When the amount of light is attenuated, for example, and propagates through an optical fiber for a long distance or when light branches from an optical fiber, the light is optically amplified by an erbium-doped optical fiber amplifier (hereinafter referred to as “amplifier”) or the like. This amplification amplifies light directly without converting the light to an electrical signal. The aforementioned amplifier is constituted by a lot of optical devices such as a lens, mirror, and filter and the like. For this reason, when the amplifier receives optical feedback reflected from the optical elements or from the light inlet/outlet end of the optical fiber inside the amplifier, the light resonates, which results in degradation of the amplification. Accordingly, an optical isolator is used for preventing the optical feedback.
Further, the light propagating through an optical fiber is influenced by external stress on the optical fiber or by the bending of the optical fiber so that polarization is not constant. Thus, the preferable optical isolators inserted between the optical fibers are polarization-independent optical isolators, which are independent of the state of polarization propagating from within the optical fiber.
As described above, polarization-independent optical isolators were designed. The isolators comprise the optical elements that are a first, second, and third birefringent element and a first and second 45-degree Faraday rotator, wherein the first 45-degree Faraday rotator is inserted between the first and second birefringent elements and the second 45-degree Faraday rotator is inserted between the second and third birefringent elements so that the optical elements are arranged in series along an optical path.
In such an arrangement, however, since the respective optical elements are arranged in series, the dimension of the optical isolator in the direction along the optical path is large and cannot be reduced in size. Further, the optical fibers must be positioned at opposite ends of the optical isolator for optically coupling this optical isolator and optical fibers. Therefore, when the aforementioned optical isolator is incorporated in an amplifier, a large area is required for routing the optical fibers within the amplifier because the optical fiber cannot be bent in a smaller radius due to the bending loss of the optical fiber, wherein the problem arises that reducing the size of the amplifier is difficult. In addition, for optically coupling the aforementioned optical isolator and the optical fibers, the optical isolator requires lenses on both the incident light side and the light-exiting side. Thus, at least two lenses are necessary for each optical isolator, which results in the problem that the number of constituent components increases.
In consideration of the problems of the in-line optical isolators as described above, in-line optical isolators have been devised wherein the constituent components have been reduced in number and size (see Patent Reference 1).
Patent Reference 1: JP06-067118A (Pages 3-4 & FIG. 1)
FIG. 13 and FIG. 14 are side views for illustrating the operation of an optical isolator 100. Specifically, FIG. 13 is a side view showing the change in the polarization and optical paths when light is transmitted in the forward direction, and FIG. 14 is a view showing the change in polarization and optical paths when light is transmitted in the reverse direction.
In the optical isolator 100 in FIG. 13 and FIG. 14, reference numerals 101 and 102 designate an optical fiber; 103 designates an optical fiber array; 104 designates a birefringent element; 105 a half-wave plate; 106 a GRIN lens; 107 a Faraday rotator; 108 a reflection member; 109 a glass plate; 110 a magnet; and 111-118 and 121-128 propagating light.
The optical fibers 101 and 102 are arranged in parallel on a glass plate and constitute the optical fiber array 3. The birefringent element 104, which is a rutile type crystal, has a half-wave plate 105 attached to half the area on one side thereof and further has the glass plate 109 having the same thickness as the half-wave plate 105 on the remaining half area on this side. The half-wave plate 105 and the glass plate 109 are positioned so that the boundary between the half-wave plate 105 and the glass plate 109 is located between the optical fibers 101 and 102, and as a result, of which only the light entering/exiting the optical fiber 102 can transmit the half-wave plate 105. The Faraday rotator 107 is attached to the end face of the Grin lens 106, and further the reflection member 108 is attached to the Faraday rotator 107. The magnet 110 is provided around the outer periphery of the Faraday rotator 107.
