JPH0429113A - Optical modulator - Google Patents

Abstract

PURPOSE:To control variation in operation point without generating a DC drift by controlling a current supplied to an electrode for heating and causing variation in refractive index due to a temperature difference between branch optical waveguides, and controlling the operation point. CONSTITUTION:This optical modulator is equipped with an optical waveguide 2 which has the branch optical waveguides 2a and 2b formed on a substrate 1 with electrooptic effect, electrodes 3a and 3b for modulation provided on the branch optical waveguides 2a and 2b, electrodes 4a and 4b for heating provided on the branch optical waveguides 2a and 2b, and a heating current application control means 5 which supplies the current to at least one of the electrodes 4a and 4b for heating. Then the current supplied to the electrodes 4a or 4b for heating is controlled to cause the variation in refractive index between the branch optical waveguides 2a and 2b owing to the temperature difference, thereby controlling the operation point. Consequently, the operation point can be controlled without generating any DC drift.

Description

[Detailed Description of the Invention] [Summary] Regarding optical modulators, the purpose of this invention is to realize a highly stable optical modulator by controlling fluctuations in the operating point of a high-speed driving Mannha-Zender type optical modulator. an optical waveguide having a branched optical waveguide formed on a substrate having a branched optical waveguide, a modulation electrode provided on each of the branched optical waveguides, and a heating electrode provided on each of the branched optical waveguides; heating current application control means for passing a current through at least one of the heating electrodes, controlling the current flowing through the heating electrode to cause a change in refractive index due to a temperature difference between the branched optical waveguides, and adjusting the operating point. A light modulator is configured to provide control.

[Industrial application field]

The present invention relates to the configuration of an optical modulator for performing high-speed and highly stable optical modulation.

In recent years, with the progress and development of optical fibers and laser light sources, various systems and devices that apply light wave technology, including optical communication, have been put into practical use and widely used. The demand has become stronger.

In particular, with the recent demand for higher speed optical communication systems, it has become necessary to modulate light at high speed in optical transmitters that transmit optical signals.

For example, in low-speed optical communication systems up to about 1.6 Gbps, a method of directly modulating a laser diode (LD) has been used. , there are limits to high speed and long distance communication due to the chirping phenomenon.

On the other hand, as the demand for large-capacity and long-distance communication will become stronger in the future, there is a need for the development of faster and more stable optical modulation systems.

[Conventional technology]

As a high-speed optical modulation method, an external modulation method in which semiconductor laser light is modulated externally is well known.

In particular, a Matsuhatsu Enda type external modulator, which is driven using a signal electrode, such as a traveling wave signal electrode, is considered to be promising, in which a branched optical waveguide is provided on a substrate having an electro-optic effect.

Figure 4 is a diagram showing an example of a Matsuhatsu Enda type external modulator.
This shows the most basic configuration.

Figure (a) is a top view (electrode and waveguide arrangement on the substrate), and figure (b) is a sectional view taken along line A-A' in figure (a).

In the figure, 1 is a substrate having an electro-optic effect, 2 is an optical waveguide, and a branched optical waveguide 2a is provided between the light input end 20 and the light output end 21.
and 2b are formed. This optical waveguide is usually made by selectively diffusing metal such as Ti on the surface of a substrate only to the optical waveguide portion, so that the refractive index of that portion is slightly larger than that of the surrounding portions.

3a and 3b are modulation electrodes, and 9 is a buffer layer for reducing absorption of light into the metal electrode layer on the optical waveguide.
A thin film of iO□ is used.

The modulation electrodes 3a and 3b are formed on the optical waveguide via the buffer layer 9 by vapor deposition or plating of a metal such as Au.

Now, the DC light from the semiconductor laser 7 is directed to the left light input end 2.
0 into the optical waveguide 2 and branched optical waveguide 2a.

The branched optical waveguide 2a is divided into two at the branch point 22 of the optical waveguide 2b.
, 2b, when a modulation signal voltage is applied from the modulation power source 6 to the modulation electrodes 3a, 3b, both lights are split by the electro-optic effect in the branch optical waveguides 2a, 2b provided on the substrate. A phase difference occurs. The configuration is such that these two lights are combined again at a combining point 23, a modulated optical signal output is taken out from the light output end 21 of the right optical waveguide 2, and the light is received by the photodetector 8 and converted into an electrical signal. has been done.

The phase difference between the two lights in the branched optical waveguides 2a and 2b is O
If a driving voltage is applied so that π and π are applied, an optical signal output is obtained as an ON-OFF pulse signal. In addition, R
A is a terminating resistor.

