In today's dense wavelength-division multiplexing (DWDM) systems, fiber Bragg gratings (FBGs) are one of many technologies that have become increasingly important. While few technologies can compete with the performance and large spectral slopes achieved by FBGs in filtering, multiplexing, and dispersion-compensation management, the use of FBGs has been difficult to achieve in practical WDM applications that require a broad spectral range.
In fiber containing a Bragg grating, the glass/air or glass/polymer coating interface causes the fiber to support a large number of cladding modes. These cladding modes create loss peaks in transmission at wavelengths below the fundamental-mode reflection band. To compensate, most commercial WDM gratings are made using high delta (Δ) fibers (that is, fibers with high-refractive-index contrast between core and cladding) to extend the width of the wavelength window unaffected by cladding-mode coupling loss. Unfortunately, the mismatch between high delta fiber and standard fiber typically results in unusually high insertion loss, and the practical operating window is 10 to 15 nm wide at best.
Significant effort has been focused on creating a method to suppress cladding modes in fiber Bragg gratings. The Corning PureMode Engineered Fiber group (Corning, NY) has demonstrated a novel fiber-based method that dramatically increases the suppression of cladding modes (see photo here and on cover). The research findings make a new range of applications practical by taking advantage of the complete operating region for WDM systems, which is expected to stretch over more than 100 nm in future systems.
The likelihood of coupling between any two modes by a grating can be calculated by an overlap integral done over the cross section of the fiber. This is a measure of how much the modes involved interact with each other and with the grating. The overlap integral, as well as grating strength and length, determines the total coupling between two modes in a grating. If the grating is made constant over the fiber cross section, the overlap integral disappears for intermodal coupling due to modal orthogonality. In a practical sense, the fundamental mode, LP01, has nonzero power only around the center of the fiber. It is generally sufficient to make the grating profile constant over the center region of the fiber. This includes the core and a small ring in the cladding near the core. The grating profile, when normalized to its peak, is called the photosensitive profile
An example of this approach is a fiber that has Δ = 0.69%, radius of 3.3 µm, photosensitive cladding thickness of 3.3 µm, and cutoff wavelength of 1460 nm. The refractive index of the photosensitive cladding is made to be the same as that of the rest of the cladding by using a combination of germanium and boron doping. The use of boron doping in the photosensitive cladding also helps achieve similar core and cladding photosensitivities. At Corning, this has been found to work for both hydrogenated fibers and nonhydrogenated fibers, although to a differing extent. The ternary glass allows independent adjustment of refractive index and photosensitivity.
The modeled tolerance in photosensitive profile to achieve a certain level of cladding-mode suppression is shown in Fig. 1. Note that 0.1-dB cladding-mode loss for a 50-dB grating cannot be achieved. The photosensitive region is not large enough in this case; a widening of the photosensitive region would be required.
To optimize fiber performance, a series of fibers with systematic changes in composition are made and their photosensitive profiles are characterized. A photosensitive profile is obtained by first writing a grating in the fiber and subsequently measuring the full cladding-mode spectrum. The modal field distributions for the fundamental mode and a large number of cladding modes are calculated from the measured fiber refractive-index profile. Next, the strength for each cladding mode is evaluated based on an assumed photosensitive profile. Then the calculated cladding-mode structure is compared with the measured one. Last, a new photosensitive profile is assumed based on the resulting discrepancies until a good estimated photosensitive profile is obtained.
The cladding-mode measurement for a fiber that is optimized for nonhydrogenation with a cladding-mode coupling loss of 0.1 dB for a 30-dB Bragg grating is shown in Fig. 2. The 0.12-dB cladding mode coupling loss in a 40-dB grating for another fiber that is optimized for hydrogenation is shown in Fig. 3. This fiber also has been optimized for reduction of splice loss to Corning SMF-28 fiber. With the aid of the fast diffusion constant for the highly doped core and cladding, an average splice loss of 0.03 dB to SMF-28 fiber has been achieved on a conventional splicer with a standard deviation of 0.006 dB, while maintaining the cladding-mode coupling suppression performance. The optimized fiber has a step-index profile with D @ 0.9% and cutoff of approximately 1350 nm.
Many industry analysts have predicted that along with other technologies, fiber Bragg gratings will play an important role in DWDM systems. The relative ease of fabrication, high performance, and potential for lower cost, combined with a method designed to suppress cladding-mode coupling loss, such as the engineered-fiber method described, make fiber Bragg gratings an interesting alternative technology platform. Practical applications could include channel add/drop filters, multiplexer/demultiplexers, gain flattening filters, band splitters, and dispersion compensators. Specialty-fiber research has demonstrated the broad working passband and potential of FBGs that may appear in future generations of products.
LIANG DONG is a senior development scientist, MICHAEL TORO is a product line manager in the PureMode Engineered Fiber group, and VINCENT RICE is a product line manager for gain equalization at Corning Inc., Photonic Technologies Division, One Riverfront Plaza, Corning, NY 14831; e-mail: [email protected], [email protected], and [email protected].