JP3426553B2 - Waveguide type attenuator and waveguide type optical modulator - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a waveguide type optical modulator, and more particularly to a waveguide type optical modulator having an attenuator portion, which can be suitably used in a wavelength division multiplexing optical network system. Regarding

[0002]

2. Description of the Related Art With the development of optical communication systems in recent years, it is required to develop a communication system having not only high speed and large capacity but also a high degree of multifunctionality. In particular, functions such as wavelength multiplexing in the same optical transmission line, switching and exchange of optical transmission lines are required. These functions are called EDF (erbium doped fiber amplifier)
Wavelength multiplexing transmission (waveleng)
th division multiplexing: hereinafter, sometimes abbreviated as WDM system).

In this WDM system, different signals are put on light waves from a plurality of light sources having different wavelengths and the signals are transmitted by a single optical fiber. That is, an arbitrary signal is taken out by an optical modulator connected to the plurality of light sources, this signal is amplified by an EDFA, and the signal is taken out through a single optical fiber. As a result, the transmission capacity of the entire system can be increased without increasing the number of optical fibers or the bit rate of each signal.

The WDM system requires that the transmission state at each wavelength be constant. However, E
Since the DFA gain has wavelength dependence and the output of each light source is not constant and changes with time, there is a problem that the light receiving level at the receiving end differs for each wavelength. In view of such a problem, it has been attempted to connect an attenuator to the optical modulator. Figure 1
[FIG. 6] is a top view showing an example of a conventional waveguide type optical modulator to which an attenuator is connected.

In the waveguide type optical modulator 1 shown in FIG. 1, a Mach-Zehnder type optical modulation optical waveguide 5 and an attenuation optical waveguide 8 made of titanium or the like are formed on the surface of a substrate 4 having an electro-optical effect by thermal diffusion or the like. Are formed so as to be connected in series. Further, a buffer layer is provided on the substrate 4 as needed. A modulation electrode composed of a first signal electrode 6 and a first ground electrode 7 is formed on the substrate 4 (or a buffer layer), and an attenuation composed of a second signal electrode 9 and a second ground electrode 10 is further formed. An electrode for use is formed.

One ends of the first signal electrode 6 and the second signal electrode 9 are connected to external power supplies 12 and 13. The other end of the first signal electrode 6 is grounded, and the other end of the second signal electrode 6 is open. Further, a polarizer 11 is provided on the entrance side and the exit side of the optical modulator 1. In addition,
The arrow in the figure indicates the traveling direction of the light wave. The light modulation optical waveguide 5, the first signal electrode 6 and the first ground electrode 7 constitute the light modulation unit 2, and the attenuation optical waveguide 8
The second signal electrode 9 and the second ground electrode 10 form a waveguide type attenuator 3. That is, the waveguide type optical modulator 1 shown in FIG. 1 has a configuration in which the waveguide type attenuator 3 is integrated. Therefore, in the following description, the waveguide type attenuator 3 is considered to be integral with the optical waveguide type optical modulator 1, and is referred to as an "attenuator section 3" for convenience.

When a light wave of wavelength λ1 on which a predetermined signal is superimposed is incident on the waveguide type optical modulator 1, it is turned on / off in the optical modulator 2 and then reaches the attenuator 3 to undergo intensity modulation. That is, the signal intensity of each wavelength in the entire communication system is made constant by forcibly attenuating the signal of the specific wavelength having a high output value by this attenuator. In addition, the intensity modulation in the attenuator unit 3 is performed by the second electrode forming the attenuation electrode.
This is performed by adjusting the frequency and voltage of the control electric signal applied to the signal electrode 9 of.

[0008]

In such a waveguide type optical modulator 1, when the attenuation in the attenuator section 3 is relatively strong, noise is particularly remarkably superimposed on the optical signal, There is a problem that good transmission characteristics cannot be obtained. As a result,
The application range of the waveguide type modulator to the optical communication system has been limited.

The present invention provides a novel waveguide modulator having a light modulator and an attenuator, which realizes good transmission characteristics regardless of the strength of attenuation in the attenuator. The purpose is to do.

