JP2005181583A - Method for adjusting phase difference of optical waveguide element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for simply adjusting a phase difference of a Mach-Zehnder type interferometer caused by an error in an optical path. <P>SOLUTION: In the method for adjusting a phase difference of an optical waveguide element equipped with the optical waveguides which constitute the Mach-Zehnder type interferometer, in a region including a section directly above the optical waveguide constructing at least an arm of the Mach-Zehnder type interferometer, circular or dome-like liquid drops composed of a liquid transparent resin with a refractive index lower than that of a material forming the optical waveguide and with shapes after application having 10-200 μm diameters are applied along the optical waveguide in such a way that they are separated one drop by one drop. Thereby the region with an effective refractive index of the optical waveguide different from that of the other optical waveguides is generated so as to adjust the phase difference of the Mach-Zehnder type interferometer. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an optical waveguide element (optical waveguide circuit) that includes an optical waveguide that constitutes a Mach-Zehnder interferometer and is used in the field of optical communication or optical information processing. In particular, the phase difference can be adjusted with high accuracy and permanently. The present invention relates to a phase difference adjusting method for an optical waveguide device.

  In recent years, research and development of optical waveguide circuits composed of optical waveguides formed on a crystalline substrate having an electro-optic effect or a thermo-optic effect have been actively conducted. In particular, an optical intensity modulator and an optical variable optical attenuator that adjust the optical output using optical interference of two light beams (both arms) like a Mach-Zehnder interferometer have been put into practical use.

  Hereinafter, a polarization-independent optical variable optical attenuator made of lithium niobate will be described as an example.

  First, as shown in FIG. 3, a Mach-Zehnder interferometer 31 is formed by diffusing Ti on a lithium niobate substrate 11 having the X axis of the substrate crystal as a normal line to form an optical waveguide parallel to the Z axis. Further, a positive electrode 15 is added to the side portions of both arms 13 and 14 in the Mach-Zehnder interferometer 31 by a metal vapor deposition film, and a negative electrode 16 is added to both arms 13 and 14.

  When a voltage is applied to the electrodes 15 and 16, the refractive index changes between the two arms 13 and 14 due to the electro-optic effect. If this refractive index change is Δn,

と表される。ここで数式（１）中、Γは光波と電場の重なりを示すパラメータ、Ｖは印加電圧、ｄは電極間距離、ｎは屈折率、ｒは電気光学定数である。また、二つのアーム１３、１４では電場の向きが逆であるため、入力ポート１２から入射した光がアームに差し掛かると位相差が生じ、両アーム間の作り込まれた位相差をδ₀とすると、

It is expressed. Here, in Equation (1), Γ is a parameter indicating the overlap of the light wave and the electric field, V is an applied voltage, d is a distance between electrodes, n is a refractive index, and r is an electro-optic constant. In addition, since the directions of the electric fields of the two arms 13 and 14 are opposite to each other, a phase difference is generated when light incident from the input port 12 reaches the arm, and the phase difference created between both arms is represented by δ ₀ . Then

と表される。数式（２）で入力パワーおよび出力パワーをそれぞれＰ_input、Ｐ_outputとする。出力側のＹ分岐における二つのアーム１３、１４間の位相差δは、電極の長さ（位相差の生じる区間の長さ）をｌとすると数式（１）の屈折率差Δｎを用いて次のように表される。

It is expressed. In Expression (2), the input power and the output power are P _input and P _output , respectively. The phase difference δ between the two arms 13 and 14 in the Y branch on the output side is expressed as follows using the refractive index difference Δn of Equation (1) where l is the length of the electrode (the length of the section where the phase difference occurs). It is expressed as

数式（１）で分かる通り、Ｅが増加すると比例してΔｎもｒの符号によって増加または減少し、δも増加または減少する。そのため、数式（２）における出力パワーＰ_outputの増減を電圧Ｖの増減によって制御することが可能である。理想的に二つのアーム長ｌが対称であるようなマッハツェンダー型干渉計では、図４に示すように印加電圧が０Ｖ（０ボルト）のとき、出力光強度は最大値をとり、徐々に電圧を上げていくと、出力光強度は徐々に減少し最小値をとる。

As can be seen from Equation (1), as E increases, Δn increases or decreases in proportion to the sign of r, and δ also increases or decreases. Therefore, the increase / decrease in the output power P _output in the equation (2) can be controlled by the increase / decrease in the voltage V. In a Mach-Zehnder interferometer in which the two arm lengths l are ideally symmetrical, when the applied voltage is 0 V (0 volt) as shown in FIG. As the value increases, the output light intensity gradually decreases and takes a minimum value.

