JP4367202B2 - Waveguide-type variable optical attenuator and attenuation gain control method - Google Patents

Description

  The present invention relates to a waveguide-type variable optical attenuator widely used in the field of optical communications and a method for controlling the attenuation gain thereof.

  In general, various optical waveguide elements are used. In particular, a waveguide type variable optical attenuator is used as an optical waveguide element for attenuating light.

  As shown in FIG. 4, the waveguide type variable optical attenuator 40 is a Mach-Zehnder type optical waveguide having two branched waveguides 44 and 45 between an input waveguide 42 and an output waveguide 43 formed on a substrate. Is formed. A thin film heater 49 serving as a phase shifter and electrode films 50 and 50 connected to both ends of the thin film heater 49 are formed on the clad surface above one branching waveguide 44. The electrode films 50 and 50 are connected to a power source via lead wires 51 and 51 in order to apply a voltage to the thin film heater 49.

  The variable optical attenuator 40 heats one branch waveguide 44 by applying a voltage to the thin film heater 49, and changes the refractive index of the branch waveguide 44. As a result, the signal light is branched by the input-side Y branching portion 47, and there is a phase difference between the signal light propagating through both branching waveguides 44 and 45, and the signal light is attenuated by optical interference at the output-side Y branching portion 48. At the design stage of the variable optical attenuator 40, the target value is designed based on the relationship between the optical path length difference between the two branch waveguides 44 and 45 in the Mach-Zehnder type optical waveguide and the temperature change (applied voltage value) by the thin film heater 49. A desired light attenuation is obtained.

  A waveguide-type variable optical attenuator that is stable with respect to environmental temperature is obtained by connecting two thin film heaters having the same absolute value of the temperature coefficient of resistance and different signs in series to one branching waveguide ( For example, see Patent Document 1).

  In addition, a plurality of waveguide type variable optical attenuators are connected in multiple stages in series, and energization of each waveguide type optical attenuator to the thin film heater is linked to each other, thereby obtaining one attenuation control characteristic as a whole. In some cases, the attenuation gain is stabilized (see, for example, Patent Document 2).

JP 2002-365597 A JP 2003-5139 A

  In the above-described waveguide variable optical attenuator 40 that controls the attenuation of signal light by using interference, the attenuation gain varies exponentially with respect to the power consumption in the thin film heater 49 due to voltage application. That is, in the range where the attenuation gain is small, the attenuation gain hardly changes with respect to the power consumption of the thin film heater 49, but in the range where the attenuation gain is large, the attenuation gain greatly changes relative to the power consumption of the thin film heater 49. Therefore, there has been a problem that it is difficult to stably perform the attenuation control.

  In addition, when a plurality of waveguide-type variable optical attenuators are connected in series in multiple stages, the overall optical attenuator length is inevitably long, and it is difficult to achieve high integration and miniaturization of the optical attenuators.

  Further, in the waveguide type optical attenuator as shown in Patent Document 1, the material of the thin film heater must be different, and in the manufacturing process, the material to be the thin film heater must be formed twice on the clad. Don't be. Since the number of film forming steps of the heater material is large, stress may be generated due to a difference in linear expansion coefficient between the clad and the thin film heater. This stress affects the polarization dependency of the optical waveguide, and in the Mach-Zehnder type waveguide, affects the deterioration of the extinction ratio. In addition, it is difficult to adjust the temperature coefficient of resistance from the film formation stage of the metal film.

  SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a waveguide type variable optical attenuator and an attenuation gain control method capable of solving the above-mentioned problems and performing signal light attenuation control accurately and stably.

