distortion and dispersion

[umer syed]

hi all,

please explain in context of optical fiber communication , what is dispersion and dispersion … difference between the two ….how they take place ansd reason…

THE QUERY IS SIMPLE TO ANSWER in terms of definite, but i am looking ahead for technical and logical anwers..

with regards

[Ankita Sharma]

hi umer syed I think you have to rearrange your query because u have mentioned dispersion and dispersion..and I don’t feel there should be any difference as u have have used same terms.
if u mean distortion and dispersion then distortion is related to signal quality and dispersion is related to fiber..distortion is common term used technically for all types of signals but for optical signals we often use dispersionand there are various reasons for the cause of dispersion..i hope u will find better answer if you try to google it.
Thanks
regards

[aasif bashir dar]

hi umar syed
dispersion ———–> technically speaking reffers to the variation of speed of wavelengths in a medium.,..

distortion….>technically speaking is DISPERSION and attenuation…

dispersions occurs becoz different wavelengths travel with different velocities in the fiber…

attenuation gennnerally for single mode fiber is 0.2dB/km… however this attenuation is also wavelength..

thus each wavelength travel with different velocity and attenuated with different magnitude leads to distortion

with regards

[ZULKARNAIN]

hi all..
i agree with aasif bashir dar and ankita sharma that dispersion is a fiber related phenomenon in which different wavelengths travel with different velocities and distortion is a signal related phenomenon.
with regards.

[ZULKARNAIN]

aasif sir can you please explain why the different wavelengths travel with different velocities even if the medium is same for every wavelength.Does it means that different wavelenghts behave differently with the medium through which they are travelling.
Also please explain the reasons behind the attenuation of the signal.Does it occur because of some defects in the fiber or the medium propertoies itself.
with regards

[Atul Sharma]

Hello Zulkarnain. Greetings.

Attenuation is common to all the systems be it wireless or optical. Attenuation is due to different factors and it widely depends on the quality of overall system design.

Regards
Atul Sharma

[aasif bashir dar]

well zulkarnain..

i guess i have answer such questions in detail as as asked by alistu in some post.
let me explain that the wavelength dependence of refractive index of medium.
i.e same medium will exibit the different refractive index for diffferent wavelengths, hence different velocities(which we call as the dispersion)

there is an equation called CAUCHY EQUATION FOR REFRACTIVE INDEX.which reveals that a medium havs different refractive index for different medium, hence different velocities

you can see that

with regards

[Dr. Dhiman Kakati]

Hi Umer Syed,
I agree with Aasif dar that distortion is an unwanted phenomena in optical communication, in general sense distortion is unwanted change in amplitude and phase while transmission, signals get distorted on an optical fiber because different frequency components undergo different attenuation and different delays.
In theoretical basis we assume that the signal transmitted (with a definite bandwidth) has same transmission characteristics but In practical cases suppose for a bandwidth of 10 GHz i.e. 0.1 nm wavelength difference (delta f) is there so for each wavelength the characteristics of the fiber is different (negligible in this case) but when transmitted long distance along the fiber this little difference becomes noticeable, i.e. each single wavelength follow different different path and arrive at the receiver with a time delay for the same signal, in this way the pulse gets broaden in time domain (and respective in frequency domain as these two are interrelated).
So dispersion may be defined as the pulse broadening due to multi-path propagation and its unit is ps/nm-km, where ps-picosecond is the pulse boradening, nm is the wavelength of the signal and km is the fiber distance.

attenuation is due to the fiber loss in terms of power.

Regards,
Dhiman

[Dr. Dhiman Kakati]

and cause of signal attenuation is due to following mechanisms.

1. Intrinsic loss in the fiber material.
2. Scattering due to micro irregularities inside the fiber.
3. Micro-bending losses due to micro-deformation of the fiber.
4. Bending or radiation losses on the fiber.

Regards,
Dhiman

[ZULKARNAIN]

thank you diman kakati for explaining the concepts.
with regards

[ZULKARNAIN]

hi damain..
i agree with you that there will be spreading of signal at the output because of different behaviour of wavelenghts.So can you explain how can we improve this dispersion in our system as in long communication we will get a very delayed reception. Can we use the optical amplifiers for long channels to decrease this dispersion or is there some other technique for decreasing this.
hope you will answer my query.
with regards.

[Dr. Dhiman Kakati]

Hi Zulkarnain,
There are various techniques though which you can compensate dispersion, the following are among them:

1. Dispersion Compensating fibers (DCFs): This is a kind of fiber which has a negative dispersion coefficient so you can use it before or after the fiber, just manually calculate the single mode fiber (SMF) dispersion and accordingly connect the required length of the DCF and eliminate dispersion effect. Pre-, post-, and symmetrical-compensation can be done by using DCF.

