JPH05224163A - Light transmitter - Google Patents

Abstract

PURPOSE:To lower the driving voltage of an optical modulator, to stabilize the operation against mark rate variation and the temperature variation and secular change of the optical modulator, and to prevent the waveform of modulation output light from deteriorating. CONSTITUTION:This transmitter is equipped with a light source 1 such as a semiconductor laser and a Mach-Zehnder type optical modulator 2 provided with signal electrodes and a bias electrode 4 which are electrically separated from two light guides, and has a modulator driving circuit 5 which applies a driving signal to one-end sides of the signal electrodes 3 and terminators 6 which are connected to the other-end side of the signal electrodes 3. Further, is provided with an operation point control circuit 7 which applies a bias voltage for operation point stabilization to the bias electrode 4, a low-frequency superposing circuit 8 which superposes a low-frequency signal for operation point stabilization on the driving signal, and a low-frequency oscillator 9 which generates the low-frequency signal and equipped with an optical branching circuit 10 which branches part of the modulation output light.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an external light modulation type optical transmitter. In recent years, in an optical communication system, an external modulation method is being developed as an optical modulation method that is less susceptible to the chromatic dispersion of an optical fiber. For example, Li
NbO ₃ Mach-Zehnder type optical modulators have attracted attention because they have excellent modulation characteristics and wavelength dispersion resistance characteristics. In the optical transmitter using such an external light modulation method, (1) operation point stabilization control of the light modulator, (2) low voltage drive of the light modulator, (3) drive circuit and light modulation There is a demand for reduction of waveform distortion due to the capacitors between the electrodes of the detector and between the electrodes and the terminator, and (4) reduction of change in modulation characteristics due to sudden change in mark ratio. It is also desired to be able to select whether or not chirping is present. For example, when dispersion can take positive or negative values, such as when performing long-distance transmission using a wavelength that is substantially the same as the zero-dispersion wavelength of the optical fiber, no chirping is suitable, and When the positive / negative of the chromatic dispersion is clear and the optical pulse compression can be generated by the chirping corresponding to the sign of the dispersion, the chirping is suitable.

[0002]

2. Description of the Related Art FIG. 11 is an explanatory view of a conventional example. Reference numeral 71 is a light source made of a semiconductor laser or the like, 72 is a Mach-Zehnder type optical modulator, 73 is a signal electrode, 74 is an optical branching circuit, and 7
5 is a modulator drive circuit, 76 is a terminator, 77 is an operating point control circuit, 78 is a low frequency superimposing circuit, 79 is a low frequency oscillator, 8
0 and 81 are bias tees, and L and C are inductances and capacitors that constitute the bias tees.

The Mach-Zehnder type optical modulator 72 is, for example, a signal for diffusing Ti into a Z-cut LiNbO ₃ substrate to form an optical waveguide having a pattern for branching and coupling, and applying an electric field to the branched optical waveguide. The electrodes 73 and grounding electrodes (not shown) are formed. Then, a drive signal is applied to one end of the signal electrode 73, and the other end is terminated by a terminator 76 of, for example, 50Ω via the capacitor C of the bias tee 81, and is in the same direction as the light propagated to the optical waveguide. It is of a traveling wave type in which an electric field due to a drive signal propagates.

In the optical modulator 72 having such a configuration, a change in temperature and a change with time occur at the operating point where the modulation operation is performed. Therefore, the low-frequency signal from the low-frequency oscillator 79 is added to the low-frequency superimposing circuit 78 to superimpose the low-frequency signal on the drive signal from the modulator driving circuit 75 according to the input signal of several Gb / s, It is applied to the signal electrode 73 via the capacitor C of the bias tee 80, and the light from the light source 71 is modulated by the drive signal on which the low frequency signal is superimposed.

The output light modulated by the optical modulator 72 is
The operating point control circuit 7 is partially branched by the optical branch circuit 74.
7 is added to the low frequency oscillator 79 and converted into an electric signal.
Is synchronously detected by the low frequency signal from. Light modulator 72
Since the operating point can be controlled by the bias voltage, the operating point control circuit 77 sends the bias voltage to the signal via the inductance L of the bias tee 81 so that the low frequency signal component of the synchronous detection output is minimized. It is applied to the electrode 73 for. Such a conventional example is disclosed, for example, in the 1990 Spring National Conference of the Institute of Electronics, Information and Communication Engineers, B-976, "Study on Automatic Bias Control Circuit for Mach-Zehnder Type Optical Modulator", or Japanese Patent Application No. 2-50189. ..

