Semiconductor optical modulator

Abstract

The present invention comprises: two waveguides (1a, 1b) that are formed on a semiconductor substrate (2), that each transmit light from a branch part (5) to a compositing part (6), and that are disposed so as to run parallel and adjacently to each other; a phase adjustment part (10) that is provided with phase adjustment electrodes (11) which apply a DC voltage for phase adjustment to the two waveguides (1a, 1b); and a modulation part (20) that is provided with modulation electrodes (21) which apply a high frequency voltage for modulation to the two waveguides (1a, 1b). The phase adjustment part (10) is positioned on both sides of a fold-back part (3) which changes the propagation direction of light 180 degrees. The fold-back part (3) is provided at at least one location. The two waveguides (1a, 1b) are each formed from an InP-based material, and have a pin structure in a cross-section perpendicular to the light propagation direction. Applied to the phase adjustment electrodes (11) is a voltage such that a forward current flows therethrough.

Description

semiconductor optical modulator

[0001] The present application relates to a semiconductor optical modulator.

[0002] The widespread use of smartphones and other devices has led to a demand for even greater capacity in optical communication systems to cope with the rapid increase in communication traffic. Therefore, multilevel optical modulation, which allows for greater capacity than intensity modulation, has attracted attention, and is now being applied to long-distance trunk lines as well as relatively short distances between large cities and data centers.

[0003] MZ (Mach-Zehnder) type semiconductor optical modulators, which are capable of multilevel phase modulation, are smaller and consume less power than conventional LN optical modulators that use LN waveguides made of lithium niobate (LiNbO3) crystal, making them an important key device for increasing capacity.

[0004] The MZ-type semiconductor optical modulator is an MZ-type interferometer formed by an optical waveguide with two arms, and has a modulation section that applies an RF electrical signal to each of the two arms, and a phase adjustment section that sets the operating point. The cross-sectional structure of the modulation section and phase adjustment section has a pin structure stacked vertically on the substrate. The semiconductor material used is, for example, an InP-based material.

[0005] The modulation section requires high-speed operation to generate large amounts of data. By applying a reverse bias to the pin structure, the refractive index is changed, and the phase of the light propagating through the arms is controlled to generate a modulated signal. A reverse bias or forward current is applied to the phase adjustment section, which changes the refractive index to set the optimal operating point, where the extinction ratio is maximized. Then, an RF signal with a reverse voltage is applied to the modulation electrodes of each arm (push-pull drive), generating an optical modulated signal.

[0006] JP 2012-163876 A

[0007] When a reverse bias is applied to the phase adjustment section, as in the semiconductor optical modulator disclosed in Patent Document 1, there is an advantage that almost no current flows, enabling low power consumption. However, as the reverse bias is increased, a sudden change in the refractive index occurs, which in turn causes a sudden increase in excess loss, making it difficult to control the operating point. Furthermore, the extinction ratio is likely to deteriorate due to the asymmetry of the loss in the arms caused by the loss occurring in one arm. The deterioration of the extinction ratio causes a deterioration of signal quality.

[0008] On the other hand, when a forward current is applied to the phase adjustment section, the changes in the refractive index and loss are gradual, making them easy to control and less likely to cause a deterioration in the extinction ratio. However, the flowing current generates heat, which can reduce the change in the refractive index. This is because the original change in the refractive index due to the forward current (carrier plasma effect) is negative, while the change in the refractive index due to heat (thermo-optic effect) is positive.

[0009] If the total change in refractive index becomes small, an extra current will be applied, causing an increase in power consumption. Furthermore, even though the current is in the forward direction, the generation of losses will increase, resulting in a deterioration of the extinction ratio due to asymmetry, just as with the application of a reverse bias.

[0010] This application aims to suppress the influence of the thermo-optic effect when the phase adjustment unit is of the forward current application type. That is, the object is to provide a forward current application type semiconductor optical modulator that is easier to control the operating point compared to the reverse bias application type and can reduce the deterioration of the extinction ratio and the increase in power consumption.

