Elemental analysis system

Abstract

We provide an elemental analysis system with improved analytical accuracy. [Solution] The elemental analysis system comprises a laser light source that outputs excitation laser light, a laser oscillator that is excited by the excitation laser light output from the laser light source and oscillates pulsed laser light, an analysis unit that analyzes the elements contained in an object based on the plasma light generated in the object irradiated by the pulsed laser light oscillated by the laser oscillator, and an optical fiber cable that guides the excitation laser light output from the laser light source to the laser oscillator. The optical fiber cable has a curved section that is bent to a bending diameter smaller than a predetermined allowable bending diameter.

Description

[Technical Field]

[0001] This invention relates to an elemental analysis system. [Background technology]

[0002] In harsh environments such as inside a nuclear reactor, there is a need for techniques to analyze the elements contained in a target object (e.g., fuel debris). One example of such a technique is laser-induced breakdown spectroscopy (LIBS), an elemental analysis method that identifies the elemental components by irradiating the target object with pulsed laser light, decomposing the wavelength of the generated plasma light with a spectrometer, and analyzing the light intensity for each wavelength (see, for example, Patent Document 1).

[0003] In such devices, a radiation-resistant probe is inserted into the reactor, and the probe is connected to a control device (e.g., a laser light source, a spectroscopic analysis unit, and a controller) located outside the reactor building via an optical fiber cable. Therefore, the total length of the optical fiber cable may exceed 100 meters in order to reach the target object. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2015-190792 [Overview of the project] [Problems that the invention aims to solve]

[0005] To maintain the accuracy of LIBS analysis, it is crucial to synchronize the oscillation of pulsed laser light relative to the object with the measurement of the plasma generated in the object. However, vibrations in the optical fiber cable increase the amplitude of the oscillation timing of the pulsed laser light (hereinafter referred to as "oscillation amplitude (jitter)"), leading to the problem of unstable analysis accuracy.

[0006] This invention has been made in view of the above circumstances, and its purpose is to provide an elemental analysis system with improved analytical accuracy. [Means for solving the problem]

[0007] To achieve the above objective, one aspect of the present invention provides an elemental analysis system comprising: a laser light source that outputs excitation laser light; a laser oscillator that is excited by the excitation laser light output from the laser light source and oscillates pulsed laser light; an analysis unit that analyzes elements contained in an object based on plasma light generated in an object irradiated with the pulsed laser light oscillated by the laser oscillator; and an optical fiber cable that guides the excitation laser light output from the laser light source to the laser oscillator, wherein the optical fiber cable has a curved portion that is curved to a bending diameter smaller than a predetermined allowable bending diameter. [Effects of the Invention]

[0008] According to the present invention, analytical accuracy can be improved. Other problems, configurations, and effects not mentioned above will be clarified by the following description of embodiments. [Brief explanation of the drawing]

[0009] [Figure 1] This is an overall diagram of the elemental analysis system. [Figure 2] This is a diagram showing the internal structure of the probe. [Figure 3] This is a schematic diagram of the laser oscillator. [Figure 4] This figure shows an example of an application for an elemental analysis system. [Figure 5] This diagram shows the relationship between the excitation laser light and the pulsed laser light. [Figure 6] (A) is a cross-sectional view of an optical fiber cable, (B) is a longitudinal section of a curved section, and (B) is an example of the profile of the excitation laser beam in the curved section. [Figure 7]This is an example of a profile of pump laser light in an optical fiber cable. [Figure 8] This figure shows the results of verifying changes in BUT (A) and laser output (B) of pulsed laser light when a curved portion is provided in an optical fiber cable. [Figure 9] These are the results of a test that confirmed the relationship between the bending diameter of a curved portion, BUT (μs), oscillation fluctuation width (μs), and laser output (mV). [Figure 10] This figure shows the oscillation characteristics of pulsed laser light when an optical fiber cable is set to a recommended bending diameter (A) and a forced bending diameter (B). [Figure 11] These are the results of a test that confirmed the relationship between the bending diameter (laying condition) of a curved portion, BUT, laser output, oscillation condition, and beam diameter. [Figure 12] This is a photograph showing the surface state of an object irradiated with pulsed laser light. [Figure 13] This is a micrograph of the surface of an object irradiated 10 times with Gaussian-distributed pulsed laser light. MODE FOR CARRYING OUT THE INVENTION

[0010] Hereinafter, embodiments of an elemental analysis system 100 according to the present invention will be described with reference to the drawings.

[0011] [Configuration of Elemental Analysis System 100] FIG. 1 is an overall configuration diagram of the elemental analysis system 100. The elemental analysis system 100 shown in FIG. 1 is used for LIBS analysis. The elemental analysis system 100 includes, for example, a laser light source 10, a spectroscopic analysis unit (analysis unit) 20, a probe 30, an optical fiber cable 40, and a controller 50. The laser light source 10 and the spectroscopic analysis unit 20 are connected to the probe 30 via the optical fiber cable 40. Further, the laser light source 10 and the spectroscopic analysis unit 20 are connected to the controller 50 via a communication cable (broken line in FIG. 1).

