JP7657447B2 - Beam forming device and measuring device - Google Patents

Description

Application of Article 30, Paragraph 2 of the Patent Act (1) Announced on May 11, 2020 at the website address https://www.osapublishing.org/abstract.cfm?uri=CLEO_AT-2020-AF3M.2 (2) Announced on May 15, 2020 at the online conference of the CLEO2020 conference (https://event.crowdcompass.com/cleo20) under presentation number AF3M-2

The present invention relates to a beam forming device for projecting light and a measurement device incorporating the same, and in particular to a beam forming device that forms a Bessel Gauss beam.

Beam forming is a fundamental technology in the fields of three-dimensional measurement and laser processing. In three-dimensional measurement based on laser scanning, the vertical and horizontal spatial resolution is determined by the beam diameter. In conventional beam forming technology, there was a trade-off between the vertical and horizontal spatial resolution and the beam propagation distance. For example, collimated light has been used in three-dimensional laser scanning. The beam diameter of such collimated light is generally on the order of centimeters, and the vertical and horizontal spatial resolution is limited to this, making high-precision three-dimensional measurement difficult. On the other hand, laser scanners such as optical coherence tomography use a focusing system, and although the beam diameter is 1 mm or less, the focal depth (propagation distance) is on the order of centimeters. This trade-off in beam forming is also a problem in laser processing. For example, when drilling a hole in silicon or metal, if the hole size is reduced by the focusing system on the exit side, the aspect ratio decreases, and if the aspect ratio is increased, the hole size increases.

A known optical processing device for optical welding focuses light emitted from the end of an optical fiber into a ring shape at a target location by sequentially making the light incident on an axicon and two collimator lenses (Patent Document 1). A known beam generating device for use in a laser microscope makes collimated light incident on one end of an optical fiber with an axicon attached, and makes the light emitted from the other flat end of the optical fiber incident on a sample via an objective lens (Patent Document 2). A known mode converter for an optical fiber amplifier is also known, which is incorporated in the output section of the optical fiber amplifier and includes an axicon facing the fiber end, and a lens and an aperture for removing highly divergent light (Patent Document 3).

The optical system in Patent Document 1 focuses light into a ring shape, and the depth of the processing area is shallow. The optical system in Patent Document 2 illuminates an area close to the other end of the optical fiber. The optical system in Patent Document 3 forms a diffraction-limited beam.

JP 2008-134468 A Japanese Patent Application Publication No. 7-261027 JP 2015-207001 A

The present invention was made in consideration of the above background technology, and aims to address the trade-off problem in laser beam forming, i.e., to keep the beam diameter narrow while increasing the propagation distance.

The beam forming device for light projection according to one aspect of the present invention comprises an optical fiber having an end that propagates laser light and emits light from the core end, and an axicon that is disposed at the emission end and receives divergent light emitted from the core end of the optical fiber and emits it from a conical surface to form a Bessel-Gauss beam. Here, divergent light means light emitted from an actual or virtual point at a predetermined divergence angle.

In the above beam forming device, a projection beam, which is a Bessel Gauss beam, is generated in front of an axicon placed at the output end. The diameter of the projection beam generated in front of the axicon can be made smaller than that of collimated light, which has a diameter that depends on the lens diameter, and the propagation distance can be made longer than the focal depth of the focusing system beam. This beam forming device has a basic structure that simply causes divergent light emitted from the core end of the optical fiber to enter the axicon, and can achieve such beam forming with an extremely simple configuration. This enables high-precision three-dimensional measurements over long distances in three-dimensional laser scanning, and makes it possible to drill extremely thin holes with high aspect ratios in the field of laser processing.

In a specific aspect of the present invention, the axicon has a plane on which the divergent light emitted from the core end of the optical fiber is incident.

In another aspect of the present invention, all of the divergent light emitted from the core end of the optical fiber is incident on the plane of the axicon. In this case, light loss can be reduced.

In another aspect of the present invention, an optical element is disposed between the core end of the optical fiber and the plane of the axicon. In this case, the state of light entering the axicon can be adjusted. Note that it is not necessary to dispose an optical element between the core end of the optical fiber and the plane of the axicon.

In another aspect of the present invention, the optical element is either a parallel plate or a lens.

