JP2015138145A - Optical modulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical modulator that enhances light-receiving sensitivity of a light-receiving element and suppresses reduction in a frequency range of the light-receiving element even when the light-receiving element is disposed on a substrate constituting the optical modulator.SOLUTION: The optical modulator includes a substrate 1, an optical waveguide 27 including a Mach-Zehnder optical waveguide formed on the substrate, and a modulation electrode (not shown in the figure) for modulating light waves propagating the optical waveguide. A light-receiving element 5 is disposed on the substrate and configured to receive either signal light propagating an output waveguide that constitutes the Mach-Zehnder optical waveguide or radiation light radiated from a multiplexing part of the Mach-Zehnder optical waveguide. In the light waves incident to a light-receiving part 50 in the light-receiving element, both of light waves L1 derived from light waves incident to the light-receiving element and directly incident to the light-receiving part and light waves L2 incident after at least one reflection or multiple times of reflection in the light-receiving element are present.

Description

  The present invention relates to an optical modulator. In particular, in an optical modulator having a Mach-Zehnder type optical waveguide, signal light propagating through an output waveguide and radiated light emitted from a multiplexing unit are detected by a light receiving element. The present invention relates to an optical modulator having a configuration.

  Various optical modulators such as an intensity modulator having a Mach-Zehnder type optical waveguide are used in the optical communication field and the optical measurement field. The intensity change of light output from the Mach-Zehnder type optical waveguide exhibits a sinusoidal characteristic with respect to the voltage applied to the modulation electrode. In order to obtain an optimum output light intensity according to the use of the optical modulator, the modulation signal applied to the modulation electrode needs to be set to an appropriate operating bias point.

  For this reason, conventionally, a part of the signal light output from the optical modulator or the radiated light emitted from the multiplexing part of the Mach-Zehnder optical waveguide is used as a monitor light by a light receiving element such as a photodetector. Detection and monitoring of the intensity state of the output light of the light modulator are performed. Based on the detection value (monitor output) of the light receiving element, the operation bias point of the modulation signal applied to the modulation electrode is adjusted (bias control).

  Patent Document 1 discloses an optical modulator as shown in FIG. 1 that monitors emitted light with a light receiving element 5 disposed outside the substrate 1. Specifically, an optical waveguide 2 including Mach-Zehnder optical waveguides (21 to 24) is formed on a substrate 1 having an electro-optic effect. A modulation electrode for modulating a light wave propagating through the optical waveguide is provided along the two branch waveguides constituting the Mach-Zehnder optical waveguide, but is omitted in the drawing. An optical fiber 4 is connected to the output waveguide 24 and is configured to guide outgoing light to the outside.

  The two radiated lights (R1, R2) emitted from the multiplexing unit 23 of the Mach-Zehnder type optical waveguide pass through the inside of the reinforcing capillary 3 for connecting the optical fiber 4 to the end of the substrate 1, and receive the light. Introduced into the element 5. Either one of the two radiated lights may be received, or both radiated lights may be received by one light receiving element as shown in FIG.

  Patent Document 2 discloses a configuration in which a light receiving element 5 is arranged on a substrate 1 constituting an optical modulator, as shown in FIGS. Specifically, an optical waveguide 2 including a Mach-Zehnder optical waveguide and a modulation electrode (not shown) for modulating a light wave propagating through the optical waveguide are formed on the substrate 1. The light receiving element 5 is disposed so as to straddle the output waveguide 24 constituting the Mach-Zehnder type optical waveguide.

  In FIG. 2, the light receiving element 5 is configured to receive both two radiated lights emitted from the multiplexing portion of the Mach-Zehnder type optical waveguide. Although the emitted light propagates through the substrate 1, in order to accurately control the position where the emitted light propagates, it is possible to provide a emitted light waveguide (25, 26) for guiding the emitted light. The light receiving element 5 is disposed so as to cover the two radiated light waveguides (25, 26).

  3 is a cross-sectional view taken along one-dot chain line X-X ′ in FIG. 2. By disposing the high refractive index films (40, 41) in contact with or in proximity to the radiation waveguide (25, 26), part of the radiation (R1, R2) is sucked up toward the light receiving element 5. , Enters the light receiving unit 50.

