twisting the second optical fiber OF 2  to the first optical fiber OF 1  and stretching the second optical fiber OF 2  with a micro-torch. In this case, the tensile distance may be adjusted while measuring a coupling degree of the pump light. 
     The first gain medium  141  may include an active material that absorbs the pump light coupled from the first optical coupler  131  and occurs an amplified spontaneous emission (ASE). The active material may contain rare earth elements. The rare earth element may absorb the pump light supplied through the second optical fiber OF 2  and may emit laser light of a single wavelength while electrons excited to a metastable state are stabilized. The rare earth element may include at least one of erbium (Er), ytterbium (Yb), and thulium (Tm). 
     The first signal light is amplified into a second signal light while passing through the first gain medium  141 , and the second signal light may transferred to the second optical coupler  132  through the first optical isolator  151  and the first filter  153 . 
     The second optical coupler  132  may couple another portion of the pump light to the first optical fiber OF 1 . The another portion of the pump light may be a portion of the pump light remaining in the second optical fiber OF 2  while passing through the first optical coupler  131 . Another portion of the pump light may be transferred from the second optical fiber OF 2  to the first optical fiber OF 1  through the second optical coupler  132 . 
     For example, when the pump light of 26 W remains in the second optical fiber OF 2  after passing through the first optical coupler  131 , 2 W, which is another portion of the pump light, may be used for amplification of the second signal light through the second optical coupler  132 . The output of the remaining 24 W may remain in the second optical fiber OF 2 . To this end, the second optical coupler  132  may be manufactured similarly to the manufacturing method of the first optical coupler  131 . 
     A tensile distance of the second optical coupler  132  may be longer than a tensile distance of the first optical coupler  131 . When two optical fibers are stretched by heating, a cross-sectional area of the two optical fibers becomes thinner. In this case, the distance from the tapering start point to the end point may be referred to as a tensile distance. In other words, the two optical fibers are stretched and tapered by heating, and the tensile distance may be a distance from a starting point of tapering to an ending point. As the tensile distance increases, the two optical fibers may be located closer together, and a degree of coupling may be increased. For example, to couple a pump light output of 1 W in the first optical coupler  131  and a pump light output of 2 W in the second optical coupler  132 , the tensile distance of the second optical coupler  132  may be manufactured to be greater than the tensile distance of the first optical coupler  131 . 
     Since the second gain medium  142  is similar to the first gain medium  141 , additional description thereof will be omitted to avoid redundancy. The second signal light is amplified into a third signal light while passing through the second gain medium  142 , and the third signal light may be transferred to the third optical coupler  133  through the second optical isolator  152  and the second filter  154 . 
     The third optical coupler  133  may couple the remaining portion of the pump light to the first optical fiber OF 1 . The remaining portion of the pump light may be the pump light remaining in the second optical fiber OF 2  while passing through the second optical coupler  132 . The remaining portion of the pump light may be transferred from the second optical fiber OF 2  to the first optical fiber OF 1  through the third optical coupler  133 . 
     For example, when the pump light of 24 W remains in the second optical fiber OF 2  after passing through the second optical coupler  132 , the remaining 24 W of the pump light may be used for amplification of the third signal light through the third optical coupler  133 . To this end, the third optical coupler may be manufactured by tensioning the second optical fiber OF 2  with a micro-torch and bonding(fusing) the second optical fiber OF 2  in parallel to the first optical fiber OF 1 . In this case, the second optical fiber OF 2  may be stretched until the cross-sectional area becomes less than or equal to a reference value. 
     Since the third gain medium  143  is similar to the first gain medium  141 , additional description thereof will be omitted. The third signal light may be amplified while passing through the third gain medium  143  and may be transferred to the outside through the end cap  160 . 
     As described above, the laser radar device  100  of  FIG. 2  may perform three-stage amplification using the single pump light source  120 . Although the multi-stage amplification is illustrated as three-stage amplification in  FIG. 2 , the number of amplification stages is not limited thereto. Accordingly, the laser radar device  100  may reduce the number of pump light sources for multi-stage amplification, thereby reducing the threshold current for driving the pump light source and increasing the efficiency of the driving board. However, the number of optical elements is still many. Hereinafter, a structure of a laser radar device capable of reducing the number of optical elements will be described. 
