JP2006243484A - Optical waveguide element and manufacturing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To adjust a zero point of an optical waveguide element by a method excellent in long term stability without applying voltage and complicating the step. <P>SOLUTION: When a light modulator 10 with a Mach-Zender type optical waveguide 12 is manufactured, in which a plurality of lights branched by a branching optical waveguide 12c on the substrate 11 are propagated through two or more separate optical waveguides 12a, 12b, respectively, and are thereafter multiplexed by a multiplexing optical waveguide 12d, the manufacturing method is characterized by performing a trimming step for removing a part of the separate optical waveguide 12a by irradiating the irradiation area 17a on the separate optical waveguide 12a with a laser beam. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an optical waveguide element and a method for manufacturing the same.

For example, in an optical waveguide device such as a light intensity modulator, a variable attenuator, or an optical switch provided with a Mach-Zehnder type optical waveguide on a substrate having an electro-optic effect, the light interference state changes depending on the applied voltage. The intensity of output light changes depending on.
Such an optical waveguide element is required to be in a predetermined light output state when the applied signal voltage is zero. Hereinafter, such a light output state when the signal voltage is zero is referred to as a zero point.
The zero point is set according to the required function and characteristics. For example, in the case of a light intensity modulator, when the signal voltage is zero, the output light intensity is set to be (1) maximum, (2) set to be minimum, and (3) between the maximum and minimum There are methods such as setting to be dots.

However, even if a plurality of optical waveguide elements are manufactured under the same manufacturing conditions, in reality, the light output state when the signal voltage is zero, due to the influence of manufacturing error, internal stress during film formation, That is, the zero point is slightly different for each element, resulting in variations with respect to the design value.
Therefore, in order to adjust such variation, a method of correcting the zero point by applying a bias voltage (DC voltage) from the outside separately from the signal voltage is known.

However, when the zero point is adjusted by applying a DC voltage in the light intensity modulator, a zero point variation called DC drift caused by the DC voltage is induced.
For this reason, in actual use, it is necessary to provide a control circuit for controlling the DC voltage while constantly monitoring the zero point, resulting in a complicated apparatus.
Therefore, a technique is desired that can control the light output state at a zero point, that is, a state where the signal voltage is zero, by a method other than applying a DC voltage.

Also, in optical elements such as wavelength filters that use Mach-Zehnder type optical waveguides and fixed optical attenuators, deviations from design values occur due to manufacturing errors and internal stresses during film formation. There are many cases where adjustment is necessary. Since such an element is of a fixed characteristic type, unlike the light intensity modulator, variable attenuator, optical switch, etc., since no electrode is provided, it is impossible to apply a bias voltage. Therefore, a method other than voltage application is required for adjustment.
In such an element, the characteristic itself of the element is a zero point.

As a method of adjusting the zero point without applying a voltage, for example, the following method has been known so far.
(A) Method of changing the physical properties of the waveguide itself; specifically, (A-1) Method of irradiating the waveguide with ultraviolet light (Patent Documents 1 and 2 below), (A-2) Refractive index control component For further diffusion (Patent Document 3 below).
(B) A method of changing the refractive index of the cladding portion on the outer periphery of the waveguide or the buffer layer portion immediately above the waveguide; specifically, (B-1) forming a high refractive index film on the waveguide, and A method of partially removing the rate film (Patent Document 4 below).
(C) A method of changing the substantial optical path length by applying stress to the waveguide and / or the substrate; specifically, (C-1) a method of forming a stress applying film (Patent Document 5 below), (C -2) There is a method of forming a stress imparting groove (Patent Document 6 below).
JP 2001-249243 A Japanese Patent Laid-Open No. 11-337892 JP 11-52315 A Japanese Patent Laid-Open No. 2001-100003 JP 2000-230955 A Japanese Patent Laid-Open No. 11-271552

However, the method (A-1) has poor long-term stability. In the method (A-2), the process is complicated and a special apparatus for local heating is required. In the method (B-1), the process is complicated, and in the partial removal of the high refractive index film, the change amount of the zero point with respect to the processing amount is small. The method (C-1) has poor long-term stability because an organic material is used as the stress imparting film. Since the method (C-2) processes the outer periphery of the waveguide, there is a problem that the change amount of the zero point with respect to the processing amount is small.
The present invention has been made in view of the above circumstances, and enables the zero point of an optical waveguide element to be adjusted by a method having excellent long-term stability without applying a voltage and causing a complicated process. For the purpose.

