KR20010073485A - WDM ring network with reduced chaotic channel power oscillation - Google Patents

Abstract

PURPOSE: A WDM(Wavelength Division Multiplexed) ring network reducing irregular vibration of channel power is provided to solve a problem of channel power vibration in the optically amplified WDM ring network, by optimizing the usage of network components. CONSTITUTION: A WDM(Wavelength Division Multiplexed) ring network includes at least more than one no-gain fixed optical amplifier and a CPE(Channel Power Equalizer). In an RS time constant, tau, of the CPE, the tau is not less than tau-1. Here, the tau-1 is an RS time constant at a time when a channel power vibration cycle, which is reduces as the tau is reduced, is firstly changed.

Description

WDM ring network with reduced chaotic channel power oscillation

The present invention relates to a WDM ring network, and more particularly, to a WDM ring network in which irregular vibration of channel power is reduced.

WDM (Wavelength Division Multiplexed) networks often generate a closed ring through reconfiguration through the addition / removal of channels, causing the power of the channel to oscillate irregularly. Such vibrations can cause fluctuations due to cross-modulation of optical amplifiers in other channels that do not form a cyclic ring and increase noise, which can cause a significant drop in the performance of the entire network. Can be. For this reason, there is a growing interest in vibration phenomena occurring in optically amplified ring networks. However, the precise analysis and research of this phenomenon has not been done yet.

Optically amplified ring networks often include non-gain fixed Erbium Doped Fiber Amplifiers ("EDFAs") and Channel Power Equalizers ("CPEs") for them. There may be various ways to solve the channel power vibration problem in the network, but the method of optimizing and solving the specifications of the devices constituting the network may be the most stable as possible.

The present inventors investigated the principle of irregular vibration occurring in the WDM ring network including the non-gain fixed EDFA and CPE, and the irregular vibration appears only when the RC time constant of the CPE is in a specific region. The area was observed to change with the length of the entire circulation ring. These results suggest that the irregular vibration in the ring is the result of the combined effect of the ring's cycle time and the CPE's RC time constant. Therefore, these facts should be considered in the design of the network.

Therefore, the technical problem of the present invention is to solve the channel power vibration problem in the optically amplified WDM ring network by optimizing the specifications of the network elements.

1 is a schematic structural diagram of a WDM ring network according to an embodiment of the present invention;

FIG. 2 is a graph of the inverse of the CPE's RC time constant versus the period of channel power oscillation as measured in the network of FIG.

3 is a graph showing irregular oscillation power in region III of FIG. 2;

4A and 4B are graphs showing trends of change in channel output power in region III of FIG. 2 when ring network lengths are 200 km and 400 km, respectively.

The WDM ring network of the present invention for solving the above technical problems is:

At least one non-gain fixed optical amplifier and a channel power equalizer therefor, wherein the RC time constant τ of the channel power equalizer is characterized in that the following equation (1).

τ≥τ1

Τ1 represents the RC time constant of the channel power equalizer when the channel power oscillation period, which decreases as τ decreases for the first time, changes rapidly.

Preferably, τ has a value that converges to τ 1 from the right.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

Referring to Figure 1, two active WADM (Wavelength Add / Drop Multiplexer; AWADM1, AWADM2) and three passive WADMs (PWADM1, PWADM2, PWADM3) in a 200km long WDM ring forming a circular optical path indicated by dotted lines Is included. Each WADM includes a multiplexer (MUX), demultiplexer (deMUX), EDFAs, and switches. Active WADM is equipped with CPE (not shown), but passive WADM is not equipped with CPE. In this embodiment, the RC time constant of the CPEs has a specific value described below with respect to the length of the WDM ring network. Meanwhile, the length between two active WADMs was 25 km. The CPEs used to maintain channel power at reference values consist of a variable attenuator controlled by a feedback loop. The input power of each EDFA was 20 dBm per channel, using all eight channels. Reference numeral WSXC, which is not described in FIG. 1, indicates a liquid crystal cross connector having a switching function.

FIG. 2 is a graph of the inverse of the CPE's RC time constant versus the period of channel power oscillation as measured in the network of FIG. Referring to FIG. 2, it can be seen that in region I, the channel power oscillation period generally decreases as the RC time constant of the CPE decreases. In addition, the power vibration appeared to be stable and regular.

