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How and where do we get pre and post FEC BER

How and where do we get pre and post FEC BER

The first thing to note is that for each frame there are two sets of 20 parity bits. One set is associated with the end to end post FEC BER. The other is used to measure the span by span raw BER. The points at which these parity bits are terminated are illustrated below.

Processing point

Process description


Calculate and insert the post FEC parity bits (those over which FEC is calculated) over the frame up to and including the MS OH.


Encode FEC over the frame up to and including the MS OH.


Calculate and insert the pre FEC parity bits (those over which FEC is not calculated) over the frame up to and including the RS OH.


Terminate the raw BER based on the pre FEC parity bits.


Re-calculate the pre FEC parity bits over the frame up to, and including, the RS OH.


Decode FEC to produce the final data.


Terminate the post FEC BER based on the post FEC parity bits.


We can use the raw BER extracted at each RS terminating point (regens and LTEs) to estimate the post FEC BER. Note that this estimate is based on an assumption of a Poisson distribution of errors. In contrast the real post FEC BER can only be extracted at the MS terminating equipment (LTEs), and this is used to feed into the PM error counts.

Following are the terminologies you will come across when referring FEC Performance parameters:

PRE-FEC BER are the bit errors caused by attenuation, ageing, temperature changes of the optical fiber. PRE-FEC indicates that the signal on the optical fiber is FEC 
encoded. The FEC decoder will recover the original signal, but depending on the PRE_FEC BER it will succeed to recover the original signal completely without errors. 
Or, if the BER on the fiber is too high, the recovered signal will  contain bit errors. 

If the signal was FEC encoded the remaining bit errors after the decoder are called POST_FEC BER. 

The NO_FEC BER are the bit errors detected when no FEC coding is used on the optical fiber. 

Uncorrected words are the word that FEC is not able to corrects.It shows that the current FEC is not able to correct anymore and we need to look for more advance FEC.

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The maximum number of erbium-doped fiber amplifiers (EDFAs) in a fiber chain is about four to  six.



The rule is based on the following rationales:

1. About 80 km exists between each in-line EDFA, because this is the approximate distance at which the signal needs to be amplified.

2. One booster is used after the transmitter.

3. One preamplifier is used before the receiver.

4. Approximately 400 km is used before an amplified spontaneous emission (ASE) has approached the signal (resulting in a loss of optical signal-to-noise ratio [OSNR]) and regeneration needs to be used.

An EDFA amplifies all the wavelengths and modulated as well as unmodulated light. Thus, every time it is used, the noise floor from stimulated emissions rises. Since the amplification actually adds power to each band (rather than multiplying it), the signal-to-noise ratio is decreased at each amplification. EDFAs also work only on the C and L bands and are typically pumped with a 980- or 1480-nm laser to excite the erbium electrons. About 100 m of fiber is needed for a 30-dB gain, but the gain curve doesn’t have a flat distribution, so a filter is usually included to ensure equal gains across the C and L bands.

For example, assume that the modulated power was 0.5 mW, and the noise from stimulated emission was 0.01 mW. The signal-to-noise ratio is 0.5/0.01 or 50. If an EDFA adds a 0.5 mW to both the modulated signal and the noise, then the modulated signal becomes 1 mW, and the noise becomes 0.501 mW, and the SNR is reduced to 2. After many amplifications,even if the total power is high, the optical signal-to-noise ratio becomes too low. This typically occurs after four to six amplifications.

Another reason to limit the number of chained EDFAs is the nonuniform nature of the gain. Generally, the gain peaks at 1555 nm and falls off on each side, and it is a function of the inversion of Er+3. When a large number of EDFAs are cascaded, the sloped of the gain becomes multiplied and sharp, as indicated in Fig. 6.3. This results is too little gain-bandwidth for a system. To help alleviate this effect, a gain flattening device often is used, such as a Mach–Zehnder or a long-period grating filter.




