JPH02166425A - Optical modulating equipment - Google Patents

Abstract

PURPOSE:To obtain the feedback system of a simple constitution which prevents the quenching ratio of a light pulse signal from degrading at a low cost by fetching one of two light outputs which are in mutually complementary relation and processed by light intensity modulation as an optical pulse signal. CONSTITUTION:The light outputs P1 and P2 which are in the mutually complementary relation and processed by the light intensity modulation are fetched partially by a coupler 11 through couplers 21a and 22a and inputted to a 1st power meter 23 and a 2nd power meter 24, and their time mean power values P1 and P2 are measured and sent to a control circuit 25. Then the control circuit 25 performs feedback control to apply a bias to a sent signal V so that the ratio of P1 and P2 is a value determined by a mark rate. Then one of the light outputs in the complementary relation is fetched as the light pulse signal and used for an optical communication. Consequently, the feedback system which prevents the quenching ratio from degrading is obtained by the low-frequency simple circuit at the low cost.

Description

[Detailed Description of the Invention] (Table of Contents) Overview Industrial Field of Application Prior Art (Figures 7 to 12) Problems to be Solved by the Invention Examples of Means and Actions for Solving the Problems (1) Principle of the Invention (Figures 1 to 3) (2) - First embodiment of the present invention (Figure 4) (3) Second embodiment of the present invention (
(Fig. 5) (4) Third embodiment of the present invention (Fig. 6) Effects of the invention [Summary] Regarding an optical modulation device, a feedback system for preventing extinction ratio deterioration of an optical pulse signal is provided with a simple configuration and The aim is to provide an optical modulation device that can be realized at low cost. Light with a constant intensity is guided into a waveguide, and the light passing through the waveguide is modulated in intensity using a transmission signal to generate an optical pulse signal. In an optical modulation device, a complementary output generating means generates two light intensity-modulated optical outputs that are complementary to each other, a power detecting means detects the time average power of these two optical outputs, and two and a control means for feedback controlling the operating point of the optical intensity modulation so that the ratio of the time average power of the optical output becomes a value corresponding to the value of the mark rate of the transmission signal. The configuration is such that one of the two modulated optical outputs is extracted as an optical pulse signal.

[Industrial application field]

The present invention relates to an optical modulation device, and particularly to an optical modulation device applied to long-distance optical fiber transmission.

In recent years, as the amount of information has increased, the bit rate of signals transmitted through optical fibers has improved. Conventionally, optical signals were generated by turning on and off semiconductor lasers, but
When the bit rate reaches the Gbps order, long-distance transmission becomes impossible due to wavelength chirping.

For this reason, instead of the above-mentioned direct modulation method, attempts have been made to perform optical intensity modulation using an external modulation method to generate an optical pulse signal. To actually use such an external modulator, it is necessary to apply feedback to prevent the operating point from changing.

[Conventional technology]

As a conventional optical modulator, particularly an external modulator, the one shown in FIG. 7, for example, is known.

In the figure, ■ is a substrate, for example, and the substrate 1 is provided with a waveguide 2 that guides light. In this case, the light used is oscillation light from a semiconductor laser, and the wavelength is suitable for optical communication. Further, as the waveguide 2, for example, one made by thermally diffusing Ti into lithium niobate is used.

The waveguide 2 branches into two parts in the middle and then merges again, and an electrode 3.4 is disposed at this branch part. Electrode 3.4
A transmission signal (1) for modulation is supplied from one end of the modulator, and a terminating resistor 5 is connected to the other end of the modulator, which is of the so-called traveling wave modulator. The terminating resistor 5 is for matching traveling waves. Furthermore, the transmission signal (2) modulates laser light with various information data for optical communication.

In such an external modulator, an optical output with an intensity as shown in Fig. 8 (Ca) is applied to the input side (input light side) of the waveguide 2,
When the transmission signal V is supplied to the electrode 3.4, the refractive index of the waveguide 2 changes depending on the magnitude and phase of the transmission signal (2).

At this time, the intensity P of the optical output has a relationship as shown in FIG. 9 with respect to the transmission signal (2). In the figure, +Vs and -Vs are positive and negative switching voltages. This is because out of the waveguide 2 that is branched into two, one has a high refractive index (and the other has a low refractive index), so a phase difference occurs between the light that propagates through the waveguide 2 and is split into two. , when they merge, a light intensity corresponding to their phase difference is obtained. Therefore, the intensity P of the light output passing through the branch part of the waveguide 2 by the electrode 3.4 changes depending on the value of the transmission signal Figure 8 (
It is extracted as an optical output (optical modulation signal) from the output side of the waveguide 2 with a size as shown in b), and is sent to an optical communication fiber to perform optical communication (particularly, transmission).

