Optical circuits, integrated optical circuits and methods for providing polarization independent output light

Abstract

An optical circuit (100) comprises: a polarization rotation / separation element (110) that spatially separates and outputs a first component being a component in a first polarization direction out of input light, and a second component obtained by converting a component in a second polarization direction orthogonal to the first polarization direction out of the input light into the first polarization direction; a multiplexer (120) that is disposed on the output side of the polarization rotation / separation element (110) and multiplexes the first component and the second component; and at least one attenuation element (131, 133) that is disposed on the output side of the polarization rotation / separation element (110) and attenuates optical power of one of the first component and the second component, both of the first component and the second component, or multiplexed light multiplexed by the multiplexer (120).

Description

[Technical field]

[0001] The present invention relates to optical circuits, optical integrated circuits and methods for providing polarization independent output light. [Background technology]

[0002] Patent Document 1 states that "input light whose polarization plane varies over time is input to orthogonal polarization separation means 12 via variable polarization rotation means 11 and separated into S polarization (vertical polarization) and P polarization (horizontal polarization)...the two optical paths are combined by multiplexing means 14...polarization rotation means 15 is inserted in the optical path of the S polarization or P polarization...to match the polarization plane of one (S polarization) with the polarization plane of the other (P polarization), and output from the multiplexing means 14 to the polarization-maintaining optical fiber as one linearly polarized wave (P polarization)" (paragraph 0008), "as the variable polarization rotation means 11, for example, a Faraday rotator is used...as the orthogonal polarization separation means 12, for example, a polarizing beam splitter or calcite is used" (paragraph 0013), and "as the polarization rotation means 15, for example, a half-wave plate is used" (paragraph 0014). [Prior art document] [Patent documents] [Patent Document 1] JP-A-9-90299 General Disclosure

[0003] In a first aspect of the present invention, an optical circuit is provided. The optical circuit may include a polarization rotation separation element that spatially separates and outputs a first component, which is a component of input light in a first polarization direction, and a second component, which is a component of the input light in a second polarization direction orthogonal to the first polarization direction, converted to the first polarization direction. The optical circuit may include a multiplexer that is disposed on the output side of the polarization rotation separation element and multiplexes the first and second components. The optical circuit may include at least one attenuation element that is disposed on the output side of the polarization rotation separation element and attenuates the optical power of one of the first and second components, both the first and second components, and the multiplexed light multiplexed by the multiplexer.

[0004] At least one attenuation element may attenuate any one of the optical powers by an amount of attenuation such that the optical power of the multiplexed light output from the optical circuit becomes a predetermined magnitude.

[0005] At least one attenuation element may be disposed between the polarization rotation separation element and the multiplexer, and may attenuate the optical power of at least one of the first and second components for which the proportion of optical power lost in the polarization rotation separation element is relatively small.

[0006] At least one attenuation element may attenuate the optical power of either the first component or the second component by an amount of attenuation such that the proportion of optical power lost from input to the polarization rotation separation element until input to the multiplexer for each of the first and second components is equal within a predetermined range.

[0007] At least one attenuating element may be disposed on the optical path of each of the first and second components between the polarization rotation separation element and the combiner.

[0008] At least one attenuation element may be arranged between the polarization rotation separation element and the multiplexer only on the optical path of the component in which the proportion of optical power lost in the polarization rotation separation element is relatively small, among the optical paths of the first component and the second component.

[0009] The optical circuit may further include an optical branching coupler arranged on an optical path between both the multiplexer and the at least one attenuation element and an output side of the optical circuit, for branching a part of the multiplexed light in a direction different from the path to the output side of the optical circuit. The optical circuit may further include a light receiving element for receiving a part of the multiplexed light branched by the optical branching coupler, in order to determine in advance an attenuation amount of the optical power by the at least one attenuation element so that the optical power of the multiplexed light output from the optical circuit has a predetermined magnitude.

[0010] The optical circuit may further include a second optical branching coupler disposed on the optical paths of the first and second components between the polarization rotation separation element and the multiplexer and branching a portion of the first and second components in a direction different from the path to the multiplexer. The optical circuit may further include a second optical receiving element receiving a portion of the first and second components branched by the second optical branching coupler, so as to determine in advance an amount of attenuation of optical power by at least one attenuation element such that the optical power of the multiplexed light output from the optical circuit has a predetermined magnitude.

[0011] At least one attenuation element may be disposed between the multiplexer and the output side of the optical circuit, and may attenuate the optical power of the multiplexed light by an amount of attenuation such that the optical power of the multiplexed light output from the optical circuit becomes a predetermined magnitude.

[0012] The optical circuit may further include an optical branching coupler disposed on an optical path between the multiplexer and the at least one attenuation element, branching a portion of the multiplexed light in a direction different from the path to the at least one attenuation element. The optical circuit may further include a light receiving element receiving a portion of the multiplexed light branched by the optical branching coupler, in order to adjust in real time an amount of attenuation of the optical power by the at least one attenuation element so that the optical power of the multiplexed light output from the optical circuit becomes a predetermined magnitude.

[0013] The optical circuit may further include at least one phase shifter between the polarization rotation separation element and the combiner, for aligning the phases of the first component and the second component with each other.

[0014] In a second aspect of the present invention, there is provided an optical integrated circuit. The optical integrated circuit may include an optical functional element. The optical integrated circuit may include any of the optical circuits described above. The optical circuit may input the multiplexed light to be output to the optical functional element.

[0015] In a third aspect of the present invention, there is provided a method. The method may include a step of spatially separating and outputting a first component, which is a component of input light in a first polarization direction, and a second component obtained by converting a component of the input light in a second polarization direction orthogonal to the first polarization direction into the first polarization direction, by a polarization rotation separation element. The method may include a step of multiplexing the first and second components by a multiplexer arranged on the output side of the polarization rotation separation element. The method may include a step of attenuating the optical power of one of the first and second components, both the first and second components, and the multiplexed light multiplexed by the multiplexer, by at least one attenuation element arranged on the output side of the polarization rotation separation element. The method may include a step of adjusting an amount of attenuation of the optical power by the at least one attenuation element so that the optical power of the output multiplexed light is a predetermined magnitude.

