Patent Publication Number: US-2003223715-A1

Title: Depolarized optical fiber, depolarized optical waveguide and fabrication method thereof

Description:
[0001] This application claims priority from Japanese Patent Application No. 2002-161973 filed Jun. 3, 2002, which is incorporated hereinto by reference.  
       BACKGROUND OF THE INVENTION  
       [0002] 1. Field of the Invention  
       [0003] The present invention relates to an optical fiber, an optical waveguide and a fabrication method thereof used for optical communication, optical sensors, optical signal processing and the like. More particularly, the present invention relates to an optical fiber and an optical waveguide capable of obtaining depolarized light whose polarization is eliminated, and to a technique of implementing an optical fiber, optical waveguide and optical component whose polarization dependence is eliminated, reduced or improved by depolarizing light propagating in them.  
       [0004] 2. Description of the Related Art  
       [0005] Generally, the polarization state of light propagating through an optical fiber is maintained for a short range. However, it can vary at random as the range increase because of fluctuations in the optical fiber core diameter, vibration or temperature variations. Since a lot of optical communication devices and sensors utilizing an optical fiber vary their characteristics depending on the polarization state of the light propagating through the optical fiber, it is sometimes necessary to control the polarization state. For example, a Raman fiber amplifier for optical fiber communication has operation characteristics greatly dependent on the polarization state of its excitation light. Accordingly, the excitation light is usually emitted into the Raman fiber amplifier after undergoing the depolarization by a depolarizer. As for the optical sensors, since the random variation in the polarization state can deteriorate their sensitivity, it is not rare to use the light passing through the depolarization by a depolarizer.  
       [0006] As optical fiber depolarizers, the following techniques have been developed and are used at present. A first method splits the light propagating through an optical fiber into two parts with an optical fiber coupler, and feeds one of them back to the optical fiber (U.S. Pat. No. 5,933,555 and U.S. Pat. No. 5,218,652); and a second method, utilizing a Lyot type depolarizer, interconnects two polarization maintaining optical fibers with the lengths of L and 2L, with their polarization maintaining planes being inclined by π/4 to each other.  
       [0007] On the other hand, an optical waveguide usually maintains the polarization state of light propagating in optical waveguide. Generally, the optical waveguide is fabricated by depositing a layer to be formed into a lower cladding on a planar substrate, followed by depositing a core layer thereon, forming the core layer to the shape of the optical waveguide by reactive ion etching, and depositing an upper cladding thereon. The optical waveguide has birefringence in many cases. This is because the optical waveguide has different thermal expansion coefficients between a silicon substrate used as its planar substrate and silica-based glass layers constituting its waveguide, and this produces stress within the glass, thereby bringing about the birefringence. Thus, the optical waveguide has the birefringence and maintains the polarization state, as well. As a result, an optical component formed in the optical waveguide such as an arrayed waveguide grating or Bragg reflection grating has a problem of polarization dependence that the optical component has different operating wavelengh depending on the polarization states, or more specifically, on its TE polarized wave and TM polarized wave.  
       [0008] As for the optical fiber, when its core has birefringence, elimination of the polarization dependence also becomes a problem.  
       [0009] To eliminate the polarization dependence from an arrayed waveguide grating, for example, a method is usually employed that makes a groove in the arrayed waveguide and inserts a waveplate into the groove (Y. Inoue et al, “Polarization mode concerter with polyimide half waveplate in silica-based planar lightwave circuits” IEEE Photonics Technol. Lett. Vol. 6, p.626, 1994). In addition, a method is proposed which eliminates the polarization dependence from a planar optical waveguide by providing an ion implanted layer on the top surface of a lower cladding layer before a core layer is deposited thereon (Japanese patent application laid-open No. 10-239547 (1998)).  
       [0010] However, as for the conventional optical fiber depolarizer, it is necessary to process the optical fiber very accurately in their fabrication. As a result, its fabrication becomes complicated and very expensive.  
       [0011] On the other hand, as for the elimination method of the polarization dependence in the arrayed waveguide grating by inserting the waveplate into the groove, the assembly process of making the groove and inserting the waveplate into the groove is necessary, which also complicates the process. In addition, the technique cannot be used when the waveplate cannot be inserted into a device such as a Bragg reflection grating.  
