JP2006305580A - Method for cutting-off wire material, and method for re-forming end face of wire material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To form a flat end face of the cut-off end face produced by cutting-off a wire material, such as photonic crystal fiber (PCF), and further to achieve an excellent optical property by preventing polishing abrasive grains from intruding into pores exposed on the end face. <P>SOLUTION: The wire material is cut off along a locus of a machining spot caused by a converged light point by forming the converged light point of a femto second laser beam on the surface portion or the inside of the wire material, such as PCF, along a required scanning path. Further, the end face of the wire material is smoothed by irradiating the end face with the femto second laser beam. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a cutting method for irradiating a laser beam to cut the wire in a direction perpendicular or oblique to the axial direction of the wire, and a method for shaping the end face of the wire, and in particular, a photonic crystal fiber (PCF), etc. The present invention relates to an optical fiber cutting method and an end face shaping method. Here, the term “wire material” is used as a term that widely expresses a long and narrow material such as a cable-shaped object such as an optical fiber or a rod-shaped object.

  Conventional optical fibers have a core-cladding structure, and the propagation principle is total reflection. In recent years, research on optical fibers that propagate light based on a principle different from the principle has been actively conducted. In an optical fiber called a photonic crystal fiber (PCF), the principle of a photonic crystal is applied to an optical fiber. PCF has periodic vacancies and is generally formed from pure glass only. There are two types of PCFs: one that propagates light by the same total reflection as before due to the decrease in the equivalent refractive index of the cladding due to holes, and the photonic band gap that results from the periodicity of holes as in the photonic crystal Some (PBG) light propagates. The latter may be referred to as a photonic band gap (PBG) type optical fiber. Since PBG-PCF does not use total reflection, it is not subject to the limitations of conventional optical fibers at various points. For example, since the reflection angle does not need to satisfy the total reflection condition, it can be bent sharply and can be used for high-density optical wiring. Also, an optical fiber in which the core itself is a hole (hollow core) can be realized. Furthermore, even with PCF using total reflection, propagation characteristics that cannot be realized with conventional optical fibers can be obtained. For example, it is possible to obtain characteristics such as a single mode in a wide wavelength band, a single mode and a large cross-sectional area, a large chromatic dispersion, a large relative refractive index, and a high nonlinearity. PCF may also be referred to as a microstructured optical fiber or holey fiber.

These PCFs are different from conventional optical fibers composed of a simple core and cladding. In order to obtain an optical transmission characteristic that is not governed by the material dispersion characteristic, an optical fiber having a higher degree of freedom in designing the transmission characteristic can be obtained by constructing a periodic structural dispersion near the center of the fiber. In recent years, PCF has been actively developed because of such characteristics. Structurally, the PCF has a structure in which a plurality of holes are concentrically arranged around the core. The transmission characteristics can be controlled by appropriately changing the position, size, number, etc. of the holes. As described above, it is superior to the conventional optical fiber in controlling the light to be transmitted, but has a drawback that the fiber is not easily cut. A conventional fiber may be simply cut, and a dedicated cutter (also called a cleaver) is also manufactured. However, since PCF and PBG have a structure with a hollow portion, it is known that the cutting operation to clean the end face is not easy with the conventional cutting method. Further, in the conventional optical fiber, when the end surface is formed obliquely (generally 8 degrees) in order to prevent reflection of the fiber end surface, it is possible to cope with a technique called polishing. However, in the holey fiber, since the abrasive enters the hole, it is basically difficult to adopt a processing method by polishing, and there is no device for cutting the holey fiber obliquely. In addition, the fiber diameters are diversified. In particular, when a large-diameter fiber is formed or when the number of holes is large, it becomes difficult to cut to form a smooth cut surface. Furthermore, it is very difficult to cut a fiber having a small diameter (fiber diameter of about 60 μm or less) by the conventional technique.

  Currently used holey fiber cutting methods employ the same cutting method as conventional optical fibers. In this method, a mechanical strain such as bending and tension is applied to generate stress strain, and a processing trace is formed by cutting or excision using a blade in a part of the stress strain generation region. The crack is propagated and cut by breaking.

