Page 697 - Introduction to Information Optics

12.2. Optical Network Elements 681
Single cavity transfer function
Multiple cavity transfer function

Center Frequency Frequency (wavelength)
Fig. 12.7. Characteristic comparison of single- and multiple-cavity dielectric thin-film filters.

12.2.3.3. Diffraction Grating
As shown in Fig. 12.9(a), a reflective diffraction grating is a mirror with
periodically corrugated lines on the surface. The diffraction grating functions
to reflect (or transmit in the case of transmission grating) light at an angle
proportional to the wavelength of incident light. The general equation for a
diffraction grating is
ml. (12.7)

where d is the period of the lines on grating surface, / is the wavelength of the
light, B i is the incident angle of the light, 0 d is the angle of diffracted light, and
m is an integer called the order of the grating. In Fig. 12.9(b), multiwavelength
light impinges on the grating at the same angle. Grating separates the
wavelengths because each different wavelength leaves the grating at a different
angle in order to satisfy Eq. (12.7). This is how diffraction grating works in the
DWDM multiplexer/demultiplexer.
The most distinguished property of this technology is that its insertion loss
is generally independent of the channel numbers. Hence, it is very attractive for
DWDM systems with a large number of channels. However, its performance
is very sensitive to polarization effects, and passband flatness is poor, with a
round shape rather than a flat top, which requires tighter wavelength control
for system operation in the field.