阅读说明：本技术 光调制元件以及光调制模块 (Optical modulation element and optical modulation module ) 是由 中田悠 宫崎徳一 及川哲 于 2019-09-26 设计创作，主要内容包括：实现一种光调制元件,不会导致高频特性以及光调制特性的劣化,且不会导致框体尺寸的增大,而能够与电子电路一同收容到同一框体内。光调制元件包含：两个马赫-曾德尔型光波导,设在基板上；分支波导,将从基板的外部输入的输入光分支为两束；两个连接波导,将经分支波导分支的光分别导向两个马赫-曾德尔型光波导；以及电极,分别控制在构成两个马赫-曾德尔型光波导的光波导中传播的光波,两个马赫-曾德尔型光波导各自的并行波导以沿着基板的一边延伸的方式而构成,分支波导以从所述一边所处的方向输入光的方式而配设,且形成为,关于向所述分支波导输入的光的传播方向呈线对称且经分支的两束光朝向与所述传播方向不同的方向输出。(Provided is an optical modulation element which can be housed in the same housing together with an electronic circuit without causing deterioration in high-frequency characteristics and optical modulation characteristics and without increasing the size of the housing. The light modulation element includes: two Mach-Zehnder type optical waveguides arranged on the substrate; a branch waveguide that branches input light input from outside of the substrate into two beams; two connecting waveguides for guiding the branched light to two Mach-Zehnder type optical waveguides; and electrodes for controlling light waves propagating through the optical waveguides constituting the two Mach-Zehnder type optical waveguides, respectively, wherein the parallel waveguides of the two Mach-Zehnder type optical waveguides are configured to extend along one side of the substrate, and the branch waveguides are arranged to input light from a direction in which the one side is located, and are formed so as to be line-symmetric with respect to a propagation direction of the light input to the branch waveguides, and so that the two branched light beams are output in a direction different from the propagation direction.)

1. A light modulation element, comprising: two Mach-Zehnder type optical waveguides arranged on the substrate; a branch waveguide that branches input light input from outside of the substrate into two beams; two connection waveguides for guiding the light branched by the branch waveguide to the two Mach-Zehnder type optical waveguides; and electrodes for controlling the light waves propagating through the optical waveguides constituting the two Mach-Zehnder type optical waveguides, respectively, wherein

Wherein the parallel waveguides of the two Mach-Zehnder type optical waveguides are configured to extend along one side of the substrate,

the branch waveguide is arranged so that light is input from a direction in which the one side is located, and

the branch waveguide is formed so as to be line-symmetric with respect to a propagation direction of light input to the branch waveguide and so that two branched lights are output in a direction different from the propagation direction.

2. The light modulation element according to claim 1, wherein

The branch waveguide includes a Y-branch optical waveguide formed line-symmetrically with respect to a propagation direction of light input to the branch waveguide.

3. The light modulation element according to claim 1 or 2, wherein

The branched waveguides are formed such that the interval between the two branched output lights when the two branched output lights are outputted from the line-symmetrically formed portions is smaller than the interval between the respective optical input ends of the two mach-zehnder type optical waveguides.

4. The light modulation element according to any one of claims 1 to 3, wherein

The electrodes are formed so as to extend along the direction in which the two Mach-Zehnder type optical waveguides extend to the other side of the substrate different from the one side.

5. The light modulation element according to any one of claims 1 to 4, wherein

The two Mach-Zehnder type optical waveguides are nested Mach-Zehnder type optical waveguides, and the nested Mach-Zehnder type optical waveguides include other Mach-Zehnder type optical waveguides in two parallel waveguides constituting the Mach-Zehnder type optical waveguides.

6. The light modulation element according to any one of claims 1 to 5, wherein

The two connection waveguides each include a linear waveguide and a curved waveguide, and are configured such that propagation losses from the light inflow portion of the branch waveguide to the light input ends of the two mach-zehnder type optical waveguides are equal to each other.

7. The light modulation element according to any one of claims 1 to 5, wherein

The two connection waveguides are configured such that optical path lengths from the light inflow portion of the branch waveguide to the light input ends of the two mach-zehnder type optical waveguides are equal to each other.

8. The light modulation element according to any one of claims 1 to 7, wherein

The substrate is rectangular with two opposing short sides and two opposing long sides longer than the short sides,

the one side is any one of the long sides,

the propagation direction of light at the time of input to the branch waveguide is a direction along the short side.

9. A light modulation module comprising:

the light modulation element according to any one of claims 1 to 8;

an electronic circuit that drives the light modulation element; and

and a housing that houses the optical modulator and the electronic circuit.

Technical Field

The present invention relates to an optical modulator and an optical modulation module.

Background

In a high-speed and large-capacity optical fiber communication system, an optical transmitter incorporating a waveguide type optical modulator is often used. Wherein LiNbO having electro-optical effect₃An optical modulation element (hereinafter also referred to as LN) used for a substrate has less optical loss and can realize optical modulation characteristics in a wide frequency band as compared with a modulation element using a semiconductor material such as indium phosphide (InP), silicon (Si), or gallium arsenide (GaAs), and is therefore widely used in high-speed and large-capacity optical fiber communication systems.

On the other hand, the modulation scheme in the optical fiber communication system is subject to the recent trend of increasing transmission capacity, and a transmission format in which multi-value modulation such as Quadrature Phase Shift Keying (QPSK) or Dual Polarization-Quadrature Phase Shift Keying (DP-QPSK) or Polarization multiplexing is introduced into the multi-value modulation is the mainstream.

Recent further increase in communication traffic due to the rapid spread of Internet services (Internet services) has also been studied, and continuous high-speed and large-capacity optical communication systems are now being studied. On the other hand, there is a constant demand for miniaturization of devices, and in addition to miniaturization of the optical modulation element itself, there is also an effort to incorporate an electronic circuit and the optical modulation element into one housing and integrate them into an optical modulation module.

For example, an optical modulation module has been proposed which is small-sized and integrated by integrally housing an optical modulation element and a high-frequency drive amplifier for driving the optical modulation element in one housing and arranging optical input/output units in parallel on one short side of the housing.

