CN1240944A - Method for optical fiber communication, and terminal device and system for use in carrying out the method - Google Patents

Abstract

The present invention relates to a method of optical fiber communication, and a terminal device and system used in the method. The purpose of the present invention is to compensate dispersion and nonlinearity. A device for inputting an optical signal with variable optical power to an optical fiber transmission line is provided. The optical signal transmitted by the transmission line is converted into an electrical signal by the optical receiver. The monitoring unit detects parameters related to the decay of the waveform of the electrical signal. The control unit controls the optical power of the optical signal output from the device so that the attenuation of the waveform is improved.

Description

Method of optical fiber communication and terminal device and system therefor

The present invention generally relates to compensation of dispersion and nonlinearity in optical fiber communication, and particularly relates to a method of optical fiber communication capable of compensating dispersion and nonlinearity in long-distance transmission, as well as a terminal device and system used in the method.

Due to the recent development of low-loss silica optical fibers, various optical fiber communication systems using such optical fibers as transmission lines have come into practical use. This fiber itself has a very wide frequency band. However, the transmission capacity of optical fiber is actually limited by the system structure. The most important limitation is the distortion of the waveform due to the dispersion present in the fiber. Furthermore, for example, the rate at which an optical signal attenuates in an optical fiber is about 0.2 dB/km. A typical example of optical signal loss due to attenuation is compensation using an optical amplifier such as an Erbium-Doped Fiber Amplifier (EDFA). The gain bandwidth of EDFA is within 1.55μm, and the loss of silica fiber in this bandwidth is the lowest.

Often referred to simply as chromatic dispersion, it is a phenomenon of group velocity dispersion of an optical signal in an optical fiber as a function of the wavelength (frequency) of the optical signal. For example, in a standard single-mode fiber, in the range of wavelengths less than 1.3 μm, longer-wavelength optical signals propagate faster than shorter-wavelength optical signals, and this resulting dispersion is often referred to as normal dispersion. In this case the dispersion (in ps/nm/km) is negative. On the contrary, in the range of wavelengths greater than 1.3 μm, optical signals with shorter wavelengths propagate faster than optical signals with longer wavelengths, and this resulting dispersion is usually called anomalous dispersion. In this case the dispersion (in ps/nm/km) is positive.

In recent years, due to the use of EDFAs, the nonlinearity of the received fiber combined with the increased power of the optical signal has attracted attention. The most important nonlinearity limiting the transmission capability is the optical Kerr effect present in optical fibers. This optical Kerr effect is a phenomenon in which the refractive index of an optical fiber changes with the power and density of an optical signal.

Changes in the refractive index modulate the phase of optical signal propagation in the fiber, resulting in frequency chirping that alters the signal spectrum. This phenomenon is called self-phase modulation (SPM). There is also the possibility that the waveform distortion caused by dispersion is further enlarged due to SMP changing the spectrum.

In this way, as the transmission distance increases, dispersion and optical Kerr effect impart waveform distortion to the optical signal. Therefore, long-distance transmission through optical fiber must control, compensate or eliminate dispersion and nonlinearity while ensuring transmission quality.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a method of optical fiber communication which compensates for dispersion and nonlinearity for long-distance transmission, and a terminal device and system for use in the method.

According to one aspect of the present invention, a method for optical fiber communication is provided. First, a device for outputting an optical signal with variable optical power to an optical fiber transmission line is provided. Second, the optical signal transmitted through the optical fiber transmission line is converted into an electrical signal. Thirdly, parameters of degradation of the electrical signal waveform (such as bit error rate or eye opening) are detected. Finally, the optical power of the optical signal output to the optical fiber transmission line is controlled according to the detected parameters so as to improve the waveform attenuation of the electrical signal.

In general, the nonlinearity or nonlinearity of an optical fiber present in an optical fiber used as an optical fiber transmission line is determined by the optical power of an optical signal output into the optical fiber. In the method of the present invention, the optical power of the optical signal is varied according to the state of the optical fiber transmission line, such as the kind of optical fiber used for the optical fiber transmission line, thereby controlling the nonlinearity of the optical fiber. Therefore, long-distance transmission is allowed while ensuring transmission quality by compensating for dispersion and nonlinearity.

According to another aspect of the present invention, there is provided a system comprising first and second terminal devices and an optical fiber transmission line connecting the first and second terminal devices. The first terminal device includes an optical transmitter that outputs an optical signal of variable optical power to the optical fiber transmission line. The second terminal device includes an optical receiver, which converts the optical signal transmitted through the optical fiber transmission line into an electrical signal, and also includes a monitoring unit for detecting the waveform distortion parameter of the electrical signal and a transmission monitoring unit for the detection parameter of the first terminal device information device. The first terminal device also includes a control unit for controlling the optical power according to the monitoring information, so as to improve the waveform attenuation of the electric signal.

