Light source

Abstract

The present invention is a light source that includes: a silicon substrate; at least one light-emitting element; and an optical waveguide circuit that comprises a quartz-based optical waveguide formed on the silicon substrate. The light-emitting element is formed as a result of being accumulated on the silicon substrate. The optical waveguide circuit is provided with a depolarization unit and a polarization rotator that is connected to the depolarization unit. The depolarization unit includes: a polarization separator that separates incoming light waves into two mutually orthogonal polarized components; a short waveguide that guides the light waves of a first polarized component; a long waveguide that guides the light waves of a second polarized component and has an optical path length longer than that of the short waveguide; and a polarized light multiplexer that multiplexes the light waves that have propagated through the short waveguide and the long waveguide. The light waves from the light-emitting element are polarized as follows. The light waves are rotated by the polarization rotator so as to have the two mutually orthogonal polarized components and thereafter are input into the depolarization unit to be separated into the two mutually orthogonal polarized components by the polarization separator, and after a delay difference has been imparted to the two separated polarized components, the polarized components are multiplexed by the polarized light multiplexer.

Description

light source

[0001] The present invention relates to a depolarized light source.

[0002] The growing demand for data communication networks, such as the Internet, has led to a demand for ever-increasing capacity in optical communication networks. To meet this growing 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] Depolarizers (hereinafter also referred to as depolarizers) are elements that depolarize light and are used as measurement light sources for optical devices and as pumping light sources for stimulated Raman fiber amplifiers. A typical existing depolarizer is a Lyot-type depolarizer, in which the polarization axis of a polarization-maintaining fiber (PMF) with a length of 1:2 is rotated 45° and fusion-spliced. However, this Lyot-type depolarizer requires the use of a long-distance fiber, which poses the problem of the device becoming large. (Non-Patent Document 1)

[0004] To address this issue, an example is known in which a depolarization device is configured as an integrated device using optical waveguide technology in order to reduce its size (see, for example, Patent Document 1). Fig. 1 is a diagram showing an example of the configuration 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 polarization-maintaining fiber (hereinafter referred to as PMF) 102 and a lens 111. In general, the light wave output from a laser diode (hereinafter referred to as LD) such as the LD 101 is linearly polarized light.

[0005] 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 input light to the degree of polarization reducer 110 is input with equal components in both a direction horizontal to the main surface of the substrate 114 and orthogonal to the propagation direction of the signal light (corresponding to the X-axis direction in FIG. 1 ), and a direction perpendicular to the main surface of the substrate 114 (X-axis direction in FIG. 1 ). In other words, in the example of FIG. 1 , the polarization axis of the PMF 102 is set at an angle of 45° with respect to the polarization direction of the LD 101, i.e., the oscillation direction of the electric field. In the degree of polarization reducer 110 shown in FIG. 1 , the pump light input as polarized light in the X-axis and Y-axis directions is separated into X-polarized light and Y-polarized light by the polarization separation waveguide 117. One of the separated pumping lights is guided to the curved waveguide 115 and the other to the straight waveguide 116, where the correlation between the orthogonal polarization components is eliminated, and the light is polarization-combined by the polarization combining waveguide section 118 and then output to the single mode fiber (hereinafter referred to as SMF) 103 via the lens 121.

[0006] The degree of polarization reducing device disclosed in Patent Document 1 solves the problem of the Lyot-type depolarizer in which the PMF is fusion-spliced, which is associated with an increase in size of the device, but still has the following problems.

[0007] (1) The output from the light source (LD 101) needs to be input to the degree of polarization reducer 110 via a polarization-maintaining fiber (PMF 102). In this case, the two polarization axes of the PMF need to be excited evenly, which increases the labor required for axial alignment.

[0008] (2) Since the lens 111 is required to input the light wave from the PMF 102 into the polarization reduction device formed on the substrate and to couple the output light to the SMF, precise optical alignment is required, which increases the number of manufacturing steps.

[0009] (3) Since the PMF 102 is used for inputting light from the light source (LD 101) to the degree-of-polarization reducing device 110, the size of the device inevitably increases due to the pigtail portion.

[0010] (4) Because a silicon waveguide is used, the loss caused by the waveguide propagation is large to begin with, and it is necessary to provide a loss waveguide 119 to balance the loss caused by the curved waveguide and the loss caused by the straight waveguide, which increases the loss of the entire device.

[0011] 2A is a diagram showing a schematic structure of an optical signal processing device newly proposed in the present disclosure that can solve the above-mentioned problems. The optical signal processing device 200 comprises an optical waveguide circuit formed of a silica-based optical waveguide fabricated by depositing glass on an optical waveguide substrate 201 made of a silicon substrate.

[0012] This optical waveguide circuit includes an input waveguide 202, a first polarization separator 203 that separates a first light wave input from an externally installed light source 215 into two orthogonal polarization components, a first short waveguide 204 that guides the first light wave of the first polarization component, a first long waveguide 205 that guides the first light wave of the second polarization component and has an optical path length longer than that of the first short waveguide 204, a first depolarization unit 220 that includes a first polarization multiplexer 206 that is connected to the output sides of the first short waveguide 204 and the first long waveguide 205 and multiplexes the first light waves that have propagated through the first short waveguide 204 and the first long waveguide 205, a first connecting waveguide 207, and a polarization The second depolarization section 230 is formed by a rotator 208, a second connecting waveguide 209, a second polarization separator 210 that separates the second light wave output from the polarization rotator 208 into two orthogonal polarization components, a second short waveguide 211 that guides the second light wave of the first polarization component, a second long waveguide 212 that guides the second light wave of the second polarization component and has an optical path length longer than that of the second short waveguide 211, a second polarization multiplexer 213 that is connected to the output sides of the second short waveguide 211 and the second long waveguide 212 and multiplexes the second light waves that have propagated through the second short waveguide 211 and the second long waveguide 212, and an output waveguide 214.

[0013] A light wave output from a light source 215 installed outside the substrate is input to an input waveguide 202 formed on a substrate 201 of the optical signal processing device 200 via an optical fiber 216. An optical fiber 217 is connected to the output waveguide 214.

