Patent Publication Number: US-2023163554-A1

Title: Optical fiber amplifier and rare earth doped optical fiber

Description:
TECHNICAL FIELD 
     The present disclosure relates to a rare-earth-added optical fiber and an optical fiber amplifier including the same. 
     BACKGROUND ART 
     For an optical fiber amplifier including a rare-earth-added optical fiber, a core excitation method and a method using clad excitation have been proposed (for example, see Non-Patent Literature 1). 
     CITATION LIST 
     Non-Patent Literature 
     Non-Patent Literature 1: Kazi S. Abedin, “Cladding-Pumped Multicore Fiber Amplifier for Space Division Multiplexing”, Handbook of Optical Fibers, Springer Nature Singapore Pte Ltd. 2018. 
     SUMMARY OF THE INVENTION 
     Technical Problem 
     A clad excitation method achieves a high efficiency in light-to-current conversion in an excitation light source as compared with a core excitation method. However, a light-to-light conversion efficiency (an efficiency of conversion from an excitation light to a signal light) would be insufficient due to a decrease in excitation light power density in a rare-earth-added optical fiber. It should be noted that a light-to-light conversion efficiency usually tends to monotonically increase with respect to an increase in excitation light power density. 
     An object of the present disclosure is to implement a clad-excitation rare-earth-added optical fiber amplifier with a high light-to-light conversion efficiency. 
     Means for Solving the Problem 
     Specifically, an optical fiber and an optical fiber amplifier according to the present disclosure each have a refractive index distribution structure provided at least at a part of a rare-earth-added optical fiber in a longitudinal direction and configured to collect an excitation light, which propagates through a clad portion, into a core portion. 
     Effects of the Invention 
     According to the present disclosure, it is possible to implement a clad-excitation rare-earth-added optical fiber amplifier with a high light-to-light conversion efficiency. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    illustrates a configuration example of an optical fiber amplifier according to Exemplary Embodiment 1. 
         FIG.  2    illustrates a configuration example of an optical fiber amplifier according to Exemplary Embodiment 2. 
         FIG.  3    illustrates a configuration example of an optical fiber amplifier according to Exemplary Embodiment 3. 
         FIG.  4    illustrates a configuration example of an optical fiber amplifier according to Exemplary Embodiment 4. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of the present disclosure will be described below in detail with reference to the drawings. It should be noted that the present disclosure is not limited to the embodiments described below. These examples of implementation are merely by way of example and the present disclosure can be implemented in embodiments provided with a variety of modifications and improvements on the basis of knowledge of those skilled in the art. It should be noted that components reference signs of which are the same herein and in the drawings are identical to each other. 
     Summary of the Present Disclosure 
     According to the present disclosure, a structure that generates a light collection effect is given to a rare-earth-added optical fiber, in which excitation light power density decreases, thereby enhancing the excitation light power density in the rare-earth-added optical fiber to improve a light-to-light conversion efficiency. In particular, the present disclosure is characterized by enabling a desired structure to be formed later using a laser or the like after a rare-earth-added optical fiber is manufactured. 
     Exemplary Embodiment 1 
       FIG.  1    illustrates an example of an optical fiber amplifier according to this exemplary embodiment. The optical fiber amplifier according to this exemplary embodiment is a clad-excitation optical fiber amplifier that uses an excitation light, which propagates through a clad portion  82 , to amplify a signal light, which propagates through a core portion  81 . Erbium is added, as a rare-earth ion to add, to the core portion  81 . 
     In this exemplary embodiment, as illustrated in  FIG.  1   , a light collector  10  that enables collecting light in a fiber longitudinal direction is provided in a rare-earth-added optical fiber  91  with the core portion  81  coated with the clad portion  82 . In the light collector  10 , light collected by a semispherical lens  12  propagates through the space instead of propagating through a waveguide. A region  11  for the space propagation is a cavity or is formed of a material having a lower refractive index than glass in a portion around the region  11 . 
     In the region  11  for the space propagation, a cavity can be formed using a femtosecond laser or a refractive index can be lowered by virtue of stress relaxation resulting from glass remelting. The lens  12  can be formed by changing refractive indexes of the core portion  81  and the clad portion  82  during the formation of the cavity portion or the low refractive index portion. 
     The core portions  81  are opposed to each other at the region  11  for the space propagation. The lens  12  is formed at one of the core portions  81  opposed to each other. The lens  12  is formed by changing the refractive indexes of the core portion  81  and the clad portion  82 . A focal length of the lens  12  is set at the other of the core portions  81  opposed to each other. This causes the excitation light and the signal light outputted from the lens  12  to be coupled to the other of the core portions  81  after propagating through the region  11 . 
