JP2017215538A - Laser and laser ultrasonic flaw detection device using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a laser emitting mid-infrared and a laser ultrasonic flaw detection device using the same.SOLUTION: The laser comprises: a first wavelength conversion element having a period polarization inversion structure (period Λ), and wavelength-converting at a temperature T a pump light (pump wavelength λ) into an idler light (idler wavelength λ) and a signal light (signal wavelength λ) to satisfy 1/λ=1/λ+1/λ; a second wavelength conversion element having a period polarization inversion structure (period Λ), and wavelength-converting at the temperature T the signal light (signal wavelength λ) into a difference frequency light (difference frequency wavelength λ) and the idler light (idler wavelength λ) to satisfy 1/λ=1/λ+1/λ; and a temperature control part heating the wavelength conversion elements to control the temperatures. The temperature control part controls the temperature T to exceed a degeneracy temperature Tat which the second wavelength conversion element parametrically oscillates.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a laser and a laser ultrasonic flaw detector using the laser. More specifically, the present invention relates to a compact laser that emits mid-infrared light using a wavelength conversion element and a laser ultrasonic flaw detector using the same.

Carbon Fiber Reinforced Plastics (CFRP) is a very unique and innovative structural material that combines strength and lightness. Its usefulness is recognized worldwide, and it is used in various fields such as aircraft and cars. Application to is starting. One of the key technologies for developing carbon composite materials such as CFRP is nondestructive inspection technology for detecting defects. Ultrasonic flaw test (LUT: Laser-Ultrasonic Testing) is to irradiate a short pulse laser to the inspection part, cause local expansion and contraction of the volume of the material, generate ultrasonic waves and analyze their propagation This is a method for nondestructively detecting defects on the material surface and inside. LUT's key technologies are diverse, but in particular, pulsed lasers for ultrasonic generation are important, and lasers that are easy to use and reliable at wavelengths optimized for CFRP inspection are extremely important. The laser used is a large, very unwieldy, low reliability CO ₂ laser.

Recently, a laser that emits mid-infrared light using optical parametric oscillation (OPO) has been developed as a laser that replaces the CO ₂ laser (see, for example, Non-Patent Document 1). Non-Patent Document 1 uses a LiTaO ₃ single crystal with a stoichiometric composition to which a periodically poled structure is formed, added Mg, performs OPO oscillation, and 3.23 μm of mid-infrared light causes laser damage to CFRP. It is reported that this is a light source with high sensitivity. However, in order to improve the accuracy of nondestructive inspection, further improvement in OPO oscillation efficiency is required.

  Another laser emitting mid-infrared light that improves the OPO oscillation efficiency has been developed (see, for example, Non-Patent Document 2 and Patent Document 1). According to Patent Document 1 and Non-Patent Document 2, a laser that emits mid-infrared light in combination of OPO and differential frequency mixing (DFM) is disclosed.

  FIG. 1 is a diagram showing the principle of a conventional laser combining OPO and DFM.

As shown in FIG. 1B, according to Patent Document 1 and Non-Patent Document 2, a configuration in which an OPO and a DFM are arranged in a resonator is disclosed. Here, in the OPO, as shown in FIG. 1A, the pump wave (ω _pump ) is converted into a signal wave (ω _signal ) and an idler wave (ω _idler ). The target mid-infrared light is an idler wave, but due to the Manley-Rowe relationship, the conversion efficiency is such that when the mid-infrared light near the wavelength of 3.2 μm is generated, the energy ratio between the idler light and the signal light is Since it is 1: 2, it is limited to a maximum of 33%. Therefore, as shown in FIG. 1B, a signal wave is converted into an idler wave and a difference frequency wave (ω _difference ) by incorporating DFM after OPO. This improves idler generation efficiency.

  Actually, Non-Patent Document 2 shows that the oscillation efficiency of idler waves in a laser in which OPO and DFM are combined is greater than that of OPO alone. However, in order to achieve such a structure, it is necessary to separately control the temperature of the OPO and the DFM, and the laser itself is required to be downsized. Also, Patent Document 1 does not disclose a specific configuration.

  On the other hand, another wavelength conversion laser device that performs wavelength conversion twice in succession has been developed (for example, see Patent Document 2). According to Patent Document 2, the first wavelength conversion element that converts light having a first wavelength different from the wavelength of incident light, the light converted by the first wavelength conversion element, and the first wavelength conversion element A wavelength conversion laser device is disclosed that includes a second wavelength conversion element that receives a fundamental wave that has not been converted and converts it to light having a second wavelength different from the first wavelength. At least one of the conversion element and the second wavelength conversion element is a fan-out structure pseudo-phase matching element or a chirp structure pseudo-phase matching element, and the first wavelength conversion element and the second wavelength conversion element are It is characterized by being controlled to the same temperature.

  However, in Patent Document 2, the first wavelength conversion element and the second wavelength conversion element both generate second harmonics and oscillate mid-infrared light, not a combination of OPO and DFM. I can't.

Japanese Patent No. 5520933 JP2015-165260A

H. Hatano et al. Optics, 17 (2015) 094011 Hideki Hatano et al., 14p-2G-9 “Development of high-efficiency mid-infrared laser light source by OPO + DFM using PP-MgSLT and application of CFRP to laser ultrasonic testing”, 76th JSAP Autumn Meeting, 2015

  An object of the present invention is to provide a small laser emitting mid-infrared light and a laser ultrasonic flaw detector using the same.