       [0012] As for the method of eliminating the polarization dependence by forming the ion implanted layer on the top surface of the lower cladding layer before depositing the core layer, it is necessary to implant the ions into almost the entire region of the deposited lower cladding. In addition, it is necessary for this method to implant nitrogen ions, oxygen ions or silicon ions by a large amount of 1×10 17 /cm 2 . These two factors offer a problem of greatly increasing the fabrication time. Furthermore, the method is likely to degenerate significantly the characteristics of the material of the lower cladding because of the ions implanted at a high concentration as mentioned above.  
       SUMMARY OF THE INVENTION  
       [0013] The present invention is implemented to solve the foregoing problems. It is therefore an object of the present invention to provide a novel depolarization method that eliminates the polarization of light propagating through an optical fiber or optical waveguide, to provide an optical fiber and optical waveguide capable of producing depolarized light, and to provide an optical fiber, optical waveguide and optical component whose polarization dependence is eliminated by depolarizing light propagating through them.  
       [0014] To accomplish the foregoing object, according to a first aspect of the present invention, there is provided an optical fiber including a core in which light propagates, and a cladding with a refractive index lower than that of the core, the optical fiber is characterized in that accelerated ions are implanted from outside into at least a partial region of the core and/or the cladding near the core such that at least part of the implanted ions stop in the partial region by the acceleration energy disappearance of themselves to depolarize the light propagating through the core.  
       [0015] It is preferable that the implantation of the ions be carried out into a part or all of the optical fiber.  
       [0016] The acceleration energy of the ions may be varied during implantation of the ions to vary thickness of the region where the ions stop by the acceleration energy disappearance of themselves.  
       [0017] The optical fiber may be rotated during implantation of the ions to vary thickness of the region where the ions stop by the acceleration energy disappearance of themselves.  
       [0018] The direction of radiation of ion beams may be altered during implantation of the ions to vary thickness of the region where the ions stop by the acceleration energy disappearance of themselves.  
       [0019] An optical component relating to the present invention is characterized in that its polarization dependence is eliminated or reduced by using the foregoing optical fiber.  
       [0020] To accomplish the foregoing object, according to a second aspect of the present invention, there is provided an optical waveguide including, on a planar substrate, a core in which light propagates, and claddings with a refractive index lower than that of the core, the optical waveguide is characterized in that accelerated ions are implanted from above the upper cladding of the optical waveguide into at least a partial region of the core and/or the claddings near the core such that at least part of the implanted ions stop in the partial region by the acceleration energy disappearance of themselves to depolarize the light propagating through the core.  
       [0021] According to a third aspect of the present invention, there is provided an optical waveguide including, on a planar substrate, a core in which light propagates, and claddings with a refractive index lower than that of the core, the optical waveguide is characterized in that after a lower cladding and a region to become the core are formed on the planar substrate, accelerated ions are implanted at least into a partial region of the region to become the core and/or the lower cladding such that at least part of the implanted ions stop in the partial region by the acceleration energy disappearance of themselves to depolarize the light propagating through the core that is embedded by an upper cladding formed thereafter on the core.  
       [0022] It is preferable that the implantation of the ions be carried out into a part or all of the optical waveguide.  
       [0023] An optical waveguide component relating to the present invention is characterized in that its polarization dependence is eliminated or reduced by using the forgoing optical waveguide.  
       [0024] To accomplish the foregoing object, according to a fourth aspect of the present invention, there is provided a fabrication method of an optical fiber including a core in which light propagates, and a cladding with a refractive index lower than that of the core, the fabrication method comprising the steps of: determining ion implantation condition such that at least part of ions which are implanted from outside into the optical fiber will stop in at least a partial region of the core and/or the cladding near the core by the acceleration energy disappearance of themselves; and depolarizing the light propagating through the core by implanting accelerated ions from outside into at least part of the optical fiber under the ion implantation condition determined.  
       [0025] To accomplish the foregoing object, according to a fifth aspect of the present invention, there is provided a fabrication method of an optical waveguide including, on a planar substrate, a core in which light propagates, and claddings with a refractive index lower than that of the core, the fabrication method comprising the steps of: determining ion implantation condition such that at least part of ions which are implanted from outside into the optical waveguide will stop in at least a partial region of the core and/or the claddings near the core by the acceleration energy disappearance of themselves; and depolarizing the light propagating through the core by implanting accelerated ions from above an upper cladding of the optical waveguide into at least part of the optical waveguide under the ion implantation condition determined.  
