JP4538721B2 - Mach-Zehnder optical modulator - Google Patents

Description

  The present invention relates to a Mach-Zehnder optical modulator that inputs signal light and outputs intensity-modulated signal light, and more specifically, temperature drift that can be used for an optical fiber communication system, an electric field sensor, or a pulse thinning-out device for repeated laser pulses. The present invention relates to a Mach-Zehnder type optical modulator with less (variation of output light due to temperature variation).

As shown in FIG. 9, a Mach-Zehnder type optical modulator manufactured using an electro-optic crystal typified by LiNbO 3 as a substrate has a single input optical waveguide 81 with a Y branch 85 and two interference arm optical waveguides 82, 82. Branched to ', and crossed again to one output optical waveguide 81' at Y branch 85 '(see, for example, Patent Document 1). Signal light I ₀ input to the input waveguide 81 is bisected by the Y branch 85 I _0/2, and the propagating interference arm waveguide 82 and 82 ', respectively. Then, it is combined at the Y branch 85 'and output from the output optical waveguide 81'. The two interference arm optical waveguides 82 and 82 'are formed to have the same length, and ideally, the signal light modulated according to the modulation curve shown by the dotted line in FIG. 10 is output. That is, if the electrode 83b is grounded and no voltage is applied to the electrode 83a, there is no phase difference in the bisected signal light propagating through the interference arm optical waveguides 82 and 82 '. Are added as they are, and the signal light output from the output optical waveguide 81 ′ is turned ON at I ₀ . On the other hand, when a Vπ voltage that causes a phase difference π to the bisected signal light is applied to the electrode 83a, when the bisected signal light is combined at the Y branch 85 ′, they cancel each other and output optical waveguides No signal light is output from 81 'and is turned off. Therefore, the ON / OFF extinction ratio when the voltage is not applied and when the voltage is applied is zero. The reason why the phase difference is generated when a voltage is applied to the electrode 83 is that the refractive index of the interference arm optical waveguides 82 and 82 'changes due to the electro-optic effect.

Ideally as described above, in practice, the two interference arm optical waveguides 82 and 82 'are exactly the same, that is, the length of the waveguide, the cross-sectional shape and the refractive index, the positional relationship with the electrode, etc. Since it cannot be the same, for example, the shift is made to a modulation curve indicated by a solid line in FIG. Therefore, the operating point shifts due to this shift, and the extinction ratio deteriorates. Moreover, the shift amount cannot be made constant due to the influence of the manufacturing process of the individual modulators. Therefore, a shift amount is adjusted by doping a portion 84 of one interference arm optical waveguide 82 'with a dopant different from that of the interference arm optical waveguide 82'. However, there is a problem in that the shift amount adjusted at the turning angle fluctuates when the environmental temperature changes, and the extinction ratio deteriorates. That is, the conventional optical modulator has a problem that it is difficult to obtain a high ON-OFF extinction ratio because the operating point tends to fluctuate when the environmental temperature changes.
Japanese Patent Laid-Open No. 11-52315

  As described above, the conventional Mach-Zehnder type optical modulator has a problem that when the environmental temperature changes, the operating point tends to fluctuate and the extinction ratio tends to deteriorate.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a Mach-Zehnder type optical modulator in which the extinction ratio does not easily deteriorate even when the environmental temperature changes.

The present invention made to solve this problem is a Mach-Zehnder type optical modulator, a substrate having an electro-optic effect, a Mach-Zehnder type optical waveguide having two interference arm optical waveguides formed on the substrate, Two pairs of temperature control bodies that make the temperature or temperature variation of the two interference arm optical waveguides constant, and the two pairs of temperature control bodies are equidistant from the respective intermediate points of the two interference arm optical waveguides They are characterized in that they are symmetrically arranged on the two interference arm optical waveguides separated from each other with the intermediate point therebetween.

  Even if the environmental temperature changes, the temperature or temperature fluctuation of the two interference arm optical waveguides is made constant by the temperature control body, so that the operating point hardly changes and the extinction ratio hardly deteriorates.

The Mach-Zehnder optical modulator may further include a heat uniform member that is between the substrate and the temperature control body and that uniformly applies heat from the temperature control body to the two interference arm optical waveguides.

