JP4917977B2 - Optical modulator with monitor photodetector - Google Patents

Description

  The present invention relates to an optical modulator with a monitor photo detector, and more particularly to an optical modulator with a monitor photo detector that facilitates mounting of the monitor photo detector and is small in size and stable in operation state.

As is well known, a substrate having a so-called electro-optic effect in which the refractive index is changed by applying an electric field such as lithium niobate (LiNbO ₃ ) in an optical modulator (hereinafter, the lithium niobate substrate is abbreviated as an LN substrate). The traveling-wave electrode type lithium niobate optical modulator (hereinafter abbreviated as LN optical modulator) having an optical waveguide and traveling-wave electrode formed thereon is 2.5 Gbit / s, 10 Gbit / s because of its excellent chirping characteristics. Applied to large-capacity optical transmission systems.

  Such an LN optical modulator has recently been studied for application to an ultra large capacity optical transmission system of 40 Gbit / s, and is expected as a key device in the large capacity optical transmission system.

[First prior art]
FIG. 12 is a perspective view showing a configuration of an LN optical modulator as an optical modulator with a monitor photodetector according to the first prior art disclosed in Patent Document 1, for example. In the figure, 1 is a z-cut LN substrate, 2 is a Mach-Zehnder type optical waveguide formed by thermal diffusion of Ti, 2a is an input optical waveguide, 2b is a Y-branch type branched optical waveguide, 2c-1 and 2c -2 is an interaction optical waveguide, 2d is a Y-branch combined optical waveguide, 2e is an output optical waveguide, and 2g is an end of the output optical waveguide.

In the figure, 3 is an electrical signal source, 4 is a central conductor of a traveling wave electrode, 5a and 5b are ground conductors, 6a and 6b are radiated light generated when the optical signal is in an OFF state, as will be described later, and 7 is A single-mode optical fiber for signal light, 8 is an optical fiber for receiving radiated light, 11 is a monitor photodetector made up of, for example, a photodiode, and 9 is a bias power supply DC based on a radiated light detection signal described later from the monitor photodetector 11. Radiation light detection means including a bias controller for adjusting the operating point and the operating point of the LN optical modulator. In order to simplify the description, the SiO ₂ buffer layer and the Si conductive layer that are usually used in the z-cut LN optical modulator are omitted.

  FIG. 13 is a diagram for explaining the operating principle of the LN optical modulator configured as shown in FIG. FIGS. 13A and 13B are diagrams for explaining the operation of the optical waveguide 2, and FIG. 13C shows a side view of the LN optical modulator.

  The operation of the LN optical modulator will be described with reference to FIGS. The light incident on the input optical waveguide 2a is divided into two in the branch optical waveguide 2b. When the electric signal from the electric signal source 3 is not applied to the central conductor 4 of the traveling wave electrode and the ground conductors 5a and 5b, as shown in FIG. 2c-2 is propagated in phase. Thereafter, the light is combined by the combined optical waveguide 2d and propagates through the output optical waveguide 2e as a fundamental mode, and finally, the light is emitted to the single-mode optical fiber 7 for signal light. This is called an ON state. A portion where the combined optical waveguide 2d is joined to the output optical waveguide 2f is referred to as a combined point 2h.

  On the other hand, when an electrical signal from the electrical signal source 3 is applied to the central conductor 4 of the traveling wave electrode and the ground conductors 5a and 5b, as shown in FIG. 2c-1 and 2c-2 are propagated in opposite phases. Thereafter, the light is multiplexed by the multiplexed optical waveguide 2d to form first-order higher-order mode light. Usually, the output optical waveguide 2e is designed so that the first-order higher-order mode light is cut off.

  Therefore, since this primary high-order mode light cannot propagate through the output optical waveguide 2e, the radiation light 6a, 6b is 0.7 degrees in the horizontal direction of the substrate, as shown in FIG. 13 (c). It is radiated into the substrate from the combining point 2h with a small angle of 0.9 degrees in the direction, and propagates while spreading in the substrate. This is called an OFF state.

  In the voltage-light output characteristics shown in FIG. 14, the curve shown by the solid line is the voltage-light output characteristics of the LN optical modulator in a certain state, and Vb is the DC bias voltage at that time.

  As shown in FIG. 14, the DC bias voltage Vb is normally set at the midpoint between the peak and bottom of the light output characteristic.

  On the other hand, when the voltage-light output characteristics change as indicated by a broken line in FIG. 14 due to some cause such as temperature fluctuation, it is necessary to change the setting of the bias point as Vb ′.

  In the first prior art, after this radiated light is received and propagated by the radiated light receiving optical fiber 8, it is converted into an electric current by being incident on a monitor photodetector 11 made of, for example, a photodiode. The synchrotron radiation detection means 9 including the bias controller detects a change in the voltage-light output characteristic based on the magnitude of the current, and finds the optimum bias point of the DC bias voltage by the bias DC power supply.

  However, the LN optical modulator configured as described above also has the following problems. As shown in FIGS. 13B and 13C, the radiated light is actually emitted from the combining point 2h at a small angle of 0.7 degrees in the horizontal direction of the substrate and 0.9 degrees in the depth direction. Since the emitted light receiving optical fiber 8 is very close to the signal light single mode optical fiber 7, it is disposed at a position slightly lower than the signal light single mode optical fiber 7. There is a need to.

