JPH1051389A - Optical transmitter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical transmitter from which a desired optical output signal is obtained at all times independently of the code of transmission line wavelength dispersion. SOLUTION: The optical transmitter is provided with a light source 1, an optical modulator 2 modulating an emitted light from the light source, a drive circuit 4 driving an optical modulator based on an input signal, and an operating point stabilizing circuit 3 which detects a drift in an optical modulation characteristic of the optical modulator to control a bias applied to the optical modulator so that the middle point of the optical modulation characteristic is overlapped with the middle of the drive signal and shifts the bias by a half period on the optical modulation characteristic based on an operating point switch signal in response to a code of a transmission line wavelength dispersion and also with a duty ratio variable circuit which identifies the input signal or its inverted signal by a different threshold level and provides an output of the result to the drive circuit based on the operating point switch signal.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an optical transmission device,
More specifically, the present invention relates to an optical transmission device using an external optical modulator. In a high-speed optical communication system, the effect of dynamic wavelength fluctuation (chirping) of an optical signal becomes large, and therefore, an optical transmission device using an external optical modulator having a small amount of chirping is used. This wavelength variation causes an improvement or deterioration of the optical output waveform depending on the sign of the chromatic dispersion of the optical fiber. Therefore, it is desired to provide an optical transmission device that does not easily cause waveform deterioration regardless of the sign of the chromatic dispersion.

[0002]

2. Description of the Related Art FIGS. 9 to 11 are diagrams (1) to (3) for explaining a conventional technology, and FIG. 9 shows a configuration of a conventional optical transmission device (Japanese Patent Laid-Open No. 4-140712). In the figure,
1 is a light source having a constant light output by a laser diode or the like, 2 is a Mach-Zehnder light modulator (MZ light modulator) that modulates light emitted from the light source 1, and 3 is a detector for detecting a drift of light modulation characteristics in the MZ light modulator 2. Controlling the bias applied to the MZ optical modulator 2 so that the center of the optical modulation characteristic overlaps the center of the drive signal, and adjusts the bias based on the operating point switching signal corresponding to the sign of the transmission path chromatic dispersion. An operating point stabilizing circuit for shifting the characteristic by a half cycle, a driving circuit for driving the MZ optical modulator 2 based on an input signal, and an input polarity inverting circuit for inverting the polarity of the input signal based on an operating point switching signal. is there.

In the MZ optical modulator 2, for example, a substrate _{Li is used.}
Using the Z-cut surface of the N _b O _3, an optical branching waveguide 2A, 2B is formed by the like for thermally diffusing T _i on this surface. A traveling wave electrode 22 is provided on the optical branching waveguide 2A, and a ground electrode 23 is provided on the optical branching waveguide 2B. One end of the traveling wave electrode 22 is terminated to ground. In such a configuration,
The light beam from the light source 1 is once branched into two by the optical branching waveguides 2A and 2B, and then combined. In this state, when a voltage _VL corresponding to the input signal = 0 is applied to the traveling wave electrode 22, the difference between the electric fields applied to the optical branch waveguides 2A and 2B at that time causes, for example, λ / 2 And the output optical signal = 0 (non-light emitting state). When a voltage V _H corresponding to the input signal = 1 is applied to the traveling wave electrode 22, 0 or λ is applied between the branched lights due to the difference in the electric field at that time.
Occurs, and the output optical signal = 1 (light emitting state).
That is, as shown in FIG. 10A, the light modulation characteristic is periodic like a sine wave.

In the operating point stabilizing circuit 3, reference numeral 31 denotes a low-frequency oscillator for generating a low-frequency signal having a predetermined frequency f ₀ ;
Is a low-frequency superimposing circuit that applies amplitude modulation to the drive signal using a low-frequency signal, 33 is an optical branching circuit that branches (extracts) a part of the optical signal output, and 34 is an optical-electrical device that converts the extracted optical signal into an electric signal. A conversion circuit 35 is a phase detection circuit that compares the phase of the low-frequency signal component included in the electric signal with the low-frequency signal of the low-frequency oscillator 31. A conversion circuit 36 is provided for the MZ optical modulator 2 based on the detected phase difference signal. A bias control circuit that generates a bias control signal such that the center of the modulation characteristic overlaps the center of the drive signal, and a low-frequency polarity inversion circuit 37 that reverses the polarity of the low-frequency signal based on the operating point switching signal.

FIG. 10 is a diagram for explaining conventional operating point stabilization control. FIG. 10A shows a case where the center of the modulation characteristic of the MZ optical modulator 2 (hereinafter referred to as an operating point A) overlaps the center of the drive signal. When amplitude modulation of low frequency f ₀ is applied to this drive signal as shown in the figure, a modulated component of frequency 2f ₀ appears in the optical output signal.