1. A. Willner and Y. Xie, “Wavelength Domain Multiplexed (WDM) Fiber-optic Communications Networks,” in Handbook of Optics, Vol. 4., M. Bass, Ed.,McGraw-Hill, New York, pp. 13–19, 2001.



Source: Optical Communications Rules of Thumb

Note:I have heard many times among optical folks discussing  maximum number of amplifiers in a link;so thought of posting this.

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Why is it preferable to put attenuator/pad at the Receive end of Optical Module?


Few analogies proving the subject:-



attenuators in fiber optic data link


  • If the distance is to short and the attenuator is too close to the transmitter, the reflected light off the attenuator will be directed back towards the Tx laser. Which will also blow your we place it at Rx.


  • Also keeping attenuator at Rx will attenuate the noise along with the signal.


  • The most important reason for putting them on the RX side is that you are protecting that which needs to be protected - the receiver in your optics. This way you know that you're not going to potentially blow the receiver in your optics by plugging in too large a signal because you assumed there was an attenuator on the TX at the far end, and there wasn't.
  • It's more convenient to test the receiver power before and after attenuation or while adjusting it with your power meter at the receiver, plus any reflectance will be attenuated on its path back to the source.



Keynote on Using Attenuators With Fiber Optic Data Links



The ability of any fiber optic system to transmit data ultimately depends on the optical power at the receiver as shown above, which shows the data link bit error rate as a function of optical power at the receiver. (BER is the inverse of signal-to-noise ratio, e.g. high BER means poor signal to noise ratio.)  Either too little or too much power will cause high bit error rates. 

Too much power, and the receiver amplifier saturates, too little and noise becomes a problem as it interferes with the signal. This receiver power depends on two basic factors: how much power is launched into the fiber by the transmitter and how much is lost by attenuation in the optical fiber cable plant that connects the transmitter and receiver.

If the power is too high as it often is in short singlemode systems with laser transmitters, you can reduce receiver power with an attenuator. Attenuators can be made by introducing an end gap between two fibers (gap loss), angular or lateral misalignment, poor fusion splicing (deliberately), inserting a neutral density filter or even stressing the fiber (usually by a serpentine holder or a mandrel wrap). Attenuators are available in models with variable attenuation or with fixed values from a few dB to 20 dB or more.

gap loss attenuators 
Gap-loss attenuators for multimode fiber

serpentine attenuator
Serpentine attenuators for singlemode fiber

Generally, multimode systems do not need attenuators. Multimode sources, even VCSELs, rarely have enough power output to saturate receivers. Singlemode systems, especially short links, often have too much power and need attenuators.

For a singlemode applications, especially analog CATV systems, the most important specification, after the correct loss value, is return loss or reflectance! Many types of attenuators (especially gap loss types) suffer from high reflectance, so they can adversely affect transmitters just like highly reflective connectors. 

attenuators in fiber optic data link

Choose a type of attenuator with good reflectance specifications and always install the attenuator ( X in the drawing) as shown at the receiver end of the link. This is because it's more convenient to test the receiver power before and after attenuation or while adjusting it with your power meter at the receiver, plus any reflectance will be attenuated on its path back to the source. 

testing attenuated power at receiver
Test the system power with the transmitter turned on and the attenuator installed at the receiver using a fiber optic power meter set to the system operating wavelength. Check to see the power is within the specified range for the receiver.

If the appropriate attenuator is not available, simply coil some patchcord around a pencil while measuring power with your fiber optic power meter, adding turns until the power is in the right range. Tape the coil and your system should work. This type of attenuator has no reflectance and is very low cost! The fiber/cable manufacturers may worry about the relaibility of a cable subjected to such a small bend radius. You should probably replace it with another type of attenuator at some point, however.

singlemode wrap attenuator
Singlemode attenuator made by wrapping fiber or simplex cable around a small mandrel. This will not work well with bend-insensitive fiber.




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Power Change during add/remove of channels on filters


The power change can be quantified as the ratio between the number of channels at the reference point after the channels are added or dropped and the number of channels at that reference point previously.We can consider composite power here and each channel at same optical power in dBm.

