JPH04140712A - Optical transmission equipment - Google Patents

Abstract

PURPOSE:To preclude wavelength deterioration due to wavelength dispersion by shifting an operation point by a half cycle on an operation characteristic curve with the use of an operation point switching signal. CONSTITUTION:The waveform of the input signal of an MZ optical modulator 6A includes a superposed low frequency signal. The modulator 6 is driven with the signal to obtain an amplitude-modulated output light signal. When a driving voltage is set to the upper half cycle of the input/output characteristic of the modulator 6A while there is no operation point drift, the low frequency signal component of the input signal is modulated in mutually opposite-phase relation. No frequency component is therefore detected in the output of a photoelectric converting circuit 18. If an operation drift is generated, the low frequency component of the input signal is modulated in in-phase relation and the output light signal is 180 deg. different. A DC signal with a level corresponding to the operation point drift is obtained in the output of a phase detecting circuit 20. A bias control circuit 22 controls the bias of the modulator 6A so that the output light signal contains no frequency component, and corrects the operation point drift to hold the operation point.

Description

[Detailed Description of the Invention] Table of Contents Overview Field of Industrial Application Conventional Technology Problems to be Solved by the Invention Means for Solving the Problems Implementation Example Summary of Effects of the Invention Concerning an optical transmitter, it is possible to solve the problem of wavelength dispersion. The purpose of the present invention is to provide an optical transmitter that can prevent waveform deterioration caused by wavelength dispersion both when the sign is positive and when the sign is negative. Modulating the light emitted from the light source according to the voltage,
an optical modulator that converts the input signal into an optical signal; and an optical modulator that detects the drift of the operating characteristic curve of the optical modulator so that the operating point is at a constant position with respect to the operating characteristic curve. The optical transmitter is equipped with an operating point stabilizing circuit that controls the operating point, and includes an operating point shift circuit that shifts the operating point by half a period on the operating characteristic curve based on an operating point switching signal.

Industrial applications The present invention relates to an optical transmitter.

In recent years, as optical communications have become faster, development of external modulation methods using optical modulators has been progressing as an optical modulation method that is less susceptible to the chromatic dispersion of optical fibers used as optical transmission lines. A Matsuhatsu Enda type optical modulator using L I Nb 03 is attracting attention as an optical modulator with excellent modulation characteristics and wavelength dispersion resistance. However, it has been confirmed that wavelength fluctuations occur even in the Matsuhatsu Enda type optical modulator, and waveform changes occur through the wavelength dispersion of the optical fiber (T, Okiyama et at, "1OGb
/s Transmission inshilarge-D
ISPMirsion Fiber Using
aTi: LiNb0:+Mach-Xen
der Modulator, l0DC'39
vol, 3°PP, 2011209). This wavelength fluctuation causes improvement or deterioration of the waveform depending on the sign of the chromatic dispersion of the optical fiber, so there is a demand for an optical transmitter that is less susceptible to waveform deterioration regardless of the sign of the chromatic dispersion.

2. Description of the Related Art Conventionally, as shown in FIG. 13, a light source 2, a drive circuit 4 that generates a drive voltage according to an input signal, and a drive circuit 4 that modulates the emitted light R2 according to the drive voltage and converts the input signal into light. An optical modulator 6 that converts into a signal, and an operating point that detects the drift of the operating characteristic curve of the optical modulator 6 and controls the optical modulator 6 so that the operating point is at a constant position with respect to the operating characteristic curve. Stabilization circuit 8
(Kuwata et al., "Study of automatic bias control circuit for Matsuhatsu Enda type optical modulator J,"
1990 IEICE Spring National Conference, B-976
>. When using such an external modulation method, the wavelength fluctuation (chirping) of the optical signal is improved compared to the case using a direct modulation method where the injection current of the semiconductor laser is modulated by the data signal, but it is especially important for high-speed transmission. In some cases, the effect of chirping in the external modulation method cannot be ignored.

FIG. 14 is a diagram showing an example of computer simulation of waveform deterioration due to optical fiber transmission when wavelength fluctuation occurs in a Matsuhatsu Enda type optical modulator. Figure (a)
, et al.), and (C) have wavelength dispersion of Ops/nm, respectively.
The simulation results of the eye pattern at 600 ps/nm and -600 ps/nm are shown.

