Optical signal processing device

Abstract

To provide an optical signal processing device with which, while suppressing an increase in the number of manufacturing processes, it is possible to suppress an increase in size attributable to a pigtail section.SOLUTION: An optical signal processing device according to the present invention comprises: a first polarization separation mechanism for separating the polarization components of first signal light; a first short waveguide and a first long waveguide by which each of first signal light separated by the first polarization separation mechanism is guided; a first polarization multiplexing mechanism for multiplexing each of first signal light having propagated through each of the first short waveguide and the first long waveguide; a polarization rotating mechanism for rotating polarization by an amount equivalent to 45° with respect to the multiplexed first signal light to obtain second signal light; a second polarization separation mechanism for separating second signal light; a second short waveguide and a second long waveguide by which each of separated second signal light is guided; and a second polarization multiplexing mechanism for multiplexing each of second signal light having propagated through each of the second short waveguide and the second long waveguide.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to an optical signal processing device, and more particularly to an optical signal processing device having a polarization canceling function. [Background technology]

[0002] With the expansion of demand for data communication networks such as the Internet, optical communication networks are expected to have even higher capacity. To meet this expanding network demand, various optical devices have been put into practical use, such as variable optical attenuators and coherent communication front-ends. These optical devices are primarily constructed using optical waveguide technology due to their high integration and manufacturability.

[0003] A polarization eliminator (hereinafter also referred to as a depolarizer) is an element that eliminates the polarization of light and is used as a measurement light source for optical devices or as a pump light source for stimulated Raman fiber amplifiers. A typical existing polarization eliminator is a Lyot-type depolarizer, which is made by rotating the polarization axis of a 1:2 polarization-maintaining fiber (hereinafter referred to as PMF) by 45° and fusion-splicing it. However, this Lyot-type depolarizer requires the use of a long-distance fiber, which poses a problem of increasing the size of the device (see, for example, Non-Patent Document 1).

[0004] To address this issue, a conventional technique for miniaturizing a polarization eliminator is known, which uses optical waveguide technology as an integrated device (see, for example, Patent Document 1). FIG. 1 is a diagram showing a configuration example of a degree of polarization reducer 110 disclosed in Patent Document 1. In the example shown in FIG. 1, excitation light output from a light source (LD) 101 is input to the degree of polarization reducer 110 via a PMF 102 and a lens 111. Generally, light waves output from a laser diode (hereinafter referred to as LD) such as the LD 101 are linearly polarized. However, in the degree of polarization reducer 110 of Patent Document 1, the output light from the LD 101 is set so as to uniformly excite the polarization axis of the PMF 102 so that the components of the input light to the degree of polarization reducer 110 are equal in both directions: a direction horizontal to the main surface of the substrate 114 and perpendicular to the propagation direction of the signal light (corresponding to the X direction in FIG. 1 ), and a direction perpendicular to the main surface of the substrate 114 (corresponding to the Y direction in FIG. 1 ). 1, the polarization axis of PMF 102 is installed at an angle of 45° with respect to the polarization direction of LD 101, i.e., the vibration direction of the electric field. In degree-of-polarization reducing device 110 shown in Fig. 1, pump light input as X- and Y-polarized waves is separated into X-polarized wave and Y-polarized wave by polarization separation waveguide 601. One of the separated signal lights is guided to curved waveguide 115 and the other to straight waveguide 602, where the correlation between the orthogonal polarization components is eliminated, and the separated signal lights are polarization-combined by polarization combination waveguide 604 and then output to a single mode optical fiber (hereinafter referred to as SMF) via lens 121. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2013 / 140521 [Non-patent literature]

[0006] [Non-Patent Document 1] Ogoshi, Nishihara, Okamoto, Kyuma, Otsu, and Hotate, "Optical Fiber Sensors," Ohmsha, pp. 41-42 (July 30, 1986) Summary of the Invention [Problem to be solved by the invention]

[0007] The depolarization device disclosed in the above-mentioned Patent Document 1 solves the problem of the Lyot-type depolarizer in which PMFs are fusion-spliced, that is, the increase in size of the device, but some problems remain.

[0008] First, in the depolarization device disclosed in Patent Document 1, the output from the light source (LD 110) needs to be input to the degree-of-polarization reducer 110 via the PMF 102, and at that time, the two polarization axes of the PMF need to be excited equally, which poses a problem in that more man-hours are required for axial alignment.

[0009] Second, the depolarization device disclosed in Patent Document 1 requires a lens 121 to input the signal light from the PMF 102 into the degree-of-polarization reducer 110 formed on the substrate and couple the output light to the SMF, which requires precise optical alignment, resulting in an increase in the number of manufacturing steps.

[0010] Thirdly, in the depolarization device disclosed in Patent Document 1, the input from the light source (LD 101) to the degree of polarization reduction device 110 uses PMF 102, which poses the problem of unavoidably increasing the size of the device due to the pigtail portion.

[0011] Fourth, the depolarization device disclosed in Patent Document 1 uses a silicon waveguide, which means that loss due to waveguide propagation is large. Therefore, a loss waveguide must be provided to balance the loss due to the curved waveguide and the loss of the straight waveguide, which poses a problem of increasing the loss of the entire device. [Means for solving the problem]

[0012] The present invention has been made in consideration of the above-mentioned problems, and its object is to provide an optical signal processing device that can suppress an increase in the number of manufacturing steps while suppressing an increase in size due to the pigtail portion.

