Patent Publication Number: US-2023152666-A1

Title: Light-emitting system

Description:
TECHNICAL FIELD 
     The present disclosure generally relates to a light-emitting system, and more particularly relates to a light-emitting system including an optical fiber. 
     BACKGROUND ART 
     A lighting fixture including a case, a projection lens, and a light source device has been proposed in the art (see, for example, Patent Literature 1). The light source device disclosed in Patent Literature 1 includes a solid-state light source and a light transmission fiber. The light transmission fiber has a first end face and a second end face, and excitation light emitted from the solid-state light source is introduced into the fiber through the first end face thereof. The light transmission fiber includes a wavelength-converting core, a light-guiding core, and a clad. The wavelength-converting core contains a wavelength-converting material that produces a population inversion state of electrons by absorbing the excitation light and that lets wavelength-converted light, falling within the visible radiation range, emerge therefrom. The light-guiding core covers the peripheral surface of the wavelength-converting core and transmits the wavelength-converted light in a direction from the first end face toward the second end face. The clad covers the peripheral surface of the light-guiding core. 
     The light transmission fiber is configured to have a stimulated emission produced by the wavelength-converted light propagating through the light-guiding core and to let not only the excitation light, emitted from the solid-state light source, but also the wavelength-converted light, amplified by the stimulated emission, emerge from the second end face. 
     It is difficult for the lighting fixture of Patent Literature 1 to increase the intensity of the wavelength-converted light. 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Literature 1: JP 2018-195627 A 
       
    
     SUMMARY OF INVENTION 
     It is therefore an object of the present disclosure to provide a light-emitting system with the ability to increase the intensity of light having a different wavelength from excitation light. 
     A light-emitting system according to an aspect of the present disclosure includes an optical fiber, a first light source unit, a second light source unit, and a light-guiding member. The optical fiber includes a wavelength-converting portion containing a wavelength-converting element. The wavelength-converting element may be excited by excitation light to produce a spontaneous emission of light having a longer wavelength than the excitation light. The wavelength-converting element may also be excited by an amplified spontaneous emission of light. The first light source unit makes the excitation light incident on the optical fiber. The second light source unit makes seed light incident on the optical fiber. The seed light causes the wavelength-converting element that has been excited by either the excitation light or the amplified spontaneous emission of light to produce a stimulated emission of light. The light-guiding member guides the light coming from the optical fiber and lets the light emerge therefrom. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    illustrates a configuration for a light-emitting system according to a first embodiment; 
         FIG.  2    is a cross-sectional view of an optical fiber included in the light-emitting system; 
         FIGS.  3 A- 3 C  illustrate an operating principle of the light-emitting system; 
         FIG.  4    illustrates a configuration for a light-emitting system according to a second embodiment; 
         FIG.  5    illustrates a configuration for a light-emitting system according to a third embodiment; 
         FIG.  6 A  is a plan view of a light-emitting system according to a fourth embodiment; and  FIG.  6 B  is a cross-sectional view illustrating a main part of the light-emitting system. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The drawings to be referred to in the following description of first to fourth embodiments are all schematic representations. That is to say, the ratio of the dimensions (including thicknesses) of respective constituent elements illustrated on the drawings does not always reflect their actual dimensional ratio. 
     First Embodiment 
     A light-emitting system  1  according to a first embodiment will be described with reference to  FIGS.  1 - 3 C . 
     (1) Overview 
     The light-emitting system  1  makes excitation light P 1  and seed light P 2  incident on an optical fiber  2  to which a wavelength-converting element (chemical element) is added as shown in  FIGS.  1  and  2   . The excitation light P 1  excites the wavelength-converting element. The seed light P 2  causes the wavelength-converting element that has been excited by the excitation light P 1  to produce a stimulated emission of light P 3  (see  FIG.  3 C ). From the optical fiber  2 , light including the excitation light P 1  and the stimulated emission of light P 3  emerges.  FIGS.  3 A- 3 C  illustrate the principle of operation of the light-emitting system  1 . In  FIGS.  3 A,  3 B, and  3 C , the ordinate represents the energy of electrons. The upward arrow shown in  FIG.  3 A  indicates absorption of the excitation light P 1 . The downward arrow shown in  FIG.  3 C  indicates transition about a spontaneous emission of light or a stimulated emission of light P 3 . In the light-emitting system  1 , an electron e −  in a ground state E 0  (including a plurality of energy levels) of the wavelength-converting element is excited to an excitation level E 2  by the excitation light P 1  that has been incident on the optical fiber  2 . Then, the electron e −  at the excitation level E 2  makes a transition to a metastable level E 1 , which is an energy level lower than the excitation level E 2 . Thereafter, the stimulated emission of light P 3  (P 32 ) is produced when the electron e −  at the metastable level E 1  is caused to make a transition to one of the highest ones of the plurality of energy levels (hereinafter referred to as a “second energy level”) of the ground state E 0  by the seed light P 2  (P 22 ), of which the wavelength corresponds to the difference in energy between the metastable level E 1  and the second energy level, for example. In addition, a stimulated emission of light P 3  (P 31 ) is also produced when the electron e −  at the metastable level E 1  is caused to make a transition to another energy level (hereinafter referred to as a “first energy level”), lower than the second energy level out of the plurality of energy levels of the ground state E 0 , by the seed light P 2  (P 21 ), of which the wavelength corresponds to the difference in energy between the metastable level E 1  and the first energy level. 