  The light receiving element 5 can be configured not only to receive radiated light but also to receive a light wave propagating through the output waveguide 24. In addition, as shown in Patent Document 1 or 2, the monitor light using the radiated light and the output emitted from the output waveguide are received by simultaneously receiving two radiated lights and adjusting the received light intensity of both. It is possible to compensate for the phase difference from light and to obtain good monitor characteristics.

  When the light receiving element 5 is arranged outside the substrate 1 as in Patent Document 1, a plurality of Mach-Zehnder type optical waveguides are integrated on the same substrate, or a Mach-Zehnder type at a position away from the end face of the substrate. When an optical waveguide is formed and another optical waveguide exists between the Mach-Zehnder type optical waveguide and the capillary, it is extremely difficult to guide only the focused light wave to the light receiving element.

  On the other hand, the configuration in which the light receiving element is arranged on the surface of the substrate 1 as in Patent Document 2 has an advantage that the focused light wave can be received accurately. However, since only a part of the light wave propagating through the optical waveguide can be received, there arises a problem that the light receiving sensitivity is low. In addition, when the area of the light receiving portion is increased to improve the light receiving sensitivity, there is a problem that the frequency band of the light receiving element is reduced in inverse proportion to the increase in the area of the light receiving portion.

Japanese Patent No. 4777789 JP2013-80009A

  The problem to be solved by the present invention is to solve the above-described problems and increase the light receiving sensitivity of the light receiving element and the frequency band of the light receiving element even when the light receiving element is arranged on the substrate constituting the optical modulator. It is an object of the present invention to provide an optical modulator that suppresses a decrease in the above.

In order to solve the above problems, the optical modulator of the present invention has the following technical features.
(1) In an optical modulator having a substrate, an optical waveguide including a Mach-Zehnder optical waveguide formed on the substrate, and a modulation electrode for modulating an optical wave propagating through the optical waveguide, the light receiving element includes: It is arranged on the substrate and receives either the signal light propagating through the output waveguide constituting the Mach-Zehnder type optical waveguide or the radiated light emitted from the combining portion of the Mach-Zehnder type optical waveguide. The light wave incident on the light receiving unit in the light receiving element includes a light wave directly incident on the light receiving element and a light wave incident on the light receiving element through at least one multiple reflection. Are both present.

(2) In the optical modulator according to (1), the light wave received by the light receiving element is the signal light, and a high refractive index structure is formed between the output waveguide and the light receiving element, A low refractive index structure is formed between the portion of the substrate where the radiated light is propagated and the light receiving element.

(3) In the optical modulator according to (1), the light wave received by the light receiving element is the emitted light, and a low refractive index structure is formed between the output waveguide and the light receiving element, A high refractive index structure is formed between the portion of the substrate where the radiated light is propagated and the light receiving element.

(4) In the optical modulator described in (3) above, the light waves received by the light receiving element are two radiated lights emitted from the multiplexing unit, and the two high refractions sandwiching the low refractive index structure The spacing of the rate structures is characterized by being at least twice the mode diameter of the light wave propagating through the output waveguide.

(5) In the optical modulator according to (1), the light wave received by the light receiving element is guided to the light receiving element side by a groove formed on the substrate or a reflecting member disposed on the substrate. Features.

(6) In the optical modulator according to (5), the light wave received by the light receiving element is two radiated lights emitted from the multiplexing unit, and the two grooves or 2 sandwiching the output waveguide The interval between the two reflecting members is at least twice the mode diameter of the light wave propagating through the output waveguide.

(7) In the optical modulator described in the above (1) or (3) to (6), the light wave received by the light receiving element is two radiated lights emitted from the multiplexing unit. By adjusting the arrangement position with respect to the substrate, the ratio of the received light amounts of the two radiated lights is adjusted.

(8) The optical modulator according to any one of (1) to (7), wherein the substrate is provided with a radiated light waveguide that guides the radiated light.

(9) In the optical modulator according to any one of (1) to (8), the thickness of the substrate is 20 μm or less.

