JP2007336458A - Optical transmission apparatus and optical transmission system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical transmission apparatus and optical transmission system capable of extending a transmission distance inexpensively. <P>SOLUTION: An analog input electric signal is branched into two signals by a distributor 2, amplified by amplifiers 3, 4, respectively and then inputted to laser diodes 5, 6. The laser diodes 5, 6 are directly modulated by the input signals and their outputs are polarization-composed by a polarization composer 7 and sent to an optical fiber 8. In this case, oscillation frequencies of the laser diodes 5, 6 are different from each other and if a band of the input electric signal 1 is about 3 GHz, the frequencies are separated by 100 GHz or more on a frequency axis. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an optical transmission system that transmits an optical signal modulated by an analog signal via an optical fiber, and an optical transmission device therefor.

In recent years, an optical analog transmission system using an optical fiber has attracted attention, and has begun to be applied as a relay system in a CATV (Cable Television) network or a mobile communication network. This type of system is sometimes referred to as a ROF (Radio Over Fiber) system.
An optical fiber can transmit an optical signal with little loss, but various noises occur in the process of transmission. Examples of noise include relative noise intensity (RIN) due to fluctuations in the output of a light source (semiconductor laser) used, shot noise generated in a receiver (such as a photodiode) of a receiver, and the like. In addition, there is thermal noise generated in the first stage amplifier of the receiver. In long-distance transmission, the thermal noise of the receiver becomes dominant, which limits the transmission distance. In order to relax this restriction and increase the transmission distance, it is necessary to increase the light output intensity on the transmission side. However, a high-power laser diode is expensive, and requires a structure and power for cooling. .
A related technique is disclosed in Patent Document 1. This document discloses a technique for improving C / N in a wavelength division multiplexing optical transmission system.
JP 2002-164868 A

As described above, in the existing optical transmission system, it is necessary to increase the light output intensity on the transmission side in order to extend the transmission distance, but it is difficult in terms of cost and structure.
The present invention has been made in view of the above circumstances, and an object thereof is to provide an optical transmission device and an optical transmission system capable of extending the transmission distance at a low cost.

  In order to achieve the above object, according to one aspect of the present invention, branch means for branching a transmission signal into two, first and second amplification sections for amplifying the branched transmission signal, and the first amplifier The first laser diode modulated and driven by the output of the amplifier, the second laser diode modulated and driven by the output of the second amplifier, and the output lights of the first and second laser diodes A polarization beam combiner that combines the polarization signals and sends them to an optical transmission line, wherein the oscillation frequency of the first laser diode and the oscillation frequency of the second laser diode are separated by at least 100 GHz or more. An optical transmission device is provided.

  By taking such a measure, the transmitted light having the intensity corresponding to the sum of the output light intensities of the first and second laser diodes reaches the receiving side, and the difference in intensity between transmission and reception can be expanded. . Therefore, the influence of noise in the transmission process can be reduced without changing the output of each laser diode, and the transmission distance can be extended without requiring an expensive device such as a cooling structure. Further, since the oscillation frequencies of the respective semiconductor lasers are separated by at least 100 GHz, the influence of the beat signal can be avoided.

  According to the present invention, an optical transmission device and an optical transmission system capable of extending the transmission distance can be provided at low cost.

[First Embodiment]
FIG. 1 is a functional block diagram showing a first embodiment of an optical transmission apparatus according to the present invention. In FIG. 1, an analog input electric signal 1 is branched into two by a distributor 2, amplified by amplifiers 3 and 4, and then input to laser diodes 5 and 6, respectively. The laser diodes 5 and 6 are directly modulated by the input signal, and their outputs are combined by the polarization combiner 7 and transmitted to the optical fiber 8. The oscillation frequencies of the laser diodes 5 and 6 are different from each other. If the band of the input electric signal 1 is about 3 GHz, the laser diodes 5 and 6 are separated from each other by at least 100 GHz on the frequency axis. As described above, by widening the interval between the two oscillation frequencies, it is possible to prevent the beat signal having the two oscillation frequencies from becoming noise.

  In the above configuration, assuming that the output light intensity of the laser diodes 5 and 6 is Po, an optical signal having an intensity of 2 Po is output from the polarization beam combiner 7 to the optical fiber 8. Thereby, the intensity difference between transmission and reception can be easily expanded, and laser diodes 5 and 6 that do not require a cooling mechanism such as a Peltier element can be used. Accordingly, it is possible to promote a reduction in cost, and it is possible to reduce power consumption of the entire apparatus by deleting power supplied to the Peltier element. Furthermore, since the time difference in propagation delay due to polarization can be ignored, the signal modulated to light can be doubled.