[0014] In this specification, as shown by the coordinate axes in FIG. 2A , for example, the direction horizontal to the main surface of the substrate 201 and orthogonal to the propagation direction of the light wave is defined as the X-axis direction, and the direction vertical to the main surface of the substrate 201 and orthogonal to the propagation direction of the light wave is defined as the Y-axis direction, and the component of the optical electric field oscillating in the X-axis direction is described as X-polarized light, and the component of the optical electric field oscillating in the Y-axis direction is described as Y-polarized light.

[0015] 2B is a diagram schematically showing the states of X-polarized light and Y-polarized light at various points in the optical signal processing device of FIG. 2. In FIG. 2B, components that are assigned the same reference numerals as in FIG. 2A are the same as in FIG. 2A. In FIG. 2B, polarization state 251 schematically shows the relationship between the X-polarized light and Y-polarized light of the light wave input to input waveguide 202, polarization state 252 schematically shows the relationship between the X-polarized light and Y-polarized light of the light wave propagating through first connecting waveguide 207, polarization state 253 schematically shows the relationship between the X-polarized light and Y-polarized light of the light wave propagating through second connecting waveguide 209, and polarization state 254 schematically shows the relationship between the X-polarized light and Y-polarized light of the light wave propagating through output waveguide 214.

[0016] 2A, signal light from a light source 215 having a linewidth Δf is input via an optical fiber 216 to an input waveguide 202 formed on an optical waveguide substrate 201. Here, the optical fiber 216 may be either an SMF or a PMF. The first light wave input to the input waveguide 202 has two orthogonal components, X-polarized light and Y-polarized light, as shown in polarization state 251 in FIG. 2B.

[0017] This light wave is separated into X-polarized light and Y-polarized light by first polarization separator 203. The separated first light wave of the first polarization component is guided to first long waveguide 205, and the separated first light wave of the second polarization component is guided to first short waveguide 204. The first light waves propagating through first long waveguide 205 and first short waveguide 204 are given a delay Δτ due to the optical path difference ΔL1 between them and are then combined by first polarization combiner 206. This combined first light wave has a state in which a delay Δτ occurs between the X-polarized light and the Y-polarized light, as shown in polarization state 252 in FIG. 2B .

[0018] The first light wave multiplexed by the first polarization multiplexer 206 is input to the polarization rotator 208 via the first connecting waveguide 207. The polarization rotator 208 has a function of rotating the polarization of the input light wave by an amount equivalent to 45°. That is, as shown by the dotted arrows in polarization state 253 of FIG. 2B , of the light wave that was X-polarized in the first connecting waveguide 207 (the dotted arrows in polarization state 252), a portion equivalent to ½ in intensity remains X-polarized, while the remaining half is converted to Y-polarized. Similarly, as shown by the solid arrows in polarization state 253 of FIG. 2B , of the light wave that was Y-polarized in the first connecting waveguide 207 (the solid arrows in polarization state 251), a portion equivalent to ½ in intensity remains Y-polarized, while the remaining half is converted to X-polarized.

[0019] The second lightwave output from the polarization rotator 208 is further separated into two orthogonal polarization components, X-polarized light and Y-polarized light, by the second polarization separator 210 via the second connecting waveguide 209. The second lightwave of the first polarization component separated by the second polarization separator 210 is guided to the second long waveguide 212, and the second lightwave of the second polarization component is guided to the second short waveguide 211. The second lightwaves of each polarization component propagating through the second long waveguide 212 and the second short waveguide 211 are multiplexed in the second polarization multiplexer 213 with a delay due to the optical path difference 2ΔL1 between them, as shown by the polarization state 254 in FIG. 2B . Finally, the lightwave is output to the output fiber 217 via the output waveguide 214.

[0020] In an optical signal processing device that operates in this manner, a light wave input from light source 215 is output as an unpolarized light wave. In other words, a light wave that was linearly polarized when output from light source 215 becomes depolarized light having two orthogonal polarization components and with reduced correlation between these orthogonal components when output to output fiber 217.

[0021] The correlation time of each of the X-polarized light and the Y-polarized light of the first light wave multiplexed by the first polarization multiplexer 206 corresponds to the delay time Δτ described above and is expressed as Δτ = nΔL1 / C, where n is an integer, ΔL1 is the optical path length difference between the first long waveguide 205 and the first short waveguide 204, and C is the speed of light.

[0022] 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 seconds and time 1 / Δf is sufficiently small. This time 1 / Δf is called the coherence time. Therefore, as can be understood by referring to FIG. 2B , if the light wave output from light source 215 at a certain time t consists of X-polarized light and Y-polarized light as shown in polarization state 251, the X-polarized light and the Y-polarized light are converted to have a delay difference as shown in polarization state 252. However, when the condition of Δτ > 1 / Δf is satisfied for this delay time, the correlation between the X-polarized light and the Y-polarized light is eliminated.

[0023] Incidentally, if the first light wave output from the light source 215 is not incident equally in the X-axis and Y-axis directions, a bias occurs in the intensity of the X-polarized light and the Y-polarized light at the position of the first connecting waveguide 207. For example, taking an extreme case as an example, if the first light wave input from the light source 215 to the input waveguide 202 happens to be X-polarized or Y-polarized, the polarization state is preserved during propagation from the first polarization separator 203 to the first polarization multiplexer 206.

[0024] However, in the optical signal processing device 200, the first lightwave output from the first depolarization unit 220 is input to the polarization rotator 208 via the first connecting waveguide 207. This polarization rotator 208 has the function of rotating the polarization state of the input lightwave, which is polarized in the X-axis or Y-axis direction, by 45° to equalize the X-axis component and the Y-axis component. That is, the lightwave component that was X-polarized in the first connecting waveguide 207 is equally separated into the X-axis component and the Y-axis component by the polarization rotator 208. For example, the lightwave component that was Y-polarized in the first connecting waveguide 207 is equally separated into the X-axis component and the Y-axis component by the polarization rotator 208. Therefore, during the propagation of the lightwave from the second polarization separator 210 to the second polarization multiplexer 213, i.e., in the second depolarization unit 230, depolarized light is generated by a mechanism similar to that of the propagation from the first polarization separator 203 to the first polarization multiplexer 206 described above.

[0025] The delay time between the polarizations from second polarization separator 210, which is second depolarization unit 230, to second polarization combiner 213 is desirably set to 2Δτ or more. The four polarization components in output waveguide 214 shown in polarization state 254 in Figure 2B are light waves simultaneously output from light source 215 and have a correlation of 1. Therefore, by setting the delay time to 2Δτ or more, the delay time between the polarization component shown by the solid line in the X-axis direction and the polarization component shown by the dotted line in the Y-axis direction in polarization state 254 becomes Δτ or more, making it possible to sufficiently depolarize the output light wave from light source 215 with linewidth Δf.