In order to achieve the above object, according to the first aspect of the present invention, a Mach-Zehnder type optical waveguide having two branch waveguides between an input waveguide and an output waveguide is formed on a substrate, and the upper side of one branch waveguide is formed. In the waveguide type variable optical attenuator for adjusting the gain attenuation of the signal light by energization of the thin film heater formed on the thin film heater, a thin film heater for coarse adjustment for coarsely adjusting the gain attenuation of the signal light is provided above one branch waveguide ; A fine adjustment thin film heater having a resistance value larger than that of the coarse adjustment thin film heater and finely adjusting the gain attenuation of the signal light, and supplying power to the coarse adjustment thin film heater and the fine adjustment thin film heater. It is a waveguide type variable optical attenuator connected individually.

According to the invention of claim 2, a Mach-Zehnder type optical waveguide having two branch waveguides is formed between an input waveguide and an output waveguide on a substrate, and a thin film heater formed above one branch waveguide is energized. In the waveguide-type variable optical attenuator for adjusting the gain attenuation of the signal light by the above, a thin film heater for coarse adjustment for roughly adjusting the gain attenuation of the signal light, and the thin film heater for coarse adjustment above the one branch waveguide A fine adjustment thin film heater having a large resistance value and finely adjusting the gain attenuation of the signal light, and connecting the coarse adjustment thin film heater and the fine adjustment thin film heater in parallel to a power source, thereby adjusting the coarse adjustment. each use a thin film heater and the fine adjustment thin film heater is a waveguide type variable optical attenuator connected in series a variable resistor.

The invention according to claim 3 is the waveguide type variable optical attenuator in which the coarse adjustment thin film heater and the fine adjustment thin film heater are formed of the same material and have different shapes.

According to the invention of claim 4, when each resistance value of the one or more coarse adjustment thin film heaters and the one or more fine adjustment thin film heaters is R, the smallest resistance value of the coarse adjustment thin film heater is R, This is a waveguide type variable optical attenuator in which the resistance value of another thin film heater is R × C ⁿ (C is a constant). .

  According to a fifth aspect of the present invention, a Mach-Zehnder type optical waveguide having two branch waveguides is formed between an input waveguide and an output waveguide on a substrate, and a thin film heater formed above one branch waveguide is energized. In the waveguide type variable optical attenuator control method for adjusting the attenuation gain of signal light, a plurality of thin film heaters having different resistance values are provided above one branch waveguide, and a power source is connected to each of the thin film heaters. Attenuation of a waveguide type variable optical attenuator that finely adjusts the attenuation gain of signal light by energizing a thin film heater having a small resistance value and coarsely adjusting the attenuation gain of the signal light by energizing a thin film heater having a large resistance value This is a gain control method.

  According to the present invention, it is possible to achieve an excellent effect that signal light attenuation control can be stably performed.

  Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.

  FIG. 1 is a top view showing a preferred embodiment of a waveguide variable optical attenuator according to the present invention.

  As shown in FIG. 1, the waveguide variable optical attenuator 10 includes a core (optical waveguide) formed on a substrate, a clad covering the core, a thin film heater, and an electrode film.

The optical waveguide is formed between the input waveguide 12, the output waveguide 13, and the input / output waveguides 12 and 13, and the two branched waveguides 14 having the same waveguide length branched by the Y branch portions 16 and 17. , 15 and Mach-Zehnder type optical waveguides. The optical waveguide 12, 13, 14, 15, on a quartz substrate using a CVD method to deposit a Ge added SiO ₂ film, optical and Ge added SiO ₂ film by using a photolithography technique and a reactive ion etching technique A waveguide (core) pattern is formed, and a clad for covering the core is formed on the optical waveguide using a flame deposition method or a CVD method.

  On the surface of the clad 11 above one branching waveguide 14, two thin film heaters 18 and 19 serving as phase shifters are formed at a predetermined distance. The thin film heaters 18 and 19 have different resistance values and are arranged so as to be orthogonal to the branching waveguide 14. In the present embodiment, the thin film heaters 18 and 19 are formed of the same metal material into rectangles having the same length, and the resistance values are different by setting the widths to different values. Here, the width of the thin film heaters 18 and 19 represents a distance in a direction perpendicular to the direction of current flow through the heater, and the length of the thin film heaters 18 and 19 represents a length in a direction parallel to the direction of current flow. The width of the plurality of thin film heaters 18 formed above the branching waveguide 14 is half that of the thin film heater 19.

