KR100376597B1 - An optical channel monitoring system using time-division multiplexed tone-frequency modulation for optical wavelength division multiplexing systems - Google Patents

Abstract

The present invention discloses an optical channel monitor system using a time division tone frequency modulation scheme in an optical wavelength division system. The system includes a plurality of blocks (3.1) for inputting a data signal of a predetermined transmission rate and a tone signal of a predetermined frequency and outputting a tone frequency modulated channel for a predetermined time, and an optical for multiplexing the sum of the output channels of the plurality of blocks. A multiplexer 3.7, an optical amplifier 3.8 for amplifying channels multiplexed by the optical multiplexer, an optical detector for detecting light by extracting only a portion of the light amplified by the optical amplifier using a tap coupler (3.10) And a monitor circuit (3.11) for processing the optical signal extracted by the optical detector to obtain a signal for the channel. Unlike the conventional method, by assigning only one tone frequency to one node, the present invention can monitor a channel without interference or distortion between tone frequencies, no matter how many channels are increased, and different tone frequencies between nodes. You can also use to find out where the channel is coming from. The present invention is easier and more economical to implement electronic circuits than conventional methods.

Description

An optical channel monitoring system using time-division multiplexed tone-frequency modulation for optical wavelength division multiplexing systems}

The present invention relates to an optical channel monitor system using time division tone frequency modulation in an optical wavelength division system. In particular, the present invention relates to a system for monitoring the presence and size of an optical channel using a time division tone frequency modulation scheme in a wavelength division multiplexing system (WDM).

Recently, due to the explosion of Internet users, the problem of increasing the capacity of the existing optical communication system is emerging. The method of increasing the capacity of an optical network is to increase the laying of a conventional time-division multiplexing system (TDM) and optical fiber. However, this method is disadvantageous and time-consuming.

In order to overcome this drawback, a system recently emerged is a wavelength-division multiplexing system (hereinafter, WDM system). This system is used in conjunction with several existing TDM systems. The outputs of the TDM systems are converted to optical wavelengths conforming to the WDM device specifications, respectively, and multiplexed, and then sent to a destination using a single optical path. At the destination, each channel is identified using an optical filter and connected to the inputs of the corresponding TDM systems. Using a WDM system, a large amount of information can be sent using existing TDM devices and installed optical paths.

However, in order to transmit a large amount of information using a WDM system, the number of channels must be large, and each optical channel needs to be monitored in an appropriate manner in order to manage these many channels. This method is divided into two types depending on whether the signal processing method of analyzing the channel is performed in the electrical domain or the optical domain.

In the electrical domain, the optical luminance of each channel is modulated to carry information on the channel. This method is already disclosed by the following documents:

US Patent No. 5,654,816, "Performance monitoring and fault location in optical transmission," by D.A. Fishman,

Y. Hamazumi and M. Koga, J. Ligthwave Technol , JLT-15, no. 2, p. 2197, 1997, and

The paper "Signal tracking and performance monitoring in multiwavelength optical networks" by F. Heismann, M. T. Fatehi, S. K. Korotky, and J. J. Veselka is ECOC'96, WeB.2.2, 1996.

In this technique, each optical channel is modulated at a different frequency representing the channel.

On the other hand, the method in the optical domain is a method of directly monitoring the optical channel using an optical filter in terms of monitoring. This method is already disclosed by the following literature:

K. Otuka et al. In an essay entitled “A high-performance optical spectrum monitor with high speed measuring time for WDM optical networks” and ECOC'97, No. 448, 1997,

As published by Hiro Suzuki et al . In Electron. Lett. Vol. 35, p. 836, 1999.

Since the present invention belongs to the electrical domain in nature, the description of the prior art is also limited to the electrical domain.

1) Configuration of the prior art

1 illustrates one example of the prior art. As can be seen in Fig. 1, the N laser diodes 1.2 of different light wavelengths are modulated with a composite signal of a 2.5 Gb / s data signal source and a tone signal source 1.3 of several tens of kHz. When there are N such building blocks 1.1, it is said that there are N channels, and the light of each channel is multiplexed to the optical multiplexer 1.4 and transmitted to the destination. That is, N channels are transmitted in one optical path. In order to know the information on the channel in the optical path anywhere in the WDM network, as shown in FIG. 1, a tap coupler 1.6, a photodetector 1.7, and a monitor circuit 1.8 are required. In the figure, a monitor is attempted at the output stage of the optical amplifier 1.5.

