JPH11225109A - Optical transmitter and optical signal sending method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress inductive Brillouin scattering by simple constitution by executing phase shifting to a wavelength multiple signal light from a transmission light source so as to send this optical signal to a transmission path. SOLUTION: An optical multiplexer 2 multiplexing the signal light of n wavelengths from a multi-wavelength optical transmitter 1 and an optical phase shifter 3 executes phase shifting to the signal light of four wavelengths to expand the spectrum width of each wavelength light. An optical branching device 4 branches rear scattered light generated by inductive Brillouin scattering and a wavelength separator 5 wavelength-separates the branched rear scattered light. Then, an optical detector 6 converts the optical power of each wavelength component of this wavelength-separated rear scattered light to an electric signal. Then, a shifter driving circuit 7 is provided with n-number of rear scattered light power detecting ports equivalent to n-number of signal light wavelength and a shifting signal output port outputting an RF signal for driving the shifter 3 and selects the rear scattered light power of a naximum value to control it to be suppressed to a desired level.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical transmission device used in an optical transmission system, and more particularly to an optical transmission device for suppressing stimulated Brillouin scattering that occurs when high power light is input to an optical transmission line.

[0002]

2. Description of the Related Art Non-linear optical effects in optical fibers are factors that limit the transmission distance and relay interval of an optical communication system.

A single-mode optical fiber used as a transmission line in an ordinary optical communication system transmits optical power in a state where the optical power is confined in a narrow space area. Spatial power density is increasing. Furthermore, since the optical fiber has a small loss in optical power and can transmit over a long distance, the interaction length between the medium constituting the optical fiber and the incident light is large. For this reason, in the optical fiber, the optical power required to generate the nonlinear optical effect is smaller than that of the bulk type optical medium, and the nonlinear optical effect is easily generated.

[0004] Among the various nonlinear optical effects, stimulated Brillouin scattering is a phenomenon in which the spectral width of input signal light is narrow.
The threshold of the optical power required for generation is small. When stimulated Brillouin scattering occurs, a part of the input signal light is backscattered during propagation in the optical fiber. Therefore, in a current long-distance optical communication system using a single-mode semiconductor laser having a high power and a narrow spectral line width as a transmission light source, the maximum optical power that can be input to an optical fiber is limited by the occurrence of stimulated Brillouin scattering. You.

[0005] The threshold value of the optical fiber input power at which stimulated Brillouin scattering occurs monotonically increases with respect to the spectral width of the input light. For a quantitative relationship between the threshold value and the spectrum width, see Japanese Patent Application Laid-Open No. Hei 4-293024 or IEEE Journal Lightwave.
Technology, Vol. 6 (1988), Vol. 5
No., pages 710 to 719, in detail. For this reason, in order to suppress stimulated Brillouin scattering, a method of expanding the spectrum width by performing frequency modulation on input light has been attempted, as described in JP-A-4-293024. Further, as a means for modulating the frequency of the signal light, as shown in FIG. 5, a method of minutely modulating the injection current of the semiconductor laser as the transmission light source has been adopted.

[0006]

However, in the above-mentioned prior art, the injection current of the semiconductor laser, which is a transmission light source, is modulated in order to perform frequency modulation. It will take.
As a result, there is a problem that the receiving sensitivity is deteriorated particularly in the optical communication apparatus of the intensity modulation system. To solve this problem, it is conceivable to select and use a semiconductor laser having a high frequency modulation efficiency. However, in this case, the yield of the semiconductor laser that satisfies the requirement is deteriorated, and the cost of the apparatus is increased.

Further, when the above-mentioned prior art is applied to a wavelength division multiplexing transmission apparatus, it is necessary to apply a minute modulation to the injection current of each of the semiconductor lasers having a plurality of signal light wavelengths, so that the apparatus becomes large-scale.

In addition, although the frequency modulation efficiency of a semiconductor laser is expected to change due to deterioration with time or the like, the above-mentioned prior art, which does not include a means for directly detecting backscattered light generated by stimulated Brillouin scattering, has been proposed. The effect of suppressing Brillouin scattering may decrease. In order to avoid this, if the frequency modulation degree is set to be large in advance in consideration of aging, the transmission characteristics may be deteriorated due to dispersion of the optical fiber. An object of the present invention is to solve the above-mentioned problems and to provide an optical transmission device including a stimulated Brillouin scattering suppression unit having a simple configuration.

[0009]

In order to achieve the above object, an optical transmission apparatus according to the present invention comprises: a transmission light source for outputting a wavelength-division multiplexed signal light; And an optical phase modulator for transmitting the optical signal.

