Broadband optical couplers based on parallel long-period fiber gratings

Optical couplers for signal distribution, such as fused tapered fiber couplers and waveguide junction couplers, are indispensable components in today’s optical fiber communication systems. For the last few years, we have been working on a new class of optical couplers. Our approach is based on using parallel long-period fiber gratings (LPFGs). An LPFG is a grating formed in a single-mode fiber; it enables light coupling from the guided mode to selected cladding modes at specific resonance wavelengths. In most applications with LPFGs, the light energy coupled to the cladding mode is lost and the LPFG functions merely as a band-rejection filter. Our studies show that the light coupled to the cladding mode can be collected efficiently by using two parallel LPFGs [1],[2]. The outputs from the two gratings are complementary to each other, one showing band-rejection characteristics and the other showing band-pass characteristics. The structure of two parallel LPFGs thus operates as a broadband optical add/drop multiplexer (OADM) [1],[2]. Using two parallel CO2-laser written LPFGs, we have achieved a peak coupling efficiency of ~86% [3], which is the highest value that has been reported so far. We have also extended the idea to three parallel LPFGs and demonstrated a total power transfer efficiency of ~85% at the resonance wavelength [4]. We can configure three parallel LPFGs to form a 1 ´ 3 coupler, a 3 ´ 3 coupler [5], or a six-port OADM [4]. LPFG couplers have emerged as a new class of all-fiber broadband couplers, which have the potential to be further developed into bandpass filters and OADMs (fixed or tunable) for coarse wavelength-division-multiplexing (CWDM) applications.The support by a grant [Project CityU 111907] from the Research Grants Council of the Hong Kong Special Administrative Region, China, is acknowledged.