In this example, it can be seen that the waveform is improved for chromatic dispersion with a positive sign, but the waveform is degraded for chromatic dispersion with a negative sign.

Problems to be Solved by the Invention When using an optical fiber with a wavelength of 1.3 μm (zero dispersion wavelength (long wavelength at which wavelength dispersion becomes 0)) for a light source with a wavelength of 1.3 μm, or when using an optical fiber with a wavelength of 1.5 μm When using an optical fiber in the zero-dispersion wavelength band, such as when using an optical fiber with a zero-dispersion wavelength of 1.5 μm for a light source, variations in the zero-dispersion wavelength of the optical fiber and the light source The sign of chromatic dispersion can be positive or negative depending on the variation in wavelength. Further, when using an optical fiber installed over a long distance, the total chromatic dispersion becomes large even if a wavelength (nearly zero dispersion) is used. In particular, in systems using optical amplifiers, extremely long distance transmission is possible by compensating the loss of the optical fiber with the gain of the optical amplifier. The influence of dispersion cannot be ignored. in this way,
According to the prior art, when the sign of chromatic dispersion can be positive or negative, there is a problem that waveform deterioration is more likely to occur due to one of the dispersions.

The present invention was created in view of such technical problems, and provides an optical transmitter that can prevent waveform deterioration caused by chromatic dispersion, whether the sign of chromatic dispersion is positive or negative. The purpose is to

Means to solve problems FIG. 1 is a diagram showing the configuration of the present invention.

The optical transmitter of the present invention includes a light source 2, a drive circuit 4 that generates a drive voltage according to an input signal, and a drive circuit 4 that modulates the light emitted from the light source 2 according to the drive voltage to convert the input signal into an optical signal. An operation of detecting the optical modulator 6 to be converted and the drift of the operating characteristic curve of the optical modulator 6, and controlling the optical modulator 6 so that the operating point is at a constant position with respect to the operating characteristic curve. The optical transmitting apparatus includes an operating point shift circuit 10 that softens the operating point by half a cycle on the operating characteristic curve based on an operating point switching signal.

Preferably, the optical modulator is a Matsuhatsu Enda type optical modulator with different phase modulation efficiencies in the optical waveguide branched into two paths.

Preferably, a polarity inversion circuit is provided that inverts the polarity of the input signal in response to the shift of the operating point by the operating point software circuit.

Effect In a Matsuhatsu Enda optical modulator with different phase modulation efficiencies in the leading waveguide that branches into two paths, if the operating point is shifted by half a period on the operating characteristic curve, the initial phase difference in the optical waveguide that branches into two paths changes. Therefore, the phase of the light can be advanced or delayed at the rise of the optical pulse. When the phase advances, the wavelength becomes instantaneously shorter, and when the phase lags, the wavelength instantaneously becomes longer, so that waveform deterioration caused by chromatic dispersion can be effectively prevented.

If an optical modulator other than the Matsuhatsu Enda optical modulator with different phase modulation efficiencies in the optical waveguide branched into two paths is used, the input signal can be adjusted by shifting the operating point by half a cycle on the operating characteristic curve. −1− and 1“0=
It becomes possible to switch the output optical signal corresponding to 1-on- and -10,000-fu.

Example The present invention will be explained in detail below.

FIG. 2 is a block diagram of an optical transmitter showing a first embodiment of the present invention. As the light source 2, a semiconductor laser driven by a steady current can be used.

In this embodiment, the optical modulator is a Matsuhatsu Enda type optical modulator (hereinafter sometimes referred to as rMl optical modulator 2).

) 6A can be used. , the configuration and operation of the MZ optical modulator 6 will be detailed later. A drive voltage is applied to the MZ optical converter 6A by the drive circuit 4, and a bias voltage is controlled via the drive circuit 4 by the function of the operating point stabilization circuit 8.

24 is a first polarity inversion circuit that inverts the polarity of the input signal in response to the shift of the operating point by the operating point shift circuit.

