JPH0327883B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention applies a modulation voltage together with a DC bias voltage to an electro-optic medium member to apply phase modulation to a transmitted light beam, and further uses a polarizer and an analyzer together to perform intensity modulation. Regarding optical modulators, in particular, it is possible to obtain a broadband and high-sensitivity optical modulator that can be easily miniaturized and formed into a thin film waveguide. (Prior Art) In conventional optical modulators using this type of electro-optic medium, in order to perform temperature compensation of optical bias, or to convert phase modulation to intensity modulation, Optical modulators configured exclusively as a differential type have been used for various purposes, such as to perform both of the above. Therefore, in this type of differential optical modulator, it is desirable that the two optical paths be modulated in opposite phases, including the case where optical beams of two polarized waves orthogonal to each other are passed through the same optical path. . Therefore, in a conventional optical modulator using a Pockels crystal as an electro-optic medium, modulation voltages with opposite phases are applied to the front and rear halves of a single Pockels crystal, or the crystal axes are reversed. Previously, two Pockels crystals were placed in cascade one behind the other, and in-phase modulation voltages were applied, resulting in equivalently anti-phase modulation, but two modulated electric fields with mutually opposite phases were prepared over a wide frequency range. It is not easy to do this, and it is also difficult to reverse the crystal axis when a thin film waveguide type Pockels crystal is used, and optical transmission loss at discontinuities is also a problem. Among the various problems with conventional optical modulators of this type as described above, the factors that limit the modulation frequency band and therefore the modulation speed include:
The main causes are electrical circuit factors and factors due to differences in propagation speeds between electrical signals and light, that is, so-called speed mismatching. Of these, the former can be alleviated considerably by configuring the electric circuit as a distributed constant traveling wave type, but the latter can be alleviated considerably by configuring the electric circuit as a distributed constant traveling wave type. Due to the inherent dispersion, the problem cannot be solved except when very narrow band modulation is applied or when the optical modulating medium member is configured with short dimensions to achieve extremely low modulation sensitivity. It was considered possible. (Problems to be Solved by the Invention) Here, the basic configuration of a conventional optical modulator using this type of electro-optic medium will be shown to clarify the conventional problems described above. The basic configuration of a conventional lumped constant optical modulator is shown, and the first
FIG. 1 shows the basic configuration of a conventional distributed constant traveling wave optical modulator. In the basic configuration of the lumped constant type shown in FIG. 10, the optical modulator itself is a capacitor in terms of an electrical circuit. That is, in the basic configuration shown in FIG. 10, a modulating electrode plate 4 and a common electrode plate 5 are attached to the opposing outer circumferential surfaces of a rectangular parallelepiped-shaped electro-optic medium block 1, and the electrode plate 4 is connected by a modulating power source 6 having an internal resistance of R ohms. .
When obtaining , the capacitance C farad between the electrode plates 4 and 5 constitutes an RC circuit with the internal resistance R ohm.
Therefore, when the modulation frequency becomes significantly higher than the CR time constant, the modulation voltage between the electrode plates 4 and 5 forming the capacitance C becomes low, and no modulation voltage is applied to the modulation element. In this way, the frequency f ₀ at which the modulated wave decreases to 1/√2 when DC voltage is applied, which is also the modulation frequency band, becomes 1/(2πRC). For example, if R = 50Ω and C = 20pF, _f0 =159MHz
becomes. On the other hand, in the basic configuration shown in FIG. 11, a modulation power source 6 is supplied via a coaxial line 17a to one end of a modulation electrode 4 and a common electrode plate 5, which are attached to the opposing outer peripheral surfaces of a long rectangular parallelepiped electro-optic medium block 1. A voltage is supplied thereto, and a load 7 is connected to the other end via a coaxial line 17b.When viewed as an electric circuit, the optical modulator forms a transmission line or a distributed constant line. For the sake of simplicity, we assume that impedance matching is achieved on both the power supply side and the load side of the optical modulator, no reflection of the modulated signal occurs, and the best modulation characteristics are obtained. The transmission speed of the modulation electric signal in the frequency domain of the electric signal is determined by the relative permittivity ε _re and relative magnetic permeability μ _re of the electro-optic medium in the frequency domain of the electric signal, and the shapes of the electrode plate and the electro-optic medium. On the other hand, when the electro-optic medium is configured as an optical waveguide, the speed of the light beam passing through the optical modulator changes somewhat depending on the shape of the optical waveguide, but mainly due to the refraction of the electro-optic medium. It depends on the rate n. Therefore, the speed of the electrical signal V _e in the optical modulator is on the order of C/√ _re with respect to the speed of light c, and the speed of light V _pp in the optical modulator is C/√ _rp = C/ It is of the order of n. However, in many electro-optic media,
Due to dispersion, the electric relative permittivity ε _re and the optical relative permittivity ε _rp are different, so the speed of the electric signal V _e and the speed of light V _pp in the optical modulator are significantly different. There is a difference. Now, if the length of the electro-optic medium block 1 in the optical modulator is l, the time required for the incident light beam to finish passing through the optical modulator is l/V _pp , and the time required for the electric signal to pass through it is l/V _e , and when the difference in transit time between the two is equal to half the period of the electrical signal, the modulating electrical signals at the input and output ends of the optical modulator are in opposite phase to each other, so that the transmitted light beam The modulation effect exerted by the device is significantly reduced. The modulation frequency f _c at which the modulated signals have opposite phases between the input and output terminals of the optical modulator is as follows.
