JP2004260542A - Optical transmission circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve an on/off ratio of an optical modulation signal from which a sufficient on/off ratio can not be obtained, wherein the output amplitude of a modulator drive circuit is not sufficient in an optical transmission circuit that uses an external modulator. <P>SOLUTION: The external modulator simultaneously modulates the intensity of continuous light of optical frequencies f<SB>1</SB>and f<SB>2</SB>(f<SB>2</SB>-f<SB>1</SB>=Δf≠0) outputted from a two-wavelength light source by an electrical signal (transmission signal), an optical modulated signal of the optical frequencies f<SB>1</SB>and f<SB>2</SB>subjected to intensity modulation is inputted to a nonlinear medium to generate four-wave mixed light of an optical frequency f<SB>3</SB>(= 2f<SB>2</SB>-f<SB>1</SB>), and only the four-wave mixed light is selected through an optical filter and outputted. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an optical transmission circuit used in an ultra-high speed / wavelength multiplex 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 method.
[0002]
[Prior art]
In recent years, the capacity of optical fiber communication systems has been accelerated due to the development of wavelength division multiplexing technology, but the signal speed per wavelength has also been increasing. At the commercial level, 1 Gbit / s per wavelength has been realized, and 40 Gbit / s per wavelength has already been researched and developed. Improvement of the signal speed per wavelength is considered to be advantageous for increasing the capacity since the frequency use efficiency can be relatively easily improved. In fact, most of the reports on transmission experiments in which the total capacity exceeds 3 Tbit / s use an optical transmission circuit of 40 to 43 Gbit / s per wavelength.
[0003]
An optical transmission circuit that generates such an ultra-high-speed optical modulation signal includes a Mach-Zehnder optical modulator (MNM manufactured by LN) using an electro-optic effect of lithium niobate as an external modulator, and an electric field absorption effect of a semiconductor. The used electro-absorption type optical modulator is used. Since the optical transmission circuit using such an external modulator has a smaller chirping or the like of the generated optical modulation signal than the direct modulation method, it is caused by the group velocity dispersion of the transmission path medium as the signal speed increases. This can suppress the influence of waveform deterioration, which is advantageous for increasing the transmission distance.
[0004]
By the way, since the voltage of the applied electric signal is relatively high at 3 to 6 Vp-p, the external modulator requires a high-output modulator drive circuit having a wide frequency characteristic. Further, 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 drive circuit needs to satisfy these three requirements of a wide band, a high output, and a high gain, and it is not easy to realize a modulator drive circuit used for a 40 Gbit / s optical transmission circuit. In the case where an optical transmission circuit that generates an optical modulation signal exceeding 100 Gbit / s will be realized in the future, the realization of a corresponding modulator driving circuit will be a major issue.
[0005]
On the other hand, if the output of the modulator drive 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 optical modulation signal is not sufficient, the transmittable distance becomes short. That is, there is a trade-off between the speedup and the long distance by using the external modulator depending on the performance of the modulator driving circuit.
[0006]
As a method of improving the on / off ratio of the optical modulation signal, the optical modulation signal and CW light are input to a nonlinear medium (dispersion shift fiber) to generate four-wave mixing light, and the four-wave mixing light is used for transmission. (Non-Patent Document 1).
[0007]
[Non-patent document 1]
A. Argyris et. al. , "Extension 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 mixing light is generated when signal light and pump light are simultaneously input to a nonlinear medium, and has the property of being 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, if the optical modulation signal is signal light and the CW light is pump light, the intensity of the generated four-wave mixing light is proportional to the square of the intensity of the optical modulation signal. The on / off ratio (extinction ratio) of the optical 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 a modulator driving circuit is insufficient and a sufficient on / off ratio cannot be obtained in an optical transmission circuit using an external modulator. It is an object of the present invention to provide an optical transmission circuit capable of performing the following.
[0010]
[Means for Solving the Problems]
Optical transmitter circuit of the present invention, at the same time strength by an electrical signal (transmission signal) optical frequency f ₁ output from the two-wavelength light source and f ₂ the continuous light _{(f 2 -f 1 = Δf ≠} 0) with an external modulator modulated, to generate four-wave mixed light of the light frequency _{f 3 (= 2f 2 -f 1} ) enter the optical modulation signal of the intensity-modulated optical frequency f ₁ and f ₂ to the nonlinear medium, an optical filter In this configuration, only four-wave mixing light is selected and output.
