Question 1 (Total Marks: 22 marks)

You are a newly employed optical engineer in a semiconductor laser company. Your first assignment is to design a Fabry Perot laser with the following specifications :

a.   If the reflectances of the end facets are the same, show that the required        reflectance of the designed laser in order to meet the specifications is 0.3646.

(4 Marks)

b.   What is the refractive index of the laser active region if there is no coating           applied to the end facets? (i.e. only laser-air interface) (3 Marks)

c.   While designing the laser, you notice that light exits via both ends of the laser.     However, only one end of the light is designed to be coupled out as the laser        output while the other half of the light is not used and is lost in the packaging.      You have decided to add a mirror at one of the laser facets to prevent the light     from exiting the laser while keeping the other facet untouched. By doing so, your design no longer meets the specification. Assuming your mirror enables 100%      reflection, calculate the new threshold current and slope of the LI curve.

(6 marks)

d.   Explain intuitively what happens to the threshold current and the slope of the LI curve with the change that you have made in part c, and why is it so.

(2 Marks)

e.   Your client is not happy with the new design as it does not meet the initial             specifications (threshold current and slope of the LI curve), although your client   does like the idea of only having light exiting via one facet only. Your client insists that the initial specifications be met in this new design (threshold current and      slope of LI curve). Find the new reflectance of the end facet where the light exits that would meet the threshold current specification without changing other         parameters. (3 Marks)

f.    Explain and show how you will go about modifying your design to attain the new reflectance that meets the client’s specifications while ensuring that light only     exits via one facet. Show all calculations that led to your answer. (4 Marks)

Question 2 (Total Marks: 21 marks)

Your company has won a tender to build a 3-node metro network based on optical fibre     infrastructure connecting three cities. After a brain-storming session, the final architecture will be based on a ring configuration as shown in the figure below.

In this architecture, each node will only be able to receive an unique wavelength assigned to it and able to then communicate with the other two nodes based on this wavelength              assignment. For instance, Node A only receives a channel at wavelength lA while able to       communicate with Node B by transmitting at wavelength lB and communicate with Node C  by transmitting at wavelength lC . With this configuration, each node will be equipped with a tunable transmitter that is able to tune to the wavelength of the node it wants to                    communicate with. The optical signals propagate through the ring in the clockwise direction only. Therefore for Node A to communicate with Node C, the optical signal at lC from Node  A will have to pass through Node B to get to Node C.

Your team has been assigned to design the optical add-drop multiplexer (OADM) within the node as shown in the figure. Here you can assume that the traffic within the ring is                controlled by the higher layer protocol such that no collisions will occur. Your task is fully     focused on the physical layer design.

a.   Using only 3-port circulators, fibre Bragg gratings (FBGs), 2x1 50:50 wavelength         independent couplers, design the optical add-drop multiplexer (OADM) in node A.    Assume that all the components that you used are ideal and operate at the correct  wavelengths. Sketch your design and clearly label the ports and components. Briefly describe how your OADM operates. (7 marks)

b.   Shown below are the losses of the components that you used to design the OADM:

Based on your design in part a, calculate the loss (in dB) that the optical signal will experience as it propagates to the:

i.     ‘Drop’ port

ii.    ‘Add’ port

iii.   ‘Through’ port (3 marks)

c.   The finance department made an error while ordering the components. Instead of     ordering 3-port circulators, they have placed an order for isolators. Due to the             urgency of the project, your team decides to use the ordered isolators in place of the 3-port circulators. Explain how you would modify your OADM design to                          accommodate for this replacement. Here you can assume that you have an                  abundance of the wavelength independent couplers and FBGs that you can use in      your modified design. Sketch your modified OADM and clearly label the

components. (6 marks)

d.   Based on your modified design and the losses of the components shown in the table below :

Recalculate the losses the signal will experience as it propagates to the

i.     ‘Drop’ port

ii.    ‘Add’ port

iii.   ‘Through’ port (3 marks)

e.   Comparing your results in part b and part d, what would be your preferred                configuration? And why? (2 marks)