In order to achieve the above object, according to the present invention, light branched into a plurality of branched optical waveguides on a substrate is propagated through a plurality of separated optical waveguides, and then multiplexed in a multiplexed optical waveguide. A method of manufacturing an optical waveguide device having a Mach-Zehnder type optical waveguide, comprising: a trimming step of irradiating the separation optical waveguide with laser light to remove a part of the separation optical waveguide An element manufacturing method is provided.
Further, the present invention has a Mach-Zehnder type optical waveguide on the substrate where the light branched into a plurality of branched optical waveguides propagates through the plurality of separated optical waveguides and then combined in the combined optical waveguide. There is provided an optical waveguide device characterized in that there is a cross-sectional area reducing portion in which a surface portion of the separation optical waveguide is removed and a cross-sectional area is reduced.

  According to the present invention, the zero point of the optical waveguide device can be adjusted by a method having excellent long-term stability without applying a voltage and without complicating the process.

Hereinafter, a first embodiment of the present invention will be described. FIG. 1 is a plan view showing an embodiment of an apparatus suitably used for carrying out the method for manufacturing an optical waveguide device of the present invention.
Reference numeral 10 in the figure denotes an optical modulator as an example of an optical waveguide element. 2 is a cross-sectional view of the optical modulator 10 taken along the line II-II in FIG.
In the optical modulator 10 of this embodiment, a Mach-Zehnder type optical waveguide 12 (hereinafter sometimes simply referred to as the optical waveguide 12) is formed on the upper surface of a LiNbO ₃ (LN) substrate 11, and a buffer layer is further formed on the LN substrate 11. A signal electrode 14 and a ground electrode 15 are provided via 13 and are schematically configured.

  The Mach-Zehnder type optical waveguide 12 includes a first optical waveguide 12a and a second optical waveguide 12b as separation optical waveguides, and a branch connected to the incident side of the first and second optical waveguides 12a and 12b. It comprises a type optical waveguide 12c and a multiplexing type optical waveguide 12d connected to the emission side of the first and second optical waveguides 12a and 12b. The central portions in the length direction of the first and second optical waveguides 12a and 12b are parallel to each other. In this example, an X-cut substrate of LN is used.

The buffer layer 13 is provided for the purpose of preventing the guided light propagating through the optical waveguide from being absorbed by the electrode, or for adjusting the traveling speed of the electric signal in the traveling wave electrode, and is generally SiO _2. Is a layer having a thickness of about 0.5 to 2 μm. In the present embodiment, the buffer layer 13 is provided on the entire surface of the substrate 11.
The signal electrode 14 is made of a conductive material such as gold (Au). The central portion 14a of the signal electrode 14 in this example extends between the first optical waveguide 12a and the second optical waveguide 12b in parallel with the central portions of the first and second optical waveguides 12a and 12b. . Both end portions 14b and 14c of the signal electrode 14 extend perpendicularly to the central portion 14a across the first optical waveguide 12a and the second optical waveguide 12b, respectively.
The ground electrode 15 is provided on both the incident side and the emission side with the signal electrode 14 interposed therebetween with a predetermined distance from the signal electrode 14 and is grounded although not shown.
In this example, the ground electrode 15 is not provided in a portion of the first and second optical waveguides 12a and 12b adjacent to the combined optical waveguide 12d.

The apparatus according to the present embodiment has a fine movement stage 1 provided with means (not shown) for fixing the optical modulator 10, and an optical waveguide 12 of the optical modulator 10 fixed on the fine movement stage 1. Thus, a signal voltage is applied to the semiconductor laser 2 as a light source for incident light, a photodiode 4 as a light receiving means for receiving output light from the optical waveguide 12 of the optical modulator 10, and a signal electrode 14 of the optical modulator 10. A function generator 3 as signal voltage input means and an oscilloscope 5 as means for monitoring the light intensity received by the photodiode 4.
The fine movement stage 1 can be finely moved in the horizontal direction, and includes driving means (not shown).
Although not shown, a laser beam irradiation device for irradiating a processing laser beam so as to be positioned above the optical modulator 10 fixed on the fine movement stage 1, and a portion irradiated with the laser beam CCD camera for observing the image is provided.

Examples of the laser light for processing include YAG laser light, CO ₂ laser light, and excimer laser light. Among these, excimer laser light is preferable. Excimer laser light has a short wavelength, and the irradiated portion is removed by ablation (decomposition and divergence into molecules and atoms), so that the smoothness of the surface of the removed portion tends to be high. A higher smoothness in the removed portion is desirable in order to suppress an increase in transmission loss due to removal of the optical waveguide.

