A group of new optical engineers is designing a 4-channel WDM network operating in the 1550 nm window as a training task
Posted: Thu Jun 09, 2022 4:35 pm
A group of new optical engineers is designing a 4-channel WDM
network operating in the 1550 nm window as a training task during
their induction. During their design, they have encountered a
number of issues that puzzled them. As their line manager, they
have approached you for advice and listed below are the issues they
have encountered. a. When testing a DFB laser which will be used as
one of the WDM transmitters (without any data modulation), they
found that the optical spectrum and power of the laser was stable
when it was biased above threshold. When biased at the recommended
bias current, the optical power was measured at 10 dBm. However,
when the DFB laser (biased at the recommended bias current) was
connected to a 500 m singlemode fibre, the optical spectrum
measured after the fibre was no longer stable, i.e. its wavelength
fluctuates with time. What do you think has caused this scenario?
How would you remove this instability? (3 marks) b. To further
investigate the phenomenon seen in part a, you suggest that they
use an optical circulator in conjunction with the DFB laser and
fibre to observe any light being reflected back to the transmitter.
Taking up your suggestion, the optical engineers measured the
optical spectrum of the backreflected light as shown in Fig. 8.
Explain what has caused this backreflection. How would you advise
the engineers to overcome this reflection? (4 marks) Fig. 8 c.
After solving the issues in part a and b, the optical engineers are
confident to put together the 4-channel WDM network. The 4 WDM
channels are located 100 GHz apart. The optical engineers have
decided to lower each transmitter power to 0 dBm to minimise the
impact of fibre nonlinearity. Based on their chosen parameters, do
you think their 4-channel WDM network will be limited by stimulated
Raman scattering (SRS) ? Please explain your answer. (3 mark
network operating in the 1550 nm window as a training task during
their induction. During their design, they have encountered a
number of issues that puzzled them. As their line manager, they
have approached you for advice and listed below are the issues they
have encountered. a. When testing a DFB laser which will be used as
one of the WDM transmitters (without any data modulation), they
found that the optical spectrum and power of the laser was stable
when it was biased above threshold. When biased at the recommended
bias current, the optical power was measured at 10 dBm. However,
when the DFB laser (biased at the recommended bias current) was
connected to a 500 m singlemode fibre, the optical spectrum
measured after the fibre was no longer stable, i.e. its wavelength
fluctuates with time. What do you think has caused this scenario?
How would you remove this instability? (3 marks) b. To further
investigate the phenomenon seen in part a, you suggest that they
use an optical circulator in conjunction with the DFB laser and
fibre to observe any light being reflected back to the transmitter.
Taking up your suggestion, the optical engineers measured the
optical spectrum of the backreflected light as shown in Fig. 8.
Explain what has caused this backreflection. How would you advise
the engineers to overcome this reflection? (4 marks) Fig. 8 c.
After solving the issues in part a and b, the optical engineers are
confident to put together the 4-channel WDM network. The 4 WDM
channels are located 100 GHz apart. The optical engineers have
decided to lower each transmitter power to 0 dBm to minimise the
impact of fibre nonlinearity. Based on their chosen parameters, do
you think their 4-channel WDM network will be limited by stimulated
Raman scattering (SRS) ? Please explain your answer. (3 mark