JPH0764031A - Optical modulator - Google Patents

Abstract

PURPOSE:To provide an optical modulator realizing a high performance transmission characteristic and suppressing electric cross-talk by constituting two modulation electrodes in the waveguide direction of an optical waveguide in tandem. CONSTITUTION:Two modulation electrodes are constituted in tandem. That is, a Z cut-LiNbO3 substrate 101, the first, second central conductors 103, 104 of the modulation electrodes modulating the refractive indexes of branched parts 102a, 102b of the optical waveguide 102 constituting a Mach-Zehnder interferometer and grounded conductors 105, 106 corresponding to respective central conductors 103, 104 are connected with each other in tandem. Then, a signal and a bias power source are impressed to the central conductors 103, 104 by first, second microwave signal sources 107, 109 and DC bias power sources 108, 110, and respective voltages of the signal sources 107, 109 and the DC bias power sources 108, 110 are adjusted, and a wavelength chirping amount of a modulation output beam P0 is made optionally adjustable. Further, gaps with the central conductors 103, 104 are made easily spreadable to several 100mum or above and the electric cross-talk is reduced sufficiently.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical modulator utilizing electro-optic effect capable of high performance operation.

[0002]

2. Description of the Related Art In an optical communication / optical information processing system, an external light intensity modulator using a ferroelectric material having an electro-optical effect, a semiconductor, an organic material or the like is widely used. In particular, in a long repeater span / large capacity optical fiber communication system, it is desirable that the wavelength chirping of the modulated light output from the transmitter is small, so that the external optical modulator is an essential device.

FIG. 3 shows a conventional optical modulator using a LiNbO ₃ crystal and an example of the control circuit configuration. In FIG. 3, for example, a z-cut-LiNbO ₃ substrate 301 having an electro-optical effect is provided.
A Mach-Zehnder interferometer type optical waveguide 302 is formed in the vicinity of the surface of the element by thermal diffusion of Ti, and a center conductor 303 and a ground conductor 304 of the modulation electrode are formed via a SiO ₂ buffer layer. When input light Pi having a constant power is incident from the incident end face of the optical waveguide 302, output light Po whose intensity (amplitude) is modulated according to the signal voltage from the signal source 305 is emitted from the emitting end face of the optical waveguide 302. In this case, due to the structural asymmetry of the central conductor 303 and the ground conductor 304 of the modulation electrode, the output light Po slightly undergoes phase modulation due to its intensity modulation and wavelength chirping occurs (output with α value ≠ 0). Become light). In an optical fiber communication system, it is generally desirable that the light source has a small α value. However, when the optical output Po has a large power, the transmission characteristic of the system is greatly affected by the chromatic dispersion characteristic and the nonlinear characteristic of the fiber. Therefore, in order to improve the receiving sensitivity and lengthen the relay span of the system, it becomes necessary to set the α value to an appropriate value. In the conventional optical modulator shown in FIG. 3, the DC bias power supply 306 is used.
The magnitude of α can be changed by adjusting the voltage V of 1. However, the modulation operating point is also changed accordingly, which causes a defect that the modulation waveform is distorted or the modulation depth becomes shallow.

FIG. 4 shows a basic configuration of a conventional LiNbO ₃ optical modulator that suppresses wavelength chirping and its control circuit. In FIG. 4, reference numeral 401 is an x-cut-LiNbO ₃ substrate, and 403 and 404 are the centers of the modulation electrodes for modulating the refractive index of the branched optical waveguide portions 402a and 402b of the optical waveguide 402 that constitutes the Mach-Zehnder interferometer. A conductor, 405 is a ground conductor, Pi is incident light, and Po is emitted light. 406,408 and
Reference numerals 407 and 409 denote first and second microwave signal sources and a DC bias power source for applying signals to the central conductors 403 and 404, respectively.

5A and 5B are diagrams for explaining the operation principle of the conventional example of FIG. 4, and FIG. 5A is applied to the central conductors 403 and 404 of FIG. 4, respectively. Voltage waveform of Fig. 5
(b) is a branched optical waveguide portion 402a, 402b of the optical waveguide 402.
, The propagation constant β for the guided light, the effective refractive index n,
The electric field strength E and the modulation electrode length L are shown, and Eo is the electric field strength of the output light Po. In FIG. 4, the incident light Pi is guided by the Mach-Zehnder interferometer incident side branch circuit to the light of intensity Eio, which is respectively guided to the optical waveguide portions 402a and 402b, and undergoes phase modulation according to the voltage of the modulation electrode. After that, it is multiplexed again by the multiplexing circuit and output. At this time, the output optical field intensity Eo is given by the following equation.