Equation (1) is obtained. f _c = (2l | 1/V _e -1/V _pp |) ^-1 (1) Also, if the difference in transit time between the electrical signal and the optical beam in the optical modulator is equal to one period of the modulating electrical signal, then While passing through the optical modulator, the light beam is completely modulated by exactly one period of change in the modulating electrical signal, and the modulation effects completely cancel each other out, resulting in no modulation at all. If we strictly analyze the above, the frequency
The change Δθ in the induced phase amount that occurs when a sinusoidal modulated voltage of f _c and amplitude E _n is applied is given by the following equation (2). Δθ=(2π/λ)(∂n/∂E)E _n l [sin {πf _c l(1/V _e -1/V _pp )} /{πf _e l(1/V _e -1/V _pp )} ×exp−j(πf _c l (1/V _e −1/V _pp )} (2) Here, λ is the wavelength of light, and (∂n/∂E) is the refractive index with unit electric field applied. An example of the frequency characteristic of this induced phase amount change is shown in Fig. 12, and an example of the change with respect to the electro-optic medium block length l is shown in Fig. 13.The modulation frequency bandwidth is
It is approximately determined by the frequency f _c in the above equation (1), and to be more precise, the −3 dB bandwidth is 0.89 f _c , and furthermore, it is inversely proportional to the electro-optic medium block length l. On the other hand, the modulation sensitivity is directly proportional to the electro-optic medium block length l.
Therefore, modulation bandwidth and modulation sensitivity have opposite tendencies with respect to the electro-optic medium length and are incompatible, so depending on the purpose or application,
We have to settle for just being okay. To address the problems of conventional optical modulators of this type, several solutions have been proposed as follows. One such conventional proposal is an inverted arrangement of equally divided crystals, and an example of its configuration is shown in FIG. In the illustrated configuration, modulation electrodes 4 are arranged on the upper and lower or left and right outer peripheral surfaces of a plurality of electro-optic crystals, four in the illustrated example, which are successively arranged with their crystal axes alternately reversed, for example, Pockels crystals 1B _a to 1B _d . and common electrode 5
A modulated voltage is applied from a modulated power source 6. However, as mentioned above, since the transmission speed of light and the modulating electric signal in the electro-optic medium are different, when light propagates in the electro-optic crystal, the depth of modulation initially increases as the light progresses. However, since the phase of the electrical signal becomes inverted with respect to the initial phase, the depth of modulation once reaches its peak and then begins to decrease. Therefore, in Equation (2) above, the value in { } is π/2, and if the modulation depth is switched to the next electro-optic crystal whose crystal axis is reversed at the position relative to the peak and propagated, the light beam Since the modulation of the electrical signal is initially in-phase, the depth of modulation can be further increased. If this process is repeated in multiple stages, even in a state of velocity mismatching where the propagation velocities of the electrical signal and light do not match, a high degree of modulation can be achieved by increasing the electro-optic medium block length. It can also be obtained in terms of frequency. In addition, in this equally divided crystal inversion arrangement, the crystal length at which the crystal axis should be inverted is determined by the modulation frequency, and conversely, when the crystal length is determined, the optimum modulation frequency is also determined. As the number of crystals increases, the modulation frequency band inevitably narrows, and as a result, it becomes suitable for high efficiency, medium to narrow band modulators at high modulation frequencies, and is suitable for DC modulation. If so, it will not work. However, a particular problem with this equally divided crystal inversion arrangement is the connections between the crystals, and it is difficult to provide a complete anti-reflection coating for the light beam, so the reflection of the light beam at the connections between the crystals is difficult. Decrease in light transmittance has become a major problem. Furthermore, when constructing a thin film optical waveguide to reduce modulation drive power, crystal axis reversal itself is technically extremely difficult. Another conventional proposal for solving the problems of this type of optical modulator described above is the inversion arrangement of unequal crystals. Although the above-mentioned equally divided crystal inversion arrangement has high efficiency, there is a problem in that the modulation becomes narrow band and it does not work for DC modulation. On the other hand, the optimal modulation frequency is similarly determined by the crystal length,
If the crystal axes of electro-optic crystals with different crystal lengths are alternately reversed and arranged in cascade to construct an optical modulator similar to the one described above, the modulation efficiency will decrease somewhat, but a relatively wide band will be achieved and direct current modulation will also be possible. . An example of the configuration of an optical modulator using such an inverted arrangement of unequal-length split crystals is shown in FIG.
