It was always exciting discussing 50ms switching/restoration time perspective for telecom circuits for every engineer who belongs to some part of telecom services including, optical, voice, data, microwave, radio, etc. I was also seeking it since the start of my telecom career, and I believe still somewhere at some point in time, engineers or telecom professionals might be hearing this term and wonder about why (“WHY”) is this? So, I researched over available knowledge pools, and using my experience, I thought of putting it into words to enlighten some of my friends like me.

 

The 50 ms idea originated from Automatic Protection-based Switching subsystems during early digital transmission systems. It was not actually based on any particular service requirement. The value persists because it is not entirely based on technical considerations which could resolve it, but has roots in historical practices and past capabilities and has been a tool of certain marketing strategies.

 

Initially, digital transmission systems based on 1:N APS typically required about 20 ms for fault detection, 10 ms for signaling, and 10 ms for the tail-end transfer relay operation, so the specification for APS switching times was reasonably set at 50 ms, allowing a 10 ms margin. 

 

 

 

For information, early generations of DS1 channel banks (1970s era) also had a Carrier Group Alarm (CGA) threshold of about 230 ms. The CGA is a time threshold for the persistence of any alarm state on the transmission line side (such as loss of signal or frame synch loss) after which all trunk channels would be busied out. But the requirement for 50 ms APS switching stayed in place, mainly because this was still technically quite feasible at no extra cost in the design of APS subsystems. 

 

The apparent sanctity of 50 ms was further entrenched in the 1990s by vendors who promoted only ring-based transport solutions and found it advantageous to insist on 50 ms as the requirement, effectively precluding distributed mesh restoration alternatives under equal consideration start of the SONET era. 

 

As a marketing strategy, the 50 ms issue served as the "mesh killer" for the 1990s as more traditional telcos were bought into this as reference.

 

On the other hand, there was also real urgency in the early 1990s to deploy some kind of fast automated restoration method relatively immediately. This lead to the quick adoption of ring-based solutions which had only incremental development requirements over 1+1 APS transmission systems. However, once rings were deployed, the effect was to only further reinforce the cultural assumption of 50 ms as the standard. Thus, as sometimes happens in engineering, what was initially a performance capability in one specific context (APS switching time) evolved into a perceived requirement in all other contexts.

But the "50 ms requirement" is undergoing serious challenges to its validity as a ubiquitous requirement, even being referred to as the "50 ms myth" by data-centric entrants to the field who see little actual need for such fast restoration from an IP services standpoint. Faster restoration is by itself always desirable as a goal, but restoration goals must be carefully set in light of corresponding costs that may be paid in terms of limiting the available choices of network architecture. In practice, insistence on "50 ms" means 1+1 dedicated APS or UPSR rings (to follow) are almost the only choices left for the operator to consider. But if something more like 200 ms is allowed, the entire scope of efficient shared-mesh architectures becomes available. So it is an issue of real importance as to whether there are any services that truly require 50 ms.

 

Sosnosky's original study found no applications that require 50 ms restoration. However, the 50 ms requirement was still being debated in 2001 when Schallenburg, understanding the potential costs involved to his company, undertook a series of experimental trials with varying interruption times and measured various service degradations on voice circuits, SNA, ATM, X.25, SS7, DS1, 56 kb/s data, NTC digital video, SONET OC-12 access services, and OC-48. He tested with controlled-duration outages and found that 200 ms outages would not jeopardize any of these services and that, except for SS7 signaling links, all other services would in fact withstand outages of two to five seconds.

Thus, the supposed requirement for 50 ms restoration seems to be more of a techno-cultural myth than a real requirement—there are quite practical reasons to consider 2 seconds as an alternate goal for network restoration. This avoids the regime of connection and session time-outs and IP/MPLS layer reactions but gives a green light to the full consideration of far more efficient mesh-based survivable architectures.

 

  A study done by Sosnosky provides a summary of effects, based on a detailed technical analysis of various services and signal types. In this study, outages are classified by their duration and it is presented how with the given different outage time, main effects/characteristics change.

 

 

Conclusive Comment

 

​Considering state-of-art technologies evolving overtimes in all aspects of telecommunication fields, switching speed is too fast, even hold-up-timer (HUT) and hold-down-timers or hold-off-timers are playing significant roles that can hold the consequent actions and avoids unavailability of service. Yes, there will definitely be some packet losses in the services which could be visible as some form of errors in the links or may increase latency sometimes but as we know it varies with the nature of services like voice, data, live stream, internet surfing, video buffering, etc. So we can say that in the recent world the networks are quite resistant to brief outages, although it could vary based on the architecture of the network and flow of the services. Even 50ms or 200ms outages would not jeopardize services (data, video, voice) and it will be based on network architecture and routing of services.

