Light receiver, light-receiving circuit, and monitoring method

Abstract

A light receiver (100) comprises a substrate (101), a first semiconductor layer (111), a second semiconductor layer (114), a first electrode (116b), a second electrode (116a), and an optical waveguide connected to the first semiconductor layer (111). The first semiconductor layer (111) has a first region (111a) having a first thickness, and a second region (111b) having a second thickness less than the first thickness. The second semiconductor layer (114) has a higher refractive index than the first semiconductor layer (111) and is provided on the upper part of the second region (111b). The first electrode (116b) is provided at a position outside the second semiconductor layer (114) with respect to the first region (111a), and the second electrode (116a) is provided on the upper part of the second semiconductor layer (114).

Description

Photoreceiver, photoreceiver circuit, and monitoring method

[0001] The present disclosure relates to a photoreceiver, a photoreceiver circuit, and a monitoring method.

[0002] With the recent spread of optical communications, there is a demand for lower costs for optical communications devices. To meet this demand, for example, there is a technology that forms optical circuits constituting optical communications devices on large-diameter wafers such as silicon wafers using micro-optical circuit technology such as silicon photonics.

[0003] Patent No. 6981365

[0004] When controlling optical circuits such as silicon photonics, it is possible to optimize the operation of the optical circuit by accurately detecting the optical power (eg, absolute value) at a particular point in the optical circuit.

[0005] Accurate optical power detection is also useful when manufacturing optical modules using optical circuits. For example, accurate optical power detection can be useful in testing processes to determine whether manufactured optical modules meet expected performance or operating conditions.

[0006] One non-limiting object of the present disclosure is to provide a photoreceiver that can monitor received light with low loss and high accuracy.

[0007] An optical receiver according to one aspect of the present disclosure includes a substrate, a first semiconductor layer provided on top of the substrate and having a first region having a first thickness and a second region having a second thickness thinner than the first thickness, a second semiconductor layer provided on top of the second region and having a refractive index higher than that of the first semiconductor layer, a first electrode provided on top of the second region and positioned outside the second semiconductor layer with respect to the first region, a second electrode provided on top of the second semiconductor layer, and an optical waveguide connected to the first semiconductor layer.

[0008] 1 is a schematic top view showing an example of the configuration of a photodetector according to a first embodiment. FIG. 2 is a cross-sectional view showing the cross section of the photodetector illustrated in FIG. 1 taken along the arrow A-A. FIG. 3 is a schematic top view showing an example of the configuration of a photodetector according to a modified example of the first embodiment. FIG. 4 is a cross-sectional view showing the cross section of the photodetector illustrated in FIG. 3 taken along the arrow A-A. FIG. 5 is a schematic top view showing an example of the configuration of a photodetector according to a second embodiment. FIG. 6 is a graph showing an example of the relationship between the received light power by the photodetector circuit according to the second embodiment and the position of the photodetector circuit relative to the propagation direction of light. FIG. 7 is a diagram showing an example of a measurement system according to the second embodiment. FIG. 8 is a flowchart showing an example of a procedure for monitoring (or estimating) input light power according to the second embodiment. FIG. 9 is a schematic top view showing an example of the configuration of a photodetector according to a fourth embodiment. FIG. 10 is a cross-sectional view showing an example of the configuration of a photodetector according to a fifth embodiment. FIG. 11 is a schematic top view showing an example of the configuration of a photodetector according to a sixth embodiment. FIG. 12 is a schematic top view showing an example of the configuration of a photodetector according to a seventh embodiment. FIG. 19 is a schematic top view illustrating a configuration example of a light receiver according to an eighth embodiment.

[0009] Hereinafter, embodiments will be described in detail with reference to the drawings. In order to avoid redundant description and to simplify and clarify the description, elements or parts with the same numbers in the drawings refer to the same or similar elements or parts unless otherwise specified.

[0010] [Summary] In order to observe the optical power in an optical circuit from the outside, a photocurrent generated in a photodetector pre-installed in the optical circuit in response to the received optical power may be measured by an ammeter installed outside the optical circuit. The photocurrent is determined by (photosensitivity R of the photodetector) x (optical power P). Therefore, by accurately determining the photosensitivity R, it is possible to accurately determine the optical power P corresponding to the photocurrent.

[0011] However, the optical sensitivity R may vary from one optical receiver to another. Therefore, the individual variation in optical sensitivity R may cause errors in the measurement of optical power. In other words, the accuracy of the optical power measurement may decrease. Furthermore, it is unrealistic to accurately determine the optical sensitivity R for each optical receiver.

[0012] Furthermore, with a normal optical receiver, for example, the amount of light absorption is too large, making it difficult to meet the demand for measuring optical power without attenuating the optical power as much as possible. To avoid such a situation or event, for example, a method may be considered in which a small portion of the optical power is branched off by an optical branching device and the branched light is received for measurement.

[0013] However, when an optical splitter is used to measure optical power, there may be individual variations in the splitting ratio of the optical splitter in addition to the optical sensitivity R of the optical receiver. Therefore, the measurement accuracy of optical power may be reduced due to the individual variations in the splitting ratio in addition to the individual variations in the optical sensitivity R.

[0014] In the exemplary embodiment of the present disclosure described below, a photodetector is used that does not use additional components such as an optical monitor and an optical branching device, but instead absorbs a portion of light propagating through a first semiconductor (e.g., an optical waveguide) using a second semiconductor and generates a current corresponding to the amount of light absorbed.

[0015] Such an optical receiver can, for example, suppress the loss of light propagating through the first semiconductor and the deterioration of the accuracy of the optical power monitor. A non-limiting example of the second semiconductor that absorbs a portion of the light propagating through the first semiconductor is germanium (Ge). Below, several exemplary embodiments of an optical receiver using Ge as a light absorption section will be described. In the following description, the term "optical receiver" may be appropriately replaced with "photodetector."

[0016] [First embodiment] Fig. 1 is a schematic top view showing an example of the configuration of a photodetector 100 according to a first embodiment, and Fig. 2 is a cross-sectional view showing the A-A cross section of the photodetector shown in Fig. 1. In Fig. 1 and Fig. 2, the X-axis, Y-axis, and Z-axis are added for convenience in explaining the three-dimensional structure of the photodetector 100.

[0017] The Z axis represents the light guide direction and corresponds to the length of the optical receiver 100 when viewed from above, the X axis represents the width of the optical receiver 100 when viewed from above, and the Y axis represents the thickness (or height) of the optical receiver 100 when viewed from the side or in a cross section. The directions represented by the X axis, Y axis, and Z axis are the same in the drawings used in the following description.

[0018] 1 and 2 , the optical receiver 100 includes, for example, an SOI (Silicon On Insulator) substrate 101 (hereinafter sometimes abbreviated as “substrate 101”), a first cladding layer 102 provided on the SOI substrate 101 (e.g., in the positive direction of the Y axis in FIG. 2 ), and a second cladding layer 103 provided on the first cladding layer 102. The first cladding layer 102 and the second cladding layer 103 may be referred to as a lower cladding layer 102 and an upper cladding layer 103, respectively, to make it easier to understand their relative positional relationship in the Y axis direction (thickness direction).

