Patent Publication Number: US-2007121684-A1

Title: Multiple wavelength laser light source using fluorescent fiber

Description:
The present application is based on Japanese patent application No. 2005-346839, the entire contents of which are incorporated herein by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a multiple wavelength laser light source using a fluorescent fiber, and more particularly to a multiple wavelength laser light source, using a fluorescent fiber, which is suitable for being used as various kinds of light sources such as a backlight light source for a liquid crystal television.  
      2. Description of Related Art  
      In recent years, a light-emitting device using a semiconductor light-emitting element such as a light-emitting diode (LED) element or a light amplification by stimulated emission of radiation (LASER) element has been widely utilized as various kinds of light sources because it is advantageous in miniaturization, an excellent power efficiency, and a long life as compared with the case of an incandescent lamp.  
      When such a light source, for example, is used as a backlight light source for a color laser display device in order to obtain an illuminating light (multiple wavelength laser light), three kinds of semiconductor light-emitting elements, i.e., red, green and blue semiconductor light-emitting elements are used.  
      Heretofore, a light source including three kinds of laser light sources, i.e., red, green and blue laser light sources as semiconductor light-emitting elements, and an optical fiber in which trivalent praseodymium ions (Pr 3+ ) excited by an excitation light emitted from at least one laser light source among these three kinds of laser light sources are added to a core has been known as this sort of light source. This light source, for example, is disclosed in the Japanese Patent Kokai No. 2001-264662.  
      In addition, an argon ion laser device having a function of exciting trivalent praseodymium ions contained in zirconium fluoride system glass constituting a core of an optical fiber by an excitation light (its wavelength is 476.5 nm) emitted from an argon ion laser has also been known as another light source. This argon ion laser device, for example, is disclosed in Optics Communications89 (1991), pp. 333 to 340.  
      However, in the case of the light source disclosed in the Japanese Patent Kokai No. 2001-264662, the three laser light sources emit the three kinds of laser beams (red, green and blue laser beams), respectively. As a result, there is encountered such a problem that not only the number of components or parts increases to swell the cost, but also the overall light source is scaled up.  
      On the other hand, in the case of the argon ion laser device disclosed in Optics Communications89 (1991), pp. 333 to 340, the core of the optical fiber is made of the zirconium fluoride system glass. As a result, there is such inconvenience that not only the mechanical strength of the optical fiber is low and it is easy to be damaged, but also the chemical durability of the optical fiber is poor and when being used in the atmosphere, the optical fiber absorbs moisture, so that it is easy to be deteriorated. In addition, the excitation light having the wavelength of 476.5 nm which is emitted from the argon ion laser is used. As a result, there is also such inconvenience that the excitation light shows a blue-green color, and thus a desired (pure) blue light can not be obtained as a light emitted through a light emission face of the optical fiber.  
     SUMMARY OF THE INVENTION  
      In the light of the foregoing, it is an object of the present invention to provide a multiple wavelength laser light source, using a fluorescent fiber, with which low cost promotion and miniaturization of the overall multiple wavelength laser light source can be realized, an optical fiber can be prevented from being damaged and deteriorated, and a desired blue light can be obtained as a light emitted from the optical fiber.  
      In order to attain the above-mentioned object, according to one aspect of the present invention, there is provided a multiple wavelength laser light source using a fluorescent fiber, including: a blue semiconductor laser element for emitting an excitation light; and an optical fiber having a first side fiber end face and a second side fiber end face, the excitation light emitted from the blue semiconductor laser element being made incident to the first side fiber end face, the excitation light thus made incident to the first side fiber end face being emitted through the second side fiber face, in which the optical fiber has dichroic mirror portions constituting a laser resonator in its first and second fiber side end faces, respectively, and a core of the optical fiber is made of a wavelength-converting member including a low phonon glass containing therein at least praseodymium ions as trivalent rare earth ions for emitting wavelength conversion lights by being excited by the excitation light.  
