JP2006350255A - Multimode optical fiber and optical module - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a multimode optical fiber and an optical module that deteriorate no output and that are essentially free from influence by return light. <P>SOLUTION: The multimode optical fiber 40 is provided with a core part 43 for the purpose of propagating in multimode a laser beam emitted from a semiconductor laser 20. The core part 43 is covered with a clad part 44 that encloses in the core part 43 the laser beam to be propagated. In an optical waveguide part 42 comprising these core part 43 and clad part 44, the end face 40A on the incident side of the laser beam is formed continuously from the core part 43 over to the clad part 44. In the end face 40A, a light reflecting part 41 is formed as well as the incident region of the laser beam. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a multimode optical fiber for propagating laser light emitted from a semiconductor laser in a multimode, and an optical module using the multimode semiconductor laser and the multimode optical fiber.

  It is generally known that semiconductor lasers are very sensitive to return light. Return light refers to the light emitted from the semiconductor laser reflected from the surface to be irradiated and the surface of an optical component such as a lens or an optical fiber provided in the optical path to the surface to be irradiated, and returned to the semiconductor laser. It is the light that comes. The surface of these external elements and the end face of the semiconductor laser constitute a new resonator, and the noise generated thereby greatly affects the characteristics of the semiconductor laser. For this reason, in the technical field where a high-power semiconductor laser having a watt-class output is used as a light source for exciting a solid laser or a light source for direct processing (laser welding, laser welding, marking, etc.) It is a problem.

  Specifically, in the field of solid-state laser excitation light sources, the absorption wavelength of the laser medium is determined, so that the wavelength of the semiconductor laser must always correspond to the absorption spectrum of the laser medium. However, when there is return light, the output light of the solid-state laser becomes unstable because the return light disturbs the wavelength and spectrum shape of the semiconductor laser. On the other hand, in the field of a light source for direct processing, since processing is performed by changing light energy into heat energy and dissolving a substance, the output of the semiconductor laser must always be stable. However, when there is return light, the semiconductor laser causes a mode hop due to the influence, and the output becomes unstable, and the near-field image changes, resulting in processing unevenness.

JP 2002-228857 A

  Therefore, as a measure to suppress the return light, conventionally, an isolator is inserted between the semiconductor laser and the irradiated surface to attenuate the return light, or the irradiated surface is tilted so that the return light is not generated from the irradiated surface. Or has been made. However, in the former measure, the output is reduced due to the loss of the isolator, and in the latter measure, there is a problem that the return light generated by the optical component in the optical path to the irradiated surface cannot be attenuated at all.

  Patent Document 1 discloses a technique related to an optical fiber with improved optical coupling efficiency with a single mode semiconductor laser used for optical communication.

  The present invention has been made in view of such problems, and an object of the present invention is to provide a multimode optical fiber and an optical module that are substantially free from the influence of return light without lowering the output.

  The multimode optical fiber of the present invention includes a core portion for propagating laser light emitted from a semiconductor laser in multimode. A clad portion that confines the propagated laser light in the core portion coats the core portion. In the optical waveguide portion composed of the core portion and the clad portion, an end face on the laser beam incident side is continuously formed from the core portion to the clad portion. A light reflecting portion is formed on the end face together with the laser light incident area.

  The optical module of the present invention is obtained by optically coupling the semiconductor laser and the multimode optical fiber.

  In the multimode optical fiber and the optical module of the present invention, the laser light emitted from the semiconductor laser is incident on the incident area of the incident-side end face and propagates through the core while being reflected at the interface between the core and the clad. And it radiates | emits from the other end surface. At this time, a part of the laser light emitted from the other end face is reflected and returned by, for example, the surface to be irradiated and the surface of an optical component provided in the optical path to the surface to be irradiated. Further, when the laser light propagating through the core portion reaches the other end face, it is reflected by the end face and returns. A part of the reflected light is reflected to the irradiated surface side by the light reflecting portion formed in the region other than the laser light incident region in the end surface on the semiconductor laser side, and does not reach the semiconductor laser. Of the reflected light, a component that reaches the incident region on the incident side end surface passes through the incident region and is incident on the active layer of the semiconductor laser mainly as a return light.