[0019] The SOI substrate 101 illustratively includes a Si substrate, a Si oxide film, and a surface Si layer. The cladding layers 102 and 103 are each formed of a cladding material (e.g., silicon dioxide, SiO) having a refractive index lower than that of silicon (Si) and germanium (Ge). Depending on the material of the substrate 101, the lower cladding layer 102 may be omitted (e.g., quartz glass).

[0020] As shown in FIG. 2, the upper cladding layer 103 is provided with a Si layer 111 located on top of the lower cladding layer 102, two Ge layers 114 located on top of the Si layer 111, and a pair of current extraction electrodes 116a and 116b corresponding to each of the Ge layers 114.

[0021] The material of the electrodes 116a and 116b is not particularly limited as long as it is a material through which a current can flow, but for example, aluminum (Al) may be used as the main material. For convenience, the pair of electrodes 116a and 116b may be referred to as the "electrode pair 116."

[0022] Illustratively, one of the electrode pair 116 is an example of a first electrode and corresponds to a cathode (negative pole), and the other of the electrode pair 116 is an example of a second electrode and corresponds to an anode (positive pole).

[0023] One of the electrode pair 116 (e.g., electrode 116a) is provided on each of the Ge layers 114. The other of the electrode pair 116 (e.g., electrode 116b) is provided on the Si layer 111 on the outer side of the Ge layer 114 in the width direction (X-axis direction).

[0024] In other words, above the lower cladding layer 102, the Si layer 111, the Ge layer 114, and the electrode pair 116 are contained within the upper cladding layer 103, and the gaps between them are filled (or buried) with a cladding material having a refractive index lower than that of Si and Ge.

[0025] Note that the surfaces of the two electrode pairs 116 in the thickness direction (positive direction of the Y axis) are, for example, exposed on the upper surface of the upper cladding layer 103. In addition, in Fig. 2, of the two sets of Ge layers 114 and electrode pairs 116 positioned symmetrically in the width direction (X axis direction), the reference numerals of the set positioned on the left side are omitted for ease of viewing in the drawing.

[0026] The Si layer 111 is a non-limiting example of a first semiconductor layer, and is, for example, a p-type Si layer obtained by doping Si with p-type impurities, which is an example of a first conductivity type. An optical waveguide 108a forming one of the optical input and optical output and an optical waveguide 108b forming the other of the optical input and optical output are connected (or coupled) to ends in the Z-axis direction of the p-type Si layer 111 (for example, a rib 111a, which will be described later), as shown in FIG.

[0027] The p-type Si layer 111 is sandwiched in the Y-axis direction between the cladding layers 102 and 103, which have a refractive index lower than that of the p-type Si layer 111, and therefore functions as a slab-shaped core layer waveguide that confines light incident from the optical waveguide 108a (or the optical waveguide 108b) and guides it to the optical waveguide 108b (or the optical waveguide 108a). One or both of the optical waveguides 108a and 108b may be connected to the p-type Si layer 111 as individual members, or may be formed integrally with the p-type Si layer 111.

[0028] Here, as illustrated in FIGS. 1 and 2 , the p-type Si layer 111 of this embodiment has, in the center when viewed from above, a rib (which may also be referred to as a “ridge”) 111 a that extends in the Z-axis direction and has a convex cross-sectional shape that protrudes in the positive direction of the Y-axis when viewed in cross section.

[0029] For example, the p-type Si layer 111 has a first region (hereinafter sometimes referred to as the "rib region 111a") having a first thickness in the Y-axis direction that corresponds to the thickness of the rib 111a, and second regions 111b on both sides of the rib 111a in the X-axis direction that have second thicknesses that are thinner than the first thickness. In Figure 2, the difference between the first thickness and the second thickness is indicated by "D".

[0030] As a non-limiting example, the difference D may be set (or designed; the same applies hereinafter) to a value on the order of several tens of nanometers to several hundred nanometers, such as 50 nm to 150 nm. The width of the rib 111a in the X-axis direction is not particularly limited, but may be set to a value on the order of several hundred nanometers to several micrometers, such as 0.4 μm to 3 μm.

[0031] The length of the rib 111a in the Z-axis direction is not particularly limited, but may be set to a value on the order of several micrometers to several tens of micrometers, for example, 5 μm to 30 μm. Note that the rib-type waveguide 111 having the rib 111a can be formed, for example, by a semiconductor deposition technique, a microfabrication technique such as photolithography, or both.

[0032] Hereinafter, the Si layer 111 having the rib 111a may be referred to as a "rib-type waveguide 111" or a "convex cross-section Si waveguide 111" for convenience. In the rib-type waveguide 111, light with a greater power propagates while being confined intensively in the rib 111a than in the two regions 111b on either side of the rib 111a.

[0033] Here, "light propagates while being intensively confined in the rib 111a" means, for example, that in the rib-type waveguide 111, the power distribution of the propagating light tends to be higher in the region closer to the rib 111a, and does not mean that the light propagates only in the rib 111a. This point also applies to the following explanation.

[0034] Ge layers 114 are provided symmetrically on the upper portions of the regions 111b on both sides of the rib 111a, for example, with the rib 111a extending in the Z-axis direction in between. Each of the Ge layers 114 is a non-limiting example of a second semiconductor, and for example, extends in the Z-axis direction with a length shorter than that of the rib-type waveguide 111, and is provided at a fixed interval in the X-axis direction from the edge of the width of the rib 111a.

[0035] As a non-limiting example, the Ge layer 114 may have a mesa shape with inclined side surfaces that define the width in the X-axis direction in the cross-sectional view of Fig. 2. However, the Ge layer 114 does not necessarily have to have a mesa shape, and may have, for example, a convex shape similar to the rib 111a in the cross-sectional view of Fig. 2.

[0036] Since the refractive index of Ge is higher than that of Si, of the light propagating through the rib-type waveguide 111, which is a Si waveguide, the light that cannot be completely confined within the rib 111a and slightly seeps into the regions 111b on both sides in the width direction (X-axis direction) (or leaks) is absorbed by the Ge layer 114. Therefore, the Ge layer 114 may be understood as an example of a light absorption portion. Therefore, in the following description, the mesa-shaped Ge layer 114 may be referred to as a "Ge mesa light absorption portion 114" for convenience.

[0037] As a non-limiting example, each of the two Ge layers 114 located on either side of the rib 111a in the width direction (X-axis direction) is formed (or stacked) by epitaxial growth on the rib-type waveguide 111. On top of each of the Ge layers 114, an n-type Ge region 115 may be formed by doping Ge with n-type impurities, which is an example of a second conductivity type, as shown in FIG.