A symmetrical two-electrode drive type Mach-Zehnder type optical modulator has been proposed in order to reduce the voltage value applied to the signal electrode of the optical modulator and to perform the chirp reduction of the modulated output light. That is, each of the branched optical waveguides is provided with a signal electrode, and a drive voltage having an opposite phase is applied to perform symmetrical modulation. For example, in 1989, "Technical Digi
est of IOOC '89 "19D4-2" Perfectly "
Chirpless Low Drive Voltage Ti: LiNb
O ₃ Mach-Zehnder Modulator with Two Travelin
g-Wave Electrodes ".

[0007]

In the conventional example shown in FIG. 11, since the drive signal is applied to the signal electrode 73 via the capacitor C of the bias tees 80 and 81, this capacitor C is used. As a result, the low-frequency component of the drive signal is blocked, and the signal waveform of the modulated output light is distorted when the mark ratio changes abruptly. The operating point control circuit 77 controls so that the operating point becomes an appropriate operating point even when the mark ratio is other than 1/2. However, since the time constant of the operating point control circuit 77 is about the same or more, it is cut off by the capacitor C. There is a drawback in that the operating point deviates from the proper point in the case of the speed of the change of the mark ratio that occurs with the time constant corresponding to the frequency to be set.
If the frequency characteristics of the capacitor C of the bias tees 80 and 81 are not sufficient, the waveform of the high-speed drive signal of several Gb / s will be distorted by the capacitor C, which causes the problem of distorted waveform of the modulated output light. It was Further, since the signal electrode 73 is formed only on one of the two branched optical waveguides, the modulation efficiency is low, and therefore the driving voltage cannot be reduced.

In order to improve the configuration of the asymmetrical modulation of the conventional example shown in FIG. 11, the symmetrical two-electrode drive type Mach-Zehnder type optical modulator is symmetrically modulated, so that the driving voltage can be reduced. However, the present situation is that the study as an optical transmitter including the stabilization of the operating point has not been sufficiently advanced. INDUSTRIAL APPLICABILITY The present invention eliminates the problem caused by the capacitor and enables stable control even against a sudden change in the mark ratio,
Moreover, the purpose is to facilitate the selection of the presence or absence of chirping.

[0009]

The optical transmitter of the present invention comprises:
Explaining with reference to FIG. 1, a signal electrode 3 for inputting light from a light source 1 such as a semiconductor laser, branching it into two optical waveguides, and applying a voltage to each of these optical waveguides independently.
A Mach-Zehnder type optical modulator 2 having a bias electrode 4 and a signal electrode 3 corresponding to two optical waveguides of the optical modulator 2 is connected by direct current, and a drive signal according to an input signal is applied. , A modulator driving circuit 5 for transmitting the modulated output light from the optical modulator 2, a terminator 6 for terminating the signal electrode 3 of the optical modulator 2 by direct current connection, and a direct current connection for the bias electrode 4. Then, an operating point control circuit 7 for controlling the operating point of the optical modulator 2 is provided.

Further, a low frequency superimposing circuit 8 for superimposing a low frequency signal on the drive signal applied from the modulator driving circuit 5 to the signal electrode 3 is provided, and an operating point control circuit 7 is provided for the output light of the optical modulator 2. The bias voltage applied to the biasing electrode 4 is controlled so that the component of the low-frequency signal included in is minimized. Further, 9 is a low frequency oscillator, and 10 is an optical branch circuit.

Further, the modulator drive circuit 5 is constituted by a differential amplifier, and the current source of this differential amplifier is modulated by a low frequency signal.

Further, the modulator driving circuit 5 is constituted by a differential amplifier, the current source of this differential amplifier is modulated by a low frequency signal, and the driving signal is directly applied to one of the signal electrodes 3 and to the other via an attenuator. Is applied to the bias electrodes,
The low frequency signal is applied together with the bias voltage.

The modulator drive circuit 5 is composed of two differential amplifiers corresponding to the signal electrodes 3, and the current sources of the two differential amplifiers are modulated by low frequency signals. Is.

