JP2001330808A - Optical transmitter - Google Patents

Abstract

(57) [Problem] To provide an optical transmitter capable of generating a transmission signal enabling high-power fiber input with a simple configuration in a long span ultra-high speed optical transmission system. A transmission light source that outputs light with a constant intensity, an optical modulator that modulates the intensity and phase of light from the transmission light source,
A modulator driving circuit for amplifying an input signal to drive an optical modulator, a phase adjusting circuit for adjusting a phase of a clock signal, and a data signal superimposed on a sine wave signal having a clock frequency of the data signal by multiplication processing or addition processing Means to do so.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical transmitter for a long span ultra high speed optical transmission system.

[0002]

2. Description of the Related Art In a long-span optical transmission system, the transmission speed has been increased in order to economically realize a large-capacity system. In recent years, high transmission speeds exceeding 10 Gbit / s have been realized due to the development of ultra-high-speed electric circuit technology.
Further, with the realization of the optical fiber amplifier, the output power of the optical transmitter and the sensitivity of the optical receiver have been improved, and the span between repeaters has been increased.

In such an ultra-high-speed optical transmission system, there is a problem that the transmission distance is limited by waveform distortion due to chromatic dispersion of the transmission fiber. In particular, the zero-dispersion wavelength currently widely used as a transmission fiber is 1.3 μm.
Using a step index type single mode fiber in the m band, 1.5μ which is the gain wavelength band of the optical fiber amplifier
When transmitting an m-band signal, the dispersion value of the fiber is +
It is a large value of about 17ps / nm / km, and the transmission speed is 40Gb
At it / s, the transmission distance is limited to a few kilometers. This waveform distortion due to chromatic dispersion must be suppressed by a dispersion equalization technique in which a dispersion medium (chirped fiber grating or the like) having a dispersion value opposite to that of the transmission fiber is equalized by inserting it before or after the transmission line. Can be.

On the other hand, in order to extend the repeater interval and increase the span, it is necessary to increase the output power of the optical transmitter. However, this requires nonlinear optical effects in the optical fiber. Since the signal light undergoes phase modulation in the fiber in proportion to the signal light intensity due to a certain self-phase modulation effect (SPM), the combined effect of the SPM and the wavelength dispersion of the transmission fiber poses a problem. In such high-power fiber transmission, even if the total dispersion of the transmission line and the dispersion equalization medium is made zero by the dispersion equalization technique described above, waveform distortion cannot be completely removed, and the transmission distance is limited. It has become a factor.

Therefore, a modulation format that is resistant to deterioration due to the non-linear effect is required. In currently introduced systems, the NRZ format is widely used as a modulation format.
On the other hand, when the transmission signal is in the RZ format, S
It is known that the tolerance to waveform distortion due to the combined effect of PM and chromatic dispersion is improved as compared with the NRZ format (D. Breuer et al., Proceeding of ECOC'96, TuD3.3).
reference). For this reason, in a long-span ultra-high-speed transmission system with a transmission speed of about 40 Gbit / s, the RZ format is being studied as a possible modulation format.

In order to further improve the resistance to fiber nonlinearity, a chirped RZ format has been proposed (NS Bergano et al., Electron. Lett., Vo
l.32, p.52, 1996).

For an optical transmitter using the RZ format and the chirped RZ format, the data electrical signal is N
Because of the RZ format, the following method is usually used to modulate in the RZ format. 1 is 2
Optical modulators are arranged in series, the first optical modulator is driven by a data signal to generate an optical signal driven in the NRZ format, and the second optical modulator is set to the clock frequency of the data signal. By driving with a sine wave signal having
Generate an R signal (A. Sano et al., Electron. Lett., Vo
l.30, p.1694, 1994). Part 2 uses a light source that oscillates pulses and modulates an optical pulse from the pulse light source with an optical modulator driven by a data signal in NRZ format (A. Sano et al., Electron. Lett., Vol. 32, p.1218,
1996). In the third method, an RZ signal is generated at the stage of an electric signal, and the optical modulator is thereby driven to generate an RZ signal (see JP-A-10-13351).