According to still another aspect of the present invention, a terminal device is provided, the terminal device includes an optical transmitter that outputs an optical signal with variable optical power to an optical transmission line; A device for receiving monitoring information of waveform distortion parameters; and a device for controlling optical power according to the monitoring information, so as to improve the waveform distortion of the optical signal.

The above and other objects, features and advantages of the present invention and the means of realizing them will become more apparent from a study of the following specification and appended claims and with reference to the preferred embodiments of the invention shown in the accompanying drawings. this invention.

Fig. 1 represents the basic configuration block diagram of the system of the present invention;

Fig. 2 shows the graph of optical dispersion characteristic;

3A to 3C are line diagrams illustrating transmission distances in the case of using DSF (Dispersion-Shifted Fiber);

4A and 4B are block diagrams illustrating DSF transmission characteristics;

5A to 5C are block diagrams illustrating transmission distances in the case of using SMF (single-mode fiber);

Fig. 6 represents the block diagram of the preferred embodiment that is applicable to the optical fiber amplifier of the present invention;

Figure 7 represents a block diagram of a preferred embodiment of the monitoring unit;

Figure 8 represents a block diagram of a preferred embodiment of the O/E converter of the monitoring unit in Figure 7;

Figure 9 shows a block diagram of a preferred embodiment of the system of the present invention;

Fig. 10 represents the block diagram of another preferred embodiment of the system of the present invention;

Figure 11 represents a modified block diagram of sending specific monitoring information in the system of Figure 10; and

Fig. 12 shows a flow chart of the control flow in the system of Fig. 10 (Fig. 11).

Some preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings. In all the figures, the same reference symbols denote the same parts.

Fig. 1 shows a block diagram of the basic configuration of the system of the present invention. The system includes a first terminal device 2 , a second terminal device 4 and an optical fiber transmission line 6 connecting the first and second terminal devices 2 and 4 .

The first terminal device 2 includes an optical transmitter 8 that outputs an optical signal with variable optical power to the optical fiber transmission line 6 from its first end 6A, and is used to control the optical signal transmitted by the optical transmitter 8 according to the provided control signal CS. Optical signal power output.

The second terminal device 4 includes an optical receiver 12 for converting the optical signal transmitted through the optical fiber transmission line 6 into an electrical signal, and a monitoring unit 14 for detecting waveform distortion parameters of the electrical signal from the optical receiver 12 . The second terminal device 4 also includes a monitoring information transmission device 16 for transmitting detected parameters of the first terminal device 2 .

The first terminal device 2 also includes a receiving unit 18 for receiving monitoring information from the second terminal device 4 . The receiving unit 18 generates the control signal CS according to the monitoring information to provide to the control unit 10, so that the waveform distortion of the electrical signal output from the optical receiver 12 is improved. For example, the receiving unit 18 generates the control signal CS to reduce the bit error of the electrical signal output from the optical receiver 12 or to increase the signal aperture of the electrical signal output from the optical receiver 12 .

The transmission of monitoring information from the transmitting unit 16 to the receiving unit 18 is performed using the optical fiber transmission line 6 , another optical fiber transmission line (not shown in FIG. 1 ), or electrical circuits or radio.

FIG. 2 shows a dispersion characteristic diagram of an optical fiber used in the optical fiber transmission line 6. As shown in FIG. In FIG. 2 , the vertical axis represents dispersion (ps/nm/km), and the horizontal axis represents wavelength (μm).

Usually a single-mode fiber (SMF) is used as the optical fiber transmission line 6, and the zero dispersion wavelength of the SMF is about 1.3 μm. In such a case, since the wavelength of the optical signal is longer than the wavelength of zero dispersion and the dispersion is in an abnormal dispersion region, it is a positive value. On the contrary, since the wavelength of the optical signal is shorter than the zero dispersion wavelength, the dispersion is in the normal dispersion region, so it is a negative value. If SMF is used as the optical fiber transmission line 6, the wavelength of the optical signal is set at 1.55 μm (eg, 1.50˜1.60 μm) to obtain the lowest loss in the SMF. Therefore, the dispersion of the optical signal always falls in the abnormal dispersion region.