       FIG. 3  is a block diagram of a laser radar device according to an embodiment of the present disclosure. Referring to  FIGS. 2 and 3 , a laser radar device  200  may include a signal light source  210 , a pump light source  220 , an optical coupler  230 , first and second gain media  241  and  242 , an optical element  250 , and an end cap  260 . Since the signal light source  210 , the pump light source  220 , the first and second gain media  241  and  242 , and the end cap  260  of  FIG. 3  are similar to the signal light source  110 , the pump light source  120 , the first and second gain media  141  and  142 , and the end cap  160 , additional description thereof will be omitted. 
     Hereinafter, the first portion of the pump light may be a portion through which the pump light is transferred from the second signal optical fiber to the first signal optical fiber through the optical coupler  230 , and the second portion of the pump light may be a portion through which the pump light is transferred from the pump optical fiber to the second signal optical fiber through the optical coupler  230 . Most of the power of the pump light may be transferred to the second signal optical fiber through the second portion. The power of the pump light from the second signal optical fiber to the first signal optical fiber may be transferred back to the first portion. 
     The signal light source  210  may output the first signal light to a first signal optical fiber  271 . The first signal light may be amplified by receiving the first portion of the pump light. The second signal light which is obtained by amplifying the first signal light may be transferred through a second signal optical fiber  272 . The second signal light may receive the second portion of the pump light, and then may be amplified and output. The second portion of the pump light may be a portion of the pump light excluding the first portion of the pump light. 
     The pump light source  220  may output the pump light to a pump optical fiber  280 . The pump optical fiber  280  may be coupled to the second signal optical fiber  272  in the optical coupler  230  to transfer the pump light to the second signal optical fiber  272 . A first portion of the pump light introduced into the second signal optical fiber  272  may be transferred to the first signal optical fiber  271  to be used for amplification of the first signal light. A second portion of the pump light introduced into the second signal optical fiber  272  may remain in the second signal optical fiber  272  to be used for amplification of the second signal light. 
     The first signal optical fiber  271  and the second signal optical fiber  272  may include a double clad optical fiber, and the pump optical fiber  280  may include a multi-mode optical fiber. 
     In the optical coupler  230 , the first signal optical fiber  271 , the second signal optical fiber  272 , and the pump optical fiber  280  may be coupled. For example, the first signal optical fiber  271  and the second signal optical fiber  272  may be bonded (tapered and fused) parallel to each other, and the pump optical fiber  280  may be bonded parallel to the second signal optical fiber  272 . In this case, the pump optical fiber  280  may be disposed to be spaced apart from the first signal optical fiber  271 . 
     The optical coupler  230  may distribute the pump light transferred from the pump optical fiber  280  to the first signal optical fiber  271  and the second signal optical fiber  272 . The pump light may be introduced into the second signal optical fiber  272  from the pump optical fiber  280 . In detail, the pump light may be coupled to the clad of the second signal optical fiber  272 . To this end, the pump optical fiber  280  may be stretched by a micro-torch and bonded in parallel to the second signal optical fiber  272 . In this case, the pump optical fiber  280  may be stretched until the cross-sectional area is equal to or less than a predetermined area. 
     The optical coupler  230  may distribute the first portion of the pump light introduced into the second signal optical fiber  272  to the first signal optical fiber  271 . In detail, the first portion of the pump light may be coupled to the clad of the first signal optical fiber  271 . To this end, the first signal optical fiber  271  and the second signal optical fiber  272  may be stretched by a micro-torch and bonded in parallel to each other. In this case, when the tensile distance is adjusted while measuring the coupling degree of the first signal optical fiber  271  and the second signal optical fiber  272 , the optical coupler  230  may distribute the pump light at a desired ratio. 
     The remaining second portion other than the first portion of the pump light may remain in the clad of the second signal optical fiber  272 . The first portion of the pump light may be less than the second portion of the pump light. For example, when the output of the pump light is 27 W, the first portion of the pump light may be 1 W. The second portion of the pump light, 26 W, may remain in the second signal optical fiber  272  to be coupled with the second signal light. The first signal light may exist in the core of the first signal optical fiber  271  through the optical coupler  230 , and the first portion of the pump light may exist in the clad of the first signal optical fiber  271 . Thereafter, the first signal light may be amplified as a second signal light by absorbing the first portion of the pump light through the first gain medium  241 . The second signal light may be transferred to the optical element  250 . The first amplifier may refer to a configuration that performs single-stage amplification and includes the first gain medium  241  and the optical element  250 . 