[0038] 2, one electrode 116a of the current extraction electrode pair 116 is provided on the upper surface of each n-type Ge region 115, with the lower surface in contact with the upper surface of the n-type Ge region 115. The other electrode 116b of the electrode pair 116 is provided on the upper portion of the rib waveguide 111, in a region that is further outward in the width direction (X-axis direction) than the Ge layer 114 with respect to the rib 111a.

[0039] For example, a p++ Si electrode portion 112 doped with a high concentration of p-type impurities is provided in the region outside the X-axis direction where the Ge layer 114 of the rib-type waveguide 111 is provided, and an electrode 116b for extracting current is provided so that the lower surface of the electrode 116b contacts the upper surface of this p++ Si electrode portion 112.

[0040] For example, the electrode 116b may be provided symmetrically in the X-axis direction with the rib 111a sandwiched between them, similar to the Ge layer 114 or the electrode 116a. In addition, one or both of the p++ Si electrode portion 112 and the n-type Ge region 115 may be unnecessary, for example, when the current detection sensitivity of the electrode pair 116 is within an acceptable range.

[0041] For example, the electrode 116a may be disposed so as to contact the upper part of the p-type Si layer 111 (e.g., the p-type second region 111b) without the p++ Si electrode portion 112. The electrode 116b may be disposed so as to contact the upper part of the n-type Ge layer 114 doped with n-type impurities without the n-type Ge region 115.

[0042] In this way, there are cases where one or both of the p++ Si electrode portion 112 and the n-type Ge region 115 may be unnecessary, and this also applies to the following explanations.

[0043] (Example of operation) In the optical receiver 100 having the above-described structure, for example, when light is incident from the optical waveguide 108a to the rib-type waveguide 111, the light propagates through the rib-type waveguide 111 while being confined intensively in the rib 111a, and both ends of the optical mode leak out slightly (for example, at a rate of a few percent or less) into the regions 111b on both sides (in the X-axis direction) of the rib 111a.

[0044] Light that leaks into regions 111b on both sides of rib 111a is absorbed in Ge layer 114, which has a higher refractive index than Si layer 111. Ge layer 114 generates charges according to the power of the absorbed light, and a current according to the generated charges flows through the pair of electrodes 116a and 116b. Therefore, for example, by connecting an ammeter to the pair of electrodes 116, it is possible to partially monitor the optical power propagating through rib waveguide 111.

[0045] Most of the optical power that is not absorbed by the Ge layer 114 propagates to the output-side optical waveguide 108b and is output from the optical receiver 100. In other words, the optical receiver 100 is a type of optical receiver that transmits input light without terminating it.

[0046] Here, by suppressing the optical absorption rate of the Ge mesa absorption portion 114 to a few percent or less, it is possible to suppress attenuation (or loss) of the optical power propagated through the rib waveguide 111 and output to the output-side optical waveguide 108b. Therefore, it is possible to realize low-loss and high-sensitivity monitoring of the optical power propagating through the rib waveguide 111.

[0047] The light absorption rate of the Ge mesa light absorption portion 114 can vary depending on, for example, the distance in the X-axis direction between the rib 111a provided in the rib waveguide 111 and the Ge mesa light absorption portion 114. For example, the shorter the distance in the X-axis direction between the rib 111a and the Ge mesa light absorption portion 114, the higher the light absorption rate of the Ge mesa light absorption portion 114, and the greater the loss of light propagating through the rib waveguide 111 tends to be.

[0048] Therefore, for example, the distance between the edge that defines the width of the rib 111a in the X-axis direction and the edge of the Ge mesa light absorption portion 114 that is closer to the rib 111a is set theoretically or experimentally so as to obtain the desired light absorption rate (for example, a few percent or less).

[0049] Such settings make it possible to realize the optical receiver 100 that can suppress the loss of propagating light and the deterioration of the accuracy of the optical power monitor. As a non-limiting example, if the optical absorption rate of the Ge mesa light absorption portion 114 is set to 5% or less, the distance between the above-mentioned edges is expected to be several μm to several tens of μm (e.g., 3 μm to 30 μm).

[0050] In the above-described embodiment, the relationship between the p-type semiconductor and the n-type semiconductor (in other words, the relationship between the first conductivity type and the second conductivity type) may be reversed. For example, the p-type Si layer (rib-type waveguide) 111 and the p++ Si electrode portion 112 may be replaced with an n-type Si layer and an n++ electrode portion, respectively, in which Si is doped with n-type impurities, and the n-type Ge region 115 may be replaced with a p-type Ge region in which Ge is doped with p-type impurities. In this case, the direction of current flowing through the pair of electrodes 116a and 116b is reversed compared to when the replacement is not performed. This reversal of the relationship between the p-type semiconductor and the n-type semiconductor also applies to the modified examples and other embodiments described below.

[0051] In the above-described embodiment, the Ge layer 114 and the electrodes 116 a and 116 b are arranged symmetrically on both sides of the rib 111 a extending in the Z-axis direction of the rib waveguide 111, but an asymmetric arrangement may also be employed. Some examples of asymmetric arrangements will be described in other embodiments below.

[0052] [Modification] Next, a modification of the first embodiment will be described with reference to Fig. 3 and Fig. 4. Fig. 3 is a schematic top view showing an example of the configuration of a photoreceiver 100A according to a modification of the first embodiment, and Fig. 4 is a cross-sectional view showing the A-A cross section of the photoreceiver 100A shown in Fig. 3.

[0053] Compared to the optical receiver 100 illustrated in FIGS. 1 and 2, the optical receiver 100A illustrated in FIGS. 3 and 4 differs from the first embodiment in part of the configuration of the Si layer 111, which is a rib-type waveguide, for example, in the material of at least the rib 113a.

[0054] As shown in non-limiting examples in FIGS. 3 and 4 , in the optical receiver 100A according to the modified example, the region of the rib 113 a having the first thickness of the Si layer 111 (hereinafter also referred to as the rib region 113 a) is a region that is not doped with p-type impurities, unlike the first embodiment.

[0055] On the other hand, a part or all of the region 111b having a second thickness thinner than the first thickness of the Si layer 111 is a region doped with p-type impurities, as in the first embodiment. However, in this modification, as shown in Figure 4, peripheral regions on both sides of the rib region 113a in the width direction (X-axis direction) of the region 111b having the second thickness are also regions that are not doped with p-type impurities.

[0056] 4, the convex region 113 including the rib region 113a is an intrinsic Si region having a higher refractive index than a region doped with p-type impurities. Hereinafter, for convenience, a rib having such a convex region 113 that is not doped with p-type impurities will be referred to as an intrinsic Si rib 113 (or an intrinsic Si region 113).

[0057] The width of the intrinsic Si rib 113 in the X-axis direction may be set theoretically or experimentally within a range in which the intrinsic Si rib 113 does not overlap with or come into contact with the Ge mesa absorption portion 114 on the second region 111b having the second thickness of the rib-type waveguide 111.

[0058] In this way, by providing the intrinsic Si rib 113 in the rib waveguide 111, it is possible to suppress unwanted light absorption caused by impurity doping more effectively than in the first embodiment. Therefore, the loss of light propagating through the rib waveguide 111 can be further reduced compared to the first embodiment.

