frequency chirp

[Aabid Baba]

I wish to ask if there is some technique to reduce frequency chirp and at the same time compensate dispersion for a system working at 60Gbps.??

responses will be appreciated

[aasif bashir dar]

hi abid,

directly modulated lasers(DMLs) have high frequency chirping.IN order to reduce frequency chirping i suggest you to use externally modulated lasers(EMLs). especially the LiNb-MZM.

how i have seen that the DMLs provide better proformance for long haul communication link than DMLs.

thus it depends on the nework you are using

with regards

[Aabid Baba]

Yeah fine. i have already used it and theoretically it is right. But i want to know what technique could we exactly use to overcome all the frequency chirps and dispersions to make our system much more optimum and performance effective. I have also noticed that for long haul directly modulated lasers give better performance than externally modulated lasers.
thanks anyway

[alistu]

Hi Aabid,

As far as I know, externally modulated lasers show better performance in general and in many aspects, as they produce higher quality optical signal with narrower spectral linewidth. The extinction ratio of them is also higher than that of directly modulated lasers. Do you agree?

Regards

[Aabid Baba]

Hi Alistu,
i completely agree with you. The reason that external modulated lasers are used is to overcome the power fluctuations and the high end chirp caused by the direct modulated lasers because of their ON-OFF operation especially when using RZ type of modulation scheme. It also limits the maximum transmitted bit rate of our systems. Undoubtedly, as you said externally modulated lasers have better performance. But is has been observed that for long reach networks, the performance is better for DMLs rather than EMLs.

regards

[alistu]

Since in long reach systems, the signal is more prone to becoming corrupted due to being disposed to more optical fiber impairments caused by the greater length of the transmission line, I wouldn’t expect such a behavior. So can you please introduce a certain paper that verifies this? Thank you.

[Aabid Baba]

Hi alistu,
I don’t remember the name of the paper right now but it was clearly mentioned that when working with high transmission powers and for long reach optical networks , direct modulated lasers give much better performance than externally modulated lasers. Also because of the feedback thing involved in externally modulated lasers, the performance degrades with high power and long reach. You may find it by googling if you want to.

regards

[aasif bashir dar]

hi aaabid
have a look on these papers.
1.https://www.osapublishing.org/abstract.cfm?uri=OFC-1986-WBB5
2.http://digital-library.theiet.org/content/journals/10.1049/el_19850755
3.http://ieeexplore.ieee.org/xpl/abstractKeywords.jsp?reload=true&arnumber=4251592

you would get some idea of reduction of the chirping of external modulators

with regards

[alistu]

Hi Aasif,

Thank you for the links and they are useful. However, as a member I’d rather see a more direct answer to the questions rather than a whole paper. I believe this way to be more convenient and informative. Otherwise, the links can usually be found by simply googling the subject. So while I appreciate you helping us all, I recommend that if you are using links in your posts, you would also incorporate a small summery of each paper or link, mentioning the main point or the novel method introduced in each.

Best regards

[Aabid Baba]

asif these paper don’t contain any substantial help. These focus on modulation schemes rather than on techniques required for reducing the frequency chirps. Any other suggestion which can be implemented in optiwave itself. I am using both techniques direct as well as external modulated laser schemes and unfortunately in both frequency chirp is introduced though less is in cooperated in external modulated laser.

[Aabid Baba]

hello asif, these paper don’t contain any substantial help. These focus on modulation schemes rather than on techniques required for reducing the frequency chirps. Any other suggestion which can be implemented in optiwave itself. I am using both techniques direct as well as external modulated laser schemes and unfortunately in both frequency chirp is introduced though less is in cooperated in external modulated laser.

Regards

[FAYIQA NAQSHBANDI]

Hi aabid

If you have designed a high capacity system and for long transmission reach,the effect of dispersion and chirp cannot be really avoided to a large extent.
And compensating both by using a single component would anyway be difficult.
Try using dispersion compensating fibers for dispersion compensation and u can use filters for reducing the effect of frequency chirp.

Regards

[Aabid Baba]

Hi fayiqa,
Thank you for the suggestion. you made a valid point that for long transmission reach,the effect of dispersion and chirp cannot be really avoided.
I though tried to use fiber bragg grating to compensate both because fiber bragg grating is said to be used for both :
– dispersion compensation using fiber bragg grating ( chirped FBG)
– avoiding frequency chirp (because FBG can be used as filter too)
But to my surprise , the performance didn’t improve a lot.

regards

[FAYIQA NAQSHBANDI]

Hi Aabid

Yes.A chirped FBG can be used for compensating the dispersion.But the same chirped FBG cannot be used for removing the frequency chirp.
Chirp compensation can be done using a component that can convert your frequency modulation to amplitude modulation. or even a component that can neutralize the effect of phase modulation can be used to remove the effect of the induced chirp.

Regards

[Aabid Baba]

Hi fayiqa,
Thanks for explaining it in a better way. You mentioned about using a component that can convert your frequency modulation to amplitude modulation or even a component that can neutralize the effect of phase modulation , Can you please tell me what component i may use in Optisystem.
Regards

[FAYIQA NAQSHBANDI]

Hi Aabid.

You can use a Mach-zender inteferometer for removing the effect of the chirp.It can be used to introduce a phase shift in the signal which is opposite to the phase shift introduced by the actual signal.Make sure one arm of the inteferometer recieves the normal input while the other input arm recives the phase shifted version of the same signal.This difference in phase shift can be used to cancel the effect of the phase modulation induced by the chirp in your circuit.
Hope its clear.