In the case of using a dispersion-shifted fiber (DSF) as the optical fiber transmission line 6, the zero dispersion wavelength of the DSF is about 1.55 μm. Also in this case, since the wavelength of the optical signal is longer than the zero-dispersion wavelength, the dispersion falls in an abnormal dispersion region, so it is a positive value. On the contrary, since the wavelength of the optical signal is shorter than the zero dispersion wavelength, the dispersion falls within the normal range, so it is a negative value. Because the loss of the DSF wavelength is the lowest at 1.55 μm, the wavelength of the optical signal is set at 1.55 μm. Therefore, whether the dispersion of the DSF falls in the abnormal dispersion region or the normal dispersion region depends on the comparison between the actual wavelength of the optical signal and the zero dispersion wavelength of the DSF.

The transmittable distance in the case of using DSF and SMF as the optical fiber transmission line 6 will be described below.

The referenced FIG. 3A shows that the system of FIG. 1 is using DSF as an important part of the optical fiber transmission line 6 . In this case, the optical transmitter 8 in the first terminal device 2 includes an E/O converter (electrical/optical converter) 20 for converting an input electrical signal into an optical signal, and an E/O converter for converting the E/O The optical signal output by the O-converter 20 is amplified with a variable gain 22. The gain of the optical amplifier 22 is adjusted by the control unit 10 (see FIG. 1 ), thereby changing the optical power of the optical signal output to the optical fiber transmission line 6 . In addition, an optical amplifier 24 is again provided as a preamplifier in the second terminal device 4 in order to increase the sensitivity of the receiver. The optical amplifier 24 is optically connected between the second end 6B of the optical fiber transmission line 6 and the optical receiver 12 (or O/E converter (optical/electrical converter).

The mentioned Figures 4A and 4B show the transmission characteristics of the DSF in the anomalous dispersion region and the normal dispersion region, respectively. The fact that should be noted here is: the optical signal in the abnormal dispersion region obtains a red-shifted chirp, the optical signal in the normal dispersion region obtains a blue-shifted chirp, and the optical signal with large optical power through the SPM The resulting chirp is always a blue-shifted chirp. Obtaining an optical signal of a blue-shifted chirp is easier than obtaining an optical signal of a red-shifted chirp. Therefore, it is assumed here that the optical signal output from the optical amplifier 8 is a red-shifted chirp.

In FIGS. 4A and 4B, the vertical axis represents the compensation signal waveform signal eye opening in the optical receiver 12, and the horizontal axis represents the transmission distance.

If the optical power of the optical signal output from the optical amplifier is relatively small, the nonlinearity of the optical fiber transmission line 6 can be ignored. Therefore, in the abnormal dispersion region, the red-shifted chirp obtained in the optical amplifier 8 and the red-shifted chirp obtained in the optical fiber transmission line 6 are combined, referring to the symbol (a ) It can be seen that the hole opening distance through the signal becomes shorter than the WDL limit of waveform decay. Conversely, in the normal dispersion region, the red-shifted chirp obtained in the optical amplifier 8 and the blue-shifted chirp obtained in the optical fiber transmission line 6 are reduced by reducing the influence of waveform compression or the pulse width of the optical signal. Canceling each other, referring to symbol (d) in Fig. 4B, it can be seen that the transmission distance is relatively long.

If the optical power of the optical signal output from the optical amplifier 8 is relatively large, the nonlinearity in the optical fiber transmission line 6 cannot be ignored. Considering the loss of the optical fiber transmission line 6, the blue-shifted chirp of the SPM is evident near the first end 6A of the optical fiber transmission line 6 (eg a 10 km long section). More particularly, if the optical power of the optical signal output from the optical amplifier 8 is relatively large, the red-shifted chirp obtained in the optical amplifier 8 and the optical fiber transmission line 6 and the blue-shifted chirp due to the SPM are in the The anomalous dispersion regions cancel each other, referring to symbol (b) in Fig. 4A, it can be seen that the transmission distance is relatively long. On the contrary, in the normal dispersion region, the blue-shifted chirp due to dispersion and the blue-shifted chirp due to SPM are combined with each other, and the transmission distance can be seen with reference to symbol (c) of FIG. 4B is relatively short.

3B and 3C show the relationship between optical power and distance converted from FIGS. 4A and 4B, respectively. The SN area of the received signal must be considered as a limiting factor for transmission. In FIGS. 3B and 3C, the SN area is indicated by the symbol SNL. Also, symbol WDL' in Figs. 3B and 3C corresponds to the waveform decay region WDL in Figs. 4A and 4B.