      In order to attain the above-mentioned object, according to another aspect of the present invention, there is provided a multiple wavelength laser light source using a fluorescent fiber, including: a blue semiconductor laser element for emitting an excitation light; and an optical fiber having a first side fiber end face and a second side fiber end face, the excitation light emitted from the blue semiconductor laser element being made incident to the first side fiber end face, the excitation light thus made incident to the first side fiber end face being emitted through the second side fiber end face, in which the optical fiber has dichroic mirror portions constituting a laser resonator in its first and second side fiber end faces, respectively, and a core of the optical fiber is made of a wavelength-converting member including a low phonon glass containing therein a phosphor for emitting wavelength conversion lights by being excited by an excitation light having a wavelength of 440 to 460 nm as the excitation light.  
      According to the present invention, the low cost promotion and miniaturization of the overall multiple wavelength laser light source can be realized, the optical fiber can be prevented from being damaged and deteriorated, and the desired blue light can be obtained as the light emitted through the optical fiber.  
      In order to attain the above-mentioned object, according to a further aspect of the present invention, there is provided a multiple wavelength laser light source using a fluorescent fiber, including: a blue semiconductor laser element for emitting a laser light; an optical fiber having a core for a wavelength-converting member containing a low phonon glass and at least praseodymium ions as trivalent rare earth ions, a first fiber end face to which the laser light is supplied, and a second fiber end face which is a light source for a multiple wavelength laser light; and first and second dichroic mirror portions, respectively, provided on the first and second fiber end faces of the optical fiber to provide a laser resonator for emitting the multiple wavelength laser light from the second fiber end face of the optical fiber. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a plan view for explaining a light-emitting device as a multiple wavelength laser light source using a fluorescent fiber according to a first embodiment of the present invention;  
       FIGS. 2A and 2B  are respectively a perspective view and a cross sectional view for explaining a blue semiconductor laser element of the light-emitting device according to the first embodiment of the present invention;  
       FIG. 3  is a cross sectional view for explaining the fluorescent fiber of the light-emitting device according to the first embodiment of the present invention;  
       FIG. 4  is a spectrum diagram of an output light emitted from the light-emitting device according to the first embodiment of the present invention; and  
       FIG. 5  is a cross sectional view for explaining a fluorescent fiber of a light-emitting device according to a second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
     First Embodiment  
       FIG. 1  is a plan view for explaining a light-emitting device as a multiple wavelength laser light source using a fluorescent fiber according to a first embodiment of the present invention.  FIGS. 2A and 2B  are respectively a perspective view and a cross sectional view for explaining a blue semiconductor laser element of the light-emitting device according to the first embodiment of the present invention. Also,  FIG. 3  is a cross sectional view for explaining the fluorescent fiber of the light-emitting device according to the first embodiment of the present invention.  
      [Overall Construction of Light-Emitting Device  1 ] 
      Referring to  FIG. 1 , a light-emitting device  1  roughly includes a blue semiconductor laser element  2  as an excitation light source, a laser resonator  3  for amplifying an excitation light (blue light) “a” emitted from the blue semiconductor laser element  2 , and wavelength conversion lights obtained through wavelength conversion by the excitation light “a” in accordance with induced emission, and an optical lens  4  interposed between the laser resonator  3  and the blue semiconductor laser element  2 .  
      [Structure of Blue Semiconductor Laser Element  2 ] 
      As shown in  FIGS. 2A and 2B , the blue semiconductor laser element  2  has a sapphire substrate  5 , a resonance ridge portion A, and a hole injection ridge portion B, and serves to emit a blue light having a wavelength of 442 nm as the excitation light a. A buffer layer  6  which has a thickness of about 50 nm and which is made of aluminum nitride (AlN) is formed on the sapphire substrate  5 . At that, GaN, GaInN or AlGaN may also be used as the material for the buffer layer  6 .  
