Experimental analysis of a discrete add-drop multiplexer

Add-drop multiplexers allow the extraction of one or more channels from a Wavelength Division Multiplexing (W.D.M.) system. This device also allows the introduction of one or more channels simultaneously. In this article, we have constructed and analyzed experimentally a discrete add-drop multiplexer which will allow the extraction/insertion of a single channel. This add-drop multiplexer is constructed by using two directional couplers and a fiber Bragg grating. INTRODUCTION A Wavelength Division Multiplexing (W.D.M.) System is composed of several channels operating at distinct wavelengths. The usage of several channels operating at several wavelengths allow the transmission of information at rates of about 1 Tbit/s (10 bits/s). This is useful for transmitting information between two large cities, but would be excessive for communication between small cities. Imagine if, between these two large cities, there is repeater station at a small city. This city could use the transmission link between these large cities, but would not use all the channels of this W.D.M. system. In this case, one might use an add-drop multiplexer, because this device allows the insertion/extraction of one or more channels without disturbing the other ones. There are many possible configurations of add-drop multiplexers. The most simple configuration is the discrete add-drop multiplexer, which is composed of two directional couplers and a fiber Bragg coupler [1]. This configuration will be described in this work. Another configuration uses a directional coupler with a uniform add-drop multiplexer in each arm of a directional coupler [2]. This configuration was constructed in a 3 dB coupler and was one of the first being implemented [3]. Another possibility is to use a grating frustrated coupler, initially proposed by Archambault et al. [4]. In this coupler, the grating is written in the fiber core of a 100% coupler. The channel to be extracted looks at an unbalanced coupler and therefore, the coupling does not occur and the signal flows through the main guide. A third configuration is based upom two asymmetrical guides and there is only one grating at the secondary arm. The Bragg grating executes the phase matching between the different propagation constants in the asymmetrical guides [5]. Also, a Mach-Zehnder interferometer can be used to construct an add-drop multiplexer, as described by Park et al. [6] The previous add-drop multiplexers are constructed in singlemode fibers. There is another scheme which uses an interferometric multimode coupler [7]. This multiplexer has four ports (two input and output ports) connected to a multimode guide. In this multimode waveguide, a fiber Bragg grating is written to allow the insertion or extraction of W.D.M. channels. Most add-drop multiplexers use a fiber Bragg grating. A fiber Bragg grating is a periodic structure with low index contrast that is written in a fiber. This device generally reflects a narrowband range of wavelengths (about 1 nm) , while the rest of the signals passes through the device without being considerably reflected. The fiber Bragg garting is written by passing a ultraviolet light in the region of 244 nm to 248 nm. EXPERIMENTAL RESULTS According to the Bragg law, the uniform fiber Bragg grating will reflect light at one wavelength, called the Bragg wavelength, which is given by, ANNALS OF OPTICS XXV ENFMC 2002 121 λB=2n, (1) where n is the effective index of the fiber and Λ is the spatial period of the grating. The grating is assumed to be uniform, with a sinusoidal modulation of the refractive index. The setup of our discrete add-drop multiplexer is illustrated in Figure 1. As mentioned before, it is composed of two 3 dB directional couplers and a fiber Bragg grating. The multi-wavelength signal enters the Input terminal of coupler 1 and is equally divided into the two output terminals of this coupler. Part of the signal goes to the fiber Bragg grating, while the other part goes to the matching layer and is completely absorbed. When the signal reaches the fiber Bragg grating, most of the channels will continue to the input terminal of coupler 2, but one channel will be reflected back to coupler 1 by the fiber Bragg grating. This reflected channel returns to the Drop terminal and back to the Input terminal. The rest of the channels will go to coupler 2 and will be directed to the output and to another terminal which is matched with oil. Figure 1: Schematic diagram of the discrete add-drop multiplexer. The fiber Bragg grating strongly reflects a single wavelength (the Bragg wavelength), which in our case, is close to 1310 nm. This can be observed in Figure 2, which shows the reflectivity spectrum of the fiber Bragg grating used in this experiment. The peak reflectivity of this grating is about 1.0 and its linewidth is about 0.9 nm. This grating was written in a photosensitive fiber. In order to test the capacity of extracting a signal, the light source was modulated by a square wave signal. The signal observed at the drop terminal is illustrated in Figure 3. It is observed that the signal at the drop terminal is weaker than the modulated signal (as expected because only a fraction of the LED signal is reflected by the grating) , but resembles the original square wave modulating the LED. The LED is modulated in amplitude by changing adequately its current. Therefore, it was observed that this multiplexer really extracted a certain portion of the original signal, as was predicted in this paper. Figure 2: Reflectivity spectrum of the fiber Bragg grating. LED (input) Fiber Bragg Gratting (λ1) 1