Amazon Web Services (AWS) is a subsidiary of Amazon providing on-demand cloud computing platforms and APIs to individuals, companies, and governments on a metered pay-as-you-go basis. Read more on https://aws.amazon.com/

Amazon keeps on hiring all round the year as they are continuously striving to look for candidates with great leadership and technical skills. The intent of this article to enable candidates to understand the process better and prepare themselves accordingly.

There are multiple ways of finding a job at AWS, and a few of my recommendations are:-

Amazon Job Portal: This job portal is the best way to explore jobs according to your skills. Just search whatever kind of job and at what locations you are looking for, and you can see the multiple posted jobs that you are always free to apply for. Some FAQs

Approach for referral: The second best way is to look for your former colleagues or acquaintance on Linkedin Contacts who can refer you for the correct job. Ask them to refer you, providing them with the Job ID mentioned on the application. This process helps you in getting some insights into the profile and the job nature.

Once your application is reviewed and found successful, somebody from HR will approach you regarding the job and ask you for your interview availability. They may ask few dates which suit your availability. Candidate is free to provide available date as per their choice.

Once the date is finalized, HR will confirm the next step, usually Phone Screen/Phone Interview.

I will explain here about Network Development Professionals. We use NDE(Network Development Engineers) as technical roles term. I will collect most of the information which is available in the public domain. This article is for experienced candidates with experience in relevant roles for some time.

Interviewing Steps :

Phone Screen/Phone Interview:

This will be a 45-60 min interview on the telephone. At scheduled time, candidate needs to join a chime bridge and you probably need to download Amazon Chime, our video-conferencing tool (the step-by-step guide can be found here). If you’re presenting, you will need to download Chime onto your desktop. The meeting ID# will be emailed to you by your recruitment point of contact. For optimal sound quality, use a headset with a microphone or with the details already provided in the email from HR.

Personal Recommendations:
-Visit Amazon Leadership Principles and go through each and recall if you have countered a situation either in your past roles or in your current role that depicts the particular leadership skills. The more the better it will lead you closer to Amazon.

– I will recommend choosing good technical experiences if you are applying for technical roles as it will be easy for both candidate and interviewer to probe more and conclude the skill.

AWS Recommendations:-

Our interviews are based on behavioural questions which ask about past situations or challenges you’ve faced and how you handled them, using the Leadership Principles to guide the discussion. We don’t use brain-teasers (e.g., “How many windows are in Manhattan?”) during the interview process. We’ve researched this approach and have found that those types of questions are unreliable when it comes to predicting a candidate’s success at Amazon. Here are some examples of behavioural questions:

• Tell me about a time when you were faced with a problem that had a number of possible solutions. What was the problem and how did you determine the course of action? What was the outcome of that choice?
• When have you ever taken a risk, made a mistake or failed? How did you respond and how did you learn from that experience?
• Describe a time when you took the lead on a project
• What did you do when you needed to motivate a group of individuals or encourage collaboration during a particular project?
• How have you leveraged data to develop a strategy?
• Keep in mind, Amazon is a data-driven company. When you answer questions, you should focus on the question asked, ensure your answer is well-structured and provide examples using metrics or data if applicable. Refer to recent situations whenever possible.

You should bear in mind that your interviewers won’t be evaluating your ability to memorise all the details of each of these topics. They will be analysing your ability to apply what you know and solve problems efficiently and effectively. Given that you sometimes have only limited time to prepare for a technical interview, I will recommend to go through the basic fundamentals with your technological stuffs .It is always recommended to looks for the lasted and most promising technology that are well proven in the technical community or different forums. This will likely yield the best results in the available time.

Other Tips for a smooth Phone interview:

Some teams at Amazon incorporate role-specific exercises or online assessments into the interview process.

• You will be notified if you are required to take a test.
• For your phone interview, find a quiet and comfortable place with no distractions.
• Use a computer with a reliable connection and access to email.
• If you’re using a mobile phone, make sure you’re in a place with good signal.
• Have a copy of your CV on hand.
• Have paper and a pen readily available.
• Come prepared with questions that you would like to explore in more detail (e.g. on initiatives/projects, the team culture, the scope of the role).
• If you need clarification on anything or have any additional follow-up questions, reach out to your recruiting point of contact.
• If you have any special requirements, questions or concerns, please reach out to us as we are committed to making reasonable provisions for all individuals.

