JP2004287029A - Optical modulator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical modulator which is given a prescribed phase change regardless of input light wavelength and is constant in diffraction angle even if a frequency shift quantity is changed. <P>SOLUTION: A diffraction grating by ruled lines 11a is constituted on the circumference of a disk 11 and the disk 11 is rotated by a motor 12. Input light is made incident on the circumference provided with the diffraction grating to diffract the incident light. The diffracted light output of the +1st order of the optical modulator fetches the light in the direction of an arrow C1. As the disk 11 moves, the phase of the diffracted light output of the +1st order can be changed in proportion to moving quantity/grating interval regardless of the input wavelength. Also, the diffraction angle depends on the wavelength of the input light and the grating interval, and therefore, if the wavelength of the input light and the grating interval are uniquely determined, the direction of the diffracted light is made constant. When the motor 12 is rotated at a uniform angular velocity in this state, the frequency shift proportional to the moving quantity/grating interval can be generated in the frequency of the diffracted light output of the +1st order. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an optical modulator that performs phase modulation and frequency modulation on a light wave.
[0002]
[Prior art]
An optical phase modulator utilizing the electro-optic effect of an electro-optic element (ferroelectric crystal) is known (see Non-Patent Documents 1 and 2). This phase light modulator changes the refractive index of the electro-optical element by an externally applied electric field, thereby changing the phase of light transmitted through the crystal. On the other hand, an optical frequency modulator using an acousto-optic effect of an acoustic medium is also known (see Non-Patent Document 1). The optical frequency modulator diffracts light transmitted through the acoustic medium by changing the refractive index of the acoustic medium by ultrasonic waves excited in the medium.
[0003]
[Non-patent document 1]
Gunta Aida, 2 other authors, "Introduction to Optical Instruments", 1st edition, Tokai University Press, September 1983, pp. 135-141
[Non-patent document 2]
Takashi Ogoshi, "Optical Fiber Sensor", 1st edition, Ohmsha, July 1986, p109-114
[0004]
[Problems to be solved by the invention]
In an optical phase modulator using an electro-optical element, it is necessary to adjust the magnitude of an externally applied electric field according to the input light wavelength in order to obtain a predetermined phase change.
[0005]
In an optical frequency modulator using an acoustic medium, when the frequency of the ultrasonic wave applied to the medium is changed to change the frequency shift amount, the traveling direction (diffraction angle) of the diffracted light also changes.
[0006]
The present invention provides an optical modulator in which a predetermined phase change is given irrespective of an input light wavelength and a diffraction angle is constant even when a frequency shift amount (modulation frequency) is changed.
[0007]
[Means for Solving the Problems]
In the optical modulator according to the first aspect of the present invention, a diffracted light generator for diffracting incident light to generate diffracted light, and a diffracted light generator of the diffracted light generator for the incident light are moved. Diffraction light generator moving means, and diffracted light generation by the diffraction light generator movement means such that the m-th order diffracted light generated by the diffracted light generator with respect to the incident light has a desired modulation frequency shift amount or modulation phase amount. Control means for controlling the movement of the unit.
According to a second aspect of the present invention, in the optical modulator according to the first aspect, the control unit controls the diffracted light generation unit moving unit to change a moving speed of the diffracted light generation unit according to the modulation frequency shift amount. It is characterized by doing.
According to a third aspect of the present invention, in the optical modulator according to the first aspect, the control unit controls the diffracted light generating unit moving unit to change a moving position of the diffracted light generating unit according to a modulation phase amount. It is characterized by the following.
According to a fourth aspect of the present invention, in the optical modulator according to any one of the first to third aspects, a diffracted light generating unit is constituted by a diffraction grating member, and the diffracted light generation unit is moved so as to move the diffraction grating member itself. It constitutes the means.
According to a fifth aspect of the present invention, in the optical modulator according to the fourth aspect, the optical modulator further comprises a motion detecting means for detecting a motion of the diffraction grating member. The movement is controlled so that the reference signal input from the device has a predetermined relationship.
According to a sixth aspect of the present invention, in the optical modulator according to any one of the first to third aspects, a plurality of lattice factors arranged side by side and each of the plurality of lattice factors are arranged for every predetermined number of lattice factors. The diffracted light generating means is configured to include a grating factor driving means to be driven, and the diffracted light generating unit moving means in this case appropriately shifts the grating factor driven by the grating factor driving means in the arrangement direction of the grating factors. Thus, the lattice factor driving means is controlled.