[0059] The intrinsic Si rib 113 in the rib-type waveguide 111 shown in this modification can also be applied to other embodiments described later.

[0060] Second Embodiment Next, a second embodiment will be described with reference to Fig. 5 to Fig. 8. Fig. 5 is a schematic top view showing an example configuration of a photodetector 100B according to the second embodiment. As shown in Fig. 5, the photodetector 100B includes a plurality of (e.g., five) Ge mesa absorption portions 114 (5 x 2 = 10 in total) arranged in series in the Z-axis direction in each of the second regions 111b on the rib-type waveguide 111 on both sides of the rib 111a.

[0061] The optical receiver 100B also includes, at the top of the rib-type waveguide 111, a plurality (e.g., five pairs) of electrode pairs 116 (a total of 10 electrode pairs, a total of 20 individual electrodes constituting the electrode pairs) corresponding to the individual Ge mesa absorption portions 114.

[0062] Various parameters such as the material, shape, and size of the Ge mesa absorption portion 114 and the electrode pairs 116 may be the same as those in the first embodiment, or may be changed as appropriate depending on the size of the optical receiver 100B (e.g., the substrate 101). Figure 5 shows an example in which five Ge mesa absorption portions 114 and electrode pairs 116 are arranged in series in the Z-axis direction in each of the two regions 111b on both sides of the rib 111a, and therefore the length of each in the Z-axis direction is shorter than in the first embodiment.

[0063] 5, a set of two Ge mesa light absorption portions 114 arranged symmetrically in the X-axis direction with the rib 111a in between and two electrode pairs 116 corresponding to these two Ge mesa light absorption portions 114 may be referred to as a "light absorption circuit" or a "light detection circuit" for convenience. In the example of Fig. 5, when an integer of 2 or greater is represented by "n", five sets of light absorption circuits #1 to #5, where n = 1 to 5, are arranged in series in the Z-axis direction along which the rib 111a extends.

[0064] The spacing in the Z-axis direction of the sets of Ge mesa absorbers 114 and electrode pairs 116 may be set, for example, based on the effect of mutual interference (hereinafter also referred to as "current crosstalk") between currents generated in the individual Ge mesa absorbers 114. For example, the spacing in the Z-axis direction may be set to a value within a range in which current crosstalk does not occur, or a value within an acceptable range in which current crosstalk does occur.

[0065] By optimizing the arrangement spacing in the Z-axis direction based on current crosstalk in this way, it is possible to, for example, reduce the size of the optical receiver 100B or improve the estimation accuracy of the input optical power (described later) by increasing the density of the Ge mesa absorption portions 114 mounted on each optical receiver 100B. The estimation of the input optical power will be described in detail below with reference to FIGS.

[0066] 5, it is possible to monitor the power of light propagating through the rib-type waveguide 111 at multiple positions or regions in the Z-axis direction (which may be conveniently referred to as a "multipoint monitor"). The other components of the optical receiver 100B may be the same as or similar to those of the first embodiment. For example, the A-A cross section in FIG. 5 (in other words, the layered structure of the optical receiver 100B in the Y-axis direction) may be understood to be the same as the A-A cross section illustrated in FIG. 2 (or FIG. 4).

[0067] For example, a lower cladding layer 102 is laminated on top of a substrate 101, and a rib waveguide 111 is laminated on top of the lower cladding layer 102. A Ge mesa absorption portion 114 is provided on top of the rib waveguide 111, and an electrode pair 116 is provided on top of the Ge mesa absorption portion 114 and the rib waveguide 111. However, in the second embodiment, the rib waveguide 111 is common to the multiple Ge mesa absorption portions 114 and the multiple electrode pairs 116 that form each of the multiple absorption circuits #1 to #5.

[0068] 5 shows only the cutting line A-A focusing on one of the light absorption circuits #1 to #5 (light absorption circuit #3), and does not show the other four sets of light absorption circuits #1, #2, #4, and #5. It may be understood that the stacking structure in the Y-axis direction of each of the other four sets of light absorption circuits #1, #2, #4, and #5 is the same as that in FIG. 2 (or FIG. 4).

[0069] In the optical receiver 100B having the above-described structure, for example, when light is incident from the optical waveguide 108a into the rib-type waveguide 111, the light propagates through the rib-type waveguide 111 while being confined intensively in the rib 111a, and both ends of the optical mode leak out slightly (for example, at a rate of a few percent or less) into the regions 111b on both sides (in the X-axis direction) of the rib 111a.

[0070] The light that leaks into the regions 111b on both sides of the rib 111a is absorbed by two Ge mesa light absorption portions 114 that are arranged symmetrically in the X-axis direction in each of the light absorption circuits #1 to #5. Then, in each Ge mesa light absorption portion 114, a charge is generated according to the power of the absorbed light, and a current according to the generated charge flows through the corresponding electrode pair 116.

[0071] In a multipoint monitor, a small portion of the light propagating through the rib-type waveguide 111 is absorbed in each of the multiple light absorption circuits #i (i is any one of 1 to n), and therefore the power of the propagating light is more likely to attenuate the further back in the propagation direction it travels. Therefore, the amount of light absorption tends to decrease in the light absorption circuits #i located further back in the light propagation direction, and the magnitude of the current flowing through the electrode pair 116 also tends to decrease.

[0072] Most of the optical power that is not absorbed in the Ge mesa light absorption portion 114 of each light absorption circuit #i is propagated to the output-side optical waveguide 108b and output.

[0073] (Monitoring Method) Next, a method for estimating the input optical power to the optical receiver 100B by utilizing the decreasing trend of the amount of optical absorption (or current value) in the above-mentioned multi-point monitor will be described with reference to Fig. 6 to Fig. 8. Fig. 6 is a graph showing an example of the relationship between the received optical power by the optical absorption circuit #i (vertical axis) and the position of the optical absorption circuit #i relative to the light propagation direction (horizontal axis).

[0074] The horizontal axis in Fig. 6 represents the position of the light absorption circuit #i relative to the light propagation direction as the light absorption circuit number (i) increases in ascending order. In other words, the horizontal axis in Fig. 6 indicates that the larger the light absorption circuit number, the greater the distance in the light propagation direction relative to the position where light enters the first-stage light absorption circuit #1.

[0075] As plotted by the black dots 601 in Figure 6, the light receiving power of each light absorption circuit #i is derived based on the current value generated in each light absorption circuit #i. As described above, each light absorption circuit #i includes two Ge mesa absorption portions 114 arranged symmetrically with the rib 111a in between. Therefore, for example, one of the current values ​​generated by each of the two Ge mesa absorption portions 114 may be selected as the current value of the single light absorption circuit #i, or the average value of the two current values ​​may be determined as the current value of the single light absorption circuit #i. The "average" may be a simple average or a weighted average.