      An n-type layer  7  which has a thickness of about 4.0 μm and which is made of a silicon (Si)-doped GaN having an electron concentration of 1×10 18  cm −3 , an n-type cladding layer  8  which has a thickness of about 500 nm and which is made of Si-doped Al 0.1 Ga 0.9 N having an electron concentration of 1×10 18  cm −3 , an n-type guide layer  9  which has a thickness of 100 nm and which is made of a Si-doped GaN having an electron concentration of 1×10 18  cm −3 , and an active layer  10  having a multi-quantum well (MQW) structure in which a barrier layer  62  which has a thickness of about 35 Å and which is made of GaN, and a well layer  61  which has a thickness of about 35 Å and which is made of Ga 0.95 In 0.05 N are alternately deposited are formed in this order on the buffer layer  6 .  
      A p-type guide layer  11  which has a thickness of about 100 nm and which is made of magnesium (Mg)-doped GaN having a hole concentration of 5×10 17  cm −3 , a p-type layer  12  which has a thickness of about 50 nm and which is made of Mg-doped Al 0.25 Ga 0.75 N having a hole concentration of 5×10 17  cm −3 , a p-type cladding layer  13  which has a thickness of about 500 nm and which is made of Mg-doped Al 0.1 Ga 0.9 N having a hole concentration of 5×10 17  cm −3 , and a p-type contact layer  14  which has a thickness of about 200 nm and which is made of Mg-doped GaN having a hole concentration of 5×10 17  cm −3  are formed in this order on the active layer  10 . At that, AlGaN or GaInN may also be used as the material for the p-type contact layer  14 .  
      An electrode  15  which has a width of 5 μm and which is made of nickel (Ni) is formed on the p-type contact layer  14 . In addition, an electrode  16  made of aluminum (Al) is formed on the n-type layer  7 .  
      The resonance ridge portion A includes the n-type cladding layer  8 , the n-type guide layer  9 , the active layer  10 , the p-type guide layer  11 , and the p-type layer  12 . In addition, the hole injection ridge portion B includes the p-type cladding layer  13 , the p-type contact layer  14 , and the electrode  15 .  
      [Construction of Laser Resonator  3 ] 
      The laser resonator  3  includes a fluorescent fiber  17  as a laser medium, and is optically connected to the blue semiconductor laser element  2  through the optical lens  4 . As described above, the laser resonator  3  serves to amplify the excitation light (blue light) “a” emitted from the blue semiconductor laser element  2 , and the wavelength conversion light obtained through the wavelength conversion by the excitation light in accordance with the induced emission.  
      As shown in  FIG. 3 , the fluorescent fiber  17  has a core  17 A and a cladding member  17 B. The fluorescent fiber  17  has one side end face (incidence face) to which the blue light from the blue semiconductor laser element  2  is made incident, and the other side end face (emission face) from which a part of the blue light is emitted as it is and for example, green, orange and red wavelength conversion lights which are obtained through the wavelength conversion of a part of the blue light within the core  17 A are emitted, respectively. The fluorescent fiber  17  is made of a fluorescent glass which does not contain therein any of ZrF 4 , HfF 4 , ThF 4  and the like, but contains therein AlF 3  as a main constituent. Thus, the stable glass is obtained which is transparent for a light range from a visible range to an infrared range, and has the excellent chemical durability and the large mechanical strength. This sort of glass has such an advantage essential to the fluorescent glass that the phonon energy is less.  
      A fiber length of the fluorescent fiber  17  is set to such a size of about 20 mm that the fluorescent fiber  17  does not absorb all the excitation light “a” from the blue semiconductor laser element  2 , but emits therefrom the green light, the orange light, and the red light in accordance with the laser oscillation. Dielectric mirrors  18  and  19  in each of which a silicon dioxide (SiO 2 ) layer and a titanium dioxide (TiO 2 ) layer are laminated and which serve as respective dichroic mirror portions constituting the laser resonator  3  are disposed in the fiber end faces of the fluorescent fiber  17 , respectively. One dielectric mirror  18  functions as an input mirror, and the other dielectric mirror  19  functions as an output mirror.  