  Here, FIG. 15 shows a state in which the optical signal is in an OFF state as viewed from the single-mode optical fiber for signal light 7 side. In FIG. 13B, for example, if the length of the output optical waveguide 2e in the optical axis direction is 4 mm, the propagation angle of the radiated light in the horizontal direction is only 0.7 degrees as described above. The distance between the single-mode optical fiber 7 for use and the radiated light 6a or the radiated light 6b is as narrow as about 50 μm. Further, the center of the power of the radiated light 6a or the radiated light 6b comes to the position of the depth H from the upper surface of the z-cut LN substrate 1, but the vertical propagation angle of the radiated light is 0.9 degrees. , H is about 63 μm. As described above, since the distance between the signal light and the center of the radiation light 6a or the radiation light 6b is small, it is very difficult to mount the signal light single-mode optical fiber 7 and the radiation light receiving optical fiber 8.

  The difficulty of this mounting will be described with reference to FIG. 16 (see, for example, FIG. 9 of Patent Document 1). In the figure, 7a is a core of a single-mode optical fiber for signal light, 8a is a core of an optical fiber for receiving radiated light, 10a is a capillary (a capillary made of a dielectric, and generally a glass material is known. Other materials such as ceramic may be used).

  A hole is formed in the capillary 10a separately from the signal light single mode optical fiber 7, and the radiated light receiving optical fiber 8 is fixed to the hole. Thus, the signal light is coupled to the core 7a of the single-mode optical fiber 7 for signal light, and the radiation 6b (or 6a) is coupled to the core 8a of the optical fiber 8 for receiving radiation. Adjust and fix each positional relationship.

  As described above, in the LN optical modulator with a monitor photodetector according to the first prior art, the distance between the signal light and the radiated light is as very small as about 50 μm, and therefore the core 7a of the single mode optical fiber 7 for signal light. It is necessary to mount the signal light and the core 8a of the radiated light receiving optical fiber 8 to couple the radiated light, which is extremely difficult to implement. Development of a vessel is desired.

  Usually, in order to avoid this difficulty in mounting, it is conceivable to increase the distance between the signal light and the emitted light.

  By the way, apart from the idea of increasing the distance between the signal light and the radiated light, an invention is disclosed in which the interference pattern is formed far from the signal light by causing the radiated light to interfere with the signal light (for example, patents). Reference 2). However, the interference between the radiated light and the signal light means that the signal light is attenuated. As a result, the loss of the signal light is increased or the signal light can be influenced because it is an interference. There is a problem that the interference pattern can be formed only in the range, that is, only in a region relatively close to the signal light, and the fact is that the above problem has not been solved.

  As described above, it is extremely difficult to mount both the single-mode optical fiber 7 for signal light and the optical fiber 8 for receiving radiated light on the capillary 10a. Therefore, consider a case where the radiated light after passing through the capillary 10a is received by the monitor photodetector 11 such as a monitor photodiode instead of using the radiated light receiving optical fiber 8.

  In this case, assuming that the refractive indices of the z-cut LN substrate 1 and the capillary 10a are 2.14 and 1.45, respectively, the emitted light passes through the capillary 10a ± 0.7 ° × 2.14 / 1.45 = ± Since it propagates at a refraction angle of 1.0 °, the emitted light propagates very close to the signal light single-mode optical fiber 7 fixed in the capillary 10a, so that it is practically difficult to mount the monitor photodetector 11. It is.

[Second prior art]
FIG. 17 shows an LN optical modulator with a monitor photodetector according to the second prior art as a structure for solving these problems. In the LN optical modulator according to the second prior art, the emitted lights 6a and 6b propagated through the z-cut LN substrate 1 are used as emitted lights 6c and 6d in the capillary 10b whose rear end is further inclined. Propagate.

  Here, by depositing a dielectric multilayer film 14 in advance on the inclined rear end surface of the capillary 10b, the light is totally reflected and emitted to the outside, and the light is received using the monitor photodetector 11 to generate a current. Convert.

  However, the LN optical modulator according to the second prior art has an important problem. This problem will be considered below. First, consider the optical path length from when the radiated lights 6a and 6b pass through the substrate end face 1a to the monitor photo detector 11 such as a monitor photo detector.

When the emitted light 6c is, after the inside of the capillary 10b propagated through the distance L ₁ after passing through the substrate end face 1a, it reflected at the dielectric multilayer film 14 of the rear inclined surface of the capillary 10a, further in the capillary 10a the propagation distance L ₂ above. Thereafter, the air propagates a distance L _3, reaches the monitor photodetector. When the refractive index of the capillary 10a and _{n c,} the optical total optical path length _{L 6c} of the radiation 6c becomes _{L 6c = n c L 1 +} n c L 2 + L 3.

On the other hand, in the case of the radiated light 6d, after passing through the substrate end face 1a and propagating through the distance L4 in the capillary 10b, it is reflected by the dielectric multilayer film 14 on the inclined rear face of the capillary 10a, and is further reflected on the capillary 10a. Propagate distance L5 up. Thereafter, the air propagates a distance L _6, reaches the monitor photodetector 11 such as a photodiode. The total optical path length L _6d of the emitted light 6d is L _6d = n _c L ₄ + n _c L ₅ + L ₆ .

  On the other hand, the radiated lights 6a and 6b propagate through the capillary 10a at different angles ± 1.0 ° and are reflected upward on the dielectric multilayer film 14 on the inclined surface at the rear end of the capillary 10a. As shown in FIG. 18, the emitted lights 6c and 6d overlap each other and interfere with each other. In FIG. 18, 6i is an overlapping part (interference part) of the radiation lights 6c and 6d.

  Here, FIG. 19 shows a state in which the phases of the emitted lights 6c and 6d are different by about 180 degrees. When the phases of the emitted light 6c and 6d are different from each other by 180 degrees, as shown in FIG. 20A, there is a portion where the power is zero in the overlapping portion.

However, since the refractive index _{n c} of the capillary 10a varies with temperature, both the optical path length _{L 6c} and _{L 6d} when synchrotron radiation 6c, 6d is incident on the monitor photo-detector 11 will vary with temperature. As a result, the phase difference between the emitted lights 6c and 6d is different from 180 degrees. Therefore, as a result of overlapping the radiation lights 6c and 6d, as shown in FIG. 20 (b), the overlapping portion 6i of the radiation lights 6c and 6d does not become zero at any location.

