Patent Publication Number: US-2021175676-A1

Title: Laser apparatus, resin degradation detection method, and detection method of optical power

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a U.S. National Stage application of International Application No. PCT/JP2019/037240 filed Sep. 24, 2019, which claims priority from Japanese patent application No. 2018-191689 filed Oct. 10, 2018. These references are incorporated herein in their entirety by reference. 
    
    
     BACKGROUND 
     Technical Field 
     The present invention relates to a laser apparatus, a resin degradation detection method, and a detection method of an optical power, and more particularly to a method of detecting degradation of a resin that fixes an optical fiber in place, for example in a laser apparatus. 
     Description of Related Art 
     There has heretofore been known a laser apparatus having a process head that delivers a laser beam, for example, from a fiber laser to a workpiece to process the workpiece (see, e.g., Patent Literature 1). With such a laser apparatus, when a workpiece is formed of a material having a high reflectivity (for example, copper or gold), a delivered laser beam is reflected from the workpiece at a high ratio. Therefore, the reflected light may return to an interior of the laser apparatus through the process head. 
     If the amount of light returning to the laser apparatus increases, one or more components within the laser apparatus (e.g., an output combiner) generates heat by the optical feedback, resulting in a damaged optical fiber or a failure such as a disconnected optical path. In order to prevent such a failure, there has been proposed a method of detecting an optical feedback propagating in a laser apparatus and stopping an operation of the laser apparatus if the amount of the optical feedback exceeds a predetermined threshold. 
     However, if such an optical feedback repetitively returns to the laser apparatus, a portion of the optical feedback is absorbed in a resin that fixes an optical fiber or the like. Thus, the resin is degraded gradually. As a result, a failure may be caused from a portion of the resin before the amount of the optical feedback detected exceeds the aforementioned threshold. 
     Therefore, it is important to detect degradation of a resin that fixes an optical fiber or the like in order to prevent a failure of a laser apparatus. However, such a resin is provided at an invisible location from an outside of the structure in most cases. Thus, it is difficult to identify the degradation of the resin. Furthermore, even if the resin is visible from an outside of the structure, some type of degradation of the resin may not be recognized by visual inspection. In such a case, it is difficult to accurately detect the degradation of the resin. 
     PATENT LITERATURE 
     Patent Literature 1: JP 2017-21099 A 
     SUMMARY 
     One or more embodiments provide a laser apparatus and a method that can effectively detect degradation of a resin that fixes an optical fiber in place. 
     According to one or more embodiments, there is provided a laser apparatus capable of effectively detecting degradation of a resin that fixes an optical fiber. The laser apparatus has an optical fiber through which a laser beam propagates, a resin that fixes the optical fiber, a sound sensor configured to detect a sound produced by the resin that shrinks when a power of light propagating through the optical fiber decreases from its peak value, a storage unit configured to store a threshold (threshold value) relating to a sound produced when the resin shrinks, and a comparison determination part operable to compare a detected value representative of the sound detected by the sound sensor to the threshold stored in the storage unit and determine that the resin has been degraded when the detected value exceeds the threshold. The laser apparatus may have at least one fiber laser connected to the optical fiber. 
     According to one or more embodiments, there is provided a method capable of effectively detecting degradation of a resin that fixes an optical fiber. This method includes setting a certain threshold, detecting a sound produced by the resin that shrinks when a power of light propagating through the optical fiber decreases from its peak value, comparing a detected value representative of the detected sound to the threshold, and determining that the resin has been degraded when the detected value exceeds the threshold. 
     According to one or more embodiments, there is provided a method capable of detecting a power of light propagating through an optical fiber. This method includes detecting a sound produced when a resin that fixes an optical fiber shrinks and, based on the detected sound, detecting that a power of light propagating through the optical fiber decreases from its peak value. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a diagram explanatory of a phenomenon in which a sound is produced by an optical feedback. 
