Mumbai University > Electronics and telecommunication > Sem 7 > optical communication and networks

Marks: 08

Years: DEC 2015

GRATINGS :

• A grating is an important element in WDM systems for combining and separating individual wavelengths. Basically a grating is a periodic structure or perturbation in a material.

• This variation in the material has the property of reflecting or transmitting light in a certain direction depending on the wavelength.

• Thus gratings can be categorized as either transmitting or reflecting. Here we will concentrate on reflection gratings, since these are widely used in optical fiber communications.

Grating principle

• Figure 4.7 defines key parameters for a reflection grating. Here $θ_i$ is the incident angle of the light, $θ_d$ is the diffracted angle, and ˄(lambda) is the period of the grating (the periodicity of the structural variation in the material).

• In a transmission grating consisting of a series of equally spaced slits, the spacing between two adjacent slits is called the pitch of the grating.

• Constructive interference at a wavelength λ occurs in the imaging plane when the rays diffracted at the angle $θ_d$ satisfy the grating equation, given by

$˄(\sin \theta_i -\sin \theta_d)= m\lambda$

• Here m is called the order of the grating. In general, only the first-order diffraction condition m=1 is considered

• A grating can separate individual wavelengths since the grating equation is satisfied at different points in the imaging plane for different wavelengths.