Virtual interviews:

If you’ve been asked to do a virtual interview, you will probably need to download Amazon Chime, our video-conferencing tool (the step-by-step guide can be found here). If you’re presenting, you will need to download Chime onto your desktop. The meeting ID# will be emailed to you by your recruitment point of contact. For optimal sound quality, use a headset with a microphone

What to Expect after Phone Screen/Phone Interview?

-Amazon HR will get back to you within two business days after your phone interview. If you haven’t heard from HR by then, feel free to give them a nudge via email.

Outcome:

–Next interview scheduling because you definitely cleared Phone Interview. Expect a call or email from HR.
–Next Phone Screen Scheduling as First Phone interview was not sufficient enough to collect data about you or due to short of time(sometimes this happens too). Second Interview Procedure is exactly same as the first Phone Interview. Expect a call or email from HR.
–Your application is not successful during this time email from HR. You are free to ask for some feedback and HR can provide some over phone maybe.

Once you are eligible for the next interview which is called On-site interview;HR will reach you again for scheduling your interviews and will send you a questionnaire with few questions . Major is Current salary (include base, bonus and equity) and Expected salary (include base, bonus and equity).Do your research and let them know the answers.

You can take your time and reply to HR with your availability. HR will send confirmation email for the interview along with the interviewers name(most of the time). Seeing current COVID-19 scenarios or sometime based on your geographical location or availability of Amazon office near your location you can opt for Video conference/In person visit to nearly Amazon office.This is generally faster else you need to wait for visa if in different country or travel arrangements.Everything is up to your choice and best available method combined.

Personal recommendation:
If you can manage a good internet speed ,do on video conference from home i.e your most comfort zone is best.

If internet speed is not reliable ,approaching nearest Amazon office is best choice.

Last should be the travel option. If job location is nearer , nothing can beat than sharing human to human interaction live.

Structure of Onsite-Interview:
Mostly Five rounds of interview schedule (45-60min each) called Loop.

Manager’s Interview-1
Members from your Team or cross team Interview -3
Bar raiser interview-1

If you are visiting the interview location there will be a lunch buddy assigned to you who will go on a quick lunch between interview with you .

Tips for the interview.

Exactly same tips as for Phone interview but expect yourself for being more patient here and you may need to sit for long hours.
Sometime it takes multiple days for the interview to complete, specially during global vacation time because of less availability of interviewers. In this case 5 interviews may get fragment to 2-3 interviews/day .

Go through Leadership principles
We use our Leadership Principles every day, whether we’re discussing ideas for new projects or finding the most effective solution to a problem. It’s just one of the things that makes Amazon peculiar. All candidates are evaluated based on our Leadership Principles. The best way to prepare for your interview is to think about how you’ve applied the Leadership Principles in your previous professional experience.

Prepare for Behavioural Interviewing
-Our interviews are based on behavioural questions which ask about past situations or challenges you’ve faced and how you handled them, using the Leadership Principles to guide the discussion. We don’t use brain-teasers (e.g., “How many windows are in Manhattan?”) during the interview process. We’ve researched this approach and have found that those types of questions are unreliable when it comes to predicting a candidate’s success at Amazon.

Here are some examples of behavioural questions:

• Tell me about a time when you were faced with a problem that had a number of possible solutions. What was the problem and how did you determine the course of action? What was the outcome of that choice?
• When have you ever taken a risk, made a mistake or failed? How did you respond and how did you learn from that experience?
• Describe a time when you took the lead on a project
• What did you do when you needed to motivate a group of individuals or encourage collaboration during a particular project?
• How have you leveraged data to develop a strategy?

Keep in mind, Amazon is a data-driven company. When you answer questions, you should focus on the question asked, ensure your answer is well-structured and provide examples using metrics or data if applicable. Refer to recent situations whenever possible.