2. Dispersion compensation by using fiber brag grating: This component allows design of apodized and chirped fiber gratings that are able to provide dispersion compensation in optical system. The idea is to create a time delay between different spectral components of the signal. Because of this different velocity of propagation of different spectral components, the pulse spreads, so we need to create fiber grating with period linearly reducing along the grating, because the higher frequencies will reflect after longer propagation in the grating a time delay between lower and higher frequency components will appear which is just opposite to this created in the SMF. Therefore propagating and reflecting our pulse in this device will allow to compensate the dispersion broadening of our pulse.

These two are the most easiest technique. and you will find samples using various techniques.

Regards,
Dhiman

[FAYIQA NAQSHBANDI]

HELLO ZULKARNAIN..

I want to clear it to you that optical amplifiers are used for amplification purpose only not for dispersion compensation.. for dispersion compensation techniques Dhiman has rightly said that you need Dispersion Compensating fibers DCFs which iskind of fiber which has a negative dispersion coefficient so you can use it before or after the fiber… just manually calculate the single mode fiber (SMF) dispersion and accordingly connect the required length of the DCF and eliminate dispersion effect…. Pre-, post-, and symmetrical-compensation can be done by using DCF…I hope it helps..

Thanks & regards

[FAYIQA NAQSHBANDI]

Also the other and trending dispersion compensation technique mentioned by dhiman can also be very useful… Dispersion compensation by using fiber brag grating: This component allows design of apodized and chirped fiber gratings that are able to provide dispersion compensation in optical system….The idea is to create a time delay between different spectral components of the signal. ..
I hope you have gotten an idea about how it works..

Thanks & regards

[Atul Sharma]

Thanks Fayiqa for the info.

[Aabid Baba]

Hello Dhiman,
The bandwidth difference point you have put forth is a valuable point. Indeed it was helpful. Thanks for sharing.
Regards

[aasif bashir dar]

dispersion can be postive or nagative based on that type of window we are using;

e.at 1550nm: dispersion is -20ps/km.nm (NAGATIVE)
at 850nm the dispersion is 80 ps ps/km.nm(positive)
NOW WHAT DOES THIS NAGATIVE OR POSITIVE SIGN INDICATES;
+VE DISPERSIONS MEANS AS THE WAVELENGTH INCRESES THE SPEED DECRESESS9i.e longer wavelength components will take longer time to reach the same destination)
-vedispersion means as the value increases the speed increases(i.e shorter wavlenght will take longer time to reach the same destination)

this time delay between the wavelength components(whether the dispersion is positive or nagative) leads to ISI.

with regards

[aasif bashir dar]

dispersion can be postive or nagative based on that type of window we are using;

e.at 1550nm: dispersion is -20ps/km.nm (NAGATIVE)
at 850nm the dispersion is 80 ps ps/km.nm(positive)
NOW WHAT DOES THIS NAGATIVE OR POSITIVE SIGN INDICATES;
+VE DISPERSIONS MEANS AS THE WAVELENGTH INCRESES THE SPEED DECRESESS9i.e longer wavelength components will take longer time to reach the same destination)
-vedispersion means as the value increases the speed increases(i.e shorter wavlenght will take longer time to reach the same destination)

this time delay between the wavelength components(whether the dispersion is positive or nagative) leads to ISI.

with regards

[Atul Sharma]

Thanks Aasif bashir for sharing so much valuable information. No needs of books Now. Seriously.

[ZULKARNAIN]

thankyou aasif sir for explaining my query related to dispersion.i understood that medium will behave differently for different wavelengths or the refractive index changes with wavelenghts.That means at the output we will get spreading of the signal as different wavelengths will reach at different times resulting loss of signal.
So please explain this if i am correct in saying this.also please explain the various techniques by which we can reduce this.
with regards.

[aasif bashir dar]

hi zulkarnain,
i want to mention here that the cauchy eqation for refractive index have not that much significance in optical fiber communication..
there is one more equation called sellmier equation which shows the sameand is most widely used equation for dispersion of materials, for convience i am attaching few links for equations for refractive index.
https://en.wikipedia.org/wiki/Sellmeier_equation
http://scienceworld.wolfram.com/physics/CauchysFormula.html
http://link.springer.com/referenceworkentry/10.1007%2F978-1-4419-6247-8_2057

you can also see video lecture on youtube for cauchy equation for refractive index.

hope i answer your question zulkarnain

with regards

[ZULKARNAIN]

thank you aasif bashir dar for explaining this.yes i will definitely go through the links which you have provided to get my doubts cleared.
with regards.

