Optical Devices and Apparatus

Abstract

To obtain an optical device and an optical unit which are novel and have further been improved so as, for example, to be constructed in smaller size.SOLUTION: The optical device comprises: a coupler that includes a first port, a second port separate from the first port in a first direction, a third port separate from the second port in a second direction intersecting the first direction, a fourth port separate from the first port in the second direction, and a multi-mode interference waveguide for guiding light inputted from the first port to the third port and guiding light inputted from the second port to the fourth port; a first waveguide for guiding light inputted to the coupler via the first port; a second waveguide for guiding light inputted to the coupler via the second port; a third waveguide for guiding light outputted from the coupler via the third port; and a fourth waveguide for guiding light outputted from the coupler via the fourth port.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an optical device and an optical apparatus. [Background technology]

[0002] Conventionally, an optical device having a multimode interference lens is known as an optical device having a waveguide (for example, Patent Document 1). In Patent Document 1, the multimode interference lens is provided at a portion where two waveguides intersect. The intersection angle of the two waveguides is set to 80° to 100° from the viewpoint of transmission quality. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] US Patent Publication No. 2005 / 0036737 Summary of the Invention [Problem to be solved by the invention]

[0004] However, in the technique of Patent Document 1, as described above, the crossing angle between the two waveguides is relatively large, ranging from 80° to 100°, which may make it difficult to miniaturize the optical device.

[0005] SUMMARY OF THE PRESENT EMBODIMENT It is therefore an object of the present invention to provide new and improved optical devices and optical arrangements, for example, that can be made smaller. [Means for solving the problem]

[0006] An optical device of the present invention includes, for example, a coupler having a first port, a second port separated from the first port in a first direction, a third port separated from the second port in a second direction intersecting the first direction, a fourth port separated from the first port in the second direction, and a multi-mode interference waveguide that guides light input from the first port to the third port and guides light input from the second port to the fourth port; a first waveguide optically connected to the first port and guides light input to the coupler via the first port, a second waveguide optically connected to the second port and guides light input to the coupler via the second port, a third waveguide optically connected to the third port and guides light output from the coupler via the third port, and a fourth waveguide optically connected to the fourth port and guides light output from the coupler via the fourth port.

[0007] In the optical device, when the length of the multimode interference waveguide in the first direction is L, the width of the multimode interference waveguide in the second direction is W, the effective refractive index of the multimode interference waveguide is n, the wavelength of light propagated in the multimode interference waveguide is λ, and m is a natural number equal to or greater than 1, the following formula (1) is satisfied: L≦4 n W 2 (2m-1) / λ (1) may also be true.

[0008] In the optical device, the multimode interference waveguide may guide 80% or more of the light from the first waveguide to the third waveguide, and guide 80% or more of the light from the second waveguide to the fourth waveguide.

[0009] The optical device may include a temperature adjustment mechanism that adjusts the temperature of the multimode interference waveguide.

[0010] In the optical device, the temperature adjustment mechanism may be an electric heater.

[0011] The optical device may have a first end in the first direction and a second end in a direction opposite to the first direction, the first waveguide extending between the second end and the first port, the second waveguide extending between the first end and the second port, the third waveguide extending between the first end and the third port, and the fourth waveguide extending between the second end and the fourth port.

[0012] The optical device may have a first end in the first direction, the first waveguide having a first curved portion bent in a generally U-shape and extending between the first end and the first port, the second waveguide extending between the first end and the second port, the third waveguide extending between the first end and the third port, and the fourth waveguide having a second curved portion bent in a generally U-shape and extending between the first end and the fourth port.

[0013] In the optical device, a first optical path length of light guided in the first waveguide, the multimode interference waveguide, and the third waveguide, and a second optical path length of light guided in the second waveguide, the multimode interference waveguide, and the fourth waveguide may be approximately the same.

[0014] In the optical device, a first signal waveguide including the first waveguide, the multimode interference waveguide, and the third waveguide, and a second signal waveguide including the second waveguide, the multimode interference waveguide, and the fourth waveguide may each guide components of a modulated signal having different phases from each other.

[0015] In the optical device, the multi-mode interference waveguide may be formed by a semiconductor mesa.

[0016] In the optical device, at least one of the first waveguide, the second waveguide, the third waveguide, and the fourth waveguide may include an amplifier section that amplifies light, and the multimode interference waveguide may be provided away from the amplifier section.

[0017] An optical apparatus of the present invention includes, for example, the optical device and an optical component optically connected to the optical device.

