What limitation of Fabry-Perot (FP) lasers is resolved by distributed feedback (DFB) lasers?

The correct choice identifies the limitation of side modes in Fabry-Perot (FP) lasers that distributed feedback (DFB) lasers effectively address.

FP lasers can emit light at multiple wavelengths due to their resonant cavity design, which leads to the generation of unwanted side modes—interference patterns that can degrade the performance of the laser in terms of spectral purity and stability. These side modes can interfere with the principal mode of operation, making the output less coherent and less effective for applications that require precise wavelength control, such as in telecommunications.

DFB lasers, on the other hand, incorporate a periodic structure in the gain medium that introduces a distributed feedback mechanism. This design significantly enhances the laser's ability to operate on a single wavelength, suppressing the side modes and thereby improving output stability and purity. As a result, the DFB laser is generally preferred in applications requiring high spectral performance, such as coherent communication systems.

In the context of the other options, while low power efficiency, high manufacturing costs, and temperature sensitivity can be relevant factors in the broader discussion of laser technologies, they do not specifically address the primary concern of side mode suppression that DFB lasers overcome.