JP2007078861A - Thermooptic variable attenuator and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To make a circuit size small by suppressing influence of thermal crosstalk on adjacent channels in a multichannel thermooptic variable attenuator constituted by arranging a plurality of variable attenuators, where heaters for shifting the phase of light by thermooptic effect are arranged in parallel on arm waveguides of a Mach-Zehnder interference circuit (MZI circuit) using two couplers. <P>SOLUTION: The multichannel thermooptic variable attenuator has a heat conductive layer made of a substance having high heat conductivity below the arm waveguide pair constituting the MZI circuit separately from a heat conductive layer provided below an adjacent MZI circuit, and also has a heat insulating layer made of a substance having lower heat conductivity than each heat conductive layer below the heat conductive layer. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a variable optical attenuator of an optical waveguide circuit used in the field of optical communication, and more specifically, a multi-channel planar optical waveguide variable optical attenuator using a thermo-optic effect (thermo-optic variable attenuator, Or a thermo-optic type VOA) and a manufacturing method thereof.

  Optical devices using planar optical circuit technology have become key devices in the field of optical communications. Among them, the role of a variable optical attenuator (VOA) that dynamically changes the light intensity is important. In particular, a planar optical waveguide variable optical attenuator (hereinafter referred to as a thermo-optic variable attenuator) using a thermo-optic effect as disclosed in References 1 and 2 is an arrayed waveguide grating multiplexer / demultiplexer. It can be integrated with other typical planar optical waveguide circuits, and has the advantage that a multi-channel integrated type can be easily achieved. Furthermore, since the thermo-optic variable attenuator does not have a driving part, it has advantages such as excellent reliability.

  A specific configuration of an optical device using such a thermo-optic effect is that thermo-optics in which light incident on the waveguide is branched by a 3 dB coupler and at least one of the branched lights is provided with a heater on the arm waveguide. This is realized by configuring a Mach-Zehnder interferometer circuit (MZI circuit) that is connected to the optical modulator and coupled again by the coupler. In this way, at least one of the branched lights is connected to the thermo-optic light modulator, and the light intensity is modulated at the emission end by giving a phase difference compared to the other light.

JP 2004-170657 A Japanese Patent No. 3337629

  FIG. 1 shows a top view of a typical conventional 8-channel integrated thermo-optic variable attenuator. A thin film heater 103 is provided on two arm waveguides 101 of a Mach-Zehnder interferometer circuit (hereinafter referred to as MZI circuit), and heat insulating grooves 105 for reducing power consumption are formed at both ends thereof. In the figure, electrodes for supplying power are not shown in order to prevent the drawing from becoming complicated. The heat insulating groove 105 is provided to prevent the heat applied from the heater 103 from spreading in the lateral direction of the substrate, and as a result, the temperature of the core of the arm waveguide 101 is increased efficiently and the power consumption is reduced. .

  The problem with such a conventional thermo-optic variable attenuator is the size of the entire circuit. Even if the thermo-optic variable attenuator is reduced in size, there is a limitation in terms of the manufacturing process or securing characteristics, and the circuit cannot be reduced.

  In order to manufacture the circuit in a small size, first, in the MZI circuit, it is effective to reduce the array waveguide interval as much as possible because the circuit width (the length in the horizontal direction in FIG. 1) can be shortened. Further, when the space from the coupling portion of the 3 dB coupler (not shown) to the arm waveguide interval is expanded by a curved waveguide (not shown), the length of the circuit at this time (the length in the vertical direction in FIG. 1). However, it can be shortened. However, when the heat insulating groove 105 is formed from the viewpoint of power consumption, the arm waveguide interval is limited by the width w of the heat insulating groove 105. At present, this heat insulating groove 105 is essential from the viewpoint of low power consumption. Even if the heat insulating groove 105 is not formed, in order to ensure the temperature difference between the cores of the pair of arm waveguides 101 constituting the MZI circuit, the arm waveguide interval must be widened as a result. Can not be converted.

  Usually, in a thermo-optic variable attenuator constituted by a quartz-based planar optical waveguide, the thickness of the clad of the arrayed waveguide is required from 40 μm to about 80 μm when it is thick from the viewpoint of power consumption. Therefore, the depth of the heat insulating groove 105 is also required to be processed to a depth of 40 to 80 μm. When the width w of the heat insulating groove is narrow, a phenomenon (loading effect) that ions cannot enter the narrow gap between the organic resists during processing of the heat insulating groove occurs, and a deep groove can be formed. The problem of disappearing occurs. Therefore, the width of the heat insulating groove 105 cannot be reduced so much. For example, in order to form a heat insulation groove having a depth of about 60 μm, a heat insulation groove width of 50 μm or more is usually required.

  As described above, since the width w of the heat insulating groove is limited due to the limitation in the manufacturing process, the distance between the arm waveguides is also limited as a result. This is because the heat insulating groove needs to be disposed between at least the pair of arm waveguides.

Now, it is assumed that the width w of the heat insulation groove 105 is a minimum width that can realize processing of the heat insulation groove depth h having a desired depth. Further, separated by insulation trenches, when the width of the ridge portion forming the arm waveguide and w _r, usually, the ridge width w _r is the mode field propagating in the core of the arm waveguide is felt insulation groove flank You don't need a size larger than the width. For example, if the relative refractive index difference Δ of about 1% of the waveguide, w _r is required about 25 [mu] m.

FIG. 2A shows an arrangement in which the MZI circuit of the multi-channel thermo-optic variable attenuator is minimized in the layout, and FIG. 2B shows the actual MZI circuit of the conventional multi-channel thermo-optic variable attenuator. The arrangement of Here, 201 is a core of the waveguide, 203 is a heater, 105 is a groove, 207 is a substrate, and 209 is a clad. As shown in FIG. 2A, the MZI circuit is ideally
d _min = 2 × (w + _wr )
The case where the Mach-Zehnder interferometers are arranged at regular intervals is the smallest in the layout and corresponds to the case where the Mach-Zehnder interferometers are arranged at high density.

In addition, in a multi-channel thermo-optic variable attenuator, a method is considered in which MZI circuits are arranged in a staggered manner, the space on the layout is narrowed, and the actual MZI circuit interval is narrowed. Even then, basically it is necessary to consider the same manner as described above ridge width w _r, this case corresponds to the case where the d _min = w + w _r.

  Of course, it is considered possible to further reduce the width of the heat insulating groove by using a more advanced processing technique than at present, and it is expected that the heat insulating groove will be improved with the development of an advanced processing process.

However, in the conventional structure, the limiting factor of the circuit size that is limited by the width of the heat insulating groove is relatively small and not the main factor. That is, even if the processing technology advances in the future, and the width of the heat insulating groove can be further narrowed, in the conventional structure, the distance d between the MZI circuits of each channel is the minimum distance d _min that is possible in the layout. Can not be placed. As a result, the circuit size is not so small. That is, in the conventional thermo-optic variable attenuator, as shown in FIG. 2B, MZI circuits must be arranged in multiple channels at a distance d far larger than the minimum MZI circuit distance d _min . This is because the interval d of the MZI circuit is determined by thermal crosstalk and cannot be reduced as much as allowed by the layout. Therefore, the limitation of the interval of the MZI circuit is a main factor that hinders downsizing of the multichannel variable attenuator. As described above, thermal crosstalk is a factor determining the circuit size more dominant than the heat insulation groove width w.