It has a 5-stage configuration, whereas it has a 5-stage configuration.
FIG. 16 shows an example of the frequency characteristics of the modulation depth when the ratio of the crystal lengths of each stage is optimally set, in comparison with the case of a single stage. It can be seen from the illustrated characteristic example that a broadband optical modulator with extremely good frequency characteristics can be realized by inverting the crystal axes and making the cascaded divided crystals have unequal lengths. Incidentally, such an inverted arrangement of unequal-length split crystals was previously proposed by the present inventor and presented at the 1975 national conference of the Institute of Electronics and Communication Engineers. However, even with this unequal-divided crystal inversion arrangement that allows broadband modulation, the loss due to light reflection at the connection plane between the electro-optic crystals and the difficulty of reversing the crystal axis when an optical waveguide configuration is used are the same as described above. This is just as big a problem as in the case of inverted placement. In addition, the optical modulators configured with the inverted arrangement of divided crystals of equal length and unequal length shown in FIGS. 14 and 15, respectively, to apply phase modulation to the light beam, are configured in an even number of stages. A half-wave plate 18 is inserted in the center of the cascaded electro-optic crystal array, and the front and rear crystal inversion arrangements are symmetrical to form a differential type, temperature compensation is performed, and the planes of polarization are perpendicular or parallel to each other. By arranging the polarizer 12 and analyzer 13 in front and behind each other, the 14th
FIGS. 17 and 18 show examples of the configuration of an optical modulator in which phase modulation applied to a light beam is converted into intensity modulation using the configurations shown in FIGS. In addition,
The half-wave plate 18 inserted in the center of the crystal inversion arrangement is
The plane of polarization of the passing light beam is rotated by 90 degrees.
This is for exchanging the two abnormal polarized waves to operate the front and rear crystal arrangements in a differential manner. However, even in such an equally divided and unequally divided crystal inversion arrangement for intensity modulation, there are still losses due to light reflection on the crystal connection surface and difficulty in inverting the crystal axis when using an optical waveguide type. This remains a major problem with this type of optical modulator. An object of the present invention is to solve the above-mentioned conventional problems, to easily form a compact and thin-film optical waveguide using an electro-optic medium such as a Kerr effect medium, and to achieve highly sensitive optical modulation in a high frequency range. It is an object of the present invention to provide a wideband sensitive optical modulator that can perform highly sensitive phase modulation and intensity modulation that exhibits flat frequency characteristics over a wide frequency band. (Means for Solving the Problems) The broadband high-sensitivity optical modulator of the present invention is an optical modulator of this type using an electro-optic medium, in particular, a wide-band and high-sensitivity optical modulator using an electro-optic crystal inverted arrangement of equal and unequal parts. The electro-optic medium member itself is integrated to solve the problems of loss due to light reflection at the crystal connection surface in a sensitive optical modulator and difficulty in reversing the crystal axis when an optical waveguide type is used. By reversing the polarity of the DC bias voltage applied to produce the electro-optic effect in accordance with the conventional crystal axis reversal, it is possible to create optical modulators with conventional equal and unequal electro-optic crystal inversion arrangements. It is possible to perform exactly the same optical modulation. That is, in the broadband high-sensitivity optical modulator of the present invention, a modulation voltage is applied together with a DC bias voltage in a direction perpendicular to the longitudinal direction of a rectangular parallelepiped electro-optic medium member to phase modulate a light beam transmitted in the longitudinal direction. In an optical modulator capable of converting phase modulation into intensity modulation by arranging polarizers and analyzers whose polarization directions are orthogonal to each other in the front and rear of the longitudinal direction, one outer peripheral surface in the longitudinal direction of the rectangular parallelepiped electro-optic medium member. It is characterized in that an electrode array consisting of a plurality of blocks extending in the longitudinal direction is applied between the electrode array and the common electrode, the polarity of which is alternately reversed for at least some of the blocks. It is something. (Function) Therefore, according to the present invention, it is possible to obtain a wide-band, high-sensitivity