[0012] The elemental analysis system 100 according to the present embodiment is used for analyzing elements contained in an object M under harsh environments (for example, environments with high radiation dose). However, it goes without saying that the elemental analysis system 100 is not limited to environments with high radiation dose, and can be applied to elemental analysis in any environment such as space environments and seabed environments.

[0013] The laser light source 10 outputs excitation laser light L1 having a predetermined frequency (for example, 808 nm). Here, as the laser light source 10, for example, it is preferable to use a semiconductor laser. This is because, compared to commonly used lasers such as near-infrared or visible pulse oscillation Nd 3+ :YAG lasers, there are advantages in the diameter of the optical fiber cable 40 that transmits the excitation laser light L1 and the applicable length. In particular, under the harsh environments described above, it is necessary to perform analysis work by remote control, and workability is regarded as important, so a semiconductor laser capable of long-distance transmission is preferable. The operation of the laser light source 10 is controlled by a controller 50.

[0014] The spectroscopic analysis unit 20 performs spectroscopic analysis on plasma light L3 generated by irradiating the object M (for example, radioactive material, fuel debris) with pulsed laser light L2. Specifically, the spectroscopic analysis unit 20 splits the plasma light L3 into wavelength and intensity, and generates spectrum information related to the wavelength and intensity of the plasma light L3. Then, the spectroscopic analysis unit 20 outputs the generated spectrum information to the controller 50. Further, based on the spectrum information acquired from the spectroscopic analysis unit 20, the controller 50 identifies elements contained in the object M and quantifies their concentrations. Since these processes are already well known, detailed description in this specification is omitted.

[0015] The probe 30 is attached to the end of the optical fiber cable 40. The probe 30 converts the excitation laser light L1 incident through the optical fiber cable 40 into pulsed laser light L2 and irradiates the target object M. The probe 30 also outputs the plasma light L3 generated in the target object M irradiated with the pulsed laser light L2 to the spectroscopic analysis unit 20 through the optical fiber cable 40. As will be described later with reference to Figure 4, the probe 30 enters the reactor vessel 61 and therefore has resistance to radiation.

[0016] Figure 2 shows the internal configuration of the probe 30. Figure 3 is a schematic diagram of the laser oscillator 32. As shown in Figure 2, the probe 30 includes, for example, a housing 31, a laser oscillator 32, a magnifying lens 33, a parallelizing lens 34, a diffractive optical element 35, a wedge prism 36, a focusing lens 37, a collecting lens 38, and a collimator lens 39. The optical components (32-39) of the probe 30 are housed in the housing 31.

[0017] As shown in Figure 3, the laser oscillator 32 is a laser medium 32a (Nd 3+ :YAG crystal, hereinafter referred to as “Nd 3+ It is sometimes written as "crystal". ) and supersaturated absorber 32b (Cr 4+ :YAG crystal, hereinafter referred to as “Cr 4+ It is sometimes referred to as a "crystal." It consists of a composite crystal of ( ). The laser medium 32a is placed on the incident side of the excitation laser light L1. The supersaturated absorber 32b(SA) is placed on the output side of the pulsed laser light L2.

[0018] Furthermore, the material of the laser medium 32a is preferably ceramic or single crystal. The ceramic is Nd YAG crystal. 3+It is excellent in doping uniformity, can achieve higher output, has high shape flexibility and is suitable for mass production, but absorption loss easily occurs due to the influence of radiation because of the presence of crystal grain boundaries. For example, in a high radiation dose environment where the gamma ray dose rate exceeds 200 Gy / h, the laser oscillation output decreases by about 20%. In contrast, a single crystal can operate normally even in a dose rate environment exceeding 5 kGy / h. Therefore, it is preferable to use a ceramic laser medium 32a in an environment where no radiation exists, and a single crystal laser medium 32a in an environment where radiation exists.

[0019] When pump laser light L1 with a wavelength of 808 nm from the laser light source 10 is quasi-continuously incident on the laser medium 32a, Nd in the crystal 3+ is excited and oscillates a laser with a wavelength of 1064 nm. On the other hand, since the saturable absorber 32b is normally opaque, it does not transmit the laser. However, as the laser continues to be absorbed by the saturable absorber 32b, the crystal becomes transparent and oscillates 1064 nm pulsed laser light L2 (<1000 ps). The laser oscillation unit 32 is also called a passive Q-switch.

[0020] Note that Nd 3+ The 808 nm incident surface of the crystal is coated with a coating having high reflectivity at 1064 nm and high transmittance at 808 nm. Also, Cr 4+ The pulsed laser emission surface of the crystal is provided with a 1064 nm partial reflection coating. These crystals and coatings form the laser oscillation unit 32.