         FIG. 1B  is a diagram explanatory of a phenomenon in which a sound is produced by an optical feedback. 
         FIG. 2  is a diagram schematically showing a laser apparatus according to one or more embodiments. 
         FIG. 3  is a diagram schematically showing an output combiner and a sound sensor in the laser apparatus illustrated in  FIG. 2 . 
         FIG. 4A  is a graph showing voltage data representative of a reference sound detected by the sound sensor illustrated in  FIG. 2  for determining a threshold. 
         FIG. 4B  is a graph showing a frequency spectrum obtained by performing a discrete Fourier transform on the voltage data illustrated in  FIG. 4A . 
         FIG. 5  is a flow chart showing an operation of the laser apparatus illustrated in  FIG. 2 . 
         FIG. 6A  is a graph showing voltage data representative of a sound detected by the sound sensor illustrated in  FIG. 2  during operation. 
         FIG. 6B  is a graph showing a frequency spectrum obtained by performing a discrete Fourier transform on the voltage data illustrated in  FIG. 6A . 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention will be described in detail below with reference to  FIGS. 1A to 6B . In  FIGS. 1A to 6B , the same or corresponding components are denoted by the same or corresponding reference numerals and will not be described below repetitively. Furthermore, in  FIGS. 1A to 6B , the scales or dimensions of components may be exaggerated, or some components may be omitted. Furthermore, in the following description, a laser apparatus using a fiber laser according to one or more embodiments will be explained as an example of a laser apparatus. Nevertheless, one or more embodiments may be applicable to any laser apparatus that outputs a laser beam. 
     The inventor has diligently studied a method of effectively detecting degradation of a resin that is caused by an optical feedback in order to prevent the aforementioned failure of a laser apparatus that would be caused by an optical feedback. As a result, the inventor has found that an optical feedback causes a resin to generate heat and to expand and that, when the power of the optical feedback decreases from its peak value, the resin shrinks so that a sound is produced. 
     As shown in  FIG. 1A , when an optical feedback propagates through an optical fiber  110 , a portion of the optical feedback is absorbed, for example, in a resin  120  that fixes the optical fiber  110  within an output combiner. Thus, the temperature of the portion  130  of the resin  120  locally increases as shown in a graph depicted at a lower side of  FIG. 1A . Accordingly, the portion  130  of the resin  120  extends by thermal expansion. Then, when the optical feedback ceases or the amount of the optical feedback decreases, the heat of the thermally expanded portion  130  of the resin  120  diffuses therearound, so that the temperature of the portion  130  decreases as shown in a graph depicted at a lower side of  FIG. 1B . Thus, the thermally expanded portion  130  shrinks back to the initial state. Such a cycle of local expansion and shrinkage of the resin  120  vibrates a structure where the resin  120  has been fixed (e.g., an output combiner). As a result, a sound is produced. In other words, a sound is produced when the power of the optical feedback propagating through the optical fiber  110  decreases from its peak value. Accordingly, whether the power of the optical feedback propagating through the optical fiber  110  decreases from its peak value can be detected by detection of a sound (hereinafter referred to as “resin shrinkage sound”) produced by shrinkage of the resin  120  that fixes the optical fiber  110 . 
     Additionally, the inventor has found that the strength of the resin shrinkage sound increases as the resin  120  is degraded. Therefore, when the strength of the resin shrinkage sound from the resin  120  that should be considered to be degraded is set as a threshold, then the degradation of the resin  120  can be determined by detecting whether or not the strength of the resin shrinkage sound produced from the resin  120  exceeds the threshold. 
     The frequency of a resin shrinkage sound being produced depends upon the natural frequency, which is defined by a location where the resin  120  shrinks and expands, a structure in the vicinity of the resin  120 , a method of fixing the resin  120 , and the like. Accordingly, if a frequency analysis is conducted on a resin shrinkage sound produced upon expansion and shrinkage of the resin  120  so as to acquire, for example, an amplitude at a specific frequency or in a specific frequency band that corresponds to the natural frequency, then the degradation of the resin  120  can be determined more accurately by comparison of the amplitude with the threshold. 