[umer syed]

hi aasif bashir dar,

thank you so much for your efforts , indeed very important concept of dispersion …. variaable velocity of different wavelengths provided by you… i had never heard of the cauchy equation before…..now i got the real concept why dispersion takes place place in the medium of single refractivre index..

greeting again
with regards

[SAHIL SINGH]

HI ALL,

As far as distortion is concerned it is an unwanted phenomena in optical communication, in general sense distortion is unwanted change in amplitude.Also dispersion can be postive or nagative based on that type of window we are using.. I would like to mention here that Dhiman has put it right that attenuation is due to the fiber loss in terms of power…

Regards
Sahil Singh

[umer syed]

hi sahil singh,

thank you so much for your efforts

greeting again
with regards

[aasif bashir dar]

hi all,
as far dispersion compensation is concerned we can do that in three sections of communication link
1. transmitter(called pre-distortion compensation)
2. inline(in the channel)
3. tranmitter(post-compensation)

for chirping signal we have two method for dispersion compensation.
1. pre-chirp(at the modulator)
2. nonlinear chirp gennerally using SOA
Chirping can be -positive, indicates the pulse is blue shifted meaning that higher frequencies are in the falling edge, and negative, indicates the pulse is red shifted meaning that lower frequencies are in the falling edge.

with regards

[Aabid Baba]

Hello Aasif,
Thanks for sharing such information. It is important to keep these things in mind while designing the system.
Regards

[aasif bashir dar]

hi all,

inline compensation can be done by;
DCF
which has a negative dispersion coefficient so you can use it before or after the fiber, just manually calculate the single mode fiber (SMF) dispersion and accordingly connect the required length of the DCF and eliminate dispersion effect. Pre-, post-, and symmetrical-compensation can be done by using DCF.
FBG
The idea is to create a time delay between different spectral components of the signal. Because of this different velocity of propagation of different spectral components, the pulse spreads, so we need to create fiber grating with period linearly reducing along the grating, because the higher frequencies will reflect after longer propagation in the grating a time delay between lower and higher frequency components will appear which is just opposite to this created in the SMF.
OPC
also called as the optical phase conjugationor midspan spectral inversion where the rays retrace the same path as they travelled upto the OPC…in other words the phaseis 180 shifted..
HOLEY FIBER
also called photonic crytal fibers and is emerging technology in terms of dispersion compensation.

with regards

[Dr. Dhiman Kakati]

Hi Aasif,
the last technique for dispersion compensation is “OPC, also called as the optical phase conjugationor midspan spectral inversion where the rays retrace the same path as they travelled upto the OPC…in other words the phaseis 180 shifted..
HOLEY FIBER
also called photonic crytal fibers and is emerging technology in terms of dispersion compensation.”

I am not aware of this technique, So please I need some more explanation, If possible refer a research paper implementing this technique. Thanking you.

Regards,
Dhiman

[aasif bashir dar]

sory copy poted becoz of hanged system
with regards

[aasif bashir dar]

hi all,
post compensation include that
microwave filtersfilter… used only for the coherent modulation schemes.hence not used so much
FFE—-feed forward equalizer
DFE—-distributed feedback equalizer..
MLSE—maximum likelihood equlizer.(best of all post compensation methods)

it must be noted here that main compensation seheme is the inline compensation… pre and post compensation are just to ehance the inline compensation…
hope this clears all the confusions regarding the dispersion compensation.

with regards

[aasif bashir dar]

sory system hang

[SAHIL SINGH]

Hi all,

you are welcome umer syed… Also thanks aasif for such useful information on dispersion compensation particularly the the pre and the post compensation… I appreciate your efforts

Regards
Sahil Singh

[aasif bashir dar]

..

[ZULKARNAIN]

thank you aasif bashir dar for your valuable explanation of the concepts.
It is very helpfull indeed.
with regards

[FAYIQA NAQSHBANDI]

inline compensation can be done by DCF which has a negative dispersion coefficient so you can use it before or after the fiber, just manually calculate the single mode fiber (SMF) dispersion and accordingly connect the required length of the DCF and eliminate dispersion effect. Pre-, post-, and symmetrical-compensation can be done by using DCF.
FBG: The idea is to create a time delay between different spectral components of the signal. Because of this different velocity of propagation of different spectral components, the pulse spreads, so we need to create fiber grating with period linearly reducing along the grating, because the higher frequencies will reflect after longer propagation in the grating a time delay between lower and higher frequency components will appear which is just opposite to this created in the SMF.
OPC: also called as the optical phase conjugationor midspan spectral inversion where the rays retrace the same path as they travelled upto the OPC…in other words the phaseis 180 shifted..
HOLEY FIBER: also called photonic crytal fibers and is emerging technology in terms of dispersion compensation.