     The optical element  250  may be an element in which the first optical isolator  151  and the first filter  153  of  FIG. 2  are combined. That is, the optical element  250  may transfer the second signal light in the traveling direction without loss and may remove noise or reflected signals incident in the opposite direction to the traveling direction. In addition, the optical element  250  may remove noise generated after amplification and may transfer the clean second signal light to the optical coupler  230 . 
     The second signal light may exist in the core of the second signal optical fiber  272  through the optical coupler  230 , and the second portion of the pump light may exist in the clad of the second signal optical fiber  272 . Thereafter, the second signal light may be amplified as a third signal light by absorbing the second portion of the pump light through the second gain medium  242 . The second amplifier may refer to a configuration that includes the second gain medium  242  to perform two-stage amplification. The third signal light may be output to the outside through the end cap  260 . 
     As described above, the laser radar device  200  of  FIG. 3  may have a smaller number of optical couplers compared to the laser radar device  100  of  FIG. 2 . That is, it may be understood that the first and second optical couplers  131  and  132  of  FIG. 2  are combined and changed into the single optical coupler  230  of  FIG. 3 . In addition, by using the optical element  250  in which the optical isolator and the filter are combined, the laser radar device  100  may be miniaturized. 
       FIGS. 4A to 4C  are diagrams illustrating the optical coupler  230  of  FIG. 3 .  FIG. 4A  is a diagram illustrating a flow of optical signals in the optical coupler  230 ,  FIG. 4B  is an enlarged view of the optical coupler  230 , and  FIG. 4C  is a cross-sectional view taken along line a-A of  FIG. 4B . 
     Referring to  FIGS. 3, 4A, and 4B , each of the first signal light, the second signal light, and the pump light may be transferred through the first signal optical fiber  271 , the second signal optical fiber  272 , and the pump optical fiber  280 . The first signal optical fiber  271  may include a first core C 1  and a first clad CL 1  surrounding the first core C 1 . The second signal optical fiber  272  may include a second core C 2  and a second clad CL 2  surrounding the second core C 2 . The pump optical fiber  280  may include a multi-mode core. For convenience of description, the pump optical fiber  280  in  FIG. 4B  is illustrated as being composed of a single layer but is not limited thereto. 
     The first signal light may be transferred through the core C 1  of the first signal optical fiber  271 , and the second signal light may be transferred through the core C 2  of the second signal optical fiber  272 . The pump light may be transferred through the pump optical fiber  280  and sequentially coupled to the second signal optical fiber  272  and the first signal optical fiber  271  in the optical coupler  230 . 
     The pump optical fiber  280  may be tapered while being stretched in the traveling direction of the pump light. The cross-sectional area of the pump optical fiber  280  may gradually decrease in the direction in which the pump light travels. When the cross-sectional area of the pump optical fiber  280  is less than a predetermined area, the pump optical fiber  280  may be heated and bonded to the second signal optical fiber  272  in parallel. Accordingly, most of the pump light may be coupled from the pump optical fiber  280  to the second clad CL 2  of the second signal optical fiber  272 . 
     The first signal optical fiber  271  and the second signal optical fiber  272  may be bonded in parallel while being stretched in the traveling direction of the first signal light. The first clad CL 1  of the first signal optical fiber  271  and the second clad CL 2  of the second signal optical fiber  272  may be fused. Accordingly, the first portion of the pump light of the second clad CL 2  may be coupled to the first clad CL 1 . A tensile distance or a fusion distance of the first clad CL 1  and the second clad CL 2  may be adjusted depending on the degree of coupling. 
     The second portion of the pump light excluding the first portion may remain in the second clad CL 2 . As a result, the first signal light may exist in the first core C 1 , the second signal light may exist in the second core C 2 , the first portion of the pump light may exist in the first clad CL 1 , and the second portion of the pump light may exist in the second clad CL 2 . 
