JP2664749B2 - Light modulator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Table of Contents] Outline Industrial application field Conventional technology (FIGS. 7 to 12) Problems to be solved by the invention Means for solving the problem Actions Embodiment (1) Principle of the present invention (Figs. 1-3) (2) First embodiment of the present invention (Fig. 4) (3) Second embodiment of the present invention (Fig. 5) (4) Third embodiment of the present invention (6) [Graph] Effect of the Invention [Overview] An object of the present invention is to provide an optical modulator that can realize a feedback system for preventing the extinction ratio of an optical pulse signal from deteriorating with a simple configuration and at a low cost. In a light modulation device that guides light of high intensity through a waveguide and performs light intensity modulation on a light passing through the waveguide by a transmission signal to generate a light pulse signal, the light modulation devices have a complementary relationship to each other, and have a light intensity modulation. Complementary output generating means for generating the two optical outputs, Power detection means for detecting the time average power of the two optical outputs, and feedback of the operating point of the light intensity modulation so that the ratio of the time average powers of the two optical outputs becomes a value corresponding to the value of the mark ratio of the transmission signal. And a control means for controlling the light output, wherein one of the two light outputs, which are in a complementary relationship to each other and are light intensity modulated, is extracted as a light pulse signal.

[Industrial applications]

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

In recent years, the bit rate of a signal transmitted through an optical fiber has been improved with an increase in the amount of information. Conventionally, an optical signal is generated by turning a semiconductor laser ON and OFF. However, when the bit rate is on the order of Gbps, long-distance transmission cannot be performed due to wavelength chirping. For this reason,
An optical pulse signal is generated by performing light intensity modulation by an external modulation method instead of the direct modulation method as described above. In order to actually use such an external modulator, it is necessary to apply feedback so that the operating point does not change.

[Conventional technology]

FIG. 7 shows an example of a conventional light modulator, especially an external modulator. In FIG. 1, reference numeral 1 denotes, for example, a substrate, and the substrate 1 is provided with a waveguide 2 for guiding light. As the light in this case, oscillation light of a semiconductor laser is used, and a wavelength suitable for optical communication is used. As the waveguide 2, for example, a waveguide made by thermally diffusing Ti into lithium niobate is used.

The waveguide 2 joins again after being branched or split in the middle, and electrodes 3 and 4 are arranged at this branch. Electrode 3,
4 is supplied with a transmission signal V for modulation from one end,
A terminating resistor 5 is connected to the other end, which is of a so-called traveling wave type modulator. Terminating resistor 5
Is for matching traveling waves. Also,
The transmission signal V modulates a laser beam with various information data for optical communication.

In such an external modulator, when an optical output having an intensity as shown in FIG. 8A is applied to the input side (input light side) of the director 2 and a transmission signal V is supplied to the electrodes 3 and 4, Transmission signal V
The refractive index of the waveguide 2 changes according to the size and phase of the light. At this time, the intensity P of the optical output with respect to the transmission signal V has a relationship as shown in FIG. In the figure, + Vs and -Vs are positive and negative switching voltages. This is because one of the two branched waveguides 2 has a higher refractive index and the other has a lower refractive index, so that the light propagates through the waveguide 2 and a phase difference occurs in the two split lights, This is because when the light beams merge, a light intensity corresponding to the phase difference is obtained. Therefore, the intensity P of the light output passing through the branch portion of the waveguide 2 is changed by the electrodes 3 and 4 depending on the value of the transmission signal V, and has a magnitude as shown in FIG. The optical output (optically modulated signal) is extracted from the optical fiber and sent to an optical fiber for optical communication (in particular, transmission).

In the case where the optical modulator is actually used as an external modulator, a square wave optical output is desirable. Therefore, voltages corresponding to the maximum value and the minimum value of the optical output as shown in FIG. To the electrodes 3 and 4. For example, the tenth
As shown in FIG. 7A, the signal “1” is set to 0, and the signal “0” is set to −
When a binary transmission signal V such as Vs is added, an optical pulse corresponding to 0 and -Vs is generated as shown in FIG.