[0076] Then, for example, a regression line 602 is derived from the received light power derived for each of the multiple light absorption circuits #i, and an intersection point 603 (a point plotted as a white circle in FIG. 6 ) between the regression line 602 and the position of the first-stage light absorption circuit #1 on the graph is determined. The optical power on the vertical axis corresponding to the intersection point 603 corresponds to the estimated value (or may also be referred to as a predicted value) of the input optical power to the first-stage light absorption circuit #1. Therefore, for example, the accuracy of estimating the input optical power to the optical receiver 100B can be improved.

[0077] The above-described derivation of the regression line and estimation of the input optical power based on the regression line may be performed using a processor 400 having arithmetic processing capabilities, such as a digital signal processor (DSP), as shown in Fig. 7. Fig. 7 is a diagram illustrating an example of a measurement system 700 according to the second embodiment.

[0078] The measurement system 700 includes an ammeter 200, an analog-to-digital converter (ADC) 300, and a processor 400. Two or more of some or all of the ammeter 200, the ADC 300, and the processor 400 may be provided. Fig. 7 shows an example of the connection relationship between the electrode pair 116 and the processor 400 when focusing on one Ge mesa light absorption portion 114 included in one light absorption circuit #i.

[0079] As shown in FIG. 7, an ammeter 200 is connected to an electrode pair 116 corresponding to one Ge mesa absorption portion 114, an analog-to-digital converter (ADC) 300 is connected to the output of the ammeter 200, and a processor 400 is connected to the output of the ADC 300.

[0080] The current flowing through the electrode pair 116 is measured by the ammeter 200, and the measured analog current value is AD converted by the ADC 300 to generate a digital current value, which is then input to the processor 400. By performing current measurements using the ammeter 200 for multiple light absorption circuits #i, a digital current value for each light absorption circuit #i is obtained and input to the processor 400.

[0081] Note that current measurement for multiple light absorption circuits #i may be performed, for example, by repeatedly changing the light absorption circuit #i (electrode pair 116) to which the ammeter 200 is connected one by one. For example, in the example shown in FIG. 7 , a switch (not shown) for switching connections between the multiple electrode pairs 116 and the ammeter 200 may be provided, and the connection target of the ammeter 200 may be switched by switch control. The switch control may be performed, for example, by the processor 400. In this case, for example, the ammeter 200, the ADC 300, and the processor 400 can be shared by multiple light absorption circuits #i, thereby reducing the scale of the measurement system 700.

[0082] Alternatively, current measurements for multiple light absorption circuits #i may be performed, for example, by connecting multiple ammeters 200 individually to each of the electrode pairs 116 of each light absorption circuit #i.

[0083] 8 is a flowchart showing an example of a procedure for monitoring (or estimating) input optical power according to the second embodiment. As shown in FIG. 8, light is input to the optical receiver 100B (e.g., the optical waveguide 108a) (S10). The light input to the optical waveguide 108a may be transmitted light from an optical transmitter having a light source, or received light from an optical receiver that receives light transmitted through an optical transmission path such as an optical fiber.

[0084] With light input to the optical receiver 100B, current measurements are performed on the plurality of light absorption circuits #i as described above (S20), and digital current values ​​that are the results of the plurality of current measurements are input to the processor 400.

[0085] The processor 400 derives a regression line 602 as shown in Fig. 6 based on the digital current values ​​obtained for each light absorption circuit #i (S30). Based on the regression line 602, the processor 400 estimates the optical power input to the first-stage light absorption circuit #1 by, for example, finding an intersection point 603 on the graph shown in Fig. 6 (S40). The processor 400 then outputs the estimated value (S50).

[0086] The destination of the estimated value may be a control circuit that feedback controls the output power of the transmitted light in the optical transmitter, or a control circuit that controls the gain of the amplifier in the optical receiver, or may be, but is not limited to, a display device such as a display, a printing device such as a printer, another processor, a storage device or storage medium such as a memory, or a communication line.

[0087] As described above, according to the monitoring method described above, with light input to the optical receiver 100B through the optical waveguide 108a, the current value is measured for each electrode pair 116 of the multiple Ge mesa absorption units 114 arranged in series in the Z-axis direction. The processor 400 then derives a regression line 602 based on each measured current value, and estimates the optical power input to the optical receiver 100B based on the regression line 602. This makes it possible to improve monitoring accuracy while suppressing loss of light propagating through the rib-type waveguide 111.

[0088] In the second embodiment described above, the input optical power to the first-stage light absorption circuit #1 is estimated, but for example, the output optical power of the last-stage light absorption circuit #n may be estimated based on the regression line 602. Also, although an example has been shown in which the number of light absorption circuits #i (in other words, the number of current measurement points along the direction in which light propagates through the rib-type waveguide 111) is n=5, n is not limited to 5 and may be any number that allows the regression line 602 to be derived, for example, n≧2.

[0089] The larger n is, the more accurate the regression line 602 is derived, and as a result, the more accurate the estimation of the input optical power is. On the other hand, the loss of light propagating through the rib-type waveguide 111 tends to increase. The value of n may be determined appropriately based on, for example, the trade-off between the desired estimation accuracy of the input optical power and the tolerable optical loss level.

[0090] The "derivation" of the regression line 602 may be a "calculation" based on an operation of a regression equation, or a "determination" that does not involve "calculation," such as by referencing data in table format (in other words, by looking up a table). Furthermore, the relationship between the light-receiving power of the light-absorbing circuit #i and the position of the light-absorbing circuit #i relative to the light propagation direction may be approximated by a method different from the method for deriving the regression line 602.

[0091] [Third Embodiment] Next, a third embodiment will be described with reference to Fig. 9 and Fig. 10. Fig. 9 is a schematic top view showing an example of the configuration of a photodetector 100C according to the third embodiment, and Fig. 10 is a cross-sectional view showing the cross section of the photodetector 100C shown in Fig. 9 taken along the line A-A.

[0092] 9 and 10 differs from the configuration of the second embodiment illustrated in Fig. 5 in that multiple (e.g., n = 5) sets of Ge mesa absorption portions 114 and electrode pairs 116 are arranged in series in the Z-axis direction in only one of two regions 111b located on either side of the rib 111a in the X-axis direction in the rib waveguide 111. As described above, the region 111b is thinner than the rib 111a in the rib waveguide 111.

[0093] The n sets of Ge mesa light absorption portions 114 and electrode pairs 116 each form light absorption circuits #1 to #n that absorb a portion of the light propagating through the rib waveguide 111 in one of the two regions 111b at a different position in the waveguide direction (Z-axis direction). Therefore, unlike the second embodiment, the light absorption circuit #i according to the third embodiment includes one Ge mesa light absorption portion 114 and one electrode pair 116. Therefore, for example, the selection or averaging of two current values ​​obtained from each light absorption circuit #i, as described in the second embodiment, is not necessary in the third embodiment.