In another aspect of the present invention, the exit axis of the optical fiber coincides with the axis of symmetry of the axicon. In this case, it is possible to reduce the disturbance of the projection beam and increase the propagation distance while suppressing an increase in the beam diameter.

In another aspect of the present invention, the wedge angle of the axicon is greater than or equal to 0.1 degrees and less than or equal to 10 degrees. The wedge angle of the axicon affects the propagation distance of the projection beam.

In another aspect of the present invention, the conical surface of the axicon can be formed as a plano-convex or Fresnel type. In the case of a plano-convex type, the effects of diffraction are less likely to occur and concentric lines do not appear. In the case of a Fresnel type, the thickness of the axicon can be reduced, making the axicon lighter.

In another aspect of the present invention, the end of the optical fiber is supported by a ferrule. In this case, it is easy to align the end of the optical fiber with high precision relative to the axicon. In addition, a commercially available optical fiber can be used, and a simple configuration can be achieved.

A measurement device according to one aspect of the present invention includes a light source that outputs intensity-modulated laser light, the above-mentioned beam forming device that receives the laser light from the light source at the incident end of the optical fiber, a light detection device that detects the reflected light that is irradiated onto the target by the beam forming device and reflected from the target, and a signal processing device that determines the distance to the target from the timing of detection of the reflected light by the light detection device and the timing of output of the laser light from the light source.

The above measurement device uses the beam forming device described above, making it possible to achieve high-precision three-dimensional measurements over long distances.

1A and 1B are conceptual diagrams illustrating a beam forming apparatus according to a first embodiment. FIG. 1A is a conceptual diagram illustrating a projection beam emitted from an axicon, and FIG. 1B is a conceptual diagram illustrating a projection beam when a condenser lens is used. FIG. 1A shows a projection beam emitted from an axicon when collimated light is incident thereon, and FIG. 1B is a conceptual diagram comparing a Bessel Gauss beam with other types of beams. 1A and 1B are diagrams illustrating a projection beam when divergent light is made incident on an axicon. 4A to 4F show an image of a cross section of the projection beam along the optical axis and the intensity distribution in the direction perpendicular to the optical axis. 4A to 4F show an image of a cross section of the projection beam along the optical axis and the intensity distribution in the direction perpendicular to the optical axis. FIG. 2 is a conceptual diagram illustrating a distance measurement device incorporating the beam forming device of FIG. 1. 8A and 8B show the measurement results obtained by the distance measuring device of FIG. 1A and 1B are charts illustrating the relationship between the refractive index of an axicon and the diameter or depth of focus of the main lobe, and 1C and 1D are charts illustrating the relationship between the wedge angle of an axicon and the diameter or depth of focus of the main lobe. 8 is a diagram illustrating a modification of the distance measuring device in FIG. 7. FIG. 11 is a conceptual diagram illustrating a beam forming device according to a second embodiment. 13A and 13B are diagrams showing a modified example of a beam forming device. FIG. 13 is a conceptual diagram illustrating a beam forming apparatus according to a third embodiment.

First Embodiment
Hereinafter, a beam forming device for light projection according to a first embodiment of the present invention will be described with reference to FIG.

As shown in FIG. 1A, the beam forming device 100 includes a light source 10, an optical fiber 20, and an axicon 30.

The light source 10 supplies laser light to the incident end 21a of the optical fiber 20. The light source 10 can be omitted, and the combination of the optical fiber 20 and the axicon 30 without the light source 10 is also called the beam forming device 100. The light source 10 is composed of various laser devices including solid-state lasers, gas lasers, semiconductor lasers, etc. and their driving circuits, but is not limited to these and can be various devices that generate laser light. The light source 10 can generate laser light of various wavelengths according to the application, for example, continuous light or pulsed light. The light source 10 may generate laser light of two or more wavelengths within a wavelength band having a predetermined spread. The optical fiber 20 propagates the laser light generated by the light source 10 with low loss, and is selected from those having propagation characteristics according to the wavelength of the laser light generated by the light source 10. The optical fiber 20 is not limited to a single mode fiber, and may be a multimode fiber. The output section (not shown) of the light source 10 and the incident end 21a of the optical fiber 20 are connected by an optical connector (not shown).