  In other words, the intensity of the light at the overlapping portion 6i of the radiated light 6c and 6d shown in FIGS. 18 to 20 changes with temperature, which hinders the DC bias control of the LN optical modulator.

  Similarly to the case of the capillary 10a in the first prior art shown in FIG. 16, in the second prior art, it is necessary to mount the single mode optical fiber 7 for signal light on the capillary 10b. Further, in order to facilitate mounting of the signal light single mode optical fiber 7 in any of the capillaries 10a and 10b, a guide larger than the outer shape of the signal light single mode optical fiber 7 is provided at the rear ends of the capillaries 10a and 10b. It is desirable to have a counterbore for use.

  However, since the radiated light propagates in the vicinity of the signal light single mode optical fiber 7 in any of the capillaries 10a and 10b, such counterbore for guiding is provided at the rear ends of the capillaries 10a and 10b. It is not possible.

[Third prior art]
FIG. 21 shows an LN optical modulator with a monitor photodetector according to the third prior art as a structure for solving these problems. In the LN optical modulator according to the third prior art, the capillary 10c is processed as shown in FIG. 21 so that only the emitted light 6c is received by the monitor photodetector 11, and the emitted light 6d does not enter the monitor photodetector 11. It is devised as follows. It is possible to guide the mounting of the signal light single-mode optical fiber 7 by cutting the capillary 10c halfway.

  However, in the structure of the LN optical modulator according to the third prior art, the capillary 10c needs to be processed into a complicated structure, and the capillary 10b of the LN optical modulator according to the second prior art shown in FIG. Similarly, it is necessary to deposit a dielectric multilayer film 15 or the like for totally reflecting light on the inclined rear end surface of the capillary 10c, and there is a problem that the manufacturing cost of the entire optical modulator becomes higher. .

[Fourth Prior Art]
FIG. 22 shows a perspective view of a fourth conventional LN optical modulator with a monitor photodetector disclosed in Patent Document 4. In FIG. Here, 50 is a z-cut LN substrate, 51 is an optical waveguide, 51A is an input optical waveguide, 51B is an input 3-dB coupler consisting of a multimode interference optical waveguide (MMI), 51C and 51D are interactive optical waveguides, 51E is an output 3-dB coupler made of a multimode interference optical waveguide (MMI), 51F is a signal light output optical waveguide, 51G is a monitor light output optical waveguide, 53 is a reflection groove, 52 is a traveling wave electrode, and 52A is A central conductor, 52B is a ground conductor, 52C is a pad, 54 is a monitor photodetector, and 55 is a block material. Reference numeral 56 denotes monitor light that exits the monitor light output optical waveguide 51 </ b> G, is reflected by the reflection groove 53, and then travels toward the monitor photodetector 54.

  FIG. 23 shows an enlarged view including the vicinity of the output 3-dB coupler 51E and the reflection groove 53 in the fourth prior art. However, the longitudinal direction is greatly reduced due to the space on the paper. As can be seen from this figure, the light emitted from the monitor light output optical waveguide 51 </ b> G is reflected by the reflection groove 53 and propagates as the monitor light 56 toward the monitor photodetector 11.

  However, this fourth prior art has a big problem. That is, since the monitor light output optical waveguide 51G is used, a 3-dB coupler is inevitably used. As this 3-dB coupler, there are a directional coupler type 3-dB coupler and a zero gap directional coupler in which the gap between two optical waveguides in the directional coupler is zero, that is, an MMI type 3-dB coupler. is there. In the fourth prior art, an MMI type 3-dB coupler for output 51E is used.

Now, if the length L _{MMI of the} output 3-dB coupler 51E deviates from the design value, a serious situation occurs in which the insertion loss increases or the ON / OFF ratio (extinction ratio) of the signal cannot be secured sufficiently. . The ideal value of the length L _MMI of the output 3-dB coupler 51E shown in FIG. 23 is defined by the width W _MMI of the output 3-dB coupler 51E.

FIG. 24 shows a design value (ideal value) of the length L _MMI of the output 3-dB coupler 51E when the width W _MMI of the output 3-dB coupler 51E is a variable. As can be seen, the length L _MMI is proportional to the square of the width W _MMI . Therefore, the width W _MMI deviates from the design value due to slight variations in the exposure conditions and development conditions in photolithography, and as a result, the length L _MMI deviates from the condition of becoming a 3-dB coupler. When such a situation occurs, as described above, the characteristics as the z-cut LN optical modulator are significantly deteriorated from the viewpoint of the insertion loss and the ON / OFF ratio of the signal. A high manufacturing accuracy of about 1 μm is required as the accuracy of the width W _MMI when the output 3-dB coupler 51E is manufactured. Therefore, it can be said that the manufacturing tolerance is severe.

  Another problem is that the output 3-dB coupler 51E has wavelength-dependent characteristics. FIG. 25 is a graph of insertion loss versus wavelength for operating the z-cut LN optical modulator. In this way, when the design is made such that the insertion loss is minimized at a certain wavelength (for example, 1.55 μm), the insertion loss increases at the other wavelengths (at the same time, the ON / OFF ratio deteriorates). Therefore, it is disadvantageous to apply the fourth prior art shown in FIG. 23 to a wavelength division multiplexing (DWDM) transmission method used in a very wide wavelength range such as full C band or full L band.