The laser according to the present invention is a first wavelength conversion element having a periodically poled structure having a _first period Λ _1, and has a pump light having a pump wavelength λ _p and an idler wavelength λ _i at a temperature T. Wavelength conversion into idler light and signal light having a signal wavelength λ _s , where the pump wavelength λ _p , the idler wavelength λ _i, and the signal wavelength λ _s are related 1 / λ _p = 1 / λ _s A first wavelength conversion element satisfying + 1 / λ _i and a second wavelength conversion element having a periodic polarization inversion structure having a _second period Λ ₂ , wherein the signal wavelength λ _s is changed at the temperature T. The signal light having a wavelength is converted into a difference frequency light having a difference frequency wavelength λ _d and an idler light having the idler wavelength λ _i , wherein the signal wavelength λ _i , the difference frequency wavelength λ _d, and the Idler The long lambda _i, satisfying the relation _{1 / λ s = 1 / λ} i + 1 / λ d, and heated a second wavelength converting element, the first wavelength conversion element and the second wavelength conversion element, A temperature control unit for controlling temperature, wherein the temperature control unit sets the temperature T of the first wavelength conversion element and the second wavelength conversion element, and the second wavelength conversion element sets the pump wavelength. pump light having a lambda _p, and wavelength-converted into a signal light having a _'idler light and the signal wavelength lambda _s having _a' idler wavelength lambda _i, wherein wherein said pump wavelength lambda _p and the idler wavelength lambda _{i '} The signal wavelength λ _{s ′} is controlled to be higher than the degenerate temperature T ₀ when the relationship 1 / λ _p = 1 / λ _{s ′} + 1 / λ _{i ′} and the relationship λ _{s ′} = λ _{i ′} are satisfied. The above-described problem is achieved.
The periodic polarization inversion structure of the first wavelength conversion element and / or the second wavelength conversion element may be a fan-out structure.
The first wavelength conversion element and / or the second wavelength conversion element is selected from the group consisting of stoichiometric lithium niobate, substantially stoichiometric lithium tantalate, Mg, Zn, Sc and I A substantially stoichiometric composition of lithium niobate containing at least one element and a substantially stoichiometric composition containing at least one element selected from the group consisting of Mg, Zn, Sc and In It may consist of a single crystal selected from the group consisting of lithium tantalate.
The first wavelength conversion element and the second wavelength conversion element may be made of a single single crystal.
The first wavelength conversion element and / or the second wavelength conversion element is a lithium tantalate having a substantially stoichiometric composition containing at least one element selected from the group consisting of Mg, Zn, Sc and In The first period Λ ₁ is in the range of 30.5 μm to 31.5 μm, and the second period Λ ₂ is in the range of 32.5 μm to 33.15 μm. May be.
The first period Λ ₁ may be in the range of 30.8 μm to 31.2 μm, and the second period Λ ₂ may be in the range of 32.7 μm to 32.9 μm.
The temperature T may be a temperature in the range of 65 ° C. or higher and 150 ° C. or lower.
A laser ultrasonic flaw detection apparatus for testing flaw detection of an object by ultrasonic waves according to the present invention has an excitation source that emits pump light having a pump wavelength λ _p, and the pump light from the excitation source has an idler wavelength λ _i . A laser that converts the wavelength into idler light and irradiates the test object with the idler light. The laser is the above-described laser, thereby solving the above-described problem.
The laser may emit mid-infrared light in which the idler light has a wavelength of 3.15 μm to 3.25 μm.
You may further provide the analysis part which detects and analyzes the ultrasonic wave which generate | occur | produced in the said test object.
The test object may be a carbon fiber reinforced plastic.
The excitation source may be an Nd: YAG laser or an Nd: YVO ₄ laser.

A laser according to the present invention includes a first wavelength conversion element having a periodic polarization inversion structure having a _first period Λ ₁ , a second wavelength conversion element having a periodic polarization inversion structure having a _second period Λ ₂ , and A temperature control unit for heating and controlling the temperature, and the temperature control unit controls the temperature T to be higher than the degenerate temperature T ₀ when the second wavelength conversion element performs OPO oscillation. To do. Thereby, even if the first wavelength conversion element and the second wavelength conversion element are operated at the same temperature T, OPO oscillation caused by the second wavelength conversion element that is not desired can be suppressed, so that high conversion efficiency can be achieved. Further, since the first wavelength conversion element and the second wavelength conversion element need only be controlled to the same temperature by a single temperature control unit, separate temperature control units are unnecessary, and the entire laser can be reduced in size. If such a laser is employed, a small and highly accurate laser ultrasonic flaw detector can be provided.

The figure which shows the principle of the laser of the combination of OPO and DFM by a prior art Schematic showing a laser according to the invention. Schematic diagram showing a laser ultrasonic flaw detector according to the present invention. Diagram showing the optical system used in the experiment The figure which shows the relationship between the oscillation wavelength of the laser of Example 1, and phase matching temperature. The figure which shows the relationship between the oscillation wavelength and phase matching temperature of the laser of Example 1 of the comparative example 2 The figure which shows the temperature dependence of the energy of the output light of the laser of Example 1. FIG. The figure which shows the relationship between the energy of the pump light of the laser of Example 1, and the energy of output light. The figure which shows the temperature dependence of the energy of the output light of the 2nd wavelength conversion element in the laser of the comparative example 2. The figure which shows the relationship between the energy of the pump light of the laser of the comparative example 2, and the energy of output light The figure which shows the relationship between the oscillation wavelength of the laser by Example 3, and phase matching temperature. The figure which shows the relationship between the oscillation wavelength of the laser by Example 4, and phase matching temperature. The figure which shows the relationship between the oscillation wavelength of the laser by Example 5, and phase matching temperature. The figure which shows the relationship between the oscillation wavelength of the laser by Example 6, and phase matching temperature. The figure which shows the relationship between the oscillation wavelength of the laser by Example 7, and phase matching temperature.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. In addition, the same number is attached | subjected to the same element and the description is abbreviate | omitted.
(Embodiment 1)
In Embodiment 1, a laser according to the present invention will be described.

  FIG. 2 is a schematic diagram showing a laser according to the present invention.

As shown in FIG. 2A, a laser 200 according to the present invention has at least a first wavelength conversion element 210 having a periodically poled structure having a _first period Λ ₁ and a second period Λ ₂ . A second wavelength conversion element 220 having a periodically poled structure, and a single temperature control unit 230 that simultaneously heats the first wavelength conversion element 210 and the second wavelength conversion element 220 to control the temperature. Prepare. Since the single temperature control unit 230 performs heating of the first wavelength conversion element 210 and the second wavelength conversion element 220, a separate temperature control unit is unnecessary, and the entire laser 200 can be downsized.