[0094] Other components of the optical receiver 100C may be the same as or similar to those of the first and second embodiments. For example, a lower cladding layer 102 is laminated on top of a substrate 101, and a rib waveguide 111 is laminated on top of the lower cladding layer 102. As shown in FIGS. 9 and 10 , a Ge mesa absorption portion 114 is provided on top of one of the two regions 111b of the rib waveguide 111, and an electrode pair 116 is provided on top of the Ge mesa absorption portion 114 and the rib waveguide 111. As in the second embodiment, the rib waveguide 111 is common to the multiple Ge mesa absorption portions 114 and the multiple electrode pairs 116 that form each of the multiple absorption circuits #1 to #5.

[0095] 9 shows only the cutting line A-A focusing on one of the light absorption circuits #1 to #5 (light absorption circuit #3), and does not show the other four sets of light absorption circuits #1, #2, #4, and #5. It can be understood that the stacked structure in the Y-axis direction of each of the other four sets of light absorption circuits #1, #2, #4, and #5 is the same as that shown in FIG.

[0096] (Example of operation) In the optical receiver 100C having the above-described structure, for example, when light is incident from the optical waveguide 108a to the rib-type waveguide 111, the light propagates through the rib-type waveguide 111 while being confined intensively in the rib 111a, and both ends of the optical mode leak out slightly (for example, at a rate of a few percent or less) into the regions 111b on both sides (in the X-axis direction) of the rib 111a.

[0097] Of the light that seeps into the regions 111b on both sides of the rib 111a, the light that seeps into one of the regions 111b where the light absorption circuits #1 to #5 are provided is absorbed by the Ge mesa light absorption portion 114 in each of the light absorption circuits #1 to #5. Then, in each of the Ge mesa light absorption portions 114, a charge is generated according to the power of the absorbed light, and a current according to the generated charge flows through the corresponding electrode pair 116.

[0098] Therefore, a multipoint monitor similar to that of the second embodiment is realized for one region 111b of the rib-type waveguide 111 in which the light absorption circuits #1 to #5 are provided. Other operational examples may be understood to be equivalent to those of the second embodiment. For example, as described with reference to FIGS. 6 to 8 , the multipoint monitor also enables the third embodiment to derive a regression line 602 based on current values ​​measured at multiple current measurement points in the Z-axis direction and to estimate the input optical power (or output optical power) based on the derived regression line 602.

[0099] The optical receiver 100C according to the third embodiment can reduce the loss of light propagating through the rib waveguide 111 more than the optical receiver 100B according to the second embodiment, in which the Ge mesa absorption portions 114 are arranged in the regions 111b on both sides of the rib 111a in the Z-axis direction. Therefore, for example, when the input optical power to the optical receiver 100C is lower than that of the second embodiment, the desired optical output power can be more easily achieved.

[0100] On the other hand, the amount of light absorption by the Ge mesa light absorption portions 114 tends to decrease, and the accuracy or sensitivity of the optical power monitor based on current measurement tends to decrease. Although there is a trade-off between the allowable loss of propagating light and the allowable level of accuracy or sensitivity of the optical power monitor, it is possible to improve the accuracy or sensitivity of the optical power monitor by, for example, increasing the size of the Ge mesa light absorption portions 114 or moving their positions closer to the ribs 111a without changing the number of Ge mesa light absorption portions 114 arranged.

[0101] [Fourth Embodiment] Next, a fourth embodiment will be described with reference to Fig. 11. Fig. 11 is a schematic top view showing an example configuration of a photoreceiver 100D according to the fourth embodiment. The photoreceiver 100D shown in Fig. 11 essentially corresponds to a configuration in which a plurality of (e.g., three) photoreceivers 100 (see Figs. 1 and 2) shown in the first embodiment are arranged in series in the Z-axis direction.

[0102] Since the optical receiver 100D includes multiple optical receivers 100 as its components, it may be referred to as a "light receiving circuit 100D" for convenience. The cross section taken along the line A-A in Fig. 11 may be understood to be the same as the cross section shown in Fig. 2 (or Fig. 4).

[0103] For example, the optical receiver 100D includes a substrate 101, a lower cladding layer 102 provided above the substrate 101 in the Y-axis direction, and an upper cladding layer 103 provided above the lower cladding layer 102 in the Y-axis direction.

[0104] Within the upper cladding layer 103, a plurality of rib-type waveguides 111-1 to 111-m are arranged in series in the Z-axis direction above the Y-axis direction of the common lower cladding layer 102. Note that m represents an integer of 2 or more, and in this example, m=3.

[0105] Adjacent rib-type waveguides 111-j (e.g., ribs 111a) in the Z-axis direction are connected by optical waveguides 108 provided above the lower cladding layer 102 in the Y-axis direction, where j represents any one integer from 1 to m.

[0106] Of the two ends in the Z-axis direction of the rib-type waveguide 111-1, the end to which the optical waveguide 108 is not connected is connected, for example, to the optical waveguide 108a provided above the lower cladding layer 102 in the Y-axis direction. Of the two ends in the Z-axis direction of the rib-type waveguide 111-3, the end to which the optical waveguide 108 is not connected is connected, for example, to the optical waveguide 108b provided above the lower cladding layer 102 in the Y-axis direction.

[0107] Therefore, for example, when light is input to the optical waveguide 108a, the input light propagates in the positive direction of the Z axis through the rib type waveguide 111-1, the optical waveguide 108, the rib type waveguide 111-2, the optical waveguide 108, and the rib type waveguide 111-3 in that order, and is output from the optical waveguide 108b. Note that light propagation in the reverse direction (negative direction of the Z axis) is also possible.

[0108] At the top of each of the rib-type waveguides 111-1 to 111-3 in the Y-axis direction, as in the first embodiment, Ge mesa light-absorbing portions 114 and electrode pairs 116 are arranged symmetrically in the X-axis direction in the regions 111b on both sides of the rib 111a in the X-axis direction.

[0109] As described above, in the fourth embodiment, the substrate 101, the lower cladding layer 102, and the upper cladding layer 103 are common to the three rib-type waveguides 111-1 to 111-3 and the two sets of Ge mesa absorption portions 114 and electrode pairs 116 provided symmetrically in the X-axis direction above the Y-axis direction of each of the rib-type waveguides 111-1 to 111-3.

[0110] On the other hand, unlike the first embodiment, the two sets of Ge mesa light absorption portions 114 and electrode pairs 116 are individually provided for each of the plurality of rib-type waveguides 111-1 to 111-3.

[0111] Therefore, current crosstalk in the Z-axis direction can be reduced more than when, for example, a plurality of sets of Ge mesa light absorption portions 114 and electrode pairs 116 (in other words, light absorption circuits #i; i = 1 to n) are arranged in series in the Z-axis direction in the same rib-type waveguide 111 as in the second embodiment (see FIG. 5 ).

[0112] Reducing current crosstalk improves the accuracy of current measurement for each light absorption circuit #i, for example. As a result, when the multipoint monitor described in the second embodiment is applied to the optical receiver 100D, the accuracy of the derived regression line 602 and the accuracy of the estimation of the input optical power (or output optical power) based on the regression line 602 improve.