[0006] Eo = Ei0 / 2 · exp ( -jβ 1 L) + Ei0 / 2 · exp (-jβ 2 L) = Ei0 · cos (ΔβL) · exp (jβL) ≡Ei · exp (j φ) (1) Here, Δβ = (β ₁ −β ₂ ) / 2 β = (β ₁ + β ₂ ) / 2 Ei = Ei 0 · cos (ΔβL) φ = βL. That is, in the optical modulator of FIG. 4, the optical intensity is modulated by changing Δβ by the voltages v ₁ and v ₂ from the microwave signal sources 406 and 408. At this time, the α value of the wavelength chirping is α = Ei · (dφ / dt) / (dEi / dt) (2) = −cot (ΔβL) · (1 + m) / (1-m) ≡A · B ( 3) m = (dn ₁ / dt) / (dn ₂ / dt) ≡Δn ₁ / Δ
n ₂ A = −cot (ΔβL) B = (1 + m) / (1-m) Here, t represents time. Δn ₁ , Δn ₂
Is the refractive index change amount of each of the optical waveguide portions 402a and 402b caused by the voltages v ₁ and v _{2 of the} microwave signal sources 408 and 410, and changes in proportion to the magnitudes of v ₁ and v ₂ , respectively. As shown in (a) of FIG. 5, when the phases of v1 and v2 are in-phase (v ₁ · v ₂ > 0), m <0, and opposite phase (v ₁ · v ₂ <
In the case of 0), m> 0. Therefore, A is the DC bias v
_1, v depending on the size of _2, B is so v _1, v can be controlled by the magnitude and phase relationships of _2, can be arbitrarily set the size of the alpha. Since the operating point is normally set to the midpoint between ON and OFF of the optical output, in this case, A becomes A = ± 1,
It can be varied within the range of −1 ≦ α ≦ 1. The above relational expression represents the characteristic of α when modulated with a minute microwave signal. In the actual operation, modulation is performed with a large signal (modulation index ˜1), so the above equation represents an approximate value in the actual operation.

[0007]

In the above conventional example, 2
Although the two modulation electrodes are arranged in parallel with the Mach-Zehnder interferometer optical waveguide, it is necessary to sufficiently separate the two modulation electrodes in order to reduce the electric crosstalk. For this reason, if the branch angle of the branch circuit of the Mach-Zehnder interferometer is made large, there is a drawback that the radiation loss of this branch portion becomes large. On the other hand, if the branch angle is kept small, the length of the branch part will increase,
There is a drawback that the propagation loss becomes large.

An object of the present invention is to realize high performance transmission characteristics by controlling wavelength chirping without distortion of the modulation waveform of light intensity in such an optical communication system. It is to provide a high-performance optical modulator that suppresses dynamic crosstalk.

[0009]

In order to solve the above problems, in the present invention, in an optical modulator using at least two modulation electrodes and an optical waveguide, and utilizing the electro-optic effect, the two modulations are provided. The electrodes were arranged in series in the waveguide direction of the optical waveguide.

[0010]

According to the present invention, in the optical modulator, the two modulation electrodes are arranged in series in the light guiding direction, so that the interval between the modulation electrodes can be widened without increasing the branch angle. It is possible to sufficiently reduce electric crosstalk.

[0011]

Embodiments of the present invention will now be described in detail with reference to the drawings.

