JP3878563B2 - Optical transmission circuit - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical transmission circuit used in an ultra-high speed / wavelength multiplexing optical communication system. In particular, the present invention relates to an optical transmission circuit that generates a high-quality optical modulation signal using an external modulation system.
[0002]
[Prior art]
In recent optical fiber communication systems, the increase in capacity has been accelerated by the progress of wavelength multiplexing transmission technology, but at the same time, the signal speed per wavelength has been increased. At the commercial level, 1 Gbit / s per wavelength has been realized, and 40 Gbit / s per wavelength has already been researched and developed. An improvement in the signal speed per wavelength is considered advantageous for increasing the capacity because the frequency utilization efficiency can be improved relatively easily. Actually, most reports of transmission experiments exceeding the total capacity of 3 Tbit / s use an optical transmission circuit of 40 to 43 Gbit / s per wavelength.
[0003]
In an optical transmission circuit that generates such an ultra-high-speed optical modulation signal, a Mach-Zehnder optical modulator (LN MZM) that uses the electro-optic effect of lithium niobate as an external modulator, or an electroabsorption effect of a semiconductor The electroabsorption optical modulator used is used. In such an optical transmitter circuit using an external modulator, the chirping of the optical modulation signal generated is smaller than that in the direct modulation method, and this is caused by the group velocity dispersion of the transmission path medium as the signal speed increases. Therefore, it is possible to suppress the influence of waveform deterioration, which is advantageous for increasing the transmission distance.
[0004]
By the way, since the external modulator has a relatively high voltage of an applied electric signal of 3 to 6 Vp-p, a high-output modulator driving circuit having a broadband frequency characteristic is required. Moreover, since the amplitude of the electric signal input to the modulator driving circuit is as small as 0.5 to 1 Vp-p, the gain must be large. The modulator driving circuit needs to satisfy these three requirements of wideband, high output, and high gain, and it is not easy to realize the modulator driving circuit used for the 40 Gbit / s optical transmission circuit. Further, in the future, when realizing an optical transmission circuit that generates an optical modulation signal exceeding 100 Gbit / s, it becomes a big problem to realize a corresponding modulator driving circuit.
[0005]
On the other hand, when the output of the modulator driving circuit is smaller than the voltage required for driving the external modulator, a sufficient on / off ratio (extinction ratio) of the output optical modulation signal cannot be obtained. If the on / off ratio of the light modulation signal is not sufficient, the transmittable distance is shortened. That is, speeding up and lengthening by using an external modulator have a trade-off relationship depending on the performance of the modulator driving circuit.
[0006]
As a method for improving the on / off ratio of this optical modulation signal, the optical modulation signal and CW light are input to a nonlinear medium (dispersion shift fiber) to generate four-wave mixed light, and the four-wave mixed light is used for transmission. The structure which performs is proposed (nonpatent literature 1).
[0007]
[Non-Patent Document 1]
A. Argyris et.al., "Extinction ratio improvement by four-wave mixing in dispersion-shifted fiber", Electronics Letters, vol.39, no.2, 23rd January 2003
[0008]
[Problems to be solved by the invention]
Four-wave mixed light is generated when signal light and pump light are simultaneously input to a nonlinear medium, and is proportional to the square of the intensity of the signal light and proportional to the intensity of the pump light. In the configuration described in Non-Patent Document 1, when the light modulation signal is signal light and the CW light is pump light, the intensity of the generated four-wave mixed light is proportional to the square of the intensity of the light modulation signal. The on / off ratio (extinction ratio) of the light modulation signal can be improved within the range, but further improvement is difficult.
[0009]
The present invention significantly improves the on / off ratio of an optical modulation signal in which an output amplitude of the modulator driving circuit is insufficient and a sufficient on / off ratio cannot be obtained in an optical transmission circuit using an external modulator. An object of the present invention is to provide an optical transmission circuit capable of performing
[0010]
[Means for Solving the Problems]
In the optical transmission circuit of the present invention, continuous light of optical frequencies f ₁ and f ₂ (f ₂ −f ₁ = Δf ≠ 0) output from a two-wavelength light source is simultaneously intensified by an electric signal (transmission signal) with an external modulator. The modulated optical signals of the optical frequencies f ₁ and f ₂ whose intensity are modulated are input to the nonlinear medium to generate four-wave mixed light having the optical frequency f ₃ (= 2f ₂ −f ₁ ). In this configuration, only the four-wave mixed light is selected and output.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
(First embodiment)
FIG. 1 shows a first embodiment of an optical transmission circuit of the present invention. In the figure, an optical transmission circuit includes a two-wavelength light source 1 that simultaneously outputs continuous light having optical frequencies f ₁ and f ₂ (f ₂ −f ₁ = Δf ≠ 0), and continuous light having optical frequencies f ₁ and f ₂ . An external modulator 2 that simultaneously modulates the intensity of the input signal, a modulator driving circuit 3 that applies an electric signal to be transmitted to the external modulator ₂ , and optical modulation of the optical frequencies f ₁ and f _{2 that} are intensity-modulated by the electric signal A nonlinear medium 4 that generates a four-wave mixed light having an optical frequency f ₃ (= 2f ₂ −f ₁ ) by inputting a signal and an optical filter 5 that selects and outputs only the four-wave mixed light having an optical frequency f _3. Composed.
