1 Introduction to OPM
As early as 2000, with the rise of the optical communications industry, OPM has attracted people's attention as a functional module for online monitoring of channel optical power, center wavelength, and optical signal-to-noise ratio (OSNR). There are also corresponding products available, but It has not been used on a large scale in actual systems. Until 2008, with the maturity of ROADM technology, the development of intelligent optical networks and the promotion of 3G networks, OPM gradually became an indispensable part of the construction of optical networks from an optional accessory, and was widely used in actual optical networks.
Figure 1 Schematic diagram of OPM application nodes
As a small spectroscopic monitoring module similar to a spectrometer, there are a variety of technical means, but there are two main types that can be promoted and applied in the market: one is a diffraction-based structure, which is mainly composed of a volume grating and an array detector. The manufacturers are Accelink (Wuhan Guangxun Technology Co., Ltd.), Bayspec, etc .; the other is an interference type structure, mainly based on TOF (Tunable opTIcal filter) technology, and its manufacturers are Axsun, Optoplex and so on. The two design schemes have their own advantages when they meet the basic optical index requirements: OPM based on diffraction type has no moving parts, and can simultaneously sample optical signals of different wavelengths within the set wavelength range. Long, good stability and fast measurement; OPM based on TOF technology can occupy certain advantages in volume and cost.
Table 1 is OPM's optical performance index (take Accelink's products as an example).
Table 1 Main performance parameters of OPM
2 Application of 40G network
The development of the optical network is based on the increasing demand for transmission capacity. There are two main means to solve this contradiction on the traditional optical fiber line: 1. Increase the transmission rate; 2. Increase the number of transmissions. The 40Gbps transmission technology has matured and is widely used. The 40Gbps optical network puts forward requirements for higher dispersion (CD) and polarization mode correlation (PMD) for many traditional optical passive devices. For OPM, The main changes are in two points: 1. Different types of code patterns bring about difficulties in signal identification; 2. Changes in the signal calculation method from the wide spread of various code patterns at high rates.
In the 10Gbps network, the main adjustment method is amplitude modulation, and the main transmission code types are NRZ (non-return to zero code) and RZ (return to zero code); while in the 40Gbps network, due to the increase in transmission rate, phase modulation has become The main means. The following table lists the modulation modes and spectral characteristics of some transmission patterns, including: NRZ (non-return to zero code), RZ-50% (return to zero code with 50% duty cycle), PSBT (phase-shaping binary transmission), NRZ-DPSK (non-return to zero-differential phase shift keying), RZ-50% DPSK (return to zero code-differential quadrature phase shift keying with 50% duty cycle), NRZ-DQPSK (non-return to zero-differential positive Cross-phase shift keying), RZ-50% DQPSK (duty cycle 50% return-to-zero code-differential quadrature phase shift keying), DP-DQPSK (dual polarization differential quadrature phase shift keying).
Table 2 Amplitude, phase and spectrum of various codes
At present, the actual 40G system uses more codes for NRZ-DPSK and NRZ-DQPSK, and with the development of optical networks, the transmission rate will be further improved, and the method of modulation and demodulation will be constantly updated. 100G will be the future development trend of transmission networks.
3 Application characteristics of OPM in 40G
In addition to the continuous improvement of the speed of optical network development, its degree of intelligence is also constantly increasing. The intelligent management requires real-time detection of the network status and signals, and thus dynamic control. For OPM, providing reliable, accurate and real-time information for optical networks has become an important part of optical network management, and its role cannot be underestimated. In the 40G optical transmission network, we first compare the test capabilities of several instruments and different transmission codes and rates to illustrate the characteristics of OPM in network applications.
Figure 2 Test block diagram
Table 3 shows that OPM's testing capabilities are consistent with spectrometers, while optical power meters can only test optical power. It is worth mentioning that for the testing of OSNR, currently OPM and OSA are based on extrapolation, and for optical signals under 40G transmission, the spectral broadening often exceeds the noise test point of DWDM, so it cannot be accurate. The OSNR value can only be used as a reference.