[0011] The semiconductor optical modulator disclosed in the present application comprises: two waveguides formed on a semiconductor substrate, arranged adjacent to each other and running parallel to each other, each transmitting light from a branching section to a combining section; a phase adjustment section having a phase adjustment electrode that applies a DC voltage for phase adjustment to each of the two waveguides; and a modulation section having a modulation electrode that applies a high-frequency voltage for modulation to each of the two waveguides, wherein the phase adjustment sections are located on both sides of a turning section that changes the traveling direction of the light by 180 degrees, and there is at least one turning section; both of the two waveguides are formed of an InP-based material and have a pin structure in a cross section perpendicular to the traveling direction of the light; and a voltage that causes a forward current to flow is applied to the phase adjustment electrode.

[0012] According to the present application, since the phase adjustment unit is of the forward current application type, it is possible to provide a semiconductor optical modulator that makes it easy to control the operating point and can reduce deterioration of the extinction ratio and increase in power consumption.

[0013] FIG. 1 is a schematic plan view showing the configuration of a semiconductor optical modulator according to a first embodiment. FIG. 2 is a line diagram showing an example of the characteristics of the semiconductor optical modulator according to the first embodiment. FIG. 3 is a line diagram for explaining the characteristics of the semiconductor optical modulator according to the first embodiment. FIG. 4 is a line diagram showing an example of the characteristics of a semiconductor optical modulator according to a comparative example. FIG. 5 is a line diagram showing another example of the characteristics of the semiconductor optical modulator according to the first embodiment. FIG. 6 is a cross-sectional view showing a schematic structure of a phase adjustment unit of the semiconductor optical modulator according to the first embodiment. FIG. 7 is a schematic plan view showing the configuration of a semiconductor optical modulator according to a second embodiment. FIG. 8 is a schematic plan view showing the configuration of a semiconductor optical modulator according to a third embodiment.

[0014] Embodiment 1. Figure 1 is a schematic plan view showing the configuration of a semiconductor optical modulator according to embodiment 1. The semiconductor optical modulator has an optical input section 7 in contact with the input end, a branching section 5, two waveguides, a first arm 1a and a second arm 1b, one or more folding sections 3 formed in each arm, one or more phase adjustment sections 10, a modulation section 20, a multiplexing section 6, and an optical output section 8. Each section is composed of a waveguide formed on a semiconductor substrate 2, and the waveguide constituting the first arm 1a and the waveguide constituting the second arm 1b are arranged adjacent to each other, including the folding section 3, and each transmits light from the branching section 5 to the multiplexing section 6.

[0015] Light is input through the optical input section 7 and branched into two at the branching section 5. In each of the first arm 1a and the second arm 1b, the light passes through a phase adjustment section 10 having a phase adjustment electrode 11 formed therein, a turning section 3 that changes the light's traveling direction by 180 degrees, and a modulation section 20 having a modulation electrode 21 formed therein. The light transmitted through the first arm 1a and the light transmitted through the second arm 1b are combined by the combining section 6 and output from the optical output section 8. The phase adjustment sections 10 are located on both sides of the turning section 3. Although two turning sections 3 are formed in FIG. 1, the effects of the present application can be achieved with at least one turning section 3. When a high-frequency voltage as an RF electrical signal is applied to the modulation electrode 21 of the modulation section 20, the refractive index of the waveguide changes, and the light transmitted through the modulation section 20 is phase-modulated, generating a phase-modulated signal.

[0016] When performing phase modulation, it is necessary to set the operating point by the phase adjustment unit 10 to an operating point at which the modulation operation of the modulation unit 20 is appropriate. In other words, when phase modulation is not performed in the modulation unit 20, a DC voltage is applied to the phase adjustment electrode 11 of the phase adjustment unit 10 to adjust the refractive index of the waveguide, thereby performing phase adjustment in the phase adjustment unit 10 so that the output light output from the optical output unit 8 becomes zero. Then, by applying RF electrical signals to the modulation electrodes 21 of the modulation units 20 of the first arm 1 a and the second arm 1 b, respectively, so that voltages in opposite directions (push-pull) are applied, a phase-modulated signal can be generated as light output from the optical output unit 8.

[0017] In the present application, the phase adjustment unit 10, which has a pin structure in a direction perpendicular to the surface of the semiconductor substrate 2, is configured as a forward current application type in which a forward current flows through the pin structure, and is configured so that the phase adjustment unit 10 is located on both sides of the folding unit 3. There is one or more folding units 3. In other words, when there are multiple folding units 3, the phase adjustment units 10 and the folding units 3 are formed alternately in the traveling direction of the light. As will be described later, the folding units 3 may also be configured to perform phase adjustment, so that the folding units 3 are included in the phase adjustment unit 10.