[0113] 11 shows only the cutting line A-A focusing on one of the three light absorption circuits #1 to #3 (for example, light absorption circuit #2), and does not show the other two sets of light absorption circuits #1 and #3. It may be understood that the stacking structure in the Y-axis direction of each of the other two sets of light absorption circuits #1 and #3 is also the same as that in FIG. 2 (or FIG. 4).

[0114] In the above example, the number of light absorption circuits #i (in other words, the number of current measurement points) is n=3, but n is not limited to 3 and may be any number that allows the regression line 602 to be derived, for example, n≧2.

[0115] Furthermore, other effects may be understood to be the same as or similar to those of the first or second embodiment.

[0116] The structures of the optical receivers exemplified in the first embodiment (including modifications) to the fourth embodiment described above may be appropriately combined and implemented. As non-limiting examples of combinations, several variations are shown below as the fifth to eighth embodiments in Figures 12 to 15.

[0117] 12 is a schematic top view illustrating an example of the configuration of a photodetector 100E according to a fifth embodiment. The photodetector 100E illustrated in FIG. 12 has, in a top view, a configuration in which, for example, a plurality of sets (e.g., five light absorption circuits #i) of Ge mesa light absorption portions 114 and electrode pairs 116 are alternately arranged at different positions in the Z-axis direction in one of two regions 111b on either side of a rib 111a in the X-axis direction.

[0118] The configuration illustrated in Fig. 12 may be understood to correspond to a configuration in which some of the multiple light absorption circuits #i arranged in series in the Z-axis direction in each of the regions 111b on both sides of the X-axis direction, with the rib 111a in between, in the configuration in Fig. 5 are thinned out. Note that in Fig. 12, the cross sections of each of the five light absorption circuits #i (for example, the cross sections taken along the arrows A-A and B-B in Fig. 12) may be understood to correspond, for example, to the cross sections shown in Fig. 10 of the third embodiment.

[0119] With this type of alternating arrangement, for example, even if the number of current measurement points in a multi-point monitor is increased, it is easy to ensure a spacing margin between the light absorption circuits that is sufficient to minimize current crosstalk in the Z-axis direction, which is expected to improve measurement accuracy and reduce the size of the photodetector.

[0120] 13 is a schematic top view showing an example of the configuration of a photodetector 100F according to a sixth embodiment. The photodetector 100F shown in Fig. 13 has a configuration in which some of the multiple rib-type waveguides 111-j are common to multiple light-absorption circuits #j in the configuration shown in Fig. 11 of the fourth embodiment.

[0121] For example, in the top view of Fig. 13, the optical receiver 100F includes two rib-type waveguides 111-1 and 111-2 on the upper part of the lower cladding layer 102, with one rib-type waveguide 111-1 being common to two light-absorption circuits #1 and #2. The other rib-type waveguide 111-2 includes one light-absorption circuit #3, similar to the example shown in Fig. 11. Note that the cross section of each light-absorption circuit #i (for example, the cross section taken along the arrows A-A in Fig. 13) may be understood to correspond to the cross section shown in Fig. 10 of the third embodiment, for example.

[0122] Such an arrangement makes it easy to selectively set a position or area in the optical receiver 100F (e.g., the upper part of the substrate 101) where the effect of reducing current crosstalk in the Z-axis direction can be obtained, and is therefore expected to improve the degree of freedom in the design of the optical receiver 100F.

[0123] Seventh Embodiment Fig. 14 is a schematic top view showing a configuration example of a photodetector 100G according to a seventh embodiment. The photodetector 100G shown in Fig. 14 corresponds to a combination of the fourth embodiment (Fig. 11) and the third embodiment (Fig. 9).

[0124] For example, in the configuration illustrated in FIG. 11, the optical receiver 100G has a configuration in which a plurality of (for example, n=3) light absorption circuits #i are arranged in series in the Z-axis direction, limited to one of two regions 111b on both sides of the rib 111a in the X-axis direction, similar to the configuration illustrated in FIG. 9.

[0125] Each light absorption circuit #i is provided individually above a plurality of (e.g., m=3) rib-type waveguides 111-j arranged individually in the Z-axis direction. A cross section of each light absorption circuit #i (e.g., a cross section taken along the line A-A in FIG. 14) may be understood to correspond to the cross section illustrated in FIG. 10 of the third embodiment, for example.

[0126] Such an arrangement can provide the effect of suppressing current crosstalk between the light absorption circuits #i and the effect of reducing the loss of light propagating through the rib-type waveguide 111 .

[0127] 15 is a schematic top view showing an example of the configuration of a photodetector 100H according to an eighth embodiment. The photodetector 100G shown in Fig. 15 has a configuration similar to that of the fifth embodiment (Fig. 12), in which, for example, in the configuration exemplified in the fourth embodiment (Fig. 11), a plurality of (e.g., n = 3) light absorption circuits #i are arranged alternately at different positions in the Z-axis direction in one of two regions 111b on both sides of the rib 111a.

[0128] Each light absorption circuit #i is provided individually above a plurality of (e.g., m=3) rib-type waveguides 111-j arranged individually in the Z-axis direction. It may be understood that the cross sections of each light absorption circuit #i (e.g., the cross sections taken along the arrows A-A and B-B in FIG. 15) correspond to the cross sections illustrated in FIG. 10 of the third embodiment, for example.

[0129] With such an arrangement, for example, the same effect as in the fourth embodiment can be obtained. In addition, as in the fifth embodiment, it is easy to ensure a spacing margin between the light absorption circuits that is sufficient to minimize current crosstalk in the Z-axis direction, and therefore improvement in measurement accuracy and reduction in the size of the photodetector can be expected.

[0130] The above is a detailed description of a non-limiting example of the present disclosure. According to the present disclosure, part of the light propagating through the rib-type waveguide 111 and seeping out from the rib 111 a is absorbed by the Ge layer 114, and the flow of charges (i.e., current) generated in accordance with the power of the light absorbed by the Ge layer 114 is measured.

[0131] Therefore, it is possible to suppress the loss of propagating light and improve the accuracy of the optical power monitor. The improved accuracy of the optical power monitor enables, for example, more accurate monitoring and control of an optical circuit including a photodetector, leading to improved performance of the optical circuit.

[0132] [Application Example] The various configurations or structures of the optical receivers (reference numerals are omitted for convenience) exemplified in the above-described embodiments may be selected to be applied or adopted depending on, for example, differences in input optical power (in other words, optical power to be monitored).

[0133] For example, when monitoring the transmission optical power of an optical transmitter using an optical receiver according to the present disclosure, it is easier to obtain the desired monitoring accuracy even if fewer light absorption circuits #i are installed than when monitoring the reception optical power of an optical receiver.

[0134] 5 and 9, the configuration shown in Fig. 9, which has a smaller number of light absorption circuits #i, may be preferentially applied to the optical receiver configuration applied to the optical transmitter. Conversely, when monitoring the received optical power of the optical receiver, the configuration shown in Fig. 5, which has a larger number of light absorption circuits #i, may be applied to the optical receiver.