So whenever we add or delete number of channels from a MUX/DEMUX/FILTER/WSS following equations define the new changed power.

For the case when channels are added (as illustrated on the right side of Figure 1 ):



 A   is the number of added channels

 U   is the number of undisturbed channels

For the case when channels are dropped (as illustrated on the left side of Figure 1):



                            D   is the number of dropped channels

                            U   is the number of undisturbed channels

                                                                            Figure 1

For example:

-           adding 7 channels with one channel undisturbed gives a power change of +9 dB;

-           dropping 7 channels with one channel undisturbed gives a power change of –9 dB;

-           adding 31 channels with one channel undisturbed gives a power change of +15 dB;

-           dropping 31 channels with one channel undisturbed gives a power change of –15 dB;


refer ITU-T G.680 for further study.

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HD-FEC & SD-FEC differences

DefinitionDecoding based on hard-bits(the output is quantized only to two levels) is called the “HD(hard-decision) decoding”, where each bit is considered definitely one or zero. Decoding based on soft-bits(the output is quantized to more than two levels) is called the “SD(soft-decision) decoding”, where not only one or zero decision but also confidence information for the decision are provided.
ApplicationGenerally for non-coherent detection optical systems, e.g.,  10 Gbit/s, 40 Gbit/s, also for some coherent detection optical systems with higher OSNR

coherent detection optical systems, e.g.,  100 Gbit/s,400 Gbit/s.

Electronics RequirementADC(Analogue-to-Digital Converter) is not necessary in the receiver.ADC is required in the receiver to provide soft information, e.g.,  coherent detection optical systems.
specificationgeneral FEC per [ITU-T G.975];super FEC per [ITU-T G.975.1].vendor specific
typical schemeConcatenated RS/BCHLDPC(Low density parity check),TPC(Turbo product code)
redundancy ratiogenerally 7%around 20%
NCGabout 5.6 dB for general FEC;>8.0 dB for super FEC.>10.0 dB
 Example(If you asked your friend about traffic jam status on roads and he replies)maybe fully jammed or free 50-50  but I found othe way free or less traffic







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Difference between OTDR and COTDR.

What is a OTDR ?

Optical Time Domain Reflectometer - also known as an OTDR, is a hardware device used for measurement of the elapsed time and intensity of light reflected on optical fiber.

How it works?

The reflectometer can compute the distance to problems on the fiber such as attenuation and breaks, making it a useful tool in optical network troubleshooting.

The intensity of the return pulses is measured and integrated as a function of time, and is plotted as a function of fiber length.

What is a COTDR?

Coherent Optical Time Domain Reflectometer - also known as a COTDR, An instrument that is used to perform out of service backscattered light measurements on optically amplified line systems.

How it works?

A fiber pair is tested by launching a test signal into the out going fiber and receiving the scattered light on the in-coming fiber.  Light scattered in the transmission fiber is coupled to the incoming fiber in the loop-back couplers in each amplifier pair in a repeater.


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A walk-through with non linear impairments in optical fiber

Non-linear interactions between the signal and the silica fibre transmission medium begin to appear as optical signal powers are increased to achieve longer span lengths at high bit rates. Consequently, non-linear fibre behaviour has emerged as an important consideration both in high capacity systems and in long unregenerated routes. These non-linearities can be generally categorized as either scattering effects (stimulated Brillouin scattering and stimulated Raman scattering) or effects related to the fibre's intensity dependent index of refraction (self-phase modulation, cross-phase modulation, modulation instability, soliton formation and four-wave mixing). A variety of parameters influence the severity of these non-linear effects, including line code (modulation format), transmission rate, fibre dispersion characteristics, the effective area and non-linear refractive index of the fibre, the number and spacing of channels in multiple channel systems, overall unregenerated system length, as well as signal intensity and source line-width. Since the implementation of transmission systems with higher bit rates than 10 Gbit/s and alternative line codes (modulation formats) than NRZ-ASK or RZ-ASK, described in [b-ITU-T G-Sup.39], non‑linear fibre effects previously not considered can have a significant influence, e.g., intra‑channel cross-phase modulation (IXPM), intra-channel four-wave mixing (IFWM) and non‑linear phase noise (NPN).