       FIG. 2  is a diagram schematically showing a laser apparatus  1  using such a resin degradation detection method. As shown in  FIG. 2 , the laser apparatus  1  has a plurality of fiber laser units  10  as laser light sources, optical fibers  12  connected to the respective fiber laser units  10 , an output combiner  20  connected to the optical fibers  12 , an optical fiber  22  connected to the output combiner  20 , a process head  30  connected to the optical fiber  22 , a controller  40  operable to control an operation of the laser apparatus  1 , and a sound sensor  50  located near the output combiner  20 . 
     Each of the fiber laser units  10  includes an optical cavity therein. Thus, each of the fiber laser units  10  is configured to output a laser beam amplified by the optical cavity. The laser beams outputted from those fiber laser units  10  propagate through the respective optical fibers  12 . Those laser beams are combined by the output combiner  20  and outputted to one optical fiber  22 . The combined laser beam is delivered through the optical fiber  22  to the process head  30  and directed as a focused laser beam L to a workpiece  100  by an optical system within the process head  30 . 
       FIG. 3  is a diagram schematically showing the output combiner  20  and the sound sensor  50 . As shown in  FIG. 3 , the output combiner  20  includes a fiber accommodation portion  26  with a groove  24  defined therein, which accommodates the input optical fibers  12  and the output optical fiber  22  therein. A bundle of the multiple optical fibers  12  extending from the fiber laser units  10  is fixed to the fiber accommodation portion  26  on an end of the groove  24  by a resin  28 . The optical fiber  22  extending to the process head  30  is fixed to the fiber accommodation portion  26  on another end of the groove  24  by a resin  29 . In one or more embodiments, the sound sensor  50  is located near the resin  28 . 
     A certain length of a coating material has been removed from an end of each of the optical fibers  12  along a longitudinal direction of the optical fiber  12 . Thus, claddings  12 A of the optical fibers  12  are exposed. Similarly, a certain length of a coating material has been removed from an end of the optical fiber  22  along a longitudinal direction of the optical fiber  22 , and a cladding  22 A of the optical fiber  22  is thus exposed. Those exposed portions of the claddings  12 A and the cladding  22 A are located between the resin  28  and the resin  29 . The diameter of the claddings  12 A of the optical fibers  12  are reduced in a tapered manner so as to match the diameter of the cladding of the optical fiber  22 . The tapered portion of the optical fibers  12  and the cladding  22 A of the optical fiber  22  are connected to each other by fusion splice. 
     For example, as shown in  FIG. 2 , when the focused laser beam L is emitted perpendicular to a surface of the workpiece  100  or the like, a portion of the focused laser beam L may be reflected from the surface of the workpiece  100  so as to return to an interior of the laser apparatus  1  from the process head  30 . Such an optical feedback introduced into the laser apparatus  1  may reach the output combiner  20 , and a portion of the optical feedback may be absorbed, for example, in the resin  28  that fixes the optical fiber  12 . Thus, the resin  28  may be degraded. In one or more embodiments, degradation of the resin  28  is detected by the aforementioned method. 
     The sound sensor  50  is located near the resin  28  and configured to detect a sound (resin shrinkage sound) produced when the resin  28  expands and shrinks due to heat caused by the optical feedback. The sound sensor  50  is configured to detect a sound at a predetermined sampling rate and externally output the detected sound, for example, as a variation of a voltage (voltage data). Any sensor capable of detecting a resin shrinkage sound can be used for the sound sensor  50 . Various kinds of sound sensors including an electrodynamic sound sensor, an electrostatic sound sensor (condenser microphone), a piezoelectric sound sensor (piezoelectric microphone), and the like may be used for the sound sensor  50 . 
