SOA-integrated optical amplifier circuit

Abstract

Provided is a compact optical amplifier in which the polarization dependent gain of an SOA can be eliminated and the number of optical components is small. Specifically, provided is an SOA-integrated optical amplifier circuit (200) comprising a planar lightwave circuit (201) and a plurality of semiconductor amplifiers (SOAs) (203, 205) integrated on the same substrate, the SOA-integrated optical amplification circuit (200) characterized by comprising a plurality of SOAs (203, 205) that amplify light, a polarization separation unit (207) that separates incident light into independent first light in a first mode and second light in a second mode, a first polarization rotation unit (203a) that converts the first mode of first light separated by the polarization separation unit (207) to the second mode, a second polarization rotation unit (203b) that converts the second mode of the first light amplified by the first SOA (203) to the first mode, and a polarization multiplexing unit (209) that multiplexes the amplified light.

Description

SOA integrated optical amplifier circuit

[0001] The present disclosure relates to an optical amplifier circuit, and more particularly to an optical amplifier circuit in which an optical circuit and an amplifier are integrated.

[0002] In recent years, the rapid increase in communication traffic has led to demands for higher capacity, higher quality, and longer transmission distances in optical networks. To achieve longer transmission distances, optical amplifiers are used to compensate for losses such as optical absorption in optical fibers. Currently, the most widely used optical amplifiers are rare-earth-doped optical fibers, in which the core of the optical fiber is doped with rare-earth elements such as erbium. However, rare-earth-doped optical fibers require electronic excitation from an external light source, which poses a challenge: the large number of components required to fabricate an optical amplifier, including the fiber itself, lasers, and optical couplers. Therefore, rare-earth-doped optical fibers require many steps to fabricate an amplifier, and there is a demand for optical amplifiers with fewer components and a simpler manufacturing process. While Raman amplifiers and semiconductor optical amplifiers (SOAs) are known as optical amplifiers other than rare-earth-doped optical fibers, SOAs are advantageous in terms of the number of components. Since the SOA uses electric current to perform electronic excitation, it does not require an external light source or optical coupler, making it possible to significantly reduce the number of parts in an optical amplification module.

[0003] Current optical communications utilize two independent, orthogonal polarization states, so optical amplifiers are required to have amplification gain with little polarization dependency. However, it is generally known that the gain of SOAs is highly polarization dependent.

[0004] Patent Document 1 discloses an example in which a polarization splitter and a polarization rotator are combined with an SOA to eliminate polarization-dependent gain in an optical amplifier using an SOA. FIG. 1 is a representative diagram of Patent Document 1. Light is incident from input port 110a into optical amplifier 100, which hybridly integrates a silica-based optical waveguide and an SOA. Optical amplifier 100 is composed of wave plate 103, SOA 105 having a U-turn optical waveguide, and silica-based planar lightwave circuits 109 and 110 having a polarization splitter 107 based on a Mach-Zehnder interferometer (MZI) circuit. Both end faces of input and output waveguides 108aa and 108ab of the SOA are connected to the silica-based planar lightwave circuit. Light mixed with two polarization states incident from input port 110a is polarization-separated by polarization splitter 107. In the polarization separation unit 107, light parallel to the surfaces of the planar lightwave circuits 109 and 110 (TE mode light) passes through 109a, and light perpendicular to the surfaces (TM mode light) passes through 109b. The TE mode light that passed through 109a passes through slit 104 in wave plate 103 and enters SOA 105 as TE mode light. The TM mode light that passed through 109b is converted to TE mode light by wave plate 103 and then enters SOA 105. In this way, since only TE mode light is entered into SOA 105, optical amplification with reduced polarization dependent gain of the SOA is possible.

[0005] In addition, Non-Patent Document 1 reports a means for reducing the polarization dependent gain of an SOA by polarization separation and rotation using spatial optical systems such as lenses and polarization separators, rather than silica-based planar lightwave circuits.