Use best proven STAR answering format:

The STAR method is a structured manner of responding to a behavioural interview question by discussing the specific situation, task, action and result of what you’re describing. Here’s what it looks like:

SITUATION

Describe the situation that you were in or the task that you needed to accomplish. Give enough detail for the interviewer to understand the complexities of the situation. This example can be from a previous job, school project, volunteer activity or any relevant event.

TASK

What goal were you working towards?

ACTION

Describe the actions you took to address the situation with an appropriate amount of detail and keep the focus on you. What specific steps did you take? What was your particular contribution? Be careful that you don’t describe what the team or group did when talking about a project. Let us know what you actually did. Use the word “I,” not “we,” when describing actions.

RESULT

Describe the outcome of your actions and don’t be shy about taking credit for your behaviour. What happened? How did the event end? What did you accomplish? What did you learn? Provide examples using metrics or data if applicable.

Consider your own successes and failures in relation to the leadership principles. Have specific examples that showcase your expertise and demonstrate how you’ve taken risks, succeeded, failed and grown over the course of your career. Bear in mind that some of Amazon’s most successful programmes have risen from the ashes of failed projects. Failure is a necessary part of innovation. It’s not optional. We understand that and believe in failing early and iterating until we get it right.

Tips for great answers
Practise using the STAR method to answer the behavioural interview questions listed above and incorporate the Amazon Leadership Principles

Ensure each answer has a beginning, middle and end. Describe the situation or problem, the actions you took and the outcome

Prepare short descriptions of a handful of different situations and be ready to answer follow-up questions in greater detail. Select examples that highlight your unique skills

Give specific examples that showcase your experience and demonstrate that you’ve taken risks, succeeded, failed and grown over the course of your career

Specifics are key: avoid generalisations. Give a detailed account of one situation for each question you answer and use data or metrics to support your example

Be forthcoming and straightforward. Don’t embellish or omit parts of the story.

Personal recommendation for technical Interviewing:

I will recommend to choose good technical experiences if you are applying for technical roles as it will be easy for both candidate and interviewer to probe more and conclude the skill.

e.g for Optical Engineers

-Prepare and revise DWDM fundamentals.
-Network Architecture and designs.
-How to solve problems through Automation skills.You can read more on my article https://mapyourtech.com/automation-in-optical-networkingif-you-are-reading-this-you-can-do-this/

-Read about latest CFPs/QSFPs/ZR etc working and fundamentals.
-Read about best problem solving techniques for issues in Network.
-Read about key performance metrices/indicators that can help you solve a problem in network.
-Read about how to collaborate with other technologies or cross platforms teams/devices to isolate and fix the problems.
-Recording solutions and creating MOP(Method of Procedure/Process) to fix issue in future.
-Think about innovative solutions that can help manage network smoothly.

Tips before you head in
Be prepared to explain what interests you about the role you’re being interviewed for and the team (or teams) you’ll be meeting with

When answering questions, be concise but detailed. We realise it’s hard to gauge how much information is too much versus not enough. An effective test is pausing after your succinct response to ask if you’ve provided enough detail or if the interviewer would like you to go into more depth

Follow-up if you need clarification. If you’re asked a question, but are not given enough information to provide a solid answer, don’t be shy about asking for more information. If additional context is not available, focus on how you would attempt to solve the problem with only limited information

For some roles, we may ask you to complete a writing sample. Why? At Amazon, we don’t use PowerPoint or any other slide-oriented presentations. Instead we write narratively structured memos and silently read one at the beginning of each meeting. These documents generally range from one to six pages and articulate the project goal(s), approach to addressing it, outcome and next steps. Given this unique aspect of our culture and the impact these papers have on what decisions we make as a company, being able to articulate your thoughts in written form is a necessary skill

We aim to hire intelligent, thoughtful and customer-obsessed people. Reflect on what motivated you to pursue a career with Amazon and be prepared to share your thought process. Although “Why Amazon?” is a standard question, it’s not just a formality for us. We genuinely want to understand what inspired you to explore an opportunity with us so we get a better sense of who you are

We try to leave a few minutes at the end of each interview to answer questions you might have, but if we don’t get to all of them, please don’t hesitate to ask your recruitment point of contact.