     The light-emitting system  1  may be used for lighting purposes, for example. However, this is only an example and should not be construed as limiting. Alternatively, the light-emitting system  1  may also be used for display and illumination purposes. The light-emitting system  1  may be applied to a facility or a moving vehicle, whichever is appropriate. Examples of facilities to which the light-emitting system  1  is applicable include warehouses, airports, single-family dwelling houses, multi-family dwelling houses, office buildings, stores, art museums, hotels, and factories. Examples of moving vehicles to which the light-emitting system  1  is applicable include automobiles, bicycles, railway trains, aircrafts, watercrafts, and drones. 
     (2) Configuration for Light-Emitting System 
     The light-emitting system  1  includes the optical fiber  2 , a first light source unit  11 , a second light source unit  12 , and a light-guiding member  6  as shown in  FIG.  1   . The first light source unit  11  makes the excitation light P 1  incident on a light incident portion  21  of the optical fiber  2 . The second light source unit  12  makes the seed light P 2 , which causes the wavelength-converting element excited by the excitation light P 1  to produce the stimulated emission of light P 3  (hereinafter referred to as “external seed light P 2 ”), incident on the light incident portion  21 . 
     (2.1) Optical Fiber 
     The optical fiber  2  includes a core  3 , a clad  4 , and a coating portion  5  as shown in  FIG.  2   . The clad  4  covers the outer peripheral surface of the core  3 . The coating portion  5  covers the outer peripheral surface of the clad  4 . A cross section, taken along a plane perpendicular to the optical axis, of the core  3  has a circular shape. The clad  4  is disposed coaxially with the core  3 . 
     The core  3  has a first end face and a second end face, which is located at the opposite longitudinal end of the core  3  from the first end face. The core  3  includes a light-transmitting material and the wavelength-converting element. The concentration of the wavelength-converting element in the core  3  may or may not be uniform along the entire length of the core  3 . The refractive index of the core  3  may be substantially equal to the refractive index of the light-transmitting material that is a main component of the core  3 . 
     The light-transmitting material may be, for example, a fluoride, an oxide, or a nitride. The fluoride may be glass fluoride, for example. The oxide may be a silicon oxide or quartz, for example. 
     The wavelength-converting element is a rare earth element. In this embodiment, the wavelength-converting element includes an element selected from the group consisting of, for example, Pr, Tb, Ho, Dy, Er, Eu, Nd, and Mn. The wavelength-converting element is contained as an ion of a rare earth element in the core  3 , e.g., contained as an ion of Pr (Pr 3+ ) or an ion of Tb (Tb 3+ ) in the core  3 . In this case, the wavelength-converting element may be excited by either the excitation light P 1  or an amplified spontaneous emission (ASE) of light. The amplified spontaneous emission (ASE) of light is produced by amplifying the spontaneous emission of light, emitted from a different wavelength-converting element other than the wavelength-converting elements itself, as internal seed light. Through such excitation, the wavelength-converting element emits not only an ASE unique to the element of the wavelength-converting element but also a stimulated emission of light having the same wavelength as the external seed light P 2 , thus emitting them as the stimulated emission of light P 3 . The wavelengths of the ASE and the external seed light P 2  are longer than the wavelength of the excitation light P 1  (which may fall within the range from 440 nm to 450 nm, for example). The wavelength of the seed light P 2  will be described later in the “(2.3) Second light source unit” section. 
     Pr 3+  is a wavelength-converting element that may emit either an ASE or amplified seed light in the cyan to red range. The intensity of the stimulated emission of light depends on the respective intensities of the internal seed light (the spontaneous emission of light) and the external seed light. If the core  3  contains Pr 3+  and Tb 3+ , then Tb 3+  is excited by absorbing an ASE from Pr 3+  and may produce an ASE having a wavelength unique to Tb 3+ . 
     The refractive index of the clad  4  is less than the refractive index of the core  3 . The clad  4  does not contain the wavelength-converting element contained in the core  3 . 
     The material of the coating portion  5  may be a resin, for example. 
     The optical fiber  2  includes the light incident portion  21 , a light emerging portion  22 , and a wavelength-converting portion  23 . 
     The light incident portion  21  is a portion on which the excitation light P 1  is incident and may include the first end face of the core  3 , for example. The light emerging portion  22  includes the second end face of the core  3 , through which light including the excitation light P 1  and the stimulated emission of light P 3  such as an ASE emerges. 
     The light incident portion  21  may include a reflection reducing portion for reducing the reflection of the excitation light P 1  incident on the light incident portion  21  from outside of the optical fiber  2 . The reflection reducing portion may be, for example, an anti-reflection coating that covers the first end face of the core  3 . 
     The wavelength-converting portion  23  is provided between the light incident portion  21  and the light emerging portion  22 . The wavelength-converting portion  23  contains a wavelength-converting element which is excited by the excitation light P 1  to emit light having a longer wavelength than the excitation light P 1 . The wavelength-converting element is an element that may absorb the excitation light P 1  and amplify, by stimulated emission, either the spontaneous emission of light or seed light having a longer wavelength than the excitation light P 1 . 
     The core  3  may have a diameter falling within the range from 25 μm to 500 μm, for example. The optical fiber  2  may have a length falling within the range from 3 m to 10 m, for example. As for the length of the wavelength-converting portion  23 , the lower the concentration of the wavelength-converting element in the wavelength-converting portion  23  is, the greater the length of the wavelength-converting portion  23  preferably is. The optical fiber  2  may have a numerical aperture of 0.22, for example. The concentration of the wavelength-converting element in the wavelength-converting portion  23  is the concentration of the wavelength-converting element in the core  3 . 