  In the multimode optical fiber and the optical module of the present invention, since the light reflecting portion is formed in a region other than the laser light incident region in the incident side end surface, the incident region in the incident side end surface of the reflected light. The component having reached 1 is mainly incident on the active layer of the end face of the semiconductor laser as incident light.

  In general, the influence of the semiconductor laser described above becomes very large when the return light is incident on a region other than the active layer, such as a clad layer or a substrate, but the influence mainly occurs when the return light is incident on the active layer. It has the property of being very limited. Therefore, if no light reflecting part is formed on the end face on the incident side, most of the reflected light propagating through the core part from the irradiated face side reaches the semiconductor laser through the end face and acts as return light. To do. In that case, since a large amount of return light is incident on a region other than the active layer of the semiconductor laser, the influence of the return light becomes very large. On the other hand, in the multimode optical fiber of the present invention, since the return light is mainly incident on the active layer as described above, the influence is very small.

  As a result, a phenomenon in which the wavelength or spectrum shape of the semiconductor laser is disturbed by the return light is suppressed, so that, for example, the stability of the output light of the solid-state laser can be improved. Further, since the phenomenon that the semiconductor laser causes a mode hop due to the return light and the output becomes unstable is suppressed, for example, it is possible to reduce the possibility that the near-field image changes and processing unevenness occurs.

  In addition, when the laser light emitted from the semiconductor laser is incident on the multimode optical fiber, the laser light can be collected in the incident area of the end face using, for example, a lens. There is no case where the light is reflected by the light reflecting portion before entering the fiber. That is, there is no laser beam incident loss due to the light reflecting portion formed on the end face.

  For these reasons, according to the multimode optical fiber and the optical module of the present invention, it is possible to reduce the influence of the return light without reducing the output.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[First Embodiment]
FIG. 1 shows a schematic configuration of an optical module 10 according to a first embodiment of the present invention. 2A shows a cross-sectional configuration in a direction parallel to the optical axis of the multimode optical fiber 40 of FIG. 1, and FIGS. 2B, 3 and 4 show an end face 40A of the multimode optical fiber 40 of FIG. 2 shows an example of a planar configuration of the side tip. The optical module 10 is intended to be used as, for example, a solid-state laser excitation light source, an endoscope light source, a gun irradiation light source, and various diagnostic light sources. An object such as a diagnostic part is arranged and irradiated with the laser beam of the semiconductor laser 20.

  The optical system of the optical module 10 includes a semiconductor laser 20, lenses 30 and 31, and a multimode optical fiber 40, and the lens 30, the multimode optical fiber 40, and the lens 31 are emitted from the semiconductor laser 20. They are arranged in this order on the optical path of the laser beam.

  The semiconductor laser 20 is a broad area type laser diode and supplies laser light for irradiating the irradiated surface S. In the semiconductor laser 20, the laser light is emitted from a light emitting point of the cleavage plane, for example, a flat light emitting point having a width of 50 μm in the slow axis (Slow axis) direction and a width of 1 μm in the fast axis (Fast axis) direction. It is like that. Here, the light emission point is a so-called light spot, which is mainly formed in the active layer (not shown). When the thickness of the active layer is very thin (for example, 1 μm or less), a part of the light spot is formed. It may also extend to the cladding layers above and below the active layer. In addition, the wavelength of the laser light changes according to changes in the temperature of the light spot and the vicinity thereof, so that, for example, even if the wavelength of the laser light is shifted by the return light, it is equal to the absorption wavelength of the laser medium. Laser light having a wavelength (for example, 808 nm) can be emitted.

  The lens 30 includes, for example, a collimating lens (not shown) and a convex lens (not shown) in this order on the optical path of the laser light. The lens 30 collimates the flat laser beam emitted from the light spot of the semiconductor laser 20 and then collects it so as to enter a predetermined region of an opening 41A (described later) of the multimode optical fiber 40. It has become. The lens 31 includes, for example, a convex lens (not shown). The lens 31 condenses the laser light emitted from the multimode optical fiber 40 and irradiates the irradiated surface S.

