Patent Publication Number: US-2013230282-A1

Title: Light guiding device and light guiding method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of International Application No. PCT/JP2012/002436, filed on Apr. 6, 2012, which is based on and claims priority to Japanese Patent Application No. JP 2011-133916, filed on 16 Jun. 2011. The disclosure of the Japanese priority application and the PCT application in their entirety, including the drawings, claims, and the specification thereof, are incorporated herein by reference. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     This invention relates to light guiding devices and light guiding methods. 
     2. Related Art 
     Previously, optical fibers had been used only with light in the near-infrared region. However, advances in optical fiber technology in recent years have resulted in expansion of the range of use, from the ultraviolet to the mid-infrared. For example, Japanese Patent Application Laid-open No. 2005-62850 (also referred to herein as “Patent Reference 1”) indicates that light at different wavelengths is guided using a photonic crystal fiber. 
     A Japanese Translation of PCT Application No. 2009-522605 (also referred to herein as “Patent Reference 2”) indicates that light generated by a plurality of light sources is guided by different optical fibers, and is then concentrated by a lens. 
     Japanese Patent Application Laid-open No. 2004-61830 (also referred to herein as “Patent Reference 3”) indicates that a single-mode optical fiber is connected to an optical element via a photonic crystal fiber. 
     Using an optical fiber, the range of light irradiation can be narrowed, and therefore an optical system can be made compact. In order to guide light at different wavelengths using an optical fiber while exploiting this feature, it is necessary to emit light at different wavelengths with substantially the same mode field diameter and having a single mode, in a collimated state, from the optical fiber. 
     Thus, as described above, there is a need in the art for an improved light guiding device. 
     SUMMARY OF THE INVENTION 
     Embodiments of the invention address this and other needs. Some embodiments emit from an optical fiber of light at different wavelengths in a collimated state, with substantially the same mode field diameter and having a single mode. 
     In some embodiments, a light guiding device includes a first single-mode optical fiber, a photonic crystal fiber, and a graded index fiber. The photonic crystal fiber is connected to the emission-side end face of the first single-mode optical fiber. The graded index fiber is connected to the emission-side end face of the photonic crystal fiber. The refractive index of the graded index fiber in a direction in which light is concentrated changes in a radial direction. 
     By way of this light guiding device, a plurality of light rays at different wavelengths pass through the first single-mode optical fiber, the photonic crystal fiber, and the graded index fiber, and are emitted. The plurality of light rays have substantially the same mode field diameter and have a single mode as a result of passing through the photonic crystal fiber. After passing through the photonic crystal fiber, the plurality of light rays further pass through the graded index fiber, and are collimated. 
     In some embodiments, first a light guiding device is prepared, including a first single-mode optical fiber, a photonic crystal fiber, and a graded index fiber. In this light guiding device, the photonic crystal fiber is connected to the emission-side end face of the first single-mode optical fiber. The graded index fiber is connected to the emission-side end face of the photonic crystal fiber. The refractive index of the graded index fiber in the direction in which light is concentrated changes in the radial direction. A plurality of light rays with different wavelengths are guided by the first single-mode optical fiber. The plurality of light rays are then subjected to single-mode conversion, and the mode field diameter is made uniform, using the photonic crystal fiber. Further, chromatic aberration correction of the plurality of light rays is performed using the graded index fiber. Then, the plurality of light rays are emitted from the graded index fiber. 
     By way of some embodiments, light at different wavelengths from an optical fiber can be emitted in a collimated state, with substantially the same mode field diameter and having a single mode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Certain features and advantageous results will become clear through the preferred embodiments described below, and the attached drawings. 
         FIG. 1  shows the configuration of the light guiding device of a first embodiment of the invention; 
         FIG. 2  is a cross-sectional view showing the configuration of a photonic crystal fiber; 
         FIG. 3  shows the configuration of the light guiding device of a second embodiment of the invention; 
         FIG. 4  shows the configuration of the light guiding device of a third embodiment of the invention; 
         FIG. 5  shows the configuration of the light guiding device of a fourth embodiment of the invention; 
         FIG. 6  shows the configuration of the optical device of a fifth embodiment of the invention; 
         FIG. 7  shows the configuration of the optical device of a sixth embodiment of the invention; and 
         FIG. 8  is a graph showing the collimator characteristic of a light guiding device of an example. 