Interview day
Check-in: arrive 15 minutes early to check in for your interview. Have your government-issued photo ID ready (e.g. driving licence, passport)

Location: detailed instructions will be sent to you via email. Some of our offices are dog-friendly. Let us know if you have any special requirements or allergies

Dress code: comfortable and casual. While safety clothing – such as closed-toed shoes – is required for some positions in our fulfilment centres, most of our office staff wear everyday clothing. We’re interested in what you have to say, not what you’re wearing

What to expect: interviews will be a mixture of questions and discussions concerning your previous experience and the challenges you’ve encountered. Come armed with detailed examples — concise, structured answers are ideal

Interviewers: depending on the role, you will meet with anywhere from two to seven Amazonians. They will likely be a mix of managers, team members, key stakeholders from related teams and a “Bar Raiser” (usually an objective interviewer from another team). All interviewers will assess potential for growth beyond the position you’re being interviewed for and focus on evaluating how well your background and skills meet core competencies, along with how they relate to Amazon’s Leadership Principles. We recommend approaching each of your interviews the same way rather than trying to tailor answers to the interviewer’s role. Interviewers will often be taking notes on their laptops. It’s important that they have precise notes of their time with you to share with other interviewers

CV: interviewers will have a copy, but feel free to bring one as well

Duration: each interview usually lasts from 45 minutes to an hour

Lunch: We will provide lunch if your interview is scheduled during the lunch hour. Let recruitment or your lunch buddy know if you have any dietary preferences

Amazon Non-Disclosure Agreement: All candidates must sign our standard Non-Disclosure Agreement. If you’re unable to print and sign prior to your arrival, we’ll have a copy available for you

Technical roles: If you’re being interviewed for a technical role, be prepared to use a whiteboard

Virtual interviews: If you’ve been asked to do a virtual interview, you will probably need to download Amazon Chime, our video-conferencing tool (step-by-step guide here). If you’re presenting, you will need to download Chime onto your desktop. The meeting ID# will be emailed to you by your recruitment point of contact. For optimal sound quality, use a headset with a microphone.

Before and after the interview
Some teams at Amazon incorporate role-specific exercises or online assessments into the interview process. You will be notified if you are required to take a test

Confirm or book arrangements if your interview requires travel. Your recruiting point of contact will either set up your travel arrangements or put you in touch with our travel agency to help you coordinate travel details and hotel stay.

An expense report should be submitted after your visit. Your recruitment point of contact will provide details on where and how to submit your report. Fill in your form clearly and ensure scanned receipts are legible – this will help prevent reimbursement delays

If you have any special requirements, questions or concerns, please contact us: we are committed to making reasonable provisions for all individuals

After your interview, be on the lookout for a quick post-interview survey via email. It is important for us to know how we did so we can continually improve our interview process. We really value your input

Expect to hear back from recruitment within five business days following your interview. If you don’t, feel free to give us a nudge.

Outcome:

Successful :
Great you have made it. Lot of Amazonians are looking forward to meet you to learn and share the best with you and from you.

Next Time:

Dont worry, you can still apply for different opportunities and its good that you got the first hand experience of interview and next time you can prepare yourself in a better way. Remember that the decision is taken in consideration with multiple parameters like current available candidates skill sets in same role ,current available racks that meets your skill sets, your future growth aspects etc.

Offer

HR will give call to explain the offer.Based on your experience, job level and skill sets there will be an competitive compensation offered to you.There is always a scope for negotiations if the explanations are convincing and agreed between candidate and the HR.

Offer consists of base salary +RSU stocks+joining bonus and other relocation benefits.It is recommended to know your worth and expectation and for that candidate can refer to online websites/different forums etc.