Further, the optical transmission device of the present invention includes a modulator control circuit for adjusting a drive signal of the optical phase modulator based on the power of return light propagating through the optical transmission path toward the transmission light source. In order to generate a signal to be input to the modulator control circuit, light having a predetermined wavelength is selected from return light propagating through the optical transmission path toward the transmission light source, and is output as selected return light. An optical filter and a photodetector for converting the selective return light into an electric signal may be included.

In addition, the optical transmission device of the present invention includes a wavelength separator that separates the return light into each wavelength component light, instead of the optical filter and the photodetector, and electrically separates each of the wavelength component lights. A plurality of photodetectors for converting the signals into signals may be included.

Further, the modulator control circuit may be configured to adjust the drive signal so that the amplitude of the output signal of the photodetector has a predetermined value.
As the drive signal, a sine wave or a digital signal can be used. The amplitude or frequency of the drive signal may be used as a parameter of the drive signal to be adjusted.

Further, the optical filter may include a Fabry-Perot interferometer, a dielectric multilayer filter, or a fiber grating. Further, the wavelength separator may include an arrayed waveguide diffraction grating formed on a quartz substrate.

Further, in order to extract return light propagating toward the transmission light source through the optical transmission line, an optical splitter or an optical circulator may be inserted into the optical transmission line.

In the present invention, the wavelength multiplexed signal light is subjected to phase modulation, and the spectral widths of all the wavelength lights constituting the wavelength multiplexed signal light are collectively expanded. By using such a configuration, it becomes possible to increase the threshold value of stimulated Brillouin scattering with respect to wavelength-multiplexed signal light with a simple configuration.
Further, the present invention employs a configuration in which backscattered light that is return light to the transmission side is monitored, and a phase modulation parameter is controlled based on the result. Even if the threshold optical power changes,
It is possible to prevent stimulated Brillouin scattering from occurring.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An optical transmission apparatus and an optical signal transmission method according to the present invention will be described in detail with reference to the drawings. FIG. 1 is a configuration diagram showing a first embodiment of the present invention. 1, an optical transmission apparatus according to the present invention includes a multi-wavelength optical transmitter 1, an optical multiplexer 2, an optical phase modulator 3, an optical splitter 4, a wavelength separator 5, a photo detector 6, and a modulator control circuit 7. Consists of

The multi-wavelength optical transmitter 1 has n wavelengths different from each other.
It has the function of sending out individual signal lights. FIG. 2 is a block diagram showing the internal configuration of the multi-wavelength optical transmitter 1. The multi-wavelength optical transmitter 1 includes n semiconductor lasers 8-1 to 8 -n as transmission light sources.
Each of the semiconductor lasers has an APC (Automatic Power Controller) for keeping the optical output constant.
ol) ATC (Automatic Te) for keeping the temperature of the circuits 10-1 to 10-n and the laser element of the semiconductor laser constant
(Performance Control) circuits 11-1 to 11-1
n stabilizes the optical output power and wavelength.

Output light (CW light) from the semiconductor laser is applied to intensity modulators 9-1 to 9-1 formed on a lithium niobate substrate.
n, is subjected to intensity modulation by a data signal (not shown), and is output as a multi-wavelength signal light. Optical multiplexer 2
Is a tree coupler composed of a fusion type optical fiber coupler, and multiplexes n-wavelength signal light transmitted from the multi-wavelength optical transmitter 1.

The optical phase modulator 3 expands the spectrum width of each wavelength light by performing phase modulation on the signal light of four wavelengths. The optical phase modulator 3 has a light input / output port and an RF signal input port. This modulator utilizes the electro-optic effect. An RF signal is applied to a lithium tantalate crystal with a high electro-optic constant via an electrode, and the input light is transmitted through the crystal to produce phase-modulated output light. Can be obtained.

The optical splitter 4 is a fusion-type optical fiber coupler, and separates backscattered light generated by stimulated Brillouin scattering. The wavelength separator 5 is an AWG (Arrayed Wa)
Veguide Grating), and separates the wavelength of the demultiplexed backscattered light. Since the wavelength of the backscattered light generated by stimulated Brillouin scattering is only about 0.1 nm smaller than the signal light wavelength, the transmission peak wavelength of the AWG is set equal to the transmission signal wavelength of the multi-wavelength optical transmitter. No problem. The photodetector 6 is composed of n photodiodes and a bias circuit, and converts the optical power of each wavelength component of the wavelength-separated backscattered light into an electric signal.

FIG. 3 shows the configuration of the modulator driving circuit 7. The modulator drive circuit 7 has n backscattered light power detection ports corresponding to n signal light wavelengths and a modulation signal output port for outputting an RF signal for driving the optical phase modulator 3. The largest backscattered light power is selected from the backscattered light powers for the respective wavelength lights by the maximum value detection circuit 77 and applied to the optical phase modulator 3 so that the backscattered light power is suppressed to a desired level. It has a function of controlling the amplitude of the modulation signal.