[0026] As described above, in the optical signal processing device according to the present disclosure shown in FIG. 2, the optical waveguide circuit is provided with a first depolarization section and a second depolarization section, and is configured so that the first depolarization section and the second depolarization section are connected via a polarization rotator. This makes it possible to depolarize the light wave regardless of the polarization state of the light wave input from the light source to the first depolarization section, thereby solving the above-mentioned problems as follows.

[0027] (1) The output from the light source (LD) may be guided without passing through a polarization-maintaining fiber, and a cheaper single-mode fiber can be used.

[0028] (2) In particular, there is no need to align the polarization axis of the polarization-maintaining fiber with that of the laser diode LD (light source) or the depolarization element substrate (optical signal processing board), eliminating the need for an additional alignment process. In this way, in addition to the price of the single-mode fiber itself, costs can be reduced by eliminating the need for this alignment process.

[0029] (3) Because a silica-based optical waveguide is used, the butt joint of optical fiber used in the implementation of silica-based optical waveguide devices can be used, simplifying the manufacturing process. In addition, in general, butt joint connections in silica-based optical waveguides are made by bonding the optical fiber and the silica-based optical waveguide with a refractive index-matched resin, which reduces the reflection loss that occurs in silicon waveguides and is superior in terms of loss, which is an important performance index for passive devices.

[0030] (4) Furthermore, silica-based optical waveguides have significantly lower optical waveguide propagation loss than silicon waveguides, satisfying important performance indicators for passive devices.

[0031] As described above, although the optical signal processing device according to the present disclosure shown in FIG. 2 can solve the above-mentioned problems, since the optical signal processing device 200 is connected to the light source 215 via the optical fiber 216, there is still a problem in miniaturizing the size of the entire device as a light source.

[0032] International Publication No. 2013 / 140521

[0033] Ogoe, Nishihara, Okamoto, Kyuma, Otsu, and Hotate, "Optical Fiber Sensor," Ohmsha, pp. 41-42 (July 30, 1986)

[0034] One object of the present disclosure is to solve the problems of the optical signal processing device shown in FIG. 2, and to provide a small, inexpensive, and low-loss depolarized light source.

[0035] The present disclosure provides a light source for achieving such an object, and one embodiment of the light source includes a silicon substrate, at least one light-emitting element, and an optical waveguide circuit consisting of a silica-based optical waveguide formed on the silicon substrate, wherein the at least one light-emitting element is integrated and formed on the silicon substrate, and the optical waveguide circuit includes at least one depolarization section and a polarization rotator connected to the depolarization section, and the depolarization section includes a polarization separator that separates an input light wave into two orthogonal polarization components, a short waveguide through which a light wave of a first polarization component is guided, a long waveguide through which a light wave of a second polarization component is guided, the long waveguide having an optical path length longer than that of the short waveguide, and a polarization multiplexer that combines the light waves propagating through each of the short waveguide and the long waveguide.

[0036] 8 is a diagram showing an example configuration of a conventional degree-of-polarization reducer 110 disclosed in Patent Document 1. FIG. 9 is a diagram showing a general structure of an optical signal processing device 200 according to the present disclosure that solves the problems of the conventional technology. FIG. 10 is a diagram showing a schematic diagram of polarization components of signal light propagating through each waveguide in the optical signal processing device 200. FIG. 11 is a diagram showing a general structure of a light source according to a first embodiment of the present disclosure. FIG. 12 is a diagram showing a schematic diagram of a polarization separator and a polarization multiplexer according to a second embodiment of the present disclosure. FIG. 13 is a diagram showing a schematic diagram of a polarization separator and a polarization multiplexer according to a third embodiment of the present disclosure. FIG. 14 is a diagram showing a schematic diagram of a polarization separator and a polarization multiplexer according to a fourth embodiment of the present disclosure. FIG. 15 is a diagram showing a schematic diagram of a polarization separator and a polarization multiplexer according to a fifth embodiment of the present disclosure. FIG. 16 is a diagram showing a schematic diagram of a polarization rotator according to a sixth embodiment of the present disclosure. FIG. 17 is a diagram showing a calculation result of the relationship between output power Io(x) and Io(y) and wavelength when a λ / 2 wave plate and a λ / 4 wave plate are used as the phase plates of the polarization rotator of FIG. 8. FIG. 18 is a diagram showing a schematic diagram of a polarization rotator according to a seventh embodiment of the present disclosure. Fig. 11 is a diagram showing calculation results of stress distribution and changes in refractive index when a stress relief groove is formed on one side surface of the optical waveguide of the polarization rotator of Fig. 10. Fig. 12 is a diagram showing a schematic structure of a light source according to an eighth embodiment of the present disclosure. Fig. 13 is a diagram showing a schematic structure of a light source of a first example embodiment according to a ninth embodiment of the present disclosure. Fig. 14 is a diagram showing a schematic configuration of a light source of a second example embodiment according to the ninth embodiment of the present disclosure.

[0037] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. Note that the following description is an example, and some configurations may be changed without departing from the gist of the present disclosure. The same or similar reference numerals indicate the same or similar elements, and repeated explanations may be omitted. Numerical values ​​in the following description are examples, and other numerical values ​​may be used in the present disclosure without departing from the gist of the present disclosure.

[0038] 1 is a diagram illustrating a schematic structure of a light source according to a first embodiment of the present disclosure. The first embodiment of the present disclosure is a light source including a silicon substrate, at least one light-emitting element, and an optical waveguide circuit formed of a silica-based optical waveguide fabricated by depositing glass on the silicon substrate, wherein the at least one light-emitting element is integrated on the silicon substrate, and the optical waveguide circuit includes at least one depolarization unit and a polarization rotator connected to the depolarization unit, and the depolarization unit includes a polarization separator that separates an input light wave into two orthogonal polarization components, a short waveguide through which a light wave of a first polarization component is guided, a long waveguide through which a light wave of a second polarization component is guided, the long waveguide having an optical path length longer than that of the short waveguide, and a polarization multiplexer that multiplexes the light waves propagating through each of the short waveguide and the long waveguide.

