Patent ID: 12259631

DESCRIPTION OF EMBODIMENTS

Hereinafter, the present invention will be described in detail by using preferred examples.

As shown inFIGS.1to9, according to the present invention, in an optical control element including a substrate1having an electro-optic effect, an optical waveguide2formed on the substrate, and control electrodes (M, B1to B2, and the like) that control a light wave propagating through the optical waveguide, an input portion (input light L1) and an output portion (output light L2) of the optical waveguide are formed on the same side of the substrate, the optical waveguide includes at least one Mach-Zehnder type optical waveguide portion (MZ) that has two branched waveguides (21,22) that are branched from one optical waveguide and combines the two branched waveguides to form one optical waveguide, and the branched waveguides have an even number of turned-back potions (A1, A2).

As the substrate1having an electro-optic effect, a substrate made of a material such as lithium niobate (LN), lithium tantalate (LT), or lead lanthanum zirconate titanate (PLZT), or a vapor deposition film of these materials or a composite substrate or the like in which these materials are bonded to different substrates may be used.

Various materials such as semiconductor materials or organic materials may also be used for optical waveguides.

As a method of forming an optical waveguide, a rib-type optical waveguide is used in which a portion of a substrate corresponding to the optical waveguide is made to protrude, such as by etching a surface of the substrate other than the optical waveguide or by forming grooves on both sides of the optical waveguide. An optical waveguide may be formed by forming a high refractive index portion on the substrate surface by using Ti or the like according to a thermal diffusion method, a proton exchange method, or the like. A composite optical waveguide may be formed, for example, by diffusing a high refractive index material in a rib-type optical waveguide portion.

A thickness of the substrate1on which the optical waveguide is formed is set to 10 μm or less, more preferably 5 μm or less in order to achieve velocity matching between a microwave of a modulation signal and a light wave.

A ratio h/t of height h of the rib-type optical waveguide (from the bottom of the groove on both sides of the rib-type optical waveguide to the top side of the rib-type optical waveguide protruding portion) to a substrate thickness t of the rib-type optical waveguide portion (from the bottom surface of the substrate to the top side of the rib-type optical waveguide protruding portion) is set to 0.8 or less. For example, in a case where the substrate thickness t is 1 μm or less, h/t is set in the range of 0.6 to 0.8. A vapor deposition film may be formed on the reinforcing substrate1, and the film may be processed into a shape of the optical waveguide as described above.

The substrate on which the optical waveguide is formed is adhered and fixed to a reinforcing substrate through direct bonding or an adhesive layer of resin or the like in order to increase mechanical strength. As the reinforcing substrate to be directly bonded, a material having a refractive index lower than that of the optical waveguide or the substrate on which the optical waveguide is formed and a thermal expansion coefficient close to that of the optical waveguide, such as quartz, is preferably used. In a case where the reinforcing substrate is bonded via an intermediate layer having a low refractive index, the same material as the substrate on which the optical waveguide is formed, for example, an LN substrate may be used as the reinforcing substrate, or a high refractive index substrate such as a silicon substrate may be used as the reinforcing substrate.

A feature of the optical control element of the present invention is that, as shown inFIG.1, the optical waveguide2formed on the substrate1has at least one Mach-Zehnder type optical waveguide portion (MZ). A feature is to minimize an optical path difference between the branched waveguides (21,22) between a branch portion20and a Y-junction23that configure the Mach-Zehnder type optical waveguide portion (MZ).

In order to minimize an optical path difference between the branched waveguides (21,22), as shown inFIG.1, an even number of turned-back potions (A1, A2) related to the branched waveguides is formed. By making the shapes of the optical waveguides at the turned-back potions A1and A2the same, not only can the optical path lengths of the branched waveguides be set to be equal to each other, but also propagation losses of the optical waveguides can be set to be the same. As a result, a loss difference between the branched waveguides is reduced, and deterioration in the on/off extinction ratio of the Mach-Zehnder type optical waveguide can be suppressed.