[Second Embodiment]
FIG. 2 is a functional block diagram showing a second embodiment of the optical transmission apparatus according to the present invention. In FIG. 2, the input electric signal 1 branched into two by the distributor 2 is delayed by a predetermined delay amount by the delay circuits 9 and 10 and then input to the amplifiers 3 and 4. The outputs of the amplifiers 3 and 4 become drive signals for the laser diodes 5 and 6, and the outputs of the laser diodes 5 and 6 are wavelength-multiplexed by the wavelength multi-wavelength waver 11 and sent to the optical fiber 8.

  In the configuration of FIG. 2, it is corrected that the output wavelengths (frequency) of the laser diodes 5 and 6 are different and the propagation time of each output light is different due to the difference in the wavelengths. In general, the chromatic dispersion of a single mode fiber is set to 0 ps / nm / km near 1310 nm, but the propagation delay increases as it deviates from this. For example, since there is a difference of about 2 ps / nm / km between 1310 nm and 1330 nm, transmission of 15 km results in a propagation delay time difference of 600 ps. Considering transmission of an analog signal of 2 GHz, the phase of 439 degrees is rotated. In simple multiplexing, the signal amplitude at the reception point is reduced by the phase shift between the two signals. In this embodiment, therefore, the input electrical signal 1 branched by the distributor 2 is delayed by the delay circuits 9 and 10 to compensate for the delay in the optical fiber 8 so that the signal strength on the receiving side is maximized. .

  FIG. 3 is a schematic diagram showing an existing optical transmitter, and a temperature control unit 13 using a Peltier element 12 or the like is required in order to increase the output of the laser diode 5 alone. For this reason, the configuration becomes complicated and the cost increases. On the other hand, in the first and second embodiments, it is possible to eliminate the need for a device related to temperature control by keeping the output of one laser diode low, simplify the configuration, and reduce the cost.

[Third Embodiment]
FIG. 4 is a functional block diagram showing an optical transmission system according to the present invention. The optical transmission device includes an oscillator 21 and an adder 22. The oscillator 21 generates a monitor signal having a frequency of 1 wavelength or less with respect to the delay time due to chromatic dispersion depending on the transmission distance, and this monitor signal is superimposed on the input electrical signal 1 by the adder 22.

  An optical signal including two wavelengths output from the optical transmission device reaches the photodiode 15 of the optical reception device via the optical fiber 8 and is converted into an electrical signal. This electric signal is input to the branching device 17 via the amplifier 16 and is divided into a branch output to the band pass filter 18 and a main signal 19. The band pass filter 18 extracts the monitor signal component 20 based on the optical signal of each wavelength.

  The intensity of the monitor signal on the receiving side is maximized when the propagation times of the two wavelengths match. Therefore, in this embodiment, the delay amounts of the delay circuits 9 and 10 on the transmission side are adjusted so that the intensity of the monitor signal component 20 is maximized. If, for example, a 1 GHz sine wave is used as the monitor signal, one wavelength is 1 nm. Therefore, the shift of about 600 ps described in the third embodiment can be identified.

[Fourth Embodiment]
FIG. 5 is a functional block diagram showing another example of the optical transmission system according to the present invention. In FIG. 5, the intensity of the monitor signal component 20 is compared with the reference signal 24 by the comparator 23, and the result is converted into an optical signal by the laser diode 25 and transmitted to the optical transmitter via the optical fiber 26. In the optical transmitter, the signal photoelectrically / electrically converted by the photodiode 27 is supplied to the controller 28, and the delay amounts of the delay circuits 9 and 10 are feedback controlled so that the intensity of the monitor signal component 20 on the receiving side is maximized. Like that. In this way, it is possible to more accurately compensate for deviations in correction amounts due to changes over time. The signal format of the optical fiber 26 may be an analog format or a digital format. Furthermore, it is possible to realize not only optical transmission but also a transmission path using another electric line (for example, telephone line).

[Fifth Embodiment]
In addition, the delay time difference can be corrected more easily. For example, in FIG. 1, the polarization multiplexer 7 is changed to a wavelength multi-wavelength multiplexer, and the output light wavelengths of the laser diodes 5 and 6 are appropriately selected. The chromatic dispersion value takes both positive and negative values before and after the zero dispersion of the optical fiber 8, but the same delay time occurs as a signal delay. Therefore, by arranging the output light wavelengths of the laser diodes 5 and 6 at the same distance from the zero dispersion wavelength of the optical fiber 8, the delay to the reception point of the signals output from the two laser diodes 5 and 6 is achieved. The time will be the same. This eliminates the need to provide the delay circuits 9 and 10 on the transmission side.

