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In Standards 4 Mins Read

Noise Concatenation in Optical Amplifier Chains

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Optical networks are the backbone of the internet, carrying vast amounts of data over great distances at the speed of light. However, maintaining signal quality over long fiber runs is a challenge due to a phenomenon known as noise concatenation. Let’s delve into how amplified spontaneous emission (ASE) noise affects Optical Signal-to-Noise Ratio (OSNR) and the performance of optical amplifier chains.

The Challenge of ASE Noise

ASE noise is an inherent byproduct of optical amplification, generated by the spontaneous emission of photons within an optical amplifier. As an optical signal traverses through a chain of amplifiers, ASE noise accumulates, degrading the OSNR with each subsequent amplifier in the chain. This degradation is a crucial consideration in designing long-haul optical transmission systems.

Understanding OSNR

OSNR measures the ratio of signal power to ASE noise power and is a critical parameter for assessing the performance of optical amplifiers. A high OSNR indicates a clean signal with low noise levels, which is vital for ensuring data integrity.

Reference System for OSNR Estimation

As depicted in Figure below), a typical multichannel N span system includes a booster amplifier, N−1 line amplifiers, and a preamplifier. To simplify the estimation of OSNR at the receiver’s input, we make a few assumptions:

Representation of optical line system interfaces (a multichannel N-span system)
  • All optical amplifiers, including the booster and preamplifier, have the same noise figure.
  • The losses of all spans are equal, and thus, the gain of the line amplifiers compensates exactly for the loss.
  • The output powers of the booster and line amplifiers are identical.

Estimating OSNR in a Cascaded System

E1: Master Equation For OSNR

E1: Master Equation For OSNR

Pout is the output power (per channel) of the booster and line amplifiers in dBm, L is the span loss in dB (which is assumed to be equal to the gain of the line amplifiers), GBA is the gain of the optical booster amplifier in dB, NFis the signal-spontaneous noise figure of the optical amplifier in dB, h is Planck’s constant (in mJ·s to be consistent with Pout in dBm), ν is the optical frequency in Hz, νr is the reference bandwidth in Hz (corresponding to c/Br ), N–1 is the total number of line amplifiers.

The OSNR at the receivers can be approximated by considering the output power of the amplifiers, the span loss, the gain of the optical booster amplifier, and the noise figure of the amplifiers. Using constants such as Planck’s constant and the optical frequency, we can derive an equation that sums the ASE noise contributions from all N+1 amplifiers in the chain.

Simplifying the Equation

Under certain conditions, the OSNR equation can be simplified. If the booster amplifier’s gain is similar to that of the line amplifiers, or if the span loss greatly exceeds the booster gain, the equation can be modified to reflect these scenarios. These simplifications help network designers estimate OSNR without complex calculations.

1)          If the gain of the booster amplifier is approximately the same as that of the line amplifiers, i.e., GBA » L, above Equation E1 can be simplified to:

osnr_2

E1-1

2)          The ASE noise from the booster amplifier can be ignored only if the span loss L (resp. the gain of the line amplifier) is much greater than the booster gain GBA. In this case Equation E1-1 can be simplified to:

E1-2

3)          Equation E1-1 is also valid in the case of a single span with only a booster amplifier, e.g., short‑haul multichannel IrDI in Figure 5-5 of [ITU-T G.959.1], in which case it can be modified to:

E1-3

4)          In case of a single span with only a preamplifier, Equation E1 can be modified to:

Practical Implications for Network Design

Understanding the accumulation of ASE noise and its impact on OSNR is crucial for designing reliable optical networks. It informs decisions on amplifier placement, the necessity of signal regeneration, and the overall system architecture. For instance, in a system where the span loss is significantly high, the impact of the booster amplifier on ASE noise may be negligible, allowing for a different design approach.

Conclusion

Noise concatenation is a critical factor in the design and operation of optical networks. By accurately estimating and managing OSNR, network operators can ensure signal quality, minimize error rates, and extend the reach of their optical networks.

In a landscape where data demands are ever-increasing, mastering the intricacies of noise concatenation and OSNR is essential for anyone involved in the design and deployment of optical communication systems.