Referring to FIG. 1B, the optical fiber 20 is a repurposed commercially available optical fiber, and is made of a coated optical fiber cord 21. The emission end 21b of the optical fiber 20 is supported by a ferrule 25. At the emission end 21b of the optical fiber 20, the fiber body 22 is passed through an insertion hole 25a provided in the ferrule 25 with the sheath removed, exposing the end face 22e and forming the core end face 22b. The core end face 22b corresponds to the end of the core 22a, i.e., the core end. An anti-reflection film is formed on the core end face 22b, but the anti-reflection film is not essential. The core end face 22b may also be inclined with respect to the emission axis.

Returning to FIG. 1A, the axicon 30 receives the divergent light L1 emitted from the core end face 22b of the optical fiber 20 and emits it from the emission face 30b to form the projection beam BP, which is a Bessel Gauss beam. The axicon 30 is formed of, for example, glass or resin with a refractive index of about 1.33 to 1.50, and the entrance face 30a is a flat face and the exit face 30b is a convex conical face. In other words, the axicon 30 is a plano-convex type. An anti-reflection film is formed on the entrance face 30a and the exit face 30b. The divergent light L1 emitted from the core end face 22b of the optical fiber 20 is entirely incident on the flat face of the axicon 30, i.e., the entrance face 30a. This reduces the loss of the divergent light L1. The axicon 30 has a holder 32, and the ferrule 25 of the optical fiber 20 is fixed to the holder 32. By using the ferrule 25, it becomes easy to align the emission end 21b of the optical fiber 20 with the axicon 30 with high accuracy. The axicon 30 is disposed at the emission end 35 of the beam forming device 100, and the projecting beam BP is formed in front of the axicon 30. The wedge angle α of the axicon 30 is about 5 degrees. In addition, the optical axis AX passing through the emission end 21b (i.e., the core end face 22b) of the optical fiber 20 coincides with the axis of symmetry of the axicon 30 and also coincides with the exit axis EX of the optical fiber 20. By aligning the optical axis AX passing through the emission end 21b of the optical fiber 20 with the axis of symmetry of the axicon 30, it is possible to reduce the disturbance of the projecting beam BP and increase the propagation distance while suppressing an increase in the beam diameter.

The material of the axicon 30 will be described in detail. The material of the axicon 30 can be BK7, an optical glass material, which has a refractive index of 1.5 at a wavelength of 1550 nm. The material of the axicon 30 can be synthetic quartz, which has a refractive index of 1.444 at a wavelength of 1550 nm. The material of the axicon 30 can be CYTOP (registered trademark) of AGC, which is a type of amorphous fluororesin, which has a refractive index of 1.3335 at a wavelength of 1550 nm. The material of the axicon 30 can be PMMA, an acrylic resin, which has a refractive index of, for example, 1.47 at a wavelength of 1550 nm, but even the same PMMA has various materials with a refractive index of 1.4. The material of the axicon 30 can be OPTEM's UV-curable resin 1.36RCM, which has a refractive index of 1.36 at a wavelength of 1550 nm, but there are various materials with refractive indices in the 1.3 range. The material of the axicon 30 can be a UV-curable resin used in photolithography 3D printers (see, for example, https://www.researchgate.net/publication/221925173_Fabrication_and_Applications_of_Microfiber/figures?lo=1), which has a refractive index of 1.36 at a wavelength of 1550 nm. Some resins that can be used to manufacture the axicon 30 have a refractive index of 1.36 at a wavelength of 1550 nm (see Zhang, Xueliang, et al. "Compact optical microfiber phase modulator." Optics letters 37.3 (2012): 320-322.).

Figure 2(A) is a conceptual diagram explaining the projection beam BP emitted from the axicon 30, and Figure 2(B) is a conceptual diagram explaining the projection beam BD when an aspheric focusing lens 3 is used instead of the axicon 30. As shown in Figure 2(A), in the case of the projection beam BP using the axicon 30, a strong central peak (main lobe) with a thin beam diameter is formed near the optical axis AX, and multiple weak peripheral peaks (side lobes) are generated at equal intervals around the main lobe. The central peak extends long along the optical axis AX, and the propagation distance corresponding to the focal depth is long. On the other hand, in the case of the projection beam BD using the focusing lens 3, a strong central peak with a thin beam diameter is formed near the optical axis AX, but it is concentrated at the focal position along the optical axis AX, and it can be seen that it can only be used in a distance range limited by the focal depth.