However, the first method requires two modulators, which is disadvantageous in terms of the cost of the apparatus, increase in power consumption and increase in loss. Further, in the second method, it is difficult to create a pulse light source that matches the clock frequency of the data signal, so that there are problems such as a decrease in the yield of the light source and an increase in cost. In the third method, R
Since the Z signal has about twice the signal band of the NRZ signal, the modulator and the modulator driving circuit require about twice as wide band components as the conventional one, but the transmission speed is over 40 Gbit / s. It is difficult to realize such components in a high-speed system.

In addition, in the case of a chirped RZ signal, a phase modulator or a dispersion medium is required in addition to the intensity modulator, so that an increase in insertion loss is a problem.

[0010]

SUMMARY OF THE INVENTION It is an object of the present invention to generate a transmission signal enabling high-power fiber input with a simple configuration in such a long-span ultra-high-speed optical transmission system. To provide a simple optical transmitter.

[0011]

In order to achieve the above object, an optical transmitter according to the present invention comprises: a transmission light source for outputting light having a constant intensity; and an optical modulator for modulating the intensity and phase of the light from the transmission light source. , A modulator drive circuit that amplifies the input signal and drives the optical modulator, a phase adjustment circuit that adjusts the phase of the clock signal, and a data signal superimposed on the sine wave signal with the clock frequency of the data signal by multiplication processing And means for performing the operation.

Further, another optical transmitter according to the present invention includes a transmission light source for outputting light having a constant intensity, an optical modulator for modulating the intensity and phase of the light from the transmission light source, and an optical modulator for amplifying an input signal to amplify an input signal. A modulator driving circuit for driving the modulator, a phase adjusting circuit for adjusting the phase of the clock signal, and a means for superimposing a sine wave signal having the clock frequency of the data signal on the data signal by addition processing. I do.

In the optical transmitter according to the present invention, the ratio of the phase change to the intensity change of the optical modulator output signal in the output state higher than the midpoint between the on level and the off level of the output signal light is the on level and the off level. It is desirable to set the bias of the modulator drive signal so as to be larger than the ratio of the phase change to the intensity change of the optical modulator output signal at the midpoint between the above.

According to such an optical transmitter of the present invention, only the light intensity is conventionally modulated by adjusting the element structure and operating point of the modulator and using the modulator under a driving condition with a small chirp coefficient. In contrast, a clock signal is superimposed on a data signal, and the operating point of the modulator is set to a condition having a large chirp coefficient so that the clock signal is efficiently converted into phase modulation by the chirp characteristics of the modulator. As a result, the phase modulation at the clock frequency is superimposed on the intensity modulation signal. As a result, a transmission signal similar to the chirped RZ signal is obtained.

According to a preferred embodiment, a semiconductor electroabsorption type optical modulator, a LiNbO ₃ Mach-Zehnder type optical modulator or the like can be used as an optical modulator in the optical transmitter of the present invention.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, embodiments of the present invention will be described with reference to the drawings.

[First Embodiment] FIG. 1 is a diagram showing the configuration of an optical transmitter according to a first embodiment of the present invention. In the figure, 1 is a transmission light source for outputting light of constant intensity, 2 is an optical modulator for modulating the intensity and phase of light from the transmission light source 1, 3 is a phase adjustment circuit for adjusting the phase of a clock signal, and 4 is data A multiplication circuit 5 for superimposing the signal and the clock signal by multiplication processing is a modulator driving circuit that amplifies the output of the multiplication circuit 4 and generates a modulator driving signal.

As the transmission light source 1, for example, a distributed feedback semiconductor laser diode is used. As the optical modulator 2, for example, an electroabsorption modulator is used. As the phase adjustment circuit 3, for example, a variable-length coaxial delay line is used. As the multiplication circuit 4, for example, a double balanced mixer is used. As the modulator driving circuit 5, for example, a combination of a high-frequency amplifier and a bias tee is used.