      The core  17 A is formed of a wavelength-converting member including a low phonon glass such as an infrared radiation transmissive fluorescent glass containing therein at least praseodymium ions (Pr 3+ ) as trivalent rare earth ions by about 500 ppm. Also, the core  17 A serves to emit the green, orange and red wavelength conversion lights by being excited by a part of the excitation light (blue light) “a” from the blue semiconductor laser element  2 . A core diameter of the core  17 A is set to a size of about 6 μm. At that, in addition to the infrared radiation transmissive fluorescent glass, a heavy metal oxide glass is used as the low phonon glass.  
      The cladding member  17 B is formed in the periphery of the core  17 A, and the overall cladding member  17 B is made of a glass or a transparent resin. A refractive index n 1  of the cladding member  17 B is set to smaller one (n 1 ≈1.45) than that n 2  (n 2 ≈1.5) of the core  17 A. A cladding diameter (an outer diameter of the fluorescent fiber  17 ) of the cladding member  17 B is set to a size of about 200 μm. A peripheral surface of the cladding member  17 B is covered with a cover member  18  made of a light-transmissive resin or a light-nontransmissive resin.  
      [Structure of Optical Lens  4 ] 
      The optical lens  4  is constituted by a double-convex lens, and is disposed between the blue semiconductor laser element  2  and the laser resonator  3  in the manner as described above. Also, the optical lens  4  serves to condense the excitation light a emitted from the blue semiconductor laser element  2  to a portion located in the incidence side end face of the dielectric mirror  18 , i.e., the input side end face of the fluorescent fiber  17  (the core  17 A).  
      [Operation of Light-Emitting Device  1 ] 
      Firstly, when a suitable voltage is applied from a power source to the blue semiconductor laser element  2 , a luminous layer of the blue semiconductor laser element  2  emits the blue light “a”, and the blue light “a” is radiated to the optical lens  4  side. The blue light “a” emitted from the blue semiconductor laser element  2  is then made incident to the dielectric mirror  18  of the laser resonator  3  through the optical lens  4 . In the laser resonator  3 , the blue light a then penetrates the dielectric mirror  18  to be made incident to the core  17 A of the fluorescent fiber  17 , and is guided to the dielectric mirror  18  while total reflection thereof is made within the core  17 A. Then, when reaching the dielectric mirror  18 , the blue light “a” is reflected by the dielectric mirror  19  to be guided to the dielectric mirror  18  while the total reflection thereof is made within the core  17 A. In this case, the blue light “a” is reflected between both the dielectric mirrors  18  and  19  within the core  17 A, and excites the praseodymium ions, whereby the green, orange and red wavelength conversion lights are emitted, respectively. After that, the blue light “a”, and the green, orange and red wavelength conversion lights penetrate the dielectric mirror  19  to be emitted in the form of a multiple wavelength output light “b” to the outside of the laser resonator  3 .  
      Next, a description will be given with respect to the results of an experiment of observing the multiple wavelength output light “b” emitted from the light-emitting device  1  according to this embodiment of the present invention.  
      This experiment was made such that the dielectric mirror  18  which transmitted the blue light “a”, but reflected the orange and red lights by 99% was prepared as an input mirror, and the dielectric mirror  19  which reflects the orange light and the red light by 90% was prepared as an output mirror, and the blue light (its wavelength was 442 nm) from the blue semiconductor laser element  2  (under the excitation conditions of 20 mW and 35 mW) was made incident to the laser resonator  3 . As a result of the experiment, the red light having a wavelength of 635 nm as the wavelength conversion light was confirmed together with the blue light having the wavelength of 442 nm as the excitation light “a” under the excitation condition of 20 mW, and the red light having a wavelength of 635 nm as the wavelength conversion light and the orange light having a wavelength of 606 nm as the wavelength conversion light were also confirmed together with the blue light having the wavelength of 442 nm as the excitation light “a” under the excitation condition of 35 mW. When the emitted light during the emission of the red and orange lights was measured, there was observed an emission spectrum having sharp emission wavelength peaks of the blue light as the excitation light, and the red and orange lights as the wavelength conversion lights. The observation results are shown in the form of a spectrum diagram in  FIG. 4 . In  FIG. 4 , an axis of ordinate represents the light intensity, and an axis of abscissa represents a wavelength.  