FIG. 10B shows a case where the operating point A of the MZ optical modulator 2 has drifted to the V _H side of the drive signal due to temperature, power supply fluctuation, and the like. In this case, a modulation component having a frequency f ₀ and substantially the same phase as the drive signal (V _H side) appears in the optical output signal. FIG. 10C shows a case where the operating point A of the MZ optical modulator 2 drifts to the _VL side of the drive signal. In this case, a modulation component having a frequency f ₀ and a phase substantially opposite to that of the drive signal (V _H side) appears in the optical output signal.

Therefore, the operating point A of the MZ optical modulator 2 is always plotted by comparing the phase of the low frequency signal of the low frequency oscillator 31 with the phase of the low frequency signal component detected from the optical output signal (for example, synchronous detection). 10 (A) can be maintained. That is, in the case of FIG. 10A, the synchronous detection output = 0, and the operating point A is settled. In the case of FIG. 10B, the synchronous detection output is greater than 0, and the operating point A at that time is returned to the center (negative direction) by the phase error. In the case of FIG. 10C, the synchronous detection output <0, and the operating point A at that time is returned to the center (positive direction) by the phase error.

FIG. 11 is a diagram for explaining waveform improvement control of an optical output signal in accordance with the sign of the chromatic dispersion of the transmission line. When the center-biased MZ optical modulator 2 is driven by a drive signal having a sharp rise and fall, the phase of the optical output signal advances at the rise of the drive signal (the wavelength λ shifts to the shorter wavelength h side), Also, at the fall of the drive signal, the phase of the optical output signal is delayed (the wavelength λ shifts toward the longer wavelength 1).

FIG. 11A shows a case where the wavelength shifts to the shorter wavelength h when the optical output signal rises, and shifts to the longer wavelength 1 when the optical output signal falls. When such an optical output signal is transmitted through an optical fiber having a negative chromatic dispersion characteristic (-Kps / nm), the short wavelength h side is delayed and the long wavelength 1 side is advanced as shown in FIG. And the eye pattern is improved. However, when this optical output signal is transmitted through an optical fiber having a positive chromatic dispersion characteristic (+ Kps / nm), the short wavelength h advances and the long wavelength 1 delays as shown in FIG. It is elongated and the eye pattern deteriorates.

FIG. 11B shows a case where the wavelength shifts to the long wavelength 1 when the optical output signal rises and shifts to the short wavelength h when the optical output signal falls. When such an optical output signal is transmitted through an optical fiber having a negative chromatic dispersion characteristic, the short wavelength h side is delayed and the long wavelength l side is advanced, as shown in FIG. Deteriorates. However, when this optical output signal is transmitted through an optical fiber having a positive chromatic dispersion characteristic, the short wavelength h side advances and the long wavelength 1 side delays as shown in FIG. Is improved.

Therefore, conventionally, when an optical output signal is transmitted through an optical fiber having negative chromatic dispersion characteristics {see FIG.
(A)｝ is a case in which the MZ optical modulator 2 is biased to the operating point A, is driven by a positive logic drive signal, and the optical output signal is transmitted through an optical fiber having a positive chromatic dispersion characteristic. FIG. 11 (B)｝ biases the MZ optical modulator 2 to an operating point B shifted by a half cycle on its characteristics,
This is driven by a negative logic drive signal. Therefore, the eye pattern can be improved irrespective of the sign of the transmission path chromatic dispersion.

[0012]

FIGS. 12 and 13 are diagrams (1) and (3) for explaining the problems of the prior art. By the way, in this type of optical transmission device, there are cases where it is desired to set the duty ratio of the optical output signal as desired. In such a case, for example, as shown in FIG. 12A, a comparator circuit (identification circuit) (not shown) is used to identify the input signal with a predetermined threshold value V _TH so that the duty ratio of the optical output signal is set to a desired value. It can be set.

However, according to the conventional method, when the polarity of the input signal (drive signal) is inverted to use the operating point B of the MZ optical modulator 2, as shown in FIG.
There is a disadvantage that the duty ratio of the optical output signal changes. In this type of optical transmission device, since the input signal is high-speed, the driving signal is, for example, as shown in FIG.
As shown in the figure, the high / low level of the drive signal differs between the case where the input signal changes at a high speed such as "101010 ..." and the case where the input signal changes at a relatively low speed such as "110011 ...". This is called intersymbol interference. For this reason, in FIG. 13A, the non-light-emitting side level of the optical output signal does not completely become 0 (non-light-emitting), and the extinction ratio and, consequently, the sensitivity when light is received by the receiver are deteriorated. This is the same in the case of FIG.

