Abstract:
There is provided a light source apparatus including at least a excitation light source and a photocoupler, a light source medium formed of one kind of a single, rare-earth element doped fiber that is excited by the excitation light source and has two terminals, and an output end formed of an optical fiber, the light source apparatus being characterized in that an optical component having at least one of reflection, branching or attenuation function is provided in anywhere of an optical circuit from the light source medium to the output terminal and that it is possible to output lights of different wavelengths derived from different spontaneous emission light peaks that are generated from rare-earth element ions added to the rare-earth element doped fiber, and that at least one of the lights to be obtained is visible light.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a multiwaveform light source capable of outputting lights of different wavelengths. 
       BACKGROUND OF THE INVENTION 
       [0002]    Rare-earth element doped fibers have been used as wavelength conversion components, for light source uses such as amplifier, spontaneous emission light (ASE) light source and laser. Hitherto, there has also been a study of a technique for obtaining lights having wavelengths shorter than the visible light by using up-conversion process, and it has become possible to obtain lights of various wavelengths by rare-earth doped fibers (see Non-patent Publication 1 and Patent Publication 1). 
         [0003]    However, in case that lights of different wavelengths are necessary, in general, it has been necessary to prepare a plurality of light source apparatuses. In this case, a plurality of lasers of different wavelengths or the like are used, but the apparatus may become large-scale. Therefore, there have been problems such as not only necessity for wide space and cooler, but also time and cost for maintenance. 
         [0004]    In contrast with this, it is known that a plurality of laser lights of different wavelengths are obtained at 1550 nm band in a fiber laser using Er 3+ . Many of these are those invented with their use as a light source for optical communication in mind, and they generate a light of a band region that is amplified by a light amplifier (see, for example, Patent Publication 2). 
         [0005]    Furthermore, there is a report that a visible light laser oscillation of blue color (492 nm) and green color (521 nm) is obtained. 
       Patent Publication 1: Japanese Patent Application Publication 2005-26475 
     Patent Publication 2: Japanese Patent Application Publication 10-209501 
     Non-patent Publication 1: Michel J. F. Digonnet; “Rare-Earth-Doped Fiber Lasers and Amplifiers”; MARCELDEKKER, INC. (1993); pp 171-242. 
       [0006]    Non-patent Publication 2: D. M. Baney, G. Rankin, K. W. Chan; Appl. Phys. Lett. 69: pp 1662-1664, (1996) 
       SUMMARY OF THE INVENTION 
       [0007]    However, many of fiber lasers using Er 3+  as described in the above Japanese Patent Application Publication 10-209501 are those invented with their use as a light source for optical communication in mind, and they generate a light of a band region that is amplified by a light amplifier. Therefore, they use ASE peaks existing in 1550 nm band of Er 3+  and generate laser lights of a plurality of wavelengths in this band region. 
         [0008]    However, in the case of thinking of simultaneously generating a plurality of laser lights by the same principal in a wavelength region from visible light to near infrared, it is not possible to obtain a multiwaveform light source in the above wavelength region by the identical method, due to no existence of ASE in a wide band region. 
         [0009]    On the other hand, in the above publication of Baney et al., visible light laser oscillation of blue light and green light of different wavelengths in visible light region has been confirmed, and the possibility of multicolor in the future has been shown, but both ends of the fiber have reflective coating, and spatial optical system is used for many of optical systems. Therefore, this method had a problem such as that it is inferior in industrial productivity and is not practical. 
         [0010]    The present invention was made in view of the above situation, and it is an object of the present invention to provide a light source apparatus for simultaneously generating lights of a plurality of wavelengths including visible region from a single apparatus. 
