JP2738078B2 - Light modulator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Summary] Regarding an optical modulator, an electro-optic effect processed into a plane is intended to prevent a fluctuation of an operating point by feedback using radiation light in a high-speed driving external light modulation. A light guide formed on the substrate and having a branch optical waveguide between a light incident end and a light output end; a signal electrode provided on one of the branch optical waveguides; A ground electrode provided on the other side of the optical waveguide, an optical fiber for signal light arranged near the light emitting end of the optical waveguide so that signal light is introduced, and a multiplexing of the branch optical waveguide. A monitor light optical fiber arranged so that radiated light radiated from a point is introduced, and a photodetector that receives the radiated light disposed at a light emitting end of the monitor light optical fiber; The output electric signal of the photodetector The optical modulator is configured to include at least a signal processing / control circuit unit that controls an operating point of the optical modulator by changing a DC bias of an input signal voltage applied to the signal electrode according to the change. .

[Industrial applications]

The present invention relates to a configuration of an optical modulator for performing high-speed and high-stability optical modulation.

In recent years, with the progress and development of optical fires and laser light sources,
While various systems and devices using optical technology, including optical communication, have been put into practical use and widely used, demands for the development of advanced technologies have been increasing.

In particular, due to the recent demand for higher speed optical communication systems,
In an optical transmitter for transmitting an optical signal, it is necessary to modulate light at high speed.

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

On the other hand, since demands for large-capacity and long-distance communication are increasing more and more in the future, there is a need to develop a higher-speed and more stable optical modulation system.

[Conventional technology]

As a high-speed light modulation method, an external modulation method for externally modulating a semiconductor laser beam is well known.

In particular, Mach-Zehnder type optical modulators in which a branch optical waveguide is provided on a substrate having an electro-optic effect and driven using signal electrodes, for example, traveling wave signal electrodes, are considered to be promising.

FIG. 5 is a diagram showing a configuration example of a Y-branch optical waveguide Mach-Zehnder type external modulator, showing the most basic configuration. FIG. 3A is a top view (electrodes and waveguide arrangement on the substrate), and FIG. 3B is a cross-sectional view taken along line AA ′ of FIG.

In the figure, 1 is a substrate having an electro-optic effect, 4 is an optical waveguide, and branch optical waveguides 4a and 4b are formed between a light input end 40 and a light output end 41. In this optical waveguide, a metal such as Ti is usually selectively diffused only into the optical waveguide portion on the surface of the substrate, and the refractive index of that portion is made slightly larger than that of the surrounding portion. 2 is a signal electrode, for example, a traveling wave signal electrode, and 3 is a ground electrode. 11 is a buffer layer for reducing the absorption of light to the metal electrode layer on the optical waveguide,
Usually, a thin film such as SiO ₂ is used.

The traveling wave signal electrode 2 and the ground electrode 3 are formed by depositing or plating a metal such as Au on the optical waveguide via the buffer layer 11.

Now, the DC light from the semiconductor laser 9 is incident on the left light incident end.
The optical waveguide 4 enters the optical waveguide 4 from 40, and the branch point 4 of the branch optical waveguides 4a and 4b
When a high-frequency modulation signal voltage is applied to the traveling wave signal electrode 2 while passing therethrough, the light is branched by the electro-optic effect in the branch optical waveguides 4a and 4b provided on the substrate. A phase difference occurs between the two lights. These two lights are merged again at the multiplexing point 43, and the modulated optical signal output is taken out from the light emitting end 41 of the right optical waveguide 4, and the photodetector 14
And is converted to an electric signal.

If a drive voltage is applied so that the phase difference between the two lights in the branch optical waveguides 4a and 4b becomes 0 and π, an optical signal output is obtained as an ON-OFF pulse signal.

Note that _RT is a terminating resistor.

However, as can be seen from the figure, the traveling wave signal electrode 2 and the ground electrode 3 usually have different areas, and
A temperature difference occurs between 4a and 4b, which causes an operating point shift or a so-called DC drift (for example, Jap. J. Appl. Phys., Vol. 20, No. 4, pp733). ~
737, 1981).

