LMC10NEJ Datasheet, PDF (9/15 Pages) Bookham, Inc. – 10Gb/s Compact InP MZ Modulator with DWDM Laser

Data Sheet
Operating the LMC10NEJ
Control Schemes for the LMC10NEJ
Arm DC Bias Voltage Control
A dynamic control scheme can be implemented to maintain the bias points on the left and right arms of the MZ modulator at the
quadrature point. This will maintain the output pulse train with a 50% eye crossing level. The control circuit needs to track any over
life change in the bias points, thereby maintaining the 50% eye crossing condition.
Dynamic Output Power Control
The power tap (tap) and variable optical attenuator (VOA) can be utilised as part of an optical power control loop. The circuit
should act to maintain a constant power tap photocurrent by changing the VOA voltage. The tap current is set to a reference value
determined during manufacturing test of the transmitter. Maintaining this value over life will ensure the power range given in this
specification can be met. It will also improve the short term power stability of the product over and above that which can be
achieved with the tap and VOA not used.
It is not intended that the VOA should be used for open loop optical attenuation other than that specified above.
Benefits of Using Control Schemes
The product can be used without the control schemes discussed above. But there are benefits of implementing the control loops:-
Maintaining the optical eye crossing to 50% through the life of the product will minimise the variation in extinction ratio and reduce
variations in link dispersion penalty.
If the tap and VOA control scheme is used then the short and long term power stability is enhanced. This is because any changes
in optical performance caused by optical coupling variations over the operational case temperature range, can be compensated for
by the power tap and VOA and overall power stability is improved. This is at the expense of lower start of life power to allow some
VOA margin with which to implement the compensation.
Summary
For more demanding applications at higher link lengths or tighter link budgets the LMC10NEJ may be used with control schemes
which will provide MZ arm bias point control for 50% optical eye crossing.
Use of the tap and VOA in a control loop can offer improved power stability compared to open loop performance.
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