Refer ITU-T G.663 for further study.

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Understanding multipliers and divisors value in calculating OTN frame rates (255,239,238,237 etc) for OPUk,ODUk and OTUk

**Multiplicative factor is just a simple math :eg. for ODU1/OPU1=3824/3808={(239*16)/(238*16)}

Here value of multiplication factor will give the number of times for rise in the frame size after adding header/overhead.

Example:let consider y=(x+delta[x])/x; In terms of OTN frame here delta[x] is increment of Overhead.

As we are using Reed Soloman(255,239) i.e we are dividing 4080bytes in sixteen frames (The forward error correction for the OTU-k uses 16-byte interleaved codecs using a Reed- Solomon S(255,239) code. The RS(255,239) code operates on byte symbols.).Hence 4080/16=255.

Try to understand using OTN frames now. I have tried to make it legible.

As we know that OPU1 payload rate= 2.488 Gbps (OC48/STM16) and is  frame size is 4*3808 as below.

*After adding OPU1 and ODU1 16 bytes overhead: Frames could be fragmented into following number of chunks.

 3808/16 = 238, (3808+16)/16 = 239

So, ODU1 rate: 2.488 x 239/238** ~ 2.499Gbps

*Now after adding  FEC bytes

  OTU1 rate: ODU1 x 255/239 = 2.488 x 239/238 x 255/239

  =2.488 x 255/238 ~2.667Gbps 


Now let’s have a small discussion over different multiplier and divisor scenarios that will make it clearer to understand.

We know that an OTU frame 4 * 4080 bytes (= 255 * 16 * 4)

 OPU representing the Payload (3824-16) * 4 * 4 = 3808 bytes (= 238 * 16 * 4) .

OPU1 is exactly the rate of STM-16.


ODU1 = (3824/3808) * OPU1 = ((16 * 239) / (238 16 *)) * OPU1 = (239/238) * STM-16 

OTU1 = (4080/3808) * OPU1 = ((255 * 16) / (238 * 16)) * OPU1 = (255/238) * STM-16 


OPU2 contains 16 * 4 = 64 bytes of fixed stuff (FS) added to the 1905 to 1920 .

OPU2 * ((238 * 16 * 4-16 * 4) / (238 * 16 * 4)) = STM-64 rate

OPU2 = 238 / (238-1) * STM-64 = 238/237* STM-64 rate 

ODU2 = (239/237) * STM-64 rate ,


 OTU2 = ( 255/237) * STM-64 rate 

 OPU3 Including 2 * 16 * 4 = 128 fixed stuff (FS) bytes added to the 1265 ~ 1280 and 2545 ~ 2560

 OPU3 * ((238 * 16 * 4-2 * 16 * 4) / (238 * 16 * 4)) = rate of STM-256

OPU3 = 238 / (238-2) * STM-256 = 238/236 * STM-256

ODU3 = (239 / 236) * STM-256

OTU3 = (255/236) * STM-256

The OTU4 was required to transport ten ODU2e signals, which have a non-SDH based clock frequency as basis. The OTU4 clock should be based on the same SDH clock as the OTU1, OTU2 and OTU3 and not on the 10GBASE-R clock, which determines the ODU2e frequency. An exercise was performed to determine the necessary divider in the factor 255/divider, and the value 227 was found to meet the requirements (factor 255/227). Note that this first analysis has indicated that a future 400 Gbit/s OTU5 could be created using a factor 255/226 and a 1 Tbit/s OTU6 using a factor 255/225.

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How to know if errors are due to linear or non-linear issue in an optical link?


When the bit error occurs to the system, generally the OSNR at the transmit end is well and the fault is well hidden.
Decrease the optical power at the transmit end at that time. If the number of bit errors decreases at the transmit end, the problem is non-linear problem.
If the number of bit errors increases at the transmit end, the problem is the OSNR degrade problem. 