FIG. 1 shows a first embodiment of the present invention in which two modulation electrodes have a tandem structure. In FIG. 1, 101 is a z-cut-LiNbO ₃ substrate, 103, 10
4 is the first and second center conductors of the modulation electrodes for modulating the refractive index of the branched portions 102a and 102b of the optical waveguide 102 that constitutes the Mach-Zehnder interferometer, and 105 and 106 are ground conductors corresponding to the respective center conductors. , Connected to each other. Pi is incident light and Po is outgoing light. 107,109 and 108,110
Are the first and second microwave signal sources and DC bias power source for applying signals to the central conductors 103 and 104. In this case, by adjusting the respective voltages v ₁ , v ₂ , V ₁ , V ₂ of the signal sources 107, 109 and the DC bias power sources 108, 110 constituting the drive circuit, the wavelength chirping amount of the modulated output light Po is adjusted as in the conventional example. Can be adjusted arbitrarily. In addition, the branched portion 102 of the Mach-Zehnder interferometer optical waveguide 102
By making the distance between a and 102b the same as the conventional size shown in FIG. 3, for example, the distance t between the first central conductor 103 and the second central conductor 104 can be easily increased by several hundreds of μm or more, so that electrical crosstalk can be achieved. Can be sufficiently reduced. Moreover, the first
Also, since the second modulation electrodes (103, 105 and 104, 106) can perform push-pull operation with respect to the optical waveguide portions 102a, 102b constituting the Mach-Zehnder interferometer, highly efficient modulation operation is also possible. Note that, in FIG. 1, even if the center conductor 104 of the second modulation electrode is formed on the optical waveguide portion 102a side and arranged in the same arrangement as the first modulation electrodes 103 and 105, the microwave signal source 109 and the DC bias Similar characteristics can be achieved by reversing the polarity of the power supply 110.

In the above, the case where the lengths of the modulation electrodes are the same for both the branched optical waveguide portions 102a and 102b has been shown.
Different lengths may be used. Also, the case where the Y branch circuit is used as the branching / multiplexing circuit of the Mach-Zehnder interferometer optical waveguide has been described, but for example, it is configured by combining a 3 dB directional coupler, an X-shaped branch circuit, or a Y branch circuit. The optical modulator and the optical switch described above can also operate with α arbitrarily set according to the same principle.

FIG. 2 shows a second embodiment according to the present invention, in which a traveling waveform asymmetrical coplanar strip line is used as the modulation electrode. First microwave signal source 20
7 is a Mach-Zehnder interferometer optical waveguide part 202a, 202b
Center conductor 203 of the first modulation electrode formed on one side 202a
In addition, the second microwave signal source 209 is connected to the central conductor 204 of the second modulation electrode formed in the optical waveguide 202. In this case, as can be seen from the definition equation (2), since the α value can be given by the ratio of the phase modulation degree of the optical output and the intensity deviation degree, the intensity modulation by the signal from the first microwave signal source 207 is performed. An arbitrary α value can be set by subjecting the generated light to phase modulation of an appropriate magnitude with a signal from the second microwave signal source 209. Note that 205 and 206 are ground electrodes, and 208 is a DC bias power supply.

In the above, the case where the Mach-Zehnder interferometer optical waveguide is used for the light intensity modulation section is shown. However, instead of the Mach-Zehnder interferometer optical waveguide, an optical waveguide such as a directional coupler or an X-shaped branch circuit is used. The effect of the present invention can be similarly applied to the optical modulator, the optical switch, and the like configured in.

Although the case where the LiNbO ₃ substrate is used has been shown above, the effect of the present invention can be obtained in an optical device using a substrate such as another dielectric or semiconductor having an electro-optical effect. Is self-evident.

[0017]

As described above, according to the present invention,
Since the two modulation electrodes are arranged in series in the waveguide direction of the optical waveguide, the electrical crosstalk can be sufficiently reduced and the wavelength chirping amount can be set to an arbitrary value by adjusting the operating voltage of the two drive circuits. Therefore, high performance transmission characteristics can be realized in the optical communication system.

[Brief description of drawings]

FIG. 1 is a diagram showing an embodiment of the present invention.

FIG. 2 is a diagram showing another embodiment according to the present invention.

FIG. 3 is a block diagram of a conventional optical modulator and control circuit.

FIG. 4 is a block diagram of a conventional optical modulator and control circuit.

FIG. 5 is a diagram illustrating the principle of wavelength chirping control.

[Explanation of reference numerals] 101,201 ... LiNbO ₃ substrate, 102,202 ... Mach-Zehnder interferometer optical waveguide, 103, 203 ... Central conductor of first modulation electrode, 104, 204 ... Central conductor of second modulation electrode, 105, 106,
205,206 ... Ground conductor, Pi ... Incident light, Po ... Emitted light, 10
7,207… First microwave signal source, 108,208… First D
C-bias power supply, 109,209 ... Second microwave signal source,
110 ... Second DC bias power supply.

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Mitsuru Naganuma 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nihon Telegraph and Telephone Corporation

Claims (1)

[Claims] 1. An optical modulator including at least two modulation electrodes and an optical waveguide, wherein the two modulation electrodes are arranged in series in a waveguide direction of the optical waveguide in an optical modulator utilizing an electro-optical effect. And an optical modulator.