  Next, the reason why the MZI circuit interval cannot be reduced due to thermal crosstalk, and therefore the circuit cannot be miniaturized will be described in detail below.

  FIG. 3 shows a cross-sectional structure of the multi-channel thermo-optic variable attenuator, and shows the adjacent channels N−1 and N + 1 with the channel N as the center. Consider a case in which power is applied to one heater 203 of channel N and driven with this configuration. As indicated by the arrows in FIG. 3, a part of the heat applied to the clad 209 by the energized heater 203 is transmitted to Si as the substrate 207 and is also transmitted to the adjacent N−1 channel and N + 1 channel. Due to the heat conducted to the adjacent channel, the optical characteristic of the adjacent channel changes, resulting in a change in transmitted light intensity.

  Therefore, in the conventional structure, as shown in FIG. 2B, the problem is avoided by arranging the Mach-Zehnder interferometer elements constituting each channel at a sufficient distance. Thus, in the conventional structure, in order to reduce the influence of thermal crosstalk, it is necessary to increase the MZI circuit interval d. As a result, the circuit size of the thermo-optic variable attenuator cannot be reduced.

  Next, how much thermal crosstalk is actually a problem will be quantitatively explained. Now, paying attention to one MZI circuit, the optical variable attenuator is driven by applying electric power to the heater on one arm waveguide. At this time, the light intensity Io appearing in the output waveguide is 1 if the input light intensity is 1.

Indicated by Here, φ is an initial phase, and Δφ is a phase difference generated when a temperature difference ΔT is given between the cores of the two arm waveguides. Δφ is a function of the temperature difference ΔT with the other core, the refractive index temperature coefficient of the core is dn / dT, and the length of the heater is L.

Indicated by. From the above equation (2), the temperature difference ΔT between the two arms necessary to obtain a certain damping intensity Io is as shown in the following equation (3).

Here, consider a case where the optical variable attenuator is driven to 25 dB. The above formula (3) is given by Io = 25.0 dB, typical quartz refractive index temperature coefficient dn / dT = 1 × 10 ⁻⁵ (1 / ° C.), heater length L = 2.0 mm The temperature difference ΔT between the two arm waveguide cores is 37.762 degrees (° C.). Similarly, the temperature required at the time of attenuation of Io = 24.9 dB and Io = 25.1 dB corresponding to the fluctuation of 0.1 dB is obtained. Table 1 below shows the obtained temperatures, and also shows the temperature differences from the respective 25 dB times. This temperature difference corresponds to an allowable temperature change amount when a variation of 0.1 dB is allowed within the range of 25 dB attenuation.

  From the above calculation, it is understood that it is necessary to suppress the temperature fluctuation to 0.016 ° C. or lower in order to suppress the fluctuation within 0.1 dB in the attenuation characteristic at the time of 25 dB attenuation.

  FIG. 4 shows a structure in which a ridge portion having a height of 60 μm and a width of 20 μm separated by a heat insulating groove 205 is driven, the heater 205 on the upper surface thereof is driven, and the center of the core 201 comes to a height of 42 μm from the substrate 207. Under the above conditions, the temperature distribution of the substrate 207 when the core center temperature is driven to achieve 25 dB attenuation (37.36 degrees higher than the core center temperature of the other arm). Show. FIG. 4 shows the result of calculation assuming that a symmetrical boundary condition is given in the vertical direction of the center of the heater and that the bottom surface of the substrate is kept at a steady 25 ° C. in order to reduce the calculation amount of the simulation. From FIG. 4, it can be seen that when the heat transmitted from the ridge portion partitioned by the heat insulating groove 205 reaches the substrate 207, the heat is transmitted and distributed in the substrate 207 in a substantially concentric semicircular shape.

FIG. 5 shows the temperature of the surface of the substrate 207 at this time by plotting (calculation results) the distance in the horizontal direction (on the line of arrow A in FIG. 4) from the heater 205 as the horizontal axis. The MZI circuit arm waveguide spacing w + w _r, when placed in a position of a certain distance d, the temperature difference ΔT of the two arm waveguides cores of the arranged MZI circuit, as shown in FIG. 5, the arm guide The temperature difference of the curve in FIG. 5 at the position where the waveguide is disposed can be approximately obtained. This calculation is based on the calculation result when there is no ridge at the location, but even if the ridge is provided at the location, the temperature of the ridge is constant at the same temperature in the steady state. Therefore, such a calculation is sufficient.

  FIG. 6 shows the temperature difference ΔT generated between the arm waveguides of the Mach-Zehnder interferometer adjacent to the driven MZI circuit, where the arm waveguide intervals w are 50 μm, 100 μm, and 250 μm, respectively. The calculation result which calculated | required w as a function is shown. Here, the curve A shows the case where the arm waveguide interval w is 50 μm, the curve B shows the case where the arm waveguide interval w is 100 μm, and the curve C shows the case where the arm waveguide interval w is 250 μm. In FIG. 6, a line (D) is drawn to 0.016 ° C., which is a temperature tolerance for satisfying the 0.1 dB tolerance for the previously obtained 25 dB attenuator. FIG. 6 means that if the temperature difference ΔT generated between the arm waveguides is lower than the straight line D, the fluctuation can be suppressed to 0.1 dB or less at the time of 25 dB attenuation.

  According to FIG. 6, it can be seen that the temperature difference ΔT between the arm waveguides can be reduced by arranging the adjacent MZI circuit at a further distance from the driven MZI circuit. This is the reason why the MZI circuit interval is widened in the conventional thermo-optic variable attenuator. Further, the smaller the arm waveguide interval w is, the closer the difference is between two points on the curve of FIG. 6, and thus the temperature difference ΔT can be reduced.

  However, as previously obtained by calculation, the temperature change capable of allowing a fluctuation of 0.1 dB when attenuated by 25 dB is 0.016 ° C. In order to satisfy this condition, even when the arm waveguide interval w is 50 μm, an adjacent MZI circuit must be arranged at a position separated by 150 μm or more. The degree of isolation is large and the spacing between MZI circuits must be wider. In the above calculation, only one Mach-Zehnder interferometer is considered to be driven. However, in the case of an actual multi-channel thermo-optic variable attenuator, not only the heat conduction from the adjacent variable attenuator but also the next, In addition, inflow of heat from the circuit of the adjacent variable attenuator can be considered. Therefore, it is necessary to further increase the distance of the MZI circuit in consideration of that amount.

  For example, in the case of an arm waveguide interval of 100 μm (curve B), at least the MZI circuit interval must be separated by 280 μm or more. This 280 μm means a theoretical minimum value that can tolerate 0.1 dB, and considering the manufacturing margin and the like, the temperature difference ΔT shown in FIG. 6 satisfies more severe conditions such as 0.01 ° C. and 0.005 ° C. So the spacing must be made. Accordingly, if the MZI circuit has an arm waveguide interval w of 100 μm, for example, in a quartz-based thermo-optic variable attenuator, it is necessary to produce the MZI circuit interval at an interval of about 500 μm. Therefore, if, for example, an 8-channel thermo-optic variable attenuator is manufactured at the MZI circuit interval, the width of the thermo-optic variable attenuator needs to be at least 4 mm in calculation. On the other hand, if an 8-channel MZI circuit is arranged with a period of 200 μm, which is twice the interval of 100 μm between the arm waveguides, the width of the thermo-optic variable attenuator can be reduced to half or less. Become.