The multimode optical fiber 40 includes an optical waveguide unit 42 including a core unit 43 and a cladding unit 44, and a light reflection unit 41. The core portion 43 is made of a material having a property transparent to the wavelength of the laser beam, for example, quartz glass (SiO ₂ ) containing a small amount of germanium dioxide (GeO ₂ ), and is a single-mode core portion for optical communication. And a cylindrical shape having a diameter 10 times or more (for example, 50 μm to 200 μm). The core portion 43 has a role as a core wire that propagates the laser light collected by the lens 30 in a multimode. The clad portion 44 is made of a material having a refractive index slightly smaller than that of the core portion 43, for example, quartz glass (SiO ₂ ) containing a small amount of fluorine (F), and has an inner diameter equal to the diameter of the core portion 43. For example, it has a cylindrical shape of 100 μm or more and 400 μm or less. The clad portion 44 covers the core portion 43 and has a role as an outer skin for confining the laser light propagating in the core portion 43 in the core portion 43.

  The optical waveguide section 42 has planar end faces 40A and 40B. In these end faces 40A and 40B, a core portion 43 is formed at the center portion, and a clad portion 44 is formed concentrically at the peripheral portion.

  On the surface of the end face 40 </ b> A, a light reflecting portion 41 that covers a part of the surface of the core portion 43 and a part or all of the surface of the cladding portion 44 is provided. The light reflection part 41 should just be comprised with the material which has a high reflectance with respect to the wavelength of a laser beam, for example, when the wavelength of a laser beam is 808 nm, it is highly reflective about 98% with respect to the wavelength 808 nm. It is composed of gold (Au) having a rate. A region where the light reflecting portion 41 is not formed in the surface of the end surface 40A is an opening 41A. The opening 41A has a shape corresponding to a predetermined region where the laser light is incident. Specifically, the opening 41A is equal to the width H μm in the Fast axis direction of the image on the end surface 40A of the laser light incident on the end surface 40A. It has a width greater than that, and has a width equal to or greater than the width L μm in the Slow axis direction of the image on the end surface 40A of the laser light incident on the end surface 40A.

  For example, as shown in FIG. 2B, the opening 41A has a width in the Fast axis direction of H μm and Slow that continuously crosses part of the surface of the core part 43 and part of the surface of the cladding part 44. It has a band-like shape whose axial width is equal to the diameter of the optical waveguide portion 42, that is, a shape in which a region (corresponding to the clad portion 44) where no laser light is incident exists. As shown in FIGS. 3 and 4, a square shape having a width in the Fast axis direction of H μm and a width in the Slow axis direction of L μm formed on a part of the surface of the core portion 43, that is, incidence of laser light. The shape may completely correspond to the area to be performed. Here, the width H and the width L vary depending on the degree of light condensing by the lens 30, but the width H is, for example, in the range of 10 μm to 25 μm, and the width L is equal to or less than the diameter (for example, 200 μm) of the core portion 43. It has become.

  The multimode optical fiber 40 having such a configuration is manufactured by, for example, the following method. First, after forming the optical waveguide 42 by a known method, for example, a photoresist is formed on the entire surface of the end face 40A. Next, by selectively exposing and developing using a mask or the like, portions other than the region where the opening 41A is to be formed are selectively removed from the formed photoresist. Further, after depositing gold (Au) from above, the remaining photoresist is removed together with gold to form the opening 41A, and the light reflecting portion 41 is formed in other regions.

  In the optical module 10 having such a configuration, the laser light emitted from the semiconductor laser 20 enters an opening 41A (incident region) provided on the end surface 40A on the semiconductor laser 20 side, and the core portion 43 and the clad portion 44. It propagates in the core part 43 while being reflected at the interface with the light and exits from the end face 40B. At this time, a part of the laser light emitted from the end face 40B is reflected and returned by the surface S to be irradiated and the surface of the optical component provided in the optical path to the surface to be irradiated. Further, when the laser light propagating in the core portion 43 reaches the end face 40B, it is reflected by the end face 40B and returns.

  A part of the reflected light is reflected to the irradiated surface S side by the light reflecting portion 41 formed in a region other than the opening 41A (incident region) in the end surface 40A on the semiconductor laser 20 side, and is reflected on the semiconductor laser. Will not reach. Of the reflected light, a component that reaches the incident region on the incident side end surface passes through the incident region and is incident on the active layer of the semiconductor laser mainly as a return light. That is, the light reflecting portion 41 acts as a diaphragm for the reflected light, but does not act as a diaphragm for the laser light emitted from the semiconductor laser 20.

