Concept and importance of optical signal-to-noise ratio (osnr)

Spectral Analysis of Optical Signal-to-Noise Ratio of optical fiber networks

The spectral analyzer (OSA) was originally used to measure the power spectrum of optical signals. After the introduction of wavelength division multiplexing (WDM), spectral analyzers were popularized because the standard power meter could not distinguish multiple wavelengths (optical power under the channel ). However, although most people are familiar with the typical application of OSA, that is, troubleshooting the network or measuring the channel power and noise levels, for various reasons, these unique measurement devices are not widely recognized in the market. One of the reasons is that the true capabilities of OSA are underestimated to some extent, especially when trying to maximize the cross-segment fiber.

　　

This article will introduce the concept and importance of the optical signal-to-noise ratio (osnr), as well as the consequences of poor network osnr. At the same time, it will introduce the appearance of OSA in the current market, finally, we will explain how to use OSA to fully develop Optical Fiber Links.

　　

SNR

 

The concept of osnr is crucial in identifying WDM Networks. It can quantitatively detect the degree of noise interference when the signal is transmitted along the optical fiber. The calculation method is to divide the total signal power by the noise power in the 0.1 nm bandwidth. Figure 1 shows a typical signal of OSA measurement, with a power of about-22 dBm and a background noise of about-46 dBm. Therefore, the osnr in this example is about 24 dB.
 

Figure 1. osnr example

Osnr importance

　　

Why is osnr measurement important? There is a direct relationship between osnr and the bit error rate (BER), in which BER is the ultimate value to measure the transmission quality. The higher the osnr, the lower the error rate, that is, the fewer transmission errors. Conversely, low (or poor) osnr may increase service vehicles and reduce service quality (QOS) (see figure 3 ).
 

Figure 2. Relationship between osnr, Ber, and QoS
 

Figure 3. Influence of Low (or poor) osnr

　　

The spectral analyzer (OSA) was originally used to measure the power spectrum of optical signals. After the introduction of wavelength division multiplexing (WDM), spectral analyzers were popularized because the standard power meter could not distinguish multiple wavelengths (optical power under the channel ). However, although most people are familiar with the typical application of OSA, that is, troubleshooting the network or measuring the channel power and noise levels, for various reasons, these unique measurement devices are not widely recognized in the market. One of the reasons is that the true capabilities of OSA are underestimated to some extent, especially when trying to maximize the cross-segment fiber.

　　

Network osnr

　

It is very interesting to explore the changes in osnr with signal propagation in optical fibers. Figure 4 shows a typical network implementation consisting of eight wavelengths reused on one optical fiber. (Please note that four Er-doped fiber amplifiers (EDFA) are used in the propagation path to increase the signal power .), Each EDFA scales up the existing signal and noise while generating noise. Therefore, the osnr will decrease after the signal passes through the amplifier. Because osnr changes with distance, osnr is usually monitored at different locations of the network, not just at the transmitter and receiver.

Figure 4. osnr evolution during Optical Fiber Propagation
 

Noise Sources in the Network

　　

EDFA is the main noise source in the network and comes from the process of "amplifying spontaneous radiation" (ASE. A typical EDFA includes a laser (known as a "pump" Source). If it operates at a wavelength of 980 nm, the Er ions are stimulated from the ground L1 to L3 (see figure 5 ); if the operating wavelength is at 1480 nm, the L1 is then stimulated to L2. Ions in L3 soon decay to L2. If a 1550 nm signal passes through the optical fiber, the signal photon will inspire L2 ions to drop to L1, and generate a new light sub with the same wavelength and the same propagation direction as the signal photon. The signal is amplified by Stimulated radiation. Er ions can also attenuation from level L2 to L1 through spontaneous radiation, which will happen randomly and generate photon. These phoins can also make Er ions produce stimulated radiation, which can be amplified, resulting in ASE noise. Correspondingly, each EDFA reduces the osnr of the enlarged signal due to its ase. If the signal passes through multiple EDFA, the first EDFA usually causes the osnr to decrease by about 3 dB, and the subsequent EDFA causes the osnr to decrease by less than 3 dB.

Figure 5. spontaneous radiation and stimulated radiation in EDFA

Current OSA in the market

　　

During Typical activation and trial run, field technicians may first use an optical fiber detector to check whether the connector is clean and then use a power meter to test the loss in the optical fiber. If the loss is greater than the pass value, on-site technical staff will use the Optical Time Domain Reflector (OTDR) to find the fault, and the last step of the test sequence is usually the BER test. The Service Activation team then opens the sender and performs OSA measurements to check the central wavelength and power level of each channel, and in some cases also checks the osnr. In this case, in the lengthy list of tests to be executed, the Osa measurement may be considered as an additional measurement of little use. In fact, this assumption ignores the true value of OSA in fully utilizing the optical network.

　　

How to Use OSA to maximize the potential of the Network

　　

Among the most critical set of indicators (Channel flat and minimum power) for network performance evaluation, OSA is one of the few measurement tools that can maximize the potential of the network. With OSA, you can perform the following three operations to optimize network performance: Increase the number of channels, increase the data rate, and test different network configurations in the lab.

　　

By measuring osnr, channel spacing, and signal spectral width, OSA allows network planners to determine whether channel numbers can be increased (figure 6 ). If the network element can handle more closely the channel spacing (for example, considering multiplexing/demultiplexing), increasing the extra channel can easily increase the bandwidth of the fiber diameter distance.

Figure 6. Use OSA to determine whether the number of channels can be increased

Second, OSA enables technicians to determine whether the data rate of the fiber diameter distance can be increased because it can measure the spectral width of the signal. As we all know, the spectral width of signals increases with the increase of data rate. For example, if the 10 gbits/s channel is black in figure 7, the data rate can be increased to 40 Gbit/s (displayed in red) without affecting network performance, as long as it is within the margin of chromatic dispersion (CD) and polarization mode dispersion (PMD. It is important to ensure that the larger spectral width does not cause overlapping channels, otherwise the BER may be increased. Therefore, a higher data rate will further optimize the use of optical fiber capacity.

Figure 7. Use OSA to determine whether data rate can be increased

The 10 Gbit/s channel is black, and the 40 Gbit/s channel is red.

　　

The third way to make full use of network potential through OSA is to allow testing of different network configurations in a lab environment. In fact, network designers/planners want to (in the Lab) evaluate the impact of the number, position and gain of the amplifier, the position of the dispersion compensator, And the ROADM insertion wavelength on the network. OSA is the only instrument that globally reflects the consequences of all these factors on the optical layer. It can also identify possible problems and areas that can be improved.

　　

Conclusion

　　

OSA is a powerful measuring device that can measure osnr and identify Fiber Links because osnr is directly related to Ber. Poor osnr will increase the number of vehicles used for maintenance and extend the downtime, which will negatively affect the network. In addition, OSA can be used to make full use of network capacity by increasing the number of channels, increasing the data rate, or testing different network configurations.