  In addition, in order to manufacture an optical fiber of good quality and efficiently perform operations such as cutting and polishing, a method of holding the optical fiber stably and performing cutting and polishing of the cut end face has also been proposed. ing. For example, in the invention described in Japanese Patent Application Laid-Open No. 2004-142016, a rotating blade is mounted on a spindle, and an angle is formed in the axial direction of a fixed optical fiber, which is relatively scanned and cut and polished. I do.

JP 2004-142016 A JP 2003-313045 A Utility Model Registration No. 3015852

  In the method of cleaving by carrying out cleavage fracture starting from the above-mentioned processing mark, if there are many holes in the peripheral part of the core like holey fiber, burrs and pleats on the edge of the hole In addition, there is a problem that the end face is not divided in a uniform plane and an irregular end face is formed. Further, in the method of cutting and polishing using a rotating blade while fixing the optical fiber, the abrasive grains enter the vacancies exposed on the end face, leading to deterioration of the optical characteristics in the vicinity of the end face. There was a problem.

  The present invention has been made in order to solve the above-mentioned problems, and it is possible to obtain a good characteristic with respect to an optical surface and the like for a cutting end surface generated by cutting a wire, and an end surface of the wire. An object is to provide a shaping method.

  In order to solve the above technical problem, according to the present invention, a laser beam of a femtosecond laser is irradiated, and a condensing point of the laser beam is formed on the surface portion or inside of the wire along a predetermined scanning path. And cut the wire. In addition, by moving a condensing point of a laser beam having a transmission wavelength along a scanning path set on a predetermined cross section in the wire, the wire is cut along the cross section. Further, the wire is cut by irradiating a laser beam from the outside of the refractive index matching medium in a state where the wire is installed in the refractive index matching medium.

  According to the present invention, the end face is flattened by irradiating the end face of the wire with a laser beam of a femtosecond laser. Further, the beam distribution intensity of the laser beam applied to the end face is made uniform. Further, a processing promoting gas is applied to the end face irradiated with the laser beam.

  According to the cutting method of the present invention, since the wire is cut by irradiating the femtosecond laser, the modified region is formed by energy absorption based on the multiphoton absorption phenomenon, and along the scanning path of the laser beam. Since the wire can be cut in the direction in which the modified region to be formed extends, a flat cut surface is formed along the processing locus. In addition, since a flat cut surface can be obtained without using abrasive grains, the abrasive grains for polishing do not enter the pores exposed on the end face of a wire such as PCF. Thereby, there is an effect that good optical characteristics can be obtained in a region in the vicinity of the cut end face.

  According to the cutting method of the present invention, the position of the condensing point is moved along a scanning path set on a predetermined cross section in the wire by using multiphoton absorption of the laser beam in the wire using the transparent material. Since the wire is cut along the cross section, the same effect as described above can be obtained, and the wire can be cut without rotating the wire such as PCF relative to the irradiation position of the laser beam. It is also possible to achieve the effect that the structure of the wire cutting device can be simplified.

  According to the cutting method of the present invention, since the laser beam is irradiated so that the condensing point is formed on a cross section having an arbitrary angle with respect to the vertical direction of the axis of the wire, the inclined end face is cut. It can be formed only by the process, and the processing time can be shortened. In addition, since it is configured to irradiate the laser beam with the wire placed in the refractive index matching medium, it is possible to prevent focusing out of focus due to the difference in the refractive index of the incident laser beam on the wire surface. Since it is possible to prevent the power density from being lowered at the point, the laser beam irradiation optical system can be simplified and the processing position accuracy can be improved.

  According to the end face shaping method according to the present invention, since the end face is irradiated with the laser beam of the femtosecond laser, the convex part processing speed by multiphoton absorption is increased and the end face is flattened. There is an effect that it is possible to flatten the end face of the wire which is difficult to polish. Here, the processing efficiency can be improved by making the beam distribution intensity of the laser beam uniform, or by applying a processing promoting gas to the end face irradiated with the laser beam.