[0135] [Supplementary Information for the Overall] The drawings used in this disclosure show an example in which the rib 111a extending in the Z-axis direction is disposed at the center in the width direction (X-axis direction) of the Si layer 111, which is an example of a first semiconductor. However, the rib 111a may be disposed at a position shifted in the X-axis direction from the center in the width direction (X-axis direction) of the Si layer 111. In this case, the areas of the two regions 111b on both sides of the rib 111a in the X-axis direction are different from each other, and therefore the size or number of the light absorption circuits #i (Ge mesa light absorption portions 114 and electrode pairs 116) to be disposed may be set according to their respective areas.

[0136] Furthermore, the drawings used in this disclosure show an example in which the multiple Ge mesa absorption portions 114 (and electrode pairs 116) are aligned at equal intervals or symmetrically in the Z-axis or X-axis direction. However, the multiple Ge mesa absorption portions 114 may be aligned at unequal intervals or asymmetrically in the Z-axis or X-axis direction. Furthermore, the sizes of the multiple Ge mesa absorption portions 114 or the distances between them in the X-axis direction and the rib 111a (or 113a) may be the same, or some or all of them may be different.

[0137] For example, when arranging multiple Ge mesa light absorption portions 114 in series in the Z-axis direction, the size of each Ge mesa light absorption portion 114 may be set so that the amount of light absorption increases (in other words, the light absorption area) as the Ge mesa light absorption portion 114 is located more downstream in the positive (or negative) direction of the Z-axis. Additionally or alternatively, when arranging multiple Ge mesa light absorption portions 114 in series in the Z-axis direction, the individual placement positions of each Ge mesa light absorption portion 114 may be set so that the more downstream in the positive (or negative) direction of the Z-axis the Ge mesa light absorption portion 114 is located, the closer it is to the rib 111a in the X-axis direction.

[0138] In these arrangements, from the perspective of multipoint monitoring, the current values ​​measured at multiple current measurement points in the Z-axis direction tend to fall within a certain range, unlike the graph illustrated in FIG. 6 . Therefore, instead of deriving the regression line 602, it is possible to estimate the optical input power (or optical output power) by, for example, averaging the measured current values ​​obtained by multipoint monitoring. However, from the perspective of the optical receiver manufacturing process or deriving a regression line using multipoint monitoring, it is useful to make the above-mentioned arrangement conditions as consistent as possible between the Ge mesa absorption portions 114.

[0139] [Additional Terminology] The term "stacked" used in the present disclosure may be understood to include not only an embodiment in which two layers are directly stacked on top of each other, but also an embodiment in which one or more additional layers are interposed between the two layers. For example, even if a photoreceiver has a structure different from the stacked structure exemplified in the present disclosure, it does not deviate from the spirit and scope of the present disclosure as long as it has a structure in which a portion of light propagating through the rib-type waveguide 111, which is an example of a first semiconductor, can be absorbed by the Ge layer 114, which is an example of a second semiconductor.

[0140] Furthermore, the term "connect" or "couple" may be understood to mean any direct or indirect "connection" or "coupling" between two or more elements. For example, the term may be understood to include an indirect "connection" or "coupling" where one or more intermediate elements are interposed between two elements that are "connected" or "coupled" to each other. A "connection" or "coupling" between elements may be either an optical or electrical "connection" or "coupling."

[0141] Any reference to an element followed by a designation such as "first...," "second...," etc. does not limit the quantity or order of those elements. These designations are merely used as a convenient way to distinguish between two or more elements. For example, a reference to a first and a second element does not imply that only two elements may be employed, nor does it imply that the first element must precede the second element in any physical quantity.

[0142] In the present disclosure, the terms "circuit" and "portion" may be interchangeable, or may be appropriately replaced with other terms such as "means" or "device." The term "detection" may be appropriately replaced with other terms such as "measurement," "measurement," "observation," or "monitor" depending on the context. The term "region" may be appropriately replaced with "portion."

[0143] Although the present disclosure has been described in detail above, it is clear to those skilled in the art that the spirit and scope of the present disclosure are not limited to the contents described throughout the present disclosure. The present disclosure can be implemented in modified and altered forms without departing from the spirit and scope of the present disclosure as defined by the claims. Therefore, the description of the present disclosure is intended for illustrative purposes only and does not have any limiting meaning on the spirit and scope of the present disclosure.

[0144] The present disclosure is useful for technologies that use optical circuits, such as optical communication systems or optical information processing systems.

[0145] 100, 100A to 100H Photodetector 101 SOI substrate 102 Lower cladding layer 103 Upper cladding layer 108, 108a, 108b Optical waveguide 111, 111-1 to 111-3 Silicon (Si) layer 111a First region (rib) 111b Second region 112 p++ Si electrode portion 113 Convex region (intrinsic Si region) 114 Germanium (Ge) layer 115 n-type Ge region 116a, 116b Electrode 200 Ammeter 300 Analog-to-digital converter (ADC) 400 Processor 700 Measurement system

Claims

1. An optical receiver comprising: a substrate; a first semiconductor layer provided on an upper portion of the substrate, the first semiconductor layer having a first region having a first thickness and a second region having a second thickness thinner than the first thickness; a second semiconductor layer provided on an upper portion of the second region, the second semiconductor layer having a higher refractive index than the first semiconductor layer; a first electrode provided on an upper portion of the second region and positioned outside the second semiconductor layer with respect to the first region; a second electrode provided on an upper portion of the second semiconductor layer; and an optical waveguide connected to the first semiconductor layer.

2. The optical receiver according to claim 1, wherein the first semiconductor layer has a region in the second region doped with one of the first and second conductivity types of impurities, and the second semiconductor layer has a region doped with the other of the first and second conductivity types of impurities.

3. The optical receiver according to claim 2, wherein the first region in the first semiconductor layer is an intrinsic semiconductor region that is not doped with an impurity of the one conductivity type.

4. The optical receiver described in claim 1, wherein a plurality of the second semiconductor layers are arranged in series above the second region along the direction in which the first region extends, the second electrode is provided on the top of each of the plurality of second semiconductor layers arranged in series, and the first electrode is provided at the outer position for each of the plurality of second semiconductor layers arranged in series.

5. The optical receiver according to claim 1, wherein the second semiconductor layer is disposed in one of two second regions located on either side of the extending first region when viewed from above.

6. The optical receiver of claim 1, wherein the first semiconductor layer is silicon and the second semiconductor layer is germanium.

7. A light receiving circuit having a configuration in which a plurality of light receiving devices according to claim 1 are connected in series.

8. A monitoring method comprising: measuring current values ​​for multiple sets of the first electrodes and the second electrodes in a state in which light is input to the optical receiver described in claim 4 through the optical waveguide; deriving a regression line based on multiple current values ​​obtained for the multiple sets; and estimating the input optical power to the optical receiver based on the regression line.

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