References

https://www.itu.int/rec/T-REC-G/e

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In Technical 3 Mins Read

Understanding Noise Figure in Amplifiers: Definition, Importance, and How to Reduce It

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When working with amplifiers, grasping the concept of noise figure is essential. This article aims to elucidate noise figure, its significance, methods for its measurement and reduction in amplifier designs. Additionally, we’ll provide the correct formula for calculating noise figure and an illustrative example.

Table of Contents

  1. What is Noise Figure in Amplifiers?
  2. Why is Noise Figure Important in Amplifiers?
  3. How to Measure Noise Figure in Amplifiers
  4. Factors Affecting Noise Figure in Amplifiers
  5. How to Reduce Noise Figure in Amplifier Design
  6. Formula for Calculating Noise Figure
  7. Example of Calculating Noise Figure
  8. Conclusion
  9. FAQs

What is Noise Figure in Amplifiers?

Noise figure quantifies the additional noise an amplifier introduces to a signal, expressed as the ratio between the signal-to-noise ratio (SNR) at the amplifier’s input and output, both measured in decibels (dB). It’s a pivotal parameter in amplifier design and selection.

Why is Noise Figure Important in Amplifiers?

In applications where SNR is critical, such as communication systems, maintaining a low noise figure is paramount to prevent signal degradation over long distances. Optimizing the noise figure in amplifier design enhances amplifier performance for specific applications.

How to Measure Noise Figure in Amplifiers

Noise figure measurement requires specialized tools like a noise figure meter, which outputs a known noise signal to measure the SNR at both the amplifier’s input and output. This allows for accurate determination of the noise added by the amplifier.

Factors Affecting Noise Figure in Amplifiers

Various factors influence amplifier noise figure, including the amplifier type, operation frequency (higher frequencies typically increase noise figure), and operating temperature (with higher temperatures usually raising the noise figure).

How to Reduce Noise Figure in Amplifier Design

Reducing noise figure can be achieved by incorporating a low-noise amplifier (LNA) at the input stage, applying negative feedback (which may lower gain), employing a balanced or differential amplifier, and minimizing amplifier temperature.

Formula for Calculating Noise Figure

The correct formula for calculating the noise figure is:

NF(dB) = SNRin (dB) −SNRout (dB)

Where NF is the noise figure in dB, SNR_in is the input signal-to-noise ratio, and SNR_out is the output signal-to-noise ratio.

Example of Calculating Noise Figure

Consider an amplifier with an input SNR of 20 dB and an output SNR of 15 dB. The noise figure is calculated as:

NF= 20 dB−15 dB =5dB

Thus, the amplifier’s noise figure is 5 dB.

Conclusion

Noise figure is an indispensable factor in amplifier design, affecting signal quality and performance. By understanding and managing noise figure, amplifiers can be optimized for specific applications, ensuring minimal signal degradation over distances. Employing strategies like using LNAs and negative feedback can effectively minimize noise figure.

FAQs

  • What’s the difference between noise figure and noise temperature?
    • Noise figure measures the noise added by an amplifier, while noise temperature represents the noise’s equivalent temperature.
  • Why is a low noise figure important in communication systems?
    • A low noise figure ensures minimal signal degradation over long distances in communication systems.
  • How is noise figure measured?
    • Noise figure is measured using a noise figure meter, which assesses the SNR at the amplifier’s input and output.
  • Can noise figure be negative?
    • No, the noise figure is always greater than or equal to 0 dB.
  • How can I reduce the noise figure in my amplifier design?
    • Reducing the noise figure can involve using a low-noise amplifier, implementing negative feedback, employing a balanced or differential amplifier, and minimizing the amplifier’s operating temperature.
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In Interviews 4 Mins Read

Raman Amplifier and Negative noise figure analysis

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In the context of Raman amplifiers, the noise figure is typically not negative. However, when comparing Raman amplifiers to other amplifiers, such as erbium-doped fiber amplifiers (EDFAs), the effective noise figure may appear to be negative due to the distributed nature of the Raman gain.

The noise figure (NF) is a parameter that describes the degradation of the signal-to-noise ratio (SNR) as the signal passes through a system or device. A higher noise figure indicates a greater degradation of the SNR, while a lower noise figure indicates better performance.