[0006] Patent Publication No. 2012-163614

[0007] K. Morito and S. Tanaka, “Record High Saturation Power (+22 dBm) and Low Noise Figure (5.7 dB) Polarization-Insensitive SOA Module” IEEE PHOTONICS TECHNOLOGY LETTERS Volume 17, Issue: 6, page: 1298 - 1300, 2005

[0008] The optical amplifiers using SOAs described in Patent Document 1 and Non-Patent Document 1 combine polarization separation and rotation functions with the SOA, allowing only TE mode light to pass through the SOA, thereby solving the problem of polarization dependent gain that SOAs have. However, the following problems remain.

[0009] The first problem is to reduce the number of components and simplify the assembly process in order to reduce the cost of optical amplifiers. However, in existing devices for eliminating polarization-dependent gain, the light entering the optical amplifier and the light emitted after amplification travel the same path in opposite directions. This requires the use of optical components such as a circulator to separate the light entering the input port of the optical amplifier from the light emitted after amplification. This leads to an increase in the number of components and the assembly process.

[0010] The second problem is miniaturizing the module to accommodate multiple optical amplifiers at high density. However, existing technologies require large optical components such as circulators and lenses, which leads to larger optical amplifiers.

[0011] The present disclosure has been made in view of such problems, and its purpose is to provide an optical amplifier that is small in size and has a small number of optical components, and that can eliminate the polarization dependent gain of an SOA.

[0012] According to one embodiment of the present disclosure, there is provided an SOA integrated optical amplifier circuit including a planar lightwave circuit and a plurality of semiconductor amplifiers (SOAs) integrated on the same substrate, the plurality of SOAs including a first SOA and a second SOA, the planar lightwave circuit including a polarization splitter configured to split incident light into independent first light of a first mode and second light of a second mode, one SOA configured to amplify the first light and a second SOA configured to amplify the second light, a first polarization rotator configured to convert the first mode of the first light separated by the polarization splitter into a second mode, and a second SOA configured to amplify the second light. and a polarization multiplexing unit configured to multiplex the first light amplified by the first SOA and converted to the first mode by the second polarization rotation unit with the second light amplified by the second SOA in the second mode, or to multiplex the first light amplified by the first SOA and converted to the first mode by the second polarization rotation unit with the second light amplified by the second SOA in the second mode, or to multiplex the first light amplified by the first SOA and converted to the second mode by the second polarization rotation unit with the second light amplified by the second SOA in the second mode.

[0013] FIG. 3 is a top view of an optical amplifier using an SOA according to the prior art; FIG. 4 is a top view of an SOA integrated optical amplifier circuit according to an embodiment; FIG. 3(a) is a top view showing the structure of a grooved waveguide used as a polarization rotation unit according to an embodiment, FIG. 3(b) shows a case where grooves are provided on both sides near the waveguide, and FIG. 3(c) is a side view showing a case where grooves are provided on only one side near the waveguide; FIG. 4(a) is a top view of a portion on a planar lightwave circuit where an SOA is fixed, and FIG. 4(b) is a side view; FIG. 4 is a top view of an SOA integrated optical amplifier circuit according to another embodiment; FIG. 4 is a top view of an SOA integrated optical amplifier circuit according to another embodiment; and FIG. 4 is a top view of an SOA integrated optical amplifier circuit according to yet another embodiment.

[0014] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. The same or similar reference numerals indicate the same or similar elements, and repeated explanations may be omitted. The numerical values ​​in the following description are examples, and other numerical values ​​may be used to implement the present disclosure without departing from the gist of the present disclosure.

[0015] (Embodiment 1) An optical amplifier circuit in which a planar lightwave circuit and an SOA are hybrid-integrated according to embodiment 1 of the present disclosure will be described with reference to Figure 2. Here, as an example of an optical amplifier circuit in which a planar lightwave circuit and an SOA are hybrid-integrated, an optical amplifier circuit in which two SOAs are integrated on a silica-based planar lightwave circuit will be described. Here, a planar lightwave circuit made of a silica-based material will be described, but the material of the planar lightwave circuit is not limited to quartz, and other materials such as silicon may be used.