[0021] Returning to FIG. 2, the expanding lens 33 expands the beam diameter of the pulsed laser light L2 oscillated by the laser oscillation unit 32. The collimating lens 34 collimates the pulsed laser light L2 expanded by the expanding lens 33. The diffractive optical element 35 converts the profile of the pulsed laser light L2 collimated by the collimating lens 34 from a Gaussian distribution to a top-hat distribution. That is, the diffractive optical element 35 is also called a homogenizer / diffuser or the like, which uniformizes the intensity distribution of the pulsed laser light L2.

[0022] The wedge prism 36 changes the angle of the top-hat distribution pulsed laser light L2 emitted from the diffractive optical element 35 and guides it to the focusing lens 37. The focusing lens 37 focuses the top-hat distribution pulsed laser light L2 onto the object M. The collection lens 38 collects the plasma light L3 generated in the object M. The plasma light L3 collected by the collection lens 38 is introduced into an optical fiber through the collimator lens 39 and guided to the spectroscopic analysis unit 20.

[0023] The optical fiber cable 40 connects the laser light source 10 and the probe 30, guiding the excitation laser light L1 output from the laser light source 10 to the probe 30. The optical fiber cable 40 also connects the probe 30 to the spectroscopic analysis unit 20, guiding the plasma light L3 output from the probe 30 to the spectroscopic analysis unit 20. The optical fiber cable 40 may include a cable connecting the laser light source 10 and the probe 30, and a cable connecting the spectroscopic analysis unit 20 and the probe 30. A well-known product can be used for the optical fiber cable 40. The following description will focus on the optical fiber cable 40 connecting the laser light source 10 and the probe 30.

[0024] As shown in Figure 1, the optical fiber cable 40 has a curved section 41 between the laser light source 10 and the probe 30. The curved section 41 is a portion of the optical fiber cable 40 that has been curved (bent) to have a bending diameter smaller than the allowable bending diameter. The "allowable bending diameter" is, for example, the bending diameter that is permitted when using the optical fiber cable 40, and is a value specified by the manufacturer of the optical fiber cable 40 (described in the instruction manual and specifications). If the manufacturer uses the term "allowable bending radius," then twice the "allowable bending radius" will be the "allowable bending diameter." Since a bending diameter smaller than the allowable bending diameter of the optical fiber cable 40 may result in performance degradation, most manufacturers set an allowable bending diameter for the optical fiber cable 40.

[0025] More specifically, the bending diameter of the curved section 41 (hereinafter sometimes referred to as the "forced bending diameter") is set to 100 to 200 times, more preferably 150 to 170 times, the diameter of the core of the optical fiber cable 40. The lower limit of the bending diameter of the curved section 41 can be set appropriately within the range in which the optical fiber cable 40 does not break. Furthermore, the position of the curved section 41 is not particularly limited as long as it is between the laser light source 10 and the probe 30. In addition, the number of times the curved section 41 is bent (more specifically, the number of turns on the reel 73, which will be described later) is not particularly limited.

[0026] The controller 50 consists of hardware including a CPU that performs various calculations, a storage device such as a ROM or HDD that stores programs for executing calculations performed by the CPU, RAM which serves as a workspace for the CPU when executing programs, and a communication interface which is an interface for sending and receiving data with other devices, and software stored in the storage device and executed by the CPU. Each function of the controller 50 is realized when the CPU loads various programs stored in the storage device into RAM and executes them.

[0027] The controller 50 controls the entire elemental analysis system 100. Specifically, the controller 50 controls the irradiation timing of the excitation laser light L1 from the laser light source 10 to the probe 30. The controller 50 also displays predetermined information on a monitor (not shown) based on the LIBS analysis results input from the spectroscopic analysis unit 20.

[0028] [Description of the uses of the elemental analysis system 100] Figure 4 shows an example of an application of the elemental analysis system 100. The elemental analysis system 100 is attached to a robotic arm 70, as shown in Figure 4, and is used for elemental analysis of an object M (e.g., fuel debris) deposited inside the reactor vessel 61.

[0029] The reactor vessel 61 is housed in the reactor building 60. The reactor building 60 is also provided with an opening 62 for accessing the reactor vessel 61 from the outside. The reactor vessel 61 stores fuel assemblies containing nuclear fuel material, control rods, etc. In such a reactor vessel 61, if an object M containing radioactive material accumulates due to an unexpected event such as an accident, elemental analysis is necessary in order to process (remove) this object M. In a harsh environment with high radiation levels, it is not possible for workers to directly visually inspect the object M and perform elemental analysis. Therefore, as shown in Figure 4, a robotic arm 70 is inserted through the opening 62 in the reactor building 60, and the robotic arm 70 is operated remotely to perform elemental analysis of the object M accumulated at the bottom of the reactor vessel 61.

[0030] The robot arm 70 comprises, for example, a plurality of arms 71 and 72. Arm 72 is rotatably supported relative to arm 71 at its tip. Arms 71 and 72 are also configured to be extendable and retractable. Furthermore, the robot arm 70 includes a reel 73 for winding and unwinding the optical fiber cable 40. The robot arm 70 then enters the interior of the reactor building 60 and the reactor vessel 61 through the opening 62.