Ｊ_０： 第０次の第１種ベッセル関数
ω_０： ビーム半径
ｎ： アキシコンの屈折率
α： アキシコンのウェッジ角
ｋ： 波数
Ｉ_０： 入射パワー
ｚ： 伝搬距離
ｒ： 半径距離 3A is a diagram for explaining a projecting beam BP formed by the axicon 30 when collimated light is incident on the axicon 30. A Gaussian beam is incident on the axicon 30, and a Bessel Gauss beam is emitted from the axicon 30 as the projecting beam BP. The Bessel Gauss beam is expressed by the following equation.

J ₀ : 0th order Bessel function of the first kind ω ₀ : Beam radius n : Refractive index of the axicon α : Wedge angle of the axicon k : Wave number I ₀ : Incident power z : Propagation distance r : Radial distance

3B is a conceptual diagram comparing a Bessel Gauss beam with other types of beams. The area AR01 on the graph is n=1, and corresponds to a Gaussian beam. The area AR02 on the graph is ω ₀ =∞, and corresponds to a Bessel beam. Furthermore, the dotted area AR00 on the graph is n=1 and ω ₀ =∞, and corresponds to a plane wave. The projector beam BP formed by the beam forming device 100 of the embodiment is a Bessel Gauss beam, n≠1 and ω ₀ ≠∞, and is included in the area AR1 indicated by small dots.

In FIG. 3A, for convenience of explanation, it is assumed that collimated light is incident on the axicon 30. In this embodiment, the divergent light L1 from the output end 21b of the optical fiber 20 is directly incident on the input surface 30a of the axicon 30. In this embodiment, the collimator lens is omitted, and the axicon 30 can be considered to have the function of a collimator lens. The reason for adopting such a configuration will be explained. The propagation distance z of the Bessel Gauss beam is given by ω ₀ /((n-1)·tan(α)), approximately ω ₀ /((n-1)·α), and can be extended by reducing the wedge angle α of the axicon 30. However, it requires a lot of effort and cost to manufacture the axicon 30 with a minute wedge angle α. On the other hand, the divergent light L1 emitted from the core end surface 22b of the optical fiber 20 becomes less bright as it moves away from the optical axis AX, but it is close to a point light source and has high uniformity. Therefore, in the beam forming device 100 of the embodiment, the optical fiber 20 and the convex axicon 30 are combined, that is, the divergent light L1 emitted from the core end face 22b of the optical fiber 20 is made incident on the convex axicon 30. More specifically, in the beam forming device 100, the optical fiber 20 is given a lens effect of negative power that generates the divergent light L1, and the exit face 30b of the axicon 30 is given a converging function, so that the spatial divergence by the optical fiber 20 and the spatial convergence by the axicon 30 are adjusted to cancel each other out, and the residual component is slightly converged. This makes it possible to generate a Bessel-Gauss beam with a long propagation distance. In a specific example, a plano-convex axicon lens (conical lens) is used as the axicon 30, and the wedge angle α is set to 5 degrees. It has been experimentally confirmed that this makes it possible to ensure a propagation distance of about 2 m. In addition, when collimated light is incident on a plano-convex axicon lens to ensure a propagation distance of about 2 m, the wedge angle of the plano-convex axicon lens needs to be very small, about 0.1 degrees, which makes the plano-convex axicon lens extremely expensive.

Figure 4 is a diagram explaining the projector beam BP when divergent light L1 from the core end face 22b of the optical fiber 20 is incident on a plano-convex axicon 30 with a large wedge angle and relatively strong convergence. The projector beam BP is a Bessel-Gauss beam, as in the case of Figure 3(A) using collimated light. The main lobe BPa of the projector beam BP is used for measurement, etc.