Would love to see viewers comment on this and further discussion.

Reference: 
Mesh-Based Survivable Networks: Options and Strategies for Optical, MPLS, SONET, and ATM Networking By Wayne D. Grover

 

 

 

Pre-FEC BER corresponding to Q.

• BER and Q can be calculated from one another according to the following relationship:

• Or, in Excel: dBQ = 20*LOG10(-NORMSINV(BER))
• dBQ = 20Log(Q)

Post-FEC BER/Q

• Post-FEC BER of <1e-15 essentially means no post-FEC errors
• This is equivalent to ~18dBQ, which is about as high as can be measured

 

FEC Limit

• This is the lowest Q (or highest BER) that can be corrected by the FEC
• Beyond this post-FEC errors will occur

e.g

Pre-FEC Calculation eg.

 

 

Assume: 219499456 : Bit Errors 0 : Uncorrectable words 6.4577198E-6 : PRE-FEC BER Assume the Time at this instant of performance was 12:05:04 which means : 304 Seconds since the last time interval.
Assume The FEC settings was : STANDARD FEC so the Rate used for 100 G transponder is : 1.1181 * 10^11
General Formula to calculate PRE_FEC:
PRE_FEC BER =    TotalErrors
------------------------------------------
(secsFromLast * (rate))  TotalErrors =((bitErrorCorrected count + (9 * (uncorrected Words count))
So Substituting the Values
219499456 / (304*1.1181 * 10^11) = 6.4577198E-6

 

Optical fiber communication professionals might have heard  about ORL (Optical Return Loss ) during design and operation on an Optical Fiber Network.Intend of this article is to share the information on this topic which could help optical fiber  engineers and professionals understanding the concept and they can utilize this knowledge to understand a network in a better way.

 

In this article we will discuss

-What is ORL?

-What are the major sources of ORL ?

-What are the implications of ORL?

-How to test and rectify ORL?

-Standards and references 

---

What is Optical Return Loss (ORL)?

Let me share few definitions so that it will be easy for every stage of engineers ;it could be student, beginner, professional or expert.

         1).  When light passes through an optical component most of it travels in the intended direction, but some light is reflected or scattered. In many applications these reflections are unwanted, because they can affect the emission characteristics of any laser in the system. In such applications, it is important to measure the reflections for the components of the system. The Return Loss is defined as the light reflected back into the input path. It is caused by scattering and reflection from optical surfaces like mirrors, lenses, and connectors or from defects, such as cracks and scratches. The backreflection is equal to the return loss with a negative quantity.

 

          2).  ORL is  defined  as the ratio (in dB) of the optical power (Pinc) traveling downstream at a system interface to the optical power reflected back upstream to the same interface. 

This includes the reflected power contributions from all system components downstream from the interface.

 

 To clarify :

 

Reflectance (dB) = P reflected (dBm) – P incident(dBm)

 

A discrete reflection will always be a negative quantity as the reflected power cannot be greater than the incident power.

 

By convention ORL is defined as:   

 

ORL(dB) = P incident (dBm) – P reflected (dBm)

 

This means that ORL will always be a positive number.The fact that we want all power to move forward and none to be reflected means that the higher the positive number, the better.

 

 

 

            3).  The reflection factor for a component is a measure of how much light the component reflects. It is a ratio of the power reflected by the device to the power incident on the  device. More normally we talk about the return loss of a component. The return loss has units of dB. Return loss is given by:

            

Return Loss(dB) = –10log(Reflection Factor) (dB)

ORL(dB) = P incident (dBm) – P reflected (dBm)

 

PPC: Back-reflected power of PC connector

PBS: Backscattered power of fiber

PR: Total amount of back-reflected power

 

ORL is measured in dB and is a positive value.Reflectance (dB) is the ratio of reflected power to incident power due to a single interface. It is defined as a negative quantity 

The higher the number, the smaller the reflection - yielding the desired result.    

 

 

 

---

 

What are the major sources of ORL ?

System components such as

• connectors,
• mechanical splices,
• attenuators,
• patch cords
• glass/air terminations

All create a change in index of refraction as seen by an optical signal. The components are reflective in nature and can contribute to system ORL.

The fiber optic cable itself creates backscatter as light propagates through it.  The amount of reflected power due to backscatter cannot be eliminated but is magnitudes smaller than the power from discrete reflections

 

Sources of loss include reflections and scattering along the fiber network. A typical Return Loss value for an Angled Physical Contact (APC) connector is about -55dB, while the RL from an open flat polish to air is typically about -14dB. High RL is a large concern in high bitrate digital or analog single mode systems and is also an indication of a potential failure point, or compromise, in any optical network.

 

 

---

 

What are the implications of ORL?