[Fifth Prior Art]
Similarly, FIG. 26 shows an LN optical modulator with a monitor photodetector according to the fifth prior art disclosed in Patent Document 4. In the fifth prior art, a Y branching type demultiplexer and a multiplexer are used instead of the input 3-dB coupler 51B and the output 3-dB coupler 51E in the fourth prior art shown in FIG. ing. The input side is also referred to as an input Y-branch or input Y-branch optical waveguide, and the output side is also referred to as an output Y-branch or output Y-branch optical waveguide. Here, 51H is an input optical waveguide, 51B ′ is a branch point of the Y branch for input, and 51I is an output optical waveguide. Reference numeral 56 ′ denotes radiated light emitted from the output Y-branch multiplexing point 51 E ′, which is reflected by the reflection groove 53 and propagates toward the monitor photodetector 54 as monitor light 56 ″. Thus, by using a Y-branch type optical waveguide instead of the 3-dB coupler made of MMI, the problems of the fourth prior art described in FIGS. 24 and 25 can be solved. The prior art No. 5 has a very large problem in practical use.

  As can be clearly seen from the figure, the reflection groove 53 is formed in the vicinity of the end face (light output side end face) 58 on the side where light is output, away from the output Y-branch multiplexing point 51E ′. . Further, a reflection groove 53 that needs to be formed at a distance that does not affect the output optical waveguide 51I from the viewpoint of insertion loss is provided between the output optical waveguide 51I and the monitor photodetector 54. Considering that the radiated light 56 'spreads only about 0.7 degrees in the horizontal direction, this also means that the reflection groove 53 is separated from the output Y-branch multiplexing point 51E' by a distance and the light output side end face It is suggested that it is formed in the vicinity of 58.

  FIG. 27 shows the depth (H in FIG. 15) where the center of the power of the radiated light 56 ′ exists with respect to the distance from the combining point 51E ′ of the output Y branch. As described in the first prior art, the ON / OFF ratio of signal light is prevented from deteriorating due to incident incident light on a signal light single mode optical fiber (see 7 in FIG. 12) (not shown). For this purpose, the length of the output optical waveguide 51I is at least 2 to 3 mm, preferably about 4 mm. When the output optical waveguide 51I has such a length, the reflection groove 53 is formed in the vicinity of the light output side end face 58, so that the z-cut LN substrate 50 is inclined at a depth of 0.9 degrees in the depth direction. Considering the propagation, as can be seen from FIG. 27, the center of the power of the radiated light 56 ′ is as deep as 30 to 40 μm even when the length of the output optical waveguide 51 I is 2 to 3 mm. When the length is 4 mm, it is as deep as 63 μm.

  From this, it is possible to immediately understand the problem of the fifth prior art. That is, in order for the monitor light 56 ″ to enter the monitor photodetector 11 efficiently, it is necessary to form the reflection groove 53 having a depth of several tens of microns. Although the etching rate of a normal dry etching apparatus depends on the apparatus, it is usually about 10 nm / min to 0.1 μm / min, and it is extremely difficult to form a deep reflection groove of several tens of microns. Even if the reflection groove is formed by dicer instead of dry etching, it is impossible to form the reflection groove without damaging the output optical waveguide because the diameter of the dicer teeth is several centimeters.

Moreover, in the fifth prior art, the reflecting surface of the reflecting groove 53 is defined as a convex curved surface as a curvature, which makes it difficult to receive the monitor light 56 ″ by the monitor photo detector 54. . In other words, the radiated light 56 ′ that has propagated several millimeters through the substrate is very wide, and if reflected on the reflecting surface having a convex curvature, the radiated light 56 ′ becomes wider than the light receiving surface of the monitor photodetector 54, and the efficiency is increased. It will deteriorate extremely.
Japanese Patent Laid-Open No. 3-145623 JP-A-10-228006 JP 2001-281507 A JP 2005-345554 A

  As described above, various optical modulators with monitor photodetectors have been proposed to control the DC bias of the LN optical modulator. However, in the first prior art, since the single-mode optical fiber for signal light is extremely close in distance, it is difficult to mount the optical fiber for receiving radiated light. In the second prior art, if the ambient temperature when using the LN optical modulator changes, the intensity of the radiated light incident on the monitor photodetector changes, causing a problem in DC bias control. In the third prior art, a single mode optical fiber for signal light is mounted, and the shape of the capillary for sending the radiated light to the monitor photodetector is complicated, which contributes to the cost increase of the LN optical modulator. It was. The fourth prior art has a problem that production tolerance is severe due to the characteristics of the output 3-dB coupler, or wavelength dependency occurs in the insertion loss and signal ON / OFF ratio (extinction ratio). Further, in the fifth prior art, a reflection groove for propagating the radiated light to the monitor photodetector as the monitor light is formed in the vicinity of the light output side end face of the LN optical modulator. Therefore, it is necessary to etch the reflection groove as deep as tens of microns, and it is extremely difficult to form the reflection groove using a dry etching apparatus. Moreover, even if a dicer is used, it is impossible to form a reflection groove without damaging the output optical waveguide that outputs signal light.

  SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an optical modulator with a monitor photo detector that solves the above-described problems of the prior art, facilitates the mounting of the monitor photo detector, and is small in size and stable in the operation state. Yes.