[0014]

Since the signal electrode 3 and the bias electrode 4 corresponding to the two optical waveguides of the optical modulator 2 are electrically separated from each other, the modulator drive circuit 5 and the signal electrode 3 are separated from each other. , And the signal electrode 3 and the terminator 6 are DC-connected to perform symmetrical modulation. Therefore, since it is not necessary to make an AC connection via a capacitor, problems such as waveform distortion due to the capacitor and a shift in the operating point due to a sudden change in the mark ratio will not occur. Further, a bias voltage can be applied to the bias electrode 4 from the operating point control circuit 7 by connecting a direct current. Therefore, the bias tee including the inductance and the capacitor can be omitted.

Further, the low frequency signal from the low frequency oscillator 9 is superposed by the low frequency superposition circuit 8 on the drive signal applied from the modulator drive circuit 5 to the signal electrode 3, and the two optical waveguides of the optical modulator 2 are provided. The light propagating in the light is modulated and combined into a modulated output light, a part of the modulated output light is branched by the optical branching circuit 10 and input to the operating point control circuit 7, and a low frequency signal included in the modulated output light is output. Biasing electrode 4 so that the component is minimized
A bias voltage is applied to. Thereby, the optical modulator 2
Will be symmetrically modulated at the proper operating point.

Further, the low frequency superimposing circuit 8 can be constructed by modulating the current source of the differential amplifier constituting the modulator driving circuit 5 with a low frequency signal. Further, by applying an input signal of opposite phase to the differential amplifier, a drive signal of opposite phase can be applied to the signal electrode 3 of the optical modulator 2 to perform symmetrical modulation.

Further, the current source of the differential amplifier which constitutes the modulator drive circuit 5 is modulated with a low frequency signal, and the drive signal of one output of the differential amplifier is directly applied to one of the signal electrodes 3 to generate The drive signal of the other output of the amplifier is applied to the signal electrode 3
Is applied to the other of the two via an attenuator. Therefore, the optical modulator 2
Is asymmetrical driving, so chirping occurs. This chirping is selected according to the code of dispersion of the optical fiber that transmits the modulated output light from the optical transmission device, which enables compression of the optical pulse and enables long-distance transmission. .. A low-frequency signal is superimposed and applied to the signal electrode 3, the low-frequency signal component is extracted from the modulated output light, and a bias voltage applied to the bias electrode 4 so that the low-frequency signal component is minimized. The operating point of the optical modulator 2 can be optimized by controlling the.

Further, the modulator driving circuit 5 is composed of two differential amplifiers corresponding to the two signal electrodes 3, and the respective low frequency signals are modulated by modulating the respective current sources with the low frequency signals. The superimposed drive signal can be applied to the signal electrode 3, and symmetric modulation and asymmetric modulation can be selected by setting the current value of the current source of each differential amplifier. That is, it is possible to select the absence or presence of chirping according to the wavelength dispersion characteristic of the optical fiber.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 2 is an explanatory view of essential parts of an embodiment of the present invention, in which 11 is an input port, 12 is an output port, and 13 and 14.
Is a branch optical waveguide, 15 and 16 are signal electrodes, and 17 and 18
Is a bias electrode, 19 and 20 are ground electrodes, 21 is a modulator driving circuit, 22 is an inverting circuit, 23 and 24 are transistors forming a differential amplifier, 25 is a current source, 26 is an operational amplifier, and 27 and 28 are transistors. Is the resistance that constitutes the terminator, 29, 3
0 is a resistance. By the input port 11, the output port 12, the branch optical waveguides 13 and 14, the signal electrodes 15 and 16, the bias electrodes 17 and 18, and the ground electrodes 19 and 20,
The Mach-Zehnder type optical modulator 2 in FIG. 1 is configured, light from a light source such as a semiconductor laser (not shown) is incident on the input port 11, and modulated output light is transmitted from the output port 12.

