JPH07107585B2 - Driving method for waveguide optical phase modulator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Outline] The present invention relates to a push-pull type waveguide optical modulator,
In order to enable accurate phase modulation without malfunction, the voltage of the light in each waveguide has the same magnitude and opposite sign, and the phase of the output light (combining the above phases) By driving with a voltage before and after the phase is inverted, the output light is discontinuously changed between two phase states, and a halfway state between them can be removed. .

[Industrial application field]

The present invention relates to a method of driving a waveguide optical phase modulator that performs phase modulation using a push-pull type waveguide optical modulator.

[Conventional technology]

The structure of a conventional waveguide optical phase modulator is shown in FIG. This phase modulator has a configuration in which a pair of electrodes 2 and 3 are provided on both sides of a linear waveguide portion 1 sandwiched therebetween. Phase modulation is applied to electrodes 2, 3
Applying a predetermined voltage between them to apply an electric field to the linear waveguide section 1 and change the refractive index by the electro-optical effect, thereby giving a binary phase change of “o” and “π” to the output light. Is going by.

[Problems to be solved by the invention]

In the above-mentioned conventional phase modulator, the voltage required for the phases of the output light to be “o” and “π”, respectively, is V ₀ (=
0) and V _π , the voltage is changed as shown in FIG. 4 (c).
When changing from V ₀ to V _π , the refractive index of the linear waveguide section 1 continuously changes according to the magnitude of the applied voltage, so that the phase of the output light also continuously changes from “o” to “π”. Will change. That is, although only “o” and “π” are required as the phase information, even a halfway phase between them is included. In this way, if the output light contains halfway phase information, an error occurs in the determination at the time of reading information, and incorrect information is transmitted.

In view of the above problems, it is an object of the present invention to provide a method for driving a waveguide optical phase modulator that eliminates a halfway phase state and enables accurate phase modulation without malfunction.

[Means for solving problems]

A method for driving a waveguide optical phase modulator according to the present invention is a push method in which two linear waveguide sections are provided between two Y-branch waveguide sections, and an electrode section is provided for each of these linear waveguide sections. It uses a pull type optical waveguide phase modulator,
Voltages such that the phases of the light in each of the above-mentioned linear waveguide sections have the same magnitude and opposite signs, and the voltages before and after the composite phase of each of the above phases are inverted are applied to the electrode section. It is characterized in that it is applied to the opposite.

[Action]

In the above-mentioned push-pull type waveguide optical modulator, when a voltage is applied to the electrode portions, electric fields in opposite directions are generated in the two linear waveguide optical portions. In response to such a change in the electric field, the phases of the light in the linear waveguide section also change in opposite directions. Therefore, in consideration of this point, if a voltage is applied such that the phases of the lights have the same magnitude and opposite signs, the phase of the output light (the light obtained by combining the above lights vectorally) is applied. Phase) of only one of "o" and "π". Moreover,
Further, the voltage before and after the phase of the output light is inverted between "o" and "π" (for example, as shown in FIGS. 4A and 4B, the phase is "0"). Based on the voltage V _{S at} which the voltage is inverted between “π”, a voltage higher than the voltage V _S by a certain amount and a voltage smaller than that (preferably, the voltages V ₀ and V _{π at which} the maximum light output can be obtained) can be obtained. If the applied voltage is set, for example, from V ₀ to V _π (or V _π
Even when V ₀ in) is varied continuously the applied voltage from the discontinuously Setsu換Ri between the output light of the phase of, for example, "o", "π", it is reliably binarized Phase information is sent out.

〔Example〕

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a configuration diagram of a waveguide optical phase modulator used in one embodiment of the present invention.

In the present embodiment, two linear waveguide portions 13 and 14 having the same length and width are provided between the two Y-branch waveguide portions 11 and 12, and on the linear waveguide portions 13 and 14, This is a push-pull type configuration in which electrodes 15 and 16 are respectively disposed on the. Further, in the Y branch waveguide portions 11 and 12, cut regions 11a and 12a extending from the vicinity of the center thereof to be gradually wider toward the branch side and having a narrower width and a lower refractive index than the respective waveguides.
Is provided. Each of the above waveguide sections 11 to 14 is formed by diffusing Ti or the like through a mask pattern with respect to a substrate 10 made of, for example, a Z plate of LiNbO ₃ which is an electro-optic crystal.