  In conventional optical waveguide type modulators using the electro-optic effect typified by lithium niobate, both of the Mach-Zehnder type interferometers in the state in which no driving voltage is applied due to an optical waveguide circuit fabrication process error. There is a problem that the phase difference between the arms cannot be made constant, and as a result, the following problems occur.

In a Mach-Zehnder interferometer in which an optical waveguide parallel to the Z axis is formed on the surface of a lithium niobate substrate having the X axis as a normal line, the electro-optic constant r has a different coefficient depending on the incident polarized light. A polarization mode having an oscillating electric field parallel to the substrate surface is called a TE mode, and a polarization mode having an oscillating electric field perpendicular to the substrate surface is called a TM mode. The electro-optic coefficient r ₁₂ acts on TE incident polarization, and the electro-optic coefficient r ₂₂ acts on TM-mode polarization. r ₁₂ and r ₂₂ have the same absolute value but opposite signs, ie

である。

It is.

  Here, the voltage-light output characteristics of each of the TE and TM modes are individually expressed.

となる。ＴＭモードについては、正の印加電圧領域だけで見れば、電圧印加によって生じる位相差δは数式（３）から、ＴＥモードの偏光とＴＭモードの偏光では見掛け上符号が反対で大きさが同一のように見える。

It becomes. Regarding the TM mode, if seen only in the positive applied voltage region, the phase difference δ caused by the voltage application is apparently opposite in sign from the TE mode polarized light and the TM mode polarized light with the same magnitude. looks like.

  FIG. 5 shows voltage-light output characteristics when the arm length l is asymmetric. When the arm length l is not ideal, the output light intensity is not maximized when the applied voltage is 0 V (0 volt), but is applied when the TE mode is incident and when the TM mode is incident. The change of the output light intensity with respect to the voltage is different. Specifically, the light output maximum that appears in the vicinity of 0 V (0 volts) shifts in the reverse direction. For this reason, there is a problem that the output light intensity changes when the polarization of the incident light changes at a certain applied voltage. As shown in FIG. 5, the difference between the TE and TM mode outputs at a certain applied voltage results in PDL (Polarization Dependent Loss) of the modulator element. Polarization-dependent loss needs to be suppressed because it leads to power fluctuation on the receiving side in the optical component located in the optical fiber communication system.

  As a method for adjusting the optical path length error resulting from the variation in the manufacturing process, a method for permanently changing the effective refractive index of the optical waveguide has been reported. As an example, a film for adjusting the operating point is loaded on one of the branching waveguides of the interferometer and the length of this film is adjusted, or a direct current is passed through a heater arranged on the optical waveguide. There is a method of heating at a high temperature (see Patent Document 1).

  Further, in Patent Document 2, a film body made of a material having a refractive index higher than that of the substrate material is formed in a part of the branch waveguide of the Mach-Zehnder interferometer, and the film thickness is ¼ or less of the waveguide wavelength. It has been reported that.

Further, in Patent Document 3, in a waveguide element having a lithium niobate crystal as a substrate, an opening is formed in a part of the SiO ₂ buffer layer, and a phase difference is adjusted by applying a cyanoacrylate polymer adhesive thereto. The method has been reported.

However, many of the methods described in Patent Documents 1 to 3 are applied to a method called a PLC (Planar Lightwave Circuit), which is a method of forming a quartz waveguide made of quartz glass containing SiO ₂ as a main component on a flat substrate. It is a thing. This PLC waveguide element is an optical waveguide element that is driven by adding a heater directly above the optical waveguide and controlling the effective refractive index of the optical waveguide or the phase difference of the interferometer by the thermo-optic effect. On the other hand, in the case of an optical waveguide element utilizing the electro-optic effect, since the current is hardly driven and only voltage is driven, the element driving power is 1 mW or less. Therefore, it is not practical to add a heater with an output of about 5 W because the feature of low power consumption cannot be utilized.