     (2.2) First Light Source Unit 
     The first light source unit  11  emits the excitation light P 1  to excite the wavelength-converting element contained in the wavelength-converting portion  23  of the optical fiber  2 . The excitation light P 1  emitted from the first light source unit  11  is incident on the light incident portion  21  of the optical fiber  2 . From the viewpoint of exciting the wavelength-converting element more efficiently, the excitation light P 1  preferably has a wavelength equal to or longer than 350 nm and equal to or shorter than 500 nm. 
     The first light source unit  11  may include a laser light source, for example. The laser light source emits a laser beam. The excitation light P 1  emitted from the first light source unit  11  (i.e., the laser beam emitted from the laser light source) is incident on the light incident portion  21 . The laser light source may be, for example, a semiconductor laser diode that emits a blue laser beam. In that case, the excitation light P 1  may have a wavelength falling within the range from 440 nm to 450 nm, for example. 
     (2.3) Second Light Source Unit 
     The second light source unit  12  emits the seed light P 2 . The seed light P 2  emitted from the second light source unit  12  is incident on the light incident portion  21  of the optical fiber  2 . 
     The light-emitting system  1  includes a plurality of (e.g., two) second light source units  12 , each of which emits, for example, seed light P 2  having a single wavelength. The seed light P 2  emitted from one of these two second light source units  12  has a different wavelength from the seed light P 2  emitted from the other of these two second light source units  12 . In the following description, one of the two second light source units  12  will be hereinafter referred to as a “second light source unit  121 ” and the other second light source unit  12  will be hereinafter referred to as a “second light source unit  122 ” for convenience sake. The second light source unit  121  may be a semiconductor laser diode that emits a green light ray, for example. The second light source unit  122  may be a semiconductor laser diode that emits a red light ray, for example. If the wavelength-converting element of the wavelength-converting portion  23  includes Pr 3+ , then the wavelength of the green seed light ray P 21  is preferably about 520 nm, for example, and the wavelength of the red seed light ray P 22  is preferably about 640 nm, for example. These second light source units  12  are light sources, each of which emits quasi-monochromatic light. As used herein, the “quasi-monochromatic light” refers to light falling within a narrow wavelength range (with a width of 10 nm, for example). The number of the second light source units  12  included in the light-emitting system  1  does not have to be two but may also be three or more or even one. If the light-emitting system  1  includes three second light source units  12 , then the light-emitting system  1  may include, as the three second light source units  12 , a semiconductor laser diode that emits a green light ray, a semiconductor laser diode that emits a red light ray, and a semiconductor laser diode that emits an orange light ray. The orange seed light ray preferably has a wavelength of about 600 nm, for example. 
     The light emitted from the second light source unit  121  is incident as a seed light ray P 2  (P 21 ) on the light incident portion  21  of the optical fiber  2 . The light emitted from the second light source unit  122  is incident as a seed light ray P 2  (P 22 ) on the light incident portion  21  of the optical fiber  2 . 
     (2.4) Light-Guiding Member 
     The light-guiding member  6  guides the light coming from the optical fiber  2  and lets the light emerge therefrom. The light-guiding member  6  has a side surface  61  and an end face  62  located opposite from the optical fiber  2 . The light-guiding member  6  lets the light coming from the (light emerging portion  22  of the) optical fiber  2  emerge from at least the side surface  61 . Optionally, the light-guiding member  6  may let the light coming from the optical fiber  2  emerge from not only the side surface  61  but also the end face  62  as well. The light-guiding member  6  is linear. The light-guiding member  6  has a circular cross-sectional shape taken along a plane that intersects at right angles with its longitudinal axis (i.e., its optical axis). The diameter of the light-guiding member  6  is preferably equal to or greater than the diameter of the core  3 , for example, and may be approximately equal to the diameter of the optical fiber  2 , for example. The light-guiding member  6  may be, but does not have to be, colorless and transparent, for example. A material for the light-guiding member  6  may be, for example, an acrylic resin. In the light-emitting system  1 , the difference in refractive index between the light-guiding member  6  and the core  3  is preferably as small as possible. Optionally, at least part of the side surface  61  of the light-guiding member  6  may have an uneven structure for controlling the distribution of the light emerging from the light-guiding member  6 . 
     The light-guiding member  6  is coupled to the optical fiber  2  to let the light coming from the optical fiber  2  incident on the light-guiding member  6 . In this embodiment, the light-guiding member  6  is connected by fusion to the optical fiber  2 . However, this is only an example and should not be construed as limiting. Alternatively, the light-guiding member  6  may be adhered to the optical fiber  2  via an adhesive which is transparent to visible radiation. In this case, the adhesive may be, for example, an epoxy resin or an acrylic resin. In this embodiment, the light-guiding member  6  has flexibility. However, the light-guiding member  6  may have no flexibility as well. 
     (2.5) Other Constituent Elements 
     The light-emitting system  1  includes a housing  10  to house the first light source unit  11  and the second light source unit  12  therein. 