Signal electrodes 15 and 16 and bias electrodes 17 and 18 which are electrically separated corresponding to the branched optical waveguides 13 and 14, respectively, are formed, and the ground electrodes 19 and 20 are
Although not shown, they are connected to each other. Further, one ends of the signal electrodes 15 and 16 and the modulator driving circuit 21 are DC-connected, and resistors 27 and 28 forming a terminator are connected to the ground electrodes 19 and 20 at the other ends of the signal electrodes 15 and 16, respectively. DC connected to. Further, the bias voltage Vb from the operating point control circuit is applied to the bias electrode 17, and the bias voltage inverted by the inversion circuit 22 is applied to the bias electrode 18. Therefore, when a drive signal is applied to the signal electrodes 15 and 16, an electric field between the signal electrodes 15 and 16 and the ground electrodes 19 and 20 is applied to the branched optical waveguides 13 and 14. Similarly,
When a bias voltage is applied to the bias electrodes 17 and 18, an electric field between the bias electrodes 17 and 18 and the ground electrodes 19 and 29 is applied to the branch optical waveguides 13 and 14.

Input signals Vin and * Vin having opposite phases are applied to the gates of the transistors 23 and 24, which form the differential amplifier of the modulator driving circuit 21, and one of the transistors 2
Since the signal electrode 15 is connected to the drain of the transistor 3 and the signal electrode 16 is connected to the drain of the other transistor 24, a drive signal having an opposite phase is applied to the signal electrodes 15 and 16. Further, since the low frequency signal Fm is applied to the current source 25 to which the voltage E is applied, the drive signal from the modulator drive circuit 21 corresponding to the input signal is the one modulated by the low frequency signal Fm, and the low frequency signal Fm. Can be symmetrically superimposed.

Therefore, the light from the light source incident on the input port 11 is phase-modulated according to the drive signal in the process of propagating through the branch waveguides 13 and 14, and becomes in-phase or anti-phase at the output port 12. Therefore, the modulated output light is intensity-modulated by being added or canceled. When the low frequency signal Fm is superimposed on the drive signal and the operating point of the optical modulator is appropriate,
The modulated output light is amplitude-modulated at a frequency that is twice the frequency fm of the low-frequency signal Fm, and thus the modulated output light does not include the component of the low-frequency signal Fm. If the operating point of the optical modulator is deviated, the low-frequency signal F
Since the m component is included, the operating point can be properly maintained by applying the bias voltage Vb so as to minimize the component of the low frequency signal Fm.

FIG. 3 is an explanatory view of an embodiment of the present invention.
The same reference numerals as those in FIG. 2 indicate the same parts, 31 is a DFB (distributed feedback type) laser as a light source, 32 is an isolator, 3
3 is an optical branch circuit, 34 is a photoelectric conversion circuit including a photodiode, 35 is an amplifier circuit, 36 is a synchronous detection circuit, 3
Reference numeral 7 is a low pass filter, 38 is an amplifier circuit, 39 is a low frequency oscillator, 40 is an operational amplifier, and 41 and 42 are resistors.
The photoelectric conversion circuit 34, the amplification circuit 35, the synchronous detection circuit 36, the low-pass filter 37, the amplification circuit 38, and the inverting circuit 22 constitute the operating point control circuit 7 of FIG. 1, and the current source of the modulator drive circuit 21. 25 configures the low frequency superimposing circuit 8 of FIG.

The light from the DFB laser 31 is incident on the input port 11 of the optical modulator through the isolator 32, and the modulated output light is output from the output port 12 of the optical modulator as a transmission optical signal. Further, a part of the light is branched by the light branching circuit 33 and added to the photoelectric conversion circuit 34, where it is converted into an electric signal. This electric signal is synchronously detected by the low frequency signal from the low frequency oscillator 39 in the synchronous detection circuit 36, the component of the low frequency signal Fm is output as a DC signal, and the amplification circuit 38 is passed through the low pass filter 37. And the amplified output becomes the bias voltage Vb. The bias voltage Vb is directly applied to the bias electrode 17 or the bias electrode 18
Are inverted by the inverting circuit 22 and applied to each of them.

FIG. 4 is a diagram for explaining the operation of one embodiment of the present invention. (A) is an input signal Vin in the case of alternating "1" and "0", and (b) is a low frequency signal Fm. (C) is a drive signal applied to one signal electrode 15, (d) is a drive signal applied to the other signal electrode 16, (e) is an output phase φ1 of one branch optical waveguide 13, ( f) is the output phase φ2 of the other branching waveguide 14, and (g) is the phase difference (φ1-φ2) of the output light of the branching optical waveguides 13 and 14 between the one and the other.
Indicates.

