Final answer:
The minimum permissible channel spacing for a WDM system using uncooled DFB lasers in the 1300 nm band over a temperature range of 15°55C is at least 4 nm, calculated based on the average temperature coefficient of wavelength drift for uncooled lasers, which is about 0.1 nm/C.
Step-by-step explanation:
The question involves a Wavelength Division Multiplexing (WDM) system, which is a technology used in fiber optic communications to combine and transmit multiple wavelengths (channels) of laser light simultaneously over the same fiber. In this scenario, a system using uncooled distributed feedback (DFB) lasers in the 1300 nm band is designed for indoor operation over a temperature range of 15°55C. The student is asking for the minimum permissible channel spacing in nanometers (nm), under the consideration of temperature-induced wavelength drift of the uncooled DFB lasers.
Laser wavelength in DFB lasers varies with temperature due to changes in the refractive index of the laser medium and the thermal expansion of the laser cavity. Although the exact channel spacing will depend on the laser's temperature coefficient, for uncooled lasers, a generally accepted average coefficient is around 0.1 nm/C. Considering the maximum temperature range of 40C (from 15C to 55C), the wavelength could drift by about 4 nm (0.1 nm/C × 40C). Therefore, the channel spacing must be at least larger than this drift to avoid channel overlap, making 4 nm the minimum permissible channel spacing.