The first wavelength conversion element 210 is a wavelength conversion element that performs parametric (OPO) oscillation by quasi phase matching. The first wavelength conversion element 210 converts the wavelength of the pump light having the pump wavelength λ _p into the idler light having the idler wavelength λ _i and the signal light having the signal wavelength λ _s at the temperature T. Here, the pump wavelength λ _p , the idler wavelength λ _i, and the signal wavelength λ _s satisfy the relationship 1 / λ _p = 1 / λ _s + 1 / λ _i .

Specifically, the first period Λ ₁ is expressed as follows using the pump wavelength λ _p , the idler wavelength λ _i, and the signal wavelength λ _s .
_{1 / Λ 1 = n p (} T) / λ p -n s (T) / λ s -n i (T) / λ i
Here, n _p (T), n _s (T), and n _i (T) are refractive indexes for the wavelengths of the pump light, the signal light, and the idler light at the temperature T, respectively.

The second wavelength conversion element 220 is a wavelength conversion element that oscillates in a difference frequency (DFM) by quasi phase matching. The second wavelength conversion element 220 converts the wavelength of the signal light having the signal wavelength λ _s into the difference frequency light having the difference frequency wavelength λ _d and the idler light having the idler wavelength λ _i at the temperature T. Here, the signal wavelength λ _i , the difference frequency wavelength λ _d and the idler wavelength λ _i satisfy the relationship 1 / λ _s = 1 / λ _i + 1 / λ _d .

Specifically, the second period Λ ₂ is expressed as follows using the difference frequency wavelength λ _d , the idler wavelength λ _i and the signal wavelength λ _s .
1 / Λ ₂ = n _s (T) / λ _s −n _d (T) / λ _d −n _i (T) / λ _i
Here, n _d (T), n _s (T), and n _i (T) are the refractive indexes for the respective wavelengths of the difference frequency light, the signal light, and the idler light at the temperature T, respectively.

The temperature control unit 230 heats the first wavelength conversion element 210 and the second wavelength conversion element 220 to the temperature T. The temperature T is a degenerate temperature T ₀ when the second wavelength conversion element 220 performs OPO oscillation. Control to be higher. When the second wavelength conversion element 220 oscillates OPO, pump light having a pump wavelength λ _p is converted from idler light having an idler wavelength λ _{i ′} (λ _i ≠ λ _{i ′} ) and a signal wavelength λ _{s ′} (λ _s ≠ λ _{s ′} ) refers to wavelength conversion to signal light having λ. Here, if the pump wavelength lambda _p and idler wavelength lambda _{i 'the} signal wavelength lambda _s' and but satisfying the relation _{1 / λ p = 1 / λ} s '+ 1 / λ i' and the relationship λ _{s '=} λ _i' temperature of is degenerate temperature T _0.

  Such a temperature control unit 230 may be a heater, a Peltier temperature controller, or the like provided with a temperature control mechanism (internal or external), as long as it can be heated to a temperature range of 40 ° C. or higher and 200 ° C. or lower.

The inventors of the present application conducted a parametric oscillation of the second wavelength conversion element 220 for the purpose of the difference frequency oscillation under the predetermined conditions by the pump light incident on the first wavelength conversion element 210, which reduced the oscillation efficiency. I found it connected. Accordingly, the temperature T of the first wavelength conversion element 210 and the second wavelength conversion element 220 described above is set to be higher than the degenerate temperature T ₀ when the second wavelength conversion element 220 performs OPO oscillation. Since the OPO oscillation of the wavelength conversion element 220 is suppressed, the overall oscillation efficiency can be improved.

More specifically, as shown in FIG. 2B, pump light having a pump wavelength λ _p and a pump frequency ω _p incident on the first wavelength conversion element 210 is converted into idler in the first wavelength conversion element 210. Wavelength conversion is performed into idler light having a wavelength λ _i and an idler frequency ω _i and signal light having a signal wavelength λ _s and a signal frequency ω _s .

Next, the wavelength-converted idler light and signal light are incident on the second wavelength conversion element 220, the idler light is transmitted through the second wavelength conversion element 220, and the signal light is transmitted through the second wavelength conversion element 220. At 220, wavelength conversion is performed into difference frequency light having difference frequency wavelength λ _d and difference frequency frequency ω _d and idler light having idler wavelength λ _i and idler frequency ω _i .

Here, since the temperature control unit 230 controls the temperature T of the first wavelength conversion element 210 and the second wavelength conversion element 220 to be higher than the degenerate temperature T ₀ , the second wavelength conversion element 220 is Even if any of pump light, idler light and signal light is incident, OPO oscillation does not occur.

As a result, the pump frequency ω _p of the pump light incident on the first wavelength conversion element 210 is equal to the ideal idler frequency ω _i of the idler light converted by the first wavelength conversion element 210 and the second wavelength. This is the sum of the idler frequency ω _i of the idler light converted by the conversion element 220 and the difference frequency frequency ω _d of the difference frequency light converted by the second wavelength conversion element 220. Here, depending on the second period lambda ₂ of the adjustment of the first period lambda ₁ and the second wavelength conversion element 220 of the first wavelength converting element 210, and a difference frequency frequency omega _d and idler frequency omega _i, It can also be made substantially equal. In such a case, the theoretical conversion efficiency is 100%.

Here, when the laser 200 is a laser that emits mid-infrared light, at least the idler wavelength λ _{i of the} idler light has a mid-infrared (for example, 3 μm to 3.5 μm). The wavelength λ _p , the first period Λ ₁ and / or the second period Λ ₂ may be designed.

The first period Λ ₁ and / or the second period Λ ₂ may be a fixed value or may have a certain width. For example, when having a certain width, the periodic polarization inversion structure of the first wavelength conversion element 210 and / or the second wavelength conversion element 220 is a fan-out structure. With the fan-out structure, the oscillation wavelength of the first wavelength conversion element 210 and / or the second wavelength conversion element 220 is variable according to the width of the first period Λ ₁ and / or the second period Λ _2. Therefore, a tunable variable wavelength laser can be provided by providing a slide mechanism or the like and selecting a desired period for the first wavelength conversion element 210 and the second wavelength conversion element 220.