Thus, when a plurality of thin film heaters are formed, the width of the other thin film heaters is designed to be 1/2, 1/4... The resistance value of the wide thin film heater is the minimum, and the resistance values of the other thin film heaters are doubled, quadrupled, and so on. Furthermore, the relationship between the resistance values of the respective thin film heaters is not limited to the other resistance value being R × 2 ⁿ times the minimum resistance value R, and other resistance values are compared to the minimum resistance value R. It suffices if the value has a relationship of R × C ⁿ times (C is a constant).

  In the present embodiment, the width of the thin film heater 18 is 30 μm and the resistance value is 400Ω, the width of the thin film heater 19 is 60 μm, and the resistance value is 200Ω.

  Electrode films 21 and 20 are formed on both ends of the thin film heater 18, an electrode film 22 is formed on one end of the thin film heater 19, and an electrode 20 is connected to the other end of the thin film heater 19. The thin film heater 18 is connected to the electrode films 20 and 21, the electrode film 20 is connected to the power source 23 via the lead wire 25, the electrode film 21 is connected to the power source 23 via the lead wire 26, and the thin film heater 19 is connected to the electrode films 20 and 22. The electrode film 20 is connected to a power source 24 via a lead wire 25, and the electrode film 22 is connected to a power source 24 via a lead wire 27. That is, the thin film heater 18 and the power source 23 constitute one closed circuit, and the thin film heater 19 and the power source 24 constitute one closed circuit.

  Three or more thin film heaters may be formed above the branching waveguide 14. The thin film heaters 18 and 19 and the electrode films 20, 21 and 22 are formed by forming a metal film on the upper surface of the clad 11, and patterning the thin film heaters 18 and 19 and the electrode films 20, 21 and 22 using a photolithography technique. Formed.

  Next, the operation of the present embodiment will be described.

  The signal light incident on the input waveguide 12 is equally branched at the Y branching portion 16, propagates through the branching waveguides 14 and 15, joins at the Y branching portion 17, and propagates to the output waveguide 13. At that time, the thin film heaters 18 and 19 are energized by the power supplies 23 and 24. Due to the heat from the thin film heaters 18 and 19, the refractive index of the branching waveguide 14 changes due to the thermo-optic effect, and the optical path length difference between the branching waveguides 14 and 15 is different. That is, the thin film heaters 18 and 19 serve as a phase shifter for the signal light propagating through the branching waveguide 14, and the signal lights propagating through the branching waveguides 14 and 15 having different optical path lengths are combined at the branching unit 17. Attenuates due to interference.

  The variable optical attenuator 10 changes the temperature of the thin film heaters 18, 19 by changing the voltage applied to the thin film heaters 18, 19, and changes the refractive index (optical path length) of the branching waveguide 14 by the thermo-optic effect. The attenuation of the signal light can be freely controlled by adjusting the heater temperature, that is, the applied voltage.

  The variable optical attenuator 10 of the present embodiment is characterized by separately adjusting the temperatures of the thin film heaters 18 and 19 having different resistance values formed on one branch waveguide 14, so that the branch waveguides 14 and 15. The optical path length difference between them is controlled to control the attenuation gain of the signal light.

  The refractive index change of the branching waveguide 14 due to the thermo-optic effect is proportional to the amount of heat generated by the thin film heaters 18 and 19. As for the heat generation amount of the thin film heaters 18 and 19, when they are within the same voltage control range, the one with the smallest resistance value (thin film heater 19) has the largest power consumption, that is, the largest heat generation amount is the largest and the resistance value is the largest. (Thin film heater 18) has the smallest maximum calorific value.