In the calculation, the case where the arm waveguide interval was 50 μm was also obtained. However, as described above, in order to realize the arm waveguide interval of 50 μm, it is necessary to make the heat insulating groove width very narrow, and the loading effect is not caused. In addition, advanced process technology that realizes deep insulation groove processing is required. At this time, when the width w _d of the ridge portion delimited by insulation grooves and 25 [mu] m, must be an adiabatic groove w also 25 [mu] m, it is required highly sophisticated manufacturing process technology.

  Further, in the previous calculation, the allowable temperature of fluctuation of 0.1 dB was estimated at the time of 25 dB attenuation. However, when the spec (specification) such as the same allowable range of 0.1 dB is required at the time of driving at a larger attenuation, 30 dB. In this case, the MZI circuit interval must be further increased. In other words, even if the size of the entire circuit is increased and the size can be reduced due to the layout, in order to solve the problem of thermal crosstalk, the MZI circuit interval cannot actually be reduced and the size is reduced. There was a point that could not be converted.

In the description so far, the quartz-based thermo-optic variable attenuator has been taken up. However, the same problems as described above can be given to variable attenuators using other thermo-optic effects. As an example, a thermo-optic variable attenuator using Si fine wires can be considered. Since Si has a refractive index temperature coefficient that is an order of magnitude greater than quartz-based materials, there is an advantage that power consumption can be reduced accordingly. Above all, the thermo-optic variable attenuator using Si wires has a small core size of about 0.3 × 0.3 μm ² and a high refractive index, so that the bending radius can be reduced and a small layout can be achieved. It is possible to make very small circuits. Therefore, if the thermo-optic variable attenuator forms only one MZI circuit, a very small circuit can be realized. However, even if a multi-channel thermo-optic variable attenuator is fabricated using a Si wire waveguide, the above-mentioned problem of thermal crosstalk is essentially the same, although the Si wire is used. However, there is a problem that the size of the entire circuit cannot be reduced so much.

  The present invention has been made in view of the above-described points, and an object thereof is to provide a thermo-optic variable optical attenuator having a small circuit size and a manufacturing method thereof, which suppresses the influence of thermal crosstalk on adjacent channels. is there.

  In order to achieve the above object, a thermo-optic variable attenuator of the present invention comprises an optical waveguide formed on a substrate having good thermal conductivity, two directional couplers constituted by the optical waveguide, and an arm waveguide pair. A plurality of Mach-Zehnder interferometer circuits (hereinafter referred to as MZI circuits) and a plurality of heaters that are arranged on the upper surfaces of the arm waveguide pairs and are thermo-optic phase shifters that change the phase of light by the thermo-optic effect. The multi-channel waveguide type optical variable attenuator is formed separately for each MZI circuit below the arm waveguide pair constituting one MZI circuit, and constitutes the clad of the arm waveguide pair. Formed between a plurality of heat conductive layers made of a material having better heat conductivity than the material, and a lower portion of each heat conductive layer and the substrate having good heat conductivity, and having a heat conductivity lower than those heat conductive layers material And having a Ranaru insulation layer.

  Here, it is preferable that the heat insulation layer under the MZI circuit is separated from the heat insulation layer of the adjacent channel.

  Moreover, the heat insulation layer under the MZI circuit may be continuous with the heat insulation layer of the adjacent channel.

  Preferably, the heat conductive layer may be formed of at least one of Si, polycrystalline Si, amorphous Si, tungsten, and tungsten silicide.

  Preferably, the heat insulating layer may be formed of at least one oxide of Si, Al, B, P, Ge, Ti, Ta, Bi or a mixture thereof.

  Further, the heat conductive layer is formed of Si, the MZI interval is arranged at a distance of 1.9 to 2.1 times the arm waveguide interval, and the thickness of the heat insulating layer is set to be an arm guide. It can be characterized in that the size is 1/4 to 1/6 times the waveguide interval.

  In order to achieve the above object, a method of manufacturing a thermo-optic variable attenuator according to the present invention includes a step of preparing a waveguide substrate in which the substrate, the heat insulating layer, and the heat conductive layer are sequentially laminated, and its heat conduction. A step of fabricating a waveguide structure, a heater and an electrode structure on the heat conductive layer without processing the layer, and a clad of a portion where a heat insulating groove is to be formed later and where the heat conductive layer is to be removed are previously formed. It has the process of removing, the process of forming a heat insulation groove | channel, and the process of removing a heat conductive layer in the location which removed the clad previously.

  Here, it is preferable to further include a step of selectively removing only the heat insulating layer remaining in the lower part of the heat insulating groove in contact with the adjacent channel in the heat insulating groove.

  According to the above-described structure of the present invention, even if the MZI circuit interval is narrowed, the effect of the thermal conduction layer and the heat insulation layer is effective to drastically suppress the characteristic variation of the thermo-optic variable attenuator due to thermal crosstalk. can get. As a result, the circuit size of the thermo-optic variable attenuator can be significantly reduced. The details of the operation will be described below.

  As shown in FIG. 4, thermal crosstalk is always generated as long as the heater is driven, and the substrate temperature in the vicinity of the heated heater rises. The temperature decreases monotonously as the distance from the heater drive location increases. Here, a temperature difference occurs between a pair of arm waveguide cores of adjacent MZI circuits, and this temperature difference causes an undesirable phase change in light propagating through both arm waveguides. Therefore, how to suppress this temperature difference is a problem. Therefore, in the present invention, as a means for solving this problem, as described above, a layer having a high thermal conductivity (this layer is referred to as a “thermal conductive layer”) separately from the substrate below the pair of arm waveguides of the MZI circuit. And a layer having a lower thermal conductivity than the thermal conductive layer (this layer will be referred to as a “heat insulation layer”) is provided between the substrate and the thermal conductive layer. It is intended to reduce the temperature difference between the waveguides.

  In the following description, a Si substrate is used as a substrate with good thermal conductivity, single crystal Si is used as a thermal conduction layer (a thermal conduction layer with high thermal conductivity), and a heat insulating layer (thermal conduction) provided between the thermal conduction layer and the substrate. A case where quartz is used as the low-rate layer) will be described as an example. Of course, the present invention is not limited to these materials, and at least the desired effect can be obtained if the magnitude relationship of the thermal conductivity constituting each layer is the same.

  FIG. 7 is a diagram for explaining the effect of the structure of the present invention. As shown in the upper part of the figure, a heat insulating layer 702 having a thickness of 10 μm is formed on a substrate 701 having a heat conductivity of 1 mm and a heat conductive layer 703 having a thickness of 10 μm is formed on the heat insulating layer. Further, in the circuit system in which a ridge structure 704 similar to that shown in FIG. 2A is formed on the partitioned heat conductive layer 703 and the heat conductive layer 703 is arranged with a width of 120 μm and an interval of 200 μm. The plot of the temperature distribution on the surface of the heat insulation layer 702 when the same power as the conditions used for obtaining the characteristics of FIG. 5 is applied to the heater 705 is plotted. It is represented by curve A. A dashed curve B in FIG. 7 shows the curve of FIG. 5 for comparison. The calculation was performed with the substrate 701 and the heat conductive layer 703 as Si, respectively, and the heat insulating layer 702 and the ridge portion 704 as quartz.