However, in reality, due to manufacturing variations and various other causes, the modulator operating point may deviate from the design value, or
It may shift during use.

The fifth figure is a diagram explaining the operating point shift, and the figure (a)
is the modulation characteristic, and (b) is the optical output pulse characteristic.

The solid line (■) in FIG. 6(a) is, for example, a normal design value characteristic, and the broken line (■) is a case where the operating point has shifted. Correspondingly, the clean output pulse waveform shown by the solid line (■) in FIG. 2 (B) changes to a waveform in which the peak is lowered and the bottom is raised, as shown by the broken line (2), that is, the extinction ratio is degraded.

Therefore, conventionally, in order to return the broken line ■ in the same figure (a) to the original normal characteristic, the solid line ■, a direct current (DC) bias voltage v
The operating point is controlled by applying I+.

[Problems to be Solved by the Invention Recently, substrates having an electro-optic effect, such as LiNb
It has been reported that when an optical modulator is applied with a DC bias when O3 is used as a substrate, the operating point gradually shifts and the extinction ratio deteriorates, a phenomenon known as DC drift. (e.g. Jap, J, Appl, P
hys,,si o1.20. No, 4. pp733-73
7, 1981).

Therefore, as mentioned above (direct current (DC) voltage V.

When trying to control the operating point by applying
There is a serious problem that C drift occurs, and a solution to this problem has been sought.

[Means to solve the problem]

The above problem is solved by an optical waveguide 2 having branched optical waveguides 2a and 2b formed on a substrate 1 having an electro-optic effect, and a modulation electrode 3 provided on the branched optical waveguides 2a and 2b.
a, 3b, heating electrodes 4a, 4b provided on the branched optical waveguides 2a, 2b, and heating electrodes 4a, 4b.
heating current application control means 5 for passing a current through at least one of the
an optical modulator configured to control the current flowing through the heating electrodes 4a, 4b to cause a change in refractive index due to a temperature difference between the branched optical waveguides 2a, 2b to control the operating point. This can be solved by

C effect] The optical modulator of the present invention uses direct current (D
C) A temperature difference is created between the two branched optical waveguides 2a and 2b without applying a bias voltage, which changes the phase difference of light based on the change in the refractive index, thereby realizing a phenomenon in which the operating point changes. It is configured to do this. Therefore, it is possible to control the operating point without causing DC drift.

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

Substrate 1 has a Li of size 50 mm x 2 mm and thickness of 1 mm.
The surface of the NbO3 Z plate was mirror polished and used.

On this substrate, Ti was vacuum-deposited to a thickness of about 1100 nm, and treated with a normal photoetching method so that Ti remained in the portion corresponding to the optical waveguide 2 including the branched optical waveguides 2a and 2b. It was heated in oxygen for 10 hours to thermally diffuse Ti into LiNbO3 to form an optical waveguide 2 with a depth of about 5 μm.

The length of the branched optical waveguide part is 40 mm, and the width of the optical waveguide is 7 mm.
It was adjusted so that it was μm. Branch optical waveguides 2a and 2
The interval b was approximately 15 μm, and the angle of the branch portion was 1°.

Next, SiO□ was formed as a buffer layer to a thickness of 500 nm by sputtering.

The modulation electrodes 3a, 3b are formed by depositing a Ti-Au alloy film on the branch optical waveguides 2a, 2b by 20 mm.
A predetermined electrode shape as shown in the figure was pattern-etched so as to overlap over the length of the electrode, and further, 8 μm thick Au was deposited thereon by plating. The terminating resistance RT was adjusted to 50Ω in accordance with the characteristic impedance of the modulation electrodes 3a and 3b.

Note that one of the modulation electrodes was used as a traveling wave signal electrode, and the other was used as a ground electrode.

4a and 4b are heating electrodes, each of which is a thin film heating element made of a resistor formed over a length of 10 to 15 mm in the remaining portions of the branched optical waveguides 2a and 2b where the modulation electrodes 3a and 3b are not provided. and are respectively connected to heating current application control means 5 (5a, 5b, 5c). As the thin film heating element forming the heating electrodes 4a and 4b, for example, a 300 nm thick Ni-Cr film is deposited by electron beam evaporation and formed into a predetermined pattern shape as shown in the figure by a known photoetching technique. .

The heating current application control means 5 of this embodiment includes the heating electrode 4a
, 4b are connected to each other.

5c and a heating current control section 5a, and the diodes 5b and 5c are connected with their polarities reversed as shown. The heating current control section 5a has a built-in variable DC power supply, and the current is controlled in response to an operating point shift detection signal (not shown).