Figures 5(A) to 5(F) and 6(A) to 6(F) show an image of the optical axis cross section of the projector beam BP, i.e., the Bessel Gauss beam, formed by the beam forming device 100, and the intensity distribution in the direction perpendicular to the optical axis of the Bessel Gauss beam. The image of the optical axis cross section was taken by photographing the projector beam BP with an InGaAs camera, and the intensity distribution in the direction perpendicular to the optical axis was measured with a scanning slit type beam profiler. In the measurements, a semiconductor laser generating light with a wavelength of 1531.8 nm was used as the light source 10, the diameter of the axicon 30 was 1 inch, and the wedge angle α was 5 degrees. 5(A) and 5(B) show the case where the distance from the apex of the axicon 30 to the measurement point of the InGaAs camera or the like is 10 cm, 5(C) and 5(D) show the case where the distance from the apex of the axicon 30 to the measurement point of the InGaAs camera or the like is 25 cm, and 5(E) and 5(F) show the case where the distance from the apex of the axicon 30 to the measurement point of the InGaAs camera or the like is 50 cm. 6(A) and 6(B) show the case where the distance from the apex of the axicon 30 to the measurement point of the InGaAs camera or the like is 100 cm, 6(C) and 6(D) show the case where the distance from the apex of the axicon 30 to the measurement point of the InGaAs camera or the like is 150 cm, and 6(E) and 6(F) show the case where the distance from the apex of the axicon 30 to the measurement point of the InGaAs camera or the like is 200 cm.

For example, when the distance from the apex of the axicon 30 to a measurement point such as an InGaAs camera is 50 cm, the width of the main lobe (i.e., the central peak) of the projector beam BP is narrower than 300 μm, and when the distance is 200 cm, the width of the main lobe of the projector beam BP is narrower than 1 mm. In other words, it is possible to suppress the diameter of the projector beam BP to 1 mm or less even at a distance of 200 cm, and it is clear that the position resolution in the lateral direction can be significantly increased.

The beam forming device 100 shown in FIG. 1(A) can be applied to laser processing. The distance measuring device 200 of the above embodiment can be used, for example, for drilling extremely fine holes with a high aspect ratio by appropriately selecting the light source 10. There are many needs for hole drilling, and the materials to be drilled are also diverse. For example, chromium molybdenum steel and high speed steel, which have excellent heat resistance, are used for engine fuel injection nozzles, stainless steel, which has excellent corrosion resistance, is used for nozzles for chemical fiber manufacturing, glass, which has excellent transparency and corrosion resistance, is used for the production of microchannels, cemented carbide is used for cold forming and drawing dies, and silicon wafers are used for through wiring for semiconductors.

Figure 7 is a block diagram illustrating a distance measurement device 200 incorporating the beam forming device 100 shown in Figure 1 (A) with appropriate modifications. The distance measurement device 200 includes a light source device 71, a beam forming device 72, a light detection device 74, a waveform comparison device 75, and a signal processing device 78.

The light source device 71 includes a laser diode 71a, an intensity modulator 71b, an optical amplifier 71c, and a signal generator 71d. The laser diode 71a is a light source and emits, for example, a laser beam with a wavelength of 1531.8 nm. The intensity modulator 71b is, for example, an optical modulator using lithium niobate crystal, and performs intensity modulation by receiving an output signal from the signal generator 71d and adjusting the transmittance of the laser beam. The intensity modulation of the laser beam is of a sine wave type, and frequencies such as 150 MHz, 300 MHz, etc. are used. The optical amplifier 71c is, for example, a fiber amplifier, and although detailed explanation is omitted, it has an erbium-doped fiber amplifier, an excitation light source, and a coupler that guides light from the excitation light source to the fiber amplifier. The signal generator 71d is an electric circuit that forms a drive signal of any waveform, including a sine wave.

The beam forming device 72 is a combination of the optical fiber 20 and the axicon 30 of the beam forming device 100 shown in FIG. 1(A). The beam forming device 72 projects the modulated light output from the light source device 71 in front of the axicon 30 as a measurement beam BM. The measurement beam BM is a Bessel Gauss beam, and can reach distances of several meters or more while maintaining a narrow beam diameter. The measurement beam BM has been subjected to sine wave type intensity modulation as described above.