[0039] According to an aspect of the present disclosure, the light-emitting element and the optical waveguide circuit having the depolarization function are integrated and formed integrally on the same substrate, which enables miniaturization compared to the case where the optical signal processing device and light source of FIG. 2 are used.

[0040] This embodiment will be described in detail below with reference to Fig. 3. The first embodiment discloses a light source 300 formed by integrating a light emitting element 301 such as a semiconductor laser element on the substrate 201 of the optical signal processing device shown in Fig. 2. The light source 300 can be used as a pumping light source for a stimulated Raman fiber amplifier, for example.

[0041] As shown in the figure, the light source 300 includes a light-emitting element 301 integrated and arranged on an optical waveguide substrate 201 made of a silicon substrate, and an optical waveguide circuit 302 formed of a silica-based optical waveguide produced by depositing glass on the substrate 201.

[0042] The optical waveguide circuit 302 is similar to that shown in FIG. 2 and includes an input waveguide 202, a first polarization separator 203 that separates a first light wave input from the light emitting element 301 into two orthogonal polarization components, a first short waveguide 204 that guides the first light wave of the first polarization component, a first long waveguide 205 that guides the first light wave of the second polarization component and has a longer optical path length than the first short waveguide 204, a first depolarization unit 220 that includes a first polarization multiplexer 206 that is connected to the output sides of the first short waveguide 204 and the first long waveguide 205 and multiplexes the first light waves that have propagated through the first short waveguide 204 and the first long waveguide 205, and a first connecting waveguide 207. a polarization rotator 208, a second connecting waveguide 209, a second polarization separator 210 that separates the second light wave output from the polarization rotator 208 into two orthogonal polarization components, a second short waveguide 211 that guides the second light wave of the first polarization component, a second long waveguide 212 that guides the second light wave of the second polarization component and has an optical path length longer than that of the second short waveguide 211, a second polarization multiplexer 213 that is connected to the output sides of the second short waveguide 211 and the second long waveguide 212 and multiplexes the second light waves that have propagated through the second short waveguide 211 and the second long waveguide 212, and an output waveguide 214.

[0043] An optical fiber 303 is connected to the output waveguide 214. In this way, the light source 300 according to the present disclosure has the light emitting element 301 integrated on the silicon substrate 201 on which the optical waveguide circuit is formed. In Figure 3, the same reference numerals as in Figure 2 are the same as those in Figure 2, and their operation is as described above, so detailed description will be omitted here.

[0044] As a method of integrating the light-emitting element 301 onto the substrate 201, for example, a groove may be formed on a glass substrate forming a quartz-based optical waveguide to expose the side surface of the input optical waveguide 202, and a light-emitting element such as a semiconductor laser element may be integrated as the light-emitting element 301 shown in the figure.

[0045] In this embodiment, an example has been described in which the optical waveguide circuit includes two first depolarizing sections and a second depolarizing section, but as will be described later, the optical waveguide circuit may include only the second depolarizing section.

[0046] According to this embodiment, since the light-emitting elements are integrated into one body, there is no need for an optical fiber or the like to transmit light from the light-emitting elements to the optical waveguide circuit, so that a small, inexpensive, depolarized light source can be obtained.

[0047] Second Embodiment In the first embodiment, the general structure of the light source having a depolarization function of the present disclosure has been described. In the second embodiment, one aspect of the specific configuration of the first and second polarization separators 203 and 210 and the first and second polarization multiplexers 206 and 213 will be described.

[0048] Fig. 4 is a diagram showing the schematic configuration of the polarization separators 203 and 210 and the polarization multiplexers 206 and 213. Fig. 4 explains the operation as a polarization separator, but it is also possible to make them operate as polarization multiplexers by propagating light waves in the opposite direction. In other words, the configuration shown in Fig. 4 can operate as the first and second polarization separators and the first and second polarization multiplexers of Fig. 3.

[0049] The polarization separator 400 shown in FIG. 4 includes a first polarization separator input waveguide 401, an optical intensity separating element 402 that separates the signal light output from the first polarization separator input waveguide 401, arm waveguides 403 and 404 through which the signal lights separated by the optical intensity separating element 402 are guided, a phase plate 405 that is placed on the arm waveguide 403 and changes the phase of each polarization component of the light wave propagating through the arm waveguide 403, a phase plate 406 that is placed on the arm waveguide 404 and changes the phase of each polarization component of the signal light propagating through the arm waveguide 404, an optical 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 separator output waveguide 408 and a second polarization separator output waveguide 409 from which the signal lights combined by the optical intensity combining element are output.

[0050] Note that 410 is a second polarization separator input waveguide, but is not used in the polarization separator. In this embodiment, the phase plates are provided on the arm waveguides 403 and 404 by cutting the optical waveguides and inserting them.

[0051] The above-described configuration is merely an example, and the polarization separator 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 element in at least one of the two arm waveguides for adjusting the phases of the polarization components. In the example shown in FIG. 4 , the polarization-dependent phase adjustment element corresponds to phase plates 405 and 406, but may also be a portion corresponding to a stress relief groove formed near the side surface of the optical waveguide, as in a fourth embodiment described later.

[0052] In the following, an example will be described in which the polarization separator 400 shown in Fig. 4 is used as the first polarization separator 203 in Fig. 3. In this case, the input waveguide 202 in Fig. 3 is connected to the first polarization separator input waveguide 401. The light wave input to the first polarization separator input waveguide 401 is branched into arm waveguides 403 and 404 by the first light intensity division element 402.

[0053] Of the branched light waves, the light waves propagating through arm waveguide 403 have the phase of the X-polarized light advanced by λ / 4 and the phase of the Y-polarized light delayed by λ / 4 by phase plate 405. On the other hand, the light waves propagating through arm waveguide 404 have the phase of the X-polarized light delayed by λ / 4 and the phase of the Y-polarized light advanced by λ / 4 by phase plate 406.

[0054] Therefore, in the optical intensity combining element 407 where the light waves that have passed through the arm waveguides 403 and 404 are combined, the interference state of the light waves is such that the phase of the X-polarized light and the Y-polarized light is shifted by an amount equivalent to λ / 2, i.e., 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 light waves, it is possible to output the light waves of X-polarized light and Y-polarized light separately to the first polarization separator output waveguide 408 or the second polarization separator output waveguide 409. In other words, it is possible to separate the X-polarized light and the Y-polarized light.