[0018] The optical device may include a modulator as the optical component. Effect of the Invention

[0019] According to the present invention, novel and improved optical devices and optical apparatus can be obtained. [Brief description of the drawings]

[0020] [Figure 1] FIG. 1 is an illustrative schematic plan view of the optical device according to the first embodiment. [Diagram 2] FIG. 2 is an illustrative schematic plan view of a multi-mode interference waveguide included in the optical device of the first embodiment. [Diagram 3] FIG. 3 is an illustrative schematic plan view of the optical device according to the second embodiment. [Figure 4] FIG. 4 is an illustrative schematic plan view of the multi-mode interference waveguide according to the third embodiment. [Diagram 5] FIG. 5 is an illustrative schematic plan view of a multi-mode interference waveguide according to the fourth embodiment. [Figure 6] FIG. 6 is an illustrative schematic plan view of the optical device according to the fifth embodiment. [Figure 7] FIG. 7 is an illustrative schematic plan view of the optical device according to the sixth embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] Exemplary embodiments of the present invention are disclosed below. The configurations of the embodiments shown below, and the actions and results (effects) brought about by the configurations are merely examples. The present invention can be realized by configurations other than those disclosed in the following embodiments. Furthermore, according to the present invention, it is possible to obtain at least one of the various effects (including derivative effects) obtained by the configurations.

[0022] The following embodiments have similar configurations. Therefore, according to the configurations of the respective embodiments, similar actions and effects based on the similar configurations can be obtained. In the following, the similar configurations are given the same reference numerals, and duplicated descriptions may be omitted.

[0023] In this specification, ordinal numbers are given for the sake of convenience to distinguish parts, directions, and the like, and do not indicate priority or order.

[0024] In each drawing, the X direction is represented by an arrow X, the Y direction is represented by an arrow Y, and the Z direction is represented by an arrow Z. The X direction, Y direction, and Z direction intersect with each other and are perpendicular to each other.

[0025] [First embodiment] [Overall configuration of optical device] 1 is a plan view of an optical device 100A (100) according to a first embodiment. The optical device 100A is formed on a semiconductor substrate by a known semiconductor process. The semiconductor substrate may be formed, for example, mainly of a III-V group semiconductor material.

[0026] 1, the optical device 100A has a rectangular appearance in which the length in the X direction is longer than the length (width) in the Y direction when viewed in a plan view, i.e., when viewed in the direction opposite to the Z direction. The optical device 100A has a predetermined thickness in the Z direction and has an overall flattened rectangular parallelepiped shape.

[0027] The optical device 100A also includes two transmission paths 20 (20I, 20Q) as transmission paths for transmitting signals. The transmission path 20I transmits an I signal, which is an optical signal, and the transmission path 20Q transmits a Q signal, which is an optical signal. The two transmission paths 20 are provided at a predetermined position in the Z direction within the optical device 100A, approximately along a virtual plane intersecting with the Z direction. The two transmission paths 20I, 20Q intersect at the coupler 10A (10). In other words, the coupler 10A is shared by the two transmission paths 20I, 20Q.

[0028] The I signal and the Q signal are an example of signals having different phases. The phase difference between the I signal and the Q signal is 90°. The I signal is an in-phase component of a complex modulated signal, and the Q signal is a quadrature component of the complex modulated signal.

[0029] The transmission path 20I is a path from the input port 21i to the output port 21o, and includes the waveguides 21a, 21b, 21c, and 21d, the coupler 10A, and the waveguides 21e and 21f. The waveguides 21a to 21f are each a section that constitutes the transmission path 20I. The input port 21i is an example of a first input port, and the output port 21o is an example of a first output port.

[0030] Moreover, the transmission path 20Q is a path from the input port 22i to the output port 22o, and includes the waveguides 22a, 22b, 22c, and 22d, the coupler 10A, and the waveguides 22e and 22f. The waveguides 22a to 22f are each a section that constitutes a waveguide path. The input port 22i is an example of a second input port, and the output port 22o is an example of a second output port. The arrangement of the transmission paths 20I and 20Q is not limited to the example in FIG. 1. For example, the transmission path 20I may be provided at the position of the transmission path 20Q in FIG. 1, and the transmission path 20Q may be provided at the position of the transmission path 20I in FIG. 1.

[0031] The waveguides 21a, 22a, 21f, and 22f are active waveguides and have a buried waveguide structure with an active layer (not shown). The active layer has a multiple quantum well (MQW) structure made of, for example, a GaInAsP-based semiconductor material or an AlGaInAs-based semiconductor material.

[0032] An electrode 23 is provided corresponding to each of the waveguides 21a, 22a, 21f, and 22f. By applying a predetermined potential difference to the electrode 23 with respect to other electrodes (not shown) and injecting a current into an active region (not shown) of the active layer, an optical amplification effect occurs, whereby the I signal or Q signal transmitted through the waveguides 21a, 22a, 21f, and 22f is amplified. That is, in this embodiment, the electrode 23 and the active layer configure an optical amplification section. That is, the optical device 100A is an example of a semiconductor optical amplifier, and may also be referred to as an array-type semiconductor optical amplifier element.

[0033] Moreover, the waveguides 21a, 22a, 21f, and 22f are curved in the opposite direction to the Y direction in the vicinity of the end face 100a in the X direction. This prevents light reflected at the input ports 21i and 22i and the output ports 21o and 22o, i.e., the end face 100a of the optical device 100A, from returning along the transmission path. In addition, the end face 100a is coated with an anti-reflection (AR) coating to prevent reflection. Except for the curved portion in the vicinity of the end face 100a, the waveguides 21a, 22a, 21f, and 22f each extend linearly substantially along the X direction and are substantially parallel to each other. The waveguides 21a, 22a, 21f, and 22f are arranged in the Y direction at a predetermined interval, for example, a constant interval, in the Y direction.

