  Chirped Bragg volume gratings (CBGs) offer a useful alternative for spectral
analysis, but increasing the bandwidth necessitates increasing the device area.
In contrast, recently developed rotated CBGs (r-CBGs), in which the Bragg
structure is rotated by $45^{\circ}$ with respect to the device facets, require
increasing only the device length to extend the bandwidth, in addition to the
convenience of resolving the spectrum at normal incidence. Here, we multiplex
r-CBGs in the same device to enable spectral analysis in two independent
spectral windows without increasing the system volume. This new device, which
we term an X-CBG, allows for compact multi-band spectroscopy in contiguous or
separated spectral windows in the visible and near-infrared for applications in
nonlinear microscopy and materials identification and sensing.