  From this result, the temperature gradient at the location where the heat conductive layer 703 exists is reduced, and it is not too much to say that the temperature is almost constant. This is because the heat conductive layer 703 is thermally insulated from the substrate 701 by the heat insulating layer 702. If the thermal conductivity of the thermal conductive layer 703 is overwhelmingly higher than that of the heat insulating layer 702 (the thermal conductivity of Si >> the thermal conductivity of quartz), the thermal conductive layer 703 partitioned for each MZI circuit is almost the same. It becomes a constant temperature. As a result, the temperature gradient at the location of the heat conductive layer 703 is significantly reduced.

  As shown in FIG. 7, in the present invention, an arm waveguide constituting the MZI circuit is formed on the partitioned heat conduction layer 703. With this configuration, even if the adjacent MZI circuit is driven and thermal crosstalk occurs, the temperature difference between the two arm waveguides formed on the heat conductive layer 703 can be made extremely small. As a result, the effect of suppressing the transmission intensity fluctuation of the adjacent channel due to the thermal crosstalk, which has been a problem in the conventional structure, can be obtained, and the distance between the adjacent Mach-Zehnder interferometers can be reduced to reduce the circuit. .

  Next, the effect of the manufacturing method of the thermo-optic variable attenuator of the present invention will be described.

  The structure of the present invention as described above is patterned and etched with a mask material in order to leave a portion that becomes the heat conductive layer 703 in advance if an attempt is made to produce the structure by a manufacturing method similar to a conventionally known structure manufacturing method. Later, it is necessary to sequentially form layers of an underclad 706, a core 707, and an overclad 708 constituting the waveguide. At that time, before depositing these layers constituting the waveguide, it is necessary to deposit the glass on the patterned thermal conductive layer, but it was deposited for the patterned thermal conductive layer. There is a disadvantage that a step is generated on the glass surface and a flat surface cannot be obtained. In particular, the flatness of the underclad 706 and the core layer 707 has a great influence on the optical characteristics. In order to eliminate the step, it is necessary to polish the surface of the glass to increase the flatness after the patterned heat conductive layer is embedded with glass. As the polishing is performed, the number of steps and the manufacturing time become longer, and as a result, there arises a disadvantage that the cost increases.

  Therefore, in order to avoid the inconvenience, in the manufacturing method of the present invention, as described above, after preparing a waveguide substrate in which a substrate, a heat insulating layer, and a heat conductive layer are sequentially laminated using a flat substrate, A waveguide structure, a heater and an electrode structure are produced on the heat conduction layer without processing the heat conduction layer, and then a cladding where a heat insulation groove is later produced and where the heat conduction layer is to be removed. Is removed in advance, and a heat insulating groove is formed. In the final step, the heat conductive layer of the unnecessary portion is removed at the portion where the clad has been previously removed in order to isolate the heat conductive layer from the adjacent MZI circuit by the heat insulating groove.

  After manufacturing the waveguide portion by a conventionally known manufacturing method, if a heat insulating groove is formed and the layer that becomes the heat conduction layer is simply removed through this heat insulating groove, the heat insulating layer is formed at the bottom of all the heat insulating groove portions. There is a possibility of removing a certain heat conductive layer. Therefore, in the present invention, before forming the heat insulating groove, a portion where the heat conductive layer is not removed is masked, and a part of the cladding above the portion to be removed later as a heat conductive layer is removed in advance. Then, after the mask of the heat insulation groove forming portion is patterned again, the heat insulation groove is excavated. In this case, the part where the part has been removed in advance reaches the heat conducting layer first, and the bottom part of the heat insulating groove at the part where it is not desired to be removed is protected by the clad without reaching the heat conducting layer. Is obtained. Then, an unnecessary portion of the heat conduction layer may be excavated and removed using the heat insulating groove. In the manufacturing method of the present invention, by manufacturing in this way, a part that becomes a waveguide circuit can be manufactured by using a generally well-known manufacturing process, and optical characteristics are not deteriorated. Further, the manufacturing method of the present invention does not require polishing, which contributes to cost reduction.

  As described above, the thermo-optic variable attenuator of the present invention solves the problem that the circuit size is increased by arranging each channel apart in order to suppress the output fluctuation of the adjacent channel due to thermal crosstalk. Thus, it is possible to reduce adjacent channel output fluctuations due to thermal crosstalk to about 1/10 of the conventional structure. Therefore, according to the present invention, the MZI circuit interval of each channel can be reduced correspondingly, and the size of the entire circuit can be dramatically reduced.

  In addition, according to the method of manufacturing a thermo-optic variable attenuator of the present invention, after forming a portion where light is guided first by a conventional process, a part of the step is selectively removed at the time of forming a heat insulating groove. Since only the lower part of the desired heat insulation groove is removed, the problem of flatness can be avoided, and the thermo-optic variable attenuator of the present invention having excellent optical characteristics can be provided at low cost without a polishing process. it can.

  Furthermore, the thermo-optic variable attenuator of the present invention is structured not only in the heat conductive layer but also in the heat insulating layer so as to be separated from the adjacent MZI circuit in the horizontal direction of the substrate, thereby suppressing the influence of thermal crosstalk and the circuit. In addition to downsizing, it is also possible to reduce power consumption. In addition, by properly selecting the thickness of the heat insulating layer, the effect of suppressing the influence of thermal crosstalk similar to that of the discrete type can be obtained even in a structure in which the heat insulating layers are connected over the entire substrate, and the process load is reduced. Can be reduced.

  The present invention is an epoch-making invention in which the temperature difference between both arms of the MZI circuit, which is almost difficult to control with the conventional structure, can be controlled with a relatively simple structure, which is once in hundreds of times. Is very big.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
(First embodiment)
Structure The structure of the thermo-optic variable attenuator according to the first embodiment of the present invention is shown in FIGS. FIG. 4A is a perspective view, FIG. 4B is a top view, and FIG. 4C is a cross-sectional structure taken along the line AA ′ in FIG. In addition, an enlarged view of one channel portion is also shown in FIG. In this figure, for simplification, electrode wiring is not shown in the drawing.

  As shown in FIGS. 8A to 8C, the thermo-optic variable attenuator 800 of this embodiment includes an optical waveguide 802 formed on a substrate 801 having good thermal conductivity, and the optical waveguide 802. A Mach-Zehnder interferometer circuit 805 (hereinafter referred to as an MZI circuit) comprising two directional couplers 803 and an arm waveguide pair 804, and disposed on the upper surface of the arm waveguide pair 804 to transmit light by a thermo-optic effect. A multi-channel waveguide-type variable optical attenuator including a plurality of heaters 806 each of which is a thermo-optic phase shifter for changing the phase, and is provided below the arm waveguide pair 804 constituting one MZI circuit 805. A plurality of heat conductive layers 810 made of a material that is formed separately for each MZI circuit and has a higher thermal conductivity than the material constituting the clad 808, 809 of the arm waveguide pair; Formed between the good substrate 801 to bottom of the heat conduction of people thermally conductive layer 810 than their thermal conductivity layer and a heat-insulating layer 811 made of substance having a low thermal conductivity. Reference numeral 807 denotes an arm waveguide core; 808, an underclad (lower clad); 809, an overclad (upper clad); and 812, a heat insulating groove.