[0034] On the other hand, the waveguides 21b, 21c, 21d, 21e, 22b, 22c, 22d, and 22e are passive waveguides and have a high mesa type waveguide structure, i.e., a semiconductor mesa structure, having a cladding layer and an optical waveguide (not shown). The cladding layer of the waveguide is made of, for example, n-type InP or i-type InP, and the core layer is made of, for example, an i-type GaInAsP-based semiconductor material with a band gap wavelength of 1300 [nm]. The high mesa type waveguide structure may also be called a deep ridge waveguide structure.

[0035] The waveguides 21b, 22b, 21e, and 22e each extend substantially along the X direction. The waveguides 21b, 22b, 21e, and 22e are arranged in the Y direction at predetermined intervals, for example, at constant intervals.

[0036] The waveguides 21d and 22c each extend substantially along the X direction. Furthermore, the waveguides 21d and 22c are arranged side by side in the Y direction at a predetermined interval, for example a constant interval, in the Y direction.

[0037] The waveguides 21d and 22b are spaced apart from each other in the X direction and are aligned in the X direction. Moreover, the waveguides 22c and 21e are spaced apart from each other in the X direction and are aligned in the X direction.

[0038] Waveguide 21c connects ends of linear waveguides 21b and 21d that are opposite to each other in the X direction, and is curved in an arc or U-shape. Waveguides 21b, 21c, and 21d form a waveguide curved in a substantially U-shape.

[0039] Waveguide 22d connects ends of linear waveguides 22c and 22e that are opposite to each other in the X direction, and is curved in an arc shape or a U-shape. Waveguides 22c, 22d, and 22e form a waveguide curved in a substantially U-shape.

[0040] The active waveguides 21a, 22a, 21f, 22f and the passive waveguides 21b, 22b, 21e, 22e are optically connected to each other by, for example, butt joint connection, etc. A conversion region may be provided between the active waveguides 21a, 22a, 21f, 22f and the passive waveguides 21b, 22b, 21e, 22e to optically connect the waveguides having different structures with low loss.

[0041] [Coupler] Fig. 2 is a plan view of the coupler 10 A. As shown in Fig. 2, the coupler 10 A has a multi-mode interference waveguide 11 and four ports 12a to 12d.

[0042] The multimode interference waveguide 11 has a quadrangular shape in plan view, and in this embodiment, has a rectangular shape with a length L in the X direction greater than a width W in the Y direction. The multimode interference waveguide 11 has a substantially constant thickness in the Z direction, and has a flat rectangular parallelepiped shape overall.

[0043] The ports 12a to 12d are optically connected to the four corners of the multimode interference waveguide 11 in a plan view. The port 12a is connected to a corner located at an end of the multimode interference waveguide 11 in the opposite direction in the X direction and the opposite direction in the Y direction. The port 12b is connected to a corner located at an end of the multimode interference waveguide 11 in the opposite direction in the X direction and the Y direction. The port 12c is connected to a corner located at an end of the multimode interference waveguide 11 in the X direction and the Y direction. The port 12d is connected to a corner located at an end of the multimode interference waveguide 11 in the opposite direction in the X direction and the Y direction. The port 12b is separated from the port 12a in the X direction, the port 12c is separated from the port 12b in the Y direction, and the port 12d is separated from the port 12a in the Y direction. The ports 12a and 12b are aligned in the X direction, and the ports 12c and 12d are aligned in the X direction. The ports 12b and 12c are arranged in the Y direction, and the ports 12a and 12d are arranged in the Y direction. The distance between the ports 12a and 12b in the X direction is approximately the same as the distance between the ports 12c and 12d in the X direction, and the distance between the ports 12b and 12c in the Y direction is approximately the same as the distance between the ports 12a and 12d in the Y direction. These ports 12a to 12d have approximately the same shape and extend in the X direction with a width Wp in the Y direction. The gap Wg between the ports 12b and 12c adjacent to each other in the Y direction is approximately the same as the gap between the ports 12a and 12d. The port 12a is an example of a first port, the port 12b is an example of a second port, the port 12c is an example of a third port, and the port 12d is an example of a fourth port.

[0044] In the multimode interference waveguide 11 of the form shown in FIG. 2, it has been found that, when the following formula (2) is satisfied, an I signal from port 12a can be guided to port 12c, and a Q signal from port 12b can be guided to port 12d. L=4 n W 2 (2m-1) / λ (2) Here, n is the effective refractive index of the multimode interference waveguide 11, and m is a natural number of 1 or more.