[Problems to be solved by the invention]

However, in such a conventional external modulator, the modulation characteristic shown in FIG. 9 (that is, the applied voltage-optical output characteristic) is normal due to the temperature change of the whole substrate as shown by the solid line in FIG. Operating point shifts from the normal state. When a transmission signal voltage as shown in FIG. 10 (a) is applied in this shift state, when the transmission signal voltage is "1" level as shown in FIG.
The ratio at the level (extinction ratio) is degraded, which adversely affects light transmission characteristics. This is because the light intensity does not completely become 0 when the level is "0", and the light intensity becomes lower than the normal state when the level is "1". As a result, the level ratio between "1" and "0" is poor. Because it becomes.

In order to solve such a problem, feedback may be performed such that a predetermined DC component is added to the transmission signal by the shift of the operating point. In this case, the extinction ratio becomes the same state as in the normal state.

However, in order to perform this feedback, a system that constantly monitors the extinction ratio is required, but it is necessary to monitor in real time at the same speed as the signal transmission speed. There is a problem that control is extremely difficult and realization of the system is difficult.

Also, even if such a feedback system can be realized, only a very complicated and expensive one can be expected.

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

[Means for solving the problem]

In order to achieve the above object, the light modulation device according to the present invention guides light of a constant intensity to a waveguide, and performs light hardness modulation on light passing through the waveguide with a transmission signal to generate a light pulse signal. In the device, they are complementary to each other,
Complementary output generating means for generating two light intensities modulated optical outputs, power detecting means for detecting the time average power of these two optical outputs, and the ratio of the time average power of the two optical outputs to the mark of the transmission signal Control means for feedback-controlling the operating point of the light intensity modulation so as to have a value corresponding to the value of the rate, and one of the two light intensity-modulated light outputs having a complementary relationship with each other is provided as an optical pulse. I take it out as a signal.

[Action]

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

Therefore, a detector with a high-speed response and an associated high-frequency circuit are not required for detecting the time-average power, and a feedback system for preventing the extinction ratio from deteriorating at a low cost with a simple circuit of a low frequency can be realized.

〔Example〕

 Hereinafter, the present invention will be described with reference to the drawings.

Explanation of Principle First, the principle of the present invention will be described. FIG. 1 is an explanatory view of the principle of the present invention. In this figure, the same components as those of the conventional example are denoted by the same reference numerals, and duplicate description will be omitted. Reference numeral 10 denotes a waveguide, and the waveguide 10 branches into two in the middle and is divided into a first waveguide 10a and a second waveguide 10b. First waveguide 10
The electrodes 3 and 4 are disposed on the a and the second waveguides 10b, respectively, as in the conventional example. On the right side of the figure, the first and second waveguides 10a and 10b are close to each other and 3 dB. A coupler (complementary output generating means) 11 is formed and then separated again.
Two optical outputs of the P ₁ and P ₂ is adapted to be removed from each end. 3dB coupler 11 has a function of shifting the phase such that the optical output P ₁ and the optical output P ₂ in complementary relationship with each other as shown in Figure 2.

In the above configuration, when actually used as an external modulator, a voltage at which the optical outputs P ₁ and P ₂ are equal (in FIG. 2, for example, a point of V = 0 corresponds) is set as an operating point,
The transmission signal is oscillated at + Vs / 2 and -Vs / 2 around this point.

For example, when using the optical pulse signal to provide an optical output P ₁ to the optical communication shake transmission signal V so as to correspond to the allowed + Vs / 2 corresponding to the signal "0" to the signal "1" to -Vs / 2. in this case,
The ratio of the time average powers P ₁ and P ₂ of the optical outputs P ₁ and P ₂ is uniquely determined if the mark ratio of the transmission signal is determined. For example,
If the mark ratio is 1/2, which is a normal value, for the above-described reason, the ratio of the time average powers P ₁ and P ₂ (hereinafter, referred to as the average power ratio) is 1. However, when the operating point of the transmission signal V shifts from point A to point B (the shift amount is Δ) as shown in FIG. 3 due to the influence of temperature or the like, the normal signal “1” is obtained even when −Vs / 2 is multiplied. The level becomes smaller by a value correlating to ΔV than the level of the signal. On the other hand, even when + Vs / 2 is applied, the level becomes larger by a value correlating to ΔV than the level of the normal signal “0”, and the extinction ratio deteriorates Eventually, the ratio of the average powers changes instead of 1.