2) Description of the prior art operation

Conventionally, each laser diode (1.2) is amplitude modulated to a tone (1.3) of several tens of kHz to distinguish each channel. The total light intensity of laser diode 1 (1.2) amplitude modulated with a 2.5 Gb / s signal and a tone of frequency f1 is shown in 1.10. In (1.10) we can see that the light intensity is weakly changing to a sinusoidal shape. In block 2, the laser diode is amplitude-modulated at the frequency f2 instead of f1. The change in light intensity would then be like (1.11). The optical signals (channels) amplitude-modulated with the tones of different frequencies are combined by the optical multiplexer 1.4. The gathered signal is amplified by the optical amplifier 1.5 to the destination. If you want to get information about the performance of each channel (for example, the strength of each channel) after passing through the optical amplifier, use a tap coupler (1.6) to extract some of the total light. The rest of the light passes through the light path (1.9). Some light branched toward (1.12) is converted into an electrical signal by the photodetector (1.7). This electrical signal has a signal component of f1, f2, ..., fN, and may have a high frequency 2.5Gb / s signal component. However, the filter removes the 2.5Gb / s signal. When the electric signal thus processed is signal processed using the monitor circuit 1.8, information about each channel is obtained. A more detailed view of the monitor circuit 1.8 is shown in FIG. 2. In FIG. 2, the electric signal changed by the photodetector 2.1 detects each tone frequency component through a band filter 2.2 connected in parallel. For example, the band filter of (2.2) passes only the electric signal whose tone frequency is f1. Each tone frequency component selected in this way is obtained by using the envelope detector of (2.2) to find the magnitude or perform other signal processing. Through this process, performance information such as strength of each channel can be found. In the monitor circuit, each band filter corresponds to each channel. That is, if the number of channels on the transmitting side is N, the number of band filters on the monitoring side should be N. This monitor circuit configuration is an example of the prior art.

3) Problems of the prior art

The prior art can be viewed as a tone frequency division multiplexing method in principle. For this to work successfully, there should be no interference between tone frequencies or distortion in the tone frequency itself. However, optical amplifiers, an essential functional block in WDM networks, exhibit some nonlinearity over the tone frequency. Thus, distortion occurs in the tone frequency as it passes through the optical amplifier. For example, if the tone frequency f1 passes through an optical amplifier, harmonics of f1 will result. These harmonics interfere with the tone frequencies on other channels, reducing the monitor's accuracy. To prevent this, the tone frequency is often raised to an odd number. For example, 7 kHz, 9 kHz, 11 kHz, 13 kHz, .... However, this method has the disadvantage that the tone frequency increases as the number of channels increases. Increasing the tone frequency as the number of channels increases can limit the performance of electronic components used in monitor circuits. To prevent this, when the tone frequency interval is narrowed and allocated, the performance of the band pass filter for filtering each tone frequency should also be excellent. In addition, the conventional method can determine the existence of a particular channel but cannot determine the source of the channel. In addition, if the number of channels increases by N times, the amount of circuit to be monitored increases by N times.

SUMMARY OF THE INVENTION The present invention has been made to solve the problems of the prior arts, and an object thereof is to provide an optical channel monitor system using a time division tone frequency modulation method in an optical wavelength division system.

1 is a block diagram of a fiber channel monitor system according to the prior art.

FIG. 2 is a detailed view of the monitor circuit of FIGS. 1 and 3.

3 is a block diagram of an optical channel monitor system according to the present invention;

4 is an output waveform diagram of an envelope detector at a monitor point and the light output shape of each channel when the tone frequency modulation time of each channel is equalized in the optical channel monitor system according to the present invention.

FIG. 5 is an output waveform diagram of an envelope detector at an optical output shape and a monitor point of each channel when the tone frequency modulation time of each channel is changed when the number of channels is 3 in the optical channel monitor system according to the present invention. FIG.

<Brief description of the main parts of the drawings>

1.1, 3.1: blocks 1.2, 3.2: laser diodes

1.3, 3.4: Nodes 1.4, 3.7: Optical Multiplexers

1.5, 3.8: optical amplifier 1.6, 3.9: tap coupler

1.7, 2.1: photodetector 1.8, 3.11: monitor circuit

2.2: Bandpass Filter 3.5: Gate

3.10: photodetector

<Configuration of the Invention>

The technical configuration of the present invention is shown in FIG. In FIG. 3, a solid line means an optical path and a dotted line means an electric signal. The laser diode 3.2 of each block 3.1 is modulated with the signal sum of the 2.5 Gbps data 3.3 and the tone signal 3.4 of frequency f1. The tone signal is applied to the laser diode 3.2 by the gate 3.5 only for a certain time 3.6. The gate control signal 3.6 applied to Block-2 is different from that of Block-1. Each channel modulated with the tone frequency only for a predetermined time is multiplexed through the optical multiplexer 3.7, amplified using the optical amplifier 3.8, and then sent to the destination.