In order to solve the above problems, a monitor photodetector with light modulator according to claim 1 of the present invention, possess an electro-optical effect, the substrate having the opposing front and rear face (1), of the substrate the front side, comprising a central conductor (4) and the ground conductor for applying a voltage for modulating the pre-Symbol light waveguide (2) for guiding light (5a, 5b) The optical waveguide includes at least an input optical waveguide (2a) for entering the light, a branched optical waveguide (2b) for branching the light incident on the input optical waveguide, and the central conductor and the ground conductor. An optical waveguide (2c-1, 2c-2) for modulating the phase of the light by applying the voltage in between, and a combined optical waveguide for combining the light propagated through the interactive optical waveguide Via the waveguide (2d) and the multiplexing point (2h) of the multiplexing optical waveguide An output optical waveguide (2f) connected to the combined optical waveguide, and a higher-order mode generated by combining phase-modulated light in the combined optical waveguide is generated by the output optical waveguide. An optical modulator that is radiated as radiated light (30a, 30b) from the multiplexing point into the substrate with little propagation through a waveguide, and the radiation that is radiated from the multiplexing point into the substrate of the optical modulator. detecting the emitted light, the monitor photodetector (11) and the monitor photodetector with light modulators comprising at a position other than the rear surface of the substrate, by reflecting the emitted light (30b), the said emitted light A reflection groove (20) for converting into monitor light (30c) propagating toward the monitor photodetector is provided, and the reflection groove installs the monitor photodetector with respect to the output optical waveguide. Is formed on the front surface of the substrate of the side opposite the monitor light said output optical waveguide cross to monitor photodetector with light modulator, characterized by propagating the below said output optical waveguide vessel.

The optical modulator with a monitor photodetector according to claim 2 of the present invention is characterized in that the multiplexing point is formed in a Y-branch optical waveguide.

The optical modulator with a monitor photodetector according to claim 3 of the present invention is characterized in that the optical waveguide is a Mach-Zehnder type optical waveguide.

The optical modulator with a monitor photodetector according to a fourth aspect of the present invention is characterized in that the monitor photodetector is provided in at least one of a side surface direction in a longitudinal direction of the substrate or a rear end direction of the substrate.

The optical modulator with a monitor photodetector according to claim 5 of the present invention is characterized in that the monitor photodetector is provided with respect to the substrate through a space.

The optical modulator with a monitor photodetector according to a sixth aspect of the present invention is characterized in that the optical waveguide is composed of two or more Mach-Zehnder optical waveguides arranged in parallel.

The optical modulator with a monitor photodetector according to claim 7 of the present invention is characterized in that the substrate is made of a semiconductor.

  In the present invention, the vicinity of the multiplexing point of the output Y-branch optical waveguide provided with the LN optical modulator has a reflection groove that can reflect the radiation emitted from the multiplexing point and propagate it to the monitor photodetector as monitor light. To form. The monitor light converted from the radiated light by the reflection groove crosses under the output optical waveguide through which the signal light propagates and travels to the monitor photodetector. In the present invention, since the reflection groove is formed in the vicinity of the multiplexing point of the output Y-branch optical waveguide, it is not necessary to etch deeply in order to form the reflection groove, and the productivity is greatly improved. There is. In addition, since the reflected light is reflected before it spreads greatly, there is an advantage that the monitor light can be generated efficiently.

  Embodiments of an optical modulator with a monitor photodetector according to the present invention will be described below. The same reference numerals as those in the first to fifth prior arts shown in FIGS. Then, description of the functional part having the same number is omitted.

[First Embodiment]
FIG. 1 is a perspective view showing a configuration of an LN optical modulator applied as a first embodiment of an optical modulator with a monitor photodetector according to the present invention. FIG. 2 is a detailed top view of a region where the emitted lights 30a and 30b of FIG. 1 are generated. That is, FIGS. 1 and 2 illustrate a state in which the optical signal is OFF. However, for easy understanding, the emitted light 30a is omitted in FIGS. In addition, the longitudinal direction is greatly reduced due to space limitations. Further, in FIG. 1, the Y branch optical waveguide is depicted in more detail than the first prior art shown in FIG. That is, the actual Y-branch optical waveguide including the input Y-branch optical waveguide has a transition region 2i in which the width of the optical waveguide is wide as shown in FIG. The length _{L Y} in the transition region 2i is usually about 1 mm to 2 mm.

  As a major feature of the present invention, the reflection groove 20 is not located far away from the combining point 2h (51E ′ in FIG. 26) of the output Y-branch optical waveguide as in the fifth prior art, but the output Y-branch light. It is provided in the vicinity of the multiplexing point 2h of the waveguide. Then, with respect to the output optical waveguide 2e, a reflection groove 20 is formed on the substrate surface opposite to the side on which the monitor photo detector 11 is installed. In other words, the output optical waveguide 2e is sandwiched between the monitor photo detector 11 and the reflection groove 20. It is an arrangement.

  The operation of the first embodiment will be described. When the optical signal is OFF, the radiated light 30b is generated from the combining point 2h of the output Y-branch optical waveguide (as described above, the radiated light 30a is actually generated, but this is omitted in FIG. 2). The radiated light 30b is reflected by the reflection groove 20, propagates as monitor light 30c across the output optical waveguide 2e (and radiated light 30a), and travels toward the substrate side surface 40 on the side where the monitor photodetector 11 is located. Then, after being emitted from the substrate side surface 40, the light enters the monitor photodetector 11. A solid line in the radiation light 30b and the monitor light 30c shown in FIG. 2 represents a locus drawn by the center of each power. Moreover, the power of light is actually spreading, and the broken line shows the spreading. Since the traveling directions of the monitor light 30c and the radiated light 30a are approximately 90 degrees different from each other, even if the monitor light 30c and the radiated light 30a intersect at the same position, they do not affect each other.

As shown in FIG. 2, the center of the power of the radiation light 30b propagates at an angle of 0.7 degrees in the horizontal direction with respect to the output optical waveguide 2e. Next, this point will be discussed in detail in FIG. Leaving a distance _{X R} from the output optical waveguide 2e when the emitted light 30b is distance _{L prop} propagation. As shown in FIG. 2, since the emitted light 30b actually has a spread, by forming a position of at least the distance X _R the reflecting groove 20 from the output optical waveguide 2e, to obtain a sufficient monitor light 30c Can do. Note that to avoid affecting the light reflecting groove 20 is propagated to the output optical waveguide 2e is 1μm or more as the distance X _R, we want to ensure about 2μm if possible.

Here, W _G in the figure is the width of the reflective grooves 20, 4 [mu] m approximately Given the retraction of photolithography and dry etching when the mask is needed. For this reason, it is difficult to form the reflection groove 20 on the z-cut LN substrate 1 between the output optical waveguide 2e and the monitor photodetector 11.

FIG. 4 is a cross-sectional view in the depth direction along AA ′ in FIG. 3 and explains the behavior of the emitted light 30b in the depth direction. As shown in FIG. 4, the center of the power of the radiated light 30b propagates toward the lower side of the z-cut LN substrate 1 with an inclination of 0.9 degrees. Here, in the same way as in FIG. 3, when the _radiated light 30b propagates the distance L _prop , it is assumed that the distance Y _{R is} separated from the output optical waveguide 2e in the depth direction.

FIG. 5 shows the value of the distance _{X R} and the distance _{Y R} for the distance _{L prop} the emitted light 30b is propagated. As can be seen, when the distance _{X R} and 2 [mu] m, distance _{Y R} becomes 2.6μm corresponding thereto (which also easily obtained as 2 [mu] m × 0.9 ° /0.7 degrees). The propagation distance _Lprop at this time is determined to be about 160 μm. That is, in the present invention, the reflection groove 20 may be formed at a distance of about 160 μm from the combining point 2h of the output Y branch optical waveguide.

Incidentally, the this about a value of 160μm may be only a shorter propagation distance _{L prop} if a is more reduced _{X R} an example, if a slightly larger distance _{X R} Conversely, the propagation distance _{L prop} is slightly longer Become. However, in the present invention, unlike the fifth prior art, the reflection groove 20 is not formed near the light output side end face of the z-cut LN optical modulator.

As described above, the length L _Y of the transition region 2i of the output Y-branch optical waveguide is usually about 1 mm to 2 mm, and the length of the output optical waveguide 2e is at least 2 to 3 mm, preferably about 4 mm. Therefore, unlike the conventional fifth conventional technique in which the reflection groove (53 in FIG. 26) is formed in the vicinity of the end face on the light output side, the present invention reflects the light near the combining point 2h of the output Y-branch optical waveguide. It can be seen whether the groove 20 is formed.

FIG. 6 is a cross-sectional view in the depth direction at BB ′ of the top view shown in FIG. Here, 1 is a z-cut LN substrate, and 61 and 62 are SiO ₂ buffer layers and Si conductive layers, which are not shown in FIG. 3, but are usually used in an LN optical modulator using the z-cut LN substrate 1. Respectively. Similarly, although not shown in FIG. 3, in practice, Au is used as the reflection mirror 63 to convert the emitted light 30b into the monitor light 30c with a high efficiency of almost 100%.

  Usually, the reflection groove 20 is formed by a process technique such as dry etching. Accordingly, in FIG. 6, the angle Θ of the wall 20 a of the reflection groove 20 is inclined at about 60 to 75 degrees instead of 90 degrees. It should be noted here that when the angle Θ of the wall 20a of the reflection groove 20 becomes too small, the monitor light 30c is directed downward at a large angle, and as disclosed in JP-A-4-24610, the monitor photo detector shown in FIG. 11 is required to be installed on a pedestal of a package housing (not shown) (hereinafter referred to as a housing pedestal) directly below the z-cut LN substrate 1. However, when the monitor photodetector 11 includes a wiring board, the size is actually as large as about 3 mm × 4 mm × 5 mm, and it is difficult to install the monitor photo detector 11 on a housing pedestal that is dimensionally limited. Also, since the housing base is usually made of metal, the wiring is likely to be short-circuited, or a monitor photo detector must be installed before fixing the LN optical modulator chip, and if the subsequent fiber fixing fails, the monitor photo detector is discarded. It can be said that the structure of Japanese Patent Application Laid-Open No. 4-24610 is not suitable for practical use.

  On the other hand, in the present invention, since the monitor light 30c is taken out from the side surface 40 of the z-cut LN substrate 1, the monitor photodetector 11 may be installed later only in the module that has been successfully fixed to the fiber, and the monitor photodetector 11 can be easily installed. Not only is this very cost effective. The angle Θ of the wall 20a of the reflection groove 20 may be 90 degrees or may exceed 90 degrees. Further, even if the monitor light 30c hits the housing pedestal (not shown) due to the shape of the reflection groove 20, the shape of the wall 21 and the way of giving the angle Θ, the side surface 40 of the z-cut LN substrate 1 is finally used. As long as it is removed, its structure belongs to the present invention.

FIG. 7 shows an example of a cross-sectional view of an interaction portion of an LN optical modulator having a ridge structure disclosed in JP-A-4-288518 as a high-performance LN optical modulator. Here, 4 is a central conductor, 5a and 5b are ground conductors, 2c-1 and 2c-2 are interactive optical waveguides, 61 is a SiO ₂ buffer layer, and 62 is a Si conductive layer. In this way, in the ridge structure, unevenness, that is, a ridge is formed near the surface of the z-cut LN substrate 1 by dry etching or the like. In the present invention, since the reflection groove 20 can be formed at the same time in the process of forming the ridge in the interaction portion, it can be said that the present invention has extremely good consistency with the manufacturing process of the conventional LN optical modulator.