 

The main effects of back-reflection  due to ORL include the following:

 

• Less light is transmitted from the transmitter.
• Increase in light source interference
• Increasing the BER in digital transmission systems
• Multi path distortion can also occur. 
• Reducing the OSNR in  transmission
• Reflections can distort the optical signal as reflections travel back and forth between reflective components.
• Strong fluctuations in the laser output power.
• Increase in transmitter noise.
• Changes central wavelength and output power.
• Permanent damage to the laser.

How does reflected power affect laser stability ?

Reflected light can provide unwanted feedback to the laser cavity which will effect:

• Frequency Modulation Response changes
• Relative Intensity Noise (RIN)
• Optical frequency variations
• Laser line-width variations

Reflection induced degradation increases with system bit-rate !The end result is higher bit error rates (BER)

 

---

 

How to test  ORL?

 

The measurement of ORL is becoming more important in the characterization of optical networks as the use of wavelength-division multiplexing increases. These systems use lasers that have a lower tolerance for ORL, and introduce elements into the network that are located in close proximity to the laser

 

The two major test methods:

   Optical Continuous Wave Reflectometry (OCWR)

            A laser source and a power meter, using the same test port, are connected to the fiber under test.

  Optical Time Domain Reflectometry (OTDR)

          The OTDR is able to measure not only the total ORL of the link but also section ORL.

 

 

To measure the ORL of a fiber span, an optical continuous wave reflectometer (OCWR) is used. The OCWR is an instrument designed to specifically measure system  and component ORL  reflectance. The OCWR launches a stable, continuous wave signal into the optical fiber and measures the strength of the time-integrated return signal. The ORL meter will return a single negative value which is the total reflectance from all reflective components seen from the point of test.  On fiber spans with multiple reflective components, discrete reflectance values cannot be determined unless component isolation is performed. The measured reflectance value is a directional value so tests should be performed on both ends of a fiber span.

 

The ORL reference measures background reflection of the fiber under test.  The reference procedure is performed each time a new test setup is required.A mandrel wrap is applied to the fiber test jumper before the point of measurement to isolate and attenuate any reflectance generators. The glass to air interface on the test connector end will be isolated from the OCWR.  The ORL zero function on the OCWR provides storage of the background reflectance level to provide the total optical return loss of the fiber test jumper. Once the mandrel wrap is taken out, the displayed ORL value represents the total ORL of the system from the point of termination.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

OTDR method is explained at https://mapyourtech.com/entries/general/otdr-optical-time-domain-reflectomer-some-know-how 

 

Typical OTDR report snapshot for reference:-

 

 

---

 

Typical Reflectance for few connectors:-

 

     PC connecters: -30dB to –40 dB

    UPC connectors: -40dB to –50dB

    APC  connectors: -60dB  

    Fiber to air interface on a PC connector: -14.7 dB

    Rayleigh backscatter for telecom fiber:  -70 dB/meter

 

The angle reduces the back-reflection of the connection.

Typical good ORL measurements range from 30-35 dB. Read more at http://mapyourtech.com/entries/general/why-it-is-good-to-have-orl-%3E30db-for-general-fiber-links-

 

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Standards and references 

 

 -Telcordia document GR-1312

R7-79 [361] The discrete reflectance seen from any ONE optical port shall be less than -27 dB.

O7-80 [362] The discrete reflectance seen from any ONE optical port should be less than –40dB.

 

-Telcordia Document GR-2918

R7-38 [35] The individual channel Optical Return Loss, ORL as defined above, shall be 24dB or more for all wavelengths used in the DWDM system.

 

All equipment and component manufactures are required to design their systems  to meet reflectance specifications set out by the ITU-T which are adopted by bodies such as Telcordia (formerly Bellcore).Their specifications are intended to minimize system degradation due to reflections and they propose:

1)Enforce reflectance requirements on individual components in a fiber span.

  R7-79 and O7-80 relate to system components.

  Taken from GR-1312, Issue 3, April 1999 

  Generic Requirements for OFAs and Proprietary DWDM systems.

 

2)Ensure system performance to have a tolerance to specified reflection values.

  R7-38[35] relates to system ORL.

  Taken from GR-2918-CORE, Issue 4, December 1999 

 

 

Note :This article is sourced from multiple informations available on internet and books.

 

 

 

 

 

 

Signal to Noise Ration (SNR)  is not an unknown terminology for Engineers and Tech professionals who are dealing with Digital or Analog form of Communication.Here we will  explore the aspect of SNR in Optical Fiber Communication  space known as Optical Signal to Noise Ratio(OSNR).

 

Warning! Keep patience while scrolling down to read as this article may seem long as this is important topic.Whole content is collected from free available trusted sources and can be downloaded or shared.Copy/Paste mistakes may be observed and can be corrected once notified.Content may seem non-structured but there is content for everyone from beginner to professional so reader can absorb what he wants.