[0031] The laser light source 10, the spectroscopic analysis unit 20, and the controller 50 are located outside the reactor building 60. The probe 30 is attached to the tip of the arm 72. The optical fiber cable 40 extends along the arms 71 and 72. The optical fiber cable 40 is wound around a reel 73 between the laser light source 10 and the spectroscopic analysis unit 20 and the probe 30. The portion of the optical fiber cable 40 wound around the reel 73 corresponds to the curved portion 41.

[0032] The robot arm 70 moves the probe 30 to a position facing the object M by extending and retracting and rotating (for example, by driving) arms 71 and 72. In this process, the reel 73 winds in and unwinds the optical fiber cable 40 in accordance with the change in the overall length of the robot arm 70. In other words, the reel 73 plays a role in maintaining the bending diameter of the curved portion 41 of the optical fiber cable 40 at a predetermined value, and also allows for the extension and retraction of the optical fiber cable 40 in accordance with the change in the overall length of the robot arm 70. This maintains an appropriate tension in the optical fiber cable 40, thereby minimizing external influences on the optical fiber cable 40. The optical fiber cable 40 is wound around the outer circumference of the cylindrical reel 73. The diameter of the reel 73 is set to be smaller than the allowable bending diameter.

[0033] Then, with the probe 30 facing the object M, the controller 50 causes the laser light source 10 to output excitation laser light L1. The probe 30 converts the excitation laser light L1, which has been incident through the optical fiber cable 40, into pulsed laser light L2 and irradiates the object M. The spectroscopic analysis unit 20 acquires the plasma light L3 generated in the object M through the probe 30 and the optical fiber cable 40, analyzes the acquired plasma light L3, and outputs the analysis results to the controller 50. In this way, the elemental analysis system 100 is suitable for performing elemental analysis of an object M, for example, referred to as fuel debris in a reactor vessel 61.

[0034] [Explanation of the relationship between the excitation laser light L1 and the pulsed laser light L2] Figure 5 shows the relationship between the excitation laser light L1 and the pulsed laser light L2. As shown in Figure 5, the controller 50 can control the oscillation timing of the pulsed laser light L2 by setting the pulse width T and current value I of the excitation laser light L1.

[0035] As an example, as shown in FIG. 5(A), when the saturable absorber 32b is excited by the excitation laser beam L1 having a current value I1, the pulsed laser beam L2 oscillates before the pulse width T1 is reached. As another example, as shown in FIG. 5(B), when the saturable absorber 32b is excited by the excitation laser beam L1 having a current value I2 (<I1), the pulsed laser beam L2 oscillates before the pulse width T2 (>T1) is reached. As still another example, as shown in FIG. 5(C), when excitation of the saturable absorber 32b is continued with the excitation laser beam L1 having a current value I1, a plurality of pulsed laser beams L2 repeatedly oscillate at predetermined intervals.

[0036] In FIG. 5, the pulse width T refers to the time period during which the saturable absorber 32b is excited by the excitation laser beam L1. In addition, the time from the start of excitation by the excitation laser beam L1 to the oscillation of the pulsed laser beam L2 is referred to as BUT (Build Up Time). As shown in FIGS. 5(A) and 5(B), the larger the current value I of the excitation laser beam L1 is, the shorter BUT becomes (BUT1<BUT2). In addition, BUT can also vary depending on the incident layout of the excitation laser beam L1 to the laser medium 32a, the type, shape, size, and the like of the laser medium 32a.

[0037] After the saturable absorber 32b emits the accumulated energy as the pulsed laser beam L2, it returns to the initial high absorption state again before the laser medium 32a forms a population inversion and the gain is recovered. For this reason, the next pulsed laser beam L2 is generated after sufficient energy is accumulated again in the laser medium 32a. It should be noted that the excitation wavelength and the oscillation wavelength differ depending on the material types of the laser medium 32a and the saturable absorber 32b.

[0038] However, even if the saturable absorber 32b is excited under the same conditions, the oscillation timing (i.e., BUT) of the pulsed laser beam L2 may fluctuate. This fluctuation in BUT is referred to as oscillation width (jitter). In order to stabilize the analysis accuracy of the elemental analysis system 100, it is important to reduce this oscillation width.

[0039] [Description of Optical Fiber Cable 40] Figure 6 shows a cross-sectional view (A) of the optical fiber cable 40, a longitudinal section (B) of the curved section 41, and an example of the profile of the excitation laser light L1 in the curved section 41. The optical fiber cable 40 is an excellent dielectric waveguide with remarkable features such as flexibility, thinness, light weight, and low loss. As shown in Figure 6(A), the optical fiber cable 40 has a two-layer structure consisting of a core 42 that serves as the propagation path for optical signals and a cladding 43 that covers the core 42. In the cross-sectional view (A) of the optical fiber cable 40, the maximum length of the perfectly circular core 42 is the core diameter. The cladding 43 is made of glass and plays the role of confining light within the core 42 and ensuring mechanical strength. In other words, the optical fiber cable 40 has a cylindrical core 42 covered with a cylindrical cladding 43.