[0045] By setting or adjusting the specifications of each part, the ratio of the I signal from the port 12a to the port 12c and the ratio of the Q signal from the port 12b to the port 12d of the multimode interference waveguide 11 can be changed to a desired value of 50% or more. In this embodiment, the ratio is preferably close to 100% and is, for example, 80% or more, and preferably 95% or more.

[0046] As an example, when λ=1.55 [μm] and n=3.2, it has been confirmed that the specifications of W=2.9 [μm], Wp=1.2 [μm], Wg=0.5 [μm], and L=70 [μm] satisfy formula (2).

[0047] Here, as shown in FIG. 1, the waveguides 21a to 21d, which are upstream of the coupler 10A in the transmission path 20I of the I signal, are optically connected to the port 12a of the coupler 10A. That is, the waveguides 21a to 21d guide the I signal input to the input port 21i and input to the coupler 10A via the port 12a. In this embodiment, the waveguide 21d immediately before the port 12a is adjacent to the port 12a in the opposite direction to the X direction and extends in the X direction, and guides the I signal in the X direction. The waveguides 21a to 21d are an example of a first waveguide. The waveguide 21c or the waveguides 21b, 21c, and 21d are an example of a first curved section.

[0048] The port 12b of the coupler 10A is optically connected to the waveguides 22a and 22b that are upstream of the coupler 10A in the transmission path 20Q of the Q signal. That is, the waveguides 22a and 22b guide the Q signal that is input to the input port 22i and is input to the coupler 10A via the port 12b. In this embodiment, the waveguide 22b immediately before the port 12b is adjacent to the port 12b in the X direction and extends in the X direction, and guides the Q signal in the opposite direction to the X direction. The waveguides 22a and 22b are an example of a second waveguide.

[0049] The waveguides 21e and 21f, which are downstream of the coupler 10A in the transmission path 20I of the I signal, are optically connected to the port 12c of the coupler 10A. The waveguides 21e and 21f guide the I signal between the port 12c and the output port 21o. That is, the waveguides 21e and 21f guide the I signal, which is output from the coupler 10A via the port 12c and is output from the output port 21o. In this embodiment, the waveguide 21e immediately after the port 12c is adjacent to the port 12c in the X direction and extends in the X direction, and guides the I signal in the X direction. The waveguides 21e and 21f are an example of a third waveguide.

[0050] The port 12d of the coupler 10A is optically connected to the waveguides 22c to 22f that are downstream of the coupler 10A in the transmission path 20Q of the Q signal. That is, the waveguides 22c to 22f guide the Q signal that is output from the coupler 10A via the port 12d and is output from the output port 22o. In this embodiment, the waveguide 22c immediately after the port 12d is adjacent to the port 12d in the opposite direction to the X direction and extends in the X direction, and guides the Q signal in the opposite direction to the X direction. The waveguides 22c to 22f are an example of a fourth waveguide. The waveguide 22d or the waveguides 22c, 22d, and 22e are an example of a second curved section.

[0051] As described above, in this embodiment, the coupler 10A having the multimode interference waveguide 11 is provided. If the coupler 10A had a multimode interference lens as in the conventional technology, instead of the multimode interference waveguide 11, it would be necessary to set the angle between the imaginary line connecting the ports 12a and 12c and the imaginary line connecting the ports 12b and 12d to 80° to 100°, and the coupler 10A would be accordingly enlarged in the Y direction. In this respect, in this embodiment, the coupler 10A has the multimode interference waveguide 11, and the intervals between the ports 12a and 12d and between the ports 12b and 12c, that is, the width W in the Y direction of the multimode interference waveguide 11 (coupler 10A), can be narrower than in the case of a multimode interference lens. Therefore, according to this embodiment, the coupler 10A and the optical device 100A can be configured to be smaller.

[0052] Furthermore, if the coupler 10A has a directional coupler instead of the multimode interference waveguide 11, the directional coupler has a greater wavelength dependency than the multimode interference waveguide 11, and so the available wavelength band may be limited. Furthermore, the directional coupler requires a smaller gap between the input and output waveguides than the multimode interference waveguide 11, and so there is a concern that the manufacturing process may be difficult or limited. Therefore, according to this embodiment, it is possible to obtain a coupler 10A, and thus an optical device 100A, that has less wavelength dependency and can be manufactured more easily.

[0053] In addition, in this embodiment, in the coupler 10A having the multimode interference waveguide 11, the transmission path 20I and the transmission path 20Q cross each other in the optical device 100A, so that the input ports 21i and 22i of the different transmission paths 20I and 20Q can be arranged side by side and the output ports 21o and 22o of the different transmission paths 20I and 20Q can be arranged side by side. Therefore, the transmission path (hereinafter referred to as the external input transmission path) optically connecting the input ports 21i and 22i to an optical component other than the optical device 100A and the transmission path (hereinafter referred to as the external output transmission path) connecting the output ports 21o and 22o to an optical component other than the optical device 100A can be arranged outside the optical device 100A without crossing each other three-dimensionally.