 

Bingo! Let's start it.

 

 

Some handy definition of OSNR to pick :-

• OSNR [dB] is the measure of the ratio of signal power to noise power in an optical channel .

                              ​

• OSNR is the short form of Optical Signal to Noise Ratio. It is key parameter to estimate performance of Optical Networks. It helps in BER calculation of Optical System.
• OSNR is important because it suggests a degree of impairment when the optical signal is carried by an optical transmission system that includes optical amplifiers.
• If we know the OSNR and the bandwidths, we can find Q and the BER
• It can be seen as the QoS at the physical layer of optical networks. OSNR is directly related to bit-error rate, which will lead to packet losses seen by higher layers.
• OSNR indirectly reflects BER and can provide a warning of potential BER deterioration.
• OSNR has long been recognised as a key performance indicator for amplified high-speed transmission networks to ensure network performance and reliability and it is related to many design parameter such as number or repeater/amplifiers ,reach ,available modulation formats etc.

Now let's explore it in more detail:-

 

Optical signal-to-noise ratio (OSNR) is used to quantify the degree of optical noise interference on optical signals. It is the ratio of service signal power to noise power within a valid bandwidth.When the signal is amplified by the optical amplifier (OA), like EDFA, its optical signal to noise ratio (OSNR) is reduced, and this is the primary reason to have limited number of OAs in a network.

The OSNR values that matter the most are at the receiver, because a low OSNR value means that the receiver will probably not detect  or recover the signal. The OSNR limit is one of the key parameters that determine how far a wavelength can travel prior to regeneration.

OSNR serves as a benchmark indicator for the assessment of performance of optical transmission systems. DWDM networks need to operate above their OSNR limit to ensure error – free operation. There exists a direct relationship between OSNR and bit error rate (BER), where BER is the ultimate value to measure the quality of a transmission.

 

The value of OSNR_out that is needed to meet the required system BER depends on many factors such as the bit rate, whether and what type of FEC is employed, the magnitude of any crosstalk or non-linear penalties in the DWDM line segments, etc.Below picture highlights OSNR as one of the important parameter  in a DWDM link.

 

Optical amplifiers such as erbium-doped fiber amplifiers (EDFAs) are normally employed in optical networks to compensate for the transmission losses over long distances. However, besides providing optical gain, EDFAs also add unwanted amplified spontaneous emission (ASE) noise into the optical signal. Furthermore, the cascading of EDFAs results in accumulation of ASE noise. ASE noise is typically quantified by OSNR and is one of the most important parameters to be monitored in optical networks since the BER is directly related to the signal OSNR Furthermore, it also plays a pivotal role in fault diagnosis and as a measure of general health of links in an optical network.

 

 

 

 

 

 

 

 

 

OSNR may change due to signal power changes or higher repeater noise levels due to aging.

 

Now lets read the Legacy method to measure it:-

 

The traditional method to measure OSNR is defined in the IEC 61280-2-9 standard and is known as the interpolation or out-of-band method, as shown in Figure-1 below

 

OSNR for a Point-to-Point Link

 

 NF_stage is the noise figure of the stage, h is Plank's constant (6.6260 × 10^-34), ν is the optical frequency 193 THz, and Δf is the bandwidth that measures the NF (it is usually 0.1 nm).

 

OSNR =158.9+ Pin.dBm −NF−10log(Br )

where OSNRdB = optical signal to noise ratio of the optical amplifier, dB

Pin.dBm = average amplifier input signal power (DWDM systems use single-channel power), dBm

NF = amplifier noise figure, dB

h = Planck's constant 6.626069 ◊ 10−34, Js

f = signal center frequency, Hz

Br = optical measurement bandwidth RBW, Hz

If the measurement optical bandwidth can be assumed to be 0.1 nm (12.48 GHz),

OSNR = 58 + Pin.dBm − NF

OSNRF.dB =158.9+Psource.dBm −Γ−NF−10log(Br)−10logN

where OSNRF.dB = final OSNR seen at the receiver, dB
Psource.dBm = average source signal power into the first span

(DWDM systems use single-channel power), dBm NF = amplifier noise figure, the same for all EDFAs, dB Br = optical measurement bandwidth, Hz
N = number of amplifiers in the fiber link excluding the booster
Γ = span loss, the same for all spans, dB

 

Above equation provides the actual mathematical calculation of OSNR. This calculation method has quite a few approximations in which we can still find the system OSNR to a great degree of accuracy. In a multichannel WDM system, the design should consider OSNR for the worst channel (the one that has the worst impairment). The worst channel is generally the first or last channel in the spectrum.

we can see that the EDFA gain factor G is not considered. That is because OSNR is a ratio, and the gain acts equally on signal and noise, canceling the gain factor in the numerator and denominator. In other words, although EDFAs alleviate the upper bound on transmission length due to attenuation, by cascading EDFAs in a series, the OSNR is continuously degraded with transmission length and ASE (from EDFAs). This degradation can be lessened somewhat by distributed Raman amplifiers (DRAs).