[0013] Furthermore, in addition to the above object, some embodiments of the present invention also have the object of providing a low-loss optical signal processing device that does not require the loss waveguide that was required in the prior art.

[0014] In order to achieve the above object, the present invention provides an optical signal processing device, comprising: a first polarization splitting mechanism that splits a first signal light into two orthogonal polarization components; a first short waveguide through which the first signal light of the first polarization component is guided; a first long waveguide through which the first signal light of the second polarization component is guided, the first long waveguide having an optical path length longer than that of the first short waveguide; a first polarization multiplexing mechanism that multiplexes the first signal lights that have propagated through the first short waveguide and the first long waveguide; and a polarization multiplexing mechanism that multiplexes the first signal lights that have propagated through the first short waveguide and the first long waveguide at an angle of 45° to the multiplexed first signal light. Provided is an optical signal processing device comprising: a polarization rotation mechanism that rotates polarization by a corresponding amount to generate a second optical signal; a second polarization separation mechanism that separates the second signal light into two orthogonal polarization components; a second short waveguide through which the second signal light of the first polarization component is guided; a second long waveguide through which the second signal light of the second polarization component is guided, the second long waveguide having an optical path length longer than that of the second short waveguide; and a second polarization multiplexing mechanism that multiplexes the second signal lights propagated through each of the second short waveguide and the second long waveguide. [Effects of the Invention]

[0015] The optical signal processing device according to the present invention is an optical signal processing device having a polarization depolarization function, and can be manufactured in a simpler process and more compact than conventional techniques. Furthermore, in some embodiments, it has lower loss than conventional techniques. [Brief explanation of the drawings]

[0016] [Figure 1]FIG. 1 is a diagram showing an example of the configuration of a degree of polarization reducing device 110 disclosed in Patent Document 1. [Figure 2] 1 is a diagram showing a schematic structure of an optical signal processing device 200 according to the present invention. [Figure 3] 1A and 1B are diagrams showing the polarization components of signal light propagating through each waveguide in the optical signal processing device 200, where (a) shows the general structure of the optical signal processing device 200, and (b) shows a diagram showing the polarization state of the signal light in the corresponding waveguide. [Figure 4] FIG. 2 is a diagram schematically illustrating a configuration of a first polarization separation mechanism 203a of an optical signal processing device 200 according to a first embodiment of the present invention. [Figure 5] FIG. 2 is a diagram schematically illustrating a configuration of a first polarization separation mechanism 203b of an optical signal processing device 200 according to the first embodiment of the present invention. [Figure 6] 2 is a diagram schematically illustrating a configuration of a polarization rotation mechanism 208a of an optical signal processing device 200 according to the first embodiment of the present invention. FIG. [Figure 7] 10A and 10B show the calculation results of the output power of the polarization rotation mechanism 208a relative to the wavelength of the signal light, where (a) shows the calculation results when the polarization rotation mechanism 208a uses a λ / 2 wave plate, and (b) shows the calculation results when the polarization rotation mechanism 208a uses a λ / 4 wave plate. [Figure 8] 10 is a diagram schematically illustrating a configuration of a first polarization separation mechanism 203c of an optical signal processing device 200 according to a second embodiment of the present invention. FIG. [Figure 9] 10 is a diagram schematically illustrating a configuration of a first polarization separation mechanism 203d of an optical signal processing device 200 according to a second embodiment of the present invention. FIG. [Figure 10] FIG. 10 is a diagram schematically illustrating a configuration of a polarization rotation mechanism 208b of an optical signal processing device 200 according to a third embodiment of the present invention. [Figure 11]10A and 10B are diagrams showing calculation results of the stress distribution and the change in refractive index in the polarization rotation mechanism 208b of the optical signal processing device 200 according to the third embodiment of the present invention, in which (a) is a contour diagram showing the in-plane distribution of the equivalent stress (Mises stress) in a cross section perpendicular to the optical axis direction in the polarization rotation mechanism 208b, and (b) is a diagram plotting the change in refractive index with respect to the X-axis and Y-axis of stress birefringence in the polarization rotation mechanism 208b. DETAILED DESCRIPTION OF THE INVENTION

[0017] Various embodiments of the present disclosure will be described in detail below with reference to the drawings. The same or similar reference numerals indicate the same or similar elements, and redundant description may be omitted. Materials and numerical values ​​are for illustrative purposes only and are not intended to limit the technical scope of the present disclosure. The following description is an example, and some configurations may be omitted or modified, or additional configurations may be added, as long as they do not deviate from the gist of one embodiment of the present invention.

[0018] (Overall configuration and operation of optical signal processing device) 2 is a diagram showing a schematic structure of an optical signal processing device 200 according to the present invention. As shown in Fig. 2, the optical signal processing device 200 is disposed on a substrate 201 and includes a first polarization separation mechanism 203 that separates a first signal light output from an externally installed light source 215 into two orthogonal polarization components, a first short waveguide 204 through which the first signal light of the first polarization component is guided, a first long waveguide 205 through which the first signal light of the second polarization component is guided, the first long waveguide 205 having a longer optical path length than the first short waveguide 204, a first polarization multiplexing mechanism 206 that multiplexes the first signal light propagating through the first short waveguide 204 and the first long waveguide 205, and a first polarization multiplexing mechanism 207 that multiplexes the first signal light multiplexed by the first polarization multiplexing mechanism 206. the second signal light is transmitted through the second short waveguide 211 and the second long waveguide 212. The ...