  As shown in FIG. 2, the relationship between the power consumption of the heater thin films 18 and 19 and the applied voltage is an exponential relationship, and the characteristics differ depending on the resistance value. For example, in the thin film heater 18 having a resistance value of 400Ω, when the same voltage is applied to the heater 19 having a resistance value of 200Ω, the power consumption of the heater 19 is half of the power consumption of the heater 18.

  It is assumed that a voltage of 0 to 10 V is applied to the power sources 23 and 24 connected to the thin film heater 18 having a resistance value of 400Ω and the thin film heater 19 having a resistance value of 200Ω, respectively. The attenuation gain is controlled by applying a voltage to the thin film heater 19 and roughly adjusting the voltage near the desired power consumption (light attenuation amount) in the range of 0 to 10 V between the power consumptions of 0 to 0.5 W. While the applied voltage of the thin film heater 19 is fixed, a voltage is applied to the thin film heater 18 and fine adjustment is performed so as to obtain a desired power consumption. The power consumption of the thin film heater 18 is 0 to 0.25 W, and the power consumption range is half that of the thin film heater 19 even in the same voltage range as the thin film heater 19. Thereby, in the thin film heater 18, even if the attenuation gain of the thin film heater 19 is large, the power consumption can be adjusted more finely than the thin film heater 19.

  Therefore, as an attenuation gain control method, the applied voltage of the thin film heater 19 having a small resistance value is adjusted to roughly adjust the attenuation gain of the signal light, and the applied voltage of the thin film heater 18 having a large resistance value is adjusted to adjust the signal light. By finely adjusting the attenuation gain, a precise attenuation gain can be obtained.

  The waveguide type variable optical attenuator generates a signal strength due to an optical amplifier such as an EDFA or an add-drop installed between optical networks, so the dynamic range of the waveguide type variable optical attenuator itself needs to be widened. There is. Further, it is necessary to make a fine adjustment in order to cope with a change in the installation environment temperature of the waveguide type variable optical attenuator. The waveguide type variable optical attenuator 10 of this embodiment can form a plurality of thin film heaters having different resistance values, and can perform coarse adjustment and fine adjustment for each thin film heater according to the range of attenuation gain. The gain control range can be widened, and stable attenuation gain control can be performed with respect to the environmental temperature.

In this embodiment, the relationship between the resistance values of the respective thin film heaters is designed so that the other resistance value is R × 2 ⁿ times the minimum resistance value R. However, depending on the dynamic range of the attenuation gain, Thus, other resistance values may be R × C ⁿ times.

  The resistance values of the thin film heaters 18 and 19 are adjusted by the heater shape. Therefore, when the thin film heaters 18 and 19 are formed, the resistance value can be changed by changing the width and length thereof, and the resistance value can be easily adjusted. Further, since the resistance values of the thin film heaters 18 and 19 are adjusted according to the shape, a plurality of thin film heaters having different resistance values can be formed using the same material, and the heater material is formed on the upper surface of the clad 11 at once. Can do. This has the effect of suppressing an increase in stress that affects the extinction ratio of the Mach-Zehnder type optical waveguide by suppressing the number of depositions of the metal film as the heater material.

  Furthermore, since the variable optical attenuator 10 of this embodiment can stably perform attenuation gain control without connecting optical attenuators in multiple stages, it can be easily integrated and miniaturized.

  Next, another embodiment of the waveguide variable optical attenuator according to the present invention will be described.

  As shown in FIG. 3, the waveguide type variable optical attenuator 30 is obtained by changing the connection form of the thin film heater and the power source of the variable optical attenuator 10. The basic components are substantially the same as those of the variable optical attenuator 10 of FIG. 1 described above, and the same components are denoted by the same reference numerals as in FIG.

  The variable optical attenuator 30 is different from the variable optical attenuator 10 in that the two thin film heaters 18 and 19 are connected in parallel to the power source 31 and the electrode films 21 and 22 connected to the thin film heaters 18 and 19 are variable. This is the point where resistors 32 and 33 are connected in series.