     The light-emitting system  1  further includes an adjustment unit  7 . The adjustment unit  7  adjusts the intensity of the seed light P 2  having at least one wavelength. In the light-emitting system  1  according to the first embodiment, the adjustment unit  7  adjusts the intensity of the excitation light P 1  and the respective intensities of seed light rays P 21 , P 22 . The adjustment unit  7  includes: a first driver circuit for driving the first light source unit  11 ; a plurality of second driver circuits, which are provided one to one for the plurality of second light source units  12  and each of which drives a corresponding one of the second light source units  12 ; and a control circuit for controlling the first driver circuit and the plurality of second driver circuits on an individual basis. In the adjustment unit  7 , the control circuit controls the first driver circuit and the plurality of second driver circuits on an individual basis, thus making the chromaticity of the light emerging from the (light emerging portion  22  of the) optical fiber  2  adjustable. In short, the light-emitting system  1  includes the adjustment unit  7 , thus enabling controlling the color of the emerging light. This allows the light-emitting system  1  to adjust the color of the light emerging from the light-guiding member  6 . In this embodiment, the adjustment unit  7  is housed in the housing  10 . However, this is only an example and the adjustment unit  7  does not have to be housed in the housing  10 . The first driver circuit and the plurality of second driver circuits are supplied with supply voltage from a first power supply circuit, for example. Meanwhile, the control circuit is supplied with supply voltage from a second power supply circuit, for example. In this embodiment, the first power supply circuit and the second power supply circuit are not counted among the constituent elements of the light-emitting system  1 . However, this is only an example and should not be construed as limiting. Alternatively, the first power supply circuit and the second power supply circuit may be counted among constituent elements of the light-emitting system  1 . 
     The light-emitting system  1  may further include a photocoupler to make the excitation light P 1  and the seed light rays P 2  incident on the light incident portion  21  of the optical fiber  2 . The photocoupler is disposed at the opening of the housing  10 . The photocoupler may be, but does not have to be, a grating. The grating may be a transmissive diffraction grating, for example. The material for the grating may be, but does not have to be, quartz, for example. 
     (3) Operation of Light-Emitting System 
     The light-emitting system  1  makes the first light source unit  11  emit the excitation light P 1  and also makes the second light source unit  12  emit the seed light P 2 . Thus, the light-emitting system  1  allows the excitation light P 1  and the seed light P 2  to be incident on the light incident portion  21  of the optical fiber  2 . Part of the excitation light P 1  incident on the light incident portion  21  emerges from the light emerging portion  22 . In the light-emitting system  1 , the light emerging from the light emerging portion  22  of the optical fiber  2  is mixed light in which the excitation light P 1 , an ASE with a wavelength of about 480 nm and produced from the wavelength-converting element, and the stimulated emission of light (P 3 ) (i.e., the light having the same wavelength as the seed light P 2 ) are mixed together. Two types of stimulated emissions of light P 31 , P 32  corresponding one to one to the multiple seed light rays P 21 , P 22  and having mutually different wavelengths may be, for example, a green ray and a red ray, respectively. In that case, the mixed light may be white light, for example. In  FIG.  3 C , the lower stimulated emission of light P 3  (P 31 ) is the green ray and the upper stimulated emission of light P 3  (P 32 ) is the red ray. 
     In the optical fiber  2 , a stimulated emission is produced by the spontaneous emission of light and the seed light P 2 , and therefore, the excitation light P 1  incident on the light incident portion  21  and the stimulated emission of light P 3  amplified by stimulated emission emerge from the light emerging portion  22 . The stimulated emission of light P 3  having the same wavelength as the seed light ray P 21  of the light emerging from the light emerging portion  22  of the optical fiber  2  has a higher intensity than the seed light ray P 21  incident from the second light source unit  121  onto the light incident portion  21 . Also, the stimulated emission of light P 3  having the same wavelength as the seed light ray P 22  of the light emerging from the light emerging portion  22  of the optical fiber  2  has a higher intensity than the seed light ray P 22  incident from the second light source unit  122  onto the light incident portion  21 . The mixed light emerging from the light emerging portion  22  of the optical fiber  2  is incoherent light. In the light-emitting system  1 , as the light incident through the light incident portion  21  of the optical fiber  2  comes closer toward the light emerging portion  22 , the stimulated emission of light P 3  increases or decreases. In the light-emitting system  1 , the chromaticity, color temperature, color rendering index, and other parameters of the light emerging from the light-guiding member  6  are determined by the respective wavelengths of the ASE and the seed light P 2 . Note that the operation of the light-emitting system  1  is different from the operation of a fiber laser that produces laser oscillation. 
     In the light-emitting system  1 , the wavelength-converting element that serves as a heat source is distributed in the core  3  of the optical fiber  2 , and therefore, an increase in temperature may be reduced while the light-emitting system  1  is being used. 
     Also, in the light-emitting system  1 , the adjustment unit  7  adjusts the intensity of the excitation light P 1  and the respective intensities of the multiple seed light rays P 2 . However, this is only an example and should not be construed as limiting. Alternatively, the adjustment unit  7  may also be configured to adjust the intensity of the seed light P 2  having at least one wavelength. 
     (4) Recapitulation 
     A light-emitting system  1  according to the first embodiment includes an optical fiber  2 , a first light source unit  11 , a second light source unit  12 , and a light-guiding member  6 . The optical fiber  2  includes a wavelength-converting portion  23  containing a wavelength-converting element. The wavelength-converting element may be excited by excitation light P 1  to produce a spontaneous emission of light having a longer wavelength than the excitation light P 1  and may also be excited by an amplified spontaneous emission of light. The first light source unit  11  makes the excitation light P 1  incident on the optical fiber  2 . The second light source unit  12  makes seed light P 2  incident on the optical fiber  2 . The seed light P 2  causes the wavelength-converting element that has been excited by the excitation light P 1  and the amplified spontaneous emission of light to produce a stimulated emission of light P 3 . The light-guiding member  6  guides the light coming from the optical fiber  2  and lets the light emerge therefrom. 
     The light-emitting system  1  according to the first embodiment enables increasing the intensity of light (i.e., the stimulated emission of light P 3 ) having a different wavelength from the excitation light P 1 . 