[0016] (Configuration of Optical Amplifier Circuit) FIG. 2 illustrates an optical amplifier circuit 200 according to the first embodiment of the present disclosure. FIG. 2 is a top view of the optical amplifier circuit 200. As shown in FIG. 2, the optical amplifier circuit 200 is configured with a planar lightwave circuit 201 and two SOAs 203 and 205 integrated on the same substrate as the planar lightwave circuit 201. The planar lightwave circuit 201 and the first SOA 203, and the planar lightwave circuit 201 and the second SOA 205 are optically connected by arranging their waveguides facing each other. In addition, the configuration of FIG. 2 uses SOAs in which input waveguides 203a and 205a to the SOAs 203 and 205 and output waveguides 203b and 205b from the SOAs 203 and 205 exist on opposing end faces of the SOAs, respectively.

[0017] The planar lightwave circuit 201 is composed of one input waveguide 201a for the planar lightwave circuit and one output waveguide 201b for the planar lightwave circuit, a polarization separation unit 207 and a polarization multiplexing unit 209 each composed of an MZI circuit, two polarization rotation units 211 and 213 each having a grooved waveguide configuration for polarization rotation, and input and output waveguides 203a, 203b, 205a, and 205b for the SOAs for optically connecting to each of the SOAs 203 and 205.

[0018] The MZIs used in the polarization splitter 207 and polarization multiplexer 209 (details not shown) are composed of two couplers with two inputs and two outputs and two waveguides that couple the couplers. One of the two inputs of the MZI in the polarization splitter 207 is connected to the input waveguide 201a for the planar lightwave circuit. One of the two outputs of the MZI in the polarization splitter 207 is connected to the input waveguide 205a of the second SOA 205, and the other of the two outputs is connected to the first polarization rotator 211 and then to the first SOA 203.

[0019] The light amplified by each SOA 203, 205 returns to the planar lightwave circuit 201, and the waveguide connected to the second SOA 205 is connected to one input of the polarization multiplexer 209, and the waveguide connected to the first SOA 203 is connected to the second polarization rotator 213. The second polarization rotator 213 is connected to the other input of the polarization multiplexer 209, and one output of the polarization multiplexer 209 is connected to the planar lightwave circuit output waveguide 201b.

[0020] The planar lightwave circuit 201 is disposed between the two output sides of the polarization splitter 207 and the two input sides of the polarization multiplexer 209, and the optical path lengths of the two different waveguides including the polarization rotator and the SOA are equal. As an example, the size of the planar lightwave circuit 201 shown in FIG. 2 is approximately 5 mm in length (Y direction) × 20 mm in width (X direction).

[0021] (Planar Lightwave Circuit) The operation of the planar lightwave circuit 201 will now be described. Light that is a mixture of two polarization states and that is input to the planar lightwave circuit 201 is input to one input waveguide of the polarization separation unit 207. The polarization separation unit 207 serves to separate the input light that is a mixture of two polarization states (for example, into TE mode light and TM mode light), and the TE mode light is output from only one output waveguide of the polarization separation unit 207, and the TM mode light is output from only the other output waveguide.

[0022] Of the polarization-separated light, the TE mode light is directly incident on the second SOA 205, and the TM mode light is converted to TE mode light by the first polarization rotation unit 211 having a grooved waveguide configuration and then incident on the second SOA 205. In this way, by converting TM mode light to TE mode light and then incident on the SOA, optical amplification with reduced polarization-dependent gain of the SOA is possible. In this specification, an example is described in which TM mode light is converted to TE mode light by the polarization rotation unit, but the converted light may also be TE mode light. In other words, the separated TE mode light and TM mode light only need to propagate through different optical paths and do not necessarily propagate through predetermined optical paths.

