  A comb spectrum generating device based on Bragg grating superstructures with
gain and loss is suggested in this paper. It includes a comprehensive analysis
of the device formulation, generation and manipulation of the comb spectrum
with a number of degrees of freedom such as duty cycle, sampling period and
gain-loss parameter. For applications such as RF traversal filters and tunable
multi-wavelength laser sources, the reflected intensities of the comb resulting
from the superstructures should have uniform intensities, and this is
guaranteed by optimizing the physical length of the device, gain and loss in
the unbroken $\mathcal{PT}$-symmetric regime. Alternatively, it can be
accomplished by reducing the duty cycle ratio of the superstructure to
extremely small values in the broken $\mathcal{PT}$-symmetric regime. Such a
customization will degrade the reflectivity of the conventional grating
superstructures, while it gives rise to narrow spectral lines with high
reflectivity in the proposed system. Remarkably, combs with an inverted
envelope are generated for larger values of gain and loss.