  Heat from the temperature control body can be applied uniformly to the two interference arm optical waveguides via the heat uniform member, and the temperature of the two interference arm optical waveguides can be made more constant even if the environmental temperature changes. . Furthermore, even if the temperature fluctuations of the two interference arm optical waveguides cannot be made completely constant, the temperature fluctuations of the two interference arm optical waveguides can be made constant. As a result, the operating point is less likely to fluctuate and the extinction ratio is less likely to deteriorate. Moreover, it can be made constant with a small temperature control body. Since it becomes more constant, it is possible to suppress the deterioration of the extinction ratio and increase the extinction ratio.

Also, it is preferable prior SL thermal uniformity member also serves as a case for accommodating the substrate.

  Since the substrate is housed in the case and shielded from the outside air, the temperature of the two interference arm optical waveguides can be made more constant even if the environmental temperature changes.

  Since the temperature control body is arranged so as to be in the same positional relationship as one and the other of the two interference arm optical waveguides, the two interference arm optical waveguides can be heated uniformly with a small temperature control body, and the temperature can be easily fixed. It is. Furthermore, even if the temperature fluctuations of the two interference arm optical waveguides cannot be made completely constant, the temperature fluctuations of the two interference arm optical waveguides can be made constant. As a result, the operating point is less likely to fluctuate, and the extinction ratio is less likely to deteriorate.

Further, the Mach-Zehnder optical modulator may further have a heat-insulating container for blocking the flow of heat with the surroundings.

  Since the heat insulating container for blocking the heat flow with the surroundings is provided, the temperatures of the two interference arm optical waveguides can be stably stabilized for a long time. Furthermore, it becomes easy to make the temperature fluctuation in the two interference arm optical waveguides constant.

  Since it has a temperature control body that makes the temperature or temperature fluctuation (temperature fluctuation given to the two interference arm waveguides) of the two interference arm optical waveguides constant, the two interference arm light guides can be used even if the environmental temperature changes. The temperature or temperature fluctuation of the waveguide is made constant by the temperature control body, the operating point does not fluctuate, and the extinction ratio does not deteriorate.

  The temperature of the two interference arm optical waveguides can be stably stabilized for a long time by having a heat insulating container that blocks heat flow from the surroundings.

  The best mode for carrying out the present invention will be described with reference to the drawings. FIG. 1 is a schematic plan view of a Mach-Zehnder optical modulator according to the first embodiment, and FIG. 2 is a cross-sectional view taken along line A1-A1 of FIG.

  The Mach-Zehnder type optical modulator is obtained by forming a Mach-Zehnder type optical waveguide 2, a signal electrode 13a, a ground electrode 13b, temperature control bodies 14, 14 ', and temperature sensors 15, 15' on a substrate 1 having an electro-optic effect.

  For the substrate 1 having the electro-optic effect, lithium niobate (LiNbO3), lithium tantalate (LiTaO3), or the like can be used. In the case of using lithium niobate, any of an X plate, a Y plate, and a Z plate can be used, but it is preferable to use an X plate in order to reduce deterioration of the extinction ratio due to temperature fluctuation.

  The Mach-Zehnder type optical waveguide 2 is obtained by branching an input optical waveguide 21 into two interference arm optical waveguides 23 and 23 'by a Y branch 22, and intersecting the output optical waveguide 21' by a Y branch 22 '.

  In order to form the Mach-Zehnder type optical waveguide 2 on the substrate 1, first, a photoresist for transferring and forming the waveguide pattern is applied by a spin coater or the like. Next, exposure / development is performed using a photomask on which waveguide patterns symmetrical to the center lines C1 and C2 (see FIG. 1) are drawn, thereby forming a waveguide pattern. Next, in order to form a waveguide, for example, titanium is vapor-deposited, and the photoresist and titanium thereon are removed. Next, the substrate 1 is heat-treated at 900 to 1100 ° C. to thermally diffuse titanium, whereby the Mach-Zehnder type optical waveguide 2 is formed.