[0011]
BEST MODE FOR CARRYING OUT THE INVENTION
(1st Embodiment)
FIG. 1 shows a first embodiment of the 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 . enter the intensity modulation at the same time as the external modulator 2, a modulator driving circuit 3 for applying an electrical signal to be transmitted to the external modulator 2, the intensity-modulated light modulated in optical frequency f ₁ and f ₂ by an electrical signal A nonlinear medium 4 for inputting a signal to generate four-wave mixed light of an optical frequency f ₃ (= 2f ₂ −f ₁ ) and an optical filter 5 for selecting and outputting only the four-wave mixed light of an optical frequency f ₃ Be composed.
[0012]
Four-wave mixing light is an extremely high-speed 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 mixing light using one of two optically modulated signals simultaneously modulated as signal light and the other as pump light, the on / off ratio (extinction ratio) of the optical modulation signal is 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 extracting the four-wave mixing light, the on / off ratio can be significantly improved (from 3 dB to 9 dB according to the optical output characteristics of FIG. 4 described later).
[0013]
FIG. 2 shows a configuration example of the two-wavelength light source 1. 2-wavelength light source 1 shown in FIG. 2 (1) includes a laser light source 11 and 12 to the output of the optical frequency f ₁ and f ₂ the continuous light, respectively, an optical multiplexer for multiplexing the output light (e.g., polarization-maintaining optical Coupler 13). The laser light sources 11 and 12 are DFB lasers or external resonance type lasers, and assume that the oscillation light frequency is stabilized.
[0014]
The two-wavelength light source 1 shown in FIG. 2B includes a laser light source 14 that outputs continuous light having an optical frequency of 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 what is referred to as CS-RZ light source, simultaneously generating continuous light of the optical frequency f ₁ and f ₂ of the optical frequency difference Delta] f.
[0015]
The double-wavelength light source 1 of this addition to concurrent continuous light of optical frequencies f ₁ and f _2, 2 and beat pulses generated mode-locked laser, supercontinuum light source or from the output light from a predetermined optical frequency comb generator A configuration that selectively outputs continuous light having two light 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 that the propagation speeds of the two optical modulation signals are the same, it is desirable that the zero-dispersion optical frequency is close to (f ₁ + f ₂ ) / 2. .
[0018]
By configuring the external modulator 2 by an electro-absorption optical modulator and configuring the nonlinear medium 4 by a semiconductor optical amplifier, they can be integrated. Further, as shown in FIG. 3, by using a two-beat pulse generating mode-locked laser as the two-wavelength light source 1, an electro-absorption optical modulator constituting the external modulator 2 and a semiconductor optical amplifier constituting the nonlinear medium 4 Can be integrated.
[0019]
FIG. 4 shows the optical output characteristics of the optical transmission circuit of the present invention when a Mach-Zehnder optical modulator is applied to the external modulator 2. The horizontal axis represents the voltage applied to the external modulator normalized by Vπ (the difference between the voltage applied to the external modulator that gives the maximum light output and the minimum light output). The vertical axis indicates the relative value of the output power. The extinction ratio depends on the applied voltage of the external modulator. When 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 is high even with a small applied voltage. It can be seen that the ratio has been obtained.
[0020]
FIG. 5 shows an experimental result for confirming the effect of the optical transmission circuit of the present invention. In the experimental system, a G. G. having a length of 20 km as the nonlinear medium 4 in the configuration of FIG. Using a 653 fiber (zero dispersion wavelength: 1551 nm), the optical wavelengths λ ₁ and λ ₂ corresponding to the optical frequencies f ₁ and f ₂ were set to 1551.5 nm and 1550.7 nm, and the optical frequency difference Δf was set to about 100 GHz. A Mach-Zehnder optical modulator (Vπ is 4.4 V) was used as the external modulator 2, the drive signal was 10 Gbit / s, and the amplitude was 2.2 Vp-p of Vπ / 2. In the experiment, in order to obtain four-wave mixing light sufficient for observation, an optical amplifier (EDFA) was arranged in front of the nonlinear medium 4 to amplify the optical modulation signal.
