Direct Modulated laser

[Ajay Vyas]

hello all
any bady used direct modulated laser system, I am facing problem in case of 64 QAM

[Ajay Vyas]

There are many examples in OptiSystem by using CW laser, Is any link fro advance modulation scheme by using the DML laser.
Specially OFDM & 64 QAM anybody develop the 128 QAM RoF link working in the same area.

[Heitor Galvao]

I am interested in this topic

[Ajay Vyas]

Thanks Heitor Galvao
I tried the edit the link of CW laser with DML but there are lots of phase noises found at detector. Many researchers worked on 64 QAM on Optisysten can we go for 128 or more as MALTAB.

Please discuss

[Ravil]

Hi Ajay,

What channel bit-rates are you using in your system?

With my understanding of this topic, we can use DML successfully for our bit-rates up to 10 Gb/s. When it becomes higher, the external modulation should be used. When we reaching 100 Gb/s, we should consider a coherent receiver.

[Ajay Vyas]

I am working on Very low bit rate like 30mbps . but in case of DML thee are lots of noise at receiving end.

[Ravil]

It’s known that, theoretically, DML shouldn’t the main reason for such a noise in Optical Comm. System. By the way, what kind of system do you have? Is it all optical or RoF, etc.? What type of laser are you using as a source at your transmitter?

[Ajay Vyas]

I am design a RoF link using CW laser & DML but in Case DML lot of Noise.
How we can overcome those noises. Please find the project file run with cw & DML

[Ravil]

Hi Ajay,

Since I’m traveling, I don’t have OptiSystem “by the hand” and not able to open your file and give you an advice…
Can you, please, send your scheme as a screen shot so I can take a look and try to help you the way I can. Taking a guess, I can assume that stability of modulated laser should play an important role, especially for complex modulation formats. If you have a chance attach your eye diagram as well.

[Ajay Vyas]

Please find the constellation diagram for both CW & DML of 64 QAM signal

[Ravil]

Hi Ajay,

Couple quick questions: was the constellation diagram that you attached measured at the end of optical link or at the end of radio link?
What type of radio channel did you use?

If I’m not mistaken, the coherent detection is used for QAM-64. In this case, my main guess to explain such amount of errors is big amount of phase noise, especially in large dynamic range. To fix this problem phase noise cancellation (PNC) circuits are used (you should be able to find papers about it).

[Ajay Vyas]

Hello Ravil
I go through some of the paper based on the phase noise in direct modulated laser in which they suggest the chirp manage laser (formed by optical filter with DML) . I was try with but there was no very good response .I try with the 54Mbps data rate

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