     A simulation for a light-to-light conversion efficiency was performed, where in an erbium-added optical fiber amplifier (EDFA) added with erbium ions as rare-earth ions, a signal light was caused to enter the core portion  81  and an excitation light was caused to enter the clad portion  82 . As a result of forming the region  11  and the lens  12  by cavity machining, the excitation light power density became 2.1 times higher and the light-to-light conversion efficiency became 1.8 times higher than those before the formation of a lens structure. In a case of the region  11  and the lens  12  being formed by changing the refractive indexes, the excitation light power density became 1.3 times higher and the light-to-light conversion efficiency became 1.2 times higher. 
     It should be noted that specifications of the optical fiber amplifier that was used were an Er addition concentration: 1000 ppm, a fiber length of the rare-earth-added optical fiber  91 : 10 m, a diameter of the core portion  81 : 4 µm, a relative refractive index difference: 2%, a wavelength of the excitation light: 980 nm, an excitation light power: 3 W, an input signal light power: -10 dBm, and a wavelength of the signal light: 1550 nm. Further, the light collector  10  was provided every 1 mm in a longitudinal direction over an entire fiber length. Regarding a size of the light collector  10 , the lens  12  was 80 µm in outer diameter and 100 µm in focal length. 
     A shape of the lens  12  is not limited to a semispherical shape and any shape enabling an excitation light to be coupled to the other of the core portions  81  can be employed. 
     Further, the rare-earth ion to add is not limited to erbium and praseodymium, ytterbium, thulium, neodymium, etc. are usable to achieve a comparable effect. 
     Exemplary Embodiment 2 
     In this exemplary embodiment, as illustrated in  FIG.  2 ( a ) , a graded index (GI) clad portion  83  that enables collecting light in a fiber longitudinal direction is provided in a rare-earth-added optical fiber  92 .  FIG.  2 ( b )  illustrates a refractive index distribution profile. As propagating in the fiber longitudinal direction, the excitation light excited by the clad portion  83  is collected toward the core portion  81 , being brought into a state close to substantial core excitation. Further, a GI structure, which is usually given by adjusting distribution of added ions during manufacturing of a fiber preform, can also be formed by inducing a change in refractive index using a femtosecond laser or the like. 
     A simulation for a light-to-light conversion efficiency was performed, where in an erbium-added optical fiber amplifier (EDFA) added with erbium ions as rare-earth ions, a signal light was caused to enter the core portion  81  and an excitation light was caused to enter the clad portion  82 . As a result, the excitation light power density became 3.2 times higher and the light-to-light conversion efficiency became 2.1 time higher than those in a case where a clad portion had no GI structure and was constant in refractive index. 
     It should be noted that specifications of the optical fiber amplifier that was used were an Er addition concentration: 500 ppm, a fiber length of the rare-earth-added optical fiber  92 : 15 m, a diameter of the core portion  81 : 4 µm, a relative refractive index difference: 2.1%, a wavelength of the excitation light: 980 nm, an excitation light power: 4W, an input signal light power: -8 dBm, and a wavelength of the signal light: 1540 nm. 
     The clad portion  83  with the GI structure may be provided across the rare-earth-added optical fiber or may be provided at a part of the rare-earth-added optical fiber. With the clad portion  83  with the GI structure being provided at least at a part of the rare-earth-added optical fiber in the longitudinal direction, a comparable effect is achievable. 
     Further, the rare-earth ion to add is not limited to erbium and praseodymium, ytterbium, thulium, neodymium, etc. are usable to achieve a comparable effect. 
     Exemplary Embodiment 3 
     In this exemplary embodiment, as illustrated in  FIG.  3   , in the rare-earth-added optical fiber  91 , a grating coupler  31  is provided that couples an excitation light caused to enter through a side surface of the rare-earth-added optical fiber  91  by using an excitation light introducer  32 , to the core portion  81  while collecting it thereinto. Further, a grating structure can be formed by inducing a change in refractive index using a femtosecond laser or the like. 
     A simulation for a light-to-light conversion efficiency was performed, where in an erbium-added optical fiber amplifier (EDFA) added with erbium ions as rare-earth ions, a signal light was caused to enter the core portion  81  and an excitation light was caused to enter through the excitation light introducer  32 . As a result, the excitation light power density became 1.8 times higher and the light-to-light conversion efficiency became 1.5 times higher than those in a typical clad-excitation EDFA. 
     Further, specifications of the optical fiber amplifier that was used were an Er addition concentration: 500 ppm, a fiber length of the rare-earth-added optical fiber  91 : 10 m, a diameter of the core portion  81 : 6 µm, a relative refractive index difference: 0.8%, a wavelength of the excitation light: 980 nm, an excitation light power: 6W, an input signal light power: -8 dBm, a wavelength of the signal light: 1550 nm, and a grating pitch: 1.3 µm. 
     Further, the rare-earth ion to add is not limited to erbium and praseodymium, ytterbium, thulium, neodymium, etc. are usable to achieve a comparable effect. 
     Exemplary Embodiment 4 