In the push-pull configuration, when a voltage is applied between the electrodes 15 and 16, for example, the lines of electric force from one electrode 15 penetrate the linear waveguide portion 13 in the depth direction and penetrate into the substrate 10. While drawing a curve while going through the other linear waveguide portion 14 toward the surface in the depth direction to reach the electrode 16,
In the linear waveguide portions 13 and 14, electric fields in opposite directions and having the same magnitude are generated. Then, accordingly, the phases of the lights propagating in the linear waveguide sections 13 and 14 also have the opposite signs and the same magnitude. Therefore, the phase of the output light (that is, the phase of the light that is vector-combined and output in the Y-branch waveguide section 12) has only one state of “o” and “π”, and is halfway. There are no phase states. That is, assuming that two lights having opposite signs and phases of the same magnitude are I ₁ and I ₂ , respectively, these phases (the phase shift with respect to the phase θ of the original input light) are α
And −α, I ₁ = I ₀ cos (θ + α), I ₂ = I ₀ cos
Since (θ−α) can be set, if I is the combined light of these two lights, I = I ₁ + I ₂ = I ₀ [cos (θ + α)
+ Cos (θ−α)] = 2I ₀ cos α · cos θ. Therefore, when 0 <α <π / 2 (when cos α <0), I = 2I
_{Since 0} · cos α · cos θ, the phase (shift of the phase with respect to θ) is “0”, while π / 2 <α <π (cos α
When <0), I = 2I ₀ · | cos α | · cos (θ−π), so the phase is “π”, that is, when α = π / 2 (I = 0 at this time) is the boundary. Therefore, the phase has only one state of "0" and "π".

In consideration of these points, in the present embodiment, as shown in FIG. 4 (a), the voltage V _{S at} which the phases of the output light are inverted between "o" and "π" is sandwiched. The two voltages V ₀ (=
0) and V _π are set, one of these two voltages V ₀ and V _π is appropriately selected and applied between the electrodes 15 and 16 to obtain binary phase information of “o” and “π”. I am trying to send out.

By modulating using such voltages V ₀ and V _π , the voltage is changed from V ₀ to V _π (V _π to V ₀ )
Even if continuously changed, the phase of the output light does not include a halfway state other than “o” and “π” as described above,
The phase can be discontinuously switched between “o” and “π”. Moreover, since the optical output becomes zero where the phase is reversed (that is, where the voltage becomes V _S ),
More convenient.

Further, in this embodiment, as shown in FIG. 1, by providing the cut regions 11a and 12a in the Y branch waveguide portions 11 and 12, it is possible to reduce the loss. In the conventional Y-branch waveguide, in order to reduce the loss at the branch, ideally,
As shown by the dotted line in the figure, the divergence angle must be made extremely small (1 ° or less), but it is difficult to form an extremely thin mask pattern for its manufacture. As shown in (3), the shape was sharp and the loss was not reduced. Therefore, by providing the cut areas 11a and 12a as in this embodiment,
Does not require an extremely thin mask pattern as in the past
You can now create a Y-branch almost as designed. As a result, the light is gradually branched along the cut regions 11a and 12a, so that the loss at the time of branching can be reduced. The phase modulator thus realized with low loss can also be used for coherent optical communication.

Next, FIG. 2 is a block diagram showing another embodiment of the present invention.

In this embodiment, instead of the linear waveguide portions 13 and 14 having the same length in FIG. 1, linear waveguide portions 23 and 24 having different lengths are provided. By changing the mutual length in this way, an optical bias is applied, and thus the fourth
As shown in FIG. 6B, the driving can be performed with the voltages V ₀ (≠ 0) and V _π . Also in this embodiment, similar to the above embodiment,
The binarized phase information of “o” and “π” can be sent out, the halfway phase can be removed, and the loss can be reduced by the cut regions 11a and 12a. Instead of changing the mutual lengths of the linear waveguides 23 and 24, the optical bias may be applied by changing their widths.