[0054] Here, it is assumed that the coupler 10A is not present, and the transmission path 20I, which is U-shaped in plan view, and the transmission path 20Q, which is also U-shaped in plan view, are arranged in the Y direction without intersecting with each other within the optical device 100. In this case, the input port 21i and the output port 21o of the transmission path 20I are arranged side by side, and the input port 22i and the output port 22o of the transmission path 20Q are arranged side by side. In such a layout, it may happen that an external input transmission path connected to one of the input ports 21i and 22i and an external output transmission path connected to one of the output ports 21o and 22o must intersect with each other three-dimensionally outside the optical device 100A. In this case, the configuration of the optical device including the optical device 100A and the external input transmission path and output transmission path may become more complicated.

[0055] In this regard, according to the present embodiment, as described above, the input ports 21i, 22i of different transmission paths 20 can be arranged side by side in a group, and the output ports 21o, 22o of different transmission paths 20 can be arranged side by side in a group. Therefore, according to the present embodiment, for example, in the optical device 100A and in an optical device including the optical device 100A, it becomes easy to optically connect the input ports 21i, 22i to other optical components, and it becomes easy to optically connect the output ports 21o, 22o to other optical components.

[0056] In addition, in this embodiment, by providing the coupler 10A having the multimode interference waveguide 11, it is possible to set the optical path lengths of the transmission paths 20I, 20Q, in which the input ports 21i, 22i and the output ports 21o, 22o are arranged together, to be approximately the same. Therefore, according to this embodiment, for example, it is possible to suppress an increase in the loss difference, i.e., the difference in amplitude or intensity, between the I signal and the Q signal, or a deviation of the phase difference between the I signal and the Q signal from a set value, depending on the difference in optical path length between the multiple transmission paths 20I, 20Q.

[0057] In this embodiment, the coupler 10A having the multimode interference waveguide 11 is provided in a portion configured as a high mesa (semiconductor mesa) and is provided away from the amplifier including the electrode 23 and the active layer. If the multimode interference waveguide 11 is provided in the active waveguides 21a, 22a, 21f, and 22f of a buried structure in which the amplifier is provided, the optical confinement is lower than that of a passive waveguide structure configured as a high mesa. For this reason, in order to prevent interference between signals, it becomes necessary to increase the Y-directional interval between the waveguides connected to the multimode interference waveguide 11. As a result, the Y-directional width W of the coupler 10A becomes larger, and accordingly the X-directional length L of the coupler 10A also becomes larger, and thus the optical device 100A becomes larger in the X-directional and Y-directional directions. In this regard, as in the present embodiment, the coupler 10A having the multimode interference waveguide 11 is provided in the high mesa and is provided away from the amplification section including the electrode 23 and the active layer, so that, for example, the increase in size of the coupler 10A and therefore the optical device 100A can be suppressed, while the various effects described above by providing the coupler 10A having the multimode interference waveguide 11 can be obtained.

[0058] In this embodiment, the input port 21i, the input port 22i, the output port 21o, and the output port 22o are provided on the end face 100a in the X direction (first direction) of the optical device 100A. With this configuration, for example, it is not necessary to provide a plurality of end faces 100a with high cleavage accuracy, and therefore the optical device 100A can be manufactured more inexpensively.

[0059] [Second embodiment] 3 is a plan view of an optical device 100B (100) of the second embodiment. The optical device 100B of this embodiment differs from the optical device 100A of the first embodiment in that it includes a temperature adjustment mechanism 24 that adjusts the temperature of the multi-mode interference waveguide 11 (coupler 10A). Except for this point, the optical device 100B has the same configuration as the optical device 100A.

[0060] The temperature adjustment mechanism 24 is, for example, a heater arranged to overlap the multimode interference waveguide 11 in the Z direction, and is, for example, an electric heater that generates heat by electrical resistance to power supplied via the wirings 24a and 24b. In this case, the heater of the temperature adjustment mechanism 24 is made of, for example, a nickel-chromium alloy. The wirings 24a and 24b are made of, for example, titanium, platinum, gold, or an alloy containing these metals. In addition, a protective film is interposed between the multimode interference waveguide 11 and the heater and wirings 24a and 24b of the temperature adjustment mechanism 24. The protective film is made of a material, such as silicon nitride, that has a lower refractive index than the core and clad of the multimode interference waveguide 11.

[0061] According to this embodiment, the temperature of the multimode interference waveguide 11 is changed by the temperature adjustment mechanism 24, thereby changing the effective refractive index n of the multimode interference waveguide 11, and thus the guiding characteristics and branching characteristics of the multimode interference waveguide 11 can be changed. In the multimode interference waveguide 11, there is a risk that the desired guiding characteristics and branching characteristics cannot be obtained due to an error in the width W and length L of the multimode interference waveguide 11 caused by manufacturing variations, or the effective refractive index n of the multimode interference waveguide 11 changes according to temperature changes. Even in such a case, according to this embodiment, the guiding characteristics and branching characteristics can be corrected by the temperature adjustment mechanism 24 so as to approach the desired characteristics. Therefore, according to this embodiment, the desired guiding characteristics and branching characteristics of the coupler 10A having the multimode interference waveguide 11 can be obtained more easily than in a configuration without the temperature adjustment mechanism 24, and the transmission quality of the light (signal) can be further improved. The temperature adjustment mechanism 24 is not limited to an electric heater, and may be, for example, a thermoelectric cooler having a thermoelectric element (Peltier element). In this case, the thermoelectric cooler may adjust the temperature of the optical device 100B as a whole, or may adjust the temperature of the multi-mode interference waveguide 11 locally.