General Causes of Bit Errors

 Performance degrade of key boards
Abnormal optical power
Signal-to-noise ratio decrease
Non-linear factor
Dispersion (chromatic dispersion/PMD) factor
Optical reflection
External factors (fiber, fiber jumper, power supply, environment and others)


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What is Optical Power Requirement and margin for a optics module's power?

Optical power tolerance: It refers to the tolerable limit of input optical power, which is the range from sensitivity to overload point.

Optical power requirement: If refers to the requirement on input optical power, realized by adjusting the system (such as adjustable attenuator, fix attenuator, optical amplifier).

Optical power margin: It refers to an acceptable extra range of optical power. For example, “–5/ + 3 dB” requirement is actually a margin requirement.


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What are main advantages and drawbacks of EDFAs?

The main advantages and drawbacks of EDFAs are as follows.


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


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

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A Lower Order-ODU or LO-ODU is an ODUk, whose OPUk transports a inside mapped client.

An Higher Order-ODU or HO-ODU is an ODUk, whose OPUk transports an inside multiplexed ODUj.

LO-ODU and HO-ODU have the same structure but with different clients.
The LO-ODU is either mapped into the associated OTUk or multiplexed into an HO-ODU.
The HO-ODU is mapped into the associated OTUk.
Please notice that, HO-ODUj multiplexed into HO-ODUk is an undesired hierarchy in one domain.

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Invalid Data Flag or Suspect Interval in PM(Performance Monitoring) counters.

The ITU standards define a “suspect internal flag” which should indicate if the data contained within a register is ‘suspect’ (conditions defined in Q.822). This is more frequently referred to as the IDF (Invalid Data Flag).

PM is bounded by strict data collection  rules as defined in standards. When the collection of PM parameters is affected then  PM system labels the collection of data as suspect with an Invalid Data Flag (IDF). For the sake of identification; some unique flag  is shown next to corresponding counter.

The purpose of the flag is to indicate when the data in the PM bin may not be complete or may have been affected such that the data is not completely reliable. The IDF does not mean the software is contingent.

Some of the common reasons  for setting the IDF include:

  • a collection time period that does not start within +/- 1 second of the nominal collection window start time.
  • a time interval that is inaccurate by +/- 10 seconds (or more)
  • the current time period changes by +/- 10 seconds (or more)
  • a restart (System Controller restarts will wipe out all history data and cause time fluctuations at line/client module;  a module restart will wipe out the current counts)
  • a PM bin is cleared manually
  • a hardware failure prevents PM from properly collecting a full period of PM data (PM clock failure)
  • a protection switch has caused a change of payload on a protection channel.
  • a payload reconfiguration has occurred (similar to above but not restricted to protection switches).
  • an System Controller archive failure has occurred, preventing history data from being collected from the line/client  cards
  • protection mode is switched from non-revertive to revertive (affects PSD only)
  • a protection switch clear indication is received when no raise was indicated
  • laser device failure (affects physical PMs)
  • loss of signal (affects receive - OPRx, IQ - physical PMs only)
  • Control Plane is booted less than 15 min period for 15-min interval and less than 24 hour period for 24-hour interval.

Suspect interval is determined by comparing nSamples to nTotalSamples on a counter PM. If nSamples is not equal to nTotalSamples then this period can be marked as suspect. 

If any 15 minute is marked as suspect or reporting for that day interval is not started at midnight then it should flag that 24 Hr as suspect.

Some of the common examples are:

  • Interface type is changed to another compatible interface (10G SR interface replaced by 10G DWDM interface),
  • Line type is changed from SONET to SDH,
  • Equipment failures are detected and those failures inhibit the accumulation of PM.
  • Transitions to/from the 'locked' state.
  • The System shall mark a given accumulation period invalid when the facility object is created or deleted during the interval.
  • Node time is changed.

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Why does coherent detection introduce pre-FEC bit errors?