In addition, when there is a buffer layer on the optical waveguide to be trimmed, there is a method of providing an opening in the buffer layer in advance and irradiating the optical waveguide in the opening with laser light, but excimer laser light is particularly used. In some cases, trimming can be performed while the buffer layer is present on the optical waveguide. This is because the LN that forms the optical waveguide absorbs excimer laser light and is ablated, whereas the SiO ₂ that forms the buffer layer transmits the excimer laser light. In other words, even when excimer laser light is irradiated in the presence of the buffer layer, the buffer layer transmits the excimer laser light, so that abrasion occurs directly in the optical waveguide below the buffer layer, and the optical waveguide portion directly It is trimmed. Here, the buffer layer in the irradiated portion is not directly affected by the excimer laser beam, but is indirectly removed because the underlying optical waveguide is removed by trimming. Therefore, an effect equivalent to that obtained when the buffer layer is removed in advance can be obtained. Further, since the removal of the buffer layer is due to an indirect action, the amount (depth) of the optical waveguide to be trimmed is constant when the power of the excimer laser light is the same regardless of the presence or absence of the buffer layer. . Therefore, it is not necessary to adjust the power of the excimer laser light according to the presence or absence of the buffer layer.
As described above, the fact that the buffer layer does not need to be provided with an opening in advance eliminates the risk of film formation failure due to the opening provided when the buffer layer is formed. The opening is provided after the buffer layer is formed. For example, there is an advantage that the labor for the operation becomes unnecessary.

The laser beam for processing is preferably irradiated onto the optical waveguide with a desired spot shape and spot size by combining a condensing unit and a shielding unit such as a mask.
The spot shape of the irradiated laser beam is not particularly limited. For example, a rectangle may be used.
Further, the spot size of the laser light preferably has a width equal to or greater than ¼ of the width of the optical waveguide irradiated with the laser light. When the width of the optical waveguide is not constant, it is preferably equal to or more than 1/4 of the minimum value of the width of the optical waveguide. More preferably, it is 1/2 or more, and more preferably 3/4 or more. In the present embodiment, the widths of the first and second optical waveguides 12a and 12b are constant and are about 10 μm.
When the laser beam spot size width is smaller than the lower limit of the above range, the optical waveguide is removed only in the width direction, leading to a decrease in the zero point variation due to the optical waveguide trimming and an increase in transmission loss. There is a risk of causing.
On the other hand, even if the spot size width is too large, there is little effect on the optical waveguide irradiated with laser light. However, considering the influence on other adjacent optical waveguides, the width of the optical waveguide in the laser light irradiation area A range not exceeding 4 times is desirable. For example, in this embodiment, the width of the spot size is preferably about 2 to 40 μm, and more preferably about 5 to 25 μm.
Further, the length of the spot size in the length direction of the optical waveguide is not particularly limited as long as it is a range in which uniform laser light intensity can be obtained in the irradiation region.

Here, the dimensions (width and depth) of the optical waveguide in this specification are the dimensions when the position of the boundary between the waveguide and the substrate can be easily confirmed and the dimensions of the optical waveguide can be clearly shown. To do. On the other hand, in the case of an optical waveguide formed of an impurity diffusion type, it is not easy to determine the boundary between the waveguide and the substrate, so that the intensity of light propagating through the optical waveguide is 1 / (maximum) of the maximum value (near the optical waveguide center). The point that has decreased to e ² is defined as the position of the boundary between the waveguide and the substrate, and the dimensions of the optical waveguide are obtained.

Hereinafter, a method for manufacturing the optical modulator 10 using the apparatus of this embodiment will be described.
First, the optical modulator 10 manufactured by an arbitrary manufacturing method is fixed on the fine movement stage 1. Light (wavelength used by the optical modulator 10) is incident on the optical waveguide 12 of the optical modulator 10 from the semiconductor laser 2, a signal voltage is applied to the electrodes, and the waveform of the light intensity received by the photodiode 4 and the applied voltage The oscilloscope 5 monitors the relationship (Lissajous waveform). This confirms the initial state of the zero point (output light intensity when the signal voltage is zero) [initial zero point confirmation step].

Separately, the blank region 16 on the substrate 11 where the optical waveguide 12 is not provided is irradiated with a processing laser beam, and the removed portion removed by the irradiation is observed [preliminary irradiation step].
The blank area 16 is preferably provided with no electrode or the like and has the same configuration as the laser light irradiation area 17a in the trimming process. The presence or absence of the optical waveguide in the blank area 16 is arbitrary, but it is more desirable to provide a dummy waveguide having the same configuration as the optical waveguide 12.
The blank area 16 is preferably separated from the optical waveguide 12 to the extent that it does not affect the light propagation characteristics of the optical waveguide 12.
The intensity of the laser beam irradiated to the optical waveguide 12 in the trimming process is determined using the result of observing the removed portion in the blank area 16 by the laser beam irradiation.
If the laser beam intensity is too weak, the removed portion becomes non-uniform. Therefore, it is preferable that the laser beam intensity used in the trimming step be equal to or higher than the minimum value (minimum irradiation intensity P) of the laser beam intensity that can obtain the uniformity of the removed portion.
The “uniformity” of the removed portion is determined by visually observing the removed portion and determining that “uniformity is obtained” when a line due to a step or the like is not seen.
On the other hand, if the laser beam intensity is too strong, the change amount of the zero point with respect to the processing amount is too large, and the controllability is deteriorated. Therefore, it is preferable that the range of the laser light intensity used in the trimming step be in the range of the minimum irradiation intensity P or more and 5 times or less of the P. A more preferable range is P or more and 2 or less of P.
Further, when the irradiation intensity of the laser beam in the trimming process is determined in advance, this preliminary irradiation process can be omitted.