         [0011]    According to the present invention, there is provided, for solving the above task, a light source apparatus comprising at least a excitation light source and a photocoupler, a light source medium formed of one kind of a single, rare-earth element doped fiber that is excited by the excitation light source and has two terminals, and an output end formed of an optical fiber, the light source apparatus being characterized in that an optical component having at least one of reflection, branching or attenuation function is provided in anywhere of an optical circuit from the light source medium to the output terminal and that it is possible to output lights of different wavelengths derived from different spontaneous emission light peaks that are generated from rare-earth element ions added to the rare-earth element doped fiber, and that at least one of the lights to be obtained is visible light. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a schematic view showing structure of a laser multicolor light source according to Example 1 of the present invention; 
           [0013]      FIG. 2  is a schematic view showing structure of an ASE multicolor light source according to Example 2 of the present invention; 
           [0014]      FIG. 3  is a schematic view showing structure of a laser and ASE mixed multicolor light source according to Example 3 of the present invention; 
           [0015]      FIG. 4  is a schematic view of a reflective mirror module in Example 3; and 
           [0016]      FIG. 5  is a schematic view of structure of a multicolor light source in the case of forming a ring-type, laser oscillator according to Example 4 of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    According to the present invention, it is possible to obtain a simple light source apparatus that makes it possible to simultaneously take out lights of a plurality of wavelengths including visible region from one light source. 
         [0018]    The present invention can be used for an applied field of optical transmission such as evaluation and measurement as well as communication system in optical communication field. 
         [0019]    In the following, the present invention is exemplarily explained in detail with reference to the attached drawings. 
         [0020]      FIG. 1  is a schematic view showing an example of a light source apparatus of the present invention. In this example, it is possible to obtain laser lights of two different wavelengths. The light source apparatus is composed of excitation light source  101 , first photocoupler  102 , rare-earth element doped fiber  103 , reflective mirror  104 , optical fiber filter  105 , first fiber Bragg grating (FBG)  106 , second FBG  107 , optical isolator  108 , second photocoupler  109 , first output end  110 , and second output end  111 . 
         [0021]    Although the excitation light source is assumed to be a semiconductor laser, it may be one in which another solid laser, gas laser, lamp, LED or the like is coupled to an optical fiber. It is ideal to use it by attaching an optical isolator. 
         [0022]    The excitation light source may have a longer wavelength or shorter wavelength than the wavelength to be obtained, and any wavelength can be used as long as an efficient excitation is possible. 
         [0023]    Wavelength and power of the excitation light are examined according to wavelength of the light to be obtained and the type of the rare-earth element doped fiber to be used. It may not necessarily be a single wavelength, and may be a mixture of lights of a plurality of wavelengths. 
         [0024]    Specification of the photocoupler is determined by the excitation light wavelength and the light source wavelength to be obtained. It is desired to be capable of supplying the excitation light to the rare-earth element doped fiber with a loss as small as possible and to have a loss as small as possible also to the light to be obtained. 
         [0025]    The rare-earth element doped fiber refers to an optical fiber that various kinds of optical fibers, such as silica, silicate, fluoride and chalcogenide series, have been doped at core or cladding with a rare-earth element such as Er, Pr, Nd, Ho and Tm. The type of rare-earth is selected according to wavelength of the light to be obtained or wavelength of the excitation light. In some cases, it is possible to conduct a codoping with other rare-earths and transition metals or a doping with different elements at cladding and core. 
         [0026]    In the present invention, one light source apparatus is equipped with one kind of a single, rare-earth element doped fiber. Herein, “one kind of a single” refers to that a fiber relating to the excitation light is singular in the light source structure. That is, it is possible in the light source of the present invention to simultaneously generate lights of a plurality of wavelengths by one kind of a single fiber excited. 
         [0027]    In the case of the present example, the reflective mirror is essentially for forming a laser oscillator structure and has a high reflectance to one or a plurality of laser wavelengths to be obtained. Although reflectance has arbitrariness in design, it is generally around 50-99.9%. 
         [0028]    It is also possible to increase reflectance to wavelength of the excitation light for the purpose of improving energy efficiency of the light source itself. With this, it is possible to introduce again the excitation light escaping out of the oscillator, into the rare-earth element doped fiber, thereby improving energy efficiency. 
         [0029]    Furthermore, there may exist a high efficiency wavelength by positively confining even a light having a wavelength other than the target wavelength in the oscillator, for improving the excitation light rate and the output at the target wavelength. 
         [0030]    Even in the case of considering an ASE light source that is different from that of the present example, it is possible to improve ASE power by using the reflective mirror. In the case of ASE light source, however, reflective mirror is not necessarily requisite. 