  Of the reflected light, the light that has reached the opening 41A (incident area) passes through the opening 41A (incident area) as it is, and enters the active layer having a light spot on the end face of the semiconductor laser 20 as return light. As shown in FIGS. 3 and 4, when the opening 41 </ b> A has a shape completely corresponding to the shape of the laser light incident region, it is incident only on the light spot.

  Next, effects of the optical module 10 of the present embodiment will be described with reference to FIGS. 5, 6, and 7.

  5, 6 and 7 show the spectrum of the laser light emitted from the semiconductor laser 20. FIG. FIG. 5 shows the spectrum when no return light is incident on the semiconductor laser 20, and FIG. 6 shows the spectrum when the light reflection portion 41 is provided on the end face 40A so that the return light is mainly incident on the active layer. FIG. 7 shows the spectrum when the light reflecting portion 41 is not provided on the end face 40A and the return light is incident on a region other than the active layer such as the cladding layer and the substrate.

  As shown in FIG. 5, when no return light is incident on the semiconductor laser 20, the semiconductor laser 20 generates no noise due to the return light. Not disturbed at all.

  As shown in FIG. 6, when the return light is mainly incident on the active layer, the oscillation wavelength of the semiconductor laser 20 is slightly shifted to the long wavelength side (low frequency side) as an influence of the return light. However, since the shape of the spectrum is not disturbed, the influence of the return light can be eliminated by changing the temperature of the semiconductor laser 20 to shift the oscillation wavelength to a predetermined value.

  On the other hand, as shown in FIG. 7, when the return light is also incident on a region other than the active layer, such as a clad layer or a substrate, the semiconductor laser 20 is severely affected by the return light, and the oscillation wavelength and spectrum It can be seen that the shape is very disturbed.

  For these reasons, by providing the light reflecting portion 41 on the end face 40A on the semiconductor laser 20 side of the optical waveguide portion 42, it becomes possible to reflect a part of the return light. As a result, it is possible to suppress disturbance in the oscillation wavelength and spectrum shape of the semiconductor laser 20 due to the return light, and thus, for example, the stability of the output light of the solid-state laser can be improved. Further, since it is possible to suppress the output from becoming unstable due to the return light causing the semiconductor laser 20 to mode hop, for example, it is possible to reduce the possibility that the near-field image changes and processing unevenness occurs. . In this way, the influence of return light can be reduced.

  In addition, when the laser light emitted from the semiconductor laser 20 is incident on the multimode optical fiber 40, the laser light can be collected in the incident area on the end face using, for example, a lens. The light is not reflected by the light reflecting portion 41 before entering the mode optical fiber 40. That is, there is no laser beam incident loss due to the formation of the light reflecting portion on the end face 40A.

  Therefore, according to the multimode optical fiber 40 of the present embodiment and the optical module 10 using the same, it is possible to reduce the influence of the return light without reducing the output.

[Second Embodiment]
Next, the optical module 11 according to the second embodiment of the present invention will be described.

  FIG. 8 shows a schematic configuration of the optical module 11 according to the present embodiment. 9A shows a cross-sectional configuration in a direction parallel to the optical axis of the multimode optical fiber 50 of FIG. 8, and FIG. 9B shows a plane of the tip on the end face 50A side of the multimode optical fiber 50 of FIG. Each of the configuration examples is shown. This optical module 11 is different from the optical module 10 provided with a multimode optical fiber 40 having a planar tip provided at both ends, in that it includes a multimode optical fiber 50 provided with a wedge-shaped tip at one end. Therefore, the description of the configuration, operation, and effects similar to those in the above embodiment will be omitted, and the multimode optical fiber 50 will be described in detail.

  The multi-mode optical fiber 50 includes an optical waveguide unit 52 composed of a core unit 53 and a cladding unit 54, and a light reflection unit 51. The optical waveguide unit 52 has a pair of end surfaces 50A and 50B. In these end faces 50A and 50B, a core portion 53 is formed at the center portion, and a clad portion 54 is formed concentrically at the peripheral portion. The end surface 50A on the semiconductor laser 20 side has a pair of inclined surfaces 50C and 50D that form a ridge line that is symmetrical with respect to the central axis of the core portion 53 and has a predetermined width H μm, and has a so-called wedge shape. . On the other hand, the end surface 50B has a planar shape.