[0016] The above summary of the invention does not list all of the features of the present invention. Also, subcombinations of these features may also be inventions. [Brief description of the drawings]

[0017] [Figure 1] FIG. 1 is a diagram illustrating an example of an optical integrated circuit 10 including an optical circuit 100 according to a first embodiment. [Diagram 2] 4 is a flowchart showing an example of an attenuation adjustment method according to the first embodiment. [Diagram 3] 4 is a flow chart showing an example of a method for providing polarization-independent output light to an optical functional device 50 according to the first embodiment. [Figure 4] FIG. 11 is a diagram illustrating an example of an optical circuit 101 according to a second embodiment. [Diagram 5] FIG. 13 is a diagram illustrating an example of an optical circuit 200 according to a third embodiment. [Figure 6] 13 is a flowchart showing an example of an attenuation amount adjusting method according to a third embodiment. [Figure 7] FIG. 13 is a diagram illustrating an example of an optical circuit 201 according to a fourth embodiment. [Figure 8] FIG. 13 is a diagram illustrating an example of an optical circuit 300 according to a fifth embodiment. [Figure 9] FIG. 13 is a diagram illustrating an example of an optical circuit 301 according to a sixth embodiment. [Figure 10] FIG. 13 is a diagram illustrating an example of an optical circuit 400 according to a seventh embodiment. [Figure 11] 13 is a flowchart illustrating an example of a method for providing polarization-independent output light to an optical functional device 50 according to a seventh embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] The present invention will be described below through embodiments of the invention, but the following embodiments do not limit the scope of the invention according to the claims. Furthermore, not all of the combinations of features described in the embodiments are necessarily essential to the solution of the invention.

[0019] 1 is a diagram showing a schematic diagram of an example of an optical integrated circuit 10 including an optical circuit 100 according to a first embodiment. The optical integrated circuit 10 is used for optical testing, for example, as an optical waveguide type optical switch, and calibrates and outputs input light input to the optical integrated circuit 10. The input light is monochromatic coherent light with a uniform phase, for example, laser light. The input light may be polarized light or may be in any polarization state. The polarization plane of the input light may change from moment to moment, or may not change.

[0020] In this embodiment, the amount of light, i.e., the magnitude of the optical power, of the input light input to the optical integrated circuit 10 is known. For example, the magnitude of the optical power is adjusted in advance on the upstream side of the optical integrated circuit 10 so that the optical power of the input light input to the optical integrated circuit 10 has a predetermined magnitude. Alternatively, the optical integrated circuit 10 may have a function of adjusting the optical power of the input light input to the optical integrated circuit 10 by itself so that the optical power of the input light is adjusted to a predetermined magnitude.

[0021] The optical integrated circuit 10 includes an optical functional element 50 and an optical circuit 100. The optical integrated circuit 10 may be an integrated circuit in which the optical functional element 50 and the optical circuit 100 are integrally mounted on the same substrate. The optical functional element 50 may be an optical waveguide type optical element, such as a semiconductor laser, an optical modulator, or an optical detector. The optical functional element 50 has characteristics that depend on the polarization state of the input light input to the optical functional element 50.

[0022] The optical circuit 100 is an optical waveguide type circuit using, for example, an optical fiber. The optical circuit 100 calibrates the input light input to the optical circuit 100 in order to provide the optical functional element 50 with output light that is not dependent on polarization. The optical circuit 100 converts at least the input light into a linearly polarized wave having one polarization direction and outputs the converted light.

[0023] Specifically, the optical circuit 100 separates the input light into a component with one polarization direction and a component with another polarization direction orthogonal to the one polarization direction converted into the one polarization direction, combines them, and outputs them. Preferably, the optical circuit 100 aligns the phases of the two separated components before combining them.

[0024] The optical circuit 100 inputs the multiplexed light to be output to the optical functional element 50. The optical circuit 100 attenuates the optical power of the input light input to the optical circuit 100 and outputs the multiplexed light. Preferably, the optical circuit 100 attenuates the optical power of the input light so that it becomes the optical power required by the optical functional element 50.

[0025] The optical circuit 100 according to this embodiment includes a polarization rotation separation element 110, a multiplexer 120, and attenuation elements 131 and 133. The polarization rotation separation element 110 spatially separates and outputs a first component, which is a component of the input light in a first polarization direction, and a second component, which is a component of the input light in a second polarization direction orthogonal to the first polarization direction, converted into the first polarization direction.

[0026] The first component is, for example, TE polarization (a plane wave in which the electric field is perpendicular to the plane of incidence; orthogonal polarization), and the component in the second polarization direction is, for example, TM polarization (a plane wave in which the magnetic field is perpendicular to the plane of incidence; parallel polarization), in which case the second component obtained by converting the component in the second polarization direction into the first polarization direction is TE polarization. Alternatively, the first component may be TM polarization, and the component in the second polarization direction may be TE polarization.

[0027] The polarization rotation separation element 110 may be, for example, a waveguide in which an optical circuit A, an optical circuit B, and an optical circuit C are connected in this order, and light is input to the optical circuit A and output from the optical circuit C. In the case where the polarization rotation separation element 110 has the function of outputting TE polarization without conversion and converting TM polarization into TE polarization and outputting it, the functions of the optical circuits A, B, and C will be described in detail below.

[0028] Optical circuit A has a mode conversion function. For TE polarized input, optical circuit A outputs the fundamental TE mode as it is. For TM polarized input, optical circuit A converts the fundamental TM mode to the first-order TE mode by using a waveguide structure that breaks the vertical symmetry, and outputs the converted mode.

