Solution:

DBR lasers are very similar to the DFB lasers described in the previous section. The major difference is that where DFB lasers have a grating within the active region of the cavity, DBR lasers have a partitioned cavity with the grating in a region that is not active (amplifying).

The reason for this structure is that the refractive index within the cavity of a laser changes during operation due to changes in temperature and electron flux.

Change of refractive index in the grating region of course changes the operational the wavelength of the device.

If you put the grating into an inactive extension of the cavity then there is a lot less wavelength variation from these causes. (Because the characteristics of the material immediately adjacent to the grating are not being changed by the laser's operation.) figure shows a schematic of this structure.

However, end mirrors are almost always used in an asymmetric configuration. Typically this is a 30% mirror on the “back” facet and either just a cleaved facet or a 4% mirror on the front (output) facet. DBR lasers typically produce a single line only with a linewidth of around .0001 nm.

The major problem is that there can be significant absorption in the inactive region near the grating(s). This causes a loss in efficiency.