  In order to form the electrodes 13a and 13b on the substrate 1, first, a photoresist for transferring the electrode pattern is applied by a spin coater, and exposure and development are performed using a photomask on which the electrode pattern is drawn to form the electrode pattern. To do. Next, for example, gold plating is performed by an electroplating method to form gold electrodes (signal electrode 13a and ground electrode 13b).

  The temperature control body 14 provided on one interference arm optical waveguide 23 and the temperature control body 14 ′ provided on the other interference arm optical waveguide 23 ′ have a film shape such as ruthenium oxide (RuO 2). A heater or a ceramic heater embedded with nichrome wire can be used. In order to form the film heater on the substrate 1, first, a photoresist for transferring the temperature control body pattern is applied, and the temperature control body pattern symmetrical to the center lines C1 and C2 (see FIG. 1) is drawn. Exposure / development is performed using a photomask to form a temperature control pattern. Next, for example, a paste prepared by kneading RuO2 powder with water is applied and fired at 700 ° C. to form temperature control bodies 14 and 14 ′. Since the temperature control bodies 14 and 14 ′ are formed on the interference arm optical waveguides 23 and 23 ′ with the center lines C1 and C2 (or C3) as the center of symmetry, the temperature control bodies 14 and 14 ′ are the interference arm light guides. This means that they are arranged in the same positional relationship as the waveguides 23 and 23 ′.

  For example, a film thermistor or the like can be used for the temperature sensors 15 and 15 ′ provided at the intersections between the interference arm optical waveguides 23 and 23 ′ and the center line C <b> 1. The film thermistor uses semiconducting (Ba, Sr) TiO3 or the like as a resistance temperature sensor.

  Reference numeral 10 denotes an input optical fiber connected to the input optical waveguide 21. Reference numeral 10 'denotes an output optical fiber connected to the output optical waveguide 21'.

  By connecting the temperature sensors 15 and 15 ′ and the temperature control bodies 14 and 14 ′ to a temperature controller (not shown), the temperature of the interference arm optical waveguides 23 and 23 ′ is made constant regardless of the environmental temperature, and the extinction ratio is set. Can be kept high. Alternatively, even if the ambient temperature changes, the extinction ratio can be maintained high by making the temperature fluctuations in the two interference arm optical waveguides constant.

  Next, a Mach-Zehnder optical modulator according to the second embodiment will be described with reference to FIGS. FIG. 3 is a schematic plan view of a Mach-Zehnder optical modulator according to the second embodiment, and FIG. 4 is a cross-sectional view taken along line A2-A2 of FIG. In addition, the same number is attached | subjected to the same component as 1st Embodiment, and description is abbreviate | omitted. Further, the electrodes 13a and 13b and the optical fibers 10 and 10 'of the first embodiment are omitted.

  In the second embodiment, the heat uniform member 17 is provided between the substrate 1 and the temperature control bodies 16 and 16 ′, and the temperature control bodies 16 and 16 ′ have a built-in temperature sensor. This is the same as the first embodiment. That is, in the first embodiment, the temperature control bodies 14 and 14 ′ are directly provided on the interference arm optical waveguides 23 and 23 ′ of the substrate 1. However, in the second embodiment, the temperature control bodies 16 and 16 ′ are heated. The uniform member 17 is provided. Further, in the first embodiment, the temperature control bodies 14 and 14 'and the temperature sensors 15 and 15' are separated, but in the second embodiment, the temperature control bodies 16 and 16 'have a built-in temperature sensor. .

  The heat uniform member 17 has, for example, a metal box shape, and is attached to the substrate 1 so as to cover the optical waveguide between the Y branch 22 and the Y branch 22 ′. As the metal, aluminum having high thermal conductivity is preferable.

  The temperature control bodies 16 and 16 'are, for example, commercially available temperature control chips with built-in heaters and thermistors, and have the same shape, size, specifications and performance. The temperature control bodies 16 and 16 'are attached to the heat uniform member 17 so that the center line C2 is the center of symmetry. The temperature control bodies 16 and 16 'are attached to the heat uniform member 17 so that the center line C1 is the center of symmetry. That is, the temperature control body 16 ′ is located at a symmetrical position with the center line C <b> 1 of the temperature control body 16 as the center of symmetry.