Therefore, if feedback is performed so that the average power ratio becomes the same as the original ratio (= 1), the original operating point (),
That is, it operates at the point where P ₁ = P ₂ at V = 0. In this case, since the detection of the time average powers P ₁ and P ₂ is merely an average value, there is no need for a high-speed response even at a high bit rate, and a slow-response, low-cost detector (power (E.g., an inexpensive power meter) may be arranged along the periphery of the optical outputs P ₁ and P ₂ . Also, a circuit associated with this detection and a feedback circuit (corresponding to a control means) may be a simple circuit of a low frequency instead of a high frequency, and therefore, a feedback system for preventing the extinction ratio from deteriorating has a simple configuration, and It can be realized at low cost.

First Embodiment Next, a first embodiment of the present invention based on the above principle will be described with reference to FIG. In describing the present embodiment, the first
Since many parts are common to those shown in FIG. 1, the same reference numerals are given to the same components to avoid repetition of the description.

In FIG. 4, a first waveguide 10a and a second waveguide 10b
Are close to each other at the 3 dB coupler 11, but then separate again, and each of the separated portions is provided with a third waveguide 21 and a fourth waveguide 22 whose ends are close to each other. One end of each of the third waveguide 21 and the fourth waveguide 22 is a coupler (directional coupler) 21a, 22a, and these couplers 21a, 22a are the first waveguide 10a and the second waveguide 10b. In order to feel the optical power transmitted from the optical disc, only the minimum necessary faint light is taken out for feedback control. At the other end of the third waveguide 21, a first power meter 23 is provided.
A second power meter 24 is provided at the other end of 22. These first power meter 23 and second power meter 24 are connected to first waveguide 10a and second waveguide 1 respectively.
The time average powers P ₁ and P ₂ of the optical output passing through 0b are measured and output to the control circuit 25. In this case, the first power meter
A low-cost detector with a slow response is sufficient for the second 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,
The end of 10b is an open end without being connected to anything. The control circuit 25 operates the light intensity modulation operation point (that is, the transmission signal operation point) such that the ratio of the time average powers P ₁ and P ₂ of each optical output becomes a value corresponding to the value of the mark rate of the transmission signal V. And generates a control signal for feedback-controlling the voltage between the electrodes 3 and 4 via the high-frequency blocking coil 27. for that reason,
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 a control current Ib flows. The transmission signal V is supplied to the electrode 3 via the 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 unit 29, and include a control circuit 25 and a high-frequency blocking coil 27.
Constitutes the control means 30.

In the above configuration, the optical outputs P ₁ , P ₁ which are complementary to each other by the 3 dB coupler 11 and are light intensity modulated
₂ is taken out through the couplers 21a and 22a, and is taken out of the first power meter 23 and the second power meter 24, respectively.
And the time average powers P ₁ and P ₂ are measured and sent to the control circuit 25. Then, the control circuit 25 determined by P ₁ and the specific mark ratio of P ₂ values such that (1 in this embodiment), i.e., so that the bias is applied to the transmission signal V such that P ₁ = P ₂ Is feedback controlled. 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 same modulation operation as the original operating point is performed so that the extinction ratio becomes the same as the original by applying the bias. And the deterioration of the extinction ratio can be prevented. Further, in preventing deterioration of the extinction ratio, a high-speed response detector and an associated high-frequency circuit are not required, and a low-frequency simple circuit can be used, and the cost is low.