[0023] The TE mode light is then amplified in intensity by the first SOA 203 and the second SOA 205. The TE mode light amplified by the first SOA 203 is incident on the second polarization rotation unit 213 having a grooved waveguide configuration, where the TE mode light is converted back to TM mode light. Next, the two polarized lights are combined by the polarization combining unit 209 and output from the output port of the planar lightwave circuit output waveguide 201b.

[0024] 2, two polarization rotation units 211, 213 are arranged one on the input side and one on the output side of first SOA 203, but second polarization rotation unit 213 arranged on the output side may also be placed on the output side of second SOA 205. That is, as long as the polarization states of the light incident on the two SOAs are the same (for example, the above-mentioned TE mode light) and the light entering the two input sides of polarization multiplexer 209 are each in an independent polarization state, second polarization rotation unit 213 can be arranged on any optical path.

[0025] (Polarization Separator and Polarization Combiner) The following describes the operating principles of the polarization separator 207 and the polarization combiner 209. Light that is a mixture of two polarized waves and that is input to one input waveguide of the polarization separator 207, which is made up of an MZI, is split into two by the first coupler 207a in the polarization separator 207. An optical path length difference or a birefringence difference is imparted to the two branched waveguides.

[0026] When the light is combined again by the second coupler 207b in the polarization splitter 207, different interference conditions are given to the TE mode light and the TM mode light due to the optical path length difference or birefringence difference between the two waveguides. In other words, by adjusting the conditions of the optical path length difference and birefringence difference, it is possible to output only TE mode light from one output waveguide of the MZI, and output only TM mode light from the other output waveguide of the MZI. In other words, light mixed with two polarized waves can be separated into each light containing only one polarized wave.

[0027] The polarization multiplexer 209 can be realized by interchanging the input waveguide and output waveguide in the polarization separator 207. That is, when TE mode light and TM mode light are input to the two input waveguides, respectively, light obtained by multiplexing the TE mode light and the TM mode light is output from one output waveguide of the MZI.

[0028] (Polarization Rotation Unit) Hereinafter, polarization rotation units 211 and 213 using grooved waveguides will be described with reference to Figures 2 and 3. The grooves serve to release stress applied to the core and cladding of planar lightwave circuit 201 resulting from the manufacturing process of planar lightwave circuit 201. Generally, planar lightwave circuit manufacturing processes include a heating process to high temperatures of 1000°C or higher. When cooling from high temperatures to low temperatures, compressive stress is applied to the silica glass in the in-plane direction due to the difference in linear thermal expansion coefficients between Si substrate 301 and core 303 and / or cladding 305 made of silica glass. On the other hand, because the stress in the perpendicular direction is small, core 303 is subjected to stresses of different magnitudes in the perpendicular direction and in-plane direction, and the refractive index in the perpendicular direction and in-plane direction differs due to the photoelastic effect.

[0029] Here, if a groove 307 is formed on only one side near the waveguide, as shown in Figure 3(c), the stress applied to the left side of the core 303 is released, and local stress is applied to the sidewall of the groove 307 and the cladding near the Si substrate 301. This stress is applied in a diagonal direction (as represented by the arrow in Figure 3(c)) when viewed from the core 303, so the stress applied to the core 303 is diagonal. Therefore, the refractive index profile of the core also becomes diagonal due to the photoelastic effect, and the axis of birefringence rotates. In other words, while the main axis of birefringence is perpendicular or horizontal to the substrate when no groove is formed, the formation of the groove tilts the main axis, and the waveguide itself begins to function as a wave plate. Therefore, by forming a groove 307 near the waveguide and optimizing the conditions of the groove 307, it is possible to give the waveguide the same functionality as a half-wave plate, enabling TE / TM conversion.