  Note that the inclination angle θ of the inclined surfaces 50C and 50D is parallel to the plane perpendicular to the central axis of the core portion 53 (FIG. 9B) in order to increase the degree of optical coupling within the optical waveguide portion 52. It is preferably larger than 0 ° and smaller than 70 °, more preferably 45 °.

  The light reflecting portion 51 is provided on the pair of inclined surfaces 50C and 50D in the end surface 50A. The light reflecting part 51 covers a part of the surface of the core part 53 and a part of the surface of the clad part 54, respectively. The opening 51A is provided in a region corresponding to the ridge line, and has a shape corresponding to a predetermined region where the laser light is incident. Specifically, as shown in FIG. 9B, the width in the fast axis direction that crosses a part of the surface of the core part 53 and a part of the surface of the cladding part 54 in the fast axis direction is H μm. The band-like shape in which the width in the slow axis (Slow axis) direction is equal to the diameter of the optical waveguide portion 52, that is, a shape in which a region (corresponding to the clad portion 54) where the laser beam is not incident exists. .

  The multimode optical fiber 50 having such a configuration is manufactured by, for example, the following method. First, the optical waveguide 42 is formed by a known method. Next, as shown in FIG. 10A, the optical waveguide portion 42 is tilted to a predetermined inclination angle, and the tip thereof is fixed to a holding portion H1 that can protrude by a predetermined height. As shown in (B), the tip is polished by the polishing section E.

  Next, as shown in FIG. 11A, the optical waveguide portion 42 is fixed to the holding portion H1 so that the sharpened tip by the above-described polishing protrudes toward the polishing portion E, and then FIG. The tip is polished by the polishing section E as shown in FIG.

  Thereafter, as shown in FIG. 12A, a high reflectivity film is formed on the tip surface that has become a wedge shape by the above-described polishing, and light is reflected on the two inclined surfaces 50C and 50D. Part 51 is formed.

  Next, as shown in FIG. 12B, the optical waveguide portion 42 having the tip coated with the light reflecting portion 51 is fixed to the holding portion H2 so as to be perpendicular to the polishing portion E, and then, FIG. As shown in (C), the tip is polished by the polishing portion E, the core portion 53 is exposed, and a ridge line having a predetermined width H μm is formed.

  As described above, when the multimode optical fiber 50 is manufactured, it is not necessary to form a mask for positioning the opening 51A. Therefore, the opening 51A is easily formed as compared with the first embodiment. can do.

  In the optical module 11 having such a configuration, the laser light emitted from the semiconductor laser 20 is incident on the opening 51A provided on the end surface 50A on the semiconductor laser 20 side, and at the interface between the core portion 53 and the cladding portion 54. The light propagates through the core 53 while being reflected, and exits from the end face 50B. At this time, part of the laser light emitted from the end surface 50B is reflected by the surface of the irradiated surface S and returns. Further, when the laser light propagating through the core portion 53 reaches the end face 50B, it is reflected by the end face 50B and returns.

  Part of the reflected light is generated by the light reflecting portion 51 formed on a region other than the opening 51A (incident region) in the end surface 50A on the semiconductor laser 20 side, specifically on the pair of inclined surfaces 50C and 50D. It is reflected to the irradiated surface S side and does not reach the semiconductor laser. Of the reflected light, the component that has reached the incident region on the incident side end surface passes through the incident region and enters the active layer of the semiconductor laser mainly as the return light. That is, the light reflecting portion 51 acts as a diaphragm for the reflected light, like the light reflecting portion 41 of the above embodiment, but does not act as a diaphragm for the laser light emitted from the semiconductor laser 20.

  Of the reflected light, the light that has reached the opening 51A (incident area) passes through the opening 51A (incident area) as it is, and enters the active layer having a light spot on the end face of the semiconductor laser 20 as return light.

  According to the multimode optical fiber 50 of the present embodiment and the optical module 11 using the same, the return light is used without reducing the output, similarly to the multimode optical fiber 40 and the optical module 10 of the above embodiment. The influence can be reduced.

