64 QAM

Could you please attach your project file? The constellation looks too ordered to be a result of a noise source.

I tried out your project and there was one or two problems I saw. First of all I didn’t get the ordered circular signal at the receiver side. I got a very noisy QAM signal with an almost random distribution. The problem here is the implementation of the Laser source. If you are going to use the measured component then it cannot be run in digital mode, because the input signal is not as simple as a digital signal. You will also need to provide an electrical bias to the input signal and attenuation so that Laser is operating above threshold and below saturation (Max power). Try playing around with the Laser source and look at the optical signal until you can get a cleaner output.

Hope this helps!

You might also want to try using a CW Laser and a Mach Zehnder modulator, so that the input signal is less distorted by chirp (and saturation/threshold effects). Once the transmission link is working you can change to directly modulated laser source.

Could you please attach your most up-to-date project file with the CW Laser?

Ok so there are few changes you should make:

1) The Mach-Zehnder Modulator automatically normalizes the electrically input, so the gain and bias components are not really doing anything. The same goes for the gain on the output. However, you can keep them there if you want.

2) The real problem is the Low Pass Filter in the Quadrature Demodulator, you should play around with this value to get better results. I was getting pretty good constellations for a cutoff frequency of 50-100 MHz.

You might also want to create your own demodulator circuit so you can see view the spectrum before the filter. Double click the Quadrature Demodulator and look at the help file to see how it is built.

Hope this helps!

The problem with using a directly modulated laser source for QAM, especially 64 QAM, is that they inherently introduce a lot of phase noise and also a chirp across each pulse. For QAM, where phase coherence is very important to the signal integrity, I don’t see a very simple solution to this problem. Now there might be a way to compensate for the chirp over each pulse, but this is not my area of expertise.

I did read an article recently on mitigating chirp in directly modulated lasers:

A. Karar, M. Yañez, Y. Jiang, J. Cartledge, J. Harley, and K. Roberts, “Electronic dispersion pre-compensation for 10.709-Gb/s using a look-up table and a directly modulated laser,” Opt. Express 19, B81-B89 (2011).

But again even this paper was for NRZ, where phase information plays a lesser role. However, if you figure out a way for reliably transmitting QAM with a directly modulated laser source, I’m sure you will have a bright future in academia!

Good luck!

hi ajay……. i m doing the research about same field, i need to share something with u…. pls help and send your mail id