 

Now see what bliss Raman Amplifier can do here:-

 

Addition of Raman and OSNR change:-

 

 

 

As we can see from above equation the factor G_RA in the numerator actually enhances the OSNR of the system.(stages) could be considered as EDFA hops here.

 

OSNR-based design essentially means whether the OSNR at the final stage (at the receiver) is in conformity with the OSNR that is desired to achieve the required BER. This also guarantees the BER requirement that is essential for generating revenue.

 

 

 

This method works well for networks up to 10G, without any Reconfigurable Optical Add-Drop Multiplexers (ROADM).

 

But traditional way of measurement don’t work anymore in High Speed Communications:-

 

However, IEC 61280-2-9 isn’t feasible for 100G+ signals as well as ROADM networks.

Figure 2 illustrates 100G channels spaced 50GHz apart, which is a common spacing in modern submarine (and terrestrial) networks. Polarization-Multiplexed (Pol-Mux) 100G+ signals are typically wider (require more optical spectrum) than legacy On-Off-Keyed (OOK) 10G signals, meaning they could overlap with neighboring channels. Accordingly, the midpoint between channels no longer consists only of noise, but rather of signal plus noise. Thus, the IEC method applied to 100G+ Pol-Mux signals will therefore lead to an overestimation of noise and inaccurate measurement date leading to incorrect decisions.

 

Figure 2: IEC 61280-2-9 Method Fails with Dense Pol-Mux 100G+ signals

Figure 3 illustrates a 100G signal that has gone through a ROADM, with the green area showing the channel bandwidth. Given filters inside a ROADM, the noise at the midpoint between channels will be carved (or filtered), leading to an underestimation of the noise level, if the IEC 61280-2-9 method is used, meaning this method is not feasible in ROADM-enabled coherent submarine networks.

Figure 3: IEC 61280-2-9 Method Fails in ROADM 100G+ Pol-Mux Networks

 

To address the issues described in Figures 2 and 3, in-band OSNR was introduced around 2009 to support OSNR measurements of 10G signals in ROADM networks and 40G OOK signals. However, this method can’t be applied to coherent, Pol-Mux 100G+ signals, because of technical reasons beyond the scope of this blog. Consequently, Pol-Mux OSNR techniques have been introduced to support 100G+ signals, which is the topic of the latest standards.

 

Appropriate standards for Pol-Mux OSNR measurements?

 

There are two standards providing relevant guidelines for OSNR measurements of Pol-Mux signals. They are the China Communications Standards Association (CCSA) YD/T 2147-2010 standard and the IEC 61282-12 standards, which was recently introduced in February 2016. Both standards provide a future-proof definition of OSNR, which can be applied to any type of signal, at any data rate, including super-channels and flexible-grid signals. Specifically, the IEC-61282-12 standard specifies that:

 

where:

• s(λ) is the time-averaged signal spectral power density, not including ASE, expressed in W/nm
• ρ(λ) is the ASE spectral power density, independent of polarization, expressed in W/nm
• Br is the reference bandwidth expressed in nm (usually 0.1nm if not otherwise stated)
• and the integration range in nm from λ1to λ2is chosen to include the total signal spectrum
•  

The only drawback of these two standards is that a careful application of their formulae requires turning off channels, to access the Amplified Spontaneous Emission (ASE) noise floor, which isn’t possible on an in-service lest we upset end-users! Fortunately, in-service Pol-Mux OSNR methods have been introduced.

below table summarizes the correct OSNR method for each type of signal.

   

Data Rate	ROADM Present?	OSNR Method	Works on in-service network?
≤10G signals	No	IEC 61280-2-9	Yes
≤10G signals	Yes	In-band OSNR	Yes
Non-coherent 40G signals	Yes or No	In-band OSNR	Yes
Coherent 100G+ signals	Yes or No	Pol-Mux OSNR (IEC and CCSA standards)	No
Coherent 100G+ Signals	Yes or No	In-Service Pol-Mux OSNR	Yes

 

Table 1: OSNR Measurement Methods for Various Signal Types

 

Using the wrong OSNR measurement method for a given signal can have a significant impact on results, as it can lead to errors ranging from a few dBs up to 10dB, potentially leading to future outages. Using the proper method guarantees the right OSNR measurements are achieved leading to accurate network modeling, link simulation, and maintenance of ongoing submarine cable network performance.