  The present invention has been described with reference to the embodiment. However, the present invention is not limited to the above embodiment, and various modifications can be made.

  For example, in the second embodiment, the opening 51A has a shape in which a region where the laser beam is not incident is partially present. However, the present invention is not limited to this, and the first Similarly to the first embodiment, the shape may completely correspond to the region where the laser light is incident.

  Moreover, in the said embodiment, although the light reflection parts 41 and 51 were the high reflectance films | membranes which consist of a material which has a high reflectance with respect to the wavelength of a laser beam, this invention is not limited to this. Instead, it may be a rough surface formed by roughly polishing the regions where the light reflecting portions 41 and 51 are formed when the tips of the optical waveguide portions 42 and 52 are polished. This is because even a rough surface can act as a diaphragm for reflecting a part of the reflected light, and the influence of return light can be reduced.

It is a schematic block diagram of the optical module which concerns on the 1st Embodiment of this invention. It is the block diagram which expanded and represented the principal part of the multimode optical fiber of FIG. It is a block diagram showing the modification of the multimode optical fiber of FIG. FIG. 7 is a configuration diagram illustrating another modification of the multimode optical fiber of FIG. 1. It is a spectrum figure when there is no return light. It is a spectrum diagram when the return light is incident mainly on the active layer. It is a spectrum figure when return light injects also into areas other than an active layer. It is a schematic block diagram of the optical module which concerns on the 2nd Embodiment of this invention. It is the block diagram which expanded and represented the principal part of the multimode optical fiber of FIG. FIG. 9 is a cross-sectional view for explaining a manufacturing process of the multimode optical fiber of FIG. 8. It is sectional drawing for demonstrating the process following FIG. FIG. 12 is a cross-sectional view for explaining a process following the process in FIG. 11.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10,11 ... Optical module, 20 ... Semiconductor laser, 30, 31 ... Lens, 40, 50 ... Multimode optical fiber, 40A, 40B, 50A, 50B ... End face, 41, 51 ... Light reflection part, 41A, 51A ... Opening Part, 42, 52 ... optical waveguide part, 43, 53 ... core part, 44, 54 ... clad part, 50C ... inclined surface, E ... polishing part, H, L ... width, H1, H2 ... holding part, S ... covered Irradiation surface, θ: inclination angle of the light reflecting portion.

Claims (9)

A laser beam incident side of a laser beam having a core part for propagating laser light emitted from a semiconductor laser in a multimode, and a clad part covering the core part and confining the propagated laser light in the core part An end face of the optical waveguide part formed continuously from the core part to the clad part,
A multimode optical fiber comprising a light reflecting portion together with a laser light incident region on an end face of the optical waveguide portion. The end face of the optical waveguide section has a band-shaped opening section that crosses the core section and the cladding section, and the opening section serves as an incident area and the area other than the opening section serves as a light reflecting section. The multimode optical fiber according to claim 1.   The end face of the optical waveguide part has a rectangular opening on the surface of the core part, the opening serves as an incident area and the area other than the opening serves as a light reflecting part. The multimode optical fiber according to claim 1. The multimode optical fiber according to claim 2, wherein the opening has a width of 10 μm or more and 25 μm or less. The multimode optical fiber according to claim 1, wherein an end surface of the optical waveguide unit is a flat surface. The end face of the optical waveguide portion has a pair of inclined surfaces that are symmetrical with respect to the central axis of the core portion and that form a ridge line with a predetermined width,
The multimode optical fiber according to claim 1, wherein the light reflecting portion is formed on an inclined surface thereof. The multimode optical fiber according to claim 1, wherein the light reflecting portion is a high reflectance film. The multimode optical fiber according to claim 1, wherein the light reflecting portion is a rough surface formed by polishing the end face. An optical module formed by optically coupling a semiconductor laser and a multimode optical fiber,
The multimode optical fiber is:
A laser beam incident side of a laser beam having a core portion for propagating laser light emitted from a semiconductor laser in a multimode, and a clad portion covering the core portion and confining the propagated laser light in the core portion An end face of the optical waveguide part formed continuously from the core part to the clad part,
An optical module comprising a light reflecting portion together with a laser light incident region on an end face of the optical waveguide portion.

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