       [0026] According to a sixth aspect of the present invention, there is provided a fabrication method of an optical waveguide including, on a planar substrate, a core in which light propagates, and claddings with a refractive index lower than that of the core, the fabrication method comprising the steps of: depolarizing, after forming a lower cladding and a region to become the core on the planar substrate, the light propagating through the core by implanting accelerated ions at least into a partial region of the region to become the core and/or the lower cladding such that the implanted ions stop in the partial region by the acceleration energy disappearance of themselves; and embedding the core by forming an upper cladding thereafter.  
       [0027] Here, the ions to be implanted may include charged particles of atoms whose atomic numbers are equal to or smaller than  36 .  
       [0028] According to the present invention, it is possible to depolarize the light propagating through the optical fiber or optical waveguide easily by implanting the ions into the optical fiber or optical waveguide.  
       [0029] Thus, according to the present invention, the optical fiber and optical waveguide that supply the depolarized light are implemented without difficulty. Accordingly, the optical fiber, optical waveguide and optical component without the polarization dependence are easily provided.  
       [0030] Moreover, since the ion implantation technique in accordance with the present invention can reduce the ion dose implanted and limit the ion implanted reign to smaller area, the fabrication of the optical waveguide or optical component without the polarization dependence can take time less than the conventional technique.  
       [0031] The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0032]FIG. 1A is a perspective view illustrating the principle of the present invention;  
     [0033]FIG. 1B is a front view of the object of FIG. 1A;  
     [0034]FIG. 2 is a perspective view illustrating a structure and fabrication method of the optical fiber of a first embodiment in accordance with the present invention;  
     [0035]FIG. 3A is a front sectional view illustrating a structure and fabrication method of an optical waveguide of a second embodiment in accordance with the present invention;  
     [0036]FIG. 3B is a perspective view of the optical waveguide of the second embodiment;  
     [0037]FIGS. 4A, 4C,  4 E and  4 G are front sectional views illustrating individual steps of a fabrication process of a variation of the second embodiment in accordance with the present invention; and  
     [0038]FIGS. 4B, 4D,  4 F and  4 H are side sectional views illustrating the individual steps. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
     [0039] The optical fiber and optical waveguide of the embodiments according to the present invention will now be described with reference to the drawings.  
     [0040] (Principle of Present Invention)  
     [0041] It is known until now that implanting accelerated ions into an optical fiber or silica glass, a material for constituting an optical fiber, brings about variations in the refractive index or density (A. L. Tchebotareva et al, “Effect of proton implantation on the photosensitivity of SMF-28 optical fiber” NIMB Vol. 148, p. 687, 1999; and M. Fujimaki et al, “Ion-implantation-induced densification in silica-based glass for fabrication of optical fiber gratings” J. Appl. Phys. Vol. 88, p. 5534, 2000). In addition to these phenomena, the inventor of the present invention discovered that implanting accelerated ions into silica-based glass for constituting an optical fiber or optical waveguide can depolarize light passing near the position where the implanted ions stop by the acceleration energy disappearance of themselves. Furthermore, the inventor also discovered that the depolarization occurs to a smaller extent in a region through which the ions passed.  
     [0042]FIGS. 1A and 1B illustrate an example of an experiment by the inventor of the present invention for explaining the principle of the present invention. FIG. 1A is a perspective view schematically illustrating the state in which boron ions  13  accelerated at 15 MeV are implanted to a dose of 5×10 13 /cm 2  into the top surface  14  of a silica glass plate over a perspective of 3 cm. In FIG. 1A, the reference numeral  15  designates a front surface of the silica glass plate,  16  designates a rear surface of the silica glass plate, and  17  designates a position where the ions stop by the acceleration energy disappearance of themselves.  
     [0043]FIG. 1B is a front view showing a polarization state when linearly polarized light is irradiated into the rear surface  16  of the silica glass plate, and the polarization state is observed at the front surface  15 . A blackened part  19  in FIG. 1B indicates a region in which the light is depolarized. The more darkness of the part  19  indicates the more depolarization.  
