<title>Four-channel multimode wavelength-division-demultiplexer (WDM) system based on surface-normal volume holographic gratings and substrate-guided waves</title>

Abstract

We demonstrate a four channel integrated wavelength division multiplexer (WDM) and demultiplexer(WDDM) based on volume holographic gratings and substrate-guided waves at near JR wavelengths. The four operating wavelengths are centered at 750, 780, 810 and 840 nm respectively. The WDM and WDDM are demonstrated using 50/125 multimode fibers. The channel-to-channel crosstalk level is measured to be less than -40 dB. The system insertion losses are -23dB, -21dB, -20dB, -22dB respectively for 750 nm, 780 nm, 810 nm and 840 nm. SYSTEM DESCRIPTIONWavelength division multiplexing (WDM) and demultiplexing (WDDM) techniques are the two key technologies for upgrading optical communication system bandwidth. The use of WDM technologies not only provides high speed optical communication links, but also provide advantages such as higher data rates, format transparency, and self-routing. Over the past twenty years, many kinds of WDDM device technologies have been developed and demonstrated [1][2][3][4]. WDDM devices using dispersive photopolymer or dichromated gelatin (DCG) volume holographic gratings have been recently reported [5][6][7]. In this paper, we report an integrated four-channel multimode fiber compatible WDDM system with four semiconductor lasers operating at 750, 780, 810 and 840 nm, respectively. The device is demonstrated using the combination of graded index (GRIN) lenses, photopolymer based holographic gratings and substrate-guided waves.A four channel WDM/WDDM is designed for multimode fiber transmission systems. Multimode fibers are widely used in short haul optical communications. The schematic of a four channel wavelength division multiplexed and demultiplexed optical transmission system is shown in Fig. 1(a). The four discrete wavelengths are provided by semiconductor laser arrays. The four wavelengths are multiplexed into one 50 xm multimode fiber for transmission. At the receiving end, a wavelength division demultiplexer is used to separate the discrete wavelengths for O/E conversion. The integrated four channel polymer holographic grating based WDDM structure is shown iii Fig. 1(b). At the input end, the surface-normal incoming multiple wavelength light is collimated by a quarter pitch GRIN lens and diffracted by the volume holographic grating into substrate guided waves. The input holographic grating is designed using the phase-matching principle. The volume holographic grating has slanted fringes induced by the refractive index modulation inside the photopolymeric film. Since the input holographic grating is dispersive, the input multiple wavelengths are diffracted into different bouncing angles. The substrate-guided lights are separated spatially as they propagate along the waveguiding plate. The device length should be long enough so that the output spots will not overlap each other [8]. The collimated spot size is determined by the numerical aperture of the fiber and the GRiN lens. At the output end, they are coupled out surface-normally by a holographic grating. The light can be detected with a ph...