Miri Blau

Switching solutions for WDM-SDM optical networks

Over the last few decades, network traffic has consistently grown at an exponential rate and was efficiently satisfied using WDM and more efficient coding schemes requiring coherent detection. There is no indication that the network traffic growth trend will cease anytime soon, and we are nearing the day when the capacity of the ubiquitous single-mode fiber will be fully exploited. Space-domain multiplexing (SDM) for high-capacity transmission is the promising solution with the scaling potential to meet future capacity demands. However, there is still a large technological gap between current WDM optical communication system designs and SDM network implementations. In this article we lay the foundation of switching node designs for future WDM-SDM optical networks.

Wavelength-selective switch with direct few mode fiber integration

The first realization of a wavelength-selective switch (WSS) with direct integration of few mode fibers (FMF) is fully described. The free-space optics FMF-WSS dynamically steers spectral information-bearing beams containing three spatial modes from an input port to one of nine output ports using a phase spatial light modulator. Sources of mode dependent losses (MDL) are identified, analytically analyzed and experimentally confirmed on account of different modal sensitivities to fiber coupling in imperfect imaging and at spectral channel edges due to mode clipping. These performance impacting effects can be reduced by adhering to provided design guidelines, which scale in support of higher spatial mode counts. The effect on data transmission of cascaded passband filtering and MDL build-up is experimentally investigated in detail.

Optimization of Spatial Aperture-Sampled Mode Multiplexer for a Three-Mode Fiber

An optimization procedure for spatial mode multiplexing from individual single-mode fibers into a three-mode fiber based on a spatial aperture sampling concept has been developed. By placing space-variant imaging elements between the single-mode and few-mode fibers, each beam aperture can be shaped for lower loss coupling and low mode-dependent losses. The optimization achieves a record theoretical -1.5-dB insertion loss, improving on the previous theoretical -2-dB record.

Spatial Aperture-Sampled Mode Multiplexer Extended to Higher Mode Count Fibers

Efficient mode division multiplexing using the spatial aperture sampled concept from single-modefibers to a few-mode fiber is extended to circular step index fibers supporting up to ten spatial modes and to annular refractive index profile fibers supporting nine optical orbital angular momentum modes (mode counts double when considering polarization states for each case). Each sampling beam aperture is spatially shaped for lower average coupling losses and mode dependent losses or a balance thereof. The optimization demonstrates the scalability and consistent low losses for the aperture-sampled mode multiplexer for increasing mode counts, as opposed to the phase hologram-based mode conversion technique. The aperture-sampling approach is also found to be robust to small input fiber alignment errors and fiber geometrical distortions.

Wavelength-selective switches with few-mode fiber integration

Mode division multiplexed (MDM) transmission is prone to mix, and necessitates that all spatial channels on a carrier wavelength remain intact, so that the information set can be unraveled at the receiver following multi-channel coherent detection and digital signal processing. This requirement is extended to the network switching gear, which should ‘jointly switch’ all modes. Wavelength-selective switches supporting joint switching are reviewed, and the interplay between mode shapes, spectral resolution, mode mixing, and channel passband are detailed.

Mode-group mixing device via complex phase masks printed on fiber tip

We introduce a novel, low loss and compact mode scrambler, printed directly on fiber-connector. The device is optimized to flatten inter-mode-group mixing level over all mode-groups at minimum loss for strong mixing in FMF links.

Wavelength demultiplexer operating over mode division multiplexed signals on ribbon fiber

We introduce a wavelength demultiplexer operating with multimode waveguides of rectangular geometry matching ribbon fibers. This eliminates the spatial-spectral mixing at output waveguide coupling. Performance is stretched between the need to separate wavelengths and modes.

Wavelength demultiplexer operating over mode division multiplexed signals on rectangular core fiber

We introduce a wavelength demultiplexer operating with multimode waveguides of rectangular geometry matching rectangular core fibers. We deal with the spatial-spectral mixing at output waveguide coupling, and offer low mode dependent loss (MDL) solutions. The design challenge the need to separate wavelengths and modes.

Induced mode mixing in SDM optical communications for reduced computation complexity

As widely discussed in literature, space domain multiplexing (SDM) signal evolution in fiber transmission is determined by the coupling mechanism between the guided modes. In this paper we demonstrate the advantages of distributed mode mixing in SDM fiber, and introduce a novel, low loss and compact mode scrambler for efficient strong mixing. The mode scrambler is optimized to have minimum loss and maximum inter-mode crosstalk by spatial phase elements, and designed to be printed directly on fiber tip.

Efficient spatial aperture-sampled mode multiplexer for ring fibers

Efficient space division multiplexing from single mode fibers to a few mode fiber is extended to specialty refractive index ring profile fibers. Optimizing the beam apertures achieves lower average coupling loss and mode dependent losses.