The operating point stabilization circuit 8 includes a low frequency oscillator 12 that outputs a low frequency signal of a predetermined frequency (-f.), and a low frequency superimposition circuit that applies amplitude modulation to an input signal using this low frequency signal and sends it to the drive circuit 4. A circuit 14, an optical branching circuit 16 that branches the optical signal output from the MZ optical modulator 6A, and an optical-electrical conversion circuit that converts one of the optical signals branched by the optical branching circuit 16 into an electrical signal. 18, the frequency component of the low frequency signal contained in this electrical signal is detected, the phase of this frequency component is compared with the phase of the low frequency signal, and the polarity is determined according to the phase difference and according to the operating point drift. A phase detection circuit 20 that outputs a DC signal at a level D, and a bias control circuit 22 that feedback controls the bias of the MZ optical modulator 6A so that the DC signal becomes D.

Figure 3 shows the waveform of the output optical signal of the optical modulator, and Figure 4 shows the occurrence of positive operating point drift (drift of the operating characteristic curve (curve showing the relationship between output optical power and drive voltage)). FIG. 5 is a diagram showing the waveform of the output optical signal when a negative operating point drift occurs.

Hereinafter, with reference to FIGS. 2 to 5, ~4Z optical modulator 6Δ
The control operation of the operating point according to the operating point drift will be explained.

The waveform of the input signal converted into an optical signal by the MZ optical modulator 6A is a low frequency signal superimposed on the input signal, as shown in FIG. 3(a). When the MZ optical modulator 6A having the input/output characteristics (operating characteristic curve) shown in FIG. 3(b) is driven with an input signal as shown in FIG. 3(c),
The frequency is 2f as shown.

An output optical signal whose amplitude is modulated by the signal is obtained.

In a state where there is no operating point drift, the drive voltage V corresponds to each logic level of the input signal. , vl are MZ optical modulators 6.
Between the upper and lower peak values (half cycle) on the input and output characteristics of
Therefore, the low frequency signal superimposed on the input signal is modulated in opposite phases to each other at the -(Level and 1-) levels of the input signal. Therefore, the low frequency signal superimposed on the input signal is , the frequency component of fo is not detected.

However, when an operating point drift occurs, as shown in FIGS. 4 and 5, the low frequency signal superimposed on the input signal is modulated in phase at the rO" level and "1" of the input signal. The average power of the output optical signal increases with frequency f due to such in-phase modulation. and its frequency component element 0
The phase differs by 180° depending on the direction of the operating point drift of the MZ optical modulator 6A. Therefore, the output of the phase detection circuit 20 contains the frequency components. A DC signal with a polarity that corresponds to the phase difference between the low-frequency signal output from the low-frequency oscillator 12 and a level that corresponds to the degree of the operating point drift is obtained.

The bias control circuit 22 adds a frequency component to the output optical signal using a signal corresponding to such an operating point drift. The bias of the MZ optical modulator 6A is controlled so as not to include, and the optimum operating point with the operating point drift corrected is maintained.

The phase detection circuit 20 includes a synchronous detection circuit to which the output signal of the optical-electrical conversion circuit 18 and the low frequency signal from the low frequency oscillator 12 are input, and a DC component of the output signal of this synchronous detection circuit. It can be configured with a low-pass filter. Further, the bias control circuit 22 can be configured using an operational amplifier having one input terminal into which the output signal of the phase detection circuit 20 is input, and the other input terminal being grounded.

In this embodiment, the operating point shift circuit inverts the polarity of the low frequency signal input to the phase detection circuit 20 based on the operating point switching signal (changes the phase of the low frequency signal by 180 degrees).
A second polarity inverting circuit 10A (changing the polarity) is used.

FIG. 6 is a diagram showing the configuration of the MZ optical modulator 10Δ. This MZ optical modulator 10A is constructed by using a Z-cut surface of LiNbo3, forming a leading waveguide by thermally diffusing T1 on this surface, and mounting electrodes on this optical waveguide. 30 is a waveguide substrate; 32 is an input end optical waveguide; 3. ,
．． 2A and 32B are optical waveguides that branch into two paths from the input end optical waveguide 32, and 33 are optical waveguides 32A and 3.
2B is the output side optical waveguide where they merge, 34 is a traveling wave electrode mainly mounted on the optical waveguide 32A, and 36 is the optical waveguide 32.
This is the ground electrode mounted on B. The earth electrode 36 is grounded, and a #-end resistor 38 is connected to the ends of the earth electrode 36 and the traveling wave electrode 34 that are closer to the output optical waveguide 33. The driving voltage is supplied to the ground electrode 36 and the side of the traveling wave electrode 34 where the terminating resistor 38 is not provided. Further, a bias voltage is applied to the traveling wave electrode 34. In the 20M Z optical modulator 10A, the traveling wave electrode 3
Since the shapes of 4 and the ground electrode 36 are asymmetrical, the strength of the electric field applied to the optical waveguides 32A and 32B is different.
There will be a difference in wavelength between the rising and falling portions of the optical signal pulse.