In this type of optical transmission device, it is expected that the reception characteristics of the partner device can be improved by reducing the intersymbol interference of the optical output signal. In such a case, for example, as shown in FIG. 14A, the operating point A of the MZ optical modulator 2 is shifted to the V _H side of the drive signal, and the non-emission side of the optical output signal is compressed, so that the other side is compressed. It is conceivable to improve the receiving characteristics of the device.

However, according to the above conventional method, when the input signal is inverted to use the operating point B of the MZ optical modulator 2, the light output side of the optical output signal is compressed as shown in FIG. As a result, there is a disadvantage that the light emission side waveform of the optical output signal changes. SUMMARY OF THE INVENTION The present invention has been made in view of the above-described problems of the related art, and an object of the present invention is to provide an optical transmission device that can always obtain a desired optical output signal regardless of the code of transmission line chromatic dispersion. It is in.

[0016]

The above-mentioned problem is solved, for example, by referring to FIG.
The problem is solved by the configuration of (A). That is, the optical transmitter of the present invention (1) comprises a light source 1, an optical modulator 2 for modulating light emitted from the light source, a driving circuit 4 for driving the optical modulator based on an input signal, Detecting the drift of the optical modulation characteristic in the optical modulator, controlling the bias applied to the optical modulator so that the center of the optical modulation characteristic overlaps the center of the drive signal, and switching the operating point in accordance with the sign of the transmission line wavelength dispersion. An operating point stabilizing circuit 3 for shifting the bias by a half cycle on the optical modulation characteristic based on a signal, wherein the input signal or its polarity inverted signal is identified by a different threshold based on the operating point switching signal. In addition, a variable duty ratio circuit 6 for outputting to the drive circuit 4 is provided.

According to the present invention (1), for example, FIG.
As shown in (1), for a positive-polarity input signal (shown by a dotted line), a desired duty ratio can be obtained in the optical output signal by identifying the input signal with the first threshold value V _TH1 . Also, as shown in FIG. 3B, for an inverted input signal of negative polarity (indicated by a dotted line), this is identified by a second threshold V _TH2 different from the above, so that an optical output signal is obtained. Can obtain a desired duty ratio. Therefore, an optical output signal having a desired, preferably the same duty ratio is always obtained irrespective of the sign of the transmission line chromatic dispersion.

The above-mentioned problem can be solved, for example, by the structure shown in FIG. That is, in the optical transmission device according to the present invention (2), in the optical transmission device based on the above premise, the drive circuit 4 changes a non-light-emitting side signal level of the input signal or its polarity inverted signal based on the operating point switching signal. It has a non-linear amplifier circuit for compression. According to the present invention (2),
For example, as shown in FIG. 5A, for a positive input signal having a high / low level that differs depending on the signal change speed, the non-light-emitting (V _L ) side of the drive signal is compressed. A desired extinction ratio (substantially complete non-emission state) can be obtained for the light output signal regardless of the signal change speed. In addition, as shown in FIG. 5B, for a negative inversion input signal having a high / low level that differs depending on the signal change speed, the non-light-emitting (V _H ) side of the drive signal is compressed. By doing so, a desired extinction ratio (substantially complete non-emission state) can be obtained for the optical output signal regardless of the signal change speed. Therefore, an optical output signal having a desired, preferably the same extinction ratio is always obtained irrespective of the sign of the transmission line chromatic dispersion.

The above problem can be solved, for example, by the structure shown in FIG. That is, the optical transmitter of the present invention (3) comprises a light source 1 and an optical modulator 2 for modulating the light emitted from the light source.
A drive circuit 4 for driving the optical modulator based on an input signal; and detecting the drift of the optical modulation characteristic in the optical modulator and performing the optical modulation so that the center of the optical modulation characteristic overlaps the center of the drive signal. Controls the bias applied to the vessel,
An operating point stabilizing circuit 3 for shifting the bias by a half cycle on the optical modulation characteristic based on an operating point switching signal corresponding to the sign of the transmission path chromatic dispersion; and inverting the polarity of the input signal based on the operating point switching signal. An input polarity inverting circuit 5 for causing the operating point stabilizing circuit 3
Comprises an offset circuit for offsetting a bias applied to the optical modulator in response to the operating point switching signal from a center of the drive signal.