[0010]

A waveguide type attenuator of the present invention comprises a substrate, an attenuation optical waveguide formed on the substrate, and an attenuation electrode for attenuating an optical signal guided in the optical waveguide. And with. Then, the effective light modulation band by the attenuation electrode is reduced to be lower than the frequency band of the high frequency noise generated from the attenuation control electric signal applied to the attenuation electrode so as to remove the high frequency noise. It is characterized by having done.

The waveguide type optical modulator of the present invention is characterized in that the above-mentioned waveguide type attenuator is integrated.

The inventors of the present invention have extensively studied the cause of noise superposition on the optical signal from the optical modulator in the attenuator. FIG. 2 is a diagram showing the relationship between the intensity of the optical output due to attenuation and the voltage applied to the attenuator section. The vertical axis represents the light output intensity log
It is represented on a scale. It is known that when a voltage is applied to the attenuator, the optical output changes due to the relationship of cos ^2. However, when this is shown on a log scale, it becomes a periodic function as shown in FIG.

When noise is added to the inputted attenuation control electric signal during the attenuation, the noise resulting from the conversion of this noise into light is also superposed on the optical output. However, when the attenuation of the attenuator is weak (point A in the figure), even if noise is added to the attenuation control electric signal, as indicated by the diagonal line in the figure,
At this level, the slope of the function for converting an electric signal into an optical signal is small, the level of the optical signal is relatively large, and the relative amount of the optical signal noise level is small. Therefore, the optical signal noise level is large. It doesn't matter.

On the other hand, when the attenuation is strong (point B in the figure), for example, when the output light is strongly attenuated by 10 dB to 20 dB, if noise is added to the attenuation control electric signal, the noise of the optical output is Since the slope of the function that converts an electrical signal into an optical signal is large as indicated by the slanted line, and the signal light level decreases due to running attenuation, the relative optical signal noise level cannot be ignored. It becomes large and deteriorates the optical transmission characteristics.

The noise on the attenuation control electric signal may be weak noise generated in the control board or weak electric waves generated around the optical modulator, which may be mixed with the attenuator of the optical modulator through an electric circuit line or directly. It is generated by mixing as radio waves. Further, this noise component is greatly amplified because the slope of the electro-optical conversion function at high attenuation is large. Therefore, it has been very difficult to solve this noise problem by taking measures to reduce the noise of the circuit itself outside the optical modulator and to take measures against the weak radio waves.

Therefore, as a result of detailed analysis of noise carried on the attenuation control electric signal, the present inventors have found that among the noise, high frequency noise generated from the attenuation control electric signal applied to the attenuation electrode is In particular, it has been found that it has a great influence on the optical signal noise. Therefore, it has been found that the noise can be effectively prevented from being superimposed on the optical signal by effectively removing the high frequency noise.

The present invention has been made as a result of such a detailed study over a long period of time. According to the present invention, the high frequency noise component generated from the control electric signal can be effectively removed, so that noise is not superimposed on the optical signal even when relatively strong attenuation is performed in the attenuator section. Therefore, in the waveguide type modulator having the optical modulator and the attenuator, good transmission characteristics can be achieved regardless of the strength of the attenuation.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described below in detail based on the embodiments of the invention. FIG. 3 is a top view showing an example of the waveguide type optical modulator of the present invention. The same parts as those of the waveguide type modulator shown in FIG. 1 are denoted by the same reference numerals. The waveguide type optical modulator 21 of the present invention shown in FIG. 3 has basically the same configuration as the conventional waveguide type optical modulator 1 shown in FIG. However, the second signal electrode 9 which constitutes an attenuation electrode in the attenuator section 3
The difference is that a capacitor 15 is provided between the ground and the ground.

The capacitor 15 corresponds to a noise removing means for high frequency noise generated from the control electric signal applied to the second signal electrode 9 of the present invention. That is, if the capacitance of the capacitor 15 is set to a predetermined value, a high frequency component corresponding to this capacitance will flow to the ground through the capacitor 15. Therefore, by appropriately adjusting the capacitance of the capacitor, the high frequency noise component generated from the control electric signal can be effectively removed.