Second Embodiment FIG. 5 is a diagram showing a second embodiment of the light modulation device according to the present invention, in which the electrodes to which the control signals are applied are separated. That is, in FIG. 5, on the right side of the electrodes 3 and 4 in the figure, new electrodes 31 and 32 are provided along the first waveguide 10a and the second waveguide 10b, respectively.
A control signal for feedback by the control circuit 25 is applied to 1 and 32. These electrodes 31, 32 and control circuit 25
Constitute the control means 33 as a whole. Others are the same as the first embodiment.

Therefore, in this embodiment, the transmission signal V is applied to the electrodes 3 and 4, and the feedback control for preventing the extinction ratio from deteriorating is performed via the electrodes 31 and 32. In this embodiment, the same effects as in the first embodiment can be obtained. In particular, the electrodes 31, 32 for feedback control are the electrodes 3,
4, the response may be slow because it is for low frequency use, and there is an advantage that the structure is simple and the cost is further reduced.

Third Embodiment FIG. 6 is a diagram showing a third embodiment of the optical modulator according to the present invention, and this embodiment is an example of a directional coupler type modulator. In FIG. 6, two substrates of the same shape and length
Two first waveguides 41 and second waveguides 42 are formed, and the first waveguide 41 and the second waveguide 42 are close to each other at the center and are separated at the end. The input light is incident on one end of the second waveguide 42, and the optical pulse signal is extracted from the other end of the first waveguide 41. Others are the same as the first embodiment.

Therefore, in the present embodiment, the same effect as in the first embodiment can be obtained, although there is a difference in the configuration for generating the complementary output.

〔The invention's effect〕

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

[Brief description of the drawings]

1 to 3 are diagrams for explaining the principle of the present invention, FIG. 1 is a diagram showing the configuration, FIG. 2 is a diagram showing characteristics of each light output, and FIG. 3 is a diagram when the operating point is shifted. FIG. 4 is a diagram showing a first embodiment of the optical modulator according to the present invention, and FIG. 5 is a diagram showing a second embodiment of the optical modulator according to the present invention. FIG. 6, FIG. 6 is a block diagram showing a third embodiment of the optical modulator according to the present invention, FIGS. 7 to 12 are diagrams showing a conventional optical modulator, FIG. The figure shows the characteristics of the input light and the optical output, FIG. 9 shows the relationship between the light intensity and the transmission signal, FIG. 10 shows the characteristics of the optical pulse signal, and FIG. FIG. 12 is a diagram showing the relationship between the light intensity and the transmission signal when the operating point is shifted. FIG. 12 is a diagram showing the characteristics of the optical pulse signal when the extinction ratio is deteriorated. 1 ... substrate, 3, 4 ... electrode, 5 ... terminal resistance, 10 ... waveguide, 10a, 41 ... first waveguide, 10b, 42 ... second waveguide, 11 ... 3dB coupler ( Complementary output generating means), 21... Third waveguide, 21a, 22a... Coupler, 22... Fourth waveguide, 23... First power meter, 24. Control circuit, 26: optical fiber, 29: power detection means, 30, 33: control means, 31, 32: electrodes, 43: coupler (complementary output generation means).

Continued on the front page (51) Int.Cl. ⁶ Identification code Agency reference number FI Technical indication H04B 10/152

Claims (2)

(57) [Claims] An optical modulation device for guiding light having a constant intensity to a waveguide and performing optical intensity modulation on light passing through the waveguide by a transmission signal to generate an optical pulse signal. A complementary output generating means for generating two light outputs modulated with light intensity; a power detecting means for detecting a time average power of these two light outputs; and a ratio of the time average power of the two light outputs to the transmission signal. Control means for performing feedback control of the operating point of the light intensity modulation so as to have a value corresponding to the value of the mark ratio of the light intensity modulation. An optical modulation device for extracting an optical pulse signal. 2. An electrode is provided along the waveguide, the light intensity modulation and the feedback control are performed via the electrode, and an electrode for applying a transmission signal and an electrode for applying a feedback signal are separated. The optical modulation device according to claim 1, wherein: