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In this step, ROADM – Reconfigurable Add/Drop Optical Multiplexer was added, which finally allowed to solve the problem with changing bandwidth requirements and future planning, which we discussed in previous articles from this series.
This concept also uses tunable wavelenght lasers in combination with ROADM’s, which offered then ultimate flexibility and capacity increased greatly.

 

Figure 1: ROADM example with tunable laser options

 

Also, like we discussed before in Steps 3, 4 and 5, testing of these links are simple  – just connect the test equipment at the both ends of optical transmission path, and test it. Most likely you would want to measure optical power at appropriate wavelength.

 

Having said that, it is also worth to mention that up to this step, all the links also had same characteristics, and design like:

 

• Number of electrical bits per each optical bit
• Methods of detection

 
And so, speaking of method of detection, during these steps we had discussed up to now, it was straightforward – optical signal was directly detected. It was simple – there is a “1” electrical bit – means optical power is high, and vice versa.

 
So, to sum it up – the amplitude of the optical signal was the one that determined, or carried information. This method is called IM – Intensity Modulation, RZ – Return To Zero,  and OOK – On/Off Keying.
 
But direction detection was sufficient for fiber links with a bandwidth up to 25 Gbit/s or 40 Gbit/s at best. Everything higher these speeds – signal detection becomes a challenge, because of chromatic dispersion and polarization mode dispersion, which distorts the shape of the signal. Thus, due to these dispersions – or distance, or bandwidth will be affected.
 
That is why, new method of the detection were introduced – coherent detection, which came to optical links from radio industry. About this – in next, last part of these article series.
 
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