The drive signals applied to the one and the other signal electrodes 15 and 16 according to the input signal Vin are (c),
As shown in (d), the phase is inverted and the low frequency signal F
Symmetric modulation is performed by superimposing m symmetrically, and the phase difference between the output light of the branch optical waveguides 13 and 14 (φ1−φ
2) is a phase difference of 0 to π, and the center of the phase difference is (φ ₁₀ −φ ₂₀ ) when the reference phases of the output light of the branch optical waveguides 13 and 14 are φ ₁₀ and φ ₂₀ . And output port 1
By being combined in 2, the intensity-modulated modulated output light is obtained.

The input / output characteristics of the optical modulator can be expressed by the following equation when the output optical power is normalized by its peak value and the driving voltage is normalized by Vπ. P (V) = [1-cos {π (V−Vd)}] / 2 (1) where P = normalized output optical power, V = normalized driving voltage, and Vd = normalized Operating point drift voltage.

Frequency f ₀ = ω ₀ for a signal of amplitude Vπ
When low frequency superimposition is performed at / 2π and the modulation factor m, the drive voltages V ₀ and V ₁ for the "0" level and the "1" level are: V ₀ = m · sin (ω ₀ t) (2) V ₁ = 1−m · sin (ω ₀ t) (3) The output optical powers P ₀ and P ₁ at this time are: P ₀ = P (V ₀ ) ≈ [1-cos (πVd) −π · m · sin (ω ₀ t), assuming that the modulation degree m is sufficiently small. sin (πVd)] / 2 (4) P ₁ = P (V ₁ ) ≈ [1 + cos (πVd) −π · m · sin (ω ₀ t) sin (πVd)] / 2 (5) .. The average power P _{2 at the} rise and fall times is P ₂ = 1 / (V ₁ −V ₀ ) ∫ ⁽ V ₁ ⁾ _{(V 0)} P (V) dV = (1/2) − [cos (πV ₀ ) sin (πVd)] / [π (1-2V ₀ )] ≈ (1/2)-[sin (πVd) / π] [1 + 2m · sin (ω ₀ t)] (6) However, “∫ ^(V1) _(V0) ” indicates the integration from V ₀ to V ₁ .

When the mark ratio of the input signal is M and the rising / falling time is r (1 / fb) (where fb = bit rate of the input signal), the cycle of the low frequency signal Fm (1 / f ₀ ).
The output optical power P _av averaged more sufficiently short time, P _av = ^{[r (1-M) 2 +} (1-r) (1-M) ] P ₀ + [rM ² + (1-r) M] P ₁ + 2r (1-M) MP ₂ ≡K ₀ P ₀ + K ₁ P ₁ + K ₂ P ₂ (7) Letting P _f0 be the component of the frequency f ₀ of the low-frequency signal Fm contained in this average output optical power P _av , P _f0 = − [{[r (1-M) ² + (1-r) (1-M )] + [RM ² + (1-r) M]} (π / 2) + 2r (1-M) M (π / 2)] × m · sin (πVd) sin (ω ₀ t) (8) Becomes

Therefore, the phase differs by 180 ° depending on the direction of the operating point drift, and this is changed to the reference frequency sin (ω
_When multiplied by ₀ t), positive and negative DC components according to the direction of the operating point drift are obtained. Therefore, the operating point can be held at an appropriate position by controlling the DC component to be zero. .. Further, from the equation (8), P _f0 = 0 is established when Vd = 0, and the mark ratio M of the input signal Vin is
In the above-described embodiment, the operating point can be controlled to the optimum position without depending on the rising / falling time r.