According to the present invention (3), for example, FIG.
As shown in the figure, for the positive input signal (drive signal), the bias of the optical modulator is changed from the central operating point A to the operating point A.
By offsetting to the 'side, a desired signal waveform that can improve, for example, intersymbol interference can be obtained in the optical output signal. For example, as shown in FIG. 8B, the bias of the optical modulator is offset from the central operating point B to the operating point B ′ for an inverted input signal (inverted drive signal) of negative polarity. In the optical output signal, a desired signal waveform that can improve, for example, intersymbol interference can be obtained. Accordingly, an optical output signal having a desired, preferably the same signal waveform is always obtained irrespective of the sign of the transmission line chromatic dispersion.

Preferably, in the present invention (4), in the above present inventions (1) to (3), the optical modulator 2 is a Mach-Zehnder type optical modulator. Also preferably, in the present invention (5), in the present invention (2), the nonlinear amplifier circuit shifts the cross point position of the eye pattern in the optical output signal to the non-light emitting side by 3 to 15% from the center thereof. Amplify nonlinearly as follows. Therefore, intersymbol interference can be improved.

Preferably, in the present invention (6), the offset circuit according to the present invention (3), wherein the cross point position of the eye pattern in the optical output signal is 3 to 15% from the center thereof to the non-light emitting side. Offset to shift. Therefore, intersymbol interference can be improved. Preferably, in the present invention (7), in the above-mentioned present invention (3), the operating point stabilizing circuit includes a low frequency superimposing circuit for amplitude-modulating the drive signal with a low frequency signal, and The modulation degree of the low-frequency superimposed component is changed accordingly. Therefore, regardless of the bias offset,
A constant degree of modulation can be maintained.

[0023]

Preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Note that the same reference numerals indicate the same or corresponding parts throughout the drawings. FIG. 2 is a diagram showing a configuration of the optical transmission device according to the first embodiment. The figure shows a duty ratio for identifying an input signal or its polarity inversion signal with different thresholds based on an operating point switching signal and outputting the same to a drive circuit. The case where a variable circuit is provided is shown.

In the figure, reference numeral 5 denotes an input polarity inversion circuit (EX
-OR circuit), 6 is a duty ratio variable circuit, CMP is a high-speed comparator circuit (identification circuit), ASW is an analog switch, A1 and A2 are operational amplifiers constituting a voltage follower, and VR1 and VR2 are variable resistors. Other than this, the operating point stabilizing circuit 3, the driving circuit 4, and the like may be the same as those in FIG.

FIG. 3 is a diagram for explaining the operation of the optical transmitter according to the first embodiment. The operation will be described below with reference to FIGS. FIG. 3A shows a case where the optical modulator 2 is biased to the operating point A (that is, a case where the sign of the transmission path chromatic dispersion is negative). When the sign of the chromatic dispersion is negative, the operating point switching signal = 0. The EX-OR circuit 5 outputs an input signal (shown by a dotted line) having the same polarity as the input signal to the comparator circuit C.
Input to + input terminal of MP. On the other hand, the analog switch ASW selectively outputs the threshold voltage V _TH1 generated by the operational amplifier A1. Thereby, the comparator circuit CMP generates a discrimination pulse signal (shown by a solid line) which becomes positive when the input signal> V _TH1 . This is linearly amplified by a driving circuit 4 at the subsequent stage, and a low frequency signal is further superimposed by a low frequency superimposing circuit 32 shown in FIG.

In this case, the threshold voltage V _TH1 is set higher (closer to V _H ) than the center of the amplitude values _{VL to} V _H of the input signal. Therefore, the duty ratio of the drive signal is set to a desired value smaller than that of the input signal. As a result, a desired duty ratio can be obtained for the optical output signal. FIG.
(B) shows a case where the optical modulator 2 is biased to the operating point B (that is, a case where the sign of the transmission path chromatic dispersion is positive).

When the sign of the chromatic dispersion is positive, the operating point switching signal = 1. The EX-OR circuit 5 outputs an inverted input signal (indicated by a dotted line) having a polarity opposite to that of the input signal to the comparator circuit CMP.
Input to the + input terminal. On the other hand, the analog switch AS
W selects and outputs the threshold voltage V _TH2 generated by the operational amplifier A2. As a result, the comparator circuit CMP generates a discrimination pulse signal (shown by a solid line) that becomes negative when the inverted input signal <V _TH2 . This is linearly amplified by a driving circuit 4 at the subsequent stage, and a low-frequency signal is further superimposed by a low-frequency superimposing circuit 32, and is added to the MZ optical modulator 2.