[0012]
Four-wave mixed light is an extremely fast nonlinear optical phenomenon that occurs when signal light and pump light are simultaneously input to a nonlinear medium, and is proportional to the square of the intensity of the signal light and proportional to the intensity of the pump light. Since the present invention generates four-wave mixed light by using one of two light modulation signals whose intensity is modulated simultaneously as signal light and the other as pump light, its on / off ratio (extinction ratio) is the light modulation signal. It is proportional to the cube of. That is, even when the on / off ratio of each optical modulation signal of the optical frequency f _1, f ₂ output from the external modulator 2 is not sufficient, as shown by the example of the eye pattern of FIG. 1, the optical frequency f ₃ By taking out the four-wave mixed light, the on / off ratio can be greatly improved (from 3 dB to 9 dB according to the light output characteristics of FIG. 4 described later).
[0013]
FIG. 2 shows a configuration example of the two-wavelength light source 1. The two-wavelength light source 1 shown in FIG. 2 (1) includes laser light sources 11 and 12 that output continuous light of optical frequencies f ₁ and f ₂ , respectively, and an optical multiplexer that combines the output light (for example, polarization maintaining light). Coupler) 13. The laser light sources 11 and 12 are DFB lasers or external resonance lasers, and the oscillation light frequency is stabilized.
[0014]
A two-wavelength light source 1 shown in FIG. 2 (2) includes a laser light source 14 that outputs continuous light having an optical frequency f ₁ + Δf / 2, and a Mach-Zehnder optical modulator that modulates the output light with a clock signal having a frequency Δf / 2. 15. This is called a CS-RZ light source, and simultaneously generates continuous light having optical frequencies f ₁ and f ₂ with an optical frequency difference Δf.
[0015]
As another two-wavelength light source 1 for simultaneously generating continuous light of optical frequencies f ₁ and f _2, a predetermined frequency is selected from output light from a mode-locked laser that generates two beat pulses, a supercontinuum light source, or an optical frequency comb generator The thing of the structure which selectively outputs the continuous light of 2 optical frequencies can be used.
[0016]
As the external modulator 2, a Mach-Zehnder optical modulator or an electroabsorption optical modulator can be used.
[0017]
As the nonlinear medium 4, an optical fiber or a semiconductor optical amplifier (SOA) can be used. However, since it is desirable that the four-wave mixing light is generated from the efficiency and the propagation speeds of the two optical modulation signals are the same, it is desirable that the zero dispersion optical frequency is in the vicinity of (f ₁ + f ₂ ) / 2. .
[0018]
The external modulator 2 can be integrated with an electroabsorption optical modulator and the nonlinear medium 4 can be integrated with a semiconductor optical amplifier. Further, as shown in FIG. 3, by using a two-beat pulse generation mode-locked laser as the two-wavelength light source 1, an electroabsorption optical modulator constituting the external modulator 2 and a semiconductor optical amplifier constituting the nonlinear medium 4 are used. These three can be integrated.
[0019]
FIG. 4 shows the optical output characteristics of the optical transmission circuit of the present invention when a Mach-Zehnder type optical modulator is applied to the external modulator 2. The horizontal axis normalizes the voltage applied to the external modulator by Vπ (difference in voltage applied to the external modulator that gives the maximum light output and the minimum light output). The vertical axis represents the relative value of output power. Although the extinction ratio depends on the applied voltage of the external modulator, if the applied voltage is 0 to Vπ / 2, the extinction ratio is 3 dB in the conventional configuration, but 9 dB in the configuration of the present invention, and high extinction is achieved even with a small applied voltage. It can be seen that the ratio is obtained.
[0020]
An experimental result for confirming the effect of the optical transmission circuit of the present invention is shown in FIG. Experimental system, in the configuration of FIG. 1, G.653 fiber length 20km as the nonlinear medium 4 (zero-dispersion wavelength: 1551 nm) using a light wavelength lambda ₁ that corresponds to the optical frequencies f _1, f _2, the lambda ₂ The optical frequency difference Δf was set to about 100 GHz at 1551.5 nm and 1550.7 nm. The external modulator 2 is a Mach-Zehnder type optical modulator (Vπ is 4.4 V), the drive signal is 10 Gbit / s, and the amplitude is Vπ / 2, 2.2 Vp-p. Further, in the experiment, in order to obtain four-wave mixed light sufficient for observation, an optical amplifier (EDFA) was placed in front of the nonlinear medium 4 to amplify the light modulation signal.
[0021]
The experimental result of FIG. 5 is a result of measuring the extinction ratio by changing the bias voltage of the external modulator 2. When the bias voltage is Vπ / 4, the theoretical values of the conventional configuration and the configuration of the present invention are 3 dB and 9 dB, respectively, and the experimental values are 2.72 dB and 7.07 dB, confirming the effect of the present invention. The reason why the experimental value of the configuration of the present invention is lower than the theoretical extinction ratio is considered to be due to the influence of the ASE noise of the optical amplifier used in the experiment.