Next, a trimming process is performed.
In the present embodiment, the irradiation region 17a including the first optical waveguide 12a is irradiated with laser light to remove a part of the first optical waveguide 12a. FIG. 3 is an enlarged cross-sectional view showing an example of a removal portion by laser light irradiation. Reference numeral 18 in the drawing indicates the removal portion. In the removed portion 18, the buffer layer 13 and the surface portion of the substrate 11 including the surface portion of the optical waveguide 12 a are removed. The planar shape of the removal portion 18 is substantially the same as the spot shape of the laser beam.
The laser light irradiation region 17a is a portion where the optical waveguide 12a is not covered with an electrode, and a combination of light propagating through the first optical waveguide 12a and light propagating through the second optical waveguide 12b occurs. It is provided closer to the incident side than the portion. In the present embodiment, a portion of the first optical waveguide 12a adjacent to the combined optical waveguide 12d is a removal portion 18.
The removed portion 18 may be a portion in which the propagation mode state of light propagating through the first optical waveguide 12a is different between the removed portion (removed portion 18) and the portion not removed. When the high refractive index portion is removed, the amount of change of the zero point due to the removal becomes larger. Preferably, the surface portion of the first optical waveguide 12a is removed as shown in FIG.

As will be described later, the amount of change at the zero point depends on the depth of the removed portion 18 in the thickness direction (X direction in the drawing) of the substrate 11. The depth of the removal portion 18 can be controlled by the intensity of the laser beam applied thereto and the irradiation time. The depth of the removal portion 18 may be in a range in which a desired zero point change amount can be obtained, but the transmission loss increases as it becomes larger (deeper). Therefore, the depth of the removed portion 18 is 15% or less of the size (depth) of the optical waveguide 12a in the depth direction of the optical waveguide 12a (in this embodiment, the X-axis direction of LN and the X direction in the figure). It is preferable that it is 9% or less.
The lower limit (minimum value) of the depth of the removed portion 18 is not particularly limited as long as a desired zero point is obtained.
The “depth of the removed portion” in the present invention is a depth based on the surface of the substrate 11.

When the surface portion of the first optical waveguide 12a is removed, the removed portion 18 has a reduced cross-sectional area of the first optical waveguide 12a compared to a portion not irradiated with laser light (non-removed portion). The cross-sectional area is reduced. The removal portion 18 preferably has a constant depth. In this case, the cross-sectional area of the first optical waveguide 12a is not the boundary between the reduced cross-sectional area (removal portion 18) and the non-removal portion adjacent thereto. It changes continuously.
When the cross-sectional area of the first optical waveguide 12a changes, the effective refractive index changes, and the same effect as when the optical path length changes substantially is obtained.
Therefore, even when the optical path lengths of the first optical waveguide 12a and the second optical waveguide 12b are designed to be equal at the time of manufacture, a reduced cross-sectional area (removed portion 18) is formed in the first optical waveguide 12a. By doing so, a difference can be provided between the optical path length of the first optical waveguide 12a and the optical path length of the second optical waveguide 12b. As a result, the phase difference between the light propagating through the first optical waveguide 12a and the light propagating through the second optical waveguide 12b, which are combined by the combining optical waveguide 12d, is controlled, and the combined optical waveguide 12d is thereby controlled. The intensity of the output light output from can be changed. That is, the zero point of the optical modulator 10 can be changed.

The amount of change of the zero point due to the trimming process and the length of the removed portion (cross-sectional area reducing portion) 18 along the light traveling direction are substantially proportional. Further, the depth of the removed portion (cross-sectional area decreasing portion) 18 and the amount of change of the zero point are also in a substantially proportional relationship. Furthermore, the amount of change at the zero point also changes depending on the width of the removed portion (cross-sectional area reducing portion) 18.
Therefore, it is preferable to irradiate the laser beam while monitoring the amount of change of the zero point and stop the laser beam irradiation when a desired zero point is obtained.