optical modulator that can be easily miniaturized and formed into a thin-film optical waveguide, particularly a high-speed wide-band optical modulator or a high-frequency, high-sensitivity optical modulator. . (Example) The present invention will be described in detail below with reference to the drawings. First, FIGS. 1 and 2 show examples of the basic configuration of the optical phase modulator of the present invention when the electro-optic medium is of a bulk type in which a light beam is incident on the entire end face of the medium member without providing any particular optical waveguide. are shown respectively. The configuration examples shown in FIGS. 1 and 2 correspond to the conventional equal-divided and unequal-divided crystal inversion arrangements shown in FIGS. 14 and 15, respectively. It has an improved form. In other words, in any of the configuration examples, the electro-optic medium member is constructed by using the Kerr effect medium member 1A whose refractive index changes in proportion to the square of the applied voltage to obtain high modulation sensitivity. It is not divided as in an optical modulator, but remains in a rectangular parallelepiped-like integral structure, and for example, among the upper electrodes attached to the upper surface facing the lower common electrode 5 attached to the lower surface shown in the figure, the DC bias voltage is applied. A bias electrode array 2 in the longitudinal direction is provided in which the bias electrodes are divided into equal or unequal lengths, and an integrated modulation electrode array 4 is provided parallel to the bias electrode array 2. To the bias electrodes 2a to 2e divided into a plurality of blocks, DC bias voltages whose polarities are alternately reversed corresponding to the conventional crystal axis inversion arrangement are applied between the bias electrodes 2a to 2e and the common electrode 5 by respective bias power supplies 3a to 3e. The modulation signal voltage from the modulation power supply 6 is connected to the common electrode 5 in the modulation electrode array 4.
, to perform a modulation operation that is substantially the same as that of a conventional crystal inversion arrangement. The configuration shown in the first diagram is an example of dividing into four equal lengths, and the configuration shown in the second diagram is an example of dividing into five unequal lengths. To explain the functions and effects of the optical modulator of the present invention with the above-mentioned basic configuration, a DC bias electric field and a modulation electric field are applied in a superimposed manner to the optical path in the Kerr effect medium member through which light propagates. Let E _bo be the DC bias electric field due to the n-th stage electrode block,
When the modulated electric field is e _n , the composite electric field E _o under the n-th electrode is expressed by the following equation (3). E _o =E _bo +e _n (3) In the Kerr effect medium to which such a composite electric field is applied, the change in refractive index due to the electric field, and therefore the change in the optical phase, is proportional to the square of the electric field, so there is a constant of proportionality. Assuming that (∂n/∂E ² ), the refractive index change Δn _o in the optical path under the n-th electrode is expressed as follows (4)
The formula becomes Δn _o = (∂n/∂E ² ) (E _bo +e _n ) ² = (∂n/∂E ² ) (E _bo ² +e _n ² +2E _bo e _n ) (4) Therefore, the DC bias electric field is modulated. It is sufficiently large compared to the electric field, E _bo ² > e _n ² , and (2π/λ)(∂n/∂E ² )e _n ² l
Under the condition of <<1, the above equation (4) can be approximated as the following equation (4'). Δn _o = (∂n/∂E ² ) (E _bo ² + 2E _bo e _n ) (4′) In other words, the refractive index change Δn _o is the product of the bias electric field squared E _bo ² and the bias electric field and the modulation electric field. times
It will be proportional to the sum of 2E _bo e _n . Therefore, among the changes in the refractive index, those caused by the modulation signal are proportional to the modulation signal, and linear modulation can be performed. On the other hand, application of a bias electric field provides the following effects. (a) Modulation sensitivity can be increased by increasing the bias electric field. (b) When the polarity of the bias electric field is reversed, the relative phase of the modulation signal and the refractive index change is reversed by 180°. Therefore, the effect of item (b) is equivalent to the reversal of the polarity of the modulated electric field or the reversal of the crystal axis in a Pockels crystal whose refractive index changes in proportion to the applied voltage. This means that it can be obtained by simply reversing the polarity of the DC bias voltage without cutting the medium member and leaving it in a single structure. Therefore, the configuration example of the optical modulator of the present invention shown in the first diagram operates in the same manner as the conventional configuration shown in FIG. 14, and the configuration example shown in the second diagram