FIGS.2and3clearly show that the same optical waveguide shape as that of the optical control element inFIG.1, and further show a modulation electrode M and bias electrodes (B1, B2) which are control electrodes. A common technical feature inFIGS.2and3is that the modulation electrode M and the bias electrodes (B1, B2) are provided in different sections for a plurality of sections of the branched waveguides (21,22) separated by the turned-back potions (A1, A2) shown inFIG.1.

InFIG.1, the sections may be classified as three sections such as a first section preceding the turned-back potion A1(a section from the branch portion20to the turned-back potion A1), a second section between the turned-back potions A1and A2(a section from the turned-back potion A1to the turned-back potion A2), and a third section after the turned-back potion A2(a section from the turned-back potion A2to the Y-junction23). InFIGS.2and3, the modulation electrode M is disposed in the first section, the bias electrode B1is disposed in the second section, and the bias electrode B2is disposed in the third section.

Since the modulation electrode M propagates a high-frequency signal through the modulation electrode M, it is preferable to reduce the bending of the electrode in order to reduce deterioration in the high-frequency signal. Therefore, the modulation electrode M is disposed to fit within one section. InFIG.2, a modulation signal S1is introduced from a direction (upper side of the substrate1) perpendicular to the extending direction (horizontal direction in the drawing) of an action portion (a portion that applies an electric field to the optical waveguide) of the modulation electrode M. Therefore, a bent portion is required to be provided in a part of a lead-in portion of the modulation electrode (a portion between the input portion of the modulation signal and the action portion). In order to suppress deterioration in a high-frequency signal due to this bent portion, as shown inFIGS.3A to3C, the modulation electrode M is formed in a linear shape from the input portion of the action portion and the modulation signal S1is introduced, and thus the bending of the lead-in portion of the modulation electrode can also be reduced to make it possible to further suppress the deterioration in a high-frequency signal.

A modulation signal S2is derived from a termination side of the modulation electrode, and the derived modulation signal S2is introduced to a terminator including a termination resistor and the like. Regarding bending of the electrode in the subsequent stage from the action portion of the modulation electrode, deterioration in a high-frequency signal such as a bending loss does not influence a frequency response of electro-optical modulation, and the design can be set with a high degree of freedom. In order to reduce the influence of signal leakage, reflection, or the like due to the bending, a terminator may be disposed on the substrate or a resistive film may be formed on the substrate.

As shown inFIGS.3A to3C, by not disposing the bias electrodes (B1, B2) in the propagation direction of the modulation signal S1in the modulation electrode M, it is possible to suppress a situation in which a leakage signal from the modulation electrode is coupled to the bias electrode, and thus an optical modulation signal becomes unstable due to the addition of high-frequency noise.

The bias electrodes (B1, B2) may be effectively disposed by using a section where the modulation electrode is not disposed. Although only one of B1or B2functions as the bias electrode, as shown inFIGS.2and3, by occupying a plurality of sections and forming a long bias electrode along the optical waveguide, a bias voltage can be lowered and this contributes to suppressing the DC drift phenomenon. Although the electro-optical efficiency is decreased, a light loss due to the bias electrode can be reduced by keeping the bias electrode away from the optical waveguide.

FIGS.2and3A to3Cshow an example in which the substrate1employs a substrate (for example, an X-cut LN substrate; hereinafter referred to as an X substrate) in which a signal electrode is disposed between optical waveguides. Of course, it goes without saying that the present invention can also be applied to an example using a substrate (for example, a Z-cut LN substrate; hereinafter referred to as a Z substrate) in which a signal electrode is disposed on an optical waveguide. The present invention can also be applied to materials other than LN, such as semiconductors, as long as the materials have the optical waveguide/electrode disposition relationship described above. InFIGS.2and3, a ground electrode is not illustrated for the sake of simplification of the drawings.

In a case where the bias electrodes are formed in two different sections, the bias electrodes are disposed, for example, as shown inFIG.2orFIG.3Asuch that signs of phase changes before and after the turned-back potion are the same.