  For example, (1) after irradiating the laser light irradiation area 17a with laser light, the same operation as the initial zero point confirmation step is performed to confirm the zero point, and the irradiation position is set in the length direction of the first optical waveguide 12a. The length of the removal portion 18 is increased by repeating a series of operations of moving in the (Y direction in the figure) and irradiating the laser beam again, and when the desired zero point is obtained, the laser beam is obtained. A method of stopping the irradiation can be used. The irradiation position can be moved by a method of moving the relative position between the laser beam irradiation apparatus and the first optical waveguide 12a in the Y direction, and preferably the fine movement stage is finely moved in the Y direction.

  Or (2) A series of laser beam irradiation to the laser beam irradiation region 17a, the zero point is confirmed by performing the same operation as the initial zero point confirmation step, and the same irradiation position is irradiated again with the laser beam. By repeating the operation, the depth of the removed portion 18 is increased, and a method of stopping the laser beam irradiation when a desired zero point is obtained can be used.

  Furthermore, both the method (1) and the method (2) may be combined. For example, the depth of the removed portion 18 can be increased to some extent by the method (2), and then the length of the removed portion 18 can be increased by the method (1) to obtain a desired zero point. As a result, the depth of the removed portion 18 may change discontinuously in the length direction.

In the case of using the method (1), the spot size of the laser beam in the length direction of the optical waveguide is usually several tens to 100 μm, whereas the actual irradiation length is usually in the order of mm. Yes, when moving and irradiating simply, several to several tens of repetitions are required. At this time, if each irradiation spot is discontinuous, the boundary between the ablation portion (removal portion) and the non-abrasion portion exists on the optical waveguide twice as many as the number of spots, and transmission loss tends to increase. Furthermore, the presence of a periodic processing part (removal part) may cause interference of transmitted light.
On the other hand, even if each irradiation spot is made continuous, it is difficult to make the irradiation spots overlap and be in close contact with each other. Therefore, a portion that is deeply ablated by multiple irradiation between the spots or an ablation residue due to poor contact occurs. Therefore, it is easy to cause an increase in transmission loss.
This problem is caused by a method of continuously moving the relative position between the laser beam irradiation apparatus and the first optical waveguide 12a in the Y direction while irradiating the laser beam, preferably by moving the XY stage while irradiating the laser beam. It can be solved by moving in the Y direction. By using this method, an ablation part having a long dimension of the order of mm in the optical waveguide direction and a uniformly continuous depth can be provided on the optical waveguide.

Further, as described above, the change amount of the shape (width, depth, and / or length) of the removed portion 18 and the change amount of the zero point are substantially proportional to each other, and the shape (width, depth, (And / or length) change amount depends on the irradiation condition of the laser beam. Therefore, if these relationships are obtained in advance by experiments, the required amount of change in the shape of the removed portion 18 is determined from the difference between the initial zero point obtained in the initial zero point confirmation step and the zero point to be obtained. It is possible to obtain the laser light irradiation condition for that purpose by calculation.
That is, first, an initial zero point confirmation step is performed, and necessary laser light irradiation conditions are calculated based on the result. After performing laser light irradiation under the conditions, the same operation as the initial zero point confirmation step is performed. It is also possible to use a method for confirming whether a desired zero point is obtained.
According to this method, unlike the methods (1) and (2), it is not necessary to perform laser beam irradiation while monitoring the zero point, and the zero point confirmation only needs to be performed twice before and after laser beam irradiation. Simplification of the process can be expected.
Even when the laser beam irradiation is performed while monitoring the zero point as in the methods (1) and (2), the shape change amount of the removed portion 18, the zero point change amount, and the laser beam irradiation condition If the relationship is obtained in advance by experiments, a preferable irradiation condition at each time of laser light irradiation can be predicted, thereby improving workability.

According to the present embodiment, the zero point can be adjusted by a simple process of removing a part of the first optical waveguide 12a by laser light irradiation. Since this method removes a part of the optical waveguide, the amount of change of the zero point with respect to the processing amount can be increased. In addition, there is no problem in terms of long-term stability because the composition of the waveguide is not changed and no organic material is used.
In general, it is often predicted that removing a part of an optical waveguide will cause an increase in transmission loss. However, according to the present invention, an increase in transmission loss, deterioration of an extinction ratio in a light intensity modulator, an optical switch, The zero point can be adjusted effectively without causing practical problems such as deterioration of the branching ratio. The better the uniformity of the removed portion (cross-sectional area reduced portion) 18, the smaller the transmission loss.

Next, a second embodiment of the present invention will be described.
The present embodiment is greatly different from the first embodiment in the first embodiment. In the first embodiment, only the first optical waveguide 12a is irradiated with laser light in the trimming process, and the removal portion (cross-sectional area reduction portion) 18 is obtained. However, in this embodiment, both the first and second optical waveguides 12a and 12b are irradiated with laser light, and a removal portion (cross-sectional area reduction portion) is formed in each of them.
According to the present embodiment, the transmission loss of each of the plurality of separated optical waveguides (the first and second optical waveguides 12a and 12b in the present embodiment) can be adjusted, whereby the extinction ratio can be improved.