operates in the same manner as the conventional configuration shown in FIG. . Note that the induced phase change amount Δθ per stage of equally divided bias electrode of the optical modulator of the present invention configured as described above using a Kerr effect medium is different from the conventional equation (2), and is expressed by the following equation (5). It becomes like the formula. Δθ=(2π/λ)(∂n/∂E ² )l [E _bo ² +2E _bo e _n (sin {πf _n l・(1/V _e −1/V _pp )} /{πf _n l(1 /V _e −1/V _pp )} ×exp−j{πf _n l1/ V _e −1/V _pp )}] (5) Next, another example of the configuration of the optical modulator of the present invention is shown in FIG. show. In the illustrated configuration example, for example, a lower common electrode 5 attached to the lower surface of the Kerr effect medium member 1A
A single row of upper electrodes deposited on the upper surface facing the electrodes is divided into a plurality of blocks 10a to 10d of equal length, and the adjacent ends of the successive electrode blocks are overlapped with each other via a dielectric material, and In this step, DC bias voltages of alternate polarity are applied from bias circuits 3a to 3d to the lower common electrode 5 to successive electrode blocks 10a to 10d which are insulated from each other. On the other hand, for modulating alternating current electric signals applied between the modulating power source 6 and the lower common electrode 5, each electrode block 10a to 10d is connected by capacitive couplings 11a to 11d formed between the end portions, and is integrally constructed. It forms a modulation electrode array. Therefore, the configuration example shown in the third diagram operates substantially in the same manner as the configuration example shown in the first diagram, and can obtain completely the same effects. Note that the upper electrode example in the configuration example shown in the third figure can be divided into unequal lengths and operated in exactly the same manner as in the configuration example shown in the second figure to obtain exactly the same effect. Next, a conventional configuration in which the conventional optical phase modulator shown in FIGS. 14 and 15 is combined with a polarizer and an analyzer to convert it into an optical intensity modulator is shown in FIGS. 17 and 18. In exactly the same way, a polarizer 12 and an analyzer 13 whose polarization planes are perpendicular or parallel to each other are arranged in front and behind each of the configuration examples of the optical modulator of the present invention shown in FIGS. 1 to 3, respectively. Examples of configurations in which the optical phase modulation is converted into the optical intensity modulation by adding the above are shown in FIGS. 4 to 6, respectively. The planes of polarization of the polarizer 12 and the analyzer 13 are arranged at an angle of 45 degrees with respect to the direction of voltage application to the Kerr effect medium. To explain the effects of the optical modulator of the present invention having the configurations shown in FIGS. 4 to 6, in which light intensity modulation is applied to the transmitted light beam as described above, the applied electric field in the Kerr effect medium member 1A The manner in which the refractive index changes due to the change in is exactly the same as in the configuration example of the optical phase modulator shown in FIGS. 1 to 3. In other words, when an electric field is applied, anisotropy occurs in the Kerr effect medium, and the refractive index changes that act on light that is linearly polarized in the direction of the electric field and light that is linearly polarized orthogonal to the direction of the electric field. The aspects of this will be different. Therefore, by making linearly polarized light tilted at 45 degrees with respect to the electric field direction enter the Kerr effect medium through a polarizer, the polarized wave component in the electric field direction and the polarized wave component perpendicular to the electric field direction are separated. If the two polarized light components are made incident, a phase difference will occur between the two polarized light components emitted from the Kerr effect medium, and as a result, the composite polarized light obtained by combining those two polarized light components will have two polarized light components. A change occurs in accordance with the phase difference generated between the wave components, and the change in the synthesized polarized light is converted into a change in light intensity by an analyzer, thereby forming a light intensity modulator. The conversion from an optical phase modulator to an optical intensity modulator is, as mentioned above, commonly performed in conventional optical modulators of this type. As is clear from the above, the structure of the modulator itself is significantly different between the present invention and the conventional one. Due to the difference in construction, the optical phase modulator and therefore the optical intensity modulator according to the present invention is free from losses due to reflections between blocks of conventional electro-optic media.