  The waveguide type variable optical attenuator 30 has the same effect as the above-described waveguide type variable attenuator 10. However, the gain attenuation control of the signal light is performed by changing the resistance values of the variable resistors 32 and 33 because the thin film heaters 18 and 19 are connected in parallel to the power supply 31 to which the voltage is applied. By separately adjusting the resistance values of the variable resistors 32 and 33 connected to the thin film heaters 18 and 19, the power consumption of the thin film heaters 18 and 19 can be controlled separately. Therefore, as with the variable optical attenuator 10 described above, the attenuation gain is roughly adjusted by the thin film heater 19 having a small resistance value, and the attenuation gain is finely adjusted by the thin film heater 18 having a large resistance value. Thus, stable attenuation gain control can be performed.

It is a top view which shows the waveguide type variable optical attenuator of this Embodiment. It is a figure which shows the relationship between the applied voltage and power consumption in a thin film heater. It is a top view which shows the waveguide type variable optical attenuator of other embodiment. It is a top view which shows the conventional waveguide type variable optical attenuator.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Waveguide-type variable optical attenuator 12 Input waveguide 13 Output waveguide 14, 15 Branched waveguide 18, 19 Thin film heater 23, 24 Power supply

Claims (5)

A Mach-Zehnder type optical waveguide having two branch waveguides is formed on the substrate between the input waveguide and the output waveguide, and the gain attenuation of the signal light is reduced by energizing the thin film heater formed above one of the branch waveguides. In the waveguide-type variable optical attenuator to be adjusted, one or more coarse adjustment thin film heaters for coarse adjustment of the gain attenuation of the signal light, and a resistance higher than that of the coarse adjustment thin film heater are provided above one branch waveguide. And one or more fine-tuning thin film heaters having a value and finely adjusting the gain attenuation of the signal light, and a power source is individually connected to the coarse adjusting thin film heater and the fine adjusting thin film heater. A waveguide type variable optical attenuator. A Mach-Zehnder type optical waveguide having two branch waveguides is formed on the substrate between the input waveguide and the output waveguide, and the gain attenuation of the signal light is reduced by energization of the thin film heater formed above one of the branch waveguides. In the waveguide type variable optical attenuator to be adjusted, a thin film heater for coarse adjustment for roughly adjusting the gain attenuation of the signal light, and a resistance value larger than that of the thin film heater for coarse adjustment are provided above one branch waveguide. A thin film heater for fine adjustment that finely adjusts the gain attenuation of the signal light, the thin film heater for coarse adjustment and the thin film heater for fine adjustment are connected in parallel to a power source, and the thin film heater for coarse adjustment and the fine adjustment waveguide type variable optical attenuator, characterized in that each variable resistor to use thin film heater connected in series. The waveguide variable optical attenuator according to claim 1 or 2, wherein the thin film heater for coarse adjustment and the thin film heater for fine adjustment are formed of the same material and have different shapes. The resistance values of the one or more coarse adjustment thin film heaters and the one or more fine adjustment thin film heaters are the resistance values of other thin film heaters, where R is the smallest resistance value of the coarse adjustment thin film heater. The waveguide type variable optical attenuator according to claim 1, wherein is R × C ⁿ (C is a constant). A Mach-Zehnder type optical waveguide having two branch waveguides is formed on the substrate between the input waveguide and the output waveguide, and the thin film heater formed above one of the branch waveguides is energized to attenuate the signal light gain. In the control method of the waveguide type variable optical attenuator for adjusting
A plurality of thin film heaters having different resistance values are provided above one branching waveguide, a power source is connected to each of the thin film heaters, and the thin film heater having a small resistance value is energized to roughly adjust the attenuation gain of the signal light, An attenuation gain control method for a waveguide type variable optical attenuator, wherein a thin film heater having a large resistance value is energized to finely adjust an attenuation gain of signal light.

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