     The line a-A of  FIGS. 4A and 4B  may indicate a point at which coupling of the first signal optical fiber  271  and the second signal optical fiber  272  starts. Alternatively, The line a-A may indicate a point at which coupling of the second signal optical fiber  272  and the pump optical fiber  280  is already completed. 
     Referring to  FIG. 4C , the cross-sectional area of the first signal optical fiber  271  and the cross-sectional area of the second signal optical fiber  272  may be the same to each other. Since the pump optical fiber  280  is stretched, the cross-sectional area of the pump optical fiber  280  may be less than the cross-sectional area of the first signal optical fiber  271  and the cross-sectional area of the second signal optical fiber  272 . The first signal optical fiber  271  and the second signal optical fiber  272  may be bonded, and the second signal optical fiber  272  and the pump optical fiber  280  may be bonded. In this case, the first signal optical fiber  271  and the pump optical fiber  280  may be spaced apart from each other. In  FIG. 4C , a center of the first signal optical fiber  271 , a center of the second signal optical fiber  272 , and a center of the pump optical fiber  280  are illustrated to lie on the same line, but the present disclosure is not limited thereto. For example, a line connecting the center of the first signal optical fiber  271  and the center of the second signal optical fiber  272  and a line connecting the center of the second signal optical fiber  272  and the center of the pump optical fiber  280  may be perpendicular to each other. 
       FIG. 5  is a flowchart illustrating an operation method of the laser radar device of  FIG. 3 . Referring to  FIGS. 3 and 5 , in operation S 110 , the first signal light may be introduced into the core of the first signal optical fiber  271  from the signal light source  210 . In operation S 120 , the pump light may be introduced into the clad of the second signal optical fiber  272  from the pump light source  220 . For example, the pump light may be introduced into the clad of the second signal optical fiber  272  at the optical coupler  230 . The introduced pump light may be distributed to the first signal optical fiber  271 . 
     In operation S 130 , the first portion of the pump light may be coupled to the clad of the first signal optical fiber  271 . For example, the first portion of the pump light may be coupled to the clad of the first signal optical fiber  271  depending on the tensile distance or the fusion distance of the first signal optical fiber  271  and the second signal optical fiber  272  at the optical coupler  230 . 
     In operation S 140 , the first signal light may be amplified by the first portion of the pump light. For example, the first gain medium  241  may absorb the first portion of the pump light to generate the second signal light which is obtained by amplifying the first signal light. As the second signal light passes through the optical element  250 , noise may be removed. The second signal light may be input back to the optical coupler  230  through the second signal optical fiber. 
     In operation S 150 , the second signal light may be amplified by the second portion of the pump light. For example, the second gain medium  242  may absorb the second portion of the pump light to generate the third signal light which is obtained by amplifying the second signal light. The third signal light may be output to the outside of the laser radar device  200 . 
       FIG. 6  is a block diagram of a laser radar device according to an embodiment of the present disclosure. Referring to  FIGS. 2, 3, and 6 , a laser radar device  300  may include a signal light source  310 , a pump light source  320 , an optical coupler  330 , and first to third gain media  341 ,  342 , and  343 , first and second optical elements  351  and  352 , and an end cap  360 . Since the signal light source  310 , the pump light source  320 , the first to third gain media  341 ,  342 , and  343 , the first and second optical elements  351  and  352 , and the end cap  360  of  FIG. 6  are similar to the signal light source  210 , the pump light source  220 , the first and second gain media  241  and  242 , the optical element  250 , and the end cap  260  of  FIG. 3 , additional description thereof will be omitted to avoid redundancy. 
     The number of amplification stages of the laser radar device  300  may be one more than the number of amplification stages of the laser radar device  200  of  FIG. 3 . In detail, the laser radar device  200  of  FIG. 3  may perform two-stage amplification, whereas the laser radar device  300  of  FIG. 6  may perform three-stage amplification. Accordingly, the signal optical fiber, the gain medium, and the optical element are further increased by one, and the coupling method of first to third signal optical fibers  371 ,  372 , and  373  and a pump optical fiber  380  in the optical coupler  330  and distribution method of the pump light may be different. Hereinafter, the laser radar device  300  of  FIG. 6  will be described based on differences from the laser radar device  200  of  FIG. 3 . 