 

 

Summarizing the importance of OSNR and its proper measurement method?

Some of the benefits of OSNR testing, including avoiding network outages, optimizing troubleshooting times, and ensuring optimal terrestrial and submarine cable performance. OSNR will become even more critical at data rates beyond 100G, because of the more stringent OSNR thresholds that will be required. Several OSNR methods have been introduced over the years, so the key takeaway is that the right OSNR measurement method must be used on a specific signal type to get accurate results.

 

 

 

Introduction to OSNR for high speed communication

 

The OSNR is the signal-to-noise ratio (SNR) measured in a reference optical bandwidth, where frequently a bandwidth Bref of 12.5 GHz is used corresponding to 0.1 nm wavelength. The OSNR relates to the Es ∕N0 and Eb ∕N0 as

 

where Bref is the previously introduced reference bandwidth, RS corresponds to the symbol rate of the transmission, r is the aforementioned rate of the code with r = k∕n, and qcorresponds to the number of bits mapped to each modulation symbol.

The use of high-order modulation formats requires a higher optical signal-to-noise ratio (OSNR), which may result in a significantly reduced achievable transmission distance .Following diagram shows the OSNR estimation stage for High Speed Optical Communication.

 

Lets talk about OSNR Penalty now:-

OSNR penalty is obtained from the BER curves and determined at a particular BER. A value of the OSNR penalty is obtained by comparing the values of OSNR before and after  the change of the parameters, which are under test, as given by

   

 

Gripple = penalty due to DWDM amplifier gain ripples

OSNRpenalties = various transmission penalties due to CD, PMD, PDL, etc. (note these penalties maybe different for 100G vs. 400G)

 

 

Calculation of Q-Factor from OSNR

The OSNR is the most important parameter that is associated with a given optical signal. It is a measurable (practical) quantity for a given network, and it can be calculated from the given system parameters. The following sections show you how to calculate OSNR. This section discusses the relationship of OSNR to the Q-factor.

The logarithmic value of Q (in dB) is related to the OSNR 

In the equation, B₀ is the optical bandwidth of the end device (photodetector) and B_c is the electrical bandwidth of the receiver filter.

Q is somewhat proportional to the OSNR

 

 

BER FORMULAS FOR THE MOST COMMON QAM SYSTEMS

Gray coding is assumed for all formats. For PM-BPSK the exact formula is:

For PM-QPSK the exact formula is:

For PM-16QAM and PM-64QAM, respectively, the following formulas are approx- imate, but their accuracy is better than ±0.05 dB of OSNRNL over the range

10−1– 10−4:

 

EDFA Noise – Why Input Power Matters

 

 

 

 

 

Optical signal suffers more than only attenuation. In amplitude, spectrally, temporally signal interaction with light- matter, light- light, light-matter-light leads to other signal disturbances

such as :-

•  Power reduction
•  Dispersion
•  Polarization
• Unbalanced amplification

Thus leading to random noise, which causes misalignments, jitter and other disturbances resulting in erroneous bits, the rate of which is known as bit-error-rate

 

 

Because of all possible influences outlined bits transmitted by source and bits arriving at the receiver may not have the same value. In actuality a threshold value is set at the receiver, above the threshold refers to a logic “one” and below threshold refers to a logic “zero”.

 

In order to measure BER in photonic regime, the optical signal is converted to electrical signal.

Example: Assuming a confidence level of 99%, BER threshold set at 10-10 and a bit rate of 2.5 Gb/s the required number n is 6.64 x 1010

 

Given the OSNR, the empirical formula to calculate BER for single fiber is

Log10 (BER) = 10.7-1.45 (OSNR)

 

 

 

In an experimental environment where factors such as loss, dispersion, and non-linear effects are excluded, if the OSNR is less than the specified threshold, the pre-FEC BER will be excessively large and uncorrectable bit errors will be generated. The OSNR  threshold in this case is called B2B OSNR tolerance.

 

 

 

Some mathematical aspects of OSNR:-

 

Calculating OSNR:

As can be seen from the definitions above, two quantities must be known to compute OSNR: the Total Signal Power, and the amount of ASE Noise Power present in a 0.1nm bandwidth.

 

Measuring the ASE power:

When the ASE noise floor is clearly visible left and right of the optical signal, the ASE Noise Power at the signal wavelength can be interpolated from two measurements made left and right of the signal.

 

Alternately, when the noise floor is not visible left and right, the optical signal needs to be removed temporarily in order to allow the measurement of the ASE Noise Power at the signal wavelength.

▪ This is the case when some filtering devices implemented somewhere in line are removing some of the noise between channels (Example = WSS).

▪ This could also be the case if the modulated signal bandwidth is so large so that the tail of adjacent signals overlaps the open space between them – masking the noise floor.