FIG. 7 is a diagram for explaining the relationship between the operating point and wavelength fluctuation of the MZ optical modulator.

In the following explanation, Eo is the amplitude of the input optical electric field, ω0
is the angular frequency of the input optical electric field, t is the time, φ,.

φB represents the phase modulated in the optical waveguides 32A and 32B by the drive voltage waveform VD (t), respectively. φ,. , φB0 are the phase changes of the optical electric fields that occur in the optical waveguides 32A and 32B when V(1(t)) is O, respectively.VB+''B is the bias voltage.

Ignoring the loss caused by the optical modulator, the optical electric field at each part is expressed as follows.

Input optical electric field; E,, (t) = EoCOS (ωat)
-11) Optical electric field at the output part of the optical waveguide 32A; L (t)=E. C03(ω.1 + φA
(VD (t))-φA,)/, l'2...(
2) Optical electric field at the output part of the optical waveguide 32B; εB (t)-EOCDS(ωOt-φn (vn (
t))+φno)/y=2...(3) Output light electric field mill. ul (1) ・(A (t)・El (t))/ , /-
2=Eo/2 ・(X2+Y2) ”” ・C03(ω
. t-tan-' (Y/X))...(4) Here, X-port O5(φA (v.(t))+φe.

)+C03(φB (VD (t))+φB0)
Y=SIN(φA(Vn(t))−φwork.

)+SIN (φB (VD (t))+φ
mo).

The wavelength fluctuation is the phase modulation term jan −'(Y/X
) is expressed as follows.

Phase component ω of the optical electric field in equation (4). To t-tan (Y/X
) as Φ(1), the angular frequency of output light ω(t) = d(Φ(t))/dt
From the wavelength λ(1)・2 πC/ω(1>, λ(t)
=2 π(, /ω(1) =2yrc/(ω.-d(tan-'(Y/X))/
dt) - (7).

On the other hand, the output light intensity P. ul (1) (Eo/2)' ・(X2=Y”) −= (8
).

Now, when the voltage applied to the traveling wave electrode 311 is increased, the phase of the light propagating through the optical waveguide 32A advances, and the phase of the light propagating through the leading waveguide 32B is delayed.

That is, due to the asymmetry of the mounted electrodes, the electric field applied to the optical waveguide 32A is stronger than the electric field applied to the optical waveguide 32B, so the phase modulation in the optical waveguide 32A is larger, and the electric field applied to the traveling wave electrode 34 is stronger. When the applied voltage is increased, the phase of the output light from this modulator always advances. on the other hand,
The change in light intensity when the voltage applied to the traveling wave electrode is increased can either increase or decrease depending on the initial phase difference of the light propagating through the optical waveguides 32A, 32B. Changing the operating point corresponds to changing the initial phase difference of the light propagating through the optical waveguides 32Δ and 32B using a bias voltage. Therefore, by changing the setting conditions of the operating point, it is possible to advance or delay the phase of the light at the rise of the output optical pulse. When the phase advances, the wavelength becomes temporarily shorter, and when the phase lags, the wavelength becomes temporarily longer. In this way, by setting the operating point, it is possible to change the wavelength fluctuation within the output optical pulse from a short wavelength to a long wavelength or from a long wavelength to a short wavelength.

In this embodiment, since the first polarity inverting circuit 24 is provided to invert the characteristics of the input signal in accordance with the softness of the operating point, the operating point is softened by half a cycle on the operating characteristic curve, and the optical pulse is When the wavelength fluctuation is changed as described above, the correspondence between the polarity of the input signal and the curve of the output light waveform does not change, as shown in FIG. In FIG. 7, operating point A is set in a region where the output light intensity decreases as the drive voltage increases, and operating point B is set in a region where the output light intensity increases as the drive voltage increases. ing.

When the optical modulator is driven at operating point A, the first half of the output optical pulse has a long wavelength and the second half has a short wavelength. Also,
When the optical modulator is driven at operating point B, the first half of the output optical waveform has a short wavelength and the second half has a long wavelength.

The optical modulator is driven at operating point A and operating point B, and the optical output at that time is determined to have wavelength dispersion of +600 ps/nm and -600 ps/nm.
FIG. 8 shows the results of the smearing of changes in the eye pattern when transmitted through a ps/nm optical fiber.

The bit rate of the input signal at this time is 10 Gb/s. If the wavelength dispersion is positive, the eye pattern is improved when driven at operating point A, while if it is negative, the eye pattern is improved when driven at operating point B. has been done. In this way, waveform deterioration due to chromatic dispersion can be prevented whether the sign of chromatic dispersion is positive or negative.

In this embodiment, a second curve inversion circuit 10A is used as the operating point shift circuit 10, which inverts the polarity of the low frequency signal input to the phase detection circuit 20 based on the operating point switching signal. FIG. 9 is a graph showing the relationship between the bias voltage of the MZ optical modulator 6A and the phase detection circuit output.

The broken line shows the case where the polarity of the low frequency signal is not inverted, and the solid line shows the case where the polarity of the low frequency signal is inverted. Due to the feedbank control by the bias control circuit 22, the bias voltage is stabilized at VBl when the polarity is not reversed, and the bias voltage is stabilized at VBl when the polarity is reversed.
Stabilized to B2. In this way, the phase detection circuit 20
By reversing the polarity of the low frequency signal input to the
The influence of chromatic dispersion can be minimized by shifting the operating point.

FIG. 10 is a block diagram of an optical transmitter showing a second embodiment of the present invention. In this embodiment, the operating point shift circuit inverts the polarity of the low frequency signal input to the low frequency superimposition circuit 14 based on the operating point switching signal, instead of the second polarity inversion circuit 10A in the first embodiment. A third polarity inversion circuit 10B is provided. In other words, the phase detection circuit 2
The operating point of the MZ optical modulator 6A is shifted by inverting the polarity of the low frequency signal input to the low frequency superimposing circuit 14 instead of inverting the polarity of the low frequency signal input to the low frequency superimposing circuit 14.

Further, FIG. 11 is a block diagram of an optical transmitter showing a third embodiment of the present invention. In this embodiment, the operating point shift circuit 10 is a fourth polarity inversion circuit that inverts the polarity of the feedback loop in the optical-to-electrical conversion circuit 18, the phase detection circuit 20, or the bias control circuit 22 based on the operating point switching signal. 10C is used.

In the second or third embodiment, as in the case of the first embodiment, the operating point of the MZ optical modulator 6A is set on the operating characteristic curve according to the wavelength dispersion characteristics of the optical fiber used as the optical transmission line. By shifting the wavelength by half a period, the influence of chromatic dispersion can be minimized.

FIG. 12 is an explanatory diagram of the effect of the embodiment of the present invention, and is a graph showing the relationship between chromatic dispersion and power penalty.

The broken line corresponds to the conventional method, the solid line corresponds to the case where the optimal operating point is set according to the sign of the wavelength dispersion in the embodiment of the present invention, and the dashed line corresponds to the case where there is no wavelength fluctuation of the light source. There is.

Here, the power penalty is defined by the following formula (power penalty) -10iog+. (Minimum received light power when there is chromatic dispersion/Minimum received light power when chromatic dispersion is 0> (dB)) From Figure 12, it can be seen that the tolerance against chromatic dispersion is significantly improved by the present invention compared to the conventional method. I understand. Also,
It can be seen that the present invention improves the tolerance to chromatic dispersion compared to the case where no change in the center wavelength occurs (the case where the phase modulation efficiency is 1:1).

As described above, according to the present invention, good transmission characteristics can be obtained regardless of whether the wavelength dispersion of the optical fiber is positive or negative, and the allowable dispersion of the system used in the zero-dispersion wavelength region of the optical fiber can be expanded. This enables longer distance transmission.

Note that even if the operating point of the optical modulator is simply switched by half a cycle on the characteristic curve without inverting the polarity of the input signal using the first polarity inversion circuit, the transmission characteristics will change in the same way as when the polarity of the input signal is inverted. is improved.

