JP3577931B2 - Optical transmitter - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an optical transmission device, and more specifically, to an optical transmission device that outputs signal light with a small spread of an optical spectrum line width.
[0002]
[Prior art]
The optical duobinary method is capable of narrowing the optical spectrum of signal light to about half that of a normal intensity modulation method, and is attracting attention as a technique for realizing high-density multiplexing in a wavelength division multiplexing optical transmission method. I have.
[0003]
As an apparatus for generating an optical duobinary waveform, for example, a method using a low-pass filter in an electric stage (for example, Ono et al., “Study of WDM high-density multiplexing using optical duobinary method”), IEICE, IEICE Gakugaku Giho OCS96-59, OPE96-109, LQE96-110 (1996-11), pp.49-54), and a method of shifting bits to perform modulation (T. Frank, et al., "Novel Duobinary Transmitter", ECOC97, 22). -25 September 1997, Conference Publication
No. 448, pp. 67-70) are known.
[0004]
[Problems to be solved by the invention]
However, in the conventional example, the degree of suppression of the spread of the optical spectrum line width is still insufficient. In long-distance and high-density wavelength division multiplexing transmission, further suppression of the spread of the optical spectrum line width is required.
[0005]
SUMMARY OF THE INVENTION It is an object of the present invention to provide an optical transmission device that can further suppress the spread of the optical spectrum line width.
[0006]
[Means for Solving the Problems]
According to the present invention, the duobinary optical signal is subjected to optical intensity modulation and optical phase modulation by the optical intensity / phase modulation means , and further polarization scrambled. Either the light intensity modulation or the optical phase modulation may be performed first.
[0007]
As a result, transmission characteristics equivalent to the transmission characteristics of the NRZ waveform can be achieved, and the spread of the signal spectrum line width can be suppressed.
[0008]
By setting the degree of modulation of the light intensity modulation to 100%, an RZ waveform can be formed, and noise can be improved.
[0009]
By using the same composition for the modulating elements to be used, adjustment of characteristics and integration can be facilitated.
[0010]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0011]
FIG. 1 shows a schematic block diagram of an embodiment of the present invention, FIG. 2 shows a timing chart thereof, and FIG. 3 shows an actually measured pulse waveform of a characteristic portion of the embodiment.
[0012]
A data generating circuit 10 generates an NRZ signal of data to be transmitted, outputs a binary data string from a Q terminal to a low-pass filter (LPF) 12a, and outputs an inverted value of the binary data string from an inverted Q output. Output to the low-pass filter (LPF) 12b . The LPFs 12a and 12b have exactly the same low-pass characteristics, and convert a binary data sequence into a duobinary data sequence by the LPFs 12a and 12b.
[0013]
For example, when the data string output from the Q output of the data generation circuit 10 has a waveform as shown in FIG. 2A, the output waveform of the LPF 12a has a three-valued waveform as shown in FIG. It becomes a waveform. The data sequence output from the inverted Q output of the data generation circuit 10 is obtained by inverting the waveform shown in FIG. 2A, and the output waveform of the LPF 12b corresponding thereto is as shown in FIG. It becomes a ternary waveform.
[0014]
The outputs of the LPFs 12a and 12b are applied to electrodes on both sides of the two-electrode optical modulator 24 via amplitude adjustment circuits 14a and 14b, amplifiers 16a and 16b, voltage adjustment circuits 20a and 20b, and delay circuits 22a and 22b. You. The amplitude adjustment circuits 14a and 14b adjust the amplitudes of the outputs of the LPFs 12a and 12b, and the amplifiers 16a and 16b amplify the outputs of the amplitude adjustment circuits 14a and 14b to a predetermined level. The voltage adjusting circuits 20a and 20b adjust the output voltages of the amplifiers 16a and 16b to a voltage level and a range suitable for optical modulation in the optical modulator 24. The delay circuits 22a and 22b are provided to match the output timings of the voltage adjustment circuits 20a and 20b with each other.
[0015]
The two-electrode type optical modulator 24 is made of a crystal of lithium niobate capable of ultra-high-speed modulation of 10 GHz or more, and outputs the CW of the laser light source 26 according to the outputs of the delay circuits 22a and 22b, that is, the complementary electric duobinary signal. The output light is subjected to light intensity and phase modulation. FIG. 2E shows the light intensity pattern of the output light from the optical modulator 24, and FIG. The optical phase shown in FIG. 2F may be opposite to that in the illustrated example. The output light intensity pattern of the optical modulator 24 (FIG. 2 (e)) is obtained by folding up and down around the level "1" of the output pattern of the LPF 12a or 12b.