EXAMPLE As an example of the present embodiment, a multi-channel waveguide type variable optical attenuator having the above-described configuration shown in FIGS. 8A to 8C is used as a heat conductive layer 810 with single crystal Si, heat insulation. The layer 811 was manufactured using quartz and evaluated.

  The relative refractive index of the used waveguide core 807 is 0.75%, the width of the waveguides 802 and 804 (waveguide width) is 6 μm, the height of the core 807 is 6 μm, the height of the under clad 808 is 40 μm, and over The height of the clad 809 is 20 μm. A directional coupler 803 constitutes a 3 dB coupler, and the arm waveguide portion 804 has a straight waveguide having a length of 2.2 mm, and a heater 806 having a length of 2.0 mm formed of Cr on the straight waveguide. And wiring (not shown) is formed on each channel with gold. In addition, heat insulation grooves 811 are formed on both sides of the heater 806. The interval between the pair of arm waveguides 804 constituting the MZI circuit 805 is 100 μm, and the interval between the MZI circuits (MZI circuit interval) is 200 μm.

A thermal conductive layer 810 made of single crystal Si and having a thickness of 10 μm is formed below a pair of arm waveguides 804 constituting one MZI circuit 805, and each thermal conductive layer 810 is formed by an adjacent Mach-Zehnder interferometer. It is formed so as to be isolated from the heat conductive layer. Further, an SiO ₂ layer having a thickness of 10 μm to be a heat insulating layer 811 is formed between the heat conductive layer 810 and the Si substrate 801, and the heat insulating layer is present over the entire surface of the Si substrate.

  A power source (not shown) was connected to an 8-channel thermo-optic variable attenuator actually manufactured under the above design conditions, and evaluation was performed. For comparison, a circuit having the same parameters except that it was produced on a simple Si substrate without a normal heat conduction layer or heat insulation layer was also measured as a conventional circuit. As an evaluation, in a state where channel # 4 (fourth channel) is attenuated by 25.0 dB, in addition to channel # 4, power that gives a variation of phase π is input, and the light intensity at the output end of channel # 4 Observed fluctuations. Since the amount of variation is very small, high-sensitivity measurement was performed by modulating the applied power and synchronizing it. FIG. 9 shows the result of measuring the influence of the thermal stroke, the horizontal axis is the number of the channel to be driven (#), and the vertical axis is the change in light intensity (dB). In FIG. 9, a solid curve A indicates the characteristics of the structure of the present invention, and a broken curve B indicates the characteristics of the conventional structure.

  According to the measurement results shown in FIG. 9, in the circuit of the conventional structure, when the adjacent channel # 3 or channel # 5 is driven, the temperature rise of the channel # 4 is a fluctuation of the light intensity with an allowable fluctuation amount of 0.1 dB or more. showed that. As described in the description of the prior art, this result qualitatively agrees with the result that the MZI circuit interval must be 280 μm or more when the arm waveguide interval is 100 μm in the conventional structure. At this time, the power required for 25 dB attenuation was 52 mW.

  On the other hand, the structure having the SOI layer divided for each channel under the pair of arm waveguides of the present embodiment as the heat conducting layer 810 showed only a fluctuation of 0.016 dB. In other words, in the structure of the present embodiment, the fluctuation was 0.1 dB or less, which is an allowable fluctuation amount even if the adjacent channel is driven at the time of 25 dB attenuation. This corresponds to 0.0028 ° C. when converted to a fluctuating temperature. This value is an order of magnitude smaller than the fluctuation amount of the conventional structure. In order to reduce the thermal crosstalk to the same extent in the conventional structure, according to the calculation shown in FIG. 6, when the arm waveguide interval is 100 μm, the MZI circuit interval must be separated by 640 μm or more in calculation.

  In the structure of the present embodiment, the power required for 25 dB attenuation was 51 mW. That is, it was confirmed that the influence of thermal crosstalk was suppressed and power consumption was not deteriorated.

  As is clear from the above, the structure of this embodiment does not cause deterioration of the power consumption value, and suppresses the characteristic variation of the adjacent channel due to thermal crosstalk, which has been a problem in the conventional structure, to about 1/10. It becomes possible. As a result, in the structure of the present embodiment, the MZI circuit interval can be arranged much closer than before, so that the circuit size of the thermo-optic variable attenuator, which has been a problem in the conventional structure, can be greatly reduced. Is possible.

Manufacturing Method Next, a manufacturing method of the multi-channel waveguide type optical variable attenuator according to this embodiment will be described with reference to the process diagrams shown in FIGS. 10 (A) to 10 (H). For simplification, the cross-sectional shape for one channel is shown for each process.

  As described above, the multi-channel waveguide type optical variable attenuator of the present embodiment is more thermally conductive than the material constituting the clads 808 and 809 below the arm waveguide pair 804 constituting one MZI circuit 805. A heat conductive layer 810 made of a material having a high efficiency, and the heat conductive layer is separated from the heat conductive layer provided in the lower part of the MZI circuit of the adjacent channel; Is also characterized by having a heat insulating layer 811 made of a material having low thermal conductivity.

  As shown in FIGS. 10A to 10H, the structure of the thermo-optic variable attenuator according to the first embodiment of the present invention described above can be manufactured as follows.

First, as shown in FIG. 10A, a 10 μm thick SiO ₂ layer 811 serving as a heat insulating layer is provided on a flat Si substrate 801 having a thickness of 1 mm, and a heat transfer layer 810 is further formed thereon. An SOI substrate on which 10 μm of single crystal Si is formed was used as a waveguide substrate. The buried oxide film layer 811 serving as a heat insulating layer is generally called a BOX layer, and the single crystal Si layer 810 is called an SOI layer.

  As step 1, first, the substrate was cleaned. Cleaning was carried out using a surfactant usually used in the semiconductor industry.

Next, as shown in FIG. 10B, as step 2, a glass layer 808 serving as an underclad and a glass 813 serving as a core were deposited on the SOI layer 810 using a flame deposition method. Here, the flame deposition method is a method in which a chloride film such as SiCl ₄ is burned in an oxyhydrogen flame to form a glass film on the substrate at high speed, and is suitable for depositing a relatively thick film. This method is widely used for manufacturing optical waveguides because of its excellent embedding characteristics. Immediately after the deposition, the glass layer 808 and the glass 813 are white because of a collection of fine particles, and a transparent film can be obtained by heat treatment. Generally, for lowering the transparent temperature, for example, glass to which an appropriate amount of P ₂ O ₅ , B ₂ O ₃ or the like is added is deposited, and GeO ₂ or the like for increasing the refractive index is further added to the core layer 813. . Here, the deposited core layer 813 has a relative refractive index difference Δ of 0.75% with respect to the cladding layer 808 and a thickness of 6 μm.

  In this embodiment, the flame deposition method is used. However, the glass deposition method is not limited to this, and various CVD methods, sputtering methods, sol-gel methods, and the like may be used.