[0019] Signal light output from an externally installed light source 215 is input to an input waveguide 202 formed on a substrate 201 of the optical signal processing device 200 via an optical fiber 216. Furthermore, the signal light multiplexed by the first polarization multiplexing mechanism 206 is input to a polarization rotation mechanism 208 via a first connection waveguide 207. Furthermore, the signal light output from the polarization rotation mechanism 208 is input to a second polarization separation mechanism 210 via a second connection waveguide 209. Furthermore, the signal light multiplexed by the second polarization multiplexing mechanism 213 is output to an output fiber 217 via an output waveguide 214.

[0020] The operation of the optical signal processing device 200 having such a configuration will be described in detail below with reference to Fig. 3. Fig. 3 is a diagram schematically showing the polarization components of signal light propagating through each waveguide in the optical signal processing device 200, where (a) shows a general structure of the optical signal processing device 200 and (b) shows a diagram schematically showing the polarization state of the signal light in the corresponding waveguide. In Fig. 2, signal light output from a light source 215 having a linewidth Δf is input to an input waveguide 202 formed on a substrate 201 of the optical signal processing device 200 via an optical fiber 216. Here, the optical fiber 216 may be either an SMF or a PMF. The polarization state of the signal light incident on the input waveguide 202 has two orthogonal components, such as a polarization state 301 shown in FIG. 3(b), in which the optical electric field oscillates in a direction horizontal to the main surface of the substrate 201 and perpendicular to the propagation direction of the signal light (the X direction in FIG. 2) (hereinafter referred to as X polarization), and a component in which the electric field oscillates in a direction perpendicular to the substrate 201 (hereinafter referred to as Y polarization).

[0021] Next, the signal light input to the input waveguide 202 is separated into X-polarized light and Y-polarized light by the first polarization separation mechanism 203. One of the separated polarization components is guided to the first long waveguide 205, and the other is guided to the first short waveguide 204. A delay Δτ occurs between the signal light propagating through the first long waveguide 205 and the first short waveguide 204 due to the optical path difference ΔL1. Thereafter, the polarization components propagating through the first long waveguide 205 and the first short waveguide 204 are multiplexed by the first polarization multiplexing mechanism 206. This multiplexed signal light has a polarization state in which a delay Δτ occurs between the X-polarized light and the Y-polarized light, as shown in polarization state 302 in FIG. 3(b).

[0022] The optical signal multiplexed by first polarization multiplexing mechanism 206 is input to polarization rotation mechanism 208 via first connecting waveguide 207. Polarization rotation mechanism 208 has a function of rotating the polarization of the input signal light by an amount equivalent to 45°. That is, as shown in polarization state 303 in FIG. 3(b), of the X-polarized light in first connecting waveguide 207, a portion equivalent to 1 / 2 in terms of intensity remains X-polarized, and the remaining portion equivalent to 1 / 2 is converted to Y-polarized light. Similarly, of the Y-polarized light in first connecting waveguide 207, a portion equivalent to 1 / 2 in terms of intensity remains Y-polarized, and the remaining portion equivalent to 1 / 2 is converted to X-polarized light.

[0023] The signal light output from the polarization rotation mechanism 208 is further separated into X-polarized light and Y-polarized light by the second polarization separation mechanism 210 via the second connection waveguide 209. One of the polarization components separated by the second polarization separation mechanism 210 is guided to the second long waveguide 212, and the other is guided to the second short waveguide 211. A delay corresponding to the optical path difference 2ΔL1 is applied to the polarization components propagating through the second long waveguide 212 and the second short waveguide 211, as shown in polarization state 304 in FIG. 3, and the polarization components are then multiplexed by the second polarization multiplexing mechanism 213. Finally, the signal light multiplexed by the second polarization multiplexing mechanism 213 is output to the output fiber 217 via the output waveguide 214.

[0024] In the optical signal processing device 200 operating in this manner, the input signal light is output as unpolarized light. In other words, the signal light is linearly polarized when it is output from the light source 215, whereas the signal light output to the output fiber 217 has two orthogonal polarization components and becomes depolarized light in which the correlation between these orthogonal components is reduced.

[0025] The correlation time of each of the X-polarized wave and the Y-polarized wave of the signal light multiplexed by the first polarization multiplexing mechanism 206 corresponds to the delay time Δτ described above, and is expressed by (Equation 1).

[0026]

number

[0027] Here, n is an integer, ΔL1 is the optical path difference between the first long waveguide 205 and the first short waveguide 204, and C is the speed of light.

[0028] Now, let us assume that the spectral linewidth of the light wave output from light source 215 is Δf. In this case, the correlation between the optical signals output at time 0 and time 1 / Δf is sufficiently small (this time 1 / Δf is called the coherence time). Therefore, as can be understood by referring to Figure 3, the signal light output from light source 215 at a certain time t is converted into X polarization and Y polarization as shown in polarization state 302, and when the condition of (Equation 2) is established for the delay time Δτ, the correlation between the X polarization and the Y polarization is eliminated.

