Dispersion supported transmission

[FAYIQA NAQSHBANDI]

Hello evryone

It is generally acknowledged that fibers with negative dispersion coefficient.(Dispersion supported fibers) can be used to reduce the effect of dispersion introduced in the optical fiber.
If instead of using two fibers..that is a normal fiber with a positive dispersion coefficient and a Dispersion supported fiber.Can we not use a single fiber and give a negative value to its dispersion coefficient ?.This could allow us to reduce the effect of dispersion without having the need to use two fibers 🙂

[aasif bashir dar]

hii fayiqa,

there is nothing like dispersion supported fiber term in practical application, but we use the term dispersion supported transmission.in such transmision we make use of special modulation schemes like optical duobinery , FSK, or others, or other dispersion compensation schemes

with regards

[aasif bashir dar]

forget to anwser your second question.
negative coeffient of dispersion does not mean it is dispersion supported fiber.

fiber with negative co-effeint is itself dispersive but this dispersion reverse of that of fiber with positive dipersion .

hence two are used simultaneously to nullify dispersion

with regards

[mohammad namera]

Hi;
you can make that theoretically by software but not in practical applications.

Regards,

[aasif bashir dar]

hi namera,
are you trying to say that unclick on the dispersion parameters and non linear parameters of the fiber component.for simulation that is correct. but results will have no practical application as we have standized fiber by ITU.G series.

with regards

[aasif bashir dar]

hi fayiqa,

i recommend you to go through ITU standards of fibers.
G.651——>multimode fibers
G.652——> single mode fbers(by default optisystem parameters are set to it)
G.653 despersion shifted fibers
and so on G.654, G655,G.656

with regards

[Aabid Baba]

Hello fayiqa,
i understood your inquiry. It is a point worth a thought. But as namera said it is an theoretical aspect. If you can find the implementation for such idea then I would suggest you to make the dispersion coefficient more negative and compare the results but yeah as asif mentioned the value should lie in the range provided ITU-T.

Regards

[FAYIQA NAQSHBANDI]

Thank you every one for your response.

Yes asif we cannot simply ignore the effect of dispersion in the fiber.So not considering the effect of dispersion is not a possible solution.

I am actually working with directly modualted lasers.These lasers are inherently chirped.This frequency chirp of the laser then interacts with the fiber dispersion and distorts the signal.What if we use a fiber with a negative dispersion coefficient.Isn’t that sufficient to reduce the positive chirp induced dispersion introduced by DML ?

[alistu]

Hi Fayiqa,

In order to compensate dispersion, the some of the dispersion produced in both pieces of fiber should become zero. If we only use one pice of optical fiber with positive dispersion or just one piece with negative dispersion, then we would not be having zero dispersion as a results.

Regards

[Aabid Baba]

Hi everyone,
Fayiqa you are mentioning to make the dispersion coefficient negative But the question is can you do it on the tool you are working on.. I believe that making dispersion coefficient theoretically seems to reduce dispersion because it is in negative domain but it might have its own repercussions on the system performance.
Can’t you use other techniques to combat dispersion in your system like using fiber bragg grating which is effectively used in dispersion compensation. You can find it in the samples files of optisystem 14.

Regards

[FAYIQA NAQSHBANDI]

Thank you alistu and aabid for responding again.

I guess I am not able to make my point clearly.What I mean is that the DML lasers introduce positive dispersion.So why to use a postive dispersive fiber to make it more positive and worsen the effect more.We can simply skip using the positive dispersive fiber and use a fiber with negative dispersive characteristics.right ?

Regards

[alistu]

Hi Fayiqa,

As you know, the dispersion is a cumulative parameter. The compensation of the dispersion should also be in the same manner. As for the DML, it produced a light with dispersion, but not dispersion slope. So it can only shift the dispersion caused by the fiber, but cannot compensate.

Regards

[aasif bashir dar]

hi fayiqa,

i unterstand your point, and is a valid one.

look for simulation purposes you can do any thing you want to.

but so far as practical consideration you have to make sure that which fibers are commercially available which has negative dispersion coefficient.Such type of fibers are ver very costy with respect to that of single mode fibers which we normally use.

a meter fiber with -38ps/nm.km dispersion will cost arround 3$.i.e your network will not be economic

second thing is that even if you use the fiber with nagative coeffeint of dispersion and laser with positive disperion(because of chirp), a point will in the channel where nagative dispersion will surpass the postive dispersion especially in long haul network.

hence then you have to again use the fiber with postive dispersion coeffint to compensate the overall dispersion

with regards

[alistu]

Hi Aasif,

Some good points are mentioned here, but I don’t think these can be the main reason why the method suggested by Fayiqa is not used in practice. Even though DCF is costly, it is widely used. Apart from that, the dispersion could have been calculated in a way so as to avoid surpassing had dispersion slope been there. But cost is also a very important factor that you brought to our attention.

Regards

[FAYIQA NAQSHBANDI]

Asif.