     [0044] As for the silica glass plate with the width of 3 cm, the polarized light usually passes through the silica glass plate with maintaining the polarization state. In fact, before the implantation of the ions  13 , it is observed that the light passes across the silica glass plate with maintaining the polarization state.  
     [0045] After the implantation of the ions  13 , the depolarization is achieved near the position  17  where the implanted ions  13  stop by the acceleration energy disappearance of themselves, that is, the position close to the depth nearly equal to the projected range of ions from the top surface  14  of the glass. In addition, as indicated by the reference numeral  19 , the depolarization is observed at a portion a few micrometers above the position  17  where the ions  13  stop. The phenomenon of the depolarization occurs when any kinds of ions are implanted.  
     [0046] (First Embodiment)  
     [0047] The inventor of the present invention invented a technique of using the foregoing phenomenon for depolarizing the light propagating through the optical fiber or optical waveguide. To depolarize the light propagating through the optical fiber using the technique, the ion implantation should be made from the outside of a cladding  22  of an optical fiber  21  to a part of the optical fiber  21  in such a manner that the ions  23  stop near a core  25  by the acceleration energy disappearance of themselves as illustrated in FIG. 2.  
     [0048] To achieve this, in the first embodiment in accordance with the present invention, hydrogen ions  23  are accelerated at 2.4 MeV, and then the accelerated ions are irradiated into the silica-based optical fiber  21  with a diameter of 125 μm from above its cladding  22 , for example. The hydrogen ions  23  thus implanted lose the acceleration energy and then stop in the core  25  of the optical fiber  21 . As a result, the light propagating through the optical fiber is depolarized. In other words, the optical fiber manufactured according to the present invention operates as a depolarizer. Here, the reference numeral  27  designates a position at which the implanted ions stop.  
     [0049] The dose of the ions  23  depends on the length L of the portion  27  where the ions stop. For the length L of the portion  27  of about 1 cm, favorable depolarization is observed in the foregoing hydrogen ion implantation when the optical fiber  21  is irradiated with the hydrogen ions to a dose of about 2×10 15 /cm 2 .  
     [0050] Implanting the ions over a wider range can further reduce the dose of the ions. The technique is expected to have higher depolarization effect by increasing the length L of the portion  27  or the dose of the ions. In addition, since the depolarization effect is achieved around the portion  27  where the ions  23  stop and at the position through which the ions  23  pass as described before, it is not necessary for the ions  23  to be implanted at the center of the core  25 . For example, the depolarization effect is produced in the core when the implanted ions pass through the core. In addition, since the electric field of the light propagating through the core  25  is expanded to the cladding  22  near the core, the depolarization effect is achieved as long as the implanted ions reach the cladding near the core, even if the ions do not reach the core itself.  
     [0051] Furthermore, varying the acceleration energy of the ions during the implantation process can heighten the depolarization effect. This is because the variation in the acceleration energy can vary the projected range of the ions into the optical fiber, thereby being able to increase the thickness of the region in which the ions stop.  
     [0052] Moreover, rotating the optical fiber around its core or varying the direction of the radiation of the ion beam during the ion implantation process has an effect of increasing the region in which the ions stop, thereby being able to heighten the depolarization effect.  
     [0053] The ion implantation may sometimes bring about point defects in the optical fiber, thereby degrading the transmission characteristic of the optical fiber. However, the point defects can be easily removed by heat treatment.  
     [0054] In the core having the depolarization effect, which is produced by the present technique, the light propagating through the core is continuously subjected to the effect of depolarization. Accordingly, polarization dependence does not occur even if the core has birefringence. Consequently, the present invention is applicable to the elimination, reduction or improvement of the polarization dependence of optical components formed in optical fibers such as Bragg reflection gratings or long-period gratings.  
     [0055] The application method is as follows. For example, as for an optical fiber including an optical component, from which polarization dependence is desired to be removed, the polarization dependence is removed by depolarizing the light propagating through the optical fiber by implanting ions into the optical fiber. Alternatively, the ions may be implanted into the optical fiber before the optical component is formed, and then the optical fiber may be used to fabricate the optical component.  
     [0056] As described before, the depolarization effect can be achieved when any kinds of ions are implanted. Thus, the implanted ions  23  may be monoatomic ions (such as H + ), molecular ions (such as H 2   + ) or cluster ions (such as C 3   + ), however the implanted ions have need to pass through the cladding  22  and stop around the core  25  of the optical fiber. Accordingly, the ion implantation requires very large acceleration energy except that the ions show large penetration depth into a material. Consequently, it is preferable that the implanted ions  23  include charged particles of atoms whose atomic numbers are equal to or smaller than  36 .  