In FIG. 3, of the n signals input to the modulator driving circuit 7, the signal having the largest amplitude is selected by the maximum value detecting circuit 77. The output of the maximum value detection circuit 77 is
The reference voltage corresponding to the backscattered light level which is the control target is compared with the comparator 78 by the comparator 78, and the difference between them is sent to the calculating unit 79. The arithmetic unit 79 calculates and outputs a control signal based on the PID control law so that the output of the comparator 78 becomes 0. The output of the operation unit 79 is input to the driver 76. The driver 76 amplifies the input signal and outputs a modulation signal for driving the optical phase modulator 3.

In order to increase the spectrum width of the signal light, the amplitude of the modulation signal may be fixed and the modulation frequency of the phase modulation may be increased. In order to change the modulation frequency, as a clock source or a sine wave oscillator of a digital signal used to generate a modulation signal, a voltage control oscillator (Voltage Control) whose frequency can be changed by an external input signal.
ed Oscillator (VCO) may be used. In this case, a VCO may be used as an oscillator or a clock source in the driver 76, and the output of the adder 75 may be supplied from a frequency adjustment terminal of the VCO. The cycle of the modulation signal needs to be set shorter than the time required for the signal light to propagate through the optical fiber on which the signal light enters. For example,
For an optical fiber of about km, it is about 0.1 ms. Therefore, for an optical fiber of this length,
The modulation frequency is set to about 10 kHz or more, which is the reciprocal of 0.1 ms.

In this embodiment, a fusion type optical fiber coupler is used as the optical branching device 4. However, an optical circulator may be used in order to reduce the optical power branching loss.

Next, a second embodiment of the present invention will be described with reference to FIG.
This will be described with reference to FIG.

In FIG. 4, the same components as those in FIG. 1 are denoted by the same reference numerals as those in FIG. The components newly introduced in FIG. 4 are the optical filter 12 and the modulator control circuit 70.

In the first embodiment, the backscattered light is separated for each wavelength, and each light power is detected. However, the signal light wavelength at which the backscattered light due to stimulated Brillouin scattering is most strongly generated may be known in advance from characteristics such as the transmission wavelength of the multi-wavelength transmission device. In this case, if the backscattered light generated at this wavelength is suppressed to a predetermined value or less, the backscattered light corresponding to all wavelengths of light will satisfy the predetermined value. Therefore, the wavelength separator 5 used in the first embodiment may be replaced with an optical filter 12 that transmits only the signal light wavelength at which the backscattered light is generated most strongly, and only the output light power of the optical filter 12 may be monitored. . For this reason, the number of photodetectors 6 prepared for the number of wavelengths in the first embodiment is reduced to only one of the photodetectors 13.

In this case, the modulator control circuit 70 has a configuration in which the maximum value detection circuit 77 is removed from FIG. The modulator control circuit 70 receives only the output electric signal of the photodetector 13 and performs the same control on this signal as in the modulator drive circuit 7 of the first embodiment.

The optical filter used in the second embodiment is a Fabry-Perot etalon in which a thin film giving a predetermined transmittance is deposited on the opposing surface, and a Fabry-type in which two partially transmitting mirrors are opposed to each other with a gap therebetween. A Perot filter, a dielectric multilayer filter, or a fiber grating can be used.

The second embodiment has an effect that the device scale of the multi-wavelength optical transmission device and the circuit size of the modulator control circuit can be reduced.

[0031]

As described above, according to the present invention, instead of frequency modulation by injection current modulation, which has been conventionally performed for spectrum expansion of a transmission light source for the purpose of suppressing stimulated Brillouin scattering, external modulation is used. It employs phase modulation by a device. For this reason, in the above-described related art, since the amplitude modulation that has occurred simultaneously with the frequency modulation is removed, it is possible to prevent the reception sensitivity from deteriorating due to the parasitic amplitude modulation.

In the conventional method of minutely modulating the injection current, especially when applied to a multi-wavelength laser light source, it is necessary to provide a frequency modulation circuit for each laser drive circuit of each wavelength, which increases the scale of the apparatus. Although inevitable, by using the present invention, the phase modulation is applied to the signal lights of all the wavelengths at once, so that the device scale can be reduced.

Further, in the present invention, the spectrum is expanded by a modulator provided outside the semiconductor laser instead of modulating the semiconductor laser main body. Since the required specifications are greatly relaxed, the effect of improving the yield of semiconductor lasers can be expected.

If the wavelength at which the backscattered light is maximum is known in advance, a configuration for monitoring only one wavelength component of the backscattered light can be adopted, and the configuration of the apparatus can be further simplified. Becomes

[Brief description of the drawings]

FIG. 1 is a configuration diagram illustrating a first embodiment of the present invention.