Thanks for helping me with the cost analysis of my project.Your point is indeed valid and i should actually consider it.
Because i am dealing with long reach networks so possibility of negative dispersion taking control over the positive dispersion of the laser can definitely be an issue again.

God Bless you.
Thanks again

[aasif bashir dar]

you are welcome.

[Aabid Baba]

you are welcome fayiqa !

[aasif bashir dar]

hi all,

talking about chirping due to modulators.

directly modulators gennerally have positive chirp.

but we can can have nagative chirp by proper selection and setting of the externally modulators.thus your convention single mode fiberlike corning-28 can be used and will lead dispersion supported transmission

with regards

[alistu]

Hi Aasif,

The chirp is a frequency distortion for as far as I am concerned, whereas dispersion broadens the signal in the time domain. Can you please verify whether I am right or not? If I am right, chirped signal cannot be used in order to compensate for the dispersion caused by the optical fiber.

Regards

[aasif bashir dar]

hi alistu ,
chirp is a frequency distortion.
but 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)

as far as chirping is considered it is one method of predistortion compensation.
with regards

[alistu]

Thank you for the explanation Aasif. Could you also provide some links indicating the use of chirped optical power source to compensate for the dispersion of the fiber? I am very much interested to read more about this. And one good link would really be enough. Thank you in advance.

Regards

[aasif bashir dar]

hi alistu,

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

[alistu]

Even if chirped signals are used, how can we say negative chirp can compensate for positive dispersion of the optical fiber (as you have stated in your former comments)? This sentence implies that chirp and dispersion can be accumulated together to make the compensation in a direct way.

Regards

[aasif bashir dar]

hi alistu,

look dispersion is all about comparative velocity of different frequency components of pulse.
longer wavelength (higher frequency component of pulse)travel slower than shoter wavlength (lower frequency component of pulseat 1550)(i guess, i may be wrong her but i am talking abouty logic how chirping migitate dispersion)

if a pulse has centre frequency f0 and fh as hiher frequency and fl as lower frequency.

then positive means order will be fl fo fh.
and negative chirp means fh fo fl.

now we have look how these frequency components will travel inside the fiber.
the explanation it would be very long, i suggest you want through chapter ( dispersion compensation method) the book by o p aggrerwal.

there is full fleged explanation and equations in that book.

with regards

[alistu]

Thank you for your explanation. However, I have doubts about some of the stuff mentioned. For example, the speed is a function of refractive index of the medium, rather than the wavelength. It is frequency and wavelength that depend on one another in the same medium (with the same refractive index).

[aasif bashir dar]

hi all
for convenience i am attaching the links;
https://books.google.com/books/about/Fiber_optic_communication_systems.html?id=NfhSAAAAMAAJ

the chapter no. is seven under the name dispersion management.

with regards

[alistu]

thanks for addressing the book. I had a look at the chapter you have mentioned but could not specifically find where it can be concluded that negative chirp can compensate for positive dispersion (I am not saying it is not there in the book, though). So I would like you to help me in this regard as you have kindly suggested the book.

Regards

[FAYIQA NAQSHBANDI]

Hi every one

Aasif,,Are you sure about the negative chirp behavior of signals using external modulators ?
I think external modulators can be used to reduce the effect of the positive chirp but i don’t think they can induce a negative chirp behavior in the circuit.

Regards

[aasif bashir dar]

hi fayiqa,

i am not soo sure but some where i have read that.

but i am sure of the nagative and positive chrip term. the link i have attached (chapter 7)discusses the impact of chirp in dispersion management. there he has analysed it for gaussion pulse . and we can genneral it for any other pulse also.

with regards

[alistu]

You are right about the negative and positive chirp Aasif. However, can you please addree part of Prof. Agrawal’s book where the discussion about dispersion compensation offers a way to compensate for positive dispersion using negative chirp? (I am not saying it is not there in the book, though)

[aasif bashir dar]

hi alistu,
i got what is your cofusion.
gennerally we say that velocity of light depends on the refractive index.

but it is not always the same, velocity may also dependent on wavelength of the travelling wave inside medium

with regards

[aasif bashir dar]

hi all ,
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 equatio called CAUCHY EQUATION FOR REFRACTIVE INDEX.

you can see that

with regards

[alistu]

Thank you for your reply Aasif. I would like to point out that you have been talking about a general application in here, and the relationship says the difference in speed due to wavelength is not that much to be considered in practical purposes such as optical systems design, when the range of wavelength change is rather narrow.

Regards

[aasif bashir dar]

hi all,
hi all there is one more equation called sellmier equation which shows the same, for convience i am attaching few links for cauchy equation 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 alistu

with regards

[alistu]

Thank you. I googled and found all the required stuff. But another thing you mentioned about the relation between speed and wavelength in your comment #31119 was that shorter wavelength have higher speed. This is not clearly consistent with the first figure shown in wikipedia Sellmeier Equation page. can you please explain it a bit more?