In this case, the threshold voltage V _TH2 is set lower (closer to V _L ) than the center of the amplitude values V _{L to} V _H of the inverted input signal. Therefore, the duty ratio of the inversion drive signal is set to a desired value smaller than that of the inversion input signal. As a result, a desired duty ratio can be obtained for the optical output signal. Thus, the duty ratio of the optical output signal shown in FIG. 3A and the duty ratio of the optical output signal shown in FIG. it can.

It is apparent that the variable duty ratio circuit 6 and the driving circuit 4 may be integrated.
Although not shown, an optical electroabsorption type optical modulator may be used instead of the MZ optical modulator 2. The voltage-light conversion characteristic of the optical electric field absorption type optical modulator does not have periodicity like the MZ light modulator 2, but has a voltage-light conversion characteristic of a negative characteristic with respect to an applied electric field, While the conversion characteristic shows nonlinearity, the amount of chirping changes with the applied voltage. For example, the 1996 IEICE General Conference B
-1103, Oi et al., "Investigation of 10 Gb / s SMF Transmission Characteristics Using Modulator Integrated DFB Laser".

When the applied voltage value is changed for chirping switching, the duty ratio becomes different before and after the chirping switching due to the non-linearity of the electric-optical conversion characteristics. A variable duty ratio circuit (of positive polarity) is provided on the input signal line, and the discrimination level is changed in accordance with the switching of chirping to change the duty ratio of the drive signal. The duty ratio can be made the same.

FIG. 4 is a diagram showing a configuration of an optical transmission apparatus according to a second embodiment. The figure shows that the driving circuit operates based on an operating point switching signal to control the input signal or its non-light-emitting signal level of the polarity-inverted signal. Is provided with a non-linear amplifier circuit for compressing. In the figure, 4 is a drive circuit according to the present embodiment, 5 is an input polarity inversion circuit (EX-OR circuit), and ASW
Is an analog switch, and T1 is an N-channel FET.
The rest of the operating point stabilizing circuit 3 and the like may be the same as in FIG.

FIG. 5 is a diagram for explaining the operation of the optical transmitter according to the second embodiment. The operation will be described below with reference to FIGS. FIG. 5A shows a case where the optical modulator 2 is biased to the operating point A. The operating point switching signal = 0, the analog switch ASW is selectively outputs the gate bias voltage V _G1 of the FETs T1. This voltage V _G1 is the FET
The gate-source voltage V _GS of T1 is changed to its threshold voltage V
It is set to shift to the _T (pinch-off) side. For this reason, the non-light-emitting side level (logic 0 level) of the input signal is subjected to signal compression (non-linear amplification). The output signal of the FET T1 is superimposed on a low-frequency signal and applied to the MZ optical modulator 2. Therefore, if the input signal is "10100110 ..."
Even if it changes, the non-light-emitting level of the drive signal is compressed to the voltage _VL side, and the extinction ratio of the light output signal is improved.

FIG. 5B shows a case where the optical modulator 2 is biased to the operating point B. The operating point switching signal = 1, the analog switch ASW is selectively outputs the gate bias voltage V _G2 of the FETs T1. This voltage V _G2 is equal to the FET T
1 gate-source voltage V _GS to its threshold voltage V _T
It is set so as to shift to the portion of the non-linear characteristic opposite to the side. Therefore, the non-light-emitting side (V _H ) of the inverted input signal is subjected to signal compression (non-linear amplification). Therefore, even if the inverted input signal changes to “01011010...” Or the like, the non-emission side level of the inverted drive signal is the voltage V
_It is compressed to the _H side, and the extinction ratio of the optical output signal is improved.
Thus, regardless of the sign of the transmission line chromatic dispersion, FIG.
And the extinction ratio of the optical output signal of FIG. 5B can be set to a desired, preferably the same, extinction ratio.

FIG. 6 is a diagram showing a configuration of an optical transmission apparatus according to a second alternative embodiment. FIG. 6 shows a drive circuit in which the non-light-emitting side signal level is compressed in advance based on an operating point switching signal. The figure shows a case where another non-linear amplifier circuit for selecting the input signal or the inverted input signal is provided. In the figure, 4 is a drive circuit according to another embodiment, I is an inverter circuit, A
SW is an analog switch, T1 is an N-channel FET, T
2 is a P-channel FET. The FET may be a MOS type or a junction type. Other operating point stabilizing circuits 3 and the like may be the same as those in FIG.