[0040] Furthermore, although it varies depending on the type of optical fiber cable 40, the refractive index n1 of the core 42 is set to approximately 1.463 to 1.467, and the refractive index n2 of the cladding 43 is set to approximately 1.45 to 1.46. In other words, the optical fiber cable 40 is composed of cladding 43 with a uniform refractive index n2, and a core 42 located in the center of the cladding 43 and having a higher refractive index n1 than the cladding 43. The optical fiber cable 40 utilizes the refractive index difference between the core 42 and the cladding 43 to conduct total internal reflection of light, allowing the signal (light) to be guided over long distances within the core 42 without leaking into the cladding 43.

[0041] The types of optical fiber cables 40 are classified according to the number of modes that can propagate, and are divided into single-mode optical fibers and multimode optical fibers. Generally, multimode optical fibers are suitable for transmitting high-power laser light, and the optical fiber cable 40 in this embodiment is also preferably a multimode optical fiber. A multimode optical fiber is defined by its core diameter and numerical aperture (NA), which can be described by the following equation. The numerical aperture NA represents the maximum incident angle θmax that can be incident on a multimode optical fiber, and is a value corresponding to the number of propagation modes. It is defined by the following equation 1 as the sine of the maximum angle that an incident light ray can have for total internal reflection within the core 42. In equation 1, n is the refractive index of the medium on the outer circumference of the optical fiber cable 40 (N=1 in the case of air).

[0042]

number

[0043] Here, if vibration is applied to the optical fiber cable 40 from the outside, optical energy is lost (attenuated). The following three points are the main factors that cause optical energy attenuation.

[0044] First, in many applications of optical fiber cables 40, light scattering is the primary cause of loss. Scattering occurs wherever there is a change in the refractive index of the medium. Such changes in refractive index can be extrinsic, such as impurities, particles, and bubbles, or intrinsic, due to variations in the density, composition, and phase state of the glass. Extrinsic scattering losses can be significantly suppressed by proper fiber cleaning, processing, and storage procedures.

[0045] Furthermore, losses caused by external changes in the optical fiber cable 40 are called bending losses. Generally, this refers to losses that occur when the optical fiber cable 40 is physically bent, for example, when it is wound into a thin coil. The guided light within the optical fiber cable 40 is spatially distributed in the core 42 and cladding 43. When the optical fiber cable 40 is bent, the speed of light near the outside must be faster than near the center in order to maintain the same spatial mode profile. And if the spatial mode profile cannot be maintained, light energy is lost to the surroundings as radiated light, so it is common to use the cable without bending it or to use a bending diameter larger than the manufacturer's recommended allowable bending diameter.

[0046] Furthermore, while most light in a multimode fiber propagates by internal total internal reflection within the core 42, higher-order modes may propagate through both the core 42 and the cladding 43 due to internal total internal reflection at the interface between the cladding 43 and the coating buffer. This is called cladding mode and generally causes losses due to its higher order.

[0047] Through trial and error, the inventors have newly discovered that by applying a bend to the optical fiber cable 40 that is significantly smaller than the allowable bending diameter (i.e., by providing a curved section 41), the oscillation of the pulsed laser light L2 becomes stable (i.e., the oscillation amplitude becomes smaller) and the output power of the pulsed laser light L2 is increased.

[0048] More specifically, it is believed that the combined action of forced mode dispersion (forcibly generating differences in the arrival time of light depending on the mode), utilization of cladding modes (utilizing cladding propagation, which is normally considered a cause of transmission loss and attenuation), and utilization of higher-order modes (actively utilizing components with slower light arrival times) allows light to leak from the core 42 into the cladding 43, and the protective layer that confines the leaked light promotes cladding propagation, resulting in a higher numerical aperture (NA). Furthermore, by primarily using slow-speed light propagation components, it is possible to homogenize the intensity distribution of the excitation laser light L1 passing through the optical fiber cable 40.

[0049] For example, as shown in Figure 6(C), when a curved section 41 smaller than the allowable bending diameter is provided (for example, a bending diameter of 110 mm, which is 183 times the core diameter), the excitation laser beam L1 in the CC in Figure 6(B) shows a top-hat type profile (i.e., a uniform emission situation).

[0050] Figure 7 shows an example of the profile of the excitation laser beam L1 inside the optical fiber cable 40. More specifically, Figure 7(A) shows the profile when the bending diameter of the curved section 41 is set to the manufacturer's recommended value. On the other hand, Figure 7(B) shows the profile when the bending diameter of the curved section 41 is set to a value smaller than the allowable bending diameter. As is clear from Figure 7, the excitation laser beam L1 follows a Gaussian distribution when the bending diameter of the curved section 41 is increased (Figure 7(A)), and follows a top-hat distribution when the bending diameter of the curved section 41 is decreased (Figure 7(B)).