     As shown in  FIG. 2 , the laser apparatus  1  includes a processing unit (processor)  42  connected to the sound sensor  50  and a storage unit (storage)  44  formed of a hard disk, ROM, RAM, or the like. The storage unit  44  stores a threshold relating to the resin shrinkage sound of the resin  28 . The details of the threshold will be described later. The voltage data are inputted to the processing unit  42  from the sound sensor  50 . 
     The processing unit  42  includes an analysis part  45  operable to perform a discrete Fourier transform on the voltage data from the sound sensor  50  for frequency analysis and a comparison determination part  46  operable to compare an amplitude (detected value) at a specific frequency in a frequency spectrum obtained by the analysis part  45  to the threshold stored in the storage unit  44 . The comparison determination part  46  is configured to determine that the resin  28  has been degraded and to send a resin degradation signal S to the controller  40  when an amplitude at the specific frequency exceeds the threshold. 
     Now the threshold stored in the storage unit  44  will be described. The threshold may be determined and stored in the storage unit  44  before the resin  28  has been degraded. For example, the threshold is determined in the following manner. 
     First, a pulsed beam having a certain power is introduced into the laser apparatus  1  from the process head  30  before the resin  28  has been degraded. Thus, the resin  28  is heated so that the temperature of the resin  28  changes. Therefore, the resin  28  expands and shrinks so as to produce a resin shrinkage sound (reference sound). The sound sensor  50  detects the reference sound at a predetermined sampling rate and inputs its voltage data to the analysis part  45  of the processing unit  42 . At that time, the sound sensor  50  acquires voltage data, for example, as illustrated in  FIG. 4A . 
     The analysis part  45  of the processing unit  42  stores the voltage data as illustrated in  FIG. 4A , which has been sent from the sound sensor  50 , for a predetermined period of time and performs a discrete Fourier transform on the data. As a result, a frequency spectrum as illustrated in  FIG. 4B  is acquired. In this frequency spectrum, a threshold is determined to be a value that exceeds an amplitude at a specific frequency (frequency of interest) that corresponds to, for example, the aforementioned natural frequency. For example, in the frequency spectrum illustrated in  FIG. 4B , an amplitude at about 2.1 kHz is about 18 mV. Therefore, a threshold is determined to be 35 mV. The threshold thus determined (35 mV) is stored in the storage unit  44 . Various factors including the aforementioned natural frequency determine what frequency is a frequency of interest and how large the threshold is as compared to an amplitude at the frequency of interest. 
     Now a normal operation of the laser apparatus  1  will be described.  FIG. 5  is a flow chart showing an operation of the laser apparatus  1 . As shown in  FIG. 5 , when the laser apparatus  1  is in normal operation, the sound sensor  50  detects a sound at a predetermined sampling rate and inputs the detected sound as voltage data to the analysis part  45  of the processing unit  42  (Step S 1 ). For example, voltage data as shown in  FIG. 6A  is acquired by the sound sensor  50  and inputted to the analysis part  45  of the processing unit  42 . The sampling rate of the sound sensor  50  needs to be higher than twice the aforementioned frequency of interest in accordance with the sampling theorem. Thus, the sampling rate of the sound sensor  50  needs to be higher than 4.2 kHz in the aforementioned example. 
     The analysis part  45  of the processing unit  42  stores the inputted voltage data for a predetermined period of time and performs a discrete Fourier transform on the voltage data (Step S 2 ). Some period of time may be enough for storing the voltage data. For example, the period of time for which the voltage data have been stored may be 10 milliseconds. This discrete Fourier transform provides a frequency spectrum. The comparison determination part  46  determines whether or not the amplitude of the frequency spectrum at the frequency of interest (2.1 kHz) exceeds a threshold stored in the storage unit  44  (35 mV) (Step S 3 ). If the amplitude at the frequency of interest does not exceed the threshold, the procedure returns to the sound sampling (Step S 1 ). If the amplitude at the frequency of interest exceeds the threshold, the comparison determination part  46  determines that the resin  28  has been degraded and sends a resin degradation signal S to the controller  40  (Step S 4 ). 