Fig. 6 is a diagram showing an example of a conventional configuration of an optical modulation element constituting a DP-QPSK modulator used as a single body before the integration as described above. The optical modulation element 600 constituting the DP-QPSK modulator includes: an input waveguide 532 formed on the substrate 530 as an LN, and receiving light input from the left in the figure; and a branch waveguide 534 which branches the inputted light into two beams having the same light amount. The optical modulation element 600 includes two modulation sections for modulating the respective light beams branched by the branched waveguides 534, i.e., so-called nest (nest) Mach-Zehnder (Mach-Zehnder) type optical waveguides 540a and 540b (each of which is a portion surrounded by a chain line shown in the figure).

Each of the nested mach-zehnder optical waveguides 540a and 540b includes: two mach-zehnder optical waveguides 544a (shown in a broken line portion), 546b (shown in a two-dot chain line portion), 544b (shown in a two-dot chain line portion), and 546b (shown in a two-dot chain line portion) which constitute two waveguide portions of a pair of parallel waveguides are provided, respectively. Thus, the nested mach-zehnder optical waveguides 540a and 540b QPSK-modulate the light input from the modulated light input ends 542a and 542b, respectively, and output the modulated light (output) to the right in the drawing from the output waveguides 548a and 548b, respectively.

These two output lights are then combined by polarization by an optical component disposed outside the substrate 530 to be converged into one light beam, input to the optical fiber terminal via, for example, a lens, and guided to the transmission path optical fiber.

On the substrate 530, signal electrodes 550a, 552a, 550b, 552b for modulating the total of four mach-zehnder optical waveguides 544a, 546a, 544b, 546b constituting the nested mach-zehnder optical waveguides 540a, 540b are provided, respectively, on the signal electrodes 550a, 552a, 550b, 552 b. The signal electrodes 550a, 552a, 550b, 552b include, for example, two ground electrodes and one center electrode sandwiched by the two ground electrodes, respectively.

The signal electrodes 550a and 552a have both ends arranged, for example, on the left and right of the longer side of the substrate 530 on the upper side in the figure. The signal electrodes 550b and 552b have opposite ends arranged, for example, on the left and right of the longer side of the lower side of the substrate 530 in the figure. The signal electrodes 550a, 552a, 550b, 552b are terminated by termination resistors (not shown) connected to the right end arrays 554a, 554b shown in the figure. Thus, the high-frequency electric signal input from the end arranged on the left side in the figure becomes a traveling wave and propagates through the signal electrodes 550a, 552a, 550b, 552 b. For example, when the optical modulator 600 modulates a transmission rate exceeding 100Gb/s, the high-frequency electrical signal has a frequency in the microwave region.

Further, on the substrate 530, bias electrodes 556a, 558a, 556b, and 558b are further provided as necessary, and the bias electrodes 556a, 558a, 556b, and 558b adjust operating points of the nested mach-zehnder optical waveguides 540a and 540 b.

Although the conventional DP-QPSK modulator having the above-described structure can function well as a single body, there is still room for improvement when integrating with the electronic circuit as described above in the direction of further speeding up. One is that since the light input/output portions (the input waveguide 532 and the output waveguides 548a and 548b) are provided on two short sides of the substrate 530 facing each other on the left and right in the drawing, the ends of the signal electrodes 550a and 552a and 550b are provided on two long sides of the substrate 530 on the upper and lower sides in the drawing, respectively, avoiding the short sides. In other words, in order to avoid the conflict of arrangement, the light input/output portions are arranged on the two short sides, and the ends of the signal electrode 550a and the like are arranged on the two long sides.

In general, in order to prevent the microwave energy propagating through the signal electrode from leaking into the air as much as possible, it is desirable that the signal electrode for propagating the microwave is configured so as not to include a bent portion or a curved portion as much as possible. In contrast, in the conventional optical modulator 600, in order to avoid the interference with the optical input/output unit in the arrangement space, the end portions of the signal electrodes 550a, 552a, 550b, and 550b are provided on the long sides of the substrate 530, and as a result, the signal electrodes 550a, 552a, 550b, and 550b may include a meandering portion including a meandering line (curved line) or the like in a section reaching the mach-zehnder optical waveguides 544a, 546a, 544b, and 546b, respectively.

As a structure for reducing the meandering portion in the signal electrode, the following structure has been proposed: as in the optical modulation element 700 shown in fig. 7, the input waveguide 632 bent in an L-shape is provided, and receives an optical input from a direction perpendicular to the extending direction of the optical waveguides of the nested mach-zehnder type optical waveguides 540a and 540b (patent document 1).

As another structure, the following structure has been proposed: as in the optical modulation element 800 shown in fig. 8, instead of the branched waveguide 534 in fig. 6, a Multimode Interference (MMI) coupler 734 is provided, and the MMI coupler 734 branches an optical input propagating in a direction orthogonal to the extending direction of the optical waveguides of the nested mach-zehnder optical waveguides 540a and 540b into two beams and outputs the two beams in the same direction as the propagation direction of the optical input (patent document 2). In this configuration, the branched lights output from the MMI coupler 734 are input to the nested mach-zehnder optical waveguides 540a and 540b, respectively, via the connection waveguide including the bent waveguide for bending the propagation direction thereof by 90 degrees.

In fig. 7 and 8, the same components as those shown in fig. 6 are denoted by the same reference numerals as those shown in fig. 6.

In the optical modulation elements 700 and 700 shown in fig. 7 and 8, since input light is input from the long side (each side on the upper side in the drawing) of the substrate 530, the signal electrodes 650a, 652a, 650b, and 652b and the signal electrodes 750a, 752a, 750b, and 750b can be linearly configured along the extending direction of the nested mach-zehnder optical waveguides 540a and 540b without meandering from one short side (each side on the left side in the drawing) of the substrate 530 to the nested mach-zehnder optical waveguides 540a and 540 b. Therefore, even if the frequency of the high-frequency signal increases with an increase in the transmission rate, the leakage of the microwave from the signal electrodes can be avoided.

