Design and manufacturing of band-rejection filters based on long period gratings for applications in next generation access networks

Abstract

ABSTRACT An engineering tool for designing LPG-based filters is reported. Band-rejection filters for telecom applications have been designed and manufactured and an automated mass production technology has been developed. The technology utilizes single-shot LPG writing with a double CO 2 laser beam. The paper discusses also the critical process parameters controlled to shape the spectral char acteristics of manufactured filters. Keywords: band rejection filter, long period grating, CO 2 Point-by-point method, LPG, FTTH, GPON 1. INTRODUCTION Fiber-based access networks have become in recent years the dominant technology for building access networks. In particular, all-fiber FTTH (Fiber to the home) networks with GPON protocol see the most growth. Most telecom operators strive to provide in such networks the Triple Play service (Internet-TV-Phone). Typically, in FTTH networks the 1310/1490 nm wavelengths are utilized for delivering the Internet and phone services (via the GPON protocol), whereas the TV service is broadcast on a dedicated 1550 nm wavelength over the same single fiber strand (an approach known as the RF overlay). Since the RF overlay technology is a typical broadcast techno logy, the TV signal at the 1550 nm wavelength reaches all subscribers, including those who have not subscribed to the service, allowing them to have a potentially unauthorized access. To prevent this, telecom operators may use 1550 nm optical blocking filters. Although filter technologies (e.g. TFF - Thin Film Filters) which allow blocking the 1550 nm band are well-known and commercially available for many years, their use involves cost of around 20 USD/subscriber, which is an unacceptable amount in most cases. An interesting and potentially very low cost alternative ar e band-rejection filters based on LPGs (Long Period Gratings). Long period gratings, and thei r filtering applications, are also known for decades, however there are currently no LPG-based filters for telecom applications available in the commercia l market. Design of such filters has to meet a variety of practical and nontrivial requirements and restrictions, such as controllable center wavelength, wide stop band, low attenuation in other spectrum bands (in the 1260-1610 nm region), and consequently, elimination of the parasitic resonances with other than the desired cladding mode. Equally important is sufficient stability of spectral characteristics in the presence of mechanical stress and temperature changes. At the same time, a cost-effective and repetitive technology is required for commercial applications. The curre nt paper presents work which resulted in designing GPON-compatible blocking filters and developing technology for their mass-production. 2. PRINCIPLE OF OPERATION The basic principle of LPG operation is to couple the fundamental core mode to co-propagating cladding modes supported by the cladding-air waveguide structure. Since the cladding modes are co-propagating, there is no reflected light, which could destabilize the laser operation in the transmitter. The energy transferred to the cladding modes typically leaks after a few centimeters. The grating is obtained by introducing periodic variations in the refractive index along the fiber. In order to achieve energy coupling it is necessary to meet the condition described by the Bragg equation (1):