     
    
    
     DETAILED DESCRIPTION 
     Below, embodiments of the invention are explained using the drawings. In general, in the drawings, the same symbols are assigned to the same constituent elements, and explanations are omitted as appropriate. 
     FIRST EMBODIMENT 
       FIG. 1  shows the configuration of the light guiding device of a first embodiment. This light guiding device comprises a first single-mode optical fiber  10 , a photonic crystal fiber  20 , and a graded index fiber (hereafter called a “GI fiber”)  30 . The first single-mode optical fiber  10  is a fiber to guide light. 
     The photonic crystal fiber  20  and the GI fiber  30  form the emission portion of the first single-mode optical fiber  10 . Specifically, the photonic crystal fiber  20  is connected to the emission-side end face of the first single-mode optical fiber  10 . The GI fiber  30  is connected to the emission-side end face of the photonic crystal fiber  20 . The refractive index of the GI fiber  30  in the direction in which light is concentrated changes in the radial direction. The lengths of the photonic crystal fiber  20  and the GI fiber  30  are designed such that the photonic crystal fiber  20  and the GI fiber  30  are accommodated within ferrules or other mounting jigs. For example, the length of the photonic crystal fiber  20  is 0.5 mm or greater and 5 mm or less, and the length of the GI fiber is 0.1 mm or greater and 1 mm or less. 
     The first single-mode optical fiber  10  is for example formed from silica glass. The first single-mode optical fiber  10  has a core  12 . The core  12  is formed by doping the body of the first single-mode optical fiber  10  with an impurity, such as Ge. The photonic crystal fiber  20  has a core  22 . The GI fiber  30  has a core  32 . The cores  12 ,  22 , and  32  are all regions in which light is guided. 
     The refractive index of the core  32  of the GI fiber  30  changes in the radial direction. This direction of change in the refractive index is the direction in which light passing through the core  32  is concentrated. For example, the impurities in the core  32  are decreased from the center toward the outside. Specifically, the impurity concentration is highest at the center of the core  32 , and is inversely proportional to the square of the distance from the center. When the GI fiber  30  is formed from silica glass, the impurity with which the core  32  is doped is for example Ge. 
     The portion connecting the first single-mode optical fiber  10  and the photonic crystal fiber  20  effects connection by for example fusion. However, the first single-mode fiber  10  and the photonic crystal fiber  20  may also be connected using an adhesive. Similarly, the photonic crystal fiber  20  and the GI fiber  30  are connected by for example fusion, but may be connected using an adhesive as well. 
       FIG. 2  is a cross-sectional view showing the configuration of the photonic crystal fiber  20 . The photonic crystal fiber  20  has a plurality of holes  24 . The holes  24  are arranged regularly within the core  22 . That is, the region in which the holes  24  are arranged is the core  22 . The plurality of holes  24  all have substantially the same diameter, and are arranged at the same intervals within the core  22 . However, holes  24  are not arranged at the center portion of the core  22 . That is, a hole  24  is absent in the center portion of the arrangement of holes  24 . On the periphery of this region in which a hole  24  is absent, three or more columns of holes  24  are arranged. In the example shown, the holes  24  are arranged in a regular hexagon. By this means, when passing through the photonic crystal fiber  20 , a plurality of light rays at different wavelengths have substantially the same mode field diameter and have a single mode. 
     Next, action and advantageous results of this embodiment are explained. The light guiding device shown in  FIG. 1  is used for example in a multi-wavelength light source device to guide a plurality of light rays at different wavelengths which have been emitted from a laser light source. The plurality of laser light rays may be made incident simultaneously on the light guiding device, or may be made incident with different timings. The laser light wavelengths are for example 490 nm or greater and 630 nm or less. 
     Of the first single-mode optical fiber  10 , the end portion on the side on which the photonic crystal fiber  20  is provided is positioned in the region to which light is to be guided, for example, above a sample. And, light is made incident on the first single-mode optical fiber  10  at the end portion on the side opposite the photonic crystal fiber  20 . 
     Light guided by the first single-mode optical fiber  10  passes through the photonic crystal fiber  20  and the GI fiber  30  and is emitted. When passing through the photonic crystal fiber  20 , the plurality of light rays at different wavelengths come to have substantially the same mode field diameter and have a single mode. Light which has passed through the photonic crystal fiber  20  further passes through the GI fiber  30 , and is collimated. 