According to a seventh aspect of the present invention, in the optical modulator according to the sixth aspect, the diffracted light generating means is provided in the optical waveguide.
[0008]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
(First embodiment)
FIG. 1 is a diagram in which the Michelson interferometer according to the first embodiment of the present invention is configured by a diffraction grating. Generally, a Michelson interferometer is used for weak light measurement and the like. In the first embodiment, a diffraction grating is used as a modulation element and an optical branching / coupling element. In FIG. 1, laser light from a light source (not shown) (for example, a high coherence light source (single mode oscillation laser)) is incident on a transmission diffraction grating 100, the 0th-order light is incident on a measurement object 101, and the + 1st-order light is The light enters the reference optical system 102, returns to the original path, and enters the transmission diffraction grating 100 again. The reference optical system 102 is configured by a reflection diffraction grating assuming a change in the wavelength of input light. The reflected light from the measurement object 101 undergoes -1st-order diffraction at the transmission diffraction grating 100, and the reflected light from the reference optical system 102 undergoes 0th-order diffraction, and the light receiving element 103 can observe two light flux interference fringes.
[0009]
The transmission diffraction grating 100 is moved left and right by applying a sine wave signal to an actuator 104 such as a piezoelectric element. Assuming that the one-sided amplitude of the movement is d and the sine wave signal is sin (ωot), the + 1st-order light undergoes a phase change represented by the following equation (1).
(Equation 1)
2π · dsin (ωot) / p (1)
Here, p is the grating pitch of the transmission diffraction grating 100 (moving grating). The -1 order light undergoes a phase change represented by the following equation (2).
(Equation 2)
-2π · dsin (ωot) / p (2)
[0010]
On the light receiving element 103, a light receiving signal proportional to the following equation (3) is obtained.
[Equation 3]
cos (θ + 4π · dsin (ωot) / p) (3)
Here, θ is the phase difference between the two light beams.
[0011]
According to the first embodiment described above, the following operational effects can be obtained.
(1) When the reflected light from the measurement object 101 is small, the received light signal of the light receiving element 103 is buried in noise and the signal cannot be distinguished. Proportional signal detection becomes possible.
[0012]
(2) Even if the wavelength of the incident light changes, the light receiving signal can be observed if the light receiving element 103 has a sufficient size, and the phase modulation amount of the grating modulation of the transmission diffraction grating 100 is a function of the wavelength. Therefore, it is possible to detect weak light with the same scale factor without adjusting the amount of phase modulation.
[0013]
(Second embodiment)
FIG. 2 is a diagram showing a main configuration of an optical modulator according to a second embodiment of the present invention. In FIG. 2, the optical modulator includes a disk 11, a motor 12, a drive circuit 13, a position sensor 14, and a PLL circuit 15. The rotation center O of the disk 11 is fixed to the rotation shaft of the motor 12. The disk 11 is composed of, for example, a glass disk having a diameter of 50 mm. The disk 11 is provided with engraved lines 11a on the circumference by chromium vapor deposition. For example, the radius r of the circumference where the score line 11a is provided is 20.4 mm, and the number N of the score lines 11a is 16000. In this case, the interval between the score lines 11a is 8 μm.
[0014]
The motor 12 is rotated by a drive signal sent from the drive circuit 13 and drives the disk 11 to rotate. The position sensor 14 is configured by, for example, a photo interrupter. The position sensor 14 includes a light emitting element (eg, an LED) and a light receiving element (eg, a PD) (not shown), and is provided so as to sandwich the marking line 11a of the disk 11 between the light emitting element and the light receiving element. The light emitted from the light emitting element is applied to the marking line 11a of the disk 11, and is received by the light receiving element when the irradiation light passes between the marking lines. The light receiving element outputs a detection signal to the PLL circuit 15 according to the intensity of the received light. When the disk 11 is rotated by the rotation of the motor 12, the light received by the light receiving element repeatedly passes through the engraved line and is obstructed by the engraved line, and repeats the strength.