[0055] When the polarization separator shown in FIG. 4 is operated as a polarization multiplexer, for example, X-polarized light may be input from the first polarization separator output waveguide 408 and Y-polarized light may be input from the second polarization separator output waveguide 409.

[0056] The light intensity splitting element 402 and the light 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 light intensity splitting element 402 when operated as a polarization splitter, or as the light intensity combining element 407 when operated as a polarization combiner.

[0057] Third Embodiment In the third embodiment, an aspect of improving the polarization extinction ratio of the first polarization separator 203 and the second polarization separator 210 in Fig. 3 will be described. If the polarization extinction ratio of the polarization separator is not good, the performance of the depolarization function of the light source in Fig. 3 may be degraded. In order to improve the polarization extinction ratio, the polarization separator of this embodiment shown in Fig. 5 can be used.

[0058] Fig. 5 discloses an embodiment in which the polarization separators described in the second embodiment are connected in multiple stages. As shown in Fig. 5, a polarization separator 500 of the third embodiment includes a first polarization separator 501 and second and third polarization separators 502 and 503. In Fig. 5, the same reference numerals as in Fig. 4 denote the same components as in Fig. 4, and therefore their explanation will be omitted here. Furthermore, as is clear from Fig. 5, the second polarization separator 502 is arranged such that the first polarization separator 501 and the third polarization separator 503 are upside down relative to the direction of the drawing.

[0059] As shown in FIG. 5, the light wave input to the polarization separator input waveguide 504 is polarized and separated by the first polarization separator 501 into the first connecting waveguide 505 and the second connecting waveguide 506, respectively. For example, the first connecting waveguide 505 is mainly composed of the X-polarized component, and the second connecting waveguide 506 is mainly composed of the Y-polarized component.

[0060] However, the polarization extinction ratio of the first polarization separator 501 is finite, and an amount of Y-polarized light component corresponding to the polarization extinction ratio remains in the first connecting waveguide 505, and an amount of X-polarized light component corresponding to the polarization extinction ratio remains in the second connecting waveguide 506. As described above, the performance of the depolarization function of the optical waveguide circuit of the light source 300 in FIG.

[0061] Therefore, in the third embodiment, second polarization separator 502 that transmits only the X-polarized component is installed downstream of first connecting waveguide 505, and third polarization separator 503 that transmits only the Y-polarized component is installed downstream of second connecting waveguide 506. Specifically, as shown in the figure, output waveguide 507, which is mainly occupied by the X-polarized component, of second polarization separator 502 connected to first connecting waveguide 505, which is the first output waveguide of first polarization separator 501, and output waveguide 508, which is mainly occupied by the Y-polarized component, of third polarization separator 503 connected to second connecting waveguide 506, which is the second output waveguide of first polarization separator 501, may be used as the first polarization separator output waveguide and second polarization separator output waveguide of polarization separator 500.

[0062] By using the polarization separator 500 of the embodiment shown in Figure 5, it is possible to obtain a polarization extinction ratio, expressed in decibels as twice that of each polarization separator alone, for output waveguide 507 corresponding to first polarization separator output waveguide 408 and output waveguide 508 corresponding to second polarization separator output waveguide 409 of the polarization separator shown in Figure 4.Therefore, when polarization separator 500 of this embodiment is used, the performance as a depolarization function of the light source of Figure 3 is improved.

[0063] Furthermore, if a higher polarization extinction ratio is required, the polarization extinction ratio may be improved by connecting more polarization separators in multiple stages. In the above, the X polarization and the Y polarization are explicitly assigned, but it should be understood that the X polarization and the Y polarization may be interchanged depending on the circuit configuration.

[0064] (Fourth embodiment) The fourth embodiment shows another aspect of the specific configuration of the polarization separators (203, 210) and polarization multiplexers (206, 213) of the light source shown in Fig. 3. In this embodiment, a portion of the optical waveguide in which stress relief grooves are formed is used as the polarization-dependent phase adjustment element.

[0065] Fig. 6 is a diagram showing the schematic structure of a polarization separator 600, which is another specific embodiment of the polarization separators (203, 210) or polarization multiplexers (206, 213) in Fig. 3. Fig. 6 explains the operation as a polarization separator, but it is also possible to operate it as a polarization multiplexer by propagating light waves in the opposite direction.

[0066] 6 includes a first polarization separator input waveguide 601, an optical intensity splitter element 602 that splits the optical wave output from the first polarization separator input waveguide 601, arm waveguides 603 and 604 through which the optical waves split by the optical intensity splitter element 602 are guided, stress relief grooves 605a and 605b formed on the substrate 201 near both side surfaces of the arm waveguide 604, an optical intensity combining element 607 that combines the optical waves that have passed through the arm waveguides 603 and 604, a first polarization separator output waveguide 608 from which the optical wave combined by the optical intensity combining element 607 is output, and a second polarization separator output waveguide 609. Reference numeral 610 denotes a second polarization separator input waveguide, which is not used in the polarization separator.

[0067] In the following description, an example will be given in which the polarization separator 600 shown in Fig. 5 is used as the first polarization separator in Fig. 3. In this case, the input waveguide 202 in Fig. 3 is connected to the first polarization separator input waveguide 601.

[0068] 6, a light wave input to a first polarization separator input waveguide 601 is branched into arm waveguides 603 and 604 by a first light intensity splitter element 602. Of the arm waveguides 603 and 604, the arm waveguide 604 has stress relief grooves 605a and 605b formed on the substrate on both sides thereof.

[0069] Generally, in a silica-based optical waveguide formed of glass or the like with very low propagation loss, a high-temperature heat treatment is performed during the manufacturing process, and a large compressive stress is applied to the glass layer due to the difference in the linear expansion coefficient between the silicon substrate and the glass layer through which the signal light is guided. Therefore, the optical waveguide has birefringence due to the photoelastic effect. This birefringence prevents the X-polarized and Y-polarized light waves input into the optical waveguide circuit of the light source disclosed herein from being converted into each other except at intended locations, providing the function of maintaining and propagating the X-polarized and Y-polarized light.