     In addition, the light-emitting system  1  according to the first embodiment lets the light emerge from the light-guiding member  6  into the external space without using any lighting devices or lenses, thus contributing to reducing the size and weight. 
     Furthermore, in the light-emitting system  1  according to the first embodiment, the light-guiding member  6  is linear. Thus, the light-emitting system  1  contributes to reducing the size and weight of the light-guiding member  6  while expanding the irradiation range of the light emerging from the light-guiding member  6 . 
     Furthermore, the light-emitting system  1  according to the first embodiment further includes an adjustment unit  7  for adjusting the respective intensities of multiple seed light rays P 2  having multiple different wavelengths, thus making the chromaticity of the light emerging from the light-guiding member  6  adjustable. 
     In addition, in the light-emitting system  1  according to the first embodiment, the wavelength-converting portion  23  contains Pr 3+  as the wavelength-converting element. The wavelength-converting element not only emits an ASE in cyan but also increases the respective intensities of stimulated emissions in green and red, because a plurality of seed light rays P 2  with mutually different wavelengths are incident onto the light incident portion  21 . This allows the light-emitting system  1  according to the first embodiment to improve the color rendering index of the light emerging from the light-guiding member  6 . Furthermore, in the light-emitting system  1  according to the first embodiment, the wavelength-converting portion  23  contains Pr 3+  and Tb 3+  as two types of wavelength-converting elements, thus enabling further improving the color rendering index of the light emerging from the light-guiding member  6 . 
     Second Embodiment 
     Next, a light-emitting system  1   a  according to a second embodiment will be described with reference to  FIG.  4   . In the following description, any constituent element of the light-emitting system  1   a  according to this second embodiment, having the same function as a counterpart of the light-emitting system  1  of the first embodiment described above, will be designated by the same reference numeral as that counterpart&#39;s, and description thereof will be omitted herein. 
     The light-emitting system  1   a  according to the second embodiment includes a light-guiding member  6   a  instead of the light-guiding member  6  of the light-emitting system  1  according to the first embodiment, which is a difference from the light-emitting system  1  according to the first embodiment. 
     The light-guiding member  6   a  includes a first light-guiding portion  65 , which is directly connected to the optical fiber  2 , and a plurality of (e.g., two) second light-guiding portions  66  branching from the first light-guiding portion  65 . 
     The light-guiding member  6   a  guides the light coming from the optical fiber  2  and lets the light emerge therefrom. The light-guiding member  6   a  has side surfaces (specifically, a side surface  651  of the first light-guiding portion  65  and respective side surfaces  661  of the plurality of second light-guiding portions  66 ) and end faces located opposite from the optical fiber  2  (specifically, the respective end faces  662  of the plurality of second light-guiding portions  66 ). The light-guiding member  6   a  lets the light coming from the (light emerging portion  22  of the) optical fiber  2  emerge from at least the side surfaces of the light-guiding member  6   a . Optionally, the light-guiding member  6   a  may let the light coming from the optical fiber  2  emerge from not only the side surfaces of the light-guiding member  6   a  but also the end faces of the light-guiding member  6   a  as well. The first light-guiding portion  65  is linear. The first light-guiding portion  65  has a circular cross-sectional shape taken along a plane that intersects at right angles with its longitudinal axis (i.e., its optical axis). The diameter of the first light-guiding portion  65  is preferably equal to or greater than the diameter of the core  3  of the optical fiber  2  (see  FIG.  3   ), for example, and may be approximately equal to the diameter of the optical fiber  2 , for example. Each of the plurality of second light-guiding portions  66  has a circular cross-sectional shape taken along a plane that intersects at right angles with its longitudinal axis (i.e., its optical axis). The light-guiding member  6   a  may be, but does not have to be, colorless and transparent, for example. A material for the light-guiding member  6   a  may be, for example, an acrylic resin. In the light-emitting system  1   a , the difference in refractive index between the light-guiding member  6   a  and the core  3  is preferably as small as possible. Optionally, at least part of the side surfaces of the light-guiding member  6   a  may have an uneven structure for controlling the distribution of the light emerging from the light-guiding member  6   a.    
     In the light-guiding member  6   a , the first light-guiding portion  65  is directly connected to the optical fiber  2  to let the light coming from the optical fiber  2  incident on the first light-guiding portion  65 . In the light-guiding member  6   a , the first light-guiding portion  65  is coupled to the optical fiber  2 . In this embodiment, the light-guiding member  6   a  is connected by fusion to the optical fiber  2 . However, this is only an example and should not be construed as limiting. Alternatively, the light-guiding member  6   a  may be adhered to the optical fiber  2  with, for example, an adhesive which is transparent to visible radiation. In this case, the adhesive may be, for example, an epoxy resin or an acrylic resin. In this embodiment, the light-guiding member  6   a  has flexibility. However, the light-guiding member  6  may have no flexibility as well. 
     The light-emitting system  1   a  according to the second embodiment includes, as well as the light-emitting system  1  according to the first embodiment, the wavelength-converting portion  23  (see  FIG.  2   ) containing the wavelength-converting element, the first light source unit  11 , the second light source unit  12 , and the light-guiding member  6   a . This allows the light-emitting system  1   a  according to the second embodiment, as well as the light-emitting system  1  according to the first embodiment, to increase the intensity of light (i.e., the stimulated emission of light P 3 ) having a different wavelength from the excitation light P 1 . 
     In addition, in the light-emitting system  1   a  according to the second embodiment, the light-guiding member  6   a  includes the first light-guiding portion  65  and the plurality of second light-guiding portions  66 , thus enabling expanding the irradiation range of the light emerging from the light-guiding member  6   a.    