[0015]
The PLL circuit 15 compares the phase of a detection signal input from the position sensor 14 with the phase of a reference clock signal input from an external device, generates a control signal corresponding to the phase difference, and outputs the control signal to the drive circuit 13. The drive circuit 13 drives the motor 12 in response to the control signal from the PLL circuit 15, so that the rotation speed of the motor 12 is feedback-controlled so that the frequency of the detection signal and the frequency of the clock signal have a predetermined relationship. The predetermined relationship is, for example, to match 1 / N (N is an integer) of the frequency of the detection signal with the frequency of the clock signal. As a result, even if the disk 11 has eccentricity, the motor 12 rotates stably without causing speed unevenness.
[0016]
In the above-mentioned optical modulator, input light from a laser light source or the like is incident on the circumference provided with the ruled line 11a in the direction of arrow A. The input light enters from an input optical system (not shown). Since the ruled line 11a forms a diffraction grating (bright and dark grating), a plurality of diffracted lights are generated. In FIG. 2, the arrow B indicates the 0th-order light (transmitted light), the arrow C1 indicates the + 1st-order diffracted light, and the arrow C2 indicates the -1st-order diffracted light. Illustration of second-order or higher order diffracted light is omitted. Such a diffraction angle θm of the m-th order diffracted light is represented by the following equation (4).
(Equation 4)
sin θm = m × λ / p (4)
Here, p is the grating (line) interval, and λ is the wavelength of the input light.
[0017]
When the disk 11 is rotated at a rotational angular speed ω (rps), the speed v of the score 11a on the circumference is expressed by the following equation (5).
(Equation 5)
v = r × 2πω (5)
Here, r is the radius of the circumference where the marking line 11a is provided.
[0018]
When the diffraction grating moves with respect to the incident light, the phase of the diffracted light changes. Since the ruled line 11a at the time t (sec) at the time of constant speed movement, that is, the displacement amount of the diffraction grating is represented by v × t, the phase of the m-th light after t seconds is represented by the following equation (6). Is done.
(Equation 6)
2π × m × v × t / p = 2π × Δf × t (6)
Here, Δf (= m × v / p) is the frequency shift amount of the m-th order diffracted light. When the disk 11 continues to rotate at the angular velocity ω, the phase change occurs continuously in the diffracted light, and as a result, the frequency of the diffracted light changes. The frequency shift amount is the difference between the frequency after the change (diffracted light) and the frequency before the change (input light). The frequency fm of the m-th order diffracted light is expressed as fm = f + Δf when represented by the frequency f of the light input to the optical modulator.
[0019]
The above described number N of the score lines 11a (the number of grids for one round of the disk 11) is expressed by the following equation (7).
(Equation 7)
N = r × 2π / p (7)
From the above equations (5) to (6), the frequency shift amount Δf of the m-th order light is expressed by the following equation (8).
(Equation 8)
Δf = m × N × ω (8)
[0020]
FIG. 3 is a table showing the frequency shift amount Δf of the + 1st-order diffracted light by the above-described optical modulator. The numerical values shown in FIG. 3 are obtained by substituting the number of lattices N and the rotational angular velocity ω into the above equation (8), and the respective frequency shift amounts at three rotational speeds of 1, 10, and 100 per second. It is a numerical example of (DELTA) f. In FIG. 3, when the frequency shift amount Δf (that is, the modulation frequency) with respect to the input light is set to 16 kHz in the optical modulator, the drive circuit 13 controls the rotation speed of the motor 12 to 60 rpm (= 1 rps).
[0021]
When extracting the + 1st-order diffracted light from the above optical modulator, an output optical system (not shown) is provided so as to extract the light diffracted in the direction of arrow C1 in FIG. In addition, when extracting the -1st-order diffracted light from the optical modulator, an output optical system (not shown) may be provided so as to extract the light diffracted in the direction of arrow C2 in FIG.
[0022]
The above-described numerical examples of the grating interval p, the number of gratings N, and the rotational angular velocity ω of the optical modulator need not be as described above. For example, when the lattice spacing p is set to 1 μm and the rotation angular velocity ω is increased to a speed corresponding to a rotation speed of 100 rps or more, the upper limit of the frequency shift amount Δf can be set to several tens of MHz.
[0023]
In the optical modulator according to the second embodiment described above, the following operation and effect can be obtained.
