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Understanding Optical Return Loss (ORL) in Optical Fiber system

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?

-Methods to help improve 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)

 

               4).  Optical return loss is the ratio of the output power of the light source to the total amount of back-reflected power 

(reflections and scattering). It is defined as a positive quantity.

 
PT: Output power of the light source
PAPC: Back-reflected power of APC connector

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:-

 

 


 

Methods to help improve ORL are as follows:

  1. Use ultra polish connectors that have low reflectance such as UPC type. APC type connectors have even better reflectance values but are not compatible with other non-APC connectors. Connection to a non-APC connector can damage the APC connector.

  2. Use fusion splices instead of mechanical connectors or mechanical splices where possible.

  3. Re-do fusion splices that are shown to have reflectance. A good fusion splice should have no reflectance.

  4. Install optical isolators at the laser to reduce back reflectance.

 

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-

 


 

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.

 

 

 

 

 

 

Automation in Optical Networking:If you are reading this you can do this !!!

 

 

Automation in Optical Networking

If you are reading this you can do this!!!

 

Google , Stackoverflow and your little efforts will do it.No worries!!!

 

This article is not for professional automation engineer but for the enthusiastic professionals who just want to start but  need some guidance to do .

 

This article is purely  written on the basis of my personal experience through out my career and the intention involved  is to help my friend and folks in understanding the basics and get a glimpse of automation in networking world , so that they can feel motivated and don't get scared with jargons used for automations.

 

In my terms:

***Automation is not replacing jobs but enabling you to live life more efficiently and with freedom.It is just an act of kindness by technology to give back to the its users and the creators.***

 

 

 The scale with which networking communication devices and its usage is  increasing, we need lot of network bandwidth and automation in place to operate, configure, predict  and manage it.Also to have a more robust  ,scalable and reliable network we need agnostic and low latency automations that can help grow the network .

 

 

Why automation is needed? Out of 1 million reasons few are as:-

  • It will make your life simpler and cheerful from monotonous and boring pieces of job.
  • It will give you time to think something more creative .
  • It will give you more flexibility as automation can be enabled or operable from remote places.
  • You can spend more time with your loved ones .
  • It can give you sense of security and confidence.
  • It can lead you to be a successful entrepreneurship  .
  • It improves efficiency and reduces errors that can be made by human.

 

What are the things you can automate? Out of 1 billion reasons few are as:-

  • Everything that you do is sooner or later can be potentially automated.
  • Device logins and configurations can be automated.
  • Metrics polling can be automated to fetch some values from device.
  • Network Management can be customised or scaled.
  • Encryption key rotation can be automated.
  • Capacity monitoring , planning or network design could be automated.
  • Fault Alarming can be automated.
  • Link routing and restoration can be automated.
  • Network self healing and fault remediation could be achieved.
  • Reporting and metrics evaluation could be achieved.
  • Multiple cronjobs could be done and automated.
  • Network discovery could be automated.
  • Regression steps can be automated.
  • On weekends, device/feature runs based testing tasks can be automated.

 

*This is just an idea that whatever you are doing in your routine job being and network engineer, almost everything can be automated.

 

 

Now the most important question: WHAT YOU NEED TO DO?

 

*THINK THAT YOU CAN DO IT.

We are blessed to have all the resources that we need to learn and groom ourself at this point of time which are available free of cost. So, even if you don’t know how to start and you have never done or written any automation till now in your career still you can do with just a believe that “YOU CAN DO IT”.

 

*RECALL THE DAYS WHEN YOU LEARED TO EARNED

WE  all have studied at some point of time from books ,newspapers, magazines ,videos etc and have achieved some benchmark in life.So at this point of time too with just a small efforts you can learn and enhance your skill using few of the greatest tools that are available free of cost which I personally recommend are:-

  • GOOGLE  : The best piece of technology platform  that a human kind have .(will recommend first to search “how to search in google search in a best way”
  • Stackoverflow  : Best spot to shadow programmers and learn new things.
  • w3schools.com  : Online school with playground to play.
  • Booksc  : The heaven for book lovers .free to download.

 

 

*ANY LANGUAGE WILL HELP BUT CHOOSE LATEST AND IN DEMAND

 

Automation  just needs an idea—>Steps to do it ——>ALGORITHM——> and any automation language will do it.

 

Never think that Automation is something which is so big that you can do it but always look for the simplest thing which you can automate and that’s the best way to start writing any automation.AUTOMATION is collection of SCRIPTS,TOOLS,PACKAGES,LIBRARIES organised in a efficient manner to collaborate and give some user consumable or readable data.

  • AUTOMATION LANGUAGES/TOOLS THAT CAN HELP NETWORK ENGINEERS
  • Python
  • TCL/Perl
  • HTML/JINJA/ANSIBLE
  • JAVA
  • NETCONF/YAML
  • SNMP/TL1/TELEMETRY /CLI
  • LINUX
  • TELNET/SSH
  • FTP/SFTP/WINSCP
  • KIBANA/GRFANA/DATABASE
  • PYCHARM/VISUAL STUDIO/SUBLIME TEXT/NOTEPAD++

Note: Do search in internet the terms if you are not aware so that you can learn more.

 

*AUTOMATION  INFOGRAPHICS 

 

 

Image credit:https://netyce.com/blog/lets-talk-about-network-automation-its-misunderstood/

 

 

Requesting every professional to help each other because this is the best way to evolve
 

 

OSNR: What does this mean;Why do we need and How to take care of it?

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 OSNRout 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

 

 NFstage 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 NF10log(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 1034, 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−NF10log(Br)10logN

where OSNRF.dB = final OSNR seen at the receiver, dB
P
source.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 GRA 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 2IEC 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 3IEC 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 = kn, 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 Bc 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 .

 

 

 

 

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Comments

Hi Pankaj,

Thanks for reading this article.I am glad that it is helpful.

Pt(Transmit) you can get from Transmit power from 6500 Amplifier modules.I believe for reflected power there is a photodiode which is attached by coupler on the Tx module.This Photodiode detects the reflected power and ORL calculation is done. This value is not visible in management interfaces like Site Manager but you can definitely see it via debug commands .Recommending to contact some Ciena Engineer who can guide you to get the reflected value.

This link will be helpful to visualize the context
https://www.rfwireless-world.com/Terminology/OCWR-method-vs-OTDR-method.html

Thanks

Thank you for this descriptive ORL article. It's really helpful

Just one query:
How can we identify the Pt and Pr (reflected power) in any optical system?
E.g. Ciena 6500

When you say passband, does that equal to MC, MEADIA CHANNEL

Hurrah, that's what I was searching for, what a data! present
here at this webpage, thanks admin of this web site.

Thanks for nice explanation.

Requests/Regards

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