  Various shapes are conceivable as the shape of the reflection groove 20 shown in FIGS. In FIG. 8, the reflection surface 41 of the reflection groove 20 that converts the emitted light 30b into the monitor light 30c is a flat surface. In FIG. 9, the reflection surface 42 of the reflection groove 21 that converts the emitted light 30b into the monitor light 30d is a convex curved surface. In the present invention, unlike the fifth prior art shown in FIG. 26, the reflection groove 21 is formed in the vicinity of the combining point 2h. Therefore, even if the reflection surface 42 is a convex curved surface, the monitor light 30d does not spread too much. Absent. On the other hand, in FIG. 10, the reflection surface 43 of the reflection groove 22 that converts the emitted light 30b into the monitor light 30e is a concave curved surface. Note that the shapes of the reflection groove and the reflection surface are not limited to these. For example, the reflection groove may be a triangle, or the reflection surface may be a more complicated shape. These are applicable to all embodiments of the present invention.

  In FIG. 1, it is assumed that the monitor photodetector 11 is installed through the side surface 40 of the z-cut LN substrate 1 or further through a space. However, the incident angle of the radiated light 30b to the reflection groove 20 in the horizontal direction is described. May be emitted to the rear end side of the z-cut LN substrate 1 (that is, the end face side from which the light is emitted), and the monitor photodetector 11 may be installed in that direction. Further, if the output optical waveguide 2e is slightly bent from the combining point 2h of the output Y-branch optical waveguide toward the substrate side surface 40 where the monitor photodetector 11 is located, the reflection groove 20 can be made closer to the radiated light 30b. Furthermore, it becomes possible to efficiently convert the emitted light 30b into the monitor light 30c. This is true for all embodiments of the invention. The monitor photodetector 11 may be arranged at the same height as the z-cut LN substrate 1, but it goes without saying that it may be arranged at a higher position or a lower position.

[Second Embodiment]
FIG. 11 is a diagram schematically showing a top view of an LN optical modulator applied as the second embodiment of the optical modulator with a monitor photodetector according to the present invention. This type of LN optical modulator is used in next-generation transmission systems such as DQPSK. Here, 70 is an input optical waveguide, 71a, 71b, 71c and 71d are interactive optical waveguides, 72a and 72b are central conductors of traveling wave electrodes, 73a, 73b and 73c are ground conductors of traveling wave electrodes, and 74a and 74b are A multiplexing point of the Y branch optical waveguide, 75a and 75b are reflection grooves, 76a and 76b are monitor lights, 77a and 77b are monitor photodetectors, and 78 is an output optical waveguide.

  As can be seen from this figure, two reflection grooves 75a and 75b convert radiated light (not shown) radiated from the two combining points 74a and 74b into monitor light 76a and 76b, and the two monitor photodetectors 77a and 77b. Receives light and performs DC bias control. As described in the first embodiment, if the two optical waveguides are partially spread from the two multiplexing points 74a and 74b toward the end face on the light emission side, reflection is achieved. Since the grooves 75a and 75b can be made closer to radiation light (not shown), the radiation light can be converted into monitor light with higher efficiency.

[Embodiments]
In the above description, the case where the LN substrate is a z-cut LN substrate has been described, but various substrates such as an x-cut substrate or a y-cut LN substrate may be used.

  Furthermore, in the above description, an asymmetric coplanar strip (ACPS) or coplanar waveguide (CPW) type traveling wave electrode is assumed as an electrode. good.

  In the above description, an LN substrate is assumed as the substrate. However, other dielectric substrates such as lithium tantalate, and further a semiconductor substrate may be used.

  As described above, the optical modulator with a monitor photodetector according to the present invention forms the reflection groove in the vicinity of the multiplexing point of the output Y-branch optical waveguide, so that it is not necessary to etch deeply to form the reflection groove. It has the advantage that the manufacturability is drastically improved, and also has the advantage that the monitor light can be generated efficiently because it is reflected before the radiated light spreads greatly. Therefore, it is useful as an optical modulator with a monitor photodetector that is small in size and stable in operation state.

The perspective view which shows the structure of the LN optical modulator applied as 1st Embodiment of the optical modulator with a monitor photodetector by this invention Partial top view of FIG. Enlarged view of FIG. Sectional view of depth direction in AA 'of FIG. The position of the center of the power of the emitted light with respect to the propagation distance of the emitted light It is sectional drawing in AA 'of FIG. 3, and is a figure for demonstrating the structure of the reflective groove 20 Sectional drawing of the interaction part of the LN optical modulator which has a ridge structure Explanatory drawing of the reflective groove whose reflective surface is a plane Explanatory drawing of a reflective groove whose reflective surface is a convex curved surface Explanatory drawing of the reflection groove whose reflection surface is a concave curved surface The top view which shows the structure of the principal part of the LN optical modulator applied as 2nd Embodiment of the optical modulator with a monitor photodetector by this invention A top view showing a configuration of an optical modulator with a monitor photodetector according to the first prior art disclosed in Patent Document 1 The figure shown in order to demonstrate the operation | movement principle of the optical modulator comprised as shown in FIG. DC bias voltage-optical output characteristic curve diagram for explaining the operation principle of the optical modulator configured as shown in FIG. The figure which shows the mode of the OFF state of the optical signal seen from the single mode optical fiber 7 side for signal light The figure shown in order to demonstrate that it is very difficult to mount the single mode optical fiber 7 for signal light, and the optical fiber 8 for light reception The top view which shows the structure of the principal part of the optical modulator with a monitor photo detector by 2nd prior art as a structure which solves the problem of the optical modulator with monitor photo detector by 1st prior art The figure shown in order to demonstrate that the emitted light 6a and 6b interfere with each other when entering the monitor photodetector 11 such as a monitor photodiode. The figure shown in order to demonstrate a mode that the phase of synchrotron radiation 6c, 6d differs about 180 degree | times When the phases of the emitted light 6c and 6d are different from each other by 180 degrees, there is a portion where the power is zero in the overlapping portion, and as a result of the temperature change, the phase difference between the emitted lights 6c and 6d is 180 degrees. FIG. 2 is a diagram for explaining that the overlapping portion of the emitted light 6c and 6d does not become zero at any position. The top view which shows the structure of the principal part of the LN optical modulator by the 3rd prior art as a structure which solves the problem of the optical modulator with a monitor photo detector by the 2nd prior art The perspective view which shows the structure of the principal part of the optical modulator with a monitor photodetector by a 4th prior art. 4 is a partially enlarged view of an optical modulator with a monitor photodetector according to the fourth prior art. The figure showing the relationship between the width W _MMI and the ideal L _MMI length of the 3-dB coupler used in the optical modulator with a monitor photodetector according to the fourth prior art The figure showing the relationship between a use wavelength and insertion loss in the optical modulator with a monitor photo detector by a 4th prior art. The perspective view which shows the structure of the principal part of the optical modulator with a monitor photodetector by a 5th prior art. The figure showing the relationship with the power center of the emitted light with respect to the distance from the multiplexing point of the optical modulator with a monitor photo detector by 5th prior art to a reflective groove