[0029] Optical circuit B functions as an asymmetric directional coupler having two adjacent waveguides, and light output from optical circuit A is input to one of the waveguides. For TE polarization input, optical circuit B outputs the TE fundamental mode input from optical circuit A from the output end of the one waveguide while keeping it as the TE fundamental mode. For TM polarization input, optical circuit B transfers power of the TE 1st mode input from optical circuit A, converted from the TM fundamental mode, from the one waveguide to the other waveguide, converts it into the TE fundamental mode, and outputs it from the output end of the other waveguide.

[0030] Optical circuit C has two waveguides whose output ends are spatially separated, and the light output from the two output ends of optical circuit B is input to each of the input ends of the two waveguides. For TE polarization input, optical circuit C outputs the TE fundamental mode input to the input end of one waveguide from optical circuit B from the output end of the one waveguide while maintaining it as the TE fundamental mode. For TM polarization input, optical circuit C outputs the TE fundamental mode converted from the TE first order mode input to the input end of the other waveguide from optical circuit B while maintaining it as the TE fundamental mode, from the output end of the other waveguide.

[0031] The multiplexer 120 is disposed on the output side of the polarization rotation separation element 110, and multiplexes the above-mentioned first and second components that are spatially separated by the polarization rotation separation element 110 and output.

[0032] Attenuator elements 131 and 133 are disposed on the output side of polarization rotation separation element 110. Attenuator elements 131 and 133 according to this embodiment are disposed between polarization rotation separation element 110 and multiplexer 120, one each on the optical path of the above-mentioned first component and second component that are spatially separated and output by polarization rotation separation element 110. Specifically, attenuator element 131 is disposed on the optical path of the TE polarized wave, which is the first component, and attenuator element 133 is disposed on the optical path of the TE polarized wave, which is the second component converted from the TM polarized wave, which is the component in the second polarization direction.

[0033] The attenuation elements 131 and 133 may attenuate the optical power of both the first component and the second component, or only one of the attenuation elements 131 and 133 may attenuate the optical power of the corresponding one of the first component and the second component. When only one of the attenuation elements 131 and 133 attenuates one of the first component and the second component, the other of the attenuation elements 131 and 133 may or may not be disposed on the optical path of the other of the first component and the second component.

[0034] The optical circuit 100 may further include an optical branching coupler 140 and a photodetector 150. The optical branching coupler 140 according to this embodiment is disposed on an optical path between the output side of the optical circuit 100 and both of the multiplexer 120 and the attenuation elements 131 and 133, and branches a part of the multiplexed light multiplexed by the multiplexer 120 in a direction different from the path to the output side of the optical circuit 100. The rest of the multiplexed light multiplexed by the multiplexer 120 travels on the path to the output side of the optical circuit 100.

[0035] The photodetector 150 receives the combined light branched by the optical branching coupler 140 out of the combined light combined by the multiplexer 120. The photodetector 150 according to the present embodiment is provided in the optical circuit 100 together with the optical branching coupler 140 for the purpose of determining in advance the amount of attenuation of the optical power by the attenuation elements 131 and 133 so that the optical power of the combined light output from the optical circuit 100 has a predetermined magnitude.

[0036] Here, if the light receiving element is arranged on the optical path upstream of at least one of the multiplexer 120 and the attenuation elements 131 and 133 together with the optical branching coupler, there is a possibility that the multiplexer 120 and the waveguide located downstream of the light receiving element and the like may cause an insertion loss of light, and in this case, the light receiving element cannot accurately detect the optical power of the output light provided to the optical functional element 50. In contrast, the light receiving element 150 according to this embodiment is arranged on the optical path between both the multiplexer 120 and the attenuation elements 131 and 133 and the output side of the optical circuit 100 together with the optical branching coupler 140. As a result, the light receiving element 150 can more accurately detect the optical power of the output light provided from the optical circuit 100 to the optical functional element 50 compared to the above-mentioned case in which the light receiving element is arranged on the optical path upstream of at least one of the multiplexer 120 and the attenuation elements 131 and 133.

[0037] In this embodiment, the optical branching coupler 140 and the photodetector 150 may be removed from the optical path of the optical circuit 100 as soon as the amount of attenuation by the attenuator elements 131 and 133 is determined. A signal indicating the magnitude of the optical power detected by the photodetector 150 may be transmitted to an external device such as a personal computer for the above purpose. In this case, a user may use the personal computer to determine the amount of attenuation of the optical power by the attenuator elements 131 and 133 so that the optical power of the multiplexed light output from the optical circuit 100 becomes a predetermined magnitude. In place of the photodetector 150, the optical circuit 100 may include an optical power detection means other than the photodetector 150.

[0038] The optical circuit 100 may further include phase shifters 161 and 163. The phase shifters 161 and 163 align the phases of the first component and the second component between the polarization rotation separation element 110 and the multiplexer 120. The phase shifters 161 and 163 may align the phases of the first component and the second component by delaying the phases of both the first component and the second component by a predetermined amount.

[0039] Alternatively, only one of the phase shifters 161 and 163 may delay the phase of the corresponding one of the first and second components by a predetermined amount, thereby aligning the phases of the first and second components. When only one of the phase shifters 161 and 163 delays the phase of one of the first and second components, the other of the phase shifters 161 and 163 may or may not be disposed on the optical path of the other of the first and second components. The above-mentioned predetermined amount of phase delay may be determined in consideration of the characteristics of the polarization rotation separation element 110, for example.

[0040] As an example, the phase shifters 161 and 163 may be heaters, etc. Instead of or in addition to the phase shifters 161 and 163, the optical circuit 100 may adjust the lengths of the optical paths in which the attenuation elements 131 and 133 are disposed, thereby aligning the phases of the first component and the second component with each other.

[0041] Fig. 2 is a flowchart showing an example of the attenuation adjustment method according to the first embodiment. The optical circuit 100 according to this embodiment executes the attenuation adjustment method shown in Fig. 2 as a preliminary step to executing a method for providing polarization-independent output light to the optical functional element 50.