     [0057] (Second Embodiment)  
     [0058] As for an optical waveguide, the depolarization effect of the light propagating through the core can be achieved by implanting ions from above the cladding into or near the core as in the optical fiber above mentioned. Thus, the optical waveguide depolarizer can be formed. The process is shown in FIGS. 3A and 3B. FIG. 3A is a front sectional view illustrating an optical waveguide of a second embodiment in accordance with the present invention, and FIG. 3B is its perspective view. The reference numeral  31  designates an optical waveguide,  33  designates an ion beam to be implanted,  34  designates a substrate,  35  designates a lower cladding,  37  designates a core, and  38  designates an upper cladding.  
     [0059] The present embodiment is similar to the first embodiment of FIG. 2 in the implantation method of the ion beam  33 , in the position within or near the core  37  where the ions stop by the acceleration energy disappearance of themselves, and in the effects and advantages at the position where the ions stop. Thus, the detailed description thereof is omitted here.  
     [0060] As described in the first embodiment, the present technique is utilized to eliminate, reduce or improve the polarization dependence. Although FIGS. 3A and 3B illustrate the case of a straight waveguide, it is obvious that the present invention is applicable to all types of optical waveguides including a curved waveguide, a branching and/or merging waveguide, or the like. The application of the present technique is easy because it is enough to implant the ions  33  into the optical waveguide. Accordingly, the present invention is easily applicable to devices with complicated configurations such as arrayed waveguide gratings, Mach-Zehnder interferometer type multiplexers &amp; demultiplexers, and directional couplers.  
     [0061] To eliminate the polarization dependence of the optical waveguide, the ions  33  may be implanted into the entire optical waveguide. However, this is not essential: Implanting ions only into a region to be depolarized can reduce the area of the region subjected to the ion implantation, and hence reduce the time required for the fabrication. For example, to depolarize only a portion where a Bragg reflection grating is formed in an optical waveguide, it is enough to implant the ions only to the portion where the Bragg reflection grating is formed.  
     [0062] (Variation of Second Embodiment)  
     [0063] As for the optical waveguide, it is also effective to implant ions under the condition without the upper cladding, as a variation of the second embodiment in accordance with the present invention. This will be described by way of example. First, an ordinary fabrication method of an optical waveguide will be described with reference to FIGS.  4 A- 4 H. As shown in FIGS. 4A and 4B, a lower cladding  35  is formed on a planar substrate  34 . This step can be omitted when the planer substrate  34  constitutes the lower cladding.  
     [0064] Subsequently, as shown in FIGS. 4C and 4D, a core layer  36  is formed on the lower cladding  35 . Then, as shown in FIGS. 4E and 4F, the core layer  36  is shaped into the core  37  of the optical waveguide.  
     [0065] Subsequently, as shown in FIGS. 4G and 4H, the optical waveguide is completed by forming an upper cladding  38  on the core  37 .  
     [0066] In the foregoing process, the depolarization effect of the optical waveguide and the elimination of the polarization dependence are implemented by implanting ions into the core layer  36  or core  37  at the step shown in FIGS. 4C and 4D or at the step shown in FIGS. 4E and 4F, that is, from the time after forming the core layer  36  to the time before forming the upper cladding  38 .  
     [0067] As described in the first embodiment, the depolarization effect takes place as long as the implanted ions pass through the core even if they do not stop in the core. Accordingly, it is permitted for the implanted ions to pass across the core layer  36  or core  37 , and to reach the lower cladding  35 .  
     [0068] In the present variation, the ion implantation is carried out under the condition without the upper cladding  38 . Thus, it offers an advantage of being able to reduce the acceleration energy of the ions to be implanted into the core layer  36 , or into the core  37 , or into the lower cladding  35  under the core. Therefore it is not necessary to restrict the ion species to be implanted in the present variation.  
     [0069] The present invention has been described in detail with respect to preferred embodiments, and it will now be apparent from the foregoing to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspect, and it is the intention, therefore, in the appended claims to cover all such changes and modifications as fall within the true spirit of the invention.