[aasif bashir dar]

hi alISTU,
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.

now alistu talking about your comment that shorter wavlength have higher speeds will be in the case of POSITIVE DISPERSION.

this can checked on any genneral book of OFC

with regards

[alistu]

Thank you Aasif. I haven’t been talking about dispersion in general in my few last comments and ISI caused by that. To avoid peripheral issues, can you please address any books or other sources where it is mentioned that lower wavelengths have higher speed, with the exact page so I can refer to?

[aasif bashir dar]

hi alistu,

i had not exact page no. right now, so you could see that lower wavelengths may have large velocity or vice versa.
but if you would have understood the the cauchy formula, that refractive index(R.I) is wavlength dependent.
AS R.i is defined as speed relative to ‘c’. hence different wavelength may different velocity but infinitesimally small, however for longer link it gets accumulated.
for convenience i will attach any pic soon.

with regards

[alistu]

It is totally accepted that wavelength and refractive index (and as a consequence speed of wave) are related. What I am questioning is the relation you mentioned for them regarding shorter wavelengths having higher speed. As I said, access to the formulas is provided for example in wikipedia.

[aasif bashir dar]

hi alistu,
i am attaching a pic of document related to cauvhy formula for refractive index;

Attachments:

1. 

   disp.png
   

[alistu]

Thank you again, but I am more interested that you please clarify about the speed increasing with the wavelength getting shorter. If actually this point about the relation between them can be found on every book, I assume finding it in the textbooks cannot be that hard. Looking forward to your answer Aasif.

Regards

[aasif bashir dar]

hi alistu,
you are taking this in wrong way.

shorter wavlengths can have more velocity THAN larger wavelengths or vice versa depending on material and the the operating wavelength (this all relative velocity).
i already pointed that i can not reffer you exact line or page no of some book.
more over it is genneral inference , after we are learning the positive and negative dispersion.

with regards

[alistu]

I assume we are talking about a case where the same medium is used for both waves whose wavelengths differ and whose velocity we are comparing. And the reason I asked for reference was the fact that you formerly mentioned in every general OFC book this can be found. I strongly believe this comparison of wavelengths is wrong and I refer you to the figure on wikipedia that I have addressed.

Regards

[aasif bashir dar]

hi alistu,
thanks for having good discussion on topic
although wikipia provides first hand information, but we can rely on wikipia, any body can edit content of articles of wikipia.
therefore there is always ambiquity in authencity and originality of any article.

with regards

[alistu]

Please refer to the file I have attached form wikipedia obviously taken from textbooks and quite consistent with other stuff discussed on the very same topic. As it can be seen, with wavelength decrease, the refractive index is increased and therefore, the speed is reduced. This example shows the results both experimental and obtained by Sellimier equation.

Attachments:

1. 

   Sellmeier-equation.png
   

[aasif bashir dar]

hi alistu,

i suggested you to go through cauchy and sellmier eqution to understand the dependece of refractive index on wavelength(as you asked for that).

now you see wikipedia graph, and you beleive it would be always true for other materials, you can not paint the every thing with same brush.this graph is for BK glass.for other materials the graph could be different.

genneral for produccing negative dispersion we introduce a heavy doping in core so to increase its refractive index, the graph is some how inverse of that.

you might have heard of dispersion slope?
what is that, it is not only we have to cnacel the dispersion but we must have match dispersion slope .

regards

[alistu]

The results obtained are for a type of glass. It is absolutely probable that silica used in optical fibers has a similar graph with the same trend but a bit different values. If you believe it is different for other materials, I suggest you introduce the graph.

If we dope the core, then we have changed refractive index by changing the medium. This has nothing to do with the issue of “the change of refraction index with the change of wavelength for the same material”.

I suggest we respectfully do not deviate from the main subject.

Regards

[aasif bashir dar]

hi alistu,

as you had already mentioned to get any document saying that lower travel with greater velocity than the hiher wavelengths or vice versa…
i am attaching the pic of document which confirm it.

Attachments:

1. 

   al.png
   

[alistu]

Thank you very much indeed Aasif. I would be more grateful if you please also introduce the book whose screen shot you have attached here, so I would have a look into it.

My above reply was an answer to you mentioning refractive index is changed by doping, which I found not so relevant. But this indeed helps.

Regards

[aasif bashir dar]

i think you should not have confusion now,

with reagards

[aasif bashir dar]

hi alistu,

the graph you have shown is of positive slope dispersion material.the nagative slope material will have different trend
i.e there refractive index will increse with the wavelength.

i suggest you to read any basic article of nagative and positive dispersion material and there grahps.then you need not my explanation.

with regards

[alistu]

Could you please explain how a laser with negative chip can be used for compensation of a fiber with positive dispersion, as you have suggested? I would be grateful if you give a brief explanation and bring an example from a paper or book like you have in your helpful reply #31418. Thank you in advance.

Regards

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