Note that when actually used as an external modulator, it is desirable to have a square wave optical output, so the voltages corresponding to the maximum and minimum values of the optical output as shown in FIG. 8(b) are used as the transmission signal V. Add to electrode 3.4 as For example, the 10th
As shown in figure (a), the signal “1” is set to 0, and the signal “0”
When a binary level transmission signal V such as -Vs is added, optical pulses corresponding to O and -Vs as shown in FIG. 2(b) are generated, and this becomes an optical signal.

[Problem to be solved by the invention]

However, in such a conventional external modulator, the modulation characteristics (i.e., applied voltage vs. optical output characteristics) shown in FIG. 9 change to normal as shown by the solid line in FIG. 11 due to changes in the temperature of the entire board. The operating point shifts from this state. When a transmission signal voltage as shown in Figure 10(a) is applied in this shifted state, the ratio (extinction ratio) between the ``1'' level and the ``0'' level deteriorates as shown in Figure 12. However, this has a negative effect on the optical transmission characteristics, because the light intensity does not completely become 0 when it is at the "0" level, and even when it is at the "1" level, the light intensity is lower than normal, and eventually " 1” and “0”
This is because the level ratio of ” becomes worse.

In order to eliminate this problem, it is sufficient to perform feedback such that a predetermined DC component is added to the transmission signal by the shift of the operating point, and in this way the extinction ratio becomes the same as in the normal state.

However, in order to perform this feedback, a system that constantly monitors the extinction ratio is required, but it must be monitored in real time at the same speed as the signal transmission speed.
At transmission speeds on the order of Gdps, there is a problem in that practical control is extremely difficult and it is difficult to realize a system.

Furthermore, even if such a feedback system were to be realized, it would only be very complex and expensive.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide an optical modulation device that can realize a feedback system for preventing extinction ratio deterioration of an optical pulse signal with a simple configuration and at low cost.

[Means to solve the problem]

In order to achieve the above object, an optical modulation device according to the present invention guides light of a constant intensity into a waveguide, performs optical intensity modulation on the light passing through the waveguide using a transmission signal, and generates an optical pulse signal. In the apparatus, complementary output generating means generates two light intensity modulated optical outputs that are complementary to each other; power detecting means detects the time average power of these two optical outputs; and a control means for feedback controlling the operating point of the light intensity modulation so that the ratio of time average power becomes a value corresponding to the value of the mark rate of the transmission signal. One of the two optical outputs is extracted as an optical pulse signal.

[Effect]

In the present invention, two light intensity modulated optical outputs are generated that are complementary to each other, the time average power of each optical output is detected, and the ratio thereof is determined as a value corresponding to the mark rate value of the transmission signal. The operating point of light intensity modulation is feedback-controlled so that Then, one of the complementary optical outputs is extracted as an optical pulse signal and provided for optical communication.

Therefore, a high-speed response detector and associated high-frequency circuit are not required when detecting time-averaged power.
A feedback system for preventing extinction ratio deterioration can be realized with a simple low-frequency circuit and at low cost.

〔Example〕

Hereinafter, the present invention will be explained based on the drawings.

First, the present invention will be described in detail. FIG. 1 is a diagram illustrating the principle of the present invention, and in this figure, the same components as in the conventional example are given the same numbers and redundant explanation will be omitted. 10 is a waveguide, and the waveguide 10 branches into two in the middle, and is separated into a first waveguide 10a and a second waveguide 10b. The first waveguide 10a and the second waveguide 10b are each provided with an electrode 3.4 as in the conventional example, but on the right side of the figure, the first waveguide 10a and the second waveguide 10b are each provided with an electrode 3.4. The wave paths 10b are brought close together to form a 3 dB coupler (complementary output generation means) 11, and then separated again so that two optical outputs P and P2 are taken out from each end.

3. (j-B coupler 11 has optical output PI and optical output P
As shown in FIG. 2, it has a function of shifting the phases of the two signals so that they are complementary to each other.