As a result, it has become possible to realize high-speed, wide-band, high-sensitivity optical modulation with high efficiency, which is faster than conventional methods. Next, FIG. 7a shows an example of the structure of the optical waveguide type optical modulator of the present invention. The illustrated configuration example is
The optical modulator of the present invention having the configuration shown in FIG. 3 is made into an optical waveguide type, and the Kerr effect medium member 1A
An optical waveguide 14 is formed in the vicinity of the upper surface by increasing the refractive index by, for example, diffusing metal ions into the optical medium and using it as a core region with respect to the surrounding cladding region. The modulation electrode 4 is applied in parallel along one side, and the optical waveguide 14 is divided into equal lengths along the other side as in the configuration shown in the third figure, and capacitive coupling 1 is formed between them.
A plurality of blocks of electrode array 10 provided with electrodes 1a to 11c.
a to 10d, and further the electrode array 10a
A ground electrode 16 is attached to the outside surrounding the area 10d, and bias electrodes 3a to 3d apply a DC bias voltage whose polarity is reversed for each block to the electrode array 10a.
10d and the ground electrode 16, and at the same time, a modulation AC electric signal is applied between the modulation electrode 4 and the ground electrode 16 by the modulation power source 6, and the electrode rows 10a to 10d connected to the ground electrode 16 are and modulation electrode 4, phase modulation is applied to the transmitted light beam within the optical waveguide 14 with the optical waveguide 14 in between, as shown in the equivalent circuit shown in FIG. 7b. The configuration example of the optical modulator of the present invention, which is made into an optical waveguide type and can be easily integrated with such a configuration, operates in the same manner as the bulk type configuration example shown in FIG. 3, and as a result, similar operations and effects can be obtained. Note that in the configuration examples using the capacitively coupled electrode rows shown in the third and fourth drawings, examples are shown in which the electrode examples 10a to 10d are divided into even numbered blocks of equal length. Of course, it is possible to similarly widen the bandwidth by dividing it into odd-numbered blocks of unequal length, as in the configuration example. In addition, in the configuration example of the optical modulator of the present invention described above, when the electrode row is divided into blocks of unequal length, regardless of phase modulation or intensity modulation, the arrangement of the block electrodes is configured symmetrically. In particular, in order to obtain broadband flat characteristics, it is preferable to set the ratio of the block lengths of each stage to an appropriate specific value. It is preferable to set the relative expansion of the modulation frequency bandwidth approximately as shown in Table 1 below when the sensitivity of modulation by application is constant.

[Table] Furthermore, in order to convert an optical waveguide type optical phase modulator into an optical intensity modulator, in the optical modulator of the present invention, the optical phase of each configuration shown in FIGS. In the same way that a polarizer and an analyzer are placed before and after the modulator to convert it into an optical intensity modulator having the configuration shown in FIGS. 4 to 6, an optical waveguide type phase modulator having the configuration shown in FIG. 7 can be obtained. In addition to placing a polarizer and an analyzer before and after the converter to convert it into an optical waveguide type optical intensity converter, we also converted it from the beginning according to the conventional Matsuhatsu Enda interferometer type optical waveguide type optical intensity converter shown in Figure 8. The optical modulator of the present invention can also be configured to act as a light intensity modulator. In the conventional optical waveguide type optical modulator of the Matsuhatsu Ender interferometer type shown in FIG. Phase modulation of mutually opposite phases is applied to each branched optical waveguide by the rows 4a to 4c, and then these two branched optical waveguides are combined by a Y-junction 15b, and the intensity modulated light obtained as a result of the phase combination is output from the output optical waveguide 14b. Conventionally, two modulation power supplies 6a and 6b, which operate in opposite phases to each other, have been required to perform phase modulation of opposite phases. On the other hand, in the optical modulator of the present invention, the functions and effects clarified from equation (4') expressing the function of the optical modulator of the present invention with the basic configuration shown in Figure 1 are described in section (b). As shown above, polarity reversal of the bias electric field changes the relative phase between the modulation signal and the refractive index change.