[0030] (SOA Integration Method) A method for integrating an SOA on a planar lightwave circuit will be described below with reference to Fig. 4. Fig. 4(a) is a top view of the portion of the planar lightwave circuit 201 where the SOAs 203 and 205 are fixed, and Fig. 4(b) is a side view. A metal layer 401 serving as a lower electrode, a solder layer 403 formed on the metal layer 401, a stopper 405, and an alignment marker 407 are formed on the planar lightwave circuit 201. A similar alignment marker 407 is also formed on the side of the SOA to be integrated.

[0031] First, a method for aligning the mounting position of the SOA in the in-plane direction will be described. Alignment of the mounting position in the in-plane direction is performed using alignment markers 407 formed on the planar lightwave circuit 201 and the SOAs 203 and 205. Markers are provided on the planar lightwave circuit 201 and the SOAs 203 and 205 at positions such that the respective waveguide positions are aligned when the alignment marker positions are aligned. Therefore, when integrating the SOAs 203 and 205 on the planar lightwave circuit 201, the positions of the alignment markers 407 on the planar lightwave circuit 201 and the SOAs 203 and 205 are observed, and the in-plane mounting position is aligned by aligning the positions of both. The shape of the alignment marker 407 can be any shape, such as a circle or a cross.

[0032] Next, a method for aligning the mounting position of the SOA in the height direction will be described. Aligning the mounting position in the height direction uses stoppers 405 formed on the planar lightwave circuit 201. The mounting height of the SOAs 203 and 205 is controlled by integrating them by pressing the lower ends of the SOAs 203 and 205 against the stoppers. By setting the stopper height so that the height of the waveguides of the planar lightwave circuit 201 and the SOA coincides when the SOAs 203 and 205 are pressed against the stoppers 405, the SOAs can be aligned in the height direction using the stoppers 405. When integrating the SOAs 203 and 205 on the planar lightwave circuit 201 using solder, the SOAs 203 and 205 are pressed against the stoppers 405 in a heated state. Therefore, the material of the stoppers can be any material that is resistant to deformation even at high temperatures (e.g., SiOx, Si, metal, etc.).

[0033] In this embodiment, an optical amplifier circuit or its optical components, which in conventional technology are configured using a spatial optical system, can be realized on a single chip on a planar lightwave circuit, thereby providing an optical amplifier circuit with a reduced number of spatial optical system components. Furthermore, unlike the configuration of FIG. 1 , this embodiment amplifies the two light beams after polarization separation using separate SOAs 203 and 205, thereby limiting the direction of travel of the light within the SOA to one direction. That is, the light only travels from left to right in FIG. 2 , and there is no light traveling in the opposite direction along the same path as in the configuration of FIG. 1 . In other words, this configuration differs from the configuration of FIG. 1 in that a circulator for separating the light beams is not required.

[0034] Furthermore, optical devices using lenses such as those described in Non-Patent Document 1 require active alignment to adjust the lens position while monitoring the light intensity, which has the drawback of long mounting time. On the other hand, this embodiment does not use lenses, so the mounting process can be significantly reduced.

[0035] Therefore, in this embodiment, by integrating two SOAs 203 and 205 on a planar lightwave circuit 201 on which a polarization splitter 207, a polarization multiplexer 209, and polarization rotators 211 and 213 are formed, what was previously realized using large optical components in conventional technology can be realized on a single chip of a planar lightwave circuit, thereby reducing the number of components, making it possible to miniaturize the optical amplifier device, and simplify the mounting process. Furthermore, by devising the layout of the planar lightwave circuit, it is possible to further miniaturize the optical amplifier circuit and reduce the number of components.

[0036] In the first embodiment, the polarization rotators 211 and 213 use grooved waveguides. In this embodiment, a configuration using a half-wave plate in the polarization rotator is disclosed as an alternative example. The positions of the polarization rotators 211 and 213 and the configuration of the optical amplifier circuit 500 are the same as in the first embodiment, so details are omitted.