  Specifically, the optical modulator 10 is fixed on the fine movement stage 1, and light (a used wavelength of the optical modulator 10) enters the optical waveguide 12 of the optical modulator 10 from the semiconductor laser 2, and a signal voltage is applied to the electrodes. The relationship between the waveform of the light intensity received by the photodiode 4 and the applied voltage (Lissajous waveform) is monitored by the oscilloscope 5. Thereby, the initial state of the extinction ratio is confirmed [initial extinction ratio confirmation step].

Apart from this, the preliminary irradiation step is performed in the same manner as in the first embodiment.
As in the present embodiment, the lower limit of the laser light intensity when irradiating laser light for the purpose of adjusting the transmission loss in the first and second optical waveguides 12a and 12b is the removed portion as in the first embodiment. It is preferable that the irradiation intensity P be equal to or higher than the minimum irradiation intensity at which the uniformity can be obtained. On the other hand, if the laser beam intensity is too strong, the amount of change in transmission loss with respect to the processing amount is too large, resulting in poor controllability. Therefore, it is preferable that the range of the laser light intensity used in the trimming step be in the range of the minimum irradiation intensity P or more and 5 times or less of the P. A more preferable range is P or more and 2 or less of P.

Next, a trimming process is performed.
First, when one of the separated optical waveguides (first optical waveguide 12a) is irradiated with laser light, a part of the first optical waveguide 12a is removed, thereby increasing transmission loss.
When the change in the extinction ratio is measured before and after the laser light irradiation, and the extinction ratio is improved by the laser light irradiation, the transmission loss of the first optical waveguide 12a is reduced to the other separation optical waveguide (second optical waveguide). It can be seen that the transmission loss is lower than 12b). Therefore, the first optical waveguide 12a is further irradiated with laser light to further increase the transmission loss. Thus, the extinction ratio improves as the difference in transmission loss between the first optical waveguide 12a and the second optical waveguide 12b is reduced. Laser light irradiation is preferably performed until a desired extinction ratio is obtained while monitoring a change in the extinction ratio.
The laser light irradiation areas in the first optical waveguide 12a and the second optical waveguide 12b are preferably set in the same manner as in the first embodiment.

  On the other hand, when a change in the extinction ratio is measured before and after the laser light irradiation, if the extinction ratio deteriorates due to the laser light irradiation, the transmission loss of the first optical waveguide 12a is reduced to the other separation optical waveguide (second optical waveguide). It can be seen that the transmission loss is higher than the optical waveguide 12b). Therefore, the transmission loss is increased by irradiating the second optical waveguide 12b with the laser light having a lower transmission loss. Thus, the extinction ratio improves as the difference in transmission loss between the first optical waveguide 12a and the second optical waveguide 12b is reduced.

  The amount of change in the extinction ratio in the present embodiment depends on the length, depth, width, and surface state of the removed portion removed by laser light irradiation. Therefore, if these relationships are obtained in advance by experiments, it is possible to obtain the necessary laser light irradiation conditions by calculation from the extinction ratio value obtained in the initial extinction ratio confirmation step.

In addition, if the initial zero point confirmation step similar to that of the first embodiment is added to the present embodiment, the laser beam in which both the zero point and the extinction ratio are preferable from the initial zero point and extinction ratio measurement results. Irradiation conditions can be calculated. Therefore, by irradiating both the first and second optical waveguides 12a and 12b with laser light, the zero point can be adjusted and the extinction ratio can be improved.
As described above, in the first embodiment, the zero point can be adjusted while suppressing an increase in transmission loss. However, a slight increase in transmission loss is unavoidable in the trimming process. It is more preferable that both the zero point adjustment and the extinction ratio can be improved.

In the above embodiment, the signal electrode 14 and the ground electrode 15 are provided on the incident side of the Mach-Zehnder type optical waveguide 12, and the laser light irradiation region 17a is provided on the emission side. 14 and the ground electrode 15 and the positional relationship between the laser light irradiation region 17a may be opposite to the above embodiment.
In the above embodiment, an example using an LN X-cut substrate is shown, but a Z-cut substrate can also be used. In the case of a Z-cut substrate, the relative positional relationship between the electrode and the optical waveguide is different from that in the above embodiment, but is the same in principle. Furthermore, not only an LN substrate but also a LiTaO ₃ (LT) substrate can be used similarly.
In the above embodiment, the laser beam for processing is irradiated in the preliminary irradiation step in the blank area 16 where the optical waveguide 12 on the substrate 11 as a processing target is not provided, but from the same material as the processing control. It may be an arbitrary region of another substrate.

