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Describe with diagram the structure of a distributed feedback laser.

Marks: 10M

Year: May2015, May2012

• The spectral display of fabry-perot laser shows that it consists of central highest amplitude of modal component and others have progressively lower amplitude.

• This is a disadvantage when used in optical communications systems because its wide spectral width significantly increases chromatic absorption, with a consequent reduction of usable transmission bandwidth.

• In DFBs, the main design objective is to generate a single line spectrum at the output, under high data rates of modulation.

• This is achieved by incorporating a corrugated layer between below the active layer of the DFB device.

• The hills and valleys type structure generate a constant change of the refractive index.

• This change in RI contributes to device’s feedback mechanism, so that a single mode is produced and the undesirable modes are suppressed.

• In order for the structure to operate as a DFB device, the grating period must satisfy the given condition:

$$g_p= \frac{λ_{mode}}η$$

$ \ \ \ $ Where,

$ \ \ \ $ $g_p$ is the grating period

$ \ \ \ $ $λ_{mode}$ is the operating wavelength

$ \ \ \ $ η is the refractive index of the effective mode

• In the DFB Laser the optical grating is usually applied over the entire active region which is pumped.

• The unpumped corrugated end regions effectively act as mirrors whose reflectivity results from the distributed feedback mechanism which is therefore dependent on wavelength.