[0037] Fig. 5 is a top view of the planar lightwave circuit 201 when a wave plate is used. The wave plate 501 is inserted into a gap that separates the waveguide of the planar lightwave circuit 201. In Fig. 5, the wave plate 501 is inserted perpendicular to the direction of light propagation in the waveguide of the planar lightwave circuit 201, but the wave plate 501 and the cross section of the waveguide at the gap may be tilted at any angle to reduce light loss due to reflection between the waveguide and the wave plate 501.

[0038] The polarization rotators 211 and 213 using the grooved waveguide described in the first embodiment require a waveguide length of approximately 3 to 4 mm, which has the disadvantage of increasing the size of the planar lightwave circuit. On the other hand, in the present embodiment using the wave plate 501, although there is the disadvantage of an increased number of components, the length of the polarization rotators 211 and 213 can be reduced to 1 mm or less. In other words, it is possible to reduce the length of the planar lightwave circuit 201 by several mm. Note that in this embodiment, as in the first embodiment, the polarization rotator 213 after optical amplification does not need to be located after the first SOA 203, and may be located after the second SOA 205, for example.

[0039] In the polarization rotators 211 and 213 using the wave plate 501, optical loss occurs due to reflection when light from the waveguide of the planar lightwave circuit 201 enters the wave plate 501 and when light that has passed through the wave plate 501 enters the waveguide of the planar lightwave circuit 201 again. In the configuration of FIG. 5 , the wave plate 501 is disposed before and after the first SOA 203, so light passing through a path including the first SOA 203 passes through the wave plate 501 twice. As a result, the amount of optical loss for light passing through the path including the wave plate 501 is greater than that for light passing through the second SOA 205, which does not pass through the wave plate 501. This difference in loss causes polarization-dependent loss in the optical amplifier circuit. Therefore, in the case of the polarization rotator 213 using the wave plate 501, it may be effective to place the polarization rotator 213, which is disposed after the first SOA 203, after the second SOA 205 in order to reduce the polarization-dependent loss of the optical amplifier circuit.

[0040] Also in this embodiment, as in the first embodiment, the optical path lengths of the two waveguides including the polarization splitter and the SOA, which connect the output side of the polarization splitter 207 and the input side of the polarization multiplexer 209, are equal.

[0041] (Embodiment 3) In the above-mentioned embodiment 1, a structure was described in which an input / output waveguide 203 a, 205 a to the SOA and an output waveguide 203 b, 205 b from the SOA are connected to opposing end faces of the SOA (for example, one of the opposing cross sections of the first SOA 203 is configured to be connected to the input / output waveguide 203 a to the SOA, and the other is configured to be connected to the output waveguide 203 b from the SOA). In this embodiment, a configuration will be described in which an SOA is used in which the waveguide inside the SOA (not shown) has a U-turn shape and the input / output waveguides are located at the same end of the SOA.

[0042] An optical amplifier circuit according to a third embodiment of the present disclosure will be described below with reference to FIG. 6 . In the third embodiment, the optical amplifier circuit 600 is also configured with a silica-based planar lightwave circuit 201 and two SOAs 203 and 205. The planar lightwave circuit 201 also includes a polarization splitter 207, a polarization multiplexer 209, and polarization rotators 211 and 213, each configured with an MZI. The mechanisms of the polarization splitter 207, the polarization multiplexer 209, and the polarization rotators 211 and 213 are similar to those in the first and second embodiments, and therefore will not be described in detail. In this embodiment, the polarization rotators 211 and 213 may be either a grooved waveguide (as described in the first embodiment) or a half-wave plate (as described in the second embodiment).

[0043] Figure 6 differs from Figure 2 in that the input and output waveguides of the SOA are located at the same end of the SOA. In the configuration in Figure 2, the input and output waveguides of the SOA are located at both ends of the SOA, so the layout of the planar lightwave circuit 201 is also adjusted accordingly, with one polarization splitter 207 and one polarization multiplexer 209 located on both sides of the SOA. This increases the horizontal length of the planar lightwave circuit 201, and as described above, the size of the planar lightwave circuit 201 is approximately 5 mm vertically (Y direction) × 20 mm horizontally (X direction).

