Page 704 - Introduction to Information Optics

688 12. Networking with Optics

A major function of the transmitter transponder is to generate highly precise
and stable wavelengths for DWDM transmission. Since a DWDM system
transmits many wavelengths carrying a variety of traffic simultaneously down
an optical fiber, it is important that these wavelengths do not drift away from
their designated value and interfere with traffic carried by other wavelengths.
This requires that a transmitter for a DWDM system has very stable
wavelength, in addition to a specific wavelength value. At 1550 nm, 100-GHz
frequency spacing is about 0.8 nm wavelength spacing. The semiconductor
laser diode (LD) wavelength has a temperature shift of about 0.1 nm/ C. An
8° difference in LD temperature will cause the wavelength to move from one
ITU grid to the next. In DWDM transmitter design, the LD of choice is the
single-frequency multiple-quantum-well (MQW)-distributed feedback (DFB)
laser. The DFB is made to the desired ITU frequency/wavelength. A thermal
electric cooler (TEC) control loop is applied to keep the LD temperature
constant. For a DWDM system with channel spacing ^100 GHz, an addi-
tional wavelength locker is also used, in combination with a TEC loop, to lock
the wavelength to the ITU grid. The most widely used wavelength locker is
based on a glass or air-gap Fabry-Perot etalon. Shown in Fig. 12.15, the
first-generation wavelength locking operation taps off 1% to 5% of the laser
output light, and feeds it both to photodiode A through a highly stable etalon
and directly to photodiode B. The etalon acts as a bandpass filter, allowing
only ITU grid wavelength to pass through. Deviations in laser wavelength
from ITU grid will result in change of optical power detected by photodiode
A. The electrical output feedback signal is generated by comparing the signals
from the two photodiodes, and is sent to the TEC of the laser to adjust the
laser chip temperature, which in turn adjusts the laser wavelength accordingly.
The thermal stability (<0.04 GHz/°C) of the etalon is a key to this wavelength

Output
Etalon
Input

TEC of DFB
Fig. 12.15. Wavelength locker for DWDM systems.