By skillfully utilizing the point of 180° inversion, as shown in FIG. 9, we have created a Matsuhatsu Enda interferometer type optical waveguide type light that operates in the same way as the conventional configuration shown in FIG. 8 using only a single modulating power source 6. An intensity modulator can be configured. That is, in the configuration example shown in FIG. 9, a single block of modulation electrode array 4 is applied between the two branched optical waveguides formed in the same manner as in the conventional configuration shown in FIG. Bias electrode rows 2a and 2b each consisting of a single block are attached to both sides, and ground electrodes 1 are further connected to each other surrounding the outside of these electrode rows.
A single modulation power supply 6 applies a modulation signal voltage between the modulation electrode row 4 and the ground electrodes 16a, 16b, and a bias power supply 3a, 3b applies the modulation signal voltage to the bias electrode row 2a, 2b.
DC bias voltages of opposite polarity are applied between the ground electrodes 16a and 16b, and the effect described in item (b) above realizes an optical waveguide type optical intensity modulator of the Matsuhatsu Enda interferometer type. (Effects of the Invention) As is clear from the above description, according to the present invention, a DC bias electric field can be applied to an electro-optic medium member exhibiting the Kerr effect in a manner superimposed on a modulated electric field, and the DC bias electric field An optical modulator that takes advantage of the fact that, by sequentially and alternately reversing the polarity of the bias electric field, the relative polarity of the change in the refractive index of the electro-optic medium due to the modulation electric field with respect to the modulation voltage can be sequentially and alternately reversed. By sequentially and alternately inverting the polarity at equal intervals or every length of a predetermined ratio, and by sequentially and alternately inverting the relative polarity of the refractive index change with respect to the modulating electric field, modulation due to speed mismatch between the modulation electric signal and the light can be achieved. A high-frequency, high-sensitivity optical modulator or a broadband flat characteristic optical modulator that eliminates the cancellation effect of modulation in an optical modulator that has a long optical path under an electric field, and enables highly sensitive optical modulation even at high modulation frequencies. can be realized. That is, according to the present invention, the Kerr effect medium member constituting the optical modulator can be integrally constructed without cutting the Pockels crystal for crystal axis inversion connection in the conventional optical modulator of this type. The same effect can be achieved simply by reversing the polarity of the applied bias electric field.As a result, the optical transmission loss associated with reflection at the conventional crystal connection plane is eliminated, and the structure is simplified and manufacturing is facilitated. In addition to facilitating the fabrication of thin-film optical waveguides, miniaturization, and integration, a particularly remarkable effect can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a perspective view showing an example of the basic configuration of the optical modulator of the present invention, FIG. 2 is a perspective view showing another example of the basic configuration, and FIG. 3 is still another example of the basic configuration. FIG. 4 is a perspective view showing a configuration example in which phase modulation/intensity modulation conversion means is applied to the basic configuration shown in FIG. 1, and FIG. FIG. 6 is a perspective view showing an example of a configuration in which a phase modulation/intensity modulation conversion means is applied to the basic configuration shown in FIG. 3, and FIGS. A perspective view and a circuit diagram respectively showing an example of an optical waveguide type configuration of a device and its equivalent circuit, FIG. 8 is a perspective view showing a conventional configuration of a Matsuhatsu Enda interferometer type optical intensity modulator, and FIG. 9 is an optical modulator of the present invention. FIG. 10 is a perspective view showing an example of the configuration of a Matsuhatsu Enda interferometer type, and FIG.
Figure 1 is a perspective view showing a schematic configuration of a traveling wave optical modulator, Figure 12 is a characteristic curve diagram showing an example of modulation frequency characteristics of an optical modulator using an electro-optic medium, and Figure 13 is a diagram showing the same optical modulation. A characteristic curve diagram showing an example of the relationship between the length of the electro-optic medium of the device and the modulation index, FIG. 14 is a perspective view showing an example of an equally divided crystal inversion arrangement of a conventional optical modulator, and FIG. 15 is a diagram of a conventional optical modulator. FIG. 16 is a perspective view showing an example of a modulator with an inverted arrangement of unevenly divided crystals. FIG. 16 is a characteristic curve diagram showing an example of the modulation sensitivity frequency characteristic due to the inverted arrangement of unevenly divided crystals. FIG. A perspective view showing another example of an inverted arrangement,
FIG. 18 is a perspective view showing another example of the unequal divided crystal inversion arrangement. 1, 1a to 1e...electro-optic medium member, 1A...