[0051] When a Gaussian-distributed excitation laser beam L1 is incident on the laser medium 32a, the excitation efficiency of the supersaturated absorber 32b differs between the peak region and other regions. Furthermore, if a temporal change in the intensity distribution occurs due to mode changes (disturbances) during extrinsic fiber transmission, the incident point of the peak region on the crystal plane shifts and the intensity distribution changes, which can lead to a decrease in excitation efficiency. As a result, the oscillation amplitude of the pulsed laser beam L2 is expected to increase.

[0052] In contrast, if the bending diameter of the optical fiber cable 40 is made smaller than the allowable bending diameter, light leaks from the core 42 to the cladding 43, and the leaked light propagates through the cladding and is emitted together with the core, resulting in a top-hat profile for the excitation laser light L1. Therefore, the light incident on the aforementioned laser oscillator 32 becomes uniform, preventing uneven distribution of energy absorption within the laser oscillator 32, stabilizing the laser oscillation, and reducing the oscillation amplitude.

[0053] The optical fiber cable 40 best suited to this embodiment is a fiber comprising a core layer, a cladding layer, and a protective layer. Preferably, the protective layer is made of a resin exhibiting a lower refractive index than the cladding layer (hereinafter referred to as "low refractive index resin-applied fiber"). Using a resin with a lower refractive index than the cladding layer in the protective layer not only confines leaked light within the optical fiber cable 40, but also enables cladding propagation, resulting in more uniform emitted light.

[0054] Specifically, examples include resins such as polytetrafluoroethylene (tetrafluoroethylene), tetrafluoroethylene copolymer (also known as Tefzel ETFE), polyvinylidene difluoride (PVDF), acrylonitrile styrene (AS), silicone resin (SI), trifluoroethylene chloride (PCTFE), and tetrafluoroethylene-hexafluoropropylene copolymer (FEP).

[0055] [Tests to confirm effectiveness] Figure 8 shows the results of verifying the changes in the BUT (A) and laser output (B) of the pulsed laser beam L2 when a curved section 41 is provided in the optical fiber cable 40. The test specimen (NA 0.22) was MF Optix STU600-20m, SUS flexible tube, with a core diameter of 0.6 mm. The measured items (LD conditions 150A, 123 μs, 1 Hz) were the BUT stretching ratio (Figure 8(A)) and the laser output (Figure 8(B)). The light emitted from the optical fiber cable 40 was focused via a pair of lenses and incident on the laser medium 32a.

[0056] The horizontal axis in Figure 8 shows the bending diameter of the curved section 41. Figure 8(A) shows the change in BUT as a percentage (%) when the bending diameter of the curved section 41 is changed, with the BUT of the optical fiber cable 40 without the curved section 41 as the reference. Figure 8(B) shows the change in laser output when the bending diameter of the curved section 41 is changed, with the BUT of the optical fiber cable 40 without the curved section 41 as the reference.

[0057] As shown in Figure 8(A), it became clear that the BUT increases as the bending diameter of the curved section 41 decreases. This indicates that the excitation laser light L1 passing through the curved section 41 changes from a Gaussian distribution to a top-hat distribution and is averaged, reducing the energy that excites the laser medium 32a, and requiring a longer excitation pulse width to oscillate the pulsed laser light L2.

[0058] Furthermore, as shown in Figure 8(B), the laser output increased 1.47 times at a bending diameter of 125 mm in the curved section 41, and 1.73 times at a bending diameter of approximately 100 mm. This confirmed that it is advantageous to set the bending diameter of the curved section 41 to 100 to 200 times, more preferably 150 to 170 times, the core diameter.

[0059] According to the literature (OPTICS EXPRESS, Vol.24, 28748-28759 (2016)), the factors contributing to the large oscillation amplitude include jitter of the drive power supply for the laser light source 10, jitter of the laser light source 10 itself, and jitter within the laser oscillator 32. It has been reported that this can be improved by increasing the output power of the drive power supply. Therefore, it is desirable to use a 150W power supply instead of the conventional 120W power supply, which has shown a large oscillation amplitude.

[0060] Figure 9 shows the results of a test that confirmed the relationship between the bending diameter of the curved section 41 and BUT (μs), oscillation amplitude (μs), and laser output (mV). In this test, the allowable bending diameter was set to 300 mm and the forced bending diameter to 95 mm. The manufacturer's recommended bending diameter is defined as a diameter (including straight sections) that is sufficiently larger than the allowable bending diameter. Furthermore, the laser output was measured using an energy meter for measuring laser light energy.

[0061] As shown in Figure 9, it can be confirmed that reducing the bending diameter of the curved section 41 increases the BUT, decreases the oscillation amplitude, and increases the laser output. In particular, when the curved section 41 is made to a forced bending diameter, the oscillation amplitude becomes smaller by more than an order of magnitude. Furthermore, it has been confirmed that the oscillation amplitude can be reduced to ±0.1 μs or less by using a high-power power supply (250 W), so combining it with a high-power power supply is even more desirable.