FIG. 6 is a diagram for explaining an operating point shift. FIG. 6A shows a modulation characteristic, and FIG. 6B shows an optical output pulse characteristic. The solid line in FIG. 7A shows normal characteristics, and the broken line shows the case where the operating point shifts. Correspondingly, a waveform in which the peak falls and the bottom rises as shown by the broken line from the clean output pulse waveform of the solid line in FIG.

FIG. 7 is a diagram for explaining DC drift. For example, when the most frequently used LiNbO ₃ is used as the substrate 1, a short-term DC
Two types, drift and long-term DC drift, have been reported, for example, in the literature and others. FIG. 4A shows the relationship between the drive voltage and time. When a constant DC voltage Vπ is applied, the light output level should be 0 as shown by the solid line in FIG. Is a phenomenon that gradually increases with the passage of time, that is, the operating point shifts.

Therefore, in order to prevent the performance of the optical modulator from deteriorating due to such shift of the operating point, monitor light is extracted from the output signal light and fed back to the input electric signal to stabilize the operating point. Has been proposed.

FIG. 8 is a diagram showing a configuration example of a conventional stabilized external modulator. In the figure, 30 is a fiber coupler, 8 'is a signal
The control circuit unit 12 'is a light detector.

The same parts as those described in the drawings are denoted by the same reference numerals, and the description of the same parts will be omitted.

That is, after a modulated optical signal output is introduced and transmitted to, for example, a single mode optical fiber 5 from a light emitting end 41 on the right side of the optical waveguide 4, the light is received by a photodetector 14, converted into an electric signal, and transmitted. Performs signal reception. During this time, a part of the signal light is branched by the fiber coupler 30 and introduced into, for example, a single-mode optical fiber 5 ', converted into an electric signal by the photodetector 12', and converted into an electric signal by the signal processing / control circuit 8 '. The shift of the operating point is detected and fed back to the input signal power supply 13 to adjust the DC bias so that the operating point is always kept at the correct operating point.

When two optical fibers are brought close to each other and coupled in parallel, a part (for example, about 1/10) of the light of one optical fiber is transferred to the other optical fiber and transmitted. It is constituted in.

[Problems to be solved by the invention]

However, in the optical modulator having such a configuration, a part of the signal light introduced into the optical fiber from the optical waveguide is branched as monitor light by the fiber coupler. Therefore, there is a serious problem that the optical signal power to be transmitted is reduced by the amount corresponding to the branch, and the transmission distance of the optical fiber is shortened by the amount corresponding thereto.

[Means for solving the problem]

The above-mentioned problem is solved by a substrate 1 having an electro-optic effect processed into a plane, and an optical waveguide 4 formed on the substrate 1 and having branching optical waveguides 4a and 4b between a light incident end 40 and a light emitting end 41. And the signal electrode 2 provided on the branch optical waveguide 4a.
A ground electrode 3 provided on the branch optical waveguide 4b;
A signal light optical fiber 5 arranged near the light emitting end 41 of the optical waveguide 4 so that signal light is introduced, and radiation emitted from a multiplexing point 43 of the branch optical waveguides 4a and 4b. A monitor light optical fiber 6 arranged so that light is introduced, a light detector 12 for receiving the radiated light provided at a light emitting end 61 of the monitor light optical fiber 6, and the light detector A signal processing / control circuit section 8 for controlling an operating point of the optical modulator by changing a DC bias of an input signal voltage applied to the signal electrode 2 in accordance with a change in an output electric signal of the optical modulator 12; The problem can be solved by configuring an optical modulator provided.

[Action]

1A and 1B are diagrams for explaining radiation light at a multiplexing point. FIG. 1A is a top view, FIG. 1B is a view taken along the line AA ′, and FIG. 1C is signal light and radiation light. This shows the relationship between the characteristics. That is, when the signal voltage is 0, the optical output is emitted from the light emitting end 41 of the 100% optical waveguide 4, and the branched optical waveguides 4a and 4b
When the light has a phase difference of Vπ or -Vπ giving λ / 2, the light output from the light emitting end 41 of the optical waveguide 4 becomes zero.