     Third Embodiment 
     Next, a light-emitting system  1   b  according to a third embodiment will be described with reference to  FIG.  5   . In the following description, any constituent element of the light-emitting system  1   b  according to this third embodiment, having the same function as a counterpart of the light-emitting system  1  of the first embodiment described above, will be designated by the same reference numeral as that counterpart&#39;s, and description thereof will be omitted herein. 
     The light-emitting system  1   b  includes a plurality of (e.g., two in the example illustrated in  FIG.  5   ) optical fibers  2 . On the other hand, the light-guiding member  6  guides the light coming from each of the plurality of optical fibers  2  and lets the light emerge therefrom. In this light-emitting system  1   b , the respective light incident portions  21  of the plurality of optical fibers  2  are arranged out of contact with each other. In the light-emitting system  1   b , the plurality of optical fibers  2  are bundled together (in other words, coupled together) at the other end opposite from the light incident portions  21  thereof and share the same light emerging portion  22  in common. 
     In this light-emitting system  1   b , the first light source unit  11  and the plurality of (e.g., two) second light source units  12  are provided for each of the plurality of (e.g., two) optical fibers  2  In other words, the light-emitting system  1   b  includes two sets of light source units, each set of which consists of one first light source unit  11  and two second light source units  12 . These two sets are housed in a single housing  10 . The adjustment unit  7  controls the first light source unit  11  and the plurality of second light source unit  12  of each set. 
     The light-emitting system  1   b  according to the third embodiment includes, as well as the light-emitting system  1  according to the first embodiment, the wavelength-converting portions  23  (see  FIG.  2   ) each containing the wavelength-converting element, the first light source units  11 , the second light source units  12 , and the light-guiding member  6 . This allows the light-emitting system  1   b  according to the third embodiment to increase the intensity of light (i.e., the stimulated emission of light P 3 ) having a different wavelength from the excitation light P 1 . 
     In addition, the light-emitting system  1   b  according to the third embodiment makes the light-guiding member  6  guide the light coming from each of the plurality of optical fibers  2  and lets the light emerge therefrom, thus enabling increasing the quantity of light emerging from the light-guiding member  6 . 
     Fourth Embodiment 
     Next, a light-emitting system  1   c  according to a fourth embodiment will be described with reference to  FIGS.  6 A and  6 B . In the following description, any constituent element of the light-emitting system  1   c  according to this fourth embodiment, having the same function as a counterpart of the light-emitting system  1  of the first embodiment described above, will be designated by the same reference numeral as that counterpart&#39;s, and description thereof will be omitted herein. Note that although not shown in  FIG.  6 A or  6 B , the light-emitting system  1   c  includes the same housing and adjustment unit as the housing  10  and the adjustment unit  7  of the light-emitting system  1  according to the first embodiment. 
     The light-emitting system  1   c  according to the fourth embodiment includes a light-guiding member  6   c  instead of the light-guiding member  6  of the light-emitting system  1  according to the first embodiment, which is a difference from the light-emitting system  1  according to the first embodiment. In addition, the light-emitting system  1   c  according to the fourth embodiment includes a plurality of (e.g., three in the example illustrated in  FIGS.  6 A and  6 B ) optical fibers  2 . 
     The light-guiding member  6   c  is a light guide plate. More specifically, the light-guiding member  6   c  includes a light guide plate body  600  in the shape of a rectangular plate and a plurality of reflective portions  603 . The light guide plate body  600  includes a plurality of light incident surfaces  604  corresponding one to one to the plurality of optical fibers  2 . Each of the plurality of light incident surfaces  604  is a recessed surface. The light guide plate body  600  has a first principal surface  601  and a second principal surface  602  intersecting with the thickness direction defined for the light guide plate body  600 . The plurality of reflective portions  603  are provided on the second principal surface  602  of the light guide plate body  600 . The first principal surface  601  of the light guide plate body  600  has an uneven structure for letting the light guided through the light-guiding member  6   c  emerge into the external space. 
     In the light-emitting system  1   c  according to the fourth embodiment, at least part of the light coming from each optical fiber  2  is allowed to emerge from the first principal surface  601  of the light guide plate body  600  of the light-guiding member  6   c.    
     The light-emitting system  1   c  according to the fourth embodiment includes the wavelength-converting portions  23  (see  FIG.  2   ) each containing the wavelength-converting element, the first light source units  11 , the second light source units  12 , and the light-guiding member  6   c . This allows the light-emitting system  1   c  according to the fourth embodiment, as well as the light-emitting system  1  according to the first embodiment, to increase the intensity of light (i.e., the stimulated emission of light P 3 ) having a different wavelength from the excitation light P 1 . 
     The light-emitting system  1   c  according to the fourth embodiment is applicable to, for example, a guidance light system or a display system. In that case, an appropriate notation may be printed on the first principal surface  601  of the light guide plate body  600 . Alternatively, a light-transmitting panel with an appropriate notation may be disposed on the first principal surface  601  of the light guide plate body  600 . 
     Other Variations 
     Note that the first to fourth embodiments described above are only exemplary ones of various embodiments of the present disclosure and should not be construed as limiting. Rather, the first to fourth exemplary embodiments may be readily modified in various manners depending on a design choice or any other factor without departing from the scope of the present disclosure. 
     For example, in the optical fiber  2 , the number of the wavelength-converting portion(s)  23  provided between the light incident portion  21  and the light emerging portion  22  does not have to be one but may also be multiple, for example. In the latter case, the plurality of wavelength-converting portions  23  are arranged side by side along the optical axis of the core  3 . 