  Further, as shown in FIG. 10C, as step 3, the core layer 813 is processed into a core shape using a standard photolithography technique and reactive ion etching, and the resulting core 807 has a width of 6 μm. It was. The pattern of the core 807 is an MZI circuit 805 including two 3 dB couplers 803 as shown in FIG. In this embodiment, an 8-channel Mach-Zehnder interferometer is used, and the interval between arm waveguides constituting the Mach-Zehnder interferometer is 100 μm, and the MZI circuit interval is 200 μm. The arm waveguide portion connecting the two 3 dB couplers was 2.5 mm.

  As step 4, as shown in FIG. 10D, a glass 809 serving as an overcladding was deposited on the processed surface of the core using a flame deposition method, and embedding was performed.

  In step 5, as shown in FIG. 10E, a heater 806 and a metal wiring portion (not shown) were formed. This process was also typically made using photolithography, vapor deposition, and lift-off process. The heater 806 was made of Cr and its length was 2 mm. The metal wiring was made using gold.

  Up to this point, there is no special process and it is a standard well-known manufacturing method.

  Next, as step 6, as shown in FIG. 10 (F), a resist to be a mask is applied and patterned, and then the insulating grooves located on both outer sides of the pair of arm waveguides are first etched by about 1 μm. . This etching portion 814 is to leave a portion to be removed and a portion not to be removed using the cladding glass as a mask when a part of the SOI layer 810 is selectively removed later. A part of the mask material was peeled off at the end of this step.

  As step 7, as shown in FIG. 10G, after applying the mask material again, a pattern to be the heat insulation groove 812 is formed. At this time, not only the outside of the pair of arm waveguides but also the heat insulating groove 812 inside is patterned and etched. Hereinafter, the heat insulating groove positioned between the pair of arm waveguides is referred to as an inner heat insulating groove 812A, while the heat insulating groove sandwiching the heater 806 is referred to as an outer heat insulating groove 812B. The outer heat insulating groove 812B reaches the SOI layer 810 first because it has been etched in step 6 first. The etching is stopped when the outer heat insulating groove 812B reaches the SOI layer 810.

Finally, as step 8, as shown in FIG. 10H, the SOI layer 810 exposed in the outer heat insulating grooves 812B of the pair of arm waveguides is removed with an SF-based gas using an ICP etching apparatus. At this time, since the bottom surface of the inner heat insulating groove 812A is covered with SiO ₂ , the SOI layer under the inner heat insulating groove 812A is not removed when removing the SOI layer. Finally, by removing the mask material used for forming the heat insulating grooves, a shape in which the SOI layer exists as the heat conductive layer 810 under the pair of arm waveguides is obtained.

  In the present embodiment, a commercially available SOI substrate is used. However, the thermal conductivity of the thermal conductive layer 810 is higher than that of the material forming the clads 808 and 809, and separates the substrate 801 and the thermal conductive layer 810. If the thermal conductivity of the material of the heat insulating layer 811 is smaller than that of the heat conductive layer 810, at least the desired effect can be obtained. Therefore, for example, a desired effect can be obtained by using a polycrystalline Si layer or various metal layers as the heat conductive layer 810 instead of the SOI layer. As the material of the heat conductive layer, a material having a large etching selectivity to a material to be a cladding is preferable.

FIGS. 11A to 11C are diagrams illustrating a method for manufacturing a waveguide substrate when polycrystalline Si is used as the substrate 801. FIG. As shown in FIG. 11A, a Si substrate 801 is prepared, and as shown in FIG. 11B, a SiO ₂ layer 811 having a predetermined thickness is deposited on the Si substrate 801 by a flame deposition method. As shown in FIG. 11C, amorphous Si indicated by reference numeral 810 was deposited by sputtering on the surface on which the SiO ₂ layer was deposited. At this point, heat treatment was performed once to polycrystallize amorphous Si. Thereafter, a thermo-optic variable attenuator may be manufactured according to the steps shown in FIGS. 10 (A) to 10 (H).

  Similarly, in this embodiment, quartz is used as the material of the heat insulating layer 811 that separates the heat conductive layer 810 and the substrate 810. However, the present invention is not limited to this, and any material having lower heat conductivity than the heat conductive layer 810 may be used. Even if it is formed of, for example, a ceramic layer such as alumina, at least one oxide of Si, Al, B, P, Ge, Ti, Ta, Bi, or a mixture thereof, at least the expected effect is can get.

  In step 8 shown in FIG. 10H, when the SOI layer 810 is removed, the etching rate is different between quartz and Si (normal selection ratio> 10), so that the surface of the BOX layer 811 below the SOI layer 810 is exposed. At this point, etching stops. At the same time, the thin cladding layer 808A remains in the inner heat insulation groove bottom 812A. Since this clad layer 808A is extremely thin, power consumption does not increase depending on the presence or absence of the clad layer 808A. The structure of this embodiment that does not remove this thin clad layer 808A is slightly degraded in power consumption (several percent) compared to the structure described later as the second embodiment of the present invention, but the effect of suppressing the influence of thermal crosstalk is Fully obtained. On the other hand, this structure has an advantage that the number of steps is small in manufacturing the thermo-optic variable attenuator of the present invention, and as a result, the manufacturing cost can be reduced.

  As described above, in the conventional manufacturing method in which the glass serving as the cladding is deposited after the heat conductive layer is processed, the flatness of the cladding surface is poor and the optical characteristics are affected. However, according to the manufacturing method of the present embodiment, the waveguide portion is manufactured by a normal well-known process before the heat conductive layer is processed. In a typical process, it is possible to remove the heat conductive layer and further the heat insulating layer, so that polishing is not required to obtain flatness, and the process load is minimized without causing deterioration of optical characteristics. Therefore, the thermo-optic variable attenuator of the present invention can be easily manufactured.

(Second Embodiment)
Next, the structure and manufacturing method of the thermo-optic variable attenuator according to the second embodiment of the present invention will be described.

  With the manufacturing method of the first embodiment of the present invention described above, a structure of a thermo-optic variable attenuator having a large effect of suppressing the influence of thermal crosstalk can be obtained. However, although the number of steps is slightly increased, further effects can be obtained by adding the following steps. FIG. 12A to FIG. 12C are diagrams showing the difference in structure obtained by the process of the second embodiment of the present invention described below.

  FIG. 12A shows a structure obtained in Step 8 shown in FIG.

  After step 8 described above, as step 9, the etching gas may be switched to a CF-based gas when the surface of the BOX layer 811 is exposed, and the cladding layer 808A remaining in the inner heat insulating groove 812A may be removed. This step 9 can be performed simply by switching the gas to remove the cladding layer 808A remaining in the inner heat insulating groove 812A, so that it is not necessary to leak the chamber of the etching apparatus and is a fully automated apparatus. If so, it will not be a burden on the manufacturing process. Etching is stopped when the SOI layer 810 in the inner heat insulating groove 812A is exposed. Thus, the structure as shown in FIG. 12B is obtained by removing the clad at the location of the inner heat insulating groove 812A. In this step 9, the BOX layer 811 exposed in the outer heat insulating groove 812B is partially etched, but this does not cause a problem.

  The cladding layer 808A remaining as a mask for the SOI layer at the bottom of the inner heat insulating groove 812A as shown in FIG. 12A slightly increases the power consumption. Therefore, in step 9, the effect of reducing the power consumption can be obtained by removing the cladding layer 808A.