     Hereinafter, the first portion of the pump light may be a portion through which the pump light is transferred from the third signal optical fiber to the first signal optical fiber through the optical coupler  330 , and the second portion of the pump light may be a portion through which the pump light is transferred from the third signal optical fiber to the second signal optical fiber through the optical coupler  330 , and the third portion of the pump light may be a portion through which the pump light is transferred from the pump optical fiber to the third signal optical fiber through the optical coupler  330 . Most of the power of the pump light may be transferred to the third signal optical fiber through the third portion. The power of the pump light from the third signal optical fiber to the first signal optical fiber or the second signal optical fiber may be transferred back to the first portion or the second portion. 
     The signal light source  310  may output the first signal light, and the pump light source  320  may output the pump light. The first signal light may be transferred to the optical coupler  330  through the first signal optical fiber  371 , and the pump light may be transferred to the optical coupler  330  through the pump optical fiber  380 . The optical coupler  330  may couple the first signal optical fiber  371 , the second signal optical fiber  372 , the third signal optical fiber  373 , and the pump optical fiber  380 , and may distribute the pump light to the first signal optical fiber  371 , the second signal optical fiber  372 , and the third signal optical fiber  373 . 
     In the optical coupler  330 , the first signal optical fiber  371 , the second signal optical fiber  372 , and the third signal optical fiber  373  may be bonded in parallel to one another. In the optical coupler  330 , the pump optical fiber  380  may be bonded parallel to the third signal optical fiber  373 . In this case, the pump optical fiber  380  may be spaced apart from the first signal optical fiber  371  and the second signal optical fiber  372 . 
     The pump light may be transferred from the pump optical fiber  380  to the third signal optical fiber  373  through the optical coupler  330 . Through the optical coupler  330 , the first portion of the pump light may be transferred from the third signal optical fiber  373  to the first signal optical fiber  371 , and the second portion of the pump light may be transferred from the third signal optical fiber  373  to the second signal optical fiber  372 . The third portion of the pump light excluding the first portion and the second portion may remain in the third signal optical fiber  373 . The output of the third portion may be greater than the sum of the output of the first portion and the output of the second portion. For example, when the pump light has an output of 27 W, each of the first portion and the second portion may have an output of 1 W, and the third portion may have an output of 25 W. 
     The first amplifier may include the first gain medium  341  and the first optical element  351  (e.g., an optical isolator and a filter). The first gain medium  341  may receive the first signal light and the first portion of the pump light from the first signal optical fiber  371  and may amplify the first signal light by transferring the first portion to the first signal light. Noise may be removed from the second signal light which is obtained by amplifying the first signal light while passing through the first optical device  351  (e.g., an optical isolator and a filter). The first optical device  351  may transfer the second signal light to the optical coupler  330  through the second signal optical fiber  372 . 
     The second amplifier may include the second gain medium  342  and the second optical element  352  (e.g., an optical isolator and a filter). The second gain medium  342  may receive the second signal light and the second portion of the pump light from the second signal optical fiber  372  and may amplify the second signal light by transferring the second portion to the second signal light. Noise may be removed from the third signal light which is obtained by amplifying the second signal light while passing through the second optical device  352 . The second optical device  352  may transfer the third signal light to the optical coupler  330  through the third signal optical fiber  373 . 
     The third amplifier may include the third gain medium  343 . The third gain medium  343  may receive the third signal light and the third portion of the pump light from the third signal optical fiber  373  and may amplify the third signal light by transferring the third portion to the third signal light. The amplified third signal light may be output to the outside through the end cap  360 . 
     Although the laser radar device  300  of  FIG. 6  performs three-stage amplification, the number of amplification stages is not limited thereto. In this case, the optical coupler may couple the signal optical fiber proportional to the number of amplifications stages and the single pump optical fiber and may distribute the pump light to a plurality of signal optical fibers. Most of the output of the pump light may be coupled to a signal optical fiber corresponding to an end amplification stage among the plurality of signal optical fibers. 