Table 3 Comparison results of OPM, spectrometer and optical power meter
OPM as an online module, its biggest advantage compared to spectrometer and optical power meter is still reflected in its low price and high degree of integration. The increasingly complex network topology and faster and faster transmission speed make the role and importance of OPM more and more important. In response to the characteristics of 40Gbps signals, OPM mainly faces the following technical difficulties and areas that need improvement:
3.1 Peak Search
No matter in any system, the first concern is whether OPM can correctly monitor the signal light. In the actual use environment, it is often the way that 10G signals are mixed with 40G signals. After the different signal light passes through erbium-doped fiber amplifiers (EDFA), optical up and down channels (OAMD) and other devices, the entire spectrum and the spectrum of signal light The type will change a lot, especially the influence of 40G signal's own broadening, which requires the peak-finding algorithm to put forward new requirements. There are two main peak-finding errors:
1. Misdetection
False detection, that is, to report an optical signal that does not actually exist. In the actual test, the main reasons for the misdetection are: 1) the noise is too large and is misjudged as a signal; 2) the side mode of the signal light is misjudged as a signal. For the 10G system, the spectral characteristics of the 10G signal itself are very different from the noise background and glitches, and it is easier to distinguish. For the 40G system, due to the broadening of the spectrum, the spectral shape of the optical signal and noise after various devices is quite close, so It is prone to misdetection.
2. Leakage
The so-called missed detection means that no actual optical signal is reported. In actual tests, the main causes of missed detection are: 1) The signal is too low, which is lower than the optical power threshold set by OPM; 2) The signal interval is too narrow, which exceeds the channel interval threshold set by OPM. For the 40G signal, due to the broadening of the signal spectrum, the signal peak and signal power are very different, so that the decision method based on the peak optical power often leads to missed detection. At the same time, due to the broadening of the spectrum, the location of the signal peak position is biased, and the channel interval calculation error is large. Once the calculation result is too small, it may also cause missed detection.
It can be seen from this that for 40G signals, the signal's own widening and the increased crosstalk of the signal to other channels make the peak-finding decision more difficult. Through the spectral characteristics of various 40G signals and the characteristics of 40G network applications, establishing perfect and accurate judgment conditions is the basic condition for ensuring OPM to correctly search for peaks.
3.2 Identification of different code types and rates
The architecture of an all-optical network and the popularization of the concept of an intelligent network ultimately require the interconnection of optical networks of different manufacturers and different specifications to achieve transparent transmission of optical signals. This poses a new topic for OPM: identifying different code types and rates.
It can be seen from Table 2 that different code patterns and rate combinations have different spectral shapes, which provides OPM with a theoretical basis for identifying different rate and code patterns.
Figure 3 shows the test spectrum of Accelink's OPM product, which compares the spectrum comparison between OSA and OPM's measured 40G NRZ and 40G NRZ-DPSK. As can be seen from the figure, in addition to the difference in dynamic range, both OSA and OPM can well reflect the spectral width of the two codes under the 40G signal, thus illustrating that through careful study of the spectral characteristics of various codes, OPM The function of identifying the pattern is achievable.
Figure 3 Spectra comparison of 40G NRZ and NRZ-DPSK tested by OSA and OPM
3.3 Calculation of center wavelength
For the calculation of the center wavelength of the 40G signal, if the original center wavelength algorithm for the 10G signal is used, the accuracy of the wavelength detection may be exceeded. This is mainly caused by the broadening of the signal spectrum at the 40G transmission rate. The limitation of the OPM's own bandwidth and the number of acquisition points leads to a further reduction in the size difference of the sampling points near the peak in the case of spectral broadening, and the condition of calculating the center wavelength is weakened, which ultimately affects the accuracy of the center wavelength.
Aiming at the differences of 40G signals with different codes, studying different calculation methods of center wavelength is a necessary method to improve the calculation accuracy of center wavelength under 40G system.