         [0031]    Alternatively, it is possible to monitor optical power with optical sensor or the like by lowering reflectance of the reflective mirror without relying on the laser light source and the ASE light source. 
         [0032]    Although an example using a mirror is shown in the present example, it is not particularly limited to reflective mirror. It is also possible to use a reflective component such as FBG as a substitute. 
         [0033]    An ordinary optical fiber is used for optical fiber filter  105 . Bending loss is generated by bending optical fiber according to its NA, bending curvature and the like. It is found on the long wavelength side. Therefore, it can be used as a long wavelength cutting filter by selecting fiber. It is also optional to suppress a long wavelength light outputted from the rare-earth element doped fiber by controlling NA, bending curvature and the like of the rare-earth element doped fiber. 
         [0034]    It is not necessary to limit the method for attenuating an unnecessary light of long wavelength to particularly this method. It is also possible to use a method using a dielectric multilayer film filter, absorptive mirror, or the like. 
         [0035]    As a method for attenuating unnecessary lights of short wavelengths, there are a method of using a dielectric multilayer film filter and a method of using an absorptive mirror or a filter or fiber doped with an absorptive substance. 
         [0036]    Characteristics of first and second FBG&#39;s are determined by laser lights of different wavelengths to be obtained. This two FBG&#39;s and the reflective mirror make respective pairs to form laser oscillators. Reflectance has arbitrariness, and it is possible to control output, efficiency and balance of two kinds of laser light powers by the change of reflectance. 
         [0037]    FBG is a light reflective device and has been used in the present example in order to form a laser oscillator. Therefore, the position of FBG is required that FBG&#39;s that make the pairs are disposed such that oscillators are constructed at both ends of the rare-earth element doped fiber. 
         [0038]    Although an example using FBG has been shown in this example, it is not limited to FBG. Basically, it suffices to form a laser oscillator. It is optional to use a reflective mirror, to form a film on the fiber edge face, or to use another method. 
         [0039]    Alternatively, in the case of obtaining a laser light source, it is also possible to form an oscillator by using a weak reflective light from the fiber edge face in place of FBG of low reflectance. 
         [0040]    The second photocoupler is for separating a laser light of a wavelength of 543 nm and a laser light of a wavelength of 850 nm, which have been generated by this light source. It is not particularly necessary in case that separation is not necessary. 
       Example 1 
       [0041]    In the following, the present invention is explained by citing examples. 
         [0042]    The present examples is a case in which laser lights of two different wavelengths based on  FIG. 1  are obtained. 
         [0043]    As rare-earth element doped fiber  103 , an erbium-doped fluoride fiber having a length of 1 m and an erbium concentration of 4000 ppm was used. 
         [0044]    As excitation light source  101 , there was used a semiconductor laser of a wavelength o 972 nm with an isolator. 
         [0045]    As first photocoupler  102 , one capable of multiplexing the excitation light and a laser light to be generated, that is, an excitation light of 972 nm and laser lights of 543 nm and 850 nm, was used. 
         [0046]    Two oscillators were formed by reflective mirror  104 , first FBG  106  and second FBG  107 . 
         [0047]    As a reflective mirror, one having a reflectance of 90% or greater at wavelengths of 543 nm and 850 nm was used. Reflectance at 972 nm of the excitation light was 45%. 
         [0048]    Optical fiber filter  105  had an NA of 0.13 and a cutoff wavelength of 730 nm. 
         [0049]    First FBG had a reflectance of 80% at a wavelength of 543 nm and a reflectance of 80% at a wavelength of 850 nm. An optical isolator capable of being used at a wavelength of 543 nm and a wavelength of 850 nm was used. 
         [0050]    Laser light of a wavelength of 543 nm and laser light of a wavelength of 850 nm were separated by second photocoupler  109 . 
         [0051]    Excitation light source  101  outputs about 400 mW of an excitation light of 972 nm. This excitation light passes through photocoupler  102 , is introduced into erbium-doped fluoride fiber  103 , and activates erbium in the erbium-doped fluoride fiber. The activated erbium emits ASE lights of some wavelength regions. Of these ASE lights, lights of 850 nm and 543 nm are subjected to laser oscillation in the oscillator, and respectively pass through first FBG and second FBG. A laser light of 543 nm is outputted from output end  110 , and a laser light of 850 nm is outputted from output end  111 . A light of long wavelength, for example, 1550 nm, of ASE light is sufficiently attenuated and is not outputted by the filter fiber. 