The operation of the optical isolator 100 will now be described. Non-polarized light incident from the optical fiber 101 is split into an ordinary ray 111 and extraordinary ray 112 by the birefringent element 104, and the optical path of the extraordinary ray 112 varies. The ordinary ray 111 and the extraordinary ray 112 are converted to parallel light rays at the end face of the GRIN lens 106 on the birefringent element 104 side. The light 113 and the light 114 out of the GRIN lens  106 are rotated 22.5 degrees in a counterclockwise direction when transmitted through the Faraday rotator 107 and further rotated 22.5 degrees in a counterclockwise direction when transmitted through the reflection member 108. As a result, the light 113 and the light 114 are rotated through 45 degrees in total. The light 113 and the light 114 are reflected by the reflection member 108, and the light 113 propagates an optical path represented by the light 115, while the light 114 propagates an optical path represented by light 116. The light 115 and light 116 from the Faraday rotator 107 enter the half-wave plate 105 while being condensed by the GRIN lens 106. The half-wave plate 105 receives linearly polarized light incident thereon at angle θ with respect to the optical axis thereof and operates to output the light as linearly polarized light at angle −θ with respect to the optical axis of the half-wave plate 105. The half-wave plate 105 is provided such that the direction of the optical axis thereof makes an angle of 22.5 degrees with respect to the extraordinary ray 112. Therefore, when the light 115 and light 116 pass through the half-wave plate 105, the polarization states change such that the polarization planes are further rotated 45 degrees in a counterclockwise direction. Since the light transmitted through the half-wave plate 105 enters the birefringent element 104, when the light 117 that is an extraordinary ray and the light 118 that is an ordinary ray are transmitted from the birefringent element 104, their optical paths are matched. Further, since the light 117 and the light 118 are condensed by the GRIN lens 106, they are effectively coupled to optical fiber 102 with a loss as low as 0.5 dB.
On the other hand, in FIG. 14, when unpolarized light is transmitted from the optical fiber 102, the birefringent element 104 separates the light into an ordinary ray 121 and an extraordinary ray 122, which propagate, respectively, along different optical paths. When transmitted through the half-wave plate 105, their polarization planes are rotated in a clockwise direction, and then converted to parallel rays by the GRIN lens 106. Before and after reflected by the reflection member 108, the light 123 and light 124, propagating within the GRIN lens 106, are transmitted through the Faraday rotator 107 twice. At this time, the polarization planes are rotated 45 degrees in total in a counterclockwise direction. The light 123 and the light 125 are reflected by the reflection member 108, and the light 123 propagates an optical path represented by the light 125, while the light 124 propagates an optical path represented by light 126. Further, the light 125 and light 126 propagate while being condensed by GRIN lens 106. When light 127 from glass plate 109 is transmitted through birefringent element 104, light 127 travels straight away because it propagates as an ordinary ray and does not enter the optical fiber 101. Since the light 128 propagates from the birefringent element 104 as an extraordinary ray, its optical path is deflected and does not enter the optical fiber 101.
In this way, an optical isolator function can be obtained in which the light transmitted from the optical fiber 101 is coupled to the optical fiber 102 while the light from the optical fiber 102 is not coupled to the optical fiber 101.
As described above, the going optical paths and returning optical paths (going-returning optical paths) are formed by providing the reflection member, the number of lenses used is only one, whereby an optical isolator constituted by the smaller number of components can be provided. In addition, compared with the aforementioned optical isolator in the in-line arrangement, the dimension in the optical path direction becomes shorter, which meets the requirement for a reduction in size. Further, the optical fibers 101 and 102 are disposed on only one side of the optical isolator 100, which provides the effect of reducing the area for routing optical fibers within the amplifier when incorporating the optical isolator in the amplifier, which contributes to a reduction in the size of the amplifier.
In the structure shown in FIG. 13 and FIG. 14, however, since only the light 114 is shifted by the birefringent element 104 and then the light 113 and light 114 are launched into the GRIN lens 106, an optical path length difference by the shifted amount is produced between the light 113 and the light 114. Thus, the optical path length difference between the light 113 and the light 114 before being transmitted into the GRIN lens 106 is not reduced to the desired numerical level. Therefore, their focal position on the reflection member 108 are not matched, and the optical paths along which the light 113 and the light 114 propagate after reflection are different. To this end, when the optical paths of the ordinary ray 118 and extraordinary ray 117 are matched by the birefringent plate 104, a polarization dependent loss (PDL) occurs, which decreases the optical coupling efficiency to the optical fiber 102 on the light-exiting side.