However, in the configuration of patent document 1 shown in fig. 7, since the L-shaped input waveguide 632 is introduced, the substrate 530 has to be elongated in the longitudinal direction by the length "a" shown in the drawing. Therefore, the demand for miniaturization of the light modulation element is violated. Further, since the branch waveguide 534 is used as it is, if the interval between the nested mach-zehnder optical waveguides 540a and 540b is increased in order to reduce the crosstalk between the two nested mach-zehnder optical waveguides 540a and 540b, the length of the connection waveguide reaching the nested mach-zehnder optical waveguides 540a and 540b from the branch waveguide 534 becomes longer, and the length of the substrate 530 becomes longer. This is because, in order to expand the distance while keeping the length of the connecting waveguide constant, the radius of curvature of the curved portion provided in the connecting waveguide needs to be reduced, but the radius of curvature cannot be reduced endlessly due to the relationship with the bending loss.

Therefore, for example, when the frequency of the high-frequency electrical signal for driving the nested mach-zehnder optical waveguides 540a and 540b is increased due to an increase in transmission rate or the like, if the crosstalk is reduced by increasing the interval, the longitudinal direction of the substrate 530 is further increased, and it becomes more difficult to meet the demand for miniaturization.

In the structure of patent document 2, an example of which is shown in fig. 8, as described above, each of the branched lights output from the MMI coupler 734 is input to the nested mach-zehnder optical waveguides 540a and 540b via the connection waveguide including the bent waveguide for bending the propagation direction thereof by 90 degrees. Therefore, as described above, since the curvature radius of the curved waveguide portion is limited, it is necessary to secure a certain area or more of the substrate portion to form the connection waveguide, and it is difficult to miniaturize the substrate 530.

On the other hand, if the branch output light of the MMI coupler 734 is output in a direction different from the propagation direction of the input light of the MMI coupler 734, the design of the mode interference section constituting the MMI coupler 734 becomes complicated, and it may be difficult to realize a desired (e.g., 1: 1) branch ratio with high accuracy.

That is, the conventional optical modulator has room for improvement in applications in which the optical modulator is housed in the same housing together with an electronic circuit.

Documents of the prior art

Patent document

Patent document 1: japanese patent laid-open No. 2014-164243

Patent document 2: japanese patent laid-open No. 2014-112219

Disclosure of Invention

Problems to be solved by the invention

In light of the above background, it is desirable to realize an optical modulation element that can be housed in the same housing together with an electronic circuit without causing deterioration of high-frequency characteristics and optical modulation characteristics and without causing an increase in the size of the housing.

Means for solving the problems

An embodiment of the present invention is a light modulation element including: two Mach-Zehnder type optical waveguides arranged on the substrate; a branch waveguide that branches input light input from outside of the substrate into two beams; two connection waveguides for guiding the light branched by the branch waveguide to the two Mach-Zehnder type optical waveguides; and electrodes for controlling light waves propagating through optical waveguides constituting the two mach-zehnder type optical waveguides, respectively, wherein the parallel waveguides of the two mach-zehnder type optical waveguides are configured to extend along one side of the substrate, and the branch waveguide is disposed to input light from a direction in which the one side is located, and is formed so as to be line-symmetric with respect to a propagation direction of the light input to the branch waveguide, and so that two branched lights are output in a direction different from the propagation direction.

According to another embodiment of the present invention, the branch waveguide includes a Y-branch optical waveguide formed line-symmetrically with respect to a propagation direction of light input to the branch waveguide.

According to another embodiment of the present invention, the branched waveguides are formed such that an interval between two branched output lights when output from the line-symmetrically formed portion is smaller than an interval between respective light input ends of the two mach-zehnder type optical waveguides.

According to another embodiment of the present invention, the electrodes are formed so as to extend along a direction in which the two mach-zehnder type optical waveguides extend to another side of the substrate different from the one side.

According to another embodiment of the present invention, each of the two mach-zehnder type optical waveguides is a nested mach-zehnder type optical waveguide, and each of the two parallel waveguides configuring the mach-zehnder type optical waveguide includes another mach-zehnder type optical waveguide.

According to another embodiment of the present invention, the two connection waveguides each include a straight waveguide and a curved waveguide, and are configured such that all propagation losses from the inflow portion of the light of the branch waveguide to the optical input ends of the two mach-zehnder type optical waveguides are equal to each other.

According to another embodiment of the present invention, the two connection waveguides are configured such that optical path lengths from the light inflow portion of the branch waveguide to the light input ends of the two mach-zehnder type optical waveguides are equal to each other.

According to another embodiment of the present invention, the substrate is a rectangle having two opposing short sides and two opposing long sides longer than the short sides, the one side is any one of the long sides, and a propagation direction of light at the time of input to the branch waveguide is a direction along the short sides.

Another embodiment of the present invention is a light modulation module comprising: any one of the light modulation elements; an electronic circuit that drives the light modulation element; and a housing that houses the optical modulation element and the electronic circuit.

In addition, the present specification includes all the contents of japanese patent application No. 2019-067621 filed on 29/3/2019.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, it is possible to realize an optical modulation element that can be housed in the same housing together with an electronic circuit without causing deterioration in high-frequency characteristics and optical modulation characteristics and without causing an increase in the size of the housing.

Drawings

Fig. 1 is a diagram showing a structure of an optical modulation module according to a first embodiment of the present invention.

Fig. 2 is a diagram showing a structure of a light modulation element used in the light modulation module shown in fig. 1.

Fig. 3 is a diagram showing a first modification of an optical modulator that can be used in the optical modulator shown in fig. 1.

Fig. 4 is a diagram showing a second modification of the optical modulator element that can be used in the optical modulator shown in fig. 1.

Fig. 5 is a diagram showing a third modification of the optical modulator that can be used in the optical modulator shown in fig. 1.

Fig. 6 is a diagram showing a structure of a conventional light modulation element that can be used as a single body.

Fig. 7 is a diagram showing a first conventional example of an optical modulation element housed in the same housing together with an electronic circuit.

Fig. 8 is a diagram showing a second conventional example of an optical modulation element housed in the same housing together with an electronic circuit.