FIG. 3 is a configuration diagram of a waveguide optical phase modulator used in still another embodiment of the present invention. In this waveguide optical phase modulator, the substrate 10 'is made of a LiNbO ₃ X plate or Y plate, and
Instead of the electrodes 15 and 16 in the figure, straight waveguide portions 13 and 14
A pair of electrodes 25a, 25b; 26 so as to sandwich each from both sides.
It has a push-pull configuration with a and 26b provided. In such an electrode configuration, one electrode 25a, 2
The voltage between 5b and the other electrode 26a, 26b has the same voltage in opposite directions and the same magnitude.
A voltage capable of switching between the two phase states may be applied to drive in the same manner as in each of the above-described embodiments. In this case, the pair of electrodes 25
a, 25b and each distance between 26a, 26b, the electrode 15 described above,
Since it can be made smaller than the distance between 16, it is possible to drive at a lower voltage.

〔The invention's effect〕

According to the method of driving a waveguide optical phase modulator of the present invention, it is possible to reliably send out binarized phase information that does not include a halfway phase state, so that there is no error in the determination at the time of reading information. It does not occur and therefore very accurate phase modulation can be achieved.

[Brief description of drawings]

FIG. 1 is a block diagram of a waveguide optical phase modulator used in one embodiment of the present invention, FIG. 2 is a block diagram of a waveguide optical phase modulator used in another embodiment of the present invention, and FIG. Is a configuration diagram of a waveguide optical phase modulator used in still another embodiment of the present invention, and FIGS. 4 (a), (b) and (c) are respectively one embodiment of the present invention and another embodiment. FIG. 5 is a diagram showing a phase modulation output in a conventional example, FIG. 5 is a configuration diagram showing a conventional Y-branch waveguide, and FIG. 6 is a configuration diagram showing a conventional waveguide optical phase modulator. 11, 12 ... Y-branch waveguide section, 11a, 12a ... Notched area, 13, 14 ... Straight waveguide section, 15, 16 ... Electrode, 23, 24
...... Linear waveguide parts, 25a, 25b; 26a, 26b …… Electrodes.

Front page continued (72) Inventor Minoru Seino 1015 Kamiodanaka, Nakahara-ku, Kawasaki-shi, Kanagawa, Fujitsu Limited (72) Inventor Toru Shiina 1015, Kamedotachu, Nakahara-ku, Kawasaki, Kanagawa (56) Reference Reference JP-A-55-134818 (JP, A) JP-A-60-28621 (JP, A)

Claims (7)

[Claims] 1. A linear waveguide section (13, 14; 23, 24) is provided between two Y branch waveguide sections (11, 12), and each of the two linear waveguide sections is provided. Electrode part (15, 16; 25a, 25
b, 26a, 26b), the method of driving a push-pull type waveguide optical phase modulator, wherein the phases of the light passing through the respective linear waveguide portions have the same magnitude and opposite signs. A method for driving a waveguide optical phase modulator, characterized in that a voltage before and after a point where the composite phase of each phase is inverted is applied to the electrode portion. 2. The Y-branch waveguide portion has cutout regions (11a, 12a) extending from the vicinity of the center thereof toward the branching side and having a narrower width and a lower refractive index than the respective waveguide portions. A method of driving a waveguide optical phase modulator according to claim 1. 3. The method for driving a waveguide optical phase modulator according to claim 2, wherein the width of the cut region gradually increases toward the branch side. 4. The electrode section according to claim 1, wherein each of the electrode sections comprises one electrode (15, 16) arranged on each of the linear waveguide sections. 1
6. A method for driving a waveguide optical phase modulator according to claim 6. 5. A pair of electrodes (25a, 25a) arranged so as to sandwich the respective linear waveguide portions from both sides thereof.
b; 26a, 26b). The method for driving a waveguide optical phase modulator according to any one of claims 1 to 3, characterized in that 6. The waveguide optical phase modulator according to any one of claims 1 to 5, wherein the combined phase is inverted between o and π. Driving method. 7. The voltage at which the composite phase is inverted,
7. The method of driving a waveguide optical phase modulator according to claim 1, wherein the optical output is minimized.