[0021]
The experimental result of FIG. 5 is a result of measuring the extinction ratio while changing the bias voltage of the external modulator 2. Assuming that 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, thereby confirming the effects 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 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. This embodiment is characterized in that an external modulator 6 is arranged between the external modulator 2 and the nonlinear medium 4, a clock signal is applied to the external modulator 6 via a modulator driving circuit 7, Is to modulate the intensity of the light modulation signal output from the device. Note that the external modulator 6 may be arranged on the input side of the external modulator 2, and the intensity of continuous light input to the external modulator 2 may be modulated by a clock signal. Thus, 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 type optical modulator or an electro-absorption type optical modulator can be used similarly to the external modulator 2. Further, the external modulators 2 and 6 may both be formed of an electro-absorption type optical modulator, and may be configured to be integrated. Further, as shown in FIG. 3, it is possible to integrate the two-wavelength light source 1 and the nonlinear medium 4 together.
[0024]
【The invention's effect】
As described above, the light transmitting circuit of the present invention, at the same time an intensity-modulated optical-modulated signal of the optical frequency f ₁ and f ₂ inputted to the nonlinear medium, four-wave mixing one signal light, and the other as a pump light This is a configuration for generating light. Since the intensity of this four-wave mixing light is proportional to the cube of the intensity of the optical modulation signal, a high extinction ratio can be obtained even if the driving voltage of the external modulator is low and the extinction ratio of the optical modulation signal input to the nonlinear medium is not sufficient. (A four-wave mixing light). In addition, since the four-wave mixing process can operate on a terabit-class optical signal, it can sufficiently cope with a high-speed signal.
[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]
1 Two-wavelength light source 2, 6 External modulator 3, 7 Modulator driving circuit 4 Nonlinear 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 having optical frequencies f ₁ and f ₂ (f ₂ −f ₁ = Δf ≠ 0);
Enter the continuous light of the optical frequencies f ₁ and f _2, and an external modulator for intensity modulation at the same time by an electric signal applied from the outside,
A modulator driving circuit for applying an electric signal to be transmitted to the external modulator;
A non-linear medium that receives light modulation signals of the light frequencies f ₁ and f ₂ intensity-modulated by the electric signal and generates four-wave mixing light of the light frequency f ₃ (= 2f ₂ −f ₁ );
The optical transmitter circuit characterized by comprising an optical filter which selects and outputs only four-wave mixed light of the light frequency f ₃ from the output light of the nonlinear medium. The optical transmission circuit according to claim 1,
The two-wavelength light source, a laser light source for outputting the optical frequency f ₁ and f ₂ the continuous light, respectively, the light transmitting circuit, characterized in that the output light is constituted by an optical multiplexer for multiplexing. The optical transmission circuit according to claim 1,
The two-wavelength light source includes a laser light source that outputs continuous light having an optical frequency of f ₁ + Δf / 2, and modulates the output light with a clock signal having a frequency of Δf / 2 to generate optical frequencies f ₁ and f ₁ + Δf (= f _2). An optical transmission circuit comprising a Mach-Zehnder type optical modulator that outputs continuous light. The optical transmission circuit according to claim 1,
An optical transmission circuit, wherein the external modulator is an electro-absorption type 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,
The optical transmission circuit, wherein the nonlinear medium is a semiconductor optical amplifier. The optical transmission circuit according to claim 1,
The optical transmission circuit, wherein the external modulator is an electro-absorption optical modulator, the nonlinear medium is a semiconductor optical amplifier, and they are integrated. The optical transmission circuit according to claim 1,
The external modulator is a first external modulator, and a second external modulator for modulating the continuous light or the optical modulation signal with a predetermined clock signal to form a pulse is provided on an input side or an output side thereof. An optical transmission circuit characterized by the above-mentioned. The optical transmission circuit according to claim 8,
An optical transmission circuit, wherein the first external modulator and the second external modulator are both electro-absorption optical modulators, and have an integrated configuration.

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