 

 

Note that power values are frequently provided in dBm by the OSA - whether the measurements is made using integrated power function or a user-specified resolution bandwidth.  To convert the values in dBm to mW, the following relation must be used:

 

 

Measuring the Total Signal Power:

When making this measurement it is important to use a bandwidth that is large enough to capture the entire signal:

▪ If using an OSA with variable resolution bandwidth, this means that the resolution bandwidth has to be set larger than the width of the signal.

▪ If using a integrated- power function between vertical markers, the markers have to be set to include the entire signal bandwidth.  

 

Note that when measuring a DWDM spectrum, the power of each DWDM signal cannot be measured independent of ASE noise present in the measurement bandwidth.

 Ie., the value that is actually measured is: Total Signal Power + ASE Noise Power.

To get the Total Signal Power only, the ASE noise content (measured separately in the previous step) must be subtracted from the measurement. Since the measurement bandwidth used to measure the noise on its own (ASE BW) may be different from the bandwidth used to measure the signal (Signal BW), a factor is added to the equation.  This removes the correct amount of noise from the measurement.

 

where:

Signal BW is the bandwidth used to measured the signal,

ASE BW is the bandwidth used to measure the ASE Noise Power only (e.g., 0.1nm).

 

Calculating OSNR from measurements:

Since OSNR must be reported as signal power with respect to 0.1nm worth of ASE noise, the denominator of the OSNR equation also includes a factor.  This adjusts the amount of ASE noise measured to an amount expected inside a 0.1nm bandwidth:

 

Thus

 

In a transmission chain, the relative evolution of the optical signal and noise levels is usually characterized by the optical signal-to-noise ratio (OSNR). The OSNR, in a given optical bandwidth, is defined as

 

 

The optical signal may be polarized, but the noise is usually not and, depending on the receiver polarization sensitivity, the noise must be considered with a single or two polarizations. Any active (or passive) linear optical device amplifies (or attenuates) simultaneously the incoming signal and the incoming noise

 

 

Accordingly with the fluctuation dissipation theorem, it also adds noise, making its output OSNR lower than its input one. This OSNR degradation is expressed in terms of noise factor (NF) defined as:

 

 

 

where G is the optical power gain (or the attenuation coefficient) of the device which is larger (or smaller) than 1. NF is denoted noise figure when it is expressed in dB. Because the relative importance of the added noise strongly depends on the input noise level, NF is only an intrinsic parameter of the optical device (i.e. independent of the input signal and noise) when an input reference noise is defined.

For polarization insensitive devices, input noise and output noise usually do depend on polarization, making noise figure independent of polarization considerations.

 

Single amplifier noise factor

OSNR at the output of an optical amplifier with an output power Pout and for an optical bandwidth B0 is expressed as:

 

where m=1 or 2 is the number of polarization modes contributing to noise. It is usual to consider the two polarization modes (m=2) of the noise and to make reference to an optical bandwidth equal to 0.1 nm corresponding to Bo = 12.5GHz at a wavelength of 1550 nm. In this particular situation, the OSNR is expressed in dB as:

 

 

 

Noise factor of a cascade of fibers and amplifiers

Let us consider now the cascade of spans displayed in Figure 3.8 and including NAMP11 optical fiber spans with an attenuation coefficient A5expð2αALÞ and NAMP lumped linear phase insensitive amplifiers with a gain net G and a noise factor NF. For each span a near compensation of the signal attenuation by the gain is assumed, making its net gain GSPAN 5 GA close to 1. Each fiber reduces the sig- nal level and reduces the noise level nearly in the same way. Since the input noise level is far above the appropriated reference level, the attenuation noise of the fiber is negligible and the OSNR is kept nearly unchanged. Each amplifier output POUT restores the input noise of the link but adds the amplifier noise.

 

 

The accumulated noise is Namp times larger than single amplifier and we have:

 

 

 

 

To bridge transoceanic distances while keeping a high OSNR (optical SNR), it is crucial to limit the noise contribution added by the successive amplifiers. The impact of the added noise on the output OSNR can be calculated with:

 

 

This equation can also be expressed in a more physical manner:

 

where N is the amplifier count, and Δλ the width of the filter where the OSNR is expressed. The number “30” at the end of Equation corresponds to the conversion of signal input power from units in watts into units scaled in milliwatts.

 

 

 

 

Note: Now try to utilise the above concepts and equations whatever way you want .

 

 

 

 

Transponder bandwidth is the product of Modulation , Baud Rate and the Polarisation.