Detailed Description

Embodiments of the present invention will be described below with reference to the drawings. Fig. 1 is a diagram showing a structure of an optical modulation module 100 according to a first embodiment of the present invention. The optical modulation module 100 includes a housing 102, an optical modulation element 104 housed in the housing 102, and an electronic circuit 106 for driving the optical modulation element 104. The optical modulation element 104 is, for example, a DP-QPSK modulator. The light modulation element 104 includes, for example, a rectangle having a pair of short sides and a pair of long sides longer than the short sides. Finally, a cover (not shown) as a plate body is fixed to the opening of the housing 102, and the inside thereof is hermetically sealed.

The light modulation module 100 further includes: signal pins 110a, 110b, 110c, 110d for inputting electrical signals used in the modulation of the light modulation element 104; and a feed-through (feed-through) section 108 for introducing the signal pins 110a, 110b, 110c, and 110d into the housing 102. In this embodiment, four electrical signals for driving four mach-zehnder modulators constituting two nested mach-zehnder modulators included in the optical modulation element 104 are input from the four signal pins 110a, 110b, 110c, and 110d, respectively.

Further, the light modulation module 100 includes: an input optical fiber 114 for inputting light into the housing 102; and an output fiber 120 that guides the light modulated by the light modulation element 104 to the outside of the housing 102.

Here, the input optical fiber 114 and the output optical fiber 120 are fixed to the housing 102 via brackets 122 and 124 as fixing members, respectively. The light input from the input optical fiber 114 is collimated by a lens 130 disposed in the holder 122, and then input to the light modulation element 104 from one long side (the long side on the upper side in the drawing) of the light modulation element 104 via a prism 132 and a lens 134. However, this is merely an example, and the light can be input to the optical modulation element 104 by, for example, introducing the input optical fiber 114 into the housing 102 via the holder 122, and disposing the end of the introduced input optical fiber 114 on one long side of the optical modulation element 104.

The light modulation module 100 has an optical unit 116, and the optical unit 116 polarization-combines the two modulated lights output from the light modulation element 104. The light polarized and combined and output from the optical unit 116 is collected by the lens 118 disposed in the holder 124 extending to the inside of the housing 102, and is coupled to the output optical fiber 120.

The high-frequency electric signal that is output from the electronic circuit 106 and drives the optical modulation element 104 is connected to one end of a signal electrode (described later) of the optical modulation element 104 directly from the substrate of the electronic circuit 106 or indirectly via a relay substrate, for example, by wire bonding or the like. The optical modulation module 100 includes two terminators 112a and 112b having a predetermined impedance in the housing 102.

Fig. 2 is a diagram showing an example of the structure of the light modulation element 104 housed in the housing 102 of the light modulation module 100 shown in fig. 1. The optical modulation element 104 is, for example, a waveguide type optical element that performs DP-QPSK modulation, and includes, for example, an optical waveguide (a dotted line of a thick line shown in the figure) formed on a substrate 230 including LN. As these optical waveguides, those formed by thermally diffusing Ti on the surface of the substrate 230 can be used, but the present invention is not limited thereto. These optical waveguides may be rib-type optical waveguides formed by making the substrate 230 thin to a thickness of several micrometers and making the effective refractive index of the line portion formed with the optical waveguides thicker than the other portions by making the thickness of the line portion thicker than the other portions, for example.

The substrate 230 is, for example, rectangular, and has: two short sides 260a, 260b extending in the vertical direction in the figure and facing each other; and two long sides 260c, 260d facing each other, extending in the illustrated left-right direction orthogonal to the short sides 260a, 260b and having a length longer than the short sides 260a, 260 b. For example, the short sides 260a and 260b of the substrate 230 face each other in parallel, the long sides 260c and 260d face each other in parallel, and the extending direction of the short sides 260a and 260b is orthogonal to the extending direction of the long sides 260c and 260 d.

The optical modulator 104 includes two nested mach-zehnder optical waveguides 240a and 240b (portions surrounded by one-dot chain lines in the figure) that perform QPSK modulation operation, respectively. The nested mach-zehnder type optical waveguides 240a and 240b are respectively included in two mach-zehnder type optical waveguides 244a (inside a rectangle shown by a dotted line), 246b (inside a rectangle shown by a two-dot chain line), 244b (inside a rectangle shown by a dotted line), and 246b (inside a rectangle shown by a two-dot chain line) provided in two waveguide sections constituting a pair of parallel waveguides. Thus, the nested mach-zehnder type optical waveguides 240a and 240b QPSK modulate the light input from the modulated light input ends 242a and 242b, respectively, and output the modulated light (output) from the output waveguides 248a and 248b, respectively.

In the present embodiment, the four mach-zehnder type optical waveguides 244a, 246b, 244b, and 246b (therefore, the parallel waveguides of the mach-zehnder type optical waveguides 244a, 246b, 244b, and 246 b) constituting the nested mach-zehnder type optical waveguides 240a and 240b are configured to extend along one side of the substrate 230, that is, for example, the long side 260c or 260 d.

On the substrate 230, signal electrodes 250a, 252a, 250b, and 252b for modulating the total four mach-zehnder optical waveguides 244a, 246a, 244b, and 246b constituting the nested mach-zehnder optical waveguides 240a and 240b are provided, respectively, on the signal electrodes 250a, 252a, 250b, and 252 b. The signal electrodes 250a, 252a, 250b, and 252b include, for example, two ground electrodes and one center electrode sandwiched between the two ground electrodes.

The signal electrodes 250a, 252a, 250b, and 252b are formed to extend along the direction in which the two nested mach-zehnder optical waveguides 240a and 240b extend, for example, to the other side of the substrate 230, i.e., the short side 260a, which is different from the one side, i.e., the long side 260c or 260 d.