FIG. 2 is a configuration diagram of a multi-wavelength optical transmitter used in the present invention.

FIG. 3 is a diagram illustrating a configuration of a modulator driving circuit used in the present invention.

FIG. 4 is a configuration diagram illustrating a second embodiment of the present invention.

FIG. 5 is a diagram illustrating a configuration of a conventional technique.

[Explanation of symbols]

 1: Multi-wavelength optical transmitter 2: Optical multiplexer 3: Optical phase modulator 4: Optical splitter 5: Wavelength separator 6: Photodetector 7, 70: Modulator control circuit 8-1 to n: Semiconductor laser 9 -1 to n: Light intensity modulator 10-1 to n: APC circuit 11-1 to n: ATC circuit 12: Optical filter 76: Driver 77: Maximum value detection circuit 78: Comparator 79: Operation unit 81: Semiconductor laser 82: Light intensity modulator 83: Oscillator 84: Mixer 85: DC current source

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code FI H04B 10/04 H04B 9/00 E 10/06 10/18 H04J 14/00 14/02

Claims (18)

[Claims] 1. A transmission light source for outputting wavelength-division multiplexed signal light, and an optical phase modulator for performing phase modulation on the wavelength-division multiplexed signal light and transmitting the optical signal to an optical transmission line. Optical transmission device. 2. The optical transmission device according to claim 1, further comprising a waveform of a drive signal of the optical phase modulator based on power of return light propagating through the optical transmission path toward the transmission light source. An optical transmission device comprising a modulator control means for adjusting the optical transmission. 3. The optical transmission device further comprises: an optical filter that selects light having a predetermined wavelength from the return light and outputs a selected return light; and a photodetector that converts the selected return light into an electric signal. The optical transmission device according to claim 2, wherein the modulator control unit detects the power of the return light based on an output signal of the photodetector. 4. The optical transmission device further includes: a wavelength separator that splits the return light into each wavelength component light; and a plurality of photodetectors that convert each of the wavelength component lights into an electric signal. 3. The optical transmission device according to claim 2, further comprising: the modulator control unit detects the power of the return light based on output signals of the plurality of photodetectors. 5. The optical transmission device according to claim 2, wherein the modulator control unit determines an amplitude of an output signal of the photodetector in advance. An optical transmission device, wherein the drive signal is adjusted to be a value. 6. The optical transmission device according to claim 2, wherein the drive signal is a sine wave. 7. The optical transmission device according to claim 2, wherein the drive signal is a digital signal. 8. The optical transmission device according to claim 2, wherein the modulator control means adjusts an amplitude of the drive signal based on an output of the photodetector. An optical transmission device characterized by adjusting. 9. The optical transmission device according to claim 2, wherein the modulator control means changes a frequency of the drive signal based on an output of the photodetector. An optical transmission device characterized by adjusting. 10. The optical transmission device according to claim 1, wherein said optical filter includes a Fabry-Perot interferometer. 11. The optical transmission device according to claim 1, wherein said optical filter includes a dielectric multilayer filter. 12. The optical transmission device according to claim 1, wherein said optical filter includes a fiber grating. 13. The optical transmission device according to claim 2, wherein said wavelength separator includes an arrayed waveguide diffraction grating formed on a quartz substrate. 14. The optical transmission device according to claim 2, wherein return light propagating through the optical transmission path toward the transmission light source is extracted, and the light detection is performed. An optical transmission device comprising: an optical splitter inserted into the optical transmission line to guide the optical transmission device to the optical transmission device. 15. The optical transmission device according to claim 2, wherein the optical transmission device is inserted into the optical transmission line and propagates through the optical transmission line toward the transmission light source. An optical transmission device comprising an optical circulator that outputs return light to the photodetector. 16. An optical signal transmission control device for controlling an optical signal to be transmitted to an optical transmission line, wherein:
An optical signal transmission control device comprising a configuration in which the transmission light source to be controlled is removed from the optical transmission device according to any one of the preceding claims. 17. An optical signal transmission method for suppressing stimulated Brillouin scattering generated when transmitting a wavelength multiplexed optical signal from a transmission light source to an optical transmission line, wherein a power of return light from the optical transmission line is predetermined. Detecting a wavelength component, and adjusting a modulation degree or a modulation frequency of the phase modulation applied to the transmission light source so that all the power of the return light is equal to or less than a predetermined value. Optical signal transmission method. 18. An optical signal transmission method for suppressing stimulated Brillouin scattering generated when a wavelength-division multiplexed optical signal of a transmission light source is transmitted to an optical transmission line, wherein the power of return light from the optical transmission line is changed for each wavelength component. And adjusting the modulation degree or the modulation frequency of the phase modulation applied to the transmission light source so that all of the power of the return light is equal to or less than a predetermined value. Optical signal transmission method.