BW=Modulation x Baud x Polarisation 

Following table will give an idea for various bit rates:-

 

Ex:

Modulation = 2 (bits/s/Hz)

Baud Rate = 32G

Polarisation= 2

 

BW= 2 x 32 x 2 =128Gbps

 

 

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 Introduction

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

 

Spectral Hole Burning (SHB)

Spectral hole burning (SHB) is a major limitation of amplified WDM systems with high channel count. The main reason lies in the fact that there is no possibility of compensating for this effect.

• Due to the inhomogeneous portion of the linewidth broadening of the dopant ions, the gain spectrum has an inhomogeneous component and gain saturation occurs, to a small extent, in an inhomogeneous manner. This effect is known as spectral hole burning because a high power signal at one wavelength can 'burn' a hole in the gain for wavelengths close to that signal by saturation of the inhomogeneously broadened ions. Spectral holes vary in width depending on the characteristics of the optical fiber in question and the power of the burning signal, but are typically less than 1 nm at the short wavelength end of the C-band, and a few nm at the long wavelength end of the C-band. The depth of the holes are very small, though, making it difficult to observe in practice.

 

• In addition, accurate predictions are very difficult to carry out. SHB acts as a selective oversaturation of specific erbium ion classes due to a precise matching of the signal wavelength with their corresponding Stark energy sublevels. Gain contributions of a given ion class to the overall amplifier gain spectrum will be dependent on the specific values of energy of the related Stark sublevel (determined by inhomogeneities in the local electric field in the glass as opposed to on the crystal) and of their population density (i.e. of the related induced saturation). Clearly, the overall gain spectrum of the amplifier may be distorted due to this SHB effect. The best-known induced distortion is the hole induced in the gain spectrum in the spectral vicinity of a saturated channel.

• This gives rise to a hole in the gain profile around the saturating channel wavelength, whose width is determined by temperature. Increasing temperature will increase this homogeneous broadening (and thus the hole width at the expense of its depth) while lower temperatures will reduce and make this hole deeper in the gain profile. . Since it is not possible to operate the amplifier at a lower temperature where the effect of homogeneous broadening vanishes, the system designer should account for the holes induced by each signal channel in the amplifier gain profile at room temperature

 

• SHB does not distort the overall gain profile because the sum of the different contributions has a flat transfer function. Problems may be encountered when some channel powers increase compared to other channels

• SHB could be seen (wrongly) at first glance as a regulating effect because the most favored channels will see a slightly lower gain due to the SHB they induce. This will indeed slightly reduces the power excursion between channels (the correcting effect being, however, much lower than the effect creating this SHB). The detrimental effect actually comes from the distortions induced in the amplifier gain spectrum due to thermal broadening. Other channels, located a few nanometers aside from the most favored ones will also see an induced reduced gain level, while such channels may not be gain favored like the channels that create the SHB effect. This will result in a decrease in the OSNR of such neighboring channels

• The SHB effect not only stresses the dynamic range of the system by increasing the burst power but it also degrades the OSNR performance. In particular, whilst the power change is limited to the beginning of the burst (during the formation of the hole), the OSNR impairment is observed at the end of burst,where the spectral holeis already completely formed by the burst.

SHB also has a limiting effect in the implementation of preemphasis of the less-favored channels. This technique consists of increasing the power of the worst channels at the transmitter side, at the expense of the best channels, leading to the same OSNR for all channels at the link output. This can be performed while keeping the EDFA output powers constant and decreasing the transmitted power of the best channels. However, the highest predistortion that can be performed at the link input in order to compensate for a given excursion in output OSNR is limited by SHB.

 

 

REF::Undersea Fiber Communication Systems by Jose Chesnoy

Here the results are after evaluating the effect of a thermal variation on the output tilt. In this particular set-up the amplifier is kept at room temperature (25 °C) and only the active fiber spool undergo a thermal cycle. Temperature of the EDFA is varied from 0 °C to 65 °C and the amplifier gain is measured at four point : 0, 25, 40 and 65 °C.

 

 

As can be seen a 65 °C temperature variation implies a 1.8 dB tilt variation. Considering a reduced temperature range (5-45 °C) the output tilt variation is about 1.1 dB.

It was  tried to investigate the origin of the temperature dependency. First  used a different EDFA with a lower erbium concentration (14 dB/m erbium peak absorption); then tried to reduce the saturation of the EDFA using lower power levels, but in both cases the output tilt variation was very similar to that of Figure .

Temperature variation has also effect on the EDFA efficiency: with high temperature the active fiber is less efficient than at low temperature.

With constant pumps power, a 65°C variation implies a 0.25 dB difference on the output power. To compensate this extra tilt we can act in two way: using the VOA ; heating the EDFA to a constant 65 °C.

First solution requires a thermal sensor to measure the EDFA temperature and a compensation table (stored in the firmware) to act on VOA attenuation.

Second solution requires a heater and special mechanics & software to store the EDFA spool and to keep their temperature constant.

 

 

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