[0070] In the polarization splitter of this embodiment, as shown in FIG. 6 , a light wave input to a first polarization splitter input waveguide 601 connected to the input waveguide 202 is split into arm waveguides 603 and 604 by a first light intensity splitter element 602. Of the arm waveguides 603 and 604, the arm waveguide 604 has stress relief grooves 605 a and 605 b formed near both side surfaces. Birefringence is eliminated in the portions of the arm waveguide 604 corresponding to the stress relief grooves 605 a and 605 b. On the other hand, the arm waveguide 603 has no stress relief grooves, so birefringence is maintained. Therefore, it is possible to set different interference states between the X-polarized component and the Y-polarized component in the light intensity combining element 607. The formulation of the interference state in this polarization splitter 600 will be described in detail below.

[0071] The refractive index of the arm waveguide 603 for the X-polarized component is n x , the refractive index for the Y-polarized component is n y and the optical path length difference between the arm waveguide 603 and the arm waveguide 604 is dL2. Furthermore, if the length of the stress relief grooves 605a and 605b along the arm waveguide 604 (in the Z-axis direction in FIG. 6) is L3 and the wavelength of the signal light is λ, the phase change amount φ of the signal light propagating through the arm waveguide 603 is expressed as the following (Equation 1) and (Equation 2) for the X-polarized component and the Y-polarized component, respectively: φ x (603) = 2π × n x ×(dL2+L3) / λ (Formula 1) φ y (603) = 2π × n y × (dL2 + L3) / λ (Equation 2) where φx (603) is the phase change amount of the X-polarized component of the signal light propagating through the arm waveguide 603, φ y (603) is the amount of phase change in the Y-polarized component of the signal light propagating through the arm waveguide 603.

[0072] On the other hand, the birefringence of the signal light propagating through the arm waveguide 604 is eliminated by the stress relief grooves 605a and 605b, 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 604 is expressed as the following (Equation 3) and (Equation 4), respectively. φ x (604)=2π×n1×L3 / λ (Formula 3) φ y (604) = 2π × n1 × L3 / λ (Equation 4) where φ x (604) is the phase change amount of the X-polarized component of the signal light propagating through the arm waveguide 604, φ y (604) is the amount of phase change in the Y-polarized component of the signal light propagating through the arm waveguide 604.

[0073] From the above (Equation 5) to (Equation 8), in order to set different interference states for the X-polarized component and the Y-polarized component in the optical intensity combining element 607, the X-polarized component of the signal light and dL2 and L3 should be set so as to satisfy the following (Equation 5) and (Equation 6), respectively. x (603)-φ x (604)=2πm (Formula 5) φ y (603)-φ y (604) = 2π(m + 1 / 2) (Equation 6) where m is an integer and n x , n y and n1 are constants determined by physical properties. Since two variables dL2 and L3 are introduced into the two equations (Equation 5) and (Equation 6), they can be solved.

[0074] Fifth Embodiment In the fourth embodiment, a polarization separator was described in which the arm waveguide portions corresponding to the stress relief grooves 605 a and 605 b formed near both side surfaces of the arm waveguide were used as polarization-dependent phase adjustment elements. However, even in this embodiment, the polarization extinction ratio can exhibit a finite value. Therefore, as the fifth embodiment, a mode in which a high polarization extinction ratio can be obtained even when the polarization separator of the fourth embodiment is used will be disclosed.

[0075] In the fifth embodiment, similar to the third embodiment, polarization separators are connected in multiple stages to increase the polarization extinction ratio. As shown in Fig. 7, a polarization separator 700 of the fifth embodiment includes a first polarization separator 701 and second and third polarization separators 702 and 703. In Fig. 7, the same reference numerals as in Fig. 6 are used to denote the same components as in Fig. 6, and therefore their explanation will be omitted here.

[0076] 7, a light wave input to a polarization separator input waveguide 704 of a polarization separator 700 of the fifth embodiment is polarized and separated by a first polarization separator 701 into a first connecting waveguide 705 and a second connecting waveguide 706, with the first connecting waveguide 705 being dominated by an X-polarized component and the second connecting waveguide 706 being dominated by a Y-polarized component. (The connecting waveguides dominated by the X-polarized component and the Y-polarized component may be reversed.)

[0077] However, the polarization extinction ratio of the polarization separator is finite, and an amount of Y-polarized light component corresponding to the polarization extinction ratio remains in the first connecting waveguide 705, and an amount of X-polarized light component corresponding to the polarization extinction ratio remains in the second connecting waveguide 706. As described above, the performance of the polarization depolarization function of the optical waveguide circuit of the light source in Figure 3 can be limited by the polarization extinction ratio of the polarization separator and polarization multiplexer.

[0078] Therefore, in the fifth embodiment, a second polarization separator 702 that transmits only the X-polarized component is installed downstream of the first connecting waveguide 705, and a third polarization separator 703 that transmits only the Y-polarized component is installed downstream of the second connecting waveguide 706. Specifically, as shown in the figure, output waveguide 707, which is mainly occupied by the X-polarized component, of second polarization separator 702 connected to first connecting waveguide 705, which is the first output waveguide of first polarization separator 701, and output waveguide 708, which is mainly occupied by the Y-polarized component, of third polarization separator 703 connected to second connecting waveguide 706, which is the second output waveguide of first polarization separator 701, may be used as the first polarization separator output waveguide and second polarization separator output waveguide of polarization separator 700.

[0079] By using the polarization separator 700 of the embodiment shown in Fig. 7, it is possible to obtain a polarization extinction ratio, expressed in decibels as twice that of each polarization separator alone, for the output waveguide 707 corresponding to the first polarization separator output waveguide 608 and the output waveguide 708 corresponding to the second polarization separator output waveguide 609 of the polarization separator 600 shown in Fig. 6, and therefore when the polarization separator 600 of this embodiment is used, the performance as a depolarization function of the optical waveguide circuit of the light source shown in Fig. 3 is improved. If a higher polarization extinction ratio is required, the polarization extinction ratio may be improved by connecting polarization separators in multiple stages.

[0080] Another feature of this embodiment is that the stress relief groove disclosed in the fourth embodiment can be formed as three stress relief grooves 711, 712, and 713, as shown in Fig. 7. Arranging stress relief groove 711 so that it is shared by second polarization separator 702 and third polarization separator 703 makes it possible to reduce the layout space of the optical circuit, which has the advantage of leading to a reduction in the device size.