  In the method of loading the operating point adjusting film described in Patent Document 1 and adjusting the length of the film, the trimming by laser irradiation, that is, the overheating of the film material is actually performed. Therefore, in this method, the waveguide substrate material is damaged at the same time as the loaded film, which may increase the light propagation loss.

  Similarly, in Patent Document 2, trimming by YAG laser irradiation is performed as a method for adjusting the length of the film body, and at the same time as the film body, the waveguide substrate material may be damaged and light propagation loss may increase. is there.

Moreover, in the method described in Patent Document 3, it is necessary to be able to apply a coating amount of a cyanoacrylate polymer adhesive or the like to be added very accurately in order to adjust a small amount of phase difference. However, in Patent Document 3, the coating method is not described, and the result cannot be reproduced based on this report.
Japanese Laid-Open Patent Publication No. 04-337707 (Claims 1, 2, paragraph number 0023) JP 2001-100193 (Claim 1, paragraph number 0028) JP 07-028006 A (Claim 6, paragraph numbers 0035 and 0036)

  The present invention has been made paying attention to such problems, and the object of the present invention is to provide a method for adjusting the phase difference of an optical waveguide element in which the drawbacks of the prior art are overcome.

  Therefore, as a result of continuous researches by the present inventors in order to solve such problems, the present invention has been completed by paying attention to the following phenomenon. That is, the present inventors pay attention to a slight amount of light leaking from the optical waveguide to the material immediately above, and do not change the refractive index of the optical waveguide itself constituting the optical circuit element, but directly above the optical waveguide. The phase difference adjustment method according to the present invention has been found by utilizing the fact that the effective refractive index for the light wave propagating through the waveguide is changed by changing the refractive index of the substance in the structure.

That is, the invention according to claim 1
Optical waveguide comprising a crystalline substrate, input and output channel optical waveguides provided on the substrate, and an optical waveguide provided between the input / output channel optical waveguides and constituting a Mach-Zehnder interferometer Based on the element phase difference adjustment method,
The region including the portion directly above the optical waveguide constituting at least one arm in the Mach-Zehnder interferometer is composed of a liquid transparent resin whose refractive index is lower than that of the material forming the optical waveguide, and the form after application is A circular or dome-shaped droplet having a diameter of 10 micrometers to 200 micrometers is divided one by one and applied along the optical waveguide, thereby generating regions having different effective refractive indexes of the optical waveguide, thereby increasing the Mach. The phase difference of the Zender interferometer is adjusted.

The invention according to claim 2
Based on the phase difference adjusting method for the optical waveguide device according to the invention of claim 1,
The transparent resin is composed of a liquid resin material having a refractive index of 1 to 2.2 and a viscosity at 25 ° C. of 50 mPa · s to 1000 mPa · s,
The invention according to claim 3
Based on the phase difference adjusting method for the optical waveguide device according to the invention of claim 1,
The crystalline substrate is composed of lithium niobate or lithium tantalate.

  The phase difference adjusting method for an optical waveguide element according to the present invention is a liquid having a refractive index lower than that of a substance forming the optical waveguide in a region including the portion directly above the optical waveguide constituting at least one arm in the Mach-Zehnder interferometer. A circular or dome-shaped droplet having a diameter of 10 micrometers to 200 micrometers, which is made of a transparent resin, is applied one by one along the optical waveguide. The phase difference of the Mach-Zehnder interferometer is adjusted by generating regions having different effective refractive indexes of the waveguide.

  Therefore, according to this phase difference adjustment method, an optical waveguide device such as a variable optical attenuator having extremely high characteristics and a high yield can be realized by a simple method. It is possible to provide the waveguide element at a low cost and with a high yield.

  The present invention will be specifically described below.