[0062] [Third embodiment] 4 is a plan view of a coupler 10C (10) of the third embodiment. The coupler 10C of this embodiment differs from the coupler 10A of the first embodiment in that it has a multi-mode interference waveguide 11 having a different shape in a plan view.

[0063] As shown in FIG. 4, in this embodiment, the multimode interference waveguide 11 has tapered portions 11t1 and 11t2 and a constant width portion 11s.

[0064] The tapered portion 11t1 is located on the side of the ports 12b and 12c and has a substantially isosceles trapezoidal shape in a plan view. That is, the width of the tapered portion 11t1 in the Y direction gradually decreases in the opposite direction to the X direction, that is, toward the ports 12a and 12d.

[0065] The constant width portion 11s is located on the opposite side of the tapered portion 11t1 in the X direction, i.e., adjacent to the ports 12a and 12d. The constant width portion 11s has a rectangular shape in a plan view. That is, the width of the constant width portion 11s in the Y direction is constant.

[0066] The tapered portion 11t2 is located on the port 12a, 12d side and has a substantially isosceles trapezoidal shape in a plan view. That is, the width of the tapered portion 11t2 in the Y direction gradually decreases toward the X direction, i.e., toward the ports 12b, 12d.

[0067] The width of the constant width portion 11s is approximately the same as the width (minimum width) of the narrowest portion of the tapered portions 11t1 and 11t2. The lengths Lt1 and Lt2 of the tapered portions 11t1 and 11t2 in the X direction are approximately the same. The length of the constant width portion 11s in the X direction is denoted as Ls.

[0068] The tapered portions 11t1, 11t2 and the constant width portion 11s are narrow portions whose average width is narrower than the widths at the ends 11e1, 11e2 in the X direction. The difference between the width W of the multimode interference waveguide 11 and the minimum width is A. In this case, the positions of the ends in the X direction and the opposite direction to the X direction of the constant width portion 11s, i.e., the ends in the width direction, are located A / 2 inward from the ends in the width direction of the maximum width portion. In this case, the average Y direction width of the tapered portions 11t1, 11t2 is (WA / 2).

[0069] The width (WA) of the constant width portion 11s is, for example, 25% or more and less than 100% of W. Moreover, the length Ls of the constant width portion 11s is, for example, 10% or more and less than 100% of the length L of the multimode interference waveguide 11.

[0070] By virtue of having such a shape, the coupler 10C having the multimode interference waveguide 11 has a high degree of freedom in designing the optical characteristics and has reduced excess loss.

[0071] As a result of the inventors' intensive research, it has been found that by appropriately adjusting the shape of the multimode interference waveguide 11 using a BPM (beam propagation method) analysis or the like, it is possible to obtain the following formula (3) in the shape of FIG. L<4 n W 2 (2m-1) / λ (3) It has been found that when the above-mentioned condition is satisfied, the I signal from port 12a can be guided to port 12c at a ratio of at least 50%, and the Q signal from port 12b can be guided to port 12d at a ratio of at least 50%.

[0072] As will be apparent from a comparison with formula (2) described in the first embodiment, this means that the length of the multimode interference waveguide 11 in the X direction can be made shorter.

[0073] That is, according to this embodiment, by using the multimode interference waveguide 11 having a configuration that satisfies formula (3), the coupler 10C having the multimode interference waveguide 11, and further the optical device 100 including the coupler 10C, can be made smaller.

[0074] In this embodiment as well, the ratio of the I signal from port 12a to port 12c and the ratio of the Q signal from port 12b to port 12d of the multimode interference waveguide 11 can be changed to a desired value of 50% or more by setting or adjusting the specifications of each part. Also in this embodiment, it is desirable that the ratio is close to 100% and is, for example, 80% or more, and preferably 95% or more.

[0075] In addition, from the first and third embodiments, the following formula (1) is obtained by combining formula (2) and formula (3): L≦4 n W 2 (2m-1) / λ (1) It is clear that by configuring in such a way that the I signal from port 12a can be guided to port 12c at a ratio of at least 50%, and the Q signal from port 12b can be guided to port 12d at a ratio of at least 50%.

[0076] [Fourth embodiment] 5 is a plan view of a coupler 10D(10) of the fourth embodiment. The coupler 10D of this embodiment differs from the coupler 10A of the first embodiment and the coupler 10C of the third embodiment in that the coupler 10D of this embodiment has a multi-mode interference waveguide 11 having a different shape in a plan view.