Example 1
Using the apparatus having the configuration shown in FIG. 1, the zero point of the optical modulator 10 was adjusted in the same procedure as in the first embodiment. An LN substrate was used as the substrate 11 of the optical modulator 10, and the Mach-Zehnder type optical waveguide 12 was formed by a titanium (Ti) diffusion method.
The width (Z direction width) of the first and second optical waveguides 12a and 12b in the optical modulator 10 is about 10 μm, and the depth (depth in the X direction) is about 7 μm. In the irradiation region 17a, the first optical waveguide 12a is covered with a buffer layer 13 (thickness: about 1 μm) made of SiO ₂ . The total length of the Mach-Zehnder type optical waveguide 12 in the optical modulator 10 (the length of the first optical waveguide 12a + the length of the branched optical waveguide 12c + the length of the combined optical waveguide 12d) is 50 mm.
As the processing laser light, KrF excimer laser light (wavelength 248 nm) was used. The spot shape of the laser light is rectangular, the spot size is 10 μm in the width direction (Z direction) of the first optical waveguide 12a, and the length in the light traveling direction (Y direction) is 100 μm. The laser beam was irradiated over a length of 2 mm in the Y direction. The intensity of the laser beam to be irradiated was set to a total power of 1 mJ, and a part of the laser beam was formed by beam shaping with an aperture.
First, after measuring the initial value of the zero point and the initial value of the transmission loss, the laser beam was irradiated a plurality of times under the same irradiation condition to one irradiation position on the first optical waveguide 12a. Each time, the zero point and transmission loss were measured after irradiation.

The measurement result of the zero point is shown in FIG. The horizontal axis of FIG. 4 indicates the depth (unit: μm) of the removed portion 18. This depth value is not an actual measurement value, but is a value calculated based on an experiment in which the depth of the removal portion per time when the laser beam is irradiated under the above conditions is obtained in advance.
The vertical axis in FIG. 4 is a value obtained by normalizing the change amount (ΔV) of the zero point with the drive voltage (Vπ), that is, a value calculated by (ΔV / Vπ) × 100 (unit:%).

The measurement result of the transmission loss is shown in FIG. The horizontal axis in FIG. 5 is the depth (unit: μm) of the removed portion 18 and is the same as the horizontal axis in FIG.
The vertical axis in FIG. 5 represents an increase amount (unit: dB) from the initial value of the transmission loss.
The allowable range of increase in transmission loss in the optical modulator of this embodiment is generally 0.4 dB or less.

From the result of FIG. 4, it can be seen that the depth of the removed portion 18 by the trimming process and the amount of change of the zero point are in a substantially proportional relationship.
From the result of FIG. 5, it can be seen that the transmission loss tends to increase as the depth of the removed portion 18 by the trimming process increases.

(Example 2)
Using the apparatus having the configuration shown in FIG. 1, the zero point of the optical modulator 10 manufactured in the same manner as in Example 1 was adjusted in the same procedure as in the first embodiment.
The spot size of the laser beam for processing was the same as in Example 1, the spot was continuously moved in the Y direction, and the laser beam was irradiated over a length of 0.5 mm in the Y direction. Others were irradiated under the same conditions as in Example 1.
First, after measuring the initial value of the zero point, the first irradiation position on the first optical waveguide 12a is irradiated with laser light, the length in the Y direction is 0.5 mm, and the width in the Z direction is 10 μm. A removal portion 18 having a depth of 0.3 μm was formed. After irradiation, the zero point was measured.
Next, the fine movement stage was moved 0.5 mm in the Y direction, and the second irradiation position adjacent to the first irradiation position was irradiated with laser light. Thus, the removed portion 18 is designed to have a length in the Y direction of 1 mm, a width in the Z direction of 10 μm, and a depth of 0.3 μm. The zero point was measured after irradiation.
Thereafter, similarly, the fine movement stage was moved 0.5 mm in the Y direction, and the length of the removal portion 18 in the Y direction was increased by 0.5 mm by irradiating laser light. The zero point was measured after each irradiation.
The zero point was measured in the same manner as in Example 1.

The measurement result of the zero point is shown in FIG. The horizontal axis in FIG. 6 indicates the length (unit: mm) of the removed portion 18. This length value is a design value. The vertical axis in FIG. 6 is the amount of change at the zero point, which is the same as the vertical axis in FIG.
From the result of FIG. 6, it can be seen that the length along the light traveling direction of the removed portion 18 in the trimming process and the amount of change of the zero point are in a substantially proportional relationship.