Other light, that is, light not emitted from the light emitting end 41 leaks out of the optical waveguide 4 as a matter of course, resulting in a loss. This leaked light is so-called radiation light 10 and is emitted from the multiplexing point 43 of the branch optical waveguides 4a and 4b.

The emitted light 10 is radiated from the multiplexing point 43 as a light beam spread into the substrate 1 slightly below the ゝ as shown in FIG. Then, the sum and phase of the optical power are shown in FIG.
As shown by the broken line, the signal light is exactly complementary to the signal light shown by the solid line.

As can be understood from the above description, according to the configuration of the present invention, when the operating point is shifted, the radiated light radiated from the multiplexing point 43 of the branch optical waveguides 4a and 4b is extracted as monitor light, and the light detector 12 And the signal processing / control circuit section 8 detects the deviation of the operating point and feeds it back to the input signal power supply 13 so that the DC bias is adjusted and always maintained at the correct operating point, so that there is no effect on the optical power of the signal light. Therefore, there is no problem that shortens the transmission distance of the optical fiber.

〔Example〕

FIG. 2 is a view for explaining the essential points of the principle of the present invention. The signal light emitted from the optical fiber 4 is introduced into the optical fiber for signal light 5 and emitted from the multiplexing point 43 of the branch optical waveguides 4a and 4b. The radiated light is introduced into the monitor light optical fiber 6, converted into an electric signal, and the DC bias of the input signal power supply is adjusted to feed back so as to stabilize the operating point.

FIG. 3 is a diagram showing the configuration of the embodiment of the present invention. Substrate 1
The surface of a Z plate of LiNbO ₃ having a size of 30 mm × 2 mm and a thickness of 1 mm was mirror-polished and used.

Ti is vacuum-deposited on this substrate to a thickness of about 100 nm, and is processed by a usual photoetching method so that Ti remains in a portion corresponding to the optical waveguide 4 including the branch optical waveguides 4a and 4b. The substrate was heated in oxygen at 10 ° C. for 10 hours to thermally diffuse Ti into LiNbO ₃ to form an optical waveguide 4 having a depth of about 5 μm.

The length of the branch optical waveguide is 20mm, and the width of the optical waveguide is 7μ.
m. The interval between the branch optical waveguides 4a and 4b was set to about 15 μm, and the angle of the branch portion was formed to 1 °.

Next, SiO ₂ was formed as a buffer layer to a thickness of 500 nm by a sputtering method.

The signal electrode (traveling wave signal electrode) 2 and the ground electrode 3 are Ti
After depositing an -Au alloy film, pattern etching is performed in a predetermined electrode shape so as to overlap the optical waveguide 4, and further,
Au having a thickness of 8 μm was formed thereon by plating.
The terminating resistance _RT was adjusted to 50Ω in accordance with the characteristic impedance of the traveling wave signal electrode 2 and the ground electrode 3.

A single mode optical fiber is used as the signal light optical fiber 5, the signal light is introduced from the light emitting end 41 of the optical waveguide 4, and a multimode optical fiber is used as the monitor light optical fiber 6. Arranged so that the divergent light 10 radiated from the multiplexing point 43 of the branch optical waveguides 4a and 4b can be introduced.

In order to connect the signal light optical fiber 5 to the light emitting end 41 and connect the monitor light optical fiber 6 to the emitted light beam emitting portion, the two optical fibers 5 and 6 are fitted and fixed at predetermined positions in the holder 7. Then, it is connected and fixed by an ordinary optical fiber connection method.

The radiated light for monitoring emitted from the light emitting end 61 of the optical fiber for monitor light 6 is received by the light detector 12 and converted into an electric signal, and then input to the signal processing / control circuit unit 8 where the light is detected. The DC bias of the input signal power supply 13 applied to the signal electrode (traveling wave signal electrode) 2 is changed in accordance with the change of the output electric signal of 12, and the operating point of the optical modulator is controlled.

The same parts as those described in the above drawings are denoted by the same reference numerals, and the description of the same parts will be omitted.