  Hereinafter, preferred embodiments of a PCF cutting method and an end face shaping method configured according to the present invention will be described in more detail with reference to the accompanying drawings.

  FIG. 1 is a diagram showing an example of a configuration for carrying out a PCF processing method using a femtosecond laser. The femtosecond laser oscillator 1 outputs a short pulse laser beam having a pulse width of 10 to 500 femtoseconds (fs). The output beam 3 is guided by using a beam transmission optical system 2 such as a mirror, and the output beam 5 is incident on the condensing optical system 6 through a dispersion compensation optical system 4 for correcting the pulse width as necessary. The condensing optical system 6 generates a convergent beam 7 from the incident laser beam 5, and irradiates the converged beam 7 to an optical fiber 8 such as a holey fiber to be processed to form a processing mark 9.

  The base 15 of the processing apparatus has an X stage 12 that moves by a predetermined distance in the X direction 13 according to a control signal 16 output from the control unit 17 and a predetermined distance in the Y direction 11 according to the control signal 16. An XY stage composed of a moving Y stage 10 is placed. The PCF, which is the object to be processed, is attached to the XY stage and can move in the XY plane, and can be rotated around the fiber axis by receiving the operation of the rotating means if necessary. The control unit 17 outputs a control signal for driving the XY stage and, if necessary, the rotating means so that the processing trace formed on the PCF draws a predetermined locus on the PCF or in the PCF. On the other hand, the control unit 17 outputs a control signal 18 to the femtosecond laser oscillator 1 so as to irradiate the processing site with the femtosecond laser having a desired energy in synchronization with the movement of the PCF. Controls oscillation start or stop of irradiation. Reference numeral A shows examples of cross sections of several holey fibers. A large number of lotus-like holes are provided in the cross section of the optical fiber, thereby forming a refractive index distribution or a photonic band gap. A femtosecond laser pulse is irradiated in a non-contact manner on a glass body having such holes to cause the PCF, which is a transparent material, to absorb energy based on a multiphoton absorption phenomenon, thereby performing processing. Although not shown in FIG. 1, it is necessary to consider the movement in the Z-axis direction perpendicular to the XY plane. The optical fiber 8 may be simply moved, but it is also possible to move by controlling the condensing optical system 6. By doing this, three-dimensionally fine processing becomes possible. By doing so, it is possible not only to process the end face of the holey fiber into a flat surface, but also to form a phase with a certain curvature and to form a lens on the end face of the optical fiber. Then, normally, the light emitted from the end face of the optical fiber is coupled to another optical component using a separate coupling lens. However, by providing the end face with the holey fiber with this lens, this extra lens is unnecessary. Become.

  FIG. 2 is a diagram illustrating a processing locus when the PCF is divided. Here, the PCF 21 is cut in a direction perpendicular to the axial direction and divided into an upper part 22 and a lower part 23, and the PCF 21 is cut in an oblique direction with respect to the axial direction to be divided into an upper part 25 and a lower part 26. An example of division will be described. When the PCF 21 is cut in a direction perpendicular to the optical axis, the laser beam is scanned along the path indicated by the broken line 24 so as to be short from the outside toward the surface portion or the inside of the transparent optical fiber 21. Irradiate a pulsed laser beam. Further, when the PCF 21 is cut in a direction oblique to the optical axis, the laser beam is scanned along a path indicated by a broken line 27, for example.

  When the fiber diameter of the PCF is thick, it is necessary to irradiate the surface or the inside of the optical fiber from the outside along the scanning path set so as to form a predetermined processing mark on the outer peripheral surface. Therefore, the fiber is rotated or the irradiation position of the laser beam is rotated around the optical fiber. By focusing the laser beam on the surface or inside of the optical fiber along the scanning path, it is possible to divide the optical fiber along the processing locus as shown by the broken line 24 or the broken line 27.