[0018] This application is characterized in that the phase adjustment unit 10 is a forward current application type. In the forward current application type, the refractive index changes linearly and gradually with respect to the phase adjustment current. FIG. 2 is a diagram showing an example of the change in output light intensity with a forward current when a forward current is applied as the phase adjustment current to the phase adjustment electrode 11 of the first arm 1a (to the right of 0 on the horizontal axis) and when a forward current is applied as the phase adjustment current to the phase adjustment electrode 11 of the second arm 1b (to the left of 0 on the horizontal axis). As shown in FIG. 2, the change in optical output with respect to the phase adjustment current is periodic. On the other hand, in the reverse bias application type, the refractive index changes abruptly from near a certain voltage, making it non-periodic, and the optical output also changes abruptly from near a certain voltage, making it difficult to control. Therefore, compared to the reverse bias application type, the forward current application type has the advantage of making it easier to control the phase adjustment.

[0019] The upper and lower graphs of Figure 3 show examples of loss characteristics (negative direction on the vertical axis indicates increased loss) for the reverse bias application type and the forward current application type, respectively. As shown in the upper graph of Figure 3, in the reverse bias application type, increasing the reverse bias voltage causes a rapid increase in loss along with a change in refractive index. This reduces the intensity of light transmitted through the arm to which the reverse bias is applied, making it difficult for the light to be extinguished by the interference of the two lights, resulting in a deterioration in the extinction ratio. On the other hand, as shown in the lower graph of Figure 3, in the forward current application type, there is no rapid increase in loss with respect to the current value. Figure 4 is a characteristic diagram showing an example of the characteristics of output light intensity versus reverse bias voltage for the reverse bias application type as a comparative example. This diagram shows the characteristics when the phase adjustment unit 10 is operated as the reverse bias application type in a configuration such as that of Figure 1. As the reverse bias on the first arm 1a increases, the extinction ratio reaches its maximum at point 1, where the extinction ratio reaches its maximum for the first time. However, the extinction ratio reaches its maximum at point 2, where the extinction ratio reaches its maximum for the second time. This is thought to be due to the influence of loss caused by the reverse bias.

[0020] An example of the output light intensity versus forward current characteristic of the forward current applied type is shown in Figure 5. In the forward current applied type, a forward current is applied to the phase adjustment electrode 11 on the first arm 1a to gradually increase the current, and of the extinction points that sequentially appear, which are local maximums of the extinction ratio, it can be seen that the extinction points with the second and third maximums (points 2 and 3) are more extinct than point 1, which is the first extinction point where the extinction ratio is maximized. Here, if the extinction ratio is defined as the ratio between the maximum and minimum of the optical output intensity when a forward current is applied, the extinction ratios are approximately 13 dB for point 1, 17 dB for point 2, and 21 dB for point 3. This is thought to be because the intensity of light transmitted toward the first arm 1a was greater than that of the second arm 1b due to manufacturing variations in the branching section 5, or because the loss in the second arm 1b was greater than that of the first arm 1a due to manufacturing variations in the waveguides of each arm.By applying a forward current to the first arm 1a side, the loss in the first arm 1a gradually increased, and the intensity of light transmitted through each arm became equal, which increased coherence and therefore improved the extinction ratio.

[0021] Incidentally, the extinction ratio deteriorates when a forward current is applied to the second arm 1b because the balance of the light intensities transmitted through each arm is further deteriorated. Because the balance of light intensities varies from one unit to another due to manufacturing variations, to improve the extinction ratio in all units, it is desirable to provide phase adjustment units in both arms, apply a forward current to the phase adjustment electrodes of each arm to obtain the extinction ratio characteristics, compare the extinction ratios, select the phase adjustment unit with the larger extinction ratio, and apply a forward current to the phase adjustment electrode of the selected phase adjustment unit to set the operating point (for example, in FIG. 2 , applying 3 mA to the first arm results in a larger extinction ratio than applying 7 mA to the second arm). Furthermore, as mentioned above, in some cases, it is desirable to set the operating point to a point with a larger extinction ratio, such as the second or third point or later. In a configuration that includes a folding section, such as the configuration disclosed in the present application, radiation loss of light occurs due to the folding section, and the magnitude of this loss is likely to vary depending on the individual unit. Therefore, in order to improve the extinction ratio, the phase adjustment section can adjust not only the phase but also the loss as described above, thereby making the transmitted light intensity of each arm section equal.