[0081] Furthermore, in the methods described in the second and third embodiments, i.e., when the phase plates 405 and 406 are installed on the optical waveguide, excessive loss occurs due to the optical signal intensity not being confined in the optical waveguide when the optical waveguide is broken. Therefore, the method of realizing a polarization separator by controlling the stress of the optical waveguide, as in the fourth embodiment and this embodiment, has an advantage in terms of loss.

[0082] Sixth Embodiment In the second to fifth embodiments, specific aspects of the polarization separators 203 and 210 and the polarization multiplexers 206 and 213 in Fig. 3 have been described. In this sixth embodiment, one specific aspect of the configuration of the polarization rotator 208 used in the optical waveguide circuit of the light source in Fig. 3 will be described.

[0083] A polarization rotator can be realized by providing an optical waveguide with a polarization-dependent phase adjustment element. Figure 8 is a diagram schematically showing the configuration of a polarization rotator 800 used as the polarization rotator 208 in Figure 3. As shown in the figure, the polarization rotator 800 of this embodiment includes a polarization rotator waveguide 801 connected between the first connecting waveguide 207 and the second connecting waveguide 209 in Figure 3, and a phase plate 802, which is a polarization-dependent phase adjustment element, installed on the polarization rotator waveguide 801.

[0084] 8, a light wave input from the first connecting waveguide 207 to the polarization rotator waveguide 801 passes through the phase plate 802 and is output from the polarization rotator waveguide 801 to the second connecting waveguide 209. Here, the phase plate 802 is a wave plate, and for example, a λ / 2 wave plate or a λ / 4 wave plate can be used.

[0085] In general, the Jones matrix of a phase plate whose optic axis is tilted at an angle θ to the direction of propagation of the light wave and whose retardation is φ is expressed by the following equation (7): In equation (7), Eo is the output electric field, and Ein is the input electric field.

[0086] Now, consider the case where the phase plate 802 is a λ / 2 wave plate. In this case, the optical axis of the phase plate 802 is set to be inclined at 22.5° with respect to the birefringence axis of the waveguide on the substrate 201. On the other hand, if the phase plate 802 is a λ / 4 wave plate, the optical axis of the phase plate 802 is set to be inclined at 45° with respect to the birefringence axis of the optical waveguide on the substrate.

[0087] Here, let us consider a case where the operating center wavelength of the optical waveguide circuit as a depolarization element is 1.55 μm and quartz crystal is used as a phase plate. By using the fact that the refractive index of quartz crystal for ordinary light is 1.5277 and the refractive index of extraordinary light is 1.5362, the polarization component Ein=(1,0) along the X-axis (X-axis direction in the figure) of the optical rotator waveguide 801 is T The output power Io is calculated when only T is input.

[0088] The calculation of the output electric field Eo to the output power Io can be obtained from the following (Equation 8) based on the above (Equation 7): * in (Equation 8) represents a complex conjugate.

[0089] FIG. 9 shows the results of calculating Io using (Equation 7) and (Equation 8) under the above conditions. FIG. 9(a) shows the relationship between wavelength and intensity of Io(x) and Io(y) when a λ / 2 wave plate is used as phase plate 802, and FIG. 9(b) shows the relationship between wavelength and intensity of Io(x) and Io(y) when a λ / 4 wave plate is used as phase plate 802. The results in FIG. 9 show that when a λ / 2 wave plate is used as phase plate 802, it operates over a wider wavelength range than when a λ / 4 wave plate is used as phase plate 802. Therefore, it is desirable to use a λ / 2 wave plate for phase plate 802 in polarization rotator 800.

[0090] In the sixth embodiment, the polarization dependent phase adjustment element of the polarization rotator is a phase plate 802 installed on the optical rotator waveguide 801. However, in this embodiment, loss is large, as in the polarization separators described in the second and third embodiments. Therefore, in the seventh embodiment, another embodiment of the specific configuration of the polarization rotator that can reduce loss compared to that of the sixth embodiment will be described.

[0091] In this embodiment, the polarization-dependent phase adjustment element provided in the polarization rotator is a portion of the polarization rotator optical waveguide 1001 that corresponds to a stress relief groove formed on the substrate 201 near one side of the polarization rotator optical waveguide 1001.

[0092] 10 is a diagram schematically illustrating the configuration of a polarization rotator 1000 according to this embodiment. As shown in the figure, the polarization rotator 1000 of this embodiment includes a polarization rotator optical waveguide 1001 connected to the output side of the first connecting waveguide 207 and the input side of the second connecting waveguide 209 in FIG. 3, and a stress relief groove 1002 formed on the substrate 201 near one side of the polarization rotator optical waveguide 1001. By forming the stress relief groove 1002 near one side of the polarization rotator optical waveguide 1001 in this way, structural asymmetry occurs in the compressive stress applied to the glass layer in the silica-based optical waveguide, thereby realizing a polarization rotator.

[0093] Figure 11(a) shows the distribution of Von Mises stress when a stress relief groove 1002 is formed near one side of the polarization rotator optical waveguide 1001 of the polarization rotator of this embodiment. Figure 11(a) shows a contour diagram showing the in-plane distribution of equivalent stress (Mises stress) in a cross section perpendicular to the optical axis direction in the polarization rotator 1000 of this embodiment, and Figure 11(b) shows a diagram plotting the change in refractive index with respect to the X-axis and Y-axis of stress birefringence in the polarization rotator 1000 of this embodiment.

[0094] As can be seen from Figure 11(a), in the polarization rotator 1000, 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 caused by the photoelastic effect will tilt.

[0095] As shown in FIG. 11( b), the difference in refractive index between the X-axis direction and the Y-axis direction is greatest at an angle of 22.5 degrees, and this angle corresponds to the major axis of the refractive index ellipse in the optical waveguide portion in which stress relief grooves 1002 are formed on only one side of the optical waveguide.

