Fiber Bragg gratings (FBGs) have been written in silica fiber codoped with boron and germanium, using a continuous-wave (CW) 244-nm laser. The thermal decay of the gratings written with and without hydrogen loading is examined over a period in excess of 5000 h. It is shown that the thermal decay of the nonhydrogen loaded gratings is well explained by the power law model proposed by Erdogan. Gratings written in hydrogen loaded fiber, however, do not obey the power law model. A new model is presented based on a log time representation which can be used to predict the decay characteristics of gratings written in hydrogen loaded fiber.

Thermal decay of fiber Bragg gratings written in boron and germanium codoped silica fiber

In-fiber devices enable a vast array of critical photonic functions ranging from signal conditioning (amplification, dispersion control) to network management (add/drop multiplexers, optical monitoring). These devices have become mainstays of fiber-optic communication systems because they provide the advantages of low loss, polarization insensitivity, high reliability, and compatibility with the transmission line. The majority of fiber devices reported to date are obtained by doping, designing, or writing gratings in the core of a single-mode fiber (SMF). Thus, these devices use the fiber only as a platform for propagating light-the device effect itself is due to some extraneously introduced material or structure (dopants for amplification, gratings for phase matching, etc.) There exists another, relatively less explored degree of freedom afforded by fibers-the ability to copropagate more than one mode. Each mode may have a uniquely defined modal dispersion and propagation characteristic. In this paper, we will describe the variety of fiber devices enabled by few-mode fibers-fibers that typically support two to four modes with suitably tailored dispersive properties. We will show that the unique dispersive properties of various modes, in conjunction with the ability to couple between them with gratings, leads to devices that offer novel solutions for dispersion compensation, spectral shaping, and polarization control, to name a few.

Dispersion-tailored few-mode fibers: a versatile platform for in-fiber photonic devices

The phenomenon of decay in UV-induced Bragg gratings is examined. Thermal stability of gratings with large and small index changes are studied and compared. The role of stabilization in enhancing the thermal stability of gratings is explained. Approaches to analyzing and predicting grating decay are reviewed and compared.

Thermal stability analysis of UV-induced fiber Bragg gratings

Practical, low-loss ( 30 dB out-of-band rejection over /spl sim/25 mm spectral widths. The approach adopted is based on the concatenation of two or more tailored broadband chirped gratings fabricated with low short-wavelength loss to maximize the rejection and stopband width in a filter with /spl sim/1 nm passband. The effect of thermal aging for similar complex structures is considered in the context of three different fiber types: standard telecommunication, high germania (Ge)-doped and boron/germania (B/Ge)-codoped. It is shown that while hydrogenated B/Ge-codoped fiber exhibits excellent photosensitivity, the resulting decrease in guiding strength and thermal stability may make germania-doped fiber the more attractive choice.

Fabrication and characterization of bandpass filters based on concatenated chirped fiber gratings

We have analyzed the effects of the fiber cladding radius on the characteristics of long-period fiber gratings. By etching the cladding of a common single-mode fiber, we verified the characteristics experimentally. When by etching we reduce the cladding radius of a common single-mode fiber on which a long-period refractive-index modulation has been imposed, the coupling strength of the core and the cladding modes increases. In addition, the difference in the propagation constants (for a fixed wavelength) between the core mode and the cladding modes increases; hence the resonant transmission dip wavelengths shift to longer wavelengths. The proposed method can be useful in making and detuning long-period fiber grating filters.

Control of the characteristics of a long-period grating by cladding etching.

Radiation-induced losses in rare-earth-doped fibers are analyzed in a novel manner. The mastercurve approach is used for analyzing stability of defects induced by radiation. This method can be applied to examine fibers for space applications.

Analysis of recovery in radiation-induced loss in rare-earth-doped fibers through master curve/demarcation energy diagrams

Summary form only given. In this work, the thermal reliability of gratings in hydrogen-loaded fibers is studied. It is known that the refractive index modulation in the fiber core is caused by UV-generated defects in the fiber core. It may, hence, be expected that a thermally induced decrease in concentration of the defects that are responsible for the refractive index change would also reduce the UV-induced index modulation, which in turn would decrease the spectral width in strong gratings.

Thermal reliability of Bragg gratings written in hydrogen-sensitized fibers

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).

S. Kannan

Papers

Analysis of recovery in radiation-induced loss in rare-earth-doped fibers through master curve/demarcation energy diagrams

Thermal reliability of Bragg gratings written in hydrogen-sensitized fibers

Reliability of long-period gratings