  When the laser beam is focused on the surface of the optical fiber, if the wavelength of the laser beam is an absorptive wavelength, heat absorption is mainly generated to form a processed groove due to melting action or a modified region due to alteration action Is done. Further, when the wavelength of the laser beam is a transmissive wavelength, multi-photon absorption is mainly generated to form a modified region due to the alteration action. When the laser beam is focused inside the optical fiber, this multiphoton absorption occurs and a modified region is formed. A compressive stress is applied to a portion where the laser beam is focused, and a tensile stress is applied to the peripheral region to generate a residual stress due to thermal stress. In this stress generation region, cracks easily propagate and the optical fiber is divided.

  As described above, in the conventional method, compared with the case where the optical fiber is divided by forming a processing trace on a part of the surface of the optical fiber and generating the cleavage fracture starting from the processing trace, the present invention Then, it became possible to divide the optical fiber along the trajectory of the processing mark formed by irradiating the femtosecond laser along the scanning path. Since the laser beam can be divided along the processing locus, the PCF is irradiated with a laser beam along a scanning path set on a plane inclined from a plane perpendicular to the optical axis of the optical fiber as indicated by a broken line 27. It became possible to form a cut end face inclined at. In addition, regarding the formation of the inclined end face, the present invention irradiates the femtosecond laser in comparison with the conventional method in which the optical fiber is polished in the inclined direction using abrasive polishing after being divided by cleavage fracture. Since the inclined end face can be formed as it is, the processing time can be shortened.

  In the present invention, since the laser beam is irradiated to divide the wire such as PCF, a measure relating to the abrasive grains entering the vacancies appearing on the end face becomes unnecessary. Therefore, the present invention can be applied not only to the PCF but also to many materials that have been difficult to process due to the problem of the shape of the cut end face obtained when cutting.

  If a femtosecond laser is used, a processed part can be formed even inside an optical fiber by forming a modified region based on multiphoton absorption even for optical materials that are transparent to the wavelength of the laser beam to be irradiated. Is possible. Therefore, even if the irradiation position of the laser beam is not relatively rotated with respect to the optical fiber along the outer circumferential direction, the condensing optical system is adjusted to adjust the converging point of the convergent beam on the surface and inside of the optical fiber. The optical fiber can be cut by appropriately moving the position. That is, by controlling the condensing optical system and moving the position of the condensing point along a scanning path appropriately set on a predetermined cross section in the optical fiber, the modified region where the residual stress occurs is surfaced. The optical fiber can be cut along the cross section.

  Since the outer peripheral surface of the optical fiber is cylindrical, in order to form a fine condensing point inside, the condensing optical system should be constructed taking into consideration that the incident surface of the laser beam is a curved surface. May be required. In this case, the convergence degree in the direction orthogonal to the optical axis direction of the optical fiber is corrected. When correction is not performed on the laser beam irradiation side, the incident surface is flattened by immersing the optical fiber in a transparent liquid having a matched refractive index, and the condensed beam is irradiated through the transparent liquid. The method is also effective. Also, by impregnating the hole with a transparent liquid whose refractive index is matched, scattering such as lens effect and laser beam reflection due to the hole can be eliminated, and the femtosecond laser reaches the center of the optical fiber. It becomes possible to make it. The impregnated liquid is removed to the extent that there is no problem in optical reliability by drying or washing after completion of processing. As a result, it is possible to prevent a reduction in power density at the laser condensing point formed inside the optical fiber, so that the laser beam irradiation optical system can be simplified, the processing efficiency can be improved, and the processing position accuracy can be improved.

  Since PCF contains many holes, spatial nonuniformity exists. Even when cutting by laser beam irradiation is performed using a wavelength that causes multiphoton absorption in PCF, a cut end face that is not flat may be formed. In such a case, it is possible to flatten the end face by irradiating the cut end face with a femtosecond laser pulse after performing the dividing process.