     When a discrete Fourier transform is performed on the voltage data shown in  FIG. 6A , a frequency spectrum as shown in  FIG. 6B  can be obtained. The amplitude of this frequency spectrum at the frequency of interest (2.1 kHz) is about 37 mV and exceeds the threshold stored in the storage unit  44  (35 mV). Therefore, the comparison determination part  46  determines that the resin  28  has been degraded and send a resin degradation signal S to the controller  40 . 
     The controller  40  that has received the resin degradation signal S stops the operation of the laser apparatus  1 , for example, by stopping an electric current supplied to the fiber laser units  10  (Step S 5 ). Thus, the operation of the laser apparatus  1  can be stopped before the laser apparatus  1  experiences a failure. Furthermore, the controller  40  may decrease an electric current supplied to the fiber laser units  10  or otherwise notify an operator of degradation of the resin  28  through another user interface (e.g., a rotating lamp, a display, or means for external communication). 
     Thus, according to one or more embodiments, degradation of a resin can be detected by using a resin shrinkage sound. Therefore, degradation that could not be detected by visual inspection can be detected. Furthermore, even if the sound sensor  50  is located outside of the output combiner  20 , the resin shrinkage sound of the resin  28  can be detected. Accordingly, even if the resin  28  is invisible from the outside of the output combiner  20 , degradation of the resin  28  can be detected. 
     Furthermore, since a threshold that reflects a state prior to degradation of the resin  28  is used in the above embodiments, the current state can be compared to the state prior to degradation of the resin  28  when the laser apparatus  1  is operated. Accordingly, degradation of the resin can be detected more accurately. 
     In the above embodiments, the comparison determination part  46  compares an amplitude of the frequency spectrum at a specific frequency to the threshold. However, an integrated value of amplitudes within a specific frequency band may be used instead of an amplitude at a specific frequency. Furthermore, in the above embodiments, the analysis part  45  of the processing unit  42  performs a discrete Fourier transform on data representative of a sound detected by the sound sensor  50  (voltage data). However, the frequency analysis using a discrete Fourier transform may not be required. A detected value, such as a voltage value representative of a sound detected by the sound sensor  50 , may be compared to the threshold. Moreover, the detected value representative of a sound detected by the sound sensor  50  may be any physical quantity including a voltage value and an electric current value. 
     Moreover, the processing unit  42 , the storage unit  44 , and the like as described above may be provided integrally with the controller  40 , which controls an operation of the laser apparatus  1 , or may be provided separately from the controller  40 . 
     The above embodiments describe examples where degradation of resin  28  in the output combiner  20  is to be detected. Nevertheless, one or more embodiments can be applied to a resin provided at any location as long as the resin may be degraded due to the laser beam. For example, one or more embodiments can be used to detect degradation of a resin that fixes an optical fiber in a structure that removes a cladding mode. 
     Although only a limited number of embodiments have been described, the scope of the present invention is not limited to the aforementioned embodiments. It should be understood that various different embodiments may be devised without departing from the scope of the present invention. 
     As described above, according to one or more embodiments, there is provided a laser apparatus capable of effectively detecting degradation of a resin that fixes an optical fiber. The laser apparatus has an optical fiber through which a laser beam propagates, a resin that fixes the optical fiber, a sound sensor configured to detect a sound produced by the resin that shrinks when a power of light propagating through the optical fiber decreases from its peak value, a storage unit configured to store a threshold relating to a sound produced when the resin shrinks, and a comparison determination part operable to compare a detected value representative of the sound detected by the sound sensor to the threshold stored in the storage unit and determine that the resin has been degraded when the detected value exceeds the threshold. The laser apparatus may have at least one fiber laser connected to the optical fiber. 