Now, for example, when the diode 5b is ON, a current flows through the heating electrode 4a and the temperature near the branched optical waveguide 2a below it increases, but since the diode 5c is OFF, no current flows through the heating electrode 4c. Therefore, the temperature near the branched optical waveguide 2b below does not rise, and a phase difference occurs between the lights propagating through the branched optical waveguides 2a and 2b, corresponding to the change in refractive index due to the temperature difference, causing a DC bias. The operating point can be shifted without applying any voltage. and,
When the current flows in the opposite direction, a phase difference also occurs in the opposite direction, and the operating point can be shifted in the opposite direction.

FIG. 2 is a diagram illustrating the operating point control of the present invention, with the vertical axis representing the optical output and the horizontal axis representing the voltage for modulation.

Now, as shown by the solid line in ■, <, -Vπ/2. +Vπ/
For example, if the refractive index change due to temperature rise is positive and the operating point shifts to the right as shown by the broken line ■, the diode 5b is turned on, that is, by passing a current through the heating electrode 4a, the operating point is shifted to the negative side by VZ+, and returned to the predetermined modulation operating characteristic curve (■).
On the other hand, when the operating point shifts to the left as shown by the broken line ■, the diode 5C is turned on, that is, current is passed through the heating electrode 4b, and the operating point is shifted by V31 to the flat side. If the control is performed to return to the predetermined modulation operating characteristic curve ■, the predetermined operating characteristic curve will automatically always be maintained.
can be maintained.

FIG. 3 is a diagram showing another embodiment of the present invention.

In this embodiment, the heating current application control means 5 is configured to apply different currents to the heating electrodes 4a and 4b, for example, a constant voltage power source 5d.

It is composed of a voltage dividing resistor 5e and a heating current control section 5a1.

Note that the same reference numerals are given to the same parts as those explained in the above drawings, and the explanation of the same parts will be omitted.

In this case, the heating current control unit 5a1 receives an operating point shift detection signal (not shown), moves the sliding contact of the voltage dividing resistor 5e, and causes current to flow through both the heating electrodes 4a and 4b at the same time. The modulator operating point is controlled by changing the phase difference between the lights propagating through both branch leading wavepaths.

In addition, in the above embodiment, Ni is used as the heating electrodes 4a and 4b.
-Cr thin film heating element was used, but other metal thin films,
Alternatively, non-metals may be used. In addition, the heating electrode 4
a, 4b are not necessarily provided directly above the branch optical waveguides 2a, 2b, but may be formed near the branch optical waveguides 2a, 2b.
It goes without saying that the same effect can be obtained even if the refractive index of the portions 2b and 2b is changed.

The embodiments described above are merely examples, and it goes without saying that preferred materials and configurations, or combinations thereof, may be used as long as they comply with the spirit of the present invention.

〔Effect of the invention〕

As explained above, according to the configuration of the present invention, in order to control the operating point of the optical modulator, there is no need to apply a direct current (DC) bias voltage between the two branch leading waveguides 2a and 2b. The structure is such that a temperature difference is generated between the two, and the optical phase difference based on the change in the refractive index changes, thereby realizing a phenomenon in which the operating point shifts.

Therefore, it is possible to control the operating point without causing DC drift, which greatly contributes to improving the performance and reliability of optical modulators for high-speed and long-distance optical communications.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an embodiment of the present invention, FIG. 2 is a diagram explaining operating point control of the present invention, FIG. 3 is a diagram showing another embodiment of the present invention, and the fourth country is a Matsuhatsu Enda type external A diagram showing an example of a modulator, and FIG. 5 is a diagram explaining an operating point shift. In the figure, 1 is a substrate, 2 is an optical waveguide, 2a and 2b are branched optical waveguides, 3a and 3b are modulation electrodes, 4a and 4b are heating electrodes, and 5 (5a, 5a', 5b, 5c, 5d, 5e) ) is a heating current application control means, and 6 is a modulation power source. Optical output Figure 1 shows the actual sales of the present invention 111 Figure 3

Claims (1)

[Claims] An optical waveguide (2) having branched optical waveguides (2a, 2b) formed on a substrate (1) having an electro-optic effect, and an optical waveguide (2) provided on the branched optical waveguides (2a, 2b). A current is applied to at least one of the modulation electrodes (3a, 3b), the heating electrodes (4a, 4b) provided on the branched optical waveguides (2a, 2b), and the heating electrodes (4a, 4b). and a heating current application control means (5) for controlling the current flowing through the heating electrodes (4a, 4b) to cause a change in refractive index due to a temperature difference between the branched optical waveguides (2a, 2b). An optical modulator characterized by controlling the operating point.