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Abstract
We show how a spatial mode can be extracted from a light beam, leaving the other orthogonal modes undisturbed, and allowing a new signal to be retransmitted on that mode. The method is self-aligning, avoids fundamental splitting losses, and uses only local feedback loops on controllable beam splitters and phase shifters. It could be implemented with Mach-Zehnder interferometers in planar optics. The method can be extended to multiple simultaneous mode extractions. As a spatial reconfigurable optical add-drop multiplexer, it is hitless, allowing reconfiguration without interrupting the transmission of any channel.
Highlights
In optical fiber telecommunications, the ability to drop and add a single wavelength channel without having to convert all the channels in and out of electronics has been very useful; reconfigurable optical add-drop multiplexers (ROADMs) have allowed convenient expansion of systems, adding channels and reconfiguring networks as needed [1]
We have shown here how to make an add-drop multiplexer for arbitrary spatial modes
A spatial reconfigurable add-drop multiplexers (SRADMs) device of this type could be implemented using any of several different approaches to planar optical circuits, including thermo-optic silicon-based technologies [37], for example, as long as appropriate mostly-transparent detectors are available
Summary
The ability to drop and add a single wavelength channel without having to convert all the channels in and out of electronics has been very useful; reconfigurable optical add-drop multiplexers (ROADMs) have allowed convenient expansion of systems, adding channels and reconfiguring networks as needed [1]. There has been growing interest in exploiting spatial modes in fibers [2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21] (see especially [2] for a recent review) and in free-space communications [22], especially with multiple overlapping modes and including angular momentum beams [22,23,24] It is possible with MIMO techniques to separate signals on different spatial modes if the mode can be spatially sampled onto different coherent detectors (e.g., [4]), but such an approach requires significant digital signal processing and does not allow one signal to be dropped or added without effectively measuring all the spatial channels. At least for modes that do not couple during propagation, for short links of low dispersion, or for modes or mode groups with low differential group delay, this kind of approach may allow arbitrary add and drop of different spatial optical channels without the necessity of MIMO calculations
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