[0016]
The configuration up to here is a conventionally known circuit configuration for generating an optical duo-binary signal from an electric duo-binary signal. The portion subsequent to the optical modulator 24 is a characteristic portion of the present embodiment. In this embodiment, the optical duobinary signal is further subjected to optical intensity modulation and phase modulation, and the polarization is scrambled and output to the optical transmission line.
[0017]
The light intensity modulator 30 modulates the output light of the light modulator 24 with the modulation factor of 100% according to the output of the drive circuit 32. That is, the NRZ signal is transformed into the RZ signal. The drive circuit 32 includes an amplitude adjustment circuit 32a for adjusting the amplitude of the clock from the data generation circuit 10, an amplifier 32b for amplifying the output of the amplitude adjustment circuit 32a, a voltage adjustment circuit 32c for adjusting the output voltage of the amplifier 32b, and a voltage adjustment. A delay circuit 32d for adjusting the time of the output of the circuit 32c is provided. The output of the delay circuit 32d is applied to the light intensity modulator 30 as a light intensity modulation signal.
[0018]
The optical phase modulator 34 optically modulates the output light of the light intensity modulator 30 according to the output of the drive circuit 36. That is, the optical phase modulator 34 applies a constant pre-chirp to the optical signal. The drive circuit 36 has the same circuit configuration as the drive circuit 32, and includes an amplitude adjustment circuit 36a for adjusting the amplitude of the clock from the data generation circuit 10, an amplifier 36b for amplifying the output of the amplitude adjustment circuit 36a, and an output voltage of the amplifier 36b. voltage adjustment circuit 36c for adjusting and consists delay circuit 36d for adjusting the output of the voltage regulating circuit 36c time, the output of the delay circuit 36d is applied to the optical phase modulator 34 as the optical phase modulation signal.
[0019]
The polarization modulator 38 modulates the polarization of the output light of the light intensity modulator 34 according to the output of the drive circuit 40, and functions as a polarization scrambler. The drive circuit 40 has the same circuit configuration as the drive circuits 32 and 36, and includes an amplitude adjustment circuit 40a for adjusting the amplitude of the clock from the data generation circuit 10, an amplifier 40b for amplifying the output of the amplitude adjustment circuit 40a, and an output of the amplifier 40b. It comprises a voltage adjustment circuit 40c for adjusting the voltage and a delay circuit 40d for adjusting the output of the voltage adjustment circuit 40c with time. The output of the delay circuit 40d is applied to the polarization modulator 38 as a polarization modulation signal.
[0020]
The light intensity modulator 30, the light phase modulator 34, and the polarization modulator 38 are made of lithium niobate crystal, like the light modulator 24.
[0021]
3 (a) shows the output optical waveform of the optical modulator 24. Figure 3 (b) shows the optical pulse waveform of the optical intensity modulator 30 a light pulse shown in FIG. 3 (a) As a result of the light intensity modulation. For reference, FIG. 3C shows an optical pulse waveform when only the optical phase modulator 34 is driven with respect to the output light of the optical modulator 24. FIG. 3 (d), an optical pulse shown in FIG. 3 (a) to modulate the light intensity by the light intensity modulator 30, further illustrating the light pulse waveform resulting from the phase modulation by the phase modulator 34. FIG. 3 ( e ) shows the result of light intensity modulation of the light pulse shown in FIG. 3 ( a ) by the light intensity modulator 30, phase modulation by the phase modulator 34, and further polarization scrambling by the polarization modulator 38. 3 shows an optical pulse waveform of FIG.
[0022]
Note that the optical pulse waveforms shown in FIGS. 3A to 3E are actually in a state in which a specified one of the optical intensity modulator 30, the optical phase modulator 34, and the polarization modulator 38 is not driven. Then, the optical pulse waveform output from the polarization modulator 38 at the time of the measurement was measured.
[0023]
Although the optical duobinary signal is already phase-modulated as shown in FIG. 2 (f), by further applying phase modulation to the entire signal light by the optical phase modulator 34, the signal spectrum can be obtained even in long-distance transmission. Spread of the line width can be suppressed. This was confirmed by orbital experiments. With the same signal band, the number of multiplexed wavelengths can be increased as compared with the NRZ signal. In addition, the light intensity modulation by the light intensity modulator 30 makes it more resistant to noise and can extend the transmission distance.