(1) The diffraction grating formed by the score line 11a is formed on the circumference of the disk 11 and the disk 11 is rotated by the motor 12. The input light is incident on the circumference provided with the diffraction grating, and the input light is diffracted. The output of the optical modulator extracts, for example, + 1st-order diffracted light as output light. As shown in the above equation (4), since the diffraction angle depends on the wavelength of the input light and the grating interval p, the direction of the diffracted light is constant unless the wavelength of the input light and the grating interval p are changed. Thus, even if the rotational angular velocity ω of the motor 12 (that is, the moving speed v of the diffraction grating) is changed, the + 1st-order diffracted light can be extracted as output light without adjusting the position of the output optical system. . Furthermore, unlike the related art (optical modulator utilizing the electro-optical effect), there is no need to apply an external electric field to obtain a predetermined phase change, so that the optical modulator can be realized with a simple configuration.
[0024]
(2) As shown in the above equation (6), the frequency shift amount Δf depends on the rotational angular velocity ω of the motor 12 (that is, the moving speed v of the diffraction grating) and the grating interval p. Therefore. The frequency shift amount Δf can be changed by changing the rotation speed of the motor 12 without changing the lattice interval p. As a result, the modulation frequency can be changed while maintaining the same diffraction angle. Furthermore, since there is no need to excite ultrasonic waves as in the related art (optical modulator utilizing an acousto-optic effect), an optical modulator can be realized with a simple configuration.
[0025]
(3) Since the rotation speed of the motor 12 is controlled by the PLL, it is possible to obtain an optical modulator having a stable frequency shift amount Δf.
[0026]
In the above description, an example in which the optical modulator is used as an optical frequency shifter by rotating the motor 12 at a constant speed has been described. However, when the motor 12 is rotated by a predetermined angle, the optical modulator can be used as an optical phase shifter. . In this case, the strong and weak phases of the detection signal input from the position sensor 14 are counted, and when the count value reaches a predetermined number, the motor 12 is stopped. Thereby, since the rotation angle of the motor 12, that is, the amount of movement of the diffraction grating can be controlled, a phase change according to the amount of movement of the diffraction grating can be obtained.
[0027]
(Third embodiment)
FIG. 4 is a diagram illustrating a main configuration of an optical modulator according to the third embodiment. In FIG. 4, the optical modulator includes a cell substrate 21 and a control circuit 22. The cell substrate 21 is provided with a cell array 21a including a plurality of micro liquid crystal switches (cells) arranged at intervals of several tens of μm. In the third embodiment, the cell array 21a functions as a diffraction grating. The cell substrate 21 is provided with an opening at a position corresponding to the minute liquid crystal switch so that the transmitted light and the diffracted light by the cell array 21a travel to the back surface of the cell substrate 21.
[0028]
The control circuit 22 sends an ON / OFF signal at a predetermined timing to each of the minute liquid crystal switches constituting the cell array 21a. For example, when the ON signal is transmitted, the liquid crystal switch is turned on to be in a transmission state, and when the OFF signal is transmitted, the liquid crystal switch is turned off to be in a light shielding state.
[0029]
FIG. 4 further shows an operation example of the cell array 21a. The cell array 21a is composed of, for example, a large number of minute liquid crystal switches. Sixteen cells constitute one block, and one block corresponds to one pitch of the lattice. Eight cells are turned on and eight cells are turned off in one block. In FIG. 4, cells in the on state are represented by white, and cells in the off state are represented by black. On / off control is performed on the on-state cell and the off-state cell such that eight cells are apparently continuous.
[0030]
FIG. 4A is a diagram showing the cell array 21a at time t1. FIG. 4B is a diagram showing the cell array 21a at time t2 (t2 = t1 + Δt). FIG. 4C is a diagram showing the cell array 21a at time t3 (t3 = t2 + Δt). In this manner, the on-state cell and the off-state cell shift one by one (rightward in the example of FIG. 4) at every predetermined time Δt.
[0031]
The control circuit 22 is configured by, for example, a shift register circuit. The control circuit 22 has 16 output signal lines D0 to D15 corresponding to each cell of the cell array 21a. The output patterns of the output signal lines D0 to D15 are the outputs of the shift register. When changing the phase (frequency) of the diffracted light, the control circuit 22 counts a clock signal generated inside the control circuit 22. Then, each time the count reaches a predetermined number (that is, the time Δt elapses), the output pattern of the output signal lines D0 to D15 is shifted by one bit by the shift register circuit. Thus, the cell array 21a is controlled on / off by the control signal of the control circuit 22.