In consideration of such PDL, composite module of optical elements having an isolator function provided by disposing a plurality of birefringent elements upstream from the lens have been devised (see, for example, Patent Reference 2).
Patent Reference 2: JP08-136859A (Page 11 & FIG. 8)
FIG. 15 is a side view for illustrating operations of such composite module of optical elements. In composite module 130 of optical elements, near a first end 136a of a GRIN lens 136, a wavelength selection filter 137, a Faraday rotator 138 and a reflection member 140 are arranged in this order from a position closer to the end 136a, and a magnet 139 is provided around the outer periphery of the Faraday rotator 138. On the other hand, near a second end 136b of the GRIN lens 136, a half-wave plate 135, a birefringent element 134, and a plurality of single mode optical fibers 131, 132, and 133 are arranged in this order from a position closer to the end 136b. The birefringent element 134 comprises first and second birefringent parts 134a and 134b. The first and second birefringent parts 134a and 134b are arranged such that their crystal axes are directed in a predetermined direction with respect to the optical axis of the GRIN lens 136 (e.g., a direction substantially orthogonal to the optical axis or a direction slanted to a predetermined angle with respect to the optical axis) with their crystal axes being directed opposite directions to each other so as to make an angle of 180 degrees. Further, the half-wave plate 135 is positioned only upstream the optical fiber 133 such that the light rays out of the optical fibers 131 and 132 are not transmitted through the half-wave plate 135.
The light of wavelength λ1 having random polarized components outgoing from the optical fiber 131 is split into two orthogonally polarized light rays by the first birefringent portion 134a. The respective light rays so split are converted to parallel light rays by the GRIN lens 136 and enter the wavelength selection filter 137. The light incident upon the wavelength selection filter 137 is reflected thereby and propagates through the GRIN lens 136 in the opposite direction, emerges from the symmetrical position with respect to the optical axis of the GRIN lens 136, and enters the second birefringent portion 134b. The second birefringent portion 134b reverses the propagation direction in the first birefringent portion 134a of the extraordinary ray so that the optical paths of the ordinary ray and extraordinary ray are matched. The optical paths so matched are launched into the optical fiber 132.
The light of wavelength λ2 from the optical fiber 132 is split into two orthogonally polarized light rays by the second birefringent portion 134b. The respective light rays so split are converted to parallel light rays by the GRIN lens 136, transmitted through the wavelength selection filter 137 and the Faraday rotator 138, and enter the reflection member 140. When transmitted through the Faraday rotator 138, the polarization planes of the incident light rays are rotated 22.5 degrees. When the light rays reflected by the reflection member 140 are re-transmitted through the Faraday rotator 138, their polarization planes are further rotated 22.5 degrees. Besides, the polarization planes of the light rays, which are transmitted through the wavelength selection filter 137 and the GRIN lens 136 and enter the half-wave plate 135, are rotated 45 degrees, and then enter the second birefringent portion 134b. The polarization state of the light at this time is that rotated 90 degrees from the state when the second birefringent portion 134b first separates the light into the ordinary ray and the extraordinary ray. Therefore, as the rays re-enter the second birefringent portion 134b, the incident ordinary ray propagates through the second birefringent portion 134b as an extraordinary ray while the incident extraordinary ray propagates thorough the second birefringent portion 134b as an ordinary ray. Thus, the optical paths of the light rays are matched at the output end of the second birefringent portion 134b, and the light enters the optical fiber 133.
