Page 700 - Introduction to Information Optics

12. Networking with Optics
star couplers are designed in such way that at one specific output port only
one particular wavelength is constructively interfered. As a result, the multiple
wavelengths from the input of the AWG are demultiplexed.
After decades of intensive worldwide research, the AWG-based DWDM
device has achieved performance suitable for commercial application. Its main
drawback is that its poor temperature coefficient requires an active tempera-
ture controller, which adds an additional bundle to network management.
Nevertheless, AWG takes advantage of silicon optical bench waveguide tech-
nology and can be manufactured in large quantities at one time. It is a
cost-effective solution for a single chip handling many channels.

12,2.3.5. Mach—Zender Interferometer-Based Interleaver
The Mach-Zender (MZ) interferometer is a basic interference device. It
typically consists of two 3 dB couplers interconnected by two optical paths of
different lengths, as shown in Fig. 12.11. The first 3 dB coupler evenly splits the
input signals into two parts, which experience different phase shift after passing
through two different paths. The two lights interfere at the second 3 dB coupler
when combined together. Because of the wavelength-dependent phase shift the
power transfer function of the device is also wavelength dependent. In matrix
form it is simply
2
sin (7i x n x AL/A)
(12.9)
2
cos (7i x n x AL/A)
where n is the refraction index of the waveguide material and AL is the path
difference,
A single input port MZ interferometer with certain value of the path
difference AL can be used as a 1 x 2 demultiplexer. Multiwavelength light
appears on the input port. When the input wavelength A { satisfies the condition
nAL/A,- = m-J2 for any positive odd integer m,-, the wavelength A,- appears on
the first output port due to constructive interference. Similarly, the wavelength
A,-, which satisfies the condition nAL/A,- = m,-/2 for any positive even integer m f .

Path difference introduction

Fig. 12.11. Schematics of an M-Z inteferometer.