GOOD EXPLANATION THAT (y)

[aasif bashir dar]

you are welcome zulkarnain, umer and dr.sahil singh

[aasif bashir dar]

hi fayiqa…
do not just copy paste the my comments….
aleast add some ideas of your own for better disscussion

with reagards

[FAYIQA NAQSHBANDI]

HELLO AASIF..

I in no way was copying you.. If you check the first two methods the DCF and FBG were already explained and to that you added some two more if i can see properly.
I complimented your expatiation if you check the words in CAPS at the end.. I guess you didn’t read it properly.. Anyways.

[SAHIL SINGH]

Hi atul sharma,

The optiwave forum is meant to share useful information… Glad to know that the information on the forum was useful… All the best..

Regards
Sahil Singh

[aasif bashir dar]

hi dhimen kakati,,

as you seems to to be interested in the dispersion compensation techniques;

OPC and holey fiber..

opc;
opc has the very long history 1951 and was first used in mirrors ,lenses .
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&cad=rja&uact=8&ved=0ahUKEwjb2-CC2fHLAhXCHx4KHfxEBicQFggtMAI&url=http%3A%2F%2Fwww.physics.iitm.ac.in%2F~cvijayan%2Fopc-review.pdf&usg=AFQjCNF3J3fM9Eg4eLI5VuKQAiIN8YSd6A&sig2=A_AS52MW-USEFt-2qFmmlQ&bvm=bv.118443451,d.dmo
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=8&cad=rja&uact=8&ved=0ahUKEwjb2-CC2fHLAhXCHx4KHfxEBicQFghEMAc&url=http%3A%2F%2Flibrary.lanl.gov%2Fcgi-bin%2Fgetfile%3F00285789.pdf&usg=AFQjCNF8UCEm3NEmJsEIv94S4GfTekPvWg&sig2=I27FeNLOKNmz5PF3-xuFOQ&bvm=bv.118443451,d.dmo

holey fibers:crytal fibers
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=16&cad=rja&uact=8&ved=0ahUKEwjCjtP42fHLAhWGmx4KHZzIBCgQFghzMA8&url=http%3A%2F%2Fwww.orc.soton.ac.uk%2Fpublications%2F23xx%2F2333.pdf&usg=AFQjCNGeBc66jJuD8fjDhKDhagTeLovj4w&sig2=Qwi9TDR_RMeptIlROQ3o4g&bvm=bv.118443451,d.dmo

with regards

[Dr. Dhiman Kakati]

Hello Aasif Bashir Dar,
Thank you So much providing with so much of information about OPC and Holey fibers, the link you have provided are very much useful in providing good knowledge abut the above mentioned topics, very much appreciated.

Regards,
Dhiman

[SAHIL SINGH]

Hi ASIF BASHIR,

Thanks for providing such valuable information on dispersion compensation techniques.. Will surely try to look for more information on the topic by consulting the posted links..Your efforts are highly appreciated..

Regards
Sahil Singh

[aasif bashir dar]

you arewelcome dhimen and sahils singh

[umer syed]

i appreciate the all the member regarding their posts on my topic…i now have the clear understading of both dispersion and distortion…

[SAHIL SINGH]

Hi Umer Syed,

I am glad to know that your query has been resolved on the forum regarding dispersion and distortion in context of optical fiber communication as well as the difference between the two… Cheers

Regards
Sahil Singh

[umer syed]

hi all,
i got my problem solved..query has been resolved on the forum regarding dispersion and distortion in context of optical fiber communication as well as the difference between the two…
how to unsubcribe the discussion here….so that i can not get further notifications from the optiwave forum on my mail.

regards

[SAHIL SINGH]

Hi Umer syed,

If you do not want further notifications from optiwave forum on this topic… Just go the dialogue box where you post your query and disable the option of “Notify me of replies via email”… You will not get any further notifications from the optiwave on this topic… Hope this helps…

Regards
Sahil Singh

[aasif bashir dar]

hi umar syed,,

just unclick the option of the””””Notify me of replies via email:”” write below the reply box
regards

[umer syed]

thus for the reply….asif…it creates excess of emails in the account …

[Ranjeet Kumar]

Hi,
Distortion, in acoustics and electronics, any change in a signal that alters the basic waveform or the relationship between various frequency components; it is usually a degradation of the signal. Straight amplification or attenuation without alteration of the waveform is not usually considered to be distortion.
A signal is said to be undistorted if it is delayed in time and scaled version of the original signal.
Why signal gets distorted while propagating on optical fiber ?
1. The loss of optical fiber is wavelength dependent. That makes a function of frequency. This is called the attenuation of the fiber.
2. Different wavelengths and different modes of the optical fiber travel with different speeds and therefore have delay which is wavelength dependent. This phenomenon is called the dispersion.