The inventors of the present invention have found that the high frequency noise component can be effectively removed by setting the modulation band of the attenuator unit 3 to 20 MHz or less. Therefore, it is preferable to suppress the modulation band in the attenuator unit 3 to 20 MHz or less by the noise removing unit composed of the capacitor 15. Further, by setting the modulation band of the attenuator unit to 10 MHz or less, the noise component can be removed more effectively.

Since the control electric signal used in the attenuator section 3 is generally around 1 MHz, the capacitance of the capacitor 15 is preferably 150 to 3000 pF, more preferably 300 to 1500 pF. Thereby, the high frequency noise component generated in such a case can be effectively removed.

Although the case where the capacitor is used as the noise removing means has been described with reference to FIG. 3, the invention is not necessarily limited to the capacitor. For example, the dielectric constant between the second signal electrode 9 and the second ground electrode forming the attenuation electrode is increased, in other words, the high frequency noise component is also removed by increasing the electric capacitance between the two electrodes. You can Furthermore, the high frequency noise component can be effectively removed by increasing the length of each of the second signal electrode 9 and the second ground electrode 10 that form the attenuation electrode. These means are based on the technical idea that the effective optical modulation band of the attenuator is intentionally reduced to be lower than the noise frequency so that the electrical noise signal is not optically modulated even if mixed in the attenuator section. It is a thing.

The substrate needs to be made of a material having an electro-optical effect, such as lithium niobate (LiNbO ₃ ), lithium tantalate (LiTa).
Ferroelectric materials such as O ₃ ) and lead lanthanum zirconate titanate (PLZT) can be exemplified. Further, any of X-cut plate, Y-cut plate, Z-cut plate and the like made of these materials can be used. Moreover, the first and second optical waveguides can be formed by a known method such as a Ti diffusion method or a proton exchange method. Furthermore, the first and second
The signal electrode and the first and second ground electrodes of
It can be formed by using a well-known film forming method such as a vacuum deposition method and a sputtering method and a plating method in combination with a material having high conductivity such as u, Ag, and Cu.

In FIG. 3, the polarizer 11 is provided on both the entrance side and the exit side of the waveguide type optical modulator 21. However, in the waveguide type optical modulator of the present invention, the polarizer is not always required. Therefore, the polarizer can be provided only on either the entrance side or the exit side of the waveguide type optical modulator. Further, even when the polarizer is not provided at all, the object of the present invention can be sufficiently achieved.

[0025]

EXAMPLES The present invention will be specifically described below with reference to examples. Example In this example, the waveguide type optical modulator 21 having the configuration shown in FIG. 3 was manufactured. X of lithium niobate as the substrate 4
A cut plate was used. The width of the first optical waveguide 5 and the second optical waveguide 8 is 9 μm due to the thermal diffusion of titanium.
Formed. Further, a buffer layer made of silicon dioxide was formed on the substrate 4 to have a thickness of 1.1 μm. Further, on the buffer layer, the first signal electrode 6 and the first ground electrode 7,
Also, the second signal electrode 9 and the second ground electrode 10 were formed to a thickness of 20 μm by gold vapor deposition and gold plating. Then, at the end of the second signal electrode, a capacitor 150
A pF capacitor 15 was provided to obtain a waveguide type optical modulator 21.

Next, an optical fiber was connected to the input / output port of the waveguide type optical modulator 21 and a light wave having a wavelength λ1 = 1550 nm was made incident, and the transmission characteristic of this light wave was investigated. Then, the frequency and applied voltage of the control electric signal applied to the second signal electrode 9 were controlled so that the output of this light wave was 1 to 10% of the maximum output. Then, when an error measuring device was used to examine the noise component at this time, it was found to be 0.4 to 0.6 dB.

Comparative Example In this comparative example, the waveguide type optical modulator 1 shown in FIG. 1 was manufactured. The waveguide type optical modulator 1 is composed of the capacitor 1
Except for not having No. 5, it was made in the same configuration as the waveguide type optical modulator shown in the example. An optical fiber is connected to the input / output port of this waveguide type optical modulator 1 and the wavelength λ1 = 15.
The transmission characteristics of this light wave were examined by making a light wave of 50 nm incident. Then, under the same conditions as the embodiment, the second
A control electric signal was applied to the signal electrode 9 of. Then, when an error measuring device was used to examine the noise component at this time, it was found to be 10.7 to 14.9 dB.