In the above configuration, when actually used as an external modulator, the operating point is the voltage at which the optical outputs P, , P are equal (corresponding to the point ■ = 0 in Figure 2, for example). , centering on this point, the operating point of the transmission signal ■ shifts from point A to point A as shown in Figure 3 due to the influence of temperature (it is more correlated to ΔV than the level of signal "1" of For example, the optical output PI is the optical pulse signal used for optical communication.In this case, the optical output P l + Pi
The ratio of the respective time average powers P, , P, is uniquely determined once the mark rate of the transmission signal is determined.

For example, for the reasons mentioned above, the mark rate is set to the normal value. If, then the time average power P,,P.

The ratio (hereinafter referred to as average power ratio) is 1.

However, the level becomes higher than the level of the normal signal "0" by a value correlated to ΔV, and the extinction ratio deteriorates, and as a result, the average power ratio is not 1 but changes.

Therefore, if we apply feedback so that the average power ratio becomes the same as the original ratio (=1), the original operating point (point),
That is, it operates at the point where V=O and P+=Pz. In this case, since the detection of the time-average parameter "y-P, ,P" is just an average value, there is no need for a high-speed response even at a high bit rate; Equivalent to a detector (power detection means):
(e.g., an inexpensive power meter) with optical output P l +P
It may be placed along the periphery of 2. In addition, the circuits and feedback circuits (corresponding to control means) associated with the second detection are not designed for high frequencies, but are simple low frequency circuits.
Therefore, a feedback system for preventing extinction ratio deterioration can be realized with a simple configuration and at low cost.

L Implementation (2) Next, a first embodiment of the present invention based on the above principle will be described with reference to FIG. In the explanation of this embodiment, since there are many parts in common with the first embodiment, the same components are given the same reference numerals to avoid duplication of explanation.

In FIG. 4, a first waveguide 10a and a second waveguide 1
0b are close to each other at the 3 dB coupler 11, but are separated again after that, and the third waveguide 21 and the fourth waveguide 22, whose ends are close to each other, are located in the separated portions.
is provided. Third waveguide 21 and fourth waveguide 2
2 has a coupler (directional coupler) 2 at one end.
1a and 22a, and these couplers 21a and 2
2a is designed to extract only the minimum amount of faint light necessary for feedback control so as to increase the optical power transmitted from the first waveguide 10a and the second waveguide 10b. A first power meter 23 is provided at the other end of the third waveguide 21, and a second power meter 24 is similarly provided at the other end of the fourth waveguide 22. These first power meter 23 and second power meter 24 are connected to the first waveguide 10a and the second waveguide 10, respectively.
The time average power PI of the optical output passing through b.

P8 is measured and output to the control circuit 25. In this case, low-cost detectors with slow response are sufficient as the first power meter 23 and the second power meter 24.

On the other hand, the optical output passing through the first waveguide 10a is guided as an optical pulse signal to the optical fiber 26 for optical communication, and the terminal end of the second waveguide 10b is not connected to anything and is an open end. The control circuit 25 calculates the time average power P, , P, of each destination output.
generates a control signal for feedback controlling the operating point of the optical intensity modulation (i.e., the operating point of the transmission signal) so that the ratio of
It is supplied between the electrodes 3 and 4 via a high frequency blocking coil 27. Therefore, the electrode 4 is grounded, a control signal from the control circuit 25 is applied to the electrode 3 through the high frequency blocking coil 27, and the control current 1b flows. Further, a transmission signal V is supplied to the electrode 3 via a capacitor 28, and the transmission signal V is applied between the electrodes 3 and 4. the third waveguide 21;
The fourth waveguide 22, the first power meter 23, and the second power meter 24 constitute a power detection means 29, and the control circuit 25 and the high frequency blocking coil 27 constitute a control means 30.

In the above configuration, each optical output P is complementary to each other and is intensity modulated by the 3 dB candela 11.
*, a part of Px is taken out through the couplers 21a and 22a and inputted to the first power meter 23 and the second power meter 24, respectively, and the time average power P+,
Pg is measured and sent to the control circuit 25. Then, the control circuit 25 sets the ratio of P and P2 to a value determined by the mark rate (1 in this embodiment), that is, P,=p,
Feedback control is performed so that a bias is applied to the transmission signal V so that . Therefore, as described in the principle of the present invention, even if the operating point of light intensity modulation shifts due to the influence of temperature, the modulation operation is similar to the original operating point so that the extinction ratio remains the same as before by applying a bias. This makes it possible to prevent deterioration of the extinction ratio. Furthermore, in order to prevent deterioration of the extinction ratio, there is no need for a fast-response detector and an accompanying high-frequency circuit, and the structure can be constructed with a simple low-frequency circuit, resulting in low cost.