  The first wavelength conversion element 210 and / or the second wavelength conversion element 220 is a ferroelectric single crystal that can form a periodically poled structure and has a 180 ° domain, but preferably has a ratio composition niobium. Lithium oxide, substantially stoichiometric lithium tantalate, substantially stoichiometric lithium niobate containing at least one element selected from the group consisting of Mg, Zn, Sc and I, and Mg, It consists of a single crystal selected from the group consisting of lithium tantalate having a substantially stoichiometric composition containing at least one element selected from the group consisting of Zn, Sc and In. If these single crystals are used, high-quality single crystals can be obtained, large-diameter elements can be provided, laser damage resistance can be provided, and high oscillation efficiency can be achieved.

In the present specification, the phrase “having a stoichiometric composition” substantially means a mole of Li ₂ O / (Ta ₂ O ₅ + Li ₂ O) or Li ₂ O / (Nb ₂ O ₅ + Li ₂ O). Although the fraction is not completely 0.50, the composition Li ₂ O / (Ta ₂ O ₅ + Li ₂ O) or (Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) closer to the stoichiometric ratio than the congruent composition. ) = 0.490 to 0.5), and the deterioration of the device characteristics due to this is such an extent that it does not become a problem in normal device design. Such a lithium tantalate or lithium niobate single crystal having a substantially stoichiometric composition can be produced, for example, by the Czochralski method using a double crucible described in Japanese Patent No. 3551242.

  In addition, by including at least one element selected from the group consisting of Mg, Zn, Sc and I, optical damage can be reduced, so that the oscillation efficiency can be improved. The addition amount is preferably in the range of 0.1 mol% or more and 3.0 mol% or less from the viewpoint of reducing photodamage.

  The periodically poled structures of the first wavelength conversion element 210 and the second wavelength conversion element 220 are manufactured by, for example, an electron beam irradiation method or an electric field application method known from Japanese Patent No. 3511204 using lithography.

  The first wavelength conversion element 210 and the second wavelength conversion element 220 may be made of a single single crystal. Thereby, since the resonator length can be shortened, further miniaturization is enabled.

The first wavelength conversion element 210 and / or the second wavelength conversion element 220 is a lithium tantalate having a substantially stoichiometric composition containing at least one element selected from the group consisting of Mg, Zn, Sc and In Preferably, the first period Λ ₁ is in the range of 30.5 μm to 31.5 μm, and the second period Λ ₂ is in the range of 32.5 μm to 33.15 μm. It is. Thereby, the laser 200 can emit mid-infrared light in the wavelength range of 3 μm to 3.5 μm when the pump wavelength λ _p has about 1 μm (exemplarily a YAG laser of 1064 nm).

More preferably, the period of the periodically poled structure of the first wavelength conversion element 210 and the second wavelength conversion element 220 is fixed, and the first period Λ ₁ is from 30.8 μm to 31.2 μm. The second period Λ ₂ is selected from the range of 32.7 μm to 32.9 μm. Thereby, the laser 200 can emit mid-infrared light having a wavelength of 3.15 μm or more and 3.25 μm or less, and is particularly advantageous for laser ultrasonic inspection of CFRP.

Further, in this case, the temperature T is controlled in a temperature range of 65 ° C. or more and 150 ° C. or less. Within this range, the second wavelength conversion element 220 becomes higher than the degenerate temperature T ₀ when OPO oscillation occurs, so that OPO oscillation can be suppressed and the oscillation efficiency of the laser 200 can be increased.

Alternatively, the period of the periodic polarization inversion structure of the first wavelength conversion element 210 is fixed, the periodical polarization inversion structure of the second wavelength conversion element 220 is a fan-out structure, and the first period Λ ₁ is 30. It is selected from the range of 8 μm or more and 30.95 μm or less, and the second period Λ ₂ satisfies the range of 32.7 μm or more and 33.15 μm or less. As a result, the laser 200 generates mid-infrared light in the wavelength range of 3 μm to 3.5 μm simply by appropriately selecting the temperature T and / or the second period Λ _{2 of} the second wavelength conversion element 220. The wavelength tuning can be performed.

Further, in this case, the temperature T is higher than the degenerative temperature T ₀ when the second wavelength conversion element 220 performs OPO oscillation in the temperature range of at least room temperature (30 ° C.) to 100 ° C. Oscillation can be suppressed and the oscillation efficiency of the laser 200 can be increased. Although the case where the periodic polarization reversal structure of the second wavelength conversion element has a fan-out structure is illustrated, the periodic polarization reversal structure of the first wavelength conversion element is a fan-out structure, and the periodic polarization of the second wavelength conversion element It goes without saying that even if the period of the inversion structure is fixed, the laser can be similarly tuned. Further, these may be combined to form a more complex tunable laser.

In FIG. 2A, the laser 200 of the present invention includes an input coupler 240 and an output coupler 250 in addition to the first wavelength conversion element 210, the second wavelength conversion element 220, and the temperature control unit 230. Thereby, a resonator can be constituted. Further, a filter 260 is installed on the light emission side of the laser 200 of the present invention, so that light having a desired wavelength (here, idler light having an idler wavelength λ _i and difference frequency wavelength λ _d is provided). Only the difference frequency light) can be taken out.

(Embodiment 2)
In Embodiment 2, a laser ultrasonic flaw detector according to the present invention will be described.
FIG. 3 is a schematic diagram showing a laser ultrasonic flaw detector according to the present invention.

The laser ultrasonic flaw detector 300 according to the present invention includes at least a pump source 310 that emits pump light having a pump wavelength λ _p , and wavelength-converts the pump light from the pump source 310 into idler light having an idler wavelength λ _i , A laser 200 for irradiating the test object 320 is provided. Here, since the laser 200 is the same as the laser 200 described in Embodiment 1, the description thereof is omitted.