The gate-source voltage V _GS is applied to the gate of the N-channel FET T1 by its own threshold voltage V
A gate bias voltage V _G1 for offsetting to the _T (pinch-off) side is applied. Therefore, the N-channel FET T1 compresses (nonlinearly amplifies) the non-light emitting side level (V _L ) of the input signal. On the other hand, P-channel FET T2
A gate bias voltage V _G2 that offsets the gate-source voltage V _GS to its own threshold voltage V _T (pinch-off) side is applied to the gate. Therefore, the P-channel FET T2 compresses (non-linearly amplifies) the non-light-emitting side level (V _H ) of the inverted input signal. T1, T
2 are configured symmetrically, and therefore, the same and symmetrical non-linear amplification characteristics are obtained.

When the operating point switching signal = 0, the analog switch ASW selects and outputs the output signal of the FET T1. Although not shown, the output signal is once again subjected to linear inversion amplification by the low frequency superimposing circuit 32, superimposed with a low frequency signal, and applied to the MZ optical modulator 2. Therefore, even if the input signal changes to "10100110 ..." or the like, the non-light-emitting level of the drive signal is compressed to the voltage _VL side, and the extinction ratio of the light output signal is improved. )reference}.

In response to the operating point switching signal = 1, the analog switch ASW selects and outputs the output signal of the FET T2. This output signal is once again subjected to linear inversion amplification by the low-frequency superimposing circuit 32, and further superimposed with a low-frequency signal.
It is applied to the light modulator 2. Therefore, when the inverted input signal is “0”
1011001... ”And the like, the non-emission side level of the inverted drive signal is compressed to the voltage V _H side, and the extinction ratio of the optical output signal is improved (see FIG. 5B).

Thus, irrespective of the sign of the chromatic dispersion of the transmission line, the extinction ratio of the optical output signal in FIG. 5A and the extinction ratio of the optical output signal in FIG. The extinction ratio can be set. At the same time, there is an effect of improving intersymbol interference. The reason will be described later. The analog switch A
If the connection logic of SW is reversed, it is obvious that the output signal does not need to be again subjected to linear inversion amplification by the low frequency superimposing circuit 32.

FIG. 7 is a diagram showing a configuration of an optical transmitter according to a third embodiment. The figure shows an operating point stabilizing circuit which applies a bias applied to an optical modulator to an optical modulator in response to an operating point switching signal. 2 shows a case in which an offset circuit for offsetting from the center of the line is provided. In the figure, 36 is a bias control circuit according to the present embodiment, A1 is an operational amplifier constituting a differential amplifier, 36a is an offset circuit, ASW is an analog switch, A2 and A3 are operational amplifiers constituting a voltage follower, and VR1 and VR2 are variable. The resistor 38 is a variable attenuator.

The phase detecting circuit 35 includes a low-frequency extracting circuit 35a for extracting a low-frequency signal component from the output signal of the optical-electrical converting circuit 34, a low-frequency signal and a low-frequency polarity inverting circuit. It comprises a synchronous detection circuit 35b for comparing the phase with the low-frequency signal output from the output 37, and a low-pass filter (LPF) 35c for integrating the detected phase difference.
The low-frequency polarity inverting circuit 37 inverts / non-inverts the polarity of the input low-frequency signal in accordance with 1/0 of the operating point switching signal, and accordingly signs the detected (converged) signal in the synchronous detection circuit 35b. Are inverted / non-inverted. Other circuit configurations may be the same as in FIG.

FIG. 8 is a diagram for explaining the operation of the optical transmitter according to the third embodiment. The operation will be described below with reference to FIGS. FIG. 8A shows a case where the optical modulator 2 is biased to the operating point A. However, in the present embodiment, the bias is biased to an operating point A ′ offset from the operating point A.

When the operating point switching signal = 0, the analog switch ASW switches the bias voltage V generated by the operational amplifier A2.
Selectively output _B1 . The bias voltage V _B1 is offset from the bias voltage _VA corresponding to the operating point _A to the V _H side by the amount by which the central operating point A is shifted to the operating point A ′. For this reason, the optical conversion characteristic of the MZ optical modulator 2 is offset to the operating point A 'side, whereby the non-light emitting side of the optical output signal is compressed, and the intersymbol interference of the non-light emitting side signal can be reduced. That is, it is possible to improve the reception characteristics of the partner device.

FIG. 8B shows a case where the optical modulator 2 is biased to the operating point B. However, in the present embodiment, the bias is biased to an operating point B ′ offset from the operating point B. In response to the operating point switching signal = 1, the analog switch ASW selectively outputs the bias voltage V _B2 generated by the operational amplifier A3. The bias voltage V _B2 is offset from the bias voltage V _B corresponding to the operating point B to the _VL side by the amount by which the central operating point B is shifted to the operating point B ′. For this reason, the non-light-emitting side of the optical output signal is compressed, and intersymbol interference of the non-light-emitting side signal can be reduced. That is, it is possible to improve the reception characteristics of the partner device. Thus, the intersymbol interference of the optical output signal of FIG. 8A and the intersymbol interference of the optical output signal of FIG. Interference characteristics can be obtained.