In other words, in the present embodiment, the signal electrodes 250a, 252a, 250b, and 252b linearly extend from the short side 260a on the left in the figure to the mach-zehnder optical waveguides 244a, 246a, 244b, and 246b constituting the nested mach-zehnder optical waveguides 240a and 240b along the extending direction of the nested mach-zehnder optical waveguides 240a and 240b (therefore, along the extending direction of the long sides 260c and 260 d). The signal electrodes 250a, 252a, 250b, and 252b are connected to signal output electrodes of the electronic circuit 106 at the ends disposed on the short side 260 a. Thus, in the optical modulator 104, the four high-frequency electrical signals input from the electronic circuit 106 at the short side 260a propagate to the four mach-zehnder optical waveguides 244a, 246a, 244b, 246b without changing the propagation direction. As a result, in the optical modulator 104, since leakage of the high-frequency electric signal input from the electronic circuit 106 is suppressed, the optical modulator 104 can be housed in the same housing 102 together with the electronic circuit 106 without causing deterioration of high-frequency characteristics.

The signal electrodes 250a and 252a are formed to extend along the extending direction of the nested mach-zehnder type optical waveguide 240a and then to be bent toward the long side 260 c. The signal electrodes 250a and 252a are connected to the terminal 112a in the end array 254a on the long side 260 c. The signal electrodes 250b and 252b are formed to extend along the extending direction of the nested mach-zehnder type optical waveguide 240b and then to be bent toward the long side 260 d. The signal electrodes 250b and 252b are connected to the terminal 112b in the end array 254b on the long side 260 d. Thus, the four high-frequency electric signals input from the electronic circuit 106 at the short side 260a propagate as traveling waves in the signal electrodes 250a, 252a, 250b, and 252b, respectively.

In the optical modulator 104, further, bias electrodes 256a, 258a, 256b, and 258b for adjusting the operating points of the nested mach-zehnder type optical waveguides 240a and 240b are provided on the substrate 230 as necessary.

The input light to the light modulation element 104 is input from the input optical fiber 114 connected to the long side 260c of the substrate 230.

In particular, the optical modulator 104 of the present embodiment does not include a branch waveguide for inputting light along the extending direction of the nested mach-zehnder optical waveguides 540a and 540b, such as the branch waveguide 534 in the conventional optical modulator 700 shown in fig. 7.

Instead, the optical modulation element 104 includes a branch waveguide 234, and the branch waveguide 234 is arranged so that light is input from a direction in which the long side 260c, which is the one side of the substrate 230, is located. The branch waveguide 234 includes a Y-branch optical waveguide formed so as to be line-symmetric with respect to the propagation direction of the light input to the branch waveguide 234 and so that the two branched lights are output in a direction different from the propagation direction. Here, in the present embodiment, the branch waveguide 234 is disposed such that the propagation direction of light input to the branch waveguide 234 from the direction in which the long side 260c is located is along the short side 260 a.

The two branched outputs outputted from the branched waveguide 234 are guided and connected to the modulated light input ends 242a, 242b of the nested mach-zehnder optical waveguides 240a, 240b by the connecting waveguides 270a, 270b, respectively.

In the present embodiment, the branched waveguide 234 is formed such that, for example, the interval W1 of the two branched output lights when output from the line-symmetrically formed portion (the portion surrounded by the rectangular broken line showing the branched waveguide 234 in fig. 2) is smaller than the interval W2 of the modulated light input ends 242a and 242b, which are the light input ends of the two nested mach-zehnder type optical waveguides 240a and 240b (i.e., W1 < W2). In addition to the positions shown in fig. 2, the modulated light input ends 242a and 242b may be defined as the positions of the branch points 280a and 280b of the two branch waveguides constituting the input units of the nested mach-zehnder type optical waveguides 240a and 240 b. At this time, the interval W2 is defined as the distance between the two branch points 280a, 280 b.

Further, in the present embodiment, as shown in fig. 2, the connection waveguides 270a and 270b include a straight waveguide and a curved waveguide. The connection waveguides 270a and 270b are configured such that all propagation losses from the light inflow portion of the branch waveguide 234 to the modulated light input ends 242a and 242b of the two nested mach-zehnder type optical waveguides 240a and 240b are equal to each other. Thus, for example, even if the branching ratio of the branching waveguide 234 is designed to be 1: even if there is a deviation 1, the degree of freedom in design can be improved by correcting the deviation by bending loss or the like of the bent waveguide portion connecting the waveguides 270a and 270 b.

As described above, since the optical modulation element 104 having the above-described structure does not use a branch waveguide for inputting light along the extending direction of the nested mach-zehnder type optical waveguides 240a and 240b, such as the branch waveguide 534 in fig. 6, it is not necessary to use the input waveguide 632 bent in an L-shape as shown in fig. 7, and an increase in the dimension in the lateral direction as shown in fig. 7 (an increase in the length indicated by "a" in the drawing) does not occur.

Further, unlike the MMI coupler 734 in the conventional optical modulation element 800 shown in fig. 8, the Y-branch waveguide constituting the branch waveguide 234 can be easily configured to equally distribute the input light with high accuracy (that is, to easily realize a 1: 1 branching ratio), even if two branched lights branched with respect to the propagation direction of the input light are output in different directions. Therefore, the light quantity of the light input to the two nested mach-zehnder type optical waveguides 240a and 240b can be made the same, and the occurrence of distortion in the modulation waveform can be easily suppressed.

Further, the Y-branch waveguide constituting the branch waveguide 234 is configured line-symmetrically with respect to the line segment of the propagation direction of the input light so as to ensure 1: a branching ratio of 1 is sufficient, and it is not always necessary to output two branched output lights in the same direction as the input light as described above. Therefore, the branch waveguides 234 may be designed such that the exit direction of one of the branch outputs is directed toward the nested mach-zehnder type optical waveguides 240a located closer to the branch waveguides 234, within a range in which the line-symmetric shape is maintained. Therefore, compared to the case of using a branch waveguide that outputs branched output light in the same direction as the input light like the MMI coupler 734 shown in fig. 8, there is no need to change the direction of light by 90 degrees, and there is no inflection point in the change in the propagation direction of light in the path from the input to the branch waveguide 234 through the connection waveguide 270a until reaching the modulated light input end 242a (that is, in the configuration of fig. 2, the change in the propagation direction of light in the path is always one direction (half-hour direction) and does not change to the opposite direction (clockwise direction) halfway). Therefore, in the optical modulator 104, the connection waveguide 270a reaching the modulated light input end 242a of the nearer nested mach-zehnder optical waveguide 240a can be formed to have a larger radius of curvature and to be shorter.