[0032]
As the clock signal, a reference clock signal input from an external device may be used. By using a clock signal having a stable clock frequency, a time span in which the output pattern of the output signal lines D0 to D15 shifts, that is, the moving speed of the diffracted light generation unit formed by the on / off controlled cell array 21a Becomes stable.
[0033]
In the optical modulator, input light from a laser light source or the like is incident on the cell array 21a in the direction of arrow A. The input light enters from an input optical system (not shown). Since the cell array 21a forms a diffraction grating (bright and dark grating), a plurality of diffracted lights are generated. When extracting + 1st-order diffracted light from the optical modulator, an output optical system (not shown) is provided so as to extract light diffracted in the direction of arrow C1 in FIG. When extracting the -1st-order diffracted light from the optical modulator, an output optical system (not shown) may be provided so as to extract the light diffracted in the direction of arrow C2 in FIG.
[0034]
The apparent moving speed of the diffracted light generator with respect to the incident light is changed by changing the count number of the clock signal counted by the control circuit 22. The movement speed decreases as the count number increases, and the movement speed increases as the count number decreases.
[0035]
As described above, in the optical modulator according to the third embodiment, similarly to the second embodiment, even if the apparent moving speed of the diffracted light generation unit is changed, the position of the output optical system can be adjusted. Without performing this operation, the + 1st-order diffracted light can be extracted as output light. Further, the frequency shift amount Δf can be changed by changing the count number of the clock signal without changing the grid interval (in this case, the number of cells constituting one pitch), so that the same diffraction angle can be maintained. The modulation frequency can be changed.
[0036]
In the above description, eight cells are turned on and eight cells are turned off to form one grid pitch. However, the number of cells to be turned on / off may be arbitrarily changed according to a required grid pitch. In order to narrow the lattice pitch, for example, three cells may be turned on, three cells may be turned off, three cells may be turned on / off alternately, and so on. On the contrary, when the lattice pitch is to be increased, for example, 16 cells may be turned on, 16 cells may be turned off, 16 cells may be turned on / off, and so on.
[0037]
(Fourth embodiment)
A configuration in which a diffraction grating is configured by a plurality of cells and a diffraction light generation unit is moved with respect to incident light may be configured on a waveguide substrate. FIG. 5 is a diagram illustrating a main configuration of an optical modulator according to the fourth embodiment. The optical modulator shown in FIG. 5 includes a cell array 31 a including a plurality of refractive index conversion units (cells) arranged at intervals of several hundred nm on a lithium niobate (LiNbO ₃ ) substrate 31. In the fourth embodiment, the cell array 31a functions as a diffraction grating (phase grating). The phase grating alternately has states with different refractive indexes.
[0038]
The control circuit (not shown) sends an ON / OFF signal at a predetermined timing to each of the refractive index conversion units constituting the cell array 31a. For example, the configuration is such that when an ON signal is transmitted, the refractive index of the refractive index conversion unit changes to a high value, and when the ON signal is not transmitted, the refractive index of the refractive index conversion unit returns to the original value. In the cell array 31a, a predetermined number of cells constitute one block, and one block corresponds to one pitch of the lattice. Half of the cells in one block are turned on, and the other half are turned off. The on / off control of the cell array 31a is performed in the same manner as in the third embodiment. Further, the number of cells to be turned on / off may be arbitrarily set according to a required grid pitch.
[0039]
In the optical modulator, input light from a laser light source or the like is incident on one point on the cell array 31a in the direction of arrow A. The input light to the waveguide substrate 31 enters from an input optical system (not shown). Since the cell array 31a forms a phase grating, a plurality of diffracted lights are generated. When taking out the + 1st-order diffracted light from the optical modulator, the output optical system 1 is provided so as to take out the light diffracted in the direction of the arrow C1 in FIG. When extracting the -1st-order diffracted light from the optical modulator, the output optical system 2 may be provided so as to extract the light diffracted in the direction of arrow C2 in FIG. When taking out the zero-order light (passing light), the output optical system 3 may be provided so as to take out the light diffracted in the direction of arrow B in FIG.