[0062] Figure 10 shows the oscillation characteristics of pulsed laser light L2 when the optical fiber cable 40 is bent to the recommended diameter (A) and the forced bending diameter (B). Of note here is the profile of the pulsed laser light L2, which is a Gaussian distribution for both the recommended bending diameter (A) and the forced bending diameter (B). In Figure 10(B), the forced bending diameter was set to 70 mm (116 times the core diameter). In both Figures 10(A) and (B), the light emitted from the optical fiber cable 40 was focused via a pair of lenses and incident onto the laser medium 32a, and the test was performed using an MF Optex STU-600 (20 m), a current value of 150 (A), and an initial pulse width of 80 (μsec). In Figure 10(B), compared to Figure 10(A), the BUT was approximately 18% longer, the laser output was approximately 50% higher, and the oscillation status was confirmed to be more stable.

[0063] Figure 11 shows the results of a test to confirm the relationship between the bending diameter (layout conditions) of the curved section 41, BUT, laser output, oscillation conditions, and beam diameter. In the test shown in Figure 11, the light emitted from the optical fiber cable 40 was directly incident on the laser medium 32a. Layout conditions "R" refer to the recommended bending diameter, layout conditions "FB" refer to the forced bending diameter, layout conditions "O" refer to on-site (conditions close to actual usage environments), and layout conditions "OFB" refer to the forced bending diameter in conditions close to actual usage environments. In addition, in layout conditions "FB" for Nos. 2 to 4, the position of the curved section 41 was changed.

[0064] As shown in Figure 11, it was confirmed that by forcing the bending diameter of the curved section 41 (i.e., reducing the bending diameter), the BUT increased, the oscillation condition stabilized, and the laser output also improved by approximately 34%, from the minimum value (0.64 mJ) to the maximum value (0.86 mJ).

[0065] Figure 12 is a photograph showing the surface condition of an object M irradiated with pulsed laser light L2. Object M can be of two types: metal and oxide. Figure 12 also compares the case where pulsed laser light L2 with a Gaussian distribution is irradiated without the diffractive optical element 35 (left side) and the case where pulsed laser light L2 with a top-hat distribution is irradiated with the diffractive optical element 35 (right side).

[0066] As shown in the left photograph, when pulsed laser light L2 with a Gaussian distribution is irradiated, perforations reflecting the Gaussian distribution are observed regardless of the material of the object M. In this case, the irradiation area changes as the irradiation time progresses, meaning that the emission volume changes over time, and as a result the plasma light L3 also changes, making high-precision measurement difficult. On the other hand, when pulsed laser light L2 with a top-hat distribution is irradiated, shallow and flat irradiation marks are produced. Because the marks are flat, the irradiation area remains constant regardless of the irradiation time, so there is no variation in the acquired data, and high-precision measurement becomes possible.

[0067] Figure 13 is a micrograph of the surface of object M(Zr) after being irradiated 10 times with pulsed laser light L2 having a Gaussian distribution. The white line indicates the shape of the A-A' cross section. Referring to Figure 13, the formation of a perforation with a diameter of approximately 33 μm and a depth of 4.3 μm, reflecting the Gaussian distribution, was confirmed at the irradiation site of the focused pulsed laser light L2. As a result, the power density of the pulsed laser light L2 differs with each irradiation, causing a large change in the intensity of the plasma light L3, which can be a factor in the large variability of the acquired data.

[0068] [Effects of this embodiment] According to the above embodiment, by providing a curved section 41 in the optical fiber cable 40, the oscillation amplitude is significantly reduced as shown in Figure 9, thereby eliminating the instability of the pulsed laser light L2 oscillation. As a result, the analysis accuracy of the elemental analysis system 100 is improved. Furthermore, since synchronization signal fibers and dichroic mirrors for synchronizing the oscillation of the pulsed laser light and the plasma measurement can be omitted, the configuration of the elemental analysis system 100 can be simplified.

[0069] Furthermore, according to the above embodiment, the laser output of the pulsed laser light L2 can be improved by providing the curved portion 41. Therefore, it is possible to oscillate a higher-power pulsed laser light L2 with the same laser light source 10 as before. Alternatively, it is possible to oscillate a pulsed laser light L2 with the same output as before with a smaller laser light source 10 than before.

[0070] Here, for optical fiber cables 40, manufacturers publish recommended values ​​such as "allowable bending ratio (≒allowable bending diameter)" (for example, representative manufacturers include Fujikura, https: / / www.optic-product.fujikura.com / optical-fibers / jp / products / s-series-high-oh / , MF Optex, https: / / www.mfoptex.com / assets / pdf / MFOPTEX_SMA_jp.pdf, and CeramOptec, https: / / www.ceramoptec.com / products / fibers / optran-uvwfs.html).

[0071] Reasons for setting an allowable bending diameter include, for example, reducing bending loss (see, for example, Japanese Patent Publication No. 2021-018362), preventing deterioration of the optical fiber cable 40, and preventing signal attenuation and errors due to differences in the arrival time of light in the field of optical communications. Furthermore, since it is common technical knowledge in this field that pulsed laser light L2 can become unstable due to external vibrations to the optical fiber cable 40, it is generally considered undesirable for stress to be applied to the optical fiber cable 40.