FIG. 2 shows the eye opening deterioration with respect to the fiber input power and the total dispersion amount when the optical transmitter of the present invention is used (a) and when the NRZ modulation signal of the conventional chirpless is used (b). It is a figure showing a calculation result of a contour line. Here, the modulator was an electroabsorption modulator. In this calculation, the optical waveform after transmission was calculated by the split step Fourier method.

Calculation conditions are as follows: transmission fiber: 1.3 μm, zero-dispersion single mode fiber 80 km, transmission speed: 40 Gb
it / s, dispersion value: 17 ps / nm / km, nonlinear coefficient: 1.3 / W
-Km, dispersion compensation: post-compensation for dispersion equalization on the receiving side. In FIG. 2, a region surrounded by a 1 dB degradation contour indicated by a thick line indicates a transmittable region.

As can be seen from FIG. 2, the signal input power is limited to +10 dBm in the conventional method, while the optical transmitter of the present invention enables a high optical input of +12 dBm or more. This is because, in the present invention, the phase modulation is superimposed at the on-level by superimposing the clock signal,
This is because the signal is equivalent to the chirped RZ signal. From this result, it can be seen that the present invention enables a longer transmission distance.

FIGS. 3A and 3B show the results obtained before calculation ((a) is the configuration of the present invention, (b) is the conventional configuration) and after transmission ((c) is the configuration of the present invention, and (d) is the conventional configuration). Configuration) is shown. As is clear from FIG. 3, in the conventional configuration, the degree of eye opening deteriorates in both the time axis direction and the intensity axis direction, whereas in the configuration of the present invention, a good eye opening is obtained.

[Embodiment 2] FIG. 4 is a diagram showing the configuration of an optical transmitter according to Embodiment 2 of the present invention. The second embodiment differs from the first embodiment in that the multiplication circuit 4 of the first embodiment is replaced by an addition circuit 6, and the other configuration is the same as that of the first embodiment. In this case, since the clock signal is also superimposed on the off level, there is a concern that the eye opening degree may deteriorate. However, since the off-level clock modulation is sufficiently suppressed in the output optical signal due to the non-linearity of the input / output characteristics of the optical modulator such as the electro-absorption modulator, the deterioration hardly occurs.

FIG. 5 shows a modulator input signal and a modulator input signal when a clock signal is modulated by a data signal on which a clock signal is superimposed by a multiplication circuit and an addition circuit when the modulation degree of the clock signal is 25% using an electro-absorption modulator. FIG. 3 is a diagram illustrating an optical signal output from an optical transmitter. As described above, it can be seen that the off-level clock signal is suppressed due to the nonlinearity of the electro-absorption modulator. On the other hand, since the phase modulation by the clock signal is superimposed on the on-level as in the case of the multiplier circuit, it goes without saying that the fiber input power is improved as described with reference to FIGS.

FIG. 6 is a diagram showing the relationship (a) between the applied voltage and the chirp coefficient and the relationship (b) between the applied voltage and the power transmittance. Although the chirp characteristic of the electroabsorption modulator depends on the element structure, as shown in FIG. 6, the chirp coefficient generally tends to increase as the bias voltage decreases (the output light intensity increases). Here, the chirp coefficient α is the ratio of the phase change to the intensity change of the output light, and is given by α = 2 (dφ / dlnI). Here, φ and I are the phase and intensity of the output optical signal. Accordingly, the phase φ of the optical signal is given by φ = (１ ／) ∫ [α (I) / I] dI. Therefore, it can be seen that the larger the chirp coefficient at the ON level, the larger the phase modulation can be obtained even if the modulation degree of the clock modulation is small.

In general, since the gain of the modulator drive circuit tends to decrease on the high frequency side, even if a clock signal is superimposed, the modulation degree of the clock signal is reduced at the output of the drive circuit, and sufficient phase modulation can be obtained. There is no problem. As described above, even when the gain band of the modulator drive circuit is insufficient, it is possible to obtain a larger phase modulation by setting the bias voltage so that the on-level chirp coefficient is increased. I understand.