  As shown in FIG. 6, since the 1.31 μm single mode fiber has a zero dispersion wavelength in the vicinity of 1310 nm, by using the semiconductor lasers of 1290 nm and 1310 nm before and after that, the propagation times on the receiving side are substantially matched. be able to. If the respective chromatic dispersion values are −2 ps / nm / km and 2 ps / nm / km, 2 ps / nm / km generates a delay time of 600 ps for each 1310 nm at a transmission distance of 15 km. However, since the mutual propagation delay times are effective as absolute values, they are the same, and a signal output in which the phases coincide with each other is obtained on the receiving side.

  Note that the present invention is not limited to the above-described embodiment as it is, and can be embodied by modifying the constituent elements without departing from the scope of the invention in the implementation stage. In addition, various inventions can be formed by appropriately combining a plurality of components disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, constituent elements over different embodiments may be appropriately combined.

1 is a functional block diagram showing a first embodiment of an optical transmission apparatus according to the present invention. The functional block diagram which shows 2nd Embodiment of the optical transmitter concerning this invention. The schematic diagram which shows the existing optical transmitter. 1 is a functional block diagram showing an optical transmission system according to the present invention. The functional block diagram which shows the other example of the optical transmission system concerning this invention. The principle figure for demonstrating the 5th Embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Input electric signal, 2 ... Divider, 3, 4 ... Amplifier, 5, 6 ... Laser diode, 7 ... Polarization synthesizer, 8 ... Optical fiber, 9, 10 ... Delay circuit, 11 ... Wavelength multiple wave multiplier , 12 Peltier element, 13 Temperature controller, 15 Photodiode, 16 Amplifier, 17 Divider, 18 Bandpass filter, 19 Main signal, 20 Monitor signal component, 21 Oscillator, 22 Addition 23 ... comparator 24 ... reference signal 25 ... laser diode 26 ... optical fiber 27 ... photodiode 28 ... controller

Claims (6)

Branching means for branching the transmission signal in two;
First and second amplifying units for amplifying the branched transmission signals, respectively;
A first laser diode modulated and driven by the output of the first amplifying unit;
A second laser diode modulated and driven by the output of the second amplifying unit;
A polarization beam combiner that combines the output lights of the first and second laser diodes into a light transmission path after combining the polarization light;
An optical transmission device characterized in that the oscillation frequency of the first laser diode and the oscillation frequency of the second laser diode are separated by at least 100 GHz. Branching means for branching the transmission signal in two;
First and second amplifying units for amplifying the branched transmission signals, respectively;
A first laser diode modulated and driven by the output of the first amplifying unit;
A second laser diode that is modulated and driven by the output of the second amplification unit and has a different oscillation frequency from the first laser diode;
A wavelength multi-polymerizer that wavelength-multiplexes each output light of the first and second laser diodes and sends it to an optical transmission line;
An optical transmission device characterized in that the oscillation frequency of the first laser diode and the oscillation frequency of the second laser diode are separated by at least 100 GHz. 3. The optical transmission apparatus according to claim 1, further comprising delay means for delaying at least one of the transmission signals branched by the branch means to compensate for a delay in the optical transmission line. The oscillation wavelength of the first laser diode and the oscillation wavelength of the second laser diode are set to a short wavelength side and a long wavelength with respect to the zero dispersion wavelength of the optical transmission line so that the delay times due to dispersion in the optical transmission line coincide with each other. The optical transmission device according to claim 1, wherein the optical transmission device is disposed on each wavelength side. In an optical transmission system comprising: an optical transmitter that transmits an optical signal to an optical transmission line; and an optical receiver that receives the optical signal via the optical transmission line.
The optical transmitter is
Branching means for branching the transmission signal in two;
First and second amplifying units for amplifying the branched transmission signals, respectively;
A first laser diode modulated and driven by the output of the first amplifying unit;
A second laser diode modulated and driven by the output of the second amplifying unit and having an oscillation frequency separated from the first laser diode by at least 100 GHz;
A polarization beam combiner that combines the output lights of the first and second laser diodes into a light transmission path by combining the output lights;
Delay means for delaying at least one of the transmission signals branched by the branch means to compensate for a delay in the optical transmission line;
Superimposing means for superimposing a monitor signal having a frequency sufficiently lower than a propagation delay time difference in the optical transmission line of each output light of the first and second laser diodes on the transmission signal;
The optical receiver is
Extraction means for extracting the monitor signal superimposed on each output light of the first and second laser diodes;
An optical transmission system comprising delay amount control means for controlling a delay amount in the delay means based on a propagation delay time difference between monitor signals extracted in the optical receiver. The delay amount control means includes:
Means for comparing the delay amount of the monitor signal extracted in the optical receiver with a reference value;
Means for transmitting the result of the comparison to the optical transmitter;
Means for feedback-controlling the amount of delay in the delay means based on the transmitted comparison result so that the arrival times of the output lights of the first and second laser diodes to the optical receiver match each other; The optical transmission system according to claim 5, further comprising:

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