Why does coherent detection introduce pre-FEC bit errors?

  • The DSP algorithm at the receive end detects and analyzes the phase and amplitude of a received signal in real time to calculate and compensate the distortion of the signal caused by factors such as CD, PMD, and nonlinearity. Because CD, PMD, and nonlinearity vary with time, the compensation amount calculated by the DSP algorithm is not so accurate and thus pre-FEC bit errors occur.
  • In practice, the transmission distance should be extended as long as possible. The nonlinearity in a long-haul system cause large changes in signal phases. The DSP algorithm is thus required to lock the phase of each signal at a large tracking step, which enables fast locking of great phase changes but has poorer compensation accuracy. As a result, background noise is introduced and further bit errors occur (error floor poorer than 1.0E-6). The background noise is the major factor that causes pre-FEC bit errors in back-to-back OSNR measurement and short-reach transmission. It is negligible compared with the noise introduced in long-haul transmission. Therefore, the large-step tracking method remarkably improves long-haul transmission performance without affecting short-reach transmission performance.
  •  The DSP algorithm is independent of optical-layer configurations, such as back-to-back configurations, the transmission distance, and the number of spans. Therefore, in a back-to-back configuration, the DSP algorithm also has a compensation error and introduces bit errors.


Important Notes on BER for Coherent

  • For 100G coherent optical modules, the pre-FEC BERs may differ when different 100G boards are connected in back-to-back manner or when WDM-side external loopbacks are performed because of differences in the optical modules. Similarly, after signals traverse spans with good OSNRs, the BERs of different 100G boards may also differ. However, the use of advanced DSP algorithm in the 100G boards ensure that all the 100G boards have the same FEC correction capability.
  • As shown in the figure below, the red and blue lines represent the test data of two different 100G boards. 


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FEC and Latency

Why does FEC introduce latency?

The addition of FEC overhead at the transmit end and FEC decoding at the receive end cause latency in signal transmission.

Relationship between latency and FEC scheme types

  • To improve the correction capability, more powerful and complex FEC codes must be used. However, the more complex the FEC codes are, the more time FEC decoding will take. As a result, the latency is increased.
  • For a non-coherent 40G signal, AFEC typically introduces a larger latency (60~120 us) than FEC, which introduces a latency of 30 us.
  • Latency of a non-coherent 40G signal is related to the ODUk mapping mode.
  • Latency is also related to the amount of overhead. More overhead means that FEC decoding will take more time.
  • Latency introduced by 100G SD-FEC with 20% overhead > Latency introduced by 100G HD-FEC with 7% overhead
  • In addition, the latency is subject to the signal coding rate. With the overhead unchanged, there will be less latency as the signal rate increases.
  • Latency introduced by 100G HD-FEC < Latency introduced by 40G AFEC

Latency specifications of OTN equipment

Data and storage services are sensitive to latency while OTN services are not. Currently, no international standards have defined how much latency that OTN signals must satisfy. Vendor equipment supports latency testing for different service rates, FEC schemes, and mapping modes. You can recommend a latency value for customers, but latency should not be an acceptance test item. You cannot make a commitment to a latency specification.


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Measuring Coherent System OSNR using integral method

For coherent signals with wide optical spectrum, the traditional scanning method using an OSA or inband polarization method (EXFO) cannot correctly measure system OSNR. Therefore, use the integral method to measure OSNR of coherent signals.

Perform the following operations to measure OSNR using the integral method:

1.Position the central frequency of the wavelength under test in the middle of the screen of an OSA. See Figure 1.
2.Select an appropriate bandwidth span for integration (for 40G/100G coherent signals, select 0.4 nm).
3.Read the sum of signal power and noise power within the specified bandwidth. On the OSA, enable the Trace Integ function and read the integral value. As shown in Figure 2, the integral optical      power (P + N) is 9.68 uW.
4.Read the integral noise power within the specified bandwidth. Disable the related laser before testing the integral noise power. Obtain the integral noise power N within the signal bandwidth      specified in step 2. The integral noise power (N) is 29.58 nW.
5.Calculate the integral noise power (n) within the reference noise bandwidth. Generally, the reference noise bandwidth is 0.1 nm. Read the integral power of central frequency within the bandwidth of 0.1 nm. In this example, the integral noise power within the reference noise bandwidth is 7.395 nW.
6.Calculate OSNR. OSNR = 10 x lg{[(P + N) - N]/n}