Figure 5 is a diagram showing a second embodiment of the optical modulation device according to the present invention, and in this embodiment, the electrodes to which control signals are applied are separated. That is, in FIG. 5, new electrodes 31.32 are provided on the right side of the electrode 3.4 along the first waveguide 10a and the second waveguide 10b. A control signal for feedback is applied from the control circuit 25. These electrodes 31.3
2 and the control circuit 25 constitute a control means 33 as a whole. The rest is the same as the first embodiment.

Therefore, in this embodiment, the transmission signal V is applied to the electrode 3.4, and the feedback control for preventing deterioration of the extinction ratio is performed via the electrodes 31.32. In addition, in particular, the electrodes 31 and 32 for feedback control can be used for lower frequencies than the electrodes 3 and 4, so the response may be slower, and the structure is simple, further reducing costs. There is an advantage to doing so.

5M plate FIG. 6 is a diagram showing a third embodiment of the optical modulation device according to the present invention, and this embodiment is an example of a directional coupler type modulator.

In FIG. 6, two first waveguides 41 and a second waveguide 42 having the same shape and length are formed on the substrate 1, and the first waveguide 41 and the second waveguide 42 are located at the center thereof. They are close to each other at the edges and separated at the ends. The input light is input to one end of the second waveguide 42, and the optical pulse signal is input to the first end of the second waveguide 42.
It is taken out from the other end of the waveguide 41. The rest is the same as the first embodiment.

Therefore, although there is a difference in the configuration for generating complementary outputs in this embodiment, it is possible to obtain the same effects as in the first embodiment.

〔Effect of the invention〕

According to the present invention, a feedback system for preventing extinction ratio deterioration of an optical pulse signal can be realized with a simple configuration and at low cost.

[Brief explanation of the drawing]

Figures 1 to 3 are diagrams explaining the present invention in detail. Figure 1 is its configuration diagram, Figure 2 is a diagram showing the characteristics of each optical output, and Figure 3 is when the operating point is shifted. FIG. 4 is a configuration diagram showing the first embodiment of the optical modulation device according to the present invention, and FIG. 5 is a configuration diagram showing the second embodiment of the optical modulation device according to the present invention. 6 is a configuration diagram showing a third embodiment of the optical modulation device according to the present invention, FIGS. 7 to 12 are diagrams showing conventional optical modulation devices, and FIG.
Figure 8 shows its configuration, Figure 8 shows its input light and optical output characteristics, Figure 9 shows its relationship between optical intensity and transmission signal, and Figure 10 shows its optical pulse signal characteristics. 11 is a diagram showing the relationship between the optical intensity and the transmission signal when the operating point shifts, and FIG. 12 is a diagram showing the characteristics of the optical pulse signal when the extinction ratio deteriorates. 22...Fourth waveguide, 23...First power meter, 24...
- Second power meter, 25... control circuit, 26... optical fiber, 29... power detection means, 30.33... control means, 31.32...Electrode, 43...Coupler (complementary output generation means) 1...Substrate, 3.4...Electrode, 5... ...terminal resistor, 10... waveguide, 10a, 41... first waveguide, 10b, 42...
...Second waveguide, 11...3dB coupler (complementary output generation means), 21...Third waveguide, 21a, 22a...Coupler, light Intensity Δ■ Figure 3 shows each first-out characteristic when the operating point of the present invention shifts. Light intensity i9 Figure

Claims (2)

[Claims] (1) In an optical modulation device that guides light of a constant intensity into a waveguide and generates an optical pulse signal by modulating the optical intensity of the light passing through the waveguide using a transmission signal, the two devices are complementary to each other, Complementary output generation means for generating two light intensity modulated optical outputs; power detection means for detecting the time average power of these two optical outputs; and a control means for feedback controlling the operating point of the light intensity modulation so that the operating point of the light intensity modulation becomes a value corresponding to the value of the ratio, and the control means is provided with a control means for feedback controlling the operating point of the light intensity modulation so that the operating point of the light intensity modulation becomes a value corresponding to the value of the light intensity. An optical modulation device that extracts signals. (2) An electrode is provided along the waveguide, and the light intensity modulation and the feedback control are performed through the electrode, and the electrode for applying the transmission signal and the electrode for applying the feedback signal are separated. The light modulation device according to claim 1.