[0042] The optical circuit 100 executes the first detection stage (step S11). Specifically, with the attenuation of the attenuation elements 131 and 133 set to 0%, the optical circuit 100 inputs only TE polarized light having known optical power to the optical circuit 100 and detects the optical power with the photodetector 150. The optical circuit 100 transmits a signal indicating the magnitude of the optical power detected by the photodetector 150 to an external device. Note that input light containing only TE polarized light and not TM polarized light travels through the optical path in which the attenuation element 131 is disposed without being converted by the polarization rotation separation element 110, and does not travel through the optical path in which the attenuation element 133 is disposed.

[0043] The optical circuit 100 executes the second detection stage (step S13). Specifically, with the attenuation of the attenuation elements 131 and 133 set to 0%, the optical circuit 100 inputs only the TM polarized light having the same optical power as in the first detection stage to the optical circuit 100 and detects the optical power with the photodetector 150. The optical circuit 100 transmits a signal indicating the magnitude of the optical power detected with the photodetector 150 to an external device. Note that the input light containing only the TM polarized light and not the TE polarized light is converted to the TE polarized light by the polarization rotation separation element 110 and travels along the optical path in which the attenuation element 133 is arranged, and does not travel along the optical path in which the attenuation element 131 is arranged.

[0044] The optical circuit 100 executes an attenuation adjustment step based on the detection results of the first detection step and the second detection step (step S15). Specifically, the optical circuit 100 determines the amount of attenuation of the optical power by the attenuation elements 131 and 133 so that the optical power of the multiplexed light output from the optical circuit 100 becomes a predetermined magnitude. The predetermined magnitude of the optical power is, for example, the optical power required by the optical functional element 50. This allows the optical circuit 100 to keep the optical power of the multiplexed light output from the optical circuit 100 constant, regardless of the ratio of the first component in the first polarization direction and the component in the second polarization direction orthogonal to the first polarization direction in the input light.

[0045] The attenuation elements 131 and 133 according to the present embodiment have a configuration in which the attenuation amount for attenuating the optical power can be adjusted by the user, for example. As an example, the attenuation amount may be a ratio of the optical power to be attenuated to the optical power of the input light expressed as a percentage, and may be settable in the range of 0 to 100%. Alternatively, the attenuation elements 131 and 133 may be selected from a plurality of attenuation elements set to different attenuation amounts.

[0046] The attenuation elements 131, 133, whose attenuation amounts are determined as described above, attenuate the optical power of both or one of the first and second components by an attenuation amount such that the optical power of the multiplexed light output from the optical circuit 100 becomes a predetermined magnitude.

[0047] Specifically, the attenuation elements 131 and 133 attenuate the optical power of at least one of the first and second components, which is relatively small in proportion to the optical power lost in the polarization rotation separation element 110. The component which is relatively small in proportion to the optical power lost in the polarization rotation separation element 110 is the second component converted from the second polarization direction to the first polarization direction by the polarization rotation separation element 110, for example, the TM polarization converted from the TE polarization.

[0048] More specifically, the attenuation elements 131 and 133 attenuate the optical power of both or one of the first and second components by an amount of attenuation that makes the proportions of optical power lost in the first and second components from when they are input to the polarization rotation separation element 110 until when they are input to the multiplexer 120 equal to each other within a predetermined range. In other words, the optical circuit 100 equalizes the proportions of insertion loss of the first and second components that occur in the optical circuit 100.

[0049] For example, suppose that the detection result of the first detection stage is that a TE polarized wave having a known optical power loses 10% of its optical power by passing through the polarization rotation separation element 110, and the detection result of the second detection stage is that a TM polarized wave having the same optical power loses 20% of its optical power by being converted to a TE polarized wave by the polarization rotation separation element 110. Also, the optical power required by the optical functional element 50 is set to 70% of the known optical power. In this case, the optical circuit 100 may determine the attenuation amount of the attenuation element 131 to be 20% and the attenuation amount of the attenuation element 133 to be 10%.

[0050] Fig. 3 is a flowchart showing an example of a method for providing polarization-independent output light to the optical functional element 50 according to the first embodiment. Before executing the method shown in the flowchart of Fig. 3, the optical circuit 100 according to this embodiment executes the attenuation adjustment method shown in the flowchart of Fig. 2 to adjust the attenuation of the optical power by the attenuation elements 131 and 133 so that the optical power of the multiplexed light output from the optical circuit 100 has a predetermined magnitude.

[0051] The optical circuit 100 executes a polarization rotation separation step (step S101). Specifically, the optical circuit 100 spatially separates a first component, which is a component of the input light in a first polarization direction, and a second component, which is a component of the input light in a second polarization direction orthogonal to the first polarization direction, into the first polarization direction, and outputs the separated components.

[0052] The optical circuit 100 executes an optical power attenuation step (step S103). Specifically, the optical circuit 100 attenuates the optical power of one or both of the first and second components by the attenuation elements 131 and 133 arranged on the output side of the polarization rotation separation element 110.

[0053] The optical circuit 100 executes a multiplexing step (step S105). Specifically, the optical circuit 100 multiplexes the first and second components by the multiplexer 120 disposed on the output side of the polarization rotation separation element 110. As a result, the optical circuit 100 outputs, for example, TE polarization as output light independent of polarization, and inputs the TE polarization to the optical functional element 50.

[0054] It is preferable that the optical circuit 100 aligns the phases of the first and second components by the phase shifters 161 and 163 after performing the optical power attenuation step and before performing the multiplexing step.

[0055] As described above, the optical circuit 100 according to the present embodiment includes a polarization rotation separation element 110 that spatially separates and outputs a first component, which is a component of the input light in a first polarization direction, and a second component, which is a component of the input light in a second polarization direction orthogonal to the first polarization direction, converted into the first polarization direction, and a multiplexer 120 that is disposed on the output side of the polarization rotation separation element 110 and multiplexes the first and second components. The optical circuit 100 further includes at least one attenuation element 131, 133 that is disposed on the output side of the polarization rotation separation element 110 and attenuates the optical power of one or both of the first and second components.