[0022] In the semiconductor optical modulator disclosed in the present application, the phase adjustment section 10 is of a forward current application type, and is formed on both sides of the folding section 3. Therefore, the overall length of the phase adjustment section 10 can be increased relative to the overall dimensions of the semiconductor optical modulator, and the current per unit length of the phase adjustment section 10 can be reduced, so that the amount of heat generated per unit length can be reduced and the influence of changes in refractive index due to heat can be suppressed.

[0023] The change in refractive index due to the carrier plasma effect in a forward current is proportional to the magnitude of the current, but the thermo-optic effect depends on the input power and is therefore proportional to the square of the current. Therefore, by lengthening the phase adjustment unit 10 and reducing the current density, the influence of the thermo-optic effect is rapidly weakened. In this way, by lengthening the phase adjustment unit 10 and sufficiently increasing the influence of the carrier plasma effect compared to the thermo-optic effect, it is possible to make it easier for the phase change to occur overall.

[0024] For example, when a forward current is applied to the phase adjustment unit 10 of the first arm 1a, if the phase adjustment unit 10 is sufficiently long, there is almost no thermal effect. However, if some heat is generated, it acts to reduce the refractive index change caused by the forward current. Furthermore, when the generated heat reaches the second arm 1b from the first arm 1a, the phase of the light passing through the second arm 1b changes. This means that if the heat generated in the first arm 1a does not reach the second arm 1b completely, the phase of the output light will not be stable. Therefore, if there is thermal interference between the first arm 1a and the second arm 1b, the response speed will be slow. Therefore, it is desirable to position the phase adjustment units 10 formed on the first arm 1a and the second arm 1b at a distance that does not cause thermal interference. It is desirable to separate the first arm 1a and the second arm 1b, for example, by 20 μm or more.

[0025] FIG. 6 shows the cross-sectional structure of the waveguide of the semiconductor optical modulator. The first arm 1a and the second arm 1b have the same cross-sectional structure. A core layer 40 is formed on an n-type lower cladding layer 45 on a semiconductor substrate 2. The core layer 40 is a semiconductor layer that changes its refractive index when a forward current is applied, thereby shifting the phase of propagating light. A p-type upper cladding layer 41 formed on the core layer 40 is a semiconductor layer with a lower refractive index than the core layer 40 to confine light within the core layer 40. A contact layer 42 formed on the p-type upper cladding layer 41 is a semiconductor layer with a lower resistance than the p-type upper cladding layer 41 to reduce the resistance when a voltage is applied to the semiconductor layer from the phase adjustment electrode 11 formed on it. The protective insulating film 44 is made of an insulating material, such as an inorganic insulating film such as an oxide film or a nitride film, or an organic insulating film such as benzocyclobutene (BCB), and serves to prevent the semiconductor from being oxidized or altered by oxygen or water in the atmosphere.

[0026] The semiconductor optical modulator shown in this embodiment has a high mesa structure in which the contact layer 42 to a portion of the n-type lower cladding layer 45 is etched by RIE or the like. In the high mesa structure shown in Fig. 6, the difference in refractive index between the core layer 40 and the etched region is large, so light can be tightly confined in the core layer 40. As a result, when the refractive index of the core layer 40 changes, the amount of phase change of the light increases, resulting in high modulation efficiency.

[0027] The semiconductor optical modulator according to the first embodiment, having the cross-sectional structure shown in Fig. 6, is manufactured as follows. On the epitaxially grown n-type lower cladding layer 45, a multi-quantum well layer made of AlGaInAs with a total thickness of 300 nm, which will become the core layer 40, a p-type InP with a thickness of 2000 nm, which will become the p-type upper cladding layer 41, and a p-type InGaAs with a thickness of 300 nm, which will become the contact layer 42, are epitaxially grown. Next, the semiconductor layers are selectively etched by RIE or the like to form the high mesa structure shown in Fig. 6, and a phase adjustment electrode 11 made of a metal such as Ti, Au, Pt, Nb, or Ni and a 300 nm thick SiO2 film are deposited on the contact layer 42 to form a thick protective insulating film 44 that protects the semiconductor surface. ２ The modulation section 20 is also formed using the same structure and manufacturing method as above.