The excitation source 310 is not limited as long as the light having the pump wavelength λ _p is converted into the light having the idler wavelength λ _i in the laser 200, but illustratively a YAG laser or Nd that emits light having the wavelength 1064 nm : YVO ₄ laser. These are available and are excellent as excitation sources. If such a laser is used for the excitation source 310, mid-infrared light having a wavelength in the range of 3 μm to 3.5 μm can be obtained as idler light. Such mid-infrared light has high sensitivity without causing damage to the test object 320 when the test object 320 is a carbon fiber reinforced plastic (CFRP), and flaw detection, flaws, cracks, etc. in the test object 320 are detected. Is preferable in order to make possible. In FIG. 3, the excitation source 310 and the laser 200 function as a pulse laser that generates vibrations in the test object 320.

  The laser ultrasonic inspection device 300 according to the present invention preferably further includes another laser 330 that detects the vibration of the test object 320 generated by the pulse laser. For example, the laser 330 can be a laser with a stable frequency, such as a He—Nd laser, a fiber laser, and a feedback semiconductor laser.

  In FIG. 3, the laser ultrasonic flaw detector 300 according to the present invention further includes an analysis unit 340 that detects and analyzes ultrasonic waves generated in the test object 320. The analysis unit 340 includes optical components used for light detection, such as a Fabry-Perot interferometer, a photodiode, and an amplifier, and a central processing unit that performs ultrasonic image analysis, etc., and visualizes the results with an oscilloscope, etc. A display unit may be provided.

  Although not shown in FIG. 3, it goes without saying that optical systems such as a mirror, a collimating lens, a condenser lens, a beam splitter, a quarter-wave plate, and a filter may be included.

  The operation of the laser ultrasonic inspection device 300 according to the present invention will be briefly described. Here, for easy understanding, the excitation source 310 is a YAG laser having a wavelength of 1064 nm, the laser 200 converts the YAG laser into idler light having an idler wavelength of 3.2 μm, and the test object 320 is CFRP. It is assumed that the laser 330 is a He—Nd laser.

The laser ultrasonic flaw detection apparatus 300, a pump light having a pump wavelength lambda _p that the excitation source 310 is emitted (YAG laser) is incident on a laser 200, a laser 200, idler light having an idler wavelength of about 3.2μm to YAG laser Convert to This idler light functions as a pulse laser.

  The pulse laser from the laser 200 is irradiated onto the surface of the test object 320 via an optical system such as a beam splitter (not shown) or a mirror (not shown). When the surface of the test object 320 is irradiated with a pulse laser, a broadband ultrasonic wave is generated on the surface of the test object 320. This ultrasonic wave propagates from the surface of the test object 320 to reach the back surface, is reflected, and returns to the surface as an echo again. A part of the pulse laser may be guided to the analysis unit 340 by a beam splitter and used as a trigger signal for the oscilloscope. Here, in the present invention, when the test object 320 is CFRP, the laser 200 emits a mid-infrared pulse laser having a wavelength of 3.2 μm, so that ultrasonic waves can be generated without causing damage to the CFRP. it can.

  On the other hand, the laser 330 functions to detect an echo that has returned to the surface of the test object 320. However, a continuous wave laser emitted from the laser 330 includes a mirror (not shown) and a beam splitter (not shown). The surface of the test object 320 is irradiated through an optical system such as a quarter-wave plate (not shown). The reflected light is again incident on the Fabry-Perot interferometer and photodiode of the analysis unit 340 through an optical system such as a quarter-wave plate, a beam splitter, and a lens (not shown), and is converted into an electrical signal. The The electric signal is appropriately amplified by an amplifier or the like to remove low frequency noise and input to an oscilloscope.

  When the surface of the test object 320 is ultrasonically vibrated by the echo reflected from the back surface of the test object 320, the spatial position of the surface is displaced with the period of the ultrasonic wave. Therefore, when the laser (light) from the laser 330 is reflected by the surface of the test object 320, the wavelength changes due to Doppler shift accompanying displacement. Such a change in wavelength can be converted into a change in intensity by passing it through, for example, a Fabry-Perot interferometer in the analysis unit 340.

  When an echo that repeatedly reflects on the front and back surfaces of the test object 320 returns to the front surface, the output signal of, for example, a photodiode of the analysis unit 340 changes greatly. If this signal is displayed on an oscilloscope, for example, the presence or absence of defects such as cavities, cracks, and impurities of the test object 320 can be determined. When the signal intensity changes at regular time intervals, it can be determined that the test object 320 has no defect. When the signal intensity changes in a time shorter than a certain time interval, it can be determined that the test object 320 has a defect and the echo is reflected at the defective part.

  In the second embodiment, the case where a Fabry-Perot interferometer is used as an echo detection method has been described. However, the present invention is not limited to this. For example, a Michelson interferometer or a dynamic hologram may be combined.

  The present invention will now be described in detail using specific examples, but it should be noted that the present invention is not limited to these examples.

[Example 1]
In Example 1, the first wavelength conversion element (210 in FIG. 2) and the second wavelength conversion element (220 in FIG. 2) are lithium tantalate single crystals (Mg: The first wavelength Λ ₁ of the first wavelength conversion element is a prismatic shape having a length of 30.9 μm, 3 mm × 3 mm × 24 mm, and the second period Λ ₂ of the second wavelength conversion element. Was a prismatic shape having a length of 32.88 μm, 3 mm × 3 mm × 20 mm, and a laser in which these were arranged in the heater as a single temperature control unit (230 in FIG. 2) was constructed. The first wavelength conversion element was an OPO element, and the second wavelength conversion element was a DFM element. In the drawing, the first wavelength conversion element may be abbreviated as OPO, and the second wavelength conversion element may be abbreviated as DFM.

  Here, the refractive index dispersion formula for Mg: SLT (see, for example, Dolev et al., Appl. Phys. B, 2009, 96, 423-432, and Lim et al., Japan Journal of Applied Physics, 52, 2013, 032601) Was used to calculate the relationship between the oscillation wavelength and the phase matching temperature when the pump wavelength was 1.064 μm. The results are shown in FIG.

  FIG. 4 is a diagram showing an optical system used in the experiment.

