Reliability of long-period gratings

Summary form only given. Ever since their conception in 1996, long-period gratings (LPGs) have come to be used in a wide variety of applications. LPGs involve coupling of core and cladding modes. The importance of thermal stabilization techniques for successful use of Bragg gratings is now well acknowledged. This is even more important in LPGs, because the wavelength at which the coupling from the core to the cladding modes takes place is directly dependent on the UV-induced refractive-index change (/spl Delta/n/sub UV/) and on n/sub core/-n/sub clad/ (/spl Delta/n). A decrease in these can greatly shift the spectra in a LPG. For example, thermal conditions that cause a 2% decrease in n/sub UV/ would have a negligible effect in a add/drop wavelength-division multiplexed (WDM) Bragg grating (/spl Delta//spl lambda/<10 picometers) but a similar decay in the case of a LPG would shift its center wavelength by about 0.5 nm, which is unacceptable for most applications (such as gain equalizers). In this paper, we present a method to analyze and predict LPG decay. Using our analysis method we have arrived at stabilization conditions yielding LPGs with predicted end-of-life (25 years) shifts of <0.02 nm at 40/spl deg/C and <0.12 nm at high aggressive conditions (60/spl deg/C).