FIGS.3B and3Cshow an example using a Z substrate, and in particular show a specific example of a disposition pattern of the bias electrodes (B1, B2). InFIG.3C, a region PR surrounded by a dotted line indicates a polarization reversal region.

In a case where an electrode is disposed on the optical waveguide, a two-electrode modulation configuration in which an electrode is disposed in each of two branched waveguides to achieve zero chirps in the modulation function of the Mach-Zehnder type optical waveguide portion or a configuration in which one modulation electrode is disposed to be switched between two branched waveguides by using polarization reversal may be employed.

FIG.4shows an example using a so-called nest type optical waveguide in which secondary sub-Mach-Zehnder type optical waveguides (MZ1, MZ2) are nested in each branched waveguide of a main Mach-Zehnder type optical waveguide. In such a configuration, a modulation electrode may be disposed in a plurality of sections (here, two sections) in order to reduce a drive voltage of the modulation electrode. In this case, a polarization reversal region is provided in a portion (one section) surrounded by the dotted line such that signs of phase changes of a light wave before and after the turned-back potion are the same.

As shown inFIG.4and subsequent figures, the optical control element of the present invention is also applicable to a case where one optical waveguide is branched into a plurality of optical waveguides, and each branched optical waveguide is provided with a Mach-Zehnder type optical waveguide portion (MZ1, MZ2). A number of times of branching of optical waveguide is not limited to one, and the optical waveguide may be branched over a plurality of tiers. The number of branched waveguides branched at one time is not limited to two, and may be three or more. The Mach-Zehnder type optical waveguide portions provided in the branched optical waveguides can be easily implemented in a state of being in parallel by arranging branched waveguides of the respective Mach-Zehnder type optical waveguide portions and providing an even number of turned-back potions for the branched waveguides.

It is preferable that a shape of the optical waveguide at each turned-back potion is also the same shape. Specifically, the radii of curvature of a plurality of branched waveguides disposed in parallel are set to R, R+r, R+2r, . . . , and R+nr (where R and r are constants, and n is a natural number) from the inside. In order to increase the radius of curvature R, a bending angle to be turned-back may be set to be more than 180 degrees (adjacent sections are parallel). (refer to the reference diagram) However, it goes without saying that it is necessary to set shapes of the different turned-back potions to be the same.

InFIG.4, two modulation electrodes (M1, M2) are disposed for sub-Mach-Zehnder type optical waveguides (MZ1, MZ2), and two modulation signals (S11, S12) are input. As for a bias electrode, a bias electrode BM is disposed for the main Mach-Zehnder type optical waveguide, and bias electrodes BS1and BS2are disposed for the sub-Mach-Zehnder type optical waveguide. Since a sufficient space can be secured for each of the bias electrodes (BM, BS1, BS2), it is possible to reduce a bias voltage.

Instead of separately providing a bias electrode such as the bias electrode BM disposed in the main Mach-Zehnder type optical waveguide and the bias electrodes (BS1, BS2) disposed in the sub-Mach-Zehnder type optical waveguide inFIG.4, a DC bias may be superimposed on a modulation signal applied to a modulation electrode so as to be applied.

FIG.5shows two nest type optical waveguides disposed in parallel. Four Mach-Zehnder type optical waveguides (MZ1to MZ4) are disposed in parallel, and modulation electrodes (M1to M4) and bias electrode (BS1to BS4) are disposed for the respective Mach-Zehnder type optical waveguides. The bias electrodes (BM1, BM2) are also provided to correspond to the main Mach-Zehnder type optical waveguide of each nest type optical waveguide.

FIG.5shows an optical control element having a polarization combining function. Input light L1is input to the optical waveguide in the substrate1through a lens30provided in an optical block3. The input light is split into two on the way and modulated by each nest type optical waveguide to output two pieces of modulation signal light. Two light waves output from the substrate1are input to an optical fiber F through lenses (31,32,36). In this case, one of the light waves is rotated on a polarization plane by a half-wave plate33, passes through reflection means34and polarization combining means35, and is combined with an other light wave to be polarization-combined into one output light.FIG.5schematically shows the case of combining light waves by spatial optics, but light waves may be polarization-combined by a waveguide type element.