[0044] On the other hand, in this embodiment, the input / output waveguides 203a, 203b, 205a, and 205b of the SOA are all present on only one side of the SOA, so the polarization splitter 207 and the polarization multiplexer 209 can be arranged on only one side of the SOAs 203 and 205. That is, the configuration of FIG. 2 can be folded in half as shown in FIG. 6 , thereby significantly reducing the size of the planar lightwave circuit 201. For example, in this embodiment, the size of the planar lightwave circuit 201 is approximately 5 mm vertically (Y direction) × 10 mm horizontally (X direction). In this configuration, since the polarization splitter 207 and the polarization multiplexer 209 are arranged on one side of the SOA, it is necessary to intersect the waveguide connecting one output side of the polarization splitter 207 to the input side of the first SOA 203 and the waveguide connecting the output side of the second SOA 205 to the input side of the polarization multiplexer 209. Also in this embodiment, the optical path lengths of the two waveguides, which connect the output side of the polarization splitter 207 and the input side of the polarization multiplexer 209 and include the polarization splitter and the SOA, are equal.

[0045] Even in a configuration in which the input and output waveguides of an SOA are present on both sides of the SOA as in FIG. 2, it is possible to place the polarization separation circuit and the polarization multiplexing circuit on only one side of the SOA. However, using an SOA in which the input and output waveguides are present on only one side, as in this embodiment, allows for a smaller size.

[0046] By providing input and output waveguides on only one side of the planar lightwave circuit, not only can the size of the planar lightwave circuit be reduced, but the number of components in the optical amplifier can also be reduced. The input and output waveguides 201a and 201b for the planar lightwave circuit are connected to optical fibers by bonding a fiber array to the end face of the planar lightwave circuit 201. In the configuration of FIG. 2, two fiber arrays are required because the input and output waveguides 201a and 201b for the planar lightwave circuit are present at both ends of the planar lightwave circuit 201. On the other hand, in the configuration of this embodiment, two input and output waveguides 201a and 201b for the planar lightwave circuit are present at each end of the planar lightwave circuit 201, so the number of fiber arrays can be reduced to one.

[0047] As described above, when an SOA is used in which the waveguide inside the SOA is U-shaped and the input and output waveguides are located at the same end of the SOA, the polarization splitter and polarization multiplexer can be located on only one side of the SOA, thereby reducing the size of the planar lightwave circuit and the number of components.

[0048] (Fourth Embodiment) Fig. 7 illustrates an optical amplifier circuit 700 according to a fourth embodiment of the present disclosure. In the fourth embodiment, another effect of the configuration in which the polarization splitter and polarization rotator described in the third embodiment are located only on one side of the SOA will be described. The fourth embodiment will be described with reference to Fig. 7. A planar lightwave circuit 201 has a polarization splitter 207, a polarization multiplexer 209, and a polarization rotator 211, each of which is made up of an MZI.

[0049] In this embodiment, the polarization rotation unit 211 is configured with a half-wave plate 501. In the third embodiment shown in FIG. 4 , the polarization rotation units 211 and 213 are disposed in the waveguide connecting the output side of the polarization splitter unit 207 and the input side of the first SOA 203, and in the waveguide connecting the output side of the second SOA 205 and the input side of the polarization multiplexer 209, respectively. Therefore, two wave plates 501 are required. On the other hand, in this embodiment, the polarization rotation unit 211 is inserted in a gap separating both the waveguide connecting the output side of the polarization splitter unit 207 and the input side of the first SOA 203, and the waveguide connecting the output side of the second SOA 205 and the input side of the polarization multiplexer 209. That is, in this embodiment, it is possible to apply polarization rotation to two waveguides using one wave plate 501, thereby reducing the number of components.