The light detection device 74 includes a light receiving optical system 74a, an optical amplifier 74b, a light selection unit 74c, and a photodiode 74d. The light receiving optical system 74a is composed of a condenser lens, and takes in the reflected light RL from the object OB and condenses and couples it to the fiber end of the optical amplifier 74b. The optical amplifier 74b is the same as the optical amplifier 71c of the light source device 71. The optical selection unit 74c selectively transmits the wavelength component corresponding to the wavelength of the measurement beam BM. The optical selection unit 74c removes background noise and unnecessary light from the optical amplifier 74b. The photodiode 74d detects the reflected light RL amplified by the optical amplifier 74b and passed through the optical selection unit 74c, and outputs a sine wave type measurement signal corresponding to the intensity of the reflected light RL.

The waveform comparison device 75 takes in the drive signal generated by the signal generator 71d of the light source device 71 from the signal branching section 71p, and takes in the measurement signal detected by the photodiode 74d of the light detection device 74. The waveform comparison device 75 compares the drive signal generated by the signal generator 71d of the light source device 71 with the measurement signal detected by the photodiode 74d of the light detection device 74, and determines the phase difference between the drive signal and the measurement signal. The phase difference between the drive signal and the measurement signal corresponds to the difference between the output timing of the transmitted light of the measurement beam BM and the detection timing of the received reflected light RL, and provides information regarding the distance from the distance measurement device 200 to the object OB.

The signal processing device 78 includes a computer and other arithmetic processing unit, and determines the distance from the distance measurement device 200 to the target OB based on the phase difference between the drive signal and the measurement signal determined by the waveform comparison device 75. The signal processing device 78 manages the operation of the laser diode 71a, the signal generator 71d, etc., as well as the operation of the waveform comparison device 75.

Figure 8 (A) is a chart showing the results of an actual distance measurement performed by operating the signal generator 71d at 300 MHz. Here, the distance measured by the distance measurement device 200 is plotted while moving an object OB of a given size in the direction of the optical axis AX to set the actual distance. Figure 8 (B) is a chart showing the results of a distance measurement performed by operating the signal generator 71d at 150 MHz. It can be seen that accurate distance measurement is possible using the intensity-modulated measurement beam BM.

Figure 9 (A) is a chart illustrating the relationship between the refractive index of the axicon 30 and the diameter of the main lobe formed by the axicon 30, and Figure 9 (B) is a chart illustrating the relationship between the refractive index of the axicon 30 and the focal depth achieved by the axicon 30. The above chart is based on a simulation in which collimated light with a beam radius of r (= 1, 2, 5, 10, 20 mm) is incident on the axicon 30, but it is believed that a similar tendency will occur in the state of the light beam depending on the refractive index of the axicon 30 even when diverging light is incident on the axicon 30. In other words, it can be seen that the lower the refractive index of the axicon 30, the longer the focal depth, i.e., the longer the propagation distance.

Figure 9(C) is a chart illustrating the relationship between the wedge angle of the axicon 30 and the diameter of the main lobe formed by the axicon 30, and Figure 9(D) is a chart illustrating the relationship between the wedge angle of the axicon 30 and the focal depth achieved by the axicon 30. In this case, it is assumed that collimated light with a beam radius of r is incident on the axicon 30. When divergent light is incident on the axicon 30 as shown in Figure 1, the axicon 30 can be considered as a basic wedge angle (e.g., about 6 degrees to 10 degrees) that offsets the divergence of the divergent light, to which a small additional wedge angle (e.g., about 1 degree) that adds a slight focusing function is added. In this case, the small additional wedge angle corresponds to the wedge angle of Figures 9(C) and 9(D) for the collimated light. In other words, it can be seen that the focal depth, i.e., the propagation distance, can be increased by setting the additional wedge angle to about 1 degree or less and approaching 0 degrees.

Figure 10 shows a distance measurement device 200 modified from the distance measurement device 200 shown in Figure 7. In this case, a transmissive beam splitting mirror 81 and a scanning mirror 82 are added to enable three-dimensional measurement. The beam splitting mirror 81 shares the optical path of the measurement beam BM with the optical path of the reflected light RL, and the scanning mirror 82 changes the deflection angle when reflecting the measurement beam BM to adjust the projection direction. Although detailed explanations are omitted, the orientation of the scanning mirror 82 is controlled by a drive mechanism 83 with respect to two orthogonal axes, and the signal processing device 78 manages the orientation of the scanning mirror 82 as angle information. The distance measurement device 200 enables three-dimensional measurement.