InFIG.5, optical path lengths of the respective optical waveguides from branching of the optical waveguide on the input side into two to input into the respective nest type optical waveguides are different. Therefore, it is necessary to precisely adjust a timing at which the modulation signals (S11, S12, S13, S14) are applied to the optical waveguides at the action portions (M1, M2, M3, M4) of the modulation electrodes. In order to achieve this, the modulation signal is output after a phase difference of the modulation signal is adjusted by using a digital signal processor (DSP; not shown), the signal is amplified by a driver circuit (not shown) and applied to the optical control element as a modulation signal.

FIG.6shows an optical control element having the same polarization combining function as inFIG.5. InFIG.6, two nest type optical waveguides, which are disposed after the optical waveguide is branched into two, are disposed on the left and right sides of an input waveguide input to which input light is input. This disposition has the drawback that a distance between the output lenses30and32in the figure is larger than the example inFIG.5and thus alignment at the time of implementation of polarization combining is difficult, but the action portions (M1to M4) of the modulation electrode can be disposed apart from each other, so that crosstalk between modulation signals can be suppressed.

FIG.7shows a modification example of the example inFIG.6, in which a position of the input light L1and positions of output light (L21, L22) are disposed to be separated. Accordingly, the modulation electrodes (M1to M4) are disposed near the input waveguide.

In this configuration, the modulation electrodes are disposed closer to each other than in the configuration inFIG.6, and thus a transmission loss of the modulation signal can be reduced until the modulation signal is input to the modulation electrodes.

FIG.7does not show a polarization combining function, but the function may be provided. FromFIG.7and the subsequent figures, only positions of action portions of the modulation electrode and the bias electrode are shown, and a lead-in portion of each electrode is not shown.

FIG.8shows a modification example of the example inFIG.7, in which an input position of the input light L1is disposed on the upper part of the substrate1, nest type optical waveguides are stacked, and one piece of output light L21is disposed near the central part of the substrate1, and another piece of output light L22is disposed on the lower part of the substrate1.

FIG.9differs from the examples up toFIG.8in that the action portions of the modulation electrode and the bias electrode are disposed in a direction perpendicular to an input direction of a light wave (horizontal direction in the figure). As described above, the optical control element of the present invention has a high degree of freedom in designing an optical waveguide, and can employ various forms.

As shown inFIG.10, by accommodating the optical control element1of the present invention in a case4made of metal or the like and connecting the optical control element1to the outside of the case via the optical fiber F, a compact optical modulation device MD can be provided. Of course, the optical fiber may be directly connected to the input portion or the output portion of the optical waveguide of the substrate1, or may be optically connected to the input portion or the output portion via a space optical system.

An optical transmission apparatus OTA can be configured by connecting an electronic circuit (digital signal processor DSP) that output a modulation signal for causing the optical modulation device MD to perform a modulation operation, to the optical modulation device MD. A driver circuit DRV is used because the modulation signal applied to the optical control element is required to be amplified. The driver circuit DRV and the digital signal processor DSP may be disposed outside the case4, or may be disposed inside the case4. In particular, by disposing the driver circuit DRV inside the case, it is possible to further reduce a propagation loss of the modulation signal from the driver circuit.

INDUSTRIAL APPLICABILITY

As described above, according to the present invention, it is possible to provide an optical control element that can minimize an optical path difference between branched waveguides while reducing a difference in structure between the branched waveguides by disposing an input portion and an output portion of an optical waveguide on the same side of a substrate on which the optical waveguide is formed. It is possible to provide an optical modulation device and an optical transmission apparatus using this optical control element.

REFERENCE SIGNS LIST

1: Substrate2: Optical waveguide21,22: Branched waveguideA1, A2: turned-back potionB1, B2: Bias electrode (action portion)M: Modulation electrode (action portion)MD: Optical modulation deviceMZ: Mach-Zehnder type optical waveguideOTA: Optical transmission apparatus