  Further, in addition to the above step 9, as a step 10, the etching is further continued without terminating the etching when the surface of the SOI layer 810 is exposed at the bottom of the inner heat insulating groove 812A. The exposed SOI layer 810 remains because it is not etched by this etching. On the other hand, the BOX layer 811 exposed in the outer heat insulating groove 812B is etched. Thereafter, the etching is stopped when the Si substrate 801 is exposed in the outer heat insulating groove 812A. The structure thus obtained is a structure in which the BOX layer 811 in the outer heat insulating groove 812B is removed as shown in FIG. As will be described later, the BOX layer 811 which is a heat insulating layer is also isolated from the BOX layer of the adjacent channel, as in the case of the heat conduction layer 810, and the effect of suppressing the influence of the thermal crosstalk is greater, and the power consumption can be further reduced. An effect is obtained.

  Next, effects of the structure shown in FIG. 12C that is separated not only in the heat conductive layer 810 but also in the heat insulating layer 811 from the adjacent MZI circuit in the substrate horizontal direction will be described. Moreover, although the optimum value exists in the thickness of the heat insulation layer 811, the optimum value is also described below.

  A broken line curve B in FIG. 13 is a thermo-optic variable attenuator having a structure in which the heat insulating layers 811 are connected throughout the substrate as shown in FIG. 12B (hereinafter referred to as “connected structure”). The graph shows how the temperature difference ΔT between the arm waveguide cores in the MZI circuit of the adjacent channel changes depending on the thickness of the heat insulating layer when power corresponding to the rotation of phase π is applied to the channel.

  On the other hand, a solid line curve A in FIG. 13 shows a structure (hereinafter referred to as “discrete structure”) that is not only in the heat conduction layer 810 as shown in FIG. In the thermo-optic variable attenuator, the power corresponding to the rotation of the phase π is applied to a certain channel, and the temperature difference ΔT between both arm waveguide cores in the MZI circuit of the adjacent channel is It shows how it varies with thickness.

  Each data in FIG. 13 is a plot of the results of the simulation, and Si (1 mm) is used for the substrate 801, Si (10 μm) is used as the heat conductive layer 810, and quartz is used as the heat insulating layer 811. The result which assumed the structure of the thermo-optic variable attenuator at the time of actually producing with the manufacturing method of this embodiment is shown.

  According to the simulation result of FIG. 13, it can be seen that if there is at least the heat insulating layer 811 in both structures shown by the solid curve A and the broken curve B, the effect of suppressing the influence of the thermal crosstalk of the present invention can be obtained. If the heat insulation layer 811 having a thickness of only 1 μm is present, the temperature difference ΔT can be suppressed to half or less compared to the case of the conventional example in which the heat conduction layer 810 and the heat insulation layer 811 are not implemented (see FIG. 6). I understand.

  As indicated by the solid curve A and the broken curve B, when the thickness of the heat insulating layer 811 increases, the temperature difference ΔT varies due to the structure. As indicated by the dashed curve B, the temperature difference ΔT increases again as the thickness of the continuous structure takes a minimum value in the vicinity of 20 μm and the thickness of the heat insulating layer 811 increases. On the other hand, as indicated by a solid curve A, the temperature difference ΔT monotonously decreases as the thickness of the heat insulating layer 811 increases in the discrete structure.

  FIG. 14 shows the relationship between the thickness of the heat insulating layer 811 and the power consumption in both the continuous structure and the discrete structure. Here, the curve A shows the power consumption characteristic in the case of the discrete structure, and the curve B shows the power consumption characteristic in the case of the connection structure. All of the data in FIG. 14 are obtained by calculating the power required to rotate the phase π in a certain channel, and are normalized with the power consumption when there is no heat insulation layer 811 as 1.

  From FIG. 14, it can be confirmed that the power consumption is not increased by adopting both structures of the present invention. As shown by the curve B, in the case of the continuous structure, the power consumption is almost constant regardless of the change in the thickness of the heat insulating layer 811, and the power consumption characteristics are worsened by the thin heat insulating layer. There is nothing. On the other hand, as shown by the curve A, in the case of a discrete structure, the thickness of the heat insulating layer 811 increases and the power consumption decreases. That is, in the discrete structure that is one of the structures of the present invention, not only can the circuit be made small while suppressing fluctuation due to thermal crosstalk, but also the effect of reducing power consumption by increasing the thickness of the heat insulating layer 811. Can also be obtained.

  In the case of the continuous structure shown in FIGS. 12 (A) and 12 (B), the effect of reducing power consumption cannot be obtained, but an additional step (step 10) for removing the heat insulating layer 811 is not necessary, so the process load is reduced. Has the advantage of being small. In other words, the effect that the heat insulating layer 811 is continuous is that the number of steps can be reduced while having the effect of suppressing the influence of thermal crosstalk compared to the case where the heat insulating layer 811 is discontinuous. In the case of the continuous structure, as described above with reference to FIG. 13 and as described later with reference to FIG. 15, there is an optimum value for the thickness of the heat insulating layer 811 due to the structure. Therefore, if the thickness of the heat insulating layer 811 is selected to an appropriate optimum value or a value close to the optimum value, the obtained effect is almost the same as that of the discrete structure.

  This effect will be further described in detail. FIG. 15 shows the difference in the optimum thickness of the heat insulating layer 811 when the thickness of the heat conductive layer 810 is changed in the above-described continuous structure. Here, the curve A is when the thickness of the heat conduction layer 810 is 5 μm, the curve B is when the thickness of the heat conduction layer 810 is 10 μm, and the curve C is 15 μm when the thickness of the heat conduction layer 810 is 15 μm. In this case, the results obtained by calculating the change in the temperature difference ΔT between the arm waveguide cores in the MZI circuit with respect to the variation in the thickness of the heat insulating layer 811 are plotted. From FIG. 15, it can be seen that even if the thickness of the heat conductive layer 810 is changed, the appropriate thickness (optimum thickness) of the heat insulating layer 811 does not change. Therefore, the thickness of the heat insulating layer 811 may be selected to an appropriate optimum value regardless of the size of the thickness of the heat conductive layer 810. The structure used in this calculation (specifically, the structure of the first embodiment in FIG. 12A or the structure of the second embodiment in FIG. 12B, arm waveguide spacing of 100 μm, MZI circuit) In the case of manufacturing using an SOI substrate with an interval of 200 μm, it is optimal if the thickness of the heat insulating layer 811 is about 20 μm. However, in this structure, as long as the thickness of the heat insulating layer 811 is in the range of 10 to 30 μm, there is no difference in the effect that can be obtained.

Further, the optimum thickness of the heat insulating layer 811 varies depending on the arrangement of the MZI circuit and the fluctuation of the arm waveguide interval. FIG. 16 shows the difference in the optimum thickness of the heat insulating layer due to the difference in the arm waveguide spacing, and in particular shows the calculation result when the adjacent Mach-Zehnder interferometer is twice the arm waveguide spacing. Here, the curve A shows the case where the arm waveguide interval is 100 μm, the curve B shows the case where the arm waveguide interval is 70 μm, and the curve C shows the case where the arm waveguide interval is 50 μm. Width w _r of the ridge structure constituting the arm waveguides 804 delimited by insulation grooves 811 in both cases, were performed calculated as 20 [mu] m. The calculation was performed assuming that the width of the heat conductive layer 810 is equal to the sum of the interval w between the arm waveguides 804 and the ridge width wr.