Recommended sites for more exploration:

https://www.levels.fyi/?compare=Amazon,Google,Facebook,Microsoft&track=Software%20Engineer

https://www.glassdoor.com/Interview/Amazon-Interview-Questions-E6036.htm?countryRedirect=true

https://dev.to/fallenstedt/three-steps-i-took-to-get-a-job-offer-from-amazon-1p23

https://www.zippia.com/advice/amazon-interview-questiond/

https://www.quora.com/How-long-does-it-take-Amazon-to-make-a-job-offer

https://fearlesssalarynegotiation.com/amazon-salary-negotiation/

https://6figr.com/in/salary/amazon

The pump wavelength used is either 980nm or 1480 nm due to the availability of these laser sources.  In the course of the explanation let see the energy level diagrams for the (Erbium) Er³⁺ ion, the absorption band of Er+, and the pump efficiency.

(a) Energy levels of erbium ions and (b) gain and attenuation spectra.

Excited State Absorption (ESA)

Ground-state absorption (GSA)

There are several states to which the erbium ions can be pumped using sources of different wavelengths, such as those operating at 1480, 980, and 800 nm. However, in practical optical systems, the pump wavelength must provide a high power to achieve a high gain per pump.

The commonly available laser diodes can operate at 800, 980, and 1480 nm, but the pump efficiency can go more than 1 dB/mW with low attenuation depending on the pump frequency.

The only pump wavelength laser sources that can give a high pumping efficiency with lower attenuations are those operating at 980 and 1480 nm.

In practice, the 980 nm pumping source is commonly used due to its high gain coefficient (4 dB/mW). The difference in the effects of these two wavelength sources is mainly caused by the absorption and emission factors.

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 and covers 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.
• OSNR is a metric for the quality assessment of received signals that are corrupted by the  ASE noise of EDFAs  OSNR is defined as the ratio of the average optical signal power to the  average optical noise power  For a single EDFA with the output power Pout and the noise  power NASE, OSNR is computed as

where
NF is the noise figure G is the amplifier gain
hf is the photon energy Δf is the optical measurement bandwidth

When addressing an OSNR value, it is important to define an optical reference bandwidth  for the calculation of OSNR  A bandwidth Δf of 12 5 GHz (or Δλ = 0 1 nm) is the typical  reference bandwidth for calculating OSNR values 

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 −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).

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.

Below context is taken from article published by Jean-Sébastien

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.

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 mentioned rate of the code with r = k∕n, and q corresponds 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 approximate, but their accuracy is better than ±0.05 dB of OSNRNL over the range 10−1 and 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)

Some mathematical aspects of OSNR:-

Assume that OSNR = 14.5 dB
ThenLog10 (BER) = 10.7-1.45 (14.5) = -10.30

Therefore BER = 10(-10.30) BER is approx 10- 10

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.

Calculating OSNR from OSA:

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:

NFdB, Noise factor of a cascade of fibers and amplifiers

Let us consider now the cascade of spans displayed in Figure  and including Namp optical fiber spans with an attenuation coefficient A 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.

Few Key Concepts to remember

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

Log10 (BER) = 10.7-1.45 (OSNR)

In DWDM links a rule of thumb would be to target an OSNR value greater than 15 dB to 18 dB at the receiver.

OSNR requirements depend on:

• Location: The required OSNR will be different for different locations in the light path. The OSNR requirement will be higher closer to the transmitter and lower closer to the receiver. This is because optical amplifiers and reconfigurable add/drop modules (ROADMs) add noise, which means that the OSNR value degrades after going through each optical amplifier or ROADM. To ensure that the OSNR value is high enough for proper detection at the receiver, the number of optical amplifiers and ROADMs needs to be considered when designing a network.
• Type of Network: For a metro network, an OSNR value of >40 dB at the transmitter might be perfectly acceptable, because there are not many amps between the transmitter and the receiver.For a submarine network, the OSNR requirements at the transmitter are much higher.
  • • • • • • • Data Rate: With the increase in the data rate for a specific modulation format, the OSNR requirement also increases.
              • Target BER: A lower target BER calls for a higher OSNR value.

The exact requirements at the receiver will vary from one manufacturer to another. Table  displays a few average OSNR figures to guarantee a BER lower than 10-8 at the receiver

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