In Raman amplification, the gain is distributed along the transmission fiber, as opposed to being localized at specific points, like in an EDFA. This distributed gain reduces the peak power of the optical signals and the accumulation of noise along the transmission path. As a result, the noise performance of a Raman amplifier can be better than that of an EDFA.

When comparing Raman amplifiers with EDFAs, it is sometimes possible to achieve an effective noise figure that is lower than that of the EDFA. In this case, the difference in noise figure between the Raman amplifier and the EDFA may be considered “negative.” However, this does not mean that the Raman amplifier itself has a negative noise figure; rather, it indicates that the Raman amplifier provides better noise performance compared to the EDFA.

In conclusion, a Raman amplifier itself does not have a negative noise figure. However, when comparing its noise performance to other amplifiers, such as EDFAs, the difference in noise figure may appear to be negative due to the superior noise performance of the Raman amplifier.

To better illustrate the concept of an “effective negative noise figure” in the context of Raman amplifiers, let’s consider an example comparing a Raman amplifier with an EDFA.

Suppose we have a fiber-optic communication system with the following parameters:

  1. Signal wavelength: 1550 nm
  2. Raman pump wavelength: 1450 nm
  3. Transmission fiber length: 100 km
  4. Total signal attenuation: 20 dB
  5. EDFA noise figure: 4 dB

Now, we introduce a Raman amplifier into the system to provide distributed gain along the transmission fiber. Due to the distributed nature of the Raman gain, the accumulation of noise is reduced, and the noise performance is improved.

Let’s assume that the Raman amplifier has an effective noise figure of 1 dB. When comparing the noise performance of the Raman amplifier with the EDFA, we can calculate the difference in noise figure:

Difference in noise figure = Raman amplifier noise figure – EDFA noise figure = 1 dB – 4 dB = -3 dB

In this example, the difference in noise figure is -3 dB, which may be interpreted as an “effective negative noise figure.” It is important to note that the Raman amplifier itself does not have a negative noise figure. The negative value simply represents a superior noise performance when compared to the EDFA.

This example demonstrates that the effective noise figure of a Raman amplifier can be lower than that of an EDFA, resulting in better noise performance and an improved signal-to-noise ratio for the overall system.

The example highlights the advantages of using Raman amplifiers in optical communication systems, especially when it comes to noise performance. In addition to the improved noise performance, there are several other benefits associated with Raman amplifiers:

  1. Broad gain bandwidth: Raman amplifiers can provide gain over a wide range of wavelengths, typically up to 100 nm or more, depending on the pump laser configuration and fiber properties. This makes Raman amplifiers well-suited for dense wavelength division multiplexing (DWDM) systems.
  2. Distributed gain: As previously mentioned, Raman amplifiers provide distributed gain along the transmission fiber. This feature helps to mitigate nonlinear effects, such as self-phase modulation and cross-phase modulation, which can degrade the signal quality and limit the transmission distance.
  3. Compatibility with other optical amplifiers: Raman amplifiers can be used in combination with other optical amplifiers, such as EDFAs, to optimize system performance by leveraging the advantages of each amplifier type.
  4. Flexibility: The performance of Raman amplifiers can be tuned by adjusting the pump laser power, wavelength, and configuration (e.g., co-propagating or counter-propagating). This flexibility allows for the optimization of system performance based on specific network requirements.

As optical communication systems continue to evolve, Raman amplifiers will likely play a significant role in addressing the challenges associated with increasing data rates, transmission distances, and network capacity. Ongoing research and development efforts aim to further improve the performance of Raman amplifiers, reduce costs, and integrate them with emerging technologies, such as software-defined networking (SDN), to enable more intelligent and adaptive optical networks.

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In Technical 4 Mins Read

A short discussion on 980nm and 1480nm pump based Erbium Doped Fiber Amplifiers (EDFA)

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The 980nm pump needs three energy level for radiation while 1480nm pumps can excite the ions directly to the metastable level .