  According to the calculation result shown in FIG. 16, the optimum heat insulating layer thickness becomes thinner as the arm waveguide interval is shorter. For example, as shown by the curve C in FIG. 16, when the arm waveguide interval is 50 μm, the optimum thickness of the heat insulating layer 811 is about 9 μm. As a result, when the MZI circuit is disposed at a distance twice as long as the arm waveguide interval, about 1/5 of the arm waveguide interval is the optimum value of the thickness of the heat insulating layer 811.

  FIG. 16 shows a calculation result of obtaining an optimum value when Si single crystal is used as the heat conduction layer 810 and quartz is used as the heat insulation layer 811. The data shown by the curve is only intended to show the optimum value. It is what. As described above, if at least the structure of the present invention is adopted, an expected temperature difference reducing effect can be obtained even if the heat insulating layer 811 is somewhat thin. Further, even if other materials are used for the heat insulating layer 811, the same effect can be obtained. For example, if a material having a thermal conductivity half that of quartz is used for the heat insulating layer 811, it can be considered that the optimum thickness of the heat insulating layer 811 is also halved.

  As described above, according to the present embodiment, an effect of suppressing the influence of thermal crosstalk can be obtained even by providing a heat insulating layer of only 1 μm. If the discrete structure of this embodiment is adopted, the influence of thermal crosstalk can be suppressed, the circuit can be miniaturized, and the power consumption can be reduced. Furthermore, in the continuous structure of this embodiment, there is an optimum value for the thickness of the heat insulating layer, so by setting the thickness of the heat insulating layer to the optimum value or its vicinity value, while reducing the process load, The same effect as the discrete structure can be obtained.

(Other embodiments)
In the above, the preferred embodiment of the present invention has been described by way of example. However, the embodiment of the present invention is not limited to the above-described example, and the constituent members thereof are within the scope of the claims. Various modifications such as replacement, change, addition, increase / decrease in number, change in shape design, and the like are all included in the embodiment of the present invention.

It is a schematic top view which shows the structural example of the conventional multichannel optical attenuator. FIG. 5 is a cross-sectional view for explaining an arrangement interval of a Mach-Zehnder interferometer circuit (MZI circuit), in which (A) shows a minimum arrangement in the layout, and (B) shows an actual arrangement. It is sectional drawing explaining the influence of the thermal crosstalk of an adjacent channel. It is explanatory drawing which shows the temperature distribution of a board | substrate when one heater is driven. It is a characteristic view which shows the temperature distribution (calculation result) of the horizontal direction of a substrate surface. It is a characteristic view which shows the relationship between the space | interval of a Mach-Zehnder interferometer, and a temperature rise. It is a characteristic view which shows the relationship between the space | interval of a thermo-optic variable attenuator which shows the effect of this invention, and temperature rise. It is the schematic which shows the structure of the thermo-optic variable attenuator of the 1st Embodiment of this invention, (A) is a perspective view, (B) is a top view, (C) is a partially expanded sectional view. It is a characteristic view which shows the result of having measured the influence of the thermal crosstalk with respect to the thermo-optic variable attenuator of this invention compared with a prior art example. It is process drawing which shows the manufacturing method of the multichannel waveguide type optical variable attenuator by the 1st Embodiment of this invention, (A)-(H) is sectional drawing, respectively. It is sectional drawing which shows the preparation methods of the board | substrate concerning the 1st Embodiment of this invention. It is a figure which shows the structure of the 2nd Embodiment of this invention compared with 1st Embodiment, (A) is the structure of 1st Embodiment, (B) is the structure of 2nd Embodiment, ( C) is a cross-sectional view showing a different structure of the second embodiment. It is a characteristic view which shows the relationship between the temperature difference (DELTA) T and heat insulation layer thickness in the connection structure concerning the 1st, 2nd embodiment of this invention, and a discrete structure. It is a characteristic view which shows the relationship between the power consumption and heat insulation layer thickness in the connection structure concerning the 1st, 2nd embodiment of this invention, and a discrete structure. It is a characteristic view which shows the optimal heat insulation layer thickness by the difference in the heat conductive layer thickness in the connection structure concerning the 1st, 2nd embodiment of this invention. It is a characteristic view which shows the optimal value of the heat insulation layer thickness by the difference in the arm waveguide space | interval concerning the 1st, 2nd embodiment of this invention.

Explanation of symbols

800 Thermo-optic variable attenuator 801 Substrate 802 Optical waveguide 803 Directional coupler 804 Arm waveguide 805 MZI circuit 806 Heater 807 Core 808 Underclad 809 Overclad 810 Thermal conduction layer 811 Thermal insulation layer 812 Thermal insulation groove

Claims (8)

A Mach-Zehnder interferometer circuit (hereinafter referred to as an MZI circuit) comprising two directional couplers and arm waveguide pairs formed by optical waveguides formed on a substrate having relatively good heat conduction, and the upper surfaces of the arm waveguide pairs A multi-channel waveguide-type variable optical attenuator including a plurality of heaters each being a thermo-optic phase shifter arranged in a thermo-optic effect to change the phase of light by a thermo-optic effect,
Each of the MZI circuits is separated from each of the MZI circuits and is formed in a lower part of the arm waveguide pair that constitutes the MZI circuit, and has a material having a higher thermal conductivity than the material that constitutes the clad of the arm waveguide pair. A plurality of heat conductive layers,
A thermo-optic variable attenuator, comprising: a heat insulating layer formed between a lower portion of each of the heat conductive layers and the substrate and made of a material having a lower thermal conductivity than the heat conductive layer.   The thermo-optic variable attenuator according to claim 1, wherein the heat insulation layer under the MZI circuit is separated from a heat insulation layer of an adjacent channel.   The thermo-optic variable attenuator according to claim 1, wherein the heat insulating layer under the MZI circuit is continuous with a heat insulating layer of an adjacent channel.   The thermo-optic variable attenuation according to any one of claims 1 to 3, wherein the thermal conductive layer is formed of at least one of Si, polycrystalline Si, amorphous Si, tungsten, and tungsten silicide. vessel.   5. The heat insulation layer according to claim 1, wherein the heat insulation layer is formed of at least one oxide of Si, Al, B, P, Ge, Ti, Ta, Bi or a mixture thereof. The described thermo-optic variable attenuator.   The thermally conductive layer is made of Si, the MZI interval is arranged at a distance of 1.9 to 2.1 times the arm waveguide interval, and the thickness of the heat insulating layer is the arm waveguide. The thermo-optic variable attenuator according to claim 3, wherein the size is 1/4 to 1/6 times the interval. A method of manufacturing a thermo-optic variable attenuator according to any one of claims 1 to 3,
Preparing a waveguide substrate in which the substrate, the heat insulating layer, and the heat conductive layer are sequentially laminated;
Producing the waveguide structure including the MZI circuit, the heater and the electrode structure on the heat conductive layer without processing the heat conductive layer;
Removing the clad in advance where the heat insulating groove is to be produced later and where the heat conductive layer is to be removed;
Forming the heat insulating groove;
And a step of removing the thermal conductive layer at a location where the cladding has been removed in advance. The manufacturing method of a thermo-optic variable attenuator according to claim 7, further comprising a step of selectively removing only the heat insulation layer remaining in a lower portion of the heat insulation groove in contact with an adjacent channel in the heat insulation groove. Method.

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