[0056] Here, as a comparative example, an optical circuit is assumed in which a Faraday rotator adjusts the optical intensity of the S-polarized and P-polarized waves of input light, a polarizing beam splitter separates both polarizations, the S-polarized wave is rotated by a half-wave plate to become a P-polarized wave, and the two P-polarized waves are combined and output. The optical circuit according to this comparative example further arranges a pair of a beam splitter and a photodetector on each of the optical paths of the two separated polarizations in order to adjust the optical intensity of the S-polarized and P-polarized waves of the input light in real time, and feeds back data indicating the optical intensity detected by the photodetector to the polarization control circuit.

[0057] In contrast, according to the optical circuit 100 of the present embodiment having the above configuration, the optical power of one or both of the first and second components having the same polarization direction output from the polarization rotation separation element 110 is attenuated by at least one attenuation element 131, 133 arranged on the output side of the polarization rotation separation element 110. Compared to the optical circuit according to the comparative example, the optical circuit 100 according to the present embodiment does not require elements such as a Faraday rotator, a polarizing beam splitter, and a half-wave plate, and a waveguide connecting these elements, and can generate the first and second components having the same polarization direction and with an adjusted amount of attenuation, using only one polarization rotation separation element 110 and at least one attenuation element 131, 133. As a result, the optical circuit 100 according to the present embodiment can be made smaller than the optical circuit according to the comparative example, and can therefore contribute to the miniaturization of the optical integrated circuit 10 in which the optical circuit 100 is implemented. As a specific example, the optical circuit 100 according to this embodiment can realize a combination of one polarization rotation separation element 110 and at least one attenuation element 131, 133 in an area of ​​100 um x 800 um or less. Moreover, the optical circuit 100 can provide polarization-independent output light to the optical functional element 50 having a characteristic that depends on the polarization state of the input light by adjusting the amount of attenuation of the optical power by the attenuation elements 131, 133 so that the optical power of the output multiplexed light has a predetermined magnitude.

[0058] 4 is a diagram showing a schematic diagram of an example of an optical circuit 101 according to the second embodiment. The optical circuit 101 according to this embodiment differs from the optical circuit 100 according to the first embodiment in that it does not include a set of an attenuation element 133 and a phase shifter 163, but only includes a set of an attenuation element 131 and a phase shifter 161. Other configurations in the optical circuit 101 are the same as the corresponding configurations in the optical circuit 100, and therefore the same reference numbers are used and duplicated descriptions are omitted. Also, for configurations corresponding to those included in the optical circuit 100 according to the first embodiment, for example, configurations having the same function but different arrangement, the same reference numbers as those in the optical circuit 100 are used and duplicated descriptions are omitted. The same applies to the multiple embodiments described below.

[0059] According to the optical circuit 101 of this embodiment, the attenuation element 131 is disposed between the polarization rotation separation element 110 and the multiplexer 120 only on the optical path of the first component and the second component, of which the proportion of optical power lost by the polarization rotation separation element 110 is relatively small. Of the first component and the second component separated and output by the polarization rotation separation element 110, the second component has a relatively large optical power loss when converted from the component in the second polarization direction by the polarization rotation separation element 110. Therefore, the optical circuit 101 attenuates only the first component by the attenuation element 131 in order to additionally attenuate the optical power of the first component so that the optical power becomes the same as the optical power of the second component, which has a relatively large loss. In other words, the attenuation element 131 attenuates the optical power of only the component of the first component and the second component, of which the proportion of optical power lost by the polarization rotation separation element 110 is relatively small.

[0060] In the optical circuit 101 according to this embodiment, only the phase shifter 161 is disposed only on the optical path where the attenuation element 131 is disposed. Alternatively, similarly to the configuration of Fig. 1, the phase shifters 161 and 163 may be disposed on their respective optical paths, or only the phase shifter 163 may be disposed only on the optical path where the attenuation element 131 is not disposed.

[0061] The optical circuit 101 according to the second embodiment described above also provides the same effects as those of the optical circuit 100 according to the first embodiment.

[0062] 5 is a diagram illustrating an example of an optical circuit 200 according to the third embodiment. The optical circuit 200 according to this embodiment differs from the optical circuit 100 according to the first embodiment in that the optical branching coupler 140 and the photodetector 150 are replaced with second optical branching couplers 241 and 243 and second photodetectors 251 and 253.

[0063] The second optical branching couplers 241, 243 are disposed on the optical paths of the first and second components between the polarization rotation separation element 110 and the multiplexer 120, and branch a portion of the first and second components in a direction different from the path to the multiplexer 120. More specifically, the second optical branching couplers 241, 243 are disposed on the optical paths of the first and second components between the phase shifters 161, 163 and the multiplexer 120. The second optical receiving elements 251, 253 receive a portion of the first and second components branched by the second optical branching couplers 241, 243.

[0064] The second photodetectors 251, 253, like the photodetector 150 in the first embodiment, are provided in the optical circuit 200 together with the second optical branching couplers 241, 243 for the purpose of determining in advance the amount of attenuation of the optical power by the attenuation elements 131, 133 so that the optical power of the multiplexed light output from the optical circuit 200 becomes a predetermined magnitude.

[0065] Here, if the light receiving element is arranged on only one of the optical paths of the first and second components together with the optical branching coupler, the optical power of the other of the first and second components cannot be detected, and therefore the above-mentioned object cannot be achieved. Also, if the light receiving element is arranged on the optical paths of the first and second components upstream of the phase shifters 161 and 163 together with the optical branching coupler, there is a possibility that insertion loss occurs due to the phase shifters 161 and 163 and the waveguide located downstream of the light receiving element, etc., and in this case, the light receiving element cannot accurately detect the optical power of the output light provided to the optical functional element 50. In contrast, the second light receiving elements 251 and 253 according to this embodiment are arranged on the optical paths of the first and second components together with the second optical branching couplers 241 and 243 between the phase shifters 161 and 163 and the multiplexer 120. As a result, the second light receiving elements 251 and 253 can avoid any of the above-mentioned problems.