With the optical system shown in FIG. 4, the relationship between the temperature and the energy of the output light and the relationship between the energy of the pump light and the energy of the output light were examined. The resonator used in the experiment was configured as a single resonance oscillator (SRO) for signal waves. The resonator mirror was a parallel plate type, and the resonator length was about 110 mm. The light source (excitation source) of the pump light was a Q-SW type YAG laser (Quantel Centurion laser). The wavelength of the pump light was 1.064 nm, the pulse width was 9 ns (FWHM), the repetition frequency was 100 Hz, and the pulse energy was 28.8 mJ. The beam of pump light had a top hat shape with a waist diameter of 2.4 mmφ (1 / e ² ).

  The pump light beam (pulse light) is incident on the resonator through an optical system for beam shaping and light amount adjustment. Next, the emitted light that has been wavelength-converted by the resonator is passed through an infrared light separation filter, passed through a Spectrogen LP2500 filter (cuts light having a wavelength shorter than 2500 nm), and only mid-infrared light is extracted. 12A-P Detected with a sensor. Part of the OPO light (1.9 to 2.4 μm band) of the second wavelength conversion element (DFM element) is taken out as reflected light of the idler separation filter. Detected with a PE50 type sensor. The results are shown in FIG. 7 and FIG.

[Comparative Example 2]
In Comparative Example 2, the first period Λ ₁ of the first wavelength conversion element is 31.1 μm, the second period Λ ₂ of the second wavelength conversion element is 32.85 μm, both of which are 3 mm × It was the same as in Example 1 except that it was a prismatic shape with a length of 3 mm × 20 mm and was arranged in a separate temperature control unit.

Similar to Example 1, the relationship between the oscillation wavelength and the phase matching temperature when the pump wavelength was 1.064 μm was calculated. Further, using the experimental system shown in FIG. 4, the relationship between the temperature and the energy of the output light and the relationship between the energy of the pump light and the energy of the output light were examined. The pulse energy of the pump light was 36 mJ, and the pump light beam had a waist diameter of 1.9 mmφ (intensity 1 / e ² ). These results are shown in FIG. 6, FIG. 9, and FIG.

[Example 3]
In Example 3, the first period Λ ₁ of the first wavelength conversion element is a prismatic shape of 3 mm × 3 mm × 24 mm length of 30.9 μm, 90.95 μm, 31.0 μm, 31.05 μm and 31.1 μm, respectively. A second wavelength conversion element having a second period Λ ₂ of 32.6 mm, 32.73 mm, 32.78 mm, 32.82 mm, 32.85 mm, and 32.88 mm, each having a length of 3 mm x 3 mm x 20 mm A laser was constructed in which these were arranged in a single temperature control unit (230 in FIG. 2). Similar to Example 1, the relationship between the oscillation wavelength and the phase matching temperature when the pump wavelength was 1.064 μm was calculated. The results are shown in FIG.

[Example 4]
In Example 4, the first period Λ ₁ of the first wavelength conversion element is a prismatic shape of 3 mm × 3 mm × 24 mm length of 30.8 μm, and the second period Λ ₂ of the second wavelength conversion element is 32. A laser having a prismatic shape of 3 mm × 3 mm × 20 mm long having a fan-out structure satisfying a range of .70 μm to 33.15 μm and arranged in a single temperature control unit (230 in FIG. 2) was constructed. Similar to Example 1, the relationship between the oscillation wavelength and the phase matching temperature when the pump wavelength was 1.064 μm was calculated. The results are shown in FIG.

[Example 5]
Example 5 was the same as Example 4 except that the _first period Λ ₁ of the first wavelength conversion element was 30.85 μm. Similar to Example 1, the relationship between the oscillation wavelength and the phase matching temperature when the pump wavelength was 1.064 μm was calculated. The results are shown in FIG.

[Example 6]
Example 6 was the same as Example 4 except that the _first period Λ ₁ of the first wavelength conversion element was 30.9 μm. Similar to Example 1, the relationship between the oscillation wavelength and the phase matching temperature when the pump wavelength was 1.064 μm was calculated. The results are shown in FIG.

[Example 7]
Example 7 was the same as Example 4 except that the _first period Λ ₁ of the first wavelength conversion element was 30.95 μm. Similar to Example 1, the relationship between the oscillation wavelength and the phase matching temperature when the pump wavelength was 1.064 μm was calculated. The results are shown in FIG.

  FIG. 5 is a graph showing the relationship between the oscillation wavelength of the laser of Example 1 and the phase matching temperature.

According to FIG. 5, at a single temperature of 80 ° C. (T), the first wavelength conversion element and the second wavelength conversion element also have a phase matching condition with an oscillation wavelength (ie, idler wavelength) of 3.24 μm. It is shown to emit idler light of 3.24 μm. Furthermore, although parametric oscillation was observed due to the second wavelength conversion element, it was found that the degenerate state occurred at 64 ° C. (T ₀ ), and no parametric oscillation occurred at higher temperatures.

In the first wavelength conversion element, the relationship between the pump wavelength λ _p (1.064 μm), the idler wavelength λ _i (3.224 μm), and the signal wavelength λ _s (1.584 μm) is 1 / λ _p = 1 / λ _s. Satisfying + 1 / λ _i , and the second wavelength conversion element has a relationship of a signal wavelength λ _i (1.584 μm), a difference frequency wavelength λ _d (3.098 μm), and an idler wavelength λ _i (3.242 μm). / Λ _s = 1 / λ _i + 1 / λ _d was satisfied. On the other hand, in the degenerate state, the relationship between the pump wavelength λ _p (1.064 μm), the idler wavelength λ _{i ′} (2.128 μm), and the signal wavelength λ _{s ′} (2.128 μm) is 1 / λ _p = 1 / λ _{s. “} + 1 / λ _i” and the relationship λ _{s ′} = λ _{i ′} were satisfied.

  FIG. 6 is a graph showing the relationship between the oscillation wavelength of the laser of Example 1 of Comparative Example 2 and the phase matching temperature.

  According to FIG. 6, when the temperature of the first wavelength conversion element is 30 ° C. and the temperature of the second wavelength conversion element is 80 ° C., the oscillation wavelength (ie, idler wavelength) is 3.23 μm and phase matching is performed. It is shown that the condition is satisfied, that is, the idler light of 3.23 μm is emitted. Furthermore, although parametric oscillation was observed due to the second wavelength conversion element, it was found that the degenerate state occurred at 71 ° C., and no parametric oscillation occurred at higher temperatures. In this case, since it is necessary to control the first wavelength conversion element and the second wavelength conversion element to different temperatures by separate heaters or the like, the laser cannot be reduced in size.