... Kerr effect medium member, 1B, 1B _a to 1B _f ... Pockels crystal member, 2a to 2e... Bias electrode, 3a to 3e... Bias power supply, 4, 4a to 4
c...Modulation electrode, 5...Lower electrode, 6, 6a, 6
b... Modulated power supply, 7... Load, 8... Input optical beam, 9... Output optical beam, 10a to 10d... Upper electrode, 11a to 11e... Capacitive coupling, 12...
Polarizer, 13...Analyzer, 14, 14a, 14b
...Optical waveguide, 15a, 15b...Y junction, 16, 16a, 16b...Earth electrode, 1
7a, 17b...coaxial cable, 18...half wave plate.

Claims (1)

[Scope of Claims] 1 A modulation voltage is applied together with a DC bias voltage in a direction perpendicular to the longitudinal direction of a rectangular parallelepiped electro-optic medium member to apply phase modulation to a light beam transmitted in the longitudinal direction. In an optical modulator capable of converting phase modulation into intensity modulation by respectively arranging a polarizer and an analyzer with polarization directions perpendicular to each other, the rectangular parallelepiped electro-optic medium member has a plurality of blocks on one outer peripheral surface in the longitudinal direction. An optical modulator, characterized in that a DC bias voltage whose polarity is alternately reversed for at least some of the blocks is applied between the common electrode and a common electrode. 2. A bias electrode array consisting of a plurality of blocks sequentially extending in the longitudinal direction and a modulation electrode array consisting of a single block extending in the longitudinal direction are arranged to face each other to form the electrode array consisting of the plurality of blocks, and the one outer circumferential surface A DC bias voltage whose polarity is alternately reversed for each block is applied to the bias electrode array between the common electrode and the common electrode attached to the other opposing outer peripheral surface, and a modulation voltage is applied to the modulation electrode array. An optical modulator according to claim 1, characterized in that: 3. The electrode array is made up of a plurality of blocks that extend sequentially in the longitudinal direction and are sequentially overlapped with each other in a non-contact manner so as to be capacitively coupled to each other. 2. The optical modulator according to claim 1, wherein a modulation voltage is applied between the optical modulator and the common electrode, and a DC bias voltage whose polarity is alternately reversed for each block is applied. 4. A longitudinal optical waveguide is provided in the vicinity of the one outer circumferential surface of the rectangular parallelepiped electro-optic medium member, and the optical waveguide is successively extended in the longitudinal direction with the optical waveguide sandwiched therebetween, and parts of the electro-optic medium member are sequentially overlapped in a non-contact manner. A bias electrode array consisting of a plurality of blocks capacitively coupled to each other and a modulation electrode array of a single block extending in the longitudinal direction are opposed to each other to constitute the electrode array consisting of the plurality of blocks, and the bias electrode array is surrounded by a plurality of blocks. and the common electrode attached to the one outer peripheral surface,
Applying a DC bias voltage whose polarity is alternately reversed for each block to the bias electrode array,
2. The optical modulator according to claim 1, wherein a modulation voltage is applied to the modulation electrode array. 5. The optical modulator according to any one of the preceding claims, wherein the plurality of blocks that extend sequentially in the longitudinal direction and constitute the electrode array have unequal lengths. 6 A two-branch optical waveguide that branches at both ends and extends in the longitudinal direction is provided near the one outer peripheral surface of the rectangular parallelepiped electro-optic medium member, and the one outer peripheral surface between and on both sides of the two-branch optical waveguide. A modulation electrode array and a bias electrode array each consisting of a single block extending in the longitudinal direction are respectively applied to the electrode array to form an electrode array consisting of the plurality of blocks, and the electrode array is surrounded by the electrode array and applied to the one outer circumferential surface. A DC bias voltage whose polarity is reversed for each block is applied to the bias electrode array between the common electrode and the common electrode, and a modulation voltage is applied to the modulation electrode array. The optical modulator according to item 1.