Method and system for obtaining variable optical attenuation with very low polarization dependent loss over an ultra wide dynamic range

#Variable optical attenuators (VOAs) serve two primary purposes in optical transmission systems: first, it is often necessary to reduce the power level of an incoming optical signal to match its intensity to the optimum operating level of a receiver component. Second, an optical attenuator is also an essential element of any equalization component, which aims at adjusting the power in the optical channels to correct signal distortions experienced throughout the transmission. The adjustment of the power level of optical signals should be done with high accuracy, repeatability and reliability. Furthermore, the present communications market requires these devices to be fabricated at low cost. Solid-state devices designed and fabricated using Planar Lightwave Circuits (PLC) technology, with no moving parts, can fulfill these demands.

#A 1×1 Mach-Zehnder interferometer (MZI), one of the most successful and useful structures built in PLC technology, can be employed as a VOA with very wide dynamical attenuation range, from total transparency (except for a small insertion loss) in its passive state, to absolute active suppression (in some cases beyond 30 dB) of the incoming signal. Nevertheless, a VOA based on an MZI configuration (as being used today) usually suffers from a serious drawback, namely its polarization dependence.

#In a simple prior art embodiment, a 1×1 MZI is based on a Y splitter and a Y combiner facing each other along the main propagation axis, and two symmetric arms connecting the inward branches of the splitter and the combiner. The Y-splitter splits the input signal into two symmetric, coherent and in-phase signals, each carrying 50% of the incoming energy. The dynamical performance of the MZI is controlled through active elements located on at least one, and preferably on both of the internal arms. Each active element can change the refractive index of the arm upon which it is located and, therefore, can induce a difference in the optical lengths of the two (otherwise symmetric) arms. Consequently, the active element introduces a controlled phase shift between the coherent signals propagating in the two arms. The combiner gathers the symmetric projection of the two shifted signals at the output of the MZI, and radiates the antisymmetric projection as losses. This yields the required attenuation.

#The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein:

#FIG. 1a shows a simple embodiment of two concatenated 1×1 MZIs according to the present invention;

#FIG. 1b shows a simple embodiment of two concatenated 2×2 MZIs according to the present invention;

#The present invention is of a novel method of operating two concatenated 1×1 MZIs normally connected by a short single-mode waveguide (typically a few millimeters long), as a VOA with ultra-wide attenuation range as well as very low PDL over the entire operational range.

#FIG. 1a shows a simple embodiment of a variable optical attenuator (VOA) 100 that can provide very low PDL and ultra wide dynamic range according to the present invention. VOA 100 includes a first MZI 102 and a second MZI 104 concatenated in tandem. Each MZI includes one splitter and one combiner. MZI 102 includes a splitter 108 and a combiner 110, while MZI 104 includes a splitter 111 and a combiner 112. Hereafter, “splitter” and “combiner” are used interchangeably since their structure is normally identical. Each splitter has a pair of symmetrical branches. Thus, splitter 108 has symmetrical branches 116, combiner 110 has symmetrical branches 118, splitter 111 has symmetrical branches 124 and combiner 112 has symmetrical branches 126. In the particular embodiments shown in FIGS. 1a and b, all symmetrical branches are symmetrical relative to a main propagation axis 130 of VOA 100. In general, as detailed below, this condition is relaxed, particularly when the two MZIs are not adjacent to each other.

#First MZI 102 has one input port P1 connected through a single-mode input waveguide 106 to splitter 108, which serves here as an input 3 dB splitter. Combiner 110, which serves here as an output combiner, is aligned with splitter 108 along main propagation axis 130, with their symmetrical branches 116 and 118 facing each other. Combiner 110 is connected through a single-mode output waveguide 132 to an output port Q1 of the MZI. Symmetrical branches 116 and 118 are connected by two well-separated, single-mode waveguide internal arms 134 and 136, which may (but do not have to) be parallel to each other and to main propagation axis 130. An active element is located on each internal arm: an element 138 on arm 134, and an element 140 on arm 136. The active elements are elements that can introduce a change in an optical property of the respective arm. The active elements may be thin film resistors or heating elements, in which case they induce, via the thermo-optical effect, a change in the local index of refraction in the arm. The dynamical performance of MZI 102 can be controlled through either one of the active elements 138 and 140.