SNL and WDL' in the left regions of Figures 3B and 3C, respectively, allow the transmission of fixed transmission capabilities. For example, if the variable range of the optical power of the optical signal output from the optical amplifier 8 is set to ΔP, the transmission distance L1 is set in the anomalous dispersion region by WDL' in FIG. 3B , and by WDL' in FIG. 3C and SNL are set to the transmission distance L2 in the normal dispersion region. That is to say, in the system of FIG. 3A, the gain of the optical amplifier 22 is adjusted by the optical power of the optical signal output from the optical transmitter 8, thus obtaining the best state of long-distance transmission dispersion and nonlinear compensation.

In the prior art, the range of the optical output power output from the optical transmitter is mostly fixed for the designed system, which takes into account the variation of the optical power within this range. Therefore, the transmission distance L3 is determined (or limited) by the worst condition. On the contrary, by controlling or adjusting the optical power of the optical signal output from the optical transmitter 8 by the method of the present invention, good transmission quality can always be obtained according to the state of the optical fiber transmission line 6, thus achieving long-distance transmission.

Different from the system shown in FIG. 3A , the system shown in FIG. 5A is characterized in that the SMF uses an optical fiber transmission line 6 . When SMF is used as the optical fiber transmission line 6, the zero dispersion wavelength is about 1.3 μm, and the optical power of the optical signal output from the optical transmitter 8 mentioned above has a wavelength of 1.55 μm, so what is obtained is only an abnormal dispersion region. Since the SMF dispersion is relatively large in the 1.55 μm band, dispersion compensating fibers (DCF) 26 and 28 are used in this system to compensate the dispersion of the optical fiber transmission line 6 . DCF 26 optically connects E/O converter 20 and optical amplifier 22 , while DCF 28 optically connects optical amplifier 24 and O/E converter 12 . Dispersion compensation of the optical fiber transmission line 6 can be performed by either of the DCFs 26 and 28 .

The absolute value of the dispersion of the fiber is much larger than that of the SMF used in DCFs 26 and 28 in order to eliminate losses. The dispersion of DCF is in the normal dispersion area, so dispersion compensation is performed.

Figures 5B and 5C correspond to Figures 4A and 3B, respectively. If DCFs26 and 28 are used, the dispersion caused by these fibers is constant. Therefore, in each case, whether the optical power of the optical signal output from the optical amplifier 8 is large or small, there is an optimum value for the maximum value of the obtained signal eye opening as shown in FIG. 5B. Therefore, if the optical power is small, the transmission distance is limited to the range between the lower distance (e) at which the signal eye opening is lower than WDL and the higher distance (f) at which the signal eye opening is lower than WDL . If the optical power is large, the transmission distance is limited to the lower distance (g) (e<g) of the signal eye opening lower than WDL and the higher distance (h) (f < h) within the range.

Therefore, the state of fixed transmission performance obtained in the system shown in FIG. 5A is determined by the WDL lines WDL(#1) and WDL(#2) on the left side with respect to the SNL shown in FIG. 5C. For example, if the variable range of the optical power of the optical signal output from the optical amplifier 8 is set to ΔP, the transmission distance with reference to symbol L4 of FIG. 5C is within a relatively wide range.

In the prior art, the optical output power of an optical transmitter is mostly fixed for varying optical powers within this range. Therefore, the transmission distance range L5 is determined (or limited) by the worst condition. On the contrary, by controlling or adjusting the optical power of the optical signal output from the optical amplifier 8 by the method of the present invention, good transmission quality is always obtained according to the state of the optical fiber transmission line 6, thus long-distance transmission or wide transmission distance range is obtained.

Figure 6 shows a block diagram of a preferred embodiment of an optical amplifier suitable for the present invention. Such an optical amplifier may be used as optical amplifier 22 (post amplifier), optical amplifier 24 (pre amplifier), or an optical amplifier including an optical repeater described below. The optical amplifier shown in Figure 6 comprises an input port 30, which provides an amplified optical signal, an output port 32 which outputs an amplified optical signal, and an amplifying unit 34 and an optical amplifying unit 34 provided along the main optical path between the input port 30 and the output port 32. Coupler 36.

The amplifying unit 34 includes an optical amplifying medium for supplying the optical signal from the input port 30, and an excitation unit for actuating the optical amplifying medium so that the optical amplifying medium provides a gain to the optical signal. If a semiconductor chip obtained by reducing the reflectivity of the opposite end faces of the laser diode is used as the optical amplification medium, the excitation unit supplies the injection current to the semiconductor chip through the current source. In this case, a gain determined according to the injection current is provided to the optical signal.