[0096] Therefore, if the length of the stress relief groove 1002 in the optical axis direction is L4, then the length should be such that the relationship in the following equation (9) is satisfied, that is, the following equation (10) is established, when the operating center wavelength of the signal light from the light source in FIG. 3 in which the polarization rotator is used is λ, relative to the difference Δn in refractive index between the X and Y axes. Here, m in the following equations (9) and (10) is an integer equal to or greater than zero and represents the order. To operate over a wider wavelength range, it is desirable to set m = 0. 2πL4Δn / λ = π / 2 + 2πm (Equation 9) L4 = (1 / 4 + m)λ / Δn (Equation 10)

[0097] Eighth Embodiment Next, as an eighth embodiment, a light source capable of outputting a plurality of different wavelengths will be described. As shown in FIG. 12 , a light source with a wide wavelength range can be achieved by integrating light-emitting elements 1201 and 1202 with a plurality of different wavelengths on a silicon substrate 201 and providing an optical multiplexer 1203 that multiplexes the output lightwaves. This light source can be used, for example, as a pumping light source for a stimulated Raman fiber amplifier. According to this embodiment, Raman gain can be achieved over a wide wavelength range. In FIG. 12 , components denoted with the same reference numerals as in FIG. 3 are the same as those in FIG. 3 , and therefore will not be described here.

[0098] The optical multiplexer 1203 used in the light source 1200 of this embodiment can be, for example, an optical multiplexer using an arrayed waveguide grating, an optical multiplexer using a Mach-Zehnder interferometer having an optical path length difference, an optical multiplexer in which Mach-Zehnder interferometers are connected in multiple stages, an optical multiplexer using a Bragg diffraction grating, etc. The light emitting elements 1201 and 1202 may be integrated by the same method as the light emitting element 301 described in the first embodiment.

[0099] The polarization separator, polarization combiner, and polarization rotator provided in the optical waveguide circuit of this light source 1200 may be the polarization separator and polarization combiner described in the second to fifth embodiments, and the polarization rotator described in the sixth and seventh embodiments, respectively.

[0100] Ninth Embodiment In the ninth embodiment, a light source is shown in a form in which the first depolarization section of the optical waveguide circuit, i.e., the sections from the polarization separator 203 to the polarization multiplexer in Fig. 3, are removed. Fig. 13 is a diagram showing a schematic structure of a light source 1300 of a first example according to the ninth embodiment. In Fig. 13, the same reference numerals as in Fig. 3 are the same as in Fig. 3, and therefore description thereof will be omitted here.

[0101] The polarization of the output lightwave of a semiconductor laser is generally biased toward one of the surfaces of the semiconductor chip. That is, the polarization of the lightwave output from the light source 1300 of this embodiment generally has a component in the X-axis direction or the Y-axis direction. In this case, the first depolarization unit, i.e., the first depolarization unit 220 in FIG. 3 , i.e., the first polarization separator 203, the first polarization multiplexer 206, and the first short waveguide 204 and the first long waveguide 205 connecting the two, may be omitted.

[0102] The output light wave from the light emitting element 1101, which is a semiconductor laser light source, is equally separated into an X-axis component and a Y-axis component by the polarization rotator 208. As a result, the light is depolarized by the second depolarization unit 230, i.e., the polarization separator 210 to the polarization combiner 213 in FIG. 12, to generate depolarized light.

[0103] In this embodiment, the number of optical circuit elements can be reduced compared to the second embodiment, and therefore a light source that is compact but has the necessary and sufficient depolarization function can be realized.

[0104] Similarly, the first depolarization unit 220, i.e., the components from the first polarization separator 203 to the first polarization multiplexer 206 in Fig. 12, can be removed from the light source 1200 of the eighth embodiment shown in Fig. 12. Fig. 14 shows a schematic structure of a light source 1400 of a second example of this embodiment.

[0105] In this embodiment, it is also possible to obtain a light source that is further miniaturized and depolarized. The polarization separator, polarization multiplexer, and polarization rotator provided in the optical waveguide circuits of light source 1300 and light source 1400 may be the polarization separator and polarization multiplexer described in the second to fifth embodiments, and the polarization rotator described in the sixth and seventh embodiments, respectively.

[0106] As described above, the present disclosure makes it possible to obtain a small, inexpensive depolarized light source in which a light emitting element and an optical waveguide circuit having a depolarization function are integrated on a substrate.

Claims

1. A light source comprising a silicon substrate, at least one light-emitting element, and an optical waveguide circuit consisting of a quartz-based optical waveguide formed on the silicon substrate, wherein the at least one light-emitting element is formed in an integrated manner on the silicon substrate, and the optical waveguide circuit comprises at least one depolarizing section and a polarization rotator connected to the depolarizing section, and the depolarizing section comprises: a polarization separator that separates an input light wave into two orthogonal polarization components, a short waveguide through which the light wave of a first polarization component is guided, and a long waveguide through which the light wave of a second polarization component is guided, the long waveguide having an optical path length longer than that of the short waveguide, and a polarization multiplexer that combines the light waves propagating through each of the short waveguide and the long waveguide.

2. The light source of claim 1, comprising a plurality of said light emitting elements of different wavelengths.

3. A light source according to claim 1 or 2, wherein the optical waveguide circuit comprises a first depolarizing section and a second depolarizing section, and the first depolarizing section and the second depolarizing section are connected via the polarization rotator.

4. The light source according to claim 1, wherein the polarization rotator includes a polarization rotator optical waveguide and a first polarization-dependent phase adjustment element for adjusting the phase of each polarization component provided in the polarization rotator optical waveguide.

5. The light source of claim 4, wherein the first polarization-dependent phase adjustment element is a portion of the polarization rotator optical waveguide that corresponds to a stress relief groove formed on the silicon substrate near one side of the polarization rotator optical waveguide.

6. The light source of claim 4, wherein said first polarization-dependent phase adjusting element is a phase plate disposed on said polarization rotator optical waveguide.

7. The light source according to claim 1, wherein the polarization separator and the polarization combiner are two-beam interference systems having two arm waveguides, and at least one of the two arm waveguides has a second polarization-dependent phase adjustment element that adjusts the phase of each polarization component.

8. The light source according to claim 7, wherein the second polarization-dependent phase adjustment element is a portion of the arm waveguide that corresponds to stress relief grooves formed on the silicon substrate near both sides of the arm waveguide.

9. The light source according to claim 7, wherein the second polarization-dependent phase adjusting element is a phase plate disposed on the arm waveguide.

10. A light source as described in claim 1, wherein the polarization of the light wave output from the light emitting element input to the optical waveguide circuit is rotated by the polarization rotator so as to have two orthogonal polarization components, then input to the depolarization section and separated into two orthogonal polarization components by the polarization separator, a delay difference is imparted to the separated first polarization component and second polarization component, and then polarization combined by the polarization combiner.

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