  By connecting the temperature control bodies 16 and 16 'to a temperature controller (not shown), the temperature of the interference arm optical waveguides 23 and 23' can be kept constant regardless of the environmental temperature, and the extinction ratio can be kept high. In addition, in the present embodiment, the interference arm optical waveguides 23 and 23 'are controlled by heating via the heat uniform member 17, so that the number of temperature control bodies can be reduced by half compared to the first embodiment, and the cost can be reduced. Can be achieved. For example, a temperature control chip incorporating a Peltier element and a thermistor may be used as the temperature control bodies 16 and 16 '. Since heating and cooling can be performed with only the temperature control chip, the time required to reach a constant temperature can be shortened.

  Next, a Mach-Zehnder optical modulator according to a third embodiment will be described with reference to FIGS. FIG. 5 is a schematic plan view of a Mach-Zehnder optical modulator according to the third embodiment, FIG. 6 is a cross-sectional view taken along line A3-A3 in FIG. 5, and FIG. 7 is a cross-sectional view taken along line A4-A4 in FIG. In addition, the same number is attached | subjected to the same component as 1st, 2nd embodiment, and description is abbreviate | omitted. Further, the electrodes 13a and 13b in the first embodiment are omitted.

  In the third embodiment, the heat uniform member 17 of the second embodiment is changed to a case 18 for housing the substrate 1, and temperature control bodies 16 and 16 ′ are attached to the upper surface 181 and the lower surface 181 ′ of the case 18. Other than that, the second embodiment is the same as the second embodiment. As shown in FIG. 6, the two temperature control bodies 16 have the same center as the center line C3 of the two interference arm optical waveguides 23 and 23 '. Similarly, as shown in FIG. 7, the two temperature control bodies 16 'also have the same center as the center line C3 of the two interference arm optical waveguides 23 and 23'. Accordingly, the temperature control bodies 16 and 16 'and the interference arm optical waveguides 23 and 23' are in the same positional relationship in the cross section. Further, as shown in FIG. 5, the temperature control bodies 16 and 16 ′ are arranged so as to have the symmetry center C1 of the interference arm optical waveguides 23 and 23 ′ as the symmetry center. The bodies 16 and 16 ′ and the interference arm optical waveguides 23 and 23 ′ are in the same positional relationship.

  By connecting the temperature control bodies 16 and 16 ′ to a temperature controller (not shown), the temperature of the interference arm optical waveguides 23 and 23 ′ can be easily made constant regardless of the environmental temperature, and the extinction ratio can be kept high. . Further, even if the temperature of the interference arm optical waveguides 23 and 23 ′ cannot be made completely constant, temperature fluctuations in the two interference arm optical waveguides 23 and 23 ′ can be easily made constant, and the extinction ratio can be kept high. it can. In addition, in the present embodiment, the substrate 1 is accommodated in the case 18, and the temperature control bodies 16 and 16 'are provided on the upper surface 181 and the lower surface 181' of the case 18, so that the extinction ratio is further reduced without being affected by environmental temperature fluctuations. Can be kept high.

  Next, a Mach-Zehnder optical modulator according to a fourth embodiment will be described with reference to FIG. FIG. 8 is a cross-sectional view of a Mach-Zehnder optical modulator according to the fourth embodiment. The Mach-Zehnder type optical modulator of the present embodiment is the one in which the Mach-Zehnder type optical modulator of the third embodiment is housed in a heat insulating container 19, and the same elements as those of the third embodiment are denoted by the same reference numerals and described. Is omitted. Although the optical waveguide 2 is omitted in FIG. 8, the temperature control bodies 16 and 16 'and the interference arm optical waveguides 23 and 23' are in the same positional relationship as in the third embodiment. The heat insulating container 19 can be made of ceramics, polystyrene foam, heat insulating sponge, or the like. Instead of storing in the heat insulating container 19, a heat insulating material may be attached to the outer peripheral surface of the case of the Mach-Zehnder type optical modulator containing the case of the third embodiment. The heat insulation container 19 of FIG. 8 is comprised with the heat insulation sponge of thickness 5mm, for example.