[0029]

number

[0030] 2 and 3, the signal light output from light source 215 is not equally incident in the X and Y directions, the intensity of the X-polarized wave and the Y-polarized wave will be biased in the signal light at the position of first connecting waveguide 207. For example, if the signal light input to input waveguide 202 happens to be X-polarized or Y-polarized, the polarization state of the signal light will be preserved during propagation from first polarization splitter mechanism 203 to first polarization multiplexer mechanism 206.

[0031] However, as described above, the optical signal processing device 200 is configured so that the signal light multiplexed by the first polarization multiplexing mechanism 206 is introduced into the polarization rotation mechanism 208. This polarization rotation mechanism 208 has the function of rotating the polarization state of the input signal light, which is biased in the X or Y direction, by 45° to equalize the X polarization and the Y polarization. That is, the light wave component that was X polarized in the first connecting waveguide 207 is separated equally into the X component and the Y component in the polarization rotation mechanism 208, and the light wave component that was Y polarized in the first connecting waveguide 207 is separated equally into the X component and the Y component in the polarization rotation mechanism 208. Therefore, in the propagation of the signal light from the second polarization splitting mechanism 210 to the second polarization multiplexing mechanism 213, depolarized light is generated by a mechanism similar to that in the propagation from the first polarization splitting mechanism 203 to the first polarization multiplexing mechanism 206 described above.

[0032] It is desirable to set the delay time between the polarizations from second polarization splitter 210 to second polarization multiplexer 213 to 2Δτ or more. The four polarization components in polarization state 304 in Fig. 3(b) are light waves output from light source 215 simultaneously and have a correlation of 1. Therefore, by setting the delay time to 2Δτ or more, the delay time between the polarization component indicated by the solid line in the X direction and the polarization component indicated by the dotted line in the Y direction in polarization state 304 becomes separated by Δτ or more, making it possible to sufficiently depolarize the output light wave from light source 215 with a linewidth Δf.

[0033] (Configuration of polarization splitting mechanism, polarization multiplexing mechanism, and polarization rotation mechanism) Next, detailed configurations and operations of the first polarization splitting mechanism 203, the first polarization multiplexing mechanism 206, the polarization rotation mechanism 208, the second polarization splitting mechanism 210, and the second polarization multiplexing mechanism 213 of the optical signal processing device 200 according to the present invention will be described below.

[0034] (First embodiment) 4 is a diagram schematically illustrating the configuration of the first polarization separation mechanism 203a of the optical signal processing device 200 according to the first embodiment of the present invention. While FIG. 4 illustrates the configuration of the first polarization separation mechanism 203a (corresponding to the first polarization separation mechanism 203 in FIG. 2), the second polarization separation mechanism 210 may have a similar configuration. Furthermore, if the configuration of the first polarization separation mechanism 203a shown in FIG. 4 is configured to propagate signal light in the opposite direction, it can also function as the first polarization multiplexing mechanism 206 and the second polarization multiplexing mechanism 213. In other words, the first polarization multiplexing mechanism 206, the second polarization separation mechanism 210, and the second polarization multiplexing mechanism 213 may all have the same configuration as the first polarization separation mechanism 203a.

[0035] As shown in FIG. 4, the first polarization splitter 203a of the optical signal processing device 200 according to the present invention includes a polarization splitter input waveguide 401 connected to the output side of the input waveguide 202, an optical intensity splitter element 402 that splits the signal light output from the polarization splitter input waveguide 401, arm waveguides 403 and 404 in which the respective signal lights split by the optical intensity splitter element 402 are guided, and an optical fiber 405 that is provided on the arm waveguide 403 and propagates through the arm waveguide 403. The optical fiber includes a phase plate 405 that changes the phase of each polarization component of the signal light, a phase plate 406 that is installed on the arm waveguide 404 and changes the phase of each polarization component of the signal light propagating through the arm waveguide 404, a light intensity combining element 407 that combines the signal lights that have passed through the phase plate 405 and the phase plate 406, and a first polarization separation mechanism output waveguide 408 and a second polarization separation mechanism output waveguide 409 from which the signal light combined by the light intensity combining element 407 is output.

[0036] 4 is merely an example, and the first polarization separation mechanism 203a may be a two-beam interferometer including two arm waveguides (corresponding to arm waveguides 403, 403 in FIG. 4), and may have a polarization-dependent phase adjustment mechanism in at least one of the two arm waveguides for adjusting the phase of each polarization component. In the example shown in FIG. 4, the polarization-dependent phase adjustment mechanism corresponds to phase plates 405 and 406, but may also be a stress relief groove as in the second embodiment described later.

[0037] In first polarization splitting mechanism 203a having such a configuration, signal light input from input waveguide 202 is split by optical intensity splitting element 402 into arm waveguide 403 and arm waveguide 404. Of the split signal light, the signal light propagating through arm waveguide 403 is advanced in phase by λ / 4 with respect to the X polarization and delayed in phase by λ / 4 with respect to the Y polarization by phase plate 405. On the other hand, the signal light propagating through arm waveguide 404 is delayed in phase by λ / 4 with respect to the X polarization and advanced in phase by λ / 4 with respect to the Y polarization by phase plate 406. Therefore, in optical intensity combining element 407 where the signal lights that have passed through phase plates 405 and 406 are combined, the interference state of the signal light is such that the phase of the X polarization and the Y polarization is shifted by an amount equivalent to λ / 2, that is, 180 degrees. In this way, by providing an appropriate optical path length difference between the arm waveguides 403 and 404 for the wavelength of the signal light, it is possible to output optical signal outputs for each of the X and Y polarizations to the first polarization separation mechanism output waveguide 408 or the second polarization separation mechanism output waveguide 409.