3.4 Calculation of optical power;
The calculation of signal optical power is one of the most stringent indicators for OPM in the 40G system and one of the key indicators for intelligent management. The calculation accuracy of OPM's signal optical power mainly depends on the stability of the data and the choice of power integration bandwidth. Compared with the method of calculating the optical power of the 10G signal, OPM calculates the optical power of the 40G signal, the main difference is the configuration of the parameters and the method of power calibration.
On the other hand, the crosstalk problem between 40G signals is also an important factor affecting the accuracy of the calculated power. In many practical 50GHz channel spacing systems, crosstalk between adjacent channel 40G signals is very serious. For 10G and 40G mixed transmission systems, the effect of 40G signals on 10G is also obvious. Therefore, for the optical power calculation of 40G signals, one of the key issues to be solved is the identification of the pattern, so as to carry out differentiated parameter configuration and processing methods.
3.5 Calculation of OSNR
For the calculation of OSNR, the traditional method is extrapolation, which is to find noise points outside the signal optical bandwidth, thereby estimating the noise level of the signal. For OPM, the signal will be widened after passing through the spectroscopic system. Before the extrapolation method is used, it needs to undergo a deconvolution operation to restore the signal. This calculation method can basically meet the current application of the actual 10G transmission system, and can more accurately calculate the OSNR of the signal.
For 40G signals, because the spectral broadening often exceeds the noise test point of DWDM, there is also a signal on the original noise test point, and the extrapolation method is no longer applicable. In this case, in-band testing can be used. Program. The in-band method, that is, the selection of noise points is within the signal bandwidth, generally uses different polarization characteristics of signal light and noise light to separate signal light and noise light, so as to accurately obtain OSNR, which will be one of the development of OPM Important technical direction.
In actual 40G transmission systems, due to the phase modulation pattern, which requires a signal OSNR at the receiving end that is much lower than the 10G amplitude modulation demodulation method, OSNR often appears as a reference index in 40G optical networks. It is no longer one of the most important parameters for evaluating system performance, and there is a moderate relaxation of the scope and accuracy of OPM testing OSNR.
4 OPM application status in 40G
At present, optical network system design manufacturers have clearly proposed 40G OPM specifications, and OPM suppliers can also meet customer needs under some commonly used code types. Taking Accelink's OPM product as an example, the product is compatible with the identification of 10G signals while supporting 40G networks, and supports the identification and calculation functions of mixed transmission of multiple code types, including DPSK and DQPSK. It is particularly worth mentioning that OPM supports the network identification function of 40Gbps signals at 50GHz intervals. It can identify up to 96 channels at the same time, and the scanning speed is less than 200ms.
Although OPM has made a breakthrough in the 40G optical network, fully implementing the 40G environment application requires further improvement of OPM's own performance indicators and research on the application environment in order to finally meet the needs of the system.
The key research issues of OPM requirements are as follows:
1. Various pattern characteristics of 40G optical transmission network;
2. The characteristics of the 40G optical transmission network, especially the analysis of the spectral characteristics under the mixed transmission of various signals;
3. Signal processing method for 50GHz interval under 40G optical transmission network;
4. Technology for testing OSNR with in-band method;
5. Low-cost solution.
In general, OPM can adapt to the development of 40G optical transmission networks and must meet the requirements of 40G optical transmission networks, regardless of technical means or application requirements. The application of the current OPM in the 40G optical transmission network has just started, and there are still many areas for research and improvement.
5 Conclusion
With the application of 40G optical networks, OPM faces many new problems, such as in-band OSNR test methods, signal identification under complex construction, smaller and smaller channel spacing, multi-pattern identification, etc., but the biggest problem It is still the cost of OPM. With the continuous development of optical networks, the increase in network speed and the degree of intelligence, the system's demand for OPM modules is increasing, and the requirements are becoming higher and higher. OPM modules can only be continuously improved and perfected to adapt to new Demand can further guarantee the security and stability of the optical network.
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