[Ranjeet Kumar]

hi,
The signal pulse then goes on spreading as it travels along the optical fiber. The pulse broadening is proportional to the distance and it is also proportional to the spectral width of the carrier. This phenomenon is the dispersion.

So we conclude that when a signal pulse travels on an optical fiber it goes on broadening due to dispersion and goes on reducing in amplitude due to attenuation. After certain distance the pulse shape is completely distorted not to resemble with the original pulse shape.
The optical communication system can be looked as a parallel multiple channel transmission of carriers spreading over the bandwidth of the carrier. One can then say that the distortion of the signal in optical communication is due to differential delay of the signal riding over different carriers within the spectral width of the carrier.

[Ranjeet Kumar]

Hi Umar,
Dispersion is a phenomenon related to the variation in velocity of different frequencies (wavelengths) or different modes. The velocity of different frequencies can be different due to intrinsic properties of the medium or due to dispersive nature of the bound structure like the optical fiber.

Due to dispersion when the optical pulses travel along the fiber they broaden as shown in Fig. As the pulses travel on the fiber due to broadening, slowly they start overlapping with each other. Consequently, the AC content of the signal reduces and the DC content of the signal increases. After certain distance the pulses barely remain distinguishable. If the signal propagates further the pulses lose their identity and the data is lost.

For given distance, to avoid overlap of the pulses, the pulse separation is to be increased that means the data rate is to be reduced. Alternatively, for a given data rate, that is for given bit separation, the distance has to be less than certain limit. The dispersion therefore has direct impact on the data rate.

[Rajguru M. Mohan]

Hi Umer,
Dispersion is the spreading of light pulse as its travels down the length of an optical fiber. Dispersion limits the bandwidth or information carrying capacity of a fiber. The bit-rates must be low enough to ensure that pulses are farther apart and therefore the greater dispersion can be tolerated.
There are three main types of dispersion in a fiber:
Modal Dispersion.
Material dispersion.
Waveguide dispersion.
Modal dispersion occurs only in Multimode fibers. It arises because rays follow different paths through the fiber and consequently arrive at the other end of the fiber at different times.
Different wavelengths also travel at different velocities through a fiber, in the same mode, as
n = c/v.
where n is index of refraction, c is the speed of light in vacuum and v is the speed of the same wavelength in the material.

Waveguide dispersion, most significant in a single- mode fiber, occurs because optical energy travels in both the core and cladding, which have slightly different refractive indices.

A signal is said to be undistorted if it is delayed in time and scaled version of the original signal.
So, in general the signals get distorted on an optical fiber because different frequency components undergo different attenuation and different delays.

Thanks

[raja irfan]

hello aasif
thanks for providing such a useful information
regards
irfan

[raja irfan]

hello
the relationship between phase and arrival time of
each frequency component is used to determine the effects
of dispersion
regards

[Rajguru M. Mohan]

Hi,
Frequency and phase modulation can still be used in optical communication systems in indirect ways to deal with noise and distortion.
For example, signals can be frequency- or phase-modulated onto a subcarrier before intensity modulating the optical carrier.
Noise performance of an analog signal will be enhanced in addition to the reduction of cross-modulation in frequency-division multiplexing strategies.
In optical fiber is signal distortion due to multiple light paths, or modes, having different distances over the length of the fiber. When light is emitted by a source, the photons (light particles) do not all travel the exact same path.
olarization mode dispersion (PMD) is caused by asymmetric distortions to the fiber from a perfect cylindrical geometry.

WM can be compared to the intermodulation distortion in standard electrical systems. When three wavelengths (λ1, λ 2, and λ 3) interact in a nonlinear medium, they give rise to a fourth wavelength (λ 4), which is formed by the scattering of the three incident photons, producing the fourth photon. This effect is known as four-wave mixing (FWM) and is a fiber-optic characteristic that affects WDM systems.

Thanks

[Aabid Baba]

Hello Umer Syed,
I agree with Aasif dar that distortion is an unwanted phenomena in optical communication. Dhiman has also rightly put it in a good way that in general sense distortion is unwanted change in amplitude and phase while transmission, signals get distorted on an optical fiber because different frequency components undergo different attenuation and different delay.
I hope this would have helped you.

Regards

[Author]

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