On the other hand, due to the symmetry of the branch point of the branch waveguide 234, the outgoing direction of the other branch output of the branch waveguide 234 is outgoing in a direction away from the distant nest-type mach-zehnder type optical waveguide 240 b. However, since the connection waveguide 270b reaching the modulated light input end 242b of the distant nested mach-zehnder type optical waveguide 240b can be configured to be longer than the connection waveguide 270a, it is possible to freely and easily design within the limit of the bending loss of the optical waveguide (i.e., the limit of the curvature radius).

As described above, the branched output light outputted from the branched waveguide 234 is connected to the modulated light input ends 242a and 242b of the nested mach-zehnder type optical waveguides 240a and 240b via the connecting waveguides 270a and 270 b. Therefore, for example, when the distance W2 between the nested mach-zehnder optical waveguides 240a and 240b is to be increased to improve crosstalk, the distance W1 between the branched output lights in the branch waveguide 234 need not be increased as long as the extension distance of the connection waveguides 270a and 270b along the short side 260a is adjusted. Therefore, in the optical modulator 104, unlike the conventional optical modulator 700 shown in fig. 7, even if the interval between the nested mach-zehnder optical waveguides 240a and 240b is increased, the length in the longitudinal direction of the substrate 230 (i.e., the direction along the long sides 260c and 260 d) does not need to be increased.

As described above, the light modulation element 104 can be housed in the same housing 102 together with the electronic circuit 106 without causing deterioration in high-frequency characteristics and light modulation characteristics and without causing an increase in size of itself (thus, without causing an increase in size of the housing 102).

Next, a modification of the light modulation element 104 that can be used in the light modulation module 100 of the first embodiment will be described.

< first modification >

Fig. 3 is a diagram showing a structure of an optical modulator 304 according to a first modification. In fig. 3, the same reference numerals as those in fig. 2 are used to denote constituent elements that are the same as those of the light modulation element 104 shown in fig. 2, and the description of fig. 2 is referred to.

The optical modulation element 304 shown in fig. 3 has the same structure as the optical modulation element 104, but differs in that an input waveguide 332 is provided instead of the input waveguide 232. While the input waveguide 232 of the optical modulator 104 guides the input light input from the long side 260c to the branch waveguide 234 without changing its propagation direction, the input waveguide 332 of the optical modulator 304 of the present modification is connected to the branch waveguide 234 by changing its propagation direction by 90 degrees after the input light input from the short side 260b propagates along the long side 260 c. That is, the light is received from the short side 260b, and is input to the branch waveguide 234 from the direction in which the long side 260c is located.

In the case where the optical modulation element 304 having the above-described structure is used by being housed in the housing 102 of the optical modulation module 100, it is not necessary to bend the input optical fiber 114 by 90 degrees toward the long side 260c inside the housing 102, unlike the case where the optical modulation element 104 is used. Therefore, when the optical modulator 304 is used, a space for routing the input optical fiber 114 in the housing 102 is not required, and for example, the size of the housing 102 in the width direction (vertical direction in the drawing) can be further reduced by changing the shape of the electronic circuit 106 while maintaining the area.

< second modification >

Fig. 4 is a diagram showing a structure of the light modulator 404 according to the second modification. In fig. 4, the same reference numerals as those in fig. 2 are used to denote constituent elements that are the same as those of the light modulation element 104 shown in fig. 2, and the description of fig. 2 is referred to.

The optical modulation element 404 shown in fig. 4 has the same structure as the optical modulation element 104, but differs in that a connection waveguide 270 a' is provided instead of the connection waveguide 270 a. The connection waveguide 270 a' has the same configuration as the connection waveguide 270a, but differs therefrom in that the light output from the branch waveguide 234 is once propagated in the same direction as the propagation direction of the input light to the branch waveguide 234, and then guided to the modulated light input end 242 a.

In the structure of fig. 4, unlike the structure in which the connection waveguide 270a is formed toward the modulated light input end 242a of the mach-zehnder type optical waveguide 240a immediately after branching at the branch waveguide 234 as in fig. 2 or 3, the connection waveguide 270 a' includes linear waveguide portions that are arranged in parallel at a narrow interval with respect to the linear waveguide portion of the connection waveguide 270 b. Thus, in the configuration of fig. 4, the branched waveguides can be connected to the plurality of mach-zehnder type optical waveguides 240a and 240b while suppressing the size of the substrate 230 in the longitudinal direction (the left-right direction in the figure), and thus the degree of freedom in the arrangement of the plurality of mach-zehnder type optical waveguides 240a and 240b can be increased.

< third modification >

Fig. 5 is a diagram showing a structure of an optical modulator 504 according to a third modification. In fig. 5, the same reference numerals as those in fig. 2 are used to denote constituent elements that are the same as those of the light modulation element 104 shown in fig. 2, and the description of fig. 2 is referred to.

The optical modulation element 504 shown in fig. 5 has the same structure as the optical modulation element 104, but differs in that a connection waveguide 470a is included instead of the connection waveguide 270 a. The connecting waveguide 470a has the same structure as the connecting waveguide 270a, but differs therefrom in that a meandering portion 472 is included in the middle thereof. Thus, the connection waveguides 470a and 270b are configured such that the optical path lengths from the light inflow portion of the branch waveguide 234 to the modulated light input ends 242a and 242b, which are the light input ends of the two nested mach-zehnder type optical waveguides 240a and 240b, are the same.

The optical modulation element 504 having the above-described structure is configured such that the optical path lengths from the end of the long side 260c of the input waveguide 232 to the modulated light input ends 242a and 242b are equal to each other, and therefore, for example, is suitable for a case where the phases of the light input to the two nested mach-zehnder type optical waveguides 240a and 240b are intended to be equal to each other at the modulated light input ends 242a and 242 b.

The present invention is not limited to the configurations of the above-described embodiments and modifications thereof, and can be implemented in various forms without departing from the spirit and scope thereof.