In this example, OSNR = 10 x log[(9.68 - 0.02958)/0.007395] = 31.156 dB


We follow integral method because Direct OSNR Scanning Cannot Ensure Accuracy because of the following reason:

A 40G/100G signal has a larger spectral width than a 10G signal. As a result, the signal spectrums of adjacent channels overlap each other. This brings difficulties in testing the OSNR using the traditional OSA method, which is implemented based on the interpolation of inter-channel noise that is equivalent to in-band noise. Inter-channel noise power contains not only the ASE noise power but also the signal crosstalk power. Therefore, the OSNR obtained using the traditional OSA method is less than the actual OSNR. The figure below shows the signal spectrums in hybrid transmission of 40G and 10G signals with 50 GHz channel spacing. As shown in the figure, a severe spectrum overlap has occurred and the tested ASE power is greater than it should be .As ROADM and OEQ technologies become mature and are widely used, the use of filter devices will impair the noise spectrum. As shown in the following figure, the noise power between channels decreases remarkably after signals traverse a filter. As a result, the OSNR obtained using the traditional OSA method is greater than the actual OSNR..

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Method to Resolve the Adjacent-Channel Interference in 100G Transmission.

In a non-coherent WDM system, each optical channel on the line side uses only one binary channel to carry service information. The service transmission rate on each optical channel is called bit rate while the binary channel rate is called baud rate. In this sense, the baud rate was equal to the bit rate. The spectral width of an optical signal is determined by the baud rate. Specifically, the spectral width is linearly proportional to the baud rate, which means a higher baud rate generates a larger spectral width.

  • Baud (pronounced as /bɔ:d/ and abbreviated as "Bd") is the unit for representing the data communication speed. It indicates the signal changes occurring in every second on a device, for example, a modulator-demodulator (modem). During encoding, one baud (namely, the signal change) actually represents two or more bits. In the current high-speed modulation techniques, each change in a carrier can transmit multiple bits, which makes the baud rate different from the transmission speed.

In practice, the spectral width of the optical signal cannot be larger than the frequency spacing between WDM channels; otherwise, the optical spectrums of the neighboring WDM channels will overlap, causing interference among data streams on different WDM channels and thus generating bit errors and a system penalty.

For example, the spectral width of a 100G BPSK/DPSK signal is about 50 GHz, which means a common 40G BPSK/DPSK modulator is not suitable for a 50 GHz channel spaced 100G system because it will cause a high crosstalk penalty. When the baud rate reaches 100 Gbaud/s, the spectral width of the BPSK/DPSK signal is greater than 50 GHz. Thus, it is impossible to achieve 50 GHz channel spacing in a 100G BPSK/DPSK transmission system.

(This is one reason that BPSK cannot be used in a 100G coherent system. The other reason is that high-speed ADC devices are costly.)

A 100G coherent system must employ new technology. The system must employ more advanced multiplexing technologies so that an optical channel contains multiple binary channels. This reduces the baud rate while keeping the line bit rate unchanged, ensuring that the spectral width is less than 50 GHz even after the line rate is increased to 100 Gbit/s. These multiplexing technologies include quadrature phase shift keying (QPSK) modulation and polarization division multiplexing (PDM).

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Basics of Coherent System.

What Is Coherent Communication?

Definition of coherent light

A coherent light consists of two light waves that:

1) Have the same oscillation direction.

2) Have the same oscillation frequency.

3) Have the same phase or maintain a constant phase relationship with each other. Two coherent light waves produce interference within the area where they meet.

Principles of Coherent Communication

Coherent communication technologies mainly include coherent modulation and coherent detection.