       FIGS. 7A to 7C  are diagrams illustrating an optical coupler of  FIG. 6 .  FIG. 7A  is a diagram illustrating a flow of optical signals in the optical coupler  330 ,  FIG. 7B  is an enlarged view of the optical coupler  330 , and  FIG. 7C  is a cross-sectional view taken along line b-B of  FIG. 7B . 
     Referring to  FIGS. 6, 7A, and 7B , the first signal light, the second signal light, the third signal light, and the pump light may be transferred through the first signal optical fiber  371 , the second signal optical fiber  372 , and the third signal optical fiber  373 , and the pump optical fiber  380 , respectively. The first signal optical fiber  371  may include the first core C 1  and the first clad CL 1  surrounding the first core C 1 . The second signal optical fiber  372  may include the second core C 2  and the second clad CL 2  surrounding the second core C 2 . The third signal optical fiber  373  may include the third core C 3  and the third clad CL 3  surrounding the third core C 3 . The pump optical fiber  380  may include a multi-mode core. 
     Since the first signal optical fiber  371 , the second signal optical fiber  372 , the third signal optical fiber  373 , and the pump optical fiber  380  of  FIGS. 7A and 7B  are similar to the first signal optical fiber  271 , the second signal optical fiber  272 , and the pump optical fiber  280  of  FIGS. 4A and 4B , except that the third signal optical fiber  373  is added, and thus additional description thereof will be omitted to avoid redundancy. 
     The first signal light may be transferred through the first core C 1  of the first signal optical fiber  371 , the second signal light may be transferred through the second core C 2  of the second signal optical fiber  372 , and the third signal light may be transferred through the third core C 3  of the third signal optical fiber  373 . The pump light may be transferred through the pump optical fiber  380  and may be coupled to the first signal optical fiber  371 , the second signal optical fiber  372 , and the third signal optical fiber  373  in the optical coupler  330 . 
     The pump optical fiber  380  may be bonded in parallel to the third signal optical fiber  373  when the cross-sectional area is equal to or less than a predetermined area while being stretched in the traveling direction of the pump light. Accordingly, the pump light may be coupled to the third clad CL 3  of the third signal optical fiber  373  from the pump optical fiber  380 . 
     The first signal optical fiber  371 , the second signal optical fiber  372 , and the third signal optical fiber  373  may be bonded in parallel to one another while being stretched in the traveling direction of the first signal light. Accordingly, the first portion of the pump light of the third clad CL 3  may be coupled to the first clad CL 1 . In addition, the second portion of the pump light of the third clad CL 3  may be coupled to the second clad CL 2 . The third portion of the pump light excluding the first portion and the second portion may remain in the third clad CL 3 . 
     As a result, the first signal light may exist in the first core C 1 , the second signal light may exist in the second core C 2 , the third signal light may exist in the third core C 3 , the first portion of the pump light may exist in the first clad CL 1 , the second portion of the pump light may exist in the second clad CL 2 , and the third portion of the pump light may exist in the third clad CL 3 . 
     The line b-B in  FIGS. 7A and 7B  may indicate a point at which coupling of the first signal optical fiber  371 , the second signal optical fiber  372 , and the third signal optical fiber  373  is started. Alternatively, the line b-B may indicate a point at which coupling of the third signal optical fiber  373  and the pump optical fiber  380  is already completed. 
     Referring to  FIG. 7C , the cross-sectional area of the first signal optical fiber  371 , the cross-sectional area of the second signal optical fiber  372 , and the cross-sectional area of the third signal optical fiber  373  may be the same to one another. The cross-sectional area of the pump optical fiber  380  may be less than the cross-sectional area of the first signal optical fiber  271  by being stretched. The first signal optical fiber  371 , the second signal optical fiber  372 , and the third signal optical fiber  373  may be bonded in parallel to one another, and the third signal optical fiber  373  and the pump optical fiber  380  may be bonded. In this case, the pump optical fiber  380  may be spaced apart from the first signal optical fiber  371  and the second signal optical fiber  372 . The arrangement of the first signal optical fiber  371 , the second signal optical fiber  372 , the third signal optical fiber  373 , and the pump optical fiber  380  is not limited to  FIG. 7C . For example, in  FIG. 7C , the first signal optical fiber  371  and the second signal optical fiber  372  are illustrated as being bonded to each other, but the present disclosure is not limited thereto and may be spaced apart. That is, the first signal optical fiber  371  and the second signal optical fiber  372  may be bonded only to the third signal optical fiber  373 . 