In this preferred embodiment, erbium-doped fiber (EDF) 38 is used as the optical amplification medium, suitable for optical signals having a wavelength of 1.55 μm. EDF 38 has a first end optically connected to input port 30 and a second end optically connected to optical coupler 36 . The excitation unit supplies excitation light of a preset wavelength to the EDF 38 through the laser diode 40 as an excitation source. For example, the wavelength of the excitation light is set at 0.98 μm or 1.48 μm. With the laser diode 40 optically connected to the first end of the EDF 38 through a WDM coupler (not shown), the optical signal and the excitation light are propagated in the same direction in the EDF 38, thereby performing forward excitation. If the laser diode 40 is optically connected to the second end of the EDF 38 through a WDM coupler (not shown), the optical signal and excitation light propagate in the opposite direction of the EDF 38, thereby performing backward excitation. A first WDM coupler is optically connected to a first end of the EDF 38 for bidirectional excitation by a first excitation source, and a second WDM coupler is optically connected to a second end of the EDF 38 for a second excitation flow.

The laser diode 40 is supplied with a driving current (DC bias current) from the driving circuit 42 so that a gain determined according to the driving current is generated in the EDF 38 . Most optical signals are amplified in the amplifying unit 34 through the optical coupler 36 and output from the output port 32 according to a given gain. The remainder of the amplified optical signal is split through an optical coupler 36 as monitor light, which is provided to a photodetector (PD) 44 such as a photodiode. The photodetector 44 outputs a signal of a voltage level which is compared with the power of the received monitor light. In most cases, the ratio split by the optical coupler 36 is not determined by the intensity of the provided optical signal, so the optical power of the optical signal output from the output port 32 is the voltage of the output signal through the optical detector 44 level to map. The output signal output from the photodetector 44 is supplied to the comparator 46 . The comparator 46 feeds back to control the driving current to be supplied to the laser diode 40 from the driving circuit 42 so that the difference between the voltage level of the output signal of the photodetector 44 and the reference voltage Vref becomes zero or a constant value.

By employing such a feedback loop, the optical power of the optical signal output from the output port 32 can be maintained at a constant value determined by the reference voltage Vref (automatic level control: ALC). Particularly in the preferred embodiment, the control unit 10 (see FIG. 1 ) is supplied by a circuit 48 for reference voltage generation. The circuit 48 generates a reference voltage Vref according to the provided control signal CS. Therefore, the target value of the ALC of the optical amplifier can be predetermined according to the control signal CS.

FIG. 7 shows a block diagram of a preferred embodiment of the monitoring unit 14 in FIG. 1 . The optical signal transmitted through the optical fiber transmission line 6 is amplified by the optical amplifier 24 as an amplifier, and the amplified optical signal is divided into a first signal wave and a second signal wave by the optical coupler 50 . The first signal wave is supplied to the first O/E converter (optical receiver) 12. The O/E converter 12 regenerates a main signal according to the received signal wave. The second signal wave is supplied to the second sensor included in the monitoring unit 14. O/E converter 52.The output signal of O/E converter 52 supplies error detection loop 54, has obtained the error information on the main signal bit error rate like this.O/E converter 52 provides different discrimination levels, signal The eye opening calculation circuit 56 can calculate the signal eye opening based on the discrimination level and error information obtained by the error detection circuit 54. The signal eye opening thus obtained is provided as signal eye opening information.

FIG. 8 referred to shows a specific structure of the O/E converter 52 in the monitoring unit 14 of FIG. 7 . The second signal wave from the optical coupler 50 (see FIG. 7 ) is supplied to the reverse biased photodiode 57 . The potential of the anode of the photodiode 57 changes with the change of the optical density or intensity modulated by the signal wave supplied to the photodiode 57, so the change of the anode potential is the output electrical signal of the photodiode. The output electrical signal is equalized-amplified by the equalizing amplifier 58 , and the equalized-amplified signal is provided to the discriminator 62 . The timing regenerator 60 regenerates a clock from the signal of the equalizing amplifier 58 . The discriminator 62 discriminates between the equalized-amplified high level and low level so that the timing from the timing regenerator 60 coincides with the timing of the discriminative level obtained.

The O/E converter 12 for the main signal can be obtained by modifying the configuration of the O/E converter 52 in FIG. 8, in which case the discrimination level supplied to the discriminator 62 becomes a constant value.

According to the preferred embodiment of FIG. 7 , parameters related to waveform distortion (bit error rate or signal eye opening) can be detected by monitoring unit 14 if the obtained main signal is from an O/E converter. Therefore, controlling the optical power to an optimal value after setting the initial value can be continuously formed under the use state of the system.