For example, in the present embodiment, the substrate 230 is, for example, a rectangle having two short sides 260a and 260b facing each other in parallel and two long sides 260c and 260d facing each other in parallel, and the extending direction of the short sides 260a and 260b is orthogonal to the extending direction of the long sides 260c and 260d, but the invention is not limited thereto. The nested mach-zehnder optical waveguides 240a and 240b extend along one long side 260c or 260d, and the branch waveguide 234 is disposed so as to allow light to flow in from the direction in which the long side 260c is located along the direction in which the other short side 260a extends, but the present invention is not limited to this.

The short sides 260a, 260b of the substrate 230 may not be parallel to each other and the long sides 260c, 260d may not be parallel to each other. The nested mach-zehnder type optical waveguides 240a and 240b, which are two mach-zehnder type optical waveguides connected to the branch waveguide 234, may be arranged such that at least the parallel waveguides of the respective optical waveguides extend along one side of the substrate 230, and the branch waveguide 234 may be arranged such that light is input from the direction in which the one side is located.

In the above-described embodiment and the modifications thereof, the optical modulation elements 104, 304, 404 perform DP-QPSK modulation by including two nested mach-zehnder optical waveguides 240a, 240b that perform QPSK modulation, respectively, but the present invention is not limited thereto. For example, instead of the nested mach-zehnder type optical waveguides 240a and 240b, two mach-zehnder type optical waveguides each performing normal amplitude modulation may be included, and after receiving light from one light source through the input waveguide 232, the light may be modulated by different high-frequency electrical signals and output to two output optical fibers.

At this time, by using the same configuration as that using the branched waveguide 234 in the optical modulation elements 104, 304, 404, the optical modulation element can be housed in the same housing together with the electronic circuit without causing deterioration of the high-frequency characteristic and the optical modulation characteristic and without causing an increase in the size of the optical modulation element itself (thus, without causing an increase in the size of the housing).

In the above-described embodiment and the modifications thereof, the ground electrodes respectively constituting the signal electrodes 250a, 252a, 250b, and 252b are provided separately from each other, but the present invention is not limited thereto. The adjacent two ground electrodes of the adjacent signal electrodes among the signal electrodes 250a, 252a, 250b, 252b may also form a conductor therebetween, thereby forming one continuous ground electrode. For example, in fig. 2, the ground electrode on the lower side of the signal electrode 250a in the drawing may be extended toward the lower side in the drawing, and thus may be combined with the ground electrode on the upper side of the signal electrode 252a in the drawing to form one ground electrode. The same applies to the ground electrode on the lower side of the signal electrode 252a and the ground electrode on the upper side of the signal electrode 252b, and the ground electrode on the lower side of the signal electrode 252b and the ground electrode on the upper side of the signal electrode 250 b.

As described above, the light modulation elements 104, 304, 404, and 504 according to the present embodiment include: two Mach-Zehnder type optical waveguides 240a, 240b, which are nest type Mach-Zehnder type optical waveguides, provided on the substrate 230; and a branch waveguide 234 that branches input light input from the outside of the substrate 230 into two beams. The light modulation elements 104, 304, 404, and 504 include: two connecting waveguides 270a and 270b for guiding the light branched by the branched waveguide 234 to the two nested mach-zehnder type optical waveguides 240a and 240b, respectively; and signal electrodes 250a, 252a, 250b, and 252b for controlling the optical waves propagating through the optical waveguides constituting the two nested mach-zehnder optical waveguides. Each of the optical waveguides constituting the two nested mach-zehnder type optical waveguides 240a and 240b is configured to extend along the long side 260c which is one side of the substrate 230. The branch waveguide 234 is disposed so as to input light from the direction in which the one side, i.e., the long side 260c, is located (for example, the direction along the extension direction of the short side 260 a). The branch waveguide 234 is formed so as to be line-symmetric with respect to the propagation direction of the light input to the branch waveguide 234, and two branched lights are output in a direction different from the propagation direction.

According to this configuration, the branched waveguide 234 is arranged so that light is input from a direction along one side (the long side 260c) of the direction in which the nested mach-zehnder optical waveguides 240a and 240b extend. Therefore, even when the signal electrodes 250a, 252a, 250b, and 252b are extended in the extending direction, light can be made incident on the substrate 230 from a direction different from the direction in which the end portions of the signal electrodes 250a, 252a, 250b, and 252b are located. Therefore, in the optical modulation elements 104, 304, 404, and 504, the signal electrodes 250a, 252a, 250b, and 252b are formed linearly along the extending direction of the nested mach-zehnder type optical waveguides 240a and 240b from the input position of the high-frequency electric signal while avoiding the collision between the input position of the high-frequency electric signal and the input position of the light toward the substrate 230, and thus, favorable high-frequency characteristics can be obtained.

In particular, in the above-described configuration, since the branch waveguide 234 is disposed so that light is input from the direction along one side (the long side 260c) of the direction in which the two nested mach-zehnder type optical waveguides 240a and 240b extend, unlike the configuration described in patent document 1, even when the interval between the two nested mach-zehnder type optical waveguides 240a and 240b is increased, the substrate 230 does not need to be increased in the extending direction. Therefore, the crosstalk between the two nested mach-zehnder optical waveguides 240a and 240b can be reduced without increasing the size of the substrate 230, and favorable optical characteristics can be obtained.

In particular, according to the above-described configuration, since the branch waveguide 234 is formed so as to be symmetrical with respect to the propagation direction of the input light and two branched light beams are output in a direction different from the propagation direction, it is not necessary to change the propagation direction of the light greatly (for example, by 90 degrees at maximum) from the branch waveguide 234 toward the nest type mach-zehnder type optical waveguide 240a located closer to the branch waveguide 234. Therefore, in the optical modulation elements 104 and 404, the size of the portion of the curved waveguide of the connection waveguide 270a reaching the nested mach-zehnder type optical waveguide 240a from the branch waveguide 234 can be reduced as compared with the structure of patent document 2 using the MMI coupler that outputs the branch light in the same direction as the input light, and therefore, the size of the substrate 230 can be reduced.