Regards

[Aabid Baba]

Hi fayiqa,
Thank you so much. It can well be quite fruitful if able to implement it on optisystem. Can you please send me a link or something where the mach-zehnder inferometer has been used for phase conjugation or anything like that. I would appreciate your efforts.
Regards

[Ranjeet Kumar]

hi,
Frequency chirping means broadening of laser linewidth it occurs due variation in laser refractive index. I think it can be reduced by fiber gratting.
https://books.google.co.in/books?id=ok0XX-3MgMoC&pg=PA355&lpg=PA355&dq=how+to+reduce+frequency+chirp&source=bl&ots=LfUexxRuqW&sig=Utj4mHhjMpn7ffk2wxtOpYxPMec&hl=en&sa=X&ved=0ahUKEwj9iYXtvrPLAhUGto4KHXoKCvcQ6AEIXDAI#v=onepage&q=how%20to%20reduce%20frequency%20chirp&f=false
https://perso.telecom-paristech.fr/~gallion/documents/free_downloads_pdf/PG_revues/PG_R11.pdf

[Aabid Baba]

Hello ranjeet,
thanks for your response and your efforts. I appreciate
I know about the theoretical aspect of the frequency chirp but what i want is an effective method that would provide me frequency and dispersion compensation simultaneously.
I had started another topic. You can refer to that because we have discussed it in detail there.

Regards

[aasif bashir dar]

hi all,
i do not agree with the definition of ranjeet that”Frequency chirping means broadening of laser linewidth it occurs due variation in laser refractive index. I think it can be reduced by fiber gratting”

chirping of optical pulse can also occur because of SOA, there it not due to refractive index variation but is due to the saturated amplification variation.

with regards

[aasif bashir dar]

2ndly , i mentioned in in previous comments on chirping and dispersion compensation that chirp can enhance or migitate the dispersion.

FBG can be used as dispersion compensator , but not CHirp migitator. it seems that migiting chirp and dispersion are same , but they are not the same.

with regards

[Aabid Baba]

Hi asif,
If you check the above comments fayiqa has pretty well explained different techniques that we can use like c we an use a Mach-zender inteferometer for removing the effect of the chirp.It can be used to introduce a phase shift in the signal which is opposite to the phase shift introduced by the actual signal.Make sure one arm of the inteferometer recieves the normal input while the other input arm recives the phase shifted version of the same signal.This difference in phase shift can be used to cancel the effect of the phase modulation induced by the chirp in your circuit.

Regards

[aasif bashir dar]

hi aabid,
i was not taking about Mach-zender inteferometer.i was commenting on the definition of chirp and its migitation by ranjeet…
what you have replied is out of the context of my comment.

but it is correct theoretical point
still thanks for reply

[Aabid Baba]

hello Asif,
i was mentioning your comment about compensating dispersion which you had put forth by saying using a fiber bragg grating and if you read through my comments i very well mentioned above COMMENTS which include all the comments above posted by fayiqa and if you go through them you will find her saying same which you mentioned about fiber bragg grating and frequency chirp compensation .
It is part of discussion too. always welcomed

regards

[aasif bashir dar]

hi aabid,

frequency chirp and dispersion are two entities, frequency chirp may icrease or reduce dispersion, you can not use frequency chirp and dispersion compensation interchangibly..
however regarding your comment, i am well aware of the use of fiber bragg grating for the dispersion compensation.

with regards

[Aabid Baba]

Hi asif ,
you didn’t get what i was actually mentioning in my comment. What you just mentioned has already been mentioned by fayiqa in the above comments if you check all the previous comments. I have no query about the frequency chirp and dispersion compensation and the difference between the two.
I had asked about different ways or methods of reducing frequency chirp and at the same time compensate dispersion for a system working at 60Gbps.
Hope it is clear.
Regards

[umer syed]

hi all,
The basic propagation equation that governs pulse evolution inside a single-mode fiber,considering expanding B to the third order of is given by (Agrawal, 2010)(see figure for equation).

Upon propagation in a linear lossless dispersive medium, a monochromatic plane wave of frequency  traveling a distance z in the z direction undergoes a phase shift and the pulse will be broadened. Figure shows the propagation of an initially unchirped Gaussian pulse
through a fiber with anomalous dispersion (B2 = GVD < 0). The pulse remains Gaussian, but its width expands and it becomes chirped with a decreasing chirp parameter (downchirped).In this case the pulse has a decreasing instantaneous frequency.

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[umer syed]

hi all,
A pulse can acquire a chirp e.g. during propagation in a transparent medium due to the effects of chromatic dispersion and nonlinearities (e.g. self-phase modulation arising from the Kerr effect). In semiconductor lasers or amplifiers, chirps can also result from refractive index changes associated with changes in the carrier density. The chirp of a pulse can be removed or reversed by propagating it through optical components with suitable chromatic dispersion.

with regards

[FAYIQA NAQSHBANDI]

Hi all,
I see that there has been a lot of discussion going about the frequency chirp and dispersion compensation techniques. Well, The spectrum of the output spectrum has a multi-peak structure and a very well- expressed red shift. Comparing this spectrum with the spectrum of the amplified Gaussian pulse, we can see that it is different. Therefore, the form of the spectrum and the amount of the red shift strongly depends on the initial pulse shape.
thanks and regards

[Aabid Baba]

Hello fayiqa,
Thanks for the reply. I appreciate.
Regards

[Manoj Kumar]

Hi Aasif,

Thank you for the links and they are useful. However, as a member I’d rather see a more direct answer to the questions rather than a whole paper. I believe this way to be more convenient and informative. Otherwise, the links can usually be found by simply googling the subject. So while I appreciate you helping us all, I recommend that if you are using links in your posts, you would also incorporate a small summery of each paper or link, mentioning the main point or the novel method introduced in each.

Best regards

[Author]

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