When the first and second signal waves are used in the preferred embodiment shown in FIG. 7, the optical coupler 50 and the O/E converter 52, which is included in the O/E converter 12 and output from the equalizing amplifier, can be omitted. The output signal can be split into a first and a second signal, the first signal being used for demodulation of the main signal and the second signal being used for error detection and calculation of signal eye opening. In this way, the intensity received by the O/E converter 12 can be increased while the number of optical components can be reduced.

Figure 9 shows a block diagram of a preferred embodiment of the system of the present invention. In this embodiment, an optical fiber transmission line 64 different from the optical fiber transmission line 6 is used for transmission of monitoring information from the second terminal device 4 to the first terminal device 2 . That is to say, the optical fiber transmission line 6 is used as a backward control flow directed from the first terminal device 2 to the second terminal device 4, and the optical fiber transmission line 64 is used as a forward control flow directed from the second terminal device 4 to the first terminal device 2 .

The optical signal containing the monitoring information output by the transmission unit 16 in the second terminal device 4 is amplified by the optical amplifier 66 as a post-amplifier, and the amplified optical signal output from the optical amplifier 66 is provided to the optical fiber transmission line by its first end 64A 64. The optical signal output from the second end of the optical fiber transmission line 64 is amplified by the optical amplifier 68 as a preamplifier, and the amplified optical signal output from the optical amplifier 68 is supplied to the receiving unit 18 of the first terminal device 2 .

The transmission unit 16 includes a monitoring information insertion circuit 70, the insertion circuit 70 inserts the monitoring information involved in the parameters detected by the monitoring unit 14 into the main signal transmitted by the forward control flow line, and the E/O converter 72 is used for The output signal from the circuit 70 is converted into an optical signal.

The receiving unit 18 including the O/E converter 74 is used to convert the optical signal amplified by the optical amplifier 68 into an electrical signal, and the monitoring information extraction circuit 76 is used to extract monitoring information from the signal output from the O/E converter 74 . The circuit 76 generates a control signal CS according to the extracted monitoring information.

According to the system of FIG. 9 , the optical power of the optical signal output from the optical transmitter 8 in the first terminal device 2 can be controlled to an optimal value through the optical fiber transmission line 64 according to monitoring information. This control will be described more explicitly below.

At the beginning of the system, in order to obtain the optical signal conditions allowed to pass through the individual optical fiber transmission lines 6 and 64 to a certain range, the incident intensity of the optical signal is set according to the type and transmission distance of the optical fiber transmission lines 6 and 64 (SMF/DSF). In the case where a combination of SMF and DCF is used as shown in FIG. 5A, the dispersion of DCF is also set.

In the next step, the optical power of the optical signal output from the optical transmitter 8 in the first terminal device 2 is changed due to the detection of a parameter involving waveform attenuation, which is monitored by the monitoring unit 14 of the second terminal device 4 detection. In this way, monitoring information is obtained from the error information, so that the obtained monitoring information is transmitted from the transmission unit 16 to the receiving unit 18 .

In the first terminal device 2, the optimum value of the optical power of the optical signal output from the optical transmitter 8 can be obtained from the correspondence between the variation of the optical power and the error information. Therefore, the control signal CS is generated due to obtaining the optimum optical power. In this way, the optical power of the optical signal output from the optical transmitter 8 to the optical fiber transmission line 6 is always kept at an optimum value, so that dispersion and nonlinearity are compensated.

In this preferred embodiment, when the optical power of the optical signal output by the optical fiber transmission line 6 is optimized, the optical power of the optical signal output by the optical fiber transmission line 64 can also be optimized by changing the functions of the terminal devices 2 and 4 . This change is easy to do in this technical field, and its dispersion can be ignored here.

For example, the optimization of such fibers must be carried out at the initial stage, when the device is introduced into a line or commissioned. However, once the system is in operation, the transmission conditions become quite fixed, so the capability for large changes in the optimum value of optical power is low. Therefore, the method of the present invention can be automatically adjusted by manually adjusting the optical power of the optical signal output from the optical transmitter 8 without using the control unit 10 of the system of FIG. 9 .

Fig. 10 shows a block diagram of another preferred embodiment of the system of the present invention. Compared with the system shown in FIG. 1 or FIG. 9, the system shown in FIG. 10 is characterized in that many optical repeaters 78 (#1) to 78 (#N) are installed along the optical fiber transmission line 6 (N is greater than 1 integer). Optical repeaters 78 (#1) to 78 (#N) are provided as linear repeaters. The linear repeater is used as a repeater for amplifying a received optical signal in the same case, and it is distinguished from a regenerative repeater for waveform shaping and the like. Each of the optical repeaters 78 (#1) to 78 (#N) has an optical amplifier 80 for amplifying a received optical signal. Particularly in this preferred embodiment, a plurality of optical repeaters 82 (#1) to 82 (#N) are provided along the optical fiber transmission line 64 . Each of the optical repeaters 82 (#1) to 82 (#N) has an optical amplifier 84 .