On the other hand, as an example in the case of asymmetrical superposition of low frequencies, if V ₀ = 0 (9) V ₁ = 1-m · sin (ω ₀ t) (10), that is, 2 When a drive signal is applied to only one of the two branched optical waveguides, P ₀ = [1-cos (πVd)] / 2 (11) P ₁ = P (V ₁ ) ≈ [1 + cos (πVd) −π · m · sin (ω ₀ t) sin (πVd)] / 2 (12) P ₂ = 1 / (V ₁ −V ₀ ) ∫ ^(V1) _(V0) P (V) dV = (1/2) − [Sin {π (V ₁ −Vd)} + sin (πVd)] / 2πV ₁ ≈ (1/2) − [sin (πVd) / π] − (m / 2) [πcos (πVd) + 2sin (πVd)] sin (ω ₀ t) − (1/2) cos (πVd) [m · sin (ω ₀ t)] ² (13) P _av = [r (1-M) ² + (1-r) (1 -M)] P ₀ + ^{[rM 2 + (1-r)} M ] _{P 1 + 2r (1-M} ) MP 2 ≡ _{0 P 0 = + K 1 P} 1 + K 2 P 2 ... (14) P f0 - _{[K 1 (π / 2) sin} (πVd) + K 2 (1 / 2π) {πcos (πVd) + 2sin (πVd)} ] × _{m · sin (ω 0 t)} = K · m · sin (πVd + θ) sin (ω 0 t) ... (15) However, K = ^{_{[(K 2/2) 2 +}} (πK 1/2 + K 2 / π) 2 ] ^1/2 (16) θ = tan ⁻¹ [πK ₂ / (π ² K ₁ + 2K ₂ )] (17)

From equation (15) corresponding to equation (8), P _f0
= 0 is when sin (πVd + θ) = 0, and the extinction ratio and the waveform are deteriorated because control is performed to an operating point that is deviated from the optimum operating point by −θ / π. Here, θ is a function of the rise / fall time r and the mark ratio M, and thus rises / rises in the case of asymmetrical modulation as compared to symmetric modulation.
It is controlled to a point deviated from the optimum operating point depending on the fall time and the mark rate.

In the above-described embodiment, the low frequency signal Fm for controlling the operating point is symmetrically superimposed by modulating the current source 25 of the differential amplifier, and the differential amplifier of the modulator driving circuit 21. Since the drive signals output from the transistors 23 and 24 are applied to the signal electrodes 15 and 16, symmetrical modulation is performed. Therefore, it is possible to reduce the drive voltage for light modulation, to prevent chirping, and to prevent the shift of the operating point even when the mark ratio suddenly changes. Further, since the bias-tee capacitor is not required, the signal waveform is not distorted, and the optical transmitter as an external modulation system for stable operation can be provided.

FIG. 5 is a diagram for explaining the principle of another embodiment of the present invention. The same symbols as those in FIG. 1 indicate the same parts, and 43 is an attenuator. This attenuator 43 attenuates the drive signal applied to one of the signal electrodes 3 to make it asymmetric with the drive signal applied to the other signal electrode 3, and uses the low frequency signal from the low frequency oscillator 9 for biasing. By adding to the electrode 4, the low frequency signal superimposed on the drive signal is compensated for being attenuated by the attenuator 43, and the low frequency signal is symmetrically superimposed. As a result, the operation of the optical modulator is stabilized even by a sudden change in the mark ratio, and the chirping is caused by the asymmetrical modulation so that the pulse width of the optical signal transmitted through the optical fiber can be compressed.

FIG. 6 is an explanatory view of another embodiment of the present invention, which shows FIG. 5 in more detail. The same reference numerals as those in FIGS. 3 and 5 indicate the same parts, 44 is a capacitor and 45 is a resistor. Is. The case where the attenuator 43 is connected between the transistor 24 of the modulator driving circuit 21 and the signal electrode 16 is shown, and the low frequency oscillator is connected to the bias electrode 18 on the signal electrode 16 side to which the attenuator 43 is connected. The low frequency signal from 39 is applied via capacitor 44. Further, the transistor 23 of the modulator driving circuit 21 and the signal electrode 17 are directly connected to each other, and the current source 25 of the transistors 23 and 24 constituting the differential amplifier is modulated by the low frequency signal Fm. The modulation operation for the optical modulator and the stabilization control of the operating point by superimposing the low-frequency signal are the same as those in the above-described embodiment, and the duplicated description will be omitted.

FIG. 7 is a diagram for explaining the operation of another embodiment of the present invention. (A) is an input signal Vin by repeating "1" and "0", (b) is a low frequency signal Fm, (c). Is a drive signal applied to one signal electrode 15, (d) is a drive signal applied to the other signal electrode 16, (e) is a low-frequency signal applied to the bias electrode 18, and (f). Is the phase φ1 of the output light of one branch optical waveguide, (g) is the phase φ2 of the output light of the other branch optical waveguide, and (h) is the phase difference (φ1- of the output light of one branch optical waveguide and the other). φ2) is shown.