[0038] As described above, if the propagation direction of the signal light is reversed in the configuration of first polarization splitter mechanism 203a, first polarization splitter mechanism 203a shown in Fig. 4 can also function as first polarization multiplexer mechanism 206 and second polarization multiplexer mechanism 213. For example, the X-polarized light may be input from first polarization splitter mechanism output waveguide 408, and the Y-polarized light may be input from second polarization splitter mechanism output waveguide 409.

[0039] The optical intensity separating element 402 and the optical intensity combining element 407 may be, for example, an optical directional coupler or a multimode interference coupler. Furthermore, a one-to-two Y-branch waveguide may be used as the optical intensity separating element 402 when operated as a polarization separating mechanism (first polarization separating mechanism 203 and second polarization separating mechanism 210) or as the optical intensity combining element 407 when operated as a polarization combining mechanism (first polarization combining mechanism 206 and second polarization combining mechanism 213).

[0040] In the above description, the first polarization separation mechanism 203 of the optical signal processing device 200 has been described as a single mechanism (performing one branch and one merge), such as the first polarization separation mechanism 203a shown in Fig. 4. However, the first polarization separation mechanism 203 of the optical signal processing device 200 may be a first polarization separation mechanism 203b having a multi-stage connection structure, as shown in Fig. 5. The first polarization separation mechanism 203b has a structure in which a second polarization separation mechanism element 203b2 and a third polarization separation mechanism element 203b3, each having a similar configuration to the first polarization separation mechanism 203a, are connected to both output waveguides of a first polarization separation mechanism element 203b1, which has a similar configuration to the first polarization separation mechanism 203a. Such a multi-stage connection configuration is not limited to the first polarization splitter mechanism 203 , but may also be applied to the second polarization splitter mechanism 210 , the first polarization multiplexer mechanism 206 , and the second polarization multiplexer mechanism 213 .

[0041] As described above, in first polarization splitter 203a, signal light input to polarization splitter input waveguide 401 is output as an optical signal output for each of X and Y polarizations to first polarization splitter output waveguide 408 or second polarization splitter output waveguide 409. For example, the X polarization component mainly occupies first polarization splitter output waveguide 408, and the Y polarization component mainly occupies second polarization splitter output waveguide 409. However, the polarization extinction ratio of first polarization splitter 203a is finite, and the Y polarization component corresponding to the polarization extinction ratio remains in first polarization splitter output waveguide 408, and the X polarization component corresponding to the polarization extinction ratio remains in second polarization splitter output waveguide 409. That is, the performance of the polarization depolarization function of the optical signal processing device 200 may be limited by the polarization extinction performance of the first polarization separation mechanism 203 and the second polarization separation mechanism 210. To address this phenomenon, by applying the configuration of the first polarization separation mechanism 203b as shown in Fig. 5, a polarization extinction ratio that is twice as high in decibels as that of a configuration in which a polarization separation mechanism is used alone (the configuration of the first polarization separation mechanism 203a) can be obtained, thereby improving the performance of the polarization depolarization function.

[0042] Fig. 6 is a diagram schematically illustrating the configuration of the polarization rotation mechanism 208a of the optical signal processing device 200 according to the first embodiment of the present invention. As shown in Fig. 6, the polarization rotation mechanism 208a is a phase plate connected to the output side of the first connecting waveguide 207 and the input side of the second connecting waveguide 209, and in this case is a wave plate such as a λ / 2 wave plate or a λ / 4 wave plate.

[0043] In general, the Jones matrix of a phase plate whose optical axis is tilted at an angle θ to the traveling direction of the signal light and whose retardation is φ is expressed by (Equation 3).

[0044]

number

[0045] where E o is the output electric field, E in is the input electric field.

[0046] Now, let us assume that polarization rotation mechanism 208a, which is a phase plate, is a λ / 2 wave plate. In this case, the optical axis of polarization rotation mechanism 208a is installed at an angle of 22.5° to the birefringence axis of the waveguide on substrate 201. If polarization rotation mechanism 208a is a λ / 4 wave plate, the optical axis of polarization rotation mechanism 208a is installed at an angle of 45° to the birefringence axis of the waveguide on substrate. Assuming that the operating center wavelength as a depolarization element is 1.55 μm and that quartz is used as a phase plate, the refractive index of ordinary light is 1.5277 and the refractive index of extraordinary light is 1.5362, so E in =(1,0) T (T stands for transposition), i.e., by inputting only the polarization component along the X axis, the output power I o can be calculated from (Equation 4).

[0047]

number

[0048] Here, * denotes the complex conjugate.

[0049] 7 shows the calculation results of the output power of the polarization rotation mechanism 208a relative to the wavelength of the signal light, obtained by such calculations. (a) shows the calculation results when the polarization rotation mechanism 208a uses a λ / 2 wave plate, and (b) shows the calculation results when the polarization rotation mechanism 208a uses a λ / 4 wave plate. As shown in FIG. 7, it can be seen that the polarization rotation mechanism 208a operates over a wider wavelength range when a λ / 2 wave plate is used as the polarization rotation mechanism 208a than when a λ / 4 wave plate is used as the polarization rotation mechanism 208a. Therefore, it can be said that it is desirable to use a λ / 2 wave plate for the polarization rotation mechanism 208a.

[0050] In optical signal processing device 200 according to the present invention having the above-described configuration, optical fiber 216 to which signal light output from light source 215 is input may be either a PMF or an SMF, and no lens (corresponding to lenses 111 and 121 in FIG. 1) is required. Therefore, the alignment process for uniformly exciting the two polarization axes of the PMF and the precise optical alignment required for lens installation, which were required in the prior art, are not required, and the manufacturing process can be simplified.

[0051] Furthermore, since the optical fiber 216 may be either a PMF or an SMF, it is possible to prevent the device from becoming large due to the pigtail portion.

[0052] In addition, if the first polarization splitter mechanism 203, the first polarization multiplexer mechanism 206, the second polarization splitter mechanism 210, and the second polarization multiplexer mechanism 213 are connected in a multi-stage configuration like the first polarization splitter mechanism 203b shown in FIG. 5, the polarization extinction ratio can be improved, and an optical signal processing device can be provided that has a polarization elimination function with lower loss than conventional technology.

[0053] (Second embodiment) An optical signal processing device according to a second embodiment of the present invention will be described in detail below with reference to the drawings. The optical signal processing device according to the second embodiment of the present invention has the same overall configuration as optical signal processing device 200 shown in Fig. 2, but differs from the first embodiment in that first polarization separation mechanism 203, first polarization multiplexing mechanism 206, second polarization separation mechanism 210, and second polarization multiplexing mechanism 213 have a configuration including stress relief grooves.

[0054] In the following description of the second embodiment, the optical signal processing device according to the present invention will be described as a silicon-based device in which a silica-based waveguide is formed on a silicon-based substrate. However, this is intended as an example, and the material system of the optical signal processing device may be other material systems (e.g., InP-based, etc.).

[0055] Fig. 8 is a diagram schematically illustrating the configuration of a first polarization separation mechanism 203c of an optical signal processing device 200 according to a second embodiment of the present invention. As in the first embodiment, Fig. 8 illustrates the configuration of the first polarization separation mechanism 203c, but the first polarization multiplexing mechanism 206, the second polarization separation mechanism 210, and the second polarization multiplexing mechanism 213 may also have the same configuration.

[0056] 8, the first polarization splitter 203c of the optical signal processing device 200 includes: a polarization splitter input waveguide 401 connected to the output side of the input waveguide 202; an optical intensity splitter element 402 that splits the signal light output from the polarization splitter input waveguide 401; arm waveguides 403 and 404 in which the signal lights split by the optical intensity splitter element 402 are guided; stress relief grooves 701a and 701b formed on the substrate 201 near both side surfaces of the arm waveguide 404; an optical intensity combining element 407 that combines the signal lights that have passed through the arm waveguides 403 and 404; and a first polarization splitter output waveguide 408 and a second polarization splitter output waveguide 409 from which the signal lights combined by the optical intensity combining element 407 are output.

[0057] Generally, glass waveguides with very small propagation loss are manufactured by high-temperature heat treatment, which generates internal stress due to the difference in linear expansion coefficient between the silicon substrate and the glass layer through which the signal light is guided. As a result, the optical waveguide has birefringence due to the photoelastic effect.

[0058] In the first polarization splitter 203c of the optical signal processing device 200 according to the second embodiment of the present invention, stress relief grooves 701a and 701b are formed near both side surfaces of the arm waveguide 404, and birefringence is eliminated in the portions of the arm waveguide 404 corresponding to the stress relief grooves 701a and 701b. On the other hand, since no stress relief grooves are formed in the arm waveguide 403, birefringence is maintained. This makes it possible to set different interference states for the X-polarized wave and the Y-polarized wave in the optical intensity combining element 407. The formulation of the interference state in the first polarization splitter 203c will be described in detail below.

[0059] The refractive index of the arm waveguide 403 for the X-polarized component is n x , the refractive index for the Y polarization component is n y and dL2 is the optical path length difference between arm waveguide 403 and arm waveguide 404. Furthermore, if L3 is the length of stress relief grooves 701a and 701b along the arm waveguide (in the Z direction in FIG. 7) and λ is the wavelength of the signal light, the amount of phase change of the signal light propagating through arm waveguide 403 is expressed by (Equation 5) and (Equation 6) for the X-polarized component and the Y-polarized component, respectively.

[0060]

number

[0061] where φ x (403) is the phase change amount of the X-polarized component of the signal light propagating through the arm waveguide 403, φ y (403) is the amount of phase change in the Y-polarized component of the signal light propagating through the arm waveguide 403.

[0062] On the other hand, the birefringence of the signal light propagating through the arm waveguide 404 is eliminated by the stress relief grooves 701a and 701b, so that the refractive index for both the X-polarized component and the Y-polarized component can be set to n1. In this case, the phase change amount of each polarization component in the arm waveguide 404 is expressed by (Equation 7) and (Equation 8), respectively.