 

 

(a) Energy level scheme of ground and first two excited states of Er ions in a silica matrix. The sublevel splitting and the lengths of arrows representing absorption and emission transitions are not drawn to scale. In the case of the 4 I11/2 state, s is the lifetime for nonradiative decay to the I13/2 first excited state and ssp is the spontaneous lifetime of the 4 I13/2 first excited state. (b) Absorption coefficient, a, and emission coefficient, g*, spectra for a typical aluminum co-doped EDF.

.The most important feature of the level scheme is that the transition energy between the I15/2 ground state and the I13/2 first excited state corresponds to photon wavelengths (approximately 1530 to 1560 nm) for which the attenuation in silica fibers is lowest. Amplification is achieved by creating an inversion by pumping atoms into the first excited state, typically using either 980 nm or 1480 nm diode lasers. Because of the superior noise figure they provide and their superior wall plug efficiency, most EDFAs are built using 980 nm pump diodes. 1480 nm pump diodes are still often used in L-band EDFAs although here, too, 980 nm pumps are becoming more widely used.

Though pumping with 1480 nm is used and has an optical power conversion efficiency which is higher than that for 980 nm pumping, the latter is preferred because of the following advantages it has over 1480 nm pumping.

  • It provides a wider separation between the laser wavelength and pump wavelength.
  • 980 nm pumping gives less noise than 1480nm.
  • Unlike 1480 nm pumping, 980 nm pumping cannot stimulate back transition to the ground state.
  • 980 nm pumping also gives a higher signal gain, the maximum gain coefficient being 11 dB/mW against 6.3 dB/mW for the 1.48
  • The reason for better performance of 980 nm pumping over the 1.48 m pumping is related to the fact that the former has a narrower absorption spectrum.
  • The inversion factor almost becomes 1 in case of 980 nm pumping whereas for 1480 nm pumping the best one gets is about 1.6.
  • Quantum mechanics puts a lower limit of 3 dB to the optical noise figure at high optical gain. 980 nm pimping provides a value of 3.1 dB, close to the quantum limit whereas 1.48  pumping gives a value of 4.2 dB.
  • 1480nm pump needs more electrical power compare to 980nm.

Application

The 980 nm pumps EDFA’s are widely used in terrestrial systems while 1480nm pumps are used as Remote Optically Pumped Amplifiers (ROPA) in subsea links where it is difficult to put amplifiers.For submarine systems, remote pumping can be used in order not to have to electrically feed the amplifiers and remove electronic parts.Nowadays ,this is used in pumping up to 200km.

The erbium-doped fiber can be activated by a pump wavelength of 980 or 1480 nm but only the second one is used in repeaterless systems due to the lower fiber loss at 1.48 mm with respect to the loss at 0.98 mm. This allows the distance between the terminal and the remote amplifier to be increased.

In a typical configuration, the ROPA is comprised of a simple short length of erbium doped fiber in the transmission line placed a few tens of kilometers before a shore terminal or a conventional in-line EDFA. The remote EDF is backward pumped by a 1480 nm laser, from the terminal or in-line EDFA, thus providing signal gain

Vendors

Following are the vendors that manufactures 980nm and 1480nm EDFAs

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In Infographics 1 Min Read

What are main advantages and drawbacks of EDFAs?

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The main advantages and drawbacks of EDFAs are as follows.

Advantages

  • Commercially available in C band (1,530 to 1,565 nm) and L band (1,560 to 1,605) and up to  84-nm range at the laboratory stage.
  • Excellent coupling: The amplifier medium is an SM fiber;
  • Insensitivity to light polarization state;
  • Low sensitivity to temperature;
  • High gain: > 30 dB with gain flatness < ±0.8 dB and < ±0.5 dB in C and L band, respectively, in the scientific literature and in the manufacturer documentation
  • Low noise figure: 4.5 to 6 dB
  • No distortion at high bit rates;
  • Simultaneous amplification of wavelength division multiplexed signals;
  • Immunity to crosstalk among wavelength multiplexed channels (to a large extent)

Drawbacks

  • Pump laser necessary;
  • Difficult to integrate with other components;
  • Need to use a gain equalizer for multistage amplification;
  • Dropping channels can give rise to errors in surviving channels:dynamic control of amplifiers is  necessary.
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