  Furthermore, according to FIG. 6, at a single temperature of 56 ° C., the first wavelength conversion element and the second wavelength conversion element also have a phase matching condition with an oscillation wavelength (that is, idler wavelength) of 3.19 μm, 3. It is shown to emit idler light of 19 μm, but in this case, the operating temperature (56 ° C.) is lower than the degenerate temperature (71 ° C.) when the second wavelength conversion element parametrically oscillates. Emits parametrically oscillated light (2.8 μm and 1.99 μm here) in addition to the 3.19 μm idler light, so the conversion efficiency to mid-infrared light is significantly reduced. Is shown to do.

  FIG. 7 is a diagram showing the temperature dependence of the energy of the output light of the laser of Example 1. In FIG.

  FIG. 7 shows the temperature dependence of the energy of idler light (output light) having an idler wavelength of 3.24 μm from the laser when the energy of the pump light is 28.8 mJ. According to FIG. 7, the idler light has the highest energy in the vicinity of the temperature of 80 ° C. This agreed well with the results shown in FIG.

  FIG. 8 is a diagram illustrating the relationship between the energy of pump light and the energy of output light of the laser according to the first embodiment.

  In FIG. 8, when the temperature controller heats the first wavelength conversion element and the second wavelength conversion element to 40 ° C. or 80 ° C., the energy of the pump light and the idler light having the idler wavelength of 3.24 μm (output light) ). According to FIG. 8, the energy of idler light at a temperature of 80 ° C. was significantly larger than that at a temperature of 40 ° C. This is in good agreement with the results shown in FIG. 5 and indicates that the first wavelength conversion element and the second wavelength conversion element are controlled at a single temperature and function as a laser emitting mid-infrared light. .

  FIG. 9 is a diagram showing the temperature dependence of the energy of the output light of the second wavelength conversion element in the laser of Comparative Example 2.

  FIG. 9 shows the temperature dependence of the energy of idler light (output light) having an idler wavelength of 3.23 μm from the second wavelength conversion element when the energy of the pump light is 36 mJ. According to FIG. 9, the idler light has the highest energy in the vicinity of the temperature of 80 ° C. This agreed well with the results shown in FIG.

  FIG. 10 is a diagram showing the relationship between the energy of the pump light and the energy of the output light of the laser of Comparative Example 2.

  In FIG. 10, energy of pump light and idler light (output light) having an idler wavelength of 3.23 μm when the first wavelength conversion element is heated to 30 ° C. and the second wavelength conversion element is heated to 80 ° C. The relationship is shown. For reference, FIG. 10 shows the result of only the first wavelength conversion element, that is, the energy of the pump light in the first wavelength conversion element when the first wavelength conversion element is maintained at room temperature (30 ° C.). And the relationship between idler light having an idler wavelength of 3.23 μm.

  According to FIG. 10, the energy of idler light is increased by using the first wavelength conversion element and the second wavelength conversion element, compared with the case of only the first wavelength conversion element, and the OPO and DFM It can be seen that photon recycling is effective. Although FIG. 8 and FIG. 10 cannot be simply compared, since a similar output energy can be obtained at a single temperature, the laser controlled at a single temperature of the present invention is effective, and separate temperature control is possible. It was shown that it is unnecessary and can be miniaturized.

From the above, in the laser according to the present invention, the first wavelength conversion element having the periodic polarization inversion structure having the first period Λ ₁ and the second wavelength conversion having the periodic polarization inversion structure having the second period Λ _2. And a single temperature controller that controls the temperature by heating the element to a temperature T. The temperature controller degenerates the temperature T when the second wavelength conversion element performs OPO oscillation. it has been shown to control to be higher than the temperature T ₀ is valid.

  FIG. 11 is a diagram illustrating the relationship between the laser oscillation wavelength and the phase matching temperature according to the third embodiment.

According to FIG. 11, the temperature (T) at which the curve of the phase matching condition of the first wavelength conversion element (OPO) and the curve of the phase matching condition of the second wavelength conversion element (DFM) intersect is set. Thus, it is understood that both the first wavelength conversion element and the second wavelength conversion element satisfy the phase matching condition and can emit mid-infrared light in the vicinity of a wavelength of 3.2 μm at a single operating temperature (T). . Furthermore, high conversion efficiency can be ensured only by selecting a phase matching condition that satisfies the condition that the operating temperature T is higher than the degenerate temperature (T ₀ ) when the second wavelength conversion element performs OPO oscillation. It was shown that it can be done.

Specifically, when both the first wavelength conversion element and the second wavelength conversion element are made of Mg: SLT single crystal, the first period Λ ₁ is in the range of 30.8 μm to 31.2 μm, It can be seen that the second period Λ ₂ may be in the range of 32.7 μm or more and 32.9 μm or less. Furthermore, in this case, the temperature T is higher than the degenerate temperature (T ₀ ) in the case of parametric oscillation of the second wavelength conversion element if the temperature is in the range of 65 ° C. or higher and 150 ° C. or lower. It has been found that a laser emitting 3.2 μm mid-infrared light can be provided.

FIG. 12 is a diagram illustrating the relationship between the laser oscillation wavelength and the phase matching temperature according to the fourth embodiment.
FIG. 13 is a diagram illustrating the relationship between the laser oscillation wavelength and the phase matching temperature according to the fifth embodiment.
FIG. 14 is a graph showing the relationship between the laser oscillation wavelength and the phase matching temperature according to Example 6.
FIG. 15 is a graph showing the relationship between the laser oscillation wavelength and the phase matching temperature according to Example 7.

