Distributed feedback laser.

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written 8.5 years ago by

teamques10 ★ 68k

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.

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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.

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