[0077] As shown in FIG. 5, in this embodiment, the side edges 11a on both sides of the multimode interference waveguide 11 in the Y direction are provided with an uneven shape including at least one of a recess and a protrusion. As in the third embodiment, the ratio of the I signal from the port 12a of the multimode interference waveguide 11 guided to the port 12c and the ratio of the Q signal from the port 12b guided to the port 12d can be changed to a desired value of 50[%] or more by appropriately adjusting the uneven shape of the side edges 11a using the BPM analysis, that is, by designing using the topology optimization method. Note that, in this embodiment, the ratio is preferably close to 100[%], for example, 80[%] or more, and preferably 95[%] or more. Note that FIG. 5 shows the Y direction enlarged more than the X direction. FIG. 5 shows a configuration example in the case of L=90[μm], W=3.1[μm], Wp=1.2[μm], and Wg=0.7[μm].

[0078] According to this embodiment, by using the multimode interference waveguide 11 having an uneven shape including at least one of a recess and a protrusion on the side edge 11a, the coupler 10C including the multimode interference waveguide 11, and further the optical device 100 including the coupler 10C, can be made smaller. Also, in the coupler 10D designed by the topology optimization method, there is an advantage that excess loss can be suppressed to a small value.

[0079] [Fifth embodiment] 6 is a plan view of an optical device 100E (100) according to a fifth embodiment. The optical device 100E according to this embodiment differs from the optical device 100A according to the first embodiment in that neither an I signal transmission path 20I nor a Q signal transmission path 20Q has a curved portion.

[0080] 6, in this embodiment, the input port 21i of the transmission path 20I is provided on the end face 100b on the opposite side in the X direction of the optical device 100E. The active waveguide 21a between the input port 21i and the passive waveguide 21d is located adjacent to the waveguide 21d in the opposite direction in the X direction.

[0081] The output port 22o of the transmission path 20Q is also provided on the end face 100b. The active waveguide 22f between the output port 22o and the passive waveguide 22c is located adjacent to the waveguide 22c in the opposite direction to the X direction.

[0082] The waveguides 21a and 22f are curved toward the Y direction as they move toward the opposite direction of the X direction near the end face 100b. This prevents light reflected at the input ports 21i and 22i, i.e., the end face 100b of the optical device 100A, from returning along the transmission path. In addition, the end face 100b is coated with an anti-reflection (AR) coating to prevent reflection. Except for the curved portion near the end face 100b, the waveguides 21a and 22f each extend linearly approximately along the X direction and are approximately parallel to each other. The waveguides 21a and 22f are arranged in the Y direction at a predetermined interval, for example, a constant interval. In addition, the linear portion of the waveguide 21a is arranged in the X direction with the waveguide 21d, and the linear portion of the waveguide 22f is arranged in the X direction with the waveguide 22c.

[0083] Further, electrodes 23 are provided corresponding to the waveguides 21a and 22f, respectively. In the present embodiment, similarly to the first embodiment, an optical amplification section is configured by the electrodes 23 and an active layer (not shown).

[0084] The optical device 100E of this embodiment also has a coupler 10A having a multi-mode interference waveguide 11. Therefore, this embodiment also has the same effect as the first embodiment by having the coupler 10A having the multi-mode interference waveguide 11.

[0085] Furthermore, according to this embodiment, since the optical device 100E does not have a U-shaped waveguide, the width in the Y direction can be made narrower accordingly.

[0086] [Sixth embodiment] 7 is a plan view of an optical device 1000 according to a sixth embodiment. The optical device 1000 according to this embodiment includes a light source 200, a modulator 300, an optical device 100F (100), and an optical device 100A (100).

[0087] The light source 200 outputs continuous waves C1 and C2. The light source 200 is, for example, a tunable laser. The continuous waves C1 and C2 output from the light source 200 are input to the modulator 300 via the transmission paths 20C1 and 20C2 (20) of the optical device 100F, respectively. The optical device 100F has a similar configuration to the optical device 100A. The substantial optical path length of the transmission paths 20C1 and 20C2 is about 4200 [μm].

[0088] The modulator 300 is, for example, a dual polarization in-phase quadrature (DP-IQ) modulator. The modulator 300 is also an InP modulator. The modulator 300 modulates continuous waves C1 and C2 to output an IQ signal. The IQ signals output from the modulator 300 are output from an output port 1000o of the optical device 1000 via transmission paths 20I and 20Q (20) of the optical device 100A. The substantial optical path length of the transmission paths 20I and 20Q is about 3200 [μm].

[0089] This embodiment also provides the same effect as the first embodiment. That is, the transmission paths of the I signal, the Q signal, and the continuous waves C1 and C2 between the optical devices 100A and 100F and the modulator 300, the transmission paths of the continuous waves C1 and C2 between the light source 200 and the optical device 100F, and the transmission paths between the optical device 100A and the output port 1000o can be arranged without crossing each other three-dimensionally. In addition, in the optical device 100A, the optical path length of the transmission path 20I of the I signal and the optical path length of the transmission path 20Q of the Q signal can be set to be approximately the same, and in the optical device 100F, the optical path length of the transmission path 20C1 of the continuous wave C1 and the optical path length of the transmission path 20C2 of the continuous wave C2 can be set to be approximately the same.