Some specific application types of the optical fiber transmission line 6 of the present invention in the system of FIG. 10 are described below.

In the first application type, the output level (optical power of the output optical signal) of each amplifier 80 is set (fixed) to a constant value. In this case, the effect of the non-linearity due to the high output level of each amplifier 80 is substantially constant. Therefore, the optical power of the optical signal output from the optical transmitter 8 in the first terminal device 2 according to the present invention is easily set at an optimum value. The reference level Vref is set at a constant value. Each optical amplifier 80 can adopt the configuration of the optical amplifier shown in FIG. 6 .

In the second application type, the output level of the optical transmitter 8 and the output level of each optical amplifier 80 are substantially equal to each other. That is, the output levels of the optical transmitter 8 and each optical amplifier 80 are adjusted according to the control signal CS (see FIG. 9).

In the third application type, the output levels of the optical transmitter 8 and the optical amplifier 80 are sequentially adjusted from the first terminal device 2 to the second terminal device 4 or from the second terminal device 4 to the first terminal device 2 .

In the second and third application types, the output level (or gain) of each optical amplifier 80 is changed according to the control signal CS. Therefore, if the optical amplifier in FIG. 6 is used as each optical amplifier 80, the reference level Vref is adjusted according to the control signal CS.

In the second and third application types, the control signal CS is required for each of the optical repeaters 78(#1) to 78(#N). Therefore, it is necessary to transmit special monitoring information including the control signal CS from the first terminal device 2 to each of the optical repeaters 78(#1) to 78(#N).

FIG. 11 shows a block diagram of a modification for sending special monitoring information in the system of FIG. 10. FIG. In this modification, the first terminal device 2 having the E/O converter 86 for converting specific supervisory information (monitoring signal) generates a supervisory light signal based on the control signal CS. The monitor optical signal output from the E/O converter 86 is combined with the main signal related to output from the optical transmitter 8 through a WDM (Wavelength Division Multiplexing) coupler 88 . Therefore, the wavelength of the monitor optical signal is different from the wavelength of the optical signal output from the optical transmitter 8 .

Numeral 78 represents each of optical repeaters 78 (#1) to 78 (#N) in FIG. 10 . In each optical repeater 78(#1) to 78(#N), the supervisory optical signal is separated by the WDM coupler 90, and the separated supervisory signal is converted into a supervisory signal by the O/E converter 92. The monitor signal output from the O/E converter 92 is supplied to a monitor circuit (SV) 94 . The monitor circuit 94 adjusts the output level (or gain) of the optical amplifier 80 according to the monitor signal. For example, if the optical amplifier in FIG. 6 is used as the optical amplifier 80, the reference level Vref is set based on the monitor signal. The monitoring signal is provided to the monitoring circuit 94 or the updated monitoring signal is converted into a monitoring optical signal through the E/O converter 96 in the monitoring circuit. The monitoring optical signal output from the E/O converter 96 is provided to the WDM coupler 98, and the optical amplifier 80 combines the monitoring optical signal and the optical signal for amplification.

In the second terminal device 4 , the monitoring optical signal is extracted through the WDM coupler 100 , and the extracted monitoring optical signal is converted into a monitoring signal through the O/E converter 102 . The monitor signal output from the O/E converter 102 is supplied to the monitor circuit 104 , and the output signal of the monitor circuit 104 is input into the forward control flow together with the output signal of the monitor unit 14 .

In the case of performing the third application type, the control flow of the system shown in FIG. 10 ( FIG. 11 ) will now be described with reference to FIG. 12 .

In step 112, initial information is entered on the transmission line. For example, the initial information includes various types of transmission lines (DSF/SMF/others), the distance between each repeater, the presence/absence of a dispersion compensator, and the existence of dispersion if there is a dispersion compensator. Transmission line parameters (loss coefficient, dispersion coefficient, and nonlinearity coefficient).

In step 114, the output level of the optical transmitter 8 and each optical amplifier 80 is set according to the input initial information, so that the optical signal of the first terminal device 2 is transmitted to the second terminal device at a level determined by the transmission quality 4.

In step 116, optimal detection and start-up setup begins. For example, step 116 is performed through a backward control flow (optical transmission line 6 ) and a forward control flow (optical transmission line 64 ).

In step 118, optimal detection and setting is performed for each backward control flow section, in other words, the optical transmitter 8 and the optical repeaters 78 (#1) to 78 (#N) are set at the same value.