[0072] In contrast, the present invention deliberately makes the bending diameter of the curved portion 41 smaller than the allowable bending diameter set by the manufacturer. In other words, the present invention assumes that the optical fiber cable 40 will be used in the field of elemental analysis, which is completely different from fields such as optical communications, and prioritizes obtaining remarkable effects such as reducing the oscillation amplitude of the pulsed laser light L2 and improving the laser output.

[0073] Furthermore, according to the above embodiment, by winding the optical fiber cable 40 onto a reel 73 smaller than the allowable bending diameter, the bending diameter of the curved portion 41 can be maintained at a desired value. As a result, the profile of the excitation laser light L1 is stabilized, and the oscillation amplitude of the pulsed laser light L2 is further stabilized.

[0074] Furthermore, according to the above embodiment, by converting the profile of the pulsed laser light L2 from a Gaussian distribution to a top-hat distribution, it is possible to suppress the time-dependent change in the amount of material evaporated in the target object M that is involved in perforation. As a result, the time-dependent change in the plasma light L3 is suppressed during elemental analysis, enabling highly accurate elemental analysis.

[0075] Furthermore, according to the above embodiment, by amplifying the pulsed laser light L2 emitted by the laser oscillation unit 32, damage to the optical elements through which the pulsed laser light L2 passes can be reduced.

[0076] It should be noted that the present invention is not limited to the embodiments described above, and various modifications are possible without departing from the spirit of the invention. All technical matters included in the technical concept described in the claims are subject to the present invention. The embodiments described above are preferred examples, but those skilled in the art can realize various alternative examples, modifications, variations, combinations, or improvements from the contents disclosed herein, and these are included in the technical scope described in the appended claims. [Explanation of symbols]

[0077] 10: Laser light source 20: Spectroscopic analysis section 30: Probe 31: Cabinet 32: Laser Oscillator 32a: Laser medium 32b: Supersaturated absorber 33: Magnifying lens 34: Parallelizing lens 35: Diffractive optical elements 36: Wedge Prism 37: Focusing lens 38: Collecting lens 39: Collimator lens 40: Fiber optic cable 41: Curved section 42: Core 43: Clad 50: Controller 60: Reactor building 61:Reactor vessel 62 :Aperture 70: Robot arm 71,72: Arm 73: Reel 100: Elemental Analysis System

Claims

1. A laser light source that emits excitation laser light, A laser oscillator unit that is excited by the excitation laser light output from the laser light source and oscillates pulsed laser light, An analysis unit analyzes elements contained in an object based on the plasma light generated in the object irradiated by the pulsed laser light emitted by the laser oscillation unit, In an elemental analysis system comprising an optical fiber cable that guides the excitation laser light output from the laser light source to the laser oscillator, The elemental analysis system is characterized in that the optical fiber cable has a curved portion that is bent to a bending diameter smaller than a predetermined allowable bending diameter.

2. In the elemental analysis system according to claim 1, An elemental analysis system characterized in that the bending diameter of the curved portion is set to 100 to 200 times the core diameter.

3. In the elemental analysis system according to claim 1, The optical fiber cable has a core layer, a cladding layer, and a protective layer. An elemental analysis system characterized in that the refractive index of the protective layer is lower than that of the cladding layer.

4. In the elemental analysis system according to claim 1, A robotic arm that supports and drives the laser oscillator at its tip, The system includes a reel that is rotatably supported on the robot arm and extends along the robot arm, and that winds the optical fiber cable in a manner that allows it to be wound up and unwound, An elemental analysis system characterized in that the diameter of the reel is smaller than the allowable bending diameter.

5. In the elemental analysis system according to claim 1, The laser oscillator unit, An elemental analysis system characterized by comprising a probe that includes a diffractive optical element that homogenizes the intensity distribution of the pulsed laser light emitted by the laser oscillator and converts the profile of the pulsed laser light from a Gaussian distribution to a top-hat distribution.

6. In the elemental analysis system described in claim 5, The aforementioned probe A magnifying lens for magnifying the pulsed laser light emitted by the laser oscillator, The system further includes a parallelizing lens that parallelizes the pulsed laser light magnified by the magnifying lens, The elemental analysis system is characterized in that the diffractive optical element converts the profile of the pulsed laser light, which has been parallelized by the parallelizing lens, into a top-hat distribution.

7. A laser light source that emits excitation laser light, A laser oscillator unit that is excited by the excitation laser light output from the laser light source and oscillates pulsed laser light, An analysis unit analyzes elements contained in an object based on the plasma light generated in the object irradiated by the pulsed laser light emitted by the laser oscillation unit, In an elemental analysis system comprising an optical fiber cable that guides the excitation laser light output from the laser light source to the laser oscillator, An elemental analysis system further comprising a diffractive optical element that homogenizes the intensity distribution of the pulsed laser light emitted by the laser oscillation unit and converts the profile of the pulsed laser light from a Gaussian distribution to a top-hat distribution.

Images