Aperiodic phase masks for inscribing complex multi-notch OH-emission filters for astronomy

We demonstrate for the first time, a new type of aperiodic phase mask (APM) for fabricating multi-channel aperiodic fiber Bragg gratings. The mask is made of individual diffraction phase gratings with discrete unequal phase-steps incorporated at periodic locations. The diffraction at the discrete phase-steps in the phase mask produces corresponding half phase-steps at periodic locations along the fiber. The accumulated phase, along with index modulation, generates the desired multinotch reflection spectrum. Complex fiber Bragg grating filters fabricated using APM, in a standard phase mask based fabrication setup, can be used to simultaneously suppress multiple aperiodic OH emission wavelengths in near infrared (NIR) existing in upper atmosphere, and increase the sensitivity of ground based telescopes.