[0063]

number

[0064] where φ x (404) is the phase change amount of the X-polarized component of the signal light propagating through the arm waveguide 404, φ y (404) is the amount of phase change in the Y-polarized component of the signal light propagating through the arm waveguide 404.

[0065] From (Equation 5) to (Equation 8), in order to set different interference states for the X polarization component and the Y polarization component in the optical intensity combining element 407, dL2 and L3 should be set so that the X polarization component and the Y polarization component of the signal light satisfy (Equation 9) and (Equation 10), respectively.

[0066]

number

[0067] In addition, n x , n y and n are constants determined by physical properties, and two variables (dL and L) are introduced into the two equations (Equation 9) and (Equation 10), so that the equations (Equation 9) and (Equation 10) can be solved.

[0068] Even in the optical signal processing device 200 having the first polarization separation mechanism 203c having such a configuration, the optical fiber 216 may be either a PMF or an SMF, as in the first embodiment, and a lens (corresponding to the lenses 111 and 121 in FIG. 1) is not required. Therefore, it is possible to simplify the manufacturing process and prevent the device from becoming large due to the pigtail portion.

[0069] Furthermore, in the explanation so far, the first polarization separation mechanism 203c has been described as being used alone (performing one branching and one joining), but similar to the first polarization separation mechanism 203b in the first embodiment, a first polarization separation mechanism 203d connected in multiple stages as shown in Figure 9 may also be applied.

[0070] 9, the first polarization separation mechanism 203d is depicted as having a configuration in which a second polarization separation mechanism element 203d2 and a third polarization separation mechanism element 203d3, each having a configuration similar to that of the first polarization separation mechanism 203c, are connected to both output waveguides of a first polarization separation mechanism element 203d1, which has a configuration similar to that of the first polarization separation mechanism 203c. However, in the third polarization separation mechanism element 203d3, stress relief grooves 701d and 701e are formed on both side surfaces of the upper arm waveguide (corresponding to the arm waveguide 403 in FIG. 8). Furthermore, the stress relief groove 701d is disposed between the second polarization separation mechanism element 203d2 and the third polarization separation mechanism element 203d3 and is configured to function as a stress relief groove for each. By arranging the stress relief grooves ce of the second polarization separation mechanism element 203d2 and the third polarization separation mechanism element 203d3 in this manner, it is possible to improve the extinction ratio while preventing the device from becoming larger.

[0071] (Third embodiment) An optical signal processing device according to a third embodiment of the present invention will be described in detail below with reference to the drawings. The optical signal processing device according to the third embodiment of the present invention has the same basic overall configuration as optical signal processing device 200 shown in Fig. 2, but differs from the first and second embodiments in that it has a configuration in which a stress relief groove is formed on one side of polarization rotation mechanism 208 on substrate 201.

[0072] In the following description of the third embodiment, the optical signal processing device according to the present invention will be described as a silicon-based element in which a silica-based waveguide is formed on a silicon-based substrate, but this is intended as an example, and the material system of the optical signal processing device may be other material systems (e.g., InP-based, etc.).

[0073] 10 is a diagram schematically illustrating the configuration of a polarization rotation mechanism 208b of an optical signal processing device 200 according to a third embodiment of the present invention. As shown in Fig. 10, the polarization rotation mechanism 208b includes a polarization rotation mechanism waveguide 1001 connected to the output side of the first connection waveguide 207 and the input side of the second connection waveguide 209, and a stress relief groove 1002 formed on the substrate 201 near one side surface of the polarization rotation mechanism waveguide 1001. In the polarization rotation mechanism 208b having such a configuration, structural asymmetry occurs in the internal stress generated in the glass layer of the silica-based optical waveguide, thereby realizing the polarization rotation mechanism.

[0074] 11A and 11B are diagrams showing calculation results of the stress distribution and the change in refractive index in the polarization rotation mechanism 208b of the optical signal processing device 200 according to the third embodiment of the present invention, in which (a) is a contour diagram showing the in-plane distribution of the equivalent stress (Mises stress) in a cross section perpendicular to the optical axis direction in the polarization rotation mechanism 208b, and (b) is a diagram plotting the change in refractive index with respect to the X-axis and Y-axis of stress birefringence in the polarization rotation mechanism 208b.

[0075] As can be seen from Figure 11(a), in polarization rotation mechanism 208b, a stress component is generated oblique to the position of the core of the optical waveguide (corresponding to the area surrounded by the dashed line in the figure), and it is expected that the axis of birefringence due to the photoelastic effect will be tilted. Furthermore, as shown in Figure 11(b), the difference in refractive index between the X and Y directions is maximized at an angle of 22.5 degrees, and this angle indicates the major axis of the refractive index ellipse of an optical waveguide in which stress relief grooves 902 are formed on only one side of the optical waveguide. Therefore, if the length of stress relief groove 902 in the optical axis direction is L, then Equation 11 holds, and L can be expressed by Equation 12, where Δn is the refractive index difference between the X and Y directions and λ is the operating center wavelength as a depolarization element.

