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The phase masks produced at INO could be divided into two main categories. "Standard" PMs were fabricated with a period as constant as possible for wavelength-division multiplexing applications. On the other hand, "chirped" PMs were fabricated with a slightly varying period with respect to position. They were used in dispersion compensation applications, dispersion being a major cause of optical signal degradation over long propagation distances.
A few clarifications
Due to the very nature of the holographic process, phase masks (PMs) are difficult to fabricate and require specialized equipment. However, PMs must be viewed as master elements useable in the production of many identical FBGs. In fact, this is their main benefit: the use of PMs allows for the main fabrication challenges to be limited to the PMs alone, and not to the multiple FBGs to be produced. Moreover, as opposed to the direct holographic recording of FBGs which would need to be realized in the UV, phase masks can be holographically recorded at visible wavelengths where good quality optical beams are more easily generated.
A phase mask must be built with a period distribution identical to that required on the corresponding FBG. When illuminated by a UV beam, the ±1st diffraction orders of the mask interfere with each other to produce interference fringes stabilized to the grating itself, which is etched in fused silica. Taking advantage of this particular point, an FBG can then simply be recorded in a photosensitized fiber brought into close contact with the PM in the UV interference field generated. This kind of FBG recording setup considerably reduces vibration, coherence, and alignment constraints. It also lessens requirements on incident UV laser beam optical quality. Essentially, the main precaution is to prevent any movement of the PM with respect to the fiber during recording.