If the bias value is offset, the low frequency superposition efficiency may change. In such a case, a variable attenuator 38 is inserted between the low-frequency oscillator 31 and the low-frequency superimposing circuit 32 so that the low-frequency superimposed component superimposed on the optical output signal is adjusted to be constant. Incidentally, the amount of intersymbol interference generally occurring on the HIGH and LOW sides of the drive signal waveform is converted into the amount of intersymbol interference on the light emission and non-light emission sides of the optical output signal waveform. In such a case, the drive signal is nonlinearly amplified, or the optical modulator 2
By offsetting the operating point from the center, it is possible to change the distribution of the intersymbol interference amount converted to the light emitting side and the non-light emitting side.

In this case, the amount of intersymbol interference on the non-light emitting side is reduced and the amount of intersymbol interference on the light emitting side is reduced accordingly, as compared with the case where the intersymbol interference amounts occurring on the light emitting side and the non-light emitting side are equally distributed. Increasing the value improves the receiving sensitivity. The reason is that, when the intersymbol interference amount exists on the light emission side of the optical output signal waveform, the optical shot noise or the beat noise with the ASE light is reduced by an amount corresponding to the decrease in the peak value due to the intersymbol interference. .

Now, the light receiving element and the optical preamplifier are combined.
Assuming an optical receiver, the noise of the optical receiver is marked
It is considered that beat noise on the (light emission) side is dominant. Optical pre
If the gain of the amplifier is large enough, the SNR of the optical receiver will be
It can be approximated as follows. S / N = 10log ｛N₁ ^Two(1-I)^Two/ 4n_spN₁B₁(1-I_1s)｝ = 10log ｛N₁ ^Two(1-I_1S-I_0S-I_R)^Two/ 4n_spN₁B₁ (1-I_1s)｝ I ≒ I_1S+ I_0S+ I_R Where N₁: Number of photons on the optical signal mark side input to the optical preamplifier I: Inter-symbol when the optical transmitter and optical receiver face
Negotiation B₁: Reception band of optical receiver n_sp: Spontaneous emission coefficient of optical preamplifier I_1s: Intersymbol interference on the optical signal mark side generated by the optical transmitter
Quantity I_0s： Optical signal space (non-light emitting) side generated by optical transmitter
Intersymbol Interference I_R: Intersymbol interference generated in the optical receiver According to the above equation, the intersymbol interference (I_1s+ I_0s) Is constant
Then, the intersymbol interference amount I _1sIt is better to increase the distribution to
It turns out that N improves.

Further, since the amount of noise on the light emitting side is larger than that on the non-light emitting side, the optimum identification level of the optical receiver is located closer to the non-light emitting side than to the center of the amplitude of the optical waveform. By shifting the cross point of the light waveform to the non-light emitting side, the eye opening in the phase direction at the optimum identification level can be widened. Thus, in the second embodiment, the non-light-emitting side of the drive signal is compressed, and in the third embodiment, the operating point of the optical modulator 2 is offset from the center, so that the optical output signal The space (non-light emitting) side inter-code interference amount is suppressed. That is, the mark side intersymbol interference amount I _1s
The distribution to is increased.

According to experiments and computer simulations, the present inventors have found that in the second embodiment, the cross point position of the eye pattern in the optical output signal is shifted from the center by 3 to 15% to the non-light emitting side. As a result, the result that the bit error rate on the optical receiver side is minimized was obtained. In the third embodiment,
By offsetting the bias so that the cross point position of the eye pattern in the optical output signal is shifted by 3 to 15% from the center to the non-light-emitting side, the code error rate on the optical receiver side is minimized as described above. I got

In the third embodiment, the bias control circuit 36 is provided with the offset circuit 36 according to the present embodiment.
Although a was provided, it is not limited to this. Offset component (control)
Are any other circuits related to the bias feedback loop in the operating point stabilizing circuit 3 (optical-electrical converting circuit 3
4, the phase detection circuit 35, the low-frequency polarity inversion circuit 37, etc.).

Although a plurality of embodiments suitable for the present invention have been described, it is to be understood that various changes can be made in the configuration, control, and combination of these components without departing from the spirit of the present invention. Not even.