As is apparent from the above examples and comparative examples, even when the attenuation in the attenuator section is set to a relatively small value of 1 to 10% of the maximum output, the capacitor as the noise removing means is provided according to the present invention. It can be seen that the noise component of the obtained optical signal is small in the example. That is, it can be seen that high-frequency noise generated from the control electric signal is effectively removed, and good transmission characteristics are achieved.

Although the present invention has been described in the embodiments of the invention with reference to specific examples, the present invention is not limited to the above-mentioned contents, and all modifications are possible without departing from the scope of the present invention. It can be modified. For example, in the above embodiment, the attenuator section 3 is composed of a Mach-Zehnder type optical waveguide, but it may be composed of a directional coupler type optical waveguide.

[0030]

As described above, in the waveguide type optical modulator of the present invention, the attenuator section is provided with the noise removing means for removing the high frequency noise generated from the control electric signal applied to the attenuation electrode. ing. Therefore, even when the attenuation in the attenuator unit is reduced to reduce the output component of the optical signal, good transmission characteristics can be obtained.

[Brief description of drawings]

FIG. 1 is a top view showing an example of a conventional waveguide type optical modulator.

FIG. 2 is a diagram showing a relationship between an optical output intensity of an attenuator and an attenuator applied voltage.

FIG. 3 is a top view showing an example of a waveguide type optical modulator of the present invention.

[Explanation of symbols]

1,21 Waveguide type optical modulator 2 Optical modulator 3 Attenuator part 4 substrates 5 First optical waveguide 6 First signal electrode 7 First ground electrode 8 Second optical waveguide 9 Second signal electrode 10 Second ground electrode 11 Polarizer 12, 13 External electrode 15 capacitors

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-9-171163 (JP, A) JP-A-10-261994 (JP, A) International publication 00/010052 (WO, A1) (58) Fields investigated (Int.Cl. ⁷ , DB name) G02F 1/03

Claims (9)

(57) [Claims] 1. A waveguide type attenuator comprising a substrate, an attenuation optical waveguide formed on the substrate, and an attenuation electrode for attenuating an optical signal guided in the optical waveguide, The effective light modulation band by the attenuating electrode is made smaller than the frequency band of the high frequency noise generated from the attenuation control electric signal applied to the attenuating electrode to remove the high frequency noise. A characteristic waveguide attenuator. By wherein increasing the capacity of the attenuation electrodes, the effective optical modulation bandwidth due to the attenuation electrodes, characterized in that reduced than the frequency band of the high-frequency noise, claim 1 In
Waveguide attenuator on board . 3. The waveguide type attenuator according to claim 2, wherein the capacitance of the attenuation electrode is increased by enlarging the attenuation electrode. 4. With respect to the attenuation electrode,
2. A noise removing means for increasing the capacitance of the attenuation electrode and reducing the effective optical modulation band of the attenuation electrode below the frequency band of the high frequency noise. Alternatively, the waveguide type attenuator described in 2. 5. The waveguide type attenuator according to claim 4, wherein the noise removing means includes a capacitor provided between the attenuation electrode and ground. 6. The capacitor has a capacity of 150 to 300.
It is 0 pF, The waveguide type attenuator of Claim 5 characterized by the above-mentioned. 7. The waveguide type attenuator according to claim 1, wherein the optical modulation band by the attenuation electrode is suppressed to 20 MHz or less. 8. A waveguide type attenuator according to any one of claims 1 to 7 is integrated.
Waveguide optical modulator. 9. The waveguide type optical modulator includes a substrate, an optical modulation optical waveguide formed on the substrate, and a modulation electrode for modulating an optical wave guided in the optical modulation optical waveguide. Having a light modulation section, the attenuation optical waveguide and the light modulation optical waveguide are connected in series,
The waveguide type optical modulator according to claim 8, wherein the optical modulator and the waveguide type attenuator are connected in series.