       FIG. 8  is a flowchart illustrating an operation method of the laser radar device of  FIG. 6 . Since an operation method of  FIG. 8  is similar to the operation method of  FIG. 6 , additional description thereof will be omitted to avoid redundancy. 
     Referring to  FIGS. 6 and 8 , in operation S 210 , the first signal light may be introduced into the core of the first signal optical fiber  371  from the signal light source  310 . In operation S 220 , the pump light may be introduced into the clad of the third signal optical fiber  373  from the pump light source  320 . The introduced pump light may be distributed to the first signal optical fiber  371  and the second signal optical fiber  372 . 
     In operation S 230 , the first portion of the pump light may be coupled to the clad of the first signal optical fiber  371 . In operation S 240 , the first signal light may be amplified by the first portion of the pump light, and may be generated as the second signal light. The second signal light may be input back to the optical coupler  330  through the core of the second signal optical fiber  372 . 
     In operation S 250 , the second portion of the pump light may be coupled to the clad of the second signal optical fiber  372 . In operation S 260 , the second signal light may be amplified by the second portion of the pump light, and may be generated as the third signal light. The third signal light may be input back to the optical coupler  330  through the core of the third signal optical fiber  373 . In operation S 270 , the third signal light may be amplified by the third portion of the pump light and may be output to the outside of the laser radar device  300  through the end cap  360 . 
       FIG. 9  is a flowchart illustrating a method of manufacturing an optical coupler according to an embodiment of the present disclosure. The optical coupler according to an embodiment of the present disclosure may include a single pump optical fiber transferring the pump light and a plurality of signal optical fibers transferring the signal light. The optical coupler may be referred to as a pump-signal optical coupler and may be configured to distribute the pump light to a plurality of signal optical fibers. 
     Referring to  FIG. 9 , in operation S 310 , a plurality of signal optical fibers may be bonded in parallel to one another. The plurality of signal fibers may be heated and stretched. Accordingly, the dads of the plurality of signal optical fibers may be fused to the clads of the adjacent signal optical fibers. In this case, a coupling degree may be controlled by adjusting the tensile distances of the plurality of signal optical fibers. 
     In operation S 320 , the pump optical fiber may be heated and stretched. Alternatively, the pump optical fiber may be stretched in a direction parallel to the plurality of signal optical fibers. That is, the pump optical fiber may be elongated in the traveling direction of the pump light, and in this case, the cross-sectional area of the pump optical fiber may gradually decrease in the traveling direction of the pump light. 
     In operation S 330 , depending on whether the cross-sectional area of the pump optical fiber is equal to or less than a predetermined area, the manufacturing method of the optical coupler may vary. When the cross-sectional area of the pump optical fiber is greater than the predetermined area, the process returns to operation S 320 , and the pump optical fiber may be further stretched. When the cross-sectional area of the pump optical fiber is less than the predetermined area, operation S 340  may be performed. 
     In operation S 340 , the pump optical fiber may be bonded in parallel to any one of the plurality of signal optical fibers. In this case, the pump optical fiber may be spaced apart from one or more signal optical fibers other than one bonded signal optical fiber among the plurality of signal optical fibers. 
     Any one signal optical fiber to which the pump optical fiber is bonded in operation S 340  may be an end signal optical fiber corresponding to an end amplification stage among the multi-stage amplification stages. That is, the pump light may be incident into the clad of the end signal optical fiber to which the pump optical fiber is bonded, and the incident pump light may be distributed to other signal optical fibers. Most of the pump light incident on the clad of the end signal optical fiber may remain in the clad of the end signal optical fiber and may be used for end amplification. 
     According to an embodiment of the present disclosure, multi-stage amplification stages may be efficiently configured with a single signal light source and a single pump light source, and accordingly, a laser radar device may be miniaturized and driven with low power consumption. 
     While the present disclosure has been described with reference to embodiments thereof, it will be apparent to those of ordinary skill in the art that various changes and modifications may be made thereto without departing from the spirit and scope of the present disclosure as set forth in the following claims.