  According to the invention of claim 1, in an optical modulator comprising a substrate, an optical waveguide including a Mach-Zehnder optical waveguide formed on the substrate, and a modulation electrode for modulating a light wave propagating through the optical waveguide, The light receiving element is disposed on the substrate, and either of the signal light propagating through the output waveguide constituting the Mach-Zehnder type optical waveguide, or the emitted light emitted from the combining portion of the Mach-Zehnder type optical waveguide The light wave incident on the light receiving portion in the light receiving element includes a light wave directly incident on the light receiving element and at least one multiple reflection inside the light receiving element. Since both of the incident light waves are present, many light waves are incident on the same light receiving portion, so that the light receiving sensitivity of the light receiving element can be increased. In addition, since it is not necessary to increase the area of the light receiving portion of the light receiving element, the frequency band of the light receiving element is not reduced.

It is a prior art example described in Patent Document 1, and is a diagram showing a state in which a light receiving element is arranged outside a substrate constituting an optical modulator. It is a prior art example described in Patent Document 2, and is a diagram showing a state in which a light receiving element is arranged on a substrate constituting an optical modulator. It is a figure which shows sectional drawing in the dashed-dotted line X-X 'shown in FIG. It is a figure which shows the outline of the light receiving element used for the optical modulator of this invention.

Hereinafter, the optical modulator of the present invention will be described in detail.
As shown in FIG. 4, the optical modulator of the present invention includes a substrate 1, an optical waveguide 27 including a Mach-Zehnder optical waveguide formed on the substrate, and a modulation for modulating an optical wave propagating through the optical waveguide. In an optical modulator having an electrode (not shown), a light receiving element 5 is disposed on the substrate, and signal light propagating through an output waveguide constituting the Mach-Zehnder type optical waveguide, or the Mach-Zehnder type A light wave that is configured to receive any one of the radiated light emitted from the multiplexing portion of the optical waveguide and that is incident on the light receiving portion 50 in the light receiving element is a light wave that is directly incident on the light wave incident in the light receiving element. Both L1 and a light wave L2 incident through at least one multiple reflection inside the light receiving element are present.

The substrate 1 may be any substrate that can form an optical waveguide, such as quartz or semiconductor. In particular, one of LiNbO ₃ , LiTaO _{3, and} PLZT (lead lanthanum zirconate titanate), which is a substrate having an electro-optic effect. Single crystals can be suitably used. In particular, a substrate suitable for the present invention has a substrate thickness of 20 μm or less. In such a thin plate, the light wave is easily confined in the substrate, and it is difficult to separate the light wave propagating in the optical waveguide from the light wave propagating in the substrate. For this reason, by arranging the light receiving element on the substrate like the optical modulator of the present invention, it becomes possible to efficiently receive the focused light wave. In addition, since unnecessary light propagates through the substrate, more accurate monitor signals can be obtained by simultaneously receiving two radiated lights as shown in Patent Document 1 or 2.

The optical waveguide 27 formed on the substrate is formed, for example, by thermally diffusing a high refractive index material such as titanium (Ti) on a LiNbO ₃ substrate (LN substrate). Further, a rib-type optical waveguide in which grooves are formed on both sides of a portion that becomes an optical waveguide and a ridge-type waveguide in which the optical waveguide portion is convex can be used. Further, the present invention can also be applied to an optical circuit in which optical waveguides are formed on different substrates such as PLC and these substrates are bonded and integrated.

The modulation electrode is composed of a signal electrode and a ground electrode, and can be formed by forming a Ti / Au electrode pattern on the surface of the substrate and using a gold plating method or the like. Furthermore, a buffer layer such as a dielectric SiO ₂ can be provided on the substrate surface after the formation of the optical waveguide, if necessary. In a region where signal light or radiated light propagating in the substrate (optical waveguide) is led out to the light receiving element side, it is difficult to efficiently derive radiated light if a buffer layer is formed. Preferably does not form a buffer layer.

  As shown in FIG. 4, the optical modulator of the present invention is configured by a photodiode (PD) or the like when monitoring a part of the light wave L0 propagating in the optical waveguide 27 formed in or on the substrate. In order to increase the light receiving sensitivity of the light receiving element 5, not only the light wave incident in the light receiving element is directly incident on the light receiving unit 50 but also the light wave incident on the light receiving unit 50 through multiple reflections at least once inside the light receiving element. That is.

  In order to multiplexly reflect the radiated light within the substrate 51 constituting the light receiving element 5, it is preferable to arrange a reflective film such as a dielectric multilayer film or a metal reflective film at a location where the light wave is reflected. For example, as shown in FIG. 4, it is reflected on the lower surface of the metal film constituting the embedded electrode, or although not shown, a dielectric multilayer film or a metal film is disposed on at least a part of the outer peripheral surface of the substrate 51, It is possible to increase the reflection efficiency. In addition, by using a dielectric multilayer film or a metal film, stable reflection is possible regardless of the surface state (dirt or the like) of the light receiving element substrate.

  Since the refractive index of the substrate 51 of the light receiving element 5 is a high refractive index of 3.16 (in the case of InP), depending on the incident angle at which signal light and radiated light to be monitored enter the substrate of the light receiving element, It is also possible to use reflection. As a specific example, if the optical waveguide 27 is an optical waveguide (n = 2.15) formed on the LN substrate 1 and the material of the substrate 51 of the light receiving element is InP (n = 3.16), the light receiving element substrate is formed. The incident angle is about 42 degrees with respect to the normal line of the LN substrate. Further, the total reflection angle on the outer peripheral surface of the light receiving element substrate (assuming the surroundings is air) is about 18 degrees. For this reason, the light wave incident on the light receiving element substrate within the range of the incident angle of 18 to 42 degrees can realize multiple reflection by total reflection in the substrate.

  In FIG. 4, the light wave L <b> 2 that is multiple-reflected is composed of three reflections in total: reflection on the lower surface of the embedded electrode, reflection on the side wall surface of the substrate 51, and reflection on the lower surface of the substrate 51. The optical modulator of the present invention is not limited to this embodiment, and for example, it may be configured such that the light wave is incident on the light receiving unit 50 after being reflected only once by the side wall surface of the substrate 51.

  When multiple reflection is included once, the light receiving sensitivity is about 1.5 times. In addition, the time delay between the light wave L1 directly incident on the light receiving portion and the light wave L2 that is multiple-reflected in the PD substrate is very small, about 2 ps. This is equivalent to 500 GHz when converted to frequency, and the frequency response speed of the light receiving unit is at most about 40 GHz, so that the waveform of the monitor signal of the light receiving element is not distorted.

  In order to adjust the light waves L1 and L2 in FIG. 4 to be incident on the light receiving unit 50, for example, the arrangement position of the light receiving element 5 is adjusted with respect to the substrate 1 (or the optical waveguide 27), and the light receiving sensitivity is the highest. The light receiving element can be set to be fixed at a certain position.

  In order to introduce a light wave to be monitored in the light receiving element, as disclosed in Patent Document 2, only a method of bringing the light receiving element substrate 51 into direct contact with a portion where the light wave such as an optical waveguide is guided. Alternatively, as shown in FIG. 4, there is a method in which a high refractive index film (42, 43) is interposed between the optical waveguide 27 and the like and the light receiving element substrate 51. In this case, the refractive index of the high refractive index film may be set higher than the refractive index of the optical waveguide 27 (or the substrate 1 guiding the light wave) and lower than the refractive index of the light receiving element substrate 51. is necessary. As described above, a high refractive index structure (a structure having a higher refractive index than that of the portion where the light wave propagates) is provided between the portion where the light wave to be monitored propagates and the light receiving element, and on the other hand, it should be monitored A low refractive index structure (a structure having a lower refractive index than the portion where the light wave is propagating) is provided between the portion where the light wave that cannot be transmitted (the light wave whose incidence on the light receiving element should be suppressed) is propagating ing. As the low refractive index structure, not only a low refractive index film but also an air layer may be interposed.

  Further, as disclosed in Patent Document 2, it is also possible to arrange a groove or a reflecting member (not shown) on the substrate 1 (or the optical waveguide 27) to guide part of the monitor light toward the light receiving element. It is.

  The case where the light waves received by the light receiving element are two radiated lights emitted from the combining part of the Mach-Zehnder type optical waveguide as shown in FIGS. 2 and 3 and the two light waves are received simultaneously will be described.

  As shown in FIG. 3, when two high-refractive index structures (40, 41) sandwiching the low-refractive index structure 60 are arranged, the interval W between the high-refractive index structures is a light wave propagating through the output waveguide 24. It is preferably at least twice the mode diameter. This is to prevent the light receiving element from sucking light waves propagating through the output waveguide. Similarly, when a groove or a reflection member is disposed on the substrate 1 and the light wave to be monitored is deflected to the light receiving element side, similarly, the interval W between the two grooves or the two reflection members sandwiching the output waveguide 24 is It is preferably at least twice the mode diameter of the light wave propagating through the output waveguide.

  Further, in the optical modulator of the present invention as well, as in Patent Document 2, the ratio of the amount of received light of two radiated lights can be adjusted by adjusting the arrangement position of the light receiving element 5 with respect to the substrate 1 (or the optical waveguides 25 and 26). It becomes possible to adjust. In addition, as shown in FIG. 2, by providing the radiated light waveguides (25, 26) that guide the radiated light, the radiated light can be more efficiently guided to the light receiving element 5.

  As described above, according to the optical modulator of the present invention, even when the light receiving element is arranged on the substrate constituting the optical modulator, the light receiving sensitivity of the light receiving element is increased and the frequency band of the light receiving element is reduced. It is possible to provide a suppressed optical modulator.

DESCRIPTION OF SYMBOLS 1 Board | substrates 2 and 27 Optical waveguide 21 Input waveguide 22 Branching waveguide 23 Combined part 24 Output waveguide 25 and 26 Waveguide for radiated light 3 Capillary 5 Light receiving element 50 Light receiving part 40-43 High refractive index structure 60 Low refractive index Construction

Claims (9)

In an optical modulator having a substrate, an optical waveguide including a Mach-Zehnder optical waveguide formed on the substrate, and a modulation electrode for modulating a light wave propagating through the optical waveguide,
The light receiving element is disposed on the substrate, and either of the signal light propagating through the output waveguide constituting the Mach-Zehnder type optical waveguide, or the emitted light emitted from the combining portion of the Mach-Zehnder type optical waveguide Is configured to receive light,
The light wave incident on the light receiving portion in the light receiving element includes both the light wave directly incident on the light receiving element and the light wave incident on the light receiving element through at least one multiple reflection. An optical modulator characterized by:   2. The optical modulator according to claim 1, wherein the light wave received by the light receiving element is the signal light, and a high refractive index structure is formed between the output waveguide and the light receiving element, A light modulator characterized in that a low refractive index structure is formed between a portion through which radiated light is propagated and the light receiving element.   2. The optical modulator according to claim 1, wherein the light wave received by the light receiving element is the radiated light, and a low refractive index structure is formed between the output waveguide and the light receiving element, A light modulator characterized in that a high refractive index structure is formed between a portion through which radiated light is propagated and the light receiving element.   4. The optical modulator according to claim 3, wherein the light wave received by the light receiving element is two radiated lights emitted from the multiplexing unit, and an interval between the two high refractive index structures sandwiching the low refractive index structure. Is at least twice the mode diameter of the light wave propagating through the output waveguide.   2. The light modulator according to claim 1, wherein the light wave received by the light receiving element is guided to the light receiving element side by a groove formed in the substrate or a reflecting member disposed on the substrate. Modulator.   6. The optical modulator according to claim 5, wherein the light wave received by the light receiving element is two radiated lights emitted from the multiplexing unit, and the two grooves or the two reflecting members sandwiching the output waveguide. The optical modulator is characterized in that the interval is at least twice the mode diameter of the light wave propagating through the output waveguide.   7. The optical modulator according to claim 1, wherein the light wave received by the light receiving element is two radiated lights emitted from the multiplexing unit, and adjusts an arrangement position of the light receiving element with respect to the substrate. Thus, the ratio of the amount of received light of the two radiated lights is adjusted.   8. The optical modulator according to claim 1, wherein a radiated light waveguide for guiding the radiated light is formed on the substrate.   9. The optical modulator according to claim 1, wherein the thickness of the substrate is 20 [mu] m or less.

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