     With this configuration, degradation of a resin can be detected by using a sound (resin shrinkage sound) produced by the resin that shrinks when a power of light propagating through the optical fiber decreases from its peak value. Thus, since degradation of the resin can be detected by using such a resin shrinkage sound, degradation that could not be detected by visual inspection can be detected. Furthermore, even if the laser apparatus has a structure where the resin is invisible from the outside of the structure, degradation of the resin can be detected. Moreover, since degradation of the resin can be detected, any necessary measures such as stop and alert can be taken before the laser apparatus experiences a failure. 
     The threshold may relate to an amplitude of a sound at a specific frequency or in a specific frequency band. The laser apparatus according to one or more embodiments may further include an analysis part operable to perform a frequency analysis on data representative of the sound detected by the sound sensor and output an amplitude at the specific frequency or in the specific frequency band as the detected value to the comparison determination part. Thus, since the comparison with the threshold employs the results obtained by a frequency analysis on data representative of the resin shrinkage sound, degradation of the resin can be detected more accurately. 
     One or more embodiments provide a method capable of effectively detecting degradation of a resin that fixes an optical fiber. This method includes setting a certain threshold, detecting a sound produced by the resin that shrinks when a power of light propagating through the optical fiber decreases from its peak value, comparing a detected value representative of the detected sound to the threshold, and determining that the resin has been degraded when the detected value exceeds the threshold. 
     According to this method, degradation of a resin can be detected by using a sound (resin shrinkage sound) produced by the resin that shrinks when a power of light propagating through an optical fiber decreases from its peak value. Thus, since degradation of the resin can be detected by using such a resin shrinkage sound, degradation that could not be detected by visual inspection can be detected. Furthermore, even if the laser apparatus has a structure where the resin is invisible from the outside of the structure, degradation of the resin can be detected. 
     The threshold may relate to an amplitude of a sound at a specific frequency or in a specific frequency band. In this case, a frequency analysis on data representative of the detected sound may be performed and an amplitude at the specific frequency or in the specific frequency band as the detected value may be compared to the threshold, upon the comparing the detected value to the threshold. Thus, since the comparison with the threshold employs the results obtained by a frequency analysis on data representative of the resin shrinkage sound, degradation of the resin can be detected more accurately. 
     Before the resin is degraded, a reference sound produced by the resin that shrinks when a power of light propagating through the optical fiber decreases from its peak value may be detected, and the threshold may be determined based on the detected reference sound. Use of such a threshold enables comparison with a state prior to degradation. Therefore, degradation of the resin can be detected more accurately. 
     According to one or more embodiments, there is provided a method capable of detecting a power of light propagating through an optical fiber. This method includes detecting a sound produced when a resin that fixes an optical fiber shrinks and, based on the detected sound, detecting that a power of light propagating through the optical fiber decreases from its peak value. 
     According to this method, the fact that a power of light propagating through an optical fiber decreases from its peak value can be detected by using a sound produced when a resin that fixes the optical fiber shrinks. 
     According to one or more embodiments, degradation of a resin that fixes an optical fiber can be detected by using a sound produced when the resin shrinks. 
     Although the disclosure has been described with respect to only a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that various other embodiments may be devised without departing from the scope of the present invention. Accordingly, the scope of the invention should be limited only by the attached claims. 
     REFERENCE SIGN LIST 
     
         
         
           
               1  Laser apparatus 
               10  Fiber laser unit 
               12  Optical fiber 
               12 A Cladding 
               20  Output combiner 
               22  Optical fiber 
               22 A Cladding 
               24  Groove 
               26  Fiber accommodation portion 
               30  Process head 
               40  Controller 
               42  Processing unit 
               44  Storage unit 
               45  Analysis part 
               46  Comparison determination part 
               50  Sound sensor 
               100  Workpiece 
             L Focused laser beam 
             S Resin degradation signal