[0022]
(Second Embodiment)
FIG. 6 shows a second embodiment of the optical transmission circuit of the present invention. A feature of the present embodiment is that an external modulator 6 is disposed between the external modulator 2 and the nonlinear medium 4, a clock signal is applied to the external modulator 6 via the modulator driving circuit 7, and the external modulator 2 The intensity of the light modulation signal output from is modulated. The external modulator 6 may be disposed on the input side of the external modulator 2 and intensity-modulated with continuous clock light input to the external modulator 2 using a clock signal. As a result, the optical modulation signals of the optical frequencies f ₁ and f ₂ output from the external modulator 2 can be RZ (Return-to-Zero) encoded.
[0023]
As the external modulator 6, a Mach-Zehnder optical modulator or an electroabsorption optical modulator can be used as in the external modulator 2. Alternatively, both the external modulators 2 and 6 may be configured by electroabsorption optical modulators and integrated. Further, as shown in FIG. 3, the two-wavelength light source 1 and the nonlinear medium 4 can be integrated.
[0024]
【The invention's effect】
As described above, the optical transmission circuit of the present invention inputs the optical modulation signals of the optical frequencies f ₁ and f ₂ that are simultaneously intensity-modulated into the nonlinear medium, and mixes four light waves using one as signal light and the other as pump light. It is the structure which generates light. Since the intensity of this four-wave mixed light is proportional to the cube of the intensity of the optical modulation signal, even if the extinction ratio of the optical modulation signal input to the nonlinear medium is low and the driving voltage of the external modulator is low, the high extinction ratio is high. Can be converted into a light modulation signal (four-wave mixed light). In addition, the four-wave mixing process can operate on terabit-class optical signals, and therefore can sufficiently handle high-speed signals.
[Brief description of the drawings]
FIG. 1 is a diagram showing a first embodiment of an optical transmission circuit of the present invention.
FIG. 2 is a diagram showing a configuration example of a two-wavelength light source 1;
FIG. 3 is a diagram showing an example of integration of the optical transmission circuit of the present invention.
FIG. 4 is a diagram showing optical output characteristics of the optical transmission circuit of the present invention.
FIG. 5 is a diagram showing experimental results for confirming the effect of the optical transmission circuit of the present invention.
FIG. 6 is a diagram showing a second embodiment of the optical transmission circuit of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 2 wavelength light sources 2, 6 External modulator 3, 7 Modulator drive circuit 4 Non-linear medium 5 Optical filter 11, 12, 14 Laser light source 13 Optical multiplexer 15 Mach-Zehnder type optical modulator

Claims (9)

A two-wavelength light source that simultaneously outputs continuous light of optical frequencies f ₁ and f ₂ (f ₂ −f ₁ = Δf ≠ 0);
An external modulator that inputs continuous light of the optical frequencies f ₁ and f ₂ and simultaneously modulates the intensity with an externally applied electrical signal;
A modulator driving circuit for applying an electric signal to be transmitted to the external modulator;
A non-linear medium for generating a four-wave mixed light having an optical frequency f ₃ (= 2f ₂ −f ₁ ) by inputting an optical modulation signal having optical frequencies f ₁ and f ₂ intensity-modulated by the electric signal;
An optical transmission circuit comprising: an optical filter that selects and outputs only the four-wave mixed light having the optical frequency f ₃ from the output light of the nonlinear medium. The optical transmission circuit according to claim 1,
The two-wavelength light source is constituted by a laser light source that outputs continuous light of optical frequencies f ₁ and f ₂ , respectively, and an optical multiplexer that multiplexes the output light. The optical transmission circuit according to claim 1,
The two-wavelength light source is a laser light source that outputs continuous light having an optical frequency f ₁ + Δf / 2, and modulates the output light with a clock signal having a frequency Δf / 2, so that the optical frequencies f ₁ and f ₁ + Δf (= f ₂ ), A Mach-Zehnder type optical modulator that outputs continuous light. The optical transmission circuit according to claim 1,
The optical transmitter circuit, wherein the external modulator is an electroabsorption optical modulator. The optical transmission circuit according to claim 1,
The optical transmission circuit, wherein the nonlinear medium is an optical fiber. The optical transmission circuit according to claim 1,
An optical transmission circuit, wherein the nonlinear medium is a semiconductor optical amplifier. The optical transmission circuit according to claim 1,
The optical transmitter circuit, wherein the external modulator is an electroabsorption optical modulator, the nonlinear medium is a semiconductor optical amplifier, and these are integrated. The optical transmission circuit according to claim 1,
The external modulator is a first external modulator, and a second external modulator that modulates and pulses the continuous light or the optical modulation signal with a predetermined clock signal is provided on the input side or output side of the external modulator. An optical transmission circuit characterized by the above. The optical transmission circuit according to claim 8, wherein
The first external modulator and the second external modulator are both electro-absorption optical modulators, and are configured to be integrated.

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