     In the optical fiber  2  described above, the wavelength-converting portion  23  is approximately as long as the core  3 . However, this is only an example and should not be construed as limiting. Alternatively, the wavelength-converting portion  23  may also be shorter than the core  3 . 
     The laser light source included in the first light source unit  11  does not have to be a semiconductor laser diode that emits a blue laser beam but may also be, for example, a semiconductor laser diode that emits a violet laser beam. Furthermore, the first light source unit  11  does not have to include the semiconductor laser diode but may have a configuration including, for example, a light-emitting diode (LED) light source and an optical system. 
     The second light source unit  121  does not have to be a semiconductor laser diode that emits a green laser beam but may also be an LED that emits a green light ray. The second light source unit  122  does not have to be a semiconductor laser diode that emits a red laser beam but may also be an LED that emits a red light ray. 
     Furthermore, the relative arrangement of the first light source unit  11  and the plurality of second light source units  12  with respect to the light incident portion  21  of the optical fiber  2  does not have to be one of the relative arrangements described for the first to fourth embodiments. For example, in the light-emitting system  1 , the relative arrangement of the first light source unit  11  and the plurality of second light source units  12  with respect to the light incident portion  21  of the optical fiber  2  may be changed by disposing a cross dichroic prism between the first light source unit  11  and the plurality of second light source units  12  and the light incident portion  21  of the optical fiber  2 . 
     Furthermore, in the light-emitting systems  1 ,  1   a ,  1   b ,  1   c , a plurality of second light source units  12  are provided for a single optical fiber  2 . However, this is only an example and should not be construed as limiting. Rather, at least one second light source unit  12  has only to be provided for a single optical fiber  2 . 
     The adjustment unit  7  only needs to adjust the intensity of seed light P 2  having at least one wavelength among the plurality of seed light rays P 2 . The adjustment unit  7  may be, for example, a liquid crystal filter which is disposed on an optical path of the seed light P 2  between the second light source unit  12  and the light incident portion  21  of the optical fiber  2  and which may adjust the transmittance of the seed light P 2 . 
     Furthermore, in the light-emitting system  1   b  according to the third embodiment, the number of the optical fibers  2  provided does not have to be two but may also be three or more. In that case, the light-emitting system  1   b  may include, for example, five optical fibers  2  and two light-guiding members  6 . Three out of the five optical fibers  2  may be connected to one light-guiding member  6  and the other two optical fibers  2  may be connected to the other light-guiding member  6 . 
     Furthermore, in the light-emitting system  1   b , the plurality of optical fibers  2  are bundled together and directly connected to the light-guiding member  6 . However, this is only an example and should not be construed as limiting. Alternatively, the light-guiding member  6  may include a plurality of first light-guiding portions which are connected one to one to the plurality of optical fibers  2  and a single second light-guiding portion which is connected to the plurality of first light-guiding portions. 
     Furthermore, in the light-emitting systems  1   b ,  1   c , the first light source unit  11  and the plurality of second light source units  12  are provided for each of the plurality of optical fibers  2 . However, this is only an example and should not be construed as limiting. Alternatively, the light-emitting system  1   b ,  1   c  may each include a first beam splitter which makes the excitation light P 1  emitted from the first light source unit  11  incident on the plurality of optical fibers  2  and a plurality of second beam splitters which make the seed light rays P 2  emitted from the plurality of second light source units  12  respectively incident on the plurality of optical fibers  2  with respect to the set of the first light source unit  11  and the plurality of second light source units  12 . 
     Aspects 
     The first to fourth embodiments and their variations described above may be specific implementations of the following aspects of the present disclosure. 
     A light-emitting system ( 1 ;  1   a ;  1   b ;  1   c ) according to a first aspect includes an optical fiber ( 2 ), a first light source unit ( 11 ), a second light source unit ( 12 ), and a light-guiding member ( 6 ;  6   a ;  6   c ). The optical fiber ( 2 ) includes a wavelength-converting portion ( 23 ) containing a wavelength-converting element. The wavelength-converting element may be excited by excitation light (P 1 ) to produce a spontaneous emission of light having a longer wavelength than the excitation light (P 1 ) and may also be excited by an amplified spontaneous emission of light. The first light source unit ( 11 ) makes the excitation light (P 1 ) incident on the optical fiber ( 2 ). The second light source unit ( 12 ) makes seed light (P 2 ) incident on the optical fiber ( 2 ). The seed light (P 2 ) causes the wavelength-converting element that has been excited by either the excitation light (P 1 ) or the amplified spontaneous emission of light to produce a stimulated emission of light (P 3 ). The light-guiding member ( 6 ;  6   a ;  6   c ) guides the light coming from the optical fiber ( 2 ) and lets the light emerge therefrom. 
     The light-emitting system ( 1 ;  1   a ;  1   b ;  1   c ) according to the first aspect enables increasing the intensity of light (stimulated emission of light P 3 ) having a different wavelength from the excitation light (P 1 ). 
     In a light-emitting system ( 1 ;  1   a ;  1   b ;  1   c ) according to a second aspect, which may be implemented in conjunction with the first aspect, the excitation light (P 1 ) has a wavelength equal to or longer than 350 nm and equal to or shorter than 500 nm. 
     The light-emitting system ( 1 ;  1   a ;  1   b ;  1   c ) according to the second aspect enables exciting the wavelength-converting element more efficiently. 