[0028] Second Embodiment Fig. 7 is a schematic plan view showing the configuration of a semiconductor optical modulator according to a second embodiment. In the semiconductor optical modulator according to the first embodiment, the influence of the thermo-optic effect is reduced by alternately arranging the phase adjustment sections 10 and the folded sections 3 to increase the total length of the phase adjustment sections 10. In the second embodiment, as shown in Fig. 7, a phase adjustment electrode 13 is also formed on the folded section 3, and by making not only the folded sections 3 but also the folded sections 3 into phase adjustment sections 10, the phase adjustment sections 10 can be further increased in length, the current density can be further reduced, and the effect can be further enhanced.

[0029] Third Embodiment. Figure 8 is a schematic plan view showing the configuration of a semiconductor optical modulator according to a third embodiment. Increasing the width of the phase adjustment electrode 11 to improve heat dissipation can reduce the influence of the thermo-optic effect. It is effective to make the width of the phase adjustment electrode formed on the waveguide of each of the first arm 1a and the second arm 1b sufficiently wider than the waveguide width. At a position where adjacent arms are the first arms 1a or the second arms 1b, as shown in Figure 8, the electrode width of the phase adjustment electrodes 11a of the adjacent arms can be increased until they are continuously connected.

[0030] Although various exemplary embodiments and examples are described in this application, the various features, aspects, and functions described in one or more embodiments are not limited to the application of a particular embodiment, but may be applied to the embodiments alone or in various combinations. Therefore, countless modifications not illustrated are contemplated within the scope of the technology disclosed in this specification. For example, this includes cases where at least one component is modified, added, or omitted, and even cases where at least one component is extracted and combined with components of another embodiment.

[0031] 1a First arm, 1b Second arm, 2 Semiconductor substrate, 3 Folding section, 5 Branching section, 6 Multiplexing section, 10 Phase adjustment section, 11, 11a, 13 Phase adjustment electrode, 20 Modulation section, 21 Modulation electrode

Claims

1. two waveguides arranged side by side and running parallel to each other, which are formed on a semiconductor substrate and transmit light from a branching section to which light is input from an optical input section arranged on one end surface of the semiconductor substrate, to a combining section that outputs light to an optical output section arranged on the one end surface of the semiconductor substrate; a phase adjustment unit including a phase adjustment electrode for applying a DC voltage for phase adjustment to each of the two waveguides; a modulation section including a modulation electrode for applying a high-frequency voltage for modulation to each of the two waveguides; and the phase adjustment units are located on both sides of a turning unit that changes the traveling direction of light by 180 degrees, and there is at least one turning unit; the combining unit is disposed at a position from the optical output unit toward the other end face of the semiconductor substrate that is located on the opposite side to the one end face, the branching section is disposed at a position facing the other end face from the optical input section, and the phase adjusting section is present at least at a position facing the other end face from the branching section, The two waveguides are both formed of an InP-based material and have a pin structure in a cross section perpendicular to the direction of light propagation, and the operating point is set by applying a voltage to one of the phase adjustment electrodes that causes a forward current to flow through the pin structure waveguide.

2. 2. The semiconductor optical modulator according to claim 1, wherein an extinction ratio at an operating point set by applying a voltage that causes a forward current to flow to a phase adjustment electrode of the phase adjustment unit of one of the phase adjustment units of the two waveguides is compared with an extinction ratio at an operating point set by applying a voltage that causes a forward current to flow to the phase adjustment electrode of the phase adjustment unit of the other waveguide, the phase adjustment unit of the waveguide having the larger extinction ratio is selected, and the operating point is set by applying a voltage that causes a forward current to the phase adjustment electrode of the phase adjustment unit of the selected waveguide.

3. 3. The semiconductor optical modulator according to claim 1, wherein the forward current is gradually increased, and an operating point is set by applying a voltage that causes the forward current to flow corresponding to one of the second and subsequent extinction points among the extinction points that appear sequentially, which are maximum points of the extinction ratio.

4. 3. The semiconductor optical modulator according to claim 1, further comprising a phase adjustment electrode at the folded portion.

5. 3. The semiconductor optical modulator according to claim 1, wherein the phase adjustment electrodes of the two waveguides, which are adjacent to each other on both sides of the folded portion, are formed so that their widths in a direction perpendicular to the light transmission direction are continuously connected at adjacent portions.