[0076]

number

[0077] Therefore, the length L of the stress relief groove 902 in the optical axis direction should be set so that Equation 12 holds. Here, m is an integer equal to or greater than zero and represents the order, but it is preferable to set m=0 to operate over a wider wavelength range.

[0078] Even in the optical signal processing device 200 having the polarization rotation mechanism 208b having such a configuration, the optical fiber 216 may be either a PMF or an SMF, as in the first and second embodiments, and a lens (corresponding to the lenses 111 and 121 in FIG. 1) is not required. Therefore, it is possible to simplify the manufacturing process and prevent the device from becoming large due to the pigtail portion. [Industrial Applicability]

[0079] As described above, the optical signal processing device according to the present invention has a configuration in which the optical fiber 216 can be either a PMF or an SMF and does not require a lens. This has the advantage of enabling a simpler manufacturing process and a more compact device compared to conventional techniques. Furthermore, in some embodiments, it is possible to achieve a lower loss configuration compared to conventional techniques. An optical signal processing device with these characteristics is expected to be applied as an optical signal processing device with polarization elimination function to a light source for measuring optical devices or a pumping light source for stimulated Raman fiber amplifiers. [Explanation of symbols]

[0080] 101LD 110 Degree of polarization reduction device 111 Lens 114 Circuit Board 115 Curved waveguide 121 Lens 200 Optical signal processing device 201 Substrate 202 Input waveguide 203 First polarization splitting mechanism 203a First polarization splitting mechanism 203b First polarization separation mechanism 203b1 First polarization separation mechanism element 203b2 Second polarization separation mechanism element 203b3 Third polarization separation mechanism element 203c First polarization separation mechanism 203d First polarization separation mechanism 203d1 First polarization separation mechanism element 203d2 Second polarization separation mechanism element 203d3 Third polarization separation mechanism element 204 First short waveguide 205 First long waveguide 206 First polarization multiplexing mechanism 207 First connecting waveguide 208 Polarization Rotation Mechanism 208a Polarization rotation mechanism 208b Polarization rotation mechanism 209 Second connecting waveguide 210 Second polarization splitting mechanism 211 Second short waveguide 212 Second long waveguide 213 Second polarization multiplexing mechanism 214 Output waveguide 215 Light source 216 Optical Fiber 217 Output Fiber 301 Polarization state 302 Polarization state 303 Polarization State 304 Polarization State 401 Polarization separation mechanism input waveguide 402 Light Intensity Separation Element 403 Arm Waveguide 404 Arm Waveguide 405 Phase plate 406 Phase plate 407 Optical Intensity Combining Element 408 First polarization splitter output waveguide 409 Second polarization splitter output waveguide 601 Polarization separation waveguide 602 straight waveguide 604 Polarization synthesis waveguide 701a Stress relief groove 701d Stress relief groove 902 Stress relief groove 1001 Polarization Rotation Mechanism Waveguide 1002 Stress relief groove

Claims

1. An optical signal processing device, a first polarization splitting mechanism that splits the first signal light into two orthogonal polarization components; a first short waveguide through which the first signal light of a first polarization component is guided; a first long waveguide through which the first signal light of a second polarization component is guided, the first long waveguide having an optical path length longer than that of the first short waveguide; a first polarization multiplexing mechanism that multiplexes the first signal lights that have propagated through the first short waveguide and the first long waveguide; a polarization rotation mechanism that rotates the polarization of the combined first signal light by an amount corresponding to 45° to generate a second signal light; a second polarization splitting mechanism that splits the second signal light into two orthogonal polarization components; a second short waveguide through which the second signal light of the first polarization component is guided; a second long waveguide through which the second signal light of the second polarization component is guided, the second long waveguide having an optical path length longer than that of the second short waveguide; a second polarization multiplexing mechanism that multiplexes the second signal lights that have propagated through the second short waveguide and the second long waveguide; An optical signal processing device comprising:

2. 2. The optical signal processing device according to claim 1, wherein a difference in optical path length between the second short waveguide and the second long waveguide is twice a difference in optical path length between the first short waveguide and the first long waveguide.

3. 2. The optical signal processing device according to claim 1, wherein the first polarization splitting mechanism, the first polarization multiplexing mechanism, the second polarization splitting mechanism, and the second polarization multiplexing mechanism are two-beam interferometers having two arm waveguides, and at least one of the two arm waveguides has a polarization-dependent phase adjustment mechanism that adjusts the phase of each polarization component.

4. 4. The optical signal processing device according to claim 3, wherein the first polarization splitting mechanism, the first polarization multiplexing mechanism, the second polarization splitting mechanism, and the second polarization multiplexing mechanism have a structure in which the two-beam interferometers are connected in multiple stages.

5. The optical signal processing device according to claim 3 , wherein the polarization-dependent phase adjustment mechanism is a phase plate.

6. 4. The optical signal processing device according to claim 3, wherein the polarization-dependent phase adjustment mechanism is a stress relief groove formed near both side surfaces of the arm waveguide.

7. 2. The optical signal processing device according to claim 1, wherein the polarization rotation mechanism is a λ / 2 wave plate or a λ / 4 wave plate.

8. The polarization rotation mechanism includes: a polarization rotation mechanism waveguide; a stress relief groove formed near one side surface of the polarization rotation mechanism waveguide; The optical signal processing device according to claim 1 , comprising:

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