Abstract
Variable optical attenuation (VOA) for three-mode fiber is experimentally presented, utilizing an amplitude spatial light modulator (SLM), achieving up to -28dB uniform attenuation for all modes. Using the ability to spatially vary the attenuation distribution with the SLM, we also achieve up to 10dB differential attenuation between the fiber's two supported mode group (LP₀₁ and LP₁₁). The spatially selective attenuation serves as the basis of a dynamic mode-group equalizer (DME), potentially gain-balancing mode dependent optical amplification. We extend the experimental three mode DME functionality with a performance analysis of a fiber supporting 6 spatial modes in four mode groups. The spatial modes' distribution and overlap limit the available dynamic range and performance of the DME in the higher mode count case.
Highlights
Mode division multiplexing (MDM) has attracted great attention in the last few years as a viable solution for increasing the information capacity of optical fibers [1]
A commercial, single mode optimized optical vector analyzer (OVA) is used in conjunction with our own free-space mode excitation multiplexer and demultiplexer based on spatial phase masks for mode conversion, with individual paths encoded by unique propagation delays and power equalized with conventional SMF-Variable optical attenuation (VOA) (Fig. 2(b))
The VOA average insertion loss (IL) is −4.3 dB, of which 2 dB are assigned to the liquid crystal on silicon (LCoS) spatial light modulator (SLM)
Summary
Mode division multiplexing (MDM) has attracted great attention in the last few years as a viable solution for increasing the information capacity of optical fibers [1]. In this work we present an adaptive attenuator that can attenuate uniformly all the fiber propagating modes, serving as a Variable Optical Attenuator (VOA), as well as selective attenuation by mode groups, realizing a Dynamic Mode group Equalizer (DME). Our FMF-VOA eliminates the modal dependence by employing an attenuation mechanism based on liquid-crystal polarization rotation, which can attenuate the reflectivity uniformly and not introduce any spatial offsets which give rise to mode mixing and MDL. Special erbiumdoping radial concentrations and pumping schemes are being employed in attempt to minimize DMG, with recent publications demonstrating 4-5 dB DMG [7,8,9,10] This DMG can be dynamically compensated with a mode-dependent loss mechanism. The study indicates that even when attempting to use more complicated attenuation patterns, high values of mode-selective attenuation cannot be obtained with a spatial masking function, and will impact the other propagating modes with undesired attenuation and mixing
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