The distance measuring device 200 shown in FIG. 10 can be applied to a three-dimensional laser scanner used for visual inspection of industrial products. Object shape measurement is in demand in many fields, and is applied to mold shape measurement, visual inspection of processed products, solder shape inspection on printed circuit boards, etc. Conventional laser scanners have used collimated light or focused light. Laser scanners using the former have a long measurement distance but poor vertical and horizontal resolution. Laser scanners using the latter have a vertical and horizontal resolution of 100 μm but a measurement distance of about 1 cm. The distance measuring device 200 of the above embodiment can achieve a vertical and horizontal resolution of 1 mm or less at a medium distance (specifically, several meters), and can be applied to high-precision visual inspection of parts, for example, on a factory production line. Until now, inspection of fine scratches, cracks, etc. on the surface of parts has been performed by engineers' touch and vision, but the practical application of the distance measuring device 200 of the above embodiment is expected to spur the realization of smart factories.

The beam forming device 100 of the first embodiment described above includes an optical fiber 20 having an exit end 21b that propagates laser light and emits light from the core end face 22b, and an axicon 30 that is disposed at the exit end 35 and receives divergent light L1 emitted from the core end face 22b of the optical fiber 20 and emits it from an exit face 30b, which is a conical surface, to form a projection beam BP or a measurement beam BM, which is a Bessel Gauss beam.

In the beam forming device 100, a projecting beam BP, which is a Bessel Gauss beam, is generated in front of the axicon 30. The diameter of the projecting beam BP generated in front of the axicon 30 can be made smaller than that of a collimated light having a diameter that depends on the lens diameter, and the propagation distance can be made longer than the focal depth of the focusing system beam. This beam forming device 100 has a basic structure that simply causes the divergent light L1 emitted from the core end face 22b of the optical fiber 20 to enter the axicon 30, and can achieve such beam forming with an extremely simple configuration. This enables high-precision three-dimensional measurement over a long distance in three-dimensional laser scanning, and enables extremely thin, high-aspect-ratio hole drilling in the field of laser processing.

Second Embodiment
The beam forming apparatus according to the second embodiment will be described below. Note that the beam forming apparatus according to the second embodiment is a partial modification of the beam forming apparatus according to the first embodiment, and a description of the common parts will be omitted.

Referring to FIG. 11, the beam forming device 2100 of the second embodiment has optical elements 41 and 42 arranged between the optical fiber 20 and the axicon 30. The optical element 41 is a rod-shaped light-transmitting member, and the optical element 42 is a parallel plate-shaped light-transmitting member. The optical element 41 is made of, for example, quartz, and can be fused to the end face 22e of the emission end 21b of the optical fiber 20 to be integrated. The optical element 42 is made of, for example, glass or resin, and is sandwiched between the emission face 41b of the optical element 41 and the incidence face 30a of the axicon 30 and joined thereto. By arranging the optical elements 41 and 42 between the optical fiber 20 and the axicon 30, the divergence angle of the light emitted from the emission end 21b of the optical fiber 20 can be suppressed or adjusted. In addition, by arranging the optical elements 41 and 42, the optical surfaces that generate reflected light can be reduced, the process of processing an anti-reflection film is unnecessary, and the loss due to the anti-reflection film can be reduced. The two optical elements 41 and 42 can be an integrated member made of a single material, rather than being separate members. Also, one of the two optical elements 41 and 42 can be omitted.

Figure 12 (A) is a diagram explaining a modified example of the beam forming device 2100 shown in Figure 11, in which a concave lens 45 is arranged instead of the optical elements 41 and 42. Also, Figure 12 (B) is a diagram explaining another modified example, in which a convex lens 46 is arranged instead of the optical elements 41 and 42. In this case, the exit surface 30b of the axicon 30 is not limited to a convex conical surface, but can also be a concave conical surface, depending on the refractive power of the convex lens 46. The concave lens 45 and the convex lens 46 are arranged detachably, making it possible to adjust the beam size and propagation distance of the Bessel Gauss beam formed by the axicon 30.