  FIG. 1 shows a schematic configuration of an intensity modulator in which an optical waveguide type Mach-Zehnder interferometer is formed on a lithium niobate substrate. That is, the lower arm portion 13 and the upper arm portion 14 of the optical waveguide are formed on the lithium niobate substrate 11 cut in the X-axis direction of the crystal by Ti vapor deposition and Ti diffusion so that light propagates in the Z-axis direction of the crystal. Formed. Here, the two arms 13 and 14 of the Mach-Zehnder interferometer are designed to be equal in length, and the intensity of light emitted from the element output port 17 when the voltage applied to the Mach-Zehnder interferometer is 0 volts. The goal is to maximize.

  Further, a Mach-Zehnder interferometer having a length of 30 mm so as to apply an electric field perpendicular to the X axis of the arms (optical waveguides) 13 and 14 and the lithium niobate substrate 11 with an interelectrode gap of 28 μm (micrometer). The positive electrode 15 and the interferometer negative electrode 16 were formed of Ti metal and Au metal.

  Then, using a single mode fiber for a wavelength of 1.55 μm from the input port 12 of the element shown in FIG. 1, light having a wavelength of 1.55 μm was incident on the input port 12 so as to be TE mode polarized light. As a result, when the applied voltage of the interferometer positive electrode 15 and the interferometer negative electrode 16 is swept from 0 to 36 volts, the maximum of the light output emitted from the element output port 17 is similar to the example shown in FIG. The voltage giving value was 1.2 volts, which was not the maximum at 0 volts. This shift was expressed as a phase shift amount in the relationship between voltage and light output, and was 0.21 radians (rad).

  Here, an acrylic ultraviolet curable resin having a refractive index of 1.4 and a viscosity coefficient of 250 mPa · s is applied from the nozzle having an inner diameter of 100 μm to the output side tapered portion 18 in the upper arm portion 14 using a piezoelectric element. It was dripped. The dropped acrylic ultraviolet curable resin was a droplet having a dome shape with a diameter of 120 μm, and the coating was applied to the output side tapered portion 18 in a length of 60 μm.

  As a result of performing the coating five times, as the length of the coating region immediately above the optical waveguide increases, the phase shift gradually decreases as shown by the black circle in the graph of FIG. 2, and the fifth coating, that is, 60 μm. The phase shift became −0.03 radians when it was applied over a length of × 5 = 3 mm.

On the other hand, after the application test, the applied uncured resin was removed from the upper arm portion 14, and the application region of the UV curable resin was increased by 60 μm in length to the output side tapered portion 19 in the lower arm portion 13. As a result, as shown by the white circles in the graph of FIG. 2, the phase shift gradually increased and the coating was applied for the fifth time, that is, 60 μm × 5 = 3 mm, as shown by the white circle in the graph of FIG. And the phase shift was about 0.39 radians. How the value of the optical output in fitting to the equation (5) that determines the Vπ and the phase shift [delta] ^TE - Here, in order to obtain the phase shift amount plotted in the graph of FIG. 2, such voltage as in FIG. 5 It was adopted.

  From this result, by applying a liquid transparent resin such as an ultraviolet curable resin directly over the optical waveguide over a length of about 3 mm, the phase shift is about ± 0.2 radians (that is, the output side taper in the upper arm portion 14). The portion 18 is “−0.03 radians−0.21 radians = −0.24 radians”, and the output side tapered portion 19 of the lower arm portion 13 is “0.39 radians−0.21 radians = + 0.18 radians”) It was confirmed that it was possible to control over the entire range. From this, a phase shift of about ± 0.07 radians per mm can be adjusted.

  The smaller the diameter of the transparent resin to be dropped, the more precise the phase adjustment becomes possible. However, even if the diameter is relatively large (120 μm), the stage on which the target optical waveguide element is placed is moved by 10 μm and applied. Can be repeated, the coating area of the transparent resin can be increased with a resolution of 10 μm. That is, the phase can be controlled with high accuracy of about ± 0.0007 radians per drop.

