Cisco Prisma II bdr Dig Rev 2 1 Dual Mux Sys for High Density Installations Installation Guide
Introducing the Enhanced Digital Reverse Modules
78-741939-01 Rev C
1-6
Functions of the Laser Module Components
Block Diagram of the Double-wide Laser Module
This is a block diagram of the double-wide laser module.
Laser and the Laser Driver
The laser driver prebiases and modulates the laser at levels set by the automatic
power control (APC) circuitry. The laser driver accepts a 2.5 Gbps digital signal and
conditions the signal to drive the laser. The 1550 laser is a nm cooled DFB laser with
a wavelength on the International Telecommunications Union (ITU) dense wave
division multiplexing (DWDM) grid.
Automatic Power Control (APC) Loop
Laser output power is controlled by monitoring the photodiode internal to the laser
package. Some of the light emitted from the laser is coupled off to the photodiode.
The current flowing through the photodiode is proportional to the amount of light
upon it. The APC loop serves to keep the prebias current at the proper level as the
laser ages. The modulation current is set to a constant current, as the slope efficiency
changes little with age. The APC loop has monitoring circuits to light the laser
power LED and send status signals to the status monitor module.
Thermo-Electric Cooler (TEC) Loop
The TEC loop controls the laser’s internal cooler and the external cooler required to
maintain a constant temperature in the node over the full temperature range of
-40°C and +65°C (-40°F to 149°F). The external TEC mounts to the base of the dual
width module, and the laser cradle mounts on the external TEC. The TEC loop
controls the temperature within ± 0.1°C. It provides a temperature status signal to
the monitoring circuit. There is a laser temperature LED that is steady when the
temperature is correct and flashes when the temperature is too high or too low.