[0040]
As described above, the optical modulator according to the fourth embodiment can adjust the position of the output optical system even if the apparent moving speed of the diffracted light generator is changed, as in the third embodiment. Without performing this operation, the + 1st-order diffracted light can be extracted as output light. Further, since the frequency shift amount Δf can be changed without changing the grating interval (in this case, the number of cells constituting one pitch), the modulation frequency can be changed while maintaining the same diffraction angle.
[0041]
Correspondence between each component in the claims and each component in the embodiment of the invention will be described. The diffracted light generating means is constituted by, for example, the disk 11 (cell arrays 21a and 31a). The diffracted light generator is constituted by, for example, a score line 11a (cell). The diffracted light generation unit moving means is constituted by, for example, the motor 12 (control circuit 22). The control means includes, for example, the drive circuit 13 and the PLL circuit 15 (control circuit 22). The movement detecting means is constituted by, for example, the position sensor 14. The lattice factor is constituted by, for example, a minute liquid crystal switch (refractive index conversion unit). The lattice factor driving means is constituted by the control circuit 22, for example. Note that each component is not limited to the above configuration as long as the characteristic functions of the present invention are not impaired. As an application field of the present embodiment, for example, a system for optical heterodyne interference or the like, and a system for a homodyne interferometer (an optical fiber gyro or the like as an application thereof) are considered.
[0042]
【The invention's effect】
According to the present invention, it is possible to provide an optical modulator in which a predetermined phase change is given irrespective of an input light wavelength and a diffraction angle is constant even when a frequency shift amount is changed.
[Brief description of the drawings]
FIG. 1 is a diagram in which a Michelson interferometer according to a first embodiment is configured by a diffraction grating.
FIG. 2 is a diagram showing a main configuration of an optical modulator according to a second embodiment of the present invention.
FIG. 3 is a diagram illustrating a frequency shift amount Δf of + 1st-order diffracted light by an optical modulator.
FIG. 4 is a diagram illustrating a configuration of a main part of an optical modulator according to a third embodiment, and an operation example of a cell array over time.
FIG. 5 is a diagram illustrating a main configuration of an optical modulator according to a fourth embodiment.
[Explanation of symbols]
11 ... disk, 11a ... engraved line,
12 ... motor, 13 ... drive circuit,
14 position sensor 21 cell substrate
21a, 31a ... cell array, 22 ... control circuit,
31 ... Waveguide substrate

Claims (7)

Diffraction light generating means for diffracting incident light to generate diffracted light,
A diffracted light generator moving means for moving a diffracted light generator of the diffracted light generator with respect to the incident light;
Movement control of the diffracted light generator by the diffracted light generator moving means such that the m-th order diffracted light generated by the diffracted light generating means with respect to the incident light has a desired modulation frequency shift amount or modulation phase amount. An optical modulator, comprising: The optical modulator according to claim 1,
The optical modulator, wherein the control unit controls the diffracted light generation unit moving unit to change a moving speed of the diffracted light generation unit according to the modulation frequency shift amount. The optical modulator according to claim 1,
The optical modulator, wherein the control unit controls the diffracted light generation unit moving unit so as to change a moving position of the diffracted light generation unit according to the modulation phase amount. The optical modulator according to any one of claims 1 to 3,
The diffracted light generating means is a diffraction grating member,
The diffracted light generator moving means moves the diffraction grating member itself. The optical modulator according to claim 4,
Further comprising a motion detecting means for detecting the motion of the diffraction grating member,
The optical modulator according to claim 1, wherein the control unit performs movement control such that a detection signal from the motion detection unit and a reference signal input from an external device have a predetermined relationship. The optical modulator according to any one of claims 1 to 3,
The diffracted light generating unit has a plurality of lattice factors arranged side by side, and a lattice factor driving unit that drives each of the plurality of lattice factors for each of a predetermined number of lattice factors,
The optical modulator according to claim 1, wherein the diffracted light generator moving unit controls the grating factor driving unit to shift a grating factor driven by the grating factor driving unit in a direction in which the grating factor is provided. The optical modulator according to claim 6,
An optical modulator according to claim 1, wherein said diffracted light generating means is provided in an optical waveguide.

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