(Example 3)
In Example 1, the following characteristics were measured for the optical modulator 10 obtained when the depth of the removed portion 18 was 0.5 μm.
(1) The insertion loss was 4.2 dB, (2) the on-off extinction ratio of DC was 28 dB, and (3) the driving voltage at the time of 2.5 Gbps signal was 3.7 V (peak to peak).

In addition, in order to confirm the transmission characteristics of the optical modulator 10, eye evaluation was performed under the following conditions.
(4) Temperature dependence of eye extinction ratio; optical modulator 10 with PRBS (Pseudo Random Binary Sequence) of 2.5 Gbps and 3.7 Vp-p in the temperature range of 1550 nm and 10 to 85 ° C. The extinction ratio of the eye waveform obtained when driving was measured. The driving voltage was constant during the measurement. The results are shown in FIG. As a result of the measurement, even when the temperature was changed in the range of 10 to 85 ° C., the extinction ratio was stable with little fluctuation of 15.6 dB to 16.2 dB.
(5) Wavelength dependence of eye extinction ratio; the extinction ratio of the eye waveform is set in the same manner as in (4) above except that the temperature is kept at 25 ° C. and the wavelength is changed in the range of 1530 to 1565 nm (C band). It was measured. The results are shown in FIG. As a result of the measurement, an extinction ratio of 15 dB or more was obtained over the entire wavelength range.
From the results of (4) and (5), the optical modulator 10 obtained in Example 1 when the depth of the removal portion 18 is 0.5 μm is the wavelength region of the entire C band and the temperature of 10 to 85 ° C. In the range, the eye waveform extinction ratio is estimated to be 14 dB or more.

It is the top view which showed the example of the apparatus used by embodiment of this invention. It is sectional drawing which showed the example of the optical modulator which concerns on embodiment of this invention. It is sectional drawing which expanded and showed the example of the removal part which concerns on embodiment of this invention. It is a graph which shows the evaluation result in the Example of this invention. It is a graph which shows the evaluation result in the Example of this invention. It is a graph which shows the evaluation result in the Example of this invention. It is a graph which shows the evaluation result in the Example of this invention. It is a graph which shows the evaluation result in the Example of this invention.

Explanation of symbols

10 Optical modulator (optical waveguide device)
11 Substrate 12 Mach-Zehnder Type Optical Waveguide 12a First Optical Waveguide (Separation Optical Waveguide)
12b Second optical waveguide (separated optical waveguide)
12c Branched-type optical waveguide 12d Combined-type optical waveguide 17a Irradiation area 18 Removal part (cross-sectional area reduced part)

Claims (9)

Manufactures an optical waveguide device having a Mach-Zehnder type optical waveguide on a substrate where light branched into a plurality of branched optical waveguides propagates through a plurality of separated optical waveguides and then combined in a combined optical waveguide A way to
A method of manufacturing an optical waveguide device, comprising: a trimming step of irradiating the separation optical waveguide with laser light to remove a part of the separation optical waveguide.   The method includes a step of confirming a zero point in a state before irradiating the separation optical waveguide with laser light, and a step of confirming a zero point in a state after irradiating the separation optical waveguide with laser light. Item 2. A method for manufacturing an optical waveguide element according to Item 1.   Before the trimming step, a preliminary irradiation step of irradiating a blank region on the substrate where the Mach-Zehnder type optical waveguide is not provided with the laser light and observing a removed portion by the irradiation is provided. The manufacturing method of the optical waveguide element of Claim 1 or 2.   4. The laser light intensity P used in the trimming step is determined to be equal to or greater than P in the preliminary irradiation step to obtain a minimum laser beam irradiation intensity P that can provide uniformity of the removed portion. The manufacturing method of the optical waveguide element of description.   5. The trimming step of irradiating two or more of the plurality of separated optical waveguides with laser light to remove a part of each separated optical waveguide, respectively. The manufacturing method of the optical waveguide element as described in any one of.   The method includes a step of confirming an extinction ratio in a state before irradiating the separation optical waveguide with laser light, and a step of confirming an extinction ratio in a state after irradiating the separation optical waveguide with laser light. Item 6. A method for manufacturing an optical waveguide device according to Item 5. An optical waveguide device having a Mach-Zehnder type optical waveguide on a substrate where light branched into a plurality of branched optical waveguides propagates through a plurality of separated optical waveguides, and then is combined in a combined optical waveguide. And
An optical waveguide device characterized in that there is a cross-sectional area reduction portion in which a cross-sectional area is reduced by removing a surface portion of the separation optical waveguide.   8. The optical waveguide device according to claim 7, wherein the cross-sectional area reducing portion exists in two or more of the plurality of separated optical waveguides. 9. The optical waveguide device according to claim 7, wherein the cross-sectional area reducing portion has uniformity.

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