On the other hand, the light of wavelength λ2 propagating in the opposite direction is split to an ordinary ray and an extraordinary ray by the second birefringent portion 134b, and the rays enter the half-wave plate 135. The half-wave plate 135 rotates 45 degrees the polarization planes of the incident light rays, and the light rays then enter the GRIN lens 136. The light rays are then converted to parallel light rays by the GRIN lens 136, the polarization plane of the light rays is rotated 22.5 degrees by the Faraday rotator 138, and the resulting light rays impinge upon the reflection member 140. The light rays reflected by the reflection member 140 are re-transmitted through the Faraday rotator 138 wherein their polarization planes are further rotated 22.5 degrees. As a result, when the light rays re-enter the second birefringent portion 134b, the incident ordinary ray propagates through the second birefringent portion 134b as an extraordinary ray while the incident extraordinary ray propagates thorough the second birefringent portion 134b as an ordinary ray, so that the optical paths of these light rays are not matched and the light rays do not enter the optical fiber 132.
As described above, the composite module 130 of optical elements is realized wherein an optical multiplexing function and an optical isolator function are provided together, and wherein a third birefringent portion 134c and a fourth birefringent portion 134d are respectively bonded to the first birefringent portion 134a and the second birefringent portion 134b such that their crystal orientations are substantially orthogonal to each other, which makes the optical path length of the optical fiber 1 equal to that of the optical fiber 2. Patent Reference 2 concludes that the aforementioned structure can reduce the degradation caused by PDL resulting from the difference in the optical path length between the ordinary ray and the extraordinary ray.
In the case of the optical isolator 100 shown in FIGS. 13 and 14, however, since the rotation angle of the Faraday rotator 107 is set at 22.5 degrees (total 45 degrees by two transmissions), when the light is reflected and re-enters the birefringent element 104, if the half-wave plate 105 is not provided, the light re-enters the birefringent element 104 with the polarization state in which the light remains rotated 45 degrees by the Faraday rotator 107. Thus, the polarization plane of the light 115 does not meet the crystal axis of the birefringent element 104, so that when the light 115 and the light 116 are transmitted from the birefringent element 104, the optical paths thereof are not matched and only light 116 is optically coupled to the optical fiber 102. As a result, reduction in coupling efficiency between the propagating light and the optical fiber 102 is caused.
Further, when the light in the opposite direction is transmitted from the optical fiber 102 and the light re-enters the birefringent element 104, the polarization plane of the light 126 is not aligned with the crystal axis of the birefringent element 104, so that the light 126 is not shifted and transmitted as it is into the optical fiber 101 as an optical feedback. As a result, the optical isolator 100 suffers from reduction in extinction ratio. The foregoing discussion also applies to the optical path providing the optical isolator function in the composite module 130 of optical elements shown in FIG. 15.
As described above, for the optical isolator using the going-returning optical paths based on reflection, a desired optical isolation property cannot be obtained without aligning the polarization plane of the extraordinary ray with the crystal orientation when the light is caused to re-enter the birefringent element when the light is sent forth along the going optical paths. Thus, the half-wave plate is an essential optical element. However, apart from the Faraday rotator, it is necessary to provide an optical element (half-wave plate) for rotating the polarization plane. Thus, the number of the kinds of the optical elements to be used becomes greater than that of the aforementioned optical isolator of the in-line arrangement (for the optical isolator of the in-line arrangement, required optical elements are two kinds: a birefringent element and a Faraday rotator). Therefore, they are not necessarily efficient means for reducing the manufacturing process and the manufacturing cost.
Further, since there is a point that the optical path passes through the half-wave plate, a new separate component of a glass plate is required to match the optical path lengths of a going path through which light is sent forth and a returning path through which light is sent back. Thus, in terms of decreasing the absolute number of components, it is not a desirable structure.
Further, since the structure shown in FIG. 15 is devised to provide both the optical multiplexing function and the optical isolator function by a single optical device, its optical path design is made more complex. Furthermore, in this structure, the birefringent elements 4c, 4d are provided for eliminating the PDL. However, it is possible to provide the optical isolator function without the third and fourth birefringent portions 134c, 134d. In other words, the optical device shown in FIG. 15 is the device having the optical paths designed to provide the third and fourth birefringent portions 134c, 134d which are not indispensable in view of providing the optical isolator function. Thus, the suggestion was not sufficient for simplification of the structure and reduction in the size and cost of the optical isolator incorporated in the amplifier.