In this region, it was observed that as the RC time constant decreases, the peak-to-peak amplitude of the oscillating channel also decreases. On the other hand, it can be seen that the channel oscillation period suddenly decreases to reach 1 ms at the boundary between the region I and the region II (transition value). It should be noted that this period corresponds to the roundtrim time of the WDM ring network. Past the transition value, reaching region II, the power fluctuations of the peaks of the oscillating channel are much larger than in the region I. In this area, the power fluctuations are unstable and confusing, sometimes in the form of pulse trains. In region III, where the RC time constant is much smaller, the pulse period again decreases with the inverse of the RC time constant. Referring to FIG. 3, which shows an irregular oscillating power in region III, the power fluctuations are more unpredictable. It can be seen that the worse. In this case, when the power vibration is magnified, it can be seen that several pulses of different magnitudes having a period of one cycle time and an integer multiple of the WDM ring network appear together.

This seemingly unpredictable behavior could be well understood when the phenomenon was analyzed in the frequency domain through the Fast Fourier Transform (FFT). 4A and 4B show Fourier transform analysis spectra obtained from trends of channel output power in region III when ring network lengths are 200 km and 400 km, respectively. In this case, other factors (such as total network loss and EDFA gain) were left unchanged except for the network cycle time. Even when the network length is 400 km, as shown in FIG. 2, it can be seen that there is a range of RC time constants in which the channel power vibration is kept constant at 2 ms, which is one cycle time. When the network length is 200 km, it was 1 ms.

In addition, one cycle time of approximately the same magnitude and the presence of higher order spectral components shown in FIG. 4B can be a clue to the interpretation of the irregular vibration. This dependence of the channel power oscillation on the circadian time suggests that the length of the WDM ring network should be considered as an important factor in the analysis of the irregular oscillations with the CPE response time. In addition, since the CPE response speed and the work cycle time at the boundary between the zone I and the zone II are almost the same value, the very large power vibration seen in the zone II is caused by the resonance between the work cycle time / frequency and the response time of the CPE. I can think of it. In region II, the circadian frequency and some higher order spectral components are the most important sources of irregular vibration, while in region III there is a transient modulation by CPE that is much faster than circadian time. In conclusion, the irregular vibration in the WDM ring network as described above can be interpreted as the mode locking phenomenon of one large laser cavity in which EDFA acts as a gain medium and loss modulation by CPE is performed.

From the above analysis, it is preferable that the RC time constant of the CPE is located in the region I of FIG. 2 but close to the boundary of the region II so as to reduce the influence of the vibration of the channel power during the design of the WDM ring network. Do. This is because the RC time constant of the CPE must have a value in this range to quickly equalize the power, obtain a stable BER (Bit Error Rate) of the channel, and enable more stable signal transmission. In Fig. 2, the boundary values of the regions I and II are the inverse of the CPE response bandwidth at the boundary, i.e., the RC time constant of the CPE, when the total gain of the EDFAs in the circular ring is about 0.2 dBm larger than the total loss. It is preferable to give at 3500-5 * L (kPa). In this case, L represents the length in km units of the shortest of the circulation rings that can be formed in the optical network.

According to the present invention, in the WDM ring network amplified by EDFA, the phenomena of irregular channel power can be reduced by optimizing the length of the WDM ring link and the RC time constant of the channel power equalizer included in the WDM ring.

Claims (6)

A WDM ring network comprising at least one non-gain fixed optical amplifier and a channel power equalizer therefor, WDM ring network, characterized in that the RC time constant τ of the channel power equalizer satisfies the following relationship, τ≥τ1, Where τ1 is the RC time constant of the channel power equalizer when the channel power oscillation period, which decreases as τ decreases for the first time, changes rapidly. 2. The WDM ring network of claim 1, wherein the channel power equalizer comprises a variable attenuator controlled by a feedback loop. 2. The WDM ring network of claim 1, wherein the τ has a value that converges from the right to τ 1, The WDM ring network of claim 1, wherein the optical amplifier is an erbium doped fiber amplifier. 5. The WDM ring network of claim 4, wherein the total gain of the optical amplifier is at least 0.2 dBm greater than the total loss. 6. The WDM ring network according to claim 5, wherein the response bandwidth of the channel power equalizer is 3500-5 X L, wherein L is the longest length of the cyclic rings that can be formed in the network. Is the length of the shortest in kilometers.