[0066] Fig. 6 is a flowchart showing an example of an attenuation adjustment method according to the third embodiment. The optical circuit 200 according to this embodiment executes the attenuation adjustment method shown in Fig. 6 as a pre-step of executing the method for providing the polarization-independent output light to the optical functional element 50 shown in the flowchart of Fig. 3.

[0067] The optical circuit 200 executes the first detection stage (step S21). Specifically, with the attenuation of the attenuation elements 131 and 133 set to 0%, the optical circuit 200 inputs only the TE polarized light having a known optical power to the optical circuit 200 and detects the optical power with the second light receiving element 251. The optical circuit 200 transmits a signal indicating the magnitude of the optical power detected by the second light receiving element 251 to an external device. Note that the input light containing only the TE polarized light and not the TM polarized light travels through the optical path in which the attenuation element 131 is arranged without being converted by the polarization rotation separation element 110, and does not travel through the optical path in which the attenuation element 133 is arranged.

[0068] The optical circuit 200 executes the second detection stage (step S23). Specifically, with the attenuation of the attenuation elements 131 and 133 set to 0%, the optical circuit 200 inputs only the TM polarized light having the same optical power as in the first detection stage to the optical circuit 200 and detects the optical power by the second light receiving element 253. The optical circuit 200 transmits a signal indicating the magnitude of the optical power detected by the second light receiving element 253 to an external device. Note that the input light containing only the TM polarized light and not the TE polarized light is converted to the TE polarized light by the polarization rotation separation element 110 and travels along the optical path in which the attenuation element 133 is arranged, and does not travel along the optical path in which the attenuation element 131 is arranged.

[0069] The optical circuit 200 executes an attenuation adjustment step based on the detection results of the first detection step and the second detection step (step S25). Step S25 is similar to step S15 shown in the flowchart of FIG. 2, so a duplicated description will be omitted.

[0070] The optical circuit 200 according to the third embodiment described above also provides the same effects as those of the optical circuit 100 according to the first embodiment.

[0071] 7 is a diagram illustrating an example of an optical circuit 201 according to the fourth embodiment. The optical circuit 201 according to this embodiment differs from the optical circuit 200 according to the third embodiment in that it does not include a pair of an attenuation element 133 and a phase shifter 163, but only includes a pair of an attenuation element 131 and a phase shifter 161.

[0072] According to the optical circuit 201 of this embodiment, similarly to the optical circuit 101 of the second embodiment, the attenuation element 131 is disposed between the polarization rotation separation element 110 and the multiplexer 120 only on the optical path of the component in which the proportion of optical power lost in the polarization rotation separation element 110 is relatively small among the optical paths of the first and second components. That is, the optical circuit 201 attenuates only the first component by the attenuation element 131 in order to additionally attenuate the optical power of the first component so that the optical power becomes equal to the optical power of the second component in which the loss is relatively large. In other words, the attenuation element 131 attenuates the optical power of only the component in which the proportion of optical power lost in the polarization rotation separation element 110 is relatively small among the first and second components.

[0073] In the optical circuit 201 according to this embodiment, similarly to the optical circuit 101 according to the second embodiment, only the phase shifter 161 is disposed only on the optical path on which the attenuation element 131 is disposed. Alternatively, similarly to the configuration of Fig. 5, the phase shifters 161 and 163 may be disposed on their respective optical paths, or only the phase shifter 163 may be disposed only on the optical path on which the attenuation element 131 is not disposed.

[0074] The optical circuit 201 according to the above-described fourth embodiment also provides the same effects as those of the optical circuit 100 according to the first embodiment.

[0075] 8 is a diagram illustrating an example of an optical circuit 300 according to the fifth embodiment. The optical circuit 300 according to this embodiment differs from the optical circuit 100 according to the first embodiment in that, in addition to the configuration of the optical circuit 100, it includes second optical branching couplers 241, 243 and second light receiving elements 251, 253. In other words, the optical circuit 300 according to this embodiment includes a combination of the configuration of the optical circuit 100 according to the first embodiment and the configuration of the optical circuit 200 according to the third embodiment.

[0076] The optical circuit 300 according to this embodiment may execute the attenuation adjustment method shown in the flowchart of FIG. 2 or the attenuation adjustment method shown in the flowchart of FIG. 6 as a preliminary step to executing the method for providing polarization-independent output light to the optical functional element 50 shown in the flowchart of FIG. 3, or may execute these two attenuation adjustment methods simultaneously or sequentially.

[0077] The optical circuit 300 according to the above-described fourth embodiment also provides the same effects as those of the optical circuit 100 according to the first embodiment.

[0078] 9 is a diagram illustrating an example of an optical circuit 301 according to the sixth embodiment. The optical circuit 301 according to this embodiment differs from the optical circuit 300 according to the fifth embodiment in that it does not include a pair of an attenuation element 133 and a phase shifter 163, but only includes a pair of an attenuation element 131 and a phase shifter 161.

[0079] According to the optical circuit 301 of this embodiment, similarly to the optical circuit 101 of the second embodiment, the attenuation element 131 is disposed between the polarization rotation separation element 110 and the multiplexer 120 only on the optical path of the component in which the proportion of optical power lost in the polarization rotation separation element 110 is relatively small among the optical paths of the first and second components. That is, the optical circuit 301 attenuates only the first component by the attenuation element 131 in order to additionally attenuate the optical power of the first component so that the optical power becomes equal to the optical power of the second component in which the loss is relatively large. In other words, the attenuation element 131 attenuates the optical power of only the component in which the proportion of optical power lost in the polarization rotation separation element 110 is relatively small among the first and second components.