FIG. 4 is a view showing a main part of the embodiment of the present invention.
Is a front view, and (b) is a side view. Holder 7
For example, a single-mode optical fiber having a diameter of 9 μmφ for introducing signal light is inserted into a hole preliminarily formed in the center of a bearing-shaped ruby bead, and a predetermined distance from the center, for example, 130 to 150 μm. For example, after inserting a multimode optical fiber having a diameter sufficient for the core 60 of 100 μmφ into a hole formed at a position where the radiation light 10 is introduced at a distance of the center of the multi-mode optical fiber, for example, Adhesively fix with ultraviolet curing epoxy resin adhesive.

In the above embodiment, the optical fiber 5 for signal light and the optical fiber 6 for monitor light are used by being fixed to the same holder 7, but it is a matter of course that light may be separately introduced.

Further, in the above embodiment, the radiation light was extracted from one side of the optical waveguide 4 by one monitor optical fiber 6, but two monitor optical fibers were used from both sides of the optical waveguide 4. The received light intensity of the emitted light may be increased to increase the sensitivity.

The embodiments described above are merely examples, and it is needless to say that materials and structures to be used can be suitably used or a combination thereof can be used as long as the purpose of the present invention is met.

〔The invention's effect〕

As described above, according to the configuration of the present invention, when the operating point shifts, the radiation emitted from the multiplexing point 43 of the branch optical waveguides 4a and 4b is extracted as monitor light, and the light detector
12 and the signal processing / control circuit unit 8 detect a shift in the operating point and feed back to the input signal power supply 13 to adjust the DC bias and always maintain the correct operating point, so that there is no effect on the optical power of the signal light. Therefore, it does not shorten the transmission distance of the optical fiber and greatly contributes to the improvement of the performance and reliability of the optical modulator for high-speed and long-distance optical communication.

[Brief description of the drawings]

FIG. 1 is a diagram for explaining radiation at a multiplexing point, FIG. 2 is a diagram for explaining essential points of the principle of the present invention, FIG. 3 is a diagram showing a configuration of an embodiment of the present invention, and FIG. FIG. 5 is a diagram illustrating a main part of the embodiment, FIG. 5 is a diagram illustrating a configuration example of a Y-branch optical waveguide Mach-Zehnder type external modulator, FIG. 6 is a diagram illustrating operating point shift, and FIG. 7 is a diagram illustrating DC drift. FIG. 8 is a diagram showing a configuration example of a conventional stabilized external modulator. In the figure, 1 is a substrate, 2 is a signal electrode, 3 is a ground electrode, 4 is an optical waveguide, 4a and 4b are branch optical waveguides, 5 is an optical fiber for signal light, 6 is an optical fiber for monitor light, 7 is a holder, 8 is a signal processing / control circuit unit, 9 is a semiconductor laser, 10 is emitted light, 11 is a buffer layer, 12 is a photodetector, 13 is an input signal power supply, 40 is a light input end, 41 is a light output end, and 42 is a light output end. The branch point 43 is a combining point.

Claims (1)

(57) [Claims] 1. A substrate (1) having an electro-optic effect processed into a plane, and a branch optical waveguide formed on the substrate (1) and disposed between a light incident end (40) and a light output end (41). An optical waveguide (4) having (4a) and (4b); and a signal electrode (2) provided on the branch optical waveguide (4a).
And a ground electrode (3) provided on the branch optical waveguide (4b).
An optical fiber for signal light (5) arranged near the light emitting end (41) of the optical waveguide (4) so as to introduce signal light; and the branch optical waveguides (4a) and ( A monitor light optical fiber (6) arranged so that radiation emitted from the multiplexing point (43) of 4b) is introduced; and a light emitting end (61) of the monitor light optical fiber (6). A photodetector (12) for receiving the emitted light, the input signal voltage being applied to the signal electrode (2) according to a change in an output electric signal of the photodetector (12). An optical modulator comprising at least a signal processing / control circuit section (8) for controlling an operating point of an optical modulator by changing a DC bias.