  FIG. 3 is a diagram showing an end face shaping method for flattening the end face of the PCF by irradiating a femtosecond laser. In this method, after the intensity distribution in the cross section of the femtosecond laser 30 is converted into a laser beam having a uniformization distribution 36 using a known beam intensity homogenization optical system, the vertical direction 31 or the optical fiber end face 34 or The laser beam is irradiated obliquely. The condensing power density of the laser beam in the irradiation area 32 is set to an appropriate level, and the relative scanning of the end face 34 of the optical fiber 33 and the irradiation beam 30 is performed in an in-plane (XY) 35 parallel to the end face 34. Then, since the processing of the convex portion formed on the end face 34 of the optical fiber proceeds faster than the processing of the concave portion, the processing end surface 34 is flattened as the irradiation time elapses. Thereby, even for an end face having a non-uniform surface shape such as a cut end face of the PCF fiber, the surface can be flattened by using the femtosecond laser.

  In order to increase the processing speed of surface flattening by laser beam irradiation, it is preferable to inject a processing promoting gas as an auxiliary gas in the vicinity of the end face of the optical fiber or to form an atmospheric gas. The end face shaping method described above is not limited to use in PCF end face shaping, but can be widely applied to shaping of the end face of a wire, such as shaping of an end face formed by cleavage fracture starting from a processing mark. .

  PCFs are ultra-compact parts such as optical communication parts, high-speed transmission in GaAs and Si (800 nm region) devices, high-speed soliton transmission in the visible light region, dispersion management in various bands, optical pulse compression, and photonics crystals. Applications are expected for coupling to devices, wiring, white light generation, optical frequency standards, nonlinear optical fiber devices, optical power transmission, low dispersion elements with air cores, and the like. Application examples of the present invention are not limited to PCF cutting and end face shaping, but can be widely applied to cutting and end face shaping of transparent wires such as ordinary optical fibers.

The figure which shows the example of a structure for enforcing the processing method of PCF using a femtosecond laser. The figure which shows the process locus at the time of dividing | segmenting PCF. The figure which shows the end surface shaping method which irradiates and planarizes the end surface of PCF by femtosecond laser.

Explanation of symbols

1: femtosecond laser oscillator, 4: dispersion compensation optical system, 6: condensing optical system (lens), 7: convergent beam, 8: object to be processed (holey fiber: PCF), 10, 12: XY stage, 17: Laser oscillation control device, 24: femtosecond laser scanning path for cutting PCF in vertical direction, 27: femtosecond laser scanning path for cutting PCF in oblique direction

Claims (9)

  The wire rod is rotated relative to the irradiation position of the laser beam of the femtosecond laser, and the focal point of the laser beam is formed on the surface or inside of the wire along a predetermined scanning path. A method of cutting a wire along the line.   A scanning path in which the condensing optical system for condensing the laser beam of the femtosecond laser irradiated to the wire is controlled, and the position of the condensing point formed on the surface portion and inside of the wire is set on a predetermined cross section The cutting method of the wire which moves along and cut | disconnects a wire along this cross section.   The wire is a photonic crystal fiber (PCF), and the PCF is irradiated with a laser beam having a wavelength in a single photon absorption region or a multiphoton absorption region, and the PCF is applied along the modified region formed inside the PCF. It cuts, The cutting method of the wire rod of Claim 1 or Claim 2 characterized by the above-mentioned.   The laser beam of a femtosecond laser is irradiated so that a condensing point is formed on a cross section having an arbitrary angle with respect to a direction perpendicular to the axis of the wire, and the wire is cut along the cross section. Or the cutting method of the wire rod of Claim 2.   The wire is cut by irradiating a laser beam from the outside of the refractive index matching medium in a state where the wire is installed in a refractive index matching medium having a refractive index that matches the refractive index of the wire. The method for cutting a wire according to claim 2.   The femtosecond laser is a mode-locked fiber laser doped with Er, Yb or the like, a fiber mode-locked laser doped with Ti, or a wavelength conversion laser of these short pulse laser outputs. The cutting method of the wire according to 2.   A method for shaping an end face of a wire, wherein the end face of the wire is flattened by irradiating the end face of the wire with a laser beam of a femtosecond laser.   8. The method of shaping an end face of a wire rod according to claim 7, wherein the beam distribution intensity of the laser beam is made uniform and the end face is irradiated with the laser beam.   8. The method of shaping an end face of a wire according to claim 7, wherein a processing promoting gas is applied to the end face irradiated with the laser beam.

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