[0080] In the optical circuit 301 according to this embodiment, similarly to the optical circuit 101 according to the second embodiment, only the phase shifter 161 is disposed only on the optical path on which the attenuation element 131 is disposed. Alternatively, similarly to the configuration of Fig. 8, the phase shifters 161 and 163 may be disposed on their respective optical paths, or only the phase shifter 163 may be disposed only on the optical path on which the attenuation element 131 is not disposed.

[0081] In addition, the optical circuit 301 may not include the second optical branching coupler 243 and the second light receiving element 253 in the configuration shown in Fig. 9. In this case, the optical circuit 301 that does not include the second optical branching coupler 243 and the second light receiving element 253 may execute the attenuation adjustment method shown in the flowchart of Fig. 2 as a preliminary step of executing the method for providing the polarization-independent output light to the optical functional element 50 shown in the flowchart of Fig. 3, or may execute the attenuation adjustment method consisting of a combination of steps S21 and S25 shown in the flowchart of Fig. 6 and step S13 shown in the flowchart of Fig. 2.

[0082] Specifically, with respect to the latter, with the attenuation of attenuation elements 131 and 133 set to 0%, optical circuit 301 inputs only TE polarized light having a known optical power to optical circuit 301 and detects the optical power at second photodetector 251. Optical circuit 301 inputs only TM polarized light having the same optical power to optical circuit 301 and detects the optical power at photodetector 150. Based on these detection results, optical circuit 301 executes the above-mentioned step S25.

[0083] It should be noted that the optical circuit 301 may include the second optical branching coupler 243 and the second photodetector 253, but may not include the second optical branching coupler 241 and the second photodetector 251. As an example of an attenuation adjustment method in this case, the optical circuit 301 inputs only TE polarized light having a known optical power to the optical circuit 301 with the attenuation of the attenuation elements 131 and 133 set to 0%, and detects the optical power with the photodetector 150. The optical circuit 301 inputs only TM polarized light having the same optical power to the optical circuit 301, and detects the optical power with the second photodetector 253. The optical circuit 301 executes the above-mentioned step S25 based on these detection results.

[0084] The optical circuit 301 according to the sixth embodiment provides the same effects as those of the optical circuit 100 according to the first embodiment.

[0085] 10 is a diagram illustrating an example of an optical circuit 400 according to the seventh embodiment. The optical circuit 300 according to this embodiment differs from the optical circuit 100 according to the first embodiment in that it does not include the attenuation elements 131 and 133, but includes an attenuation element 430 and a control circuit 470.

[0086] According to the optical circuit 400 of this embodiment, the attenuation element 430 attenuates the optical power of the multiplexed light multiplexed by the multiplexer 120. More specifically, the attenuation element 430 is disposed between the multiplexer 120 and the output side of the optical circuit 400, and attenuates the optical power of the multiplexed light by an attenuation amount that makes the optical power of the multiplexed light output from the optical circuit 400 a predetermined magnitude.

[0087] The control circuit 470 controls the amount of attenuation by the attenuation element 430 in real time based on the magnitude of the optical power detected by the photodetector 150 via the optical branching coupler 140. According to the optical circuit 400 of this embodiment, the optical branching coupler 140 is disposed on the optical path between the multiplexer 120 and the attenuation element 430, and branches a part of the multiplexed light in a direction different from the path to the attenuation element 430. The photodetector 150 receives the multiplexed light branched by the optical branching coupler 140 from the multiplexed light multiplexed by the multiplexer 120. The photodetector 150 according to this embodiment is provided in the optical circuit 400 together with the optical branching coupler 140 for the purpose of adjusting the amount of attenuation of the optical power by the attenuation element 430 in real time so that the optical power of the multiplexed light output from the optical circuit 100 becomes a predetermined magnitude. Thus, in this embodiment, the optical branching coupler 140 and the optical receiving element 150 are required to implement the method for the optical circuit 400 to provide polarization-independent output light to the optical functional element 50 .

[0088] FIG. 11 is a flow chart illustrating an example of a method for providing polarization independent output light to the optical functional device 50 according to the seventh embodiment.

[0089] The optical circuit 400 according to the present embodiment executes a polarization rotation separation step (step S201). Specifically, the optical circuit 400 spatially separates a first component, which is a component of the input light in a first polarization direction, and a second component, which is a component of the input light in a second polarization direction orthogonal to the first polarization direction, into the first polarization direction, and outputs the separated components.

[0090] The optical circuit 400 executes a multiplexing step (step S203). Specifically, the optical circuit 400 multiplexes the first component and the second component by the multiplexer 120 arranged on the output side of the polarization rotation separation element 110. After executing the polarization rotation separation step, the optical circuit 400 preferably aligns the phases of the first component and the second component by the phase shifters 161 and 163 before executing the multiplexing step.

[0091] The optical circuit 400 executes an attenuation adjustment step (step S205). Specifically, the optical circuit 400 adjusts the attenuation of the optical power by the attenuation element 430 so that the optical power of the output multiplexed light becomes a predetermined magnitude. More specifically, the control circuit 470 of the optical circuit 400 adjusts in real time the attenuation of the optical power by the attenuation element 430 based on the magnitude of the optical power detected by the light receiving element 150 via the optical branching coupler 140 so that the optical power of the multiplexed light output from the optical circuit 100 becomes a predetermined magnitude.

[0092] The optical circuit 400 executes an optical power attenuation step (step S207). Specifically, the optical circuit 400 attenuates the optical power of the multiplexed light multiplexed by the multiplexer 120 by using the attenuation element 430 arranged on the output side of the polarization rotation separation element 110. As a result, the optical circuit 400 outputs, for example, TE polarization as output light independent of polarization, and inputs the TE polarization to the optical functional element 50.