  12 to 15 show the results in the range where the phase matching temperature is 30 ° C. or higher and 100 ° C. or lower. 12 to 15 show that the temperature of the first wavelength conversion element and the second wavelength conversion element (that is, the phase matching temperature in the figure) is the parametric oscillation of the second wavelength conversion element. It is designed to be higher than the degenerate temperature in the case of performing the calculation, and is calculated under the condition that the parametric oscillation of the second wavelength conversion element is not seen. 12 to 15, the idler wavelength is a condition for both light having an idler wavelength based on parametric oscillation by the first wavelength conversion element and light having an idler wavelength due to difference frequency oscillation by the second wavelength conversion element. Meet.

12 to 15, the first period of the first wavelength conversion element is fixed to a value selected from the range of 30.8 μm or more and 30.95 μm or less, and the second period of the second wavelength conversion element is set. Is a fan-out structure having a width satisfying a range of 32.7 μm or more and 33.15 μm or less, so that at least a range of 3 ° C. Can emit outside light. The laser according to the present invention emits mid-infrared light in the wavelength range of 3 μm or more and 3.5 μm or less simply by appropriately selecting the temperature T and / or the second period Λ ₂ of the second wavelength conversion element. It was shown that wavelength tuning is possible.

  If a temperature at which the wavelength of the idler light and the wavelength of the difference frequency light intersect is adopted, a conversion efficiency of 100% can be achieved theoretically. However, for example, when CFRP is used as a test object, the difference frequency light also has a mid-infrared wavelength range, so that it does not significantly affect the accuracy in laser ultrasonic flaw detection, so strict control is required. There may be no.

  In Examples 4 to 7, the periodic polarization inversion structure of the second wavelength conversion element is designed to have a fan-out structure, but it goes without saying that the laser can be similarly tuned even if the reverse is true. . Further, these may be combined to form a more complex tunable laser.

  The laser according to the present invention is particularly advantageous for oscillation of mid-infrared laser light using wavelength conversion and is a small laser because only one temperature control unit is required. Such a laser is applied to an ultrasonic flaw detector and is preferable for detecting a defect in CFRP.

DESCRIPTION OF SYMBOLS 200 Laser 210 1st wavelength conversion element 220 2nd wavelength conversion element 230 Temperature control part 240 Input coupler 250 Output coupler 260 Filter 300 Laser ultrasonic flaw detector 310 Excitation source 320 Test object 330 Another laser 340 Analysis part

Claims (12)

A first wavelength conversion element having a periodically poled structure having a _first period Λ _{1, wherein} at a temperature T, pump light having a pump wavelength λ _p is converted into idler light having an idler wavelength λ _i and signal wavelength λ wavelength conversion to signal light having _s , where the pump wavelength λ _p , the idler wavelength λ _i, and the signal wavelength λ _s satisfy the relationship 1 / λ _p = 1 / λ _s + 1 / λ _i A first wavelength conversion element;
A second wavelength conversion element having a periodic polarization reversal structure having a _second period Λ ₂ , wherein the signal light having the signal wavelength λ _s at the temperature T is converted into a difference frequency having a difference frequency wavelength λ _d. Wavelength conversion to light and the idler light having the idler wavelength λ _i , where the signal wavelength λ _i , the difference frequency wavelength λ _d, and the idler wavelength λ _i are related 1 / λ _s = 1 / a second wavelength conversion element satisfying λ _i + 1 / λ _d ;
A temperature control unit that heats the first wavelength conversion element and the second wavelength conversion element and controls temperature;
The temperature control unit converts the temperature T of the first wavelength conversion element and the second wavelength conversion element, and pump light having the pump wavelength λ _p by the second wavelength conversion element to the idler wavelength λ wavelength conversion to idler light having _{i ′} and signal light having signal wavelength λ _{s ′} , where the pump wavelength λ _p , the idler wavelength λ _{i ′,} and the signal wavelength λ _{s ′} have the relationship 1 / A laser that controls to be higher than the degenerate temperature T ₀ when satisfying λ _p = 1 / λ _{s ′} + 1 / λ _{i ′} and the relationship λ _{s ′} = λ _{i ′} .   2. The laser according to claim 1, wherein the periodic polarization inversion structure of the first wavelength conversion element and / or the second wavelength conversion element is a fan-out structure.   The first wavelength conversion element and / or the second wavelength conversion element is selected from the group consisting of stoichiometric lithium niobate, substantially stoichiometric lithium tantalate, Mg, Zn, Sc and I A substantially stoichiometric composition of lithium niobate containing at least one element and a substantially stoichiometric composition containing at least one element selected from the group consisting of Mg, Zn, Sc and In The laser according to claim 1, comprising a single crystal selected from the group consisting of lithium tantalate.   The laser according to claim 1, wherein the first wavelength conversion element and the second wavelength conversion element are made of a single single crystal. The first wavelength conversion element and / or the second wavelength conversion element is a lithium tantalate having a substantially stoichiometric composition containing at least one element selected from the group consisting of Mg, Zn, Sc and In Consisting of a single crystal consisting of
The first period Λ ₁ ranges from 30.5 μm to 31.5 μm,
The laser according to claim 2, wherein the second period Λ ₂ is in a range of 32.5 μm to 33.15 μm. The first period Λ ₁ ranges from 30.8 μm to 31.2 μm,
The laser according to claim 5, wherein the second period Λ ₂ is in a range of 32.7 μm to 32.9 μm.   The laser according to claim 6, wherein the temperature T is a temperature in a range of 65 ° C. or higher and 150 ° C. or lower. A laser ultrasonic flaw detector for testing flaw detection of a test object by ultrasonic waves,
An excitation source emitting pump light having a pump wavelength λ _p ;
A laser that converts the wavelength of the pump light from the excitation source into idler light having an idler wavelength λ _i and irradiates the test object with the idler light;
The laser ultrasonic flaw detector, which is the laser according to claim 1.   The laser ultrasonic flaw detector according to claim 8, wherein the laser emits mid-infrared light having a wavelength of 3.15 μm to 3.25 μm.   The laser ultrasonic flaw detector according to claim 8, further comprising an analysis unit that detects and analyzes ultrasonic waves generated in the test object.   The laser ultrasonic flaw detector according to claim 9, wherein the test object is a carbon fiber reinforced plastic. The laser ultrasonic flaw detector according to claim 8, wherein the excitation source is an Nd: YAG laser or an Nd: YVO ₄ laser.