[0090] Although the embodiment of the present invention has been illustrated above, the above embodiment is merely an example and is not intended to limit the scope of the invention. The above embodiment can be implemented in various other forms, and various omissions, substitutions, combinations, and modifications can be made without departing from the gist of the invention. In addition, the specifications of each configuration, shape, and the like (structure, type, direction, model, size, length, width, thickness, height, number, arrangement, position, material, etc.) can be appropriately changed and implemented. [Explanation of symbols]

[0091] 10, 10A, 10C, 10D...Coupler 11...Multimode interference waveguide 11a…Side edge 11e1,11e2...end 11s…equal width part 11t1, 11t2...Tapered section 12a…Port (first port) 12b…Port (second port) 12c…Port (third port) 12d…Port (fourth port) 20, 20C1, 20C2, 20I, 20Q...Transmission path 21a...Waveguide (first waveguide) 21b~21d...Waveguide (first waveguide, first curved part) 21e, 21f...Waveguide (third waveguide) 21i…Input port (first input port) 21o…Output port (first output port) 22a, 22b...Waveguide (second waveguide) 22f...Waveguide (fourth waveguide) 22c~22e...Waveguide (fourth waveguide, second curved part) 22i…Input port (second input port) 22o…Output port (second output port) 23...Electrode 24…Temperature adjustment mechanism 24a, 24b…Wiring 100, 100A, 100B, 100E, 100F...Optical devices 100a...End face (first end) 100b...End face (second end) 200...Light source 300…Modulator 1000...Optical device 1000o…Output port C1, C2...Continuous wave I…Signal Q…signal X…direction (first direction) Y…direction (second direction) Z…direction

Claims

1. A first end in a first direction, the first end being provided with a first input port into which light is input, a second input port into which light is input from the first input port in a second direction intersecting the first direction, a first output port from which light is output from the second input port in the second direction, and a second output port from which light is output from the first output port in the second direction; a coupler having a high mesa structure, the coupler including: a first port for inputting light in the first direction; a second port for inputting light in the first direction away from the first port in a direction opposite to the first direction; a third port for outputting light in the first direction away from the second port in the second direction; a fourth port for outputting light in the first direction away from the first port in the second direction in the direction opposite to the first direction; and a multimode interference waveguide for guiding light from the first port to the third port and for guiding light from the second port to the fourth port; a first waveguide having a high mesa structure, the first waveguide being adjacent to and optically connected to the first port, the first waveguide having a curved portion convex in a direction opposite to the first direction, and guiding light input from the first input port and input to the coupler via the first port; a second waveguide having a high mesa structure adjacent to and optically connected to the second port, guiding light input from the second input port and input to the coupler via the second port; a third waveguide having a high mesa structure adjacent to and optically connected to the third port, which guides light output from the coupler via the third port and output from the first output port; a fourth waveguide having a high mesa structure that is adjacent to and optically connected to the fourth port, has a curved portion that is convex in a direction opposite to the first direction, and guides light that is output from the coupler via the fourth port and output from the second output port; Equipped with a first optical path length of light guided in the first waveguide, the multimode interference waveguide, and the third waveguide, and a second optical path length of light guided in the second waveguide, the multimode interference waveguide, and the fourth waveguide are substantially equal to each other.

2. When the length of the multimode interference waveguide in the first direction is L, the width of the multimode interference waveguide in the second direction is W, the effective refractive index of the multimode interference waveguide is n, the wavelength of light propagated in the multimode interference waveguide is λ, and m is a natural number equal to or greater than 1, the following formula (1) is satisfied: L≦4・n・W ２ ・(2m-1) / λ ・・・(1) The optical device according to claim 1 , wherein

3. 3. The optical device according to claim 1, wherein the multimode interference waveguide guides 80% or more of light from the first waveguide to the third waveguide and guides 80% or more of light from the second waveguide to the fourth waveguide.

4. 4. The optical device according to claim 1, further comprising a temperature adjustment mechanism for adjusting a temperature of the multimode interference waveguide.

5. The optical device according to claim 4 , wherein the temperature adjustment mechanism is an electric heater.

6. 6. The optical device according to claim 1, wherein a first signal waveguide including the first waveguide, the multimode interference waveguide, and the third waveguide, and a second signal waveguide including the second waveguide, the multimode interference waveguide, and the fourth waveguide, respectively guide components of a modulated signal having different phases from each other.

7. an amplifier section configured to amplify light in at least one of the first waveguide, the second waveguide, the third waveguide, and the fourth waveguide; 7. The optical device according to claim 1, wherein the multi-mode interference waveguide is provided apart from the amplifier section.

8. An optical device according to any one of claims 1 to 7; an optical component optically connected to the optical device; An optical device comprising:

9. The optical device according to claim 8 , further comprising an optical modulator as the optical component.

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