      According to the first embodiment as has been described so far, the following effects are obtained.  
      (1) Since the single laser light source (the blue semiconductor laser element  2 ) outputs the multiple wavelength laser light, the number of components or parts can be reduced, and thus the low cost promotion and miniaturization of the overall light-emitting device can be realized.  
      (2) Since the fluorescent fiber  17  is made of the low phonon glass including the fluoride glass which does not contain therein any of ZrF 4 , HfF 4 , ThF 4  and the like, but contains therein AlF 3  as the main constituent, the mechanical strength and chemical durability of the fluorescent fiber  17  are enhanced, and thus the fluorescent fiber  17  can be prevented from being damaged and deteriorated.  
      (3) Since the blue light having the wavelength of 442 nm is used as the excitation light “a”, the desired (pure) blue light can be obtained as the light emitted through the light emission face of the fluorescent fiber  17 .  
     Second Embodiment  
       FIG. 5  is a cross sectional view for explaining a fluorescent fiber of a light-emitting device according to a second embodiment of the present invention. In  FIG. 5 , the same members as those shown in  FIG. 3  are designated with the same reference numerals, and its detailed description is omitted here.  
      As shown in  FIG. 5 , the feature of the light-emitting device  1  (as shown in  FIG. 1 ) in the second embodiment is that the light-emitting device  1  includes a fluorescent fiber  50  having a cladding member  51  including a first cladding member  51 A which is formed adjacently to the peripheral surface of the core  17 A, and a second cladding member  51 B which is formed adjacently to a peripheral surface of the first cladding member  51 A.  
      For this reason, a refractive index n 1  of the first cladding member  51 A is set to one (n 1 ≈1.48) that is smaller than that n 2  (n 2 ≈1.50) of the core  17 A, but is larger than that n 3  (n 3 ≈1.45) of the second cladding member  51 B.  
      According to the second embodiment as has been described so far, in addition to the effects (1) to (3) of the first embodiment, the following effect is obtained.  
      The first cladding member  51 A can function as an optical waveguide. Also, the green, orange and red wavelength conversion lights can be obtained by deriving the excitation light “a” guided to the first cladding member  51 A into the core  17 A.  
      While the light-emitting device of the present invention has been described in accordance with the above-mentioned first and second embodiments, it should be noted that the present invention is not intended to be limited to the above-mentioned first and second embodiments, and can be implemented in the form of various kinds of aspects without departing the gist thereof. For example, the following changes can be made.  
      (1) While in the first and second embodiments, the description has been given with respect to the case where the dichroic mirror portions constituting the laser resonator  3  are formed by disposing the dielectric mirrors  18  and  19  in the fiber end faces of the fluorescent fiber  17 , respectively, the present invention is not limited thereto. That is to say, the dichroic mirror portions may also be formed by evaporating reflecting films onto the fiber end faces of the optical fiber, respectively. In addition, the dichroic mirror portions may also be formed by disposing reflecting mirrors in positions facing the fiber end faces of the fluorescent fiber through collimate lenses, respectively.  
      (2) While in the first and second embodiments, the description has been given with respect to the case where the blue light having the wavelength of 442 nm is used as the excitation light “a” emitted from the blue semiconductor laser element  2 , the present invention is not limited thereto. That is to say, the blue light having the high excitation efficiency, and having a wavelength falling within the range of 440 to 460 nm in which that blue light can be used as the output light as it is may be used as the excitation light “a”.  
      (3) While in the first and second embodiments, the description has been given with respect to the case where a content m of the trivalent praseodymium ions (Pr 3+ ) is set to 500 ppm, the present invention is not limited thereto. That is to say, the content m of the trivalent praseodymium ions may be set to one falling within the range of 100 ppm≦m≦10,000 ppm. In this case, when the content m is less than 100 ppm, neither of the wavelength conversion lights is obtained within the core  17 A. On the other hand, when the content m is more than 10,000 ppm, the light-transmissive property within the core  17 A becomes poor.