FIG. 7 shows a case where the optical transmitter of the present invention is used (a) and a conventional chirpless NR when a LiNbO ₃ Mach-Zehnder type optical modulator is used as the modulator.
It is a figure which shows the calculation result of the eye opening deterioration contour with respect to fiber input power and the total dispersion amount at the time of using a Z modulation signal (b). In the case (a) of the present invention, only one electrode of the Mach-Zehnder optical modulator is driven.

In this calculation, as in FIG. 2, the optical waveform after transmission was calculated by the split step Fourier method.
The calculation conditions were the same as in FIG.
Zero dispersion single mode fiber 80km, transmission speed: 40
Gbit / s, dispersion value: 17 ps / nm / km, nonlinear coefficient: 1.3 /
W · km, dispersion compensation: post-compensation. In the drawing, a region surrounded by a 1 dB degradation contour line shown by a bold line indicates a transmittable region. From FIG. 7, it is possible to confirm that the present invention is effective even if the Mach-Zehnder type optical modulator is used, the fiber input power can be up to about +12 dBm.

[0029]

As described above, according to the present invention,
With a simple configuration, it is possible to efficiently suppress the waveform distortion due to the interaction between the self-phase modulation effect and the chromatic dispersion in the fiber, so that the input power to the fiber can be increased and the error rate can be increased. And a high-quality transmission system with a low transmission rate, and an economical transmission system can be realized by increasing the interval between repeaters.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a configuration of an optical transmitter according to a first embodiment of the present invention.

FIG. 2 is a diagram showing calculation results of eye opening deterioration contours in transmission using an electro-absorption modulator.

FIG. 3 is a diagram illustrating examples of eye patterns before and after transmission.

FIG. 4 is a diagram showing a configuration of an optical transmitter according to a second embodiment of the present invention.

FIG. 5 is a diagram illustrating calculation results of modulator input and transmitter output eye patterns when a multiplication circuit and an addition circuit are used.

FIG. 6 is a diagram showing the applied voltage dependence of the chirp coefficient and the power transmittance of the electro-absorption modulator.

FIG. 7 is a diagram illustrating calculation results of eye opening deterioration contours in transmission using a LiNbO ₃ Mach-Zehnder type optical modulator.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Transmission light source 2 Optical modulator 3 Phase adjustment circuit 4 Multiplication circuit 5 Modulator drive circuit 6 Addition circuit

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H04B 10/142 10/04 10/06 10/28 10/26 10/14 F-term (Reference) 2H079 AA02 AA12 AA13 BA01 CA04 EA05 EA07 FA03 HA11 5K002 AA02 BA13 CA01 CA15 CA16 FA01

Claims (5)

[Claims] 1. A transmission light source for outputting light having a constant intensity, an optical modulator for modulating the intensity and phase of light from the transmission light source,
A modulator driving circuit for amplifying an input signal to drive an optical modulator, a phase adjusting circuit for adjusting a phase of a clock signal, and a means for superimposing a sine wave signal having a clock frequency of a data signal on a data signal by a multiplication process An optical transmitter, comprising: 2. A transmission light source for outputting light having a constant intensity, an optical modulator for modulating the intensity and phase of light from the transmission light source,
A modulator driving circuit that amplifies an input signal and drives an optical modulator; a phase adjustment circuit that adjusts the phase of a clock signal; and a unit that superimposes a sine wave signal having a clock frequency of the data signal on the data signal by addition processing. An optical transmitter, comprising: 3. The optical modulator according to claim 1, wherein the ratio of the phase change to the intensity change of the optical modulator output signal in the output state higher than the middle point between the on level and the off level of the output signal light is the optical modulation at the middle point between the on level and the off level. 3. The optical transmitter according to claim 1, wherein a bias of the modulator drive signal is set to be larger than a ratio of a phase change to an intensity change of the modulator output signal. 4. The optical transmitter according to claim 1, wherein the optical modulator is a semiconductor electro-absorption type optical modulator. 5. The optical transmitter according to claim 1, wherein the optical modulator is a LiNbO ₃ Mach-Zehnder optical modulator.