Since the sum of the amplitudes of (c) and (d) of FIG. 7 needs to be equal to the sum of the amplitude of (c) and (d) of FIG. 4, the sum of (c) of FIG. The amplitude needs to be larger than the amplitude shown in FIG. The drive signal applied to the signal electrode 16 is attenuated by the attenuator 43 as shown in (d). Therefore, as shown in (g), the phase change of the output optical phase φ2 of the branch optical waveguide 14 is smaller than the phase change of the output optical phase φ1 of the branch optical waveguide 13, and a low frequency signal is applied to the bias electrode 18. Since the signals are superimposed and applied, the phase changes corresponding to the low frequency signal. Phase difference of the output light of the branched optical waveguides 13 and 14 (φ1-
φ2) is a phase change between 0 and π as shown in (h), and low frequency is superimposed symmetrically on the phase difference of the output light due to the low frequency superimposition on the bias electrode 18. .. Then, intensity-modulated modulated output light is obtained from the output port 12.

FIG. 8 is an explanatory view of the principle of still another embodiment of the present invention. The same reference numerals as those in FIG.
Reference numeral b is a drive circuit, and 8a and 8b are low frequency superposition circuits. In this embodiment, the modulator drive circuit 5 is provided with drive circuits 5a and 5b corresponding to the signal electrodes 3 of the optical modulator 2.
By controlling the drive signals from the drive circuits 5a and 5b, symmetrical modulation and asymmetrical modulation can be selected.

FIG. 9 is an explanatory view of still another embodiment of the present invention, which shows FIG. 8 in more detail, and shows a drive circuit 5a.
Includes transistors 51 and 52 and a current source 53 that form a differential amplifier, and the drive circuit 5b includes transistors 54 and 55 and a current source 56 that form a differential amplifier. The low-frequency signal Fm is input to the current sources 53 and 56, respectively, and the low-frequency superimposing circuits 8a and 8b in FIG. 8 are configured.

An input signal Vin is applied to the gate of one transistor 51 of the drive circuit 5a, and an inverted input signal * Vin is applied to the gate of the other transistor 52,
The signal electrode 15 is connected to the drain of the transistor 51. The input signal Vin is applied to the gate of one transistor 54 of the driving circuit 5b, and the inverted input signal * Vin is applied to the gate of the other transistor 55, and the signal electrode 16 is connected to the drain of the transistor 55. Therefore, if the drive circuits 5a and 5b have the same characteristics,
Symmetrical modulation is performed by the drive signal on which the low-frequency signal Fm is superimposed, and the same operation as in the embodiment shown in FIG. 3 is performed. In addition, the characteristics of the drive circuits 5a and 5b are compared with those of the current sources 53 and 56.
By changing the current value depending on the current value and the like, it is possible to generate asymmetric modulation and generate chirping. The control of stabilizing the operating point of the optical modulator is performed by detecting the superimposed low frequency signal, as in the above-described embodiments.

FIG. 10 is a diagram for explaining the operation of still another embodiment of the present invention. (A) is an input signal in which "1" and "0" are alternately repeated, (b) is a low frequency signal Fm, (C) is a drive signal applied to the signal electrode 15, (d) is a drive signal applied to the signal electrode 16, and (e) is a branched optical waveguide 1.
3 shows the output light phase φ1, (f) shows the output light phase φ2 of the branch optical waveguide 14, and (g) shows the output light phase difference (φ1−φ2) of the branch optical waveguides 13 and 14, respectively. It shows a case where asymmetrical modulation is performed with different characteristics. In this case, the same low frequency signal F is applied to the current sources 53 and 56.
Since m is added as a modulation input, the low frequency signal Fm will be symmetrically superimposed. Therefore, it is possible to control the stable operating point even when the mark ratio suddenly changes.

The present invention is not limited to the above-mentioned embodiments, but it is possible to add a function of selecting symmetrical modulation and asymmetrical modulation.