  In the present embodiment, since the Mach-Zehnder type optical modulator containing the case of the third embodiment is housed in the heat insulating container 19, the flow of heat between the interference arm optical waveguide and the outside air is blocked, and the environmental temperature varies. Also, the temperature of the interference arm optical waveguide can be made constant with high accuracy.

  The Mach-Zehnder type optical modulator of this example is the optical modulator of the third embodiment shown in FIGS. 5 to 7, and the temperature control chips 16 and 16 ′ with built-in heaters and thermistors are thermally conductive adhesives. The case 18 is fixed to the upper surface 181 and the lower surface 181 ′. Two temperature control chips 16, 16 'were connected to a temperature controller (not shown) to control the temperature of the chips 16, 16' to 40 ° C. When the ambient temperature was varied in the range of 10 to 40 ° C. while controlling the temperature of the chips 16 and 16 ′ to 40 ° C., the maximum temperature variation distribution error on the outer peripheral surface of the case 18 was 0.1 to 0. It was 2 ° C.

  While changing the environmental temperature to 10 to 40 ° C., a modulation voltage is applied between the electrodes 13a and 13b (see FIG. 1) omitted in FIGS. 5 to 7, and signal light is incident from the input fiber 10 to output light. When the signal light output from the fiber 10 ′ was detected by a photodetector, an extinction ratio of 25 db was obtained.

  The Mach-Zehnder type optical modulator of this example is the optical modulator of the fourth embodiment shown in FIG. That is, the light modulator of Example 1 is housed in the heat insulating container 19.

  While changing the environmental temperature to 10 to 40 ° C., a modulation voltage is applied between the electrode 13a and the electrode 13b (see FIG. 1) omitted in FIG. When the signal light output from 'was detected by a photodetector, an extinction ratio of 40 db was obtained.

[Comparative Example 1]
The Mach-Zehnder type optical modulator of this comparative example is obtained by removing the temperature control chips 16 and 16 'from the optical modulator of the first embodiment.

  While changing the environmental temperature to 10 to 40 ° C., a modulation voltage is applied between the electrodes 13a and 13b (see FIG. 1) omitted in FIGS. 5 to 7, and signal light is incident from the input fiber 10 to output light. When the signal light output from the fiber 10 ′ was detected by a photodetector, only an extinction ratio of 15 db was obtained at the maximum.

1 is a plan view of a Mach-Zehnder optical modulator according to a first embodiment. It is A1-A1 sectional drawing of FIG. It is a top view of the Mach-Zehnder type optical modulator of 2nd Embodiment. It is A2-A2 sectional drawing of FIG. It is a top view of the Mach-Zehnder type optical modulator of 3rd Embodiment and 1st Example. It is A3-A3 sectional drawing of FIG. It is A4-A4 sectional drawing of FIG. It is sectional drawing of the Mach-Zehnder type | mold optical modulator of 4th Embodiment and 2nd Example. It is a top view of the conventional Mach-Zehnder type | mold optical modulator. It is a modulation curve which shows the relationship between a modulation voltage and output light.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ..... Substrate 2 ..... Mach-Zehnder type optical waveguides 14, 14 ', 16, 16' .... Temperature control body 17 · · · · · · Heat uniform member 18 ··········································· '・ ・ ・ ・ ・ ・ ・ ・ ・ Interference arm optical waveguide

Claims (4)

A substrate exhibiting an electro-optic effect;
A Mach-Zehnder type optical waveguide having two interference arm waveguides formed on the substrate;
Two pairs of temperature control bodies that make the temperature or temperature fluctuation of the two interference arm optical waveguides constant, and
The two pairs of temperature control bodies are symmetrically arranged on the two interference arm optical waveguides equidistant from the intermediate points of the two interference arm optical waveguides with the intermediate point therebetween, respectively. A characteristic Mach-Zehnder type optical modulator.   2. The Mach-Zehnder according to claim 1, further comprising a heat uniforming member between the substrate and the temperature control body, which uniformly applies heat from the temperature control body to the two interference arm optical waveguides. Type optical modulator.   The Mach-Zehnder optical modulator according to claim 2, wherein the heat uniform member also serves as a case for housing the substrate.   The Mach-Zehnder optical modulator according to claim 1, further comprising a heat insulating container that blocks a flow of heat from the surroundings.

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