     A light-emitting system ( 1 ;  1   a ;  1   b ;  1   c ) according to a third aspect, which may be implemented in conjunction with the first or second aspect, further includes an adjustment unit ( 7 ). The adjustment unit ( 7 ) adjusts intensity of the seed light (P 2 ). 
     The light-emitting system ( 1 ;  1   a ;  1   b ;  1   c ) according to the third aspect enables adjusting the chromaticity of the light emerging from the light-guiding member ( 6 ;  6   a ;  6   c ). 
     A light-emitting system ( 1 ;  1   a ;  1   b ;  1   c ) according to a fourth aspect, which may be implemented in conjunction with any one of the first to third aspects, includes a plurality of the second light source units ( 12 ). The plurality of the second light source units ( 12 ) respectively emit multiple rays of the seed light (P 2 ). The multiple rays of the seed light (P 2 ) emitted from the plurality of the second light source units ( 12 ) have mutually different wavelengths. 
     The light-emitting system ( 1 ;  1   a ;  1   b ;  1   c ) according to the fourth aspect allows light, including multiple rays of stimulated emission of light (P 3 ) corresponding one to one to the multiple rays of the seed light (P 2 ), to emerge from the light-guiding member ( 6 ;  6   a ;  6   c ), thus contributing to improving the color rendering index. 
     In a light-emitting system ( 1 ;  1   a ;  1   b ) according to a fifth aspect, which may be implemented in conjunction with any one of the first to fourth aspects, the light-guiding member ( 6 ;  6   a ) has a side surface ( 61 ;  651 ,  661 ) and an end face ( 62 ;  662 ) opposite from the optical fiber ( 2 ). The light-guiding member ( 6 ;  6   a ) lets at least part of the light coming from the optical fiber ( 2 ) emerge through the side surface ( 61 ;  651 ,  661 ) of the light-guiding member ( 6 ;  6   a ). 
     In a light-emitting system ( 1 ;  1   b ) according to a sixth aspect, which may be implemented in conjunction with any one of the first to fifth aspects, the light-guiding member ( 6 ) is linear. 
     The light-emitting system ( 1 ;  1   b ) according to the sixth aspect contributes to reducing the size and weight of the light-guiding member ( 6 ) while expanding the irradiation range of the light emerging from the light-guiding member ( 6 ). 
     In a light-emitting system ( 1   a ) according to a seventh aspect, which may be implemented in conjunction with any one of the first to sixth aspects, the light-guiding member ( 6   a ) includes: a first light-guiding portion ( 65 ) directly connected to the optical fiber ( 2 ); and a plurality of second light-guiding portions ( 66 ) branching from the first light-guiding portion ( 65 ). 
     The light-emitting system ( 1   a ) according to the seventh aspect enables expanding the irradiation range of the light emerging from the light-guiding member ( 6   a ). 
     A light-emitting system ( 1   b ;  1   c ) according to an eighth aspect, which may be implemented in conjunction with any one of the first to seventh aspects, includes a plurality of the optical fibers ( 2 ). The light-guiding member ( 6 ;  6   c ) guides the light coming from at least two optical fibers ( 2 ), out of the plurality of the optical fibers ( 2 ), and let the light emerge therefrom. 
     The light-emitting system ( 1   b ;  1   c ) according to the eighth aspect enables increasing the quantity of light emerging from the light-guiding member ( 6 ;  6   c ). 
     In a light-emitting system ( 1 ;  1   a ;  1   b ;  1   c ) according to a ninth aspect, which may be implemented in conjunction with any one of the first to eighth aspects, the first light source unit ( 11 ) includes a laser light source. 
     The light-emitting system ( 1 ;  1   a ;  1   b ;  1   c ) according to the ninth aspect enables increasing the intensity of the excitation light (P 1 ). 
     In a light-emitting system ( 1 ;  1   a ;  1   b ;  1   c ) according to a tenth aspect, which may be implemented in conjunction with any one of the first to ninth aspects, the second light source unit ( 12 ) includes a laser light source. 
     The light-emitting system ( 1 ;  1   a ;  1   b ;  1   c ) according to the tenth aspect enables increasing the intensity of the seed light (P 2 ). 
     In a light-emitting system ( 1 ;  1   a ;  1   b ;  1   c ) according to an eleventh aspect, which may be implemented in conjunction with any one of the first to tenth aspects, the wavelength-converting element includes one or more elements selected from the group consisting of Pr, Tb, Ho, Dy, Er, Eu, Nd, and Mn. 
     The light-emitting system ( 1 ;  1   a ;  1   b ;  1   c ) according to the eleventh aspect enables, when the wavelength-converting element includes two or more elements, for example, excitation caused by the amplified spontaneous emission of light from at least one element to produce an amplified spontaneous emission of light from another element at a different wavelength. 
     REFERENCE SIGNS LIST 
     
         
           1 ,  1   a    1   b ,  1   c  Light-Emitting System 
           2  Optical Fiber 
           21  Light Incident Portion 
           22  Light Emerging Portion 
           23  Wavelength-Converting Portion 
           6 ,  6   a ,  6   c  Light-Guiding Member 
           61  Side Surface 
           62  End Face 
           65  First Light-Guiding Portion 
           651  Side Surface 
           66  Second Light-Guiding Portion 
           661  Side Surface 
           662  End Face 
           7  Adjustment Unit 
           11  First Light Source Unit 
           12  Second Light Source Unit 
           121  Second Light Source Unit 
           122  Second Light Source Unit 
         P 1  Excitation Light 
         P 2  Seed Light 
         P 21  Seed Light Ray 
         P 22  Seed Light Ray 
         P 3  Stimulated Emission of Light