     In this exemplary embodiment, as illustrated in  FIG.  4   , a Fresnel lens  41  that uniformizes the power density of the excitation light in the clad portion  82  and the semispherical lens  12  that can collect light in the fiber longitudinal direction are provided in a rare-earth-added multicore optical fiber  93  including a plurality of core portions  81 A,  81 B, and  81 C. A region  11  for the space propagation is a cavity or is formed of a material having a lower refractive index than glass in a portion around the region  11  as in Exemplary Embodiment 1. 
     The excitation light in the clad portion  82  is trapped in the clad portion  82  by virtue of total reflection on an outer interface of the core portion  82 , so that an intensity of the excitation light near a fiber periphery tends to be lower than an intensity of the excitation light near a fiber center. Accordingly, it is of concern that in the multicore optical fiber  93  for amplification, the intensity of the excitation light in the core portions  81 A and  81 C located outside relative to the central core portion  81 B decreases with a gain lowered. 
     The Fresnel lens  41 , which converts the excitation light at an outer side in the clad portion  82  to a parallel light, has a function to reduce an influence of the total reflection on the outer interface of the clad portion  82  to keep the intensity of the excitation light high at the outer side in the clad portion  82 . As a result, in this embodiment, localization of the excitation light in the clad portion  82  can be eliminated with a difference in gain between the cores improved. Further, the Fresnel lens  41  can be formed by inducing a change in refractive index using a femtosecond laser or the like. 
     A simulation for a light-to-light conversion efficiency was performed, where in an erbium-added six-core optical fiber amplifier (EDFA) added with erbium ions as rare-earth ions, a signal light was caused to enter each of the core portions  81  and an excitation light was caused to enter the clad portion  82 . As a result, the average excitation light power density of the plurality of core portions  81  became 1.5 times higher and the light-to-light conversion efficiency became 1.3 times higher than those in a typical clad-excitation six-core EDFA. Further, the maximum gain deviation between the cores was improved from 2 dB to 0.5 dB. 
     Further, specifications of the optical fiber amplifier that was used were an Er addition concentration: 500 ppm, a fiber length of the rare-earth-added optical fiber  91 : 10 m, a diameter of the core portion  81 : 5 µm, a relative refractive index difference: 1.2%, a wavelength of the excitation light: 980 nm, an excitation light power: 8W, an input signal light power: -8 dBm, and a wavelength of the signal light: 1550 nm. Further, the Fresnel lens  41  and the lens  12  are each provided every 2 mm in the longitudinal direction over a fiber entire length. 
     The Fresnel lens  41  may be formed across a cross section of the clad portion  82  of the multicore optical fiber  93  as illustrated in  FIG.  4    or may be formed only in the clad portion  82  near an outer periphery of the multicore optical fiber  93 . For example, the Fresnel lens  41  may be formed only at outer peripheral sides relative to the core portions  81 A and  81 C located at the outer side in the multicore optical fiber  93 . 
     Further, the rare-earth ion to add is not limited to erbium and praseodymium, ytterbium, thulium, neodymium, etc. are usable to achieve a comparable effect. 
     Effect of the Present Disclosure 
     According to the present disclosure, a clad-excitation rare-earth-added optical fiber amplifier is also allowed to achieve a light-to-light conversion efficiency comparable to that of a core-excitation one. Further, a multicore optical fiber amplifier has an effect to reduce a difference in gain between cores. 
     Point of the Present Disclosure 
     A structure that collects a clad excitation light in a longitudinal direction of a rare-earth-added optical fiber is provided and, consequently, a light-to-light conversion efficiency can be improved. 
     INDUSTRIAL APPLICABILITY 
     The present disclosure is applicable to information communication industries. 
     
       
         
           
               
               
               
             
               
                 Reference Signs List 
               
             
            
               
                   
                 
                   10 
                 
                 Light collector 
               
               
                   
                 
                   11 
                 
                 Region 
               
               
                   
                 
                   12 
                 
                 Lens 
               
               
                   
                 
                   31 
                 
                 Grating coupler 
               
               
                   
                 
                   32 
                 
                 Excitation light introducer 
               
               
                   
                 
                   41 
                 
                 Fresnel lens 
               
               
                   
                   81 ,  81 A,  81 B,  81 C Core portion 
               
               
                   
                   82 ,  83  Clad portion 
               
               
                   
                   91 ,  92  Rare-earth-added optical fiber 
               
               
                   
                 
                   93 
                 
                 Multicore optical fiber