Coherent modulation uses the signals that are propagated to change the frequencies, phases, and amplitudes of optical carriers. (Intensity modulation only changes the strength of light.)

 Modulation detection mixes the laser light generated by a local oscillator (LO) with the incoming signal light using an optical hybrid to produce an IF signal that maintains the constant frequency, phase, and amplitude relationships with the signal light.

The motivation behind using the coherent communication techniques is two-fold.

First, the receiver sensitivity can be improved by up to 20 dB compared with that of IM/DD systems.

Second, the use of coherent detection may allow a more efficient use of fiber bandwidth by increasing the spectral efficiency of WDM systems

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Why heterodyne detection technique is used in Coherent technology receivers rather homodyne?

As we know that either homodyne or heterodyne detection can be used to convert the received optical signal into an electrical form. In the case of homodyne detection, the optical signal is demodulated directly to the baseband. Although simple in concept, homodyne detection is difficult to implement in practice, as it requires a local oscillator whose frequency matches the carrier frequency exactly and whose  phase is locked to the incoming signal. Such a demodulation scheme is called synchronous and is essential for homodyne detection. Although optical phase-locked loops have been developed for this purpose, their use is complicated in practice.

Heterodyne detection simplifies the receiver design, as neither optical phase locking nor frequency matching of the local oscillator is required. However, the electrical signal  oscillates rapidly at microwave frequencies and must be demodulated from the IF bandto the baseband using techniques similar to those developed for microwave communication systems. Demodulation can be carried out either synchronously or asynchronously. Asynchronous demodulation is also called incoherent in the radio communication literature. In the optical communication literature, the term coherent detection is used in a wider sense.

A lightwave system is called coherent as long as it uses a local oscillator irrespective of the demodulation technique used to convert the IF signal to baseband frequencies.


*In case of homodyne coherent-detection technique, the local-oscillator frequency is selected to coincide with the signal-carrier frequency.

*In case of heterodyne detection the local-oscillator frequency  is chosen to differ from the signal-carrier frequency.

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Basic understanding on Tap ratio for Splitter/Coupler

Fiber splitters/couplers divide optical power from one common port to two or more split ports and combine all optical power from the split ports to one common port (1 × N coupler). They operate across the entire band or bands such as C, L, or O bands. The three port 1 × 2 tap is a splitter commonly used to access a small amount of signal power in a live fiber span for measurement or OSA analysis. Splitters are referred to by their splitting ratio, which is the power output of an individual split port divided by the total power output of all split ports. Popular splitting ratios are shown in Table below; however, others are available. Equation below can be used to estimate the splitter insertion loss for a typical split port. Excess splitter loss adds to the port's power division loss and is lost signal power due to the splitter properties. It typically varies between 0.1 to 2 dB, refer to manufacturer’s specifications for accurate values. It should be noted that splitter function is symmetrical.

where IL = splitter insertion loss for the split port, dB

Pi = optical output power for single split port, mW

PT = total optical power output for all split ports, mW

SR = splitting ratio for the split port, %

Γe = splitter excess loss (typical range 0.1 to 2 dB), dB

Common splitter applications include

• Permanent installation in a fiber link as a tap with 2%|98% splitting ratio. This provides for access to live fiber signal power and OSA spectrum measurement without affecting fiber traffic. Commonly installed in DWDM amplifier systems.

• Video and CATV networks to distribute signals.

• Passive optical networks (PON).

• Fiber protection systems.

Example with calculation:

If a 0 dBm signal is launched into the common port of a 25% |75% splitter, then the two split ports, output power will be −6.2 and −1.5 dBm. However, if a 0 dBm signal is launched into the 25% split port, then the common port output power will be −6.2 dBm.


Launch power=0 dBm =1mW                   

Tap is  25%|75%

 so equivalent mW power which is linear  will be


and after converting them ,dBm value will be

-6.02dBm| -1.24dBm


Some of the common split ratios and their equivalent Optical Power is available below for reference.

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