In step 120, it is determined whether setup has been completed for all lines. If it is confirmed that all lines have not been completed, return to step 116 to start optimal detection and setting of forward control flow.

For example, when all line settings are completed, the program proceeds to step 122, and the setting information is recorded in the memory device connected to the CPU provided in the first terminal device 2 .

The setting operation from the upstream line of the first terminal device 2 to the second terminal device 4 is performed sequentially by the control flow shown in FIG. 12 , and the setting operation may also be performed in reverse order. The setting operation of the upstream line can also be performed similarly.

According to the above description of the present invention, a fiber optic communication method capable of compensating dispersion and nonlinearity for long-distance transmission can be provided, as well as a terminal device and a system used in the method. This effect is achieved by the particularly preferred embodiments of the present invention described above, and other descriptions are omitted here.

The invention is not limited to the preferred embodiments described in detail above. The scope of the present invention is defined by the appended claims, and various changes and changes made by the present invention are included in the scope of the claims.

Claims (17)

1. A method comprising the steps of: (a) providing means for outputting an optical signal having variable optical power to an optical fiber transmission line; (b) converting said optical signal transmitted through said optical fiber transmission line into an electrical signal; (c) detecting the parameters involved in the attenuation of the electrical signal waveform; and (d) controlling the optical power according to the detected parameters so that the waveform attenuation of the electrical signal is improved. 2. The method according to claim 1, wherein: said means is provided by an optical amplifier contained in an optical repeater disposed along said optical fiber transmission line; Said step (d) includes the step of adjusting the gain of said optical amplifier. 3. A method according to claim 1, wherein said means is provided by an optical transmitter connected to one end of said optical fiber transmission line. 4. The method according to claim 3, wherein: The optical transmitter includes an electrical/optical converter for converting an input electrical signal into an optical signal and an optical amplifier for amplifying the optical signal; Said step (d) includes the step of adjusting the gain of the optical amplifier. 5. The method according to claim 1, wherein said parameter is a bit error rate of the electrical signal. 6. The method according to claim 1, wherein the parameter is eye opening of said electrical signal. 7. The method according to claim 1, wherein said device is provided by an optical transmitter connected to one end of the optical fiber transmission line, and provided by an optical amplifier included in each optical repeater arranged along said optical fiber transmission line . 8. The method according to claim 7, wherein said step (d) includes the step of satisfying a condition that the output power of the optical transmitter is substantially equal to the output power of said optical amplifier. 9. The method according to claim 7, wherein said step (d) includes the step of sequentially adjusting the output power of the optical transmitter and the optical amplifier. 10. A system comprising: first and second terminal devices; and an optical fiber transmission line connected to said first and second terminal devices; The first terminal device includes an optical transmitter for outputting an optical signal with variable optical power to the optical fiber transmission line; The second terminal device includes an optical receiver that converts the optical signal transmitted by the optical fiber transmission line into an electrical signal, a monitoring unit that detects parameters related to the attenuation of the electrical signal waveform, and transmits monitoring information of the detected parameters to the a device of the first terminal device; The first terminal device further includes a control unit for controlling the optical power according to the control monitoring information so as to improve the waveform attenuation of the electrical signal. 11. The system of claim 10, wherein: The optical transmitter includes an electrical/optical converter for converting an input electrical signal into an optical signal, and an optical amplifier for amplifying the optical signal; and The control unit adjusts the gain of the optical amplifier. 12. The system according to claim 10, wherein said second terminal device further comprises an optical amplifier for amplifying the optical signal to be received by the optical receiver. 13. The system of claim 10, wherein said fiber optic transmission line comprises a dispersion-shifted fiber having a dispersion-shifted wavelength near 1.55 [mu]m zero dispersion wavelength. 14. The system of claim 10, wherein said fiber optic transmission line comprises a single mode fiber having a zero dispersion wavelength near 1.3 [mu]m. 15. The system of claim 14, further comprising a dispersion compensating fiber compensating for dispersion existing in the fiber transmission line. 16. The system of claim 10, further comprising a second optical fiber transmission line connected to the first and second terminal devices; The monitoring information is sent from the second terminal device to the first terminal device through the second optical fiber transmission line. 17. A terminal device comprising: An optical transmitter that outputs an optical signal with variable optical power to an optical fiber transmission line; means for receiving monitoring information of detected parameters related to the attenuation of the waveform of the optical signal transmitted by the optical fiber transmission line; and The device for controlling the optical power according to the monitoring information, so that the waveform attenuation of the optical signal is improved.

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