[0024]
When the transmission characteristics were evaluated by loop transmission experiments, good values similar to those of the normal NRZ waveform were obtained. For example, for a transmission rate of about 10 Gbit / s, an optical duobinary signal alone has a transmission distance of about 3,000 km, but a light intensity modulation (light intensity modulator 30) of 6,000 to 6,800 km. The transmission distance was reached, and the optical phase modulation (optical phase modulator 34) and the polarization scrambler (polarization modulator 38) were added to obtain a Q value of 15.9 dB after 9,000 km transmission. Incidentally, the Q value when the polarization modulator 38 is omitted is 15.5 dB, which means that the polarization scramble by the polarization modulator 38 has improved 0.4 dB.
[0025]
In the above embodiment, the optical phase modulator 34 is arranged after the optical intensity modulator 30. On the contrary, the optical phase modulator may be arranged after the optical phase modulator. By using modulation elements having the same composition for each of the modulators 24, 30, 34, and 38, characteristics can be easily adjusted and integration can be easily performed.
[0026]
The receiving device may have the same configuration as a conventional receiving device corresponding to an RZ signal.
[0027]
Summarizing the above, in the present embodiment, the transmission distance can be lengthened because the time waveform is hardly deteriorated even when transmitted. Since the spread of the spectral line width can be suppressed by the additional phase modulation, the wavelength multiplexing interval in the wavelength division multiplexing transmission system can be narrowed, and the number of wavelength multiplexing can be increased even in the same wavelength band.
[0028]
【The invention's effect】
As can be easily understood from the above description, according to the present invention, since the spread of the signal spectrum line width can be suppressed, the number of wavelength multiplexing in the wavelength division multiplexing transmission system can be increased, and the transmission capacity can be increased. it can. Further, the transmission distance can be lengthened.
[Brief description of the drawings]
FIG. 1 is a schematic block diagram of an embodiment of the present invention.
FIG. 2 is a timing chart until an optical duobinary signal is generated in the embodiment.
FIG. 3 is an example of an actually measured light pulse waveform showing the effects of light intensity modulation, phase modulation, and polarization modulation of the present embodiment.
[Explanation of symbols]
10: Data generation circuits 12a, 12b: Low-pass filter (LPF)
14a, 14b: amplitude adjustment circuits 16a, 16b: amplifiers 20a, 20b: voltage adjustment circuits 22a, 22b: delay circuit 24: two-electrode optical modulator 30: light intensity modulator 32: drive circuit 32a: amplitude adjustment circuit 32b: Amplifier 32c: Voltage adjustment circuit 32d: Delay circuit 34: Optical phase modulator 36: Drive circuit 36a: Amplitude adjustment circuit 36b: Amplifier 36c: Voltage adjustment circuit 36d: Delay circuit 38: Polarization modulator 40: Drive circuit 40a: Amplitude Adjustment circuit 40b: Amplifier 40c: Voltage adjustment circuit 40d: Delay circuit

Claims (7)

Signal light generating means for generating a duobinary optical signal;
Light intensity / phase modulation means for light intensity modulation and optical phase modulation of the output light of the signal light generation means ,
Polarization scrambler for polarization scrambling the output light of the light intensity / phase modulation means
An optical transmission device comprising: The light intensity / phase modulation means comprises light intensity modulation means for modulating the light intensity of the output light of the signal light generation means, and optical phase modulation means for optically modulating the output light of the light intensity modulation means. 2. The optical transmission device according to 1. The light intensity / phase modulation means comprises an optical phase modulation means for optically modulating the output light of the signal light generation means, and a light intensity modulation means for optically modulating the output light of the optical phase modulation means. 2. The optical transmission device according to 1. The optical transmitter according to claim 2, wherein the light intensity modulating unit modulates the input light with a modulation degree of 100%. The signal light generating means, an electric signal generating means for generating an electric duobinary signal, a light source for generating light having a signal wavelength, and the output light of the light source according to the duobinary signal generated by the electric signal generating means. The optical transmission device according to claim 1, further comprising an optical modulation unit that performs intensity and phase modulation. 6. The optical transmission device according to claim 5, wherein the light modulating means and the light intensity / phase modulating means are composed of modulating elements having the same composition. The polarization scrambler, optical transmitter according to any one of claims 1 to 6 consisting of the modulation device having the same composition as the optical intensity and phase modulation means.

Images