Explanation of symbols

DESCRIPTION OF SYMBOLS 1, 50: z-cut LN board | substrate 1a: Board | substrate end surface 2: Mach-Zehnder type optical waveguide 2a 51I: Output optical waveguide 2b: Y branch type branched optical waveguide 2c-1, 2c-2: Interaction optical waveguide 2d: Y Branching type combined optical waveguide 2e, 2f: Output optical waveguide 2g: Output optical waveguide end 2h, 51E ': Combined optical waveguide point 2i: Transition region of Y-branched branched optical waveguide 3: Electricity Signal source 4: Center conductor of traveling wave electrode 5a, 5b: Ground conductor 6a, 6b, 6c, 6d, 6e, 6f: Radiated light 7: Single mode optical fiber for signal light 7a: Single mode optical fiber for signal light Core 8a: Synchrotron radiation receiving optical fiber 8: Synchrotron radiation receiving optical fiber core 9: Synchrotron radiation detecting means 10a, 10b, 10c, 10d: Capillary 11, 54: Monitor photo detector 12: Mirror 3: Glass block 14, 15: Dielectric multilayer film 20, 21, 22 formed as a reflection surface on the inclined surface at the rear end of the capillary 20, 21: Reflection groove 20 a: Walls 30 a, 30 b, 56 ′ of reflection groove 20 30d, 30e, 56, 56 ″: Monitor light 40: Side surface 41, 42, 43 of the z-cut LN substrate 1: Reflecting surface 51: Optical waveguide 51A: Input optical waveguide 51B: 3-dB coupler for input 51B ′: Branch point of branch optical waveguide 51C, 51D: interaction optical waveguide 51E: output 3-dB coupler 51F: signal light output optical waveguide 51G: monitor light output optical waveguide 51H: input optical waveguide 51I: output optical waveguide 52: traveling-wave electrode 52A: central conductor 52B: ground conductor 52C: pad 53: reflecting groove 55: block member 58: the light output end face 61: SiO ₂ buffer layer 2: Si conductive layer 63: Reflection mirror 70: Input optical waveguides 71a, 71b, 71c, 71d: Interaction optical waveguides 72a, 72b: Center conductors 73a, 73b, 73c: Ground conductors 74a, 74b: Combined points 75a, 75b : Reflection groove 76a, 76b: Monitor light 77a, 77b: Monitor photo detector 78: Optical waveguide for output

Claims (7)

Have a electro-optical effect, the substrate (1) having opposed front surface of the back side, the front side of the substrate, an optical waveguide for guiding light (2) and before Symbol A central conductor (4) for applying a voltage for modulating light and a ground conductor (5a, 5b), and the optical waveguide includes at least an input optical waveguide (2a) for incident light; A branched optical waveguide (2b) for branching light incident on the input optical waveguide, and an interactive optical waveguide for modulating the phase of the light by applying the voltage between the central conductor and the ground conductor ( 2c-1, 2c-2), a combined optical waveguide (2d) for combining the light propagating through the interaction optical waveguide, and the combining point (2h) of the combined optical waveguide. Output optical waveguide (2f) connected to the wave optical waveguide, and phase modulated The higher order mode generated by combining the light in the combined optical waveguide is emitted as radiated light (30a, 30b) from the combined point into the substrate without substantially propagating through the output optical waveguide. An optical modulator;
Detecting said emitted light emitted from said merging point in the substrate of the optical modulator, the monitor photodetector (11) and the monitor photodetector with light modulators comprising at a position other than the back surface of the substrate ,
A reflection groove (20) that converts the radiation light into monitor light (30c) that propagates toward the monitor photodetector by reflecting the radiation light (30b) is provided, and the reflection groove is formed with respect to the output optical waveguide. The monitor light is formed on the front surface of the substrate opposite to the side where the monitor photodetector is installed, and the monitor light propagates crossing the output optical waveguide below the output optical waveguide. An optical modulator with a monitor photodetector. 2. The optical modulator with a monitor photodetector according to claim 1, wherein the multiplexing point is formed in a Y-branch optical waveguide . 3. The optical modulator with a monitor photodetector according to claim 1, wherein the optical waveguide is a Mach-Zehnder type optical waveguide . 4. The optical modulation with a monitor photo detector according to claim 1, wherein the monitor photo detector is provided in at least one of a side surface direction in a longitudinal direction of the substrate or a rear end direction of the substrate. vessel. 4. The optical modulator with a monitor photodetector according to claim 1, wherein the monitor photodetector is provided with a space with respect to the substrate . 6. The optical modulator with a monitor photodetector according to claim 1, wherein the optical waveguide includes two or more Mach-Zehnder type optical waveguides configured in parallel . 7. The optical modulator with a monitor photodetector according to claim 1, wherein the substrate is made of a semiconductor .

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