[0093] The optical circuit 400 according to the seventh embodiment provides the same effects as those of the optical circuit 100 according to the first embodiment.

[0094] Although the present invention has been described above using the embodiments, the technical scope of the present invention is not limited to the scope described in the above embodiments. It is clear to those skilled in the art that various modifications and improvements can be made to the above embodiments. It is clear from the claims that such modifications and improvements can also be included in the technical scope of the present invention.

[0095] It should be noted that the order of execution of each process, such as operations, procedures, steps, and stages, in the devices, systems, programs, and methods shown in the claims, specifications, and drawings is not specifically indicated as "before," "prior to," etc., and may be realized in any order unless the output of a previous process is used in a later process. Even if the operational flow in the claims, specifications, and drawings is explained using "first," "next," etc. for convenience, it does not mean that it is essential to perform the process in this order. [Explanation of symbols]

[0096] 10 Optical Integrated Circuits 50 Optical functional elements 100, 101, 200, 201, 300, 301, 400 optical circuit 110 Polarization Rotation Splitter 120 Multiplexer 131, 133, 430 Damping element 140 Optical Branch Coupler 150 Photodetector 161, 163 Phase shifter 241, 243 Second optical branch coupler 251, 253 Second light receiving element 470 Control Circuit

Claims

1. a polarization rotation separation element that spatially separates and outputs a first component, which is a component of an input light in a first polarization direction, and a second component obtained by converting a component of the input light in a second polarization direction orthogonal to the first polarization direction into the first polarization direction; a combiner disposed on an output side of the polarization rotation separation element and configured to combine the first component and the second component; at least one attenuation element that is arranged on an output side of the polarization rotation separation element and attenuates the optical power of one of the first component and the second component, both of the first component and the second component, and the multiplexed light multiplexed by the multiplexer; An optical circuit comprising:

2. the at least one attenuation element attenuates any one of the optical powers by an attenuation amount such that the optical power of the multiplexed light output from the optical circuit becomes a predetermined magnitude. The optical circuit according to claim 1 .

3. the at least one attenuation element is disposed between the polarization rotation separation element and the multiplexer, and attenuates the optical power of at least one of the first component and the second component, the proportion of optical power lost in the polarization rotation separation element being relatively small.

3. The optical circuit according to claim 1 or 2.

4. the at least one attenuation element attenuates the optical power of either the first component or the second component by an amount of attenuation such that a ratio of optical power lost from inputting the first component to inputting the second component to inputting the first component to the second component is equal to each other within a predetermined range. The optical circuit according to claim 3 .

5. the at least one attenuating element is disposed on each of the optical paths of the first component and the second component between the polarization rotation separation element and the combiner; 5. The optical circuit according to claim 3 or 4.

6. the at least one attenuation element is disposed between the polarization rotation separation element and the multiplexer only on an optical path of a component in which a proportion of optical power lost in the polarization rotation separation element is relatively small, among the optical paths of the first component and the second component.

5. The optical circuit according to claim 3 or 4.

7. an optical branching coupler that is disposed on an optical path between both the multiplexer and the at least one attenuation element and an output side of the optical circuit, and branches a part of the multiplexed light in a direction different from a path to the output side of the optical circuit; a light receiving element that receives a part of the multiplexed light branched by the optical branching coupler so as to determine in advance an amount of attenuation of optical power by the at least one attenuation element so that the optical power of the multiplexed light output from the optical circuit has a predetermined magnitude; The optical circuit according to claim 1 , further comprising:

8. a second optical branching coupler disposed on each of the optical paths of the first component and the second component between the polarization rotation separation element and the multiplexer, the second optical branching coupler branching a portion of the first component and the second component in a direction different from the path to the multiplexer; a second light receiving element that receives a part of the first component and the second component branched by the second optical branching coupler so as to determine in advance an amount of attenuation of optical power by the at least one attenuation element so that the optical power of the multiplexed light output from the optical circuit has a predetermined magnitude; The optical circuit according to claim 1 , further comprising:

9. the at least one attenuation element is disposed between the multiplexer and an output side of the optical circuit, and attenuates the optical power of the multiplexed light by an attenuation amount such that the optical power of the multiplexed light output from the optical circuit becomes a predetermined magnitude.

3. The optical circuit according to claim 1 or 2.

10. an optical branching coupler that is disposed on an optical path between the multiplexer and the at least one attenuation element and branches a part of the multiplexed light in a direction different from a path to the at least one attenuation element; a light receiving element that receives a part of the multiplexed light branched by the optical branching coupler, in order to adjust in real time an attenuation amount of the optical power by the at least one attenuation element so that the optical power of the multiplexed light output from the optical circuit becomes a predetermined magnitude; The optical circuit of claim 9 further comprising:

11. and further comprising at least one phase shifter between the polarization rotation separation element and the combiner for aligning the phases of the first component and the second component with each other. An optical circuit according to any one of claims 1 to 10.

12. The polarization rotation separation element comprises: a waveguide structure for converting a polarization mode; Two adjacent waveguides functioning as an asymmetric directional coupler; 12. The optical circuit according to claim 1, further comprising a waveguide coupled thereto.

13. An optical functional element; An optical circuit according to any one of claims 1 to 12; Equipped with The optical circuit inputs the output combined light to the optical functional element. Optical integrated circuits.

14. a step of spatially separating and outputting a first component, which is a component of an input light in a first polarization direction, and a second component obtained by converting a component of the input light in a second polarization direction orthogonal to the first polarization direction into the first polarization direction, by a polarization rotation separation element; a combiner disposed on an output side of the polarization rotation separation element, combining the first component and the second component; attenuating optical power of one of the first component and the second component, both the first component and the second component, and the combined light combined by the combiner, by at least one attenuation element disposed on an output side of the polarization rotation separation element; adjusting an amount of attenuation of optical power by the at least one attenuation element so that the optical power of the output multiplexed light has a predetermined magnitude; A method for providing the above.

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