     Hence, by means of this embodiment, light from a single optical fiber at different wavelengths can be emitted in a collimated state, with substantially the same mode field diameter and having a single mode. By using the light guiding device of this embodiment, the optical system necessary for light guiding is decreased, and a multi-wavelength light source device can be made compact. 
     The emission-side end face of the GI fiber  30  may be treated with an anti-reflection coating. An anti-reflection coating is for example a thin film with refractive index lower than that of the GI fiber  30 . By forming an anti-reflection coating, when light is emitted from the GI fiber  30 , reflection of light at the interface between the GI fiber  30  and the outside can be suppressed. 
     SECOND EMBODIMENT 
       FIG. 3  shows the configuration of the light guiding device of a second embodiment. This light guiding device has the same configuration as the light guiding device of the first embodiment, except for the fact that the GI fiber  30  has a concave portion  34 . 
     The concave portion  34  is provided in the emission-side end face of the GI fiber  30 . The concave portion  34  has a concave lens shape, and is provided at least over the entirety of the end face of the core  32 . The concave portion  34  has the function of correcting chromatic aberration in light emitted from the GI fiber  30 . 
     The concave portion  34  may be formed by polishing, or may be formed by etching. The impurity concentration in the core  32  is highest at the center of the core  32  and falls in moving toward the outside. The strength of the GI fiber  30  is inversely proportional to the impurity concentration. Hence, if the end face of the core  32  is polished or etched, the core  32  becomes deepest at the center, and becomes shallower toward the outside. Further, the impurity concentration in the core  32  is inversely proportional to the square of the distance from the center. Hence, the concave portion  34  has a concave lens shape. When etching the core  32 , for example an HF system solution is used as the etching liquid. 
     When the concave portion  34  is formed by polishing, only minimal equipment investments are necessary. Moreover, a plurality of light guiding devices can be treated simultaneously, so that productivity is improved. When on the other hand the concave portion  34  is formed by etching, the shape of the concave portion  34  during fabrication can be monitored, so that the precision of fabrication of the concave portion  34  is improved. 
     By way of this embodiment also, advantageous results similar to those of the first embodiment can be obtained. Further, a concave portion  34  with a concave lens shape is formed in the emission-side end face of the GI fiber  30 . Hence even if a lens is not provided on the outside, when a plurality of light rays at different wavelengths are emitted from the GI fiber  30 , the occurrence of chromatic aberration can be suppressed. 
     THIRD EMBODIMENT 
       FIG. 4  shows the configuration of the light guiding device of a third embodiment. The light guiding device of this embodiment has the same configuration as the light guiding device of the second embodiment, except for the structure of the end portion  14  of the first single-mode optical fiber  10 . 
     In this embodiment, the core  12  of the first single-mode optical fiber  10  gradually expands at the end portion  14 . Such a structure is obtained by heat treatment of the end portion  14  (TEC (Thermally Expanded Core) treatment), causing thermal diffusion of impurities in the core  12 . The mode field diameter at the face joined with the photonic crystal fiber  20  of the first single-mode optical fiber  10  is the same as the mode field diameter of the photonic crystal fiber  20 . In this embodiment, a concave portion  34  may not be provided. 
     By means of this embodiment also, advantageous results similar to those of the second embodiment can be obtained. Further, the core  12  of the first single-mode optical fiber  10  expands gradually at the end portion  14 . At the face joined with the photonic crystal fiber  20 , the core  12  has the same diameter as the core  22  of the photonic crystal fiber  20 . Hence at the face joining the first single-mode optical fiber  10  and the photonic crystal fiber  20 , the occurrence of optical losses arising from mismatches of mode field diameters can be suppressed. 
     FOURTH EMBODIMENT 
       FIG. 5  shows the configuration of the light guiding device of a fourth embodiment. The light guiding device of this embodiment has the same configuration as the light guiding device of the second embodiment, except for the fact of comprising a second single-mode fiber  40 . 