  On the other hand, the influence of the uneven thickness of the droplet is that the size of the light propagating through the waveguide oozes upward from the waveguide substrate is 0.5 μm or less, so the radius of the droplet is much larger than that. If this is the case, the variation in size of the droplets can be ignored. The liquid transparent resin is cured after the required coating amount is determined. In this case, when the liquid transparent resin is composed of an ultraviolet curable resin, curing shrinkage is caused by irradiation with ultraviolet rays. As a countermeasure against this, if a material having a shrinkage ratio after curing of 10% or less is selected, the resin is cured. Subsequent phase shift can be almost suppressed. Furthermore, the influence of the refractive index change of the resin itself accompanying the curing can be suppressed by dividing the “coating / curing” step into two steps, for example. That is, in the first step, the liquid transparent resin is applied to, for example, 90% of the length corresponding to the target phase correction amount, and then cured. Next, after confirming the phase value achieved so far, the remaining phase value is applied and cured in the second step. It has also been confirmed that the loss of the optical waveguide does not increase by the application of these liquid transparent resins.

  Then, when the phase adjustment was performed on the above-described polarization-independent variable optical attenuator using the phase difference adjustment method according to the present invention, the PDL (polarization-dependent loss) at the time of 10 dB attenuation was 5 dB. Decreased to 1.5 dB, and PDL at 20 dB attenuation decreased from 12.3 dB to 2.1 dB.

  Although the present invention has been described by taking a Z-directional propagation modulator as an example on an X-directional crystal, the present invention is of course not limited to this orientation and modulator.

  Also in the interference type element (also called a balanced bridge type modulator) in which both the input side and the output side of the Mach-Zehnder type interferometer of FIG. 1 or one Y-branch is replaced with a directional coupler type 3 dB coupler. It is clear that the present invention is effective.

  Further, in the above description, the output side taper portions 18 and 19 of the upper arm portion 14 and the lower arm portion 13 are exemplified as the region to which the liquid droplet made of the liquid transparent resin is applied. The same effect can be obtained even when applied to the tapered portion, and the same effect can be obtained when applied to the non-tapered region between the electrodes 15 and 16.

  Further, although an ultraviolet curable resin is applied as a liquid transparent resin, a thermosetting resin or the like can also be applied.

Explanatory drawing for demonstrating the phase difference adjustment method of the optical waveguide element which concerns on this invention. The graph which shows the relationship between the frequency | count of application | coating of a droplet, and a phase shift amount in the phase difference adjustment method of the optical waveguide element which concerns on this invention. Explanatory drawing of an optical waveguide element provided with the optical waveguide which comprises a Mach-Zehnder type interferometer. The graph which shows the relationship between the applied voltage and optical output in an optical waveguide element (optical waveguide type Mach-Zehnder interferometer) provided with the optical waveguide which comprises a Mach-Zehnder type interferometer. The graph which shows the relationship between the applied voltage and optical output in an optical waveguide type | mold Mach-Zehnder interferometer in case the phase difference built between arms exists.

Explanation of symbols

11 Lithium niobate substrate (crystalline substrate)
DESCRIPTION OF SYMBOLS 12 Input port 13 Lower arm 14 Upper arm 15 Positive electrode 16 Negative electrode 17 Output port 18 Output side taper part of upper arm 19 Output side taper part of lower arm

Claims (3)

Optical waveguide comprising a crystalline substrate, input and output channel optical waveguides provided on the substrate, and an optical waveguide provided between the input / output channel optical waveguides and constituting a Mach-Zehnder interferometer In the element phase difference adjustment method,
The region including the portion directly above the optical waveguide constituting at least one arm in the Mach-Zehnder interferometer is composed of a liquid transparent resin whose refractive index is lower than that of the material forming the optical waveguide, and the form after application is A circular or dome-shaped droplet having a diameter of 10 micrometers to 200 micrometers is divided one by one and applied along the optical waveguide, thereby generating regions having different effective refractive indexes of the optical waveguide, thereby increasing the Mach. A method of adjusting a phase difference of an optical waveguide device, wherein the phase difference of a Zender interferometer is adjusted.   2. The optical waveguide according to claim 1, wherein the transparent resin is composed of a liquid resin material having a refractive index of 1 to 2.2 and a viscosity at 25 ° C. of 50 mPa · s to 1000 mPa · s. Method for adjusting the phase difference of the element. 2. The phase difference adjusting method for an optical waveguide element according to claim 1, wherein the crystalline substrate is made of lithium niobate or lithium tantalate.

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