In this case, since the polarity of the signal is reversed, the receiving end or the like is provided with a separate function to adjust the polarity of the signal.

Furthermore, according to the present invention, the polarity of the optical signal can be reversed by shifting the operating point of the optical modulator by half a cycle on the characteristic curve. This method is also useful simply as a method for reversing the polarity of an optical signal.

As described in detail, according to the present invention, it is possible to provide an optical transmitter that can prevent wavelength deterioration caused by chromatic dispersion both when the sign of chromatic dispersion is positive and when the sign of chromatic dispersion is negative. be effective.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the configuration of the present invention, FIG. 2 is a block diagram of an optical transmitter showing a first embodiment of the invention, FIG. 3 is a diagram showing the waveform of the output optical signal of the optical modulator, and FIG. Figure 4 shows the waveform of the output optical signal when operating point drift occurs in the positive direction. Figure 5 shows the waveform of the output optical signal when operating point drift occurs in the negative direction. Figure 6 shows MZ optical modulation. 7 is a diagram showing the relationship between the operating point and wavelength fluctuation. FIG. 8 is a diagram illustrating the selection of the operating point according to the sign of wavelength dispersion. FIG. 9 is an explanatory diagram of the stable point of the bias voltage, FIG. 10 is a block diagram of an optical transmitter showing a second embodiment of the present invention, and FIG. 11 is a block diagram of an optical transmitter showing a third embodiment of the present invention. 12 is a diagram for explaining the effects of the embodiment of the present invention, FIG. 13 is an explanatory diagram of the prior art, and FIG. 14 is a diagram showing an example of a change in an eye pattern due to optical fiber transmission. 2... Light source, 4... Drive circuit, 6... Optical modulator, 8... Operating point stabilization circuit, 10... Operating point stabilizer circuit.

Claims (1)

[Claims] 1. A light source (2); a drive circuit (4) that generates a drive voltage according to an input signal; An optical modulator (6) that converts an input signal into an optical signal, and detects the drift of the operating characteristic curve of the optical modulator (6) so that the operating point is at a constant position with respect to the operating characteristic curve. and an operating point stabilizing circuit (8) that controls the optical modulator (6), the operating point is shifted by half a period on the operating characteristic curve based on an operating point switching signal. Operating point shift circuit (10)
An optical transmitter comprising: 2. The optical modulator (6) according to claim 1, wherein the optical modulator (6) is a Mach-Zehnder optical modulator (6A) with different phase modulation efficiencies in the optical waveguide (32A, 32B) branched into two paths. Optical transmitter. 3. Claim 2, further comprising a first polarity inversion circuit (24) that inverts the polarity of the input signal in response to the shift of the operating point by the operating point shift circuit (10).
The optical transmitter described in . 4. The operating point stabilization circuit (8) includes a low frequency oscillator (12) that outputs a predetermined low frequency signal, and the low frequency signal applies amplitude modulation to the input signal to the drive circuit (4). A low frequency superimposition circuit (14) for sending out, an optical branching circuit (16) for branching the optical signal output from the Mach-Zehnder optical modulator (6A), and an optical signal branched by the optical branching circuit (16). an optical-to-electrical conversion circuit (18) that converts one of the signals into an electrical signal; A phase detection circuit (20
); and a bias control circuit (22) for feedback controlling the bias of the Mach-Zehnder optical modulator (6A) so that the DC signal becomes zero. Transmitting device. 5. The operating point shift circuit (10) includes a second polarity inversion circuit (1) that inverts the polarity of the low frequency signal input to the phase detection circuit (20) based on the operating point switching signal.
5. The optical transmitter according to claim 4, wherein the optical transmitter is 0A). 6. The operating point shift circuit (10) includes a third polarity inversion circuit (10) that inverts the polarity of the low frequency signal input to the low frequency superimposition circuit (14) based on the operating point switching signal.
10B). The optical transmitter according to claim 4. 7. The operating point shift circuit (10) inverts the polarity of the feedback loop in the opto-electric conversion circuit (18), phase detection circuit (20), or bias control circuit (22) based on the operating point switching signal. 5. The optical transmitter according to claim 4, wherein the optical transmitter is a fourth polarity reversing circuit (10C).