Where the channel is desired to be monitored, only a portion of the multiplexed light is extracted using the tap coupler (3.9) and sent to the optical detector (3.10). The output of the photodetector 3.10 is signaled by the monitor circuit 3.11 to obtain information about the channel. The configuration of the monitor circuit 3.11 is as shown in FIG.

Detailed Description of the Invention

Comparing the present invention with the prior art of Fig. 1, the present invention differs from Fig. 1 in that the tone frequency used in each channel is constant at f1. In addition, unlike the prior art, the tone frequency modulation is performed only for a certain time. The laser diode 3.2 of each block 3.1 is modulated with the signal sum of 2.5 Gbps 3.3 and the tone signal 3.4 of frequency f1. The tone signal is applied by the gate 3.5 to the laser diode 3.2 only for a certain time 3.6. The gate control signal 3.6 applied to Block-2 also differs from the prior art.

In the prior art, channels are distinguished by assigning different tone frequencies to each channel. However, in the present invention, the tone frequencies are the same, and the channels are distinguished by different time lengths for modulating the tone frequencies in each channel. For example, channel 1 only modulates the tone frequency f1 during T1 (3.12). Then, after a predetermined time T_blank (3.13), channel 2 is tone modulated for T2 (3.14) time. In this way, the optical signals for channels 1 and 2 become (3.15) and (3.16). Where the channel is monitored, the circuit as shown in Fig. 2 is constructed and the f1 signal component is extracted using the f1 band filter. The extracted signal is input to the envelope detector, and the output is in the form of a square wave. Each square wave represents its own channel. By measuring the length of the square wave, the presence or absence of each channel can be known, and by measuring the size of the square wave, the size of each channel is known. Or simply count the number of square waves coming in over a period of time to determine the number of channels present. Or, specify the length to be modulated in the lowest numbered channel, and make the other channels equal. The monitoring side detects the channel with the lowest number and measures the square wave coming in at regular time intervals based on this to find out the channel number and strength at the same time.

FIG. 3 is a schematic diagram of the present invention, in which each channel is tone-modulated by time-division multiplexing, but also by pulse width modulation (PWM). Channels from other nodes are different from f1. In this case, each band filter examines each node in Fig. 2. In the conventional method, each band filter is compared to that corresponding to each channel. Monitoring also makes it easy to see where each channel is coming from and how many channels it is.

<Example 1>

FIG. 4 applies the present invention to a system with three channels, in which the tone frequency modulation times of each channel are equalized. At this time, the shape of the light output of each channel and the output of the envelope detector at the monitor point are shown. By monitoring the output of the envelope detector 4.4 for a period of time, information on the number of channels can be obtained.

<Example 2>

FIG. 5 shows the present invention applied to a system having three channels, in which the tone frequency modulation time of each channel is different, that is, pulse width modulation. At this time, the shape of the light output of each channel and the output of the envelope detector at the monitor point are shown. The difference from <Example 1> is that not only the number of channels but also the channel number can be known.

Unlike the conventional method, by assigning only one tone frequency to one node, the channel can be monitored without any interference or distortion between the tone frequencies no matter how many channels are increased.

It is also possible to find out where the channel is coming from by using different tone frequencies between each node.

The present invention is easier and economical to implement an electronic circuit than the conventional method. It is also very economical compared to the channel monitor method using the optical filter. This is because the method using the optical filter does not know the source of the channel.

Claims (5)

A plurality of blocks for inputting a data signal of a predetermined transmission rate and a tone signal of a predetermined frequency to output a tone frequency modulated channel for a predetermined time; An optical multiplexer for summing and multiplexing output channels of the plurality of blocks; An optical amplifier for amplifying a channel multiplexed by the optical multiplexer; An optical detector for extracting only a part of the light amplified by the optical amplifier using a tap coupler to detect light; And And a monitor circuit for processing the optical signal extracted by the optical detector to obtain a signal for a channel. The method of claim 1, wherein the monitor circuit A photodetector for detecting light input by the photodetector; A plurality of band pass filters for detecting the tone frequency of each block from the light detected by the photo detector; A plurality of envelope detectors for outputting square wave channels by inputting tone frequencies detected by the band filters; And And a signal processor for inputting the output signals of the plurality of envelope detectors to perform signal processing. 3. The optical channel monitor system according to claim 2, wherein a tone frequency different from the tone frequency of each block is allocated to channels from other nodes. 3. The optical channel monitor system according to claim 2, wherein the length of the square wave is checked to confirm the existence of each channel. 2. The optical channel monitor system according to claim 1, wherein different tone frequencies are used between each node to determine where the source of the channel is for a plurality of channels.