[0051]

As described above, according to the present invention, a desired optical output signal can be always obtained in an optical transmitting apparatus irrespective of the code of chromatic dispersion of a transmission line, and the receiving side has improved receiving sensitivity characteristics. it can.

[Brief description of the drawings]

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

FIG. 2 is a diagram illustrating a configuration of an optical transmission device according to a first embodiment.

FIG. 3 is a diagram illustrating an operation of the optical transmission device according to the first embodiment.

FIG. 4 is a diagram illustrating a configuration of an optical transmission device according to a second embodiment.

FIG. 5 is a diagram illustrating an operation of the optical transmission device according to the second embodiment.

FIG. 6 is a diagram illustrating a configuration of an optical transmission device according to a second other embodiment.

FIG. 7 is a diagram illustrating a configuration of an optical transmission device according to a third embodiment.

FIG. 8 is a diagram illustrating an operation of the optical transmission device according to the third embodiment.

FIG. 9 is a diagram (1) for explaining a conventional technique;

FIG. 10 is a diagram (2) for explaining a conventional technique;

FIG. 11 is a diagram (3) illustrating a conventional technique;

FIG. 12 is a diagram (1) illustrating a problem of the related art.

FIG. 13 is a diagram (2) illustrating a problem of the related art.

FIG. 14 is a diagram (3) for explaining a problem of the related art;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Light source 2 Mach-Zehnder type optical modulator 2A, 2B Optical branch waveguide 3 Operating point stabilization circuit 4 Drive circuit 5 Input polarity inversion circuit 6 Duty ratio variable circuit 22 Traveling wave electrode 23 Earth electrode 31 Low frequency oscillator 32 Low frequency superposition circuit Reference Signs List 33 optical branch circuit 34 optical-electrical conversion circuit 35 phase detection circuit 36 bias control circuit 37 low frequency polarity inversion circuit 38 variable attenuator

Continued on the front page (51) Int.Cl. ⁶ Identification number Agency reference number FI Technical display location H04B 10/02 10/18

Claims (7)

[Claims] 1. A light source, an optical modulator for modulating light emitted from the light source, a driving circuit for driving the optical modulator based on an input signal, and detecting a drift of an optical modulation characteristic in the optical modulator. The bias applied to the optical modulator is controlled so that the center of the optical modulation characteristic overlaps the center of the drive signal, and the bias is adjusted based on the operating point switching signal according to the sign of the transmission path chromatic dispersion. An operating point stabilizing circuit that shifts by a half cycle in the optical transmission device. An optical transmission device, comprising: 2. A light source, an optical modulator for modulating light emitted from the light source, a driving circuit for driving the optical modulator based on an input signal, and detecting a drift of an optical modulation characteristic in the optical modulator. The bias applied to the optical modulator is controlled so that the center of the optical modulation characteristic overlaps the center of the drive signal, and the bias is adjusted based on the operating point switching signal according to the sign of the transmission path chromatic dispersion. An operating point stabilizing circuit for shifting the half-period by a half-period, wherein the driving circuit is configured to compress a non-light-emitting side signal level of the input signal or its polarity inverted signal based on the operating point switching signal. An optical transmission device comprising: 3. A light source, an optical modulator for modulating light emitted from the light source, a driving circuit for driving the optical modulator based on an input signal, and detecting a drift of an optical modulation characteristic in the optical modulator. The bias applied to the optical modulator is controlled so that the center of the optical modulation characteristic overlaps the center of the drive signal, and the bias is adjusted based on the operating point switching signal according to the sign of the transmission path chromatic dispersion. An optical transmission device comprising: an operating point stabilizing circuit that shifts by a half cycle at; and an input polarity inverting circuit that inverts the polarity of the input signal based on the operating point switching signal. An optical transmission device, comprising: an offset circuit that offsets a bias applied to the optical modulator from a center of the drive signal in response to a switching signal. 4. The optical transmission device according to claim 1, wherein the optical modulator is a Mach-Zehnder type optical modulator. 5. The nonlinear amplification circuit according to claim 2, wherein the nonlinear amplification circuit performs nonlinear amplification such that the cross point position of the eye pattern in the optical output signal is shifted from the center by 3 to 15% to the non-light-emitting side. Optical transmitter. 6. The light according to claim 3, wherein the offset circuit offsets such that the cross point position of the eye pattern in the optical output signal is shifted from the center by 3 to 15% to the non-light emitting side. Transmission device. 7. The operating point stabilizing circuit includes a low-frequency superimposing circuit for amplitude-modulating a drive signal with a low-frequency signal, and changes a modulation degree of a low-frequency superimposed component according to a bias offset amount. The optical transmission device according to claim 3, wherein