The axicon 30 can be replaced with an optical device that can change the effective wedge angle (see European Patent Publication No. 2194404). By using such an optical device, the convergence effect of the Bessel Gauss beam emitted from the axicon 30 can be changed.

Third Embodiment
The beam forming apparatus according to the third embodiment will be described below. Note that the beam forming apparatus according to the third embodiment is a partial modification of the beam forming apparatus according to the first embodiment, and a description of common parts will be omitted.

Referring to FIG. 13, in the beam forming device 3100 of the third embodiment, the exit surface 130b of the axicon 30 is a Fresnel type optical surface and functions as a conical surface equivalent to the exit surface 30b shown in FIG. 1(A). By making the exit surface 130b of the axicon 30 a Fresnel surface, the axicon 30 can be made thin and lightweight.

[Variations and Others]
The present invention is not limited to the above-described embodiment, and can be implemented within the scope of the present invention without departing from the spirit and scope of the present invention. For example, the following modifications are also possible.

For example, in the distance measurement device 200 shown in FIG. 10, the light source device 71 can be replaced with a wavelength sweep type pulse generator, and a spatial dispersion device such as a diffraction element can be placed at the output stage of the beam forming device 72. In this case, the scanning mirror 82 can be omitted, or the number of rotation axes of the scanning mirror 82 can be reduced.

In FIG. 7, the light source device 71, the light detection device 74, the waveform comparison device 75, etc. are merely examples and can be replaced by various elements.

The wedge angle α of the axicon 30 is set to 0.1 degrees or more and 10 degrees or less in order to ensure the propagation distance while narrowing the beam diameter, but it can also be set to a range of 10 degrees to 40 degrees.

10...light source, 20...optical fiber, 21...optical fiber cord, 21a...entrance end, 21b...exit end, 22...fiber body, 22a...core, 22b...core end face, 25...ferrule, 25a...insertion hole, 30...axicon, 30a...entrance surface, 30b...exit surface, 32...holder, 35...exit end, 71...light source device, 71a...laser diode, 71b...intensity modulator, 71c...optical amplifier, 71d...signal generator, 71p...signal branching section, 72...beam forming device, 74...optical detection device, 74a...light receiving optical system, 74b...optical amplifier, 74c...light selection section, 74d...photodiode, 75...waveform comparison device, 78...signal processing device, 81...wave-combining/branching mirror, 82...scanning mirror, 83...driving mechanism, 100...beam forming device, 200...distance measuring device, AX...optical axis, BD...projection beam, BM...measurement beam, EX...emission axis, L1...divergent light, RL...reflected light

Claims (8)

an optical fiber that propagates laser light and has an end portion that emits light from a core end;
an axicon disposed at an exit end, which receives divergent light emitted from the core end of the optical fiber and emits the divergent light from a conical surface to form a Bessel-Gauss beam for projection;
an optical element disposed between the core end of the optical fiber and the axicon;
The optical element increases the divergence angle of the divergent light. The beam forming device according to claim 1, wherein the axicon has a plane on which the divergent light emitted from the core end of the optical fiber is incident via the optical element. 3. The beam forming device according to claim 2 , wherein all of the divergent light emitted from the core end of the optical fiber is incident on the plane of the axicon. The beam forming device according to any one of claims 1 to 3, wherein the exit axis of the optical fiber coincides with the axis of symmetry of the axicon. The beam forming device according to any one of claims 1 to 4, wherein the wedge angle of the axicon is 0.1 degrees or more and 10 degrees or less. The beam forming device according to any one of claims 1 to 5, wherein the conical surface of the axicon is formed as a plano-convex or Fresnel type. The beam forming device according to any one of claims 1 to 6, wherein the end of the optical fiber is supported by a ferrule. a light source that outputs intensity-modulated laser light;
a beam forming device according to any one of claims 1 to 7, wherein the laser light from the light source is received at an end of an incident side of the optical fiber;
a light detection device that detects light irradiated onto an object by the beam forming device and reflected from the object;
a signal processing device that determines a distance to the target based on a detection timing of the reflected light by the light detection device and an output timing of the laser light from the light source.

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