[0050] Also in this embodiment, the wave plate 501 may be installed at an angle to reduce reflection loss when light enters the wave plate 501 from the waveguide. Furthermore, although the wave plate 501 is inserted on the left side of the intersection of the waveguides in FIG. 6 , the wave plate may be inserted on the right side of the intersection. Furthermore, also in this embodiment, the optical path lengths of the two waveguides connecting the output side of the polarization splitter and the input side of the polarization multiplexer are equal.

[0051] Additional Considerations The foregoing description of embodiments of the present invention has been presented for purposes of illustration and is not intended to be exhaustive or to be limited to the precise form disclosed. Those skilled in the art will recognize that many modifications and variations are possible in light of the above disclosure.

[0052] Finally, the language used herein has been selected primarily for readability and instructional purposes, and may not have been selected to delineate or limit the subject matter of the invention. Accordingly, it is intended that the scope of the invention be limited not by this detailed description, but rather by the appended claims. Accordingly, the disclosure of embodiments of the invention is intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the claims.

[0053] According to the present disclosure, it is possible to provide an optical amplifier with a reduced number of parts and a simpler mounting process and / or a compact optical amplifier.

[0054] 100 Optical amplifier 110a Incident port of MZI 103 Wave plate 105 SOA 107 Polarization splitter 108aa, 108ab Incident / exit waveguide of SOA 109, 110 Silica-based planar lightwave circuit 109a, 109b Exit port of MZI 200, 500, 600, 700 Optical amplifier circuit 201 Planar lightwave circuit 201a, 201b Incident / exit waveguide of planar lightwave circuit 203 First SOA 205 Second SOA 203a, 205a Incident waveguide to SOA 203b, 205b Exit waveguide from SOA 207 Polarization splitter 209 Polarization multiplexer 211 First polarization rotator 213 Second polarization rotator 301 Si substrate 303 Core 305 Cladding 307 Groove 401 Metal layer 403 Solder layer 405 Stopper 407 Alignment marker 501 Wave plate

Claims

1. An SOA integrated optical amplifier circuit comprising a planar lightwave circuit and a plurality of semiconductor amplifiers (SOAs) integrated on the same substrate, wherein the plurality of SOAs include a first SOA and a second SOA, and the planar lightwave circuit comprises: a polarization splitter configured to split incident light into independent first light of a first mode and second light of a second mode, wherein the first SOA is configured to amplify the first light and the second SOA is configured to amplify the second light; a first polarization rotation unit configured to convert the first mode of the first light separated by the polarization splitter to the second mode; and a second polarization rotation unit configured to convert the second mode of the first light amplified by the first SOA to the first mode or convert the second mode of the second light amplified by the second SOA to the first mode. and a polarization multiplexing unit configured to multiplex the first light amplified by the first SOA and converted to the first mode by the second polarization rotation unit with the second light in the second mode amplified by the second SOA, or to multiplex the first light in the second mode amplified by the first SOA with the second light amplified by the second SOA and converted to the second mode by the second polarization rotation unit.

2. The SOA integrated optical amplifier circuit according to claim 1, characterized in that the first polarization rotation unit and the second polarization rotation unit include a waveguide and a groove formed on one side near the waveguide.

3. The SOA integrated optical amplifier circuit according to claim 1, characterized in that the first polarization rotation unit and the second polarization rotation unit include a waveguide and a half-wave plate inserted in a gap separating the waveguide.

4. The SOA integrated optical amplifier circuit according to claim 1, characterized in that the first SOA and the second SOA have their input and output at the same end, a waveguide connecting the polarization splitter and the input of the first SOA intersects with a waveguide connecting the output of the second SOA and the polarization multiplexer, and the polarization splitter and the polarization multiplexer are arranged on the end side where the input and output of the first SOA and the second SOA are located.

5. The SOA integrated optical amplifier circuit according to claim 4, characterized in that the first polarization rotation unit and the second polarization rotation unit are a single shared half-wave plate, and the single shared half-wave plate is inserted into a gap separating the waveguide connecting the polarization splitter unit and the input of the first SOA and the waveguide connecting the output of the second SOA and the polarization multiplexer unit.

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