As a result of these effects, the light modulation elements 104 and 404 can be housed in the housing 102 together with the electronic circuit 106 without causing deterioration in the high-frequency characteristics and the light modulation characteristics and without causing an increase in the size of the housing 102.

In the optical modulation elements 104, 304, 404, and 504, the branch waveguide 234 includes a Y-branch optical waveguide formed in line symmetry with respect to the propagation direction of the light input to the branch waveguide 234. According to this structure, for example, it is possible to easily form a film that can realize 1: a line symmetric branching waveguide 234 with a branching ratio of 1.

In the optical modulation elements 104, 304, 404, and 504, the branched waveguides 234 are formed such that the interval W1 of the two branched output lights when output from the line-symmetrically formed portions is smaller than the interval W2 of the optical input ends of the two nested mach-zehnder type optical waveguides 240a and 240 b.

According to this structure, for example, even when the interval between the two nested mach-zehnder optical waveguides 240a and 240b is increased, the substrate 230 does not need to be increased in size. Therefore, the optical modulation elements 104, 304, 404, 504 can be configured compactly while effectively reducing crosstalk between the two nested mach-zehnder optical waveguides 240a, 240 b.

In order to ensure the branching ratio of the branching waveguide 234, the symmetry of the branching waveguide 234 must be ensured until the two branched output lights are separated to a degree that allows the resulting influence (twice or more the light intensity distribution of the light wave propagating through the branching waveguide). For this reason, it is more desirable that the interval W1 of the two branch output lights satisfies the condition.

In the optical modulation elements 104, 304, 404, and 504, the signal electrodes 250a, 252a, 250b, and 252b are formed so as to extend along the direction in which the two nested mach-zehnder optical waveguides 240a and 240b extend to another side (for example, the short side 260a) different from the one side (for example, the long side 260c) of the substrate 230.

With this configuration, the high-frequency electric signal input from the other side (short side 260a) can be linearly guided to the positions of the two nested mach-zehnder type optical waveguides 240a and 240b along the direction in which the two nested mach-zehnder type optical waveguides 240a and 240b extend by the signal electrodes 250a, 252a, 250b and 252 b. Therefore, the optical modulation elements 104, 304, 404, and 504 can realize good high-frequency characteristics.

In the optical modulation elements 104, 304, 404, and 504, the two nested mach-zehnder type optical waveguides 240a and 240b include the other mach-zehnder type optical waveguides 244a and 246a, and 244b and 246b, respectively, in the two parallel waveguides constituting the nested mach-zehnder type optical waveguides 240a and 240b, respectively.

With this configuration, the DP-QPSK modulator can be compactly configured on the substrate 230.

In the optical modulation elements 104 and 304, the two connection waveguides 270a and 270b each include a straight waveguide and a curved waveguide, and are configured such that all propagation losses from the light inflow portion of the branch waveguide 234 to the modulated light input ends 242a and 242b of the two nested mach-zehnder type optical waveguides 240a and 240b are equal to each other.

With this configuration, the propagation loss or the bending loss of each of the linear waveguides and the curved waveguides constituting the connection waveguides 270a and 270b can be adjusted so that the amounts of light input to the two nested mach-zehnder type optical waveguides 240a and 240b become equal, thereby improving the degree of freedom in design.

In the optical modulator 504, the two connecting waveguides 470a and 270b are configured such that the optical path lengths from the light inflow portion of the branch waveguide 234 to the modulated light input ends 242a and 242b of the two nested mach-zehnder type optical waveguides 240a and 240b are equal to each other.

With this configuration, the phases of the light input to the two nested mach-zehnder optical waveguides 240a and 240b can be accurately matched.

In the light modulation elements 104, 304, 404, and 504, the substrate 230 has a rectangular shape having two opposing short sides 260a and 260b and two opposing long sides 260c and 260d longer than the short sides 260a and 260 b. The one side is one of the long sides 260c and 260d, and the propagation direction of light entering the branched waveguide 234 is a direction along the short sides 260a and 260 b.

According to this configuration, for example, in the rectangular substrate 230, the high-frequency electric signal is input from the end portion extending from the signal electrodes 250a, 250b, 252a, and 252b along the long side 260c to the short side 260a, and light from the outside can be arranged on the side other than the short side 260a (for example, the long side 260c or the short side 260b) toward the incident position of the substrate 230. This makes it possible to easily realize a structure in which the signal electrodes 250a, 250b, 252a, and 252b extend linearly to the two nested mach-zehnder optical waveguides 240a and 240b while avoiding collision between the input position of the high-frequency electric signal and the input position of light.

Further, the light modulation module 100 according to the embodiment includes: any of the light modulation elements 104, 304, 404, 504; an electronic circuit 106 that drives the light modulation element; and a housing 102 that houses the light modulation element and the electronic circuit 106.

With this configuration, the optical modulation module 100 having excellent high-frequency characteristics and optical modulation characteristics can be compactly configured.

Description of the symbols

100: light modulation module

102: frame body

104. 304, 404, 504, 600, 700, 800: light modulation element

106: electronic circuit

108: piercing part

110a, 110b, 110c, 110 d: signal pin

112a, 112 b: terminal device

114: input optical fiber

116: optical unit

118: lens and lens assembly

120: output optical fiber

122. 124: support frame

230. 530: substrate

232. 332, 532, 632, 732: input waveguide

234. 534: branched waveguide

240a, 240b, 540a, 540 b: nest type Mach-Zehnder type optical waveguide

242a, 242b, 542a, 542 b: modulated light input

244a, 244b, 246a, 246b, 544a, 544b, 546a, 546 b: Mach-Zehnder type optical waveguide

248a, 248b, 548a, 548 b: output waveguide

250a, 250b, 252a, 252b, 550a, 550b, 552a, 552b, 650a, 650b, 652a, 652b, 750a, 750b, 752a, 752 b: signal electrode

254a, 254b, 554a, 554 b: end arrangement

256a, 256b, 258a, 258b, 556a, 556b, 558a, 558 b: bias electrode

260a, 260 b: short side

260c, 260 d: long side

270a, 270b, 470 a: connecting waveguide

472: snake part

734: MMI coupler

280a, 280 b: branch point