     The second single-mode fiber  40  is provided between the first single-mode optical fiber  10  and the photonic crystal fiber  20 . The second single-mode fiber  40  is a low-N.A. (Numerical Aperture) fiber. That is, the diameter of the core  42  of the second single-mode fiber  40  is larger than that of the core  12  of the first single-mode optical fiber  10 . That is, the mode field diameter of the second single-mode fiber  40  is greater than the mode field diameter of the first single-mode optical fiber  10 . However, the mode field diameter of the second single-mode fiber  40  is equal to or smaller than the mode field diameter of the photonic crystal fiber  20 . Further, the difference in refractive indices of the core  42  and the cladding portion of the second single-mode fiber  40  is smaller than the difference in refractive indices of the core  12  and cladding portion of the first single-mode optical fiber  10 . In the first embodiment, a second single-mode fiber  40  may be provided. 
     By means of this embodiment also, advantageous results similar to those of the second embodiment can be obtained. Further, a second single-mode fiber  40  is positioned between the first single-mode optical fiber  10  and the photonic crystal fiber  20 . Hence the mode field diameter of light guided by the first single-mode optical fiber  10  expands during propagation in the second single-mode fiber  40 , and thereafter is incident on the photonic crystal fiber  20 . Hence at the face joining the first single-mode fiber  10  and the photonic crystal fiber  20 , the occurrence of optical losses arising from mismatches of mode field diameters can be suppressed. 
     FIFTH EMBODIMENT 
       FIG. 6  shows the configuration of the optical device of a fifth embodiment. The optical device of this embodiment mounts the emission-side end portion of a light guiding device according to any one of the first to fourth embodiments on a ferrule  60 . In the example shown in the figure, the light guiding device of the fourth embodiment is shown. 
     Specifically, the first single-mode optical fiber  10  is covered by a covering member  50 . However, of the first single-mode optical fiber  10 , the covering member  50  is not provided on the emission-side end portion. The emission-side end portion of the first single-mode optical fiber  10 , together with the end portion of the covering member  50 , is inserted into an insertion opening  62  of the ferrule  60 . And, the end portion of the first single-mode optical fiber  10 , the second single-mode fiber  40 , the photonic crystal fiber  20 , and the GI fiber  30  are held by the ferrule  60 . 
     SIXTH EMBODIMENT 
       FIG. 7  shows the configuration of the optical device of a sixth embodiment. In the optical device of this embodiment, a plurality of the light guiding devices according to any one of the first to the fourth embodiments are held by a holding member  70 . In the example shown in the figure, light guiding devices of the fourth embodiment are shown. 
     A plurality of V-shape grooves are provided in parallel in the holding member  70 . The end portions of first single-mode optical fibers  10 , second single-mode fibers  40 , photonic crystal fibers  20 , and GI fibers  30  are fitted into the grooves. By this means, the holding member  70  can hold a plurality of light guiding devices in parallel. 
     EXAMPLE 
     The light guiding device shown in  FIG. 4  was fabricated. A visible light fiber with a cutoff wavelength of 430 nm was used as the first single-mode optical fiber  10 . The photonic crystal fiber  20  used had a mode field diameter of 15 μm. The GI fiber  30  used had a core of 62.5 nm. 
     First, the end portion  14  of the first single-mode optical fiber  10  was subjected to heat treatment. Then, the first single-mode optical fiber  10  and the photonic crystal fiber  20  were heat-fused. Further, the photonic crystal fiber  20  and the GI fiber  30  were heat-fused. Thereafter a concave portion  34  was formed by HF etching of the GI fiber  30 . 
       FIG. 8  shows collimator characteristics of the light guiding device of the example. The vertical axis indicates the beam diameter of emitted light, and the horizontal axis indicates the distance from the concave portion  34 . As shown in the figure, satisfactory collimation characteristics were obtained both for light of wavelength 540 nm and for light of wavelength 560 nm. The beam diameters were substantially the same at these two wavelengths. 
     In the above, embodiments of the invention have been explained referring to the drawings, but the embodiments are merely examples of the invention, and various configurations other than those described above can be adopted. 
     Examples of specific embodiments are illustrated in the accompanying drawings. While the invention is described in conjunction with these specific embodiments, it will be understood that it is not intended to limit the invention to the described embodiments. On the contrary, it is intended to cover alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims. In the above description, specific details are set forth in order to provide a thorough understanding of embodiments of the invention. Embodiments of the invention may be practiced without some or all of these specific details. Further, portions of different embodiments and/or drawings can be combined, as would be understood by one of skill in the art.