[0043]

As described above, according to the present invention, the signal electrode 3 and the bias electrode 4 corresponding to the two optical waveguides of the optical modulator 2 are electrically separated from each other. The example bias tee can be omitted. Further, since the low frequency signal can be symmetrically superimposed, the driving signal for modulation is applied to the two optical waveguides.
The drive voltage of the optical modulator 2 can be reduced, the operating point can be stabilized, the fluctuation of the mark ratio can be stabilized, and the deterioration of the modulated output light waveform can be prevented. Accordingly, it is possible to provide a stable operation optical transmitter that modulates with a high-speed input signal of several Gb / s or more.

Further, it becomes easy to perform the symmetrical modulation, and therefore, the modulation without chirping can be performed. When asymmetrical modulation is performed by applying a drive signal to one of the signal electrodes through an attenuator, modulation with chirping is performed, which corresponds to the sign of dispersion of the optical fiber transmitting the optical signal. By causing the chirping described above, the optical pulse width can be compressed, and long-distance transmission can be performed.

When the modulator drive circuit 5 is composed of two differential amplifiers, the same structure can be used for both symmetrical modulation and asymmetrical modulation, and an optical fiber for transmitting an optical signal can be realized. There is an advantage that it is easy to optimize the characteristics of the optical transmission device in the optical communication system in consideration of the characteristics and the like.

[Brief description of drawings]

FIG. 1 is a diagram illustrating the principle of the present invention.

FIG. 2 is an explanatory view of a main part of one embodiment of the present invention.

FIG. 3 is an explanatory diagram of an embodiment of the present invention.

FIG. 4 is an operation explanatory diagram of the embodiment of the present invention.

FIG. 5 is a diagram illustrating the principle of another embodiment of the present invention.

FIG. 6 is an explanatory diagram of another embodiment of the present invention.

FIG. 7 is an operation explanatory diagram of another embodiment of the present invention.

FIG. 8 is a principle explanatory view of still another embodiment of the present invention.

FIG. 9 is an explanatory diagram of still another embodiment of the present invention.

FIG. 10 is an operation explanatory diagram of still another embodiment of the present invention.

FIG. 11 is an explanatory diagram of a conventional example.

[Explanation of symbols]

 1 Light Source 2 Optical Modulator 3 Signal Electrode 4 Biasing Electrode 5 Modulator Drive Circuit 6 Terminator 7 Operating Point Control Circuit 8 Low Frequency Superimposing Circuit 9 Low Frequency Oscillator 10 Optical Branch Circuit

Claims (5)

[Claims] 1. A signal electrode (3) and a bias electrode for inputting light from a light source (1), branching the light into two optical waveguides, and independently applying a voltage to the two optical waveguides. (4) Mach-Zehnder type optical modulator having (2)
And a direct current connection to the signal electrodes (3) corresponding to the two optical waveguides of the optical modulator (2), applying a drive signal according to the input signal, and modulating from the optical modulator (2). A modulator drive circuit (5) for sending the output light, a terminator (6) for terminating the signal electrode (3) by direct current connection, and a direct current connection for the bias electrode (4). An optical transmitter comprising: an operating point control circuit (7) for controlling an operating point of the optical modulator (2). 2. A low frequency superimposing circuit (8) for superimposing a low frequency signal on a drive signal applied from the modulator driving circuit (5) to the signal electrode (3), the operating point control circuit. (7) is configured to control the bias voltage applied to the bias electrode (4) so that the component of the low-frequency signal included in the output light of the optical modulator (2) is minimized. The optical transmission device according to claim 1, wherein: 3. The light according to claim 2, wherein the modulator driving circuit (5) is composed of a differential amplifier, and a current source of the differential amplifier is modulated by the low frequency signal. Transmitter. 4. The modulator drive circuit (5) is composed of a differential amplifier, the current source of the differential amplifier is modulated by the low frequency signal, and directly on one of the signal electrodes (3). Apply a drive signal to the other via an attenuator,
The optical transmitter according to claim 2, wherein the low frequency signal is applied to a bias electrode together with a bias voltage. 5. The modulator driving circuit (5) is composed of two differential amplifiers corresponding to the signal electrodes (3), and respective current sources of the two differential amplifiers are the low frequency. The optical transmitter according to claim 2, wherein the optical transmitter is configured to be modulated by a signal.