JP4977788B1 - Light modulator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical modulator capable of compensating a phase difference between output light and monitor light of the optical modulator and having a smaller size with a simple structure. <P>SOLUTION: An optical modulator includes: a substrate 1 having an electro-optical effect; an optical waveguide 2 including Mach-Zehnder optical waveguides (21 to 24) formed on the substrate; a modulation electrode for modulating the light wave propagating along the optical waveguide; an optical fiber 4 for guiding the emission light from the optical waveguide; and a reinforcement capillary 3 for connection with the optical fiber at an end of the substrate. The reinforcement capillary is provided with reflection means (31 and 32) that reflects two emission light rays (R1 and R2) emitted from the Mach-Zehnder optical waveguides toward one light-receiving element 5, so that the two emission rays reflected on the reflection means intersect with each other. <P>COPYRIGHT: (C)2012,JPO&amp;INPIT

Description

  The present invention relates to an optical modulator, and more particularly, to an optical modulator having a configuration in which radiated light from a Mach-Zehnder type optical waveguide is detected by a light receiving element.

  In the optical communication field and the optical measurement field, an optical modulator such as an intensity modulator having a Mach-Zehnder type optical waveguide is frequently used. The Mach-Zehnder type optical waveguide has a configuration in which an input waveguide is branched into two, and the two branched waveguides are coupled to be connected to an output waveguide. Depending on the type of optical modulator, only one Mach-Zehnder type optical waveguide is used, or another Mach-Zehnder type optical waveguide is nested in the middle of each branching waveguide of one Mach-Zehnder type optical waveguide. Various forms exist, for example, in the case of incorporating into the device.

  When the light wave propagated through each branch waveguide of the Mach-Zehnder type optical waveguide is coupled in the same phase in the combined portion, the light wave output to the output waveguide is in the On state, and in the case of the opposite phase, the optical waveguide Are introduced into a radiation waveguide disposed so as to sandwich the radiation or output waveguide in the substrate on which the substrate is formed, and the output of the output waveguide operates so as to be in the off state. Hereinafter, a light wave output from the output waveguide in the On state is referred to as On light, and a light wave emitted from the combined portion in the Off state is referred to as Off light or radiated light.

  The intensity change of the light output from the Mach-Zehnder type optical waveguide shows a sinusoidal characteristic, so that it propagates through the Mach-Zehnder type optical waveguide to obtain the optimum output light intensity according to the application of the optical modulator. It is necessary to set the modulation signal applied to the modulation electrode for modulating the light wave to be set to an appropriate operation bias point.

  For this reason, conventionally, a part of output light (On light) derived from an optical fiber connected to the optical modulator, or Off light is detected as a monitor light by a light receiving element such as a photodetector, and light modulation is performed. Monitoring the state of the intensity of the output light of the vessel is 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).

  Even when the bias control is performed by the monitor as described above, in order for the output from the optical modulator to be appropriate, the output function of the optical fiber output of the optical modulator and the monitor output becomes the applied voltage to the modulation electrode. On the other hand, it is required to have an in-phase or anti-phase relationship and no phase difference therebetween. For this reason, a structure that prevents unnecessary light from being mixed into the monitor light and a structure that uses two off lights have been proposed.

  In conventional optical communication control, even if a slight deviation of the bias point has occurred in the monitor output, it has not been a serious problem. This is because the light level detected as a signal is the maximum transmission or minimum transmission level of the output function of the intensity modulator having a Mach-Zehnder type optical waveguide, and in this case, the waveform is shaped by the nonlinearity of the output function. A phase difference of a few percent was acceptable.

  On the other hand, with the increase in communication capacity in recent years, for example, when a multilevel modulation format such as differential quadrature phase shift keying (DQPSK) is used, the 1/2 strength point of the output function is used. Must be set so that becomes the output light level. In this case, since the bias point is set at a point sensitive to a change in light intensity, the operation bias point of the optical modulator is strictly set, for example, with respect to the half-wave voltage Vπ in order to keep the output signal quality good. It is necessary to perform control with an accuracy of 1% or less.

  By the way, as the structure of the Mach-Zehnder type optical waveguide, in the Y branch structure of the multiplexing unit, when light waves are input from the two branching waveguides in the same phase to the multiplexing unit, most of them are the output waveguides. It is changed to the basic mode and output as On light. However, some of the light is emitted in the same phase as the On light on both sides of the output waveguide as a conversion loss.

  In addition, when light waves are input from the two branching waveguides in opposite phases to the multiplexing unit, the output waveguide is usually designed to guide only the fundamental mode. Light having different phases (reverse phase) is emitted on both sides, and becomes Off light. As a result, not only the off-light (reverse phase) but also a part of the conversion loss light (changed to the same phase as the on-light) is mixed in the emitted light. However, a phase difference deviating from the reverse phase state is generated.

  For this reason, as in Patent Document 1, in the configuration in which only one of the radiated light is detected as the monitor light, a state shifted from the normal phase of the Off light is detected. It is difficult to accurately adjust the bias to a point.

  Further, in order to improve the performance of the optical modulator, when a thin plate structure having a substrate thickness of 20 μm or less is used, it is necessary to provide a radiated light waveguide at the combined portion as in Patent Document 2. In such a case, unnecessary light propagates without spreading through the thin plate substrate having the characteristics of a slab waveguide, so that various unnecessary light is likely to be mixed into the monitor output. It becomes easy to produce a phase difference between the main output which is an output and the monitor output.

  Furthermore, as shown in Patent Document 3, it has been proposed to improve the monitor characteristics by using two radiated lights radiated on both sides of the output waveguide as monitor lights. This is because the direction in which the phase difference between the radiated light occurs is different in sign between the monitor outputs, so that the deviation can be corrected by using both radiated lights.

  However, as in Patent Document 3, in order to obtain a monitor output, it is necessary to use a photodetector having a large light receiving surface or two photodetectors. In the former case, a photodetector having a large light receiving diameter has a large component size. Moreover, there is a problem that the high-speed frequency response of the monitor output is inferior. In the latter case, the number of parts is increased, the structure and connection are complicated, and this causes an increase in size and cost.

JP 2001-281507 A JP 2010-237376 A US Pat. No. 6,795,620

  The problem to be solved by the present invention is to solve the above-described problems, compensate for the phase difference between the output light of the optical modulator and the monitor light, and have a configuration that can be downsized with a simple configuration. An optical modulator is provided.

  In order to solve the above-mentioned problems, in the invention according to claim 1, a substrate having an electro-optic effect, an optical waveguide including a Mach-Zehnder type optical waveguide formed on the substrate, and a light wave propagating through the optical waveguide are modulated. An optical modulator having a modulation electrode, an optical fiber for guiding light emitted from the optical waveguide, and a reinforcing capillary for connecting to the optical fiber at an end of the substrate, the Mach-Zehnder type The reinforcing capillary is provided with reflecting means for reflecting two radiated light emitted from the optical waveguide toward one light receiving element, and the two radiated lights reflected by the reflecting means are configured to intersect each other. It is characterized by.

  According to a second aspect of the present invention, in the optical modulator according to the first aspect, the light receiving element is arranged at a position deviated from an intersection where the two radiated lights intersect.

  According to a third aspect of the present invention, in the optical modulator according to the second aspect, the light receiving element has a position where an optical path interval between the two radiated lights is 0.5 to 2 times a light receiving diameter of the light receiving element. It is characterized by being arranged in.

  According to a fourth aspect of the present invention, in the optical modulator according to any one of the first to third aspects, the substrate is provided with a radiated light waveguide for guiding the radiated light. To do.

  The invention according to claim 5 is the optical modulator according to any one of claims 1 to 4, wherein the thickness of the substrate is 20 μm or less.

  According to the first aspect of the present invention, a substrate having an electro-optic effect, an optical waveguide including a Mach-Zehnder type optical waveguide formed on the substrate, a modulation electrode for modulating a light wave propagating through the optical waveguide, 2 is emitted from the Mach-Zehnder type optical waveguide in an optical modulator having an optical fiber that guides light emitted from the optical waveguide and a reinforcing capillary connected to the optical fiber at the end of the substrate. The reinforcing capillary is provided with reflecting means for reflecting one radiated light toward one light receiving element, and the two radiated lights reflected by the reflecting means are configured to intersect each other. Using one small light receiving element, two radiated lights can be received simultaneously. The two radiated lights have characteristics that are opposite to each other in the positive and negative phases with respect to the output light of the optical modulator. Since the respective light intensities are added by simultaneous reception and the output characteristics are compensated, Monitor light (monitor output) having a phase opposite to that of the output light can be easily obtained.

  According to the second aspect of the present invention, the light receiving element is disposed at a position deviated from the intersection where the two radiated lights intersect, so that the two radiated lights are incident on the light receiving element only by adjusting the position of the light receiving element. The ratio of the light intensity can be easily adjusted, and the monitor output detected by the light receiving element can be compensated for an appropriate phase difference.

  According to the invention of claim 3, the light receiving element is disposed at a position where the optical path interval between the two radiated lights is 0.5 to 2 times the light receiving diameter of the light receiving element. The monitor output detected by the light receiving element can be compensated for an appropriate phase difference simply by moving the sensor in a plane perpendicular to the direction of propagation of the radiated light, resulting in high light receiving sensitivity and fluctuations in light receiving sensitivity. Can be reduced.

  According to the fourth aspect of the present invention, the substrate is provided with a radiated light waveguide that guides the radiated light, so that the position and direction of the radiated light emitted to the reinforcing capillary can be adjusted. By cooperating with the reflection means of the reinforcing capillary, it becomes possible to easily guide the two radiated lights to an appropriate position.

  According to the invention of claim 5, since the thickness of the substrate is 20 μm or less, even when there is a lot of unnecessary light confined in the thin plate substrate, it is possible to obtain accurate monitor light using the two radiated lights.

It is a figure explaining the 1st Example of the optical modulator of this invention. It is a figure explaining the relationship between the output light and radiated light in the optical modulator of this invention. It is a figure explaining the positional relationship of the propagation direction of a radiated light, and a light receiving element in the optical modulator of this invention. It is a figure explaining the relationship between the cross section of the emitted light in the optical modulator of this invention, and the light-receiving diameter of a light receiving element. It is a figure explaining the 2nd Example of the optical modulator of this invention. It is a figure explaining the 3rd Example of the optical modulator of this invention.

Hereinafter, the present invention will be described in detail using preferred examples.
FIG. 1 shows a first embodiment of the optical modulator of the present invention.
The present invention includes a substrate 1 having an electro-optic effect, an optical waveguide 2 including Mach-Zehnder optical waveguides (21 to 24) formed on the substrate, and a modulation electrode for modulating a light wave propagating through the optical waveguide (Not shown), an optical modulator having an optical fiber 4 for guiding light emitted from the optical waveguide, and a reinforcing capillary 3 for connecting to the optical fiber at an end of the substrate. Reflecting means (31, 32) for reflecting two radiated lights (R1, R2) emitted from the Zehnder type optical waveguide toward one light receiving element 5 is provided in the reinforcing capillary. The phase difference between the two radiated lights when they reach the light receiving element is not zero, and the two radiated lights reflected by the reflecting means intersect each other.

  In the present invention, by changing the cutting angle of the end portion of the reinforcing capillary 3 as shown in FIG. 1, two light beams (R1, R2) generated in the combined portion 23 are reduced to one light receiving element having a small light receiving diameter. 5 can receive light. Thereby, the phase difference between the monitor light and the optical fiber output light S can be compensated, and good monitor characteristics can be obtained.

  Specifically, the light intensity of the outgoing light S of the optical fiber is displayed by a graph A in FIG. The horizontal axis of the graph represents the bias voltage (input voltage) applied to the modulation electrode. The light intensities of the two emitted lights (R1, R2) are denoted by B and C. In general, when only the off-light is used, the intensity of the emitted light changes in a phase opposite to that of the output light, and the electric field amplitudes of the two emitted lights are opposite to each other. If a part of light that is a conversion loss in On light (in-phase state with On light) is mixed with this, the light intensities of the two radiated lights (R1, R2) are displayed as B and C in FIG. , They will shift in opposite directions to produce a phase difference. When the radiated lights B and C are simultaneously received, the influence of part of the light that causes the conversion loss is canceled, and the graph D can be obtained as the monitor output. Thus, the output characteristics of the emitted light are corrected, and a monitor output having no phase difference with respect to the output light output from the optical fiber can be obtained. That is, the monitor output D is in a phase opposite to that of the output light A. The height of each graph is standardized.

  Actually, the light intensity distribution when the radiated lights R1 and R2 in FIG. 1 reach the light receiving element is not the same. This means that the amplitude values of B and C in the graph of FIG. 2 are different. For this reason, even if both are incident on one light receiving element and the light intensity change is simply added, the output of the graph D cannot be obtained.

  For this reason, as shown in FIG. 3, the propagation directions of the two radiation lights R1 and R2 are set to intersect each other, and the light receiving surface of the light receiving element 5 is arranged in a predetermined range h while avoiding the intersection X. is doing. The “intersection” in the present invention is not limited to the case where the propagation direction of the radiated light is on the same plane.

  The light receiving element 5 can be installed with its position adjusted in the vertical and horizontal directions in FIG. As a result, as shown in FIG. 4, the position of the light receiving surface PD can be changed relative to the beam shape of the radiated light R1 and the beam shape of the radiated light R2, and the radiated light R1 incident on the light receiving element can be changed. And the light intensity of R2 can be changed.

  The position of the light receiving element 5 is such that the optical path interval d (peak interval of light intensity distribution) of the two radiated lights (R1, R2) is 0.5 to 2 times the light receiving diameter of the light receiving element (the diameter of the light receiving surface PD). It is preferable to arrange in the position. If it is less than 0.5 times, even if the light receiving element is moved in the left-right direction in FIG. 3 or FIG. It is difficult to efficiently receive both radiated light simultaneously with one light receiving element.

  The light receiving surface is not limited to a circular shape, and may be a rectangular shape. In this case, the “light receiving diameter” corresponds to the length in the direction corresponding to the arrow d with the light receiving surface PD of FIG.

  3 indicates a range in which the optical path interval d between the two radiated lights (R1, R2) is 0.5 to 2 times the light receiving diameter of the light receiving element. Further, the range h is arranged at a position deviating from the intersection X where two radiated lights intersect. By arranging the light receiving element at a position away from the intersection X, the ratio of the amount of received light of the two monitor lights can be adjusted while minimizing sensitivity fluctuations due to positional deviation, and the phase difference of the monitor output is adjusted. be able to.

  The reflecting means provided in the reinforcing capillary 3 may be a flat surface as shown in FIG. 1 or a curved surface. Further, by configuring the reflection means to totally reflect the emitted light, the emitted light can be efficiently directed toward the light receiving element. The means for total reflection is realized by setting the end face angle of the capillary so that the relationship between the optical axis of the radiated light and the reflection surface satisfies total reflection, or by forming a mirror on the capillary reflection surface using a metal film. it can.

  By installing the light receiving element 5 at a point where the optical axis of the monitor light is spaced about the light receiving diameter of the light receiving element by the optical modulator as shown in FIG. Output can be obtained. In addition, since the optical power input to the light receiving element is increased as compared with the conventional case of receiving one side of the emitted light, the sensitivity of the light receiving element for monitoring can be increased. Furthermore, a light receiving element having a small light receiving diameter can be used, and a monitor output excellent in frequency response can be obtained. For example, by setting the light receiving diameter to 100 μm or less, high-speed monitor light response characteristics of about several hundred M to several GHz can be realized.

  Next, as shown in FIG. 5, in order to guide the radiated light emitted from the combined portion of the Mach-Zehnder type optical waveguide, the radiated light waveguide 6 disposed so as to sandwich the output waveguide 24 is provided. It is preferable. In particular, when a thin plate having a substrate thickness of 20 μm or less is used, it is possible to efficiently separate it from output light by providing a radiation waveguide.

  In addition, by adjusting the shape of the radiated light waveguide 6, the position and direction in which the radiated light (R 1, R 2) is emitted to the reinforcing capillary 3 can be adjusted. In cooperation with the means (31, 32), it becomes possible to easily guide the two emitted lights to the proper positions.

  Next, as shown in FIG. 6, it is also possible to provide a notch 33 at the center of the reinforcing capillary. With this configuration, it is possible to suppress the decoupling light L of the optical fiber, which is emitted from the connection portion between the output waveguide 24 and the optical fiber 4 in the substrate, from entering the light receiving element, and to deteriorate the extinction ratio of the monitor output. Can be suppressed.

  In addition, as shown in FIG. 6, the substrate and the optical fiber or the reinforcing capillary are joined at an angle satisfying Snell's law, and it is also possible to prevent the light wave from being regularly reflected on the joining surface and going backward. is there.

  Furthermore, in the optical modulator of the present invention, the light receiving element is disposed before or after the propagation direction of the emitted light intersects as shown in FIG. Therefore, an optical modulator having an optimum phase difference can be obtained even when there is a manufacturing error.

  As described above, according to the optical modulator of the present invention, the optical modulator can compensate for the phase difference between the output light of the optical modulator and the monitor light and can be downsized with a simple configuration. Can be provided.

DESCRIPTION OF SYMBOLS 1 Substrate having electro-optic effect 2 Optical waveguide 21 Input waveguide 22 Branching waveguide 23 Combined portion 24 Output waveguide 3 Reinforcing capillaries 31 and 32 Reflecting means 33 Cut section 4 Optical fiber 5 Light receiving element 6 For emitted light Waveguide R1, R2 Synchrotron radiation (Off light, etc.)

Claims (5)

A substrate having an electro-optic effect, an optical waveguide including a Mach-Zehnder optical waveguide formed on the substrate, a modulation electrode for modulating a light wave propagating through the optical waveguide, and light emitted from the optical waveguide are guided. In an optical modulator having a waved optical fiber and a reinforcing capillary for connecting to the optical fiber at an end of the substrate,
Reflecting means for reflecting two radiated lights emitted from the Mach-Zehnder type optical waveguide toward one light receiving element is provided in the reinforcing capillary,
An optical modulator characterized in that the two radiated lights reflected by the reflecting means intersect each other.   2. The optical modulator according to claim 1, wherein the light receiving element is disposed at a position deviated from an intersection where the two radiated lights intersect.   3. The optical modulator according to claim 2, wherein the light receiving element is disposed at a position where an optical path interval between the two radiated lights is 0.5 to 2 times a light receiving diameter of the light receiving element. An optical modulator.   4. The optical modulator according to claim 1, wherein a radiation wave guide for guiding the emitted light is formed on the substrate.   5. The optical modulator according to claim 1, wherein the substrate has a thickness of 20 [mu] m or less.

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