         [0052]    The outputs upon this were 2.5 mW with a laser light of 543 nm and 1.1 mW with a laser light of 850 nm. 
       Example 2 
       [0053]    The present example is an example of a light source apparatus having a structure from which first FBG and second FBG have been removed. In this example, it is possible to take out ASE lights of 550 nm band and 850 nm band from two different output ends. 
         [0054]    Example 1 was conducted in the same way except the omission of first FBG and second FBG. 
         [0055]    An ASE light of a wavelength of 550 nm band and an ASE light of a wavelength of 850 nm band were respectively obtained from output end  110  and output end  111 . 
       Example 3 
       [0056]      FIG. 3  is a schematic view showing a light source apparatus. In the light source apparatus in this example, laser lights of three different wavelengths are obtained, and furthermore a laser light and an ASE light of one wavelength band are simultaneously emitted from one port. 
         [0057]    The light source apparatus is composed of excitation light source  201 , first photocoupler  202 , rare-earth element doped fiber  203 , first reflective mirror module  204 , second reflective mirror module  205 , first FBG  206 , second FBG  207 , third FBG  208 , second photocoupler  209 , optical fiber filter  210 , first optical isolator  211 , second optical isolator  212 , third optical isolator  213 , first output end  214 , second output end  215 , and third output end  216 . 
         [0058]    As rare-earth element doped fiber  203 , a praseodymium and ytterbium co-doped fluoride fiber having a length of 1 m, a praseodymium concentration of 1000 ppm and an ytterbium concentration of 10000 ppm was used. 
         [0059]    A laser light of 840 nm obtained by semiconductor laser and a laser light of 1015 nm obtained by fiber laser were used for excitation light source  201 . 100 mW of excitation light of 840 nm was outputted, and 400 mW of laser light of 1015 nm was outputted. These excitation lights were multiplexed by the photocoupler, and furthermore an isolator was attached thereto. This was used as an excitation source. 
         [0060]    As first and second reflective mirror modules  204  and  205 , modules were used which have a structure in which a laser light in optical fiber  301  is once made into a collimated light by collimate lens  302 , followed by passing through or reflection from filter mirror  303  and then respectively making light incident on the opposite side or the original optical fiber  301  ( FIG. 4 ). 
         [0061]    As the filter used in first reflective mirror module  204 , there was used a filter having reflections respectively to wavelengths of 635 nm and 600 nm respectively outputted from second and third output ends and having small reflectances to a blue light and a light of 521 nm outputted from first output end. Reflectances to wavelengths of 635 nm and 600 nm were 95%. On the other hand, reflectances to the blue light and the light of 521 nm were less than 3%. 
         [0062]    As the filter used in second reflective mirror module  205 , there was used a filter having reflections respectively to a blue light and a light of 521 nm outputted from first output end and having small reflectances to lights of 600 nm and 635 nm outputted from second and third output ends. Reflectances to the blue light and a wavelength of 521 nm were 93%. On the other hand, reflectances to the lights of 600 nm and 635 nm were less than 4%. The second reflective mirror is not necessarily required to have a reflection to blue color light. 
         [0063]    First FBG  206  and second reflective mirror module  205  make a pair, thereby forming a laser oscillator to a wavelength of 521 nm. Reflectance was 80%. 
         [0064]    Second FBG  207  and first reflective mirror module  204  make a pair, thereby forming a laser oscillator to a wavelength of 635 nm. Reflectance was 80%. 
         [0065]    Third FBG  208  and first reflective mirror module  204  make a pair, thereby forming a laser oscillator to a wavelength of 600 nm. Reflectance was 80%. 
         [0066]    Second photocoupler is for separating laser lights of 635 nm and 600 nm. 
         [0067]    For two optical fiber filters  210 , 2 m of a fiber having an NA of 0.13 and a cutoff wavelength of 0.57 μm was used, and it was wound to have a diameter of 5 cm. With this, a light having a wavelength longer than about 0.9 μm is attenuated and is not outputted from the first to third output ends. 
         [0068]    An excitation light of 100 mW at 840 nm and 400 mW at 1015 nm was outputted from the excitation light source. This excitation light passes through photocoupler  202 , then is introduced into praseodymium and ytterbium co-doped fluoride fiber  203 , and then activates praseodymium in praseodymium and ytterbium co-doped fluoride fiber  203 . The activated praseodymium emits ASE lights of some wavelength regions. Of these ASE lights, lights of 635 nm, 600 nm and 521 nm are subjected to laser oscillation in the oscillator. They are respectively outputted from second output end  215 , third output end  216  and first output end  214  via second optical isolator  212 , third optical isolator  213  and first optical isolator  211 . A blue color ASE light at around 490 nm is reflected from second reflective mirror module  205  or directly passes through first optical isolator  211  and then is outputted from first output end  214 . 
         [0069]    Relative to laser light, first to third optical isolators can prevent incidence of light from the outside of the oscillator, thereby stabilizing laser output. Furthermore, it is possible to prevent an unexpected laser oscillation, which is generated by forming an oscillator in ASE light source with a reflective object of the outside of the apparatus. 
         [0070]    Of the excitation lights, a light not absorbed by praseodymium and ytterbium co-doped fluoride fiber  203  was attenuated by first reflective mirror module  204  and first optical isolator  211  and therefore was not outputted. 
         [0071]    Although ASE light is observed at 1.3 μm band in praseodymium and ytterbium co-doped fluoride fiber, it was attenuated by optical fiber filter  210  and therefore was not outputted. 
         [0072]    As the light source, there were obtained about 10 μW of 490 nm blue-color ASE light, 1 mW of 521 nm laser light and 8 mW of 635 nm laser light. 
       Example 4 
       [0073]      FIG. 5  is a schematic view showing a light source apparatus. In the present example, a ring-type laser oscillator was formed, and a multicolor light source was prepared. 
         [0074]    The light source apparatus is composed of excitation light source  401 , first photocoupler  402 , rare-earth element doped fiber  403 , second photocoupler  404 , third photocoupler  405 , first output end  406 , and second output end  407 . 
         [0075]    As rare-earth element doped fiber  403 , there was used an erbium-doped fluoride fiber having a length of  1   m  and an erbium concentration of 2000 ppm. 
         [0076]    A semiconductor laser was used as excitation light source  401 . Output was 400 mW at a wavelength of 972 nm. 
         [0077]    First photocoupler  402  was installed for supplying excitation light to the ring-type oscillator. 
         [0078]    Second photocoupler  404  and third photocoupler  405  were installed for respectively taking 543 nm laser light and 850 nm laser light out of the oscillator. Regarding branching ratio, branching ratio of 550 nm light was 50% in second photocoupler  404 . It was 30% with 850 nm light in third photocoupler  40 . 
         [0079]    The excitation light outputted from excitation light source  401  activates erbium ions of rare-earth element doped fiber  403 . The activated erbium ions emitted various ASE lights. By their circulation in the ring oscillator, gain was obtained successively, thereby reaching laser oscillation. 
         [0080]    In the present example, 543 nm laser light was branched at second photocoupler and then its laser light was outputted from first output end  406 . 850 nm laser light was branched at third photocoupler  405  and then outputted from second output end  407 . By changing branching ratio of photocoupler, it becomes possible to control output. 
         [0081]    543 nm laser light of 3 mW was outputted from first output end  406 , and 850 nm laser light of 2.5 mW was outputted from second output end  407 . 
         [0082]    By selecting fiber used for the ring oscillator, it is possible to give a sufficiently large fiber bending loss, with respect to a long wavelength light such as 1550 nm band light. Therefore, laser oscillation of 1550 nm band light was not found. 
         [0083]    Although light of short wavelength region was not problematic in the present example, it is optional in some cases to use a filter module as shown in Example 3. By using absorptive filter, it becomes possible to control gain at various wavelength regions and to control output. 
         [0084]    Although optical isolator was not used in the present example, it becomes possible to get stabilization and higher output by setting the direction of light circulating through the ring oscillator to a single direction by using isolator.